The Cf <sub> 4 </ Sub> In Several Metal Oxides On The Decomposition Reaction

Perfluoro- carbons (PFCs) such as CF₄, C2F6 are representative greenhouse gases. They have extremely high global-warming potential (GWP) and long atmospheric lifetimes, for example, CF₄ has a GWP value of 6500 relative to CO₂ and lifetime of 50000 years. The Kyoto Protocol on climate change required the industrialized countries to reduce the total output of six specific greenhouse gases including PFCs. In China, the major source of CF₄ is the aluminum production industry. The disposal PFCs, mainly CF₄, if not destructed, will bring serious pollution effect after discharged into the atmosphere.In the present thesis, CF₄ decomposition in the absence of water on some metal oxides has been performed to change CF₄ into solid metal fluorides. On the basis of the experimental data, the following conclusions have been drawn.(1) Of the single metal oxides,γ-Al₂O₃ is the best effective de-fluorinated agent. MgO and CaO are inactive for CF₄ decomposition due to structural sintering and consequently the decrease in surface area at high reaction temperatures.(2) The conversion of CF₄ was tested over bareγ-Al₂O₃ at different temperatures of 825, 850 and 900℃, it was found that the initial activity of Al₂O₃ increased in the order of 825℃< 850℃< 900℃, unfortunately the CO₂ yield declined quickly with time on stream because the un-reactedγ-Al₂O₃ was transformed into inactiveα-Al₂O₃ phase that resulted from the highly exothermic effect of Al₂O₃-fluorination reaction.(3) In the process of CF₄ decomposition over modified Al₂O₃, the order in the fluorination reaction activity of different metals is as follows: Mg, Ba, La, Ce > Al > Zn, Ni. In the case of MgO-Al₂O₃, fluorination of Mg species and the highly exothermic effect produced led to the collapse of MgAl₂O₄ structure, in which the magnesium element was fluorinated into MgF2. Al₂O₃ modified with P or Ni could inhibit the transformation of un-reactedγ-Al₂O₃ intoα-Al₂O₃ during CF₄ decomposition, and consequently CF₄ conversion reached a stable level with time on stream.(4) Over the ternary de-fluorinated agents consisting of metals oxides (M_xO_y), NaF and Si, CF₄ conversion reached 80% or higher levels at 850℃of reaction temperature. As for the reaction mechanism, NaF acted as a catalyst in ternary de-fluorinated agents to produce sodium vapor and break up the C-F bond of CF₄, then the fluorine element was fixed by M_xO_y, so the formation of inert phases of Na₃AlF₆ in NaF-Si-Al₂O₃ and NaMgF3 in NaF-Si-MgO resulted in the loss of free NaF and thus the decrease of CF₄ conversion with time on stream isothermally at 850oC.