Effect of heating profile on desorption curve in temperature programmed desorption analysis: case study of acid sites distribution of SAPO-34

摘要

Comparison of the traditional linear heating method of TPD with an original stepwise heating scheme was reported for the first time. Stepwise heating TPD was carried out by keeping the temperature constant as soon as ammonia desorption signal rises until the signal returns to the baseline. More ammonia desorption peaks on a SAPO-34 catalyst were identified using TPD with stepwise heating. The effect of temperature ramp on desorption peak broadening in TPD curve was also addressed. The more distinct ammonia desorption peaks in stepwise TPD indicates that ammonia adsorbs in about five or six different ways on SAPO-34, and attribution of different adsorptions may be explained considering some known features of SAPO's acidity; including adsorption on the bridge hydroxyl groups Br0nsted acid sites with different acid strength resulting from different silicon environment of SAPO molecular sieves, adsorption on terminal hydroxyl groups Br0nsted acid sites, and by multiple adsorption of ammonia on acid sites.