Adsorption and reactivity of nitrogen oxides (NO2, NO, N2O) on Fe-zeolites

International audienceNitrous oxide decomposition and temperature programmed desorption tests on Fe-ZSM-5 and Fe-silicalite show that the catalytic conversion mechanism of N2O into N-2 and O-2 over Fe-zeolites is more complex than expected. Nitrogen oxides are formed as byproducts of the catalytic process with the major part consisting in NO2 species adsorbed on the iron sites. FTIR spectroscopy of adsorbed N2O, NO, and NO2 has been used to investigate the structure and environment of the iron active species of the Fe-MFI catalysts before and after atomic oxygen deposition. The interactions of NO and N2O probes on activated Fe-ZSM-5 have evidenced two families of mononuclear Fe(II) centers (FeA and FeB) differing in the coordination state of Fe. N2O also interacts with Bronsted sites of Fe-ZSM-5 via hydrogen bonding. This type of interaction is nearly absent in Fe-silicalite. Polynuclear species (clusters) and iron oxide particles, whose concentrations are strongly influenced by the iron content and by the preparation methods are also present. When oxidized samples (by N2O) are considered, the ability of FeA and FeB centers to adsorb N2O and NO is strongly depressed. On the contrary, the surface chemistry of iron particles is not appreciably influenced. These results represent an indirect proof of the preferential presence of adsorbed oxygen on isolated Fe centers. NO titration of oxidized Fe-ZSM-5 results in the formation of a complex network of interplaying neutral (NO, NO2, N2O4) and ionic species (NO+, NO2-, NO3-). The cooperation of sites between Bronsted and iron active sites is demonstrated. The last observation is fully confirmed by the experiments performed using NO2 probe that titrates both Bronsted and iron sites. On the basis of the comparison of catalytic results of N2O decomposition and of spectroscopic results concerning the titration of surface sites with N2O, NO, and NO2 obtained on the same samples (which form the main scope of the paper), it clearly emerges that mononuclear sites characterized by lowest coordination are the most active in N2O decomposition. Under the adopted conditions, low or negligible activity is shown by FexOy clusters and Fe2O3 particles

Tailoring Metal-Organic Frameworks for CO(2) Capture: The Amino Effect

Carbon dioxide capture from processes is one of the strategies adopted to decrease anthropogenic greenhouse gas emissions. To lower the cost associated with the regeneration of amine-based scrubber systems, one of the envisaged strategies is the grafting of amines onto high-surface-area supports and, in particular, onto metal-organic frameworks (MOFs). In this study, the interaction between CO(2) and aliphatic and aromatic amines has been characterized by quantum mechanical methods (MP2), focusing attention both on species already reported in MOFs and on new amine-based linkers, to inspire the rational synthesis of new high-capacity MOFs. The calculations highlight binding-site requisites and indicate that CO(2) vibrations are independent of the adsorption energy and monitoring them in probe-molecule experiments is not a suitable marker of efficient adsorption

Tailoring Metal-Organic Frameworks for CO2 Capture: The Amino Effect

INGENIERIE+MS

Stability and Reactivity of Grafted Cr(CO)(3) Species on MOF Linkers: A Computational Study

The possibility to modulate Cr(CO)(3) properties by grafting it onto metal-organic framework (MOF) linkers of different natures has been investigated using density functional methods. MOF linkers were modeled using clusters constituted by benzene rings doubly substituted in the para position. The effect of the electron-donor or electron-acceptor nature of benzene substituents on the stability of the (eta(6)-arene)Cr(CO)(3) adduct and on the shift of the CO bands has been considered. Different electron-donor (-NH2, -CH3, -OH, -COONa) and electron-acceptor (-F, -COOH, -CN, -CF3) substituents have been used and the results compared with the bare benzene. C6H4(COOZnOH)(2) and C6H4(Zn4O13C6H5)(2) clusters have also been adopted as models of the MOF-5 benzene rings, The possibility of modulating the stability and the reactivity of Cr(CO)(3) species by grafting them to MOFs with different organic linkers was verified. In particular, this study indicates that electron-acceptor (e.g., C6H4(COOH)(2)) substituted MOF linkers facilitate the substitution of CO by incoming molecules, whereas the use of electron-donor ones (e.g., C6H4(OH)(2)) would improve the stability of the Cr(CO)(3) adduct and the ring acidity. Furthermore, an almost linear dependence of the Cr(CO)(3) binding energies on the calculated structural and vibrational features of the tricarbonyl was found, suggesting that the stability of the Cr(CO)(3) adduct can be inferred experimentally from vibrational and diffraction data. In the end, on the basis of the results obtained, it was possible to successfully explain the experimental shift of the CO IR stretching features of grafted Cr(CO)(3) on the UiO-66, CPO-27-Ni, and MOF-5 aromatic linkers and on the benzene rings of poly(ethylstyrene-co-divinylbenzene). The sign of the Delta(v) over tilde (CO) shift with respect to C6H6Cr(CO)(3) has been found to be strongly dependent on higher/lower electron density on the ring

The role of surfaces in hydrogen storage

This review deals with the main materials employed so far for hydrogen storage and it is specifically focused on the role of surface phenomena in their storage performance. Surface properties are relevant in all classes of materials: when dihydrogen is stored in the form of hydrides, the structure, texture and reactivity of the surfaces have large influence on the kinetics of charge/discharge cycles. In the storage of molecular hydrogen, surface-molecule interactions are responsible for the storage properties of the materials