Synthesis Of Carbon Supported Molybdenum Nitride As Electrode For Electrocatalytic Nitrogen Reduction To Ammonia

As an important chemical raw material and carrier of hydrogen,there are about 175 million tons of ammonia have been used in the production of chemical fertilizers,explosives,medicines and other industrial products every year at a world scale.In addition,ammonia is supposed to replace hydrogen and fossil to be a new generation of internal combustion engine fuel due to its high energy density and advantages in transportation and storage.At present,90%of ammonia in the world comes from Haber-Bosch process which is contrary to the current tendency in the context of carbon neutrality and sustainable development due to its disadvantages of high energy consumption.Electrocatalytic reduction synthesis of ammonia（NRR）has attracted extensive attention from researchers in recent years for its advantages of simple process conditions（room temperature and pressure）,high selectivity and high utilization rate.However,the NRR reaction system has some challenges:（1）nitrogen molecules have strong thermodynamic stability,and（2）the NRR reaction is accompanied by hydrogen evolution,so the Faraday efficiency of the NRR reaction is low.Reasonable design,construction of cheap,efficient and stable electrocatalyst is the key to solve the above problems.In order to develop high activity catalyst,the composition and surface interface properties of catalyst were regulated.In addition,the inhibition of hydrogen evolution reaction was realized by optimizing the reaction system and changing the composition of electrolyte to control the local environment of catalyst surface interface,which greatly improved the Faraday efficiency and ammonia formation rate of NRR.The main aspects of this research are as follows:（1）Hydrothermal method was adopted to synthesize the composite material of molybdenum trioxide and reduced graphene oxide（MoO₃/rGO）.Then,the composite material of molybdenum nitride and reduced graphene oxide（Mo₂N/rGO）was obtained by annealing in ammonia with MoO₃/rGO as the precursor.Annealing temperature was controlled to adjust the nitriding level of Mo₂N/rGO.Meanwhile the conductivity of Mo₂N/rGO was regulated by controlling the amount of rGO in the precursor.In methanol-water solution,the Mo₂N/rGO obtained when the amount of rGO in the precursor was 20 mg and the annealing temperature was 650℃showed the best NRR performance that the NH₃yeild rate was 20.55±1.56μg h^-1mg^-1_cat.with the corresponding Faraday efficiency of 66.6±3.15%,which achieved at the potential of-1.1 V（vs.Ag/Ag Cl）.Morover,there is no obvious degradation after 9 cycles.（2）MoO₃nanorods were firstly synthesized via hydrothermal method,and then the fabricated MoO₃was coated with glucose-derived carbon（MoO₃@C）by another hydrothermal process.Finally,MoO₃@C was annealed in ammonia to get glucose-derived carbon coated molybdenum nitride（Mo₂N@C）.The annealing temperature was controlled to adjust the active sites on the catalyst.The results showed that Mo₂N@C can be obtained only at the annealing temperature of 650℃,and its highest NH₃yield was103.5±1.3μg h^-1mg^-1_cat.with the corresponding Faraday efficiency of75.9±0.6%at-1.1 V（vs.Ag/Ag Cl）in methanol-aqueous solution,which were nearly 7 times and 3 times compared to molybdenum nitride with no carbon（Mo₂N）,respectively.