Study On The Performance Regulation And Mechanism Of Non-thermal Plasma Assisted CO₂-CH₄ Reforming

Human production and living as well as the burning of fossil fuels emit a large amount of carbon dioxide（CO₂）,resulting in the greenhouse effect of global warming,causing environmental and climate problems.The use of clean energy methane（CH₄）can effectively mitigate carbon dioxide emissions,but CH₄ is also a greenhouse gas,its greenhouse effect is 25 times that of CO₂,and irrational exploitation and use will aggravate global warming.The rational use of CO₂ and CH₄ has attracted more attention.Reforming of CO₂ and CH₄ to produce syngas and higher value-added chemicals is of great significance to alleviate greenhouse effect and energy problems.With the development of plasma technology,non-equilibrium plasma has been used more and more widely in CO₂-CH₄ reforming reaction due to its obvious non-thermodynamic equilibrium characteristics,which mainly includes two forms:One is to use plasma for catalyst preparation,improve the microstructure of catalyst,and improve the catalytic performance.The other one is to directly use the plasma for catalytic reforming,which converts CO₂-CH₄into syngas（H₂ and CO）,low carbon hydrocarbons（C₂-C₄）and liquid products（alcohols and acids）in one step at normal temperature and pressure.Therefore,these two application ways have been widely concerned by domestic and foreign scholars in the field of CO₂-CH₄ reforming.In this paper,we design the experimental platforms of the two application ways,and study the performance of CO₂-CH₄ reforming.The results are as follows:（1）The Ni-based catalyst supported by CeO₂was prepared by impregnation method,and then treated by calcine or plasma for the use of CO₂-CH₄ reforming to produce syngas.The performance of the catalysts obtained by different treatment methods were evaluated by performance test and stability test.The test results show that the P-Ni/CeO₂ catalysts treated by plasma alone had the best catalytic performance,the P-Ni/CeO₂ catalysts combined with plasma heat treatment had the second performance,and the traditional heat treatment catalysts Ni/CeO₂ had poor performance.At 700 ^oC,the conversion rates of CO₂ and CH₄for P-Ni/CeO₂ catalysts were 83.4%and 73.1%,respectively,while for Ni/CeO₂ catalysts only 79.4%and63.0%,indicating that the plasma treatment can significantly improve the performance of the catalysts.The results of SEM and TEM characterization of the catalysts show that the particle size of Ni in P-Ni/CeO₂catalysts after plasma treatment was smaller and the dispersion was improved,which is the main reason for its excellent performance.The SEM and TEM results of the catalyst showed that the Ni particle size increased and carbon deposition existed in the catalyst after reaction,which led to the degradation of the catalyst performance.It was found that the aggregation of P-Ni/CeO₂ catalyst was weak and the carbon accumulation was less,so it had good catalytic performance.The results show that the plasma treatment catalyst can improve the catalyst structure and catalytic performance,and has a good application prospect in the catalytic reaction.（2）The dielectric barrier discharge（DBD）plasma could directly conversion of CO₂-CH₄ into syngas and liquid chemicals at normal temperature and pressure.Nickel foam（NF）supported Nickel-aluminum hydrotalc（Ni Al-LDH/NF）were successfully designed by hydrothermal synthesis method.Ni O/NF,Ni/NF and Ni Ga/NF catalysts were obtained through different treatment methods,which were used for plasma catalytic CO₂-CH₄ conversion.Ni/NF and Ni Ga/NF catalysts containing more Ni⁰ showed higher total liquid selectivity（>30%）and higher selectivity of CH₃COOH（>15%）.The Ni²⁺of Ni O/NF catalysts promoted the formation of CH₃OH（8.9%）and CH₃COOH（9.6%）.The Ni Al-LDH/NF catalysts containing Ni²⁺showed the highest selectivity of CH₃OH（12.3%）,possibly because the more-OH groups on the surface of the catalysts.In addition,the characterization of these catalysts confirmed that the consumption of-OH was closely related to the formation of CH₃OH during the reaction.Therefore,we proposed an-OH reservoir mechanism,that is,a large number of-OH groups in Ni Al-LDH/NF catalysts can react with interfacial CH_x*free radicals to form CH₃OH.The results showed that the selectivity of liquid products in the plasma catalytic CO₂-CH₄ reforming was closely related to the microstructure,surface valence state and reducibility of the catalyst.（3）Plasma catalysis has great potential to directly convert CO₂-CH₄ into H₂ and high value-added chemicals at low temperature.In order to effectively regulate the product distribution and further reveal the catalytic role of active sites in plasma catalysis,we exploited 5A zeolite supported catalysts with varied Cu/Fe active sites and further investigated the effects of Ar/H₂O additives in this reaction to tune the products distribution.The results showed that Cu/5A catalyst was favorable to the formation of CH₃OH（18.0%）,and Fe/5A promoted the formation of CH₃COOH（7.9%）.In addition,the conversion of CO₂（30.3%）and CH₄（55.6%）can be improved by adding Ar to CO₂-CH₄,while the addition of H₂O significantly improves the selectivity of H₂（from 36.1%to 56.6%）.The adsorption processes of key free radicals on the catalyst surface and the plasma-catalyst collaborative transformation routes are proposed by systematically measuring the catalysts,combining with In situ scatter infrared Fourier transform spectroscopy（In situ DRIFTS）and kinetic modeling.This work provides new insights into the design of highly selective catalysts for tuning the products distribution in the plasma-catalytic co-conversion of CO₂-CH₄.（4）The experimental study of DBD reactor with water as medium was carried out to promote the generation of liquid products.The effects of circulating water velocity,temperature,concentration and rising and falling time of nanosecond pulsed plasma on the product distribution were investigated.The results show that the selectivity of gas and liquid in the product is greatly affected by the reaction conditions,and low temperature and low discharge power are favorable to the formation of liquid.The plasma catalyzed CH₃OH transformation experiment and the change of catalysts filling position confirmed the transformation of liquid product to gas product in the plasma catalyzed CO₂-CH₄ coversion.When the catalysts were filled at the bottom,the selectivity of liquid reached 30.1%and that of CH₃OH was 20.1%.When the catalysts were filled at the top of the discharge area,the selectivity of liquid was 24.2%,while that of CH₃OH was only 9.7%,indicating that the catalysts filling at the bottom was conducive to the formation of liquid products.In addition,the results of plasma-catalyst coordination experiment show that Cu₂Ni₁Al catalysts promoted the formation of liquid products（29.7%）,Ni Al catalysts promoted the formation of gas products（H₂:45.2%,CO:55.1%）,indicating that Cu promotes the formation of liquid products,while Ni was favorable to the formation of gas products.In plasma catalysis,appropriate plasma generation conditions and the combination of plasma and catalyst can improve the catalytic performance and regulate the product distribution.