An Artificial Photosynthesis System Based On CeO₂ As Light Harvester And N-doped Graphene Complex As Artificial Metalloenzyme For CO₂ Reduction To Methanol Fuel

Large emission of carbon dioxide into airs brings great pressure to atmospheric environment.The reduction of carbon dioxide is not only beneficial to improve the quality of the environment, but it also could become the resource by reducing carbon dioxide to fuels. In the conversion of CO2, it is widely concerned that the photocatalytic reduction of carbon dioxide to fuel molecules using artificial photosynthesis systems. This paper reports the fabrication of an artificial photosynthesis system using cerium oxide as light harvestor, N-doped graphene as photosensitive agent and matrix, metal ions as artificial metalloenzyme through forming coordination bonds between metal ions and nitrogen atoms. Research on the reduction of carbon dioxide under light irradiation was conducted as follows.First, urea((NH2)2CO) is used as the raw material for N-doped graphene(NG). The combination of NG with cerium oxide(CeO2) semiconductor yielded a hybrid photocatalyst CeO2-NG, The hybrid photocatalyst CeO2-NG was characterized by X-ray diffraction(XRD), transmission electron microscopy(TEM), Ultraviolet-visible(UV-vis) diffuse reflectance spectroscopy(DRS), Raman spectroscopy and X-ray photoelectron spectroscopy(XPS). The hybrid photocatalyst containing nitrogen atoms coordinates with Cu(II) ions to fabricate an artificial photosynthesis system. The results demonstrate the coordination state of Cu(II) to nitrogen atoms plays the vital role in reducing CO2 to fuel methanol.The production rate of CO2 to methanol approached 507.3μmol?g-1 cat. ?h-1 for CeO2-NG-Cu2+ artificial photosynthesis system in 80 min, whereas the production rate is only 5.8 μmol?g-1 cat. ?h-1 and 15.00μmol?g-1 cat. ?h-1 for bare CeO2-NG and CeO2-GO photocatalyst without metal-enzyme. It shows that the production rate of CO2 to methanol is higher in the presences of nitrogen atoms.Second, imdazole is used as the second raw material for N-doped graphene. The combination of imdazole-nitrogen graphene(NGi) with cerium oxide(CeO2) semiconductor yielded a hybrid photocatalyst CeO2-NGi, The hybrid photocatalyst CeO2-NGi was characterized by XRD, TEM, UV-vis DRS, Raman spectroscopy and XPS. The hybrid photocatalyst containing nitrogen atoms coordinates with Cu(II) ions to fabricate an artificial photosynthesis system. The results demonstrate the coordination state of Cu(II) to nitrogen atoms plays the vital role in reducing CO2 to fuel methanol.The production rate of CO2 to methanol approached 385.8μmol?g-1 cat. ?h-1 for CeO2-NGi-Cu2+ artificial photosynthesis system in 80 min, whereas the production rate is only 3.57 μmol?g-1 cat. ?h-1 and 15.00μmol?g-1 cat. ?h-1 for bare CeO2-NGi and CeO2-GO photocatalyst without metalloenzyme. It shows that the production rate of CO2 to methanol is higher in the presences of nitrogen atoms.Third, the combination of g-C3N4 with CeO2 semiconductor yielded a hybrid photocatalyst Ce O2-g-C3N4. The hybrid photocatalyst CeO2-g-C3N4 was characterized by XRD, TEM, UV-vis DRS, Raman spectroscopy and XPS. The hybrid photocatalyst containing nitrogen atoms coordinates with Cu(II) ions to fabricate third artificial photosynthesis system. The results demonstrate the coordination state of Cu(II) to nitrogen atoms plays the vital role in reducing CO2 to fuel methanol.The production rate of CO2 to methanol approached 257.22μmol?g-1 cat. ?h-1 for CeO2-g-C3N4-Cu2+ artificial photosynthesis system in 80 min, whereas the production rate is only 2.14 μmol?g-1 cat. ?h-1 and 15.00μmol?g-1 cat. ?h-1 for bare CeO2- g-C3N4 and CeO2-GO photocatalyst without metalloenzyme. It shows that the artificial metalloenzyme, instead of free metal ions, plays the very vital role in reducing CO2 to methanol.