(356f) Thermodynamic and Kinetic Factors Influential in the Redispersion of Pt-Group Metal Nanoparticles to Ion-Exchanged Cations in Zeolites

I am interested in areas of computational chemistry, materials research, and applied research in environmental catalysis, using density functional theory calculations, ab-initio molecular dynamics and simulations, thermodynamic and kinetic modeling, and machine learning-based techniques.

Abstract

The structural transformation of metal nanoparticles to isolated atoms/ ions on oxide supports affects their reactivity, deactivation, and regeneration, and is significant for Pt-group metals (PGM) ion-exchanged or supported in zeolites which are being explored for applications like Wacker, and methane oxidation, and passive NOx adsorption (PNA) in diesel engines. Sintering of ion-exchanged Pd cations (active sites for PNA and Wacker oxidation) to nanoparticles in reducing environments results in deactivation. However, redispersion of particles to ion-exchanged cations via NO, or hydrothermal treatment with air and steam can facilitate regeneration. Conversely, an oxidizing environment causes redispersion of Pt nanoparticles causes loss of catalytic activity for methane oxidation, while reduction treatments allow cation agglomeration and catalyst reactivation. An in-depth understanding of the structural interconversion between cations and particles is thus essential for assessing stability and regeneration. In this study, we investigate the thermodynamic and kinetic factors dictating the redispersion of Pd particles to ion-exchanged cations in CHA zeolites, and the influence of parameters like particle size and distribution on the support, and external gas conditions, using experiments and computational modeling. From density functional theory calculations and thermodynamic analysis, we evaluated the extent of ion-exchange as a function of particle size and reaction conditions (temperature, O₂ and H₂O pressures), and found that complete conversion of Pd particles of all sizes to cations is thermodynamically feasible under dry air treatments. However, the experimentally observed incomplete conversion of large nanoparticles implies kinetic barriers to oxidation of Pd particles to PdO agglomerates, and the subsequent redispersion to cations. Higher H₂O pressure shifted the thermodynamic equilibrium towards the formation of PdO agglomerates from cations at T<800K, indicating H₂O in exhaust stream can facilitate deactivation of materials for PNA applications. Consistent with theory, wet air treatments of synthesized samples resulted in decreased cation exchange; moreover, when cations were initially present in the sample, agglomeration of cations to PdO was seen. Redispersion rate measurements exhibited size-dependence, with smaller particles exchanging to cations faster. We constructed kinetic Monte Carlo simulations (kMC) for a support-mediated Ostwald ripening mechanism, where monomers ejected from particles diffuse on the zeolite and are subsequently trapped at the Brønsted acid sites, and found it to be consistent with the observed size-dependent kinetics. Our kMC results showed that monodispersed particles converted to cations faster compared to log-normal distribution due to the presence of a long tail of larger sized particles that disintegrated the slowest. Analogous theoretical study of Pt-exchanged zeolites revealed similar thermodynamic results; we studied redispersion of Pt nanoparticles present on the external zeolite surface (“supported particles”) and nanoclusters encapsulated in the zeolite pores (“unsupported particles”), and observed that the extent of ion-exchange increased as the particles become smaller and the temperatures increased. Akin to Pd, presence of H₂O promoted agglomeration of both supported and unsupported Pt nanoparticles, although the stabilization of the encapsulated “unsupported particles” due to the zeolite confinement effects influences the extent of redispersion. We found our kinetic model for gas-phase Ostwald ripening followed by atom trapping to be qualitatively consistent with previously reported experimental studies for Pt nanoparticle redispersion. Our findings illustrate the influence of factors such as gas conditions, particle size and distribution, on the thermodynamics and kinetics of particle redispersion, and can be extended to similar redispersion of PGM paticles on other metal oxide supports containing atom trapping sites or defects.