Studies on ethylene adsorption on Ni,Pd and Pt films and the reactivity of the selected forms of the adsorbate with hydrogen

A rapidly-recording, static-capacitor method was used to study surface potential changes that occur during ethylene adsorption on Ni, Pd, and Pt films at 298 K. A precise dosing system for the introduction of gases and a careful determination of the roughness factor allowed us to relate the surface potential to the concentration of the various adsorbed species. Using this information we propose a mechanism for ethylene selfhydrogenation. Ethylene admolecules adsorbed in the second layer react readily with hydrogen whereas those from the first layer do not. On platinum films, only the electropositively-polarised β⁺ hydrogen adatoms are reactive with ethylene, but with Ni and Pd films only the electronegatively-polarised β^? hydrogen adatoms are reactive.