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On the distribution of aluminium and magnesium oxides in wustite catalysts for ammonia synthesis
Applied Catalysis A: General 247 (2003) 9–15
On the distribution of aluminium and magnesium oxides in wustite catalysts for ammonia synthesis Michał J. Figurski∗ , Walerian Arabczyk a , Zofia Lendzion-Bielu´n a , Ryszard J. Kale´nczuk a , Stanisław Lenart b a
Institute of Chemical and Environment Engineering, Technical University of Szczecin, 70-322 Szczecin, ul. Pułaskiego 10, Poland b Institute of Materials Engineering, Technical University of Szczecin, 70-322 Szczecin, ul. Pułaskiego 10, Poland Received 7 October 2002; received in revised form 3 January 2003; accepted 7 January 2003
Abstract The distribution of aluminium and magnesium oxides in wusite catalysts for ammonia synthesis was studied using the selective etching method, energy dispersive spectroscopy (EDS) and nitrogen chemisorption. Aluminium oxide was found to occur in three forms in the wustite catalyst’s precursor: one in the solid solution with wustite, the other two in the spaces between wustite grains, being distinguished by solubility in hydrochloric acid. The rising content of CaO in the precursor causes an increase of the HCl-soluble form of Al2 O3 on behalf of the non-HCl-soluble form. The amount of Al2 O3 dissolved in the wustite phase was the same in all investigated wustite samples, and amounted to 0.3 wt.%. The magnesium oxide is well dispersed in the wustite phase, and it has no influence on the forms of occurrence of Al2 O3 . Considering the surface area of the reduced catalyst, the exclusive promotion with Al2 O3 is twice as effective as exclusive promotion with MgO, while catalysts promoted with both oxides possess much larger surface areas, an indication of a synergistic effect. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Iron catalyst; Wustite; Promoter distribution; Surface area
1. Introduction Recently a new method for the investigation of the distribution of the promoters in the oxidised form of an iron catalyst has been developed [1]. This method was successfully applied to elucidate the distributions of promoters in the iron catalyst precursors with differing Fe2+ /Fe3+ ratios (denoted as “R”), i.e. enriched in the wustite or maghemite phase [2]. The results from these works indicate that the potassium oxide distri∗ Corresponding author. Present address: Department of Preventive Medicine, Pomeranian Academy of Medicine, 70-111 Szczecin, ul. Powstancow Wielkopolskich 72, Poland. E-mail address: [email protected] (M.J. Figurski).
bution (activating promoter) does not depend on R— potassium oxide occurs in the glass phase between the iron oxide crystallites [2,3]. It was also found, that the distributions of calcium and aluminium oxides (structural promoters) depend strongly on R, and that there is a much lower content of these promoters incorporated into iron oxides in the wustite catalyst than in the magnetite catalysts [2]. The content of structural promoters incorporated into the iron oxides determines the surface properties, particularly the quantity of surface area of the catalyst resulting from the oxide precursor, and in this way it influences the catalytic activity. The activity of iron catalysts of various R were investigated [2,4,5]. There is an agreement that the relationship between the R and activity of the catalysts
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M.J. Figurski et al. / Applied Catalysis A: General 247 (2003) 9–15
shows two maxima—first for R around 0.5 (magnetite) and second for R around 5–7 (wustite). However, some of the authors [4] say that the catalysts based on wustite are about 15% more active than their magnetite counterparts, while others [2] report that the wustite catalysts are about 10% less active. It’s worth noticing, that those results were obtained for catalysts of different promoter composition—authors of work [4] have investigated the catalysts of low content of structural promoters, and the authors of work [2] have investigated the catalysts of the same promoter content as in the industrial, magnetite-based catalyst. This indicates, that the optimal composition of structural promoters for the wustite catalyst is different than that for the magnetite catalyst. When considering quantity of specific surface area of the catalyst, aluminium oxide is the structural promoter of greatest importance [6]. It is in great part incorporated into the magnetite phase and only in very small amounts in the wustite phase of the catalyst. Calcium oxide is partially soluble in magnetite and wustite phases [2]. In our investigation we turned to magnesium oxide, because it is highly soluble both in magnetite and wustite [3]. Comparing the effect on the quantity of specific surface area, magnesium oxide is a second structural promoter, after aluminium oxide and before calcium oxide [6]. We tried to incorporate the magnesium oxide into the wustite catalyst, instead of calcium oxide, as this should result in better solubility of structural promoters in the iron oxides. The aim of this work is to investigate the distribution of magnesium and aluminium oxides in the wustite catalyst precursors and its influence on the surface properties of the resulting catalysts.
(K1–K4), as well as one wustite catalyst doped with calcium and aluminium oxides in the same proportions as in the standard industrial catalyst (K0). In order to obtain the desired oxidation degree of the precursor, graphite was added, as a reducing agent. The mixture was melted, then tapped into a steel container and cooled to the room temperature to obtain a metastable wustite phase [8]. Chemical composition of the samples was analysed using an inductively-coupled plasma atomic emission spectroscopy (ICP-AES, Jobin Yvon Ultrace 238), and X-ray fluorescence (XRF). The oxidation degree of the samples was determined by manganometry and X-ray powder diffraction (XRD, Philips X-pert) [9]. Distribution of the promoters in the investigated samples was determined with the selective etching method [1] and X-ray microanalysis (EDS, Oxford ISIS 300 coupled with Jeol scanning microscope JSM 6100). The specific surface area was determined with BET method (Micromeritics ASAP 2010) after reduction and after overheating of the catalysts. The reduction was carried in a six-channel integral reactor with purified hydrogen–nitrogen mixture (3:1) under 0.1 MPa and the space velocity of about 20,000 h−1 . The temperature program for reduction was as follows: 200 ◦ C, 1 h; 350 ◦ C, 2 h; 400 ◦ C, 12 h; 450 ◦ C, 48 h; 500 ◦ C, 2 h. The high-temperature reduction experiments were also performed to simulate the behaviour of catalyst during a long-term period of working in ammonia synthesis conditions [10]. High-temperature reduction was carried along the same temperature program as standard reduction with exception to an additional step of temperature 650 ◦ C for 17 h. The reduced samples were handled under inert atmosphere, to avoid passivation.
2. Experimental The catalysts used in this study were prepared using the laboratory installation for continuous fusing [7] from the magnetite ore, calcium, aluminium and magnesium oxides and potassium nitrate. The amount of magnesium, calcium and aluminium oxides was calculated so that the total content of structural promoters (in moles of atoms of calcium, aluminium and magnesium) in all the resulting catalysts was equal. Four wustite catalysts doped with magnesium and aluminium oxides in various proportions were prepared
3. Results and discussion The chemical composition of the investigated catalysts is presented in Table 1. The contents of potassium oxide and silica in these catalysts were 0.66 ± 0.04 and 0.45 ± 0.03 wt.%, respectively. Only the reflections from wustite phase are present on the X-ray diffraction patterns for the samples K0–K4, which is consistent with results of [5]. This is because the Fe3+ ions present in the sample does not
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Table 1 Chemical composition, oxidation degree and phase content of promoters of the investigated catalysts Catalyst
R
MgO (wt.%) (ctotal and css )
CaO (wt.%)
Al2 O3 (wt.%)
ctotal
css
ctotal
css
ci-sol
ci-non
Total css (wt.%)
Specific surface area (m2 /g) Standard reduction
High-temperature reduction
Industriala K0
0.63 4.5
0.53 0.57
2.9 2.9
1.0 0.73
3.3 3.3
2.7 0.35
0.58 2.5
– 0.45
4.3 1.6
22.0 10.7
12.3 5.4
K1 K2 K3 K4
3.7 3.9 4.5 7.3
4.0 2.3 1.4 0.37
0.43 0.69 0.34 0.28
0.12 0.35 0.13 0.12
0.45 1.6 2.7 5.1
0.30 0.34 0.32 0.33
0.15 0.96 1.0 1.0
– 0.30 1.4 3.8
4.4 3.0 1.8 0.82
8.7 21.1 16.7 15.0
2.7 11.4 7.2 7.1
a
Reference data [1].
form a separate magnetite phase, but are dissolved in the highly non-stoichiometric wustite phase. From the Fe–O phase system we find, that the wustite phase is stable at the melting temperature for R ranging from 1.9 to 11. The R of all of the obtained wustite catalysts fits in this range. In the way of fast cooling the samples to the room temperature (rapid crystallisation), the metastable wustite phase was obtained. Results of analysis of promoters distribution in the phases of the investigated catalyst precursors, obtained
in the way of selective etching, are shown on Figs. 1 and 2. The common relationship of etching degree of promoter to the etching degree of iron for the wustite catalyst is a curve that has two approximately linear segments. The right-hand segment, beginning at about 3–10% of iron etching degree, represents the etching of promoter dissolved in the wustite. The extrapolation of this segment to the value of 0 and 100% of etching degree of iron allows for determination of the content of promoter in solid solution with the wustite. The
Fig. 1. Relationship of etching degree of potassium, magnesium and calcium to the etching degree of iron.
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Fig. 2. Relationship of weight percent of aluminium oxide etched from the precursor (related to the mass of the precursor before etching) to the etching degree of iron. Total contents of Al2 O3 in the precursors are marked on the right side of the chart area. For the catalysts K0 and K4 the total content of Al2 O3 (see Table 1) exceeds the scale of y-axis.
left-hand segment represents the process of etching of the promoter present in the spaces between wustite grains. The intersection of the former segment with y-axis (at 100% of etching degree of iron) determines the content of HCl-soluble promoter. In some cases the total content of particular promoter is larger than that determined with extrapolation of the etching line, therefore the differential content of the promoter is non-soluble in HCl solutions in the conditions applied [1]. The relationship of etching degree of potassium, magnesium and calcium to the etching degree of iron, shown on Fig. 1, is common for all wustite catalysts. The distribution of promoters in the phases of the industrial magnetite catalyst can be found in [1]. Potassium is 100% etched below 3% of etching degree of iron. This result demonstrates that potassium is located in the intergranular spaces (glass phase) and is consistent with the results of other investigators [1,2,11]. Conversely, the etching degree of magnesium rises proportionally to the etching degree of iron, which
means that magnesium is near evenly incorporated into the wustite phase. Residual calcium oxide is present at about 40%, being located in the intergranular spaces, while the rest forms a solid solution with wustite [1]. The example of a promoter occurring in the non-HCl-soluble form is aluminium oxide. Fig. 2 shows the relationship of weight percent of aluminium oxide etched from the precursor (related to the mass of the precursor before etching) to the etching degree of iron. The linear segments of etching curves for above 10% etching degree of iron are parallel to one another. This is an indication that the content of aluminium oxide in solid solution with wustite (css ) is equal for all wustite samples and amounts around 0.3 wt.%, despite the different total content of aluminium oxide (ctotal ) and other promoters. Also the content of aluminium oxide etched at 100% of etching degree of iron, i.e. HCl-soluble, is nearly the same for the catalysts K2, K3 and K4. ctotal and css , as well as the contents of HCl-soluble (ci-sol ) and non-soluble (ci-non ) aluminium oxide in the intergranular spaces,
M.J. Figurski et al. / Applied Catalysis A: General 247 (2003) 9–15
are shown in Table 1. The way in which the values of css , ci-non and ci-sol have been read from Fig. 3 is shown as an example for catalyst K3 on the right side of that figure. Furthermore, in the calcium-oxide-doped catalyst K0, aluminium oxide occurs exclusively in HCl-soluble form, while in all the catalysts doped with magnesium oxide, the content of HCl-soluble aluminium oxide is almost equal, amounting to 1 wt.%. The only exception is catalyst K1, which contains less than 1 wt.% of total aluminium oxide. The possible known aluminium oxide compounds of very low HCl solubility, include pure Al2 O3 (corundum), MgO·Al2 O3 (spinel), and silicates. In order to locate the presence of such compounds in the investigated catalysts, the EDS mappings and pointwise X-ray microanalysis experiments were performed. The Fig. 3 shows EDS mappings of the catalyst K0 and catalyst K3. Because the magnesium mapping for catalyst K3 is close to uniform, which is consistent with the result from selective etching, the image was omitted from this paper. From the catalyst K0 images on the left one can see that the promoters in the intergranular spaces occur in the forms of Al2 O3 ·nK2 O, Al2 O3 ·nCaO, pure CaO or small quantities of the above compounds with silica. The compounds of aluminium and calcium oxides are highly soluble in HCl. In the catalyst K3, there is a highly visible grain of Al2 O3 , indicated with number “1” on the right-hand SEM image, which is not in compound with any other promoter. The results of pointwise analysis (Table 2) indicate, that aluminium oxide is at least partially bound with iron oxide. The form of aluminium-iron oxide compound ensuing from these results is Al2 O3 ·FeO + pure Al2 O3 (in molar proportions).
Table 2 Results of pointwise X-ray microanalysis in (wt.%) of respective oxides, points indicated on SEI image from Fig. 3
Mg Al Si K Ca Fe
Point 1 (wt.%)
Point 2 (wt.%)
Point 3 (wt.%)
4.8 53.0 0.9 0.0 0.8 40.4
0.0 26.0 37.4 32.0 0.6 4.0
2.7 1.0 0.4 0.0 0.0 95.9
13
Some amount of magnesium oxide present there exists in solid solution with the above compound, so that these species may constitute the non-HCl-soluble form of aluminium oxide found with selective etching. There are also some compounds of aluminium oxide with K2 O and with residual CaO in the catalyst K3. Pointwise analysis in the point numbered 2 shows typical composition of the species in the intergranular spaces—the results of this analysis almost stoichiometrically indicate the compound K2 O · SiO2 · 1/2Al2 O3 . This compound is highly soluble in hydrochloric acid, and should constitute the HCl-soluble form of aluminium oxide. Point 3 of the pointwise analysis exhibits the composition of wustite phase. As it was found with selective etching, the magnesium oxide is dissolved in the wustite, together with small amounts of aluminium oxide and silica. The amount of aluminium oxide dissolved in the wustite, calculated from pointwise analysis (about 1.0 wt.%), is different from that found with selective etching (0.3 wt.%). The possible reason for that difference may be the displacement of promoters present in glass phase during mechanical treatment in the procedure of sample preparation for EDS analysis. These promoters, then, appear as if being present in the wustite phase. The selective etching method gives an averaged result, where the promoters from intergranular spaces are etched in the beginning of the etching process. Consequently, the result from that method (0.3 wt.% of Al2 O3 in solid solution with wustite) is more reliable. The results of specific surface area determination for the reduced catalysts are shown in Table 1. Higher reduction temperature also causes decrease of specific surface area by a factor of about two for all catalysts except catalyst K1, whose area decreased over three times. This catalyst is less temperature resistant than the others, which is probably due to the lack of aluminium oxide in this catalyst. It is characteristic that the surface area of the wustite catalyst K0 is half that of the industrial catalyst, though these two catalysts have the same promoter composition. Results from Table 1 indicate, that the change of the specific surface area is caused by the change of the content of structural promoters in a solid solution with the iron oxides [2]. The catalyst K1 may be considered as doublypromoted (with addition of K2 O and MgO), and has the specific surface area 8.7 m2 /g. The other
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Fig. 3. SEM images and EDS mappings of the wustite catalyst K0 (left-hand images) and the wustite Mg doped catalyst K3 (right-hand images).
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doubly-promoted catalyst containing Al2 O3 as the structural promoter (K4) has a surface area of about 15 m2 /g. Considering that these two catalysts have approximately the same molar content of Mg and Al atoms, we find that the wustite catalyst promotion with Al is about twice as effective as promotion with Mg when the surface area development is taken into account. The triply-promoted catalysts containing both of these structural promoters have specific surface areas higher than would be expected from a simple proportion of the contents of these promoters. This suggests the existence of a synergistic effect of the aluminium and magnesium oxides. Another approach would be to focus solely on the aluminium oxide content, as it is the “strongest” structural promoter. We find that increasing the fraction of Al2 O3 (catalysts K2, K3 and K4) has led to the decrease of specific surface area. It is already known that the larger amounts of structural promoters tend to decrease the surface area of magnetite catalysts [12]. This means that the amounts above 2 wt.% of aluminium oxide are too much for wustite catalysts, and the optimal amount of aluminium oxide for the development of surface area of wustite catalyst lies in the vicinity of 1.6 wt.% For the magnetite catalysts, however, the addition of Al2 O3 in amounts up to 12 wt.% results in an increase of the surface area [13]. It is possible that only the HCl-soluble form of Al2 O3 is capable of development of surface area. Meanwhile, the excess aluminium oxide causes its decrease, as the catalyst K2, containing the smallest amount of insoluble Al2 O3 , possesses the highest surface area of all the aluminium-oxide-promoted catalysts. Based on the results of the distribution of Al2 O3 from Table 1, we find that the optimal amount of aluminium oxide for the development of surface area of wustite catalyst should be about 1.3 wt.%, which comprises of 0.3 wt.% dissolved in wustite phase plus about 1 wt.% of HCl-soluble form of Al2 O3 .
4. Conclusions Our results confirm that the magnesium oxide is incorporated uniformly in the wustite phase. The potassium oxide, together with silica, is located in the intergranular spaces of all investigated catalysts. We have found that the Al2 O3 occurs in three forms
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in the wustite catalyst precursor: one form is the solution in the wustite phase, the other two are located in the intergranular spaces of the catalyst. The content of Al2 O3 in the solid solution with wustite phase amounts to about 0.3 wt.% (from selective etching), and does not depend on the quantity of Al2 O3 added nor on the addition of other promoters. Content of HCl-soluble Al2 O3 in the intergranular spaces depends on the addition of CaO. When only traces of CaO are present, the HCl-soluble Al2 O3 exists at 1 wt.% in the wustite catalysts, generally in the form of K2 O · SiO2 · 1/2Al2 O3 and Al2 O3 ·FeO. The remaining pure Al2 O3 and spinel comprise the non-HCl-soluble form of aluminium oxide. The surface area of the catalyst promoted with Al2 O3 is about twice that of the catalyst promoted with MgO. The higher surface areas of catalysts containing both promoters may result from either the synergistic effect of MgO and Al2 O3 , or quite low optimal Al2 O3 content for wustite catalysts. Presuming that only the HCl-soluble Al2 O3 is capable of surface area development, its optimal content for wustite catalysts not containing calcium oxide should be around 1.3 wt.%. References [1] Z. Lendzion-Bielu´n, W. Arabczyk, Appl. Catal. A 207 (1–2) (2001) 37. [2] Z. Lendzion-Bielu´n, W. Arabczyk, M. Figurski, Appl. Catal. A 227 (2002) 255. [3] H.C. Chen, R.B. Anderson, J. Coll. Int. Sci. 38 (2) (1972) 535. [4] H.Z. Liu, X.N. Li, Z.N. Hu, Appl. Catal. A 142 (1996) 209. [5] H.Z. Liu, X.N. Li, Ind. Eng. Chem. Res. 36 (2) (1997) 335. [6] M.E. Dry, J.A.K. Du Plessis, G.M. Leuteritz, J. Catal. 6 (1966) 194. ´ [7] W. Arabczyk, J. Ziebro, K. Kałucki, R. Swierkowski, M. Jakrzewska, Chemik 1 (1996) 22. [8] A. Pattek-Janczyk, B. Miczko, Appl. Catal. A 124 (1995) 253. [9] D. Szczuko, W. Arabczyk, Chemia Analityczna 45 (2000) 273. [10] Z. Kowalczyk, S. Jodzis, Appl. Catal. 58 (1990) 29. [11] M.E. Dry, L.C. Ferreira, J. Catal. 7 (1967) 352. [12] P.H. Emmett, S. Brunauer, J. Am. Chem. Soc. 59 (1937) 1553. [13] L.M. Dmitrienko, Osnovy predvidlenya kataliticheskovo deistvia, Trudy IV Miezhdunarodnovo Kongresa po Katalizu, Nauka 1 (1970) 328.
On the distribution of aluminium and magnesium oxides in wustite catalysts for ammonia synthesis Michał J. Figurski∗ , Walerian Arabczyk a , Zofia Lendzion-Bielu´n a , Ryszard J. Kale´nczuk a , Stanisław Lenart b a
Institute of Chemical and Environment Engineering, Technical University of Szczecin, 70-322 Szczecin, ul. Pułaskiego 10, Poland b Institute of Materials Engineering, Technical University of Szczecin, 70-322 Szczecin, ul. Pułaskiego 10, Poland Received 7 October 2002; received in revised form 3 January 2003; accepted 7 January 2003
Abstract The distribution of aluminium and magnesium oxides in wusite catalysts for ammonia synthesis was studied using the selective etching method, energy dispersive spectroscopy (EDS) and nitrogen chemisorption. Aluminium oxide was found to occur in three forms in the wustite catalyst’s precursor: one in the solid solution with wustite, the other two in the spaces between wustite grains, being distinguished by solubility in hydrochloric acid. The rising content of CaO in the precursor causes an increase of the HCl-soluble form of Al2 O3 on behalf of the non-HCl-soluble form. The amount of Al2 O3 dissolved in the wustite phase was the same in all investigated wustite samples, and amounted to 0.3 wt.%. The magnesium oxide is well dispersed in the wustite phase, and it has no influence on the forms of occurrence of Al2 O3 . Considering the surface area of the reduced catalyst, the exclusive promotion with Al2 O3 is twice as effective as exclusive promotion with MgO, while catalysts promoted with both oxides possess much larger surface areas, an indication of a synergistic effect. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Iron catalyst; Wustite; Promoter distribution; Surface area
1. Introduction Recently a new method for the investigation of the distribution of the promoters in the oxidised form of an iron catalyst has been developed [1]. This method was successfully applied to elucidate the distributions of promoters in the iron catalyst precursors with differing Fe2+ /Fe3+ ratios (denoted as “R”), i.e. enriched in the wustite or maghemite phase [2]. The results from these works indicate that the potassium oxide distri∗ Corresponding author. Present address: Department of Preventive Medicine, Pomeranian Academy of Medicine, 70-111 Szczecin, ul. Powstancow Wielkopolskich 72, Poland. E-mail address: [email protected] (M.J. Figurski).
bution (activating promoter) does not depend on R— potassium oxide occurs in the glass phase between the iron oxide crystallites [2,3]. It was also found, that the distributions of calcium and aluminium oxides (structural promoters) depend strongly on R, and that there is a much lower content of these promoters incorporated into iron oxides in the wustite catalyst than in the magnetite catalysts [2]. The content of structural promoters incorporated into the iron oxides determines the surface properties, particularly the quantity of surface area of the catalyst resulting from the oxide precursor, and in this way it influences the catalytic activity. The activity of iron catalysts of various R were investigated [2,4,5]. There is an agreement that the relationship between the R and activity of the catalysts
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shows two maxima—first for R around 0.5 (magnetite) and second for R around 5–7 (wustite). However, some of the authors [4] say that the catalysts based on wustite are about 15% more active than their magnetite counterparts, while others [2] report that the wustite catalysts are about 10% less active. It’s worth noticing, that those results were obtained for catalysts of different promoter composition—authors of work [4] have investigated the catalysts of low content of structural promoters, and the authors of work [2] have investigated the catalysts of the same promoter content as in the industrial, magnetite-based catalyst. This indicates, that the optimal composition of structural promoters for the wustite catalyst is different than that for the magnetite catalyst. When considering quantity of specific surface area of the catalyst, aluminium oxide is the structural promoter of greatest importance [6]. It is in great part incorporated into the magnetite phase and only in very small amounts in the wustite phase of the catalyst. Calcium oxide is partially soluble in magnetite and wustite phases [2]. In our investigation we turned to magnesium oxide, because it is highly soluble both in magnetite and wustite [3]. Comparing the effect on the quantity of specific surface area, magnesium oxide is a second structural promoter, after aluminium oxide and before calcium oxide [6]. We tried to incorporate the magnesium oxide into the wustite catalyst, instead of calcium oxide, as this should result in better solubility of structural promoters in the iron oxides. The aim of this work is to investigate the distribution of magnesium and aluminium oxides in the wustite catalyst precursors and its influence on the surface properties of the resulting catalysts.
(K1–K4), as well as one wustite catalyst doped with calcium and aluminium oxides in the same proportions as in the standard industrial catalyst (K0). In order to obtain the desired oxidation degree of the precursor, graphite was added, as a reducing agent. The mixture was melted, then tapped into a steel container and cooled to the room temperature to obtain a metastable wustite phase [8]. Chemical composition of the samples was analysed using an inductively-coupled plasma atomic emission spectroscopy (ICP-AES, Jobin Yvon Ultrace 238), and X-ray fluorescence (XRF). The oxidation degree of the samples was determined by manganometry and X-ray powder diffraction (XRD, Philips X-pert) [9]. Distribution of the promoters in the investigated samples was determined with the selective etching method [1] and X-ray microanalysis (EDS, Oxford ISIS 300 coupled with Jeol scanning microscope JSM 6100). The specific surface area was determined with BET method (Micromeritics ASAP 2010) after reduction and after overheating of the catalysts. The reduction was carried in a six-channel integral reactor with purified hydrogen–nitrogen mixture (3:1) under 0.1 MPa and the space velocity of about 20,000 h−1 . The temperature program for reduction was as follows: 200 ◦ C, 1 h; 350 ◦ C, 2 h; 400 ◦ C, 12 h; 450 ◦ C, 48 h; 500 ◦ C, 2 h. The high-temperature reduction experiments were also performed to simulate the behaviour of catalyst during a long-term period of working in ammonia synthesis conditions [10]. High-temperature reduction was carried along the same temperature program as standard reduction with exception to an additional step of temperature 650 ◦ C for 17 h. The reduced samples were handled under inert atmosphere, to avoid passivation.
2. Experimental The catalysts used in this study were prepared using the laboratory installation for continuous fusing [7] from the magnetite ore, calcium, aluminium and magnesium oxides and potassium nitrate. The amount of magnesium, calcium and aluminium oxides was calculated so that the total content of structural promoters (in moles of atoms of calcium, aluminium and magnesium) in all the resulting catalysts was equal. Four wustite catalysts doped with magnesium and aluminium oxides in various proportions were prepared
3. Results and discussion The chemical composition of the investigated catalysts is presented in Table 1. The contents of potassium oxide and silica in these catalysts were 0.66 ± 0.04 and 0.45 ± 0.03 wt.%, respectively. Only the reflections from wustite phase are present on the X-ray diffraction patterns for the samples K0–K4, which is consistent with results of [5]. This is because the Fe3+ ions present in the sample does not
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Table 1 Chemical composition, oxidation degree and phase content of promoters of the investigated catalysts Catalyst
R
MgO (wt.%) (ctotal and css )
CaO (wt.%)
Al2 O3 (wt.%)
ctotal
css
ctotal
css
ci-sol
ci-non
Total css (wt.%)
Specific surface area (m2 /g) Standard reduction
High-temperature reduction
Industriala K0
0.63 4.5
0.53 0.57
2.9 2.9
1.0 0.73
3.3 3.3
2.7 0.35
0.58 2.5
– 0.45
4.3 1.6
22.0 10.7
12.3 5.4
K1 K2 K3 K4
3.7 3.9 4.5 7.3
4.0 2.3 1.4 0.37
0.43 0.69 0.34 0.28
0.12 0.35 0.13 0.12
0.45 1.6 2.7 5.1
0.30 0.34 0.32 0.33
0.15 0.96 1.0 1.0
– 0.30 1.4 3.8
4.4 3.0 1.8 0.82
8.7 21.1 16.7 15.0
2.7 11.4 7.2 7.1
a
Reference data [1].
form a separate magnetite phase, but are dissolved in the highly non-stoichiometric wustite phase. From the Fe–O phase system we find, that the wustite phase is stable at the melting temperature for R ranging from 1.9 to 11. The R of all of the obtained wustite catalysts fits in this range. In the way of fast cooling the samples to the room temperature (rapid crystallisation), the metastable wustite phase was obtained. Results of analysis of promoters distribution in the phases of the investigated catalyst precursors, obtained
in the way of selective etching, are shown on Figs. 1 and 2. The common relationship of etching degree of promoter to the etching degree of iron for the wustite catalyst is a curve that has two approximately linear segments. The right-hand segment, beginning at about 3–10% of iron etching degree, represents the etching of promoter dissolved in the wustite. The extrapolation of this segment to the value of 0 and 100% of etching degree of iron allows for determination of the content of promoter in solid solution with the wustite. The
Fig. 1. Relationship of etching degree of potassium, magnesium and calcium to the etching degree of iron.
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Fig. 2. Relationship of weight percent of aluminium oxide etched from the precursor (related to the mass of the precursor before etching) to the etching degree of iron. Total contents of Al2 O3 in the precursors are marked on the right side of the chart area. For the catalysts K0 and K4 the total content of Al2 O3 (see Table 1) exceeds the scale of y-axis.
left-hand segment represents the process of etching of the promoter present in the spaces between wustite grains. The intersection of the former segment with y-axis (at 100% of etching degree of iron) determines the content of HCl-soluble promoter. In some cases the total content of particular promoter is larger than that determined with extrapolation of the etching line, therefore the differential content of the promoter is non-soluble in HCl solutions in the conditions applied [1]. The relationship of etching degree of potassium, magnesium and calcium to the etching degree of iron, shown on Fig. 1, is common for all wustite catalysts. The distribution of promoters in the phases of the industrial magnetite catalyst can be found in [1]. Potassium is 100% etched below 3% of etching degree of iron. This result demonstrates that potassium is located in the intergranular spaces (glass phase) and is consistent with the results of other investigators [1,2,11]. Conversely, the etching degree of magnesium rises proportionally to the etching degree of iron, which
means that magnesium is near evenly incorporated into the wustite phase. Residual calcium oxide is present at about 40%, being located in the intergranular spaces, while the rest forms a solid solution with wustite [1]. The example of a promoter occurring in the non-HCl-soluble form is aluminium oxide. Fig. 2 shows the relationship of weight percent of aluminium oxide etched from the precursor (related to the mass of the precursor before etching) to the etching degree of iron. The linear segments of etching curves for above 10% etching degree of iron are parallel to one another. This is an indication that the content of aluminium oxide in solid solution with wustite (css ) is equal for all wustite samples and amounts around 0.3 wt.%, despite the different total content of aluminium oxide (ctotal ) and other promoters. Also the content of aluminium oxide etched at 100% of etching degree of iron, i.e. HCl-soluble, is nearly the same for the catalysts K2, K3 and K4. ctotal and css , as well as the contents of HCl-soluble (ci-sol ) and non-soluble (ci-non ) aluminium oxide in the intergranular spaces,
M.J. Figurski et al. / Applied Catalysis A: General 247 (2003) 9–15
are shown in Table 1. The way in which the values of css , ci-non and ci-sol have been read from Fig. 3 is shown as an example for catalyst K3 on the right side of that figure. Furthermore, in the calcium-oxide-doped catalyst K0, aluminium oxide occurs exclusively in HCl-soluble form, while in all the catalysts doped with magnesium oxide, the content of HCl-soluble aluminium oxide is almost equal, amounting to 1 wt.%. The only exception is catalyst K1, which contains less than 1 wt.% of total aluminium oxide. The possible known aluminium oxide compounds of very low HCl solubility, include pure Al2 O3 (corundum), MgO·Al2 O3 (spinel), and silicates. In order to locate the presence of such compounds in the investigated catalysts, the EDS mappings and pointwise X-ray microanalysis experiments were performed. The Fig. 3 shows EDS mappings of the catalyst K0 and catalyst K3. Because the magnesium mapping for catalyst K3 is close to uniform, which is consistent with the result from selective etching, the image was omitted from this paper. From the catalyst K0 images on the left one can see that the promoters in the intergranular spaces occur in the forms of Al2 O3 ·nK2 O, Al2 O3 ·nCaO, pure CaO or small quantities of the above compounds with silica. The compounds of aluminium and calcium oxides are highly soluble in HCl. In the catalyst K3, there is a highly visible grain of Al2 O3 , indicated with number “1” on the right-hand SEM image, which is not in compound with any other promoter. The results of pointwise analysis (Table 2) indicate, that aluminium oxide is at least partially bound with iron oxide. The form of aluminium-iron oxide compound ensuing from these results is Al2 O3 ·FeO + pure Al2 O3 (in molar proportions).
Table 2 Results of pointwise X-ray microanalysis in (wt.%) of respective oxides, points indicated on SEI image from Fig. 3
Mg Al Si K Ca Fe
Point 1 (wt.%)
Point 2 (wt.%)
Point 3 (wt.%)
4.8 53.0 0.9 0.0 0.8 40.4
0.0 26.0 37.4 32.0 0.6 4.0
2.7 1.0 0.4 0.0 0.0 95.9
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Some amount of magnesium oxide present there exists in solid solution with the above compound, so that these species may constitute the non-HCl-soluble form of aluminium oxide found with selective etching. There are also some compounds of aluminium oxide with K2 O and with residual CaO in the catalyst K3. Pointwise analysis in the point numbered 2 shows typical composition of the species in the intergranular spaces—the results of this analysis almost stoichiometrically indicate the compound K2 O · SiO2 · 1/2Al2 O3 . This compound is highly soluble in hydrochloric acid, and should constitute the HCl-soluble form of aluminium oxide. Point 3 of the pointwise analysis exhibits the composition of wustite phase. As it was found with selective etching, the magnesium oxide is dissolved in the wustite, together with small amounts of aluminium oxide and silica. The amount of aluminium oxide dissolved in the wustite, calculated from pointwise analysis (about 1.0 wt.%), is different from that found with selective etching (0.3 wt.%). The possible reason for that difference may be the displacement of promoters present in glass phase during mechanical treatment in the procedure of sample preparation for EDS analysis. These promoters, then, appear as if being present in the wustite phase. The selective etching method gives an averaged result, where the promoters from intergranular spaces are etched in the beginning of the etching process. Consequently, the result from that method (0.3 wt.% of Al2 O3 in solid solution with wustite) is more reliable. The results of specific surface area determination for the reduced catalysts are shown in Table 1. Higher reduction temperature also causes decrease of specific surface area by a factor of about two for all catalysts except catalyst K1, whose area decreased over three times. This catalyst is less temperature resistant than the others, which is probably due to the lack of aluminium oxide in this catalyst. It is characteristic that the surface area of the wustite catalyst K0 is half that of the industrial catalyst, though these two catalysts have the same promoter composition. Results from Table 1 indicate, that the change of the specific surface area is caused by the change of the content of structural promoters in a solid solution with the iron oxides [2]. The catalyst K1 may be considered as doublypromoted (with addition of K2 O and MgO), and has the specific surface area 8.7 m2 /g. The other
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Fig. 3. SEM images and EDS mappings of the wustite catalyst K0 (left-hand images) and the wustite Mg doped catalyst K3 (right-hand images).
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doubly-promoted catalyst containing Al2 O3 as the structural promoter (K4) has a surface area of about 15 m2 /g. Considering that these two catalysts have approximately the same molar content of Mg and Al atoms, we find that the wustite catalyst promotion with Al is about twice as effective as promotion with Mg when the surface area development is taken into account. The triply-promoted catalysts containing both of these structural promoters have specific surface areas higher than would be expected from a simple proportion of the contents of these promoters. This suggests the existence of a synergistic effect of the aluminium and magnesium oxides. Another approach would be to focus solely on the aluminium oxide content, as it is the “strongest” structural promoter. We find that increasing the fraction of Al2 O3 (catalysts K2, K3 and K4) has led to the decrease of specific surface area. It is already known that the larger amounts of structural promoters tend to decrease the surface area of magnetite catalysts [12]. This means that the amounts above 2 wt.% of aluminium oxide are too much for wustite catalysts, and the optimal amount of aluminium oxide for the development of surface area of wustite catalyst lies in the vicinity of 1.6 wt.% For the magnetite catalysts, however, the addition of Al2 O3 in amounts up to 12 wt.% results in an increase of the surface area [13]. It is possible that only the HCl-soluble form of Al2 O3 is capable of development of surface area. Meanwhile, the excess aluminium oxide causes its decrease, as the catalyst K2, containing the smallest amount of insoluble Al2 O3 , possesses the highest surface area of all the aluminium-oxide-promoted catalysts. Based on the results of the distribution of Al2 O3 from Table 1, we find that the optimal amount of aluminium oxide for the development of surface area of wustite catalyst should be about 1.3 wt.%, which comprises of 0.3 wt.% dissolved in wustite phase plus about 1 wt.% of HCl-soluble form of Al2 O3 .
4. Conclusions Our results confirm that the magnesium oxide is incorporated uniformly in the wustite phase. The potassium oxide, together with silica, is located in the intergranular spaces of all investigated catalysts. We have found that the Al2 O3 occurs in three forms
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in the wustite catalyst precursor: one form is the solution in the wustite phase, the other two are located in the intergranular spaces of the catalyst. The content of Al2 O3 in the solid solution with wustite phase amounts to about 0.3 wt.% (from selective etching), and does not depend on the quantity of Al2 O3 added nor on the addition of other promoters. Content of HCl-soluble Al2 O3 in the intergranular spaces depends on the addition of CaO. When only traces of CaO are present, the HCl-soluble Al2 O3 exists at 1 wt.% in the wustite catalysts, generally in the form of K2 O · SiO2 · 1/2Al2 O3 and Al2 O3 ·FeO. The remaining pure Al2 O3 and spinel comprise the non-HCl-soluble form of aluminium oxide. The surface area of the catalyst promoted with Al2 O3 is about twice that of the catalyst promoted with MgO. The higher surface areas of catalysts containing both promoters may result from either the synergistic effect of MgO and Al2 O3 , or quite low optimal Al2 O3 content for wustite catalysts. Presuming that only the HCl-soluble Al2 O3 is capable of surface area development, its optimal content for wustite catalysts not containing calcium oxide should be around 1.3 wt.%. References [1] Z. Lendzion-Bielu´n, W. Arabczyk, Appl. Catal. A 207 (1–2) (2001) 37. [2] Z. Lendzion-Bielu´n, W. Arabczyk, M. Figurski, Appl. Catal. A 227 (2002) 255. [3] H.C. Chen, R.B. Anderson, J. Coll. Int. Sci. 38 (2) (1972) 535. [4] H.Z. Liu, X.N. Li, Z.N. Hu, Appl. Catal. A 142 (1996) 209. [5] H.Z. Liu, X.N. Li, Ind. Eng. Chem. Res. 36 (2) (1997) 335. [6] M.E. Dry, J.A.K. Du Plessis, G.M. Leuteritz, J. Catal. 6 (1966) 194. ´ [7] W. Arabczyk, J. Ziebro, K. Kałucki, R. Swierkowski, M. Jakrzewska, Chemik 1 (1996) 22. [8] A. Pattek-Janczyk, B. Miczko, Appl. Catal. A 124 (1995) 253. [9] D. Szczuko, W. Arabczyk, Chemia Analityczna 45 (2000) 273. [10] Z. Kowalczyk, S. Jodzis, Appl. Catal. 58 (1990) 29. [11] M.E. Dry, L.C. Ferreira, J. Catal. 7 (1967) 352. [12] P.H. Emmett, S. Brunauer, J. Am. Chem. Soc. 59 (1937) 1553. [13] L.M. Dmitrienko, Osnovy predvidlenya kataliticheskovo deistvia, Trudy IV Miezhdunarodnovo Kongresa po Katalizu, Nauka 1 (1970) 328.