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

Three Ru/CeO2 catalysts with different morphologies of CeO2 (cube spheres, microspheres and nano rods) were used to evaluate the support-morphology-dependent ammonia synthesis activity. Catalytic experiments show that the Ru/CeO2-CS catalyst has higher catalytic activity (27,000 mu molg(-1)h(-1)) than Ru/CeO2-MS (21,000 mu molg(-1)h(-1)) and Ru/CeO2-NR (15,000 mu molg(-1)h(-1)) under the reaction conditions of 450 degrees C, 3MPa, H-2/N-2=3:1 (60mLmin(-1)). The transmission electron microscopy analysis showed that the dispersion of active metal Ru is affected by the morphology of CeO2. Brunauer-Emmett-Teller indicates that the 3-5nm the pore size of CeO2 supports contributes to the active metal Ru enters the pores of the CeO2 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 CeO2. XPS and CO2-TPD demonstrated that the Ru/CeO2-CS catalyst exhibited higher surface oxygen vacancies, higher basic site density and lower Ru binding energy, indicating that Ru nanoparticles in Ru/CeO2-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 CeO2 supports on the Ru/CeO2 catalyst is related to the pore size distribution and the surface oxygen vacancies of the CeO2 supports, ratio of Run+, basic site density.