Methane, activation

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

Catalysts. The methanation of CO and C02 is catalyzed by metals of Group VIII, by molybdenum (Group VI), and by silver (Group I). These catalysts were identified by Fischer, Tropsch, and Dilthey (18) who studied the methanation properties of various metals at temperatures up to 800°C. They found that methanation activity varied with the metal as follows ruthenium > iridium > rhodium > nickel > cobalt > osmium > platinum > iron > molybdenum > palladium > silver. 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

A. Hausberger As we mentioned earlier, the light hydrocarbons do not seem to affect catalyst activity, and they do reform into methane. However, you can increase the hydrocarbon content to levels where they do depress the methanation activity. If the hydrocarbons are high enough in unsaturation, they will form carbon when they get to a certain level. As far as hydrogen cyanide and ammonia are concerned, we don t expect them to affect the nickel methanation catalyst. 

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... 

Figure 2.42. Relative steady-state methanation activity profiles for Ni ( ), Co (A), Fe ( ), and Ru (O) as a function of gas-phase H2S concentration. Reaction conditions 100 kPa, 400°C, 1% CO/99%H2 for Co, Fe, and Ru, 4% CO/96% H2 for Ni.131 Reprinted with permission from Academic Press.

Figure 1.13 Transition-state configuration of methane activation on Ru(1120) surface bond (eo) ...

The BEP a value for methane activation is close to 1. As a consequence of the BEP value for hydrogenation of adsorbed methyl, the reverse reaction should be nearly zero. 

Table 1.3 Methane activation on edge and corner atoms (kilojoules per mol).

When the selectivity of a reaction is controlled by differences in the way molecules are activated on different sites, the probability of the presence of different sites becomes important. An example again can be taken from the activation of CO. For methanation, activation of the CO bond is essential. This will proceed with low barriers at step-edge-type sites. If one is interested in the production of methanol, catalytic surfaces are preferred, which do not allow for easy CO dissociation. This will typically be the case for terrace sites. The selectivity of the reaction to produce methanol will then be given by an expression as in Eq. (1.29a) ... 

We discussed that for methane activation this leads to lowering of the activation energy compared to the reactivity of terrace, edge, or corner atoms successively. 

Whereas now the bond cleavage reaction is nonsurface dependent, the reverse reaction clearly is. The stronger the NH2 and NH fragments bind, the higher the barrier for the recombination reaction. In the case of methane activation we found the reverse situation. Both situations are consistent with microscopic reversibihty. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Surface Characterization and Methanation Activity of Catalysts Derived from Binary and Ternary Intermetallics... 

Muller RP, Phihpp DM, Goddard WA. 2003. Quantum mechanical-rapid prototyping apphed to methane activation. Top Catal 23 81-98. 

Decomposition of methane to H2 and carbon over Ni/Si02 was carried out in a membrane reactor (membrane 90Pd-10Ag) [106]. The use of the membrane reactor allowed increasing the H2 yield by shifting the reaction equilibrium toward the products. An excellent review of the literature data on nonoxidative methane activation over the surface of transition metals was recently published by Choudhary et al. [107]. 

Fig. 1. Unified scheme (similar to that presented in Ref. (56)) for protonation of platinum(II) methyl compounds and for methane activation. L is a general two-electron donor ligand. The ligands L on Pt need not be identical, and charges are not shown.

A similar conclusion, that the alternative pathways sometimes differ only slightly in the energy barrier, was reached in several more recent theoretical papers which investigated methane activation in ( N2 )PtCl2 systems relevant to the Catalytica process. It has been demonstrated experimentally that compared with (bipyrimidine)PtCl2,... 

Fig. 4. Relevant structures for the discussion of methane activation by (bipyrimi-dine)PtCl2 Methane complex of Pt(II) (A) methyl(hydrido)platinum(IV) complex, the product of the oxidative addition (B) transition state for intramolecular deprotonation of the methane complex ( cr-bond metathesis , sometimes also called electrophilic , C) intermolecular deprotonation of the methane complex ( electrophilic pathway , D).

As a result of disproportionation of CO on small particles, the selectivity of the CO-H2 reactions shifts from methanol on large particles to methane on small ones. The methanation activity increases as the metal particle size decreases, indicating that methanation is a structure-sensitive reaction on palladium. 

Treatment under vacuum at 150 °C of this hydride leads to a reversible evolution of about 0.2 equiv. H2 as well as a reversible decrease of the v(Ta-H) band of about 40% these observations suggest the existence of a mixture of tantalum mono-(-80%) and tris-hydrides (-20%). This assertion is strengthened by the over-stoi-chiometric hydrogen evolution (>1H2/Ta-CH3) during methane activation on these [(=SiO)2Ta(H)J hydrides. By following the IR spectra-time evolution of this reachon at 150 °C, a heterogeneous reactivity of various [Ta-H] sites is observed, the less active needing a temperature of 250 °C to react completely with methane. 

Mechanistically, the unusual reactivity of the starting tantalum hydrides [(sSiO)2Ta H] and [(=SiO)2Ta (H)3] towards ammonia to yield the amido imido complex [(=SiO)2Ta(=NH)(NH2)] can be fully rationalized in terms of classical molecular organometallic elementary steps. Scheme 2.21 offers an example of a such sequence of elementary steps with the noteworthy close analogy with methane activation by the same hydrides described above. 

Moreover, stepwise analysis of the IR spectra corroborated the NMR evidence, allowed an understanding of the methane activation process on lb and also permitted the novel identification of v(ZrH) for la, lb and for a transient species 2i at 1633, 1649 and 1622, and 1609 cm, respectively (Figure 3.2). 

Several authors have proposed that CH4 combustion over PdO occurs via a redox mechanism [82-85]. Methane activation through assisted hydrogen extraction is generally regarded as the rate-determining step, although there is not a general consensus on the nature of the adsorption sites. Further, desorption of H2O by decomposition of surface hydroxyls has been reported to play a key role in reaction kinetics at temperatures below 450 °C [67, 86]. 

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... 

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