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This article was published as part of the

In-situcharacterization of heterogeneous

catalysts themed issue

Guest editor Bert M. Weckhuysen

Please take a look at the issue 12 2010table of contentsto

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4928 Chem. Soc. Rev., 2010, 39, 49284950 This journal is c The Royal Society of Chemistry 2010

Analysing and understanding the active site by IR spectroscopyw

Alexandre Vimont, Fre de ric Thibault-Starzyk and Marco Daturi*

Received 7th June 2010

DOI: 10.1039/b919543m

IR spectroscopy is a technique particularly adapted for understanding the mechanism of catalytic

reactions, being able to probe the surface mechanisms at the molecular level. In this critical review

the main advances in the field are presented, both under the aspects of the in situ and operando

approaches. A broad view of the most authoritative literature of the domain is given, based

largely on the experience built up at the LCS laboratory in the last decades. After having presented

the general methodology to observe a potential active site directly or by probe molecule adsorption,

several examples illustrate the qualitative and quantitative analysis of the physicalchemical

properties of the surface entities. The last part of the review is dedicated to the discrimination

of the role of the active site and its links with the catalytic steps; the hot problem of the reaction

intermediates and their visibility via spectroscopic techniques is critically addressed (138 references).

Introduction

Heterogeneous catalysis is finding an increasing importance in

everyday life. The industrial heterogeneous catalysts are

shaped multifunctional devices intended to offer to the reacting

agents a multitude of active sites, distributed on the external

surface or inside the porosity of the materials, in order to

optimise the contact between the reacting molecule and the

transformation centre in the sense of the Sabatiers principle.

On these sites the reacting molecules are adsorbed, the inter-

mediates are generated and the products formed. Physically,

the active centres are the sites where the reaction crosses the

energy barrier, as classically represented in Fig. 1.Therefore, the active sites are intrinsically linked with the

intermediate species and they can be affected by the poisons

which can be formed during the reaction. An accurate design

of the active sites (in terms of quality, strength, position, . . .) is

obviously the key to obtain the optimum catalyst. On the way

to catalyst optimisation and rational design, we have to

thoroughly characterize the active sites, particularly when in

action, considering that they are not static entities, but they

undergo modifications depending on the reaction conditions

and surface restructuration phenomena. For this purpose, IR

spectroscopy is one of the most adapted tools, being extremely

sensitive to the molecular vibrations and able to discriminate

the different geometrical distortions of the molecules accordingto the adsorption state on a site.

In this contribution, we will present how it is possible to

evidence the adsorption sites on a catalyst, particularly using

Laboratoire Catalyse et Spectrochimie, ENSICAEN,

Universitede Caen, CNRS, 6 Bd Marechal Juin, F-14050 Caen,France. E-mail: [email protected]; Fax: +33-231452822;Tel: +33-231452730w Part of the themed issue covering recent advances in the in-situcharacterization of heterogeneous catalysts.

Alexandre Vimont

Alexandre Vimont (born 1972,

France) received his PhD in

2000 from the University of

Caen in the field of in situ

and operando infrared spectro-

scopy applied to catalysis,

under the supervision of Jean-

Claude Lavalley and FredericThibaut-Starzyk. He then

joined the laboratoire Catalyse

et Spectrochimie as a permanent

CNRS research engineer. His

current research interests focus

on the comprehension at the

molecular scale of the adsorp-

tion sites in acid materials and

in particular in metal organic frameworks, using in situ and

operando Infrared spectroscopy.

Fre de ric Thibault-Starzyk

Frederic Thibault-Starzyk was

born in Saint-Lo (France) in

1965. He received his PhD

in synthetic organic chemistry

in 1992 from the University of

Caen. After post-doctoral

work with Pierre Jacobs at

the University of Leuven, hebecame Chargede Recherche

(1995) and Director of Research

in the CNRS (2009) at the

Catalysis and Spectrochemistry

Laboratory. In 20034, he was

an overseas fellow, Churchill

College, working with David

King at the Chemistry Depart-

ment, University of Cambridge. His research interests are in

heterogeneous catalysis and infrared spectroscopy, including

operando spectroscopy, time resolved measurements, and new

spectroscopic approaches for the characterisation of solids and

zeolites.

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This journal is c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 49284950 4929

probe molecules. We will describe the most accurate methodo-

logies to obtain information on physicalchemical properties

of the sites, as well as on their concentration and strength. We

will show then how to differentiate an active site from a bare

adsorption centre by coupling FT-IR with catalytic evidences.

We will finally discuss briefly the problem of the detection

limit of intermediate species (intrinsically linked to active sites)

using infrared spectroscopy.

Surface adsorption site identification on a catalyst

The best identification of the potential active sites by IR

spectroscopy is the direct detection of their IR fingerprint.The best examples are probably those given by the zeolites, for

which the strong Brnsted-acid sites are often identified

through the n(OH) band of hydroxyl groups. If this is not

possible, specific methods must be employed to obtain a

spectroscopic response for the sites. The most common is the

adsorption of probe molecules, which provides IR spectra

specific to the interaction with a single site. From the spectro-

scopic point of view, several reviews have given criteria about

the choice of the right molecule,35 However, without any

further considerations, it is worthwhile noticing that whatever

the probe molecule used, it generates a perturbation of the

surface of the catalysts: indirect perturbations such as electron

withdrawing effect, or direct perturbations like chemical reactions

(protonation, electron transfer, decomposition, . . .). These

phenomena can be considered as invasive, but according to

the probe used and the chemical reaction considered, this can

also give information on the dynamic response of a surface to

the molecule adsorption. Therefore, it can be concluded that

whatever the probe molecule used, the best one is the reactant

itself, providing the same perturbation as during the chemical

catalytic reaction. In this view, the best conditions for a study

appear to be those during the reaction, i.e. the operando

conditions (real in situ);6 we will develop this point towards

the end of the review.

The spectroscopic study of the adsorption sites on a catalyst

surface requires a good knowledge of the surface state itself, as

well as an excellent reproducibility of the characterisation, to

fix the parameters and the limits of the surface description. In

this view, it is necessary to be aware of the presence and nature

of surface impurities, to establish how to clean the surface and

make the sites accessible.

Impurities and surface activation

IR is a very sensitive technique for detecting surface impurities

such as water, carbonates (formed by catalyst contact with

ambient atmosphere), organics and other residual species after

synthesis, such as nitrates, sulfates, templates, etc., because

these species present characteristic bands in the IR spectra.

Although many of these species do not represent a real

problem for catalytic reactions, being often eliminated at the

temperature at which the process takes place, or even taking a

beneficial role in the reaction itself, their presence may inhibit

(at least partially) probe adsorption, or simply disturb the

correct spectral interpretation by band overlapping. Band

position and intensity can vary depending on the sample

nature; an abundant literature exists describing these spectra

(see for example ref. 5). Thermal stability of these species

strongly depends on acidbase properties of the material: on

relatively acidic compounds (alumina, zirconia, . . .) carbonate

and nitrate species can be removed at mild temperatures. On

basic oxides, such as MgO, carbonate impurities can still be

observed after a thermal treatment at 750 1C. In the case of

sulfates, a reducing treatment at higher temperature is even

necessary to clean the surface, but traces of sulfur can remain.7

Nevertheless, we should consider the fact that these residual

species are essentially bulk moieties, almost inert during the

catalytic process; therefore it is better to leave them in

place rather than to heat the sample at very high temperature

(with the aim to clean all the impurities), so producing a

drastic sintering of its surface.

It is much more difficult to detect the presence of alien

anionic entities such as sulfur, Cl and F, or cationic species

Fig. 1 reaction profile and intermediates in the simple case of a

process going through two elemental reaction steps A - I- B: a is

the profile for a non catalytic thermal reaction; b is a catalytic reaction

using a good catalyst; c is a catalytic reaction using a bad catalyst.

If I is an intermediate with a low stability, it is hard to detect; if I 0 is a

stable intermediate it will easily be detected. The transition state A* or

I* is not detectable.1,2

Marco Daturi

Marco Daturi was born in

Genoa (Italy) in 1964. After

having obtained a Master in

Physics, he studied Chemistry,

receiving a PhD in Chemical

Engineering in 1996 from the

University of Genoa (director:Prof. G. Busca). After post-

doctoral work with Dr J.-C.

Lavalley at the LCS Catalysis

and Spectrochemistry Labo-

ratory, he became Lecturer

(1998) then Professor (2002)

at the University of Caen,

where he teaches thermo-

dynamics and spectroscopy.

His research topics deal with in situ and operando IR spectro-

scopy, heterogeneous catalysis, material surface investigations

and design. He applies fundamental studies to the domains of

pollutants abatement and environmental protection.

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such as alkaline cations in the zeolites, due to the absence of

specific bands. An indirect method consists in the observation

of the n(OH) range of the catalyst spectrum: OH/Cl, OH/F

substitutions and OH/ONa+ alkali exchange strongly modify

the intensity and the position of the n(OH) bands. These

impurities and contaminants strongly influence the acidbase

properties of the samples. As an example, Cl, F and sulfate

species can increase the strength of the acid sites on alumina,8

whereas sodium (even if present in as small quantities as

300 ppm) poisons the strongest Lewis-acid sites,9,10 Traces of

chloride also hinder oxygen mobility on cerium-based com-

pounds11 and inhibit the oxidation properties of the supported

metals.12

Effect of the temperature of activation

The increase of the temperature of activation for metal oxides

not only removes the impurities and molecularly adsorbed

water but also leads to surface reconstruction, due to the

elimination of hydroxyls, according to the mechanism involving

two MOH groups: 2 MOH - MOM + H2O (with the

possible creation of a vacancy). In extreme conditions, if the

activation temperature exceeds the calcination conditions

during sample synthesis, sintering phenomena can also occur.13

The thermal treatment leads to surface reconstruction, and

the amounts of exposed OH groups, cationic, anionic and

defective sites, which govern their acidbase and redox properties,

are not predictable, but can be estimated by IR absorption

experiments of probe molecules at various temperatures. The

most complete IR studies about the effect of the temperature

of activation on the surface modification spectroscopy probably

deal with MgO and alumina.1417

Direct observation of the active sites

Case of Brnsted-acid sites.Direct observation of the poten-

tial active sites is possible when Brnsted-acid sites are con-

sidered and this possibility is well illustrated by the zeolites or

silica, for which the hydroxyl groups can be directly observed

through their n(OH) bands. The spectrum of hydroxyl groups

of steamed HY zeolite (Fig. 2) allows one to observe the acidic

OH group located in different sites: bridged hydroxy groups in

supercages (3625 cm1), or in sodalite cages (3545 cm1).

Perturbation of bridged OH groups by extraframework

entities generates highly acidic species with specific n(OH)

band (3602 cm1 for those in the supercages, 3520 in the

sodalite cages). For this zeolite, the presence of the 3602 cm1

band is often related to the activity of the catalyst in strongly

demanding reactions like n-hexane cracking,18,19 The OH

bands in the dealuminated Y zeolites have been studied in

detail and a new model has been proposed recently.20

Such a

sensitivity to the environment is well illustrated with the IR

spectra of the mordenite, which presents three nOH bands,

depending on the location (Fig. 2b).21 The spectra of the

sample during the conversion of xylene show that only the

hydroxy groups present inside the main channels act as active

sites towards the isomerisation reaction. Sites located in the

narrow side pockets are not catalytically active, but have a

strong influence on selectivity. This case illustrates nicely the

fact that the direct observation of the potential sites can give

valuable indications on the reaction protagonists.22

Thermally activated divided metal oxides present residual

OH groups having IR stretching frequencies related to the

nature of surface cations. The multiplicity of the n(OH) bands

of oxides such as alumina has been mainly explained invoking:

(i) the multifold coordination of the hydroxyls themselves

(linear species giving rise to n(OH) bands at a higher frequency

than those characterising bridged species), (ii) the coordination

number of the cation to which OH groups are bound, (iii) the

presence of morphologic defects on the surface (edges, corners,

etc.). In the case of many oxides such as Al2O3, Ga2O3, CeO2,

ZrO2, the higher n(OH) wavenumber component presents

a basic rather than acidic character, as shown by CO2experiments.

2325Their relation to defect crystallographic

sites has also been invoked considering the sensitivity of this

component to the presence of coordinated species in the

neighborhood.24 This shows that the co-existence of several

types of sites on the surface complicates the relation between a

n(OH) band and a well defined Brnsted surface sites in the

case of metal oxides.

Case of Lewis-acid sites. Direct IR observation of a

Lewis-acid site itself is not possible since a coordinatively

unsaturated site is not a vibrator. Considering the Lewis site

and its first coordination sphere, the vibrations (metaloxygen

vibrations for metal oxides) are generally coupled with the

very intense bands from the bulk vibration modes and only

studied by mixing the samples with KBr; therefore only

hydrated solids are generally studied. However in few cases,

defect sites are detected in the IR spectrum of activated

samples by specific IR bands, mainly in the low frequency

range: bands situated at 3782/880 cm1

on beta zeolite26

and

around 960 cm1 on Ti-silicalite27 can be considered as a

fingerprint of Lewis-acid sites and have been partly related

to the enhanced activity of these Lewis-acid catalysts in the

catalyzed MeerweinPondorfVerley (MPV) reaction.28

Surface AlO species (at ca. 1050 cm1) on alumina29 and

strained siloxane bridges on highly dehydroxylated silica

(880940 cm1) are dissociation sites for water, alcohols, H2S.30

However, the number of cases in which direct characterisation

Fig. 2 IR spectra of the hydroxy groups of a steamed Y zeolites

(a) and of a mordenite zeolite (b) after activation at 450 1C.

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of Lewis acidity is possible is limited, and complementary IR

adsorption experiments using probe molecules are often

necessary.

Probe molecule use.The direct detection of adsorption sites

by IR spectroscopy is often not possible, therefore specific

methods must be employed to obtain a spectroscopic response

of the sites. The most common is the adsorption of probe

molecules which gives IR spectra specific to the interaction

with the site, as mentioned above. Concerning the choice of

the appropriate probe molecule, Lercher et al.3 gave special

emphasis to the criteria that have to be met to arrive at a

characterization of materials that are useful for catalytic

application, selecting the right molecule for the right site. The

choice of the adapted probe will depend on many parameters:

the chemical function providing the interaction with the site

under study; the size of the molecule, depending on the site

accessibility; the optimum of the interaction (sufficient to

furnish valuable information, not excessive so as to limit

surface modifications); the spectral response, producing a signal

intense enough, with band positions sensitive to the interaction;

stability on the surface catalyst to avoid decomposition; and

sufficient vapour pressure to be easily introduced in an IR cell.

The investigated sites are very often cations, acting as Lewis

centres, but infrared spectroscopy has also been widely used

in order to characterize the metallic centres in oxides and

deposited metal complex. Even if direct investigations on

metaloxygen vibrations are reported3133 most of the studies

related with catalysis are dealing with the adsorption of probe

molecules. Among these molecules, N2, methanol, NO and CO

are frequently used and the latter two are also by far the most

common in the literature. In 2002 a very complete review

concerning the infrared spectra of chemisorbed carbon

monoxide as a characterization tool for the cationic sites of

oxides34 was published. A specific property of CO is that the

slightly antibonding HOMO 5s orbital is occupied. This

orbital is very important for the electron-donating properties

of CO, because a decrease of electron occupation on this

orbital leads to stabilization of the entire molecule and thus

to an increase of the n(CO) wavenumber (compared to the

n(CO) for the gas phase at 2143.5 cm1

). On the contrary, the

addition of electron density from a metal d orbital to one of

the 2p* LUMO orbitals (so called p back donation) leads to a

substantial decrease of the vibrational frequency of CO, i.e.to

a weakening of the CO bond. As far as the red-ox properties

are concerned and in the ideal case, the expected information

with CO as a probe are the following:

oxidation state of the cations on the surface,

coordination state of these cations,

location of the cations on flat planes or other surface

structures,

location of the active phase on the support,

surface phase analysis,

existence of strong oxidizing agents on the surface.

The characterization of the various sites on mixed oxides

can be advantageously carried out by CO adsorption at

various equilibrium pressures at low temperature, followed

by evacuation at increasing temperatures to obtain infor-

mation about the stabilities of the various species. Although

the CO stretching frequency is the most informative parameter,

the data determining the stabilities of the various species can

be decisive for the assignment of the bands. Multiple carbonyls

adsorbed on the same metal cation are possible and in order to

identify them, isotopic mixtures should be used. This was the

case, for example, of a PtNamordenite sample, where the

use of a 12CO13CO isotopic mixture combined with analysis

of the second derivatives of the spectra was very useful

for proving the polycarbonyl structures.35 Again, using iso-

topic labeled 13CO and 15NO molecules mixed with their

most abundant analogues, it was possible to describe the

multiplicity and symmetry of CO and NO ligands on a

complex coordinated with Rh2+ in a Rh-ZSM-5 sample;

geometrical structures and band assignments were supported

by DFT computational results.36 However, sometimes the

polycarbonyls are very stable and in this case, if 12CO is

adsorbed first and then 13CO introduced, mixed species may

not form at ambient temperature.

Concerning NO, Hadjiivanov37 reported that the coordi-

nation of the NO molecule to a cationic site via the nitrogen

atom is accompanied by a partial charge transfer from the

5sorbital together with an increase in the bond order, just as

in the case of CO. Formation of a p-back-bond, although not

as easy as with CO, is also possible, and this results in a

decrease in the NO stretching modes. The different surface

mononitrosyls absorb in a wide spectral range: 19661710 cm1.

When only a s bond is formed, a frequency above that of

gaseous NO (1876 cm1

) is expected, whereas with low-valent

cations, rich in d-electrons, p-back donation is possible and

the NO stretching modes can fall below 1876 cm1. Cations

having no d-electrons produce mononitrosyls only; on the

contrary, dimeric molecules are very often the principal

adspecies on transition metal cations. This is the case of NO

adsorption on V-, Cr-, Mo-, W-, Fe-, Cu- and Co-containing

oxide systems where the metal cations are not in their highest

oxidation state. Thus, it is clear that a p-back donation

stabilizes dinitrosyls. Examples of complementary information

obtained by CO and NO coadsorption are also available.38

Acid sites. A review by Busca39 describes the bases of IR

spectroscopic methods for the characterization of the surface

acidity (both of Lewis and Brnsted type), on different mixed

oxides. A systematization is proposed associating the surface

acidity with the ionicity/covalency of the elementoxygen

bond, mainly affected by the size and charge of the cation.

In a parallel work, the results obtained for the characterization

of the Lewis-acid strength of more than 30 binary and ternary

mixed oxides are interpreted on the basis of the different

polarizing powers of the involved cations.40

Recent progress

on acidity characterization is reviewed elsewhere and

described to be related to the broadening of the spectral range

(investigation of overtones, combination bands, and low-

frequency modes) and to the adsorption of new non-traditional

probe molecules for identification of acid sites.41,42

Some examples on oxide and zeolitic compounds.The choice

of the probe molecule is crucial to obtain an overall view of the

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acidity. Its size has to be small enough to interact with all

available sites and to avoid confinement effects43 but its basic

strength has to be strong enough to interact even with the

weakest acidic sites. Ammonia seems to be a good candidate

for this but due to the high polarity of the NH bonds,

hydrogen bonding with basic entities governs the coordination

of adsorbed species and direct conclusions about acidic

strength are not straightforward.44 That is why most often

the adsorption of several probe molecules is required. For

example, the FAU and the MOR structures are made of big

and small cavities in which some acidic hydroxyls are out of

reach of basic molecules such as pyridine. Co-adsorbing the

strongly basic trimethylamine (TMA) and NH3(see Fig. 3), we

were recently able to give for the first time an infrared evidence

of three distinct acidic hydroxyls in defect-free HY,45 to give

an assignment for the corresponding wavenumbers and to

characterize their respective acidic strength.46 Moreover,

TMA desorption associated with the recovery of hydroxyls

at 3656 and 3638 cm1

(and two corresponding n(NH) bands

reveal the presence of at least two distinct acidic strengths for

the hydroxyls located inside the supercages. For the same site

location, the local chemical factor should then play a role: the

aluminium distribution in the framework is not necessarily

homogeneous, and the number of Al next-nearest neighbours

influences the acidic strength of a given site. Another explana-

tion for the unusual 3656 cm1 component could be that part

of the O4

crystallographic sites is a proton holder for this low

Si/Al ratio HY sample; in such a case, all the four theoretically

forecasted sites in the zeolitic FAU structure would have been

observed by IR spectroscopy.46

The combined use of these

two molecules moreover helped us to better characterize the

various coordinated NH4+ and determine the activity ranking

between the ammonium species and coordinated ammonia

over Lewis sites during NOx

SCR.47

In the zeolites, some strong Lewis-acid sites can be obtained

during steaming leading to the formation of extra-framework

aluminium species. Mild Lewis sites may be naturally present

(in the alkaline form) or generated upon ionic exchange with

transition metals which are necessary for NOx

SCR with

hydrocarbons. For over-exchange level, some Lewis species

may remain on the external surface and coadsorption of the

bulky ortho-toluonitrile and CO was recently reported to

identify the different Con+ species and their location in a Co

H-MFI zeolite.48 In this respect, the use of the nitrile probe

provided more precise evidences about the distribution of Co

species in active CH4-SCR Co-HMFI than those arising

from previous UVvis, EXAFS and XRD data.4953 The

oTN (ortho-toluonitrile) and NO co-adsorption allowed to

determine that a significant amount of cobalt species is at the

external surface, mostly in the form of divalent cobalt. On

the other hand, in the internal surface part of Co species are

trivalent, together with predominant divalent Co ions. These

observations coupled with those coming from the operando

study were valuable for reactivity explanation, inferring that

the active sites for CH4-SCR in Co-MFI are Co3+ species

(presence of a nitrosyl n(NO) band at 1930 cm1) located in

the cavities, but likely in non-classical cation positions, which

are able to convert NO to an adsorbed bridging nitrate species,

that can be later decomposed to give gas phase NO2.54

Moreover, the cavity may contribute to the stabilization of

aggregates containing trivalent cobalt. At the same time, the

presence of Co-isocyanates involved in the SCR suggests that

a possible route for the reaction implies the reduction of

nitrate-like species by methane, forming H2O and isocyanates,

which could later react with NO producing N2 and CO2. On

the contrary, it seemed that substitutional Co2+ ions did

not play a key role in the reaction, being very likely almost

redox-inactive. Co2+-dinitrosyls formed on them being

decomposed well below the reaction temperature, they did

not seem to be involved in the reaction.54

These considerations

linking the active site with the reactivity of the species

coordinated on it and with the possible intermediates can be

generalized to all the reactions and will be discussed later in

this review.

Metalorganic framework: the ideal case for spectroscopic

identification of adsorption sites. In the case of the metal

oxides, surface relaxation and reconstruction phenomena

might not allow to accurately identify the nature of the

Lewis-acid sites (coordination, oxidation degree, unsaturation

degree) via the study of spectra after the adsorption of probe

molecules. On the contrary, this is not the case for the majority

of hybrid organicinorganic solids such as MOFs (metalorganic

frameworks), which are crystalline nanoporous solids, with a

framework of inorganic units (clusters, chains or planes) and

organic linkers (phosphonates, carboxylates, sulfonates). The

majority of MOFs present framework metal sites that can

exhibit coordinative vacancies upon solvent removal; such

sites represent Lewis-acid centers of well defined symmetry

and oxidation degree. As has been shown in the case of

MIL-10055 and H-KUST56,57 it is possible to assign in a clear

and indisputable way the bands due to the adsorbed species,

like those (still debated nowadays) for carbonyls or nitrosyls

coordinated on cations such as chrome, copper and iron. CO

adsorption on MIL-100/101(Cr) is an example of the con-

tribution of IR to the identification of the potential active sites

in MOF.55,58,59 Three n(CO) bands are observed at 2207,

2200 and 2193 cm1 (Fig. 4), showing that Cr3+ sites are

Fig. 3 Infrared spectra of the HY sample upon TMA adsorption and

NH3saturation evidencing OH in the supercages at 3637 cm1, OH in

the sodalite units at 3548 cm1 and OH in the hexagonal prism at

3501 cm1 (from ref. 45). Reproduced by permission of the American

Chemical Society.

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not equivalent. This heterogeneity is attributed to the possible

presence of fluoride ions (synthesis being performed in the

presence of hydrofluoric acid) on the metallic trimers, with 2, 1

or no fluorine ion, respectively, in the neighborhood of the

coordinatively unsaturated (CUS) Cr3+ considered site.55

Quantitatively, the number of free Cr3+ sites in the activated

compound is exactly that expected, considering that onecorner over the three octahedra is occupied by one anion.

On MIL-101(Cr), this methodology allows identifying these

sites as the grafting centres of catalytically active sites.59

Relationship between probe molecules and activity: the case of

acetonitrile. Two possible aspects of acidity are generally

considered. The first one is the hydrogen bond that can be

established between a Brnsted site and the basic probe

molecule (for example when carbon monoxide is interacting

with acidic zeolites at liquid-nitrogen temperature). The second

one is the extent of proton transfer, or more exactly the

amount of protonated probe molecule (e.g. pyridine) on the

surface of the catalyst at room temperature. These two facets

of acidity, however, can not reliably be used for explaining the

catalytic activity in acid-catalyzed reactions. The H-bond

between a basic molecule and the various acid sites in a solid

is strongly influenced by temperature. The linear relation-

ship between the strength of a H-bond with CO at 100 K

(often used for comparing solid catalysts) and the activation of

proton transfer to a hydrocarbon in a chemical reactor at

700 K is far from being established. Acetonitrile has been

increasingly used as an infrared probe molecule for solid

catalysts, and might well provide an integrated approach of

acidity. Both H-bond and protonation can be observed, and it

can be used to probe the actual proton transfer in reaction

conditions.

In the absence of water, heating acetonitrile on an acidic

zeolite leads to the reversible protonation of the probe

molecule.60

Protonation of acetonitrile on zeolite Brnsted

sites at high temperature has been used to build an acidity

scale agreeing with catalytic activities in the conversion of

saturated hydrocarbons: acidity is determined under conditions

near to that of catalytic reactions (high temperature), leading

to a more relevant parameter for the prediction of catalytic

activity than H-bonds. Acid catalysis involves protonation

of the reactant, and a scale built on actual protona-

tion, preferably in reaction conditions, is more interesting.

The protonation temperature was measured on a series of

zeolites, and depends very much on the pore size.61 The nearer

the pore size is to the size of the adsorbed molecule, the lower

the protonation temperature. For example, in mordenite, two

main locations exist for the Brnsted site: in the main channels

and in the side pockets. From the point of view of CO at

low temperature, a stronger H-bond is created in the main

channels, and the stronger site would therefore be in the main

channels. However, protonation has only been observed in the

small side pockets, where maximum confinement takes place.

The measurement of the protonation temperature is a way to

know how easy the proton transfer is from the acid catalyst

to the basic adsorbed molecule. It is therefore a new spectro-

scopic measurement for the acidity of the solid, one that

does not only involve the interactions strength between the

adsorbed molecule and the surface, but the actual proton

transfer, the real nature of acidity. The probe is here not

the adsorbed molecule itself, but rather the actual catalytic

protonation reaction. The new scale obtained between various

zeolites was compared to the activity in catalytic conversion of

saturated hydrocarbons at high temperature, a reaction where

activity is linked to the acid strength of very strong sites.

Contrarily to what was observed with H-bonding of CO or

with pyridine protonation at room temperature, the scale of

acetonitrile protonation temperature was perfectly linked with

the catalytic activity.62

Modification of the basicity of the probe molecule by the acid

site.Acetonitrile has also shown that the basicity of the probe

is influenced by the adsorption on the solid, especially on

zeolites where confinement is important. During molecular

dynamics simulations of acetonitrile on mordenites,63 the

electric dipole on the molecule was significantly modified in

the move from the main channel to the inside of the small

lateral cavity. In the purely siliceous mordenite used for the

simulation, the molecular dipole of acetonitrile was enhanced

from 3 D (in the large channels or in the gas phase) to 4 D in

the small cavities. Such enhanced dipole could also imply that

the basicity of acetonitrile would be increased by a third on

entering the small cavities. Moreover, the probe molecule is

locked in the cavity, and does not escape easily. It is thus

kept in close distance to the OH group, thus enhancing the

probability for the proton transfer (Fig. 5).

When the size of the molecule compares with that of the

cavity, electron densities in the basic molecule are modified,

the basicity of the probe can be enhanced, and the proton

transfer can happen more easily. All these parameters show

that the basicity of the probe molecule can not be considered

without the solid, and that it is always a pair at work.

Modification of the acidity of the site by the probe molecule.

In some cases, the probe molecule itself modifies the surface

acid sites: during comparative study of the Lewis acidity

between SiO2B2O3 and alumina by adsorption of pyridine

(py), CD3CN (MeCN) and CO, no coordination of carbon

monoxide has been observed on silicaboria even at low

temperature whereas CO strongly coordinates on alumina.64

Similarly, coordinated pyridine and acetonitrile show a

much lower thermal stability on SiO2B2O3 than on Al2O3,

Fig. 4 Left: CO adsorption sites of the trimers of chromium

octahedra in MIL-100(Cr). Right: n(CO) bands of CO adsorbed on

MIL-100(Cr).55

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indicating that silicaboria presents much weaker Lewis-acid

sites than g-Al2O3. On the other hand, coordinated pyridine

and acetonitrile species show that infrared frequency shifts

(n8a, n19b and n(CN), respectively) are larger on B2O3SiO2than on Al2O3, suggesting that charge transfer from these

probe molecules is more important on the B3+ than on the

Al3+ Lewis acid (Fig. 6). DFT calculations of the interaction

of these probe molecules with models representing Al3+ and

B3+ Lewis-acid sites adequately reproduce these experimental

observations (Fig. 7). The weakness of the B3+

Lewis-acid

sites is ascribed to the p-character of BO bonds, which

disfavours the conversion of boron from a trigonal planar

conformation to a tetrahedral conformation upon adsorption

of probe molecules and decreases the adsorption energy of

pyridine and acetonitrile despite a strong charge transfer.

The absence of interaction noted during the adsorption of

CO on SiO2B2O3 has been explained by its basicity, not

strong enough to compensate for the energy required for the

conformational change of the B3+ Lewis-acid centre.

Transformation LewisBrnsted sites. Brnsted acidity can

be generated by water on the surface of crystalline or amorphous

solids. On metal oxides, it is well known that water can

transform Lewis into Brnsted acidity by dissociative water

adsorption on the Md+Od acidbase pairs, with the con-

sequent creation of MOH acidic groups. On aluminium

fluorides, water addition transforms Lewis-acid sites into

Brnsted sites on activated compounds.65

Due to the strong

Lewis acidity, water is strongly coordinated on fluorides and

generates strong Brnsted sites, as evidenced by CO adsorp-

tion. The absence of sufficiently strong basic sites on fluorides,

as shown by an unpublished study of propyne adsorption,

inhibits dissociative adsorption of water on such materials.

The consequences of the adsorption of protic molecules

(water and alcohols) on the acidity of MOFs, have been well

identified by IR spectroscopy:55,58 adsorption of water on

activated MIL-100(Cr) leads to the formation of coordinated

species well characterized by two narrow n(OH) bands at

about 3700 and 3580 cm1. CO adsorption at 100 K shows

that coordinated water induces the creation of Brnsted-acid

sites with a strength close to that reported in the case of

phosphated silica. The successive addition of CO molecules on

hydrated MIL-100(Cr) shows that each coordinated water

molecules creates two Brnsted-acid sites (see Scheme 1).

This LewisBrnsted acid conversion was used to modulate

the strength of the created Brnsted-acid sites according to the

Fig. 5 Molecular dynamic simulation of acetonitrile in a purely

siliceous mordenite. The Ycoordinate corresponds to the distance of

the probe molecule from the centre of the main channel. The Z

coordinate corresponds to the distance along the main channel.

(A) Positions of the centre of gravity of acetonitrile during the

simulation, showing it can go from the main channels to the side

pockets. (B) Molecular dipole and (C) location of the molecule during

the simulation vs. distance from the main channel in the mordenite

structure along the axis of the side pocket (ZB 2 A ) (adapted from

ref. 63). Reproduced by permission of the American Chemical Society.

Fig. 6 IR spectra of activated Al2O3(top) and boriasilica (bottom)

after introduction of pyridine at room temperature (left) and CO at

100 K (right-spectra at various coverage).64

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nature of the adsorbate used: the comparison of the results

deduced from the grafting of CH3OH, H2O, CF3CH2OH and

(CF3)2CHOH shows that the stronger the acidity of the

adsorbate, the higher the acidity of the generated Brnstedsites. Their strength is directly related to the nature of the

grafted molecules and can reach that of the zeolitic Brnsted-

acid sites when fluorinated alcohols are used (see Fig. 8).58

Basic sites. Heterogeneous catalysis using basic solids has

been much less studied than acidic catalysis. It seems even that

a concern exists about the definition of basicity itself. It may be

useful to recall here that, in spite of the common use, there is

not a physical difference between the so-called Brnsted and

Lewis basicity, because for both it is due to electrons on

oxygen atoms. In fact, for Brnsted, an acid site is a proton

donor (i.e. an hydroxyl on heterogeneous catalysts), whereas

Fig. 7 Evolution of the total interaction energy (DEtot, ), deformation energy of the Lewis-acid center (DEdef(A), ), deformation energy of the

probe molecule (DEdef, --), and interaction energies of the probe with the Lewis center at their geometry in the complex (DEint, -- -) for six acidbase

complexes as a function of the ML distance (M = B or Al; L = C or N). 64 The deformation energy is the difference between the energies of the

isolated species at their equilibrium geometries and the energy of the isolated species at their geometry in the complex.

Scheme 1 Interaction of CO with coordinated water molecules in

MIL-100(Cr).58 Reproduced by permission of the American Chemical

Society.

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for Lewis it is an electron pair acceptor (i.e. a cation); but

looking at basicity definitions, in the sense of Brnsted a basic

site is a proton acceptor, while for Lewis it is an electron pair

donor, therefore an oxygen site in both cases, or an halogen

atom for halogen based catalysts. Some simple concepts

addressing this problem have also been mentioned by

Zecchinaet al.66

Concerning the difficulties in studying surface basicity, we

might explain it with the fact that FT-IR is a technique

sensitive to molecular bonds: in the case of acidity, when using

a probe molecule to characterize Lewis sites, the molecular

deformations of the molecule induced by the cationic polarizing

effect are measured. In the case of a basic site, the surface is

donating electrons and the back-donation effect on a molecule

is a more complicated phenomenon to quantify. When studying

proton donation or acceptance, the spectra are always com-

plicated by broad and badly resolved features. From a general

point of view, anyway, protonic molecules seem particularly

adapted for probing basic sites, taking into account the

basicity definition itself (and impacting them only by weak

interactions).

As an acid site is always associated to its conjugated basic

site, reviews dealing with characterization of acidity by infrared

also report interesting data about basicity (see for example

ref. 41) and even describe some typical probe molecule inter-

actions: CH-acids such as chloroform (Cl3CH(D)), acetylene

(C2H2) and methylacetylene (CH3C2H) are shown to be

potentially suitable probe molecules for basic properties using

the H-bonding method.67 All three molecules undergo

Oz2 HC hydrogen bonding and the induced red-shift of

the CH stretching frequency permits a ranking of the base

strength of a given series of materials. Many other probe

molecules were tested for the specific study of the surface

basicity of divided metal oxides, and Lavalley reviewed some

years ago the infrared spectrometric studies of the surface

basicity of metal oxides and zeolites using adsorbed probe

molecules.4 Results obtained from carbon monoxide (CO),

carbon dioxide (CO2), sulfur dioxide (SO2), pyrrole (C4H5N),

chloroform (CHCl3), acetonitrile (CH3CN), alkanes, thiols,

boric acid tri-Me-ether, ammonia (NH3), and pyridine

(C5H5N) were discussed in that well cited review. As we

already noticed in the case of the acidity study, the author

reminds us that no probe can be used universally. CO2 for

weakly basic metal oxides and for basic OH groups, CO for

the characterization of highly basic structural defects on metal

oxides activated at high temperature and pyrrole in the case of

alkaline zeolites, appear to be quite suitable probes. Moreover,

both NH3 and pyridine (generally used as probes for the

measure of the acidity of catalysts) are also described to

adsorb on basic oxides via dissociative chemisorption. In this

respect we should stress that a strongly interacting probe (such

as CO2 and NO2, for example) highly modifies the surface

behaviour, often leading to false interpretations on the

strength of the basic site. Moreover, a probe which dissociates

is an unfriendly tool for site concentration characterisation

when using volumetric methods. Again, a non-dissociating,

weakly interacting protonic probe would appear as a most

adapted tool for surface basic site characterization.

A pioneering review on the basic properties of zeolites was

proposed by Barthomeuf.68 More recently, NO2disproportio-

nation on alkaline zeolites was used to generate nitrosonium

(NO+) and nitrate ions whose infrared vibrations are shown

to be very sensitive to the cation chemical hardness and to the

basicity of zeolitic oxygen atoms.69

Recently, Michalska et al.70 have pointed out that propyne

is an excellent probe for the study of oxygen basicity. Addi-

tionally, they have verified that probe dissociation does not

depend on the site strength, but can be due to the presence

of Lewis-acid sites coupled with the basic ones: the formation

of the hydrogen bond weakens the bond betweenRC and H.

Fig. 8 Brnsted-acid strength of OH groups from various grafted species on MIL-100(Cr) measured by CO adsorption: correlation between the

n(OH) shifts, the H0 values and the n(CO) position.58

The y axis label should be read as |Dn(OH)|. Reproduced by permission of the AmericanChemical Society.

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In the presence of an acid site, which can host the CH3CRC

moiety, the surface protonation is therefore easily achieved.

Therefore, propyne dissociation is a good probe for the

presence of acidbase pairs on a surface.70

Basicity case study. Among the catalysts having basic

properties, ceria is probably the most common owing to its

properties in the domains of oxidation and hydrogen produc-

tion. It is the base material for car exhaust control devices,

notably for Three Way Catalyst (TWC) applications. For

these reasons an extensive characterisation of its properties

has been undertaken for at least two decades. Pyrrole adsorp-

tion on CeO2leads to dissociative adsorption characterized by

stretching ring vibrations at 1444 and 1367 cm1 typical of

pyrrolate ions and n(OH) vibration at 3628 cm1 typical

of surface hydroxyls formed upon proton transfer.71

This

complete dissociation of C4H5N is indicative of the high

basicity of CeO2 surface O2 ions but does not allow an

investigation of its variation upon reducing ceria. CO2 was

further adsorbed as it acts as a Lewis acid toward either O2

surface ions (with the production of carbonates) or residual

basic OH surface species (with the production of hydrogen

carbonates (HC)). The study extended to CeZr mixed oxides72

indicates that hydrogen-carbonates (indicative for basic OH

groups) are mainly observed for rich ceria compounds and

that the intensity of carbonate species is directly proportional

to the cerium content, as expected according to the basic

properties of this element. Identification of the spectral

features typical of each species arising from CO2 adsorption

was clarified by studying the splitting of the n3 band of

carbonates and their thermal stability.

Nowadays, basicity is becoming a more and more important

parameter, and basic materials are involved in numerous

industrial processes, such as fine chemical productionviagreen

chemistry routes (replacing homogeneous by heterogeneous

procedures, for example in esterification reactions in the

absence of any solvent), or environmental catalysis. One of

the most investigated methods for nitrogen oxide removal is

NOx

-trapping. In this process, NOx

are stored and concen-

trated in highly basic compounds, before being submitted to a

reduction. The materials used in this respect are typically

barium, alkaline metals, alkaline-earths and rare earth

oxides.73 In such a case CO2 and NO2 are the most adopted

probe molecules, giving rise to carbonates and nitrates,

respectively. The storing materials are obviously submitted

to severe surface and bulk restructuration, but in the same way

they will be under service, exposed to the reacting lean and

reach flows, containing NOx

and high concentrations of CO2.

General indications on nitrate characterization and their

coordination on a number of solids can be found in the

excellent review by Hadjiivanov.37

A comparative study of

barium and potassium based formulation74 indicated upon

NO2adsorption the formation of both ionic and covalent-like

NO3 species over PtRh/Ba/Al2O3, whereas only very stable

ionic potassium nitrates (sharp peak at 1373 cm1) were

detected over Pt/K/Mn/CeAl2O3. This is due to the higher

basicity of the potassium sites which furthermore enlarges

the adsorbing temperature window and delays the nitrate

release during the rich step impeaching NO sudden outlet.

A comparison of the latter composition with a great number

of other NSR catalysts was reported in ref. 75, where the exact

chemistry of nitrite and nitrate formation was investigated,

and their coordination on specific structural sites of the oxide

determined thanks to the parallel use of TEM analyses.

Another interesting example is the synthesis of phyto-

sterol esters from transesterification of a fatty methyl ester

(dodecanoate) with b-sitosterol carried out in the presence of

basic solid catalysts, such as lanthanum oxides.76 The

acidbase properties of La2O3 were characterized by IR

spectroscopy, which revealed the presence of residual unidentate,

bidentate, polydentate, and mineral carbonate species inside

all of the solids even after activation, suggesting different basic

and catalytic characteristics of the samples. The carbonate

strengths were determined by propyne adsorption, using the

shift of the n(CRC) stretching mode for the adsorbed species:

the lower the position of that vibration, the greater the basicity

of the corresponding site. A ranking of the basic strength of

the surface carbonates species of the lanthanum oxycarbonate

samples was thus possible and it was correlated to the catalytic

activity: the lower the basicity of carbonates, the higher the

phytosterol ester yield. Moreover, a thorough spectral

characterization of KBr-diluted samples indicated that the

higher the intensity of unidentate carbonate bands (1499 and

1382 cm1), the higher the sterol ester yield, suggesting those

moieties of medium basic strength play a central role in the

catalytic mechanism.

It was shown in our laboratory that COS hydrolysis77 and

CS2 hydrolysis78 could be used as test-reactions for metal

oxide hydroxyl basicity. However, the use of IR spectroscopy

should add value to this form of characterisation: adsorption

of CS2 on a series of metal oxides (Al2O3, ZrO2, ZnO and

CeO2) gave rise to a specific interaction with O2 sites, leading

to the formation of xanthate (COS2

)2 species characterized

by bands in the 12001000 cm1 range whose intensities

correlate well with the relative basicity of the analyzed oxides.

Co-adsorption experiments of CS2with either CO2or pyridine

showed that its adsorption sites are mainly those giving rise to

bidentate carbonates, showing that this new probe is more

specific than CO2. Moreover, the transformation of xanthate

into carbonate species at low temperature could account also

for the surface oxygen mobility.79

Metallic and redox sites. Transition metal oxides, rare-earth

oxides and various metal complexes deposited on their surface

are typical catalytic phases leading to redox properties. For

each of these phases, complementary tools exist for an appro-

priate characterization of the metal coordination number,

oxidation state or nuclearity. Among all the techniques, IR

provides information by characterizing the characteristic

vibrations of intrinsic (hydroxyls) or extrinsic (methanol,

CO, . . .) probes.

We have already discussed the principles of the use of probe

molecules for the characterisation of surface species in the

section concerning probe molecule use. CO and NO can

provide highly valuable information on the supported metal

dispersion and coordination, as well as on the oxidation degree

of such moieties. Typical examples can be found in the

works of Binet8083 and Bazin.84,85 This methodology appears

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particularly adapted to the case of redox supports, where the

alternative hydrogen chemisorption will lead to imprecise

results. Another positive point for the use of such probes

is their small size, allowing site accessibility even in small

cavities. Moreover, the fair interaction energy between the

probe and the site is a guarantee for a minor perturbation

of the surface state upon interaction. A useful volumetric/

spectroscopic CO adsorption combined method allows

metal dispersion calculation in a simple and reliable way, by

integrating the bands relative to CO adsorption on the metal

sites vs. the molar amount of the introduced probe, as shown

in Fig. 9.85,86 In such a way CO adsorption mode, sites and

quantity are continuously monitored by IR spectra upon

calibrated doses introduction.

Additionally, carbon monoxide and nitrogen monoxide

adsorption can provide very useful information on the coordi-

nation mode of such molecules (of primary importance for

environmental issues) and on the complexes formed with

the metal particles, potential sites for pollutant abatement.

For example, it was found that the adsorption of CO on a non-

reduced Pt/TiO2sample reveals the existence of Pt3+ and Pt2+

cations as well as some amount of metallic platinum. However,

Pt4+ species are also present on the sample but, being

coordinatively saturated, cannot adsorb CO, while NO forms

nitrosyl species with bare Pt2+ and Pt0 sites, but it is not

coordinated to Pt3+ ions. Therefore, it appears that NO is a

more sensitive probe than CO for testing the state of Pt2+

cations. Moreover it must be underlined that probe adsorption

is not totally innocent: adsorbed CO slowly reduces the

platinum cations, whereas NO oxidizes metallic platinum

even at ambient temperature.87

Similar results were found on

Rh-ZSM-5, where a new kind of rhodium gem-dicarbonyls

was discovered. The shift of the n(CO) vibration allowed under-

standing Rh position in the porous structure, its oxidation

state and the capacity to host different chemical species having

different stability, especially in the presence of water. These

data are fundamental for understanding the mechanism of

different catalytic reactions.88 According to this methodology,

CO and NO adsorption on zeolithe supported Rh nano-

particles containing different promoter elements permitted to

both characterise the effect of the additive and the catalytic

activity of the noble metal.89

Many studies were carried out on copper oxidation state,

due to its importance when inserted in ZSM-5 zeolites for NO

reduction. Its characteristic carbonyl band at 2158 cm1

provides quantitative results on integrating its molar extinc-

tion coefficient.90 Hadjiivanov et al.91 described the water

effect (which is always present during deNOx real conditions)

and reported that bands at 2158 and 2134 cm1 may charac-

terize CO bound to dry and wet Cu+ centres, respectively,

the latter being also possibly assigned to a CuO-like phase.

However a comparative study using both Cu-ZSM-5 and

CuO/Al2O3 allowed Praliaud et al. to propose the n(CO)

at 21232133 cm1 to be due to non-isolated Cu+ species

(Cu+

surrounded by Cu2+

ions) arising from the partial

reduction of bulk CuO. Whereas, the band at 21522157 cm1

would characterize isolated Cu+ ions, which are described to

be responsible for the high activity in NO reduction into N2.92

Concerning NO, its adsorption even at room temperature may

lead to Cu+ oxidation to Cu2+ and therefore its use for the

determination of copper oxidation state distribution is rather

problematical. However the formation of Cu+ mono-

and dinitrosyls is observable for high temperature (770 K)

Cu-ZSM-5 activated under vacuum and Datka et al.93 even

reported the possible existence of two distinct Cu+

NO

species at 1812 and 1825 cm1

, associated to two distinct

Cu+ sites differing in the density of oxygen packing. A

discussion around Cu+ carbonyls can be also found in

the Strauss94,95 and Zecchina96 reports. Further detailing the

characterization of Cu+ by NO, it has been shown that the

zeolitic structure also influences the symmetry of the dinitrosyl

species and for a CuMOR sample the Cu+NO (1813 cm1)

transformation into Cu+(NO)2 leads to different spectral

feature depending on the Cu+ location: a doublet with

ns and nas at 1828 and 1730 cm1 in the main channels

and another doublet with ns and nas at 1870 and 1785 cm1

in the constrained side-pockets.97

The use of both probes can also be interesting when each one

is sensitive to an oxidation state of an element, as in the case of

copper, probed by NO in the Cu2+ state, whereas CO adsorp-

tion would be more specific to the Cu+ state (since CO com-

plexes with Cu2+ are only stable at very low temperature).98

Another example concerns the adsorption of both CO and

NO as informative for the determination of the oxidation state

of vanadium on vanadiatitania catalysts. Although the

carbonyl bands for V4+

CO, V3+

CO and Ti4+

CO species

almost coincide,99 the fact that NO forms dinitrosyls with Vn+

but not with Ti4+ allows the effective use of NO as a probe

molecule.100

Iron-containing ZSM-5 catalysts were also studied for their

potential application in deNOx by hydrocarbons. Interesting

results identifying different Fe2+ species as active sites for

NOx

SCR with propene can be found in ref. 101 and 102. Fe3+

characterization in the Y structure was also previously studied

Fig. 9 Integrated intensity of the n(CO) bands bound to platinum as

a function of introduced CO amount (reproduced from ref. 85). The

abscissa of the line intersection corresponds to a CO monolayer on

the exposed metal surface. This allows calculating the number ofaccessible Pt atoms,i.e.the metal dispersion. Reproduced by permission

of Elsevier.

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using CO adsorption,103 even if IR spectroscopy of probe

molecules is more suitable for characterization of Fe2+ than

Fe3+ cations. More recently, we tried to resolve this inter-

pretation conflict present in the specialized literature, reporting

NO adsorption followed by infrared spectroscopy to charac-

terize iron cations in Fe-ferrierite.104 Different iron sites

forming mononitrosyl species were identified. The comple-

mentary use of Mo ssbauer spectroscopy enabled us to deter-

mine that the iron oxidation state is essentially +2. For low Fe

loading (by ion exchange), the main fraction of Fe2+ cations is

suggested to be located in highly accessible positions of the

ferrierite, where ionic exchange takes place in the easiest

way. When the amount of Fe is increased, a second site in a

less accessible position is detected. When an oxidative

pretreatment is applied, only the iron cations in the confined

positions lead to the formation of Fe3+OH species. More-

over, NO appears to be able to form polynitrosyl species with

these confined Fe2+ cations. It thus appears that both the

oxidation and the coordination states of confined Fe2+ may

change easily, which makes them excellent candidates for

active redox sites.104 Combining CO and NO as molecular

probes, we were able to go into very fine detail for site

characterisation in Fe-FER. It was ascertained that type I

sites are the most populated and the Fe2+ ions located in the

so-called G-positions are the most symmetric ones. Their

unique geometry allows two guest molecules approaching

the site from different cages, resulting in the formation of

monocarbonyls (2195 cm1

), converted then, at low tempera-

tures, into dicarbonyls (2188 cm1). Type I Fe2+ cations are

hardly sensitive to oxidizing treatment and show little tendency

to yield Fe3+ cations. Type II sites are less populated and less

symmetric: Fe2+ ions in these B-positions form, with CO,

monocarbonyls (2189 cm1) and, with NO, mononitrosyls

(1880 cm1) practically coinciding in wavenumber with the

nitrosyls formed with type I Fe2+ ions. These cations are

sensitive to oxidizing treatment and are easily oxidized to

Fe3+ ions most probably associated to the formation of

a-oxygen species. When the iron concentration in the samples

increases, a third site (F-site) is occupied. Iron ions in this

position change easily and reversibly their oxidation state from

Fe2+ to Fe3+ thus forming Fe3+OH or Fe3+O species.

When in the Fe2+ state, iron ions form the most stable

carbonyl species (2196 cm1) which can be converted, at

low temperature, into di- (B2188 cm1) and tricarbonyls

(B2180 cm1). With NO these Fe2+ ions form nitrosyls

absorbing at 1895 cm1. With time, in NO atmosphere, the

Fe2+ cations are displaced from their original positions in

order to form tetranitrosyl species.105

The specific structure of MOF compounds gives particular

adsorption, separation and catalytic properties to these

materials. For example, the controlled reduction of a large-pore

iron(III) trimesate with unsaturated iron sites, MIL-100(Fe),

strongly increases the strength of interaction with unsaturated

gas molecules, such as propylene and CO, that have either a

double or triple bond. Therefore, this property leads to a

dramatic improvement of not only preferential gas sorption

but also separation performance for the investigated hybrid

compound: it could be used for the removal of CO impurity to

protect the deactivation of Pt electrodes in low temperature

fuel cells, for example, or the removal of CO from CO2

-rich

mixtures arising from the production of hydrogen from

biomass. The use of mixed-valent MIL-100(Fe) may be also

considered for further applications involving the selective

separation or purification of olefins and acetylenes from

hydrocarbon mixtures. The reducibility of MIL-100(Fe) to

form FeII CUS has been unambiguously shown by in situ IR

spectroscopic analysis using CO as a probe, which also

allowed to quantify the concentration of Fe2+/Fe3+ sites as

a function of the activation treatment (see Scheme 2 and

Fig. 10). Moreover it provided evidences that unsaturated

sites can be created not only through the removal of water

but also to a much lesser extent from the departure of other

Scheme 2 Representations of MIL-100(Fe): (a) one unit cell, (b) two types of mesoporous cages shown as polyhedra, (c) formation of

FeIII CUS and FeII CUS in an octahedral iron trimer of MIL-100(Fe) by dehydration and partial reduction from the departure of anionic ligands

(X

= F

or OH

) (from ref. 106). Reproduced by permission of Wiley-VCH.

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molecules, such as trimesic acid or anionic ligands (F and OH)

coordinated on terminal sites of FeIII.106

As already mentioned, ceria and ceriazirconia are catalyst

components of primary importance, notably for their red-ox

properties. This arises from the ability of the Ce cation

oxidation number in ceria and ceria-zirconia to easily change

between 3+ and 4+. The surface state (reduced or oxidized)

and composition (ratio between cerium and zirconium cations)

is available using methanol (CH3OH) adsorption.107 Its dissocia-

tion leads to cation coordinated methoxy and hydroxyl

formation. Both then(CO) wavenumbers associated to methoxy

species108,109 and the n(OH) associated to surface hydroxyls110

depend on the cerium oxidation state. The Ce4+/Ce3+ surface

ratio is thus available from the quantitative study of the

corresponding methoxy intensities. Fig. 11 shows the con-

secutive adsorption of oxygen (O2) calibrated doses after

methanol dissociation over pre-reduced ceriumzirconium

mixed oxide which enable the determination of the oxygen

storage capacity of the sample.

Moreover, the methoxy species is very sensitive to the local

coordination site and it allows discriminating between the

different cations on a surface. Concerning ceriazirconia solid

solutions, for example, methoxys on Ce4+ and Zr4+ surface

sites present specific IR fingerprints. Therefore, the surface

cationic composition can easily been obtained upon methanol

adsorption at room temperature.25 Rare-earth compounds

also present electronic transitions, arising for ions in internal

structural defects; for ceria, at temperatures above 523 K

a new band appears at 2120 cm1 and is attributed to the2

F5/2-2

F7/2electronic transition of Ce3+

, thus indicating the

beginning of bulk oxide reduction.111

Accessibility of sites. To be active in a catalytic reaction, a

surface site must be accessible to reactants and products.Isotope labelling is here again a very useful tool. Deutera-

tion of the surface OH groups is only possible if they are

accessible to the deuterated molecule used for the exchange. In

silica, for example, internal silanol groups are distinguished

from external ones based on accessibility to deuterated water

molecules.112

An accessibility index (ACI) was derived from infrared

spectroscopy of substituted alkylpyridines with different sizes

(pyridine: 0.57 nm, 2,6-lutidine: 0.67 nm, collidine: 0.74 nm)

over hierarchical ZSM-5 crystals. The samples were preparedby selective silicon extraction of a parent commercial sample in

alkaline medium (desilication) and contained different degrees

of intracrystalline mesoporosity. The enhanced accessibility of

acid sites in the hierarchical zeolites was shown. A relatively

bulky molecule such as collidine, which probes practically no

acid sites of the parent medium-pore MFI structure, can access

up to 40% of the Brnsted sites in the mesoporous sample.

The ACI is a powerful tool to standardize acid site accessibility

in zeolites and can be used to rank the effectiveness of

synthetic strategies towards hierarchical zeolites (mesoporous

crystals, nanocrystals, and composites).113

Time resolved studies of probe molecules: 2D-pressure jumpspectroscopy. Time-resolved IR spectroscopy can be used in

probe molecule studies. Microsecond infrared spectroscopy

can be used to monitor adsorbed probe molecules after a

pressure jump on the surface of a porous catalyst. The analysis

of the time behavior of the adsorbed molecule can be obtained

by a Fourier Transform of the IR spectra over time. A 2D map

is obtained, showing frequency response vs. IR spectra for the

adsorbed probe molecule on the catalyst. This technique has

Fig. 10 (a) IR spectra of MIL-100 under a stream of 10% CO at 25 1C after activation under a helium flux at various temperatures over 3 or 12 h.

(b) Amount of FeIII CUS and FeII CUS detected by IR analysis upon CO adsorption at 173 1C on MIL-100(Fe) activated under high vacuum at

different temperatures (from ref. 106). Reproduced by permission of Wiley-VCH.

Fig. 11 Progressive reoxidation of a mixed Ce0.80Zr0.20O2compound

by introduction of small doses of O2 after reduction under hydrogen

at 673 K.

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been used to study platinum catalysts supported on MFI

zeolites. The 2D map can be read as a map showing loca-

tion vs. size for the metal particles. Small particles can be

distinguished from large ones, and their location can be

determined in the pores or on the outer surface of the zeolite.

The role of the pores was demonstrated for the protection

of small particles during ageing of the catalyst, and sintering

was limited by the pore diameter where the particle was

located.86

Quantitative analysis by coupling IR with gravimetry.Thermo-

gravimetric analysis (TGA) allows monitoring weight changes

in the sample. It has been combined with IR to give new

information on surface sites. Reliable quantitative information

is the key to understanding the catalytic role of surface sites,

and this combination of techniques offers just that. It was used

to determine the molar absorption coefficients for IR bands of

OH groups on silica and HY zeolites, as well as for adsorbed

probe molecules.112 Important information was obtained

about the quantity of the OH groups located in the different

cages of HY zeolite and also about the quantity of inner andsurface silanol groups on silica. The number of silanol groups

accessible to water molecules was shown to be constant

whatever the sample of precipitated silica, as well as the ratio

of water/silanol groups under room atmosphere. Combined

thermogravimetry and IR was also applied to operando con-

ditions, and denoted as AGIR (Analysis by Gravimetry and

IR): a modified microbalance was used to follow mass changes

(mg accuracy) inside a catalytic reactor equipped with infrared

windows. The spectroscopic response of water and ammonium

ions coadsorbed together on zeolites was shown to vary depending

on the conditions. The molar absorption coefficients for d(H2O)

andd(NH4+) at 1640 and 1540 cm1 for water and ammonia on

a HY zeolite were studied under dry gas flow at variabletemperature. It showed the influence of coverage on the infrared

response of adsorbed species in zeolites. Adsorption sites change

with coverage, and bands are shifted and their shape and

intensity are modified. Other interesting facts were observed:

water modifies strongly the aspect of the d(NH4+) vibration

band in ammoniated zeolites, without changing the absorption

coefficient. Measuring the sample mass while at the same time

recording its infrared spectrum showed the key importance of

conditions under which the measurement is done. The presence

of co-adsorbed species (water in particular) strongly modifies the

spectrum of surface species. Under reaction condition, this new

technique is especially important for a correct assignment of

infrared features and catalytic behaviour, and above all it makesIR measurements really quantitative.114

Other coupled techniques. As we have mentioned in the

introduction, the purpose of this review is to critically discuss

the characterization of the active site in catalytical processes

by using IR analysis compared with catalytic tests. Sometimes,

additional information can be provided by the coupling of

complemental techniques able to yield enhanced insight into

material local properties (structural, textural, physical, . . .)

and reactional events at the nano-scale. For more information

about this point we refer the reader to specific literature,

notably within the present themed issue. We just want to

briefly mention a few examples:

Numerous techniques have been coupled with IR, thus

broadening the experimental data set for subsequent mecha-

nistic considerations. Of course, complementary information

can be obtained by parallel experiments; nevertheless, to be

sure that results are comparable, it is often preferred that the

investigation is carried out in a single setup.115

The most natural technique to associate with IR is certainly

Raman, to cover the totality of the vibrational spectrum. The

firstin situRaman and FTIR spectroscopic characterization of

a catalytic system under reaction conditions using a single

bench-top instrument with a dedicated cell was published in

2003 by Payen et al.116 Following both the evolution of the

molecular structure of the active phase by Raman spectro-

scopy and the nature of the different surface adsorbed species

by IR and Raman spectroscopy during deNOx reaction they

were able to provide an exhaustive identification of the

adsorbed species and the corresponding coordination sites.

Similar information can also be obtained by coupling

DRIFT with hard X-ray diffraction, which can highlight

phenomena taking place in nanoparticulate catalysts, such as

the formation of simultaneous surface and bulk species and

their evolution.117

Also combining, at high time resolution, a transmission

based structural probes, dispersive EXAFS, with diffuse

reflectance infrared spectroscopy, can highlight fundamental

steps occurring during gassolid interactions, like that of

oxidation and reduction of alumina-supported Rh at 573 K

using NO and H2, and the structuralreactive role of linear

Rh-nitrosyl species within these processes.118

Chemometric tools.Chemometrics is of course a very efficient

way of decomposing complex bands, when they are progressivelychanging during reaction or during progressive adsorption. CO

adsorption on Pt sites leads to complex bands on zeolites where

different metal particle sizes can exist.86 Small particles lead to a

stronger influence of corners and edges compared to faces, and

large particles produce bands with a stronger influence of large

faces. Chemometrics allows separating contributions, and the

spectra of CO on small and large particles can be distinguished

quantitatively. This was revealed to be very important for

studying the influence of ageing of the catalyst on the catalytic

sites.86

Active sites intermediates and spectators(discrimination of the aforementioned sites

by using catalytic evidences)

Intermediates, active sites and spectator species are well

defined concepts in catalysis. We will summarize them again

here for a clear discussion of their spectroscopic characteri-

zation. Heterogeneous catalysis involves adsorption of

reactants on the surface, their reaction on the active site, with

the possible formation of intermediates species, and the

formation of final products (with a possible change of adsorp-

tion site), and the further desorption of the products which will

go to the exit of the reactor.

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During the adsorption step, several sites can participate.

Some of them will indeed be the place for the reaction

(and these are the active sites) while some others will only be

adsorption sites on which no reaction will take place. These

non-active sites can adsorb strongly reactants or products

and block any movement of surface species, resulting in a

deactivation of the catalyst. Several reactions can take place

on the surface. Some species will have no role in the reaction

under study, and will thus be spectator species. Some other

species can hamper or slow down the reaction, either by

staying adsorbed on the active site or by blocking the circula-

tion of surface species, and will be poison species. Another

type of species will be formed on the surface but will be later

transformed into the final expected product, and these are

intermediate species. Intermediates are stable species on the

surface (at least for a short duration), and should not be

confused with transition states. Transition states are postulated

between two chemical species on the surface, but do not actually

exist since they have no actual lifetime. They are only pathways

between two states on the surface (on an active site).

Correlation between probe studies and activity

In situ analyses of a surface provide photographs of the

material state, detailing the concentration of the superficial

entities directly by their corresponding IR spectra or via the

spectral response of a molecule adsorbed on the site and their

relative strengths vs. the applied probe, as aforementioned.

The most general way to link catalytic activity to a specific site

is the correlation method. It consists of measuring activities of

a series of materials (starting from the initial reaction rate, or

the conversion at the steady state), and to compare this value

to the number of sites estimated by the intensity of a specific

IR band. In general, this site concentration corresponds to the

entities able to adsorb probe molecules. If a linear correlation

between these two sets of data is found, the activity can

reasonably be attributed to the presence of such adsorption

sites. Nevertheless, all the sites able to adsorb a molecule are

not necessarily active: the presence of the correlation means

that the ratio between the active and the adsorption sites

remains constant when their global amount varies. Additionally,

we should consider the limitations intrinsic to the probe

molecule (probe sensitivity to the strength, nature and number

of sites) and the difficulties to obtain for both spectroscopic

and catalytic measurements in similar activation conditions.

The use of appropriate probe molecules permits nevertheless

to formulate reliable hypotheses on the nature of the corres-

ponding sites. Moreover, when the quantitative analysis of the

sites is possible, a calculation of the turn over frequency (TOF)

can also been performed. A demonstrative example concerns

active site study via CO adsorption in hydrotreating reactions

such as hydrodesulfurization (HDS). They are usually related

to anionic vacancies (coordinatively unsaturated sites, CUS)

located on the edges of mixed sulfide nanosized particles

(CoMoS or NiMoS) supported on high specific surface

area alumina. Nitrogen monoxide has been the most employed

but a partial oxidation of the sulfide phase may occur, even at

very low temperatures. Infrared spectra of CO adsorbed on

the sulfided promoted CoMo/Al2O3catalysts displays a strong

nCO band at 2070 cm1 which is correlated to the HDS

catalytic activity.119 It has been assigned to CO interacting

either with a Co atom or with a Mo atom adjacent to a Co

atom. Similar correlations between the intensity of specific nCO

bands and the activity in HDS catalytic activity have been also

found with Mo/Al2O3catalyst. Due to the drastic procedure of

activation of sulfide catalyst, it is worth noting that some

researchers of our laboratory have designed a new IR cell,

called CellEx, in order to characterize in situ sulfided catalysts

under a pertinent H2S/H2flow, with pressure varying from 0.1

up to 4.0 MPa. This cell allows obtaining similar sulfidation

procedure for both IR characterizations and catalytic tests.120

Another significant example can be mentioned in the case of

the NOx

selective reduction by hydrocarbons on oxide-based

catalysts. Working on Ag/Al2O3 samples for NOx reduction

by ethanol, we observed the formation of cyanide and

isocyanate species. The IR band assignment was not straight-

forward: contrarily to what is usually reported in the litera-

ture, n(NCO) of AgI(NCO) species is located at 2204 cm1

(and not at 2230 cm1

), whereas that of Ag0

(NCO) species is

at 2243 cm1. Thus, during SCR reaction of NO on silver/

alumina catalyst, the isocyanate species generally observed as

intermediate compounds around 2230 and 2260 cm1 are not

linked to silver sites but to the support, the main role of silver

being to favour NCO formation and concentration on the

support. The hypothesis that the two observed bands are due

to different alumina coordination sites (Alocta and Altetra) for

isocyanates was determined as the most probable by isotopic

substitutions.121 This information was useful for the compre-

hension of the NO SCR pathway. In fact comparing the

selective catalytic reduction of NOx

in excess of oxygen using

ethanol as reducing agent on silver/alumina and on bare

alumina showed the connection between the presence of silver

and isocyanate species on a catalyst. Then, a detailed investi-

gation concerning these groups was undertaken to understand

their formation, their localization, and their reactivity in order

to propose the pathway of the NOx

SCR into N2. Three

elemental sequences were suggested explaining, first, the

formation of silver cyanide and its transformation into

Al3+NCO, then isocyanate hydrolysis into ammonia, and

finally the reaction of the latter species with NO in the

presence of oxygen giving rise to nitrogen (Fig. 12).122

However, a working surface can have a totally different

behaviour, with sites changing their nature. The real valence of

a site can be ascertained only catching it in full action, using

operando techniques.

Operando studies on Fe-FER indicated that these samples

present interesting NOx

SCR efficiency for temperature as low

as 433 K. Our investigations also indicate that for such

low reaction temperature ammonia and nitrogen monoxide

compete for adsorption onto the Fe2+

species (whose fine

characterisation has been reported in the section Metallic and

redox sites), which are active for the NO-to-NO2 oxidation.

Our results are also consistent with this last reaction being

the rate determining step of the global SCR process. We finally

concentrated our study on the effect of SO2on this NO-to-NO2reaction. We must conclude that for the 2.5 wt% Fe-FER

sample, the majority of iron sites are poisoned by sulfate

formation, although some Fe2+ sites are thio-resistant.123

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Operando IR spectroscopy shows out all its added values

when very complicated and multifuncti

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