FACTORS FOR MAINTENANCE OF A HIGH-SURFACE-AREA OF SILICA-COATED ALPHA-ALUMINA AFTER HEATING GREATER-THAN 1573-K

In order to maintain a high surface area after heating at > 1573 K, porous aluminas (TM-100 and ALO-4) were coated with a thin layer of silica. Without coating, both the aluminas were transformed completely to the alpha-phase at 1473 K. Coating TM-100 with 5.2 wt.% silica caused no change in the theta-phase at 1573 K, while almost complete transformation to alpha-alumina occurred for 2.7 wt.% silica. For 3.6 wt.% silica, both the theta- and alpha-phases coexisted at 1573 K. As a result of the incomplete phase transformation, the surface area of TM-100 was high, i.e. 46 and 37 M2 g-1 for 5.2 and 3.6 wt.% coating silica, respectively. After they had been heated at 1673 K, all silica-coated TM-100s were transformed to alpha-alumina and mullite was also formed for 3.6 and 5.2 wt.% coating silica. Alpha-Phase TM-100 with 2.7 wt.% coating silica exhibited a surface area of 17 M2 g-1. However, a large amount of coating silica did not lead to the maintenance of a high surface area, probably owing to the formation of mullite. Electron micrographs revealed that, in accordance with a high surface area, silica coating markedly suppressed the crystal growth. Although similar effects of silica coating were also observed for ALO-4, these effects were less pronounced. On heating ALO-4 at 1573 K, the phase transformation was almost completed even for the large amount of coating silica, and surface area was lower than that of TM-100. The highly dispersive character of the starting TM-100 may explain the observed high surface area of the silica-coated alpha-alumina. The difference in the effect of silica coating between TM-100 and ALO-4 may also be ascribed to the difference in their dispersive characteristics.