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

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