INTERACTION OF THE Fe/γ-Al2O3 CATALYTIC SYSTEM WITH PROBE MOLECULES III. STUDY OF THE INTERACTION OF AMMONIA WITH γ-Al2O3 OXIDE AND THE Fe/γ-Al2O3 SYSTEM

Abstract

The work is devoted to the study of the interaction of heterogeneous catalytic systems with adsorbedmolecules. It presents the results of the interaction of the initial γ-Al2O3 oxide and the Fe/γ-Al2O3 system withammonia obtained using IR spectroscopy and the method of temperature programmed desorption.Lewis and Bronsted acid centers were detected on the surface of both γ-Al2O3 oxide and the Fe/γ-Al2O3 systemduring their interaction with ammonia in the temperature range 293-773K using IR spectroscopy. The relativeintensity of the absorption bands corresponding to these centers depends on the temperature of interaction withammonia.It was found by ammonia TPD that adsorbed NH3 is desorbed in the form of five peaks from the γ-Al2O3 surfaceon a temperature scale, and in the form of seven peaks from the surface of the Fe/γ-Al2O3 system. It was shown thatthe total amount of desorbed ammonia decreases markedly with increasing adsorption temperature. Moreover, forindividual temperature peaks, the amount of adsorbed ammonia can either decrease, pass through an extremum, orremain approximately constant. The appearance of additional desorption peaks for the Fe/γ-Al2O3 system isassociated with iron deposited on alumina.Proceeding from the temperature range of existence of desorption peaks for γ-Al2O3 oxide and the Fe/γ-Al2O3system it was established that they contain weak acid centers (desorption temperature up to 523K), acidic centers ofmedium strength (desorption temperature from 523 to 613K) and strong acid sites (desorption temperature above613K). The volume of desorbed ammonia in the indicated temperature ranges can serve as a quantitative measure ofvarious types of acid sites.