Relationship between Precursor Phase Composition and Performance

From the hump curve, it is obvious that, within the range of Fe2+/Fe3+ < 1, the relationship between the activity and Fe2+/Fe3+ is in agreement with t...
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Ind. Eng. Chem. Res. 1997, 36, 335-341

335

Relationship between Precursor Phase Composition and Performance of Catalyst for Ammonia Synthesis Liu Huazhang* and Li Xiaonian Department of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P.R. China

The relationships among catalytic activity, reduction performance, and phase composition in precursor of melten-iron catalyst for ammonia synthesis are investigated with high-pressure testing equipment, thermogravity, and XRD. It is found that the monophase of the iron oxide in precursor is an essential condition for the high activity and easy reduction of the iron catalyst for ammonia synthesis. The activity of the catalyst is consistent with its reduction performance, both of which have similar regularity change to phase composition ratios in the precursor. The easier the catalyst is reduced, the higher the activity. FeO-based catalyst with wustite phase structure has the highest activity and the easiest reduction of all the melten-iron catalysts for ammonia synthesis. The experimental results can be explained by phase composition ratio f, which was determined by the authors. Introduction Melten-iron catalyst for ammonia synthesis developed successfully in 1909 is a milestone in the history of catalysis development; it not only propelled the development of inorganic chemical industry and organic synthetic industry but also produced a series of theories and concepts and propelled the development of catalytic theory.1 The catalyst has been researched and improved widely for more than 80 years. It was commonly believed that the catalyst has the best activity when its chemical composition and crystal structure are most similar to those of magnetite2-9 and has a volcano curve (single peak) (Figure 1) between the activity and Fe2+/ Fe3+ of the catalysts that seems to be an undoubted classical conclusion up to now. However, this conclusion has not been interpreted theoretically. Although the activity of catalyst has been somewhat increased through the variable functions of the promoters, there has been no breakthrough over the performances of the catalyst. In 1986, one of the authors (L.H.) for the first time used FeO with wustite structure as the precursor of iron catalyst and found it has extremely high ammonia synthesis activity and rapid reduction rate, which led to the invention of FeO-based catalyst for ammonia synthesis.13 In the present work, the relationships among the activity, reduction performance, and phase composition of the precursor have been studied systematically, and it is found that the monophase of iron oxide in precursor is an essential condition for the high activity and the easy reduction of the iron catalyst and that FeO-based catalyst has the highest activity and the most rapid reduction rate of all the iron catalysts for ammonia synthesis. It negated the classical conclusion that the Fe3O4-based catalyst has the highest activity. Experimental Section Catalyst Preparation. The samples were prepared by melting the mixture of refined magnetite and a certain amount of reducing agent and promoters (metal oxides, nitrates, or carbonates of aluminum, potassium, calcium, etc.). After the melt is cooled in a cooler, the solidifier material is crushed and sieved in the required particle size as samples. The wustite, expressed as Fe1-xO in the sample precursor, is formed from the chemical reaction of the reductant and magnetite at a high temperature during S0888-5885(96)00072-3 CCC: $14.00

Figure 1. Classical volcano shape activity curve. Promoters: Al2O3, K2O. Temperature: 450 °C. Space velocity: 10 000 h-1. Pressure: 100 atm.4

the melting process, in which the components are obtained by monitoring and adjusting the ratio of Fe2+ and Fe3+. The chemical composition and its Fe2+/Fe3+ were analyzed according to HG1-1430-81. XRD spectra were acquired using a Philips PW1732 X-ray diffractometer. These data were obtained by using a Co source operated at 40 kV and 25 mA and by step scanning through 2θ angles with (1/30)° increments. Reduction Evaluation. The reduction of the catalyst was studied by using a commercial thermogravimetric system (Shimadzu DT-40). The range of thermogravity (TG) is 6 mg. The temperature program is as follows: (1) The catalyst is heated at a constant rate of 10 °C/min from room temperature to 200 °C and maintained for 60 min at 200 °C for the desorption of physically adsorbed water. (2) The catalyst is heated at a constant rate of 3 °C/min from 200 to 650 °C. The particle size of the catalyst is 0.034-0.044 mm; the mass of the catalyst in each experiment is about 20 mg. The reduction process is carried out in a flowing stream of hydrogen (175 mL/min at STP). Activity Evaluation. The activity of the catalyst was measured in the high-pressure testing equipment (20 MPa).14 The stoichiometric hydrogen-nitrogen mixture was obtained by decomposition of ammonia. The ammonia synthetic reaction was carried out at the laboratory fixed-bed reactor of 14 mm in diameter. After the catalyst was reduced according to the given procedure, the percentage by volume of ammonia (NH3 %) of the reactor outlet under the given conditions was measured by the method of sulfuric acid neutralization. © 1997 American Chemical Society

336 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 Table 1. Precursor Composition of Samplesa sample no.

Fe2+/Fe3+ atom ratio

1 2 3 4 5 6 7 8 9 10 11

0.32 0.52 1.09 1.60 2.22 3.33 4.65 5.66 7.58 8.22 8.97

chemical composition (% by wt) Fe2O3 FeO 72.75 64.02 47.30 38.40 31.25 23.44 18.08 15.38 11.98 11.16 10.33

20.95 29.68 46.40 55.30 62.45 70.27 75.63 78.32 81.72 82.54 83.37

phase (according to XRD) major phase minor phase Fe3O4 Fe3O4 Fe3O4 FeO FeO FeO FeO FeO FeO FeO FeO

FeO FeO Fe3O4 Fe3O4

phase compositionb (relative molecule ratio) FeO Fe3O4 X in Fe1-xO 0.56 (Fe2O3) 0.03 1 1 1 1 1 1 1 1 1

1 1 0.85 0.45 0.29 0.18 0.12 0.10 0.07 0.06 0.059

0.104 0.081 0.070 0.055 0.051 0.48

a The promoters include Al O , K O, CaO, and other metallic oxides; the total content is 6.3% by weight. b According to the data of 2 3 2 chemical composition.

Figure 3. Mossbauer spectra obtained using AME-50 analyzer at room temperature for samples 2 and 6. The source, 50 mci of 57Co, diffused into a Pd matrix. The Doppler velocity was calibrated with a 12.7-µm metallic iron foil.

Figure 2. XRD spectra of samples. Fe2+/Fe3+ of samples as follows: 1, 0.32; 2, 0.52; 3, 1.09; 4, 1.60; 5, 2.22; 6, 3.33; 7, 4.65; 8, 5.66; 9, 7.58; 10, 8.22; 11, 8.97.

Results and Discussion XRD Research. The chemical compositions of the samples are listed in Table 1. The Fe2+/Fe3+ varies within the range of 0.32-8.97. The XRD spectra of the samples are shown in Figure 2. From the XRD spectra, it can be seen that the Fe3O4 phase decreases slowly when Fe2+/Fe3+ increases from 0.32 to 2.22 and completely disappears when Fe2+/Fe3+ reaches about 3.33; also the FeO phase increases gradually as Fe2+/Fe3+ varies from 0.52 to 3.33, and only the Fe1-xO peaks appear when Fe2+/Fe3+ is about 3.33. Sample 1 whose Fe2+/Fe3+ is 0.32 ought to have the Fe2O3 phase according to the results of chemical analysis in Table 1; however, no peaks of the Fe2O3 phase were detected in the XRD spectrum. The presumable reason for this is that the formation of nonstoichiometric magnetite (the structure formula may be indicated as follows: Fe3-yO4 or the formation of γ-Fe2O3 which is an isomorphous compound with Fe3O4). Sample 2

contains a very small amount of FeO in addition to the dominant phase Fe3O4, which is in agreement with the XRD result of studying the conventional Fe3O4-based catalyst. In the case of sample 3, whose Fe2+/Fe3+ is 1.09, it was observed that Fe3O4 (its main peak I/I0 ) 100; 2θ is 41.44) coexisted with the FeO phase (its main peak I/I0 ) 71; 2θ is 49.26), although each exists with its independent phase. After Fe2+/Fe3+ > 0.5, with an increase of Fe2+/Fe3+, the FeO phase increases gradually and finally becomes the dominant phase, whereas the Fe3O4 phase decreases gradually and becomes the minor phase. When Fe2+/Fe3+ > 3.33, there are only three FeO peaks in the XRD spectra (2θ are 49.26, 72.24, and 42.34, respectively), the Fe3O4 phase disappears completely, and the precursor of catalyst becomes pure wustite (Fe1-xO). From the standpoint of chemistry, the Fe3O4 phase ought to have existed when Fe2+/Fe3+ ) 3.33, which was not found from the XRD spectrum, either. This indicates that the iron oxide in the precursor has completely formed to wustite with a defect lattice of iron ion and nonstoichiometry. The results mentioned above are in agreement with the room temperature Mossbauer spectroscopies of samples 2 and 6 shown in Figure 3. The Mossbauer spectroscopy of sample 2 as a conventional catalyst consists of two typical hexafinger peaks of Fe3O4, and that of sample 6 consists of one typical dissymmetrical double peak of Fe1-xO. The concentrations of the defect lattice of iron ion (x value in Fe1-xO) of the samples calculated from

Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 337

Figure 4. CAMEL’S hump activity curve. Promoters: Al2O3, K2O, CaO, and others. Pressure: 15 MPa. Space velocity: 30 000 h-1. Temperature: b, 425 °C; 2, 400 °C.

the data of chemical analysis have been listed in the right column of Table 1. When Fe2+/Fe3+ > 8.2, a R-Fe phase is present in the Fe-O equilibrium phase diagram and the content of the R-Fe phase is 1-3% by chemical analysis, but no R-Fe phase was observed in the XRD spectra. Relationship between the Catalyst Activity and Fe2+/Fe3+. The relationship between the catalyst activity and Fe2+/Fe3+ is shown in Figure 4. As Fe2+/Fe3+ changes, a hump (double peaks) curve was detected for the catalyst activity. From the hump curve, it is obvious that, within the range of Fe2+/Fe3+ < 1, the relationship between the activity and Fe2+/Fe3+ is in agreement with the study results of the traditional catalyst composed of magnetite (see Figure 1). As Fe2+/Fe3+ increases from 0.5 to 1, the catalyst activity decreases; it is also consistent with the result of Artyukh, who studied the activity of catalysts with 3.5% (by weight) Al2O3 as the only promoter and at various FeO contents of 41.8-52.8%; i.e., the Fe2+/Fe3+ is 0.8-1.2.10-12 It is discovered from the present experimental result that the catalyst activity gradually increases again when it skips the low point at which Fe2+/Fe3+ is 1; when the precursor begins to form the wustite phase structure Fe1-xO, the activity of the catalyst is obviously higher than that of the traditional catalyst, the Fe2+/Fe3+ of which is 0.5; after Fe2+/Fe3+ reaches about 5 and the catalyst forms a complete wustite structure in the precursor, the activity reaches the highest value. Moreover, the activity, reduction performance, and other performances are much better than those of the traditional catalyst.14 The experimental result has broken through the classical conclusion followed for over 80 years, which held that the catalyst would be of the highest activity only when the composition was magnetite, and thus established a new catalyst systemswustite Fe1-xO system that can improve the performance of iron catalyst. Relationship between Reduction Performance and Fe2+/Fe3+. The TG and DTG spectra of the catalyst reduction are shown in Figures 5 and 6. The reduction reactions of catalyst with H2 are as follows:

Figure 5. TG curves of catalysts under H2 reduction conditions (instrument, Shimadzu TGA-41; catalyst size, 0.034-0.044 mm; flow of H2, 175 mL/min; rate of rising temperature, 3 °C/min). Fe2+/ Fe3+ of samples as follows: 1, 0.32; 2, 0.52; 3, 1.09; 4, 1.60; 5, 2.22; 6, 3.33; 7, 4.65; 8, 5.66; 9, 7.58; 10, 8.22; 11, 8.97.

Fe2O3 + 3H2 ) 2Fe + 3H2O

(1)

Fe3O4 + 4H2 ) 3Fe + 4H2O

(2)

FeO + H2 ) Fe + H2O

(3)

Fe1-xO + H2 ) (1 - x)Fe + H2O

(4)

Figure 6. DTG curves of catalysts under H2 reduction conditions (instrument, Shimadzu TGA-41; catalyst size, 0.034-0.044 mm; flow of H2, 175 mL/min; rate of rising temperature, 3 °C/min). Fe2+/ Fe3+ of samples as follows: 1, 0.32; 2, 0.52; 3, 1.09; 4, 1.60; 5, 2.22; 6, 3.33; 7, 4.65; 8, 5.66; 9, 7.58; 10, 8.22; 11, 8.97.

Because Fe3O4 and Fe1-xO can be expressed as Fe2O3 + FeO, the theoretical weight loss of the catalyst reduced completely can be calculated according to the

data of chemical analysis in Table 1 and formulas (1) and (3). The practical weight loss of the catalyst under the reduction conditions is obtained from the TG curve. The reduction degree is the ratio of the practical weight

338 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 Table 2. Reduction Degree of Catalysts sample no. theoretical weight loss, % practical weight loss, % reduction degree, %

1

2

3

4

5

6

7

8

9

10

11

26.53 25.03 94.35

23.85 24.73 95.67

24.55 23.45 95.51

23.86 22.76 95.17

23.30 21.67 93.00

22.69 22.35 98.50

22.27 21.09 94.70

22.06 21.33 96.69

21.80 21.60 97.29

21.74 20.47 94.16

21.67 20.94 96.63

Figure 7. Relation curve between the initial reduction temperature and Fe2+/Fe3+ of catalysts.

Figure 9. Relation curves between temperature of the fastest reduction rate and Fe2+/Fe3+ of catalysts.

Figure 8. Relation curves between terminal reduction temperature and Fe2+/Fe3+ of catalysts.

loss and the theoretical weight loss; the experimental results are listed at Table 2. From Table 2, we find the one reason of easy reduction of FeO-based catalyst is that the theoretical weight loss decreases with an increase of the FeO content of the catalyst, but the reduction degree of all samples did not reached 100% at H2 and 650 °C. The reason for this may be that the solid solution with difficult reduction is formed among Fe3O4 or FeO and Al2O3 or K2O and others. Phase Composition and Reduction Temperature. There is an obvious change law between phase composition (Fe2+/Fe3+) and reduction temperature in Figures 5 and 6. The three intrinsic temperatures during the period of reduction, which are the beginning reduction temperature (Tb), the terminal reduction temperature (Tt), and the temperature with the fastest reduction rate (Tm), are discussed. Beginning Reduction Temperature (Tb). The relationship curve between Tb and Fe2+/Fe3+ is shown in Figure 7. The beginning reduction temperatures of samples have no obvious differences and are between 340 and 360 °C besides sample 1 of Fe2+/Fe3+ ) 0.32. This is probably because the catalyst is a nonporous solid before reduction. Reduction Terminal Temperature (Tt). The relationship between Fe2+/Fe3+ and Tt is shown in Figure 8. From Figure 8, we see that sample 3 of Fe2+/Fe3+ ) 1.09 has the highest Tt; when Fe2+/Fe3+ > 1.09, Tt

decreases with an increase of Fe2+/Fe3+; after Fe2+/Fe3+ reaches 3.33, the change of Tt becomes small and maintains about 520 °C. While Fe2+/Fe3+ < 1.09, Tt decreases quickly with a decrease of Fe2+/Fe3+. The change law of Tt with Fe2+/Fe3+ is determined by the phase composition of the precursor of the catalyst. The Tt of a traditional catalyst in which Fe2+/Fe3+ is 0.52 is 619 °C. With the increase of Fe2+/Fe3+ from 0.52, the FeO content increases and forms a mixture of Fe3O4 and FeO and Tt rises. When Fe2+/Fe3+ reaches 1.0, Fe3O4 and FeO coexist with the same moles and Tt is maximum, 645 °C. This is because of the result of the competitive reduction in the two-phase region (for details, see below). The conclusions can be obtained from those above as follows: (1) The reduction of Fe1-xO based catalyst is easier than that of Fe3O4-based catalyst; its terminal reduction temperature is lower by about 100 °C than that of Fe3O4-based catalyst. (2) If the FeO phase is joined into the Fe3O4 phase or the Fe3O4 phase is joined into the FeO phase, the reduction terminal temperature will rise. When the molecule ratio of Fe3O4 and FeO is 1:1 and the two phases coexist, the reduction terminal temperature reaches the highest point. Temperature with the Fastest Reduction Rate (Tm). The relationship between the temperature of the fastest reduction rate (Tm) and Fe2+/Fe3+ is shown in Figure 9. As seen from Figure 9, the Tm of the traditional catalyst with Fe3O4 is 525 °C, and the Tm of the Fe1-xO catalyst is 480 °C; the latter is 45 °C lower than that of the former. From Figures 8 and 9, we see that Tm of the Fe3O4 catalyst is matched to Tt of the Fe1-xO catalyst; that is, when the traditional catalyst reaches the temperature of the fastest reduction rate, the Fe1-xO catalyst has been reduced completely. Note that especially in Figure 9 there are two Tm’s when the samples Fe2+/Fe3+ ) 1.09, 1.60, and 2.22, respectively. This is because the precursors of the three samples are composed of two phases, e.g., FeO and Fe3O4, and both exist in independent phases (as Table

Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 339

Figure 10. XRD spectra of catalysts after the first reduction stage: (a) Fe2+/Fe3+, 1.09; the temperature stopped reduction, 450 °C; cooling time, 1 min; (b) Fe2+/Fe3+, 1.60; the temperature stopped reduction, 510 °C; cooling time, 1 min; (c) Fe2+/Fe3+, 2.22; the temperature stopped reduction, 500 °C; cooling time, 1 min.

Figure 11. Relation curve between reduction rate and Fe2+/Fe3+ of catalysts.

1); the reduction process is divided into several steps according to phases made in order of FeO and then Fe3O4; i.e., FeO is reduced first, and when the reduction is nearly finished, reduction of Fe3O4 begins, which obeys the competitive mechanism. FeO and Fe3O4 are both reduced to R-Fe directly. This is proved by the experimental result shown in Figure 10. After the end of first stage reduction, there are only R-Fe and Fe3O4 peaks, and the FeO peaks have disappeared in the XRD spectra. This indicated that FeO has been reduced fully at the first stage, and the unreduced part is Fe3O4. This also is proved by the fact that the second peaks (Fe3O4 reduction peak) become smaller with a decrease of the Fe3O4 content in Figure 6. This is in agreement with the conclusion obtained by Clausen studying a commercial catalyst reduction with Mossbauer.15 Phase Composition and Reduction Rate. The average reduction rate of the samples can be calculated by the total weight loss of each sample divided by the reduction time. The relationship between the average reduction rate and Fe2+/Fe3+ is shown in Figure 11. Comparing Figure 11 with Figure 8, we found that the shape of the curve between the reduction rate and Fe2+/

Fe3+ is just opposite to that of the curve of Tt and Fe2+/ Fe3+. The reduction rate of the sample requiring a high reduction temperature is slow, and the reduction rate of the sample requiring a low reduction temperature is fast. The shape of the curve between the reduction rate and Fe2+/Fe3+ is similar to that of the curve between f (for definition, see below) and Fe2+/Fe3+ shown in Figure 12. Under the certain promoters and the same manufacturing method, the reduction rate is determined by precursor phase composition ratio f. When the precursor composition consists of FeO and Fe3O4, the reduction rate will be reduced; the larger the extent of mixing (f value is reduced), the lower the rate of the reduction. Besides this, comparing Figures 4 and 11, we found that the activity is consistent with its reduction rate. The more easily the catalyst is reduced, the higher is its activity. In the above figures, it is noted that nonstoichiometric Fe3-yO4 (as sample 1) and Fe1-xO (such as sample 11) phases have similar reduction properties and higher reduction rates and lower reduction temperatures than those of a stoichiometric crystal (such as sample 2). So, we thought that the reduction rate and reduction temperature of the catalyst are related with not only its precursor phase composition but also the iron ion defect concentration in the iron oxides of the precursor. The Fe1-xO and Fe3-yO4 phases with iron ion defects have faster reduction rates and lower reduction temperatures. Relationship between the Catalyst Performances and the Ratio of the Precursor Phase Composition. Generally, the change of the promoters has some effect on the catalyst performances. Yet, studies done in this field for several decades indicated that it would not cause the pivotal change in the catalyst activity. Because iron is a variable-valence transition element, the activity of the iron catalyst with R-Fe as the activity phase should have certainly some inherent relation to the phase of its iron oxide in the precursor. In Figure 4, we have noted that, with the change of the phase composition, the two points of the high activity of the catalyst exist in a single phase (Fe3O4 or Fe1-xO). It can be further discovered that when the molecule ratio of the Fe3O4 and FeO phases is 1:1, the activity of the catalyst reduces to its lowest point. Besides, when the molecule ratio of these two phases changes to 1:0, the activity rises again to its high point. These phenomena indicate that, if the FeO phase is joined into the Fe3O4 phase (see the segment of Fe2+/Fe3+ within 0.5-1.0 in Figure 4) or the Fe3O4 phase is joined into the FeO phase (see the segment of Fe2+/Fe3+ within 1.0-7), the activity of the catalyst will be decreased. At the same time, it is worth noting that when Fe2+/Fe3+ > 8.2, the decrease of the activity of the catalyst to some degree is just due to the presence of a new phase R-Fe which causes the phase composition not to be single. Therefore, we thought that the monophase of the iron oxide in the precursor is an essential condition of the high activity of the iron catalyst. The unicity of the phase composition is expressed by the ratio, f, of the precursor phase composition in this paper. The physical meaning of f is the molecule ratio of different iron oxides (Fe2O3, Fe3O4, and FeO) in the precursor (R ) Fe2+/Fe3+ is the atom ratio of Fe2+/Fe3+) and is defined as follows:

f)

[major phase] [major phase] + [minor phase]

340 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997

Figure 12. Relation between Fe2+/Fe3+ and ratio of phase composition.

where [ ] is the number of moles, and there are relationships between f and R in different regions of phases as follows:

When 0 E R E 0.25 (Fe2O3(major)-Fe3O4(minor) phase region) f1 )

[Fe2O3] [Fe2O3] + [Fe3O4]

) 1 - 2R

When 0.25 E R E 0.5 (Fe3O4(major)-Fe2O3(minor) phase region) f2 )

[Fe3O4] [Fe3O4] + [Fe2O3]

) 2R

When 0.5 E R E 1 (Fe3O4(major)-FeO(minor) phase region) f3 )

[Fe3O4] [Fe3O4] + [FeO]

)

1 2R

When 1 E R E 8.2 (FeO(major)-Fe3O4(minor) phase region) f4 )

[FeO] 1 )12R [FeO] + [Fe3O4]

When R > 8.2 (FeO(major)-R-Fe(minor) phase region) f5 )

[FeO] [FeO] + [R-Fe]

The theoretical values of f for the above samples are calculated according to the Fe2+/Fe3+ values in Table 1, respectively. The relationship curve of f and Fe2+/ Fe3+ is shown in Figure 12. After comparison of Figures 4 and 12, we obviously found that the f-Fe2+/Fe3+ curve is extremely similar to the NH3 %-Fe2+/Fe3+ curve. In the segments whose Fe2+/Fe3+ < 1, there are bigger slopes of both activity curve and f curve, while in the segments whose Fe2+/Fe3+ < 1, there are bigger slopes of both activity curve and f curve, while in the segments whose Fe2+/Fe3+ > 1, the slopes of the two curves become gentle. Therefore, it can be seen that the variation law of catalyst activity with Fe2+/Fe3+ can be well explained by the molecule ratio f of the phases which have different crystal structures. At the same

time, it gives a theoretical interpretation to the study of results (see Figure 1) of the conventional catalyst. Therefore, it can be thought that what determines the essentiality of the variation in the activity of the iron catalyst for ammonia synthesis is the molecule ratio value (f) of the iron oxides (Fe3O4, Fe2O3, and FeO) which have different crystal structures rather than the atom ratio value (R) of iron which has different valences. When the molecule ratio value f equals 1 (i.e., only one kind of iron oxide and one crystal structure), the catalyst has a high activity; when f equals 0.5 (i.e., two kinds of iron oxides coexist with two different crystal structures), the activity is at a low point. The mixture of any two phases causing f < 1 leads to the activity decrease. According to the experiment result, we presume that another high point and low point of activity will appear at Fe2+/Fe3+ ) 0 (pure Fe2O3, f ) 1) and Fe2+/Fe3+ ) 0.25 (Fe2O3:Fe3O4 ) 1:1, f ) 0.5), respectively. Analysis of Activity and Reduction Performance in the Two-Phase Region. Studies of the traditional catalyst for ammonia synthesis in the past decades have shown that the uniform distribution of the promoters is the key of the high activity of the catalyst. The singlephase composition in the precursor is favorable to the uniform distribution of the promoters. In the case of the coexistence (or mixture) of the two phases, since different phases have different physical and chemical properties and crystal structures, for example, different phases have different melting points (that of Fe3O4 is 1597 °C, while that of FeO is 1377 °C), there would occur the phenomenon of fractional crystallization or peritection at the cooling and congelation stages during the preparing procedure of catalyst, which leads to the nonuniform distribution of the iron oxide in the precursor. Also, the phases which have different crystal structures and different properties have different requirements to the metallic ion valence of promoters and have different solubility to promoters, which would influence the uniform distribution of promoters. In a word, when the precursor of the catalyst is composed of two different phases, not only is the nonuniform distribution of the promoters intensified but also the nonuniform distribution of the iron oxides in the precursor will occur, which is thought to be the main cause of the decrease of the activity in the two-phase region. At the same time, the reduction process of catalyst in the two-phase region is made by stages in order of different phases. As known for us, reduction of the iron catalyst is a reversible reaction. In the case of reduction by stages in order of different phases, the R-Fe obtained from the reduction of FeO by H2 at first stage will be oxidized easily by H2O produced at the second reduction

Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 341

stage. The redox repeatedly can make a R-Fe microcrystal grow and decreases the activity of catalyst after reduction. So we come to the conclusion that the uniform distribution of the components (including the iron oxide and the promoters) in the catalyst is the key to preparing the high activity catalyst. Only when the phase composition of the catalyst in the precursor is single, the precursor system of the traditional catalyst with Fe3O4 is changed, and the promoters are uniformly distributed can a new melten-iron catalyst for ammonia synthesis which has better performances be obtained. Conclusion 1. The hump (double peaks) curve of activity in the research of melten-iron catalyst for ammonia synthesis was discovered for the first time. From this curve we can learn that, in all melten-iron catalysts, catalyst reduced from nonstoichiometric Fe1-xO, wustite, is of the highest activity, the fastest reduction rate, and the lowest reduction temperature, which has refuted the classical conclusion adopted for more than 80 years that “melten-iron catalyst whose precursor chemical components are similar to those of magnetite is of the highest activity”. 2. The performance of melten-iron catalyst for ammonia synthesis has a close relation to the original composition of the iron oxide in the precursor. The precursor of the best melten-iron catalyst ought to have only one phase of iron oxide. Any mixture of two phases will result in the decrease of the performance of the catalyst. The larger the degree of mixing, the more the decrease of the performance. The regularity of such changes can be explained by phase composition ratio f which was put forward by the authors in the paper. 3. The activity of the ammonia synthesis catalyst is consistent with its reduction performance, both of which have a similar regularity of change to phase composition ratio of f. The more easily the catalyst is reduced, the higher its activity. 4. When the precursor of the catalyst is composed of two different phases, not only is the nonuniform distribution of the promoters intensified but also the nonuniform distribution of the iron oxides in the precursor will occur, which decreases the activity of the catalyst. In addition, in pure hydrogen (H2), the reduction process is divided into several steps according to different

phases, which results in the decrease of the activity of the catalysts after reduction. Acknowledgment This work was supported by the Zhejiang Province Natural Science Foundation. Literature Cited (1) Timm, B. The synthesis of ammonia and heterogeneous catalysis. 8th International Congress on Catalysis, Berlin, 1980. (2) Bosch, C.; Mittasch, A.; Stern, G.; et al. DRP 249447, DRP 258146, 1910. (3) Almquist, J. A.; Crittenden, E. D. A study of pure-iron and promoted-iron catalysts for ammonia synthesis. Ind. Eng. Chem. 1926, 18, 1307-1309. (4) Bridger, G. I.; et al. Chem. Eng. Prog. 1947, 43, 291. (5) Curtis, H. A. Fixed Nitrogen; ACS Monographs 59; American Chemical Society: Washington, DC, 1932; Chapter IX. (6) Hinnchs, H. The catalyst for the synthesis of ammonia and its production. Br. Chem. Eng. 1967, 12 (11), 1745-1746. (7) Kuzetsov, I. D. Optimum chemical composition of a fourfold promoted catalyst for ammonia synthesis. Chem. Technol. 1963, 15 (4), 211-214. (8) Nielsen, A. An investigation on promoted iron catalysts for the synthesis of ammonia, 3rd ed.; Gjellerup: Copenhagen, 1968. (9) Slack, A. V.; James, G. R. Ammonia (in four parts); Marcel Dekker: New York, 1980; Part III. (10) Stre’tsov, O. A.; Dvornik, O. S.; Lytkin, V. P. Study of ammonia synthesis catalyst with different iron(II) content. Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 1977, 20 (11), 16221626. (11) Artyukh, Yu. N.; Fedun, O. S.; Zyuzya, L. A. Activity of once-promoted catalysts of ammonia synthesis containing a different amount of iron(II). Katal. Katal. 1980, 18, 20-23. (12) Strel’sov, O. A.; Fedun, O. S.; Artyukh, Yu. N.; Et al. Effect of the valence state of iron in the starting catalyst on their properties in an ammonia synthesis reaction. Tezisy dokl.-Ukr. Resp. Konf. Fiz. Khim. 1977, 12, 159-160. (13) Huazhang, L.; et al. CN86108528.0, 1986. (14) Huazhang, L.; Zhangneng, H.; Xiaonian, L.; et al. A study of ZA-3 ammonia synthesis catalyst-1 laboratory research. J. Zhejiang Inst. Technol. 1993, 2, 15-19. (15) Clausen, B. S.; Morup, S.; Topsoe, H.; Candia, R. J. Phys. Colloq. 1976, 37, C6-C245.

Received for review February 2, 1996 Revised manuscript received October 21, 1996 Accepted October 25, 1996X IE960072S

X Abstract published in Advance ACS Abstracts, December 15, 1996.