Adlayers of Keggin Type Polytungstate Anions on Platinum: Negligible Electrochemical Signatures and Manifestations of “Molecular UPD”

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-).

Co-adsorbtion of Cu and Keggin type polytungstates on polycrystalline Pt: interplay of atomic and molecular UPD

Second harmonic generation (SHG), electrochemical quartz microbalance (EQCM), and cyclic voltammetry are applied to clarify the structure and properties of Cu adlayers formed in the presence of Keggin polytungstate anions. For 0.02-10 mM CuSO4 solutions, no pronounced suppression of underpotential copper deposition (Cu UPD) by 0.1-10 mM H3PW12O40 (PW12) or H4SiW12O40 (SiW12) is observed in electrochemical experiments. Moreover, coadsorption with polyanions results in an increase of charge in the Cu UPD region. EQCM data demonstrate high surface coverage with polytungstate in the overall potential range and their pronounced co-adsorption with Cu2+ cations under open circuit. The unusual potential dependence of EQCM response of polytungstates is discovered and discussed in terms of anion interactions with adsorbed hydrogen. The SHG response of Cu UPD demonstrates a non-linear dependence on Cu surface coverage, which is interpreted in terms of discontinuous submonolayers consisting of 2D Cu islands. The additives of PW12 or SiW12 decrease copper SHG response at low and high CuSO4 concentrations, with minor effect for a mid range of concentrations. In all mixed solutions, the potential dependence of the SHG response remains typical for Cu UPD, not for polytungstates. SHG transients measured under potential step mode demonstrate that the initial non-steady-state SHG behavior of the adlayer is more close to the behavior of polytungstates, but typical copper features appear at longer wavelength. These facts favor the hypothesis of Cu adatom penetration through anionic adlayers and formation of a metal submonolayer at the vacant areas between large quasi-spherical polyanions, with subsequent transformation into a Pt/Cu/polytungstate layered structure.