Four octamolybdate compounds with properties of organic dye adsorption and photocatalytic reduction of Cr(VI).

Hydrothermal synthesis was used to create four different POM-based compounds, namely {[Co(Hptpm) ₂ (β-Mo ₈ O ₂₆ ) _0.5 (ξ-Mo ₈ O ₂₆ ) _0.5 ]}·H ₂ O (1), [Co(Hptpm) ₂ (δ-Mo ₈ O ₂₆ )]·H ₂ O (2), [Co(Hptpm) ₂ (β-Mo ₈ O ₂₆ )] (3) and [Zn(Hptpm) ₂ (β-Mo ₈ O ₂₆ )] (4) (ptpm = 4-[3-(3-pyridine-2-yl-[1,2,4]triazol-4-yl)-propyl]-morpholine). 1-4 containing different octamolybdate isomers were characterized. Compounds 1-4 showed good electrochemical performance and can be utilized as bifunctional sensors for NO ₂ ^- , H ₂ O ₂ , Cr(VI) and Fe(III). Taking compound 1 as an example, the detection limits are 0.081 μM for NO ₂ ^- , 0.072 μM for H ₂ O ₂ , 0.054 μM for Cr(VI) and 0.063 μM for Fe(III), respectively. Compounds 1-4 have good capacitance. Moreover, compounds 1-4 also show good adsorption properties for organic cationic dyes. The cationic dyes include methylene blue (MB), crystal violet (CV) and neutral red (NR). In addition, 1-4 have excellent characteristics that can reduce Cr(VI) to Cr(III) by photocatalytic technology. Within 30 min, the reduction rates were 95.85% for 1, 93.99% for 2, 90.29% for 3 and 88.18% for 4.