氨(NH3)作为重要的化学品和能源储存介质,需求量与日俱增.本文旨在通过电化学硝酸根还原反应(NO3-RR),将NO3-转化为NH3,不仅解决了NO3-引起的环境污染问题,又可以满足对NH3的迫切需求.然而,NO3-RR涉及多个电子和质子转移过程,其中,NO2-是NO3-活化转化和深度还原合成NH3的重要中间体.酞菁铜(CuPc)能够高效地活化转化NO3-为NO2-,但在低过电位时无法有效地将NO2-还原为NH3,难以获得较高的氨法拉第效率(FENH3)和分电流密度.而氮配位的铁单原子催化剂(FeNC)则有较好的NO2-吸附活化特性.因此,利用双组分催化剂之间的协同作用以实现高效NO3-RR的活性和选择性是本文的主要研究思路.本文设计了 CuPc/FeNC串联催化剂,利用CuPc和FeNC对NO3-和NO2-的吸附活化能力的差异,实现了高效的协同催化转化.X射线衍射、高角环形暗场扫描透射电镜、X射线光电子能谱及X射线吸收谱结果表明,FeNC催化剂中Fe原子均匀分布于ZIF-8热解后的基底.通过将FeNC和CuPc负载于气体扩散电极,在流动电解池中完成NO3-RR.CuPc/FeNC催化剂在较低电势区间中能够实现接近100％的NH3法拉第效率,同时在-0.57 V vs.RHE时达到273 mA cm-2的NH3分电流密度,并且在整个电势范围内有效地抑制了NO2-聚集.与单组分催化剂CuPc和FeNC对比结果表明,在-0.53 V vs.RHE时,CuPc/FeNC催化剂表现出较高的FE(NH3)/FE(NO2-)比值,是CuPc催化剂的50倍;同时CuPc/FeNC催化剂上NH3分电流密度是FeNC催化剂的1.5倍.进一步研究了NO3-RR中的串联反应机制,其中FeNC催化剂表现出较高的NO2-RR活性,并且有效抑制了析氢反应.此外,CuPc/FeNC催化剂和FeNC催化剂在NO2-RR中表现出类似的NH3分电流密度,这表明在NO3-RR中,CuPc/FeNC催化剂性能的提高来源于FeNC位点能够进一步还原CuPc位点产生的NO2-.理论计算结果表明,FeNC比CuPc表现出更强的NO2-吸附活化能力,说明NO2-在FeNC上更容易进行加氢还原.NO3-RR反应全路径分析结果表明,对于*NO3还原到*NO2过程,CuPc相对于FeNC位点具有明显降低的反应自由能,说明CuPc有利于NO2-的生成;而FeNC位点在后续的*NO2还原合成*NH3过程中具有更低的反应自由能,这与实验结果一致.一系列非原位和原位表征证明了CuPc催化剂在高电位下存在少量金属颗粒析出,与CuPc催化剂在高电位下NH3分电流密度快速增加结果一致.综上,本工作中CuPc和FeNC催化剂之间的协同作用弥补了各自的不足,通过串联反应机制,在低过电位下有效增加了NH3的法拉第效率和电流密度,实现了高效的协同催化转化,为设计和合成高效催化剂提供了新思路.
Synergistic catalytic conversion of nitrate into ammonia on copper phthalocyanine and FeNC two-component catalyst
Cu-based catalysts have been extensively studied to enhance the performance of the electrochemi-cal nitrate reduction reaction(NO3-RR),while it is still a challenge to balance high ammonia(NH3)current density and Faradaic efficiency.Here,we incorporated nitrogen coordinated iron single atom catalyst(FeNC)with copper phthalocyanine(CuPc),denoted as CuPc/FeNC,for NO3-RR.Compared with the two individual catalysts,this two-component catalyst increases NH3 Faradaic efficiency and current density at low overpotentials,achieves efficient synergistic catalytic conver-sion.Experiments and theoretical calculations reveal that the enhanced electrochemical perfor-mance of CuPc/FeNC catalyst comes from the tandem process,in which NO2-is produced on CuPc and then transferred to FeNC and further reduced to NH3.In this exceptional tandem catalyst sys-tem,an outstanding NH3 Faradaic efficiency close to 100％was achieved at potentials greater than-0.35 V vs.RHE,coupled with a peak NH3 partial current density of 273 mA cm-2 at-0.57 V vs.RHE,effectively suppressing NO2-production across the entire potential range.This strategy provides a design platform for the continued advancement of NO3 RR catalysts.
Electrochemical reduction of nitrateto ammoniaSynergistic catalytic conversionTandem catalysisTwo-component catalystOperando characterization
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维普
