Effect of steps on the decomposition of CH₃O at PdZn alloy surfaces

The decomposition of methoxide (CH₃O) on a PdZn alloy is considered to be the rate-limiting step of steam re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir 2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-free flat (111) and (100) PdZn surfaces to promote C-H or C-O bond breaking of CH₃O. Thus, we applied the same 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 facilitated on (221)^Pd: the calculated activation energy is dramatically reduced, to ∼50 kJ mol^-1 from ∼90 kJ mol ^-1 on flat PdZn surfaces, increasing the rate constant by a factor of 10⁸. The lower barrier is mainly due to a weaker interaction of the reactant CH₃O and an enhanced interaction of the product CH ₂O with the substrate. The activation energy for C-O bond scission did not decrease on the (221)^Pd step. On the (221)^Zn step, the calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the stronger adsorption of CH₃O. Steps (and other defects) appear to be crucial for methanol steam re-forming on Pd/ZnO catalyst; the stepped surface PdZn(221)^Pd is a realistic model for studying the reactivity of this catalyst.