Moving reaction front

OS 93] [R 31] [P 73] Using a simplified modeling approach, it was shown that axial dispersion changes the direction and shape of moving reaction fronts and also affects the interplay between dispersion and migration in an electrical field [145]. 

A third possibility, is that the reaction is taking place according to a moving reaction front model, i.e., the reaction rate is so fast, that the adsorption and reaction take place immediately when the gas pulse arrives at the catalyst bed. This model is described in detail in the next paragraph. 

Figure 6. Schematical representation of the moving-reaction-front model. A pulsing CO over O-precovered platinum. B Pulsing CO (excess) and Oj alternately over initial O-covered platinum. Gray O-covered White CO covered or clean platinum. The numbers denote the pulse cycle number.

This moving reaction front behavior implies that not the reaction rate as a function of a varying oxygen surface coverage is measured, as was plarmed, but the reaction rate as a function of the location of the reaction front in the reaction. 

At higher temperature the CO oxidation in Multitrack takes place according to a moving reaction front pattern. 

The moving-reaction-front model is able to describe the occurrence of the double pulse responses obtained in measurements. 

The TPO experiment (Fig. 18) showed that NO desorbs from platinum from about 423 K, but only at high oxygen surface coverage. In Fig. 14, a drastic decrease of nitrogen and N2O formation is observed, which can be explained in terms of the moving reaction front through the catalyst bed. As the reaction zone arrives at the last positions, N2O cannot decompose anymore, since there is no fresh platinum surface left. As the last positions are deactivated, the catalyst s activity sharply decreases and the surface remains mainly covered with NH and NH2. This is supported by XPS N(ls) measurement and indirectly by NO pulse experiments. 

The occurrence of SHS reactions is predicated upon the ability of a reaction front to produce heat at a magnitude and a rate which can raise the temperature of the adjacent layer to the ignition point, and thus sustain the process in the form of a moving reaction front or combustion wave. However, for thermodynamic and kinetic reasons this process may not be possible, as is the case for the synthesis of many important materials. Referring to Eq. (1), a thermodynamic limitation is operative when the reaction enthalpy, Q, is low and this is the general basis for the use of the empirical limit of the adiabatic temperature discussed in an earlier... 

L. K. Bieniasz and C. Bureau. Use of dynamically adaptive grid techniques for the solution of electrochemical kinetic equations Part 7. Testing of the finite-difference patch-adaptive strategy on example kinetic models with moving reaction fronts, in one-dimensional space geometry, J. Electroanal. Chem. 481, 152-167 (2000). 

For a more general strategy, the use of dynamically adaptive grids has been proposed by several authors [4, 5] as a more suitable and (in some cases) more efficient solution for the problem of moving reaction fronts. The... 

Also in the initial period of regeneration, a heat front moves through the bed, heating up the bed behind the moving reaction front from the initial temperature To to the maximum temperature Traax. The velocity of the heat front (here about 1.1 m h ) is much higher than that of the reaction front (0.05 m h ), and is calculated by the heat balance ... 

In this case, the reaction-modified phase arises within the region containing the diffusing molecules, while the unreacted phase remains outside the region where diffusion takes place. These distinct phases are separated by a moving reaction front. 

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