Mechanisms of CO oxidation on high entropy spinels

The CO oxidation reaction on (Co,Mg,Mn,Ni,Zn)(Al,Co,Cr,Fe,Mn)₂O₄ and (Cr,Mn,Fe,Co,Ni)₃O₄ high entropy spinel oxides was studied for what concerns its mechanism by means of operando soft X-ray absorption spectroscopy. In the (Cr,Mn,Fe,Co,Ni)₃O₄ high entropy spinel, CO oxidation starts at ca. 150 °C, and complete conversion to CO₂ is obtained at ca. 300 °C. For the (Co,Mg,Mn,Ni,Zn)(Al,Co,Cr,Fe,Mn)₂O₄ spinel oxides, in contrast, the reaction starts at ca. 200 °C, and complete conversion needs temperatures of the order of 350 °C. Concerning the reaction mechanism, we found that, in both cases, Mn is the active metal, via the Mn(II)/Mn(III) redox couple. CO is found to adsorb on surface Mn(III) sites and reduces them to Mn(II). The Mn(III) oxidation state is recovered by O₂, yielding CO₂. All the other transition metals are found to be inactive and act just as spectators. These findings are discussed in the framework of the present knowledge on high entropy oxides.