Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Carbon monoxide component
Reexamination Certificate
2001-10-29
2004-09-07
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Carbon monoxide component
C422S177000, C422S211000, C502S304000
Reexamination Certificate
active
06787118
ABSTRACT:
BACKGROUND OF THE INVENTION
Hydrogen powered polymer electrolyte membrane fuel cells (PEMFCs) are useful for power generation due to their high efficiency and power density, low emissions, low operating temperature, small size, and portability. Because of these characteristics, hydrogen PEMFCs are being developed for both mobile and stationary applications. Natural gas or methanol are more practical fuels than hydrogen for commercial devices because of storage and transportation issues. If natural gas or methanol is used as fuel sources for PEMFCs, a reforming step is first used to convert the fuel to hydrogen prior to entering the cell. Unfortunately, the carbon monoxide which is produced along with hydrogen in the reforming step degrades PEMFC performance.
The vast majority of published work addressing the catalytic oxidation of CO has not considered selectivity in the presence of hydrogen. In the early 1980s, Stark et al. reported that Pt supported on SnO
x
was an effective CO oxidation catalyst. Hoflund et al. also studied the Pt/SnO
x
system and compared it to Au/MnO
x
. They reported that Au/MnO
x
was more active and had a longer lifetime than Pt/SnO
x
. The most significant results reported were that the Au/MnO
x
catalyst produced nearly 60% conversion of CO to CO
2
at only 35° C. and increasing the temperature to 55° C. produced over 80% conversion to CO
2
. Haruta et al. also have performed extensive testing on supported Au catalysts for oxidation of CO
2
. Using ultra-fine gold particles dispersed on the oxides of Fe, Co and Ni, they reported activity for the oxidation of CO at only −70° C. Since Au alone is known to be inactive towards oxidation, and the metal oxides used in the above studies have very limited activity, the results obtained by Hoflund et al. and Haruta et al. underscore the synergistic effects between the metal and support material.
The catalytic properties of Ce-based catalysts also have been studied. CeO
2
without any dopants or supported metals has been shown to be active for oxidation of CO with light-off temperatures above 300° C. By adding La to promote oxygen vacancies, and a small quantity of Cu (1 at. %), either as bulk CuO or incorporated into the structure, the light-off temperatures for CO oxidation were reported to be reduced to less than 100° C. Strong interactions between transition metal dopants and metal oxides are believed to be largely responsible for the enhancement in catalytic activity. Active sites formed by highly dispersed clusters of transition metal atoms and ions may promote oxidation at the phase boundary between the cluster and the metal oxide.
Selective CO oxidation in the presence of H
2
has not been as thoroughly investigated as low-temperature oxidation activity. Oh and Sinkevitch studied performance for noble metals (Ru, Rh, Pt) supported on Al
2
O
3
. At high temperatures (~170° C.) Ru/Al
2
O
3
and Rh/Al
2
O
3
were reported to demonstrate nearly 100% conversion of CO with approximately 5% conversion of H
2
for optimum inlet O
2
concentrations. These results were superior to those reported for Pt/Al
2
O
3
, which was reported to produce almost 20% conversion of H
2
with 100% conversion of CO. At lower temperatures (~140° C.) the better reported performance of Ru and Rh relative to Pt was even more pronounced, and Ru was significantly more selective than Rh. Kahlich et al. observed a similar trend between Ru/Al
2
O
3
and Pt/Al
2
O
3
; however, the authors reported that Au/Fe
2
O
3
generated comparable activities and selectivities at temperatures 70° C. lower. Haruta et al. also noted high activity of Au/Fe
2
O
3
, as well as Au supported on Co
3
O
4
, NiO, Be(OH)
2
, and Mg(OH)
2
. Moreover, the authors reported that Au supported on selected metal oxides was more selective for CO oxidation relative to H
2
than Pt and Pd catalysts. Furthermore, Bethke and Kung recently reported that selectivity for CO oxidation could be improved by decreasing Au particle size, and the authors suggest that the optimal particle size is between 5-10 nm. Finally, Sekizawa et al. reported that the water-gas shift catalyst Cu/Al
2
O
3
—ZnO has promise for selective removal of CO from methanol reformate under appropriate conditions. Despite these apparent successes, current catalysts for selective CO oxidation lack sufficient activity and selectivity at temperatures compatible with PEMFCs (i.e., around 90-110° C.) and require careful control of O
2
concentration in the feed for optimum performance.
Methods for removal of carbon monoxide from hydrogen fuel include adsorption, reduction and oxidation. Adsorption methods remove carbon monoxide by trapping it on a suitable substrate. Although this method is effective, a portion of the purified hydrogen stream must be used as a sweep gas to regenerate the adsorbent, which decreases the amount of fuel available for the cell. Furthermore, large quantities of adsorbent are needed and heat must be applied to the adsorbent to liberate carbon monoxide during regeneration. Alternatively, carbon monoxide can be removed by catalytic reduction to methane. Unfortunately, catalysts that promote this reaction also promote undesirable side reactions that result in generation of more carbon monoxide.
There is a need for catalysts which selectively remove CO from hydrogen-containing gases. One particular use for such catalysts is to remove CO from a reformate gas feedstream with minimal reduction in hydrogen content.
SUMMARY OF THE INVENTION
The catalysts in this invention are multi-component metal oxides with or without noble metals. These catalysts are useful to selectively remove carbon monoxide in the presence of hydrogen. The hydrogen may be present in a large excess. One specific application of the materials of the invention is improving fuel quality for hydrogen PEMFCs.
Catalysts described in this invention are believed to remove carbon monoxide by selectively oxidizing it to carbon dioxide. Carbon dioxide does not affect the performance of hydrogen PEMFCs. Furthermore, the catalysts described in this invention are distinct from other reported selective carbon monoxide oxidation catalysts since they do not rely on noble metals or metals supported on traditional carriers such as Al
2
O
3
. Some significant and distinct aspects of catalysts in this invention are that they are multi-component metal oxides tailored to give selective oxidation of carbon monoxide in the presence of hydrogen, including a large excess of hydrogen, and the compositions do not require expensive noble metals to be effective.
This invention relates to the composition, synthesis, and use of distinct metal oxide catalysts either with or without catalytic noble metals for selective oxidation of carbon monoxide in the presence of hydrogen.
The preferred catalyst compositions of the invention have the formula:
nN/Ce
1−(x+y+z)
A
x
A′
y
A″
z
O
2−&dgr;
where A, A′, A″ are independently selected from the group consisting of: Zr, Gd, La, Sc, Sr, Co, Cr, Fe, Mn, V, Ti, Cu and Ni; N is one or more members of the group consisting of Pt, Pd and Au;
n is a weight percent between 0 and 25;
x, y and z are independently 0 to 0.9;
x+y+z is 0.1 to 0.9; and
&dgr; is a number which renders the composition charge neutral. These catalyst compositions are also useful in the methods of the invention.
Other preferred catalyst compositions of the invention have the formula:
nN/(MO
x
)
y
(CeO
2−&dgr;
)
1−y
,
where
M is one or more members of the group selected from: Zr, Co, Cr, Fe, Mn, V, Ti, Ni and Cu; N is one or more members of the group consisting of Pt, Pd and Au;
n is a weight percent between 0 and 25;
y is 0.1 to 0.9;
and x and &dgr; make the compositions charge neutral. These catalyst compositions are also useful in the methods of the invention.
Catalyst compositions of the invention may comprise mixed oxides, single-phase materials, or multi-phase materials. Catalyst compositions further comprising a supporting material are also provided and are useful in the me
Roark Shane E.
White James H.
Eltron Research
Greenlee Winner and Sullivan P.C.
Medina Maribel
Silverman Stanley S.
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