Elementary surface reaction steps methane activation

In this study it was found that dissociation of CO2 and CH4 is an elementary step in the CO2 reforming of methane and that an active site for the dissociation of CO2 and CH4 (eqns. (1) and (2)) is metallic Ni on the KNiCa catalyst. Ni surface of KNiCa/ZSI catalyst was mostly occupied by adsorbed C and O species as intermediates during the reaction. Surface reaction of these species produced carbon monoxide and simultaneously rejuvenated nickel species (eqn. (5)), which was considered to be rate-determining step under the following reaction scheme. 

It appears that a significant amount of energy is required for CH activation, the primary elementary step of the hydrocarbon conversion reaction. If one chemisorbs methane at low temperature on a transition metal surface, it desorbs before reaction can occur. Aliphatic hydrocarbons can dissociate from a preadsorbed state, if they contain enough carbon atoms in their chain to induce a high heat of adsorption whereby CH dissociation can take place at a rate large compared to the desorption rate. So far this has only been found for surfaces containing highly active metal atoms, such as the reconstructed Ir (110) surface or stepped surfaces l. As we will see, both electronic and steric effects may play a role. 

Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of the active species may be reduced by direct interaction with methane or from the reaction with H, Hg, C or CO. The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. A catalytic material and support which does not easily form or stabilise OH species is therefore desirable. Another essential property for the formation of Hg and CO as primary products is a low surface coverage of intermediates, such that the probability of O-H, OH-H and CO-O interactions is reduced. ... 

Based on such studies, it has been concluded that the lateral facets are active for the oxidation of methane to formaldahyde [10] and the oxidative ammonalysis of toluene [7], while the (010) facet is active for the conversion of methanol to formalahyde [1]. Studies of the oxidation of propene to acrolein illustrate that it is not always easy to relate overall activities or selectivities to the presence of a single face [3, 5, 8, 9]. Since the overall reaction is composed of several elementary steps, it is possible that different steps occur on different facets. For example, it has been proposed that the mechanism for the oxidation of propene to acrolein begins with the activation to an allyl intermediate on a lateral facet and ends with the addition of O on a basal facet [5]. The (210) facet, which is thought to consist of terraces with (010) character and ledges with (100) character, should be able to perform both elementary steps. This explanation has been used to rationalize the observation that the (210) surface is especially active for the conversion of propene of acrolein [9]. Using similar... 

Bond Order Conservation-Morse Potential (BOC-MP) formalism is used to study the change in activation energy for different elementary steps involving all possible chemical species on any metallic smrface in order to identify optimum catalysts for the production of methane, methanol or ethylene from synthesis gas. All possible steps are analyzed for the methanation reaction. It is assumed that the reaction occurs on pure metallic surfaces, thus, the influence of the support or promoters is not taken into account. The method suggests that Ni is a more selective for methanation reaction, in agreement with the fact that Ni/SiOz is the real catalyst used industrially. On the other hand on the surface of Fe or W the reaction may residt in the formation of adsorbed C, also in agreement with experiment. 

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