Development Of Graphene Supported Ruthenium Catalysts And Their Application To Ammonia Synthesis

Ammonia plays a vital role in our daily life.Ammonia is regared as an ideal hydrogen carrier,which could coordinate the energy and environment issues.Developing an active catalyst for ammonia synthesis is theoretically and practically valuable from both the viewpoints of reducing energy consumption by the conventional Haber-Bosch process and shifting our society to renewable energies in the future.As the second-generation catalyst for ammonia synthesis,ruthenium catalyst shows high catalytic activity under milder condition compared with iron-based catalysts.However,the catalytic performane of ruthenium catalyst for ammonia synthesis was support-dependent and structure-sensitive.Therefore,controllable synthesis of Ru nanocrystallites grown on appropriate support is of tremedous importance for developing highly efficient catalytic systems for ammonia synthesis.In this thesis,polyol method was applied to synthesize Ru nanocrystallites.The size of Ru nanocrystallites can be successfully tailored by varying the research parameters.Additionally,the abundance of B5 sites of ruthenium nanocrystallites is supposed to be enhanced by tuning the size and shape of ruthenium crystallites grown epitaxially on the surface of layered graphene support having high specific surface area.Based on the results of controllable synthesis of ruthenium nanocrystallites,the same approaches were utilized in construction of graphene supported ruthenium catalysts.The main research contents and conclusions are shown as follow:1.Polyol method is an effective way to fabrication of Ru nanocrystallites.The size of Ru nanocrystallites can be successfully tailored by varying the reduction ability of polyols,initial concentration of precursors,or by adding capping agents.The Ru nanocrystllites derived from ethylene glycol have the largest average size(d)of 8.42 nm with a standard deviation(σ)of 3.10.Replacement of ethylene glycol with 1,3-propanediol,a stronger reductant,significantly accelerates the reduction of Ru,thus increases the nucleation rate and the number of Ru nuclei,resulting in smaller particles(d = 3.61 nm)and a narrower size distribution(σ = 0.76).As expected,oleic acid capping agent substantially hindered the growth of Ru particles(d = 1.77 nm,σ = 0.74).Raising the concentration of Ru precursor by ten times accelerated the abrupt nucleation of Ru atoms,resulting in nanoparticles having a relatively smaller average size(d =1.75 nm)and a narrower size distribution(σ =0.98).2.Graphene was successfully prepared using oxidation-reduction method.The variation of concentration of graphite oxide dispersion,sonication time and the kind of reducing agent has the significant effect on the specific surface area of prepared graphene.When using the ethylene glycol as reductant,different sonication time was applied to sonicate graphene oxide with different concentration to obtain the graphene with the highest specific surface area.For different kinds of reducing agent,the graphene derived from 1,2-propanediol has the largest specific surface area.3.Graphene oxides and Ru presusor can be simultaneously reduced by polyols to prepare the graphene supported ruthenium catalysts.Based on the results of controllable synthesis of Ru nanocrystallites,the Ru nanocrystallites with different morphology grown on graphene substrate were produced by tuning the research parameters.The remained oxygen functionalities of reduced graphene oxide provide anchor sites for Ru3+.The zeta potential of graphene oxide in polyols is found to play a key role in tailoring the morphology and oxidation state of Ru nanocrystallites on graphene substrate.Negatively charged graphene oxides in polyols can coordinate with Ru precursors via negatively charged carboxylic acid groups to anchor ruthenium complexes on the substrate of graphene oxides.Flat and hexagonal ruthenium particles of 3-5 nm bonded to the graphene substrate via C-O-Ru bonds epitaxially grow in 1,3-propanediol,which shows the highest catalytic performance(24 mmol NH3/gRu · h)for ammonia synthesis among all the prepared samples.