Stability is often the most important factor in catalyst discovery and improvement. In this seminar I discuss our groups recent work modeling the stability of two important classes of materials and sets of conditions: transition metal carbides during synthesis, and zeolite supported metal cations and nanoparticles under different hydrothermal conditions of practical interest.
Transition metal carbides are attractive catalysts because they have catalytic properties similar to Pt-group metals. However, their synthesis is complex, often traversing through multiple metastable phases during nucleation. We use quantum chemical calculations, thermodynamic analyses, and comparison to a large body of experimental data, to show that control over nanoparticle size is a key factor in phase selection during carbide synthesis.
Next, we apply similar computational machinery to the more complex system of Pt and Pd cations and nanoparticles supported on zeolites, which are used as catalysts for a wide range of chemical reactions, with various and distinct active site requirements. Consequently, sintering and redispersion processes that interconvert cations and nanoparticles underpin activation and deactivation of these catalysts, yet the influence of the nanoparticle size distribution, gas conditions, zeolite topology, and cation identity on the thermodynamic and kinetic factors influencing such interconversion are not well-understood. We use a combination of computational analyses and experimental validation to elucidate the conditions that generate a favorable thermodynamic driving force for agglomeration or redispersion and use kinetic Monte Carlo simulations to estimate the rates of interconversion. Our results show that water, a ubiquitous molecule in practical applications of these materials, promotes agglomeration of cations into nanoparticles for both Pt and Pd, even under high-temperature oxidizing conditions.