Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO2, WO3 and ZnO)

Hydrogen peroxide (H2 O2 ) has received increasing attention because it is not only a mild and environmentally friendly oxidant for organic synthesis and environmental remediation but also a promising new liquid fuel. The production of H2 O2 by photocatalysis is a sustainable process, since it uses water and oxygen as the source materials and solar light as the energy. Encouraging processes have been developed in the last decade for the photocatalytic production of H2 O2 . In this Review we summarize research progress in the development of processes for the photocatalytic production of H2 O2 . After a brief introduction emphasizing the superiorities of the photocatalytic generation of H2 O2 , the basic principles of establishing an efficient photocatalytic system for generating H2 O2 are discussed, highlighting the advanced photocatalysts used. This Review is concluded by a brief summary and outlook for future advances in this emerging research field.

Production of Hydrogen Peroxide by Photocatalytic Processes

In this work, the application of advanced oxidation processes (AOPs) for the removal of antibiotics from water has been reviewed. The present concern about water has been exposed, and the main problems derived from the presence of emerging pollutants have been analyzed. Photolysis processes, ozone-based AOPs including ozonation, O3/UV, O3/H2O2, and O3/H2O2/UV, hydrogen peroxide-based methods (i.e., H2O2/UV, Fenton, Fenton-like, hetero-Fenton, and photo-Fenton), heterogeneous photocatalysis (TiO2/UV and TiO2/H2O2/UV systems), and sonochemical and electrooxidative AOPs have been reviewed. The main challenges and prospects of AOPs, as well as some recommendations for the improvement of AOPs aimed at the removal of antibiotics from wastewaters, are pointed out.

Advanced Oxidation Processes for the Removal of Antibiotics from Water. An Overview

In this study, carbon nanodots (C-dots)/WO3 photocatalysts were prepared via a two-step hydrothermal method. The morphologies and optical properties of the as-prepared materials were investigated. Compared with the prepared WO3 and C-dots, the C-dots/WO3 possessed stronger photocatalytic capability and excellent recyclability for photocatalytic elimination of Rhodamine B. For example, the achieved first order reaction rate constant of 0.01942 min-1 for C-dots/WO3 was ∼7.7 times higher than that of the prepared WO3. The enhanced photocatalytic activity of C-dots/WO3 was attributed to the enhanced light harvesting ability and efficient spatial separation of photo-excited electron-hole pairs resulting from the synergistic effect of WO3 and C-dots. The high photocatalytic activity of C-dots/WO3 remained unchanged even after 3 cycles of use. Meanwhile, a possible mechanism of C-dots/WO3 for the enhanced photocatalytic activity was proposed.

Two-step hydrothermally synthesized carbon nanodots/WO3 photocatalysts with enhanced photocatalytic performance

Understanding the hydrogen peroxide electrochemistry on platinum can provide information about the oxygen reduction reaction mechanism, whether H(2)O(2) participates as an intermediate or not. The H(2)O(2) oxidation and reduction reaction on polycrystalline platinum is a diffusion-limited reaction in 0.1 M HClO(4). The applied potential determines the Pt surface state, which is then decisive for the direction of the reaction: when H(2)O(2) interacts with reduced surface sites it decomposes producing adsorbed OH species; when it interacts with oxidized Pt sites then H(2)O(2) is oxidized to O(2) by reducing the surface. Electronic structure calculations indicate that the activation energies of both processes are low at room temperature. The H(2)O(2) reduction and oxidation reactions can therefore be utilized for monitoring the potential-dependent oxidation of the platinum surface. In particular, the potential at which the hydrogen peroxide reduction and oxidation reactions are equally likely to occur reflects the intrinsic affinity of the platinum surface for oxygenated species. This potential can be experimentally determined as the crossing-point of linear potential sweeps in the positive direction for different rotation rates, hereby defined as the "ORR-corrected mixed potential" (c-MP).

Hydrogen peroxide electrochemistry on platinum: Towards understanding the oxygen reduction reaction mechanism

Anatase TiO2 is the most studied environmental photocatalyst, but its efficiency is still not high enough to enable application Herein we report a positive effect of Ni2+ ions on the photocatalytic reaction in neutral aqueous solution On addition of 1 mM Ni(ClO4)2, the rates of phenol oxidation on TiO2 and 05 wt% Pt/TiO2 increased by factors of 21 and 80, respectively Meanwhile, the formation of organic intermediate was reduced, and the removal of total organic content was enhanced Interestingly, Ni2+ concentration did not change with irradiation time Moreover, a maximum rate of phenol degradation was observed with Ni2+ ions, occupying half the surface of TiO2 or Pt/TiO2, where Ni(OH)+ was more active than Ni(OH)2 X-ray photoelectron spectroscopy identified a Ni3+ species produced in absence of phenol A (photo)electrochemical study revealed that Ni2+ oxidation was favored over H2O oxidation, whereas Ni2+ and Pt inhibited and facilitated O2 reduction, respectively It is proposed that the adsorbed Ni(OH)+ on TiO2 is oxidized to Ni3+ by the photogenerated holes of TiO2, followed by regeneration through phenol oxidation The Ni2+-catalyzed organic oxidation then promotes the Pt-catalyzed O2 reduction, and vice visa, resulting into the greatly improved efficiency of charge separation This work highlights the possibility of Ni2+ ions as the hole mediator of organic oxidation over a semiconductor

Role of Ni2+ ions in TiO2 and Pt/TiO2 photocatalysis for phenol degradation in aqueous suspensions

Various methods that aim to improve the photocatalytic activity of TiO2 have been reported in the literature. Herein, we report that addition of CuWO4 into the aqueous suspension of TiO2 can result in significant enhancement in the rate of phenol degradation. As the amount of CuWO4 increased, the rate of phenol degradation increased and then decreased. A maximum rate of phenol degradation observed with 2 wt % CuWO4 was about 2.83 times that in the absence of CuWO4. A similar result was also observed with CuO. However, six consecutive tests showed that CuWO4/TiO2 was much more stable than CuO/TiO2, due to the very high stability of CuWO4 against photocorrosion. The improved activity of TiO2 is not due to CuWO4 and CuO themselves and also does not match their solubility in aqueous solution. Moreover, for the generation of OH radicals, and for the decomposition of H2O2 in aqueous solution, CuWO4/TiO2 was also more active than TiO2. Through a (photo) electrochemical measurement, a possible mechanism is propos...

Improved Photocatalytic Activity of TiO2 on the Addition of CuWO4

Improved photocatalytic degradation of chlorophenol over Pt/Bi 2 WO 6 on addition of phosphate

Improved visible light photocatalytic activity of WO3 through CuWO4 for phenol degradation

UV irradiation of Au/TiO2 photocatalysts in the presence of borate and phosphate anions can produce H2O2 at a millimolar level in alkaline water solution. The positive effect of the anions is ascribed to the anion-mediated hole transfer from Au/TiO2 to an electron donor which thus accelerates the two-electron reduction of O2 to H2O2.

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Xianqiang Xiong

Papers

Role of Ni2+ ions in TiO2 and Pt/TiO2 photocatalysis for phenol degradation in aqueous suspensions

Improved Photocatalytic Activity of TiO2 on the Addition of CuWO4

Improved photocatalytic degradation of chlorophenol over Pt/Bi 2 WO 6 on addition of phosphate

Improved visible light photocatalytic activity of WO3 through CuWO4 for phenol degradation

Sustained production of H2O2 in alkaline water solution using borate and phosphate-modified Au/TiO2 photocatalysts