FTIR Study of CO Adsorption on Ni−ZSM-5

Adsorption of CO on Ni−ZSM-5 reveals the existence of two kinds of Ni2+ ions, with the respective stretching frequencies being at 2220 and 2212 cm-1. The carbonyl complexes are resistant to evacuation at room temperature, which is explained by the high electrophilicity of cations in a ZSM matrix. At 85 K and in the presence of CO, part of the Ni2+−CO species are converted into Ni2+(CO)2 dicarbonyls (2204 cm-1) which are characterized by a very low stability and easily lose one of their CO ligands. Interaction of Ni−ZSM-5 with CO at 673−773 K results in a preferential reduction of the Ni2+ ions which form the carbonyls characterized by the band at 2220 cm-1. CO adsorption at room temperature on the reduced sample leads to the formation of Ni+(CO)2 gem dicarbonyls (νs at 2136 cm-1 and νas at 2092 cm-1) which are converted, during evacuation, to linear Ni+−CO species (2109 cm-1). The dicarbonyl structure is proved by 12CO−13CO coadsorption, and the mixed Ni+(12CO)(13CO) species are characterized by ν(12CO) at 2123 cm-1 and ν(13CO) at 2058 cm-1. At low temperature, a third CO molecule is coordinated to the Ni+ cations, thus producing tricarbonyls (2156, 2124, and 2109 cm-1). The latter species lose one of their CO ligands during evacuation at 85 K and are converted back into dicarbonyls. The Ni+ sites are easily oxidized in the presence of O2. However, the Ni2+ ions produced differ in properties from the initially deposited cations and are easily reduced to Ni+ even at ambient temperature.