2013 Vol. 26, No. 3

Article
Content
2013, 26(3): 0-0. doi: 10.1088/1674-0068/26/3/0-0
Pyrolysis of benzene at 30 Torr was studied from 1360 K to 1820 K in this work. Synchrotron vacuum ultraviolet photoionization mass spectrometry was employed to detect the pyroly-sis products such as radicals, isomers and polycyclic aromatic hydrocarbons, and measure their mole fraction profiles versus temperature. A low-pressure pyrolysis model of benzene was developed and validated by the experimental results. Rate of production analysis was performed to reveal the major reaction networks in both fuel decomposition and aromatic growth processes. It is concluded that benzene is mainly decomposed via H-abstraction reaction to produce phenyl and partly decomposed via unimolecular decomposition reac-tions to produce propargyl or phenyl. The decomposition process stops at the formation of acetylene and polyyne species like diacetylene and 1,3,5-hexatriyne due to their high thermal stabilities. Besides, the aromatic growth process in the low-pressure pyrolysis of benzene is concluded to initiate from benzene and phenyl, and is controlled by the even carbon growth mechanism due to the inhibited formation of C5 and C7 species which play important roles in the odd carbon growth mechanism.
We investigate the fluorescence quenching of Rhodamine 6G (R6G), a well known laser dye with a high fluorescence quantum yield, by as-synthesized graphene oxide (GO) in aqueous solution, which is found to be rather efficient. By means of steady-state and time-resolved fluorescence spectroscopy combined with detailed analysis about the linear absorption variation for this R6G-GO system, the pertinent quenching mechanism has been elucidated to be a combination of dynamic and static quenching. Possible ground-state complexes between R6G and GO during the static quenching have also been suggested. Furthermore, the direction of photoinduced electron transfer between R6G and GO has been discussed.
Metastable 40Ar* atoms are produced in the two metastable states 3p54s[3/2]2 and 3p54s′[1/2]0 in a pulsed DC discharge in a beam, and are subsequently excited to the even-parity autoionizing resonance series 3p5np′ [3/2]1,2, 3p5np′ [1/2]1, and 3p5nf′ [5/2]3 using single photon excitation with a pulsed dye laser. The excitation spectra of the even-parity autoion-izing resonance series from the metastable 40Ar* are obtained by recording the autoionized Ar+ ions with time-of-flight ion detection in the photon energy range of 32500-35600 cm-1 with an experimental bandwidth of <0.1 cm-1. A wealth of autoionizing resonances are newly observed, from which more precise and systematic spectroscopic data of the level energies and quantum defects are derived.
In the wavelength range of 231-275 nm, we have studied the mass-resolved dissociation spectra of OCS+ via B2+←X23/2(000) and B2+←X21/2(000, 001) transitions by preparing OCS+ ions in the well-defined spin-orbit states. The spectroscopic constants ofυ1(CS stretch)=828.9 (810.4) cm-1, υ2(bend)=491.3 cm-1 and υ3(CO stretch)=1887.2 cm-1 for OCS+(B2+) are deduced. The observed dependence of the υ2(bend) mode excitation of B2+ on the spin-orbit splitting of X2∏(Ω=1/2, 3/2) in the B2+←X2∏ transition can be attributed to the K coupling between the (000)21/2 and (010)2+1/2 vibronic levels of X2∏ state, which makes the B2+(010)←X21/2(000) transition possible.
A new pulsed helium nano droplets machine has been constructed. The droplets were gener-ated by expansion of the pure helium through the cryogenic valve attached to a closed-cycle cryostat. The mean size of helium droplets can be controlled between 103 and 105 helium atoms by tuning the backing pressure (10-40 bar) and temperature (10-30 K). Compared with the continuous-flow beam source, the density of droplet is at least one order of magni-tude higher, which offers the opportunity to combine the system with the commercial pulsed laser to study chemical reactions inside of the superfluid helium at ultra-low temperature. The performance for the system has been checked by studying the photodissociation of CH3I doped droplets at 252 nm with the velocity map imaging technique. The photofragments, CH3, were detected by (2+1) resonance enhanced multiphoton ionization. The speed and angular distributions derived from resulting images show clear evidence of the relaxation effect by the surrounding helium atoms. The pulsed helium droplets depletion spectroscopy was also demonstrated. The depletion spectrum of benzene doped helium droplets indicatesthat less than 3% depletion can be observed with the newly constructed apparatus.
To understand better the molecular-level details of ≡Si+(SC) or ≡SiO-(SOA) ion group to -NH2 teminated poly(amido-amine) dendrimers in the gas phase, density functional theory is used to optimize the minimum energy and transition state structures with UB3LYP/6-311G(d) and HF/6-31G levels. The tertiary amine nitrogen and the amide oxygen are found to be the most favorable binding sites. The activation energies of the different active sites and the reaction steps of SC and/or SOA ion group and the amide sites are also analyzed. The stable compounds are formed via the electrostatic interaction and the coordination effect. The orientation of the amide O and the rotation of the branches minimizes the energy of the whole system.
To investigate the non-covalent interaction between cyclodextrins (CD) and lithium ion, a stoichiometry of α-CD, β-CD, heptakis(2,6-di-O-methyl)-β-CD (DM-β-CD), or heptakis(2,3,6-tri-O-methyl)-β-CD (TM-β-CD) was mixed with lithium salt, respectively, and then incubated at room temperature for 10 min to reach the equilibrium. In posi-tive mode, the electrospray ionization mass spectrometry (ESI-MS) results demonstrated that lithium ion can conjugate to α-, β-, DM-β- or TM-β-CD and form 1:1 stoichiometric non-covalent complexes. The binding of the complexes was further confirmed by collision-induced dissociation. The dissociation constants Kd1 of four complexes (Li+α-CD, Li+β-CD, Li+DM-β-CD, and Li+TM-β-CD) were determined by mass spectrometric titration. The results showed Kd1 were 18.7, 26.7, 33.6, 30.5 μmol/L for the complexes of Li+ with α-CD, β-CD, DM-β-CD, and TM-β-CD, respectively. Kd1 for the Li+ complexes of β-CD is smaller than that of DM-β-CD due to its steric effect of the partial substituted -CH3. The Kd1 for the Li+ complexes of DM-β-CD is nearly in agreement with that of TM-β-CD, indicating Li+ is more likely to locate in the small rim of DM-β-CD's hydrophobic cavity. The DFT results showed through electrostatic interaction, one Li+ can strongly conjugate to four neighboring oxygen atoms. For the (α-CD+Li)+ complex, one Li+ may also situate the small rim of α-CD's hydrophobic cavity to form a non-specific host-guest complex.
Adsorption reactions between surfaces of nanodiamond and nanosilica with diameter of 100 nm prepared as suspension solutions of 0.25 μg/μL and lysozyme molecule with different concentrations of 7 mmol/L PPBS at pH=7, 9, 11, and 13 have been investigated by fluores-cence spectroscopy. Adsorption reaction constants and coverages of lysozyme with different concentrations of 0-1000 nmol/L under the influences of different pH values have been ob-tained. Helicities and conformations of the adsorbed lysozyme molecules, free spaces of every adsorbed lysozyme molecule on the surfaces of nanoparticles at different concentrations and pH values have been deduced and discussed. The highest adsorption capabilities for both sys-tems and conformational efficiency of the adsorbed lysozyme molecule at pH=13 have been obtained. Lysozyme molecules can be prepared, adsorbed and carried with optimal activity and helicity, with 2 and 10 mg/m2 on unit nanosurface, 130 and 150 mg/g with respect to the weight of nanoparticle, within the linear regions of the coverages at around 150-250 nmol/L and four pH values for nanodiamond and nanosilica, respectively. They can be prepared inthe tightest packed form, with 20 and 55 mg/m2, 810-1680 and 580-1100 mg/g at threshold concentrations and four pH values for nanodiamond and nanosilica, respectively.
Magnetic and optical properties of ZnO co-doped with transition metal and carbon have been investigated using density functional theory based on first-principles ultrasoft pseudopoten-tial method. Upon co-doping with transition metal (TM) and carbon, the calculated results show a shift in the Fermi level and a remarkable change in the covalency of ZnO. Such cases energetically favor ferromagnetic semiconductor with high Curie temperature due to p-d exchange interaction between TM ions and holes induced by C doping. The total en-ergy difference between the ferromagnetic and the antiferromagnetic configurations, spatial charge and spin density, which determine the magnetic ordering, were calculated in co-doped systems for further analysis of magnetic properties. It was also discovered that optical prop-erties in the higher energy region remain relatively unchanged while those at the low energyregion are changed after the co-doping. These changes of optical properties are qualitatively explained based on the calculated electronic structure. The validity of our calculation in comparison with other theoretical predictions will further motivate the experimental inves-tigation of (TM, C) co-doped ZnO diluted magnetic semiconductors.
Developing photosensitizers suitable for the cobalt electrolyte and understanding the structure-property relationship of organic dyes is warranted for the dye-sensitized solar cells (DSSCs). The DSSCs incorporating tris(1,10-phenanthroline)cobalt(II/III)-based redox elec-trolyte and four synthesized organic dyes as photosensitizers are described. The photovoltaic performance of these dyes-sensitized solar cells employing the cobalt redox shuttle and the influences of the π-conjugated spacers of organic dyes upon the photovoltage and photocur-rent of mesoscopic titania solar cells are investigated. It is found that organic dyes with thiophene derivates as linkers are suitable for DSSCs employing cobalt electrolytes. DSSCs sensitized with the as-synthesized dyes in combination with the cobalt redox shuttle yield an overall power conversion efficiency of 6.1% under 100 mW/cm2 AM1.5 G illumination.
The mechanism and kinetics of electrocatalytic oxidation of formic acid at Pt electrodes is discussed in detail based on previous electrochemical in-situ ATR-FTIRS data [Langmuir 22, 10399 (2006) and Angewa. Chem. Int. Ed. 50, 1159 (2011)]. A kinetic model withformic acid adsorption (and probably the simultaneous C-H bond activation) as the rate determining step, which contributes to the majority of reaction current for formic acid oxi-dation, was proposed for the direct pathway. The model simulates well the IR spectroscopic results obtained under conditions where the poisoning effect of carbon monoxide (CO) is negligible and formic acid concentration is below 0.1 mol/L. The kinetic simulation predicts that in the direct pathway formic acid oxidation probably only needs one Pt atom as active site, formate is the site blocking species instead of being the active intermediate. We review in detail the conclusion that formate pathway (with either 1st or 2nd order reaction kinetics) is the direct pathway, possible origins for the discrepancies are pointed out.
n-Heptane is the most important straight chain paraffin in the fossil-fuel industry. In this work, pyrolysis behavior of n-heptane at high temperature is investigated by a se-ries of ReaxFF based reactive molecular dynamics simulations. Temperature effects on then-heptane pyrolysis and related products distributions have been detailedly analyzed. The simulation results indicate that the temperature effect is characterized in stages. High tem-perature can accelerate the decomposition of n-heptane, but the influence becomes small after it reaches a certain level. According to the different reaction behaviors, pyrolysis of n-heptane could be divided into three stages. The variation trends of the mass fraction evolu-tion of ethylene (C2H4), C3, and C4 calculated from reactive molecular dynamics simulations are in good agreement with the previous experimental results. The apparent activation en-ergy extracted from the first-order kinetic analysis is 53.96 kcal/mol and a pre-exponential factor is 55.34×1013 s-1, which is reasonably consistent with the experimental results.
Flowerlike LiFePO4 particles self-assembled by plate-like crystals with about 200 nm thickness were prepared by the poly(ethylene glycol)-assisted hydrothermal synthesis. Poly(ethylene glycol) in the hydrothermal system played an important role in reducingthe thickness of the plate-like LiFePO4 crystals as a co-solvent and forming the flower-like structure as a soft template. The flowerlike LiFePO4 exhibits high discharge capacity of 140 mAh/g and shows quite good cycling performance in the lithium-ion batteries. Con-sidering that the conductive carbon in the obtained LiFePO4 is negligible, the excellent cellperformance suggests that the flowerlike LiFePO4 is a promising cathode material for the lithium-ion batteries.
Monoclinic BiVO4 hollow nanospheres were successfully prepared via template-free method using citric acid (C6H8O7) as chelating agent and characterized by X-ray diffraction patterns, transmission electron microscope, UV-Vis DRS, and TG-DTA technique. C6H8O7 played an important role in the formation of hollow spheres. Morphology observations revealed that when appropriate amount ofC6H8O7 was introduced, the cavity with the diameter of 40 nm was obtained in BiVO4 nanospheres. UV-Vis diffuse reflectance spectra indicated that the samples had absorption in both UV and visible light region. The photocatalytic activities were evaluated by the degradation of methylene blue under Xe lamp irradiation. Hollow spheres endow BiVO4samples with greatly improved photocatalytic activity. A possible formation mechanism of hollow spheres was proposed.
Bimetallic CoCu nanocomposites were synthesized in polyol by using Ru as heterogeneous nucleation agent and stearic acid as surfactant, and their catalytic properties were investi- gated by hydrogenolysis of glycerol to propanediols. It was found that the surfactant could induce Co nanocrystals to form nanowires as structure-directing agent, while it's ineffective for Cu because only spherical Cu particles were produced under the same condition. When Co2+ and Cu2+ coexist in polyol, Cu2+ is firstly reduced and forms the spherical particles, and then the Cu particles afford surface for the subsequential reduction of Co2+ and growth of Co nanocrystals to form the nanorods, obtaining the urchin-like CoCu nanocomposites. The catalytic performance in selective hydrogenolysis of glycerol to propanediols proposed that the CoCu urchin-like nanocomposites was superior to the Co nanowires possibly due to that the synergistic effect between Co and Cu component promoted conversion of glyc-erol and obtained the higher propanediol yields based on the specific surface areas of the catalysts.
The conversion of cellulose to 5-hydroxymethylfurfural (HMF) has been investigated by a one-pot consecutive reaction. At first, cellulose was depolymerised into glucose via a fast degradation of cellulose in molten ZnCl2 in the presence of hydrochloric acid, and the yield of glucose is 75% in 120 s at reaction temperature of 95 oC. Then, DMSO was used as solvent and different kinds of metal chloride were added as catalysts, and the conversion was carried out continuously at 110-130 oC for 0.5-4 h. The yield of HMF was 53% when CrCl3 were used as catalyst. The one-pot two steps conversion was carried out at atmosphere pressure, and it is a simple route to prepare HMF from lignocellulosic feedstock on a large scale.
Dummy molecularly imprinted polymers (DMIPs) for 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) were produced using three structural analogues as dummy template molecules. The chosen analogues were 4-(acetymethylamino)-1-(3-pyridyl)-butanol, 4-(methylamino)-1-(3-pyridyl)-1-butanol, and 1-(3-pyridyl)-1,4,-butanediol. The molecular recognition characteristics of the produced polymers were evaluated by X-ray photoelec-tron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). Interactionsbetween NNAL and methacrylic acid should be cooperative hydrogen bonds while the ni-trogen atom of the pyridine ring and the oxygen atom of the nitroso group in NNAL are two of the hydrogen-bond acceptors. It was further demonstrated that DMIP synthesized by4-(acetymethylamino)-1-(3-pyridyl)-butanol had the best binding performance by XPS and FT-IR. Then dummy molecularly imprinted solid phase extraction (DMISPE) was developed for the determination of the analyte using the hit polymer as the sorbing material. Underoptimal conditions, the recovery of NNAL dissolved in standard solution reached 93%. And the investigated polymer exhibited much higher binding of NNAL when nicotine was acted as the competitive molecule. Also the proposed method was applied to the measurement of NNAL spiked in blank urine samples with recoveries ranging from 87.2% to 101.2%.