An investigation of the adsorption of (R,R)-tartaric acid on oxidised Ni{1 1 1} surfaces

Despite extensive research into the heterogeneously catalysed enantioselective hydrogenation of beta-ketoesters over chirally modified Ni-based catalysts, the surface chemistry underlying enantioselective promotion is still poorly understood and hotly debated. In recent years, surface scientists have made considerable strides in understanding how chiral modifiers such as (R,R)-tartaric acid interact with metal surfaces. However, all of the research to date has concentrated on clean metal surfaces. Since modification of Ni nanoparticles occurs from aqueous solution and since the metallic nanoparticles are normally exposed to air prior to modification, it is likely that the adsorption of chiral modifiers occurs on Ni surfaces covered by a passivating oxide/hydroxide film. In this paper we investigate, with RAIRS and TPD, the adsorption of (R,R)-tartaric acid on Ni {111}; {111}-p(2 x 2)O and NiO{111} surfaces. We show that the thermal stability of the adsorbed tartrate species is strongly enhanced by oxidation of the Ni surface. Indeed, the thermal decomposition of the film is shown to tend towards that expected for nickel(R)tartrate. We investigate the effect of preoxidising and/or thermal processing of the Ni surfaces on the vibrational spectra of the adsorbed chiral modifier. The implications of this work for promoting the understanding of enantioselective heterogeneous catalysis are discussed. (C) 2004 Elsevier B.V. All rights reserved.