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The exploration of high-efficiency photocatalysts to drive the conversion of highly toxic heavy metal hexavalent chromium (Cr(VI)) in wastewater to low-toxic trivalent chromium (Cr(III)) is of great significance for purifying water that contains emerging contaminants. Herein, four hourglass-type phosphomolybdate-based hybrid networks—(H2bpe)2[M(H2O)3]2{M[P4Mo6O31H7]2}·8H2O (M = Mn for 1, Co for 2) and (Hbpe)(H2bpe)Na[M(H2O)3]2{M[P4Mo6O31H7]2}·9H2O (M = Mn for 3, Co for 4; {M[P4Mo6O31H7]2}8− (abbr. M{P4Mo6}2); bpe = 1,2-di(4-pyridyl)ethylene)—were hydrothermally synthesized as heterogeneous photocatalysts for Cr(VI) reduction. A structural analysis showed that the four hybrids 14 exhibited two-dimensional inorganic sheet-like structures with a 3,6-connected kgd topology built of hourglass phosphomolybdate clusters having different central metal ions, which further interacted with organic bpe cations via abundant hydrogen-bonding interactions to extend the structure to a three-dimensional (3D) supramolecular network. The four hybrids displayed excellent redox properties and wide visible-light absorption. When used as heterogeneous photocatalysts, hybrids 14 exhibited excellent photocatalytic activity for Cr(VI) reduction under 10 W white light irradiation, with reduction rates of 91% for 1, 74% for 2, 90% for 3, and 71% for 4, respectively, within 80 min. The Cr(VI) reduction reaction over hybrids 14 followed the pseudo first-order kinetics model with reaction rate constants k of 0.0237 min−1 for 1, 0.0143 min−1 for 2, 0.0221 min−1 for 3 and 0.0134 min−1 for 4, respectively. The Mn{P4Mo6}2-based hybrids 1 and 3 showed better photocatalytic performance than the Co{P4Mo6}2-based hybrids 2 and 4, along with excellent recycle stability. This mechanism study shows that the different central metals M in the M{P4Mo6}2 cluster have a considerable impact on photocatalytic performance due to their regulation effect on the electronic structure. This work provides evidence for the important role of the central metal in hourglass-type phosphomolybdate in the regulation of photocatalytic performance, and it brings inspiration for the design of highly efficient photocatalysts based on polyoxometalates.


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Photoactive hourglass-type M{P4Mo6}2 networks for efficient removal of hexavalent chromium

Show Author's information Xiao-Yu YinHao-Xue BiHao SongJing-Yan HeYuan-Yuan Ma ( )Ting-Ting FangZhan-Gang Han ( )
Hebei Key Laboratory of Organic Functional Molecules, National Demonstration Center for Experimental Chemistry Education, Hebei Key Laboratory of Inorganic Nanomaterials, College of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang 050024, China

Abstract

The exploration of high-efficiency photocatalysts to drive the conversion of highly toxic heavy metal hexavalent chromium (Cr(VI)) in wastewater to low-toxic trivalent chromium (Cr(III)) is of great significance for purifying water that contains emerging contaminants. Herein, four hourglass-type phosphomolybdate-based hybrid networks—(H2bpe)2[M(H2O)3]2{M[P4Mo6O31H7]2}·8H2O (M = Mn for 1, Co for 2) and (Hbpe)(H2bpe)Na[M(H2O)3]2{M[P4Mo6O31H7]2}·9H2O (M = Mn for 3, Co for 4; {M[P4Mo6O31H7]2}8− (abbr. M{P4Mo6}2); bpe = 1,2-di(4-pyridyl)ethylene)—were hydrothermally synthesized as heterogeneous photocatalysts for Cr(VI) reduction. A structural analysis showed that the four hybrids 14 exhibited two-dimensional inorganic sheet-like structures with a 3,6-connected kgd topology built of hourglass phosphomolybdate clusters having different central metal ions, which further interacted with organic bpe cations via abundant hydrogen-bonding interactions to extend the structure to a three-dimensional (3D) supramolecular network. The four hybrids displayed excellent redox properties and wide visible-light absorption. When used as heterogeneous photocatalysts, hybrids 14 exhibited excellent photocatalytic activity for Cr(VI) reduction under 10 W white light irradiation, with reduction rates of 91% for 1, 74% for 2, 90% for 3, and 71% for 4, respectively, within 80 min. The Cr(VI) reduction reaction over hybrids 14 followed the pseudo first-order kinetics model with reaction rate constants k of 0.0237 min−1 for 1, 0.0143 min−1 for 2, 0.0221 min−1 for 3 and 0.0134 min−1 for 4, respectively. The Mn{P4Mo6}2-based hybrids 1 and 3 showed better photocatalytic performance than the Co{P4Mo6}2-based hybrids 2 and 4, along with excellent recycle stability. This mechanism study shows that the different central metals M in the M{P4Mo6}2 cluster have a considerable impact on photocatalytic performance due to their regulation effect on the electronic structure. This work provides evidence for the important role of the central metal in hourglass-type phosphomolybdate in the regulation of photocatalytic performance, and it brings inspiration for the design of highly efficient photocatalysts based on polyoxometalates.

Keywords: photocatalysis, polyoxometalate, phosphomolybdate, Cr(VI) reduction

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Publication history

Received: 01 December 2022
Revised: 04 February 2023
Accepted: 23 February 2023
Published: 23 March 2023
Issue date: June 2023

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© The Author(s) 2023. Polyoxometalates published by Tsinghua University Press.

Acknowledgements

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (Nos. 21871076 and 21901060), the Natural Science Foundation of Hebei Province (Nos. B2020205008, B2022205005, B2019205074, and B2016205051), the Science and Technology Project of Hebei Education Department (No. BJ2020037), the China Postdoctoral Science Foundation funded Project (No. 2021TQ0095), the Science Foundation of Hebei Normal University (No. L2019B15), the Innovation Capability Improvement Plan Project of Hebei Province (No. 22567604H), and College Student Science and Technology Innovation Project of Hebei Province (No. 202210094002).

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