Dissociation of carbon monoxide

It is also called dissociative because one of the rate-determining steps is the dissociation of carbon monoxide. The cycle is started by the dissociation of a ligand, which results in the release of the planar 16 electron species (I). In analogy to the cobalt mechanism (see Wiese KD and Obst D, 2006, in this volume), the next step is the addition of an olefin molecule to form the r-complex (II). This complex undergoes a rearrangement reaction to the corresponding reaction steps decide whether a branched or a linear aldehyde is the product of the hydroformylation experiment. The next step is the addition of a carbon monoxide molecule to the 18 electron species (IV). Now, the insertion of carbon monoxide takes place and... 

The fact that surface structure, in particular steps and coordinatively unsaturated sites, has an influence on the state and reactivity of carbon monoxide is entirely in keeping with the empirical correlation (Fig. 6) between heat of adsorption, electron binding energies, and molecular state. Elegant studies by Mason, Somorjai, and their colleagues (32, 33) have established that with Pt(lll) surfaces, dissociation occurs at the step sites only, and once these are filled carbon monoxide is adsorbed molecularly (Fig. 7). The implications of the facile dissociation of carbon monoxide by such metals as iron, molybdenum, and tungsten for the conversion of carbon monoxide into hydrocarbons (the Fischer-Tropsch process) have been emphasized and discussed by a number of people (32,34). 

The major direction taken with Fischer carbenes, however, has been annulation reactions (e.g., Eqs. 11-13) rather than cyclopropanation and insertion. Here, the dissociation of carbon monoxide initiates the sequence of events that lead to product (e.g., Eq. 42). °° Alternatively, an unsaturated unit conjugated with the carbene... 

Calculation of the detonation velocity of the mixture C2N2 + 02 is related to the calculation of the dissociation of carbon monoxide with the formation of a free carbon atom. The chemical constants of the substances which participate in the reaction (CO, C, 0, 02) are sufficiently well known. However, there still exists great uncertainty about the magnitude of the dissociation energy of carbon monoxide, and discussion continues even now. The magnitude of the dissociation heat of carbon monoxide is also of much interest because it is related by simple thermochemical relations to the evaporation heat of carbon and to the energy necessary to break up any organic molecules into their component atoms. 

In the literature the following values for the heat of dissociation of carbon monoxide are cited (kcal/mole) 166—Schmid (1938), 210—Herzberg (1937), 256—Kohn (1920), Kinch and Penney (1941). In his last note, Herzberg concluded that the dissociation heat is not more than 221 kcal/mole. 

Studies of the behavior of supported ruthenium systems has been stimulated because of the finding that ruthenium appears to be the most active element, based on exposed surface atoms, for carbon monoxide hydrogenation (ref. 65). A number of workers have studied the dissociation of carbon monoxide and subsequent buildup of carbidic carbon on the various crystallographic faces of ruthenium (refs. 66-70). It has been shown that the carbidic carbon is easily hydrogenated and is thought to be a precursor for the hydrocarbon products, while the less reactive graphitic carbon is associated with catalyst deactivation (refs. 34,71-72). 

The dissociation of carbon monoxide, however, even at extremely high temperatures is immeasurably small, and we have thus to measure A3 indirectly—by combining two other reactions whose individual affinities can be calculated Let us take the reactions—... 

J. W. Rich and R. C. Bergman, Dissociation of Carbon Monoxide by Optically Initiated Vibration-Vibration Pumping, Calspan Corporation Report WG-6005-A-l, Buffalo New York, 14221. 

While the hydrogenation of the active surface carbon that forms from CO dissociation appears to be the predominant mechanism of CH4 formation, it is not the only mechanism that produces methane. Poutsma et al. [85] have detected the formation of CH4 over paliadium surfaces that do not readily dissociate carbon monoxide. They also observed methane formation over nickel surfaces at 300 K under conditions in which only molecular carbon m.onoxide appears to be present on the catalyst surfaces [81]. Vannice [86] also reported the formation of methane over platinurh, palladium, and iridium surfaces, and independent experiments indicate the absence of carbon monoxide dissociation over these transition-metal catalysts in most cases. It appears that the direct hydrogenation of molecular carbon monoxide can also occur but that this reaction has a much lower rate than methane formation via the hydrogenation of the active carbon that is produced from the dissociation of carbon monoxide in the appropriate temperature range. 

Another mechanism, proposed by Pichler [87] and Emmettt [88, 89], involves the direct hydrogenation of molecular carbon monoxide to an enol species, followed by dehydration and further hydrogenation to produce methane. It is likely that this mechanism provides an additional reaction channel that may compete over certain transition-metal catalysts with CH4 formation via the dissociation of carbon monoxide. Recent studies of methane formation over molybdenum indicate positive CO and H2 pressure dependencies of the reaction rate... 

The growth mechanism appears to be the same irrespective of type of hydrocarbon and whether it is the endothermic dissociation of methane or the exothermic dissociation of carbon monoxide (8). However, the resulting morphology and degree of graphitization depends on parameters such as type of hydrocarbon, metal, particle size, and temperature. Hence, there might not be a unique growth mechanism for the formation of carbon fibers and nanotubes. 

The addition of oxygen also resnlts in the formation of CO in the gas phase. Further redistribution of radicals in the system is influenced in this case by the following reactions O + H2 H + OH, OH + H2 H + H2O, and H2 + CO H + HCO. Deposition of diamond Aims can be also performed using CO as a major gas-phase source of carbon atoms (see, for example, Aithal Subramaniam, 2002). The formation of carbon atoms in this case is due to different non-equilibrium mechanisms of CO disproportioning (see Section 5.7), as well as by direct dissociation of carbon monoxide by electron impact. The thermal mechanism is not effective in non-equilibrium systems, because it requires temperatures exceeding 3000 K which are not present in the non-thermal plasma discharges under consideration. 

Carbyne complexes have also been generated by migration of a substituent at the car-bene to an open coordination site at the metal. For example, dissociation of carbon monoxide from a Fischer carbene complex has been induced photochemically, and this generation of an open coordination site at the metal led to migration of tire alkoxy substituent on the carbene to the metal to form a neutral carb5me complex (Scheme 13.8, bottom). - ... 

Dissociation of carbon monoxide from an acylcobalt tetracarbonyl yield a 16-electron acylcobalt tricarbonyl complex which is the active intermediate in the decarbonylation, oxidative addition, and in various ligand substitution reactions. 

After dissociation of carbon monoxide (or a phosphine) the unsaturated intermediates 3c, 3t, 10c, and lOt may form, which will subsequently... 

Sulphur also inhibits the dissociation of carbon monoxide (the Boudouard reaction). TGA studies [385] on nickel catalysts showed that methanation as well as carbon from the Boudouard reaction were strongly inhibited with increasing sulphur coverage. Other studies [186] found that sulphur eliminates the dissociation of carbon monoxide at a H2S/Ni coverage above 0.33. 

Sulphur was also reported [314] to promote the conversion of the reactive a-carbon formed by dissociation of carbon monoxide into die less active P-carbon as also seen for giun formation [108] (refer to Section 5.3.3). 

It may be noticed that while the observed values of the dissociation constants and .3 differ from the hypothetical values by a factor less than 10, the observed value of ,4 is 3 X 10 times smaller than the corresponding value expected in the absence of heme-heme interaction. This large discrepancy suggests the stabilization due to some possible structural change when the hemoglobin molecule is fully oxygenated. Similar discrepancy was found in the dissociation of carbon monoxide hemoglobin. 

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