Ammonia adsorption on alumina

Thus, adsorption of NH3 on alumina resembles that of water in many respects. Both molecules are adsorbed molecularly at low temperatures but are chemisorbed dissociatively at higher temperatures. Ammonia is held strongly on A1203 surfaces and cannot be removed completely even on desorption at 500°C. Various species occur simultaneously, their relative importance being determined by the OH content of the surface. Furthermore, displacement adsorptions may take place. Thus, NH2" ions readily replaced chloride ions on surfaces of chlo-rided aluminas (166). One has, therefore, to conclude that ammonia retention on aluminas cannot be an acceptable measure of surface acidity and can hardly be related to catalytic activity. Ammonia adsorption on aluminas as studied by infrared spectroscopy, perhaps combined with TPD experiments (173), gives ample information on surface properties but ammonia cannot be used as a specific poison on alumina. 

Peri (17) in his report on the ammonia adsorption on alumina suggested that the reaction... 

The differential heats of adsorption of ammonia and sulfur dioxide on amphoteric alumina show the coexistence of strong acid sites and basic sites on its surface [94,95]. The heats of ammonia adsorption on alumina are typical of a strong acidic surface the initial heat increases and the adsorption capacity decreases with increasing pretreatment temperature [11]. On the other hand, magnesia, which is a basic oxide, displays only strong basic sites and no acidity. 

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