Active Sites for Passive NOx Adsorption (PNA) on Pd/H-SSZ-13

Unmesh  Menon, University of Houston

Most engine exhaust after-treatment catalytic processes such as selective catalytic reduction (SCR) are effective at temperatures above 200°C. During vehicle startup or low load operation temperatures can be below this threshold and the majority of NO_x and hydrocarbons (HCs) slip through the after-treatment system. To meet emission standards, NO_x and HC may be trapped during this period and released at higher temperatures when downstream catalysts are active.  The passive NOx adsorber (PNA) is emerging as a technology to accomplish this task for NO_x. However, the nature and role of the active sites in the PNA are inadequately understood. In this investigation, we characterize the active sites of a model PNA, Pd-exchanged H-SSZ-13, using Temporal Analysis of Products (TAP), a method that employs rapid pulsing at ultralow pressure. The data are used to develop a working mechanism comprising reaction steps that describe the NO_x trapping and release over a range of temperatures. The TAP data are interpreted with the help of an atmospheric pressure flow reactor, diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT).

The types of active Pd sites present in the catalyst are referred to as Pd^II, Pd^I, Pd^II-OH and Pd^IIO. Experiments were conducted at temperatures between 100°C and 450°C with mixtures of isotopic and non-isotopic NO, with and without O₂ and CO under dry and wet conditions to study the participation of different Pd species and the effects of H₂O, CO on NO_x uptake and release.

A DFT investigation on the stability of these sites and the associated NO binding energies suggest that NO binds strongly on Pd^I sites, the formation of which can be explained through the two-step site modification mechanism

2 Z [Pd^IIOH] → Z [Pd^I – O –Pd^I] Z + H₂O → 2 ZPd^I

Product distribution from the isotopic experiments with labelled NO and O₂ is consistent with the formation of the Pd^I sites from two adjacent Pd^IIOH sites.

The substantial reduction of NO_x storage in the presence of H₂O is primarily attributed to the loss of NO uptake on Brønsted acid sites, which are inhibitied by H₂O. In contrast, NO uptake on Pd^I is less impeded by bound H₂O, as suggested by DFT results. Considering that NO_x uptake over Pd zeolites is enhanced in the presence of CO, our results suggest that the oxidation or reduction potential of the gas phase alters the relative availability of active storage sites. In particular, the strongly exothermic nature of CO oxidation renders the reduction from Pd^IIOH to Pd^I thermodynamically favorable. Comparable results are obtained with C₂H₄ as the reductant.

This study aims to identify the various active sites for passive NO_x adsorption using Pd/H-SSZ-13 catalyst, which is the most promising material to date. An atomic-level understanding of the ad-/desorption and reaction steps on these active sites will greatly accelerate the design and development of improved PNA materials.