Novel cerium oxide-type high entropy rare earth oxides for photocatalytic CO2 hydrogenation

Mohit Yadav,a Dalibor Tatar,b Igor Djerdj,b András Sápi,a Tamás Gyulavári,a Zsolt Pap,a Ákos Kukovecz,a Zoltán Kónya,c

a Department of Applied and Environmental Chemistry, Interdisciplinary Excellence Centre, University of Szeged, H-6720, Rerrich Béla Sqr. 1, Szeged, Hungary
b Department of Chemistry, University of Osijek, Cara Hadrijana 8/A, HR-31000 Osijek, Croatia
c ELKH-SZTE Reaction Kinetics and Surface Chemistry Research Group, University of Szeged, H-6720, Rerrich Béla Sqr. 1, Szeged, Hungary

Carbon dioxide (CO2) is a double-edged sword. Although it helps create a warm environment on Earth, the excessive burning of fossil fuels has led to a continuous rise in CO2 concentration in the atmosphere, resulting in irreversible climate changes [1, 2]. High entropy materials, which consist of various elements in single-phase compounds, are known for their unique properties and crystal structures due to their high configurational entropy. The recent research trend has focused on utilizing nanostructured ceria (CeO2) in various applications due to its availability, affordability, and stability. It has been found that this rare earth oxide has the potential to be used in photocatalytic applications, including energy production, hydrogen generation, oxygen evolution, and storage capacity enhancement [3, 4].
In our research, we prepared six ceria-based rare earth high-entropy oxides (HEOs) with fluorite structure and examined their photocatalytic behavior toward CO2 hydrogenation. The cationic site in the fluorite lattice consists of five equimolar elements, including Sm, Ce, Pr, La, and Nd (rare earth elements) and Y and Zr (transition metals). The HEOs exhibit band gaps ranging from 2.65 to 3.37 eV and appropriate valence and conduction band positions for CO2 reduction. The samples possess high photocatalytic activity, which can be attributed predominantly to the accessibility of more active sites, resulting in more photogenerated electrons. The materials produced carbon monoxide as the main product, but some methane and methanol were also generated. The photocatalytic performance of all studied HEOs surpasses single fluorite oxides or equivalent mixed oxides. The Ce0.2Zr0.2La0.2Nd0.2Sm0.2O2 (CZLNS) showed the highest photocatalytic conversion of CO2 (29.7 %) and formation rate for CO (1256.1 nmol) among the HEO samples and its pristine CeO2 counterpart (6.6 %). The best-performing photocatalyst was investigated further by theoretical modeling using density functional theory.



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