SURFACE-DIFFUSION OF THE CYCLOHEXADIENYL RADICAL ADSORBED ON SILICA AND ON A SILICA-SUPPORTED PD CATALYST STUDIED BY MEANS OF ALC-MU-SR

The dynamic behaviour of the cyclohexadienyl radical adsorbed on plain silica and on a silica supported Pd catalyst is investigated by means of the avoided-level-crossing muon spin resonance technique. The infl uence of benzene coverage and of surface texture on the hyperfine coup ling constants, on the signal amplitudes and on the mobility of the ra dical is studied. Information about surface diffusion is obtained usin g a theoretical model based on a stochastic Liouville formalism. The r adical mobility increases with coverage of the surface with benzene. A rrhenius parameters are discussed within the framework of a hopping mo del based on transition-state theory for surface diffusion. It is sugg ested that the dynamics of the surface hydroxyl groups play a crucial role in the diffusion process.