Methane catalytic combustion

A convenient method to produce porous surfaces is the anodic oxidation of aluminum plates. Such microstructured aluminum platelets have been coated by wet impregnation with Pt-, V- and Zr-precursors [35], and tested under catalytic methane combustion conditions. The conversion rate of oxygen followed directly the platinum content in the catalysts. These data were well reproducible even after five different runs. 

Catalytic Methane Combustion and Methods for Sample Preparation... 

In the case of catalytic methane combustion, aluminum was chosen as an appropriate material for the catalyst wafers since anodic oxidation of aluminum can be used to obtain porous surfaces. Such micro structured aluminum platelets were coated by wet impregnation with Pt, V and Zr precursors [50],... 

Figure 13.5. Catalytic methane combustion of methane on a) aA),20i9 issued from sol-gel process, b) BaAlijOt issued from reverse microemulsion method and c) CeOj-BaAliPty composite issued from microemulsion method (Reprinted from Letters to Nature. Ref. 72).

Catalytic methane combustion and WGSs are coupled. Combined SRE and WGS reaction process produced syngas with H2/CO of 30 and H2/EtOH ratio 5 at 99% EtOH conversion. The catalyst was stable during the lOOh onstream. 

Numerous studies concerning hydrocarbons combustion have been experienced and reported in catalysis literature. Trovarelli et al. used the introduction of Zr (and Hf as well) for methane combustion [67]. For solids prepared by coprecipitation and calcined at 930°C BET area increases from 6 m /g (ceria) to 26-29 mVg for ceria-zirconia (Ceo,8Zro,202). For the catalytic methane combustion T50 was lowered by 130°C. Rate calculated at 450°C was multiplied by a factor of 8 when using solid solution. Moreover authors noted no CO formation for either ceria or solid solution. Finally it was concluded that catalytic activity was related, at least partially, to higher oxygen mobility at lower temperature for solid solution and easier Ce VCe switching. 

In the introduction we mentioned that the two applications for the catalytic methane combustion were in power generation and in the abatement of methane emissions in engine exhausts. In the first application, the amount of NO produced is in fact very small because the presence of a catalyst reduces the operating temperature. However, in the second type of application, the amounts of NO present can be substantial (e.g. several thousand ppm). In addition, the so-called selective catalytic reduction of NO can be effected by using methane as a reductant. In this case, the methane combustion takes place simultaneously with the NO reduction. Therefore, it is important to understand how the presence of NO affects the methane combustion under typical exhaust conditions. In the presence of NO, methane can undergo two different reactions, the reduction of NO and the total oxidation ... 

Figure 11.9 The first one tenth of the computational grid used in the 2D simulation of the microchannel reactor for coupling catalytic methane combustion and i-octane steam reforming.

Xiang et al [29] performed catalytic methane combustion tests over Lai-j Ce. ... 

Eguchi and coworkers [86] investigated the influence of the A-site cation (A La, Pr, Sm, and Nd) on AMnAliiOi9 a. Both the specific area and the catalytic activity increased with increasing ionic radius of the lanthanides. La was found to have the most beneficial effect on catalytic methane combustion activity. The activities of the La-based catalysts were further enhanced upon substitution of Al with Mn and Cu. Interestingly, the activity of these compounds was found to be greater than the B-site substituted Ba-hexaaluminate catalysts. Based on this, the authors concluded that the A-site cations could exert a significant influence on the oxidation state of the catalytically active B ions. 

Fig.l Surface area and catalytic activity for methane combustion of AMnAlii-Oi9methane conversion level is 10%. Reaction condition CH4,1 vol% air, 99 vol% space velocity, 48 OOOh ... 

The measurement of catalytic activity of PdPt bimetallic nanoparticles over methane combustion showed that the difference in activity with increasing and decreasing reaction temperatures disappeared probably due to the synergestic effect of the formation of the PdPt bimetallic nanoparticles [176]. 

The results of the catalytic activity for methane combustion are summarised in Table 1 and fig. 1. The methane conversions of the Pd2HZSHe catalyst are higher than those of the Pd2HZIHe sample. In fact, the ignition temperatures T10% (temperature necessary to have 10% of methane conversion) are respectively 355 and 371°C. This result suggests that the catalyst prepared by solid-exchange method is more active than that prepared by impregnation. 

Leanza, R Rossetti, 1 Fabbrini, L Oliva, C Fomi, L. Perovskite catalysts for the catalytic flameless combustion of methane Preparation by flame-hydrolysis and charaeterisation by TPD-TPR-MS and EPR. Appl. Catal, B Environmental, 2000, Volume 28, Issue 1, 55-64. 

Baiker, A Marti, PE Keusch, P Fritsch, E Reller, A. Influence of the A-site cation in AC0O3 (A = La, Pr, Nd, and Gd) perovskite-type oxides on catalytic activity for methane combustion. J. Catal, 1994, Volume 146, Issue 1, 268-276. 

Ciambelli, P Cimino, S Lisi, L Faticanti, M Minelli, G Pettiti, 1 Porta, P. La, Ca and Fe oxide perovskites preparation, characterization and catalytic properties for methane combustion. Appl Catal, B Environmental, 2001, Volume 33, Issue 3, 193-203. 

The concepts discussed so far indicate that the major challenge in asymmetric operation is correct adjustment of the loci of heat release and heat consumption. A reactor concept aiming at an optimum distribution of the process heat has been proposed [25, 26] for coupling methane steam reforming and methane combustion. The primary task in this context is to define a favorable initial state and to assess the distribution of heat extraction from the fixed bed during the endothermic semicycle. An optimal initial state features cold ends and an extended temperature plateau in the catalytic part of the fixed bed. The downstream heat transfer zone is inert, in order to avoid any back-reaction (Fig. 1.13). 

Fig. 1.15. Coupling of propane dehydrogenation and methane combustion in a four-step catalytic fixed-bed process [28].

The kinetics and mechanism of methane combustion have been the subject of many investigations, e.g.. Refs. 43-47, because of the importance of natural gas as a potential fuel for catalytic combustors. Under conditions expected in catalytic combustors, i.e., excess oxygen, a first order in methane is generally observed [48], whercas a variety of orders has been observed for other hydrocarbons [13]. The actual mechanism appears to be quite complex and depends on the fuel used. For instance, inhibiting effects are observed for the products carbon dioxide and water in methane combustion over supported palladium catalysts [49,50]. The inhibition of methane adsorption and the formation of a surface palladium hydroxide were proposed to explain the observation. 

Very similar results were also obtained by Farrauto et al. [51] from a study of the high-temperature catalytic chemistry of supported Pd for the combustion of methane. Palladium oxide supported on alumina decomposes in two distinct steps in air at atmospheric pressure. The first step occurs between 750 and 800X and is believed to be a decomposition of Pd-O species dispersed on bulk Pd metal, designated (PdO /Pd). The second decomposition occurs between 800 and 850" C, and it behaves like crystalline palladium oxide (PdO). To form the oxide once again, metallic Pd has to be cooled down to 650°C, thus causing a hysteresis gap of 150°C. Above 500°C, catalytic methane oxidation can occur only as long as the palladium oxide phase is still present. Above 650 C, metallic Pd cannot chemisorb oxygen, and hence it is catalytically inactive toward methane oxidation. 

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