Quantitative DRIFTS investigation of possible reaction mechanisms for the water-gas shift reaction on high-activity Pt- and Au-based catalysts

Article abstract of DOI:10.1016/j.jcat.2007.09.003

The present work emphasizes the importance of including a full quantitative analysis when in situ operando methods are used to investigate reaction mechanisms and reaction intermediates. The fact that some surface species exchange at a similar rate to the reaction product during isotopic transients is a necessary but not sufficient criterion for participation as a key reaction intermediate. This is exemplified here in the case of highly active low-temperature water-gas shift (WGS) catalysts based on gold and platinum. Operando DRIFTS data, isotopic exchanges, and DRIFTS calibration curves relating the concentration of formate species to the corresponding DRIFTS band intensity were combined to obtain a quantitative measure of the specific rate of formate decomposition. Despite displaying a rapid isotopic exchange rate (sometimes as fast as that of the reaction product CO₂), the concentration of formates seen by DRIFTS was found to account for at most only 10% of the CO₂ produced under the experimental conditions reported herein. These new results obtained on Au/CeZrO₄ and Pt/CeO₂ preparations (which are among the most active low-temperature WGS catalysts reported to date), led to the same conclusions regarding the minor role of IR-observable formates as those obtained in the case of less active Au/Ce(La)O₂ and Pt/ZrO₂ catalysts.

Full text of DOI:10.1016/j.jcat.2007.09.003

Journal of Catalysis 252 (2007) 18–22
www.elsevier.com/locate/jcat
Quantitative DRIFTS investigation of possible reaction mechanisms
for the water–gas shift reaction on high-activity Pt- and Au-based catalysts
F.C. Meunier a,∗, A. Goguet a, C. Hardacre a, R. Burch a, D. Thompsett b
a
CenTACat, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, BT9 5AG, Northern Ireland, UK
b
Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG4 9NH, UK
Received 6 July 2007; revised 5 September 2007; accepted 10 September 2007
Available online 4 October 2007
Abstract
The present work emphasizes the importance of including a full quantitative analysis when in situ operando methods are used to investigate
reaction mechanisms and reaction intermediates. The fact that some surface species exchange at a similar rate to the reaction product during
isotopic transients is a necessary but not sufficient criterion for participation as a key reaction intermediate. This is exemplified here in the case
of highly active low-temperature water–gas shift (WGS) catalysts based on gold and platinum. Operando DRIFTS data, isotopic exchanges, and
DRIFTS calibration curves relating the concentration of formate species to the corresponding DRIFTS band intensity were combined to obtain a
quantitative measure of the specific rate of formate decomposition. Despite displaying a rapid isotopic exchange rate (sometimes as fast as that
of the reaction product CO ), the concentration of formates seen by DRIFTS was found to account for at most only 10% of the CO produced
2
2
under the experimental conditions reported herein. These new results obtained on Au/CeZrO and Pt/CeO preparations (which are among the
4
2
most active low-temperature WGS catalysts reported to date), led to the same conclusions regarding the minor role of IR-observable formates as
those obtained in the case of less active Au/Ce(La)O and Pt/ZrO catalysts.
2
2
© 2007 Elsevier Inc. All rights reserved.
Keywords: Operando; In situ; SSITKA; DRIFT; FTIR; Spectroscopy; Water–gas shift; Formate; Surface species; Gold; Ceria; Spectator
1. Introduction
isotopic exchange and laser Raman spectroscopy [4], and UV–
vis–NIR and Raman spectroscopies [5].
Operando techniques are growing in importance with the
realization that ex situ methods can be misleading. We have
recently reported that the reactivity of surface species, such
as carbonates and formates, adsorbed over redox oxide-based
catalysts can dramatically depend on the experimental proce-
dure used [1]. However, although operando techniques can in
principle provide evidence about reaction mechanisms and piv-
otal reaction intermediates, it is important to be aware that the
conclusions may be misleading in the absence of a full quanti-
tative analysis of the results. There have been relatively few in
situ and operando quantitative investigations of surface species.
A few examples can be found especially when characterization
techniques were combined: transmission FT-IR and gravimetry
[2], gravimetry and temperature-programmed techniques [3],
Diffuse reflectance FT-IR spectroscopy (DRIFTS) is in-
creasingly being used as a means to investigate the reactivity of
surface species under reaction conditions, but it is usually con-
sidered only a qualitative technique. However, we have demon-
strated that DRIFTS spectroscopy can be an accurate quanti-
tative tool for operando studies, providing that an appropriate
analytical transformation of the diffused intensity is used (i.e.,
in most cases, the pseudo-absorbance rather than the Kubelka–
Munk function [6]) and that a calibration curve relating band
intensity to adsorbate concentration is available [7]. We also
showed that an appropriately modified DRIFTS cell reactor led
to reaction rates identical to those measured in a linear quartz
tube plug flow reactor [7].
The combination of DRIFTS or transmission FTIR and
steady-state isotopic transient kinetic analysis (SSITKA [8]) is
a powerful combination for investigating the reactivity of sur-
face species under reaction conditions. The operando DRIFTS–
*
Corresponding author. Fax: +44 28 90 974890.
E-mail address: f.meunier@qub.ac.uk (F.C. Meunier).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.09.003

F.C. Meunier et al. / Journal of Catalysis 252 (2007) 18–22
19
SSITKA method used in the present study relies on using a sin-
gle catalytic bed, which allows DRIFTS characterization of the
surface of the very same catalyst particles that are responsible
for the catalytic activity measured at the exit of the cell by gas
chromatography or mass spectrometry [9–11]. This methodol-
ogy is similar to that developed earlier for transmission FTIR
by Chuang et al. [12,13]. These techniques derived from the so-
called “isotopic jump” technique of Tamaru et al. [14], which
relied on a two-bed IR cell.
In examples dealing with the water–gas shift (WGS) re-
action over Pt/ZrO2 [15] and Pt/CeO2 [16], we have shown
that the formates adsorbed on the catalysts were exchanged
on a significantly longer time scale than that of the reaction
product CO2, clearly stressing that formates were essentially
spectators in these cases. However, it was also observed that
formates were exchanged on the same time scale as that of
CO2 at higher temperatures over the Pt/CeO2. The fact that sur-
face species exchange as rapidly as the main reaction product
during a SSITKA–DRIFTS experiment cannot yet be taken as
unambiguous evidence that these species belong to the main re-
action pathway. This was clearly exemplified in our study of the
WGS reaction over a catalyst that exhibited a rather low activ-
ity (0.6 at% Au/Ce(La)O2 [17]). Despite the fact that formates
and CO2 exchanged on similar time scales, the specific rate of
formate decomposition (reported to the catalyst mass) was ac-
tually found to be more than an order of magnitude lower than
the rate of CO2 production [7].
In relation to the interest that low-temperature WGS on
noble metal-based catalysts has received over the past years
and the recurrent debate on the role of formates determined
by IR spectroscopy [18–43], here we report new quantitative
SSITKA–DRIFTS data showing that the IR-observable for-
mate species are not important in the reaction mechanism for
high-activity Au/CeZrO4 [44,45] and Pt/CeO2 catalysts [16].
Because the SSITKA–DRIFT analysis of the Pt-based mater-
ial has been reported elsewhere [16,46], only the quantification
of the formate concentration and the corresponding decompo-
sition rate calculation are developed here for this sample.
in the case of the Au/CeZrO4 and 115 5 mg in the case of
the denser Pt/CeO2. The purity of the gases used (i.e., H2, Kr,
CO, and Ar, supplied by BOC) was >99.95%. The 13CO was
99% pure (supplied by Cambridge Isotope Laboratories). The
DRIFTS cell was connected to the feed gas cylinders through
low-volume stainless-steel lines. The gas flows were controlled
by Aera mass flow controllers, which were calibrated regularly.
A 4-way valve was used to allow rapid switching between two
reaction feeds when appropriate. High-purity water was intro-
duced using a single saturator that delivered a constant and ac-
curate water level. Note that the water delivery was completely
unaffected by the valve switching (as determined by MS or GC
analyses [16]).
Unless stated otherwise, the catalyst was brought up to re-
action temperature under Ar, after which water was added to
the Ar stream. A reference DRIFTS single scan was recorded
after the signal was stabilized. The reaction mixture was sub-
sequently introduced at a total flow rate of 100 ml min−1. The
reaction flow was going down the reactor bed; therefore, the
upper layer of the catalyst (that mostly probed by the DRIFTS
technique) was at the front of the bed.
Steady-state conditions in terms of the concentration of the
surface species measured by DRIFTS were reached in less than
30 min. The assignment of the IR bands and the integration
method are described in Section 3. The IR data are reported
as log1/R, with R = I/I0, where R is the sample reflectance,
I0 is the intensity measured on the sample after exposure to
the CO-free feed, and I is the intensity measured under reac-
tion condition. The pseudo-absorbance log1/R gives a better
linear representation of the band intensity against sample sur-
face coverage than that given by the Kubelka–Munk function
for strongly absorbing media, such as those based on CeO2 [6].
The calibration standards used to relate the formate DRIFTS
signal to the concentration of these species over the catalysts
were prepared by incipient wetness impregnation of sodium
formate (Aldrich, 99.9%) solutions over the corresponding sup-
ports. The samples were dried in ambient air in an oven at 80 ◦C
after impregnation, after which the DRIFTS spectra of the stan-
dards were collected under pure Ar at 80 ◦C. The supports free
of noble metals were used to prevent any formate decompo-
sition by the metals during the analysis [1]. The difference in
the mid-IR scattering and absorption properties of the materials
with and without noble metal was negligible in the present case,
as was found previously for SiO2 and Pt/SiO2 [6]. Several sam-
ples of the same standard were reloaded in the DRIFTS cell,
and the measurements were reproducible within 10%.
2. Experimental
The experimental setup consisted of an in situ high tem-
perature diffuse reflectance IR cell (from Spectra-Tech) fitted
with ZnSe windows. The DRIFTS cell was located in a Bruker
Equinox 55 spectrometer operating at a resolution of 4 cm−1
.
The reactor crucible was modified to ensure plug-flow condi-
tions throughout the catalyst bed. The interface between the ce-
ramic reactor and the metallic base plate was sealed with PTFE
tape to prevent any sample bypass. The original porous bed to
support the sample was replaced by an inert metallic mesh. The
WGS reaction rates measured with this cell were equal to those
measured in a conventional tubular plug-flow reactor [7]. No re-
actant conversion was observed when the crucible was heated
up to the reaction temperature in the absence of catalyst.
3. Results and discussion
The typical in situ DRIFT spectrum obtained during the
WGS reaction over the highly active Au/CeZrO4 is shown in
Fig. 1. The 12C-spectrum exhibited a band associated with
formate species at 2841 cm−1 (C–H stretching [47]) and car-
bonyl at 2097 cm−1 (CO adsorbed on Au0 edge or defect
sites [48–50]). The other bands in the 1700–1200 cm−1 region
were due to formates and carbonates. The support hydroxyl
groups also were observed at 3673 cm−1 (free hydroxyls) and
The catalysts used were a 2 wt% Au/CeZrO4 and a 2 wt%
Pt/CeO2, the details of which have been reported previously
[45,46]. The catalyst mass placed in the crucible was 60 3 mg

20
F.C. Meunier et al. / Journal of Catalysis 252 (2007) 18–22
12
Fig. 3. Logarithm of the relative intensity of the C-formate DRIFT band re-
lating to the data reported in Fig. 2.
Fig. 1. In situ DRIFT spectrum of the Au/CeZrO at steady-state under a stream
4
12
of 2.5% CO + 7% H O + 13% H in Ar.
2
2
pared with that of CO2 (ca. 0.6 min) was a strong indication
that formates detected by DRIFTS were not the main reaction
intermediates in the formation of CO2, whereas the rapidly ex-
changing carbonyl was the potential main intermediate over the
Au/CeZrO4.
The comparison of the CO2 and formate-exchange curves
was more intriguing in the case of the Pt/CeO2 material [16].
The formate exchange was significantly slower than that of CO2
at 160 and 180 ◦C, suggesting that formates were unimportant
reaction intermediates at these temperatures. However, the ex-
change of these two species was essentially identical at 220 ◦C,
suggesting that formates could potentially be a main reaction
intermediate under these conditions [16].
The relevance of the formates seen by DRIFTS in this work
(Fig. 1) and those in Ref. [16] in the formation of CO2 was as-
certained by a quantitative comparison of the specific rate of
CO2 formation (measured by GC analysis of the DRIFTS cell
effluents) and the specific rate of formate decomposition. The
latter was calculated as the product of the formate concentration
by the pseudo-first-order rate constant of formate decomposi-
tion, denoted as k [7],
Fig. 2. Comparison of the relative exchange of the gas-phase CO and CO
2
and surface formate species during an isotopic exchange over the Au/CeZrO .
4
◦
13
T = 125 C. Feed: 2.5% CO, 7% H O, 13% H in Ar. The sample was ini-
2
2
12
tially at steady-state under the corresponding non-labeled feed: 2.5% CO, 7%
H O, 13% H in Ar.
2
2
3397 cm−1 (H-bonded hydroxyls). The in situ spectrum ob-
tained over the Pt/CeO2 material was similar and is described
in detail elsewhere [46,47].
Using SSITKA techniques allows us to assess the chemical
reactivity of the formate species with respect to the formation of
the reaction product CO2 under reaction conditions. This tech-
nique involves replacing one of the reactants (here 12CO) by an
isotopomer (here 13CO) during the reaction and following si-
multaneously the exchange of the labeled reaction product (here
13CO2) by mass spectrometry and the surface species (e.g.,
13C- or 12C-containing carbonyl and formates) by DRIFTS.
The DRIFTS bands of the surface species typically shift to
lower wavenumbers during the analysis, and various integration
methods can be used to accurately quantify the surface species
exchange (see [7,9,46,47] for the description of the integration
methods and typical 12C and 13C spectra obtained over similar
catalyst formulations).
The carbonyl exchange (not shown) over these two highly
active catalysts was complete in about 0.1 min, which was as
fast as that of CO(g) and that of the Kr tracer (not shown), in-
dicating that the exchange time of these species was limited
by the purge of the system. In contrast, the exchange times of
CO2 and the formate species over the Au/CeZrO4 were signif-
icantly longer than the purge time of the DRIFTS cell (Fig. 2).
The longer half-exchange time of formates (ca. 2 min) com-
rate of formate decomposition = k[formate].
The value of k was obtained during a 12CO–13CO isotopic ex-
change under WGS conditions by measuring the slope of the
line representing the logarithm of the formate concentration as
a function of time following the isotopic switch (see Fig. 3 for
Au/CeZrO4 at 125 ◦C and Ref. [16] for Pt/CeO2).
Calibration curves (Fig. 4) were also drawn to accurately
quantify the concentration of formates. The calibration curves
were obtained by measuring the intensity of the CH stretch-
ing DRIFTS band centered at around 2841 cm−1 of standard
samples. Exemplar in situ spectra measured over the Au and
Pt ceria-based catalysts and spectra of the calibration stan-
dards are shown in Fig. 5. The integration ranges were 2890–
2800 cm−1 for the Au/CeZrO4 and 2880–2800 cm−1 for the
Pt/CeO2. The calibration curves provide a simple and quanti-
tative measure of the amount of surface formates produced at
steady state under reaction conditions. It is important to note
that the slope of the calibration curve was different for each cat-
alyst, stressing that each calibration curve is material-dependent
and cannot be extrapolated to other catalysts. This observation
results from the fact that the sample scattering and absorptiv-

F.C. Meunier et al. / Journal of Catalysis 252 (2007) 18–22
21
Fig. 4. Calibration plots relating the area of the formate CH stretching DRIFT
band to the nominal formate loading on the (Q) CeZrO and (") CeO supports
4
2
impregnated with sodium formate.
Fig. 6. (a) Rate of CO production and rate of formate decomposition over the
2
Au/CeZrO at three different temperatures under 2.5% CO, 7% H O, 13% H
4
2
2
in Ar. (b) Rate of CO production and rate of formate decomposition over the
2
Pt/CeO at three different temperatures under 2% CO + 7% H O in Ar.
2
2
The experimental conditions used here are similar to those
used in most other papers reported so far. However, it should be
stressed that Davis and co-workers, who proposed that the for-
mates seen by IR were main reaction intermediates, typically
used much higher concentrations of water (up to 89% [40]).
A higher partial pressure of water could accelerate formate de-
composition significantly; therefore, a full quantitative analysis
under the conditions used by Davis et al. would be necessary to
determine the relevance of the formates detected by DRIFTS in
this case.
Our work does not allow us to determine the exact nature of
the main reaction intermediates. It is possible that the main re-
action intermediates (not excluding some highly reactive hypo-
thetical formates) have very low steady-state surface concentra-
tions and are not observable by IR. Recent theoretical work on
Au/CeO2 has suggested that an associative mechanism involv-
ing a carboxyl species HOCO* (rather than a formate HCOO∗∗)
occurs [52,53], although the synergistic role of the support (pos-
sibly involved in the activation of water) remains incompletely
understood.
The unambiguous conclusions obtained here on the minor-
ity role of formates detected by IR do not contradict a statement
from our earlier paper [16] regarding the Pt/CeO2 catalysts sug-
gesting that the formates seen by DRIFTS are only potential
main reaction intermediates at the higher temperatures. The
positive conclusions reported here supercede the nonconclu-
sive statements reported previously [16], stressing yet again that
nonfully quantitative work is merely speculative.
◦
Fig. 5. (a) DRIFT spectrum recorded at 80 C in Ar over CeZrO impregnated
4
with 0.11 wt% of formate ions and (b) in situ DRIFT spectrum recorded over
Au/CeZrO under 2.5% CO, 7% H O, 13% H in Ar at 125 C. (c) DRIFT
spectrum recorded at 80 C in Ar over CeO impregnated with 0.67 wt% for-
mate ions and (d) in situ DRIFT spectrum recorded over Pt/CeO under 2% CO
◦
4
2
2
◦
2
2
◦
+ 7% H O in Ar at 160 C.
2
ity will depend to a great degree on its composition, texture,
and porosity. Because calibration curves relating to DRIFTS
analyses are also known to show nonlinear behavior [51], it is
important to not extrapolate outside the range of standards mea-
sured.
The decomposition rate of the formate species yielded the
upper limit of the rate of formate decomposition to CO2, be-
cause it is likely that formates were not only decomposing to
give CO2, but instead gave a mixture of CO and CO2 [24]. Even
assuming that formates decomposed solely to CO2, the rate of
formate decomposition for our highly active Au/CeZrO4 and
Pt/CeO2 WGS catalysts accounted for less than 5% and 10%
of the total rate of CO2 formation in the range of temperatures
and experimental conditions used here, respectively (Fig. 6).
Therefore, the formates detected by DRIFTS are minor reac-
tion intermediates even with these very active catalysts.
4. Conclusion
The formates seen on DRIFTS are not part of the major reac-
tion pathway leading to CO2 over our highly active Au/CeZrO4

22
F.C. Meunier et al. / Journal of Catalysis 252 (2007) 18–22
and Pt/CeO2 WGS catalysts under the experimental conditions
used in the present work. The proportion of CO2 formed by
a route involving the formates seen by DRIFTS is <5% for
the Au/CeZrO4 and <10% for the Pt/CeO2. This work further
demonstrates that with careful experimentation, it is possible to
perform fully quantitative operando DRIFTS, and that unam-
biguous conclusions can be drawn from these data.
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Acknowledgment
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This work was supported in part by the EPSRC, under the
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