Potentiometric hydrocarbon gas sensing characteristics of sodium ion conducting zeolite ZSM-5

The sodium ion conducting zeolite Na-ZSM-5 is used as a novel sensitive material in a potentiometric gas sensor with a Na0.85CoO2 reference electrode. At 723 K, a fast voltage response to alkane (C1–C4) partial pressure changes (1–100 kPa) in O2/CO2/N2 base gas is observed. The sensing effect is quantitatively described by a model assuming a change of the Na+ ion activity in the zeolite due to sorption of alkanes in the micropores. The sensor signal is logarithmically dependent on alkane partial pressure at high concentrations and shows a levelling-off behaviour at low partial pressures. The slopes in the logarithmic range are between 99 and 144 mV/decade. The lower detection limits are dependent on the alkane chain length and vary from 12 ppm for n-butane to 1.1% for methane, for an observed standard deviation of 3.3 mV. Cross-sensitivities to O2, H2, and CO2 have different response behaviour and are ascribed to a superimposed effect involving changes of the Na activity at the zeolite/gold interface. Response times (t90) are in the order of 5 min.