Metal Phosphate Catalysts Effective for Degradation of Sulfur Hexafluoride

Abstract

SF6 decomposed stably over AlPO4, CePO4, YPO4, and Zr3(PO4)4 at 800−1000 K, but Ca2P2O7, Mg2P2O7, and Sr2P2O7 were less active. Active catalysts crystallized during the reaction. SO3, SO2F2, and HF were obtained as major products. SO3 was selectively formed over YPO4 and Zr3(PO4)4, but SO2F2 formed over AlPO4 and CePO4 in a significant selectivity at lower temperatures. Decomposition activity was independent of the concentration of oxygen, but it was dependent on the concentration of water vapor, suggesting that hydrolysis is the essential reaction. Catalytic activity was independent of the acidity of the catalysts. Catalytic activity was independent of the amount of surface hydroxyls, but it was correlated with the concentration of surface hydroxyls. From an analogy of hydrolysis of CCl2F2 on AlPO4, degradation may proceed via a similar bidentate surface intermediate, OsurfaceH···F(SF4)F···HOsurface, in which two fluorine atoms of SF6 interacted with two surface hydroxyls. AlPO4 showed a steady activity and high selectivity at 843 K for 25 h, suggesting a long catalyst life. AlF3 was significantly less active than metal phosphates. This suggests that development of the catalysts which are not transformed into metal fluorides is important.

Catalytic activity of rare earth phosphates for SF6 decomposition and promotion effects of rare earths added into AlPO4

“…In a previous paper, the authors of the present study studied the decomposition of SF 6 over various metal phosphates [6] and estimated a reaction mechanism similar to that for CCl 2 F 2 decomposition via a bidentate surface intermediate in which two fluorine atoms of the SF 6 molecule simultaneously interacted with two surface hydroxyls as follows: O surface H· · ·F(SF 4 )F· · ·HO surface .…”

“…Therefore, it was concluded that the essential reaction of SF 6 decomposition is hydrolysis and that decomposition is carried out through a bidentate intermediate [6]. The decomposition of SF 6 over RE phosphates may proceed by a reaction mechanism similar to that over metal phosphates other than RE phosphates.…”

“…SF 6 has an enormously large global warming potential of 23,900; therefore the development of a safe and low-cost degradation method is eagerly awaited. The decomposition of this compound gives HF and/or F 2 , which are strongly corrosive when water is present.…”

“…The decomposition of this compound gives HF and/or F 2 , which are strongly corrosive when water is present. Many studies have been performed on the thermal decomposition of SF 6 in electric discharges in connection with plasma etching [1][2][3] and photoreductive degradation [4]. In the decomposition of SF 6 by atmospheric pressure plasma, the removal efficiency was found to be more than 90%, and the production of toxic by-products, including SF 4 , SO 2 F 2 , S 2 F 10 , SOF 2 , and SOF 4 , was reported [5].…”

“…A study of the catalytic decomposition of waste SF 6 has yet to be carried out. The authors studied the catalytic decomposition of SF 6 and found that various metal phosphates, including AlPO 4 , CePO 4 , YPO 4 , and Zr 3 (PO 4 ) 4 , are active for SF 6 decomposition at 800-1000 K [6]. In these catalysts, SO 2 F 2 and SO 3 were obtained as products containing sulfur.…”