Controlling the Bonding of CO on Cobalt Clusters by Coadsorption of H₂

Abstract

Reactions of small molecules with transition metal nanoparticles have attracted considerable interest over the past decades since they can provide a conceptual framework for applications such as heterogeneous catalysis and hydrogen storage [1][2][3][4] . The reaction of H 2 and CO with Fe and Co nanoparticles has been widely studied due to the relevance to the Fischer-Tropsch process in which a mixture of H 2 and CO is converted into long chain hydrocarbons. An important step in this process is the breaking of the strong C-O bond. Upon adsorption of CO on a suitable catalyst (Fe, Ru or Co), the electron density in orbitals that are antibonding with respect to the C-O bond is increased by transfer of electron density from the metal, ultimately leading to the weakening, or activation, of the C-O bond. This activation and therefore the reactivity of CO towards hydrogenation can be influenced by controlling the electron density of the supported metal nanoparticle.Here we report on the co-adsorption of H 2 and CO on small cationic Co clusters (Co n + , n=4-20) in the gas-phase, specifically addressing the effect on the C-O bond. We discuss how H 2 adsorption modifies the C-O bond strength by controlling the electron density available for back donation. We show that each adsorbed H atom formally withdraws 0.09 -0.25 electrons, depending on the size of the cluster. This is of general interest as it gives quantitative insights into metal-ligand interactions.

Platinum Group Metal Clusters: From Gas‐Phase Structures and Reactivities towards Model Catalysts

“…The frequency shifts as a function of cluster charge and size are consistent with the charge delocalization that is expected for a conducting sphere 71. The CO stretch can also act as a probe for co‐adsorbate induced changes in electronic structure of the cluster 72. These studies provide a direct link to surface science, where measurements of the CO stretch frequency have been used as a probe for the properties of surface sites 73.…”

H2 Adsorption on 3d Transition Metal Clusters: A Combined Infrared Spectroscopy and Density Functional Study

“…It has indeed been shown that the cluster charge is delocalized over the surface of the cluster. 57,58 However, a combination of a theoretical and more detailed spectroscopic study is needed to identify the reason for the absence of a D-D stretch absorption band for the nickel hydride complexes with more than 10 nickel atoms. 6.…”

The adsorption of CO on transition metal clusters: A case study of cluster surface chemistry

“…Co-adsorption of hydrogen and carbon 14/27 monoxide onto 3d TM clusters has been investigated in more detail [80,81] inspired by the relevance of these systems in heterogeneous catalysis, e.g., for Fischer-Tropsch synthesis. Coadsorbed H can affect the interaction of CO with the cluster via site blocking leading to a stabilization of molecular CO on an early transition metal like vanadium [81], but it is also found to alter the C-O bond strength.…”