Geometric and electronic structures of hydrogenated transition metal (Sc, Ti, Zr) clusters

We report first-principles electronic structure calculations of hydrogen adsorption and saturation on M13 (M=Sc,Ti,Zr) clusters of icosahedral (Ih) and cuboctahedral (Oh) symmetries. Hydrogen saturation of the Ih metal clusters yields energetically stable M13 H20 and M13 H30 systems compared to M13 H14 and M13 H24 systems for Oh clusters. In all these clusters, the hydrogen adsorption involves dissociative chemisorption of H2 molecules. Upon initial hydrogenation, the dissociated hydrogen atoms lie above either the triangular or quadrangular face of the metal cluster. A further increase in hydrogen saturation leads to the formation of bridged hydrogen bond between adjacent metal atoms. The role of the unfilled d orbitals in imparting stability to the hydrogenated clusters is explored by examining their density of states (DOS) near the Fermi level. It has been found that the inverse correlation between the d -band center and the chemisorption energies is valid only at low hydrogen concentrations. At higher hydrogen coverages, the trend is reversed. The results illustrate that at high adsorbate concentrations or surface coverage, the correlation of the chemisorption energies with the d -band center of the pure metal cluster or nanoparticle may not be realistic but one has to take into account the changes in the d -orbital DOS due to the presence of coadsorbed molecules. To obtain further insights into the initial steps involved in hydrogen adsorption and dissociation, the transition states and activation energies of the M13 H2 system have been determined and found to conform with the empirical Brønsted-Evans-Polanyi relationship. A highly intense infrared band in the 1000-1500 cm-1 region, associated with the adsorbed hydrogens in these hydrogenated metal clusters, may be used to experimentally monitor hydrogen coverage.