Coordination Chemistry of Fe²⁺Ions in Fe,H-ZSM-5 Zeolite as Revealed by the IR Spectra of Adsorbed CO and NO

Abstract

An Fe,H-ZSM-5 sample (Si/Al ratio of 13.5, Fe concentration of 1.18 wt %) was prepared by liquid ion exchange from solution of FeCl 2 under nitrogen atmosphere at 370 K. When the sample is treated with oxygen at 673 K, iron is mainly in the Fe 3+ state, including (i) Fe 3+ −OH hydroxyl groups monitored in the IR spectra by a band at 3672 cm −1, (ii) oligonuclear species, and (iii) small iron oxide clusters. Some iron ions are stabilized as Fe 2+ and resist oxidation with oxygen even at 673 K. Adsorption of CO at 100 K reveals the existence of two families of coordinatively unsaturated Fe 2+ ions monitored by Fe 2+ −CO bands at 2200 and 2194 cm −1 . In the presence of CO in the gas phase, the 2194 cm −1 species are converted into dicarbonyls (2188 cm −1 ). A sharp increase in number of the CO adsorption sites is observed after reduction of the sample with CO at 673 K. Interaction of the sample with NO at room temperature leads to reduction of some Fe 3+ sites to Fe 2+ . The reduction of Fe 3+ −OH groups is a fast process, while Fe 3+ ions from another fraction are more slowly reduced. Mononitrosyl bands at 1882 cm −1 and 1896 cm −1 are formed immediately after NO adsorption. With time, Fe 2+ (NO) x polynitrosyls are produced by (i) consumption of the 1896 cm −1 species and (ii) with the participation of freshly reduced Fe 2+ ions. Thus, the number of Fe 2+ sites monitored by NO exceeds the number of sites detected by CO. The formation of polynitrosyls is an activated process but is not suppressed by lowering the temperature down to 100 K. The decomposition of these species follows two different routes: (i) via dinitrosyls and (ii) via mononitrosyls. After NO + O 2 coadsorption, nitrates are formed, and their thermal decomposition leads to restoration of the Fe 3+ −OH groups. The possible location of Fe 2+ cations is also discussed.