NaY Zeolite-Supported Rhodium and Iridium Cluster Catalysts: Characterization by X-ray Absorption Spectroscopy during Propene Hydrogenation Catalysis

Abstract

Clusters of rhodium and of iridium were synthesized in the cages of the sodium form of the faujasite zeolite Y by decarbonylation of zeolite-supported [Rh6(CO)16] or [Ir4(CO)12] in H2at 200 or 300°C, respectively. Larger clusters (aggregates) of rhodium or iridium consisting of 10 to 20 atoms each on average were formed by treatment of zeolite-supported [Rh6(CO)16] or [Ir6(CO)16] in H2at 250 or 300°C. The samples were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy under vacuum and in (a) H2, (b) propene, and (c) H2/propene mixtures (with catalytic hydrogenation taking place). The EXAFS data indicate that the clusters or aggregates were intact, with the metal frames changed undetectably in the presence of any of the gases, even during catalysis and at temperatures up to 140°C. In the presence of propene only, rhodium clusters showed changes in their X-ray absorption near-edge spectra, but the iridium clusters did not. When propene was present with H2during catalysis, the near-edge spectra were essentially the same as those characterizing the metal clusters (or aggregates) in H2only. The data are consistent with the inference that the metal clusters, approximated as predominantly Ir4or Rh6(the latter not fully decarbonylated) are catalytically active themselves, as are larger aggregates of those metals.