Experimental assessment of SFNM04 perovskite for chemical looping CO2 dissociation

For decades, the increase in anthropogenic CO2 emissions has represented one of the main problems to be solved or, at least, to stem. Over the years, the growth of this factor has inevitably caused a significant environmental impact leading to global warming and an increase in energy demand. The rise in energy production allowed to satisfy this demand mainly exploiting fossil fuels (coal, natural gas, etc.) as raw material in power plants. However, there have been several international agreements that aimed to reduce and mitigate the CO2 emission and, consequently, to keep the rise in global average temperature under control. Strong measures have been adopted with the purpose of making the power plant processes more efficient. Nevertheless, an alternative and more promising way to mitigate the emissions issue would be the energy transition to renewable source by following a decarbonization pathway. Carbon capture, as attractive process, has been widely discussed in this work considering, however, the increasing importance of the CO2 re-utilization for the production of synthetic fuels or chemicals by exploiting thermochemical redox cycle processes. Chemical looping for two-step thermochemical CO2 splitting involves the use of an oxygen carrier or redox material that reacts alternately with oxidizing and reducing gas mixtures in order to produce a synthetic gas. Generally, syngas (CO/H2 mixture) production is the main objective since it is the starting point for the production of multiple chemicals (methanol, ethanol, DME, etc.). Many different materials have been used as oxygen carrier in chemical looping processes, starting from metal oxides, then exploiting more promising materials such as cerium oxides and perovskites. In particular, this work comes from an international collaboration project involving Università di Udine, Massachusetts Institute of Technology (M.I.T.), and Politecnico di Torino which aspires to propose a new perovskite-based oxygen carrier for the conversion of the captured CO2 to carbon monoxide. The material in question is 〖Sr〗_2 Fe〖Ni〗_0.4 〖Mo〗_0.6 O_(6-δ) (SFNM-04) and it was synthetized by Università di Udine, it has been extensively investigated in order to assess its redox ability and stability through microreactor tests. An important property of this material is the formation of Fe-Ni alloys by exsolution process when subjected to a reducing environment. This phenomena allows to Fe2+ and Ni3+, when reduced, to exsolve from the lattice to the surface of the sample acting as catalysts for the oxidation step. Moreover, this phenomena permits to generate a large number of oxygen vacancies which are fundamental for the redox reaction allowing a larger production of CO. The experimental section of this work is focused on the CO production achieved during the oxidation reactions in which a gas mixture containing different concentrations of carbon dioxide reacts with the SFNM-04 sample. Both the oxidation and reduction conditions such as operating temperatures, gas mixture compositions, and reaction times have been modified and changed in order to assess the perovskite response and the consequent CO global yield, CO maximum production rate, and the CO2 conversion. Through all the experiments it was noticed a negative impact on the redox ability of the sample when reducing the operating temperature of both oxidation and reduction. 850°C was the maximum temperature value imposed in the thermochemical cycles performed