Co-Pt bimetallic catalysts for the selective oxidation of carbon monoxide in hydrogen-containing mixtures

Article abstract of DOI:10.1134/S0023158407020127

The performance of a Co-Pt powder and of Co-Pt catalysts supported on γ-Al₂O₃ and on the graphite-like carbon material Sibunit in selective CO oxidation in hydrogen-containing mixtures is considered. Fine particles of metal-metal solid solutions and intermetallides were obtained by the decomposition of a Co- and Pt-containing double complex salt in a hydrogen atmosphere at ～400°C. As compared to their Pt and Co monometallic counterparts, the bimetallic catalysts are more active and allow the CO concentration in hydrogen-containing mixtures to be reduced from 1 to 10^-3 vol %. This effect is likely due to the formation of bimetallic particles of a Co-Pt solid solution on the support surface.

Full text of DOI:10.1134/S0023158407020127

ISSN 0023-1584, Kinetics and Catalysis, 2007, Vol. 48, No. 2, pp. 276–281. © MAIK “Nauka /Interperiodica” (Russia), 2007.
Original Russian Text © P.V. Snytnikov, K.V. Yusenko, S.V. Korenev, Yu.V. Shubin, V.A. Sobyanin, 2007, published in Kinetika i Kataliz, 2007, Vol. 48, No. 2, pp. 292–297.
Co–Pt Bimetallic Catalysts for the Selective Oxidation
of Carbon Monoxide in Hydrogen-Containing Mixtures
P. V. Snytnikova, K. V. Yusenkob, S. V. Korenevb, Yu. V. Shubinb, and V. A. Sobyanina,c
a Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
b Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
c Novosibirsk State University, Novosibirsk, 630090 Russia
e-mail: pvsnyt@catalysis.nsk.su
Received November 7, 2005
Abstract—The performance of a Co–Pt powder and of Co–Pt catalysts supported on γ-Al2O3 and on the graph-
ite-like carbon material Sibunit in selective CO oxidation in hydrogen-containing mixtures is considered. Fine
particles of metal–metal solid solutions and intermetallides were obtained by the decomposition of a Co- and
Pt-containing double complex salt in a hydrogen atmosphere at ~400°C. As compared to their Pt and Co mono-
metallic counterparts, the bimetallic catalysts are more active and allow the CO concentration in hydrogen-con-
taining mixtures to be reduced from 1 to 10–3 vol %. This effect is likely due to the formation of bimetallic par-
ticles of a Co–Pt solid solution on the support surface.
DOI: 10.1134/S0023158407020127
In recent years, the design and commercialization of verted into a hydrogen-containing mixture by steam,
power plants based on proton exchange membrane fuel air, or autothermal reforming followed by the steam
cells (PEMFCs) have attracted keen interest from conversion of the resulting carbon monoxide. The mix-
researchers all over the world. This interest stems from ture usually consists of H2, CO2, CH4, N2, H2O, and
the already demonstrated and recognized advantages of ~0.5–2 vol % CO. However, this mixture cannot be fed
fuel cells over conventional power sources. These into a PEMFC because CO concentrations above
advantages include high chemical-to-electric energy 0.001–0.01 vol % (10–100 ppm) poison the fuel cell
conversion efficiency, noiseless operation, and modu- electrode [1–3]. For this reason, the second step is CO
larity. Due to their modularity, PEMFCs can readily be reduction to 10 ppm by fine purification of the hydro-
combined into stacks to vary the electric power within gen-containing gas. The most promising purification
the given technology. The fuel in a PEMFC is hydro- method is catalytic CO oxidation in the presence of H2.
gen, and the oxidizer is atmospheric oxygen. The only
atmospheric emission resulting from the operation of a
PEMFC is water vapor, so PEMFCs meet the most
stringent environmental regulations. Although PEM-
FCs are still rather expensive, many countries consider
them to be promising for improving power supply sys-
tems in a variety of applications ranging from mobile
phones and vehicles to dwellings.
This purification technique involves two simulta-
neous catalytic reactions, namely, CO oxidation and H2
oxidation. Therefore, an efficient catalyst must be both
highly active and selective toward CO oxidation in the
presence of H2.
Numerous supported metal catalysts have been sug-
gested for CO oxidation in the presence of H2, includ-
ing active and selective Pt- and Ru-containing catalysts
supported on γ-Al2O3 and carbon materials [4–12].
Nevertheless, it is vital to enhance the activity of these
catalysts and to reduce their noble metal content. A way
of solving this problem is by employing bimetallic cat-
alysts and metal catalysts promoted with metal oxides.
Igarashi et al. [13] suggested the bimetallic catalyst
Pt–Ru/mordenite. This catalyst was found to be more
active than Pt/mordenite or Ru/mordenite. It was
assumed that Pt–Ru alloy particles form on the support
surface during synthesis. The fact that the bimetallic
catalyst Pt–Ru/mordenite is more active than the Pt and
Ru monometallic catalysts is explained in terms of the
However, there are difficulties preventing PEMFCs
from gaining widespread use. These include the
absence of a developed hydrogen infrastructure and the
challenging problem of safe hydrogen storage. In view
of this, researchers' attention is now focused on devel-
oping a fuel processor capable of producing the neces-
sary amount of a hydrogen-containing gas from hydro-
carbons (natural gas, gasoline, methanol, ethanol, etc.)
just at the PEMFC operation site. It is this problem that
has recently occupied the foreground of research in the
field of fuel cells, as is indicated by the large number of
relevant publications [1–3].
Hydrogen production in a fuel processor involves following “bifunctional mechanism”: the CO mole-
several steps. Initially, the hydrocarbon stock is con- cules, adsorbed on Pt, are oxidized by the nearest oxy-
276

Co–Pt BIMETALLIC CATALYSTS FOR THE SELECTIVE OXIDATION
Properties of the catalysts
277
Before the reaction
After the reaction
Catalyst
SBET, m2/g
Vpor, cm3/g
SBET, m2/g
Vpor, cm3/g
2 wt % (Co–Pt)/C; Co/Pt = 1
2 wt % (Co–Pt)/γ-Al2O3; Co/Pt = 1
Co–Pt (powder); Co/Pt = 1
Pt/γ-Al2O3
300
150
1
0.3
0.5
–
300
150
1
0.3
0.5
–
150
150
0.5
0.5
150
150
0.5
0.5
Co/γ-Al2O3
gen atoms, which are mainly adsorbed on Ru. A num- sists largely of a solid solution with a face-centered lat-
ber of studies have been devoted to Pt/γ-Al2O3 and tice and its composition is Co0.5Pt0.5. The possibility of
Pt/mordenite catalysts promoted with iron oxide [14– the formation of Pt–Co bimetallic particles was proved
17]. This promoter enhances the activity and selectivity in another work [23].
of the platinum catalysts. This effect arises from the
The supports in (Co–Pt)/C and (Co–Pt)/γ-Al2O3
strong interaction between platinum and iron oxide
[15]. Part of the platinum surface in these catalysts is
covered with iron oxide. This causes a decrease in the
energy of the bonding between Pt and the adsorbed CO
molecules, thereby favoring CO oxidation to CO2 by
oxygen of the iron oxide. The reduced iron oxide is then
reoxidized by oxygen from the gas phase.
were, respectively, Sibunit (graphite-like carbon) and
(Co–Pt)/γ-Al2O3 as spherical granules ~1 mm in diam-
eter. Sibunitt had SBET = 300 m2/g and a pore volume of
0.3 cm3/g, and γ-Al2O3 had SBET = 150 m2/g and a pore
volume of 0.5 cm3/g.
The (Co–Pt)/ë and (Co–Pt)/γ-Al2O3 catalysts were
prepared in several steps from aqueous solutions of the
complexes [CÓ(NH3)5NO2]Cl2 and ä2[Pt(NO2)4]. The
catalyst preparation procedure was discussed in an ear-
lier publication [24]. The support was impregnated with
a ä2[Pt(NO2)4] solution by the incipient-wetness
method and was then air-dried at 110°ë. The resulting
material was impregnated with a [CÓ(NH3)5NO2]Cl2
solution by the incipient-wetness method and was again
air-dried at 110°ë. These operations produce the dou-
ble complex salt [Co(NH3)5NO2][Pt(NO2)4] and KCl in
the pores of the support. Next, the solids were reduced
in a hydrogen atmosphere at 400°ë for 2 h. The total
metal content of a catalyst was 2 wt %, and the Pt/Co
atomic ratio was close to unity. For KCl removal, the
catalyst granules were brought into warm distilled
water and, after 5–10 min, the supernatant solution was
poured out. This procedure was repeated until the
supernatant was free of chloride ions.
Earlier, we suggested highly active, Al2O3-sup-
ported, Co–Pt bimetallic catalysts [18]. The activity of
these catalysts in CO oxidation in the presence of
hydrogen is higher than the activity of the Al2O3-sup-
ported Co and Pt catalysts. This finding was confirmed
by a later study [19]. A Co–Pt/TiO2 catalyst [20] was
also reported. Again, its activity in CO oxidation in the
presence of hydrogen was found to be higher than the
activity of the corresponding monometallic catalysts
Co/TiO2 and Pt/TiO2. Based on indirect data, it was
assumed that the higher activity of the Co–Pt catalysts as
compared to the Pt and Co monometallic catalysts is due
to the formation of a Co–Pt bimetallic phase [19, 20].
Here, we report the properties of a Co–Pt powder
and of Co–Pt catalysts supported on γ-Al2O3 and on the
graphite-like carbon material Sibunit [21] in the selec-
tive oxidation of carbon monoxide in hydrogen-con-
taining gas mixtures. The powder and supported cata-
lysts were prepared by the decomposition of a double
Co–Pt complex salt in a hydrogen atmosphere at mod-
erate temperatures (~400°ë), a procedure allowing fine
particles of metal–metal solid solutions and intermetal-
lides to be obtained [22].
For comparison, we prepared the monometallic cat-
alysts Pt/γ-Al2O3 and Co/γ-Al2O3 by impregnating
γ-Al2O3 with [CÓ(NH3)5NO2]Cl2 and ä2[Pt(NO2)4]
solutions by the incipient-wetness method. The metal
content of these catalyst was 2 wt %.
The supported catalysts and the Co–Pt powder were
characterized by a variety of physicochemical methods.
EXPERIMENTAL
The specific surface area (SBET) and the pore volume
The Co–Pt powder was obtained by the decomposi- (Vpor) of the supports and catalysts were derived from
tion
of
the
double
complex
salt the complete 77-K nitrogen adsorption isotherms
[Co(NH3)5NO2][Pt(NO2)4] · 1.5H2O in a hydrogen atmo- (ASAP-2400 instrument, Micrometrics, United States).
sphere at 400°C. This salt, which is poorly soluble in These parameters of the catalysts were the same before
water, was synthesized by mixing aqueous solutions of and after the reaction (Table 1). Moreover, the SBET and
[CÓ(NH3)5NO2]Cl2 and ä2[Pt(NO2)4]. It will be demon- Vpor values of the catalysts were equal to the SBET and
strated below that the Pt–Co powder thus obtained con- Vpor values of their respective supports.
KINETICS AND CATALYSIS Vol. 48 No. 2 2007

278
SNYTNIKOV et al.
The X-ray powder diffraction patterns of the cata-
[CO] – [CO]
2 [O2]0 – [O2]out
1
0
out
----------------------------------------
S =
× 100%,
lysts for phase analysis were recorded on a DRON–
Seifert-RM4 diffractometer (CuKα radiation, graphite
monochromator in the reflected beam, amplitude-dis-
crimination scintillation detector, 2θ = 5°–135°, scan-
ning with 0.02° increments). The data obtained were
compared to JCPDS standard data (electronic version
PCPDFWIN) [25]. The diffraction data were processed
using the PowderCel 2.4 program, which allowed the
phase composition, unit-cell parameters, and the coher-
ent-scattering domain size (D) to be determined.
where [CO]0 and [O2]0 are the inlet CO and O2 concen-
trations and [CO]out and [O2]out are the outlet CO and O2
concentrations.
RESULTS AND DISCUSSION
The oxidation of CO in hydrogen-containing mix-
tures was initially studied for the Co–Pt powder.
Figure 1 shows the temperature dependences of the
outlet CO concentration, O2 conversion, and selectivity
for this system.
For the initial oxygen concentration 1.5 vol %, the
outlet CO concentration is 300 ppm at 100–110°C, the
O2 conversion is 100%, and S is about 30%. Raising the
O2 concentration in the reaction mixture to 2.5 vol %
makes it possible to reduce [CO]out to <10 ppm at 100–
110°C. In this case, the oxygen conversion is again
complete, but the selectivity is decreased to ~20%.
Thus, the Co–Pt powder is an active catalyst for selec-
tive CO oxidation in hydrogen-containing mixtures.
The phase composition of the Co–Pt powder before
and after the selective oxidation of CO in the presence
of H2 was determined by X-ray diffraction. The diffrac-
tion patterns for this powder and for some Co–Pt solid
solutions and intermetallides are presented in Fig. 2.
The strongest reflections from the Co–Pt powder before
the reaction (diffraction pattern 1) are broad and are
assigned to the fcc lattice of the solid solution with a
unit-cell parameter of a = 3.771(2) Å. This unit-cell
parameter implies that the mean volume per atom of the
Kinetic measurements for CO oxidation in hydro-
gen-containing gas mixtures were detailed in an earlier
paper [9]. The reaction was carried out at atmospheric
pressure in a flow quartz reactor. The reactor was a U-
shaped tube with a length of 40 cm, an inner diameter
of 8 mm, and a wall thickness of ~1 mm and had a ther-
mocouple well with an outer diameter of 3 mm at its
central axis. A weighed portion of the catalyst (granule
diameter, ≤1 mm) or the Co–Pt powder mixed with
powdered quartz (particle size, ~1 mm) was charged
into the reactor. The total height of the catalyst bed was
12 cm. The reactor inlet and outlet were fitted with a fil-
ter to prevent the small catalyst particles entrained by
the flowing gas from entering the gas inlet and outlet
capillary piping. The reaction temperature was mea-
sured with a chromel–alumel thermocouple placed in
the center of the catalyst bed. The catalyst was given no
pretreatment before the experiment.
The reactant and product concentrations at the reac-
tor inlet and outlet were identified using a Kristall-2000
chromatograph (Russia) equipped with a thermal-con-
ductivity detector, a flame-ionization detector, and a solid solution is Vmean = 13.41 Å3. This value coincides
methanizer accessory using a nickel catalyst (NKM-4 closely with the half-sum of the atomic volumes of
cobalt and platinum (VCo = 11.02 Å3, VPt = 15.09 Å3).
Therefore, the main phase of the Co–Pt powder is the
Co0.5Pt0.5 solid solution. The coherent-scattering
domain size of this phase derived from diffraction pro-
files is 50–70 Å.
brand). The combination of the methanizer and the
flame-ionization detector allowed any hydrocarbon
present in the gas mixture along with CO and CO2 to be
quantified with high sensitivity. The gas mixture being
analyzed was separated in a column packed with the
molecular sieve NaX (for the thermal-conductivity
detector) or with Porapak Q (for the flame-ionization
detector). The sensitivity of this method makes possible
measurement of CO, CH4, and CO2 concentrations
Along with the strong reflections from the Co0.5Pt0.5
phase, weak superstructure peaks are present in the dif-
fraction pattern of the Co–Pt powder before the reac-
tion. These peaks occur in nearly the same positions as
the reflections from the CoPt intermetallide (tetragonal
lattice with a = 3.803 Å and c = 3.701 Å, according to
JCPDS file no. 43-1358) [25]. However, their low
intensity and the absence of tetragonal splitting suggest
only that the solid solution is incompletely ordered.
Note that the main reflections are asymmetric and
broadened on their small-angle side. This is explained
by the fact that the Co–Pt powder before the reaction
contains a small amount of a solid solution with a
higher platinum content.
down to ~10–4 vol % and é2 concentrations down to
10−3 vol %. The oxidation of carbon monoxide in
hydrogen-containing mixtures was characterized in
terms of CO conversion (Xëé), é2 conversion (XO ),
2
and selectivity (S), which were calculated using the
equations
[CO]0 – [CO]out
--------------------------------------
XCO
=
× 100%,
[CO]0
The profiles and intensities of all reflections from
the Co–Pt powder after the reaction (diffraction pattern 2)
are nearly the same as the reflection profiles and inten-
sities observed before the reaction. Note only that the
[O2]0 – [O2]out
-----------------------------------
[O2]0
XO
=
× 100%,
2
KINETICS AND CATALYSIS Vol. 48 No. 2 2007

Co–Pt BIMETALLIC CATALYSTS FOR THE SELECTIVE OXIDATION
279
[CO]out, ppm
104
Intensity
(‡)
103
102
101
1
2
1
XO , %
2
(b)
100
80
60
40
20
2
2
3
1
4
5
0
S, %
100
20
30
40
50
60
70
2θ, deg
(c)
Fig. 2. (1, 2) X-ray diffraction patterns from the Co–Pt pow-
der (1) before and (2) after CO oxidation in the hydrogen-
containing mixture. (3–5) Theoretical diffraction patterns
80
60
40
20
for (3) the Co Pt solid solution, (4) the Co Pt solid
0.5 0.5
0.1 0.9
solution, and (5) the CoPt intermetallide.
ence of H2. Since the Co–Pt powder and the supported
Co–Pt catalysts were prepared in similar ways, we
believe that particles of the Co–Pt solid solution form
on the support surface, as in the case of the powder.
Figure 3 illustrates the catalytic properties of the
supported bimetallic ((Co–Pt)/γ-Al2O3 and (Co–Pt)/ë)
and monometallic (Pt/γ-Al2O3 and Co/γ-Al2O3) cata-
lysts in CO oxidation in the presence of hydrogen.
Clearly, the activity of the Co/γ-Al2O3 catalyst is low.
Even at a higher reaction temperature of 230°ë, the CO
concentration decreased only from 1 to 0.73 vol % and the
O2 conversion was as low as ~20%. The Pt/γ-Al2O3,
(Co−Pt)/ë, and (Co–Pt)/γ-Al2O3 catalysts are substan-
tially more active. The temperature dependences of
1
2
0
70
90
110
T, °C
130
150
Fig. 1. Temperature dependences of (a) the outlet CO con-
centration, (b) O conversion, and (c) selectivity for CO oxi-
dation in the presence of H on the Co–Pt powder catalyst.
2
2
Feed composition (vol %): CO, (1) 1.5 and (2) 2.5; O , 65;
2
H , 10; H O, 20; CO and He, the balance. GHSV =
2
2
2
–1
8000 h .
[CO]out, XO , and SCO for these catalysts are similar and
2
set of weak reflections is better pronounced in this dif-
fraction pattern. These reflections arise from an fcc
solid solution with a = 3.898(5) Å, whose composition
is, accordingly, Co0.1Pt0.9. The amount of this solid solu-
tion in the powder is no larger than 5 wt %.
are only shifted relative to one another along the tem-
perature axis. The highest activity is shown by the
bimetallic catalysts (Co–Pt)/ë and (Co−Pt)/γ-Al2O3;
the lowest activity, by the monometallic catalyst
Pt/γ-Al2O3. The Co–Pt catalysts reduce the CO concen-
tration in the mixture to a lower level than the monome-
tallic catalyst Pt/γ-Al2O3. With the bimetallic catalysts
(Co–Pt)/C and (Co–Pt)/γ-Al2O3, CO reduction to
The above data suggest that the Co–Pt powder,
which consists largely of an fcc solid solution, is an
active catalyst for selective CO oxidation in the pres-
KINETICS AND CATALYSIS Vol. 48 No. 2 2007

280
[CO]out, ppm
SNYTNIKOV et al.
this low CO level is not usually achieved, the outlet CO
concentration being 80–150 ppm. Again, the oxygen
conversion is complete and selectivity is ~30%.
(‡)
104
103
102
101
These results suggest that the supported Co–Pt cat-
alysts are active and selective in CO oxidation in hydro-
gen-containing mixtures. It is important that the
(ëÓ−Pt)/C and (Co–Pt)/γ-Al2O3 catalysts, as well as the
Co–Pt powder, show stable performance under the
reaction conditions. No decrease in activity was
observed for these catalysts throughout a ~50-h-long
experiment.
Unfortunately, the X-ray diffraction characteriza-
tion of the (ëÓ–Pt)/C and (Co–Pt)/γ-Al2O3 catalysts did
not allow us to identify any Co–Pt bimetallic phase.
The integrated intensity of the reflections from the met-
als is low because of the low metal content (2 wt %) of
the catalysts, and the small size of the metal particles
causes reflection broadening, thus making the diffrac-
tion pattern difficult to interpret. At the same time,
based on the data obtained for the Co–Pt powder and on
the fact that the bimetallic catalysts (ëÓ–Pt)/C and
(Co−Pt)/γ-Al2O3 are more active than the monometallic
catalysts Co/γ-Al2O3 and Pt/γ-Al2O3, we believe that the
active component in the supported Co–Pt catalyst is a
Co–Pt solid solution.
1
2
3
4
XO , %
2
(b)
100
80
60
40
20
0
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, grant no. 03-03-33036.
S, %
(c)
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