- Email: [email protected]
Surface properties of wustite based iron-cobalt catalysts for ammonia synthesis reaction
Catalysis Communications 136 (2020) 105907
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Surface properties of wustite based iron-cobalt catalysts for ammonia synthesis reaction
T
⁎
Zofia Lendzion-Bieluń , Artur Jurkowski West Pomeranian University of Technology in Szczecin, Faculty of Chemical Technology and Engineering, Piastów Ave. 42, 71-065 Szczecin, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Ammonia synthesis Wustite Cobalt oxide TPD-H2
In this work, surface properties of wustite-based iron and iron‑cobalt catalysts were investigated. Measurements of hydrogen temperature-programmed desorption (TPD-H2) and surface area of active forms of catalysts were conducted after reduction of catalyst precursors (16 h) in pure hydrogen at various temperatures (450, 475, 500, 550 and 600 °C). The amount of cobalt in the catalyst structure and the number of adsorption sites relative to hydrogen were correlated with the activity for ammonia synthesis reaction. The presence of cobalt increased thermal resistance of the catalysts and at the same time their activity in ammonia synthesis reaction.
1. Introduction Many scientists believe that everything on catalyst development for ammonia synthesis has already been done and that the only way to improve iron catalyst activity is to change the promoters used. However, the role of the precursor was omitted. This perception resulted from the assumption of a classical volcano-type curve of the catalyst activity versus the ratio of Fe2+/Fe3+ ions (denoted as R) in a precursor form. According to this assumption, the most active catalyst is obtained as a result of reduction of iron oxide with R equal to 0.5 [1]. This value corresponds to the magnetite form of Fe3O4. Increasing or decreasing R causes a decrease in the activity of the obtained iron catalyst. The change in the approach to the precursor form led to Liu Huazhang's team results [1]. The authors studied precursors with R higher than 0.5. They found that precursors with a high proportion of Fe2+ ions, called wustite, showed higher activity than traditional iron catalysts. In many publications the Chinese team proved that the wustitebased catalyst is characterized by higher catalytic activity and more advantageous parameters of the reduction process parallel to almost the same thermostability and resistance to CO poisoning in comparison with the conventional magnetite-based iron and iron‑cobalt catalysts [2]. In the following years of studies on the wustite iron catalyst, many scientist described the influence of the phase composition of the precursor on the catalytic activity and the reduction process [3], the impact of a kind of the oxide precursor on the nitrogen desorption process [4], hydrogen desorption [3,5], the in situ course of reduction of
⁎
wustite and magnetite catalysts [6], the influence of Fe2+/Fe3+ ratio and promoter additions on the microstructure of the iron fused catalyst for the ammonia synthesis [7], the influence of Nb2O5 addition, as a potential promoter, on the reduction and activity of the wustite catalyst in ammonia synthesis [8]. A peculiar resume of the collected experimental results and general knowledge on ammonia synthesis catalysts was published by Huazhang Liu as a book [9] and a research paper [10]. The distribution of promoters in the catalyst precursor structure affects distribution of promoters in the form of an active iron catalyst for ammonia synthesis. The uniform distribution of promoters over the surface of the active form of a catalyst determines high activity and stability of catalyst structure. The results of research on the distribution of lithium oxide in the catalyst structure [11] support that statement. The etching method [12] allows us to examine how individual promoters are distributed in the oxidized form of the iron fused catalyst. The effect of the molar R (Fe2+/Fe3+) ratio in iron catalyst precursors on the distribution of promoters tested with the etching method is described in [13]. According to the results presented there, potassium oxide is found entirely in the intergranular spaces regardless of R. Aluminium oxide is present in the iron catalyst precursors in three forms: one directly bound to the grain magnetite or wustite, and two forms located in intergranular spaces, soluble as well as insoluble in HCl. Calcium oxide occurs in intergranular spaces, both in magnetite and wustite grains. It was found that when R increases the concentration of aluminium decreases while the concentration of calcium increases in wustite grains. In our recent work [14] we stated that the introduction of cobalt
Corresponding author. E-mail address: [email protected] (Z. Lendzion-Bieluń).
https://doi.org/10.1016/j.catcom.2019.105907 Received 15 July 2019; Received in revised form 10 December 2019; Accepted 11 December 2019 Available online 12 December 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 136 (2020) 105907
Z. Lendzion-Bieluń and A. Jurkowski
into the wustite‑iron catalyst structure increased the activity of the catalyst in the ammonia synthesis reaction. The purpose of the present work is to determine the effect of the addition of cobalt into the ironbased wustite catalyst structure on the distribution of promoters in the precursors, and correlation of the activity of these catalysts in the reaction of ammonia synthesis with hydrogen adsorption on active surface sites. 2. Experimental 2.1. Preparation of catalysts Iron and iron‑cobalt precursors of catalysts (an oxidized form of catalysts) were obtained by melting in a laboratory installation as described elsewhere [15]. Magnetite ore, oxides of cobalt, calcium and aluminium, as well as potassium nitrate were used for melting. Metallic iron powder was used as a reducing agent. The proportions of the individual components were selected so as to obtain the assumed concentration of promoters and cobalt, as well as the appropriate R of the catalysts obtained. The melting process was continued for approximately 50 min and then the lava of catalyst precursors was transferred to a water-cooled crystallizer. A batch of catalyst precursor obtained in one melting cycle was roughly 10 kg. The catalyst precursor was then shredded and screened. Precursor grains of 1.0–1.2 mm were used in catalytic tests. The catalyst precursor without the addition of cobalt was designated W-8 while the catalyst precursors with various amounts of added cobalt W-11, W-14 and W-15, respectively.
Fig. 1. Powder XRD patterns of obtained catalyst precursors W-8, W-11, W-14 and W-15.
of promoters embedded directly in the wustite structure and the catalysts in the intergranular spaces of the tested catalysts was assessed. The activity of the obtained catalyst precursors (oxidized form of catalysts) in the ammonia synthesis reaction was tested using a sixchannel reactor. A stoichiometric mixture of 3H2:N2 obtained from decomposition of ammonia was used as a reducing agent and synthesis gas. An analyzer and an interferometer were used to measure the concentration of ammonia at the outlet. The exact conditions of the reduction and synthesis of ammonia reaction were presented elsewhere [11,14].
2.2. Characterization of catalysts The phase composition of the obtained catalyst precursors was examined by X-ray powder diffraction analysis using the Philips X'Pert Pro apparatus. The source of X-ray radiation used was a CoKα lamp. The phase composition and positions of 2θ reflections were determined using the X'Pert HighScore Plus software. The ICP-OES analytical method (Perkin Elmer Optime 5300DV spectrometer) was used to determine the quantitative composition of the tested catalyst precursors. The ratio R of Fe2+/Fe3+ ions was determined after manganometric titration. Before titration, samples of the catalyst precursors were dissolved in hydrochloric acid under an nitrogen gas atmosphere. Measurements of the temperature-programmed hydrogen desorption (TPD-H2) and specific surface area of reduced catalysts were carried out using the Autochem II 2920. (Micromeritics). The analyses were conducted after 16-h reduction of the catalyst precursors in a pure hydrogen gas atmosphere (flow 70 cm3/min) at five various different temperatures, ca. 450, 475, 500, 550 and 600 °C. Before the start of TPD-H2, an adsorption of hydrogen (100 vol%) at 20 °C for 1 h over the surface of the pre-reduced catalysts was conducted. The specific surface area (SSA, m2/g) of the catalysts in the reduced form was determined by one-point low-temperature nitrogen adsorption at 77 K. The distribution of promoters was determined by the selective hydrochloric acid etching method described elsewhere [12]. Weighted portions of the catalyst precursors of 0.50 g and 50 cm3 of an aqueous hydrochloric acid solution of appropriate concentration were placed in 200 cm3 Erlenmeyer flasks. The resulting suspensions were placed in a shaker where samples were dissolved at room temperature. Various contact times (5–240 min) and various concentrations of HCl solutions (0.9–36 vol%) were used to obtain different degrees of iron oxide and promoter dissolution. Subsequently, the solutions were gravitationally filtered out with a funnel using a paper filter. The obtained filtrates were tested for iron, aluminium, calcium, potassium and cobalt content using the ICP-OES method. Based on the results of the conducted analyses (graph of the percentage of promoter etching vs. the degree of iron etching), the content
3. Results and discussion The obtained X-ray diffraction patterns of catalyst precursors (oxidized form of catalysts) shown in Fig. 1 confirm the presence of the wustite phase (JCPSD 00–006-0615). No reflections due to cobalt(II) oxide, oxides of promoters and other compounds of these elements are observed in the diffraction patterns. This result may prove that these compounds were embedded in the structure of iron oxide and/or formed very fine crystalline phases. After examining the exact position of reflections, a slight shift in the peak maxima can be seen. Comparing the precursor of the catalysts W-8 without addition of cobalt (II) oxide and W-11 with the highest content of that promoter, it could be seen that the position of reflex shifted slightly towards higher angles for W11 precursor (insert Fig. 1). This is due to the incorporation of cobalt ions into the wustite structure. Co2+ions have a smaller ion radius than Fe2+ ions. The consequence of this is a decrease in the interplanar distance (d) which can be estimated from powder XRD results. According to Bragg's law, the smaller interplanar distance would cause a shift of the observed diffraction towards higher 2theta angles. This could provide evidence for the embedding of cobalt ions into the structure of the wustite. Table 1 presents analysis results of the Table 1 The ratio R and percentage composition of iron-based and iron‑cobalt based wustite catalyst precursors. Sample name
R
Oxide content [wt%] Al2O3
W-8 W-11 W-15 W-14 a
2
6.4 4.9 6.2 6.8
CaO a
1.8/0.8 3.4/1.1a 2.1/0.8a 2.1/0.7a
Content in catalyst wustite grain.
a
1.6/1.0 2.1/0.8a 2.1/1.0a 1.8/0.6a
K2O
CoO
0.4 0.7 0.4 0.4
– 3.6/2.30a 2.4/2.10a 1.4/0.75a
Catalysis Communications 136 (2020) 105907
Z. Lendzion-Bieluń and A. Jurkowski
the function of the structural promoter, responsible for their thermostability. Therefore, in the case of catalyst precursors with a wustite structure and with the concentration of this oxide being lower than in the magnetite grains [13], the function of the structural promoter that increases thermostability is taken over by calcium oxide. Based on the presented results of XRD and etching analysis method, we can conclude that cobalt was embedded into the wustite structure and partly assumed the role of a structural promoter, increasing the thermostability of catalysts. Based on our earlier findings [14] and data reported by others [16], the addition of cobalt to an iron catalyst is known to increase its reducibility. Moreover, the addition of cobalt affected the increase of catalyst activity in the ammonia synthesis reaction. Our predictions on the cobalt role in the wustite catalyst structure were confirmed by the following hydrogen chemisorption/TPD results. According to the literature [17], TPD-H2 curves for iron catalysts consist of two peaks. The first desorption peak corresponds to atomic hydrogen bonded on the iron surface, while the second peak to hydrogen dissolved in the bulk of iron catalyst. The TPD-H2 profiles of two example catalysts, without cobalt addition (W-8) and with cobalt addition (W-14) are shown in Electronic Supporting Information (ESI, Fig. S1). For other tested catalysts, TPD-H2 profiles look similar, where each profile shows two peak maxima. TPD-H2 measurements were carried out over catalyst surfaces after reduction at various temperatures and results are presented in Fig. 3 along with catalytic activity results. More precisely, the relationship of the volume of desorbed hydrogen (TPD-H2) with catalytic activity at 450 °C, the latter expressed as a rate constant for the ammonia synthesis reaction, k [gNH3∙MPa0.5/gCat∙h], derived from Temkin-Pyzhew equation, is presented as a function of the temperature used in the reduction process for the cobalt catalysts. The W-14 and W-15 catalysts show the highest activity (based on the k value) in the ammonia synthesis reaction as shown in Fig. 3A and B, after reducing the precursors at 500 °C. The volume of desorbed hydrogen is also the largest for these catalysts after reduction at 500 °C. The W-11 catalyst with the highest cobalt content in the wustite grain, reached its maximum activity after reduction at 450 °C as illustrated in Fig. 3C. The lowest reduction temperature after which the catalyst with the highest content of cobalt (W11) achieved its maximum activity confirm that cobalt in the catalyst grains accelerated the reduction process. The catalyst W-8, without cobalt, achieved the maximum volume of hydrogen desorbed after reduction at 475 °C as depicted in Fig. S2 (ESI). At the same time, the catalyst achieved its maximum activity after reduction at this temperature. However, both the activity and the amount of desorbed hydrogen were lower for this catalyst than for W-11, W-14 and W-15 catalyst samples after reduction at all temperatures tested. The amount of adsorbed hydrogen on the reduced catalyst surface and which was determined by the TPD-H2 technique (Fig. 3), could be correlated with the number of active sites on the catalyst surface for hydrogen activation. The decrease in the amount of hydrogen adsorbed on the surface of the reduced catalyst with the increase of reduction temperature could be related to the process of recrystallization and sintering of catalyst crystallites. The fact that the recrystallization process proceeded is also evidenced by the change in the specific surface area of the catalysts after the reduction process at various temperatures (Fig. 4). The largest decrease in specific surface area along with the increase in reduction temperature was found for the W-8 catalyst without the cobalt addition. The percentage reduction of the specific surface area for that catalyst after reduction at 500 °C with respect to the initial value obtained after reduction at 450 °C, was about twice as high as that for the W-11 and W-15 catalysts, and five times higher compared to the W-14 catalyst. It proves that the addition of cobalt affected the improvement of resistance to sintering of wustite catalysts. Fig. 5 shows the change in the reaction rate constant k of ammonia synthesis at 400 °C with the content of cobalt oxide in the grain of
Fig. 2. Dependence of etching degree (%) of cobalt on the etching degree (%) of iron in the catalyst precursors W-11, W-14 and W-15.
quantitative composition and R values of the tested catalyst precursors. The catalyst precursors obtained with the addition of cobalt differed by the molar ratio R, the content of promoters (Al2O3, CaO, K2O) and cobalt (CoO). The catalyst precursor W-11 had the lowest R ratio (4.9) and the highest content of promoter oxides and cobalt oxide. The catalyst precursors W-14 and W-15 had similar values of R and the content of promoter oxides, while the content of cobalt oxide varied. The catalyst precursor W-8, without cobalt, had a similar value of R (6.4) to the W-15 and W-14 catalyst precursors. The oxidized form of the iron catalyst consists of intergranular spaces and wustite grains for R > 4.5 [9]. For R < 4.5, magnetite grains occur additionally. On the basis of the previous research [12–15], it is known that the structure of iron catalyst and its activity in ammonia synthesis depend on the distribution of promoters in grains of the oxidized form of the catalyst (magnetite or wustite). Promoter content in wustite grains was determined with the selective etching method. In the first stage of etching, iron ions and promoter elements from intergranular spaces migrated into solution and next wustite grains were dissolved [12]. Fig. 2 shows the dependence of the etching degree of cobalt on the iron etching degree in the process of selective etching of the precursor catalysts in hydrochloric acid solution. It can be seen that some of the cobalt, ca. from 20% to 40%, passed into the solution with a small degree of iron dissolution (maximum at 15%). It was cobalt, located in intergranular spaces of the oxidized form of the catalyst. The remaining cobalt is dissolved together with iron that forms wustite grains. This finding can also confirm that cobalt is embedded into the wustite structure. On the basis of cobalt content in the catalyst precursors and the slope of dissolution curves (Fig. 2), the cobalt content in wustite grains was calculated. In the same way, the contents of CaO and Al2O3 promoters in the wustite grain were calculated on the basis of dissolution curves. The calculated values are presented in Table 1. As in the case of catalyst precursor of the magnetite catalyst [12], > 90% of potassium oxide was found in intergranular spaces of the studied wustite precursors. The content of aluminium oxide in the wustite grains was found to depend on R. The Al3+ ions, due to the same valence as Fe3+, integrate seamlessly into the Fe3O4 structure to form a solid solution (FeAl2O4) with the same crystallographic structure as Fe3O4 [16]. In the wustite, Al2O3 created a substitutional solid solution that builds up to a lesser extent due to larger obstacles caused by various oxidation states. Therefore, when R increased in the tested oxidized forms of catalysts, the concentration of Al2O3 dropped from 1.1 to 0.7 wt%. The aluminium oxide in the structure of magnetite catalyst precursors performs 3
Catalysis Communications 136 (2020) 105907
Z. Lendzion-Bieluń and A. Jurkowski
Fig. 3. Volume of desorbed hydrogen (TPD-H2) and rate constant k (activity measurements of ammonia synthesis reaction) as a function of reduction temperature over: A) catalyst W-11, B) catalyst W-14, C) catalyst W-15. Reduction temperatures: 450, 475, 500 and 600 °C.
Fig. 4. Change of specific surface area (m2/g) with reduction temperature for the catalysts W-8, W-11, W-14 and W-15.
Fig. 5. Dependence of the specific activity of catalyst (k) measured at 400 °C on the amount (wt%) of cobalt oxide in catalyst wustite grain.
wustite for different reduction temperatures. Catalysts with the addition of cobalt reach the maximum activity after reduction at 500 °C. The W-15 catalyst, where the concentration of CoO in the wustite phase is 2.1 wt%, reaches the highest value of k constant. A further increase in CoO content in the catalyst W-11 causes a slight decrease in k-value. Comparing the k-values for catalysts after reduction at 600 °C, cobalt addition increases the k rate constant value. Based on these results we can conclude that, just like in the magnetite catalyst precursors doped with cobalt [18], there is also a certain optimum content of cobalt in the wustite catalyst precursors.
The activity of the catalyst with cobalt oxide content of 2.1 wt% in catalyst grains after reduction at 500 °C was 2.1 times higher than that of the catalyst without the addition of cobalt (II) oxide. The result is similar to the theoretical calculations presented by Jin Qian [19]. The presented paper calculated the activity of iron catalyst, whose 25% of the atoms in the top layer were replaced with cobalt atoms. That type of catalyst had 2.3 times higher activity at 400 °C than the catalyst without the addition of cobalt.
4
Catalysis Communications 136 (2020) 105907
Z. Lendzion-Bieluń and A. Jurkowski
4. Conclusions
[4] S. Guan, H.Z. Liu, Effect of an Iron oxide precursor on the N2 desorption performance for an Ammonia synthesis catalyst, Ind. Eng. Chem. Res. 39 (2000) 2891–2895, https://doi.org/10.1021/ie990695g. [5] L. Huazhang, L. Caibo, L. Xiaonian, C. Yaqing, Effect of an Iron oxide precursor on the H2 desorption performance for an ammonia synthesis catalyst, Ind. Eng. Chem. Res. 42 (2003) 1347–1349, https://doi.org/10.1021/ie0202524. [6] Y.F. Zheng, H.Z. Liu, Z.J. Liu, X.N. Li, In situ X-ray diffraction study of reduction processes of Fe3O4- and Fe1−xO-based ammonia-synthesis catalysts, J. Solid State Chem. 182 (2009) 2385–2391, https://doi.org/10.1016/j.jssc.2009.06.030. [7] Z. Pu, Y. Zheng, H. Liu, X. Li, Influence of promoter and Fe2+/Fe3+ ratio on microstructure of fused iron catalysts for ammonia synthesis, Indian J. Chem. 50A (2011) 156–162 http://nopr.niscair.res.in/handle/123456789/11014. [8] W. Han, S. Huang, T. Cheng, H. Tang, Y. Li, H. Liu, Promotion of Nb2O5 on the wustite-based iron catalyst for ammonia synthesis, Appl. Surf. Sci. 353 (2015) 17–23, https://doi.org/10.1016/j.apsusc.2015.06.049. [9] L. Huazhang, Ammonia Synthesis Catalysts- Innovation and Practice, Word Scientific, 2013. [10] H. Liu, W. Han, Wüstite-based catalyst for ammonia synthesis: structure, property and performance, Catal. Today 297 (2017) 276–291, https://doi.org/10.1016/j. cattod.2017.04.062. [11] R. Jędrzejewski, Z. Lendzion-Bieluń, W. Arabczyk, The activity of fused-iron catalyst doped with lithium oxide for ammonia synthesis, Polish J. Chem. Tech. 18 (2016) 78–83, https://doi.org/10.1515/pjct-2016-0032. [12] Z. Lendzion-Bieluń, W. Arabczyk, Method for determination of the chemical composition of phases of the iron catalyst precursor for ammonia synthesis, Appl. Catal. A Gen. 207 (2001) 37–41, https://doi.org/10.1016/S0926-860X(00)00614-1. [13] Z. Lendzion-Bieluń, W. Arabczyk, M. Figurski, The effect of the iron oxidation degree on distribution of promotors in the fused catalyst precursors and their activity in the ammonia synthesis reaction, Appl. Catal. A Gen. 227 (2002) 255–263, https://doi.org/10.1016/S0926-860X(01)00938-3. [14] Ł. Czekajło, Z. Lendzion-Bieluń, Wustite based iron-cobalt catalyst for ammonia synthesis, Catal. Today 286 (2017) 114–117, https://doi.org/10.1016/j.cattod. 2016.11.013. [15] R. Jędrzejewski, Z. Lendzion-Bieluń, Reduction process of iron catalyst precursors for ammonia synthesis doped with lithium oxide, Catal. Today 8 (2018) 494, https://doi.org/10.3390/catal8110494. [16] R. Brown, M.E. Cooper, D.A. Whan, Temperature programmed reduction of alumina-supported iron, cobalt and nickel bimetallic catalysts, Appl. Catal. 3 (1982) 177–186, https://doi.org/10.1016/0166-9834(82)80090-0. [17] W. Arabczyk, I. Jasińska, R. Pelka, Measurements of the relative number of active sites on iron catalyst for ammonia synthesis by hydrogen desorption, Catal. Today 169 (2011) 97–101, https://doi.org/10.1016/j.cattod.2010.09.003. [18] R.J. Kaleńczuk, Effect of cobalt on the morphology and activity of fused iron catalysts for ammonia -synthesis, Appl. Catal. A General 112 (2) (1994) 149–160, https://doi.org/10.1016/0926-860X(94)80216-5. [19] J. Qian, A. Fortunelli, W.A. Goddard, Effect of co doping on mechanism and kinetics of ammonia synthesis on Fe(111) surface, J. Catal. 370 (2019) 364–371, https:// doi.org/10.1016/j.jcat.2019.01.001.
In this work, wustite-based catalyst precursors with various concentrations of cobalt and without cobalt addition were investigated for ammonia synthesis. The use of cobalt promoter improved the thermal resistance and specific surface area loss under H2 treatment of precursor catalyst. The same results were observed for the activity in ammonia synthesis after reduction of the catalyst precursor at 600 °C. These features can significantly improve lifetime of that type of catalysts. The best activity improvement was observed for the catalyst obtained from the precursor with cobalt (II) oxide content of 2.4 wt%, of which 90% was present in the wustite grains. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by The Polish Centre for Research and Development Centre No. Tango2/340001/NCBR/2017. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.105907. References [1] L. Huazhang, L. Xiaonian, Relationship between precursor phase composition and performance of catalyst for ammonia synthesis, Ind. Eng. Res. 36 (1997) 335–341, https://doi.org/10.1021/ie960072s. [2] H.Z. Liu, X.N. Li, Z.N. Hu, Development of novel low temperature and low pressure ammonia synthesis catalyst, Appl. Catal. A Gen. 142 (1996) 209–222, https://doi. org/10.1016/0926-860X(96)00047-6. [3] A. Jafari, A. Ebadi, S. Sahebdelfar, Effect of iron oxide precursor on the properties and ammonia synthesis activity of fused iron catalysts, React. Kinet. Mech. Catal. 126 (2019) 307–325, https://doi.org/10.1007/s11144-018-1498-6.
5
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Surface properties of wustite based iron-cobalt catalysts for ammonia synthesis reaction
T
⁎
Zofia Lendzion-Bieluń , Artur Jurkowski West Pomeranian University of Technology in Szczecin, Faculty of Chemical Technology and Engineering, Piastów Ave. 42, 71-065 Szczecin, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: Ammonia synthesis Wustite Cobalt oxide TPD-H2
In this work, surface properties of wustite-based iron and iron‑cobalt catalysts were investigated. Measurements of hydrogen temperature-programmed desorption (TPD-H2) and surface area of active forms of catalysts were conducted after reduction of catalyst precursors (16 h) in pure hydrogen at various temperatures (450, 475, 500, 550 and 600 °C). The amount of cobalt in the catalyst structure and the number of adsorption sites relative to hydrogen were correlated with the activity for ammonia synthesis reaction. The presence of cobalt increased thermal resistance of the catalysts and at the same time their activity in ammonia synthesis reaction.
1. Introduction Many scientists believe that everything on catalyst development for ammonia synthesis has already been done and that the only way to improve iron catalyst activity is to change the promoters used. However, the role of the precursor was omitted. This perception resulted from the assumption of a classical volcano-type curve of the catalyst activity versus the ratio of Fe2+/Fe3+ ions (denoted as R) in a precursor form. According to this assumption, the most active catalyst is obtained as a result of reduction of iron oxide with R equal to 0.5 [1]. This value corresponds to the magnetite form of Fe3O4. Increasing or decreasing R causes a decrease in the activity of the obtained iron catalyst. The change in the approach to the precursor form led to Liu Huazhang's team results [1]. The authors studied precursors with R higher than 0.5. They found that precursors with a high proportion of Fe2+ ions, called wustite, showed higher activity than traditional iron catalysts. In many publications the Chinese team proved that the wustitebased catalyst is characterized by higher catalytic activity and more advantageous parameters of the reduction process parallel to almost the same thermostability and resistance to CO poisoning in comparison with the conventional magnetite-based iron and iron‑cobalt catalysts [2]. In the following years of studies on the wustite iron catalyst, many scientist described the influence of the phase composition of the precursor on the catalytic activity and the reduction process [3], the impact of a kind of the oxide precursor on the nitrogen desorption process [4], hydrogen desorption [3,5], the in situ course of reduction of
⁎
wustite and magnetite catalysts [6], the influence of Fe2+/Fe3+ ratio and promoter additions on the microstructure of the iron fused catalyst for the ammonia synthesis [7], the influence of Nb2O5 addition, as a potential promoter, on the reduction and activity of the wustite catalyst in ammonia synthesis [8]. A peculiar resume of the collected experimental results and general knowledge on ammonia synthesis catalysts was published by Huazhang Liu as a book [9] and a research paper [10]. The distribution of promoters in the catalyst precursor structure affects distribution of promoters in the form of an active iron catalyst for ammonia synthesis. The uniform distribution of promoters over the surface of the active form of a catalyst determines high activity and stability of catalyst structure. The results of research on the distribution of lithium oxide in the catalyst structure [11] support that statement. The etching method [12] allows us to examine how individual promoters are distributed in the oxidized form of the iron fused catalyst. The effect of the molar R (Fe2+/Fe3+) ratio in iron catalyst precursors on the distribution of promoters tested with the etching method is described in [13]. According to the results presented there, potassium oxide is found entirely in the intergranular spaces regardless of R. Aluminium oxide is present in the iron catalyst precursors in three forms: one directly bound to the grain magnetite or wustite, and two forms located in intergranular spaces, soluble as well as insoluble in HCl. Calcium oxide occurs in intergranular spaces, both in magnetite and wustite grains. It was found that when R increases the concentration of aluminium decreases while the concentration of calcium increases in wustite grains. In our recent work [14] we stated that the introduction of cobalt
Corresponding author. E-mail address: [email protected] (Z. Lendzion-Bieluń).
https://doi.org/10.1016/j.catcom.2019.105907 Received 15 July 2019; Received in revised form 10 December 2019; Accepted 11 December 2019 Available online 12 December 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 136 (2020) 105907
Z. Lendzion-Bieluń and A. Jurkowski
into the wustite‑iron catalyst structure increased the activity of the catalyst in the ammonia synthesis reaction. The purpose of the present work is to determine the effect of the addition of cobalt into the ironbased wustite catalyst structure on the distribution of promoters in the precursors, and correlation of the activity of these catalysts in the reaction of ammonia synthesis with hydrogen adsorption on active surface sites. 2. Experimental 2.1. Preparation of catalysts Iron and iron‑cobalt precursors of catalysts (an oxidized form of catalysts) were obtained by melting in a laboratory installation as described elsewhere [15]. Magnetite ore, oxides of cobalt, calcium and aluminium, as well as potassium nitrate were used for melting. Metallic iron powder was used as a reducing agent. The proportions of the individual components were selected so as to obtain the assumed concentration of promoters and cobalt, as well as the appropriate R of the catalysts obtained. The melting process was continued for approximately 50 min and then the lava of catalyst precursors was transferred to a water-cooled crystallizer. A batch of catalyst precursor obtained in one melting cycle was roughly 10 kg. The catalyst precursor was then shredded and screened. Precursor grains of 1.0–1.2 mm were used in catalytic tests. The catalyst precursor without the addition of cobalt was designated W-8 while the catalyst precursors with various amounts of added cobalt W-11, W-14 and W-15, respectively.
Fig. 1. Powder XRD patterns of obtained catalyst precursors W-8, W-11, W-14 and W-15.
of promoters embedded directly in the wustite structure and the catalysts in the intergranular spaces of the tested catalysts was assessed. The activity of the obtained catalyst precursors (oxidized form of catalysts) in the ammonia synthesis reaction was tested using a sixchannel reactor. A stoichiometric mixture of 3H2:N2 obtained from decomposition of ammonia was used as a reducing agent and synthesis gas. An analyzer and an interferometer were used to measure the concentration of ammonia at the outlet. The exact conditions of the reduction and synthesis of ammonia reaction were presented elsewhere [11,14].
2.2. Characterization of catalysts The phase composition of the obtained catalyst precursors was examined by X-ray powder diffraction analysis using the Philips X'Pert Pro apparatus. The source of X-ray radiation used was a CoKα lamp. The phase composition and positions of 2θ reflections were determined using the X'Pert HighScore Plus software. The ICP-OES analytical method (Perkin Elmer Optime 5300DV spectrometer) was used to determine the quantitative composition of the tested catalyst precursors. The ratio R of Fe2+/Fe3+ ions was determined after manganometric titration. Before titration, samples of the catalyst precursors were dissolved in hydrochloric acid under an nitrogen gas atmosphere. Measurements of the temperature-programmed hydrogen desorption (TPD-H2) and specific surface area of reduced catalysts were carried out using the Autochem II 2920. (Micromeritics). The analyses were conducted after 16-h reduction of the catalyst precursors in a pure hydrogen gas atmosphere (flow 70 cm3/min) at five various different temperatures, ca. 450, 475, 500, 550 and 600 °C. Before the start of TPD-H2, an adsorption of hydrogen (100 vol%) at 20 °C for 1 h over the surface of the pre-reduced catalysts was conducted. The specific surface area (SSA, m2/g) of the catalysts in the reduced form was determined by one-point low-temperature nitrogen adsorption at 77 K. The distribution of promoters was determined by the selective hydrochloric acid etching method described elsewhere [12]. Weighted portions of the catalyst precursors of 0.50 g and 50 cm3 of an aqueous hydrochloric acid solution of appropriate concentration were placed in 200 cm3 Erlenmeyer flasks. The resulting suspensions were placed in a shaker where samples were dissolved at room temperature. Various contact times (5–240 min) and various concentrations of HCl solutions (0.9–36 vol%) were used to obtain different degrees of iron oxide and promoter dissolution. Subsequently, the solutions were gravitationally filtered out with a funnel using a paper filter. The obtained filtrates were tested for iron, aluminium, calcium, potassium and cobalt content using the ICP-OES method. Based on the results of the conducted analyses (graph of the percentage of promoter etching vs. the degree of iron etching), the content
3. Results and discussion The obtained X-ray diffraction patterns of catalyst precursors (oxidized form of catalysts) shown in Fig. 1 confirm the presence of the wustite phase (JCPSD 00–006-0615). No reflections due to cobalt(II) oxide, oxides of promoters and other compounds of these elements are observed in the diffraction patterns. This result may prove that these compounds were embedded in the structure of iron oxide and/or formed very fine crystalline phases. After examining the exact position of reflections, a slight shift in the peak maxima can be seen. Comparing the precursor of the catalysts W-8 without addition of cobalt (II) oxide and W-11 with the highest content of that promoter, it could be seen that the position of reflex shifted slightly towards higher angles for W11 precursor (insert Fig. 1). This is due to the incorporation of cobalt ions into the wustite structure. Co2+ions have a smaller ion radius than Fe2+ ions. The consequence of this is a decrease in the interplanar distance (d) which can be estimated from powder XRD results. According to Bragg's law, the smaller interplanar distance would cause a shift of the observed diffraction towards higher 2theta angles. This could provide evidence for the embedding of cobalt ions into the structure of the wustite. Table 1 presents analysis results of the Table 1 The ratio R and percentage composition of iron-based and iron‑cobalt based wustite catalyst precursors. Sample name
R
Oxide content [wt%] Al2O3
W-8 W-11 W-15 W-14 a
2
6.4 4.9 6.2 6.8
CaO a
1.8/0.8 3.4/1.1a 2.1/0.8a 2.1/0.7a
Content in catalyst wustite grain.
a
1.6/1.0 2.1/0.8a 2.1/1.0a 1.8/0.6a
K2O
CoO
0.4 0.7 0.4 0.4
– 3.6/2.30a 2.4/2.10a 1.4/0.75a
Catalysis Communications 136 (2020) 105907
Z. Lendzion-Bieluń and A. Jurkowski
the function of the structural promoter, responsible for their thermostability. Therefore, in the case of catalyst precursors with a wustite structure and with the concentration of this oxide being lower than in the magnetite grains [13], the function of the structural promoter that increases thermostability is taken over by calcium oxide. Based on the presented results of XRD and etching analysis method, we can conclude that cobalt was embedded into the wustite structure and partly assumed the role of a structural promoter, increasing the thermostability of catalysts. Based on our earlier findings [14] and data reported by others [16], the addition of cobalt to an iron catalyst is known to increase its reducibility. Moreover, the addition of cobalt affected the increase of catalyst activity in the ammonia synthesis reaction. Our predictions on the cobalt role in the wustite catalyst structure were confirmed by the following hydrogen chemisorption/TPD results. According to the literature [17], TPD-H2 curves for iron catalysts consist of two peaks. The first desorption peak corresponds to atomic hydrogen bonded on the iron surface, while the second peak to hydrogen dissolved in the bulk of iron catalyst. The TPD-H2 profiles of two example catalysts, without cobalt addition (W-8) and with cobalt addition (W-14) are shown in Electronic Supporting Information (ESI, Fig. S1). For other tested catalysts, TPD-H2 profiles look similar, where each profile shows two peak maxima. TPD-H2 measurements were carried out over catalyst surfaces after reduction at various temperatures and results are presented in Fig. 3 along with catalytic activity results. More precisely, the relationship of the volume of desorbed hydrogen (TPD-H2) with catalytic activity at 450 °C, the latter expressed as a rate constant for the ammonia synthesis reaction, k [gNH3∙MPa0.5/gCat∙h], derived from Temkin-Pyzhew equation, is presented as a function of the temperature used in the reduction process for the cobalt catalysts. The W-14 and W-15 catalysts show the highest activity (based on the k value) in the ammonia synthesis reaction as shown in Fig. 3A and B, after reducing the precursors at 500 °C. The volume of desorbed hydrogen is also the largest for these catalysts after reduction at 500 °C. The W-11 catalyst with the highest cobalt content in the wustite grain, reached its maximum activity after reduction at 450 °C as illustrated in Fig. 3C. The lowest reduction temperature after which the catalyst with the highest content of cobalt (W11) achieved its maximum activity confirm that cobalt in the catalyst grains accelerated the reduction process. The catalyst W-8, without cobalt, achieved the maximum volume of hydrogen desorbed after reduction at 475 °C as depicted in Fig. S2 (ESI). At the same time, the catalyst achieved its maximum activity after reduction at this temperature. However, both the activity and the amount of desorbed hydrogen were lower for this catalyst than for W-11, W-14 and W-15 catalyst samples after reduction at all temperatures tested. The amount of adsorbed hydrogen on the reduced catalyst surface and which was determined by the TPD-H2 technique (Fig. 3), could be correlated with the number of active sites on the catalyst surface for hydrogen activation. The decrease in the amount of hydrogen adsorbed on the surface of the reduced catalyst with the increase of reduction temperature could be related to the process of recrystallization and sintering of catalyst crystallites. The fact that the recrystallization process proceeded is also evidenced by the change in the specific surface area of the catalysts after the reduction process at various temperatures (Fig. 4). The largest decrease in specific surface area along with the increase in reduction temperature was found for the W-8 catalyst without the cobalt addition. The percentage reduction of the specific surface area for that catalyst after reduction at 500 °C with respect to the initial value obtained after reduction at 450 °C, was about twice as high as that for the W-11 and W-15 catalysts, and five times higher compared to the W-14 catalyst. It proves that the addition of cobalt affected the improvement of resistance to sintering of wustite catalysts. Fig. 5 shows the change in the reaction rate constant k of ammonia synthesis at 400 °C with the content of cobalt oxide in the grain of
Fig. 2. Dependence of etching degree (%) of cobalt on the etching degree (%) of iron in the catalyst precursors W-11, W-14 and W-15.
quantitative composition and R values of the tested catalyst precursors. The catalyst precursors obtained with the addition of cobalt differed by the molar ratio R, the content of promoters (Al2O3, CaO, K2O) and cobalt (CoO). The catalyst precursor W-11 had the lowest R ratio (4.9) and the highest content of promoter oxides and cobalt oxide. The catalyst precursors W-14 and W-15 had similar values of R and the content of promoter oxides, while the content of cobalt oxide varied. The catalyst precursor W-8, without cobalt, had a similar value of R (6.4) to the W-15 and W-14 catalyst precursors. The oxidized form of the iron catalyst consists of intergranular spaces and wustite grains for R > 4.5 [9]. For R < 4.5, magnetite grains occur additionally. On the basis of the previous research [12–15], it is known that the structure of iron catalyst and its activity in ammonia synthesis depend on the distribution of promoters in grains of the oxidized form of the catalyst (magnetite or wustite). Promoter content in wustite grains was determined with the selective etching method. In the first stage of etching, iron ions and promoter elements from intergranular spaces migrated into solution and next wustite grains were dissolved [12]. Fig. 2 shows the dependence of the etching degree of cobalt on the iron etching degree in the process of selective etching of the precursor catalysts in hydrochloric acid solution. It can be seen that some of the cobalt, ca. from 20% to 40%, passed into the solution with a small degree of iron dissolution (maximum at 15%). It was cobalt, located in intergranular spaces of the oxidized form of the catalyst. The remaining cobalt is dissolved together with iron that forms wustite grains. This finding can also confirm that cobalt is embedded into the wustite structure. On the basis of cobalt content in the catalyst precursors and the slope of dissolution curves (Fig. 2), the cobalt content in wustite grains was calculated. In the same way, the contents of CaO and Al2O3 promoters in the wustite grain were calculated on the basis of dissolution curves. The calculated values are presented in Table 1. As in the case of catalyst precursor of the magnetite catalyst [12], > 90% of potassium oxide was found in intergranular spaces of the studied wustite precursors. The content of aluminium oxide in the wustite grains was found to depend on R. The Al3+ ions, due to the same valence as Fe3+, integrate seamlessly into the Fe3O4 structure to form a solid solution (FeAl2O4) with the same crystallographic structure as Fe3O4 [16]. In the wustite, Al2O3 created a substitutional solid solution that builds up to a lesser extent due to larger obstacles caused by various oxidation states. Therefore, when R increased in the tested oxidized forms of catalysts, the concentration of Al2O3 dropped from 1.1 to 0.7 wt%. The aluminium oxide in the structure of magnetite catalyst precursors performs 3
Catalysis Communications 136 (2020) 105907
Z. Lendzion-Bieluń and A. Jurkowski
Fig. 3. Volume of desorbed hydrogen (TPD-H2) and rate constant k (activity measurements of ammonia synthesis reaction) as a function of reduction temperature over: A) catalyst W-11, B) catalyst W-14, C) catalyst W-15. Reduction temperatures: 450, 475, 500 and 600 °C.
Fig. 4. Change of specific surface area (m2/g) with reduction temperature for the catalysts W-8, W-11, W-14 and W-15.
Fig. 5. Dependence of the specific activity of catalyst (k) measured at 400 °C on the amount (wt%) of cobalt oxide in catalyst wustite grain.
wustite for different reduction temperatures. Catalysts with the addition of cobalt reach the maximum activity after reduction at 500 °C. The W-15 catalyst, where the concentration of CoO in the wustite phase is 2.1 wt%, reaches the highest value of k constant. A further increase in CoO content in the catalyst W-11 causes a slight decrease in k-value. Comparing the k-values for catalysts after reduction at 600 °C, cobalt addition increases the k rate constant value. Based on these results we can conclude that, just like in the magnetite catalyst precursors doped with cobalt [18], there is also a certain optimum content of cobalt in the wustite catalyst precursors.
The activity of the catalyst with cobalt oxide content of 2.1 wt% in catalyst grains after reduction at 500 °C was 2.1 times higher than that of the catalyst without the addition of cobalt (II) oxide. The result is similar to the theoretical calculations presented by Jin Qian [19]. The presented paper calculated the activity of iron catalyst, whose 25% of the atoms in the top layer were replaced with cobalt atoms. That type of catalyst had 2.3 times higher activity at 400 °C than the catalyst without the addition of cobalt.
4
Catalysis Communications 136 (2020) 105907
Z. Lendzion-Bieluń and A. Jurkowski
4. Conclusions
[4] S. Guan, H.Z. Liu, Effect of an Iron oxide precursor on the N2 desorption performance for an Ammonia synthesis catalyst, Ind. Eng. Chem. Res. 39 (2000) 2891–2895, https://doi.org/10.1021/ie990695g. [5] L. Huazhang, L. Caibo, L. Xiaonian, C. Yaqing, Effect of an Iron oxide precursor on the H2 desorption performance for an ammonia synthesis catalyst, Ind. Eng. Chem. Res. 42 (2003) 1347–1349, https://doi.org/10.1021/ie0202524. [6] Y.F. Zheng, H.Z. Liu, Z.J. Liu, X.N. Li, In situ X-ray diffraction study of reduction processes of Fe3O4- and Fe1−xO-based ammonia-synthesis catalysts, J. Solid State Chem. 182 (2009) 2385–2391, https://doi.org/10.1016/j.jssc.2009.06.030. [7] Z. Pu, Y. Zheng, H. Liu, X. Li, Influence of promoter and Fe2+/Fe3+ ratio on microstructure of fused iron catalysts for ammonia synthesis, Indian J. Chem. 50A (2011) 156–162 http://nopr.niscair.res.in/handle/123456789/11014. [8] W. Han, S. Huang, T. Cheng, H. Tang, Y. Li, H. Liu, Promotion of Nb2O5 on the wustite-based iron catalyst for ammonia synthesis, Appl. Surf. Sci. 353 (2015) 17–23, https://doi.org/10.1016/j.apsusc.2015.06.049. [9] L. Huazhang, Ammonia Synthesis Catalysts- Innovation and Practice, Word Scientific, 2013. [10] H. Liu, W. Han, Wüstite-based catalyst for ammonia synthesis: structure, property and performance, Catal. Today 297 (2017) 276–291, https://doi.org/10.1016/j. cattod.2017.04.062. [11] R. Jędrzejewski, Z. Lendzion-Bieluń, W. Arabczyk, The activity of fused-iron catalyst doped with lithium oxide for ammonia synthesis, Polish J. Chem. Tech. 18 (2016) 78–83, https://doi.org/10.1515/pjct-2016-0032. [12] Z. Lendzion-Bieluń, W. Arabczyk, Method for determination of the chemical composition of phases of the iron catalyst precursor for ammonia synthesis, Appl. Catal. A Gen. 207 (2001) 37–41, https://doi.org/10.1016/S0926-860X(00)00614-1. [13] Z. Lendzion-Bieluń, W. Arabczyk, M. Figurski, The effect of the iron oxidation degree on distribution of promotors in the fused catalyst precursors and their activity in the ammonia synthesis reaction, Appl. Catal. A Gen. 227 (2002) 255–263, https://doi.org/10.1016/S0926-860X(01)00938-3. [14] Ł. Czekajło, Z. Lendzion-Bieluń, Wustite based iron-cobalt catalyst for ammonia synthesis, Catal. Today 286 (2017) 114–117, https://doi.org/10.1016/j.cattod. 2016.11.013. [15] R. Jędrzejewski, Z. Lendzion-Bieluń, Reduction process of iron catalyst precursors for ammonia synthesis doped with lithium oxide, Catal. Today 8 (2018) 494, https://doi.org/10.3390/catal8110494. [16] R. Brown, M.E. Cooper, D.A. Whan, Temperature programmed reduction of alumina-supported iron, cobalt and nickel bimetallic catalysts, Appl. Catal. 3 (1982) 177–186, https://doi.org/10.1016/0166-9834(82)80090-0. [17] W. Arabczyk, I. Jasińska, R. Pelka, Measurements of the relative number of active sites on iron catalyst for ammonia synthesis by hydrogen desorption, Catal. Today 169 (2011) 97–101, https://doi.org/10.1016/j.cattod.2010.09.003. [18] R.J. Kaleńczuk, Effect of cobalt on the morphology and activity of fused iron catalysts for ammonia -synthesis, Appl. Catal. A General 112 (2) (1994) 149–160, https://doi.org/10.1016/0926-860X(94)80216-5. [19] J. Qian, A. Fortunelli, W.A. Goddard, Effect of co doping on mechanism and kinetics of ammonia synthesis on Fe(111) surface, J. Catal. 370 (2019) 364–371, https:// doi.org/10.1016/j.jcat.2019.01.001.
In this work, wustite-based catalyst precursors with various concentrations of cobalt and without cobalt addition were investigated for ammonia synthesis. The use of cobalt promoter improved the thermal resistance and specific surface area loss under H2 treatment of precursor catalyst. The same results were observed for the activity in ammonia synthesis after reduction of the catalyst precursor at 600 °C. These features can significantly improve lifetime of that type of catalysts. The best activity improvement was observed for the catalyst obtained from the precursor with cobalt (II) oxide content of 2.4 wt%, of which 90% was present in the wustite grains. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by The Polish Centre for Research and Development Centre No. Tango2/340001/NCBR/2017. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.105907. References [1] L. Huazhang, L. Xiaonian, Relationship between precursor phase composition and performance of catalyst for ammonia synthesis, Ind. Eng. Res. 36 (1997) 335–341, https://doi.org/10.1021/ie960072s. [2] H.Z. Liu, X.N. Li, Z.N. Hu, Development of novel low temperature and low pressure ammonia synthesis catalyst, Appl. Catal. A Gen. 142 (1996) 209–222, https://doi. org/10.1016/0926-860X(96)00047-6. [3] A. Jafari, A. Ebadi, S. Sahebdelfar, Effect of iron oxide precursor on the properties and ammonia synthesis activity of fused iron catalysts, React. Kinet. Mech. Catal. 126 (2019) 307–325, https://doi.org/10.1007/s11144-018-1498-6.
5