NO2 decomposition

A second possible mechanism for NO2 decomposition starts with a bimolecular reaction. When two fast-moving NO2 molecules collide, an oxygen atom may be transferred between them to form molecules of NO3 and NO. Molecules of NO3 are unstable and readily break apart into NO and O2. This reaction sequence can be summarized as Mechanism It for NO2 decomposition ... 

Both proposed mechanisms for NO2 decomposition contain chemical species produced in the first step and consumed in the second step. This is the defining characteristic of an intermediate. An intermediate is a chemical species produced In an early step of a mechanism and consumed in a later step. Intermediates never appear in the overall chemical equation. Notice that neither the O atoms of Mechanism I nor the NO3 molecules of Mechanism II appear In the balanced chemical equation for NO2 decomposition. 

The most direct way to test the validity of a mechanism is to determine what intermediates are present during the reaction. If oxygen atoms were detected, we would know that Mechanism I is a reasonable description of NO2 decomposition. Likewise, the observation of NO3 molecules would suggest that Mechanism II is reasonable. In practice, the detection of intermediates is quite difficult because they are usually reactive enough to be consumed as rapidly as they are produced. As a result, the concentration of an intermediate in a reaction mixture is very low. Highly sensitive measuring techniques are required for the direct detection of chemical intermediates. 

Because rates change continuously with time, accurate rate determinations must use small time intervals. At the outset of the NO2 decomposition experiment shown in Figure 15-6. the time interval between measurements is 5 seconds. For the first 5 seconds, the average rate of O2 production is as follows ... 

Notice that over the same period the rate of O2 formation is only half the rate of NO2 consumption. This follows from the molecular view of the mechanism and from the stoichiometry of the reaction. The rate relationship among the three species involved in NO2 decomposition is given by the following expression ... 

Remember that the overall rate of a reaction is determined by the rate of the slowest step, hi other words, no reaction can proceed faster than the rate-determining step. Any step that comes after the rate-determining step cannot influence the overall rate of reaction. In both proposed mechanisms, the first step of each mechanism is rate-determining. That is, each proposed mechanism predicts an overall rate of NO2 decomposition that is the same as the rate of the first step in the mechanism. 

The two proposed mechanisms for this reaction predict different rate laws. Whereas Mechanism I predicts that the rate is proportional to NO2 concentration. Mechanism II predicts that the rate is proportional to the square of NO2 concentration. Experiments agree with the prediction of Mechanism II, so Mechanism II is consistent with the experimental behavior of the NO2 decomposition reaction. Mechanism I predicts rate behavior contrary to what is observed experimentally, so Mechanism I cannot be correct. 

Every reaction has its own characteristic rate constant that depends on the intrinsic speed of that particular reaction. For example, the value of k in the rate law for NO2 decomposition is different from the value of k for the reaction of O3 with NO. Rate constants are independent of concentration and time, but as we discuss in Section 15-1. rate constants are sensitive to temperature. 

Every rate law must be determined experimentally. A chemist may imagine a reasonable mechanism for a reaction, but that mechanism must be tested by comparing the actual rate law for the reaction with the rate law predicted by the mechanism. To determine a rate law, chemists observe how the rate of a reaction changes with concentration. The graph of the data for the NO2 decomposition reaction shown in Figure 15-6 is an example of such observations. 

Comparisons between reactants and metals reveal several interesting trends. NO2 decomposition is very fast and becomes flux limited above 800K with a constant reaction probability of 0.06 on both metals. NH3 decomposition is also Identical on both metals to within a factor of 2. 

Similar results have been obtained over Pt-Ba/Al2O3 (1 20 100 w/w), but in this case the NO outlet concentration is related to NO2 disproportionation and to NO2 decomposition (Figure 13.11) in fact, O2 formation was observed which originates... 

As in the absence of CO2, the adsorption of NO2 occurs with evolution of NO via reaction (13.37) and is accompanied by NO2 decomposition on Pt sites, with evolution of NO and O2 (Figure 13.14a). The release of CO2 was observed in quantitative agreement with the following stoichiometry ... 

Increased-valence structure 36 for N20(CooV) is analogous to increased-valence structure 7 for isoelectronic HCNO. It has been used recently to develop a VB representation for the N20(CooV) - N20(cyclic,C2V) —> NON(Dooh) isomerization process [32]. In Ref. [10], comparisons are made between 36 and other VB structures with apparently quinquevalent nitrogen atoms, and in Section 4.3, 36 provides the VB representation for N2O when it is formed in NCO-NO and OCN-NO2 decomposition reactions. 

The photolysis of mixtures of NO, NO2, and O2 by Ford (1) shows that the quantum yields for NO2 decomposition are inversely proportional to the O2 pressure, and the effect is mainly attributed to the formation of O3 when O atoms react with O2. 

Hydroxyl radical OH Photolysis of hydroxo or other Fe(III) complexes of NOJ, NO2 decomposition of O3, photolysis of H2O2... 

On Fig. 9 the kinetic curves of NO2 decomposition into N2 are shown over the reduced form of the catalyst, via N2O as intermediate. In this reaction only the catalysts pretreated in reductive atmosphere are active, the oxidative pretreatment leads to deactivation of the catalyst independently either on the metallic form or the ion-exchange procedure. 

On the basis of ESR investigations and the catalytic measurements performed at a relatively high molar ratio of the assumed active sites to the reactants, the following statements can be made for the initial steps of the NO and NO2 decomposition ... 

NO oxidation reached a maximum of 10 % NO conversion around 450 °C, while at higher temperamres thermodynamic equilibrium was approached. Rnally, the inverse of NO oxidation, namely NO2 decomposition to NO, was found to become significant only at temperatures greater than 350 °C. 

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