468673 Material Prediction for Carbon Dioxide Conversion to Carbon Monoxide Using Reverse Water Gas Shift Chemical Looping (RWGS-CL) Process
In general, the solar thermochemical CO2 reduction is a two-step cycle, where CO2 is converted to CO over metal oxides, while utilizing the solar energy to provide for the high temperature requirements of the process. Over the years, perovskite oxides have proven worthy for this reaction. In this process, a perovskite oxide of the form (ABO3) is first reduced to ABO3-δ at high temperatures, while in the second step, it is oxidized back to its stoichiometric form while the CO2 is converted to CO. This CO can then be further hydrogenated to hydrocarbon fuels. A modified approach of this process is the use of renewable hydrogen during the reduction of the perovskite oxides. This process is named as reverse water gas chemical looping (RWGS‑CL) and has the ability to create oxygen vacancies at much lower temperatures (~500 oC). The success of these CO2 conversion processes lies greatly on the perovskite oxide’s oxygen vacancy formation capacities. The oxygen vacancy formation characteristic is greatly dominated by the materials composition. Perovskite oxides have the ability to accommodate one or more elements on the ‘A’ and ‘B’ sites, thereby forming structures like A1(1-x)A2xBO3, AB1(1-y)B2yO3 and A1(1-x)A2xB1(1-y)B2yO3 along with ABO3. It is this oxygen vacancy forming ability of these materials that determines their CO2 conversion capabilities.
We hence, undertook a screening study to predict the appropriate materials for this purpose. We chose four different elements like lanthanum, strontium, barium and calcium for the ‘A’ site while the 3d transition metals along with aluminum and gallium for the ‘B’ sites. Using bulk oxygen vacancy formation energy as the descriptor for this process, we were able to predict the materials that can convert CO2 to CO using RWGS-CL process. For this purpose, we performed density functional theory based calculations using Vienna an-initio Simulation Package (VASP). We restricted ourselves to only cubic structures, as it had been shown that oxygen vacancy formation trends across different compositions were invariant of crystal structures. For the experimental validation, we performed temperature programmed experiments to observe the formation of oxygen vacancies and carbon monoxide in the two steps of RWGS-CL process. Intrinsic material property driven models to predict oxygen vacancy formation and the CO2 conversion capability are also being developed.
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