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)

Effect of Pore Size, Solvation, and Defectivity on the Perturbation of Adsorbates in MOFs: The Paradigmatic Mg2(dobpdc) Case Study

Mg2(dobpdc) (H4-dobpdc = 4,4\u2032-dihydroxy-(1,1\u2032-biphenyl)-3,3\u2032-dicarboxylic acid) is an attractive metal organic framework (MOF) because of its unique performances in carbon dioxide capture when combined with aliphatic amines. We have adopted this material as a paradigmatic case for the study of the effect of pore size, availability of open metal sites, and structural defectivity on the sorptive properties of MOFs, with the aim of enabling the design of better sorbents and better tools for their characterization. In this study, the adsorption of CO2, CO, and N2 has been investigated by means of infrared spectroscopy and high-quality periodic quantum mechanical B3LYP-D* calculations. Comparison with literature data on the isomorph MOF-74-Mg, characterized by the shorter dobdc linker, allowed the verification of a small, although appreciable effect of the pore size on the perturbation of the adsorbates. Although it is generally observed that the interaction energy with adsorbates decreases with increasing pore size, Mg2(dobpdc) represents an exception in the IRMOF-74 family, and its interaction energy with adsorbates is greater than that of the smaller pore member of the family. The origin of this counterintuitive behavior was found in the increase of the dispersion energy component and in the lower framework deformation. The other typical aspects that can influence the interaction with guest molecules were investigated: the presence of residual solvents competing for adsorption and structural damage. For what concerns solvation, the affinity for solvents with different polarities was tested. Selective capping of the main adsorption sites (Mg2+) was achieved by preadsorbing CH3OH on the open metal sites. IR spectroscopy of CO adsorption at 100 K was revealed to be able to detect the presence of molecules precoordinated to Mg2+ and then to check the quality of activation procedures for MOFs with open Mg2+ sites. Because of Mg2(dobpdc) air-sensitivity, a full IR characterization was also performed after damage by exposure to a water-saturated atmosphere. Surprisingly, in spite of the drastic structural collapse verified by nitrogen volumetry (decrease of 83% in the surface area), the effect of damage on the infrared spectrum of the MOF was negligible. Similarly, the only change observed in the spectra of the probe molecules was a slight decrease in their intensity after damaging. This means that IR spectroscopy is not a reliable technique to evidence the degradation of this MOF, unlike what reported for other systems

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

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

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