Ab initio calculation of chemisorption systems: H on Pd(001) and Pd(110)

Atomic-scale selectivity of hydrogen for storage sites in Pd nanoparticles at atmospheric pressure

Hydrogen-storage materials are important carriers for a viable hydrogen economy. Despite palladium being the most studied storage material, the hydrogen-storage mechanism of Pd remains ambiguous owing to the lack of atomic-scale evidence of the diffusion and storage of H atoms in its lattice. In the study reported here, this classical process was investigated on the atomic scale using an in situ transmission electron microscope equipped with an atmospheric-pressure sample holder. The expansion of the Pd interplanar spacings was found to comprise three distinct stages during the diffusion of H atoms. Moreover, the expansion in d-spacing of Pd{111} was markedly different from that of Pd{220}. First-principles calculations indicate that H atoms mainly occupy the centers of the tetrahedral cages in the Pd unit cells during the diffusion stage, and they eventually occupy the octahedral cage centers in the equilibrium state. Moreover, H atoms were detected in substantially high densities in defects such as stacking faults and twin boundaries. These observations on the preferred hydrogen-storage domains can help clarify the hydrogen-storage mechanism and offer guidelines on the future design of higher-capacity hydrogen-storage materials.

Dipalladium(I) terphenyl diphosphine complexes as models for two-site adsorption and activation of organic molecules

A para-terphenyl diphosphine was employed to support a dipalladium(I) moiety. Unlike previously reported dipalladium(I) species, the present system provides a single molecular hemisphere for binding of ligands across two metal centers, enabling the characterization and comparison of the binding of a wide variety of saturated and unsaturated organic molecules. The dipalladium(I) terphenyl diphosphine toluene-capped complex was synthesized from a dipalladium(I) hexaacetonitrile precursor in the presence of toluene. The palladium centers display interactions with the π-systems of the central ring of the terphenyl unit and that of the toluene. Exchange of toluene for anisole, 1,3-butadiene, 1,3-cyclohexadiene, thiophenes, pyrroles, or furans resulted in well-defined π-bound complexes which were studied by crystallography, nuclear magnetic resonance (NMR) spectroscopy, and density functional theory. Structural characterization shows that the interactions of the dipalladium unit with the central arene of the diphosphine does not vary significantly in this series allowing for a systematic comparison of the binding of the incoming ligands to the dipalladium moiety. Several of the complexes exhibit rare μ-η(2):η(2) or μ-η(2):η(1)(O or S) bridging motifs. Hydrogenation of the thiophene and benzothiophene adducts was demonstrated to proceed at room temperature. The relative binding strength of the neutral ligands was determined by competition experiments monitored by NMR spectroscopy. The relative equilibrium constants for ligand substitution span over 13 orders of magnitude. This represents the most comprehensive analysis to date of the relative binding of heterocycles and unsaturated ligands to bimetallic sites. Binding interactions were computationally studied with electrostatic potentials and molecular orbital analysis. Anionic ligands were also demonstrated to form π-bound complexes.

Equation of State for PdH by a New Tight Binding Approach

We present an extension of a newly proposed tight-binding method of calculating total energies and related quantities to the study of binary compounds. As an example, we discuss the equation of state of the palladium-hydrogen system.