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PubMed2001

Ultrasonic degradation of trichloroethylene and chlorobenzene at micromolar concentrations: kinetics and modelling.

Although most papers in the field of sonochemical degradation of volatile organics in aqueous media describe experiments at the millimolar concentration range, this study focuses on the degradation kinetics of chlorobenzene (CB) and trichloroethylene (TCE) in the micromolar range. It was found that the reaction kinetics increase with decreasing initial substrate concentrations. For example, the pseudo-first-order reaction rate constant of CB increases by a factor of 14.3, if the initial concentration drops from 3440 to 1 microM. Previous work in the millimolar range has shown that the degradation of these volatiles is mainly due to pyrolytic reactions. The enhancement of the reaction kinetics at lower concentrations, in this work, could no longer be explained by this mechanism, even by taking into account the effect of the concentration of the solutes on the reaction temperature. Therefore, a new model was developed, incorporating gas phase OH radical induced degradation, next to pyrolysis. The model, fitting the experimental results, illustrated that at micromolar concentrations the OH radical induced degradation becomes significant. Simulations showed that at initial concentrations of CB > 1000 microM degradation is due to pyrolysis for over 99.97%, but it was also demonstrated that at concentrations between 1 and 5 microM, the OH radical mechanism contributed 48.5% of the total degradation.

Journal Article↗

PubMed2002

Sonolysis of surfactants in aqueous solutions: an accumulation of solute in the interfacial region of the cavitation bubbles.

The sonolysis of surfactants (such as sodium dodecylbenzenesulfonate (DBS), sodium dodecylsulfate (SDS), and polyethylene glycol monostearate), sodium 4-toluenesulfonate (STS), and 1-hexanol in aqueous solutions was investigated under an argon atmosphere with ultrasound of 200 kHz in order to compare the scavenging efficiency of the hydroxyl radical and the accumulation in the gas-liquid interfacial region of the cavitation bubbles. The degradation rate of the solute follows the order 1-hexanol > DBS and SDS > STS. The scavenging efficiency of the hydroxyl radical by non-volatile surfactants was much greater than that of the non-volatile and hydrophilic solute (e.g., STS). The surfactant was accumulated in a relatively high ratio in the interfacial region. The degradation of surfactants occurred by reaction with the hydroxyl radical and also by pyrolysis at high temperature. On the other hand, STS, due to its non-volatile and hydrophilic properties, was principally present in the bulk solution and the degradation by pyrolysis was not observed at the investigated concentration ranges.

Journal Article↗

PubMed2004

Sonochemistry of degrading p-chlorophenol in water by high frequency ultrasound.

The degradation process and mechanism of p-chlorophenol in water using high frequency ultrasound (1.7 mHz) was investigated in this paper. The p-chlorophenol was degraded successfully in the high frequency ultrasonic device with low consumption of energy. The degradation effect was improved by increasing the solution concentration. No products or intermediate products were detected in the reaction mixture by the analytic methods of MS and 1H-NMR after ultrasonic irradiation. The dominant degradation mechanism is high temperature pyrolysis in ultrasonic cavities rather than free radical oxidation, and the experimental results can be explained completely according to pyrolysis mechanism.

Chlorophenols↗

PubMed2003

Artifact production in the assay of anhydroecgonine methyl ester in serum using gas chromatography-mass spectrometry.

The detection of the pyrolysis product anhydroecgonine methyl ester (AEME, methylecgonidine) after cocaine smoking using gas chromatography-mass spectrometry is hampered by the artifactual production of AEME. The amount of AEME increases with the amount of cocaine used producing false positive values in authentic samples. A method for the correction of quantitative values was established using calibration of pyrolysis and estimation of the artifactual AEME. Authentic AEME in serum was differentiated from the artifact above 3.5 microg/l, 99% prediction limits of the quantitation were +/-3.1 microg/l. In 16 serum samples and five postmortem blood samples, cocaine and AEME were detected, but after application of the correction method only ten were truly positive for AEME.

Artifacts↗

PubMed2001

Electroanalytical performance of carbon films with near-atomic flatness.

Physicochemical and electrochemical characterization of carbon films obtained by pyrolyzing a commercially available photoresist has been performed. Photoresist spin-coated on to a silicon wafer was pyrolyzed at 1,000 degrees C in a reducing atmosphere (95% nitrogen and 5% hydrogen) to produce conducting carbon films. The pyrolyzed photoresist films (PPF) show unusual surface properties compared to other carbon electrodes. The surfaces are nearly atomically smooth with a root-mean-square roughness of <0.5 nm. PPF have a very low background current and oxygen/carbon atomic ratio compared to conventional glassy carbon and show relatively weak adsorption of methylene blue and anthraquinone-2,6-disulfonate. The low oxygen/carbon ratio and the relative stability of PPF indicate that surfaces may be partially hydrogen terminated. The pyrolyzed films were compared to glassy carbon (GC) heat treated under the same conditions as pyrolysis to evaluate the electroanalytical utility of PPF. Heterogeneous electron-transfer kinetics of various redox systems were evaluated. For Ru(NH3)6(3+/2+), Fe(CN)6(3-/4-), and chlorpromazine, fresh PPF surfaces show electron-transfer rates similar to those on GC, but for redox systems such as Fe3+/2+, ascorbic acid, dopamine, and oxygen, the kinetics on PPF are slower. Very weak interactions between the PPF surface and these redox systems lead to their slow electron-transfer kinetics. Electrochemical anodization results in a simultaneous increase in background current, adsorption, and electron-transfer kinetics. The PPF surfaces can be chemically modified via diazonium ion reduction to yield a covalently attached monolayer. Such a modification could help in the preparation of low-cost, high-volume analyte-specific electrodes for diverse electroanalytical applications. Overall, pyrolysis of the photoresist yields an electrode surface with properties similar to a very smooth version of glassy carbon, with some important differences in surface chemistry.

Journal Article↗

PubMed2001

On-line high-precision stable hydrogen isotopic analyses on nanoliter water samples.

We describe a new on-line chromium reduction technique for the measurement of stable hydrogen (deltaD) isotopes in waters using continuous-flow isotope ratio mass spectrometry. The on-line Cr reduction method has low intersample memory effects (< 1%) and excellent precision and accuracy for deltaD (+/-0.5% and was used to analyze waters samples as small as 50 nL. The on-line Cr method has a number of significant advantages over conventional offline Zn and U reduction and on-line carbon-based pyrolysis techniques. A single Cr reactor can be used to analyze approximately 1,000 water samples using an injection volume of 0.5 microL, with an individual sample analysis time of 4 min. Intersample memory effects are negligible. The Cr reactor temperature of 1050 degree C is easily attainable on standard elemental analyzers and so does not require the specialized and costly high-temperature furnaces of carbon-based pyrolysis reactors. Furthermore, hydrogen isotopes in extremely small water samples in the 100-nL range or less can be easily measured; hence, this new method opens up a number of exciting application areas in earth and environmental sciences, for example, natural abundance deltaD measurements of individual fluid inclusions in geologic materials using a laser source and measurements of body fluids in physiological and metabolic research.

Algorithms↗

PubMed2001

An in vivo probe based on mechanically strong but structurally small carbon electrodes with an appreciable surface area.

Physically small carbon electrodes were fabricated by pyrolyzing acetylene in a nitrogen atmosphere using pulled quartz capillaries as the supporting substrate. A carbon disk geometry was obtained when a parallel flow of acetylene (50 kPa) and nitrogen (10 mL min(-1)) was introduced into the system. Further, carbon was found to deposit at the tip and on the shank of the quartz capillaries when the nitrogen flow rate was increased (80 mL min(-1)), yielding an approximately cylindrical geometry. A series of electrochemical and spectroscopic analyses was carried out to examine the type of carbon surface obtained by pyrolysis of acetylene. The results suggested that a surface consisting of an almost defect-free highly oriented pyrolytic graphite type structure was formed by the pyrolyzed acetylene. However, this contradicts the kinetically reversible electron transfer observed for dopamine oxidation at these electrodes. Meanwhile, the nonpolar and relatively oxygen-free characteristics indicate that these electrodes also behave similarly to a hydrogenated carbon surface. The formation of a hydrogenated carbon-type surface may be plausible as a result of the attack on the carbon surface by a surplus of hydrogen produced by the pyrolysis of acetylene to form graphitic carbon. These characteristics are expected to aid in reducing electrode fouling, which is often encountered in electrochemical detection of neurotransmitters in vivo. In conjunction with a miniature physical dimension, their appreciable surface area and enhanced mechanical strength make these carbon electrodes well suited to the detection of neurotransmitters in vivo.

Acetylene↗

PubMed2004

Ultraflat carbon film electrodes prepared by electron beam evaporation.

A facile method for the preparation of thin-film carbon electrodes by electron beam evaporation onto highly doped silicon is presented. The physical and electrochemical properties of these films both before and after postdeposition pyrolysis are investigated. Raman spectroscopy establishes the amorphous structure of the nonpyrolyzed carbon films and confirms the formation of graphitic carbon after pyrolysis at 1000 degrees C. Scanning force microscopy reveals the root-mean-square roughness of nonpyrolyzed films to be approximately 1 A, while pyrolyzed films exhibit an increased roughness of approximately 4 A. The electrochemical behavior of the electrodes resembles glassy carbon, with measured heterogeneous electron-transfer rate constants among the highest measured for thin carbon films. These carbon film electrodes will potentially find applications in such fields as molecular electronics and scanning probe microscopy of adsorbed species.

Journal Article↗

PubMed1998

Direct mass spectrometric analysis of in situ thermally hydrolyzed and methylated lipids from whole bacterial cells.

Fatty acid methyl esters (FAMEs) were generated in situ, during pyrolysis, from whole-cell bacterial samples and analyzed by mass spectrometry (MS). The FAME profiles obtained by an in situ thermal hydrolysis methylation (THM) step were compared with gas chromatography (GC) and MS analyses of the chemically extracted and methylated fatty acids. This correlation was based on the ability of each technique to differentiate a representative group of 15 bacteria at the species level as predicted by principal component analysis. All three analyses, GC/FAME, pyrolysis-MS/FAME, and in situ THM-MS/FAME differentiated the studied bacterial sample set into three discrete clusters. The bacteria comprising each cluster were the same for all three analyses, showing that taxonomic information of the lipid profiles was preserved in the Py-MS/FAME and in situ THM-MS/FAME analyses of whole cells. Contributions from saturated, unsaturated, cyclopropyl, and branched bacterial fatty acids to the differentiation of microorganisms were identified for all three analyses. The in situ THM-MS/FAME approach is simple, requires small samples (approximately 2 x 10(6) cells/profile), and is rapid, with a total analysis time under 5 min/sample.

Esters↗

PubMed2005

Denitrification mechanism in combustion of biocoal briquettes.

Pulp black liquor (PBL), an industrial waste from paper production, has been previously shown to be an effective binder and denitrification agent for coal briquettes. This study investigated the denitrification mechanism of PBL in both the volatile combustion and char combustion stages of coal briquettes. X-ray diffraction and ion chromatography were used to analyze the residual ashes of combustion. The exhaust gas was analyzed by a flue gas analysis system and a Q-mass spectrometry system. The denitrification mechanism of PBL in the volatile combustion stage was found to result from the emission of NH3. The denitrification of PBL in the char combustion stage was associated with the NaOH contained in PBL. The direct reaction of NaOH with NO gas was examined, and some interesting phenomena were observed. Pure carbon or pure NaOH showed only limited reaction with NO. However, the mixture of NaOH and carbon (NaOH + C) significantly enhanced the reaction. This mixture increased the NO removal up to 100%. Subsequently, denitrification lasted for a long time period, with about 25% of NO removal. The pyrolysis characteristic of NaNO3, a compound resulting from denitrification, was also affected by the presence of carbon. In the presence of carbon, the NOx emission resulting from the pyrolysis of NaNO3 was reduced by a factor of 6. Since the denitrification phenomena appeared only in the absence of oxygen, a model of oxygen distribution in a burning coal briquette was employed to explain the reactions occurring in real combustion of coal briquettes.

Air Pollutants↗

PubMed2005

Thermal growth and decomposition of methylnaphthalenes.

Diesel particulate matter (DPM) has been recognized as carcinogenic due to its respirable sizes and toxic compositions. It is essential to understand its formation mechanisms to effectively reduce DPM emissions. Studies have indicated that resonance stabilize radicals can result in more soot and PAC (polycyclic aromatic compounds) formation in combustion processes. As the largest single polycyclic aromatic constituent in petroleum fuels, 1- and 2-methylnaphthalenes can readily form resonance-stabilized radicals in combustion, and yet the reaction mechanisms are not well understood. In this paper, product formation mechanisms of these compounds are experimentally investigated in a flow reactor. The pyrolysis of 1- and 2-methylnaphthalene was studied from 800 to 1000 degrees C, and the oxidation of 2-methylnaphthalene was studied from 650to 950 degrees C. The PACs were quantified by GC/MS and HPLC, and soot was quantified gravimetrically. Naphthalene is the most abundant product and is formed from methyl loss by H displacement. Isomerization in pyrolytic conditions results in the second most abundant product. The methyl radicals recombine with the reactants to form ethylnaphthalenes and dimethylnaphthalenes. 2-Naphthaldehyde is formed from the oxidation of the naphthylmethyl radicals. 1-Methylnaphthalene is of slightly higher reactivity in comparison to its isomer. The experimental results indicate that the PAC formation from the pyrolysis and oxidation of the two methylnaphthalenes are likely to be governed by similar mechanisms. The lack of carbon dimerization products in significant quantities may be due to their further growth into soot.

Air Pollutants↗

PubMed2004

Mechanisms and inorganic byproducts of trihalomethane compounds sonodegradation.

Organic pollutants can be sonodegraded by two pathways: pyrolysis, oxidation by free radicals, or the combination of both. The sonolytic degradation mechanisms and byproducts formation of aqueous trihalomethanes (THMs) were investigated at acoustic frequency of 20 kHz. The main sonodegradation mechanism of the chloroform, dichlorobromomethane, dibromochloromethane, and bromoform was found to be pyrolysis. The sonolysis degradation pathway of iodoform is free radicals oxidation. Hydrogen peroxide, nitrate, chloride, bromide, iodide, and iodate ions were detected and quantified as the inorganic products of the THMs sonication. A total of 48% TOC removal was achieved after a 180-min sonication of the THMs mixture.

Bromides↗

PubMed2006

An experimental and kinetic modeling study of the reaction of CHF3 with methane.

The gas-phase reaction of CHF3 with CH4 has been studied experimentally and computationally. The motivation behind the study is that reaction of CHF3 with CH4 provides a possible route for synthesis of CH2=CF2 (C2H2F2). Experiments are carried out in a plug flow, isothermal alpha-alumina reactor at atmospheric pressure over the temperature range of 973-1173 K. To assist in understanding the reaction mechanism and the role of the reactor material involved in the reaction of CHF3 with CH4, the reaction of CHF3 with CH4, pyrolysis of CH4, and pyrolysis of CHCIF2 have been studied in the presence of alpha-alumina or alpha-AIF3 particles under various conditions. Under all conditions studied for the reaction of CHF3 and CH4, the major products are C2F4, C2H2F2, and HF. Minor products include C2H2, C2H4, C2H3F, C2HF3, C3F6, CO2, and H2. C2H6, CH2F2, and CHF2CHF2 are detected in trace amounts. The initial step is the gas-phase unimolecular decomposition of CHF3, producing CF2 and HF. It is proposed that CF2 decomposes on the surface of alpha-alumina, producing F radicals that are responsible for the activation of CH4. A reaction scheme developed on the basis of the existing NIST HFC and GRI-Mech 3.0 mechanisms is used to model the reaction of CHF3 with CH4. Generally satisfactory agreement between experimental and modeling results is obtained on the conversion levels of CHF3 and CH4 and rates of formation of major products. Using the software package AURORA, the reaction pathways leading to the formation of major products are elucidated.

Chlorofluorocarbons, Methane↗

PubMed2002

Practical synthesis of an open geodesic polyarene with a fullerene-type 6:6-double bond at the center: diindeno[1,2,3,4-defg;1',2',3',4'-mnop]chrysene.

Diindeno[1,2,3,4-defg;1',2',3',4'-mnop]chrysene (1), the smallest possible alkene-centered C60 substructure with a curved pi-system, is obtained in 25-35% yield by flash vacuum pyrolysis of the twisted 1,1'-dibromobifluorenylidene (2) on a 100 mg scale at 1050 degrees C. At 1200 degrees C, the bowl-shaped hydrocarbon 1 rearranges to the planar isomer diindeno[5,6,7,1-defg;5',6',7',1'-lmnop]chrysene (14) by a double 5/6 ring-expansion/ring-contraction. X-ray crystallography establishes that the central carbon atoms of 1 are nearly 80% as pyramidalized as the carbon atoms of C60 (POAV angles = 9.0 degrees and 11.6 degrees for 1 and C60, respectively). A four-step synthesis has been developed to prepare the pyrolysis precursor (2) as a mixture of (E)- and (Z)-isomers in 39% overall yield from commercially available 9-fluorenone-1-carboxylic acid (10).

Journal Article↗

PubMed2002

Nanostructured carbon arrays from block copolymers of polyacrylonitrile.

Arrays of graphitic carbon nanoclusters were obtained by pyrolysis of nanoscale phase-separated block copolymers of polyacrylonitrile and poly(n-butyl acrylate). Upon heating in an inert atmosphere to temperatures ranging from approximately 400 to 1200 degrees C, polyacrylonitrile domains were converted into carbon nanoclusters, maintaining the overall shape and spacing, whereas the poly(n-butyl acrylate) phase was sacrificed. Preservation of the original nanoscale morphology of a block copolymer was possible only if pyrolysis was preceded by oxidation at temperatures of approximately 230 degrees C, in analogy with thermal stabilization of polyacrylonitrile precursor in the process used in the manufacturing of carbon fibers. Preorganization of the carbon precursor through self-assembly in block copolymers of polyacrylonitrile appears to be an attractive and robust strategy for templated synthesis of well-defined nanostructured carbon materials.

Journal Article↗

PubMed2002

Beyond the Roger Brown rearrangement: long-range atom topomerization in conjugated polyynes.

Long-range carbon atom topomerization in a 1,3-diyne has been demonstrated for the first time. 1-Phenyl-4-p-tolyl-1,3-butadiyne, (13)C-enriched at C-1, was synthesized and subjected to flash vacuum pyrolysis. At 800 degrees C and 0.01 Torr, this resulted in nearly complete (13)C label equilibration between C-1 and C-2, as seen by NMR analysis. Pyrolysis at 900 degrees C further led to ca. 35% of the label migrating about equally to C-3 and C-4. These results demonstrate that both intrabond and interbond atom exchange processes are operative, with the former having a lower activation barrier. DFT and Moller-Plesset calculations support a mechanism that passes through Brown rearrangement (1,2-shift), closure to trialene (bicyclo[1.1.0]-1,3-butadiene), bond-shift isomerization to exchange C-2 and C-3, and ring opening. The resulting vinylidene can rearrange to a butadiyne with the isotopic label at C-3 or C-4. Consistent with earlier calculations, trialene is predicted to have alternating peripheral bonds, with a weak central sigma bond and significant diradical character. Trialene is predicted [(B3LYP/6-311+G(2d,p)] to lie 64.6 kcal/mol above butadiyne, with barriers of 2.2 and 4.4 kcal/mol, respectively, for ring opening or bond-shift isomerization. Other potential rearrangement mechanisms which pass through tetrahedrene (E(rel) = 167.2 kcal/mol) or 1,2,3-cyclobutatriene (E(rel) = 161.1 kcal/mol) lie at much higher energies.

Journal Article↗

PubMed2003

Free-radical reactions under diffusional constraints: orientation does matter in hydrogen transfer.

Pyrolysis of silica-immobilized 1,3-diphenylpropane at 375 degrees C has been examined in the presence of a series of isomeric (by point of attachment) co-attached hydroaromatic spacer molecules. Under the diffusional constraints, the pyrolysis rate is sensitive to the orientation of the spacer molecule, which must transfer hydrogen to intermediate benzylic radicals on the surface. Spacer molecules possessing a meta-orientation of the benzylic hydrogens with respect to the surface linkage are able to attain a better geometry for the hydrogen transfer on the surface resulting in faster reaction rates.

Journal Article↗

PubMed2004

3,5-Pyridyne--a heterocyclic meta-benzyne derivative.

3,5-Pyridyne (3) has been generated by flash vacuum pyrolysis of 3,5-diiodopyridine (20) and 3,5-dinitropyridine (21) and characterized by IR spectroscopy in cryogenic argon matrices. The aryne can clearly be distinguished from other side products by its photolability at 254 nm, inducing a rapid ring-opening presumably to (Z)-1-aza-hex-3-ene-1,5-diyne. As byproducts of the pyrolysis, HCN and butadiyne were identified, together with traces of acetylene, cyanoacetylene, (E)-1-aza-hex-3-ene-1,5-diyne, and the 3-iodo-5-pyridyl radical (from 20). Several pathways for rearrangements and fragmentations of 3 and of the parent meta-benzyne (1) have been explored computationally by density functional theory and ab initio quantum chemical methods. The lowest energy decomposition pathway of biradicals 1 and 3 is a ring-opening process accompanied by hydrogen migration, leading to (Z)-hex-3-ene-1,5-diyne [(Z)-10] and (Z)-3-aza-hex-3-ene-1,5-diyne [(Z)-24], respectively. Both reactions require activation energies of 45-50 kcal mol(-1). Mechanisms leading from (Z)-24 or directly from 3 to the experimentally observed byproducts are discussed. Upon replacement of the C(5)H moiety by N in meta-benzyne, high-level calculations predict a modest shortening of the interradical distance by 5-7 pm and a reduction of the singlet-triplet energy splitting by 3 kcal mol(-1), in good agreement with isodesmic equations, according to which the singlet ground state of 3 is destabilized relative to 1 by 3-4 kcal mol(-1). In contrast to 3,5-borabenzyne (2), which is found to be doubly aromatic, nucleus-independent chemical shifts of 3 are almost identical to that of pyridine, indicating the absence of paramagnetic ring current effects that may be associated with "in-plane antiaromaticity". As compared with 1, the overall perturbation caused by the nitrogen atom in 3 is weak, and four electron, three center interaction is of minor importance in this molecule.

Journal Article↗