Combining photocatalytic water treatment and fuel synthesis: improving the activity of TiO2 by cobalt - doping and Ag/Ag2O - deposition

Abstract

Water treatment and solar fuel synthesis are nowadays considered to be the two major concerns for achieving a sustainable living environment. With the growing demand for advanced environmental remediation of polluted water and as the cost of fossil fuels increases, photocatalysis has received increasing attention in recent years. In the present work, modified TiO2 photocatalysts are used for both photocatalytic processes. Initially, TiO2 (P25) is modified with different mass ratios of self-prepared Ag/Ag2O employing two synthetic methods, namely, a mechanical mixture and a precipitation method. After the characterization, the photocatalytic activity of the seven prepared materials including Ag/Ag2O, Ag/Ag2O ⁄⁄ TiO2 mixtures (TM), and Ag/Ag2O ⁄⁄ TiO2 composites (TC) is evaluated by methylene blue (MB) bleaching under UV-vis and only visible light illumination. The results reveal that the light-induced bleaching of aqueous MB in the presence of TM and TC under both UV-vis and visible illumination is enhanced. Interestingly, Ag/Ag2O shows the best photocatalytic activity under both illumination conditions. However, the observed photobleaching of MB is found to be rather initiated by an interfacial electron transfer from the photo-excited MB adsorbed at the surface of Ag2O to the latter′s conduction band. Moreover, XRD and XPS data confirm that Ag2O performs as an electron acceptor in the light-induced reaction of MB, and Ag+ is reduced to Ag(0). On the other hand, photocatalytic H2 generation from methanol reforming under solar light using all-synthesized photocatalysts is also studied. The experimental results indicate that bare Ag/Ag2O is not able to transfer an electron to H+, which is also confirmed by the flatband potentials measurements. Nevertheless, with increasing the mass fraction of TiO_2 in the TC and TM, the evolved amounts of H2 are observed to increase implying that the fraction of photons absorbed by Ag2O being inactive decreases. Hence, it is concluded that Ag/Ag2O is not a (photo)stable material since the photoreduction of Ag+ is experimentally confirmed to proceed yielding Ag. Afterward, the photocatalytic degradation of oxytetracycline hydrochloride (OTC HCl) is investigated in the presence of cobalt-doped TiO2 (Co-TiO2) photocatalysts. Co-TiO2 were synthesized by two different solvothermal methods, namely, reflux (Co-TiO2-R) and hydrothermal (Co-TiO2-HT) synthesis. The characterization of Co-TiO2-R and Co-TiO2-HT samples by means of XRD and Raman spectroscopy reveals that Ti4+ was substituted by Co2+ in the TiO2 crystal structure. The observed initial reaction rates of the light-induced degradation of OTC HCl over both Co-doped TiO2 upon UV-vis light are found to be higher than those of pureTiO2, and the commercial P25. However, the photolysis of OTC HCl still can take part in the overall mechanism. Besides, the photocatalytic H2 formation from aqueous methanol employing the Co-TiO2-R and Co-TiO2-HT under solar irradiation is studied. The experimental results evince that cobalt doping has no additional advantage regarding the photocatalytic activity since the evolved amounts of H2 are close to the detection limit. Mechanistic investigation using EPR confirms the recombination of photo-generated charge carriers. In order to suppress the fast charge carriers recombination, the surface of Co-TiO2 is modified with Pt. Higher formation rates of H2 are observed due to the improved electrons and holes separation. Therefore, Pt seems to compensate the decrease of the photocatalytic activity due to Co-doping.