Modeling transition states for selective catalytic hydrogenation paths on transition metal surfaces

Density functional theory (DFT) quantum chemical calculations have been used to analyze the reaction coordinate for β-hydride elimination of ethyl on Pd(111), as a model for general C-H bond activation and C=C bond hydrogenation. The DFT computed activation barrier of +69 kJ/mol is comparable to the activation energy of 40-57 kJ/mol measured experimentally by Kovacs and Solymosi [1]. The role of electron-withdrawing substituents, such as -OH and -F, on the structure and energetics of adsorption and selective hydrogenation for a series of different substituted ethylene intermediates were examined in an effort to construct structure-reactivity relationships. Strong electron-withdrawing substituents were found to reduce the adsorption energy of the di-σ binding mode. These substituents were also found to raise the activation barrier for β-hydride elimination of the corresponding β-substituted-ethyl intermediates. The reaction mechanism and transition state structures for various other C-H bond activation reactions are compared. The results indicate that there is a noticeable similarity between the transition state structures for various C-H bond activation reactions. This suggests that there may be a universal mechanism that governs a series of relevant selective hydrogenation reactions.