HEAT OF WATER CHEMISORPTION ON ALPHA-AL2O3 AT 200-400 DEGREES-C

The differential heats and the adsorption isotherms of water on alpha- Al2O3 have been measured at five different temperatures in the range 2 00-400 degrees C using a microcalorimetric-volumetric apparatus. The a mounts of water adsorbed and integral heats decreased with increasing temperature. At all temperatures the differential heats decreased with increasing amount of chemisorbed water, suggesting an ordinary surfac e heterogeneity. At very low coverages, the heats of adsorption at all temperatures are >200 kJ mol(-1), at higher coverages they are <200 k J mol(-1). Two types of adsorption were considered: dissociative adsor ption and coordination of molecular water. We also carried out some th ermokinetic investigations on heat emission as a function of time at i ncreasing equilibrium water pressures at 200 degrees C and 400 degrees C, and at different adsorption temperatures at 0.05 Torr equilibrium water pressure.