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FTIR Study of CO and NO Adsorption and Coadsorption on Ni-ZSM-5 and Ni/SiO 2

2002; American Chemical Society; Volume: 18; Issue: 11 Linguagem: Inglês

10.1021/la015739g

1520-5827

Mihail Mihaylov, Konstantin Hadjiivanov,

Gas Sensing Nanomaterials and Sensors

Ni-ZSM-5 and Ni/SiO2 samples have been characterized by the IR spectra of adsorbed and coadsorbed NO and CO. Adsorption of CO at room temperature on Ni/SiO2 leads to formation of Ni2+−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 Ni2+−CO species on Ni/SiO2 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 (νs at 2136 cm-1 and νas 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/SiO2 sample. NO adsorption on Ni/SiO2 results in formation of Ni2+−NO species (1870 cm-1) which are converted, under NO equilibrium pressure, into Ni2+(NO)2 dinitrosyls (νs at 1870 cm-1 and νas 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/SiO2 sample NO replaces preadsorbed CO. However, the situation with Ni-ZSM-5 is quite different. Here, mixed Ni2+(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 Ni2+ 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.