Heptanuclear Cadmium Borovanadate Containing a Sandwich-Type Anionic Partial Structure {B20V12} as a Heterogeneous Photocatalyst for the Reduction of CO2 into CH4

The novel heptanuclear cadmium borovanadate [Cd­(en)3]­{[Cd­(H2O)­(en)]6[B20V12O60(OH)2]}·8H2O (en = ethylenediamine) has been prepared under hydrothermal condition. It exhibits a 2D layered architecture that is built up from the [Cd­(H2O)­(en)]2+ coordination cations and new sandwich-type borovanadate anions [B20V12O60(OH)2]14‑ constructed by BO3, BO4, BO3(OH), and VO5 units. It features the first heptanuclear cadmium borovanadate containing the single valence of V4+ cations and unique B/V ratio (B/V = 20/12). Its thermal, magnetic, XPS, and optical properties have been systematically studied. The E g (2.60 eV), E VB (1.69 eV), and E CB (−0.91 eV) were well determined by the UV–vis diffuse reflectance spectrum, UPS curve, and the formula E CB = E VB – E g, respectively. The photocatalytic experiments revealed that the borovanadate could be selected as an effective and stable heterogeneous catalyst for the reduction of CO2 to CH4 by means of visible light (the amount for CH4 gas could reach 50.57 μmol·g–1·h–1 when the reaction time was 3 h). The recycling experiment and photocatalytic mechanism were also well investigated and established in detail.