Group VIII metals methanation activity

The metal catalysts active for steam reforming of methane are the group VIII metals, usually nickel. Although other group VIII metals are active, they have drawbacks for example, iron rapidly oxidizes, cobalt cannot withstand the partial pressures of steam, and the precious metals (rhodium, ruthenium, platinum, and palladium) are too expensive for commercial operation. Rhodium and ruthenium are ten times more active than nickel, platinum, and palladium. However, the selectivity of platinum and palladium are better than rhodium [1]. The supports for most industrial catalysts are based on ceramic oxides or oxides stabilized by hydraulic cement. The commonly-used ceramic supports include a-alumina, magnesia, calcium-aluminate, or magnesium-alu-minate [4,8]. Supports used for low temperature reforming (< 770 K) are... 

The methanation reactions are the reverse of the reactions for steam reforming. Nickel and other Group VIII metals are active. Both CO2 and CO are converted, meaning that the syngas should have a module M=3 (refer to Table 1.6). 

Throughout these studies, no product other than propane was observed. However, subsequent studies by Sinfelt et al. [249—251] using silica-supported Group VIII metals (Co, Ni, Cu, Ru, Os, Rh, Ir, Pd and Pt) have shown that, in addition to hydrogenation, hydrocracking to ethane and methane occurs with cobalt, nickel, ruthenium and osmium, but not with the other metals studied. From the metal surface areas determined by hydrogen and carbon monoxide chemisorption, the specific activities of... 

Arrhenius parameters for the methanation reaction on alumina-supported Group VIII metals (227b) were close to the line for cracking reactions on several metals (Table III, A). Activity was based on the numbers of surface metal atoms and a compensation relation was described from these data we calculate c = 0.1185 0.0117, B = 15.216 1.068, and oL = 0.491. 

These activities were based on activities per unit metallic area rather than on activities per gram, as in the results of Fischer and Tropsch. Rostrup-Nielsen suggests that the low activity of the cobalt may be due to the fact that the reaction was carried out under conditions when the cobalt can be oxidized by the steam of the reaction mixture. For the methanation reaction, Fischer, Tropsch, and Dilthey49 give the following order of activities for the Group VIII metals ... 

Direct experimental verification of very highly dispersed bimetallic clusters is complicated by limitations in the ability of physical methods to obtain structural information on such systems. In such a system, however, a catalytic reaction can serve as a sensitive probe to obtain evidence of interaction between the atoms of the two metallic components. For supported bimetallic combinations of a Group VIII and a Group IB metal, the hydrogenolysis of ethane to methane is a useful reaction for this purpose. In the case of unsupported bimetallic systems of this type, as discussed previously, the interaction between the Group VIII metal and the Group IB metal results in a marked suppression of the hydrogenolysis activity of the former. 

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. 

In order to produce methanol the catalyst should only dissociate the hydrogen but leave the carbon monoxide intact. Metals such as copper (in practice promoted with ZnO) and palladium as well as several alloys based on noble group VIII metals fulfill these requirements. Iron, cobalt, nickel, and ruthenium, on the other hand, are active for the production of hydrocarbons, because in contrast to copper, these metals easily dissociate CO. Nickel is a selective catalyst for methane formation. Carbidic carbon formed on the surface of the catalyst is hydrogenated to methane. The oxygen atoms from dissociated CO react with CO to CO2 or with H-atoms to water. The conversion of CO and H2 to higher hydrocarbons (on Fe, Co, and Ru) is called the Fischer-Tropsch reaction. The Fischer-Tropsch process provides a way to produce liquid fuels from coal or natural gas. 

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. 

CH4 reactions with CO2 or H2O on group VIII or noble metals (Ru, Rh, Pd, Ir, Pt) [1] form synthesis gas which is the precursor to valuable fuels and chemical compounds, as lirst shown by Fischer and Tropsch [2]. Due to the cost and availability of the nickel, compared to noble metals, Ni catalysts are used industrially. However, Ni-based catalysts tend to form inactive carbon residues that bloek the pores as well as the active sites of catalyst, and whose main activity is die formation of carbon filaments [3]. Therefore, the industrial methane steam reaction is usually performed under an excess of water to maintain the catalyst activity. Another alternative is the modification of the composition of the catalyst (generally Ni/Al203) by addition of a basic compound like MgO [4]. It is well known that the formation of NiO-MgO solid solution is easily favoured by calcining the mixed oxide at high temperatures [5] and much attention was devoted to its specific properties [6]. Parmaliana and al. 

A large number of studies have been performed for the development of active and coke-resistant catalysts for the dry reforming of methane. The common catalysts are composed of a metal from group VIII to X supported over an oxide. Noble metals are well known for their high catalytic activities in reforming reactions, but nickel-based catalysts are widely used due to its lower cost... 

Methanation. Table VIII shows those carbonyl catalysts which when activated at 250 °C were found to be more active than a reduced oxide. Again, with the exception of Os the list is limited to the difficult to reduce metals. As in the case of ethylene hydrogenation, after activation at 500 °C in H. a mutually exclusive group is formed, consisting mostly of noble metals. 

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