Kinetics of methane oxidation

It is clear that the concept of formaldehyde as the relatively stable intermediate responsible for degenerate branching is a fundamental one in elaborating the kinetics of methane oxidation. Indeed the statement has been made (64) that the combustion of methane may be regarded as the formation and oxidation of formaldehyde. 

Webley PA, Tester JW. Fundamental kinetics of methane oxidation in supercritical water. Energy Fuels 1991 5 411 -19. 

These data show that the copper concentration available to growing cells not only determines whether the pMMO will be expressed, but strongly influences the kinetics of methane oxidation by whole cells containing the pMMO (Table III). This information has potentially important environmental implications. If methanotrophs were copper-limited in nature, we would expect to observe high whole-cell Ks values because of expression of sMMO and the copper-deficient pMMO. This situation should result in a poor ability of natural populations to survive at low (submicromolar) methane concentrations. Alternatively, if they are not copper-limited, the Ks values should be lower, improving the ability of these cells to grow at submicromolar methane concentrations. 

In summary, the information available to date indicate that copper is a major regulatory parameter for methane oxidation in methanotrophs. Not only does it regulate the expression of sMMO and pMMO in those strains that contain both, it also controls the kinetics of methane oxidation by the pMMO. 

P. A. Webley and J. W. Tester, Fundamental Kinetics of Methane Oxidation in Supercritical Water, Energy Fuels, 5, 411-419 (1991). 

A third way to describe the kinetics of methane oxidation is the descriptive power law ... 

H., Yang, Y.L., and Jacobson, A.J. (1997) Reaction kinetics of methane oxidation over LaCri. ijeOa perovskite catalysts. 

Belessi, V.C., Ladavos, A.K., Armatas, G.S., and Pomonis, P.J. (2001) Kinetics of methane oxidation over La-Sr-Ce-Fe-O mixed oxide solids. Phys. Chem. Chem. Phys., 3, 3856-3862. 

Simchenko VP, Shcherbakov PM, Vedeneev VI, Arutyunov VS. The kinetics of methane oxidation at high... 

Bender, M., Conrad, R. 1993. Kinetics of methane oxidation in oxic soils. Chemosphere 26(1 ) 687-696. 

Walkiewicz, A., Bulak, R, Brzezihska, M., Wodarczyk, T., Polakowski C., 2012. Kinetics of methane oxidation in selected mineral soils. Int. Agrophys. 26 401-406. 

Another related problem is associated with over-oxidation of the substrate, in the extreme case resulting in complete combustion. In the case of methane oxidation by FeO", for example, the activation of methane occurs with about 10% of the gas-kinetic collision rate, whereas those of the putative oxygenation products CH3OH, CH2O, and HCOOH occur on every collision [60]. With regard to applied catalysis this would imply that the oxidation products are oxidized faster by about one order of magnitude compared to methane as the initial substrate. In the particular context of heterogeneous oxidation... 

Thus, kinetic analysis of the free-radical mechanism of methane oxidation to methanol with hydrogen peroxide under pressure shows that under current conditions the complex cyclic scheme consisting of several chain reactions is reduced to a short scheme. 

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 ... 

Creighton ( 3,5) has shown that the induction period of methane oxidation is described by Semenov s model. Analysis of the results of numerical calculations using a detailed chemical kinetics reaction scheme showed that about eight reactions were dominant, and that the rate of creation and consumption of... 

Not much information is published on the kinetics of partial oxidation. The methane concentration is about 8 times higher than indicated by reaction (71), but as expected increasing pressure promotes methane formation. From a mere thermodynamical point of view no soot should be present at 1300 °C and O/C > 1. However, the raw sythesis gas contains more or less soot, depending on feedstock. Gasification of heavy oils fractions yields about 1-2% soot, but with methane the soot content is close to zero. 

Dunfield P. and Knowles R. (1995) Kinetics of inhibition of methane oxidation by nitrate, nitrite, and ammonium in a Humisol. Appl. Environ. Microbiol. 61, 3129-3135. 

Hydrogen Oxidation Kinetics. Shea et al. (22) studied the kinetics of methane fermentation by an enrichment culture of lithotrophic (autotrophic) hydrogen oxidizing methanogenic bacteria at 37 °C. Reported values of the kinetic coeflScients are as follows (1) Y = 0.043 mg volatile suspended solids per mg of hydrogen COD removed, (2) b = —0.009 day"S (3) k = 24.8 mg hydrogen COD removed per mg volatile suspended solids per day and (4) Ks = 569 mm of mercury, hydrogen pressure. 

Drake, M. C., Kinetics of Nitric Oxide Formation in Laminar and Turbulent Methane Combustion, Report no. GRI-85/0271, Gas Research Institute, Chicago, 1985. 

Further development of kinetic models for the OCM process followed the path of addition of a limited number of heterogeneous steps (first of all— initiation or generation of primary methyl radicals) to homogeneous schemes of methane oxidation (Aparicio et al, 1991 Hatano et al, 1990 McCarty et al, 1990 Shi et al, 1992 Vedeneev et al, 1995 Zanthoff and Baerns, 1990). There was certain logic in such an approach since the most efficient OCM catalysts are almost exclusively oxides with no transition metal ions (some Mn-contain-ing oxide systems are the only exception), any reactions in adsorbed layers at such temperatures can be neglected. In the framework of such models some substantial features of the process could be described. For instance, they predicted the limit in the C2-hydrocarbon yield close to that reliably observed experimentally over the most efficient catalysts (20-25%). 

Kinetics of methane conversion to desirable products, as in reactions (4)-(6), is a vital consideration. The selective conversion of methane to any of a variety of products has been shown to proceed slowly, except for total oxidation. Thus, the design of active yet selective catalysts for various reactions of methane has presented a formidable challenge for many years. 

N. Bahlawane, Kinetics of methane combustion over CVD-made cobalt oxide catalysts Appl. Catal. B-Environ., 2006, in press. 

McCarty [125] used an annular reactor to evaluate kinetics of methane combustion over PdO-supported catalysts. The design of the apparatus had a small gap between cylinders (0.1-0.3 mm) and a thin coating (10 pm). Using high flow rates and dilute methane and oxygen in helium, the author claims to have measured the intrinsic rate of methane oxidation up to 900°C, without contributions from gas-phase reactions. Groppi et al. 

The kinetics of methane combustion on ceramic perovskites can be almost always described by the standard equation corresponding to bimolecular Rideal-Eley mechanism. To our knowledge, the full-term bimolecular Langmuir-Hinshelwood model has not been observed dimng methane oxidation. In the RE case, oxygen molecules are adsorbed in dissociative form on the surface metal ions while methane reacts with them from the gaseous phase or from a very weakly adsorbed state, the distinction been elusive. The equation describing those results reads... 

Researchers at the Politecnico di Milano [40-44] have demonstrated the interest of this kind of reactor for the kinetic study of methane oxidation. They have shown additional advantages of this reactor configuration the small annular zone leads to short diffusion distances the gas flow is laminar and theoretically based correlations can be used to describe mass transfer and high radiation losses lead to more isothermal conditions. However, its main advantages are the direct measurement of the catalyst temperature, thus eliminating any heat... 

In our opinion, based on the results of a kinetic modeling of the process, secondary oxygen is formed by the decomposition of hydrogen peroxide, a species that, at the time of complete conversion of oxygen and actual stop of the branched-chain process of methane oxidation, may contain up to 20% of the oxygen originally present in the mixture. 

The data on the kinetics of the oxidative condensation of methane are more numerous than on the other reactions of its oxidation. They show that the yield of C2 hydrocarbons in this reaction does not exceed 25%. As for the aforementioned processes, reports of higher yields, up to 30% or even higher, have not been confirmed. It has been shown that the reaction proceeds via a redox mechanism involving the interaction of methane with 0 or C>2 oxygen centres on the catalyst surface and the formation of CH3 radicals and surface OHg... 

---