Methane dissociation

For example, consider the dissociative adsorption of methane on a Ni(lOO) surface. If the experiment is performed above 350 K, methane dissociates into carbon atoms and hydrogen that desorbs instantaneously. Consequently, one determines the uptake by measuring (e.g. with Auger electron spectroscopy) how much carbon is deposited after exposure of the surface to a certain amount of methane. A plot of the resulting carbon coverage against the methane exposure represents the uptake curve. 

Figure 8.3. Potential energy diagram based on DFTcalculations. Notice how the reaction pathway is strongly modified by the presence of atomic steps on the Ni (112) surface. First of all, steps lowerthe barrier for the initial methane dissociation. Although this barrier...

We could also have assumed that methane dissociation is rate limiting. Write up the rate again, assuming that oxygen is the MARI. 

The attached figure displays the uptake of carbon on a Ni(lOO) surface when exposed to methane at different temperatures. The methane dissociates and dehydrogenates on the surface, resulting in the carbon overlayer. The reaction is assumed to be... 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

Barnett and co-workers recently reported that it might be possible to utilize hydrocarbons directly in SOFC with Ni-based anodes. " ° First, with methane. they observed that there is a narrow temperature window, between 550 and 650 °C. in which carbon is not as stable. The equilibrium constant for methane dissociation to carbon and Hz is strongly shifted to methane below 650 °C. and the equilibrium constant for the Boudouard reaction, the disproportionation of CO to carbon and COz, is shifted to CO above 550 °C. Therefore, in this temperature range, they reported that it is possible to operate the cell in a stable manner. (However, a subsequent report by this group showed that there is no stable operating window for ethane due to the fact that carbon formation from ethane is shifted to lower temperatures. ) In more recent work, this group has suggested that, even when carbon does form on Ni-based anodes, it may be possible to remove this carbon as fast as it forms if the flux from the electrolyte is sufficient to remove carbon faster than it is formed.Observations by Weber et al. have confirmed the possibility of stable operation in methane. Similarly, Kendall et al. showed that dilution of methane with COz caused a shift in the reaction mechanism that allowed for more stable operation. 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

Figure 4.16. Calculated variations in the activation energy for methane dissociation over a number of different surfaces. The results are shown as a function of the energy of the d states coupling to the transition state methane molecule. Adapted from Ref. [57].

Although the HRTEM movies provide important information about the atomic-scale surface dynamics, they capture only the result of the atomic diffusion events and not directly the individual atomic displacements. To understand the origin of the transport mechanisms, the observations have been complemented with results based on DFT calculations. The calculations show that step edges facilitate methane dissociation and that carbon atoms bind more strongly to step edges than... 

Some molecular species are known to undergo dissociation upon adsorption, especially on metal surfaces. For example, molecular H2 dissociates on a metal surface into two surface-adsorbed H(s) atoms, and similarly methane dissociates into CH3(s) and H(s). Such dissociative adsorption is usually assumed to require two surface open sites, and the process is considered to be concerted that is, the adsorption and breaking-apart of the molecule are taken to occur in a single step. 

Figure 2. Evolution of DOS along methane dissociation pathway on GaO —total DOS A - Ga contribution x - O contribution.

A similar study by Schoofs et al. [43] of methane dissociation on the Pt(l 11) surface produced qualitatively similar results An exponential increase in the value of S0 with increasing normal energy and a weak dependence of S0 on surface temperature, Ts. Further, like Rettner et al. [40], Schoofs and coworkers find these trends consistent with a hydrogen tunneling mechanism through a one-dimensional parabolic-shaped activation barrier. 

Values ofS0,t determined experimentally from bulb experiments and those determined from calculations using molecular beam data atrg = 300 K. Also included, are calculated contributions toSG from the trapping mediated and direct mechanisms for methane dissociation on Ir(l 1 0)-(l x 2). 

The reaction sequence for methane dissociation on the Ni-surface may be written as ... 

The equilibrium pressure of the methane dissociation is only slightly higher than that of the carbide formation. In order to suppress excessive free-carbon formation, the CH4 pressure has to be maintained above the minimum pressure for the carburization, but below that resulting in free-carbon formation. 

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