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) [PdML/Re(0001)], PdML/Ru(0001), Pd(111), and PdML/Au(111). The computed (□3×□3) di-σ binding energy for ethylene on PdML/Re(00001), PdML/Ru(0001), Pd(111), and PdML/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 PdML/Re(00001), PdML/Ru(0001), Pd(111), and PdML/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 σCH* 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.
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We would like to thank Professor Jens Nørskov, Professor Bjørk Hammer, and Dr. Lars Hansen for helpful discussions as well as for the use of their plane-wave pseudopotential program. We also thank Dr. Victor S. Lusvardi, Dr. Kathy Saturday, Dr. Jan Lerou, Dr. Bruce Smart, and Professor Robert J. Davis for their helpful discussions. The DuPont Chemical Company (USA) and the NSF (Career Award CTS-9702762) are acknowledged for financial support. Acknowledgment is made to the donors of The Petroleum Research Fund (Grant 31342G5), administered by the ACS, for support of this research. V.P. expresses gratitude to the University of Minnesotta-IBM Shared Research Project, ACS, and the NSF (CDA-9502979) for support through the IBM Graduate Student Award in Computational Chemistry.
- Bimetallic overlayers
- Density functional theory
- Evans-Polanyi relation