Catalysts methane reaction rates

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

Fischer-Tropsch Process. The Hterature on the hydrogenation of carbon monoxide dates back to 1902 when the synthesis of methane from synthesis gas over a nickel catalyst was reported (17). In 1923, F. Fischer and H. Tropsch reported the formation of a mixture of organic compounds they called synthol by reaction of synthesis gas over alkalized iron turnings at 10—15 MPa (99—150 atm) and 400—450°C (18). This mixture contained mostly oxygenated compounds, but also contained a small amount of alkanes and alkenes. Further study of the reaction at 0.7 MPa (6.9 atm) revealed that low pressure favored olefinic and paraffinic hydrocarbons and minimized oxygenates, but at this pressure the reaction rate was very low. Because of their pioneering work on catalytic hydrocarbon synthesis, this class of reactions became known as the Fischer-Tropsch (FT) synthesis. 

Fischer Tropsch synthesis is catalyzed by a variety of transition metals such as iron, nickel, and cobalt. Iron is the preferred catalyst due to its higher activity and lower cost. Nickel produces large amounts of methane, while cobalt has a lower reaction rate and lower selectivity than iron. By comparing cobalt and iron catalysts, it was found that cobalt promotes more middle-distillate products. In FTS, cobalt produces... 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

For the first assumption, the value of Kw for the shift appears to be too high. It must be this high because it is necessary to make C02 appear while both C02 and CO are being consumed rapidly by methanation. The data may be tested to see if the indicated rate appears unreasonable from the standpoint of mass transfer to the gross catalyst surface. Regardless of the rate of diffusion in catalyst pores or the surface reaction rate, it is unlikely that the reaction can proceed more rapidly than material can reach the gross pill surface unless the reaction is a homogeneous one that is catalyzed by free radicals strewn from the catalyst into the gas stream. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

Fig. 6. Activities of copper-nickel alloy catalysts for the hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene. The activities refer to reaction rates at 316° C. Ethane hydrogenolysis activities were obtained at ethane and hydrogen pressures of 0.030 and 0.20 atm., respectively. Cyclohexane dehydrogenation activities were obtained at cyclohexane and hydrogen pressures of 0.17 and 0.83 atm, respectively (74).

The apparent reaction order of carbon-catalyzed methane decomposition reaction was determined to be 0.6 0.1 for AC (lignite) and 0.5 0.1 for CB (BP2000) catalysts. Thus, the rate equation for carbon-catalyzed decomposition of methane can be written as follows ... 

FIGURE 9.18 Influence of pressure on reaction rate, olefin content in the C3 fraction, and methane selectivity with cobalt as the catalyst for FT synthesis. Catalyst 100Co-18Th02-100 Si02 (Kieselguhr), H2/CO = 1.8, 175°C. 

The model presented in this paper will display the concentration gradients in a cylindrical, fully wax-filled pore of the catalyst. For simplification reasons, only the reactants CO and H2 as well as the reaction product H20 will be considered. C02 formation is disregarded in the model due to its comparatively low concentration in FT products and because compared to steam, C02 has no impact on the main reaction rate. Methane formation (usually not more than 5% of total CO conversion) was also neglected. 

Fig. 4. Methane production rate at 62S K over a Ni(100) catalyst as a function of surface carbon coverage at various reaction conditions. (From /te/ 12.)...

Chemical/Physical. The estimated hydrolysis half-life in water at 25 °C and pH 7 is 274 yr (Mabey and Mill, 1978). Hydrogen gas was bubbled in an aqueous solution containing 18.8 pmol dibromochloromethane. After 24 h, only 18% of the dibromochloromethane reacted to form methane and minor traces of ethane. In the presence of colloidal platinum catalyst, the reaction proceeded at a much faster rate forming the same end products (Wang et al., 1988). 

The production of 1-alkenes from ethylene oligomerization was carried out with high selectivity in ionic liquids in the presence of a cationic nickel complex catalyst (ri -methallyl)-[bis(diphenylphosphino)methane-monoxide-K -P,0]nickel(II) hexafluoroantimonate, [(mall)-Ni(dppmo)]Sbp6 (240). The overall reaction rate of... 

The batch reactor, above described, permits both to operate at quasi-zero conversion per pass and to evaluate the cat ytic activity at finite values of the reagents conversion. A typical test performed on Si02 catalyst at 600°C is presented in Figure 1. It is remarkable how in our approach the product selectivity is unaffected by the methane conversion. A special care was taken to avoid oxygen-limiting conditions and, hence, methane conversion data obtained for oxygen conversions below 20% only have been used for the calculation of reaction rates. 

The amount of the products formed over the studied catalysts, in the presence and absence of molecular O2, are listed in Table III. It is evident that the formation of the oxidation products is associated with the gas phase oxygen supply. Then, as the reaction rates in the mixture of reactant and in separate steps differ (19), these data exclude the participation of lattice oxygen in the partial oxidation of methane via a two step redox mechanism as main reaction pathway proving the occurrence of a "concerted mechanism". 

The difference in H2 selectivity between Pt and Rh can be explained by the relative instability of the OH species on Rh surfaces. For the H2-O2-H2O reaction system on both and Rh, the elementary reaction steps have been identified and reaction rate parameters have been determined using laser induced fluorescence (LIF) to monitor the formation of OH radicals during hydrogen oxidation and water decomposition at high surface temperatures. These results have been fit to a model based on the mechanism (22). From these LIF experiments, it has been demonstrated that the formation of OH by reaction 10b is much less favorable on Rh than on Pt. This explains why Rh catalysts give significantly higher H2 selectivities than Pt catalysts in our methane oxidation experiments. 

---