(691a) Modifying the Entropy of Adsorbates Bound to Atomically Dispersed Metals By Support Functionalization

Organic functionalization of heterogeneous catalysts provides a method for modifying reactivity beyond changing the catalyst structure, composition, or support. Previous work has focused on adsorbing organic groups on active metal surfaces. This approach has drawbacks, such as low stability, blocking of active sites, changes in metal electronic structure, and introduction of active site heterogeneity. An alternative to directly functionalizing the active metal is to functionalize the oxide support surrounding an active metal site. In this work we demonstrate that atomically dispersed Rh on a phosphonic acid functionalized γ-Al₂O₃ support exhibits increased activity and selectivity for the ethylene hydroformylation reaction. Octylphosphonic acid and 1H,1H,2H,2H-perfluorooctanephosphonic acid were chosen as functional groups as they allowed for investigation of the contribution of electronic interactions to observed reactivity. Samples with different coverages (0, 0.8, 2.1, and 3.2 P/nm²) of each phosphonic acid were characterized in order to explore the effects of steric hindrance. The influence of support functionalization on characteristics of the Rh active site was characterized through probe molecule Fourier transform infrared spectroscopy, temperature programmed desorption, kinetic studies, and X-Ray photoelectron spectroscopy measurements. The observed changes in catalytic reactivity of Rh due to surrounding support functionalization are attributed to an increase in confinement of adsorbed species on functionalized surfaces. Increased confinement of adsorbates at the atomically dispersed metal site resulted in a decreased entropy of bound species and greater entropic driving forces, while enthalpic driving forces were not significantly changed. This work provides a perspective on controlling the catalytic properties of atomically dispersed metals by modifying the catalyst away from the binding site.