Adsorption and dissociation of oxygen on Pt(335)

Oxygen adsorption and dissociation on Pt(335) have been studied with high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption. Five vibrational peaks are found: from atomic 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 molecular 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 210 K, terrace O at 750 K and step O at 850 K. Our analysis of the HREELS intensities suggests that the step species are bound on the exposed step edges, not in the trough as suggested by previous studies. Dissociation occurs almost exclusively at the steps: At saturation coverage about 95% of the step O-2 dissociates before desorbing, while 95% of the terrace O-2 desorbs directly. Dissociation of the edge O-2 occurs in part directly and in part through the metastable 700 cm(-1) state. Direct dissociation produces step O and terrace O with equal probabilities, while the 700 cm(-1) state produces predominantly step O. Equilibration between step and terrace O occurs only above 400 K.