(582d) Computational Study of Active Site Structure and NOx Storage in Pd-Exchanged Zeolites

Growing concerns over the adverse effects of air pollution caused by vehicular NO_x emissions are driving efforts to develop new abatement technologies using passive NO_x adsorbers (PNA). PNA materials enable a further reduction of cold-start NO_x emissions by trapping NO_x at low temperatures and retaining it until the emission control system has reached its optimal working temperature (>200°C). Pd-exchanged zeolites have emerged as promising substrates for PNA applications. However, the nature of the active sites and the mechanism underlying NO_x storage in these materials are still under debate. In this study, we use quantum chemical simulations to elucidate the structure of Pd species exchanged into H-CHA and to examine the interaction of NO with these potential adsorption sites. A QM/MM approach was applied to a large cluster model representing the CHA topology to limit the computational cost. An extensive configurational search was performed to identify the lowest free energy configurations of various putative Pdⁿ⁺ species (n = 0–2) in exchange sites at isolated Al atoms and proximate Al pairs. Subsequently constructed phase diagrams show that ionically dispersed Pd⁺ or Pd²⁺ are the most favorable, depending on the operating conditions and the location of the Al atoms. Finally, NO adsorption was modeled on the prevailing Pd cation sites. Pd²⁺ sites at NNNN Al pairs were found to be most favorable thermodynamically, but these appear to be poor NO binding sites. NO adsorption was found to be favorable up to 150 ºC on Pd²⁺ sites at NNN Al pairs, and up to 350–450 ºC on Pd⁺ sites at NNN Al pairs, suggesting that these sites may be responsible for respectively the low-temperature and high-temperature desorption peaks observed in TPD experiments on Pd/H-CHA. These insights provide an important first step towards understanding the storage mechanism for NO_x in Pd-exchanged zeolites.