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Gradient-corrected density-functional theory ͑DFT-GGA͒ periodic slab calculations have been used to analyze the binding of atomic hydrogen on monometallic Pd͑111͒, Re͑0001͒, and bimetallic Pd ML /Re(0001) ͓pseudomorphic monolayer of Pd͑111͒ on Re͑0001͔͒ and Re ML /Pd(111) surfaces. The computed binding energies of atomic hydrogen adsorbed in the fcc hollow site, at 100% surface coverage, on the Pd͑111͒, Re͑0001͒, Pd ML /Re(0001), and Re ML /Pd(111) surfaces, are Ϫ2.66, Ϫ2.82, Ϫ2.25, and Ϫ2.78 eV, respectively. Formal chemisorption theory was used to correlate the predicted binding energy with the location of the d-band center of the bare metal surfaces, using a model developed by Hammer and Nørskov. The DFTcomputed adsorption energies were also analyzed on the basis of the density of states ͑DOS͒ at the Fermi level for the clean metal surfaces. The results indicate a clear correlation between the d-band center of the surface metal atoms and the hydrogen chemisorption energy. The further the d-band center is from the Fermi level, the weaker is the chemisorption bond of atomic hydrogen on the surface. Although the DOS at the Fermi level may be related to the location of the d-band, it does not appear to provide an independent parameter for assessing surface reactivity. The weak chemisorption of hydrogen on the Pd ML /Re(0001) surface relates to substantial lowering of the d-band center of Pd, when it is pseudomorphically deposited as a monolayer on a Re substrate.

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DFT-GGA periodic slab calculations were used to examine the adsorption and hydrogenation of ethylene to a surface ethyl intermediate on the Pd(111) surface. The reaction was examined for two different surface coverages, corresponding to (2×3) [low coverage] and ( 3× 3)R 30°[high coverage] unit cells. For the low coverage, the di-σ adsorption of ethylene (-62 kJ/mol) is 32 kJ/mol stronger than the π-adsorption mode. The intrinsic activation barrier for hydrogenation of di-σ bonded ethylene to ethyl, for a (2×3) unit cell, was found to be +88 kJ/mol with a reaction energy of +25 kJ/mol. There appeared to be no direct pathway for hydrogenation of π-bonded ethylene to ethyl, for low surface coverages. At higher coverages, however, lateral repulsive interactions between adsorbates destabilize the di-σ adsorption of ethylene to a binding energy of -23 kJ/mol. A favorable surface geometry for the ( 3× 3)R 30°coverage is achieved when ethylene is π-bound and hydrogen is bound to a neighboring bridge site. At high coverage, the hydrogenation of di-σ bound ethylene to ethyl has an intrinsic barrier of +82 kJ/mol and a reaction energy of -5 kJ/mol, which is only slightly reduced from the low coverage case. For a ( 3× 3)R 30°unit cell, however, the more favorable reaction pathway is via hydrogenation of π-bonded ethylene, with an intrinsic barrier of +36 kJ/mol and an energy of reaction of -18 kJ/mol. This pathway is inaccessible at low coverage. This paper illustrates the importance of weakly bound intermediates and surface coverage effects in reaction pathway analysis.