ADSORPTION AND DISSOCIATION OF OXYGEN ON PT(335)

Oxygen adsorption and dissociation on Pt(335) have been studied with h igh-resolution electron energy loss spectroscopy (HREELS) and temperat ure-programmed desorption. Five vibrational peaks are found: from atom ic oxygen on the terraces at 470 cm(-1) and on the steps at 560 cm(-1) , from molecular oxygen on the terraces at 860 cm(-1) and on the steps at 830 cm(-1), and, under limited conditions, from a distinct molecul ar step species at 700 cm(-1). Four desorption features correspond to the first four vibrational peaks: terrace O-2 at 125 K, step O-2 at 21 0 K, terrace O at 750 K and step O at 850 K. Our analysis of the HREEL S intensities suggests that the step species are bound on the exposed step edges, not in the trough as suggested by previous studies. Dissoc iation occurs almost exclusively at the steps: At saturation coverage about 95% of the step O-2 dissociates before desorbing, while 95% of t he terrace O-2 desorbs directly. Dissociation of the edge O-2 occurs i n part directly and in part through the metastable 700 cm(-1) state. D irect dissociation produces step O and terrace O with equal probabilit ies, while the 700 cm(-1) state produces predominantly step O. Equilib ration between step and terrace O occurs only above 400 K.