Supported Rhodium Catalysts With Controlled Nuclearity

Varying the nuclearity (number of metal atoms) of catalytic sites in supported rhodium catalysts can leads to significant changes in catalyst performance, especially when the nuclearities are low and the catalytic species consist of only one or a few metal atoms. Catalysts initially incorporating Rh(C₂H₄)₂ complexes were synthesized from Rh(C₂H₄)₂(acetylacetonate) on various metal oxide and zeolite supports. These complexes were treated under a range of conditions (e.g., reduction in the presence of H₂) to form clusters, some as small as dimers, as indicated by EXAFS and infrared spectra. The mononuclear species have low activities for reactions such as hydrogenation of ethylene and oxidation of CO, but the activities increase as the rhodium nuclearity increases. The results show that formation of Rh–Rh bonds in the catalytic sites triggers the onset of new catalytic properties. These results have led to the design of supported rhodium catalysts with optimized nuclearities for hydrogenation of 1,4-butadiene and oxidation of CO.