Atomic diffusion steady-state

Considering natural stratospheric ozone pro-duction/destruction as a balanced cycle, the NO.v reaction sequence is responsible for approximately half of the loss in the upper stratosphere, but much less in the lower stratosphere (Wennberg et al, 1994). Since this is a natural steady-state process, this is not the same as a long term O3 loss. The principal source of NO to the stratosphere is the slow upward diffusion of tropospheric N2O, and its subsequent reaction with O atoms, or photolysis (McElroy et ai, 1976). 

Diffusion into the electrode. If the surface radical is H, there may be diffusion into the electrode and this may cause a change in the character of the surface and the atoms immediately beneath it. Hence, for surface-catalyzed reactions on real surfaces, finding the steady state in the i—t curve at constant potential may show complexities (Fig. 7.44). Where is the steady state in Fig. 7.44(b) It becomes a matter of judgment The best plan is to take the first time-invariant section and to reject the further variations, which simply indicate a nonconstant surface.44... 

A corollary to the conclusion of the above discussion is that the number of lattice oxygen available for reaction at the active site depends not only on the atomic details of the site, but also on the rate of lattice diffusion compared with the rate of surface reaction. If the rate of lattice diffusion is slow relative to surface reaction, then the number of lattice oxygen atoms available for the reaction during the residence of an intermediate could be estimated from the atomic structure of the site, as in the examples above. On the other hand, if lattice diffusion is fast, a lattice oxygen atom will be replenished immediately after its removal. Then the behavior of the catalyst no longer depends as much on the stoichiometry of the active site, but on the degree of reduction of the active site at steady state, which would determine the heat of removal (or the reactivity) of the lattice oxygen. 

Atoms are diffusing into the boundary laterally from its edges and can diffuse out through its front face into the forward grain. At the same time, atoms will be deposited in the backward grain in the wake of the boundary. In the quasi-steady state in a coordinate system fixed to the moving boundary, the diffusion flux in the forward grain is J = — DXL(dc/dx) — vc and the diffusion equation is... 

The situation becomes quite different when the a//3 interface is no longer capable of maintaining the concentration of B atoms in its vicinity at the equilibrium value c 0. If the concentration there rises to the value ca0, the instantaneous quasi-steady-state current of atoms delivered to the particle by the diffusion field (obtained from Eq. 13.22) will be given by... 

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