Boudouard reaction decomposition

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

Hall et a/. pointed out that carburisation is controlled by three independent processes, i.e. carbon deposition, carbon ingress (through the protective scale) and carbon diffusion through the matrix. Carbon deposition usually occurs by decomposition of CH4 adsorbed on the surface or the catalytic decomposition of CO (Boudouard reaction). Hydrogen... 

In the C02 reforming of methane, carbon formation can occur via two possible pathways CH4 decomposition and CO disproportionation (the Boudouard reaction). Carbon formation by CH4 decomposition is a structure-sensitive reaction (158,159). Specifically, the Ni(100) and Ni(110) surfaces are more active in the decomposition of CH4 to carbon than the Ni(lll) surface (158). The CO disproportionation,... 

Figure 11-1 Equilibrium Constants (Partial Pressures in MPa) for (a) Water Gas Shift, (b) Methane Formation, (c) Carbon Deposition (Boudouard Reaction), and (d) Methane Decomposition (J.R. Rostrup-Nielsen, in Catalysis Science and Technology, Edited by J.R. Anderson and M. Boudart, Springer-Verlag, Berlin GDR, p.l, 1984.)... 

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. 

CO, and therefore we exclude the presence of these bulk metal carbonyls. The dissociation of CO by the Boudouard reaction (or the decomposition of palladium carbonyls) would lead to carbon deposition (Kung et al., 2000 McCrea et al., 2001) and should produce a feature at 284.0 eV characteristic of graphite or at 284.4 eV characteristic of amorphous carbon. In the case of carbide species, a feature at lower BE (<283.5 eV) would appear. Even if carbon dissolved in the palladium bulk near the surface region, the escape depth of the Cls electrons (about 2 nm) should have been sufficient to allow its detection. The absence of any carbon-related signals indicates that CO does not dissociate at 400 K and approximately 1 mbar, even over the course of several hours. This result is an important argument in the discussion about the possibility of CO dissociation at high pressure. 

CO2 reforming of methane (equation 1) has been proposed as one of the most promising technologies for utilization of these two greenhouse gases, and this synthesis gas is suitable for Fischer-Tropsch synthesis and oxygenated chemicals. A serious problem is carbon deposition via Boudouard reaction (equation 2) and/or methane decomposition (equation 3). 

WGS), reverse water-gas shift (RWGS), CO disproportionation (Boudouard reaction), and methane decomposition reactions as described in Equations 22-2.5 ... 

The high temperatures and low pressures needed for this endothermic reaction are conditions conducive to deactivation due to coke deposition through the Boudouard reaction and the decomposition of CH4 [12]. Current research has focused on Group VIII metals [13] on a variety of supports. Most of the work has been on Rh and Ni supported on AI2O3, Ti02, Si02, and MgO [1, 8, 14-17] with some interest in Pt and Pd [5, 10, 18-19]. High activity and low carbon deposition have been observed on Rh catalysts [20-23], but the limited supply of this metal when compared to others reduces the potential for it to be commercially feasible [6]. Many studies have been done on Ni [24-28] which show that the support has a profound effect on the activity and deactivation of the catalyst. Severe deactivation due to carbonaceous deposits was found to... 

CH4 may be converted to synthesis gas by steam reforming or by reaction with Oj through secondary reforming and partial oxidation [1]. Steam may also be replaced by COj. As for all processes where hydrocarbons or carbon oxides are exposed to high temperamres, carbon deposition is a possible problem in the production of synthesis gas. In the production of synthesis gas, carbon may be formed by decomposition of methane (and higher hydrocarbons) or by the Boudouard reaction [2] ... 

Steam reforming involves the risk for carbon formation by the decomposition of methane and other hydrocarbons or by the Boudouard reaction (ref. 1). 

The whisker carbon has a higher energy than graphite, which is reflected in lower equilibrium constants for the reversible decomposition reactions of carbon monoxide (Reaction R7 in Table 5.2, the Boudouard reaction) and of methane (Reaction R6 in Table 5.2) as shown for the two reactions in Figures 5.9 and 5.10, respectively [49] [149] [150] [378] [381]. 

For afixed gas composition of H2, H2O, CO, CO2, and CH4 there is a temperature, Tb, below which the exothermic Boudouard reaction is thermodynamically favored, and a temperature, Tm, above which carbon formation by the endothermic decomposition of CH4 is thermodynamically favored more extensive details on carbon deposition are found elsewhere (22, 59, 60, 61). 

Methane reforming Eq. (2.36) is the simplest example of steam reforming (SR). This reaction is endothermic at MCFC temperatures and over an active solid catalyst the product of the reaction in a conventional reforming reactor is dictated by the equilibrium of Eq. (2.36) and the water-gas shift (WGS) reaction Eq. (2.37). This means that the product gas from a reformer depends only by the inlet steam/ methane ratio (or more generally steam/carbon ratio) and the reaction temperature and pressure. Similar reaction can be written for other hydrocarbons such as natural gas, naphtha, purified gasoline, and diesel. In the case of reforming oxygenates such as ethanol [125, 126], the situation is in some way more complex, as other side reactions can occur. With simple hydrocarbons, like as methane, the formation of carbon by pyrolysis of the hydrocarbon or decomposition of carbon monoxide via the Boudouard reaction Eq. (2.38) is the only unwanted product. 

The main issue of catalyst deactivation is carbon deposition originated from methane decomposition (7.60) and the Boudouard reaction (7.61). 

Carbon Formation. Steam reforming involves the risk of carbon formation by the decomposition of methane and other hydrocarbons or by the Boudouard reaction (reactions (7) -(10)). Reactions (7) - (8) are catalyzed by nickel (Rostrup-Nielsen, 1984a). The carbon grows as a fibre (whisker) with a nickel crystal at the tip. The methane or carbon monoxide is adsorbed dissociatively on the nickel surface (Alstrup, 1988). Carbon atoms not reacting to gaseous molecules are dissolved in the nickel crystal, and solid carbon nucleates at the non-exposed side of the nickel crystal, preferably from Ae dense (111) surface planes. Reaction (10) results in pyrolytic carbon encapsulating the catalyst. 

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