Formation of surface and subsurface hydrogen states on thin palladium and platinum films precovered with methane

Abstract

The interaction of hydrogen with methane-precovered thin palladium and platinum films at 298 K has been studied by means of surface-potential change (ΔSP) measurements and thermal desorption mass spectrometry (TDMS). It has been observed that adsorption of hydrogen on thin Pd and Pt films is complicated by interaction with preadsorbed methane adspecies. Most of the methane adsorbate is replaced by hydrogen but part of it remains on the surface affecting the electrical character of the hydrogen adsorbate on thin Pd and Pt film surfaces. The ΔSP measurements reveal the ability of thin Pd films precovered with strongly adsorbed methane and hydrogen adspecies at 298 K to form palladium hydride at 78 K. The TD spectra indicate the existence of a subsurface state of hydrogen on methane-precovered Pd film at 298 K. The origin of ΔSP due to hydrogen adsorption and reaction with preadsorbed methane adspecies on thin Pd and Pt films at 298 K is carefully analyzed.