Design And Preparation Of CeO₂ Support Metal Catalysts And Their Performances Of Ammonia Synthesis

Catalytic ammonia synthesis from hydrogen and nitrogen is one of the most important inventions in the 20th century.Because of the high energy-comsumption of Fe c.atalysts and the insufficient stability of Ru/C catalysts,there is an urgent need for the development of a highly stable and efficient catalyst for ammonia synthesis.In this thesis,in order to investigate the factors influencing the performances of ammonia synthesis,we changed the morphology of ceria,the kind and loading of promoter and the loading of Ru metal to prepare a series of metal catalysts,which used CeO2 as support,cobalt and ruthenium as active metal.The main results and conclusions were obtained as follows:(1)The ceria morphology(polyhedra,nanorods and hexagonal shapes)affects the reduction of Co oxides and CeO2,A larger amount of CeO2 is reduced for polyhedral CeO2-supported Co catalyst,which leads to higher Ce3+ concentration and lower binding energy value of Co species,therefore,the sample shows highest catalytic activity of ammonia synthesis.The presence of a small amount of Ba promoter increases the amount of adsorbed H2 and N2,while the high Ba loading enhances N2 adsorption along with the decrease of H2 adsorption.The addition of K promoter inhibits the sites available for hydrogen and nitrogen adsorption/desorption at low temperature.Consequently,Ce02 supported Co catalyst with an appropriate amount of Ba promoter shows high activity of ammonia synthesis,and the samples with high Ba loading or K promoter exhibit low rates.(2)On CeO2 rods,CeO2 polyhedrons,the Ru species are relatively low in crystallinity and enriched with Ru4+.Whereas on Ce02 cubes,there is a higher percentage of metallic Ru,and the Ru species are larger in particle size.The addition of Ru may increase the amount of oxygen vacancies not only by the conversion of Ce4+ to Ce3+,but also via the formation of Ru-O-Ce bonds.For Ru catalysts supported on CeO2 rods and CeO2 polyhedrons,the low crystallinity of Ru species and high concentration of oxygen vacancies greatly enhance the adsorption of hydrogen and nitrogen,and also lead to desorption of surface hydrogen in the form of H2.On the contrary,for the Ru/CeO2 cubes catalyst with lower concentration of oxygen vacancies and larger Ru particle size,the adsorption of hydrogen and nitrogen is less favorable,and most of the adsorbed hydrogen desorbs as H2O,and then Ru/CeO2 cubes shows low ammonia synthesis activities.(3)The differences in the ammonia synthesis activity do not directly correlate with the changes of the surface area and the am ount of oxygen vacancies for varuous Ru/CeO2 catalysts.Most Ru metal are avaliable to the adsorption of CO molecules for Ru/Ce02 catalyst with low Ru loading(1 wt.%),and thus the average size of Ru particles estimated from TEM images is close to the value obtained from CO chemisorption.Some Ru particles would agglomerate with each other to form Ru clusters with large size for Ru/CeO2 catalyst with high Ru loading.The large size of Ru clusters is not accessible to gas phase CO molecules or other gases.As a consequence,the ammonia synthesis rates based on the weight of Ru metal is very low for Ru/CeO2 catalyst with high Ru loading.In this thesis,the influence factors on the properties and the ammonia synthesis activities of CeO2 supported metal catalysts have been studied.The result provides valuable guildines for the preparation of an efficient catalyst for ammonia synthesis.