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ISBN 978-952-62-1283-8 (Paperback)ISBN 978-952-62-1284-5 (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 2016

C 578

Ari Väliheikki

RESISTANCE OF CATALYTIC MATERIALS TOWARDS CHEMICAL IMPURITIESTHE EFFECT OF SULPHUR AND BIOMATERIAL-BASED COMPOUNDS ON THE PERFORMANCE OF DOC AND SCR CATALYSTS

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

C 578

ACTA

Ari Väliheikki

C578etukansi.kesken.fm Page 1 Friday, June 3, 2016 9:19 AM

A C T A U N I V E R S I T A T I S O U L U E N S I SC Te c h n i c a 5 7 8

ARI VÄLIHEIKKI

RESISTANCE OF CATALYTIC MATERIALS TOWARDS CHEMICAL IMPURITIESThe effect of sulphur and biomaterial-based compounds on the performance of DOC and SCR catalysts

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in the Arina auditorium (TA105), Linnanmaa, on9 September 2016, at 12 noon

UNIVERSITY OF OULU, OULU 2016

Copyright © 2016Acta Univ. Oul. C 578, 2016

Supervised byProfessor Riitta KeiskiDocent Tanja KolliDocent Mika Huuhtanen

Reviewed byProfessor Luca LiettiProfessor Pascal Granger

ISBN 978-952-62-1283-8 (Paperback)ISBN 978-952-62-1284-5 (PDF)

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

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2016

OpponentProfessor Magnus Skoglundh

Väliheikki, Ari, Resistance of catalytic materials towards chemical impurities. Theeffect of sulphur and biomaterial-based compounds on the performance of DOCand SCR catalystsUniversity of Oulu Graduate School; University of Oulu, Faculty of TechnologyActa Univ. Oul. C 578, 2016University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

Exhaust gas emissions, e.g. nitrogen oxides (NOx), hydrocarbons (HCs) and carbon monoxide(CO), are harmful to human health and the environment. Catalysis is an efficient method todecrease these emissions. Unfortunately, the fuels and lubricant oils may contain chemicalimpurities that are also present in exhaust gases. Thus, catalytic materials with high activity andchemical resistance towards impurities are needed in the abatement of exhaust gas emission.

In this thesis, the aim was to gain new knowledge about the effects of chemical impurities onthe behaviour and activity of the catalysts. To find out these effects, the impurities existing in theexhaust gas particulate matter after combustion of biofuels and fossil fuels were analysed. Thestudied zeolite (ZSM-5), cerium-zirconium mixed oxides (CeZr and ZrCe) and silicon-zirconiumoxide (SiZr) based catalysts were also treated with impurities to simulate the poisoning of thecatalysts by, e.g. potassium, sodium, phosphorus and sulphur, using gas or liquid phase treatments.Several characterization techniques were applied to find out the effects of impurities on catalysts’properties. The activity of catalysts was tested in laboratory-scale measurements in CO and HCoxidation and NOx reduction using ammonia (NH3) and hydrogen (H2) as reductants.

The results revealed that the CeZr based catalysts had a high activity in NOx reduction by NH3and moderate activity by H2. Sulphur was proven to enhance the activity of CeZr catalysts in NOxreduction. This is due to an increase in chemisorbed oxygen after the sulphur treatment on thecatalyst surface. Instead, in HC and CO oxidation reactions, sulphur had a negligible impact onthe activity of the SiZr based diesel oxidation catalyst. Thus, both CeZr and SiZr based catalystscan be utilized in exhaust gas purification when sulphur is present. ZSM-5 based catalysts wereproven to be resistant to potassium and sodium. Alternatively, the activity of SiZr based catalystsdecreased due to phosphorus. Thus, the removal of biomaterial-based impurities from the exhaustgases is needed to retain high catalyst activity in the exhaust gas after-treatment system.

Keywords: catalyst, cerium-zirconium mixed oxides, diesel oxidation catalyst,phosphorus, poisoning, potassium, selective catalytic reduction, silicon-zirconiumoxide, sodium, sulphur, tungsten, zeolite

Väliheikki, Ari, Katalyyttisten materiaalien vastustuskyky kemiallisiaepäpuhtauksia vastaan. Rikin ja biomassapohjaisten polttoaineidenepäpuhtauksien vaikutus DOC- ja SCR-katalyyttien toimintaanOulun yliopiston tutkijakoulu; Oulun yliopisto, Teknillinen tiedekuntaActa Univ. Oul. C 578, 2016Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Pakokaasupäästöissä olevat typen oksidit (NOx), hiilivedyt (HCs) ja hiilimonoksidi (CO) ovathaitallisia ihmisten terveydelle ja ympäristölle. Katalyysi on tehokas menetelmä vähentää näitäpäästökomponentteja. Polttoaineet ja voiteluöljyt sisältävät epäpuhtauksia, jotka siirtyvät myöspakokaasuihin. Tästä johtuen pakokaasupäästöjen hallinnassa tarvitaan katalyyttimateriaaleja,joilla on hyvä vastustuskyky myrkyttymistä vastaan.

Tavoitteena oli saada uutta tietoa kemiallisten epäpuhtauksien vaikutuksesta katalyyttien toi-mintaan. Biopolttoaineiden sisältämät mahdolliset epäpuhtaudet selvitettiin analysoimalla fossii-lisen ja biopolttoaineen palamisessa muodostuvia partikkeleita ja vertaamalla niitä polttoainei-den hivenaineanalyysiin. Tutkimuksessa käytetyt zeoliitti (ZSM-5), cerium-zirkonium-sekaoksi-di (CeZr) ja pii-zirkonium-oksidipohjaiset (SiZr) katalyytit käsiteltiin epäpuhtauksilla (kalium,natrium, fosfori ja rikki) kaasu- ja nestefaasissa. Tutkimuksessa käytettiin useita karakterisointi-tekniikoita, joiden avulla selvitettiin epäpuhtauksien vaikutuksia katalyyttien ominaisuuksiin.Katalyyttien toimintaa testattiin laboratoriomittakaavan kokeissa CO:n ja HC-yhdisteiden hape-tuksessa sekä NOx:ien pelkistyksessä käyttäen ammoniakkia (NH3) tai vetyä (H2) pelkistimenä.

Tulokset osoittavat, että CeZr-pohjaisten katalyyttien aktiivisuus NOx:ien pelkistyksessä olihyvä käytettäessä pelkistimenä NH3:a ja kohtalainen käytettäessä vetyä. Rikki paransi CeZr-katalyyttien aktiivisuutta NOx:ien pelkistyksessä, mikä johtui kemiallisesti sitoutuneen hapenosuudesta katalyyttien pinnoilla. Vastaavasti hiilivetyjen ja CO:n hapetusreaktioissa rikki ei vai-kuttanut SiZr-pohjaisten dieselhapetuskatalyyttien aktiivisuuteen. Sekä CeZr- ja SiZr-pohjaisiakatalyytteja voidaan siten käyttää rikkiä sisältävien pakokaasujen puhdistuksessa. SiZr-pohjais-ten katalyyttien aktiivisuus laski fosforin vuoksi. ZSM-5-pohjaiset katalyytit olivat vastustusky-kyisiä kaliumille ja natriumille. Kestäviä katalyyttejä on siten kehitettävä, mikäli biopolttoainei-den sisältämien epäpuhtauksien poistaminen polttoaineista ei ole mahdollista.

Asiasanat: cerium-zirkonium-sekaoksidi, dieselhapetuskatalyytti, fosfori, kalium,katalyytti, myrkyttyminen, natrium, pii-zirkoniumoksidi, rikki, selektiivinenkatalyyttinen pelkistys, volframi, zeoliitti

To my parents

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9

Acknowledgements

The research work presented in this thesis was conducted in the Environmental and

Chemical Engineering (ECE) research group between 2012 and 2016. During this

time, I had an opportunity to work with numerous of people who have inspired me

to complete this thesis.

First of all, I want to express my deepest gratitude to my principal Supervisor,

Prof. Riitta Keiski for the opportunity to work in her research group and the support

she has given me throughout my doctoral studies. I also want to express my

warmest thanks to my advisors, Doc. Tanja Kolli and Doc. Mika Huuhtanen for

their assistance to complete this work. Thanks for all the support and guidance

during the past few years!

During my doctoral studies, I was also privileged to do a research visit to the

University of Cyprus (Nov-Dec 2012). I would like to thank Prof. Angelos M.

Efstathiou and his group members Dr. Klito Petallidou and Dr. Christos Kalamaras

for their hospitality and sharing their expertise in catalyst research during the visit.

I acknowledge the Finnish Funding Agency for Technology (Tekes), Academy

of Finland and many industrial partners for funding the research projects where I

have worked during my doctoral studies. I acknowledge the University of Oulu

Graduate School and Oulu University Scholarship Foundation for the financial

support. I also appreciate Dinex Ecocat Ltd for providing catalyst materials for this

work.

I want to express my gratitude to Prof. Luca Lietti from Politecnico di Milano

and Prof. Pascal Granger from Université de Lille for the pre-examination of this

thesis and their valuable comments on it.

My warmest thanks to all my colleagues in the ECE group for supporting me

to complete this work. Especially, I want to thank Mrs. Marja Kärkkäinen and Mrs.

Anna Valtanen for the friendship and good collaboration during the research work.

I would also like to thank Mr. Jorma Penttinen and Mr. Markus Riihimäki for their

assistance in laboratory work. I want to thank my office mate Mr. Ari Vuokila for

many cheerful conversations during the past few years. Many thanks to the former

ECE group members, Dr. Kati Oravisjärvi and Mrs. Mari Pietikäinen, for good

collaboration in the article writing process. I would also like to thank other co-

authors of articles, especially Dr. Mari Honkanen from Tampere University of

Technology, Mr. Olli Heikkinen from Aalto University, and Prof. Seppo Niemi

from Turku University of Applied Sciences, for good collaboration. I also want to

10

express my warmest thanks to Dr. Teuvo Maunula and Dr. Kauko Kallinen, from

Dinex Ecocat Ltd, for their valuable advice throughout the thesis work.

Finally, I express my warmest gratitude to my family. I warmly thank my

parents Irma and Seppo and my sister Marika for their continuous support and care.

Last but not least, I express my deepest gratitude to my fiancée Tytti. You help me

to get my thoughts away from the work and bring love to my life!

Oulu, May 2016 Ari Väliheikki

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List of symbols and abbreviations

Latin symbols

c concentration [mol/l]

p pressure [Pa]

p/po relative pressure

SBET specific surface area [m2/g]

t time [s], [min] or [h]

T temperature [K] or [°C]

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

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

TM melting point of material [K]

vol% volume percentage

wt% weight percentage

∆Hr° standard enthalpy of reaction [kJ/mol]

Abbreviations

AAS Atomic absorption spectroscopy

BEA Beta polymorph A

BET Brunauer–Emmett–Teller method

BJH Barrett–Joyner–Halenda method

CeZr Ce-rich cerium-zirconium mixed oxide

DFO Diesel fuel oil

DOC Diesel oxidation catalyst

DPF Diesel particulate filter

DRIFT Diffuse Reflectance Infrared Fourier Transform

ECA Emission Control Area

EDS Energy dispersive spectroscopy

ELPI Electric low pressure impactor

EPA Environmental Protection Agency of the United States of America

EU European Union

FE-SEM Field emission scanning electron microscopy

FGD Flue gas desulfurization

FTIRS Fourier transform infrared spectroscopy

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GHG Greenhouse gas

GHSV Gas hourly space velocity [h-1]

HC Hydrocarbon

ICP-OES Inductively coupled plasma atomic emission spectroscopy

IMO International Maritime Organization

IR Infrared

MARPOL Maritime Pollution

MOR Mordenite

NIST National Institute of Standards and Technology

NOx Nitrogen oxides

OSC Oxygen storage capacity [µmol O/g]

PM Particulate matter

ppm Parts per million (in volume)

RSO Rape seed oil

SCR Selective catalytic reduction

SEM Scanning electron microscopy

SiZr Silicon-zirconium oxide

SO2 Sulphur dioxide

TEM Transmission electron microscopy

TPD Temperature programmed desorption

TUAS Turku University of Applied Sciences

TUT Tampere University of Technology

TWC Three-way catalyst

UCyprus the University of Cyprus

ULSD Ultra-low sulphur diesel

UNFCCC United Nations Framework Convention on Climate Change

UOulu the University of Oulu

USY Ultrastable Y

UV-Vis Ultraviolet–visible spectroscopy

XPS X-ray photoelectron spectroscopy

XRF X-ray fluorescence

ZrCe Zr-rich cerium-zirconium mixed oxides

ZSM-5 Zeolite Socony Mobil-5

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Original publications

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

the text by their Roman numerals:

I Pietikäinen M, Väliheikki A, Oravisjärvi K, Kolli T, Huuhtanen M, Niemi S, Virtanen S, Karhu T & Keiski RL (2015) Particle and NOx emissions of a non-road diesel engine with an SCR unit: the effect of fuel. Renewable Energy 77: 377–385.

II Väliheikki A, Kolli T, Huuhtanen M, Maunula T, Kinnunen T & Keiski RL (2013) The effect of biofuel originated potassium and sodium on the NH3-SCR activity of Fe-ZSM-5 and W-ZSM-5 catalysts. Topics in Catalysis 56: 602–610.

III Väliheikki A, Petallidou KC, Kalamaras CM, Kolli T, Huuhtanen M, Maunula T, Keiski RL & Efstathiou AM (2014) Selective catalytic reduction of NOx by hydrogen (H2-SCR) on WOx-promoted CezZr1-zO2 solids. Applied Catalysis B: Environmental 156–157: 72–83.

IV Väliheikki A, Kolli T, Huuhtanen M, Maunula T & Keiski RL (2015) Activity enhancement of W-CeZr oxide catalysts by SO2 treatment in NH3-SCR. Topics in Catalysis 58: 1002–1011.

The manuscripts for the publications (Papers II-IV) were written by the author of

the thesis. In Paper I, the author was responsible for the chemical analyses of the

particulates and the interpretation of these results. In addition, the manuscript for

Paper I was written in close collaboration with the other authors. Furthermore, the

manuscript for article: “The impact of sulphur, phosphorus and their co-effect on

Pt/SiO2-ZrO2 diesel oxidation catalysts”, done by Väliheikki A, Kolli T, Honkanen

M, Heikkinen O, Kärkkäinen M, Kallinen K, Huuhtanen M, Vippola M, Lahtinen

J, Keiski RL, has been accepted for publication in Topics in Catalysis (In Press).

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15

Contents

Abstract

Tiivistelmä

Acknowledgements 9 List of symbols and abbreviations 11 Original publications 13 Contents 15 1 Introduction 17

1.1 Background ............................................................................................. 17 1.2 Aim of the work ...................................................................................... 19

2 Environmental catalysis 23 2.1 Active materials and promoters .............................................................. 23 2.2 Support materials .................................................................................... 24

3 Emission control techniques 27 3.1 Selective Catalytic Reduction of NOx ..................................................... 27

3.1.1 NOx reduction by ammonia .......................................................... 28 3.1.2 NOx reduction by hydrogen .......................................................... 29 3.1.3 NOx reduction by hydrocarbons and carbon monoxide ................ 30

3.2 Diesel Oxidation Catalysts ...................................................................... 30 4 Catalyst deactivation 33

4.1 General .................................................................................................... 33 4.2 Chemical deactivation ............................................................................. 34

4.2.1 The effect of sulphur .................................................................... 34 4.2.2 The effect of biomaterial-based impurities ................................... 36

5 Experimental 37 5.1 Studied catalyst materials ........................................................................ 37 5.2 Catalyst poisoning treatments ................................................................. 38

5.2.1 Gas phase treatments .................................................................... 39 5.2.2 Treatment by wet impregnation .................................................... 40

5.3 Material characterization ......................................................................... 40 5.3.1 Particulate analysis ....................................................................... 41 5.3.2 Elemental analyses ....................................................................... 43 5.3.3 Physisorption analyses.................................................................. 43 5.3.4 Surface characterization ............................................................... 44 5.3.5 Desorption and surface reaction studies ....................................... 45 5.3.6 Acidity studies .............................................................................. 45

16

5.3.7 Activity measurements ................................................................. 46 6 Results and discussion 49

6.1 Chemical impurities on exhaust gas particulate matter ........................... 49 6.2 Effect of impurities on catalyst properties ............................................... 52

6.2.1 Sulphur ......................................................................................... 52 6.2.2 Biomaterial-based impurities (P, K and Na) ................................. 58

6.3 Catalyst activity studies ........................................................................... 62 6.3.1 NOx reduction ............................................................................... 62 6.3.2 CO and HC oxidation ................................................................... 68

6.4 Temperature programmed studies ........................................................... 70 6.4.1 Catalyst acidity ............................................................................. 70 6.4.2 The surface reaction and desorption of NO .................................. 72

7 Summary and conclusions 75 List of references 79 Original publications 89

17

1 Introduction

1.1 Background

Exhaust gas emissions e.g. nitrogen oxides (NOx), hydrocarbons (HCs), carbon

monoxide (CO) and particulate matter (PM) from mobile and stationary sources

are of major concern, due to their effect on human health and the environment. NOx,

HCs and CO compounds are present in exhaust gases because of an incomplete

combustion of fuels (EPA 1994). NOx can also be formed by an oxidation reaction

of N2 at temperatures above 1000°C. NOx emissions cause respiratory problems in

humans. In addition, NOx causes acid rain, photochemical smog and a depletion of

the ozone layer. (Roy et al. 2009, Skalska et al. 2010) The health effects due to CO

exposure can be lethal, as CO binds to haemoglobin and decreases the oxygen

carrying capacity of blood (Ernst & Zibrak 1998, Raub et al. 2000). HCs can form

many toxic compounds that cause cancer and other dangerous health effects. (EPA

1994)

Therefore, more and more stringent legislation for reducing gaseous emissions

from stationary, mobile, and marine sources has been set by the European Union

(EU) and other countries over the last decades. According to the Euro 6 (in Europe)

and Tier 3 (in the USA) targets, strict limits are set for NOx, HC, CO and PM

emissions from diesel and gasoline cars (EU 2007, EPA 2014a). For non-road diesel

engines, stringent limits for these emissions are set based on the Tier 4 targets (EPA

2014b). Similarly, International Maritime Organization (IMO) MARPOL Annex

VI has adopted the Tier 2 and 3 targets to decrease the NOx emissions from ships

globally and especially tight NOx limits for large ships constructed after 2016 will

be adopted in the North America’s Emission Control Areas (ECAs) and later in the

Baltic Sea ECA (IMO 2015, ICCT 2014). In addition, the greenhouse gas (GHG)

emissions, e.g. carbon dioxide (CO2) and methane (CH4), have been regulated

globally in the Kyoto Protocol since 1997 (UNFCCC 1998). Recently, the global

GHG emissions were under discussion in the Paris Agreement (UNFCCC 2015),

where the target was to prevent the impacts of climate change. The EU has also set

strict regulations for carbon dioxide (CO2) emissions beyond 2020 (EU 2014). The

public interest in emission control has also recently increased e.g. due to violations

in emission tests by car manufacturers.

The use of biofuels has been increasing over the last decades due to the EU

legislation and the wide interest towards renewable fuels (EU 2009). In the

18

European Union, the share of energy from renewable sources to total energy

consumption has increased from 8.3 to 15.0% in 2004–2013. The main renewable

sources were wood and other bio-based fuels, which covered more than half of the

renewable energy produced, whereas the remainder was covered with wind, solar

and hydro power, geothermal energy and waste from renewable sources (Eurostat

2015). Unfortunately, the bio-based fuels contain several elements e.g. phosphorus

(P), calcium (Ca), magnesium (Mg), potassium (K) and silicon (Si). The amounts

of these elements in bio-based fuels are dependent on the quality of raw materials.

For example, biochar and wood consist a lot of alkali metals. (Phyllis2 2016,

Obernberger et al. 2006, Andersson et al. 2007, Suominen 2012). A part of these

elements entrain to exhaust gases and, thus, the catalysts need to be resistant

towards these impurities. In addition, sulphur dioxide (SO2) is present in the

exhaust gases of several applications e.g. coal and petroleum combustion, as well

as shipping (Bowman 1991, Smith et al. 2011, Boscarato et al. 2015). The use of

sulphur containing fuels is a major challenge in developing countries. For example

in India, the total SO2 emissions have increased by 40% between the years 2005–

2010. The growth in SO2 emissions is notable in industrial and energy sectors,

where coal combustion has increased and the current legislation does not require

an installation of flue gas desulfurization units (FGD). (Klimont et al. 2013)

Catalysis is a mature method to reduce exhaust gas emissions. Selective

catalytic reduction (SCR) by ammonia (NH3), urea ((NH2)2CO) and hydrogen (H2)

has proven to be an efficient method in NOx abatement (Granger & Parvulescu

2011). Diesel oxidation catalysts (DOC) are widely used in the removal of

unburned CO and HCs. Unfortunately, some of the catalyst materials that are used

currently, have several drawbacks, e.g. poor thermal stability, low tolerance

towards biomaterial-based compounds (e.g. P, Ca, K) and the toxicity of vanadium

compounds (Chapman 2011, Klimczak et al. 2010, Lagerkvist & Oskarsson 2007),

and the price of platinum and other noble metals (Liu et al. 2015b). Thus, novel

catalyst materials with high resistance towards impurities for emission control are

needed.

Due to health, environmental and legislative issues, there is a demand for

catalyst materials with high activity, selectivity and stability. Catalytic materials

active at low temperatures are needed to avoid cold-start emissions from vehicles.

The cold-start emissions are especially problematic when the ambient temperature

is below 0 °C and the catalyst is not warmed-up (Weilenmann et al. 2009, Dardiotis

et al. 2013). In addition, catalysts are also required for high temperature conditions

e.g. for stationary emission sources where a high activity and long-term stability of

19

catalysts are needed. The increasing use of biofuels in industrialized countries and

the growing number of vehicles that are emitting more SO2 in developing countries

are the driving forces for this study. Thus, the effects of impurities on the catalyst

activity are studied.

1.2 Aim of the work

The main objective of this thesis is to find out catalyst materials that have high

resistance towards chemical impurities existing in exhaust gases. Exhaust gases can

contain impurities originating from both biofuels and fossil fuels due to combustion

of these fuels simultaneously in mobile and stationary applications. In the thesis, it

is clarified how the impurities originating from biofuels, i.e. potassium, sodium and

phosphorus, and fossil fuels i.e. sulphur, affect the catalytic properties and activity

of the studied catalysts. The studied catalyst materials should be non-toxic, they

should meet the tightening emission limits in exhaust gas purification, and have

long-term stability, and high activity and selectivity. The objectives of the thesis

are the following ones:

– to determine the chemical impurities in the particulate matter (PM) existing in

biofuel and fossil fuel exhaust gases;

– to find out the effect of biomaterial-based impurities, potassium (K), sodium

(Na) and phosphorus (P), on the zeolites and SiO2-ZrO2 -based catalysts during

the exhaust gas abatement;

– to find out the effect of sulphur on the SiO2-ZrO2 and CezZr1-zO2 -based

emission abatement catalysts; and

– to gain new knowledge about the suitability of novel catalytic materials for

DOC and SCR applications.

The linkages between the above-mentioned objectives of the thesis and the

appended Papers I-IV and the study by Väliheikki et al. (2016), are shown in Figure

1.

20

Fig. 1. The linkage between the objectives of this thesis.

In Paper I, the chemical composition of impurities that are present in biofuel and

fossil fuel exhaust gases was determined. To find this out, exhaust gas particles of

untreated rape seed oil (RSO) and diesel fuel oil (DFO) are examined. In addition,

the number and particle size distribution of exhaust gas particles is presented. The

conclusions of Paper I give the background information for the following Papers

(II, IV and Väliheikki et al. 2016), in which the effects of chemical impurities on

catalyst performance are examined.

Papers II and Väliheikki et al. (2016) give information about the effects of

biomaterial-based impurities such as phosphorus (P), potassium (K) and sodium

(Na), on catalysts activity in exhaust gas purification. The effects of these

impurities are examined because the bio-based fuels are used to replace fossil fuels

in the transport sector. In Paper by Väliheikki et al. (2016), an accelerated

laboratory scale treatment method is used to simulate the long-term exposure of

catalysts to exhaust gas impurities. This method was based on the previous studies

done at the University of Oulu (UOulu) (Kröger 2007), but the method has been

modified and developed further for this study. In Paper II, a wet impregnation

method is used to simulate the poisoning effect of biomaterial-based impurities on

catalyst performance.

Papers IV and Väliheikki et al. (2016) reveal the potential of cerium-zirconium

(CezZr1-zO2-) mixed oxide and silicon-zirconium oxide (SiO2-ZrO2) based catalysts

Chemical impurities on particulate matter in

exhaust gas

(PAPER I)

Suitability of novelcatalysts for DOC and

SCR applications

(PAPERS III, IV and Väliheikki et al. 2016)

The effect of sulphuron exhaust gas

abatement catalysts

(PAPERS IV and Väliheikki et al. 2016)

The effect of biofuelimpurities on exhaust

gas abatement catalysts

(PAPERS II and Väliheikki et al. 2016)

The resistance of catalytic materialstowards chemical impurities

21

in stationary and mobile applications, where exhaust gases contain sulphur dioxide

(SO2). These kinds of materials are needed, for example, in emerging markets, due

to the risk that the emission abatement catalysts are exposed to exhaust gases

having a high sulphur content.

Papers III and IV discuss the suitability of cerium-zirconium mixed oxide

based catalysts for exhaust gas purification. In Paper IV, tungsten-cerium-

zirconium oxide (WOx-CezZr1-zO2) catalysts were studied in Selective Catalytic

Reduction (SCR) of NOx, using ammonia (NH3) as the reducing agent. Besides,

Paper III provides new information about the performance of the same catalyst with

a novel NOx control technology (H2-SCR). This is one of the first papers ever

published in the topics area where metal oxide catalysts without any noble metal

are used in H2-SCR. Thus, this thesis provides new knowledge on the performance

of SCR catalysts using NH3 and H2 as a reductant in order to find out the most

efficient reductant for the non-noble metal catalysts used in the SCR technology.

The studied catalysts were prepared using facilities provided by the

Environmental and Chemical Engineering research unit at UOulu and also facilities

provided by industry. Industrial partner in cooperation, Dinex Ecocat Oy, provided

the cerium-zirconium mixed oxide support and silicon-zirconium oxide based

catalysts for this study. The catalysts were characterized, using several

characterization methods, and tested in laboratory scale activity tests in model gas

mixtures. The experimental work has mainly been done at the University of Oulu

and a part of the characterization studies has been done at the University of Cyprus

(UCyprus), Tampere University of Technology (TUT), Aalto University (Aalto)

and Turku University of Applied Sciences (TUAS).

This work has been structured as follows. The catalysts used for the emission

control are presented in Chapter 2. In Chapter 3, the types of catalyst deactivation

are discussed. Chapter 4 presents a brief overview about the emission control

techniques used in this thesis. The experimental part containing the studied

catalysts, catalyst poisoning treatment and activity test procedures are presented in

Chapter 5. The research results and discussion are presented in Chapter 6. Finally,

the conclusions of this thesis are summarized in Chapter 7.

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2 Environmental catalysis

Catalysis is an effective method to increase the rate of a chemical reaction. In

catalytic reactions, catalysts decrease the activation energy needed for a reaction.

In principle, catalysts interact with reactants during the reaction, but catalysts

remain unaffected after the reaction. Thus, catalysts can be further used in chemical

reactions. When catalysts are used, the conversions of reactants are higher, since

less energy is required during the reaction and, the yields of products are typically

higher, since catalysts are more selective.

In this thesis, the catalysts studied are in powder and monolith forms. The

catalytic materials consist of active metals, support material and, in some cases,

promoters and their roles are explained in this chapter. In addition, in this chapter,

widely-used catalysts and their most important properties for the catalyst

performance in selected chemical reactions are summarised. Especially, the

catalytic materials used in this study are highlighted.

2.1 Active materials and promoters

Active materials are responsible for the chemical reactions over catalysts. Both

metal oxides and noble metals are used as active materials in catalysts. The

selection of a suitable catalyst is dependent on the reaction and the application.

(Richardson 1989, 26) Noble metals, e.g. platinum (Pt) and palladium (Pd), have

proven to have high activity in the simultaneous oxidation of CO and HCs

(Herreros et al. 2014, Caporali et al. 2014, Kärkkäinen et al. 2015, Arvajova et al. 2016). In addition, platinum containing catalysts have been used in the reduction

of NO, using HCs or H2 as a reductant (Costa et al. 2001, Huuhtanen 2006,

Olympiou & Efstathiou 2011).

Vanadium pentoxide (V2O5) is the most used active material in SCR processes.

Unfortunately, the V2O5-based catalysts have some disadvantages, such as poor

thermal durability (Chapman 2011, Casanova et al. 2012) and some vanadium

compounds (especially V2O5) are described as toxic materials (Domingo 1996,

Lagerkvist & Oskarsson 2007). In the NH3-SCR process, vanadium-based catalysts

have low N2 selectivity and harmful nitrous oxide (N2O) can be produced at

temperatures above 500 °C (Kröcher & Elsener 2008).

Iron (Fe) as an active component has been reported to have good redox

properties. The redox couple (Fe2+/Fe3+) enables the formation of oxygen vacancies,

which are important in the formation of chemisorbed oxygen on a catalyst (Zhu et

24

al. 2015). Iron-zeolite catalysts have shown high activity in NO oxidation, which

is important for the NH3-SCR process (Devadas et al. 2007).

The role of a promoter is to increase the performance of catalytic materials.

Promoters have proven to affect the catalyst activity, selectivity and stability.

Promoters can be divided into four groups: (1) structural promoters increase the

catalyst selectivity by reducing the number of possible reactions, (2) textural

promoters prevent catalyst sintering and increase the hydrothermal stability of a

catalyst, (3) electronic promoters alter the chemical binding of reactants due to e.g.

enhanced acidity, and (4) anti-poison compounds prevent catalyst poisoning caused

by impurities. (Weckhuysen & Wachs 2001)

Some materials can perform both as an active compound and a promoter. For

example, tungsten oxide (WO3) has shown promising results in the NH3-SCR

process when used as an active metal on CeO2 and cerium-zirconium oxides. (Ma

et al. 2012, Chang et al. 2013a, Li et al. 2008). In addition, tungsten is a widely

used promoter in vanadium-based catalysts, in which tungsten has an important

role in maintaining the catalyst structure (Seo & Choi 2015).

2.2 Support materials

The catalysts require support materials that have certain properties e.g. a high

surface area and porosity, good dispersion of the active component and good

interaction between the support and the active component. Depending on the

application, the suitable chemical properties, e.g. acidity and redox, and thermal

stability of the support are also required. (Richardson 1989, 28–35)

Aluminium oxide (Al2O3) based catalysts are proven to be efficient materials

in CO oxidation and NO reduction (Kolli 2006). Al2O3 is a widely used support for

DOCs, due to its good thermal stability and interaction with the active metal, i.e.

Pt (Kim et al. 2013). Unfortunately, Al2O3 based catalysts have shown low

resistance towards chemical impurities such as sulphur and phosphorus (Kröcher

et al. 2009, Kolli et al. 2010, Park et al. 2012, Kärkkäinen et al. 2015).

Silicon dioxide (SiO2) has attractive features when used as a support material,

e.g. a small particle size, high specific surface area and high porosity (Wang et al. 2010). SiO2 also has low basicity that improves the catalyst resistance towards

impurities e.g. sulphur dioxide (Kim et al. 2013). Pure SiO2 has no acid sites on its

surface, but the addition of zirconium oxide (ZrO2) on the SiO2 support creates

strong acid sites on the interface of ZrO2 and SiO2. Zirconium oxide also increases

the catalyst hydrothermal stability as well as the dispersion of the active metal, e.g.

25

platinum, due to a stronger metal-support interaction compared to bare SiO2. (Datka

et al. 1992, Park et al. 2012, Kim et al. 2013).

Zeolites, such as zeolite Socony Mobil-5 (ZSM-5), mordenite (MOR) and beta

polymorph A (BEA), have gained interest in some applications due to their high

activity in the removal of NOx and CO, high surface area and high acidity. Zeolites

are microporous crystalline aluminosilicates composed of silicon, aluminium and

oxygen atoms having a unique and well-defined uniform pore structure (Nath &

Bhakhar 2011). The properties of zeolites are dependent on the molar ratio of Si/Al.

The incorporation of aluminium on the silicon dioxide framework makes the zeolite

framework negatively charged and attracts the extra framework ions. (Payra &

Dutta 2003) For example, iron and copper are used as active substances on zeolites

in SCR reactions (Kröcher & Elsener 2008, Gao et al. 2015). In addition, one

promising alternative for high temperature process conditions is ultrastable Y (USY)

zeolite, which has shown high hydrothermal stability (Ochońska et al. 2012).

Cerium-zirconium mixed oxide (CezZr1-zO2) support materials have shown a

great performance in several applications, e.g. in SCR, three-way catalysis (TWCs),

and diesel oxidation catalysis (DOC), which is mainly due to its high oxygen

storage capacity (OSC) (Li et al. 2008, Shanmuga Priya et al. 2014). Cerium oxide

(CeO2) has reported to have good redox properties (Ce4+ ↔ Ce3+) (Graham et al. 2000, Daturi et al. 2001, Adamowska et al. 2007) and good interaction with other

transition metals to create more active sites e.g. for NOx adsorption (Chen et al. 2014). CeO2 can improve the catalytic activity and thermal stability of traditional

vanadium and zeolite based SCR catalysts (Chen et al. 2014). The role of zirconium

oxide (ZrO2) is to increase the reducibility of CeO2 (Zhang et al. 2010, Wang et al. 2012, Atribak et al. 2010). In comparison with other catalytic materials, the

relatively low price of cerium-zirconium mixed oxides also favours their use in

emission control catalysts. (Liu et al. 2015a).

One of the most commonly used support material is titanium dioxide (TiO2).

As a support material, TiO2 has good properties e.g. chemical stability and safety

concerning both humans and the environment (Hashimoto et al. 2005, Gupta &

Tripathi 2011). Titanium dioxide (TiO2) with V2O5 (as an active metal) is an

efficient catalyst used in NOx reduction reactions. TiO2 increases the acidity of a

vanadium-based catalyst and improves its resistance towards sulphur. (Seo & Choi

2015) However, at high temperatures (<700 °C), TiO2 can transform from anatase

to rutile, which decreases the surface area and activity of TiO2 based catalysts

(Casanova et al. 2012). Alternatively, at low temperatures (<250 °C), the small

26

amount of rutile can improve the activity of V2O5/TiO2 catalysts in NH3-SCR.

(Zhang et al. 2014)

27

3 Emission control techniques

One of the most important applications for catalysts is their use in emission control.

The amount of exhaust gas emissions from stationary and mobile sources can be

decreased by improving the quality of fuel and lubricant oils, developing motor

technology, and using catalysts in exhaust gas after-treatment systems. (Neste Oil

2007) The requirements for the after-treatment system are dependent on the exhaust

gas properties. The after-treatment system in spark-ignition engine (gasoline)

vehicles has to operate in stoichiometric conditions, where rich conditions, i.e.

excess of fuel, and lean conditions i.e. excess of oxygen, are fluctuating. Three-

way catalysts (TWCs) are efficient in HCs and CO oxidation in rich conditions as

well as in NOx reduction in lean conditions. (Song 2015)

Alternatively, the catalyst materials required for compression-ignition engine

(diesel) vehicles where the exhaust gas after-treatment system is operating in lean

conditions continuously, must be developed (Song 2015). In diesel vehicles, the

exhaust gas after-treatment systems contain selective catalytic reduction (SCR)

units and diesel oxidation catalysts (DOCs), among other units, such as diesel

particulate filters (DPF) for PM. SCR units are also used in the purification of

exhaust gases emitted from stationary sources, e.g. power plants. The role of the

SCR unit is to decrease NOx (NO and NO2) emissions by using NH3, H2 or HCs as

a reducing agent. Typically, the share of NO2 in total NOx is 5–15% in diesel

engines emissions before the after-treatment system (Dieselnet 2011). The role of

DOC is to oxidize HCs and CO to CO2 and H2O. In addition, DOC also oxidizes

NO to NO2, which improves the removal of NOx in the following SCR unit and

oxidation of exhaust gas particles in the DPF unit.

In this chapter, the chemical reactions taking place in SCR and DOC units are

presented. The values for the standard enthalpies of reactions (∆Hr°) (T = 25 °C,

p = 101 325 Pa) were calculated, based on the data obtained from National Institute

of Standards and Technology (NIST) Chemistry WebBook (NIST 2015).

3.1 Selective Catalytic Reduction of NOx

Selective catalytic reduction (SCR) is one of the best-known methods in decreasing

NOx emissions. In the SCR process, different reducing agents, such as ammonia

(NH3) and urea ((NH2)2CO), as well as hydrocarbons (HCs), carbon monoxide (CO)

and hydrogen (H2), have been applied to reduce NOx.

28

3.1.1 NOx reduction by ammonia

The most commonly used reducing agent in SCR is ammonia. The NH3-SCR

technology has been applied since the late 1970s, firstly in Japan from where it

spread to Europe and USA (Forzatti 2001). In the NH3-SCR process, NH3 reacts

with NOx emissions and converts these to harmless compounds, nitrogen and water.

As NH3 storage is difficult, urea is widely used as the source for NH3. Urea will

decompose to NH3 and water at temperatures above 200 °C. (Guan et al. 2014)

This technology is utilized in stationary engines, heavy-duty diesel vehicles, and

ships.

The desired overall SCR reactions and their ∆Hr° values per mole of NOx

(NO+NO2) can be presented, as follows (Koebel et al. 2000, Brandenberger et al. 2008, Guan et al. 2014):

4NH + 4NO + O → 4N + 6H O ∆Hr° = −407.1 kJ mol⁄ NO (1)

4NH + 6NO → 5N + 6H O ∆Hr° = −301.5 kJ mol⁄ NO (2)

4NH + 2NO + 2NO → 4N + 6H O ∆Hr° = −378.5 kJ mol⁄ NO (3)

4NH + 3NO → 3.5N + 6H O ∆Hr° = −341.7 kJ mol⁄ NO (4)

Equation 1 shows the standard path of the SCR reaction, because >90% of NOx is

in the form of NO. The reaction described by Eq. 2 without oxygen is slower than

the reaction shown in Eq. 1. Reaction, where both NO and NO2 are present (Eq. 3)

is faster than the main reaction (Eq. 1). Reaction between NO2 and NH3 (Eq. 4) is

much slower than reactions described by Eqs. 1 and 3. (Koebel et al. 2000, Guan

et al. 2014).

In addition to the SCR reactions, the unwanted side reactions are also possible.

Equations 5 and 6 show that N2O is formed through the formation and

decomposition of ammonium nitrate (NH4NO3) (Guan et al. 2014):

2NH + 2NO → NH NO +N +H O ∆Hr° = −446.8 kJ mol⁄ NH NO (5)

NH NO → N O+ 2H O ∆Hr° = −171.0 kJ mol⁄ NH NO (6)

The reaction (Eq. 5) occurs at temperatures below 200 °C in the excess of NO2

(>50% of total NOx). The decomposition reaction (Eq. 6) takes place at

temperatures below 250 °C (Guan et al. 2014).

The formation of N2O is also possible as presented by reactions shown in Eqs.

7–11 (Devadas et al. 2006, Brandenberger et al. 2008):

2NH + 2NO → N O+N + 3H O ∆Hr° = −308.9 kJ mol⁄ NH (7)

29

3NH + 4NO → 3.5N O + 4.5H O ∆Hr° = −265.3 kJ mol⁄ NH (8)

4NH + 4NO + O → 4N O + 6H O ∆Hr° = −267.9 kJ mol⁄ NH (9)

2NH + 2O → N O + 3H O ∆Hr° = −275.8 kJ mol⁄ NH (10)

4NH + 4NO + 3O → 4N O + 6H O ∆Hr° = −325.1 kJ mol⁄ NH (11)

At temperatures below 350 °C, N2O can be formed in the presence of NH3 and N2O

as shown in Eqs. 7–9. At temperatures above 350 °C, the formation of N2O is

possible via the reactions shown in Eqs. 10–11. (Devadas et al. 2006,

Brandenberger et al. 2008)

NH3 can oxidize directly to N2 (Eq. 12) (Brandenberger et al. 2008, Guan et al. 2014):

4NH + 3O → 2N + 6H O ∆Hr° = −316.8 kJ mol⁄ NH (12)

At high temperatures (>500 °C), NH3 is oxidised to NO as presented in Eq. 13

(Koebel et al. 2000, Guan et al. 2014):

4NH + 5O → 4NO + 6H O ∆Hr° = −226.6 kJ mol⁄ NH (13)

3.1.2 NOx reduction by hydrogen

Hydrogen can be used as an alternative reducing agent in the SCR process. The H2-

SCR process can operate at lower temperatures, compared to the NH3-SCR process.

H2-SCR has shown a great potential in NOx reduction over noble metal catalysts.

In H2-SCR, noble metal (Pt and Pd) based catalysts are proven to be efficient

catalysts at temperatures below 200 °C. H2-SCR is also a potential technology to

avoid the problems that are possible when using the urea- or NH3-SCR technology,

e.g. ammonia slip. Similarly to NH3, hydrogen is also a non-carbon-containing

reductant that is needed to decrease carbon dioxide (CO2) emissions. The H2-SCR

technology can especially be used in industrial sites, where H2 gas is available e.g.

petrochemical plants and oil refineries (Olympiou & Efstathiou 2011, Elsherif et al. 2015).

In H2-SCR, the desired NO reduction reaction shown by Eq. 14 (Costa et al. 2007, Olympiou & Efstathiou 2011) is:

2NO + 4H + O → N + 4H O ∆Hr° = −574.0 kJ mol⁄ NO (14)

However, hydrogen can be consumed by the unwanted side reactions (Eqs. 15–17).

In addition, the formation of N2O is possible (Eqs. 16 and 17). (Costa et al. 2007,

Olympiou & Efstathiou 2011):

30

H + 0.5O → H O ∆Hr° = −241.8 kJ mol⁄ H (15)

2NO + H → N O+H O ∆Hr° = −170.2 kJ mol⁄ NO (16)

2NO + 3H + O → N O+ 3H O ∆Hr° = −412.0 kJ mol⁄ NO (17)

3.1.3 NOx reduction by hydrocarbons and carbon monoxide

Apart from NH3, urea and H2 reductants, carbon monoxide (CO) and hydrocarbons

(HCs) have been studied in NOx reduction. Both CO and HCs (e.g. alkenes, alkanes

and aromatic) are present in diesel exhaust gas. (Diehl et al. 2010) Three-way-

catalysis (TWC) is a commercially used technology that uses both reducing agents

HCs and CO in NOx abatement. However, the excess amount of oxygen inhibits

NOx reduction (Kašpar et al. 2003) Thus, the solution for the HC-SCR technology

could be the extra injection of reductants directly to the diesel exhaust gas stream

(Shelef 1995). Recently, Toyota has developed a new NOx reduction system, where

high catalytic performance is achieved by improving the injection of HC reductant

(Uenishi et al. 2014). The positive aspect with this technology is that the installation

of additional containers for reductants, e.g. urea, are not needed. For the HC-SCR

reactions, several catalyst materials have been studied e.g. Al2O3, TiO2 and zeolite

based catalysts (Huuhtanen 2006, Maunula 2007). However, the HC-SCR

technology increases the fuel consumption and CO2 emissions, and thus, the focus

of studies should be more on other technology solutions and reducing agents.

3.2 Diesel Oxidation Catalysts

Diesel oxidation catalysts (DOCs) are used in the oxidation of hydrocarbons (HCs),

carbon monoxide (CO) and nitrogen monoxide (NO). However, the diesel exhaust

gas contains tens of different kinds of HCs, including alkanes, alkenes and aromatic

hydrocarbons (Siegl et al. 1999, Diehl et al. 2010). Thus, the number of HC

oxidation reactions are numerous in the real life emission abatement. The desired

reactions over DOC are presented in Eqs. 18 and 19, where HCs are marked as

HyCx. (Twigg & Webster 2005):

CO + 0.5O → CO ∆Hr° = −283.0 kJ mol⁄ CO (18)

H C + x + y 4⁄ O → xCO + y/2H O (19)

In this thesis, propene (C3H6) was selected to simulate the hydrocarbons present in

the exhaust gas stream, as it is commonly agreed to be the HC model compound

31

(Burch & Watling 1998, Maunula 2007). A substantial number of reactions between

C3H6, O2 and NOx are possible (Huuhtanen 2006) and, thus, for simplicity, the

following reaction (Eq. 20) presents the complete oxidation of C3H6.

C H + 4.5O → 3CO + 3H O ∆Hr° = −1926.5 kJ mol⁄ C H (20)

DOC also oxidizes NO to NO2 (Eq. 21) (Twigg & Webster 2005):

NO + 0.5O → NO ∆Hr° = −57.2 kJ mol⁄ NO (21)

The oxidation of NO to NO2 (Eq. 21) is also desired because an equal amount of

NO and NO2 in the exhaust gas improves the catalyst activity in SCR (Rahkamaa-

Tolonen et al. 2005). The higher amount of NO2 is also beneficial for the operation

of a diesel particulate filter (DPF), due to the excellent ability of NO2 to oxidize

particles.

32

33

4 Catalyst deactivation

4.1 General

Catalysts can lose their activity over time and the phenomenon is called

deactivation. This is an undesired phenomenon, because replacing or regenerating

the deactivated catalysts in industrial applications is time-consuming and expensive.

The reasons for the catalyst deactivation can be mechanical, chemical and thermal.

Mechanical deactivation is caused by a failure or fouling of catalyst particles.

Particle failure means the crushing or attrition of a catalyst. This can results in the

channelling of a catalyst bed and plugging of pores. Particle failure also increases

the risk of more serious deactivation phenomena e.g. coking or thermal

deactivation. Another type of mechanical phenomenon, fouling, means the

mechanical deposition of particles from a gas stream on the catalyst surface.

Fouling causes pore plugging and the loss of active sites. (Richardson 1989, 191–

192, Bartholomew 2001).

Thermal deactivation can be due to volatilization of the active component,

formation of in-active compounds, phase transformation and sintering of the

catalyst. Component volatilization occurs due to gas molecules, e.g. O2, and their

reactions with the active component of a catalyst to produce a volatile compound.

In the case of platinum, the formation of a volatile compound i.e. PtO2 occurs at

temperatures above 700°C. This causes the loss of an active component, which is

flushed away in the gas stream. Compound formation means that the catalysts react

with gas molecules, producing less active compounds on catalysts. Compound

formation is not a surface phenomenon, but may occur in the catalyst bulk.

Sintering means structural changes on a catalyst or substrate that decreases the

catalyst surface area. (Bartholomew 2001, Richardson 1989, 191–195) This

phenomenon can occur at Tammann temperatures (0.5·Tm, where Tm is the melting

point of material, in the absolute scale), where the ions of the bulk have enough

thermal energy for bulk-to-surface migrations. Furthermore, at Hüttig temperatures

(0.3·Tm in the absolute scale), the surface species have sufficient mobility to

agglomerate and sinter (Edwards et al. 2002). Sintering is also accelerated in the

presence of water (Bartholomew 2001). Phase transformations can decrease the

catalyst’s activity by reducing the catalyst surface area. (Richardson 1989, 193).

For example, the activity of vanadium-based SCR catalysts decreases due to the

TiO2 support phase transformation from anatase to rutile at high temperatures

34

(>700 °C) (Casanova et al. 2012). The phase transformation from anatase to rutile

can be reduced by using additives, e.g. CeO2, on TiO2 (Glen et al. 2011).

Chemical deactivation is due to coking or poisoning. Coking is due to the

carbon containing residues, which cover the catalyst surface. Coking causes pore

plugging and the loss of active sites. Catalyst activity can be retained by

regenerating by burning or replacing the coked catalyst. (Richardson 1989, 210)

This thesis focuses on chemical deactivation, which is discussed in the following

chapter.

4.2 Chemical deactivation

The activity of a catalyst can be decreased due to the chemical deactivation e.g.

poisoning. Poisoning is caused by a strong chemisorption of impurities on the

active sites of a catalyst. The impurities can e.g. block the active sites or affect the

reactants’ adsorption on catalysts. The impurities can also change the chemical

nature of active sites or cause the formation of new compounds. (Forzatti & Lietti

1999)

Poisoning can be selective or non-selective. Selective poisoning means that the

impurities concentrate on certain surface sites, depending on e.g. catalyst acidity.

For example, sulphur and alkali metals (K and Na) can selectively adsorb on the

active sites of catalyst materials. Non-selective poisoning means that the impurities

adsorb uniformly on the catalyst surface. For instance, the formation of K and Na

sulphates/phosphates is possible and these compounds can deposit on the surface

of the catalysts. (Argyle & Bartholomew 2015, Forzatti & Lietti 1999) In addition,

catalyst poisoning can be reversible or irreversible. Reversible poisoning means

that the impurities can be removed from the catalyst surface and catalyst activity is

restored. Eliminating impurities from the feed stream is the best way to avoid

poisoning. In the case of irreversible poisoning, the impurities remain on the

catalyst and the catalyst is deactivated permanently. (Forzatti & Lietti 1999)

4.2.1 The effect of sulphur

Sulphur dioxide (SO2) is present in exhaust gases e.g. coal and petroleum

derivatives’ (gasoline, diesel) combustion and shipping, as well as in flue gases

originated e.g. from metal smelting (Bowman 1991, Smith et al. 2011). In the

industrial processes, sulphur mainly forms gaseous SO2, but it can also oxidize to

sulphates (SO42-) in the presence of alkali and alkaline earth metals. Formed

35

sulphates can then react with fly ash particles present in flue gases and deposit on

the surfaces of processing units e.g. heat exchangers. (Obernberger et al. 2006).

Furthermore, the sulphur species can also adsorb on the commercial DOC, as well

as on SCR catalysts which can lead to the formation and deposition of ammonium

sulphates ((NH4)2SO4). (Kröcher et al. 2009)

Sulphur has proven to decrease the activity of both ZSM-5 (e.g. de Oliveira et al. 2009a, de Oliveira et al. 2009b, Cheng et al. 2010) and Al2O3 based catalysts

(e.g. Kröcher et al. 2009, Kolli et al. 2010, Park et al. 2012, Kärkkäinen et al. 2013).

The activity of Al2O3 based catalysts can decrease due to the formation of

aluminium sulphates (Al2(SO4)3).

In addition, sulphur has a twofold effect on the activity of vanadium (V)-based

catalysts. In the vanadium-based catalysts, titanium oxide (TiO2) is normally used

as a support. These catalysts have been reported to have a good resistance towards

SO2 in the absence of water (Huang et al. 2002, de Oliveira et al. 2009a, de Oliveira

et al. 2009b, Magnusson et al. 2012). Instead, some studies reveal that the activity

of vanadium-based catalysts in NH3-SCR decreases due to SO2 in the presence of

water. (Huang et al. 2002, Magnusson et al. 2012).

The SiO2 support is inert towards sulphur poisoning (Dawody et al. 2005).

However, sulphur can poison the Pt/SiO2 catalyst by sintering the active metal (Pt),

which is possible due to the weak metal-support interaction (Kim et al. 2013).

Sulphur has also proven to decrease the activity of Pt/SiO2-ZrO2 based catalysts in

NO reduction (Park et al. 2012), as well as in CO and C3H6 oxidation (Kim et al. 2013).

In the case of CeO2, sulphur has proven to form surface and bulk sulphates

(Ce(SO4)2) (Luo & Gorte 2004, Ryou et al. 2015). Sulphate species on CeO2

increases e.g. NH3 adsorption that can increase the activity of the catalyst in NOx

reduction (Gu et al. 2010, Chang et al. 2013b). Instead, in the case of ZrO2, the

formation of surface sulphates on ZrO2, e.g. ZrO(SO4) has been observed (Luo &

Gorte 2004). The addition of sulphur on Pt/ZrO2 during the catalyst preparation has

proven to increase its activity in propane oxidation, which is due to an increased

acidity of the support (Yazawa et al. 2002). In addition, sulphur can also affect the

activity of the cerium-zirconium oxide based catalysts. Sulphur on cerium-

zirconium oxide increases acidity and improves the catalyst resistance towards

alkali impurities (Gao et al. 2014).

36

4.2.2 The effect of biomaterial-based impurities

Due to legislative and environmental reasons, an increased share of total energy

consumption will be covered by using biofuels. The source materials for biofuels

e.g. straw, grain and wood contain, however, impurities such as potassium, sodium,

calcium and phosphorus (Cuiping et al. 2004, Obernberger et al. 2006). These

impurities remain during the combustion of biofuels and, thus, entrain to the

exhaust gas stream. Thus, these impurities are present in after-treatment systems

and are possible reasons for the poisoning of a catalyst.

Potassium and sodium exist in straw and grain (Obernberger et al. 2006).

Potassium decreases the activity of vanadium-based catalysts, which is due to the

decreased ammonia adsorption capacity (Nicosia et al. 2008, Klimczak et al. 2010,

Liu et al. 2015b). The effect of potassium can be hindered by an addition of sulphur

to vanadium-based catalyst, which increases the acidity of the catalyst (Putluru et al. 2012). Several iron and copper containing zeolite (e.g. ZSM-5, MOR and BEA)

catalysts have better resistance towards potassium than vanadium-based catalysts.

However, potassium also decreases the acidity and activity of iron- and copper-

zeolite catalysts. (Kern et al. 2010, Putluru et al. 2012) Potassium can also cause

agglomeration of active metal species, via forming clusters on copper-zeolite

catalysts (Ma et al. 2015).

Similarly, sodium decreases the activity and acidity of both vanadium-based

and zeolite-based catalysts (Klimczak et al. 2010, Kern et al. 2010). Sodium has

proven to decrease the activity of CeO2/TiO2 catalysts due to decreased acidity and

inhibition of the Ce4+/Ce3+ redox cycle (Wang et al. 2013a).

Phosphorus can originate from biofuels and lubricant oils (Obernberger et al. 2006, Andersson et al. 2007, Winkler et al. 2008). Phosphorus forms Zn, Mg or Ca

phosphates with other oil-derived contaminants that can contaminate the catalyst

surface. Phosphorus can also react with the catalyst washcoat and form compounds

e.g. aluminium phosphates (AlPO4). (Rokosz et al. 2001) Phosphorus also

decreases the activity of a zeolite-based SCR catalyst by lowering the catalyst

surface area. Phosphates also hinder the redox properties of active sites, which are

responsible for the NO oxidation to NO2. (Kern et al. 2010) In the case of cerium

containing catalysts, phosphorus can form cerium phosphates (CePO4). These

compounds decrease the oxygen storage capacity (OSC) and the catalyst activity

(Xu et al. 2004, Christou et al. 2011). Phosphorus also decreases the specific

surface area of a cerium-zirconium mixed oxide catalyst (Christou et al. 2011),

which indicates catalyst deactivation by sintering.

37

5 Experimental

5.1 Studied catalyst materials

The studied catalysts were in a powder form (SCR catalysts) and as metallic

monoliths (DOC). Figure 2 presents examples of the studied catalysts.

Fig. 2. The examples of studied powder and metallic monolith catalysts.

In the case of powder form catalysts, the target amount of a catalytically active

metal (added on the support via wet impregnation) was 3 wt% of iron (Fe) in the

case of ZSM-5 or tungsten (W) in the case of cerium-zirconium mixed oxide

support. The Ce-rich cerium-zirconium mixed oxide (marked as CeZr) and Zr-rich

cerium-zirconium mixed oxides (marked as ZrCe) were provided by Dinex Ecocat

Oy. The zeolite (NH4-ZSM-5, marked as ZSM-5) support was provided by Zeolyst

International, and iron nitrate (Fe(NO3)3 × 9 H2O,) and ammonium metatungstate

hydrate ((NH4)6H2W12O40 × xH2O) precursor materials for active metal (Fe or W)

were purchased from Sigma Aldrich.

At first, the precursor materials were dissolved in distilled water. The solution

was added on a wetted support. The resulting solution of a precursor and a support

was mixed with a magnetic stirrer for 5 hours (Fe-ZSM-5) or overnight (for W-

CeZr and W-ZrCe). Then, the samples were dried overnight at 110–120 °C. Finally,

38

the catalysts were calcined in a muffle oven for 1 h at 150 °C, 2 h at 350 °C and 1

h at 600 °C with a heating rate of 5 °C/min between the steps. (Papers II, III, IV)

The studied monolith-type catalyst contained 0 or 0.5 wt% of platinum (Pt) as

the active material on a silicon-zirconium oxide (SiO2-ZrO2, marked as SiZr)

support. The support material consisted of SiO2-ZrO2 mixed oxide (80 wt%) with

SiO2 as a binder (20 wt%), which resulted in the SiO2/ZrO2 weight ratio of 30/70

in the final coating. Both the Pt/SiZr catalyst and the SiZr support monoliths were

provided by Dinex Ecocat Oy and tested as diesel oxidation catalysts (DOCs).

(Väliheikki et al. 2016) The list of the studied catalysts and their compositions are

presented in more detailed in Table 1.

Table 1. The list of studied catalyst materials.

Catalyst Active metal Support (weight ratio) Used in Paper

ZSM-5 - SiO2/Al2O3 (93/7) II

Fe-ZSM-5 Fe 3.3 wt% SiO2/Al2O3 (93/7) II

W-ZSM-5 W 2.1 wt% SiO2/Al2O3 (93/7) II

ZrCe - CeO2/ZrO2 (17/83) III

CeZr - CeO2/ZrO2 (85/15) III, IV

W-ZrCe W 3 wt% CeO2/ZrO2 (17/83) III

W-CeZr W 3 wt% CeO2/ZrO2 (85/15) III

W-CeZr W 3.6 wt% CeO2/ZrO2 (85/15) IV

SiZr - SiO2/ZrO2 (30/70) Väliheikki et al. (2016)

Pt/SiZr Pt 0.5 wt% SiO2/ZrO2 (30/70) Väliheikki et al. (2016)

The composition of ZSM-5 was calculated, based on the molar ratio information

given by a catalyst manufacturer, Zeolyst International. The composition of CeZr,

ZrCe and SiZr were given by Dinex Ecocat Oy.

5.2 Catalyst poisoning treatments

The impacts of selected biomaterial-based impurities and sulphur on catalyst

performance were examined. Therefore, the studied catalysts were treated, using

two accelerated laboratory scale deactivation methods: a gas phase treatment and

wet impregnation. These methods were used to simulate the long–term exposure of

impurities present in the exhaust gas stream.

39

5.2.1 Gas phase treatments

Gas phase treatments were done for both powder and monolith form catalysts. The

W-CeZr catalyst or CeZr support in a powder form was placed in a vertically

positioned tubular quartz reactor between layers of inert quartz and glass wool. The

mass of sample was 1 g. (Paper IV). The monolith type Pt/SiZr catalyst (diameter

1.0 cm, length 3.7 cm, mass 1.5 g) was able to retain its position without wools

inside the reactor (Väliheikki et al. 2016). At first, the reactor was heated to 400 °C

(10 °C/min) in a gas mixture of 10 vol% of synthetic air and 90 vol% of N2. The

treatments were then done for 5h at 400 °C, using water (H2O), sulphur dioxide

(SO2) or aqueous ammonium phosphate ((NH4)2HPO4). The aqueous solution with

the feeding rate of 0.069 cm3/min was fed using a peristaltic pump. These gas phase

treatments were done, according to the following compositions:

1. S-treatment: 100 ppm SO2, 10% air, and balance N2

2. S+H2O-treatment: 100 ppm SO2, 10% H2O, 10% air, and balance N2

3. P+H2O-treatment: 10% H2O containing c((NH4)2HPO4)=0.13M, 10% air, and

balance N2

After the treatment, the reactor was cooled down to room temperature under a gas

mixture of 10 vol% of synthetic air and 90 vol% of N2. The flow was kept constant

(1 dm3/min) throughout the experiment. A schematic picture of the experimental

set-up is shown in Figure 3. (Paper IV and Väliheikki et al. 2016)

40

Fig. 3. Schematic picture of the experimental set-up for the gas phase treatments.

5.2.2 Treatment by wet impregnation

At first, potassium and sodium treatments were tested, using the equipment for

gaseous phase poisoning presented in our earlier studies (Kröger 2007). The treated

samples were analysed and the amounts of potassium (K) and sodium (Na) were

below the detection limits. Thus, the studied zeolite-based catalysts were treated by

alkali metals, using a wet impregnation method (Paper II).

K- and Na-treatments were done in the liquid phase using potassium nitrate

(KNO3) or sodium nitrate (NaNO3) as a precursor. The desired amounts were 1 wt%

of K or Na on the catalyst. KNO3 or NaNO3 was dissolved in 10 ml of distilled

water. Then, the precursor solution was mixed with the wetted catalyst and stirred

for 5h at room temperature. After the treatment, the catalyst was dried in static air

at 110 °C overnight. Finally, the catalysts were calcined in a muffle oven at 150 °C

for one hour, at 350 °C for two hours and at 600 °C for one hour.

5.3 Material characterization

Several techniques and methods were used for material characterization. The

characterizations were carried out at the University of Oulu (UOulu), the University

of Cyprus (UCyprus), Tampere University of Technology (TUT) and Aalto

41

University (Aalto) and Turku University of Applied Sciences (TUAS). In Table 2,

the places where the experiments were done and the papers in which the results are

published are presented.

Table 2. The material characterization and activity testing methods used.

Analyzing method Used in Paper Place where experiment was done

AAS II UOulu

Activity

NH3-SCR II, IV UOulu

H2-SCR III UCyprus

DOC V UOulu

BET/BJH II – IV, Väliheikki et al. (2016) UOulu

DRIFTS IV, Väliheikki et al. (2016) UOulu

ELPI I TUAS

ICP-OES II UOulu

NH3-DRIFTS III UCyprus

NH3-TPD III, IV UCyprus, UOulu

NO-TPD III, Väliheikki et al. (2016) UCyprus, UOulu

SEM-EDS I, Väliheikki et al. (2016) UOulu, TUT

TEM-EDS II, Väliheikki et al. (2016) UOulu, TUT

UV-VIS III UCyprus

XPS IV, Väliheikki et al. (2016) UOulu, Aalto

XRF IV UOulu

The SEM-EDS, TEM-EDS and XPS analyses were done using facilities provided

by the Center of Microscopy and Nanotechnology at UOulu. In addition, AAS and

ICP-OES analyses were done at UOulu by the laboratory personnel of the Faculty

of Technology and the Trace Element Laboratory in the Faculty of Science,

respectively.

5.3.1 Particulate analysis

The chemical composition and size distribution of the particulate matter (PM) in a

diesel engine exhaust gas stream were studied (Paper I). This study was done in

order to gain knowledge about impurities present in the exhaust gas stream. PMs

were collected during the ISO 8178 E2 driving cycle using a non-road diesel engine.

The fuels used were diesel fuel oil (DFO) and untreated rape seed oil (RSO). The

concentrations of trace elements in the RSO and DFO fuels are presented in Table 3.

42

Table 3. The concentrations of trace elements in the RSO and DFO fuels (Paper I).

Element

(mg/kg)

Mg K Na Ca P Al Cu Fe Mn Si V Zn

RSO 55 13 1.3 324 366 0 0 0.5 0.4 0 0 0.8

DFO <dl <dl <dl <dl <dl n.a. n.a. n.a. n.a. n.a. n.a. n.a.

<dl = under detection limit (1 ppm), n.a. = not analysed

The concentrations of all the analysed trace elements in DFO were below the

detection limits (Table 3). The duration of the particle measurement was 40 min

and the experiments were done with and without an SCR catalyst. Aqueous urea

solution (AdBlue) was injected into the exhaust gas stream before the SCR catalyst

unit.

An Electric Low Pressure Impactor (ELPI) system was used to collect the

exhaust gas particles. The ELPI sampling line was connected to the engine exhaust

gas stream through a two-stage diluter. At first, the exhaust gas was diluted with air

at 200 °C, followed by dilution with air at room temperature. The total flow to the

ELPI system was 10.3 dm3/min and the exhaust gas particles (mean diameter 0.02–

6 μm) were collected on aluminium foils (Figure 4). The particle size distribution

of the exhaust gas particles was analysed using the ELPI system.

Fig. 4. The aluminium foil used for exhaust gas particles. (Paper I)

After the engine test procedure, the concentrations of chemical elements on

aluminium foils were analysed, using a Field Emission Scanning Electron

43

Microscope (FESEM, Zeiss ULTRA plus) equipped with an Energy-Dispersive X-

ray Spectroscopy (EDS). The analyses were done from 3 different points from the

aluminium foil. The concentrations of chemical impurities (P, Ca, K, Mg, Zn, S, Si,

Fe and Cu) on aluminium foils were analysed.

5.3.2 Elemental analyses

Atomic adsorption spectrometer (AAS) was used to analyse the potassium and

sodium concentrations on the studied ZSM-5 catalysts (Paper II). Prior to the

analysis, the catalysts were diluted to aqua regia. Then, the experiments were

carried out with a Perkin Elmer Analyst 400 apparatus, using the flame method for

atomizing the sample.

Inductively coupled plasma optical emission spectrometry (ICP-OES) was

used to analyse the amount of an active substance, e.g. iron and tungsten, on the

ZSM-5 catalysts (Paper II). Before the analyses, catalysts were diluted by aqua

regia. The experiments were carried out, using Perkin Elmer Optima 5300 DV.

An X-ray fluorescence spectrometer (XRF) was used to obtain information

about the element concentrations on the fresh and treated W-CeZr catalyst and the

CeZr support (Paper IV). These experiments were carried out, using a PANalytical

Axios mAX 4kW spectrometer with an Omnian standardless analysis method. The

studied sample was mixed with a flux (Fluxana FX-X65-2) in Pt-Au crucible and

melted in a furnace at 1150 °C. The molten sample was poured into a mould and

cooled to room temperature in static air before the measurement.

5.3.3 Physisorption analyses

The specific surface areas (SBET), pore sizes and pore volumes of all the studied

catalysts were determined by nitrogen adsorption at -196 °C, using a Micrometrics

ASAP2020 analyser (Papers II-IV, Väliheikki et al. 2016). At first, the catalysts

were pre-treated in a vacuum for 30 min at 150 °C and followed by evacuation for

120 min at 300 °C. The reactor chamber was then cooled to room temperature and

filled with nitrogen gas. Before analysis, gaseous N2 was evacuated from the

reactor chamber. The specific surface areas (m2/g) were determined by nitrogen

adsorption at -196 °C at specific pressure range (p/po=0.06–0.20) according to the

Brunauer-Emmett-Teller (BET) method. Pore volumes and pore sizes characteristic

were determined by N2 adsorption at certain partial pressures (p/po=0.14–0.99),

according to the Barrett-Joyner-Halenda (BJH) theory.

44

5.3.4 Surface characterization

X-ray photoelectron spectroscopy (XPS) was used to analyse chemical states and

concentrations of chemical elements on the surface of W-CeZr and Pt/SiZr catalysts.

These experiments were carried out using two spectrometers: Thermo Fisher

Scientific ESCALAB 250Xi spectrometer in the Center of Microscopy and

Nanotechnology, UOulu (Paper IV), and Surface Science Laboratories SSX-100 at

Aalto University (Väliheikki et al. 2016). Both analysers were equipped with a

monochromatic Al Kα (1486.6 eV) X-ray beam that was used as the radiation source.

The samples were put on the indium substrate and then transferred into the

measurement chamber. Specific software was used in data analysis and mixed

Gauss-Lorentz functions in fitting the core level spectra. The binding energy values

were calibrated, using C1s peak as a reference (284.6–284.8 eV).

Ultraviolet–visible spectroscopy (UV-VIS) studies were done to characterize

chemical states of elements and energy band gap of W-CeZr and W-ZrCe catalysts

(Paper III). The experiments were done in the wavelength range of 270–800 nm,

using a Perkin-Elmer Lambda 950 spectrometer equipped with a high-

temperature/high-pressure temperature controllable DRS cell (Harrick, Praying

Mantis). Before the analysis, the catalysts were pre-treated at 150 °C for 1 hour in

an argon flow (Ar, 60 cm3/min) and then cooled to 25 °C before the data acquisition

was performed. The high reflectance material (FluorilonTM) was used as a reference

material.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was

used to characterize the surface molecules on fresh and treated W-CeZr and Pt/SiZr

catalysts prior and after the treatments (Paper IV, Väliheikki et al. 2016). These

experiments were carried out, using a Bruker Vertex V80 vacuum FTIR

spectrometer with a Harrick Praying MantisTM DRIFT equipment. The

measurements were done in atmospheric pressure at 25 °C, using a mirror as a

reference. The spectra were recorded in the range of 4000–400 cm-1 with a

resolution of 4 cm-1.

Field emission scanning electron microscopy (FESEM) studies were done in

order to analyse the sulphur and phosphorus concentrations on the Pt/SiZr catalyst

surface (Väliheikki et al. 2016). The analyses were done using a Zeiss ULTRAplus

microscope equipped with an energy dispersive X-ray spectrometer (EDS, INCA

Energy 350, Oxford Instruments). The cross-sectional samples were carbon coated

to avoid sample charging during the analysis.

45

Transmission electron microscope (TEM) studies were carried out to analyse

changes in morphology, particle size and elemental concentrations of the zeolite-

based catalyst before and after the treatments. The experiments were done, using

two spectrometers: a LEO 912 OMEGA Energy filtered TEM was used for the

powdered samples at UOulu (Paper II) and a Jeol JEM-2020 TEM equipped with

an Noran Vantage Energy dispersive X-ray spectrometer in TUT for the powdered

and cross-sectional samples (Väliheikki et al. 2016).

5.3.5 Desorption and surface reaction studies

Temperature programmed desorption (TPD) of NO was done to find out the active

sites on catalysts for NO adsorption. The studied Pt/SiZr catalyst was pre-treated at

300 °C (Väliheikki et al. 2016) under a N2 flow. Then, the catalyst was cooled to

room temperature. The NO adsorption (1% NO/N2) was done for 30 min at 25°C,

followed by the NO-TPD run in a N2 flow at 25–500 °C, using a heating rate of

20 °C/min. These studies were carried out at UOulu, using a GasmetTM FT-IR to

analyse the concentrations of gaseous components. In addition, NO-TPD studies

were also done for W-CeZr and W-ZrCe catalysts (Paper III). First, the catalysts

were pre-treated at 600 °C. Then, the catalyst was cooled to 25°C in a helium (He)

flow, followed by a treatment with 0.1% NO/He for 30 min. Thereafter, the NO-

TPD run was done at 25–600 °C, using a heating rate of 30°C/min in a He flow.

These studies were carried out, using an on-line quadrupole mass spectrometer

(Omnistar, 1-300 amu, Balzers) for data analysis at UCyprus.

Temperature programmed surface reaction (TPSR) studies were done to find

out the active sites for NO in the H2-SCR reaction (Paper III). First, the W-CeZr or

W-ZrCe catalyst was pre-treated at 600 °C under a 20% O2/He flow. Then, the

catalyst was cooled down to room temperature and the catalyst was treated with

0.1% NO/10% O2/He for 30 min. After that, the reactor was purged with He. The

TPSR run was done in a gas mixture of 1% H2/5% O2/He at 25–600 °C, using a

heating rate of 30°C/min. These experiments were done at UCyprus with the same

apparatus that was used for the NO-TPD studies.

5.3.6 Acidity studies

Studies of the temperature programmed desorption (TPD) of NH3 were done to

obtain information about the acidity of W-CeZr and W-ZrCe catalysts (Papers III

and IV). Before the experiments, the materials were pre-treated in-situ at high

46

temperature to remove the moisture. Then, the studied catalyst was treated in a 1%

NH3/N2 flow for 30 min. Then, the reactor was purged with inert gas (He or N2).

Thereafter, NH3 desorption was done under an inert gas flow at 25–600 °C, using

a heating rate of 30 °C/min at UCyprus and 20 °C/min at UOulu. The NH3-TPD

experiments were done using the on-line quadrupole mass spectrometer (Omnistar,

1-300 amu, Balzers) at UCyprus and GasmetTM FT-IR gas analyser at UOulu.

In situ NH3-DRIFTS studies were done to find out the catalyst active sites for

NH3 adsorption (Paper III). These experiments were done, using a Perkin-Elmer

Frontier FT-IR spectrometer equipped with a specific DRIFT cell (Harrick, Praying

MantisTM). At first, the catalyst was put in a ceramic cup of a DRIFTS cell. The

catalyst was pre-treated at 600 °C in a 20%O2/He flow followed by purging in an

Ar flow. The background spectra were recorded at desired temperatures (25–

600 °C), while the catalyst was cooled to room temperature in an Ar flow. Then,

the NH3 adsorption was done at 25 °C in a 0.55%NH3/Ar/He gas flow. After that,

the reactor was purged with Ar. The spectra were measured in a Ar flow at desired

temperatures (25–600 °C) and subtracted from the background spectra.

5.3.7 Activity measurements

The activity measurements were done to study the activity of DOC and SCR

catalysts. At UOulu, the activity of a monolith-type SiZr based DOC, as well as the

NH3-SCR activity of Fe-ZSM-5 or W-CeZr catalysts, were studied. At UCyprus,

the studied catalysts for H2-SCR were W-CeZr and W-ZrCe. The catalyst activity

was studied under the reaction gas mixtures that simulate the lean diesel exhaust

gas conditions with H2 or NH3 as the reducing agent (Table 4).

Table 4. The used reaction gas mixtures studied in activity measurements.

Activity NO NO2 NH3 H2 O2 H2O CO C3H6 CO2 N2 He

measurement [%] [%] [%] [%] [%] [%] [%] [%] [%] [%] [%]

Oulu

Mix. 1 0.1 - - - 12 10 0.05 0.03 - bal. -

Mix. 2 0.09 0.01 0.1 - 10 10 - - - bal. -

UCyprus

Mix. 3 0.047 0.005 - 1 5 - - - 10 - bal.

Mix. 4 0.047 0.005 - 1 5 7 - - 10 - bal.

Mix. 5 0.047 0.005 - 0.6 5 - - - 10 - bal.

Mix. 6 0.042 0.010 - 1 10 - - - 10 - bal.

47

At UOulu, the catalyst powder studied was packed in a quartz tube reactor as thin

layers between quartz wool (Papers II and IV). The studied monolith catalyst was

also packed into a quartz tube reactor between two quartz wools to retain its

position (Väliheikki et al. 2016). The masses of DOC and SCR catalysts were 0.10g

and 0.25g, respectively. The DOC activity was measured in the temperature range

of 125–300 °C, using the gas mixture 1 (Table 4). The SCR catalyst activity was

measured at 150–600 °C, using the gas mixture 2 (Table 4). Water was fed via a

peristaltic pump at 125 °C. The concentrations of gas components (CO, C3H6, NO,

NO2, N2O, NH3, and H2O) were analysed, using a GasmetTM FT-IR gas analyser.

The volume of O2 was analysed using a paramagnetic oxygen analyser (Magnos

106, ABB Advanced Optima). The total flow was 1000 cm3/min. The gas hourly

space velocity (GHSV) was 34 000 h-1 for DOC and between 60 000–180 000 h-1

for NH3-SCR.

In the H2-SCR experiments, the W-CeZr or W-ZrCe catalyst was placed as one

layer in a quartz tube reactor (Paper III). At first, the catalyst was pre-treated at

600 °C for 1 hour in a 20% O2/He flow. Then, the catalyst was cooled down in He

flow. The catalyst activity was measured at 10 different temperatures from 150–

600°C, using several reaction gas mixtures (Table 3). The temperature was kept

constant in each measurement point until steady-state achieved. The NO and NO2

concentrations were analysed using a NOx chemiluminescence analyser (Thermo

Electron Corporation), H2 was analysed, using a mass spectrometer (Balzers

Omnistar), N2O using an infrared analyser (Teledyne Analytical Instruments). The

total flow (300 cm3/min) and the gas hourly space velocity (GHSV) (51 000 h-1)

were kept constant and, thus, the amount of catalyst was varied (0.33-0.66 g).

48

49

6 Results and discussion

6.1 Chemical impurities on exhaust gas particulate matter

The diesel fuel oil (DFO) and rape seed oil (RSO) were combusted in a non-road

diesel engine and the exhaust gas particles were collected on aluminium foils

(Paper I). These foils were analysed, in order to find out the number and chemical

composition of exhaust gas particles. Figure 5 shows the particle number (1/cm3)

in the exhaust gas of DFO and RSO fuelled engines as a function of the particle

size. The experiments were done with and without the SCR catalyst.

Fig. 5. The particles number in the exhaust gas of DFO and RSO fuelled engines as a

function of particle size. (Logarithmic scale) (Paper I, published by permission of

Elsevier)

Figure 5 shows that the exhaust gas from both RSO and DFO fuelled engines

contained the highest amount of particles in the size range of 0.04–0.20 µm. In

addition, the RSO exhaust gas contained a 10 to 100 times higher amount of

particles, compared to the DFO exhaust gas (size >0.04 µm). When DFO was used

as the fuel, the total number of particles in exhaust gases was 1.0 · 108 and 1.3 · 108

in the presence and absence of the SCR catalyst, respectively. Instead, the number

of particles in the RSO fuelled exhaust gases were 2.4 · 109 and 4.0 · 109 with and

without the SCR catalyst, respectively. Thus, the SCR catalyst decreases the total

50

number of particles in the case of both fuels. The decrease in the particle number

was observed in the case of all particle sizes expected with the smallest particles

(the mean size of 0.02 µm). However, as reported in the study by Marjamäki &

Keskinen (2001), the evaporation of semi-volatile particles (<0.03 µm) can affect

the results of the smallest particle fraction.

Figure 6 presents the chemical composition (wt%) of particles (the mean size

of 0.04 and 0.20 μm) in the exhaust gas of DFO and RSO fuelled engines with and

without the SCR catalyst.

Fig. 6. Chemical compositions (wt%) of particles (mean size of 0.04 μm) in the exhaust

gases of DFO and RSO fuelled engines. (Data adopted from Paper I, published by

permission of Elsevier)

no SCR SCR no SCR SCR0

20

40

60

80

100

Ch

em

ica

l co

mp

os

itio

n (

wt%

)

DFO RSO

Others Ca K P Mg O C

Particle size: 0.04 μm

51

Fig. 7. Chemical compositions (wt%) of particles (mean size of 0.20 μm) in the exhaust

gases of DFO and RSO fuelled engines. (Data adopted from Paper I, published by

permission of Elsevier)

According to the results in Figures 6 and 7, the carbon concentration was higher on

the DFO samples compared to the RSO samples. Instead, the concentrations of

phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) were higher in

the RSO exhaust gas, compared to the DFO exhaust gas. The addition of the SCR

catalyst in the engine test had only a minor impact on the chemical composition of

particles. The elemental analyses of the RSO exhaust gas particles were in good

correlation with the trace element analysis from the rape seed oil (Table 3). In

addition, the low amounts of sulphur (S) (≈ 1 wt%) were found on the DFO exhaust

gas particles (the mean size of 0.2–1.2 μm), but not from the RSO exhaust gas

particles. Based on the particulate matter analysis, the composition of particulates

is dependent on raw material which is used for biofuels and diesel fuel oil.

Similar to these results, Betha & Balasubramanian (2011) have found higher

concentrations of K and Mg on the exhaust gas particles of biodiesel, compared to

fossil fuel. In their studies, the studied fuels, ultra-low sulphur diesel (ULSD) and

waste cooking oil-derived biodiesel, were combusted in a diesel back-up power

generator (4.5 kW). In addition, Popovicheva et al. (2015) have found Ca, P and S

impurities from the exhaust gas particles when conventional diesel or refined rape-

seed oil were combusted in the John Deere’s engine. However, in their study, the

no SCR SCR no SCR SCR0

20

40

60

80

100

Ch

emic

al c

om

po

siti

on

(w

t%)

DFO RSO

Others Ca K P Mg O C

Particle size: 0.20 μm

52

diesel exhaust gas contained higher amounts of these impurities, which could be

due to a better biofuel quality used in the experiments.

Based on the elemental analysis and other relevant studies (Popovicheva et al. 2015, Betha & Balasubramanian 2011), the source and quality of fuel have a clear

effect on the chemical impurities on exhaust gas particles. Due to the presence of

impurities, the effect of biomaterial-based impurities and sulphur on catalyst

performance in emission control needs to be studied.

6.2 Effect of impurities on catalyst properties

The impact of sulphur and several biomaterial-based impurities (P, K and Na) on

the performance of the catalyst materials was examined. Therefore, the studied

catalysts were treated by using two accelerated laboratory scale poisoning

procedures: a wet impregnation method and a gas phase treatment. The latter

procedure was based on our earlier studies in UOulu (Kröger et al. 2006), but the

procedure was further developed. The effect of impurities on the studied materials

were examined, using several characterizations methods and in laboratory-scale

activity tests by using model gas mixtures.

6.2.1 Sulphur

The effect of sulphur (S) on the studied catalyst was examined after the treatments

in the absence and presence of 10 vol% water vapour (marked as S-, and S+H2O-

treatments, respectively). The sulphur concentrations on the cerium-zirconium

mixed oxide (marked as CeZr) support and tungsten on the cerium-zirconium

mixed oxide (marked as W-CeZr) catalyst were analysed, using XRF (Paper IV).

XPS analysis was carried out to analyse the amount of sulphur on the surface of the

silicon-zirconium oxide (marked as SiZr) support and platinum on the silicon-

zirconium oxide (marked as Pt/SiZr) catalyst. The XPS analyses were taken from

both the inlet and outlet sections of the Pt/SiZr and SiZr monoliths to find out

whether sulphur is accumulated on the catalyst surface (Väliheikki et al. 2016). In

Figure 8, the results of sulphur concentrations on the studied catalysts and supports

after S- and S+H2O-treatments are summarized.

53

Fig. 8. Sulphur concentrations on the S- and S+H2O treated W-CeZr and Pt/SiZr catalysts

as well as on the CeZr and SiZr supports. (Data adopted from Paper IV and Väliheikki et

al. 2016)

From Figure 8, it can be seen that the sulphur concentration was higher on the CeZr

based catalysts, compared to the SiZr based catalysts. Tungsten, as an active metal,

had a minor effect on the amount of sulphur on the CeZr based catalysts. Water was

observed to increase the sulphur adsorption on the CeZr and SiZr support slightly,

except in the case of the W-CeZr catalyst. However, the differences in the sulphur

concentrations due to water are minor. (Paper IV)

In the case of Pt/SiZr catalysts and SiZr support, the sulphur concentration on

the samples surface was analysed from the inlet and outlet of the monolith. The

results show that sulphur was adsorbed, both on the inlet and outlet sections of the

Pt/SiZr catalyst. In addition, higher sulphur concentrations were found on the

Pt/SiZr catalysts, compared to the SiZr support. Water increased the sulphur

adsorption on SiZr based catalysts slightly. (Väliheikki et al. 2016)

Sulphur compounds formed on the CeZr and SiZr based catalysts were

identified, using DRIFT and XPS. Figures 9 and 10 show the DRIFT spectra over

the fresh, S- and S+H2O-treated W-CeZr catalysts and the CeZr support,

respectively.

1.6

1.8

0.4

0.9

0.2 0.2

1.9

1.4

0.40.5

0

0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

S c

on

cen

trat

ion

(w

t%)

W-CeZr CeZr Pt/SiZr Pt/SiZr SiZr SiZr

S S+H

2O

(inlet) (outlet) (inlet) (outlet)

54

Fig. 9. The DRIFT spectra over fresh, S- and S+H2O-treated W-CeZr catalysts. (Modified

from Paper IV, published by permission of Springer)

Fig. 10. The DRIFT spectra over fresh, S- and S+H2O-treated CeZr support. (Modified

from Paper IV, published by permission of Springer)

Figures 9 and 10 show a strong adsorption band at 900–600 cm-1, which

corresponds to the Ce–O–Ce and Zr–O–Zr bonds (Wang et al. 2013b). In the case

of the fresh W-CeZr catalyst, a small peak located at 930 cm-1 can be attributed to

2000 1600 1200 800 400

1146

1309

Ab

sorb

anc

e (a

.u.)

Wavenumber (cm-1)

1630

1128

930

688

0

50

100

150

200

S

S+H2O

Fresh

2000 1600 1200 800 400

Ab

sorb

ance

(a.

u.)

Wavenumber (cm-1)

10631311

1630

679

1556

0

50

100

150

200

S

S+H2O

Fresh

55

the Ce2(WO4)3 compound (Burcham & Wachs 1998, Wang et al. 2013b). Figures 9

and 10 also reveal a wide peak at 1250–1000 cm−1 after the S+H2O and S-

treatments. In the case of the S+H2O-treated W-CeZr catalyst and CeZr support,

the peak maximum is located at 1128 and 1063 cm-1, respectively. These peaks

correspond to the S=O and S–O vibration modes of surface bidentate SO42− and/or

HSO4-species (Bazin et al. 2009, Mao et al. 2009). In the case of the S-treated W–

CeZr catalyst, the peak maximum is observed at 1146 cm−1, which can be attributed

to sulphate species attached to ceria (Luo & Gorte 2004, Bazin et al. 2009). The

SO4 species can form highly thermally stable Ce(SO4)2 and Ce2(SO4)3, that can

inhibit catalytic reactions (Xu et al. 2009, Wang et al. 2013b). Similarly, the XPS

results showed that sulphur species exist on both S- and S+H2O-treated W-CeZr

catalyst and CeZr support. The XPS spectra of the S2p core level revealed a broad

peak between 168–171eV that corresponds to SO42- (Descostes et al. 2000, Gu et

al. 2010). (Paper IV)

In the case of Pt/SiZr catalyst and SiZr support, the XPS spectra of the S2p

core level showed the peak maximum at 168–170 eV, which can be interpreted as

the SO42- species. Instead, the DRIFT results did not reveal any sulphur compounds

on the Pt/SiZr catalyst and the SiZr support, which is probably due to the relatively

low sulphur concentration. (Väliheikki et al. 2016)

Figure 11 shows the deconvolution of the XPS O1s core level spectra of fresh,

S-, and S+H2O-treated W-CeZr catalysts.

56

Fig. 11. The deconvolution of XPS O1s core level spectra of fresh, S- and S+H2O-treated

W–CeZr catalysts. Oα = lattice oxygen; Oβ = chemisorbed oxygen. (Modified from Paper

IV, published by permission of Springer)

The deconvolution of the O1s spectra showed two peaks on the W-CeZr catalyst

(Figure 11). The first peak maximum was detected at 529.2–529.8 eV, which

corresponds to the lattice oxygen (Oα). The maximum of the second peak was

located at 529.9–531.2 eV, which can be attributed to chemisorbed oxygen (Oβ).

(Kang et al. 2007, Gu et al. 2010) The ratio of the chemisorbed oxygen to the total

oxygen (Oβ/(Oβ + Oα)) was calculated from the areas of both peaks. According to

the results, the portion of chemisorbed oxygen over fresh, S- and S+H2O-treated

W-CeZr catalysts was 46, 59 and 66%. Thus, S- and S+H2O-treatments increased

the ratio of chemisorbed oxygen to the total oxygen species on the W-CeZr catalysts.

Similarly, S- and S+H2O-treatments increased the portion of chemisorbed oxygen

in the case of CeZr support. (Paper IV) According to several studies (Busca et al. 1998, Notoya et al. 2001, Gu et al. 2010), chemisorbed oxygen species have proven

to be active in the NH3-SCR reactions.

However, in the case of SiZr based catalysts, S- and S+H2O-treatments had no

effect on the chemical states of elements (Si or Zr). In addition, XPS results

revealed that oxygen was bound on silicon or zirconium and differences between

the types of oxygen species (chemisorbed or lattice) were not found on the catalyst.

The reason could be that the SiZr support did not have a similar redox ability as

cerium.

536 534 532 530 528 5260

20000

40000

60000

80000

00000

20000

Οα

Co

un

ts /

s

Binding energy (eV)

Οβ

S

Fresh

S+H2O

57

The effect of sulphur on the catalyst surface area and porosity was studied. The

specific surface areas and BJH pore volumes of fresh, S-, S+H2O-treated W-CeZr

and Pt/SiZr catalysts, as well as CeZr and SiZr supports, are shown in Figures 12

and 13, respectively.

Fig. 12. Specific surface area (SBET, m2/g) of fresh, sulphur (S) and sulphur+water

(S+H2O) treated catalysts and supports. (Data adopted from Paper IV and Väliheikki et

al. 2016)

6574

108 108

63

79

106113

95

107

119113

0

20

40

60

80

100

120

140

160

BE

T s

urf

ace

are

a (

m2 /g

)

W-CeZr CeZr Pt/SiZr SiZr

Fresh S S+H

2O

58

Fig. 13. BJH pore volumes (cm3/g) of fresh, sulphur (S) and sulphur+water (S+H2O)

treated catalysts and supports. (Data adopted from Paper IV and Väliheikki et al. 2016)

According to the results, sulphur decreases the specific surface area of W-CeZr

catalysts and CeZr support by 26–34% (Figure 12). In addition, sulphur decreases

the BJH pore volumes of CeZr support and W-CeZr catalyst by 25–27% (Figure

13). Based on the BET and BJH results, sulphur is covering the surface and

plugging the pores of the W-CeZr catalyst and the CeZr support. The effect of water

on the surface areas was minor, because the SBET values and BJH pore volumes

over S- and S+H2O-treated W-CeZr catalysts and CeZr support decreased similarly

in the presence and absence of water. (Paper IV)

In the case of Pt/SiZr catalyst and SiZr support, sulphur had a negligible effect

on the specific surface areas and BJH pore volumes (Figures 12 and 13). These

results are in good agreement with the elemental analysis, because only minor

amounts of sulphur were found on the S- and S+H2O-treated Pt/SiZr catalysts and

the SiZr support. In addition, the SBET and BJH values were almost similar to each

other after S- and S+H2O-treatments. Thus, the presence of water has a negligible

effect on the SBET values and pore volumes. (Väliheikki et al. 2016)

6.2.2 Biomaterial-based impurities (P, K and Na)

The effects of selected biomaterial-based impurities, e.g. phosphorus, potassium

and sodium, on the catalyst’s behaviour were investigated. The phosphorus

0.110.12

0.21

0.19

0.110.12

0.22

0.20

0.150.16

0.22

0.20

0.00

0.05

0.10

0.15

0.20

0.25

0.30

BJH

po

re v

olu

me

(cm

3/g

)

W-CeZr CeZr Pt/SiZr SiZr

Fresh S S+H

2O

59

concentrations on the gas phase treated Pt/SiZr catalyst and SiZr support were

analysed by XPS (Väliheikki et al. 2016). Both the inlet and outlet sections of the

catalyst were analysed and the results are shown in Figure 14.

Fig. 14. Phosphorus concentrations (wt%) on the inlet and outlet of the

phosphorus+water (P+H2O) treated Pt/SiZr catalyst and SiZr support. (Data adopted

from Väliheikki et al. 2016)

Based on the results, phosphorus was mainly accumulated on the inlet of the Pt/SiZr

catalyst. The results also showed that phosphorus concentration was higher on the

Pt/SiZr catalysts (6.6–7.9 wt%), compared to the parent SiZr support (0.2–1.9 wt%).

Thus, Pt increases the P concentration on the catalyst. These results are in

agreement with the FESEM-EDS results, which showed that the amount of

phosphorus was circa 5 wt% throughout the P+H2O-treated Pt/SiZr catalyst.

Similarly, in a study done by Alfaya et al. (2000), the phosphorus concentration

was 1.9–4.5 wt% on silicon-zirconium mixed oxides after the phosphorus

adsorption. In their study, the higher concentration of zirconium on the sample

increased the amount of phosphorus on the mixed oxide.

Figure 15 shows the DRIFT spectra over the fresh and P+H2O treated Pt/SiZr

catalyst and the SiZr support.

7.9

6.6

0.2

1.9

0

1

2

3

4

5

6

7

8

9

P c

on

cen

tra

tio

n (

wt%

)

Pt/SiZr Pt/SiZr SiZr SiZr

P+H2O

(inlet) (outlet) (inlet) (outlet)

60

Fig. 15. DRIFT spectra over the fresh and P+H2O-treated Pt/SiZr catalyst and the SiZr

support.

The phosphorus species on the Pt/SiZr catalyst and SiZr support can be detected

from the DRIFT results (Figure 15). The P+H2O-treatments shifted the broad band

maxima from 1300–1140 cm-1 to higher wave numbers, which is probably due to

the asymmetric stretching mode of phosphate (PO4) (Lee et al. 2000, Mami et al. 2008). Similarly, phosphates can also be detected from the XPS results of the

P+H2O-treated Pt/SiZr catalyst and the SiZr support. The XPS spectra of the P2s

core level showed a peak located at 190–191 eV, which corresponds to phosphates

(PO43-) (NIST 2015). In addition, the XPS studies also revealed that the P+H2O-

treatment had no effect on the state of Pt on the catalyst, because only metallic Pt

was found in the case of fresh and P+H2O-treated Pt/SiZr catalysts.

The potassium (K) and sodium (Na) concentrations on zeolite-based catalysts

(Fe-ZSM-5, W-ZSM-5 and ZSM-5) after the wet impregnation were detected by

AAS. The results revealed that the K and Na concentrations were in the range of

0.9–1.0 wt%, which was the target amount of K and Na on the catalyst. (Paper II)

The effect of biomaterial-based impurities, e.g. potassium, sodium and

phosphorus, on the specific surface area (SBET) of catalysts was studied. The SBET

values of fresh, K, and Na treated Fe-ZSM-5, W-ZSM-5 and ZSM-5 catalysts, as

well as fresh and P+H2O treated Pt/SiZr and SiZr catalysts and support, are shown

in Figure 16. The results of the BJH pore volumes are shown in Figure 17.

2000 1800 1600 1400 1200 1000 800 600 400

-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.50.00.51.01.52.0

Pt-SiZr

SiZr

SiZr

P+H2O

Ad

sob

ran

ce (

a.u

.)

Wavenumber (cm-1)

Fresh 1630

1300-1140

818

736

1092

P+H2O

Fresh

Pt-SiZr

61

Fig. 16. Specific surface area (SBET, m2/g) of fresh, potassium (K), sodium (Na) treated

Fe-ZSM-5, W-ZSM-5 catalyst and ZSM-5 support, as well as phosphorus+water (P+H2O)

treated Pt/SiZr catalyst and SiZr support. (Data adopted from Paper II and Väliheikki et

al. 2016)

Fig. 17. BJH pore volumes (cm3/g) of fresh, potassium (K), sodium (Na), and

phosphorus+water (P+H2O) treated catalysts and supports. (Data adopted from Papers

II and Väliheikki et al. 2016)

324 322

345340323

355347 352

324

119 113

72

50

0

50

100

150

200

250

300

350

400

BE

T s

urf

ace

are

a (m

2 /g)

Fe-ZSM-5 W-ZSM-5 ZSM-5 Pt/SiZr SiZr

Fresh K Na P+H

2O

0.180.17

0.180.180.17

0.190.19 0.19

0.17

0.22

0.2

0.17

0.11

0.00

0.05

0.10

0.15

0.20

0.25

0.30

BJ

H p

ore

vo

lum

e (

cm

3/g

)

Fe-ZSM-5 W-ZSM-5 ZSM-5 Pt/SiZr SiZr

Fresh K Na P+H

2O

62

Based on the SBET results, the SBET values were in the range of 320–360 m2/g with

the fresh, K and Na-treated ZSM-5 based catalysts and the support. Potassium and

sodium were proven to have a minor impact on the SBET values of zeolite-based

catalysts. In addition, potassium and sodium decreased the BJH pore volumes

slightly. In the case of ZSM-5 support, the SBET values and pore volumes increased

slightly, but the difference was quite small.

Instead, phosphorus decreased the specific surface area of the Pt/SiZr catalyst

and the SiZr support by 39% and 56%, respectively. Similar kinds of trends were

also observed in the BJH pore volumes, which decreased by 23% and 45%,

respectively. Similarly to these results, the studies done by Christou et al. (2011)

and Kärkkäinen et al. (2015) reveal that phosphorus also decreases the surface area

of cerium-zirconium and aluminium oxide based catalysts.

The effect of phosphorus on Pt/SiZr catalysts was also studied, using TEM-

EDS, but no notable differences between the fresh and P+H2O-treated Pt/SiZr

catalysts were observed (Väliheikki et al. 2016). In addition, the TEM images for

fresh, K- and Na-treated Fe-ZSM-5 catalysts were morphologically quite similar

(Paper II).

6.3 Catalyst activity studies

6.3.1 NOx reduction

Fe-ZSM-5, W-ZSM-5 and W-CeZr catalyst samples were studied to find out their

catalytic performance in NOx reduction by NH3. The formation of unwanted N2O

was also studied. Based on the activity test results, the most active NH3-SCR

catalysts were Fe-ZSM-5 and W-CeZr. Thus, both Fe-ZSM-5 and W-CeZr catalysts

were selected for further studies in which the effects of impurities (potassium,

sodium and sulphur) on catalyst activity in NH3-SCR were studied. Figures 18 and

19 show the NOx and NH3 conversions over the fresh, K- and Na-treated Fe-ZSM-

5 catalyst and the ZSM-5 support as a function of temperature. (Paper II)

63

Fig. 18. The NOx conversion over the fresh, K- and Na-treated Fe-ZSM-5 catalyst, and

the ZSM-5 support in NH3-SCR. (Modified from Paper II, published by permission of

Springer)

Fig. 19. The NH3 conversion over the fresh, K- and Na-treated Fe-ZSM-5 catalyst, and

the ZSM-5 support in NH3-SCR. (Modified from Paper II, published by permission of

Springer)

200 300 400 500 6000

20

40

60

80

100

NO

x c

on

vers

ion

(%

)

Temperature (oC)

Fe-ZSM-5 (Fresh) Fe-ZSM-5 (K) Fe-ZSM-5 (Na) ZSM-5 (Fresh) ZSM-5 (K) ZSM-5 (Na)

200 300 400 500 6000

20

40

60

80

100

NH

3 c

on

vers

ion

(%

)

Temperature (oC)

Fe-ZSM-5 (Fresh) Fe-ZSM-5 (K) Fe-ZSM-5 (Na) ZSM-5 (Fresh) ZSM-5 (K) ZSM-5 (Na)

64

The activity test results revealed that the fresh Fe-ZSM-5 catalyst achieved over

95% NOx conversion at 360–500 °C (Figure 18). The effect of K- and Na-

treatments on the activity of Fe-ZSM-5 was negligible, because the NOx and NH3

conversions remained at the same level as with the fresh Fe-ZSM-5 catalyst in the

studied temperature range. NH3 was completely converted to other products at

temperatures above 400 °C (Figure 19). Thus, the NH3 slip catalyst is not needed

at high temperatures. In addition, the NH3 conversions over the fresh, as well as the

K- and Na-treated Fe-ZSM-5 catalyst, were higher than the NOx conversions at

temperatures above 500 °C. This proves that NH3 is consumed in other reactions

forming NO and N2 (Eqs. 12 and 13). The formation of N2O over the fresh, K- and

Na-treated Fe-ZSM-5 was not detected in the studied temperature range.

The ZSM-5 support had a moderate activity in NOx reduction and the

maximum of 50% NOx conversion was achieved at 600 °C. K- and Na-treatments

had a small positive effect on the NOx and NH3 conversions over the ZSM-5

support. The ZSM-5 support achieved the maximum NOx conversion of 55% after

the studied treatments. The reason for the improved activity is probably due to the

formation of NOx storage sites that has been observed with zeolites after exposure

to potassium (Shwan et al. 2015). In addition, the potassium treatments have

proven to decrease the amount of acid sites on zeolites (Putluru et al. 2012, Shwan

et al. 2015). In the case of Fe-zeolites, potassium affects the catalyst redox

properties by increasing the relative amount of Fe3+ which has proven to be the

reason for the lower NOx reduction activity of catalysts after potassium treatments

(Shwan et al. 2015). However, in this thesis, the catalyst activity remained high

after potassium treatments and, thus, the effect of potassium on catalyst redox

properties were negligible. In addition, the activity of the W-ZSM-5 catalysts was

studied in NOx reduction. The NOx and NH3 conversions over the W-ZSM-5

catalysts were similar to the ZSM-5 support. The effect of tungsten was negligible

on the activity of the ZSM-5 support because of the lack of redox active

components on the catalyst.

Figures 20 and 21 show the NOx and NH3 conversions over the fresh, S- and

S+H2O-treated W-CeZr catalysts and the CeZr support.

65

Fig. 20. The NOx conversion over the fresh, S- and S+H2O-treated W-CeZr catalyst, and

the CeZr support in NH3-SCR. (Modified from Paper IV, published by permission of

Springer)

Fig. 21. The NH3 conversion over the fresh, S- and S+H2O-treated W-CeZr catalyst, and

the CeZr support in NH3-SCR. (Modified from Paper IV, published by permission of

Springer)

200 300 400 500 6000

20

40

60

80

100

NO

x co

nve

rsio

n (

%)

Temperature (oC)

W-CeZr (Fresh) W-CeZr (S) W-CeZr (S+H

2O)

CeZr (Fresh) CeZr (S) CeZr (S+H

2O)

200 300 400 500 6000

20

40

60

80

100

NH

3 co

nve

rsio

n (

%)

Temperature (oC)

W-CeZr (Fresh) W-CeZr (S) W-CeZr (S+H

2O)

CeZr (Fresh) CeZr (S) CeZr (S+H

2O)

66

Figure 20 shows that the fresh W-CeZr catalyst achieved the maximum NOx

conversion of 85% at 350 °C. Based on the results, sulphur was proven to increase

the catalyst activity. The NOx conversion over the S- and S+H2O-treated W-CeZr

catalysts was more than 95% at 350–410 °C. In addition, the S+H2O- and S-

treatments had a negligible effect on the NH3 conversion over the W-CeZr catalyst

(Figure 21). In the case of fresh, as well as S- and S+H2O-treated W-CeZr, NH3

was completely converted to other products at temperatures above 400 °C.

However, the NH3 conversions over the W-CeZr catalysts were higher than the NOx

conversions at temperatures above 450 °C, which indicates that NH3 is consumed

in unwanted side reactions forming e.g. NO and N2O (Eqs. 5–11,13) or in oxidation

reaction to N2 (Eq. 12). In the case of the fresh W-CeZr catalyst, the maximum N2O

formation of 30 ppm was detected at 500–520 °C. The S- and S+H2O-treatments

of the W-CeZr catalyst decreased the formation of N2O slightly (not presented).

The fresh CeZr support was not active in the NOx reduction, because it

achieved only 14% NOx conversion at 370 °C (Figure 20). The fresh CeZr support

had the NH3 conversion of 100% at temperatures above 550 °C (Figure 21). In the

case of S- and S+H2O-treated CeZr support, the NOx and NH3 conversions were

both over 95% at temperatures of around 400 °C. At temperatures above 400°C,

the NOx conversion started to decrease, whereas NH3 was converted to other

products.

The activity enhancement is probably due to the increased amount of

chemisorbed oxygen on the W-CeZr catalyst and the CeZr support after the S- and

S+H2O-treatments shown by the XPS results. In NH3-SCR, the role of chemisorbed

oxygen is to activate adsorbed NH3 to amide species. According to studies by Busca

et al. (1998) and Gu et al. (2010), amide species can react with gaseous NO,

forming the NH2NO intermediate that can decompose to N2 and H2O. The other

possible mechanism is that the chemisorbed oxygen reacts with the adsorbed NO,

forming NO2. Then the adsorbed NO2 reacts with NH4 and forms the NH4NO2

intermediate, which then decomposes to N2 and H2O. (Notoya et al. 2001). In

addition, the acidities of fresh and sulphur treated CeZr catalysts were studied using

temperature programmed desorption of NH3. However, these results showed that

sulphur decreased slightly NH3 desorption on CeZr catalysts. Thus, the minor

changes in the acidity of catalysts was not the main reason for the activity

enhancement after the sulphur treatment.

The use of H2 as an alternative reducing agent was also tested in the NOx

reduction (Paper III). Both Ce-rich (W-CeZr) and Zr-rich (W-ZrCe) materials were

used as catalysts in order to find out the effect of the Ce:Zr ratio on the catalyst

67

activity. Figure 22 shows the NOx and H2 conversions over W-CeZr and W-ZrCe

catalysts in H2-SCR.

Fig. 22. The NOx and H2 conversions over the W-CeZr and W-ZrCe catalysts in H2-SCR.

(Modified from Paper III, published by permission of Elsevier)

The Ce:Zr ratio (wt% ratios for CeO2/ZrO2 : 85/15 (CeZr) or 17/83 (ZrCe)) was

proven to affect the catalyst activity in H2-SCR. Figure 22 shows that the W-ZrCe

catalysts achieved 8–20 percentage points higher NOx conversion than the W-CeZr

catalyst at temperatures of 150–250 °C and 400–600 °C. Instead, the W-CeZr

catalyst had 10 percentage points higher NOx conversion at temperatures of 300–

350 °C. The maximum NOx conversions over the W-CeZr and W-ZrCe catalysts

were 46% (at 250 °C) and 54% (at 300 °C), respectively. At higher temperatures,

the NOx conversion decreases which is due to the lack of the H2 reductant. More

than 95% of H2 is oxidized with both the studied catalysts at temperatures above

400 °C.

In the H2-SCR activity tests, the GHSV was kept at 51000 h-1 and the total flow

was 300 cm3/min. To meet these conditions, the amount of catalyst was varied. The

mass of the W-CeZr catalyst in the activity tests was 0.66g and the mass of the W-

ZrCe catalyst was 0.33g. The differences between the NOx conversions over the W-

CeZr and W-ZrCe catalysts were minor, compared to the mass of the catalyst used

in the tests. This proves that the site activity of the W-ZrCe catalyst was clearly

higher, compared to the W-CeZr catalyst if it was assumed that the catalyst site

density is almost equal.

200 300 400 500 6000

20

40

60

80

100

W-ZrCe (H2)

W-CeZr (H2)

W-ZrCe (NOx)

W-CeZr (NOx)

Temperature (oC)

Co

nve

rsio

ns

(%)

68

The formation of N2O was also studied. The W-CeZr catalyst had the

maximum N2O formation of 15–20 ppm at 200–350 °C. In the studied temperature

area, the formation of N2O over the W-ZrCe catalyst was ca. 5–10 ppm lower,

compared to the W-CeZr catalyst (not presented).

6.3.2 CO and HC oxidation

The activity of the Pt/SiZr diesel oxidation catalyst (DOC) was studied in CO and

C3H6 oxidation (Väliheikki et al. 2016). The impact of sulphur and phosphorus on

the activity of the Pt/SiZr catalyst is shown in Figure 23.

Fig. 23. The CO and C3H6 conversions over the fresh, S+H2O and P+H2O treated Pt/SiZr

catalyst as a function of temperature.

From the results, it can be seen that the fresh Pt/SiZr catalyst achieved 100% CO

and C3H6 conversion at 205 and 230 °C, respectively (Figure 23). S+H2O-treatment

had a negligible effect on the CO and C3H6 conversions over the Pt/SiZr catalyst.

(Väliheikki et al. 2016) The good resistance towards SO2 is probably due to the

SiZr support. According to a study by Kim et al. (2013), SiO2 has low basicity that

can prevent SO2 poisoning. In addition, ZrO2 enhances the interaction between Pt

and the support that improves the stability of the catalyst towards sulphur poisoning.

(Kim et al. 2013)

140 160 180 200 220 240 260 280 3000

20

40

60

80

100

P+H2O (CO)

Fresh (C3H

6)

P+H2O (C

3H

6)

S+H2O (C

3H

6)

Co

nve

rsio

n (

%)

Temperature (oC)

Fresh (CO)

S+H2O (CO)

69

Instead, P+H2O-treatment decreases the activity of the Pt/SiZr catalyst in CO

and C3H6 oxidation during the whole activity test. The activity loss is probably due

to the formation of phosphates (PO4) shown by XPS results. These compounds are

probably blocking the accessibility of the reacting molecules to the catalyst pores

and the active sites. (Väliheikki et al. 2016)

Besides the CO and C3H6 oxidation reactions, NO oxidation over the Pt/SiZr

catalyst was studied. Figure 24 shows the NO conversions and N2O formation over

the fresh, as well as the S+H2O and P+H2O-treated Pt/SiZr catalyst.

Fig. 24. The NO conversions (marked as circles) and N2O formation (marked as dash

and dotted lines) over the fresh, as well as the S+H2O and P+H2O-treated Pt/SiZr catalyst.

The results show that the fresh Pt/SiZr catalyst achieved the maximum NO

conversion of 45%. The S+H2O-treatment decreased the NO conversions by 10

percentage points at temperatures above 240 °C (Figure 24). In addition, the NO-

TPD results confirmed that sulphur decreases the amount of NOx species adsorbed

on Pt/SiZr catalyst at temperatures above 200 °C. The loss of these species could

be related to the explanation presented in a study by Law et al. (2010) that showed

that the reaction between basic hydroxyl of ZrO2 and sulphates (SO4) inhibits the

formation of stable nitrates. In this study, the lower amount of NOx species might

be limiting the NO conversion in the activity tests. Instead, the S+H2O-treatment

had a negligible effect on the N2O formation.

140 160 180 200 220 240 260 280 3000

10

20

30

40

50 Fresh

(NO)

S+H2O (NO)

P+H2O (NO)

Fresh (N2O)

S+H2O (N

2O)

P+H2O (N

2O)

Temperature (oC)

NO

Co

nv

ers

ion

(%

)

0

40

80

120

160

200

N2O

fo

rmat

ion

(p

pm

)

70

The P+H2O-treatment was proved to decrease the NO conversion over the

Pt/SiZr catalyst by 20 percentage points at temperatures above 230 °C. The activity

loss is probably due to the formation of phosphates (PO4) determined by DRIFT

and these species are decreasing the specific surface area and pore volumes of the

catalyst. Thus, the accessibility of reactants to the active sites is hindered. This

observation is in good agreement with the NO-TPD results, which reveals that the

P+H2O treatment decreases the amount of desorbed NOx species. The lower

amount of these species could be limiting to the NO oxidation to NO2 in activity

tests. These results are in good agreement with a study by Kern et al. (2010), which

revealed that phosphorus species decreases NO oxidation to NO2 in the presence of

Fe-zeolite catalysts.

6.4 Temperature programmed studies

6.4.1 Catalyst acidity

The acidity studies were carried out to find the effect of the Ce:Zr ratio (wt% ratios

for CeO2/ZrO2:85/15 (CeZr) or 17/83 (ZrCe)) on the catalyst acidity. Figure 25

presents the NH3 desorption on W-CeZr and W-ZrCe catalysts during the TPD run.

71

Fig. 25. The NH3 and N2 desorption over W-CeZr (wt% ratio for CeO2/ZrO2 = 85/15) and

W-ZrCe (wt% ratio for CeO2/ZrO2 = 17/83) catalysts during NH3-TPD. N2 desorption traces

were multiplied by factor five (x5). (Paper III, published by permission of Elsevier)

Figure 25 shows that in the case of the W-ZrCe catalyst, three major NH3 desorption

peaks were found at 130, 260 and 375 °C. In the case of the W-CeZr catalyst, three

major NH3 desorption peaks were detected at 125, 242 and 360 °C. No notable

differences on the amount of desorbed NH3 were observed at temperatures below

250 °C. However, at temperatures above 250 °C, the amount of desorbed NH3 from

the W-CeZr was significantly lower, compared to the W-ZrCe catalyst. (Paper III)

Furthermore, only a small amount of N2 was detected to be desorbed in the

NH3-TPD experiment over both studied catalysts. The N2 desorption over the W-

ZrCe and W-CeZr catalysts was observed at 70–560 °C and 70–480 °C, respectively.

The maximum of the N2 desorption rate over both the studied catalysts was

observed at 375 °C. (Paper III) These N2 desorption peaks are due to the reaction

between NH3 and the lattice oxygen (Wang et al. 2012). The formation and

desorption of N2 was three times higher with the W-ZrCe catalyst, in comparison

to the W-CeZr catalyst. Because the amount of NH3 desorption at temperatures

higher than 300 °C was higher over the W-ZrCe catalyst, it can be concluded that

the strongly adsorbed NH3 was oxidized to N2.

Figure 26 presents the NH3-DRIFTS spectra of the W-ZrCe and W-CeZr

catalysts after the NH3 adsorption at 25 °C followed by a TPD run in an Ar flow.

100 200 300 400 500 6000,0

0,2

0,4

0,6

0,8

1,0

Co

nc

en

tra

tio

n (

mo

l%)

Temperature (oC)

A: NH3 (W-ZrCe)

B: NH3 (W-CeZr)

C: N2 (W-ZrCe)

D: N2 (W-CeZr)

25

D(x5)C(x5)

A

B

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Fig. 26. The NH3-DRIFTS spectra of the W-ZrCe (a) and W-CeZr (b) catalysts after 0.55%

NH3/He chemisorption at 25 °C for 15 min, followed by TPD in an Ar flow. (Paper III,

published by permission of Elsevier)

In the case of the W-ZrCe catalyst (Figure 26a), the spectra presents the infrared

(IR) bands at 1584 and 1190 cm-1, which can be assigned to NH3 adsorbed on Lewis

acid sites (Chen et al. 2010, Gu et al. 2013). Instead, the broad bands with a peak

maximum at 1460 and 1682 cm-1 correspond to the NH4+ species on Brönsted acid

sites (Gu et al. 2013, Chen et al. 2011). The intensities of all these bands decrease

when the temperature is increased. Similarly, Figure 26b shows that the IR bands

of the W-CeZr catalyst decrease when the temperature is increased to 600°C. The

IR bands completely disappear at 600 °C and 400 °C in the case of the W-ZrCe and

W-CeZr catalysts. Based on the NH3-DRIFTS results, NH3 is more strongly

adsorbed on the W-ZrCe catalyst than the W-CeZr catalyst. These results are in

good agreement with the NH3-TPD results, which also indicated that more NH3 is

adsorbed on W-ZrCe than on the W-CeZr catalyst at temperatures above 200 °C.

(Paper III)

6.4.2 The surface reaction and desorption of NO

Figure 27 presents the NO and NO2 desorption over the W-CeZr and W-ZrCe

catalysts during the H2/O2-TPSR run.

73

Fig. 27. NO and NO2 desorption over W-CeZr and W-ZrCe catalysts in H2/O2-TPSR. NO

chemisorption conditions: 0.1% NO/ 10% O2/He flow (100 cm3/min), T = 25 °C, t = 30 min.

TPSR run: 1% H2/5% O2/He flow (30 cm3/min), Heating rate 30 °C/min. (Paper III,

published by permission of Elsevier)

The NO desorption peaks over the W-ZrCe catalyst were detected at 139 °C and

374 °C, respectively, and the second peak has a shoulder peak at around 440 °C. In

the case of the W-CeZr catalyst, three NO desorption peaks were observed at 130,

376 and 437 °C. The total amount of NO desorbed from the W-ZrCe catalyst was

24%, lower compared to the W-CeZr catalyst. In addition, two NO2 desorption

peaks over the W-ZrCe and W-CeZr catalysts were detected at around 160 °C and

400 °C. The NO2 desorption peak over the W-ZrCe catalyst at 400 °C is three times

higher than the NO2 desorption peak at 160 °C. In the case of the W-CeZr catalyst,

the NO2 desorption peak at 400 °C is ten times higher than the NO2 desorption peak

at 160 °C. No significant amounts of other gases (NH3, N2O, N2) desorbed during

the H2/O2-TPSR experiment. (Paper III)

Figure 28 presents NO desorption over the W-CeZr and W-ZrCe catalysts

during the NO-TPD run.

74

Fig. 28. NO desorption over W-CeZr and W-ZrCe catalysts in NO-TPD. NO chemisorption

conditions: 1% NO/He flow (30 cm3/min), T = 25 °C, t = 30 min. NO desorption run: He

flow (30 cm3/min), Heating rate 30 °C/min. (Paper III, published by permission of Elsevier)

In the case of W-ZrCe catalyst, three NO desorption peaks were found at 114 °C,

243 °C and 445 °C (Figure 28). The highest NO desorption peak was detected at

445 °C, which was 40% higher than the NO desorption peak at 243 °C. In the case

of W-CeZr catalyst, two NO desorption peaks were found at 245 °C and 445 °C

with a large shoulder at lower temperatures than the first NO desorption peak. The

NO desorption peak at 445 °C was approximately 15% lower than the NO

desorption peak at 245 °C. It was also noteworthy that no significant amounts of

other gases (NH3, N2, NO2, N2O) were formed during the NO-TPD experiments.

These results are also in good agreement with the activity tests in NH3-SCR.

The W-CeZr catalyst was not active in NOx reduction at temperatures higher than

500 °C. Similarly, desorption of NO or NO2 was not observed at temperatures

above 500 °C either in the NO-TPD or the H2/O2-TPSR experiment. (Paper III)

75

7 Summary and conclusions

The use of biofuels has increased over decades. The source materials for biofuels

contain impurities that can entrain to fuel combustion processes and further to

exhaust gas after-treatment systems. This causes many challenges for the exhaust

gas purification units. In order to understand how biomaterial-based impurities

affect the catalytic materials used in the purification units, the chemical

composition of exhaust gas particles was analysed. The engine bench test

procedures were done using fossil fuel (diesel fuel oil, DFO) and biofuel (untreated

rape seed oil, RSO) as a fuel. The results revealed that RSO contains ten times more

exhaust gas particles than DFO. In addition, the amount of phosphorus, calcium,

magnesium and potassium was clearly higher in RSO, compared to DFO. These

results were also consistent with the trace element analysis taken from the studied

fuels. Based on these results and other studies reported, the effect of several

impurities (phosphorus, potassium, sodium and sulphur) on the properties and

activity of catalytic materials used in the exhaust gas after-treatment systems were

analysed.

In this study, several fresh and biomaterial-based impurity-treated support and

catalytic materials used in the exhaust gas purification units, i.e. selective catalytic

reduction (SCR) and diesel oxidation catalyst (DOC) units, were analysed. The

studied materials need to have good properties, e.g. high surface area and high

acidity, to be resistant to impurities originating from fuels and lubricant oils. The

effect of alkali metals (potassium and sodium) was negligible on the properties of

iron-zeolite (Fe-ZSM-5) catalysts. The reason could be the high surface area of the

zeolite support. In addition, potassium and sodium did not affect the properties and

activity of other studied zeolite-based catalysts, although these materials were not

as active as Fe-ZSM-5 in NOx reduction. Instead, phosphorus, which is a

biomaterial-based impurity, decreased the activity of the platinum/silicon-

zirconium oxide (Pt/SiZr) catalyst in C3H6, CO and NO oxidation. The activity loss

was probably due to the formation of phosphates that were blocking the

accessibility of reacting molecules to active sites. The amount of phosphorus was

relatively high on the surface of SiZr.

Sulphur has been proven to be the cause for the deactivation of many

commercial catalytic materials. In this thesis, the effect of sulphur was studied on

the deactivation of cerium-zirconium mixed oxide (CeZr) and SiZr based catalysts.

The amount of sulphur on the catalysts’ surfaces was higher on the CeZr based

catalysts, compared to the SiZr based catalysts. Sulphur increased the activity of

76

the W-CeZr catalyst and CeZr support in NH3-SCR. The reason for the improved

activity was probably the higher portion of chemisorbed oxygen on the CeZr based

catalysts. In addition, the effect of sulphur on the activity of SiZr based catalysts

was negligible in the DOC applications. In the case of SiZr based catalysts, the

good resistance towards SO2 poisoning was due to the good features of the support

in terms of the acidity and basicity.

The studied catalysts should have high activity, stability and selectivity in the

SCR process. The most active SCR catalysts in activity tests were Fe-ZSM-5 and

W-CeZr. These catalysts were considered to be promising alternatives for the

commercial NH3-SCR catalysts, which can contain toxic compounds such as

vanadium pentoxide (V2O5). Contrary to the studies presented in the literature

about the V2O5-based catalysts, the Fe-ZSM-5 catalyst had high tolerance towards

biomaterial-based compounds. In addition, the Fe-ZSM-5 catalyst had high activity

in NOx reduction at high temperatures (350–500 °C) and it did not form any

harmful N2O, which is often a challenge in the SCR processes. Furthermore, in this

thesis, both ammonia and hydrogen were used as reductants in the SCR with metal

oxide catalysts. In the case of W-CeZr catalyst, higher NOx conversion was

achieved with NH3, compared to H2. However, these results were not fully

comparable, because the activity tests in NH3- and H2-SCR were done in different

process conditions (flow rate of reactants, catalyst packing and reactor size) with

two experimental set-ups. The results still prove that the W-CeZr catalyst has good

potential to be used in the H2-SCR process. More catalyst development work is

needed to achieve as high purification results as the noble metal catalysts have in

H2-SCR.

In the case of Pt/SiZr diesel oxidation catalyst, the activity was moderate in

CO and C3H6 oxidation. In addition, the catalyst was also active in the oxidation of

NO to NO2, which is beneficial for the operation of the following exhaust gas after

treatment unit i.e. diesel particulate filter (DPF) and SCR. The catalyst activity

remained high after treatments with sulphur. Thus, this catalyst is a promising

alternative for the commercial Pt/Al2O3 diesel oxidation catalysts.

This thesis provides new information on the suitability of Fe-ZSM-5, W-CeZr

and Pt/SiZr catalysts for exhaust gas purification, as well as catalyst resistance

towards chemical impurities. The sulphur-resistant catalyst materials are needed,

especially in emerging markets, due to a risk of catalyst exposures to high sulphur

fuel exhaust gases. In addition, catalyst materials with high resistance towards

biomaterial-based compounds are needed e.g. after-treatment systems in stationary

application, where bio-based fuels are combusted. The information gained in this

77

study can be utilized in developing efficient and environmentally friendly catalyst

materials for vehicles and stationary applications. By developing and using

sustainable catalyst materials, the air quality can be improved and, thus, human

health can be enhanced.

This thesis gives an overview about the resistance of catalytic materials

towards impurities, whilst also providing ideas for further studies. Stability tests

for treated catalysts are needed to find out the effect of impurities on catalysts

during long-term exposure to high temperatures. Alternatively, the challenges in

NOx control during the cold-start of an engine in winter conditions need to be

resolved. One solution could also be the new application in which a SCR catalyst

is combined with a NOx trap. The other option is to study new catalysts materials

that have shown high activity at low temperatures e.g. copper on various supports.

In addition, one challenge that also needs to be resolved is the optimization of the

space needed for purification units, e.g. DOC and SCR, and a possible integration

of these two systems. Thus, studies concerning the suitable catalyst material for

hybrid systems are needed.

78

79

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Original publications

I Pietikäinen M, Väliheikki A, Oravisjärvi K, Kolli T, Huuhtanen M, Niemi S, Virtanen S, Karhu T & Keiski RL (2015) Particle and NOx emissions of a non-road diesel engine with an SCR unit: the effect of fuel. Renewable Energy 77: 377–385.

II Väliheikki A, Kolli T, Huuhtanen M, Maunula T, Kinnunen T & Keiski RL (2013) The effect of biofuel originated potassium and sodium on the NH3-SCR activity of Fe-ZSM-5 and W-ZSM-5 catalysts. Topics in Catalysis 56: 602–610.

III Väliheikki A, Petallidou KC, Kalamaras CM, Kolli T, Huuhtanen M, Maunula T, Keiski RL & Efstathiou AM (2014) Selective catalytic reduction of NOx by hydrogen (H2-SCR) on WOx-promoted CezZr1-zO2 solids. Applied Catalysis B: Environmental 156–157: 72–83.

IV Väliheikki A, Kolli T, Huuhtanen M, Maunula T & Keiski RL (2015) Activity enhancement of W-CeZr oxide catalysts by SO2 treatment in NH3-SCR. Topics in Catalysis 58: 1002–1011.

Reprinted with permission from Elsevier (I, III) and Springer (II, IV).

Original publications are not included in the electronic version of the dissertation.

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577. Ohenoja, Markku (2016) Computational methods for exploiting image-based datain paper web profile control

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