Field-emission studies on nickel. Part 1 .—Adsorption and interaction of ethylene and hydrogen

Abstract

The adsorption of H₂ and C₂H₄ on clean nickel surfaces was studied by field-emission microscopy, (FEM). The preparation of nickel emitters is described. Initial C₂H₄ adsorption at 20°C on bare nickel is dissociative as shown by close similarities in FEM patterns and the increase in work function ϕ, for H₂ or C₂H₄ adsorption. Subsequent C₂H₄ adsorption caused enhanced emission from areas around the (112) planes, accompanied by a reduction in ϕ to 4.8 eV; comparative surface potential measurements were made over nickel films. Enhanced emission is ascribed to dimerization of adsorbed ethylene on specific sites identified as step-edges plus terraces of (111) plane. Dimerization is discussed in terms of the direction of emergence of e_g orbitals at the surface. Hydrogen preadsorbed at pressures >5 × 10^–5 Torr inhibited the dimerization process. When the characteristic enhanced emission in the (112)“regions” was present after C₂H₄ adsorption, heating to 350–370°C formed “crystallites”, possibly graphite, due to aggregation of the two-dimensional surface carbide left after dehydrogenation.