Adsorption Of Ch4, C2h6 And Neo-C5h12 On (111), (100) And (110) Faces Of Nickel Observed By Leed And By Photoelectric Work-Function Measurement

Low-energy electon diffraction and photoelectric work-function measurements have been made with(111), (110) and (100) nickel surfaces, and the results of adsorption of methane, ethane and neopentane on these surfaces have been studied. Diffraction patterns from ordered structures were observed in all cases of hydrocarbon absorption at 20 °C; however, these were also accompanied by appreciable diffuse scattering from disordered adsorption. The ordered arrays formed initially at 20 °C were from various (2×2) structures, and it is concluded that in all cases the adsorbed entities in the ordered arrays were C 1 units, probably CH 2 . Treatments at higher exposures and higher temperatures resulted in a sequence of surface structures of which the high temperature (> 350 °C) end products were: for (111) and (110), ‘pseudo-clean’ surfaces, where the term ‘pseudo-clean’ refers to a surface covered with carbon atoms in a P (1×1) structure forming the same mesh as that of the surface nickel atoms, and with the carbon atoms probably in the normal surface crystallographic sites; for (100), P (2×2) where the adsorbed entities were again probably carbon atoms. Some apparently complex structures formed on (111) and (110) before the ‘pseudo-clean’ surfaces were probably coincidence lattice structures. Hydrocarbon adsorption resulted in all cases in work-function reductions. From an interpretation of these data it is concluded that an (isolated) adsorbed CH 2 residue has a surface dipole of about 0.2 to 0.3 D (Debye unit) on (100), while on (110) and (111) the corresponding figures are both about 0.7 to 0.8 D. The origin of this dipole is discussed in terms of the likely surface bonding. For the ultimate high temperature surfaces (i. e. 350 °C) the work-functions were about 0.2 to 0.3 eV below the clean surface values, and this is interpreted as resulting from carbon atoms with a net excess positive charge situated a little above the plane of the outermost nickel atoms.