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Al2O3 catalyst for reverse-water-gas-shift reaction of CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process
Applied Catalysis A: General 211 (2001) 81–90
Development of ZnO/Al2 O3 catalyst for reverse-water-gas-shift reaction of CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process Sang-Woo Park b , Oh-Shim Joo a,∗ , Kwang-Deog Jung a , Hyo Kim b , Sung-Hwan Han a a b
Catalysis Laboratory, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul, South Korea Department of Chemical Engineering, University of Seoul, 90 Chonnong-dong, Tongdaemun-gu, Seoul, South Korea Received 21 August 2000; received in revised form 27 October 2000; accepted 30 October 2000
Abstract ZnO and ZnO/Al2 O3 catalysts were studied for a reverse-water-gas-shift reaction (RWReaction). The catalytic activities depended on the compositions of Zn and Al at the temperature range of 673–973 K and GHSV of 15,000. The activities were close to the equilibrium conversion at temperatures above 873 K. The catalysts were characterized by using BET, TPR, XRD, SEM, and TEM. The ZnO/Al2 O3 catalysts were mixtures of ZnO and ZnAl2 O4 phases, and the particle size of the ZnO was strongly dependent on its composition in the ZnO/Al2 O3 catalysts. ZnO/Al2 O3 (Zn:Al = 1:1) catalyst has the smallest particle size of ZnO and its conversion of CO2 at 873 K and GHSV of 150,000 was 43%. The stability of ZnO/Al2 O3 catalysts increased in the presence of the large particles of ZnO. Hence, ZnO/Al2 O3 (Zn:Al = 4:1) catalyst was more stable than the ZnO/Al2 O3 (Zn:Al = 1:1) catalyst. The conversion of CO2 on the ZnO/Al2 O3 (Zn:Al = 1:1) catalyst decreased from 43 to 17% in 48 h. The ZnO in ZnO/Al2 O3 catalysts was reduced to the Zn metal during the RWReaction, which contributed to the deactivation of the ZnO/Al2 O3 catalysts. Meanwhile, the activity of ZnAl2 O4 catalyst was stable for 100 h at 873 K and GHSV of 150,000. The ZnAl2 O4 catalyst was developed for the RWReaction of the CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process for methanol formation from CO2 . © 2001 Elsevier Science B.V. All rights reserved. Keywords: CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process; Reverse-water-gas-shift reaction; ZnO/Al2 O3 ; ZnO; Al2 O3 ; ZnAl2 O4
1. Introduction The conversion of CO2 to chemical resources has been studied by several methods; such conversion would mitigate the greenhouse effects [1–3]. Especially, the catalytic hydrogenation of CO2 to form methanol is one of the efficient processes to treat a
∗ Corresponding author. Fax: +82-2-958-5219. E-mail address: [email protected] (O.-S. Joo).
large quantity of CO2 , minimizing the hydrogen loss compared with that of hydrocarbon production. We have reported CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process to convert CO2 into methanol; this process consisted of a reverse-water-gas-shift reaction (RWReaction) and a methanol synthesis reaction [4]. In the CAMERE process, carbon dioxide and hydrogen were converted to CO and H2 O by the RWReaction, and then the mixture gas of CO/CO2 /H2 was fed into the methanol reactor after removing the water.
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The higher the conversion of CO2 to CO was in the RWReaction, the higher the methanol productivity was in the CAMERE process [5]. The conditions of the RWReaction were reductive with excess of H2 . Oxide catalysts could be reduced to the metal under such a strong reductive condition, which caused deactivation of the catalysts and decreased the selectivity of CO by producing CH4 [6–9]. Besides, the reaction temperature for the RWReaction in the CAMERE process should be high enough to maintain the conversion of CO2 as high as possible, which accelerated the reduction of the catalysts. Hence, the development of a stable and active catalyst for the RWReaction is a critical point for the optimization of the CAMERE process to form methanol from CO2 . The RWReaction is the reverse reaction of a watergas-shift reaction. The water-gas-shift reaction has been studied intensively for the last several decades in order to adjust the H2 /CO ratio in the synthesis gas. On the other hand, the reverse-water-gas-shift reaction (RWReaction) has attracted little attention because of little demand. In the CAMERE process, the RWReaction was an important reaction maximizing the methanol productivity and minimizing the effect of water in the CO2 hydrogenation process to form methanol [5]. Herein, we report the development and the characterization of ZnO/Al2 O3 catalysts for the RWReaction. Especially, the activity of ZnAl2 O4 catalyst was stable for 100 h without any deactivation for the RWReaction of the CAMERE process.
2. Experimental The ZnO/Al2 O3 catalysts of various compositions were prepared by a co-precipitation of the corresponding metal nitrates [10]. The catalysts were calcined at the temperatures of 773 or 1123 K for 3 h. The BET surface areas of the catalysts were measured by ASAP 2000 (Micromeritics). The catalyst charged in a tubular reactor (a quartz reactor of 3/8 in.) was heated up to the reaction temperature in the presence of CO2 and H2 before the RWReaction. The RWReaction was carried out at atmosphere pressure and the ratio of H2 /CO2 was 3 in the reactant gas. The catalytic activities were studied at the temperature range of 673–973 K and GHSV (ml/gcat h) of 15,000, while the durability was tested at 873 K and GHSV of 150,000
to screen a stable catalyst for the RWReaction under a severe reaction condition. The ZnO/Al2 O3 catalysts were characterized by using BET, XRD, TPR, TEM, and SEM. The reducibility of ZnO was investigated by a temperature-programmed reduction (TPR), which was carried out at the heating rate of 10 K/min and the flow rate of 50 ml/min of 5% H2 in argon. The hydrogen consumption for the reduction was monitored by a TCD. X-ray powder diffraction patterns were recorded by a Rigaku D-Max-IIIA diffractometer with Cu K␣ radiation to examine the bulk structure of ZnO/Al2 O3 catalysts. SEM images were obtained by a Hitachi S-4200 field emission scanning electron microscopy. The samples were coated with Pt–Pd just before the examination. TEM images were obtained by a Phillips CM-30 scanning transmission electron microscopy. The specimens were prepared by a suspension formation on holes of a carbon grid.
3. Results and discussion 3.1. Activity of ZnO/Al2 O3 catalysts calcined at 1123 K The activities of ZnO/Al2 O3 catalysts were investigated in the temperature range of 673–979 K and GHSV of 15,000. Fig. 1 shows the dependency of the CO2 conversion on the composition of Zn and Al versus temperature. The ZnO/Al2 O3 catalysts of 1:1, 2:1, and 4:1 in the molar ratio of Zn to Al show the activity close to the equilibrium conversion at the temperature above 873 K, while ZnO and ZnO/Al2 O3 (Zn:Al = 1:2) show lower activity than those of other ZnO/Al2 O3 catalysts. It is worth noting that the activities depended on the amounts of ZnO in the ZnO/Al2 O3 catalysts. 3.2. Thermal treatment of ZnO TPR spectra of the ZnO calcined at the temperatures of 773 and 1123 K were monitored to investigate the effect of calcination temperature on the reducibility of ZnO. Fig. 2 shows the spectra at the temperature range of 300–1273 K depending on the calcination temperature. The ZnO calcined at 773 K exhibited two reduction peaks at 573 and 723 K (Fig. 2a).
S.-W. Park et al. / Applied Catalysis A: General 211 (2001) 81–90
Fig. 1. CO2 conversion vs. reaction temperature on ZnO/Al2 O3 catalysts calcined at 1123 K; (---): equil.; (䊐): ZnAl (1:1); (4): ZnAl (2:1); (×) ZnAl (4:1); (䉫): ZnO; (–): ZnAl (1:2).
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shift of the reduction temperature meant a significant change of the morphology in ZnO structure. The morphology change of ZnO by the heat treatment at 773 and 1123 K was observed by a field-emission scanning-electron microscope. The heat treatment at 773 K produced needle-like morphology with poor crystallinity. As shown in Fig. 3b, the ZnO was agglomerated by the heat treatment at 1123 K, increasing the particle size. The average particle size increased to 0.5–2 m with the formation of the large particles. It was expected that the agglomeration to large particles of ZnO would accelerate with continuous heat treatment. The particle size of ZnO calcined at 1123 K became bigger than that of the ZnO calcined at 773 K. The size increment of ZnO particle suppressed its reducibility, as shown in the TPR experiment (Fig. 2b), indicating that the reducibility of ZnO in ZnO/Al2 O3 catalysts is reversely proportional to the particle size in a ZnO-containing catalyst.
3.3. Characterization of ZnO/Al2 O3 catalysts As the calcination temperature increased to 1123 K, those peaks shifted to 839 and 909 K (Fig. 2b). The reducibility of ZnO suppressed by the calcination at the high temperature of 1123 K. The shift of the reduction temperature was about 200 K. Such a large
Fig. 2. TPR spectra for ZnO. Heating rate = 10 K/min; 5% H2 in argon = 50 ml/min; (a) calcined at 773 K; (b) calcined at 1123 K.
The ZnO/Al2 O3 catalysts were examined by XRD to analyze their structure. Fig. 4 shows the XRD spectra of the ZnO/Al2 O3 catalysts calcined at 1123 K. The ZnO/Al2 O3 catalysts having Zn:Al = 1:1, 2:1, and 4:1 show the presence of ZnO and ZnAl2 O4 phases. As the ratio of Zn to Al decreased, the intensity of ZnO peaks gradually decreased. In the ZnO/Al2 O3 (Zn:Al = 1:2) catalyst, the peaks from ZnO completely disappeared, and only peaks from ZnAl2 O4 remained. Also, the ZnO/Al2 O3 (Zn:Al = 1:2) catalyst was totally a spinel structure of Zn and Al. It was well known that Zn and Al easily formed a spinel structure when the oxide precursor was calcined at temperature above 773 K [11]. In the normal spinel structure of ZnAl2 O4 , Zn occupied octahedral sites, while Al occupied tetrahedral sites [12]. The Zn in the ZnAl2 O4 catalyst located in a different environment compared to the Zn in ZnO. The activity of ZnO was similar to that of ZnAl2 O4 for RWReaction, even though the surface area of the ZnO was 66 times smaller than that of the ZnAl2 O4 . The Zn in ZnO showed better performance than the one in ZnAl2 O4 . The ZnO exhibited much better performance per unit BET surface area than that of ZnAl2 O4 catalyst. The Zn in both ZnO and ZnAl2 O4 can be a key catalytic element for the RWReaction.
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Fig. 3. SEM images of ZnO depending on the calcination temperature.
The BET surface areas of ZnO/Al2 O3 catalysts were measured by ASAP 2000 (Table 1). With the heat treatment at 1123 K, the BET surface area of ZnO and ZnO/Al2 O3 catalysts considerably decreased. Al2 O3 showed poor activity for RWReaction even with the high BET surface area of 107.5 m2 /g. The catalysts having the high concentration of ZnO indicate lower surface area than the catalysts having the high concentration of Al2 O3 except ZnAl2 O4 . Interestingly, ZnO showed nearly the same activity as that of ZnAl2 O4
(Fig. 1), although their surface areas were remarkably different (Table 1). Fig. 5 shows the TEM images of ZnO, ZnO/Al2 O3 (Zn:Al = 2:1), and ZnO/Al2 O3 (Zn:Al = 1:1) catalysts. The particle size of the catalysts decreased in the presence of ZnAl2 O4 . As the Al content increased, the particle size of the catalyst became smaller. Moreover, the ZnO/Al2 O3 catalysts show dark and white spots in the TEM images. The EDS (energy dispersive spectroscopy) spectra of ZnO/Al2 O3 (Zn:Al = 2:1) catalyst indicates that the bulk composition was not uniform (Fig. 6). The dark spots in the TEM image mainly consisted of Zn (Fig. 6B), while the white parts consisted of Al and small amounts of Zn. The white spots were mainly ZnAl2 O4 phase. It is interesting to note that the size
Table 1 BET surface areas of ZnO, Al2 O3 , and ZnO/Al2 O3 catalysts depending on the calcination temperature No.
Fig. 4. XRD spectra of the fresh catalysts of ZnO and ZnO/Al2 O3 . (×): ZnO; ( ): ZnAl2 O4 ; (a) ZnO; (b) ZnAl (4:1); (c) ZnAl (2:1); (d) ZnAl (1:1); (e) ZnAl (1:2).
1 2 3 4 5 6
Scat (m2 /g)
Samples
ZnO ZnO/Al2 O3 ZnO/Al2 O3 ZnO/Al2 O3 ZnO/Al2 O3 Al2 O3
(Zn:Al = 4:1) (Zn:Al = 2:1) (Zn:Al = 1:1) (Zn:Al = 1:2)
Calcined at 773 K
Calcined at 1123 K
22.0 113.0 125.1 114.1 192.5 227.4
1.5 15.6 21.4 25.9 100.9 107.5
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Fig. 5. TEM images of ZnO and ZnO/Al2 O3 catalysts calcined at 1123 K.
of dark spots dramatically decreased as the Al content increased. The presence of ZnAl2 O4 prohibited the agglomeration of ZnO and kept them in a small particle state. Due to the small particle size of ZnO and the increment of Al content, the ZnO/Al2 O3 (Zn:Al = 1:1) catalyst had a relatively large surface area. However, the ZnO/Al2 O3 catalysts of ZnAl (1:1), ZnAl (2:1), and ZnAl (4:1) showed similar catalytic activity at GHSV of 15,000. The activity of the ZnO/Al2 O3 catalysts except ZnAl2 O4 at 723 K was dependent on the BET surface area of the catalysts.
3.4. Stability of ZnO/Al2 O3 catalysts for RWReaction of CAMERE process The stability of the ZnO/Al2 O3 catalysts was investigated under the reaction condition of 873 K and GHSV of 150,000. The high space velocity was applied to confirm the stability in a short period of time. Fig. 7 shows the conversion of CO2 versus reaction time. The stability was very dependent on the amounts of ZnO in the ZnO/Al2 O3 catalysts. The ZnO/Al2 O3 (Zn:Al = 1:1) catalyst shows a
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Fig. 6. TEM image and EDS spectra of ZnO/Al2 O3 (Zn:Al = 2:1) catalyst.
high initial activity of 43%. But the ZnAl2 O4 shows a very low CO2 conversion of 10%, although the BET surface area was four times larger than that of ZnO/Al2 O3 (Zn:Al = 1:1) catalyst. It means that ZnO is more active than ZnAl2 O4 for the RWReaction. The catalysts of ZnO/Al2 O3 (Zn:Al = 4:1) and ZnO/Al2 O3 (Zn:Al = 1:1) show the same deactiva-
tion tendency in the initial 10 h. The catalyst of ZnAl (4:1) was stable up to 40 h and then slowly deactivated. On the other hand, the catalytic activity of ZnAl (1:1) steadily decreased from 43 to 17% in 48 h. In the meantime, ZnAl2 O4 catalyst was very stable, though the activity was not so high. Moreover, the activity of ZnAl2 O4 catalyst was stable for 240 h even at 973 K.
Fig. 7. CO2 conversion vs. reaction time at GHSV of 150,000 on the ZnO/Al2 O3 catalysts calcined at 1123 K. (a) ZnAl (4:1); (b) ZnAl (1:1); (c) ZnAl (1:2).
S.-W. Park et al. / Applied Catalysis A: General 211 (2001) 81–90
Fig. 8. XRD spectra of ZnO/Al2 O3 (Zn:Al = 1:1) catalyst. (×): ZnO; ( ): ZnAl2 O4 ; (a) fresh catalyst; (b) used catalyst.
Interestingly, the deactivation rate of the ZnO/Al2 O3 (Zn:Al = 4:1) was different from that of ZnO/Al2 O3 (Zn:Al = 1:1). The deactivation rate depended on the amount of ZnO in the ZnO/Al2 O3 catalysts. In order to understand the reason for the deactivation of ZnO/Al2 O3 catalysts, the ZnO/Al2 O3 (Zn:Al = 1:1) catalyst was investigated by XRD after the RWReaction for 48 h at 873 K and GHSV of 150,000. Fig. 8
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shows the XRD spectra of the fresh and the deactivated catalysts. The peaks from ZnO entirely disappeared after the durability test, while the peaks from ZnAl2 O4 remained unchanged. Such results strongly suggest that ZnO was reduced to the Zn metal and this was followed by evaporation during the RWReaction. Actually, the white metallic Zn metal was deposited on the wall of cold parts of the RWReactor. The deactivation rate might depend on the particle size of ZnO in the ZnO/Al2 O3 catalysts. The dramatic deactivation (Fig. 7b) of ZnO/Al2 O3 catalyst came from the small particle size of ZnO, because the ZnO with small particle size could be easily reduced. On the other hand, ZnAl2 O4 showed no deactivation under the same reaction condition of 873 K and GHSV of 150,000. Fig. 9 shows the TEM images of ZnO/Al2 O3 (Zn:Al = 1:2) catalysts calcined at the temperatures of 773 and 1123 K. The ZnO/Al2 O3 (Zn:Al = 1:2) catalyst calcined at 773 K consisted of the separate phases of ZnO, Al2 O3 , and trace amounts of ZnAl2 O4 (from XRD data), in which the ZnO was segregated from the Al2 O3 phase (Fig. 9A). Meanwhile, the catalyst calcined at 1123 K showed the uniform phase of ZnAl2 O4 (Fig. 9B). The SEM image of the ZnO/Al2 O3 (Zn:Al = 1:2) catalyst calcined at 773 K also shows similar morphology to that of ZnO (Figs. 10A and 3A). The formation of needle-like morphology indicated the presence of free ZnO. The
Fig. 9. TEM images of the ZnO/Al2 O3 (Zn:Al = 1:2) catalyst.
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Fig. 10. SEM images of ZnO/Al2 O3 (Zn:Al = 1:2) catalyst.
calcination at 773 K was not enough to form the spinel structure of ZnAl2 O4 . The ZnO/Al2 O3 (Zn:Al = 1:2) catalyst calcined at 773 K showed similar activity to that of ZnO calcined at 773 K for the RWReaction, showing deactivation. The ZnO/Al2 O3 (Zn:Al = 1:2) catalyst calcined at 1123 K consisted of the uniform phase of ZnAl2 O4 spinel structure (Fig. 9B), and had different morphology from ZnO (Fig. 10B). The ZnAl2 O4 was investigated by XRD to observe the structure transformation after the RWReaction (Fig. 11). The XRD spectra were basically similar before and after the RWReaction at the temperature range of 873–973 K and GHSV of 150,000. It means that the structure of ZnAl2 O4 catalyst was not changed under the RWReaction conditions. The ZnAl2 O4 catalyst was thermally stable during the RWReaction without structure change. Moreover, the ZnAl2 O4 catalyst showed good activity close to the equilibrium conversion at 873 K and GHSV of 5,000, which was a condition for the practical operation of the CAMERE process. The ZnAl2 O4 would be a good catalyst for the RWReaction of the CAMERE process.
ter were oxidizing agents preventing the reduction of a metal oxide. The reducing power of H2 and CO was dependent on the partial pressure of the gases and on the reaction temperature. During the RWReaction, the ZnO in ZnO/Al2 O3 catalysts can be reduced to the Zn metal by CO and H2 as follows: ZnO + H2 = Zn + H2 O
(1)
ZnO + CO = Zn + CO2
(2)
3.5. Thermodynamics for reduction of ZnO In the RWReaction of the CAMERE process, carbon dioxide and excess of hydrogen reacted to give CO and H2 O. The reactant H2 and the produced CO were strong reducing agents, which can reduce a metal oxide to the metal. Meanwhile, the CO2 and the wa-
Fig. 11. XRD spectra of ZnAl2 O4 catalyst. ( ): ZnAl2 O4 ; (a) fresh catalyst; (b) used catalyst.
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Fig. 12. Reducibility of ZnO to Zn depending on the partial pressure of CO, H2 , and temperature.
Gibbs energy equations of Eqs. (1) and (2) were obtained using the data in the literature [13,14]: 1G1 = 27761.2 + 11.333T log T − 1.36 × 10−3 T 2 −67250/T − 46.3405T
(3)
1G2 = 16440.9 + 4.4256T log T − 1.08 × 10−3 T 2 −24250/T − 15.2327T
(4)
CZn PH2 O , CZnO PH2 CZn PCO2 K2 = CZnO PCO 1Gi Ki = Exp − RT At equilibrium, K1 =
(5) (6)
The Zn concentration (CZn ) could be calculated from Eqs. (3)–(6). Fig. 12 indicates that the ZnO phase can be reduced to the Zn metal depending on the partial pressures of H2 and CO and on the reaction temperature. The line for changing from ZnO to ZnO + Zn metal phase means that 0.01 mol% of ZnO was reduced to the Zn metal under the partial pressure of H2 and CO. In a reaction condition range below the line, ZnO can be reduced to the Zn metal. In addition, an equation of −1G1 /RT = ln(PH2 O /PH2 ) + ln(CZn /CZnO ) could be derived from Eqs. (5) and (6). For a ZnO molecule to be reduced at the same partial pressures of H2 and CO, the line shifts toward an even higher positive value of the ln(PH2 O /PH2 ) and the ln(PCO2 /PCO ). It meant that ZnO could be reduced at a condition having the positive values of the ln(PH2 O /PH2 ) and the ln(PCO2 /PCO ). The values of the ln(PH2 O /PH2 =
Table 2 The calculated values for −1Gi , Ki , and PCi /PCi at C Zn = 0.01 mol% T (K)
−1G1 (cal/mol)
K1
PH2 O /PH2
−1G2 (cal/mol)
K2
PCO2 /PCO
770 873 970
16374.0 15289.9 14295.6
2.25E-5 1.49E-4 6.00E-4
0.225 1.49 6.0
13939.2 13710.2 13495.7
1.10E-4 3.70E-4 9.10E-4
1.10 3.70 9.10
90
S.-W. Park et al. / Applied Catalysis A: General 211 (2001) 81–90
0.25) and the ln(PCO2 /PCO = 0.67) were −1.39 and −0.41, respectively, under the reaction condition for the RWReaction of the CAMERE process. The values in the line for ZnO (0.01 mol%) reduction at 873 K were 0.40 and 1.31 (Fig. 12). Table 2 indicates the calculated values for −1Gi , Ki , and PCi /PCi at C Zn = 0.01 mol% and 873 K. ZnO can be reduced at the values of PH2 O /PH2 or PCO2 /PCO lower than those at the table. Those indicate that ZnO in the ZnO/Al2 O3 catalysts should be reduced to the Zn metal under the RWReaction conditions of the CAMERE process.
4. Conclusion The stable catalyst of ZnAl2 O4 was developed for the RWReaction of the CAMERE process. The ZnAl2 O4 catalyst showed no deactivation for 240 h for the RWReaction carried out at the temperature range of 873–973 K and GHSV of 150,000. The activities of ZnO and ZnO/Al2 O3 catalysts were investigated over the temperature range of 673–979 K and GHSV of 15,000, while the durability was tested at 873 K and GHSV of 150,000. The ZnO/Al2 O3 catalysts were mixtures of ZnO and ZnAl2 O4 , in which the amounts were dependent on the composition of Zn and Al. The ZnAl2 O4 showed stable performance for the RWReaction of the CAMERE process. Meanwhile, the ZnO/Al2 O3 catalysts were deactivated due to the reduction of ZnO, even though its activity was higher than that of ZnAl2 O4 . The ZnO was agglomerated by the heat treatment at 1123 K, which retarded the reduction of the ZnO.
Acknowledgements This research was performed for the Greenhouse Gas Research Center, one of the Critical Technology-21 Programs, funded by the Ministry of Science and Technology of Korea. References [1] P.G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 95 (2) (1995) 259. [2] J.M. Lehn, R. Ziessel, J. Organomet. Chem. 382 (1990) 157. [3] W. Leitner, Angew. Chem. Int. Engl. 34 (1995) 2207. [4] O.S. Joo, K.D. Jung, I. Moon, A. Ya Rozovskii, G.I. Lin, S.H. Han, S.J. Uhm, Ind. Eng. Chem. Res. 38 (5) (1999) 1808. [5] S.W. Park, O.S. Joo, K.D. Jung, Y. Chung, H. Kim, S.H. Han, Ind. Eng. Chem. Res., submitted for publication. [6] K.D. Jung, O.S. Joo, S.H. Han, S.J. Uhm, I.J. Jung, Catal. Lett. 35 (1995) 303. [7] K.D. Jung, O.S. Joo, S.H. Han, Catal. Lett. 68 (2000) 49. [8] S.W. Park, O.S. Joo, K.D. Jung, Y. Chung, H. Kim, S.H. Han, Catal. Lett., in press. [9] M.V. Twigg, Catalyst Handbook, Wolfe, London, 1989. [10] O.S. Joo, K.D. Jung, S.H. Han, S.J. Uhm, D.K. Lee, S.K. Ihm, Appl. Catal. A: Gen. 135 (1996) 273. [11] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173. [12] R.W. Grimes, A.B. Anderson, A.H. Heuer, J. Am. Chem. Soc. 111 (1989) 1. [13] L.B. Prankrafts, Thermodynamic properties of elementa and oxides, Bur. Mines Bull. 672. [14] J.M. Smith, H.C. Van Ness, Introduction to Chemical Engineering Thermodynamics, 3rd Edition, McGraw-Hill, New York, 1975.
Development of ZnO/Al2 O3 catalyst for reverse-water-gas-shift reaction of CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process Sang-Woo Park b , Oh-Shim Joo a,∗ , Kwang-Deog Jung a , Hyo Kim b , Sung-Hwan Han a a b
Catalysis Laboratory, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul, South Korea Department of Chemical Engineering, University of Seoul, 90 Chonnong-dong, Tongdaemun-gu, Seoul, South Korea Received 21 August 2000; received in revised form 27 October 2000; accepted 30 October 2000
Abstract ZnO and ZnO/Al2 O3 catalysts were studied for a reverse-water-gas-shift reaction (RWReaction). The catalytic activities depended on the compositions of Zn and Al at the temperature range of 673–973 K and GHSV of 15,000. The activities were close to the equilibrium conversion at temperatures above 873 K. The catalysts were characterized by using BET, TPR, XRD, SEM, and TEM. The ZnO/Al2 O3 catalysts were mixtures of ZnO and ZnAl2 O4 phases, and the particle size of the ZnO was strongly dependent on its composition in the ZnO/Al2 O3 catalysts. ZnO/Al2 O3 (Zn:Al = 1:1) catalyst has the smallest particle size of ZnO and its conversion of CO2 at 873 K and GHSV of 150,000 was 43%. The stability of ZnO/Al2 O3 catalysts increased in the presence of the large particles of ZnO. Hence, ZnO/Al2 O3 (Zn:Al = 4:1) catalyst was more stable than the ZnO/Al2 O3 (Zn:Al = 1:1) catalyst. The conversion of CO2 on the ZnO/Al2 O3 (Zn:Al = 1:1) catalyst decreased from 43 to 17% in 48 h. The ZnO in ZnO/Al2 O3 catalysts was reduced to the Zn metal during the RWReaction, which contributed to the deactivation of the ZnO/Al2 O3 catalysts. Meanwhile, the activity of ZnAl2 O4 catalyst was stable for 100 h at 873 K and GHSV of 150,000. The ZnAl2 O4 catalyst was developed for the RWReaction of the CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process for methanol formation from CO2 . © 2001 Elsevier Science B.V. All rights reserved. Keywords: CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process; Reverse-water-gas-shift reaction; ZnO/Al2 O3 ; ZnO; Al2 O3 ; ZnAl2 O4
1. Introduction The conversion of CO2 to chemical resources has been studied by several methods; such conversion would mitigate the greenhouse effects [1–3]. Especially, the catalytic hydrogenation of CO2 to form methanol is one of the efficient processes to treat a
∗ Corresponding author. Fax: +82-2-958-5219. E-mail address: [email protected] (O.-S. Joo).
large quantity of CO2 , minimizing the hydrogen loss compared with that of hydrocarbon production. We have reported CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process to convert CO2 into methanol; this process consisted of a reverse-water-gas-shift reaction (RWReaction) and a methanol synthesis reaction [4]. In the CAMERE process, carbon dioxide and hydrogen were converted to CO and H2 O by the RWReaction, and then the mixture gas of CO/CO2 /H2 was fed into the methanol reactor after removing the water.
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The higher the conversion of CO2 to CO was in the RWReaction, the higher the methanol productivity was in the CAMERE process [5]. The conditions of the RWReaction were reductive with excess of H2 . Oxide catalysts could be reduced to the metal under such a strong reductive condition, which caused deactivation of the catalysts and decreased the selectivity of CO by producing CH4 [6–9]. Besides, the reaction temperature for the RWReaction in the CAMERE process should be high enough to maintain the conversion of CO2 as high as possible, which accelerated the reduction of the catalysts. Hence, the development of a stable and active catalyst for the RWReaction is a critical point for the optimization of the CAMERE process to form methanol from CO2 . The RWReaction is the reverse reaction of a watergas-shift reaction. The water-gas-shift reaction has been studied intensively for the last several decades in order to adjust the H2 /CO ratio in the synthesis gas. On the other hand, the reverse-water-gas-shift reaction (RWReaction) has attracted little attention because of little demand. In the CAMERE process, the RWReaction was an important reaction maximizing the methanol productivity and minimizing the effect of water in the CO2 hydrogenation process to form methanol [5]. Herein, we report the development and the characterization of ZnO/Al2 O3 catalysts for the RWReaction. Especially, the activity of ZnAl2 O4 catalyst was stable for 100 h without any deactivation for the RWReaction of the CAMERE process.
2. Experimental The ZnO/Al2 O3 catalysts of various compositions were prepared by a co-precipitation of the corresponding metal nitrates [10]. The catalysts were calcined at the temperatures of 773 or 1123 K for 3 h. The BET surface areas of the catalysts were measured by ASAP 2000 (Micromeritics). The catalyst charged in a tubular reactor (a quartz reactor of 3/8 in.) was heated up to the reaction temperature in the presence of CO2 and H2 before the RWReaction. The RWReaction was carried out at atmosphere pressure and the ratio of H2 /CO2 was 3 in the reactant gas. The catalytic activities were studied at the temperature range of 673–973 K and GHSV (ml/gcat h) of 15,000, while the durability was tested at 873 K and GHSV of 150,000
to screen a stable catalyst for the RWReaction under a severe reaction condition. The ZnO/Al2 O3 catalysts were characterized by using BET, XRD, TPR, TEM, and SEM. The reducibility of ZnO was investigated by a temperature-programmed reduction (TPR), which was carried out at the heating rate of 10 K/min and the flow rate of 50 ml/min of 5% H2 in argon. The hydrogen consumption for the reduction was monitored by a TCD. X-ray powder diffraction patterns were recorded by a Rigaku D-Max-IIIA diffractometer with Cu K␣ radiation to examine the bulk structure of ZnO/Al2 O3 catalysts. SEM images were obtained by a Hitachi S-4200 field emission scanning electron microscopy. The samples were coated with Pt–Pd just before the examination. TEM images were obtained by a Phillips CM-30 scanning transmission electron microscopy. The specimens were prepared by a suspension formation on holes of a carbon grid.
3. Results and discussion 3.1. Activity of ZnO/Al2 O3 catalysts calcined at 1123 K The activities of ZnO/Al2 O3 catalysts were investigated in the temperature range of 673–979 K and GHSV of 15,000. Fig. 1 shows the dependency of the CO2 conversion on the composition of Zn and Al versus temperature. The ZnO/Al2 O3 catalysts of 1:1, 2:1, and 4:1 in the molar ratio of Zn to Al show the activity close to the equilibrium conversion at the temperature above 873 K, while ZnO and ZnO/Al2 O3 (Zn:Al = 1:2) show lower activity than those of other ZnO/Al2 O3 catalysts. It is worth noting that the activities depended on the amounts of ZnO in the ZnO/Al2 O3 catalysts. 3.2. Thermal treatment of ZnO TPR spectra of the ZnO calcined at the temperatures of 773 and 1123 K were monitored to investigate the effect of calcination temperature on the reducibility of ZnO. Fig. 2 shows the spectra at the temperature range of 300–1273 K depending on the calcination temperature. The ZnO calcined at 773 K exhibited two reduction peaks at 573 and 723 K (Fig. 2a).
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Fig. 1. CO2 conversion vs. reaction temperature on ZnO/Al2 O3 catalysts calcined at 1123 K; (---): equil.; (䊐): ZnAl (1:1); (4): ZnAl (2:1); (×) ZnAl (4:1); (䉫): ZnO; (–): ZnAl (1:2).
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shift of the reduction temperature meant a significant change of the morphology in ZnO structure. The morphology change of ZnO by the heat treatment at 773 and 1123 K was observed by a field-emission scanning-electron microscope. The heat treatment at 773 K produced needle-like morphology with poor crystallinity. As shown in Fig. 3b, the ZnO was agglomerated by the heat treatment at 1123 K, increasing the particle size. The average particle size increased to 0.5–2 m with the formation of the large particles. It was expected that the agglomeration to large particles of ZnO would accelerate with continuous heat treatment. The particle size of ZnO calcined at 1123 K became bigger than that of the ZnO calcined at 773 K. The size increment of ZnO particle suppressed its reducibility, as shown in the TPR experiment (Fig. 2b), indicating that the reducibility of ZnO in ZnO/Al2 O3 catalysts is reversely proportional to the particle size in a ZnO-containing catalyst.
3.3. Characterization of ZnO/Al2 O3 catalysts As the calcination temperature increased to 1123 K, those peaks shifted to 839 and 909 K (Fig. 2b). The reducibility of ZnO suppressed by the calcination at the high temperature of 1123 K. The shift of the reduction temperature was about 200 K. Such a large
Fig. 2. TPR spectra for ZnO. Heating rate = 10 K/min; 5% H2 in argon = 50 ml/min; (a) calcined at 773 K; (b) calcined at 1123 K.
The ZnO/Al2 O3 catalysts were examined by XRD to analyze their structure. Fig. 4 shows the XRD spectra of the ZnO/Al2 O3 catalysts calcined at 1123 K. The ZnO/Al2 O3 catalysts having Zn:Al = 1:1, 2:1, and 4:1 show the presence of ZnO and ZnAl2 O4 phases. As the ratio of Zn to Al decreased, the intensity of ZnO peaks gradually decreased. In the ZnO/Al2 O3 (Zn:Al = 1:2) catalyst, the peaks from ZnO completely disappeared, and only peaks from ZnAl2 O4 remained. Also, the ZnO/Al2 O3 (Zn:Al = 1:2) catalyst was totally a spinel structure of Zn and Al. It was well known that Zn and Al easily formed a spinel structure when the oxide precursor was calcined at temperature above 773 K [11]. In the normal spinel structure of ZnAl2 O4 , Zn occupied octahedral sites, while Al occupied tetrahedral sites [12]. The Zn in the ZnAl2 O4 catalyst located in a different environment compared to the Zn in ZnO. The activity of ZnO was similar to that of ZnAl2 O4 for RWReaction, even though the surface area of the ZnO was 66 times smaller than that of the ZnAl2 O4 . The Zn in ZnO showed better performance than the one in ZnAl2 O4 . The ZnO exhibited much better performance per unit BET surface area than that of ZnAl2 O4 catalyst. The Zn in both ZnO and ZnAl2 O4 can be a key catalytic element for the RWReaction.
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Fig. 3. SEM images of ZnO depending on the calcination temperature.
The BET surface areas of ZnO/Al2 O3 catalysts were measured by ASAP 2000 (Table 1). With the heat treatment at 1123 K, the BET surface area of ZnO and ZnO/Al2 O3 catalysts considerably decreased. Al2 O3 showed poor activity for RWReaction even with the high BET surface area of 107.5 m2 /g. The catalysts having the high concentration of ZnO indicate lower surface area than the catalysts having the high concentration of Al2 O3 except ZnAl2 O4 . Interestingly, ZnO showed nearly the same activity as that of ZnAl2 O4
(Fig. 1), although their surface areas were remarkably different (Table 1). Fig. 5 shows the TEM images of ZnO, ZnO/Al2 O3 (Zn:Al = 2:1), and ZnO/Al2 O3 (Zn:Al = 1:1) catalysts. The particle size of the catalysts decreased in the presence of ZnAl2 O4 . As the Al content increased, the particle size of the catalyst became smaller. Moreover, the ZnO/Al2 O3 catalysts show dark and white spots in the TEM images. The EDS (energy dispersive spectroscopy) spectra of ZnO/Al2 O3 (Zn:Al = 2:1) catalyst indicates that the bulk composition was not uniform (Fig. 6). The dark spots in the TEM image mainly consisted of Zn (Fig. 6B), while the white parts consisted of Al and small amounts of Zn. The white spots were mainly ZnAl2 O4 phase. It is interesting to note that the size
Table 1 BET surface areas of ZnO, Al2 O3 , and ZnO/Al2 O3 catalysts depending on the calcination temperature No.
Fig. 4. XRD spectra of the fresh catalysts of ZnO and ZnO/Al2 O3 . (×): ZnO; ( ): ZnAl2 O4 ; (a) ZnO; (b) ZnAl (4:1); (c) ZnAl (2:1); (d) ZnAl (1:1); (e) ZnAl (1:2).
1 2 3 4 5 6
Scat (m2 /g)
Samples
ZnO ZnO/Al2 O3 ZnO/Al2 O3 ZnO/Al2 O3 ZnO/Al2 O3 Al2 O3
(Zn:Al = 4:1) (Zn:Al = 2:1) (Zn:Al = 1:1) (Zn:Al = 1:2)
Calcined at 773 K
Calcined at 1123 K
22.0 113.0 125.1 114.1 192.5 227.4
1.5 15.6 21.4 25.9 100.9 107.5
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Fig. 5. TEM images of ZnO and ZnO/Al2 O3 catalysts calcined at 1123 K.
of dark spots dramatically decreased as the Al content increased. The presence of ZnAl2 O4 prohibited the agglomeration of ZnO and kept them in a small particle state. Due to the small particle size of ZnO and the increment of Al content, the ZnO/Al2 O3 (Zn:Al = 1:1) catalyst had a relatively large surface area. However, the ZnO/Al2 O3 catalysts of ZnAl (1:1), ZnAl (2:1), and ZnAl (4:1) showed similar catalytic activity at GHSV of 15,000. The activity of the ZnO/Al2 O3 catalysts except ZnAl2 O4 at 723 K was dependent on the BET surface area of the catalysts.
3.4. Stability of ZnO/Al2 O3 catalysts for RWReaction of CAMERE process The stability of the ZnO/Al2 O3 catalysts was investigated under the reaction condition of 873 K and GHSV of 150,000. The high space velocity was applied to confirm the stability in a short period of time. Fig. 7 shows the conversion of CO2 versus reaction time. The stability was very dependent on the amounts of ZnO in the ZnO/Al2 O3 catalysts. The ZnO/Al2 O3 (Zn:Al = 1:1) catalyst shows a
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Fig. 6. TEM image and EDS spectra of ZnO/Al2 O3 (Zn:Al = 2:1) catalyst.
high initial activity of 43%. But the ZnAl2 O4 shows a very low CO2 conversion of 10%, although the BET surface area was four times larger than that of ZnO/Al2 O3 (Zn:Al = 1:1) catalyst. It means that ZnO is more active than ZnAl2 O4 for the RWReaction. The catalysts of ZnO/Al2 O3 (Zn:Al = 4:1) and ZnO/Al2 O3 (Zn:Al = 1:1) show the same deactiva-
tion tendency in the initial 10 h. The catalyst of ZnAl (4:1) was stable up to 40 h and then slowly deactivated. On the other hand, the catalytic activity of ZnAl (1:1) steadily decreased from 43 to 17% in 48 h. In the meantime, ZnAl2 O4 catalyst was very stable, though the activity was not so high. Moreover, the activity of ZnAl2 O4 catalyst was stable for 240 h even at 973 K.
Fig. 7. CO2 conversion vs. reaction time at GHSV of 150,000 on the ZnO/Al2 O3 catalysts calcined at 1123 K. (a) ZnAl (4:1); (b) ZnAl (1:1); (c) ZnAl (1:2).
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Fig. 8. XRD spectra of ZnO/Al2 O3 (Zn:Al = 1:1) catalyst. (×): ZnO; ( ): ZnAl2 O4 ; (a) fresh catalyst; (b) used catalyst.
Interestingly, the deactivation rate of the ZnO/Al2 O3 (Zn:Al = 4:1) was different from that of ZnO/Al2 O3 (Zn:Al = 1:1). The deactivation rate depended on the amount of ZnO in the ZnO/Al2 O3 catalysts. In order to understand the reason for the deactivation of ZnO/Al2 O3 catalysts, the ZnO/Al2 O3 (Zn:Al = 1:1) catalyst was investigated by XRD after the RWReaction for 48 h at 873 K and GHSV of 150,000. Fig. 8
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shows the XRD spectra of the fresh and the deactivated catalysts. The peaks from ZnO entirely disappeared after the durability test, while the peaks from ZnAl2 O4 remained unchanged. Such results strongly suggest that ZnO was reduced to the Zn metal and this was followed by evaporation during the RWReaction. Actually, the white metallic Zn metal was deposited on the wall of cold parts of the RWReactor. The deactivation rate might depend on the particle size of ZnO in the ZnO/Al2 O3 catalysts. The dramatic deactivation (Fig. 7b) of ZnO/Al2 O3 catalyst came from the small particle size of ZnO, because the ZnO with small particle size could be easily reduced. On the other hand, ZnAl2 O4 showed no deactivation under the same reaction condition of 873 K and GHSV of 150,000. Fig. 9 shows the TEM images of ZnO/Al2 O3 (Zn:Al = 1:2) catalysts calcined at the temperatures of 773 and 1123 K. The ZnO/Al2 O3 (Zn:Al = 1:2) catalyst calcined at 773 K consisted of the separate phases of ZnO, Al2 O3 , and trace amounts of ZnAl2 O4 (from XRD data), in which the ZnO was segregated from the Al2 O3 phase (Fig. 9A). Meanwhile, the catalyst calcined at 1123 K showed the uniform phase of ZnAl2 O4 (Fig. 9B). The SEM image of the ZnO/Al2 O3 (Zn:Al = 1:2) catalyst calcined at 773 K also shows similar morphology to that of ZnO (Figs. 10A and 3A). The formation of needle-like morphology indicated the presence of free ZnO. The
Fig. 9. TEM images of the ZnO/Al2 O3 (Zn:Al = 1:2) catalyst.
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Fig. 10. SEM images of ZnO/Al2 O3 (Zn:Al = 1:2) catalyst.
calcination at 773 K was not enough to form the spinel structure of ZnAl2 O4 . The ZnO/Al2 O3 (Zn:Al = 1:2) catalyst calcined at 773 K showed similar activity to that of ZnO calcined at 773 K for the RWReaction, showing deactivation. The ZnO/Al2 O3 (Zn:Al = 1:2) catalyst calcined at 1123 K consisted of the uniform phase of ZnAl2 O4 spinel structure (Fig. 9B), and had different morphology from ZnO (Fig. 10B). The ZnAl2 O4 was investigated by XRD to observe the structure transformation after the RWReaction (Fig. 11). The XRD spectra were basically similar before and after the RWReaction at the temperature range of 873–973 K and GHSV of 150,000. It means that the structure of ZnAl2 O4 catalyst was not changed under the RWReaction conditions. The ZnAl2 O4 catalyst was thermally stable during the RWReaction without structure change. Moreover, the ZnAl2 O4 catalyst showed good activity close to the equilibrium conversion at 873 K and GHSV of 5,000, which was a condition for the practical operation of the CAMERE process. The ZnAl2 O4 would be a good catalyst for the RWReaction of the CAMERE process.
ter were oxidizing agents preventing the reduction of a metal oxide. The reducing power of H2 and CO was dependent on the partial pressure of the gases and on the reaction temperature. During the RWReaction, the ZnO in ZnO/Al2 O3 catalysts can be reduced to the Zn metal by CO and H2 as follows: ZnO + H2 = Zn + H2 O
(1)
ZnO + CO = Zn + CO2
(2)
3.5. Thermodynamics for reduction of ZnO In the RWReaction of the CAMERE process, carbon dioxide and excess of hydrogen reacted to give CO and H2 O. The reactant H2 and the produced CO were strong reducing agents, which can reduce a metal oxide to the metal. Meanwhile, the CO2 and the wa-
Fig. 11. XRD spectra of ZnAl2 O4 catalyst. ( ): ZnAl2 O4 ; (a) fresh catalyst; (b) used catalyst.
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Fig. 12. Reducibility of ZnO to Zn depending on the partial pressure of CO, H2 , and temperature.
Gibbs energy equations of Eqs. (1) and (2) were obtained using the data in the literature [13,14]: 1G1 = 27761.2 + 11.333T log T − 1.36 × 10−3 T 2 −67250/T − 46.3405T
(3)
1G2 = 16440.9 + 4.4256T log T − 1.08 × 10−3 T 2 −24250/T − 15.2327T
(4)
CZn PH2 O , CZnO PH2 CZn PCO2 K2 = CZnO PCO 1Gi Ki = Exp − RT At equilibrium, K1 =
(5) (6)
The Zn concentration (CZn ) could be calculated from Eqs. (3)–(6). Fig. 12 indicates that the ZnO phase can be reduced to the Zn metal depending on the partial pressures of H2 and CO and on the reaction temperature. The line for changing from ZnO to ZnO + Zn metal phase means that 0.01 mol% of ZnO was reduced to the Zn metal under the partial pressure of H2 and CO. In a reaction condition range below the line, ZnO can be reduced to the Zn metal. In addition, an equation of −1G1 /RT = ln(PH2 O /PH2 ) + ln(CZn /CZnO ) could be derived from Eqs. (5) and (6). For a ZnO molecule to be reduced at the same partial pressures of H2 and CO, the line shifts toward an even higher positive value of the ln(PH2 O /PH2 ) and the ln(PCO2 /PCO ). It meant that ZnO could be reduced at a condition having the positive values of the ln(PH2 O /PH2 ) and the ln(PCO2 /PCO ). The values of the ln(PH2 O /PH2 =
Table 2 The calculated values for −1Gi , Ki , and PCi /PCi at C Zn = 0.01 mol% T (K)
−1G1 (cal/mol)
K1
PH2 O /PH2
−1G2 (cal/mol)
K2
PCO2 /PCO
770 873 970
16374.0 15289.9 14295.6
2.25E-5 1.49E-4 6.00E-4
0.225 1.49 6.0
13939.2 13710.2 13495.7
1.10E-4 3.70E-4 9.10E-4
1.10 3.70 9.10
90
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0.25) and the ln(PCO2 /PCO = 0.67) were −1.39 and −0.41, respectively, under the reaction condition for the RWReaction of the CAMERE process. The values in the line for ZnO (0.01 mol%) reduction at 873 K were 0.40 and 1.31 (Fig. 12). Table 2 indicates the calculated values for −1Gi , Ki , and PCi /PCi at C Zn = 0.01 mol% and 873 K. ZnO can be reduced at the values of PH2 O /PH2 or PCO2 /PCO lower than those at the table. Those indicate that ZnO in the ZnO/Al2 O3 catalysts should be reduced to the Zn metal under the RWReaction conditions of the CAMERE process.
4. Conclusion The stable catalyst of ZnAl2 O4 was developed for the RWReaction of the CAMERE process. The ZnAl2 O4 catalyst showed no deactivation for 240 h for the RWReaction carried out at the temperature range of 873–973 K and GHSV of 150,000. The activities of ZnO and ZnO/Al2 O3 catalysts were investigated over the temperature range of 673–979 K and GHSV of 15,000, while the durability was tested at 873 K and GHSV of 150,000. The ZnO/Al2 O3 catalysts were mixtures of ZnO and ZnAl2 O4 , in which the amounts were dependent on the composition of Zn and Al. The ZnAl2 O4 showed stable performance for the RWReaction of the CAMERE process. Meanwhile, the ZnO/Al2 O3 catalysts were deactivated due to the reduction of ZnO, even though its activity was higher than that of ZnAl2 O4 . The ZnO was agglomerated by the heat treatment at 1123 K, which retarded the reduction of the ZnO.
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