Investigation of manganese-doped iron ammonia synthesis catalysts

Applied Catalysis A: General 266 (2004) 11–20

Investigation of manganese-doped iron ammonia synthesis catalysts Michał J. Figurski a,∗ , Walerian Arabczyk a , Zofia Lendzion-Bielu´n a , Stanisław Lenart b a

Institute of Chemical Inorganic Technology and Environment Engineering, Technical University of Szczecin, 70-322 Szczecin, ul. Pulaskiego 10, Poland b Institute of Materials Engineering, Technical University of Szczecin, 70-310 Szczecin, al. Piastów 19, Poland Received 17 September 2003; received in revised form 22 January 2004; accepted 27 January 2004 Available online 25 March 2004

Abstract The iron catalysts for ammonia synthesis doped with manganese in amounts up to 3 wt.% has been investigated. The addition of manganese oxide did not influence the distribution of other promoters in the catalyst precursors, manganese oxide itself was entirely incorporated into phases of iron oxides, its concentration in the wustite phase was about twice that in the magnetite phase. At concentrations of manganese below 0.4 wt.% the activity of manganese-doped catalyst was about 10% higher than that of standard industrial catalyst in the temperatures 500 and 450 ◦ C after reduction, and in 400 ◦ C after overheating. At higher concentrations the manganese acted as a catalyst’s poison. The cause for the higher activity of catalysts of low manganese content is surface area development, poisoning effect is due to occupation of active sites by manganese atoms at manganese content exceeding 0.4 wt.%. The corresponding surface coverage calculated according to Langmuir–Maclean’s model of segregation amounted to θ = 0.25. The estimated value of free energy of segregation of manganese in iron in the overheated catalysts amounted to approximately −15 kJ/mol. Before overheating no segregation process was observed. The surface coverage exceeding θ ≈ 0.3 caused formation of a new phase of manganese oxide on the surface of catalyst. Little effect of manganese addition on specific surface area and mean iron crystallite size is observed after reduction. After overheating manganese caused the rise of specific surface area due to higher energy of manganese–oxygen bond than that of iron–oxygen bond. Together with increasing surface area the mean diameter of iron crystallites also increased. © 2004 Elsevier B.V. All rights reserved. Keywords: Iron catalyst; Ammonia synthesis; Manganese; Active surface model

1. Introduction Since the very beginning of the last century the ammonia has been produced utilising the iron ammonia synthesis catalyst with addition of small quantities of promoters. Much work has been done up to this moment to investigate the promoting effect of many metals and metal oxides on iron catalyst. As a result, the optimal promoter composition of that catalyst was established. The industrially utilised iron catalyst usually contains totally about 7 wt.% of aluminium, calcium, silicon and potassium oxides. Some industrial catalysts also contain cobalt [1]. There is, however, a number of elements, whose promoting effect on iron catalyst is still not clearly known—one such element is manganese. ∗ Corresponding author. Present address: Department of Preventive Medicine, Pomeranian Academy of Medicine, ul. Powsta´nc´ow Wielkopolskich 72, 70-111 Szczecin, Poland. Tel.: +48-91-4661631; fax: +48-91-4661628. E-mail address: [email protected] (M.J. Figurski).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.01.032

There are very few reports in the literature, concerning ammonia synthesis on catalysts containing manganese. The works [2,3] indicate that manganese could be a valuable promoter for the iron catalyst. The catalysts fused or impregnated with manganese content up to 1 wt.% were investigated, however, these catalysts were of variable potassium oxide content at the same time. As it is known from other works, the concentration of potassium oxide in the iron catalyst is crucial for its activity in ammonia synthesis [4], so that evidence indicating higher activity of the manganese-doped catalysts is very unclear. The manganese containing iron catalysts for Fisher– Tropsch process were also investigated [5,6]. These works were generally focused on the structure and physical properties of the catalysts. The authors reported that manganese oxide is well soluble in magnetite and causes proportional increase in its lattice constant. In the reduced samples, the rising manganese content causes the increase in the specific surface area and free iron surface area. Based on that results the authors considered the manganese to be a

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medium-effective structural promoter [6]. Though one must take into account that manganese oxide under reducing atmosphere and in temperature of 500 ◦ C might, unlike other structural promoters, partially undergo reduction and incorporate into ␣-Fe phase.

2. Experimental The catalysts used in this study were prepared using the laboratory installation for continuous fusing [7] from industrial catalyst with addition of amounts of manganese dioxide up to 6 wt.% (pure, Riedel de Haen). In order to maintain the Me2+ /Me3+ ratio in the precursors on the same level as in the industrial catalyst, where Me stands for iron and manganese ions, the graphite was added, as a reducing agent. Particularly, the amount of graphite was calculated to reduce the MnO2 to Mn3 O4 with slight excess, so that the ratio of Mn2+ to Mn3+ ions was the same as Fe2+ to Fe3+ ions in substrate industrial catalyst. The mixture of the above ingredients was melted and cooled to the room temperature, then crushed and sieved. Chemical composition of the samples was analysed using ICP-AES method (Jobin Yvon Ultrace 238). The iron oxidation degree of the samples was determined by manganometry, the phase composition was analysed with X-ray diffraction method (Phillips X-pert with cobalt X-ray source). Distribution of the promoters in the investigated samples was determined with selective etching method [8]. In order to confirm the results from selective etching, the EDS mappings were performed utilising Oxford Isis 300 equipment, coupled with Jeol JSM 6100 scanning microscope. The catalytic activity of the investigated samples was determined. The 2 g samples of catalyst grains 1–1.2 mm were primarily reduced with hydrogen–nitrogen mixture (3:1), in the six-channel integral reactor. The space velocity of reducing gas was about 20,000 h−1 , pressure 0.1 MPa, and the temperature program was as follows: 200 ◦ C for 1 h, 350 ◦ C for 2 h, 400 ◦ C for 12 h, 450 ◦ C for 24 h, 500 ◦ C for 24 h. The final stage of reduction was carried over under 10 MPa pressure in temperature 500 ◦ C, until stable level of concentration of output ammonia was reached (usually 3–4 h). The rate of ammonia synthesis reaction was determined subsequently for each catalyst under 10 MPa pressure, in the temperatures 350, 400, 450 and 500 ◦ C. The ammonia concentration on the reactor’s outlet was determined using an interferometer. The rate constant k calculated from the Temkin–Pyzhew equation in its integral form, as given by Kuzniecow et al. [9], was assumed to be a measure of the catalyst activity. Thermostability of the investigated catalysts was evaluated by overheating the catalysts under 0.1 MPa pressure over a period of 17 h at temperature 650 ◦ C [10]. After overheating the complete activity determination test was repeated.

The samples were then handled under inert atmosphere in order to avoid passivation and the surface area was determined for each catalyst with low-temperature nitrogen adsorption method utilising Micromeritics ASAP 2010 equipment. The samples were subsequently passivated by allowing the air to slowly diffuse into sample containers and the XRD patterns of Fe(2 1 1) peak were collected for each sample in order to calculate the mean size of iron crystallites with Scherrer method.

3. Results and discussion The content of manganese oxide in the investigated catalysts is shown in Table 1. It was difficult to determine whether manganese oxide occurs in the fused catalyst in the form of MnO or Mn3 O4 , so that we decided to express its concentration as weight percent of metallic manganese. The composition of other promoters in all the catalysts was as follows: 2.8 ± 0.3 wt.% CaO, 3.3 ± 0.3 wt.% Al2 O3 , 0.70 ± 0.08 wt.% K2 O and 0.36 ± 0.06 wt.% SiO2 . The iron oxidation degree “R” defined as Fe2+ /Fe3+ ratio was in range 0.60–0.66 for all samples. Fig. 1 shows an exemplary XRD spectra of the unreduced catalyst mn-7 containing 2.16 wt.% of manganese. It was impossible to distinguish the phases composed of iron and manganese oxides separately up to the content of 3 wt.% of manganese by means of X-ray diffraction. It indicates that the phases combined of both oxides were formed as previously observed by Dry and Ferreira [6]. The most intensive reflections came from the magnetite-like (Fe, Mn)3 O4 phase. The reflections from wustite-like phase (Fe, Mn)O were also observed, as well as other small reflections on the diffraction pattern, which may come from CaO·FeO and K2 FeO2 . On the diffraction pattern of the reduced sample mn-7 (not shown) only reflections from Fe were observed. Fig. 2 shows the etching relationship common for all the investigated manganese-doped catalysts—data points of all of these catalysts are included in that figure. Basing on that relationship the distributions of promoters in the phases of Table 1 Oxidation degree, manganese oxide content and unit surface activity of the investigated catalysts Sample

Industrial mn-1 mn-2 mn-3 mn-4 mn-5 mn-6 mn-7 mn-8 a

R (−)

0.63 0.66 0.59 0.65 0.66 0.63 0.65 0.60 0.64

Manganese concentration (wt.%) Total

In magnetite

In wustite

0.038 0.092 0.26 0.34 0,41 0.57 1.68 2.16 2.99

0.038 0.088 0.25 0.32 0.39 0.54 1.56 1.99 2.73

0.060 0.14 0.50 0.52 0.62 0.93 2.58 3.90 4.71

Data for overheated catalysts only.

Unit surface activitya (a.u.) 1.00 1.06 1.06 1.08 0.52 0.68 0.45 0.35 0.43

M.J. Figurski et al. / Applied Catalysis A: General 266 (2004) 11–20

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Fig. 1. Diffraction spectra of a sample mn-6. Reflections from magnetite are denoted as “M”; reflections from wustite are denoted as “W”; reflection from CaFeO2 is denoted as “Ca”; and reflection from K2 FeO2 is denoted as “K”.

the precursors of investigated catalysts were evaluated, according to the selective etching method [8]. The promoters not bound with iron, occurring in the glass phase between the grains of iron oxides, undergo dissolution in the initial stage of etching. The potassium oxide in the investigated catalysts is 100% etched for the etching degree of iron below 3%, that means its total amount is present in the intergranular spaces.

About 60% of total content of calcium oxide is present in the intergranular spaces and the remaining amount is dissolved in iron oxides. The stage of etching of CaO from intergranular spaces may also be split into two substages: first up to 2% of iron etching degree, and second in the range of 2–20% of iron etching degree. The first substage is dissolution of CaO contained in the intergranular spaces in the form non-bound with iron—around 20% of total CaO. Second

Fig. 2. The relationship between the manganese, potassium, calcium and aluminium etching degree and the etching degree of iron.

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substage is dissolution of a compound of calcium and iron oxides, probably CaFeO2 , as it was found on XRD pattern, which comprises about 40% of total CaO. Through extrapolation of further two substages of CaO etching line, the values of CaO concentration in magnetite and wustite phase were found. The concentration of CaO amounts to 4 wt.% in the wustite phase and to around 0.4–0.6 wt.% in the magnetite phase, depending upon a sample. Aluminium oxide in about 13% occurs in the intergranular spaces in the form not bound with iron oxides, the rest is dissolved in magnetite. Bending of the Al2 O3 etching line, defined as the intersection point of two straight lines approximating the less and more inclined segments of the etching curve, appears in the investigated catalysts at about 65% of etching degree of iron while in standard catalyst (not containing manganese) it occurs at about 50%. This point might be interpreted as an endpoint of etching of wustite [8,11]. For comparison in the industrial catalyst there is 64% of CaO and 9% of Al2 O3 located in the intergranular spaces. The CaO concentration in the wustite phase is 4.9 wt.%, the aluminium oxide concentration in this phase is negligible. In the magnetite phase the concentration of CaO is 0.63 wt.% and the rest of Al2 O3 is located there [8]. From the above,

one can see that the distribution of promoters in the standard industrial catalyst is similar to that in manganese-doped catalysts, the existing differences are within range of experimental error and may occur due to variations of R among investigated samples. This shows that manganese does not compete with other promoters, moreover its distribution is identical to iron, thus it cannot be considered merely a structural promoter, as the works of Dry et al. [5,6] suggested. Addition of manganese does not influence the distribution of other promoters, though it affects the kinetics of wustite phase dissolution. On the EDS images shown in Fig. 3 there are iron oxide grains visible: the big grain is about 300 ␮m in diameter, the small grains are about 10 ␮m in diameter. The areas between the iron oxide grains are occupied generally by calcium—shown on the mapping image, potassium oxide and silica, as it was previously reported by Chen and Anderson [12]. The concentration of manganese is obviously diversified: in the small grains of iron oxides it is higher, and in the big one it is smaller. Pointwise investigation of the chemical composition of the big and small grain revealed that the big grain (point 1) contains dissolved Al2 O3 , so it is the magnetite grain (see Table 2, points indicated in Fig. 3),

Fig. 3. SEM image and EDS mappings of Fe, Mn and Ca for a sample mn-8. Crosses with numbers on the SEM image indicate pointwise analysis.

M.J. Figurski et al. / Applied Catalysis A: General 266 (2004) 11–20 Table 2 Results of pointwise EDS analysis of the sample mn-8 (in wt.%), points indicated in Fig. 3 Element

Point 1

Point 2

Al2 O3 CaO Mn3 O4 Iron oxides

3.10 0.64 4.81 91.44

0.00 1.38 7.99 90.64

and the small grain (point 2) contains dissolved CaO and no dissolved Al2 O3 , so it is the wustite grain. The manganese concentration is about two-fold higher in the wustite than in the magnetite grain. The results of calculation of manganese oxide concentration in the phases of the catalyst precursors based on the etching data are shown in Table 1. The chemical composition of the active phase of catalysts was not investigated, although it is known that the reduction process does not affect the promoters of very high enthalpies of formation such as oxides of calcium and aluminium. XRD investigation of the reduced sample mn-7 showed no presence of iron/manganese oxides. Results of the catalytic activity investigation are shown in Figs. 4 and 5 in the form of relationships of relative activity versus the content of manganese after reduction and after overheating, respectively. The relative activity is a ratio of the value of rate constant k of a particular catalyst to the value of rate constant k0 of reference catalyst at each temperature. In Fig. 4 in the temperatures between 500 and 450 ◦ C the relative activities of manganese-doped catalysts are higher than that of industrial catalyst if the manganese content does not exceed around 0.4 wt.%. At 400 ◦ C the relative activity of investigated catalysts decreases monotonously, starting from the smallest additions of manganese. Catalysts of the manganese content in range 0.04–0.4 wt.% possess also

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higher apparent activation energy (around 180 kJ/mol) compared to the industrial catalyst (165 kJ/mol). The relative activities of the overheated samples in relation to the manganese content are shown in Fig. 5. The relative activity decreases monotonously with addition of manganese in 500 ◦ C, though at 450 ◦ C and mostly at 400 ◦ C it is higher than 1 in the range 0.038–0.4 wt.% of manganese. That is a change compared to before overheating, when these catalysts were most active in high temperatures. The altered temperature pattern of activities indicates also that these catalysts after overheating possess lower apparent activation energy than the standard one—it amounted to about 170 kJ/mol. The specific surface area and the mean iron crystallite diameter are shown in Fig. 6 related to the content of manganese—solid lines represent data after reduction, dashed lines represent data after overheating. Right after reduction of catalysts only little influence of manganese addition is observed, both in case of the specific surface area and mean iron crystallite diameter. After overheating in the range of manganese content below 1 wt.% the increase in surface area is observed, then the specific surface area reaches an approximately steady level. The mean iron crystallite diameter increases linearly with increasing content of manganese. These results indicate that the standard reduction procedure may be insufficient for complete reduction of manganese oxide and a higher reduction temperature is required for manganese to influence the surface area and mean iron crystallite size. Our results after overheating are consistent with results of Dry et al. [5]. In the double-layer model of catalyst’s active surface [13] it was assumed that the quantity of specific surface area of the catalyst is proportional to amount of oxygen adsorbed on that surface, or more precisely: to the overall energy of bonding of the oxygen atoms to atoms of iron on the surface

Fig. 4. The relationship of the relative activity in 500, 450 and 400 ◦ C to the manganese content in the catalysts—after reduction.

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Fig. 5. The relationship of the relative activity in 500, 450 and 400 ◦ C to the manganese content in the catalysts—after overheating.

of catalyst. So that one can influence the quantity of specific surface area by replacing the iron atoms with atoms of higher oxygen binding energy, for example manganese. Hence the manganese-doped catalyst should possess larger surface area. The experimental results of overheated catalysts presented above indicate that indeed the addition of manganese caused the rise of specific surface area, even despite that the mean size of iron crystallites has also risen with manganese content. That is very unusual, as the opposite relationship was expected. Fig. 7 shows that the specific surface area rises proportionally to the rise of mean crystallite diameter. The possible explanation of this is that the addition of manganese causes the increase in the roughness of surface and therefore allows for surface area development

in spite of increasing crystallite size. Note that this relationship was observed for overheated catalysts only. By meaning the relative activities of the respective catalyst in all temperatures the value of mean relative activity for that catalyst was obtained. The ratio of mean relative activity and the specific surface area of a catalyst gives the unit surface activity of the catalyst—it is almost equal for the samples of manganese content below 0.4 wt.% (see Table 1). The unit surface activity may be considered as a measure of amount of active sites per square meter of catalyst’s surface. This means that the rise of catalytic activity is connected exclusively with the development of surface area, as the concentration of active sites on the surface of these catalysts is about constant. That would suggest the role of manganese

Fig. 6. The relationship of specific surface area and mean crystallite size to the manganese content in the catalysts.

M.J. Figurski et al. / Applied Catalysis A: General 266 (2004) 11–20

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Fig. 7. Relationship between the surface area and the mean iron crystallite size.

is structural promotion. For the catalysts containing more than 0.4 wt.% of manganese the unit surface activity drops down significantly—this indicates that the number of active sites has decreased. The manganese at concentrations above 0.4 wt.% exhibited a “poisoning” effect on the reduced catalysts. According to the double-layer model, larger contents of manganese may lead to decrease in catalytic activity due to occupation of active sites by manganese and/or oxygen atoms. If the concentration of manganese exceeds a specific value, the manganese or manganese oxide particles start to occupy the iron positions in the active sites which cause the observed decrease in the concentration of active sites. Along the way presented in [14] we have assumed that the decrease in activity of a poisoned catalyst is proportional to decrease in number of active sites due to poisoning. The activity measurements therefore allows for intermediate estimation of concentration of poison atoms on the surface of the catalyst. In these calculations we used surface-area-weighted activity instead of measured mass-weighted activity in order to eliminate the influence of changing surface area on result of calculations. Under assumption that one manganese/manganese oxide particle blocks one active site, the surface coverage θ could be calculated using the equation: θ =1−

k S0 , k0 S

where k and S represent, respectively, the relative activity and specific surface area of catalysts, while index “0” denotes the reference catalyst. The amount of poisoning atoms on iron surface is most probably controlled by the segregation equilibrium, so that the values of θ calculated for each catalyst and in each temperature were used to assess

the value of G (free energy of segregation) of manganese, according to Langmuir–Maclean’s equation in the form  

−G 1 xb θ . = + ln ln 1−θ R T 1 − xb The values of G were evaluated from the slopes of lines on the Arrhenius plot of the three catalysts of highest manganese content (mn-6, mn-7 and mn-8), and amounted to around 0 kJ/mol after reduction and about −15 kJ/mol after overheating. The values of xb in the above calculations were assumed to be constant, as the bulk content of manganese in the catalysts of concern was high enough to remain virtually unaffected by change of surface coverage. This result confirms that the mechanism of manganese action is different before and after overheating. Further calculations were performed for overheated catalysts only. Using the iterative numeric method and equal G = −15 kJ/mol for all overheated catalyst the values of θ were calculated at the temperatures in range 350–500 ◦ C from the Langmuir–Maclean’s equation. The value of xb was calculated using the equation given below xb =

[Mn] − (12 × 1018 · SBET · θ −1 N) , [Fe] + [Mn]

where [Mn] and [Fe] are the total molar concentrations of respective elements, 12 × 1018 is the mean number of iron atoms on the most common plane (1 0 0) in the catalyst, SBET is the measured specific surface area, θ is the surface coverage and N is the Avogadro’s number. The starting condition for xb = 0.99 was chosen. Results of calculations in the form of relationship between the calculated θ and relative activity are shown in Fig. 8. The shape of the above relationship can be better explained on a model relationship, based on the double-layer

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Fig. 8. The relative activity vs. the surface coverage by manganese of the manganese-doped catalysts.

active surface model by Arabczyk et al. [13]. The proposed model relationship between relative activity and surface coverage by any segregating species is shown in Fig. 9. We assumed that the surface coverage value can exceed 1 if more than one layer is formed on the surface of catalyst.

Assuming that the segregating manganese occurs in the catalyst in two phases only, i.e. in the bulk of ␣-Fe grain and in the two-dimensional layer on the surface of that grain the increasing overall content of manganese would cause the increase in both θ and xb up to the moment of reaching

Fig. 9. The model relationship between activity and surface coverage.

M.J. Figurski et al. / Applied Catalysis A: General 266 (2004) 11–20

the permanent surface coverage, θ = 1. Then only the bulk content of manganese would increase causing no further change in surface coverage. The model change of activity of such catalyst is represented by the solid line in Fig. 9, under assumption that one atom of manganese replaces one iron atom. Therefore the optimal concentration of manganese should be 50% of surface coverage, causing the increase in surface area but occupying no active sites. Further increase in surface coverage would cause the activity to fall to almost zero because of decrease in concentration of active sites. Another possibility is that there occurs a process of formation of another (third) phase at higher manganese concentrations. This implies that the surface coverage by manganese is limited to a certain level above which the new phase starts to exist. The balance between the bulk content and surface coverage is still determined by the free energy of segregation, according to Langmuir–Maclean’s model, though the excess manganese is absorbed to form a three-dimensional structures on the surface of iron. These structures occupy little of surface area, thus the activity at high manganese contents reaches an approximately steady level. The model change of activity in this case is depicted in Fig. 9 by the dashed line. The formation of the new phase on the surface of iron most probably starts through the nucleation process. For catalysts of manganese content within a certain range it is possible that this process starts or not due to many factors, such as reduction conditions, local overheating during reduction, local content of other promoters and many others. Therefore, the catalysts of manganese concentration within that special range may exhibit unusual or “discontinuous” relationship between manganese content and activity. This situation is demonstrated on the model relationship from Fig. 9 by the dotted line. We claim that the real shape of this re-

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lationship should be the combination of the three mentioned lines in their respective areas of manganese concentration. It is possible to distinguish the three model areas in Fig. 8, though the values of surface coverage are different than that on the model relationship. The combination of Fig. 8 and adjusted Fig. 9 is shown in Fig. 10. The measurements underlying the calculated data presented in the above figures are not of very high precision, though they allow for general estimation of some values. The limiting value for the solid line from Fig. 9, representing two-phase segregation relationship is approximately 0.25—the respective value on the model relationship was 1, which suggest that one manganese or manganese oxide particle poisons four active sites on the surface of catalyst. The surface coverage optimal for activity seems to be in range of θ from 0.05 to 0.2, approximately 0.13, which is about half of the limiting value, θ = 0.25. The area of discontinuity extends from θ approximately 0.2 to about 0.35, and the steady level of catalyst’s activity is about 40% of the activity of reference catalyst. Fig. 11 shows a proposed model based on the double-layer model of catalysts active surface of the standard catalyst and manganese-doped one, in which the manganese concentration is in the range 0.13–0.3θ and some manganese atoms block active sites. This model is intended to be valid for overheated manganese-doped catalysts, where the total amount of manganese oxide is reduced. The oxygen atoms remaining after reduction in the form of K–O groups [15] are “shared” among the manganese and iron atoms in the crystallite, though the probability of occurrence of oxygen bound with manganese atom is significantly higher, as the enthalpy of formation of iron oxides is about 100 kJ/mol lower than that of manganese oxides. So that there would

Fig. 10. Interpretation of the real measured data with model from Fig. 10.

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Fig. 11. Model of active surface of standard and manganese-doped catalyst.

be manganese atoms binding K–O groups and therefore affecting the quantity of surface area.

4. Conclusions The addition of manganese oxide did not influence the distributions of other promoters in the iron catalyst for ammonia synthesis. The manganese oxide distribution in iron oxides was not uniform, and it occurred in the wustite phase in higher concentration than in the magnetite phase. Manganese oxide promotion affects the activity of catalysts both after reduction and after overheating, though higher reduction temperature (overheating) is necessary for manganese to affect the quantity of surface area and the mean iron crystallite size. The process of manganese segregation was also observed after overheating only. The cause for observed rise of relative activity of catalysts of manganese content below 0.4 wt.%, is most probably the development of surface area. The manganese may be therefore considered as a structural promoter. The cause for decrease in relative activity of catalysts of higher manganese content was occupation of the active sites on the surface of the catalyst, so that manganese acted as a catalyst’s poison. A model of active surface of manganese-doped catalysts was proposed to explain the observed phenomena, where the surface coverage by manganese determines the behaviour of catalyst. If it is below approximately 0.13 the manganese atoms occupy the positions of iron in the Fe–O–K complexes on the surface thus leading to increase in surface area, but does not cause the poisoning effect. If surface coverage is above 0.13, the manganese occupy the positions of iron in the active sites causing decrease in activity. At surface coverages higher than 0.3 the formation of a new phase on the catalyst’s surface starts.

In the investigated range of concentrations the manganese caused the increase in both the surface area and mean iron crystallite size of overheated catalysts, which is a relationship unique for this kind of catalysts. It is possible that this effect is caused by roughening of surface of crystallites of growing size caused by addition of manganese. Acknowledgements We wish to gratefully acknowledge the financial support of Polish Research Committee under grant no. 3T09B 03719, in the years 2000–2003. References [1] U. Zordi, A. Antonioni, Nitrogen 122 (1977) 33. [2] X.X. Guan, C.L. Qin, Y.H. Liu, C.F. Zhang, W.X. Wang, Gongye Cuihua 6 (4) (1998) 33. [3] Y.F. Shen, K.H. Huang, React. Kinet. Catal. Lett. 46 (1) (1992) 115. [4] Z. Kowalczyk, S. Jodzis, Przemysł Chemiczny 66 (6) (1987) 279. [5] M.E. Dry, J.A.K. Du Plessis, G.M. Leuteritz, J. Catal. 6 (1966) 194. [6] M.E. Dry, L.C. Ferreira, J. Catal. 7 (1967) 352. ´ [7] W. Arabczyk, J. Ziebro, K. Kałucki, R. Swierkowski, M. Jakrzewska, Chemik 1 (1996) 22. [8] Z. Lendzion-Bielu´n, W. Arabczyk, Appl. Catal. A 207 (1–2) (2001) 37. [9] L.D. Kuzniecow, L.M. Dmitrienko, P.D. Rabina, U.A. Sokolinski, Sintiez Ammiaka, Chimia, Moscow, 1982. [10] Z. Kowalczyk, S. Jodzis, Appl. Catal. 58 (1990) 29. [11] Z. Lendzion-Bielu´n, W. Arabczyk, M. Figurski, Appl. Catal. A 227 (2002) 255. [12] H.C. Chen, R.B. Anderson, J. Catal. 28 (1973) 161. [13] W. Arabczyk, U. Narkiewicz, D. Moszy´nski, Langmuir 15 (18) (1999) 5785. [14] W. Arabczyk, U. Narkiewicz, Appl. Surf. Sci. 196 (2002) 423. [15] W. Arabczyk, U. Narkiewicz, K. Kałucki, Vacuum 45 (2–3) (1994) 267.