Reversible Metal Aggregation and Redispersion Driven by the Catalytic Water Gas Shift Half-Reactions: Interconversion of Single-Site Rhodium Complexes and Tetrarhodium Clusters in Zeolite HY

Rhodium gem-dicarbonyl complexes, Rh(CO)2, bonded within the pore structure of zeolite HY and formed by the reaction of Rh(CO)2(acac) (acac = acetylacetonato) with OH groups on the zeolite surface were converted in >95% yield to Rh4(CO)12 by reaction with CO + water at 308 K, and the process was reversed by treatment of the supported clusters in helium at 353 K. The chemistry of these reactions was characterized by IR and X-ray absorption spectra recorded during the changes and by density functional theory. The cluster formation is driven by the water gas shift half-reaction, leading to generation of CO2 and zeolite surface protons, and the reverse reaction proceeds via the half-reaction that completes the cycle of the water gas shift reaction. Thus, the overall process is cyclic–catalytic. The yield in the synthesis of Rh4(CO)12 is the highest reported, and the high selectivity is facilitated by the confining environment for the clusters in the zeolite supercages and the low density of OH groups on the zeolite surface (the zeolite Si:Al atomic ratio was 30). The results provide insights into the first steps of sintering of atomically dispersed metals on supports