Abstract: Titania sols were prepared by acid hydrolysis of a TiCl4 precursor and mixed oxides SiO2TiO2,ZrO2TiO2,WO3TiO3 and MoO3TiO2 as well as Pt/TiO2 catalysts for the photocatalytic oxidation of formaldehyde were obtained by usin g the method of the modification of sol particles.The influence of the additives on the photocatalytic activity was investigated.Comparing with pure TiO2,the ad dition of silica or zirconia increased the photocatalytic activity,while the add ition of Pt and MoO3 decreased the activity,and the addition of WO3 had little e ffect on the activity.Among these catalysts,the activity of the SiO2TiO2 catal yst was the best while that of the MoO3TiO2 catalyst was the worst.It is of gr eat significance that the conversion of formaldehyde was increased up to 94％ ov er the SiO2TiO2 catalyst.The increased activity of the SiO2TiO2 catalyst was due to the higher surface area and porosity,the smaller crystallite size and th e stronger adsorption of UV light.A comparison of our catalyst compositions with those in the literature suggested that the difference in activity due to the ad dition of a second metal oxide maybe caused by the surface chemistry of the cata lysts,particularly the acidity.

Key words: Sol-gel, Photocatalysis, Formaldehyde, MxOy-TiO₂