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-SiVW11O405- (SiVW11), was more readily adsorbed than nonsubstituted silicotungstate, α-SiW12O404- (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-SiV3W9O407-, 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-SiVIIIW11O407-).