Kinetics carbon monoxide activation energy

Just as in the case of the H2-D2 exchange on ZnO, two mechanisms are also discernible for the carbon monoxide oxidation [stage (b)] on nickel oxide below 300°C. There is a low-temperature mechanism operative between 100° and 180°C. characterized by a low activation energy of 2 kcal./mole and a high-temperature mechanism, above 180°C., with a higher activation energy of 13 kcal./mole. The kinetics are different and are respectively ... 

Table 10.10 summarizes the kinetic data on SCWO of organic compounds, organic mixtures, ammonia, and carbon monoxide. M and n are the constants in Equation (10.25). These kinetic data can fit the pseudo first-order reaction models proposed by Wightman (1981). The activation energy from... 

Experimental results supported the assumption that this temperature was necessary to gain the PdZn alloy on the catalyst surface. No catalyst deactivation was detectable during the experiments. At 300 °C full conversion was achieved at a 100 ms residence time [32] and 5% and lower carbon monoxide selectivity. First order kinetics were determined, revealing 7.04 1013 h 1 for the pre-exponential factor and 92.8 kj mol 1 for the activation energy. 

The reason why the minimum steam ratio goes down with temperature is not known with certainty. One possibility is that the competing reactions of carbon production and consumption have such kinetics that the rate of coke consumption increases faster with temperature than the rate of coke generation, which suggests that the carbon-steam reaction has a higher activation energy than the methane cracking and carbon monoxide disproportionation reaction. 

Values of Activation Energies of Methanol Synthesis from Carbon Monoxide, Efk), and from Carbon Dioxide, E,(k ), and Adsorption Enthalpies AH and Entropies AS Derived from the Kinetic Model Utilizing Constants in Table IX ... 

Catalysis relies on changes in the kinetics of chemical reactions. Thermodynamics acts as an arrow to show the way to the most stable products, but kinetics defines the relative rates of the many competitive pathways available for the reactants, and can therefore be used to make metastable products from catalytic processes in a fast and selective way. Indeed, cafalysis work by opening alternative mechanistic routes with lower activation energy barriers than those of the noncatalyzed reactions. As an example, Figure 1 illustrates how the use of metal catalysts facilitates the dissociation of molecular oxygen, and with that the oxidation of carbon monoxide. Thanks to the availability of new pathways, catalyzed reactions can be carried out at much faster rates and at lower temperatures than noncatalyzed reactions. Note, however, that a catalyst can shorten the time needed to achieve thermodynamic equihbrium, but caimot shift the position of that equihbrium, and therefore cannot catalyze a thermodynamicaUy unfavorable reaction. ... 

Trifluoroacetic acid at 300-390 °C produces mainly carbon dioxide, difluoro-methyl trifluoroacetate, carbon monoxide and trifluoroacetyl fluoride. Blake and Pritchard propose that the decomposition proceeds through the elimination of hydrogen fluoride, followed by the formation of difluorocarbene which largely adds to trifluoroacetic acid to form the difluoromethyl ester. The kinetic order is about 0.5 and the overall activation energy for the formation of carbon dioxide and the difluoromethyl ester is about 45 kcal.mole" ... 

The activation energy for reductive elimination from [NiR2(bipy)] decreases on coordination of an electron-deficient alkene , and second-order kinetics is observed in the phosphine-induced elimination from the same complex, indicating that the rate-determining step is formation of a five-coordinate intermediate . In general, reductive elimination from cis-dialkylnickel(II) complexes may be promoted by addition of CO, phosphines, or alkenes, and it proceeds by an associative mechanism, whereas the corresponding trans complexes are more resistant to elimination Carbon monoxide induced reductive elimination may also produce ketones ... 

Bryce and Greenwood studied the kinetics of formation of the major volatile fraction from potato starch, and its components. They limited their interest to the temperature range from 156 to 337 and to the formation of water, as well as of carbon mon- and di-oxide. The results revealed the following facts. Stability toward pyrolysis within the first 20 minutes of the process falls in the order amylose < starch < amylopectin < cellulose. Autocatalysis is absent, as shown by Puddington. Both carbon mon- and di-oxide are evolved as a consequence of each of two first-order reactions. The initial one is fast, and the second is slow. The reasons are not well understood, but they probably involve some secondary physical effects. The amount of both carbon oxides is a direct function of the quantity of water produced from any polysaccharide, which, furthermore, is independent of the temperature. The activation energy for the production of carbon mon-and di-oxide reaches 161.6 kJ/mol, and is practically independent of the polysaccharide formed. At the limiting rates, the approximate ratios of water carbon dioxide carbon monoxide were found to be 16 4 1 for amylopectin, 13 3 1 for starch, 10 3 1 for amylose, and 16 5 1 for cellulose. 

A final deficiency in the model relates to the detailed kinetics of the adsorption and decomposition of carbon dioxide to carbon monoxide and an adsorbed oxygen atom. Experimentally we have shown that the decomposition of carbon dioxide on Cu is precursor state mediated, having an activation energy of 3 k cal mol" for the decomposition of the weakly held precursor state CO2 (C02(a)) which is less than the heat of adsorption of the weakly held precursor state by 1 k cal mol" (7). However, the model (table 1 and figure 2) shows an activation energy for the decomposition of the weakly held precursor state of 16.3 k cal mol". This value of 16.3 k cal mol", required by the combination of the kinetics and thermodynamics involved in the forward reaction 2... 

The turnover frequencies measured at different temperatures were comparable to those found for hydroformylations using other strategies [1,3,33,35]. To develop the kinetic model, the effects of the linalool and the catalyst concentration and of the total carbon monoxide and hydrogen pressure on the outcome of the hydroformylation were investigated. In all cases, the reaction rate was enhanced by a first-order dependence. The results were in good agreement with the calculated model. The activation energy was found to be 14.5 kcal mol h... 

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