Structural and mechanistic requirements for methane activation and chemical conversion on supported iridium clusters.

Chemical conversion of CH4 into larger molecules is practiced by initially forming H2–CO mixtures over metal catalysts. These reactions use CO2 or H2O as co-reactants in processes which also produce H2, that is then used in refining and petrochemical processes and in fuel cells. The strong bonds in CH4 (439 kJmol 1 ) and the endothermic nature of reforming reactions call for high temperatures and stable catalysts. Supported Ir clusters catalyze CO2–CH4 reactions without detectable carbon formation, with turnover rates higher than those found with other Group VIII metals, except Pt. Experimental and theoretical studies on model surfaces 9] have suggested that C H bond activation is sensitive to surface structure and requires coordinatively unsaturated sites. 11] The effects of cluster size and of concomitant changes in surface coordination on re-forming turnover rates remain unexplored for Ir catalysts. Contradictory conclusions about the kinetic relevance of C H activation steps within catalytic sequences remain, and catalytic supports have often been claimed to be required for the activation of CO2 or H2O co-reactants. 7]