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 PdII. 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.