Development of Passive HC/NOx Trap Catalysts for Low Temperature Gasoline Applications

Abstract

Current vehicles generate a large fraction of their total tailpipe emissions during the beginning of the cold start test. For a current 3.5L GTDI (gasoline turbocharged direct injection) production vehicle, the majority (>60%) of CO, HC, and NOx are emitted in the first 60 seconds before the three-way catalyst reaches operating temperature of 350°C. Clearly, a means of storing HC and NO during cold starts would have a meaningful, measurable impact on vehicle tailpipe emissions. Against this background, this project aimed to develop fundamental understanding of the chemistry of NO/HC adsorption and reaction in Pd-zeolites so as to facilitate the rational design of passive NOx adsorber (PNA) catalysts. Given that numerous publications exist concerning HC trapping by Pd-zeolites, emphasis was placed on identifying the factors controlling NOx adsorption in Pd-zeolites. The approach adopted combined both experimental and computational methods, which together allow a deeper understanding of the governing chemistry than the use of either method alone. The workflow began with Pd/H-CHA and Pd/H-BEA catalyst synthesis and characterization, in which the Si/Al ratio and extent of Al pairing were systematically varied. This was followed by catalyst evaluation using temperature-programed adsorption/desorption methods, as well as in situ diffuse reflectance UV-vis spectroscopy (DRUVS), X-ray absorption near edge spectroscopy (XANES), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and transmission IR spectroscopic measurements to probe the nature of the NO adsorption sites and the form of the adsorbed NO. In parallel, the adsorption of NO and other relevant species (H2O, CO, HCs) was studied by means of quantum chemical calculations in order to rationalize the experimental data and provide additional insights. Catalyst aging studies were also performed with the aim of elucidating the mechanism(s) of catalyst degradation. Finally, the insights gained in this project were applied to the preparation of an optimized PNA catalyst, the performance of which was validated using exhaust gas from an engine dynamometer. Key findings included the observation that the dominant mechanism of formation for ion-exchanged Z2Pd and Z[PdOH] species in Pd-CHA is solid-state ion-exchange, and Z2Pd species are likely the most thermodynamically stable ion-exchanged structure after air treatments (>400 °C). CO-DRIFTS showed the presence of Pd+-CO bands at low Pd loadings on Pd-BEA with increasing Pd2+ content at higher loadings. Reduction of ionic Pd and PdO species by H2 was found to generate large quantities of small, pore-confined Pd metal particles, H2-TPR experiments showing that Pd-CHA has a greater ability to maintain Pd in an ionic state than does Pd-BEA and achieves complete recovery of ionic Pd species by re-oxidation treatments regardless of the reductant employed. However, reduction in CO was shown to produce larger Pd metal particles than reduction in H2; moreover, a lower degree of Pd re-dispersion upon re-oxidation was also observed for CO-reduced Pd-BEA compared to the corresponding H2-reduced sample. Quantum mechanical calculations showed that ionically dispersed Pd species (Pd+ and Pd2+) are thermodynamically most favorable in Pd-CHA over a wide range of oxidizing and reducing conditions. Calculated Gibbs free energies for NO adsorption show that NO generally binds more strongly on Pd+ sites than on Pd2+ sites, suggesting that the former are responsible for the high-temperature desorption peak observed in TPD experiments, while the latter contribute to the observed low-temperature feature. When evaluated in simulated exhaust gas, the presence of reductants (C2H4, C3H6, CO/H2) improved the NOx storage performance of Pd/zeolite passive NOx adsorbers under lean conditions but not stoichiometric conditions and mitigated the inhibitory effects of water. Against this, repeated cold start tests showed progressive decline in NO storage capacity, which could be linked to facile reduction of Pd by CO and H2O. For CHA, increasing the percentage of paired Al sites in the zeolite was beneficial for improving robustness to repeated cold starts and improved thermal durability. However, the low intrinsic storage efficiency (at best 1 NO per Pd) and the continuing high cost of palladium, along with remaining durability challenges, create an unfavorable business case for implementing Pd/zeolite passive NOx adsorbers on vehicles at the current time.