Kinetics methane activation energy

The kinetic expression was derived by Akers and White (10) who assumed that the rate-controlling factor in methane formation was the reaction between the adsorbed reactants to form adsorbed products. However, the observed temperature-dependence of the rate was small, which indicates a low activation energy, and diffusion was probably rate-controlling for the catalyst used. 

Kim et al. [123] conducted the kinetic study of methane catalytic decomposition over ACs. Several domestic (South Korea) ACs made out of coconut shell and coal were tested as catalysts for methane decomposition at the range of temperatures 750-900°C using a fixed-bed reactor. The authors reported that no significant difference in kinetic behavior of different AC samples was observed despite the differences in their surface area and method of activation. The reaction order was 0.5 for all the AC samples tested and their activation energies were also very close (about 200 kj/mol) regardless of the origin. The ashes derived from AC and coal did not show appreciable catalytic effect on methane decomposition. 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

Kinetic measurements over a Ni(lOO) catalyst containing well-controlled submonolayer quantities of potassium show a general decrease in the steady-state methanation rate with little apparent change in the activation energy associated with the kinetics (Fig. 22). However, the potassium did change the steady-state coverage of active carbon on the catalyst. This carbon level changed from 10% of a monolayer on the clean catalyst to 30% on the potassium covered catalyst. 

The induction time data and density profiles pf detonations in oxy-hydrogen and oxy-methane mixtures were analyzed on the basis of the kinetic data obtained by the reflected-wave technique and similar methods. A plot of the ignition delay vs 1/T in oxy-ammonia mixtures gave a straight line with a slope corresponding to an activation energy of 42.5 kcal/mole. In these mixtures the induction zone is not uniform, but the shock front is flat and end of the reaction zone is clearly discernible. Onedimensional detonation waves of low Mach number but relatively stable were obtained in a gas preheated to 600-1800°K ahead of the shock front... 

Carbon-based catalysts have also been considered for the methane decomposition reaction. Yoon and co-workers have recently investigated the kinetics of methane decomposition on activated carbons as well as on carbon blacks.In case of activated carbons the authors observed mass transport effects in the catalyst particles and also significant pore mouth plugging. The reaction order was found to be 0.5 and the activation energy was found to be 200 kJ/mol for the different activated carbon samples. On the other hand, for... 

Figure 10.14 and Figure 10.15 show the correlation of kinetic rates and activation energy vs. Elumo. The dataset used was 2-butanone, acetic acid, methane, and acetamide ... 

The reason why the minimum steam ratio goes down with temperature is not known with certainty. One possibility is that the competing reactions of carbon production and consumption have such kinetics that the rate of coke consumption increases faster with temperature than the rate of coke generation, which suggests that the carbon-steam reaction has a higher activation energy than the methane cracking and carbon monoxide disproportionation reaction. 

Activation energies for methanation over Ni and Ru were the same for both the fresh and the aged catalysts (Table XVII). In contrast, activation energies for methanation over aged Co and Fe were lower by 50 and 25 kJ/mol, respectively as compared to fresh Co and Fe (Table XVII). In the case of Co the CO partial pressure dependence changed from a negative order in the upper pseudosteady state to a positive-order dependence in the lower pseudosteady state (Table XVIII). The dependence on H2 partial pressure was positive one-half order in both upper and lower pseudosteady states. For Fe the kinetic behavior was not investigated. 

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