Publication

Monomeric Fe Species in Square Planar Geometry Active for Low Temperature NH3-SCR of NO

Davide Ferri
2022
Journal paper
Abstract

Monomeric Fe species in zeolites are considered to be the active sites for the low temperature activity toward ammonia-assisted selective catalytic reduction of nitrogen oxides (NH3-SCR) in exhaust gases. Herein, we report on a preparation method to synthesize single-site Fe/ZSM-5 by combination of dealumination with the use of a bulky iron complex to introduce Fe. Transient in situ XAS experiments at the Fe K-edge under dynamic NH3-SCR reaction conditions demonstrated the involvement of all iron atoms in the redox cycle and, with that, suggest that Fe species in the studied Fe/ZSM-5 catalyst are monomers. Simulation of experimental X-ray absorption data revealed that the Fe species in the sample obtained by this approach adopt a square planar geometry, which changes into fivefold coordination at low temperature by strongly binding either NO or NH3 as the ligand. The presence of monomeric Fe species in this Fe/ZSM-5 conveys molecular level insights into the temperature-dependent NH3-SCR activity and might prove useful in the study of other reactions over monomeric Fe species, such as methane partial oxidation or dehydroaromatization reactions.

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Related concepts (32)
Ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous waste, particularly among aquatic organisms, and it contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to 45% of the world's food and fertilizers. Around 70% of ammonia is used to make fertilisers in various forms and composition, such as urea and diammonium phosphate.
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack, or propelling nozzle. It often disperses downwind in a pattern called an exhaust plume. It is a major component of motor vehicle emissions (and from stationary internal combustion engines), which can also include crankcase blow-by and evaporation of unused gasoline.
Selective catalytic reduction
Selective catalytic reduction (SCR) means of converting nitrogen oxides, also referred to as NOxNOx with the aid of a catalyst into diatomic nitrogen (N2), and water (H2O). A reductant, typically anhydrous ammonia (NH3), aqueous ammonia (NH4OH), or a urea (CO(NH2)2) solution, is added to a stream of flue or exhaust gas and is reacted onto a catalyst. As the reaction drives toward completion, nitrogen (N2), and carbon dioxide (CO2), in the case of urea use, are produced.
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