Active sites and reactive intermediates in the hydrogenolytic cleavage of C-C bonds in cyclohexane over supported iridium

Hydrogenolysis of cyclohexane has been explored over supported Ir catalysts. The kinetic data are combined with modeling results to assess the structural requirements and the nature of catalytically relevant surface intermediates for endocyclic C-C bond cleavage. The turnover frequency (TOF) for cyclohexane hydrogenolysis showed complex dependence on Ir particle size, while the selectivity to the primary ring opening product, n-hexane, decreased monotonically with decreasing Ir dispersion. The decreasing TOF as the Ir dispersion decreased from 65% to 52% originates principally from the diminishing abundance of low-coordination Ir atoms at particle surfaces. The increase of the TOF with further Ir particle growth is attributed to an increased fraction of terrace planes, or step sites, and a less unsaturated nature of the most abundant reactive intermediate. Selectivities for multiple C-C bond cleavage, yielding C _<6 alkanes, varies with the relative abundance of coordinatively unsaturated Ir atoms and terrace planes. The multiple hydrogenolysis depends additionally upon H ₂ pressure, because single and multiple C-C bond scissions are mediated by surface intermediates with different H-deficiencies.