Pseudopotential multireference single and double excited configuration interaction calculations of nickel-containing molecules. 4. HNiSiH3 and HNiAlH3 as minimum models of hydrogen atoms chemisorption on a supported nickel atom

Abstract

The chemisorption of atomic H on a Ni atom bound to SiH{sub 3} or AlH{sub 3} supporting fragments has been investigated by means of pseudopotential CI calculations. The two systems investigated, H-Ni-SiH{sub 3} and H-Ni-AlH{sub 3}, can be considered as minimum models of strong (SMSI) and weak (WMSI) metal-support interactions, respectively. It is found that the presence on the SiH{sub 3} support of a singly occupied dangling bond, not present on AlH{sub 3}, is the key factor determining the strong metal-support interaction. By this interaction, the Ni atom binds the adsorbed H atom less strongly than in the free NiH molecule. The chemisorption of H decreases also the Ni-substrate bond strength. This result, however, is found only after inclusion of correlation effects. Uncorrelated calculations show the opposite trend, that is, a reinforcement of both Ni-H and Ni-support bonds by H chemisorption. This contradictory result is probably the consequence of the different SCF and CI energy separation between the Ni 3d{sup 8}4s{sup 2} and 3d{sup 9}4s{sup 1} configurations. The formation of two covalent bonds, as in H-Ni-SiH{sub 3}, implies that the metal atom assumes a 3d{sup n}4s{sup 2}-like configuration more suitable to form directional 4sp hybrid orbitals. The case of Nimore » is compared with the limiting situations of Fe (3d{sup 6}4s{sup 2}) and Cu (3d{sup 10}4s{sup 1}) to generalize the conclusions about the electronic mechanisms with the limiting situations of Fe (3d{sup 6}4s{sup 2}) and Cu (3d{sup 10}4s{sup 1}) to generalize the conclusions about the electronic mechanisms of SMSI. For the model of WMSI, the authors found that in their system the interaction of a supported Ni with the approaching H atom induces the rupture of the Ni-substrate bond and the formation of a Ni-H molecule which easily desorbs from the surface.« less