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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) 2M (H2O) 32MP4Mo6O31H72·8H2O (M = Mn for 1, Co for 2) and (Hbpe) (H2bpe) NaM (H2O) 32MP4Mo6O31H72·9H2O (M = Mn for 3, Co for 4; MP4Mo6O31H728− (abbr. MP4Mo62) ; 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 1–4 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 1–4 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 1–4 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 MnP4Mo62-based hybrids 1 and 3 showed better photocatalytic performance than the CoP4Mo62-based hybrids 2 and 4, along with excellent recycle stability. This mechanism study shows that the different central metals M in the MP4Mo62 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.
Yin et al. (Fri,) studied this question.