Activation energy, apparent reactions

These approximations also provide elegant physicochemical insights into the mechanism as summarized in two recent papers by Stoltze [2] and Dumesic [3]. Thus, as shown by Stoltze [2], under the RDS approximation many characteristics of the microkinetic mechanisms, e.g., rate equations (De Donder relations), apparent activation energies, apparent reaction orders, etc., may be naturally partitioned into a sum of contributions associated with a special class of reactions involving only one surface species. Dumesic [3] extended the idea to general systems. 

Thus, the radius of the atom carrying the free valence has a substantial influence on the activation barrier to the addition reaction the greater the radius of this atom, the higher the activation energy. Apparently, this effect is due to the repulsion in the transition state, which is due to the interaction between the electron shells of the attacked double bond and the atom that attacks this bond. 

As mentioned above, in the absence of O2 all observed OH removal appears to be via the abstraction route, i.e. Reaction la. Apparently, Reaction -lb is very fast compared to the time scale of our experiments. However, the adduct lifetime must be long enough that it can be scavenged by 02 in competition with decomposition back to reactants. The dramatic dependence of kQbs on temperature is qualitatively consistent with the above mechanism. The activation energy for Reaction -lb is expected to be quite large, so the fraction of adduct molecules scavenged by 02 can increase dramatically over a relatively small temperature range. 

Apparent activation energies of reactions may be negative, corresponding to a decrease in rate with increase in temperature, if the pathway contains at least one reverse step with an activation energy that is high compared with those of the forward steps. Also, the rates of a few reaction steps of free radicals decrease with increasing temperature in a non-Arrhenius fashion. 

For pore diffusion resistances in reactions having moderate heat evolution, the following phenomena characteristically hold true in industrial ammonia synthesis [212] in the temperature range in which transport limitation is operative, the apparent energy of activation falls to about half its value at low temperatures the apparent activation energy and reaction order, as well as the ammonia production per unit volume of catalyst, decrease with increasing catalyst particle size [211], [213]-[215]. For example at the gas inlet to a TVA converter, the effective rate of formation of ammonia on 5.7-mm particles is only about a quarter of the rate measured on very much smaller grains (Fig. 13) [157]. 

The apparent first-order rate constant of n-alkane cracking over ZSM-5 at 380°C. [Reprinted from J. Wei, Adsorption and Cracking of N-Alkanes over ZSM-5 Negative Activation Energy of Reaction, Chem. Eng. Sci., 51 (1996) 2995, with permission from Elsevier Science.]... 

Sullivan has studied the H2-I2 and D2-I2 systems quite extensively inducing the formation of the iodides both thermally and photochemically. The initial thermal studies of the H2-I2 system in the temperature range 667-800 °K confirmed the calculations of Benson and Srinivasan. Their calculations indicated that the apparent temperature dependence of the activation energy for reaction involving H2 observed by Bodenstein could be accounted for if an atomic mechanism produced HI in parallel with the molecular mechanism. 

The mechanism and kinetics of the NO + CO reaction on Rh(lll) have been discussed in detail by Zhdanov and Kasemo (108). They showed that simulations based on surface science data obtained at low pressures reproduce the scale of the reaction rate at the pressure regime of interest for the TWC but fail to predict accurately the apparent activation energy and reaction orders. 

Where A/ and / are the apparent frequency factors and activation energy of reaction / and are given in Table 6.23 for the six reactions. (Sheel and Crowe, 1969 and Crowe, 1989). 

Finally, is observed that the rcw increases with the pcH4 [16-18] and with the reduction temperature, while it diminishes with the reaction temperature. The influence of the ph2 has no clear effect. The effect of the reaction temperature is due to the fact that high reaction temperatures favor gasification over the growth of coke filaments [15,19,26,27] which would explain the negative value of the activation energy apparent for the filament growth. 

Calorimetric measurements were used to study the effect of a plasticizer on the cm-ing rate of epojgr oligomers. It was found that the plasticizer inhibits curing reaction because it forms complexes with proton donors. A plasticizer also decreases apparent activation energy of reaction (Figure 15.5) because it changes the reaction mechanism." ... 

E,E, Intrinsic activation energy, apparent activation energy of reaction j J mol"2... 

Rosner DE. The apparent chemical kinetics of surface reactions in external flow systems Diffusional falsification of activation energy and reaction order. AIChE Journal 1963 9 321-331. 

To minimize diffusion effects, established kinetic practice requires that samples should be as small as possible (thin layers spread on multistory crucible [574,689]). However, the smaller the sample, the greater is the ratio of its surface to its bulk and this may overemphasize surface reactions and make correlation with large-scale processes poorer. Experience shows, however, that even very small samples (less than 1 mg) are far from being small enough to be free of diffusion inhibition. To justify the obvious errors, the adjectives "apparent", "formal" and "procedural" are used in conjunction with the otherwise strictly-defined terms of activation energy and reaction order as established in homogeneous chemical kinetics. 

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