Infrared study of CO and 2-butenal co-adsorption on Zn modified Pt/CeO2-SiO2 catalysts

Article abstract of DOI:10.1039/b209446k

Infrared spectra of CO alone and CO followed by the introduction of 2-butenal (crotonaldehyde) have been recorded for Pt/CeO2-SiO2 and Zn doped Pt/CeO2-SiO2 catalysts which had been pre-reduced at 473, 623 and 773 K. Samples treated at the highest reducti

Full text of DOI:10.1039/b209446k

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Infrared study of CO and 2-butenal co-adsorption on Zn
modified Pt/CeO2–SiO2 catalysts
b
Joaquin Silvestre-Albero, A. Sepulveda-Escribano, F. Rodrıguez-Reinoso and
b
b
´
´
James A. Anderson*a
a
Catalysis and Surface Chemistry group, Division of Physical and Inorganic Chemistry,
The University, Dundee, Scotland, UK DD1 4HN
b
´
´
Departamento de Quımica Inorganica, Universidad de Alicante, Aptdo. 99,
E-03080 Alicante, Spain
Received 27th September 2002, Accepted 5th November 2002
First published as an Advance Article on the web 26th November 2002
Infrared spectra of CO alone and CO followed by the introduction of 2-butenal (crotonaldehyde) have been
recorded for Pt/CeO2–SiO2 and Zn doped Pt/CeO2–SiO2 catalysts which had been pre-reduced at 473, 623 and
773 K. Samples treated at the highest reduction temperature showed the greatest selectivity toward the
unsaturated alcohol although no clear spectroscopic evidence for the formation of an SMSI state was obtained.
Although 773 K reduced Zn containing catalysts showed the greatest yield, there was no IR spectroscopic
evidence to support the formation of significant quantities of a Pt–Zn alloyed phase or that the addition of Zn
enhanced Pt-support interactions beyond increasing the amount of surface reduced ceria. Results show that,
contrary to popular opinion, the development of an SMSI state which enhances the density of support-metal
interface sites is not essential in order to increase the yield of unsaturated alcohol and the presence of reduced
cationic sites at the interface zone may be the crucial factor. Although IR spectra in the nCO region were
dominated by dipole coupling effects, enhanced intensity of carbonyl bands in the 2030–1950 cmꢀ1 region for
773 K reduced samples and for Zn containing samples reduced at lower temperatures suggests that sites
responsible for these carbonyl bands may be the active sites in the selective hydrogenation reaction.
involving the co-adsorption of CO and 2-butenal (crotonalde-
Introduction
hyde) may be used to derive information relevant to the both
the metal-support interaction and the interaction of these
components with the a,b-unsaturated aldehyde.10,11 Previous
studies using co-adsorption have been used to show the
presence of electronic effects and redox reactions of crotonal-
dehyde in cases where the active metal is readily oxidized or
reduced and the support may be influential in inducing selec-
tivity of the reaction.10 A recent study12 suggests that the addi-
tion of Zn to supported platinum catalysts may increase the
metal support interaction and thus modify the catalyst beha-
vior for 2-butenal hydrogenation. In the present study, this
same series of catalysts have been studied by FTIR spectro-
scopy to determine if further evidence is available to support
such conclusions.
The selective hydrogenation of organic molecules over hetero-
geneous catalysts is a reaction that has received significant
attention in recent years due to its relevance in producing
appropriate reagents for the fine chemicals industry. Of these
hydrogenation reactions, those involving a,b-unsaturated alde-
hydes such as crotonaldehyde has been selected as a test reac-
tion to model systems where more than one reducible
functional group is present.1–4 Clearly, the modes of adsorp-
tion of the a,b-unsaturated aldehyde and the nature of its pre-
ferred adsorption sites are paramount in controlling selectivity
in the reaction. Theoretical5,6 and experimental studies7,8 have
been conducted using platinum single crystal surfaces in both
monometallic and alloyed forms, although fewer studies at a
fundamental level have been performed of the adsorption
modes on supported metal catalyst surfaces9–11 and fewer spe-
cifically deal with catalysts in an SMSI state.11,12 The use of
supported catalyst to study adsorption modes involves many
complications, among which is that adsorption of the C=C
or C=O double bond parallel to the surface as a prerequisite
to hydrogenation renders the stretching modes IR inactive as
a consequence of the surface selection rule. Additionally, the
spectra may be dominated by bands due to carboxylates and
molecularly adsorbed aldehyde at exposed Lewis acid centers
on the surface of the support.10,11 In such cases, the IR spec-
trum may be dominated by spectator species which make the
detection of adsorption on the active catalyst component more
difficult. Infrared spectroscopy of adsorbed CO may be used
to provide information regarding size and morphology of
metallic particles and the extent of interaction with the sup-
port, other promoters, and co-adsorbates.13–17 Experiments
Experimental
Pt/CeO2–SiO2 and PtZn/CeO2–SiO2 catalysts were prepared
by impregnation of a 20 wt.% CeO2/SiO2 support using an
acetone solution of [Pt(NH3)4](NO3)2 and Zn(NO3)2ꢁ6H2O
(from Aldrich). A ratio of 10 cm3 of impregnating solution
per gram of support was used in both cases. Precursors were
selected to avoid the presence of chlorine in the final catalysts.
The excess solvent was removed by flowing N2 through the
suspension. The remaining solid was dried overnight at 373
K and calcined in air at 673 K for 4 h. A more detailed descrip-
tion of the preparation procedure can be found elsewhere.12
Platinum content was 1.1 wt.% in both samples and the zinc
content in the bimetallic catalyst was 0.6 wt.%, as determined
by AAS. These values correspond to a Zn/Pt atomic ratio of
208
Phys. Chem. Chem. Phys., 2003, 5, 208–216
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DOI: 10.1039/b209446k

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1.6. BET surface areas were 211 and 228 m2 gꢀ1 for the Zn-free
and Zn containing catalysts, respectively.
FTIR experiments were conducted using a quartz infrared
cell fitted with CaF2 windows and an external furnace. The
IR chamber was glass-blown to a conventional vacuum system
with rotary and oil diffusion pumps achieving pressures of
down to 1 ꢂ 10ꢀ3 N mꢀ2. Catalyst samples were prepared as
16 mm diameter self-supporting disks by pressing 30 mg of
loose powder between polished steel dies at 20 MN mꢀ2. Prior
to IR measurements, samples were reduced in situ under a
hydrogen flow (50 cm3 minꢀ1) at 473, 623 or 773 K for 1 h.
After the reduction treatment, samples were out-gassed at
the reduction temperature for 20 min to a final pressure of
ꢃ10ꢀ5 Torr before cooling to ambient temperature. Pulses of
CO and crotonaldehyde were introduced at 298 K using cali-
brated volumes and an Edwards active strain gauge to measure
the size of the pulse. IR spectra were recorded using a PC con-
trolled Perkin-Elmer 1710 FTIR spectrometer operating at 4
cmꢀ1 resolution and averaging 100 scans per spectrum. Croto-
naldehyde was purified by repeated distillation under reduced
pressure. For the co-adsorption measurements, 25 Torr doses
of CO were initially introduced followed by evacuation at
298 K for 20 min prior to exposure to a crotonaldehyde pulse
corresponding to low (2.8 mmol gcatꢀ1) and high (41.5 mmol
gcatꢀ1) coverage of the surface. These values correspond with
8 ꢂ 10ꢀ3 (low) and 0.12 molecules nmꢀ2 (high) for the Zn-free
sample and 7.4 ꢂ 10ꢀ3 and 0.11 molecules nmꢀ2 for the Zn-
containing samples.
Fig. 1 Infrared spectra of a Pt/CeO2–SiO2 catalyst reduced at (A)
473 K, (B) 623 K and (C) 773 K and exposed to (---) 0.5 Torr CO,
(
) 25 Torr CO and then (—) outgassed at 298 K for 40 min.
Prior to determining the catalytic behavior, catalysts were
reduced by heating in situ under flowing hydrogen (50 cm3
minꢀ1) to 473, 623 or 773 K at 5 K minꢀ1 and then holding
the final temperature for 1 h. The vapor-phase hydrogenation
of crotonaldehyde was performed at atmospheric pressure and
at a temperature of 353 K. The reduced catalysts (ca. 100 mg)
were contacted with a reaction mixture (total flow: 50 cm3
minꢀ1; H2/aldehyde ratio of 26) formed by flowing hydrogen
through a thermostabilized saturator (293 K) containing the
aldehyde ( > 99.5% purity, Fluka). The reaction products were
analyzed by on line gas chromatography, using a Carbowax
20M 58/90 25–30 m capillary column.
greater increase. This increase in intensity was also accompa-
nied by a blue shift of both bands to 2072 cmꢀ1 and 2086
cmꢀ1. At this maximum pressure a shoulder centered at 2003
cmꢀ1 was also discernible. Evacuation treatment for 40 min
produced a red shift of the linear CO bands to 2080 cmꢀ1
and 2065 cmꢀ1. IR spectra (Fig. 1C) of the Pt/CeO2–SiO2 cat-
alyst reduced at the highest temperature (773 K) showed a
broad band with a maximum at 2071 cmꢀ1 together with a
high frequency shoulder at 2080 cmꢀ1 and some low frequency
contribution to the overall band envelope. Those low fre-
quency contributions were more prominent following an
increase in CO pressure, this condition also favoring the
appearance of the 2003 cmꢀ1 maximum, which was more
dominant than for sample reduced at 623 K. Evacuation pro-
duced an increase in intensity at 2063 cmꢀ1 accompanied by
reduced intensity at 2003 and ca. 1850 cmꢀ1
.
Results
Fig. 2 shows IR spectra for CO adsorbed on the bimetallic
PtZn/CeO2–SiO2 catalyst after low (A: 473 K), medium (B:
623 K) and high (C: 773 K) temperature reduction. All spectra
exhibit bands in the 2080–2057 cmꢀ1 region which correspond
to linear adsorbed CO species on the different metallic sites.
After reduction at 473 K and exposure to a low pressure (0.5
Torr) of CO, spectra exhibited a broad band with a maximum
Adsorption of CO on reduced Pt/CeO2–SiO2 and
PtZn/CeO2–SiO2 catalysts
FTIR measurements of adsorbed CO on Pt/CeO2–SiO2 cata-
lyst reduced at (A) 473 K, (B) 623 K and (C) 773 K are shown
in Fig. 1. In all cases, samples at 298 K were exposed to
increasing pressures of CO up to a maximum of 25 Torr fol-
lowed by evacuation for 40 min. For the sake of brevity, only
spectra at 0.5 and 25 Torr are shown. Spectra of the mono
metallic catalyst reduced at 473 K (A) exhibits a broad band
centered at 2067 cmꢀ1 which became more defined on increas-
ing CO pressure. This increased pressure also produces a high
frequency shoulder at 2080 cmꢀ1 together with a broad contri-
bution at 2169 cmꢀ1 associated with CO species in the gas
phase. A broad maximum of low intensity at ca. 1850 cmꢀ1
due to bridge bound CO was detected at the higher pressure.
Evacuation at 298 K red-shifted the 2067 cmꢀ1 band by 5
cmꢀ1 without significant change in intensity. Reduction at
623 K (B) again produced two contributions at 2067 and
2080 cmꢀ1, however the relative intensities were reversed with
respect to the 473 K reduced surface with the former band now
present as a low frequency shoulder on the latter. The band
due to bridge bound CO was very weak under these conditions
but grew on increasing the pressure. An increase in the CO
pressure also led to increases in intensity of both of the linear
features with the lower frequency contribution undergoing the
centered at 2072 cmꢀ1 together with a shoulder at 2055 cmꢀ1
.
Exposure to higher CO pressures produced an increase in both
contributions to leave a doublet at 2079 and 2065 cmꢀ1 of
similar intensities. At this higher pressure, features at ca.
1850 and 2003 cmꢀ1 were also discernible. Note that the latter
feature was not present at this reduction temperature for the
Zn-free sample. Subsequent evacuation treatment led to a
decrease in the intensity of both main peaks and a red shift
in both maxima to 2074 and 2061 cmꢀ1. This was accompanied
by significant intensity loss at 2000 and 1850 cmꢀ1. A sample
reduced at 623 K and exposed to a low CO pressure produced
a spectrum with the main feature centered at 2067 cmꢀ1 with
associated shoulders at 2046 and 2003 cmꢀ1. An increase in
CO pressure shifted the main peak to 2071 cmꢀ1 (with a poorly
defined shoulder at 2082 cmꢀ1) and the aforementioned contri-
bution at 2003 cmꢀ1 and the bridge bound band at 1850 cmꢀ1
were both increased in intensity. Subsequent evacuation for
40 min removed adsorbed CO responsible for the high fre-
quency band but the band at 2071 cmꢀ1 was largely unaffected.
Phys. Chem. Chem. Phys., 2003, 5, 208–216
209

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by evacuation and the introduction of low and high coverage
doses of crotonaldehyde over the CO covered surface. Irre-
spective of the catalyst reduction treatment (473 K (A), 623
K (B) and 773 K (C)), interaction of crotonaldehyde with
the CO covered surface produced a shift to lower wavenum-
bers of the maxima due to linearly adsorbed CO. Similar
observations were made when CO and crotonaldehyde were
co-adsorbed on Pt/TiO2 (11). The sample reduced at 473 K
(Fig. 3A) and treated with 25 Torr CO followed by evacuation
exhibited bands at 2078 and 2062 cmꢀ1 as observed in previous
spectra (Fig. 1A). This observation confirms the degree of
reproducibility of the experiments. A low coverage crotonalde-
hyde pulse introduction to the CO covered, 473 K reduced sur-
face, produced a slight shift of both contributions to 2073 and
2059 cmꢀ1, respectively. At higher crotonaldehyde coverage,
the intensity of the higher frequency band was most affected
while the lower frequency component retained its intensity
and its FWHM but was red shifted to 2049 cmꢀ1. A sample
reduced at 623 K (B) showed similar behavior. Following
CO exposure and brief evacuation, the IR spectrum (Fig.
3B) exhibited two maxima at 2080 and 2067 cmꢀ1 as before
(Fig. 1B) together with some low frequency shoulders which
gave rise to the observed tail around 2030 cmꢀ1. Exposure to
a low coverage pulse of crotonaldehyde produced a decrease
in intensity of the high frequency component without signifi-
cant displacement, whereas the lower frequency band under-
went a 5 cmꢀ1 red shift to 2062 cmꢀ1 but with a much lesser
intensity loss. This effect was further pronounced for a higher
coverage crotonaldehyde pulse where the 2080 cmꢀ1 band
intensity was significantly reduced again without appreciable
frequency shift while the main maximum was further displaced
to 2050 cmꢀ1 without appreciable intensity loss or change in
the FWHM. After a high temperature reduction treatment,
the Zn-free catalyst exposed to 25 Torr CO exhibited an
intense band at 2071 cmꢀ1 together with a shoulder at 2083
cmꢀ1. Exposure to a low coverage crotonaldehyde pulse pro-
duced minor changes to the spectra with a slight decrease in
the intensity of both bands and a red-shift in the lower fre-
quency component by 2 cmꢀ1. However, exposure to higher
crotonaldehyde vapor pressure produced more significant
changes in the spectrum with the high frequency component
being significantly attenuated without undergoing a shift in
frequency and the lower frequency component was now pre-
sent at 2058 cmꢀ1 and exhibited marginally less intensity.
Fig. 4 shows spectra obtained following the co-adsorption of
CO and crotonaldehyde over the PtZn/CeO2–SiO2 catalyst
Fig. 2 Infrared spectra of a PtZn/CeO2–SiO2 catalyst reduced at (A)
473, (B) 623 and (C) 773 K and exposed to (---) 0.5 Torr CO, ( ) 25
Torr CO and then (—) outgassed at 298 K for 40 min.
This treatment also modified the appearance the lower fre-
quency range with a tail of notable intensity between 2050
and 1950 cmꢀ1 with maxima around 2047 and 2027 cmꢀ1
.
After a high temperature (773 K) reduction treatment the IR
spectrum after low CO pressure exhibited two main bands at
2079 and 2065 cmꢀ1 which became more prominent and
showed shifts to higher frequencies under higher CO pressures.
These two bands were accompanied by several lower frequency
contributions in the 2050–1950 cmꢀ1 range which were
retained after evacuation at 298 K. Bridge bound CO was less
significant for samples reduced at 773 K.
Co-adsorption of CO and crotonaldehyde
Fig. 3 displays IR spectra corresponding to the adsorption of
25 Torr CO on the Zn-free Pt/CeO2–SiO2 catalyst followed
Fig. 3 Infrared spectra of a Pt/CeO2–SiO2 catalyst reduced at (A)
473, (B) 623 and (C) 773 K, exposed to 25 Torr CO followed by out-
gassing at 298 K for 20 min ( ) and exposure to a low coverage pulse
of crotonaldehyde (—) and a high coverage crotonaldehyde pulse (---).
Fig. 4 Infrared spectra of a PtZn/CeO2–SiO2 catalyst reduced at (A)
473, (B) 623 and (C) 773 K, exposed to 25 Torr CO followed by out-
gassing at 298 K for 20 min ( ) and exposure to a low coverage pulse
of crotonaldehyde (—) and a high coverage crotonaldehyde pulse (---).
210
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after reduction treatment at 473 K (A), 623 K (B) and 773 K
(C). As observed in the case of the Zn-free catalyst, and irre-
spective of the reduction treatment applied, interaction of cro-
tonaldehyde with a CO covered surface led to a red shift of the
overall band envelope of linearly adsorbed CO. Additionally,
although less readily observed due to the relatively low inten-
sity, addition of crotonaldehyde led to a red shift of the band
centered at ca. 1850 cmꢀ1 due to bridge bound carbonyls. The
sample reduced at 473 K and pre-covered with CO exhibited a
broad band centered at 2068 cmꢀ1 together with considerable
asymmetry towards low frequencies. Exposure to a low cover-
age pulse of crotonaldehyde produced a definition of this
broad band into two contributions at 2071 and 2057 cmꢀ1
,
with the former suffering the greater loss in intensity. At a
higher vapor pressure of crotonaldehyde there was a further
intensity decrease in the high frequency component without
significant shift in frequency while the lower frequency maxi-
mum was now located at 2042 cmꢀ1 and exhibited a FWHM
(79 cmꢀ1) which was of greater magnitude than prior to croto-
naldehyde exposure (FWHM ¼ 50 cmꢀ1). The tail on this
broad band presented a poorly resolved maximum at 2008
cmꢀ1. Similar experiments performed on a sample reduced at
623 K showed equivalent behavior. The sample exposed to
CO alone exhibited a maximum intensity at 2067 cmꢀ1
together with a shoulder at 2052 cmꢀ1. Both contributions
were better defined after the introduction of a low coverage
pulse of crotonaldehyde which led to a slight red shift of the
overall band envelope due to linearly adsorbed CO. Exposure
to a high coverage pulse of crotonaldehyde produced a signifi-
cant decrease in intensity of the high frequency component (ca.
2067 cmꢀ1) band together with a shift to lower wavenumber of
the main peak. As observed for the 473 K reduced sample (Fig.
4A), this broad maximum was centered at 2042 cmꢀ1 and
showed evidence for a low frequency contribution around
2008 cmꢀ1 which contributed to the low frequency shoulder
tail. A reduction treatment at the highest temperature (773
K) on the Zn containing catalyst followed by exposure to
CO produced a maximum with evidence for two components
centered at 2069 and 2078 cmꢀ1, and a long tail between
2050 and 1950 cmꢀ1. Exposure of the CO covered surface to
low dose of crotonaldehyde produced only minor changes in
the overall band envelope but with an indication of reduced
intensity of the main peaks and gained intensity in the tail.
This modification was enhanced by the addition of further cro-
tonaldehyde which left the main maximum at 2072 cmꢀ1 with
an equivalent intensity as the component which made up the
Fig. 5 FTIR spectra of PtZn/CeO2–SiO2 catalyst reduced at 773 K
and exposed to increasing vapor pressures of crotonaldehyde at 298 K.
methyl group. The 1307 cmꢀ1 band corresponds with the
–CH= in-plane deformation (1303 cmꢀ1 in the liquid phase
spectrum). The 1643 cmꢀ1 band is due to the C=C stretching
mode, while the band at 1668 cmꢀ1 is due to the nC=O. The
red shift in the latter and the unperturbed nature of the former
indicate that the dominant mode of adsorption at low coverage
involved the molecule adsorbed via the carbonyl rather than
the vinyl function, probably ligated to exposed cationic sites
acting as Lewis acid centers in the support surface. However,
as the XPS atomic ratios12 were only ca. 8.3 : 1 for Ce : Pt,
and neglecting the possibility of chemisorbed aldehyde on
the exposed silica surface, it is quite possible that crotonalde-
hyde adsorbed at exposed Pt sites contributed to the IR spectra
(Fig. 5). The growth of the feature at 1678 (sh) cmꢀ1 at higher
coverage can be attributed to the nC=O of the aldehyde, again
adsorbed via the carbonyl function, but at weaker adsorption
sites, possibly involving hydrogen bonding interactions with
surface hydroxyl species. An additional feature, also observed
at higher coverage was a band at 1602 cmꢀ1 which was very
clearly observed for the Zn containing catalyst reduced at
773 K (Fig. 5), less clear, but still a resolved feature for Zn-free
catalyst reduced at 773 K (Fig. 6), and absent for both samples
reduced at 473 and 623 K. This feature may be attributed to
nC=C of a p bound species.
tail, now centered at ca. 2030 cmꢀ1
.
Adsorption of crotonaldehyde
Figs. 5 and 6 display spectra of crotonaldehyde exposed at
increasing vapor pressures to a representative Pt/CeO2–SiO2
and Pt/Zn/CeO2–SiO2 catalysts, respectively, both reduced
at 773 K. The third and last spectra in each case represent
the ‘‘low’’ and ‘‘high’’ coverage pulses, respectively. All cata-
lysts displayed very similar spectra with the exception of the
473 K reduced Zn-free sample which showed no band at
1560 cmꢀ1. This feature was common to spectra recorded of
the Zn containing samples regardless of reduction temperature
and the Zn-free sample when reduced at 623 or 773 K. Other
features were bands at 1678 (sh), 1668, 1643, 1446, 1399,
1378 and 1307 cmꢀ1. These features grew with increased expo-
sure at low vapor pressures indicating that these were all
modes of a single adsorption mode which dominated at low
coverage. The only exception was the low temperature (473
K) reduced Zn-free sample where the dominant feature at
low coverage was located at ca. 1630 cmꢀ1 but which became
dominated by the species giving the bands listed above features
as coverage was increased. Bands at 1446 and 1378 cmꢀ1 are
due to the asymmetric and symmetric deformations of the
Fig. 6 FTIR spectra of Pt/CeO2–SiO2 catalyst reduced at 773 K and
exposed to increasing vapor pressures of crotonaldehyde at 298 K.
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Hydrogenation of crotonaldehyde
2065 cmꢀ1 often appearing as the most dominant feature,
and a tail between 2050 and 1950 cmꢀ1, probably containing
many distinct contributions but mostly showing a maximum
at 2003 cmꢀ1. Each will be discussed in turn, firstly in terms
of what they reveal regarding the characteristics of the two cat-
alysts and then, in the second section in terms of what they
reveal regarding the interactions of the active catalyst compo-
nents with the crotonaldehyde. The first band corresponds
with a frequency at which the dominant maxima were found
for Pt/TiO2 catalysts11 although in that study a further, higher
frequency contribution at 2094 cmꢀ1 was observed which was
absent in this study. The 2094 cmꢀ1 band was assigned to CO
adsorbed on close-packed terrace sites such as {111} patches
where the carbonyls were exposed to extensive coupling inter-
actions. The band at 2084 cmꢀ1 was attributed11 to carbonyls
on less extended terrace sites and/or where the metal atoms
were in less densely packed arrangements such as in {100} type
facets and such an assignment is accepted here to explain the
highest frequency component observed for both Pt/CeO2–
SiO2 and PtZn/CeO2–SiO2 . The absence here of a feature at
ca. 2094 cmꢀ1 could be interpreted in terms of the dominance
of more spherical Pt particles over CeO2–SiO2 while flat-raft
like particles with extended flat surfaces were favored over
the titania support. Copper was also envisaged to prefer a
two-dimensional raft-like distribution when supported on tita-
nia.10 In addition to the progressive loss of CO and hydrogen
chemisorption as a function of increasing reduction tempera-
ture, evidence for the formation of an SMSI state for Pt/
TiO2 catalysts was the progressive loss of the sites giving the
2094 cmꢀ1 band.11 Spectra for the Zn-free Pt/CeO2–SiO2 sam-
ple show an increase in the contribution made by the 2080
cmꢀ1 feature as the reduction temperature was increased from
473 to 673 K (Fig. 1A and B); however, the band was very
weak for the 773 K reduced sample despite the overall increase
in area of the overall band envelope. Zn containing Pt/CeO2–
SiO2 on the other hand showed no overall increase in band
envelope on increasing the reduction temperature from 623
to 773 K, however, in contrast to Zn-free sample, the band
at 2080 cmꢀ1 was still present as an intense feature for the
773 K reduced sample. One interpretation, made on the basis
Table 1 reports the activity and selectivity for crotonaldehyde
hydrogenation at 353 K over Pt/CeO2–SiO2 and PtZn/CeO2–
SiO2 reduced in flowing H2 at different temperatures (473, 623
and 773 K) for 1 h. The initial data are taken after 4 min on
stream, when the carbon balance was achieved. Prior to this,
a given amount of reactant and products were being adsorbed
on the catalysts and then, no reliable measurements could be
done. The comparation between the initial data (t ¼ 4) and
those measured after 200 min on stream the degree of catalyst
deactivation and its effect on selectivity to be evaluated.
The Zn-free samples did not show a great deal of sensitivity
to the reduction temperature. Activity increased slightly when
passing from 473 to 623 K and then decreased again slightly
after reduction at 773 K. However, the selectivity to crotyl
alcohol was strongly affected, mainly after reduction at this
highest temperature. It should be noted that selectivity to buta-
nol was nil, and to light hydrocarbons was lower than 5% in all
cases. The increase in selectivity to crotyl alcohol with increas-
ing reduction temperature indicates the creation of active sites
on which the hydrogenation of the carbonyl bond is carried
out at the expense of sites which are active for the hydrogena-
tion of the C=C bond. Similar results were obtained for Pt/
TiO2 when reduced at temperatures between 473 and 773
K11 although, in that case, the catalyst was strongly deacti-
vated after reduction at 773 K. The Zn-containing catalyst,
on the other hand, showed both increased activity and crotyl
alcohol selectivity as the reduction temperature was increased.
The selectivity to crotyl alcohol was significant here even after
reduction at only 473 K. The lower selectivity for the PtZn cat-
alyst reduced at 773 K is related to the high amounts of buta-
nol produced during the initial stages of reaction. Indeed, at
t ¼ 10 min the selectivity to butanol was only 3% and the
selectivity to crotyl alcohol had increased to 36%. All catalysts
showed a rapid deactivation with time on stream but reached a
stable conversion level after ca. 75 min. Results at t ¼ 200 are
also shown in Table 1, where it can be seen that the degree of
deactivation increased with reduction temperature, this effect
being more pronounced for the Zn-containing catalyst.
11
of previous results for Pt/TiO2 would be that the loss or
depletion of the 2080 cmꢀ1 feature in Pt/CeO2–SiO2 at the
highest reduction temperature was the consequence of cover-
age of the sites responsible by sub-oxides of cerium although
higher temperatures are usually required for the formation of
an SMSI state for ceria than titania.18 In applying the same
reasoning to the Zn containing sample, it would have to be
argued that the presence of Zn, present as the oxide, hindered
the platinum from entering into such a state given the strong
presence of the 2080 cmꢀ1 band for this sample even after
Discussion
Adsorption of CO
Spectra of adsorbed CO in the region characteristic15 of line-
arly adsorbed species show three main characteristics indepen-
dent of the presence of Zn and the reduction temperature.
These are: a high frequency component at 2080 cmꢀ1, often
present as a shoulder or an unresolved feature, a band at ca.
Table 1 Catalytic activity and selectivity for crotonaldehyde hydrogenation at 353 K over Pt/CeO2–SiO2 and PtZn/CeO2–SiO2 catalysts
Selectivity (%)
Reduction
temperature/K
Time on
stream/min
Activity/
mmol (g Pt)ꢀ1 sꢀ1
Catalyst
Butanal
Crotyl alcohol
Conversion (%)
Pt/CeO2–SiO2
473
623
773
473
623
773
4
200
4
204
157
227
144
189
83
92
95
72
94
56
88
74
85
58
84
63
78
2
0
10
8
9
11
7
200
4
2
40
9
8
200
4
4
PtZn/CeO2–SiO2
55
20
22
5
2
200
4
1
178
25
35
10
15
22
8
200
4
1
667
47
27
2
200
212
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773 K reduction. An alternative suggestion would be that the
presence of Zn hindered a morphological transition of the Pt
catalysts between 623 and 773 K which in its absence resulted
in a change in the distribution of the exposed sites such that the
centers responsible for the 2080 cmꢀ1 band became less signifi-
cant and were replaced by sites giving rise to CO absorption
bands at a lower frequency. Arguments against a significant
change in particle size/shape and inherent changes to mor-
phology for the Pt/CeO2–SiO2 catalysts as the reduction tem-
perature was increased was the invariance in the Pt/Si atomic
ratios obtained by XPS for 473 and 773 K reduced samples.12
The balance of the argument would be that the presence of
Zn hinders processes which lead to the loss of the sites giving
the 2080 cmꢀ1 band and which occur in the absence of Zn
when the Pt/CeO2–SiO2 catalysts are reduced between 623
and 773 K.
does not show such a tendency and thus provides further evi-
dence against the formation of bimetallic clusters under these
conditions.
Unlike spectra of CO adsorbed on single crystal or extended
surfaces, spectra of linearly adsorbed CO on supported Pt
always exhibit a main band which displays considerable asym-
metry towards lower frequencies11,14–16,19 indicating the pre-
sence of sites which are a unique feature of a supported
catalyst. This was also a feature for the Pt/CeO2–SiO2 catalyst
(with or without Zn) studied here. Although the range of fre-
quencies was similar for Zn-containing and Zn-free catalysts
(to 1950 cmꢀ1), the intensity was of significance for both the
773 K, and in particular, for the Zn-containing sample (Fig.
2C). An additional component of this tail was a resolved fea-
ture at 2003 cmꢀ1. This was not detected in previous studies
19
of Pt/CeO2 although the maximum reduction temperature
One further point to add regarding the sites giving the 2080
cmꢀ1 band involves the proposal12 that 773 K reduction of
PtZn/CeO2–SiO2 catalysts leads to the formation of a Pt–Zn
‘‘alloy.’’ FTIR results here are not consistent with such a pro-
posal. The incorporation of regular arrays of Zn atoms into an
otherwise contiguous array of Pt atoms would be expected to
have a significant effect on the frequency of nCO. Such carbo-
nyl species when forming part of an extended array of mole-
cules with similar singleton frequency as its nearest neighbors
exhibit high stretching frequencies mainly as a consequence
of the number of dipole–dipole coupling interactions. Dilution
of such an array by the addition of Zn atoms, assuming the
absence of an exactly compensating electronic effect by Zn,
would lead to reduced frequency of the Pt carbonyls exhibiting
the 2080 cmꢀ1 band, a result which is not observed for the 773
K reduced PtZn/CeO2–SiO2 catalyst.
Assuming that the frequency of the Pt carbonyls are mainly
a consequence of the coordination number on the adsorbing
atom and the number of dipole–dipole interactions with neigh-
boring adsorbed carbon monoxide molecules rather than being
dominated by the influence of support-metal interactions, then
the band appearing at ca. 2065 cmꢀ1 can be attributed as
before11 to sites of lower coordination such as atoms located
at edges and kinks and sites at the peripheries of extended
facets.15,16,19 Support for the initial statement is made above
where it is reasoned that the particles here may be spherical
(or hemispherical) rather than raft-like, the latter favoring
metal–support interaction. Further evidence that the particles
are spherical/hemispherical in nature come from the fact the
2065 cmꢀ1 band is the dominant feature in the majority of
the spectra recorded here but higher frequency carbonyl bands
dominated spectra of similarly loaded Pt/TiO2 catalysts where
flatter particles with more extended facets were thought to
dominate. Although the 2065 cmꢀ1 feature already dominated
spectra of CO adsorbed on the samples employed here, inten-
sity transfer from low frequency to high frequency species11,14
as a consequence of dipole coupling interactions between car-
bonyls at adjacent but dissimilar sites, is likely to result in the a
concomitant loss of intensity at 2065 cmꢀ1 and equivalent
intensity gain at 2080 cmꢀ1 assuming that the two types of sites
are both present within the same particle. The observed inten-
sity at 2065 cmꢀ1 might therefore underestimate (in relative
terms) the number of sites present although in a similar man-
ner, this band intensity might be enhanced by transfer from yet
lower frequency maxima. The formation of inter-metallic com-
pounds involving Pt and an additional metal with a lower
enthalpy of sublimation is expected to produce particles in
which the other metal, Zn in this case, should occupy the
higher energy (i.e. lower coordinated sites) to reduce the over-
all surface free energy of the particle. Should reduction treat-
ment at 773 K favor12 the formation of such a bimetallic
cluster, then the expectation in terms of modified IR spectrum
of adsorbed CO would be to diminish the contribution of the
2065 cmꢀ1 feature. Comparison of spectra in Fig. 2A and 2C
employed in that study was 673 K. Spectra here for the Zn-free
system are not inconsistent with this result as the 2003 cmꢀ1
feature was only observed as a resolved feature for sample
reduced at 773 K. On the other hand, this feature was common
to spectra of the Zn containing catalysts for all three reduction
temperatures. One plausible interpretation is that the band is
due to the Pt bound CO where the oxygen interacts with
exposed, possible reduced exposed cerium ions, in the support
surface. Such sites would only be created at the metal particle-
support interface and would be most abundant in cases where
small particles were present or where isolated Pt atoms were
distributed over the support. The appearance of the 2003
cmꢀ1 maximum irrespective of reduction temperatures for cat-
alyst containing Zn, but only after 773 K reduction for Zn-free
catalysts and an assignment involving reduced, exposed Ce
sites is supported by TPR results.12 These experiments show12
that Pt is reduced below 450 K, which is consistent with the
failure to detect bands here between 2150 and 2100 cmꢀ1
which are characteristic of CO adsorbed at oxidized platinum
sites.15,16,19,20 A second reduction peak, attributed to surface
reduction of ceria and centered at 640 K was only complete
for Zn-free samples at 700 K so only the IR experiments per-
formed on a 773 K reduced sample would involve an exten-
sively reduced ceria surface. This peak was absent for the Zn
containing sample12 but was replaced by a broadened first
peak where both the Pt and the surface ceria was reduced in
a step with a maximum at 377 and shoulder at 447 K. i.e. in
the case of the Zn containing catalyst, IR experiments per-
formed at all reduction temperatures involved predominantly
reduced ceria surfaces. In spite of this surface reduction of
ceria, samples, even after 773 K reduction, did not show clear
signs of an SMSI state, which, in the case of Pt/TiO2 cata-
lysts11 was evidenced by significantly reduced CO uptake and
a correspondingly smaller IR band envelope. Results are con-
sistent with recent findings for model Pt/CeO2 systems18 where
temperatures above 800 K were required before modifications
to the CO chemisorption properties were observed.
A proposed scenario consistent with enhanced ceria reduci-
bility in the presence of Zn12 and the possible role of ZnO in
suppressing the onset of an SMSI state (loss of 2080 cmꢀ1
band) would involve a bimodal distribution of particles. Lar-
ger particles (giving the 2080 and much of the 2065 cmꢀ1 band)
are located on deposits of ZnO where they are effectively iso-
lated from the ceria component. Similar particles are present
for Zn-free sample but these are in direct interaction with
the ceria support and show some signs (low activity in reac-
tion, loss of the 2080 cmꢀ1 band) of entering into the initial
stages of an SMSI state after 773 K reduction. Ceria reduction
at lower temperatures, facilitated by the presence of reduced
Zn rather than reduced Pt for the Pt–Zn catalyst does not
lower the onset temperature for the SMSI state of the larger
particles as they are isolated from the reduced ceria. Smaller
clusters of Pt and even isolated Pt atoms, are in direct contact
with areas of ceria, however the creation of Pt–Ce3+ interface
Phys. Chem. Chem. Phys., 2003, 5, 208–216
213

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sites where tilted CO molecules are adsorbed (2003 cmꢀ1 band)
is only possible when reduction of the ceria surface occurs, a
process which is apparently12 facilitated by the presence of
zinc.
effects of crotonaldehyde on adsorbed CO were summarized10
as; a geometric blocking effect which reduced the surface cov-
erage of CO, an electronic effect due to the electron donating
nature of crotonaldehyde acting as a Lewis base and a redox
effect which resulted in the oxidation of crotonaldehyde and
the reduction of Cu(I) to Cu(0). On the other hand, Pt/TiO2
catalysts exhibited effects which were interpreted11 in terms
of the displacement of CO from the weakest adsorption sites
which as a consequence of reduced dipole coupling inter-
actions between CO on different adsorption sites, diminished
the intensity transfer phenomenon14 which then led to
enhanced intensity at lower frequencies. In both cases the tita-
nia support was considered to play a significant role in the
overall surface chemistry although only in the case of sup-
ported platinum catalyst11 was it envisaged as acting in a man-
ner characteristic of an SMSI effect. Supported copper did not
display the usual characteristics of a catalyst in an SMSI
state.10
Note that although the close proximity of Pt and Zn is envi-
saged in the above model, the suggestion12 that Pt-Zn alloy
formation occurs following 773 K reduction is not consistent
with the IR results obtained. Although Pt-Zn alloy formation
has been observed for Pt/ZnO catalysts reduced at tempera-
tures as low as 473 K21–23 one would expect considerable dif-
ferences between spectra of the 773 K reduced samples in the
presence and absence of Zn. These differences, such as reduced
overall band intensity,23 lower frequency of the nCO maxi-
mum, invariance in nCO as a function of CO coverage and loss
of intensity of bands due to bridge-bonded species as a result
of isolation of Pt sites,22 were not apparent.
Adsorption of crotonaldehyde
Unlike Cu/TiO2 ,10 the possibility of redox behavior during
co-adsorption involving crotonaldehyde can be ruled out.
Although XPS results12 provide some evidence for Pt states
which are reduced to a greater extent following high tempera-
ture treatment, the IR results provide no evidence for the gen-
eration of new, additional sites following adsorption of
crotonaldehyde. The complete band envelope for adsorbed
CO in the presence of the aldehyde fell within the same overall
wavenumber range as in the presence of CO alone (although
the distribution of intensity at each wavenumber was chan-
ged). Neither do results indicate that the presence of aldehyde
acts only to reduce the surface coverage of COad (geometric
effect) as this would eventually lead to spectra akin to those
observed at low coverage with a band of reduced intensity
right across the range of nCO, i.e. the observed effect would
be akin to a desorption experiment in the presence of CO
alone.
If a Pt dispersion of 50% is assumed, then sufficient aldehyde
molecules would be present in the initial pulse to interact with
10% of all exposed Pt atoms assuming a 1 : 1 configuration.
Addition of the first pulse of crotonaldehyde had very little
effect on the spectra, resulting in slight loss at the maxima,
and an equivalent gain in intensity on the low-frequency tail.
It is clear that the vast majority of the molecules in the initial
dose are adsorbed by the support, most likely at exposed Lewis
acid centers of the support as suggested by spectra in Fig. 5.
Increased amounts of aldehyde (equivalent to 1.5 molecules
of aldehyde per Pt surface atom at 50% dispersion) caused a
red shift in the band envelope of both linear and bridged spe-
cies but without significant loss in the overall integrated inten-
sity. As no obvious loss in bridge-bonded species accompanied
this process, it is unlikely that the loss of CO adsorption sites
by displacement by crotonaldehyde is compensated by conver-
sion of bridging to linear sites. It is possible however that loss
of CO adsorption sites without significant loss of band inten-
sity is a consequence of compensation by the creation of mod-
ified sites (e.g. with aldehyde at an adjacent site) where the
adsorbed CO has a higher absorption coefficient. Again this
scenario is unlikely to be the dominant effect as exact compen-
sation would appear somewhat fortuitous.
With the exception of the 473 K reduced Zn-free sample, all
other catalysts exhibited a weak feature at 1560 cmꢀ1 which
can be attributed to nas(COO) vibrations of adsorbed croto-
nate anions. Carboxylate species were not formed at low sur-
face coverage and appeared to be only a minority species or
absent for samples with limited or no surface ceria reduction
(e.g. Zn-free 473 K reduced sample). As in the case of Cu/
TiO2 ,10 the formation of carboxylate species was facilitated
by an increase in sample temperature. The dominant mode
of adsorption involved interaction of the carbonyl group act-
ing as a Lewis base with exposed surface sites. These were
probably mainly exposed Ce4+ sites as the band appeared
for all samples regardless of reduction temperature and for
samples with no evidence12 of significant surface reduction.
In all cases the effect of increasing coverage shifted the maxi-
mum from 1664–1668 cmꢀ1. At highest coverages, the carbo-
nyl group was involved in weaker interactions, probably
involving hydrogen bonds with exposed hydroxyls. As indi-
cated above, as the Ce : Pt XPS atomic ratios were only ca.
8.3 : 1 it is possible that the spectra contained contribution
from crotonaldehyde adsorbed at exposed Pt sites. However
if crotonaldehyde favored a di-sco adsorption arrangement
as in the case of Pt {111}3 neither the nCO nor the nC=C
would be observed by IR, unless these were adsorbed at inter-
facial sites where the surface selection rule would no longer
apply. In any case, adsorption on the support clearly made
the major contribution, given the limited displacement of CO
which occurred from the metal under conditions which gave
intense spectra due to adsorbed crotonaldehyde shown in the
last spectrum in the series shown in Figs. 5 and 6 and repre-
senting the ‘‘high coverage’’ pulse. The 473 K reduced Zn-free
catalyst gave evidence that the s-complex involving ligation
via the carbonyl O-atom was not the major species at low cov-
erage. The absence, or low intensity of the carbonyl stretching
mode under these conditions and the perturbation of the nC=C
would be consistent with the p di-sco species proposed by Eng-
lisch et al.3 for adsorption at interface sites. However, as
adsorption under these conditions (low coverage pulse) dis-
placed very little CO from the surface (Fig. 4A) it is unlikely
in this case to involve adsorption sites located on the metal sur-
face. The difference between the dominant low coverage mode
of adsorption over 473 K reduced Zn-free samples and the
others, is probably a consequence of the low extent of surface
reduction12 experienced by this sample.
Co-adsorption of CO with unsaturated hydrocarbons or
Lewis bases on supported platinum catalysts has previously
been shown to produce red-shifts in the maxima due to
nCO24,25 which has been interpreted in terms of electronic
effects which are expected to increase the extent of back dona-
tion into the carbon monoxide p* orbitals, thus reducing the
carbon-oxygen bond order. These shifts were ca. 40 cmꢀ1 for
benzene,24,25 37 cmꢀ1 for ethene25 and 67 cmꢀ1 for ammonia.25
It is tempting, therefore to interpret shifts in the main linear
carbonyl bands (listed for increasing reduction temperature)
of 14, 18 and 15 cmꢀ1 for Zn-free catalyst and 21, 22 and
ca. 30 cmꢀ1 for Zn containing catalyst to electronic effects
Co-adsorption
Previous studies10,11 involving co-adsorption of CO and croto-
naldehyde over titania supported metal catalysts have shown
that several effects are observed which depend on the nature
of the metal adsorbent. For Cu/TiO2 catalysts, the principle
214
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induced by the presence of crotonaldehyde. However, Stoop
et al.26 using 12CO and 13CO mixtures, were able to show that
co-adsorbed ethene exhibited only a very small electronic effect
and that the majority of the ca. 35 cmꢀ1 shift in nCO was the
consequence of separation of the dipoles (i.e. a geometric
effect). One scenario which would account for the limited elec-
tronic effects26 and the red-shift in nCO in the presence of
co-adsorbates24,25 would be that aldehyde (or unsaturated
hydrocarbon) was adsorbed at the interface sites, primarily on
the support sites but bridging the interface such that CO at the
boundary Pt atoms were displaced. The groups of Vannice1,27
and Lercher3 both envisage these locations as being the active
sites for hydrogenation based on activities using different sup-
ports and varying reduction temperatures. While not at odds
with the model of the active site, FTIR results of the co-
adsorption studies here are not consistent with this picture of
initial CO displacement. Should the aldehyde adsorb at
exposed cation sites in the support–metal interface, then the
presence of the adsorbed aldehyde would eliminate any inter-
action between CO molecules which were simultaneously inter-
acting with Pt atoms and exposed cationic sites at the interface.
As argued above, such sites might be responsible for the
resolved feature at 2003 cmꢀ1. This band, however, was absent
following evacuation at 298 K and so the adsorption of alde-
hyde at such sites is not proven experimentally. However, what
is shown quite clearly both here and in previous studies of sup-
ported Pt catalysts11 is that these carbonyls giving rise to the
highest frequency component of the band envelope are those
which are initially displaced by the aldehyde. These species
exhibiting high nCO are unlikely to be located at interface sites
as in such a location they would experience fewer dipole inter-
actions and would exhibit lower stretching frequencies than
their central neighbors located on metal atoms within an
equivalent facet, but surrounded by more nearest neighbor
dipoles. Note that an electronic effect induced by aldehyde
adsorbed on the support and influencing the metal support
interaction is also unlikely to explain results since a shift in
the entire band envelope without loss of intensity at a specific
frequency would be expected, or, if only the smaller particle
are influenced, then only the lower frequency carbonyls would
be shifted. This is not consistent with the observed results
where a shift in the whole band envelope was observed with
selective loss in intensity at the highest frequency.
The loss of the highest frequency carbonyls as aldehyde is
introduced reflects the relative ease by which these CO mole-
cules are displaced and is consistent with the fact that these
sites are the last to be filled on increasing coverage (Figs. 1
and 2). As these sites (see above) are due to CO on more
extended facets, displacement of CO results in a shift to
lower frequency as the number of dipole–dipole interactions
is reduced. This does not, however, infer anything about the
most active site for crotonaldehyde adsorption/reaction. The
selective displacement of CO from these sites rather than a
more random desorption process which accompanies outgas-
sing, maintains regionally high densities of CO in other
facets of the metal crystallite. The elimination from adjacent
facets of carbonyls which exhibit higher stretching frequen-
cies, eliminates intensity transfer14 resulting in the growth in
the lower frequency region of the spectra shown in Figs.
3 and 4. Note that the separation (Dn ꢄ 30 cmꢀ1) between
the frequency at which intensity is lost (ca. 2080 cmꢀ1 in
Fig. 3 or 2070 cmꢀ1 in Fig. 4) and where the maximum is
gained (ca. 2050 cmꢀ1 in Fig. 3 or 2042 cmꢀ1 in Fig. 4) is
equivalent to the expected separation between a molecule
on a terrace and a defect site14,28 and is small enough to
allow dipole coupling interactions.14 The large FWHM of
the Zn containing catalysts in the presence of crotonalde-
hyde compared to Zn-free sample, is a consequence of the
greater intensity contribution at low frequencies with the
initial CO band envelope although both samples show a
return to background absorption at the same frequency
(ca. 1950 cmꢀ1).
One possible link between the spectroscopic characterization
and reaction data, would be that high temperature treatments
or the addition of Zn result in the formation of small clusters
of Pt atoms with low coordination and as a consequence of
their size, have a large interface zone. These sites are evidenced
by the low frequency tail exhibited by all samples here but of
greater significance for the 773 K reduced samples and specifi-
cally, the Zn-containing samples. This spectroscopic feature is
not present for single crystal surfaces and so can be assigned to
sites which are a unique feature of a supported catalyst. The
small size of the cluster and their inherent close contact with
support makes these sites highly sensitive to pretreatment
effects and the addition of modifiers to the carrier. Further-
more, as the presence of Zn would appear to enhance the redu-
cibility of ceria,12 this increased number of Ce3+ sites, when
located in the interfacial zone close to platinum atoms could
be expected to facilitate activation of the aldehyde carbonyl
bond and lead to the observed enhanced yields of crotyl alco-
hol. Although qualitatively, agreement exists between the
activity and the magnitude of the band intensity between
2050 and 1950 cmꢀ1, an attempt to show a quantitative rela-
tionship between integrated intensity of the nCO band in this
region and the yield to crotyl alcohol proved unsuccessful,
probably as a consequence of the number of other contributing
factors, such as activity of other centers (selective or otherwise)
and deactivation of sites as a function of time on stream.
Conclusions
IR spectra of high temperature treated Zn containing and Zn-
free PtCeO2/SiO2 samples do not, unlike Pt/TiO2 catalysts,
show significant signs of an SMSI state although enhanced
selectivity to the unsaturated alcohol was observed for such
samples and for low temperature reduced Zn containing sam-
ples. The presence of Zn facilitated the reduction of surface
ceria, thereby augmenting the potential number of Ce3+-Pt
metal interface sites, particularly in cases where small Pt parti-
cles were located in ceria rich zones of the support. Such Pt
sites were defined by low frequency (2030–1950 cmꢀ1) carbonyl
species. The presence of Zn enhanced the number of sites that
gave such low frequency carbonyl species, lending to the pro-
posal that such Pt atoms may be the active sites for the hydro-
genation reaction. Co-adsorption experiments provide limited
evidence for the active sites as spectra are dominated by dipole
coupling effects. Introduction of crotonaldehyde to a CO cov-
ered surface leads to CO displacement from the weakest
adsorption sites which had the effect of enhancing intensity
at lower frequencies as a consequence of the removal of inten-
sity transfer phenomena.
Acknowledgements
We thank the M.E.C. (Spain) for a travel grant (to J.S.A)
which financed a trip to the University of Dundee during
which the FTIR measurements were conducted. Financial sup-
port from C.I.C.Y.T (Project BQU 2000-0467) is also grate-
fully acknowledged.
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