Coordination of Two N₂ Molecules to One Ni⁺ Site in Ni–ZSM-5: An FTIR Spectroscopy Study

Ni⁺ cations were produced in a Ni–ZSM-5 zeolite by partial reduction with CO, and the ability of the Ni⁺ and Ni²⁺ sites to coordinate ¹⁴N₂, ¹⁵N₂, and/or CO molecules was studied by FTIR spectroscopy. With Ni⁺ cations, CO produces mono-, di-, and tricarbonyl species while only mono- and dicarbonyls are formed with Ni²⁺. Adsorption of ¹⁴N₂ at 100 K results in formation of Ni²⁺–¹⁴N₂ adducts (band at 2343 cm^–1) and geminal Ni⁺(¹⁴N₂)₂ complexes (ν_s at 2287 and ν_as at 2270 cm^–1). The Ni⁺(¹⁴N₂)₂ complexes lose their ligands stepwise during evacuation at 100 K, and two kinds of monoligand Ni⁺–¹⁴N₂ species are formed (2252 and 2238 cm^–1). After ¹⁵N₂ adsorption, the Ni⁺(¹⁵N₂)₂ complexes were observed at 2211 and 2194 cm^–1: the two Ni⁺–¹⁵N₂ species at 2178 and 2163 cm^–1, and the Ni²⁺–¹⁵N₂ adducts at 2264 cm^–1. To prove the geminal structure, coadsorption of ¹⁴N₂ and ¹⁵N₂ was studied. It resulted in formation of Ni⁺(¹⁴N₂)­(¹⁵N₂) species that were characterized by ¹⁴N–¹⁴N and ¹⁵N–¹⁵N stretchings at 2277 and 2201 cm^–1, respectively. These values are in excellent agreement with those calculated on the basis of the approximate force field model. Experiments on coadsorption of ¹⁴N₂ and CO have shown formation of Ni⁺(CO)­(¹⁴N₂) complexes that were characterized by CO and ¹⁴N–¹⁴N stretchings at 2098 and 2305 cm^–1, respectively. The reasons for simultaneous coordination of two ¹⁴N₂ (or one ¹⁴N₂ and one CO) molecules to one Ni⁺ site and the nature of the bonds in the different complexes are discussed.