Preparation of the photocatalysts based on TiO2 using titynylsulphate

Abstract

This work focuses on preparation of photocatalytic TiO2 and its doped forms from cheap and industrially used precursor titanyl sulphate. TiO2 and its doped forms were synthetized using thermal hydrolysis method. In order to increase the photocatalytic performance of the TiO2 product it was doped with zirconium and synthetized mixed oxides were crystallized using pressurized hot (subcritical) water. Three sets of composites/oxides were synthetized and processed. The first set consisted of five ZrxTi1-xOn mixed oxides with the wide range of Zr loading (the Zr content ranged from 10 to 90 mol.% of Zr) and parent oxides (TiO2 and ZrO2). The second set consisted of eleven Zr0.1Ti0.9On mixed oxides which served for investigation of the effect of various processing conditions of pressurized hot (subcritical) water crystallization on the mixed oxides photoactivity. The third set consisted of five Zr/TiO2 oxides with Zr doping ranging from 0.5 to 7.5 mol.% of Zr. Prepared materials were characterized by selected complementary characterization methods to reveal the relation between physico-chemical properties of composites/oxides and their photocatalytic activity. The selected characterization methods were: X-ray fluorescence spectrometry (XRF), nitrogen physisorption, UV-vis diffuse reflectance spectroscopy (DRS), X-ray powder diffraction (XRD), photoelectrochemical measurement, photoluminescence spectroscopy (PL) and transmission electron microscopy analysis (TEM). The photocatalytic activity was determined by the decomposition of model azo-dye Acid orange 7. To increase the photocatalytic activity the zirconium content and crystallization conditions were optimized. The first set was investigated for the optimal Zr content (the Zr content ranged from 10 to 90 mol.% of Zr) and was tested whether the application of pressurized hot (subcritical) water crystallization is viable. The crystallographic phase composition was determined to be the main factor influencing the photocatalytic activity of mixed oxides. The optimal Zr content in ZrxTi1-xOn mixed oxide was found to be 10 mol.% of Zr. The second set served for the optimization of conditions of the pressurized hot (subcritical) water crystallization. Three key processing parameters were selected and optimized: temperature, pressure and the volume of water used as a crystallization medium. The optimal crystallization conditions leading to the most photoactive Ti0.1Zr0.9On mixed oxide were T = 250°C, p = 10 MPa and VH2O = 2 L. The third set further optimized the Zr content (below 10 mol.% of Zr). The Zr content in TiO2 ranged from 0.5 to 7.5 mol.%. The optimized crystallization conditions found in the second set were applied. The resulting Zr/TiO2 oxides showed improved photocatalytic activity. The photocatalytic activity of 2 mol.% Zr/TiO2 surpassed the photocatalytic activity of the undoped TiO2. The Zr dopation and use of pressurized hot (subcritical) water crystallization improved the photocatalytic activity of the TiO2 produced from the titanyl sulphate precursor. The key parameters affecting the photocatalytic performance were the pressurized hot (subcritical) water processing conditions and the crystallographic phase composition. The processing conditions had an effect on the formation of surface defects in Zr0.1Ti0.9On mixed oxide and Zr/TiO2 oxides. The applied pressure had the most notable effect on the formation of defects. The surface defects played a key role in photoinduced formation of reactive radicals. All of the mixed oxides and parent oxides showed photogenerated current under irradiation with the wavelengths 320 nm and higher. The best photocatalytic activity results in Acid orange 7 decomposition were achieved with the third set, the Zr/TiO2 oxides. 2 mol.% Zr/TiO2 oxide which achieved 99 % conversion of AO7 after 3 h was the most notable photocatalyst from all of them.