Oxygen Vacancy Concept in Catalysts
1. Oxygen Vacancies Concept – What is Oxygen Vacancies?
The concept of Oxygen Vacancies OVS was first proposed in 1960 (Superficial chemistry and solid imperfections. Nature, 1960, 186: 3-6) for the study of the mechanism of gas interaction with solid metal oxides. The study of the mechanism of interaction between gases and solid metal oxides. A specific external environment (e.g., high temperature) causes oxygen detachment from the lattice, leading to oxygen deficiency and the formation of oxygen vacancies, and the defect equation can be expressed as.
MOx-ʎO lattice = V0+ MOx-1+ʎ/2O2
In short,Oxygen vacancies are defects formed in the metal oxide lattice when oxygen is removed from one oxygen atom.
In the case of metal oxides, the oxygen vacancy is a type of defect (point defect). In metal oxides, the electronegativity of other elements is generally less than oxygen, so when the loss of oxygen, the equivalent of taking an oxygen atom plus two positively charged electrons – holes, if the two electrons – holes are bound in the oxygen vacancies on the oxygen vacancies that are generally positively charged.
2. What is the role of oxygen vacancies?
a. Regulate the electronic structure of metal oxides
a. Regulate the electronic structure of metal oxides
The presence of oxygen vacancies causes the Fermi energy level of the oxide to move upward, resulting in the appearance of defect energy levels in the band gap and thus reducing the width of the energy band and improving the light absorption performance.
Promoting carrier separation
Oxygen vacancies promote the conversion of excitons into carriers and accelerate the surface reduction half-reaction to promote carrier separation. oVS defects generate unsaturated coordination sites on the oxide surface (edges, corners or terraces).
b. As active sites
Oxygen vacancies optimize the adsorption energy of reactants on the catalyst surface, thereby lowering the reaction energy barrier and promoting molecular activation. In catalysts OVS act synergistically with nearby active metal sites.
2. how to create oxygen vacancies in metal oxides?
3. What kind of oxides are in a position to create oxygen vacancies?
Oxides can be divided into two categories based on their chemical behavior: non-reducible and reducible oxides. Reducible oxides change their oxidation state due to the corresponding metal cation, and non-reducible oxides consist of materials that do not readily lose oxygen. Since oxygen is in the O-2 oxidation state, the excess electrons that remain on the material by removing the neutral O atoms cannot be accommodated in the excessively energetic cation holes, which leads to the formation of conduction bands in the material. These oxides, such as SiO2, MgO, Al2O3, etc., belong to this class. Typically these materials are characterized by separated valence bands (VB) separating very large band gaps (typically >3 eV).
Therefore oxygen ions in oxides are difficult to separate. When oxygen atoms are removed in the form of O2 or H2O, the excess electrons left on the material are trapped in specific positions (e.g. oxygen vacancies) and create new defect states in the band gap. This process is energetically very expensive and therefore these non-reducible oxides are highly stoichiometric, stable and chemically inert. Conversely reducible oxides are characterized by their ability to exchange oxygen with relative ease. This is due to the fact that the empty state available on the material consists of cationic d orbitals, which are not too energetic relative to VB. These oxides typically have semiconducting properties with a band gap <3 eV. the removal of oxygen leads to redistribution of excess electrons across the cationic null energy levels, thus transforming their oxidation state from Mn to M(n-1). Transition metal oxides such as TiO2, WO3, NiO, Fe2O3, CeO2 , Co3O4, etc.
i.e. Mars and van Krevelen mechanism: refers to the reaction process as a reaction of reactants with catalyst lattice oxygen ions. The first step is that the reactants and catalyst produce oxygen vacancies that are reduced. The second step is that the catalyst is regenerated by reoxidizing the catalyst by replenishing the oxygen vacancies with dissociated adsorbed oxygen. Since the first step is the reduction of the oxide catalyst and the second step is the oxidation of the catalyst, this mechanism is also known as the redox mechanism.
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