Pulse-response TAP studies of the reverse water-gas shift reaction over a Pt/CeO2 catalyst

Article abstract of DOI:10.1016/j.jcat.2005.10.020

The temporal analysis of products (TAP) technique was successfully applied for the first time to investigate the reverse water-gas shift (RWGS) reaction over a 2% Pt/CeO₂ catalyst. The adsorption/desorption rate constants for CO₂ and H₂ were determined in separate TAP pulse-response experiments, and the number of H-containing exchangeable species was determined using D₂ multipulse TAP experiments. This number is similar to the amount of active sites observed in previous SSITKA experiments. The CO production in the RWGS reaction was studied in a TAP experiment using separate (sequential) and simultaneous pulsing of CO₂ and H ₂. A small yield of CO was observed when CO₂ was pulsed alone over the reduced catalyst, whereas a much higher CO yield was observed when CO₂ and H₂ were pulsed consecutively. The maximum CO yield was observed when the CO₂ pulse was followed by a H₂ pulse with only a short (1 s) delay. Based on these findings, we conclude that an associative reaction mechanism dominates the RWGS reaction under these experimental conditions. The rate constants for several elementary steps can be determined from the TAP data. In addition, using a difference in the time scale of the separate reaction steps identified in the TAP experiments, it is possible to distinguish a number of possible reaction pathways.

Full text of DOI:10.1016/j.jcat.2005.10.020

Journal of Catalysis 237 (2006) 102–110
www.elsevier.com/locate/jcat
Pulse-response TAP studies of the reverse water–gas shift reaction
over a Pt/CeO2 catalyst
A. Goguet a, S.O. Shekhtman a, R. Burch a, C. Hardacre a,∗, F.C. Meunier a, G.S. Yablonsky b
a
CenTACat, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AG, Northern Ireland, UK
b
Washington University, Department of Chemical Engineering, St Louis, MO 63130, USA
Received 12 September 2005; revised 14 October 2005; accepted 17 October 2005
Available online 28 November 2005
Abstract
The temporal analysis of products (TAP) technique was successfully applied for the first time to investigate the reverse water–gas shift (RWGS)
reaction over a 2% Pt/CeO catalyst. The adsorption/desorption rate constants for CO and H were determined in separate TAP pulse-response
2
2
2
experiments, and the number of H-containing exchangeable species was determined using D multipulse TAP experiments. This number is
2
similar to the amount of active sites observed in previous SSITKA experiments. The CO production in the RWGS reaction was studied in a
TAP experiment using separate (sequential) and simultaneous pulsing of CO and H . A small yield of CO was observed when CO was pulsed
2
2
2
alone over the reduced catalyst, whereas a much higher CO yield was observed when CO and H were pulsed consecutively. The maximum
2
2
CO yield was observed when the CO pulse was followed by a H pulse with only a short (1 s) delay. Based on these findings, we conclude that
2
2
an associative reaction mechanism dominates the RWGS reaction under these experimental conditions. The rate constants for several elementary
steps can be determined from the TAP data. In addition, using a difference in the time scale of the separate reaction steps identified in the TAP
experiments, it is possible to distinguish a number of possible reaction pathways.
2005 Elsevier Inc. All rights reserved.
Keywords: Temporal analysis of products; Ceria; Water–gas shift; Pt
1. Introduction
tracted much attention, with ceria-supported catalysts in partic-
ular showing similar or higher activity than the more conven-
tional Cu/ZnO/Al2O3 materials under some reaction conditions
[1–19]. However, the activity and stability of ceria-based for-
mulations depends markedly on the preparation method used
[8,20–24]. Therefore, there is a need for a better understanding
of the reaction mechanism over these types of catalysts, to en-
able further improvement in their formulation and preparation.
To date, two main reaction mechanisms have been proposed:
a redox process (Scheme 1) and an associative “formate” mech-
anism (Scheme 2). In the redox mechanism, CO adsorbs on re-
duced metal sites and reacts with an oxygen atom coming from
The water–gas shift (WGS) reaction is a key step in ap-
plications such as H2 production via reforming processes for
fuel cell applications, particularly for mobile sources. In such
applications, a successful low-temperature WGS catalyst must
retain high activity and structural stability over a wide range of
reaction conditions, including both gas composition and tem-
perature. To date, although a number of highly active cata-
lysts have been reported, the catalysts often deactivate with
temperature, in particular. The importance of this process is
exemplified by the numerous research programs aimed at dis-
covering more stable high-activity catalysts. In this regard, the
activity of platinum group metals (PGM) supported on rare
earth oxides as catalysts to promote the WGS reaction has at-
Pt + CO ↔ Pt–CO,
(I)
(II)
Pt–CO + 2CeO ↔ CO + Ce O + Pt,
2
2
2
3
H O + Ce O ↔ 2CeO + H .
(III)
2
2
3
2
2
*
Corresponding author. Fax: +44 (0) 28 9097 4687.
E-mail address: c.hardacre@qub.ac.uk (C. Hardacre).
Scheme 1.
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2005.10.020
2005 Elsevier Inc. All rights reserved.

A. Goguet et al. / Journal of Catalysis 237 (2006) 102–110
103
CO + σ ↔ CO
,
(I)
ad
fraction of the support close to the metal particles could be in a
partially reduced state and would correspond to the active part
of the support.
H O + 2σ ↔ H + OH
,
ad
(II)
(III)
(IV)
(V)
2
ad
CO + OH ↔ HCOO + σ,
ad
ad
ad
The reaction mechanism is further complicated by the fact
that the surface composition, and thus the main reaction steps,
can dramatically depend on the experimental procedure used
during the reaction. Based on DRIFTS analysis combined with
the use of isotopic tracers, formates were found to be less labile
than carbonyl and carbonate species under steady-state reaction
conditions over a 2% Pt/CeO2 catalyst, whereas the carbon-
ates were found to be most stable during the desorption-type
non-steady-state experiments carried out in an inert purge gas
[39]. Furthermore, combining mass spectrometry and DRIFTS
analysis during SSITKA experiments under operating condi-
tions over a Pt/CeO2 catalyst showed that it was possible to
discard formate species as a major reaction intermediate under
RWGS reaction conditions [40,41]. Quantification of the num-
ber of active sites showed that the Pt-carbonyl species could not
be the only reaction intermediates and that for this catalyst un-
der these conditions, the RWGS reaction proceeds via surface
carbonate intermediates, possibly followed by the formation of
Pt-bound carbonyls. In this reaction mechanism, the direct re-
oxidation of the support by the CO2 is proposed, and the active
sites involved in this reaction step are distributed as a circular
zone around the Pt particles.
Although much less studied than steady-state reaction data,
non-steady-state kinetic information has proven to be a power-
ful tool in revealing the detailed mechanism and roles of differ-
ent surface intermediates for a wide range of catalytic reactions
including the WGS process [10,13]. For example, Gorte and
Zhao used normal pressure-pulse studies to observe the reaction
steps in the proposed redox mechanism for WGS over Pd/CeO2
catalyst [25]. Therein, the catalyst was repeatedly exposed to
two 250-s CO pulses followed by two H2O pulses. Both the ini-
tial pulses of CO and H2O led to production CO2 and H2, with
the H2 thought to originate from the decomposition of surface
hydroxyls and the CO2 from removal of surface carbonates.
Consequently, reactions between the pulsed gas reactants and
surface intermediates must occur to explain these non-steady-
state kinetic data. Although this study provides insight into the
reaction mechanism, at normal pressures the surface composi-
tion is not generally well defined, and it changes significantly
during each pulse. This is clearly shown by the large change
in response to CO and H2O after the first and second pulses of
each gas. This changing surface state hinders interpretation and
quantification of the non-steady-state normal pressure data.
Over the last two decades, the temporal analysis of prod-
ucts (TAP) technique, developed by Gleaves and co-workers
[42,43], has been successfully applied to non-steady-state ki-
netic characterization of model and multicomponent industrial
catalysts in many areas of chemical kinetics and engineering
[44]. The technique provides an opportunity to control the cata-
lyst composition so that it does not change significantly during
a single-pulse experiment, but can be changed in a controlled
manner in a long trail of low-intensity pulses [45].
HCOO + σ ↔ CO
+ H
,
ad
ad
2,ad
CO
↔ CO + σ,
2,ad
2
2H ↔ H + 2σ.
(VI)
ad
2
Scheme 2.
the ceria to form CO2. The reduced ceria is subsequently reoxi-
dized by water, releasing hydrogen [8,10,20,21,25]. In the asso-
ciative mechanism, the main reaction intermediate is a bidentate
formate, produced by reaction of CO with terminal hydroxyl
groups on the oxide support, which decomposes to form H2
and a monodentate carbonate [3,26–31]. In Scheme 2, σ refers
to sites on the oxide support.
Along with whether a redox or an associative reaction mech-
anism is responsible for the WGS activity over oxide-supported
PGM catalysts, the role of the PGM in the surface reaction
has also been addressed. Flytzani–Stephanopoulos and cowork-
ers recently reported that on removal of all transmission elec-
tron microscopy (TEM)-observable PGM particles, the activity
of ceria-supported gold and platinum catalysts remained un-
changed compared with that of the as-prepared catalysts [32–
34]. These authors proposed that active metal, in the form of
cations, diffuses into the ceria and is anchored at surface de-
fects. These ions increase the reducibility of the support, with
any remaining zerovalent metal considered unimportant in the
WGS reaction. This mechanism is a modification of the normal-
support activation (NSA) effect summarized by Golunski et al.
[35] and originally proposed by Frost [36] for methanol syn-
thesis catalysts. This junction effect modification between the
PGM and oxide support leads to a dramatic increase in the num-
ber of oxygen vacancies within the support, which become the
active sites—a reversal of the conventional roles of the oxide
and the PGM.
This may be contrasted with a recent in situ EXAFS and
DFT study on similar catalysts demonstrating that the state
of the catalyst under reaction conditions may be significantly
different than that of the prepared catalyst [37]. Although
Au/CeZrO4 catalysts exhibit cationic gold in the fresh cata-
lyst, under mild WGS reaction conditions, rapid reduction of
the gold and agglomeration into 1- to 2-nm particles occurred.
The EXAFS and XANES experimental results combined with
DFT calculations point to a model in which a metallic gold clus-
ter containing about 50 atoms is in intimate contact with the
oxide support to the extent that up to 15% of the gold “atoms”
at the interface with the support may be located at cation va-
cancies. Such gold atoms would be expected to carry a small
positive charge (Auδ+), which may be very important in creat-
ing the active site for the WGS reaction. However, it is clear
that for these catalysts, metallic gold particles, not isolated gold
ions (e.g., Au3+), are present under reaction conditions [37].
Similarly, there is strong evidence for involvement of the sup-
port in the RWGS reaction mechanism over Pt/CeO2 catalysts
[38]. It has been proposed that during the reaction, a limited
This paper demonstrates for the first time that the TAP tech-
nique can be applied to the RWGS reaction to provide unique

104
A. Goguet et al. / Journal of Catalysis 237 (2006) 102–110
information on the nature and reactivity of key surface inter-
mediates that may be relevant to the reaction mechanism for
a 2% Pt/CeO2 catalyst. The reaction between the specific gas
reactants and a catalyst that has been conditioned to have a
particular surface composition was measured to identify the
separate elementary steps of the reaction mechanism.
3. Vacuum pulse-response TAP experiments using (i) single
reactant pulses, (ii) simultaneously pulsing two reagents,
or (iii) pulsing two reagents with a time delay.
The exit flow-rate time dependence for reactants and prod-
ucts was extracted from the mass spectrometer data using stan-
dard fragmentation patterns and sensitivity factors normalized
to the inert (argon).
2. Experimental
In the Knudsen diffusion flow regime, the product pulse
shape is independent of the pulse intensity [42]. Therefore, to
maintain the Knudsen conditions and to avoid altering the cata-
lyst state on pulsing, the number of reactant molecules in each
TAP pulse was only ∼7 × 10−9 mol. This may be compared
with an estimate of 3 × 10−6 mol for the amount of active in-
termediates or oxygen on the catalyst surface from the CO/CO2
released by the sample in a SSITKA experiment [40]. Thus,
a large number of reactant pulses are required to change the
catalyst composition significantly.
The catalyst used in this study was a 2% Pt/CeO2 provided
by Johnson Matthey. The specific surface area was measured
by BET (Micromeretics ASAP 2010) and was found to be
180 m2 g−1. The Pt dispersion was measured by H2 chemisorp-
tion (Micromeretics Autochem 2910) and was found to be
17%. Note that the dispersion measurements were performed
at −80 ◦C to minimize the potential spillover of H2 onto the
support [46]. For all of the experiments described below, the
gases (H2, Ar, CO, and CO2) were of >99.9% purity, supplied
by BOC. Before each reaction, the catalyst was activated under
flowing 20% oxygen in nitrogen for 1 h, followed by reduc-
tion in pure hydrogen for 1 h at 300 ◦C and 1 atm pressure. In
addition, CO2 pretreatment after the oxidation-reduction cycle
was performed at 300 ◦C and 1 atm pressure using flowing pure
CO2 for 1 h.
The TAP pulse-response experiments were performed in a
TAP-I reactor (Autoclave Engineers) using a stainless steel mi-
croreactor (41 mm long, 5.5 mm i.d.). In all experiments, the
thin-zone TAP reactor (TZTR) concept developed by Shekht-
man and Yablonsky was used, where uniformity of gas and
surface concentration in the catalyst zone is achieved by mak-
ing the active zone thin compared with the length of the reactor
[47]. In the TZTR, diffusion dampens the nonuniformity in the
concentration created by reaction to 15–20% over a wide do-
main of conversions (up to 80%) [48–50]. Another key feature
of the TZTR is that the gas concentration in the catalyst zone
can be approximated as being proportional to the observed re-
sponses, which allows the correlation of the product response
with those associated with the reactants. The details of this ap-
proximation will be reported elsewhere [51].
The TZTR was packed with two 17-mm-long beds of silicon
carbide sandwiching 1 mm of catalyst. All particles were 250–
450 µm in size. The temperature of the reactor was measured
by a thermocouple positioned in the center of the catalyst bed.
Reactants and products were recorded at the reactor outlet by a
UTI 100C quadrupole mass spectrometer. The temperature was
kept constant during the normal pressure treatment and vacuum
pulse response experiments at 300 ◦C. The response collection
time was 15 s. In all experiments, reactant pulses were diluted
by argon in a ratio of 1:1, and masses characteristic of all re-
actants and possible products (CO, CO2, H2, CH4, and H2O)
were followed. The following procedure was used for all exper-
iments:
3. Results and discussion
3.1. CO2 adsorption
The interaction of CO2 with the catalysts as a function of the
pretreatment was studied by consecutive pulsing of CO2 over
the Pt/CeO2 catalyst. The following observations were made
(Fig. 1):
1. Over the reduced and CO2 pretreated catalyst, fast re-
versible adsorption mainly on ceria sites was found, result-
ing in broad peak responses. Fig. 1 shows a typical CO2
response.
2. Very small amounts of CO were produced.
3. Not all injected CO2 molecules were observed at the outlet
of the reactor in the single-pulse experiment.
4. No significant change of CO2 responses was observed
when the pretreated catalysts were exposed to a large num-
ber of CO2 pulses (up to 10,000) in a multipulse experi-
ment, irrespective of the pretreatment.
Observation 3 indicates that CO2 forms surface intermedi-
ates that do not decompose within the time scale of the single-
pulse experiment, which is 15 s. It is clear from observation 4
1. Pretreatment of the catalyst by exposing to different gases
at 1 atm pressure.
◦
2. Evacuation of the reactor to 10−6 Torr.
2
2
Fig. 1. A typical broad CO response over the 2% Pt/CeO catalyst at 300 C.

A. Goguet et al. / Journal of Catalysis 237 (2006) 102–110
105
(a)
◦
Fig. 2. A typical H response over the 2% Pt/CeO catalyst at 300 C. The
2
2
broad tail is plotted on a magnified scale.
that these intermediates are desorbed, and hence they do not
accumulate on the catalyst. It was not possible to completely
saturate the surface even after thousands of CO2 pulses over
hours. Given the relative time scales for the TAP experiments
and the delay between pulses, it can be concluded that these sur-
face intermediates decompose on the minute time scale, that is,
faster than the timescale of the multipulse TAP experiments but
slower than the time scale of the single-pulse TAP experiment.
Such decomposition contributes to the baseline and cannot be
monitored as a pulse response.
Analysis of the reversible fast interaction of CO2 with the
catalyst from a single-pulse response TAP experiment shows
that it may be described in terms of apparent adsorption and
desorption rate constants, ka = 170 s−1 and kd = 1.3 s−1, re-
spectively.
(b)
Fig. 3. (a) H (P), DH (1) and D (E) response intensities versus pulse num-
2
2
ber and (b) D–H exchange rate constant versus the amount of D consumed
2
observed following deuterium pulsing over hydrogen treated 2% Pt/CeO cat-
2
alyst.
with the amount of H-containing species located on or around
the platinum particle. Remarkably, this number agrees very
well with the amount of exchangeable sites observed with a
SSITKA experiment using isotopically labeled carbon mole-
cules (2.2 × 10−4 molg−1) [40], which may indicate that the
active intermediate or transient state complex contains both car-
bon and hydrogen.
3.2. Hydrogen adsorption
The following were observed for H2 pulsed over the reduced
or CO2-treated Pt/CeO2 catalyst (see Fig. 2):
Fig. 3b shows how the rate constant for surface D–H ex-
change varies as a function of the amount of D2 exchanged.
A clear linear trend is found indicating a first-order kinetic de-
pendence with respect to H-containing active species.
1. Fast reversible adsorption on the catalyst, resulting in a
sharp peak (due to fast H2 diffusion) and a long tail, as
shown in Fig. 2.
2. Little change in the H2 responses as a function of pulse
number in multipulse experiments, indicating a reversible
interaction of H2 with the catalyst.
3.3. Pulsing of CO2 and H2
Over a preoxidized and then prereduced catalyst, 13CO2 and
H2 were pulsed simultaneously or with an increasing time de-
lay between the reactants; the latter sequence is illustrated in
Fig. 4. The results of these experiments compared with those
with 13CO2 and H2 pulsed separately are shown in Fig. 5. As
described above, when CO2 was pulsed alone, only a small
yield of CO (<0.5%) was observed. It should be noted that
this was only quantifiable using 13CO2, due to the increased
sensitivity of the 29-amu signal compared with the 28-amu sig-
nal, which is masked to a degree by the background gas signal.
As expected, no CO was observed on pulsing hydrogen alone.
However, when 13CO2 was repeatedly pulsed followed by the
hydrogen pulse with different time delays, (i.e., consecutive
pulsing), the observed CO yield was significantly higher and
decreased with increasing time delay. The CO response profiles
As was found for CO2, the reversible interaction of H2 with
the catalyst observed in a TAP experiment can be described in
terms of an apparent adsorption and desorption rate constant,
ka = 110 s−1 and kd = 0.9 s−1, respectively.
To probe the dissociative adsorption and desorption process
for the hydrogen, deuterium was pulsed over a hydrogen-treated
catalyst and the responses for D2, DH, and H2 were monitored.
As shown in Fig. 3a, the intensity of the D2 responses increases
with pulse number, whereas the intensity of the DH and H2
responses decrease indicating a decrease in the of amount of
surface hydrogen available for the D–H exchange with each
progressive pulse of D2. The total amount of D2 molecules
that underwent D–H exchange in this experiment was esti-
mated as 3 × 10−4 molgc−a1t . This number can be associated

106
A. Goguet et al. / Journal of Catalysis 237 (2006) 102–110
13
13
Fig. 4. CO , H and CO responses observed in the consecutive pulsing TAP experiment using a collection time of 15 s with a time delay of 2 s between the
2
2
13
CO and H pulses.
2
2
showed a sharp increase in CO production coincident with the
hydrogen pulse and superimposed on a broad CO peak with a
slowly decaying tail.
around 1 s to produce CO. In comparison, the sharp leading
edge of the second peak corresponds to a relatively fast reac-
tion between pulsed H2 and a catalyst surface with adsorbed
CO2 intermediates.
The broad CO2 responses indicate that CO2 was present in
the catalyst zone during most of the 15-s time interval, with
a maximum at approximately 0.13 s after the pulse. Accord-
ing to the hydrogen response, the concentration of hydrogen
in the catalyst zone sharply increased once it was pulsed, then
dropped quickly by more than an order of magnitude, as shown
in Fig. 4. No significant change in hydrogen response was ob-
served with time delay. According to the CO responses, the
amount of CO produced increases sharply after the hydrogen
response and decays slowly thereafter. This may indicate that
H2 quickly adsorbs and reacts with a surface intermediate that
then subsequently releases CO. This behavior is not observed
when 13CO2 and H2 are pulsed simultaneously, in which case
the CO response does not rise sharply but reaches a maximum
at 0.63 s after the pulses, later than the maximum reached by ei-
ther the H2 or CO2. This indicates that the CO2 reacts to form
an active surface intermediate on a time scale of seconds.
The CO responses may be deconvoluted into two peaks for
CO production. The position of the first peak, shown by the
arrow in Fig. 5b, does not change with the H2 pulse time delay,
with a maximum occurring after 0.6–1.0 s. A small decrease is
observed in the intensity of the peak with increasing time delay
of the H2. The sharp second peak closely follows the time of the
H2 pulse, the intensity of which monotonously decreases with
increasing time delay.
The constant position and broad front of the first peak are
consistent with a relatively slow reaction between CO2 pulsed
at t = 0 s and a catalyst surface with some coverage of ad-
sorbed hydrogen. The decreasing intensity with increasing time
delay of the H2 reflects the fact that the adsorbed hydrogen must
transform into an active species before it can react with the CO2
pulse. As this takes a finite amount of time, and an increasing
time delay between the CO2 pulse and the next H2 pulse re-
sults in a decreasing time delay between the H2 pulse and the
next CO2 pulse, the concentration of active hydrogen decreases
leading to the lower CO yield at t = 0 s. Remarkably, it takes
Preliminary CO multipulse experiments have shown that CO
desorption is fast, with a desorption rate constant kd ≈ 1.5 s−1
,
and hence the reactions are not CO desorption rate limited at
this temperature. The sharp increase in CO yield after the H2
pulse may reflect the absence of limiting steps within this path-
way. Thus, the rate of slow CO release must be limited by the
formation of the CO2-containing surface intermediate. The log-
arithmic plot of CO responses with respect to time shown in
Fig. 6 indicate that the decrease in CO production after each
of the two peaks is governed by a common exponential decay
constant, kexp = 0.2 s−1, which can be explained by decompo-
sition of the same intermediate, that is, whether the reaction is
initiated by the CO2 pulse or the H2 pulse. However, there is
a significant difference in the time for formation of this inter-
mediate and hence the broad and sharp response on CO2 and
H2 pulsing, respectively. This may be indicative of the higher
rate of surface diffusion and reduced adsorption strength of H2
compared with CO2 over the catalyst.
Fig. 7a shows the CO/CO2 response intensities normalized
to argon versus the H2 pulse time delay. The plot also shows
partial contributions to the CO response from the time intervals
after the hydrogen pulse. The intensity of the CO2 response
decreases with increasing time delay, reflecting the influence
of the H2 pulse on the CO2–catalyst interaction. In these vac-
uum pulse response TAP experiments, the observed CO yield is
<4%, and it exhibits a maximum at 1 s delay of the hydrogen
pulse, then decreases with increasing time delay. This decrease
is associated with the reduced CO production after the hydrogen
pulse and may be associated with the amount of intermediate
available for a fast reaction with adsorbed hydrogen. Fig. 7b
shows a logarithmic plot of the CO yield after the hydrogen
pulse, which exhibits an exponential decay with a rate constant
k ≈ 0.24 s−1. This decay can be attributed to decomposition
of the active CO2-containing intermediate with which adsorbed
H2 reacts. As the time delay for the hydrogen increases beyond

A. Goguet et al. / Journal of Catalysis 237 (2006) 102–110
107
(a)
(b)
Fig. 5. (a) CO and (b) CO responses observed when CO and H were pulsed alone or consecutively with different time delays for H , in each case the
13
13
13
2
2
2
2
13
13
13
CO is pulsed at 0 s and the H is pulsed after the corresponding time delay. The dotted lines define the base lines for the corresponding CO and CO
2
2
2
responses.
2 s, the surface concentration of this intermediate drops and
hence the CO yield decreases.
The fact that a small CO yield (<0.5%) is observed when
CO2 is pulsed alone over reduced catalyst compared with the
CO yield (up to 3.5%) in consecutive pulsing (Fig. 5b) clearly
proves that the presence of both reactants is critical for CO
production. Thus, under the TAP conditions used herein, an
associative mechanism is primarily operating for the RWGS re-

108
A. Goguet et al. / Journal of Catalysis 237 (2006) 102–110
13
13
Fig. 6. CO responses versus time observed when CO and H were pulsed consecutively with different H time delays plotted on the logarithmic scale. The
2
2
2
−0.2t
dashed line shows the exponential fitting of the decay curve, y = e
.
−1
−1
,
CO + σ ↔ CO
,
k = 170 s , k = 1.3 s
(I)
(II)
a
2
2,ad
d
−1
−1
H
+ 2Pt ↔ 2Pt–H,
k = 110 s , k = 0.9 s
,
a
2
d
−1
−1
,
CO
↔ I
,
not fast, k ≈ 0.2 s , k ≈ 0.2 s
(III)
(IV)
(V)
nterm
2,ad
f
b
−1
I
+ Pt–H ↔ Pt–CO + OH
,
fast, k ≈ 1.0 s [52],
k ≈ 1.5 s [52],
nterm
ad
b
−1
Pt–CO ↔ CO + Pt,
d
OH + Pt–H ↔ H O + Pt + σ.
(VI)
ad
2
Scheme 3.
ated primarily with the ceria support. Step (II) reflects the fast
adsorption/desorption observed for H2, most likely associated
with the metal sites. Step (III) describes the formation of a
specific C-containing intermediate, Interm, with which adsorbed
hydrogen can react. Comparing the calculated rate constants
for the processes, step (III) is clearly the rate-determining step
in this reaction mechanism. Because hydrogen adsorption oc-
curs on the platinum, it is likely that Interm is also located in the
vicinity of these particles due to the fast reaction occurring on
pulsing hydrogen, as shown in step (IV). Thus, step (III) may
also include diffusion of adsorbed CO2 to the active site for the
reaction. The forward rate constant in step (III) for the forma-
tion of Interm was estimated from the exponential decay curve
for CO production (Fig. 6). The reverse rate constant in step
(III) was estimated from Fig. 7b. Step (V) is associated with
desorption of CO from platinum [52]. Step (VI) is added to de-
scribe H2O production from platinum to complete the reaction
[53,54]. This mechanism does not include the second form of
adsorbed CO2 identified in the TAP experiments, because this
decays on a time scale that is too long to be part of this reaction
scheme.
(a)
(b)
13
13
Fig. 7. (a) Normalized response intensities for total CO ("), total CO×10
2
13
(E) production and CO production measured after hydrogen pulse ×10 (P)
13
versus the H pulse time delay and (b) CO yield after hydrogen pulse plotted
2
on a logarithmic scale versus the time delay.
action. Scheme 3 shows a tentative reaction mechanism for the
fast reaction observed under TAP conditions.
Step (I) reflects the fast adsorption/desorption observed for
CO2. Here σ is an adsorption site for CO2, probably associ-
Scheme 3 may be compared with Scheme 2 due to the fact
that they are both associative mechanisms. There are a number
of fundamental differences between the two schemes, however.
In Scheme 2, adsorbed hydrogen reacts with adsorbed CO2 to

A. Goguet et al. / Journal of Catalysis 237 (2006) 102–110
109
form the formate intermediate; in comparison, in Scheme 3 the
References
formation of the carbon-containing intermediate is slow and
does not require the presence of adsorbed hydrogen on the sur-
face. If adsorbed hydrogen were necessary, then a different de-
pendence on the H2 pulse time delay would be observed. There-
fore, it is likely that the intermediate formed is not a simple
formate species; it may be a surface carbonate. However, this
is only a possibility, because the form of this carbon-containing
intermediate cannot be directly identified from the TAP data.
Subsequently, the reaction of this carbon-containing intermedi-
ate with adsorbed hydrogen is very fast, as shown in step (IV)
of Scheme 3.
It should be noted that reaction Scheme 3 refers only to the
associative pathway that dominates the CO yield under the con-
ditions reported herein and does not exclude the high probabil-
ity of other reaction pathways existing under different process
conditions. The proposed mechanism provides a starting point
for evaluating the possible contributions of redox and associa-
tive mechanisms for the WGS and RWGS reactions in future
steady-state and transient studies taken over a range of temper-
atures and conditions.
[1] B.I. Whittington, C.J. Jiang, D.L. Trimm, Catal. Today 26 (1995) 41.
[2] J. Barbier Jr., D. Duprez, Appl. Catal. B 4 (1994) 105.
[3] T. Shido, Y. Iwasawa, J. Catal. 141 (1993) 71.
[4] J.T. Kummer, J. Phys. Chem. 90 (1986) 4747.
[5] E.C. Su, W.G. Rothschild, J. Catal. 99 (1986) 506.
[6] R.K. Hertz, J.A. Sell, J. Catal. 94 (1985) 166.
[7] K.C. Taylor, in: J.R. Anderson, M. Boudard (Eds.), Automobile Catalytic
Converters, in: Catalysis: Science and Technology, vol. 5, Springer, Berlin,
1984.
[8] T. Bunluesin, R.J. Gorte, G.W. Raham, Appl. Catal. B 15 (1998) 107.
[9] J.M. Zalc, V. Sokolvskii, D.G. Löffler, J. Catal. 206 (2002) 169.
[10] S. Hilaire, X. Wang, T. Luo, R.J. Gorte, J. Wagner, Appl. Catal. A 215
(2001) 271.
[11] X. Wang, R.J. Gorte, Catal. Lett. 73 (2001) 15.
[12] Q. Fu, A. Weber, M. Flytzani-Stephanopoulos, Catal. Lett. 77 (2001) 87.
[13] T. Luo, R.J. Gorte, Catal. Lett. 85 (3–4) (2003) 139.
[14] X. Wang, R.J. Gorte, J.P. Wagner, J. Catal. 212 (2002) 225.
[15] G. Jacobs, P.M. Patterson, L. Williams, E. Chenu, D. Sparks, G. Thomas,
B.H. Davis, Appl. Catal. A 262 (2004) 177.
[16] T. Tabakova, F. Boccuzzi, M. Manzoli, J.W. Sobczak, V. Idakiev, D. An-
dreeva, Appl. Catal. B 49 (2004) 73.
[17] G. Jacobs, E. Chenu, P.M. Patterson, L. Williams, D. Sparks, G. Thomas,
B.H. Davis, Appl. Catal. A 258 (2004) 203.
[18] Y. Li, Q. Fu, M. Flytzani-Stephanopoulos, Appl. Catal. B 27 (2000) 179.
[19] G. Jacobs, S. Khalid, P.M. Patterson, D.E. Sparks, B.H. Davis, Appl. Catal.
A 268 (2004) 255.
[20] T. Bunluesin, R.J. Gorte, G.W. Graham, Appl. Catal. B 14 (1997) 105.
[21] H. Cordatos, T. Bunluesin, J.S. Stubenrauch, J.M. Vohs, R.J. Gorte,
J. Phys. Chem. 100 (1996) 785.
[22] D. Andreeva, T. Tabakova, V. Idakiev, D. Christov, R. Giovanoli, Appl.
Catal. A 169 (1998) 9.
[23] V. Idakiev, T. Tabakova, Z.Y. Yuan, B.L. Su, Appl. Catal. A 270 (2004)
135.
The WGS/RWGS reaction is a challenge for TAP studies
because of its global reversibility. However, using the large dif-
ference in the time scales of the separate steps and appropriate
pulse sequences, it is possible to extract the various aspects of
the mechanism and determine the rate constants of several ele-
mentary steps. To the best of our knowledge, the present work
is the first paper to illustrate this property using the TAP tech-
nique [55].
[24] M. Haruta, Gold Bull. 37 (2004) 27.
[25] R.J. Gorte, S. Zhao, Catal. Today 104 (2005) 18.
[26] G. Jacobs, L. Williams, U. Graham, D. Sparks, B.H. Davis, J. Phys. Chem.
B 107 (2003) 10398.
[27] G. Jacobs, E. Chenu, P.M. Patterson, L. Williams, D. Sparks, G. Thomas,
B.H. Davis, Appl. Catal. A 258 (2004) 203.
[28] G. Jacobs, B.H. Davis, Appl. Catal. A 284 (2005) 31.
[29] G. Jacobs, A.C. Crawford, B.H. Davis, Catal. Lett. 100 (2005) 147.
[30] G. Jacobs, U.M. Graham, E. Chenu, P.M. Patterson, A. Dozier, B.H. Da-
vis, J. Catal. 229 (2005) 499.
[31] G. Jacobs, P.A. Patterson, U.M. Graham, D.E. Sparks, B.H. Davis, Appl.
Catal. A 269 (2004) 63.
[32] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science 301 (2003) 935.
[33] Q. Fu, W. Deng, H. Saltsburg, M. Flytzani-Stephanopoulos, Appl. Catal.
B 56 (2005) 57.
[34] Q. Fu, S. Kudriavtseva, H. Saltsburg, M. Flytzani-Stephanopoulos, Chem.
Eng. J. 93 (2003) 41.
[35] S. Golunski, R. Rajaram, N. Hodge, G.J. Hutchings, C.J. Kiely, Catal.
Today 72 (2002) 107.
[36] J.C. Frost, Nature 334 (1988) 577.
4. Conclusion
The TAP technique was successfully applied for the first
time to study RWGS reaction studies. The adsorption/desorp-
tion rate constants for CO2 and H2 were determined in sepa-
rate TAP pulse-response experiments over reduced and CO2-
pretreated catalysts. In addition, the amount of H-containing
active species was determined using D2 multipulse TAP ex-
periments. Remarkably, this number agrees with the amount of
active sites observed in a previous SSITKA experiment. An as-
sociative reaction mechanism is thought to dominate for the fast
RWGS reaction under TAP conditions, and a tentative reaction
scheme has been proposed. The mechanism includes fast ad-
sorption of CO2 and H2, slow formation of the CO2 intermedi-
ate available for reaction, fast reaction of the CO2 intermediate
with adsorbed hydrogen, and release of adsorbed CO and water.
In addition, a second species associated with CO2 was found
that desorbs on a time scale of minutes.
[37] A. Amieiro Foncesca, R. Burch, Y. Chen, J. Fisher, A. Goguet, C. Har-
dacre, P. Hu, D. Thompsett, D. Tibiletti, J. Phys. Chem. B, submitted for
publication.
[38] A. Goguet, F. Meunier, J.P. Breen, R. Burch, M.I. Petch, A. Faur Ghenciu,
J. Catal. 226 (2004) 382.
[39] D. Tibiletti, A. Goguet, F.C. Meunier, J.P. Breen, R. Burch, Chem. Com-
mun. (2004) 1636.
Acknowledgments
[40] A. Goguet, D. Tibiletti, F.C. Meunier, J.P. Breen, R. Burch, J. Phys. Chem.
B 108 (2004) 20240.
[41] F.C. Meunier, D. Reid, S. Shekhtman, D. Tibiletti, A. Goguet, C. Hardacre,
R. Burch, unpublished results.
[42] J.T. Gleaves, J.R. Ebner, T.C. Kuechler, Catal. Rev. Sci. Eng. 30 (1988)
49.
The authors thank Johnson Matthey for supplying the cat-
alyst, the EPSRC (under the CARMAC project) for providing
funding, and the Centre for Theory and Application of Catalysts
(CenTACat) for providing a visiting research professorship to
GSY.

110
A. Goguet et al. / Journal of Catalysis 237 (2006) 102–110
[43] J.T. Gleaves, G.S. Yablonsky, P. Phanawadee, Y. Schuurman, Appl. Catal.
A 160 (1997) 55.
[49] S.O. Shekhtman, G.S. Yablonsky, J.T. Gleaves, R. Fushimi, Chem. Eng.
Sci. 59 (2004) 5493.
[44] G.S. Yablonsky, M. Olea, G.B. Marin, J. Catal. 216 (2003) 120.
[45] S.O. Shekhtman, G.S. Yablonsky, J.T. Gleaves, R. Fushimi, Chem. Eng.
Sci. 58 (2003) 4843.
[46] S. Bernal, F.J. Botana, J.J. Calvino, M.A. Cauqui, G.A. Jobacho, J.M. Pin-
tado, J.M. Rodriguez-Izquierdo, J. Phys. Chem. 97 (1993) 4118.
[47] S.O. Shekhtman, G.S. Yablonsky, J.T. Gleaves, S. Chen, Chem. Eng.
Sci. 54 (1999) 4371.
[50] S.O. Shekhtman, G.S. Yablonsky, Ind. Eng. Chem. Res. 44 (2005)
6518.
[51] S.O. Shekhtman, G.S. Yablonsky, paper in preparation.
[52] S.O. Shekhtman, A. Goguet, C. Hardacre, F.C. Meunier, R. Burch, G.S.
Yablonsky, in preparation.
[53] K. Beduerfig, S. Voelkening, Y. Wang, J. Winterllin, K. Yacobi, G. Ertl,
J. Chem. Phys. 111 (24) (1999) 11147.
[54] A. Norman, V. Perrichon, Phys. Chem. Chem. Phys. 5 (2003) 3557.
[55] TAP Workshop, 6–8 December 2004, Leipzig, Germany.
[48] P. Phanawadee, S.O. Shekhtman, C. Jarungmanorom, G.S. Yablonsky,
J.T. Gleaves, Chem. Eng. Sci. 58 (2003) 2215.