2152
Langmuir 2002, 18, 2152-2157
Propene Hydrogenation Catalyzed by γ-Al2O3-Supported Ir4 Clusters: Inhibition by Dehydrogenated Propene Derivatives on Ir4 Andrew M. Argo and Bruce C. Gates* Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 Received July 30, 2001. In Final Form: November 27, 2001 Ir4 clusters supported on MgO and on γ-Al2O3 were prepared from supported [Ir4(CO)12] and used as catalysts for hydrogenation of propene. Extended X-ray absorption fine structure spectra of the working catalysts identify the Ir4 clusters themselves as the catalytically active species. Infrared spectra of Ir4/ γ-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 Ir4 clusters are supported on MgO instead of γ-Al2O3. The results suggest that the more basic MgO support is a better electron donor to the clusters than γ-Al2O3, causing the strength of the interaction between the clusters and propene to be less for Ir4/MgO than for Ir4/γ-Al2O3.
Introduction
Experimental Section
The surfaces of many metal catalysts become covered with carbonaceous species as a result of exposure to alkenes or other unsaturated hydrocarbons, and the carbonaceous species may greatly affect the performance of metals as catalysts.1 Often the carbonaceous species are ill-defined, but some are simple and well-characterized ligands, exemplified by ethylidyne on metals such as Pt2-4 and Rh,5 formed during ethene hydrogenation catalysis. Ethylidyne is usually regarded as a spectator species in catalysis,5-8 and Cremer et al.,4,9 using sum frequency generation, found 0.25 ML (monolayer) of ethylidyne on Pt(111) during ethene hydrogenation catalysis, whereas the coverage by π-bonded ethene, the principal reaction intermediate, was only 0.04 ML. Our goal was to characterize carbonaceous species on supported metal catalysts that are structurally well defined and strongly influenced by supports. The catalysts were chosen to be extremely small metal clusters, Ir4, on γ-Al2O3 or MgO. We report extended X-ray absorption fine structure (EXAFS) spectra characterizing the clusters and infrared (IR) spectra characterizing the carbonaceous species on them during propene hydrogenation catalysis at room temperature. The data identify propylidyne as a strong reaction inhibitor and show that its formation on the clusters is influenced strongly by the support.
Materials, Sample Preparation, and Handling. The syntheses of Ir4 supported on γ-Al2O3 (Ir4/γ-Al2O3)10 and of Ir4/ MgO,11 described elsewhere, were carried out on a vacuum line or in a drybox (Vacuum Atmospheres HE-63-P) purged with N2 that was recirculated through O2- and moisture-scavenging traps (supported Cu particles and zeolite 4A, respectively). The drybox was equipped with O2 and moisture detectors, indicating concentrations
