Characterization and evaluation of the photocatalytic activity of oxides based on TiO₂ synthesized by hydrolysis controlled by the use of water/acetone mixtures

Abstract

New photocatalysts based on TiO2 were synthesized and characterized. The synthesis involved the controlled hydrolysis of titanium tetraisopropoxide using water containing different proportions of acetone. X-ray diffraction analyses combined with Raman spectroscopy revealed crystalline oxides characterized by the coexistence of the anatase and brookite phases. The Rietveld refinement of diffractograms showed that the presence of acetone in the synthesis process influenced the composition of these crystalline phases, with the proportion of brookite growing from 13% to 22% with the addition of this solvent in the synthesis process. The BET isotherms revealed that these materials are mesoporous with surface area approximately 12% higher than that of the oxide prepared from hydrolysis using pure water. The photocatalytic potential of these oxides was evaluated by means degradation tests using the dyes Ponceau 4R and Reactive Red 120 as oxidizable substrates. The values achieved using the most efficient photocatalyst among the synthesized oxides were, respectively, 83% and 79% for mineralization, and 100% for discoloration of these dyes. This same oxide loaded with 0.5% of platinum and suspended in a 5:1 v/v water/methanol mixture, produced 56 mmol of gaseous hydrogen in 5 h of reaction, a specific hydrogen production rate of 138.5 mmol h−1g−1, a value 60% higher than that achieved using TiO2 P25 under similar conditions.

Hydrogen production by photocatalysis using new composites based on SiO2 coated by TiO2

“…By analyzing the diffractograms shown in Fig. 2, and confronting with information reported in literature (Machado & Machado, 2020;Neto et al, 2017) and with the JCPDS 21-1272 (anatase) and 29-1360 (brookite) crystallographic files, it is possible to observe the coexistence of two crystalline phases in all the synthesized oxides, being anatase the main phase and brookite the secondary. It is worth mentioning the similarity of the diffraction profiles of these materials, with emphasis on the well-defined diffraction peaks for the uncoated material (W50), suggesting a significant increase in the crystallinity as a consequence of the heat treatment applied during the synthesis (El-Sheikh et al, 2017).…”

“…The standard photocatalyst, called W50, was obtained by the sol-gel method following a description done in a previous study (Machado & Machado, 2020). This process consists in the solubilization of titanium tetraisopropoxide in isopropanol at 3 C under ultrasonic stirring for 20 min, followed by its hydrolysis by the addition, by dripping, of a water/acetone 50% v/v mixture, and precipitation under ultrasonic stirring.…”

“…The coating of SiO 2 by TiO 2 involved the previous dispersion of silica, in the estimated amount by stoichiometric calculation, in 150 mL of 2-propanol under magnetic stirring for 1 h, followed by the rapid addition of 10 mL of titanium isopropoxide to the suspension. This mixture was maintained under vigorous magnetic stirring for 19 h. Subsequently, in the hydrolysis of the titanium precursor, a water/acetone (50% v/v) mixture was added drop by drop to the mixture, which was kept under magnetic stirring for 1 h. Finally, the resulting colloidal suspension was centrifuged, being the precipitate separated and submitted to the same heat treatment provided to the photocatalyst used as reference (Machado & Machado, 2020).…”

“…The photocatalytic system used in bench scale (Machado & Machado, 2020) is based in a reactor built in borosilicate glass with total volume of 1.5 L, possessing a cooling jacket also made in borosilicate glass. It is connected to a thermostatic bath, which keeps the reaction medium at 20 C. The reactor was positioned on a magnetic agitator, used to promote homogenization of the aqueous suspension containing the catalyst and reactive species.…”

“…In this sense, the present study proposes two simultaneous fronts to minimize trends that compromise TiO 2 performance, seeking to maximize the photocatalytic production of H 2 : First, to promote structural modifications in TiO 2 with the creation of a biphasic (anatase/brookite) material. This mixture of phases tends to favor photocatalytic activity, minimizing the recombination of photoinduced charge carriers, since the cathode potential of the conduction band of brookite is approximately 0.14 eV lower than that of anatase (Patrocinio et al, 2015;Machado & Machado, 2020). Second, the immobilization of TiO 2 on silica (SiO 2 ) surface, giving rise to core@shell (SiO 2 @TiO 2 ) type composites with different proportions of both species.…”