(252c) Spatio-Temporal Features of NOx and Hydrocarbons Trapping and Conversion in a PNA+HCT+OC Sequential Monolith Configuration

In the past 10 years there has been considerable progress in addressing NO_x (x = 1, 2) and CO/hydrocarbon (HC) emissions from diesel vehicles using urea-SCR¹ technology and DOC technology², respectively. However, these catalysts only achieve high NO and CO/HC conversions (>90%) once the exhaust temperature has exceeded 200 ^oC. During the “cold start” period a large fraction of the exhaust HCs and NO_x remain unreacted and are emitted to the environment. Precious metal dispersed zeolites (Pd/SSZ-13, Pt+Pd/BEA) have been shown to be effective for trapping NO and HCs at low temperature, followed by release at high temperature and subsequent conversion. To date there have been few studies of the combined trap. In this study we investigate the combination of PNA, HCT (HC trap) and OC (oxidation catalyst) to address a number of questions about material selection, device configuration, and operation strategy to achieve emission performance targets. Using spatially-resolved mass spectrometry, this is the first study to provide insight into the spatiotemporal features of transient behavior of sequential PNA, HCT and OC. Conventional measurements of the inlet and outlet values provide only a partial picture of the reactor behavior. The instantaneous profiles provide insight into the storage and reduction dynamics and the mass coupling between the three catalysts.

Materials and Methods

For the spatial-temporal measurements, PNA monolith containing 1 wt. % Pd/CHA, HCT monolith containing 1 wt% Pt/BEA and OC monolith containing (0.5 wt. % Pd + 0.5 wt. % Pt)/CeO₂ (each 2cm in length with 8x8 Channels in a 400 CPSI monolith) were installed in sequence in a quartz tube reactor. All three catalysts were provided by Johnson Matthey Inc. The catalysts were degreened at 600 ^oC for 4 h in 12% O₂ and 6% H₂O before usage. The feed gas mixture contained 400 ppm NO, 12% O₂, 6% H₂O and balance Ar. NO uptake was conducted for 30 min over 100^oC followed by a temperature ramp up to 600 ^oC for 25 min. Outlet Species concentrations were analyzed by an online Thermo Scientific 6700 Nicolet gas analyzer while the transient spatial profiles were obtained via a mass spectrometer (Hiden Analytic Ltd., HPR-20) with multiple capillary sampling system (Hiden Analytic Ltd³). The spatial system comprises of a rail mounted inlet assembly and HPR-20 QIC benchtop gas analysis system. The system is capable of acquiring data from 8 capillaries and 8 thermocouples. Three capillaries probes (OD: 363 and ID 220) and three thermocouples (OD: 250, type K) were employed for this study with three monoliths. Each capillary and thermocouple was positioned 2 cm apart starting at the entrance of PNA catalyst.

Results and Discussion

Figure 1. a - c shows typical performance of each catalyst using a simulated diesel exhaust feed. Common exhaust components C₂H₄/CO have an incremental benefit on NO uptake. Significant formation of NO₂ in a sequential PNA+HCT configuration is beneficial for downstream SCR. The Pt-Pd based OC shows low temperature oxidation performances for CO and hydrocarbons. The Pt-based oxidation catalyst used as a conventional DOC is more prone to CO poisoning, while adding Pd to Pt in the catalytic system induces slower particle sintering and moderating CO inhibition compared to monometallic Pt-based catalyst⁴. Figure 1d shows transient NO concentration at three different positions in the PNA along with the transient effluent NO (measured by FTIR). The spatial profiles provide detail on NO uptake in the PNA. The spatially resolved measurements can help to unravel the complex dynamical behavior that is obviously inaccessible without the spatial probes.

Significance

Profiles spanning the entire PNA+HCT+OC device will be will be further explored for application in the optimum design of multi-functional catalytic system. Such measurements help to optimize the multi-functional device design and operating strategy for achieving high adsorption and conversion of NOx and HCs species during the cold start period. The Spaci Mass Spectrometer method provides a more accurate and comprehensive picture of structured catalysts under operating conditions which will help us in accurately model and scale up the system.

References

1. Zheng, D. Luss, M.P. Harold, SAE Int. J. Engines. 7 (2014).
2. Mukai, K., Kanesaka, H., Akama, H. and Ikeda, T., SAE, 0148, (2004).
3. Sá, D.L.A. Fernandes, F. Aiouache, A. Goguet, C. Hardacre, D. Lundie, et al.,Analyst 135 (2010) 2260–2272.
4. Kang, S.B., Hazlett, M., Balakotaiah, V., Kalamaras, C., Applied Catalysis B: Environmental, 223, 67-75 (2018)

Acknowledgements

We acknowledge the financial support of this research by the Department of Energy (DE- EE0008233). We also thank Johnson Matthey, Inc. for supplying the monolith samples.