G-C ₃ 4 And Its Photocatalytic Reduction Of Hexavalent Chromium In Water

In recent years, the hexavalent chromium pollution incidents have happened occasionally in our country. These pollution incidents have caused great trouble to the life of the local people. Many researchers have attempted to use semiconductor photocatalysis technology to treat the hexavalent chromium pollutant, and have got some achievement. However, the lack of efficient, stable, nontoxic and cheap visible-light-activated photocatalysts has limited the practical applications of the photocatalysis technology. Graphite carbon nitride (g-C3N4) is a metal-free semiconductor material. It has a small bandgap (2.7eV), which enables it to have a good light-absorbing ability in the visible region. Nonetheless, g-C3N4 prepared by the routine thermal condensation methods exhibited only low photocatalytic activity, due to its large particles size, small specific surface area, and high recombination rate of photogenerated electrons and holes. Hence, in this thesis, we try to develop some modification methods to enhance the photocatalytic activity of g-C3N4 for the reduction of aqueous hexavalent chromium. The main contents completed are summarized as follows:1. First, g-C3N4 was synthesized via thermal condensation of melamine at 580℃ for 2 h; then, g-C3N4 was modified by hydrothermal treatment in different concentrations (0.1,0.2 and 0.3 mol/L) of NaOH aqueous solution at 80-140℃ for 3-12 h. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (SEM) were used to characterize the composition and structure of the samples. Nitrogen adsorption-desorption was tested to determine the specific surface area and pore structure of the samples. UV-vis DRS diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) were used to study the optical properties of the samples. Electrochemical impedance spectroscopy (EIS) and photocurrent tests were employed to study the electrochemical properties of the samples. Finally, the photocatalytic properties of the modified g-C3N4 samples were tested in the reduction of hexavalent chromium in 50 mg/L K.2Cr2O7 aqueous solution under visible-light irradiation, and compared with those of g-C3N4. The results showed that:(1) after hydrothermal treatment in dilute NaOH aqueous solution, the particle size of g-C3N4 became smaller, the specific surface area and pore volume of g-C3N4 became larger, and the separation and transfer of photogenerated charges were enhanced; (2) when the hydrothermal treatment temperature was 80℃ and time was 6 h,0.2 mol/L was the optimum concentration of NaOH aqueous solution to obtain the most efficient modified g-(3) while the concentration of NaOH aqueous solution was 0.2 mol/L and the hydrothermal treatment time was 6 h, the photocatalytic activity of the modified g-C3N4 increased when the hydrothermal treatment temperature was increased from 80℃ to 120℃, but decreased when the hydrothermal treatment temperature was further increased from 120℃ to 140℃, indicating 120℃ was the optimum hydrothermal treatment temperature; and (4) the longer the hydrothermal treatment time, the higher photocatalytic activity of the modified g-C3N4;2. First, g-C3N4 was synthesized via thermal condensation of melamine at 560℃ for 3 h; then, g-C3N4 was modified by hydrothermal treatment in deionized water at 140-190℃ for 8 h. XRD, FT-IR, XPS, and TEM were used to characterize the composition and structure of the samples. Nitrogen adsorption-desorption was tested to determine the specific surface area and pore structure of the samples. UV-vis DRS diffuse reflectance spectroscopy and PL were used to study the optical properties of the samples. EIS, Zeta potential measurement and photocurrent tests were employed to study the electrochemical properties of the samples. Finally, the photocatalytic properties of the modified g-C3N4 samples were tested in the reduction of hexavalent chromium in 50 mg/L K2Cr2O7 aqueous solution under visible-light irradiation, and compared with those of g-C3N4. The results demonstrated that:(1) after hydrothermal treatment in deionized water, the particle size of g-C3N4 became smaller, the specific surface area and pore volume of g-C3N4 became larger, the surface charge of g-C3N4 changed gradually from negative to positive, and the separation and transfer of photogenerated charges were enhanced; (2) the higher hydrothermal treatment temperature, the higher photocatalytic activity of the modified in the reduction of hexavalent chromium; (3) the effects of the initial pH value of aqueous solution on the photocatalytic activity of the modified g-C3N4 were complex: the photocatalytic activity of the modified g-C3N4 increased when the initial pH value of K2Cr2O7 aqueous solution was reduced from 7 to 3, but decreased when the initial pH value of K2Cr2O7 aqueous solution was further decreased from 3 to 1, indicating pH=3 was the optimum initial pH value of K2Cr2O7 aqueous solution; (4) after the photocatalytic reactions, hexavalent chromium in the K2Cr2O7 aqueous solution has been reduced to trivalent chromium.3. First, g-C3N4 was synthesized via thermal condensation of melamine at 580℃ for 2 h; then, was modified by hydrothermal treatment in the presence of different amounts of SnCl4-5H2O at 180℃ for 5 h, and a series of g-C3N4/SnO2 composites (g-C3N4/SnO2-1, g-C3N4/Sn02-2, g-C3N4/SnO2-3) were obtained. XRD, TG, FTIR, TEM, HRTEM and XPS were used to characterize the composition and structure of the samples. Nitrogen adsorption-desorption was tested to determine the specific surface area and pore structure of the samples. UV-vis DRS diffuse reflectance spectroscopy and PL were used to study the optical properties of the samples. EIS was employed to study the electrochemical properties of the samples. Finally, the photocatalytic properties of the modified g-C3N4 samples were tested in the reduction of hexavalent chromium in 50 mg/L K2Cr2O7 aqueous solution under visible-light irradiation, and compared with those of g-C3N4. The results demonstrated that:(1) SnO2 had no absorption in the visible region, g-C3N4 and g-C3N4/Sn02 composite displayed obvious absorption in the visible region; (2) g-C3N4/Sn02-2 containing 26.5 mass/o of SnO2 exhibited the highest photocatalytic activity; (3) the matched band structure between g-C3N4 and SnO2-2 as well as the heterojunction structure of the as-prepared g-C3N4/Sn02 composites were beneficial to the separation of electrons and holes; and (4) the charge separation and transport efficiency of g-C3N4/Sn02-2 was the highest, contributing to its highest photocatalytic activity.