Experimental and Modeling Studies of Passive NOx Adsorbers

To meet the future more stringent regulations for nitrogen oxides (NOx) emission from vehicle’s exhaust, improved technologies which can further reduce NOx emissions during cold start period are needed. The passive NOx adsorbers (PNA) have been gaining attention recently as a technology that has the potential to abate cold-start NOx emissions. A combined experimental and modeling study of the PNA is presented that advances the understanding and prediction of the effects of various operating parameters and material properties of H-ZSM-5, Pd/H-ZSM-5 and Pd/SSZ-13. Experiments reveal that the presence of H2O in the feed significantly lowers NOx uptake at lower temperatures as a result of competition for sites between H2O and NOx species and the extent of inhibition is a function of temperature. A one-dimensional two-phase transient monolith model is developed to predict and validate NOx uptake and temperature programmed desorption (TPD) data for H-ZSM-5, Pd/H-ZSM-5 and Pd/SSZ-13. The model is extended for NO uptake and release over Pd/SSZ-13 with H2O, CO, C2H4 and C12H26 in the feed. The microkinetic schemes for each of the feeds involves multi-site NO adsorption on multiple types of Pd cations: Z-[PdOH]+, Z-Pd2+Z-, and Z-Pd+ , with a key feature being the reduction of a pair of Z-[PdOH]+ sites to two Z-Pd+ sites. The reduced sites bind NO the strongest. The Pd(II) to Pd(I) reduction generates NO2, CO2 or C2H4O depending on the feed constituents. For NO-only feed, this endothermic reaction occurs at temperature ~120℃ and generates NO2. In the presence of CO and C2H4 the exothermic reduction occurs at lower temperatures and generates CO2 and C2H4O respectively. The presence of C12H26 is shown to impact the NO uptake and/or release despite its own limited uptake. When a co-feed containing C12H26 and NO is supplied to an unsaturated (with C12H26) sample, the NO uptake is unaffected but during the subsequent temperature ramp the release of trapped NO is delayed from 175℃ to over 220℃. The release delay is beneficial for PNA performance as the primary NOx aftertreatment technology Selective Catalytic Reduction (SCR) is ineffective at temperatures below 200℃. Oxidation of C12H26 leads to the generation of partial oxidation product CO at lower temperatures (< 250℃) and complete oxidation product CO2 at higher temperatures. Carbon monoxide binds strongly to Pd sites and can delay NO release. Model tuning utilizes a combination of diffuse reflective infrared Fourier transform spectroscopy (DRIFTS) data, density functional theory (DFT) estimates of energy barriers and a fit of constant temperature uptake and temperature-ramped desorption and conversion data. The tuned model is validated at different uptake temperatures, desorption ramp rates, and feed flowrates. The model helps to interpret the data features and trends and is used to identify operating conditions to meet application-relevant performance metrics including NOx trapping efficiency and NOx release temperature.