Effect of thermal treatment on the structure and texture of titania

Anatase and rutile pigments, from two sources (B and T) were thoroughly purified from sulphate and chloride contaminants, thermally treated in the temperature range 150 to 550 ° C, and investigated using thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), nitrogen and organic vapour adsorption techniques. TG analysis reveals two main dehydration steps, the first results from physically adsorbed water and the second from structural and ligand water. The number of ligand water molecules released through a unit surface area (nm²) is in the range 4.50 to 5.15, being evolved in the temperature range 250 to 300 ° C. Two dehydroxylation endotherms appear for the anatase samples in the temperature range 350 to 420 ° C which seem to arise from the presence of two types of hydroxyls. No transformation from the anatase to rutile sructure occurred in the temperature range investigated. Estimation of crystallite sizes showed a marked increase at temperatures >250 ° C for anatase (B), being greatest for the (101) plane, and >400 ° C for rutile (B), where the three planes (101), (110) and (111) increased distinctly. Maximum anisotropy was observed for the anatase heated at 550 ° C. Nitrogen adsorption data revealed a marked decrease in the specific area and total pore volume by thermal treatment 400 ° C for anatase and 250 ° C for rutile whereby it retains a nearly stable value with an average range of 2.62 nm. The anatase (B) samples are composed of a mixture of both meso- and micropores whereas for rutile (B) microporosity appeared only for the sample heated at 150 ° C, becoming predominantly mesoporous at higher temperatures. Most heated samples exhibited two group sizes in the mesopore range resulting from their existence in the form of particles constituted from a collection of small particulates. The rutile (B) products are generally characterized by possessing a wider pore system than those from anatase. The anatase (T) samples are predominantly microporous at 150 ° C and become mesoporous at 550 ° C — the reverse is true for rutile (T). Cyclohexane and benzene adsorption measure only a fraction of the nitrogen area. Specific interaction (H-bonding) is believed to exist in some cases between the cyclohexane molecules and titania surface hydroxyls, as well as some enhanced adsorption which is believed to occur with benzene adsorbate.