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.
Visible‐light‐driven photocatalytic Cr(VI) reduction is a promising pathway to moderate environmental pollution, in which the development of photocatalysts is pivotal. Herein, three hourglass‐type phosphomolybdate‐based hybrids with the formula of: (H2bpe)3[Zn(H2PO4)][Zn(bpe)(H2O)2]H{Zn[P4Mo6O31H6]2} ⋅ 6H2O (1) Na6[H2bz]2[ZnNa4(H2O)5]{Zn [P4Mo6O31H3]2} ⋅ 2H2O (2) and (H2mbpy) {[Zn(mbpy)(H2O)]2[Zn(H2O)]2}{Zn[P4Mo6O31H6]2} ⋅ 10H2O (3) (bpe=trans‐1,2‐bi(4‐pyridyl)‐ethylene; bz=4,4′‐diaminobiphenyl; mbpy=4,4’‐dimethyl‐2,2’bipyridine) were synthesized under the guidance of the functional organic moiety modification strategy. Structural analysis showed that hybrids 1–3 have similar 2D layer‐like spatial arrangements constructed by {Zn[P4Mo6]2} clusters and organic components with different conjugated degree. With excellent redox properties and wide visible‐light absorption capacities, hybrids 1–3 display favourable photocatalytic activity for Cr(VI) reduction with 79%, 70% and 64% reduction rates, which are superior to that of only inorganic {Zn[P4Mo6]2} itself (21%). The investigation of organic components on photocatalytic performance of hybrids 1–3 suggested that the organic counter cations (bpe, bz and mbpy) can effectively affect the visible‐light absorption, as well as the recombination of photogenerated carriers stemmed from {Zn[P4Mo6]2} clusters, further promoting their photocatalytic performances towards Cr(VI) reduction. This work provides an experimental basis for the design of functionalized photocatalysts via the modification of organic species.
The exploration of high-efficient photocatalyst 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 water purification from 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), (Hbpe)(H2bpe)Na[M(H2O)3]2{M[P4Mo6O31H7]2}•9H2O (M = Mn for 3, Co for 4) [bpe= 1,2-di(4-pyridyl)ethylene], were hydrothermally synthesized as heterogeneous photocatalysts for Cr(VI) reduction. Structural analysis shows that four hybrids 1-4 exhibit two-dimensional inorganic sheet-like structures with 3,6-connected kgd topology constructed by hourglass phosphomolybdate clusters with different central metal ions, which further interact with organic bpe cations via abundant hydrogen -bonding interactions to extend the structure into(3-D) supramolecular network. Four hybrids displayed excellent redox properties and wide visible-light absorption. When using as heterogeneous photocatalysts, hybrids 1-4 exhibited excellent photocatalytic activities 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 within 80 minutes, respectively. The Cr(VI) reduction reaction over hybrids 1-4 follow 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. Among them, Mn{P4Mo6}2-based hybrids 1 and 3 show better photocatalytic performance than Co{P4Mo6}2based hybrids 2 and 4 along with excellent recycle stability ({M[P4Mo6O31H7]2} 8-(abbr. M{P4Mo6}2)). The mechanism study shows that the different central metal M in M{P4Mo6}2 cluster has great impact on the photocatalytic performance due to their regulation effect on electronic structure. This work evidences the important role of central metal in hourglass -type phosphomolybdate on the regulation of photocatalytic performance, also provides inspiration for the design of highly efficient photocatalysts based on polyoxometalates.
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