Clarification of copper species over Cu-SAPO-34 catalyst by DRIFTS and DFT study of CO adsorption

Abstract

In this work, the nature, location and evolution of Cu⁺ ions in Cu-SAPO-34 are investigated by diffuse reflectance infrared Fourier transform spectrum (DRIFTS) of CO adsorption and density functional theory (DFT) calculation. By combination with DFT results, characteristic Cu⁺–CO bands located at 2154 and 2136 cm⁻¹ are attributed to CO adsorbed on Cu⁺ ions located at sites I (in the plane of six-membered ring connected to the large cages) and site II (in the eight-membered ring cages near the tilted four membered ring) in the framework of H-SAPO-34 zeolite. Subsequently, both the influences of Cu loading and preparation method are considered and discussed. By varying the Cu loading, the site-occupation preference of Cu⁺ ions on site I is confirmed, especially at low Cu loadings. Through elevating the desorption temperature, migration of Cu⁺ ions is revealed because of the adsorption-induced effect. Furthermore, a facile and more efficient approach to introduce Cu⁺ ions into CHA zeolite, compared with solid-state ion exchange with CuCl and conventional ion exchange in aqueous solution, and the different preparation methods also result in different occupations of Cu⁺ ions.