Chlorination in the Adsorbed State

The much higher collision rate considerably increases the tolerable activation energy of interaction compared with the reaction in gas. According to Eq. 3.5, the 

Its enthalpy change must be more positive than in gas (cf. Fig. 3.4) by about 130kJ mol-1, which is the difference of the sublimation enthalpies of ZrCh and Z1CI4. With such a correction, the enthalpy of this step of chlorination of Zr atoms, which happens to be -132 kJ mol-1 in gas (see Fig. 3.4), may be about zero on the surface. Provided that Eq. 3.6 is still valid, chlorination on the surface is fast enough for the purpose. It is illustrated by the data of Table 3.3. 

So far, we have left aside the question whether the change in the structure of the fragment modifying the surface might also bring a contribution. We would expect it to have an opposite sign than that due ZrCL-ZrCIq. Let us consider an obviously extreme situation. It would be the reaction 

The enthalpy of this reaction would evidently gain one more, this time negative contribution, approximately equal to the difference between the sublimation energies of the two Ti compounds. In the particular case, this would markedly compensate the contribution from the zirconium chlorides. Thus, when a priori neglecting the possible contribution from the surface, we obtain a safer margin to judge whether the reaction on surface is fast enough. 

Let us go to the allowable reaction enthalpies. We have seen from the concrete example that the Ar// value for a reaction on surface can be much less negative than for the essentially similar reaction in gas. Very probably, with other combinations of the reaction partners, some of the halogenation steps may have positive ArH. Hence, it is not any more guaranteed that they are fast enough. Evidently, ATH 

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