Stepwise reaction, determining standard

In this case, Sj is determined by the standard thermodynamic parameters of both the transition states and the initial and final reactants but not of the thermalized intermediates. Therefore, equations (1.36) and (1.40) con elude that the stationary rate of a complex stepwise reaction comprising an arbitrary combination of intermediate linear transformations is indepen dent of the standard values of thermodynamic parameters of the intermedi ates. It is determined only by the difference of thermodynamic rushes of the initial reactant and the product, as well as by the standard thermo dynamic parameters of the transition states between various intertrans forming reaction groups. 

Thus, the rate-determining parameters here are the standard Gibbs energy of the formation of the transition state in elementary reaction 1 (scheme 4.4) and the standard Gibbs potentials of the formation of "external" reactants R as well as of P and intermediate Ki. The apparent activation energy of the stepwise reaction is... 

The first step is ammonia adsorption/desorption and this step was tuned to an ammonia TPD experiment described in an earlier section (Eq. 12.21, see Fig. 12.4). The second step is ammonia oxidation (Eq. 12.22) and the third step NO oxidation (Eq. 12.23). The parameters for these three steps were determined by separate experiments. In Eq. (12.24), the reaction for standard SCR is described and in Eq. (12.25), the rapid SCR with equimolar amounts of NO and NO2 is covered. NH3 also reacts with NO2 alone, which is described in Eq. (12.26). The final step (see Eq. 12.27) is N2O production from a reaction between adsorbed ammonia and NO2. The results from exposing the Cu-ZSM-5 catalyst to 500 ppm NO, 500 ppm NH3, 8 % O2 and 5 % H2O, while increasing the temperature stepwise are shown in Fig. 12.14 [10]. Initially, there is a total uptake of ammonia for an extended time period, but due to minor NO storage, NO breaks through immediately. The temperature is then increased to about 150 °C, while loosely bound ammonia is released. At this temperature, the standard SCR reaction is active and equimolar consumption of NO and NH3 is seen. At 200 °C, the conversion of NOx is close to 100 %. At high temperature, the NOx conversion again decreases due to the ammonia oxidation. Also, some NO2 is produced, since the back part of the catalyst is not exposed to ammonia, NO oxidation may occur. The global model described... 

Again, while the stationary rate of stepwise noncatalytic reactions is independent of the standard thermodynamic parameters of thermafized intermediates, the rate determining parameters of catalytic reactions may depend on the standard thermodynamic parameters of thermafized catalytic intermediates and, as a consequence, influence considerably the apparent activation energy of the overall process. 

The main evidence for this cyclic mechanism over an alternative stepwise mechanism lies in the failure of Maruyama et al. to observe any partial reactions catalysed by PEPC. The mechanism is also consistent with the stereochemistry of carboxylation at vinyl carbon determined by Rose et al. (1969). O Leary et a/. (1981) have studied the kinetic (carbon) isotope effect associated with PEPC-catalysed carboxylation of PEP, employing isotopi-cally equilibrated HC03 /C02. Comparison of these KIEs with those for standard C—C bond forming and breaking processes led to the proposal of a stepwise mechanism, involving the intermediacy of carboxyphosphate (Scheme 42b). 

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