Hydrazine decomposition over a supported iridium catalyst

Abstract

Adsorption and desorption of hydrazine and its decomposition products (H 2, N 2, NH 3) on an indium alumina catalyst have been studied by temperature programmed desorption and ir spectroscopy in order to determine the mechanism of the decomposition and the nature of the active centers. Hydrogen is chemisorbed on two types of adsorption sites. Activation energies of desorption have been calculated for these sites and they are 10.5 and 25 kcal mole −1. Sites of lower energy are attributed to the existence of an Ir-alumina complex and these of higher energy to bulk iridium. Nitrogen is not adsorbed from the gaseous phase between 25 and 700 °C. Ammonia is adsorbed on metal and alumina at room temperature, its decomposition begins at 200 °C. Desorption spectra, recorded after adsorbing and decomposing hydrazine, show a new desorption peak, which is attributed to nitrogen remaining adsorbed on iridium. It corresponds to a desorption activation energy of 14 kcal mole −1, and it confirms that the first step of decomposition is the formation of a bond between the nitrogen atoms of the hydrazine molecule and the unfilled d orbitals of the metal. Apparently, at 300 °K, the decomposition is carried out according to the reaction 3N 2H 4 → 4NH 3 + N 2. The ir spectra of ammonia and hydrazine adsorbed on the catalyst exhibit a band at 1220 cm −1, besides the adsorption bands characteristic of ammonia adsorbed on alumina; this band could be attributable to ammonia chemisorbed on iridium.