Central Metals to Guide the Bandgap of Hourglass-type Polyoxometalate Hybrids as Photocatalyst for the Reduction of Cr(VI)

Abstract

Visible-light photocatalytic reduction of hexavalent chromium Cr(VI) has become one of the most challenging topics in the field of environmental remediation. The key is to explore a stable and efficient photocatalyst. In this work, a series of highly reduced hourglass-type phosphomolybdate hybrids with molecular formula of (H 2 bpe) 3 {M-[P 4 Mo 6 O 31 H 8 ] 2 }•8H 2 O (M = Na for 1, Ca for 2, Cd for 3, Mn for 4, Zn for 5; bpe = trans-1,2-bi(4-pyridyl)-ethylene) were synthesized by hydrothermal methods and used to reduce Cr(VI) under visible-light conditions. The experimental results showed that hybrids 1−5 were of isomorphic structure and constructed by organic bpe cations and hourglass phosphomolybdate clusters with different metal centers via hydrogenbonding interactions to extend the structure into supramolecular network. With the excellent redox properties and extensive visible light absorption, hybrids 1−5 displayed good photocatalytic activity for Cr(VI) reduction with the reduction conversion rates of 87.84%, 80.31%, 89.07%, 90.92%, and 92.83% within just 10 W white-light irradiation of 20 min at room temperature. Among them, Zn-centered hybrid 5 showed the best photocatalytic performance and recycle stability. The mechanism study showed that the different central metal M can regulate the band gap of hybrid photocatalysts due to its different electronic property, thus affecting their photocatalytic performance. This work provided a promising way to design efficient polyoxometalate-based photocatalysts via the molecular-level composition modulation strategy.

Keggin Cluster Modulated Photocatalytic Activity of Aryl Sulfonium Polyoxometalate Hybrids toward Dichromate Reduction

“…19,20 Apart from this, several hourglass-type POMbased catalysts have also been reported for photocatalytic Cr(VI) reduction. 4,21,22 The efficiency and recovery of POM-based photocatalysts for Cr(VI) reduction depend largely on (a) the prevention of the recombination of the photogenerated e − −h + pair and (b) the heterogeneous nature of the catalyst in water. In this study, we decided to use aryl sulfonium counterions (ASCs) to address the above two aspects simultaneously, i.e., to stabilize the photogenerated e − −h + pair on the cluster and to control the solubility of the hybrids in water.…”

“…Earlier reports on photocatalytic Cr(VI) reduction suggest that the electrons in the conduction band, generated after photoexcitation of the catalytic materials, are mainly responsible for photoreduction. 45,46 Therefore, in the present study, an experiment was conducted with AgNO 3 as an electron-trapping agent to shed more light on the mechanism of the photocatalytic reaction using hybrid 1 as a model catalyst. It was found that the presence of AgNO 3 in the reaction medium results in quenching the photocatalytic activity of hybrid 1.…”

“…One approach to making heterogeneous POM catalysts is to use large countercations. Inorganic cations like Ag + and Fe 3+ have been applied as countercations to develop heterogeneous Keggin cluster-based photocatalysts for Cr­(VI) reduction. , Apart from this, several hourglass-type POM-based catalysts have also been reported for photocatalytic Cr­(VI) reduction. ,, …”

Four Anderson-type [TeMo6O24]6−-based metal–organic complexes with a new bis(pyrimidine)-bis(amide): multifunctional electrochemical and adsorption performances

“…POMOCs combine the properties of POMs and MOCs, which can provide good cycling stability. 19,20 Therefore, syntheses and development of novel POMOCs is still fairly demanded and important.…”

Two three-dimensional Fe(ii) frameworks based on {P4Mo6} tetrameric clusters exhibiting efficient visible-light photocatalytic properties for the degradation of Cr(vi) and methylene blue

“…Benefiting from the brilliant redox properties, strong electron transfer capability and tunable band gap structures, {P 4 Mo 6 }-based compounds have been applied in photocatalytic systems, such as CO 2 reduction, [28][29][30][31][32] dye degradation, 33,34 and Cr(VI) reduction. [35][36][37][38] In these reactions, the rapid and reversible multi-electron transfer characteristics of the {P 4 Mo 6 } units play an important role in improving the photocatalytic performance. Consequently, {P 4 Mo 6 }-based high-dimensional frameworks with highly insoluble structures and a broad spectral absorption range are expected to be excellent candidates for heterogeneous photocatalytic degradation of pollutants.…”