Sol–gel derived mesoporous Pt and Cr-doped WO<Subscript>3</Subscript> thin films: the role played by mesoporosity and metal doping in enhancing the gas sensing properties

Mesoporous Cr or Pt-doped WO₃ thin films to be employed as ammonia gas sensors were prepared by a fast one-step sol–gel procedure, based on the use of triblock copolymer as templating agent. The obtained films were constituted by aggregates of interconnected WO₃ nanocrystals (20–50 nm) separated by mesopores with dimensions ranging between 2 and 15 nm. The doping metals, Pt and Cr, resulted differently hosted in the WO₃ mesoporous matrix. Chromium is homogeneously dispersed in the oxide matrix, mainly as Cr(III) and Cr(V) centers, as revealed by EPR spectroscopy; instead platinum segregated as Pt (0) nanoparticles (4 nm) mainly included inside the WO₃ nanocrystals. The semiconductor layers containing Pt nanoclusters revealed, upon exposure to NH₃, remarkable electrical responses, much higher than Cr-doped and undoped layers, particularly at low ammonia concentration (6.2 ppm). This behavior was attributed to the presence of Pt nanoparticles segregated inside the semiconductor matrix, which act as catalysts of the N–H bond cleavage, decreasing the activation barrier in the ammonia dissociation. The role of the mesoporous structure in influencing the chemisorption and the gas diffusion in the WO₃ matrix appeared less decisive than the electronic differences between the two examined doping metals. The overall results suggest that a careful combination between mesoporous architecture and metal doping can really promote the electrical response of WO₃ toward ammonia.     

Influence of oxidation state of sulfur on the dissociation of [Tz-(CH2)n-S(O)m-(CH2)n-Tz + Na+] adducts generated by electrospray ionization (Tz = tetrazole ring; n = 2, 3; m= 0, 1, 2)

Sodium adducts of six organosulfur‐α,ω‐ditetrazole compounds (Tz‐(CH₂)_n‐S(O)_m‐(CH₂)_n‐Tz; where Tz = tetrazole ring; n = 2, 3; m = 0, 1, 2) were generated via electrospray ionization (ESI) and their fragmentation pattern assessed via collision‐induced dissociation (CID). Two main dissociation channels were observed: (a) losses of N₂ and HN₃ from the tetrazole rings; (b) cleavage of the C–S bond. The sulfoxides pass predominantly through the second fragmentation pathway, but for the sulfides and sulfones the tetrazole ring fragmentation occurs. Theoretical calculations at the B3LYP/6‐31 + G(d,p) level indicate that for all the adducts (sulfide, sulfoxide, and sulfone) the dissociation pathway that leads to product ions arising from loss of N₂ was the most exothermic. Based on these results and assumptions, it was postulated that the dissociation of the sulfoxide adducts occurs under kinetic control (N₂‐loss pathway via a much more energetic transition state). For the sulfide and sulfone adducts, on the other hand, the dissociation process takes place via a thermodynamically controlled process. Copyright © 2011 John Wiley & Sons, Ltd.     

Quantum mechanics calculations, basicity and crystal structure: the route to transition metal complexes of azahelicenes

Quantum mechanics density functional calculations provided gas-phase electron distributions and proton affinities for several mono- and diaza[5]helicenes; computational results, together with experimental data concerning crystal structures and propensity to methylation of the nitrogen atom(s), provide a basis for designing azahelicene complexes with transition metal ions.     

Conformational studies of ethylene glycol and its two methyl ether derivatives

As a theoretical analysis of the conformational equilibria of ethylene glycol, methoxyethanol and dimethoxyethane, the energy of each stable conformational isomer (rotamer) of these molecules was calculated for various temperatures and solvent dielectric constants. Classical semi-empirical potential functions were used. Besides intrinsic potentials for rotation about single bonds, intramolecular dispersion and repulsive interaction, dipole-dipole interaction and hydrogen bonding energies were included. Interaction with the solvent was considered only in terms of a continuous dielectric medium interacting with the local dipoles and quadrupoles of the molecule. For each rotamer, the dihedral angles giving the lowest energy were determined. From the energies of each rotamer, Boltzmann distributions of populations were obtained, and total concentrations were calculated in various physically distinguishable states, e.g. those with and without internal hydrogen bonds, or those in which the central C-C bond takes a trans or a gauche conformation. It is shown that the equilibrium constants, K _HB and K _TG, for these two cases are not identical. While changes in the dielectric constant may alter strongly the geometries and energies of individual rotamers, their effect on the average geometries and on the two equilibrium constants is small. The same is true of temperature changes, and is due to the presence of several rotamers in each of the physical states considered. Thus the small temperature dependence of some observed physical properties is shown to be consistent with the distribution of molecules over several conformational states. In solution, the fraction of ethylene glycol molecules with two free OH groups (i.e. without an intramolecular hydrogen bond) is predicted to be at least 20 per cent. This shows that the presence of three-dimensional hydrogen-bonded structures in the liquid, which we propose, is possible in principle.     

Enhanced Aryltetralin Lignans Production in Linum Adventi-Tious Root Cultures

Lignans are the main secondary metabolites synthetized by Linum species as plant defense molecules. They are also valuable for human health, in particular, for their potent antiviral and antineoplastic properties. In this study, the adventitious root cultures of three Linum species (L. flavum, L. mucronatum and L. dolomiticum) were developed to produce aryltetralin lignans. The effect of two elicitors, methyl jasmonate and coronatine, on aryltetralin lignans production was also evaluated. The adventitious root cultures from L. dolomiticum were obtained and analyzed for the first time and resulted as the best producer for all the aryltetralins highlighted in this system: Podophyllotoxin, 6-methoxypodophyllotoxin and 6-methoxypodophyllotoxin-7-O-β-glucoside, the last showing a productivity of 92.6 mg/g DW. The two elicitors differently affected the production of the 6-methoxypodophyllotoxin and 6-methoxypodophyllotoxin-7-O-β-glucoside.     

Studies for a new generation of acrylic binders for exterior wood coatings

Modern acrylic binders for water-based exterior wood coatings should give films which are tack-free, hard and blocking resistant and at the same time very flexible to guarantee a long service life. This study shows that multiphase acrylic emulsions with controlled particle morphology give a means to overcome these contradictory requirements. Binder parameters such as polarity, particle size and surface functionality as well as the type and quantity of surfactants used in the process also have a strong influence on fundamental wood coating properties, such as water protection, blushing resistance, viscosity, wet adhesion and durability.     

Refining the submesh strategy in the two‐level finite element method: application to the advection–diffusion equation

We introduce a new submesh strategy for the two‐level finite element method. The numerical results show that the new submesh is able to better capture the boundary layer which is caused by the choice of bubble functions. The effect of an improved approximation of the residual free bubbles is studied for the advective–diffusive equation. Copyright © 2002 John Wiley & Sons, Ltd.