Anybody can help me to determine whether Ethylene gas is a reduction gas or oxidation gas when it reacts with metal oxide sensor? Or is it possible Ethylene gas can be either reduction or oxidation depending on metal oxide the material is exposed to?
Usually, the ethylene gas is a reduction gas. You can measure the relationship between resistance of the gas sensor when measure the gas with temperature, then a conclusion can be obtained.
Usually the reducing gases are donating electrons to the metal oxide and depending on the type of the oxide either n-doped or p-doped MOx the response will be different and not the character of the gas itself. An n-type MOx is one where the majority charge carriers are electrons, and upon interaction with a reducing gas an increase in conductivity occurs (while, an oxidizing gas serves to deplete the sensing layer of charge carrying electrons, resulting in a decrease in conductivity). A p-type semiconductor is a material that conducts with positive holes being the majority charge carriers; so, the opposite effects are observed with the material and showing an increase in conductivity in the presence of an oxidizing gas (where the gas has increased the number of positive holes). A resistance increase with a reducing gas is observed, where the negative charge introduced in to the material reduces the positive (hole) charge carrier concentration.
Generally, one considers ethylene (C2H4) as a reducing gas BUT it depends on with which background gas you are using to measure it. For example, if you use an innert gas (like Argon) which do not react with ethylene then it will react as reducing as. If you use oxygen or Air or oxygen rich background gas then there are chances that C2H4 will convert into CH3CHO, having oxidizing nature.
In mixed-potential type sensors, it is considered similar to CO or propene, reducing gas. I hope this would be same in the resistive type as well.
Dear And Rashid,
As Azhar mentioned, it depends on the background gas and its possible interaction with Ethylene, especially if the sensing material has also catalytic activity. If there is no side reaction, it is considered as an electron donor (reducing).
However, if there is possible side reactions, i.e. presence of oxygen in the background gas, ethylene can be oxidized to different species (CH3CHO, CO, H2, OH, CO2, …) depending on the catalytic performance of the sensing material. Then, the sensing response will depend on the population of these species at the surface of the sensing material, which in turn, depends on the desorption temperature of these species.
Regards
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