MnOx/g-C3N4光热协同催化净化NO的性能增强和反应机理

引用本文: 陈鹏, 董帆, 冉茂希, 李佳芮. MnO_x/g-C₃N₄光热协同催化净化NO的性能增强和反应机理[J]. 催化学报, 2018, 39(4): 619-629. doi: 10.1016/S1872-2067(18)63029-3 shu

Citation: Peng Chen, Fan Dong, Maoxi Ran, Jiarui Li. Synergistic photo-thermal catalytic NO purification of MnO_x/g-C₃N₄: Enhanced performance and reaction mechanism[J]. Chinese Journal of Catalysis, 2018, 39(4): 619-629. doi: 10.1016/S1872-2067(18)63029-3 shu

MnO_x/g-C₃N₄光热协同催化净化NO的性能增强和反应机理

重庆工商大学环境与资源学院, 重庆市催化与环境新材料重点实验室, 重庆 400067
基金项目:
国家重点研发计划（2016YFC02047）；国家自然科学基金（51478070，21501016，21777011）；重庆市高校创新团队（CXTDG201602014）；重庆市自然科学基金（cstc2017jcyjBX0052）.

摘要: 许多研究表明，MnO_x和g-C₃N₄均有催化氧化NO的活性，并且探索了它们各自的转化机理.然而，MnO_x/g-C₃N₄复合材料的光热催化机理仍然是一个未解决的问题.我们通过室温沉淀法直接合成不同摩尔比的MnO_x/g-C₃N₄，并发现其表现出良好的光热协同催化氧化NO的性能.MnO_x/g-C₃N₄催化剂在g-C₃N₄表面含有不同价态的MnO_x.通过原位红外光谱在60℃下研究了紫外-可见光诱导的MnO_x热催化NO的机理以及MnO_x/g-C₃N₄光热协同催化NO的机理.结果表明，光照对MnO_x热催化NO的过程几乎没有影响，但对MnO_x/g-C₃N₄光热协同催化NO产生积极作用并且形成重要的催化循环机制.具体过程是光生电子（e^-）转移到MnO_x上参与光热协同的还原循环（Mn⁴⁺→Mn³⁺→Mn²⁺），且低价Mn离子易给出电子（e^-）与光生空穴（h⁺）相结合而诱导逆向的循环（Mn²⁺→Mn³⁺→Mn⁴⁺），使活性氧空位再生.通过MnO_x（Mn⁴⁺/Mn³⁺/Mn²⁺）变价而产生的活性氧（O^-）可将中间产物（NOH和N₂O₂^-）氧化为终产物（NO₂^-和NO₃^-）.这将为开发更好的净化NO_x的催化剂提供重要的指导意义.
XRD表征结果表明，MnO_x/g-C₃N₄复合催化剂的结晶度较低.TEM和XPS表征结果表明，g-C₃N₄表面含有多种低结晶度的MnO_x，主要含有MnO，MnO₂和Mn₂O₃.此外，通过对比MnO_x和1：5 MnO_x/g-C₃N₄催化净化NO的XPS结果，发现反应后的MnO_x含有大量Mn-Nitrate且Mn³⁺和Mn⁴⁺大幅度减少；同时，反应前后1：5 MnO_x/g-C₃N₄的Mn²⁺，Mn³⁺和Mn⁴⁺的含量变化微弱.BET-BJH测试结果显示，MnO_x/g-C₃N₄复合催化剂的比表面积和孔容均高于纯g-C₃N₄.UV-VisDRS测试结果显示，MnO_x/g-C₃N₄复合催化剂显示了良好的可见光吸收能力.紫外-可见光催化去除NO的测试结果表明，1：5 MnO_x/g-C₃N₄（44%）的光催化活性明显高于MnO_x（28%）和g-C₃N₄（36%）.ESR测试结果表明，参与反应的主要活性物种为·O₂^-自由基.EPR测试结果表明，1：5 MnO_x/g-C₃N₄的氧空位明显多于MnO_x，丰富的活性氧空位更有利于电子的迁移且促进Mnⁿ⁺（n=2，3和4）的变价而诱导O₂分子形成活性氧（O^-）.以上结果清晰地表明1：5 MnO_x/g-C₃N₄表现出不同的理化特性.
可见光催化氧化NO的原位红外光谱表明，光照前后MnO_x催化氧化NO的过程没有明显的变化，表明其属于典型的热催化过程，综合上述表征结果发现MnO_x的氧缺陷是Mnⁿ⁺（n=3和4）变价的活性位点，可诱导O₂产生活性氧催化氧化NO为硝酸盐吸附在MnO_x上；光照前后1：5MnO_x/g-C₃N₄催化氧化NO的过程有明显不同，光照前主要表现为g-C₃N₄表面MnO_x的热催化过程，而光照后1：5 MnO_x/g-C₃N₄为光热协同催化NO的过程.具体过程是g-C₃N₄的光生电子（e^-）转移到MnO_x上参与光热协同的还原循环（Mn⁴⁺→Mn³⁺→Mn²⁺），且低价Mn离子易给出电子（e^-）与光生空穴（h⁺）相结合而诱导逆向的循环（Mn²⁺→Mn³⁺→Mn⁴⁺）使活性氧空位再生.通过MnO_x（Mn⁴⁺/Mn³⁺/Mn²⁺）变价而产生的活性氧（O^-）可将中间产物（NOH和N₂O₂^-）氧化为终产物（NO₂^-和NO₃^-）.

关键词:

English

Synergistic photo-thermal catalytic NO purification of MnO_x/g-C₃N₄: Enhanced performance and reaction mechanism

Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, China
Received Date: 22 December 2017
Revised Date: 19 January 2018
Fund Project:
This work was supported by the National Key R&D Plan (2016YFC02047), the National Natural Science Foundation of China (51478070, 21501016, 21777011), the Innovative Research Team of Chongqing (CXTDG201602014), and the Natural Science Foundation of Chongqing (cstc2017jcyjBX0052).

Abstract: Both MnO_x and g-C₃N₄ have been proved to be active in the catalytic oxidation of NO, and their individual mechanisms for catalytic NO conversion have also been investigated. However, the mechanism of photo-thermal catalysis of the MnO_x/g-C₃N₄composite remains unresolved. In this paper, MnO_x/g-C₃N₄ catalysts with different molar ratios were synthesized by the precipitation approach at room temperature. The as-prepared catalysts exhibit excellent synergistic photo-thermal catalytic performance towards the purification of NO in air. The MnO_x/g-C₃N₄ catalysts contain MnO_x with different valence states on the surface of g-C₃N₄. The thermal catalytic reaction for NO oxidation on MnO_x and the photo-thermal catalytic reaction on 1:5 MnO_x/g-C₃N₄ were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The results show that light exerted a weak effect on NO oxidation over MnO_x,and it exerted a positive synergistic effect on NO conversion over 1:5 MnO_x/g-C₃N₄. A synergistic photo-thermal catalytic cycle of NO oxidation on MnO_x/g-C₃N₄ is proposed. Specifically, photo-generated electrons (e^-) are transferred to MnO_x and participate in the synergistic photo-thermal reduction cycle (Mn⁴⁺→Mn³⁺→Mn²⁺). The reverse cycle (Mn²⁺→Mn³⁺→Mn⁴⁺) can regenerate the active oxygen vacancy sites and inject electrons into the g-C₃N₄ hole (h⁺). The active oxygen (O^-) was generated in the redox cycles among manganese species (Mn⁴⁺/Mn³⁺/Mn²⁺) and could oxidize the intermediates (NOH and N₂O₂^-) to final products (NO₂^- and NO₃^-). This paper can provide insightful guidance for the development of better catalysts for NO_x purification.

Key words:

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沈阳化工大学材料科学与工程学院沈阳 110142

MnO_x/g-C₃N₄光热协同催化净化NO的性能增强和反应机理

English

Synergistic photo-thermal catalytic NO purification of MnO_x/g-C₃N₄: Enhanced performance and reaction mechanism

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