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 <i>E</i>_g (2.60 eV), <i>E</i>_VB (1.69 eV), and <i>E</i>_CB (-0.91 eV) were well determined by the UV-vis diffuse reflectance spectrum, UPS curve, and the formula <i>E</i>_CB = <i>E</i>_VB - <i>E</i>_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.