(43c) Earth Abundant Perovskite Oxides for Low Temperature CO2 Conversion

The current energy and environmental scenario demands acute attention towards generation of energy in a sustainable fashion. Over the last decade, significant efforts have been spent for the reduction of carbon dioxide emissions and its impact on global climate concerns. Carbon dioxide capture and sequestration (CCS) has garnered a lot of interest leading to several CCS plants being commissioned. However, the scale of CCS (~ 35 Mtpa, in early 2017) is still way below the scale of global CO₂ emissions (~ 35 Gtpa, in 2015). The other promising approach to reduce the atmospheric CO₂ content is thus to repurpose waste CO₂ emissions. Amongst the several possible routes being researched, the solar thermochemical cycles (STC) presents one of the highest sun-to-fuel conversion efficiency.¹ The disadvantages of this process are however high temperatures of operation (more than 1000 °C) and stability of the materials. Reverse water gas shift chemical looping (RWGS-CL) is a modified approach of STC that can convert CO­₂ to CO (100% selective) at much lower temperatures of ~ 500 °C.^2-3 The RWGS-CL process involves two consecutive steps of reduction and oxidation. The first step consists of reducing mixed metal oxides like perovskite oxides (ABO₃) to their oxygen deficient forms (ABO_3-δ) by heating in presence of hydrogen, while the second step involves the conversion of CO₂ to CO over these oxygen vacant materials along the with latter getting converted to their original stoichiometric forms. Since these materials are capable of converting CO₂ to CO, a core component of syngas (a mixture of CO, H₂), they present an opportunity to close a synthetic carbon cycle, enabling carbon footprint reduction of several carbon-positive processes.

An intrinsic parameter that describes the CO₂ conversion capability of these perovskite oxides is their oxygen vacancy formation energy (E_vac).^3-4 Since this E_vac is closely related to the elemental composition of these perovskites, it is quite rational to tune the perovskite composition for predicting materials with desired CO­₂ conversion performance. Perovskite oxide type materials (ABO₃) are perfect platforms for material composition tuning as they can accommodate a variety of elements in their respective ‘A’ and ‘B’ sites. Density functional theory (DFT) calculated E_vac is thus used to screen different perovskites oxides of the form ABO₃, A1_0.5A2_0.5BO₃, AB1_0.5B2_0.5O₃, and A1_0.5A2_0.5 B1_0.5B2_0.5O₃ (A = La, Ca, Sr, Ba and B = 3d elements from Cr to Ni along with Al and Ga). La_0.75Sr_0.25FeO_3­ (a perovskite oxide with E_vac of ~3.4 eV) exhibited an isothermal CO₂ conversion process via RWGS-CL. Hence, any materials with E_vac values close to 3.4 eV were hypothesized to have CO₂ conversion capabilities. Based on this principle, we predicted several perovskite oxides. These were synthesized via Pechini method and then subsequently tested for CO₂ conversion. Amongst many of the predicted materials that demonstrated successful conversion of CO₂ to CO, the earth abundant lanthanum and calcium based oxides showed the best performance till date. We could thus have the highest CO₂ conversion rates at lowest temperatures of ~450 °C by RWGS-CL. Several consecutive cyclic experiments revealed long term stability of these materials enabling potential industrial implementation. An empirical model has also been developed correlating the DFT-calculated oxygen vacancy formation energy of a material to its intrinsic properties like enthalpy of formation and bond-dissociation energies. This model may allow future prediction of E_vac of similar perovskite oxides without rigorous DFT calculations. This RWGS-CL process and earth abundant perovskite material combination can pave the way for sustainable production of CO enabling thermal integration with Fischer Tropsch synthesis (FTS) reaction for the generation of high value hydrocarbons.

References:

1. Chueh, W. C.; Falter, C.; Abbott, M.; Scipio, D.; Furler, P.; Haile, S. M.; Steinfeld, A., High-Flux Solar-Driven Thermochemical Dissociation of CO₂ and H₂O Using Nonstoichiometric Ceria. Science 2010, 330, 1797-1801.

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. Industrial & Engineering Chemistry Research 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. Catalysis Today 2015, 258, 2, 691-698.

4. Maiti, D.; Daza, Y. A.; Yung, M. M.; Kuhn, J. N.; Bhethanabotla, V. R., Oxygen vacancy formation characteristics in the bulk and across different surface terminations of La_(1-x)Sr_xFe_(1-y)Co_yO_(3-d) perovskite oxides for CO₂ conversion. Journal of Materials Chemistry A 2016, 4 (14), 5137-5148.