(6ix) Low temperature carbon dioxide reduction to carbon monoxide on perovskite-type oxides

In the US, around 6000 million mega tonnes of CO₂ are released yearly to the atmosphere, yet less than 300 mega tonnes are re-utilized in industrial processes such as urea and salicylic acid synthesis. Here, an alternative to CO₂ utilization is proposed, where CO₂ can be transformed to high-value chemicals that could be used as a source of C1 chemistry for synthetic fuels synthesis.  Being a very stable molecule, CO₂ requires extreme conditions (very high temperatures or reducing environments) for its activation. The reverse water gas shift (RWGS) reaction can convert CO₂ to CO at ~ 600 °C but requires excess hydrogen, is equilibrium limited and commonly produces undesired CH₄. In spite of its disadvantages, it is cost competitive and the incorporation of renewable energy has been proven promising [1].

In this work, the Reverse Water Gas Shift Chemical Looping (RWGS-CL) process is demonstrated [2,3,4]. Here, the RWGS is intensified to avoid its most common drawbacks (use of excess hydrogen, equilibrium limitations and side reactions) using mixed oxides with high oxygen mobility. The process consists on a hydrogen pulse, which creates oxygen vacancies in the materials that are later re-oxidized by a flow of carbon dioxide, while yielding CO [2,3,4].

Perovskites (ABO₃) are ideal for RWGS-CL because they have the ability to form oxygen vacancies while maintaining their crystalline structure and their catalytic properties can be tailored by partial substitution of the A and B site metals. Various concentrations of lanthanum and strontium [2], and cobalt and iron [3] were tested in the A and B site of the perovskite respectively to design a process that allows maximum CO formation, at the lowest temperatures whit high CO yields. The tested materials proved stable during the five cycles studied.

The RWGS-CL process transforms CO₂ to CO starting at 500 °C (~100 °C lower than conventional RWGS) [3], which could allow for a temperature integration of the process for CO₂ conversion and fuels synthesis through Fischer-Tropsch, lowering overall energy use and environmental impact if heat is provided by renewable sources.

References

[1] Mallapragada, D. S.; Singh, N. R.; Curteanu, V.; Agrawal, R. Sun-to-Fuel Assessment of Routes for Fixing CO2 as Liquid Fuel. Ind. Eng. Chem. Res. 2013, 52, 5136−5144.

[2] Daza, Y.A., Kent, R.A., Yung, M.M., Kuhn, J.N., Carbon dioxide conversion by reverse water gas shift chemical looping on perovskite-type oxides. Ind. Eng. Chem. Res. 2014, 53, 5828−5837.

[3] Daza, Y.A., Maiti, D., Kent, R.A., Bhethanabotla, V.R., Kuhn, J.N., Isothermal reverse water gas shift chemical looping on La_0.75Sr_0.25Co_(1-Y)Fe_YO₃ perovskite-type oxides. In press Catalysis Today, 2015.

[4] Daza, Y.A., Maiti, D., Hare, B.J., Bhethanabotla, V.R., Kuhn, J.N., More Cu, more problems: Decreased CO₂ conversion ability by Cu-doped La_0.75Sr_0.25FeO₃ reduced perovskite oxides. Submitted to Surface Science for a special issue honoring Gabor Somorjai.