Characterization of Ag/Al2O3 de-NOxcatalysts by probing surface acidity and basicity of the supporting substrate

Abstract

Ag/Al₂O₃ catalysts with various Ag contents were characterized by thermal gravimetry (TG), differential thermal analysis (DTA), temperature-programmed desorption (TPD), NH₃ and CO₂ 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 μmol g⁻¹ decreases the Lewis acid sites but increases the Brønsted acid sites with medium NH₃ adsorption energy. A greater Ag loading induces interaction between NH₃ molecules and Ag oxides, leading to reduction of Ag oxides and dissociation of adsorbed NH₃. 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.