High Catalytic Activity of Pd-1/ZnO(10(1)over-bar0) toward Methanol Partial Oxidation: A DFT plus KMC Study

We perform density functional theory (DFT) calculations of the energetics for several pathways associated with methanol partial oxidation (MPO) reaction on singly distributed Pd on ZnO (Pd-1/ZnO) and use them in kinetic Monte Carlo (KMC) simulations for elucidating reaction mechanism. We compare these results for Pd-1/ZnO with those obtained for the same set of reactions on a 32-atom Pd16Zn16 nanocluster. Our KMC simulations show that Pd,/ZnO offers high, temperature dependent selectivity (similar to 93%) for H-2 production and a moderate one (similar to 76%) for CO2, in good agreement with experiment (which reports 90 and 85%, respectively). On the other hand, Pc16Zn16 yields no selectivity for H-2 but almost perfect, temperature independent selectivity (-100%) for CO2 and H2O, leading to full oxidation of methanol. The high activity of Pd-1/ZnO for MPO can be credited to the singly distributed Pd sites and to the Pd -modified geometric and electronic structures of the neighboring Zn sites, and its high H-2 selectivity may be related to the abundant supply of H atoms resulting from methanol decomposition on the surface. Pd loading has a decisive impact on adsorption and dissociation of methanol and oxygen. With higher Pd loadings, the activity of the Zn site alters in such a way that it provides weaker binding to methanol and stronger binding to O-2, thereby resulting in facile O-2 dissociation. Singly distributed Pd atoms not only serve as a more stable binding site for methanol than does Pd in Pd16Zn16 but also induce spontaneous CO2 formation and nearly spontaneous dissociation of H2O. In an alternate but slower pathway for production of CO2 involving HCOO* intermediate on Pdi/ZnO, the rate-limiting step is dissociation of H2COO*, followed by decomposition of HCOO* into CO2* and H*.