Effect of calcination temperature on CO₂ methanation performance of LaCoO₃ perovskite catalyst precursors

Abstract

A series of lanthanum cobaltites (LaCoO₃) calcined at different temperatures (600, 700, 800, and 900 °C) were investigated as catalyst precursors for the CO₂ methanation reaction. Characterization data revealed that samples prepared at low calcination temperatures (i.e., 600 °C) exhibited a slightly distorted rhombohedral crystal structure, higher BET surface area, enhanced reducibility, and lower oxygen vacancy concentration as compared with catalysts calcined at higher temperatures. After additional reductive treatment at 400 °C following the calcination, the trend in oxygen vacancy concentrations was reversed, although the bulk crystal structure remained unchanged. Results of CO₂-temperature-programmed desorption measurements indicated that the reduced samples, especially those calcined at low temperatures, exhibited better CO₂ adsorption affinity, which is crucial for CO₂ activation. The catalytic activity of the reduced samples was evaluated under both differential and high CO₂ conversion conditions. Arrhenius plots showed little variation in the apparent activation energy, confirming XPS results that differences in the catalytic performance were attributed to the number of active sites rather than significant changes in the nature of the active sites. After successful activation of LaCoO₃ prior to the reaction, the r-LaCoO₃-600 catalyst demonstrated superior activity, achieving 73% CO₂ conversion and 95% CH₄ selectivity at a space velocity of 12,000 mL CO₂ gcat⁻¹ h⁻¹, at 350 °C and 40 bar, using a CO₂:H₂ ratio of 1:4. Additionally, a 72 h stability test of the r-LaCoO₃-600 catalyst under the same conditions showed slight deactivation, limited with only approximately 10% decrease in CO₂ conversion while maintaining high CH₄ selectivity. The methane space-time yield of 5959 gCH₄ kgcat⁻¹ h⁻¹ offered by the r-LaCoO₃-600 catalyst surpasses those of most of the ABO₃-type perovskites. This high performance is linked to its higher surface area, increased oxygen vacancy concentration after H₂ reduction, and greater cobalt dispersion post reaction. In contrast, samples calcined at higher temperatures developed larger Co species after reaction, attributed to the lower oxygen vacancy concentration in the reduced catalyst.