Methanol gas-sensing properties of CeO_2-Fe_2O_3 thin films

摘要

Ce-doped iron oxide thin films have been prepared by a liquid-phase method (LPD) and tested as methanol gas sensors. Sensing tests have shown that the addition of Ce to Fe_2O_3 increases the response to methanol at low temperature ( < 350 ℃). The optimum loading of Ce was 50% in moles. At operating temperatures > 400 ℃, the response of CeO_2-doped Fe_2O_3 sensors is instead lower than that of pure Fe_2O_3. To find properties-sensitivity relationships, the microstructure, acid-base properties and reactivity of CeO_2-Fe_2O_3 powders were investigated by X-ray diffraction (XRD), temperature-programmed desorption of adsorbed CO_2 (TPD-CO_2) and gas-phase oxidation of methanol. Ce-doped iron oxides powders have shown the formation of a Fe_xCe_(1-x)O_2 solid solution, smaller grain size and an increase of the number of basic sites compared to pure Fe_2O_3. Moreover, the addition of Ce to Fe_2O_3 was found to promote the adsorption of methanol and its subsequent oxidation to CO_2. Correlation between the amount of basic sites, oxidation activity and the response to methanol were reported. According to the obtained results, the role played by Ce on modifying the methanol-sensing properties of iron oxide thin films was related to its capability to: (a) form a solid solution and reduce the grain size; (b) enhance the number of basic sites and methanol adsorption; (c) promote the methanol oxidation activity of the sensing layer. This latter was found to be the factor limiting the sensors response at higher temperatures.