# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 1 Introduction
The reduced particles are stored in a reservoir so that the flow of particles to the oxidation reactor (discharging step) is controlled and maintained constant for heat/electricity production despite variation in the solar energy input. The oxygen carrier is then regenerated in air and then stored in another reservoir after being cooled. Figure 1: Scheme for the Chemical Looping Combustion (CLC) process. Among the metal oxides with oxygen uncoupling capability, cobalt, manganese, and copper oxides are the most promising redox systems. Ca-Mn perovskites with Mg doping followed by La-based perovskites have also emerged as an adequate option [11]. Compared to cobalt- and copper-based systems, manganese-based oxide systems present the advantage of being inexpensive and harmless. Manganese oxide has several stable phases: MnO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Mn\({}_{3}\)O\({}_{4}\), and MnO. Reduction of MnO\({}_{2}\) to Mn\({}_{2}\)O\({}_{3}\) is an irreversible reaction. Mn\({}_{2}\)O\({}_{3}\)/Mn\({}_{3}\)O\({}_{4}\) transformation releases oxygen according to reaction 2 (r2):  
\[6\ \text{Mn}_{2}\text{O}_{3}\leftrightarrow 4\ \text{Mn}_{3}\text{O}_{4}+\text{O}_{2}\] (r2)  
While Mn\({}_{2}\)O\({}_{3}\) reduction takes place at high temperatures, Mn\({}_{3}\)O\({}_{4}\) exhibited limited re-oxidation at temperatures higher than 800 \({}^{\circ}\)C, resulting in a dense and highly sintered structure. In order to narrow this hysteresis loop, manganese oxides have been combined to produce mixed oxides, mainly with iron, copper, magnesium, and silicon oxides [12]. The addition of iron oxide to manganese oxide has proven to be one of the most effective methods for improving the reoxidation of manganese oxide. The addition of iron weakens the Mn-O bond, thus promoting an increase in the reactivity of the material [8]. Through the different studies performed, it can be concluded that the addition of iron oxide (10 to 40 wt.%) reduced the thermal hysteresis, allowed for a lower sintering degree, and also improved the cycling response of the mixed oxides [13; 14]. The addition of copper oxide to Mn-Fe materials can further improve the thermal hysteresis since the incorporation of Cu decreases the reduction temperature [15]. Manganese and iron mixed oxides have also been tested in CLOU applications with promising results [16; 17]. Magnesium addition to manganese oxide has also demonstrated a good potential for thermochemical energy storage [18]. However, the temperature window for reduction-oxidation (1000-1500 \({}^{\circ}\)C) is considered excessively high for solar concentrated applications [19]. Compared to mixed manganese-iron oxides, the addition of SiO\({}_{2}\) to manganese oxide results in materials with more stable redox behavior, also increasing the temperature at which the material can be reoxidized [20] and the reoxidation rate [21]. The combined system Mn/Mg/Si has also recently been tested as a potential oxygen carrier for CLOU with promising results [22]. Other combinations, such as Al\({}_{2}\)O\({}_{3}\)[23; 24] or ZrO\({}_{2}\)[24] with manganese oxide, did not improve the thermochemical properties of the original manganese oxide and therefore were discarded. Recently, the effect of adding TiO\({}_{2}\) to manganese oxide was also considered by the authors. Manganese oxide particles including different mass fractions of titanium oxide ranging from 0 to 50 wt.% were prepared by pelletization [25]. After calcining under certain temperature conditions, the presence of a new solid phase (pyrophanite, MnTiO\({}_{3}\)) was  
Figure 2: Scheme for the solar Chemical Looping Air Separation (Sol-CLAS) system. observed together with the Mn\({}_{2}\)O\({}_{3}\)-Mn\({}_{3}\)O\({}_{4}\) redox pair.