Methane rate constant with

The type of carbon formed depends on the reaction conditions, whereby higher reaction temperatures favour the formation of the more inert-graphitic carbon nanotubes. Based on kinetic and isotopic investigations on the mechanism of DRM reaction over Ni/MgO catalysts, Wei and Iglesia [189] observed a similarity in turnover rates and first-order rate constants with methane decomposition. Hence, they concluded that the dissociation is the kinetically relevant step for the DRM reaction and that the Ni behavior resembles that of supported noble metal catalysts (Rh, Pt, Ir, Ru). 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

The occurrence of proton transfer reactions between Z)3+ ions and CHa, C2H, and NDZ, between methanium ions and NH, C2HG, CzD , and partially deuterated methanes, and between ammonium ions and ND has been demonstrated in irradiated mixtures of D2 and various reactants near 1 atm. pressure. The methanium ion-methane sequence proceeds without thermal activation between —78° and 25°C. The rate constants for the methanium ion-methane and ammonium ion-ammonia proton transfer reactions are 3.3 X 10 11 cc./molecule-sec. and 1.8 X 70 10 cc./molecule-sec., respectively, assuming equal neutralization rate constants for methanium and ammonium ions (7.6 X 10 4 cc./molecule-sec.). The methanium ion-methane and ammonium ion-ammonia sequences exhibit chain character. Ethanium ions do not undergo proton transfer with ethane. Propanium ions appear to dissociate even at total pressures near 1 atm. 

The mixture used in the present simulation is stoichiometric methane-air. Table 3.2.1 shows the chemical reaction schemes for a methane-air mixture, which has 27 species, including 5 ion molecules such as CH% CHO% F130+, CH3+, and C2IT3O and electron and 81 elementary reactions with ion-molecule reactions [9-11]. The reaction rate constants for elementary reaction with ion molecules have been reported in Refs. [10,11]. 

In the M. trichosporium OB3b system, a third intermediate, T, with kmax at 325 nm (e = 6000 M-1cm 1) was observed in the presence of the substrate nitrobenzene (70). This species was assigned as the product, 4-nitrophenol, bound to the dinuclear iron site, and its absorption was attributed primarily to the 4-nitrophenol moiety. No analogous intermediate was found with the M. capsulatus (Bath) system in the presence of nitrobenzene. For both systems, addition of methane accelerated the rate of disappearance of the optical spectrum of Q (k > 0.065 s-1) without appreciatively affecting its formation rate constant (51, 70). In the absence of substrate, Q decayed slowly (k 0.065 s-1). This decay may be accompanied by oxidation of a protein side chain. 

If k2 > kt, then the amount of methane produced will always exceed that expected on the basis of the kx/k2 level. Thus, in agreement with experiment, only C3 products and above will follow the kx /k2 prediction and in the experimentally found kx /k2 ratio, kx refers to propagation from C2 onward and only after C2 is the single-termination rate constant. k2, applicable. 

Sauer, Sustmann and coworkers59 have reported second-order rate constants for the reaction of trans-1 -substituted 1,3-butadienes with tetracyanoethylene (TCNE) in dichloro-methane at 20 °C their values are X, log 6 OMe, 7.935, vinyl, 5.456 Ph, 5.814 Me, 5.243 H, 3.228. The data were correlated with the CR equation the best regression equation is ... 

Many extensive models of the high-temperature oxidation process of methane have been published [20, 20a, 20b, 21], Such models are quite complex and include hundreds of reactions. The availability of sophisticated computers and computer programs such as those described in Appendix I permits the development of these models, which can be used to predict flow-reactor results, flame speeds, emissions, etc., and to compare these predictions with appropriate experimental data. Differences between model and experiment are used to modify the mechanisms and rate constants that are not firmly established. The purpose here is to point out the dominant reaction steps in these complex... 

The observation that essentially the same rate constants are measured in methane and propane at 40 and 100 °C demonstrates that the starting oxonium ion 35 is in thermal equilibrium with the bulk gas and that its unimolecular rearrangement depends exclusively on the reaction temperature. 

Howard CJ, Evenson KM. 1976. Rate constants for the reactions of OH with CH4and fluorine, chlorine, and bromine substituted methanes at 296K. J Chem Phys 64 197-202. 

Table 16.3 Names, abbreviations, pseudo-first-order rate constants, and half-lives of polyhalo-genated alkanes in Fe(II)/goethite suspension. Experimental conditions 25 m L" goethite, pH 7.2, tgq>24 h. Fe(II) = 1 mM. b Standard deviation, c number of replicates, d t =5 h. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalo-genated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741. Copyright 2002 American Chemical Society...

The value of for attachment to SFg given in Fig. 8 is 2.1 x reaction has also been studied in other high mobility liquids including methane (/id = 400 cm /Vs), argon = 400 cm /Vs), and xenon = 2000 cm /Vs) [127-129], and the rate constant is nearly constant at 3 1 x lO sec h This has been explained by Warman [106] and others as due to the fact that the residence time, td, of an electron within a reaction radius, R, is short compared to the attachment time, ta. Thus rate constants would be expected to fall off with increasing mobility according to the equation ... 

The simulation results of the electron ion recombination rate constant obtained in Ref. 39 are plotted in Fig. 5. The figure shows that the rate constant becomes lower than the Debye-Smoluchowski value when the electron mean free path exceeds —O.Olrc. At higher values of X, the ratio kjk further decreases with increasing mean free path. The simulation results are found to be in good agreement with the experimental data on the electron ion recombination rate constant in liquid methane, which are also plotted in Fig. 5. 

Hsu, K. J., and W. B. DeMore, Rate Constants and Temperature Dependences for the Reactions of Hydroxyl Radical with Several Halogenated Methanes, Ethanes, and Propanes by Relative Rate Measurements, J. Phys. Chem., 99, 1235-1244 (1995). 

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