FTIR study of CO and NOxadsorption and co-adsorption on Cu/silicalite-1

Abstract

Testing of two different samples of Cu/silicalite-1 with CO reveals the existence of Cu⁺ sites typical of oxide-supported copper. CO adsorbed on these sites is characterized by a band at 2137 cm⁻¹. Heating the samples in a CO atmosphere leads to reduction of particular Cu²⁺ cations and the Cu⁺ sites formed behave as Cu⁺ cations exchanged in ZSM-5. Adsorption of CO on these sites results in the formation of dicarbonyls (ν_s at 2177 cm⁻¹ and ν_as at 2150 cm⁻¹) which, after evacuation, are converted into monocarbonyls (ν_CO at 2157 cm⁻¹). In the presence of water, mixed Cu⁺(H₂O)(CO) species are formed (ν_CO at 2130 cm⁻¹). NO adsorption on Cu/silicalite-1 results in the formation of Cu²⁺(NO) complexes which are characterized by a broad band centered at 1888 cm⁻¹. Addition of a small amount of oxygen to the system results in the formation of surface nitrates typical of supported copper catalysts (1630, 1608 and 1578 cm⁻¹). In addition, weakly bonded N₂O₃ , N₂O₄ and NO₂ are formed. The reasons for the appearance of Cu⁺ sites in cationic positions are discussed.