Nanocomposites and nanomaterials

Acidic-basic properties of the commercial TiO2 powders
Kutuzova A.S.1, Dontsova T.A.1
1 Department of Technology of Inorganic Substances, Water Purification and General Chemical Technology, National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”. Prospect Peremohy, 37, Kyiv-03056, Ukraine. 
E-mail: kutuzova.anasta@gmail.com

Commercially available TiO2 powders (P25 TiO2 AEROXIDE® and P90 TiO2 AEROXIDE®, Evonik, Germany) are widely used in photocatalytic activity studies as a reference of synthesized TiO2. It is well known that photoactivity of the material depends on its sorption properties. The latter, in turn, can be explained by acidic-basic properties of the material surface. So, the aim of the study was to evaluate the amount and nature of the active sites by Hammet indicator method as described in [1].
Hammet indicator method of assessing acidity is based on the selective adsorption of acid-base indicators with certain pKa of the acid-base transition and further spectrophotometric determination of the optical density change of solutions. According to the obtained results, commercial TiO2 samples Р25 and Р90 have almost the same amount of active sites: 16.8 µmol/g and 15.7 µmol/g at pKa -0.29 respectively. However, Brønsted basic sites prevail in Р25 sample (8.2 µmol/g at pKa 8.8), while in P90 sample Brønsted acidic sites (4.8 µmol/g at pKa 5.25) predominate. However, P25 sample has more active sites than P90 sample.
Thus, commercial samples, which have both acidic and basic sites of Brønsted type, are simultaneously active regarding both cationic and anionic dyes. In addition, sorption and photocatalytic properties of P25 and P90 powders are in a good agreement with their acidic-basic properties. Sample P25, containing a larger amount of basic Brønsted sites than P90 sample, is more active regarding methylene blue (cationic dye). Sample P90, in turn, has a larger number of acidic Brønsted sites and therefore is more active regarding congo red (anionic dye) compared to P25 sample.

1.	Dontsova T. A., Yanushevskaya E. I., Nahirniak S. V., Makarchuk O. V., Ivanets A. I., Roshchina M. Y., Kutuzova A. S, Kulikov L. M. Directional Control of the Structural Adsorption Properties of Clays by Magnetite Modification // J. Nanomater.-2018.-2018.-P. 1-9.