Graphite Carbon Nitride Composites For Hydrogen Production By Photocatalytic Water Splitting

The conversion of solar energy on the earth into clean carbon-neutral hydrogen fuel by photocatalytic water splitting has become one of the important ways to solve the energy crisis in the world.Finding cheap,efficient and stable semiconductor photocatalysts has become the key to realize the goal.Graphite phase carbon nitride?g-C₃N₄?is one of the most promising photocatalysts with its suitable band gap and'band positions.However,problems such as low separation efficiency and fast recombination rate of photo generated electron-hole pairs limit the practical application of g-C₃N₄.Therefore,design and modification of g-C₃N₄-based photocatalysts are considered to be important research topics in the field of energy and environmental chemistry.In this thesis,three g-C₃N₄-based photocatalysts were designed and synthesized to achieve photocatalytic overall water splitting,including nickel oxide/graphite phase carbon nitride composite system?NiO/g-C₃N₄?,copper oxide/graphite phase carbon nitride composite system?CuO/g-C₃N₄?,living chlorella and carbon micro co-modified graphite phase carbon nitride composite system?C-N-g-C₃N₄?.We have characterized these photocatalysts,evaluated the catalytic activities of them and discussed their mechanisms.The detailed research contents are as follows:1.In order to use a suitable cocatalyst to realize the photocatalytic overall water splitting of g-C₃N₄,amorphous NiO modified g-C₃N₄?NiO/g-C₃N₄?was designed and synthesized.It has been confirmed that the optimal NiO/g-C₃N₄ photocatalyst can simultaneously produce 1.41 ?mol·h^-1 H₂ and 0.71 ?mol·h^-1 O2 by visible light irradiation???420 nm?without any sacrificial agent.Further experiments show that the photocatalytic ability of NiO/g-C₃N₄ is attributed to the fact that amorphous NiO acts as a cocatalyst to promote electron-hole pairs separation,provide active sites for hydrogen production and decompose H₂O₂ produced by g-C₃N₄.2.In order to construct a Z-scheme system to realize the photo catalytic overall water splitting of g-C₃N₄,CuO modified g-C₃N₄?CuO/g-C₃N₄?was designed and synthesized.Experiments have shown that the optimal CuO/g-C₃N₄ photocatalyst can simultaneously produce 0.46 ?mol·h^-1 H₂ and 0.22 ?mol·h^-1 O2 by visible light irradiation???420 nm?without any sacrificial agent.The CuO/g-C₃N₄ Z-scheme system can take full advantage of the advantages of g-C₃N₄ and CuO,wherein g-C₃N₄ can provide an appropriate energy level for hydrogen production and act as a reduction site,while CuO with narrow band gap can improve the visible light response,as an oxidation site,provide a suitable energy level for the production of oxygen and decompose H₂O₂ produced by g-C₃N₄.In addition,the formation of a Z-scheme photocatalytic system can effectively improve charge transfer efficiency and improve photocatalytic performance.3.The by-product of hydrogen peroxide?H₂O₂?produced by g-C₃N₄ in the process of water splitting by two electron pathway is an important chemical raw material.In order to achieve the simultaneous production and retention of H₂O₂ and H₂ in a photocatalytic system,a living chlorella and carbon micro co-modified graphite phase carbon nitride composite system?C-N-g-C₃N₄?was designed and synthesized.Experiments show that by coupling biological H₂O₂ formation and photocatalytic water splitting,the optimal C-N-g-C₃N₄ photocatalyst can produce 0.98 ?mol·h^-1 H₂O₂ and 0.84 ?mol·h^-1 H₂ simultaneously by visible light irradiation???420 nm?without any sacrificial agent.In this system,needle coke can be used as a good co-catalyst to enhance the separation of photo-generated electron-hole pairs in g-C₃N₄ and act as a reduction site,while g-C₃N₄ can provide an oxidation site for the formation of partial H₂O₂,living chlorella is responsible for increasing the production of H₂O₂ under the protection of g-C₃N₄.