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

The recent discovery of a new class of materials, named high entropy oxide (HEO) lead to attractive applications. HEOs are a class of materials composed by multi-elements system (generally, five components or more) in a particular crystal structure which is usually, different from that of the parent compounds. In this way the resulting effect is to increase the total configuration entropy. In this study 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. This type of HEO has a rock-salt structure in which all the cations are formally in the M(II) oxidation state. The catalytic activity of the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O HEO at quite low temperature (250°C-300°C) toward the CO oxidation reactions make it as a good candidate for this kind of application but at the same time lay questions concerning the mechanisms of the catalytic circle, as none of the parent oxides owns such a reactivity. The interesting queries involve the information on the local electronic structure of the transition metals (Co, Ni, Cu, and Zn), their oxidation states, the nature of the active surface site, and possible changes in all of these properties during the reaction course. For this purpose, the operando soft X-ray absorption spectroscopy (soft-XAS) experiments at the L2,3edges of the transition metals (TMs) was performed. The oxidation state of Cu, Co and Ni is confirmed to be 2+, as visible from the L2,3-edges collected at 1 bar of He flux at room temperature in Figures 1 and 2. During the heating at 250°C in the stoichiometric CO + 1/2Ogas mixture (1 bar of pressure), a peak in the Cu spectrum at ca. 934.8 eV starts to appear indicating the formation of Cu(I) sites (Figure 1).
 

Figure 1.     (a) Cu L2,3- edge XAS spectra of the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O HEO material under different conditions. The inset shows on an enlarged scale the Cu(I) peak at ca. 934.8 eV. (b) Cu L2,3-edge XAS spectra of CuO under different conditions and Cu2O at room temperature. In this panel, non-normalized spectra are shown.

 

The COformation reached the maximum rate at 250°C, as detected by a gas sensor located in the output of the cell reactor. The Cu(II) oxidation state can be recovered by treatment of the sample with O2 at 250°C. This result shows unequivocally that CO oxidation on the HEO proceeds via adsorption of CO on the Cu sites at the surface. This adsorption causes a charge transfer from CO to Cu, thus leading to Cu(I). Then, if the temperature is high enough to allow the oxidation of adsorbed CO by O2, CO2leaves the surface and the majority of Cu(I) is reoxidized to Cu(II). At the same time the Ni and Co sites act as spectators, indeed any changes at the Co, Ni L2,3edges were observed during the course of the reaction (Figure 2). This experiment demonstrates that HEO is stable during the CO oxidation while similar materials such as CuO undergoes an heavily reduction of Cu(II) that started at 150°C (Figure 1b) and that is not reversible. Thus, this evidence suggests 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.The availability of this information and the possibility of applying this measurement protocol systematically, on the one hand will enable knowledge-based material and cell design, and, on the other hand, will provide rational guidelines for the definition of optimal charge/discharge policies.
 

Figure 2.     (a) Co and (b) Ni L2,3-edge XAS spectra of the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O HEO material under different conditions.

 

This research was conducted by the following research team:

Martina Fracchia1, Paolo Ghigna1, Tommaso Pozzi1, Umberto Anselmi Tamburini1,Valentina Colombo2, Luca Braglia3, Piero Torelli3

 

Dipartimento di Chimica, Università di Pavia, Italy
Dipartimento di Chimica, Università di Milano, Italy
CNR-IOM, Trieste, Italy


Contact persons:

Paolo Ghigna, e-mail: paolo.ghigna@unipv.it
Piero Torelli, e-mail: piero.torelli@elettra.eu

 

Reference

Martina Fracchia, Paolo Ghigna, Tommaso Pozzi, Umberto Anselmi Tamburini, Valentina Colombo, Luca Braglia, Piero Torelli "Stabilization by Configurational Entropy of the Cu(II) Active Site during CO Oxidation on Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O", The Journal of Physical Chemistry Letters, 11, 9, 3589 (2020); doi: 10.1021/acs.jpclett.0c00602



Last Updated on Tuesday, 07 July 2020 14:41