(349c) Thermodynamic Analysis of the Reversible Interconversion of Platinum Cations to Nanoparticles in SSZ-13 Zeolites

Metal ions exchanged on zeolites offer an elegant solution to achieve high metal dispersion and combat sintering. However, under reducing environments, exchanged metal cations form metal nanoparticles, which through Ostwald ripening and/or coalescence form even larger nanoparticles, thus resulting in a loss of catalytic activity. Recent studies show that oxidizing conditions may reversibly transform nanoparticles encapsulated in zeolites, or on the external surface of the zeolite crystallite, to smaller particles or single metal cations, but the lack of molecular level knowledge regarding the thermodynamics and kinetics of this process limits exploiting the full potential of interconversion for practical applications. In this study, we analyze the reaction conditions under which the reversible interconversion of Pt-exchanged SSZ-13 zeolites is feasible, using density functional theory (DFT) calculations, and first-principles thermodynamic modeling. Our results establish that under high-temperature and oxidizing conditions, Pt and Pt-oxide particles disintegrate into Pt^II cations that populate aluminum pairs located in the six-membered rings of SSZ-13 zeolites. Our thermodynamic model demonstrates that the conditions (temperature and O₂ pressure) required for interconversion to Pt cations depend sensitively on the particle sizes and the corresponding free energies of Pt and Pt-oxide nanoparticles. Our model predicts that, under conditions of practical interest, the temperature for the interconversion between Pt-oxides and Pt cations increases with particle size, approaching the Pt-oxide bulk disintegration temperature at approximately 630 K. Our model results are consistent with the complete interconversion of Pt and Pt-oxide nanoparticles of variable sizes to Pt cations at conditions reported in literature. However, our results also predict that for a subset of larger Pt-oxide particle sizes, complete ion exchange should be achievable at temperatures lower than those experimentally reported as limiting, implicating kinetic effects at lower temperatures as rate-limiting in the interconversion process.