Field-emission study of face-centred cubic group VIII transition metals. Part 2.—Adsorption of hydrogen, ethylene and acetylene on palladium

Abstract

The adsorption of H₂, C₂H₄ and C₂H₂ on a Pd surface has been studied by means of field emission microscope. The adsorption of ethylene at 295 K initially caused a decrease in work function. On continued exposure, the work function reached a minimum 1 min after the initial introduction and then increased to the same level as that for the hydrogen-adsorbing surface, indicating that C₂H₄ dissociates, releasing hydrogen atoms on the surface. In contrast, the adsorption of C₂H₂ was mainly non-dissociative and produced enhanced electron emision from the stepped area about the (111) face with an accompanying decrease in work function of ca. 1 eV. Thermal dehydrogenation of adsorbed C₂H₂ occurred above 370 K and the carbon left on the surface was converted to surface carbide on the (211) and (311) faces at ca. 700 K. Excess carbon produced graphite crystallites. The features of these adsorbed and decomposed layers are discussed in comparison with those on a nickel emitter.