Synthesis of cadmium sulfide pillared layered compounds and photocatalytic reduction of nitrate under visible light irradiation

Layered compound/CdS nanocomposites incorporating CdS pillars in the interlayer of layered compounds such as hectorite, H₂Ti₄O₉ and H₄Nb₆O₁₇ were prepared by the stepwise ion exchange reactions of the layered compounds with C₃H₇NH₃⁺ and Cd²⁺ followed by the reaction with H₂S gas. The pillar height was less than 1 nm and the band gap energy was slightly larger than that of normal crystalline CdS. Although the host layered compounds were insulators or wide band gap semiconductors, CdS pillared layered compounds showed photocatalytic activity under visible light irradiation. Nitrate ion was photochemically reduced to nitrite, nitrogen and ammonia by irradiating visible light (λ>400 m) from a high pressure mercury lamp to nitrate solution containing H₂Ti₄O₉/CdS and H₄Nb₆O₁₇/CdS nanocomposites in the presence of a sacrificial hole acceptor such as methanol, whereas no photoreduction of nitrate proceeded in the presence of unsupported CdS and hectorite/CdS nanocomposite, indicating that the incorporation of CdS pillars in the interlayer of a semiconducting layered compound such as H₂Ti₄O₉ and H₄Nb₆O₁₇ was much more efficient in enhancing the nitrate reduction activity than when an insulator such as hectorite was used.