Methane decomposition catalysts

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. 

Bimetallic Catalysts and Promoters. Shah and co-workers compared the methane decomposition activities and stabilities for monometallic (Pd, Mo or Ni) and bimetallic M-Fe (M = Pd, Mo or Ni) catalyst above 673 Their studies showed that the bimetallic M-Fe catalysts produced hydrogen at significantly higher rates than the monometallic (M) catalysts. The Pd-Fe catalyst was found to be the most active methane decomposition catalyst at 973 K. 

The energy input requirements for TCD are significantly less than that of steam methane reforming (37.8 and 63.3 kJ/mol H2, respectively). Due to the absence of oxidants (e.g., H2O and/or O2), no carbon oxides are formed in the reaction. The choice of an efficient and durable methane decomposition catalyst is vital for the development of a TCD process. Two major problems associated with existing catalysts relate to their rapid deactivation (due to carbon deposition) and coproduction of large amounts of CO2 during the catalyst regeneration step. The successful development of efficient and stable carbon-based catalysts for a methane decomposition process can solve both the catalyst deactivation and CO2 emission problems. 

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. 

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... 

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 ... 

Kim et al. [123] conducted the kinetic study of methane catalytic decomposition over ACs. Several domestic (South Korea) ACs made out of coconut shell and coal were tested as catalysts for methane decomposition at the range of temperatures 750-900°C using a fixed-bed reactor. The authors reported that no significant difference in kinetic behavior of different AC samples was observed despite the differences in their surface area and method of activation. The reaction order was 0.5 for all the AC samples tested and their activation energies were also very close (about 200 kj/mol) regardless of the origin. The ashes derived from AC and coal did not show appreciable catalytic effect on methane decomposition. 

Takenaka, S. et al., Methane decomposition into hydrogen and carbon nanofibers over supported Pd-Ni catalysts,. Catal., 220, 468,2003. 

Piao, L. et al., Methane decomposition to carbon nanotubes and hydrogen on an alumina supported nickel aerogel catalyst, Catal. Today, 74,145, 2002. 

Reshetenko, T. et al., Carbon capacious Ni-Cu-Al2Os catalysts for high-temperature methane decomposition, Appl. Catal. A General, 247, 51, 2003. 

Ermakova, M. and Ermakov, D., Ni/SiOz and Fe/Si02 catalysts for production of hydrogen and filamentous carbon via methane decomposition, Catal. Today, 77, 225, 2002. 

In contrast, the activity of supported rhodium catalysts is determined principally by the concentration of accessible surface Rh atoms, which catalyze methane decomposition, followed by CO2 reduction (186). As a result, the support plays a minimal role in the rhodium-containing catalysts. 

A substantial difficulty in ethanol SR is a too rapid catalyst deactivation due to coking. This can occur by several reactions, such as methane decomposition (19) or the Boudouard reaction (20), but primarily the polymerization of ethylene is thought to cause the problems (21). Unlike the situation for methane SR, it appears that for ethanol SR the deactivation by coke formation is lower at high temperatures. 

The decomposition of methane is an important process since it can produce two valuable products hydrogen and carbon filaments. Wayne Goodman (Texas A M University) and Tushar Choudhary (ConocoPhillips) show that methane decomposition may be a viable alternative to conventional steam reforming as a source of hydrogen, without the formation of COx as a byproduct. The authors examine the effects of catalyst support and promoters, as well as the inevitable regeneration of the catalyst. The formation of carbon fibers, under certain conditions, makes this process an attractive one. 

Carbon-based catalysts have also been considered for the methane decomposition reaction. Yoon and co-workers have recently investigated the kinetics of methane decomposition on activated carbons as well as on carbon blacks.In case of activated carbons the authors observed mass transport effects in the catalyst particles and also significant pore mouth plugging. The reaction order was found to be 0.5 and the activation energy was found to be 200 kJ/mol for the different activated carbon samples. On the other hand, for... 

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