Oxidation Of Acetylene Research Articles

Formation of soot particles in pyrolysis and oxidation of aliphatic and aromatic hydrocarbons: Experiments and detailed kinetic modeling

Experiments on pyrolysis and oxidation of rich mixtures of various aliphatic and simple aromatic hydrocarbons in reflected shock waves have been performed. The mixtures C2H2/Ar, C2H6/Ar, C2H4/Ar, C2H4/O2/Ar, CH4/Ar, CH4/O2/Ar, C3H8/Ar, C3H6/Ar, toluene/Ar, and benzene/Ar were studied. The yield of soot and the temperature of soot particles were determined experimentally by the double-beam absorption emission method. The kinetic model of soot formation during the pyrolysis and oxidation of rich mixtures of aliphatic and aromatic hydrocarbons complemented with a set of nucleations of soot particles from both polyaromatic fragments and unsaturated aliphatic hydrocarbons was suggested. This kinetic model of soot formation was successfully tested. It describes the experimental literature data on the yield of the products of pyrolysis and oxidation of acetylene and diacetylene in a shock tube. The results of our experiments and kinetic calculations of the time, temperature, and concentration dependences are in good agreement for all hydrocarbons under study. All the kinetic parameters of the model remained strictly constant.

Influence of dimethyl ether addition on the oxidation of acetylene in the absence and presence of NO

Dimethyl ether (DME) is a promising diesel fuel additive for reducing soot and NOx emissions, because of its interesting properties and the possibility of a renewable production. An experimental and modeling study of the oxidation of acetylene (C2H2, considered as an important soot precursor) and DME mixtures has been performed under well-controlled flow reactor conditions. The influence of temperature, air excess ratio (λ) and presence of NO on the oxidation process has been analyzed. Under fuel-rich conditions, the presence of DME in these mixtures modifies the radical pool delaying the acetylene consumption. C2H2 and DME, and the radicals generated in their conversion, interact with NO achieving different levels of NO concentration diminution depending upon the operating conditions. Under fuel-lean conditions, the presence of DME in the mixtures increases the NO diminution, whereas for the other values of λ considered, the maximum NO decrease reached is lower than that obtained in the case of pure acetylene.

Inhibiting the combustion of air–acetylene mixtures

The effect propane, methane, and a mixture of 18 vol % С3Н6–40 vol % С3Н8–42 vol % С4Н10 have on the combustion of air–acetylene mixtures is investigated experimentally. The upper concentration limit of flame propagation, maximum explosion pressure, and maximum rate of rise of explosion pressure are determined. It is found that propane and a mixture of 18 vol % С3Н6–40 vol % С3Н8–42 vol % С4Н10 are strong inhibitors of combustion of acetylene in its concentration ranges of 2–8 vol %. The inhibition effect becomes weaker as the acetylene content in the mixture increases. It disappears completely at C2H2 concentrations exceeding 15 vol %. The above experimental findings are explained using the proposed scheme of acetylene oxidation.

Modeling of NO formation in low pressure premixed flames

In this paper a detailed sub-mechanism NOMecha2.0 for NOx chemistry at high temperature is validated on a large experimental database obtained in flames, a jet-stirred reactor (JSR) and a plug-flow reactor under sub-atmospheric and atmospheric conditions. This sub-mechanism mainly based on the one implemented in the GDFkin®3.0_NCN model for prompt-NO formation has been thoroughly updated. Prompt-NO formation results from the reaction between CH and N2 yielding NCN and H. Then, NCN radicals react with H and O-atoms yielding HCN + N and CN + NO, respectively. Care was taken in the choice of the enthalpy of formation of the N-species, especially for NCN radicals for which the enthalpy values reported in the literature differ by more than 10 kJ mol−1. Critical rate constant values for the two main reactions (CH + N2 → NCN + H and NCN + H → HCN + N) and the branching ratio of the reactions NCN + H → products are discussed. Validation focuses mainly on prompt-NO formation in low pressure flames stabilized with different fuels (CH4, C2H6, C3H8 and C2H2) using data from the literature. It draws in particularly on the extended experimental database, including species profiles (CH, NCN, CN, HCN, NCO and NO) previously obtained in Lille in CH4 and C2H2 flames, which was extended in this work. Validation also considers experiments dedicated to HCN oxidation in a JSR or in flames, and to HNCO oxidation performed in a plug-flow reactor. In order to improve the prediction of HNCO, HCN/HNC isomerization has been implemented in NOMecha2.0. Finally the NOMecha2.0 sub-mechanism was satisfactorily coupled to a detailed chemical mechanism from the literature dedicated to the oxidation of acetylene at high temperature to predict NO formation.

Study of acetylene poisoning of Pt cathode on proton exchange membrane fuel cell spatial performance using a segmented cell system

Acetylene is a welding fuel and precursor for organic synthesis, which requires considering it to be a possible air pollutant. In this work, the spatial performance of a proton exchange membrane fuel cell exposed to 300 ppm C2H2 and different operating currents was studied with a segmented cell system. The injection of C2H2 resulted in a cell performance decrease and redistribution of segments' currents depending on the operating conditions. Performance loss was 20–50 mV at 0.1–0.2 A cm−2 and was accompanied by a rapid redistribution of localized currents. Acetylene exposure at 0.4–1.0 A cm−2 led to a sharp voltage decrease to 0.07–0.13 V and significant changes in current distribution during a transition period, when the cell reached a voltage of 0.55–0.6 V. A recovery of the cell voltage was observed after stopping the C2H2 injection. Spatial electrochemical impedance spectroscopy (EIS) data showed different segments' behavior at low and high currents. It was assumed that acetylene oxidation occurs at high cell voltage, while it reduces at low cell potential. A detailed analysis of the current density distribution, its correlation with EIS data and possible C2H2 oxidation/reduction mechanisms are presented and discussed.

Investigation of secondary formation of formic acid: urban environment vs. oil and gas producing region

Abstract. Formic acid (HCOOH) is one of the most abundant carboxylic acids in the atmosphere. However, current photochemical models cannot fully explain observed concentrations and in particular secondary formation of formic acid across various environments. In this work, formic acid measurements made at an urban receptor site (Pasadena) in June–July 2010 during CalNex (California Research at the Nexus of Air Quality and Climate Change) and a site in an oil and gas producing region (Uintah Basin) in January–February 2013 during UBWOS 2013 (Uintah Basin Winter Ozone Studies) will be discussed. Although the VOC (volatile organic compounds) compositions differed dramatically at the two sites, measured formic acid concentrations were comparable: 2.3 ± 1.3 in UBWOS 2013 and 2.0 ± 1.0 ppb in CalNex. We determine that concentrations of formic acid at both sites were dominated by secondary formation (> 99%). A constrained box model using the Master Chemical Mechanism (MCM v3.2) underestimates the measured formic acid concentrations drastically at both sites (by a factor of > 10). Compared to the original MCM model that includes only ozonolysis of unsaturated organic compounds and OH oxidation of acetylene, when we updated yields of ozonolysis of alkenes and included OH oxidation of isoprene, vinyl alcohol chemistry, reaction of formaldehyde with HO2, oxidation of aromatics, and reaction of CH3O2 with OH, the model predictions for formic acid were improved by a factor of 6.4 in UBWOS 2013 and 4.5 in CalNex, respectively. A comparison of measured and modeled HCOOH/acetone ratios is used to evaluate the model performance for formic acid. We conclude that the modified chemical mechanism can explain 19 and 45% of secondary formation of formic acid in UBWOS 2013 and CalNex, respectively. The contributions from aqueous reactions in aerosol and heterogeneous reactions on aerosol surface to formic acid are estimated to be 0–6 and 0–5% in UBWOS 2013 and CalNex, respectively. We observe that air–snow exchange processes and morning fog events may also contribute to ambient formic acid concentrations during UBWOS 2013 (~ 20% in total). In total, 53–59 in UBWOS 2013 and 50–55% in CalNex of secondary formation of formic acid remains unexplained. More work on formic acid formation pathways is needed to reduce the uncertainties in the sources and budget of formic acid and to narrow the gaps between measurements and model results.

Proton exchange membrane fuel cell cathode contamination – Acetylene

Acetylene adsorption on PEMFC electrodes and contamination in single cells are investigated with 300 ppm acetylene at a cathode held at 80 °C. The results of adsorption experiments suggest that acetylene adsorbs readily on electrodes and is reduced to ethylene and ethane under an open circuit potential of H2/N2, as the adsorbates can be electro-oxidized at high potentials. The cell voltage response shows that 300 ppm acetylene results in a cell performance loss of approximately 88%. The voltage degradation curve is divided into two stages by an inflection point, which suggests that potential-dependent processes are involved in acetylene poisoning. These potential-dependent processes may include acetylene oxidation and reduction as well as accumulation of intermediates on the electrode surface. Electrochemical impedance spectroscopy analysis suggests that acetylene affects the oxygen reduction reaction and may also affect mass transport processes. Acetylene also may be reduced in the steady poisoning state of the operating cell. After neat air operation, the cyclic voltammetry results imply that the cathode catalyst surface is almost completely restored, with no contaminant residues remaining in the MEA. Linear scanning voltammetry measurements show no change in hydrogen crossover caused by contamination, and polarization curves confirm complete recovery of cell performance.

Soot formation during the pyrolysis and oxidation of acetylene and ethylene in shock waves

Experimental and computational studies of soot formation during the pyrolysis and oxidative pyrolysis of hydrocarbons with multiple bonds have been performed on ethylene and acetylene as examples. The experiments have been carried out behind reflected shock waves over the temperature range from 1700 to 2800 K at a pressure of 3 to 5 bar. The process of soot formation was recorded using the absorption-emission method, which enables one to simultaneously measure the mass of soot particles per unit volume and their temperature. The experimental data are interpreted within a new kinetic mechanism that takes into account the nucleation of soot particles from both polyaromatic hydrocarbon fragments and unsaturated aliphatic hydrocarbons. The proposed kinetic model of soot formation closely reproduces the time dependence of the soot yield and the temperature of soot particles measured in the present work, as well as the concentrations of several key components formed in the early stages of pyrolysis and oxidation of acetylene and ethylene, reported in a number of works by other authors.

Catalytic complete oxidation of acetylene and propene over clay versus cordierite honeycomb monoliths without and with chemical vapor deposited cobalt oxide

The catalytic oxidation of acetylene (C2H2) and propene (C3H6), selected as model unsaturated hydrocarbons, was investigated over uncoated- and Co3O4-coated natural clay honeycomb monoliths and their performance was compared with commercial cordierite. Co3O4 was deposited using chemical vapor deposition (CVD) and chosen based on its recently demonstrated high activity. The catalytic behavior of the clay and cordierite-based monolithic samples has been investigated for the determination of the temperature-dependent conversion yield, specific reaction rate and related activation energy. The obtained results revealed a promising intrinsic catalytic performance of the uncoated clay monoliths. Furthermore, a synergetic interaction between the clay and the Co3O4 film has enabled an improved performance relative to the coated cordierite. The present paper reveals a promising potential application of the natural clay as catalyst and/or catalyst-support through the association of the advantages offered by the chemical composition and honeycomb monolithic structure of clay.

Experimental and kinetic studies of acetylene flames at elevated pressures

The kinetic effects of CO2 and H2O dilution on the laminar flame speed of acetylene at elevated pressures have been investigated experimentally using outwardly propagating spherical flames in a nearly constant pressure chamber from 1 to 20atm. The flame speeds of C2H2/air mixtures at atmospheric pressure agree with recent measurements reasonably well. Detailed analysis on the combustion chemistry of acetylene reveals that C2H2+O, HCCO+O2, HCO+O2, CH3+HO2, H+C2H3, CO+OH, CH2(S)+C2H2, and HCO decomposition are among the most important reactions, which leads to a new kinetic model (HP Mech) that incorporates the recent understanding of elementary reactions. The effects of CO2 dilution on acetylene flame speeds are found to be small for both fuel rich and lean conditions due to the direct CO2 formation pathway (HCCO+O2) in acetylene oxidation. Water dilution effects are more pronounced, especially at lean conditions, because the radical pool composition is altered by shifting the equilibrium of H2O+O=OH+OH. Comparing to USC Mech II, HP Mech has much better performance compared to the current experimental measurements as well as the shock tube and flow reactor data.

A Simplified Global Model to Describe the Oxidation of Acetylene Under Nanosecond Pulsed Discharges in a Complex Corona Reactor

The objective of this paper is to analyse the oxidation of acetylene under nanosecond pulsed N2/O2 discharges generated in a complex multi-pin-to-plane (MPP) corona reactor in the frame of Yan’s generic chemical kinetic model. We made use of the results obtained from the detailed kinetic model published previously (Redolfi et al. in Plasma Chem Plasma Process 29(3):173–195, 2009) in order to propose a global reactor models based on Yan’s generic chemical model and taking into account the non-homogeneous and non-stationary character of the discharges. This enables us expressing the energy cost in terms of physical and kinetic parameters of the discharge. We checked the model validity by comparing predicted and measured energy cost-values for acetylene in MPP reactor. The methodology presented may be adapted to predict the energy cost in other complex corona reactor provided the model parameters are determined experimentally.

Vibrational Excitation Can Control Tropospheric Chemistry

Which products form during the oxidation of acetylene by hydroxyl radicals and oxygen depends on the internal excitation of a radical intermediate.

Interception of Excited Vibrational Quantum States by O 2 in Atmospheric Association Reactions

Bimolecular reactions in Earth's atmosphere are generally assumed to proceed between reactants whose internal quantum states are fully thermally relaxed. Here, we highlight a dramatic role for vibrationally excited bimolecular reactants in the oxidation of acetylene. The reaction proceeds by preliminary adduct formation between the alkyne and OH radical, with subsequent O(2) addition. Using a detailed theoretical model, we show that the product-branching ratio is determined by the excited vibrational quantum-state distribution of the adduct at the moment it reacts with O(2). Experimentally, we found that under the simulated atmospheric conditions O(2) intercepts ~25% of the excited adducts before their vibrational quantum states have fully relaxed. Analogous interception of excited-state radicals by O(2) is likely common to a range of atmospheric reactions that proceed through peroxy complexes.

The Iron‐Catalyzed Oxidation of Alkynes—1,2‐Dione Formation Versus Oxidative Cleavage—A Matter of Temperature

AbstractThe iron‐catalyzed oxidation of a variety of alkynes with hydrogen peroxide as the oxidant was examined in detail. Various reaction parameters were investigated in the oxidation of diphenyl acetylene. The best catalytic performance was observed for a combination of FeCl3 with 4‐dimethylamino pyridine (DMAP) as the ligand, yielding various 1,2‐diones in excellent yields and selectivities at room temperature. Moreover, by subsequently increasing the reaction temperature to 60 °C, a selective oxidative cleavage was observed, resulting in carboxylic acid derivatives.

Electrochemical acetylene sensor based on Au/MWCNTs

Gold nanoparticles supported on multi-walled carbon nanotubes (Au/MWCNTs) were prepared as electrocatalysts for an electrochemical sensor of acetylene. The catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical experiments showed that the catalysts exhibited a distinctly higher activity for the acetylene electro-oxidation than Au-NPs and Au/C catalysts. In typical cyclic voltammograms, the onset potential of Au/MWCNTs for the acetylene oxidation is 0.73 V, which is 0.19 V more negative than that of Au-NPs. The acetylene sensor test showed good linear response towards acetylene concentration in a wide range from 10 ppm to 50 ppm with a steady response current for 300 s, good sensitivity of 80 nA/ppm and fast 90% response time ( t 90) of 25 s to 50 ppm acetylene. All the results indicate a good potential application of this sensor in the detection of acetylene.

Effect of operating conditions on NO reduction by acetylene–ethanol mixtures

An experimental and theoretical study of the influence of different operating conditions on NO reduction by acetylene–ethanol mixtures has been carried out. The present investigation includes the evaluation of the impact of adding different amounts of ethanol for NO reduction by an acetylene–ethanol mixture, and the evaluation of the capacity of acetylene–ethanol mixtures to reduce different amounts of NO. The experiments were conducted in a quartz flow reactor at atmospheric pressure in the 775–1375 K temperature range operating under different oxygen concentrations, from fuel-rich to fuel-lean conditions, considering the influence of these operating variables. The experimental results have been simulated and interpreted in terms of a literature detailed gas-phase kinetic mechanism. The present results show that ethanol addition to acetylene shifts the onset for NO consumption to higher temperatures; however, the maximum NO reduction levels reached by pure acetylene oxidation are not significantly modified by its addition. The initial concentrations of NO and oxygen are important parameters affecting NO reduction by acetylene–ethanol mixtures oxidation, since they are closely coupled to the fate of hydrocarbon radicals responsible of NO consumption.

Role of support in the oxidation of acetylene over gold catalysts

The influence of the carrier (CeO 2, TiO 2 or ZrO 2) on the reaction of acetylene oxidation over a 2% gold catalyst has been investigated in the presence or in the absence of CO. XPS reveals that gold is metallic in every case and the percentage of gold on the surface is always higher than that in the bulk confirming the wide dispersion as evaluated by TEM. There is a significant effect of the support on the activity in the absence of CO, but the presence of CO affects the reactivity differently according to the support. In a mixture of CO and C 2H 2, the oxidation of CO is systematically inhibited by C 2H 2 whereas the behaviour is completely different for the oxidation of C 2H 2. CO has no effect upon Au/CeO 2, acts as an inhibitor with Au/TiO 2 and as a promoter with Au/ZrO 2. Kinetic study reveals that the order towards C 2H 2 remains unchanged whereas the order towards O 2 clearly depends upon the support. These effects on activity and kinetics may result from various factors discussed in this paper.

Photocatalytic efficiencies of self-cleaning glasses. Influence of physical factors

Two commercial types of self-cleaning glass (SCG) have been tested to confirm the real photocatalytic nature of their properties. This was done by using four photocatalytic tests: (i) in the gas phase with the total oxidation of acetylene; (ii) in water with the total degradation of malic acid, (iii) in water with the total degradation of methylene blue, and (iv) in the solid phase with the total oxidative degradation of a layer of stearic acid deposited on the self-cleaning glass surface, in contact with the superficial titania coating. The influence of various factors (temperature, humidity, wavelength, radiant flux, presence of inorganic particles stuck at the glass surface) was explained in line with the fundamentals of photocatalysis. The results helped to understand the behaviour of self-cleaning glass.

Ignition times for a branched-chain thermal explosion chemistry with heat loss

The ignition times for a branched-chain thermal explosion involving simplified initiation, branching, and termination of chains, as well as heat exchange with the surroundings, but assuming that consumption of fuel is negligible for the combustion of acetylene oxidation chemistry that covers nth Arrhenius kinetics for the chain-branching step, is studied. An approximate solution is presented in the form of temperature as a function of time in terms of the physical parameters in the special case when the order of branching reaction is zero. The effect of heat loss on the problem studied is addressed numerically and we showed different qualitative effects of varying the dimensionless parameters.

Clusters as model systems for investigating nanoscale oxidation catalysis

Gas-phase cluster reactivity experiments, combined with theoretical calculations, provide insight into how factors such as size, stoichiometry and charging effects influence reactions promoted by metal and metal oxide catalysts. In this Letter, we describe the use of clusters to investigate the role of different types of oxygen centers in catalytic oxidation reactions. First, we focus on recent results from our laboratory concerning the oxidation of carbon monoxide by gold clusters and how the charge state of the cluster determines the mechanism of oxidation. We then discuss how radical oxygen centers in zirconium oxide clusters promote the oxidation of carbon monoxide, ethylene and acetylene and how charge state may be used to tune the selectivity of these reactions. An overview of future directions for the research is given in conclusion.