Thick-film impedance based hydrocarbon detection based on chromium(III) oxide/zeolite interfaces

Selective hydrocarbon (HQ gas sensors can help to further optimize on-board monitoring of catalysts in automotive exhaust gas after treatment systems. A highly selective hydrocarbon gas sensor is obtained by covering Au interdigital electrodes (IDE) with zeolite and chromium(III) oxide thick-films. As a novel arrangement, the zeolite does not cover the semiconducting metal oxide as a catalytically active filter layer. A Cr2O3-thick-film is applied on top of an ion-conducting Pt-doped ZSM-5 zeolite thick-film. Impedance spectra of such sensors show high-frequency semicircles, which are assigned to ionic mobility in the zeolite bulk material, a hydrocarbon dependent semicircle in the medium-frequency range and low-frequency contributions. Measurements at fixed frequency in the gas dependent range show high sensitivity to different hydrocarbon gases and no cross sensitivity toward CO and H-2. This new sensing mechanism is attributed to the interface between zeolite and Cr2O3. A typical filter effect or the known resistive sensing mechanism of p-type Cr2O3 were ruled out as main effects. (c) 2006 Elsevier B.V. All rights reserved.