In situ EXAFS and FTIR studies of the promotion behavior of Pt-Nb2O5/Al2O3 catalysts during the preferential oxidation of CO

Article abstract of DOI:10.1016/j.jcat.2008.12.008

The promotional effect of Nb to Pt/Al₂O₃ supported catalysts during the preferential oxidation of CO (PROX) was studied using various spectroscopic techniques. Addition of small amounts of Nb (<5%) stabilizes 40% of the loaded platin

Full text of DOI:10.1016/j.jcat.2008.12.008

Journal of Catalysis 262 (2009) 102–110
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Journal of Catalysis
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In situ EXAFS and FTIR studies of the promotion behavior of Pt–Nb2O5/Al2O3
catalysts during the preferential oxidation of CO
S. Guerrero a, J.T. Miller b,c, A.J. Kropf c, E.E. Wolf a,∗
a
Chemical Engineering Department, University of Notre Dame, Notre Dame, IN 46556, USA
BP Research Center, E-1F, 150 W. Warrenville Rd., Naperville, IL 60563, USA
Chemical Engineering Division, Argonne National Laboratory, 9700 S. Cass Avenue, IL 60439, USA
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The promotional effect of Nb to Pt/Al2O3 supported catalysts during the preferential oxidation of CO
(PROX) was studied using various spectroscopic techniques. Addition of small amounts of Nb (<5%)
stabilizes 40% of the loaded platinum as Pt2+, which remains oxidized even after reduction treatments.
This Nb-promoted catalyst is very active and selective for the PROX reaction. On Pt/Nb2O5 and at high Nb
loading for the Pt/Nb/Al2O3 catalysts, the selectivity to CO2 decreases and the selectivity for H2 oxidation
increases opposite to the selectivity observed at low Nb loadings. The increase CO2 selectivity due to Nb
promotion is ascribed to the inhibition of CO at low temperature which decreases hydrogen oxidation.
Received 27 September 2008
Revised 8 December 2008
Accepted 9 December 2008
Available online 7 January 2009
Keywords:
PROX reaction
Pt supported catalyst
Nb promoter
Operando FTIR results indicate the presence of adsorbed CO as well as carbonates, bicarbonates and
−1
formates during the PROX reaction. An IR band at 968 cm
indicates the presence of Nb=O moieties at
low Nb loadings. At higher Nb loadings, IR suggests the formation of three-dimensional Nb2O5 aggregates.
The surface of the Nb containing catalysts is complex containing reduced and oxidized Pt which is
modified by NbOx species either surrounding the Pt crystallites or decorating them.
© 2008 Published by Elsevier Inc.
1. Introduction
must be minimized in a PROX catalyst. One alternative is to use
promoters that allow decreasing Pt loading while maintaining high
The increased demand for fossil fuels have led to a renewed
interest in alternative fuels, among them, the use of proton ex-
change membrane (PEM) fuel cells, utilizing hydrogen as energy
source. The use of an upstream reformer of a hydrocarbon, such
as methanol or ethanol, is among several processes investigated to
produce hydrogen for PEM fuel cells. Such processes can provide a
H2-rich stream having an approximate composition of 45–75% H2,
15–25% CO2, 0.5–2% CO, 10–20% H2O and N2 [1,2]. The presence
of CO in the reformer effluent stream, however, constitutes a prob-
lem since it acts as a poison of Pt used on the anode of the fuel
cell, which requires reducing the CO concentration in the effluent
below 20 ppm. This can be achieved by preferentially oxidizing CO
to CO2 in a H2-rich stream upstream the fuel cell. The preferen-
tial oxidation of CO (PROX) requires highly selective catalyst for CO
oxidation and at the same time unselective to the unwanted oxi-
dation of H2.
activity and selectivity. Previous works have shown that combin-
ing Pt and Sn supported on a carbon support resulted in a better
PROX activity than on an unpromoted Pt/Al2O3 catalyst [6,12]. This
effect was attributed to the promotion of oxygen adsorption on Sn
sites and to the competing adsorption of CO and H2 on Pt sites [6].
It has also been suggested that promoters can positively affect the
catalyst selectivity by blocking the spillover of adsorbed oxygen
onto the support and/or hydrogen species, both involved in hy-
drogen oxidation [10,13–15]. Minemura et al. [16] reported that
during the PROX reaction, a 2% Pt supported on Al2O3 promoted
with potassium could reach 100% CO conversion and 50% CO2 se-
◦
lectivity at around 90 C.
In a previous paper [17], we reported an interesting promotion–
inhibition effect of adding Nb to Al2O3-supported Pt catalysts dur-
ing the PROX reaction. At low Nb loadings the catalysts were very
active and selective for CO oxidation. As Nb loading increased, this
effect reversed and in the limit of Nb loading, i.e. Pt supported on
niobia, the CO selectivity of Pt was inhibited during the PROX re-
action. Interestingly, this latter catalyst is very active for hydrogen
oxidation in the presence of significant amounts of CO, i.e. it be-
comes resistant to CO poisoning.
Pt is known for being an active catalyst in the PROX reac-
tion [3–10] as it was first reported for such reaction to purify
hydrogen used in ammonia synthesis [11]. Fuel cells, however, al-
ready utilize significant amounts of Pt, and therefore the Pt loading
In this work, we present in situ and operando (under reaction
conditions) FTIR and EXAFS/XANES spectroscopy characterization
Corresponding author. Fax: +1 574 631 8366.
E-mail address: ewolf@nd.edu (E.E. Wolf).
*
0021-9517/$ – see front matter © 2008 Published by Elsevier Inc.
doi:10.1016/j.jcat.2008.12.008

S. Guerrero et al. / Journal of Catalysis 262 (2009) 102–110
103
results from Pt/Nb/Al2O3 catalysts using various Nb loadings to ex-
plain the promotion–inhibition effect previously reported.
(Alltech) and a molecular sieve (Alltech) columns, operating in par-
allel, and maintained at 50 C using He as a carrier at 120 cc/min.
◦
This column arrangement allows separating H2, CO, O2, CO2, and
CH4. The latter gas was not detected in any experiment. Selectivity
is defined as the ratio between oxygen consumed in the oxidation
of CO versus the total amount of oxygen consumed and it was cal-
culated as: Se = Pcoou2t/2(Pin − Pout), where Pi corresponds to the
2. Experimental
2.1. Catalysts preparation
o2
o2
Several methods of catalyst preparation were used to add Nb
to the Al2O3 support, but the results proved to be independent
of the way Nb was loaded [17]. In all cases, Pt was loaded onto
Nb containing supports by incipient wetness impregnation using
tetraammineplatinum(II) nitrate (Aldrich) followed by calcination
partial pressure of the component i.
2.3. FTIR measurements
IR measurements were obtained in both transmission and dif-
fuse reflectance mode (DRIFTS). Transmission infrared spectra of
pressed disks (∼20 mg) were collected in situ in an IR reactor cell
◦
in air at 300 C for 4 h. Initially, Nb was loaded on the Al2O3 sup-
port (X%Nb–Al2O3, X = 0, 1, 5, 10, 20 is the niobium loading) by
co-precipitation of ammonium niobium oxalate (Aldrich) and col-
loidal alumina sol (Criterion Catalyst and Technologies) with aque-
placed in a Mattson FTIR spectrometer (Galaxy 6020). Data were
−1
recorded at a resolution of 2 cm
and 30 scans per spectrum.
◦
ous ammonia. These supports were calcined at 500 C for 12 h.
The IR cell is equipped with CsI windows, and has connections
for inlet and outlet gas flows, and has two thermocouples con-
nected to a temperature controller to monitor and control the
cell and catalyst temperature. Spectra were recorded in absorbance
mode after subtraction of the background spectrum of the cat-
alyst’s disk under helium at the corresponding temperature. The
samples were pretreated at various conditions prior to study ad-
sorption and reaction. After pretreatment, the catalyst was cooled
to room temperature in He, and under operando or reaction con-
ditions, the reaction mixture was introduced in the cell at a total
flow rate of 195 cc/min and a composition of 0.8% CO, 0.8% O2,
and 51% H2, with He as balance. Spectra were collected at differ-
Catalysts were also prepared by impregnation of Al2O3 with Nb
oxalate followed by calcination at 300 C for 2 h. Catalysts with
◦
Nb loading lower than 20% are referred as low Nb loading. Higher
loadings of Nb of 30 and 50% (1%Pt/Y%Nb–Al2O3, Y = 30, 50) were
attained by the successive impregnation of Nb using ammonium
niobium oxalate followed by impregnation of tetraammineplat-
◦
inum(II) nitrate and calcination at 300 C for 4 h. The Nb2O5, used
as support for the 1%Pt/Nb2O5 catalyst, was prepared by a micro-
emulsion of a solution of ammonium niobium oxalate diluted in
10 g of pluronic acid L64 (BASF), which was strongly stirred un-
til the formation of a foam was observed. Precipitation of Nb2O5
was induced by adding a solution of NH4OH, 0.5 M (100 ml) to the
foam. After filtration, the solid was washed with ethanol and the
remaining solvent was extracted from the solid by using boiling
acetone. After drying overnight at room temperature, the resulting
◦
ent temperatures as it increased at a rate of 1 C/min. Transmission
experiments in which no reaction was carried out (i.e. in situ), such
as CO adsorption and desorption, showed different intensities and
frequencies than under reaction or operando conditions and thus
are not shown.
◦
solid was calcined at 300 C for 2 h.
The DRIFTS experiments were carried out to obtain spectra at
low frequencies related to support species which could not be ob-
tained under transmission mode. DRIFTS results were obtained in
a special IR reaction cell equipped with ZnSe windows and a pray-
ing mantis optical geometry (Harrick, 3-3S) placed in a Bruker FTIR
spectrometer (Equinox 55). The cell includes a heating element
and a thermocouple, which provides the feedback to a tempera-
ture controller to maintain the temperature constant at a set value.
The gases flow upwards through a shallow bed of catalysts reach-
ing the dome of the cell and flowing out near its base. A total
2.2. Activity-selectivity measurements
Catalysts’ activity was measured in a quartz tubular flow re-
actor of 1 cm ID and 30 cm long, using 200 mg of catalyst. The
catalyst powders were first pelletized and the resulting pellets bro-
ken up into a powder that was sieved to yield agglomerates with
average particle sizes of about 1 mm. The reactants flow rates
were kept constant by electronic mass flow controllers. A temper-
ature controller, using a feedback from a thermocouple placed in a
thermo-well located in the center of the catalyst bed, controlled an
of 250 scans were recorded per spectrum, which was displayed in
electrical heater and maintained the reactor temperature constant
−1
Kubelka–Munk units, over the range of 4000–500 cm
at a res-
◦
(
0.1 C). The reactor is also equipped with an external diaphragm
olution of 2 cm−1. To improve the reflectivity of the samples, the
catalysts were diluted with KBr at ratio of KBr:catalyst of 10:1. In
every experiment, 50 milligrams of catalysts sample were reduced
pump (Thomas Industries) to provide an external recycle loop. The
recycle ratio (recycled flow rate/outlet flow rate) was about 20 to
ensure conditions equivalent to perfect mixing and to eliminate
diffusional mass and heat-transport effects during reaction. The
perfect mixing conditions allowed the direct calculation of reac-
tion rates in a much broader range of reaction conditions (up to
100% conversion) that cannot be attained under differential reac-
tion conditions normally used in single pass plug flow reactors (i.e.
at less than 10% conversion).
◦
in 15 cc/min of H2 at 200 C for 1 h. Pure KBr was used as back-
ground after the same conditions of the reduction pretreatment
were used, which was then subtracted from the IR spectrum of
the sample to correct for temperature effects.
2.4. EXAFS data collection and analysis
◦
Prior to each run, the catalysts were reduced in situ at 200 C
Measurements using extended X-ray absorption fine-structure
(EXAFS) spectroscopy were made on the insertion-device beam
line at the advanced photon source (APS), Argonne National Labo-
ratory (ANL). Measurements were made in transmission mode with
ionization chambers optimized for the maximum current with lin-
ear response (∼1010 photons detected/s). A cryogenically cooled
double-crystal Si (111) monochromator with resolution (ꢀE) better
than 2.5 eV at 13272.6 keV (Pt L2 edge) was used in conjunction
with a Rh-coated mirror to minimize the presence of harmon-
ics [18]. The integration time per data point was 1–3 s, and three
in a stream of pure hydrogen flowing at 50 cc/min for 1 h. When
higher reduction temperatures were used, the same activity results
were obtained. After cooling the reactor to room temperature and
purging with helium, the reaction mixture with a composition of
0.8% CO, 0.8% O2, and 51% H2, with He as balance, was added at a
total flow rate of 195 cc/min. Experiments were conducted by in-
◦
creasing linearly the reactor temperature at 1 C/min until reaching
◦
200 C.
Samples from the gaseous effluent were analyzed on stream
using a gas chromatograph (Varian 3700) equipped with a CTR I

104
S. Guerrero et al. / Journal of Catalysis 262 (2009) 102–110
scans were obtained for each processing condition. Standard pro-
cedures based on WINXAS97 software [19] were used to extract
the EXAFS results [20]. Phase shifts and backscattering amplitudes
were obtained from EXAFS results using Na2Pt(OH)6 as a reference
compound for Pt–O parameters and a Pt foil for Pt–Pt parameters.
The sample was pressed into a cylindrical holder with a thick-
ness chosen to give an absorbance (ꢀμx) of about 1.0 in the Pt
edge region, corresponding to approximately 30 mg of catalyst.
The sample holder was centered in a continuous-flow in situ EX-
AFS reactor tube (18 in. long, 0.75-in. diameter) fitted at both ends
with polyimide windows and valves to isolate the reactor from
the atmosphere. The catalysts were first reduced outside the EX-
AFS data acquisition room by flowing a mixture of 4%H2/He. After
the prescribed treatment the sample was cooled to room temper-
ature in He, and then the cell was isolated by closing the valves
fitted at both ends of the tube and moved to the EXAFS data ac-
quisition room. Experiments were conducted in situ in a stream
one reaching 100% conversion at room temperature. CO oxidation
on Pt/Nb2O5, however, exhibited high activity when hydrogen was
not present in the feed, albeit lower than on Pt supported on
alumina with or without niobia. Table 1 also lists the Pt disper-
sion (D) for each catalyst showing that the Pt/Nb2O5 catalyst has
smaller Pt dispersion than that of the unpromoted and Nb pro-
moted Pt/Al2O3 catalysts.
3.2. XAS characterization
The EXAFS and XANES data were obtained in various gases (CO,
H2 only as well as under PROX conditions) and at different temper-
atures on the 1%Pt/X%Nb–Al2O3 (X = 0, 1, 5, 10) catalysts. EXAFS
fits of the k2-weighted Fourier transform data are given in Ta-
ble 2, for the unpromoted and reduced Pt/Al2O3, while fits for the
◦
1%Pt/X%Nb–Al2O3 (X = 0, 1, 5, 10) catalysts reduced at 250 C are
presented in Table 3. Table 4 gives the results for the 1%Pt/5%Nb–
Al2O3 catalyst reduced at different temperatures. For the unpro-
moted 1%Pt/Al2O3 catalyst (Table 2), the Pt–Pt scattering (bond)
distance and coordination number (CN) remain approximately the
same with the gas atmosphere (H2, CO, PROX) or temperature
◦
of either 1%CO/He or in 4%H2/He at 110, 155, and 205 C. As in
the case of FTIR studies, results were obtained under various in
situ and operando conditions. The later were carried out by flow-
ing 200 cc/min of a mixture of 1%CO/He, 1%O2/He, and 4%H2/He,
◦
◦
balance He, at 110, 155, and 205 C and showed the effect of the
used (25–205 C). The Pt–Pt scattering distance values are slightly
reaction on the surface.
3. Results
shorter than that for Pt foil (2.77 Å), which is consistent with the
presence of small particles on the supported catalyst [21]. The ab-
sence of Pt–O during PROX reaction indicates that there is little
Pt oxidation despite the presence of O2 in the gas mixture. As it
was previously reported using TEM, the particle size distribution
for these catalysts is in the range of 10–20 Å [17].
3.1. Activity
XANES spectra of the reduced Pt/Al2O3 catalysts, in the pres-
ence or absence of CO at various temperatures, are shown in Fig. 1.
The height of the Pt edge (white line) varies with the amount of
adsorbed CO and with temperature (see inserts in Fig. 1). An ap-
proximate estimate of the corresponding CO coverage at different
temperatures can be made by fitting a linear combination of the
reduced (no CO) and CO-saturated Pt XANES white line [22,23].
The same procedure was used to fit the XANES spectra in hydro-
gen at different temperatures (Table 4) [24]. The XANES estimate
of the percentage of CO saturation is also listed in Table 2.
Table 1 summarizes previous key activity results for selected
catalysts and at conditions emphasizing the niobia promotion–
inhibition behavior of the Pt supported catalysts. Details for other
catalysts and conditions used are listed in [17]. In short, Pt pro-
moted with low Nb loadings (5–20%) increased the PROX reac-
tion activity and selectivity, and as loading increased, the activity-
selectivity resembled the unpromoted catalyst. When Pt was sup-
ported on Nb2O5 however, the PROX activity was inhibited signif-
icantly. Also listed in Table 1 are the conversion for CO oxidation
and hydrogen oxidation only, the later reaction being the fastest
Fig. 1A also shows the XANES spectra of the Pt/Al2O3 catalyst
◦
at 110, 155, and 205 C in the presence of flowing CO. The white
Table 1
◦
line intensity at 110 and 155 C is very similar to that of a surface
completely covered by CO (see Table 2). The white line intensity
decreases slightly when the sample is heated to 205 C in flow-
ing CO, corresponding to an estimated decrease in CO coverage of
about 85% of its saturation value. When the same experiment was
Conversion and selectivity for the PROX reaction on selected catalysts. CO and H2
oxidation only are also included.
◦
Catalysts
(D = dispersion)
Temp. PROX
CO
oxidation
conversion conversion
(%)
H2
oxidation
◦
( C)
CO
CO2
conversion selectivity
(%) (%)
(%)
◦
repeated at 155 C in 1% CO with 4% H2/He (Table 2), the white
Pt/Al2O3 (74%D)
Pt/5%Nb–Al2O3 (74%D) 150
Pt/Nb2O5 (27%D)
150
23 100
100
100
23
100
100
100
100
line estimate indicated that CO decreased to about 65% of the sat-
uration coverage, possibly due to competitive adsorption between
H2 and CO. The CO coverage estimate from the white line intensity
on the unpromoted Pt/Al2O3 catalyst was also made under PROX
85
5
73
5
150
180
10
4
100
Table 2
◦
EXAFS and XANES fits: 1%Pt/Al2O3 under various experimental conditions. Prior each experiment the catalyst was reduced at 250 C in 4%H2/He.
Conditions
CO saturation
(%)
CN
R (Å)
ꢀσ2
(×103 Å2)
E0
(eV)
Scatter
(
10%)
(
0.02)
◦
Temperature ( C)
Gasa
25
110
205
4% H2
4% H2
4% H2
–
–
–
Pt–Pt
Pt–Pt
Pt–Pt
8.9
8.9
8.9
2.75
2.74
2.73
3.0
3.4
3.8
−4.2
−4.4
−4.2
110
155
155
205
1% CO
1% CO
100
100
65
Pt–Pt
Pt–Pt
8.7
8.6
2.75
2.76
3.4
3.6
−3.8
−3.7
◦
1% CO + 4% H2
CO desorbs in flowing H2 at 155
Pt–Pt
C
1% CO
85
8.4
2.75
3.8
−3.9
110
155
205
0.8% CO + 0.8% O2 + 4% H2
0.8% CO + 0.8% O2 + 4% H2
0.8% CO + 0.8% O2 + 4% H2
100
75
55
Pt–Pt
Pt–Pt
Pt–Pt
9.0
9.0
9.2
2.76
2.75
2.75
3.4
3.6
3.8
−3.4
−4.3
−4.2

S. Guerrero et al. / Journal of Catalysis 262 (2009) 102–110
105
Table 3
EXAFS and XANES fits. 1%Pt/X%Nb–AI2O3 catalyst prereduced at 250 C.
and 70% is Pt0. By contrast, the unpromoted Pt/Al2O3 catalyst is
fully reduced metallic Pt. While only a portion of the Pt is reduced
◦
◦
◦
ꢀσ2
E0
in H2 at 250 C for Pt/Nb–Al2O3 catalysts, air oxidation at 205 C,
leads to complete oxidation to PtO, i.e., Pt2+ (Fig. 2B). Fig. 3A
shows that Pt LII XANES intensity of Pt/5%Nb–Al2O3 increases upon
Pt/X%Nb–AI2O3 Fraction Fraction Scatter CN
R (Å)
(
Nb loading, %
Pt2 +
Pt0
(
10%)
0.02) (×103 Å2) (eV)
0
1
–
0.4
1.0
0.6
Pt–Pt
Pt–O
Pt–Pt
Pt–O
Pt–Pt
Pt–O
Pt–Pt
8.9
1.2
3.6
1.1
4.3
1.1
3.4
2.75
2.01
2.70
2.00
2.71
2.00
2.70
3.0
3.0
5.0
3.0
5.0
3.0
5.0
−4.2
−2.0
−6.6
−2.1
−4.6
−2.0
−5.2
◦
◦
CO adsorption at 110 C. At 205 C, the XANES intensity decreases
to about 70% of the CO saturation coverage.
5
0.4
0.3
0.6
0.7
◦
Under PROX reaction conditions at 110 C, Fig. 3B, the XANES
intensity is the same as with 100% CO coverage, i.e., pure CO (see
10
◦
Fig. 3A). At 155 C there is a decrease in the white line intensity,
and it is estimated as 80% of the CO saturation coverage, while at
◦
205 C, the white line intensity indicates 60% CO coverage. As ob-
Table 4
served for Pt/Al2O3, the CO coverage is lower under PROX reaction
conditions, and is consistent with the observation that H2 is com-
peting with CO for Pt sites during the PROX reaction.
EXAFS and XANES fits on l%Pt/5%Nb–Al2O3. Data collected under He at 25, 110, and
◦
205 C right after reduction in 4% H2 at the same temperatures.
R (Å)
ꢀσ2
(×103 Å2)
Conditions
Temperature ( C) Gas
H2 saturation
(%)
CN
E0
(eV)
Scatter
The XANES intensity also increases upon adsorption of hydro-
gen on the catalyst surface. The linear variation of the white line
intensity with the amount of adsorbed hydrogen can be used to es-
timate the degree of hydrogen coverage [24]. The reduced catalysts
without adsorbed and fully saturated with hydrogen were used
as XANES references to estimate intermediate H2 surface cover-
age (Table 4). Note that although the Nb catalysts adsorb H2, Pt–O
bonds are still observed by EXAFS. Further increase in temperature
(
10%)
(
0.02)
◦
25
110
205
205
4% H2 100
4% H2 90
Pt–O
Pt–Pt
Pt–O
Pt–Pt
Pt–O
Pt–Pt
Pt–O
2.0
3.9
2.2
3.8
2.0
4.0
3.8
2.04
2.71
2.02
2.69
2.01
2.69
2.04
3.1
6.3
6.0
6.5
7.9
5.0
0.2
−0.7
−1.2
−3.7
−6.0
−2.9
−5.6
−2.1
4% H2
Air
0
–
◦
leads to 90% H2 saturation at 110 C and total H2 desorption at
◦
205 C. These results indicate that while H2 competes for adsorp-
reaction conditions. Fig. 1B shows that under reaction conditions,
the white line intensity decreases with temperature indicating that
CO saturation gradually decreases when increasing temperature.
tion sites with CO, at high temperature, there is little chemisorbed
hydrogen.
In summary, the EXAFS results show that on the unpromoted
Pt/Al2O3 only metallic Pt is present under PROX reaction condi-
tions. On the promoted catalysts with low Nb loadings, EXAFS in-
dicates the presence of only partially reduced Pt, while a portion
of the Pt remains partially oxidized as PtO. On these catalysts, ad-
sorption of H2 competes with CO adsorption leading to lower CO
coverage in the presence of H2. Due to X-ray absorption by the nio-
bia support, transmission XAS experiments could not be conducted
on Pt supported on niobia.
◦
When the reaction was carried out at 110 C, the white line in-
tensity corresponded to a surface completely saturated with CO
(Table 2). A further increase in the reaction temperature to 155 C
led to an estimate of 75% of CO saturation coverage. The lowest
CO saturation coverage estimate corresponded to 55% at 205 C.
◦
◦
These results indicate that, under reaction conditions CO coverage
is lower than when only CO is present due to the competitive ad-
sorption of CO, H2, and oxygen. Fits of the EXAFS data indicate
little change in the coordination number (CN) and scattering dis-
tance (R) between the reduced 1%Pt/Al2O3 with adsorbed CO or
3.3. Transmission infrared results
◦
under PROX reaction at 205 C. This indicates that the Pt parti-
cle size does not change with either flowing CO or under PROX
reaction conditions and that the Pt remains fully metallic under
reaction conditions and partial conversion.
The catalysts were also analyzed by operando transmission FTIR
and in situ DRIFTS under various conditions. Although the flow
patterns and mass of catalysts were different between these IR
reactors and the flow tubular reactor in which activity was mea-
sured, the conversions followed similar trends.
Fig. 2A shows the XANES spectra for the Pt/X% Nb–Al2O3 (X =
0, 5, 10, 20) catalysts, i.e., those with low Nb loading. The results
indicate that with low Nb loading Pt remains partially oxidized
◦
even after reduction in H2 at 250 C. Estimates made by fitting the
3.3.1. PROX reaction
normalized XANES indicate that the Pt/5%Nb–Al2O3 and Pt/10%Nb–
Al2O3 catalysts are approximately 40% Pt2+ and 60% as Pt0 (see
Table 3). Similarly, for Pt/20%Nb–Al2O3, 30% of the platinum is Pt2+
Operando transmission FTIR spectra of Pt/Al2O3, Pt/5%Nb–Al2O3,
and Pt/Nb2O5 catalysts were obtained under PROX reaction con-
ditions (0.8% CO, 0.8% O2, and 51% H2, balance He) and at dif-
◦
Fig. 1. Normalized LII XANES spectra for the reduced Pt/Al2O3 catalyst at 110, 155, and 205 C under: (A) flowing 1% CO balance He, and (B) PROX conditions: 0.8% CO, 0.8%
O2, and 4% H2 balance He. The inserts are a magnified section where the normalized absorption is maximum.

106
S. Guerrero et al. / Journal of Catalysis 262 (2009) 102–110
◦
Fig. 2. Normalized LII XANES spectra: (A) Pt/X%Nb–Al2O3 catalysts (X = 0, 5, 10, 20) in flowing He after being reduced at 250 C, and (B) Pt/5%Nb–Al2O3 and Pt/Al2O3 catalyst
◦
◦
reduced at 250 C and oxidized at 205 C. The insert in (A) shows the area were the normalized absorption is maximum.
Fig. 3. Normalized LII XANES spectra for the reduced Pt/5%Nb–Al2O3 catalyst from 13.24 to 13.30 keV. Spectra taken in: (A) flowing 1% CO balance He, and (B) PROX
conditions: 0.8% CO, 0.8% O2, and 4% H2 balance He.
ferent temperatures. Conversions were measured at steady state
by online gas chromatography. The IR spectra of linear CO (L–CO)
frequency of water is located at 1600 cm−1, and its rotational–
vibrational modes appear as a noise-like baseline around that fre-
quency.
The IR spectra of Pt/Nb2O5, Fig. 4C, shows a lower L–CO ab-
sorbance, due to its lower dispersion, and no formates are ob-
adsorbed in the presence of hydrogen (not shown) were broad
−1
and located at 2036, 2018 and 2071 cm
on Pt/Al2O3, Pt/5%Nb–
Al2O3 and Pt/Nb2O5 respectively. Adsorbed CO IR spectra under
served with only adsorbed bicarbonates appearing at 1619 and
PROX reaction conditions (Fig. 4), were different than in pure CO
−1
1414 cm−1. These bands on Pt/Nb2O5 are about 26 cm
lower
correspond-
−1
flow showing additional bands appearing from 1700 to 1200 cm
.
−1
−
than those on Pt/Al2O3. The weak band at 2078 cm
The new bands are attributed to formates (HCO2 ) at 1590, 1393
and 1373 cm−1, carbonates (CO23−) at 1510 cm−1, and bicarbonates
ing to CO adsorbed on Pt2+, remains almost unchanged as temper-
ature increases, indicating no change in the CO surface coverage
with increasing temperature, while for the Pt/Al2O3 and Pt/5%Nb–
Al2O3 there is a decrease in CO coverage at high temperature. No
bridge bonded CO was detected at any temperature and the (gas
phase) CO2 band for Pt/Nb2O5 was also much smaller than for the
other catalysts in agreement with the lower activity of Pt/Nb2O5
towards CO oxidation during the PROX reaction [17]. The effluent
from the reactor was analyzed in an IR gas-cell (1 in. OD, 6 in.
long). For all the catalysts, the only PROX reaction products were
water, CO2, and CO gas.
−
−1
(HCO3 ) at 1645, 1440 and 1230 cm
[7,25–35] adsorbed on the
supports. Fig. 4A shows the IR spectra on Pt/Al2O3 in the temper-
◦
ature range of 50–200 C, along with the corresponding CO con-
◦
version at each temperature. At 50 C, the L–CO band is located at
−1
−1 ◦
2052 cm
and gradually red-shifts to 2035 cm
at 200 C (and
a conversion of 15%). A small broad band corresponding to bridge
−1
bonded CO species is observed at 1819 cm
and its intensity de-
creases slightly as temperature increases. Adsorbed formate species
−1
give rise to an IR absorption band at 1591 cm [7], which appears
◦
around 120 C and increases with temperature and increasing CO
conversion. A double peak observed at 1392 and 1373 cm
−1
asso-
3.3.2. CO oxidation only (without H2)
ciated with formates also increases in intensity with temperature.
Similar bands positions are shown in Fig. 4B for the Pt/5%Nb–Al2O3
catalyst, although at the same temperatures, CO conversions were
higher than on Pt/Al2O3, consistent with the increase rates of re-
action measured in the recycle reactor [17]. At higher conversions,
Previously we reported that the selectivity to H2 oxidation on
Pt/Nb2O5 during the PROX reaction was favored whereas the CO2
selectivity was drastically decreased, which is also shown in Ta-
ble 1 [17]. Operando transmission FTIR for oxidation of CO in the
absence of H2 (0.8% CO and 0.8% O2, balance He) was conducted
on Pt/Nb2O5 and Pt/5%Nb–Al2O3 (Fig. 5).
−1
the formate bands at 1589 cm−1, 1373 cm
and carbonate bands
are more intense on Pt/5%Nb–Al2O3 than on Pt/Al2O3. Such accu-
mulation of surface formates on a Pt/Al2O3 catalyst under PROX
reaction was also reported by Schubert et al. [7]. The absorption
The results show that on Pt/Nb2O5, the position of the L–CO
band is 2083 cm−1, slightly higher than during the PROX reac-
tion. The intensity of the adsorbed CO band increases with in-

S. Guerrero et al. / Journal of Catalysis 262 (2009) 102–110
107
◦
Fig. 4. FTIR spectra during PROX reaction conditions after reduction in H2 at 250 C for 1 h on (A) 1%Pt/Al2O3, (B) 1%Pt/5%Nb–Al2O3, and (C) 1%Pt/Nb2O5. Gas mixture: 0.8%
CO, 0.8% O2, 51% H2, balance He. Total flow: 195 cc/min.
◦
Fig. 5. FTIR spectra during the CO oxidation reaction after reduction in H2 at 250 C for 1 h on (A) 1%Pt/Nb2O5, and (B) 1%Pt/5%Nb–Al2O3, gas.0.8% CO, 0.8% O2, balance He.
Total flow: 195 cc/min.
creasing temperature before ignition. The increased intensity may
indicate some oxidation of the reduced Pt under reaction con-
ditions (Fig. 5A) since the position of the CO band does not
change. For Pt/5%Nb–Al2O3 the frequency of the L–CO band is
2055 cm−1, significantly higher than the main CO only adsorp-
tion band (2018 cm−1). The higher frequency IR absorption band
is assigned to CO adsorption on oxidized Pt. With increasing tem-
perature, the CO IR absorption slowly decreases with increasing
and 1438 cm−1. Other adsorbed carbonates species on Pt/Nb2O5
were not observed due to the low intensity of those bands. The
−1
band at 1416 cm
is absent on the Pt/Nb2O5 catalyst after igni-
◦
tion, Fig. 5A. CO ignition is observed at 245 and 240 C for the
Pt/Nb2O5 and Pt/5%Nb–Al2O3 catalysts, respectively. For Pt/5%Nb–
Al2O3, Fig. 5B, the bands at 1647 (bicarbonate), 1586 (formate),
−1
1438 (bicarbonate), 1393 (formate), and 1371 cm
(formate) are
still present after ignition. However, these bands disappeared when
oxygen alone was passed over the catalysts at the ignition temper-
ature (not shown).
temperature consistent with the XANES spectra. On the Pt/Nb2O5
−1
catalyst, there is only a very small bicarbonate band at 1416 cm
;
while on Pt/5%Nb–Al2O3 there are a set of bands corresponding to
In summary, operando transmission FTIR spectra for CO oxida-
tion show adsorbed CO only, very different than during the PROX
formates at 1586, 1393, and 1371 cm−1, and bicarbonates at 1647,

108
S. Guerrero et al. / Journal of Catalysis 262 (2009) 102–110
−1
versus frequency in the range of 1800 to 1100 cm
on catalysts
with different Nb loadings. It can be seen that on most catalysts
CO2 adsorption gives a set of bands similar to those observed un-
der PROX conditions. The peaks observed in Fig. 6 are the superpo-
sition of absorbance bands from different species: bicarbonates at
1646, 1438, 1229 cm−1, and formates at 1372 cm−1. The shoulder
−1
near 1700 cm
on catalysts with low Nb loadings indicates the
possible formation of carbonates [33]. It is readily observed that
the catalysts loaded with 1, 5, and 10% Nb exhibit the highest in-
tensities due to bicarbonates which decreases as loading increases
and no bands are detected on Pt/Nb2O5.
The catalyst promoted with 20% Nb does not show a band
−1
at 1372 cm
(formate), whereas such band is still observed on
Pt/Al2O3. While formates and carbonates appeared in the PROX
oxidation on promoted and unpromoted Pt, the intensity is much
higher than during the CO2 adsorption experiment even though
CO2 concentrations are much lower during the PROX operando
studies. Carbonates are seen only at high conversion during CO
oxidation and none is detected on Pt/Nb2O5. So the formation of
the low frequency bands observed during the PROX reaction is not
only due to CO2 adsorption on the support but also due to species
generated during reaction, i.e. in the presence of hydrogen and for-
mation of H2O.
Fig. 6. CO2 adsorption on Pt/X%Nb–Al2O3 catalysts (X = 1, 5, 10, 20, 30, 50, and
100%, X = 0 corresponds to the unpromoted Pt/Al2O3 catalyst). The adsorption was
◦
◦
carried out at 50 C after reduction at 200 C in H2.
reaction wherein additional bands ascribed to carbonates and for-
mates are detected. The band of adsorbed L–CO appears at higher
wave numbers on Pt/Nb2O5 than on promoted and unpromoted
catalysts, indicating that the Pt surface is mainly populated by
weakly adsorbed linear CO on semi-oxidized platinum. The pres-
ence or Pt2+ under reducing conditions as shown previously by
the EXAFS/XANES results, does not inhibit the adsorption of CO in
semi-oxidized platinum and neither the CO oxidation activity of
the promoted catalyst. However, in the absence of hydrogen the
Pt/Nb2O5 catalyst shows a CO ignition similar to that of Pt/5%Nb–
Al2O3.
3.4.2. Nb=O infrared absorbance
Burcham et al. [37] using Raman and IR, studied the Nb=O
symmetric stretching mode of Nb2O5 supported in different ox-
−1
ides. They found that Nb=O absorbed at 980 cm
when Nb2O5
was supported either on Al2O3 or TiO2. We monitored the Nb=O
◦
band using DRIFTS after reducing the catalysts in H2 200 C for 1 h,
then cooling to room temperature in He. Spectra were collected at
room temperature and the spectrum of Pt/Al2O3 was subtracted
from each spectra of the Nb promoted Pt/Al2O3 catalysts.
Fig. 7A shows the frequency of the Nb=O absorption band with
increasing Nb loading, and Fig. 7B shows the normalized integrated
intensity (IA) of the Nb=O band vs Nb loading (highest integrated
area used as normalization factor). The absorption frequency of the
3.4. Diffuse reflectance (DRIFTS) FTIR
DRIFTS experiments were conducted to probe the state of the
Nb=O species and to investigate the spectra of adsorbed CO2
−1
which originated bands in the 1800–1100 cm
the ones observed during the PROX reaction.
range similar to
Nb=O band shifts to higher values as Nb loading increases, level-
−1
ing off at 980 cm
at the higher loadings (Fig. 7A), whereas the
3.4.1. CO2 adsorption
normalized intensity of the Nb=O absorption band increases with
the Nb loading (Fig. 7B). It has been reported that when Nb=O
coverage is exceeded, polymerization of Nb occurs [17,38,39]. The
change in the Nb=O frequency together with the increase in its
integrated absorbance, suggest that polymerization of finely dis-
persed Nb species occurs as Nb loading increases.
It is well known that CO2 can form a variety of adsorbates on
hydroxylated Al2O3 [32–35,40–44], therefore we studied its ad-
sorption using DRIFTS. The catalysts were first reduced in H2 at
200 C for 1 h and then cooled to 50 C, followed by introduction
of CO2 at that temperature. The DRIFTS results are presented in
Fig. 6 for all the catalysts in terms of Kubelka–Munk units (KM)
◦
◦
Fig. 7. DRIFTS on the Pt/X%Nb–Al2O3 (X = 0, 1, 5, 10, 20, 30, 50, and 100%) catalysts. (A) Change in the absorption frequency of the Nb=O bond as a function of Nb loading.
(B) Relative integrated absorbance from (A).

S. Guerrero et al. / Journal of Catalysis 262 (2009) 102–110
109
4. Discussion
in the activity/selectivity results obtained in our catalysts, but this
pathway cannot be completely ruled out either.
No significant differences were observed between the operando
PROX-IR spectra of L–CO on Pt/Al2O3 and Pt/5%Nb–Al2O3 catalysts
(Fig. 4). This further supports the hypothesis that the partially oxi-
dized Pt species increase the oxidation of CO favoring CO2 selectiv-
ity, although how this pathway take place is not known. There is
a significant difference in both intensity and frequency of the CO
spectra on Pt/Nb2O5, a catalyst with low CO2 selectivity. CO ab-
sorbance is smaller, consistent with the lower dispersion of Pt on
this catalyst, and the L–CO frequency is higher, indicating a weaker
bond onto a more oxidized surface. Thus one possibility to explain
the low CO2 selectivity of Pt/Nb2O5 is that this weakly adsorbed
CO does not inhibits the rate of H2 and oxygen dissociative ad-
sorption, resulting in higher hydrogen oxidation and lower CO2
selectivity. We hypothesize that the additional species formed in
the presence of hydrogen during the PROX reaction, are responsi-
ble for the decrease in both amount and strength of CO adsorption
decreasing its inhibition of hydrogen oxidation.
FTIR spectra during the PROX reaction show several additional
bands corresponding to formates and bicarbonates. The CO2 ad-
sorption DRIFTS experiments show that bicarbonate and carbon-
ates bands also appear, indicating that the additional carbonate
and formate species could originate from CO2 formed during CO
oxidation. The intensity of the formate band produced from water
and CO2 during the operando PROX FTIR experiments increase with
conversion. While band intensities in transmission and KM-units
in DRIFTS experiments are similar but not completely equivalent,
the intensity of these low frequency bands is larger during the
PROX reaction even though CO2 concentration is lower. This sug-
gests that the carbonate, bicarbonate and formate bands cannot be
ascribed only to CO2 adsorbed on the support, and that the re-
action pathway is not just the simple competitive adsorption and
reaction between adsorbed CO, hydrogen, and oxygen. Additional
species related to these low frequency bands must also form from
the adsorbed reactants. The formation of hydroxyls both, on Pt, and
the support are known to play a role. In fact detailed kinetics sim-
ulations of the PROX reaction in our group indicate that the rate
of formation of adsorbed hydroxyls is the fastest one followed by
its reaction with adsorbed hydrogen to form water and with ad-
sorbed CO to form CO2 [58]. These adsorbed hydroxyls can form
other species with adsorbed CO that might be responsible for the
low frequency bands observed during the PROX reaction. We spec-
ulate that these additional species may play a role in reducing CO
adsorption and its inhibition of H2 oxidation on Pt/Nb2O5. In the
operando CO oxidation only FTIR experiments, very small low fre-
quency bands are detectable on Pt/5%Nb–Al2O3, while there are
no such bands on the Pt/Al2O3 catalyst. Thus, in the absence of
hydrogen, these additional species are not present and CO readily
adsorbs on semi-oxidized Pt and form CO2 at a rate comparable to
Pt/Al2O3 which explains why during CO oxidation only is high on
Pt/Nb2O5 but low during the PROX reaction.
Activity results show that on Pt supported catalysts, hydrogen
oxidation only is the fastest reaction igniting at room tempera-
ture, whereas during the PROX reaction CO inhibits hydrogen and
oxygen dissociative adsorption at low temperature favoring CO2 se-
lectivity. As temperature increases, CO desorption increases, oxygen
adsorption increases leading to hydrogen oxidation and a decrease
in CO2 selectivity. CO inhibition is further promoted by the pres-
ence of niobia at low concentration, but it decreases as niobia
loading increases. On Pt/Nb2O5, however, CO adsorption is lower
and weaker inhibiting less H2 adsorption and oxidation resulting
in low CO2 selectivity. Interestingly on this catalyst CO oxidation is
high when H2 is not present. Below we attempt to correlate these
activity-selectivity results with structural and FTIR results.
The EXAFS results show that, under PROX reaction conditions,
on the unpromoted Pt/Al2O3 only metallic Pt is present, while on
catalysts promoted with low Nb loadings, the Pt is partially re-
duced with some partially oxidized Pt2+. The presence of Pt2+
under reducing conditions, as shown by EXAFS/XANES, does not
inhibit the adsorption of CO on semi-oxidized platinum. It is likely
that these oxidized Pt species play a role on increasing the CO2 se-
lectivity under PROX reaction, thus explaining the promoting effect
of Nb.
The presence of semi-oxidized Pt on the promoted catalysts
suggests a complex surface made of Pt crystallites and PtO species
affected by niobia moieties surrounding the Pt crystallites or dec-
orating them. IR results indicate that Nb moieties can have dif-
ferent structural characteristics depending on loading. The lower
frequency Nb=O band observed during the DRITFS experiments
on low Nb loadings catalysts indicates formation of isolated Nb=O
moieties. As the Nb loading increases, two-dimensional Nb islands
appear and thus the intensity of Nb=O increases. Above the mono-
layer coverage (20%), the frequency of Nb=O remains constant at
−1
980 cm
which is consistent with the presence of Nb2O5 aggre-
gates [36,37]. In our previous work, we reported that ex situ XPS
showed an abrupt increase in the Nb 3d binding energy when
the Nb loading reached a monolayer coverage, also indicative of
the formation of three-dimensional bulk niobia oxide at higher Nb
loadings [17]. This structural change is in agreement with the blue
shift of the Nb=O absorption frequency observed in Fig. 6A. Under
these conditions several scenarios can develop with Nb moieties in
close proximity to the Pt crystallites, or decorating them at high
loadings or both.
Increasing the loading of Nb changes the support environment
and it is expected that such changes will also affect the interaction
between Pt and the reacting species. The niobia-support interac-
tion has been discussed in the literature [45–49], indicating that
support surface hydroxyls play a fundamental role in the type of
species formed. The formation of Nb species on the Al2O3 support
not only occurs by impregnation of the support, but it also occurs
by titration of surface hydroxyls [50]. It is not clear how these sup-
port hydroxyls affect the reaction however. DRIFTS spectra of the
5. Conclusion
various supports after reduction (not shown), showed broad bands
−1
between 3700 and 2750 cm
corresponding to surface hydroxyls
The present work combines XAFS and IR characterization tech-
niques to better understand the promotion effect of Nb on Pt/Al2O3
catalysts for the PROX reaction. Addition of small amounts of Nb
(<5%) stabilizes 40% of the loaded platinum as Pt2+ even after re-
duction treatments. Increasing loadings of Nb forms NbOx species
that first forms isolated islands, then a monolayer, and three-
dimensional oxides at very high loadings. These NbOx species may
be surrounding the Pt crystallites affecting the interface between
Pt and the alumina support and the species adsorbed on these
metal–support interfaces. In addition, some Nb may be decorating
the Pt surface affecting the oxidation state. These interactions are
and chemisorbed water, which varied with Nb loading. A possi-
ble effect of the niobia support is that hydrogen adsorbed on Pt
can spillover to the support [26,52–56] and react with the support
hydroxyls [51,53,57]. Therefore a lower concentration of hydrox-
yls, caused by niobia titration, could decrease hydrogen oxidation
and favor CO2 selectivity. In fact, Pedrero et al. [10] found that
adding Cs progressively to Pt supported catalysts titrated surface
hydroxyls, which improved the PROX selectivity by restricting the
hydrogen spill-over pathway. We have no evidence that spillover
occurs on our catalysts, however, and we cannot ascertain its role

110
S. Guerrero et al. / Journal of Catalysis 262 (2009) 102–110
complex, thus it is difficult to ascertain uniquely which factors are
responsible for the changes in the observed activity and selectiv-
ity. Nonetheless it is clear that the formation of partially oxidized
Pt species increases CO2 activity and selectivity.
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The PROX reaction involves both CO and H2 oxidation as well
as the formation of intermediates associated with the presence
of hydroxyls, CO2 and H2O on the catalyst’s surface. Adsorbed CO
inhibits the much faster H2 oxidation reaction and this effect in-
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very low. It is suggested that the low CO2 selectivity results from
the decrease in the amount and strength of CO adsorption due to
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We gratefully acknowledge partial support of this work by
a grant from Companhia Brasileira de Metalurgia e Mineração,
CBMM; a Bayer Postdoctoral Fellowship in Environmental Chem-
istry through the Center for Environmental Science and Technology
at the University of Notre Dame, and NSF Grant CTS 0138070. Use
of the Advanced Photon Source was supported by the US Depart-
ment of Energy, Office of Basic Energy Sciences, Office of Science
(DOE-BES-SC), under Contract No W-31-109-Eng-38. The MRCAT is
funded by the member institutions and DOE-BES-SC under con-
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