Test of the water adsorption model of organic electrocatalysis in the carbon monoxide–silver system in alkaline medium

Abstract

A test has been carried out of the model of T. Iwasita, X.H. Xia, H.-D. Liess, W. Vielstich [J. Phys. Chem. B. 101 (1997) 7542], according to which the maximum at about the same potential in both the positive and negative sweeps in cyclic voltammograms (CVs) of small organic molecules on Pt is due to the concurrence of two processes with opposite potential dependences, adsorption of the organic compound and electrooxidation of its intermediates, which decrease and increase, respectively, with increasing potential. In turn, the decrease with increasing potential of the adsorption of the undissociated organic is due to its increasing displacement by molecular water, this competition occurring because the two molecular compounds have similar, low values of the adsorption energy. According to the model of T. Iwasita, X.H. Xia, H.-D. Liess, W. Vielstich [J. Phys. Chem. B. 101 (1997) 7542], with CO on Pt no anodic currents are observed in the negative sweep because of the high adsorption energy of CO on Pt, which precludes its displacement by water. Therefore, the model has been tested with the CO–Ag system, for which anodic currents should be observed in the negative sweep, since the adsorption energy of CO on Ag is very low. Effectively, this has been found to be the case, which indicates that the model is indeed applicable to the CO–Ag system over the tested pH range, 10–14. At pH⩽11 adsorbed CO was displaced from the surface of Ag by N 2 bubbling, while it was not at pH⩾12. However, even at pH⩾12 the adsorption energy of CO on Ag should be rather weak, since anodic currents appeared in the negative sweep in CO-saturated solution over the whole pH range tested, 10–14.