Interaction of Carbon Monoxide with Anatase Surfaces at High Temperatures: Optimization of a Carbon Monoxide Sensor

Abstract

Sensing the presence of particular gases in harsh environments, such as at high temperatures, poses challenges in the choice of materials as well as in measurements of the appropriate sensing-related property of the material. In this study, we examine the sensing of carbon monoxide (CO) in a nitrogen background at temperatures up to 600 °C using the anatase phase of TiO2 as the sensing material. In particular, the change in resistance of anatase is used to detect the presence of CO. Copper oxide (CuO) is added to anatase to increase the sensitivity toward CO detection. However, the presence of CuO led to partial transformation of anatase to rutile at temperatures of 800 °C used for bonding the sensor material to the sensing platform. By adding La2O3 to the CuO/anatase, the anatase phase is maintained under all thermal treatments. Diffuse reflectance infrared spectroscopy is used to examine the mechanism of CO oxidation. Interaction of lanthanum with the anatase increased the reactivity of the anatase surface toward CO. In addition, the presence of CuO led to increased adsorption of CO as well as enhanced desorption of CO2, explaining the enhancement of the sensitivity of the CuO-containing anatase toward sensing of CO. Electron microscopy has provided information on the microstructure of the sensor material. An effective medium approximation theory is used to model the observed resistivity data over the temperature range 400−600 °C. The energies of adsorption of CO and the reaction of CO with adsorbed oxygen to form CO2 are extracted. These values are consistent with the role of CuO acting as a catalyst. This study demonstrates that anatase doped with lanthanum along with the presence of surface/CuO is an effective sensor for CO at temperatures as high as 600 °C.