Preparation and Properties of the Thermostable Alumina Mixed Oxides for Combustion Catalysts

Abstract

Various alumina mixed oxides were prepared intending to obtain thermostable supports for catalyst. And the influences of preparation procedures, added components, their added amounts and raw materials on sintering of the mixed oxides were investigated by measuring their specific surface areas, X-ray diffractions, thermal gravimetric and differential thermal analyses.
The alumina mixed oxides prepared from alkoxides by a chemical mixing procedure showed higher specific surface areas than the corresponding ones prepared from -the same alkoxides by a coprecipitation procedure, when their calcination temperatures were less than 1200°C. But these mixed oxides prepared by the above procedures showed almost the same va lues in their specific surface areas one another, being calcined above 1200°C. When SrO, BaO, La₂O₃ and ZrO₂ were added into Al₂O₃ with 10 wt% proportion by the chemical mixing procedure, sintering of the mixed oxides was found to be depressed and the -Mixed oxides substantially showed higher specific surface areas than alumina alone. On the contrary, additions of MgO, CaO, and CeO₂ into Al₂O₃ with 10 wt% proportion accelerated sintering of the mixed oxides and lowered their specific surface areas. The alumina mixed oxides prepared from alkoxides generally showed higher specific surface areas and kept their amorphisms by higher temperatures than the corresponding ones prepared from nitrates. Among the additives used, BaO was the most effective for depressing the sintering of the alumina mixed oxides and the decrease of their specific surface areas. But, the composition of BaO/Al₂O₃ mixed oxides showing the highest specific surface areas was not constant at all calcination temperatures used but changed in the range of 10-20 wt% BaO according to the calcination temperatures. Both 10 wt% BaO/Al₂O₃ mixed oxides calcined for 100 h at 1000°C and 3 h at 1200°C were amorphous, and showed high surface areas of 139 m²/g and 96 m²/g, respectively.