Dissociative adsorption of carbon

Alternatively, an intermediate formation of an adsorbed methylene on the catalyst surface through the dissociative adsorption of carbon monoxide has been considered ... 

Non-dissociative, dissociative. If a molecule is adsorbed without fragmentation, the adsorption process is non-dissociative. Adsorption of carbon monoxide is frequently of this type. If a molecule is adsorbed with dissociation into two or more fragments both or all of which are bound to the surface of the adsorbent, the process is dissociative. Chemisorption of hydrogen is commonly of this type. 

When the carbon monoxide molecule dissociates upon adsorption, it is referred to as the dissociative adsorption of carbon monoxide. As in the case of molecular adsorption, the rate of adsorption here is proportional to the pressure of carbon monoxide in the system because this rate governs the number of gaseous collisions with the surface. For a molecule to dissociate as it adsorbs, however, two adjacent vacant active sites are required rather than the single site needed when a substance adsorbs in its molecular form. The probability of two vacant sites occurring adjacent to one another is proportional to the square of the concentration of vacant sites. These two observations mean that the rate of adsorption is proportional to the product of the carbon monoxide partial pressure and the square of the vacant-site concentration, Pco u-... 

The C-0 bond can be more easily activated when the CO molecule interacts with more than two metal atoms. Recently, the dissociative adsorption of carbon monoxide by polynuclear metal complexes, such as [(silox)2TaH2]2 (Eq. 57) [126, 127] and [(silox)2WCl]2 (Eq. 58) [126-129], and tetratungsten alkoxides [129] has been achieved. Hydrogenation of CO to give hydrocarbons promoted by metal clusters has been reviewed [130]. 

Experimental results on single crystals show that at temperatures below about 250 K, dissociative adsorption of carbon monoxide on the surface does not proceed. Molecular adsorption of carbon monoxide occurs on different sites, each showing different heats of adsorption. 

The volcano curve is bounded by the rate of dissociative adsorption of CO and hydrogenation of adsorbed carbon. This is illustrated in Figure 1.9. 

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 7.2. The uptake of carbon by dissociative adsorption of methane on Ni(lll) follows f rst-order kinetics. The experiment involved dosing the surface with a supersonic beam of molecular methane at the indicated...

In our previous work [11], it has been shown that the reduction of NO with CH4 on Ga and ln/H-ZSM-5 catalysts proceeds through the reactions (1) and (2), and that CH4 was hardly activated by NO in the absence of oxygen on these catalysts. Therefore, NO2 plays an important role and the formation of NO2 is a necessary step for the reduction of NO with CH4. In the works of Li and Armor [17] and Cowan et al. [18], the rate-determining step in NO reduction with CH4 on Co-ferrierite and Co-ZSM-5 catalysts is involved in the dissociative adsorption of CH4, and the adsorbed NO2 facilitates the step to break the carbon-hydrogen bond in CH4. It is suggested that NO reduction by use of CH4 needs the formation of the adsorbed NO2, which can activate CH4. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

A dissociative adsorption of methanol forming surface methoxy groups is suggested as the initial step. This is followed by the slow step, the formation of some form of adsorbed formaldehyde species. Evidence.for the bridged species is not available, experiments with °0 labeled methanol are expected to clarify this. Continued surface oxidation leads to a surface formate group and to carbon monoxide. All the byproducts can be obtained by combination of the appropriate surface species. 

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

In 1999, Binet et al.395 published a review on the response of adsorbed molecules to the oxidized/reduced states of ceria. In light of recent infrared studies on ceria, the assignments for OH groups, methoxy species, carbonate species, and formates are highly instructive. The OH and methoxy species have been briefly discussed. Characteristic band assignments of carbonate and formate species are provided below, the latter formed form the dissociative adsorption of formic acid, the reaction of CO with H2-reduced ceria surface, or via selective oxidation of methanol. Formate band intensities were a strong function of the extent of surface reduction of ceria. 

Correlations between surface species and emitted secondary ions are based on characterization of the surface adlayer by adsorption and thermal desorption measurements. It is shown that the secondary ion ratios RuC+/Ru+ and R CTVRuJ can be quantitatively related to the amount of nondesorbable surface carbon formed by the dissociative adsorption of ethylene. In addition, emitted hydrocarbon-containing secondary ions can be directly related to hydrocarbon species on the surface, thus allowing a relatively detailed analysis of the hydrocarbon species present. The latter results are consistent with ejection mechanisms involving intact emission and simple fragmentation of parent hydrocarbon species. 

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