Renewable Energy Global Innovations features: Highly Efficient Photoelectrocatalytic Reduction of Hexavalent Chromium based on the Cascade Energy Transfer towards Using no Semiconducting Photocatalysts

Significance Statement

The modern society especially the developing and undeveloped countries is suffering from the increasingly severe environmental pollution mostly caused by hazardous matters, such as toxic heavy metal ion Cr(VI). The most desirable way to remove Cr(VI) is suggested lie on the use of solar irradiation which is cheap and endless. The photocatalytic removal of Cr(VI) based on semiconductor oxide photocatalyst has been provided, however, due to the low efficiency, high cost, and difficulty of recycling photocatalyst powder from the aqueous solution, this method cannot be popularized at the economical level. Hence, it is of urgent priority to design novel methods to utilize the solar light and then lead to the efficient removal of toxic metal ions. Here, we construct an energy relay structure based on citric acid, Ti anode, and Cr(VI), whereby the Cr(VI) are efficiently reduced to Cr(III) under the UV irradiation. As shown in the image, upon the photoexcitation of the citric acid, the electron transfer from citric acid to Ti anode occurs efficiently, mostly because the positively biased Ti is able to facilitate the separation of electrons from holes left in citric acid. Then, the photogenerated electrons in Ti anode spontaneously flow to Cr(VI), which is a energetically favorable process, leading to the efficient reduction of Cr(VI) under the condition that no semiconductor oxide photocatalyst is present in the whole reaction. The method we provide is simple, easy-to-set up, and highly efficient, offering a big step towards the purification of waste water at the commercial level.

About The Author

Jing Shang is an associate professor at the department of environmental science and engineering at Peking University since the year of 2004. She received her Ph.D in environmental science from Jilin University, Changchun city in China, in 2001. From 2001 to 2003, she conducted postdoc research in the department of chemistry at Tsinghua University. She develops novel photoelectrochemical technologies to efficiently remove organic and inorganic pollutants, and is now working on the atmospheric chemistry as well.

Journal Reference

Electrochimica Acta, Volume 188, 2016, Pages 752–756.

Xiang Feng, Jing Shang, Tong Zhu

State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China

Abstract

Highly efficient photoelectrocatalytic (PEC) reduction of Cr(VI) has been realized in the absence of semiconducting photocatalysts. In the novel-configuration cell using Ti anode, Pt cathode, Cr(VI), and citric acid, the rate constant of the photoelectrocatalytic reduction of Cr(VI) at a bias of 1.5 V was almost 3 times than that in the conventional-configuration cell using ITO/TiO₂ anode, Ti cathode, Cr(VI), and citric acid. It was mostly because the citric acid, Ti anode, and Cr(VI) formed an energy-relay cascade structure, in which the photogenerated electrons in the citric acid were transferred to the positively biased Ti and then from anode to Cr(VI), leading to the very efficient Cr(VI) reduction. We develop a simple photoelectrocatalytic method to reduce Cr(VI) over the Ti anode sensitized by photoexcited organic dye in no need of metal oxide photocatalysts, which can be considered as an important advance towards the cost-effective, environmentally friendly treatment of waste water.

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