(433h) A DFT Study for the Mechanistic Understanding of Passive NOx Adsorption on Pd/H-BEA Under Dry Condition

With constantly changing fuel sources and improvements in engine technology the treatment of vehicle emissions at low temperature poses a formidable challenge. Although there has been a success in addressing NO_x emissions at temperatures above 200ºC with the aid of selective catalytic reduction (SCR) technology, emission control during vehicle cold start (temperatures < 200ºC) remains arduous. Metal-exchanged zeolites functioning as passive NO_x adsorber have been proposed as a possible solution. Their role is to adsorb NO_x from the engine exhaust at low temperature and release them when the downstream SCR reaches operating temperature.

In the present work, we have used density functional theory to gain a mechanistic understanding of NO_x adsorption on Pd/H-BEA, and have proposed a sequence of feasible elementary steps that can explain observations from flow reactor experiments for NO_x trapping.¹ Due to the complex nature of palladium speciation in zeolite pores, the structure of the active site remains elusive. For zeolites with atomically dispersed Pd in the absence of water the most likely active sites are [Z₂Pd^II], [ZPd^IIOH], and [ZPd^I]. On the [Z₂Pd^II] site NO can be stored by reversible molecular adsorption or in the form of nitrate (Figure 1). The [ZPd^IIOH] site catalyzes low energy pathways for NO and also CO oxidation, while [ZPd^I] can be dynamically formed and provides the strongest NO binding site. The presence of CO as stronger reductant was found to be beneficial for NO_x storage. The active site assignments are corroborated by vibrational analysis and comparison to observed experimental diffuse reflectance spectra.¹

Capturing the temperature dependent evolution and role of active sites in Pd/H-BEA upon NO exposure will expedite the development of suitable PNA materials to achieve desired emission control performance at low temperature.

Reference:

1. Malamis, Sotirios A., Michael P. Harold, William S. Epling, Industrial & Engineering Chemistry Research 58,22912-22923 (2019).