Catalytic oxidation of C(3) hydrocarbons: In situ mechanistic studies on platinum and supported platinum surfaces.

Abstract

Catalytic hydrocarbon oxidation plays an important role in catalytic emission control, in catalytic combustion, and in detection of hydrocarbons. Despite the importance of hydrocarbon oxidation, molecular understanding of the fundamental surface processes involved remains limited. In this dissertation propylene and propyne oxidation processes have been studied using synchrotron-based in-situ soft X-ray and traditional surface science methods. The combination of methods provides unprecedented information regarding the concentrations, stoichiometries and bonding of carbon containing intermediates over a broad range of temperatures and pressures. Oxydehydrogenated C₃ intermediates are dominant under both steady state and transient conditions below 330 K. The formation of 1-methylvinyl and propyne intermediates in propylene oxidation clearly indicates that olefinic hydrogens are removed preferentially under reaction conditions. Above 330 K rapid skeletal oxidation removes these organic intermediates from the surface. The activation energies for surface oxidation range from 15 to 20 Kcal/mol. During steady state temperature cycling experiments, large thermal hysteresis loops are observed as a result of competition between adsorption/oxydehydrogenation and skeletal oxidation. Propylene and propyne oxidation on a 100 A Pt/Al₂O₃ thin film is very similar to oxidation on the Pt(111) surface. Taken together these results provide important new insight into surface oxidation processes and highlight the new perspective provided by in-situ soft X-ray methods.