Cyanate and Cyanurate Adsorption at Silver Electrodes in Neutral Solutions: In Situ ATR-SEIRAS and DFT Studies

Abstract

The adsorption and reactivity of cyanate at silver electrodes is studied spectroelectrochemically in sodium perchlorate solutions and compared to those of cyanuric acid (1,3,5-triazine-2,4,6-trione, C3H3N3O3). The surface enhanced infrared reflection absorption spectra obtained under attenuated total reflection conditions (ATR-SEIRAS) for low cyanate concentrations show that some amount of adsorbed CO is formed at the silver electrode surface at low electrode potentials. Adsorbed cyanate species are formed at higher potentials and predominate at the electrode surface. However, for cyanate concentrations above 1 mM, the ATR-SEIRA spectra show potential-dependent absorption features between 1800 and 1300 cm–1, which are similar to those obtained after dosing cyanuric acid to the sodium perchlorate solution. This observation suggests that adsorbed cyanuric acid related species are formed from cyanate anions at the silver surface sites as the result of an electroless trimerization reaction. The infrared spectra collected in the cyanuric acid containing solutions also show a higher amount of adsorbed CO at low potential when compared to that observed in cyanate-containing solutions, thus indicating certain instability of cyanuric acid at the silver surfaces. Density functional theory (DFT) calculations have allowed the study of the effect of crystallographic orientation and bonding geometry on the adsorption energy and vibrational frequencies of adsorbates. Also, the effect of adsorbate coverage on the vibrational frequencies has been investigated. In the case of cyanate, DFT calculations show that the most favorable adspecies is N-bonded cyanate adsorbed in 4-fold hollow (100) sites. The experimental spectra in cyanuric acid solutions agree with the formation of adlayers of specifically adsorbed triketo-monocyanurate species that adsorb, in a tridentate configuration, perpendicular to the electrode surface. The polyoriented nature of the silver surface used in the spectroelectrochemical experiments, together with the eventual existence of cyanate in domains with different local coverages, can explain the broadness of the band for the asymmetric OCN stretching. For adsorbed triketo-monocyanurate, high local coverages are at the origin of the band observed experimentally at ca. 1790 cm–1 that can be associated with the in-phase collective CO stretching.