Structural Modulation And Catalytic Performance Of The Ru/CeO₂ Catalysts For Ammonia Synthesis Under Mild Conditions

Ammonia is not only the basic chemical raw material for the production of fertilizers,plastics and fibers,but also an important energy carrier in the hydrogen economy due to its high hydrogen density?17.6 wt%?and easy storage and transportation.Industrial N₂ fixation,the Haber-Bosch process,which requires high temperatures?400-500??and high pressures?20-40 MPa?.In the modern environment of green chemistry,it has become an inevitable trend to find a ruthenium-based catalyst with high activity under mild conditions instead of the traditional iron-based catalyst in the Haber-Bosch process.In this thesis,the effect of the morphology of CeO₂ support and support prepared by different precipitants on the catalytic performance of ammonia synthesis was studied.CeO₂ support of different morphologies affects the dispersion of active metal Ru,surface oxygen vacancies,basic site density and Ru binding energy,thus affecting the catalytic performance of ammonia synthesis.The difference in oxygen vacancy concentration on the surface of the CeO₂support prepared by different precipitants is a key factor affecting the catalytic performance of ammonia synthesis.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 the2.5%Ru/CeO₂-CS catalyst has higher catalytic activity(27000?mol·g^-1·h^-1)than2.5%Ru/CeO₂-MS(21000?mol·g^-1·h^-1)and 2.5%Ru/CeO₂-NR(15000?mol·g^-1·h^-1)under the reaction conditions of 450?,3 MPa,H₂/N₂=3:1(60 mL·min^-1).The turnover frequency?TOF?value of the 2.5%Ru/CeO₂-CS catalyst is the highest(108.86 h^-1)at the 3 MPa,450?.The 2.5%Ru/CeO₂-CS catalyst showed no significant loss of activity after continuous reaction at 3 MPa and 450?for100 h,and it had high thermal stability.HRTEM indicates that the dispersion of active metal Ru is obviously affected by the morphology of CeO₂.XPS and CO₂-TPD demonstrates that the Ru/CeO₂-CS catalyst exhibits high surface oxygen vacancies,high basic site density and low Ru binding energy,which promotes the electron enrichment of Ru nanoparticles,thus accelerating the dissociation of the rate-limiting step N?N in ammonia synthesis.Three Ru/CeO₂ catalysts with different surface oxygen vacancies were prepared by changing the precipitants species C₁₂H₂₉NO?TPAOH?,C₂H₈N₂?EDA?and NaOH in the synthesis of CeO₂ support.According to the H₂-TPR and XPS characterization,the CeO₂-TPAOH support prepared by the TPAOH precipitant had the highest surface oxygen vacancy concentration.Catalytic performance results showed that 2.5%Ru/CeO₂-TPAOH(22000?mol·g^-1·h^-1)has higher ammonia synthesis activity than 2.5%Ru/CeO₂-EDA(19000?mol·g^-1·h^-1)and2.5%Ru/CeO₂-NaOH(16500?mol·g^-1·h^-1)under the reaction conditions of450?,3 MPa,H₂/N₂=3:1(60 mL·min^-1),and has high thermal stability of 100h.The 4%Cs-2.5%Ru/CeO₂-TPAOH catalyst has the highest ammonia synthesis activity(32000?mol·g^-1·h^-1)and high thermal stability of 100 h at 3 MPa and450?.The high Ce³⁺/Ce⁴⁺and high O^-/O^2-in the Ru/CeO₂-TPAOH catalyst indicate that it has a large number of oxygen vacancies.For Ru/CeO₂-EDA and Ru/CeO₂-NaOH catalysts with low oxygen vacancy concentrations,the ammonia synthesis catalytic activity is relatively low.