Detection and differentiation of C4 hydrocarbon isomers over the Pd–SnO2 compressed powder sensor

The detection and differentiation of C4 hydrocarbon gases, including n-butane, 1-butene, iso-butylene, cis- and trans-2-butene are investigated over a palladium-promoted tin oxide sensor using AC impedance. It is found that 1% Pd–SnO2 gives a good linear response and good sensitivity with respect to a wide range of n-butane concentrations (0–10,000 ppm). At high operation temperatures (≥400°C), the tin oxide-based sensor offers no differentiation on different C4 gases and is consistent with the literature that this type of sensor is rather non-selective. However, at ≤300°C, the sensor gave broadly two different types of sensitivity towards the C4 gases of the same concentration: the sensitivities of n-butane and trans-2-butene are analogous, while 1-butene, iso-butylene and cis-2-butene all show sensitivities a factor of 20–30 higher. The sensitivity difference apparently depends on the chemical reactivity of the particular C4 molecules, which in turn depends on the presence and position of their double bond. Hence, high sensitivity is obtained if the positions of their double bond are more susceptible for attack by surface oxygen species leading to total surface combustion of the C4 hydrocarbons. On the other hand, if there is an absence or a surface inaccessible double bond as in the cases of n-butane and the trans-2-butene, surface attack to the hydrocarbon gases involves a kinetically more difficult hydrogen abstraction reaction, hence giving a lower sensitivity.