Adsorption of CO and H2 on Transition Metal Clusters : insights from Vibrational Spectroscopy and Density Functional Theory

Abstract

Adsorption of hydrogen (H2) and carbon monoxide (CO) molecules on transition metals is of paramount importance for several (catalytic) processes. These include the purification of H2 streams and the Fischer-Tropsch reaction, in which a mixture of H2 and CO is converted to synthetic fuels. As a consequence, many studies into the binding of these two molecules on transition metals have been performed. Nevertheless, fundamental questions regarding the effects of metal particle size and electron density on adsorbate binding geometry, as well as on the effects of co-adsorption, remain. The research described in this Ph.D. thesis explores these questions, both experimentally and theoretically, by focusing on well-defined transition metal clusters in the gas-phase as model systems. The first part of the thesis deals with the adsorption H2 on a variety of transition metal clusters. It is shown that the hydrogen binding sites are highly dependent on the metal, as well as on cluster size and that they are different for small clusters compared to extended surfaces. In addition, it is found that the first H2 molecule to bind on a relatively unreactive Ni4+ cluster, can bind molecularly, while it binds exclusively dissociatively on the reactive Ni5+ and Ni6+ clusters. In the second part of the thesis, investigations into the effects associated with co-adsorption of H2 and CO are presented. These effects were studied by focusing on an early (vanadium) and a late (cobalt) transition metal as case studies. In case of vanadium it is found that co-adsorption of H2 leads to a stabilization of CO against dissociation. This is shown to be predominantly a structural effect. In contrast, co-adsorption of H2 is demonstrated to have a significant electronic effect on cobalt clusters. To explain the experimental observations, a model describing the cluster size and charge dependence of the binding of CO on transition metal clusters was extended to incorporate the co-adsorption of H2. Each adsorbed hydrogen atoms lowers the electron density of the metal particle by 0.09 - 0.25 of an electron, depending on cluster size. In the last part of the thesis, the dependence of the adsorbate binding geometry on the charge state of the metal cluster is explored. For these studies, rhodium, cobalt, and nickel carbonyls were used as model systems. In case of the rhodium and cobalt carbonyls, the removal of an electron from a neutral complex leads to a destabilization of bridge bound CO ligands. This destabilization is shown to be due to the removal of an electron from an orbital that is bonding with respect to the bridge bound carbonyl groups.