Gas-phase Reactions Of Radicals Research Articles

Mechanistic Insights into Radical-Induced Selective Oxidation of Methane over Nonmetallic Boron Nitride Catalysts

Boron-based nonmetallic materials (such as B2O3 and BN) emerge as promising catalysts for selective oxidation of light alkanes by O2 to form value-added products, resulting from their unique advantage in suppressing CO2 formation. However, the site requirements and reaction mechanism of these boron-based catalysts are still in vigorous debate, especially for methane (the most stable and abundant alkane). Here, we show that hexagonal BN (h-BN) exhibits high selectivities to formaldehyde and CO in catalyzing aerobic oxidation of methane, similar to Al2O3-supported B2O3 catalysts, while h-BN requires an extra induction period to reach a steady state. According to various structural characterizations, we find that active boron oxide species are gradually formed in situ on the surface of h-BN, which accounts for the observed induction period. Unexpectedly, kinetic studies on the effects of void space, catalyst loading, and methane conversion all indicate that h-BN merely acts as a radical generator to induce gas-phase radical reactions of methane oxidation, in contrast to the predominant surface reactions on B2O3/Al2O3 catalysts. Consequently, a revised kinetic model is developed to accurately describe the gas-phase radical feature of methane oxidation over h-BN. With the aid of in situ synchrotron vacuum ultraviolet photoionization mass spectroscopy, the methyl radical (CH3•) is further verified as the primary reactive species that triggers the gas-phase methane oxidation network. Theoretical calculations elucidate that the moderate H-abstraction ability of predominant CH3• and CH3OO• radicals renders an easier control of the methane oxidation selectivity compared to other oxygen-containing radicals generally proposed for such processes, bringing deeper understanding of the excellent anti-overoxidation ability of boron-based catalysts.

No Evidence for a Significant Impact of Heterogeneous Chemistry on Radical Concentrations in the North China Plain in Summer 2014

The oxidation of nitric oxide to nitrogen dioxide by hydroperoxy (HO2) and organic peroxy radicals (RO2) is responsible for the chemical net ozone production in the troposphere and for the regeneration of hydroxyl radicals, the most important oxidant in the atmosphere. In Summer 2014, a field campaign was conducted in the North China Plain, where increasingly severe ozone pollution has been experienced in the last years. Chemical conditions in the campaign were representative for this area. Radical and trace gas concentrations were measured, allowing for calculating the turnover rates of gas-phase radical reactions. Therefore, the importance of heterogeneous HO2 uptake on aerosol could be experimentally determined. HO2 uptake could have suppressed ozone formation at that time because of the competition with gas-phase reactions that produce ozone. The successful reduction of the aerosol load in the North China Plain in the last years could have led to a significant decrease of HO2 loss on particles, so that ozone-forming reactions could have gained importance in the last years. However, the analysis of the measured radical budget in this campaign shows that HO2 aerosol uptake did not impact radical chemistry for chemical conditions in 2014. Therefore, reduced HO2 uptake on aerosol since then is likely not the reason for the increasing number of ozone pollution events in the North China Plain, contradicting conclusions made from model calculations reported in the literature.

Online analysis of gas-phase radical reactions using vacuum ultraviolet lamp photoionization and time-of-flight mass spectrometry

A home-made vacuum ultraviolet photoionization time-of-flight mass spectrometer has been developed and coupled to an atmospheric simulation chamber operated at atmospheric pressure and to a fast flow tube at low pressure (1-10 Torr). Gas sampling from the simulation chamber is realized directly via a capillary effusive beam, and sampling from the flow tube is via a continuous molecular beam inlet. Both devices are connected simultaneously to the ionization chamber of the mass spectrometer and can be switched in-between within minutes to study gas-phase radical reactions of atmospheric interest in a large range of reaction conditions and reaction times (from milliseconds in the flow tube to hours in the simulation chamber). A cage-shaped photoionization source combined with a commercial 10.6 eV krypton lamp has been developed to provide a high ion collection efficiency along the long light path in the cage. This way, a multiplexed detection with high sensitivity down to the sub-parts per billion volume concentration range, e.g., a limit of detection of 0.3 ppbv with an accumulation time of 60 s for benzene and 1.3 ppbv for the methyl radical, is obtained. The performance and suitability of the setup are illustrated by the study of the chlorine-initiated oxidation reaction of toluene in the atmospheric simulation chamber and in the fast flow tube. Stable products and reactive intermediates have been well-determined and their reaction dynamics are discussed.

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection.

For reactive gas-phase environments, including combustion, extraterrestrials atmospheres and our Earth's atmosphere, the availability of quality chemical data is essential for predictive chemical models. These data include reaction rate coefficients and product branching fractions. This perspective overviews recent isomer-resolved production detection experiments for reactions of two of the most reactive gas phase radicals, the CN and CH radicals, with a suite of small hydrocarbons. A particular focus is given to flow-tube experiments using synchrotron photoionization mass spectrometry. Coupled with computational studies and other experiment techniques, flow tube isomer-resolved product detection have provided significant mechanistic details of these radical + neutral reactions with some general patterns emerging.

Radical Reactions in the Gas Phase: Recent Development and Application in Biomolecules

This review summarizes recent literature describing the use of gas phase radical reactions for structural characterization of complex biomolecules other than peptides. Specifically, chemical derivatization, in-source chemical reaction, and gas phase ion/ion reactions have been demonstrated as effective ways to generate radical precursor ions that yield structural informative fragments complementary to those from conventional collision-induced dissociation (CID). Radical driven dissociation has been applied to a variety of biomolecules including peptides, nucleic acids, carbohydrates, and phospholipids. The majority of the molecules discussed in this review see limited fragmentation from conventional CID, and the gas phase radical reactions open up completely new dissociation channels for these molecules and therefore yield high fidelity confirmation of the structures of the target molecules. Due to the extensively studied peptide fragmentation, this review focuses only on nonpeptide biomolecules such as nucleic acids, carbohydrates, and phospholipids.

Carbon and Hydrogen Isotopic Composition of Methane and C2+ Alkanes in Electrical Spark Discharge: Implications for Identifying Sources of Hydrocarbons in Terrestrial and Extraterrestrial Settings

The low-molecular-weight alkanes--methane, ethane, propane, and butane--are found in a wide range of terrestrial and extraterrestrial settings. The development of robust criteria for distinguishing abiogenic from biogenic alkanes is essential for current investigations of Mars' atmosphere and for future exobiology missions to other planets and moons. Here, we show that alkanes synthesized during gas-phase radical recombination reactions in electrical discharge experiments have values of δ(2)H(methane)>δ(2)H(ethane)>δ(2)H(propane), similar to those of the carbon isotopes. The distribution of hydrogen isotopes in gas-phase radical reactions is likely due to kinetic fractionations either (i) from the preferential incorporation of (1)H into longer-chain alkanes due to the more rapid rate of collisions of the smaller (1)H-containing molecules or (ii) by secondary ion effects. Similar δ(13)C(C1-C2+) and δ(2)H(C1-C2+) patterns may be expected in a range of extraterrestrial environments where gas-phase radical reactions dominate, including interstellar space, the atmosphere and liquid hydrocarbon lakes of Saturn's moon Titan, and the outer atmospheres of Jupiter, Saturn, Neptune, and Uranus. Radical recombination reactions at high temperatures and pressures may provide an explanation for the combined reversed δ(13)C(C1-C2+) and δ(2)H(C1-C2+) patterns of terrestrial alkanes documented at a number of high-temperature/pressure crustal sites.

Reactions of Hydrocarbon Radicals and Biradicals

Experimental measurements of the rate coefficients for several types of gas phase radical reactions are reviewed. They include radical isomerization and dissociation, radical + radical association, and unimolecular reactions of peroxy radicals. Some reactions of methylene in its lowest singlet state are also considered. The links to theories of chemical reaction rates and especially of capture rates for radical + radical reactions are examined. Many of the reactions involve formation of adducts, which can isomerize and dissociate. Such reactions frequently involve energy distributions of the adducts that are far from Boltzmann, and the interpretation of measurements requires the use of master equation techniques. The basis of these methods and the use of matrix diagonalization and eigenvector/eigenvalue analysis to extract phenomenological rate coefficients are discussed. The relevance of the measurements to applications in atmospheric chemistry and, especially, in combustion is briefly considered.

Halocarbons and other trace heteroatomic organic compounds in volcanic gases from Vulcano (Aeolian Islands, Italy)

Adsorbent-trapped volcanic gases, sublimates and condensates from active vents of the La Fossa crater on the island of Vulcano (Aeolian Islands, Italy) as well as ambient and industrial air were quantitatively analyzed by Short-Path Thermal Desorption–Solid Phase Microextraction–Cryotrapping–Gas Chromatography/Mass Spectrometry (SPTD–SPME–CF–GC–MS). Among the over 200 detected and quantified compounds are alkanes, alkenes, arenes, phenols, aldehydes, carboxylic acids, esters, ketones, nitriles, PAHs and their halogenated, methylated and sulfonated derivatives, as well as various heterocyclic compounds including thiophenes and furans. Most compounds are found at concentrations well above laboratory, ambient air, adsorbent and field blank levels. For some analytes (e.g., CFC-11, CH2Cl2, CH3Br), concentrations are up to several orders of magnitude greater than even mid-latitudinal industrial urban air maxima. Air or laboratory contamination is negligible or absent on the basis of noble gas measurements and their isotopic ratios.The organic compounds are interpreted as the product of abiogenic gas-phase radical reactions. On the basis of isomer abundances, n-alkane distributions and substitution patterns the compounds are thought to have formed by high-temperature (e.g., 900°C) alkyl free radical reactions and halide electrophilic substitution on arenes, alkanes and alkenes. The apparent abiogenic organic chemistry of volcanic gases may give insights into metal transport processes during the formation and alteration of hydrothermal ore deposits, into the natural volcanic source strength of ozone-depleting atmospheric trace gases (i.e., halocarbons), into possibly sensitive trace gas redox pairs as potential early indicators of subsurface changes on volcanoes in the state of imminent unrest, and into the possible hydrothermal origin of early life on Earth, as indicated by the presence of simple amino acids, nitriles, and alkanoic acids.

Catalytic Conversion of Methane to Methanol Using Cu-Zeolites

The conversion of methane to value-added liquid chemicals is a promising answer to the imminent demand for fuels and chemical synthesis materials in the advent of a dwindling petroleum supply. Current technology requires high energy input for the synthesis gas production, and is characterized by low overall selectivity, which calls for alternative reaction routes. The limitation to achieve high selectivity is the high C-H bond strength of methane. High-temperature reaction systems favor gas-phase radical reactions and total oxidation. This suggests that the catalysts for methane activation should be active at low temperatures. The enzymatic-inspired metal-exchanged zeolite systems apparently fulfill this need, however, methanol yield is low and a catalytic process cannot yet be established. Homogeneous and heterogeneous catalytic systems have been described which stabilize the intermediate formed after the first C-H activation. The understanding of the reaction mechanism and the determination of the active metal sites are important for formulating strategies for the upgrade of methane conversion catalytic technologies.

A kinetic model for the oxidative coupling of methane over Na2WO4/Mn/SiO2

A kinetic model of the oxidative coupling of methane was developed by considering ethane, ethylene, carbon monoxide, and carbon dioxide production in such a way that the reactor was designed for the reactions to take place in the catalyst-loading region as much as possible. The effectiveness of the reactor specification was validated by low conversion of methane in the blank test. A mechanism for catalytic reactions on the surface of Na2WO4/Mn/SiO2 gel and gas-phase radical reactions was suggested based on a literature survey. Reaction rates were developed by applying rapid equilibrium for adsorption and the quasi steady state approximation to the intermediate on the catalytic surface. Kinetic parameters were estimated by fitting experimental data with temperature, space velocity, and CH4/O2 ratio varied in full factorial manner. The validity of the model was corroborated by comparing its simulated results with experimental data. The effectiveness of the estimated parameters was discussed with respect to reported values. The effects of operating conditions were assessed using the developed model.

OH Radical Gas Phase Reactions with Aliphatic Ethers: A Variational Transition State Theory Study

A theoretical study of the mechanism and kinetics of the OH radical reactions with aliphatic ethers is presented. Several methods were evaluated using the 6-311++G(d,p) basis set, with dimethyl ether as a test molecule. On the basis of the dimethyl ether results, the M05-2X functional was selected for the rest of the calculations. All the possible H abstraction paths have been modeled, and the importance of differentiating among H atoms bonded to the same C atom, according to their orientation with respect to the O atom in the ether, is analyzed. The rate coefficients are calculated using interpolated variational transition-state theory by mapping (IVTST-M), within the IVTST-M-4/4 scheme, in conjunction with small-curvature tunneling (SCT) corrections. The discussion is focused on the 280-400 K temperature range, but additional information is provided for an extended range (280-2000 K). Our analysis suggests a stepwise mechanism involving the formation of H bonded complexes in the entrance and exit channels. The vicinity of the O atom was found to increase the relative site reactivity. In fact, it was found to influence reactivity to a larger extent than the nature of the carbon site (primary, secondary, or tertiary). The overall agreement between the calculated and the available experimental data is very good and supports the reliability of the rate coefficients and the branching ratios proposed here for the first time. It also supports the performance of the M05-2X functional and the IVTST-M-4/4 scheme for kinetic calculations.

Differences Between Al2O3 and Ag/Al2O3 for Lean Reduction of NO x with Dimethyl Ether

Lean reduction of NOx with DME occurs with high selectivity to N-2 over Al2O3 between 300 degrees C and 550 degrees C with a maximum of 47% at 380 degrees C, and with lower selectivity over Ag/Al2O3 between 250 degrees C and 400 degrees C due to the catalysts' sensitivity to gas phase radical reactions and activity for NOx reduction with methanol.

Generalized Topological Indices. Modeling Gas-Phase Rate Coefficients of Atmospheric Relevance

We develop the idea that the use of ad hoc molecular descriptors in QSAR/QSPR studies is not an optimal solution. Instead, we propose to optimize these descriptors for the specific properties under study. In the case of topological indices (TIs) we propose the use of the generalized topological indices (GTIs), which account for several of the classical TIs in one single graph invariant. GTIs represent points in a six-dimensional space of topological parameters, which can be optimized for describing a specific property. The situation shows some resemblance with the geometry optimization procedures used to minimize molecular energy. Here we study the reaction rate coefficients for the gas-phase radical reactions between alkanes and cycloalkanes with OH, Cl, and NO3 radicals. These reaction rate coefficients were studied recently by using some "classical" TIs, such as Balaban and Randić indices. Despite the fact that the Randić index produces acceptable QSPR models for some of these reactions, the models developed are still far from being optimal. Using the GTI approach we have improved these QSPRs by reducing the standard deviation by almost 50%. In addition, the current approach permits the illustration of the similarities and differences among the different descriptors studied, indicating possible directions for searching new optimal molecular descriptors.

Gas temperature and surface heating in plasma enhanced chemical-vapour-deposition

The gas temperature in plasma enhanced chemical-vapour-deposition is a key parameter to determine the properties of deposited films because it influences gas-phase reactions of radicals and surface reactions of precursors. In this paper, we report the measurement results of the gas temperature determined from optical emission spectroscopy of molecular spectra and discuss heating of the growing surface of the film. It is found that the gas temperature linearly increases with the plasma density and the surrounding wall temperature. The surface heating is dominated by collisions with high-temperature gas molecules in a plasma generated at a high gas pressure condition. The measurements are performed in a hydrogen diluted silane plasma generated by very-high-frequency discharge under a high-rate growth condition of hydrogenated micro-crystalline silicon for photovoltaic applications.

Molecular Origins of Selectivity in the Reduction of NOx by NH3

The fundamental principle underlying the selective catalytic reduction (SCR) of NOx to N2 is the promotion of reactions of reductant with NOx over competing, and thermodynamically preferred, reactions with a large excess of O2. A similar competition between NOx and O2 exists in the noncatalytic, thermal reduction of NOx with NH3. In this work, density functional theory calculations are used to elucidate the origins of the remarkable selectivity in thermal deNOx. Thermal deNOx is initiated by the conversion of NH3 into the active reductant, NH2 radical. NH2 radical reacts with NO at rates typical of gas-phase radical reactions to produce a relatively strongly bound H2NNO adduct that readily rearranges and decomposes to N2 and H2O. In contrast, NH2 radical reacts exceedingly slowly with O2: the H2N−OO adduct is weakly bound and more readily falls apart than reacts to products. The pronounced discrimination of NH2 against reaction with O2 is unusual behavior for a radical but can be understood through comparison of the electronic structures of the H2NNO and H2NOO radical adducts. These two key elements of thermal deNOxreductant activation and kinetic inhibition of reactions with O2are similarly essential to successful catalytic lean NOx reduction, and are important to consider in evaluating and modeling NOx SCR.

Recent progress in infrared absorption techniques for elementary gas-phase reaction kinetics.

Sensitive and precise measurements of rate coefficients, branching fractions, and energy disposal from gas-phase radical reactions provide information about the mechanism of elementary reactions as well as furnish modelers of complicated chemical systems with rate data. This chapter describes the use of time-resolved infrared laser absorption as a tool for investigating gas-phase radical reactions, emphasizing the exploitation of the particular advantages of the technique. The reaction of Cl atoms with HD illustrates the complementarity of thermal kinetic measurements with molecular beam data. Measurements of second-order reactions, such as the self-reactions of SiH3 and C3H3 radicals, and determinations of product branching fractions in reactions such as CN + O2 rely on the wide applicability of infrared absorption and on the straightforward relationship of absorption to absolute concentration. Finally, investigations of product vibrational distributions, as in the CN + H2 reaction, provide additional insight into the details of reaction mechanisms.

Oxidative dehydrogenation of light paraffins in novel short contact time reactors. Experimental and theoretical investigation

A previous study on the selective oxidation of propane over a commercial Pt/Al 2O 3 catalyst is herein extended to the oxidative dehydrogenation (ODH) of ethane. ODH tests were performed in an annular reactor, wherein catalytic experiments could be performed at varying temperatures on a wider operating field than that characteristic of autothermal reactors; besides, gas-phase experiments were performed for comparison. Together with the results of a theoretical analysis on homogeneous ethane oxidative pyrolysis, the bulk of data indicated that the high-temperature production of ethylene observed in the presence of the catalyst could be well explained by the single contribution of gas-phase radical reactions. The catalyst was proved to be active, instead, in the non-selective oxidation of paraffins to CO x . Further tests in an insulated millisecond contact time reactor demonstrated that the catalytic combustion reactions could be exploited to ignite the gas-phase reactions; comparison with the simulations of a purely homogeneous adiabatic reactor showed that at the highest temperatures the observed ethylene yields were close to the maximum obtainable values.

New data on gas-phase radical reactions in the steam reforming of methane in the presence of catalysts: I. Nickel catalysts

Methane pyrolysis and steam reforming were studied over a series of nickel catalysts (Ni-Al2O3, Ni/MgO, and Ni/LiAlO2) under the same conditions (650-750°C, PCH4 = 0.001-0.03 MPa). Unlike heterogeneous reaction of pyrolysis, some of the steps of steam reforming of methane occur in the gas phase. When gasphase reactions were suppressed, the rate and activation energy of steam reforming are close to the corresponding kinetic characteristics for pyrolysis. Hypothetically, the rate-limiting step of the process is the dissociative adsorption of methane on nickel in this case.

An Experimental and Theoretical Study of Gaseous Products in the Radiolysis of Germane/Ethylene Mixtures

Gas-phase radical reactions of germane/ethylene mixtures have been investigated using theoretical calculations and radiolysis techniques. Ab initio quantum chemical calculations have been performed on reactions starting from GeHx (x = 0–3) radicals and C2H4. The geometrical structure and relative stability of isomeric GeC2Hn (n = 4–7) radicals have been investigated and restricted to the species in which a C–C bond is present, at the MP2(FROZEN) level of calculation with double-ζ quality basis sets. To better evaluate the relative stability of the investigated species, single-point calculations at the QCISD(T) level were performed with a 6-311G(3df,2p) basis set. Reaction enthalpies and heats of formation of the species have also been determined using theoretical calculations. From irradiation of GeH4/C2H4 mixtures, condensed products and volatile species were obtained. Composition, amount, and characteristics of the condensed phase products are reported. The volatile compounds were identified and their amounts determined. From experimental results and theoretical calculations some hypotheses on the reaction mechanisms are presented.

Rate constants of OH radical reactions in gas phase with some fluorinated compounds: A correlation with molecular parameters

For a number of hydrofluorocarbons (HFCs), EHez has been found to have a linear correlation with each of the following: (i) log (k/n); (ii) A/n; and (iii) Ea/R, where EH = HOMO energy of the molecule, z = average fractional positive charge on the abstractable hydrogen atom in the molecule, k = rate constant of the gas-phase H abstraction reaction of the molecule with OH radical at 298 K, n = number of abstractable H atoms in the molecule, A = preexponential factor, and Ea/R = activation temperature of the said reaction. These correlations have been used to estimate the temperature dependent rate constants for the reactions of OH radical with CF3CF2CH2CH2CF2CF3, CF3CH2CF2CH2CF3, CF3CF2CH2CH2F, CF3CH2CH3, CF3CH2CHF2, CF3CHFCH2F, and CHF2CHFCHF2 as {6.97 × 10−13 exp(1481/T)}, {5.43 × 10−13 exp(1754/T)}, {7.95 × 10−13 exp(l308/T)}, {8.0 × 10−13 exp(1300/T)}, {7.03 × 10−13 exp(1470/T)}, {7.33 × 10−13 exp(1417/T)}, and {8.09 × 10−13 exp(1285/T)}, respectively. These have not yet been measured experimentally. Linear correlation between EHez and log (k/n) has also been observed for nine halogen substituted acetaldehydes. On the other hand, EH is found to have a better linear correlation with log (k/n) than EHez in the case of fluorinated ethers and alcohols where the available experimental data are at present limited. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 187–194, 1997.