The geometrical structure of small nickel clusters is probed via molecular adsorption of nitrogen on their surfaces. Nitrogen uptake patterns can be rationalized with the proposed structures if it is assumed that N2 binds to every exposed nickel atom, that the binding energies decrease with increasing metal—metal coordination, and that atoms that are four or less coordinate can bind two nitrogen molecules. In some cases nitrogen adsorption causes a change in cluster structure, usually to one that can accommodate more nitrogen molecules. Cluster structures are proposed for all clusters (bare and nitrogenated) in the 3–15-atom size range except Ni4 and Ni11. The nitrogen uptake for Ni4 is consistent with virtually any structure, and the data for Ni11 could not be interpreted in terms of a specific structure. In general, nickel cluster structures are different from those found for rare gas clusters as well as those derived from bulk packing. A comparison of the experimental results with existing theoretical calculations is presented.
Methane activation by nickel cluster cations, Ni n + (n=2-16): Reaction mechanisms and thermochemistry of cluster-CH x (x=0-3) complexesThe molecular adsorption of nitrogen on nickel clusters is used to probe the clusters' geometrical structures. The application of nitrogen binding rules derived from earlier studies of both larger and smaller nickel clusters allows a determination of structure from nitrogen uptake patterns. In the 16and 28-atom size region cluster structure is dominated by local pentagonal symmetry, a consequence of a preference for close packing of atoms on clusters with curved surfaces. In most cases, the structures that result can be derived from the 13-atom icosahedron, the polyicosahedral 19-, 23-, and 26-atom clusters, and the 55-atom icosahedron, by adding or removing atoms. Icosahedral and polyicosahedral clusters often have substantial surface strain, which in some cases is relieved by deviations from the ideal geometry. Structures are proposed for all clusters in the Ni 16 to Ni 28 size range, with the exception of Ni 27 . Generally, there is no evidence for structural changes as a consequence of nitrogen binding, so that the proposed structures are those of the bare as well as the nitrogenated clusters. Where possible, comparison with existing theoretical calculations of nickel cluster structure is made.
We applied periodic density-functional theory (DFT) to investigate the dehydrogenation of ethanol on a Rh/CeO2 (111) surface. Ethanol is calculated to have the greatest energy of adsorption when the oxygen atom of the molecule is adsorbed onto a Ce atom in the surface, relative to other surface atoms (Rh or O). Before forming a six-membered ring of an oxametallacyclic compound (Rh-CH2CH2O-Ce(a)), two hydrogen atoms from ethanol are first eliminated; the barriers for dissociation of the O-H and the beta-carbon (CH2-H) hydrogens are calculated to be 12.00 and 28.57 kcal/mol, respectively. The dehydrogenated H atom has the greatest adsorption energy (E(ads) = 101.59 kcal/mol) when it is adsorbed onto an oxygen atom of the surface. The dehydrogenation continues with the loss of two hydrogens from the alpha-carbon, forming an intermediate species Rh-CH2CO-Ce(a), for which the successive barriers are 34.26 and 40.84 kcal/mol. Scission of the C-C bond occurs at this stage with a dissociation barrier Ea = 49.54 kcal/mol, to form Rh-CH(2(a)) + 4H(a) + CO(g). At high temperatures, these adsorbates desorb to yield the final products CH(4(g)), H(2(g)), and CO(g).
With quantum-chemical calculations, we investigated the hydrogenation of a CO2 molecule on Fe(111) and W(111) surfaces using the density functional theory (DFT) with the projector-augmented wave (PAW) approach in periodic boundary condition. The structures and geometric parameters of the hydrogenation products, and the potential-energy surfaces, were calculated. It was shown that similar reaction paths for the hydrogenation of CO2 on Fe(111) and W(111) surfaces were found but with disparate energies. The rate-controlling energy barriers from M-CO2 (M = Fe, W) plus H atom to form formate (HCOO) and carboxyl (COOH) on a Fe(111) surface are 0.37 and 1.69 eV, respectively, but 0.54 and 2.79 eV, respectively, on a W(111) surface. The most probable path for the hydrogenation of a CO2 molecule on either the Fe(111) or W(111) surface is the formation of a formate-vertical structure. To understand the interaction between adsorbates and surfaces, we calculated the Bader charges and analyzed the local densities of states.
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