THEORETICAL ASPECTS OF PHOTOINITIATED CHEMISORPTION, DISSOCIATION, AND DESORPTION OF O2 ON PT(111)

The chemisorption of molecular oxygen on a Pt(111) surface of a three layer slab model is examined using tight-binding extended Huckel calculations. On-top, 2-fold-bridge, and 3-fold-capped sites are considered. Chemisorption of O2 appears to be more favorable at the 2-fold site. The stabilization is due to the better overlap of the O2 2-sigma(u) orbital with the Pt surface. Photoinitiated dissociation and desorption mechanisms of O2 are studied using an extended surface and a cluster model. Dissociation begins by stretching the O-O bond as the O2 pi* becomes occupied via electronic excitation. This results in a lowering in energy of the O2 sigma*, which is subsequently filled partially by electrons. Desorption is attributed to a pi*z(Pt-O) to pi*z(Pt-O)* excitation. Photodesorption and photodissociation of O2 on Pt(111) proceed via direct excitation of the adsorbate-substrate complex.