LaCoO3 as a catalyst precursor for CO2 hydrogenation to methane: Effects of calcination temperature on catalytic properties

A series of lanthanum cobalt oxides (LaCoO3) calcined at different temperatures (600, 700, 800, and 900 °C) have been investigated as catalyst precursors for the CO2 methanation reaction. Structural characteristics of the as-prepared samples were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence spectroscopy (XRF), temperature programmed desorption of CO2 (CO2-TPD), temperature programmed reduction (H2-TPR), N2 adsorption-desorption, and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) techniques. Data showed that the samples prepared at lower calcination temperatures were observed to have a slightly distorted rhombohedral crystalline structure, higher BET surface area, enhanced reducibility, and lower oxygen vacancy concentration. After the reductive treatment under 400 °C, the trend associated with the changes in the oxygen vacancy concentrations was reversed, whereas the crystal structure remained unchanged. The CO2-TPD results indicated that the reduced samples with decreasing calcination temperatures resulted in a better affinity to CO2, which is crucial for CO2 activation. The catalytic activity of reduced samples on CO2 methanation was measured at differential and high CO2 conversion conditions. Arrhenius plots showed no drastic variation in the apparent activation energy, confirming with XPS results that the difference between catalytic performances within each sample could result from the number of active sites rather than the changes in the identity of active sites. During CO2 methanation, the perovskite structure was destroyed and converted into mainly La2CO3OH, especially for the best-performing catalyst LaCoO3-600. With the increasing calcination temperature of the perovskite, the formation of La2O2CO3, La(OH)3, and metallic cobalt phases on the spent catalyst was observed. With successful activation of the LaCoO3 prior to the reaction, the superior activity of 73% CO2 conversion and 95% CH4 selectivity was observed at a space velocity of 12000 mlCO2 gcat-1 h-1 at 350°C and 40 bar using a CO2:H2 ratio of 1:4. Moreover, stability test of the LaCoO3-600 catalyst for a time-on-stream of 72-h under the identical conditions demonstrated that the methane selectivity remained unaffected with only a 10 % CO2 conversion decrease. To the best of our knowledge, the methane production rate (5959 gCH4kgcat-1h-1) that we measured on LaCoO3-600 catalyst is superior to that of all other ABO3-type perovskites, and it is attributed to the enhanced oxygen vacancy concentration of the reduced catalysts.