Rhodium Cluster Catalysts Enable Efficient Catalytic Hydroformylation in Mild Conditions: Cluster Size Dependence.

The size of the reactive metal domain, termed an 'ensemble', in supported metal catalysts critically influences their catalytic activity. To maximize mass-activity (MA), a deep understanding of the optimal catalytic environment is vital for the rational catalyst design concept, yet this remains challenging due to difficulties in synthesis and analysis especially in the sub-nanometer regime. Herein, we have successfully synthesized a series of supported Rh species ranging from single-atoms to (sub-)nanometer clusters and nanoparticles. Their catalytic performance in the hydroformylation of styrene was evaluated. We discovered that the optimal structures are the Rh clusters, achieving a MA as high as 305.3 mol gRh-total -1 h-1, a value that surpasses previously reported values for both heterogeneous catalysts and the homogeneous Wilkinson's catalyst. Kinetic and theoretical analyses reveal that nanometer-sized clusters provide a specific reaction environment that facilitates the balanced co-adsorption of styrene, CO, and H thereby promoting hydroformylation. Our study highlights that a profound understanding of the optimal active structure is the key to maximizing the MA and advancing beyond simplistic atom efficiency considerations.