【作者】 王静；

【导师】 褚克；

【作者基本信息】 兰州交通大学 ， 材料学， 2021， 硕士

【摘要】 氨（NH₃）是一种重要的化工原料,它能够应用于化肥等诸多领域。对于农业而言,NH₃是氮肥的主要成分,而氮肥可以提高农作物的产量。NH₃还能够用于生产多种化学药品,包括炸药、塑料、合成纤维、树脂和工业制冷剂等。在多种新能源中,NH₃被认为是有吸引力的清洁高效的能源。如今,Haber-Bosch工艺是工业产NH₃的主要工艺,该工艺会产生大量的温室气体,容易造成环境污染。因此,寻找一种清洁、可持续的NH₃生产工艺是非常有必要的。电催化氮还原（NRR）被认为是在环境条件下生产NH₃的一种高效、清洁和可持续的方法。然而,这种电化学方法的发展在实践中受到很大的阻碍。由于N₂分子断裂N≡N三键很困难以及竞争的析氢反应（HER）的存在导致NRR反应效率低下。目前最大的挑战是怎样在环境条件下开发出高NRR活性的催化剂。许多实验和理论研究表明Mo具有类固氮酶催化机理和高催化加氢能力,因此Mo基纳米材料能够成为潜在的NRR催化剂。本文主要研究两种Mo基催化剂:MoO₂/RGO（还原氧化石墨烯）和Fe Mo₃S₄。通过XRD、TEM、SEM和XPS对所合成催化剂的形貌、结构进行表征,通过LSV、CV和计时电流进行电化学性能测试,通过密度泛函理论（DFT）计算,研究了催化剂的NRR的催化机理。（1）以钼酸铵和氧化石墨烯为前驱体,通过微波法合成了MoO₂/RGO纳米复合材料。实验结果表明,MoO₂纳米粒子均匀密集地负载在了RGO表面。电催化性能测试显示,MoO₂/RGO纳米复合材料的NH₃产率为37.4μg h^-1mg^-1,法拉第效率（FE）为6.6%,优于单独的RGO和MoO₂。MoO₂/RGO纳米复合材料同时具有很好的长时稳定性和循环稳定性。DFT计算表明,与单独的MoO₂相比,MoO₂/RGO纳米复合材料与*N₂H具有更强的电子相互作用,并且从活性Mo位向*N₂H贡献了更多的电子,从而大大降低了决速步骤*N₂→*N₂H的形成能垒,加速了NRR反应的进行。（2）以FeCl₃、Na₂MoO₄和硫代乙酰胺为前驱体,通过两步水热法合成了Fe Mo₃S₄纳米棒。实验结果表明,合成的Fe Mo₃S₄纳米棒具有高的结晶度。电催化性能测试显示,Fe Mo₃S₄纳米棒的NH₃产率为65.3μg h^-1 mg^-1,FE为19.2%。Fe Mo₃S₄纳米棒同时具有良好的长时稳定性和循环稳定性。DFT计算表明:Fe Mo₃S₄上暴露于表面的Fe_3c位点是N₂吸附、激活、加氢反应的主要活性位点,并且能有效阻止HER副反应的发生,提高了N₂→NH₃的转换效率。更多还原

【Abstract】 Ammonia（NH₃）is an important chemical raw material,which can be used in many fields such as fertilizers.For agriculture,NH₃ is the main component of nitrogen fertilizer,and nitrogen fertilizer can increase the yield of crops.NH₃ can also be used to produce a variety of chemicals,including explosives,plastics,synthetic fibers,resins and industrial refrigerants.Among a variety of new energy sources,NH₃ has been considered to be an attractive,clean and efficient energy source.Nowadays,the Haber-Bosch process is the main process for industrial production of NH₃.This process generates a large amount of greenhouse gases and easily causes environmental pollution.Therefore,it is highly necessary to search for a clean and sustainable NH₃ production process.Electrocatalytic nitrogen reduction（NRR）has been considered to be an efficient,clean and sustainable method to produce NH₃ under ambient conditions.However,the development of this electrochemical method has been greatly hindered in practice.It is difficult for N₂ molecules to break the N≡N triple bond and the existence of the competing hydrogen evolution reaction（HER）leads to the inefficiency of the NRR process.The biggest challenge at present is how to develop a catalyst with high NRR activity under ambient conditions.Many experiments and theoretical studies have shown that Mo possesses a nitrogenase-like catalytic mechanism and a high catalytic hydrogenation ability,thus Mo-based nanomaterials are the potential NRR catalysts.This thesis mainly studied two Mo-based catalysts:MoO₂/RGO（reduced graphene oxide）and Fe Mo₃S₄.The morphology and structure of the synthesized catalysts were fully characterized by XRD,TEM,SEM and XPS.The electrochemical performance of the catalysts was tested by LSV,CV and chronoamperometry.Density functional theory（DFT）calculations were used to systematically study the NRR catalytic mechanism of MoO₂/RGO and Fe Mo₃S₄ catalysts.（1）Using ammonium molybdate and graphene oxide as precursors,MoO₂/RGO nanocomposites were synthesized by a microwave method.The experimental results showed that MoO₂ nanoparticles were evenly and densely loaded on the surface of RGO.The electrocatalytic performance test showed that the NH₃ yield of the MoO₂/RGO nanocomposite was 37.4μg h^-1 mg^-1,and the Faraday efficiency（FE）was 6.6%,which were both better than that of RGO and MoO₂ alone.MoO₂/RGO nanocomposites also had a good electrocatalytic stability.DFT calculations showed that,compared with MoO₂ alone,MoO₂/RGO nanocomposite had a stronger electronic interaction with*N₂H,and contributed more electrons from the active Mo site to*N₂H,thereby greatly reducing energy barrier for the rate-determining step,leading to the accelerated progress of the NRR reaction.（2）FeCl₃,Na₂MoO₄ and thioacetamide were used as precursors to synthesize Fe Mo₃S₄nanorods by a two-step hydrothermal method.The experimental results showed that Fe Mo₃S₄nanorods with high crystallinity were synthesized.The electrocatalytic performance test showed that the NH₃ yield of Fe Mo₃S₄ nanorods was 65.3μg h^-1 mg^-1,and the FE was 19.2%.Fe Mo₃S₄ nanorods also had both good cycle stability and electrocatalytic stability.DFT calculations showed that Fe_3c sites exposed on the surface of Fe Mo₃S₄ were the main active sites for N₂ adsorption,activation,and hydrogenation reactions,and could effectively prevent the occurrence of the HER side reaction and improve the conversion efficiency of N₂→NH₃.更多还原