Oxidation Of Acetylene Research Articles

Elucidating high-pressure chemistry in acetylene oxidation: Jet-stirred reactor experiments, pressure effects, and kinetic interpretation

Acetylene plays a crucial role as an intermediate in the combustion of complex hydrocarbons, such as, e.g., jet fuels. In this study, high-pressure acetylene oxidation has been investigated by a combination of kinetic and experimental methods using a jet-stirred reactor (JSR) in the range of 497–910 K at p = 24 atm. The study covered three fuel-equivalence ratios: φ = 0.5 (fuel-lean), φ = 1.0 (stoichiometric), and φ = 3.0 (fuel-rich). Mole fraction profiles of 10 species, including CO, CO2, CH4, C2H4, C2H6, C3H6, C3H8, CH3OH, CH3CHO, and C3H4O, were identified and quantified using gas chromatography (GC) and gas chromatography-mass spectrometry (GC–MS). A kinetic model for describing the high-pressure chemistry of acetylene oxidation is proposed, which well characterizes important experimental findings, such as the fuel oxidation reactivity and the speciation of crucial products. At high pressures (p = 24 atm), acetylene exhibits a higher fuel consumption than at lower pressures (p = 1 and 12 atm) because of the increased sensitivity of dehydrogenation reactions by OH radicals accelerating the oxidation of the fuel at low temperatures. Furthermore, fuel-specific intermediates are observed, including acetaldehyde, propanal, small alkanes, and alkenes. These species mainly result from H-abstraction reactions by OH following O2-addition reactions from triple carbon bond moieties in acetylene. The formation of further products, such as carbon monoxide and carbon dioxide, is closely related to the consumption of these fuel-specific species. In particular, the formation of aromatics, such as e.g., benzene and toluene, are detected at trace levels in the current experiments due to the rapid formation and decomposition process occurring at high pressures. By analyzing the distinct kinetic behavior, it was found that acetylene was almost completely depleted at higher system pressure (p = 24 atm). Some intermediates are rather active and can react with O2 and peroxides. Consequently, the high-pressure oxidation of acetylene mainly proceeds along the pathway of CC → CHCHOH → HOCHO/OCHCHO → CO → CO2 at the high-pressure chemistry (p = 24 atm). Overall, this study provides valuable insights into the pressure-dependent combustion behavior of acetylene and its implications for optimizing jet fuel combustion processes.

Mode-Specific Dynamics Studies for the Multichannel C2H2 + OH Reaction.

The C2H2 + OH reaction is a key elementary reaction in acetylene oxidation, and the products forming in different reaction channels, such as C2H and CH3 radicals, are also important for subsequent reaction processes in the combustion process. In this work, we investigated the dynamics of the C2H2 + OH reaction with specific vibrational mode excitations and analyzed the mode specificity based on quasi-classical trajectory calculations on a recently developed full-dimensional potential energy surface. It is found that exciting OH stretching mode can promote the production of H + OCCH2 and CO + CH3, while the excitation of C-H symmetric/antisymmetric stretching mode of C2H2 can facilitate the H2O + C2H channel. Based on the prediction of vibrationally adiabatic and sudden vector projection models, the mode specificity in the C2H2 + OH reaction can be attributed to the difference in the degree of coupling between the initial motion mode and the reaction coordinate of each reaction path, which ultimately leads to the changes in rate constants and the product branching ratios. These findings can offer theoretical insights to regulate the branching ratio of the multichannel C2H2 + OH reaction.

Dynamics studies for the multi-well and multi-channel reaction of OH with C2H2 on a full-dimensional global potential energy surface.

The C2H2 + OH reaction is an important acetylene oxidation pathway in the combustion process, as well as a typical multi-well and multi-channel reaction. Here, we report an accurate full-dimensional machine learning-based potential energy surface (PES) for the C2H2 + OH reaction at the UCCSD(T)-F12b/cc-pVTZ-F12 level, based on about 475 000 ab initio points. Extensive quasi-classical trajectory (QCT) calculations were performed on the newly developed PES to obtain detailed dynamic data and analyze reaction mechanisms. Below 1000 K, the C2H2 + OH reaction produces H + OCCH2 and CO + CH3. With increasing temperature, the product channels H2O + C2H and H + HCCOH are accessible and the former dominates above 1900 K. It is found that the formation of H2O + C2H is dominated by a direct reaction process, while other channels belong to the indirect mechanism involving long-lived intermediates along the reaction pathways. At low temperatures, the C2H2 + OH reaction behaves like an unimolecular reaction due to the unique PES topographic features, of which the dynamic features are similar to the decomposition of energy-rich complexes formed by C2H2 + OH collision. The classification of trajectories that undergo different reaction pathways to generate each product and their product energy distributions were also reported in this work. This dynamic information may provide a deep understanding of the C2H2 + OH reaction.

The role of predissociation states in the UV photooxidation of acetylene

We study the photo-induced oxidation of acetylene in two different spectral areas, around 220 and 320 nm. In both spectral areas, the dominant reaction pathways involve the rupture of the acetylene C−H bond, thus primarily generating C2H and H fragments. The released fragments react in O2 presence resulting in glyoxal and other molecular species formation. We observe strong variations in the photooxidation rates, which point to the increased participation of excited, predissociation states, in the oxidation process. Such effects of increased excited state participation in photooxidation should be observed in a variety of photo-induced reactions, affecting the reaction spectral response and eventually the reaction cycles in terrestrial or planetary environments.

Оценка возможностей современных кинетических механизмов окисления ацетилена для моделирования нестационарных процессов горения

Acetylene is characterized by high reactivity and appears to be one of the promising gas fuels. However, possible combustion regimes of such fuels require a comprehensive study to be widely introduced in practice. This work is devoted to analyzing the modern kinetic mechanisms of acetylene oxidation. Current approaches to numerical analysis of the gas-dynamic flows in chemically active gas mixtures are a powerful tool in solving many industrial and energy problems. Obtaining positive results of numerical simulation of the non-stationary combustion and detonation processes is impossible without the use of reliable and efficient kinetic mechanisms. Kinetic mechanisms were considered describing the acetylene oxidation. Eight most optimal mechanisms were studied to identify the possibility of their implementation in detailed simulation of the non-stationary combustion processes, in particular, in flame acceleration and transition to detonation. Ignition delay time and laminar burning velocity were calculated using a complete model of the reacting medium gas dynamics. To evaluate correctness of the ignition and combustion parameters obtained values, they were compared with the available experimental data. Based on the obtained results analysis, conclusions were made on the possibility of applying the kinetic mechanisms under consideration, taking into account the combustion parameters accuracy and the computational efficiency

Experimental and Modeling Evaluation of Dimethoxymethane as an Additive for High-Pressure Acetylene Oxidation.

The high-pressure oxidation of acetylene–dimethoxymethane (C2H2–DMM) mixtures in a tubular flow reactor has been analyzed from both experimental and modeling perspectives. In addition to pressure (20, 40, and 60 bar), the influence of the oxygen availability (by modifying the air excess ratio, λ) and the presence of DMM (two different concentrations have been tested, 70 and 280 ppm, for a given concentration of C2H2 of 700 ppm) have also been analyzed. The chemical kinetic mechanism, progressively built by our research group in the last years, has been updated with recent theoretical calculations for DMM and validated against the present results and literature data. Results indicate that, under fuel-lean conditions, adding DMM enhances C2H2 reactivity by increased radical production through DMM chain branching pathways, more evident for the higher concentration of DMM. H-abstraction reactions with OH radicals as the main abstracting species to form dimethoxymethyl (CH3OCHOCH3) and methoxymethoxymethyl (CH3OCH2OCH2) radicals are the main DMM consumption routes, with the first one being slightly favored. There is a competition between β-scission and O2-addition reactions in the consumption of both radicals that depends on the oxygen availability. As the O2 concentration in the reactant mixture is increased, the O2-addition reactions become more relevant. The effect of the addition of several oxygenates, such as ethanol, dimethyl ether (DME), or DMM, on C2H2 high-pressure oxidation has been compared. Results indicate that ethanol has almost no effect, whereas the addition of an ether, DME or DMM, shifts the conversion of C2H2 to lower temperatures.

An experimental and modeling study of acetylene-dimethyl ether mixtures oxidation at high-pressure

The oxidation of acetylene (as soot precursor) and dimethyl ether (DME, as a promising fuel additive) mixtures has been analyzed in a tubular flow reactor, under high-pressure conditions (20, 40 and 60 bar), in the 450–1050 K temperature range. The effect of varying the air excess ratio (λ≈0.7, 1 and 20) and the percentage of DME with respect to acetylene (10 and 40%) has been analyzed from both experimental and modeling points of view. The addition of DME modifies the composition of the radical pool, increasing the production of OH radicals which cause a shift in the onset temperature for C2H2 conversion to lower temperatures; the higher the amount of DME, the lower the temperature. The presence of DME favors the oxidation of C2H2 towards products such as CO and CO2, eliminating carbon from the paths that lead to the formation of soot. On the other hand, in the presence of C2H2, DME begins to be consumed at temperatures higher than those required for the high-pressure oxidation of neat DME, around 175–200 K more. Consequently, the negative temperature coefficient (NTC) region characteristic of this compound at low temperatures is not observed under those conditions. However, an additional analysis of the influence of DME inlet concentration (at 20 bar and λ=1) indicates that, if the amount of DME in the mixture is increased to 500 ppm and more (700 or 1000 ppm), the reaction pathways responsible for this high DME reactivity at low temperatures become more relevant and the NTC region can now be observed.

Theoretical insight into the effect of impurity on the acetoxylation of ethylene to form vinyl acetate: A DFT study of acetylene coversion on PdAu(1 0 0) surface

Vinyl acetate (VA) is an important organic chemical material, which is mainly obtained from ethylene acetoxylation. The raw material, ethylene, usually contains a small amount of acetylene. To understand the effect of acetylene on vinyl acetate system, we used density functional theory (DFT) to explore the pathways and the main products of acetylene conversion on PdAu(100) surface. DFT calculation results suggest that acetylene is quite active on PdAu(100) surface and the Pd-Au catalyst can catalyze the oxidation, hydrogenation and polymerization reaction of acetylene. In all reactions, oxidation reaction is the most likely to occur due to low energy barrier. CO2 and aldehydes as oxidation products will exist simultaneously in the reaction system. Hydrogenation and polymerization of acetylene have a mute effect on vinyl acetate system. We can draw a preliminary conclusion that trace acetylene has little effect on vinyl acetate system.

High-temperature oxidation of acetylene by N2O at high Ar dilution conditions and in laminar premixed C2H2 + O2 + N2 flames

High-temperature oxidation of acetylene (C2H2) is studied behind reflected shock waves and in laminar flames. Atomic resonance absorption spectroscopy (ARAS) is employed to record oxygen atom concentration profiles for the mixture of 10 ppm C2H2 + 10 ppm N2O + argon and temperatures from 1688 K to 3179 K, extending the range of such data available from the literature. Laminar burning velocity of C2H2 in a diluted oxidizer with 11–13% O2 in the O2 + N2 mixture is measured using the heat flux method and compared to the literature data for the 13% O2 mixture. An updated detailed kinetic mechanism is presented to model and analyze the results, and the selection of rate constants in the C2H2 sub-mechanism, whose importance was identified by the sensitivity analysis, is discussed. The performance of the new model is compared against several reaction schemes available from the literature, and kinetic differences between them are outlined. The new shock-wave data helped to improve the performance of the present model compared to its previous version. For the laminar flames, a particular importance of reactions involving C2H3 is identified, however, the reasons for the observed differences in model predictions are to a large extent located outside the C2H2 sub-mechanism, which were also identified.

Photocatalytic Activity of Multicompound TiO2/SiO2 Nanoparticles

Multicompound TiO2/SiO2 nanoparticles with a diameter of 50–70 nm were generated using a liquid flame spray (LFS) nanoparticle deposition in a single flame. Here, we study the photocatalytic activity of deposited multicompound nanoparticles in gas-phase via oxidation of acetylene into carbon dioxide that gives new insight about the multicompound nanoparticle morphology. A small addition of SiO2 content of 0.5%, 1.0% and 3.0% significantly suppressed the photocatalytic activity by 33%, 44% and 70%, respectively, whereas 5.0% SiO2 addition completely removed the activity. This may be due to a formation of a thin passivating SiO2 layer on top of the of the TiO2 nanostructures during the LFS nanoparticle deposition. Surface wetting results support this hypothesis with a significant increase in water contact angle as the SiO2 content is increased.

Development of soot formation sub-model for Scania DC-9 diesel engine in the steady-state condition

The purpose of the present study is to provide a sub-model for the formation and oxidation of soot based on chemical kinetics for the DC-9 Scania diesel engine. The present model is a phenomenological soot model, consisting of nine main stages in the formation of soot including acetylene formation, precursors formation, particle inception, coagulation, surface growth, oxygen, and hydroxyl surface oxidation and oxidation of acetylene and precursors. Acetylene is considered as the main ingredient in the formation of soot, and together with the carbon precursors, the two middle species of the soot mechanism control the soot formation process. Also, in this sub-model, the hydroxyl surface oxidation step has been added. The present sub-model has advantages over two-stage models and is capable of predicting mass growth of soot, precursors, particle diameter and its number. The following inputs of the soot sub-model, including pressure, temperature, volume and unburned fuel content, are derived from a zero-dimensional combustion model developed in this study. Also, an equilibrium sub-model has been used to calculate the concentration of some of the participating species in the soot sub-model. A comparison of the theoretical results with relevant experimental results shows that the present sub-model has been able to predict the pure amount of produced soot accurately. The average particle diameter is about 40 nm, which is very small in the range of particulate matter. The sub-model also predicts that the number of particles to be 1019 particles per unit volume.

Rate Inhibition and Enhancement on Ceria-Promoted Pd Monolith Catalysts: Oxidation of Acetylene, Ethylene, and Propylene and Their Mixtures

The catalytic oxidations of acetylene, ethylene, and propylene under transient and steady-state conditions were studied using model Pd/Al₂O₃ and Pd/CeO₂–ZrO₂ monolith catalysts. Kinetics and light-off measurements highlight the differences between the oxidation of each hydrocarbon. Acetylene is strongly self-inhibiting and −1 order while the reaction order with respect to ethylene is +1, both at temperatures below light-off. Ethylene oxidation is moderately inhibited by oxygen at low temperatures and ethylene at high temperatures. Mixture light-off experiments reveal acetylene inhibits the oxidations of ethylene, propylene, and CO. Light-off of the other species follow acetylene oxidation, which is nearly unaffected by the other species. The Pd/CeO₂–ZrO₂ catalyst gives consistently higher conversion compared with Pd/Al₂O₃ at the same Pd loading for each of the individual oxidation reactions, although the negative-order kinetics of acetylene is sustained. Oxidation of acetylene and mixtures containing acetylene are enhanced with Pd/CeO₂–ZrO₂ compared to Pd/Al₂O₃. The effects of oxidative or reductive pretreatment of the catalysts on species oxidations are also evaluated. Mechanistic implications for the reactions are described to guide kinetic model development. The findings highlight and clarify inhibition effects during single and co-oxidation and offer insights into optimizing the catalyst composition and its operation.

Dissection of the multichannel reaction of acetylene with atomic oxygen: from the global potential energy surface to rate coefficients and branching dynamics.

The O(3P) + C2H2 reaction is the first step in acetylene oxidation. The accurate kinetic data and the understanding of the reaction dynamics is of great importance. To this end, a full-dimensional global potential energy surface (PES) for the ground triplet state of the O(3P) + C2H2 reaction is constructed based on approximately 85 000 ab initio points calculated at the level of explicitly correlated unrestricted coupled cluster single, double, and perturbative triple excitations with the explicitly correlated polarized valence triple zeta basis set (UCCSD(T)-F12b/VTZ-F12). The PES is fit using the permutation invariant polynomial-neural network (PIP-NN) approach with a total root mean square error of 0.21 kcal mol-1. The key topographic features of the PES, including multiple potential wells and saddle points along different reaction pathways, are well represented by this fit PES. The kinetics and dynamics of the O(3P) + C2H2 reaction are investigated using the quasi-classical trajectory (QCT) method. The calculated rate coefficients are in good agreement with experimental data over a wide temperature range, especially when the temperature is lower than 1500 K. The product branch ratio has also been determined, which indicates the H + HCCO channel as the dominant reaction pathway at 298-3000 K, accounting for 80-90% of the overall rate coefficient, in agreement with experimental observations. The dynamics of the reaction is analyzed in detail.

Tolerance and mitigation strategies of proton exchange membrane fuel cells subject to acetylene contamination

Acetylene contamination in proton exchange membrane fuel cells (PEMFCs) significantly depends on cell operating conditions. In this work, acetylene contamination is studied by varying the acetylene concentration, cathode potential and temperature. Hysteresis in the cell performance response is observed during cycling tests with variations in acetylene concentration and cathode potential and is attributed to the potential dependency of acetylene redox reactions. The tolerance of PEMFCs to acetylene is established at approximately 23 ppm for a commercially available catalyst-coated membrane. For a cell poisoned by acetylene at a concentration above the tolerance value, performance losses are eliminated by bringing the ohmic compensated cell voltage into the acetylene oxidation (cell voltage increase) or hydrogenation (cell voltage decrease) regions.

New experimental insights into acetylene oxidation through novel ignition delay times, laminar burning velocities and chemical kinetic modelling

The oxidation of acetylene is central to the oxidation of virtually all hydrocarbon fuels. It is also important for commercial industry, due to its wide range of applications such as flame photometry, atomic absorption, welding etc. In this study, ignition delay times (IDTs) for acetylene oxidation were measured at elevated pressures (10–30 bar) and temperatures (700–1300 K) in a high-pressure shock tube (HPST) and in a rapid compression machine (RCM). The range of pressures, temperatures and mixture compositions studied are at conditions never previously investigated in the literature. The new measurements highlight some major shortcomings in our understanding of the oxidation mechanism of acetylene. The importance of these findings is accentuated, considering the fundamental nature of acetylene chemistry in modelling larger hydrocarbons. These data are complemented by new laminar burning velocity (LBV) experiments, independently performed in two different laboratories. As commercial cylinders commonly contain acetylene gas dissolved in acetone, we have also tested the influence of acetone on acetylene reactivity. It was found that the measured LBVs in both laboratories decreased when acetylene dissolved in acetone was used versus when the pure acetylene was used. The IDTs displayed no such sensitivity. When compared to the literature data, the new LBVs for pure acetylene displayed a pronounced increase in the fuel-rich regime, and the peak flame speeds from TAMU and RWTH increased by about 21 and 14 cm/s, respectively. The kinetic models, with one exception, over-predict the measured IDTs by an order of magnitude at temperatures below ∼1000 K. The reaction of acetylene with hydroperoxyl radicals is critical in accurately predicting the low-temperature, high-pressure IDT data. The experimental findings together with the interpreted models highlight the need for further work to better understand acetylene oxidation.

Ethanol as a Fuel Additive: High-Pressure Oxidation of Its Mixtures with Acetylene

An experimental and modeling study of the oxidation of acetylene–ethanol mixtures under high-pressure conditions (10–40 bar) has been carried out in the 575–1075 K temperature range in a plug-flow reactor. The influence on the oxidation process of the oxygen inlet concentration (determined by the air excess ratio, λ) and the amount of ethanol (0–200 ppm) present in the reactant mixture has also been evaluated. In general, the predictions obtained with the proposed model are in satisfactory agreement with the experimental data. For a given pressure, the onset temperature for acetylene conversion is almost the same independent of the oxygen or ethanol concentration in the reactant mixture but is shifted to lower temperatures when the pressure is increased. Under the conditions of this study, the ethanol presence does not modify the main reaction routes for acetylene conversion, with its main effect being the modification of the radical pool composition.

Exploring possible reaction pathways for the o-atom transfer reactions to unsaturated substrates catalyzed by a [Ni-NO2 ] ↔ [Ni-NO] redox couple using DFT methods.

The (nitro)(N-methyldithiocarbamato)(trimethylphospane)nickel(II), [Ni(NO2 )(S2 CNHMe)(PMe3 )] complex catalyses efficiently the O-atom transfer reactions to CO and acetylene. Energetically feasible sequence of elementary steps involved in the catalytic cycle of the air oxidation of CO and acetylene are proposed promoted by the Ni(NO2 )(S2 CNHMe)(PMe3 )] ↔ Ni(NO2 )(S2 CNHMe)(PMe3 ) redox couple using DFT methods both in vacuum and dichloromethane solutions. The catalytic air oxidation of HC≡CH involves formation of a five-member metallacycle intermediate, via a [3 + 2] cyclo-addition reaction of HC≡CH to the Ni-N = O moiety of the Ni(NO2 )(S2 CNHMe)(PMe3 )] complex, followed by a β H-atom migration toward the Cα carbon atom of the coordinated acetylene and release of the oxidation product (ketene). The geometric and energetic reaction profile for the reversible [Ni( κN1-NO2 )(S2 CNHMe)(PMe3 )] ⇌ [Ni( κO,O2-ONO)(S2 CNHMe)(PMe3 )] linkage isomerization has also been modeled by DFT calculations. © 2017 Wiley Periodicals, Inc.

Mesoporous Co3O4 supported Pt catalysts for low-temperature oxidation of acetylene

0.6Pt/meso-Co3O4 exhibits the highest catalytic activity of acetylene oxidation at 120 °C and good stability for at least 60 h.

Shock Tube and Modeling Study of Chemical Ionization in the Oxidation of Acetylene and Methane Mixtures

ABSTRACTChemical ionization during the oxidation of methane and acetylene is experimentally and theoretically studied behind reflected shock waves over a wide temperature range and atmospheric pressure. The results of experimental measurements of the concentration of free electrons by microwave interferometry and electric probe method during the oxidation of acetylene and methane behind reflected shock waves are presented. A detailed kinetic model of the process of chemical ionization was developed. The results of experimental measurements and kinetic simulations are in good qualitative and quantitative agreement. The kinetic model of chemical ionization makes it possible to improve the kinetic description of the experimentally measured time histories of free electrons for the hydrocarbons studied.

New insights in the low-temperature oxidation of acetylene

This work presents new experimental data of C2H2 low-temperature oxidation for equivalence ratios Φ= 0.5–3.0 in a newly designed jet-stirred reactor over a temperature range of 600–1100K at atmospheric pressure with residence time corresponding from 1.94 to 1.06s. Mole fraction profiles of 17 intermediates including aromatic compounds such as toluene, styrene and ethylbenzene were quantified. A detailed kinetic mechanism involving 295 species and 1830 reactions was established to predict the oxidation of C2H2 and formation of PAH. In developing the mechanism, particular attention was paid to reactions of the vinyl radical and to steps involved in the sequence C2H2→iC4H5→fulvene→C5H5CH2→C6H6. In general, the peak concentrations of intermediates gradually increase and peak locations tend to shift toward high temperatures with Φ increasing. Flux analysis indicates that the addition of H and the reaction with O are the two major channels governing C2H2 consumption. At temperatures below 1000K, benzene is mainly formed through the C2+C4 channels:C2H2+iC4H5→fulvene→C5H5CH2 isomers→C6H6.The C1+C5 pathway: CH3+C5H5→C5H5CH3→(fulvene and C5H5CH2 radicals)→C6H6 tends to be the dominant route for benzene formation at temperatures above 1000K. In addition to the present data, the model predicts well ignition delay times reported in literature.