Catalytic Decomposition of Ammonium Nitrate in Superheated Aqueous Solutions

• Published in: Journal of the American Chemical Society 1997-10, Vol.119 (41), p.9738-9744
• Main Authors: MacNeil, Joseph H, Zhang, Hai-Tao, Berseth, Polly, Trogler, William C
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja971618k

The decomposition of aqueous ammonium nitrate at elevated temperatures and pressures is examined as a function of chloride, nitrate, and total acidity. Catalysis requiring both chloride and acid was observed in solutions containing 20% (w/w) NH4NO3 at 180 °C. Nitrous oxide and dinitrogen were generated in a 4:1 ratio below 0.2 M H+. Dinitrogen formation correlated with the production of additional acidity by the reaction 5NH4NO3 → 4N2 + 2HNO3 + 9H2O. The second-order dependence of the decomposition reaction on [H+] is consistent with the reversible formation of NO2 +. Incorporation of 18O into the N2O product, as well as the inverse deuterium isotope effect, supports this conclusion. A novel mechanism based on the intermediacy of NO2Cl is proposed for the chloride catalysis and contrasted to the radical-based pathways operational in molten NH4NO3 decompositions. Isotope-labeling experiments using 15NH4NO3 lead to the formation of 15NNO-labeled nitrous oxide and the dinitrogen products 15N⋮15N and N⋮15N in a 1:3 ratio. Decomposition of NH4 15NO3 produces only N15NO and N⋮15N. This agrees with the reaction sequences:  NO2Cl + NH4 + → {O2N−NH3}+ + H+ + Cl-, {O2N−NH3}+ → N2O + H3O+ and 3NO2Cl + 3NH3 → 3NH2Cl + 3NO2 - + 3H+, 3NH2Cl → N2 + NH4 + + 3Cl- + 2H+, 3NO2 - + 3NH4 + → 3N2 + 6H2O. These results bear on the industrial preparation of NH4NO3 and suggest conditions under which nitrous oxide emissions might be important to the global N2O budget.