Abstract
Abstract Heterogeneous catalysts generally have a variety of active-site structures due to the innate heterogeneity of the surface, resulting in complicated correlations between activity and active-site structure. Single site heterogeneous cobalt catalysts with a uniform catalytic surface were utilized as a platform to probe surface sensitive reactions; in this case CO2 hydrogenation. It was found that atomically isolated cobalt single sites, which exist solely in the tetrahedral Co2+ coordination, exclusively form CO under typical CO2 methanation conditions, while cobalt clusters yielded the highest rate of CO2 reaction and began to form methane. Utilizing the principles of Ostwald Ripening to probe the ensemble effects for CO2 hydrogenation, the transition from atomic isolation to small clusters of atoms to nanoparticles was explored. The chemical structure of the cobalt was elucidated primarily via X-Ray Absorption Spectroscopy (XANES/EXAFS) and X-Ray Photoelectron Spectroscopy (XPS).