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 200-400 degrees C using a microcalorimetric-volumetric apparatus. The amounts 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 surface heterogeneity. At very low coverages, the heats of adsorption at all temperatures are >200 kJ mol(-1), at higher coverages they are <200 kJ mol(-1). Two types of adsorption were considered: dissociative adsorption and coordination of molecular water. We also carried out some thermokinetic investigations on heat emission as a function of time at increasing equilibrium water pressures at 200 degrees C and 400 degrees C, and at different adsorption temperatures at 0.05 Torr equilibrium water pressure.