Based on quantum mechanical calculations, MIT researchers have developed pictures showing how sulfur dioxide on a platinum catalyst converts to sulfur trioxide, a compound that poisons advanced catalytic converters for new fuel-efficient engines. Platinum atoms are blue spheres, oxygen atoms are black, and the sulfur atom is white.
Based on quantum mechanical calculations, MIT researchers have developed pictures showing how sulfur dioxide on a platinum catalyst converts to sulfur trioxide, a compound that poisons advanced catalytic converters for new fuel-efficient engines. Platinum atoms are blue spheres, oxygen atoms are black, and the sulfur atom is white.

"Removing sulfur from fuel is difficult and costly, so we need to develop a sulfur-resistant catalytic converter that will work with the lean-running engines now being designed," says Bernhardt Trout, associate professor of chemical engineering.

With excess oxygen present, sulfur dioxide in the exhaust reacts on the platinum catalyst to form sulfur trioxide. The sulfur trioxide prevents the device from capturing nitrogen oxides. "Our goal is to stop the reactions that turn sulfur dioxide into sulfur trioxide, but without interfering with the reactions that clean up carbon monoxide and hydrocarbons," says Trout. "That's challenging because all of those reactions involve the same process -- adding an oxygen atom to an existing molecule."

Researchers are now working on "selective oxidation" using quantum mechanical calculations to determine the reaction process by which sulfur trioxide forms. Calculating the behavior of all electrons during the reactions is computationally intensive and takes place on supercomputers at the National Computational Science Alliance at the University of Illinois at Urbana-Champaign.

Based on their findings, the team has developed a series of pictures that show step-by-step how a single oxygen atom on a platinum surface approaches and eventually joins a sulfur-dioxide molecule to form sulfur trioxide. So far, large-scale simulations based on this atomic-level understanding suggest that oxygen atoms will cluster together. Further research will delve into trying to understand the clustering process and whether steps to either encourage or discourage it may prevent the formation of sulfur trioxide.