Theoretical modeling of mechanisms of phenylacetylene and styrene hydrogenation on the Pd(100) surface

Abstract

Various routes of phenylacetylene and styrene hydrogenation on the Pd(100) surface were studied by the DFT-PBE density functional method. The three-layer model of the Pd₅₃ cluster was used for the description of the Pd(100) surface. The Gibbs activation energy of hydrogenation of a phenylacetylene molecule adsorbed according to the di-σ type was shown to be 21.7 kcal mol⁻¹, whereas that of a styrene molecule was 23.4 kcal mol⁻¹. The Gibbs activation energy for the strongly adsorbed di-μ-structure of phenylacetylene is significantly higher (35.2 kcal mol⁻¹). The difference in Δ^≠G₂₉₈ values of the reaction of phenylacetylene and styrene hydrogenation explains the enhanced selectivity to styrene, but this difference is lower than the earlier calculated value for the Pd(111) surface. According to the calculations, a low coverage of the Pd(100) surface should favor the formation of stable fourfold site di-μ-structures of adsorbed phenylacetylene, which can result in a sharp decrease in the phenylacetylene hydrogenation rate and a decrease in the selectivity of hydrogenation to styrene.