Applications of Catalytic Ammonia Chemistry: From Low-Pressure Ammonia Synthesis to the Decomposition of Tritiated Ammonia as a By-Product of Magnetically Confined Nuclear Fusion
Abstract
Ammonia is the 2nd most produced chemical in the world with vital importance for its use in fertilizer as the global population has grown in accordance with the industrialization of NH3. While 80% of NH3 produced goes to fertilizer, it has gained attention recently as a hydrogen storage and transportation vessel. Industrially, NH3 is produced at high temperatures and pressures which in turn require high electrical input. Turning to a green NH3 cycle with low temperature and pressure membrane reactors for both NH3 synthesis and decomposition would allow for much lower energy input as well as a renewable energy to power the membrane reactors and the processes needed for hydrogen separation from water and nitrogen separation from air. However, there is need for efficient catalysts to ensure high rates of low-pressure NH3 synthesis and complete decomposition of NH3 at low temperatures to avoid thermal degradation of the membrane and to minimize poisoning of the membrane with unreacted NH3. We report an investigation into a catalyst discovery for the decomposition of small concentrations of tritiated NH3 as a by-product of magnetically confined nuclear fusion reactions. Previous work from our group discovered a RuYK/Al2O3 that had excellent activity in 100% NH3. However, this catalyst would need to be optimized for <5% NH3 as well as removing the electron withdrawing groups from the formulation. A precursor study was conducted to determine the effect of electron withdrawing groups and make the catalyst abide by the DOE Tritium Handling Standards. A statistical design of experiments was used to determine the optimal weight loadings to completely decompose the tritiated NH3. A 6.9 wt-% Ru, 4.3 wt-% Y and 11.9 wt-% K catalyst was determined to be the optimal formulation for this application at the warranted conditions. The optimized catalyst achieved 85% conversion of 5% NH3/Ar at 250 ºC. Thermal and chemical stability of the catalyst were investigated through thermal cycling and exposure to other primary by-products of nuclear fusion. Additionally, an expansion of the materials discovery work done previously in our group was conducted with the addition of lanthanides as secondary metals. This led to a new catalyst formulation, RuLaK/Al2O3, that reached 52% conversion of pure NH3 at 300 °C which is double the activity from the previous formulation. Next, different support materials were investigated for low pressure NH3 synthesis. Oxides of praseodymium, cerium, and magnesium were synthesized via precipitation method and 5 wt-% Ru was loaded onto the oxide supports using wetness impregnation method. These synthesized catalysts were tested for NH3 synthesis activity in a single channel plug flow reactor where the CeO2 and Pr2O3 showed similar activity. STEM imaging was done to get information about the size of the Ru on the surface of the catalysts. While the CeO2 and MgO catalysts had small nanoparticles, the Pr2O3 catalyst showed to be a single Ru atom catalyst