Cf <sub> 4 </ Sub> And Nf <sub> 3 </ Sub> Decomposition Reaction

Perfluoro- compounds (PFCs), such as CF₄, NF3 have been widely used as etching and cleaning agents in semiconductor industry. They have high global warming potential (GWP) and long lifetime; as an example, CF₄ has a GWP of 6500 relative to that of CO₂ and a lifetime of 50000 years. CF₄ and NF3 decomposition over some metal oxides in the absence of water to change into the metal fluorides is an effective method for the destruction of PFCs exhausted from industrial processes.In the present thesis, CF₄ decomposition without water over Al₂O₃, P/Al₂O₃, and the de-fluorinated agent of NaF-Si-MO, a ternary mixture of NaF, Si and MO, where MO refers to Al₂O₃, La₂O₃, CeO₂, MgO, CaO, SrO, and BaO, were investigated. We also investigated NF3 decomposition over Al₂O₃ based de-fluorinated agents in the absence of water. Some conclusions have been drawn as follows:(1) The conversion of CF₄ temperature-programmed decomposition over Al₂O₃ reached the highest level at 700°C dependent upon the gas hourly space velocity (GHSV). At the isothermal reaction stage,γ-Al₂O₃ exhibited a good structural stability and high de-fluorinated activity for CF₄ thermal decomposition at 850°C, while the un-reactedγ-Al₂O₃ was transformed intoα-Al₂O₃ at 900°C in CF₄ decomposition atmosphere and led to a sharp decrease in CF₄ conversion. After the impregnation of H3PO4 solution intoγ-Al₂O₃ particles, the AlPO4 phase was formed onγ-Al₂O₃ surface after heat-treatment. For the CF₄ decomposition over P/Al₂O₃ at 900°C, Al₂O₃ is active species, the AlPO4 formed in heat-treatment process acted as a protective layer forγ-Al₂O₃ phase to inhibit the formation ofα-Al₂O₃, enhanced the structural stability ofγ-Al₂O₃ phase and hence the de-fluorinated activity of Al₂O₃.(2) CF₄ conversion over the de-fluorinated agents of NaF-Si-MO was remarkably increased compared to the corresponding metal oxide. On NaF-Si-MgO, NaF-Si-CaO and NaF-Si-CeO₂, the CF₄ conversion reached almost 100% at 850°C of reaction temperature. After reaction, the chemical structure of MO in NaF-Si-MO has a large change, Al₂O₃, MgO was transformed into Na3AlF6 and NaMgF3; CaO, SrO, BaO into CaF2, SrF2, and BaF2; La₂O₃, CeO₂ into LaF2 and CeF3; CeO₂, MgO, CaO, SrO, BaO into Ce4.67(SiO4)3O, Mg2SiO4, Ca3Si2O7, SrSiO3, Ba2SiO4, respectively. On the basis of XRD and XPS characterization results, the chemical equation of CF₄ decomposition reaction over each NaF-Si-MO mixture was presented as follows,①12NaF + 3Si + 2Al₂O₃ + 3CF₄ = 3SiO₂ + 4Na3AlF6 + 3C,②3Si + 2La₂O₃ + 2CF₄ = 3SiO₂ + 4LaF2 + 2C,③72Si + 156CeO₂ + 33CF₄ = 24Ce4.67(SiO4)3O + 44CeF3 + 33C,④2NaF + Si + 4MgO+ CF₄= Mg2SiO4 + 2NaMgF3 + C,⑤2Si + 7CaO + 2CF₄ = Ca3Si2O7 + 4CaF2 + 2C,⑥Si + 3SrO+ CF₄ = SrSiO3 + 2SrF2 + C,⑦Si + 4BaO+ CF₄ = Ba2SiO4 + 2BaF2 + C(3) NF3 was easily decomposed than CF₄ in the absence of water. Of the single metal oxides,γ-Al₂O₃ was the best effective de-fluorinated agent and 100% conversion of NF3 could keep 8.5h. Alkaline earth metal modified Al₂O₃ could not increase the activity. The effectiveness of transition metal modified Al₂O₃ for promoting NF3 decomposition followed the order of Co/Al₂O₃ > Fe/Al₂O₃ > Ni/Al₂O₃ > Cu/Al₂O₃≈Al₂O₃ >Zn/Al₂O₃.