Preliminary redox assessment study on double perovskite structure Sr2FeNi0.4Mo0.6O6-d by experimental investigation in TGA and microreactor

The ever-increasing demand for energy is generating a consequent increase of the energy production in the years to come. This currently comes mostly from power plants that use fossil fuels as their main fuel. Natural gas or coal-based power plants continue to improve from the point of view of efficiency, reducing emissions and wastes per unit of energy more and more. With a view to following international agreements in limiting the rise in global average temperatures, mitigation actions against CO2 emissions are becoming increasingly fundamental. Carbon capture and further uses are one of the leading themes of scientific research, especially when retrofitting pre-existing plants. A possible route consists in chemical looping processes that involve the use of an oxygen carrier that alternates between a reduced state and an oxidized state creating a syngas as a useful product. Metal oxides are the family of materials that is more used in chemical looping processes, but recently many studies have been conducted to find alternatives that can achieve better performance in terms of CO2 capture and consequently higher syngas yields. These materials must avoid deactivation problems that are common during redox cycle processes. Carbon deposition creates a surface layer that prevents oxygen ions from penetrating inside the matrix and thus occupying the vacancies previously created in the reduction step. Perovskites have a structure very favorable to the transport of oxygen, especially if in the presence of doping that weakens the link between sites B and oxygen allowing the greater creation of vacancies. It has been seen that working with a sub-stoichiometric structure further favors this process. Doping in B sites results in exsolution phenomena on cations by moving from the bulk to the surface thus increasing performance and acting as a catalyst for CO2 capture. This thesis presents the first results obtained on a new material with composition Sr2FeNi0.4Mo0.6O(6-d) (SFNM04) through TGA tests and subsequent microreactor tests. It can be seen from the first results how the effect of CO2 is evident in the oxidation stage, but still reversible in the presence of a reductive atmosphere with hydrogen. Nevertheless, the material present problems related to the formation of carbonates with an oxidating environment in the simultaneous presence of carbon compounds such as CO2 at different concentration. Post-mortem analysis of the sample shows the presence of strontium carbonate formation SrCO3 and strontium molybdate SrMoO4. The first inhibits the carbon monoxide formation as it is created from the interference of the CO2 molecule with the SrO oxide. The second phase, has insulating characteristics, generally inhibiting the re-oxidation of the material. These compounds were seen disappearing by subjecting the material to air treatment at 800 °C for one hour. The formation of Fe-Ni alloys by exsolution phenomena has only been achieved for reductions in H2 perpetuated for more than 30 minutes in TGA. This suggests that more time is needed to allow the Fe3+ and Ni3+ ions to be reduced to Fe2+ and Ni2+ and exsolve on the surface of the sample catalyzing the oxidation. In conclusion, this work is part of a preliminary study to determine the reproducibility of redox cycles for SFNM-04 perovskite and to form the basis of information for more in-depth and targeted analysis with the prospect of opening new pathway towards systems with less expensive materials.