The influence of Au on the adsorption of methylacetoacetate on Ni—a study with medium energy ion scattering and reflection absorption infrared spectroscopy

Abstract

The hydrogenation of β-ketoesters over chirally modified Ni catalysts is one of the most successful examples of enantioselective heterogeneous catalysis. One of the factors limiting the enantioselectivity of this process is the fact that bare (achiral) Ni sites are active for the racemic hydrogenation reaction. In this work, we investigate the interaction of the simplest β-ketoester, methylacetoacetate, with bimetallic Ni/Au surfaces in order to examine whether the addition of Au to the Ni catalyst has the potential to enhance the enantioselectivity of the catalytic reaction. Using reflection absorption infrared spectroscopy (RAIRS), we probe the molecular conformation of methylacetoacetate on the bimetallic surface as a function of pre-annealing temperature. We show that the presence of Au strongly influences the adsorption of methylacetoacetate which adsorbs predominantly as the enolate. Using medium energy ion scattering (MEIS), we found that once the surface composition of Ni reaches ∼20% or above, the sticking probability of methylacetoacetate increases significantly and methylacetoacetate adsorption results in further segregation of Ni to the surface. The implications for enantioselective heterogeneous catalysis are discussed.