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.