Condition-Dependent Pd Speciation and NO adsorption in Pd/Zeolites

Christopher  Paolucci, University of Virginia

Pd-SSZ-13 zeolites are promising materials for passive NOx adsorption (PNA), but Pd cation speciation in the presence of H₂O, CO, and NO, ubiquitous gases in pollution remediation technologies, remains largely unknown. Here, we interrogate the molecular structure of Pd cations in SSZ-13 zeolites and their interaction with H₂O and NO using experimental and computational analyses. Density functional theory (DFT) and spectroscopic characterization establish that Pd cations preferentially populate two Al (2Al) sites in the six-membered ring as Pd^II. In situ spectroscopic and kinetic analyses follow Pd coordination environment and reactivity as a function of environmental conditions, and molecular structures are rationalized through ab initio molecular dynamics, CCSD(T) calculations, and first-principles thermodynamic modeling. Experiment and computational modeling together reveal that at temperatures < 573 K Pd ions are solvated and mobilized by H₂O molecules, forming molecular complexes akin to their Pd homogeneous analogues. Exposure to NO promotes transformation from 2Al to 1Al charge-compensated H₂O-solvated Pd-nitrosyl complexes, that desorb NO at higher temperatures, and inhibit CO oxidation rates. Comparison with Pd-BEA and Pd-ZSM-5 zeolites evidence a heterogeneous distribution of Pd-NO complexes under dry conditions that coalesce into homogeneous H₂O-solvated Pd-nitrosyl complexes upon exposure to H₂O.