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 M₁₃H₂₀ and M₁₃H₃₀ systems compared to M₁₃H₁₄ and M₁₃H₂₄ 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 M₁₃H₂ system have been determined and found to conform with the empirical Br{}nsted-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.