Preparation Of Novel Non-Noble Metal Based Materials And Electrochemical Synthesis Of Ammonia

Ammonia is the main component of nitrogen fertilizer,is an important raw material for drugs,plastics,resins and explosives,and is one of the most productive chemical industrial products at present.Synthetic ammonia plays an important role in industry,agriculture and people’s production and life.Nitrogen（N₂）is the most widely used and cheapest nitrogen source for ammonia synthesis.The traditional Haber ammonia synthesis process requires the harsh conditions of high temperature and high pressure to promote the coupling of nitrogen and hydrogen,which has high energy consumption and large carbon dioxide emission.Electrochemical catalytic nitrogen reduction reaction（NRR）,N₂ directly reacts with H₂O to synthesize ammonia under normal temperature and pressure,the reaction conditions are mild and the energy consumption is low.However,due to the high stability of N≡N and the competition of hydrogen evolution reaction（HER）,the development of NRR depends on the development of highly active and highly selective catalysts.On the other hand,nitrate is one of the pollution sources of water.Using nitric acid as a nitrogen source and"turning waste into treasure"to synthesize ammonia by electrochemical catalytic reduction of nitric acid（NO₃RR）is also an important way to artificially fix ammonia.In this paper,the surface modification and active site regulation of nano-carbon materials and transition metal materials were carried out by chemical and physical methods to improve the activity and selectivity of electrochemical catalytic reduction of ammonia synthesis,providing direction and basis for the development of electrochemical nitrogen fixation catalysts.（1）Graphene/anthraquinone composite catalyzed ammonia synthesis Anthraquinone（AQ）,1-hydroxy-anthraquinone（1-AQ）and 2,6-dihydroxy-anthraquinone（2-AQ）were combined with graphene（r GO）byπ-πinteraction to improve the catalytic performance of r GO on NRR.The results show that from r GO to anthraquinone modified graphene（AQG）,to 1-hydroxy-anthraquinone modified graphene（1-AQG）,and then to 2,6-dihydroxy-anthraquinone modified graphene（2-AQG）,the surface area,conductivity and ion transfer rate of the catalytic material increase with the increasing of oxygen-containing functional groups on the surface of the composite.In 0.1 M Na₂SO₄,the ammonia yield of NRR increased from 4.49μg·h^-1·mg_cat^-1in r GO to 5.35（AQG）,8.91（1-AQG）and 12.15μg·h^-1·mg_cat^-1（2-AQG）.The Faraday efficiency gradually increased from 8.0%to 8.9%,15.2%and 26.4%.Therefore,using oxygen-containing compounds to modify the surface of carbon materials with non-covalent bonds is an effective way to improve the surface area and electrical conductivity of carbon materials and improve their NRR catalytic performance.（2）Laser irradiation improves the catalytic performance of nickel foam The surface functional groups and structure of nickel foam（NF）were changed by laser irradiation at a fixed point,and the adsorption of NO₃^-and H₂O on the catalyst surface was enhanced,thus improving the catalytic performance of NF for NO₃RR and oxygen evolution reaction（OER）.The results showed that the catalytic OER overpotential(50 m A·cm^-2)of nickel foam irradiated by laser for3 min（L-NF 1）and 5 min（L-NF 2）decreased from 434 m V to 390 m V and 368 m V respectively.The ammonia production of NO₃RR increased from 385.2μg·h^-1·cm_cat^-2 in NF to 516.5 and 549.1μg·h^-1·cm_cat^-2 in L-NF 1 and L-NF 2.This work shows that a short time of laser irradiation can introduce hydrophilic groups on the surface of nickel foam,improve its hydrophilicity,increase its electrochemical surface area,enhance the adsorption of reactants on the catalyst surface,and improve the catalytic performance of the material.Therefore,laser irradiation is a simple and effective way to improve the catalytic performance of transition metals.（3）Preparation and properties of Ni Fe-LDH/NF catalytic materials Nickel-iron bimetallic hydroxide（Ni Fe-LDH/NF）,nickel carbonate（Ni CO₃/NF）and ferric oxide（Fe₃O₄/NF）were grown in situ on nickel foam with nickel salts and/or iron salts as metal sources,respectively.The process of in-situ growth of bimetallic hydroxide（LDH）on nickel foam substrate improved the characteristics of easy aggregation of LDH and obtained a unique structure of two-dimensional layer vertical matrix distribution.It not only increases the active area of the catalyst,improves the utilization rate of the catalytic active site,but also increases the effective contact between the catalyst and the electrolyte,and improves the material transport capacity of the catalytic material,thus effectively improving the catalytic performance of Ni Fe-LDH/NF.Ni Fe-LDH/NF has multiple catalytic active centers,and NO₃RR and OER have better catalytic performance than Ni CO₃/NF and Fe₃O₄/NF with single catalytic active centers.In 0.1 M HCl,the NHF-LDH/NF catalyzed the ammonia production of NO₃RR to 1121.36μg·h^-1·cm_cat^-2,the Faraday efficiency reached 85%（-0.9V vs.RHE）,and the overpotential of OER catalyzed in 1M KOH was as low as 250 m V.Therefore,it is an effective way to obtain high-efficiency electrochemical catalytic materials to construct synergistic dual-catalytic active centers and control the distribution state and distribution structure of catalysts on conductive substrates.