Ammonia: a review of mechanistic models of ammonia volatilization and theoretical studies of the catalytic reduction of dinitrogen to ammonia by a boron cation, a boron dihydride cation, and beryllium dihydride
AuthorBell, Cary JaneAnne
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In order to maintain the food production necessary for sustaining the world's ever increasing population, millions of tons of ammonia fertilizer are produced each year. Studies have been performed to investigate how ammonia fertilizers can be used efficiently and how they can be made more efficiently. A significant amount of nitrogen fertilizer applied to crops is lost through ammonia volatilization. Many investigations have been implemented to study the factors and processes controlling ammonia volatilization including the models that are reviewed. At the core of these models is the calculation of the rate of ammonia volatilization, which is a function of a mass transfer coefficient and the difference between the concentration of gaseous ammonia at the slurry/soil surface and that of the free air, which is input. In general, the mass transfer coefficient equation is a function of friction wind velocity, aerodynamic roughness length, the distance the ammonia is estimated to travel, and in the case of the advection model method, the length of the field. The concentration of gaseous ammonia at the slurry/soil surface is a function of the equilibrium constant, Henry's Law constant, pH, concentration of total ammoniacal nitrogen (ammonia and ammonium), time, depth of ammonia infiltration into the soil, soil water content, porosity, and bulk density, and is dependent on physical and chemical processes such as adsorption, convection, and diffusion. In industrial ammonia synthesis, high temperatures and pressures are required to cleave the bond between the nitrogen atoms of dinitrogen prior to hydrogenation. This study theoretically investigates catalytic reductions of dinitrogen to ammonia by B+, BH2+, Be, and BeH2, in which dinitrogen cleavage is avoided through sequential hydrogenation of dinitrogen. The stationary points and transition states of these catalytic systems are located and characterized at the MP2 and CCSD(T) levels of theory with 6-31g* and double and triple zeta basis sets. Each catalyzed reduction of a nitrogen species (dinitrogen, substituted diimide and hydrazine) occurs by hydrogenating one nitrogen atom and then the other. When dihydrogen is restricted, B+ is the most effective in activating dinitrogen. However, BeH2 is calculated to be the most efficient catalyst.