Monopropellant decomposition catalysts: II. Sintering studies on Ir/Al2O3 catalysts, influence of chloride anions Original Research Article

Supported Ir/Al2O3 catalysts with low (1 wt.% Ir) and high load contents (29 wt.% Ir) have been subjected to sintering through water vapor treatment at different temperatures. They were characterized by iridium dispersion measurements, X-rays diffraction and transmission electron microscopy. The sintering process takes place at temperatures higher than 450°C and the surface chloride ions play an important role by accelerating the growing rate of the iridium particles. The chloride ions allow the formation of iridium oxychloride or hydroxychloride species in presence of steam, leading to a particle growing through migration of molecular species. For dechlorinated or chlorideless samples, the rate of this process is lowered and the sintering can be due to migration of iridium crystallites. The final dispersion and particle distribution size depend only on the treatment temperature, whatever the type of alumina, the initial dispersion, the content of chloride or the loading level of iridium.Two ways permit to prevent the sintering: (i) the presence of hydrogen in gas flow which stabilizes the metal particles and (ii) the presence of hydrogen chloride which blocks the growing process. A way is proposed to stabilize the catalyst against ageing effects : removal of the chloride ions by treatment under a gas flow containing water vapor and hydrogen.