Methane combustion reactions

Application of the method for the deposition of gold on Pd alumina catalyst for the methane combustion reaction... 

The material synthesized by HIP method has been investigated for its catalytic activity towards methane combustion reaction using a pure gas, steady state catalyst evaluation assembly, equipped with precise gas flow control and heating system. Gas analysis was carried out using an auto-sampling PC controlled, MTI-P-200, GC system (16). The feed used was Methane=1.5% + O2 =18% and balance He. The W/F space velocity used for the various reactions was approximately 0.15 g.s./Ncm. ... 

In the temperature range of the methane combustion reaction, on the other hand, a oxygen has already been desorbed and this may explain the similar catalytic activities observed for these mixed oxides. 

Time-dependent Catalytic Activity - The strong variation of activity as a function of time on stream is a typical feature of the methane combustion reaction on most Pd catalysts. Several different transient phenomena have been reported. In some cases, the activity is initially low, or even zero, but then it increases with time on stream. In other cases, the activity starts high but then it drops to a lower steady state value. The approach to the steady state has also been found to vary greatly. In some cases, it reaches a relatively constant value in a few minutes. In other cases, the activity still changes after several hours on stream. 

Although the explanations of the role of oxygen in the methane combustion reaction are diverse, a common concept that emerges in most studies is the realization that different forms of oxygen can be present in the catalyst and that they have different reactivities. The following sections analyse the methodology that has been used to characterize the various Pd-O species and their role in activity. 

The Pd-O bond also varies with the extent of oxidation of Pd. During the methane combustion reaction, the catalyst surface is a non-equilibrium, kineti-cally controlled structure. The oxygen concentration profile in the particle results from a combination of particle reconstruction, oxygen adsorption, bulk diffusion, and oxygen removal. This concentration profile varies as a function of time, and as the oxygen content increases, the Pd-O bond strength decreases. This increase is accompanied by an increase in the specific activity. The most widely accepted reaction pathway is the Mars and van Krevelen redox mechanism, which involves lattice oxygen and uneoordinated Pd centers as active species. Inhibition by products (H2O and CO2) and impurities (SO2) is a major drawback for low temperature combustion. The effect of sulfur is particularly important for catalytic converters for NGV applications because it drastically reduces the methane combustion activity. 

The coefficients in the chemical reactions of combustion may be interpreted as the number of moles of the substances required for the reactions to occur. For example, in the methane combustion reaction, 1 mole of methane reacts with 2 moles of oxygen to form 1 mole of carbon dioxide and 2 moles of water. Although the number of atoms of each element must be conserved during a reaction, the total number of moles or molecules need not. Because the number of atoms of each element cannot change, it follows that the mass of each element and... 

Si02 powder used in the impregnation method was the sol-gel prepared Si02 atvarious pH, corresponding to the pH values used for sol-gel prepared catalysts (SG). From the results in Table 22-2, it was found that the pH of gelation strongly modified the porosity and surface area of the catalysts, but did not affect so much upon the dispersion of Pd atoms in the catalysts. The catalysts were conducted to methane combustion reaction, and the Pd/Si ratios at the surface vicinities of catalysts were measured by XPS before and after... 

Demoulin O, Navez M, Ruiz P. Investigation of the behaviour of a Pd/Y-Al203 catalyst during methane combustion reaction using in situ DRIFT spectroscopy. Appl Catal A. 2005 295 59. 

Figure 6-13 shows three different paths for the combustion reaction of methane. One path, indicated with the blue arrow, is the path that might occur when natural gas bums on a stove burner. As CH4 and O2 combine in a flame, all sorts of chemical species can form, including OH, CH3 O, and so on. This is not a convenient path for calculating the energy change for the net reaction, because the process involves many steps and several unstable chemical species. 

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]. 

Figure 2.19 provides the thermodynamic equilibrium data for methane decomposition reaction. At temperatures above 800°C, molar fractions of hydrogen and carbon products approach their maximum equilibrium value. The effect of pressure on the molar fraction of H2 at different temperatures is shown in Figure 2.20. It is evident that the H2 production yield is favored by low pressure. The energy requirement per mole of hydrogen produced (37.8 kj/mol H2) is significantly less than that for the SMR reaction (68.7 kj/mol H2). Owing to a relatively low endothermicity of the process, <10% of the heat of methane combustion is needed to drive the process. In addition to hydrogen as a major product, the process produces a very important by-product clean carbon. Because no CO is formed in the reaction, there is no need for the WGS reaction and energy-intensive gas separation stages.

Heinzel et al. [77] compared the performance of a natural gas autothermal reformer with that of a steam reformer. The ATR reactor was loaded with a Pt catalyst on a metallic substrate followed by a fixed bed of Pt catalyst. In the start-up phase, the metallic substrate was electrically heated until the catalytic combustion of a stoichiometric methane-air mixture occurred. The reactor temperature was increased by the heat of the combustion reaction and later water was added to limit the temperature rise in the catalyst, while the air flow was reduced to sub-stoichiometric settings. With respect to the steam reformer, the behavior of the ATR reactor was more flexible regarding the start-up time and the load change, thus being more suitable for small-scale stationary applications. 

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