Epitaxial growth of ultrathin palladium films on Re{0001}

Ultrathin bimetallic layers create unusual magnetic and surface chemical effects through the modification of electronic structure brought on by low dimensionality, polymorphism, reduced screening, and epitaxial strain. Previous studies have related valence and core-level shifts to surface reactivity through the d-band model of Hammer and Nørskov, and in heteroepitaxial films this band position is determined by competing effects of coordination, strain, and hybridization of substrate and overlayer states. In this study we employ the epitaxially matched Pd on Re{0001} system to grow films with no lateral strain. We use a recent advancement in low-energy electron diffraction to expand the data range sufficiently for a reliable determination of the growth sequence and out-of-plane surface relaxation as a function of film thickness. The results are supported by scanning tunneling microscopy and X-ray photoemission spectroscopy, which show that the growth is layer-by-layer with significant core-level shifts due to changes in film structure, morphology, and bonding

The importance of transient states at higher coverages in catalytic reactions

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 fur two different surface coverages, corresponding to (2x3) [low coverage] and (root 3 x root 3)R 30 degrees [high coverage] unit cells. For the low coverage, the di-sigma adsorption of ethylene (-62 kJ/mol) is 32 kJ/mol stronger than the rr-adsorption mode. The intrinsic activation barrier for hydrogenation of di-sigma bonded ethylene to ethyl, for a (2x3) 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 pi-bonded ethylene to ethyl, fur low surface coverages. At higher coverages, however, lateral repulsive interactions between adsorbates destabilize the di-a adsorption of ethylene to a binding energy of -23 kJ/mol. A favorable surface geometry for the (root 3x root 3)R 30 degrees coverage;is achieved when ethylene is It-bound and hydrogen is bound to a neighboring bridge site, Al high coverage, the hydrogenation of di-sigma 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 (root 3x root 3)R 30 degrees unit cell, however, the more favorable reaction pathway is via hydrogenation of pi- 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 analysi

A density functional theory analysis of the reaction pathways and intermediates for ethylene dehydrogenation over Pd(111)

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 C2Hx 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) surfac