DFT and spectroelectrochemical study of cyanate adsorption on gold single crystal electrodes in neutral medium

Abstract

The adsorption of cyanate anions at Au(111) and Au(100) single crystal electrodes has been studied spectroelectrochemically in neutral solutions. Potential-dependent in situ InfraRed Reflection Absorption spectra obtained below the onset of cyanate oxidation were compared with previously published data and analyzed on the basis of periodical Density Functional Theory (DFT) calculations. The calculated adsorption energies for cyanate and related species suggest that cyanic and isocyanic acid adsorb weakly at the studied gold surfaces and, thus, seems not to be at the origin of any of the adsorbate bands in the experimental infrared spectra collected in the cyanatecontaining solutions. The latter features can be clearly ascribed to the asymmetric OCN stretching of N-bonded cyanate species. The observation of absorption bands in a wide spectral region, including features above 2200 cm -1 , agrees with the coexistence of N-bonded cyanate species with different adsorption sites and tilting angles. DFT calculations have revealed that although these adspecies can have significantly different frequencies, their adsorption energies are rather close. In addition, the existence of collective in-phase vibrations at relatively high cyanate coverages also contributes to the widening of the absorption bands.

Spectroelectrochemical detection of specifically adsorbed cyanurate anions at gold electrodes with (111) orientation in contact with cyanate and cyanuric acid neutral solutions

“…infrared absorption by adsorbates and the removal of interferences due to the bulk solvent characteristic of the ATR-SEIRAS technique [13,14] helped in the observation of new absorption bands in the range between 1800 and 1300 cm -1 , not detected previously in the external reflection experiments with gold single crystal electrodes [11]. These bands appear far from the typical wavenumbers for adsorbed cyanate (either adsorbed or in solution), thus suggesting the formation of new adsorbed chemical species.…”

“…In addition, the existence of collective in-phase vibrations at relatively high cyanate coverages also contributes to the widening of the absorption bands. The calculated adsorption energies [11] for cyanic and isocyanic acid indicate that these species adsorb weakly at the studied gold surfaces and, thus, seem not to be at the origin of any of the adsorbate bands.…”

“…Cyanate is a simple anion species whose adsorption and oxidation have been studied in the past at various electrode materials [1][2][3][4][5][6][7][8][9][10][11]. Published papers include in situ spectroscopic studies (either infrared [1][2][3][4][5][6][7][8][9][10][11] or Raman [7] ) dealing with the identification of the adsorbed species formed from cyanate-containing solutions as well as the corresponding cyanate oxidation products. The adsorption of cyanate was detected as a potential-dependent process as witnessed by the observation of a band around 2220 cm -1 , ascribed by most authors to N-bonded cyanate anions [2][3][4][5][6]10].…”

“…This latter process would be induced by the protons released upon the electrochemical oxidation of the electrode surface. Taking into account the calculated optimized adsorbate geometries, adsorption energies and vibrational harmonic frequencies obtained from DFT calculations for cyanate and related species adsorbed at gold [11],…”

“…In the papers commented above, no bands below 1800 cm -1 were reported for in situ infrared spectra, which were collected under external reflection conditions [1][2][3][4][5][6][7][8][9][10][11], thus indicating that adsorbed cyanate was the only adsorbate formed from dissolved cyanate anions. However, in a more recent paper, we have reported the first ATR-SEIRAS study of cyanate adsorption at Au(111)-25 nm [12] that changes the picture described above.…”