BIMETALLIC CATALYSTS FROM ALUMINA-SUPPORTED MOLYBDENUM IRIDIUM CLUSTERS

The heterometallic clusters CpMoIr3(CO)11 and Cp2Mo2Ir2(CO)10 (Cp = η5-C5H5) were deposited onto fumed alumina (Degussa Al2O3-C, 0.2 wt % Ir) as catalyst precursors. Comparable materials were prepared from Ir4(CO)12 and Cp2Mo2(CO)6 as well as from a stoichiometric mixture (Ir4 + 2Mo2) of these homometallic compounds. The supported compounds were activated by heating to 500°C in flowing H2. Methane evolution profiles observed during activation were characteristic for each heterometallic cluster precursor and were distinct from profiles associated with the homometallic compounds. H2 and CO chemisorption on the activated Ir-containing materials indicated high dispersion (H/Ir ≥ 1, CO/Ir ∼ 1). The IR-containing materials, [Mo2Ir2], [MoIr3], [Ir4], and [Ir4 + 2Mo2], were active catalysts for the hydrogenolysis of n-butane at 215°C, whereas [Mo2] was inactive. The [MoIr3] catalyst exhibited enhanced activity (5-10 times) over the [Ir4] and [Ir4 + 2Mo2] catalysts, but the selectivity toward ethane production, 70-75%, was the same for all these samples. In contrast, the [Mo2Ir2] catalyst exhibited a change in ethane selectivity to ca. 50% but had catalytic activity comparable to the [Ir4 + 2Mo2] material. The differences in catalytic properties displayed by the heterometallic-cluster-derived catalysts are attributed to bimetallic interactions maintained in the activated materials. Mo K edge X-ray absorption spectra were collected in order to characterize the metal-metal interactions. The near-edge spectra (XANES) showed unique features for [MoIr3] and [Mo2Ir2] in comparison with [Ir4 + 2Mo2]; the spectrum of the latter was identical with that of [Mo2] alone. A XANES feature at 20010 eV, indicative of Mo=O moieties, was apparent in oxygen-contaminated samples, but it could be removed by in situ activation. Fourier transforms of the EXAFS data collected on in situ activated samples of [MoIr3], [Mo2Ir2], and [Ir4 + 2Mo2] showed that each sample had structurally unique molybdenum sites. In particular, both heterometallic-cluster-derived samples, [MoIr3] and [Mo2Ir2], displayed evidence for Mo-M interactions at R′ = 2.5-2.6 Å, but the sample derived from the stoichiometric mixture, [Ir4 + 2Mo2], did not. © 1990 American Chemical Society.