Pt3Co and PtCu intermetallic compounds: Promising catalysts for preferential oxidation of CO in excess hydrogen

Article abstract of DOI:10.1016/j.jcat.2008.06.030

Pt-based intermetallic compounds (IMCs) supported on silica were studied as a catalyst for the preferential oxidation (PROX) of CO in excess hydrogen to obtain the active catalyst at low temperatures. Pt₃Co/SiO₂ and PtCu/SiO₂ were more active than Pt/SiO₂ and Pt/Al₂O₃ in the temperature range of 373-453 K. Both IMC catalysts were stable during the reaction at 473 K without the irreversible segregation at the surface. Infrared (IR), X-ray photoelectron spectroscopy (XPS), and adsorption measurements revealed a significant electron transfer from Pt atoms to Co or Cu atoms. The electron-deficient Pt atoms adsorbed CO weakly compared with Pt metal, accelerating the adsorption of oxygen on the IMC surface. Kinetic studies indicated that Co and Cu atoms in IMCs also adsorb oxygen. This improved oxygen adsorption likely resulted in the high PROX activity at low temperatures. The effects of hydrogen and water on the rate of CO₂ formation also were studied.

Full text of DOI:10.1016/j.jcat.2008.06.030

Journal of Catalysis 258 (2008) 306–314
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Journal of Catalysis
www.elsevier.com/locate/jcat
Pt3Co and PtCu intermetallic compounds: Promising catalysts for preferential
oxidation of CO in excess hydrogen
∗
Takayuki Komatsu , Asako Tamura
Department of Chemistry, Tokyo Institute of Technology, 2-12-1-E1-10 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Pt-based intermetallic compounds (IMCs) supported on silica were studied as
a catalyst for the
Received 14 April 2008
Revised 2 June 2008
Accepted 2 June 2008
preferential oxidation (PROX) of CO in excess hydrogen to obtain the active catalyst at low temperatures.
Pt3Co/SiO2 and PtCu/SiO2 were more active than Pt/SiO2 and Pt/Al2O3 in the temperature range of 373–
453 K. Both IMC catalysts were stable during the reaction at 473 K without the irreversible segregation at
the surface. Infrared (IR), X-ray photoelectron spectroscopy (XPS), and adsorption measurements revealed
a significant electron transfer from Pt atoms to Co or Cu atoms. The electron-deficient Pt atoms adsorbed
CO weakly compared with Pt metal, accelerating the adsorption of oxygen on the IMC surface. Kinetic
studies indicated that Co and Cu atoms in IMCs also adsorb oxygen. This improved oxygen adsorption
likely resulted in the high PROX activity at low temperatures. The effects of hydrogen and water on the
rate of CO2 formation also were studied.
Keywords:
Preferential oxidation
Carbon monoxide
Hydrogen
Intermetallic compound
Platinum
Cobalt
Copper
© 2008 Elsevier Inc. All rights reserved.
1. Introduction
Au supported on zeolite-A with an Pt/Au atomic ratio of 2 has
been found to be the most active Pt–Au/A catalyst [3]. In this cat-
alyst, Pt and Au appeared to be in separated phases. In the case
of Pt–Pd/CeO2 [4], the most active catalyst had the composition
of Pt/Pd = 1/7. Pt–Ni/Al2O3 prepared by co-impregnation proved to
be a better catalyst than that prepared by successive impregna-
tion [5]. EDX revealed the presence of Pt–Ni bimetallic particles
with a Pt/Ni ratio of 0.5–2 and monometallic Ni particles. Co-
impregnation tended to create the bimetallic particles, which may
be the active species at lower temperatures. The foregoing com-
binations of Pt and other metals have solid solution phases with
widely ranging compositions [6].
Preferential oxidation of CO in an excess amount of hydro-
gen (PROX) is an important process for obtaining CO-free hydro-
gen for proton exchange membrane fuel cells (PEMFCs). Hydrogen
produced through the steam reforming of hydrocarbons, such as
naphtha and natural gas, always contains CO as a byproduct at a
level of 10–20%. The water–gas shift reaction is applied to reduce
the CO concentration in hydrogen. But a significant amount of CO
(ca. 1%) remains in hydrogen to deactivate the platinum electrodes
of PEMFCs. The PROX of CO further reduces the proportion of CO
to ꢀ10 ppm, providing sufficient life for PEMFCs.
Intermetallic compounds (IMCs) are the stoichiometric com-
pounds between two or more metal elements. Compared with
typical alloys, which are solid solutions with the same crystal
structures as their component metals, various IMCs have different
specific crystal structures than those of their component metals.
Therefore, some IMCs are known to have unique bulk proper-
ties, such as superconductivity, hydrogen storage ability, and shape
memory effects. The catalytic properties of IMCs have been little
studied to date, except in the so-called “hydrogen storage alloys,”
such as LaNi5 [7]. We have previously studied the catalytic proper-
ties of IMCs compared with those of pure metals [8–16]. When Pt-
based IMCs were used for H2–D2 equilibration, Pt3Ge, Pt2Ge, and
PtGe gave much lower activity than Pt [9], and Pt3Ti and PtTi3 gave
much higher activity than Pt [13]. Pt3Ge was highly selective for
the partial hydrogenation of 1,3-butadiene into butenes [9]. Pt3Ti
was more active than Pt for the hydrogenation of ethylene [13].
Supported platinum catalysts have been widely studied for the
PROX reaction. Oh and Sinkevitch [1] reported that Pt was the
most active among various metals supported on alumina at higher
temperatures (around 440 K). However, from an economical stand-
point, drawbacks of Pt catalysts include their low activity at tem-
peratures around 370 K (the operating temperature of PEMFCs)
and their low selectivity to CO2 due to the higher oxygen concen-
tration to increase the activity. Consequently, bimetallic catalysts
containing Pt and a second element have been studied in efforts
to improve the activity of Pt catalysts. The activity of Pt–Ru/SiO2
was found to be between the activities of Pt/SiO2 and Ru/SiO2 [2],
although the addition of Pt improved the dispersion of Ru. Pt–
Corresponding author. Fax: +81 3 5734 2758.
*
E-mail address: komatsu@chem.titech.ac.jp (T. Komatsu).
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.06.030

T. Komatsu, A. Tamura / Journal of Catalysis 258 (2008) 306–314
307
Pt3Sn/H-SAPO-11 was more selective than Pt/H-SAPO-11 for the
dehydroisomerization of butane into isobutene [16]. The formation
of Pt3Sn retarded the hydrogenolysis to methane without decreas-
ing the dehydrogenation activity, resulting in increased selectivity
to isobutene. The similar retardation of hydrogenolysis also was
observed for the aromatization of butane on Pt3Ge/H-ZSM-5 com-
pared with Pt/H-ZSM-5 [11].
In the foregoing cases, the formation of IMCs drastically
changed the adsorption behavior from pure Pt metal through
changes in the atomic distance between the Pt atoms and the elec-
tronic state of Pt. Therefore, on the surface of IMCs, the adsorption
of CO and O2, as well as hydrogen, can be altered by forming IMCs
with other elements. This will change the catalytic properties of Pt
for the PROX reaction. To date, however, the catalytic properties of
single-phase IMCs for PROX have not been reported.
In recent years, Pt-based bimetallic catalysts, in which Pt and
the other element can have IMC phases without forming the wide-
range solid solution phases, have been studied for the PROX re-
action. Schubert et al. [17] have reported the higher activity and
selectivity of Pt–Sn/carbon compared with Pt/Al2O3 at 353 K. XPS
and IR measurements revealed that most of the tin was present
in the oxidized state, SnOx, although some was reduced to form
a Pt–Sn solid solution. It was suggested that this SnOx species
adsorbed oxygen instead of Pt. In the case of Pt–Sn/Nb2O5 [18],
however, the addition of tin did not enhance the PROX activity.
Among the Pt–Co catalysts, Pt–Co/TiO2 was reportedly more ac-
tive than Pt/TiO2 at low temperatures [19]. XPS and TPR results
demonstrated Co species in various oxidation states. Pt–Co/Al2O3
with Pt/Co = 1 also was more active than Pt/Al2O3 [20]; however,
the active species were not identified by XRD measurements. The
addition of sodium on Pt–Co/Al2O3 promoted the formation of a
Pt–Co bimetallic phase, inhibiting cobalt spinel formation on the
surface [21]. On a YSZ support [22], the catalyst with Pt/Co = 1/5
was the most active. TEM of this catalyst revealed isolated Pt–Co
bimetallic particles, as well as cobalt particles. Watanabe et al. [23]
found that Pt–Fe/mordenite was more active than Pt/mordenite
and Pt/Al2O3. They also found that the active catalyst contained
metal particles inside the pores of mordenite, where XRD did not
reveal the presence of Pt–Fe IMCs [24]. The addition of a large
amount of iron onto Pt/TiO2, where the iron loading corresponded
to the comparable amount of TiO2, also increased the activity at
temperatures below 373 K [25]; however, iron was in an oxidized
state without the formation of IMCs. In the foregoing studies, the
catalytic properties of Pt-based IMCs for PROX were not clarified,
although the surface formation of IMCs may contribute to the gen-
eration of new active sites.
reactor, into which hydrogen (99.9995%) was fed at a flow rate of
60 ml min−1. After the catalyst was heated at 403 K for 1 h, the
temperature was raised to 673 K and kept at this temperature for
2 h to reduce platinum. The catalyst was cooled to room tempera-
ture with flowing helium (99.999%) and kept in a drying desiccator.
IMC catalysts, PtnMm/SiO2 (M = Tl, Cu and Fe), were prepared
by successive impregnation onto Pt/SiO2. In the case of Pt3Tl2/SiO2,
a known amount of Pt (3 wt%)/SiO2 was placed into an evap-
orating dish, and an aqueous solution of thallium(I) nitrate was
added onto the Pt/SiO2 in an amount corresponding to Pt/Tl = 3/2
(atomic ratio) with the pore-filling procedure. This was sealed
with plastic film, left overnight at room temperature, and then
reduced similarly to the preparation of Pt/SiO2 described earlier.
In the cases of PtCu/SiO2 and Pt3Fe/SiO2, aqueous solutions of
copper(II) nitrate and iron(III) nitrate, respectively, were impreg-
nated onto Pt/SiO2. The reduction temperatures were 873 K for
Pt3Tl2/SiO2 and Pt3Fe/SiO2 and 1073 K for PtCu/SiO2. The prepa-
ration of Pt–Cu/SiO2 with various Pt/Cu ratios was carried out
through a similar impregnation onto Pt (3 wt%)/SiO2 using a spe-
cific amount of copper(II) nitrate solution. Pt3Co/SiO2 was prepared
by a co-impregnation method. An aqueous solution of tetraam-
mineplatinum(II) acetate and cobalt(II) nitrate was added to silica
gel with the pore-filling procedure to achieve the platinum load-
ing of 3 wt% and Pt/Co = 3. The mixture was sealed by the plastic
film overnight at room temperature. It was reduced at 673 K by
the flowing hydrogen. The preparation of Pt–Co/SiO2 with various
Pt/Co ratios was done by a similar co-impregnation to obtain the
Pt loading corresponding to Pt (3 wt%)/SiO2.
Pt3Sn/SiO2 and PtGe/SiO2 were prepared by a chemical vapor
deposition (CVD) method with Pt (3 wt%)/SiO2. Pt/SiO2 stored in
air was placed into a CVD reactor and reduced with flowing hy-
drogen at 673 K for 30 min. In the case of Pt3Sn/SiO2, Sn(CH3)4
(Soekawa Chemicals) vaporized at 273 K was introduced with a
hydrogen carrier (30 ml min−1) at the CVD temperature of 453 K
for 1 h to obtain a Pt/Sn ratio of ca. 3. In the case of PtGe/SiO2,
Ge(CH3)4 (Soekawa Chemicals) vaporized at 273 K was used for
CVD treatment onto Pt/SiO2 at the CVD temperature of 553 K for
1 h to obtain a Pt/Ge ratio of ca. 1.
Unsupported IMC catalysts were prepared by melting the mix-
ture of stoichiometric amounts of two component metals in an
arc-melting apparatus under 53 kPa of argon. The ingots thus ob-
tained were crushed in air and filtered into particles with diame-
ters <90 μm.
2.2. Characterization
In the present study, we applied some single-phase Pt-based
IMCs to the catalyst for the PROX reaction. Our aim was to clar-
ify the catalytic properties of Pt-based IMCs for the PROX reaction
and to obtain IMC catalysts that are active and selective for the
formation of CO2 at lower temperatures.
The crystal structure of supported metal particles was exam-
ined by powder X-ray diffraction (XRD) using a Rigaku RINT2400
diffractometer with a CuKα X-ray source. Images were recorded
with a JEOL JEM-2010 transmission electron microscope. XPS spec-
tra were recorded with a PerkinElmer PHI 5600 spectrometer. The
catalyst was pressed into a pellet and placed into a quartz reactor,
where it was reduced under flowing hydrogen. The reduced cata-
lyst was transferred to the spectrometer without being exposed to
air. Spectra were obtained with an AlKα X-ray source using C 1s
as a reference for binding energy.
2. Experimental
2.1. Catalyst preparation
Pt/SiO2 was prepared by a pore-filling impregnation method. An
aqueous solution of tetraammineplatinum(II) acetate (N.E. Chemcat
Corp.) was added to silica gel (Cariact G-6, Fuji Silysia) that had
been previously dried at 403 K and cooled in air to room temper-
ature. The amount of Pt solution was calculated to fill the pores
of silica gel and to achieve a Pt loading of 3 wt%. The mixture
was sealed with a piece of plastic film overnight at room tempera-
ture, then dried on a hot water bath with stirring and placed into
an oven. The temperature was raised in air to 403 K for 6 h and
673 K for 4 h. The Pt/SiO2 thus prepared was placed into a quartz
IR spectra of adsorbed CO were measured with a JASCO FT/IR-
430 spectrometer in transmission mode. A self-supporting wafer
(ca. 10 mg cm−2) of catalyst was placed in a quartz cell with CaF2
windows and attached to a glass circulation system. The catalyst
was reduced at 673 K for 30 min with 15 kPa of circulating hydro-
gen through a cold trap kept at 77 K. Then the catalyst was cooled
in vacuo to 298 K, and CO (2 kPa) was introduced for 10 min. After
the evacuation at 298 K for 10 min, a spectrum was recorded.
The amount of adsorbed CO was measured with a pulse re-
action system. A known amount of catalyst was placed into the

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T. Komatsu, A. Tamura / Journal of Catalysis 258 (2008) 306–314
quartz reactor and reduced at 673 K for 1 h with flowing hydrogen.
The catalyst was purged by flowing helium at 673 K for 30 min.
Pulses of CO (5.0%)/He were introduced with a helium carrier at
room temperature, and the outlet gas was analyzed with a ther-
mal conductivity detector to measure the amount of irreversibly
adsorbed CO.
2.3. Catalytic reaction
PROX reaction was carried out with a continuous-flow reaction
system under atmospheric pressure. A specific amount (0.050 g) of
catalyst mixed with 0.1 g of α-alumina was placed into a 17-mm-
i.d. quartz tubular reactor. Before the catalytic run, the catalyst was
reduced with flowing hydrogen at 673 K for 30 min, then purged
by flowing helium at 673 K. The reaction was started at a specific
temperature by supplying a reactant gas composed of CO (2.0%),
O2 (1.0–2.0%), H2 (35%), and He (balance) with a total flow rate of
140 ml min−1. The reaction temperature was raised stepwise, with
a product analysis conducted at each temperature. Gaseous prod-
ucts were analyzed with an online gas chromatograph (Shimadzu,
GC-14B) with a thermal conductivity detector and a column (2 mm
i.d., 3 m long) of active carbon (GL Science). CO2 selectivity was
calculated based on the total amount of oxygen converted into CO2
and H2O.
Fig. 1. XRD patterns of Pt (a), Pt–Co (Pt/Co = 3) (b), Pt–Fe (Pt/Fe = 3) (c), Pt–Sn
(Pt/Sn = 3) (d), Pt–Cu (Pt/Cu = 1) (e), Pt–Tl (Pt/Tl = 3/2) (f) and Pt–Ge (Pt/Ge = 1)
(g) supported on SiO2.
3. Results and discussion
3.1. PROX reaction on Pt-based IMC catalysts
The PROX reaction was carried out with various Pt-based IMCs
supported on silica gel. To obtain particles of single-phase IMC,
the interaction between metal and support cannot be too strong,
because an overly strong interaction will prevent two metal com-
ponents from forming their compound. Therefore, we selected sil-
ica gel (which usually has a very weak interaction with metals)
as the support material, even though Pt supported on alumina has
been more widely used for oxidation reactions compared with Pt
on silica. Fig. 1 shows XRD patterns of Pt–M/SiO2 (M = Ge, Tl, Cu,
Sn, Fe and Co) catalysts prepared with an Pt/M atomic ratio ad-
justed to the composition of a typical IMC between Pt and M. The
parent Pt/SiO2 (a) showed two diffraction peaks attributed to Pt
metal shown in dotted lines. Among Pt–M/SiO2 catalysts, Pt–Fe
(c), Pt–Cu (e) and Pt–Tl (f) on SiO2 prepared by the successive
impregnation gave the single-phase IMCs Pt3Fe, PtCu, and Pt3Tl2,
respectively. The co-impregnation gave the single-phase Pt3Co on
SiO2 (b). On the other hand, these impregnation methods could
not obtain single-phase Pt–Sn and Pt–Ge IMCs, whereas the CVD
method provided the single-phase Pt3Sn (d) and PtGe (g). Although
the preparation method was not the same, XRD revealed that all
of the Pt–M/SiO2 catalysts had single-phase IMC particles and were
free from Pt particles.
These catalysts were tested for the PROX reaction at various
temperatures, as shown in Fig. 2 together with the results for
Pt/SiO2. All of the catalysts had the same amount of Pt (1.5 mg).
On Pt/SiO2, the oxidation of CO occurred at temperatures above
420 K. CO conversion increased with reaction temperature, reach-
ing 100% at 493 K. All of the IMC catalysts demonstrated greater
CO conversions than Pt/SiO2 at 453 K and below. Pt3Co/SiO2 and
PtCu/SiO2 exhibited conversions of about 60% at 373 K and 100%
and 94%, respectively, at 453 K, whereas CO conversion did not
increase at temperatures above 453 K on Pt3Tl2/SiO2, Pt3Fe/SiO2,
and Pt3Sn/SiO2. These results indicate that Pt3Co and PtCu IMCs
are better catalysts than Pt for the PROX reaction. In fact, a sepa-
rate run on Pt (3 wt%)/γ -Al2O3 gave CO conversions of only 1% at
373 K and 21% at 413 K.
Fig. 2. PROX of CO on Pt (×), Pt3Co ("), PtCu (!), Pt3Tl2 (P), Pt3Fe (Q), Pt3Sn (F)
and PtGe (E) supported on SiO2. Reactant mixture: CO (2.0%), O2 (2.0%), H2 (35%)
and He (balance).
3.2. PROX reaction on Pt3Co/SiO2 and PtCu/SiO2
As discussed in the previous section, Pt3Co and PtCu supported
on silica were found to be excellent catalysts for the PROX re-
action. Because the combinations of both Pt–Co [26] and Pt–Cu
[27] have several IMC phases in their phase diagrams, we evalu-
ated the effect of composition in Pt–Co/SiO2 and Pt–Cu/SiO2 on
catalytic activity. Fig. 3 shows the CO conversions for Pt–Co/SiO2
catalysts with Pt/Co atomic ratios of 1/4, 1, 3, 4, and 9. It is
clear that all of the Pt–Co/SiO2 catalysts showed higher CO con-
versions than Pt/SiO2. At temperatures below 400 K, Pt–Co/SiO2
with Pt/Co = 1 gave the highest conversion. But the conversion de-
creased at higher temperatures, indicating that the selectivity for
CO2 formation must be low at these temperatures. The Co-rich
catalyst, Pt–Co/SiO2 (Pt/Co = 1/4), also exhibited low conversion at
high temperatures, although Pt–Co/YSZ (Pt/Co = 1/5) has been re-

T. Komatsu, A. Tamura / Journal of Catalysis 258 (2008) 306–314
309
Fig. 5. PROX of CO on Pt–Cu/SiO2 with Pt/Cu atomic ratios of 0.3 (P), 1 (!), 2 (1)
and 5 (E) and on Pt/SiO2 (×). Reactant mixture: CO (2.0%), O2 (1.5%), H2 (35%) and
He (balance).
Fig. 3. PROX of CO on Pt–Co/SiO2 with Pt/Co atomic ratios of 1/4 (!),
1 (Q),
3 ("), 4 (2) and 9 (F) and on Pt/SiO2 (×). Reactant mixture: CO (2.0%), O2 (1.5%),
H2 (35%) and He (balance).
Fig. 6. XRD patterns of Pt/SiO2 (a) and Pt–Cu/SiO2 with Pt/Cu atomic ratios of 5 (b),
2 (c), 1 (d) and 0.3 (e).
Fig. 4. XRD patterns of Pt/SiO2 (a) and Pt–Co/SiO2 with Pt/Co atomic ratios of 9 (b),
4 (c), 3 (d), 1 (e) and 1/4 (f).
Based on the results shown in Figs. 3 and 4, we conclude that
Pt3Co was the most effective IMC for the PROX reaction in a wide
temperature range, whereas PtCo had greater activity but lower se-
lectivity. The high activity of Pt–Co bimetallic catalysts has already
been reported on TiO2 [19], Al2O3 [20], and YSZ [22]; however, the
contribution of Pt3Co IMC was first revealed in the present study.
We also studied the combination of Pt–Cu in a manner simi-
lar to that for Pt–Co described earlier. Fig. 5 shows the results of
the reaction on Pt–Cu/SiO2 with Pt/Cu ratios of 0.3, 1, 2, and 5.
Pt–Cu/SiO2 with Pt/Cu = 1 gave the highest conversion at al-
most all temperatures studied, whereas Pt–Cu/SiO2 with Pt/Cu = 5
gave a much lower conversion, comparable to that on Pt/SiO2.
Cu (5 wt%)/SiO2 also gave low activity, with 18% CO conversion
at 450 K. Fig. 6 shows XRD patterns for Pt–Cu/SiO2. The addi-
tion of Cu on Pt/SiO2 (a) shifted the diffraction peak to a greater
angle. Because Pt and Cu will form a solid solution with any com-
ported to be active [22]. In contrast, Pt–Co/SiO2 (Pt/Co = 3) gave
higher conversions at 413 and 453 K than any other catalysts,
with significant conversions even at lower temperatures. A sep-
arate run on Co (5 wt%)/SiO2 gave much lower activity, with 16%
CO conversion, at 453 K. Based on these findings, we conclude that
Pt–Co/SiO2 (Pt/Co = 3) is the most effective Pt–Co/SiO2 catalyst for
the PROX reaction.
To identify the active IMCs, XRD patterns were obtained for the
Pt–Co/SiO2 catalysts, as shown in Fig. 4. The addition of Co onto
Pt/SiO2 (a) gradually shifted the peak position to greater angles
in (b) and (c), suggesting the formation of a solid solution be-
tween Pt and Pt3Co. At a Pt/Co ratio of 3 (d), the peak appeared
◦
at almost the same position as that of Pt3Co (40.5 ), as indicated
by the dotted line. Further addition of Co (Pt/Co = 1) generated the
diffraction from PtCo (e). The Co-rich catalyst (Pt/Co = 1/4) showed
no significant XRD pattern, indicating the absence of an IMC phase.

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T. Komatsu, A. Tamura / Journal of Catalysis 258 (2008) 306–314
Fig. 7. CO2 selectivity (solid symbols) and CO conversion (open symbols) of
Pt3Co/SiO2 (", !), PtCu/SiO2 (2, 1) and Pt/SiO2 (Q, P). Reactant mixture: CO (2.0%),
O2 (1.3%), H2 (35%) and He (balance).
position at higher temperatures, the solid solution between Pt–Cu
alloy and PtCu IMC may be formed in these catalysts. Pt–Cu/SiO2
with Pt/Cu = 1 (d) gave the diffraction in almost the same position
◦
as that of PtCu (41.0 ). Pt–Cu/SiO2 with Pt/Cu = 0.3 (e) gave the
diffraction from PtCu3. Based on these findings, we conclude that
PtCu was the most effective IMC between Pt and Cu for the PROX
reaction at a wide temperature range.
Fig. 8. XRD patterns of Pt3Co/SiO2 (1) and PtCu/SiO2 (2) before (B) and after (A) the
PROX of CO at 413 K for 4 h.
Fig. 7 shows the CO2 selectivity of the active catalysts, Pt3Co/
SiO2 and PtCu/SiO2, together with that of Pt/SiO2 at various tem-
peratures. Pt3Co/SiO2 showed almost 100% selectivity to CO2 at low
temperatures, indicating that oxygen was used only for the oxida-
tion of CO; that is, no hydrogen consumption occurred on Pt3Co.
Although CO2 selectivity decreased at higher temperatures, it re-
mained around 70% at 453 K, where CO conversion was >95%.
PtCu/SiO2 gave lower selectivity than Pt3Co/SiO2 even at low tem-
peratures. Its selectivity was comparable to that of Pt/SiO2 at a
similar conversion level. Clearly, due to its high selectivity and high
activity, Pt3Co/SiO2 is a better catalyst than PtCu/SiO2 for the PROX
reaction.
The surface of IMCs, such as LaNi5 and ThNi5, changes during
the hydrogenation of CO from IMC phase to Ni metallic particles
and a thin layer of La2O3 or ThO2 [28]. When the reactant con-
tains oxygen atoms, such segregation into pure metal and oxide
phases can occur if one component element has a greater affin-
ity to oxygen than the other. Moreover, the complete oxidation
into oxide phases can occur if both elements have a high affinity.
Consequently, we studied the stability of Pt3Co/SiO2 and PtCu/SiO2
under the stated reaction conditions. Fig. 8 shows XRD patterns
of Pt3Co/SiO2 and PtCu/SiO2 before and after the PROX reaction at
a constant temperature of 413 K for 4 h. At this temperature, CO
conversion on both catalysts exceeded 80%. The diffraction peaks
of Pt3Co (1-A, B) and PtCu (2-A, B) did not change significantly,
and no diffractions from other oxide species were observed, indi-
cating no segregation in bulk of IMC phases during the reaction.
To clarify the surface segregation during the PROX reaction at high
temperatures, the reaction was repeated on the same catalyst, as
shown in Fig. 9. The reaction was carried out from 333 to 473 K in
the first run, after which the temperature was decreased to 333 K
in the flow of reactant and the reaction was repeated to 473 K
in the second run. For both catalysts, CO conversion in the sec-
ond run closely replicated that in the first run. The Pt3Co and PtCu
surfaces did not change irreversibly during the reaction at higher
temperatures, suggesting that no surface segregation of these IMCs
Fig. 9. CO conversion in the first run (open symbols) and the subsequent second
run (solid symbols) on Pt3Co/SiO2 (", !) and PtCu/SiO2 (2, 1). Reactant mixture:
CO (2.0%), O2 (1.3%), H2 (35%) and He (balance).
occurred and that Pt3Co and PtCu were stable under the reaction
conditions.
3.3. Characterization of Pt3Co/SiO2 and PtCu/SiO2
In this section, we explore why Pt3Co/SiO2 and PtCu/SiO2 ex-
hibited greater activity than Pt/SiO2 for the PROX reaction, pre-
senting some characterization results. First, we performed TEM on
these catalysts to determine the particle sizes of the metals and
IMCs. Fig. 10 shows TEM images of three catalysts obtained with
the same magnification. Pt/SiO2 (a) was composed of a few large
particles with numerous highly dispersed small particles. The par-
ticle size distribution, shown in Fig. 11 in solid bars, was mainly
1–3 nm. Fig. 10b shows that PtCu/SiO2 comprised larger parti-
cles than Pt/SiO2, with most particles in the range of 2–5 nm.
Pt3Co/SiO2 had larger particles than PtCu/SiO2. Based on the XRD

T. Komatsu, A. Tamura / Journal of Catalysis 258 (2008) 306–314
311
Fig. 10. TEM images of Pt/SiO2 (a), PtCu/SiO2 (b) and Pt3Co/SiO2 (c).
longer than that on pure Pt. This could be evidence of the for-
mation of IMC phases on the surface of Pt3Co and PtCu particles
as well as in their bulk, as was confirmed by XRD (see Figs. 4
and 6). The disappearance of bridged CO was also found for Ni3Sn,
Ni3Sn2, and Ni3Sn4 on SiO2 [29]. Moreover, the intensity of the
linear CO band on the IMC catalysts was significantly lower than
that on the Pt catalysts. The adsorption of CO is weakened by
IMC formation. The strong adsorption of CO on Pt hinders the
adsorption of O2, resulting in low PROX activity at low temper-
atures. The weaker adsorption of CO on IMCs may be the reason
why Pt3Co/SiO2 and PtCu/SiO2 exhibited greater PROX activity than
−1
Pt/SiO2. The amount of CO adsorbed at 298 K was 86 μmol g
on
−1
−1
Pt/SiO2, 11 μmol g
on Pt3Co/SiO2, and 10 μmol g
on PtCu/SiO2.
As expected based on the TEM results (Fig. 10), the uptakes on the
IMC catalysts were much lower than those on Pt/SiO2. We calcu-
lated the TOFs for the formation of CO2 based on the CO uptake.
The TOFs on Pt3Co/SiO2 and PtCu/SiO2 were 0.84 and 0.52 s−1, re-
Fig. 11. Distribution of metal particle size in Pt/SiO2 (solid bars), PtCu/SiO2 (open
bars) and Pt3Co/SiO2 (shaded bars).
spectively, at the reaction temperature of 353 K. These values are
−1
much higher than the 0.032 s
PROX reaction at 363 K [30].
reported for K–Pt/Al2O3 in the
The IR peak position of linear CO shifted to a lower wavenum-
ber when some of the adsorbed CO molecules were desorbed by
evacuation. The final peak position after successive evacuation was
almost the same (ca. 2048 cm−1) for the three catalysts. Because
the electronic state of the surface Pt atoms could not be clarified
accurately because of the repulsive interaction between adsorbed
CO molecules, we studied the electronic (ligand) effect through the
formation of IMCs by XPS. Fig. 13 shows XPS spectra of Pt 4f (a),
Co 2p (b), and Cu 2p (c) in pure metal foils and also of unsup-
ported Pt3Co (d) and PtCu (e). Because the intensity of Co 2p peaks
in Pt3Co/SiO2 was too weak to allow us to estimate the oxida-
tion state and surface composition, here we used unsupported IMC
powders prepared by the arc-melting technique [9]. Table 1 gives
the binding energies obtained from the XPS spectra. The Pt 4f7/2
peaks in Pt3Co and PtCu gave slightly higher binding energies than
that seen for Pt foil. Pt atoms on the surface of Pt3Co and PtCu
were found to be positively charged compared with those on pure
Pt metal. Pt 4f7/2 peaks in Pt3Co/SiO2, PtCu/SiO2, and Pt/SiO2 ap-
peared at 74.1, 74.2, and 73.9 eV, respectively. The similar positive
shifts in Pt 4f7/2 binding energy for unsupported IMCs also were
found for SiO2-supported IMCs; however, these binding energies
were greater than those for bulk IMCs due to the small particle
size of the supported catalysts. The electron-deficient Pt atoms on
IMCs weakly adsorbed CO compared with those on pure Pt due
to the decreased back-donation from Pt to CO. This corresponds to
the results for CO adsorption obtained by IR and CO uptake mea-
surements. The ligand effect of IMC formation was found to affect
the catalytic activity for the PROX reaction, as well as the ensemble
effect revealed by the IR spectra. As shown in Table 1, the Cu 2p3/2
peak appeared at a lower binding energy in PtCu than in Cu foil,
indicating the electron transfer from Pt to Cu in PtCu, although the
appreciable change in Co 2p3/2 peak position was not observed in
Fig. 12. IR spectra of CO adsorbed on Pt/SiO2 (a), Pt3Co/SiO2 (b) and PtCu/SiO2 (c).
patterns, the crystallite sizes of these catalysts were estimated as
<4 nm for Pt/SiO2 and PtCu/SiO2 and 6 nm for Pt3Co/SiO2. The
rough agreement between particle size determined by TEM and
crystallite size determined by XRD would exclude the presence
of amorphous metal particles and polycrystalline particles on sil-
ica. Based on these results, we conclude that the higher activity
of Pt3Co/SiO2 and PtCu/SiO2 compared with that of Pt/SiO2 is not
associated with the greater dispersion of these IMCs; rather, the
intrinsic activity of Pt3Co and PtCu must be higher than that of Pt.
We also investigated the catalyst surfaces by evaluating the
IR spectra of adsorbed CO. As shown in Fig. 12, CO adsorbed on
Pt/SiO2 at 298 K gave a main absorption band attributed to linear
−1
CO at around 2050 cm
and a weak band attributed to bridged
CO at around 1850 cm−1. Pt3Co/SiO2 and PtCu/SiO2 gave only a
−1
band at around 2050 cm
attributed to linear CO adsorbed on
Pt atoms in IMCs. The disappearance of bridged CO indicates that
the distance between adjacent Pt atoms on the surface of IMCs is

312
T. Komatsu, A. Tamura / Journal of Catalysis 258 (2008) 306–314
Fig. 13. XPS spectra of Pt 4f, Co 2p3/2 and Cu 2p3/2 in foils of Pt (a), Co (b) and Cu (c) and unsupported Pt3Co (d) and PtCu (e).
Table 1
Results of XPS measurement on unsupported catalysts
Catalyst
Binding energy (eV)
Surface composition
Pt 4f7/2
Co 2p3/2
Cu 2p3/2
Pt/Co
Pt/Cu
Pt foil
Co foil
Cu foil
Pt3Co
PtCu
70.9
–
–
71.3
71.2
–
–
–
–
–
–
4
–
–
–
–
–
778.1
–
778.1
–
932.5
–
932.1
1.1
Fig. 15. Effect of O2 partial pressure on the rate of CO2 formation at 413 K on Pt/SiO2
(×) and at 333 K on Pt3Co/SiO2 (!) and PtCu/SiO2 (P) with H2 (a) and without
H2 (b).
for Pt/SiO2 and to 333 K for the IMC catalysts to obtain CO con-
versions of 5–10%. In the case of Pt/SiO2, the reaction order was
estimated to be 0 with respect to P(CO) and 1 with respect to
P(O2), irrespective of the presence or absence of hydrogen. The
CO adsorption must be saturated on the surface of Pt; therefore,
the reaction may proceed mainly through the Rideal mechanism
with the reaction between the adsorbed CO and gaseous O2. In
contrast, the reaction order with respect to P(CO) was not 0 for
both IMC catalysts, indicating that CO adsorption is not saturated.
The reaction proceeded through the Langmuir–Hinshelwood mech-
anism, as has been proposed on Pt–Fe/mordenite [32]. In the case
of Pt3Co/SiO2, the reaction order with respect to P(O2) was positive
both with and without hydrogen. CO adsorption was stronger than
O2 adsorption irrespective of the presence or absence of hydro-
gen. Oxygen was adsorbed mainly on Co, because the IR spectra
did not indicate CO adsorption on Co. The CO molecules on Pt
would be oxidized by oxygen atoms activated on Co, resulting in
higher activity of Pt3Co compared with Pt metal. Other researchers
have reported on the adsorption site for oxygen in the PROX reac-
tion, for example, Sn or SnOx in Pt–Sn/Al2O3 [17], Nb2O5 support
in Pt–Sn/Nb2O5 [18], and Fe in Pt–Fe/mordenite [32]. In the case
of PtCu/SiO2, however, the reaction order with respect to P(O2)
was negative in the presence of hydrogen (Fig. 15a) but posi-
Fig. 14. Effect of CO partial pressure on the rate of CO2 formation at 413 K on
Pt/SiO2 (×) and at 333 K on Pt3Co/SiO2 (!) and PtCu/SiO2 (P) with H2 (a) and
without H2 (b).
the formation of Pt3Co. Lee et al. [31] reported a positive shift in
Pt 4f7/2 and negative shifts in Co 2p3/2 and Cu 2p3/2 for bulk Pt3Co
and PtCu, but no significant shift of Pt 4f7/2 in PtCu. In the case of
Pt3Ge, however, we detected the electron transfer from Ge to Pt by
XPS [9]. The composition near the surface of IMCs was estimated
from the XPS spectra, as shown in Table 1. On both IMCs, the sur-
face compositions were comparable to the bulk ones, Pt/Co = 3
and Pt/Cu = 1. The enrichment in one element on the surface did
not occur on both IMCs. This also provides evidence of the forma-
tion of IMC phase on the surface of Pt3Co and PtCu, in addition to
the disappearance in bridged CO.
We carried out kinetic studies to help clarify the adsorption of
reactant molecules under the stated reaction conditions. Figs. 14
and 15 show the effect of partial pressures of CO and O2, respec-
tively, on the rate of CO2 formation in the presence (a) or absence
(b) of hydrogen. The reaction temperatures were adjusted to 413 K

T. Komatsu, A. Tamura / Journal of Catalysis 258 (2008) 306–314
313
Fig. 16. Effect of H2 (35%, a) and H2O (10%, b) on the oxidation of CO at 353 K on Pt3Co/SiO2 (!) and PtCu/SiO2 (P) with the reactant mixture of CO (2.0%), O2 (1.0%) and
He (balance).
tive without hydrogen. As a result, the adsorptions of CO and O2
were almost comparable in the presence of hydrogen. Hydrogen
enhanced the adsorption of O2. Comparing CO2 formation rates
obtained with and without hydrogen reveals that the activity of
Pt and Pt3Co increased slightly and that of PtCu increased signifi-
cantly with the addition of hydrogen. Hydrogen contributed to the
high PROX activity of PtCu/SiO2 at low temperatures.
high activity is related to the ready adsorption of oxygen under
the reaction conditions compared with the surface of pure Pt. The
formation of IMCs induced an extension of the Pt–Pt atomic dis-
tance (geographic effect) and an electron transfer from Pt to Co or
Cu (electronic effect). Both of these effects resulted in the weaker
adsorption of CO on the IMC surface compared with on the Pt sur-
face. Pt3Co was found to have high selectivity for CO2 formation.
PtCu had a slightly lower selectivity, because on PtCu the forma-
tion of small amounts of water through the oxidation of hydrogen
is necessary for the oxidation of CO at low temperatures.
We also studied the enhancing effect of hydrogen on the PROX
reaction in the steady-state reaction. Fig. 16 shows the effect of
adding hydrogen and water to the reactant containing CO and O2.
It should be noted that the O2 concentration was lower than that
in the cases shown in Figs. 2, 3, and 5. On Pt3Co/SiO2, CO conver-
sion was increased slightly by the addition of hydrogen but was
not increased by the addition of water. The weak enhancing ef-
fect was caused by the hydrogen itself. On PtCu/SiO2, however, CO
conversion increased by 30% with the addition of hydrogen and
even further by the addition of water. The enhancing effect of
hydrogen reflected the contribution of water produced by the ox-
idation of hydrogen during the PROX reaction. As shown in Fig. 7,
the oxidation of hydrogen occurred on PtCu/SiO2 at all tempera-
tures studied, supplying the water molecules necessary to enhance
CO2 formation. The formation of CO2 in the presence of water
could be caused by the CO shift reaction. We carried out the re-
action of CO (2%), H2O (10%), and H2 (35%) in a helium carrier on
PtCu/SiO2 and found no CO2 formation in the temperature range of
333–473 K. The CO shift reaction is negligible during the PROX re-
action. The increase in CO2 formation rate by the addition of water
results from the acceleration of O2 adsorption to oxidize CO. Such
an enhancing effect of water has been reported for Pt–Pd/CeO2 [4]
and FeOx/Pt/TiO2 [33]. The water molecules adsorbed on Cu may
react with adjacent CO on Pt to form a carboxyl group [33], which
may accelerate the adsorption of oxygen to oxidize CO into CO2.
The participation of water molecules in the PROX reaction on PtCu
is the reason for its lower selectivity compared with Pt3Co. The
change in CO conversion was fast for both the addition and re-
moval of hydrogen; therefore, PtCu was stable in the absence of
hydrogen, without the phase segregation caused by the oxidation
of copper.
Acknowledgments
This work was supported in part by Grant-in-Aid 17560680
from the Japanese Ministry of Education, Science, Sports and Cul-
ture. The authors thank Professor Takeyama for IMC preparation,
Professor Yano for the XPS measurements, and Dr. Takagi and Mr.
Genseki (Center for Advanced Materials Analysis, Tokyo Institute of
Technology) for the TEM measurements.
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