FTIR spectroscopic and cyclic voltammetric study of the influence of resonant guanidonium cations on HSO4- adsorption in the HUPD region at Pt(111) and (100) surfaces

Elsewhere we have reported how resonant cations such as guanidonium (G(+)) and N,N-dimethylguanidonium (DMG(+)) have major surface-specific influences on H underpotential deposition (UPD) profiles in voltammetry at Pt(hkl) su rfaces. We suggested that the effects of G(+) (or DMG(+)) were largely elec trostatic in origin, leading to stabilisation of adsorbed anions by ion pai ring in the double-layer interphase with G(+) or DMG(+) cations, and thus t he range of potentials over which they influence UPD of H. This paper repor ts in situ FTIR, single-reflectance, spectroscopic studies of ion adsorptio n at Pt(111) and Pt(100) surfaces in the HSO4- presence of G(+) resonant ca tions. These experiments were conducted for the purpose of identifying the molecular basis of effects of co-adsorption of G(+) on the states of such a dsorbed bisulfate anions through changes of vibrational spectra. The spectr oscopic results provide evidence about the cooperative chemisorption of G() cations with the ions on the Pt single-crystal surfaces and provide suppo rt for the previously HSO4- reported idea of electrostatic ion pairing betw een the cations and anions, but in the chemisorbed state at the Pt surface. On account of the structural similarity of urea and guanidine, and because of previously published evidence for chemisorption of urea at Pt, results are also reported, comparatively, for effects of urea on the electrosorptio n of and its consequent effect on UPD of H. A mechanism of cooperative chem isorption HSO4- of G(+) cations with the ions co-adsorbed at the Pt single- crystal surfaces is proposed. Explanations are HSO4+ also given for the pre viously reported, in some cases appreciable, increases that arise in the to tal voltammetric charges, measured within the potential range of UPD of H.