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oxygen layer on properties of iron surfaces
Applied Catalysis A: General 182 (1999) 379±384
In¯uence of potassium/oxygen layer on properties of iron surfaces W. Arabczyk, U. Narkiewicz, D. MoszynÂski* Institute of Inorganic Chemical Technology, Technical University of Szczecin, Puøaskiego 10, 70-322, Szczecin, Poland Received 30 July 1998; received in revised form 27 January 1999; accepted 28 January 1999
Abstract In¯uence of oxygen concentration in the iron bulk on desorption of potassium from the iron surface has been studied. Desorption of potassium from the iron surface, clean and precovered with oxygen, has been examined. It has been found that the higher the oxygen concentration in the bulk the higher the temperature required to decompose the potassium/oxygen layer existing on the iron surface. Suf®cient oxygen concentration in the iron bulk makes potassium/oxygen layer to be stable on iron surface even at the temperature of ammonia synthesis. Diffusion of promoters from the surface of the iron catalyst onto a clean iron foil has also been studied. Only potassium tends to diffuse from the iron catalyst to the clean surface of iron. Considering the results of those experiments, model of the active surface of iron catalyst has been proposed. An adlayer consisting of equal amount of potassium and oxygen atoms is formed on the iron surface. The free sites for dinitrogen adsorption and ammonia formation are located under potassium layer in voids in the oxygen layer. Oxygen atoms bridge iron and potassium atoms and increase thermal stability of potassium on the iron surface. Based on that model, in¯uence of potassium on ammonia decomposition has been interpreted. Some other catalyst properties have been derived from the model. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Iron catalyst; Potassium; Desorption; Ammonia
1. Introduction The industrial process of ammonia synthesis from elements was developed more than 80 years ago. The catalyst used in this process (fused and reduces iron oxides with addition of potassium, calcium and aluminium oxides) has remained almost unchanged since then [1]. Numerous studies carried out during these years helped to answer many questions but many of them are still unanswered. Some results and conclusions are still incomplete or controversial. *Corresponding author. Fax: +48 91 433 0352; e-mail: [email protected]
The dissociative chemisorption of dinitrogen is known as a rate-limiting step in ammonia synthesis [2]. Dinitrogen adsorption [3] as well as ammonia synthesis [4] on the clean iron surfaces are structuresensitive reactions, for example, dinitrogen dissociation on Fe(1 1 1) is about 60 times faster than on Fe (1 0 0). Adsorbed potassium signi®cantly changes properties of the iron surface and in¯uences its activity. After potassium adsorption the difference in activity observed for potassium-free surfaces is eliminated [5]. The active phase of the iron catalyst is metallic iron covered with KO layer. Ratio of potassium to oxygen atoms was found to be 1:1 [6]. Strongin and
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(99)00034-4
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Somorjai [7] found only small amount of potassium and oxygen after ammonia synthesis on iron single crystals though the iron surface had been precovered with potassium monolayer. However, TPD studies showed that potassium completely disappears from the iron surface not before the temperature of 900 K [8,9]. A simple model of the iron catalyst surface was proposed earlier [10]. It is assumed that potassium atoms are adsorbed on the iron surface and the active sites capable to adsorb non-metal atoms are located between potassium and iron atoms. Part of these sites are occupied by oxygen atoms which increase the thermal stability of potassium on the catalyst surface. Under ammonia synthesis conditions voids existing in oxygen layer are occupied by nitrogen atoms and ammonia synthesis process performs on them. This paper is intended to explain the discrepancies between reported experimental results on behaviour of potassium on the iron surface. An attempt to enhance previous model of the surface of the iron catalyst has also been made. Based on the model, decomposition of ammonia over the iron catalyst is interpreted. 2. Experimental 2.1. Potassium desorption Studies of adsorption and desorption of potassium and oxygen on the surface of a single crystal Fe(1 1 1) were carried out in the UHV chamber. A base pressure was maintained below 10ÿ8 Pa. Composition of surface was examined by Auger electron spectroscopy (AES). Auger spectra were acquired using CMA analyser. The spectrometer was operated in the d(N(E))/dE mode. Primary electron energy of 2.5 keV and primary beam current ± 10 mA was used. Modulation amplitude was 4 V peak-to-peak. Oxygen adsorption was performed at a pressure of 10ÿ6 Pa. Potassium was produced by heating a zeolite K ion emitter. During experiments a sample with various oxygen concentration in the iron bulk should have been obtained. Low oxygen concentration in the bulk was achieved by subsequent sample heating followed by Ar bombardment. Fe(1 1 1) sample was then enriched with oxygen by subsequent oxygen adsorp-
Fig. 1. Placement of the iron foil in the reactor and catalyst bed.
tion followed by heating at the temperature of 520 K. With growing oxygen concentration in the bulk the temperature should have been gradually increased up to 670 K. 2.2. Promoters surface diffusion The experiment on promoters diffusion during catalyst reduction was carried out in a six-channel integral reactor described elsewhere [11]. A sample of a thin (0.1 mm) pure polycrystalline iron foil was put in a bed of an industrial catalyst so as half of it remained without contact with catalyst grains (Fig. 1). The iron catalyst was reduced under atmospheric pressure in the ¯ow of stoichiometric hydrogen±nitrogen mixture. The temperature was gradually increased up to 770 K. Then the ammonia synthesis process was performed for 40 h under the pressure of 10 MPa and at the temperature of 720 K. Afterwards the catalyst was cooled down and passivated and the iron foil was transferred into UHV chamber. Surface composition was examined by Auger electron spectroscopy. 2.3. Ammonia decomposition The reduced industrial catalyst PS3-INS containing: 3.3 wt% Al2O3, 3.2 wt% CaO and 0.8 wt% K2O was used in the ammonia decomposition experiment. Catalytic activity was measured for the unmodi®ed catalyst and for the catalyst from which potassium compounds were washed out by hot water. After washing, potassium content decreased to 0.05 wt% K2O. Contents of calcium and aluminium compounds remained unchanged. Kinetic measurements of
W. Arabczyk et al. / Applied Catalysis A: General 182 (1999) 379±384
ammonia decomposition were carried out in a reactor described elsewhere [12]. It was performed under atmospheric pressure and in the temperature range between 620 and 770 K. 3. Results and discussion Potassium was adsorbed on the clean Fe(1 1 1) sample with low oxygen concentration in the bulk. The surface is saturated with potassium when 3.81018 mÿ2 of atoms is adsorbed which is in agreement with the results obtained previously [8]. Heating of the sample leads to the complete desorption of potassium at the temperature of 470 K (Fig. 2, curve (a)). This observation indicates that potassium could not occur in the metallic form on the catalyst surface under ammonia conditions. Potassium adsorption on the sample precovered with oxygen (Fig. 2, curve (b)) shows that maximal number of potassium atoms in monolayer increases from 3.81018 to 5.61018 mÿ2. It is due to interactions between potassium and oxygen which leads to the reduction of potassium ion radius. Presence of oxygen atoms also increases the thermal stability of potassium on the iron surface. Its total desorption is observed at 550 K. At the ®rst stage of heating only oxygen concentration decreases until the ratio of nK/nO reaches 1:1. Then, with raising temperature, oxygen and potassium disappear simultaneously (Fig. 2, curves (b) and (b0 )). Oxygen
Fig. 2. Desorption curves of potassium and oxygen (with primes) measured by AES: (a) low oxygen concentration in the iron lattice and no detectable oxygen on the surface; (b) and (b0 ) low oxygen concentration in the iron lattice and oxygen on the surface; (c) and (c0 ) high oxygen concentration in the iron lattice and oxygen on the surface.
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dissolves in the iron lattice and potassium atoms desorb from the surface. Potassium adsorption/desorption cycles were performed on the iron sample which bulk was gradually enriched with oxygen (only one experiment is shown in Fig. 2). There is no signi®cant difference between potassium adsorption on the samples with various oxygen concentration in the bulk. However, the differences during desorption process are observed. Similarly to the curve (b) at the ®rst stage of heating, an excess of oxygen is removed from the iron surface. The ratio of nK/nO reaches 1:1 at higher temperature (about 600 K). Then potassium and oxygen disappear simultaneously (curve (c) and (c0 )). Potassium is completely removed from the iron surface at the temperature of 900 K. Comparison of the experiments performed over samples with low and high oxygen concentration in the bulk suggests that the temperature of thermal decomposition of the KO layer existing on the iron surface considerably increases with an increase of oxygen concentration in the bulk. It is probably due to dissolution of oxygen atoms in the iron lattice. Low oxygen concentration in iron favours the diffusion of surface oxygen into iron lattice which leads to depletion of surface from oxygen and consequently causes potassium desorption. On the sample containing more dissolved oxygen the diffusion of surface oxygen is much slower and requires higher temperature. In that case potassium can exist on the iron surface at higher temperature. Those conclusions enable one to explain a split of opinions existing in the literature. Strongin and Somorjai [7] report that after their experiments of ammonia synthesis over iron single crystals, potassium concentration on the surface is only 0.15 ML. The rest is clean iron surface. Other papers point out that in the industrial catalyst 90±96% of the iron surface is covered with promoters [13,14], mainly by potassium. This controversy probably arises from different samples preparation. UHV experiments are usually carried out over very pure single crystal. The concentration of oxygen in its bulk is usually very low and the surface is also cleaned very carefully. During experiments carried out under ammonia synthesis conditions, it leads to complete desorption of potassium due to lack of `oxygen anchors' and obtained results are similar to those observed for a clean iron
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surface. On the other hand industrial catalysts even in reduced form always contain some amount of oxygen. It prevents the complete potassium removal. The studies of reduction of the iron catalyst show that potassium, which is initially located on the grain boundaries, diffuses to the iron crystallites surface [15]. Aluminium and calcium which are also contained in the catalyst could also cover the surface of iron crystallites. There is a question: Is any of promoters privileged in any way and cover the surface mostly? To answer this question the experiment on diffusion of promoters from the catalyst onto iron foil was carried out. The clean iron foil was intended to simulate a newly emerging (during reduction process) iron surface and catalyst grains play a role similar to that of grain boundaries during real reduction process. Prolonged heating of the iron foil together with the catalyst should have induced a diffusion of promoters onto clean foil. However, neither calcium nor aluminium was observed during AES measurements. Entire surface was covered with the layer consisting of potassium and oxygen though only half of the sample was in direct contact with catalyst grains. This effect and lack of aluminium and calcium on the surface suggest that in this system potassium has the strongest tendency to cover the iron catalyst surface. The results presented above allow to expand the model of the catalyst surface described in the introduction. Mentioned model proposes an existence of a potassium layer over a layer of oxygen atoms and indicates that voids in the oxygen layer are free sites capable to adsorb dinitrogen molecules. The model does not describe the way leading to the formation of this active layer and does not enable to obtain quantitative description of surface structures. New model assumes that during reduction of the iron catalyst potassium diffuses from grain boundaries onto iron crystallites surface. Probably it leads to the partial or total removal of aluminium and calcium from that surface. Afterwards the equilibrium between potassium and oxygen is achieved when the ratio nK/ nO reaches 1:1. Taking into account that during reduction there is an excess of oxygen atoms, it could be assumed that iron surface could be covered with potassium in maximal possible degree. It was earlier shown that maximal number of potassium atoms on the iron surface precovered with
oxygen is 5.61018 mÿ2. Assuming the nK/nO ratio as 1:1 the number of 5.61018 mÿ2 oxygen atoms is required to stabilise potassium atoms. The maximal number of oxygen atoms adsorbed in the monolayer on Fe (1 1 1) and Fe(1 0 0) is found to be 141018 and 121018 mÿ2, respectively [16,17]. Those numbers are considered as a maximal number of adsites on those surfaces in two-dimensional structure. According to the new model, potassium does not occupy the adsites in the oxygen layer but is located over oxygen atoms so as the number of voids in the oxygen layer can be calculated as 14ÿ5.8)8.2 1018 mÿ2 for Fe(1 1 1) and 12ÿ5.8)6.21018 mÿ2 for Fe(1 0 0). It was earlier described that dinitrogen molecules can adsorb on the free adsites under potassium layer [10]. The number of free adsites determines some properties of the iron catalyst. The active surface could be taken as a ratio of the number of free sites (8.21018 mÿ2 for Fe(1 1 1) and 6.21018 mÿ2 for Fe(1 0 0)) to the total number of adsites (141018 mÿ2 for Fe(1 1 1) and 121018 mÿ2 for Fe(1 0 0)) and it gives 8.2/1458% and 6.2/1252%, respectively. The active area of industrial iron catalyst is measured by CO chemisorption. CO and N2 are isoelectronic molecules. Both adsorb only on free iron atoms. This area for typical industrial iron catalysts is in the range of 40±60% [18,19] and is in line with the numbers calculated from the model. It is assumed that there is an optimal amount of potassium needed to form active surface. This amount is connected with the surface area of catalyst. Typical surface area of the industrial iron catalysts is about 10 m2/g [20] and in that case the potassium oxide concentration, calculated from the model, is 0.47 wt%. Because about 10% of potassium contained in the catalyst is permanently bound with other promoters [21] and does not take part in the formation of active surface [15] this number can be increased to 0.52 wt%. This number is in good agreement with results of the studies of in¯uence of potassium concentration in the catalyst on its activity carried out by Kowalczyk [22] and presented in Fig. 3. It shows that optimal concentration of potassium is about 0.5 wt%. Typical industrial catalysts contain 0.5±0.8 wt% of K2O. On the basis of present model some kinetic results can be explained. It was earlier found that potassium impregnation of the iron catalyst leads to decrease in
W. Arabczyk et al. / Applied Catalysis A: General 182 (1999) 379±384
Fig. 3. Dependence of the catalytic activity of the fused iron catalyst on the potassium concentration [22].
the rate of ammonia decomposition [23]. In our experiments, ammonia decomposition was carried out over typical iron catalyst and over this catalyst without potassium. The simple Arrhenius plot (Fig. 4) shows that decomposition rate is about four times higher for catalyst without potassium though apparent activation energy for these cases differ less than 10%. It suggests that the main reason is the change of preexponential factor of reaction rate equation.
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Fig. 5. Top-view of the Fe(1 0 0) surface covered with oxygen and potassium according to the model.
This effect could be explained in the light of the present model. The preexponential factor is in¯uenced, among others, by surface structure. The surface of the iron catalyst is covered by potassium oxide and only in the voids in the oxygen layer free iron atoms can adsorb ammonia molecules. Ammonia dissociation leads to the formation of adsorbed nitrogen atoms, localised under potassium atoms. With the statistical presentation of the surface (Fig. 5) it can be expected
Fig. 4. The dependance of ammonia decomposition rate on the temperature for the catalyst with potassium and catalyst without potassium.
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that each adsorbed nitrogen atom is surrounded by oxygen and potassium atoms. The diffusion of nitrogen atoms under the monolayer of potassium atoms with regard to the geometrical considerations resembles the diffusion in the solid state. It can be expected that it will be signi®cantly slower in comparison with diffusion proceeding on the clean surface, therefore, the number of collisions leading to the formation of dinitrogen molecules decreases. On the surface free of potassium, this effect will be eliminated. 4. Conclusions The results presented in this paper suggest signi®cant in¯uence of oxygen concentration in the bulk on the temperature of decomposition of KO layer existing on the iron surface and consequently on the desorption of potassium. High concentration of oxygen in iron bulk makes possible that stable form of potassium exists on the iron surface even under ammonia synthesis conditions. It was also found that potassium has the tendency to cover the iron surface. It leads to the formation of potassium/oxygen layer even in presence of other promoters like calcium or aluminium. The model of the active surface of the iron catalyst was proposed. Potassium atoms are bound to iron surface through oxygen bridges which increases their thermal stability. There are voids in the oxygen layer which play as free sites for dinitrogen or ammonia adsorption. Based on the model, the value of the active surface (about 58%) and optimal concentration of potassium oxide in the catalyst (about 0.5 wt%) was calculated. Those values are in good agreement with experimental results. The in¯uence of potassium on the rate of ammonia decomposition is due to geometrical obstacles build from oxygen and potassium atoms. Those structures
block the recombination of dinitrogen molecules and it lowers overall rate of ammonia decomposition. References [1] K. Tamaru, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Fundamental and Practice, Plenum Press, New York, 1991. [2] A. Ozaki, H. Taylor, M. Boudart, Kinetics and mechanism of the ammonia synthesis, Proc. R. Soc. London, A 258 (1960) 47. [3] F. Bozso, G. Ertl, M. Grunze, M. Weiss, J. Catal. 49 (1977) 18. [4] D.R. Strongin, J. Carrazza, S.R. Bare, G.A. Somorjai, J. Catal. 103 (1987) 213. [5] G. Ertl, S.B. Lee, M. Weiss, Surf. Sci. 114 (1982) 527. [6] W. Arabczyk, K. Kaøucki, Proceedings of the 10th International Congress of Catalysis, Part C, Budapest, 1992, p. 2539. [7] D.R. Strongin, G.A. Somorjai, J. Catal. 109 (1988) 51. [8] S.B. Lee, M. Weiss, G. Ertl, Surf. Sci. 108 (1981) 357. [9] Z. PaaÂl, G. Ertl, S.B. Lee, Appl. Surf. Sci. 8 (1981) 231. [10] W. Arabczyk, U. Narkiewicz, K. Kaøucki, Vacuum 45(2±3) (1994) 267. [11] R.J. KalenÂczuk, J. Chem. Tech. Biotechnol. 59 (1994) 73. [12] K. Kaøucki, R.J. KalenÂczuk, W. Arabczyk, Z. SÂpiewak, Przem. Chem. 65 (1986) 532. [13] D.C. Silverman, M. Boudart, J. Catal. 77 (1982) 208. [14] G. Ertl, D. Prigge, R. SchloÈgl, M. Weiss, J. Catal. 79 (1983) 359. [15] P.D. Rabina, T.Y. Malysheva, L.D. Kuznetsov, V.A. Batyrev, Kinet. Katal. 11 (1970) 1243. [16] S. Nakanishi, T. Horiguchi, Proceedings of the Seventh International Vacuum Congress and Third International Conference on Solid Surfaces, p. A2727, 1977. [17] K.O. Legg, F. Jona, D.W. Jepsen, P.M. Marcus, Phys. Rev. B 16 (1977) 5271. [18] R. Brill, J. Catal. 19 (1970) 236. [19] V. Solbakken, A. Solbakken, P.H. Emmet, J. Catal. 15 (1969) 90. [20] D.R. Strongin, G.A. Somorjai, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Fundamental and Practice, Plenum Press, New York, 1991. [21] K. Kaøucki, W. Arabczyk, Z. Janecki, K. Stoøecki, I Kongres Technologii Chemicznej, Szczecin, 1995, p. 466. [22] Z. Kowalczyk, S. Jodzis, Przem. Chem. 66 (1987) 279. [23] K.S. Love, P.H. Emmet, J. Am. Chem. Soc. 63 (1941) 3297.
In¯uence of potassium/oxygen layer on properties of iron surfaces W. Arabczyk, U. Narkiewicz, D. MoszynÂski* Institute of Inorganic Chemical Technology, Technical University of Szczecin, Puøaskiego 10, 70-322, Szczecin, Poland Received 30 July 1998; received in revised form 27 January 1999; accepted 28 January 1999
Abstract In¯uence of oxygen concentration in the iron bulk on desorption of potassium from the iron surface has been studied. Desorption of potassium from the iron surface, clean and precovered with oxygen, has been examined. It has been found that the higher the oxygen concentration in the bulk the higher the temperature required to decompose the potassium/oxygen layer existing on the iron surface. Suf®cient oxygen concentration in the iron bulk makes potassium/oxygen layer to be stable on iron surface even at the temperature of ammonia synthesis. Diffusion of promoters from the surface of the iron catalyst onto a clean iron foil has also been studied. Only potassium tends to diffuse from the iron catalyst to the clean surface of iron. Considering the results of those experiments, model of the active surface of iron catalyst has been proposed. An adlayer consisting of equal amount of potassium and oxygen atoms is formed on the iron surface. The free sites for dinitrogen adsorption and ammonia formation are located under potassium layer in voids in the oxygen layer. Oxygen atoms bridge iron and potassium atoms and increase thermal stability of potassium on the iron surface. Based on that model, in¯uence of potassium on ammonia decomposition has been interpreted. Some other catalyst properties have been derived from the model. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Iron catalyst; Potassium; Desorption; Ammonia
1. Introduction The industrial process of ammonia synthesis from elements was developed more than 80 years ago. The catalyst used in this process (fused and reduces iron oxides with addition of potassium, calcium and aluminium oxides) has remained almost unchanged since then [1]. Numerous studies carried out during these years helped to answer many questions but many of them are still unanswered. Some results and conclusions are still incomplete or controversial. *Corresponding author. Fax: +48 91 433 0352; e-mail: [email protected]
The dissociative chemisorption of dinitrogen is known as a rate-limiting step in ammonia synthesis [2]. Dinitrogen adsorption [3] as well as ammonia synthesis [4] on the clean iron surfaces are structuresensitive reactions, for example, dinitrogen dissociation on Fe(1 1 1) is about 60 times faster than on Fe (1 0 0). Adsorbed potassium signi®cantly changes properties of the iron surface and in¯uences its activity. After potassium adsorption the difference in activity observed for potassium-free surfaces is eliminated [5]. The active phase of the iron catalyst is metallic iron covered with KO layer. Ratio of potassium to oxygen atoms was found to be 1:1 [6]. Strongin and
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(99)00034-4
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Somorjai [7] found only small amount of potassium and oxygen after ammonia synthesis on iron single crystals though the iron surface had been precovered with potassium monolayer. However, TPD studies showed that potassium completely disappears from the iron surface not before the temperature of 900 K [8,9]. A simple model of the iron catalyst surface was proposed earlier [10]. It is assumed that potassium atoms are adsorbed on the iron surface and the active sites capable to adsorb non-metal atoms are located between potassium and iron atoms. Part of these sites are occupied by oxygen atoms which increase the thermal stability of potassium on the catalyst surface. Under ammonia synthesis conditions voids existing in oxygen layer are occupied by nitrogen atoms and ammonia synthesis process performs on them. This paper is intended to explain the discrepancies between reported experimental results on behaviour of potassium on the iron surface. An attempt to enhance previous model of the surface of the iron catalyst has also been made. Based on the model, decomposition of ammonia over the iron catalyst is interpreted. 2. Experimental 2.1. Potassium desorption Studies of adsorption and desorption of potassium and oxygen on the surface of a single crystal Fe(1 1 1) were carried out in the UHV chamber. A base pressure was maintained below 10ÿ8 Pa. Composition of surface was examined by Auger electron spectroscopy (AES). Auger spectra were acquired using CMA analyser. The spectrometer was operated in the d(N(E))/dE mode. Primary electron energy of 2.5 keV and primary beam current ± 10 mA was used. Modulation amplitude was 4 V peak-to-peak. Oxygen adsorption was performed at a pressure of 10ÿ6 Pa. Potassium was produced by heating a zeolite K ion emitter. During experiments a sample with various oxygen concentration in the iron bulk should have been obtained. Low oxygen concentration in the bulk was achieved by subsequent sample heating followed by Ar bombardment. Fe(1 1 1) sample was then enriched with oxygen by subsequent oxygen adsorp-
Fig. 1. Placement of the iron foil in the reactor and catalyst bed.
tion followed by heating at the temperature of 520 K. With growing oxygen concentration in the bulk the temperature should have been gradually increased up to 670 K. 2.2. Promoters surface diffusion The experiment on promoters diffusion during catalyst reduction was carried out in a six-channel integral reactor described elsewhere [11]. A sample of a thin (0.1 mm) pure polycrystalline iron foil was put in a bed of an industrial catalyst so as half of it remained without contact with catalyst grains (Fig. 1). The iron catalyst was reduced under atmospheric pressure in the ¯ow of stoichiometric hydrogen±nitrogen mixture. The temperature was gradually increased up to 770 K. Then the ammonia synthesis process was performed for 40 h under the pressure of 10 MPa and at the temperature of 720 K. Afterwards the catalyst was cooled down and passivated and the iron foil was transferred into UHV chamber. Surface composition was examined by Auger electron spectroscopy. 2.3. Ammonia decomposition The reduced industrial catalyst PS3-INS containing: 3.3 wt% Al2O3, 3.2 wt% CaO and 0.8 wt% K2O was used in the ammonia decomposition experiment. Catalytic activity was measured for the unmodi®ed catalyst and for the catalyst from which potassium compounds were washed out by hot water. After washing, potassium content decreased to 0.05 wt% K2O. Contents of calcium and aluminium compounds remained unchanged. Kinetic measurements of
W. Arabczyk et al. / Applied Catalysis A: General 182 (1999) 379±384
ammonia decomposition were carried out in a reactor described elsewhere [12]. It was performed under atmospheric pressure and in the temperature range between 620 and 770 K. 3. Results and discussion Potassium was adsorbed on the clean Fe(1 1 1) sample with low oxygen concentration in the bulk. The surface is saturated with potassium when 3.81018 mÿ2 of atoms is adsorbed which is in agreement with the results obtained previously [8]. Heating of the sample leads to the complete desorption of potassium at the temperature of 470 K (Fig. 2, curve (a)). This observation indicates that potassium could not occur in the metallic form on the catalyst surface under ammonia conditions. Potassium adsorption on the sample precovered with oxygen (Fig. 2, curve (b)) shows that maximal number of potassium atoms in monolayer increases from 3.81018 to 5.61018 mÿ2. It is due to interactions between potassium and oxygen which leads to the reduction of potassium ion radius. Presence of oxygen atoms also increases the thermal stability of potassium on the iron surface. Its total desorption is observed at 550 K. At the ®rst stage of heating only oxygen concentration decreases until the ratio of nK/nO reaches 1:1. Then, with raising temperature, oxygen and potassium disappear simultaneously (Fig. 2, curves (b) and (b0 )). Oxygen
Fig. 2. Desorption curves of potassium and oxygen (with primes) measured by AES: (a) low oxygen concentration in the iron lattice and no detectable oxygen on the surface; (b) and (b0 ) low oxygen concentration in the iron lattice and oxygen on the surface; (c) and (c0 ) high oxygen concentration in the iron lattice and oxygen on the surface.
381
dissolves in the iron lattice and potassium atoms desorb from the surface. Potassium adsorption/desorption cycles were performed on the iron sample which bulk was gradually enriched with oxygen (only one experiment is shown in Fig. 2). There is no signi®cant difference between potassium adsorption on the samples with various oxygen concentration in the bulk. However, the differences during desorption process are observed. Similarly to the curve (b) at the ®rst stage of heating, an excess of oxygen is removed from the iron surface. The ratio of nK/nO reaches 1:1 at higher temperature (about 600 K). Then potassium and oxygen disappear simultaneously (curve (c) and (c0 )). Potassium is completely removed from the iron surface at the temperature of 900 K. Comparison of the experiments performed over samples with low and high oxygen concentration in the bulk suggests that the temperature of thermal decomposition of the KO layer existing on the iron surface considerably increases with an increase of oxygen concentration in the bulk. It is probably due to dissolution of oxygen atoms in the iron lattice. Low oxygen concentration in iron favours the diffusion of surface oxygen into iron lattice which leads to depletion of surface from oxygen and consequently causes potassium desorption. On the sample containing more dissolved oxygen the diffusion of surface oxygen is much slower and requires higher temperature. In that case potassium can exist on the iron surface at higher temperature. Those conclusions enable one to explain a split of opinions existing in the literature. Strongin and Somorjai [7] report that after their experiments of ammonia synthesis over iron single crystals, potassium concentration on the surface is only 0.15 ML. The rest is clean iron surface. Other papers point out that in the industrial catalyst 90±96% of the iron surface is covered with promoters [13,14], mainly by potassium. This controversy probably arises from different samples preparation. UHV experiments are usually carried out over very pure single crystal. The concentration of oxygen in its bulk is usually very low and the surface is also cleaned very carefully. During experiments carried out under ammonia synthesis conditions, it leads to complete desorption of potassium due to lack of `oxygen anchors' and obtained results are similar to those observed for a clean iron
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surface. On the other hand industrial catalysts even in reduced form always contain some amount of oxygen. It prevents the complete potassium removal. The studies of reduction of the iron catalyst show that potassium, which is initially located on the grain boundaries, diffuses to the iron crystallites surface [15]. Aluminium and calcium which are also contained in the catalyst could also cover the surface of iron crystallites. There is a question: Is any of promoters privileged in any way and cover the surface mostly? To answer this question the experiment on diffusion of promoters from the catalyst onto iron foil was carried out. The clean iron foil was intended to simulate a newly emerging (during reduction process) iron surface and catalyst grains play a role similar to that of grain boundaries during real reduction process. Prolonged heating of the iron foil together with the catalyst should have induced a diffusion of promoters onto clean foil. However, neither calcium nor aluminium was observed during AES measurements. Entire surface was covered with the layer consisting of potassium and oxygen though only half of the sample was in direct contact with catalyst grains. This effect and lack of aluminium and calcium on the surface suggest that in this system potassium has the strongest tendency to cover the iron catalyst surface. The results presented above allow to expand the model of the catalyst surface described in the introduction. Mentioned model proposes an existence of a potassium layer over a layer of oxygen atoms and indicates that voids in the oxygen layer are free sites capable to adsorb dinitrogen molecules. The model does not describe the way leading to the formation of this active layer and does not enable to obtain quantitative description of surface structures. New model assumes that during reduction of the iron catalyst potassium diffuses from grain boundaries onto iron crystallites surface. Probably it leads to the partial or total removal of aluminium and calcium from that surface. Afterwards the equilibrium between potassium and oxygen is achieved when the ratio nK/ nO reaches 1:1. Taking into account that during reduction there is an excess of oxygen atoms, it could be assumed that iron surface could be covered with potassium in maximal possible degree. It was earlier shown that maximal number of potassium atoms on the iron surface precovered with
oxygen is 5.61018 mÿ2. Assuming the nK/nO ratio as 1:1 the number of 5.61018 mÿ2 oxygen atoms is required to stabilise potassium atoms. The maximal number of oxygen atoms adsorbed in the monolayer on Fe (1 1 1) and Fe(1 0 0) is found to be 141018 and 121018 mÿ2, respectively [16,17]. Those numbers are considered as a maximal number of adsites on those surfaces in two-dimensional structure. According to the new model, potassium does not occupy the adsites in the oxygen layer but is located over oxygen atoms so as the number of voids in the oxygen layer can be calculated as 14ÿ5.8)8.2 1018 mÿ2 for Fe(1 1 1) and 12ÿ5.8)6.21018 mÿ2 for Fe(1 0 0). It was earlier described that dinitrogen molecules can adsorb on the free adsites under potassium layer [10]. The number of free adsites determines some properties of the iron catalyst. The active surface could be taken as a ratio of the number of free sites (8.21018 mÿ2 for Fe(1 1 1) and 6.21018 mÿ2 for Fe(1 0 0)) to the total number of adsites (141018 mÿ2 for Fe(1 1 1) and 121018 mÿ2 for Fe(1 0 0)) and it gives 8.2/1458% and 6.2/1252%, respectively. The active area of industrial iron catalyst is measured by CO chemisorption. CO and N2 are isoelectronic molecules. Both adsorb only on free iron atoms. This area for typical industrial iron catalysts is in the range of 40±60% [18,19] and is in line with the numbers calculated from the model. It is assumed that there is an optimal amount of potassium needed to form active surface. This amount is connected with the surface area of catalyst. Typical surface area of the industrial iron catalysts is about 10 m2/g [20] and in that case the potassium oxide concentration, calculated from the model, is 0.47 wt%. Because about 10% of potassium contained in the catalyst is permanently bound with other promoters [21] and does not take part in the formation of active surface [15] this number can be increased to 0.52 wt%. This number is in good agreement with results of the studies of in¯uence of potassium concentration in the catalyst on its activity carried out by Kowalczyk [22] and presented in Fig. 3. It shows that optimal concentration of potassium is about 0.5 wt%. Typical industrial catalysts contain 0.5±0.8 wt% of K2O. On the basis of present model some kinetic results can be explained. It was earlier found that potassium impregnation of the iron catalyst leads to decrease in
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Fig. 3. Dependence of the catalytic activity of the fused iron catalyst on the potassium concentration [22].
the rate of ammonia decomposition [23]. In our experiments, ammonia decomposition was carried out over typical iron catalyst and over this catalyst without potassium. The simple Arrhenius plot (Fig. 4) shows that decomposition rate is about four times higher for catalyst without potassium though apparent activation energy for these cases differ less than 10%. It suggests that the main reason is the change of preexponential factor of reaction rate equation.
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Fig. 5. Top-view of the Fe(1 0 0) surface covered with oxygen and potassium according to the model.
This effect could be explained in the light of the present model. The preexponential factor is in¯uenced, among others, by surface structure. The surface of the iron catalyst is covered by potassium oxide and only in the voids in the oxygen layer free iron atoms can adsorb ammonia molecules. Ammonia dissociation leads to the formation of adsorbed nitrogen atoms, localised under potassium atoms. With the statistical presentation of the surface (Fig. 5) it can be expected
Fig. 4. The dependance of ammonia decomposition rate on the temperature for the catalyst with potassium and catalyst without potassium.
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that each adsorbed nitrogen atom is surrounded by oxygen and potassium atoms. The diffusion of nitrogen atoms under the monolayer of potassium atoms with regard to the geometrical considerations resembles the diffusion in the solid state. It can be expected that it will be signi®cantly slower in comparison with diffusion proceeding on the clean surface, therefore, the number of collisions leading to the formation of dinitrogen molecules decreases. On the surface free of potassium, this effect will be eliminated. 4. Conclusions The results presented in this paper suggest signi®cant in¯uence of oxygen concentration in the bulk on the temperature of decomposition of KO layer existing on the iron surface and consequently on the desorption of potassium. High concentration of oxygen in iron bulk makes possible that stable form of potassium exists on the iron surface even under ammonia synthesis conditions. It was also found that potassium has the tendency to cover the iron surface. It leads to the formation of potassium/oxygen layer even in presence of other promoters like calcium or aluminium. The model of the active surface of the iron catalyst was proposed. Potassium atoms are bound to iron surface through oxygen bridges which increases their thermal stability. There are voids in the oxygen layer which play as free sites for dinitrogen or ammonia adsorption. Based on the model, the value of the active surface (about 58%) and optimal concentration of potassium oxide in the catalyst (about 0.5 wt%) was calculated. Those values are in good agreement with experimental results. The in¯uence of potassium on the rate of ammonia decomposition is due to geometrical obstacles build from oxygen and potassium atoms. Those structures
block the recombination of dinitrogen molecules and it lowers overall rate of ammonia decomposition. References [1] K. Tamaru, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Fundamental and Practice, Plenum Press, New York, 1991. [2] A. Ozaki, H. Taylor, M. Boudart, Kinetics and mechanism of the ammonia synthesis, Proc. R. Soc. London, A 258 (1960) 47. [3] F. Bozso, G. Ertl, M. Grunze, M. Weiss, J. Catal. 49 (1977) 18. [4] D.R. Strongin, J. Carrazza, S.R. Bare, G.A. Somorjai, J. Catal. 103 (1987) 213. [5] G. Ertl, S.B. Lee, M. Weiss, Surf. Sci. 114 (1982) 527. [6] W. Arabczyk, K. Kaøucki, Proceedings of the 10th International Congress of Catalysis, Part C, Budapest, 1992, p. 2539. [7] D.R. Strongin, G.A. Somorjai, J. Catal. 109 (1988) 51. [8] S.B. Lee, M. Weiss, G. Ertl, Surf. Sci. 108 (1981) 357. [9] Z. PaaÂl, G. Ertl, S.B. Lee, Appl. Surf. Sci. 8 (1981) 231. [10] W. Arabczyk, U. Narkiewicz, K. Kaøucki, Vacuum 45(2±3) (1994) 267. [11] R.J. KalenÂczuk, J. Chem. Tech. Biotechnol. 59 (1994) 73. [12] K. Kaøucki, R.J. KalenÂczuk, W. Arabczyk, Z. SÂpiewak, Przem. Chem. 65 (1986) 532. [13] D.C. Silverman, M. Boudart, J. Catal. 77 (1982) 208. [14] G. Ertl, D. Prigge, R. SchloÈgl, M. Weiss, J. Catal. 79 (1983) 359. [15] P.D. Rabina, T.Y. Malysheva, L.D. Kuznetsov, V.A. Batyrev, Kinet. Katal. 11 (1970) 1243. [16] S. Nakanishi, T. Horiguchi, Proceedings of the Seventh International Vacuum Congress and Third International Conference on Solid Surfaces, p. A2727, 1977. [17] K.O. Legg, F. Jona, D.W. Jepsen, P.M. Marcus, Phys. Rev. B 16 (1977) 5271. [18] R. Brill, J. Catal. 19 (1970) 236. [19] V. Solbakken, A. Solbakken, P.H. Emmet, J. Catal. 15 (1969) 90. [20] D.R. Strongin, G.A. Somorjai, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Fundamental and Practice, Plenum Press, New York, 1991. [21] K. Kaøucki, W. Arabczyk, Z. Janecki, K. Stoøecki, I Kongres Technologii Chemicznej, Szczecin, 1995, p. 466. [22] Z. Kowalczyk, S. Jodzis, Przem. Chem. 66 (1987) 279. [23] K.S. Love, P.H. Emmet, J. Am. Chem. Soc. 63 (1941) 3297.