Designed Synthesis And Surface Properties Modification Of Carbon Nitride-based Nanomaterials For Photo-/Electrocatalytic CO₂ Reduction

The excessive exploitation and burning of fossil fuels caused a sharp rise in the concentration of carbon dioxide,which has triggered off severe energy crisis and the"greenhouse effect".Using solar energy or renewable electric energy as input energy to upgrade carbon dioxide into hydrocarbon fuel and high value-added chemical products is considered to be a clean and efficient carbon dioxide utilization technology,which can alleviate the energy and climate problems caused by excessive carbon dioxide emission.Carbon nitride-based catalysts have attracted considerable attention on photo-/electrocatalytic carbon dioxide reduction(CO₂RR)due to their advantages of simple synthesis,cheap and easy to obtain,excellent stability and controllable surface structure.However,carbon nitride-based catalysts synthesized by thermal-polymerization generally suffered from low surface area,limited active sites and severe charge carriers recombination,thus resulted in poor products selectivity and low conversion efficiency of CO₂RR.As a consequence,it is of great significance to explore carbon nitride-based catalysts with superior activity and high selectivity for CO₂RR.Herein,several strategies have been employed to construct carbon nitride-based catalysts with unique morphological features,adjust electronic structure and regulate surface properties,aiming to enhance CO₂adsorption and promote charge carriers separation on catalysts,thus improving the photo/electrocatalytic performance of carbon dioxide reduction.The main research contents are presented as follows:(1)A porous nitrogen-doping carbon grafted carbon nitride(N-doping carbon/CN)has been fabricated via the temperature-programmed thermal polymerization of urea with carboxyl functionalized polystyrene nanospheres as template and carbon source.BET-BJH analysis and CO₂adsorption-desorption isotherms verified that CO₂sorption reached 0.22mmol g^-1,5.5 times that on bulk g-C₃N₄(0.04 mmol g^-1),due to the unique nanoporous structure and the lager surface area.In addition,the markedly increased intensity of graphitic sp²C and a small new characteristic peak of N-sp²C confirmed that N-doping carbon was grafted into g-C₃N₄network.The photoelectric performance proved accelerated photogenerated electron transport effectively inhibited the recombination of charge carriers on the surface of N-doping carbon/CN.The porous N-doping carbon/CN exhibited prominent photocatalytic activity on CO₂reduction with water vapor under visible light.The average CO production rate reached 15.4?mol g^-1h^-1,in comparison of bulk g-C₃N₄(2.2?mol g^-1h^-1).The porous structure,larger surface area and rapid transfer of electrons on the surface of N-doping carbon/CN resulted in a significantly improved activity of photocatalytic CO₂RR.(2)The phosphorus-doped carbon nitride nanotubes(P-g-C₃N₄)were synthesized through the one-step thermal reaction of melamine and sodium hypophosphite monohydrate.Phosphine gas generated from the thermal decomposition of Na H₂PO₂induced the curling of g-C₃N₄nanosheets to form the unique mesoporous structure.And the FT-IR spectra indicated more amino groups existed on the surface of P-g-C₃N₄.Therefore,the adsorption capacity of CO₂was notably enhanced,contributed to the unique mesoporous structure and more surface amino groups.The photoelectronic performance characterization and electronic structure analysis of p-g-C₃N₄revealed the interstitial doping phosphorus not only down shifted the conduction band and valence band positions and narrowed the bandgap further to broaden the visible light region,but efficiently enhanced separation efficiency of photoexcited charge carriers.The production of CO and CH₄from photocatalytic CO₂reduction on P-g-C₃N₄increased to 3.10 and 13.92 times that on g-C₃N₄in gas-solid system under visible light,respectively.The total evolution ratio of CO/CH₄dramatically decreased to 1.3 for P-g-C₃N₄from 6.02 for g-C₃N₄,indicating a higher selectivity of CH₄product on P-g-C₃N₄.(3)Carbon nitride nanosheets were successfully cut into highly dispersed nanodots(CN-dots)via the hydrothermal method using hydrazine monohydrochloride as the shear molecule.HRTEM graphs showed that the average size of CN nanodots was about 16.13 nm.The solid¹³C MAS NMR spectrum,XANES spectra and XPS spectra manifested that hydrazine group may broke the C=N bond by attacking the CN heterocyclic ring in the g-C₃N₄and linked with the C atom into the CN nanodots structure.The unique CN nanodots structure possessed narrowing bandgap,intensiveadsorption in entire UV-Vis light range,as well as enhanced charge transfer.CN-dots displayed superior selectivity and activity for photocatalytic reduction of HCO₃^-to CO under visible light.In CO₂saturated HCO₃^-solution(c=1.14 mol L^-1),the CO yield on CN-dots was 15.18?mol h^-1g^-1,which was 21.9 times higher than that of the pristine g-C₃N₄(0.84?mol h^-1g^-1).Density functional theory calculation results showed CN-dots remarkably improved the adsorption capacity of HCO₃^-,and the?G of CO desorption was less than 0,resulting in highly active and selective reduction of HCO₃^-to CO on CN-dots.(4)Carbon nitride material(C₁₀N₉)with good crystallinity and high nitrogen content was synthesized by controlled pyrolysis treatment of purine as the raw material.HRTEM photographs revealed C₁₀N₉showed a tightly packed lamellar structure with high crystallinity.XPS spectra indicated the atomic ratio of pyridinic-N,pyrrolic-N and graphitic-N was 8.5:3.5:1.0(C₁₀N₉),while after hydrogeneration treatment,which was significantly reduced to 1.8:1.0:1.0(H-C_xN_y),respectively.The nitrogen content of H-C_xN_ydecreased to 16.98%from46.52%(C₁₀N₉).At-0.47 V vs.RHE,C₁₀N₉showed high activity on selective reduction of CO₂to acetic acid(70.1?mol g^-1h^-1),whereas main H₂product was dected on H-C_xN_ywith low N content.We speculate that the activity and selectivity of electrocatalytic CO₂reduction may be strongly dependent on the crystallinity,chemical structure and the distribution of N active sites of C₁₀N₉materials.