Dissociatively adsorbed

Only recently has a mechanism been proposed for the copper-cataly2ed reaction that is completely satisfactory (58). It had been known for many years that a small amount of carbon dioxide in the feed to the reactor is necessary for optimum yield, but most workers in the field beHeved that the main reaction in the formation of methanol was the hydrogenation of carbon monoxide. Now, convincing evidence has been assembled to indicate that methanol is actually formed with >99% selectivity by the reaction of dissociated, adsorbed hydrogen and carbon dioxide on the metallic copper surface in two steps ... 

Hydrogen is dissociatively adsorbed on two catalyst sites, indicated by an asterisk. Sites need not necessarily be on different atoms. ... 

The first equation was derived by assuming that the rate-controlling step is the reaction of one molecule of adsorbed C02 with two molecules of dissociated adsorbed hydrogen. The second equation, which correlates almost as well, is based on the assumption that the rate-determining step is the reaction of one molecule of adsorbed C02 with two molecules of adsorbed hydrogen. This indicates that, in this particular case, it was not possible to prove reaction mechanisms by the study of kinetic data. 

Such simple considerations led Scholten and Konvalinka to confirm the form of the dependence of the reaction velocity on the pressure, as had been observed in their experiments. Taking into account a more realistic situation, on the polycrystalline hydride surface with which a hydrogen molecule is dealing when colliding and subsequently being dissociatively adsorbed, we should assume rather a different probability of an encounter with a hydride center of a /3-phase lattice, an empty octahedral hole, or a free palladium atom—for every kind of crystallite orientation on the surface, even when it is represented, for the sake of simplicity, by only the three low index planes. 

Kinetic studies of the decomposition of metal formates have occasionally been undertaken in conjunction with investigations of the mechanisms of the heterogeneous decomposition of formic acid on the metal concerned. These comparative measurements have been expected to give information concerning the role of surface formate [522] (dissociatively adsorbed formic acid) in reactions of both types. Great care is required,... 

As already mentioned in section 2.5.1.4, oxygen is dissociatively adsorbed on most metals even below room temperature. Thus under conditions of technological interest (e.g. in the NH3 oxidation reaction) the... 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Comparable patterns are followed by other organic substances such as formaldehyde and formic acid. All these substances are dissociatively adsorbed on platinum [4] and it was suggested that they build the same adsorption product [35]. 

TPD and static secondary ion mass spectrometry (SSIMS) data suggest that methanol dissociatively adsorbs at Ob-vacs and molecularly at the Ti5c sites [52, 53]. There is also some evidence that methanol also dissociates at other sites apart from Ob-vacs, presumably Tisc sites [53-55]. Similar conclusions have been reached for a series of short-chain (C2-C8) aliphatic alcohols [56-58]. 

The temperature dependence of the extent of adsorption was not interpreted, except that the results were considered to be consistent with the magnetic measurements of Selwood (see Section II,C) which indicate that the number of carbon-metal bonds between adsorbed species and the surface increases threefold between 120°and 200°C due to extensive dissociative chemisorption. The authors proposed that two forms of chemisorbed benzene exist at the nickel surface, (i) an associatively adsorbed form which can be displaced by further benzene, and which may be w- or hexa-removal from the surface. 

These results are similar to those with propylene insofar as they indicate dissociative adsorption of the olefin. The hydrogen that yields the hydroxyl has not been identified but it seems reasonable to suppose that, once again, the allylic hydrogen is lost. Results with butene, however, do differ from those with propylene in two respects first, the dissociation (as evidenced by the OH band) is rapid but not instantaneous as found for propylene second, dissociatively adsorbed butene is more easily removed by room temperature evacuation than dissociatively adsorbed propylene. These facts suggests that steric effects are present hence, the kinetic behavior of these two species may be quite different. 

It is known [4] that methylacetylene can be adsorbed dissociatively or not. The dissociation of methylacetylene is characterized by the appearance of a typical v(OH) vibration. Therefore, the absence of any zeolitic v(OH) band for the different LSX samples indicates that methylacetylene adsorbs mainly without dissociation The presence of non-dissociated adsorbed methylacetylene is also evidenced by the detection of specific v(C=C) and v(=CH) vibrations band. As expected, their wavenumbers decrease with increasing basicity. Moreover, the complex shape of the v(=CH) band reveals different environments of basic sites. 

Current opinion is that CO dissociatively adsorbs on the catalyst surface, and that both C and O are subsequently hydrogenated, yielding CH2 and H20. 

Cl2 adsorbs without dissociation, adsorbs very little. 

It is likely that both mechanisms are active and dependent on potential. At low potentials (<200 mV) on PtRu, the bifunctional mechanism is not active because Ru is unable to dissociate adsorbed H2O to produce OH. However, above 250 mV, this does occur and CO oxidation by adsorbed OH becomes the dominant reaction in achieving CO tolerance. This is strongly related to the use of PtRu as a MeOH oxidation catalyst because CO oxidation is also the rate-determining step for this reaction. 

The kinetic data indicate that the rate equation of DPM could be expressed as a surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen as mentioned above. Applying this result to the asym DAM, and taking the above discussions into account, the reaction scheme of asym DAM could be drawn as in Fig. 3. 

These findings suggest that the rate equation of asym DAM can be expressed by the sum of the reaction rates between the adsorbed individual aryl group and dissociatively adsorbed hydrogen as follows ... 

The kinetic studies of the hydrogenolysis of DPM indicate that both the DPM and hydrogen are adsorbed on the same kind of active sites on the catalyst. Also, the rate-determining step of the hydrogenolysis is a surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen. When the rate equation for DPM is applied to asym DAMs, their reactivities can be satisfactorily explained, and it is suggested that the product selectivity is proportional to the ratio of the adsorption equilibrium constants of the two aryl groups. 

Figure 5.4 STM image and models of CH3S on Au(l 1 1). (a) STM micrograph of stacks of CH3S-AU-SCH3 on Au(l 1 1) recorded atT=5K. Structure was obtained by dissociating adsorbed CH3SSCH3 at 300 K. (b) Model of proposed structure with gold adatoms labelled as Au.

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