(294c) Mechanistic Origins of Strong Metal Support Interaction

The strong metal support interaction (SMSI) is among the best-known classes of oxide-metal inter-facial interactions in heterogeneous catalysis. Historically SMSI has been associated with deactivation of catalytic sites. Recent developments suggest, in certain circumstances, SMSI can modify active sites to yield improved catalytic properties. In the current study, density functional theory (DFT) calculations are used, with surface science experiments, to study SMSI. The structural, thermodynamic and electronic properties of encapsulating layers can be studied in the form of ultrathin film models supported on single crystal metal substrates; microscopic images of SMSI show that Moiré patterns of sub-oxide films on nanoparticles are similar to films grown on single crystal substrates. As a starting point, we analyze the Pd/ZnO system. It’s a potential methanol synthesis catalyst, and since ZnO is an irreducible oxide, it provides a test of the traditional hypothesis that reduction of support cations is necessary for SMSI. Using DFT calculations and a lattice matching algorithm to minimize strain in periodic overlayer structures, the properties of Zn hydroxy films on Pd(111), surface Pd-Zn alloys, as well as PdO with varying stoichiometries, were determined. To compare our calculations with experiments, where the films aren’t in equilibrium with bulk species, we developed a mixed canonical-grand canonical phase diagram scheme. The resulting predictions match well with experimentally observed structures and validate the predicted properties of the various Zn-O-H-Pd surface phases. With changes in the thermodynamic ensemble, the results can then be extrapolated to understand stability of films at realistic SMSI conditions. Using this formulation, we have discovered that ZnOH films, which don’t exist in bulk, may form and promote SMSI in irreducible oxides. The framework developed in the Pd/ZnO system is expanded to conduct thermodynamic study for different oxide-metal pairs. These systematic trends-based studies are used to suggest broader conceptual explanations of SMSI.