Some Underlying Principles of Activation

Catalyst activation is a complex sequence of gas-solid reactions of 

Wustite is a nonstoichiometric microheterogeneous solid. Its idealized structure is rather similar to the magnetite structure. Both lattices are built from a cubic, close packed, oxygen lattice with the iron ions located only in octahedral voids for wustite, and in both octahedral and tetrahedral voids for magnetite. Wustite is thermodynamically metastable below 833 K but can be easily supercooled to room temperature. The composition of the material is variable and always deficient in iron Fei j,0 with x varying from 4.5% to 11%. Electroneutrality is preserved by the presence of Fe ions. These ions and the holes in the iron sublattice are not statistically distributed throughout the material but form a variety of clusters centered around the ferric ions placed in tetrahedral interstitial sites.  

The other ternary oxides also occur as spinel superstructures such as CaFe204 (ferrite) or potassium iron oxides which contain the spinel structure as a building block. These complex structures of the beta alumina type consist of the formal composition (K2O x 11 Fc203) and can be described as a layer structure with unit cells of spinel linked by layers of potassium ions octahedrally coordinated to the (4 -h 4) oxygen ions of two adjacent spinel units. 

A section of the iron-oxygen phase diagram is shown in Fig. 2.7. The dashed line represents a typical path for the manufacture and activation of an ammonia synthesis catalyst. All effects due to the promoter are omitted. 

The phase diagram shows some further relevant details. The solubility of oxygen in iron is very low (about 0.03 wt% up to 600 °C) which is a consequence of the high affinity of iron for oxygen to form oxides. Furthermore, we see in the whole range of compositions between the precursor oxides and the final catalyst, complete stability in all mixtures of metal and the oxide, magnetite. An intermediate phase with a composition between that of iron metal and wustite is thermodynamically unstable, from which it is concluded that an activation process under equilibrium conditions would involve a direct transition of magnetite into iron metal. 

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