Studio della formazione di retinature in una lega d’alluminio 6061 anodizzata

Nel presente lavoro sono esaminate le possibili cause del fenomeno delle retinature superficiali che si possono presentare su particolari in lega di alluminio sottoposti ad anodizzazione e fissaggio. In particolare sono stati presi in esame diversi parametri influenzanti il fenomeno, quali la tipologia del fissaggio, lo spessore dello strato d’ossido, le condizioni di temperatura ed umidità in cui i campioni sono conservati. Sono state inizialmente indagate le proprietà strutturali e meccaniche dello strato d’ossido superficiale, rispettivamente mediante misure di diffrazione di raggi X a fascio parallelo e prove di microdurezza. Successivamente sono state condotte prove mediante riscaldamenti controllati su campioni conservati a temperature pari od inferiori a quella ambiente e su provini conservati in assenza di umidità (ovvero sotto vuoto)

Theoretical characterization of dihydrogen adducts with halide anions

The interaction between a hydrogen molecule and the halide anions F-, Cl-, Br-, and I- has been studied at different levels of theory and with different basis sets. The most stable configurations of the complexes have a linear geometry, while the t-shaped complexes are saddle points on the potential energy surface, opposite to what is observed for alkali cations. An electrostatic analysis conducted on the resulting adducts has highlighted the predominance of the electrostatic term in the complexation energy and, in particular, of the quadrupole- and dipole-polarizability dependent contributions. Another striking difference with respect to the positive ions, is the fact that although the binding energies have similar values (ranging between 25 and 3 kJ/mol for F- and I-, respectively), the vibrational shift of the (v) over tilde (H-H) and in general the perturbation of the hydrogen molecule in complexes are much greater in the complexes with anions (Delta(v) over tilde (H-H) ranges between -720 and -65 cm(-1)). Another difference with respect to the interaction with cations is a larger charge transfer from the anion to the hydrogen molecule. The Delta(v) over bar is the result of the cooperative role of the electrostatics and of the charge transfer in the interaction. The correlation between binding energies and vibrational shift is far from linear, contrary to what is observed for cation complexes, in accordance with the higher polarizability and dynamic polarizability of the molecule along the molecular axis. The observed correlation may be valuable in the interpretation of spectra and thermodynamic properties of adsorbed H-2 in storage materials. (c) 2006 American Institute of Physics

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

Theoretical characterization of dihydrogen adducts with alkaline cations

As part of an extended fundamental study on the interaction of molecular hydrogen with the surfaces of potential storage materials, this work describes the interaction of dihydrogen with bare alkaline cations (Li+, Na+, K+, Rb+) by means of ab initio and density functional calculations. The effects of method and basis set are evaluated, and an estimate of the electrostatic contributions to the binding energy is evaluated. All methods predict a t-shaped coordination geometry, in accord with the quadrupolar nature of the molecule. The binding energies at the MP2/aug-cc-pVQZ (MP2-Moller-Plesset method truncated at second order) level ranges between 24 kJ/mol for Li+ and 5 kJ/mol for Rb+. Basis set size has minor effect on the binding energy of the complexes, although the poorer sets have severe limitations in the description of the electrostatics of the isolated molecule. The electrostatic contribution to the binding energy calculated from the electrostatic properties of the isolated molecule are in fact strongly basis set dependent: Their careful analysis shows that the predominant terms of the interaction are those due to the quadrupole and dipole-polarizability terms. The vibrational spectra of the adducts are evaluated and compared with a large set of experimental and theoretical results,from the literature. This review highlights a linear correlation between the frequency shift and the binding energy, which is valuable in the understanding of adsorption phenomena by means of spectroscopic methods. (c) 2005 American Institute of Physics

Theoretical maximal storage of hydrogen in zeolitic frameworks

Physisorption and encapsulation of molecular hydrogen in tailored microporous materials are two of the options for hydrogen storage. Among these materials, zeolites have been widely investigated. In these materials, the attained storage capacities vary widely with structure and composition, leading to the expectation that materials with improved binding sites, together with lighter frameworks, may represent efficient storage materials. In this work, we address the problem of the determination of the maximum amount of molecular hydrogen which could, in principle, be stored in a given zeolitic framework, as limited by the size, structure and flexibility of its pore system. To this end, the progressive filling with H-2 of 12 purely siliceous models of common zeolite frameworks has been simulated by means of classical molecular mechanics. By monitoring the variation of cell parameters upon progressive filling of the pores, conclusions are drawn regarding the maximum storage capacity of each framework and, more generally, on framework flexibility. The flexible non-pentasils RHO, FAU, KFI, LTA and CHA display the highest maximal capacities, ranging between 2.86-2.65 mass%, well below the targets set for automotive applications but still in an interesting range. The predicted maximal storage capacities correlate well with experimental results obtained at low temperature. The technique is easily extendable to any other microporous structure, and it can provide a method for the screening of hypothetical new materials for hydrogen storage applications

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