Ind. Eng. Chem. Res. 2000, 39, 2891-2895
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Effect of an Iron Oxide Precursor on the N2 Desorption Performance for an Ammonia Synthesis Catalyst† Sheng Guan‡ and Hua Zhang Liu* Catalysis Institute of Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China
The effects of an iron oxide precursor of an ammonia synthesis catalyst on the N2 desorption performance and catalytic activity were studied by means of N2 temperature-programmed desorption. The results show that there are close relationships among the N2 adsorption/ desorption performance on a catalyst surface, catalytic activity, and the type and composition of the iron oxide precursor. The N2 desorption activation energy, the temperature for desorption, and the amounts of desorption species are apparently different because of a different oxide precursor. Among the three precursors being studied, which are Fe3O4, Fe1-xO, and their mixture, the order of the N2 desorption activation energy is as follows: Fe3O4 + FeO > Fe3O4 > Fe1-xO. The desorption temperature has similar changes, but the desorption volume is just the opposite. The N2 desorption activation energy is similar to the reaction activation energy for ammonia synthesis. Furthermore, their sequences for different iron oxide precursors are the same. This proves that the dissociative adsorption of N2 is the rate-determined step for an ammonia synthesis reaction. The experimental results indicate that the precursor has a notable effect on the N2 adsorption/desorption performance and catalytic activity, and the Fe1-xO-based catalyst has the highest activity among all of the fused iron catalysts for ammonia synthesis. 1. Introduction The Fe3O4-based ammonia synthesis catalyst was first developed by Harber et al. early last century. Thereafter, this catalyst has been studied deeply by many researchers all over the world (summarized in refs 1-4). Most of the earlier work is limited in the study of the promoters. In 1947, Bridger et al.5 investigated the relationship between the catalytic activity of a Al2O3K2O doubly promoted catalyst and the ratio of Fe2+/Fe3+ in the oxide precursor and got the classic volcano-shaped curve as shown in Figure 1. It was commonly believed that the catalyst has the best activity when its chemical composition and crystal structure of the oxide precursor are most similar to those of nature magnetite. Up to now, except in the A301 made in China, the oxide precursors of the ammonia synthesis catalysts produced all over the world are all Fe3O4. Their difference is only the kind and content of the promoters. In 1986, Huazhang et al.6 broke through the classic conclusion that the Fe3O4-based catalyst has the highest activity. The Fe1-xO-based catalyst (A301) was first developed successfully. The relationship called CAMEL’s hump activity (specific rate) curve between the catalytic activity and oxide precursor (represented in Fe2+/Fe3+) was obtained as shown in Figure 2. Figure 2 shows that the traditional volcano-shaped activity curve only represents the part where Fe2+/Fe3+ in the oxide precursor is smaller than 1. When Fe2+/ Fe3+ is larger than 1 and especially when it is larger than 3, the activity of the iron catalyst will enter into a new high activity area, where the activity is higher than * To whom correspondence should be addressed. Telephone and fax: 0086-571-8320-063. E-mail:
[email protected]. † The project was financially supported by the Natural Science Foundation of Zheijiang Province. ‡ Present address: Zheijiang NHU Company Ltd.
Figure 1. Classical volcano-shaped activity curve (5): promoters, Al2O3, K2O; temperature, 450 °C; space velocity, 10 000 h-1; pressure, 100 atm.
Figure 2. CAMEL’s hump activity (specific rate) curve: promoters, Al2O3, K2O, CaO, etc. (see Table 1); pressure, 15.0 MPa; temperature, 425 °C (9) and 400 °C (b); space velocity, 30 000 h-1; catalyst volume, 2 mL; particle size, 1.0-1.4 mm.
that of the traditional catalyst with Fe2+/Fe3+ of about 0.5. The A301 catalyst has the highest activity with the precursor as pure Fe1-xO. However, whether the oxide precursor is Fe3O4, Fe1-xO, or a mixture of two iron oxides such as Fe3O4 and FeO, the active state after
10.1021/ie990695g CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000
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reduction will be all R-Fe. Figure 2 indicates that the activity of the iron catalyst is relevant to the property and composition of the iron oxide precursor. Before the extensive surface science studies were undertaken, much was already known about the mechanism of ammonia synthesis over iron. It was generally believed that ammonia synthesis proceeded through elementary steps in which both nitrogen and hydrogen adsorb and dissociate on the surface, atomic hydrogen adds stepwise to atomic nitrogen, and ammonia desorbs from the surface, as follows:
1. N2 + * h *N2 2. *N2 h 2*N 3. *N + *H h *NH + * 4. *NH + *H h *NH2 + * 5. *NH2 + *H h *NH3 + * 6. *NH3 h NH3 + * 7. H2 + 2* h 2*H where * signifies a vacant site on the surface. Nitrogen adsorption and/or dissociation is the ratedetermined step, while others step very quickly and are almost in the balance.7 So it can be imagined that there shall be difference in the rate-determined step of N2 adsorption (desorption) between the two R-Fe reduced from Fe1-xO and Fe3O4. Temperature-programmed desorption (TPD) is an effective method on the study of N2 adsorption/desorption on the catalyst surface. This paper uses nine fused iron catalysts with different precursors as its study objects. It uses TPD as its main method in the study of the relationship of the catalyst precursor and its activity. The illustration of the humped activity curve (Figure 2) of fused iron catalysts is its ultimate goal. 2. Experimental Section 2.1. Preparation and Analysis of the Sample. The samples are prepared by melting the mixture of refined magnetite and a certain amount of reducing agent (such as pure iron) and promoters (metal oxides, nitrates, or carbonates of aluminum, potassium, calcium, etc.). The chemical composition and the crystal phase (expressible as Fe2+/Fe3+) of samples are controlled by adjusting the ratio of magnetite and the reducing agent. The chemical composition is analyzed by chemical analysis. The crystal phases of samples are distinguished by a MXP series X-ray diffractometer, Co KR radiation, 40 kV, 25 mA, and 4 °C/min continuous scanning. The chemical composition and the crystal phases of samples are listed in Table 1. From Table 1, it is known that the precursor of samples 1 and 2 is Fe3O4. Fe2O3 was not detected, although it exists when Fe2+/Fe3+ < 0.5. The precursor of samples 3-5 is the mixture of Fe3O4 and FeO, whose molar ratios of FeO-Fe3O4 are 1:1.28, 1:0.82, and 1:0.30, respectively. The precursors of samples 6-9 is only FeO. The Fe3O4 phase was not detected, although it exists when Fe2+/Fe3+ > 0.5. This indicates that the iron oxide in the precursor has completely formed to a nonstoichiometric ferrous oxide Wustite8 expressed as Fe1-xO
Table 1. Chemical Composition and Crystal Phase of Samples
sample no. Fe2+/Fe3+ 1 2 3 4 5 6 7 8 9
0.313 0.50 0.89 1.11 2.16 3.15 4.62 6.52 7.54
chemical composition (wt %)a
crystal phase (by XRD)
FeO
Fe2O3
major phase
minor phase
20.81 29.39 42.12 47.33 62.52 70.10 76.31 80.91 82.54
73.89 65.31 52.58 47.37 32.17 24.69 18.39 13.79 12.16
Fe3O4 Fe3O4 Fe3O4 FeO FeO FeO FeO FeO FeO
not detected not detected FeO Fe3O4 Fe3O4 not detected not detected not detected not detected
a The promoters of the samples are 2.4% Al O , 0.7% K O, 1.8% 2 3 2 CaO, and 0.4% SiO2 in samples before reduction.
with a defect lattice of iron ion (x represents the absent degree of iron ion). 2.2. TPD Experiments. The N2 TPD experiments were carried out in a gas chromatograph 1104GC with a quartz reactor of 5 mm internal diameter and using 0.8 g of catalyst of 200-300 standard mesh. First the catalyst is reduced by pure hydrogen. Then it is reacted with N2 at 350 °C and at atmospheric pressure for 2 h and blown with He for 2 h to remove the adsorbed hydrogen on the catalyst surface. After this, N2 is adsorbed for 2 h at the same temperature. To make the N2 adsorption reach saturation, after the N2 adsorption the temperature of the catalyst bed should be slowly decreased from 350 to 40 °C in 2 h. Again it should be blown with He at the same temperature for 2 h to remove the nonchemical adsorbed nitrogen on the catalyst surface. The TPD is going on at the stipulated heating rate in the range of 40-700 °C at 25 mL/min of He and 101 kPa of pressure. 3. Results and Discussion 3.1. Qualitative Analysis of the TPD Spectrum. The N2 TPD experiments were done at heating rates (β) of 10, 20, and 30 °C/min, respectively. Figure 3 shows spectra when β is 20 °C/min. For most of the samples, there are two desorption peaks on the N2 TPD spectrum and peak I is greater than peak II. The N2 TPD data of the peak temperature and peak area with different heating rates are listed in Table 2. Boszo et al.9,10 had used the flash desorption method to study the N2 desorption, which is dissociatively adsorbed on the iron single-crystal surface. They discovered that the N2 TPD peaks on Fe(111), Fe(110), and Fe(100) crystal faces are located at 587, 627, and 707 °C, respectively, at a heating rate of 6-10 °C/s. As is known, the heating rate has a great influence on the TPD peak temperature. The concrete influence degree is also relative to the order of the desorption. Commonly, the peak temperature will be moved to the higher temperature side when the heating rate is greater. From Table 2, the temperature of peak I at 30 °C/min is about 550-600 °C. This indicates that peak I should be the N2 desorption peak under the experiment conditions. It is probably generated by the desorption of N-* which is dissociatively adsorbed on the Fe(111) crystal face. So it can be inferred that N-* can react with hydrogen to form NH3. This is confirmed by the TPD spectra before and after the adsorbed N-* reaction with hydrogen at 350 °C for 2 h, as shown in Figure 4.
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Figure 5. Ed relation to Fe2+/Fe3+ in the oxide precursor. Table 3. Ed of N2 Desorption of Catalysts
Figure 3. N2 TPD spectra when β ) 20 °C/min. The numbers indicate the code of the samples as described in Table 1. a
Figure 4. TPD spectra before (2) and after (1) H2 reaction with adsorbed N-*.
Ed (kJ/mol)
Ra
1 2 3 4 5 6 7 8 9
185.3 155.3 215.9 183.8 267.3 164.2 168.5 152.1 167.9
0.890 0.999 0.840 0.996 1.000 0.996 1.000 0.996 0.999
Linear correlation coefficient.
According to the areas of peaks I and II, the adsorption state represented by peak I is not only the major N2 adsorption state on the industrial catalyst surface but also the major intermediate species taking part in the ammonia synthesis reaction. So the structure of the major activity center on the industrial catalyst surface is probably similar to Fe (111). According to the TPD theory,12 under the conditions that the inner and outer diffusions are eliminated and N2 adsorption activation energy Ed is irrelative to the coverage (θ) of N2, the following equation can be deduced:
2 log Tm - log β )
Table 2. N2 TPD Data of Peak I peak temp (°C)
sample no.
peak area (arb. unit)
sample no.
10 °C/ min
20 °C/ min
30 °C/ min
10 °C/ min
20 °C/ min
30 °C/ min
1 2 3 4 5 6 7 8 9
519.0 525.5 574.4 556.1 548.5 524.6 530.4 531.0 529.5
525.0 549.0 577.6 574.2 562.8 543.5 550.5 551.7 551.6
545.0 561.3 597.8 589.3 571.0 558.9 564.3 568.5 563.0
94.5 66.0 4.0 2.3 22.0 66.2 42.1 80.3 42.9
84.8 65.2 1.2 6.8 10.9 44.1 30.6 57.6 29.0
22.3 14.6 1.8 1.7 5.9 53.5 21.0 44.0 35.3
As Figure 4 illustrates, after the adsorbed N-* reacts with hydrogen, peak I disappears and a new desorption peak appears at about 150 °C. According to the literature,11 the desorption peak of NH3 on the traditional catalyst surface is at about 140 °C. So it can be concluded that the new peak is the desorption peak of the NHx-* species generated by the reaction of H2 and the adsorbed N-*. The experiment strongly proves that peak I is N2-adsorbed on the surface and is probably generated by the N-* species, which is adsorbed on the Fe (111) crystal face.12,13
Ed Ed 1 + log 2.303R Tm νnRnθmn-1
(1)
If we maintain the initial coverage of the adsorbate at a constant for every experiment, it can be regarded that the coverage θm at peak maximum is almost the same as that in each desorption. So the last item on the right of eq 1 turns into a constant. From plots 2 log Tm - log β versus 1/Tm, Ed values calculated by the slope of the linear correlation coefficient R are listed in Table 3. It can be seen from Table 3 that the coefficients R approach 1, except in samples 1 and 3. This indicates that the experiment conditions are in accordance with the hypothesis of eq 1 and that the interaction among the N-* adsorbed on the surface is very weak. 3.2. Relationship between the Desorption Activation Energy Ed and the Oxide Precursor. The relationship between the N2 desorption activation energy and the iron oxide precursor is shown in Figure 5 and Table 3. The effect of the oxide precursor on Ed in Figure 5 is very similar to the relationship of the activity and the oxide precursor in Figure 2. The higher the activity, the lower the N2 desorption activation energy Ed.
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From the above results, mean desorption activation energies Ed in different phase regions of iron oxide precursors are obtained respectively as follows:
Fe3O4
170.3 kJ/mol
Fe3O4 + FeO
222.3 kJ/mol
Fe1-xO
163.2 kJ/mol
The order of N2 desorption activation energies Ed which were measured through TPD is as follows: twophase region (Fe3O4 + FeO) > Fe3O4 > Fe1-xO. This is just opposite to the order of the catalytic activity.2 In other words, the order of the N2 desorption activation energy is in accordance with the reaction activation energy of the ammonia synthesis on the surface. The phenomenon shows that the activity of the fused iron catalyst is mainly determined by the dissociation ability of the N2 molecule. This is just because the dissociation ability of N2 changes with the oxide precursor has a “hump-type” curve (opposite to the Ed changing sequence) and the activity of the fused iron catalyst has a “hump-type” curve as shown in Figure 2. The Temkin-Pyzhev expression14 of ammonia synthesis is as follows:
Figure 6. N2 TPD peak temperature relation to Fe2+/Fe3+ in the oxide precursor when β ) 10 °C/min.
r ) k1PN2(PH23/PNH32)R - k-1(PNH32/PH23)β ) k-1[KPN2(PH23/PNH32)R - (PNH32/PH23)β]; K ) k1/k-1 ) k-1.0e-Ea,-1/RT[KPN2(PH23/PNH32)R - (PNH32/PH23)β] The N2 desorption is the inverse reaction of the ratedetermined step of the ammonia synthesis reaction, Ea,-1 ) Ed. So theoretically, the activation energy of the ammonia synthesis reaction should be close to the N2 desorption activation energy. When R ) β ) 0.5 according to the result of Liu,15 the reaction activation energies of A110-2 commercial catalyst with Fe3O4 as the precursor and A301 commercial catalyst with Fe1-xO as the precursor are 173.85 and 164.25 kJ/mol, respectively15 (reaction conditions were as described with Figure 2). These values are very close to the values of the mean desorption activation energy of N2 with Fe3O4 and Fe1-xO as the precursors which resulted in this paper. The activity of the fused iron catalyst is closely relative to the desorption performance of N2 on the surface. The high activity of A301 mainly comes from the Fe1-xO precursor and not the adjusting effect of the promoters because the promoters in the samples are the same in this experiment. 3.3. Relationship between the N2 Desorption Temperature (TPD Peak Temperature) and the Oxide Precursor and Their Activity. The relationship between the N2 desorption temperature on the surface and the oxide precursor is shown in Figure 6. It can be seen in Figure 6 that the higher the catalytic activity, the lower the N2 TPD peak temperature; conversely, the lower the catalytic activity, the higher the N2 TPD peak temperature. Especially, when the oxide precursor is a mixture of Fe3O4 and FeO, the catalytic activity decreases and the N2 desorption temperature increases with an increase of their mixed degree. When the mixed degree reaches the maximum,
Figure 7. N2 TPD peak area relation to Fe2+/Fe3+ in the oxide precursor when β ) 10 °C/min.
that is, the molar ratio of Fe3O4 and FeO is close to 1, the catalytic activity decreases to the minimum (Figure 2) and the N2 desorption temperature reaches the maximum. So it is evident that the oxide precursor influences the catalytic activity and the N2 desorption temperature. In fact, the values of the peak temperature obtained in the same experimental condition also reflect the values of the activation energies of N2 desorption on the surface. The higher the peak temperature, the more difficult it is for the nitrogen in such adsorption state to be desorbed, and so the activation energy rises; the adsorption and dissociation of the iron to N2 is more difficult, and so the catalytic activity decreases. Figures 5 and 6 reflect the N2 TPD peak temperature, and Ed values have the same tendency with a change of the oxide precursor. 3.4. Relationship between the N2 Desorption Volume (Peak Area) and the Oxide Precursor. The peak area measured by N2 TPD represents the number of dissociatively adsorbed N-* on the catalyst surface. Figure 7 gives the relationship between the peak area and the oxide precursor (Fe2+/Fe3+) when β ) 10 °C/ min. From Figure 7, it can be seen that the relationship between the peak area and Fe2+/Fe3+ is very similar to the relationship of the catalytic activity and Fe2+/Fe3+ except in sample 1. When the oxide precursor is only Fe3O4 or FeO (Fe2+/Fe3+ ) 0.5 or larger than 3.15), the number of the dissociatively adsorbed N-* on the surface is more, and so the catalytic activity is higher. When
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the oxide precursor is a mixture of Fe3O4 and FeO (Fe2+/ Fe3+ is in the range of 0.5-3.15), the N2 TPD peak area drops rapidly to the minimum, and so the catalytic activity also decreases to the minimum. This proves strongly that the dissociative adsorption of N2 is the rate-determined step of ammonia synthesis on the iron surface. The higher the catalytic activity, the higher the number of dissociatively adsorbed N-* on the surface, the faster the reaction rate of ammonia synthesis. Literature Cited (1) Nielsen, A. Investigation on promoted iron catalyst for the synthesis of ammonia, 3rd ed.; Jul Giellerup’s Forlag.: Copenhagen, Denmark, 1968. (2) Ertl, G. Surface Science and catalysissstudies on the Mechanism of ammonia synthesis: The P. H. Emmett Award Address. Catal. Rev.-Sci. Eng. 1980, 21, 201. (3) Ertl, G. In Catalytic Ammonia Synthesis, Fundamentals and practice; Jennings, J. R., Ed.; Plenum Press: New York, 1991; p 22. (4) Nielsen, A. Ammonia synthesis: Exploratory and applied research. Catal. Rev.-Sci. Eng. 1981, 23 (1 & 2), 17. (5) Bridger, G. L.; Pole, G. R.; Beinlich, A. W.; et al. Production and performance of ammonia synthesis catalyst. Chem. Eng. Prog. 1947, 43 (6), 291. (6) Liu, H.; Li, X. The precursor phase composition of iron catalyst and discovery of FeO based catalyst for ammonia synthesis. Sci. China, Ser. B 1995, 38 (5), 529-537.
(7) Aparicio, L. M.; Dumesic, J. A. Ammonia synthesis kinetics: Surface chemistry, rate expressions and kinetics analysis. Top. Catal. 1994, 1, 233. (8) Schiitze, J.; Mahdi, W. On the structure of the activated iron catalyst for ammonia synthesis. Top. Catal. 1994, 1, 195. (9) Boszo, F.; Ertl, G.; Grunze, M.; Weiss, M. Interaction of nitrogen with iron surfaces I. Fe(100) and Fe(111). J. Catal. 1977, 49, 18. (10) Boszo, F.; Ertl, G.; Weiss, M. Interaction of nitrogen with iron surfaces. II. Fe(110)1. J. Catal. 1977, 50, 519. (11) Fastrup, B. Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Top. Catal. 1994, 1, 273. (12) Cvetanovic, R. J.; Amenomiya, Y. Application of a temperature-programmed desorption technique to catalyst studies. Adv. Catal. 1967, 17, 103. (13) Muhler, M.; Rosowski, F.; Ertl, G. The dissociative adsorption of N2 on a multiply promoted iron catalyst used for ammonia synthesis: A temperature-programmed-desorption study. Catal. Lett. 1994, 24, 317. (14) Temkin, M.; Pyzhev, V. Acta Physicochim. USSR 1940, 12, 327. (15) Liu, H.; Hu, Z.; Li, X.; et al. A study of ZA-3 ammonia synthesis catalystsI Laboratory research. J. Zhejiang Inst. Technol. (in Chinese). 1993, 2, 15.
Received for review September 16, 1999 Revised manuscript received March 30, 2000 Accepted April 24, 2000 IE990695G
