| The influence of different precursors such as urea,thiourea,dicyandiamide,melamine,etc.on the formaldehyde degradation rate of g-C3N4 sample under visible light at room temperature was studied,and then the g-C3N4 sample was combined with the metal oxide MxOy(M=Co,Mn,Fe,Cu)to prepare Co3O4/g-C3N4 marerial.The preparation technology conditions of the Co3O4/g-C3N4 and CeO2/Co3O4/g-C3N4 samples were optimized by using orthogonal test and Box-Behnken design method.The photocatalytic formaldehyde degradation mechanism of samples were investigated by X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),Fourier infrared spectrometer(FT-IR),UV-Vis diffuse reflectance spectrometer(UV-Vis DRS),N2-adsorption/desorption,X-ray energy spectrometer(XPS)and photoluminescence spectrophotometer(PL)The main conclusions are as follows:(1)The formaldehyde removal rate of g-C3N4 samples prepared with urea,thiourea,dicyandiamide and melamine as precursors were 44.6%,38.3%,32.2%and 30.3%,respectively.The specific surface area of samples was 64.19 m2/g,35.07 m2/g,28.68 m2/g,7.35 m2/g,and the pore volume of samples was 0.37cm2/g,0.05 cm2/g,0.05 cm2/g,0.04 cm2/g,respectively.(2)When the molar ratio of MxOy/g-C3N4 was 1:1,g-C3N4 was 0.5g,potassium carbonate was 0.25g,hydrothermal temperature was 130℃,hydrothermal time was 6h,calcination temperature was 400℃,and calcination time was 3h,the formaldehyde removal rates of Co3O4/g-C3N4,Mn2O3/g-C3N4,CuO/g-C3N4 and Fe2O3/g-C3N4 samples were 79.16%,77.11%,56.45%and 51.51%,respectively.(3)The order of the influence of technology factors on the formaldehyde removal performance of Co3O4/g-C3N4 sample was:calcination temperature>hydrothermal time>hydrothermal temperature>calcination time>cobalt loading;The interaction between calcination temperature and hydrothermal temperature or hydrothermal time,calcination time and hydrothermal temperature or hydrothermal time,hydrothermal temperature and hydrothermal time was significant,while the interaction between calcination temperature and calcination time was not significant.(4)The regression equation of Co3O4/g-C3N4 sample HCHO removal rate is:Y=82.5+0.3225A+0.221667B+1.28583C+0.768333D+0.11AB-0.19AC-0.2825AD+0.2925BC+0.1425BD+3.045CD-7.2925A2-7.35625B2-5.78C2-5.48875D2(R2=0.9778,P value=0.0001).The optimized preparation technology conditions were obtained:g-C3N4 was 0.5g,cobalt nitrate was 5.0 mmol,potassium carbonate was 0.25g,hydrothermal temperature was 131.36℃,hydrothermal time was 8.02h,calcination temperature was 307.64℃,calcination time was 1.55h,the formaldehyde removal rate of the sample was 80.9%～83.5%,the mean value was 82.10%,the maximum deviation from the predicited value(82.50%)was 1.71%,and the average deviation was 0.5%,which means the model was reliable.(5)The Co3O4/g-C3N4 sample presented a flaky structure,and was composed of Co3O4 and g-C3N4,without other impurities.Its specific surface area was 166.64m2/g,which was 2.6 times than that of the g-C3N4 sample.The average particle size and pore volume of Co3O4/g-C3N4 sample were 5.50 nm and 0.370 cm3/g,respectively,which were 4.2 and 26.4 times less than those of the g-C3N4 sample.(6)The regression equation of the formaldehyde removal rate of CeO2/Co3O4/g-C3N4 sample is:Y=82.5+0.3225A+0.221667B+1.28583C+0.768333D+0.11AB-0.19AC-0.2825AD+0.2925BC+0.1425BD+3.045CD-7.2925A2-7.35625B2-5.78C2-5.48875D2(R2=0.9778).The optimized preparation technology conditions were obtained:g-C3N4 was 0.5g,cobalt nitrate was 5.0 mmol,cerium nitrate was 3.047×10-4mol,hydrothermal temperature was 130.27℃,hydrothermal time was 8.21h,calcination temperature was 319.22℃,calcination time was 1.50h,the formaldehyde removal rate of the sample was 88.50～91.38%,the average deviation from the predicted value was 2.36%,which means that the predication result of the model was relatively accurate.(7)The CeO2/Co3O4/g-C3N4 sample presented a flaky structure,and was composed of CeO2,g-C3N4 and Co3O4 crystals and amorphous.The OⅢ/OⅠ、Co2+/Co3+and Ce3+/Ce4+on the surface of CeO2/Co3O4/g-C3N4 sample were 0.38,0.60,and 0.19,respectively.Its specific surface area was 129.15m2/g,which was 64.96 m2/g more than that of the g-C3N4 sample.The average pore size and pore volume of CeO2/Co3O4/g-C3N4 sample were 4.81 and 0.018 cm3/g,which were 18.47nm and 0.352 cm3/g less than those of the g-C3N4 sample,respectively.(8)In the tolerance experiment of CeO2/Co3O4/g-C3N4 sample,the initial formaldehyde removal rate of the sample was 85.15%,which slowly increased to 91.49 within 9h,and remained 90.56%after the reaction to 21h.When the reaction lasted for 24h,the formaldehyde removal rate of the sample was still 87.47%,which was 16.91%higher than that of the Co3O4/g-C3N4 sample.(9)The process of CeO2/Co3O4/g-C3N4 degrading formaldehyde is:the holes in the valence band of Co3O4 react with the OH-on the surface to produce·OH,while the e-in the conduction band of CeO2 react with the O2 molecules adsorbed on the surface of the material to form ·O2-with strong oxidizing properties can oxidize formaldehyde molecules to degrade formaldehyde. |
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