Heptanuclear\nCadmium Borovanadate Containing a Sandwich-Type\nAnionic Partial Structure {B<sub>20</sub>V<sub>12</sub>} as a Heterogeneous\nPhotocatalyst for the Reduction of CO<sub>2</sub> into CH<sub>4</

The novel heptanuclear cadmium borovanadate [Cd(en)<sub>3</sub>]{[Cd(H<sub>2</sub>O)(en)]<sub>6</sub>[B<sub>20</sub>V<sub>12</sub>O<sub>60</sub>(OH)<sub>2</sub>]}·8H<sub>2</sub>O (en\n= ethylenediamine)\nhas been prepared under hydrothermal condition. It exhibits a 2D layered\narchitecture that is built up from the [Cd(H<sub>2</sub>O)(en)]<sup>2+</sup> coordination cations and new sandwich-type borovanadate\nanions [B<sub>20</sub>V<sub>12</sub>O<sub>60</sub>(OH)<sub>2</sub>]<sup>14‑</sup> constructed by BO<sub>3</sub>, BO<sub>4</sub>, BO<sub>3</sub>(OH), and VO<sub>5</sub> units. It features the first\nheptanuclear cadmium borovanadate containing the single valence of\nV<sup>4+</sup> cations and unique B/V ratio (B/V = 20/12). Its thermal,\nmagnetic, XPS, and optical properties have been systematically studied.\nThe <i>E</i><sub>g</sub> (2.60 eV), <i>E</i><sub>VB</sub> (1.69 eV), and <i>E</i><sub>CB</sub> (−0.91\neV) were well determined by the UV–vis diffuse reflectance\nspectrum, UPS curve, and the formula <i>E</i><sub>CB</sub> = <i>E</i><sub>VB</sub> – <i>E</i><sub>g</sub>, respectively. The photocatalytic experiments revealed that\nthe borovanadate could be selected as an effective and stable heterogeneous\ncatalyst for the reduction of CO<sub>2</sub> to CH<sub>4</sub> by\nmeans of visible light (the amount for CH<sub>4</sub> gas could reach\n50.57 μmol·g<sup>–1</sup>·h<sup>–1</sup> when the reaction time was 3 h). The recycling experiment and photocatalytic\nmechanism were also well investigated and established in detail.