Preparation Of Ru,Cu,Fe-Based Transition Metal Phosphide For Electrocatalytic Nitrogen Reduction Reaction

The human beings are pushed into a new era by the rapid development of science and technology,the electronic information age,which requires the coordinated development of relevant basic industries,such as energy,materials and information to drive the prosperity and development of society.As one of the important factors in social development,energy plays a critical role in human development.However,with the aggravating environmental problems and the exhausted fossil fuels,people need to develop new energy substances to drive social development and prosperity.Ammonia(NH₃),as one of most produced chemicals in the world,plays a critical role in the agriculture,medicine as well as industry.For instance,NH₃ is the key raw material for the synthesis of materials,such as fertilizers,medicaments,dyes,explosives,which are closely related to human daily life.In addition,with considerable hydrogen content and high energy density,NH₃ is also functioned as a promising candidate for the carbon-free energy carrier.At present,the industrial NH₃fabrication is still dominated by the traditional Haber–Bosch method,which is conduction under harsh conditions(high temperature and high pressure),resulting in abundant of energy consumption.Moreover,as one of the reactants,H₂ stems form the steam-methane reforming,accompanied with plenty of CO₂ emission.To settle with the increasingly serious environmental issue and energy crisis,a sustainable approach is required for the realization of artificial NH₃synthesis.Electrochemical N₂ reduction is treated as a promising alternative to take the place of the traditional Haber–Bosch method based on the reaction between N₂ and H₂O under ambient conditions.However,due to the high sluggishness of the N₂ molecule,efficient electrocatalysts are needed to accelerate the N₂ reduction reaction.Moreover,due to the competitive relationship with hydrogen evolution,efficient electrocatalysts also should possess the capality to suppress the hydrogen evolution.The N₂ reduction reaction for NH₃ formation on the surface of catalysts mainly experiences two processes:(i)the adsorption of N₂on the surface of catalysts;(ii)the hydrogenation of N₂ for the NH₃synthesis.For the N₂ adsorption,catalysts should possess empty d orbitals for the acceptance of electrons from the?orbitals with full electrons of N₂.For the hydrogenation of N₂,catalysts should possess the d orbitals with full electrons for the back donation to the empty?orbitals of N₂ molecules.Transition metals,due to their special electronic structures,contains both empty d orbital and full d orbital,so they can be used to electrochemically catalyze N₂ reduction reaction for NH₃ formation at ambient conditions.Among these metals,Ru,Cu and Fe-based catalysts are the most promising candidates for the conversion of N₂ to NH₃ under ambient conditions due to the reason that Ru and Fe-based catalysts are the efficient catalysts in the traditional Haber–Bosch method to accelerate the transformation of N₂ to NH₃,while,Cu is the normal and cheap element.However,studies shows that the interaction between transition metals and intermediates is too strong,resulting in that the generated NH₃ cannot be released in time from the surface of the catalyst,thus slowing down the process of cyclic reaction and reducing the rate of NH₃ synthesis.It is worth noting that it is an effective pathway to modulate the electronic structure of transition metals through introducing the electronegative nonmetals of P.Therefore,it is reasonable to speculate that Ru,Cu and Fe-based transition metal phosphates can be used as effective catalysts to accelerate the process of N₂ reduction reaction for NH₃ formation under ambient conditions.In this dissertation,we focus on the experimental synthesis of transition metal phosphates and systematically investigate their morphology,electronic structure as well as the NRR performance under ambient conditions,as summarized following:(1)Ru₂P nanoparticle-reduced graphene oxide hybrid was synthesized by the two-step reaction,in which the precursor of hybrid was prepared by the hydrothermal method followed by the phosphorization under high temperature condition.Morphological characterization indicates that the Ru₂P nanoparticles were uniformly anchored on the surface of rGO.Tested in 0.1 M HCl,such hybrid exhibits excellent capacity for NH₃synthesis with a high rate of 32.8?g h^-1 mg^-1_cat.and a high Faradaic efficiency(FE)of13.04%at-0.6 V vs.reversible hydrogen electrode(RHE).Moreover,during the test procedure,the hybrid also shows superior durability and selectivity for artificial NH₃synthesis under ambient conditions.The superior performance of Ru₂P nanoparticle-reduced graphene oxide hybrid is derived from the introduction of P,which can transfer some electrons from Ru to itself,resulting in the decrease of adsorption capacity to NRR intermediates,thus accelerating the whole NRR process.At the same time,the presence of P can inhibit the competitive hydrogen evolution reaction and improve the performance of Ru₂P nanoparticles for NRR,resulting in the improvement of electron utilization.Furthermore,the introduction of rGO can improve the dispersion of Ru₂P nanoparticles,making more active sites exposed for NRR.At the same time,the introduced rGO can reduce the impedance of electron transfer in the process of NRR catalyzed by Ru₂P nanoparticles and improve the kinetics of electron transfer in the process of NRR.According to the theoretical calculation results,Ru atoms in the Ru₂P nanoparticles are the active sites for the NRR and rGO can enhance the electron transfer kinetics for NRR.in addition,the Ru₂P nanoparticle-reduced graphene oxide hybrid is an effective catalyst for NRR due to its low energy barrier.(2)FeP nanorods were prepared via two-step reaction:(i).hydrothermal method for the precursor,(ii).phosphorization of the precursor under high temperature condition.Tested in acidic environment(0.1 M HCl solution),FeP nanorods shows superior NRR performance,in which the NH₃ yield rate can be reached to 36.5?g h^-1 mg^-1_cat.and the FE is 14%at-0.45 V vs.RHE under ambient conditions.Furthermore,the excellent durability and selectivity of FeP nanorods also indicates that FeP nanorods are efficient electrochemical catalysts for artificial NH₃ synthesis under ambient conditions.The results of this work indicates that the excellent catalytic performance of FeP nanorods of N₂ reduction reaction for NH₃ formation can be attributed to the introducing the nonmetal element of P,which can transfer some electrons from Fe to itself,resulting in the decrease of adsorption capacity to NRR intermediates,thus accelerating the whole NRR process.At the same time,the presence of P can inhibit the competitive hydrogen evolution reaction and improve the performance of FeP nanorods for NRR,resulting in the improvement of electron utilization.(3)Cu₃P nanoparticle-reduced graphene oxide hybrid was synthesized by the phosphorization of the precursor under high temperature condition.Morphological characterization indicates that the Cu₃P nanoparticles were uniformly anchored on the surface of rGO.Tested in 0.1 M HCl,such hybrid exhibits excellent capacity for NH₃synthesis with a high rate of 26.38?g h^-1 mg^-1_cat.and a high FE of 10.11%at-0.45 V vs.RHE.Moreover,during the test procedure,the hybrid also shows superior durability and selectivity for artificial NH₃ synthesis under ambient conditions.The superior performance of Cu₃P nanoparticle-reduced graphene oxide hybrid can be attributed to the introducing of P,which decrease the charge density of Cu nanoparticles,resulting in the enhanced adsorption of N₂ on the surface of the catalysts and the accelerated rate of NH₃formation.At the same time,the presence of P can inhibit the competitive hydrogen evolution reaction and improve the performance of Cu₃P nanoparticles for NRR,resulting in the improvement of electron utilization.Furthermore,the introduction of rGO can improve the dispersion of Ru₂P nanoparticles,making more active sites exposed for NRR.At the same time,rGO can accelerate the electron transfer during the reduction reaction process,resulting in the enhanced rate of NH₃ formation.According to the theoretical calculation results,Cu₃P is an efficient catalyst for the N₂ reduction and the first protonation process is the potential-determining step for the whole N₂ reduction process.