High Value Carbon Materials Derived From Straw For Electrocatalytic Nitrate Reduction Applications

In recent years,nitrogen fertilizer has made an important contribution to the improvement of crop yield and national food security in China.Ammonia（NH₃）is one of the main raw materials in the fertilizer industry and organic chemical production.The NH₃ synthesis process has become an important process component in the production of nitrogen fertilizer.At present,NH₃ is mainly synthesized by the Haber-Bosch process,which catalyzes the synthesis of NH₃ from N₂ and H₂ at high temperatures and pressure.The reaction process has harsh conditions,complex reaction devices,and serious energy consumption,which is not in line with the concept of sustainable development.Meanwhile,there is runoff waste of nitrogen fertilizer in farmland,and the nitrate（NO₃^-）contained in nitrogen fertilizer is discharged into rivers and lakes with rainwater.In addition,NO₃^-is also emitted during the production of many processes.These NO₃^-emissions cause serious pollution to the environment and can also be a serious health hazard to wildlife and humans.Electrocatalytic NO₃^-reduction can recover NO₃^-under mild conditions and produce NH₃,which can be used as a raw material for nitrogen fertilizer and is an economical and green means of the ammonia synthesis process.In the electrochemical NO₃^-reduction process,the catalytic performance of the electrode depends mainly on the composition and structure of the catalyst,so the development of efficient electrocatalytic NO₃^-reduction catalysts is the key.Therefore,a variety of catalysts have been developed for use as cathode materials,such as noble metal,non-precious metal,and non-metallic catalysts,and significant differences were found in the performance of different materials for electrocatalytic nitrate reduction.Among them,carbon materials have the advantages of low cost,easy functionalization,and high stability.They also have great potential for application in the electrocatalytic reduction of NO₃^-.Agricultural straw,as a widely distributed renewable agricultural waste,has the advantages of high carbon content and low cost.The use of straw as a carbon precursor to prepare carbon materials for ammonia catalysts can not only reduce the pollution caused by burning straw to the atmosphere but also turn waste into treasure.In this paper,transition metals and their nitrogen oxides were loaded or modified on carbon materials synthesized from corn stover as carbon precursors.They are used as catalysts for electrocatalytic nitrate reduction reaction（NO₃RR）.The catalysts were tuned by defect engineering as well as interface engineering strategies to enhance the catalyst performance and obtain NO₃RR electrocatalytic catalysts with high activity and stability.The details are as follows:（1）Carbon dots（CDs）/Ta ON microspheres with abundant oxygen vacancies were synthesized by spray drying,annealing,and microwave methods for electrocatalytic NO₃RR.Spray drying method is simple,short synthesis cycle and continuous batch preparation of catalyst precursors is possible,which greatly saves time and cost.The introduction of OVs can improve the adsorption of NO₃^-ions and weaken the N=O bonding energy in NO₃^-,thus facilitating the deoxygenation and hydrogenation of NO₃^-and thus the synthesis of NH₃.adsorption,making more reaction sites available for the catalyst.Transient photovoltage tests showed that CDs/Ta ON had higher electron transport capacity compared to Ta₂O₅ and Ta ON.The maximum NH₃ yield of the CDs/Ta ON catalyst was 2839.2 mmol g_cat.^-1h^-1 at a potential of-0.6 V vs.RHE in a mixed electrolyte solution of 1 M KOH and 0.1 M KNO₃,corresponding to a Faraday efficiency of 86.25%.The high yield and Faraday efficiency of the catalyst after 5 cycles of stability test and 10 h long time stability test indicated that it has good stability.（2）The mesoporous biomass carbon material can effectively enrich NO₃^-in the electrolyte to increase the concentration of NO₃^-around the catalyst,make Fe₃O₄ uniformly dispersed,reduce the agglomeration effect of Fe₃O₄ nanoparticles,expose more active sites and facilitate the adsorption of NO₃^-for electrocatalytic NO₃RR.Finally,the electrocatalytic NO₃RR was promoted.The ratio of Fe³⁺and Fe²⁺in Fe₃O₄ nanoparticles was adjusted by regulating the annealing temperature,and the highest catalyst performance was achieved at an annealing temperature of 700°C.The XPS results showed that the percentage of Fe²⁺in Fe₃O₄ increased relatively with the increase of annealing temperature,probably due to the enhancement of the reduction of Fe₃O₄ nanoparticles by biomass carbon under a high-temperatures environment;The transient photovoltage test results showed that the time was taken for electrons to recover from the excited state to the ground state in the catalyst gradually became shorter,indicating that the electron transport in the catalyst was more favorable after high-temperature annealing.The Fe₃O₄/BC-700 catalyst had the highest NH₃ yield and Faraday efficiency at-0.5 V vs.RHE potential in 1 M KOH and 0.1 M KNO₃ solutions with 4384.33 mmol g_cat.^-1h^-1 and 93.42%,respectively.After five cycles of stability test and 24 h long time test,the catalyst still had a good performance,indicating that the catalyst has good electrochemical stability.（3）Straw and ZIF-67 Co/Biomass Carbon composite catalyst were prepared by co-carbonization by hydrothermal method and annealing method for electrocatalytic NO₃RR.The metal-organic framework structure can well disperse the catalyst components,giving them a larger specific surface area,which is favorable for the adsorption of NO₃^-and provides more reactive sites.The introduction of biomass carbon can also enhance the conductivity of the catalyst and facilitate electron transfer.The best electrocatalytic NO₃RR performance was achieved by electrochemical testing with Co/BC-100.It has an NH₃ yield of 3588.92 mmol g_cat.^-1h^-1 and a Faraday efficiency of 97.01%at-0.5 V vs.RHE potential.