Strong Ruδ+–Ce3+ electronic interaction induced by a CeOy overlayer for enhanced low-temperature N2-to-NH3 conversion

Abstract

Ammonia (NH₃) synthesis with hydrogen (H₂) produced from renewable energy is a promising route to reduce carbon footprints and the dependence on fossil fuels. However, the development of highly efficient and stable catalyst for NH₃ synthesis under mild conditions is a challenge. Herein, a series of boron nitride (BN)-supported Ru catalysts were synthesized, and the NH₃ synthesis rate was enhanced significantly after the introduce of Ce. The developed RuCe_2.4/BN catalyst with a Ce/Ru molar ratio of 2.4 exhibited the optimal NH₃ synthesis rate of 14.6 mmol_NH3 g_cat⁻¹ h⁻¹ at 400 °C and 1.0 MPa, which was ∼21-fold that of the catalyst prepared without Ce. In situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption and X-ray photoemission spectroscopy coupled with electron energy loss spectroscopy revealed that the d–d electronic interaction between Ru and CeO₂ can be differentially adjusted by a change in the Ce/Ru molar ratio. An appropriate amount of Ce could enhance the electronic interaction between the CeO_y overlayer and Ru entities. Also, the CeO_y overlayer acted as an “electron shuttle” and enhanced the charge transfer from Ce to the surface of Ru atoms. Finally, electron donation occurred from the surface abundancy of Ru atoms to N₂ molecule through a d–π* interaction. Moreover, the introduction of excessive Ce could block Ru active sites, weaken the electron transfer and, therefore, decrease the NH₃ synthesis rate.