Electrocatalytic N₂ Reduction To NH₃ By Non-noble Metal Based Nanocatalysts

As an important industrial raw material,NH₃ plays a key role in the global economy.At present,industrial-scale NH₃ production mainly relies on the Haber-Bosch process,but this method is energy consuming and pollution serious.Therefore,it is of great significance to find an environmentally benign and low energy consumption alternative strategy.Electrochemical N₂ reduction（NRR）can synthesize ammonia with the existence of electrocatalysts under mild conditions,which has aroused great research interest as an attractive technology due to the advantages of no greenhouse gas and emissions energy-effective.Noble metal-based catalysts perform actively,however,their wide application is limited by the scarcity and high price.While the development and modification of non-precious metal nanomaterials such as transition metals,lanthanide series and carbon-based catalysts,which are widely available and cheap,have attracted great attentions.Based on the review of recent progress in electrochemical nitrogen reduction catalysts,three new and efficient non-noble metal-based catalysts have been developed and their NRR properties have been investigated.The details are as follows:1.According to synergistic catalysis and the good effect of Mn²⁺on nitrogenase,a new catalyst（MnS-Mn₃O₄）for NRR was synthesized.The surface of MnS is partly oxidized into Mn₃O₄,which serve as the surface protection layer and avoid further oxidation of MnS,thus improving the stability.When tested in 0.1 M Na₂SO₄,it performs a large NH₃ yield of17.19μg h^-1 mg^-1_cat.and faradaic efficiency（FE）of 6.04%.There is no N₂H₄ during the process,indicating an excellent selectivity of N₂ to NH₃fixation at ambient conditions.Moreover,MnS-Mn₃O₄ exhibits excellent selectivity of N₂ to NH₃ and strong long-term electrochemical durability.2.Inspired by the fact that F^-can inhibit hydrogen evolution reaction（HER）in certain reduction reactions,we developed hexagonal LaF₃nanoplates which can inhibit the competing HER in NRR,and synthesized La₂O₃ as a control material in a similar way.In 0.5 M Li ClO₄,this catalyst offers a high FE（16.0%）and a large NH₃ yield(55.9μg h^-1 mg^-1_cat.)at-0.45 V,much higher than those of La₂O₃(1.4%;12.5μg h^-1 mg^-1_cat.).The inhibition of HER of hexagonal LaF₃ nanoplates was proved by density functional theory calculation.This study is not only the first use of metal fluoride in electrochemical ammonia fixation,but also provides an exciting new way for the rational design catalysts of efficient N₂ fixation.3.In order to avoid the pollution of metal ions to the environment,perylene-3,4,9,10-tetracarboxylic acid（PTCA）,which has the advantages of low cost,manufacture and high electron mobility,was combined with rGO via ultrasonic treatment as synergistic catalyst.When tested in 0.1 M HCl,this catalyst achieves a large NH₃ yield of 24.7μg h^-1 mg^-1_cat.with a high FE of 6.9%at-0.50 V,and is higher than PTCA and rGO.Furthermore,it exhibits a satisfactory electrochemical and structural stability.DFT calculations suggest that the NRR over PTCA-rGO takes place via both distal associative and partially alternative routes,and both PTCA and rGO synergistically enhance the NRR activity.This work not only provides an earth-abundant electrocatalyst that co-catalyzes the NRR,but would open up an exciting new road to the rational design and development of organic nanostructures/rGO hybrid catalysts for NRR application.