Propene Hydrogenation Catalyzed by γ-Al2O3-Supported Ir4 Clusters: Inhibition by Dehydrogenated Propene Derivatives on Ir4

Ir 4 clusters supported on MgO and on γ-Al 2 O 3 were prepared from supported [Ir 4 (CO) 12 ] and used as catalysts for hydrogenation of propene. Extended X-ray absorption fine structure spectra of the working catalysts identify the Ir 4 clusters themselves as the catalytically active species. Infrared spectra of Ir 4 /γ-Al 2 O 3 during catalysis indicate propylidyne and highly dehydrogenated propene derivatives (approximated as C 3 H 2 and C 2 H) bonded to the clusters; they are inhibitors of catalysis and not reactive intermediates. Infrared spectra also show that the formation of propylidyne and highly dehydrogenated propene derivatives is greatly reduced when the Ir 4 clusters are supported on MgO instead of γ-Al 2 O 3 . The results suggest that the more basic MgO support is a better electron donor to the clusters than γ-Al 2 O 3, causing the strength of the interaction between the clusters and propene to be less for Ir 4 /MgO than for Ir 4 /γ-Al 2 O 3 .