The influece of water vapour on carbon monoxide sensitivity of α-Fe 2O 3 microporous ceramic sensors

Abstract

The influence of humidity on the carbon monoxide (CO) sensitivity of α-Fe 2O 3 porous ceramic sensors was investigated by precision volt-amperometric and impedance spectroscpy techniques in the 0-300 ppm CO range. The humidity varied from 0 to 95% r.h. The sensors were prepared by sintering a 130 m 2/g specific area α-hematite powder at 800, 850 and 900 °C, respectively. The microstructural properties were characterized by X-ray diffraction, N 2 adsorption and mercury intrusion porosimetric techniques. The variation of the resistance and conductivity activation energy were evaluated in high vacuum, dry and wet air conditions, as a function of the temperature. The resistance versus temperature relationship was explained, according to literature data, as thermally activated adsorption/desorption mechanisms of the surface oxygen-related species like O -, O 2- and OH -. α-Fe 2O 3 reacts as an n-type semiconductor when exposed to CO and water vapour atmospheres. The maximum Co and water vapour sensitivity, expressed as Δ R/R variations, were found at 300 °C for both gases. Cross-sensitivity tests carried out at 300 and 400 °C activation temperatures showed that water vapour interferes with Co increasing the Δ R/R variation. A catalytic action of the water vapour at 300 ° C is proposed.