FTIR study of CO and NO adsorption and coadsorption on a Cu/SiO2 catalyst: Probing the oxidation state of copper

Abstract

At room temperature CO is adsorbed strongly on Cu⁺ sites (band at 2131 cm⁻¹) whereas NO is preferably adsorbed on Cu²⁺ sites (band at 1882 cm⁻¹). Coadsorption of CO and NO allows simultaneous and selective detection of both kinds of cations. This observation is used to follow the changes on the sample surface occurring in the presence of oxygen. Addition of small amounts of O₂ to the CO–NO–Cu/SiO₂ system first leads to the oxidation of the Cu⁺ sites to Cu²⁺. This process is followed by formation of surface nitrates which block the Cu²⁺ sites for NO adsorption. Adsorption of CO at 85 K allows detection of Cu²⁺ cations (ca. 2200 cm⁻¹) in addition to the Cu⁺, CO is replaced by NO from these sites. Cu⁰ sites form carbonyls which, when copper is highly dispersed, can absorb at the same frequency at which Cu⁺–CO carbonyls are detected. In this case both kinds of species could be distinguished by their stability: the Cu⁰–CO species are easily destroyed during evacuation.