Activation And Transformation Of CO₂ And CH₄ On Ag-or Ni-based Catalysts

Industrial production and human activities have caused a sharp increase in greenhouse gases,which causing a lot of environmental problems.In recent years,the effective utilization of greenhouse gases has gradually become a hot spot of scientific research,in which the reaction of CO₂ and CH₄ converted to high value-added chemicals is not only expected to alleviate the increasingly serious situation of the current greenhouse effect,but also can realize the effective and comprehensive utilization of resources.However,the chemical properties of CO₂ and CH₄ are extremely stable with a strict reaction conditions and high energy consumption.Therefore,the research on the green and mild photocatalysis technology for this reaction is of great significance.In this paper,local surface plasmon resonance?LSPR?of silver was used to design and prepare a series of high-efficiency photocatalysts.The crystal structure,surface morphology and optical properties of the material were studied,and the efficiency of photocatalytic conversion of CH₄ and CO₂ at room temperature was systematically researched.The composite photocatalyst was prepared by using Ag as the active catalyst and TiO₂?P25?as the active carrier,in which Ag has strong plasma effect.Silver was reduced on the surface of TiO₂ with a simple chemical reduction method.SEM?TEM?UV-Vis?PL?and XPS analysis results show that Ag?4 nm?was loaded on the surface of TiO₂ uniformly,plasma resonance effect peak appeared at 450 nm wavelength.What's more,with the increase of the amount of Ag,the separation of photogenic electrons and holes will be more efficient,and attain to the optimum when 1.0 wt%Ag/TiO₂,this conclusion is consistent with the experimental results.XPS results indicate that silver exists in metallic state.The CO and C₂H₄ was produced at photocatalytic conversion reaction with the 1.0 wt%Ag/TiO₂as the catalysis,and the yield of CO and C₂H₄ has reached 1149 mol·g^-1·h^-1 and 686mol·g^-1·h^-1,respectively.The reaction mechanism of photocatalytic conversion of CO₂ and CH₄ by Ag/TiO₂plasma catalyst has also been speculated and verified by experiments.TPD test imply that the CH₄ is prefer to adsorb on Ag,while the CO₂ tend to adsorb on TiO₂,which was of great significance to determine the reactivity sites of CO₂ and CH₄.The reactions under different light sources?visible light,ultraviolet light and uv-visible light?were carried out and the changes in the chemical state of Ag were detected by XPS.The results showed that the light source have a great influence on the reaction,and the catalytic activity could reach the highest under the simulated sunlight including visible light and ultraviolet light.Isotope labeling experiments confirmed that C in CO₂ was transferred to CO instead of C₂H₄,and in-situ FTIR was used to capture the intermediate species·HOCO and·CH₂=CH₂.Based on the above characterization,we proposed a novel reaction mechanism to generate ethylene from coupling effect between LSPR effect under visible light and semiconductor photoelectric effect under ultraviolet light.P-type and wide-band gap semiconductor NiO was introduced to improve the photocatalytic efficiency.NiO-TiO₂ heterostructure was prepared by hydrothermal calcining and Ag was supported by chemical reduction.The results of SEM?UV-Vis?PL and XPS analysis expressed that the NiO and TiO₂ particles were distributed uniformly and established well contact surfaces with each other.Besides,there appeared new light response between 200 nm-300 nm in the deep ultraviolet region and 700-800nm in the near infrared region.The separation efficiency was improved with the loading amount increased.After the addition of NiO,the chemical state of Ti and Ag did not change in XPS,indicating that NiO and TiO₂ were heterogeneous structures rather than doped,the results of Ni 2p and O 1s both proved that Ni existed in the catalyst in the form of Ni²⁺.In the photocatalytic activity test,Ag/20%NiO-TiO₂ was the optimal catalyst,the yield of CO?C₂H₆?C₂H₄?C₃H₈ and C₃H₆ were reached 1730?mol·g^-1?1153?mol·g^-1?634?mol·g^-1?852?mol·g^-1 and 301?mol·g^-1 after 2 h reaction,respectively.