Nitrogen oxides represent one of the main threats for the environment. Despite decades of intensive research efforts, a sustainable solution for NOx removal under environmental conditions is still undefined. Using theoretical modelling, material design, state-of-the-art investigation methods and mimicking enzymes, it is found that selected porous hybrid iron(II/III) based MOF material are able to decompose NOx, at room temperature, in the presence of water and oxygen, into N2 and O2 and without reducing agents. This paves the way to the development of new highly sustainable heterogeneous catalysts to improve air quality.

Air pollution is an epochal concern, particularly in urban areas, and is linked to combustion processes 1 . The emission of nitrogen oxides (NOx) constitutes a critical environmental problem, and it can affect severely human health2,3,4 . At ambient temperature and pressure NOx decomposition is thermodynamically favoured; however, this process is kinetically inhibited, owing to a high activation energy5,6 . To date, no reported catalysts have had the required properties to lower the activation energy of this process without the help of coreacting agents and high temperatures7,8,9 . Here, we show that NO conversion to molecular nitrogen can be achieved at room temperature in the presence of O2 and H2O vapour, and in the absence of any further reducing agent, using iron-based Metal-Organic Frameworks (MOFs). Further, we demonstrate that MOFs work similarly to enzymes, but are stable in environments unfriendly to living matter. These findings open large perspectives on the solution of stringent problems in chemistry, such as the removal of pollutants or the activation of highly stable molecules.

The effect of CO2 and H2O on the NO/O-2 storage over a Pt-Ba/Al2O3 LNT catalyst was investigated in the temperature range 150-350 degrees C by combining FT-IR spectroscopy and microreactor flow experiments. It is found that the presence of water shows negligible effect on the NO storage at all the temperatures studied. Otherwise, the presence of CO2 inhibits the "nitrite route" (initial formation of surface nitrites and their subsequent evolution to nitrates), whereas the "nitrate route" (involving NO oxidation to NO2 over Pt and its subsequent adsorption on Ba in the form of nitrates) proceeds as in the absence of CO2. The inhibition effect of CO2 on nitrite formation increases on increasing the temperature. This behavior is likely related to the different thermal stability of nitrites and carbonates and, in particular, to the decreasing ability of nitrites to displace carbonates on increasing temperature. Simultaneously, the presence of CO2 in the feed significantly shortens or eliminates the dead time in the NO breakthrough at the reactor outlet. As a consequence, the presence of dead time in the absence of CO2 has been correlated to the occurrence of the nitrite route. (C) 2014 Elsevier B.V. All rights reserved.

Nitrites dominate! Nitrites and nitrates are formed on the catalyst surface upon storage of NOx over a model Pt–Ba/Al2O3 NOx storage–reduction catalyst. Nitrites are formed through the oxidation of NO at the Pt–Ba interface (nitrite route), while nitrates are formed upon nitrite oxidation and/or NO2 adsorption following NO oxidation at Pt sites. The nitrite route is the unique pathway responsible for the storage of NOx at low temperatures, but it also dominates at high temperatures.

Investigations of the aging behaviour of a commercial lean NOx trap (LNT) are reported in this paper. Two aging processes were tested and compared: a hydrothermal aging at 800°C and a vehicle aging corresponding to a 80,000km use. Samples were characterized with Operando FTIR, XRF, XRD, SEM-EDX, TEM, and BET analyses. LNT functionalities were evaluated using a flow reactor capable of performing NOx storage and conversion measurements, and correlated with the LNT structure evolution due to aging. The results highlighted that Pt sintering had an impact on the NO oxidation functionality and also deteriorated the Pt/Ba interface. It was hence partly responsible for the NSC decrease. Both aging processes had a similar impact on the LNT NOx storage capacity (NSC) although their structural evolutions were different: BaAl2O4 formation on the hydrothermal aged catalyst versus barium poisoned by sulphur on the vehicle aged catalyst.

The work described in this paper focuses on the impact of thermal aging on NOx trap structure and functions. They were evaluated on a Synthetic Gas Bench (SGB) and correlated with the analysis of the structural and chemical evolution of the catalyst. A FTIR Operando study allowed to further analyse the mechanisms occurring on the catalyst surface and highlight the most critical points. NOx trap samples were hydrothermally aged in a furnace up to 900°C under an oxidising flow. The main following impacts on the material were highlighted: reduction of the surface area, sintering of Pt yielding a decrease of the NO oxidation efficiency and hence of the NOx storage capacity, and a loss of CO and HC conversion, barium structural evolution into BaAl2O4 also being partly responsible for the loss of NOx storage capacity. Other possibilities for loss of NSC are: the transition of -Al2O3 to -Al2O3 and the evolution of Ba crystalline structure which needs further analysis. Both XRD and surface IR Operando studies show that hydrothermal aging has a rather homogeneous impact on all materials: alumina phase transition and BaAl2O4 production, which all lead to a partial decrease of all storage sites. L’étude suivante porte sur l’impact du vieillissement thermique sur la structure et les fonctions d’un piège à NOx. Les essais ont été réalisés sur un banc gaz synthétiques (BGS) et les résultats ont été corrélés à des analyses structurale et chimique du catalyseur. Une étude FTIR Operando a permis de mieux analyser les mécanismes se produisant sur la surface du catalyseur et de mettre en évidence les points les plus critiques. Des échantillons ont été vieillis hydrothermiquement à 900 °C sous un flux oxydant. Les principaux impacts ont été les suivants : la réduction de la surface spécifique, le frittage du Pt qui mène à une diminution de l’efficacité d’oxydation de NO, CO, HC et de la capacité de stockage, la formation de BaAl2O4 est en partie responsable de la perte de capacité de stockage de NOx. D’autres possibilités pour la perte de NSC sont : la transition de la -Al2O3 à la -Al2O3 et l’évolution de la structure cristalline du baryum qui demande des analyses approfondies. Les études DRX et Operando IR montrent que le vieillissement hydrothermique a un impact assez homogène sur tous les matériaux : la transition de phase de l’alumine et la formation de BaAl2O4, qui toutes mènent à une diminution partielle de tous les sites de stockage.

Diesel Lean NOx-Trap Thermal Aging and Performance Evolution Characterization The work described in this paper focuses on the impact of thermal aging on NOx trap structure and functions. They were evaluated on a Synthetic Gas Bench (SGB) and correlated with the analysis of the structural and chemical evolution of the catalyst. A FTIR Operand study allowed to further analyse the mechanisms occurring on the catalyst surface and highlight the most critical points. NOx trap samples were hydrothermally aged in a furnace up to 900 degrees C under an oxidising flow. The main following impacts on the material were highlighted: reduction of the surface area, sintering of Pt yielding a decrease of the NO oxidation efficiency and hence of the NOx storage capacity, and a loss of CO and HC conversion, barium structural evolution into BaAl2O4 also being partly responsible for the loss of NOx storage capacity. Other possibilities for loss of NSC are: the transition of gamma-Al2O3 to delta-Al2O3 and the evolution of Ba crystalline structure which needs further analysis. Boils XRD and surface IR Operando studies show that hydrothermal aging has a rather homogeneous impact on all materials: alumina phase transition and BaAl2O4 production, which all lead to a partial decrease of all storage sites.

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.

The effect of H2S upon the acidity of the MgO surface has been investigated using FTIR spectra of adsorbed dimethylpyridine (2,6-DMP) and CO as test molecules. In the presence of weakly bound H2S, the spectra of 2,6-DMP reveal Bronsted acidity. Adsorption of CO perturbs OH groups from MgO only in the presence of H2S or of aprotic molecules such as CO2 or SO2. This infers that these are the surface OH groups activated by acidic molecules which act as the source of protons. Thus, weak adsorption of acidic compounds, such as H2S, SO2, or CO2, can lead to dramatic enhancement of the acidity of the MgO surface.

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.

Iron was introduced by ionic exchange inside the FER structure in order to yield a Fe-FER series with increasing metal loading. Characterization of the Fe2+ cations by adsorption of CO at liquid nitrogen temperature followed by infrared spectroscopy allowed to identify three distinct sites for iron. The most abundant iron species are located on easily accessible sites of the FER structure, whereas high metal loading is required to observe more confined Fe2+ species. According to the CO adsorption results, the main iron species appears to be coordinatively unsaturated whereas isotopic labelling upon NO adsorption indicates that two distinct iron sites almost give rise to the same mononitrosyl infrared signature. Studying the catalyst upon interaction with NO and O2 in operando conditions leads to the observation of these mononitrosyl species who behave as reaction intermediates for the NO oxidation into NO2. All our Fe-FER samples presenting these mononitrosyl complexes are active not only in NO-to-NO2 reaction but also in the NOx selective catalytic reduction with ammonia. The effects of both NH3 and SO2 as adsorption competitor during the low temperature NH3-SCR are also discussed.

In heterogeneous catalysis acidity has a very important influence on activity and selectivity: correct determination of acidic properties is a base to improve industrial processes. The aim of this work was to study trimethylamine (TMA) as a probe molecule able to distinguish between the different Brønsted acid sites in zeolitic frameworks. Our work mainly focused on faujasite-type zeolites because the HY zeolite is one of the most used acidic catalysts in industrial processes. In this paper, typical IR bands assigned to TMA-protonated species (formed in supercages) are detected in the HY zeolite. TMA interacting by hydrogen bonding with the acid sites located in the sodalite units is also observed. The wavenumbers of some typical IR bands assigned to TMA-protonated species appear to depend on the acidic strength, and a complementary study with ZSM-5 and X-FAU samples confirms this proposition.

The catalytic oxidation of volatile organic compounds (VOCs) with different functional groups: propane, propene, hexane, cyclohexane, methylcyclohexane, benzene, toluene, ethanol, propan-2-ol, propanal, acetone, ethyl acetate, isopropyl acetate, MEK are studied, individually or in two-component mixtures over La0.8Sr0.2MnO3+x perovskite in air. Total conversion to CO2 and H2O occurs at temperature below 623K. However, some by-products are detected in the flue gas, especially during the oxy-derivative compounds oxidation. For the mixtures, the comportment of the VOCs is modified. A “mixture effect” is observed depending on the composition of the mixture.