Three-way catalyst for air pollution control and Na-zeolite for CO2 capture

Abstract

Individual reductant species in a complicated reaction network have a strong influence on NH3 and N2O emissions over commercially aged three-way catalysts (TWC). The generation of NH3 was maximized under rich circumstances (0.97 ≤ λ < 1.00) at high temperature ranges of 350 to 500 ℃ under simulated complete mixture feed compositions comprising CO, H2, and a hydrocarbon mixture of C3H6 and C3H8 as reductants. At low temperatures between 250 and 350 ℃, the production of N2O was more evident under a stoichiometric condition (λ = 1.00). We conducted basic feed experiments using a single reductant (H2, CO, or C3H6) and NO to assess the contribution of particular reductant species in NH3 and N2O emissions described from full feed circumstances. When H2 is available in the feed gas under stoichiometric conditions, NH3 is mainly generated by the NO-H2 reaction through -NH2 intermediates. The involvement of H2 in the formation of N2O, on the other hand, is primarily limited to low-temperature reactions below 200 ℃. The CO-NO reaction, which forms surface NCO- species at warmer temperatures, contributes the most to N2O formation, however steam reforming of C3H6 can also produce additional surface CO and H2 species. This work also investigated the impact of the Pt-substitution in typical Pd/Rh TWC formulation on the comprehensive catalytic performance. TWC performance was systematically evaluated in a packed-bed reactor under conditions corresponding to different catalytic converter configurations, namely the warmup catalytic converter (WCC) and the under-floor catalytic converter (UCC). The WCC front, WCC rear and UCC configurations were tested under simulated exhaust conditions including fuel-rich, stoichiometric, and fuel-lean (0.99 ≤ λ ≤ 1.01). Pt-substituted TWCs outperformed Pd-based counterparts, regardless of the converter type, in CO, C3H6 and C3H8 oxidation and NO reduction under the exhaust conditions studied. Specifically, NO conversion is clearly increased from 14–77% to 23–87% with the Pt-substituted TWC in the temperature range of 200–400 ℃. The Pt-substituted TWC also demonstrated increased N2 selectivity by 15–75% from suppressing byproduct formation such as N2O and NH3. Moreover, Pt-substituted TWCs exhibited significant durability upon hydrothermal aging at 1050 ℃. After aging, the N2 selectivity over Pd-based TWCs drastically dropped from 70–80% to 15–35%, however Pt-substituted TWCs showed milder loss in N2 selectivity from 80–100% to 60–80%. The key finding from this study is that Pt incorporation in a Pd/Rh TWC is an improved formulation for emissions control from gasoline vehicles with improved selectivity and durability. As a first step for the development of a CCU (Carbon Capture and Utilization) technology catalyst for carbon neutrality, the CO2 capture performance was confirmed using cheap and easily available zeolite. The CO2 capture experiment was performed under 400 ppm conditions, the same as atmospheric CO2 concentration, as a He balance. H-form zeolite did not show any CO2 adsorption performance. The CO2 adsorption performance was improved through Na ion exchange, an alkali metal, and the contents of Na ion exchange and the CO2 adsorption and desorption performance were compared according to the channel frameworks of zeolite and the Si/Al ratio. Although the CO2 adsorption performance was not high, it is sufficient to find the tendency of the CO2 adsorption performance according to the frameworks and the Si/Al ratio. Na-Y with the largest pore size and Na-SSZ-13 with the smallest did not show CO2 physical adsorption. However, SSZ-13 showed the highest amount of CO2 adsorbed per mole of high Na. And Na-ZSM-5 showed the best CO2 adsorption performance, and two chemical adsorption sites were also confirmed through CO2-TPD. In addition, the CO2 adsorption performance of ZSM-5 with different Si/Al ratios was compared. As a result, there was no CO2 adsorption performance of H-form ZSM-5, and it was confirmed that the low Si/Al ratio increased the amount of ion-exchanged Na, contributing to the improvement of the CO2 adsorption performance. These results and data are thought to provide guidelines for improving the CO2 capture performance of zeolite.