Relation between catalytic properties and structure of metal films: I. Deuterium exchange of methane, ethane, and propane over nickel

Abstract

To assess the influence of surface structure on the mode of adsorption of saturated hydrocarbons, deuterium exchange reactions with propane, ethane, and methane were studied over various ultra-high-vacuum-deposited nickel films. Reactions were studied over {111} and {100} surfaces, prepared by nickel evaporation onto mica and sodium chloride, respectively, at elevated substrate temperatures, as well as over sintered and unsintered random polycrystal films. Reactions with propane and ethane gave initial product distributions from which the following conclusions are drawn: (i) at exchange temperatures, it is very unlikely that a monoadsorbed hydrocarbon residue will be retained on {111} or {100} surfaces because of the ready formation of a double bond between the carbon and the metal; (ii) surface sites for the retention of a monoadsorbed hydrocarbon residue are present on lowtemperature random polycrystal films, but they are thermally unstable in that they are much reduced by sintering, and are completely absent in {111} or {100} surfaces; (iii) exchange by a process of rapid interconversion between monoadsorbed and α,β-diadsorbed surface species does not occur on {111} or {100} surfaces and, ipso facto, is unlikely on sintered surfaces; (iv) multiple exchange occurs mainly by desorption from extensively dehydrogenated surface residues. It is suggested that the thermally unstable sites suitable for monoadsorption are associated with atomically rough areas of surface, either as high index planes or facets, or at quasimacroscopic features such as steps and terrace edges. Although the product distributions were sensitive to surface structure, activation energies and frequency factors were not. Methane exchange did not show discrimination with respect to surface structure, and reasons for this are discussed. The possible unreliability of exchange data obtained under conditions of poor surface cleanliness is emphasized.