Theoretical investigation of novel two-step decomposition of nitric oxide over Fe(II) ion-exchanged zeolites using DFT calculations

Abstract

A new method for the direct catalytic decomposition of nitric oxide (NO) is proposed in order to overcome several well-known problems toward its practical use. In this novel methodology, NO is first selectively adsorbed on Fe(II) ion-exchanged zeolites (LTA, MFI, and FAU) in the presence of excess oxygen; the concentrated NO in the zeolite microchannels is then decomposed by low-energy irradiation. The NO molecule is selectively adsorbed on Fe(II)–zeolites under the excess oxygen conditions, and that NO bond cleavage is more likely to occur for NO on Fe(II) than in the gas phase, because Fe(II) supplies an electron into an orbital of the NO molecule. A dinitrosyl species formed on Fe(II) sites in zeolites is decomposed via a N2O intermediate with lower activation energy. Achieving a closer distance between the two nitrogen atoms of two NO molecules by concentrating them on Fe(II) in zeolitic microcages is key to accelerate the decomposition with greatly reduced energy requirements. Based on these results, a catalytic redox cycle of Fe(II) in zeolites for NO decomposition is proposed, where the largest activation energy (the rate-determining step) is for the reaction 2NO→N2O+O (ca. 160kJ/mol), which is higher than that of NO decomposition on Cu-ZSM-5 (ca. 135kJ/mol). It is proposed that imitating the molecular mechanism of fungal nitric oxide reductase (P450nor) by utilizing an adjacent proton on Fe(II) ion-exchanged proton-form zeolite, the required energy for 2NO→N2O+O decreases further (ca. 40kJ/mol), allowing for successful reaction initiation with low-energy irradiation such as microwaves.