Low-Temperature CO Oxidation over Combustion Made Fe- and Cr-Doped Co₃O₄ Catalysts: Role of Dopant’s Nature toward Achieving Superior Catalytic Activity and Stability

Co₃O₄ with a spinel structure shows unique activity for CO oxidation at low temperature under dry conditions; however the active surface is not very stable. In this study, two series of Fe- and Cr-doped Co₃O₄ catalysts were prepared by a single-step solution combustion technique. Fe was chosen because of its redox activity corresponding to the Fe²⁺/Fe³⁺ redox couple and compared to Cr, which is mainly stable in the Cr³⁺ state. The catalytic activity of new materials for low-temperature CO oxidation was correlated to the nature of the dopant. As a function of dopant concentration, the temperature corresponding to the 50% CO conversion (<i>T</i>₅₀) demonstrated significant differences. The maximal activity was achieved for 15% Fe-doped Co₃O₄ with <i>T</i>₅₀ of −85 °C and remained almost constant up to 25% Fe. In the case of Cr, the activity was observed to be maximum for 7% of Cr with <i>T</i>₅₀ of −42 °C and significantly decreased for higher Cr loadings. Similarly, there was a contrasting behavior in catalyst stability too. 100% CO conversion was achieved below −60 °C for 15% Fe/Co₃O₄ catalyst and remained unchanged even after calcination at 600 °C. In contrast, Co₃O₄ or 15% Cr/Co₃O₄ catalysts strongly deactivated after the same treatment. These differences were correlated to the oxidation states, coordination numbers, the nature of surface planes, and the redox properties. We observed that both Cr and Fe were typically present in the +3 oxidation state, occupying octahedral sites in the spinel structure. The catalysts were mainly exposed to (111) and (220) planes on the surface. H₂-TPR indicated clear differences in the redox activity of materials due to Fe and Cr substitutions. The reducibility of surface Co³⁺ species remained similar in all Fe-doped Co₃O₄ catalysts in contrast to nonreducible Cr-doped analogs, which shifted the reduction temperature to the higher values. As the Fe³⁺/Fe²⁺ redox couple partly substituted the Co³⁺/Co²⁺ redox couple in the spinel structure, similar bond strength of Fe–O keep redox activity of Co³⁺ species almost unchanged leading to higher activity and stability of Fe/Co₃O₄ catalysts for low-temperature CO oxidation. In contrast, nonreducible Cr³⁺ species characterized by strong Cr–O bond substituting active Co³⁺ sites can make the Cr/Co₃O₄ surface less active for CO oxidation