Periodic density functional study of Rh and Pd interaction with the (100)MgO surface

The adsorption geometry and electronic properties of palladium and rhodium atoms deposited on the regular (100)MgO surface were analyzed by means of periodic DFT calculations using local, gradient-corrected and hybrid (B3LYP) functionals. Spin-polarized computations revealed doublet spin state of Rh atom to be the most stable electronic state for the adsorbed rhodium atom on (100)MgO. The preferred adsorption site of the metal (Pd and Rh) atoms was found to be the site on top of the surface oxygen atoms. A relatively stable geometry for the adsorption of the Pd and Rh in a bridge position above the two surface oxygens was found as well. The electronic structures suggested partly covalent bonding with contribution from electrostatic attraction between the metal and the oxygen atoms for both optimized structures. Small charge transfer was obtained from the support to the Pd and Rh metal atoms. The calculations showed that rhodium was bound stronger to the substrate probably due to stronger polarization effects

Polarization-dependence of palladium deposition on ferroelectric lithium niobate (0001) surfaces

We investigate the effect of ferroelectric polarization direction on the geometric properties of Pd deposited on the positive and negative surfaces of LiNbO$_3$ (0001). We predict preferred geometries and diffusion properties of small Pd clusters using density functional theory, and use these calculations as the basis for kinetic Monte Carlo simulations of Pd deposition on a larger scale. Our results show that on the positive surface, Pd atoms favor a clustered configuration, while on the negative surface, Pd atoms are adsorbed in a more dispersed pattern due to suppression of diffusion and agglomeration. This suggests that the effect of LiNbO$_3$ polarization direction on the catalytic activity of Pd [J. Phys. Chem. \textbf{88}, 1148 (1984)] is due, at least in part, to differences in adsorption geometry. Further investigations using these methods can aid the search for catalysts whose activities switch reversibly with the polarization of their ferroelectric substrates

The adsorption structure of furan on Pd(1 1 1)

The structure of molecular furan, C4H4O, on Pd(1 1 1) has been investigated by O K-edge near-edge X-ray absorption fine structure (NEXAFS) and C 1s scanned-energy mode photoelectron diffraction (PhD). NEXAFS shows the molecule to be adsorbed with the molecular plane close to parallel to the surface, a conclusion confirmed by the PhD analysis. Chemical-state specific C 1s PhD data were obtained for the two inequivalent C atoms in the furan, the α-C atoms adjacent to the O atom, and the β-C atoms bonded only to C atoms, but only the PhD modulations for the α-C emitters were of sufficiently large amplitude for detailed evaluation using multiple scattering calculations. This analysis shows the α-C atoms to be located approximately 0.6 Å off-atop surface Pd atoms with an associated C–Pd bondlength of 2.13 ± 0.03 Å. Two alternative local geometries consistent with the data place the O atom in off-atop or near-hollow locations, and for each of these local structures there are two equally-possible registries relative to the fcc and hcp hollow sites. The results are in good agreement with earlier density functional theory calculations which indicate that the fcc and hcp registries are equally probable, but the PhD results fail to distinguish the two distinct local bonding geometries

Electronic stress tensor analysis of hydrogenated palladium clusters

We study the chemical bonds of small palladium clusters Pd_n (n=2-9) saturated by hydrogen atoms using electronic stress tensor. Our calculation includes bond orders which are recently proposed based on the stress tensor. It is shown that our bond orders can classify the different types of chemical bonds in those clusters. In particular, we discuss Pd-H bonds associated with the H atoms with high coordination numbers and the difference of H-H bonds in the different Pd clusters from viewpoint of the electronic stress tensor. The notion of "pseudo-spindle structure" is proposed as the region between two atoms where the largest eigenvalue of the electronic stress tensor is negative and corresponding eigenvectors forming a pattern which connects them.Comment: 22 pages, 13 figures, published online, Theoretical Chemistry Account

Electronic structure, electron-phonon coupling and superconductivity of isotypic noncentrosymmetric crystals Li2_2Pd3_3B and Li2_2Pt3_3B

Electronic structure of recently discovered isotypic ternary borides Li$_2$Pd$_3$B and Li$_2$Pt$_3$B, with noncentrosymmetric crystal structures, is studied with a view to understanding their superconducting properties. Estimates of the Fermi-surface averaged electron-phonon matrix element and Hopfield parameter are obtained in the rigid ion approximation of Gaspari and Gyorffy [Phys. Rev. Lett. {\bf 28} (1972) 801]. The contribution of the lithium atoms to the electron-phonon coupling is found to be negligible, while both boron and palladium atoms contribute equally strongly to the Hopfield parameter. There is a significant transfer of charge from lithium, almost the entire valence charge, to the B-Pd(Pt) complex. The electronic structure and superconducting properties of Li$_2$Pd$_3$B, thus, can be understood from the viewpoint of the compound being composed of a connected array of B-Pd tetrahedra decoupled from the backbone of Li atoms, which are connected by relatively short bonds. Our results suggest that conventional s-wave electron-phonon interaction without explicit consideration of SO coupling can explain qualitatively the observed $T_c$ in Li$_2$Pd$_3$B. However, such an approach is likely to fail to describe superconductivity in Li$_2$Pt$_3$B.Comment: 14 pages, 4 figures An erroneous statement following Eq. 6 in version 1 has been deleted. A statement regarding the possible inadequacy of Eq. 6 has been added following Eq. 6. At two places in the discussion Refs. 37,39 has been changed to 37-39, as it should b

Fabrication of stable Pd nanowire assisted by hydrogen in solution

We have mechanically fabricated a Pd nanowire in solution under electrochemical potential control. A clear feature appeared in the conductance histogram when the electrochemical potential of the Pd wire was kept at the hydrogen evolution potential. Conductance traces showed the Pd wire was stretched 0.4 nm in length just before breaking, suggesting that at least two Pd atoms might contribute to the formation of the Pd wire. The results indicate that a certain atomic configuration of the Pd nanowire is stabilized by hydrogen. We discuss the stabilization mechanism due to changes in bond strengths caused by hydrogen adsorption or incorporation.Comment: 4 pages, 3 figures, Appl. Phys. Lett., in pres