Effect of the first row transition metal cations on the mode of decomposition of ammonium nitrate supported on alumina-aluminum phosphate and the final products obtained

Abstract

The mode of decomposition of ammonium nitrate in mixtures with 80% Al 2O 3 and 20% of first row transition metal oxides MOx (M = Mn, Cr, Fe, Co, Ni, Cu, Zn) and containing different amounts of phosphorus (Al:M:P = 100:20:18 and 100:20:144) has been examined under non-isothermal conditions in a thermogravimetric balance. Pure ammonium nitrate is decomposed endothermically while addition of Al 2O 3 shows a stabilizing action of this endo effect. Substitution of 20% of aluminum by a first row transition metal (M = Mn, Cr, Fe, Co, Ni, Cu and Zn) shows that zinc does not alter the route of decomposition while the rest show a tendency to decompose ammonium nitrate exothermically. The catalytic action of the cations toward such an exothermic route is Fe = Ni < Cu < Mn < Co < Cr, the last cations driving the decomposition to explosion. Addition of phosphorus to ratios P/(Al + M) = 0.15 (Al:M:P = 100:20:18) and 1.2 (Al:M:P = 100:20:144) leads to a gradual inhibition of the catalytic action of the transition metal cations towards exothermic explosion. The reasons controlling the catalytic action of the cations for the explosive decomposition of NH 4NO 3 are primarily related to electrochemical Gibbs free energy Δ G = − nFE of the process M oxidized/M reduced. On top of this the crystal field stabilization effect for each particular cation should be taken into account. The extent of exothermic decomposition of the ammonium nitrate seems to influence the specific surface area of the final product in a positive manner in the absence of phosphorus but in a negative manner in its presence. The reasons underlying this effect should be attributed to the stabilizing action of phosphorus on the decomposition route.