Methane catalyst formation

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

Intimate mixing of the components can lead to the formation of compounds or of solid solutions of the components which are difficult to reduce at 300°C but which, when reduced, contain well dispersed and well stabilized nickel. Methanation catalysts in practice therefore are compromises which combine optimum reducibility with activity and stability. As an example of compound formation, alumina readily forms with nickel... 

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... 

Agrawal et al.33 performed studies of Co/A1203 catalysts using sulfur-free feed synthesis gas and reported a slow continual deactivation of Co/A1203 methanation catalysts at 300°C due to carbon deposition. They postulate that the deactivation could occur by carburization of bulk cobalt and formation of graphite deposits on the Co surface, which they observed by Auger spectroscopy. 

Snoeck, J.-W., Froment, G. F., and Fowles, M. 2002. Steam/C02 reforming of methane. Carbon formation and gasification on catalysts with various potassium contents. Ind. Eng. Chem. Res. 41 3548-56. 

The ratio of rates of formation and removal (by H2) of firmly bound species ( carbon ) is different with different metals. Evidently, Pt and Pd keep more carbon on their surfaces than do the good methanation catalysts such as Ni, Ru, or Rh. The surface of, say, Pt is better blocked and thus protected against hydrogenolysis than are surfaces of other metals. The often-found particle size sensitivity of hydrocarbon reactions on Pt (less on other metals) might be related to this. 

Rhodium is a unique metal since it can catalyze several transformations.222,223 It is an active methanation catalyst and yields saturated hydrocarbons on an inert support. Methanol is the main product in the presence of rhodium on Mg(OH)2. Transition-metal oxides as supports or promoters shift the selectivity toward the formation of C2 and higher oxygenates. 

Zeolites also appear to be suitable supports for the formation of small bimetallic clusters between components which are immiscible as bulk metals. The Ru/Ni zeolite Y methanation catalyst is an interesting example (234) where the properties of a single active metal component could be advantageously modified. 

In 1974, the oil supply crisis stimulated research throughout the world on the Fischer-Tropsch Synthesis (FTS) of fuels. Surprisingly, the first result of this was evidence concerning the mechanism with typical FTS and methanation catalysts — Fe, Co, Ni (Ru) — the initiation step is the dissociation of CO [8] and not the formation of hydroxycarbene. 

It is now widely accepted that the activation of CO is highly structure sensitive (II). The activation of CO on most of the transition metals has been investigated. The computational results for cobalt (6) and ruthenium (5) are of particular relevance to us because these elements in the metallic state are active for the Fischer-Tropsch reaction. These results can be compared with those obtained for rhodium (40), which selectively catalyzes the formation of alcohols from CO and H2, and for nickel (30), which is a methanation catalyst. 

In H2 production from methane, carbon formation usually takes place in the form of fibres or whiskers, with a small Ni particle at the top of a fibre.5,6 Carbon formation may lead to the breakdown of the catalyst, and carbon deposits and degraded catalyst may cause partial or complete blockage of the reformer tubes. Uneven flow distribution is responsible for localised overheating of the hot tubes. Accordingly, carbon formation must be avoided in tubular reformers. There are two major reactions responsible for carbon formation ... 

Ruthenium Catalyst. The ruthenium catalyst studied was 0.5% ruthenium on 1/8-in. alumina pellets. Data in Table IV show that at low temperatures and low feed rate/catalyst weight ratios, this is an effective methanation catalyst. Higher hydrocarbon formation is, again, lower than expected, possibly owing to hydrogenolysis. Trace yields of oils containing paraffins, olefins, and aromatics were obtained. 

With y-alumina, silica and Na-Y zeolite as supports, catalysts derived from [PPN][Ru3Co(CO)i3][ I and other Ru-Co clustersl were studied in the hydrogenation of CO to methane. The formation of CO2 suggested the occurrence of the WGSR and the possibility, at low CO/H2 ratios (1 4), of there being some CO2 methanation. Under these conditions the best yields were obtained with alumina as support. 

For a long time Cu has been considered as the only metal active in the methanol synthesis, while Pd, Pt, and Ir have been regarded as poor methanation catalysts.It was therefore rather a surprise when Poutsma et al reported that Pd, and to a lesser extent also some other Group VIII metals, can be very good methanol synthesis catalysts. Later it appeared that Poutsma et al. were quite lucky in the choice of their silica not aU silicas are equally good as supports.Besides the important possible effects of the different defect structures of various silicas, one has to be aware of the role that minute contaminations can play. The prominent role of the support in making Pd active for methanol synthesis is now established quite clearlythe best supports are those which can form an intermediate compound with Pd precursor, and the best promoters also form such compounds. It is also known that formation of these intermediate compounds decreases the reducibility of the compound (i.e., PdClj) which is used as the primary precursor of the catalyst. 

Fe4S4(SR)4] , are good catalysts for reduction of CO2. Recently it was found that other compounds also facilitate such reduction. Electrolytic reduction of CO2 in acetonitrile in the presence of [Rh(dppe)2]Cl leads to the formation of The rhenium(I) compound [ReCl(CO)3 (bipy)] catalyzes electrolytic reduction of CO2 to carbon monoxide. The electrolytic reduction of CO2 in some cases probably proceeds via the radical anion C02 its formation explains various reduction products see scheme (13.241). Palladium complexes, for instance, [Pd2Cl2(dppm)2], [Pd(dppm)2], and [PdCl2(dppm)], slowly catalyze reduction of CO2 to methane, ethyl formate, and traces of ethyl oxalate. 

A particular issue is the deactivation of methanation catalysts by carbon formation. Kuijpers et al. [345] observed significant carbon formation over a nickel/kieselgur catalyst containing 54wt.% nickel when exposed to a mixture of 10 vol.% carbon monoxide, 15 wt.% hydrogen, with a balance of nitrogen at 0.6 bar pressure and a 250 °C reaction temperature. Carbon filaments were found, which contributed to 10 wt.% of the catalyst mass at the inlet of the fixed bed. A nickel/silica catalyst showed practically no coke formation for 1000 h duration under the same conditions. 

Methane reforming Eq. (2.36) is the simplest example of steam reforming (SR). This reaction is endothermic at MCFC temperatures and over an active solid catalyst the product of the reaction in a conventional reforming reactor is dictated by the equilibrium of Eq. (2.36) and the water-gas shift (WGS) reaction Eq. (2.37). This means that the product gas from a reformer depends only by the inlet steam/ methane ratio (or more generally steam/carbon ratio) and the reaction temperature and pressure. Similar reaction can be written for other hydrocarbons such as natural gas, naphtha, purified gasoline, and diesel. In the case of reforming oxygenates such as ethanol [125, 126], the situation is in some way more complex, as other side reactions can occur. With simple hydrocarbons, like as methane, the formation of carbon by pyrolysis of the hydrocarbon or decomposition of carbon monoxide via the Boudouard reaction Eq. (2.38) is the only unwanted product. 

---