Kinetics carbon dioxide activation energy

Carbon dioxide decomposes behind a shock front in accordance with the kinetics expected of a unimolecular reaction in its low-pressure region31-37. The second-order rate coefficients obtained by a number of experimentalists in the temperature range 2500-11000 °K are in reasonable agreement, but there is a considerable spread in the values derived for the Arrhenius activation energy (Table 3). Furthermore, even the highest of these values31 is much smaller than the endothermicity (D = 125.8 kcal.mole-1) of... 

In many cases, redox reactions that are favorable from a thermodynamic point of view may not actually take place sometimes, the activation energy barriers for such reactions are too high to allow fast transformation, according to the preferred thermodynamic considerations. For example, the complete oxidation of any organic molecule to carbon dioxide and water is thermodynamically favorable. However, such oxidation is not favorable kinetically, which implies that organic molecules— including all forms of living species—are not oxidized immediately this fact explains the ability to sustain life. The reason for this difference between kinetic and thermodynamic considerations, for redox reactions, is partly because redox reactions are relatively slow compared to other reactions and partly due to the fact that, in many cases, reactions are poorly coupled because of slow species diffusion... 

Despite the fact that carbon dioxide (C02) is used in a great number of industrial applications, it remains a molecule of low reactivity, and methods have still to be identified for its activation. Both thermodynamic and kinetic problems are connected with the reactivity of C02, and few reactions are thermodynamically feasible. A very promising approach to activation is offered by its coordination to transition metal complexes, as both stoichiometric reactions of C-C bond formation and catalytic reactions of C02 are promoted by transition metal systems. Efforts to enhance the yield of hydrogen in water gas-shift (WGS) reactions have also been centered on C02 interactions with transition metal catalysts. The coordination on metal centers lowers the activation energy required in further reactions with suitable reactants involving C02, making it possible to convert this inert molecule into useful products. 

These effects have been observed by Margolis and co-workers in their detailed kinetic study of the catalytic oxidation of hydrocarbons (219-222). The extensive oxidation of hydrocarbons to carbon dioxide over catalysts of the spinel type has been studied by a number of investigators. It was possible for Margolis and co-workers to establish the effect of additives upon the basic kinetic constants, namely the activation energy of oxidation and the frequency factor for this 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 ... 

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" ... 

FIGURE 9.14 Dependence of the diffusion activation energy of carbon dioxide, methane, and propane in organosihcon polymers on the kinetic diameter of the penetrant molecules (designations of polymers are the same as in Figure 9.9). (From analysis of results presented in Semenova, S.I., J. Membr. Sci., 231, 189, 2004. With permission.)... 

Equation 2.78 predicts that the rate of SRM reaction can be determined mainly by the partial pressure of methanol. For water, the rate shows a weak reverse dependence. The adsorption of carbon dioxide is competitive to that of methanol, water, and the oxygenate intermediates and thereby inhibiting the overall reaction. The apparent activation energy calculated based on these kinetic studies for various Cu-based catalysts are in the range between 70 and 90kJ/mol. The rate expressions and activation energies for the SRM reaction over a few Cu-based catalysts reported in the recent literature are summarized in Table 2.25.188,210-213... 

Benito and Searcy [4] argue that when large reactant samples are used, approximately equilibrium pressures of carbon dioxide may be established at regions within the sample bulk and the rate of outward COj difiusion measured in kinetic work is determined by local pressure gradients. The apparent value of is then close to the enthalpy of dissociation. During decompositions at low pressures, however, the value of the activation energy is greater because local internal pseudoequilibrium is not attained [4]. 

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. 

The low activation energies for aldehydes and carbon dioxide formation might be due to the fact that these reactions occur in the diffusion or transient regions. The activation energies for the formation of aldehydes and acids from propene over V206 were measured in the kinetic region by Kutzeva and Margolis (147) and were found to be 13 and 14 kcal/ mole, respectively. 

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... 

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