Zirconia-based oxygen transport

Platinum supported on ceria-zirconia-based oxides shows high activity and selectivity in selective oxidation of hydrocarbons to syngas. The oxygen transport from the support to the metal phase plays a key role in the mechanism of this reaction and therefore deserves to be investigated carefully for advanced kinetics and mechanistic pathways. 

At high oxygen pressures, oxide phases show defect electron (hole) conduction (oxidation semiconduction) and at low oxygen pressures excess electron conduction (reduction semiconduction). The transport number of excess electrons in Zro.ssCao.isOi.ss as a function of the oxygen partial pressure could be determined by measurements with a Ca,CaO/air cell [79]. The hole conduction of zirconia-based solid electrolytes was noticed for the first time when cells with Ni,NiO reference electrodes for gas potentiometry [44,91] were tested in air. The harmful oxygen permeability was measured potentiometrically in 1965 [92]. 

In general Zr02 oxygen sensors consist of a tube-like solid-state Zr02 electrolyte where the electronic conductivity is based on oxygen ion charge carrier transport. The inner and outer surface of the yttrium-doped and stabilized zirconia tube is covered by porous platinum electrodes. 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

The investigations of SOEs with oxygen ion conductivity (mainly based on zirconia, ceria and gallates) and with protonic conductivity (mainly cerates and zirconates) included detailed studies of their ceramic and transport properties, contact resistance of grains in SOE, ageing processes depending on the mode of fabrication, temperature, composition of material, impurities, gas atmosphere, and other factors. 

In Section 13.63, it was stated that oxygen-conductive membranes are under development. These membranes are suitable for advanced power generation requiring oxygen for combustion or gasification and are based on zirconia and perovskite where oxygen is transported throngh the material as 0 . 

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