Diesel engines when compared with gasoline engines produce higher amount of NOx due the operating conditions. Among the NOx produced NO represents the great majority while, under air excess conditions, the removal of NOx is necessarily catalytic and still challenging. In the ammonia based SCR, it is often admitted that, when dealing with the original formulation developed for stationary sources (V/TiO2), the fast SCR proceeds in presence of an equimolar mixture of NO and NO2. Most of the catalytic formulations developed up to now (except Cu based zeolites) thus suffer the same rate determining step consisting in the NO to NO2 oxidation. Aiming at improving the catalytic efficiency through the catalyst composition, it is fundamental to understand the reaction mechanism at its elementary steps in order to properly design the active sites. The choice regarding the distinct possible hypothetical mechanisms was thus here guided by the nature of the active sites. It is indeed worth knowing, for a better tuning of the catalyst formulation, whether isolated, oligomeric or dual iron sites are involved in the catalytic loop. The approach in this work consists in a systematic study of the influence of the inlet NO concentration under O-2 excess on both the initial reaction rate (NO to NO2 conversion level kept below 10%) and the NO coverage level onto iron measured by operando FTIR. Isothermal experiments were performed over an aged FeFER catalyst at three distinct temperatures. The whole set of data was then processed and compared to the expected evolution derived from five possible mechanisms. The best mechanism determined in the frame of this study involves isolated iron sites onto which NO and O-2 co-adsorb and the corresponding rate determining step consists in the dissociation of the so-formed Fe_NOO2 intermediate species. The associated activation energy and reaction enthalpy values are then evaluated.

Fe–FER zeolites were characterized by FTIR spectroscopy of adsorbed CO and NO. Two aged samples (Fe–FER-1 and Fe–FER-4 with Fe content of 1.1 and 3.7wt%, respectively) and one freshly prepared 57Fe rich sample (57Fe–FER, designed for Mössbauer studies and containing 1.5wt% Fe) were studied. Both CO and NO are adsorbed onto Fe2+ cations and have different sensitivities to their location and/or coordination state. CO adsorption on Fe–FER-1 reveals two kinds of Fe2+ sites with the respective complexes observed at 2195 and 2189cm−1. The principal carbonyls (2195cm−1) are easily converted into dicarbonyls (2188cm−1) at low temperature and high CO equilibrium pressure. NO is less sensitive than CO to the environment of Fe2+ ions and NO adsorption gives rise to a single band at 1878cm−1. However, careful analysis reveals that this band consists of two closely located components. With the Fe–FER-4 sample a third family of iron sites was detected by CO at 2196cm−1. These carbonyl species are stepwise converted to di- (∼2188cm−1) and tricarbonyls (∼2180cm−1). With these sites NO forms another nitrosyls clearly detected at 1895cm−1. The latter are converted with time into polynitrosyls. These new sites are very sensitive to the preliminary treatment and easily change their oxidation state, forming Fe2+/Fe3+ redox couples. The sample preliminary treated with oxygen at 673K is characterized by Fe3+–OH groups (3674cm−1) and reactive oxygen that produces carbonates when reacting with CO, and NO+ when interacting with NO. Adsorption of NO on a freshly prepared 57Fe–FER sample confirms the presence of the three distinct Fe2+ sites which is consistent with Mössbauer data. Finally, summarizing all the data, location of the different sites inside the FER structure is proposed. The results obtained are discussed in relation with the catalytic performance of Fe–FER.

Adsorption properties of Fe-containing dealuminated BEA zeolites were investigated by FTIR spectroscopy of adsorbed CO and NO. Two Fe-containing SiBEA zeolite samples were prepared by a two-step post-synthesis method: creation of vacant T-atom sites (T = Si, Al) by dealumination of tetraethylammonium BEA zeolite with nitric acid followed by impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO3)(3). The two samples differed in iron content (0.9 and 4.2 wt.%, for Fe(0.9)SiBEA and Fe-4.2-SiBEA, respectively). The parent SiBEA sample was characterized by IR bands at 3735 cm(-1) (isolated internal silanols), 3705 and 3515 cm-1 (associated with hydroxyl nests at vacant T-atom sites). Upon the impregnation step, the bands at 3705 and 3515 cm(-1) practically disappeared, indicating consumption of the corresponding hydroxyls and incorporation of iron into the framework of SiBEA zeolite (also confirmed by XRD). In agreement with this, the IR spectra of the two samples revealed acidic bridging hydroxyls of a Fe3+-O(H)-Si type characterized by a band at 3632 cm(-1) in higher concentration for Fe(4.2)SiBEA. The 3632 cm(-1) band shifted to 3352 cm(-1) after low-temperature CO adsorption (Delta nu similar to 280 cm(-1)) evidencing a high acidity of the bridging OH groups.Low-temperature CO adsorption experiments revealed the presence of mainly two families of Fen+ sites, evidenced by carbonyl bands at 2215 and 2186 cm(-1), respectively. The latter sites were in higher concentration for Fe(4.2)SiBEA. In addition, a minor fraction of iron sites were found to be able to form tricarbonyls (bands at 2155, 2123 and 2115 cm(-1)). It was also deduced that the majority of iron introduced was in a Fe3+ state and the majority of these ions did not interact with probe molecules. Adsorption of NO leads to appearance of different mononitrosyls (1901, 1869 and 1842 cm(-1)). With time and in the presence of NO, polynitrosyls (1920 and 1815 cm(-1)) were also formed. Experiments on coadsorption of CO and NO reveal that the iron sites producing the 2215 cm(-1) carbonyls form nitrosyl species absorbing at 1901 cm(-1). It is suggested that highly electrophilic Fe3+ ions act as adsorption sites in this case.Treatment of the samples with CO at 673 K generated new Fe2+ sites monitored by CO at 2183, 2174 and 2166 cm(-1). NO adsorption revealed different mono-, di- and polynitrosyl species. A peculiarity in this case was that interconversion between poly- and dinitrosyl species was well observed. The amount of reduced iron was much higher for Fe(4.2)SiBEA than for Fe(0.9)SiBEA. (C) 2009 Elsevier Inc. All rights reserved.

FTIR spectroscopy was applied to the investigation of the nitrosyl complexes formed with the extra-framework iron species in Fe-ferrierite (Fe-FER). To clarify the loading effect on the nature of the species formed, six ferrierites with Fe loadings varying between 0 and 3.7 wt % were prepared via ionic exchange and investigated. A Fe/SiO2 and a Fe-FER sample containing iron oxide were also studied for comparison purposes. Adsorption of NO on Fe-FER (containing no iron oxide species) gives different nitrosyl species, and different iron sites can be evidenced depending on the iron concentration. The bands are assigned to mononitrosyl on iron whose oxidation state is determined to be +2 thanks to Mossbauer spectroscopy. In particular, one specific Fe2+ cation (typical of a highly loaded sample) appears to be easily converted to Fe3+ upon oxygen treatment, which makes these sites excellent candidates for catalytically active redox sites.