Monopropellant decomposition catalysts. I. Ageing of highly loaded Ir/Al2O3 catalysts in oxygen and steam. Influence of chloride content

Alumina supported iridium catalysts used in satellite thrusters are prepare d several years before satellite launch and the ageing of such catalysts is an important parameter for the lifetime of the satellite. The ageing has b een studied using two procedures and different characterization methods: me tal dispersion through hydrogen chemisorption followed by oxygen and hydrog en titrations, powder X-ray diffraction, transmission electron microscopy a nd catalytic activity in ammonia decomposition. Long term storage during si x months and one year under static argon, air or hydrogen atmosphere led to the same slight decrease of the metallic dispersion. After accelerated age ing using dynamic oxygen or water vapor flow, the samples display the same dispersion drop and the same evolution of the crystallite size distribution with a drop of the smallest crystallites due to sintering effects or to fo rmation of oxichloride surface species. The catalytic activity discloses a difference between the ageing treatments: the treatment with oxygen leads t o an increase of the measured activity while a treatment with water vapor g ives the opposite behavior. The removal of surface chloride ions stabilizes the catalyst against water vapor ageing effects without any modification o f dispersion, size distribution and catalytic activity. (C) 1999 Elsevier S cience B.V. All rights reserved.