FTIR Study of Low-Temperature CO Adsorption on Cu-ZSM-5: Evidence of the Formation of Cu2+(CO)2 Species

CO adsorption at ambient temperature leads to formation of the well-known Cu+–CO species (2158 cm−1) and Cu+(CO)2 dicarbonyls (2177 and 2150 cm−1). In addition, Cu2+–CO complexes (2179 cm−1) appear under equilibrium CO pressure and are very easily destroyed by evacuation. The CO stretching frequency almost coincides in position with νs of the Cu+(CO)2 species. Lowering the temperature in the presence of gas-phase CO leads to formation of Cu2+(CO)2 geminal dicarbonyls (2170 cm−1) and a small amount of Cu+(CO)3 tricarbonyls (evidenced by a band at 2191 cm−1). The results demonstrate the high coordinative unsaturation of both Cu+ and Cu2+ ions in Cu-ZSM-5.