Transition metal catalysis of the heterogeneous decomposition of hydrazine: adiabatic kinetics by accelerating rate calorimetry

Abstract

The thermochemical kinetics of the transition-metal-catalyzed decomposition reaction of hydrazine have been studied using accelerating rate calorimetry. The reaction stoichiometry was determined by both product analysis and thermochemical balance. In the range 350–515 K when both liquid and vapor hydrazine are present, the decomposition proceeds according to: N 2H 4(1)→ 1 3 N 2(g)+ 4 3 NH 3(g) with Δ H rxn = −123.3 kJ mol −1 (515 K). In the range 515–590 K when only vapor is present in the reaction system, Δ H rxn = −154.8 kJ mol −1 (515 K). The reaction shows apparent zero-order kinetics and the rates were found to be proportional to the surface area of added metals in the form of powders or foils. Rates and activation parameters are reported for ten transition metals and the Fe : Ni : Cr alloy, 304L stainless steel. The activation energies and relative specific activities at 353 K are: Co (43 kJ mol −1, 3900); Ni (93 kJ mol −1, 310); Mo (76 kJ mol −1, 94); V (85 kJ mol −1, 66); Fe (119 kJ mol −1, 33); W (68 kJ mol −1, 11); Au (74 kJ mol −1, 7.8); Ti (95 kJ mol −1, 1.0); 304L (100 kJ mol −1, 0.43); Cr (105 kJ mol −1, 0.099); Ta (106 kJ mol −1, 0.06). The Arrhenius activation parameters for the various metals (and different samples of the same metal) show a linear compensation effect: In A = (1/θ) E a/ R + 1n k o with a compensation temperature, θ, of 445 K. Different samples of molybdenum powders showed θ = 432 K and an isokinetic temperature of 430 K, at which the rates for all molybdenum samples were equal. The relative activities are explained in terms of the binding energies of N-adatoms to the metal surface and an empirical thermochemical relationship is proposed. Additionally it is noted that the least catalytically reactive metals are those whose ambient-atmosphere oxide films are not reduced by hydrazine.