Synthesis And Photocatalytic Properties Of Graphitic Carbon Nitride And Its Iron Composites

Graphitic carbon nitride(g-C3N4),with high nitrogen content,high physicochemical stability and suitable band gap,is a cheap and readily available non-metallic photocatalytic material.It has been widely applied in various photocatalytic reactions and as a support of metal catalysts.However,g-C3N4 prepared by conventional pyrolysis usually has bulk structure with low specific surface area,weak light absorption and low charge separation efficiency,resulting in limited metal loading and photocatalytic performance,which greatly limits its practical application.g-C3N4 with nanosheet or other specific morphologies and pore structure can be prepared by the hard-template method,which increases the specific surface area and significantly improves the photocatalytic performance.However,the subsequent removal of the template is complicated,and the usage of fluorine-containing toxic agents causes environmental pollution.The development of soft-template and template-free methods(including self-assembly)is still very challenging.In this thesis,based on the template-free pyrolysis approach,a series of Fe/g-C3N4 catalysts with high-density metal sites and g-C3N4 nanosheets with high crystallinity were prepared via tuning metal precursors,g-C3N4 precursors and pyrolytic temperatures.These catalysts exhibit excellent performance in photo-Fenton reaction,photocatalytic hydrogen production and CO2 reduction.The main contents are as follows:A Fe/g-C3N4 catalyst with high metal-loading(up to 18.2 wt%)was synthesized by a one-step pyrolysis of highly compatible precursors which constituted of an Fe-imidazole coordination compound and melamine.Iron species are dispersed in g-C3N4 in the form of ultra-small clusters and atomically dispersed(USCAD)Fe sites,generating high-density Fe-Nx catalytically active centers.Density functional theory(DFT)calculations demonstrated that USCAD Fe sites can be stabilized on g-C3N4.The distribution of Fe clusters and single atoms in the layer and interlayer of g-C3N4 are revealed by DFT calculations and HAADF-STEM,that is,Fe clusters could only be stabilized between layers,while Fe single atoms are stable between layers and in single layer.The high-density Fe-Nx sites are stabilized by g-C3N4 with the formation of Fe-N bonds,which accelerates the transfer of photogenerated electrons in g-C3N4 to Fe3+ and promotes the cycle between Fe3+ and Fe2+.When applied in photo-Fenton reaction,the catalysts exhibit excellent activity and stability in degrading various organic pollutants with a low iron leaching of 0.69 mg/L.Aided by certain amount of sulfur sources in Fe salts/g-C3N4 precursors,the porous alveolate Fe/g-C3N4 catalysts with high-density USCAD Fe sites(iron loading up to 17.7 wt%)were synthesized via a sulfur-assisted method.Sulfur acts as a "sacrificial carrier" to increase the dispersion of Fe species through Fe-S coordination and eventually generate porous alveolate structure by escaping in the form of SO2 during pyrolysis,eliminating the subsequent template removal.This sulfur-assisted method exhibits good feasibility in a variety of S-species(thiourea,S powder and NH4SCN)and Fe salts to synthesize USCAD Fe/g-C3N4 catalysts with porous alveolate structure.The porous alveolate structure is conducive to expose more USCAD Fe sites to reactants.It also promotes light absorption and accelerates the transfer of photogenerated electrons in g-C3N4 to Fe3+and the generation of Fe2+.The S-Fe-salt/CN catalysts exhibit greatly promoted activity and reusability for degrading various organic pollutants in photo-Fenton reaction compared to the corresponding Fe-salt/CN catalysts.A series of highly crystalline g-C3N4 nanosheets were synthesized via a high-temperature pyrolytic strategy(with the pyrolytic temperature up to 690℃).The size and interlayer spacing can be controlled by changing the pyrolytic temperature.The highly crystalline g-C3N4 nanosheets with larger size and smaller interlayer spacing can be synthesized at 670 0C(U400-670).The U400-670 has a highly expanded conjugated π-dectron system,leading to the narrowed band gap(2.45 eV)and easier π→π*and n→π*transitions,which greatly promotes the visible light absorption and the separation efficiency of photogenerated carriers in g-C3N4.When applied in photocatalytic hydrogen production,the hydrogen evolution rate of U400-X(except U400-690)are above 4000 μmol/g/h.U400-670 exhibits the highest hydrogen evolution rate as high as 12020 μmol/g/h.It also has excellent photocatalytic CO2 reduction performance with high CO selectivity.The initial rate of CO formation was as high as 1414 μmol/g/h,and the total amount of CO produced in 5 hours reaches 4447 μmol/g.