Reactions Boudouard reaction

Chemistiy of Coal Utilization, 2d suppl. voL, Lowry [ed.], Wiley, New York, 1963) that there are three fundamental reactions the Boudouard reaction (27-17), the heterogeneous water-gas reaction (27-18), and the hydrogasification reaction (27-15). The eqmlibrium constants for these reactions are sufficient to calculate all the reactions listed. 

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... 

The reaction involving only carbon dioxide, carbon and carbon monoxide is popularly known as the Boudouard reaction. Its occurrence in the forward direction is favored at high temperatures. The reverse reaction, namely, carbon precipitation, occurs at lower temperatures. The precipitation of carbon is, however, known to occur only slowly at temperatures below 700 °C in the absence of a catalyst. 

Step 4 Side reaction such as CO dissociation (Eqn. 6), the Boudouard reaction (Eqn. 7) or the water gas shift (WGS) reaction (Eqn. 8, with surface OH species) may also occur ... 

Figure 4.1 summarizes the different routes that can potentially lead to carbon deposition during FTS (a) CO dissociation occurs on cobalt to form an adsorbed atomic carbon, which is also referred to as surface carbide, which can further react to produce the FT intermediates and products. The adsorbed atomic carbon may also form bulk carbide or a polymeric type of carbon. Carbon deposition may also result (b) from the Boudouard reaction and (c) due to further reaction and dehydrogenation of the FTS product (what is commonly called coke), a reaction that should be limited at typical FT reaction conditions. Carbon formed on the surface of cobalt can also spill over or migrate to the support. This is reported to readily occur on Co/A1203 catalysts.43 The chemical nature of the carbonaceous deposits during FTS will depend on the conditions of temperature and pressure, the age of the catalyst, the chemical nature of the feed, and the products formed. 

Two forms of carbon (carbidic and graphitic) have been observed by x-ray photoelectron spectroscopy (XPS) on polycrystalline cobalt foil during the disproportionation of CO by Nakamura et al.57 The dissociation of adsorbed CO occurred at temperatures higher than 60°C, and carbidic carbon and adsorbed oxygen were formed on the cobalt surface. After the surface is covered with adsorbed carbon and oxygen, no further dissociation of adsorbed CO occurs. Contrary to the dissociation of adsorbed CO, the deposition of carbon by the concerted Boudouard reaction continues on the carbidic carbon-deposited surface. The deposition of carbon increases... 

Note that the gas mixture in the lower right corner in Figure 4.27(b) is unstable due to the Boudouard reaction... 

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,... 

The first reaction (Boudouard equilibrium) favors carbon formation at lower temperatures compared to POX. The hydrogen concentrations attained depend on the fuel used in POX but it never reaches the theoretical level [27],... 

When this happens, the oxygen flux to the particle surface is cut-off because of its consumption in oxidizing CO, and C02 becomes the relevant gasification agent for the particle surface according to the Boudouard Reaction ... 

Which has been studied on supported Ni catalysts and on Ni films . Studies such as those described here show that methane can be catalytically synthesized over Ni by an active (carbidic) carbon formed via the Boudouard reaction and its subsequent hydrogenation to methane. However, to demonstrate that this surface carbon route is the major reaction pathway, kinetic measurements of both carbon formation from CO and its removal by H2 were carried out . 

Increasing the operating pressure of MCFCs results in enhanced cell voltages because of the increase in the partial pressure of the reactants, increase in gas solubilities, and increase in mass transport rates. Opposing the benefits of increased pressure are the effects of pressure on undesirable side reactions such as carbon deposition (Boudouard reaction) ... 

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.)... 

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