NOx uptake and release on Pd/SSZ-13: Impact Of Feed composition and temperature

Abstract

The Passive NOx Adsorber (PNA) is an emerging technology to abate cold-start NOx emissions from lean combustion vehicles. Binding sites on Pd-exchanged zeolites adsorb NO at low temperature (< 150 °C) and release NO and NO2 at higher temperature, enabling their downstream reduction. The impacts of other exhaust components such as H2O, hydrocarbons (HCs) and CO, along with operating conditions including temperature and flowrate must be quantified and understood in order to develop effective PNA materials. We have conducted a systematic set of experiments of the NOx uptake and release features on Pd-exchanged SSZ-13 in the absence and presence of several diesel exhaust components. The NO uptake approaches NO/Pd ∼ 1 for a range of conditions, and along with the generation of NO2 during NO feed and CO2 (CH3CHO) during NO + CO (NO + C2H4) co-feed, suggests the reduction of well-dispersed Pd cations. The total NO uptake between 50 and 150 °C is nearly constant but occurs in both lower temperature (< ∼120 °C) and higher temperature (>120 °C) regimes, indicating competition of NO uptake with H2O. The additions of CO and C2H4 increase the lower temperature uptake and higher temperature release of NO, both features desirable for application. In contrast, H2, dodecane, and toluene have a negligible effect on the NO uptake. During NO uptake the co-generation of NO2, CO2 and CH3CHO, respectively with reductants NO, CO, and C2H4, provides evidence for the reduction of Pd2+ in the form of [Pd(OH)]+ bound to zeolitic anionic sites [Al-O-Si]− and/or PdO2 clusters. Supporting DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) measurements identify several potential surface species that are linked to the multiple NO + NO2 desorption peaks. Two prospective mechanistic schemes are proposed that are consistent with the uptake/release and DRIFTS data. © 2020 Elsevier B.V.