Hump-type activity curve

The above-mentioned experimental results indicated that, in the ranges of Fe +/Fe + 1 i.e., the first peak, the relation between catalytic activity and the Fe +/Fe + ratio is consistent with those results of traditional catalysts, in which the precursor is magnetite phases (Fig. 3.27). The facts of the decreasing activity with increasing of Fe +/Fe + ratio from 0.5 to 1, also coincides well with the results 

Sample No. Pe2+ /Fe3+ ratios Crystal phase (XRD) X values in Fei xO Lattice constant/nm Fe3 O4 FeO Reaction rate / (mmol- h ) 

In other words, the activity of fused iron catalysts with iron oxides as a precusor relates to not only the content of FeO, but also, more importantly, to its crystal structure of wiistite. When the Fe +/Fe + ratio is smaller than one, although the content of FeO increases the activity decreases, because the crystal structure of wiistite is not yet formed. When the Fe +/Fe + ratio is smaller than 3.15 where the catalyst precursor begins to come to an incomplete structure of wiistite, the activity increases and surpasses strikingly that of the traditional catalyst with Fe +/Fe + at about 0.5. After the Fe +/Fe + reaches five, catalyst precursor forms a complete wiistite structure, while the fused iron catalysts shows its highest activities. Both the activity and reduction behavior are enhanced significantly compared to that of the traditional catalysts. 

Thereout, it is found that the catalyst has the highest activity among all the fused iron catalysts for ammonia synthesis when its chemical composition and crystal structure of the precmsor are those of wiistite (Fei xO)- It is called Fei xO or wiistite based ammonia sjmthesis catalysts, where the defect concentration x of iron ion is 0.04 x 0.10. These experimental results break through the classical conclusion that lasted for more than 80 years, namely the catalyst has the best activity when its chemical composition and crystal structure of the precursor are most close to those of magnetite. It also provides a new approach for a novelcat-alytic system — wiistite Fei xO system for improving the performances of the fused iron catalysts. 

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Fig. 3.28 Hump type activity curve for iron catalyst Experimental conditions pressure 15.0 MPa temperature 425°C ( ), 400°C ( ) Space velocity 3 X 10 h l promoters AI2O3, K2O, CaO...

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. 

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. 

Whereas, no matter what kind of iron oxide acts as precursor, the active phase after reduction is all of a-Fe. Why is the catalytic activity able to be improved markedly when the precursor changes from Fc304 to Fei xO Why does the catalytic activity cause the hump-type activity curve along with the variation of iron oxide precursor How such different types of activities are aroused All of these new subjects and the hidden scientific hints derived from the discovery of wustite based catalysts need an extensive and deeper investigation further. 

In 1986, Liu et at found that the iron catalyst with wiistite as the precursor has extremely high ammonia synthesis activity and rapid reduction rate, which led to the invention of wiistite (Fei xO) based catalyst for ammonia synthesis. The relationship between the activity and the iron oxides (Fe304, FeO and Fe203) and their mixtures in the precursor were studied systematically, and a hump type curve was found between the activity and the ratio (Fe +/Fe +). It was speculated that the monophase of iron oxide phase in the precursor is an essential condition for high activity of the catalyst and a uniform distribution of iron oxide phase and promoters is a key to make a better performance of catalyst. The hump type curve was interpreted by the ratio of phase compositions in the precursor, that is, the activity change of the fused iron catalyst depends essentially on the molecule ratio of different iron oxides but not on the atomic ratio of Fe + and Fe +, or Fe +/Fe +, in the precursor under certain promoters. Thus we found that Fei xO based catalyst with wiistite phase structure (Fei xO, 0.04 < x < 0.10) for ammonia synthesis has the highest activity among all the fused iron catalysts for ammonia synthesis. 

Concept of molecular ratio of iron oxides. Why does the activity show hump)-type curve (binary peak) with the ratio of Fe +/Fe + At first, someone suggested that it is possible the result of the increasing contents of iron in the catalysts. However, it is seen from Fig. 3.28 that the activity displays the hump-type curve along with the increasing Fe +/Fe + and this kind of difference of activity is not caused by the changing contents of the iron. Because the contents of the iron increase monotonicaUy with the increasing Fe /Fe " ", while the activity does not. 

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