Catalysts methanation rate over

Figure 2. A comparison of the rate (turn-over frequency) of methane synthesis over single crystal and supported ruthenium catalysts. Total reactant pressure for the single crystal studies was 120 Torr.

Fig. 2. (a) A comparison of the rate of methane synthesis over single crystal nickel catalysts and supported Ni/AliO, catalysts at 120 torr total reactant pressure. (From Rtf. 12.) (b) Atomic configuration of a Ni(100) surface, (c) Atomic configuration of a... 

Kinetic measurements over a Ni(lOO) catalyst containing well-controlled submonolayer quantities of potassium show a general decrease in the steady-state methanation rate with little apparent change in the activation energy associated with the kinetics (Fig. 22). However, the potassium did change the steady-state coverage of active carbon on the catalyst. This carbon level changed from 10% of a monolayer on the clean catalyst to 30% on the potassium covered catalyst. 

Fig. 22. A comparison of the rate of methane synthesis over a clean single crystal Ni(100) catalyst with the corresponding rate over a potassium-doped catalyst. Total reactant pressure is 120 torr, Hj/CO = 4/1. (From Ref. 148.)...

Synthesis gas production. Alqahtany et al.92 have studied synthesis gas production from methane over an iron/iron oxide electrode-catalyst. Although the study was essentially devoted to fuel cell operation, for purposes of comparison some potentiometric work was performed at 950°C. It was found that under reaction conditions Fe, FeO or Fe304 could be the stable catalyst phase. Hysteresis in the rates of methane conversion were observed with much greater rates over a pre-reduced surface than over a pre-oxidised surface possibly due to the formation of an oxide. 

Similar observations were reported by other researchers. For example, it was demonstrated that the values for the rate constants for methane decomposition over Ni, Co and Fe catalysts declined as the run proceeded.28 At the temperatures below 1000°C the experimental data followed the kinetic equation ... 

Rostrup-Nielsen and Pedersen (209) recently studied sulfur poisoning of supported nickel catalysts in both methanation and Boudouard reactions by means of gravimetric and differential packed-bed reactor experiments. In their gravimetric experiments a synthesis mixture (H2/CO/He = 5/7/3) containing 1-2 ppm H2S was passed over a catalyst pellet of 13% Ni/Al203-MgO at 673 K and 1 atm. The rates of Boudouard and methanation reactions were determined from weight increases and exit methane concentrations respectively. In the presence of 2 ppm H2S a factor of 20 decrease was observed in both methanation and Boudouard rates over a period of 30-60 min. However, the selectivity or ratio of the rates for Boudouard and methanation reactions was constant with time. From these results the authors concluded that the methanation and Boudouard reactions involve the same intermediate, carbon, and that sulfur blocks the sites for the formation of this intermediate. 

If the same concentration of CO is used but with H2 instead of He, there will be steady-state methanation over the Ni catalyst. This rate is measured by the rate of production of CH4, essentially the only hydrocarbon produced. Now the same isotopic tracing experiment can be done to determine the coverage of CO present on the surface during the reaction. A correction for the CO reacted must be included, although it may be small in a differential reactor 17). 

We have presented evidence that the strong metal-support interactions observed with titania and niobia are due to an oxide layer over the metal catalyst. This layer interacts chemically with the metal, as evidenced by the fact that the titania layers on Pt, Rh, and Pd do have slightly different properties. The fact that the methanation rates for a titania-covered and a niobia-covered Pt foil are identical indicates that the reason for enhanced methanation activity on the oxide-covered surface is likely due to geometric and not electronic cons id er at ions. 

Hargreaves et al. (1990) also observed the shift from CO to C2 hydrocarbons during methane oxidation over MgO catalyst at varying flow rate. However, in this case authors explained this effect in terms of the difference in kinetic orders of two reactions—OCM and oxidation to CO with respect to CH3 radical concentration, which in turn is changing at varied flow rate. 

The rate controlling step for reaction involves methane adsorption. Catalyst structure has a marked effect on the kinetics of the reaction. Thus, under certain conditions the rate of reaction over a Ni/Kieselguhr catalyst at 911 K is first order with respect to the partial pressure of CH4 and independent of H2O and product partial P, while for other nickel catalysts the rate depends on the partial P of H2O, H2, and CO. ... 

While the hydrogenation of the active surface carbon that forms from CO dissociation appears to be the predominant mechanism of CH4 formation, it is not the only mechanism that produces methane. Poutsma et al. [85] have detected the formation of CH4 over paliadium surfaces that do not readily dissociate carbon monoxide. They also observed methane formation over nickel surfaces at 300 K under conditions in which only molecular carbon m.onoxide appears to be present on the catalyst surfaces [81]. Vannice [86] also reported the formation of methane over platinurh, palladium, and iridium surfaces, and independent experiments indicate the absence of carbon monoxide dissociation over these transition-metal catalysts in most cases. It appears that the direct hydrogenation of molecular carbon monoxide can also occur but that this reaction has a much lower rate than methane formation via the hydrogenation of the active carbon that is produced from the dissociation of carbon monoxide in the appropriate temperature range. 

Another mechanism, proposed by Pichler [87] and Emmettt [88, 89], involves the direct hydrogenation of molecular carbon monoxide to an enol species, followed by dehydration and further hydrogenation to produce methane. It is likely that this mechanism provides an additional reaction channel that may compete over certain transition-metal catalysts with CH4 formation via the dissociation of carbon monoxide. Recent studies of methane formation over molybdenum indicate positive CO and H2 pressure dependencies of the reaction rate... 

Tn recent years ultrahigh vacuum methods have been applied to cata-lytic studies on initially clean metal surfaces having low surface area. In several instances (the hydrogenolysis of cyclopropane over platinum (I) and the catalytic methanation reaction over rhodium (2) and nickel (3)) a link between ultrahigh vacuum methods and conventional catalytic measurements was established. That is, specific reaction rates over low area cm ) catalyst samples agreed with specific reaction... 

The effect of added manganese is to raise the rate of product formation by a rhodium catalyst by approximately one order of magnitude. Despite this, neither the temperature nor the pressure dependence of the reaction rate is changed significantly by the addition. Furthermore, the temperature and pressure dependences of the rate of product formation in these experiments are quite close to those determined by Vannice (II) for methane formation over a Rh/Al203 catalyst at atmospheric pressure. Vannice reports that his methanation results can be represented by ... 

Vannices data (11) on rate of methane formation over a Rh/Al203 catalyst can be interpreted in terms of Equation 3, which is equivalent to Equation 4 with m = 2, as well as in terms of the mechanisms suggested by Vannice (J5) and Vannice and Ollis (16). Substitution of Equation 6 in Equation 4 yields ... 

Nonetheless, rate expressions more complex than a simple power law are sometimes useful. For example, a power law expression does not provide any insight into the reasons for changing reactant order (i.e., a changing value of a ) with temperature or organic reactant concentration. However, such effects are frequently observed in oxidation reactions and are often consistent with more fundamentally based rate expressions. Consider, for example, what one would suppose to be the simple oxidation of methane. Golodets (p. 445) states that methane oxidation over metal oxide catalysts may be interpreted by the following mechanism ... 

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