Sensing small molecules with amperometric sensors

Amperometric sensors based on stabilized zirconia are well established with respect to the detection of oxygen. Recent trends aim at the detection of oxygen containing gases like NO, or combustible gases such as CO, H-2 and hydrocarbons. Concepts with multi-electrode amperometric sensors are able to detect O-2 and NO or O-2 and combustibles at the same time. However, the clear separation of different electrode reactions is often a problem. Highly selective electrodes for specific reactions are desired to improve sensor performance. In this context, the understanding of the mechanisms of electrode reactions is important for the optimization of sensor properties. Electrochemical as well as catalytic reactions determine the electrode properties. NO-reduction at La1-xSrxMnO3-electrodes proceeds via electrochemically generated oxygen vacancies. Silver under the same conditions has no activity for NO-reduction. In situ surface spectroscopic investigations combined with electrochemical methods under applied potentials provide deeper insight in the processes at the electrode surfaces. Work function changes detected with ultraviolet photoelectron spectroscopy (UPS) on evaporated Ag electrodes could be attributed to the potential dependent formation of subsurface oxygen. The latter was identified by X-ray photoelectron spectrosopy (XPS). Electrochemical investigations reveal an influence of oxygen desorption kinetics from the surface. Frequency dependent measurements further indicate that the desorption kinetics is further modified by a slow process. (C) 2002 Elsevier Science B.V. All rights reserved.