A New Approach to Studying Cobalt's Surface Chemistry Using Cobalt Nanoparticles Grown on Copper

C-C bond forming reactions are ubiquitous processes that are vital for systems ranging from petroleum refining to the Calvin Cycle. Cobalt is an attractive catalyst for C-C formation, as it displays a rich set of chemical properties and is an earth-abundant material with low toxicity. Co-based heterogeneous catalysts are found to be active for a variety of reactions, most notably the Fischer-Tropsch synthesis (FTS), the process that converts syngas into liquid fuels. Homogeneous Co catalysts are less popular than their heterogeneous counterparts, but they have recently gained attention for their unique selectivity in forming C-C bonds in the synthesis of organic molecules that are central to the pharmaceutical and other industries. By understanding the fundamental molecular processes that govern these reactions on Co, these catalysts can be developed into viable, efficient candidates for industrial use.;This thesis describes a surface science approach to exploring two C-C bond forming reactions, FTS and Ullmann coupling, on a model catalyst surface in which Co is deposited on Cu(111). The Co/Cu(111) surface is well suited for these studies, as Co exhibits a unique growth mode that results in bilayer Co nanoparticles on the Cu surface. These nanoparticles allow for the examination of both Co single crystal basal planes, as well as the geometrically restricted features of a nanoparticle. The results described herein provide important mechanistic details about the two studied reactions that are crucial in the rational design of future catalysts.;In the study of FTS, details regarding the interaction of CO and H with the Co surfaces are uncovered, as well as information regarding the interaction of the two species when co-adsorbed. Specifically, we observe that the Co-Cu interface provides unique pathways for H to adsorb onto the Co nanoparticles, allowing for dense H overlayers to be observed. We also see in these experiments that H can be forced to spill over onto the Cu support either by large H exposures or via phase-segregated CO adlayers that destabilize the H by exerting a two-dimensional pressure. These experiments provide new insights into how these reactants interact on Co at catalytically relevant coverages and may inspire new ideas for catalyst design given the limited CO-H interface.;Experiments on a Co-free Cu surface elucidate that the reaction intermediate in the Ullmann coupling of bromobenzene is an organometallic complex in which two phenyl groups are bound to a single Cu atom that is extracted from the surface. We also show that the selectivity for the formation of this intermediate is dictated by Cu steps, which are the active sites for C-Br bond cleavage. Ullmann experiments on the Co/Cu(111) surface show that Co has unique C-C bond forming properties that differ from the Cu support, including the ability to couple sp3 benzylic carbons, which suggests that new catalysts based on Co may be interesting to explore for these important reactions.