Reoxidized

Reoxidized is a term used in chemistry and materials science to describe a substance that has undergone oxidation again after a prior reduction or after an initial oxidation, resulting in the formation or restoration of an oxide layer.

In practice, reoxidation occurs when a reduced metal, oxide, or reduced oxide is exposed to an oxidizing environment, typically oxygen or air, at sufficient temperature. The oxide layer that forms can protect the material from further degradation, modify surface chemistry, or alter catalytic activity.

Common contexts include ceria- and zirconia-based materials used in catalysis and solid oxide fuel cells, where the oxide lattice hosts oxygen vacancies that can be replenished by reoxidation. For example, ceria (CeO2) can be reduced to CeO2-δ during operation and then reoxidized to CeO2 in air, regenerating its oxygen storage capability. In metal processing, metals such as iron or copper develop oxide layers upon exposure to air; reoxidation can restore protective oxides or, conversely, change corrosion behavior.

Reoxidation kinetics depend on temperature, oxygen partial pressure, and the diffusion of oxygen through the existing oxide; some oxides exhibit nonstoichiometric compositions (for example CeO2-δ) that adjust during redox cycling. Proper control of reoxidation is important in catalytic regeneration, corrosion resistance, and energy storage applications.

See also oxidation and reduction, oxide layer.