CF₄ Decomposion By Combination Of Low Temperature Plasma And Catalysis

Carbon tetrafluoride（CF₄）is one of the six greenhouse gases strictly controlled by the "Kyoto Protocol".The control and treatment of CF₄ in industrial exhaust gas are important aspects of greenhouse gas emission reduction.The decomposition of CF₄ molecules usually requires a high temperature of more than 2000 K,and the end treatment is extremely difficult.It is urgent to develop a simple and efficient end treatment method.Plasma can generate a large number of high-energy electrons and active particles at room temperature,and can produce significant synergistic effects with catalysts,and has great potential to decompose organic pollutants at low temperature.Focusing on the goal of efficient plasma decomposition of CF₄,this paper carried out a series of studies on the influencing factors of plasma decomposition of CF₄,the decomposition of CF₄ by plasma/catalysis system,the catalyst modification and the kinetics of CF₄ decomposition.The main conclusions are as follows:（1）Aiming at the problem that the dominant factors affecting the decomposition of CF₄ in plasma are unclear,experimental studies on the decomposition of CF₄ under different conditions of reduced electric field,gas flow rate,initial concentration,carrier gas and added gas in plasma alone were carried out.The results show that the plasma has different electron energy distributions under different conditions,which is the key factor for the decomposition of CF₄.The decomposition efficiency of35% and the energy efficiency of 6.03 g/k Wh were achieved under the conditions of 40 Td reduced electric field,50 ml/min gas flow,20000 ppm initial concentration and Ar as carrier gas;water vapor absorbed more electron energy than oxygen,The decomposition efficiency and energy efficiency decreased to 22% and 3.79 g/k Wh when 0.5%concentration of water vapor was added,while the decomposition efficiency and energy efficiency were increased to 44% and 7.58 g/k Wh by adding 0.5% concentration of oxygen,respectively.It is revealed that CF₄ is mainly decomposed by collision with high-energy electrons in plasma,the contribution of free radicals to the decomposition is much lower than that of high-energy electrons.（2）Aiming at the low decomposition efficiency of CF₄ by plasma alone,a plasma/catalysis synergistic system was developed,and a systematic study on the catalytic decomposition of CF₄ by low temperature plasma coupled with γ-Al₂O₃ was carried out.The results show that under the same experimental conditions,the addition of the catalyst increases the decomposition rate of CF₄ from 35% in the plasma alone to 69%,when adding 0.5% oxygen concentration,the decomposition efficiency is further increased to 74%,and when adding0.5% concentration water vapor,the decomposition efficiency of CF₄ decreased to 43%;the CF₄ decomposition stability experiment found that after the decomposition efficiency reached the max value in the synergistic system,the efficiency decreased.The stable CF₄ decomposition efficiency without gas addition,oxygen addition and water vapor addition are 53%,61% and 31%,respectively.It is revealed that in the synergistic system,the strong acid and Lewis acid sites on the catalyst surface are the key active sites for the decomposition of CF₄.The reason for the decrease in the decomposition rate is that the catalyst participates in the decomposition process of CF₄,and the formation of Al F3 on the catalyst surface reduces the surface acid sites.The addition water vapor promotes the formation of Al F3.（3）Aiming at the issue of insufficient surface acidity and easy deactivation of γ-Al₂O₃,the Zr/γ-Al₂O₃ catalysts with more surface acid sites were prepared by homogeneous coprecipitation method,and applied in CF₄ decomposition.The results showed that after modification,the amounts of surface strong acid and Lewis acid sites of the catalyst increased from 416 μmol/g and 176.9 μmol/g to 1168 μmol/g and 246.4μmol/g,respectively.Under the same conditions,the CF₄ decomposition efficiency over the Zr/γ-Al₂O₃ system increased from 69% to 92% of that of the γ-Al₂O₃ system,and remained 1168 μmol/g and 203.7 μmol/g acid sites after the reaction for 50 min,respectively.Successfully enhanced the CF₄ decomposition efficiency and stability in synergistic system.It is revealed that the CF₄ decomposition efficiency is positively correlated with the number of acid sites in the synergistic system.The mechanism of the acidity enhancement on the catalyst surface is that electrons transfer from Zr to Al;the plasma enhanced the adsorption-diffusion process of CF₄ and intermediates in the catalyst surface and pores.（4）Aiming at the issue that the decomposition path of CF₄ in plasma is unclear,the research on the decomposition kinetics of CF₄ in plasma is carried out by experiments combined with numerical simulation.The results show that under the reduced electric field of 40 Td,the addition of water vapor and oxygen increases the average electron energy in the plasma from 5.85 e V to 6.88 e V and 8.17 e V,respectively.The O radicals generated from oxygen combine with intermediate products to prevent the recombination of C and F,thereby promoting the decomposition of CF₄;adding water vapor increase the average electron energy of the plasma,but also absorb a large amount of electron energy,resulting in the decreased of CF₄ decomposition efficiency.It is clarified that,the C-F bonds break through the collide between CF₄ molecules and high-energy electrons,and then the generated CF_x groups undergo chain reactions with free radicals such as O,OH and H,to generate final product such as CO₂,F₂,HF.66 figures,28 tables and 220 references...