The strong metal support interaction (SMSI) is among the best known classes of oxide-metal interfacial interactions in heterogeneous catalysis. The SMSI was first investigated forty years ago by Tauster et al., and in the intervening years, electron microscopy has confirmed the existence of an encapsulating layer consisting of modified support oxides on top of the metal nanoparticles in various related cases. Although, historically it has been associated with deactivation of catalytic sites, recent developments suggest that, in certain circumstances, the SMSI can modify catalytic active sites to yield improved catalytic properties. Here, density functional theory (DFT) calculations, along with surface science experiments, are employed to systematically study the molecular-level underpinnings of the SMSI. . As a starting point, we analyze the Pd/ZnO system. This is 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 to exhibit SMSI. Using detailed DFT calculations, the properties of ultrathin zinc hydroxy films on Pd(111), are determined. To compare the calculations with surface science experiments, where the ultrathin films are not in equilibrium with bulk species, we developed a novel mixed canonical – grand canonical phase diagram scheme. With simple changes in the thermodynamic ensemble and chemical potential reservoirs, the results are subsequently extrapolated to determine stability of ultrathin films in realistic SMSI conditions. The results demonstrate that stable ZnOH films, which do not exist in bulk, may in fact form and promote SMSI in these irreducible oxides. This behavior is traced to both hydrogen incorporation in the films and strong stabilization of the films by the Pd substrates. Finally, the computational framework developed in the Pd/ZnO system is expanded to conduct thermodynamic study for different metal-metal oxide pairs. To facilitate comparisons between disparate systems, we introduce new scaling relationships between oxide film formation and combination of oxide cation and anion adsorption energies. Further, the scaling relationships hold for different surface terminations. These systematic trends-based studies are used to suggest broader conceptual explanations of the SMSI phenomena and to provide insights to predict the existence of special boundary sites that may be active for heterogeneous catalysis.

Mechanistic origins of strong metal support interaction

ACS Spring 2022

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