An in situ electrical conductivity study of LaCoFe perovskite-based catalysts in correlation with the total oxidation of methane

• LaCo1−xFexO3 catalysts were partially reduced under methane–air reaction mixture.
• LaCo1−yO3 catalysts were not reduced even under pure methane.
• Redox mechanism with consumption of surface lattice oxygen on LaCo1−xFexO3 catalysts.
• Suprafacial mechanism involving only adsorbed oxygen species on LaCo1−yO3.
• The suprafacial mechanism is related to the metallic conductivity state of LaCo1−yO3.

LaCo1−yO3 (y = 0 and 0.2) and LaCo1−xFexO3 (x = 0.6 and 1) perovskites were used as catalysts for the total oxidation of methane in the temperature range 300–800 °C and characterized by in situ electrical conductivity measurements. Their electrical conductivity was studied as a function of temperature and oxygen partial pressure and temporal responses during sequential exposures to air, methane–air mixture (reaction mixture) and pure methane in conditions similar to those of catalysis were analyzed. The intrinsic activity of the perovskites at 550 °C followed the order: LaFeO3 < LaCo0.8O3 < LaCo0.4Fe0.6O3 < LaCoO3. LaCo1−yO3 perovskites were in a metallic conductivity state in the reaction temperature range while LaCo1−xFexO3 perovskites appeared to be p-type semiconductors both under air and under reaction mixture with positive holes as the main charge carriers. A transition from semiconducting state to the metallic state was observed for LaCo0.4Fe0.6O3 in the reaction temperature range. Their electrical conductivity at 550 °C increased following the order: LaFeO3 < LaCo0.8O3 < LaCo0.4Fe0.6O3 < LaCoO3. Correlations between their conducting behavior and redox properties, on one hand, and their catalytic behavior, on the other hand, have been evidenced. It was demonstrated that the reaction mechanism involves surface lattice O− species and can be assimilated to a Mars and van Krevelen mechanism for LaFeO3, while it involves only adsorbed oxygen species for LaCo1−yO3, being assimilated to a suprafacial mechanism. Both suprafacial and redox mechanisms take place on LaCo0.4Fe0.6O3 perovskite.

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