Comparisons of propylene and propyne catalytic oxidation on a 100 Å Pt/Al2O3 thin film using in situ soft X-ray fluorescence methods

Propyne and propylene deep oxidation to form CO2 and H2O in flowing oxygen pressures up to 0.01 Torr has been studied on a 100 Angstrom Pt/Al2O3 thin film catalyst using a combination of synchrotron based soft X-ray techniques. In situ temperature-programmed and isothermal kinetic experiments have been used to evaluate reaction mechanisms and energetics. Propyne oxidation on the thin film occurs in a simple, one-step process with a C3H4 (propyne) intermediate. Propylene oxidation on this surface is more complex. Initial propylene desorption and oxydehydrogenation results in the formation of a C3H5 intermediate (1-methylvinyl). Skeletal oxidation begins with increasing temperature and additional oxydehydrogenation results in the formation of a C3H4 propyne-like intermediate. Thus, as observed previously on the Pt(111) surface, skeletal oxidation of both propyne and propylene above 370 K proceeds through a C3H4 intermediate. Even for this complex, supported Pt thin film sample, the oxidation mechanisms for propyne and propylene remain stable over the entire range of oxygen pressures studied. The propyne and propylene oxidation temperatures on the Pt film are quite similar to the Pt(111) surface. Propyne adsorbs on the 100 Angstrom Pt/Al2O3 thin film through the Tc system nearly parallel to the surface, in a manner very similar to the Pt(111) surface. However, molecular adsorption of propylene on the Pt film surface is more complex. A new weakly adsorbed bonding mode is observed on the Pt film in addition to a strongly adsorbed form similar to propylene bonding on the Pt(111) surface. This weakly bound species desorbs between 200 and 270 K, while the more tightly bound propylene species that remains on the surface undergoes oxydehydrogenation and skeletal oxidation as outlined above. (C) 2004 Published by Elsevier B.V.