Structured Ammonia Carriers for Selective Catalytic Reduction


Air quality has been one of the long-term focuses in society and has raised people’s concern regarding its amelioration 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 in most countries due to their devastating impact on the environment and human health. The transport sector, as the primary source of NOx, therefore, is regulated with ever-evolving NOx emission standards for vehicles. One typical approach to abate NOx from vehicle exhaust is using ammonia (NH3) to reduce NOx and produce environmentally friendly nitrogen (N2) and water (H2O) by selective catalytic reduction (SCR). Conventional urea-based SCR systems using urea as an indirect ammonia source have presented a series of problems, including low conversion efficiency with the lowering of exhaust temperature, freezing of urea solution in low-temperature regions, and emission of carbon dioxide (CO2) as a by-product. Solid SCR systems have emerged as a new direction for NOx reduction (DeNOx) in both industry and research, owing to their high NOx converting efficiency at low exhaust temperatures with direct ammonia dosing. In solid SCR systems, the ammonia storage and delivery unit is a critical part influencing DeNOx performance. The most popular ammonia carriers in solid SCR systems are alkaline earth metal halides (AEMHs), such as MgCl2, CaCl2, and SrCl2. AEMHs face two main challenges as ammonia carriers: (1) low kinetics of ammonia absorption and desorption for urban driving and engine idle scenarios; (2) poor structural stability in terms of thermal melting spread due to heat accumulation and dramatic volume expansion/shrinkage during ammonia absorption-desorption cycles. In this thesis, various physisorbents and chemisorbents, including metal-organic frameworks (MOFs), zeolites, and carbon-reinforced AEMHs, are designed, fabricated, and evaluated as optimized ammonia carriers for solid SCR systems. MOFs [M2(adc)2(dabco)] (M = Co, Ni, Cu, Zn) in this research have demonstrated physisorption of ammonia and superior kinetics of adsorption and desorption compared to MgCl2. Among the synthesized MOFs, Ni2(adc)2(dabco) possessed the highest ammonia uptake capacity, resulting from its high specific surface area. Ni2(adc)2(dabco) released 6 times the mole fraction of ammonia in the first 10 minutes compared to Mg(NH3)6Cl2, indicating that physisorbents can offer a solution to shorten the buffer time for ammonia dosing in SCR. To combine the physisorption of microporous materials with the chemisorption of AEMHs, SrCl2-impregnated zeolite granules as well as three-dimensional (3D) printed zeolite units to carry AEMHs were designed. By optimizing the parameters in the ion exchange and impregnation process, the fabricated SrCl2-impregnated zeolite granules showed two stages of ammonia sorption, including a rapid adsorption stage from the zeolites and an abundant absorption stage from SrCl2. The SrCl2-impregnated zeolite A granules retained 73% of the compressive strength of the pristine CaA granules after ammonia cycles, indicating excellent structural stability of the composite granules. The feasibility of applying 3D printing technology to co-structure AEMHs and zeolites was examined by designing zeolite NaX units to carry MgCl2. A 3D-printed NaX scaffold was successfully fabricated with an optimal formulation of zeolite NaX ink after rheological studies. Carbon materials were selected to form composites with AEMHs, including graphite (Gt), graphene nanoplatelets aggregates (GNA) as additives, and graphene networks as the scaffold. The pelletized carbon-MgCl2 composites containing 20 wt% Gt/GNA presented high structural integrity up to 800 °C above the melting point of MgCl2. Besides, the introduction of nanopores from GNA could promote ammonia diffusion in the MgCl2, resulting in enhanced kinetics of ammonia sorption and desorption. A porous SrCl2 structure scaffolded by graphene networks was fabricated by freeze-casting. The optimized porous SrCl2 with 80 wt% SrCl2 loading maintained its macro- and micro-structure, accommodating the volume swing after 20 ammonia sorption–desorption cycles without disintegration. Furthermore, the porous SrCl2 demonstrated superior kinetics of ammonia sorption and desorption by possessing more surface sites for ammonia adsorption and a shorter diffusion length in the SrCl2 particles. This 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 enhanced structural stability. Potential directions for optimizing the ammonia carriers are suggested, such as combining physisorbents (MOFs, zeolites, etc.) and chemisorbents (AEMHs) in flexible networks and optimizing the volumetric ammonia uptake capacity while maintaining the structural stability of the ammonia carriers.

Doctoral Thesis