Catalytic activity of rare earth phosphates for SF 6 decomposition and promotion effects of rare earths added into AlPO 4

Abstract

SF 6 was selectively hydrolyzed to SO 3 over rare earth (RE) phosphates above 800 K. CePO 4 was the most active catalyst, followed by GdPO 4, YbPO 4, DyPO 4, ErPO 4, SmPO 4, PrPO 4, TbPO 4, NdPO 4, and LaPO 4. The middle RE phosphates were found to be more active than the light RE phosphates, but the reason for the high activity of CePO 4, which belongs to the light RE group, is not clear. The catalytic activity was independent of the specific surface area (SSA), acid amount, and acid concentration of the catalysts. RE phosphates were single-phase, and a broad inversely proportional relation was observed between the crystallite size and SSA, except in the case of CePO 4. The combination of highly active AlPO 4 and CePO 4 creates synergetic effects in the catalytic activity and SO 3 selectivity over all ranges of composition. Binary catalysts were a mixture of small crystalline AlPO 4 and CePO 4. The addition of Ce promoted the crystallization of AlPO 4, which was controlled to about 10 nm at 10–50% Ce content. The turnover frequency for SF 6 decomposition was proportional to the surface concentration of hydroxyls of binary catalysts. Therefore, synergy effects may come from the number of hydroxyl (OH) pair sites on which a bidentate intermediate of hydrolysis of SF 6 may be formed by the moderate crystallization of AlPO 4. The addition of Ce, Pr, or Y to AlPO 4 brings about a small promotion effect for SF 6 decomposition, but the addition of La, Nd > Gd > Yb diminishes the activity. The addition of Gd, Pr, or Nd greatly improved the SO 3 selectivity. A linear relationship between catalytic activity and the concentration of surface hydroxyls of the catalysts supports a reaction mechanism in which two F atoms of an SF 6 molecule interact with two surface hydroxyls to form a bidentate intermediate.