THE INFLUENCE OF WATER-VAPOR ON CARBON-MONOXIDE SENSITIVITY OF ALPHA-FE2O3 MICROPOROUS CERAMIC SENSORS

The influence of humidity on the carbon monoxide (CO) sensitivity of alpha-Fe2O3 porous ceramic sensors was investigated by precision volt-amperometric and impedance spectroscopy 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 m2/g specific area alpha-hematite powder at 800, 850 and 900-degrees-C, respectively. The microstructural properties were characterized by X-ray diffraction, N2 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-, O2- and OH-. Alpha-Fe2O3 reacts as an n-type semiconductor when exposed to CO and water vapour atmospheres. The maximum CO and water vapour sensitivity, expressed as DELTAR/R variations, were found at 300-degrees-C for both gases. Cross-sensitivity tests carried out at 300 and 400-degrees-C activation temperatures showed that water vapour interferes with CO increasing the DELTAR/R variation. A catalytic action of the water vapour at 300-degrees-C is proposed.