Diesel soot oxidation by nitrogen dioxide,oxygen and water under engine exhaust conditions: Kinetics data related to the reaction mechanism

Experimental studies on diesel soot oxidation under a wide range of conditions relevant for modern diesel engine exhaust and continuously regenerating particle trap were performed. Hence, reactivity tests were carried out in a fixed bed reactor for various temperatures and different concentrations of oxygen, NO₂ and water (300–600 °C, 0–10% O₂, 0–600 ppm NO₂, 0–10% H₂O). The soot oxidation rate was determined by measuring the concentration of CO and CO₂ product gases. The parametric study shows that the overall oxidation process can be described by three parallel reactions: a direct C–NO₂ reaction, a direct C–O₂ reaction and a cooperative C–NO₂–O₂ reaction. C–NO₂ and C–NO₂–O₂ are the main reactions for soot oxidation between 300 and 450 °C. Water vapour acts as a catalyst on the direct C–NO₂ reaction. This catalytic effect decreases with the increase of temperature until 450 °C. Above 450 °C, the direct C–O₂ reaction contributes to the global soot oxidation rate. Water vapour has also a catalytic effect on the direct C–O₂ reaction between 450 °C and 600 °C. Above 600 °C, the direct C–O₂ reaction is the only main reaction for soot oxidation. Taking into account the established reaction mechanism, a one-dimensional model of soot oxidation was proposed. The roles of NO₂, O₂ and H₂O were considered and the kinetic constants were obtained. The suggested kinetic model may be useful for simulating the behaviour of a diesel particulate filter system during the regeneration process.     

The oxidation of hydrocarbons by water vapor behind high-temperature shock-waves

The oxidation of acetylene by water vapor was studied behind the reflected shock in a single-pulse shock tube. Computer simulation experiments reproduced the experimental results in the temperature range of 1500 to 2000°K. The kinetic scheme suggested here involves three major processes, (1) production of hydrogen atoms by the sequence of reactions which lead from acetylene to carbon; (2) production of OH radicals, mainly by the reaction H + H₂O → H₂ + OH, and (3) fast oxidation of the acetylene and other C/H species by the available oxidants in the system. The experimental results of methane oxidation suggest that methane is converted to acetylene prior to its oxidation. The implication of the experimental results to processes occurring in planetary atmospheres as a result of thunder shock waves is briefly discussed.     

The influence of the active component and support nature, gas mixture composition on physicochemical and catalytic properties of catalysts for soot oxidation

Physicochemical and catalytic properties of compositions Fe(Ce)–Mn–O/support (gamma-, theta-, alpha-Al₂O₃, SiO₂ as the support) and Pt/CeO₂/theta-Al₂O₃ for oxidation of soot were characterized. It was established that the phase composition of the initial catalysts depended mainly on the nature of the active component and preparation conditions. Non-isothermal treatment of the soot–catalyst compositions at the temperature up to 1000 °C resulted in a change in the phase composition depending mainly on the final treatment temperature. The catalyst surface area was determined by the support nature. It was established that catalyst activities for oxidation of soot are determined by both catalyst nature and composition of gas mixture. The process of the soot oxidation is thought to involve oxygen from the catalyst surface. The higher proportion of weakly bound surface oxygen, the higher was the catalyst activity. An increase in the oxygen concentration from 5% O₂/N₂ to 15% O₂/N₂ is shown to lead to a decrease of the temperature of the soot oxidation. The influence of the oxygen concentration on the process of soot oxidation becomes weaker in the presence of water vapor. Results showed that the presence of NO in the gas mixture favors a decrease in the oxidation temperature of the soot, the higher being the nitrogen oxide concentration, the more pronounced effect. Introduction of SO₂ in amount of 50 ppm in the gas mixture has no noticeable effect on the process of the soot oxidation. Among the catalysts under study, Fe–Mn–K–O/gamma-Al₂O₃ is most effective to oxidation of the soot at otherwise identical conditions.     

Carbon Dynamics on the Molybdenum Carbide Surface during Catalytic Propane Dehydrogenation

The effect of the gas‐phase chemical potential on surface chemistry and reactivity of molybdenum carbide has been investigated in catalytic reactions of propane in oxidizing and reducing reactant mixtures by adding H₂, O₂, H₂O, and CO₂ to a C₃H₈/N₂ feed. The balance between surface oxidation state, phase stability, carbon deposition, and the complex reaction network involving dehydrogenation reactions, hydrogenolysis, metathesis, water‐gas shift reaction, hydrogenation, and steam reforming is discussed. Raman spectroscopy and a surface‐sensitive study by means of in situ X‐ray photoelectron spectroscopy evidence that the dynamic formation of surface carbon species under a reducing atmosphere strongly shifts the product spectrum to the C₃‐alkene at the expense of hydrogenolysis products. A similar response of selectivity, which is accompanied by a boost of activity, is observed by tuning the oxidation state of Mo in the presence of mild oxidants, such as H₂O and CO₂, in the feed as well as by V doping. The results obtained allow us to draw a picture of the active catalyst surface and to propose a structure–activity correlation as a map for catalyst optimization.     

Hydrogen Production From Crude Bio-oil and Biomass Char by Electrochemical Catalytic Reforming

We reports an efficient approach for production of hydrogen from crude bio-oil and biomass char in the dual fixed-bed system by using the electrochemical catalytic reforming method. The maximal absolute hydrogen yield reached 110.9 g H₂/kg dry biomass. The product gas was a mixed gas containing 72%H₂, 26%CO₂, 1.9%CO, and a trace amount of CH₄. It was observed that adding biomass char (a by-product of pyrolysis of biomass) could remarkably increase the absolute H₂ yield (about 20%-50%). The higher reforming temperature could enhance the steam reforming reaction of organic compounds in crude bio-oil and the reaction of CO and H₂O. In addition, the CuZn-Al₂O₃ catalyst in the water-gas shift bed could also increase the absolute H₂ yield via shifting CO to CO₂.     

Oxidization of SO2 by Reactive Oxygen Species for Flue Gas Desulfurization and H2SO4 Production

A strong ionization dielectric barrier discharge was used to produce a high concentration of reactive oxygen species that were then injected into a simulated flue gas in a duct to remove SO₂ by oxidation. Sulfuric acid (H₂SO₄) was produced through the following two reactions: (1) O₃ oxidation of SO₂–SO₃, which then reacted with H₂O to produce H₂SO₄; and (2) reaction of O₂ ⁺ with H₂O to produce ·OH radicals, which then rapidly and non-selectively oxidized SO₂–H₂SO₄. When the molar ratio of reactive oxygen species to SO₂ was 4:1, the SO₂ removal efficiency was 94.6%, the energy consumption per cubic meter of flue gas was 13.3 Wh/m³, the concentration of recovered H₂SO₄ was 4.53 g/l, and the H₂SO₄ recovery efficiency was 28.8%. The H₂O volume fraction in the simulated flue gas affected the SO₂ removal efficiency, whereas the O₂ and CO₂ volume fractions did not. These results prove that oxidation by reactive oxygen species is a feasible method for flue gas desulfurization.     

Insights and Activation Energy Surface of the Dehydrogenation of C2HxO Species in Ethanol Oxidation Reaction on Ir(100)

Dehydrogenation of an organic compound is the first and the most fundamental elementary reaction in many organic reactions. In ethanol oxidation reaction (EOR) to form CO₂, there are a total of 46 pathways in C₂H_xO (x=1–6) species leading to the removal of all six hydrogen atoms in five C−H bonds and one O−H bond. To investigate the degree of dehydrogenation in EOR under operando conditions, we performed density function theory (DFT) calculations to study 28 dehydrogenation steps of C₂H_xO on Ir(100). An activation energy surface was then constructed and compared with that of the C−C bond cleavages to understand the importance of the degree of dehydrogenation in EOR. The results show that there are likely 28 dehydrogenations in EOR under fuel cell temperatures and the last two hydrogens in C₂H₂O are less likely cleaved. On the other hand, deep dehydrogenation including 45 dehydrogenations can occur under ethanol steam reforming conditions.     

Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts

An 48‐step sur face reaction mechanism with thermodynamic consistent kinetic data is presented for the catalytic conversion of the gaseous chemical system H₂/O₂/H₂O/CO/CO₂/CH₄ over Rh/Al₂O₃ catalysts. Total and partial oxidation as well as steam reforming and dry reforming of methane over Rh catalysts is studied experimentally and numerically at varying temperature and composition. The results are used to extend the kinetic schemes we developed for H₂ oxidation, CO oxidation kinetics, and the water‐gas‐shift reactions in former studies. Aside from the experiments in a stagnation‐flow reactor presented here, we modeled a number of experiments from the literature to test the newly established kinetic scheme.     

Ethylene pyrolysis and oxidation: A kinetic modeling study

Ethylene oxidation and pyrolysis was modeled using a comprehensive kinetic reaction mechanism. This mechanism is an updated version of one developed earlier. It includes the most recent findings concerning the kinetics of the reactions involved in the oxidation of ethylene. The proposed mechanism was tested against ethylene oxidation experimental data (molecular species concentration profiles) obtained in jet stirred reactors (1–10 atm, 880–1253 K), ignition delay times measured in shock tubes (0.2–12 atm, 1058–2200 K) and ethylene pyrolysis data in shock tube (2–6 atm, 1700–2200 K). The general prediction of concentration profiles of minor species formed during ethylene oxidation is improved in the present model by using more accurate kinetic data for several reactions (principally: HO₂ + HO₂ → H₂O₂ + O₂, C₂H₄ + OH → C₂H₃ + H₂O, C₂H₂ + OH → Products, C₂H₃ → C₂H₂ + H).     

Maximum Hydrogen Production by Autothermal Steam Reforming of Bio-oil With NiCuZnAl Catalyst

Autothermal steam reforming (ATR) of bio-oil, which couples the endothermic steam reform-ing reaction with the exothermic partial oxidation, offers many advantages from a technical and economic point of view. Effective production of hydrogen through ATR of bio-oil was performed at lower temperature with NiCuZnAl catalyst. The highest hydrogen yield from bio-oil reached 64.3% with a nearly complete bio-oil conversion at 600 oC, the ratio of steam to carbon fed (S/C) of 3 and the oxygen to carbon ratio (O/C) of 0.34. The reaction con-ditions in ATR including temperature, O/C, S/C and weight hourly space velocity can be used to control both hydrogen yield and products distribution. The comparison between the ATR and common steam reforming of bio-oil was studied. The mechanism of the ATR of bio-oil was also discussed.     

Continuous high-temperature fluidized bed pyrolysis of coal in complex atmospheres: Product distribution and pyrolysis gas

This study is devoted to investigating the continuous coal pyrolysis in a laboratory fluidized bed reactor that fed coal and discharged char continuously at temperatures of 750–980 °C and in N₂-base atmospheres containing O₂, H₂, CO, CH₄ and CO₂ at varied contents. The results showed that the designed continuous pyrolysis test provided a clear understanding of the coal pyrolysis behavior in various complex atmospheres free of and with O₂. The effect of adding H₂, CO, CH₄ or CO₂ into the atmosphere on the tar yield was related to the O₂ content in the atmosphere. Without O₂ in the atmosphere, adding H₂ and CO₂ decreased the pyrolysis tar yield, but the tar yield was conversely higher with raising the CO and CH₄ contents in the atmosphere. In O₂-containing atmospheres, the influence from varying the atmospheric gas composition on the product distribution and pyrolysis gas composition was closely related to the oxidation or gasification reactions occurring to char, tar and the tested gas.     

Oscillations during partial oxidation of methane to synthesis gas over Ru/Al2O3 catalyst (Special column of methane transformation, Article)

Oscillations in temperatures of catalyst bed as well as concentrations of gas phase species at the exit of reactor were observed during the partial oxidation of methane to synthesis gas over Ru/Al₂O₃ in the temperature range of 600 to 850 °C. XRD, H₂-TPR and in situ Raman techniques was used to characterize the catalyst. Two types of ruthenium species, i.e. the ruthenium species weakly interacted with Al₂O₃ and that strongly interacted with the support, were identified by H₂-TPR experiment. These species are responsible for two types of oscillation profiles observed during the reaction. The oscillations were the result of these ruthenium species switching cyclically between the oxidized state and the reduced state under the reaction condition. These cyclic transformations, in turn, were the result of temperature variations caused by the varying levels of the strongly exothermic CH₄ combustion and the highly endothermic CH₄ reforming (with H₂O and CO₂) reactions (or the less exothermic direct partial oxidation of methane to CO and H₂), which were favored by the oxidized and the metallic sites, respectively. The major pathway of synthesis gas formation over the catalyst was via the combustion-reforming mechanism.     

Thermodynamic possibilities and constraints of pure hydrogen production by a chromium,nickel, and manganese-based chemical looping process at lower temperatures

The reduction of chromium, nickel, and manganese oxides by hydrogen, CO, CH₄, and model syngas (mixtures of CO + H₂ or H₂ + CO + CO₂) and oxidation by water vapor has been studied from the thermodynamic and chemical equilibrium point of view. Attention was concentrated not only on the convenient conditions for reduction of the relevant oxides to metals or lower oxides at temperatures in the range 400–1000 K, but also on the possible formation of soot, carbides, and carbonates as precursors for the carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of very stable Cr₂O₃ to metallic Cr by hydrogen or CO at temperatures of 400–1000 K is thermodynamically excluded. Reduction of nickel oxide (NiO) and manganese oxide (Mn₃O₄) by hydrogen or CO at such temperatures is feasible. The oxidation of MnO and Ni by steam and simultaneous production of hydrogen at temperatures between 400 and 1000 K is a difficult step from the thermodynamics viewpoint. Assuming the Ni—NiO system, the formation of nickel aluminum spinel could be used to increase the equilibrium hydrogen yield, thus, enabling the hydrogen production via looping redox process. The equilibrium hydrogen yield under the conditions of steam oxidation of the Ni—NiO system is, however, substantially lower than that for the Fe—Fe₃O₄ system. The system comprising nickel ferrite seems to be unsuitable for cyclic redox processes. Under strongly reducing conditions, at high CO concentrations/partial pressures, formation of nickel carbide (Ni₃C) is thermodynamically favored. Pressurized conditions during the reduction step with CO/CO₂ containing gases enhance the formation of soot and carbon-containing compounds such as carbides and/or carbonates.     

Soot formation from C2H2 and C2H4 pyrolysis at different temperatures

A study of the pyrolysis of two hydrocarbons, C₂H₂ and C₂H₄, at different temperatures has been carried out in order to compare their behaviour in terms of soot and gas yields and gas composition. Pyrolysis experiments have been performed in the same conditions for both hydrocarbons: an inlet hydrocarbon concentration of 15,000 ppmv and a temperature range of 1000–1200 °C. For C₂H₂ and C₂H₄ pyrolysis tests, the results present the same trend when increasing the temperature: an increase in soot yield, a decrease in gas yield and a similar evolution of the outlet gases. Comparatively, it can be observed that acetylene is a more sooting hydrocarbon than ethylene for a given temperature. Additionally, the study of soot reactivity with O₂ and NO shows that the soot samples obtained from ethylene show a slightly higher reactivity towards O₂ and NO than the soot samples formed from acetylene.     

Peculiarities of TiH<Subscript>2</Subscript> decomposition

The eventual competition between the dehydrogenation and oxidation of TiH₂ powder in the argon, oxygen, and static air atmospheres has been examined in the regime of linear heating using differential thermal analysis (DTA), X-ray diffraction, transmission and scanning electron microscopy coupled to energy dispersive spectroscopy techniques. Three stages of dehydrogenation, R1, R2, and R3, and two different oxidation reactions, O1 and O2, have been identified. The kinetics of individual reactions has been investigated by the Kissinger and Suriñach curve fitting procedures. While the kinetics of O1, O2, and R1 reactions depends on the diffusivities of the external reagent gas, being oxygen, and internal product gas, being hydrogen, the main dehydrogenation stage R2 is controlled by the combined nucleation-and-growth and coalescence kinetics. The effect of pre-annealing of TiH₂ on its subsequent degradation has also been studied. Due to the huge overheating accompanying both oxidation reactions in the DTA set-up, substantial thermal losses have been demonstrated and the method to obtain the true heat of oxidation reactions in metals has been described.     

An experimental and modeling study of ethanol oxidation kinetics in an atmospheric pressure flow reactor

Experimental profiles of stable species concentrations and temperature are reported for the flow reactor oxidation of ethanol at atmospheric pressure, initial temperatures near 1100 K and equivalence ratios of 0.61–1.24. Acetaldehyde, ethene, and methane appear in roughly equal concentrations as major intermediate species under these conditions. A detailed chemical mechanism is validated by comparison with the experimental species profiles. The importance of including all three isomeric forms of the C₂H₅O radical in such a mechanism is demonstrated. The primary source of ethene in ethanol oxidation is verified to be the decomposition of the C₂H₄OH radical. The agreement between the model and experiment at 1100 K is optimized when the branching ratio of the reactions of C₂H₅OH with OH and H is defined by (30% C₂H₄OH + 50% CH₃CHOH + 20% CH₃CH₂O) + XH. As in methanol oxidation, HO₂ chemistry is very important, while the H + O₂ chain branching reaction plays only a minor role until late in fuel decay, even at temperatures above 1100 K.     

Two‐Dimensional Spatial Resolution of Concentration Profiles in Catalytic Reactors by Planar Laser‐Induced Fluorescence: NO Reduction over Diesel Oxidation Catalysts

Planar laser‐induced fluorescence (PLIF) enables noninvasive in situ investigations of catalytic flow reactors. The method is based on the selective detection of two‐dimensional absolute concentration maps of conversion‐relevant species in the surrounding gas phase inside a catalytic channel. Exemplarily, the catalytic reduction of NO with hydrogen (2 NO+5 H₂→2 H₂O+2 NH₃) is investigated over a Pt/Al₂O₃ coated diesel oxidation catalyst by NO PLIF inside an optically accessible channel reactor. Quenching‐corrected 2D concentration maps of the NO fluorescence above the catalytic surface are obtained under both, nonreactive and reactive conditions. The impact of varying feed concentration, temperature, and flow velocities on NO concentration profiles are investigated in steady state. The technique presented has a high potential for a better understanding of interactions of mass transfer and surface kinetics in heterogeneously catalyzed gas‐phase reactions.     

The kinetics of gasification of char derived from sewage sludge

Gasification of char derived from sewage sludge was studied under different oxidizing atmospheres containing CO₂, O₂ or H₂O. The gasification tests were carried out in thermobalance at different temperatures and oxidizing reagent concentrations. The most efficient were the gaseous mixtures containing oxygen. The reaction took place at temperature 400–500 °C, whilst in the case of CO₂ and steam much higher temperatures (700–900 °C) were necessary to complete the conversion. Two rate models for gas–solid reaction were applied to describe the effect of char conversion on reaction rate. The shrinking core model for reaction-controlled regime was found to be the best for predicting the rate of char gasification in CO₂ and O₂ atmosphere. The experimental data for steam gasification of the char were fitted best by the first-order kinetics. The kinetic parameters estimated from the experimental data are in accordance with the literature for lignocellulosic char gasification and are the first published for sewage sludge char gasification.     

Selective CO oxidation on a Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in the surface ignition regime: 1. Fine purification of hydrogen-containing gases

Selective CO oxidation in a mixture simulating the methanol steam reforming product with an air admixture was studied over Ru/Al₂O₃ catalysts in a quasi-adiabatic reactor. On-line monitoring of the gas temperature in the catalyst bed and of the residual CO concentration at different reaction conditions made it possible to observe the ignition and quenching of the catalyst surface, including transitional regimes. A sharp decrease in the residual CO concentration takes place when the reaction passes to the ignition regime. The evolution of the temperature distribution in the catalyst bed in the ignition regime and the specific features of the steady-state and transitional regimes are considered, including the effect of the sample history. In selective CO oxidation and in H₂ oxidation in the absence of CO, the catalyst is deactivated slowly because of ruthenium oxidation. In both reactions, the deactivated catalyst can be reactivated by short-term treatment with hydrogen. A 0.1% Ru/Al₂O₃ catalyst is suggested. In the surface ignition regime, this catalyst can reduce the residual CO concentration from 0.8 vol % to 10–15 ppm at O₂/CO = 1 even in the presence of H₂O and CO₂ (up to ～20 vol %) at a volumetric flow rate of ～100 1 (g Cat)^?1 h^?1, which is one magnitude higher than the flow rates reported for this process in the literature.     

Role of CH, CH3, and OH Radicals in Organic Compound Decomposition by Water Plasmas

Decomposition of acetone, methanol, ethanol, and glycerine by water plasmas at atmospheric pressure has been investigated using a direct current discharge. At torch powers of 910–1,050 W and organic compound concentrations of 1–10 mol%, the decomposition rate of methanol and glycerine was over 99%, while those of acetone and ethanol was 95.4–99%. The concentrations of H₂ obtained were 60–80% in the effluent gas for any compounds by pyrolysis. Based on the experimental results, the decomposition mechanism of organic compounds in water plasmas was proposed and the roles of intermediate species such as CH, CH₃, and OH have been investigated; CH radical generated from organic compounds decomposition was easily oxidized to form CO; incomplete oxidation of CH₃ leads to C₂H₂ generation as well as soot formation; and negligible amount of soot observed from glycerine decomposition even at high concentration indicated that oxidation of CH_×(×:1–3) was enhanced by OH radical.