Kinetic modeling of nitrous oxide decomposition on Fe-ZSM-5 in the presence of nitric oxide based on parameters obtained from first-principles calculations

Abstract

A variety of experiments for the N₂O decomposition over Fe-ZSM-5 catalysts have been simulated in the presence and absence of small amounts of nitric oxide and water vapor. A comprehensive reaction mechanism over mononuclear iron sites was considered, and all elementary reaction rate constants were obtained from density functional theory and transition state theory. Various experimental observations, such as the acceleration of the N₂O decomposition in the presence of nitric oxide at low temperatures, can be described with the studied reaction mechanism on mononuclear iron sites. No other iron species, such as binuclear iron, are necessary for explaining experimental observations. At high temperatures, Z⁻[FeO]⁺ sites are active for N₂O decomposition, forming either Z⁻[OFeO]⁺ and Z⁻[FeO₂]⁺ sites on which a second N₂O can decompose to form Z⁻[OFeO₂]⁺ sites from which O₂ can rapidly desorb. At low temperatures, nitric oxide activates water poisoned Z⁻[Fe(OH)₂]⁺ sites to form active Z⁻[FeOH]⁺ species. The catalytic cycle on Z⁻[FeOH]⁺ involves N₂O dissociation to form Z⁻[OFeOH]⁺ sites that are inactive for a second N₂O decomposition. Instead, NO adsorbs and NO₂ desorbs in order to regenerate the Z⁻[FeOH]⁺ sites. On all active iron sites, the first N₂O dissociation step is the most critical rate-controlling step. The concentration of nitric oxide and water vapor together determine at which temperature the switch between the most dominant active site (hydroxo- versus oxo–iron species) occurs. Overall, this study motivates the investigation of Z⁻[OFeOH]⁺ sites as potential α-oxygen species that can oxidize various hydrocarbons at low temperatures.