# Direct Solar Thermal Water-Splitting Using Iron and Iron Oxides at High Temperatures: A Review
## 5 Fluidized Bed Reactors for Thermochemical Water Splitting and Their Materials  
It is important to highlight that iron oxide is selected because it is an environmentally friendly base material that is relatively low-cost and is commonly used in redox reactions due to its high oxygen-by-weight ratio, which ensures sufficient reaction time [54]. According to the reviewed scientific literature, temperatures above 1300 \({}^{\circ}\)C can be achieved using a thermochemical solar reactor with metallic materials such as cerium oxide or zinc oxide in a fluidized bed [77; 79]. The thermal outputs obtained in reactors using water steam and various materials in a fluidized bed are presented in Table 5. Table 5 presents the materials used in the production of H\({}_{2}\) via fluidized beds, along with pertinent details such as (i) thermal potential, (ii) temperature ranges within the reactors, (iii) type of fluidized material, and (iv) particle size. Additionally, it includes information on the concentration relationships referenced in the literature. The iron-oxide materials investigated for water separation in the oxidation stage at the laboratory level are detailed in Table 6. Table 6 displays the materials used, focusing on iron-based compounds such as magnetite, which have been doped with materials including zircon, nickel, caesium, and gadolinium. It also details the sample type, partial pressure, environment type, and exposure time. [PERSON] et al. [39] observed that the rate of hydrogen production after steam injection reached a peak of 12.3 Ncm\({}^{3}\)/min at 25 min, after which point the rate of hydrogen production rapidly decreased. This demonstrates that hydrogen production through a fluidized bed is feasible. Table 7 below presents a list of thermochemical solar reactors that have utilized fluidized-bed materials and different gases for fluidization, considering the geometry of the reactor and the materials used in its construction. Additionally, the concentration ratio is provided  
Figure 6: Operating temperature of typical thermochemical water-splitting cycles in the reduction stage. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Thermal Power**} & **Temperatures** & \multirow{2}{*}{**Thermal Fluid**} & **Fluidized Bed** & **Particle Size** & **Radiation or** & \multirow{2}{*}{**Refs.**} \\  & **Reached or** & **Required** & & **Material** & **(\(\mu\)m)** & **Ratio** & \\ \hline \multirow{4}{*}{1 kW\({}_{\text{th}}\)} & \multirow{4}{*}{1300 \({}^{\circ}\)C} & \multirow{4}{*}{Water/Steam} & \multirow{4}{*}{Coal-coke} & 140 & \multirow{4}{*}{477 W/cm\({}^{2}\)} & \multirow{4}{*}{[74; 80; 81; 82]} \\  & & & & (200–300) & & \\ \cline{1-1}  & & & & (300–500) & & \\ \cline{1-1}  & & & & (500–710) & & \\ \hline \multirow{2}{*}{10 to 20 kW\({}_{\text{th}}\)} & \multirow{2}{*}{2200 \({}^{\circ}\)C} & \multirow{2}{*}{Water/Steam} & ZnO reagent & \multirow{2}{*}{1–5} & \multirow{2}{*}{-} & [83] \\  & & & powder & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Temperatures reached in a thermochemical solar reactor with a fluidized bed. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Iron Oxides Material** & \begin{tabular}{c} **Sample/Particle** \\ **Size (\(\upmu\)m)** \\ \end{tabular} & \begin{tabular}{c} **Temp. (\({}^{\circ}\)C)** \\ **(Time)** \\ \end{tabular} & \
\begin{tabular}{c} **Partial Vapour** \\ **Pressure** \\ \end{tabular} &
\begin{tabular}{c} **Thermal Fluid** \\ **(Flow)** \\ \end{tabular} & **Refs.** \\ \hline Iron oxide with & & & & & \\ various & & & & & \\ support materials, & & Fe\({}_{2}\)O\({}_{3}\)/ZrO\({}_{2}\) & & & & \({}_{2}\)O/Ar & \\ ZrO\({}_{2}\), CeO\({}_{2}\), & Fe\({}_{2}\)O\({}_{3}\)/CeO\({}_{2}\) & 150–300 & 550 & - & (prop. 5:95) & [70] \\ yttria-stabilized & Fe\({}_{2}\)O\({}_{3}\)/YSZ & & & & (300 mL/min) & \\ zirconia (YSZ), and & Fe\({}_{2}\)O\({}_{4}\)/GDC & & & & \\ gadolinium-doped & & & & & \\ ceria (GDC) & & & & & \\ \hline \end{tabular}
\end{table} 
Table 6: _Cont._