Preparation Of Sulfides And G-C₃N₄ Based Composite Catalysts For Photocatalytic Reduction Of CO₂

Energy shortages and environmental degradation are more and more serious problems that human beings are facing in the 21 st century,bringing great challenges to the survival of mankind.Global warming is one of the environmental problems that need to be treated seriously.CO₂ is the main gas cause the "greenhouse effect",at the same time is an abundant carbon resource.Then,how to reduce and utilize CO₂ have been the subject of the researchers.Solar energy is a clean,abundant and renewable new energy,solar energy conversion of CO₂ into fuels can not only alleviate environmental pressure,but also be a promising way to promote the realization of the carbon cycle and deal with energy crisis.In this paper,we explored the performance of the conventional catalysts and studied their activity of photocatalytic conversion CO₂ into methyl formate in methanol solution,the following contents were investigated:Firstly,sulfides（CdS,ZnS）were synthesized by hydrothermal method,and the influence of different reaction conditions on the catalyst performance were explored.The as-prepared samples were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,UV-vis absorption spectroscopy（UV-vis）,fluorescence spectroscopy（PL）,etc.CdS catalysts with various morphologies（including leaf-like,flower-like,microsphere,etc）were obtained,and showed higher catalytic activity.ZnS microspheres made from different sulfur sources had significant difference in their surface properties,and also brought great effects on their photocatalytic activities.Among them,the ZnS samples made from thioacetamide had the highest catalytic performance for photocatalytic reduction of CO₂,with the formation rate of methyl formate being 1928.13 μmol/gcat/h.Secondly,polycondensation method was used to prepare the graphite phase carbonic nitrogen（g-C₃N₄）semiconductor.We took urea and melamine as precursors,and they had remarkable effect on the morphology and properties of the g-C₃N₄ catalysts.The g-C₃N₄ sample made from urea had better performance.Thirdly,CdS/g-C₃N₄ composite photocatalysts with various mass ratios were synthesized by hydrothermal method in a round-bottom flask under continuous stirring.The two semiconductor formed Z-scheme,which promoted the separation of photo-generated electrons and holes,improving the photocatalytic performance of the catalysts.The activity of the composite catalysts changed with the different mass ratio of CdS in CdS/g-C₃N₄,and when the mass ratio was 1:2,the photocatalytic activity reached the highest.Compared with the pure CdS,the photo-stability of the composites got improved.Finally,based on the theory of conventional band gap theory,the mechanism of photocatalytic reduction of CO₂ in the methanol was analyzed.