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

Abstract

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) surfaces. We suggested that the effects of G⁺ (or DMG⁺) were largely electrostatic in origin, leading to stabilisation of adsorbed anions by ion pairing in the double-layer interphase with G⁺ or DMG⁺ cations, and thus the range of potentials over which they influence UPD of H. This paper reports in situ FTIR, single-reflectance, spectroscopic studies of HSO₄⁻ ion adsorption at Pt(111) and Pt(100) surfaces in the presence of G⁺ resonant cations. These experiments were conducted for the purpose of identifying the molecular basis of effects of co-adsorption of G⁺ on the states of such adsorbed bisulfate anions through changes of vibrational spectra. The spectroscopic results provide evidence about the cooperative chemisorption of G⁺ cations with the HSO₄⁻ ions on the Pt single-crystal surfaces and provide support for the previously reported idea of electrostatic ion pairing between 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 electrosorption of HSO₄⁻ and its consequent effect on UPD of H. A mechanism of cooperative chemisorption of G⁺ cations with the HSO₄⁻ ions co-adsorbed at the Pt single-crystal surfaces is proposed. Explanations are also given for the previously reported, in some cases appreciable, increases that arise in the total voltammetric charges, measured within the potential range of UPD of H.