(544de) Theoretical Investigation of the Decomposition of Cyclohexane on Ir and Pt Surfaces

Cyclohexene decomposition and the reverse reaction of benzene hydrogenation have been studied extensively on Pt surfaces as model reactions of de/hydrogenation catalysis by Pt.^1,2 The corresponding processes catalyzed by Ir have received less attention due to the greater hydrogenolysis activity of Ir. However, Ir has been shown to possess high regioselectivity in cleaving secondary-secondary C-C bonds, in the context of selective ring opening (SRO) of cycloparaffins, a key step in the conversion of naphthenic compounds to those more suited as diesel fuels.³ We have theoretically investigated and compared the decomposition of cyclohexane on the close-packed Ir(111) and Pt(111) surfaces as a first step toward understanding the ability of Ir for promoting SRO. Detailed density functional theory calculations have allowed us to identify the more facile decomposition pathways on each surface. In the low coverage limit, repeated dehydrogenation of cyclohexane at C centers closer to the surface is kinetically preferred on Ir(111) and results in the formation of carbene-type intermediates and rapid C-C cleavage, so that cyclohexene and benzene are not major products. Similar decomposition pathways also exist on Pt(111) although they are less competitive than the cyclohexene --> benzene⁴ pathway. Kinetic and vibrational analyses provide further support for these previously unrecognized decomposition pathways for cyclohexane on the two metals. Our findings are consistent with the literature suggestion that Ir readily catalyzes C-C cleavage via the dicarbene mechanism,^3,5-7 and sheds light on the intrinsic difference in the reactivity of Ir vs. Pt toward hydrocarbon decomposition.

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