Catalytic conversion measurements

I n general, the target gases are reacting with the surface of the heated MOX gas sensor and are consequently converted into different products, depending on the gas itself and the background conditions. In normal air conditions, for example, reducing gases like CO and H₂ are oxidized into CO₂ and H₂O by reacting with pre-adsorbed oxygen species at the surface of the sensor (generally considered to be at the origin of gas sensor response). A powerful and simple method that allows getting an idea about the reaction paths and the mechanisms taking place during these reactions is, therefore, to measure the changes in the composition of the ambient atmosphere determined by the sensing. Practically speaking, one monitors the gas composition before and after the sensor test chamber. For the different kind of gases several analyzing techniques can be applied:

• Spectroscopic gas analyzers (Innova 1312 and 1301)

• A hygrometer for monitoring water production

• An electrochemical oxygen gas analyzer (Zirox SGM 400)

• A paramagnetic oxygen analyzer (ABB)

• A chemiluminescence analyzer (ML 9841B) for the detection of NO_x

• A mass spectrometer (EI) with an inlet capillary for gases

• CO consumption of Pd doped SnO₂ based sensors, J. Kappler, A. Tomescu, N. Barsan, U. Weimar, Thin Solid Films, 391, 2001, 186-191.

• Sensing of hydrocarbons with tin oxide sensors: Possible reaction path as revealed by consumption measurements, W. Schmid, N. Barsan, U. Weimar, Sensors and Actuators B, 89, 2003, 232-236.

• Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO₂ sensors in the presence of humidity, N. Barsan and U. Weimar, J. Phys. Condens. Matter, 15, 2003, R813-R83.

• CO sensing with SnO₂ thick film sensors: role of oxygen and water vapour, S. Hahn, N. Barsan, U. Weimar, S.G. Ejakov, H.J. Visser, R.E. Soltis,, Thin Solid Films, 436, 2003, 17-24.