Metal Phosphate Catalysts Effective for Degradation of Sulfur Hexafluoride

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.