MoS2 Nanoparticles Decorating Titanate-Nanotube Surfaces: Combined Microscopy, Spectroscopy, and Catalytic Studies.

MoS2/TNTs composites have been obtained by impregnation of titanate nanotubes (TNTs) with a centrifuged solution of nanosized MoS2 particles in isopropyl alcohol (IPA). The characterization has been performed by combining UV-vis-NIR, Raman, AFM, and HRTEM analyses, before and after impregnation. HRTEM images show that the contact between single-layer MoS2 nanoparticles and the support is efficient, so justifying the decoration concept. The volatility of IPA solvent allows the preparation of composites at low temperature and free of carbonaceous impurities. MoS2 nanoparticles have strong excitonic transitions, which are only slightly shifted with respect to the bulk because of quantum size effects. Concentrations of MoS2, less than 0.1 wt %, are enough to induce strong absorption in the visible. Photodegradation of methylene blue (MB) has been performed on TNTs and MoS2/TNTs to verify the effect of the presence of MoS2. The first layer of adsorbed MB is consumed first, followed by clustered MB in the second and more external layers. The presence of low concentrated MoS2 nanoparticles does not substantially alter the photocatalytic properties of TNTs. This result is due to poor overlapping between the high frequency of MoS2 C, D excitonic transitions and the TNTs band gap transition.

Low temperature activation and reactivity of CO2 over a Cr(II)-based heterogeneous catalyst: a spectroscopic study.

A new heterogeneous catalyst for CO(2) activation was identified in the Cr(II)/SiO(2) Phillips catalyst, one of the most important catalysts used industrially for olefin polymerization. Interestingly, it was found that Cr(II)/SiO(2) strongly activates CO(2) already at room temperature, making it available for chemicals synthesis. A preliminary attempt in this direction was done by following the reaction of CO(2) with ethylene oxide at room temperature by means of FT-IR spectroscopy, which showed the formation of ethylene carbonate. Besides non-reductive CO(2) activation, Cr(II)/SiO(2) showed good performances in catalytic reduction of CO(2) to CO, when heated under mild conditions or irradiated with UV-Vis light. Both, in situ FT-IR and UV-Vis spectroscopy, were applied to highlight the redox process occurring at the Cr centres. These results open interesting perspectives to be developed in the field of CO(2) chemical fixation.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy.

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

FTIR spectroscopy and thermodynamics of CO and H(2) adsorbed on gamma-, delta- and alpha-Al(2)O(3).

The adsorption of CO and H(2) at the surface of transitional (gamma and delta) and corundum (alpha) phases of Al(2)O(3) is studied by means of FTIR spectroscopy at temperature variable in the 293-60 K (CO) and 293-20 K (H(2)) intervals with the aims of better clarifying the nature of the surface Lewis centres and evaluate the thermodynamics of the adsorption process.

Storage of hydrogen as a guest of a nanoporous polymeric crystalline phase.

A mechanism of H(2) uptake, based on adsorption in the ordered cavities of nanoporous polymeric crystalline phases rather than on disordered amorphous polymeric surfaces, has been clearly established, for aerogels of syndiotactic polystyrene (s-PS) exhibiting the nanoporous δ phase. An ordered arrangement of the H(2) molecules is proven by FTIR spectra while the inclusion of H(2) is assessed by gravimetric measurements and molecular simulations.

Direct evidence of adsorption induced Cr(II) mobility on the SiO(2) surface upon complexation by CO.

In situ, temperature dependent, XAFS proved that Cr(II) grafted on SiO(2) is extracted from the surface upon CO adsorption at 100 K: DeltaR(Cr-O) = +0.08 A. CO adsorption evolves into two coverage dependent steps: (i) displacement of weak siloxane ligands (non-classical carbonyls); (ii) relaxation of R(Cr-O) surface bond, R(Cr-CO) optimization and transformation into classical carbonyls.

Response of CPO-27-Ni towards CO, N2 and C2H4.

Coordinatively unsaturated Ni(2+) atoms in CPO-27-Ni form linear adducts with molecular nitrogen. The framework responds to the adsorption-modifying vibrational properties and local structure around adsorbing sites. The present paper deals with a fundamental infrared (IR) study of the interaction of gases on a microporous adsorbent metallorganic framework CPO-27-Ni containing, after solvent removal, coordinatively unsaturated Ni(2+) atoms [Dietzel et al., Chem. Commun. 2006, 959]. CO, N(2) and C(2)H(4) have been chosen. Notwithstanding the relative medium (CO and C(2)H(4)) and weak (N(2)) adsorption enthalpies and the low equilibrium pressures adopted (100-10(-3) mbar) the CPO-27-Ni framework responds promptly and reversibly to the adsorption process, modifying significantly both vibrational properties and local structure around Ni(2+) adsorbing sites as determined by a parallel EXAFS investigation locating the N(2) molecule 2.27 +/- 0.03 A apart from Ni(2+). For both N(2) and C(2)H(4), IR spectra have been discussed and carefully compared with literature data. Isosteric heat of adsorption of the Ni(2+)...N(2) complex formation has been evaluated from temperature dependent IR study to be -DeltaH(ads) = 17 kJ mol(-1).

Modeling CO and N2 adsorption at Cr surface species of Phillips catalyst by hybrid density functionals: effect of Hartree-Fock exchange percentage.

In this article, we present a computational study of the structure and vibrational properties of the species formed by the interaction between Cr sites of Phillips catalyst and probe molecules (CO, N(2)). The vibrational properties of these surface species, intensively investigated in the past, form a very rich and ideal set of experimental data to test computational approaches. By adopting the X(4)Si(2)O(3)Cr (X = H, OH, F) cluster as a simplified model of the ([triple bond]SiO)(2)Cr(II) species present at the surface of the real catalyst, we found that the B3LYP hybrid functional (containing 20% of Hartree-Fock exchange), when applied to this model, is unable to reproduce with reasonable accuracy the currently available experimental data (principally coming from IR spectroscopy). Better agreement is obtained when the percentage of Hartree-Fock exchange is increased (up to 35-40%).

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 (-NH(2), -CH(3), -OH, -COONa) and electron-acceptor (-F, -COOH, -CN, -CF(3)) substituents have been used and the results compared with the bare benzene. C(6)H(4)(COOZnOH)(2) and C(6)H(4)(Zn(4)O(13)C(6)H(5))(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., C(6)H(4)(COOH)(2)) substituted MOF linkers facilitate the substitution of CO by incoming molecules, whereas the use of electron-donor ones (e.g., C(6)H(4)(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 nu(CO) shift with respect to C(6)H(6)Cr(CO)(3) has been found to be strongly dependent on higher/lower electron density on the ring.

Spectroscopic investigation of the encapsulation and the reactivity towards NO of a Co(II)-porphyrin inside a cross-linked polymeric matrix.

A highly dispersed cobalt(II) porphyrin (PhCo(II)) encapsulated inside a highly cross-linked poly(4-ethylstyrene-co-divinylbenzene) matrix (St-DVB) is investigated by means of FTIR, Raman, UV-Vis and EPR spectroscopies. The adoption of the highly porous St-DVB support is the key factor in reaching an optimum porphyrin dispersion in undiluted conditions ( approximately 10 wt% of porphyrin). It is demonstrated that the encapsulated PhCo(II) are characterized by the absence of any axial interactions involving the polymeric matrix, that can be considered a "non-coordinating" solvent. Finally, the permanent porosity of the encapsulating matrix allows gaseous molecules to reach the Co(II) cations also in the absence of swellable solvents. In particular, the reactivity of the isolated Co(II) porphyrin species towards nitric oxide (NO) is investigated, with possible implications in the understanding of the crucial role played by NO in biological systems.

Chromocene in porous polystyrene: an example of organometallic chemistry in confined spaces.

In this work, we present an innovative approach to investigate the structure and the reactivity of a molecularly dispersed organometallic compound. The poly(4-ethylstyrene-co-divinylbenzene) microporous system (PS) is used as "solid solvent" able to molecularly disperse CrCp2, allowing: (i) its full characterization by means of spectroscopic techniques; (ii) the pressure and temperature dependent study of its interaction towards simple molecules like CO freely diffusing through the pores; (iii) the accurate determination of the reaction enthalpies by both direct microcalorimetric measurements and by an indirect spectroscopic approach. The experimental results are compared with quantum-mechanical calculations adopting the DFT approximation with two different functionals (namely BP86 and B3-LYP), showing the limitations and the potentialities of DFT methods in predicting the properties of open shell systems. It is concluded that modern DFT methods are able to give a coherent view of the vibrational properties of the CrCp2 molecule (and of the complex formed upon CO adsorption) that well match the experimental results, while the energetic predictions should be taken with care as they are significantly dependent on the functionals used.

Photo-degradation of yperite over V, Fe and Mn-doped titania-silica photocatalysts.

The photocatalytic decomposition of yperite (bis(2-chloroethyl)sulfide), a chemical warfare agent, was achieved by using titania-silica catalysts doped with several transition metal ions. The preparation of these catalysts was achieved by impregnation of a titania-silica mixed oxide previously synthesized using a sol-gel route with salts of the doping elements (vanadium, iron, manganese). The above catalysts were characterized using several spectroscopic techniques: FTIR, Raman, DR-UV-Vis, and XPS. The band gap energy was measured for each photocatalytic system. The reaction was carried out in two different types of reactors, i.e. naturally aerated and a closed quartz tube aerated under a constant flow, and using two types of irradiation, UV-Vis and Vis. The investigated systems proved to be extremely active, leading to an almost complete degradation of yperite in 2 h of irradiation. An excellent correlation between the photocatalytic performances and the band gap has been found. Based on the characterization data and on the temporal evolution of the reaction products, a reaction mechanism has been suggested. This mechanism considers two distinct pathways for the decomposition of yperite, namely the C-S bond cleavage and the S oxidation.

Role of exposed metal sites in hydrogen storage in MOFs.

The role of exposed metal sites in increasing the H2 storage performances in metal-organic frameworks (MOFs) has been investigated by means of IR spectrometry. Three MOFs have been considered: MOF-5, with unexposed metal sites, and HKUST-1 and CPO-27-Ni, with exposed Cu(2+) and Ni(2+), respectively. The onset temperature of spectroscopic features associated with adsorbed H2 correlates with the adsorption enthalpy obtained by the VTIR method and with the shift experienced by the H-H stretching frequency. This relationship can be ascribed to the different nature and accessibility of the metal sites. On the basis of a pure energetic evaluation, it was observed that the best performance was shown by CPO-27-Ni that exhibits also an initial adsorption enthalpy of -13.5 kJ mol(-1), the highest yet observed for a MOF. Unfortunately, upon comparison of the hydrogen amounts stored at high pressure, the hydrogen capacities in these conditions are mostly dependent on the surface area and total pore volume of the material. This means that if control of MOF surface area can benefit the total stored amounts, only the presence of a great number of strong adsorption sites can make the (P, T) storage conditions more economically favorable. These observations lead to the prediction that efficient H2 storage by physisorption can be obtained by increasing the surface density of strong adsorption sites.

FTIR spectroscopy and thermodynamics of hydrogen adsorbed in a cross-linked polymer.

The adsorption of H(2) in a cross-linked poly(styrene-co-divinylbenzene) (St-DVB) microporous polymer (BET surface area 920 m(2) g(-1)) is studied by volumetric and gravimetric methods, FTIR spectroscopy at variable temperature (300-14 K) and ab initio calculations. At 77 K the polymer reversibly stores up to 1.3 mass% H(2) at a pressure of 1 bar and 1.8 mass% at 10 bar. The adsorption process involves the specific interaction of H(2) with the structural phenyl rings through weak dispersive forces. The interacting molecules become IR active and give rise to vibrational and rotational-vibrational manifestations which are affected by the temperature, the contact time and the H(2) equilibrium pressure. The spectra of the H(2)/St-DVB system reported here represent the first IR evidence of the adsorption of hydrogen on unsaturated molecules. The adsorption enthalpy is evaluated by the VTIR (variable temperature IR spectroscopy) method (C. Otero Areán et al., Phys. Chem. Chem. Phys., 2007, DOI: 10.1039/b615535a) and compared with the results of ab initio calculations for the H(2)/benzene interaction and with literature data.

Functionalization of zeolitic cavities: grafting NH2 groups in framework T sites of B-SSZ-13--a way to obtain basic solids catalysts?

Insertion of B atoms into an Al-free zeolitic framework with CHA topology results in the formation of B-SSZ-13 zeotype with Si/B = 11. B K-edge NEXAFS testifies that B forms [B(OSi)4] units in a Td-like geometry (sp3-hybridized B atoms). According to B K-edge NEXAFS and IR, template burning results in the formation of [B(OSi)3] units in a D3h-like geometry (sp2-hybridized B atoms) with a break of a B-O-Si bond and the formation of a Si-OH group. The activated material contains B(III) Lewis acid centers able to specifically coordinate bases like NH3. Such [B(OSi)3] units are reactive toward ammonia, resulting in the formation of B-NH2 surface functionality inside the pores of B-SSZ-13 already under mild conditions, i.e., 35 mbar of NH3 at 373 K for 30 min and without crystallinity degradation. A minor fraction of Si-NH2 cannot be excluded owing to the presence of two IR doublets at 3500 and 3430 cm-1 and at 1600 and 1550 cm-1. Ab initio B3LYP/6-31+G(d,p) calculations on a cluster model, supported by a single-point MP2 on B3LYP/6-31+G(D,P) optimized structures, found the break by NH3 of a B-O-Si bond of the [B(OSi)3] unit with formation of [SiOH] and [H2N-B(OSi)2] species to be energetically favored. Comparison between experimental and computed frequency shifts shows them to be in semiquantitative agreement. The high stability of the B-NH2 surface functionality is probed by N K-edge NEXAFS spectra collected under UHV conditions. These findings can open a new route in the preparation of shape selective solid basic catalysts.

Direct observation and modelling of ordered hydrogen adsorption and catalyzed ortho-para conversion on ETS-10 titanosilicate material.

Hydrogen physisorption on porous high surface materials is investigated for the purpose of hydrogen storage and hydrogen separation, because of its simplicity and intrinsic reversibility. For these purposes, the understanding of the binding of dihydrogen to materials, of the structure of the adsorbed phase and of the ortho-para conversion during thermal and pressure cycles are crucial for the development of new hydrogen adsorbents. We report the direct observation by IR spectroscopic methods of structured hydrogen adsorption on a porous titanosilicate (ETS-10), with resolution of the kinetics of the ortho-para transition, and an interpretation of the structure of the adsorbed phase based on classical atomistic simulations. Distinct infrared signals of o- and p-H2 in different adsorbed states are measured, and the conversion of o- to p-H2 is monitored over a timescale of hours, indicating the presence of a catalyzed reaction. Hydrogen adsorption occurs in three different regimes characterized by well separated IR manifestations: at low pressures ordered 1:1 adducts with Na and K ions exposed in the channels of the material are formed, which gradually convert into ordered 2:1 adducts. Further addition of H2 occurs only through the formation of a disordered condensed phase. The binding enthalpy of the Na+-H2 1:1 adduct is of -8.7+/-0.1 kJ mol(-1), as measured spectroscopically. Modeling of the weak interaction of H2 with the materials requires an accurate force field with a precise description of both dispersion and electrostatics. A novel three body force field for molecular hydrogen is presented, based on the fitting of an accurate PES for the H2-H2 interaction to the experimental dipole polarizability and quadrupole moment. Molecular mechanics simulations of hydrogen adsorption at different coverages confirm the three regimes of adsorption and the structure of the adsorbed phase.

Structure and nuclearity of active sites in Fe-zeolites: comparison with iron sites in enzymes and homogeneous catalysts.

Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe(2+) homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe(2+) grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe(2+) presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N(2)O decomposition on isolated sites, whose properties are more similar to those of the (FeO)(2+) in cationic complexes (included the [(H(2)O)(5)FeO](2+)"brown ring" complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.