Understanding the Role of Reaction Conditions and Zeolite Properties on Interconversion Between Cations and Nanoparticles in Pd/Zeolites
Keka Mandal, University of Virginia
Zeolite supported metal catalysts are ubiquitous in industrial applications such as chemical synthesis and separations, pollution abatement technologies, hydrogenation, oxidation, and carbon-carbon coupling reactions. Zeolites are aluminosilicates made up of interconnected SiO4 and AlO4 tetrahedra, forming well-defined, nanoporous cage-like structures. Synthesis protocols and gas treatments, along with compositional parameters such as Si/Al ratio, and metal loading, determine the speciation, location and spatial distribution of the metal species in the zeolites. Metals in zeolites can exist as (a) cations exchanged in the zeolitic framework, (b) clusters (< 2 nm) encapsulated in the zeolite cages, and (c) extra-crystalline nanoparticles located on the outer surfaces of zeolites. The metal speciation in zeolites is sensitive to the reaction environment, and certain conditions facilitate cations to nanoparticles interconversion in zeolites.
In this study, we investigate Pd/zeolites at a molecular level by employing computational modeling and tools such as density functional theory (DFT), wave function theory (WFT), ab initio molecular dynamics (AIMD) simulations, thermodynamic models, aided by spectroscopic and kinetic measurements. Our study examined the speciation of Pd cations and their complexing with H2O and NO, which are a critical parameters in the optimal design and synthesis in their implementation as passive NOx adsorbers (PNAs) in diesel engines. Our results characterization demonstrate the proclivity of the exchanged Pd ions to charge-compensate two Al (2Al) sites in the six-membered ring of SSZ-13 as PdII, similar to their Cu and Co analogues. PdII ions at the 2Al sites are solvated and mobilized by H2O under conditions of practical interest for PNAs, which upon exposure to NO evolve to H2O-solvated PdII-NO– complexes, a behavior that transcends across different zeolite topologies. Moreover, in the presence of H2O, NO forms H2O-solvated Pd-nitrosyl complex at the 1Al site facilitating 2Alà1Al site transformation. H2O-solvated Pd-nitrosyl complexes also inhibits competitive CO adsorption that has been reported to contribute to irreversible degradation of PNAs. In the presence of CO and H2O, Pd cations aggregate to form Pd0 clusters (extra-crystalline and encapsulated), which oxidize to form PdO agglomerates which do not adsorb NO, thereby leading to a loss in NO storage capacity. We formulated thermodynamic phase diagrams which establish that factors such as nanoparticle size, density of proximal Al sites in the zeolite, and O2and H2O pressures, sensitively influence the maximum extent of ion exchange. We demonstrate through computational modeling and experiment that, contrary to prior proposals, H2O inhibits conversion of PdO nanoparticles to Pd cations. A non-elementary mechanism for Pd redispersion proposed in literature suggests the formation of mobile, neutral gas-phase Pd(OH)2 species via interfacial detachment and migration in the presence of H2O. We investigate plausible intermediates and activation barriers that may facilitate cation exchange, and determine the role of H2O in the interconversion mechanism using DFT calculations.