# Direct Solar Thermal Water-Splitting Using Iron and Iron Oxides at High Temperatures: A Review
## 2 Theoretical Background
[PERSON] and [PERSON] et al. [23] indicated that the oxidation of FeO in the presence of water vapour is a spontaneous reaction (\(\Delta G<0\)) at temperatures below 1000 K.  
In the use of iron (Fe) and its oxides at high temperatures for hydrogen (H\({}_{2}\)) production, there are various mechanisms that would allow material agglomeration to occur. The adhesion or clumping of particles notably affects the efficiency and stability of the thermochemical process. Agglomeration is influenced by three factors: (i) at high temperatures, it is normal for Fe and Fe\({}_{\text{n}}\)O\({}_{\text{m}}\) particles to fuse together, forming larger particles; (ii) changes in the crystalline structure of Fe and Fe\({}_{\text{n}}\)O\({}_{\text{m}}\) at high temperatures cause physical changes that lead to agglomeration; and (iii) the thermal expansion of these materials causes mechanical stresses that facilitate agglomeration. During the chemical loop process, the morphology and support materials of the iron oxides can influence the stability and effectiveness of the process, as oxygen carriers play a crucial role during reactions, causing changes in their physical structures across several high-temperature redox cycles. Moreover, factors such as the reducing atmosphere and the behaviour of specific iron oxides such as hematite and magnetite under certain high-temperature conditions can contribute to different agglomeration behaviours, which affect the mechanism of hydrogen evolution [24; 25]. [PERSON] et al. [26] suggested that various metal oxides can form volatile compounds through direct reactions with water vapour and that hydroxides and oxy-hydroxides can be produced by hydration [27]. On the other hand, [PERSON] et al. [28] demonstrated the existence of a volatile iron hydroxide at temperatures exceeding 1300 \({}^{\circ}\)C. Compared to non-volatile metal oxides, volatile metal oxides exhibit a higher oxygen-exchange capacity, which directly correlates with their hydrogen-production capacity [29]. Iron hydroxides modify their reactive properties under specific conditions at high temperatures. The volatile iron hydroxides are (i) Fe(OH)\({}_{2}\) and (ii) Fe(OH)\({}_{3}\). These hydroxides can form under reducing conditions. Fe(OH)\({}_{2}\) is less stable than Fe(OH)\({}_{3}\) and can easily dehydrate or decompose at high temperatures, potentially forming iron oxides and releasing water vapour. On the other hand, Fe(OH)\({}_{3}\) can also undergo dehydration or decomposition when it is subjected to high temperatures. The stability, reactivity, and transformations of these hydroxides under elevated thermal conditions are key to optimizing the efficiency of hydrogen production in thermochemical cycles [30]. Table 1 displays the thermodynamic parameters (\(\Delta\)G\({}^{\circ}\), \(\Delta\)H\({}^{\circ}\), \(\Delta\)S\({}^{\circ}\), and T) used to determine the activation energy for the reaction of water vapour with iron and the products of vapour dissociation into molecules, ions, and iron-oxidation products (oxides and hydroxides). Table 2 presents the typical thermodynamic parameters generated during the oxidation/reduction processes of metals in the presence of H\({}_{2}\)O\({}_{\text{(g)}}\) at high temperatures.