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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.

DFT-GGA periodic slab calculations are used to examine ethylene dehydrogenation paths over Pd(111). The most favorable adsorption modes along with their corresponding binding energies for all C 2 H x intermediates (acetylene, acetylidene, ethylene, ethyl, ethylidene, ethylidyne, vinyl, and vinylidene) are analyzed for 0.25 monolayer coverage on Pd(111). The binding energies are used to calculate the overall reaction energies for a number of elementary C-H bond activation and isomerization pathways that are likely involved in the decomposition of ethylene to ethylidyne over the well-defined Pd(111) surface. The intrinsic activation barrier for the dehydrogenation of ethylene to vinyl is determined using transition state search calculations. The stability of the surface vinyl species relative to ethylidyne is assessed by computing the activation barriers for the two-step conversion of vinyl to ethylidyne, via an ethylidene surface intermediate. Calculations indicate that the barrier for the conversion of vinyl to ethylidyne over Pd( 111) is 84 kJ/mol, which is 67 kJ/mol lower than the computed barrier for vinyl formation from ethylene (151 kJ/mol). This is in agreement with UHV experimental literature that have consistently identified ethylidyne, but have not detected the vinyl species, during the thermal reactions of ethylene on the Pd(111) surface.

Gradient corrected periodic density functional theory ͑DFT-GGA͒ slab calculations were used to examine the chemisorption of atomic hydrogen on various Pd-Re alloyed overlayers and uniformly alloyed surfaces. Adsorption was examined at 33% surface coverage, where atomic hydrogen preferred the three-fold fcc sites. The binding energy of atomic hydrogen is observed to vary by as much as 0.7 eV due to Pd-Re interactions. The computed adsorption energies were found to be between Ϫ2.35 eV ͓for monolayer Pd-on-Re, i.e., Pd ML /Re͑0001͔͒ and Ϫ3.05 eV ͓for Pd 33 Re 66 /Pd͑111͔͒. A d-band weighting scheme was developed to extend the Hammer-Nørskov surface reactivity model ͓Surf. Sci. 343, 211 ͑1995͔͒ to the analysis of bimetallic Pd-Re alloyed systems. The hydrogen chemisorption energies are correlated linearly to the surface d-band center, which is weighted appropriately by the d-band coupling matrix elements for Pd and Re. The farther the weighted d-band center is shifted below the Fermi energy, the weaker is the interaction of atomic hydrogen with the alloyed Pd-Re surface.

Nonlocal density functional theory (DFT) calculations were performed to analyze the di-σ, π, and atop chemisorption modes of maleic anhydride on the close-packed Pd(111), Re(0001), and pseudomorphic monolayers of Pd on Re(0001) [Pd ML /Re(0001)] and Pd on Mo(110) [Pd ML /Mo(110)] surfaces. The DFTcomputed binding energies for maleic anhydride in the atop, π, and di-σ modes on Pd(111) are -28, -34, and -83 kJ/mol, respectively. The calculated adsorption energy and vibrational frequencies for di-σ adsorption are in good agreement with the UHV-TPD and HREELS data of Xu and Goodman (Langmuir 1996(Langmuir , 12, 1807(Langmuir -1816. The atop, π, and di-σ adsorptions of maleic anhydride on Re(0001) are significantly stronger than on Pd(111), with binding energies of -38, -210, and -200 kJ/mol, respectively. The π adsorption mode of maleic anhydride on Re(0001), in particular, is much stronger than that on Pd(111), due to additional interactions of the carbonyl groups of maleic anhydride with the Re(0001) surface. Adsorption energies for maleic anhydride on the bimetallic Pd/Re(0001) and Pd/Mo(110) surfaces are 10-20 kJ/mol weaker than on monometallic Pd(111). This decrease in the adsorption energy is explained on the basis of formal chemisorption theory. Using the analysis of Hammer and Nørskov (Nature 1995, 376, 238, and In Chemisorption and ReactiVity on Supported Clusters and Thin Films; Lambert, R. M., Pacchioni, G., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997; pp 285-351), the di-σ adsorption energies have been correlated to the location of the d band center for the clean metal surface. Calculations indicate that the strong Pd-Re and Pd-Mo interactions in the bimetallic surfaces shift the d band center for the surface Pd layer away from the Fermi level. This weakens the interaction of the olefinic group of maleic anhydride with the bimetallic Pd ML /Re(0001) and Pd ML /Mo(110) surfaces. The only exception is that for π-bound maleic anhydride on the Pd ML /Mo(110) surface, where the larger Pd-Pd spacing, relative to Pd(111), allows for better overlap of the carbonyl group π-orbitals with the metal d-orbitals. The π-mode binding energy on the Pd ML /Mo(110) surface is, therefore, stronger than the corresponding binding energy on Pd(111). The results are consistent with the experimental observations of Xu and Goodman (Langmuir 1996(Langmuir , 12, 1807(Langmuir -1816.

Nonlocal gradient corrected periodic density functional theory (DFT) was used to investigate the effect of water on the dissociation of acetic acid to the acetate anion and its corresponding proton on the Pd(111) surface. In the gas phase, the homolytic dissociation of acetic acid into acetate and hydrogen radicals (+468 kJ/mol) is clearly favored over its heterolytic dissociation into the acetate anion and proton (+1483 kJ/mol). In the presence of water, however, the heterolytic dissociation of acetic acid was found to be thermoneutral. The charged products (acetate ion and proton) are strongly stabilized by water. The metal surface acts to lower the endothermicity of the dissociation step. The energy of dissociation of acetic acid over Pd(111) was found to be +28 kJ/mol in the vapor phase. An analysis of the charge indicates that the dissociation of acetic acid over Pd(111) in the vapor phase is homolytic, forming products which are free radical like. The dissociation of acetic acid over Pd in the presence of water molecules, however, was found to be more heterolytic than in the vapor phase, forming products that have ionic characteristics. The dissociation of acetic acid over Pd(111) in the presence of solvating water molecules was calculated to be +37 kJ/mol. The metal surface stabilizes the acetate species but to a relatively weaker extent than the stabilization provided by the water solvent. The acetate anion was found to be 57 kJ/mol more stable when completely solvated by water molecules than on Pd(111). In the vapor phase, the acetate anion binds with an energy of over −198 kJ/mol on Pd(111). The surface acts as a “solvent” to shield the negative charge of the ion. In the presence of a solvent, however, the interaction between the acetate ion and the Pd(111) surface is weakened considerably. The interaction between acetate and the surface, however, is nevertheless attractive at −114 kJ/mol. While the acetate anion is thermodynamically more stable when completely solvated by water molecules, there appears to be a barrier for it to desorb from the metal surface.