(758a) Condition-Dependent Pd Speciation and NO Adsorption in Pd/Zeolites

Metal exchanged zeolites and their interaction with other compounds typify a behavior distinct from both the zeolites’ heterogeneous properties and the metals’ homogeneous inorganic chemistry, which is still not well-understood. This poses a challenge in the optimal design and synthesis of these materials for applications such as emissions control where the underlying association of the exchanged metal cations with gases like H₂O, NO, etc. is critical. In this study, we examine Pd cations exchanged in SSZ-13 zeolites and their complexing with H₂O and NO at a molecular level, employing experimental and computational modeling techniques. Density functional theory (DFT) analyses aided by spectroscopic characterization show that exchanged Pd ions preferentially charge-compensate two Al (2Al) sites in the six-membered ring of SSZ-13 as Pd^II, akin to similarly sized Cu^II and Co^II analogues. We further probe the reaction environment-dependent Pd coordination chemistry and reactivity using in-situ spectroscopy and kinetic studies, along with ab initio molecular dynamics, coupled-cluster calculations, and first-principles based thermodynamic analyses to arrive at probable molecular structures. Computational modeling and experimental results establish solvation and enhanced mobility of Pd ions at 2Al sites by H₂O below 573 K, forming four-fold coordinated square planar complexes as observed in homogeneous Pd compounds, detached from the zeolite framework. Addition of NO facilitates transformation from 2Al to 1Al sites, resulting in H₂O-solvated Pd-nitrosyl complexes in the form of [Pd^II(NO^-)(H₂O)₃]⁺ that interact only electrostatically with the zeolite framework. These complexes exhibit higher desorption temperatures than their dehydrated counterparts, while also inhibiting competitive CO adsorption. Moreover, in-situ infrared spectra of zeolites with different topologies, such as Pd/BEA and Pd/ZSM-5, demonstrate formation of similar H₂O-solvated Pd-nitrosyl complexes, reinforcing the role of H₂O in solvation and homogenization of Pd ions in the presence of NO across zeolites of varying frameworks.