Kinetics of the platinum catalyzed hydrazoic acid decomposition in acidic media

The kinetics and the mechanism of the hydrazoic acid decomposition over the 1 % Pt/SiO2 catalyst in the solutions of perchloric and nitric acids were studied at [HN3](0) = 5 x 10(-3) to 5 x 10(-2) M; [H+] = 0.5-9 M (HClO4 or HNO3); T = 44-75 degrees and a solid-to-liquid ratio S/L = 0.05-0.15 g/ml. It was shown that in 0.5-8 M HClO4 and 0.5-5 M HNO3 the hydrazoic acid catalytic decomposition proceeds as a heterogeneous disproportionation of HN3 molecules with a formation of the ammonium ions and the gaseous nitrogen as reaction products. The reaction deceleration and the increase of the apparent activation energy with the increase of the solution acidity is explained by the competitive adsorption of the hydrogen ions on active catalyst centers. In the concentrated (>6 M) nitric acid, the decomposition of hydrazoic acid proceeds due to a catalytic oxidation of adsorbed HN3 molecules by nitronium ions NO2+ existing in the nitric acid solutions in some equilibrium concentration. Mechanisms of the both process are postulated. (C) 2003 Elsevier B.V. All rights reserved.