Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N₂ Activation

Abstract

In this work, a relatively new class of materials, rare earth (RE) based high entropy oxides (HEO) are discussed in terms of the evolution of the oxygen vacant sites (O v ) content in their structure as the composition changes from binary to HEO using both experimental and computational tools; the composition of HEO under focus is the CeLaPrSmGdO due to the importance of ceriarelated (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO 2 , the latter as representative nonreducible oxide, were used and compared as supports for Ru (6 wt % loading). The role of the O v in the HEO is highlighted for the ammonia production with particular emphasis on the N 2 dissociation step (N 2(ads) → N ads ) over a HEO; the latter step is considered the rate controlling one in the ammonia production. Density functional theory (DFT) calculations and 18 O 2 transient isotopic experiments were used to probe the energy of formation, the population, and the easiness of formation for the O v at 650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPS probed the Ce−O chemical environment at different length scales. In particular, it was found that the particular HEO composition eases the O v formation in bulk, in medium (Raman), and in short (localized) order (EPR); more O v population was found on the surface of the HEO compared to the binary reference oxide (CePrO). Additionally, HEO gives rise to smaller and less sharp faceted Ru particles, yet in stronger interaction with the HEO support and abundance of Ru−O−Ce entities (Raman and XPS). Ammonia production reaction at 400 °C and in the 10−50 bar range was performed over Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO 2 catalysts; the Ru/HEO had superior performance at 10 bar compared to the rest of catalysts. The best performing Ru/HEO catalyst was activated under different temperatures (650 vs 800 °C) so to adjust the O v population with the lower temperature maintaining better performance for the catalyst. DFT calculations showed that the HEO active site for N adsorption involves the O v site adjacent to the adsorption event.