Analysis and Characterization of Sr2FeMo0.6Ni0.4O6-δ Double Perovskite in Reverse Water-Gas Shift Chemical Looping Process

The present work investigates the redox performance of Sr2FeMo0.6Ni0.4O6-δ (SFMN) double perovskite in the Reverse Water-Gas Shift Chemical Looping (RWGS-CL) reaction in the medium temperature range (700 °C). Previous studies conducted on the same material revealed that H2-reduction at temperatures above 800 °C leads to the exsolution of bimetallic Ni-Fe alloy particles and the formation of a highly oxygen deficient Ruddlesden-Popper (RP) phase. In the same study, Thermogravimetric (TGA) and Temperature Programmed Reduction (TPR) and Oxidation (TPO) analyses showed that the optimal reduction and oxidation temperatures for maximizing the CO yield are around 850 °C and 750 °C, respectively, and that the cycled material could work steadily under isothermal conditions at 850 °C. The goal of this thesis is to conduct a further TGA investigation on SFMN, to knowingly assess the fuel production capability pre- and post-exsolution in the medium temperature range (700 °C), which would be competitive against the conventional RWGS process. Ex-situ X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM) characterization studies have been implemented to observe the morphological and structural evolution of the OC upon H2/CO2 cycling. More in detail, the objectives that we tried to achieve in this work are the following: i.??Assess the SFMN redox performance at low CO2 concentrations to limit the formation of carbonates on the material surface, which would be a limiting factor for both catalyst activity as well as fuel yield estimation procedure. ii.??Observe the exsolution mechanism via XRD and FE-SEM analysis at 700 °C and 850 °C and evaluate its impact on the material reactivity in the RWGS-CL reaction. iii.??Assess the stability of the OC performance over 50 cycles at 700 °C, and compare the results with the experiments conducted at 850 °C. Ex-situ XRD analysis showed that the exsolution of Ni-Fe nanoparticles to the material surface already starts to occur at 700 °C (well below 800 °C). From SEM micrographs we verified that, after 2h in a 2.5% H2/Ar reducing atmosphere at 700 °C and 850°C, the exsolution of round-shaped nanoparticles effectively occurred, with an average size distribution of 12,1 ± 3,0 nm and 20,4 ± 4,8 nm respectively. This is coherent with the literature, in which the higher reduction temperature is reported to induce exsolved particles of larger size. From TGA experiments, the material proved that it can steadily work over 50 cycles under isothermal conditions at 700 °C, in which the material showed a better resistance to carbonates deposition phenomena compared to the case at 850 °C. However, a CO production yield of 439 ± 13 μmol/gSFMN was recorded from the stability test at 700°C, which turned out to be significantly lower than the 763 ± 18 μmol/gSFMN recorded at 850 °C. TGA experiments were carried out at 𝐶𝑂2 Circle Lab in the Environment Park (Turin), while the material characterization via XRD and FE-SEM was carried out at the Department of Applied Science and Technology (DISAT), at Polytechnic of Turin.