Afonja, O; (2013) Use of zeolites to effect discrimination in metal oxide semiconductor gas sensors. Doctoral thesis (PhD), UCL (University College London).

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Abstract

Composite zeolite-semiconducting metal oxide gas sensors have been produced using standard screen printing techniques. Zeolites A, ferrierite, ZSM-5, mordenite, ! and Y, in their acid form, have been incorporated as overlayers or admixtures (ZSM-5, mordenite and !) to tungsten trioxide (WO3) and titanium doped chromium oxide (CTO) thick films screen printed on interdigitated electrode substrates. These composite sensors, in addition to unmodified control sensors, were evaluated for selectivity to specific concentrations of CO, NH3, NO2 and C2 – C4 alkanes, alkenes and alcohols. A new gas sensing rig (AA Rig) capable of housing 8 sensors in a gas-tight enclosure with capability for sequential delivery of test gases over the sensors, control of sensor operating temperature via integrated heater track including DC resistance sensor conductivity measurements was designed and built. Arrays of sensors in batches of 8 were operated at a stable temperature of 400 °C and their responses to low concentration of test gases was monitored and recorded with view to the assembly of array of sensors possessing biased specificity for simple or complex gas mixtures. The dynamic responses of 16 sensors comprising unmodified WO3 and CTO controls (2 different thickness layers per oxide), zeolites H-ZSM-5, H-Mordenite and H-! overlaid and admixed sensors was shown to exhibit degrees of variance and gas specific patterns in their gas responses. The pattern of response of the 8 WO3 sensor array was found to exhibit repeatable and reproducible ‘fingerprints’ for 30 ppm CO, 25 ppm NH3 and 0.5 ppm NO2. Additionally, this WO3 array exhibited distinctive selectivity to 50 ppm alkenes and alcohols for good gas specific fingerprints with primary and secondary alcohol discrimination. Array comprising WO3 and CTO control, H-ZSM-5 and H-Mordenite admixed sensors exhibited significant NO2 selectivity in the binary mixture of CO and NO2, thus demonstrating capability for environmental monitoring.

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