Efficient visible-light-driven reduction of hexavalent chromium catalyzed by conjugated organic species modified hourglass-type phosphomolybdate hybrids

Abstract

The exploration of stable and efficient visible-light-driven photocatalysts for wastewater treatment has been recognized as one of the most challenging issues in environmental remediation. To reveal the structure–activity relationship of catalysts at a molecular level, here four conjugated organic species modified hourglass-type phosphomolybdate hybrids as photocatalysts were prepared by a hydrothermal method with a formula of (H₂L1)₂(H₂L2)₂[Na(H₂O)]₂[M(H₂O)₂] {M[P₄Mo₆O₃₁H₅]₂}·7H₂O (M = Fe for 1, Co for 2, Mn for 3 and Zn for 4; L1 = 4,4′-diaminobiphenyl; L2 = 2,2′-dimethyl-6,6′-biquinoline). Experimental results indicated that crystals 1–4 possess 0-D + 1-D → 3-D supramolecular isomorphic structures, and show wide visible light absorption capacities accompanied with suitable energy band structures. When used as photocatalysts for Cr(VI) reduction, all crystals exhibited favorable photocatalytic performance towards Cr(VI) reduction with removal rates of 86%, 82%, 76% and 72% after 10 W white light irradiation for 120 min. The reaction follows pseudo first-order kinetics in Cr(VI) concentration with a reaction rate constant k of 0.016 min⁻¹ for 1, 0.014 min⁻¹ for 2, 0.011 min⁻¹ for 3 and 0.010 min⁻¹ for 4, respectively. Among them, crystal 1 exhibits the best photocatalytic performance and recycling stability, and it still displayed good photocatalytic performance towards Cr(VI) reduction in real water sources, highlighting its significant practical potential. The mechanism investigation revealed that the central metal M can affect their photocatalytic performance due to their different electronic properties. The synergistic effect among polyanions and conjugated organic species results in the efficient performance of 1. This work provides a promising way to design efficient polyoxometalate-based photocatalysts by adjusting the structural composition.