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 eg 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.
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