The Strong Metal Support Interaction (SMSI) phenomena, first observed for metals supported on reducible oxides, is now seen across a range of materials like irreducible oxides, carbides, and borides. Although the growth of support overlayers on metal particles prevents sintering, these overlayers decrease reaction rates; since the metal active sites are passivated. Our previous study showed that noble metals exhibit SMSI phenomena on TiB2 supports with the nanoparticles being sintering resistant. To understand stability-reactivity-selectivity trade-offs engendered by SMSI phenomena, we use HCOOH decomposition as a probe reaction. We build a first principles microkinetic model constructed across a library of candidate active sites: Pd(111), TiB2(0001) supports, monolayers of TiB2(0001) supported on Pd, and partially complete monolayers that represent intermediate stages of SMSI. We consider both oxidized and boron-terminated support overlayers, since XPS experiments suggest surface oxidation. We verify that the structural details and oxidation states of the computational models match experiments. We then determine the transition state energies for HCOOH dehydrogenation to CO2/H2 and HCOOH dehydration to CO/H2O across the library of active sites. We find that the different binding modes of C1 oxygenates on TiB2 overlayers decreases the C-H/O-H dissociation barriers of HCOOH relative to Pd(111). TiB2 overlayers, however, exhibit a 0.8 eV higher barrier for H-H recombination, the rate determining step. These competing effects result in the overlayers formed through SMSI exhibiting reduced catalytic turnovers for HCOOH dissociation as compared to Pd(111). Partially complete overlayers exhibit higher rates than complete overlayers, because these structures expose coordinatively unsatured Ti sites where H-H recombination proceeds with a barrier comparable to Pd(111). The microkinetic analysis indicates that while Pd sites are selective to both dehydration (CO + H2O) and dehydrogenation (CO2 + H2), the partial or complete overlayers of TiB2 are highly selective to dehydrogenation. Using the microkinetic model, we discuss how SMSI influences kinetic metrics like apparent activation energies, rate orders, and degrees of rate control. This microkinetic study highlights the catalytic consequences of the structural evolution of a catalyst undergoing SMSI phenomena.

Microkinetic study of how strong metal support interaction phenomena influences reaction rates and selectivity

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