Ammonia Synthesis over Complex Metal Hydrides: A Lab Scale Study

Ammonia synthesis is the process where N₂ and H₂ are reacted to produce NH₃, a key chemical for fertilizer production. The industrial process uses iron based catalysts and requires high temperatures. The objective of this thesis is to investigate complex transition metal hydrides as low temperature catalysts for ammonia synthesis, with respect to reaction kinetics and stability. In order to synthesize the complex transition hydrides and carry out kinetic studies, a high pressure lab scale experimental set up was designed and constructed. Three complex metal hydrides (Li₄RuH₆, LiMg₂RuH₇ and Mg₂FeH₆) were synthesized and characterized by using techniques including XRD, BET, TEM imaging and Raman spectroscopy. Catalytic activity tests were conducted in a differential plug flow reactor and kinetic parameters were obtained. The activation energies for Li₄RuH₆, LiMg₂RuH₇ and Mg₂FeH₆ were found to be 82 kJ/mol, 88 kJ/mol and 114 kJ/mol respectively for 3:1:2 (H₂:N₂:Ar) stoichiometric ratio. Lowest temperatures at which ammonia synthesis was detected were 250 °C for Li₄RuH₆, 240 °C for LiMg₂RuH₇ and 350 °C for Mg₂FeH₆. LiMg₂RuH₇ showed the highest ammonia synthesis rate at 1 bar, 3:1:2 (H₂:N₂:Ar) stoichiometric ratio compared to the other catalysts tested in this study, including a commercially bought Ru/Al₂O₃ used for benchmark. A reaction rate of 100 μmol/gcat · h at 240 °C was measured over LiMg₂RuH₇. The study also revealed that the ternary hydrides are not stable during reaction conditions, and decompose into two phases, where the two phase composition is also active. Spent Li₄RuH₆ sample was found to be consisting of Ru and LiOH, whereas the activated material resulting from LiMg₂RuH₇ was found to be consisting of LiMgN and Ru. Initial tests showed promising reaction rates over the latter and requires further investigations to determine the roles of each phase.