Passive NOx adsorbers (PNA) has garnered significant attention as a potential solution for NOx and hydrocarbon (HC) slip that occurs from automobile engine exhaust emissions below 200C where most after-treatment catalytic processes are not effective. 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) along with an atmospheric pressure flow reactor, diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) to develop a working mechanism that describes the NOx trapping and release over a range of temperatures. The types of active Pd sites present in the catalyst are referred to as PdII, PdI, PdIIOH and PdIIO. Experiments were conducted at temperature range 100°C-450°C with mixtures of isotopic and non-isotopic NO, O2 and CO under close to dry and wet conditions to study the participation of different Pd species and the effect of H2O and CO on NOx uptake and release. NO2 was detected at lower temperatures suggesting NO oxidation. A DRIFTS characterization showed evidence of PdII, PdI and a DFT investigation on the stability of the sites and the associated NO binding energies suggest that NO binds strongly on PdI sites. The formation of PdI from PdIIOH can be explained via a four-step site modification redox mechanism on [Pd-O-Pd]II. The facile reoxidation thermodynamics of PdI to [Pd-O-Pd] II confirmed the formation of NO2. The substantial reduction of NOx storage in the presence of H2O is primarily attributed to the inhibition of the Brønsted acid sites by H2O. In contrast, NO uptake on PdI is less impeded by bound H2O, as suggested by DFT results. Considering that NOx 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 PdIIOH to PdI thermodynamically favorable. This study aims to identify the various active sites for passive NOx 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.

Active sites for passive NOx adsorption (PNA) on Pd/H-SSZ-13: Role of dimeric sites

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.