Temperature-Programmed Desorption of Ammonia on Zeolites. Influence of the Experimental Conditions on the Acidity Measurement

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Abstract
The Influence of the experimental conditions of the TPD (Temperature-Programmed Desorption) of ammonia on the measurement of acidity of mordenites and Y zeolites was studied. The results were applicable to the measurements of the strength and the number of acid sites of zeolites. For contact times of the carrier gas from 10−2 to 10−6 g min ml−1 defined as W/F (W, sample weight; F, flow rate of the carrier gas), the peak maximum temperature was related to the parameter (W/F). The relationship was explained on the basis of a theoretical equation derived by Cvetanovic and Amenomiya under the conditions of readsorption. Ammonia is readsorbed on zeolites under these conditions, and one has to use the experimental conditions of the same contact time in order to evaluate the acid strength of zeolite from the peak maximum temperature. On the other hand, the ratio of intensity of low temperature peak (1-peak) to that of the high temperature peak (h-peak) increased with increasing the contact time of the carrier gas. This relation suggested that the 1-peak intensity depended upon the degree of evacuation of the weakly-held ammonia. The 1-peak is ascribable to very weakly or physically adsorbed ammonia on zeolites, and only the h-peak has to be accounted for to measure the number of acid sites.