Synthesis And Properties Of Ru-based Catalyst For Ammonia Synthesis With CeO₂ As Support

Ammonia is widely used in industry and agriculture and its demand is increasing.At present,the industry mainly uses the Haber-Bosch method to synthesize ammonia based on iron-based catalysts.This method causes high costs,high energy consumption,and high pollution due to the high temperature and high pressure conditions.The key to solving this problem is to find a catalyst with high activity under mild conditions to replace the iron-based catalyst used in the Haber-Bosch process.Based on the above background and existing problems,the in-depth performance and mechanism of synthetic ammonia were studied by selecting a metal ruthenium as an active center and using a metal oxide CeO₂ as a support.The aim is to synthesize a ruthenium-based catalyst with high activity and high stability in synthesizing ammonia,and to achieve the purpose of synthesizing ammonia under mild conditions.In this thesis,we studied the morphology and surface acidity and basic of CeO₂ on the ammonia synthesis reaction.According to the recently reports,the different morphology of CeO₂ supports will lead to different catalytic activity of ammonia synthesis.Different morphologies will expose different numbers of{110}and{100}crystal faces,which will produce different amounts of oxygen vacancies,affecting the performance of ammonia synthesis catalytic reactions.The stronger the basicity of the surface of the support?the smaller the electronegativity?,the better the catalytic performance of the ammonia synthesis.According to the method in the literature,a relatively uniform,exposes{110}and{100}crystal faces with more oxygen vacancies and nano-spherical structure of CeO₂ support was obtained.A series of catalysts were prepared by impregnation with different amounts of acetylacetonate ruthenium.The catalytic performance test results show that the TOF was highest and showing high catalytic activity when the amount of ruthenium was 2.5%,the activity of2.5%Ru/CeO₂ catalyst reaches 8940?mol/?g·h?at 450°C,3.8 MPa,H₂/N₂=3?60mL/min?,which is higher than that of commercial cerium oxide nearly 2.5 times.After adding different assistant?K,Cs,Ba?,under the same conditions,the activity of the 2%Ba-2.5%Ru/CeO₂ catalyst reached 11400?mol/?g·h?.According to H₂-TPR and XPS analysis,the 2%Ba-2.5%Ru/CeO₂ catalyst has better reducibility and more oxygen vacancies,and due to the strong interaction between Ba species and CeO₂,it is more conducive to the dispersion of Ru.The catalytic activity is greatly increased.The CeO₂-11 synthesized under the condition of pH=11,and the catalytic performance of the x%Ru/CeO₂-11 catalyst prepared by the impregnation method,catalytic performance test results show,At 450°C,3.8 MPa,H₂/N₂=3?60 mL/min?,The results show that the catalytic activity of 1.25%Ru/CeO₂-11 catalyst was7040?mol/?g·h?.The catalyst was higher than that of CeO₂ nanospheres under the same conditions.As can be seen from the CO₂-TPD,the 1.25%Ru/CeO₂-11catalyst is more basic;the 2%Cs-1.25%Ru/CeO₂ catalyst with 2%Cs assistant has a catalytic activity of 11070?mol/?g·h?.At the same time,it can be seen from the H₂-TPR and XPS characterization that 2%Cs-1.25%Ru/CeO₂ have more oxygen vacancies,which increase the electron density around Ru,thus promoting N₂activation and dissociation.Therefore,the catalytic activity is also improved.