Adsorption and reactivity of resonant guanidonium-type cations and aliphatic oximes in the hydrogen UPD region at single-crystal surfaces of platinum.

Abstract

The direction of the research described in this thesis originates from earlier work in this laboratory on the reactive chemisorption of CH3 CN at Pt single-crystal surfaces, studied by means of cyclic voltammetry and integrated adsorption current transients, coupled with in situ IR reflectance spectroscopy. It was found that resonant guanidonium cations, +NH 2 = C(NH2)2 (G+), have major surface-specific influences on H underpotential deposition (UPD) voltammeric profiles at Pt(hkl) surfaces. Cyclic voltammetry, adsorbate displacement by CO, adsorption charge-transient, in situ FTIR spectroscopy, and a.c. impedance spectroscopy techniques were employed complimentarily to study the adsorption behaviour of G+ cations at Pt single-crystal surfaces. Based on the results of adsorption charge-transients obtained by introduction of 1% solution of G+ in 0.1 mol dm-3 NaOH at the Pt(111) surface, it is concluded that G+ becomes chemisorbed on the Pt surface in a 2-electron transfer process, with resulting electrooxidation of the H's dissociated from --NH2 groups upon their chemisorption. In situ FTIR experiments were conducted in aq. 0.05 mol dm-3 H2SO4 for the purpose of identifying the molecular basis of effects of co-adsorption of G + on the states of adsorbed HSO4- anions through changes of vibrational spectra. The spectroscopic results revealed a broad band in the spectra over the 1500--1600 cm-1 range which was assigned to the >C = NH2+ stretch of G+. Supported by the voltammetric behaviour, they also provided evidence about the co-operative chemisorption of G+ cations with the HSO4- ions on the Pt single-crystal surfaces, where both species are in the chemisorbed state at the Pt surface. This is an attractive interaction which leads to the observed substantial, voltammetric shift of potential for HSO4- desorption towards the RHE potential. A mechanism of this co-operative chemisorption (or ion-pairing) between the G+ cations and the HSO4- (also ClO4- and OH- in aq. HClO4 and NaOH solutions, respectively) ions, co-adsorbed at the Pt single-crystal surfaces, is proposed. Explanations are also given for the reported, in some cases appreciable (for Pt(100) and (511) surfaces), increases that arise in the total voltammetric charges, measured within the potential range for UPD of H. No significant diffusion control for the process of adsorption of G+ at the Pt surface was observed. Thus, the recorded response currents in the voltammograms are directly associated with surface electrochemical processes. (Abstract shortened by UMI.)