Sulfur is a common impurity in fossil fuels. Upon combustion, sulfur by-products such as H2S can contribute to air pollution and acid rain. To keep these pollutants out of the atmosphere, catalytic converters and smokestack scrubbers typically contain metal-oxide catalysts that adsorb the pollutants - that is, cause them to stick to the catalyst surface.
"But many of these devices were designed on a trial-and-error basis, without understanding the detail of how they actually work," says Brookhaven chemistry department research associate Andrea Freitag. Furthermore, the catalysts typically used are made from expensive metals like platinum and rhodium. Over time, some of these catalysts lose their ability to adsorb pollutants, "like a Slinky that loses its springiness due to overuse," says John Larese, a senior scientist in Brookhaven1s chemistry department.
Freitag and Larese are working to develop new catalysts based on less-expensive metals like magnesium and zinc, and are conducting molecular-level studies of how the pollutants and catalysts interact. Their findings may help engineers build a better pollution trap.
For example, the scientists have used X-ray diffraction techniques at Brookhaven1s National Synchrotron Light Source and neutron scattering at facilities in Europe to take molecular level "pictures" of how the pollutant molecules adhere to the catalyst crystals. In accompanying thermodynamic studies, in which the catalyst is held at a constant temperature while the pressure of a pollutant gas above it increases, the scientists calculate the capacity of the catalyst to adsorb the pollutant.
"These studies enable us to match what1s happening thermodynamically with what1s happening microscopically," Larese says. For example, the team has shown that when H2S adsorbs on MgO, two or three uniform layers form at distinct intervals as the pressure increases.
The studies also show that the adsorption process can be reversed if the catalyst is heated to release the adsorbed gases. That means the catalyst can be used over and over, eliminating the "worn-out Slinky" problem.
The Brookhaven scientists are also looking at ways to tailor-make MgO and other catalyst crystals to increase their adsorbing capacity, for example, by altering the crystals'size or shape to increase their surface area, or by doping the crystals'surfaces with other reactive metals such as zinc, nickel, chromium and copper.
The structural and thermodynamic studies are the key, Larese says; "You can make a better catalyst if you understand the process."
This paper will be presented at session V15 on March 23, 2000, at 2:30 p.m. in room 205B of the Minneapolis Convention Center.
The U.S. Department of Energy1s Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.