Metal ions exchanged on zeolites represent a unique bridge between heterogeneous solid materials and homogeneous inorganic chemistry. The complexing of exchanged metal ions with H2O or NO, is of particular relevance for a number of reactions, including the ubiquitous presence of both gases in pollution remediation technologies. 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 the Pd coordination environment and reactivity as a function of environmental conditions, and molecular structures are rationalized through ab initio molecular dynamics 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, promoting catalytic CO oxidation, and form molecular complexes akin to their Pd homogeneous analogues. Exposure to NO promotes transformation from 2Al → 1Al charge-compensated H2O-solvated Pd-nitrosyl complexes, which desorb NO at higher temperatures and inhibit CO adsorption and oxidation. A comparison with Pd-BEA and Pd-ZSM-5 zeolites demonstrates a heterogeneous distribution of Pd-NO complexes under dry conditions that coalesce into homogeneous H2O-solvated Pd-nitrosyl complexes upon exposure to H2O.

Palladium cations and nanoparticles supported on oxides and zeolites are used as catalysts and adsorbents for a wide range of chemical reactions of practical importance, with various and distinct active site requirements. Consequently, sintering and redispersion processes that interconvert site-isolated Pd cations and agglomerated nanoparticles underpin catalyst activation and deactivation phenomena, yet the influence of Pd nanoparticle size distribution and gas conditions on the thermodynamic and kinetic factors influencing such interconversion is imprecisely understood. Here, we prepare Pd nanoparticles of different particle size and distribution (normal, log–normal) supported on high-symmetry crystalline zeolites to access well-defined materials whose Pd site distributions can be quantitatively characterized by experiments and modeled accurately by theory. Ab initio thermodynamic modeling and isothermal (593–973 K) wet and dry air treatments of Pd-zeolites reveal that the widely observed deactivation in low-temperature hydrous environments reflects site interconversion thermodynamics that favor the agglomeration of isolated Pd cations into nanoparticles. Under high-temperature anhydrous conditions, experimental kinetic measurements and kinetic Monte Carlo simulations evince the preeminence of kinetic factors on Pd nanoparticle redispersion into cations, which proceeds at rates that are strongly influenced by the initial Pd particle size distribution and via a substrate diffusion-mediated Ostwald ripening process whereby Pd monomers are captured in an atom trapping process at anionic exchange sites (framework Al) in the zeolite support. These findings resolve longstanding questions regarding the roles of H2O and support interactions in Pd redispersion processes and identify strategies to enhance or suppress Pd site interconversion by modifying oxide supports and gas conditions.