Effect of niobium addition in support catalysts applied in satellite propulsion

Catalysts composed of iridium as the active phase dispersed in aluminum oxide (Ir/Al2O3) are used in propulsion systems that employ hydrazine as monopropellant in the control of satellite orbit and attitude. The aluminum oxide (Al2O3) utilized as support must present high values of specific surface area, pore volume, and crush strength. The niobium effect was evaluated in this work, in its oxide form (Nb2O5), by 3 different methods: with the employment of a NbCl5 precursor solution, by wet impregnation and dry impregnation of an alumina obtained from a mixture of gibbsite and boehmite and by physical mixing of gibbsite and hydrated niobium oxide, both autoclaved separately. Aluminum oxides were prepared in both cases containing Nb2O5 contents of 10, 20, and 30% w/w. The acid impregnating NbCl5 solution in the wet impregnation method caused a strong attack to the Al2O3 support, altering and compromising its initial structure and morphology. This process did not occur in the supports prepared by dry impregnation. However, results indicated that the use of this methodology with Nb2O5 contents of 20% and 30%, caused an extensive coverage of the support by Nb2O5, modifying the nature and amount of alumina sites responsible for anchorage of the iridium precursor. In the case of supports prepared through physical mixture (Nb2O5-Al2O3) from aluminum hydroxide and niobium acid precursor compounds, with both being previously autoclaved separately, the 20% and 30% Nb2O5 contents presented the most promising properties, since the binder effect caused by amorphous Nb2O5 increased the crush strength of the support, without compromising the aluminum oxide morphology and texture. Despite of existence of stronger acid sites due to the addition of niobium oxide to aluminum oxide, no increase in the acidity of the materials was observed due to the presence of niobium oxide. This can be attributed to the high calcination temperature used, as well as, to the procedures used for the preparation of such materials.