Pd-exchanged zeolites, in particular Pd/H-CHA, have shown promise as passive NOx adsorbers (PNAs) for capturing pollutants emitted during the cold-start period of gasoline-powered vehicles. However, the nature of the adsorption sites and the mechanisms underlying NOx storage on this material remain a topic of ongoing debate. While Pd is originally introduced into H-CHA as Pd2+ cations, the speciation of Pd inside the zeolite pores ultimately depends on the temperature and gas composition applied in the preparation of the PNA. Additionally, different complexes can be formed upon NO adsorption depending on the operating conditions. In this study, quantum chemical calculations using a QM/MM approach were used to predict the speciation of Pd at isolated Al atoms and proximate Al pairs located in the 6-rings and 8-rings of the CHA framework, and to elucidate the nature of the NO complexes formed on the available adsorption sites. These insights are then used to aid the interpretation of TPD experiments and in situ IR spectroscopy measurements. A large cluster model containing 696 tetrahedral atoms (T696) was used to model the CHA framework. A smaller cluster of T-atoms containing the active site and the adsorbates are treated quantum mechanically (QM), at the ωB97X-D/def2-SV(P) level of theory for geometry optimizations and frequency calculations, and at the ωB97M-V/def2-TZVPD level of theory for energy refinements. The remainder of the model is treated by molecular mechanics (MM) using a CHARMM-type force field with the P2 parameter set. Whether Pd+ or Pd2+ is thermodynamically most favorable is found to depend heavily on the location of the Al atoms within the framework. NO adsorption free energies show that NO generally binds more strongly on Pd+ sites than on Pd2+ sites, suggesting that the former are responsible for the high-temperature desorption peak observed in the TPD experiments, while the latter contribute to the low-temperature feature. Calculated NO stretching frequencies reveal how various NO adsorption complexes, on both Pd+ and Pd2+ sites at different Al pairs, contribute to the different bands observed in IR spectra at different temperatures.

Quantum chemical study of NO adsorption on Pd/H-CHA

ACS Spring Meeting 2021

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