Propene hydrogenation catalyzed by γ-Al2O3-supported Ir4 clusters:: Inhibition by dehydrogenated propene derivatives on Ir4

Ir-4 clusters supported on MgO and on gamma-Al2O3 were prepared from supported [lr(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/ gamma-Al2O3 during catalysis indicate propylidyne and highly dehydrogenated propene derivatives (approximated as C3H2 and C2H) 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 gamma-Al2O3. The results suggest that the more basic MgO support is a better electron donor to the clusters than gamma-Al2O3, causing the strength of the interaction between the clusters and propene to be less for Ir-4/MgO than for Ir-4/gamma-Al2O3.