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Ammonia synthesis over a Ba and Ce-promoted carbon-supported cobalt catalyst. Effect of the cerium addition and preparation procedure
Journal of Catalysis 303 (2013) 130–134
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Research Note
Ammonia synthesis over a Ba and Ce-promoted carbon-supported cobalt catalyst. Effect of the cerium addition and preparation procedure _ Magdalena Karolewska a, Elzbieta Truszkiewicz a, Maria Ws´ciseł a, Bogusław Mierzwa b, Leszek Ke˛pin´ ski c, a,⇑ Wioletta Raróg-Pilecka a b c
Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Low Temperature and Structure Research of the Polish Academy of Sciences, Okólna 2, 50-950 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 14 January 2013 Revised 6 March 2013 Accepted 6 March 2013
Keywords: Ammonia synthesis Cobalt catalyst Cerium promoter Catalyst preparation Cobalt dispersion
a b s t r a c t Carbon-supported cobalt catalysts promoted with barium and cerium were synthesized, characterized and tested in NH3 synthesis. Two different methods of addition of the cerium promoter were used, i.e., subsequent impregnation and co-impregnation. The method of addition of the cerium promoter proved to be a crucial factor in the preparation of efficient Co–Ce–Ba/C catalysts. The co-impregnation method is more beneficial and leads to obtaining a well-dispersed catalytic material with the highest activity. The carbon-supported cobalt catalyst promoted with barium and cerium is more active than the magnetitebased commercial catalyst, especially at high conversion degrees, which means that it is less inhibited by ammonia. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Ammonia synthesis is a process of great importance both for science and for industry. The simple reaction of hydrogen and nitrogen has contributed to the development of many fundamental concepts for catalysis (e.g., structural sensitivity of a reaction, promoters, poisons) [1]. The industrial process of ammonia synthesis has a significant value for today’s society. It is estimated that without ammonia produced in the industrial process, only 60% of the global population could be nourished [2]. This problem would be even more serious in the next 40 years due to the predicted fast rate of global population increase. For these reasons, the improvement of the ammonia synthesis efficiency has been attracting researchers from both industrial centers and academia. Multiple studies on the development of new catalysts with catalytic properties much more favorable than those of the two catalysts commonly used in ammonia plants all over the world, the conventional magnetite-based catalyst [3,4] and the modern Ru/C catalyst [5–7] implemented in the Kellogg Advanced Ammonia Process (KAAP technology), have been reported. Among the investigated systems, Co-based catalysts were found to be promising materials in terms of their activity in ammonia synthesis [8–19]. The long-lasting research on Co-based catalytic systems was ⇑ Corresponding author. Fax: +48 22 628 27 41. E-mail address: [email protected] (W. Raróg-Pilecka). 0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2013.03.005
focused on Co-based catalysts supported on carbon, among others [9,10,15,16]. Although cobalt exhibits low activity in the ammonia synthesis reaction [20], recent studies revealed that addition of selected elements to cobalt systems improves their general catalyst properties, mainly activity and thermostability [10,17–19]. In the literature, many potential modifications of cobalt catalysts have been described. Hagen [9,10] has shown that barium is a very effective promoter for cobalt. Our previous studies [17–19] revealed that the addition of cerium to a Co + Ba type catalyst as a second promoter gives favorable results in the case of the unsupported cobalt catalysts obtained both by the templating method [17] and by coprecipitation [18,19]. In our previous studies [15,16], the application potential of the Ba–Co/carbon catalysts was shown. Comparison between the Ba– Co/carbon catalysts and magnetite-based commercial catalyst (KM I) indicates that the reaction rate over cobalt was more sensitive to temperature (higher activation energy for Co than for Fe) and less sensitive to ammonia concentration in the gas phase [16]. However, because of low cobalt dispersions in these catalytic systems [15,16], the weight-based reaction rate with respect to the mass of metal (Co) is rather low. According to our suggestions in previous papers [15,16], further experimental work, aiming at increasing the Co dispersion, among others, is necessary. Improvement of this parameter would increase the practical potential of this new cobalt catalyst.
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The present work studies the effect of the addition of cerium to carbon-supported Co–Ba materials on the Co dispersion and catalytic activity of these systems. It also aims to answer the question of how the preparation procedure affects the catalytic properties of the obtained Co–Ce–Ba/C catalysts in ammonia synthesis. For this reason, two different methods of cerium addition were carried out. The dispersion of cobalt was characterized by hydrogen chemisorption (H2 TPD) and X-ray diffraction (XRD). Some transmission electron microscopy (TEM) experiments were also performed. The rates of NH3 synthesis on the obtained catalysts were examined under an industrially relevant pressure (9.0 MPa).
Table 1 Chemical composition of the prepared catalysts.
a
Catalyst symbol
Co loading (wt.%)a
Ba loading (mmol/gC+Co)
Ce loading (mmol/gC+Co)
Method of cerium addition
Co–Ba/C (Co– Ce)S– Ba/C (Co– Ce)T– Ba/C
9.51 9.44
0.84 0.85
– 0.074
9.34
0.89
0.071
– Subsequent impregnation (method 1) Co-impregnation (method 2)
In relation to the mass of unpromoted material (C + Co).
2. Experimental 2.1. Carbon support A commercial activated carbon (GF4O, Norit) was used as a support for the active phase. The material was subjected to a modification procedure according to the following steps: (1) flushing in HCl and water, (2) high-temperature treatment (1900 °C, Ar), and (3) subsequent gasification at 865 °C for 5 h in a stream containing Ar and water vapor up to 32.2% mass loss. The support thus prepared was a microporous adsorbent, as indicated by the nitrogen physisorption experiments. The total surface area (total area of micropores and macropores) was 1610.8 m2/g, of which 1381 m2/ g was attributed to the area of micropores. 2.2. Catalyst preparation The obtained carbon support (designed C in the catalyst symbols) was impregnated with alcoholic solutions of the active phase and the cerium promoter precursors. Two different methods of cerium addition were carried out: 1. cobalt and cerium precursors were added subsequently (subsequent impregnation—indicated by the index S next to Ce in the catalyst’s symbol), or 2. precursors of the active phase (Co) and the promoter (Ce) were put together into carbon from one solution (co-impregnation— indicated by the index T next to Ce in the catalyst’s symbol). Depending on the method used, the support was first dipped either into an alcoholic solution of Co(NO3)26H2O (method 1) or into a mixture of Co(NO3)26H2O and Ce(NO3)36H2O (method 2). Next, the solvent was evaporated and the samples were calcined for 24 h in flowing Ar at 200 °C. During the next step, the prepared materials were calcined in air (200 °C, 24 h) to convert the Co precursor to its oxide form (Co3O4). When method (1) was used, after the Co precursor addition and calcination in Ar, the solution of cerium nitrate was put into the calcined material, and the calcination procedure described above was repeated. In both cases (methods 1 and 2), the weight-based ratio of cerium to cobalt reached a value of 1:10. To obtain the Ba-promoted systems, the preliminarily calcined samples were impregnated with a 130 g/dm3 aqueous solution of Ba(NO3)2 at 90 °C for 24 h. After separation of the solid materials from the solution and air-drying at 90 °C for 24 h, the samples were crushed and sieved to obtain a 0.2–0.63 mm fraction. The chemical compositions of the resulting catalysts are presented in Table 1. 2.3. Characterization methods The obtained Ba-promoted catalysts were characterized by several methods, including H2 chemisorption (H2 TPD), XRD, and TEM.
X-ray powder diffraction data were collected using a Geigerflex diffractometer (Rigaku-Denki). The average size of cobalt crystallites was determined using Scherrer’s equation. The microstructure of the catalysts was studied using TEM. The TEM images were recorded with a Philips CM20 Super Twin microscope, which provides a 0.25-nm resolution at 200 kV. The H2 chemisorption studies were carried out in a fully automated PEAK-4 instrument [21] equipped with a TCD cell and supplied with high-purity (6 N) gases. A detailed description of the measurement procedure can be found elsewhere [16]. After reduction in a hydrogen–argon stream (80:20) at 500 °C (20 h), the sample was flushed with argon and cooled to 150 °C. Then, the Ar stream was replaced with hydrogen, and the sample was cooled in H2 to 0 °C. Finally, the specimen was heated (20 °C/min) in a flow of argon (40 ml/min), and the concentration of desorbed hydrogen in the outlet stream was observed. The amount of preadsorbed H2 was determined by integrating the TPD response. 2.4. Catalytic activity studies The activity measurements of the catalysts in NH3 synthesis were carried out in a tubular flow reactor supplied with a very pure (>6.5 N) hydrogen and nitrogen mixture (H2:N2 = 3:1). The detailed description of the apparatus and experimental procedure can be found elsewhere [22]. In brief, under steady-state conditions of pressure (9.0 MPa), temperature (400 °C), gas flow rate (70 dm3 [STP]/h), and (x1), the increments in NH3 content (x2 x1) due to the reaction over a small catalyst layer (0.15–0.45 g) were measured. Based on such data, the reaction rates corresponding to the mean values of x = (x1 + x2)/2 were determined. Prior to kinetic tests, the samples were reduced in a stoichiometric H2:N2 stream at 0.1 MPa according to the following temperature program: 470 °C for 40 h and then 520 °C for 20 h. 3. Results and discussion 3.1. Catalyst characterization XRD data collected for the post-NH3 synthesis samples of the prepared catalysts are presented in Fig. 1. The results show that all tested materials are poorly crystallized. Metallic cobalt can be identified as a fcc phase. The strongest signals characteristic for fcc Co displays the pattern corresponding to the Co–Ba/C sample. Smaller Co peaks were observed for (Co–Ce)S–Ba/C. The weakest signals derived from the fcc Co phase were noted in the pattern for the (Co–Ce)T–Ba/C sample. Because of the small contribution of the crystalline cobalt phase in the pattern recorded for the (Co–Ce)T–Ba/C sample, a detailed estimation of the average Co crystallite diameters for this material could not be performed. The values of this parameter (dXRD) calculated for the two other
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Fig. 1. XRD patterns of the post-NH3 synthesis promoted cobalt catalysts.
samples (Co–Ba/C and (Co–Ce)S–Ba/C) are listed in Table 2. As can be seen, the average Co crystallite diameters are similar for both materials (ca. 15 nm). However, when the integrated areas under (1 1 1), (2 0 0), and (2 2 0) of cobalt signals are compared, it is suggested that the cobalt fcc phase is highly strained. Additional signals, which were obtained for the Co–Ba/C sample at 2h 24° and 34° (see Fig. 1), were assigned to BaCO3. For the sample (Co– Ce)S–Ba/C, in which cerium was added by subsequent impregnation, two peaks appeared at about 2h 28° and 50.7°, corresponding to Ba(CeO3). No signals from a cerium- and barium-containing phase have been detected in the diffraction pattern of the (Co– Ce)T–Ba/C sample. This is mainly due to the fact that this sample is particularly well dispersed, as mentioned above. TEM studies were performed to determine the morphology and homogeneity of the catalysts tested previously in NH3 synthesis. Examples of TEM images are presented in Fig. 2. As can be seen, Co forms ‘‘core–shell’’ type particles where crystallites of metallic fcc cobalt are covered with an amorphous layer of the oxide phase (CoO) (see Fig. 2a—sample Co–Ba/C). The presence of CoO on the surface of the cobalt crystallites can be explained by the fact that measurements were carried out ex situ after exposure of the samples to air. Barium exists in the tested samples in the form of BaO or BaCO3, both crystalline and amorphous. As it is depicted in Fig. 2b—sample (Co–Ce)T–Ba/C, the phases containing barium show a characteristic, ribbon-shaped morphology. The SAED pattern, presented in the inset in Fig. 2b, illustrates rings derived from C (support) and BaO. Visible individual spots can be assigned to BaCO3. Fig. 2c - sample (Co–Ce)S–Ba/C, shows that the second promoter, which is cerium, is accompanied by barium and forms a single phase Ba(CeO3). This type of particle was identified in the (Co–Ce)S–Ba/C sample, where cerium was added by subsequent impregnation. Significant homogeneity of the (Co–Ce)T–Ba/C sample (see Fig. 2d) makes analysis very difficult. All phases present in this sample are well dispersed, and probably for this reason, crystallites of metallic cobalt were not detected, but only their oxidized forms. For all the prepared materials, it was difficult to distinguish
Table 2 Dispersion of cobalt in the obtained catalysts. Catalyst symbol
Co–Ba/C (Co–Ce)S–Ba/C (Co–Ce)T–Ba/C a
Chemisorption data FE
dH (nm)
0.18 0.17 0.37
7.0 7.3 3.4
dXRD (fcc) (nm)a
15 17 –
Determined for the post-NH3 synthesis samples of the obtained catalysts.
the occurring phases due to their similar structure and lattice parameters. Therefore, a detailed estimation of the Co particles’ size distribution could not be performed. Nevertheless, the TEM results, indicating the highest dispersion of the (Co–Ce)T–Ba/C sample, are in good agreement with the XRD data. The chemisorption studies (H2 TPD) were conducted for all the catalysts to investigate the dispersion of the active phase (cobalt). The results, collected in Table 2, indicate that for the Co–Ba/C and (Co–Ce)S–Ba/C catalysts the dispersion of cobalt, expressed in terms of FE (fraction exposed, defined as the number of surface cobalt atoms referred to the total number of Co atoms), is similar. As a result, the average Co crystallite diameter (dH) is about 7 nm for both materials. The addition of cerium together with the active phase precursor from one solution (the co-precipitation method), leads to a remarkable—more than twofold—increase in dispersion (see Table 2) from 0.18 for Co–Ba/C or 0.17 for (Co–Ce)S–Ba/C to 0.37 for (Co–Ce)T–Ba/C. This fact is consistent with the TEM and XRD results indicating that the (Co–Ce)T–Ba/C catalyst is the most dispersed material among all the studied systems. The discrepancy between the values of dH and dXRD can be explained by the fact that the latter experiments were performed ex situ and that small Co particles remain undetected due to oxidation on exposure to air. Hence, the average Co crystallite size (dXRD) can be overestimated. Summing up this part of the research, it should be stated that cerium improves the dispersion of cobalt. However, the technique of cerium addition to these catalytic systems is substantial.[Author: Is this the intended meaning?] The second method, in which cobalt and cerium are added from one solution, is more beneficial and leads to well-dispersed catalytic material. 3.2. Catalyst activity The results of NH3 synthesis reaction rates based on the weight of the catalyst as a function of ammonia concentration for the prepared catalysts are shown in Fig. 3. Data referring to the commercial magnetite-based catalyst KM I are presented for comparison. As seen, the activities of Co–Ba/C and (Co–Ce)S–Ba/C catalysts are comparable. The addition of the cerium promoter to the Co–Ba/C system by subsequent impregnation does not affect the activity of the catalyst. An entirely different effect was observed for the (Co–Ce)T–Ba/C catalyst. The addition of cerium to the catalyst simultaneously with the precursor of the active phase (the coimpregnation method) leads to an activity about 2 times higher than the activities of the cobalt system promoted only with barium (Co–Ba/C), as well as the cobalt catalyst in which cerium was added by subsequent impregnation ((Co–Ce)S–Ba/C), in the whole range of studied NH3 concentrations. Good agreement of the catalyst activity results (Fig. 3) and the chemisorption data (Table 2) is observed. In fact, the (Co–Ce)T–Ba/C catalyst proved to have higher dispersion (FE = 0.37) than the other studied systems, which means that the surface of the active phase (Co) is larger. Consequently, a higher activity in ammonia synthesis was observed. The presented data confirm that the method of cerium addition is a crucial factor influencing the catalytic properties of the Co– Ce–Ba/C type catalysts. Addition of cerium can have a positive effect on the activity of catalysts, provided that an appropriate preparation procedure is applied. The addition of two compounds (Co and Ce) together from one solution (according to method 2) proved to be more advantageous. Furthermore, the (Co–Ce)T–Ba/C is much more active than the magnetite-based commercial catalyst (KM I) and, what is important, its advantage increases with increasing NH3 concentration (conversion degree)—see Fig. 3. At low NH3 content (1%), the former is 2.5 times more active than the latter. At high NH3 content (11%), the reaction rate is more than 8 times higher over the cobalt system. The difference between Co and Fe will be significantly larger at still higher, but industrially important
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Fig. 2. TEM micrographs of the post-NH3 synthesis promoted cobalt catalysts after exposure to air: (a) image of a cobalt crystallite covered with an amorphous layer of CoO for the Co–Ba/C sample; (b) the ribbon-shaped morphology of phases containing barium with a SAED pattern (inset) for the (Co–Ce)T–Ba/C sample; (c) image of the Ba(CeO3) particle with FFT pattern (inset) for the (Co–Ce)S––Ba/C sample; and (d) general view showing well-dispersed (Co–Ce)T–Ba/C catalyst sample.
conversion degrees. For this reason, the cobalt catalysts supported on carbon are not so much an alternative to the alloy iron catalyst, but rather its complement, and they should be applied at high conversion degrees. 4. Conclusions
Fig. 3. Ammonia partial pressure dependencies of the reaction rates at 400 °C and 9.0 MPa for the promoted cobalt catalysts and the industrial iron catalyst KM I.
conversions (15–20%). In addition, the activity of the Co–Ce–Ba/C catalyst will be higher when the content of the active phase increases and when high dispersion is maintained by the presence of cerium. However, it should be remembered that the Co–Ce– Ba/C catalysts have a bulk density about five times less than the bulk density of the iron catalyst. This is why the advantage, defined by the reaction rate calculated on the basis of the catalyst bed volume, of the cobalt system over the iron system is seen only at high
In summary, cerium was added to a barium-promoted supported cobalt catalyst (Co–Ba/C) to evaluate the effect of a cerium promoter on the cobalt dispersion and the catalytic properties of the obtained materials. The catalytic studies in NH3 synthesis revealed that addition of cerium enhances the activity of the Co– Ba/C catalyst, provided that an appropriate preparation procedure is applied. The (Co–Ce)T–Ba/C catalyst, in which cerium was added together with the precursor of the active phase (the co-impregnation), exhibited higher dispersion and higher activity in ammonia synthesis than the (Co–Ce)S–Ba/C catalyst, in which cerium was added by subsequent impregnation, or than the Co–Ba/C catalyst promoted only with barium. Hence, the selection of the cerium addition method can be considered as an essential factor influencing the properties and activity of the Co–Ce–Ba/C catalysts. Moreover, the (Co–Ce)T–Ba/C catalyst is much more active than the conventional iron catalyst, and its advantage increases with the increase in ammonia concentration in the gas phase. Acknowledgment This work was financially supported by the Warsaw University of Technology.
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References [1] K. Tamaru, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis: Fundamentals and Practice, Plenum Press, New York, 1991, pp. 1–18. [2] V. Smil, Am. J. Agric. Econ. 83 (2001) 790–792. [3] R. Schlögl, Angew. Chem. Int. Ed. 42 (2003) 2004–2008. [4] S.R. Tennison, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis: Fundamentals and Practice, Plenum Press, New York, 1991, pp. 303–364. [5] R.B. Strait, Nitrogen Methanol 238 (1999) 37–43. [6] R.B. Strait, International Conference and Exhibition Nitrogen ’99, Caracas 28.02-02.03. 1999, pp. 43–59. [7] A.I. Foster, P.G. James, U.S. Patent 4163775 (1979). [8] R. Kalen´ czuk, Appl. Catal. A 112 (1994) 149–160. [9] S. Hagen, R. Barfod, R. Fehrmann, C.J.H. Jacobsen, H.T. Teunissen, K. Stahl, I. Chorkendorf, Chem. Commun. 11 (2002) 1206–1207. [10] S. Hagen, R. Barfod, R. Fehrmann, C.J.H. Jacobsen, H.T. Teunissen, I. Chorkendorf, J. Catal. 214 (2003) 327–335. [11] C.J.H. Jacobson, Chem. Commun. 12 (2000) 1057–1058.
[12] R. Kojima, K. Aika, Chem. Lett. 5 (2000) 514–515. [13] R. Kojima, K. Aika, Appl. Catal. A 219 (2001) 157–170. [14] D. Moszyn´ ski, R. Je˛drzejewski, J. Ziebro, W. Arabczyk, Appl. Surf. Sci. 256 (2010) 5581–5584. [15] W. Raróg-Pilecka, E. Mis´kiewicz, M. Matyszek, Z. Kaszkur, L. Ke˛pin´ ski, Z. Kowalczyk, J. Catal. 237 (2006) 207–210. [16] W. Raróg-Pilecka, E. Mis´kiewicz, L. Ke˛pin´ ski, Z. Kaszkur, K. Kielar, Z. Kowalczyk, J. Catal. 249 (2007) 24–33. [17] W. Raróg-Pilecka, E. Mis´kiewicz, Z. Kowalczyk, Catal. Commun. 9 (2008) 870– 873. [18] W. Raróg-Pilecka, M. Karolewska, E. Truszkiewicz, E. Iwanek, B. Mierzwa, Catal. Lett. 141 (2011) 678–684. [19] M. Karolewska, E. Truszkiewicz, B. Mierzwa, L. Ke˛pin´ ski, W. Raróg-Pilecka, Appl. Catal. A 445–446 (2012) 280–286. [20] G. Rambeau, A. Jorti, H. Amariglio, J. Catal. 94 (1985) 155–165. [21] W. Raróg-Pilecka, E. Mis´kiewicz, D. Szmigiel, Z. Kowalczyk, J. Catal. 231 (2005) 11–19. [22] Z. Kowalczyk, Catal. Lett. 37 (1996) 173–179.
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Research Note
Ammonia synthesis over a Ba and Ce-promoted carbon-supported cobalt catalyst. Effect of the cerium addition and preparation procedure _ Magdalena Karolewska a, Elzbieta Truszkiewicz a, Maria Ws´ciseł a, Bogusław Mierzwa b, Leszek Ke˛pin´ ski c, a,⇑ Wioletta Raróg-Pilecka a b c
Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Low Temperature and Structure Research of the Polish Academy of Sciences, Okólna 2, 50-950 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 14 January 2013 Revised 6 March 2013 Accepted 6 March 2013
Keywords: Ammonia synthesis Cobalt catalyst Cerium promoter Catalyst preparation Cobalt dispersion
a b s t r a c t Carbon-supported cobalt catalysts promoted with barium and cerium were synthesized, characterized and tested in NH3 synthesis. Two different methods of addition of the cerium promoter were used, i.e., subsequent impregnation and co-impregnation. The method of addition of the cerium promoter proved to be a crucial factor in the preparation of efficient Co–Ce–Ba/C catalysts. The co-impregnation method is more beneficial and leads to obtaining a well-dispersed catalytic material with the highest activity. The carbon-supported cobalt catalyst promoted with barium and cerium is more active than the magnetitebased commercial catalyst, especially at high conversion degrees, which means that it is less inhibited by ammonia. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Ammonia synthesis is a process of great importance both for science and for industry. The simple reaction of hydrogen and nitrogen has contributed to the development of many fundamental concepts for catalysis (e.g., structural sensitivity of a reaction, promoters, poisons) [1]. The industrial process of ammonia synthesis has a significant value for today’s society. It is estimated that without ammonia produced in the industrial process, only 60% of the global population could be nourished [2]. This problem would be even more serious in the next 40 years due to the predicted fast rate of global population increase. For these reasons, the improvement of the ammonia synthesis efficiency has been attracting researchers from both industrial centers and academia. Multiple studies on the development of new catalysts with catalytic properties much more favorable than those of the two catalysts commonly used in ammonia plants all over the world, the conventional magnetite-based catalyst [3,4] and the modern Ru/C catalyst [5–7] implemented in the Kellogg Advanced Ammonia Process (KAAP technology), have been reported. Among the investigated systems, Co-based catalysts were found to be promising materials in terms of their activity in ammonia synthesis [8–19]. The long-lasting research on Co-based catalytic systems was ⇑ Corresponding author. Fax: +48 22 628 27 41. E-mail address: [email protected] (W. Raróg-Pilecka). 0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2013.03.005
focused on Co-based catalysts supported on carbon, among others [9,10,15,16]. Although cobalt exhibits low activity in the ammonia synthesis reaction [20], recent studies revealed that addition of selected elements to cobalt systems improves their general catalyst properties, mainly activity and thermostability [10,17–19]. In the literature, many potential modifications of cobalt catalysts have been described. Hagen [9,10] has shown that barium is a very effective promoter for cobalt. Our previous studies [17–19] revealed that the addition of cerium to a Co + Ba type catalyst as a second promoter gives favorable results in the case of the unsupported cobalt catalysts obtained both by the templating method [17] and by coprecipitation [18,19]. In our previous studies [15,16], the application potential of the Ba–Co/carbon catalysts was shown. Comparison between the Ba– Co/carbon catalysts and magnetite-based commercial catalyst (KM I) indicates that the reaction rate over cobalt was more sensitive to temperature (higher activation energy for Co than for Fe) and less sensitive to ammonia concentration in the gas phase [16]. However, because of low cobalt dispersions in these catalytic systems [15,16], the weight-based reaction rate with respect to the mass of metal (Co) is rather low. According to our suggestions in previous papers [15,16], further experimental work, aiming at increasing the Co dispersion, among others, is necessary. Improvement of this parameter would increase the practical potential of this new cobalt catalyst.
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The present work studies the effect of the addition of cerium to carbon-supported Co–Ba materials on the Co dispersion and catalytic activity of these systems. It also aims to answer the question of how the preparation procedure affects the catalytic properties of the obtained Co–Ce–Ba/C catalysts in ammonia synthesis. For this reason, two different methods of cerium addition were carried out. The dispersion of cobalt was characterized by hydrogen chemisorption (H2 TPD) and X-ray diffraction (XRD). Some transmission electron microscopy (TEM) experiments were also performed. The rates of NH3 synthesis on the obtained catalysts were examined under an industrially relevant pressure (9.0 MPa).
Table 1 Chemical composition of the prepared catalysts.
a
Catalyst symbol
Co loading (wt.%)a
Ba loading (mmol/gC+Co)
Ce loading (mmol/gC+Co)
Method of cerium addition
Co–Ba/C (Co– Ce)S– Ba/C (Co– Ce)T– Ba/C
9.51 9.44
0.84 0.85
– 0.074
9.34
0.89
0.071
– Subsequent impregnation (method 1) Co-impregnation (method 2)
In relation to the mass of unpromoted material (C + Co).
2. Experimental 2.1. Carbon support A commercial activated carbon (GF4O, Norit) was used as a support for the active phase. The material was subjected to a modification procedure according to the following steps: (1) flushing in HCl and water, (2) high-temperature treatment (1900 °C, Ar), and (3) subsequent gasification at 865 °C for 5 h in a stream containing Ar and water vapor up to 32.2% mass loss. The support thus prepared was a microporous adsorbent, as indicated by the nitrogen physisorption experiments. The total surface area (total area of micropores and macropores) was 1610.8 m2/g, of which 1381 m2/ g was attributed to the area of micropores. 2.2. Catalyst preparation The obtained carbon support (designed C in the catalyst symbols) was impregnated with alcoholic solutions of the active phase and the cerium promoter precursors. Two different methods of cerium addition were carried out: 1. cobalt and cerium precursors were added subsequently (subsequent impregnation—indicated by the index S next to Ce in the catalyst’s symbol), or 2. precursors of the active phase (Co) and the promoter (Ce) were put together into carbon from one solution (co-impregnation— indicated by the index T next to Ce in the catalyst’s symbol). Depending on the method used, the support was first dipped either into an alcoholic solution of Co(NO3)26H2O (method 1) or into a mixture of Co(NO3)26H2O and Ce(NO3)36H2O (method 2). Next, the solvent was evaporated and the samples were calcined for 24 h in flowing Ar at 200 °C. During the next step, the prepared materials were calcined in air (200 °C, 24 h) to convert the Co precursor to its oxide form (Co3O4). When method (1) was used, after the Co precursor addition and calcination in Ar, the solution of cerium nitrate was put into the calcined material, and the calcination procedure described above was repeated. In both cases (methods 1 and 2), the weight-based ratio of cerium to cobalt reached a value of 1:10. To obtain the Ba-promoted systems, the preliminarily calcined samples were impregnated with a 130 g/dm3 aqueous solution of Ba(NO3)2 at 90 °C for 24 h. After separation of the solid materials from the solution and air-drying at 90 °C for 24 h, the samples were crushed and sieved to obtain a 0.2–0.63 mm fraction. The chemical compositions of the resulting catalysts are presented in Table 1. 2.3. Characterization methods The obtained Ba-promoted catalysts were characterized by several methods, including H2 chemisorption (H2 TPD), XRD, and TEM.
X-ray powder diffraction data were collected using a Geigerflex diffractometer (Rigaku-Denki). The average size of cobalt crystallites was determined using Scherrer’s equation. The microstructure of the catalysts was studied using TEM. The TEM images were recorded with a Philips CM20 Super Twin microscope, which provides a 0.25-nm resolution at 200 kV. The H2 chemisorption studies were carried out in a fully automated PEAK-4 instrument [21] equipped with a TCD cell and supplied with high-purity (6 N) gases. A detailed description of the measurement procedure can be found elsewhere [16]. After reduction in a hydrogen–argon stream (80:20) at 500 °C (20 h), the sample was flushed with argon and cooled to 150 °C. Then, the Ar stream was replaced with hydrogen, and the sample was cooled in H2 to 0 °C. Finally, the specimen was heated (20 °C/min) in a flow of argon (40 ml/min), and the concentration of desorbed hydrogen in the outlet stream was observed. The amount of preadsorbed H2 was determined by integrating the TPD response. 2.4. Catalytic activity studies The activity measurements of the catalysts in NH3 synthesis were carried out in a tubular flow reactor supplied with a very pure (>6.5 N) hydrogen and nitrogen mixture (H2:N2 = 3:1). The detailed description of the apparatus and experimental procedure can be found elsewhere [22]. In brief, under steady-state conditions of pressure (9.0 MPa), temperature (400 °C), gas flow rate (70 dm3 [STP]/h), and (x1), the increments in NH3 content (x2 x1) due to the reaction over a small catalyst layer (0.15–0.45 g) were measured. Based on such data, the reaction rates corresponding to the mean values of x = (x1 + x2)/2 were determined. Prior to kinetic tests, the samples were reduced in a stoichiometric H2:N2 stream at 0.1 MPa according to the following temperature program: 470 °C for 40 h and then 520 °C for 20 h. 3. Results and discussion 3.1. Catalyst characterization XRD data collected for the post-NH3 synthesis samples of the prepared catalysts are presented in Fig. 1. The results show that all tested materials are poorly crystallized. Metallic cobalt can be identified as a fcc phase. The strongest signals characteristic for fcc Co displays the pattern corresponding to the Co–Ba/C sample. Smaller Co peaks were observed for (Co–Ce)S–Ba/C. The weakest signals derived from the fcc Co phase were noted in the pattern for the (Co–Ce)T–Ba/C sample. Because of the small contribution of the crystalline cobalt phase in the pattern recorded for the (Co–Ce)T–Ba/C sample, a detailed estimation of the average Co crystallite diameters for this material could not be performed. The values of this parameter (dXRD) calculated for the two other
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Fig. 1. XRD patterns of the post-NH3 synthesis promoted cobalt catalysts.
samples (Co–Ba/C and (Co–Ce)S–Ba/C) are listed in Table 2. As can be seen, the average Co crystallite diameters are similar for both materials (ca. 15 nm). However, when the integrated areas under (1 1 1), (2 0 0), and (2 2 0) of cobalt signals are compared, it is suggested that the cobalt fcc phase is highly strained. Additional signals, which were obtained for the Co–Ba/C sample at 2h 24° and 34° (see Fig. 1), were assigned to BaCO3. For the sample (Co– Ce)S–Ba/C, in which cerium was added by subsequent impregnation, two peaks appeared at about 2h 28° and 50.7°, corresponding to Ba(CeO3). No signals from a cerium- and barium-containing phase have been detected in the diffraction pattern of the (Co– Ce)T–Ba/C sample. This is mainly due to the fact that this sample is particularly well dispersed, as mentioned above. TEM studies were performed to determine the morphology and homogeneity of the catalysts tested previously in NH3 synthesis. Examples of TEM images are presented in Fig. 2. As can be seen, Co forms ‘‘core–shell’’ type particles where crystallites of metallic fcc cobalt are covered with an amorphous layer of the oxide phase (CoO) (see Fig. 2a—sample Co–Ba/C). The presence of CoO on the surface of the cobalt crystallites can be explained by the fact that measurements were carried out ex situ after exposure of the samples to air. Barium exists in the tested samples in the form of BaO or BaCO3, both crystalline and amorphous. As it is depicted in Fig. 2b—sample (Co–Ce)T–Ba/C, the phases containing barium show a characteristic, ribbon-shaped morphology. The SAED pattern, presented in the inset in Fig. 2b, illustrates rings derived from C (support) and BaO. Visible individual spots can be assigned to BaCO3. Fig. 2c - sample (Co–Ce)S–Ba/C, shows that the second promoter, which is cerium, is accompanied by barium and forms a single phase Ba(CeO3). This type of particle was identified in the (Co–Ce)S–Ba/C sample, where cerium was added by subsequent impregnation. Significant homogeneity of the (Co–Ce)T–Ba/C sample (see Fig. 2d) makes analysis very difficult. All phases present in this sample are well dispersed, and probably for this reason, crystallites of metallic cobalt were not detected, but only their oxidized forms. For all the prepared materials, it was difficult to distinguish
Table 2 Dispersion of cobalt in the obtained catalysts. Catalyst symbol
Co–Ba/C (Co–Ce)S–Ba/C (Co–Ce)T–Ba/C a
Chemisorption data FE
dH (nm)
0.18 0.17 0.37
7.0 7.3 3.4
dXRD (fcc) (nm)a
15 17 –
Determined for the post-NH3 synthesis samples of the obtained catalysts.
the occurring phases due to their similar structure and lattice parameters. Therefore, a detailed estimation of the Co particles’ size distribution could not be performed. Nevertheless, the TEM results, indicating the highest dispersion of the (Co–Ce)T–Ba/C sample, are in good agreement with the XRD data. The chemisorption studies (H2 TPD) were conducted for all the catalysts to investigate the dispersion of the active phase (cobalt). The results, collected in Table 2, indicate that for the Co–Ba/C and (Co–Ce)S–Ba/C catalysts the dispersion of cobalt, expressed in terms of FE (fraction exposed, defined as the number of surface cobalt atoms referred to the total number of Co atoms), is similar. As a result, the average Co crystallite diameter (dH) is about 7 nm for both materials. The addition of cerium together with the active phase precursor from one solution (the co-precipitation method), leads to a remarkable—more than twofold—increase in dispersion (see Table 2) from 0.18 for Co–Ba/C or 0.17 for (Co–Ce)S–Ba/C to 0.37 for (Co–Ce)T–Ba/C. This fact is consistent with the TEM and XRD results indicating that the (Co–Ce)T–Ba/C catalyst is the most dispersed material among all the studied systems. The discrepancy between the values of dH and dXRD can be explained by the fact that the latter experiments were performed ex situ and that small Co particles remain undetected due to oxidation on exposure to air. Hence, the average Co crystallite size (dXRD) can be overestimated. Summing up this part of the research, it should be stated that cerium improves the dispersion of cobalt. However, the technique of cerium addition to these catalytic systems is substantial.[Author: Is this the intended meaning?] The second method, in which cobalt and cerium are added from one solution, is more beneficial and leads to well-dispersed catalytic material. 3.2. Catalyst activity The results of NH3 synthesis reaction rates based on the weight of the catalyst as a function of ammonia concentration for the prepared catalysts are shown in Fig. 3. Data referring to the commercial magnetite-based catalyst KM I are presented for comparison. As seen, the activities of Co–Ba/C and (Co–Ce)S–Ba/C catalysts are comparable. The addition of the cerium promoter to the Co–Ba/C system by subsequent impregnation does not affect the activity of the catalyst. An entirely different effect was observed for the (Co–Ce)T–Ba/C catalyst. The addition of cerium to the catalyst simultaneously with the precursor of the active phase (the coimpregnation method) leads to an activity about 2 times higher than the activities of the cobalt system promoted only with barium (Co–Ba/C), as well as the cobalt catalyst in which cerium was added by subsequent impregnation ((Co–Ce)S–Ba/C), in the whole range of studied NH3 concentrations. Good agreement of the catalyst activity results (Fig. 3) and the chemisorption data (Table 2) is observed. In fact, the (Co–Ce)T–Ba/C catalyst proved to have higher dispersion (FE = 0.37) than the other studied systems, which means that the surface of the active phase (Co) is larger. Consequently, a higher activity in ammonia synthesis was observed. The presented data confirm that the method of cerium addition is a crucial factor influencing the catalytic properties of the Co– Ce–Ba/C type catalysts. Addition of cerium can have a positive effect on the activity of catalysts, provided that an appropriate preparation procedure is applied. The addition of two compounds (Co and Ce) together from one solution (according to method 2) proved to be more advantageous. Furthermore, the (Co–Ce)T–Ba/C is much more active than the magnetite-based commercial catalyst (KM I) and, what is important, its advantage increases with increasing NH3 concentration (conversion degree)—see Fig. 3. At low NH3 content (1%), the former is 2.5 times more active than the latter. At high NH3 content (11%), the reaction rate is more than 8 times higher over the cobalt system. The difference between Co and Fe will be significantly larger at still higher, but industrially important
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Fig. 2. TEM micrographs of the post-NH3 synthesis promoted cobalt catalysts after exposure to air: (a) image of a cobalt crystallite covered with an amorphous layer of CoO for the Co–Ba/C sample; (b) the ribbon-shaped morphology of phases containing barium with a SAED pattern (inset) for the (Co–Ce)T–Ba/C sample; (c) image of the Ba(CeO3) particle with FFT pattern (inset) for the (Co–Ce)S––Ba/C sample; and (d) general view showing well-dispersed (Co–Ce)T–Ba/C catalyst sample.
conversion degrees. For this reason, the cobalt catalysts supported on carbon are not so much an alternative to the alloy iron catalyst, but rather its complement, and they should be applied at high conversion degrees. 4. Conclusions
Fig. 3. Ammonia partial pressure dependencies of the reaction rates at 400 °C and 9.0 MPa for the promoted cobalt catalysts and the industrial iron catalyst KM I.
conversions (15–20%). In addition, the activity of the Co–Ce–Ba/C catalyst will be higher when the content of the active phase increases and when high dispersion is maintained by the presence of cerium. However, it should be remembered that the Co–Ce– Ba/C catalysts have a bulk density about five times less than the bulk density of the iron catalyst. This is why the advantage, defined by the reaction rate calculated on the basis of the catalyst bed volume, of the cobalt system over the iron system is seen only at high
In summary, cerium was added to a barium-promoted supported cobalt catalyst (Co–Ba/C) to evaluate the effect of a cerium promoter on the cobalt dispersion and the catalytic properties of the obtained materials. The catalytic studies in NH3 synthesis revealed that addition of cerium enhances the activity of the Co– Ba/C catalyst, provided that an appropriate preparation procedure is applied. The (Co–Ce)T–Ba/C catalyst, in which cerium was added together with the precursor of the active phase (the co-impregnation), exhibited higher dispersion and higher activity in ammonia synthesis than the (Co–Ce)S–Ba/C catalyst, in which cerium was added by subsequent impregnation, or than the Co–Ba/C catalyst promoted only with barium. Hence, the selection of the cerium addition method can be considered as an essential factor influencing the properties and activity of the Co–Ce–Ba/C catalysts. Moreover, the (Co–Ce)T–Ba/C catalyst is much more active than the conventional iron catalyst, and its advantage increases with the increase in ammonia concentration in the gas phase. Acknowledgment This work was financially supported by the Warsaw University of Technology.
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