(607e) How Structural Evolution during Strong Metal Support Phenomena Influences Reaction Rates and Selectivity

The Strong Metal Support Interaction (SMSI) phenomena, first observed for metals supported on reducible oxides, is now seen on 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 twelve active sites: Pd(111), TiB₂(0001) supports, monolayers of TiB₂(0001) supported on Pd, and partially complete monolayers representing intermediate stages of SMSI. We consider both oxidized and boron-terminated support overlayers, since XPS experiments suggest surface oxidation. 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. The different binding modes of C₁ oxygenates on TiB₂ overlayers stabilizes key oxygenates like monodentate formate relative to Pd(111). The presence of monodentate formate is also confirmed using DRIFTS. TiB₂ overlayers, however, exhibit a 0.8 eV higher barrier for H-H recombination, the rate determining step. These competing effects result in SMSI overlayers exhibiting 10⁶ times slower catalytic turnovers for HCOOH dissociation relative to Pd(111). Partially complete overlayers however, 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). Structures exhibiting SMSI phenomena have significantly higher selectivity towards dehydration (CO + H₂) versus Pd sites. 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.