Tuning metal hydride thermodynamics via size and composition: Li-H, Mg-H, Al-H, and Mg-Al-H nanoclusters for hydrogen storage

Nanoscale Li and intermetallic Al-Mg metal hydride clusters are investigated as a possible hydrogen storage material using the high-level quantum Monte Carlo computational method. Lower level methods such as density functional theory are qualitatively, not quantitatively accurate for the calculation of the enthalpy of absorption of H(2). At sizes around 1 nm, it is predicted that Al/Mg alloyed nanoparticles are stable relative to the pure compositions and the metal composition can be tuned in tandem with the size to tune the hydrogen absorption energy, making this a promising route to a rechargeable hydrogen storage material.     

Strategies for designing energy-efficient clusters-based WSN topologies

Wireless Sensor Networks are used in several practical applications such as environmental monitoring and risk detection. In this work, we deal with the problem of organizing the network topology into clusters in order to minimize the total energy consumption. The problem is modeled as an Independent Dominating Problem with Connecting requirements. We first present a state-of-the-art on the problems to optimize energy consumption in WSN. Then, we propose a mixed integer linear programming formulation, constructive heuristics, a local search procedure, and a GRASP-based metaheuristic. Results are provided for large scale WSN instances.     

Culturing and investigation of stress-induced lipid accumulation in microalgae using a microfluidic device

There is increasing interest in using microalgae as a lipid feedstock for the production of biofuels. Lipids used for these purposes are triacylglycerols that can be converted to fatty acid methyl esters (biodiesel) or decarboxylated to “green diesel.” Lipid accumulation in most microalgal species is dependent on environmental stress and culturing conditions, and these conditions are currently optimized using slow, labor-intensive screening processes. Increasing the screening throughput would help reduce the development cost and time to commercial production. Here, we demonstrated an initial step towards this goal in the development of a glass/poly(dimethylsiloxane) (PDMS) microfluidic device capable of screening microalgal culturing and stress conditions. The device contained power-free valves to isolate microalgae in a microfluidic growth chamber for culturing and stress experiments. Initial experiments involved determining the biocompatibility and culturing capability of the device using the microalga Tetraselmis chuii. With this device, T. chuii could be successfully cultured for up to 3 weeks on-chip. Following these experiments, the device was used to investigate lipid accumulation in the microalga Neochloris oleabundans. It was shown that this microalga could be stressed to accumulate cytosolic lipids in a microfluidic environment, as evidenced with fluorescence lipid staining. This work represents the first example of microalgal culturing in a microfluidic device and signifies an important expansion of microfluidics into the biofuels research arena.     

Recognition properties of acyclic glycoluril oligomers

The fragmentation reaction of bis-nor-seco-CB[10] with 3,5-dimethylphenol (3) delivers methylene bridged glycoluril pentamer 5 in 81% yield. The host-guest recognition properties of the previously known tetramer 4 and those of pentamer 5 and hexamer 6 toward cationic guests in water are used to delineate some important features of the binding of acyclic CB[n]-type receptors.     

Synthesis of hydrochloric acid solution for total mercury determination in natural waters

Total mercury (Hg_T) determination requires the addition of concentrated hydrochloric acid solution (≥10 mol L⁻¹ HCl) in relatively high amounts to preserve the samples and to prepare reagent solutions. A method for the preparation of concentrated HCl with Hg_T concentration of lower than 5 ng L⁻¹ is described in this article. It is based on the well-known chemical reaction: 2 NH₄Cl + H₂SO₄ → (NH₄)₂SO₄ + 2 HCl. This method is validated thanks to the US Environmental Protection Agency method 1631 and standard reference materials BCR-579 (mercury in coastal seawater).     

Optical energy storage properties of Sr<Subscript>2</Subscript>MgSi<Subscript>2</Subscript>O<Subscript>7</Subscript>:Eu<Superscript>2+</Superscript>,R<Superscript>3+</Superscript> persistent luminescence materials

The details of the mechanism of persistent luminescence were probed by investigating the trap level structure of Sr₂MgSi₂O₇:Eu²⁺,R³⁺ materials (R: Y, La-Lu, excluding Pm and Eu) with thermoluminescence (TL) measurements and Density Functional Theory (DFT) calculations. The TL results indicated that the shallowest traps for each Sr₂MgSi₂O₇:Eu²⁺,R³⁺ material above room temperature were always ca. 0.7 eV corresponding to a strong TL maximum at ca. 90 °C. This main trap energy was only slightly modified by the different co-dopants, which, in contrast, had a significant effect on the depths of the deeper traps. The combined results of the trap level energies obtained from the experimental data and DFT calculations suggest that the main trap responsible for the persistent luminescence of the Sr₂MgSi₂O₇:Eu²⁺,R³⁺ materials is created by charge compensation lattice defects, identified tentatively as oxygen vacancies, induced by the R³⁺ co-dopants.     

Effect of protonation and deprotonation on surface charge density of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>

The protonation and deprotonation of the Nb₂O₅ surface has been followed in order to understand the reactions of surface of this catalyst. The simultaneous potentiometric and conductometric titrations had been carried by using 50 mL of water suspension of Nb₂O₅ 40 g L⁻¹. The oxide was entirely deprotonated when adding 0.4 mL NaOH 1 mol L⁻¹, and later titrated with 0.1 mol L⁻¹. The titration had supplied K ₁ and K ₂ and the obtained values were 3.24 × 10⁻³ and 4.17 × 10⁻⁸, respectively. The zero point charge was pH_pcz = 4.94. The thermodynamic studies were carried out by using 50 mL of a 40 g/L Nb₂O₅ aqueous suspension with the pH adjusted to pH_PZC value. The suspension was titrated with 0.5 mol/L of HNO₃ or NaOH for protonation or deprotonation studies, respectively, in an isoperibol calorimeter CSC ISC-4300. Thus, the obtained thermodynamic values of the protonation and deprotonation of Nb₂O₅ were Δ_dp G = −37.60 kJ/mol, Δ_dp H = −23.72 kJ/mol and Δ_dpS = 47 J/(mol K).     

Cross-section energy dependence of the [C6H6-M]+ adduct formation between benzene molecules and alkali ions (M = Li, Na, K)

The association reactions of benzene molecules with alkali ions M(+) (Li(+), Na(+) and K(+)) under single collision conditions have been studied using a radiofrequency-guided-ion-beam apparatus and mass spectrometry characterization of the different adducts. Cross-section energy dependences for [M-C(6)H(6)](+) adduct formation have been measured at collision energies up to 1.20 eV in the center of mass frame. All excitation functions decrease when collision energy increases, showing the expected behaviour for barrierless reactions. From ab initio chemical structure calculations at the MP2(full) level, the formation of the adducts makes evident the alkali ion-benzene non-covalent chemical bond. The cross-section energy dependence and the role of radiative cooling rates and unimolecular decomposition on the stabilization of the energized collision complex are also discussed.     

A 3D-analysis of cluster formation and dynamics of the X(-)-benzene (X = F, Cl, Br, I) ionic dimer solvated by Ar atoms

The specific influence of X(-) ions (X = F,Cl, Br, I) in the solvation process of halide-benzene (X(-)-Bz) ionic heterodimers by Ar atoms is investigated by means of molecular dynamic (MD) simulations. The gradual evolution from cluster rearrangement to solvation dynamics is discussed by considering ensembles of n (n = 1-15 and n = 30) Ar atoms around the X(-)-Bz stable ionic dimers. The potential energy surfaces employed are based on an atom/ion-atom and atom/ion-bond decomposition, which has been developed previously by some of the authors. The outcome of the dynamics is analyzed by employing radial distribution functions (RDF) and tridimensional (3D) probability densities.     

Ultraviolet photodissociation dynamics of the benzyl radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 4(2)B(2) electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C(7)H(6) product translational energy distributions, P(E(T))s, are derived from the H-atom TOF spectra. The P(E(T)) distributions peak near 5.5 kcal mol(-1), and the fraction of average translational energy in the total excess energy, , is ～0.3. The P(E(T))s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C(6)H(5)CD(2) and C(6)D(5)CH(2) are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.