Improvement of Toluene Selectivity via the Application of an Ethanol Oxidizing Catalytic Cell Upstream of a YSZ-Based Sensor for Air Monitoring Applications

The sensing characteristics of a yttria-stabilized zirconia (YSZ)-based sensor utilizing a NiO sensing-electrode (SE) towards toluene (C(7)H(8)) and interfering gases (C(3)H(6), H(2), CO, NO(2) and C(2)H(5)OH) were evaluated with a view to selective C(7)H(8) monitoring in indoor atmospheres. The fabricated YSZ-based sensor showed preferential responses toward 480 ppb C(2)H(5)OH, rather than the target 50 ppb C(7)H(8) at an operational temperature of 450 °C under humid conditions (RH ≃ 32%). To overcome this limitation, the catalytic activity of Cr(2)O(3), SnO(2), Fe(2)O(3) and NiO powders were evaluated for their selective ethanol oxidation ability. Among these oxides, SnO(2) was found to selectively oxidize C(2)H(5)OH, thus improving C(7)H(8) selectivity. An inline pre-catalytic cell loaded with SnO(2) powder was installed upstream of the YSZ-based sensor utilizing NiO-SE, which enabled the following excellent abilities by selectively catalyzing common interfering gases; sensitive ppb level detection of C(7)H(8) lower than the established Japanese Guideline value; low interferences from 50 ppb C(3)H(6), 500 ppb H(2), 100 ppb CO, 40 ppb NO(2), as well as 480 ppb C(2)H(5)OH. These operational characteristics are all indicative that the developed sensor may be suitable for real-time C(7)H(8) concentration monitoring in indoor environments.