Structured Ammonia Carriers for Selective Catalytic Reduction

Abstract: Air quality has been one of the long-term focuses in the society and raising people’s concern to ameliorate it in the post coronavirus disease 2019 (COVID-19) pandemic era. Nitrogen oxides (NOx) including NO and NO2, as one of the most harmful air pollutants, have been stringently monitored on their concentration in most countries due to their devastating impact on the environment and human health. Transport as the main source of NOx, therefore, was regulated with ever-evolving NOx emission standards for the vehicles. One of the most common approaches to abate NOx from the vehicle exhaust is using ammonia (NH3) to reduce NOx and produce environmental-friendly nitrogen (N2) and water (H2O) by selective catalytic reduction (SCR). Conventional urea SCR systems using urea as an indirect ammonia source have presented a series of problems, including low conversion efficiency with exhaust cooling trend, freezing of urea solution in low-temperature regions, emission of CO2 as a byproduct, etc.Solid SCR systems have emerged as a new direction on NOx reduction (deNOx) in both industry and research owing to their high NOx converting efficiency at low exhaust temperature with a direct ammonia dosing. In solid SCR systems, the ammonia storage and delivery unit is one of the most key parts influencing the deNOx performance. The most popular ammonia carriers in current solid SCR systems are alkaline earth metal halides (AEMHs), such as MgCl2, CaCl2, SrCl2. AEMHs demonstrate two shortcomings as ammonia carriers, including (1) low kinetics in ammonia absorption and desorption for urban driving and engine idle scenarios; (2) poor structural stability in terms of thermal melting spread due to the heat accumulation, and the dramatic volume expansion and shrinkage during ammonia absorption-desorption cycles.In this thesis, various materials including metal-organic frameworks (MOFs), zeolites, and carbon-reinforced AEMHs, were designed, fabricated, and evaluated as optimized ammonia carries for solid SCR systems.The MOFs [M2(adc)2(dabco)](M = Co, Ni, Cu, Zn) synthesized in this research have demonstrated superior kinetics in ammonia adsorption and desorption compared to MgCl2. Among the synthesized MOFs, Ni2(adc)2(dabco) possessed the highest ammonia uptake capacity of 12.1 mmol g−1, resulting from its high surface area, 772 m2 g−1. Ni2(adc)2(dabco) presented six times ammonia dosing (6 mmol g−1) in the first 10 min compared to the Mg(NH3)6Cl2, indicating that physisorbents can offer a solution to shorten the buffer time for ammonia dosing in SCR. To combine the physisorption from porous materials and chemisorption from AEMHs, AEMHs-impregnated zeolite granules and three-dimensional (3D) printed zeolite- AEMHs units were designed. By optimizing the parameters in the ion-exchange and impregnation process, the fabricated AEMHs-impregnated zeolite granules, offered 2 stages of ammonia sorption, including a rapid adsorption stage from the zeolites and an abundant absorption stage from AEMHs. The SrCl2-impregnated zeolite A granules retained 73% compressive strength of the raw CaA granules after ammonia cycles, indicating a promising method of structuring the AEMHs with zeolite granules. The feasibility of applying 3D printing technology to co-structure AEMHs and zeolites was examined by designing a zeolite NaX-MgCl2 unit.  A 3D-printed NaX scaffold was successfully fabricated with an optimal formulation of the zeolite NaX ink after rheological studies.Carbon materials, including graphite (Gt), graphene nanoplatelets aggregates (GNA), and graphene networks were selected to enhance AEMHs as additives and scaffold, respectively. The pelletized carbon-MgCl2 composites containing 20 wt% Gt/GNA presented high structural integrity up to 800 ◦C, which was above the melting point of MgCl2.  Furthermore, the nanopores from GNA could promote the ammonia diffusion in the MgCl2, resulting in an enhancement in the kinetics of ammonia absorption and desorption. A porous SrCl2 structure scaffolded by graphene networks was fabricated by the freeze-casting process.  The optimized porous SrCl2 with 80 wt% SrCl2 loading maintained its macro- and micro-structure accommodating the volume swing after 20 ammonia absorption–desorption cycles without disintegration. Moreover, the porous SrCl2 demonstrated superior kinetics in ammonia absorption and desorption by having more surface adsorption sites and shorter diffusion length. The structuring approach was verified with other AEMHs, including MgCl2 and CaCl2.The results from this thesis offer several solutions to structure AEMHs and their composites as ammonia carriers for SCR, with rapid kinetics and structural stability. The potential directions of optimizing the ammonia carriers are suggested, such as combining physisorption and chemisorption in flexible networks and enhancing the volumetric ammonia uptake capacity in a macroporous structure.