While there is a lot of interest in Platinum group metal single atom catalysts and subnanometric clusters for CH4 oxidation, Wacker oxidation, and passive NOx adsorption (PNA), the dynamic transformation between metal particles and isolated atoms is not well understood. Under reducing conditions, Pd cations exchanged in zeolites sinter to particles, causing the loss of active sites for Wacker oxidation and PNA, but regeneration is feasible via redispersion of Pd agglomerates by NO or hydrothermal treatment. Redispersion of Pt particles in oxidizing conditions causes loss of catalytic activity for CH4 oxidation, while reduction treatments facilitate agglomeration. An in-depth understanding of the structural interconversion between cations and particles is thus essential for assessing stability and regeneration of these materials. Here, we use experiments and computational modeling to explore the thermodynamic and kinetic factors influencing the redispersion of Pd and Pt particles in zeolites. Using density functional theory calculations and thermodynamic models, we analyzed the effect of particle size and reaction environment (gas pressures, temperature) on the extent of ion-exchange. In dry air, it is thermodynamically feasible to completely redisperse Pd particles of all sizes. However, experiments showed partial redispersion of larger particles, suggesting kinetic limitations. Increased H2O pressures led to a shift in the thermodynamic equilibrium, favoring PdO formation from cations at T&lt; 800K, evincing its deteriorating effect for PNA. Results from wet air treatments of synthesized Pd particles showed reduced cation exchange as well; also, we noted that samples with exchanged cations showed a propensity for PdO agglomeration. Redispersion rate experiments revealed particle-size dependence, with small particles redispersing faster. Kinetic Monte Carlo (kMC) simulations for Pd redispersion via support-mediated Ostwald ripening and subsequent atom trapping at Brønsted acid sites, accurately captured the observed size-dependent kinetics. Our kMC results indicated faster redispersion of monodispersed particles than those with log-normal distribution where the long tail of larger particles disintegrated the slowest. Analogous theoretical study of Pt/zeolites revealed similar thermodynamic results, with our kinetic model for gas-phase Ostwald ripening and atom trapping being qualitatively consistent with previously reported experimental studies.

Kinetic and Thermodynamic Factors Influencing Palladium and Platinum Nanoparticle Redispersion into Mononuclear Cations in Zeolite Supports

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