FTIR Study of CO and NO Adsorption and Coadsorption on Ni-ZSM-5 and Ni/SiO2

Ni-ZSM-5 and Ni/SiO 2 samples have been characterized by the IR spectra of adsorbed and coadsorbed NO and CO. Adsorption of CO at room temperature on Ni/SiO 2 leads to formation of Ni 2+ −CO species (ν(CO) at 2192 cm - 1 ) that are easily destroyed by evacuation. The carbonyls formed on Ni-ZSM-5 after CO adsorption are two types and characterized by bands at 2220 and 2212 cm - 1 . In line with this high frequency, both species are much more stable than the Ni 2+ −CO species on Ni/SiO 2 and are highly resistant toward evacuation. Reduction of Ni-ZSM-5 with CO results in the appearance of Ni + ions. With CO these ions form Ni + (CO) 2 dicarbonyls (ν s at 2136 cm - 1 and ν as at 2092 cm - 1 ), which lose one of their CO ligands during evacuation and are thus converted into Ni + −CO linear species (2109 cm - 1 ). In contrast, no Ni + ions are produced after reduction of the Ni/SiO 2 sample. NO adsorption on Ni/SiO 2 results in formation of Ni 2+ −NO species (1870 cm - 1 ) which are converted, under NO equilibrium pressure, into Ni 2+ (NO) 2 dinitrosyls (ν s at 1870 cm - 1 and ν as at 1842 cm - 1 ). Similar species are formed on Ni-ZSM-5: the mononitrosyls are characterized by bands at 1905−1895 cm - 1, whereas the bands typical of dinitrosyls are at ca. 1900 cm - 1 and 1874−1862 cm - 1 . After evacuation only mononitrosyls are present on the sample. With the Ni/SiO 2 sample NO replaces preadsorbed CO. However, the situation with Ni-ZSM-5 is quite different. Here, mixed Ni 2+ (CO)(NO) species are clearly detected (ν(CO) at 2147 cm - 1 and ν(NO) at 1863 cm - 1 ). In these complexes the bond between Ni 2+ and the ligands is weakened and, as a result, CO is easily removed by evacuation. The reasons for the different properties of nickel cations in different matrixes are discussed. It is proposed that the low coordination number of cations in ZSM-5 is the main reason for the formation of mixed complexes.