Catalytic Properties of Zinc Manganites for Carbon Oxidation

Abstract

This paper presents a comparative study of the catalytic properties of the ZnMn₂O₄ and ZnMnO₃ manganites for carbon oxidation. The manganites have been prepared by a sol–gel process using reaction mixtures with different ratios of the constituent oxides. The particles of the two manganites were comparable in size (200–500 nm) and specific surface area (34–42 m²/g) and ensured similar behaviors of the catalytic reaction in the temperature range 260–470°C. The catalytic activity of the perovskite phase ZnMnO₃ can be accounted for by the high absolute oxygen nonstoichiometry, δ = 0.29, as determined by iodometry. Contact interaction between spinel ZnMn₂O₄ and carbon leads to the reduction of Mn³⁺ to Mn²⁺ and is accompanied by Mn₃O₄ formation. The change in the relationship between the constituent oxides in the surface layer of the ZnMn₂O₄ particles leads to the formation of the active perovskite phase ZnMnO₃, which initiates catalytic reaction.