Methane formation rate

Methane formation rate (mole CH4/g-cat/s) C02 formation rate (mole C02/g-cat/s)... 

Table 28.1 shows the weight percent carbon in the catalyst as a function of activation and reaction. The analyzed carbon includes carbidic and free carbon. The calculated carbon content for the various carbide phases are 6.7% in Fe3C, 7.9% in x-Fe2.5C, 8.9% in e -Fe2.2C and 9.7% in -Fe2C. From the table, it can be inferred that activation for 2 h at 543 K induces a greater extent of carbide formation than activation at 523 K. This correlates well with the observed methane formation rate as well as TEM and XRD as described below. The BET surface areas of the samples after activation, activation followed by 10 h of reaction at 523 K, and after activation followed by 45 h of reaction at 523 K are shown in Table 28.2. 

Moreover, they reported that methane formation rates from thermolysis of such molecules are significantly greater than... 

Fig. 3. Methane formation rates recorded at the beginning and after 24h of continuous electrolysis runs performed at increasingly cathodic potentials. Activated Cu electrode, 0.5 M-KHCO3/CO2 solution, 22°C.

The catalyst in each reactor section can be unloaded without mixing and its coke content determined by a highly sensitive TPO technique [4], using a modified Altamira temperature-programmed unit (Model AMI-1). In this modification, the gas exiting the reaction cell enters a methanator where CO2 and CO are converted to methane over a Ru catalyst with a constant supply of hydrogen. The methane formation rate is measured by an FID detector. 

Figure 3. Average methane formation rate versus total electrolysis time (closed symbols reagent grade Na2S04 open symbols 99.999% Na2S04>.

Figure 5. Plot of methane formation rate and current vs potential for electrochemical reduction of CO2 at Ru electrodes. Hydrogen couple formal potential, -0.36 V vs SCE.

Figure 5. Integral methane formation rate for pretreated Pittsburgh seam bituminous coal...

Reaction Rate. We compared the integral methane formation rates from our pilot plant tests with those reported by others (1, 2, 6,14). To do so on the same basis, we took the reaction rate to be pseudo-first order with respect to the hydrogen partial pressure. The calculated reaction rate constant for each run is plotted against carbon gasification in Figure 5. Several observations can be made ... 

Kinetics. Extensive studies of the kinetics of methane synthesis were reported by White and co-workers (10,11, 12, 13, 14, 15). They studied the reaction between CO and hydrogen over a reduced nickel catalyst on kieselguhr at 1 atm and 300°-350°C (10). They correlated the rate of methane formation by the equation ... 

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. 

It is highly active but easily poisoned by sulfur and not particularly selective to methane. Oddly enough, carbon monoxide appears to inhibit the rate of methane formation. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

Interestingly, one can easily deduce an expression for the relative rate of coke formation as compared to that of methanation. The rate of initial coke formation depends on the combination probability of carbon atoms and hence is given by... 

The relative rate of coke versus methane formation then follows from... 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

Figure 2. The dependence of the reaction rate for methane formation on surface composition.

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. 

The model presented in this paper will display the concentration gradients in a cylindrical, fully wax-filled pore of the catalyst. For simplification reasons, only the reactants CO and H2 as well as the reaction product H20 will be considered. C02 formation is disregarded in the model due to its comparatively low concentration in FT products and because compared to steam, C02 has no impact on the main reaction rate. Methane formation (usually not more than 5% of total CO conversion) was also neglected. 

In reality, not only the main reaction (the Fischer-Tropsch reaction) leading to the formation of higher hydrocarbons (Equation 12.1), but also methane formation (Equation 12.2) and the water-gas shift reaction (Equation 12.3) have to be considered. The rate equations for these three reactions on a commercial Fe-catalyst were determined by Popp8 and Raak2 and summarized by Jess et al.9 However, to simplify matters, just the Fischer-Tropsch reaction forms the basis of the approach presented here ... 

Some of these same experiments have been done using 10% Fe/Al203 rather than the fused iron catalyst (53). Figure 22 shows the result of a switch from H2 to 10% CO in H2 over a freshly reduced catalyst. Here a large initial rate of methane formation is observed and water does not appear until most of the initial peak has passed. The probable explanation for the presence of the CHi peak is that water produced by methanation is adsorbed on the initially dry y-Al203 support (100 m2/g). Thus the iron remains briefly in a relatively reduced state. For the CCI catalyst the AI2O3 promoter is not sufficient to prevent the water from rising quickly as shown in Fig. 19. The H/0 ratio on the surface is reduced, and carburization occurs more rapidly than methanation, as for the unsupported catalyst. 

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