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ISBN 978-952-62-1722-2 (Paperback)ISBN 978-952-62-1723-9 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

OULU 2017

C 632

Marja-Liisa Kärkkäinen

DEACTIVATION OF OXIDATION CATALYSTS BY SULPHUR AND PHOSPHORUS IN DIESEL AND GAS DRIVEN VEHICLES

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF TECHNOLOGY

C 632

AC

TAM

arja-Liisa Kärkkäinen

C632etukansi.kesken.fm Page 1 Monday, October 30, 2017 11:54 AM

ACTA UNIVERS ITAT I S OULUENS I SC Te c h n i c a 6 3 2

MARJA-LIISA KÄRKKÄINEN

DEACTIVATION OF OXIDATION CATALYSTS BY SULPHUR AND PHOSPHORUS IN DIESEL ANDGAS DRIVEN VEHICLES

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in Kuusamonsali (YB210), Linnanmaa, on 8December 2017, at 12 noon

UNIVERSITY OF OULU, OULU 2017

Copyright © 2017Acta Univ. Oul. C 632, 2017

Supervised byProfessor Riitta KeiskiDocent Mika Huuhtanen

Reviewed byProfessor Louise OlssonProfessor Mika Suvanto

ISBN 978-952-62-1722-2 (Paperback)ISBN 978-952-62-1723-9 (PDF)

ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2017

OpponentProfessor Lars J. Pettersson

Kärkkäinen, Marja-Liisa, Deactivation of oxidation catalysts by sulphur andphosphorus in diesel and gas driven vehicles. University of Oulu Graduate School; University of Oulu, Faculty of TechnologyActa Univ. Oul. C 632, 2017University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

The combustion of fuels in motor vehicles is one of the most significant causes of air emissions.The use of oxidation catalysts in exhaust gas emission treatment can reduce hydrocarbons (HCs)and carbon monoxide (CO) emissions by more than 90%. Fuels and engine lubricants containimpurities like sulphur (S) and phosphorus (P), which can have a significant effect on the activityand durability of oxidation catalysts.

This thesis aims at increasing the current knowledge of the deactivation phenomena caused bysulphur and phosphorus in diesel and natural/bio gas oxidation catalysts. Accelerated laboratoryscale sulphur, phosphorus and thermal treatments in gas-phase conditions were carried out foralumina (Al2O3) based platinum (Pt) and platinum-palladium (PtPd) metallic monolith diesel andnatural gas oxidation catalysts. In addition, a vehicle-aged natural gas oxidation catalyst and anengine-bench-aged diesel oxidation catalyst were studied and used as a reference for thelaboratory-scale-aged catalysts. BET-BJH, FESEM, TEM, XPS and DRIFT were used ascharacterization techniques to determine changes on the catalysts. The effect of accelerateddeactivation treatments on the catalyst activity was determined using laboratory scalemeasurements in CO, HC and nitric oxide (NO) oxidation.

Sulphur and phosphorus were found to cause morphological and chemical changes on thestudied catalysts. Sulphur was found to be adsorbed vertically throughout the entire catalystsupport from the catalyst surface to the metallic monolith, while phosphorus accumulated on thesurface region of the precious metal containing catalysts. Both, sulphur and phosphorus, slightlyincreased the average size of the precious metal particles size and are adsorbed onto the aluminaby chemical bonds. In addition, a partial transformation from PdO to Pd and a change in the shapeof the precious metal particles due to phosphorus were detected. Due to the detected structural andchemical changes on the catalysts, sulphur and phosphorus treatments reduced the catalyticactivity of the studied diesel and natural-gas-oxidation catalysts. Correspondence between realand simulated ageing was found and thus the used accelerated laboratory scale aging method canbe stated to be a good tool to simulate sulphur and phosphorus exposure.

Keywords: deactivation, diesel oxidation catalysts, methane, natural gas oxidationcatalyst, palladium, phosphorus, platinum, poisoning, sulphur

Kärkkäinen, Marja-Liisa, Diesel- ja kaasuajoneuvoissa käytettävien hapetus-katalyyttien deaktivoituminen rikin ja fosforin vaikutuksesta. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Teknillinen tiedekuntaActa Univ. Oul. C 632, 2017Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Moottoriajoneuvot ovat merkittäviä ilmapäästöjen aiheuttajia. Hapetuskatalyyttejä käyttämällähiilimonoksidi- ja hiilivetypäästöistä pystytään poistamaan yli 90 %. Polttoaineet ja voiteluai-neet sisältävät epäpuhtauksia kuten rikkiä ja fosforia, jotka voivat merkittävästi heikentää hape-tuskatalyyttien aktiivisuutta ja kestävyyttä.

Väitöskirjan tavoitteena on tuottaa uutta tietoa rikin ja fosforin aiheuttamasta diesel- ja maa-kaasukatalyyttien deaktivoitumisesta. Metalliseen monoliittiin tuettuja alumiinioksidipohjaisiaplatina- ja palladiumkatalyytteja ikäytetiin tekemällä niille rikki-, fosfori- ja lämpökäsittelyjä.Maantieikäytettyä maakaasuhapetuskatalyyttiä ja moottoripenkki-ikäytettyä dieselhapetuskata-lyyttiä käytettiin laboratorioikäytettyjen katalyyttien referensseinä. Ikäytyskäsittelyjen aiheutta-mat muutokset analysoitiin BET-BJH-, FESEM-, TEM-, XPS- ja DRIFT-menetelmillä. Käsitte-lyjen vaikutus katalyyttien hiilimonoksidin, hiilivetyjen ja typenoksidien hapetusaktiivisuuteentutkittiin laboratoriomittakaavan aktiivisuuslaitteella.

Rikki ja fosfori aiheuttivat rakenteellisia ja kemiallisia muutoksia tutkittuihin katalyytteihin.Rikki adsorboitui koko tukiaineeseen (tukiaineen pinnalta pohjalle), kun taas fosfori adsorboituivain pinnan alueelle. Sekä rikki että fosfori kasvattivat jalometallipartikkeleiden kokoa sekämuodostivat alumiinioksidin kanssa yhdisteitä. Lisäksi fosforikäsittelyjen havaittiin osittain pel-kistävän PdO:n Pd:ksi ja muuttavan jalometallipartikkelien muotoa. Havaitut rikin ja fosforinaiheuttamat rakenteelliset sekä kemialliset muutokset laskivat diesel- ja maakaasukatalyyttienhapetusaktiivisuutta. Laboratorioikäytyksillä havaittiin olevan hyvä korrelaatio todellisissa olo-suhteissa tehtyjen ikäytysten kanssa ja tästä syystä työssä käytetyn laboratoriomittakaavan ikäy-tysmenetelmän voidaan todeta olevan hyvä työkalu simuloimaan rikin ja fosforin aiheuttamaadeaktivoitumista.

Asiasanat: deaktivoituminen, dieselhapetuskatalyytti, fosfori, maakaasuhapetus-katalyytti, metaani, myrkyttyminen, palladium, platina, rikki

To my beloveds

8

9

Acknowledgements

This doctoral thesis was conducted in the Environmental and Chemical

Engineering (ECE) research unit of the Faculty of Technology at the University o

f Oulu, Finland. I wish to express my appreciation to all the people who have taken

part of the research behind this study and people who have supported me during

this work.

First and foremost, I would like to express the sincerest gratitude to my

principal supervisor Prof. Riitta Keiski for giving me the opportunity to carry out

the thesis work in her research unit. I highly appreciate her expert knowledge on

catalysis, and her solid support and encouragement have been very valuable to me

during the years. I also want to extend my warmest gratitude to my supervisor Doc.

Mika Huuhtanen for all advices and support during the years, and especially very

valuable comments throughout the writing process. Thanks are addressed to Doc.

Satu Ojala, who gave me a hint of the open position in Prof. Riitta Keiski’s research

unit and supported me during the thesis work. I would like to acknowledge also Dr.

Satu Pitkäaho for valuables advices during these years, and Dr. Piia Juholin, Dr.

Tiina Pääkkönen and Mrs. Auli Turkki for helping with all the practical issues at

the final stage of this thesis.

Warm compliments to Prof. Louise Olsson from Chalmers University of

Technology and Prof. Mika Suvanto from University of Eastern Finland for their

efforts in pre-examination and very valuable comments given regarding this thesis.

I am grateful to Prof. Marja Lajunen for acting as a chair of my follow-up group as

well as the members of the follow-up group: Dr. Katariina Rahkamaa-Tolonen and

Dr. Niina Halonen. Thanks are due to Mr. Gary Attwood and Mr. Tomi Louhelainen

for editing the language of my thesis and articles.

The study reported here has been carried out in collaboration with Tampere

University of Technology, Aalto University and Dinex Ecocat Ltd. Warm

compliments goes to the group members and co-authors Doc. Tanja Kolli, Dr. Mari

Honkanen, Dr. Olli Heikkinen, Lic. Sc. Ville Viitanen, Dr. Kauko Kallinen, Dr. Ari

Väliheikki, Mrs. Anna Valtanen, Prof. Minnamari Vippola, Dr. Jouko Lahtinen for

the very fruitful co-operation and valuable comments. Special thanks are addressed

to Doc. Tanja Kolli for her assistance in writing of Papers III and V, Dr. Mari

Honkanen for an exceptionally good collaboration with very fruitful discussions

and to Dr. Kauko Kallinen - your invaluable and honest comments, and advices are

well appreciated.

10

I would like to thank Mrs. Anna Valtanen, Dr. Ari Väliheikki, Mr. Esa

Turpeinen and Mr. Jorma Penttinen for patiently teaching me, giving support and

help with laboratory equipment and many other practical issues. I am very grateful

for all of you! I am also thankful for assistance with analyses of Mr. Jorma

Penttinen and Lic. Sc. Kaisu Ainassaari. Mr. Frazer Cumming and Ms. Nazia

Hussain are acknowledge for helping me experimental work in the laboratory.

For the financial support, thanks are due to the Academy of Finland, which was

the source for ACABIO project funding and Council of Oulu Region from the

European Regional Development Fund via Sulka project. In addition, I am

gratefully acknowledging the Oulu University Scholarship Foundation, Tauno

Tönning Foundation, the Finnish Foundation for Technology Promotion and

University of Oulu Graduate School for supporting my studies via grants.

Before starting my doctoral studies, I had a chance to work at Dinex Ecocat

Ltd, which gave me a solid background for the doctoral studies. In particular, I want

to acknowledge Dr.h.c. Matti Härkönen, Dr. Teuvo Maunula, Lic. Sc. Auli

Savimäki and Dr. Kauko Kallinen. From you I learned so much during those years.

Further, I want to appreciate all current and past members of ECE unit for very

good collaboration and so many nice moments in the coffee room as well as on the

trips. During the years, I have had chance to share the room with many co-workers.

Thanks, Bouchra Darif, Johanna Niemistö, Sanna Antikainen, Piia Juholin, Ritva

Isomäki, Hanna Valkama, Reeta Tolonen and Auli Turkki for cosy conversations

and making working more pleasurable. Special tanks go to Minna Pirilä, Satu Ojala,

Kaisu Ainassaari and Bouchra Darif for your friendship.

Many thanks to my friends and brothers Mika and Tomi and their families for

their support and taking my mind out of the work. I am very grateful my mother

Liisa. She has believed me and has been taking care our children when needed. In

addition, parents-in-law Aune and Jukka have also helped taking care of the

children, so thank you! My warmest expression of gratitude goes to my beloved

husband Keijo. Without your endless support and patient this work wouldn’t have

been possible. Finally, I would like to thank to my children Sanni, Konsta and Otso

for bringing my live so much joy and happiness. My beloveds, I love you so much!

Oulu, October 2017 Marja Kärkkäinen

11

Abbreviations

API American Petroleum Institute

AsB Angular selective backscatter

BET Brunauer-Emmett-Teller method

BJH Barrett-Joyner-Halenda method

CNG Compressed natural gas

DOC Diesel oxidation catalyst

DRIFT Diffuse Reflectance Infrared Fourier Transform

EB Engine-bench

EC European Community

EDS Energy dispersive spectrometer

EEA European Environment Agency

EELS Electron energy loss spectroscopy

EPA Environmental Protection Agency of the United States of America

EU European Union

FESEM Field emission scanning electron microscopy

FTIR Fourier transform infrared spectroscopy

GHG Greenhouse gas

GHSV Gas hourly space velocity [h-1]

HC Hydrocarbon

HPW High phosphorus concentration treatment

ICP-OES Inductively coupled plasma- optical emission spectroscopy

ILSAC International Lubricants Standardization and Approval Committee

LPW Light phosphorus concentration treatment

MCT Mercury Cadmium Telluride detector

NGOC Natural gas oxidation catalyst

PM Particulate matter

PF Particulate filter

RT Room temperature

SAED Selected area electron diffraction

SOF Soluble organic fraction

SW Sulphur dioxide-water treatment

TA Thermal aged

TEM Transmission electron microscopy

T50 Temperature of 50% conversion of the feed component [°C]

T90 Temperature of 90% conversion of the feed component [°C]

12

TUT Tampere University of Technology

TPD Temperature programmed desorption

USA United State of America

VOC Volatile organic compound

W H2O-treatment

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

XRF X-ray fluorescence

ZDDP Zinc dialkyldithiophosphate

Chemical compounds:

Al2O3 Aluminium oxide, alumina

AlPO4 Aluminium phosphate

Al2(SO4)3 Aluminium sulphate

CeO2 Cerium(IV) oxide, ceria

CH4 Methane

C3H6 Propene

CO Carbon monoxide

CO2 Carbon dioxide

H2S Hydrogen sulphide

NO Nitric oxide

NO2 Nitrogen dioxide

NOx Nitrogen oxides

PO3- Phosphite

SiO2 Silicon dioxide, silica

SO2 Sulphur dioxide

SO3 Sulphur trioxide, sulphite

SO42 Sulphate

TiO2 Titanium dioxide, titania

ZrO2 Zirconium dioxide, zirconia

13

Original publications

This thesis is based on the following publications, which are referred to throughout

the text by their Roman numerals:

I Kärkkäinen M, Honkanen M, Viitanen V, Kolli T, Valtanen A, Huuhtanen M, Kallinen K, Vippola M, Lepistö T, Lahtinen J & Keiski RL (2013) Deactivation of diesel oxidation catalysts by sulphur in laboratory and engine-bench scale aging. Topics in Catalysis 56: 672–678.

II Honkanen M, Kärkkäinen M, Kolli T, Heikkinen O, Viitanen V, Zeng L, Jiang H, Kallinen K, Huuhtanen M, Keiski RL, Lahtinen J, Olsson E & Vippola M (2016) Accelerated deactivation studies of the natural-gas oxidation catalyst – Verifying the role of sulfur and elevated temperature in catalyst aging. Applied Catalysis B: Environmental 182: 439–448.

III Kärkkäinen M, Kolli T, Honkanen M, Heikkinen O, Huuhtanen M, Kallinen K, Lepistö T, Lahtinen J, Vippola M & Keiski RL (2015) The effect of phosphorus exposure on diesel oxidation catalysts – Part I: Activity measurements, elementary and surface analyses. Topics in Catalysis 58: 961–970.

IV Honkanen M, Kärkkäinen M, Heikkinen O, Kallinen K, Kolli T, Huuhtanen M, Lahtinen J, Keiski RL, Lepistö T & Vippola M (2015) The effect of phosphorus exposure on diesel oxidation catalysts – Part II: Characterization of structural changes by transmission electron microscopy. Topics in Catalysis 58(14): 971–976.

V Kärkkäinen M, Kolli T, Honkanen M, Heikkinen O, Väliheikki A, Huuhtanen M, Kallinen K, Lahtinen J, Vippola M, & Keiski RL (2016) The influence of phosphorus exposure on a natural-gas-oxidation catalyst. Topics in Catalysis 59: 1044–1048.

Kärkkäinen was the first and corresponding author of Papers I, III and V. In Papers

II and IV, Kärkkäinen’s contribution was in conducting laboratory scale water,

sulphur and phosphorus treatments, activity tests and DRIFT tests as well as in

interpreting the obtained data. She also participated in the planning of the contents

of the publication with the first author and in writing the article.

14

15

Contents

Abstract

Tiivistelmä

Acknowledgements 9 

Abbreviations 11 

Original publications 13 

Contents 15 

1  Introduction 17 

1.1  Objectives and scope ............................................................................... 19 

2  Oxidation catalysis 23 

2.1  Oxidation catalysts .................................................................................. 23 

2.1.1  Diesel vehicle application ............................................................. 24 

2.1.2  Natural gas vehicle applications ................................................... 25 

2.2  Materials in oxidation catalysts ............................................................... 26 

2.2.1  Active compounds ........................................................................ 26 

2.2.2  Support materials .......................................................................... 26 

3  Catalyst deactivation 29 

3.1  General .................................................................................................... 29 

3.1.1  Mechanical failures ...................................................................... 29 

3.1.2  Thermal transformations .............................................................. 30 

3.1.3  Deactivation due to impurities ...................................................... 31 

3.2  Impurities and their effects ...................................................................... 31 

3.2.1  The origin of impurities ................................................................ 31 

3.2.2  Sulphur contamination.................................................................. 33 

3.2.3  Phosphorus contamination ............................................................ 34 

4  Materials and methods 37 

4.1  Materials ................................................................................................. 38 

4.2  Deactivation treatment procedures .......................................................... 38 

4.2.1  Laboratory scale treatments .......................................................... 38 

4.2.2  Vehicle and engine-bench-aged treatments .................................. 40 

4.3  Catalyst characterization ......................................................................... 40 

4.3.1  Bulk analyses ................................................................................ 40 

4.3.2  Physisorption analyses.................................................................. 40 

4.3.3  Electron microscopy, spectroscopy and diffraction

analyses ........................................................................................ 41 

4.3.4  Activity measurements ................................................................. 43 

16

5  Results and discussion 45 

5.1  Characterizations ..................................................................................... 45 

5.1.1  The vehicle-aged oxidation catalyst ............................................. 45 

5.1.2  The fresh catalysts and effect of water ......................................... 47 

5.1.3  The effect of sulphur ..................................................................... 48 

5.1.4  The effect of phosphorus .............................................................. 54 

5.2  Activity measurements ............................................................................ 64 

5.2.1  Fresh and water treated diesel oxidation catalysts ........................ 64 

5.2.2  The effect of sulphur on diesel oxidation catalysts ....................... 67 

5.2.3  The effect of phosphorus on diesel oxidation catalysts ................ 70 

5.2.4  The effect of sulphur and phosphorus on a natural gas

oxidation catalyst .......................................................................... 74 

6  Summary and conclusions 77 

List of references 81 

Original publications 93 

17

1 Introduction

The combustion of fuels in motor vehicles is one of the most significant causes of

air emissions. The emissions caused by the incomplete burning of fuels in engines

have numerous harmful effects both on the environment and on people’s health.

Thus, emission reduction from vehicle engines is a top of priority all over the world.

Oxidation catalysts have been successfully used in the vehicles since the 1970s to

oxidise carbon monoxide (CO) and hydrocarbons (HCs) to release less harmful

carbon dioxide (CO2) and water. In 1980, three-way catalysts were developed and

taken into use in stoichiometric conditions (Farrauto & Hoke 2010, Twigg 2011).

After that, oxidation catalysts have been used in lean burn vehicles such as trucks

in the USA since 1994 and in European diesel light-duty passenger cars and vans

since Euro 2 (1996) (Farrauto & Hoke 2010, Lemon 2014). Emission limits for

passenger and heavy-duty vehicles have been tightened continuously (EC 2007, EC

2009 c, EPA 2007, EU 2017) together with long-term durability requirements (EC

2009 c). Currently, to meet the emission limits set by the Euro VI (7 years or

700 000 km for heavy-duty vehicles), one key part of the diesel exhaust gas after-

treatment system for heavy-duty trucks is the diesel oxidation catalyst (DOC)

(Lemon 2014).

Petrol and diesel vehicles are still the most generally used vehicles, but for

example, the number of natural gas vehicles is constantly increasing. In January

2017, around 23 million natural gas vehicles existed worldwide (NGV Global 2017)

and the forecast is that the number of natural-gas-vehicles (NGVs) will increase to

50 million vehicles by 2020 (Kakaee et al. 2014). Advantages of natural gas

compared to petrol and diesel fuels are NG’s lower particulate and nitrogen oxides

emissions (Gélin et al. 2003, Kakaee et al. 2014, Korakianitis et al. 2011,

Poompipatpong et al. 2011, Papagiannakis et al. 2010). Since natural gas consists

mainly of methane, it also has a lower carbon content (Korakianitis et al. 2011,

Kakaee et al. 2014) resulting in a reduction in CO2 emissions (Papagiannakis et al.

2004, Korakianitis et al. 2011, Anderson 2015). Natural gas can be used in

stoichiometric and lean burn engines (Korakianitis et al. 2011, Kakaee et al. 2014).

Natural gas lean burn engines are used mainly in heavy-duty vehicles such as buses

and trucks (Klingstedt et al. 2001). In diesel engines, diesel fuel can be replaced

totally by compressed natural gas (CNG) by carrying out engine modifications. The

other possibility is to replace the diesel fuel partly by methane (dual-fuel

combustion). It is possible to use a dual-fuel system without any modification to

diesel engines and CNG is used as the main fuel and the role of the diesel (biodiesel

18

or other high cetane number fuels can also be used) is to start the combustion and

ignite the flame of the CNG in the combustion chamber. (Kakaee et al. 2014,

Korakianitis et al. 2011).

The reduction of greenhouse gases (GHGs), e.g. carbon dioxide (CO2) and

methane (CH4), has been regulated globally in the Kyoto Protocol in 1997

(UNFCCC 1998) and currently strengthened further in the Paris agreement in 2015

(UNFCCC 2015). In addition, the EU has the short-term goal to reduce greenhouse

gases by 20% from 1990 levels by the year 2020 (EC 2009 a) and a further target

to reduce greenhouse gas emissions by 80% from the 1990 levels by the year 2050

(EEA 2016). The main strategies for reducing GHG emissions of traffic are the

development of more fuel-efficient engines, the increased use of the plug-in hybrids

and electric vehicles, as well as an increase in the use of biofuels (EEA 2016,

REN21). Biofuels mean liquid or gaseous fuels for transport produced from

biomass. For example, the European Union’s target for the usage of the share of

biofuels in the transportation sector is 10% by 2020 (EC 2009 b).

Biodiesel is considered to be the a mostly carbon neutral fuel and it is usually

produced by catalytic transesterification of natural oils, such as soybean oil, palm

oil and rape-seed oil with low molecular weight alcohols, such as methanol and

ethanol (Lee & Shah 2013, He 2016). Biodiesel is used in several countries like

Brazil, Argentina, the United States, Germany and France, and can be used in pure

form or can be blended with petroleum diesel in different proportions (Amais et al.

2015, Lee & Shah 2013). The disadvantage in the use of edible oils is their high

price and competition over food consumption. Therefore, further research has been

focused on the development of biodiesel manufacturing using inexpensive waste

and non-edible oils (Baskar & Aiswarya 2016, Bhuiya et.al 2016). Biodiesel’s fuel

properties vary depending on the source of feedstocks and have a remarkable effect

on engine performance and emissions. In general, biodiesel reduces CO2, PM, CO,

and HCs emissions, but increases NOx emissions slightly compared to petroleum

diesel fuel (Baskar & Aiswarya 2016, Lee & Shah 2013, Suh et al. 2016).

Biomethane/biogas is a biofuel, which can be manufactured locally from

organic waste like the by-products from the anaerobic treatment of wastewaters,

organic fractions of municipal solid wastes, livestock residues or organic agro

industrial wastes (Munoz et al. 2015, Anderson 2015). Current biomethane

production is concentrated primarily in Europe (REN21). The challenge in the use

of biomethane as a transportation fuel is in the required standard to purify the fuel

in a cost effective and environmental friendly way (Munoz et al. 2015, Korakianitis

et al. 2011). As biomethane production increases, it could be blended more with

19

natural gas, and finally and it could even totally substitute natural gas (Anderson

2015).

The role of oxidation catalysts differs in diesel and natural gas vehicles. The

main role of the diesel oxidation catalyst (DOC) is to oxidise CO and unburned

HCs and the soluble organic fraction (SOF) of diesel particulates, but often the

effective oxidation of nitrogen oxide (NO) to nitrogen dioxide (NO2) is also

desirable (Russell & Epling 2011, Kröcher et al. 2009). In natural gas vehicles, the

main challenge for natural gas oxidation catalysts (NGOC) is effective oxidation

of unburned methane (CH4). CH4 is more difficult to oxidize compared to other

hydrocarbons and therefore a higher amount of precious metals and higher

temperatures are needed (Gélin & Primet 2002, Barbier & Duprez 2014).

Fuels and engine lubricants contain impurity and additive compounds like

phosphorus (P), calcium (Ca), zinc (Zn), magnesium (Mg) and sulphur (S). These

compounds are catalyst poisons and they can have a significant effect on the

activity and the durability of a catalytic converter (Winkler et al. 2009, Galisteo et

al. 2004 and Bartholomew 2001). Sulphur and phosphorus compounds are typically

attributed as the main reasons for the exhaust gas catalyst’s chemical deactivation

(Rokosz et al. 2001). Both diesel and natural gas fuels contain sulphur. In addition,

biogas can contain sulphur. Over the years, it has been possible to reduce the

sulphur content in diesel fuel significantly. For example, ultra-low sulphur diesel

(CS 10 ppm) is already supplied in all European countries, the USA, Canada,

China and Russia (EC 2003, Dieselnet 2017). On the other hand, in many countries

in South America, Africa and Asia, high sulphur content diesel (CS 350 ppm) is

still available (Global Fuels Specifications 2016, Dieselnet 2017). Phosphorus is a

typical additive in engine lubricant oil, but it also a common impurity in biodiesel

(Johnson & Hils 2013, Lyra et al. 2009, Sarin 2012, Pietikäinen et al. 2015).

The increasing use of various biofuels and blended fuels can increase the

accumulation of impurities on the catalyst surface and thus reduce the long-term

durability remarkably. Therefore, there is a demand to develop catalysts with a high

resistance to impurities as well as to improve their regeneration potentiality. Thus,

it is essential to gain more detailed understanding of the effects of the impurities on

the catalyst performance.

1.1 Objectives and scope

The main goal of this thesis is to increase the knowledge of the deactivation

phenomena caused by sulphur and phosphorus in diesel and bio/natural gas

20

oxidation catalysts. Various characterization techniques such as BET-BJH, FESEM,

TEM, XPS and DRIFT analyses are used to determine the morphological surface

changes and the adsorbed sulphur and phosphorus compounds formed. Based on

the information acquired, similarity and differences in the sulphur and phosphorus

exposure are identified. In addition, the effect of sulphur and phosphorus treatments

on catalytic oxidation activity is studied. The objectives of the thesis can be

summarized as follows:

– To define the structural and chemical changes on the catalyst surface due to

sulphur and phosphorus, separately;

– To illustrate the effect of sulphur and phosphorus, separately, on the activity of

diesel and gas oxidation catalysts;

– To evaluate the effect of precious metal composition on the oxidation activity

of fresh catalysts as well as on sulphur and phosphorus treated DOC/Pt and

DOC/PtPd catalysts;

– To evaluate the correlation between the laboratory-aging and the engine-aging

as reliable ageing methods in catalyst deactivation studies.

Fig. 1. The contributions of the orginal papers in the thesis.

Paper I deals with the sulphur exposure of the diesel oxidation catalysts. In this

paper comparative studies on the characteristic properties and catalytic activity of

sulphur on Pt- and PtPd-based catalysts are presented. The activity tests for fresh,

water and sulphur treated catalysts are performed with a gas mixture without nitric

21

oxide. In addition, a comparison between laboratory and engine-bench-aged Pt-

based diesel oxidation catalysts are presented. In addition, un-published DRIFT and

activity tests results using a gas mixture with NO are presented in the thesis in

Chapters 5.1.3 and 5.2.2.

Paper II covers sulphur exposure and thermal aging of PtPd natural gas

oxidation catalysts in varied combinations. In this study, the focus is to determine

the role of sulphur with and without thermal treatment. The results of a solely

thermal treated sample are used as a reference. The characterization information

from the vehicle-aged PtPd natural gas oxidation catalyst is used as a background

for the treatments done on a laboratory scale. These results are presented in this

thesis in Chapters 5.1.1.

Papers III and IV focus on obtaining knowledge about phosphorus exposure on

Pt- and PtPd-based diesel oxidation catalysts. In both papers, phosphorus treatment

results using two different phosphorus concentrations (high and low) are presented.

Paper III covers the characterization and performance results except cross-sectional

transmission electron microscopy (TEM) characterization results, which are

reported in Paper IV.

Paper V presents the effect of the phosphorus exposure on natural gas oxidation

catalysts using two different phosphorus concentrations (high and low). The

information provided by the characterization and performance results provides new

insights into the phosphorus deactivation phenomena for alumina based PtPd

catalysts. The NGOC support characterizations are presented in Chapter 5.1.4.

22

23

2 Oxidation catalysis

During oxidation, catalysis compounds are oxidized using catalysts. The role of a

catalyst is to reduce the activation energy which is needed for the oxidation reaction.

Consequently, the desired product can be produced faster, using fewer resources,

and generating less waste (Teles et al. 2015, Rothenberg 2010). Oxidation catalysts

are widely used in applications such as exhaust gas purification, partial oxidation

and in the production of industrial inorganic chemicals like sulphuric acid and nitric

acid (Prins et al. 2016). Both, homogeneous and heterogeneous catalysts are used

in oxidation catalysis and the catalyst forms vary a great deal. For example, pellets,

gauzes and monoliths are used (Twigg & Webster 2006).

The most typical reactions in exhaust gas oxidation catalysis are shown in

Equations 1-4. Usually, the CO reaction begins first and subsequently the HC and

nitrogen oxide (NOx) reactions take place (Heck et al. 2002). Oxidation of CO, HC

and the soluble organic fraction SOF to CO2 and H2O can be described as follows

(Hochmuth et al. 2013, Twigg 2011, Heck et al. 2002):

0.5 → (1)

0.25 → 0.5 (2)

→ (3)

The oxidation of NO to nitrogen dioxide (NO2) follows the reaction (Martinez-

Arias et al. 2012, Olsson & Fridell 2002):

0.5 ⇌ (4)

2.1 Oxidation catalysts

Heterogeneous monolith-type oxidation catalysts are used widely in air pollution

control and for emission mitigation purposes in vehicles, as well as in stationary

applications (Twigg & Webster 2006). Oxidation catalysts are mainly used for the

oxidation of hydrocarbons and carbon monoxide in vehicle applications and

oxidation of volatile organic compounds (VOCs) in stationary applications (Barbier

& Dubrez 2014). Two types of monolith substrates are commonly used, i.e. ceramic

and metallic monoliths. Ceramic monolith substrates are made of cordierite, i.e.

2MgO.2Al2O3, and they have a good thermal shock resistance and low coefficient

of thermal expansion (Williams 2001). Even though ceramic monoliths are the most

widely used monolith substrates because of their low price, good corrosive

24

resistance and good catalyst adhesion, a substantial number of metallic substrates

are also used (Gulati et al. 2006, Lemon 2014). Ni and Cr as well as ferritic alloys

containing Al are widely used as metallic substrate materials. The advantages of

metallic substrates are their higher mechanical resistance, thinner walls and higher

thermal conductivity, which allows faster light-off times (Avila et al. 2005, Gulati

et al. 2006, Farrauto 2010). In addition, metallic monolith substrates are widely

used in the oxidation of VOCs, since with metallic framework materials it is

possible to prepare converters on a larger scale and with special monolith shapes

(Twigg & Webster 2006, Avila et al. 2005).

The monolithic framework is coated with a thin layer of catalytic material,

which consists of highly dispersed active metals on a high surface area support and

appropriate promoters (Twigg & Webster 2006). The catalytic materials, which are

used especially in DOC and NGOC catalysts are explained in more detail in Section

2.2.

2.1.1 Diesel vehicle application

Diesel emissions from vehicles contain solid, liquid and gaseous components.

Solids released from diesel engines are basically soot (carbon-rich particles) which

is referred to often as particulate matter (PM). Liquids are generally referred to as

the soluble organic fraction (SOF) and are unburned diesel fuel and engine

lubricants. SOF can also contain sulphates to some extent, which are formed during

combustion from the sulphur compounds present in the diesel fuel. The gaseous

components in diesel exhaust gas include carbon monoxide (CO), hydrocarbons

(HCs) with a carbon number between C2-C15, and nitrogen oxides (NOx).

(Hochmuth et al. 2013, Martinez-Arias et al. 2012, Corro 2002) The composition

of diesel exhaust gas is typically: 200-1000 ppm NOx,10-330 ppm total

hydrocarbons, 150-1200 ppm CO, 5-15 vol% O2, 1-7 vol% H2O, 3-13 vol% CO2,

10-100 ppm sulphur oxides, and 50-400 mgm-3 particulates (Martinez-Arias et al.

2012). Since diesel engines operate with a large excess of air, exhaust gas

purification must be done in lean conditions, not in stoichiometric conditions as it

is possible to do in petrol engine applications. Therefore, the exhaust gas

temperature of the diesel engine is much cooler than that of a stoichiometrically

operated petrol engine (100-500 °C vs. 300-1100 °C) and the exhaust gas

temperature in a diesel engine can be around 120–150 °C in urban driving

conditions. (Hochmuth et al. 2013 and Twigg 2011)

25

Currently, a heavy-duty diesel exhaust gas after-treatment system typically

contains a diesel oxidation catalyst (DOC), a particulate filter (PF) and units for

NOx mitigation. The main goal of the DOCs is to oxidize CO, HCs, and the SOF

of particulates completely (Russell & Epling 2011, Lemon 2014, Resitoglu et al.

2015). The oxidation of NO to NO2 in DOC has also become indispensable. In the

diesel exhaust gas, NO and NO2 are not in thermodynamic equilibrium and the

share of NO2 is only about 5-10%. In diesel particulate filters, NO2 accelerates soot

oxidation at low temperatures (Kröcher et al. 2009, Piumetti et al. 2015 Khazanchi

et al. 2017). Alternative methods for NOx reduction are selective catalytic reduction

(SCR) and NOx storage reduction (NSR). In NOx reduction, the presence of NO2

increases the NOx conversion efficiency in the urea-SCR, making a fast SCR

reaction possible and in the NSR system NO2 improves the ability to store NOx as

nitrates (Twigg 2011, Kröcher et al. 2009, Rahkamaa-Tolonen et al. 2005).

2.1.2 Natural gas vehicle applications

Lean burn natural gas vehicles (NGV) have lower particulate (PM) and NOx

emissions than comparable diesel engines. However, NGVs can emit relatively

high levels of unburned methane (CH4), which is a strong greenhouse gas.

Therefore, natural gas oxidation catalysts (NGOCs) are necessarily needed to

oxidize the unburned CH4. At the same time CO is oxidized to a less harmful

compound, i.e. CO2. (Cho & He 2007, Gélin & Primet et al. 2002, Korakianitis et

al. 2011, Choudhary et al. 2002). Typically, exhausts of lean NGVs have 500–5000

ppm of CH4, a maximum 15 vol% CO2, 10-15 vol% H2O and ppm levels of SOx

and NOx (Gélin & Primet et al. 2002, Zanoletti et al. 2009). CH4 has a higher

stability than the other hydrocarbon molecules and therefore it is the most difficult

hydrocarbon to oxidize (Barbier & Duprez 2014). The exhaust gas temperature of

lean burn natural gas vehicles is relatively low (300–500 °C) and therefore methane

oxidation across a natural gas oxidation catalyst is challenging (Liu et al. 2001,

Abbasi et al. 2012).

26

2.2 Materials in oxidation catalysts

2.2.1 Active compounds

Precious metals (Pt, Pd and Rh) as well as transition metal oxides such as copper

(Cu), chromium (Cr), nickel (Ni), cobalt (Co) and manganese (Mn) are known to

be active in the oxidation of hydrocarbons and carbon monoxide (Neyestanaki et

al. 2004, Farrauto 2010). From these, platinum and palladium are the most used

active compounds in CO and HCs oxidation in light-duty applications (Farrauto

2010, Russell & Epling 2011). For example, Yu Yao (1980) and Gololobov et al.

(2009) have found that Pt has a higher oxidation activity than Pd towards heavier

alkanes C3H8, whereas Pd has higher activity towards the oxidization of alkanes

(CH4 and C2H6), and CO than Pt. In addition, Yu Yao (1980) found that propene

oxidation is much faster than propane oxidation over Pd, and the tendency was the

opposite over Pt. If NOx oxidation activity in the diesel oxidation catalyst is

desirable, Pt is the preferable choice for that purpose (Watanabe et al. 2007,

Wiebenga et al. 2012). Thus, Pt is the most active in diesel applications and Pd is

the most active in methane oxidation in lean conditions. Nevertheless, the higher

thermal stability of DOC in an oxidative atmosphere is possible when PtPd alloys

are used (Kaneeda 2009, Morlang 2005). In methane oxidation, higher resistance

against sulphur, water and Pd sintering can be achieved using a PdPt catalyst

compared to a Pd only catalyst (Narui et al. 1999, Pieck et al. 2002, Persson et al.

2007, Ozawa et al. 2004). It is widely agreed that PdO is the active phase while

metallic palladium is much less active (Farrauto et al. 1995, Burch & Urbano 1995,

Datye et al. 2000, Antony et al. 2013). In proportion, metallic Pt is reported to be

more active than PtO especially in NO oxidation (Hauff et al. 2012).

2.2.2 Support materials

In the oxidation catalyst, the active compounds are typically deposited on a support

(carrier) (Neyestanaki et al. 2004). The support and active compounds form a

coating, generally referred to as the catalysed washcoat, on the monolith substrates

(Farrauto 2010). The support has several roles and it influences the catalyst activity,

selectivity and durability (Heck et al. 2002, Neyestanaki et al. 2004). The main role

of the support is to provide the surface to disperse the catalytic substance to

maximise the catalytic surface area and increase the thermal stability of the active

component. The properties of the support material such as the support surface area,

27

pore volume, surface acidity and reactivity, thermal conductivity, and thermal

expansion, have a strong effect on the performance of the final catalyst. (Heck et

al. 2002, Neyestanaki et al. 2004) The support material is quite often a complex

mixture of different metals or oxides, which all have a separate role in the support

material. The support materials used in CO and HCs oxidation catalysts can consist

of several oxides such as alumina (Al2O3), titania (TiO2), ceria (CeO2), zirconia

(ZrO2), silica (SiO2) and vanadia (V2O5). (Martinez-Arias et al. 2012)

γ-Alumina (γ-Al2O3) is commonly the main component in the support material

because it has a large surface area, good thermal stability and good interaction with

precious metals like Pt (Heck et al. 2002, Kim et al. 2013). γ-Alumina is

manufactured from monohydrate (Boehmite) and, to obtain the γ-alumina phase,

Boehmite needs to be heated to ~500 °C (Hayes & Kolaczkowski 1997, Prins et al.

2016). Concerning alternative materials, silica-based materials have been

investigated to replace γ-alumina. The advantage of silica and SiO2ZrO2 are in their

inertness towards sulphur (e.g. Kim et al. 2016, Väliheikki et al. 2017).

The support material often contains additives such as promoters and stabilizers.

In the γ-alumina based support materials typical additives include metal oxides (e.g.

ceria, zirconia and titania). Ceria can have several roles in the oxidation catalyst.

Ceria can act as a stabilizer inhibiting the phase transition of alumina (Heck et al.

2002) or it can enhance CO oxidation at the metal-support interface (Barbier &

Duprez 2014). Zirconia has been also used to improve the thermal stability of

alumina (Russell & Epling 2011). Typically, ceria-zirconia mixed oxide is used as

an additive in commercial catalysts (Kim et al. 2016). Other typical thermal

stabilizers for alumina are barium oxide (BaO) and lanthanum oxide (La2O3)

(Hayes & Kolaczkowski 1997, Russell & Epling 2011). Titania is a sulphur

resistant material and for that reason it has also been introduced in oxidation

catalysts in vehicle applications (Yang et al. 2017).

28

29

3 Catalyst deactivation

3.1 General

Catalyst deactivation, the loss of catalytic activity and/or selectivity over time, is a

general problem in heterogeneous oxidation catalysis. Typically, the deactivation

of automobile oxidation catalysts occurs slowly (Heck et al. 2002, Chorkendorff &

Niementsverdriet 2007). The deactivation process is physical or chemical in its

nature, but quite often it can be classified as chemical, thermal, or mechanical

deactivation. In practice, more than one process or type of deactivation are often

present at the same time (Neyestanaki et al. 2004, Bartholomew 2001, Argyle &

Bartholomew et al. 2015, Moulijn et al. 2001, Richardson 1989). In addition,

deactivation can be classified according to its mechanism. There are several

different classifications for this, but Moulijn et al. (2001) and Murzin (2013) have

grouped the main mechanisms in heterogenous catalysis as follows: poisoning,

fouling, thermal degradation, mechanical damage, and attrition. Thermal

degradation, mechanical damage and attraction are typically irreversible, but

poisoning and fouling are at least partly reversible (Moulijn et. al. 2001). The

principles of various types of mechanical failures, thermal transformations, and

deactivation due to impurities are discussed in more detail in the following chapters.

3.1.1 Mechanical failures

Catalyst deactivation by mechanical damage or attrition can inhibit catalyst activity

in several ways. First, mechanical damage, e.g. monolith failure, blocks the pores

and inhibits or even prevents catalyst functionality. In automobile applications,

monolith failure can be caused by a sudden mechanical impact or thermal shock.

(Bartholomew 2001, Moulijn et al. 2001) Secondly, high gas flow velocity and

rapid temperature changes (thermal stress) can cause attrition, leading, for example,

to the loss of the support and precious metals. This can happen in particular when

a metal substrate is used, since the thermal expansion difference between the

support material and substrate can cause the loss of support material. (Heck et al.

2002, Moulijn et al. 2001, Argyle & Bartholomew et al. 2015)

30

3.1.2 Thermal transformations

Thermally induced deactivation is a physical process, which can be caused by

volatilization, sintering, or phase transformation. Due to elevated temperatures, the

active component can react with the surrounded medium and form volatile

compounds. (Moulijn et al. 2001) Sintering can cause the loss of the active material

surface due to particle growth of the catalytic phase (catalyst sintering) and/or the

loss of the support area due to collapse of the support (support sintering). The

sintering process takes place at elevated temperatures and is accelerated in

hydrothermal conditions. (Bartholomew 2001, Argyle & Bartholomew et al. 2015)

Sintering is a strongly temperature dependent phenomenon and the underlying

mechanism is surface diffusion or mobility of large aggregates (crystal growth).

Hüttig and Tamman temperatures can be used to estimate the temperature at which

sintering could occur. At the Hüttig temperature (THüttig= 0.3Tmelting) atoms at

defects become mobile indicating surface diffusion whereas at the Tamman

temperature (TTamman= 0.5Tmelting) atoms of the bulk exhibit mobile indicating

crystal growth. (Moulijn et al. 2001) In practice, the real temperature where a solid

becomes mobile depends on several factors such as texture, particle size and the

morphology of the material. Generally, highly porous materials are more sensitive

to sintering than less porous materials and small particles are more sensitive to

sintering than large particles. In addition, structural promoters (or impurities) may

inhibit the surface migration of atoms of active particles (Moulijn et al. 2001).

In the case of active material sintering, it is generally proposed that metal

particle growth can happen by crystallite migration and due to Ostwald ripening

(atom migration) (Forzatti et al. 1999, Hansen et al. 2013). In crystallite migration

crystallites migrate along the support surface, and the collision and coalescence of

two crystallites forms a large particle. In Ostwald ripening metal atoms escape from

one crystallite and migrate along the support and are captured by larger crystallites

(Hansen et al. 2013, Neyestanaki et al. 2004, Sehested et al. 2004, Forzatti et al.

1999).

Support sintering can be accompanied by phase transformations. A good

example of this is Al2O3, where γ-alumina is gradually transformed via δ-alumina

and θ-alumina to α-alumina at high temperatures (over 1000°C) resulting in a loss

of surface area (Honkanen et al. 2017, Neyestanaki et al. 2004). The normal

operating temperature in diesel and lean-natural-gas engines are moderate

compared to petrol engines and thus thermal deactivation in normal operation

conditions (100-500 °C) is not a crucial problem for DOC or NGOC (Heck et al.

31

2002). Nevertheless, the repeated misfiring or particulate filter regeneration in the

case of a diesel engine can lead to sudden temperature increases and unexpectedly

high values (up to 900 °C) in the oxidation catalyst (Cavataio et al. 2009,

Chorkendorff & Niementsverdriet 2007).

3.1.3 Deactivation due to impurities

Selective or non-selective poisoning can cause a loss of catalyst activity and

selectivity caused by the adsorption of impurities (Heck et al. 2002). Selective

poisoning (poisoning) is a typical form of chemical deactivation and arises from

the chemisorption of species on catalytic sites inhibiting or altering the sites for

catalytic reactions (Hayes & Kolaczkowski 1997, Bartholomew 2001). Usually,

chemisorption is strong and causes a permanent loss of the active site (irreversible

poisoning). In some cases, it is possible that the poison is desorbed over time once

the impurity in the feed is reduced (reversible poisoning) (Hayes & Kolaczkowski

1997).

Non-selective poisoning (fouling) arises as a result of physical adsorption of

species/impurities on catalytic sites blocking access to active sites (Hayes &

Kolaczkowski 1997, Heck et al. 2002). Fouling is classified as mechanical

deactivation (Richardson 1989, Bartholomew 2001, Argyle & Bartholomew et al.

2015). The most common example of fouling is carbon deposition on the catalyst

surface (coking), but it is a relevant mechanism also in the case of the impurities

like Mg, Ca and Zn, S and P (Bartholomew 2001, Winkler et al. 2009).

In summary, impurities such as sulphur and phosphorus can cause both

poisoning and fouling. Fouling can be irreversible or reversible, like poisoning.

Typically, carbon fouling is reversible, since carbon deposition can be burned off

the catalyst surface in oxygen rich conditions at elevated temperatures.

(Bartholomew 2001, Hayes & Kolaczkowski 1997, Moulijn et al. 2001)

3.2 Impurities and their effects

3.2.1 The origin of impurities

A catalyst’s impurities originate from engine lubricant additives and fuels. Engine

lubricants contain several types of oil additives, which aim to improve the

performance and extend the lifetime of the lubricant (Finnish National Board of

32

Education 2017). Detergents are surface-active materials and are used in engine

lubricants to keep engine parts clean. Typically, detergents contain alkaline earth

metals like calcium (Ca), sodium (Na) and magnesium (Mg). To prevent foaming

of engine lubricants silicon containing anti-foaming additives are primarily used.

(Finnish National Board of Education 2017, Winkler et al. 2010) Antiwear

resistance is a critical feature in engine lubricants. Zinc dialkyldithiophosphate

(ZDDP) has been used as an antiwear resistance additive from the mid-1940s and

it is still commonly used, because of ZDDP’s very good antiwear features and low

price (Faring & Jao 2017, Johnson & Hils 2013, Spikes 2004). ZDDP is a metal

containing phosphate ester, which contains Zn, P and S (Johnson & Hils 2013). The

phosphorus concentration in engine lubricants differs from 600 ppm to 2000 ppm.

The International Lubricants Standardization and Approval Committee (ILSAC) is

the organization responsible for setting the lubricant specifications for passenger

cars and the current standard GF-5 (introduced in 2010) sets the phosphorus

concentration between 600 ppm minimum and 800 ppm maximum for passenger

car engine lubricants (Faring & Jao 2017, ILSAC 2017). Generally, the phosphorus

concentration limits in different standards are higher for heavy duty engine oils

(Faring & Jao 2017). The sulphur concentration in engine oils can be up to ~ 4000

ppm (Wiebenga et al. 2012), but engine oils with very low sulphur concentrations

(~ 15 ppm) are also available (API 2017). Sulphur concentration limits for engine

oils have been set, for example, by the American Petroleum Institute (API) (API

2017).

The use of ultra-low sulphur diesel is spreading all over the world, so sulphur’s

significance as a catalyst impurity in diesel vehicles will diminish gradually

(Global Fuels Specifications 2016). Natural gas fuel always contains sulphur, even

if the SOx content is very low (ppm level) (Kakaee et al. 2014, Gélin & Primet

2002). Feedstocks for biogas (sewage sludge, livestock manure or agro-industrial

biowaste) can contain inorganic and organic sulphur, which is converted into

dissolved sulfides and are transferred to biogas mainly in the form of hydrogen

sulphide (H2S) (Arrhenius et al. 2017, Munoz et al. 2015). The European standard

on automotive fuel specifications for natural and biomethane gas specifies 20 mgm-

3 without odorization and 30 mgm-3 with odorization as maximum limits for sulphur

concentrations; even though the goal should be 10 mgm-3 with odorization (EN

2017). Currently, the quality that the gas industry can provide is 30 mgm-3 with

odorization and therefore that value is within the standard (EN 2017).

In biodiesel, phosphorus, calcium and magnesium are typical impurities,

whereas the sulphur content is typically very low (Amais et al. 2015, Lyra et al.

33

2009). Phosphorus originates from the feedstocks used in biodiesel production,

such as phospholipids in vegetable oils or animal fats or alternatively from the

inorganic salts in used frying oil (Lyra et al. 2009, Sarin 2012). For example, low-

quality, untreated rapeseed oil can contain very high amounts of phosphorus (~350

mgkg-1) (Pietikäinen et al. 2015). Currently the maximum permissible

concentration of phosphorus set by biodiesel standards in Europe is 4 mgkg-1 (EN

2012) and in the USA 10 mgkg-1 (Amais et al. 2015).

3.2.2 Sulphur contamination

Typically, sulphur is in the exhaust in the form of SO2. In oxygen rich conditions

and in the presence of precious metal containing catalysts, SO2 is further oxidized

to SO3 at temperatures above 300 °C (Neyestanaki et al. 2004, Corro 2002).

Sulphur compounds are bound tightly on the active sites of the catalyst forming

stable precious metal surface sulphates (NMSO4, in which NM is the precious

metal). The formed sulphates inhibit reactants from adsorbing on the active sites

(Neyestanaki et al. 2004). Even very small amounts of sulphur reduce the methane

oxidation activity over Pd-based catalysts. A low sulphur concentration also

inhibits ethane, propane, and CO oxidations but to a lesser extent than the oxidation

of methane (Gélin & Primet 2002). In methane oxidation, one way to prevent

adsorption of sulphur on the Pd surface species is to use a bimetallic PtPd catalyst.

The role of metallic Pt0 surface species is to adsorb sulphur compounds and

preventing the adsorption of sulphur on the Pd surface species leaves the Pd surface

available for methane oxidation (Corro et al. 2010).

The support materials can be sulphur sensitive like alumina (Al2O3) and

sulphur resistant like silica (SiO2). It is generally agreed that SO2 inhibition of Pd

has a different mechanism on sulphur sensitive and sulphur resistant support

materials (Lampert et al. 1997, Colussi et al. 2010, Konsolakis et al. 2013, and

Mowery & McCormick 2001). In both cases, Pd first oxidizes SO2 to SO3. With a

sulphur sensitive support, SO3 is adsorbed on both Pd and the support material, but

with a sulphur resistant support, SO3 is adsorbed only on Pd. Therefore, the sulphur

sensitive support acts like a sink and prevents the rapid deactivation of Pd. Recently,

Hamzehlouyan et al. (2016) have studied adsorption and desorption of SO2 over γ-

alumina and Pt/γ-alumina. Based on DRIFT and temperature programmed

desorption (TPD) data, they suggest the following reaction steps for the SO2

adsorption and desorption on/from γ-alumina: 1) SO2 is molecularly adsorbed on

the Al sites; 2) SO2 adsorbs onto coordinated oxygen atoms on the alumina; 3) the

34

adsorbed SO2 forms surface sulphites/sulphates; and 4) the formation of bulk

aluminium sulphites occurs. In addition, they found that Pt has an increasing effect

on the sulphate formation and the migration of the surface sulphates to the bulk

alumina. The migration of sulphates from the bulk back to the alumina surface was

found to happen at lower temperatures in the case of Pt supported alumina

compared to bare alumina (Hamzehlouyan et al. 2016). Nevertheless, Pt only or Pt-

rich catalysts are found to be less effective in sulphate formation than the Pd only

or Pd-rich catalysts (Wilburn & Epling 2017). Konsolakis et al. (2013) reported

that the acidity of Pd/γ- alumina catalyst changes during SO2-treatment. When fresh

Pd/γ-alumina catalysts have only Lewis acid sites, SO2-treated catalysts also have

a significant amount of Brönsted acid sites on the Pd/γ-alumina support. This in

particular can have a diminishing effect on the methane oxidation activity (Wischert

et al. 2011).

The presence of water alone has been reported to have a decreasing effect

especially on the methane conversion by forming Pd(OH)2 (Burch & Urbano 1995,

Roth et al. 2000). In addition, Mowery & McCormick et al. (2001) have shown

that the joint effect of water and sulphur increases the formation of PdSO4, since

the spill over of SO2 from PdO to the alumina is inhibited. Sulphur poisoning is

typically reversible and SOx from the catalyst surface can desorbed already at

temperatures below 400 °C (Kröcher et al. 2009). The decomposition of more

stable aluminium sulphate at lean conditions (excess of O2) takes place at 700-

750 °C (Arosio et al. 2006, Kröcher et al. 2009).

Regeneration can be achieved using short reductive pulses (e.g., CH4) and, for

example, Arosio et al. (2006) have reported that complete regeneration is already

possible at a temperature of 600 °C. During regeneration, sulphur can quickly and

directly be desorbed from the Pd surface with a sulphur resistant support. With a

sulphur sensitive support, regeneration is much more difficult because of the SO3

spill over from the support on the Pd surface during the regeneration (Lampert et

al. 1997). According to Kinnunen et al. (2017), a sulphur poisoned catalyst can be

regenerated by decomposing PdSO4 to PdSO3 followed by the release of oxygen

and SO2. As a result of the decomposition of PdSO4, metallic Pd is formed, which

is an inactive phase for methane combustion (Kinnunen et al. 2017).

3.2.3 Phosphorus contamination

In phosphorus contamination, a phosphate-containing layer can be formed on the

support (fouling), which blocks the support pores and covers the precious metals

35

(Matam et al. 2012, Mowery et al. 1999, Winkler et al. 2009). In addition,

phosphate compounds can form bonds and compounds directly with the support

elements (poisoning) (Matam et al. 2012). With an alumina support, phosphate

typically forms aluminium phosphate (AlPO4) compounds (Galisteo et al. 2004,

Kröger 2007, Rokosz 2001, Matam et al. 2013), which can modify the chemical

properties of the surface as well as the electronic properties of the Pt particles

(Ramis et al. 1989). Phosphorus has been observed to also form compounds with

Ce, Ze and Mg forming CePO4, Zn2P2O7 and Mg3(PO4)2, respectively (Larese et al.

2003, López et al. 2005, Winkler et al. 2009). It is possible that a glassy phosphorus

containing layer, which blocks active sites, is formed on the catalyst surface

(Klimczak et al. 2010) Phosphorus has been mostly detected in the front part of

vehicle-aged DOC and NGOC catalysts (Angove & Cant 2000, Lanzerath et al.

2009, Mowery et al. 1999, Wiebenga et al. 2012, Winkler et al. 2009).

Regeneration of a catalyst that has been contaminated with phosphorus is much

more difficult than that of a sulphur contaminated catalyst, since phosphorus cannot

desorb or can be evacuated through heating. However, Rokoz et al. (2001) and

Christou & Efstathiou (2007), for example, have found phosphorus compound

removal by oxalic acid to be a good method for phosphorus regeneration.

36

37

4 Materials and methods

Individual impurities (sulphur and phosphorus) were studied using a laboratory

scale deactivation procedure. Bench- and engine-aged catalysts were used as

references for the laboratory scale-aged catalysts. This work was done in the project

funded by the Academy of Finland, and in close co-operation with three universities

and one industrial partner, Dinex Ecocat Ltd. Figure 2 illustrates the tasks and the

roles of the partners in this work. Dinex Ecocat Ltd provided the fresh catalyst

materials. The laboratory scale deactivation treatments used in this study were done

at the University of Oulu (UOulu). X-ray fluorescence (XRF) and Inductively

Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) analyses were done as

purchased services. The other characterizations and analyses were done in co-

operation with UOulu, Tampere University of Technology (TUT) and Aalto

University (Aalto).

Fig. 2. Tasks and roles of the partners in this work. University of Oulu (UOulu), Tampere

University of Technology (TUT), Aalto University (Aalto).

38

4.1 Materials

The catalysts used in this study were three precious metal loaded catalysts mounted

on a metallic foil (monolith type catalysts) (Figure 3), two being diesel oxidation

catalysts (marked as DOC/PtPd and DOC/Pt) and one a natural gas oxidation

catalyst (marked as NGOC/PtPd). DOC/PtPd and DOC/Pt catalysts were used in

Papers I, III and IV and NGOC/PtPd catalysts were used in Papers II and V. In

addition, the DOC support material without precious metals was used to indicate

the roles of Pd and Pt for phosphorus exposure (Paper III). Detailed information

about the used catalysts is presented in Table 1.

Table 1. Composition of catalysts.

Catalyst Support material Active materials Active material content

of catalyst (gdm-3)

DOC support γ-alumina with additives (zeolites

and e.g. Ce, Zr and Ti oxides)

- -

DOC/Pt see above Pt 1.8

DOC/PtPd see above Pt:Pd (4:1) 1.8

NGOC support γ-alumina - -

NGOC/PtPd see above Pt:Pd (1:4) 8.8

Fig. 3. Picture of a catalyst sample.

4.2 Deactivation treatment procedures

4.2.1 Laboratory scale treatments

Gas phase sulphur, phosphorus and water treatments of the studied catalysts were

performed at UOulu using a laboratory scale tubular quartz reactor. The diameter

of the sample monolithic catalyst was 1.0 cm and the length was 3.8 cm. The

conditions of the laboratory scale treatments using water (W), sulphur dioxide-

water (SW), low phosphorus-water (LPW), high phosphorus-water (HPW) and

39

thermal aged (TA) are shown in Table 2. All the treatments were completed at

400 °C for 5 h with the total gas flow of 1 dm3. The gas hourly space velocity

(GHSV) was 20 000 h-1 during the treatments. Water and SO2-treatments (Papers I

and II) were performed using the setup shown in Figure 4. 0.065 M and 0.13 M

aqueous P-solutions or pure water were added through a peristaltic pump into the

gas feed before the tube furnace, and the solution was evaporated in a furnace

before entering the catalyst bed (monolith). The gaseous compounds were

controlled by mass flow controllers (Brooks 5280S). Water and P-treatments

(Papers III-V) were done with a similar setup but the furnace, reactor pipe and the

sample inside were aligned vertically. In addition, the thermal treatments were done

at TUT for the NGOC/Pt catalyst (Paper II).

Table 2. Laboratory scale treatments and corresponding markings used.

Marking Treatment

W 10 vol% H2O, 10 vol% air and N2 (bal.) at 400 °C 5h

SW 100 ppm SO2, 10 vol% H2O, 10 vol% air and N2 (bal.) at 400 °C 5h

LPW 10 vol% 0.065 M (NH4)2HPO4 (aq.), 10 vol% air and N2 (bal.) at 400 °C 5h

HPW 10 vol% 0.13 M (NH4)2HPO4 (aq.), 10 vol% air and N2 (bal.) at 400 °C 5h

TA 100 vol% synthetic air (80 % N2 and 20 % O2) at 1000 °C 5h

TA+SW TA-treatment followed SW-treatment

Fig. 4. Schematic of the SO2 treatment equipment.

Control unit

Peristaltic pump

Solution

Control unit

Furnace

Quartz reactor

Temperature measurement

Mass flow controllers

Gas containers

Flow direction

40

4.2.2 Vehicle and engine-bench-aged treatments

The long-term field ageing for the NGOC/PtPd catalyst was done in a lean-burn

natural gas heavy duty vehicle for 160 000 km. The engine-bench-aging for the

DOC/Pt catalyst was performed with a diesel passenger car engine for ~30 h using

a diesel fuel with a high sulphur content (CS ≥ 350 ppm). For the engine-bench-

aged catalyst the accumulated soot was burned in air for 4 h at 600 °C before

activity experiments and analyses. The vehicle and engine-bench-aged samples

were provided by Dinex Ecocat Ltd.

4.3 Catalyst characterization

4.3.1 Bulk analyses

Inductively coupled plasma optical emission spectroscopy (ICP-OES)

The phosphorus content analysis in bulk was done on an ICP-OES device (Thermo

Fisher Scientific, iCAP 6500 Duo) (Papers III, IV). The catalyst powders from two

monoliths were diluted with a solution of HNO3 and HCl and then digested using

microwaves. After digestion, the samples were diluted with water and analysed.

The ICP-OES analyses were done as a purchased service by Suomen

Ympäristöpalvelu.

X-ray photoelectron fluorescence (XRF)

The sulphur content of sulphur treated catalyst samples was analysed by a Philips

MagiX apparatus containing Super Q Analytical software (Paper I). The amount of

catalyst in the analyses was ~30 mg. XRF analyses were done as a purchased

service by Kemiantutkimuspalvelut Oy.

4.3.2 Physisorption analyses

To determine the effect of deactivation treatments on the specific surface area, pore

size and pore volume, physisorption analyses were carried out using a Micrometrics

ASAP 2020 device (Papers I-III and V). Specific surface areas (m2g-1) were

measured from the N2 adsorption isotherms at -196 °C according to the BET

41

(Brunauer-Emmett-Teller) method. The pore size and pore volume distributions of

catalysts were calculated from N2 desorption isotherms using the BJH (Barrett-

Joyner-Halenda) method. The measurements were performed using catalyst

powders, so before the analyses the catalysts were scraped from the metal foil.

4.3.3 Electron microscopy, spectroscopy and diffraction analyses

Field-emission scanning electron microscopy (FESEM)

The microstructures of the fresh and treated monolith catalysts were studied with

an FESEM (Zeiss ULTRA plus) equipped with an energy dispersive spectrometer

(EDS, INCA Energy 350 with INCAx-act silicon-drift detector (SDD), Oxford

Instruments) using cross-sectional FESEM samples (Papers II, III and V). A

conventional metallographic sample preparation technique (including the moulding

of a catalyst monolith in a resin, grinding and polishing) was used in the sample

preparation. Finally, the sample was coated by carbon to avoid sample charging in

the FESEM studies. The images were taken with an angular selective backscatter

(AsB) detector to maximize the Z-contrast.

Transmission electron microscopy (TEM)

TEM was used to characterize the microstructure and composition of the samples

as well as to determine structural changes in the support and the precious metal. A

TEM (Jeol JEM-2010) equipped with an energy dispersive X-ray spectrometer

(EDS, Noran Vantage with Si(Li) detector Thermo Scientific) was used for imaging

and elemental analysis. In addition, a high resolution TEM (HRTEM, FEI Titan 80-

300) was used for high-resolution imaging, electron energy loss spectroscopy

(EELS), and elemental analysis. In Paper I, the TEM samples used were catalyst

powders scraped from the metallic foil. First, the powder was crushed between

glass slides and then the crushed powder was dispersed with ethanol onto a carbon-

covered Cu grid. Cross-sectional samples were used in Papers II, IV and V. The

sample preparation was started so that two small pieces of the catalyst monolith

were attached to catalyst layers face-to-face to a titanium grid by carbon glue. The

grid was pre-thinned by hand to a thickness of ~100 μm and then thinning was

continued with a dimple grinder (Model 656, Gatan Inc.) to the thickness of ~20

42

µm. The final thinning was done with a precision ion polishing system (PIPS,

Model 691, Gatan Inc.).

X-ray photoelectron spectroscopy (XPS)

Chemical states and atomic concentrations of phosphorus and noble metals were

characterized with an XPS (Surface Science Laboratories SSX-100) using

monochromatic Al Kα radiation (Papers I-III and V). Samples from the fresh

catalyst and the inlet and outlet sections of the treated catalysts were measured. The

samples were made by scraping the catalyst surface and pressing the scraped

powder against indium films. This resulted in samples where the charging was

small. The binding energy was calibrated by setting carbon 1s line at 284.6 eV.

During the quantitative analysis, Shirley background subtraction was applied

(Shirley 1972, Aronniemi et al. 2005). The contribution from carbon and indium

was omitted in the compositional analysis.

X-ray diffraction (XRD)

XRD was used for identifying the phase composition of fresh and treated catalysts.

In the XRD analyses scraped catalyst powders were also used. The XRD patterns

were recorded with a Siemens D5000 diffractometer, and nickel-filtered Cu Kα

radiation was applied (Papers III and V). The patterns were acquired in the angle

diffraction range of 15° ≤ 2θ ≤ 90°, with a 5s count accumulation per step.

Diffraction patterns were assigned using a PDF database supplied by the

International Centre of Diffraction Data. For the XRD analysis, the phosphorus

exposure to the support material NGOC was applied using 0.2 M phosphorus

feeding in order to obtain better detectability. In addition, PANanalytical Empyrean

diffractometer using Cu Kα radiation with the PIXcel3D detector was used in Paper

II. Crystallite sizes were determined from the XRD patterns using the HighScore

plus software based on the Scherrer equation (shape factor 0.9). Diffraction patterns

were assigned using the database (PDF-4 + 2014) from the International Centre of

Diffraction Data.

Diffuse reflectance infrared Fourier transform (DRIFT)

An FTIR spectrometer (Bruker Vertex V80) equipped with a DRIFT unit and liquid

nitrogen-cooled Mercury Cadmium Telluride (MCT) detector were utilized to

43

determine the bonding of the compounds on the scraped catalyst powder (Papers II,

III and V). The DRIFT analyses were performed at room temperature under

atmospheric conditions. A mirror was used as a background spectrum. The spectra

were recorded by using the resolution of 4 cm-1.

4.3.4 Activity measurements

Activity tests were performed on a laboratory scale using gas mixtures which

simulate the exhaust gases of diesel (Papers I and III) and natural gas (Papers II, V)

vehicles. In addition, two different gas mixtures, with and without nitric oxide (NO)

were employed for the diesel oxidation catalysts. The used gas mixtures are

presented in Figure 5. The tests were carried out for the fresh and treated metallic

monolith catalysts at atmospheric pressure in a horizontally aligned tubular quartz

reactor (i.d. 0.8 cm). In the diesel activity tests, the gas mixture was inserted into

the reactor at room temperature (RT), while in the natural gas activity tests, the

mixture was added at 100 °C. Water feeding was started for all the experiments at

110 °C using a peristaltic pump. The sample was heated at a heating rate of 5 °Cmin-

1 for the DOC and 10 °C min-1 for the NGOC up to the desired temperature (300 °C

and 600 °C for the DOC and the NGOC, respectively). The catalyst was kept at

300 °C and 600 °C for 15 min for the DOC and NGOC, respectively, and after that

the temperature was reduced to RT under an N2 flow. The procedure was repeated

to determine the stability of the catalyst during the test and to ensure the reliability

of the results. Gas flow rates were controlled with mass flow controllers (Brooks

5280S). The outlet gas composition was measured by an FTIR gas analyser

(GasmetTM CR2000). The oxygen concentration was determined with a

paramagnetic oxygen analyser (ABB Advanced Optima). The total gas flow was 1

dm3, resulting in a gas hourly space velocity (GHSV) of 31 000 h-1 for a monolith.

44

Fig. 5. The gas compositions and conditions in the activity tests.

DIESELRT‐300 °C5 °C /min

NATURAL GASRT‐600 °C10 °C /min

Without NO:500 ppm CO, 300 ppm C3H6, 12 vol% O2 and 10 vol% H2O, and 

N2 as the balance gas (Paper I)

With NO:1000 ppm NO, 500 ppm CO, 300 ppm C3H6, 12 vol% O2 and 

10 vol% H2O, and N2 as the balance gas (Paper III)

600 ppm CH4, 500 ppm CO, 10 vol% CO2, 12 vol% O2 and 10 vol% H2O and N2 as the balance gas (Papers II and V)

45

5 Results and discussion

5.1 Characterizations

5.1.1 The vehicle-aged oxidation catalyst

Aging in a real application using an actual size catalyst provides realistic

information about the overall deactivating effect caused by oil- and fuel-derived

contaminants and high operating temperatures. Therefore, a vehicle-aged natural

gas oxidation catalyst (NGOC/PtPd) was studied (Honkanen et al. 2013) and used

as a reference for the laboratory-scale-ageing.

It was found that the vehicle-aged NGOC/PtPd catalyst had been exposed to

thermal deactivation together with poisoning and fouling. Information on the

accumulation of sulphur, phosphorus, calcium and zinc on the support was

collected using FESEM-EDS. In the inlet part of the catalyst (Figure 6a), the

surface was totally covered by a Ca- and S-rich layer detected by FESEM-EDS.

Phosphorus had also penetrated the top layer of the catalyst. The following

contaminants were found in the inlet part of the vehicle-aged catalyst with XPS

measurements: calcium (~19 wt%), sulphur (12 wt%), phosphorus (8 wt%) and

zinc (5 wt%). In the outlet part the amount of these impurities was significantly

lower: calcium (~0.8 wt%), sulphur (1.3 wt%), phosphorus (0.2 wt%) and zinc (0.1

wt%) (Honkanen et al. 2013). The FESEM-EDS line analysis (Figures 6b-c) shows

that sulphur accumulated throughout the support (from the surface to the bottom),

but phosphorus was located mainly in the region of 10 μm from the surface. Similar

behaviour as with phosphorus was observed with calcium, but zinc accumulated

mainly on the surface, at a penetration depth of ~5 μm. In Figure 6b-c, it can also

be seen that the sulphur concentration (from the 10 μm region) increased while the

concentration of phosphorus decreased.

46

Fig. 6. Cross-sectional FESEM AsB-image, and sulphur and phosphorus contents over

vehicle-aged NGOC/PtPd catalysts: a) cross-sectional image, b) sulphur content, and c)

phosphorus content.

Binding energies were measured by XPS to determine the chemical compositions

of the elements. S 2p was found with a binding energy value of ~168.9 eV (Figure

7a). This suggests that sulphur appeared on the vehicle-aged catalyst or the support

surface mainly in the form of sulphates (SO42-) (Moulder et al. 1992). P 2s was

observed with a binding energy of ~190.4 eV (Figure 7b) indicating phosphorus

mainly in the form of phosphate (PO43-) (Naumkin et al. 2012). The results are in

good agreement with the results by Galisteo et al. (2004), Wiebenga et al. (2012)

and Andersson et al. (2007).

47

Fig. 7. XPS spectra and binding energies a) S 2p, b) P 2s for the inlet and outlet parts of

the vehicle-aged NGOC/PtPd catalyst.

5.1.2 The fresh catalysts and effect of water

Structures of the fresh diesel and natural gas oxidation catalysts were studied by

cross-sectional transmission electron microscopy (TEM). The fresh diesel

oxidation catalysts’ cross-sectional TEM characterization is reported in Paper IV.

The fresh DOC/PtPd had very small-grained (< 5 nm) Pt/Pd particles supported on

a fine grained γ-alumina, titania, ceria and zirconia containing support material.

Furthermore, monocrystalline alumina-rich and amorphous silica-rich areas

without Pt or Pd particles were found. The fresh DOC/Pt consisted of small (5-10

nm) Pt particles supported on fine grained γ-alumina and titania containing areas

(Paper IV). The fresh NGOC/PtPd catalyst had a small-grained γ-alumina support

with well distributed, small-grained (< 5 nm) Pt and Pd particles (Paper II).

The chemical composition of elements of the fresh diesel and gas oxidation

catalysts was studied by XPS (Papers I and II). According to the binding energies

of Pt (Table 3), platinum was found in a metallic form (Pt) in the fresh DOC/Pt

samples (Moulder et al. 1992, Serrano-Ruiz et al. 2006, Olsson & Fridell 2002,

Talo et al. 1995). In the case of fresh DOC/PtPd and NGOC/PtPd, platinum and

palladium were found in oxide form (PtO and PdO, respectively), which is in

agreement with results by Brun et al. (1999) and Shyu & Otto (1988).

160 165 170 175 180

inlet

Inte

nsi

ty (

a.u

.)

Binding energy (eV)

outlet

168.9

168.7

a)S (2p)

180 185 190 195 200

inlet

Inte

nsi

ty (

a.u

.)

Binding energy (eV)

190.4

outlet

b)P (2s)

48

Table 3. Binding energies (eV) measured by XPS over fresh, H2O (W-) and SO2+H2O (SW-)

treated DOC/PtPd, DOC/Pt and NGOC/PtPd catalysts. In addition, results of engine-

bench-aged (EB-aged) DOC/Pt catalyst as well as thermal aged (TA-) and TA-treatment

followed by SW-treatment (TA+SW) NGOC/PtPd catalyst (Modified from Paper I).

Catalyst Treatment Pd

3d5/2

Pt

4d5/2

S

2p3/2

Al

1s

O

1s

DOC/PtPd Fresh 336.4 315.5 - 119.2 531.2 W 337.0 315.4 - 119.4 531.3 SW 337.2 315.5 169.1 119.2 531.5

DOC/Pt Fresh - 314.8 - 119.7 531.4

W - 314.1 - 119.3 531.0

SW - 314.7 169.7 119.5 531.8

EB - 314.4 169.2 118.9 531.0

NGOC/PtPd Fresh 336.3 315.3 - 118.3 530.2

W n.d n.d n.d n.d n.d

SW 336.8 315.5 169.5 119.3 531.2

TA 340 - - 119.0 530.9

TA+SW 340 - 169.1 119.0 530.9

n.d.= not determined

The laboratory scale sulphur and phosphorus treatments were carried out in the

presence of water. In order to determine the role of water during the sulphur and

phosphorus treatments, water treatments of DOC/PtPd, DOC/Pt and NGOC/PtPd

catalysts were conducted (Paper I and Paper V). It was observed that the water

treatment did not have any significant influence on the SBET value, total pore

volume or average pore size of the studied catalyst (Table 4). This suggests that

hydrothermal sintering did not take place. Based on the XPS results, the water

treatment did not cause any significant changes in the oxidation stage of platinum

or palladium on the DOC catalyst (Table 3). For the water treated NGOC catalyst

the XPS measurement was not done.

5.1.3 The effect of sulphur

According to the XRF results, the sulphur content in the SW-treated DOC/PtPd and

DOC/Pt catalysts was determined to be 1.9 and 1.8 wt%, respectively (Table 4,

Paper I). The XPS analyses results support the XRF results although the sulphur

concentrations were systematically higher due to differences in the surface versus

bulk analyses. The concentration of sulphur on the SW-treated NGOC/PtPd catalyst

was detected by XPS (Table 4, Paper II) to be ~ 4 wt% and a little bit less (~3.5

49

wt%) on the TA+SW-treated NGOC/PtPd catalyst. Based on the XPS results, the

catalyst surface adsorbed quite the same amount of sulphur over all the studied

catalysts. According to the physisorption results (Table 4, Papers I and II), the SW-

treatment reduced the specific surface area (SBET) and the pore volume of the

DOC/PtPd and DOC/Pt catalysts. The SBET values of the DOC/PtPd and DOC/Pt

catalysts decreased by 13% and 15%, respectively. The SBET value of the SW-

treated NGOC/PtPd decreased only slightly (5%) and no changes in pore volume

were detected. The TA-treatment of the NGOC/PtPd catalyst caused a significant

change to the catalyst support structure reducing the specific surface area by ~40%

and increasing the average pore size by ~40% compared to the fresh NGOC/PtPd

catalyst (Table 4, Paper II). The SBET, total pore volume and average pore size

values were identical between the TA and TA+SW-treated NGOC/PtPd catalyst, so

the SW-treatment was not observed to have any decreasing effect on these values

after thermal treatment.

Table 4. Sulphur contents (wt%) measured by XRF and XPS, specific surface area (SBET),

total pore volume and average pore size values of fresh, H2O (W-) and SO2+H2O (SW-)

treated DOC/PtPd, DOC/Pt and NGOC/PtPd catalysts. In addition, the results are

presented for the engine-bench-aged (EB-aged) DOC/Pt catalyst as well as thermal aged

(TA-) and TA-treatment followed by SW-treatment (TA+SW) NGOC/PtPt catalysts

(Modified from Paper I and II).

Catalyst Treatment S

XRF

S

average XPS

Specific

surface area

Total pore

volume

Average pore

size

(wt%) (wt%) (m2g-1) (cm3g-1) (nm)

DOC/PtPd Fresh - - 222 0.38 7.0 W - - 223 0.39 7.1 SW 1.9 4.6 194 0.31 6.5

DOC/Pt Fresh - - 225 0.41 7.5

W - - 218 0.40 7.3

SW 1.8 4.0 192 0.36 7.6

EB 1.0 2.3 218 0.38 7.1

NGOC/PtPd Fresh - - 154 0.40 10.0

W - - 160 0.39 10.0

SW n.d. 3.8 147 0.40 10.0

TA n.d. - 96 0.34 14

TA+SW n.d. 3.3 96 0.33 14

n.d. = not determined

50

Figure 8 presents the TEM images of the fresh and SW-treated DOC/PtPd catalysts.

No significant changes were observed in the alumina-based support, or the sizes of

precious metal particles (palladium or platinum). More detailed FESEM-EDS and

TEM-EDS analyses showed that in the SW-treated NGOC/PtPd catalyst, sulphur

accumulated uniformly throughout the whole catalyst structure horizontally from

the inlet part to the outlet part and vertically from catalyst surface to the metallic

monolith (Paper II). In addition, electron energy loss spectroscopy (EELS) maps

indicated that sulphur accumulated mainly close to the precious metal particles

(Paper II).

A slight increase in the precious metal particles was detected by TEM as well

as in the PdO crystal size (from 3 to 6 nm) detected by XRD for the NGOC/PtPd

catalyst and these are noted in (Paper II). The amount of platinum in the fresh and

SW-treated samples were, however, too small to be detected by XRD. Lee et al.

(1999) found that SO2 induced sintering of Pt/Al2O3 particles. They explained that

a reaction of SO2 with oxygen at the Pt-O-support interface reduces the Pt-O

interaction, which promotes the migration and sintering of Pt particles. Similar

phenomena could also take place with Pd particles. Significant morphological

changes due to the TA-treatment were observed when compared to the fresh and

SW-treated NGOC/PtPd catalysts. The phase transformation of alumina (from γ-

alumina to α-alumina) and formation of the PdPt alloy was detected by XRD and

TEM and significant increases in the precious metal particle size (from ~5 nm to

50-60 nm) were observed by FESEM and TEM (Paper II). According to the XRD,

FESEM and TEM studies, no further morphological changes were detected after

the TA+SW-treatment compared to the TA-treatment. Based on the EELS maps

(Paper II), sulphur was found to be distributing randomly in the TA+SW-treated

NGOC/PtPd catalyst in contrast to the SW-treated NGOC/PtPd catalyst.

51

Fig. 8. TEM images of a) the fresh, b) SO2+H2O (SW-) treated, and c) engine-bench-aged

(EB-aged) DOC/Pt catalysts (Paper I, published by permission of Springer).

Fig. 9. XPS spectra and binding energies S 2p for the SO2+H2O (SW-) treated DOC/PtPd,

DOC/Pt and NGOC/PtPd catalysts.

Based on the XPS-measurements, the SW-treatment did not cause significant

changes in the chemical compositions of platinum, palladium or the support

material in the DOC/PtPd, DOC/Pt or NGOC/PtPd catalysts compared to the fresh

catalysts (Table 3). The chemical composition of sulphur on the surfaces of all the

SW-treated catalysts was observed to be the same form as that of the vehicle-aged

catalyst, namely SO42- (Figure 9, Figure 7a).

The DRIFT spectra of the fresh and SW-treated catalysts are presented in

Figure 10. The SW-treatment had an effect on the binding of the compounds on the

surface of the catalyst. The SW-treated DOC/Pt has a very broad band at around

160 165 170 175 180

NGOC/PtPd SWDOC/Pt SW

Inte

nsity

(a.

u.)

Binding energy (eV)

DOC/PtPd SW

169.1

169.7

169.5

52

1300 cm-1 most probably representing the surface sulphate species on alumina

(Konsolakis et al. 2013, Abdulhamid et al. 2006) or alternatively physisorped SO2

on alumina (Şentürk et al. 2012, Pieplu et al. 1998, Hamzehlouyan 2016, Mitchell

& Sheinker 1996). The broad band at around 1280 cm-1 of the SW-treated

DOC/PtPd can be attributed to sulphate species interacting with the surface OH-

groups (Yao et al. 1981, Şentürk et al. 2012, Gracia et al. 2005). The SW and

TA+SW-treated NGOC/PtPd catalysts (Figure 10) have a broad band at around

1200–1100 cm−1 indicating the bulk aluminium sulphate species (Paper II,

Abdulhamid et al. 2006, Mitchell & Sheinker 1996, Şentürk et al. 2012). No

sulphur compounds on palladium (Pd-SO4) or platinum (Pt-SO4) were observed,

because an absorbance band at 1435 cm−1 (Paper II, Hoyos et al. 2010, Mowery &

McCormick 2001) was missing. The absence of SO42-

species on the Pd with an

alumina support has also been reported by Colussi et al. (2010), Yu & Shaw (1998)

and Konsolakis et al. (2013). As mentioned in the theory part of this paper, this is

due to the nature of alumina, which acts as a sulphur sink inhibiting the sulphating

of Pd sites (Konsolakis et al. 2013, Lampert et al. 1997). It can thus be concluded

that SO2 is first oxidized to SO3 over the Pd entities and then it migrates to the

alumina support forming Al2(SO4)3 (Colussi et al. 2010, Konsolakis et al. 2013).

The band at 1640 cm−1 in all spectra indicates the presence of surface-bound water

(Yu & Shaw 1998, Konsolakis et al. 2013). The bands at 1240 and 1190 cm-1 of the

DOC catalysts as well as the band at 1033 cm-1 of the NGOC catalysts originate

from the catalytic material.

53

Fig. 10. DRIFT spectra of the fresh and SO2+H2O (SW-) treated DOC/PtPd, DOC/Pt and

NGOC/PtPt catalysts. In addition, the spectra of the thermal aged (TA-) and TA-treatment

followed by SW-treatment (TA+SW) NGOC/PtPt catalyst (Modified from Paper III).

In summary, sulphur caused minor morphological and chemical changes on

DOC/PtPd, DOC/Pt and NGOC/PtPd catalysts. Based on XPS analysis, the

quantity of the accumulated sulphur was detected to be quite similar (3.3-4.6 wt%)

in the SW-treated DOC/PtPd, DOC/Pt and NGOC/PtPd catalysts as well as in the

TA+SW-treated NGOC/PtPd catalysts. Sulphur was found to adsorb vertically

throughout the whole catalyst support material from the catalyst surface to the

metallic monolith, and not only on the surface of the catalytically active material.

Only a minor (~15%) decrease in the SBET and pore volume values was observed

over the SW-treated DOC/Pt and DOC/PtPd catalysts, and no notable changes on

the NGOC/PtPd catalysts were detected. According to the detailed structural

inspection of the SW-treated NGOC/PtPd catalyst, a slight increase in the size of

the precious metal particles was observed. In addition, sulphur accumulation

mainly close to the precious metal particles was detected. The TA-treatment was

found to cause significant morphological changes on the NGOC/Pt catalyst, but no

6008001000120014001600180020002200

1640 960

1240

1302

1190

1245

1018

NGOC/PtPd TA+SW (x0.5)

NGOC/PtPd TA (x0.25)

Abs

orba

nce

(a.

u.)

Wavenumber (cm-1)

DOC/Pt Fresh

DOC/Pt SW

DOC/PtPd Fresh

DOC/PtPd SW

NGOC/PtPd Fresh (x0.5)

NGOC/PtPd SW (x0.5)

1156991

1033

1282

54

further morphological changes were detected after the TA+SW-treatment compared

to the TA-treatment. Based on the DRIFT results, sulphate species on the alumina

and surface OH-groups in DOC/Pt and DOC/PtPd catalysts, were found

respectively, as well as bulk aluminium sulphate in the SW- and TA+SW-treated

NGOC/PtPd catalysts.

Correlation between laboratory and engine aged samples (Paper I)

The correlation between the laboratory exposed and engine-bench-aged DOC/Pt

catalyst was evaluated. The content of sulphur on the engine-bench-aged catalyst

by XRF was found to be ~1 wt%, which is a bit lower than the SW-treated sample

(Table 4). The difference in the sulphur amount is probably caused by different

conditions during the accelerated deactivation of the catalyst compared to the

engine-bench-aged procedure. In the laboratory scale accelerated procedure, the

sulphur exposure was done in stable conditions using a higher sulphur content than

in the engine-bench-aging. No substantial amounts of other impurities were

identified. Table 4 shows that the engine-bench-aged catalyst’s specific surface area

results correlate well with the results of the SW-treated catalyst. Based on the XPS

results, sulphur was also found in the form of sulphate on the engine-bench-aged

catalyst and it did not cause any significant changes in the chemical composition

of platinum (Table 3). Instead, minor growth in Pt particles, i.e. sintering, on the

engine-bench-aged catalyst was observed (Figure 8c). The size of the largest

detected particle was approximately 20 nm. No growth in particle size was detected

on the SW-treated DOC/Pt samples (Figure 8b). Based on the characterization

results, the results of the laboratory scale SW-treated catalysts correspond well to

the results of the engine-bench-aging.

5.1.4 The effect of phosphorus

Diesel oxidation catalyst (Paper III and IV)

The phosphorus accumulation of DOC/PtPd and DOC/Pt catalysts and their bare

support material was studied. The phosphorus content of the DOC samples was

measured by implementing an ICP-OES, XPS and FESEM-EDS analyses. Based

on ICP-OES results, the accumulation of phosphorus was higher on the DOC/Pt

and NGOC/PtPd catalysts compared to the DOC/PtPd catalyst (Table 5). In

55

addition, the phosphorus content was found to be substantially higher for both the

DOC and NGOC bare support materials than the correspondingly treated precious

metal containing DOC and NGOC catalysts. The systematically higher phosphorus

contents (Table 5) measured by XPS compared to those from the ICP-OES analysis

can be explained by the surface sensitivity of XPS indicating that phosphorus

accumulates mainly on the surfaces of the catalyst particles.

Table 5 also presents the results of the N2-physisorption analyses. The results

show that the phosphorus treatment had a diminishing effect on the SBET and total

pore volume values for both DOC and NGOC catalysts. The LPW-treatment

reduced the SBET and the total pore volume of the DOC/PtPd, DOC/Pt and

NGOC/PtPd catalysts by ~13%, ~28% and ~16%, respectively. After the HPW-

treatment, the reduction in the SBET values of the DOC/PtPd, DOC/Pt and

NGOC/PtPd catalysts were ~26%, ~39% and ~31%, respectively. The decrease of

the SBET values from the phosphorus treatments was higher on the DOC and NGOC

bare support materials compared to the samples with precious metals, which may

be due to the higher phosphorus concentration of the bare support materials versus

samples with precious metals. The decrease in the SBET and the total pore volume

values are in good agreement with the amount of phosphorus accumulated on the

DOC/PtPd and DOC/Pt catalysts. As supposed, the specific surface area (SBET)

values for the fresh samples with precious metals were slightly lower in comparison

to the bare support material.

56

Table 5. Phosphorus contents (wt%) measured by ICP-OES and XPS, specific surface

area (SBET), total pore volume and average pore size of fresh, low (LPW) and high (HPW)

P-solution treated DOC/PtPd, DOC/Pt and NGOC/PtPd catalysts. In addition, the results

are presented for the fresh and HPW-treated DOC-support material as well as fresh,

LPW- and HPW-treated NGOC support (Modified from Paper III).

Catalyst Treatment P

bulk ICP-OES

P

average XPS

Specific

surface area

Total pore

volume

Average

pore size

(wt%) (wt%) (m2g-1) (cm3 g-1) (nm)

DOC/PtPd Fresh - - 222 0.38 7.0

LPW 1.7 8 194 0.34 6.9

HPW 3.5 8 165 0.25 6.9

DOC/Pt Fresh - - 225 0.41 7.5

LPW 3.6 5 163 0.30 7.4

HPW 4.7 9 137 0.26 7.4

DOC/Support Fresh - - 233 0.41 7.1

LPW n.d. n.d. n.d. n.d. n.d.

HPW 7.3 16 124 0.22 7.1

NGOC/PtPd Fresh - - 154 0.37 10.1

LPW 3.3 8 129 0.30 9.3

HPW 6.2 7 107 0.25 9.3

NGOC/Support Fresh - - 182 0.43 9.5

LPW 5.7 12 133 0.35 10.5

HPW 8.8 14 111 0.27 9.6

n.d. = not determined

Based on the cross-sectional FESEM analyses, most of the phosphorus is

accumulated on the catalyst surface layer and the phosphorus decrease is evident

through the catalyst layer from top to bottom of the phosphorus treated DOC/PtPd,

DOC/Pt and NGOC/PtPd catalysts (Paper III and V). Figure 11 illustrates the cross-

sectional FESEM images and phosphorus content extracted from the selected

measurement points (around 5 μm interval) on the LPW- and HPW-treated

DOC/PtPd and DOC/Pt catalysts (Paper III). Most of the phosphorus, ~13-15 wt%

in the HPW-treated DOC/PtPd and DOC/Pt catalysts, accumulated down to 10 μm

from the surface over the HPW-treated PtP (Figure 11a-b, Paper III). The

phosphorus concentration is similar (15-20 wt%) throughout the bare support

material layer from top to bottom (Figure 11c, Paper III). The amount of

phosphorus on the catalyst surface was ~10 wt% on the LPW-treated NGOC/PtPd

catalyst and ~15 wt% on the HPW-treated NGOC/PtPd (Paper V). The amount of

phosphorus decreased almost linearly through the catalyst from top to bottom. The

results correlate well with the phosphorus accumulation results over the vehicle-

57

aged NGOC/PtPd catalyst (Figure 6c). The phosphorus concentration was higher

on the surface region of the bare support (~20 wt%) of the NGOC than on the

NGOC/PtPd catalyst, and the phosphorus concentration decreased only slightly

from the surface region towards the metallic monolith, which is concurrent with

the DOC results.

Fig. 11. Cross-sectional FESEM images and phosphorus content information from the

selected measurement points: a) DOC/PtPd catalyst after high (HPW) and low (LPW) P-

solution treatment, b) DOC/Pt catalyst after HPW- and LPW-treatment, and c) support

material after HPW-treatment (Paper III, published by permission of Springer).

The detailed structural characterization was done by TEM. Paper IV reports the

cross-sectional TEM results for the LPW- and HPW-treated DOC/Pt and DOC/PtPd

58

catalysts. The deactivation behaviour of phosphorus for both catalysts was found

to be similar. LPW- and HPW-treatments increased the precious metal particle size

slightly and changed the shape of the particles from irregular to spherical.

Phosphorus accumulated mainly in the alumina-rich areas. In the case of HPW-

treated DOC catalysts, the phosphorus content was even 20 wt% in the alumina-

rich areas and below 5 wt% in the silica rich areas. After the HPW-treatment of

both the DOC catalysts, the amorphous form of alumina rich area was detected in

the selected area electron diffraction (SAED) pattern. The cross-sectional TEM

studies (Figure 12) of the HPW-treated NGOC/PtPd catalyst, reported in Paper V,

indicate a denser structure in comparison to the fresh catalyst. On the surface of the

HPW-treated NGOC/PtPd catalyst, the size of the PtPd particles increased slightly

compared to the fresh catalyst. Based on the SAED patterns, the nanocrystalline γ-

alumina in the fresh NGOC catalyst changed to an amorphous structure in the

HPW-treated catalyst (Figure 12). The results are in good agreement with the DOC

results.

Fig. 12. Cross-sectional TEM images and SAED patterns of a) the fresh NGOC/PtPd

catalyst and b) the high P-solution (HPW) treated NGOC/PtPd catalyst (Paper V,

published by permission of Springer).

In order to determine the chemical compositions of elements before and after

phosphorus exposure, XPS, DRIFT and XRD analyses were done. In Table 6,

binding energies of the fresh and phosphorus treated DOC and NGOC samples are

presented. In all the phosphorus treated DOC and NGOC samples, P 2s was found

in a binding energy range between 190.1 eV and 191.8 eV. This suggests that

phosphorus is attached to the precious metal or to the support surface in the form

59

of phosphate form (PO43-) (Naumkin et al. 2012). The phosphate form of

phosphorus was also detected in the vehicle-aged sample (Figure 7b). The

phosphorus treatments did not change the platinum or palladium oxidation states

(Table 6).

Table 6. Binding energies (eV) measured by XPS for fresh, low (LPW) and high (HPW)

P-solution treated DOC/PtPd, DOC/Pt and NGOC/PtPd catalyst. In addition, the results

are presented for the fresh and HPW-treated DOC-support material as well as fresh,

LPW- and HPW-treated NGOC support (Modified from Paper III).

Catalyst Treatment Pd Pt P Al O

3d5/2 4d5/2 2s 1s 1s

DOC/PtPd Fresh 336.4 315.5 - 119.2 531.2

LPW 337.3 315.4 191.2 119.2 531.7

HPW 336.9 315.0 190.1 118.1 530.8

DOC/Pt Fresh - 314.8 - 119.7 531.4

LPW - 314.2 191.4 119.2 531.4

HPW - 313.2 190.4 118.2 530.3

DOC/Support Fresh - - - 119.1 531.2

LPW n.d. n.d. n.d. n.d. n.d.

HPW - - 191.3 118.5 531.3

NGOC/PtPd Fresh 336.3 315.3 - 118.3 530.2

LPW 336.9 315.0 191.0 119.0 531.1

HPW 336.6 315.3 190.9 118.7 531.4

NGOC/Support Fresh - - - 118.6 530.9

LPW - - 191.3 119.1 531.8

HPW - - 191.8 119.5 532.0

n.d. = not determined

The DRIFT spectra of the fresh and HPW-treated DOC and NGOC catalysts are

presented in Figure 13 (Papers III and V). For the fresh DOC samples, two broad

bands were identified at 1190 cm-1 and 960 cm-1 and for the fresh NGOC sample

only one sharp band was observed at 1033 cm-1. These bands can be attributed to

be material originated and, thus, are not due to the phosphorus adsorption. The band

at 1640 cm−1 in all spectra shows the presence of surface-bound water (Yu & Shaw

1998, Konsolakis et al. 2013). In the case of the HPW-treated DOC and NGOC

catalysts, a very broad band is detected at ~1300 cm-1, and the band at 1190 cm-1

for the DOC samples and the band at 1033 cm-1 for the NGOC sample have almost

disappeared. A very broad band at ~1300 cm-1 was observed also for the HPW-

treated NGOC bare support material (Paper III). The band in the area around 1300

60

cm-1 can be attributed to phosphite (PO3-) and especially aluminium metaphosphate

Al(PO3)3 (Nyquist et al. 1971). Wiebenga et al. (2012) have found that phosphorus,

in the form of PO3-, is typically found in vehicle-aged catalyst samples.

Fig. 13. DRIFT spectra of fresh and high P-solution (HPW) treated DOC/PtPd, DOC/Pt,

NGOC/PtPd and DOC/Support (Modified from Paper III).

The XRD patterns of the fresh and HPW-treated DOC/support and DOC/PtPd

catalyst are presented in Figure 14. The XRD analyses for the fresh DOC/PtPd and

DOC/support have main peaks at 23.5° (peak b), 25.5° (peak c) and around 29°

(peak d) indicating a silica phase, anatase phase of titania and CexZr1-xO2 phase,

correspondingly (Rashad et al. 2013, Wang 2013, Wu et al. 2017). In addition to

these peaks, a low shoulder in the phosphorus treated catalysts at around 20° was

detected, which most probably indicates the presence of an amorphous aluminium

phosphate (AlPO4) phase (Galisteo et al. 2004, Poojary et al. 1993, PDF-2 Powder

Diffraction File Database). In the cross-sectional TEM analysis, alumina was found

in an amorphous form in the HPW-treated DOC/PtPd and DOC/Pt catalysts. In

addition, phosphorus was found to accumulate mainly on the alumina-rich areas.

The results gained by TEM suggest the formation of dense and amorphous AlPO4.

6008001000120014001600180020002200

Ab

sorb

an

ce (

a.u

.)

Wavenumber (cm-1)

DOC/Support HPW

DOC/Support Fresh

DOC/Pt Fresh

13001640

1055

1190 960

NGOC/PtPd HPW

NGOC/PtPd Fresh

DOC/Pt HPW

DOC/PtPd Fresh

DOC/PtPd HPW

61

Therefore, the TEM results correspond and support the XRD results well. The

formation of amorphous AlPO4 is very likely, since the transformation of AlPO4

from an amorphous to a crystalline phase has been earlier reported (Scaccia et al.

2003, Youssif et al. 2004) to happen at >500 °C and the temperature in our aging

procedure was only 400 °C.

Fig. 14. The XRD pattern of (1) the fresh DOC support material, (2) the DOC support

material after treatment with high P-solution (HPW), (3) the fresh DOC/PtPd catalyst and

(4) the DOC/PtPd catalyst after the HPW treatment; a = AlPO4, b = SiO2, c = TiO2, d =

CexZr1-xO2 (Paper III, published by permission of Springer).

The XRD analyses for the fresh NGOC/PtPd catalyst shows the main peaks at 33°

(peak b) and 46° (peak d) (Figure 15) indicating the PdO phase and the ɣ-alumina

phase, correspondingly (PDF-2 Powder Diffraction File Database). The detection

of the phosphorus compounds of the samples was possible only from the high

phosphorus concentration samples. Therefore, only the HPW-treated NGOC/PtPd-

catalyst and the 0.2 M P-solution-treated bare support material results with fresh

samples are presented in Figure 15. The quite broad peak at around 20° (peak a) in

the phosphorus treated catalysts indicates the presence of an amorphous AlPO4

phase (PDF-2 Powder Diffraction File Database, Galisteo et al. 2004). The result

corresponds well with the TEM results as well as the results obtained with the

10 20 30 40

a

Inte

nsity

(a.

u.)

2-Theta (deg)

(1)

(2)

(3)

(4)a

b c d

62

HPW-treated DOC/PtPd catalyst. In addition, the found amorphous AlPO4 phase

explains why such a high amount of phosphorus is needed until it can be detected

by XRD. When analogous samples with precious metals are compared, it can be

seen, that they both have a peak b at 2θ=34°, which corresponds to PdO (PDF-2

Powder Diffraction File Database). However, the peak is lower in the HPW-treated

NGOC/PtPd catalyst compared to the fresh sample. Furthermore, the HPW-treated

NGOC/PtPd catalyst shows a peak at 2θ=40° (peak c), indicating the presence of

elemental Pd. Usually the transformation of PdO to elemental Pd has been reported

to occur by sintering (Lampert et al. 1997, Persson et al. 2007, Euzen et al. 1999,

Gholami & Smith et al. 2015a). In the study by Honkanen et al. (2017), it was

found that PdO sintering to Pd in air starts at 700 °C in the NGOC/PtPd catalyst

and the temperature used in the aging procedure was only 400 °C. It is therefore

possible, that some form of interaction with phosphorus caused this partial

reduction of PdO, as AlPO4 was formed. Based on N2 physisorption, CO

chemisorption and STEM-analysis, Matam et al. (2012) concluded that mono- to

multilayer amorphous aluminium phosphate-like species are formed at 700 °C.

Phosphorus species can potentially affect the strength of interactions between PdOx

nanoparticles and the support and hence, can alter the electronic and catalytic

properties of the surface Pd sites.

63

Fig. 15. XRD pattern of the catalysts samples, (1) the fresh NGOC support material, (2)

the NGOC support material after the treatment with 0.2 M P-solution, (3) the fresh

NGOC/PtPd catalyst and (4) the NGOC/PtPd catalyst after the treatment with high P-

solution (HPW); a = AlPO4, b = PdO, c = Pd, d = γ-Al2O3.

In summary, based on the characterization results, phosphorus treatments (LPW-

and HPW) change the morphological and chemical structures of DOC/PtPd,

DOC/Pt and NGOC/PtPd catalysts. It was found, that when more phosphorus was

present in the feed-stream, higher amounts of phosphates were found on the studied

catalysts. In the case of the precious metal containing catalyst (DOC/Pt, DOC/PtPd

and NGOC/PtPd), phosphorus was detected to accumulate only on the surface

region of the catalyst. The phosphorus accumulation on the catalyst surface reduces

the SBET and pore volume values, and this phenomenon was more severe after

HPW- than after the LPW-treatment. Phosphorus treatments were found to slightly

increase the average size of precious metal particles, and the shape of the metal

particles changed from an irregular form to a mainly spherical form. Phosphorus

was adsorbed mainly on the alumina by chemical bonds and most likely formed

dense amorphous aluminium phosphate. It was also found, that phosphorus can

cause the partial transformation from PdO to Pd. The deactivation behaviour of

phosphorus differed between precious metal containing catalysts and bare support

20 30 40 50 60 70

a ccd

b

b

Inte

nsity

(a.

u.)

2-Theta (deg)

(1)

(2)

(3)

(4)

a

b

cd

64

materials. The amount of accumulated phosphorus was found to be significantly

higher in the bare support material than in the precious metal containing catalysts.

In addition, the accumulation of phosphorus on the bare support materials was

almost uniform from the surface towards the bottom.

5.2 Activity measurements

5.2.1 Fresh and water treated diesel oxidation catalysts

The laboratory scale activity measurements for fresh and water (W-) treated

DOC/PtPd and DOC/Pt catalysts were done using a gas mixture without and with

NO (Papers I and III, respectively). In this study, all of the activity tests were done

in the presence of water. The used gas compositions are presented in Figure 5. C3H6

and CO conversions over the fresh DOC/PtPd and DOC/Pt catalysts of the gas

mixture without NO and with NO are presented in Figure 16. In addition, in Figure

16, NO conversions as well as NO2 and N2O formations are shown for the fresh

DOC/PtPd and DOC/Pt catalysts using a gas mixture with NO. The light-off

temperatures (T50 and T90) over the fresh and water treated DOC/PtPd and DOC/Pt

catalysts are summarized in Table 7. The activity measurement results of the water

treated catalysts indicate that the water treatment has no effect on the activity of the

studied catalysts. The only exception was the C3H6 oxidation temperature increase

by ~20 °C for the DOC/PtPd catalyst in the case when the gas mixture without NO

was used (Table 7, Papers I and III). This could be due to the formation of Pd(OH)2,

which inhibits the hydrocarbon oxidation at low temperatures (<190 °C) (Burch &

Urbano 1995, Roth et al. 2000, Gholami et al. 2015b).

In the case of fresh DOC/Pt and DOC/PtPd when a gas mixture without NO

was used (Figure 16a and c), the C3H6 oxidation started at around 110 °C and at

135 °C, respectively. Both catalysts adsorbed and desorbed C3H6 at low

temperatures (below 120 °C), which was a consequence of the presence of zeolites

in the DOC catalysts (Paper I). For example, Kröger et al. (2007) and Huuhtanen

et al. (2005) have reported that the fresh zeolite samples have the ability to adsorb

propene at low temperatures. Above 135 °C, the C3H6 oxidation activity of the fresh

DOC/Pt catalyst was only slightly better compared to the fresh DOC/PtPd catalyst:

the T50-values were equal, but the T90-value of DOC/Pt was slightly (~15 °C) lower

compared to that of the DOC/PtPd catalyst (Figure 16a and c, and Table 7). A small

difference can be seen in CO oxidation where the T50-value of the fresh DOC/Pt

65

was ~20 °C and T90 10 °C lower compared to the fresh DOC/PtPd catalyst (Figures

16a and c, and Table 7). CO adsorption on Pt and Pd was very strong even at low

temperatures (<100 °C) (Zhou et al. 2015), which explains the remarkable CO

conversions (~20%) already at 110 °C for the fresh DOC PtPd and Pt catalysts.

(Paper I)

Fig. 16. Conversion as a function of temperature for different gas mixtures: a) and c)

without NO (w/o NO) and b) and d) with NO over the fresh DOC/PtPd and DOC/Pt

catalysts. For a gas mixture with NO, NO2 and N2O formation is also presented (Paper I

and III, published by permission of Springer).

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

a) DOC/PtPd Fresh w/o NO

Con

vers

ion

(%)

Temperature (°C)

CO C3H6

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

Con

vers

ion

(%)

Temperature (°C)

CO C3H6 NO

b) DOC/PtPd Fresh with NO

0

100

200

300

400

500

NO

2 an

d N2 O

form

atio

n (ppm)

NO2 N2O

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

Con

vers

ion

(%)

Temperature (°C)

CO C3H6 NO

d) DOC/Pt Fresh with NO

0

100

200

300

400

500

NO2 N2O

NO

2 an

d N2 O

form

atio

n (ppm)

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

Co

nve

rsio

n (

%)

Temperature (°C)

CO C3H6

c) DOC/Pt Fresh w/o NO

66

Table 7. Light-off temperatures (T50 and T90) for C3H6 and CO oxidation and maximum

conversion of NO over the fresh, H2O- (W-) treated DOC/PtPd and DOC/Pt catalysts.

(Modified from Papers I and III).

Catalyst Treatment T50 (°C) T90 (°C) Maximum conversion (%)

C3H6 CO C3H6 CO NO

Without NO

DOC/PtPd Fresh 144 138 169 153 -

W 166 140 191 182 -

DOC/Pt Fresh 141 122 151 143 -

W 135 120 143 132 -

With NO

DOC/PtPd Fresh 192 178 212 193 13

W 194 182 219 203 14

DOC/Pt Fresh 176 129 186 160 50

W 180 117 192 172 48

For the fresh DOC/PtPd and DOC/Pt catalysts, the presence of NO in the gas

mixture shifted CO and C3H6 oxidation to higher temperatures (Figure 16b and d,

and Table 7). In addition, NO increased the temperature difference of CO and C3H6

oxidation activity over the fresh DOC/Pt catalyst compared to the fresh DOC/PtPd

catalyst (Paper III). A closer inspection of the fresh DOC/PtPd and DOC/Pt

catalysts using a gas mixture with NO (Figure 16b and d) indicates that the

oxidation of CO started first, followed by that of C3H6. NO2 formation started after

the CO was totally converted and C3H6 had converted by more than 90% (Paper

III). The observations are in good correspondence with the results reported e.g. by

Khosravi et al. (2014) and Hauff et al. (2012). Above 140 °C, a small amount (~10

ppm) of N2O was observed, which was caused by the reaction of C3H6 with NO

based on Equation (5) (Khosravi et al. 2014):

2 4 → 3 3 (5)

A comparison between the fresh DOC/PtPd and DOC/Pt catalysts illustrates that

the activity for CO, C3H6 and NO oxidation over the fresh DOC/Pt catalysts was

higher than over the PtPd catalysts for the entire temperature range. In the NO

conversions, the difference between the fresh catalysts was the most significant.

The maximum conversion of NO over the DOC/Pt catalyst was 50% and over the

DOC/PtPd catalyst only 13%. Based on the XPS measurements (Table 3), Pt was

found in a metallic form (Pt) on the fresh DOC/Pt catalyst and in an oxidized form

(PtO) on the DOC/PtPd catalyst. Metallic Pt has been reported to be more active

than oxidized Pt especially in NO oxidation (Hauff et al. 2012). Thus, the detected

67

PtO in this study most probably explains the lower NO conversion over the

DOC/PtPd catalyst compared to the DOC/Pt catalysts (Paper III).

5.2.2 The effect of sulphur on diesel oxidation catalysts

The gas mixtures without and with NO were also used to verify the role of sulphur

dioxide-water (SW-) treatment on the oxidation activity of DOC/PtPd and DOC/Pt

catalysts. In the case of SW-treatment and a gas mixture without NO (Paper I), the

C3H6 oxidation activity decreased over the DOC/PtPd and DOC/Pt catalysts

(Figure 17a and c, and Table 8). Desorption of C3H6 after the water feeding was

significantly stronger over the SW-treated DOC/PtPd catalyst than the fresh

DOC/PtPd catalyst and therefore propene oxidation did not start until a temperature

of ~170 °C. Nevertheless, the light-off temperatures (T50 and T90) were equal in the

C3H6 oxidation over the SW-treated DOC/PtPd and DOC/Pt catalysts (Paper II).

This means that the deactivation effect of the SW-treatment is considered equal (an

increase of 32 °C) at the light-off temperature T50 for the activity of DOC/PtPd and

DOC/Pt catalysts. However, the effect is stronger for the activity over the DOC/Pt

(35 °C) catalysts than that of DOC/PtPd (18 °C) at the light-off temperature of T90.

Quite similar effects can be seen for the CO oxidation activity (Figure 17a and c,

and Table 8) (Paper I).

68

Fig. 17. Conversion as a function of temperature for gas mixtures: a) and c) without (w/o

NO) and b) and d) with NO over the SO2+H2O- treated (SW) DOC/PtPd and DOC/Pt

catalysts. For a gas mixture with NO, NO2 and N2O the formation is also presented.

(Figures a and c modified from Paper I).

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

Con

vers

ion

(%)

Temperature (°C)

CO C3H6 NO

b) DOC/PtPd SW with NO

0

100

200

300

400

500

NO2 N2O

NO

2 an

d N2 O

form

atio

n (ppm)

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

CO C3H6

a) DOC/PtPd SW w/o NO

Co

nve

rsio

n (

%)

Temperature (°C)

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

Con

vers

ion

(%)

Temperature (°C)

CO C3H6 NO

d) DOC/Pt SW with NO

0

100

200

300

400

500

NO2 N2O

NO

2 and

N2

O form

ation (pp

m)

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

c) DOC/Pt SW w/o NO

Co

nve

rsio

n (

%)

Temperature (°C)

CO C3H6

69

Table 8. Light-off temperatures (T50 and T90) for C3H6 and CO oxidation, and the

maximum conversion of NO over the fresh, H2O- (W-), SO2+H2O- (SW-) DOC/PtPd and

DOC/Pt catalysts. In addition, the results are presented for the engine-bench-aged (EB-

aged) DOC/Pt catalyst (Modified from Papers I and III).

Catalyst Treatment T50 (°C) T90 (°C) Maximum conversion (%)

C3H6 CO C3H6 CO NO

Without NO

DOC/PtPd Fresh 144 138 169 153 -

SW 176 170 187 180 -

DOC/Pt Fresh 141 122 151 143 -

SW 173 143 186 180 -

EB-aged 162 152 182 179 -

With NO

DOC/PtPd Fresh 192 178 212 193 13

SW 194 172 226 192 17

DOC/Pt Fresh 176 129 186 160 50

SW 190 147 201 185 46

The SW-treatment of the gas mixture with NO influences the oxidation activity of

DOC/Pt more than DOC/PtPd as shown in Table 8 and Figure 17b and d. The CO

oxidation is identical over the fresh and SW-treated DOC/PtPd catalysts. C3H6

oxidation over the SW-treated DOC/PtPd (Figure 17b) starts at the same

temperature as over the fresh DOC/PtPd (Figure 16b) and the SW-treatment does

not decrease the C3H6 oxidation activity until the higher temperatures are reached

(above 190 °C). No change in the NO oxidation activity compared to the fresh

DOC/PtPd catalyst was observed. For the DOC/Pt catalyst, the SW-treatment shifts

the CO oxidation temperature to higher temperature (by 10-20 °C) compared to the

fresh catalyst (Figure 17d and 16d). Therefore, the oxidation of C3H6 and NO over

the SW-treated DOC/Pt catalyst shifts the T50 values by ~20 °C and ~10 °C,

respectively, to higher temperatures than over the fresh DOC/Pt catalyst.

Based on the results, the SW-treatment was found to especially shift the

propene light-off temperature (T50 and T90) to higher temperatures. Since only a

minor reduction in the SBET and pore volume values and no change in the pore size

were detected over the SW-treated DOC/PtPd and DOC/Pt catalysts, it can be

assumed that the loss of the active sites is the main reason for the higher oxidation

temperature needed (Heck et al. 2002).

70

Correlation between laboratory and engine aged samples

The correlation between the laboratory and engine aged samples was studied using

sulphur dioxide-water (SW) treated and engine-bench-aged DOC/Pt catalysts

(Paper I). Figures 18 presents the CO and C3H6 oxidation activity over the fresh,

SW-treated and EB-aged DOC/Pt catalysts. In addition, the T50 and T90 values are

presented in Table 8.

Fig. 18. a) CO and b) C3H6 conversions over the fresh, SO2+H2O- (SW-) treated and

engine-bench aged (EB-aged) DOC/Pt catalysts using a gas mixture without NO

(Modified from Paper I).

The results show that the laboratory scale SW-treatment corresponds relatively well

to the EB-aged results. In Figure 18a, it can be seen, that there are no significant

differences in the CO oxidation activity between the EB-aged and the SW-treated

catalysts. C3H6 oxidation over the SW-treated sample (Figure 18b) starts at ~35 °C

higher temperatures compared to the EB-aged catalysts, but only slight (10 °C) and

not significant differences in the light-off temperatures T50 and T90, respectively,

were observed. The slightly better activity of EB-aged catalysts in C3H6 oxidation

can be explained by the XRF results, which show that the sulphur accumulation on

the bulk analysis of the EB-aged DOC/Pt is lower than on the SW-treated catalyst

(S content 1 wt% and 2 wt%, respectively).

5.2.3 The effect of phosphorus on diesel oxidation catalysts

To determine the effect of phosphorus on the DOC/PtPd and DOC/Pt catalyst

activity, two different phosphorus concentrations, low (marked as LPW) and high

(HPW), were used in activity tests (Paper III). The activity tests were done with a

120 140 160 180 200 220 240 260 280 300

0

20

40

60

80

100 b)

C3H

6 C

onv

ers

ion

(%)

Temperature (°C)

DOC/Pt Fresh

DOC/Pt SW DOC/Pt EB-aged

120 140 160 180 200 220 240 260 280 300

0

20

40

60

80

100

CO

Co

nve

rsio

n (%

)

Temperature (°C)

DOC/Pt Fresh

DOC/Pt SW

DOC/Pt EB-aged

a)

71

gas mixture with NO (Figure 5) C3H6, CO and NO conversions. The NO2 and N2O

formation over the fresh and phosphorus treated (LPW and HPW) DOC/PtPd and

DOC/Pt catalysts are shown in Figure 19.

The LPW-treatment had a negligible effect on the DOC/PtPd and DOC/Pt

catalysts’ CO oxidation activity and the oxidation of C3H6 started at the same

temperature as it did with the fresh catalysts. Propene conversion of ~100% was

observed over the fresh DOC/PtPd, fresh and LPW-treated DOC/Pt catalysts and

~90% for the LPW-treated DOC/PtPd catalysts. A decrease in the C3H6 oxidation

activity compared to the fresh catalysts can be seen at above 170 °C. The light-off

temperature T90 increased by ~50 °C and by ~20 °C for the LPW-treated DOC/PtPd

and DOC/Pt catalyst, respectively, compared to the similar fresh catalysts. Based

on the cross-sectional TEM analysis, a minor precious metal particle size increase

and a change in the shape of the particles over the LPW-treated DOC/PtPd and

DOC/Pt catalysts could be one reason for the observed changes. In addition, only

a slight decrease in the SBET, pore size and pore volume values over the LPW-

treated DOC/PtPd catalyst was seen. Thus, most probably, the main reason for the

activity loss of the LPW-treated DOC/PtPd catalysts is the reduced accessibility of

reactants to the catalyst pores and on the active surface.

72

Fig. 19. Conversion as a function of temperature for the gas mixture with NO over the

fresh, low (LPW) and high (HPW) P-solution treated DOC/PtPd and DOC/Pt catalysts

(Paper III, published by permission of Springer).

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100C

onv

ersi

on

(%)

Temperature (°C)

CO C

3H

6

NO

a) DOC/PtPd Fresh

0

100

200

300

400

500

NO2

N2O

NO

2 and N

2 O fo

rmation

(ppm

)

120 140 160 180 200 220 240 260 280 3000

20

40

60

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100

Con

vers

ion

(%

)

Temperature (°C)

CO C

3H

6

NO

b) DOC/Pt Fresh

0

100

200

300

400

500

NO2

N2O

NO

2 and N

2 O form

ation (pp

m)

120 140 160 180 200 220 240 260 280 3000

20

40

60

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100

Con

vers

ion

(%

)

Temperature (°C)

CO C

3H

6

NO

c) DOC/PtPd LPW

0

100

200

300

400

500

NO2

N2O

NO

2a

nd N2

O form

ation (p

pm)

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

Con

vers

ion

(%

)

Temperature (°C)

CO C

3H

6

NO

d) DOC/Pt LPW

0

100

200

300

400

500

NO2

N2O

NO

2a

nd N2

O form

ation (p

pm)

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

Co

nver

sio

n (%

)

Temperature (°C)

CO C

3H

6

NO

f) DOC/Pt HPW

0

100

200

300

400

500

NO2

N2O

NO

2 and N

2 O fo

rmation

(ppm

)

120 140 160 180 200 220 240 260 280 3000

20

40

60

80

100

Con

vers

ion

(%

)

Temperature (°C)

CO C

3H

6

NO

e) DOC/PtPd HPW

0

100

200

300

400

500

NO2

N2O

NO

2 a

nd N2

O form

ation (p

pm)

73

Table 9. Light-off temperatures (T50 and T90) for the C3H6 and CO oxidation and the

maximum conversion of NO over the fresh, low (LPW) and high (HPW) P-solution treated

DOC/PtPd and DOC/Pt catalysts (Modified from Paper III).

Catalyst Treatment T50 (°C) T90 (°C) Maximum conversion (%)

C3H6 CO C3H6 CO NO

DOC/PtPd Fresh 192 178 212 193 13

LPW 196 170 267 196 11

HPW 207 173 - 288 0

DOC/Pt Fresh 176 129 186 160 50

LPW 179 145 203 171 50

HPW 197 168 295 196 17

The results in Figure 19 and Table 9 show that the HPW-treatment has a more

severe effect on the DOC/PtPd and DOC/Pt catalysts’ oxidation activity than the

LPW-treatment. The HPW-treatment reduced the C3H6 conversions so that the

maximum C3H6 conversions were 78% and 90% (at 300 °C) for the DOC/PtPd and

DOC/Pt catalysts, respectively. In addition, the HPW-treatment reduced the

maximum CO conversion over the HPW-treated DOC/PtPd catalyst by ~10%

compared to the fresh catalyst. Furthermore, the HPW-treatment caused a

significant reduction in the maximum conversions of NO over the DOC/PtPd and

DOC/Pt catalysts (reduced by 67% and 100%, respectively). It is possible that the

uncompleted oxidation reactions of CO and C3H6 over the PtPd catalyst inhibit the

NO reactions in the studied temperature area (Khosravi et al. 2014, Hauff et al.

2012).

For both the studied catalysts, based on their characterizations, the HPW-

treatment caused chemical and structural changes on the catalyst surface by

forming amorphous AlPO4, increasing the particle sizes of the precious metals

slightly and changing their shape from irregular to mainly spherical. It is suggested

that phosphorus may cover some of the surface sites and may partly block the pores.

It is worth noting, that based on the ICP-OES, XPS and physisorption analyses

(Table 5), the phosphorus concentration was lower and changes in SBET and pore

volume were minor in the DOC/PtPd catalyst after the HPW-treatment in

comparison to the correspondingly treated DOC/Pt catalyst. However, the HPW-

treatment similarly or even more severely affects the DOC/PtPd catalyst’s

oxidation activity than that of the DOC/Pt catalyst. It is most probable, that in the

case of the HPW-treated DOC/PtPd catalyst, oxidized Pt together with

morphological and chemical changes caused the lower activity for the DOC/PtPd

catalyst.

74

5.2.4 The effect of sulphur and phosphorus on a natural gas

oxidation catalyst

The effect of water (W-), sulphur (SW-), low (LPW-) and high (HPW-) phosphorus

treatments on the CH4 and CO oxidation activity over the NGOC/PtPd catalyst was

studied (Paper II and V). Experiments were also done to find out the combined

effect of the thermal aging (TA) followed by sulphur exposure (SW) on the

NGOC/PtPd catalyst’s activity in CH4 and CO oxidation (Paper II). As a reference,

the activity of the TA-treated NGOC/PtPd catalyst was also studied (Paper II). The

light-off temperatures (T50 and T90) of CO and CH4 over the fresh and treated

NGOC/PtPd catalysts are summarized in Table 10 and the CH4 conversions are

presented in Figure 20. Water treatment was found to have a negligible effect on

the CO and CH4 oxidation activity.

Figure 20 clearly shows the diminishing effect of the SW-treatments on the

methane oxidation activity of the studied catalyst. Over the SW-treated catalyst,

methane oxidation started at ~120 °C higher temperatures compared to the fresh

catalyst and the light-off temperatures (T50) were observed to increase by 50 °C

compared to the fresh catalyst. For the fresh catalyst, ~100% CH4 conversion was

achieved and after the SW-treatment the maximum conversion was around 90%.

The SW-treatment was found to increase the carbon monoxide T90 value by over

50 °C of the NGOC (Table 10). However, 90% CO conversion was still achieved

at low temperatures (~150 °C) (Paper II). Based on a physisorption analysis, no

notable changes in the SBET and pore volume values of the NGOC/PtPd catalysts

were detected (Table 4). Higher sulphur concentrations were detected by STEM-

EELS near the precious metal particles than in those regions without Pt and Pd

particles. A slight increase in precious metal particles analysed by XRD was also

observed. Thus, the reduced activity detected by the temperature shift in the light-

off curves to higher temperatures was most probably caused due to a combination

of structural changes (particle transformation) and chemical deactivation

(sulphates).

TA-treatment was carried out to determine the effect of sulphur exposure on a

thermally aged NGOC/PtPd catalyst. Characterizations indicate that the TA-

treatment caused drastic sintering of the precious metals and support material.

Figure 20 and Table 10 show that the effect of the TA-treatment alone on the CH4

oxidation activity is quite similar to the SW-treated NGOC/PtPd catalyst.

Nevertheless, the SW-treatment following the TA-treatment (TA+SW)

significantly reduces the CH4 oxidation activity compared to the TA-treated catalyst.

75

Based on the XRD, FESEM and TEM studies, morphology changes on the catalyst

after the TA- and TA+SW-treatments were observed to be similar, as well as

changes in the SBET-values and the pores sizes compared to the fresh catalyst.

Therefore, it seems that the formation of aluminium sulphate distributed randomly

over the TA+SW-treated catalyst surface in addition to the sintered catalyst was the

reason for the dramatic catalyst deactivation.

All the activity tests included two similar temperature ramps (run 1 and run 2).

Significant differences were only noticed with the TA+SW-treated NGOC/PtPd

catalyst, between run 1 and 2 and therefore these results are included in Figure 20.

The difference in the CH4 oxidation activity between run 1 and run 2 indicates that

partial regeneration of the catalyst took place during the first run removing sulphur

compounds from the catalyst (Paper II).

Fig. 20. Conversion of CH4 as a function of temperature for the NGOC/PtPd catalyst

after different treatments: fresh, H2O- (W-), SO2+H2O- (SW-), low (LPW) and high (HPW)

P-solution, thermal aged (TA) and TA-treatment followed by SW-treatment (TA+SW)

(Modified from Papers II and V).

150 200 250 300 350 400 450 500 550 6000

20

40

60

80

100

Co

nve

rsio

n (

%)

Temperature (°C)

Fresh W SW LPW HPW TA TA+SW run 1 TA+SW run 2

76

Table 10. Light-off temperatures (T50 and T90) for the CH4 and CO oxidation over the fresh

NGOC/PtPd catalyst with the following treatments: H2O- (W-), SO2+H2O- (SW-), low (LPW)

and high (HPW) P-solution, thermal aged (TA-) and TA-treatment followed by SW-

treatment (TA+SW) (Modified from Paper II).

Catalyst Treatment T50 (°C) T90 (°C)

CH4 CO CH4 CO

NGOC/Pt Fresh 390 < 110 488 < 110 W 393 < 110 493 < 110

SW 437 < 110 550 147

LPW 420 < 110 - 136

HPW 503 < 110 - 232

TA 445 < 110 570 115

TA+SW run 1 564 148 - 205

TA+SW run 2 498 < 110 - 114

The phosphorus treated NGOC has a lower methane oxidation activity compared

to the fresh natural gas oxidation catalyst (Figure 20). For the fresh catalyst, ~100%

CH4 conversions were achieved, but after the LPW- and HPW-treatments the

maximum conversions were only 86% and 63%, respectively. The deactivation

effect of the HPW-treatment on the NGOC/PtPd catalysts is stronger compared to

the LPW-treatment in the whole temperature range. The increase in the light-off

temperature (T50) of CH4 was moderate (~30 °C) over the LPW-treated catalyst and

significant (~110 °C) over the HPW-treated catalyst (Table 10). In addition, both

phosphorus treatments increased the T90-values of the CO conversion, resulting in

a difference of ~25 °C and ~100 °C for the LPW- and HPW-treated catalyst,

respectively, compared to the fresh catalyst (Paper V). In the case of the LPW-

treated NGOC/PtPd catalyst the significant decrease in the maximum CH4

conversion and smaller pore sizes compared to the fresh catalyst suggest that the

pore diffusion could be hindered. For the HPW-treated NGOC/PtPd catalyst, the

pore size was similar, but the accumulated phosphorus content was higher than for

the LPW-treated catalyst. This is most probably caused by formation of a fouling

layer on the catalyst surface hindering further CH4 oxidation.

77

6 Summary and conclusions

Emission limits for passenger and heavy-duty vehicles have been tightened

continuously together with the long-term durability requirements. Fossil fuels and

engine lubricants contain impurities which can have a significant effect on the

activity and durability of a catalytic converter. In addition, the increasing use of

various biofuels and blended fuels can increase the accumulation of impurities on

the catalyst surface and thus remarkably reduce the long-term durability.

In this thesis, deactivation phenomena due to sulphur and phosphorus in diesel

and natural gas oxidation catalysts were studied with the aim to increase knowledge

about the effects of impurities on the catalyst performance. The main objectives of

the research were to define the structural and chemical changes on the catalyst

surface caused by sulphur and phosphorus, as well to illustrate the effect of these

impurities on the activity of diesel and natural gas oxidation catalysts. In addition,

the goal was to evaluate the effect of precious metal composition (Pt vs. PtPd) on

the oxidation activity of diesel oxidation catalysts, and to define the correlation

between real and simulated ageing.

The sulphur exposure was studied in Papers I and II, respectively. Sulphur was

found to especially shift the C3H6 light-off temperature (T50 and T90) to higher

temperatures on diesel and natural gas oxidation catalysts. The analyses indicated

that sulphur was adsorbed vertically throughout the entire catalyst support material

from the catalyst surface to the metallic monolith. Only a minor or negligible

reduction in the SBET and pore volume values were observed. Sulphate species were

detected on alumina or they were chemically bound as bulk aluminium sulphate,

not on the precious metals. A more detailed inspection of the sulphur treated natural

gas oxidation catalysts indicated a sulphur accumulation close to the noble metal

particles and a slight increase in precious metal particle sizes. Thus, reduced

activity is most probably caused by combination of minor structural changes and

chemical deactivation (sulphates).

The thermally aged natural gas oxidation catalyst was also sulphur treated

(Paper III). The effect of the thermal aging alone on the CH4 oxidation activity is

quite similar to the sulphur treated natural gas oxidation catalyst. Nevertheless, the

sulphur treatment following the thermal aging significantly reduces the CH4

oxidation activity compared to the thermal aged catalyst alone. Based on the

analyses, thermal deactivation caused drastic sintering of the precious metal and

support material, but no further morphology changes were observed after the

sulphur treatment. Therefore, the formation of aluminium sulphate distributed

78

randomly over the thermal aged and the sulphur treated catalyst surface in addition

to the sintered catalyst is most probably the reason for the drastic catalyst

deactivation.

Phosphorus exposure was studied in Papers III-V. The phosphorus treatments

were done using two different phosphorus concentrations (low and high). Both

treatments reduced the catalysts activity, but the high phosphorus concentration

treatment was found to have a more significant decreasing effect on the CO and

hydrocarbons (C3H6 and CH4) oxidation activity than the low phosphorus

concentration treatment. In the case of diesel oxidation catalysts, the most drastic

decrease was detected in the maximum conversion of NO. Based on the analyses,

the phosphorus accumulation on the catalyst surface was higher and the decrease

in the SBET and pore volume was more severe after the high-concentration

phosphorus treatment than after the low-concentration phosphorus treatment.

Phosphorus was observed to accumulate only on the surface region of the precious

metal containing catalysts, forming a fouling layer on the catalyst surface, which

hindered the surface reactions. In addition, phosphorus was mainly found to adsorb

on the alumina by forming chemical bonds and was most likely to form dense,

amorphous aluminium phosphate. Phosphorus treatments were found to slightly

increase the average size of the precious metal particles, and the shape of the metal

particles changed from an irregular form to a mainly spherical form. Therefore,

these significant structural changes together with chemical deactivation

(aluminium phosphates) can be assumed to reduce oxidation activity.

In Papers I, III and IV, the effect of precious metal composition (Pt vs. PtPd)

on the oxidation activity on fresh as well as on sulphur and phosphorus treated

diesel oxidation catalysts (DOC/PtPd and DOC/Pt catalysts) was studied. Two

different gas mixtures, with and without NO were used in the diesel oxidation

catalyst tests. For both gas mixtures, the oxidation activity of fresh and treated

catalysts was higher than that of the DOC/PtPd catalysts over the entire temperature

range. The difference in the oxidation activities between the DOC/PtPd and

DOC/Pt catalysts with a gas mixture without NO was much smaller than with the

gas mixture with NO. It is worth noting that the deactivation effect on the C3H6

oxidation activity with both gas mixtures was found to be more severe on the

sulphur treated DOC/Pt catalyst than the DOC/PtPd catalyst. The phosphorus

treatments reduced the CO and C3H6 oxidation activity more on the DOC/PtPd

catalyst than on the DOC/Pt catalyst especially at higher temperatures (above

170 °C). According to TEM, XPS, XRD and DRIFT analyses, the structural and

chemical changes between the DOC/PtPd and DOC/Pt catalysts were observed to

79

be similar after sulphur and phosphorus treatments. Nevertheless, based on the ICP-

OES results, the amount of the accumulated sulphur was determined to be identical

in the DOC/PtPd and DOC/Pt catalysts, but the amount of phosphorus was higher

in the DOC/Pt catalyst than in the DOC/PtPd catalyst. Thus, in the case of fresh,

sulphur and phosphorus treated DOC/PtPd catalysts, oxidized Pt most probably

lowers the activity for the DOC/PtPd catalyst and is at least one reason for the lower

activity of the DOC/PtPd catalyst compared to the DOC/Pt catalyst.

The correlation between laboratory aging and engine-aging was studied using

the sulphur treated and engine-bench-aged DOC/Pt catalysts (Paper I). The C3H6

and CO oxidation activity of the sulphur treated DOC/Pt catalysts corresponds

relatively well to the results of the engine-bench-aged catalyst. Based on the XPS

results, sulphur was detected to appear mainly as sulphate (SO42-) on both catalysts.

In addition, the SBET and total pore volume values had a relatively good correlation

between the laboratory and engine-bench-aged DOC/Pt catalysts. In addition, good

correlation between the characterization results (chemical state results by XPS and

phosphorus accumulation depth by FESEM-EDS) of the vehicle and laboratory

aged natural gas oxidation catalyst can be seen. In summary, it can be concluded

that the laboratory scale deactivation aging method is a useful tool for simulating

deactivation phenomena caused by sulphur and phosphorus.

To conclude, this thesis provides valuable knowledge about the effect of

sulphur and phosphorus on Pt or PtPd containing alumina based catalysts. This

information can be used to develop more impurity resistant catalytic materials. In

addition, the information gained, the developed catalytic materials and accelerated

ageing procedures can further be used in other applications to abate methane

emissions from fugitive emissions, flue gases, methane filling stations, storing fuels,

and from exhaust gases originating from transportation.

Nevertheless, there is still a need for further research into catalyst deactivation.

In this thesis, exposures to impurities (sulphur and phosphorus) were studied

separately and the next step would be to investigate the combined effect of

impurities on the catalyst. Besides experimental studies, modelling studies could

also bring forth new knowledge that would beneficial in understanding the

deactivation phenomena and the regeneration possibilities of these complicated

materials.

80

81

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Original publications

I Kärkkäinen M, Honkanen M, Viitanen V, Kolli T, Valtanen A, Huuhtanen M, Kallinen K, Vippola M, Lepistö T, Lahtinen J & Keiski RL (2013) Deactivation of diesel oxidation catalysts by sulphur in laboratory and engine-bench scale aging. Topics in Catalysis 56: 672–678.

II Honkanen M, Kärkkäinen M, Kolli T, Heikkinen O, Viitanen V, Zeng L, Jiang H, Kallinen K, Huuhtanen M, Keiski RL, Lahtinen J, Olsson E & Vippola M (2016) Accelerated deactivation studies of the natural-gas oxidation catalyst – Verifying the role of sulfur and elevated temperature in catalyst aging. Applied Catalysis B: Environmental 182: 439–448.

III Kärkkäinen M, Kolli T, Honkanen M, Heikkinen O, Huuhtanen M, Kallinen K, Lepistö T, Lahtinen J, Vippola M & Keiski RL (2015) The effect of phosphorus exposure on diesel oxidation catalysts – Part I: Activity measurements, elementary and surface analyses. Topics in Catalysis 58: 961–970.

IV Honkanen M, Kärkkäinen M, Heikkinen O, Kallinen K, Kolli T, Huuhtanen M, Lahtinen J, Keiski RL, Lepistö T & Vippola M (2015) The effect of phosphorus exposure on diesel oxidation catalysts – Part II: Characterization of structural changes by transmission electron microscopy. Topics in Catalysis 58(14): 971–976.

V Kärkkäinen M, Kolli T, Honkanen M, Heikkinen O, Väliheikki A, Huuhtanen M, Kallinen K, Lahtinen J, Vippola M, & Keiski RL (2016) The influence of phosphorus exposure on a natural-gas-oxidation catalyst. Topics in Catalysis 59: 1044–1048.

Reprinted with permission from Springer (I, III, IV, V) and Elsevier (II).

Original publications are not included in the electronic version of the dissertation.

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