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Illinois chemists spray their way to better catalysts
James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
7/11/2005
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CHAMPAIGN, Ill.
–– Using a technique called ultrasonic spray pyrolysis,
researchers at the University of Illinois at Urbana-Champaign have created
an improved catalyst for removing smelly sulfur-containing compounds
from gasoline and other fossil fuels. The improved catalyst is a form
of molybdenum disulfide, most commonly recognized as the black lubricant
used to grease automobiles and machinery.
Molybdenum disulfide is made of long flat layers of molybdenum metal
atoms sandwiched above and below by single atomic layers of sulfur.
The interactions between sulfur-sulfur planes are weak, so they can
easily slide past one another, providing excellent high-temperature
lubrication.
Molybdenum disulfide’s other important commercial application
is as a catalyst used by the petroleum industry to remove ecologically
damaging sulfur-containing compounds in gasoline. When burned, these
sulfur compounds cause the formation of acid rain.
“The flat planes of molybdenum disulfide that make it a good lubricant
also decrease its ability to react with fuels to remove sulfur,”
said Ken Suslick, the Marvin T. Schmidt Professor of Chemistry at Illinois and a researcher at the Beckman
Institute for Advanced Science and Technology. “This is because
all the reactions necessary for sulfur removal occur on the edges of
the long planes, and the bigger the planes, the less relative edge there
is.“
Using ultrasonic spray pyrolysis, Suslick and graduate student Sara
Skrabalak discovered a way to make a highly porous network of molybdenum
disulfide that preferentially exposes the catalytic edges. The researchers
describe their work in a paper that has been accepted for publication
in the Journal of the American Chemical Society, and posted on its Web
site. Funding was provided by the National Science Foundation.
Using an ordinary household ultrasonic humidifier, Suslick and Skrabalak
spray small droplets of precursor solutions into micron-sized droplets.
The droplets are then carried by a gas stream into a furnace, where
the solvent evaporates and dissolved substances react to form a product.
This spray-synthesis technique has allowed for the continuous, inexpensive
production of spherical powders of varying composition. Research efforts
are expanding this technique to the production of nanoparticles and
industrially important catalysts.
The new form of molybdenum disulfide is made by spraying droplets of
a water solution of ammonium tetrathiomolybdate (a molybdenum disulfide
precursor) and colloidal silica (very fine sand). As the droplets are
heated in the furnace, water evaporates and a molybdenum disulfide/silica
composite is formed. The composite is then treated with hydrofluoric
acid, which etches away the silica and leaves a network of molybdenum
disulfide behind.
“This treatment leaves pores where the silica used to be and exposes
the catalytically active edge of molybdenum disulfide,” Suslick
said. “Molybdenum disulfide is the standard industrial catalyst
for hydrodesulfurization (the removal of sulfur from fuels using hydrogen),
but this unique form of molybdenum disulfide has superior catalytic
properties when compared to conventionally synthesized molybdenum disulfide.”
In addition, the new spray-synthesis route to catalytic materials is
simple, easily scaled-up, and can be adapted to other industrial materials.
Editor’s note:
To reach Ken Suslick, call 217-333-2794; e-mail: ksuslick@illinois.edu.
