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

Omer Elmutasim; Aseel G Hussien; Abhishek Sharan; Sara AlKhoori; Michalis A Vasiliades; Inas Magdy Abdelrahman Taha; Seokjin Kim; Messaoud Harfouche; Abdul-Hamid Emwas; Dalaver H Anjum; Angelos M Efstathiou; Cafer T Yavuz; Nirpendra Singh; Kyriaki Polychronopoulou

doi:10.1021/acsami.3c16521

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

ACS Appl Mater Interfaces. 2024 Apr 29;16(18):23038-23053. doi: 10.1021/acsami.3c16521. Online ahead of print.

Authors

Omer Elmutasim^{1

2}, Aseel G Hussien^{1

2}, Abhishek Sharan^{2

3}, Sara AlKhoori^{1

2}, Michalis A Vasiliades⁴, Inas Magdy Abdelrahman Taha³, Seokjin Kim⁵, Messaoud Harfouche⁶, Abdul-Hamid Emwas⁷, Dalaver H Anjum^{2

3}, Angelos M Efstathiou⁴, Cafer T Yavuz⁵, Nirpendra Singh^{2

3}, Kyriaki Polychronopoulou^{1

2}

Affiliations

¹ Mechanical Engineering Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates.
² Center for Catalysis and Separation (CeCaS), Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates.
³ Physics Department, Khalifa University, P.O. Box 127788, Abu Dhabi, United Arab Emirates.
⁴ Department of Chemistry, Heterogeneous Catalysis Laboratory, University of Cyprus, 1 University Avenue, University Campus, 2109 Nicosia, Cyprus.
⁵ Oxide & Organic Nanomaterials for Energy & Environment (ONE) Laboratory, Advanced Membranes & Porous Materials (AMPM) Center, and KAUST Catalysis Center (KCC), Physical Science & Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia.
⁶ Synchrotron-Light for Experimental Science and Applications in the Middle East (SESAME), Allan 19252, Jordan.
⁷ Core Laboratories, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.

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 ceria-related (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO₂, 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 N₂ 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 ¹⁸O₂ 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/SiO₂ 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.

Keywords: DFT; EPR; Synchrotron EXAFS; ammonia; high entropy oxides; isotopic exchange; oxygen vacancies.