Study On Electrocatalytic Nitrogen Fixation By Oligoatomic Catalysis

Electrochemical catalytic synthesis of ammonia is a green and sustainable technology,and has the most potential to replace the method of industrial ammonia production.The green and highly efficient synthesis of ammonia is n ot only a necessary chemical product to solve the development of human life,but also a key carrier to solve the bottleneck of hydrogen energy storage and transportation.Since the development and utilization of clean energy to replace traditional energy,hydrogen energy has been regarded as the most important clean energy in the 21st century,with irreplaceable core strategic position.Therefore,the electrochemical catalytic ammonia synthesis method not only has a great application prospect,but also has the time significance.Since the invention of electrocatalytic ammonia synthesis technology,it has faced many difficulties and challenges.So far,the problems to be solved in electrocatalytic ammonia synthesis technology include finding ideal electrocata lysts,unifying experimental testing standards,standardizing experimental testing methods,and exploring the characterization technology of mechanism.It is an important way to accelerate the industrial application of electrocatalytic ammonia production m ethod to design and screen the ideal electrocatalysts by the first-principles method.In this paper,a series of monatomic catalysts and diatomic catalysts were designed by density functional theory（VASP）for electrocatalytic nitrogen reduction.The catal ytic mechanism of monatomic and diatomic catalysts in the process of electrocatalytic ammonia production was studied.First,we investigated the electrocatalytic nitrogen reduction performance and catalytic mechanism of transition metal-doped graphene plane by using the common two-dimensional material graphene as catalyst substrate and loading transition metal atoms.The special d orbital electron distribution of transition metal is an important factor to realize the catalytic nitrogen reduction reaction.T he occupied d orbital of the transition metal can provide electrons to the anti-bonding vacant orbital-the lowest unoccupied molecular orbital（LUMO）of the N₂ molecule,so as to realize the activation of the nitrogen molecule.On the other hand,the transition metal’s unoccupied d orbital can accept lone pair electrons from N ₂ molecules,which can enhance the adsorption capacity of nitrogen molecules.Theoretic al calculation shows that Mo doped graphene plane can not only effectively adsorb nitrogen molecules,but also rapidly catalyze nitrogen reduction reaction.When the N ₂ molecule followed the distal reaction mechanism in the Mo doped graphene plane,it had the optimal reduction reaction performance,and its free energy of reaction was only 0.67 eV.In addition,the adsorption capacity of N₂ molecules on the catalyst surface is stronger than that of H,indicating that Mo doped graphene plane has a better ammo nia producing selectivity.Therefore,Mo doped graphene plane has good catalytic performance and has great application value.Then,we have found that the transition metal-doped graphene plane can accelerate the nitrogen reduction reaction.However,the ed ge effect of two-dimensional materials often plays a key role in catalytic performance.Therefore,this chapter further investigates the application of transition metal doped graphene edge in electrocatalytic nitrogen reduction reaction.Three screening cr iteria were introduced to determine the catalytic selectivity of transition metal loading on the edge of graphene.（1）calculate the adsorption energy of N₂ molecule at the reaction site of catalyst.The stronger the adsorption energy is,the more favorabl e it is for the subsequent reaction.（2）calculate the adsorption of the intermediate substance（N ₂H*）for the first hydrogenation of nitrogen molecules.The stronger the adsorption energy,the better the reaction will be.（3）calculate the destabilization effect of the intermediate substance（NH₂*）.The weaker the adsorption energy is,the faster the whole reaction will be.Through the above three screening criteria,we determined that the Co doped graphene edge has the optimal catalytic nitrogen reduction capability.When the N₂ molecule was reduced at the edge of the Co doped graphene by the side-end reaction mechanism,it had the best performance,with the free energy of 0.88 eV.Different transition metal atoms are selective to the loading position of n anomaterials.Therefore,the design of special atomic structure,morphology,is the key to reflect the catalytic effect of the catalyst.Next,we tried to introduce Fe atom load on two-dimensional material black phosphorene as an electrocatalytic nitrogen reduction catalyst,investigate the catalytic performance of the catalyst,and analyze the mechanism of nitrogen reduction reaction process.Black phosphorene belongs to a class of non-monatomic layer two-dimensional materials with special electronic struc ture.At the same time,the structure of black phosphorene can continuously provide electrons for the nitrogen reduction reaction process,which is a potential nitrogen fixation catalyst.The theoretical results show that the electron-rich black phosphorene structure is not conducive to the adsorption of nitrogen molecules,but the black phosphorene loaded with Fe atoms has a strong adsorption effect on nitrogen molecules.Subsequently,the nitrogen molecules in the adsorbed state can undergo fast hydrogena tion and electron coupling process,and finally release ammonia gas.When the nitrogen molecule follows the enzyme reaction mechanism,it is more conducive to the process of reduction reaction,and its reaction free energy is 0.81 eV.The electron density of states shows that when N₂molecules are adsorbed in terminal manner,the degree of activation of nitrogen molecules is low.On the contrary,when N ₂ molecules are adsorbed in a side-end manner,their activation degree is higher,which is also more condu cive to the subsequent reduction reaction.Finally,through summarizing the calculation results of the first three chapters,we found that the selectivity of monatomic catalyst in the nitrogen reduction reaction was too simple to satisfy the multi-proton electron coupling process.Therefore,in this chapter,the synergistic effect of double transition metal atoms in the catalytic reaction process is investigated through the design of diatomic catalyst-Fe,V co-doped carbon nitride（FeV@C₂N）.The theoretical calculation results show that the co-doped carbon nitride catalyst with iron and vanadium can overcome the complexity of the nitrogen reduction reaction and accelerate the nitrogen reduction reaction with the synergistic effect of two atoms.When nitrogen molecules follow the enzymatic reaction mechanism,it is most conducive to the reduction process of nitrogen,and its free energy of reaction is only 0.17 eV,which is comparable to the dissociation performance of N₂ molecules at the step of Ru metal.The refore,this chapter proposes that the synergistic effect of diatomic catalysts can make up for the single selectivity problem of monatomic catalysts and solve the multi-selectivity problem of complex nitrogen fixation reaction paths,thus providing a theo retical basis for the design of more ideal catalysts in the future.