Bonding and chemistry of hydrocarbon monolayers on metal surfaces

Abstract

Results from ultra-high vacuum surface analysis techniques over the last ten years have dramatically increased our understanding of how unsaturated hydrocarbons react with metal surfaces. We review here what has been learned, utilizing results from our laboratory on the bonding and chemistry of ethylene and benzene to illustrate general principles. Several points deserve emphasis. First, clean surfaces of most transition metals (as might be expected from their coordinative unsaturation) are highly reactive towards unsaturated hydrocarbons, sequentially dehydrogenating these adsorbates over the temperature range of ∼200–800K to evolve hydrogen and deposit carbon. Throughout this temperature range, partially dehydrogenated decomposition intermediates can be isolated and identified. The bonding (and also possibly the chemistry) of these surface fragments is strikingly similar to that of hydrocarbon ligands in multinuclear organometallic clusters. Studies which combine reactions at atmospheric pressure with surface analysis in UHV show that these stable surface fragments are present on active hydrocarbon catalysts but are not reaction intermediates. It appears, however, that these surface organometallic compounds are important for promoting catalytic turnover of reactive species by tempering the surface reactivity through adsorbate-adsorbate interactions. Studies (under ultra-high vacuum conditions) of adsorbates at high surface coverages and in the presence of coadsorbates show dramatic effects of attractive and repulsive adsorbate-adsorbate interactions on the surface bonding and chemistry.