P. Sautet

Ab initio study of the bare and hydrated (001) surface of tetragonal zirconia

The (001) surface of tetragonal zirconia, both bare, and fully hydrated or hydroxylated has been theoretically studied. A periodic Hartree-Fock method has been used, by simulating the adsorbent with a thin slab. Pseudopotentials have been employed for core electrons, and a “split-valence” set for valence electrons. Several geometries have been considered for surface water or OH groups. In the two cases, one preferential structure has been individuated, corresponding to a similar adsorption energy (∼ 15 kcal/mol with respect to isolated water molecules); the corresponding electronic structure has been analyzed. The vibrational frequencies of adsorbed OH groups have been calculated; they differ by more than 350 cm−1. Similar differences in OH vibrational frequencies have not been observed in infrared studies of zirconia, which, however, concern the monoclinic phase.

NO chemisorption on a magnetic alloy surface: a density-functional periodic study of Pd3Mn(100) compared with Pd(100)

Self-consistent calculations based on density functional theory with gradient corrections are used to compare the adsorption of NO on Pd(100) and Pd3Mn(100). There are two types of Pd3Mn(100) surfaces, one with only Pd atoms (A) and one with Pd and Mn atoms alternately ordered (B). For adsorption on surface A, the adsorption sites are in the same stability order as for palladium, with the atop site less stable than the bridge and the hollow ones, but all the binding energies are slightly weaker. For adsorption on surface B, the stability order is totally different, the atop Mn site being the most stable one. Therefore, NO prefers to adsorb on a magnetic Mn atom rather than on a Pd atom. This result is interpreted in terms of orbital interactions by the existence of a strong interaction between the partially filled 2π NO orbitals and the empty d spin-orbital of Mn. NO keeps a large magnetic moment when adsorbed on surface B (0.6μB).

Density functional study of the structural and electronic properties of RuS2(111). I. Bare surfaces

Trends in the surface relaxation and electronic properties of RuS2(111) as a function of the sulfur concentration at the surface have been investigated by ab initio density functional calculations. Consistent with experimental and previous theoretical studies, sulfur-enriched terminations without exposed unsaturated metal atoms (anionic vacancies) turn out to be more stable compared to the stoichiometric and highly reduced surfaces. Terminations involving the simultaneous occurrence of a full S2 perpendicular-pair and a single S-atom from a broken tilted pair are not energetically favored, as predicted by the calculated formation enthalpies. The observed SRu bond strengthening at the surface for terminations with anionic vacancies is related to the increased hybridization between sulfur 3p-π and ruthenium 4d bands.

Scanning tunneling microscopy study of model catalysts obtained by cluster beam deposition of palladium onto highly oriented pyrolitic graphite

Cluster beam deposition is used to obtain model catalysts (palladium on highly oriented pyrolitic graphite). The supported particles are characterized by transmission electron microscopy (TEM) and scanning tunneling microscopy. Decoration of graphite steps and formation of groups of palladium particles on the graphite surface are imaged by TEM. High spatial resolution observations show periodic (√3×√3)R30° charge density modulations of graphite around the supported particles. Such modulations are explained following the work of Mizes and Foster [Science 244, 559 (1989)] that take into account the fact that the particles act as perturbations to the graphite wave functions. On the basis of this explanation, the interactions of palladium and platinum (Xhie et al., Phys. Rev. B 43, 8917 (1991)] with the HOPG are compared. The results of this comparison are in complete agreement with the different catalytic behavior of the two metals in presence of reactions of hydrogenation of unsaturated hydrocarbons.

Density functional study of the structural and electronic properties of RuS2(111): II. Hydrogenated surfaces

Abstract Hydrogen chemisorption on RuS2(111) surfaces with distinct sulfur coverages has been investigated by a periodic density functional approach. Consistent with experimental and previous theoretical studies, dissociative hydrogen chemisorption is an exothermic process. A sulfur enriched termination (without anionic vacancies) with a hydrogen coverage of four hydrogen atoms per unit cell is found to be energetically favored compared to the stoichiometric and highly reduced surfaces. This structure involves exclusively protonic SH species at the surface. Additional reduction with hydrogen is endothermic. Generation of hydridic RuH species requires an under-stoichiometric surface termination and enhances the potential reactivity of the ruthenium sites. Hydrogen adsorption is observed to compensate the relaxation of RuS2(111) bare surfaces, restoring a bulk-like structure. The calculated relaxation trends on hydrogen adsorption are discussed in terms of the changes in hybridization between sulfur 3p-π and ruthenium 4d bands, induced by mixing with the hydrogen s-states.