CO and NO Adsorptions on Different Iron Sites of Fe-ZSM-5 Clusters: A Density Functional Theory Study

Density functional theory (DFT) calculations were carried out in a study of CO and NO adsorptions on different iron sites of Fe-ZSM-5. The adsorption energies and distances were similar for CO adsorption on both small and large cluster calculations. The adsorption energies (ΔE) and enthalpies (ΔH) found for NO adsorption on [FeO]1+-ZSM-5 and [Fe(OH)2]1+-ZSM-5 clusters were in line with the previous theoretical values. Some additional energy was required for NO and CO adsorptions on [FeO]2+-ZSM-5 (NO adsorption only), [FeOH]2+-ZSM-5, and [Fe(OH)2]2+-ZSM-5 clusters because of their positive ΔG values. The scaled vibrational frequencies for adsorbed CO and NO molecules on Fe2+-ZSM-5 cluster were computed as 1870 and 2149 cm−1, respectively. These values are in good agreement with the experimental values for the Fe2+−CO and Fe2+−NO bands (1882 and 2140 cm−1, respectively). Moreover, the calculated vibrational frequency (1897 cm−1) for NO adsorption on the [HO−FeOFe−OH]2+-ZSM-5 cluster matches well with the experimental value (1892 cm−1).