Stabilization by Configurational Entropy of the Cu(II) Active Site during CO Oxidation on Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O.

Martina Fracchia,Umberto Anselmi Tamburini,Piero Torelli,Luca Braglia,Valentina Colombo,Tommaso Pozzi,Paolo Ghigna

doi:10.1021/acs.jpclett.0c00602

Abstract

The mechanisms of CO oxidation on the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O 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 O2 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 T > 100 °C. Thus, the main conclusion of this work is that the high configurational entropy of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O stabilizes the rock-salt structure and permits the oxidation/reduction of Cu to be reversible, thus permitting the catalytic cycle to take place.

Highlights

The Cu(II) oxidation state can be but slowly recovered by treatment of the sample with O2 at ca. 250 °C
Co3O4 spinel is the most active for CO oxidation,[5] but it is severely deactivated by trace amounts of moisture that are usually present in the feed gas
This rationale suggests that the presence of transition metals in high oxidation states is a prerequisite for finding effective catalysts for the CO oxidation reaction; all of the oxide catalysts for the CO oxidation reaction do have metals in high or mixed oxidation or valence states

Summary

■ EXPERIMENTAL METHODS

The sample holder was fixed with screws onto the titanium base of the cell, which was floating from ground and connected with a coaxial cable In this geometry, the X-ray beam passes through the membrane and the gas layer and hits the sample and generates the secondary emission, which is collected by a picoammeter connected to the sample and measuring the drain current. Comparison of the experimental spectra with theoretical calculations (Figure S1), CO oxidation rate in the temperature range of interest (Figure S2), Cu L2,3-edge spectra of the high-entropy oxide in the CO + O2 mixture at temperatures below the start of the CO oxidation reaction (Figure S3), determination of the Cu(I) fraction (Figure S4 and Table S1), discussion of the choice of soft-XAS as a mechanistic probe for the CO oxidation over the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O HEO, and details of the powder X-ray diffraction analysis (PDF).

■ ACKNOWLEDGMENTS

■ REFERENCES