Stabilization by Configurational Entropy of the Cu(II) Active Site during CO Oxidation on MgCoNiCuZnO.

The mechanisms of CO oxidation on the MgCoNiCuZnO high-entropy oxide were studied by means of operando soft X-ray absorption spectroscopy. We found that Cu is the active metal and that Cu(II) can be rapidly reduced to Cu(I) by CO when the temperature is higher than 130 °C. Co and Ni do not have any role in this respect. The Cu(II) oxidation state can be easily but slowly recovered by treatment of the sample with O at ca. 250 °C. However, it should be noted that CuO is readily and irreversibly reduced to Cu(I) when it is treated with CO at > 100 °C. Thus, the main conclusion of this work is that the high configurational entropy of MgCoNiCuZnO stabilizes the rock-salt structure and permits the oxidation/reduction of Cu to be reversible, thus permitting the catalytic cycle to take place.