Rates of methane formation

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 ... 

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... 

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. 

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. 

Methanol Reduction at Ruthenium. The reduction of methanol to methane does occur as shown by the data in Table III. The data for each electrode are presented in the order that they were collected. Rates can be higher for methanol reduction compared to carbon dioxide reduction though faradaic efficiencies are lower. Unlike carbon dioxide reduction, the rate of methane formation is extremely... 

Figure 3. The effect of pH on the average rate of methane formation from methanol. In 0.2 M Na2S04 at 60 °C and at constant over potential (see table 3).

Water addition leads to an increased formation rate of C5 +, but the rate of methane formation is constant, hence the difference in selectivity with water addition. 

Intrinsic to interpreting catalytic poisoning and promotion in terms of electronic effects is the inference that adsorption of an electropositive impurity should moderate or compensate for the effects of an electronegative impurity. Recent experiments have shown this to be true in the case of CO2 methanation where the adsorption of sulfur decreases the rate of methane formation significantly. The adsorption of potassium in the presence of sulfur indicates that the potassium can neutralize the effects of sulfur. 

Although addition of Sn to Raney Ni catalysts greatly reduces the rate of methane formation, to achieve the highest selectivities for production of H 2 it is also essential to minimize the partial pressures of the gases produced and their residence time in the reactor. In contrast, over unpromoted Raney Ni catalysts it is impossible to achieve this high selectivity under any conditions [292]. 

Reaction 22a is important only with cobalt acetate catalyst and accounts for the fast rate of methane formation during the reaction of peracetic with acetaldehyde. It can also explain how methane is produced only from the methyl group of peracetic acid. This reaction path is more important with cobalt probably because of the higher oxidation potential of the cobalt (III)-cobalt (II) couple relative to that of the manganese (III) -manganese (II) couple. 

It was shown later that ATP synthesis can be coupled to methanogenesis at very low A/iH values (—90 to —lOOmV) [103] Proton potentials of defined magnitude, adjusted with K gradient in the presence of valinomycin, were applied to cells of Methanosarcina barkeri and Methanobacterium thermoautotrophicum, and the ATP pool and the rates of methane formation from Hz/methanol and from Hz/COz were followed as a function of... 

The fact that there is a significant increase in the rate of methane formation shows that the NO is providing some entirely different mechanism for CH4 production. In this connection, it is interesting that in the ethane pyrolysis these is no HCN, which is formed in significant amounts in the acetaldehyde decomposition. A likely source of HCN is... 

Table I. Relative Rates of Methane Formation from Various Substrates...

In the presence of excess hydrogen, the rate of methane formation through the methanation reaction increases by increasing the operating pressure, while at the same time carbon dioxide will react with hydrogen to produce carbon monoxide through the reversed shift reaction. The same trend is observed for the release rate of ethylene and... 

Figure 4. Plot of the rate of methane formation at a Ru electrode as a function of pH at 60-63 C and a constant overpotential.

The rate of methane formation for pretreated Pittsburgh coal is not slowed by the presence of methane in the feed gas. This agrees with Zielke and Gorin (14) in their study of hydrogasmcation of Disco char. 

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