Catalysts for methanation

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

The catalyst losses in either system are moderate and not excessively costly when inexpensive iron catalyst is used (as for production of liquid hydrocarbons). It is questionable, however, whether comparable losses of expensive nickel catalysts (for methanation) could be tolerated. For this reason, it is quite likely that the fluidized catalyst system will be used for methanation only after a cheap methanation catalyst is developed. 

Design of stable catalysts for methane-carbon dioxide reforming... 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Pt/H-MCM-22 catalysts for methane combustion have been prepared by ion-exchange of a highly crystalline H-MCM-22 zeolite using [Pt(NH3)4](N03)2. The activation procedure of the catalyst precursor has been optimized and all steps monitored by HRTEM, SEM and FTIR of CO adsorbed. The preliminary decomposition/calcination of the ion exchanged sample is very crucial in that influence the final properties of platinum active species. 

Modulation of zeolite acidity by post-synthesis treatments in Mo/HZSM-5 catalysts for methane dehydroaromatization... 

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 ... 

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. 

Lercher, J. et al., Design of stable catalysts for methane carbon dioxide reforming, Stud. Surf. Sci. Catal., 101,463,1996. 

Figure 4. Differential spectra of CO chemisorbed on alumina-supported Ni particles both before and after heating to 425 K. Very little surface hydrocarbon is seen to form on the Ni particles. This lack of surface hydrocarbon reflects the selectivity of such catalysts for methanation over Fisher-Tropsch synthesis.

C. Bozo, N. Guilhaume, and J.-M. Herrmann, The role of the ceria-zirconia support in the reactivity of platinum and palladium catalysts for methane total oxidation under lean conditions, J. Catal. 393, 393 06 (2001). 

Co2(CO)g has been used to obtain encapsulated cobalt clusters in Y-faujasite, which have been used as model catalysts for methane homologation [152]. The gas phase adsorption of Co2(CO)8 under N2 rendered predominately encaged Co4(CO)i2 species whereas Co,s(CO)iis was obtained when the impregnation of Co2(CO)8 was carried out under a CO/H2 atmosphere [152, 155], Samples were oxidized at 80°C, subsequently reduced at 400 °C and then structurally characterized by EXAFS. Clusters of two and three cobalt atoms were formed from encaged Co4(CO)i2 and COis(CO)iis, respectively. Higher methane conversion and selectivity to C2+ products in the CH4 homologation reaction have been obtained for the two atoms-size cluster sample the results were discussed using a DFT model [152]. 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

Poisoning ofPalladium Catalysts for Methane Oxidation. Applied Catalysis, 70, 87-100. 

Find et al. [42] developed a nickel-based catalyst for methane steam reforming. As material for the micro structured plates, AluchromY steel, which is an FeCrAlloy (see Section 2.10.7) was applied. This steel forms a thin layer of alumina on its surface, which is less than 1 pm thick. This layer was used as an adhesion interface for the catalyst. I ts formation was achieved by thermal treatment of micro structured plates for 4 h at 1 000 °C. 

It was found that almost all transition metals (d-metals) exhibit catalytic activity toward methane decomposition reaction to some extent, and some demonstrate remarkably high activity. It should be noted, however, that there is no universal agreement among different groups of researchers regarding the choice of the most efficient metal catalyst for methane decomposition. For example, it was demonstrated that the rate of methane activation in the presence of transition metals followed the order Co, Ru, Ni, Rh > Pt, Re, Ir > Pd, Cu, W, Fe, Mo.20 Other researchers have found Pd to be the most active catalyst for methane decomposition,18-21 whereas still others found Ni was the catalyst of choice,22 or Fe and Ni.23-24 Finally, Co catalyst demonstrated highest activity in methane decomposition reaction.25... 

Using feed gas containing stoichiometric amounts of methane and oxygen Nakamura [14] obtained at 900 K synthesis gas with the selectivity higher than 90 % Simultaneously Nakamura et al. [14] as well as Rostrup-Nielsen and Bak Hansen [15] found that at temperatures 900 K and higher ruthenium is a good catalyst for methane reforming by water as well as by carbon dioxide. Therefore, Nakamura at al. [14] claim that the synthesis gas is produced due to reaction of methane with carbon dioxide and water which are formed earlier. On the other hand, Choudhary et... 

THE INFLUENCE OF SUPERACIDIC MODIFICATION ON Zr02 AND Fe203 CATALYSTS FOR METHANE COMBUSTION... 

Influence of Superacidic Modification on Zr02 and Fe20 Catalysts for Methane Combustion 153... 

Influence of Superacidic" Modification on Zr02 and Fe203 Catalysts for Methane Combustion... 

Zamar F., Trovarelli A., Leitenburg C. and de Dolcetti G., Ce02-based solid solution with the fluorite structure as novel and effective catalysts for methane combustion. J. Chem. Soc. Chem. Commun. (1995) pp. 965-966. 

These findings show that the newly formed surface sites in low coordination on the Li-doped MgO are more active than those on the undoped catalyst, which explains the high activity of the Li-doped MgO catalyst for methane oxidative coupling. Ilo et al (99) detected the (Li-O ) or O sites on the Li-doped MgO catalyst by EPR and suggested that these sites play a significant role in the formation of CH3 radicals from CH4. It is unclear whether the newly produced surface sites in low coordination are directly associated with the existence of such active sites. 

J.G. McCarty and H. Wise, Perovskite catalysts for methane combustion, CataL Today 5 231 (1990). 

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