Correlation between deactivation and Pt-carbonyl formation during toluene hydrogenation using a H2/CO2 mixture

Article abstract of DOI:10.1016/j.jcat.2010.12.002

H₂/CO₂ mixtures derived from biomass degradation may be regarded as a renewable source of hydrogen that could be used for hydrogenation reactions. The practicality of directly using a CO ₂-containing H₂ feed was evaluated during the hydrogenation of toluene to methylcyclohexane at 75 °C and ambient pressure. An operando DRIFTS study was carried out to unravel some of the mechanistic features of this reaction over a 0.88% Pt/Al₂O₃ catalyst, in particular to rationalize the loss of activity observed in the presence of CO₂. We report, for the first time, a quantitative link between the loss of activity for hydrogenation and the formation of carbonyls adsorbed on the platinum, while the deactivation could not be correlated to the species formed onto the alumina, e.g. carbonates, hydrogenocarbonates and formates.

Full text of DOI:10.1016/j.jcat.2010.12.002

Journal of Catalysis 278 (2011) 153–161
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Correlation between deactivation and Pt-carbonyl formation during toluene
hydrogenation using a H /CO mixture
2
2
Aqeel Ahmad Taimoor , Isabelle Pitault , Frédéric C. Meunier b,
a
a,
⇑
⇑
a
University of Lyon, CNRS, Laboratoire de Génie des Procédés Catalytiques, CPE Lyon, 43 bd du 11 novembre 1918, BP 82077, 69616 Villeurbanne Cedex, France
Laboratoire Catalyse et Spectrochimie, University of Caen, CNRS, ENSICAEN, 6 Boulevard du Marechal Juin, 14050 Caen Cedex, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
H /CO mixtures derived from biomass degradation may be regarded as a renewable source of hydrogen
2
2
Received 15 October 2010
Revised 5 December 2010
Accepted 7 December 2010
Available online 8 January 2011
2 2
that could be used for hydrogenation reactions. The practicality of directly using a CO -containing H feed
was evaluated during the hydrogenation of toluene to methylcyclohexane at 75 °C and ambient pressure.
An operando DRIFTS study was carried out to unravel some of the mechanistic features of this reaction
2 3
over a 0.88% Pt/Al O catalyst, in particular to rationalize the loss of activity observed in the presence
of CO . We report, for the first time, a quantitative link between the loss of activity for hydrogenation
2
Keywords:
Biogas
and the formation of carbonyls adsorbed on the platinum, while the deactivation could not be correlated
to the species formed onto the alumina, e.g. carbonates, hydrogenocarbonates and formates.
Ó 2010 Elsevier Inc. All rights reserved.
CO
2
H
2
CO
Pt
Toluene
Hydrogenation
Reverse water–gas shift
Operando
1
. Introduction
are commonly used in hydrogenation reactions. The dissociative
2
chemisorption energy of H on the Pt(1 1 1) surface is typically
Biogas produced by dark fermentation of organic wastes may
comprised between 68 and 80 kJ/mol [9–11], while the CO binding
energy on the same surface ranges from 144 to 173 kJ/mol [12,13].
Therefore, CO molecules bind more strongly to the Pt atoms than
dissociating H atoms and may block the sites required for hydrogen
adsorption and strongly decrease the hydrogenation rate. The poi-
soning effect will be worse if the molecule to be hydrogenated
can only adsorb on the metal sites, hence be in competition with
CO. Note that the adsorption enthalpy of toluene on Pt(1 1 1) is
about 65 kJ/mol [14].
consist of a mixture of mostly dihydrogen and carbon dioxide [1,2].
The possibility to directly use this mixture at low temperature
and pressure as a renewable feedstock for hydrogenation reactions
would be valuable. However, a possible difficulty associated with
this feedstock is the potential reactivity of carbon dioxide. CO
2
is usually considered as an inert molecule, but it can lead, for
instance, to carbon monoxide through the reverse water–gas shift
reaction (RWGS, Eq. (1)) when an appropriate catalyst such as a
Pt-group metal is present [3–5].
2
The poisoning of Pt catalysts by CO formed from CO was actu-
ally reported by several authors. Baiker and co-workers showed by
in situ IR spectroscopy that Pt, Ru, Rh and Pd were all gradually
ꢀ
H
2
þ CO
2
! H
2
O þ CO
D
r
H ð298 KÞ ¼ þ42 kJ=mol
ð1Þ
covered with carbonyl species under 90 bars of supercritical CO
2
The concentration of CO produced is small at low temperatures,
first, because of thermodynamic limitations and second, because
the reaction rates are minute below 250 °C for catalysts based on
Pt [5–8]. However, even if the CO concentrations formed are low
and possibly not detectable in the reactor effluent, CO traces may
lead to a dramatic change of the activity of Pt-group metals, which
(
scCO ) in the presence of 1% H at 50 °C, with the Pt-based sample
2
2
displaying the highest rate of carbonyl formation [15]. Baiker and
co-workers [16,17] had reported earlier on the deactivation and
low activity of a 5 wt.% Pt/Al
2
O
3
2 2
in scCO (80 bar of CO ) at ca.
3
5 °C during ethyl pyruvate hydrogenation, which was ascribed
to the reduction of CO
surface. Xu et al. [18] have used a CO
2
to CO, the latter species poisoning the metal
-containing medium for poly-
2
⇑
Corresponding authors.
styrene ring hydrogenation and reported a deactivation due to CO
formed from the RWGS (Eq. (1)) over a Pd-based catalyst at 150 °C,
E-mail addresses: isabelle.pitault@lgpc.cpe.fr (I. Pitault), frederic.meunier@
ensicaen.fr (F.C. Meunier).
0
021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.12.002

1
54
A.A. Taimoor et al. / Journal of Catalysis 278 (2011) 153–161
while the deactivation of a Ni-based catalyst (observed at 180 °C)
hydrogenation reactions carried out in the presence of CO
2
, as
was ascribed to water formed from the CO
2
methanation (Eq. (2))
recently discussed by Roberts and co-workers [18].
To better assess the possible role of the RWGS (Eq. (1)) in hydro-
ꢀ
4
H
2
þ CO
2
! 2H
2
O þ CH
4
D
r
H ð298 KÞ ¼ ꢁ165 kJ=mol
ð2Þ
genation reactions carried out in the presence of CO , we report
2
here an investigation of the hydrogenation of toluene over a Pt/
The involvement of the RWGS was also noted in the case of the
operation of low-temperature fuel cells. Van Zee and co-workers
have shown that the reformate (i.e. CO , N and H ) used in a proton
2 2 2
exchange membrane fuel cell (PEMFC) at 70 °C led to the formation
of some CO that poisoned the Pt electrode [8].
In situ diffuse reflectance FTIR spectra of a Pt/C catalyst under a
2 2
CO /H 10/1 mixture under atmospheric conditions revealed the
presence of a broad peak Pt-carbonyl, which was suggested to be
able to modify the sample activity for the hydrogenation of haloni-
troaromatics [19]. Arunajatesan et al. reported the formation of
carbonyl species over a Pd/alumina under different conditions
2 3 2 2
Al O catalyst using a simulated biogas, i.e. a H + CO mixture.
Our primary aim was to determine whether CO poisoning could oc-
cur on Pt-based catalysts used at very low temperature (i.e. 75 °C)
and atmospheric pressure for a gas–solid system. This set of
conditions would be of interest for any commercial hydrogenation
reaction using biogas. Toluene was selected as a test molecule,
the hydrogenation of which is often considered as non-structure
sensitive [28,29].
Preliminary reactivity tests conducted using a classical stainless
steel tubular reactor showed that a strong deactivation occurred in
2
the presence of CO . An in-depth operando diffuse reflectance FTIR
(
P = 138 bar; T = 342 K; molar CO
2
/H
2
= 19) [20]. Based on the pres-
spectroscopy (DRIFTS) study was then carried out to attempt to re-
late the deactivation to observable surface species. Discriminating
between surface species is important because toluene hydrogena-
tion has been shown to proceed also on the support in the case of
many oxide-supported Pt catalysts, supposedly due to hydrogen
spill-over onto the support and hydrogenation of toluene on the
support sites [30]. It appeared that the deactivation observed in
our case could be quantitatively linked only to the formation of
Pt-carbonyl species.
ence of water peaks at reaction conditions, these authors proposed
that the reverse water–gas shift reaction is the most plausible
mechanism for CO formation.
The use of different metals, supports or substrates may allow
overcoming the formation of carbonyls and/or catalyst poisoning
under CO
the hydrogenation of phenol with H
in situ IR spectroscopy [21]. The formation of Rh-carbonyl species
was observed over Rh–Al and Rh/C. The nature and intensity of
2
-containing feeds. Recently, Fujita et al. investigated
2
under high-pressure CO by
2
2 3
O
the carbonyl bands were quite different on each sample and only
the alumina-based material showed a complete deactivation.
2
. Experimental
.1. Catalyst preparation
The Pt/Al catalyst was prepared using c-alumina particles
It must be stressed that beneficial effects due to CO
have also been reported. Wang and co-workers have reported that
CO used as a gas diluent during the dehydrogenation of ethyl ben-
2
reactivity
2
2
2
O
3
zene to styrene also acted as a promoter for the reaction [22].
These authors proposed that ethylbenzene dehydrogenation was
coupled with the RWGS (Eq. (1)); hence, the hydrogen formed
was effectively being scavenged by the CO . Note that this benefi-
2
cial coupling was observed at a high temperature, i.e. 550 °C, over
non-precious transition metals supported on alumina.
impregnated by a platinum acetyl-acetonate solution. The size of
the alumina particles (from Sasol Chemie) was in the range 150–
2
2
50
l
m with a specific surface area of 155 m /g. One hundred
and fifty grams of alumina powder was mixed with 200 ml toluene
and 3.13 g of platinum acetyl-acetonate. The mixture was stirred
for 3 h at 60 °C. The solvent was then removed by evaporation at
Arai and co-workers reported that both conversion and selectiv-
ity increased during the hydrogenation of unsaturated aldehydes
8
0 °C under reduced pressure. The resulting powder was then
dried at 120 °C for one night. This catalyst was then calcined in
ambient air at 500 °C for 4 h using a temperature ramp of 3 °C/min.
After preparation, the platinum content on the catalyst was about
2 3 2
to unsaturated alcohols over a 1 wt.% Pt/Al O at 50 °C in scCO
[
23]. The improvement in activity was suggested to arise from (i)
the change of the dielectric constant of the reaction medium, (ii)
the increased solubility of reagents and/or (iii) increased mass
transport coefficient, although no evidence was provided. The pos-
sible formation of CO was not mentioned. Zhao et al. have used
0
.88 wt.% and the Pt dispersion, measured by H
2
chemisorption
(
see sample pre-treatment details in the Supplementary material),
was about 54%.
2
scCO medium to enhance selective hydrogenation of nitro aro-
matics over Pt/C [24,25]. These authors reported no deactivation
and similar conversions as those measured in ethanol were ob-
2.2. Catalytic tests in a tubular stainless steel reactor
tained. The reaction in scCO
2
was found to be structure insensitive,
The standard procedure for catalyst activation consisted in an
in situ reduction at 350 °C (heating ramp of 3 °C/min) for 4 h under
hydrogen flow of 80 ml/min (STP). The dinitrogen, carbon dioxide
and dihydrogen used were provided by Air Liquide under the pur-
while the opposite was true in the case of using the ethanol sol-
vent. Again, no mention of a possible role of carbon monoxide on
the activity of the sample was mentioned.
Arai and co-workers [26] showed that strong molecular
interactions were taking place between organic substrates and
CO , particularly at high CO pressures. It is possible that such
2 2
interactions could be one of the factors affecting the catalyst activ-
ity (rate and/or selectivity) during the conversion of the substrate
ity category of Alpha Gaz 1, specifically containing H
< 2 ppm and CnHm < 0.5 ppm. The toluene was obtained from
Sigma Aldrich with a purity above 99.5%.
The feed composition consisted of a mixture made of 10%
toluene + 45% (N or CO ) in H . The catalytic experiments were
2
O < 3 ppm,
O
2
2
2
2
in the presence of CO
2
.
conducted in a packed-bed stainless steel tube reactor (10 cm in
height, 6 mm internal diameter). The 10-cm-long reactor was in-
serted in a 20-cm-long copper shell, the thickness of which was
5 cm. The copper shell was then wrapped with heating jackets.
The absence of any temperature gradient within the reactor length
was confirmed by measurement using a thermocouple. The reactor
was loaded with 2 g of catalyst and the resulting bed was ca. 5 cm
in height. The bed was held between two 5 mm layers of quartz
particles, those being located between two layers of SiC.
While the increased reagent solubility and transfer coefficient
via overcoming of the liquid–gas interface) [27] are likely contrib-
(
2
utors to improved catalytic properties when working in scCO , the
evidence provided by many authors [15–17,19,21] also suggests
that selective site poisoning could be, at least in part, responsible
for the improved performance of metal catalysts when using
2 2
scCO and H . There seems actually to be some controversy on
whether CO formation can be effective and induce poisoning for

A.A. Taimoor et al. / Journal of Catalysis 278 (2011) 153–161
155
The gas analysis was carried out using a HP Series II 5890 Gas
chromatograph fitted with a thermal conductivity (TCD) detector.
Two columns PoraBond Q (length = 25 m, diameter = 0.32 mm
The availability of the IR spectrum of the gas-phase measured at
the exit of the DRIFTS cell enabled correcting the DRIFTS spectrum
from any gas-phase contribution. A correcting factor of 0.25 was
applied, which corresponded to the ratio between the optical path-
length in the DRIFTS cell (which depends partly on the catalyst bed
properties) and that in the gas-cell. This ratio was determined after
the spectrokinetic experiment by flowing pure CO over the catalyst
and rationing the area of the gas-phase signal of CO measured in
the DRIFTS cell to that obtained in the gas-cell. The effective path-
length in the DRIFTS cell (through the void above the bed + multi-
ple reflections in the catalyst bed) was found to be about 6.75 cm.
An example of a gas-phase-corrected DRIFTS spectrum and the cor-
responding original spectrum are given in Fig. S2.
and thickness = 5
lm) and Molsieve 5A (length = 10 m, diame-
ter = 0.32 mm and thickness = 10
lm) provided by Varian were
employed. The gas samples were injected online with helium as
carrier gas. The oven temperature was maintained at 50 °C for
2
min to allow peak separation. After each injection, the column
temperature was increased to 200 °C to get rid of any water and
carbon dioxide adsorbed on the molecular sieve.
2.3. Operando diffuse reflectance FTIR analysis
A high-pressure high-temperature diffuse reflectance reaction
The DRIFTS cell loaded with inert SiC presented a non-negligible
activity for toluene hydrogenation to methylcyclohexane (about 4%
cell from Spectra-Tech was used. The cell was modified, mostly
as described elsewhere [31], to eliminate bed by-pass. The only dif-
ference with the modification described in reference [31] is that a
quartz wool plug was used in place of a metallic mesh to hold the
sample. This was done to limit toluene hydrogenation over the
metallic components of the reaction cell (vide infra). The modified
conversion at 75 °C in the absence of CO
2
and 3.6% in the presence
of CO ). Most of the exposed naked heating wires that were
2
present in the original cell were covered with a ceramic coating,
but with no effect. The observed activity could possibly due to a
reaction occurring on remaining exposed heating wires and/or
the thermocouple guide. The conversion due to the cell (i.e. 4%)
was subtracted from that measured with the catalyst when
calculating TOF numbers. Note that the effect of the cell on toluene
conversion would certainly depend on the experimental conditions,
in particular the total gas flow rate, therefore requiring to measure
the cell residual activity under appropriate conditions.
cell yielded a full conversion of CO in the presence of O
2
after the
reaction light-off over the Pt/Al catalyst, showing that no by-
2 3
O
pass occurred (Fig. S1, Supplementary material). No CO oxidation
activity was recorded over a cell loaded with SiC.
The DRIFTS assembly was fitted in a Nicolet Magna 550 FTIR
spectrometer, and a MCT detector cooled with liquid nitrogen
2 2 2
was used. Gases (i.e. high purity Ar, O , CO and H from Air Liqu-
ide) were fed through low volume-heated stainless steel lines to
the cell. Toluene was introduced via a saturator kept at 25 °C.
The feed composition was different from that used in the kinetic
3. Results and discussion
setup and consisted of 1.6% toluene + 40% H
2
+ 2 or 40% CO
2
(if
3.1. Kinetic measurements in the tubular stainless steel reactor
The toluene conversions obtained for various W/F over the Pt/
present) in Ar. A low partial pressure of toluene was used here to
prevent condensation on the DRIFTS cells walls, which were held
at around room temperature. The tests were made at atmospheric
pressure and at a total flow rate of 25 ml/min. The DRIFTS data are
reported as log(1/R), where R is the sample reflectance. The pseu-
do-absorbance log(1/R) gives a better linear representation of the
band intensity against sample surface coverage than that given
by the Kubelka–Munk function for strongly absorbing media such
as those based on alumina [32].The contribution of gas-phase tol-
uene and cyclohexane was subtracted from the surface DRIFTS sig-
nal (vide infra).
Al
steel tubular reactor are given in Table 1. The catalyst was active
for toluene hydrogenation in the absence of CO . The turnover fre-
quency (TOF, Table 1) calculated on the basis of Pt surface atoms
2 3
O catalyst at 75 °C and atmospheric pressure in the stainless
2
ꢁ1
(and for conversions <100%) was about 0.11 ± 0.01 s . Note that
these TOF were the average values from the TOF changing along
the catalyst bed in the reactor, because of the integral flow nature
of the reactor.
Toluene hydrogenation on Pt is usually reported as a structure-
insensitive reaction [28,29], and therefore, the TOF measured here
should be similar to those reported in the literature. Rousset et al.
The reaction cell crucible was filled first with a layer of SiC and
then topped with 19 mg of catalyst powder. The corresponding ra-
tio between catalyst weight and toluene molar flow rate (noted W/
have reported that the toluene hydrogenation TOF of a 0.06 wt.%
ꢁ1
ꢁ1
F) was about 1.04 kgcat s mol . The cell thermocouple was located
Pt/Al
2
O
3
was about 0.12 s
at 75 °C, using 2.5% toluene in H
were
measured [33], the TOF of Rousset et al. extrapolated to our condi-
2
at the interface between the SiC and catalyst layers. The sample
was first oxidized at 350 °C for 15 min under 20% O
[33]. Since a zeroth-order in toluene and an order of 0.5 in H
2
2
in Ar before
ꢁ1
being reduced for 30 min under 40% H in Ar at the same
2
tions should be around 0.08 s . Assuming that the same kinetic
orders apply, the TOF values reported by Lin and Vannice [30] for
a 0.78 wt.% Pt/Al O would roughly correspond to 0.03 s trans-
2 3
ꢁ
1
temperature (heating ramp was about 30 °C min ). The reference
backgrounds were collected at 75 °C over the activated sample
ꢁ
1
ꢁ1
using 128 scan at a 4 cm resolution. A background using a totally
reflecting aluminium mirror was also recorded. During the operan-
do analysis, 8, 16 or 32 scans were collected, depending on the
time resolution sought.
lated into our conditions. Our TOF values measured in the tubular
stainless steel reactor are therefore of the same order of magnitude
as those reported in the literature, the differences noted could pos-
sibly be due to the error introduced during the extrapolation or
experimental errors associated with the measurements. It must
be made clear that the point of our work was not to investigate
whether toluene hydrogenation on Pt is truly a structure-insensi-
tive reaction.
Toluene and methylcyclohexane gas-phase concentrations were
measured after the DRIFTS cell using an IR gas-cell, the optical
pathway of which was about 27 cm. Toluene conversion was
calculated on the basis of the concentrations of toluene and
methylcyclohexane measured, assuming that no other reaction
product was formed. This assumption was reasonable since the
No toluene conversion to methylcyclohexane was obtained over
the Pt/Al
indicating that the hydrogenation activity of the sample was strongly
inhibited by CO . In another experiment (Fig. 1), the conversion
was measured following a series of CO pulses into the feed and
resulted in a gradual and irreversible deactivation. The reduction
of the catalyst using the standard procedure (4 h under H at
2 3
O catalyst when carbon dioxide was present (Table 1),
ꢁ
1
gas-phase IR spectra in the 3200–2800 cm
region could be
accurately decomposed using the spectra of pure toluene and
methylcyclohexane only. Note that no CO gas-phase could ever
be observed, although it might have been present at level not
detectable with our gas-cell (ca. 200 ppm).
2
2
2

1
56
A.A. Taimoor et al. / Journal of Catalysis 278 (2011) 153–161
Table 1
Catalytic activity in toluene hydrogenation measured in the microkinetic setup at 75 °C and atmospheric pressure over the Pt/Al
2
O
3
. The last line reports the activity of the some
catalyst measured in the DRIFTS cell.
TOF (s 1
ꢁ
)
Feed composition (vol.%)
Catalyst weight/
Toluene
toluene molar flow (kgcat s/mol)
conversion (%)
H
2
N
2
CO
2
Toluene
4
4
4
4
4
5
5
5
5
0
45
45
45
0
0
0
45
0
10
10
10
10
1.6
50
30
20
50
100
74.9
60.2
0
0.08 ± 0.01
0.10 ± 0.01
0.12 ± 0.01
0
0
58.4a
1.04
8.6
b
0.054 ± 0.01
a
2
Ar was used in place of N .
This value has been corrected for the conversion measured over the cell loaded with SiC (ca. 4%).
b
not least the relative error on the conversion measurement esti-
mated at 10%. A TOF corrected value of 0.057 s was calculated
100
ꢁ1
to allow a comparison with the TOF values obtained with the par-
tial pressures used in the case of the tubular reactor data discussed
in the previous section (assuming a 0.5 reaction order for H and
2
8
6
4
2
0
0
0
0
0
o CO2-free experiment
x Experiment with CO2 pulses
zeroth-order in toluene). The corrected value of the TOF measured
ꢁ1
in the DRIFTS cell (i.e. 0.057 ± 0.010 s ) appeared to be significantly
lower than those obtained in the case of the tubular stainless steel
ꢁ1
reactor, i.e. 0.11 ± 0.01 s (Table 1).
This discrepancy could be due in part to the fact that about 26%
of the Pt sites were still covered by carbonyl species after an insuf-
ficiently long reduction in the DRIFTS cell (vide infra, the presence
of these carbonyls will actually be useful in understanding the sur-
face reactivity of the catalyst). Another possible reason for the dis-
crepancy could be due to a lower effective temperature in the
sample zone. Assuming that the apparent activation energy of
the reaction was about 50 kJ/mol as reported by Lin and Vannice
on similar materials [30], a temperature drop from 75 to 70 °C
would lead to a TOF loss of about 22%. It is possible that the
temperature of the bed was lower, due to the thermal coupling
between the reaction cell walls (kept at room temperature) and
the catalyst bed in the presence of large concentrations of H2,
which exhibits a high thermal conductivity. These two possible
effects could by themselves alone explain the ca. 50% lower TOF
measured in our DRIFTS cell. Despite these shortcomings, the
DRIFTS results discussed in the following sections will bring reliable
0
2
4
6
8
Time on stream /h
Fig. 1. Conversion of toluene in the reactor used for kinetic studies with (x) and
with no (s) CO pulses. Feed: 10% toluene + 45% H in N . W/F = 50 kgcat s/mol.
2
2
2
ꢁ
6
T = 75 °C. The arrows indicate the various times at which 3.5 ꢂ 10 moles of CO
2
were injected into the feed.
3
50 °C) did not allow regaining any activity in the absence of CO
However, the Pt/Al treated with air at 300 °C and then reduced
using the standard procedure allowed full regeneration (data not
shown).
The inhibition of toluene hydrogenation in the presence of CO
could be the result of several effects, for instance (i) a simple com-
petitive adsorption between CO and one of the reactants (which
2
.
2 3
O
2
2
should yet be a reversible effect), (ii) the formation of poisonous
CO through the RWGS and/or (iii) the formation of poisonous water
through CO
2
methanation. The relevance of these various effects
and valuable insights into the effect of CO on the hydrogenation of
2
will be assessed by the spectrokinetic data collected over the
DRIFTS cell described in the following sections.
toluene over this catalyst.
3.3. In situ DRIFTS study of the sample activation
3.2. Kinetic measurements in the modified DRIFTS cell
In situ DRIFTS analyses were conducted to monitor the state of
Spectroscopic cells are not ideal kinetic reactors, and therefore,
the catalyst surface during the sample activation. Physisorbed water
ꢁ1
it is crucial to check that the reaction rate or TOF measured in these
reactors is consistent with that obtained with traditional non-
spectroscopic units. Published work reporting agreeing reaction
rates between conventional reactors and spectroscopic cells (all
spectroscopies being considered) are actually very few [33–37]. A
recent review discusses these examples as well as the many pitfalls
that the spectroscopic cell experimentalist may face (in particular
difficulties with temperature control) while attempting to deter-
mine reaction rates in such cells [38].
(band at 1645 cm ) and various carbonate/formate/carboxylate
ꢁ1
species (broad ill-defined bands in the 1700–1200 cm region)
were observed on the fresh sample (Fig. S3, Supplementary
ꢁ1
material). The broad band centred around 2070 cm was due to
the so-called ABC contour associated with combination bands of
hydroxyl groups [39], i.e. it was not a carbonyl species.
During oxidation at 350 °C, sharper bands were present in the
ꢁ1
1600–1200 cm region and no bands could be observed in the
carbonyl stretching region (Fig. S3). The intensity of the formate/
The modified DRIFTS cell used here presented no bed by-pass,
as evidenced by the total CO conversion obtained during CO
oxidation above the light-off temperature (Fig. S1, Supplementary
material). Unfortunately, the cell itself (simply loaded with inert
SiC) presented a non-negligible activity for toluene hydrogenation,
which needed to be taken into account when calculating the
catalyst TOF (see experimental section).
2
carbonate bands decreased significantly when H was introduced
ꢁ1
at 350 °C and a new band was observed at ca. 2020 cm (Figs. S3
and 2A), which can be assigned to Pt-carbonyl species adsorbed
on Pt-defective site of high index planes or small Pt particles
[40–42]. These carbonyls were formed from the reduction of the
various surface species above described, directly at the surface or
ꢁ
1
via gas-phase CO
gradually decreased with increasing time under H
low-intensity band appeared at ca. 2090 cm (Fig. 2A).
2
[43]. The intensity of the band at 2020 cm
, while a new
The initial TOF measured over our Pt/Al
O
2 3
catalyst at 75 °C and
2
ꢁ1
atmospheric pressure in the DRIFTS cell reactor was
ꢁ1
ꢁ1
0
.054 ± 0.010 s (Table 1). The precision margin was estimated
The nature of the 2090 cm
band is unclear, and tentative
from the various sources of error involved in the TOF calculation,
assignments are discussed below. On-top Pt–H species exhibit

A.A. Taimoor et al. / Journal of Catalysis 278 (2011) 153–161
157
occupied after reduction with H
2
at 350 °C was approximately 26%.
2020
A
It is unlikely that Pt sites of extended dense planes with high
coordination would be fully occupied before most defect sites are
covered, even by considering the minor energetic gain that could
be associated with island formation. It is therefore unlikely that
Abs.
0.05
ꢁ1
the growth of the 2090 cm band was related to Pt sintering.
2
090
In another experiment also at 75 °C, a reduced sample (never
exposed to CO) was left under pure Ar after removing the 40% H
present. The carbonyl bands showed a minor yet significant red
shift upon removing H , showing that hydrogen adatoms were
present on the Pt particles and interacted with the adsorbed CO
Fig. S4). The fact that the carbonyl IR bands blue shift when H
2
2
2
200
2100
2000
1900
1800
(
2
Wavenumbers /cm-1
is present can be due to an increased local concentration of carbon-
yls (e.g. island formation) due to repulsive interactions between CO
and H adatoms, as shown for the case of Pt(1 1 1) surfaces by Hoge
Abs.
.1
2
049
ꢁ1
B
et al. [46]. The 2090 cm signal could thus correspond, somewhat
0
unexpectedly, to small defective particles that were saturated with
adsorbed CO, possibly involving multicarbonyl species [47–50].
Strong dipole coupling, neighbouring H adatoms (which are rarely
present in most IR studies investigating the nature of Pt-carbonyls)
and reduced electron back-donation from the Pt atoms to the car-
bonyls would all lead to a blue shift of the carbonyl wavenumber.
More work would clearly be needed to ascertain this assignment.
Whatever the exact nature of the sites leading to the bands at
2029
2
090
ꢁ1
ꢁ1
ꢁ1
around 2020 cm (shifting to 2049 cm ) and at 2090 cm , our
data show that various Pt surface sites were present and that some
of those (ca. 26%) were covered with carbonyl following the 30-
2
200
2100
2000
1900
1800
1700
Wavenumbers /cm-1
2
min reduction in H 350 °C. It is likely that using a longer reduction
period (e.g. 4 h, as for the kinetic data collected over the tubular
reactor) could have led to a full removal of these carbonyls. The
presence of these residual carbonyls will yet prove useful in under-
standing further the adsorption properties of the metal.
Fig. 2. (A) In situ DRIFTS spectra of the Pt/Al
at 350 °C under 40% H . The spectra were recorded 1 min (dotted line), 15 min (thin
solid line) and 30 min (thick solid line) after introducing the H at 350 °C. The
spectrum recorded over a mirror was used as background in all cases. (B) In situ
DRIFTS spectra of the Pt/Al catalyst after reduction at 350 °C under 40% H . The
spectra were recorded under H (middle line), at equilibrium condition under
00 ppm of CO (top line) and subsequently under O (bottom line). T = 75 °C. The
spectrum recorded over a mirror was used as background in all cases.
2 3
O catalyst during the reduction stage
2
2
2
O
3
2
2
8
2
3.4. In situ DRIFTS study of toluene hydrogenation
2 3 2
The Pt/Al O sample was exposed to toluene and H at 75 °C,
weak IR bands at 2120 cm 1 and at ca. 2060 cm , but these bands
ꢁ
ꢁ1
following the reduction at 350 °C. New strong bands were ob-
are almost impossible to observe when carbonyls are also present
44,45]. Bands at 2090 cm are commonly assigned to CO ad-
ꢁ1
served, in particular in the 3110–2700 cm region, corresponding
ꢁ1
[
to the stretching vibration of the CH bonds of adsorbed toluene
Fig. 3, middle spectrum). A difference spectrum between the tolu-
ene-containing and toluene-free cases is given in Fig. 4 for the sake
sorbed on highly coordinated Pt atoms of low index planes. It must
be stressed that the dispersion of the Pt was 54% and therefore the
average crystallite size is expected to be around 2 nm. The gradual
apparition of 2090 cm band would, at first, suggest the sintering
of some of the small Pt particles to form larger less defective
crystals. The occurrence of sintering rationalizes the fact that the
(
ꢁ1
Abs.
ꢁ1
carbonyl associated with the defective sites (2020 cm
would be vanishing while others associated with low index planes
band)
0.3
ꢁ1
(
2090 cm
band) would be formed. The former sites typically
exhibit much larger CO adsorption enthalpies and are therefore
expected to be preferentially covered, when both sites are present.
However, the observations described in the following paragraph
question this sintering hypothesis.
The sample obtained after H reduction at 350 °C was subsequently
2
exposed to 800 ppm of CO at 75 °C for a few minutes leading to
surface saturation with carbonyl species (Fig. 2B). The band at ca.
ꢁ
ꢁ
1
2
020–2029 cm grew markedly in intensity, also blue shifting
1
3730
up to 2049 cm at saturation. Such blue shift is expected when
carbonyl surface concentration increases, because of increased
dipole–dipole interactions and a decrease in the average Pt electron
back-donation [42]. Bridged carbonyls were also observed below
3500
3000
2500
2000
1500
ꢁ1
1
2
900 cm . On the contrary, no intensity change was observed at
090 cm , indicating that the corresponding sites were already
Wavenumbers /cm-1
ꢁ1
Fig. 3. In situ DRIFTS spectra of the Pt/Al
under 40% H (bottom), subsequently exposed to the toluene/H
then 40 s following the addition of 40% CO in the feed (top). T = 75 °C, the spectrum
recorded over a mirror was used as background in all cases. The spectra were offset
for the sake of clarity. The contribution of gas-phase species was subtracted.
2
O
3
catalyst: after reduction at 350 °C
at saturation before exposure to the 800 ppm of CO. The spectrum
obtained under O is also shown, showing that all these bands
were removed by oxidation at 75 °C (Fig. 2.B). Incidentally, these
2
2
feed (middle), and
2
2
data indicate that the proportion of CO adsorption sites that were

1
58
A.A. Taimoor et al. / Journal of Catalysis 278 (2011) 153–161
Abs.
The carbonyl bands were still present following toluene
adsorption, although those were modified (mostly shifted) when
compared to the case of the sample under H after reduction (Figs. 3
3
648
1496
0
.05
2
and 4). A zoom-in of the carbonyl region is also given in Fig. 5
for the sake of clarity. The modification of the Pt-bound carbonyl
signal clearly indicates that the hydrocarbon adsorbed also on
the Pt. The two main carbonyl bands shifted towards lower wave-
number upon toluene introduction. The shifts were reversible
when toluene was removed, although the intensity of the
3
027
3059
1603
2
923
2
877
CO2(g)
2
1960
1872
2
740
1453
ꢁ
1
2
020 cm band was significantly decreased (data not shown). This
097
2
030
shows that toluene competed with CO for adsorption on the metal,
particularly for the sites associated with the carbonyl band at
3
741
ꢁ
1
2
020 cm . The carbonyl red shift observed in the presence of
3
500
3000
2500
2000
1500
toluene was most likely due to an increased back-donation of Pt
Wavenumbers /cm-1
ꢃ
electronic density into the
p
anti-bonding orbital of CO, due to
Fig. 4. DRIFTS spectrum of Pt/Alumina after 30 min under H
spectrum of the activated sample was recorded at the same temperature under Ar/
and used as background. The contribution of gas-phase species was subtracted.
The observed CO (g) was ambient CO , not in the system.
2
/toluene. T = 75 °C, the
an increased electronic density of the Pt particles following the
adsorption of the electron-rich toluene.
H
2
2
2
3.5. Operando DRIFTS study of the effect of CO
2
ꢁ
1
of clarity. The negative bands at 3741 cm indicated that some of
the alumina surface hydroxyl groups were in interaction with tol-
uene molecules, resulting in the broad H-bonded hydroxyl band at
The effects of the addition of CO into the toluene/H feed on
2
2
the catalyst surface were studied by operando DRIFTS, while the
toluene conversion to methylcyclohexane was monitored using
the FTIR gas-cell located at the DRIFTS cell exit. The evolution of
the conversion of toluene is shown in Fig. 6 over various periods
with different feed compositions.
ꢁ1
3
648 cm (Fig. 4). Toluene was therefore adsorbed in significant
quantities on the support. All the other bands indexed in Fig. 4
except for those around 2000 cm ) can be assigned to adsorbed
ꢁ1
(
toluene, the spectrum of which is similar – but not identical – to
that of the gas-phase molecule [51].
An apparent slow deactivation was first noted under the
CO -free feed, that is for reaction times t < 0 s. The origin of this
2
Gas-phase methylcyclohexane [52] exhibits two major bands at
930 and 2860 cm , which overlap with those of toluene, so it is
deactivation is unclear. A minor increase (around noise level) in the
DRIFTS carbonyl signal suggests that this slow deactivation could
be due to the incomplete reduction of the sample (e.g. alumina-
supported carbonates still being reduced) or traces of CO or CO
2
contaminant in the system. The deactivation actually slowed down
on a later reaction test (1800 s < t < 7300 s), supporting the view
2
that the cause for the deactivation under CO -free conditions was
transient in nature. The subsequent data will suggest that any CO
contamination level was well below 3 ppm (vide infra).
ꢁ1
2
difficult to assess whether the bands of this compound were also
present on the spectrum reported in Fig. 4. The adsorption enthal-
py of methylcyclohexane on the support but also on Pt is expected
to be much lower than that of toluene [53]. By comparison, on
ꢁ1
Pt(1 1 1), the adsorption enthalpy of benzene is 71 kJ mol , while
ꢁ1
that of cyclohexane is only 27 kJ mol [14]. It is therefore ex-
pected that methylcyclohexane desorption would be extremely
fast once formed and the corresponding steady-state coverage on
the catalyst should be negligible.
The toluene conversion dropped dramatically as soon as 40%
CO was added to the feed at t = 0 s, reaching the residual level
2
of activity of the cell after about 1300 s. The DRIFTS spectrum
exhibited marked changes already 40 s after the introduction of
CO
2
(Figs. 3 and 5). New large bands appeared around 1650 and
ꢁ
1
1
431 cm , which could be indicative of the presence of water,
carbonate and/or hydrogenocarbonate species. The carbonyl band
signal also increased, with the most prominent band absorbing
Abs.
Toluene Conversion /%
0.04
o
O
2
then H
2
at 75
C
+
40%
2
+2%
CO
CO
2
1
0
5
0
2200
2100
2000
1900
1800
-2000
0
2000
4000
Time on stream /s
6000
8000
10000
Wavenumbers /cm-1
Fig. 5. Magnification of the carbonyl stretching region related to Fig. 3. In situ
DRIFTS spectra of the Pt/Al catalyst: after reduction at 350 °C under 40% H
bottom), subsequently exposed to the toluene/H feed (middle), and then 40 s
following the addition of 40% CO in the feed (top). T = 75 °C, the spectrum recorded
over a mirror was used as background in all cases.
2 3 2
Fig. 6. Toluene conversion over the Pt/Al O . The concentration of added CO is
indicated on the plot for the periods during which this molecule was added to the
feed. The conversion due to the DRIFTS cell reactor filled only with SiC is indicated
by the dotted lines. Feed: 1.6% toluene + 40% H in Ar. Total pressure = 1 bar, W/
2
F = 1.04 kgcat s/mol.
2
O
3
2
(
2
2

A.A. Taimoor et al. / Journal of Catalysis 278 (2011) 153–161
159
Abs.
.04
1652
0
1431
2
024
1
394
1593
1
367
Time
under
1229
CO
2
2
008
2200
2000
1800
1600
1400
1200
Wavenumbers /cm-1
2 3 2 2
Fig. 7. Operando DRIFTS spectra of the Pt/Al O under a toluene/H feed for various times on stream with additional 40% CO : 81, 176, 273, 470, 847, 1244 and 3645 s.
T = 75 °C, the spectrum of the sample recorded at the same temperature under toluene/H was used as background.
2
1
1
0
9
8
7
6
5
4
0
0
0
0
.8
.6
.4
.2
0
0
500
1000
1500
Time on stream /s
0
5000
10000
Fig. 8. Correlation of the relative loss of toluene conversion ( ) with the relative
increase in intensity of the species seen by DRIFTS over the Pt/Al following the
introduction of 40% CO in the toluene/H feed: carbonyl (d), formate (N),
hydrogenocarbonates (4) and all the bands in the region 1800–1550 cm (s).
Pt-carbonyl band area /a.u.
2 3
O
2
2
Fig. 10. Correlation between the toluene conversion and the DRIFTS band area of
Pt-carbonyl species formed under 2% (s) and 40% (d) CO in the toluene/H feed.
ꢁ
1
2
2
1
1
3
1
9
7
5
3
signal comes from species adsorbed on the alumina, this stresses
that there was essentially no competitive adsorption between tol-
uene and CO -derived species formed on the support under our
2
experimental conditions.
2
Difference spectra were calculated for the CO -containing data
by subtracting the spectrum collected just before the introduction
of CO (this is equivalent to taking the sample under toluene/H as
background). This allowed observing more easily the modifications
occurring upon CO introduction (Fig. 7). The structure of the new
2
2
2
growing carbonyl band appeared more clearly as essentially a
single peak, which was growing with time and shifting towards
ꢁ1
higher wavenumbers from 2008 to 2024 cm as the concentration
of Pt–CO increased. The blue shifting of this band was probably
related to the increasing surface concentration of carbonyl through
0
500
1000
1500
Time on stream /s
(
i) the decrease in the average electronic density of Pt available for
ꢃ
back-donation into the carbonyl
p
anti-bonding orbital, (ii) the
Fig. 9. Effect of the CO
The conversion due to the DRIFTS cell reactor filled only with SiC is indicated by the
dotted line. Feed: 1.6% toluene + 40% H + 2 (s) or 40 (d) % CO in Ar. Total
2 2
pressure = 1 bar, W/F = 1.04 kgcat s/mol.
2 2 3
concentration on the toluene conversion over the Pt/Al O .
increase in CO(ads) dipole–dipole interactions [42,54] and (iii)
the likely displacement of some or all the (electron-rich) toluene
adsorbed on the metal.
The difference spectra (Fig. 7) made more obvious the
1229 cm band, which is assigned to the bending mode of the
ꢁ
1
ꢁ1
now at 1997 cm (Fig. 5). Interestingly, the surface concentration
of toluene did not appear to change, as shown by the constant area
hydroxyl group of the hydrogenocarbonate species (the other
ꢁ1
ꢁ1
of the band at 3030 cm (Fig. 3). Since most of the toluene DRIFTS
corresponding bands being at 1652 and 1431 cm ) [55]. The

1
60
A.A. Taimoor et al. / Journal of Catalysis 278 (2011) 153–161
hydrogenocarbonate species therefore appeared to be the main
surface species formed under CO and was probably formed from
the reaction of CO and an alumina surface hydroxyl group [56].
Additional bands with a positive growth were observed at 1589,
The true level of CO(g) is likely to be lower, because CO desorption
might become insignificant at low temperatures. In conclusion,
the concentration level of CO that was possibly present and
deactivated the catalyst under the 2% CO -containing feed was
2
likely to be less than 3 ppm. This also shows that the level of CO
impurity in our system was far less than 3 ppm of CO, since the
2
2
ꢁ
1
1
394 and 1369 cm , which corresponds to formate species, prob-
ably formed by the reaction of CO with alumina surface hydroxyl
groups [56]. It is likely that there were also other carbonate-type
species and water present, the signal of which would overlap with
the major bands already discussed.
deactivation rate measured in the absence of CO
2
was much slower
than that observed under 2% CO
2
(Fig. 6, t > 2000 s).
The possible link between the decrease in the toluene conver-
sion and the various surface species observed by DRIFTS was
assessed by plotting the relative variation of the area of the
corresponding signals (Fig. 8). The species correlating best with
the loss of conversion appeared to be the newly formed carbonyl,
followed by the formates. The hydrogenocarbonate signal was
not correlated with the decrease in the reaction product signal
and can be discarded as the main origin of the deactivation. It is
interesting to note that full deactivation appeared to be reached
before the saturation of the Pt sites with carbonyl (Fig. 8). This
could be rationalized by the fact that an ensemble of sites is actu-
ally needed to obtain toluene conversion, e.g. requiring adsorption
3.6. Discussion
2 3
Our Pt/Al O catalyst was active for the hydrogenation of tolu-
ene at 75 °C. It is interesting to note that our pre-reduction proce-
dure led to the formation of Pt-carbonyl species (Figs. 2, 3 and 5),
derived from the reduction of the carbonate-type species initially
present on the sample (Fig. S2). The carbonyl band was broad
and resulted from the overlap of several contributions spanning
ꢁ
1
from 2100 to 1900 cm
.
This stresses that different reduction temperatures, duration
and gas flow rate could lead to different residual coverage of Pt
with CO and potentially different extent of Pt poisoning. This
observation may explain some of the discrepancies reported in
the literature regarding the hydrogenation activity patterns of
aromatics over metals [58]. The temperature at which the
hydrogenation is conducted used may also lead to different levels
of CO removal from the Pt, hence complicating the determination
of accurate TOF values.
2
sites for both H and toluene. The number of available sites in a do-
main has been suggested to be crucial in determining selective
hydrogenation pathways over Pd-based catalysts [57].
Similar conclusions on the deactivating role of CO
in the case of exposing the sample to a much lower concentration
of CO , i.e. 2%. The corresponding toluene conversion data are
reported in Fig. 6, for t > 7300 s. The conversion decay appeared
to be slower when compared to the case of the 40% CO , as con-
2
were obtained
2
Operando FTIR work by Ferri et al. carried out in supercritical
CO in the presence of H over a model Pt/Al O catalyst [59]
2 2 2 3
2
firmed by the data replotted in Fig. 9. The growth of the carbonyl
and hydrogenocarbonate species was also slower in the case of
showed that the presence of large concentration of carbonates on
the support surface resulted in the formation of atypical carbonyl
species that could not be formed by simple exposure to CO. These
the feed with the least CO
that the formate signal was negligible in the case of the deactiva-
tion occurring under the feed containing the 2% CO , stressing that
formates were not the origin of the loss of activity.
2
(Figs. S5 and S6). It must be stressed
ꢁ1
unusual carbonyl exhibited bands at ca. 1965 and 1756 cm and
were assigned to linear and bridged carbonyl adsorbed at Pt-alu-
2
ꢁ1
mina interfacial sites. The band observed here at 1976 cm (Figs. 3
and 5) in the presence of toluene and hydrogen could possibly be
ꢁ1
Therefore, the carbonyls formed at ca. 2000 cm were the only
observable surface species that could be linked to the decreasing
activity in toluene hydrogenation. The relation between catalyst
activity and the area of the DRIFTS band of the carbonyls is plotted
ꢁ1
related to the 1965 cm band observed by Ferri et al. [59] on their
catalyst. However, upon exposure to CO (Fig. 5) or even CO (Fig. 2),
this carbonyl band appeared to shift from ca. 1976 up to 2049 cm
2
ꢁ
1
in Fig. 10 for both CO
an unequivocal relation between the carbonyl band intensity and
the level of conversion remaining, independently of the CO partial
2
concentrations. It is striking that there was
at saturation. It is therefore reasonable in our case to assign this
band to Pt sites that are not interfacial with alumina.
2
Our data clearly showed that H
2
and toluene adsorbed on some
pressure. These data therefore prove that the poisoning of Pt by
strongly bound carbonyl was the main reason for the loss of tolu-
Pt sites in the absence of CO , because of the shift that these spe-
cies produced on the residual carbonyl signal (Fig. S4 and Figs. 4
and 5). We have yet no indication whether the poisoning by car-
2
ene hydrogenation activity in the presence of CO
It must be noted that the catalyst could be fully regenerated
in situ at 75 °C by passing an O feed for a few seconds (Fig. 6). This
2
.
bonyl inhibited the adsorption of toluene or H
CO is a well-known poison of Pt sites needed for H
in the case of the Pt-based catalysts used in H -fuel cells, H
2
2
or both. While
chemisorption
can yet
2
2
is clearly not sufficient for improving the applicability of the
present material and clearly other formulations will have to be
developed to provide stable conversions.
It is not clear whether any gas-phase CO was actually produced
under our reaction conditions. It is possible that a minute amount
2
compete better for adsorption with CO whenever the concentra-
tion of the latter is very low [60], as it is the case for our data
([CO] expected to be lower than 3 ppm in the case of the 2%
2 2
CO -containing feed). It is therefore reasonable to suggest that H
of CO
2
would decompose to CO(ads) and O(ads) over the sample,
activation was not significantly hindered in the present case and
that the loss of activity was solely due to the inhibition of toluene
adsorption on the metal. In support to this view, Rioux et al. [61]
showed that CO was in competition for adsorption with ethylene
with no CO ever leaving the catalyst surface (the O can be scav-
enged by hydrogen to form water). To investigate this matter,
the rate of the reverse water–gas shift (RWGS) reaction was mea-
sured over our DRIFTS cell. No CO could be observed below 200 °C
and not H
over Pt nanoparticles. More work would yet be required to unam-
biguously confirm that H adsorption was not hindered in our case.
Our Pt-alumina would clearly not be of any interest for the
hydrogenation of many aromatic hydrocarbons using a H + CO
mixtures. Other catalytic formulations could surely lead to
improved stability against CO poisoning, either because CO disso-
2
in the case of the CO-modified ethylene hydrogenation
(
our CO(g) detection limit was about 200 ppm). Significant concen-
tration of CO and methane was obtained above 250 °C (Fig. S7) and
an arrhenius-type plot was constructed using these data points
2
(Fig. S8) to estimate back the theoretical level of CO that could
2
2
be formed at 75 °C, if the linear relationship were to hold. A level
of ca. 3 ppm of CO could thus be expected from a RWGS feed with
2
concentrations of H
were similar to that used the toluene hydrogenation experiment
described in Fig. 6 for t > 7300 s.
2
and CO
2
of 50% and 2%, respectively, which
ciation would be hindered under relevant experimental conditions
or the sample would not be poisoned by CO. Metals such as Ru and
Ni supported on alumina did not allow observing any carbonyl

A.A. Taimoor et al. / Journal of Catalysis 278 (2011) 153–161
161
species suggesting that these catalysts may be better suited than
Pt-based catalysts for hydrogenation in supercritical CO [20]. Also,
rhodium supported on carbon support was shown to be less en-
cline to CO poisoning than an alumina-supported Rh, as showed
by Fujita et al. [21].
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[
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. Conclusions
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and operando DRIFTS spectroscopy in investigating the mechanis-
tic features of toluene hydrogenation over a Pt-alumina catalyst.
The following main conclusions can be drawn:
[
(
[
20] V. Arunajatesan, B. Subramaniam, K.W. Hutchenson, F.E. Herkes, Chem. Eng.
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1
. Some Pt sites were permanently covered with carbonyls
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poisoned sites may vary depending on sample pre-treatment
and operating conditions, which may affect significantly the
determination of accurate TOF values.
[
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2
3
. Toluene adsorbed both on the metal and on the alumina under
our experimental conditions.
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[
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2 3
of our Pt/Al O . An unequivocal relation was obtained between
the level of toluene conversion and the Pt-carbonyl band
intensity.
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