In-situ infrared studies of the Cd-UPD mediated reduction of nitrate on gold

Abstract

Underpotential-deposited cadmium on Ag, Au and Pt has been found to exhibit electrocatalytic properties for the reduction of nitrate in aqueous media [1,2]. Insight into the mechanism associated with this process has been recently obtained from experiments involving rotating ring-disk electrodes [2]. An unambiguous identification of reaction products, however, requires the use of in situ methods with molecular and atomic specificity, an area that has experienced extraordinary progress in recent years [3]. Particularly relevant to the study of the electrochemical properties of simple nitrogen containing compounds is the work of Bruckenstein and co-workers who pioneered the use of on-line mass spectrometric techniques for the detection and identification of gas phase products [4]. One of the main objectives of this work is to elucidate the mechanistic pathways associated with the reduction of nitrate on Au mediated by Cd-UPD by using in situ Fourier Transform Infrared Reflection Absorption Spectroscopy, FTIRRAS, for monitoring solution phase species. As will be shown, the results obtained have provided evidence that for [H+] < [NO;] the electrocatalytic process yields nitrite as the only detectable product. At least two well defined isosbestic points could be identified in plots of potential difference spectra as a function of the applied voltage. This indicates that to the level of sensitivity of in situ FTIRRAS the nitrate/nitrite conversion is quantitative. Additional evidence in support of this view was provided by the fact that the magnitude of the ratio of the integrated absorption bands of nitrate and nitrite was found to be constant over a wide potential region.