Mesomorphic behavior of polydiethylphosphazenes as described by13C,14N,and2H NMR

The existence of mesomorphism in polydiethylphosphazene was recently established by MAS NMR and X-ray diffraction characterization. In the present work the mechanism of motion of the ethyl side groups in the high-temperature polymorph tabove 45 C) is identified and compared to the arrangement of side groups in the low-temperature polymorph. For this purpose a few NMR active nuclei (¹³C,¹⁴N, and²H) were exploited to define the side-chain motions occurring at transition. Experiments performed at varying temperatures close to the onset of solid transition suggest the presence of jumps between two conformations in the pretransition state. Rotor-synchronized triple-resonance NMR of the high-temperature phase determined the average distances between the carbons and the nitrogens in the polymorphs. The theoretical prediction of the dipolar interaction between the nuclei supports the hypothesis that ethyl groups can undergo a complete rotation about the P CH₂ bond by jumping across a conformational barrier. The mechanism of motion of the ethyl groups must be cooperative and the collapse of the rigid shell around the main chain is described at the transition.     

Aerobic Oxidation of Glucose with Gold Catalyst: Hydrogen Peroxide as Intermediate and Reagent

Careful analytical determinations show that the gold‐catalysed aerobic oxidation of glucose occurs through a two‐electrons mechanism leading to gluconate and hydrogen peroxide. This latter decomposes before reaching the critical concentration for competing with O₂ in glucose oxidation. A mechanism of glucose oxidation on gold nanoparticles is presented.     

Step-by-step observation of the hydration of C3S by magic-angle spinning 29Si nuclear magnetic resonance: the masking effect of D2O

High-resolution solid-state ²⁹Si nuclear magnetic resonance (NMR) characterization of hydrated C₃S provides a means of selectively observing the hydrogen-containing phases. In particular, by applying the cross-polarization (CP) technique, we demonstrated that a contrast can be obtained, not only for hydrated against anhydrous phases, but also for hydrogen-hydrated against deuterium-hydrated phases. Selectively deuterated samples were thus prepared by a H₂O/D₂O alternated hydration procedure. The progress of the polycondensation of orthosilicates as derived by H₂O was followed, demonstrating that a separate microphase of anhydrous C₃S prevailingly reacted at any step. An induction period, showing a low reactivity and mainly formation of Q^o hydrated was observed during the first hydration steps of selectively deuterated samples. The efficiency of the masking effect of D₂O was also indirectly proven by comparison with calorimetric measurements.     

Propane partial oxidation over M-substituted vanadyl phosphates dispersed on titania and silica

Mono-substituted M³⁺ compounds of vanadyl phosphate dihydrate VOPO₄·2H₂O, with formula [M(H₂O)]_xVO_1−xPO₄·nH₂O (M=Al, Fe, Cr, x=0.15–0.20, n=2–2.40), and a di-substituted compound with formula Fe_0.08Cr_0.08(H₂O)_0.16VO_0.84PO₄·2.9H₂O, either pure or supported, were characterised by XRD, EDAX, TG/DTA, physisorption and chemisorption measurements, and tested as catalysts in the partial oxidation of propane. Incorporation of the M³⁺ cation into VOPO₄ produces a marked increase in surface area, pore volume, and reducibility, with subsequent enhancement of the catalytic activity. Upon adsorption of these compounds on titania, a homogeneous distribution of highly dispersed species is obtained, whereas on silica small conglomerates of crystalline phases of VOP and FeVOP are formed, presumably by polymerisation on the acidic surface sites of the support. The titania-supported samples exhibit higher catalytic activity and better selectivity to partial oxidation products (acetic acid and propene), compared to silica-based materials; these effects are attributed to the higher dispersion and reducibility of the surface species. Propane oxidation over the supported materials undergoes transition to the ignited state, in which surface temperatures up to 900 K are attained, and homogenous reactions yield mainly propene and CO.     

Mesomorphic behavior of polydiethylphosphazenes as described by13C,14N, and2H NMR

The existence of mesomorphism in polydiethylphosphazene was recently established by MAS NMR and X-ray diffraction characterization. In the present work the mechanism of motion of the ethyl side groups in the high-temperature polymorph tabove 45 C) is identified and compared to the arrangement of side groups in the low-temperature polymorph. For this purpose a few NMR active nuclei (¹³C,¹⁴N, and²H) were exploited to define the side-chain motions occurring at transition. Experiments performed at varying temperatures close to the onset of solid transition suggest the presence of jumps between two conformations in the pretransition state. Rotor-synchronized triple-resonance NMR of the high-temperature phase determined the average distances between the carbons and the nitrogens in the polymorphs. The theoretical prediction of the dipolar interaction between the nuclei supports the hypothesis that ethyl groups can undergo a complete rotation about the P CH₂ bond by jumping across a conformational barrier. The mechanism of motion of the ethyl groups must be cooperative and the collapse of the rigid shell around the main chain is described at the transition.Presented at the 1st Italian Workshop on Cyclo- and Poly (phosphazene) Materials. February 15 16, 1996, at the CNR Research Area in Padova, Italy.     

Comparison of gold supported catalysts obtained by using different allotropic forms of titanium dioxide

Gold catalysts were prepared on different allotropic phases of TiO₂ using the colloidal deposition method. The supports were chosen in order to study the influence of the support structure on the catalytic activity of the final material. Furthermore, for the same allotropic modification of titania, materials with a different particle size distributions have been used to study the influence of the grain size of the support on the deposition of the colloid. Our results indicate that the activity of the final catalyst is not much affected by the variation of the titania structure, though the situation becomes different when the catalyst is calcined at different temperatures. In this case, pure anatase and rutile supported catalysts showed a lower thermostability than the one prepared using P25 titanium oxide (Degussa). Concerning the colloid immobilization on the support it was found that the most important parameter is the grain size of the support. In particular, the deposition of the colloidal gold particles is greatly enhanced in the case of supports composed of particles of few nanometers in size.     

Effect of the titania morphology on the Au/TiO2-catalyzed aerobic epoxidation of stilbene

We use a colloidal deposition method to prepare gold nanoparticles with similar size distributions centered at 3 nm over various anatase titania supports. All UV100, PC500 and AK350 titanias are loaded with similar amount of gold (1.0 ± 0.2 wt.%) which is in similar electronic and optical environments, as shown by X-ray photoelectron spectroscopy (XPS) and UV–vis. This allows us to assess the effect of the titania crystallization, morphology and chemical composition on the catalytic properties of gold in the aerobic epoxidation of trans-stilbene. We find that Au/UV100 is more active than Au/PC500 and Au/AK350 but that selectivities are similar on all materials. Epoxide yields on the other hand critically depend on the support functionalization and surface composition. TG–DTA characterization of the bare titania powders reveals indeed that AK350, which leads to the least active catalyst, is slightly less hydroxylated than PC500 and UV100. This indicates that surface titanol groups might be involved in the epoxidation of trans-stilbene. The presence of boron oxide on Au/UV100 (XPS), due to reaction of UV100 with the NaBH₄ reductant during the synthesis, is also thought to promote the epoxide-forming mechanism. This chemical promotion effect appears to compensate for the specific and beneficial gold–P25 interaction. As a result, Au/UV100 is more efficient than the reference Au/P25 catalyst for this reaction.     

Nanocrystalline silicon film grown by LEPECVD for photovoltaic applications

This work deals with the characterization of nanocrystalline (nc) silicon films, grown using the plasma enhanced chemical vapour deposition (PECVD) process based on a low-voltage–high-current arc discharge plasma named LEPECVD (low-energy PECVD).The structural, electrical and chemical properties of the LEPECVD grown films have been studied as a function of the deposition parameters (substrate temperature, growth rate, silane dilution). The results show that the films consist of elongated nanocrystals along the 1 1 1direction, embedded in an amorphous matrix. The crystallite size along the 1 1 1 direction is in the range of 9-20 nm. The volume fraction of crystallinity (χ_c) varies between 51% and 78%, depending on preparation conditions. Conductivity values of the order of 10⁻⁶ Ω⁻¹ cm⁻¹ for the layers were measured, making the material suitable for the p–i–n junction application.     

Engineering Porous Emitting Framework Nanoparticles with Integrated Sensitizers for Low‐Power Photon Upconversion by Triplet Fusion

The conversion of low‐energy light into photons of higher energy based on sensitized triplet–triplet annihilation (sTTA) upconversion is emerging as the most promising wavelength‐shifting methodology because it operates efficiently at excitation powers as low as the solar irradiance. However, the production of solid‐state upconverters suited for direct integration in devices is still an ongoing challenge owing to the difficulties concerning the organization of two complementary moieties, the triplet sensitizer, and the annihilator, which must interact efficiently. This problem is solved by fabricating porous fluorescent nanoparticles wherein the emitters are integrated into robust covalent architectures. These emitting porous aromatic framework (ePAF) nanoparticles allow intimate interaction with the included metallo‐porphyrin as triplet sensitizers. Remarkably, the high concentration of framed chromophores ensures hopping‐mediated triplet diffusion required for TTA, yet the low density of the framework promotes their high optical features without quenching effects, typical of the solid state. A green‐to‐blue photon upconversion yield as high as 15% is achieved: a record performance among annihilators in a condensed phase. Furthermore, the engineered ePAF architecture containing covalently linked sensitizers produces full‐fledge solid‐state bicomponent particles that behave as autonomous nanodevices.