Role of Cluster Size in Catalysis:  Spectroscopic Investigation of γ-Al2O3-Supported Ir4 and Ir6 during Ethene Hydrogenation

• Published in: Journal of the American Chemical Society 2003-06, Vol.125 (23), p.7107-7115
• Main Authors: Argo, Andrew M, Odzak, Josip F, Gates, Bruce C
• Format: Article
• Language: English
• Subjects: Catalysis; Catalytic reactions; Chemistry; Exact sciences and technology; General and physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja027741f

γ-Al2O3-supported Ir4 and Ir6 were prepared by decarbonylation of tetra- and hexanuclear iridium carbonyls, respectively, and compared as catalysts for ethene hydrogenation at atmospheric pressure and temperatures in the range 273−300 K. Rates of the reaction were determined along with extended X-ray absorption fine structure (EXAFS) and IR spectra characterizing the clusters in the working catalysts. EXAFS data show that the Ir4 and Ir6 cluster frames remained intact during catalysis. Di-σ-bonded ethene and π-bonded ethene on the clusters were identified by IR spectroscopy and found to compete as the principal reaction intermediates, with the former predominating at ethene partial pressures less than about 200 Torr and the latter at higher ethene partial pressures. Hydrogen on the clusters is inferred to form by dissociative adsorption of H2; alternatively, it is provided by OH groups of the support. The rate of ethene hydrogenation on Ir4 is typically several times greater than that on Ir6.