First-principles Study On The Regulation Of Electrocatalytic Ammonia Synthesis Performance In Graphitic Carbon Nitride-based Materials

As the increasing of the population,the demand for energy and fertilizer is exploding.Therefore,how to synthesize ammonia efficiently and environmentally has been paid more and more attention.The traditional Haber-Bosch process for ammonia synthesis consumes huge energy and causes a large amount of carbon emissions.Therefore,how to find an alternative synthetic ammonia process has become an urgent problem to be solved.Electrochemical catalytic synthesis of ammonia is considered to be the most promising ammonia synthesis method because of its simple synthesis equipment,energy saving,environmental protection,and high conversion efficiency.However,there is no efficient and stable catalyst available for industrial production.In order to solve the problems encountered in the catalyst design at present,more and more researchers begin to pay attention to the catalytic mechanism of electrochemical ammonia synthesis（NRR）.In this paper,we mainly studied the NRR performance of transition metal atoms（Nb）anchored on the graphitic carbon nitride（g-C₃N₄）with different sizes（Nb₁,Nb₂,Nb₃）by the density functional theory calculations.;And the NRR performance of metal and non-metal co-doped in the graphitic carbon nitride containing N defects.The study of theoretical calculations reveals the mechanism of the NRR reaction of the catalyst and the origin of the NRR activity,which will provide theoretical guidance for experimental work.In summary,our research results are as follows:（1）Graphitic carbon nitride doped with different sizes of Nb atoms exhibit different NRR properties.And as the size of metal atoms changes,or the number of active sites changes,we have a new understanding of the reaction path.On the basis of the study of Nb atoms,the general rule of polyatomic NRR catalysis is generalized,that is,the adsorption of H at the active site in the reaction to form*NH₂+*H intermediates is beneficial to reduce the direct hydrogenation of*NH₂ to form*NH₃ with high energy barrier.Under this path,the best catalyst we get is Nb₂@g-C₃N₄ with only 0.25V overpotential,which is lower than most of the metal-based catalysts that have been reported.（2）On the basis of the previous research,we are interested in the origin of catalyst activity.How to adjust the adsorption energy of the active center to the key intermediate of the reaction is the key to improving the performance of a catalyst.Therefore,we can further reveal the origin of the catalytic activity of the active center by adjusting the coordination of atoms near the active center,hoping to have a deeper insight into the reaction mechanism of NRR.On this basis,we first screened the NRR performance of TM-doped graphite phase carbon nitride,and fully analyzed the origin of the NRR activity of V@g-C₃N₄.Subsequently,non-metallic doping was used to regulate the catalytic activity of V atoms,and successfully reduced the NRR overpotential from0.43V of V single atom to 0.28V,indicating that the activity of the active center can be artificially adjusted to an optimal value,and summarized the catalyst design plan.Here,we hope that our ideas can provide some guidance for experimental workers.The research results of this paper show that the graphitic carbon nitride material doped with Nb diatoms has very excellent NRR performance and follows the new hydrogenation path found in our research to react.With the continuous upgrading of analysis and testing equipment,Real-time observation of catalytic reaction pathways is becoming more and more possible;non-metal regulating the activity of active sites can be an effective solution for catalyst modification.The advantage is that it will not introduce new active sites like metal doping,but It only plays a minor role in changing the coordination environment of the active center.However,the impact of this small change on the reaction rate can be huge.We hope that our research work can inspire more experiments or theoretical workers to develop more efficient nitrogen reduction catalysts.