IR Studies of the Cu Ions in Cu-Faujasites

Abstract

The properties of Cu ions in dealuminated faujasite-type zeolites (Si/Al = 31) containing 1, 2, and 5 wt.% of Cu were investigated by IR spectroscopy with CO and NO as probe molecules. Cu was introduced by impregnation into zeolites in both protonic (HFAU) and sodium (NaFAU) forms of zeolite. Four kinds of Cu species were found: Cu⁺_exch., Cu⁺_oxide, Cu²⁺_exch. (square, planar, and square pyramidal), and Cu²⁺_oxide (CuO). The proportions between these four kinds of Cu depended on the amount of Cu and on the form of zeolite to which Cu was introduced (HFAU or NaFAU). Zeolites with 1 wt.% of Cu introduced to HFAU (denoted as Cu(1)HFAU) contained only Cu⁺_exch., whereas other forms of Cu were present in zeolites of higher Cu contents. The concentration of Cu⁺_exch. was determined by quantitative IR studies of CO adsorption. According to the IR results, some Cu ions were situated inside hexagonal prisms and/or cuboctahedra, and were inaccessible to adsorbed molecules. IR studies also evidenced that Cu ions in oxide forms-Cu⁺_oxide and Cu²⁺_oxide (CuO)-were better electron donors than Cu in exchange positions (Cu⁺_exch. and Cu²⁺_exch).

Keywords: CO adsorption; Cu sites; IR spectroscopy; NO adsorption.