(641g) Condition and Particle Size Effects on the Interconversion between Nanoparticles and Cations in Pd-Exchanged SSZ-13 Zeolites

Metal-exchanged zeolites, though often exhibiting enhanced resistance to reduction and sintering in comparison to other supported metal cation counterparts, may gradually lose their potency in reducing environments. Optimization of zeolite synthesis and process conditions can thus be aided by molecular level knowledge of the mechanism for metal or metal-oxide nanoparticles to regenerate and convert to metal cations. This is of particular importance for Pd-exchanged SSZ-13 zeolites, whose ability to adsorb NO at low temperatures make it a promising candidate for emissions control. Computational and experimental studies have established that Pd cations are responsible for NO adsorption. In the presence of CO and H₂O, Pd cations aggregate to form Pd⁰ clusters, which oxidize to form PdO nanoparticles, leading to a loss in NO adsorption capacity. Here, employing density functional theory (DFT) calculations, and first-principles thermodynamic modeling, we formulate phase diagrams that demonstrate that nanoparticle size, gas conditions, and zeolite composition, dictate the conditions under which the regeneration of PdO particles to cations can occur. Our results substantiate the experimentally observed difficulty to solid-state ion exchange Pd into zeolites from large PdO nanoparticles at temperatures < 800 K. Further, we show that factors such as nanoparticle size, density of proximal Al sites in the zeolite, and O₂ and 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. We show that interconversion thermodynamics do not tell the complete story; ion exchange is kinetically limited for larger particles. Prior studies have proposed a non-elementary mechanism whereby PdO particles eject mobile Pd(OH)₂ precursors via interfacial detachment and migration. We report DFT calculations that investigate plausible intermediates and barriers that may facilitate cation exchange, and determine the role of H₂O in the interconversion mechanism.