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

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 bo th kinds of cations. This observation is used to follow the changes on the sample surface occurring in the presence of oxygen. Addition of small amoun ts of O-2 to the CO-NO-Cu/SiO2 system first leads to the oxidation of the C u+ 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 all ows detection of Cu2+ cations (ca. 2200 cm(-1)) in addition to the Cu+, CO is replaced by NO from these sites. C sites form carbonyls which, when copp er is highly dispersed, can absorb at the same frequency at which Cu+-CO ca rbonyls are detected. In this case both kinds of species could be distingui shed by their stability: the Cu-0-CO species are easily destroyed during ev acuation.