Adsorption and oxidation of propane and cyclopropane on IrO2(110)

Abstract

We investigated the adsorption and oxidation of n-propane and cyclopropane (C₃H₈ and c-C₃H₆) on the IrO₂(110) surface using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. We find that the activation of both C₃H₈ and c-C₃H₆ is facile on IrO₂(110) at low temperature, and that the dissociated alkanes oxidize during TPRS to produce CO, CO₂ and H₂O above ∼400 K. Propane conversion to propylene is negligible during TPRS for the conditions studied. Our results show that the maximum yield of alkane that oxidizes during TPRS is higher for c-C₃H₆ compared with C₃H₈ (∼0.30 vs. 0.18 monolayer) and that pre-hydrogenation of the surface suppresses c-C₃H₆ oxidation to a lesser extent than C₃H₈. Consistent with the experimental results, DFT predicts that C₃H₈ and c-C₃H₆ form σ-complexes on IrO₂(110) and that C–H bond activation of the complexes as well as subsequent dehydrogenation are highly facile via H-transfer to O_br atoms (bridging O-atoms). Our calculations predict that propane conversion to gaseous propylene is kinetically disfavored on IrO₂(110) because HO_br recombination makes O_br atoms available to promote further dehydrogenation at lower temperatures than those needed for the adsorbed C₃H₆ intermediate to desorb as propylene. We also present evidence that that the ability for c-C₃H₆ to activate via ring-opening is responsible for cyclopropane attaining higher reaction yields during TPRS and exhibiting a weaker sensitivity to surface pre-hydrogenation compared with n-propane.