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Catalyst support structures facilitate high-temperature fuel reforming
James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
7/27/2005
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CHAMPAIGN, Ill.
–– The catalytic reforming of liquid fuels offers an attractive
solution to supplying hydrogen to fuel cells while avoiding the safety
and storage issues related to gaseous hydrogen. Existing catalytic support
structures, however, tend to break down at the high temperatures needed
to prevent fouling of the catalytic surface by soot.
Now, researchers at the University of Illinois at Urbana-Champaign have
developed porous support materials that can withstand the rigors of
high-temperature reforming of hydrocarbon fuels.
“These novel materials show great promise for the on-demand reforming
of hydrocarbons such as diesel fuel into hydrogen for portable power
sources,” said Paul Kenis, a professor of chemical
and biomolecular engineering at Illinois and a corresponding author
of a paper to appear in the August issue of the journal Advanced Functional
Materials.
To be useful for hydrocarbon fuel reforming, a catalyst support must
have a high surface area, be stable at high temperatures, and possess
a low pressure drop.
“Our new materials satisfy all three key requirements,”
said Kenis, who also is a researcher at the Beckman Institute for Advanced
Science and Technology. “They have a large surface area created
by a network of interconnected pores. They can operate at temperatures
above 800 degrees Celsius, which prevents the formation of soot on the
catalytic surfaces. And they have a low pressure drop, which means it
takes less pressure to push the fuel through the catalyst.”
To fabricate the supports, the researchers begin by placing a polydimethylsiloxane
(PDMS) mold onto a flat surface, forming a channel about 500 microns
wide that is open at both ends. A slurry containing polystyrene spheres
50 nanometers to 10 microns in diameter is then allowed to flow into
the channel from one end by capillary action.
“Once the slurry reaches the other end of the channel, the spheres
begin to pack together as a result of solvent evaporation, and the packing
process continues toward the inlet end,” Kenis said. “After
the packing process is completed, we remove any remaining solvent, which
leaves a sacrificial template consisting of a bed of closely packed
spheres.”
Next, the researchers fill the spaces between the spheres with a low-viscosity,
preceramic polymer-based liquid. After low-temperature curing, the mold
is removed, leaving a stable, freestanding structure.
Lastly, the cured ceramic precursor is pyrolyzed at 1,200 degrees Celsius
for two hours in an inert atmosphere. “The polystyrene spheres
decompose during the pyrolysis process,” Kenis said. “The
end result is a silicon carbide or silicon carbonitride replica with
a tailored structure of interconnected pores.”
The overall size of the replica can be precisely tailored through the
dimensions of the mold, Kenis said, while the pore size can be tailored
independently by the size of spheres used in the sacrificial template.
To demonstrate the use of these materials as catalyst supports, the
researchers coated samples of the porous structure with ruthenium. The
structure was then incorporated within a stainless steel housing, where
it successfully stripped hydrogen from ammonia at temperatures up to
500 degrees Celsius. In work not yet published, Kenis and his colleagues
incorporated the structure in a ceramic housing, which enabled the successful
decomposition of ammonia at operating temperatures up to 1,000 degrees
Celsius.
The researchers also showed that the silicon carbide and silicon carbonitride
structures are stable at temperatures as high as 1,200 degrees Celsius
in air, thus showing their promise to perform fuel reforming at temperatures
where fouling of the catalyst by soot does not occur.
While the demonstration was performed on a microscale reformer, the
material could be used for large-scale reformers, Kenis said, with improvements
in the fabrication processes.
The research team included Kenis, visiting faculty Dong-Pyo (Don) Kim,
visiting graduate student In-Kyung Sung, and two Illinois graduate students
Michael Mitchell and Christian. Funding was provided by the U.S. Department
of Defense, Army Research Office, Korean National Research Lab, National
Science Foundation and the University of Illinois.
Editor’s
note: To reach Paul Kenis, call 217-265-0523; e-mail: kenis@illinois.edu. Christian
is the entire name of the graduate student named in the last paragraph.
