Classical volcano-type activity curve

During their initial studies about the airmionia synthesis catalysts, Bosch et alA found that the catalysts obtained from the reduction of natural magnetite are better than the catalysts from other iron compoimds. Almquist et alA studied the relation between the activity of iron catalyst and the oxidization degree before its reduction and found that those catalysts, of which the ratio of Fe +/Fe + is closer to be 0.5 and compositions closer to magnetite, has the highest activity. Bridger et alA further studied the fused iron catalysts promoted with binary promoters AI2O3-K2O, 

27 Classical volcano type activity curve. Promoters AI2O3 K2O temperature 450°C space velocity 10,000 pressure 100 atm 

Fe + /Fe + Activity before heat-resistant /NH3% Activity after heat-resistant /NH3% Fe2+/Fe + Activity before heat-resistant /NH3% Activity after heat-resistant /NH3% 

Notes Testing conditions for activity space velocity of 30,000 h pressure of 30 MPa, H2 N2 of 3 1, temperature of 475°C for heat-resistant temperature is 525°C for 24 h. 

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It will be seen from above discussion that the activity of ammonia synthesis catalyst correlates not only with the chemical compositions, but also the crystal types and crystal structure of iron oxides. The relationships between the activity and the Fe +/Fe + ratio can be interpreted perfectly by the molecular ratio / of the iron oxides, which have the different crystal structures in their precursors. At the same time, it also gives the theoretical explanation for those results of the classical catalysts (Fig. 3.27). For example, for the classical volcano-type activity curve, when Fe +/Fe + = 0.5, then / = f (Eqs. 3.16 and 3.17), so the catalyst has the good activity both sides at Fe +/Fe + = 0.5, due to / < 1, so the activity of the catalyst decreases. 

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