Morphology Effect of Ceria on the Ammonia Synthesis Activity of Ru/CeO2 Catalysts

Three Ru/CeO 2 catalysts with different morphologies of CeO 2 (cube spheres, microspheres and nano rods) were used to evaluate the support-morphology-dependent ammonia synthesis activity. Catalytic experiments show that the Ru/CeO 2 –CS catalyst has higher catalytic activity (27,000 µmol g −1  h −1 ) than Ru/CeO 2 –MS (21,000 µmol g −1  h −1 ) and Ru/CeO 2 –NR (15,000 µmol g −1  h −1 ) under the reaction conditions of 450 °C, 3 MPa, H 2 /N 2  = 3:1 (60 mL min −1 ). The transmission electron microscopy analysis showed that the dispersion of active metal Ru is affected by the morphology of CeO 2 . Brunauer–Emmett–Teller indicates that the 3–5 nm the pore size of CeO 2 supports contributes to the active metal Ru enters the pores of the CeO 2 support, which improves the dispersion of Ru and prevents the sintering and agglomeration of Ru in some extent. TPR studies shown that the reduction of ruthenium oxide is influenced by the morphology of CeO 2 . XPS and CO 2 -TPD demonstrated that the Ru/CeO 2 –CS catalyst exhibited higher surface oxygen vacancies, higher basic site density and lower Ru binding energy, indicating that Ru nanoparticles in Ru/CeO 2 –CS is more electron-rich and are more capable of back-donating electrons to adsorbed N 2 and subsequently activate N 2 . Our results indicate that the morphology effect of the CeO 2 supports on the Ru/CeO 2 catalyst is related to the pore size distribution and the surface oxygen vacancies of the CeO 2 supports, ratio of Ru n+ , basic site density. Graphical Abstract The CeO 2 –CS (cube spheres) morphology facilitated the dispersion of the active metal Ru and the 3 nm the pore size of CeO 2 –CS contributed to the Ru enters the pores of the support. And the Ru/CeO 2 –CS catalyst exhibited higher surface oxygen vacancies and lower Ru binding energy, which enhanced the adsorption of hydrogen and nitrogen species and thus weakened the N≡N bond.