We report on the preparation of a new photocatalyst by impregnation of size-selected TiO2 nanoparticles into monolithic ultraporous alumina of theta and alpha polymorphs. The alumina matrix is prepared by selective surface oxidation of liquid Hg-alloys in a humid atmosphere at room temperature with subsequent annealing at 1100-1300 degrees C. An additional treatment with trimethylethoxysilane was used to mechanically reinforce the ultraporous structure. Monodispersed titanium-oxo-alkoxy nanoparticles were generated in a sol-gel reactor with rapid micromixing, then deposited by liquid colloid impregnation and thermally treated above 350 degrees C to form anatase TiO2. High-resolution TEM images show evidence of non-aggregated 5-nm nanoparticles in theta-alumina matrix at a mass loading of 26 wt.%, which conserve their anatase crystalline structure after the thermal treatment at temperatures as high as 1000 degrees C. The rutile phase appears only at T >= 1100 degrees C when particle size increases to similar to 17 nm due to the aggregation. The photocatalytic activity (ethylene decomposition) of the non-aggregated nanoparticles is conserved over a wide temperature range, and it decreases as soon as the aggregation takes place. (C) 2011 Elsevier B.V. All rights reserved.