Kinetics of the platinum catalyzed hydrazoic acid decomposition in acidic media

Abstract

The kinetics and the mechanism of the hydrazoic acid decomposition over the 1% Pt/SiO 2 catalyst in the solutions of perchloric and nitric acids were studied at [HN 3] 0=5×10 −3 to 5×10 −2 M; [H +]=0.5–9 M (HClO 4 or HNO 3); T=44–75 °C and a solid-to-liquid ratio S/L=0.05–0.15 g/ml. It was shown that in 0.5–8 M HClO 4 and 0.5–5 M HNO 3 the hydrazoic acid catalytic decomposition proceeds as a heterogeneous disproportionation of HN 3 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 HN 3 molecules by nitronium ions NO 2 +, existing in the nitric acid solutions in some equilibrium concentration. Mechanisms of the both process are postulated.