Investigation Of The Preparation, Characterization And Catalytic Performance Of Iridium Catalysts

Iridium has been usually included in systematic studies on the catalytic application of group Ⅷtransition metals. Because of iridium resources are rare and expensive, it has been investigated as catalyst only for limited reactions: de-NOx for automobile exhaust gases, hydrazine decomposition for rocket thruster and the hydrogenation of unsaturated hydrocarbons. The catalytic decomposition of hydrazine on alumina supported iridium catalysts was developed and applied for the propulsion, attitude and orientation controlof satellites. At present, the Shell-405 catalysts are used for the main catalyst of hydrize decomposition. But the catalyst does not satisfy the development of high technology and the field of high acceleration and wallop. So many people are engaged in the modification research of iridium catalysts. Therefore, the basal research during the preparation of catalysts is very important and necessary. In this paper, the methanol decomposition was used for model reaction. Firstly, the preparation parameter of catalysts including to calcination temperature, calcinations atmosphere and reduction temperature were investigated. Concerned with the TPR, TPD and FT-IR charaterization, the effects of different preparation conditions on the reduction ability and adsorption-desorption performance of catalysts were discussed in detail. It was found that the catalysts calcined with 400 ℃and hydrogen atmosphere for 4 h had higher catalytic activity. The main active specie was zero value state iridium. Comparing with the catalyst calcined under air atmosphere, hydrogen atmosphere could reduce the interaction between iridium and hydrogen or carbon monoxide. Secondly, alumina-supported iridium catalysts were prepared by glow discharge plasma method and with different metal oxide modified alumina. It was found that the dispersion of catalyst samples prepared by plasma higher than that of catalyst samples by conventional impregnation, methanol conversion improved. The amount of hydrogen consumption and desorption were made to quantify by TPR and TPD peak area respectively. The hydrogen spillover was found on iridium-supported catalyst. The hydrogen spillover increase in linearity followed with the MgO content. The plasma technology can improve the hydrogen spillover of iridium catalyst. From FT-IR and XPS characterization, it was also concluded that the plasma technology could improve the surface acdic sites and iridium state. The addition of different promoter could improve remarkably the selectivity of Ir/Al2O3 catalysts for methanol decomposition to carbon monoxide and hydrogen. Both La2O3 and ZrO2 were very good promoter for methanol decomposition. At last, promoting effect of cerium oxide in silica-supported copper-iridium bimetallic catalysts was studies. From the results of the performance of methanol decomposition and TPR, TPD, XRD and XPS characterizations, it was found that the main active phases of catalysts after reduction at 400 ℃for 3 h were Cu19Ir and a small amount of Cu0. Convenient ratio of copper and iridium or convenient cerium oxide content could improve the content of Cu19Ir. Cerium oxide promoter could improve the dispersion, reductivity and satbility of the catalysts. The ceria promoted catalyst (8%CeO2) maintained a conversion of more than 97% after 10h's reaction, it was much more stable than the catalyst without promoter.