Isopolytungstate Adsorption on Platinum: Manifestations of Underpotential Deposition

Metatungstate isopolyanion adsorption on smooth polycrystalline Pt and carbon-supported dispersed platinum in sulfuric acid medium is studied by cyclic voltammetry. Manifestations of both reversible and irreversible adsorption are found, with the features typical for heteropolytungstates of the same Keggin structure. When (NH4)(6)H2W12O40 is added to sulfuric acid solution, H underpotential deposition (UPD) charge starts to redistribute: it decreases at high hydrogen coverage but increases at higher potentials, with appearance of additional reversible redox feature in "double layer region." The total charge value in diluted metatungstate solutions remains the same as for pure Pt. However, this value decreases with metatungstate concentration. At any fixed (NH4)(6)H2W12O40 concentration, the total charges in the hydrogen region are practically the same in 0.1 and 0.5 M sulfuric acid. Strongly bonded metatungstate species can be kept at the surface after the transfer to supporting solution. These irreversibly adsorbed species are redox-inactive and demonstrate stronger suppression of H UPD. Excursion to oxygen adsorption region does not result in complete desorption, but redistribution of charge takes place again. Our observations can be assigned (by analogy with Keggin anions adsorption on Au and Ag) to slow adsorbate-metal bonding, with subsequent increase of the number of bonded oxygen atoms. General trends are qualitatively the same for both Pt materials, but dispersed Pt on glassy carbon favors irreversible adsorption. The data confirm that adsorbed metatungstate can undergo reversible reduction at potentials more positive than the equilibrium potential of its first W(V/VI) redox transition (molecular UPD). Comparative electrocatalytic tests are presented for nitrate electroreduction on bare Pt, Pt modified with irreversibly adsorbed metatungstate, and Pt in metatungstate solution. The latter system, containing both forms of tungstate adsorbate, demonstrates high steady-state electrocatalytic activity at potentials below ca. 0.1 V reversible hydrogen electrode (RHE).