Structured Carbon-Alkaline Earth Metal Halides Composites for Ammonia Storage

Abstract: NOx (NO, NO2) is one of the most harmful air-pollutants from exhaust, resulting in series of environmental problems as well as severe healthy issues for human beings. Selective catalytic reduction (SCR) system is a common approach to eliminate NOx onboard by using ammonia as a reductant. However, ammonia storage unit has been one of the restriction factors for the NOx conversion efficiency because of insufficient ammonia dosing rate and the corrosive and hazardous nature of ammonia. Thus, a reliable ammonia storage and delivery system is of high scientific and commercial desire.In this thesis, novel composites were fabricated and studied based on MgCl2 and SrCl2, two commercial alkaline earth metal halides (AEMH) for ammonia storage. In order to reduce the melting issue and enhance the kinetics of the ammonia sorption, carbon materials, graphite (Gt) and graphene nanoplatelets aggregates (GNA) were added to MgCl2 at 1 wt.%, 10 wt.% and 20 wt.%. With ball milling and hydraulic pressing, the aforementioned carbon-MgCl2 composites were structured into pellets for various characterization. With real-time recording in the tube furnace at 1073 K, we observed that with 20% carbon additives, the pelletized composites maintained their structure with 95% mass retention, while the pure MgCl2 completely melted and disintegrated. According to the SEM images, carbon materials separated MgCl2 so that the molten MgCl2 cannot form large droplet to spread out. Furthermore, the 20 wt.% GNA-80 wt.% MgCl2 (GNA20) composites demonstrated enhanced kinetics in both absorption and desorption of ammonia, which is 83% faster in ammonia absorption and 73% faster in desorption in the first two minutes compared to the pure MgCl2. The BET surface area and mercury intrusion porosimetry results explains the kinetic elevation by the GNA by introducing extra reaction surface and nanopores as the diffusion path for ammonia. The enhancement of both structural stabilityand kinetics make the GNA20 composite a robust ammonia carrier.During the chemical absorption process, SrCl2 uptakes 8 ammonia molecules resulting in 4 times volume expansion. This dramatic expansion and shrinkage during the absorption and desorption will destroy the structure and disintegrate the SrCl2 into powder, which could bring the dust explosion risk for many applications. Based on the carbon-salts composites, a novel porous SrCl2 structure is designed and fabricated with graphene oxide as skeleton by freeze casting process. Porous SrCl2 structure is feasible for various geometries with different molds at a wide SrCl2 load from 0 wt.% to 96 wt.%. The ammonia capacity of the porous SrCl2 is linear proportional to the SrCl2 load. During the ammonia absorption and desorption cycles, the graphene oxide skeleton could self-adjust along with the volume swing to within its flexibility. This porous SrCl2 demonstrates excellent tolerance of volume swing and enhanced kinetics as a promising ammonia storage material. Our approach and results may cast light on the obstacles of structuring self-expansion and shrinkage materials as well as on enhancing the gas sorption properties.