Preferential oxidation of CO in H2 by aqueous polyoxometalates over metal catalysts

Article abstract of DOI:10.1002/anie.200461601

Stream cleaning: CO in CO/H₂ mixtures is oxidized preferentially at room temperature with an aqueous polyoxometalate (POM) solution over gold catalysts (see scheme). The rate of H₂ oxidation is slow and is inhibited by CO. This process can be used to remove CO efficiently from H ₂ gas streams. The solution containing protons and reduced POM can be used to produce electrical energy at a fuel-cell anode through re-oxidation of the reduced POM.

Full text of DOI:10.1002/anie.200461601

Communications
Heterogeneous Catalysis
Preferential Oxidation of CO in H2 by Aqueous
Polyoxometalates over Metal Catalysts**
Won Bae Kim, Tobias Voitl, Gabriel J. Rodriguez-
Rivera, Steven T. Evans, and James. A. Dumesic*
Production of H2 for fuel cells is usually accomplished by a
multistep process, which starts with catalytic reforming of
hydrocarbons[1] or oxygenated hydrocarbons over metal
catalysts[2,3] to produce a mixture of H2, CO, and CO2.
These reformate gases are subsequently treated by several
steps, such as water–gas shift (WGS) (CO + H2O!CO2 +
H2)[4,5] and preferential oxidation of CO in the H2-rich gas
stream (PROX; CO + 1/2O2!CO2),[6,7] for applications
involving proton-exchange membrane (PEM) fuel cells, this
oxidation of CO is necessary owing to the strong poisoning
effects of CO on Pt-based anodes.[8] While these methods for
removing CO from H2 gas stream are well established, they
suffer from several limitations. For example, the WGS
reaction is slow at the low temperatures (e.g., 500 K) required
to achieve favorable thermodynamics for this reaction,[9] and
PROX requires the injection of O2 (or air) into the H2 gas
stream and consumes a fraction of the H2 as well. Moreover,
these processes for CO conversion often involve the use of
platinum or platinum-alloy catalysts, which are expensive and
compete with fuel-cell electrodes for the limited supply of this
precious metal, compared with the abundant holdings of gold
in the world.[10]
Recently, we reported the discovery of a room-temper-
ature process for production of electrical energy using a fuel
cell containing a carbon anode and operating with an aqueous
solution of a polyoxometalate (POM) compound that had
been reduced with pure CO over gold-nanotube catalysts.[11]
This process for the oxidation and utilization of CO involves
the reaction of CO and liquid water with a reducible POM,
such as [H3PMo12O40], which serves as an oxidizing agent for
reaction of CO with liquid water and as an energy-storage
agent for electrons; the process takes advantage of the high
catalytic activities of gold for CO oxidation,[12–14] especially in
the presence of liquid water, as shown experimentally using
gold-nanotube membrane catalysts[15] and as predicted by
density functional theory calculations.[16,17] A representative
[*] W. B. Kim, T. Voitl, G. J. Rodriguez-Rivera, S. T. Evans,
Prof. J. A. Dumesic
Chemical and Biological Engineering Department
University of Wisconsin
Madison, WI 53706 (USA)
Fax: (+1)608-262-5434
E-mail: dumesic@engr.wisc.edu
[**] This work was supported by the National Science Foundation and
Daimler-Chrysler, and the U.S. Department of Energy. Fellowships
were provided by the Camille and Henry Dreyfus Foundation and
the Graduate Engineering Scholars program. We thank the World
Gold Council for providing gold catalysts, and we thank M.
Mavrikakis, J. K. Nørskov, and Bjerne S. Clausen for valuable
discussions.
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DOI: 10.1002/anie.200461601
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Angewandte
Chemie
stoichiometric reaction for this process is shown in Equa-
tion (1).
3ꢁ
COðgÞ þ H2OðlÞ þ ½PMo12O40ꢀðaqÞ
!
ð1Þ
5ꢁ
CO2ðgÞ þ 2 HþðaqÞ þ ½PMo12O40ꢀðaqÞ
The aqueous solution of protons and reduced POM
formed in this reaction can be delivered to a PEM fuel cell
to generate electrical energy by using a carbon anode, thereby
regenerating the POM in its initial oxidized state.[11] The POM
species thus functions as a redox shuttle, thereby eliminating
the need to convert CO with H2O into gaseous H2 and CO2
for the production of electrical energy.
Herein, we demonstrate that this new process for CO
oxidation using aqueous POM solutions can be carried out at
high rates at room temperature over various metal catalysts,
such as nanoparticles of Au, Pt, Pd, and Ir supported on
carbon. Importantly, we show that catalysts containing Au
nanoparticles are unique because they selectively and rapidly
catalyze the oxidation of CO, in H2-rich gas mixtures
containing CO concentrations from 0.1 to 10%, using a
reversible POM oxidant, in contrast to the irreversible O2
oxidant that is employed in the conventional PROX process.
Accordingly, our room-temperature process using aqueous
POM solutions can be employed effectively for PROX
applications without consuming significant amounts of H2,
which is of particular importance in the production and
purification of H2 for the PEM fuel-cell applications.
Rates of CO oxidation by aqueous POM at 298 K are
presented in Figure 1 for different supported metal catalysts.
All catalysts tested herein showed a monotonic increase of
turnover frequency (TOF, sꢁ1) with respect to CO pressure
from 3.4 to 15.8 bar, the TOF in this study is defined as the
number of CO2 molecules produced per second normalized
by the number of surface metal sites (see caption of Figure 1).
All reaction kinetics measurements were made repeatedly,
under conditions where the rates were not limited by mass-
transport effects, by working at initial conversions of POM
lower than 20% (i.e., less than 1 electron transferred per
POM) and by decreasing the amount of catalyst and the metal
loading until the measured rate per gram of catalyst became
independent of the amount of catalyst and the metal loading
(i.e., 1 wt%). The values of TOF measured at 3.4 and 15.8 bar
of CO pressure are compared over the different metals in
Figure 1, showing that the TOF for oxidation of pure CO
decreases in the order Pd > Au > Pt > Ir> Rh > Ru at the
higher pressure of CO (15.8 bar), with a negligible rate for
Ag/C catalyst. The Au/C catalyst showed the highest TOF
compared to the other metals at the lower CO pressure of
3.4 bar.
Figure 1. Rate of CO oxidation by aqueous [H3PMo12O40] (0.05m) over
~
*
carbon-supported metal catalysts, ( ) TOF at 3.4 bar, ( ) TOF at
15.8 bar, and associated energy changes: (black bar) energy change for
dissociative adsorption of water (H2O!OHads +0.5H2), (light gray
bar) energy change for adsorption of CO, (dark gray bar) energy
change for dissociative adsorption of H2 (H2!2Hads). Values for
energy changes were determined by DFT calculations for stepped sur-
faces.[19] To determine the TOF value, the number of surface sites for
Au/C was estimated from the TEM-determined metal particle size, for
Pd/C, Pt/C, Ir/C, Rh/C, and Ru/C the number of surface sites was
measured by CO adsorption at 298 K, and for Ag/C the number of sur-
face sites was determined by O2 adsorption at 423 K as is established
elsewhere.[22] The metal dispersion (the number of surface metal
atoms normalized by the total number of metal atoms) is 0.12 for Au/
C, 0.39 for Pt/C, 0.23 for Pd/C, 0.13 for Ir/C, 0.18 for Rh/C, 0.55 for
Ru/C, and 0.09 for Ag/C.
dissociative adsorption of CO and H2, respectively).[18,19]
A
low value (that is, a positive energy change) of the binding
energy for OH species is desirable to ensure that the surface
of the metal does not become oxidized and thereby poisoned
by the aqueous POM solution. Thus, metals such as Au, Pd,
and Pt display high activities because they bind OH species
sufficient weakly. A sufficiently high binding energy (that is, a
negative energy change) for CO and H-atoms is required for
these species to interact with the metal surface during the
steps that eventually lead to the formation of protons and
electrons. Thus, Ag shows very low catalytic activity because
of the very weak interactions of adsorbed CO and H atoms
with this metal. Finally, very high (negative) binding energies
of CO on the metal surface may be undesirable, since such
high values could lead to poisoning of the surface by strongly
adsorbed CO at room temperature. Thus, the weaker binding
of CO on Au compared to Pt may explain the higher catalytic
activity of Au.
The metal loadings of the supported Au, Pd, Pt, and Ir
catalysts were determined by inductively coupled plasma
(ICP) analysis before and after their use for CO oxidation in
aqueous POM solutions. The gold loading of the Au/C
catalyst remained unchanged after exposure to reaction
conditions, and dissolved gold species were not observed in
the aqueous solution within the detection limit of the ICP
High catalytic activity for oxidation of CO by aqueous
POM solutions may be related to low binding energy for the
adsorption of OH groups on the metal surface (as given in
Figure 1 (black bars) by the energy change for the dissociative
adsorption of water to give adsorbed OH species and gaseous
H2), combined with sufficiently high binding energies for the
adsorption of CO and H-atoms (as given in Figure 1 (light and
dark gray bars) by the energy changes for the molecular and
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Communications
analysis (ꢂ 1 ppm). Moreover transmission electron micro-
aqueous POM solutions with gold catalysts can be used to
remove CO efficiently from H2 gas streams even at room
temperature without significant consumption of H2. Indeed,
for a lower CO concentration of 0.1% CO in H2, the ratio of
the rate of CO versus H2 oxidation was 0.10–0.15 at total
pressures of the gas mixture from 14.8 to 49.3 bar, corre-
sponding to a first-order rate constant (see Figure 3B) that is
still at least 100 times faster for CO than for H2 oxidation. The
TOF for CO oxidation is not affected by the presence of H2,
and the rate is first order with respect to CO pressure, that is,
all the TOF for CO data in Figure 3 A fall on the same line
regardless of whether measurements were made using pure
CO or using gas mixtures of CO in H2. In contrast, the TOF
for H2 oxidation is suppressed by the presence of CO, showing
a significant decrease of the rate from the case using pure H2
to when using 99.9% H2 (with 0.1% CO), and the TOF
decreases further when using 99% H2 (with 1% CO;
Figure 3 A). Note, when the gas mixture containing 90% H2
with 10% CO is employed, there is no measurable rate of H2
oxidation over the gold catalyst (see Figure 2B).
It is clear from Figure 2 and Figure 3 that Au/C catalyzes
the selective oxidation of CO (compared to H2) by aqueous
POM solutions, and the presence of CO inhibits the oxidation
of H2. In contrast, the rate of H2 oxidation over Pt/C is faster
than the rate of CO oxidation, and although the presence of
CO inhibits the oxidation of H2, metals such as Pt, Pd, and Ir
are not effective catalysts for selective oxidation of CO,
although these metals show high activity for the oxidation of
CO without H2.
scopy (TEM) measurements did not show any changes in
average metal particle sizes upon exposure of supported gold
catalysts to reaction conditions. We observed 10–20% loss of
the initial catalytic activity after first use, but stable activity
was maintained, at levels of at least 50% of the initial activity,
after exposure of the catalyst to 0.05m POM solution for
3 months. In contrast, Pd was leached from Pd/C into the
aqueous solution under the reaction conditions, which led to
dissolved species, such as Pd2+, that might serve as a
homogeneous catalyst.[20] Slow leaching of metal may take
place upon exposure of carbon-supported Pt and Ir catalysts
to reaction conditions, ICP analyses detected 10–20 ppm of Pt
and 5 ppm of Ir species in 0.05m POM solutions contacted for
24 h with the Pt/C and Ir/C catalysts, respectively.
Rates of CO versus H2 oxidation by aqueous POM
solution were measured on Au/C, Pt/C, Pd/C, and Ir/C
catalysts using pure CO and pure H2 gases (Figure 2 A), as
well as gas mixtures containing 10% CO in H2 (Figure 2B)
and 1% CO in H2 (Figure 2C). The rate of CO oxidation over
Au/C was at least 10-times faster than the rate of H2 oxidation
when both gases were passed separately, whereas Pt/C
showed a much higher rate of H2 oxidation than CO
oxidation. For a CO:H2 gas mixture containing 10% CO
(Figure 2B), the rate of CO oxidation was approximately
0.8 sꢁ1 on Au/C, and the rate of H2 oxidation was negligible.
The other catalysts also showed an inhibition of the rate of H2
oxidation by CO when 10% CO in H2 was used compared to
using pure H2. When the concentration of CO was decreased
to 1% CO in H2 (Figure 2C), the Au catalyst maintained its
effectiveness for PROX, showing a value for the rate ratio
(defined as TOF(CO)/TOF(H2)) of about 9. In contrast, the
other catalysts showed low ratios for the rate of CO versus H2
oxidation under these conditions.
Figure 4 shows a possible reaction scheme for CO
oxidation by an oxidized POM species in aqueous condition
on a gold catalyst (the structure of the POM anion is a
simplified representation of the Keggin structure[21] in which
one PO4 tetrahedron is surrounded by 12 MoO6 octahedra).
Carbon monoxide and water adsorb, albeit weakly, on the
gold surface. In contrast, dissociative adsorption of H2 on gold
is energetically unfavorable compared to the adsorption of
CO and is accompanied by a high energy barrier.[18] The
results in Figure 2 and Figure 3 indicate that this more
favorable adsorption of CO inhibits the activation of H2 on
The rate ratio of 9 for the Au catalyst using the 1% CO in
H2 gas mixture corresponds to a first-order rate constant of
CO oxidation that is 900-times higher than the rate constant
for H2 oxidation at 14.8 bar, assuming that the rates are
proportional to the respective reactant pressures. These
results indicate that our process for CO oxidation using
Figure 2. Rates of CO and H2 oxidation with aqueous [H3PMo12O40] (0.05m) over carbon-supported metal catalysts in different gas mixtures.
(Black bar) CO oxidation, (white bar) H2 oxidation, (gray bar) ratio of rates for CO versus H2 oxidation (TOF(CO)/TOF(H2)).
780
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Angew. Chem. Int. Ed. 2005, 44, 778 –782

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Chemie
dissociates eventually to produce CO2, which results in the
formation of a proton in liquid water and transfer of an
electron to the gold surface, this electron is subsequently
transported to a reducible POM species.
Experimental Section
In a typical experiment, a batch reactor loaded with 20 cm3 of 0.05m
POM ([H3PMo12O40], purchased from Aldrich) solution and catalyst
was purged three times with CO and then filled with CO at specific
pressures. The reactor was a stainless steel vessel (approximately
350 cm3 volume), in which the POM solution and catalyst were placed
in a glass liner with a magnetic stirrer. For the oxidation of CO in H2,
the reactor was purged three times with N2 and then filled with a
CO:H2 mixture. Gas-phase products were analyzed at specific times
by releasing pressure to an online gas chromatograph equipped with a
thermal conductivity detector. The rate of H2 oxidation was measured
by monitoring the color change of the POM solution using a UV/Vis
spectrometer operating at 500 nm.
CO and mixtures of CO:H2 were supplied from AGA. Carbon-
supported gold catalyst (Au/C) was supplied from the World Gold
Council (Type D, Lot No. 4D), which was characterized by the
supplier as 0.8 wt% Au with an average Au particle diameter of
10.5 nm. Catalysts consisting of 1 wt% Pt/C and 1 wt% Pd/C were
purchased from Aldrich and 1 wt% Ir/C was purchased from Alfa. A
catalyst containing 1 wt% Rh on carbon (Vulcan carbon) was
prepared using a solution reduction method of RhCl3 by NaBH4,
followed by rinsing with deionized water and drying in a vacuum
desiccator. Preparations of 1 wt% Ru and Ag on carbon were carried
out by impregnation of RuCl3 and
Figure 3. Rates of CO and H2 oxidation with aqueous [H3PMo12O40]
*
(0.05m) over 0.8% Au/C. A) ( ) TOF for CO with respect to CO pres-
sure in measurements using pure CO and gas mixtures of CO (10%,
&
1%, and 0.1%) in H2, (&) TOF for H2 using pure H2, ( ) TOF for H2
~
with respect to pressure of H2 in 99.9% H2 with 0.1% CO and ( ) in
&
*
99% H2 with 1% CO. B) ( ) TOF for CO and ( ) TOF for H2 with
respect to the total pressure of 0.1% CO:H2 mixture, (&) TOF of H2
using pure H2.
AgNO3, followed by reduction at 623 K
with flowing H2. All catalysts were
characterized by ICP to determine the
actual metal loadings on the catalysts.
Received: August 10, 2004
Published online: December 23, 2004
Keywords: carbon monoxide · gold ·
.
heterogeneous catalysis · hydrogen ·
polyoxometalates
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