A REFLECTION ABSORPTION INFRARED-SPECTROSCOPY STUDY OF THE ADSORPTION OF DIOXYGEN SPECIES ON A SILVER SURFACE

Two, adsorbed molecular-oxygen species are identified on a polycrystalline silver surface using reflection-adsorption infrared spectroscopy (RAIRS). One oxygen species is a peroxo (O2(2-)) species which is identified from its vibrational frequency (983 cm-1) and which stands up on the surface. Its heat of adsorption, measured from adsorption isosteres, varies with oxygen coverage and is given by DELTA-H(ads) = 14.5 - 18.7-theta kcal/mol where theta is the coverage and theta = 1 is measured from the integrated intensity of the 983 cm-1 RAIRS peak for a sample temperature of 90 K in the presence of 145 Torr of oxygen. The second oxygen species is oriented with its O-O bond parallel to the surface, has an O-O stretching frequency of 622 cm-1 and is postulated to be the precursor to dissociative oxygen adsorption. Its heat of adsorption is also coverage dependent being given by DELTA-H(ads) = 14.0 - 16.3-theta kcal/mol where here theta = 1 is taken as the maximum integrated peak intensity for an oxygen pressure of 250 Torr at a sample temperature of 90 K. The variation in coverage for both species as a function of oxygen pressure is well described by a Temkin isotherm in accord with their coverage-dependent heats of adsorption.