Spectroscopic Investigation of the Adsorption and Oxidation of Thiourea on Polycrystalline Au and Au(111) in Acidic Media

Abstract

In the present paper, a systematic electrochemical investigation on thiourea (TU) electrooxidation was developed on polycrystalline and (111) single-crystal gold electrodes in 0.1 M perchloric acid. The combination of cyclic voltammetry with in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry techniques have allowed the nature of the species formed during the electroadsorption and electrooxidation of TU to be established. FTIRS experiments were performed in D2O to clean up the region of the H2O bending around 1600 cm(-1). It was concluded that TU adsorbs tilted on the surface in the 0.05-0.40 VRHE potential range. A dual-path reaction mechanism was evidenced in the oxidation process. The first pathway takes place from adsorbed TU at E > 0.40 VRHE and implies the formation of [Au(I)-(TU)2]+, which is oxidized to NH2CN and S0 at E > 0.80 VRHE. In a following oxidation step at E > 1.20 V, N2, CO2, and HSO4-/SO4(2-) were produced. The second parallel reaction occurs from TU in solution at E > 0.50 VRHE to form (TU)2(2+). All these species were characterized from the spectroscopic experiments. Similar results were obtained for both surfaces.

On the electrochemical behavior of formamidine disulfide on gold electrodes in acid media

“…The optimized adsorption geometry for TU on clusters modeling the Au(111) surface was found to correspond to a unidentate configuration for adsorbed TU on top surface sites via the sulfur atom and with the C-S bond tilted by 45º from the normal. This result confirmed the conclusions derived from the observation of the asymmetric NCN band in the in situ infrared spectra [6,17].…”

“…This latter band would be infrared active but it must be kept in mind that the significant deviation of the molecular planes (around 45º) with respect to the surface normal would make difficult the observation of such adsorbate, as the z-component of the transition dipole moments would be significantly lower as compared to the case where these molecular planes are perpendicular to the surface. As in most of the in-situ external reflection infrared experiments reported for TU, deuterium oxide was used as the solvent in order to avoid interferences from the bending bands of water [6,17]. As a similar spectroelectrochemical behavior has been observed for Au(111) and Au(100) electrodes in the FDS-containing solution, we will describe in the following only the infrared spectra obtained with Au(111).…”

“…In the case of gold electrodes, the formation of FDS is paralleled by the oxidative dissolution of gold that takes place at high TU concentrations via the formation of gold-TU complexes [5][6][7].…”

“…The presence of adsorbed TU and FDS species at metal electrodes and their role in the processes described above is a point still open to discussion. As expected from the strong sulfur-metal interaction, a variety of surface techniques has allowed the detection of irreversible adsorbed TU on gold and platinum electrodes in contact with TUcontaining solutions [4,6,[12][13][14][15][16]. Surface-Enhanced Raman Scattering (SERS) spectroscopy experiments proved the bonding of TU to the metal surface through the sulfur atom [12,13,[16][17][18].…”

“…Moreover, both external reflection infrared spectra [6] and surface-enhanced infrared reflectionabsorption spectra obtained under attenuated total reflection conditions (ATR-SEIRA) [17] showed bands assigned to the symmetric and asymmetric NCN stretching modes of adsorbed TU. The observation of the latter band implies a significant tilting of the S-C axis with respect to the surface normal.…”