Iron oxides and the surface textures of catalysts

In order to reveal the intrinsic relation between the surface properties and textures with the hump-type activity curve, the specific surface area of both the ammonia synthesis fused iron catalyst with different iron oxides as precursors and their active components were measured by the means of low temperature physical adsorption of N2 and selective chemisorptions of CO, CO2 as shown in Table 3.17. 

Notes (i) The cont-ent and type of promoters (AI2O3, K2O, CaO etc.) is the same in all samples, (ii) is the total surface area measured by N2 physisorption at 77.4 K (BET) s the surface area of active a-Fe measured by chemisorption of CO at 77.4 K  

It is clearly seen from Fig. 3.38 that the ratio (Sa/Sr) of sm-face coverage of acid and base also shows two peaks along with change of Fe +/Fe +, which is well consistent with the hump-type activity curve (Fig. 3.28). The activity increases first followed by decrease and then increase again with the increasing Sa/Sr value, reaches the maximum when Sa/Sr is in the range of 1.1 1.2, and is very low when Sa/Sr 0.9. Therefore, the high activity of Fei xO based catalyst is related with their surface acid-base cooperative effect. 

This kind of acid-base synergistic effect relates with the precursor of catalysts (to be expressed by Fe +/Fe +), but not directly with the absolute amounts of acidic ad basic oxides. It can be seen from Fig. 3.39 that the ratio of acid and base surface area of the highest active catalyst is about 1.1 and the catalyst precursor phase is 

39 Cooperative effect of surface of acid and base. Conditions for activity testing Pressure of 15 MPa, Temperature of 698 K ( ) and 673K (o), Space velocity of 10,000 h-i 

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