b919543m
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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
access other reviews in this themed issue
View Online
http://pubs.rsc.org/en/Journals/JournalIssues/CS#/issueID=CS039012&Type=Current&issnprint=0306-0012http://pubs.rsc.org/en/Journals/JournalIssues/CS#/issueID=CS039012&Type=Current&issnprint=0306-0012http://dx.doi.org/10.1039/b919543mhttp://pubs.rsc.org/en/Journals/JournalIssues/CS#/issueID=CS039012&Type=Current&issnprint=0306-0012http://pubs.rsc.org/en/Journals/JournalIssues/CS -
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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.
CRITICAL REVIEW www.rsc.org/csr | Chemical Society Reviews
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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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4930 Chem. Soc. Rev., 2010, 39, 49284950 This journal is c The Royal Society of Chemistry 2010
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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4932 Chem. Soc. Rev., 2010, 39, 49284950 This journal is c The Royal Society of Chemistry 2010
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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4936 Chem. Soc. Rev., 2010, 39, 49284950 This journal is c The Royal Society of Chemistry 2010
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