【作者】 孙杰；

【导师】 赵国华；

【作者基本信息】 同济大学 ， 化学， 2023， 博士

【摘要】 氮循环作为地球上最重要的物质循环之一,近些年来由于人类活动的破坏而严重失衡。氮平衡被破坏最直观的表现就是自然水体中硝酸盐含量的快速累计,这会导致水体富营养化等一系列环境问题,更严重的结果是会对人类健康造成严重威胁,如如高铁血红蛋白血症和非霍奇金淋巴瘤。因此,将水体中的硝酸盐高效选择性地转化为无毒无害、环境友好的氮气（N₂）对解决生态环境问题和人类生命健康问题具有重要的研究意义。与传统的反硝化方法相比,电化学还原硝酸盐具有温和可控的操作条件及高转化率、高N₂选择性的去除效果,被视为一种具有前景的反硝化技术。硝酸盐还原反应（NRR）过程可以被分为硝酸盐（NO₃^-）转化为亚硝酸盐（NO₂^-）的决速步骤及NO₂^-转化为N₂或氨（NH₃）的选择性决定步骤,因此我们选择将促进剂金属（Cu、Fe）和加氢金属（Pd、Ni）进行组合制备出了一系列金属双原子催化剂（DACs）。借助于DACs易于控制的催化活性位点和便于调节的级联催化作用,有效提高了光电催化NO₃^-的还原效率和N₂的选择性,并通过原位手段从原子级角度对NO₃^-还原过程中的催化活性位点、中间产物、及催化作用机制进行了探索,最终借助于密度泛函理论（DFT）计算从热力学角度阐明了整个反应过程的变化趋势。主要获得的研究成果如下:（1）基于金属有机框架（MOFs）结构特有的大比表面积和可调控结构单元的特性,在ZIF-8中通过离子替换法引入了FeNi双金属,并在高温碳化后构筑了具有特定Fe-Ni-N₆配位结构的Fe₃Ni-N-C双原子催化剂,用作电催化NO₃^-还原选择性产N₂的阴极材料,实现了97.9%的NO₃^-转化率,99.3%的N₂选择性,以及52.1%的法拉第效率。通过一系列原位电化学表征方法表明优异的电催化选择性能归因于多孔碳电催化剂中Fe-Ni催化位点的接力催化作用,其中Fe位点促进了NO₃^-在阴极上的吸附活化以及向NO₂^-的还原,Ni位点进一步吸附了中间产物NO₂^-并通过表面活性[H]实现了NO₂^-到N₂的持续还原过程（NO₂^-→NO→N₂O→N₂）。特别是在较宽的初始pH值范围和NO₃^-浓度范围内,Fe₃Ni-N-C电催化剂都保持了较高的催化活性及稳定性,展示了其在实际水体净化中的巨大潜力。（2）为了进一步探究双金属异相催化在NO₃^-还原为N₂反应中的协同效应是如何在原子尺度上发挥作用的,首次使用2D黑磷纳米片作为巨大的P配体来锚定高密度的PdCu双原子,在原位紫外光还原作用下形成了独特的PdCu-P₄配位结构。将该催化剂用作光电化学NO₃^-还原双室电解体系的光电阴极材料,实现了96.3%的NO₃^-转化率,95.2%的N₂选择性,特别是84.5%的法拉第效率和8.474 g N g^-1_PdCu h^-1的NO₃^-去除效率超过了先前报道的双金属催化剂。DFT计算结合一系列原位光电化学表征方法表明PdCu双原子与相邻的P原子间形成的类共价键在热力学上促进了NO₃^-的吸附,Pd原子对N的强吸附增大了*Cu-O-NO₂中N-O键的键长（1.24?→1.27?）从而易于发生断裂,同样Cu原子对O的强吸附也会增大*Pd-NO₂中N-O键的键长（1.24?→1.28?/1.29?）从而发生断裂。最终Pd原子表面的两个*Pd-N在热力学作用下完成快速N≡N成键过程。这种双原子催化剂在NO₃^-还原中的协同作用机制可以为异核双原子催化剂在其他催化过程中的应用提供指导,拓展异核双原子催化剂的设计和应用。（3）电催化NRR在低反应物浓度下受到传质能力差的影响,导致NO₃^-的完全还原需要长达数小时的处理时间。因此我们通过将负载在N掺杂碳上的CuNi双原子锚定在碳纳米管交织的框架中（CuNi-NC@CNT）,制备了具有高导电性、渗透性和柔性的电催化膜用作双室流动电解池的阴极膜材料。该膜在单程电过滤中实现了对10 ppm NO₃^-电解液的98.5%的NO₃^-转化率和96.2%的高N₂选择性。一系列电化学表征证明Cu与Ni复合会导致NRR半波电位(E_1/2)的正移,这一发现可以在给定电位下增强催化活性。通过具有高电极表面积的电催化膜（EM）的对流可以增强NO₃^-富集,传质并最大限度地提高催化剂利用效率,潜在地解决NO₃^-浓度梯度小和N₂选择性低的挑战。原位生成的活性[H]的有效利用和反应物在受限膜孔中的富集也显著提高了N₂选择性。此外,考虑到潜在的安培级电流密度,EM可以很容易地集成到现有系统中作为水处理的使用设备。（4）阴极NRR受限于阳极析氧反应（OER）中O-O键形成所需高能垒导致的大驱动过电位,呈现了反应动力学缓慢、选择性差和能量利用率低的弊端。因此我们设计利用热力学更有利的氨氧化反应（AOR）取代传统的阳极OER,开发了一种负载在微孔氮掺杂碳上的双功能原子级分散Fe-Ni催化剂（FeNi-NC）,并应用于硝酸盐和氨的共电解。在1.8 V的槽压下,FeNi-NC双功能电极在阴极和阳极分别实现了90.3%和99.4%的N₂法拉第效率,在共电解系统中用AOR代替OER可节省约19.4%的总功耗。DFT计算结合原位X射线吸收近边结构谱证明了在Ni-NC中引入Fe不仅使Ni 3d的分波态密度（PDOS）远离了费米能级,而且抑制了Ni位点与表面N₂的3d-2π*耦合,导致了N₂易于解吸而不会过度氧化,这最终降低了选择性决定步骤中N₂逸出的能垒。这项工作提供了一种可行的策略来调节原子分散催化剂的电子构型,以提高活性位点对于含氮污染物的双功能催化活性,并促进自然氮循环的平衡。更多还原

【Abstract】 The nitrogen cycle,as one of the most important material cycles on earth,has been seriously imbalanced in recent years due to the destruction of human activities.The most obvious manifestation of the disruption of nitrogen balance is the rapid accumulation of nitrate in natural water bodies,which can lead to a series of environmental problems,and as a more serious result,it can pose a serious threat to human health,such as methemoglobinemia and non-Hodgkin’s lymphoma.Therefore,the efficient and selective conversion of nitrate in water bodies into non-toxic,non-hazardous and environmentally friendly nitrogen（N₂）is of great research significance in solving environmental problems as well as human health issues.Compared with the traditional denitrification methods,electrochemical reduction of nitrate is regarded as a promising denitrification technology because of its mild and controllable operating conditions,high conversion rate,and high N₂ selectivity.The nitrate reduction reaction（NRR）process can be divided into a rate-determining step for the conversion of nitrate（NO₃^-）to nitrite（NO₂^-）and a selectivity-determining step for the conversion of NO₂^-to N₂ or ammonia（NH₃）.Therefore,we chose to prepare a series of metal diatomic catalysts（DACs）by combining promoter metals（Cu,Fe）and hydrogenation metals（Pd,Ni）.The photoelectrocatalytic NO₃^-reduction efficiency and N₂ selectivity are effectively improved with the easily controllable catalytic active sites and easily adjustable cascade catalysis of DACs.The catalytic active sites,intermediates,and catalytic mechanism in the NO₃^-reduction process are explored by in situ electrochemical infrared spectroscopy,in situ differential electrochemical mass spectrometry,in situ X-ray absorption spectroscopy.The trend of the whole reaction process is finally elucidated thermodynamically by means of density functional theory calculations.The main research results are as follows:（1）Based on the characteristic large specific surface area and tunable structural units of metal-organic frameworks（MOFs）structures,FeNi species were introduced into ZIF-8 by ion substitution,and Fe₃Ni-N-C diatomic catalysts with a specific Fe-Ni-N₆ coordination structure were constructed after high-temperature carbonization,which were used as electrocatalytic NRR cathode material.Fe₃Ni-N-C achieved 97.9%NO₃^-removal rate,99.3%N₂ selectivity,and 52.1%Faraday efficiency.A series of in situ electrochemical characterization methods indicated that the excellent electrocatalytic selectivity performance was attributed to the relay catalytic effect of Fe-Ni catalytic sites in the porous carbon electrocatalysts.The Fe sites promoted the adsorption activation of NO₃^-on the cathode and its reduction to NO₂^-,while the Ni sites further adsorbed the intermediate product NO₂^-and realized the continuous reduction process from NO₂^-to N₂ through the surface active[H]（NO₂^-→NO→N₂O→N₂）.In particular,the Fe₃Ni-N-C electrocatalyst maintained high catalytic activity and stability over a wide range of initial pH and NO₃^-concentration,demonstrating its great potential in practical water purification.（2）To further investigate how the synergistic effect of bimetallic heterogeneous catalysis in the NO₃^-reduction to N₂ reaction works on the atomic scale,2D black phosphorus nanosheets were used for the first time as giant P ligands to anchor the high-density PdCu diatoms,and a unique PdCu-P₄ coordination structure was formed under in situ UV reduction.The catalyst was used as a photoelectrocathode material for a photoelectrochemical NO₃^-reduction two-compartment electrolysis system,achieving96.3%NO₃^-removal rate,95.2%N₂ selectivity,and especially 84.5%Faraday efficiency and 8.474 g N g^-1_PdCu h^-1 NO₃^-removal efficiency exceeded those previously reported for bimetallic catalysts.DFT calculations combined with a series of in situ photoelectrochemical characterization methods indicate that the covalent-like bonds formed between the PdCu diatom and the neighboring P atoms thermodynamically promoted the adsorption of NO₃^-.The strong adsorption of N by Pd atoms increases the bond length of the N-O bond in*Cu-O-NO₂（1.24?→1.27?）and thus is prone to break,and similarly the strong adsorption of O by Cu atoms increases the bond length of the N-O bond in*Pd-NO₂（1.24?→1.28?/1.29?）and thus is prone to break.Eventually the two*Pd-N on the surface of Pd atoms complete the fast N≡N bond formation process driven by thermodynamics.This mechanism of synergistic effect of diatomic catalysts in NO₃^-reduction can provide guidance for the application of heteronuclear diatomic catalysts in other catalytic processes,and expand the design and application of heteronuclear diatomic catalysts.（3）Electrocatalytic NRR suffers from poor mass transfer capability at low reactant concentrations,resulting in treatment times of up to several hours for the complete reduction of NO₃^-.Therefore,we prepared a free-standing carbonaceous membrane（CuNi-NC@CNT）with high electrical conductivity,permeability,and flexibility to be used as a cathode membrane material for a two-compartment flow-through electrolyzer by anchoring CuNi diatoms loaded on N-doped carbon in an interwoven framework of carbon nanotubes.The membrane achieved 98.5%NO₃^-conversion and 96.2%high N₂selectivity for 10 ppm NO₃^-electrolyte in single-pass electrofiltration.A series of electrochemical characterizations demonstrated that Cu complexed with Ni leads to a positive shift in the NRR half-wave potential(E_1/2),a finding that enhances catalytic activity at a given overpotential.The convection through electrified membranes（EM）with high electrode surface area could enhance NO₃^-enrichment,mass transfer and maximize the efficiency of catalyst utilization,potentially addressing the challenges of small NO₃^-concentration gradient and low N₂ selectivity.The efficient utilization of in situ generated reactive[H]and the enrichment of reactants in the confined membrane pores also significantly improved the N₂ selectivity.In addition,given the potential ampere-level current density,the EM can be easily integrated into existing systems as a utilization device for water treatment.（4）The cathodic NRR is limited by the large driving overpotential due to the high energy barrier required for O-O bond formation in the anodic oxygen-removal reaction（OER）,presenting the disadvantages of slow reaction kinetics,poor selectivity and low energy utilization.Therefore,we designed to utilize the thermodynamically more favorable ammonia oxidation reaction（AOR）to replace the conventional anodic OER,and developed a bifunctional atomically dispersed Fe-Ni catalyst loaded on microporous nitrogen-doped carbon（FeNi-NC）for the co-electrolysis of NO₃^-and NH₃.The FeNi-NC bifunctional electrode achieved 90.3%and 99.4%N₂ Faraday efficiencies at the cathode and anode,respectively,at a cell voltage of 1.8 V.The replacement of OER by AOR in the co-electrolysis system saved≈19.4%overall electricity consumption.DFT calculations combined with in situ X-ray absorption near-edge structure spectroscopy demonstrated that the introduction of Fe into Ni-NC not only shifted the Ni 3d partial density of states（PDOS）away from the Fermi energy level,but also suppressed the 3d-2π*coupling of the Ni sites to surface N₂,resulting in easy desorption of N₂ without over-oxidation,which eventually lowered the energy barrier for N₂ escape in the selectivity determining step.This work provides a promising strategy to regulate the electronic configuration of atomically dispersed catalysts to enhance the bifunctional catalytic activity of the active sites for nitrogenous pollutants.更多还原