Reaction kinetics Methanation

The rate is near first order in methane and zero order in oxygen for oxygen to methane ratios higher than 1. Also, the reaction kinetics remain unaffected upon polarization conditions. The kinetic data indicate weak bonding of methane and strong bonding of oxygen on the catalyst surface. 

The kinetics of H-O recombination is very important in the reforming reaction of methane to produce CO and H2. When more weakly bonded O js recombines with Hads (preferred on Pt), the main product next to CO will be H2. On planar Rh with a stronger M-O bond interaction, this reaction is suppressed and therefore H2 is the main product [23]. Clearly this selectivity will be dramatically affected by the presence of surface steps. 

Steady state and non steady state kinetic measurements suggest that methane carbon dioxide reforming proceeds in sequential steps combining dissociation and surface reaction of methane and CO2 During admission of pulses of methane on the supported Pt catalysts and on the oxide supports, methane decomposes into hydrogen and surface carbon The amount of CH, converted per pulse decreases drastically after the third pulse (this corresponds to about 2-3 molecules of CH< converted per Pt atom) indicating that the reaction stops when Pt is covered with (reactive) carbon CO2 is also concluded to dissociate under reaction conditions generating CO and adsorbed... 

For transition-metal catalyzed hydroxylation of alkane C-H bonds, the reactions of alkanes with platinum(II) complexes were the most successful. In an aqueous solution of hexachloroplatinic acid and Na2PtCl4, alkanes were converted into a mixture of isomeric alkyl chlorides, alcohols, and ketones, and the platinum(IV) is reduced to platinum(II).7 The kinetics of the reaction with methane as the alkane have been described in detail.8... 

In contrast to the results obtained for dehydrogenation reactions, kinetic energy release distributions for alkane elimination processes can usually be fit with phase space theory. Results for the loss of methane from reaction 9 of Co + with isobutane are shown in Figure 10b. In fitting the... 

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

In many gaseous state reactions of technological importance, short-lived intermediate molecules which are formed by the decomposition of reacting species play a significant role in the reaction kinetics. Thus reactions involving the methane molecule, CH4, show the presence of a well-defined dissociation product, CH3, the methyl radical, which has a finite lifetime as a separate entity and which plays an important part in a sequence or chain of chemical reactions. 

Calculate the reaction kinetics of a methane oxygen mixture diluted with... 

The hydrogenolysis of neopentane occurs primarily by breaking of a single C—C bond to produce methane and isobutane as the major reaction products Small amounts of ethane and propane were also produced. The reaction kinetics were similar on the two surfaces, indicating a similar mechanism for both surfaces. 

Megalofonos, S.K., Rapayaimakos, N.G. 1997. Kinetics of catalytic reaction of methane and hydrogen sulphide over MoS2. J Appl Catal A General 165 249-258. 

Several authors have proposed that CH4 combustion over PdO occurs via a redox mechanism [82-85]. Methane activation through assisted hydrogen extraction is generally regarded as the rate-determining step, although there is not a general consensus on the nature of the adsorption sites. Further, desorption of H2O by decomposition of surface hydroxyls has been reported to play a key role in reaction kinetics at temperatures below 450 °C [67, 86]. 

The exchange reactions of methane with deuterium over several metals (21, 39,146,147,195, 215, 216, 229) exhibited compensation behavior (Table III, G), as has been pointed out previously (3). No such relationship was found in the data available for exchange reactions of C2-C6 hydrocarbons. This is consistent with discussion given above (see Section III, B, 4) in which it was concluded that the differences in the kinetic behavior of the exchange reactions between methane and the other hydrocarbons varied with the individual metals concerned. 

McKee (21, 195) and McKee and Norton (219, 249, 250) have reported compensation effects in the exchange reactions of methane on several pairs of binary noble metal alloy catalysts. For each combination of elements kinetic measurements were made at a number of different compositions. Although the compensation behavior was generally very similar, there were perceptible differences in the values of B and e calculated for the various alloy combinations. The parameters found, by use of the formulas given in Appendix II, are summarized in Table IV, A-E, and are subject to the following comments. In consideration of data for the Pd-Rh alloys, the point for... 

Most carbocations are too reactive to be directly observable in ordinary solvents, and until relatively recently evidence has been obtained indirectly, primarily through the study of reaction kinetics and trapping processes, experiments discussed in Sections 5.1, 5.2, and 5.4. Nevertheless, a few types of compounds have long been known to produce observable concentrations of positive ions relatively easily. The triarylmethyl derivatives were the first of this type to be investigated the halides ionize readily in non-nucleophilic solvents such as sulfur dioxide,70 and the alcohols yield solutions of the ions in concentrated sulfuric acid. Early observations by the freezing-point depression technique (see Section 3.2, p. 130) established that each mole of triphenyl carbinol yields 4 moles of ions in sulfuric acid, the reaction presumably being by way of Equation 5.14.71 Results in methane-sulfonic acid are similar.72... 

Gupta, A., Methane Hydrate Dissociation Measurements and Modeling The Role of Heat Transfer and Reaction Kinetics, Ph.D. Thesis, Colorado School of Mines, Golden, CO (2007). 

Figure 3.35 (See color insert following page 390.) X-ray CT imaging shows radial dissociation of a hydrate core. Image number 1 -8 (top number on each image) recorded over 0-245 min (bottom number on each image). The cell pressure was decreased from 4.65 to 3.0 MPa over 248 min. The hydrate core temperature decreased from 277 to 274 K with time, following the three-phase methane hydrate equilibrium line. (From Gupta, A., Methane Hydrate Dissociation Measurements andModeling The Role of Heat Transfer and Reaction Kinetics, Ph.D. Thesis Colorado School of Mines, Golden, CO (2007). With permission.)...

Dicks A.L., Pointon K.D., Siddle A., 2000. Intrinsic reaction kinetics of methane steam reforming on a nickel/zirconia anode. Journal of Power Sources 86, 523-530. 

P-site (125) is favored by 3.3 kcal/mol over protonation at the C-site (122). However, protonation at the carbon is favored for cations 120 and 121. All three dicationic species 123, 124, and 125, are predicted to be kinetically stable with significant energy barriers towards homolytic or heterolytic fragmantation reactions. In studies of isodesmic reactions with methane, the dicationic species are shown to be far more reactive (eqs 36-37). 

The gas composition is optimized with DOFs outside the CO2 scrubbing system with regard to inert composition (methane and argon) and hydrogen to nitrogen ratio since the levels of these components affect downstream (ammonia synthesis) reaction kinetics. Improved kinetics at lower inert levels are achieved at the expense of using more fuel or feedstock, since lower inerts can be achieved by firing the primary... 

Then, a survey of micro reactors for heterogeneous catalyst screening introduces the technological methods used for screening. The description of microstructured reactors will be supplemented by other, conventional small-scale equipment such as mini-batch and fixed-bed reactors and small monoliths. For each of these reactors, exemplary applications will be given in order to demonstrate the properties of small-scale operation. Among a number of examples, methane oxidation as a sample reaction will be considered in detail. In a detailed case study, some intrinsic theoretical aspects of micro devices are discussed with respect to reactor design and experimental evaluation under the transient mode of reactor operation. It will be shown that, as soon as fluid dynamic information is added to the pure experimental data, more complex aspects of catalysis are derivable from overall conversion data, such as the intrinsic reaction kinetics. 

On the other hand, the determinant equation (5.28) allows the study of complex reaction kinetics with incompletely studied mechanisms neglecting the assumptive stationary concentration method. Let us assume that the most probable mechanism of methane oxidation to methanol with hydrogen peroxide is unknown. Then equations (5.29) and (5.30) should be presented in the form that discloses the conjugation mechanism of these two reactions ... 

Industrial chemical reactions are often more complex than the earlier types of reaction kinetics. Complex reactions can be a combination of consecutive and parallel reactions, sometimes with individual steps being reversible. An example is the chlorination of a mixture of benzene and toluene. An example of consecutive reactions is the chlorination of methane to methyl chloride and subsequent chlorination to yield carbon tetrachloride. A further example involves the chlorination of benzene to monochlorobenzene, and subsequent chlorination... 

The results show that the specificities of catalyst deactivation and it s kinetic description are in closed connection with reaction kinetics of main process and they form a common kinetic model. The kinetic nature of promotor action in platinum catalysts side by side with other physicochemical research follows from this studies as well. It is concern the increase of slow step rate, the decrease of side processes (including coke formation) rate and the acceleration of coke transformation into methane owing to the increase of hydrogen contents in coke. The obtained data can be united by common kinetic model.lt is desirable to solve some problems in describing the catalyst deactivation such as the consideration of coke distribution between surfaces of metal, promoter and the carrier in the course of reactions, diffusion effects etc,. 

Engleman, V. S. "Survey and Evaluation of Kinetic Data on Reactions in Methane/Air Combustion" Report No. EPA-600/2-76-003, 1976. 

Liu, K.E., Valentine, A.M., Wang, D., Huynh, B.H. Edmondson, D.E. Salifoglou, A. Lippard, SJ. (1995) Kinetic and spectroscopic characterization of intermediates and component interactions in reactions of methane monooxygenase from Methylococcus capsulatus (Bath), J. Am. Chem. Soc. 117, 10174-10185. 

Zhang, X.-Y., and Lipscomb, J. D., 1999, Kinetic studies on electron transfer reactions in methane monooxygenase from M. trichosporium OB3b, J. Inorg. Biochem. 74 349. 

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