Efficient ammonia synthesis over a core–shell Ru/CeO2 catalyst with a tunable CeO2 size: DFT calculations and XAS spectroscopy studies

Abstract

In the present work, Ru@CeO₂ catalysts similar in their core–shell morphology but different in nanoparticle sizes (95, 150 and 225 nm) were obtained by varying the hydrothermal time for the preparation of colloidal carbon spheres to be used as templates. The surface areas of these catalysts are in the range of 111–146 m² g⁻¹. After in situ H₂ treatment at 450 °C for 4 h, the Ru@CeO₂-9 catalyst with a nanoparticle size of 225 nm has an ammonia synthesis rate of 13 504 μmol g_NH3⁻¹ h⁻¹ at 1 MPa (N₂ : H₂ = 1 : 3) and 425 °C. The results of XAS, Raman and XPS investigations reveal the active sites in the form of Ru–hydride and Ru⁰ clusters of the Ru@CeO₂ catalysts for ammonia synthesis. Moreover, H₂ treatment results in the strengthening of the ruthenium–ceria interaction, which is beneficial for ammonia formation. It is demonstrated that the size of the nanoparticle has a strong effect on the strength of the ruthenium–ceria interaction, which influences the reduction degree of ruthenium and cerium and the amount of the generated Ru–hydride species. The outcomes of DFT calculations provide evidence that the strength of the ruthenium–cerium interaction is associated with the N₂ adsorption energy. In comparison with Ru/CeO₂-CP synthesized by the co-precipitation method, the core–shell structure of Ru@CeO₂ affords a larger surface area, stronger metal–support interactions and more reductive species, and the consequence is a higher ammonia synthesis rate.