Nanoparticulate TiO2-Al2O3 Photocatalytic Media: Effect of Particle Size and Polymorphism on Photocatalytic Activity

Impregnation of size-selected titanium-oxo-alkoxy nanoparticles into monolithic ultraporous alumina (UPA) permits nanoparticulate photocatalytic media with an extended activity into the high-temperature range up to 1000 degrees C, which is explained by anatase TiO2 phase stability due to the inhibited nanoparticles aggregation. In this Article we report on the effect of the nanoparticle polymorphism and size on the photocatalytic ethylene gas decomposition. Use of UPA supports of gamma, theta, and alpha polymorphs, covered with silica or not, and thermal treatment at different temperatures allow modification of the nanoparticles size and crystalline composition. In all cases, the interaction between titania and UPA support is found to affect the polymorph stability. In particular, a separating layer of silica increases the temperature of the anatase-rutile transformation. The main conclusion is that anatase nanoparticles exhibit the higher activity compared to rutile and composite anatase/rutile nanoparticles. The rutile activity strongly decreases with size 2R >= 5 nm, while that of anatase nanoparticles does not appreciably change for sizes 5 nm <= 2R <= 10 nm. The material activity strongly decreases when rutile phase is nucleated onto the anatase one.