催化学报 ›› 2018, Vol. 39 ›› Issue (4): 619-629.DOI: 10.1016/S1872-2067(18)63029-3

• 论文 • 上一篇    下一篇

MnOx/g-C3N4光热协同催化净化NO的性能增强和反应机理

陈鹏, 董帆, 冉茂希, 李佳芮   

  1. 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
  • 收稿日期:2017-12-22 修回日期:2018-01-19 出版日期:2018-04-18 发布日期:2018-04-08
  • 通讯作者: 董帆
  • 基金资助:

    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).

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

Peng Chen, Fan Dong, Maoxi Ran, Jiarui Li   

  1. Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, China
  • Received:2017-12-22 Revised:2018-01-19 Online:2018-04-18 Published:2018-04-08
  • Contact: 10.1016/S1872-2067(18)63029-3
  • Supported by:

    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).

摘要:

许多研究表明,MnOx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,MnOx/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MnOx/g-C3N4,并发现其表现出良好的光热协同催化氧化NO的性能.MnOx/g-C3N4催化剂在g-C3N4表面含有不同价态的MnOx.通过原位红外光谱在60℃下研究了紫外-可见光诱导的MnOx热催化NO的机理以及MnOx/g-C3N4光热协同催化NO的机理.结果表明,光照对MnOx热催化NO的过程几乎没有影响,但对MnOx/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MnOx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MnOx(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NOx的催化剂提供重要的指导意义.
XRD表征结果表明,MnOx/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MnOx,主要含有MnO,MnO2和Mn2O3.此外,通过对比MnOx和1:5 MnOx/g-C3N4催化净化NO的XPS结果,发现反应后的MnOx含有大量Mn-Nitrate且Mn3+和Mn4+大幅度减少;同时,反应前后1:5 MnOx/g-C3N4的Mn2+,Mn3+和Mn4+的含量变化微弱.BET-BJH测试结果显示,MnOx/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,MnOx/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 MnOx/g-C3N4(44%)的光催化活性明显高于MnOx(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 MnOx/g-C3N4的氧空位明显多于MnOx,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 MnOx/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MnOx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MnOx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MnOx上;光照前后1:5MnOx/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MnOx的热催化过程,而光照后1:5 MnOx/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MnOx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MnOx(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: MnOx, g-C3N4, 协同催化, 光热协同, 原位红外光谱, NO氧化

Abstract:

Both MnOx 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 MnOx/g-C3N4composite remains unresolved. In this paper, MnOx/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 MnOx/g-C3N4 catalysts contain MnOx with different valence states on the surface of g-C3N4. The thermal catalytic reaction for NO oxidation on MnOx and the photo-thermal catalytic reaction on 1:5 MnOx/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 MnOx,and it exerted a positive synergistic effect on NO conversion over 1:5 MnOx/g-C3N4. A synergistic photo-thermal catalytic cycle of NO oxidation on MnOx/g-C3N4 is proposed. Specifically, photo-generated electrons (e-) are transferred to MnOx 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 NOx purification.

Key words: MnOx, g-C3N4, Synergistic catalysis, Photo-thermal, In situ DRIFTS, NO oxidation