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Within the framework of the GGA+U implementation of density functional theory, we investigate atomistic and electronic structures of Au adsorbed on the stoichiometric and the defective CeO2{111} surfaces, in the latter of which either O or Ce vacancies are presented. We show that on the stoichiometric surface, the most stable adsorption site of Au is not on the top of the outermost O atoms, as previously suggested, but on a bridgelike site in which the Au directly binds to two O atoms. We suggest that on both sites, the original empty Ce 4f states near the Fermi level facilitate the oxidation of the Au; the preference of the Au for being on the bridgelike site is due to the larger O 2p-d(Au) mixing, accompanied by more significant electron redistributions. On the reduced surface with O vacancies, the most stable adsorption site of Au is near the vacancy position. Unlike that on the stoichiometric surface, strong ionic bonding character exists between Au and Ce, as the former becomes Auδ- due to the occupation of the 6s(Au) orbitals. Upon substitution for one of the Ce atoms in the lattice, the Au possesses a much stronger positive charge than that in other cases. We find that although Au is strongly bonded when it is at the Ce vacancy site, the overall binding (i.e., with the Ce vacancy formation energy being taken into account) is weaker than that for Au adsorbed at the stoichiometric surface. [ABSTRACT FROM AUTHOR]

We have performed first-principles calculations for H absorption, H2 dissociation, and H diffusion in bulk BC3, a graphitelike layered structure. We show that in bulk BC3, H2 can dissociatively (and exothermically) absorb at low coverages. Several low-energy pathways have been computed for the dissociation process. The dissociation barriers are on the order of 0.2 eV or less. This presents a striking contrast to the situation in pure graphite and also on monolayer BC3(0001), where H2 dissociative absorption is considerably more difficult. Hydrogen absorption at high coverages has also been investigated; we find that the absorption enthalpy becomes moderately more exothermic at higher coverages. The calculated range of H binding energies is close to the desired value for reversible hydrogen storage. [ABSTRACT FROM AUTHOR]

The behavior of adsorbed water on oxides is of fundamental interest in many areas. Despite considerable attention received recently, our understanding of water chemistry is still short of needs and expectations, particularly on the topic of the coadsorption of water and other species. In this study we carry out density functional theory calculations to investigate the coadsorption of water and oxygen on the TiO2(110) surface. We show that oxygen exerts profound influences on the water adsorption, altering the mechanism of water dissociation. On the one hand, the possible dissociation route along [-110] is prohibited due to the weakening of the H bond between water and the lattice bridging oxygen in the presence of the coadsorbed oxygen, and on the other hand the coadsorbed oxygen induces dissociation along [001]. These results lead to a consistent interpretation of experiments. Furthermore, several possible final states and the related formation mechanisms are discussed in detail. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We apply first-principles molecular dynamics and static calculations in the study of several layers of water adsorbed at an oxide surface. Our aim is to bridge the gap between the well-explored monolayer and the complex, little-understood oxide–water interface by probing this middle ground where there is still contact with vacuum surface experiments. By examining coverages up to three monolayers on rutile TiO[sub 2](110) we emerge with three major conclusions. First, there is a trend with increasing coverage for water near the surface to become more molecular in nature, viz. the coordination of H to O for all near-surface molecules tends to lower values as the coverage increases. This means the hydroniumlike and other partially dissociated structures predicted up to 1.5 monolayers become less favorable as the coverage increases. Second, the templating influence of the surface disrupts the structure of the second and third layers at 3 ML coverage. Third, we can make a very satisfactory interpretation of experimental results (HREELS and TPD) gaining some new insights in the process. The assignment of TPD peaks to layer-by-layer desorption is shown to be open to question, as some second-layer molecules may contribute to the same peak as do those in the third layer. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]