(232a) Cation and Nanoparticle Interconversion in Metal-Exchanged Zeolites

Metal cations and nanoparticles supported on oxides and zeolites 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 agglomerated nanoparticles underpin catalyst activation and deactivation phenomena, 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. Here, we use density functional theory calculations and mathematical modeling 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. We show that redispersion of Pd nanoparticles proceeds through a different mechanism than for Pt nanoparticles, and cationic Pt and Pd have different stability windows with respect to gas conditions and zeolite topology. 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.