Infrared study of the NO reduction by hydrocarbons over iron sites with low nuclearity: Some new insight into the reaction pathway

To understand at a molecular level the nature of iron-catalyzed selective reduction of NO by hydrocarbons (HC-SCR), binuclear μ-hydroxo-bridged iron clusters with a well-defined structure were constructed in cavities of Y zeolite by surface organometallic chemistry of ferrocene and were characterized by extended X-ray absorption fine structure spectroscopy and NO adsorption monitored by infrared (IR) spectroscopy. IR spectroscopy was used to study in detail the SCR of NO by propylene, ethylene, and methane over these well-defined iron sites. The results reveal that the NO reduction undergoes different reaction pathways, depending on the type of hydrocarbon reductants and the reaction temperature. In the absence of O₂, propylene can reduce NO coadsorbed over Fe sites by a NO adduct mechanism at room temperature, while at high reaction temperatures greater than 150 °C, NO₂ produced by the disproportionation reaction of NO contributes in major part to the SCR reaction. NH₃ produced by the sequential reactions of olefins, acetaldehyde, acetic acid, and nitro-compounds with NO₂ was found to be a key intermediate for the formation of N₂. The nitro-compounds formed by coadsorption of reductant and oxidant over Fe sites may be the key intermediates initiating the reduction process at high temperature. Methane as a reductant represents a NO₂ reduction pathway different from propylene and ethylene. It is oxidized to CO₂, CO, and H₂O probably via the nitromethane pathway, while NO₂ is directly reduced to N₂. The whole reaction network of the iron-catalyzed HC-SCR in the absence of O₂ was proposed and discussed by a combination of literature with some important intermediates observed such as R-CN, HNCO, -COO^-, and NH₃. The catalytic results show that the presence of O₂ greatly aids in the iron-catalyzed reduction of NO to N₂, but the intrinsic reduction pathway is not changed.