Heterogeneous decomposition of hydrazine on tungsten films

Abstract

The decomposition, in the temperature range 195–455 K, of hydrazine chemisorbed on tungsten films at 195 K has been studied under isothermal conditions, and also when the temperature of decomposition was increased linearly with time. Decomposition occurs by two main processes: (i) by dissociative chemisorption to form adsorbed amide radicals, followed by further surface dissociation, thereby producing adsorbed imide radicals (or even nitrogen adatoms) and mobile hydrogen adatoms; these latter combine and subsequently desorb as hydrogen molecules; (ii) by associative chemisorption of the hydrazine molecule to surface tungsten atoms; the activated surface complex so formed breaks down to give ammonia and nitrogen, largely by the reaction: 3N₂H₄→ 4NH₃+ N₂. A study of the decomposition on tungsten films (i) contaminated by the products of a previous decomposition; (ii) presaturated with hydrogen; (iii) partially saturated by both presorption and postsorption of either carbon monoxide or ammonia, indicated that the molecular adsorption takes place on the (110) plane and that the other planes are active for the dissociative adsorption. On the (110) plane, the hydrazine molecule appears to be attached to a single tungsten atom; on the other planes, it occupies four adjacent surface sites. This dual mechanism is consistent with previous results on the heterogeneous decomposition of hydrazine on a wide variety of different surfaces.