# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 2 Materials and Methods
### Thermogravimetric Analyzer  
Five redox cycles were carried out in a thermogravimetric analyzer (TGA), CI Electronics type. 50 mg of each sample were loaded in a platinum-wired mesh basket and heated up in air atmosphere to the desired temperature (850 \({}^{\circ}\)C or 940 \({}^{\circ}\)C). The selection of this temperature range is related to the study of the phase diagram of the Mn-Ti-O system presented in the previous work from the authors [25]. In this work, the theoretical composition of the Mn-Ti samples prepared was analyzed according to this phase diagram, and it was concluded that these temperatures were adequate to carry out the oxidation of the samples in the presence of the Mn\({}_{2}\)O\({}_{3}\) or Mn\({}_{3}\)O\({}_{4}\)/MnTiO\({}_{3}\) mixture. All the tests were done with a total flow of 25 L\({}_{\text{N}}\)/h. More detailed descriptions of the TGA and the procedure can be found elsewhere [25].  
First, the capacity to release gaseous oxygen and regenerate in air of the different Mn-Ti mixed oxides at a determined temperature was evaluated. In the reduction step, highly pure N\({}_{2}\) (<2 ppm O\({}_{2}\)) was supplied. Then, the reactivity of the Mn-Ti samples with the main combustion gases (CO, H\({}_{2}\), and CH\({}_{4}\)) was evaluated, considering that, in this case, reduction to MnO was reached. In the reduction step, the composition of the gaseous reactants was 15% H\({}_{2}\), 15% CO + 20% CO\({}_{2}\), and 15% CH\({}_{4}\) + 20% H\({}_{2}\)O, respectively.  
Some tests with 200 redox cycles were also performed to evaluate the mechanical and chemical stability of selected Mn-Ti oxides. In this case, the temperature was set to 950 \({}^{\circ}\)C. The first experiment was performed using N\({}_{2}\) in the reduction (2 min) and air in the oxidation (8 min), so that the CLOU capacity of the material could be evaluated, i.e., redox transformation Mn\({}_{2}\)O\({}_{3}\)/Mn\({}_{3}\)O\({}_{4}\). In the second experiment, the redox transformation Mn\({}_{2}\)O\({}_{3}\)/MnO was evaluated. Thus, reduction conditions were 2% H\({}_{2}\) during 55 s and oxidation with 10% O\({}_{2}\) during 1 min.  
The oxygen transport capacity, R\({}_{\text{OC}}\), was defined as the mass fraction of the oxygen carrier that can be transferred as oxygen:  
\[R_{\text{OC}}=\frac{m_{o}-m_{r}}{m_{oxid}} \tag{1}\]  
where \(m_{o}\) is the initial mass of the sample in the oxidized state, \(m_{r}\) the mass of the sample in the reduced state, and \(m_{oxid}\) the mass of the fully oxidized sample.  
Another parameter to be calculated from the TGA results is the reduction conversion (\(X_{r}\)) and the oxidation conversion (\(X_{o}\)):  
\[X_{r}=\frac{m_{0}-m}{R_{OC}m_{oxid}} \tag{2}\]  
\[X_{O}=\frac{m-m_{r}}{R_{OC}m_{oxid}} \tag{3}\]  
Finally, the rate index for the reaction considered can also be calculated as:  
\[Rate\ Index\left(\frac{\%}{min}\right)=60\cdot 100\cdot R_{OC}\cdot\left(\frac{ dX_{i}}{dt}\right)\bigg{|}_{norm} \tag{4}\]  
where \(\left(\frac{dX_{i}}{dt}\right)\bigg{|}_{norm}\) is the normalized reaction rate expressed as s\({}^{-1}\). In the reaction with the main combustion gases (H\({}_{2}\), CO, CH\({}_{4}\)), the normalized rate at a fuel gas concentration of 15 vol.% is considered, while in the reaction with O\({}_{2}\), gas concentration is normalized to 10 vol.%.