Direct verification of a high-translational-energy molecular precursor to oxygen dissociation on Pd(111)

Adsorption of high-translational-energy oxygen on Pd(lll) was examined with electron energy-loss spectroscopy (EELS) and molecular beam techniques. EEL spectra indicate that a direct molecular chemisorption mechanism holds at high kinetic energies (up to 1.27 eV) on a 77 K substrate. The initially intact peroxo-like species proceeds to dissociation or desorption only after sufficient heating - direct dissociation from the gas phase appears to be negligible. We estimate the thermal activation barrier for conversion from the peroxo-like species to be approximately 0.32 eV. Initial adsorption probability measurements in this high-kinetic-energy regime support a molecular chemisorption-initiated mechanism. (C) 1998 Published by Elsevier Science B.V. All rights reserved.