CO2 Conversion Performance of Perovskite Oxides Designed with Abundant Metals

Integration of Materials and Process Informatics: Metal Oxide and Process Design for CO2 Reduction

In materials informatics, a mathematical model constructed between the synthesis conditions of materials and their properties and activities is used to design synthesis conditions in which the properties and activities have the desired values. In process informatics, a mathematical model constructed between the process conditions for devices and industrial plants and product quality and cost is used to design process conditions that can produce the desired products. In this study, we propose a method to simultaneously design the synthesis conditions of materials and the process conditions of products by integrating materials and process informatics in the reverse water-gas shift chemical looping (RWGS-CL) reaction, which produces CO from CO₂ using metal oxides via the RWGS-CL process. Four methods: Gaussian process regression-Bayesian optimization (GPR-BO), Gaussian mixture regression-Bayesian optimization (GMR-BO), GMR-BO-multiple, and GPR-GMR-BO were investigated for the optimization. All four proposed methods outperformed the results of a random search. GPR-BO achieved the highest performance and proposed 27 promising candidates for the synthesis conditions and metal oxides. The selected metals did not include Cu and Ga, which tended to have high predicted CO₂ and H₂ conversion rates, but Fe and La, which had slightly lower predicted CO₂ and H₂ conversion rates. These results indicate that a combination of metal oxides with lower predicted CO₂ and H₂ conversion rates and optimized process conditions was important for the optimization of both materials and processes, which was achieved by integrating materials and process informatics via the proposed method. Thus, we confirmed that it is possible to simultaneously optimize the combination of metals, composition ratios, synthesis conditions of the material or the metal oxide, and the process conditions using experimental datasets, process simulations, and machine learning, such as GPR, GMR, BO, and multiobjective optimization with a genetic algorithm.

Emerging natural and tailored perovskite-type mixed oxides–based catalysts for CO2 conversions

The rapid economic and societal development have led to unprecedented energy demand and consumption resulting in the harmful emission of pollutants. Hence, the conversion of greenhouse gases into valuable chemicals and fuels has become an urgent challenge for the scientific community. In recent decades, perovskite-type mixed oxide-based catalysts have attracted significant attention as efficient CO2 conversion catalysts due to the characteristics of both reversible oxygen storage capacity and stable structure compared to traditional oxide-supported catalysts. In this review, we hand over a comprehensive overview of the research for CO2 conversion by these emerging perovskite-type mixed oxide-based catalysts. Three main CO2 conversions, namely reverse water gas shift reaction, CO2 methanation, and CO2 reforming of methane have been introduced over perovskite-type mixed oxide-based catalysts and their reaction mechanisms. Different approaches for promoting activity and resisting carbon deposition have also been discussed, involving increased oxygen vacancies, enhanced dispersion of active metal, and fine-tuning strong metal-support interactions. Finally, the current challenges are mooted, and we have proposed future research prospects in this field to inspire more sensational breakthroughs in the material and environment fields.

Efficient CO2 conversion to CO using chemical looping over Co-In oxide

CO₂ conversion to CO by reverse water-gas shift using chemical looping (RWGS-CL) can be conducted at lower temperatures (ca. 723-823 K) than the conventional catalytic RWGS (>973 K), and has been attracting attention as an efficient process for CO production from CO₂. In this study, Co-In₂O₃ was developed as an oxygen storage material (OSM) that can realize an efficient RWGS-CL process. Co-In₂O₃ showed a high CO₂ splitting rate in the mid-temperature range (723-823 K) compared with previously reported materials and had high durability through redox cycles. Importantly, the maximum CO₂ conversion in the CO₂ splitting step (ca. 80%) was much higher than the equilibrium conversion of catalytic RWGS in the mid-temperature range, indicating that Co-In₂O₃ is a suitable OSM for the RWGS-CL process.

Design and Analysis of Metal Oxides for CO2 Reduction Using Machine Learning, Transfer Learning, and Bayesian Optimization

We aim to achieve resource recycling by capturing and using CO₂ generated in a chemical production and disposal process. We focused on CO₂ conversion to CO by the reverse water gas shift-chemical looping (RWGS-CL) reaction. This reaction proceeds in two steps (H₂ + MO _x ⇆ H₂O + MO _x-1; CO₂ + MO _x-1 ⇆ CO + MO _x ) via a metal oxide that acts as an oxygen carrier. High CO₂ conversion can be achieved owing to a low H₂O concentration in the second step, which causes an unwanted back reaction (H₂ + CO₂ ⇆ CO + H₂O). However, the RWGS-CL process is difficult to control because of repeated thermochemical redox cycling, and the CO₂ and H₂ conversion extents vary depending on the metal oxide composition and experimental conditions. In this study, we developed metal oxides and simultaneously optimized experimental conditions to satisfy target CO₂ and H₂ conversion extents by using machine learning and Bayesian optimization. We used transfer learning to improve the prediction accuracy of the mathematical models by incorporating a data set and knowledge of oxygen vacancy formation energy. Furthermore, we analyzed the RWGS-CL reaction based on the prediction accuracy of each variable and the feature importance of the random forest regression model.

Reverse Water Gas Shift by Chemical Looping with Iron-Substituted Hexaaluminate Catalysts

The Fe-substituted Ba-hexaaluminates (BaFeHAl) are active catalysts for reverse water-gas shift (RWGS) reaction conducted in chemical looping mode. Increasing of the degree of substitution of Al3+ for Fe3+ ions in co-precipitated BaHAl from 60% (BaFeHAl) to 100% (BaFe-hexaferrite, BaFeHF), growing its surface area from 5 to 30 m2/g, and promotion with potassium increased the CO capacity in isothermal RWGS-CL runs at 350–450 °C, where the hexaaluminate/hexaferrite structure is stable. Increasing H2-reduction temperature converts BaFeHAl to a thermally stable BaFeHF modification that contains additional Ba-O-Fe bridges in its structure, reinforcing the connection between alternatively stacked spinel blocks. This material displayed the highest CO capacity of 400 µmol/g at isothermal RWGS-CL run conducted at 550 °C due to increased concentration of oxygen vacancies reflected by greater surface Fe2+/Fe3+ ratio detected by XPS. The results demonstrate direct connection between CO capacity measured in RWGS-CL experiments and calculated CO2 conversion.

Perovskites in the Energy Grid and CO2 Conversion: Current Context and Future Directions

CO2 emissions from the consumption of fossil fuels are continuously increasing, thus impacting Earth’s climate. In this context, intensive research efforts are being dedicated to develop materials that can effectively reduce CO2 levels in the atmosphere and convert CO2 into value-added chemicals and fuels, thus contributing to sustainable energy and meeting the increase in energy demand. The development of clean energy by conversion technologies is of high priority to circumvent these challenges. Among the various methods that include photoelectrochemical, high-temperature conversion, electrocatalytic, biocatalytic, and organocatalytic reactions, photocatalytic CO2 reduction has received great attention because of its potential to efficiently reduce the level of CO2 in the atmosphere by converting it into fuels and value-added chemicals. Among the reported CO2 conversion catalysts, perovskite oxides catalyze redox reactions and exhibit high catalytic activity, stability, long charge diffusion lengths, compositional flexibility, and tunable band gap and band edge. This review focuses on recent advances and future prospects in the design and performance of perovskites for CO2 conversion, particularly emphasizing on the structure of the catalysts, defect engineering and interface tuning at the nanoscale, and conversion technologies and rational approaches for enhancing CO2 transformation to value-added chemicals and chemical feedstocks.