Palladium/Zeolites as Model Catalysts for Cold Start Passive NOx Adsorption

Reducing NOx emissions is challenging during engine or aftertreatment system warm up due to the low exhaust temperatures and thus limited reaction kinetics. NOx emitted during this period is often referred to as cold start NOx emissions and represents one of the main obstacles in emissions regulations compliance. Passive NOx adsorbers (PNA) have been proposed to help address cold start NOx emissions via low temperature NOx trapping and subsequent thermal release, such that NOx generated and trapped during cold start can ultimately be converted by a downstream NOx reduction catalyst. Several groups of potential PNAs have been evaluated for low temperature NOx storage and subsequent release, of which the laboratory-scale experimental testing results appear to be promising and point to Pd/zeolites as potentially the best PNA material. In this study, the nature of Pd within the zeolite framework and Pd speciation as a function of reaction conditions were first investigated using Pd/SSZ-13 model PNAs. Results show that Pd cations are responsible for the NOx storage and tend to be solvated by H2O under cold start conditions. Once solvated, Pd detaches from the zeolite and forms homogeneous Pd-nitrosyl species. This effect of H2O solvation appears to be a general phenomenon regardless of zeolite topology. A similar study on a Pd/ZSM-5 commercial PNA confirmed the solvated nature of Pd and revealed key reaction and adsorption pathways under realistic NO adsorption conditions. A simplified NO adsorption and desorption mechanism that is able to capture the characteristics of NO adsorption in the presence of H2O and CO was proposed.
Additionally, a CO-induced irreversible NOx storage degradation mode was observed on Pd/BEA model PNAs during cyclic NO adsorption and temperature programmed desorption (TPD) experiments. Characterization of the degraded catalyst indicates that the Pd migration from ion-exchanged positions to particles that reside on the external surface of the zeolite leads to the observed degradation. The degradation mechanism was further studied using in situ X-ray absorption spectroscopy (XAS) and temperature programmed reduction (TPR) experiments. Results show that CO leads to the reduction of Pd cations and high temperature accelerates the agglomeration of PdO nanoparticles to prevent them from oxidative regeneration.
As a final topic, Pd/SSZ-13 PNA was integrated with a Pt/Al2O3 oxidation catalyst, of which the results show enhancement in system durability and more ideal NOx release temperature. The first is due to the limited high temperature CO exposure. The more ideal NOx release temperature was found to correlate to the early onset of NO2 formation as a result of enhanced oxidation activity.