On highly active partially oxidized platinum in carbon monoxide oxidation over supported platinum catalysts

Article abstract of DOI:10.1016/j.jcat.2009.02.010

In situ X-ray absorption spectroscopy identified the role of oxidized platinum species in generating a high-activity state in the oxidation of carbon monoxide over Pt/Al₂O₃, Pt/TiO₂ and Pt/SiO₂. Two activity reg

Full text of DOI:10.1016/j.jcat.2009.02.010

Journal of Catalysis 263 (2009) 228–238
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Journal of Catalysis
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On highly active partially oxidized platinum in carbon monoxide oxidation over
supported platinum catalysts
a
a
b
b
a,∗
E.M.C. Alayon , J. Singh , M. Nachtegaal , M. Harfouche , J.A. van Bokhoven
a
Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland
Paul Scherrer Institute, General Energy Research Department, Laboratory for Energy and Materials Cycles, 5232 Villigen PSI, Switzerland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In situ X-ray absorption spectroscopy identified the role of oxidized platinum species in generating a
high-activity state in the oxidation of carbon monoxide over Pt/Al2O3, Pt/TiO2 and Pt/SiO2. Two activity
regimes were identified: low-activity at high carbon monoxide concentration and low temperature, and
high activity at low carbon monoxide concentration and high temperature. The change between the two
regimes occurred suddenly; there was ignition when going from low to high activity, and extinction
from high to low activity. Ignition only occurred when the concentration of oxygen was higher than that
of carbon monoxide. The catalyst was metallic and covered with carbon monoxide at low activity and
partially oxidic at high activity. Carbon monoxide poisoned the catalyst at low activity. The partially
oxidized platinum catalysts showed high activity. The partially oxidized catalyst had a lower oxygen
Received 24 November 2008
Revised 5 February 2009
Accepted 12 February 2009
Available online 17 March 2009
Keywords:
In situ XAS
Carbon monoxide oxidation
Supported platinum catalysts
Active phase
coordination number and shorter Pt–O bond length as expected for bulk PtO , suggesting that a strongly
2
defected platinum oxide was formed with a possibly square planar coordination. The catalyst structure
showed a dynamic behavior and the amount of oxide depended on the concentration of reactants in
the gas phase. Smaller particles of Pt/Al2O3 reached high activity at lower temperature than the larger
particles. Pt/TiO2 reached high activity at the lowest temperature while Pt/SiO2 required the highest
temperature. Parallel with the temperature of ignition, Pt/SiO2 was the most oxidized while Pt/TiO2 was
the least oxidized.
©
2009 Elsevier Inc. All rights reserved.
1
. Introduction
structural analysis under catalytically relevant conditions [2,3]. The
Pt L3 whiteline, which is the first intense feature in the spectrum,
is sensitive to the oxidation state of the catalyst as it reflects the
empty d-density of states. The Pt L3 X-ray absorption near-edge
structure (XANES) is also sensitive to the local geometry and to
the presence of adsorbates on the surface of the nanoparticles and
the shape of the whiteline reveals the mode of adsorption [4–6].
The extended X-ray absorption fine structure (EXAFS) function is
dependent on the local structure of the absorbing platinum atom.
It provides coordination number, inter-atomic distance, and Debye–
Waller factor [2].
The catalytic oxidation of carbon monoxide is the key reac-
tion during the preferential oxidation of carbon monoxide in a
hydrogen feed gas mixture in fuel cells, where it is crucial to de-
crease the carbon monoxide concentration below 50 ppmv [7]. It
is also one of the reactions occurring in a three-way catalytic con-
verter, where the pollutant gases are transformed into less harmful
ones. Despite the seeming simplicity of carbon monoxide oxida-
tion, the structure of the catalyst and the mechanism at high ac-
tivity are still debated. Structural transformation of the platinum
catalyst during carbon monoxide oxidation, both on single crystals
and supported particles, has been reported. On a single crystal of
The catalytic properties of platinum remain one of the most
fascinating subjects of study. While it belongs to the family of
noble metals because of its rarity in the earth’s crust and its re-
sistance to corrosion, it has a different chemical behavior than the
other noble metals copper, silver, and gold. Its extended d-orbitals
enable chemisorption of adsorbates that subsequently react and
desorb to complete the catalytic cycle. In spite of the wide impor-
tance platinum has gained in industry, and the extensive research
performed on its catalytic behavior, the fundamental aspects of ad-
sorption, reaction, and desorption that occur at its surface continue
to be debated. Understanding what is happening at the catalyst
surface and what the structure of the catalytically active phase is,
are important for controlling the kinetics of the reaction in terms
of activity, selectivity, and stability. Structural characterization is
most relevant when performed under reaction conditions, which is
generally at high pressure and temperature [1]. X-ray absorption
spectroscopy (XAS) is an element-specific technique that enables
Corresponding author.
E-mail address: j.a.vanbokhoven@chem.ethz.ch (J.A. van Bokhoven).
*
0
021-9517/$ – see front matter © 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.02.010

E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
229
Pt(100) [8,9], a transformation occurs from a hexagonal phase to a
reaction and the influence of particle size in Pt/Al2O3 and deter-
mine their structure under catalytic conditions using in situ Pt L3
XAS.
(
1 × 1) phase with islands of c(2 × 2) when going from a low to
a high carbon monoxide surface concentration. This surface phase
transition [8] has been identified to be the driving force for the ki-
netic oscillations in catalytic carbon monoxide oxidation because
the adsorption properties of the surface switch with the transi-
tion. The experiments were carried out on clean single crystals
under ultra-high vacuum in a system equipped with LEED, AES,
and work function measurement facilities. A possible formation of
an oxide was linked to the presence of surface impurities, thus the
surface of the catalyst was repeatedly cleaned to suppress oxide
formation. Later experiments performed with scanning tunneling
microscopy [10] and surface X-ray diffraction [11] on a single crys-
tal of Pt(110) surface near atmospheric pressure showed a rough-
ening of the surface coinciding with a sudden increase in catalytic
activity for carbon monoxide oxidation. This switching of surfaces
was attributed to a change in the surface composition. When the
carbon monoxide concentration dropped below a particular value,
the surface oxidized. This oxidized surface showed higher cat-
alytic activity than the metallic surface [10,11]. To mimic industrial
supported catalysts, nanometer-sized metal particles of platinum
were deposited on silica, titania and alumina in ordered arrays by
electron beam lithography. Carbon monoxide oxidation was per-
formed at a few hundred Torr in various carbon monoxide and
oxygen ratios and the total pressure brought to atmospheric pres-
sure with helium. The reaction became sustainable above an igni-
tion temperature, at which new carbon monoxide species, which
were active for oxidation, were identified [12]. However, because
the oxidic species has less affinity for carbon monoxide adsorption
and the enhancement of conversion occurred at high temperature,
metallic platinum was identified as the active surface for carbon
monoxide oxidation [13,14]. Oxidation was also associated with
catalyst deactivation during oxidation rate oscillations. These os-
cillations corresponded with periodic oxidation and reduction of
the silica-supported platinum catalyst [15]. From previous results
the low surface concentration of carbon monoxide at high temper-
ature enabled oxygen to react with the platinum surface, which
switched the reaction from being limited by desorption of the car-
bon monoxide to a regime with a different rate limiting step. The
result was a higher rate of reaction and a partially oxidized cat-
alyst. The enhanced conversion of carbon monoxide depleted the
gas phase carbon monoxide concentration further, which decreased
the surface concentration of carbon monoxide. This increased the
extent of surface oxidation and caused an autocatalytic ignition of
the reaction [16].
2. Experimental
2.1. Catalyst preparation and characterization
2
wt% Pt/Al2O3 was prepared by incipient-wetness impregna-
tion. A solution of 0.197 g Pt(NH3)4(NO3)2 in 2.55 mL water was
mixed with 5 g of Al2O3, which was dried overnight at 120 C.
The mixture was shaken for 30 min and dried at room tempera-
ture for 4 h. The precursor was calcined in a flow of air at 0.6 bar
in two stages: at 200 C for 4 h and then at 400 C for 4 h. The
catalyst was reduced in a flow of hydrogen at 450 C for 2 h and
then cooled in a flow of helium. Two sets of this catalyst were
synthesized. 3 wt% Pt/TiO2 was also prepared by incipient-wetness
impregnation. A solution of 0.3 g of H PtCl6 in 70 mL water, with
the pH adjusted to 4–5 using NaOH, was added to 4.0 g of TiO .
◦
◦
◦
◦
2
2
◦
The solution was stirred for 2 h at 70 C. The obtained paste was
filtered and washed with 1 L of warm water (70 C) and dried un-
der vacuum. The catalyst was calcined in a flow of air: heated at
◦
◦
◦
5 C/min to 120 C, remained at this temperature for 4 h, heated
◦
◦
again at 5 C/min to 400 C, remained at this temperature for an-
other 4 h, and cooled to room temperature in a flow of air. The
catalyst was reduced in a flow of 50 mL/min hydrogen for 2 h
◦
at 200 C and cooled down to room temperature in 10 mL/min of
flowing helium. Another 2 wt% Pt/SiO2 catalyst was synthesized by
incipient-wetness impregnation. 40 mL water was added to 0.197 g
of Pt(NH3)4(NO3)2. NaOH was added to adjust the pH to 10. The
solution was mixed with 5 g of dried SiO2 and stirred for 2.5 h,
◦
was filtered, washed with 1 L of warm water (70 C), and dried at
◦
100 C for 5 h. The calcination was done in a similar way as for
Pt/TiO2 described above. The catalyst was reduced in 50 mL/min
of flowing hydrogen for 2 h at 250 C and cooled to room temper-
ature in hydrogen flow.
The size and shape of the platinum nanoparticles were ob-
served by transmission electron microscopy (TEM) with a Tecani
F30 microscope operating at 300 kV. A detector attached to
the Tecani microscope using energy-dispersive X-ray spectroscopy
(EDX) confirmed the tiny bright spots as the platinum particles.
These particles were measured, counted, and translated into a par-
ticle size frequency distribution.
◦
2.2. Kinetics and XAS measurement
The size of supported platinum particles strongly affects their
catalytic performance [13,17,18]. Moreover, the type of support and
the interface between particles and support influence the catalytic
turnover [19–25]. Thus, the important factors controlling catalytic
activity over platinum are particle size, support, and the active
phase. It is our goal to determine the structure of the active cat-
alyst under reaction conditions, and to determine the effect of
particle size and that of the support. We compare the behavior of
nano-sized platinum on Al2O3, TiO2 and SiO2 during the catalytic
Each of the catalysts was sieved to 63–125 μm, before loading
in a plug flow (tubular packed bed) reactor, which was 1.6 mm
in internal diameter, and which could accommodate 18 to 21 mg
of the supported platinum catalysts depending on their density.
The reactor also functioned as a transmission and/or fluorescence
cell for X-ray absorption spectroscopy [26]. In addition, 2 wt%
Pt/Al2O3 catalyst was loaded in a thin bed reactor [27], which ac-
commodated 150 mg of the catalyst. This cell also functioned as
Table 1
Summary of catalytic properties of supported platinum catalysts in the oxidation of carbon monoxide.
Catalyst
Particle
size
wt%
platinum
Amount of
catalyst
(mg)
Platinum
loading
(mg)
Ignition
Extinction
Hysteresis
( C)
◦
temperature
temperature
◦
◦
(nm)
( C)
( C)
s-Pt/Al2O3
b-Pt/Al2O3
b-Pt/Al2O3a
Pt/TiO2
0.9
2.0
2.0
1.3
1.9
2
2
2
3
2
18.4
18.0
150.0
21.2
0.4
0.4
3.0
0.6
0.4
204
226
169
188
299
195
191
115
145
281
9
35
40
43
18
Pt/SiO2
18.0
a
Measured in thin bed reactor.

230
E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
Table 2
ble temperature and flow conditions, EXAFS scans were recorded
Gas treatment and reaction conditions.
from 11.350 to 12.200 keV. It took about 30 min to complete one
EXAFS scan. Multiple scans were averaged to improve the signal to
noise ratio.
Step
Gas
Flow
mL/min)
Temperature ramp
( C/min)
Temperature
( C)
◦
◦
(
Reduction sequence
1
2
3
4
5
6
He
10% H2/He
10% H2/He
He
He
He
30.0
30.0
30.0
30.0
30.0
30.0
–
5
–
–
5
–
35
–
200
200
–
2.4. Data treatment
The traces for carbon monoxide, oxygen, and carbon dioxide,
recorded by the mass spectrometer in terms of current, were di-
vided by the helium signal for normalization. They were further
processed to conversion curves against time or temperature. Con-
version in carbon monoxide, (XCO), was defined as
35
Heating to maximum CO conversion
7
10% O2/He
CO
He
10.0
1.0
19.0
2
0
2
ramp
–
Φ
− Φv,CO,out
v,CO,in
v,CO,in
XCO
=
,
Φ
Increasing CO flow
10% O2/He
8
10.0
1.25
18.75
where Φv,CO,in and Φv,CO,out were the flowrates of carbon monox-
ide at the inlet and outlet of the reactor respectively. The inlet
flowrate was controlled by the mass flow controller. The outlet
flowrate was derived from a calibration line assuming that the
normalized signal at maximum conversion corresponded to 100%
conversion, and that the normalized signal in the absence of con-
version was the flowrate set at the inlet.
CO
He
Cooling to room temperature
9
10% O2/He
CO
He
10.0
1.0
19.0
ramp
Data reduction for the XAS spectra was done using standard
procedures [2,3]. It involved subtraction of the pre-edge back-
ground, edge energy determination, subtraction of the background
signal from the spectra, and normalization of the edge jump to
one. The data were processed using XDAP [28]. Previously gener-
ated reference files [29] were used to fit the EXAFS data. These
were generated from the known unit cell parameters of platinum
metal and platinum oxide using Atoms [30], calculated using the
FEFF8 code [31] and fitted to actual XAS measured data of plat-
inum foil and platinum oxide. Multiple shell fitting was performed
in R-space (1.5 < R < 3.5) after Fourier transformation (2.5 < k <
12), and using a k-weighting of three for platinum–platinum scat-
terer pairs and two for the spectra that contained a mixture of
platinum–oxygen and platinum–platinum pairs.
a transmission cell for XAS. Table 1 summarizes the catalyst load-
ings. All gases used were from Messer and were of high purity
(
>99.995 vol%). Mass flow controllers were calibrated with a digi-
tal flow meter. A mass spectrometer (GSD 300 O2, OmniStar, from
Pfeiffer Vacuum) was attached at the end of the reactor for prod-
uct gas analysis. Before a reaction, the catalyst was reduced in
a sequence shown in Table 2. The total gas flowrate was always
30 mL/min corresponding to a space velocity of 0.06 s through
the catalyst bed or a weight hourly space velocity (WHSV) of 15
−1
−1
gcat depending on the gas mixture and catalyst
to 26 ggas h
loading in the plug flow reactor. After the pretreatment, a reac-
tant gas mixture consisting of 10 mL/min of 10% oxygen in helium,
1
mL/min of carbon monoxide, and 19 mL/min of helium was fed
to the reactor. This corresponds to a three percent carbon monox-
ide feed and an oxygen/carbon monoxide ratio of one. Temperature
was ramped up using 2 C/min until a 100% conversion in car-
3. Results
◦
bon monoxide was observed from the mass spectrometer traces
of carbon monoxide, oxygen, and carbon dioxide. At two to five
degrees higher than the temperature of maximum conversion, the
carbon monoxide flow was increased to 1.25 mL/min, yielding a
four percent carbon monoxide in the feed, while maintaining the
total flowrate at 30 mL/min. This resulted in a conversion in car-
bon monoxide of less than 100%. Flowrates were always returned
to their initial values before the temperature was ramped down at
3.1. Particle size
Fig. 1 shows TEM micrographs of the supported platinum cat-
alysts. The little bright dots are the platinum particles and the
larger light gray areas are the metal oxide supports. The platinum
particles were well dispersed over all supports except silica. The
small platinum on alumina particles (from here on referred to as
s-Pt/Al2O3) were mainly 0.9 nm; the bigger platinum on alumina
(b-Pt/Al2O3) particles 2.0 nm; the platinum on titania particles
◦
2
C/min to room temperature.
1.3 nm, and the platinum on silica particles 1.9 nm. The particles
2
.3. In situ X-ray absorption spectroscopy
on the latter support showed a significant fraction of larger parti-
cles.
XAS measurements were performed at the X10DA (superXAS)
beamline at the Swiss Light Source, Villigen, Switzerland. Energy
scans were performed with a Si-111 double crystal monochromator.
Beam size was approximately 100 by 115 μm. Spectra were col-
lected either in fluorescence or transmission geometry depending
on the support. The first ionization chamber was filled with he-
lium to detect the initial intensity; the second ionization chamber
with a mixture of helium and nitrogen to detect the transmit-
ted X-rays. A platinum foil was placed between the second ion
chamber and a silicon diode for internal energy calibration. In flu-
orescence mode, a 13-channel germanium detector was used. Dur-
ing ramping up and down the temperature, XANES spectra were
recorded from 11.545 to 11.600 keV. One XANES scan took four
minutes, which corresponds to an eight degree span from start
to finish using a ramp of two degrees per minute. Under sta-
3.2. Kinetic data
Table 1 summarizes the results of the kinetic measurements.
Fig. 2 shows the normalized intensity signals from the mass spec-
trometer traces of carbon monoxide, oxygen, and carbon diox-
ide coming out of the reactor during carbon monoxide oxida-
tion over s-Pt/Al2O3, and the temperature of the reactor. Fig. 3
displays the corresponding carbon monoxide conversion curve as
function of temperature. The points marked with letters A to E in
Fig. 2 correspond to those in Fig. 3. Starting at room temperature,
the conversion of carbon monoxide increased exponentially with
◦
temperature. At 204 C (B), a sudden jump in carbon monoxide
conversion was observed and the catalyst reached a high activ-
ity state, at which the conversion in carbon monoxide was about

E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
231
Fig. 1. TEM images of the supported platinum catalysts: (a) s-Pt/Al2O3, (b) b-Pt/Al2O3, (c) Pt/TiO2, and (d) Pt/SiO2.
Fig. 2. Mass spectrometer traces of carbon monoxide (black), oxygen (red), and carbon dioxide (blue) normalized to the helium signal and the temperature profile (green)
during carbon monoxide oxidation over s-Pt/Al2O3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2
1
−1
−1
5
4 mL/min/gcat or 1.3 × 10 molecules min gcat. We call this
Ignition curves for carbon monoxide oxidation over a polycrys-
talline platinum wire were previously reported [32], but emphasis
was more on the evolution of catalyst temperature with time. More
recently, ignition and extinction during carbon monoxide oxidation
over platinum were also identified [33], however these terms were
used to identify the temperature at which 50% of carbon monox-
ide converted. This 50% conversion point is referred to as light-off
temperature in literature. Carbon monoxide conversion curves with
temperature, including a similar oxygen/carbon monoxide ratio
were presented earlier [18] but only light-off conditions were iden-
jump ignition. At 21,200 s (C), the carbon monoxide flow was in-
creased to 1.25 mL/min and the flow of helium was decreased to
maintain the total flow at 30 mL/min. The resulting conversion was
about 97%. At 28,800 s (D), the carbon monoxide and helium flows
were returned to 1 and 18 mL/min respectively and the conversion
returned to 100%. Upon decreasing temperature, a sudden jump to
2% conversion was observed at 195 C (E). We call this reverse
jump extinction and the catalyst returned to the low activity state.
Hysteresis of 9 C between the two activity regimes was observed.
◦
4
◦

232
E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
Fig. 3. Conversion plot of carbon monoxide with temperature over s-Pt/Al2O3.
Fig. 4. Comparison of the heating (a) and cooling (b) curves for the different supported platinum catalysts: Pt/TiO2 (pink), s-Pt/Al2O3 (black), b-Pt/Al2O3 (red), and Pt/SiO2
green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(
tified and not jumps in conversion. Ignition and extinction were
indicated in the carbon monoxide conversion plots [34] where the
relationships among the kinetic parameters under conditions of
catalyst surface ignition during selective carbon monoxide oxida-
tion on Pt/Ru/ceramic support were investigated.
The conversion of carbon monoxide was also determined with
b-Pt/Al2O3 mounted in a thin bed reactor (data not shown). The
switch from low to high activity and the inverse switch from high
to low activity occurred at lower temperatures than when the
same catalyst was mounted in the plug flow reactor (Table 1). After
ignition, the conversion reached only about 70% probably because
of unfavorable flow conditions. The total flowrate was double that
of the plug flow by increased helium flow. The different observa-
tions cannot be attributed solely to the different flows and flow
scheme in the two types of reactors, since the thin bed reactor
contained much more catalyst than the plug flow reactor, 150 mg
versus 18.0 mg.
For all catalysts reported in this manuscript, the trend is similar
to that observed for the s-Pt/Al2O3 particles, however with dis-
tinctly different ignition and extinction temperatures. Fig. 4 shows
the conversion plot against temperature during heating and cool-
ing over the b-Pt/Al2O3, Pt/TiO2, and Pt/SiO2. During heating, the
platinum catalyst supported on TiO2 reached the high activity re-
◦
gion at the lowest temperature. Ignition was at 188 C, which was
already near maximum conversion. The first two breaks in the con-
version curve for Pt/TiO2 correspond to the points where the tem-
perature was held constant for about 1.5 h for taking EXAFS scans.
3.3. XANES spectra
◦
Pt/TiO2 switched to low conversion at 145 C, which is the low-
Fig. 5 shows the Pt L3 XANES spectra of platinum in the metal-
lic and in the Pt4 oxidized state. In the metallic state, the white-
line showed a maximum absorption of 1.25, less compared to that
in the oxidic state of 2.25. The metallic spectrum showed two
bands at 11.580 and 11.596 keV immediately after the whiteline
region. The oxidic spectrum showed a characteristic dip after the
whiteline, and showed fine structure at 11.582 and 11.590 keV.
Figs. 6a and 6b compare the Pt L3 XANES spectra measured
while heating and cooling s-Pt/Al2O3 under the flow of oxy-
gen/carbon monoxide with a ratio of one at various temperatures.
The indicated temperatures refer to the beginning of each scan.
+
est extinction temperature of all catalysts measured here. Pt/SiO2
◦
required the highest temperature, 299 C, to switch to the high
activity region. Pt/SiO2 jumped back to low activity also at the
◦
highest temperature, 281 C. The b-Pt/Al2O3 and s-Pt/Al2O3 cata-
lysts were intermediate between Pt/TiO2 and Pt/SiO2 in terms of
activity and the temperatures required for ignition and extinction.
The temperatures of ignition and extinction showed some variation
upon varying the amount of catalyst, flow rate, and temperature
ramps (data not shown). This indicates that ignition and extinction
occur in an unstable reaction regime. Normal temperature behav-
ior, without ignition and extinction, was observed when using a
stoichiometric ratio of CO and O (data not shown).
◦
The spectrum at 145 C was taken in an atmosphere of helium af-
ter reduction and removal of hydrogen. The spectrum showed a

E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
233
whiteline intensity similar to that of the platinum foil in Fig. 5.
The bands at energies above the whiteline were subdued com-
pared to that of the platinum foil spectrum but the characteristic
feature of the wiggles was the same. This spectrum shows that the
small platinum particles were in a reduced state after reduction
◦
and removal of hydrogen. The spectrum at 150 C was obtained
in a mixed carbon monoxide and oxygen atmosphere. Compared
to the one recorded in helium, there was an increase in intensity
and broadening of the whiteline to the high energy side, which
is indicative of platinum with carbon monoxide adsorbed on the
surface [6,16]. The features after the absorption edge overlapped
those of the spectrum in helium, indicating the metallic charac-
ter of the catalyst under these conditions. The spectrum measured
◦
◦
at 174 C shared the same features as the one at 150 C but with
a slightly decreased broadening of the whiteline indicating that
part of the carbon monoxide was desorbed from the surface. The
◦
spectrum taken at 198 C, which was at 46% conversion, started
Fig. 5. Pt L3 XANES of references: α-PtO2 (red), and platinum foil (black). (For in-
terpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
with a shape that resembled that of metallic platinum with car-
bon monoxide adsorbed on the surface, however, at 11.571 keV, it
suddenly changed to that of an oxide. This corresponds to the tem-
◦
◦
◦
◦
◦
Fig. 6. Pt L3 XANES spectra recorded during heating (a) at 145 C in helium (black), 150 C in oxygen and carbon monoxide (red), 174 C (blue), 198 C (green), and 211
C
◦
◦
◦
◦
◦
(
pink), during cooling (b) at 207 C (black), 199 C (red), 183 C (blue), and 83 C (green), and at the region of high activity (c) below ignition at 190 C (black), above ignition
at 211 C and 100% conversion (red), and above ignition at 210 C and 97% conversion (blue) of s-Pt/Al2O3; ignition occurred at 204 C, extinction at 195 C. (For interpretation
of the references to color in this figure legend, the reader is referred to the web version of this article.)
◦
◦
◦
◦

234
E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
◦
perature of ignition of 204 C, which correlates to point B in Figs. 1
and 2. The rapid change in rate of reaction is paralleled by fast
structural changes of the platinum, as observed with time resolved
ature and in flowing helium for s-Pt/Al2O3 showed a first shell
dominated by a Pt–Pt backscattering pair, with coordination num-
ber of 5.5. This correlates to metallic platinum particles with an
average size of about 1 nm [35]. The Pt–Pt distance was 2.72 Å,
smaller than the 2.76 Å of bulk platinum, indicating that the plat-
inum cluster contracts when the particle size is very small [35–38].
The EXAFS spectrum taken at a slightly higher temperature in the
presence of oxygen and carbon monoxide shows an increased Pt–Pt
distance to 2.77 Å and equal coordination number within the mar-
gin of error. Bond length relaxation occurred after the adsorption
of carbon monoxide as supported by the XANES similar to what
occurs after adsorption of hydrogen [36,38,39]. Above ignition, an
additional Pt–O scatterer with a Pt–O distance of 1.99 Å was ob-
served that agrees well with the Pt–O distances of the platinum
◦
XAS [16]. The spectrum taken at 211 C, which is above ignition,
had characteristic features of the spectrum of oxidized platinum in
Fig. 5. The maximum intensity of the whiteline was 1.8, which is
less than that of the Pt4 spectrum, indicating that either the oxi-
dation state is less than +4 or that there are still metallic platinum
species left in the catalyst under these conditions. During cooling,
+
◦
the shape of the spectrum taken at 207 C and above extinction
showed an intense whiteline and the characteristic dip after the
absorption edge. The scan taken at 199 C with a conversion of
00% started with the intense whiteline feature of an oxide and
◦
1
at 11.573 keV shifted to the wiggles of the metallic spectrum af-
ter the edge, which again corresponds to the point of extinction at
oxides, especially PtO and Pt O4 [40]. These fitting parameters are
also in agreement with the XANES spectra in Fig. 6 that showed
3
◦
195 C, when the conversion jumped down to 41%, which is point E
in Figs. 1 and 2. The other two scans recorded at lower tempera-
characteristic oxidic features of platinum above ignition. The Pt–Pt
coordination number decreased to 2.0 and the Pt–Pt distance de-
creased to 2.71 Å. This Pt–Pt contribution appeared to be a com-
bination of Pt–Pt contributions from the remaining metal core and
from the oxide and was thus difficult to fit separately.1 This Pt–Pt
◦
◦
tures 183 C (26% conversion) and 83 C (2% conversion) showed
low whiteline intensity and small features above the edge. They
also showed the broadened whiteline region characteristic of car-
bon monoxide adsorbed on the surface.
Fig. 6c shows the XANES spectrum taken above ignition at 97%
conversion. The whiteline intensity is lower than that of the spec-
trum at above ignition at 100% conversion, yet higher than that
at below ignition. The post-edge line is lower than the one at be-
low ignition, with features similar to the post-edge dip and subtle
curves of that at 100% conversion. The overall features of this spec-
trum show a shape similar to the oxidic spectrum with a reduced
whiteline intensity, which suggests a reduced extent of oxidation.
In summary, the changes in the shapes of the spectra during
cooling were opposite to those observed during heating. Above the
temperature of ignition, the spectra showed elements of oxidized
platinum, even after increasing the carbon monoxide concentra-
tion, such that no full conversion was reached. Below the ignition
and extinction temperatures the spectra resembled those of carbon
monoxide adsorbed platinum nanoparticles.
Fig. 7 shows the XANES spectra taken at increasing and de-
creasing temperature over b-Pt/Al2O3, Pt/TiO2, and Pt/SiO2. The
same trends are observed. All spectra below ignition temperature
showed the less intense and broadened whiteline of metallic plat-
inum with carbon monoxide adsorbed on the surface, the amount
of which decreases with temperature. Above ignition, the whiteline
increased intensity similar to that of an oxidic species. The spec-
tra were of metallic character at low activity and then switched to
having oxidic features at high activity. The change to the oxidic
features was a function of the temperature of ignition. The in-
crease in the whiteline intensity was most pronounced for Pt/SiO2,
which ignited at the highest temperature, and the lowest increase
was observed for Pt/TiO2, which ignited at the lowest temperature.
Upon decreasing the temperature, just above extinction, the con-
version decreased for b-PtAl2O3 and Pt/TiO2, which was paralleled
by a decrease in intensity of the spectral whiteline and thus lower
extent of oxidation.
bond length was shorter as expected for PtO . At increased carbon
monoxide flow and 97% conversion, the fit still showed the Pt–Pt
2
and Pt–O contributions with a slight increase in the Pt–Pt coordi-
nation to 3.0 and a slight decrease in the Pt–O coordination to 1.2
in agreement with features in the XANES (Fig. 6c). The EXAFS fit
of the spectrum that was recorded at room temperature again was
dominated by Pt–Pt scattering. The coordination number of 5.6
was similar to the starting value of 5.5 when it was in helium.
The distance was 2.76 Å compared to 2.72 Å when measured in
helium, which is again attributed to the presence of adsorbed car-
bon monoxide that slightly expands the cluster. The Debye–Waller
factor showed higher values at higher temperature and returned
to almost its original value when cooled down to room tempera-
ture.
The b-Pt/Al2O3 catalyst, measured in the plug flow reactor at
◦
3
5 C in flowing helium, had a Pt–Pt coordination number of 7.5.
This translates to a size of about 2 nm. The Pt–Pt distance in-
creased slightly from 2.74 to 2.75 Å after introducing oxygen and
◦
carbon monoxide at 125 C. The increase in bond length is less
pronounced for the big particles than for the small particles. Fits
of spectra recorded above ignition showed the presence of a Pt–O
◦
scatterer and decreased Pt–Pt contribution. Cooling down to 42 C,
the spectrum returned to the Pt–Pt dominated fit. In comparison to
◦
the values at 35 C and in helium, the coordination number is the
same, 7.5, but the Pt–Pt distance stayed at 2.75 Å because of the
influence of adsorbed carbon monoxide. For the same catalyst but
mounted in a different reactor, the fit parameters above and below
ignition were similar to that in the plug flow reactor, in agreement
with the XANES spectra (not shown).
◦
Fits of the Pt/TiO2 EXAFS recorded at 35 C in helium atmo-
sphere showed a coordination number of 7.0, which corresponds
to a size of about 1.3 nm. The 2.71 Å Pt–Pt distance was smaller
than the bulk value of 2.76 Å, again indicative of contraction when
3
.4. EXAFS fitting results
Fig. 8 shows the averaged and k -weighted EXAFS function and
◦
decreasing particle size. Heating to 132 and 178 C, in an oxygen
and carbon monoxide environment, the Pt–Pt distance increased
to 2.75 Å, the Debye–Waller factor increased, and the Pt–Pt co-
ordination number remained 7.0. Above ignition, the Pt–Pt coor-
dination number decreased to 6.1 along with a decrease in the
Pt–Pt distance to 2.72 Å. A small Pt–O contribution with coordina-
tion number of 0.7 at 1.97 Å was observed. After cooling to room
temperature, the Pt–Pt dominated spectrum returned: the coordi-
3
corresponding Fourier transform of the EXAFS spectra taken at
room temperature under a flow of helium for b-Pt/Al2O3 big parti-
cles after reduction and removal of hydrogen. The fits (dotted lines)
of these functions provided coordination number (7.5), Debye–
2
Waller factor (0.0040 Å ), inter-atomic Pt–Pt distance (2.72 Å), and
edge shift (1.7 eV) (Table 3).
The structural parameters for the fits of all the EXAFS spec-
tra taken during the stages of the carbon monoxide oxidation runs
are presented in Table 3. The spectrum taken at room temper-
1
Pt–Pt contribution could also be fitted with combined contributions from a
short Pt–Pt and a long Pt–O–Pt bond.

E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
235
◦
◦
◦
◦
◦
◦
◦
Fig. 7. Pt L3 XANES spectra recorded during heating (a) at 125 C (black), 194 C (red), 210 C (blue), 218 C (green), and 231 C (pink), and cooling (b) at 232 C (black), 213
C
◦
◦
◦
◦
◦
(
(
red), 185 C (blue), 156 C (green), and 111 C (pink) of b-Pt/Al2O3; ignition occurred at 226 C, extinction occurred at 191 C; Pt L3 XANES spectra recorded during heating
c) at 68 C (black), 134 C (red), 180 C (blue), and 190 C (green), and cooling (d) at 177 C (black), 145 C (red), 114 C (blue), and 36 C (green) of Pt/TiO2; ignition occurred
at 188 C, extinction occurred at 145 C; Pt L3 XANES spectra recorded during heating (e) at 40 C (black), 140 C (red), 240 C (blue), 294 C (green), and 306 C (pink), and
cooling (f) at 296 C (black), 286 C (red), 236 C (blue), and 106 C (green) of Pt/SiO2; ignition occurred at 299 C, extinction occurred at 281 C. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
nation number became 6.9, which is similar to the values below
ignition at various temperatures. The Pt–Pt distance also returned
to 2.75 Å.
For Pt/SiO2 at 35 C in helium, coordination number was 7.6
and the Pt–Pt distance was 2.77 Å, which were quite similar to
those of b-Pt/Al2O3 (2.0 nm) under the same conditions. Above

236
E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
3
3
−1
Fig. 8. (a) k -Weighted chi function and (b) corresponding Fourier transform (k -weighted, 2.5 < k < 12 Å ) of b-Pt/Al2O3 experimental data (solid line) and fit (dashed
line) in flowing helium at room temperature.
Table 3
EXAFS fitting results.
face. The higher the temperature of ignition, the larger was the
fraction of oxidic platinum. Upon cooling to below the tempera-
ture of extinction, the structure of all catalysts returned to their
original structure.
DW (Å2)
Conditions
Ab–Sc pair
Pt–Pt
O2/CO Pt–Pt
O2/CO Pt–O
Pt–Pta
N
R (Å) ꢀE0 (eV) Goodness
◦
of fit
Temp. ( C) Gas
s-Pt/Al2O3 (0.9 nm)
4
. Discussion
35
He
5.5 0.0040
5.7 0.0047
2.4 0.0025
2.0 0.0050
1.2 0.0025
3.0 0.0050
5.6 0.0038
2.72
2.77
1.99
2.71
1.98
2.71
2.76
1.7
0.5
3.2
5.0
14.9
104.3
129
211
4.1. Generating high activity
−1.4
210
O2/CO Pt–O
Pt–Pt
O2/CO Pt–Pt
1.7
51.3
11.2
In the oxidation of carbon monoxide over supported platinum
catalysts, two activity regimes are observed: a low-activity regime
and a high-activity regime (Figs. 3 and 4). The low activity region
occurs at low temperature and low carbon monoxide conversion.
The high activity regime occurs at high temperature and high
carbon monoxide conversion. Similar results had been reported
during carbon monoxide oxidation on various forms of platinum-
based catalysts: [10–12,33,41–44]. Some studies on carbon monox-
ide oxidation have been made by varying the oxygen and carbon
monoxide partial pressures at fixed temperatures [10,11,44]. The
changes in conversion in our study have been achieved by chang-
ing temperature at fixed inlet oxygen and carbon monoxide partial
pressures.
2.6
1.9
38
b-Pt/Al2O3 (2.0 nm) in plug flow reactor
He Pt–Pt
35
7.5 0.0004
7.8 0.0010
1.3 0.0025
6.7 0.0047
1.4 0.0025
6.7 0.0047
7.5 0.0004
2.74
2.75
1.96
2.72
1.97
2.72
2.75
0.3
0.7
3.6
1.6
3.9
1.9
0.4
18.0
32.3
36.7
125
O2/CO Pt–Pt
O2/CO Pt–O
Pt–Pt
O2/CO Pt–O
Pt–Pt
231
232
42
33.5
17.6
O2/CO Pt–Pt
Pt/TiO2
35
He
Pt–Pt
7.0 0.0018
7.0 0.0037
7.0 0.0040
0.7 0.0026
6.1 0.0050
6.9 0.0018
2.71
2.75
2.75
1.97
2.72
2.75
1.6
1.1
1.1
3.6
1.7
1.8
13.4
21.0
20.0
22.9
1
32
78
90
O2/CO Pt–Pt
O2/CO Pt–Pt
O2/CO Pt–O
Pt–Pt
1
The switch between high and low activity regimes is charac-
terized by discontinuous changes in carbon monoxide conversion.
The jump to high activity is called ignition, the reverse extinction.
These switches are reversible but show hysteresis. The temperature
of ignition is higher than the temperature of extinction. One rea-
son may be the local heating of the platinum particles because of
the exothermicity of the reaction. During the oxidation of methane
over Pt/Al2O3, a comparison of the calculated and observed conver-
sions had been made [45] and it appeared that platinum particles
1
36
O2/CO Pt–Pt
26.8
Pt/SiO2
35
O2/CO Pt–Pt
O2/CO Pt–O
Pt–Pt
O2/CO Pt–O
Pt–Pt
7.6 0.0035
3.1 0.0032
2.3 0.0070
3.7 0.0032
2.0 0.0070
2.77
1.98
2.71
1.98
2.69
0.2
2.1
23.0
320.2
305
−1.8
305
2.9
310.4
−1.1
a
◦
Pt–Pt could also be fitted with two contributions of a short Pt–Pt at 2.60 Å and
could be between 75 and 100 C hotter than the average temper-
that of a long Pt–O–Pt at 3.15 Å.
ature detected by the thermocouple in the catalyst bed. A simi-
lar effect likely occurs in the carbon monoxide oxidation (Figs. 3
and 4). Hysteresis of the ignition and extinction temperature can
also be attributed to different states of the surface of the catalyst
at high and low activity.
ignition, the Pt–Pt coordination decreased to 2.3 and the Pt–Pt dis-
tance likewise decreased to 2.70. The Pt–O contribution appeared
with a coordination number of 3.1 at 1.98 Å.
In summary, all catalysts showed an increase in Pt–Pt distance
after switching from a flow of helium to one containing carbon
monoxide. The platinum particles were in the metallic state at
low conversion values as indicated by a Pt–Pt backscattering pair
dominating the EXAFS functions and Fourier transforms. Above ig-
nition, a Pt–O contribution appeared at a relatively short distance.
The jump in conversion indicates that there is a change in the
mechanism of the reaction. This is also shown by varying activa-
tion energy below and above ignition of 176 and 59 kJ/mol re-
spectively [12]. At low activity, the conversion of carbon monoxide
increases exponentially with temperature. The reaction proceeds
via the Langmuir–Hinshelwood mechanism [42,46] where both car-
bon monoxide and oxygen adsorb on the surface of the catalyst to
react to form carbon dioxide. The presence of carbon monoxide at
the surface of platinum was reflected in the XANES spectra and
EXAFS analysis and is generally observed by infra red spectroscopy
2
A Pt–Pt contribution remained, but with a shorter Pt–Pt distance
than in a typical PtO2; it resembled the Pt–Pt distance of metal-
lic platinum. Thus the catalyst at above ignition consists of mixed
oxide-metal phases, with the oxide most likely located at the sur-
[
13,47,48]. Desorption of carbon monoxide is required to enable
oxygen to adsorb and dissociate on the surface. The partial reac-
tion order in carbon monoxide is close to −1 and for oxygen close
2
See footnote 1.

E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
237
to +1 for the carbon monoxide covered surface, which reflects the
inhibiting effect of carbon monoxide on the rate of reaction [18,49].
With increasing temperature, there is enhanced production of car-
bon dioxide and decreasing concentration of carbon monoxide. The
decrease in the coverage of carbon monoxide with temperature
was reflected in the XANES spectra and is also well known in lit-
erature based on infra red spectroscopy [13,47,48].
cover its surface with an oxide layer, but it was just enough for
b-Pt/Al2O3, which could therefore have metal sites exposed. The
particles of s-Pt/Al2O3 deeply oxidized as did those of Pt/SiO2. The
amount of oxide on Pt/TiO2 is insufficient to cover its surface com-
◦
pletely. When increasing the amount of carbon monoxide at 207 C
over s-Pt/Al2O3, resulting in about 97% conversion, the XANES and
EXAFS showed that there was a slight decrease in amount of ox-
ide, but that the system did not switch to the low-activity regime.
This shows that the structure of the catalyst is very dynamic and
depends on the exact gas composition.
The sudden switch in activity was paralleled by a change in
the structure of the catalyst. The XANES spectra and EXAFS analy-
sis showed a switch from the metallic to the partially oxidic state
upon ignition and the reverse upon extinction. The low activity in
the original state of the catalyst originates from the poisoning of
the surface by carbon monoxide [12,47]. The high activity is a re-
sult of the exposure of surface sites of the catalyst after depletion
of surface carbon monoxide. The low carbon monoxide concen-
tration on the surface enabled oxygen to react to it forming an
oxide layer. The result is a more active catalyst compared to the
carbon monoxide poisoned catalyst. During ignition, the enhance-
ment in conversion is paralleled by the formation of the oxide [16],
a process, which proceeds auto-catalytically. The structural trans-
formation of the nanoparticles we observe is, however, different
from the surface reconstruction of the Pt(100) surface from a car-
bon monoxide-covered 1 × 1 phase to a hex phase saturated with
adsorbed oxygen [8,9], which occurs under UHV conditions. Both
these surfaces are still metallic. Our EXAFS analysis showed break-
ing of platinum–platinum bonds. SXRD measurements performed
during carbon monoxide oxidation on a Pt(110) crystal [11] re-
ported that the surface became covered with a distorted α-PtO2;
a commensurate and an incommensurate type, with the two types
of oxides having a significantly higher reaction rate than the metal-
lic surface. Recently, a monolayer of surface oxygen on rhodium,
palladium, and platinum showed a hyperactivity, about two to
three orders of magnitude greater than the surfaces that were poi-
soned by carbon monoxide [50]. However, our data do not suggest
the presence of the α-PtO2 phase or a chemisorbed layer of oxy-
gen on our nanoparticles (vide infra).
In general, no or very little carbon monoxide adsorbed on a
platinum catalyst in the high activity state is observed using infra
red spectroscopy [13], which correlates to the partially oxidic state
of platinum. The reactivity towards carbon monoxide may be so
high that the concentration of carbon monoxide on either reduced
or oxidized surface is too low to be detected. In contrast to the low
activity regime, the reaction order is half in carbon monoxide and
oxygen and the activation energy decreased to 59 kJ/mol [12]. This
has also been attributed to a diffusion-limited surface reaction be-
tween adsorbed carbon monoxide and oxygen [43]. The change in
the reaction order and activation energy indicates that the rate is
not anymore limited by carbon monoxide desorption and subse-
quent oxygen activation. A high reactivity of a ruthenium surface
oxide has often been described, and Mars van Krevelen [51] and
Langmuir–Hinshelwood mechanisms [52] have been proposed.
Using core level spectroscopy and density functional theory cal-
culations [55], it was proposed that platinum ridges undergo 1-di-
mensional oxidation. The 1-D oxide stripes were found to be highly
reactive in carbon monoxide oxidation due to the favorable transi-
tion states at the phase boundary. Because platinum nanoparticles
have more edges, they are likewise expected to undergo 1-D oxi-
dation as precursor to platinum oxidation. When a monolayer of
oxygen adsorbs on the catalyst, it cannot desorb from the sur-
face and platinum begins to oxidize [44]. Theory suggested that
the surface of Pt(111) cannot be fully covered by oxygen [40,56],
and that the highest coverage attainable with an O2 oxidant is 0.5
monolayer [57]. The extent of oxidation strongly depends on the
oxygen partial pressure [10]. The dependence of the heat of ox-
ide formation to particle size has been studied [53] by calorimetric
measurements. The evolved heat of oxide formation increased with
decreasing particles size and the stoichiometry of the oxide ap-
proached that of PtO2. These studies indicate that smaller particles
tend to show more oxidation than single crystals. From SXRD [11]
the surface oxide structure over Pt(110) during reaction conditions
was found to be a thin distorted α-PtO2, exhibiting a compressive
and tensile strain relative to the bulk α-PtO2 lattice. Bulk PtO2
however displays low catalytic activity. The most oxidized spec-
trum at high activity (Pt/SiO2) we obtained, had hints of post-edge
features of β-PtO2 [58] but with a whiteline intensity intermediate
between the α-PtO2 and β-PtO2. These post-edge features were
absent for the less oxidized spectra of Pt/Al2O3 and Pt/TiO2. These
observations suggest that the platinum oxide lacks long-range or-
der, is distorted, and has a different structure than either α- or
β-PtO2. The Pt–Pt parameters obtained from fitting the oxidized
s-Pt/Al2O3 spectrum appeared to be resulting from more than one
Pt–Pt contribution. It could have come from the Pt–Pt contribu-
tions of the remaining metal core and of the oxide. The short Pt–O
distance and low Pt–O coordination suggest a coordination that is
lower than octahedral, such as the square-planar configuration in
PtO and Pt3O4 [59]. Calculations have predicted that Pt3O4 is active
for oxidation, as carbon monoxide binds strongly to the undercoor-
dinated oxygen at the surface and that PtO2 can be active, when it
contains many surface defects [41]. Likewise, in-situ XRD showed
that oxygen covered particles during carbon monoxide oxidation
were composed of combined metallic, PtO, and Pt3O4 clusters [15].
However, the platinum oxide that we observed is characterized by
disorder and a significant fraction of defects. Our oxide structure
does not resemble a surface that is partially covered with oxygen
that showed hyperactivity [50], nor does it resemble bulk PtO2 that
showed low activity [60]. Under-coordination at the surface of the
nanoparticles, strain, and partial oxidation are the key phenomena
responsible for the high catalytic activity.
4
.2. Structure of the active oxide
Both XANES and EXAFS showed that the extent of oxidation,
upon ignition, strongly depended on particle size [23,53] and sup-
port [24,25] as shown in Figs. 6 and 7. The higher the temperature
of ignition was, the higher the extent of oxidation of the platinum
particles. Thus Pt/TiO2 was the least and Pt/SiO2 the most oxidized.
Pt/Al2O3 showed the opposite trend. With increasing temperature
the solubility of oxygen in the metal increases [54]. With oxida-
tion likely on the surface, our EXAFS analysis consistently showed
that there was still a metal core left in all the oxidized catalysts.
A larger metallic platinum core was left in the bigger particles.
Based on the coordination numbers correlated to particle size [35],
the extent of oxidation on s-Pt/Al2O3 was sufficient to completely
5. Conclusion
Carbon monoxide oxidation on supported platinum catalysts ex-
hibits two activity regimes: low activity at high carbon monoxide
concentration and low temperature and high activity at low car-
bon monoxide concentration and high temperature. There is igni-
tion when going from low to high activity and there is extinction
when going from high to low activity. The catalyst at low activity

238
E.M.C. Alayon et al. / Journal of Catalysis 263 (2009) 228–238
is metallic covered with carbon monoxide, which hampers oxygen
from reacting. At high activity, the particles are partially oxidic.
Our data suggest that a disordered surface oxide has formed that
is rich in defects and with a platinum coordination that is possibly
square planar. The depletion of carbon monoxide at high activity
enables oxygen to interact with the surface, and enhances the cat-
alytic activity by changing the rate-limiting step. The thickness of
the disordered oxide layer strongly depends on particle size and
the support and could be partially related to the temperature at
which it formed. Among the supported catalysts, Pt/TiO2 reached
high activity at the lowest temperature and Pt/SiO2 needed the
highest temperature to reach high activity. Pt/Al2O3 showed an in-
termediate temperature.
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