Surface methane intermediates

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Figure 1 Typical SSITKA transients with area between CH4 and Ar determining t, surface residence time of methane intermediates...

The model of deactivation describes the transformations of two boundary forms of the carbonaceous deposits during the catalytic hydrogenation of CO2. These are the hydrocarbon-formed active deposit (CH) and the graphitic inactive one (C)n. Thus deactivation is based on dehydrogenation of the active deposit into the inactive one that blocks active centers for hydrogenation. The active deposit, a product of polymerization of surface methane precursors (CH ), is simultaneously their consumer and producer. The mass balance of the active intermediates derived from the model assumptions gave the kinetic equation which quantitatively describes the deactivation. 

As was shown above, the time dependence of semi-logarithmic decay of C in the product reveals information about the reaction mechanism. The downward convexity observed for both Co/Ti02 (Figure 51.9B) and Co sponge (not shown see figure 6.35 in Reference [21]) catalysts at different H2/CO ratios characterizes the presence of at least two pools of methane intermediates on the catalyst surface (C and Cp), that in turn, is heterogeneous toward methane formation. Thus, Ti02 does not seriously influence the formation mechanism of hydrocarbon reaction products, and the methane formation route via CFI Oads species on the support has an inappreciable contribution. 

The catalyst surface is heterogeneous with respect to the methane intermediate. Two methane intermediates are defined Ca,ads and C 3,ads. 

Both methane intermediates are O-free surface species. 

Numerical Modeling of Transient Isotope Responses On the first step, the authors analyzed in detail five possible heterogeneous methanation models based on two gas phase (CO, CH4) and three surface components (COads, Ca,ads, and Cp,ads) that follow from qualitative analysis of CO labeling data [19,21]. These models contained either a buffer step or parallel routes of methane formation. The homogeneous model having one type of methane intermediate was also considered. A methanation reaction was modeled separately from the entire set of reactions included in the Fischer-Tropsch ... 

There are several studies related to the formation of intermediate species. Calculations of the binding energies of species and surfaces can help verifying the possible formation of species during the chemical reaction. In particular, Van Santen et al. [4] showed in this way the dissociation of methane (intermediates CH. where x = A nH) on ruthenium surface (1120) by applying the density functional theory (DFT) [4]. 

In the proposal for the methane-formaldehyde mechanism, methane and formaldehyde are formed from the surface methoxy intermediates and react to form ethanol [83-86]. Tajima et al. [83] used theoretical calculations to indicate that CHyHCHO are also potential reaction intermediates, van Santen with Blaszkowski [86] have theoretically found a transition state for hydride transfer between methanol and surface methoxy to form methane and potentially formaldehyde. Because methane is very slow to react, almost all conventional experimental and theoretical studies have supposed methane to be independent of the first C-C bond... 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

The influence of Zn-deposition on Cu(lll) surfaces on methanol synthesis by hydrogenation of CO2 shows that Zn creates sites stabilizing the formate intermediate and thus promotes the hydrogenation process [2.44]. Further publications deal with methane oxidation by various layered rock-salt-type oxides [2.45], poisoning of vana-dia in VOx/Ti02 by K2O, leading to lower reduction capability of the vanadia, because of the formation of [2.46], and interaction of SO2 with Cu, CU2O, and CuO to show the temperature-dependence of SO2 absorption or sulfide formation [2.47]. 

Figure 4. Schematic potential energy surface for the reaction of FeO" " with methane. The sohd line indicates the sextet surface the quartet surface is shown with a dotted line, in each case leading to the production of Fe + CH3OH. The dashed line leads to formation of FeOET + CH3. The pathway leading to the minor FeCH2" + H2O channel is not shown. Schematic structures are shown for the three minima the [OFe CHJ entrance channel complex, [HO—Fe—CH3] insertion intermediate, and Fe" (CH30H) exit channel complex. See text for details on the calculations on which the potential energy surface is based.

The catalyst performance depends on the H2 to CCI2F2 feed ratio. The selectivities to CH2F2 and CHCIF2 are influenced by the H2 to CCI2F2 feed ratio, while the selectivity to methane is independent of this ratio. We have previously proposed a reaction mechanism with serial reactions on the catalyst surface and minor readsorption of the intermediate products, which is depicted in figure 8 [4,5]. Thus the kinetics of the reaction follows mainly parallel reaction pathways, in which the selectivities are not influenced by the conversion, and a... 

Finally, the change in selectivity for the methane/pentane couple for the two different substrates (18% for hexane, 56% for cyclohexane) can be explained as follows in the case of cyclohexane, the Ci to C5 products are formed through the second carbon-carbon bond cleavage via the hexyl surface intermediate D whereas in the case of hexane, the initial carbon-hydrogen bond activation step can lead to any of three alkyl surface intermediates (D, E, and F) before arriving at the key metallacychc intermediates... 

While this scheme is useful in helping to predict products from di-rr-methane rearrangements, all evidence indicates that the structures drawn are not intermediates in the reaction. That is, they do not represent energy minima on the potential energy surface leading from the excited state of the reactant to the ground state of the product. 

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