Reaction–diffusion effects and systematic design of gas-sensitive resistors based on semiconducting oxides

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Abstract
A method which can be used for the systematic design of gas-sensitive resistors is presented for quantitative evaluation of the kinetic parameters for the catalysed decomposition of certain gases on the surface of semiconducting oxides. The method is based on the measurement of the response to the presence of the gas of the electrical resistivity of the material, measured at different spatially separated regions. The analysis is presented for a disc geometry, with electrodes at the centre and periphery of the disc. For two different oxides, SnO2 and Ba6Fe1.5Nb8.5O30, a transition with increasing gas concentration from first-order to zeroth-order kinetics for combustion of propane and carbon monoxide is shown and the results are related to the variation of the surface coverage of adsorbed oxygen species. Rate constants for oxygen adsorption are derived. The surface oxygen species which controls the combustion rate is not the same as that which mediates the conductivity change.