Direct oxidation of methane to methanol at low temperature and pressure in an electrochemical fuel cell

Article abstract of DOI:10.1002/anie.200703928

(Chemical Equation Presented) Fuel for thought? The direct oxidation of methane to methanol occurs at atmospheric pressure between 50 and 250°C in a fuel-cell-type reactor (see picture). The efficiency of the electrochemical activation of oxygen is higher than that for the catalytic activation of oxygen.

Full text of DOI:10.1002/anie.200703928

Communications
DOI: 10.1002/anie.200703928
Methane Oxidation
Direct Oxidation of Methane to Methanol at Low Temperature and
Pressure in an Electrochemical Fuel Cell
Atsuko Tomita, Junya Nakajima, and Takashi Hibino*
Methane is an abundantly available fuel whose use is mainly
limited to that of a primary energy source due to its low
reactivity. Methanol, on the other hand, is a useful inter-
mediate material in many chemical manufacturing processes
as well as a safe-to-handle liquid fuel for transportation and
storage. There is therefore a long-standing industrial interest
in producing methanol from methane effectively. The
conventional synthesis of methanol from methane involves
multi-step processes, including the steam reforming of
methane and subsequent catalytic reaction. These processes,
however, require high temperatures (< 7008C) and pressures
Although the direct oxidation of hydrocarbons has been
demonstrated in solid-oxide fuel cells (SOFCs) with stabilized
[
10,11]
[12]
zirconia
and perovskite-type oxide electrolytes, these
electrolytes require high temperatures above 5008C to show
high ionic conductivities. We have recently reported that
3
+
10 mol% In -doped SnP O (Sn In P O ) shows high
2
7
0.9
À1
0.1
À1
2
7
[1]
proton conductivities of above 10 Scm between 100 and
3508C under water-free conditions. This material has also
been used as an electrolyte in fuel cells, and herein we
report the selective oxidation of methane to methanol in a
hydrogen-oxygen fuel cell containing Sn In P O as electro-
[
13]
[14]
0
.9
0.1
2
7
(
200–300 atm) operations, respectively, which lead to high
running costs.
The direct oxidation of methane to methanol has received
lyte.
Sn In P O was prepared as reported previously. Pd/
[13]
0
.9
0.1
2
7
C, Pt/C, Rh/C, Au/C, and PdAu/C (10 wt.% metal) catalysts
were tested as the cathodes. The weight ratio of Pd to Au in
PdAu/C was in the range from 1 to 8. These catalysts were
prepared by an impregnation method starting from the
corresponding metal salts (PdCl , H PtCl ·6H O, RhCl , and
much attention as the next step in methanol production since
it avoids the above multi-step processes. However, this
oxidation is regarded as a very difficult reaction, especially
in the gas phase at low pressure, because of the need to
operate at high temperatures (> 4008C), where methanol is
2
2
6
2
3
HAuCl ·4H O). Thus, carbon powder (Black Pearls) was
4
2
[2–7]
quickly oxidized to formaldehyde and COx.
One approach
suspended in an ethanol/water solution and the metal salt
added at about 608C. After drying and grinding, the catalyst
powders were reduced under an H /Ar (10 vol.% H )
for oxidizing methane at lower temperatures is to apply an
electrochemical cell to the reaction system. Otsuka and
Yamanaka et al., for example, have reported the selective
oxidation of light alkanes to oxygenates by the electrochemi-
2
2
atmosphere at 4508C for 4 h. A commercial Pt/C anode
(60 wt.% Pt) was purchased from E-TEK inc.
[
8,9]
2
cally activated oxygen species
that are generated at the
The anode and cathode (area: 0.5 cm ) were attached on
cathode in polymer electrolyte (PEFCs) and phosphoric acid
fuel cells (PAFCs) [Eqs. (1) and (2)].
opposite sides of the electrolyte (thickness: 1.0 mm) and two
gas chambers were set up by placing the cell assembly
between two alumina tubes. The cathode and anode were
supplied with a 50 vol.% methane–50 vol.% oxygen mixture
and hydrogen, respectively, at a flow rate of 30 mLmin . Gas
analysis was performed with online flame-ionization detector
Anode : H ! 2 H þ 2 eÀ
þ
ð1Þ
ð2Þ
2
À1
Cathode : O þ 2 H þ 2 eÀ ! O* þ H O
þ
2
2
(FID) and thermal-conductivity detector (TCD) gas chroma-
tographs. The whole gas line and the alumina tube were
heated to a temperature of approximately 808C and the gas
concentrations were obtained after the reaction steady state
had been attained. Current/voltage curves for the fuel cell
were measured with a galvanostat.
The ability to produce methanol from methane was first
investigated with the Pd/C, Pt/C, Rh/C, and Au/C cathodes at
2508C. The common feature of these fuel cells is their open-
circuit voltage (OCV; 715–915 mV), which is lower than the
theoretical OCV of 1.1 V in all cases. This result is not due to
the presence of methane in the cathode gas, since there is no
difference in the OCVs in the presence and absence of
methane. At least two factors are responsible for the lower
OCVs: 1) physical leakage of gas through the electrolyte and
These active oxygen species can directly oxidize various
hydrocarbons, such as ethane and propane, to oxygenates at
low temperatures (< 808C), however, there has been no
report on the selective oxidation of methane to methanol in
fuel-cell systems. This is because the operating temperature is
too low to oxidize methane. Clearly, therefore, a fuel cell
capable of working above 808C is necessary for methane
oxidation.
[*] J. Nakajima, Prof. T. Hibino
Graduate School of Environmental Studies, Nagoya University
Fro-cho, Chilkusa-ku, Nagoya 464-8601 (Japan)
Fax : (+81)52-789-4206
E-mail: hibino@urban.env.nagoya-u.ac.jp
2
) partial electron/hole conduction in the electrolyte, which
A. Tomita
causes an internal short circuit. These fuel cells also show a
large difference in the short-circuit current (140–250 mA).
Materials Research Institute for Sustainable Development
National Institute of Advanced Industrial Science and Technology
Simosidami, Moriyama-ku, Nagoya, 463-8560 (Japan)
1
462
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Angewandte
Chemie
For this reason, all catalyst tests were performed at a constant
current of 200 mA, unless otherwise stated.
As shown in Figures 1a and 1b, methanol and carbon
dioxide are produced by polarization of the Pd/C cathode. No
other product was observed in our GC analysis. Small
amounts of methanol and carbon dioxide are also produced
at the Pd/C cathode under open-circuit conditions (Fig-
ures 1a,b). As described above, a small amount of hydrogen
permeates through the electrolyte from the anode to the
cathode, and we assume that this permeating hydrogen reacts
with oxygen at the cathode and activates it catalytically.
Furthermore, while the Pt/C and Au/C cathodes catalyze the
production of carbon dioxide rather than methanol, the Rh/C
cathode shows almost no activity. It has been reported that
some noble metals, notably Pd, show high catalytic activity for
the formation of hydrogen peroxide from hydrogen and
[
4,5,15–17]
oxygen,
which suggests that the difference in the rate
of methanol formation among the tested cathodes reflects
their efficiency in producing hydrogen peroxide or derivatives
electrochemically.
Although the direct oxidation of methane to methanol is
possible at the Pd/C cathode, the rate of methanol formation
[
18,19]
is far from satisfactory. Landon et al.
et al.,
and Ishihara
[
20]
for example, have reported that the catalytic
formation of hydrogen peroxide from hydrogen and oxygen
is enhanced by the addition of Au to Pd. This method was
therefore applied in the present study. It can be seen from
Figures 1c and 1d that the highest rate of methanol formation
is achieved at a Pd/Au ratio of 8:1. Another important result is
that the rate of carbon dioxide formation is relatively low at
this Pd/Au ratio. The resulting selectivity toward methanol
reached 6.03%, which is about three times higher than the
value obtained for the Pd cathode.
To better understand these reactions at the PdAu/C
cathode, both fuel cell and catalyst tests were performed
under various conditions. Figure 2 shows the cell voltage–
Figure 2. Cell voltage–current density curves of the fuel cell with a
PdAu/C (Pd/Au=8:1) cathode between 50 and 2508C: H2,
Pt/CjSn0.9In0.1P O (1 mm)jPdAu/C, 50% O +50% CH .
2
7
2
4
current density curves of the fuel cell with the PdAu/C
cathode in the temperature range 50–2508C. A positive effect
on the cell performance is seen upon addition of Au to Pd. For
example, the OCV at 2508C is 727 mV for the Pd/C cathode
and 740 mV for the PdAu/C cathode, and the short-circuit
current at 2508C is 140 mA for the Pd/C cathode and 230 mA
for the PdAu/C cathode. This effect is ascribed to a reduction
of the polarization resistance of the cathode. Figures 3a and
3b show the rates of methanol and carbon dioxide formation,
Figure 1. Rates of a) methanol and b) carbon dioxide formation at
various metal/C cathodes at 2508C. Rates of c) methanol and
d) carbon dioxide formation at PdAu/C cathodes with various compo-
sitions. A mixture of 50% methane and 50% oxygen was supplied at a
À1
rate of 30 mLmin to all the cathodes.
Angew. Chem. Int. Ed. 2008, 47, 1462 –1464
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1463

Communications
hydrogen electrode (NHE)), which is more negative than the
oxidation potential of hydrogen peroxide (0.68 V vs. NHE).
Thus, it is possible that hydrogen peroxide is more stable
under polarization conditions than under open-circuit con-
ditions, which results in an enhancement of methanol
formation.
In conclusion, this study has demonstrated that methane
can be converted directly into methanol by raising the
reaction temperature to above 808C. However, as shown in
Figure 3, methanol is produced even at 508C, which suggests
that low-temperature proton conductors, such as Nafion,
could also be used as the electrolyte under similar conditions.
Indeed, an electrochemical cell containing Nafion produces
methanol and carbon dioxide at 508C, although only about
half the amount produced with Sn In P O . This result
0
.9
0.1
2
7
means that the Sn In P O electrolyte has no specific effect
0
.9
0.1
2
7
on methanol production other than allowing high proton
conductivities above 808C and also suggests that this PdAu/C
electrode can be utilized with various electrolytes.
Received: August 27, 2007
Revised: October 15, 2007
Published online: January 17, 2008
Keywords: electrochemistry · fuel cells · methane · methanol ·
oxidation
Figure 3. Rates of a) methanol and b) carbon dioxide formation at a
.
PdAu/C (Pd/Au=8:1) cathode between 50 and 2508C. This figure
includes the results for the catalytic reaction between gas-phase
hydrogen, oxygen, and methane at the open-circuit voltage.
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conversion: 0.012%) at 508C.
It would be useful to compare the electrochemical
activation of oxygen with the catalytic activation of oxygen,
therefore gaseous hydrogen was added to the cathode gas
outside the system. A hydrogen concentration of 4.5% is
equivalent to the amount of hydrogen produced by passing a
current of 200 mA through the cell. The results of these
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[
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Angew. Chem. Int. Ed. 2008, 47, 1462 –1464