Hunting the active species in gas sensing processes - operando studies on tin dioxide based sensors

摘要

The understanding of the gas sensing mechanism at a fundamental level implies the knowledge of the state of preadsorbed surface species. Some question marks on the commonly accepted ideas were raised by the recorded higher sensitivity of sensors to CO in nitrogen and by the fact that the combustion of CO was observed in air and in humid nitrogen. These facts question the monopoly of oxygen ions as the reaction partners for CO and they were the driving force for thereby presented investigations. DRIFT Spectroscopy and resistance measurements have been simultaneously applied to discriminate between the species that are actively taking part in the sensing processes and spectators. The comparison between the different sensors has been focused on verifying whether the observed phenomena are general or whether they depend on the technology. It was observed that for SnO_2 sensors, the reaction of oxygen, with water results in the formation of terminal hydroxyls and the release of an electron to the conduction band. It indicates that water compete with reducing gases for the oxygen ions. This phenomenon was independent of the technology and thus it could be SnO_2 characteristic. It was shown that CO reacts preferentially with ionosorbed oxygen at the surface of tin dioxide. In the case of lack of oxygen different scenarios are possibly dependent on hydration state of the surface.