Structural studies of reaction products between paramolybdate anion and some cationic [metal(II) and (III)-(ethylenediamine)n] complexes utilized as new photocatalysts under UV–VIS light illumination

Five cationic transition metal (ethylenediamine) complexes (M=Cr(III), Co(III), Ni(II), and Cu(II)):paramolybdate anion (Mo₇O₂₄^6?) have been synthesis and characterized via their elemental analysis, magnetic susceptibility (μ_eff), thermal analysis (TG and DTA), FTIR spectra, and X-ray diffraction (XRD). The FTIR study suggests that the compounds prepared be of the ion-pair type (M(en)_n]_m·Mo₇O₂₄). Thermal study showed that molybdenum in the Cr(III), and Co(III) compounds is reduced from oxidation state (VI) to (V) at high temperature. The stoichiometries of the resulting mixed oxides at elevated temperatures (500–750°C) are: Cr₂O₃·7MoO_2.5, Co₂O₃·7MoO_2.5, 6CoOCl·7MoO_2.5, 3NiO·7MoO₃ and 3CuO·7MoO₃. Above 750°C the molybdenum oxide in the ion-pair compounds start the sublimation process. X-ray diffraction of Cr(en)₃]·Mo₇O₂₄, Co(en)₃]·Mo₇O₂₄, and Cu(en)₂(H₂O)₂]₃·Mo₇O₂₄ shows that these complexes are crystalline solids with a similar structure, while the Ni(en)(H₂O)₄]₃·Mo₇O₂₄, and Co(en)(H₂O)₂Cl₂]₆·Mo₇O₂₄ ion-pair compounds display a different structure.

A novel technique based on photocatalysis to eliminate Cr(VI) ions, a toxic pollutant in the environment, was applied. The photoreduction of Cr(VI) to Cr(III) ion in aqueous suspensions using new-mixed oxides as photocatalysts (Cr₂O₃·MoO_2.5, Co₂O₃·MoO_2.5, NiO·MoO₃, and CuO·MoO₃) under air-equilibration and irradiation by a medium pressure mercury lamp (UV–VIS) was investigated.