Models of Structural Promotion

Single crystals of Fe with evaporated overlayers of K and/or A1 have been studied as models for the structure of the industrial catalyst. After heating of an Fe(llO) crystal with an A1 overlayer, the surface looks rough under the electron microscope [227]. The reaction leading to a dispersion of A1 and Fe is primarily a reaction between oxidic phases [221]. The activity increases after reaction between the crystal and the Al-overlayer [227], possibly due to the formation of facets [227]. After heating of an Fe(llO) crystal in 5 atm of N2 to 450 C, no increase is found in the activity of the crystal [227]. The presence of A1 stabilizes the Fe(lll) and Fe(211) surfaces during NH3 synthesis at 20 atm [221]. The heating of Fe(llO) and Fe(lOO) with an A1 overlayer causes a recrystallization of 

A large number of other catalyst models have been studied. The mostly frequently studied models are Fe/C [297-316]. Fe/Al203 [94,108, 137, 317-321], Fe/MgO [108,167, 319, 320, 322-330], Fe/SiO [319-321,331,339] Fe/Ti02 [340,342], FeTi intermetallics [343,349] and FeZr intermetallics [350, 354]. 

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