(660e) Pseudomorphic Monolayer Catalysts for Denox Applications

One of the great challenges of our time is the efficient and equitable consumption of fossil fuels. One of the consequences of our need to conserve our finite petroleum resources is a transition to more efficient diesel engines and lean burning gasoline engines. Associated with the switch to these lean burning engines is an increase in NOx production which results in a need to improve exhaust catalysts. Surface science studies of epitaxially grown metal layers on top of a different metal single crystal have revealed that these pseudomorphic monolayers have novel adsorption properties and therefore could potentially serve as a whole new class of catalysts. Specifically, the use of pseudomorphic monolayers of Pt over other transition metals will be examined for their ability to perform in both reducing and oxidizing environments. From these calculations, it is hoped that suitable candidates are identified for replacement of monometallic Pt catalysts thereby simultaneously improving performance and reducing cost. One such example is the oxygen reduction catalyst, as demonstrated by Adzic et al. who have made a breakthrough discovery in their attempt to improve fuel cell catalysts based upon computational work by Mavrikakis and co-workers. Mavrikakis' group predicted that a monolayer Pt skin above another transition metal may have an equal or better performance as pure Pt for oxygen reduction, resulting in the potential to dramatically reduce the amount of Pt in the electrode and was subsequently confirmed experimentally. One of the exciting implications of this work is that other systems in which expensive platinum group metals are involved may lend themselves to similar improvements. Therefore, we have focused our efforts on one of the largest sources of platinum demand: the catalytic converter. As a starting point, the oxidation of NO to NO2 was examined for a variety of potential candidate catalysts. It is important to recognize that the stable surfaces in the oxidizing environment may in fact be surface oxides. Furthermore, core-shell systems like Pt/Cu may be reversed in the presence of oxygen such that a surface oxide of copper forms above a Pt core depending upon the temperature and partial pressure of oxygen. Density functional theory based calculations were performed using the program VASP with plane wave basis set and ultrasoft pseudopotentials with periodic supercells. Transition state calculations for the evaluation of kinetics will use the climbing nudged elastic band method as described by Jonsson and coworkers.