NF ₃ And SF ₆ In Some Metal Oxide Decomposition Reaction

NF3has been used as an etching gas in semiconductor industry, and SF6has beenutilized as an ignition inhibitor for several decades. Both of them have high globalwarming potential (GWP) and long lifetime; as an example, SF6has a large globalwarming potential of22800relative to that of CO2and a lifetime of3200years. In theabsence of water, NF3and SF6decomposition over some metal oxides to change intometal fluorides is an effective method for the destruction of NF3and SF6.In the present thesis, NF3conversion over MgO and Al2O3based de-fluorinatedreagents, and SF6decomposition over the de-fluorinated reagents of NaF-Si-MO withoutwater were investigated. BET, XRD, SEM and EDX techniques have been used tocharacterize these reagents. The conclusions are drawn as follows:(1) NF3can be decomposed over MgO, V2O5, Mn2O3and Cr2O3at400C, and thereactivity follows the order of MgO> Mn2O3> V2O5> Cr2O3.Mn/MgO is a good de-fluorinated reagent and more active than bare MgO for NF3decomposition, as an example, the time of NF3full conversion over10%Mn/MgO reachedas long as460min at400C; while the reactivity of Cr/MgO reagents is lower than that ofMgO. On the basis of structural characterization and thermodynamic calculation data, it issuggested that the fluorine/oxygen exchange reaction took place between the surface layerMnF3and MgO substrate in Mn/MgO during NF3decomposition, thus promoted thefluorination of MgO and improved the de-fluorinated reactivity. On the contrary, thefluorine/oxygen exchange reaction between the surface CrF3and MgO substrate inCr/MgO does not exist, in addition, the coverage of MgO by CrF3with low surface areaand pore volume limited the diffusion of NF3into MgO pores and subsequentdecomposition reaction, therefore the de-fluorinated reactivity of Cr/MgO decreased.(2) V/Al2O3and Mn/Al2O3are effective reagents for NF3decomposition without water,especially the time of NF3full conversion over2~10%Mn/Al2O3reached as long as660min at400C. It is thought that the fluorine/oxygen exchange reaction took place betweenthe surface layer MnF3and Al2O3substrate in Mn/Al2O3during NF3decomposition,therefore promoted the fluorination of Al2O3and improved the de-fluorinated reactivity.As a contrast, the reactivity of10%Mn/Al2O3prepared by impregnation of C4H6MnO4solution onto Al2O3is better than10%Mn/Al2O3prepared by using Mn(NO3)2as precursor.(3) The ternary mixture consisting of NaF, silicon and one metal oxide such as Al2O3,La2O3, CeO2, MgO and CaO can effectively destruct SF6in the absence of water,especially the reagent of NaF-Si-CaO exhibited a high reactivity with90%conversion ofSF6lasting for310min.The plausible paths of SF6decomposition over NaF-Si-Al2O3, NaF-Si-La2O3,NaF-Si-CeO2, NaF-Si-MgO, NaF-Si-CaO, and NaF-Si-BaO are proposed as follows,①5NaF+3Si+5Al2O3+2SF6=AlF3+3Al2(SiO4)O+Na5Al3F14+2S，②18Si+26La2O3+10SF6=3La9.31(Si1.04O4)6O2+12LaF3+12LaF2+10S，③36Si+78CeO2+11SF6=12Ce4.67(SiO4)3O+22CeF3+11S，④6NaF+3Si+12MgO+2SF6=3Mg2SiO4+6NaMgF3+2S，⑤6Si+21CaO+4SF6=3Ca3Si2O7+12CaF2+4S，⑥3Si+12BaO+2SF6=3Ba2SiO4+6BaF2+2S...