First Zeldovich reaction

A detailed quasiclassical trajectory (QCT) study of the first Zeldovich reaction (reaction 5a, Table 1)... 

It was assumed that the first Zeldovich reaction occurs with the products and the reactants in their ground electronic states. 

The first and most prominent source is known as thermal NO or Zeldovich-NO. The label thermal refers to the high temperatures required to break the N2 triple bond in its reaction with O atom and its location of appearance in a flame. 

Although Bowman and Seery s results would, at first, seem to refute the suggestion by Fenimore that prompt NO forms by reactions other than the Zeldovich mechanism, one must remember that flames and shock tube-initiated reacting systems are distinctively different processes. In a flame there is a temperature profile that begins at the ambient temperature and proceeds to the flame temperature. Thus, although flame temperatures may be simulated in shock tubes, the reactions in flames are initiated at much lower temperatures than those in shock tubes. As stressed many times before, the temperature history frequently determines the kinetic route and the products. Therefore shock tube results do not prove that the Zeldovich mechanism alone determines prompt NO formation. The prompt NO could arise from other reactions in flames, as suggested by Fenimore. 

Attention was first drawn to reaction (49) by Zeldovich,462 following a suggestion by Semenov, in order to explain the high-temperature oxidation of nitrogen in explosions. Indications of the very rapid rate of this reaction came from the almost simultaneous work of Glick, Klein, and Squire,167 Harteck and Dondes,181 and Kistiakowsky and Volpi.253... 

The first estimate of kinetic parameters for the thermal decomposition of nitric oxide were made by Zeldovich and Frank-Kamenetsky . From a study of nitric oxide formation in H2-O2-N2 flames these workers proposed a value of 82+10 kcal. mole for the activation energy of decomposition. Vetter studied the reaction over the temperature range 1200-1900 K. He found a small rate increase on addition of oxygen and postulated a chain process involving the reactions... 

Nitric oxide is formed in combustion engines by the interaction of oxygen and nitrogen in air at the high temperatures reached during the combustion cycle. The percentages of NO found in the exhaust gas is close to that calculated from the chemical equilibrium N2+02 = 2N0 at peak temperatures near 2500 K. Once NO is formed, its abundance seems to be effectively frozen in. The mechanism of NO formation is not precisely known. Most authors have adopted the reaction chain first proposed by Zeldovich et al. (1947) ... 

Some of the first considerations of the problem of diffusion and reaction in porous catalysts were reported independently by Thiele [E.W. Thiele, Ind. Eng. Chem., 31, 916 (1939)] Damkohler [G. Damkohler, Der Chemie-Ingenieur, 3, 430 (1937)] and Zeldovich [Ya.B. Zeldovich, Acta Phys.-Chim. USSR, 10, 583 (1939)] although the first solution to the mathematical problem was given by Jiittner in 1909 [F. Jiittner, Z. Phys. Chem., 65, 595 (1909)]. Consider the porous catalyst in the form of a flat slab of semi-infinite dimension on the surface, and of half-thickness W as shown in Figure 7.3. The first-order, irreversible reaction A B is catalyzed within the porous matrix with an intrinsic rate (—r). We assume that the mass-transport process is in one direction though the porous structure and may be represented by a normal diffusion-type expression, that there is no net eonveetive transport eontribution, and that the medium is isotropic. For this case, a steady-state mass balance over the differential volume element dz (for unit surface area) (Figure 7.3), yields... 

The first reaction in the chain is the limiting reaction of the Zeldovich mechanism and its stimulation by vibrational excitation of N2 molecules, discussed in Section 6.2. The second reaction is stimulated by vibrational excitation of CO molecules due to the relath ely high energy barrier of the reaction and the absence of activation energy of the reverse reaction (see Sections 2.7.2 and 2.7.3). 

The stability of this solution depends on the parameters of the problem. We first note that the Zeldovich number Z can be interpreted as the ratio of the diffusion time scale to and the reaction time scale U. Thus, for Z sufficiently small all the heat released in the reaction can be diffused away from the interface. However, for Z sufficiently large this is not the case. The heat released in the reaction which exceeds the amount of heat that can be carried away by diffusion necessarily raises the temperature at the interface. This, in turn, leads to an increase in the thermal gradient into the fresh fuel (leading to the interface speeding up), which lowers the interface temperature (leading to the interface slowing down), and the process repeats. This is the mechanism for the onset of oscillatory propagation as Z exceeds a critical value Zc. 

Moreover, consider the description of the structure of a premixed flame developed by Zeldovich and Kamenetzki-Frank [13]. The reaction rate is of first order with respect to fuel and oxygen, and is given by... 

In his study of the cellular structure of flames, Zeldovich [6] is among the first who has stressed the importance of this mechanism. He has indeed shown that a plane flame front can undergo an instability if the diffusivity of the species limiting the reaction (inhibitor) exceeds the thermal diffusivity (activator) of the gas mixture. 

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