Skeletal mechanism of inhibition and suppression of a methane-air flame by addition of trimethyl phosphate

A skeletal mechanism of inhibition and quenching of methane flames by addition of trimethyl phosphate was developed. It includes a mechanism of methane oxidation consisting of 19 elementary steps involving 15 species (including N₂), and four elementary reactions involving three phosphorus-containing species (PO₂, HOPO, and HOPO₂). The developed skeletal mechanism adequately predicts the burning velocity of flames with added inhibitor over a range of equivalence ratio of 0.7–1.4 and can be used to model fire suppression.     

HCN synthesis from methane and ammonia over platinum

The mechanism of the HCN formation from ammonia and methane over Pt black was investigated using a temporal analysis of products (TAPs) reactor system. At 1173 K the hydrogen cyanide production rate depends on the order of introducing the reactants. HCN is formed rapidly on the methane pulse just after introducing ammonia. However, a slow formation of HCN is observed on the ammonia pulse that follows a methane pulse. Moreover the form of the HCN response resembles closely that of the nitrogen and hydrogen also released during the ammonia pulse. Thus, the rate-determining step for the formation of HCN is the decomposition rate of ammonia. A reaction sequence based on elementary steps is proposed for the HCN synthesis. The formation of HCN after pulsing H₂ points to a pool of surface intermediate species that are hydrogenated to HCN.     

A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts

Catalytic partial oxidation of methane has been reviewed with an emphasis on the reaction mechanisms over transition metal catalysts. The thermodynamics and aspects related to heat and mass transport is also evaluated, and an extensive table on research contributions to methane partial oxidation over transition metal catalysts in the literature is provided.Presented are both theoretical and experimental evidence pointing to inherent differences in the reaction mechanism over transition metals. These differences are related to methane dissociation, binding site preferences, the stability of OH surface species, surface residence times of active species and contributions from lattice oxygen atoms and support species.Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of active species may be reduced by direct interaction with methane or from the reaction with H, H₂, C or CO.The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. Another essential property for the formation of H₂ and CO as primary products is a low surface coverage of intermediates, such that the probability of O–H, OH–H and CO–O interactions are reduced.The local concentrations of reactants and products change rapidly through the catalyst bed. This influences the reaction mechanisms, but the product composition is typically close to equilibrated at the bed exit temperature.     

Non-stationary catalytic cracking of methane over ceria-based catalysts: Mechanistic approach and catalyst optimization

The non-stationary cracking of methane over various noble metal/CeO₂-doped catalysts at 400 and 600 °C was followed by DRIFT spectroscopy and on the basis of the identified elementary steps a simplified kinetic modeling is proposed. The production of H₂ by direct decomposition of CH₄ on the noble metal is improved by the capacity of ceria to store carbonaceous surface species thanks to: (i) the spillover of carbonyls from noble metal particles towards basic hydroxyls formed on partially reduced Ce sites and (ii) the reverse spillover of ceria oxygen towards metal to oxidize the carbon issued from methane cracking. The resulting formate adspecies are in turn oxidized into carbon dioxide during the regeneration step. Doping the ceria with basic lanthanide oxides and replacing Pt by more efficient and eventually better dispersed metals for methane decomposition like Rh and Ir lead to significant improvements in the hydrogen productivity.     

How and Why Do Cluster Size,Charge State,and Ligands Affect the Course of Metal-Mediated Gas-Phase Activation of Methane?

Recent progress in the gas-phase activation of methane is discussed. We demonstrate that cluster size, charge state, and ligands crucially affect both the reactivity and selectivity of metal-mediated bond activation processes. We outline the important role that relativistic effects and spin densities play and discuss the paradigm of two-state reactivity in thermal reactions. State-of-the-art mass-spectrometry based experiments, in conjunction with electronic structure calculations, permit identification of the elementary steps at a strictly molecular level and thus allow to uncover mechanistic features for four types of reactions: (i) metal-mediated dehydrogenation of methane, (ii) ligand-switch processes of the type ML + CH₄ → M(CH₃) + HL, (iii) hydrogen-atom abstraction as the crucial step in the oxidative coupling of methane, and (iv) the mechanism of the challenging CH₄→CH₃OH conversion.     

Kinetics of the heterogeneous catalytic decomposition of methane

Experiments have clearly demonstrated that dissociation of CH₄ on (supported) metal catalyst (e.g. Pt, Ru, Rh, Ir, Pd) occurs to give hydrogen, a small amount of ethane and surface carbonaceous species. For this catalytic decomposition of methane and its conversion into higher hydrocarbons (especially to ethane and surface carbon) model has been developed to investigate the kinetics. Rate constants of the elementary steps have been estimated. The problem with experimental data (especially for the surface species CH_m-s) is also treated for the sake of future improvement in the kinetics studies. A comparison with catalytic hydrogenolysis of ethane kinetics is also outlined.     

APPLICATION OF A SYSTEMATIC APPROACH FOR DETERMINING POSSIBLE REACTION MECHANISMS: METHANOL SYNTHESIS AND ANODIC OXIDATION OF ZINC

A systematic enumeration of possible reaction mechanisms consistent with a given set of chemical species and elementary steps is useful in guiding research in heterogeneous reaction systems which are encountered in catalysis and electrochemistry. In this paper, the synthesis of methanol by catalytic hydrogenation of carbon monoxide and the anoidic oxidation of zinc in alkaline solutions are presented as two examples of the usefulness of such an approach. The technique utilized to generate the reaction mechanisms is based upon combinatorial analysis, linear algebra and the principle of microscopic reversibility.

In the case of methanol synthesis, it is shown that in addition to the two currently proposed mechanisms, there exist two alternate reaction pathways each of which involves the hydrogenation of a formyl species to form a surface methoxide. One of these mechanisms has recently been proposed for a ZnO catalyst, however there remains an additional mechanism for consideration. For the case of anodic zinc oxidation, it is shown that mechanisms which involve cathodic elementary steps are also possible. To the authors knowledge, these mechanisms have not been previously discussed in the literature.     

Methane Steam Reforming Kinetics on a Ni/Mg/K/Al2O3 Catalyst

The kinetics of methane steam reforming were studied on a Ni/Mg/K/Al₂O₃ catalyst that was developed for conditioning of biomass-derived syngas. Reactions were conducted in a packed-bed reactor while the concentrations of reactants (methane and steam) and products (hydrogen, carbon monoxide, and carbon dioxide) were varied at atmospheric pressure, with the effects of temperature (525–700 °C) and residence time also being investigated. A power law rate model was developed using nonlinear regression to provide a predictive capability for the rate of methane conversion over this catalyst, to be used for reactor design and technoeconomic analysis of process designs. In order to provide some mechanistic insight, and to compare this catalyst to other non-promoted Ni/Al₂O₃ catalysts reported in the literature, a reaction mechanism consisting of five elementary steps, using a Langmuir–Hinshelwood type approach, was also considered. These five steps included: (i) CH₄ adsorption, (ii) H₂O adsorption, (iii) surface reaction of adsorbed CH₄ and H₂O to form CO and H₂, (iv) CO desorption, and (v) H₂ desorption. Nonlinear regression was then used to fit each of the rate laws to the experimental data. From these results, the model that assumed CH₄ adsorption to be the rate determining step provided the best fit of the experimental data. This finding is consistent with literature studies on non-promoted Ni/Al₂O₃ catalysts, in which methane adsorption has been proposed to be the rate determining step during catalytic methane steam reforming. Both the power rate laws and the rate law assuming CH₄ adsorption to be the rate determining step can be used as predictive tools for determining methane conversion for a given set of process conditions. Additionally, a rate expression that assumed the rate was only a function of methane partial pressure was considered, namely, $rate = k*P_{{CH_{4} }}$ rate = k ? P CH 4 , where $k = k_{0} *e^{{^{{ - {\text{Ea}}/{\text{RT}}}} }}$ k = k 0 ? e ? Ea / RT , with P_CH4 in units of Torr. This first-order-methane rate expression fit the data well, yielding an apparent activation energy over this catalyst of E_a = 93 kJ/mol and the pre-exponential rate constant of k₀ = 7.67 × 10⁵ mol/(g-cat s Torr CH₄).     

催眠镇静药甲基三唑安定的合成

以对氯硝基苯为起始原料,经六步反应合成了催眠镇静药８－氯－１－甲基－６－苯基－４Ｈ－［１,２,４］三唑并［４,３－α］［１,４］苯并二氮杂。各步收率均高于参考文献报道收率。总收率由老工艺的２０．８％提高到现在的３４．６％。所得产品经元素分析,ＵＶ、ＩＲ、１ＨＮＭＲ、ＭＳ等光谱分析,证明结构正确。     

Modeling Negative Temperature Coefficient region in methane oxidation

Standard kinetic models are essential tools for predicting and interpreting the evolution of oxidation processes and obtain useful information for designing and dimensioning practical combustion facilities. Quite often a large part of the development work consists in the determination of the most suited chemical kinetics scheme to use in numerical simulations. This step is even more critical in the case of innovative technologies. In fact, in this case, models are required to work in extrapolative conditions, i.e. in range of parameters outside the ones for which they have been optimized. This is the case of prediction methane autoignition at atmospheric pressure, in diluted conditions, corresponding to MILD combustion conditions, where no experimental data are available. The aim of the present work is to compare the efficacies in predicting the existence of Negative Temperature Coefficient (NTC) behavior of ignition time of methane at atmospheric pressure of several kinetic models available in the literature. Such phenomenology is extensively described in the literature for high molecular weight paraffin but few experimental evidences are reported about its occurrence in methane oxidation. Methane autoignition time in dependence of temperature, reaction pathways with rate of production, sensitivity and flow diagram analysis have been exploited in order to highlight the kinetic controlling steps of methane autoignition at different temperature ranges. It has been shown that the prevalence of either the oxidation or the recombination results in a speeding or a slowing down of the reactive process. In this reactive network, a key role is covered by the active oxidation pathway. At the same time, in dependence of working temperature, the branching routes of H₂/O₂ reaction mechanism supply a great part of radicals needed for ignition. Thus, numerical results presented in the paper clearly show that the Negative Temperature Coefficient region in the Arrhenius plot of methane ignition delay marks the shift from one principal reaction route to the others.     

Detailed surface reaction mechanism for reduction of NO by CO

In order to meet the stringent regulatory norms of NO_x and CO emitted by automobiles, reduction of these pollutants has become an intense field of research. Various catalysts like Pt, Rh, Ir, Cu, and Fe have been found to possess high activity for the reduction of NO. However, the available detailed surface reaction mechanisms are not satisfactory in clarifying all the aspects of the simultaneous reduction of NO and oxidation of CO. Here we have developed a quantitative surface reaction mechanism based on elementary steps, in order to comprehend the phenomena of catalytic reduction of NO by CO. Eleven elementary steps are proposed for the NO–CO and NO–CO–O₂ systems on Pt group catalysts. The elementary reaction mechanism is coupled with the continuously stirred tank reactor/packed bed reactor models and the simulation results are validated against literature experiments for the NO–CO reaction on Pt, and the NO–CO–O₂ reaction on Ir catalyst. Despite the simplicity, the CSTR model is able to capture the observed phenomena well on Pt and Ir catalysts. The effect of O₂ on the activity of CO for NO reduction is also analysed in detail through the simulations.     

APPLICATION OF A SYSTEMATIC APPROACH FOR DETERMINING POSSIBLE REACTION MECHANISMS: METHANOL SYNTHESIS AND ANODIC OXIDATION OF ZINC

A systematic enumeration of possible reaction mechanisms consistent with a given set of chemical species and elementary steps is useful in guiding research in heterogeneous reaction systems which are encountered in catalysis and electrochemistry. In this paper, the synthesis of methanol by catalytic hydrogenation of carbon monoxide and the anoidic oxidation of zinc in alkaline solutions are presented as two examples of the usefulness of such an approach. The technique utilized to generate the reaction mechanisms is based upon combinatorial analysis, linear algebra and the principle of microscopic reversibility.

In the case of methanol synthesis, it is shown that in addition to the two currently proposed mechanisms, there exist two alternate reaction pathways each of which involves the hydrogenation of a formyl species to form a surface methoxide. One of these mechanisms has recently been proposed for a ZnO catalyst, however there remains an additional mechanism for consideration. For the case of anodic zinc oxidation, it is shown that mechanisms which involve cathodic elementary steps are also possible. To the authors knowledge, these mechanisms have not been previously discussed in the literature.     

甲烷/乙烷与甲烷/丙烷混合燃料着火特性

利用激波管与CHEMKIN软件研究了不同初始条件下乙烷和丙烷的掺混对甲烷着火延迟时间的影响规律,并从化学动力学角度分析了掺混乙烷和丙烷对甲烷着火延迟时间造成影响的原因。实验与模拟研究表明乙烷和丙烷的掺混会造成甲烷着火延迟时间的大幅度缩短,但随着温度的升高,其对甲烷着火延迟时间的影响逐渐变小。通过敏感性分析发现无论是甲烷/乙烷混合燃料还是甲烷/丙烷混合燃料,对着火促进最大的基元反应都是H+O₂=O+OH（R1）,在甲烷/乙烷和甲烷/丙烷混合燃料的着火反应中对着火抑制最大的两个基元反应是CH₄+H=CH₃+H₂（R128）和CH₄+OH=CH₃+H₂O（R129）。通过路径分析发现在甲烷/乙烷与甲烷/丙烷混合燃料中,随着混合燃料中乙烷与丙烷比例的增加,甲烷的主要反应路径基本不发生变化,主要影响了CH₃的消耗速率。     

OH spillover from a γ-Al2O3 support in the partial oxidation of methane over Ru/Al2O3

The microkinetics of the oxidation of methane on Ru/Al₂O₃ is developed using a TAP reactor. The shapes of the response curves of reactants and products are used to identify the elementary reactions and their rate parameters. The support, which strongly adsorbs H₂O, participates in the reaction sequence through its supply of hydroxyl to metal particles by spillover, and this step is of kinetic significance. The relative rates of the elementary reactions that give overall partial oxidation or total oxidation are discussed.     

Chemical aspects of coal metamorphism and the formation of low-molecular volatiles

The change in elementary composition of coal’s organic mass is analyzed. The state of the carbon dioxide, water and methane in coals beds of different metamorphic development is considered.     

Autothermal reforming of methane on rhodium catalysts: Microkinetic analysis for model reduction

Methane autothermal reforming has been studied using comprehensive, detailed microkinetic mechanisms, and a hierarchically reduced rate expression has been derived without apriori assumptions. The microkinetic mechanism is adapted from literature and has been validated with reported experimental results. Rate determining steps are elicited by reaction path analysis, partial equilibrium analysis and sensitivity analysis. Results show that methane activation occurs via dissociative adsorption to pyrolysis, while oxidation of the carbon occurs by O(s). Further, the mechanism is reduced through information obtained from the reaction path analysis, which is further substantiated by principal component analysis. A 33% reduction from the full microkinetic mechanism is obtained. One-step rate equation is further derived from the reduced microkinetic mechanism. The results show that this rate equation accurately predicts conversions as well as outlet mole fraction for a wide range of inlet compositions.     

Catalytic solutions for fugitive methane emissions in the oil and gas sector

Global warming attributed in part to the release of the so-called greenhouse gases is becoming an increasing concern, and steps are being implemented to mitigate such emissions. The two most significant emissions are carbon dioxide and methane. A significant fraction of emissions is emitted by oil and gas production and transportation facilities. Because methane has at least 21 times the greenhouse gas potential of carbon dioxide, it is advantageous to convert methane to carbon dioxide via combustion, even if the carbon dioxide is vented to the atmosphere. Fugitive methane combustion does however present certain difficulties in its combustion. This paper presents an overview of the problem and suggests some possible catalytic reactor technologies appropriate for the solution.     

Rational modeling of the CPO of methane over platinum gauze: Elementary gas-phase and surface mechanisms coupled with flow simulations

The high-temperature catalytic partial oxidation of methane (CPOM) over a platinum gauze reactor was modeled by integrating 3D numerical simulations of the flow field coupled with heat transport as well as detailed micro-kinetic models including gas-phase and surface reaction mechanisms. Model results describe well CPO experiments over Pt-gauzes in the literature. The conversions of CH₄ and O₂ increase with an increased contact time and were constant in the temperature range of 1000–1200 K. The selectivity to CO linearly increases with temperature. H₂ was only observed above 1200 K, below this temperature H₂O was the only hydrogen-containing product. The contribution of heterogeneous steps in the overall process is prominent, but in the later stages of the reactor, gas-phase reactions become important at certain conditions of temperature, pressure and residence time. Simulations predicted significant gas-phase production of ethane and ethylene via methane oxidative coupling upon increasing the total pressure and residence time. Consequently, homogeneous and heterogeneous processes should be simultaneously implemented in order to accomplish a solid reactor modeling.     

CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM-BASED CATALYSTS

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.