Electrochemical quartz crystal microbalance determination of nickel formal partial charge number during nickel-underpotential deposition on platinum in sulphate media

We studied nickel-underpotential deposition (UPD) on polycrystalline platinum in sodium sulphate media using Electrochemical Quartz Crystal Microbalance (EQCM). EQCM enables to separate hydrogen and nickel adsorption/desorption contributions, which was impossible solely with electrochemistry. Thus, nickel coverage can be determined: nearly 0.8 Ni-UPD monolayer over platinum is deposited above nickel Nernst potential for pH 5.5. Coupling the gravimetric signal with nickel stripping peak coulometry yields the nickel formal partial charge number: (l)(Ni,EQCM) = 1.3 +/- 0.13. Electrochemical measurements using Swathirajan and Bruckenstein's method give: (l)(Ni) = 1.5 +/- 0.17. Within the errors bars, both methods give similar results and (l)(Ni) lower than (z)(Ni2+) : nickel adatoms apparently remain partially charged upon UPD on platinum following anion and nickel co-adsorption. (c) 2007 Elsevier B.V. All rights reserved.