Stable Rhodium Pair Sites on MgO: Influence of Ligands and Rhodium Nuclearity on Catalysis of Ethylene Hydrogenation and H–D Exchange in the Reaction of H2 with D2

Understanding of supported metal pair-site catalysts is largely lacking, because most are structurally nonuniform or incorporate stabilizing ligands that inhibit catalysis. We synthesized rhodium pair-sites on MgO from Rh2(μ-OMe)2(COD)2 (OMe = methoxy; COD = cyclooctadiene), characterizing the surface species with infrared and X-ray absorption spectroscopies. Catalytic properties for ethylene hydrogenation and H–D exchange in the H2 + D2 reaction show that surface pair sites were retained when organic ligands were removed, giving catalysts that facilitate H2 dissociation much more rapidly than single-site rhodium and catalyze ethylene hydrogenation 2 orders of magnitude faster than single-site rhodium on MgO at 298 K.