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perovskites in steam reforming of methane
Applied Catalysis A: General 286 (2005) 23–29 www.elsevier.com/locate/apcata
Catalytic activities and coking resistance of Ni/perovskites in steam reforming of methane Kohei Urasaki, Yasushi Sekine, Sho Kawabe, Eiichi Kikuchi, Masahiko Matsukata * Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan Received 5 October 2004; received in revised form 7 February 2005; accepted 17 February 2005 Available online 12 April 2005
Abstract Steam reforming of methane for the purpose of hydrogen production was performed using nickel catalysts supported on a variety of perovskites, including LaAlO3, LaFeO3, SrTiO3, BaTiO3, La0.4Ba0.6Co0.2Fe0.8O3d, to compare the catalytic activity and resistance to coking of these catalysts to those of the conventional Ni/a-Al2O3 catalyst. Ni/LaAlO3 and Ni/SrTiO3 showed high catalytic activities among the Ni/ perovskites and longer-term stabilities than the conventional catalyst. Temperature programmed oxidation of carbon deposited on used catalysts revealed that inactive carbon species detected on Ni/a-Al2O3 were not formed in the case of Ni/LaAlO3. The results of temperature programmed reduction confirmed that consumption and recovery of the lattice oxygen in perovskites occurred during the reaction, and that the reducibility of perovskites had a great impact on the steam reforming activity. The lattice oxygen in perovskites is considered to play important roles in promoting the oxidation of CHx fragments adsorbed on metallic nickel. # 2005 Published by Elsevier B.V. Keywords: Steam reforming of methane; Lattice oxygen; Coking; Hydrogen; Ni/LaAlO3; Ni/LaFeO3; Ni/SrTiO3; Ni/BaTiO3; Ni/La0.4Ba0.6Co0.2Fe0.8O3d
1. Introduction Steam reforming of methane, which has been used for large-scale hydrogen production in industrial processes such as petroleum refinery and ammonia synthesis, has become increasingly important recently because of its potential applications in fuel cells. As the steam reforming reaction is strongly endothermic, the process requires reaction temperatures higher than 1000 K and an excess amount of steam to prevent coking on the catalyst. However, high reaction temperatures and the use of excess steam cause increases in the amount of energy consumed and consequently in the hydrogen production cost. The development of new, more active steam reforming catalysts with high durability against coking is thus desirable. * Corresponding author. Tel.: +81 35286 3850; fax: +81 35286 3850. E-mail addresses: [email protected] (K. Urasaki), [email protected] (M. Matsukata). 0926-860X/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.apcata.2005.02.020
The basic reaction in the steam reforming of methane is as follows: CH4 þ H2 O ! CO þ 3H2
0 1 DH298 K ¼ 206 kJ mol
Although rhodium and ruthenium are more active catalytically, nickel has generally been used as the catalyst in steam reforming processes, mainly for practical reasons. Specifically, nickel supported on inexpensive, thermally stable aalumina or alumina–magnesia has been widely employed as the conventional industrial steam reforming catalysts [1,2]. As previous researchers have often pointed out, however, nickel is more susceptible to coking and tends to deactivate [3,4]. From an industrial point of view, the development of nickel catalysts with greater resistance to coking is thus an attractive research goal. An effective approach to developing such nickel catalysts is to focus on the selection and modification of catalyst supports. It is widely accepted that the addition of alkali,
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alkali earth oxides and rare earth metal oxides to the aAl2O3 support or the use of basic metal oxides as the support improves resistance to coking. This positive effect is understood to result from the enhancement in steam adsorption, in the oxidation rate of CHx fragments adsorbed on metallic nickel and/or the reduction of methane activation and dissociation [2]. Although improvements of the support greatly impact catalytic activity and/or resistance to coking, limited attempts have been made to apply oxygen-ion conducting oxides, for instance perovskite-type oxides, to the steam reforming catalysts. It is mechanistically expected that oxidation of CHx fragments adsorbed on metallic nickel would be promoted by the lattice oxygen in oxygen-ion conducting oxides and that the consumed lattice oxygen would be regenerated by steam. Such a mechanism has specifically been proposed for CO oxidation [5,6] and dry reforming of methane [7–9]. Huang and coworkers have evaluated the activity of nickel supported on ceria-based ionconducting oxides for dry reforming of methane and have pointed out that the lattice oxygen in modified ceria may play some positive roles in the activation of methane and carbon dioxide [10–12]. Takehira et al. have reported that the perovskite-type oxides such as SrTiO3, CaTiO3, BaTiO3 that contain a small amount of nickel in the titanium sites show high catalytic activities with high resistance to coking in both partial oxidation of methane [13–15] and dry reforming of methane [16], due to the high dispersion of nickel; these researchers also examined oxygen mobility in perovskites, and found that the high resistance to coking might be partly due to the migration of mobile oxygen from the perovskite support to the metallic nickel particles. In the present study, the catalytic activity and resistance to coking of nickel catalysts supported on a variety of the perovskite-type oxides (LaAlO3, LaFeO3, SrTiO3, BaTiO3, La0.4Ba0.6Co0.2Fe0.8O3d) are compared to those of the conventional Ni/a-Al2O3 catalyst for steam reforming of methane. To investigate differences in catalytic activity among the Ni/perovskite catalysts examined, we estimated the dispersion and reduction properties of nickel by means of CO chemisorption and temperature programmed reduction, respectively. Finally, the roles of the lattice oxygen on the catalytic activity and carbon deposition will be discussed.
2. Experimental 2.1. Catalyst preparation Some perovskite-type oxides including LaAlO3, LaFeO3, SrTiO3, BaTiO3, and La0.4Ba0.6Co0.2Fe0.8O3d(LBCF) as well as a-Al2O3 were prepared for use as catalyst supports as follows. LaAlO3, LaFeO3 and LBCF were prepared by the sol–gel method. The metal nitrates obtained from Kanto Chemical Co., Inc., were dissolved into water; then excess amounts of
citric acid and ethylene glycol were added to the solutions. The molar ratio of total metal ion:citric acid:ethylene glycol was 1:3:3. The obtained solutions were evaporated until gellike materials were formed. These materials were then calcined successively at 673 K for 2 h and at 1123 K for 11 h. SrTiO3 and BaTiO3 were prepared by physically mixing TiO2 (anatase; Idemitsu Co., Ltd.) and SrCO3 or BaCO3 (Kanto Chemical Co., Inc.). The mixtures were then calcined at 1423 K for 10 h. The a-Al2O3 support was formed by calcining g-Al2O3 (Nishio Industry Co., Ltd.) at 1573 K for 2 h. Nickel catalysts were prepared by the impregnation method using these supports and an aqueous solution of Ni(NO3)26H2O (Kanto Chemical Co., Inc.) to produce a Ni loading of 10 wt.%. The resulting compounds were dried overnight at 393 K, and calcined at 773 K for 1 h. 2.2. Activity test Steam reforming of methane using the various nickel catalysts was carried out in a continuous flow system with a fixed bed of catalyst at atmospheric pressure. After a catalyst was reduced in a hydrogen stream, at 1073 K for 1 h, the carrier gas was switched from hydrogen to a mixture of methane and steam with a molar H2O/CH4 ratio of 1 or 2 at 1073 K and 0.1 MPa. The products were analyzed by means of a gas chromatograph equipped with a thermal conductivity detector using Shimadzu GC-8A and an active charcoal separation column. Catalytic activity was evaluated in terms of methane conversion, calculated on the basis of the following formula: CCH4 methane conversionð%Þ ¼ 1 CCH4 þ CCO2 þ CCO 100 where, C is the molar fraction in the outlet gas. 2.3. Characterization The reduction of each catalyst was monitored by means of temperature programmed reduction (TPR) in an H2 flow from ambient temperature to 1173 K. The weight loss of each catalyst was determined by means of thermogravimetry (TG) using a Shimadzu TG-50A. The amount of deposited carbon on each catalyst after the reaction was quantified and characterized by temperature programmed oxidation (TPO) in a stream of 10% O2/He from ambient temperature to 1073 K; the amount of CO2 formed was measured by use of a quadrupole mass spectrometer. X-ray diffraction (XRD) patterns of samples were recorded with a Rigaku RINT-2000 X-ray diffractometer employing Cu Ka radiation filtered by nickel. BET specific surface areas of supports were determined by nitrogen adsorption at 77 K using a BEL JAPAN BELSORP 28SA.
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The active surface area of metallic nickel after the reduction procedure was determined by means of CO chemisorption using Quantachrome Autosorb-1-C. Before measurement, samples were reduced in situ in a hydrogen stream at a temperature of 1073 K for 1 h, evacuated to vacuum at the same temperature and cooled to 303 K in the vacuum. Afterwards, the CO adsorption isotherm, including both physisorption and chemisorption, was measured. After evacuation at 303 K, a second isotherm including only physisorption was measured. Chemisorbed CO uptakes were determined by examining the difference between the two isotherms. The dispersion of nickel was calculated under the
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assumption that the stoichiometry of CO/Ni(surface) was equal to 2.
3. Results and discussion 3.1. Structures of Ni/perovskite catalysts Fig. 1 shows the XRD patterns of Ni/LaAlO3, Ni/LaFeO3, Ni/SrTiO3, Ni/BaTiO3, and Ni/LBCF before and after reduction, and after activity tests. Every Ni/perovskite catalyst showed the diffraction from the corresponding perovskite structures as well as the diffraction from nickel oxide. After reduction and activity tests, LaAlO3, SrTiO3 and BaTiO3 maintained their perovskite structures. On the other hand, the structures of LaFeO3 and LBCF changed drastically. After reduction of these two compounds, the reflection peaks related to perovskites were weakened and broadened and other peaks related to some oxides, metals or alloys produced by reduction of LaFeO3 and LBCF were observed. After the activity tests, the reduced LaFeO3 and LBCF were reoxidized by steam and their perovskite structures were recovered. 3.2. Catalytic activities of Ni/perovskite catalysts To examine the effects of perovskites on catalytic activities, the activities of both the various Ni/perovskite catalysts and the Ni/a-Al2O3 catalyst in steam reforming of methane were tested under the conditions of 1073 K and 0.1 MPa with a molar H2O/CH4 ratio of 2. Fig. 2 shows the time on stream changes of CH4 conversion for the various
Fig. 1. XRD patterns of (A) fresh catalysts, (B) reduced catalysts and (C) used catalysts: (*) perovskite, (&) nickel, (!) nickel oxide. Catalyst; (a) Ni/LaAlO3, (b) Ni/SrTiO3, (c) Ni/LaFeO3, (d) Ni/BaTiO3, (e) Ni/LBCF.
Fig. 2. Catalytic activities of supported Ni catalysts. Catalyst; (*) Ni/ LaAlO3, (&) Ni/SrTiO3, (*) Ni/LaFeO3, (~) Ni/BaTiO3, (!) Ni/LBCF, (^) Ni/a-Al2O3. Reaction conditions: temperature, 1073 K; molar H2O/ CH4 ratio, 2; W/F, 1.58 g h mol1.
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Fig. 3. Effect of dispersion on catalytic activities of Ni/perovskites. Reaction conditions: temperature, 1073 K; molar H2O/CH4 ratio, 2; W/F, 1.58 g h mol1.
Ni/perovskite and the Ni/a-Al2O3 catalysts. The catalytic activities of the catalysts tested varied considerably. Ni/ LaAlO3 and Ni/SrTiO3 showed high activities and the CH4 conversion levels of 91.7 and 88.4 % at 1 h of run, respectively. The activities of Ni/LaAlO3 and Ni/SrTiO3 were comparable to that of Ni/a-Al2O3. On the other hand, Ni/LaFeO3 showed a lower activity and the activities of Ni/ BaTiO3 and Ni/LBCF were extremely low. The reaction products produced by all these catalysts were composed of CO, CO2, and H2; no other products were detected. The gas composition of the outlet gas for all catalysts was in equilibrium, as in most cases of steam reforming of methane using nickel catalysts, where the rate of steam reforming of methane is much slower than that of water-gas-shift reaction at high reaction temperatures [1,2]. For the purpose of examining such large differences in catalytic activities among perovskite supported nickel catalysts tested, we determined the active surface area of metallic nickel by means of CO chemisorption. Table 1 lists nickel particle size and dispersion of nickel, calculated from CO uptakes. Fig. 3 shows the relation between catalytic activity and dispersion of nickel, calculated by the CO uptakes. As there was a clear correlation between CH4 conversion and dispersion, we concluded that nickel particle size, which varied significantly among the Ni/perovskite Table 1 Dispersion, particle size and specific surface area of Ni/perovskites Catalysts
Dispersion (%)
Particle size (nm)
SAa m2/g-cat1
Ni/LaAlO3 Ni/SrTiO3 Ni/LaFeO3 Ni/BaTiO3 Ni/LBCF
1.6 1.2 0.48 0.36 0.22
64 82 210 280 460
15.9 13.4 15.9 15.0 6.0
a
Specific surface area calculated by BET method.
Fig. 4. Stabilities of Ni/a-Al2O3 and Ni/LaAlO3. Reaction conditions: temperature, 1073 K; molar H2O/CH4 ratio, 1; W/F, 1.58 g h mol1. Catalyst; (*) Ni/LaAlO3, (*) Ni/a-Al2O3.
catalysts, had a considerable impact on catalytic activity. The surface area of supports might reasonably be expected to be a factor in the dispersion of nickel; however, the differences in the BET surface areas of the perovskites tested did not affect the dispersion of nickel, as shown in Table 1. Although it is still not clearly understood why the dispersion of nickel changes depending on the kind of perovskite supports, nickel particle size may be determined by an affinity between the precursors of nickel species and the perovskites. As shown in Table 1, the nickel particle sizes obtained by CO chemisorption in Ni/LaFeO3, Ni/BaTiO3 and Ni/LBCF were larger than 200 nm. Such extremely large values obtained by CO chemisorption may result from the encapsulation of nickel clusters caused by the partial reduction of supports through strong metal support interaction (SMSI) [17]. The effects of the properties of supports on the dispersion of nickel will be studied further. 3.3. Stabilities of Ni/LaAlO3 To compare the stabilities of Ni/LaAlO3 and Ni/a-Al2O3, we continued activity tests for 24 h under the conditions of 1073 K, 0.1 MPa and a molar H2O/CH4 ratio of 1. The results are shown in Fig. 4. In the case of Ni/a-Al2O3, CH4 conversion decreased from 83% at 2 h to 75% at 24 h, while Ni/LaAlO3 was stable, maintaining CH4 conversion levels of 77–80% throughout the run. Deposition of coke is one of the major causes of catalyst deactivation in steam reforming of hydrocarbons. The quantity and quality of carbon deposited on Ni/LaAlO3 and Ni/a-Al2O3 after reaction for 24 h were thus evaluated by means of TPO measurements. The results are shown in Fig. 5. In the case of Ni/a-Al2O3 which was deactivated gradually during the activity test, three peaks
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Fig. 6. Time programmed reduction using TG for (A) fresh catalysts and (B) supports. Catalysts; (A) (a) Ni/LaAlO3, (b) Ni/SrTiO3, (c) Ni/LaFeO3, (d) Ni/BaTiO3, (e) Ni/LBCF, (B) (a) LaAlO3, (b) SrTiO3, (c) LaFeO3, (d) BaTiO3, (e) LBCF. Fig. 5. Temperature programmed oxidation for (a) Ni/LaAlO3 and (b) Ni/aAl2O3 after reaction for 24 h.
were observed: around 623, 773 and 923 K, and the total amount of deposited carbon was 3.96 mg g-cat1. In contrast, Ni/LaAlO3 gave only one peak at around 623 K and the amount of deposited carbon was 3.39 mg g-cat1, slightly less than that deposited on Ni/a-Al2O3. The amount of deposited carbonaceous species oxidized at lower temperatures, designated as Ca, was approximately the same for both Ni/a-Al2O3 and Ni/LaAlO3; the formation of Ca species may therefore be considered to have no influence on catalyst deactivation, and the Ca species may be understood to be reactive, reacting with steam to yield CO or CO2 that is removed immediately from the surface of the catalyst. In contrast, carbonaceous species oxidized at higher temperatures, designated as Cb for temperatures around 773 K and as Cg around 923 K, were formed only on Ni/a-Al2O3. The formation and accumulation of Cb and Cg species can therefore be understood to cause catalyst deactivation by covering the surface of the active metallic nickel. Hence, the Cb and Cg species are considered to be less reactive and cannot be easily eliminated with steam. It can be inferred that Ni/LaAlO3 suppresses the formation of the inactive Cb and Cg carbon species, maintaining catalytic activity because the lattice oxygen in LaAlO3 enhances the oxidation of Cb and Cg species. This inference is supported by the fact that the TPO peak temperature of the Ca in Ni/ LaAlO3 was shifted towards values lower by ca. 50 K than that in Ni/a-Al2O3. 3.4. Temperature programmed reduction To understand the oxidation–reduction properties of Ni/ perovskites, we performed TPR measurements using TG in hydrogen flow. Fig. 6 shows the weight loss of nickel catalysts supported on perovskites as well as unsupported
perovskite supports as a function of temperature. Every Ni/ perovskite showed a decrease in weight starting around 600 K. The observed weight loss can be considered to correspond to the reduction of nickel oxide. However the amount of weight loss was larger than the 26.5 mg g-cat1 loss caused by the reduction of nickel oxide. The excess weight loss is caused by the reduction of perovskites. In the cases of Ni/LaAlO3, Ni/SrTiO3 and Ni/BaTiO3 which maintain their perovskite structures after reduction procedure, the excess weight loss can be understood to result from the release of the lattice oxygen in perovskites and the formation of oxygen vacancies. On the other hand, the greater weight loss of Ni/LaFeO3 and Ni/LBCF can be understood to result both from the release of lattice oxygen and the reduction and decomposition of the supports because these compounds did not maintain their structures during reduction, as demonstrated by the results of XRD measurements, shown in Fig. 1(B). When the same experiments were performed using unsupported perovskites, the weight of every sample decreased. Compared with Ni/ perovskites, however, the temperature at which the reduction started was much higher and the reduction degree of supports was smaller. The nickel promoted the reduction of perovskites. Similar phenomena have been reported in the case of other metal supported catalysts [7–9] and some reports have mentioned that such promotion could result from the hydrogen spill-over from metallic nickel [18]. TPR measurements were also performed on the Ni/ LaAlO3, Ni/SrTiO3 and Ni/BaTiO3, which kept the perovskite structure of supports after reduction, after activity tests at 1073 K and 0.1 MPa with a molar H2O/CH4 ratio of 2; the purpose was to verify that the lattice oxygen released during the reduction procedure was regenerated in the course of steam reforming. The results are shown in Fig. 7. Weight loss was observed with every catalyst. Because XRD measurements of used catalysts (Fig. 1(C)) revealed that the nickel is not oxidized during reactions, all of the weight loss
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Fig. 7. Temperature programmed reduction using TG for (a) used catalysts and (b) fresh catalysts. Catalyst; (A) Ni/LaAlO3, (B) Ni/SrTiO3, (C) Ni/ BaTiO3.
determined by use of measuring the difference between the weight loss obtained from the results of TPR measurements and that caused by the 26.5 mg g-cat1 loss caused by reduction of nickel oxide. ‘‘Regeneration ratio’’ means the ratio of the amount of released lattice oxygen in the fresh catalyst to that in the used catalyst. The lattice oxygen which had been released during reduction was not wholly recovered during reaction in any of the catalysts tested. These results suggest that some lattice oxygen is consumed during the reaction. The lattice oxygen seems to participate in the oxidation of CHx fragments adsorbed on nickel, contributing to the suppression of forming inactive carbonaceous species. The amount of lattice oxygen released during TPR measurements varied considerably from one catalyst to another. Fig. 8 shows the correlation for each catalyst between the catalytic activity and the regeneration ratio. Compared to Ni/LaAlO3 and Ni/SrTiO3, greater amounts of lattice oxygen were removed from Ni/BaTiO3 in either reduction with hydrogen or reaction in steam reforming. The small regeneration in this catalyst means that little lattice oxygen is recovered during the reaction. This seems to explain why the catalytic activity of Ni/BaTiO3 is lower than the activities of Ni/LaAlO3 and Ni/SrTiO3. Namely, it implies that Ni/LaAlO3 and Ni/SrTiO3 showed much higher catalytic activities than Ni/BaTiO3 because they have larger amounts of lattice oxygen near the surface and metallic nickel and they have more frequent participation of lattice oxygen in steam reforming reactions. Lattice oxygen which closely adjoins active nickel sites can be easily transferred to CHx fragments adsorbed on metallic nickel, contributing to the oxidation of CHx fragments. As mentioned above, the catalytic activities of Ni/perovskite were related to the dispersion of their metallic nickel. Thus, we concluded that the catalytic activity of Ni/perovskite is strongly dependent
can be regarded as arising from the reduction of perovskite supports. As this weight loss was caused by the release of lattice oxygen, it can be confirmed that the lattice oxygen released during reduction procedure has been recovered by steam during reaction. Table 2 shows the amount of the lattice oxygen released from fresh and used catalysts during TPR measurements. The amount of lattice oxygen released in fresh catalysts was Table 2 Weight loss of fresh and used Ni/perovskites measured by TPR Catalyst
Ni/LaAlO3 Ni/SrTiO3 Ni/BaTiO3
Weight of released lattice oxygen (mg g-cat1) Fresh
Used
12.8 42.4 104
6.0 14.8 22.3
Regeneration ratio (–)
0.47 0.35 0.21
Fig. 8. The correlation between the regeneration ratio and the catalytic activity. Catalyst; (^) Ni/LaAlO3, (&) Ni/SrTiO3, (*) Ni/BaTiO3.
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on the reducibility of perovskite supports, which is significantly influenced by the particle size of metallic nickel.
4. Conclusions The steam reforming of methane employing various Ni/ perovskite catalysts including Ni/LaAlO3, Ni/SrTiO3, Ni/ LaFeO3, Ni/BaTiO3 and Ni/LBCF was studied under the conditions of 1073 K, atmospheric pressure and a molar H2O/CH4 ratio of 2 for the purpose of enhancing the catalytic activity and decreasing the coking on the nickel catalyst. The study confirmed that utilization of the lattice oxygen in LaAlO3 and SrTiO3 effectively promotes the reaction of CHx fragments adsorbed on metallic nickel with steam. At a molar H2O/CH4 ratio of 1, Ni/LaAlO3 showed a high methane conversion stably for 24 h, while the conventional Ni/a-Al2O3 was deactivated due to carbon deposition. Because the inactive carbon species observed to form on the Ni/a-Al2O3 were not detected on Ni/LaAlO3, LaAlO3 can be understood to prevent the formation of inactive carbon species. To elucidate the roles of the perovskites in detail, we performed TPR measurements performed on Ni/perovskites before and after use in the reactions. The results showed that the lattice oxygen in perovskites is consumed and regenerated repeatedly during the reaction. Both the reducibility and the particle size of nickel were found to be related to catalytic activities. These results suggest that the lattice oxygen in LaAlO3 and SrTiO3 accesses CHx fragments adsorbed on nickel readily due to the large amount of lattice oxygen near the surface of the perovskite, and interfaces between the nickel particles and
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the support. Thus, Ni/LaAlO3 and Ni/SrTiO3 have high catalytic activities. The lattice oxygen in LaAlO3 and SrTiO3, thus plays a positive role both in both promoting the oxidation of CHx fragments adsorbed on metallic nickel and in hindering the production of inactive carbon species.
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Catalytic activities and coking resistance of Ni/perovskites in steam reforming of methane Kohei Urasaki, Yasushi Sekine, Sho Kawabe, Eiichi Kikuchi, Masahiko Matsukata * Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan Received 5 October 2004; received in revised form 7 February 2005; accepted 17 February 2005 Available online 12 April 2005
Abstract Steam reforming of methane for the purpose of hydrogen production was performed using nickel catalysts supported on a variety of perovskites, including LaAlO3, LaFeO3, SrTiO3, BaTiO3, La0.4Ba0.6Co0.2Fe0.8O3d, to compare the catalytic activity and resistance to coking of these catalysts to those of the conventional Ni/a-Al2O3 catalyst. Ni/LaAlO3 and Ni/SrTiO3 showed high catalytic activities among the Ni/ perovskites and longer-term stabilities than the conventional catalyst. Temperature programmed oxidation of carbon deposited on used catalysts revealed that inactive carbon species detected on Ni/a-Al2O3 were not formed in the case of Ni/LaAlO3. The results of temperature programmed reduction confirmed that consumption and recovery of the lattice oxygen in perovskites occurred during the reaction, and that the reducibility of perovskites had a great impact on the steam reforming activity. The lattice oxygen in perovskites is considered to play important roles in promoting the oxidation of CHx fragments adsorbed on metallic nickel. # 2005 Published by Elsevier B.V. Keywords: Steam reforming of methane; Lattice oxygen; Coking; Hydrogen; Ni/LaAlO3; Ni/LaFeO3; Ni/SrTiO3; Ni/BaTiO3; Ni/La0.4Ba0.6Co0.2Fe0.8O3d
1. Introduction Steam reforming of methane, which has been used for large-scale hydrogen production in industrial processes such as petroleum refinery and ammonia synthesis, has become increasingly important recently because of its potential applications in fuel cells. As the steam reforming reaction is strongly endothermic, the process requires reaction temperatures higher than 1000 K and an excess amount of steam to prevent coking on the catalyst. However, high reaction temperatures and the use of excess steam cause increases in the amount of energy consumed and consequently in the hydrogen production cost. The development of new, more active steam reforming catalysts with high durability against coking is thus desirable. * Corresponding author. Tel.: +81 35286 3850; fax: +81 35286 3850. E-mail addresses: [email protected] (K. Urasaki), [email protected] (M. Matsukata). 0926-860X/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.apcata.2005.02.020
The basic reaction in the steam reforming of methane is as follows: CH4 þ H2 O ! CO þ 3H2
0 1 DH298 K ¼ 206 kJ mol
Although rhodium and ruthenium are more active catalytically, nickel has generally been used as the catalyst in steam reforming processes, mainly for practical reasons. Specifically, nickel supported on inexpensive, thermally stable aalumina or alumina–magnesia has been widely employed as the conventional industrial steam reforming catalysts [1,2]. As previous researchers have often pointed out, however, nickel is more susceptible to coking and tends to deactivate [3,4]. From an industrial point of view, the development of nickel catalysts with greater resistance to coking is thus an attractive research goal. An effective approach to developing such nickel catalysts is to focus on the selection and modification of catalyst supports. It is widely accepted that the addition of alkali,
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alkali earth oxides and rare earth metal oxides to the aAl2O3 support or the use of basic metal oxides as the support improves resistance to coking. This positive effect is understood to result from the enhancement in steam adsorption, in the oxidation rate of CHx fragments adsorbed on metallic nickel and/or the reduction of methane activation and dissociation [2]. Although improvements of the support greatly impact catalytic activity and/or resistance to coking, limited attempts have been made to apply oxygen-ion conducting oxides, for instance perovskite-type oxides, to the steam reforming catalysts. It is mechanistically expected that oxidation of CHx fragments adsorbed on metallic nickel would be promoted by the lattice oxygen in oxygen-ion conducting oxides and that the consumed lattice oxygen would be regenerated by steam. Such a mechanism has specifically been proposed for CO oxidation [5,6] and dry reforming of methane [7–9]. Huang and coworkers have evaluated the activity of nickel supported on ceria-based ionconducting oxides for dry reforming of methane and have pointed out that the lattice oxygen in modified ceria may play some positive roles in the activation of methane and carbon dioxide [10–12]. Takehira et al. have reported that the perovskite-type oxides such as SrTiO3, CaTiO3, BaTiO3 that contain a small amount of nickel in the titanium sites show high catalytic activities with high resistance to coking in both partial oxidation of methane [13–15] and dry reforming of methane [16], due to the high dispersion of nickel; these researchers also examined oxygen mobility in perovskites, and found that the high resistance to coking might be partly due to the migration of mobile oxygen from the perovskite support to the metallic nickel particles. In the present study, the catalytic activity and resistance to coking of nickel catalysts supported on a variety of the perovskite-type oxides (LaAlO3, LaFeO3, SrTiO3, BaTiO3, La0.4Ba0.6Co0.2Fe0.8O3d) are compared to those of the conventional Ni/a-Al2O3 catalyst for steam reforming of methane. To investigate differences in catalytic activity among the Ni/perovskite catalysts examined, we estimated the dispersion and reduction properties of nickel by means of CO chemisorption and temperature programmed reduction, respectively. Finally, the roles of the lattice oxygen on the catalytic activity and carbon deposition will be discussed.
2. Experimental 2.1. Catalyst preparation Some perovskite-type oxides including LaAlO3, LaFeO3, SrTiO3, BaTiO3, and La0.4Ba0.6Co0.2Fe0.8O3d(LBCF) as well as a-Al2O3 were prepared for use as catalyst supports as follows. LaAlO3, LaFeO3 and LBCF were prepared by the sol–gel method. The metal nitrates obtained from Kanto Chemical Co., Inc., were dissolved into water; then excess amounts of
citric acid and ethylene glycol were added to the solutions. The molar ratio of total metal ion:citric acid:ethylene glycol was 1:3:3. The obtained solutions were evaporated until gellike materials were formed. These materials were then calcined successively at 673 K for 2 h and at 1123 K for 11 h. SrTiO3 and BaTiO3 were prepared by physically mixing TiO2 (anatase; Idemitsu Co., Ltd.) and SrCO3 or BaCO3 (Kanto Chemical Co., Inc.). The mixtures were then calcined at 1423 K for 10 h. The a-Al2O3 support was formed by calcining g-Al2O3 (Nishio Industry Co., Ltd.) at 1573 K for 2 h. Nickel catalysts were prepared by the impregnation method using these supports and an aqueous solution of Ni(NO3)26H2O (Kanto Chemical Co., Inc.) to produce a Ni loading of 10 wt.%. The resulting compounds were dried overnight at 393 K, and calcined at 773 K for 1 h. 2.2. Activity test Steam reforming of methane using the various nickel catalysts was carried out in a continuous flow system with a fixed bed of catalyst at atmospheric pressure. After a catalyst was reduced in a hydrogen stream, at 1073 K for 1 h, the carrier gas was switched from hydrogen to a mixture of methane and steam with a molar H2O/CH4 ratio of 1 or 2 at 1073 K and 0.1 MPa. The products were analyzed by means of a gas chromatograph equipped with a thermal conductivity detector using Shimadzu GC-8A and an active charcoal separation column. Catalytic activity was evaluated in terms of methane conversion, calculated on the basis of the following formula: CCH4 methane conversionð%Þ ¼ 1 CCH4 þ CCO2 þ CCO 100 where, C is the molar fraction in the outlet gas. 2.3. Characterization The reduction of each catalyst was monitored by means of temperature programmed reduction (TPR) in an H2 flow from ambient temperature to 1173 K. The weight loss of each catalyst was determined by means of thermogravimetry (TG) using a Shimadzu TG-50A. The amount of deposited carbon on each catalyst after the reaction was quantified and characterized by temperature programmed oxidation (TPO) in a stream of 10% O2/He from ambient temperature to 1073 K; the amount of CO2 formed was measured by use of a quadrupole mass spectrometer. X-ray diffraction (XRD) patterns of samples were recorded with a Rigaku RINT-2000 X-ray diffractometer employing Cu Ka radiation filtered by nickel. BET specific surface areas of supports were determined by nitrogen adsorption at 77 K using a BEL JAPAN BELSORP 28SA.
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The active surface area of metallic nickel after the reduction procedure was determined by means of CO chemisorption using Quantachrome Autosorb-1-C. Before measurement, samples were reduced in situ in a hydrogen stream at a temperature of 1073 K for 1 h, evacuated to vacuum at the same temperature and cooled to 303 K in the vacuum. Afterwards, the CO adsorption isotherm, including both physisorption and chemisorption, was measured. After evacuation at 303 K, a second isotherm including only physisorption was measured. Chemisorbed CO uptakes were determined by examining the difference between the two isotherms. The dispersion of nickel was calculated under the
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assumption that the stoichiometry of CO/Ni(surface) was equal to 2.
3. Results and discussion 3.1. Structures of Ni/perovskite catalysts Fig. 1 shows the XRD patterns of Ni/LaAlO3, Ni/LaFeO3, Ni/SrTiO3, Ni/BaTiO3, and Ni/LBCF before and after reduction, and after activity tests. Every Ni/perovskite catalyst showed the diffraction from the corresponding perovskite structures as well as the diffraction from nickel oxide. After reduction and activity tests, LaAlO3, SrTiO3 and BaTiO3 maintained their perovskite structures. On the other hand, the structures of LaFeO3 and LBCF changed drastically. After reduction of these two compounds, the reflection peaks related to perovskites were weakened and broadened and other peaks related to some oxides, metals or alloys produced by reduction of LaFeO3 and LBCF were observed. After the activity tests, the reduced LaFeO3 and LBCF were reoxidized by steam and their perovskite structures were recovered. 3.2. Catalytic activities of Ni/perovskite catalysts To examine the effects of perovskites on catalytic activities, the activities of both the various Ni/perovskite catalysts and the Ni/a-Al2O3 catalyst in steam reforming of methane were tested under the conditions of 1073 K and 0.1 MPa with a molar H2O/CH4 ratio of 2. Fig. 2 shows the time on stream changes of CH4 conversion for the various
Fig. 1. XRD patterns of (A) fresh catalysts, (B) reduced catalysts and (C) used catalysts: (*) perovskite, (&) nickel, (!) nickel oxide. Catalyst; (a) Ni/LaAlO3, (b) Ni/SrTiO3, (c) Ni/LaFeO3, (d) Ni/BaTiO3, (e) Ni/LBCF.
Fig. 2. Catalytic activities of supported Ni catalysts. Catalyst; (*) Ni/ LaAlO3, (&) Ni/SrTiO3, (*) Ni/LaFeO3, (~) Ni/BaTiO3, (!) Ni/LBCF, (^) Ni/a-Al2O3. Reaction conditions: temperature, 1073 K; molar H2O/ CH4 ratio, 2; W/F, 1.58 g h mol1.
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Fig. 3. Effect of dispersion on catalytic activities of Ni/perovskites. Reaction conditions: temperature, 1073 K; molar H2O/CH4 ratio, 2; W/F, 1.58 g h mol1.
Ni/perovskite and the Ni/a-Al2O3 catalysts. The catalytic activities of the catalysts tested varied considerably. Ni/ LaAlO3 and Ni/SrTiO3 showed high activities and the CH4 conversion levels of 91.7 and 88.4 % at 1 h of run, respectively. The activities of Ni/LaAlO3 and Ni/SrTiO3 were comparable to that of Ni/a-Al2O3. On the other hand, Ni/LaFeO3 showed a lower activity and the activities of Ni/ BaTiO3 and Ni/LBCF were extremely low. The reaction products produced by all these catalysts were composed of CO, CO2, and H2; no other products were detected. The gas composition of the outlet gas for all catalysts was in equilibrium, as in most cases of steam reforming of methane using nickel catalysts, where the rate of steam reforming of methane is much slower than that of water-gas-shift reaction at high reaction temperatures [1,2]. For the purpose of examining such large differences in catalytic activities among perovskite supported nickel catalysts tested, we determined the active surface area of metallic nickel by means of CO chemisorption. Table 1 lists nickel particle size and dispersion of nickel, calculated from CO uptakes. Fig. 3 shows the relation between catalytic activity and dispersion of nickel, calculated by the CO uptakes. As there was a clear correlation between CH4 conversion and dispersion, we concluded that nickel particle size, which varied significantly among the Ni/perovskite Table 1 Dispersion, particle size and specific surface area of Ni/perovskites Catalysts
Dispersion (%)
Particle size (nm)
SAa m2/g-cat1
Ni/LaAlO3 Ni/SrTiO3 Ni/LaFeO3 Ni/BaTiO3 Ni/LBCF
1.6 1.2 0.48 0.36 0.22
64 82 210 280 460
15.9 13.4 15.9 15.0 6.0
a
Specific surface area calculated by BET method.
Fig. 4. Stabilities of Ni/a-Al2O3 and Ni/LaAlO3. Reaction conditions: temperature, 1073 K; molar H2O/CH4 ratio, 1; W/F, 1.58 g h mol1. Catalyst; (*) Ni/LaAlO3, (*) Ni/a-Al2O3.
catalysts, had a considerable impact on catalytic activity. The surface area of supports might reasonably be expected to be a factor in the dispersion of nickel; however, the differences in the BET surface areas of the perovskites tested did not affect the dispersion of nickel, as shown in Table 1. Although it is still not clearly understood why the dispersion of nickel changes depending on the kind of perovskite supports, nickel particle size may be determined by an affinity between the precursors of nickel species and the perovskites. As shown in Table 1, the nickel particle sizes obtained by CO chemisorption in Ni/LaFeO3, Ni/BaTiO3 and Ni/LBCF were larger than 200 nm. Such extremely large values obtained by CO chemisorption may result from the encapsulation of nickel clusters caused by the partial reduction of supports through strong metal support interaction (SMSI) [17]. The effects of the properties of supports on the dispersion of nickel will be studied further. 3.3. Stabilities of Ni/LaAlO3 To compare the stabilities of Ni/LaAlO3 and Ni/a-Al2O3, we continued activity tests for 24 h under the conditions of 1073 K, 0.1 MPa and a molar H2O/CH4 ratio of 1. The results are shown in Fig. 4. In the case of Ni/a-Al2O3, CH4 conversion decreased from 83% at 2 h to 75% at 24 h, while Ni/LaAlO3 was stable, maintaining CH4 conversion levels of 77–80% throughout the run. Deposition of coke is one of the major causes of catalyst deactivation in steam reforming of hydrocarbons. The quantity and quality of carbon deposited on Ni/LaAlO3 and Ni/a-Al2O3 after reaction for 24 h were thus evaluated by means of TPO measurements. The results are shown in Fig. 5. In the case of Ni/a-Al2O3 which was deactivated gradually during the activity test, three peaks
K. Urasaki et al. / Applied Catalysis A: General 286 (2005) 23–29
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Fig. 6. Time programmed reduction using TG for (A) fresh catalysts and (B) supports. Catalysts; (A) (a) Ni/LaAlO3, (b) Ni/SrTiO3, (c) Ni/LaFeO3, (d) Ni/BaTiO3, (e) Ni/LBCF, (B) (a) LaAlO3, (b) SrTiO3, (c) LaFeO3, (d) BaTiO3, (e) LBCF. Fig. 5. Temperature programmed oxidation for (a) Ni/LaAlO3 and (b) Ni/aAl2O3 after reaction for 24 h.
were observed: around 623, 773 and 923 K, and the total amount of deposited carbon was 3.96 mg g-cat1. In contrast, Ni/LaAlO3 gave only one peak at around 623 K and the amount of deposited carbon was 3.39 mg g-cat1, slightly less than that deposited on Ni/a-Al2O3. The amount of deposited carbonaceous species oxidized at lower temperatures, designated as Ca, was approximately the same for both Ni/a-Al2O3 and Ni/LaAlO3; the formation of Ca species may therefore be considered to have no influence on catalyst deactivation, and the Ca species may be understood to be reactive, reacting with steam to yield CO or CO2 that is removed immediately from the surface of the catalyst. In contrast, carbonaceous species oxidized at higher temperatures, designated as Cb for temperatures around 773 K and as Cg around 923 K, were formed only on Ni/a-Al2O3. The formation and accumulation of Cb and Cg species can therefore be understood to cause catalyst deactivation by covering the surface of the active metallic nickel. Hence, the Cb and Cg species are considered to be less reactive and cannot be easily eliminated with steam. It can be inferred that Ni/LaAlO3 suppresses the formation of the inactive Cb and Cg carbon species, maintaining catalytic activity because the lattice oxygen in LaAlO3 enhances the oxidation of Cb and Cg species. This inference is supported by the fact that the TPO peak temperature of the Ca in Ni/ LaAlO3 was shifted towards values lower by ca. 50 K than that in Ni/a-Al2O3. 3.4. Temperature programmed reduction To understand the oxidation–reduction properties of Ni/ perovskites, we performed TPR measurements using TG in hydrogen flow. Fig. 6 shows the weight loss of nickel catalysts supported on perovskites as well as unsupported
perovskite supports as a function of temperature. Every Ni/ perovskite showed a decrease in weight starting around 600 K. The observed weight loss can be considered to correspond to the reduction of nickel oxide. However the amount of weight loss was larger than the 26.5 mg g-cat1 loss caused by the reduction of nickel oxide. The excess weight loss is caused by the reduction of perovskites. In the cases of Ni/LaAlO3, Ni/SrTiO3 and Ni/BaTiO3 which maintain their perovskite structures after reduction procedure, the excess weight loss can be understood to result from the release of the lattice oxygen in perovskites and the formation of oxygen vacancies. On the other hand, the greater weight loss of Ni/LaFeO3 and Ni/LBCF can be understood to result both from the release of lattice oxygen and the reduction and decomposition of the supports because these compounds did not maintain their structures during reduction, as demonstrated by the results of XRD measurements, shown in Fig. 1(B). When the same experiments were performed using unsupported perovskites, the weight of every sample decreased. Compared with Ni/ perovskites, however, the temperature at which the reduction started was much higher and the reduction degree of supports was smaller. The nickel promoted the reduction of perovskites. Similar phenomena have been reported in the case of other metal supported catalysts [7–9] and some reports have mentioned that such promotion could result from the hydrogen spill-over from metallic nickel [18]. TPR measurements were also performed on the Ni/ LaAlO3, Ni/SrTiO3 and Ni/BaTiO3, which kept the perovskite structure of supports after reduction, after activity tests at 1073 K and 0.1 MPa with a molar H2O/CH4 ratio of 2; the purpose was to verify that the lattice oxygen released during the reduction procedure was regenerated in the course of steam reforming. The results are shown in Fig. 7. Weight loss was observed with every catalyst. Because XRD measurements of used catalysts (Fig. 1(C)) revealed that the nickel is not oxidized during reactions, all of the weight loss
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Fig. 7. Temperature programmed reduction using TG for (a) used catalysts and (b) fresh catalysts. Catalyst; (A) Ni/LaAlO3, (B) Ni/SrTiO3, (C) Ni/ BaTiO3.
determined by use of measuring the difference between the weight loss obtained from the results of TPR measurements and that caused by the 26.5 mg g-cat1 loss caused by reduction of nickel oxide. ‘‘Regeneration ratio’’ means the ratio of the amount of released lattice oxygen in the fresh catalyst to that in the used catalyst. The lattice oxygen which had been released during reduction was not wholly recovered during reaction in any of the catalysts tested. These results suggest that some lattice oxygen is consumed during the reaction. The lattice oxygen seems to participate in the oxidation of CHx fragments adsorbed on nickel, contributing to the suppression of forming inactive carbonaceous species. The amount of lattice oxygen released during TPR measurements varied considerably from one catalyst to another. Fig. 8 shows the correlation for each catalyst between the catalytic activity and the regeneration ratio. Compared to Ni/LaAlO3 and Ni/SrTiO3, greater amounts of lattice oxygen were removed from Ni/BaTiO3 in either reduction with hydrogen or reaction in steam reforming. The small regeneration in this catalyst means that little lattice oxygen is recovered during the reaction. This seems to explain why the catalytic activity of Ni/BaTiO3 is lower than the activities of Ni/LaAlO3 and Ni/SrTiO3. Namely, it implies that Ni/LaAlO3 and Ni/SrTiO3 showed much higher catalytic activities than Ni/BaTiO3 because they have larger amounts of lattice oxygen near the surface and metallic nickel and they have more frequent participation of lattice oxygen in steam reforming reactions. Lattice oxygen which closely adjoins active nickel sites can be easily transferred to CHx fragments adsorbed on metallic nickel, contributing to the oxidation of CHx fragments. As mentioned above, the catalytic activities of Ni/perovskite were related to the dispersion of their metallic nickel. Thus, we concluded that the catalytic activity of Ni/perovskite is strongly dependent
can be regarded as arising from the reduction of perovskite supports. As this weight loss was caused by the release of lattice oxygen, it can be confirmed that the lattice oxygen released during reduction procedure has been recovered by steam during reaction. Table 2 shows the amount of the lattice oxygen released from fresh and used catalysts during TPR measurements. The amount of lattice oxygen released in fresh catalysts was Table 2 Weight loss of fresh and used Ni/perovskites measured by TPR Catalyst
Ni/LaAlO3 Ni/SrTiO3 Ni/BaTiO3
Weight of released lattice oxygen (mg g-cat1) Fresh
Used
12.8 42.4 104
6.0 14.8 22.3
Regeneration ratio (–)
0.47 0.35 0.21
Fig. 8. The correlation between the regeneration ratio and the catalytic activity. Catalyst; (^) Ni/LaAlO3, (&) Ni/SrTiO3, (*) Ni/BaTiO3.
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on the reducibility of perovskite supports, which is significantly influenced by the particle size of metallic nickel.
4. Conclusions The steam reforming of methane employing various Ni/ perovskite catalysts including Ni/LaAlO3, Ni/SrTiO3, Ni/ LaFeO3, Ni/BaTiO3 and Ni/LBCF was studied under the conditions of 1073 K, atmospheric pressure and a molar H2O/CH4 ratio of 2 for the purpose of enhancing the catalytic activity and decreasing the coking on the nickel catalyst. The study confirmed that utilization of the lattice oxygen in LaAlO3 and SrTiO3 effectively promotes the reaction of CHx fragments adsorbed on metallic nickel with steam. At a molar H2O/CH4 ratio of 1, Ni/LaAlO3 showed a high methane conversion stably for 24 h, while the conventional Ni/a-Al2O3 was deactivated due to carbon deposition. Because the inactive carbon species observed to form on the Ni/a-Al2O3 were not detected on Ni/LaAlO3, LaAlO3 can be understood to prevent the formation of inactive carbon species. To elucidate the roles of the perovskites in detail, we performed TPR measurements performed on Ni/perovskites before and after use in the reactions. The results showed that the lattice oxygen in perovskites is consumed and regenerated repeatedly during the reaction. Both the reducibility and the particle size of nickel were found to be related to catalytic activities. These results suggest that the lattice oxygen in LaAlO3 and SrTiO3 accesses CHx fragments adsorbed on nickel readily due to the large amount of lattice oxygen near the surface of the perovskite, and interfaces between the nickel particles and
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the support. Thus, Ni/LaAlO3 and Ni/SrTiO3 have high catalytic activities. The lattice oxygen in LaAlO3 and SrTiO3, thus plays a positive role both in both promoting the oxidation of CHx fragments adsorbed on metallic nickel and in hindering the production of inactive carbon species.
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