Methane formation activity

Methane. The methane evolution profiles for all five shale samples are surprisingly similar, but occur at significantly higher temperatures than has been observed (2) for the Green River shale. Although some methane evolution accompanies the oil formation, the major part is formed in the secondary pyrolysis region. At least three major processes with maxima in the vicinity of 500, 580 and 700°C appear to contribute to the total methane formation. Activation energies for these processes were determined for Condor carbonaceous shale and are summarised in Table 6. 

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... 

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. 

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

Similarly, Bond et al. [4] confirmed that the microwave stimulation of methane transformation reactions in the presence of a number of rare earth basic oxides to form C2 hydrocarbons (ethene, ethane) was achieved at a lower temperature and with the increased selectivity. Microwave irradiation resulted in an increase of the ethene/ethane ratio, which was desirable. The results obtained were explained by the formation of hot spots (Sect. 10.3.3) of higher temperature than the bulk catalyst. This means that methane is activated at these hot spots. 

Figure 7 shows the results of TPR of CO adsorbed on nickel supported on activated carbon, y-alumina and silica gel, respectively. For Ni/Y Al20 and Ni/Si02/ only CO was desorbed at low temperature and methane (and CO2) were formed at higher temperature. In the case of Ni/A.C., however, almost all of adsorbed CO was desorbed below 150 C. It has been generally accepted that the first step of methane formation by hydrogenation of CO is the dissociation of C-0 bond (Equation 8) (8). The resultant and then react with either hydrogen or CO as... 

Copper based catalysts have long been considered as the only effective methanol synthesis catalysts. However, Poutsma et al. (7) showed that palladium catalysts were active in methanol synthesis from CO-H. This latter metal had been previously considered as either almost inactive or active only for methane formation (8). Furthermore it is now known that both activity and selectivity can change drastically with the support. Vannice (9) observed that the methanation activity of a Pd/Al O was enhanced eighty and forty times compared to palladium black or Pd/SiO (or Pd/TiO ) respectively. The support effect on the selectivity was pointed out by many authors even at atmospheric pressure when the reaction temperature... 

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. 

Tonkovich et al. [81] compared the performance ofa commercial ruthenium/zirconia powder catalyst from Degussa with a laboratory-made ruthenium/zirconia catalyst prepared on a nickel foam monolith for the water-gas shift reaction. Methane formation occurred for the powder catalyst, which was much less pronounced for the monolith. The selectivity towards methane could be reduced at shorter residence times. However, the activity of the laboratory-made catalyst was lower, which was partially attributed to the lower catalyst mass (modified residence time). 

Some papers have been published that examine Ru/SiC>2 as a catalyst in the Methanation step. These papers looked at the effects of hydrogen and temperature as well as how a Cl-modified Ru/SiC)2 catalyst performs. The Cl decreased catalytic activity, but it enhanced selectivity for methane formation -even though it was present on the catalyst only during the initial stages of the reaction77, 8. For extremely low temperature applications (i.e., < 180°C), one company offers a catalyst with 0.3% ruthenium on alumina. This catalyst does not contain any NiO or CaO166. 

Potassium promoters have also been investigated for other catalyst metals. Thus, the addition of KNO3 to Ni(OM)j/SiOi led to a sharp decrease in methane formation [59] and a Ru3(C0)i /K2C03/AJ2 03 catalyst was shown to be highly active forC2 --C5 olefin formation [60]. 

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