Visible-Light-Driven Efficient Photocatalytic Reduction of Cr(VI) in a Wide pH Range by Two New Isopolymolybdate-Based Ni(II)-Complexes

The development of new catalytic materials for the fast and efficient photocatalytic reduction of toxic Cr(VI) to less toxic Cr(III) across a wide pH range of water environment has become one of the most challenging topics. Herein, two new isopolymolybdate-based Ni(II)-complexes [NiII(bcbpy)2(beta-Mo8O26)0.5 (H2O)] & sdot; 4H2O (BHU-5) and [NiII(bcbpy)2[theta-Mo8O26)0.5] & sdot; 4H2O (BHU-6, Hbcbpy=1-(4-carboxybenzyl)-4,4 '-bipyridinium) were synthesized via a hydrothermal reaction with the mixture of Na2MoO4 & sdot; 2H2O, NiCl2 & sdot; 6H2O and HbcbpyCl ligands. Single-crystal X-ray diffraction analysis displays that BHU-5 and BHU-6 contain binuclear nickel-bcbpy units, which in association with different isopolymolybdates, and forming distinct 1D chain and 2D layer network structures, respectively. When used as photocatalysts for the reduction of Cr(VI), BHU-5 and BHU-6 exhibited distinct photocatalytic activities, achieving Cr(VI) removal efficiencies of 92 % and 98 % under visible light irradiation within 45 min. These efficiencies surpass those of the pristine HbcbpyCl, Na2MoO4 & sdot; 2H2O, NiCl2 & sdot; 6H2O and most reported polymolybdate-based catalysts. Moreover, BHU-5 and BHU-6 demonstrated high efficiencies in removing Cr(VI) and maintained high structural stability across a wide pH range of 3.5-11.5. The influence of different structures on catalytic efficiency was further studied. The photocatalytic mechanism and reaction kinetics were investigated in detail.