Decomposition methane

The structure-sensitive character of methane decomposition reaction has been confirmed, as has the importance of the metal dispersion on the catalyst performance over different supported Ni catalysts. TEM analyses of the spent catalysts reveal that both filamentous and encapsulating carbon species were formed under isothermal conditions at 823 K, the latter being responsible for catalyst deactivation. 

Li et reported a novel method of obtaining nickel oxide particles with controlled crystalline size and fibrous shape, highly dispersed on in situ produced carbon, inhibiting further growth of Ni particles. On the other hand, Ni/CFC (filamentous carbon) catalysts were shown to have sufficient efficiency in low-temperature methane decomposition. Thus, the use of CFG, whose textural properties can be modified by their activation with Hg or COg, opens up the possibility of its application as a support in heterogeneous catalysis. Methane decomposition over Ni-loaded activated carbon (AC) was also investigated. XRD results showed absence of NiO with only Ni metal crystallites formed in the catalyst even if calcined in Ar, which eliminates the inevitable reduction step with other supports. However, the formation of NisC during the process leads to deactivation of the catalysts. Filamentous carbon formation is 

A mixture of Ni°/NiO, produced by thermal decomposition of nickel acetate, dispersed on either silica or cordierite supports, was found to be catalytically active for the decomposition of methane without the need for any pre-treatment. Other authors used Ni catalysts supported on zirconia to produce H2 and a high yield of multiwalled carbon nanotubes. Raman spectroscopy suggested that carbon nanotubes formed at temperatures higher that 973 K had more graphite-like structure than those obtained at lower temperatures. They also reported that feed gas containing methane and hydrogen caused slow deactivation of the catalyst, and carbon yield increased with increasing Hg partial pressure in the feed gas. For a commercial Ni catalyst (65% wt Ni supported on a mixture of silica and alumina) it was found that catalyst deactivation depends on the 

Bonura et alf studied several supported Ni catalysts for methane decomposition and found that both filamentous and encapsulating carbon species were formed under isothermal conditions at 823 K, the latter being responsible for catalyst deactivation. They also confirmed the structure-sensitive character of methane decomposition. The efficient use of these catalysts implies a high dispersion of metal phases which can be achieved by controlled segregation of the active phase. Different Ni mixed oxides such as Ni-Al hydrotalcite, Ni-La perovskites and Ni-Al spinels as catalysts precursors allow a high degree of Ni dispersion, of which that derived from hydrotalcite mixed oxide showed the highest activity for Hg production by methane decomposition.  

Cobalt molybdenum carbide was also found to be active as a catalyst for the methane decomposition for hydrogen production. 

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As in the steam/TCR analysis the Boudouard reaction is ignored here, together with direct methane decomposition. 

Methane decomposition is the most important reaction step, especially for high-temperature operations. Thus, carbon deposition occurs commonly and is a major problem, especially with the Ni-based anode. However, carbon deposition may not deactivate the anode [10, 11]. In some cases, the anode activity increases due to carbon deposition whieh increases the electrical conductivity of the low-Ni-content anode [II]. 

It could be concluded that thermal plasma process for methane decomposition is very effective for the production of high purity of the hydrogen as well as synthesis of the carbon black. 

The effect of catalyst supports on methane conversions and hydrogen yield in the methane decomposition at 998 K and GHSV of2700 h at steady state. 

Subsequent admission of oxygen in pulses indicated that carbon deposited by methane decomposition could be removed quantitatively by oxidation. The carbon remaining on the catalysts could also be quantitatively removed in the presence of Pt by CO2 CO was the only reaction product... 

Let us now use the sequence of elementary steps to explain the activity loss for some of the catalysts The combination of hydrogen chemisorption and catalytic measurements indicate that blocking of Pt by coke rather than sintering causes the severe deactivation observed in the case of Pt/y-AljOj The loss in hydrogen chemisorption capacity of the catalysts after use (Table 2) is attributed mainly to carbon formed by methane decomposition on Pt and impeding further access. Since this coke on Pt is a reactive intermediate, Pt/Zr02 continues to maintain its stable activity with time on stream. 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

Ogata, A., Mizuno, K., Kushiyama, S. and Yamamoto, T. (1998) Methane Decomposition in a Barium Titanate Packed-Bed Nonthermal Plasma Reactor, Plasma Chem. Plasma Process 18, 363-73. 

The photochemistry of Titan s atmosphere can be summarized as follows the unsaturated compounds are formed from HCN and C2H2, which is derived from CH4. Methane decomposition leads to further ethane formation. 

Thermodynamic equilibrium data for methane decomposition reaction at atmospheric pressure. 

Methane decomposition reaction is a moderately endothermic process ... 

Figure 2.19 provides the thermodynamic equilibrium data for methane decomposition reaction. At temperatures above 800°C, molar fractions of hydrogen and carbon products approach their maximum equilibrium value. The effect of pressure on the molar fraction of H2 at different temperatures is shown in Figure 2.20. It is evident that the H2 production yield is favored by low pressure. The energy requirement per mole of hydrogen produced (37.8 kj/mol H2) is significantly less than that for the SMR reaction (68.7 kj/mol H2). Owing to a relatively low endothermicity of the process, <10% of the heat of methane combustion is needed to drive the process. In addition to hydrogen as a major product, the process produces a very important by-product clean carbon. Because no CO is formed in the reaction, there is no need for the WGS reaction and energy-intensive gas separation stages.

Because methane decomposition reaction requires high temperatures, there have been attempts to use catalysts to reduce the temperature of thermal decomposition of methane. Figure 2.21 summarizes reported literature data on different catalysts for methane decomposition and the preferred temperature range. It can be seen that transition metals... 

Summary of literature data on methane decomposition catalysts and preferred temperature range. Catalysts 1 = nickel, 2 = iron, 3 = carbon, and 4 = other transition metals (Co, Pd, Pt, Cr, Ru, Mo, W). The dotted line arbitrarily separates heterogeneous (catalytic) and homogeneous (noncatalytic, gas phase) temperature regimes of the methane decomposition reaction. 

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 ... 

A series of kinetic studies on the carbon filament formation by methane decomposition over Ni catalysts was reported by Snoeck et al. [116]. The authors derived a rigorous kinetic model for the formation of the filamentous carbon and hydrogen by methane cracking. The model includes the following steps ... 

The use of carbon-based catalysts offers certain advantages over metal catalysts due to their availability, durability, and low cost. In contrast to the metal-based catalysts, carbon catalysts are sulfur resistant and can withstand much higher temperatures. Muradov [98,99] screened a variety of carbon materials and demonstrated that the efficient catalytic methane decomposition can be accomplished over high surface area carbons at temperatures... 

The apparent reaction order of carbon-catalyzed methane decomposition reaction was determined to be 0.6 0.1 for AC (lignite) and 0.5 0.1 for CB (BP2000) catalysts. Thus, the rate equation for carbon-catalyzed decomposition of methane can be written as follows ... 

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