Synthesis Of Boron-or Nitrogen-doped Graphene And Their Applications In Ruthenium Catalysts For Ammonia Synthesis

The industry of ammonia synthesis plays an important role in the national economy and national defense,because ammonia is the main raw material for making fertilizers and promoting food production,the main source of activated nitrogen required by biology,and also an important renewable fuel and ideal hydrogen carrier that can coordinate the problems of energy shortage and environmental pollution.Compared with the traditional iron-based ammonia synthesis catalysts,which require higher reaction pressure and temperature,ruthenium-based catalysts,as the second-generation ammonia synthesis catalysts,have higher catalytic activity for ammonia synthesis at low temperature and pressure.The supports and promoters can significantly change the structure and electron distribution of ruthenium active sites,thus greatly affecting the performance of ruthenium based ammonia catalysts.So developing an ideal support for ruthenium catalysts is important for the construction of highly active ruthenium based catalysts.Graphene is widely used as a catalyst support due to its large specific surface area and strong electron transmission capacity.Studies have shown that doping hetero atoms into graphene can not only retain its original basic properties,but also adjust its electronic structure,increase the number of active sites,and then improve its catalytic activity.Therefore,in this study,doped graphene was prepared by hydrothermal synthesis method,and doped graphene was used as a ruthenium-based catalyst support,and barium nitrate was added as an promoter,so as to improve the activity of ruthenium-based ammonia synthesis catalyst supported by graphene.The main research contents and conclusions are shown as follows:1.Doped graphene was synthesized by the hydrothermal method with saturated boric acid solution or 25%ammonia solution as source of boron or nitrogen and graphene oxide dispersion.The morphology,structure and components of doped graphene was characterized by XPS,FT-IR,SEM,Raman and XRD,the results show that boric acid or ammonia can effectively reduce the graphene oxide,and doped into graphene.In doped graphene,the boron and nitrogen content is respectively between 0.22%-0.54%(atomic fraction)and 7.26-8.69%(atomic fraction).2.When nitrogen-doped graphene was used as the support of ruthenium-based catalysts,the loss of catalysts was large.Compared with the element content before and after the reaction,the content of nitrogen is significantly reduced,so it is inferred that C-N bond in the support was destroyed under the condition of synthesis of ammonia,so nitrogen-doped graphene is not suitable for the support of ruthenium-based catalysts.When boron-doped graphene was used as the support of ruthenium-based catalyst without promoter,there is no ammonia synthesis activity,and the ammonia synthesis rate is significantly enhance after adding promoters,in this catalytic system the optimal ruthenium-loading is 1wt%,the best pretreatment temperature is 300?and the optimal promoter content is Ba:Ru=8.In this condition,the ammonia synthesis rate of8Ba-1%Ru/BG-12 is the best(0.601 mmol NH₃ g^-1 _cat h^-1),about twice as much as that of the catalyst supported by undoped reduced graphene(0.340 mmol NH₃ g^-1 _catat h^-1).3.When boron-doped graphene is used as the ruthenium-based catalyst support,the content of boron has a certain effect on the optimal ammonia synthesis rate of the catalyst.With the increase of boron content,the ruthenium of the active component is easier to be reduced.At the same time,the adsorbed nitrogen of the catalyst increases,which makes the contact between the active component ruthenium and nitrogen increase,promotes the dissociation of the three bonds of nitrogen and nitrogen,and increases the ammonia synthesis rate of the catalyst.