ABSTRACT
Density functional theory calculations were performed to examine the effect of a C vacancy on the physisorption of H(2) onto Ti-functionalized C(60) fullerene when H(2) is oriented along the x-, y-, and z-axes of the fullerene. The effect of the C vacancy on the physisorption modes of H(2) was investigated as a function of H(2) binding energy within the energy window (-0.2 to -0.6 eV) targeted by the Department of Energy (DOE), and as functions of a variety of other physicochemical properties. The results indicate that the preferential orientations of H(2) in the defect-free (i.e., no C vacancy) C(60)TiH(2) complex are along the x- and y-axes of C(60) (with adsorption energies of -0.23 and -0.21 eV, respectively), making these orientations the most suitable ones for hydrogen storage, in contrast to the results obtained for defect-containing fullerenes. The defect-containing (i.e., containing a C vacancy) C(59)TiH(2) complex do not exhibit adsorption energies within the targeted energy range. Charge transfer occurs from Ti 3d to C 2p of the fullerene. The binding of H(2) is dominated by the pairwise support-metal interaction energy E(i)(Cn...Ti), and the role of the fullerene is not restricted to supporting the metal. The C vacancy enhances the adsorption energy of Ti, in contrast to that of H(2). A significant reduction in the energy gap of the pristine C(60) fullerene is observed when TiH(2) is adsorbed by it. While the C( n ) fullerene readily participates in nucleophilic processes, the adjacent TiH(2) fragment is available for electrophilic processes.
ABSTRACT
We have analyzed, by means of density functional theory calculations and the embedded cluster model, the adsorption and spin-state properties of Cr, Ni, Mo, and Pt deposited on a MgO crystal. We considered deposition at the Mg(2+) site of a defect-free surface and at Li(+) and Na(+) sites of impurity-containing surfaces. To avoid artificial polarization effects, clusters of moderate sizes with no border anions were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The interaction between a transition metal atom and a surface results from a competition between Hund's rule for the adsorbed atom and the formation of a chemical bond at the interface. We found that the adsorption energies of the metal atoms are significantly enhanced by the cation impurities, and the adsorption energies of the low-spin states of spin-quenched complexes are always more favorable than those of the high-spin states. Spin polarization effects tend to preserve the spin states of the adsorbed atoms relative to those of the isolated atoms. The metal-support interactions stabilize the low-spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. Spin quenching occurs for Cr and Mo complexes at the Mg(2+) site of the pure surface and at Li(+) and Na(+) sites of the impurity-containing surfaces. Variations of the spin-state properties of free metals and of the adsorption and spin-state properties of metal complexes are correlated with the energies of the frontier orbitals. The electrostatic potential energy curves provide further understanding of the nature of the examined properties.
