The Effects of Long Term Sulfur Exposure on Three Way Catalyst Performance in Passive Selective Catalytic Reduction Systems
Calvin Thomas, University of South Carolina
Conventional gasoline-powered engines operate with a stoichiometric air-fuel ratio (AFR), but greater fuel economy is achieved in lean burn engines by increasing the AFR . However, a traditional three-way catalyst (TWC) system is unable to fully reduce nitrogen oxide (NOX) emissions in the oxidizing exhaust of these engines. Passive selective catalytic reduction (SCR) is a promising approach for the control of NOX emissions in lean burn systems. By periodically decreasing the AFR and operating fuel-rich, ammonia (NH3) can be produced over a TWC and stored on a downstream SCR catalyst. Then, during lean operation, stored NH3 is used to reduce the lean NOX emissions. Previous work conducted by Prikhodko et al. incorporated both TWC and SCR catalysts into a lean gasoline engine system, showing a projected fuel efficiency benefit of up to 10%, illustrating the viability of Passive SCR . This work is focused on the long term viability of the system, examining the effects of hydrothermal aging and realistic concentrations of sulfur dioxide (SO2) on the performance of the TWC in a passive SCR system.
Materials and Methods
Two commercially formulated catalysts were studied: A Pd-only TWC, and a NOX storage TWC (NS-TWC) catalyst, containing Ce for oxygen storage capacity (OSC) and Ba for NOX storage capacity (NSC). These catalysts were hydrothermally aged for 100 hours at 900°C prior to sulfation. During sulfation and evaluation, the catalysts are exposed to constant cycling between λ values of 0.97 (rich) and 2.0 (lean), where λ is a measure of AFR normalized to stoichiometric such that λ > 1 is fuel lean, and λ < 1 is fuel rich. This cycling is controlled by monitoring the outlet NOX in the lean phase and the outlet NH3 in the rich phase, and switching the phase when the integrated effluent reaches a threshold value. During both sulfation and evaluation of the catalyst the outlet concentrations of key gases are measured using a combination of gas-phase FTIR and mass spectrometry. The nitrogen species NOX, NH3, and nitrous oxide (N2O) are monitored, while the reductants hydrogen (H2), carbon monoxide (CO), and propane (C3H8) are also measured.
The catalysts are evaluated in the temperature range 350°C – 650°C in simulated exhaust conditions. After exposure to 2ppm SO2 for 12.5 hours, another evaluation is conducted. Catalysts were evaluated a final time after desulfation at 650°C. Individual reactions were investigated to determine more clearly determine the deactivation of specific reaction pathways in the simulated exhaust experiments. Variation of the rich phase AFR was also investigated as a potential route for mitigation of the effect of sulfur. The rich phase λ was varied in the range 0.92 – 0.98 with 2ppm SO2 in the stream at both low temperature (350°C) and high temperature (650°C).
Results and Discussion
Sulfation caused an increase in outlet C3H8, CO, and N2O, while the outlet H2 and the production of NH3 were both decreased. In each case, the initial catalyst performance was recovered through operation at 650°C for 3 hours. The decrease in H2 could indicate a deactivation of water gas shift (WGS) or steam reformation reactions, which can play a large part in the production of NH3 . The reaction probe experiments provided evidence that when H2 is used as a reductant, production of NH3 is not significantly affected. However, when CO or C3H8 is used, the production of NH3 was deactivated. Furthermore, both WGS and steam reforming were found to be significantly deactivated by sulfation. However, despite low H2 production from WGS at 350°C, NH3 production with CO as a reductant remained high, indicating that it may be formed through an alternative reaction pathway. Finally, decreasing the rich phase λ was shown to be effective in increasing the production of NH3, shortening time spent in the rich phase. However, this decrease would also lead to an increase in fuel consumption during rich operation.
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