# Direct Solar Thermal Water-Splitting Using Iron and Iron Oxides at High Temperatures: A Review
## 3 Mechanisms of High-Temperature Oxidation
The general reaction for the production of H\({}_{2}\) is shown below:  
\[2H_{2}O+2e^{-}\to 2 OH^{-}+H_{2} \tag{15}\]  
The mechanism of evolution of H\({}_{2}\) (HER) would occur on the surface of the pores and micropores, where the hydroxyls (OH\({}^{-}\)) formed by the dissolution of the vapour are discharged through a cathodic reaction according to Figure 3.  
[PERSON] et al. [62] proposed that the oxide layer is still thin and flexible in the first step the of oxidation process. In the presence of H\({}_{2}\)O\({}_{(\mathrm{g})}\), the transport mechanisms are likely to occur specifically through cavity or pore formation [63]. The mechanism is represented by Figure 4.  
Figure 3: Diagram of the electrochemical dissolution of iron. The symbol “?” means that the question remains about the interaction between metal (Fe) and water vapor. [PERSON] et al. [37] proposed that the H\({}_{2}\)O\({}_{\text{(g)}}\) is involved in the transport processes that lead to scale growth in the oxide layer. The researchers indicate that the Fe cations (Fe\({}^{2+}\)/Fe\({}^{3+}\)) can simultaneously diffuse towards the outer surface in this process. Based on the knowledge of the structure and diffusion of iron oxides, the oxidation mechanism of pure iron above 570 \({}^{\circ}\)C in an iron\(-\)oxygen systems can be better understood. In particular, in the large pores in the inner part of the scale of FeO, a circulation mechanism is assumed that consists of the oxidation of Fe in contact with H\({}_{2}\)O\({}_{\text{(g)}}\), releasing H\({}_{2}\) to can move back in the pore towards the outer part of the scale, where it reduces the oxide, thus forming an H\({}_{2}\)O\({}_{\text{(g)}}\) molecule again [64]. The first reaction that occurs is from the formation of OH\({}^{-}\) ions, which would increase cation vacancies and would also be the main diffusion species [11; 12; 52]; this reaction is expressed as follows:  
\[Fe+2H_{2}O\ \to Fe^{2+}+2 OH^{-}+H_{2} \tag{16}\]  
[PERSON] et al. [52] suggested the formation of ferrous oxide (FeO) as shown in Figure 5; that is, iron in the presence of water vapour at high temperatures will precipitate FeO and Fe\({}_{3}\)O\({}_{4}\)[52] or Fe\({}_{2}\)O\({}_{3}\)[36], as follows:  
\[H_{2}O+G\leftrightarrow H+OH+G \tag{11}\]0  
\[H_{2}O+G\leftrightarrow H+OH+G \tag{11}\]1 The mechanism in O\({}_{2}\)-H\({}_{2}\)O vapour mixtures after long periods (adapted from [62]). \[H_{2}O+G\leftrightarrow H+OH+G \tag{11}\]2 (a) Cross-sectional SEM image of oxides formed on iron after 1 h of oxidation at 650 \({}^{\circ}\)C and (b) cross-sectional SEM image of oxide scale formed on iron after 10 h of steam oxidation at 650 \({}^{\circ}\)C, where the outermost white layer is nickel-plated (from [52]). However, FeO in contact with Fe(OH)\({}_{2}\)[65] is generated at temperatures above 1300 \({}^{\circ}\)C [28] and can form Fe\({}_{2}\)O\({}_{3}\) in conjunction with the release of H\({}_{2}\)[52]. [PERSON] et al., in 1965 [62], proposed the existence of pores at the Fe/FeO interface during the oxidation process, where they form oxide bridges from the metal to the scale. This structure allows for further oxidation of the metal without substantial inhibition. A mixture of H\({}_{2}\)/H\({}_{2}\)O is formed in these pores, and through an oxidation/reduction mechanism, O\({}_{2}\) is transported to the Fe surface. An oxide of the Fe\({}_{n}\)O\({}_{m}\) type will typically contain a variety of defects. These defects are crucial for the transport of material through the oxide layer and thus play a critical role in the oxidation process [66; 12]. [PERSON] et al. [52] suggested that during the initial period (before 5 h), the presence of a single layer of columnar grains may facilitate relatively fast transport of OH\({}^{-}\) ions across grain boundaries. It is therefore reasonable to assume that the hydroxyl ions interact with the surface hematite and magnetite, forming FeO. However, certain aspects remain under investigation. [PERSON] et al.