Dissociative Hydrogen Adsorption on Close-Packed Cobalt Nanoparticle Surfaces

Abstract

The dissociative adsorption of hydrogen on cobalt is central to a number of catalytic reactions, yet to date there are relatively few studies examining this important process. Here we utilize Co nanoparticles grown on Cu(111), instead of the traditional planar Co single crystals, to study a more catalytically relevant form of Co. We present scanning tunneling microscopy images of different phases of H on the close-packed Co nanoparticle surfaces with a range of densities. Our data reveal a so-far unreported high coverage phase of H with a (1 × 1) structure and elucidate the importance of spillover from step edges in H adsorption. We also illustrate that, in contrast to the low density phases, the H-(1 × 1) structure can only be formed at an intermediate temperature, indicating that compression to this higher-density phase is activated. Density functional theory calculations yield energies for each of the H overlayer structures, as well as their preferred geometries. This work is the first to report on higher coverage (>0.75 ML) phases of H on Co, which are undoubtedly important in catalytic systems at elevated pressure. Finally, through the use of epitaxial Co nanoparticle growth on Cu(111), we illustrate the importance of step edges in H2 activation and the formation of dense H phases.