The mechanism of operation of tin(iv) oxide carbon monoxide sensors

Abstract

The electrical properties of tin(iv) oxide render it an excellent material for the detection of very low levels of carbon monoxide, and several devices based on sintered pellets of SnO₂ are commercially available^1,2. Investigations of the operation of such sensors have generally been restricted to studies of the electrical behaviour of the oxide and little attention has been paid to the relationship between the surface adsorption phenomena and electrical changes in the bulk oxide. These studies³ demonstrated the consistency of a model in which conductance is effectively controlled by the population of negatively charged oxygen adsorbates. The chemical reactions occurring at the oxide surface, although unsubstantiated, have been assumed to involve CO adsorption, desorption of CO₂ (which produces an increase in conductance) and replenishment of the resulting surface oxygen vacancy by adsorption of molecular oxygen⁴. Here we describe experiments in which the nature of the surface species adsorbed into tin(iv) oxide from atmospheres containing low levels of carbon monoxide is monitored using transmission infrared spectroscopy while simultaneously measuring the electrical changes produced in the bulk oxide. The data demonstrate the effect of heat treatment on the formation of adsorbate species and the mutual interdependence of adsorbate species and bulk oxide electrical conductance. This relationship is important both for sensor operation and heterogeneous catalysis.