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

ACS Catalysis2020Vol. 10(21), pp. 12801–12818

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Keka Mandal, Yuntao Gu, Karl S. Westendorff, Sichi Li, Josh A. Pihl, Lars C. Grabow, William S. Epling, Christopher Paolucci

Abstract

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.

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Abstract

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