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

Abstract

We report first-principles electronic structure calculations of hydrogen adsorption and saturation on ${M}_{13}$ $(M=\text{Sc},\text{Ti},\text{Zr})$ clusters of icosahedral $({I}_{h})$ and cuboctahedral $({O}_{h})$ symmetries. Hydrogen saturation of the ${I}_{h}$ metal clusters yields energetically stable ${M}_{13}{\text{H}}_{20}$ and ${M}_{13}{\text{H}}_{30}$ systems compared to ${M}_{13}{\text{H}}_{14}$ and ${M}_{13}{\text{H}}_{24}$ systems for ${O}_{h}$ clusters. In all these clusters, the hydrogen adsorption involves dissociative chemisorption of ${\text{H}}_{2}$ 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 ${M}_{13}{\text{H}}_{2}$ system have been determined and found to conform with the empirical Br\o{}nsted-Evans-Polanyi relationship. A highly intense infrared band in the $1000--1500\text{ }{\text{cm}}^{\ensuremath{-}1}$ region, associated with the adsorbed hydrogens in these hydrogenated metal clusters, may be used to experimentally monitor hydrogen coverage.