Combined voltammetry and in situ infrared spectroscopy of tetramethylthiourea on gold in aqueous acid solutions

The voltammetric behaviour of tetramethylthiourea (TMTU) dissolved in aqueous perchloric and sulphuric acid solutions on polycrystalline gold combined with in situ Fourier transform infrared reflection absorption spectroscopy (FTIRRAS) was investigated. Conventional and triangular modulated voltammetry and rotating disc electrode data demonstrate the occurrence of two redox surface processes in the potential ranges −0.1≤E≤−0.3  V and 0.7≤E≤0.8  V (versus SHE), respectively. These processes would involve the participation of TMTU adsorbates. The redox surface process observed in the range − 0.1 to − 0.3 V can be related to the electrosorption of TMTU. The absence of infrared absorption bands from these adsorbates agrees with a flat, almost parallel adsorption of TMTU molecules on gold that has been already reported. In the range 0.4 V ≤E≤0.8  V both electrochemical and spectroscopic data indicate the anodic formation of soluble gold-TMTU complex and tetramethylformamidine disulphide (TMFDS2+) ions presumably via Au(TMTU)+ adsorbates. The kinetics of soluble gold-TMTU complex ion formation is under diffusion control, whereas the formation of TMFDS2+ ions behaves as a rather irreversible process with the anodic slope, derived from current-potential curves, ∂E/∂log⁡I=0.120  V decade−1 at 298 K. At E>1.2  V, the global electro-oxidation of both TMTU and TMFDS2+ species yields carbon dioxide, sulphate ions and carbonyl-containing compounds, as indicated by IR spectra. These reactions involve the participation of oxygen-containing adsorbates produced from water electro-oxidation on gold.