Aging of Ir/Al2O3 catalysts for the decomposition of hydrazine

Abstract

Alumina supported iridium catalysts used in satellite thrusters are prepared several years before satellite launch and the aging of such catalysts is an important parameter for the lifetime of the satellite. The aging has been followed using two procedures and different characterization methods : metal dispersion through hydrogen chemisorption followed by oxygen and hydrogen titration's, powder X-rays diffraction, transmission electron microscopy and catalytic activity in decomposition of ammonia. Long lasting storage's during 6 months or one year under static argon, air or hydrogen atmosphere lead to the same slight decrease of the metallic dispersion. After speeded aging under 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 formation of oxichloride surface species. The catalytic activity discloses a clear difference between the aging treatments concerning the modifications of the active centers. Oxygen leads to an increase of activity whereas steam treatment displays the opposite behavior. The removal of surface chloride ions stabilizes the catalyst against steam aging effects and manifests no modification of dispersion, size distribution and catalytic activity.