Alumina-supported catalysts, methanation activity

Whereas the effect of water on deactivation and on the overall activity of the FTS varies with the support, similar effects of water on the selectivity is reported for all catalysts, to a certain degree independent of the support, promoter and conditions. The effect can be summarized as an increase in C5 + selectivity, a decrease in methane selectivity, and in some instances a weak enhancement of the C02 selectivity is observed. Fig. 4 illustrates the effect on the C5 + and methane selectivity of adding water to cobalt catalysts supported on alumina, silica and titania, and both unpromoted and Re-promoted catalysts are shown. At the outset these selectivities are strong functions of the conversion, the C5 + selectivity increasing and the methane decreasing with increasing conversion, as illustrated by the trendlines in the figures. The points for methane are below, and C5 + -selectivity is above the line when water is added. Similar results were reported by many authors for alumina-supported catalysts,16-19 23 30 silica-supported catalysts,30 37 46-48 and titania-supported catalysts.19 30... 

Vannice and co-workers (176) found that methanation over Pd catalysts is essentially a surface-insensitive reaction however, the activity depends upon the support used. Alumina-supported and silica/alumina-supported catalysts were 10 times more active than the silica-supported ones, resembling very much the corresponding relation reported for benzene... 

Fig. 25. Activity versus time for alumina-supported catalysts during methanation at 525 K, 1 atm in 10 ppm H2S (H2/CO = 99) (Ref. 194).

A comparative study of the alumina-supported catalysts prepared from [H2FeOs3(CO)i3], [H20s3Rh(acac)(CO)io], and [Rli4(CO)i2] was performed and each catalyst was found to be active in the conversion of CO -I- H2. The major product observed in each experiment was methane and the hydrocarbon products were formed in approximately a Schulz-Flory-Anderson distribution. The heterogeneous [Os3Rh] catalyst was two orders of magnitude more active at 543 K than the [FeOs3] catalyst, but showed a lower selectivity for ether formation. ... 

The morphology and properties of the carbon deposition of the nickel-based catalysts for carbon dioxide reforming of methane are investigated. Silica supported nickel catalysts were more facile to carbon deposition than alumina supported catalysts. The decomposition of methane resulted in the formation of at least three kinds of surface carbon species on supported nickel catalysts. Carbidic C , carbonaceous Cp and carbidic clusters Cy surface carbon species formed by decomposition of methane showed different thermal stability and reactivity. The carbidic carbon was a very active and important intermediate in the carbon dioxide reforming of methane and the carbidic clusters Cy species might be the precursor of the surface carbon deposition. The partially dehydrogenated Cp species can react with H2 or CO2 to form CH4 or CO. 

NASA conducted studies on the development of the catalysts for methane decomposition process for space life-support systems [94], A special catalytic reactor with a rotating magnetic field to support Co catalyst at 850°C was designed. In the 1970s, a U.S. Army researcher M. Callahan [95] developed a fuel processor to catalytically convert different hydrocarbon fuels to hydrogen, which was used to feed a 1.5 kW FC. He screened a number of metals for the catalytic activity in the methane decomposition reaction including Ni, Co, Fe, Pt, and Cr. Alumina-supported Ni catalyst was selected as the most suitable for the process. The following rate equation for methane decomposition was reported ... 

Alumina-supported Ru catalysts derived from supported ruthenium carbonyls have been reported to be effective for carbon dioxide methanation, showing higher activity than other catalysts prepared from RUCI3. The catalytic activity depended on the nuclearity of the carbonyl precursor [111]. 

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

Methane (99.99 vol%) (Air Products and Chemicals, Inc.) was used without further purification. Activated alumina, Ni(NO3)26H20 and Fe(N03)39H20 (Fisher Scientific) were used without further purification. Alumina-supported Ni and Fe catalysts were synthesized according to the procedures described in the literature.38 Samples of activated carbon and graphite were obtained from Fisher Scientific and Aldrich, respectively. Preparation of POT photocatalysts and related experimental procedures were described previously.14... 

Comparison of Methanation Activities for Zeolite and Alumina Supported Nickel Catalysts... 

Silver-alumina type catalysts are by far the most widely used, especially since they are the main catalytic source in the epoxidation of ethylene. Therefore, they are readily available and already have undergone extensive studies. Many systems have sought to utilize the presence of NO (another harmful environmental species) in gas feeds. In this case, the NO species would be reduced to N2, causing oxidation of the hydrocarbon with the support of the catalyst. Studies have helped to elucidate the active species on the catalyst surface at varying temperatures and species leading to the desired products (31). Results from a recent study point to the active silver species being a [Ag O Al] bound intermediate that leads to N2 formation (32). If the silver is present in nanoparticle form, it is simply believed to be a spectator. Other work showed mixed results on the benefit of silver-based alumina systems for the oxidation of methane and higher hydrocarbons. The effect is dependent on the type of reactor system prepared (33,34). 

It is known that supported palladium catalysts are the most active for the total oxidation of methane [3], and there are many studies focusing on the alumina supported ones [4 and references cited therein] However, alumina is not stable at the temperatures commonly used for methane oxidation. To avoid this problem, other authors [5] have suggested the use of zirconia-based supports, which are considered as more thermally stable. In this way, these supports were found to present very different properties, depending on the synthesis method and the presence of additives. 

As shown above, oxidized diamond exhibited considerable activity in the oxidative dehydrogenation of alkanes, hence further studies on the oxidized diamond supported catalysts were exploited. Nickel-loaded alumina is generally used for the partial oxidation of methane (reaction 5). However, carbon deposition onto the nickel is the major problem in the commercialization of this process. 

Methane oxidation at mild or low temperatures can be catalyzed by platinum group metals. Palladium is one of the most efQdent metals (1) and has been studied over mai supports (2-6). This particular metal, when supported on alumina, b ins to show an increase in its activity between 350 and 420°C. At these conditions a general increase in the active spedes particle size is observed. Piimet and Briot (7,8) defined two states for the Pd/Al203 supported catalyst a state I, obtained after simple reduction and a state n after the catalyst had reacted at 600°C for 14 h under 02/CH4=4A. State II was more active than state I and showed a lower binding oietgy of oxygen with palladium. However, the state of the active phase was not clear. The diffoences in activity, also observed by others, have also been related to the formation/decomposition of PdO (9), to the oxygen adsorbed on metallic Pd (2), to the modification of Pd surface spedes (3), and to the reconstruction of PdO crystallites (4, 10). One of the hypotheses for the activation of the Pd catalysts was the establishment of an epitaxy between the metal and the support (8, 11). 

An increase in the activity of alumina supported Pd catalysts under conditions of methane complete oxidation was once again observed in the present work. Even catalysts which were treated under the reaction mkture at 900°C, an elevated temperature even for a combustion reaction, were activated. The samples can only be activated under reaction conditions and at high temperatures. Furthermore, the increase in activity was accompanied by an increase in the particle size. However, the catalytic activity could not be correlated directly with the particle size as already observed by others (4, 5). The absence of a correlation is not surprising since it was observed that the active catalyst contained both metallic and oxidized palladium. 

---