Adsorption of Keggin-Type Polyoxometalates on Rh Metal Particles under Reductive Conditions

The adsorption of POMs on Rh/SiO₂ in water solvent under strongly reductive conditions was investigated. Aqueous solutions of α-Keggin type silicotungstate and silicovanadotungstates were mixed with Rh/SiO₂ at 393-473 K under 1 MPa of H₂. Monovanadium-substituted silicotungstate, α-SiVW₁₁O₄₀^5- (SiVW₁₁), was more readily adsorbed than nonsubstituted silicotungstate, α-SiW₁₂O₄₀^4- (SiW₁₂). After adsorption at 433 K, SiVW₁₁ was desorbed from Rh/SiO₂ by oxidation with Br₂ water without change of the Keggin structure, as evidenced by ⁵¹V NMR. Trivanadium-substituted silicotungstate, α-1,2,3-SiV₃W₉O₄₀^7-, was not stable, and the desorbed species from Rh/SiO₂ by oxidation with Br₂ did not maintain the Keggin structure. The very high temperature for adsorption (473 K) also led to the decomposition of the Keggin structure of SiVW₁₁. An increase in the concentration of SiVW₁₁ in the liquid phase gave a saturation of the amount of desorbable SiVW₁₁, up to five SiVW₁₁ anions per one Rh particle with a 3 nm size. The elemental analysis and W L₃-edge extended X-ray absorption fine structure of Rh/SiO₂ after the adsorption of SiVW₁₁ showed that a part of SiVW₁₁ was decomposed and irreversibly adsorbed as metallic W species incorporated into the surface of Rh metal particles. The amount of decomposed SiVW₁₁ was almost the same as that of SiVW₁₁ adsorbed as the original Keggin structure. The desorbable SiVW₁₁ was probably bonded on the W atom incorporated on the Rh metal particles as the two-electron-reduced form (α-SiV^IIIW₁₁O₄₀^7-).