Enhanced chemical looping CO2 conversion activity and thermal stability of perovskite LaCo1−xAlxO3 by Al substitution

Abstract

The reverse water–gas shift chemical looping (RWGS-CL) process that utilizes redox reactions of metal oxides is promising for converting CO₂ to CO at low temperatures. Metal oxides with perovskite structures, particularly, perovskite LaCoO₃ are promising frameworks for designing RWGS-CL materials as they can often release oxygen atoms topotactically to form oxygen vacancies. In this study, solid solutions of perovskite LaCo_1−xAl_xO₃ (0 ≤ x ≤ 1), which exhibited high CO production capability and thermal stability under the RWGS-CL process, were developed. Al-substituted LaCo_0.5Al_0.5O₃ (x = 0.5) exhibited a 4.1 times higher CO production rate (2.97 × 10⁻⁴ CO mol g⁻¹ min⁻¹) than that of LaCoO₃ (x = 0; 0.73 × 10⁻⁴ CO mol g⁻¹ min⁻¹). Diffuse reflectance infrared Fourier transform spectroscopy studies suggested that an increase in CO₂ adsorption sites produced by the coexistence of Al and Co was responsible for the enhancement of CO production rate. Furthermore, LaCo_0.5Al_0.5O₃ maintained its perovskite structure during the RWGS-CL process at 500 °C without significant decomposition, whereas LaCoO₃ decomposed into La₂O₃ and Co⁰. In situ X-ray diffraction study revealed that the high thermal stability was attributed to the suppression of phase transition into a brownmillerite structure with ordered oxygen vacancies. These findings provide a critical design approach for the industrial application of perovskite oxides in the RWGS-CL processes.