Support functionalization as an approach for modifying activation entropies of catalytic reactions on atomically dispersed metal sites

Oxide supported atomically dispersed late transition metal catalysts exhibit unique catalytic reactivity and selectivity compared to metal clusters. For this class of catalysts, efforts have been devoted to understand how variations in the active site or support composition influence catalytic properties. Here, we examine the influence of phosphonic acid functionalization of γ-Al₂O₃ surrounding atomically dispersed Rh active sites on their chemical and catalytic reactivity. It was observed that support functionalization modifies the activation entropies of elementary steps (CO desorption) and catalytic cycles (ethylene hydroformylation) on atomically dispersed Rh sites, with minimal influence on reaction enthalpies. Changes in activation entropy were attributed to support functionalization causing a decrease in the translational degrees of freedom associated with mobile atomically dispersed Rh(CO)₂ species on the γ-Al₂O₃ support. Changes to the activation entropies on functionalized catalysts caused increases in turn over frequencies and selectivity for ethylene hydroformylation in gas phase steady state conditions. The observed breaking of enthalpy–entropy compensation suggests that support functionalization may enable tuning of catalytic reactivity beyond the confines of linear free relationships for catalysts consisting of mobile active sites.