Characterization of Ag/Al_2O_3 de-NO_x catalysts by probing surface acidity and basicity of the supporting substrate

摘要

Ag/Al_2O_3 catalysts with various Ag contents were characterized by thermal gravimetry (TG), differential thermal analysis (DTA), temperature-programmed desorption (TPD), NH_3 and CO_2 adsorption calorimetry, FT-IR spectroscopic method, and diffuse reflectance spectra (DRS). It was found that hydroxy groups on alumina have a close relationship with Ag loading either through ion-exchange with the proton of an acidic -OH or bonding with oxygen of a basic-OH species. An Ag loading of less than 240 #mu#mol g~(-1) decreases the Lewis acid sites but increases the Bronsted acid sites with medium NH_3 adsorption energy. A greater Ag loading induces interaction between NH_3 molecules and Ag oxides, leading to reduction of Ag oxides and dissociation of adsorbed NH_3. A large amount of Ag loading produces greatly enhanced basicity. The changes in the loaded form of silver on alumina are discussed. The appropriate acid-base property plays an important role in de-NO_x activity using propane as the reductant.