Stabilization by Configurational Entropy of the Cu(II) Active Site during CO Oxidation on Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O

Abstract

The mechanisms of CO oxidation on the Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O 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 2 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 Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O stabilizes the rock-salt structure and permits the oxidation/reduction of Cu to be reversible, thus permitting the catalytic cycle to take place.

Is Configurational Entropy the Main Stabilizing Term in Rock-Salt Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O High-Entropy Oxide?

“…E1 shows a considerable broadening of all the reflections, except for the 111 family. This broadening is consistent with a tetragonal distortion of the rock-salt structure [6,7]. The broadening decreases significantly by decreasing the number of components, and therefore the configurational entropy.…”

High-entropy materials for catalysis: A new frontier

“…1F ). Identifying the catalytic active sites in HEMs can also be realized by monitoring the change of the oxidation states of different elements with operando XAS measurements ( 48 ). On the other hand, x-ray photoelectron spectroscopy (XPS) analyzes the chemical environment including elemental composition and oxidation states on the surface and subsurface regions.…”

“…( C ) In situ Cu L 2,3 edge XAS spectra of the Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O catalyst in CO at 235°C (red to blue with a 60-min interval) and then switched to the O 2 atmosphere at the same temperature (black). Reproduced with permission from the American Chemical Society ( 48 ). ( D ) Thermochemical water splitting performance of (FeMgCoNi)O x , NiFe 2 O 4 , and CoFe 2 O 4 and the calculated thermodynamic limit of CeO 2 .…”

“…Some function as active sites that account for the adsorption and conversion of molecular species, while the others act as neighboring spectators that permit electronic or geometric regulation. To identify the active species in the rock salt HEO (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O) for CO oxidation, Fracchia et al ( 48 ) used operando soft XAS to monitor the evolution of the L 2,3 edge of Cu, Co, and Ni in different atmospheres. While XAS is typically considered as a bulk technique, the total electron yield mode endows the capability to probe the 3d states of the transition metal species on the surface and subsurface regions, which is particularly useful for catalytic studies.…”

Chemical reactions on surfaces for applications in catalysis, gas sensing, adsorption-assisted desalination and Li-ion batteries: opportunities and challenges for surface science

“…Experimental methods have drastically improved in their resolution and capabilities, with the possibility to follow in real-time surface chemical reactions using both microscopic 43,44 and spectroscopic 45,46 tools. Similarly, the advent of operando techniques [47][48][49][50][51] is abating the long-standing hurdle related to the ''pressure gap'' 52 between in situ experiments in an ultrahigh vacuum and industrial catalytic processes. 53 Correspondingly, theoretical models accomplished a huge leap with the unceasing upgrading of computational capabilities, [54][55][56][57][58] including corrections for taking into account weak forces, 59,60 now enabling the prediction of chemical properties of surfaces and interfaces.…”