INTERACTION OF HYDROGEN WITH POLYCRYSTALLINE PALLADIUM FILMS

The kinetics of hydrogen interaction with thin palladium films at 195, 298, 348 and 360 K was studied by means of surface-potential change ( ASP) measurements and thermal desorption mass spectrometry (TDMS). Dis sociative adsorption occurs at all temperatures, forming mobile states of the hydrogen adsorbate. The kinetics of adsorption at 298 K, at co verage theta greater-than-or-equal-to 0.8, is strongly influenced by i nteraction within the adsorbate layer. Two TDMS peaks, well described by second-order desorption kinetics, were detected when hydrogen adsor ption was carried out at 195 K. However, a small part of the hydrogen adsorbate desorbs according to the first-order kinetic equation. This effect can be attributed to the existence of interactions between adso rbed hydrogen atoms, subsurface hydrogen, or can be related to structu ral imperfections of the Pd film surface. A detailed analysis of the i sothermal desorption of the weakly adsorbed hydrogen species was perfo rmed. The results obtained from TDMS and from isothermal desorption ar e compared.