Theoretical Study On Catalytic Performance Of Graphitic Carbon Nitride Derivatives For CO₂ Reduction Reaction

Graphitic carbon nitride（g-C₃N₄）has attracted extensive interdisciplinary attention in the field of solar energy conversion and environmental remediation due to its advantages such as medium band gap,good physicochemical stability,low cost and easy preparation.However,the catalytic efficiency of the original g-C₃N₄ is still limited due to its low visible light efficiency,fast charge recombination,insufficient light capture ability and insufficient electrical conductivity.Therefore,in order to improve the catalytic performance of g-C₃N₄,several modification methods such as structure optimization,element doping and semiconductor synthesis have been adopted.Fossil fuel is the main energy source and material of various chemical products.However,the use of fossil fuels has led to the increasing accumulation of the greenhouse gas CO₂,causing serious environmental problems.Therefore,to find an effective way to reduce CO₂emissions is the key to solve this problem.Solar energy is a kind of inexhaustible,inexhaustible clean energy,more and more people turn their eyes to solar photocatalysis.It was found that g-C₃N₄ derivatives have good catalytic activity for carbon dioxide.The cost of designing and synthesizing new catalysts will be greatly reduced by exploring the microscopic reaction mechanism and predicting the catalytic performance of catalysts through quantum chemical calculation methods.In this paper,the density functional theory（DFT）method is used to study the application of g-C₃N₄ derivatives in photocatalytic CO₂ reduction（CO₂RR）.The paper is divided into five chapters.Chapter one is introduction,which mainly introduces the derivatives of g-C₃N₄ and applications in the field of photocatalysis.The second chapter is theoretical background of computational simulation.The third to fifth chapters are the main contents of the study,which are summarized as below:1.The electronic and optical properties of g-C₃N₄ derivatives[C₆N₇（C₆H₄）_1.5]_n（systems 1 and 2）,and[C₆N₇(C₁₂H₈)_1.5]_n（system 3）were studied using first principles,and the optimal reaction path for CO₂ reduction in system 1 was explored.The band gap results show that compared with g-C₃N₄,the band gaps of the three carbon nitride derivatives decreased,and the absorption intensity in the visible regionincreased,indicating that these derivatives generated more electrons under visible light irradiation.The highest occupied orbital（HOMO）and the lowest unooccupied orbital（LUMO）of the three systems showed that system 1 could better separate the photogenerated e^-/H⁺pairs,accelerate the carrier migration rate and improve the photocatalytic efficiency The results show that CH₃OH is the main product of CO₂ reduction catalyzed by system 1.The optimal path is CO₂→COOH*→HCOOH and CO₂→COOH*→CO→HCO*→HCHO→CH₂OH*→CH₃OH,the rate decision step is CO₂→COOH*,ΔG is 1.22 e V,less than the rate decision step of g-C₃N₄ catalytic CO₂RR,forecasting system 1 is expected to become efficient photocatalyst.2.The study on electronic structure and optical properties of g-C₃N₄ derivative[C₆N₇（CN₂）_1.5]_n were studied by DFT,and the CO₂ reaction mechanism of[C₆N₇（CN₂）_1.5]_nas photocatalyst reduction was investigated.The results show that the optimal path of CO₂reduction using[C₆N₇（CN₂）_1.5]_n as photocatalyst is CO₂→COOH*→HCOOH→HCO*→HCHO→CH₂OH*→CH₃OH.Compared with g-C₃N₄,the derivative changed rate decision step（CO₂→COOH*）,the gibbs free energy change（ΔG）（g-C₃N₄）fell from 1.43e V to 0.89 e V.It is predicted that[C₆N₇（CN₂）_1.5]_n has better catalytic activity than g-C₃N₄ in CO₂ reduction.3.The band structure,differential charge density,absorption spectrum and the best reaction path of photocatalytic CO₂ reduction of Ni-C₃N₅,Co-C₃N₅,and Fe-C₃N₅were calculated by first-principles method of photocatalytic CO₂ reduction.Compared with g-C₃N₅,Ni-C₃N₅,Co-C₃N₅,and Fe-C₃N₅ have narrower band gaps,which are predicted to increase its photocatalytic activity significantly.Charge-density difference analysis showed that a large number of electrons were transferred from Ni,Co or Fe to CO₂ through M-C and M-O bonds during the adsorption process.The absorption spectra of Ni-C₃N₅,Co-C₃N₅,and Fe-C₃N₅ have a large area of overlap with sunlight in the range of visible light.It is speculated that they have good catalytic activity in visible light region.Our calculation shows that for Ni-C₃N₅,the rate-determining step is CO*→HCO*,ΔG is 0.34 e V,and the main products is CH₄.For Co-C₃N₅,the rate-determining step is CO*→HCO*,ΔG is 0.58e V,and the main product is CH₄.For Fe-C₃N₅,the rate-determining step is CH₃O*→CH₃OH*,ΔG is 0.64 e V,and the main product is CH₄.Therefore,Ni-C₃N₅,Co-C₃N₅,and Fe-C₃N₅ are expected to be highly efficient photocatalysts for CO₂ reduction.