Photocatalytic Cr(VI) reduction by metal-free photocatalysts under visible-light irradiation

The photocatalytic reduction of hazardous Cr(VI) to nontoxic Cr(III) by metal free catalytic system under visible light irradiation is a promising approach to suppress Cr(VI) pollution, while the efficiency is inevitably tied to the performance of catalytic system. Metal-free materials with semiconductor properties have aroused wide concern for solar driven Cr(VI) reduction because of their satisfactory visible-light harvesting, diverse synthetic approaches and ready functionality. These distinctive features have attracted ever-increasing attention, prompting the rapid development of a variety of metal-free-based photocatalysts with interesting structures and properties, but there is an urgent need to summarize the research status and in-depth insights the intrinsic structure-activity relationship of photocatalytic Cr(VI) reduction. This review mainly focused on the state-of-the-art development process of metal-free-based photocatalyst for photocatalytic Cr(VI) reduction under visible light irradiation. The representative research work of photocatalytic Cr(VI) reduction by various metal-free-based photocatalytic systems, containing graphite carbon nitride (g-C₃N₄), covalent organic frameworks (COFs), covalent triazine frameworks (CTFs) and conjugated microporous polymers (CMPs) was also highlighted. The strategies to optimize the structures and properties of metal-free-based photocatalysts at the molecular level have been presented for enhancing the photocatalytic performance, and 100 % Cr(VI) reduction was observed under visible light irradiation. The reaction mechanism has also been discussed, which is conducive to gaining valuable insights of specific solar energy utilization in Cr(VI) reduction. Furthermore, the summarization and generalization of structure-activity relationship offer guidance for the design and synthesis of metal-free-based photocatalytic systems.