Nature of nitrogen adsorbed on transition metal surfaces as revealed by electron spectroscopy and cognate techniques

Abstract

The nature of the chemisorbed states of nitrogen on various transition metal surfaces is discussed comprehensively on the basis of the results of electron spectroscopic investigations augmented by those from other techniques such as LEED and thermal desorption. A brief discussion of the photoemission spectra of free N₂, a comparison of adsorbed N₂ and CO as well as of physisorption of N₂ on metal surfaces is also presented. We discuss the chemisorption of N₂ on the surfaces of certain metals (e.g. Ni, Fe, Ru and W) in some detail, paying considerable attention to the effect of electropositive and electronegative surface modifiers. Features of the various chemisorbed states (one or more weakly chemisorbed γ states, strongly chemisorbed α states with bond orders between 1 and 2, and dissociative chemisorbed β states) on different surfaces are described and relations between them indicated. While the γ state could be a precursor of the α state, the α state could be the precursor of the β state and this kind of information is of direct relevance to ammonia synthesis. The nature of adsorption of N₂ on the surfaces of some metals (e.g. Cr, Co) deserves further study and such investigations might as well suggest alternative catalysts for ammonia synthesis.