| Abstract
| - The decomposition of methoxide (CH3O) on a PdZn alloy is considered to be the rate-limiting step of steamre-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-freeflat (111) and (100) PdZn surfaces to promote C−H or C−O bond breaking of CH3O. Thus, we applied thesame DF periodic slab-model approach to investigate these two routes of CH3O decomposition on PdZn(221)surfaces that expose Pd, (221)Pd, and Zn, (221)Zn, steps. C−H bond cleavage of CH3O is greatly facilitatedon (221)Pd: the calculated activation energy is dramatically reduced, to ∼50 kJ mol-1 from ∼90 kJ mol-1 onflat PdZn surfaces, increasing the rate constant by a factor of 108. The lower barrier is mainly due to aweaker interaction of the reactant CH3O and an enhanced interaction of the product CH2O with the substrate.The activation energy for C−O bond scission did not decrease on the (221)Pd step. On the (221)Zn step, thecalculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of thestronger adsorption of CH3O. Steps (and other defects) appear to be crucial for methanol steam re-formingon Pd/ZnO catalyst; the stepped surface PdZn(221)Pd is a realistic model for studying the reactivity of thiscatalyst.
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