水钠锰矿光电催化降解亚甲基蓝及其机理研究

Researches on Photocatalytic Degradation of Methylene Blue by Birnessite and Reaction Mechanism

水钠锰矿在自然界分布广泛、容易制备、光谱响应宽,受到越来越多的关注并被应用于环境治理领域。该研究通过电沉积法高效快速制备了水钠锰矿电极,研究其在可见光下对亚甲基蓝的光催化降解效果,通过添加空穴(h+)捕获剂甲酸和羟基自由基(·OH)捕获剂异丙醇分析光催化降解机理。结果表明,通过X射线衍射(XRD)和拉曼光谱证实合成物质为水钠锰矿,并且无其他矿物结晶相。紫外可见漫反射吸收谱显示水钠锰矿电极可吸收300～700 nm的光。通过Tauc方程计算得水钠锰矿电极直接带隙为2.8 V,间接带隙为2.4 V。在1.0 V恒定电压下,黑暗中反应3.5 h后亚甲基蓝的吸附率为11.0%,而光照下亚甲基蓝的降解率为67.0%,并且12h后降解率可达98.5%。光照下亚甲基蓝的降解率显著提高,印证了水钠锰矿具有良好的表面光催化性质。分别向反应体系中添加0.7%的甲酸和1.0 mmol/L的异丙醇后,亚甲基蓝的降解率降低到47.7%和34.6%;经动力学计算得出甲酸对h+的淬灭率为35.944%,异丙醇对·OH的淬灭率为56.760%。通过加入捕获剂发现水钠锰矿主要通过光生h+和·OH对亚甲基蓝进行氧化降解作用,其中·OH的氧化作用强于h+。

Based on its wide distribution,easy preparation and broad spectral response,birnessite has been paid more and more attention and applied to environmental protection.Birnessite electrode was prepared with electrodeposition method and its photocatalytic degradation of methylene blue under visible light were investigated.The photocatalytic degradation mechanism was analyzed by adding formic acid as hole(h+)scavenger and isopropanol as hydroxyl radical(·OH)scavenger.The results indicated that X-ray diffraction(XRD)and Raman spectroscopy confirmed that the electrode mineral phase was birnessite with no other phase.The UV-Vis diffuse reflectance spectra illustrated that the birnessite electrode had a significant absorption of visible light from 300 to 700 nm,which direct and indirect band gap were 2.8 eV and 2.4 eV,resperctively,based on Tauc plots calculation.At a constant voltage of 1.0 V conditions,the adsorption rate of methylene blue was 11.0%in the darkness after 3.5 h,while the degradation rate of methylene blue was 67.0%and the degradation rate was 98.5%after 12 h with light illumination.This suggested that birnessite had obvious surface photocatalytic capacity.After adding 0.7%formic acid and 1.0 mmol/L isopropanol to the reaction system,the degradation rate of methylene blue decreased to 47.7%and34.6%,respectively.According to the kinetics calculations,the quenching rate of formic acid to h+was 35.944%,and the quenching rate of isopropanol to·OH was 56.760%.The above results showed that the h+and·OH played important role in photocatalytic degradation of methylene blue and the oxidation of·OH was stronger than that of h+.