Surface dehydroxylation of amorphous and crystalline silicas (quartz dust) has been investigated from the standpoint of the development of hydrophobicity upon thermal treatment. Hydrophobicity occurs when only siloxane bridges and isolated silanols (IR band at ca. 3750 cm-') are present and is monitored by an enthalpy of adsorption of water lower than the latent heat of liquefaction (44 kJ mol-'). This calorimetric method allows the evaluation of the extent of hydrophilic and hydrophobic patches when both are present at the surface. All silicas develop hydrophobicity upon thermal treatment in vacuo, but quartz is much less easily dehydroxylated than amorphous materials. It is still mainly hydrophilic after outgassing at 1073 K, whereas pyrogenic silicas (Aerosil) become hydrophobic upon outgassing at T < 673 K. Quartz is also characterized by a few very reactive sites (q 9 90 kJ mol-'), absent on the amorphous specimens. Both these facts might be related to the specific quartz pathogenicity. Rehydroxylation at room temperature of dehydroxylated silicas occurs to a very limited extent. Hydrophilic patches exhibit a marked heterogeneity towards water with an enthalpy of adsorption decreasing from 90 to 44 kJ mol-'. The enthalpy of adsorption approaches 44 kJ mol-' corresponding to the addition of multilayers of adsorbed water.

In this work we present an exhaustive, experimental and theoretical, investigation on the structural, vibrational, electronic, and energetic properties of TS-1 catalyst. The perturbation induced by adsorption of water and ammonia ligands, which represent two key molecules in the industrial processes catalyzed by TS-1, is also discussed in great detail on both experimental and computational ground. Theory and experiments allow us to present a picture able to describe in a satisfactory way the interaction of both molecules with Ti(IV) sites and with the hosting siliceous matrix.

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A systematic analysis of the thermodynamic and spectroscopic features of a vast set of data on carbonyl-like complexes present in the literature, combined with some new experimental data here reported, allowed us to attempt to single out the electrostatic σ- and π-contributions in the formation of carbonyl bonding for the so-called “nonclassical carbonyls” hosted inside zeolitic nanocavities. In particular, the room-temperature adsorption of CO on well-defined Cu(I) and Ag(I)−ZSM-5 systems (as testified by X-ray absorption experiments) was studied by means of the joint use of IR spectroscopy and adsorption microcalorimetry. The formation in the zeolite pores of heterogeneous [Cu(CO)2]+ and [Ag(CO)]+ complexes, the stoichiometry of which is in good agreement with the homogeneous nonclassical carbonyls, was monitored as a function of increasing p CO. In the early stage of the interaction, strong and irreversibly bound monocarbonyl species characterized by ν̃COads > ν̃COgas are formed on Cu(I) and Ag(I) sites. Conversely, labile adducts (electrostatic in nature) are formed on Na+ and K+ sites hosted in the same zeolite pores. The zero-coverage enthalpy of CO adsorbed on Cu(I) and Ag(I) sites (−Δads H ∼ 120 and ∼100 kJ/mol, respectively) is much larger than the −Δads H values measured for the two alkaline-metal adducts (∼35 and ∼28 kJ/mol for Na+ and K+, respectively), despite the closeness of the charge/radius ratios of the two sets of metal cations [Na+ and Cu(I); K+ and Ag(I)]. The high −Δads H values and the irreversible nature of (a fraction) of the d-block metal carbonyls suggest the onset of a π-back-donation reinforcing the carbonyl bond with respect to a plain σ-coordination. A clear deviation from an empirical rule, which linearly correlate Δν̃CO and −ΔH ads quantities for a large set of non-d/d 0 /d 10 metal carbonyls, was observed in the case of copper-and silver−carbonyls, confirming the interplay of σ- and π-back-donation contributions for such species, otherwise defined nonclassical carbonyls. The Δν̃CO versus −Δads H empirical rule was found of general validity, in that it allows to infer the enthalpy values from the blue-shift of the C−O stretching frequency (and vice-versa) in the case of non-d/d 0 metal carbonyls, whereas it allows to roughly estimate by the deviation from the line the extent of the π-back-donation in the case of d-block metal carbonyls. Further, the spectroscopic and thermodynamic features of carbonyl species formed on (partially) reduced copper sites have shown that in the absence of strong electrostatic plus σ-coordinative components the carbonyl bond is surprisingly weak, despite the presence of π-back-donation.

Hydroxyapatite is the mineral component of human bones and teeth enamel and is used as synthetic biomaterial. It also grows outside bioglasses as a response of their incorporation in body fluids. The focus is then on understanding the microscopic steps occurring at its surfaces as this allows researchers to understand the key features of biomolecular adhesion. This perspective article deals with in silico simulations of these processes by quantum-mechanical methods based on density functional theory using the hybrid B3LYP functional and Gaussian basis functions.

The joint use of microcalorimetric and computational approaches has been adopted to describe H2O interaction with cus Al(III) Lewis and Si(OH)+ Al- Brønsted acidic sites within H-BEA and H-MFI zeolites (both with approximately 6 Al/unit cell). Adsorption data obtained at 303 K were compared to experimental model systems, such as all-silica zeolites, amorphous silica, and silico-alumina, transition alumina. In parallel, ab initio molecular modeling was carried out to mimic, in a cluster approach, Lewis and Brønsted acidic sites, as well as a variety of Si-OH species either with H-bonding interacting (nests and pairs) or isolated. H-BEA and H-MFI water affinity values were found to be almost equivalent, in both quantitative and energetic terms, in that dominated by Al-containing sites population, more than by nanocavity topology or by acidic site nature. Both H-zeolites, saturated with approximately 5 Torr of H2O vapor, bind approximately 4 H2O per Al site, almost one of which is tightly bound and not eliminated by RT pumping-off. A 160 < q(diff) < 80 kJ/mol interval was measured for the adsorption up to 1H2O/Al. The zero-coverage heat of adsorption (q0 approximately 160 kJ/mol, for both H-zeolites) was assigned to H2O/Lewis complex formation, which dominates the early stage of the process, in agreement with the ab initio computed H2O/Lewis sites binding energy. The rather broad q(diff) interval was interpreted as due to the simultaneous adsorption of H2O on both structural Brønsted sites and strongly polarized H2O already adsorbed on Lewis sites. For this latter species, BE = 74 kJ/mol was computed, slightly higher than BE = 65 kJ/mol for H2O/Brønsted sites interaction, showing that H2O coordinated on cus Al(III) Lewis sites behaves as a structural Brønsted site. The investigated all-silica zeolites have been categorized as hydrophilic in that the measured heat of adsorption (100 < q(diff) < 44 kJ/mol) was larger than the heat of liquefaction of water (44 kJ/mol) in the whole coverage examined. Indeed, polar defects present in the hydrophobic Si-O-Si framework do form relatively stable H2O adducts. Crystalline versus amorphous aluminosilicate q(diff) versus n(ads) plots showed that the measured adsorption heat is lower than expected, due to the extraction work of Al atoms from the amorphous matrix to bring them in interaction with H2O. On the contrary, such an energy cost is not required for the crystalline material, in which acidic sites are already in place, as imposed by the rigidity of the framework. Modeling results supported the experimental data interpretation.

Surface heterogeneity of both amorphous (Aerosil) and crystalline polymorphs of silica (a-quartz and a-cristobalite) has been studied by measuring the heat of reversible adsorption of water, ammonia, methanol and terl-butyl alcohol as a function of coverage and through the comparison with ab initio results on cluster models. The adsorption of terf-butyl alcohol is rather insensitive to both crystallinity and degree of dehydration, being largely due to nonspecific dispersive interactions. Water, ammonia, and methanol reveal structural heterogeneity, though to a different extent; a peculiar induced heterogeneity transmitted through H-bonding of interacting silanols is evidenced by the adsorption of ammonia. The most dehydrated sample studied is the high-temperature treated Aerosil: the heat of adsorption of water reveals some heterogeneity because of the different role of isolated and geminal silanols; ammonia shows heterogeneity also due to the species originated by its dissociation on strained bridges, during its previous contact with the surface. On mildly dehydrated surfaces, patches of silanols are present: heat of adsorption of water distinguishes between such hydrophilic patches and hydrophobic ones. Extensively hydrated samples exhibit large patches of silanols, interacting with one another through H-bonds: adsorption of ammonia reveals that the H-bonding strength of the terminal hydroxyl increases with the size of the patch. Rupture of H-bonds among adjacent SiOH groups occurs with increasing ammonia coverage, revealed by a linear decrease of the heat of adsorption over a very wide range of coverage: accordingly, the experimental isotherm follows the Temkin model. Ab initio calculations on chains of interacting silanols fully confirm such a picture.

The oxidation state of Cu species dispersed in a Cu-ZSM-5 zeolite obtained by a nonconventional gas-phase CuCl exchange, and nominally containing only Cu(I) species, was studied by x-ray photoelectron spectroscopy (XPS) and x-ray absorption near edge structure (XANES) analyses. The oxidation of Cu(I) species to Cu(II) by simple exposure to the atmosphere and subsequent reduction by thermal activation in vacuo was monitored. The quantitative and energetic aspects of the formation of carbonyl-like and amino-complexes at the metallic sites was studied by means of adsorption microcalorimetry. CO and NH3 were used as probe molecules in order to assess the coordinative unsaturation of the Cu(I) cations. Adsorption heats comprised in the 130–40 kJ mol−1 interval were obtained for the formation of both type of complexes. The perturbation induced on the Cu centers and/or on the zeolite matrix by the adsorption of the probe molecules was monitored by parallel experiments of XPS, IR, and XANES. A significant fraction of CO and NH3 molecules are irreversibly held on Cu(I) sites even after outgassing at room temperature (RT) at a final dynamic vacuum of 10−5 Torr. On the contrary, no evidence of Cu(I)/CO or of Cu(I)/NH3 complexes was observed by XPS, indicating that such adducts are totally destroyed upon outgassing at 10−9 Torr. This fact implies a reconsideration of what was previously considered as a “stable adduct.” XPS allowed to reveal the existence of ammonia adsorbed on defective Al(III) species, and to explain the chemical nature of species formed at the earliest stages of NH3 dosage and characterized by a heat of adsorption as high as 180 kJ mol−1. By comparing the quantitative XPS and volumetric-calorimetric data it was inferred that a significant gradient of defects amount is present in the system. Finally, from the whole set of XPS measurements here reported and from parallel blank experiments on the ZSM-5 zeolite before Cu-exchange, a calibration scale for the N(1s) peak of various nitrogen species in the different zeolite samples is proposed.