Platinum supported on doped alumina catalysts for propulsion applications. Xerogels versus aerogels

Different supports and catalysts have been prepared for the decomposition of aqueous 79 wt% HAN solutions (HAN = hydroxylammonium nitrate NH ₃OHNO ₃ ). The supports (silica-doped alumina) were prepared by sol–gel synthesis using subcritical drying for xerogel samples or CO ₂ supercritical drying for aerogel samples, then calcined at 1200 °C for 5 h. The active phase was introduced by impregnation from aqueous platinum precursor solutions or by one-step addition of the precursor before the sol formation. Characterization (X-ray diffraction, BET surface area, transmission electron microscopy) and evaluation of the catalysts (thermal analysis apparatus, constant volume reactor) were performed. Aerogels present much better thermal stability at high temperature and are more homogeneously dispersed than xerogels. Nevertheless, aerogels lead to less efficient catalysts for the decomposition of HAN solutions at low temperatures. The one-step procedure leads to an increase of the thermal stability for the aerogel samples. The impregnated catalysts always display the best activity, which correlates with the smaller Pt crystallite size that they support. The possibility to decompose aqueous HAN solutions catalytically at low temperatures (<40 °C) with a short ignition delay (less than 1 s) has been demonstrated.