Theoretical density functional study of the hydrogenation of maleic acid over Pd and Re surfaces

Abstract

Non-local density functional (DFT) quantum chemical calculations were used to calculate energies of reaction ( ΔE r× n ), enthalpies of reaction ( ΔH r× n ) and free energies of reaction ( ΔG r× n ) for vapor-phase CC bond hydrogenation of maleic acid, ethylene, 1,2-ethenediol, vinyl alcohol, 1,1-difluoroethylene and acrolein. The DFT-computed reaction enthalpies were found to be within 10–20 kJ/mol of experiment. While the SCF-based energies of reaction ( ΔE r× n ) were observed to systematically overestimate the enthalpies of reaction ( ΔH r× n ), the calculated correction term was 29±1 kJ/mol for all the reactions of the present homologous set. The computed di- σ binding energy of maleic acid on the Pd(1 1 1) and Re(0 0 0 1) surfaces were found to be −48 and −107 kJ/mol, respectively. These values are consistent with expected trends from experiment. The di- σ adsorption of maleic anhydride on Pd(1 1 1), at two different surface coverages, Θ=0.11 and 0.2 ML, were examined to determine the effect of lateral interactions between the adsorbates. Lateral repulsive interactions predominate as the coverage of di- σ bound maleic anhydride on Pd is increased from Θ=0.11 to 0.2 ML. This results in significant decrease of the binding energy from −84 kJ/mol to −12 kJ/mol. The role of solvent on maleic acid hydrogenation kinetics was investigated, using the β-hydride elimination of ethyl on a Pd 2 cluster as a model for CC bond hydrogenation. An activation barrier of +75 kJ/mol and an overall energy of reaction of +15 kJ/mol for vapor-phase β-hydride elimination of ethyl on the Pd dimer were found to be in reasonable agreement with our earlier results on larger Pd 19 clusters (Neurock, 1997). The solvent medium, modeled as a dielectric continuum ( ε=20) surrounding a solute cavity of radius 10 Å, was observed to decrease the activation barrier by less than 1 kJ/mol and decrease the overall endothermicity by 9 kJ/mol, for the β-hydride elimination reaction.