Preparation Of Palladium Modified Carbon-based Catalyst And Its Properties Toward Electrochemical Ammonia Synthesis

Ammonia plays an important role in industrial and agricultural production,as well as energy storage and conversion.At present,the industrial synthesis of ammonia mainly relies on Haber-Bosch method,which has complicated process flow,low conversion rate,large energy consumption and serious environmental pollution.Although many efforts have been devoted to developing novelty catalysts to ameliorate this dilemma,the extreme reaction conditions still cannot be fundamentally changed,and it is challenging to break through the thermodynamic limit of ammonia synthesis.Therefore,it is imperative to explore an environmental friendly technique to synthesize ammonia for the replacement of traditional method.Electrochemical ammonia synthesis is a new strategy for ammonia synthesis.It utilizes abundant water and nitrogen as raw materials to achieve nitrogen reduction reaction at ambient temperature and pressure with renewable electric energy,which overcomes the limitation of thermodynamics.However,the electrochemical synthesis of ammonia is faced with problems of low yield and efficiency due to poor solubility of nitrogen in water,strong N≡N,the existence of competitive hydrogen evolution reaction（HER）and other factors,so it is very important to develop high-performance catalyst.Related research shows that the formation ofα-PdH phase can effectively balance the competitive adsorption between nitrogen and proton,which is conducive to nitrogen hydrogenation.Carbon material exhibits weak hydrogen adsorption ability,which can suppress competitive hydrogen evolution reaction on a certain extent.And modified carbon-based material has remarkable performance in NRR.In this paper,palladium nanoparticles modified carbon matrix composites were prepared by electrospinning and chemical reduction methods.The nitrogen reduction performance was evaluated properly through the systematic physical characterization and the comprehensive electrochemical test.The influence of structure-activity relationship on the performance of NRR was explored deeply and the reaction mechanism of nitrogen reduction was revealed from the atomic scale by density functional theory（DFT）calculation.（1）Palladium nanocrystals chelated carbon fibers（PdNCs@CNFs）was prepared by electrospinning and carbonization at high temperature.The abundant defect sites on the surface of one-dimensional carbon fibers and the stress effect induced by the fiber localization are favorable for nitrogen adsorption and activation.In 0.1 M Na₂SO₄ solution,PdNPs@CNFs showed high nitrogen reduction performance.The ammonia yield of the catalyst was 4.4μg h^-1mg^-1 at-0.2 V vs.RHE,and the corresponding Faraday efficiency was up to 14.8%.Meanwhile,the catalyst showed superior stability during the long-time test.The mechanism of NRR was discussed through DFT calculation.The model optimization results showed that nitrogen molecules adsorbed on the catalyst surface in the form of end adsorption.Differential charge analysis showed that surface palladium atoms transferred electrons to theπorbital of nitrogen,which promoted the polarization of nitrogen molecules and activated N≡N.NRR was simulated by the remote hydrogenation path,and the rate-determining step（RDS）was the first protogenation（*NN→*NNH）of N₂.（2）Palladium particles loaded on commercial graphene composite powder（PdNPs@GCP）were prepared by simple sodium borohydride reduction.The ammonia yield was 5.2μg h^-1 mg^-1 at-0.2 V vs.RHE,the Faraday efficiency of ammonia synthesis reached 9.77%at-0.1 V vs.RHE,and showed good durability and selectivity in all constant potential polarization.This is due to the special structure activity relationship between GCP and palladium particles:the two-dimensional structure of GCP improves the electron transport efficiency,providing a large specific surface area for NRR,and its hydrophobic interface can inhibit the competitive reaction;meanwhile,the metal-carrier interaction fine-tunes the electronic structure of the Pd particles,optimizing the adsorption and desorption of intermediate products to promote the NRR process.（3）The porous carbon spheres（PCS）were used as carriers to support palladium particles.The localized effect of microchannels was used to promote the adsorption of nitrogen molecules.And N≡N is activated by indirect hydrogenation of the inner palladium particle.Moreover,the large specific surface area provided by the microporous structure is not only conducive to the dispersion of palladium particles,but also beneficial to the charge and electrolyte ions transfer,thus improving the performance of the catalyst.The ammonia yield was 5.1μg h^-1 mg^-1 at-0.2V vs.RHE,and the Faraday efficiency was up to 16.4%at-0.1 V vs.RHE.