Electronic Factors Governing Ethylene Hydrogenation and Dehydrogenation Activity of Pseudomorphic Pd_ML/Re(0001), Pd_ML/Ru(0001), Pd(111), and Pd_ML/Au(111) Surfaces

DFT-GGA periodic slab calculations were used to examine the chemisorption, hydrogenation, and dehydrogenation of ethylene on pseudomorphic monolayers of Pd(111) on Re(0001) [Pd_ML/Re(0001)], Pd_ML/Ru(0001), Pd(111), and Pd_ML/Au(111). The computed (□3×□3) di-σ binding energy for ethylene on Pd_ML/Re(00001), Pd_ML/Ru(0001), Pd(111), and Pd_ML/Au(111) are -10, -31, -62, and -78 kJ/mol, respectively. Hydrogen chemisorption follows trends very similar to the adsorption of ethylene with calculated dissociative adsorption energies of +2, -6, -78, and -83 kJ/mol, on the Pd_ML/Re(00001), Pd_ML/Ru(0001), Pd(111), and Pd_ML/Au(111) surfaces, respectively. The elementary reactions of ethylene hydrogenation to form a surface ethyl intermediate and the dehydrogenation of ethylene to form a surface vinyl species were examined as model reactions for metal-catalyzed coupling and adsorbate bond-breaking reactions, respectively. Activation barriers and energies of reaction were computed for these elementary C-H bond-forming and C-H bond-breaking reactions over all the aforementioned surfaces. Calculations indicate that the activation barriers for the C-H bond breaking of surface-bound ethylene and ethyl intermediates correlate linearly with the corresponding overall energies of reaction for different Pd overlayer surfaces, with a slope of 0.65. The C-H bond activation barriers appear to be lower on surfaces where the reaction is more exothermic, consistent with the Evans-Polanyi postulate. Finally, we demonstrate that both the trends in the adsorption energy of ethylene and the activation barriers for hydrogenation/dehydrogenation of ethylene are correlated to the intrinsic electronic properties of the bare metal surface. Using concepts derived from frontier-orbital theory, we extend the simple surface-activity model developed by Hammer and Nørskov (Surf. Sci.343, 211 (1995)) to predict the chemisorption and surface reactivity of both ethylene and ethyl on different surfaces. The d-band for the bare Pd overlayer is observed shifting closer to the Fermi energy as the substrate metal is changed from a reactive metal such as Re to a noble metal such as Au. Since C-H bond activation of ethyl and ethylene is primarily guided by electron-backdonation to the antibonding σ_CH* orbital, the activation barriers for C-H bond breaking were found to be lower on surfaces where the d-band is closer to the Fermi level. The converse is true for the microscopic reverse, C-H bond formation reaction.