# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 3 Results
### Granulated Particles: Comparison with Results from Particles Produced by Pelletization
Finally, it should be said that in both granulated particles and those obtained from pellets, in the sample with Ti/(Mn+Ti) = 0.49, pyrophosphate was the only phase identified. Manganese-titanium particles produced from pellets showed an improvement with respect to pure manganese oxide regarding oxygen release and regeneration in air at temperatures higher than 800 \({}^{\circ}\)C. This behavior was also investigated with the particles produced by granulation by performing successive N\({}_{2}\)-air cycles in the TGA at 850 \({}^{\circ}\)C and 940 \({}^{\circ}\)C to determine the oxygen transport capacity (\(R_{\mathrm{OC}}^{out}\)) for oxygen release. Figure 5 shows an example of a typical TGA experiment for determination of \(R_{\mathrm{OC}}^{out}\). Figure 6 compares the evolution with the Ti/(Mn+Ti) molar ratio of the values of oxygen transport capacity (\(R_{\mathrm{OC}}^{out}\)) for oxygen release obtained for pellet-produced [25] and granulated particles at both 850 and 940 \({}^{\circ}\)C obtained from TGA curves similar to that in Figure 5 using Equation (1). In all the calculations, the fifth cycle was considered. The values of \(R_{\mathrm{OC}}^{out}\) in the case of particles produced from pellets were similar for the two temperatures tested, i.e., 850 \({}^{\circ}\)C and 940 \({}^{\circ}\)C. A maximum in the oxygen transport capacity for the oxygen release (\(R_{\mathrm{OC}}^{out}\)) was reached at both temperatures for Ti/(Mn+Ti) molar ratios between 0.15 and 0.2. The same behavior is observed for particles produced by granulation, and a maximum in (\(R_{\mathrm{OC}}^{out}\)) is detected for similar Mn-Ti compositions. However, there are some differences between particles obtained from pellets and granulated particles. At 850 \({}^{\circ}\)C, similar values of \(R_{\mathrm{OC}}^{out}\) were obtained for the two types of particles (maximum value close to 1.4%), but at 940 \({}^{\circ}\)C, the \(R_{\mathrm{OC}}^{out}\) values observed with the granulated particles were lower than those corresponding to particles prepared by pelletization. At 940 \({}^{\circ}\)C, the \(R_{\mathrm{OC}}^{out}\) values for the MnTi_15 granulated samples were 1.0%, and for MnTi_20, they were 0.8%, while for the particles obtained from pellets, the corresponding values were 1.4% and 1.2%,  
Figure 5: Reduction and oxidation cycles in TGA at 850 \({}^{\circ}\)C for the MnTi_20 sample. Reduction was performed in N\({}_{2}\) and oxidation in air during 30 min in both cases. respectively. In the previous work from the authors [25], it was found that pyrophanite (MnTiO\({}_{3}\)) was formed but it was not involved in the chemical reactions for oxygen release, which was exclusively attributed to Mn\({}_{2}\)O\({}_{3}\) reduction to Mn\({}_{3}\)O\({}_{4}\). It was speculated that the role of the MnTiO\({}_{3}\) formed was to facilitate the oxidation from Mn\({}_{3}\)O\({}_{4}\) to Mn\({}_{2}\)O\({}_{3}\) at higher temperatures compared to pure Mn\({}_{3}\)O\({}_{4}\). Due to this fact, Mn-Ti pellets were capable of releasing oxygen at 940 \({}^{\circ}\)C and being oxidized back. The same behavior seems to be taking place in the particles produced by granulation. But maybe the lower amount of pyrophanite in the granulated particles compared to the pellets affected the Mn\({}_{3}\)O\({}_{4}\) reoxidation capacity at 940 \({}^{\circ}\)C, making it lower. Thus, calcination conditions should be carefully controlled and optimized in order to favor that most of the TiO\({}_{2}\) added is included in the manganese oxide matrix in the form of pyrophanite. The experiments with the samples with 15% and 20% TiO\({}_{2}\), both at 850 and 940 \({}^{\circ}\)C, were repeated for further analysis, and after the first reduction step, the samples were not regenerated but cooled down in nitrogen. The reduced samples were also taken to XRD analysis in order to determine the evolution of the solid phases during the reduction step.