(292c) Tuning the Rate and Selectivity of Formic Acid Decomposition through Strong Metal Support Interaction Phenomena in Transition Metal Borides.

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 increases resistance to sintering, these overlayers decrease reaction rates; since the metal active sites are passivated. Our previous study¹ showed that noble metals exhibit SMSI phenomena on TiB₂ supports with the nanoparticles being sintering resistant. To understand stability-reactivity trade-offs engendered by SMSI phenomena, we use HCOOH decomposition as a probe. 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 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). Relative to Pd(111), however, TiB₂ overlayers 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 however, exhibit higher rates than complete overlayers, because these structures expose low-coordinated Ti sites where H-H recombination proceeds with a barrier comparable to Pd(111). Using a microkinetic model, we discuss how SMSI influences apparent activation energies, rate orders, and degrees of rate control. This microkinetic study highlights that controlling the extent of SMSI can create active sites with high rates while remaining resistant to sintering.

1. Advanced Materials, 2021, https://doi.org/10.1002/adma.202101536.