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Hydrogenation of CO2 to methanol over In2O3 catalyst
Journal of CO2 Utilization 12 (2015) 1–6
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Hydrogenation of CO2 to methanol over In2O3 catalyst Kaihang Suna,b , Zhigang Fana,b , Jingyun Yea,b , Jinmao Yana,b , Qingfeng Gea,c, Yanan Lia,d , Wenjun Hea,d , Weimin Yanga,d , Chang-jun Liua,b,* a
Tianjin Co-Innovation Center of Chemical Science & Engineering, Tianjin University, Tianjin 300072, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, IL 62901, USA d SINOPEC Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 June 2015 Received in revised form 31 August 2015 Accepted 12 September 2015 Available online 3 October 2015
The superior catalytic activity of In2O3 for CO2 hydrogenation to methanol is demonstrated here. The experimental results demonstrate that the reaction temperature and pressure have a significant influence on methanol yield. The conversion of CO2 over In2O3 increases with the increase of reaction temperature and pressure. The yield and formation rate of methanol also increase with the increase of reaction pressure. However, they increase firstly with the increase of reaction temperature but start to decrease when the temperature rises above 330 C. At 330 C and 4 MPa, the yield of methanol reaches 2.82%, while the methanol production rate reaches 3.69 mol h1 kgcat1, higher than many other reported catalysts, which normally show very low selectivity of methanol at such high temperature. This confirms the previous theoretical study that In2O3 inhibits the reverse water gas shift, a competitive endothermic reaction for methanol synthesis from CO2 hydrogenation. The mechanism for CO2 hydrogenation to methanol over In2O3 catalyst has been discussed. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: In2O3 CO2 Hydrogenation Methanol
1. Introduction With the increasing concern on the emission control of carbon dioxide, CO2 utilization has recently received great attention worldwide [1–5]. For example, the theme of the 2014 CO2 forum [1], held in Lyon last year, is the large scale utilization of carbon dioxide. From 2015, the international conference on CO2 utilization (ICCDU), initiated at the beginning of 1990s, will be organized every year. For large scale utilization of carbon dioxide, two major problems need to be solved: one is how to get clean energy or CO2-free hydrogen resources to convert carbon dioxide. The other is how to efficiently activate CO2 molecule due to its thermodynamic and chemical stability. From the literature, a rapid increasing publication can be found on the topics of hydrogen production via solar cell or solar fuel. The first problem can be expected to be solved in the near future with many promising progresses in the renewable energy. However, the progress in CO2 activation is not impressed. Much less papers can be found with CO2 activation, compared to hydrogen production. Regarding the
* Corresponding author at: Tianjin Co-Innovation Center of Chemical Science & Engineering, Tianjin University, Tianjin 300072, China. E-mail addresses: [email protected], [email protected] (C.-j. Liu). http://dx.doi.org/10.1016/j.jcou.2015.09.002 2212-9820/ ã 2015 Elsevier Ltd. All rights reserved.
activation and conversion of carbon dioxide, several reaction options have been exploited, including CO2 reforming [6–8] and CO2 hydrogenation [9,10]. Among all these options, hydrogenation of carbon dioxide to methanol has attracted more attentions by far. The economic feasibility for this process has been established [9,10]. A CO2 based renewable plant, named in the honor of Prof. George Olah, was opened in April 2012 in Grindavik, Reykjanes, Iceland [9]. Geothermal energy was applied for hydrogen generation. This plant is expected to reclaim about 4500 t/year of CO2 from the Earth’s atmosphere and to produce about five million liters of methanol per year [9]. It should be enough to meet about 2.5% of the total gasoline market in Iceland [9]. Many catalysts have been investigated but the most studied one is Cu/ZnO based catalyst [10,11] for CO2 hydrogenation to methanol. However, the activity and methanol selectivity of developed catalysts still need to be improved. A significant effort has been made towards the development of new catalysts. Recently, during the density functional theory (DFT) studies of CO2 activation over Al2O3 [12,13], Ga2O3 [14,15] and In2O3 [16,17], we found that In2O3 theoretically possesses high activity for CO2 hydrogenation to methanol. Before that, no reported work can be found in the literature with high activity towards CO2 hydrogenation to methanol over a single oxide only. From the DFT studies [16,17], the perfect In2O3 crystal possesses high activity towards adsorption and activation of CO2 [16]. The defective In2O3 surface with oxygen
2
K. Sun et al. / Journal of CO2 Utilization 12 (2015) 1–6
vacancies, however, shows even higher activity for methanol synthesis from CO2 hydrogenation [17]. Hydrogenation of CO2 to HCOO on the defective In2O3 surface is both thermodynamically and kinetically favorable, although there are other routes for CO2 hydrogenation. The step for HCOO hydrogenation to H2CO is slightly endothermic with a barrier of 0.57 eV. Subsequently, H2CO hydrogenation to CH3O is more favorable than protonation to H2COH for methanol formation. The DFT studies [17] demonstrate that the oxygen vacancy on the calculated In2O3 surface assists CO2 activation and hydrogenation. It also stabilizes the key intermediates involved in methanol formation. In addition, methanol formation replenishes the oxygen vacancy sites whereas H2 helps to generate the vacancies. The cycle between the perfect and defective states of the surface catalyzes the formation of methanol from CO2 hydrogenation. Such unusual chemistry is not found over Ga2O3 [14,15]. The experimental study reported by Collins et al. [18] has showed that pure Ga2O3 has no activity for hydrogenation of CO2 to methanol. This confirms the results of our DFT studies [14,15]. To demonstrate the results of previous DFT studies, CO2 hydrogenation to CO (reverse water gas shift, RWGS) on In2O3 [19] and In2O3 based catalysts [20,21] at atmospheric pressure was firstly conducted with 1:1 feed ratio of CO2 and H2. These experimental studies have confirmed that In2O3 and related catalysts are indeed superior catalysts for CO2 activation [19–21]. In the present work, we aim to further experimentally confirm the excellent activity of In2O3 for hydrogenation of CO2 to methanol at elevated pressures. The previous DFT studies [16,17] suggested that In2O3 inhibits the RGWS reaction at the condition for methanol synthesis. This significantly helps methanol synthesis. We will also confirm it in this work. 2. Experimental 2.1. Catalysts preparation The In2O3 catalyst was prepared by calcination of commercial indium (III) oxide powders (Guangfu, 99.9%, Tianjin, China) at 500 C for 5 h under air. 2.2. Activity test The test for CO2 hydrogenation over the In2O3 catalyst was conducted in a tubular micro-reactor. 0.2 g of In2O3 catalyst (40–60 mesh), mixed with 1.0 g of SiC (40–60 mesh) as diluent, was loaded into the reactor. Prior to the catalytic test, the catalyst was firstly purged by N2 (30 mL/min) at room temperature for 0.5 h under atmospheric pressure. The reactant (H2/CO2/N2 = 3/1/1, molar ratio) was then fed into the reactor with a flow rate of 150 mL/min. After the pressure reached the targeted pressure, the flow rate of reactant gas was decreased to 50 mL/min. The gaseous hourly space velocity (GHSV) was 15 000 cm3 h1 gcat1. After that, the temperature was increased to 250 C at a heating rate of 10 C/min. The catalytic activities started to be recorded from 250 C to 350 C. The effluent was analyzed by an online gas chromatograph (Agilent 4890D) equipped with a two-column system connected to a flame ionized detector (FID) and a thermal conductivity detector (TCD), respectively. All the post-reactor lines and valves were heated to 140 C to prevent the condensation of the products. The CO2 conversion (X(CO2)), CH3OH yield (Y(CH3OH)) and formation rate of CH3OH (R(CH3OH)) were calculated according to the following equations: XðCO2 Þ ¼
nðCOÞout þ nðCH3 OHÞout þ nðCH4 Þout 100 nðCOÞout þ nðCH3 OHÞout þnðCH4 Þout þ nðCO2 Þout
Y ðCH3 OHÞ ¼
XðCO2 Þ SðCH3 OHÞ 100
RðCH3 OHÞ ¼
Y ðCH3 OHÞ GHSV 2 22:4
n represents the molar amount of the molecule in the effluent. S means the selectivity of methanol. 2.3. Catalyst characterization 2.3.1. Thermal gravimetric analysis (TGA) The TGA analysis was carried out under a mixed-gas atmosphere (total flow: 25 mL/min, O2/N2 = 1/4, molar ratio) at a constant rate of 10 C/min, using a Netzsch STA 449 F3 system. The sample (10 mg) was loaded into an alumina crucible and heated from room temperature to 1000 C. 2.3.2. X-ray diffraction (XRD) The phase structure of the catalyst was analyzed by X-ray diffraction, using a Rigaku D/max 2500 v/pc diffractometer with Cu Ka radiation (40 kV, 200 mA) at a scanning rate of 4 /min within the 2u range of 10–90 . The phase identification was made by comparison with the Joint Committee on Powder Diffraction Standards (JCPDSs). 2.3.3. High resolution transmission electron microscope (HR-TEM) TEM analysis was performed on a Philips Tecnai G2 F20 system operated at 200 kV. The sample was firstly suspended into ethanol and then dispersed ultrasonically for 10 min. A drop of the suspension was deposited on a copper grid coated with carbon to prepare the sample for HR-TEM analysis. 3. Results and discussion 3.1. Catalytic performance Fig. 1 shows the conversion of CO2 versus the temperature under different pressures. Obviously, the conversion of CO2 over In2O3 increases with the increase of reaction temperature and pressure. As shown in Fig. 2, the formation rate and yield of methanol have similar changing patterns. This indicates that CO2 hydrogenation on In2O3 has a similar trend of methanol yield and formation rate with the temperature at different pressures. The best yields and formation rates of methanol all appear at 330 C. With the increasing reaction pressure, the activities of all samples for methanol synthesis are enhanced. For example, the yield and formation rate of methanol at 4 MPa are higher than those at lower pressures at every temperature point. Because of the limitation of the reactor applied, reactions at higher pressures (higher than 4 MPa) have to be conducted in the future. We expect the methanol yield will be further improved with further increasing reaction pressures. Only CO, methanol and trace methane were detected. No other hydrocarbons can be identified. When the reaction temperature is above 330 C, the conversion of CO2 keeps increasing but the formation rate and the yield of methanol decrease. This is because high temperature is favored for RWGS, an endothermic reaction. CO2 hydrogenation to methanol is an exothermic reaction. With the rising temperature, more and more CO is generated by RWGS on In2O3 whereas In2O3 surface does not adsorb CO strongly according to our previous works [17]. Herein, CO2 tends to transform into CO rather than methanol at high temperature. The generated CO cannot be further converted to intermediate on In2O3
K. Sun et al. / Journal of CO2 Utilization 12 (2015) 1–6
3
Fig. 1. CO2 conversion over In2O3.
so that most of methanol should be produced by CO2. Consequently, the methanol yield and formation rate decrease beyond 350 C is due to the production of CO by RWGS. In order to clarify the activity of In2O3 for methanol synthesis via CO2 hydrogenation, a comparison of catalytic activity has been made with those reported studies in the literature. The comparative results have been shown in Table 1. As shown in Table 1, the methanol formation rate of In2O3 catalyst is 0.78 mol h1 kgcat1 at 270 C but reaches 3.69 mol h1 kgcat1 at 330 C. The methanol formation rate at 330 C is higher than that over most of developed catalysts at lower reaction temperatures [18,22–34]. Only Cu/ZnO/ ZrO2/Al2O3 [25], Cu/ZnO/Ga2O3 [24] and Ga2O3-Pd/SiO2 [33,34] shows higher methanol formation rate. The methanol formation rate at 270 C on In2O3 is sufficiently high because it is achieved with the pure oxide. All those reported catalysts require
combinations of catalyst with co-catalyst, or with promoter and supporting material. As mentioned before, no pure oxide except In2O3 shows high activity for methanol synthesis from CO2 hydrogenation. For the copper based catalysts [22–29], co-catalyst, promoter(s) and support have to be applied in order to possess a good enough activity with high CO2 conversion, good CH3OH selectivity and high CH3OH formation rate. To prepare these catalysts, wet impregnation and co-precipitation are common methods. After extensive studies, further improvement in the copper based catalysts has become difficult. Since the methanol formation reaction has to compete with the endothermic RGWS reaction at higher temperatures, the reported works on CO2 hydrogenation to methanol over Cu/ZnO based and Pd based catalysts were normally performed at temperatures below 300 C. It is very difficult for those reported catalysts to
Fig. 2. CH3OH formation rate (a) and yield (b) at different pressures.
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Table 1 The comparison of In2O3 and other catalysts for CO2 hydrogenation to methanol. Catalyst
Temperature ( C)
Pressure (MPa)
CO2 conversion (%)
CH3OH selectivity (%)
CH3OH formation rateb (mol h1 kgcat1
Reference
CuO/ZnO Cu/ZnO/ZrO2 Cu/ZnO/Al2O3 Cu/ZnO/ZrO2/Al2O3 CuO/ZrO2 Cu/Ga2O3/ZrO2 Cu/B2O3/ZrO2 Cu-Zn-Ga/SiO2 Cu-Zn/SiO2 Cu/ZnO/Ga2O3 Ga2O3–Pd/SiO2 Pd/Al2O3 PdMgAl PdZnAl PdMgGa Ni5Ga3 Pd(0.34)–Cu In2O3 In2O3 b-Ga2O3
250 230 230 230 240 250 250 270 270 250 250 250 250 250 250 200 250 270 330 250
5 3 3 3 2 2 2 2 2 8 3 5 3 3 3 0.1 4.1 4 4 3
11.7 19.3 18.7 23.2 6.3 13.71 15.83 2.1 1.8 / / 3.4 0.3 0.6 1 / 6.6 1.1 7.1 /
36.1 48.6 43 60.3 48.8 75.59 67.26 96.6 99.1 83 70 29.9 4 60 47 60a 34 54.9 39.7 /
/ 2.51 2.15 3.75 / 1.93 1.8 / 3.58 5.94 7.9 / 0.018 0.546 0.63 2.5 1.116 0.78 3.69 /
[22] [25] [25] [25] [28] [23] [23] [29] [26,27] [24] [33,34] [22] [30] [30] [30] [31] [32] This work This work [18]
a b
The CH3OH selectivity of Ni5Ga3 includes DME and CH3OH. The formation rate of CH3OH was calculated by the catalytic data in the references if not given directly.
maintain sufficiently high selectivity of methanol at temperatures higher than 300 C. The reported works in Table 1 were all conducted at temperatures below 270 C [18,22–34]. As abovementioned, the DFT studies indicated that In2O3 can inhibit the RGWS reaction [16,17]. This is the reason that In2O3 shows an unusual high selectivity of methanol (39.7%) at 330 C with a very high methanol formation rate. Catalytic activities of some alloyed catalysts [30–32] are also given in Table 1. Moreover, compared with pure Ga2O3 [18], In2O3 shows a much higher activity towards CO2 hydrogenation to methanol. Pure Ga2O3 does not show any activity for CO2 hydrogenation to methanol as above-indicated [18]. It has to combine with catalyst (like Cu/ZnO [24] or Pd [33,34]) to present a nice activity. These results indicate that In2O3 possesses a great potential as catalyst, promoter or supporting material. More important, the preparation of In2O3 catalyst is easier than other catalysts in Table 1. The products over In2O3 catalyst are simple. Only
methanol, CO and trace methane are produced. These products can be easily separated by condensation. CO can be recycled into the reaction for further methanol synthesis with modification of the catalyst. 3.2. Characterization To decide the temperature of thermal treatment, the commercial In2O3 powder was analyzed by TGA. As shown in Fig. 3, the weight loss (1%) of sample does not change when the temperature increases beyond 500 C. This weight loss is caused by the impurity (like moisture) of commercial In2O3 powders. Therefore, we chose 500 C to treat the commercial powder. From 300 C to 600 C, there is an endothermic peak in the DSC curve, which is induced from the endothermic decomposition of the impurity. XRD patterns of the samples are illustrated in Fig. 4. The peaks for these four used samples (after reactions at 350 C as shown in (Figs. 1 and 2) and pure In2O3 (fresh) appear at 2u = 21.498 , 30.580 ,
Fig. 3. TGA curves of commercial In2O3 sample.
K. Sun et al. / Journal of CO2 Utilization 12 (2015) 1–6
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Fig. 4. XRD patterns of fresh and used catalysts.
35.466 , 45.691, 51.037 and 60.676 , which are assigned to the diffractions from the (2 11), (2 2 2), (4 0 0), (4 3 1), (4 4 0) and (6 2 2) planes, belonging to Ia3(2 0 6) space group, according to PDF#653170. As shown in the diffraction patterns, the In2O3 peaks for all samples are similar, indicating that the reaction at various pressures and temperatures has no influence on the crystal structure of In2O3. This suggests In2O3 remains stable after the reaction within the temperature range tested. The average size of fresh In2O3 particles and the used samples is 25.8 nm and 28.1 nm, according to the Scherrer equations [20]. There is a minor increase in the particle size after reaction. To understand the morphological
and the structural changes of all samples, HR-TEM analyses were performed. Fig. 5 shows HR-TEM images of the fresh catalyst and the used samples. For fresh In2O3 particles, some rectangular and hexagonal holes can be observed. No lattice fringes of pure indium can be found over the used samples, suggesting that the catalyst exists in the form of In2O3 and has good crystallinity of In2O3. Meanwhile, (2 11), (2 2 2) and (4 11) lattice planes can be easily found on the fresh In2O3 and all the used samples. In particular, the particle sizes of all the used samples are close to that of the fresh In2O3. In2O3 particles possess good structural stability in the reaction within the
Fig. 5. HR-TEM of fresh and used catalysts.
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conditions tested. All these results are consistent with the XRD results. 4. Conclusions This work confirms the previous theoretical studies that In2O3 is an excellent catalyst for CO2 hydrogenation to methanol. It also demonstrates the micro-kinetic modeling that the increasing reaction pressure can increase the formation rate of methanol [35,36]. The catalyst characterization using TGA, XRD and HR-TEM indicates that In2O3 has good thermal and structural stability for the reactions below 500 C. Higher pressure favors the methanol synthesis. For the temperature effect, there exist a maximum for methanol yield. Beyond it, the yield of methanol will be reduced. With the reaction condition tested, the highest methanol yield (2.82%) is achieved at 330 C and 4 MPa with a CO2 conversion of 7.13%. The highest formation rate of methanol is 3.69 mol h1 kgcat1, higher than most of the other reported catalysts in Table 1. Compared with other catalysts, In2O3 catalyst can be easily prepared and the products of reaction are mainly methanol and CO with trace methane. For future possible applications, CO can be easily recycled into the reaction system for methanol synthesis together with CO2 hydrogenation (after some modification of the catalyst). The present investigation is very helpful for the future development of In2O3 based new catalysts for CO2 hydrogenation to methanol. Especially, In2O3 based catalyst is a promising photocatalyst [37–40]. One can expect a rapid progress in the CO2 activation and conversion over In2O3 based catalysts. Acknowledgments This work was supported by SINOPEC (with contract No. 413109) and the National Natural Science Foundation of China (with contract No. 91334206). References [1] http://CO2forum.cpe.fr. [2] M.B. Ansari, S.E. Park, Energy Environ. Sci. 5 (2012) 9419–9437. [3] R.X. Yuan, Y. Li, H.B. Yan, H. Wang, J. Song, Z.S. Zhang, W.B. Fan, J.G. Chen, Z.W. Liu, Z.T. Liu, Chin. J. Catal. 35 (2014) 1329–1336. [4] B. Yu, Z.F. Diao, C.X. Guo, L.N. He, J. CO2 Utilization 1 (2013) 60–68. [5] S. Saeidi, N.A.S. Amin, M.R. Rahimpour, J. CO2 Utilization 5 (2014) 66–81.
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Hydrogenation of CO2 to methanol over In2O3 catalyst Kaihang Suna,b , Zhigang Fana,b , Jingyun Yea,b , Jinmao Yana,b , Qingfeng Gea,c, Yanan Lia,d , Wenjun Hea,d , Weimin Yanga,d , Chang-jun Liua,b,* a
Tianjin Co-Innovation Center of Chemical Science & Engineering, Tianjin University, Tianjin 300072, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, IL 62901, USA d SINOPEC Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 June 2015 Received in revised form 31 August 2015 Accepted 12 September 2015 Available online 3 October 2015
The superior catalytic activity of In2O3 for CO2 hydrogenation to methanol is demonstrated here. The experimental results demonstrate that the reaction temperature and pressure have a significant influence on methanol yield. The conversion of CO2 over In2O3 increases with the increase of reaction temperature and pressure. The yield and formation rate of methanol also increase with the increase of reaction pressure. However, they increase firstly with the increase of reaction temperature but start to decrease when the temperature rises above 330 C. At 330 C and 4 MPa, the yield of methanol reaches 2.82%, while the methanol production rate reaches 3.69 mol h1 kgcat1, higher than many other reported catalysts, which normally show very low selectivity of methanol at such high temperature. This confirms the previous theoretical study that In2O3 inhibits the reverse water gas shift, a competitive endothermic reaction for methanol synthesis from CO2 hydrogenation. The mechanism for CO2 hydrogenation to methanol over In2O3 catalyst has been discussed. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: In2O3 CO2 Hydrogenation Methanol
1. Introduction With the increasing concern on the emission control of carbon dioxide, CO2 utilization has recently received great attention worldwide [1–5]. For example, the theme of the 2014 CO2 forum [1], held in Lyon last year, is the large scale utilization of carbon dioxide. From 2015, the international conference on CO2 utilization (ICCDU), initiated at the beginning of 1990s, will be organized every year. For large scale utilization of carbon dioxide, two major problems need to be solved: one is how to get clean energy or CO2-free hydrogen resources to convert carbon dioxide. The other is how to efficiently activate CO2 molecule due to its thermodynamic and chemical stability. From the literature, a rapid increasing publication can be found on the topics of hydrogen production via solar cell or solar fuel. The first problem can be expected to be solved in the near future with many promising progresses in the renewable energy. However, the progress in CO2 activation is not impressed. Much less papers can be found with CO2 activation, compared to hydrogen production. Regarding the
* Corresponding author at: Tianjin Co-Innovation Center of Chemical Science & Engineering, Tianjin University, Tianjin 300072, China. E-mail addresses: [email protected], [email protected] (C.-j. Liu). http://dx.doi.org/10.1016/j.jcou.2015.09.002 2212-9820/ ã 2015 Elsevier Ltd. All rights reserved.
activation and conversion of carbon dioxide, several reaction options have been exploited, including CO2 reforming [6–8] and CO2 hydrogenation [9,10]. Among all these options, hydrogenation of carbon dioxide to methanol has attracted more attentions by far. The economic feasibility for this process has been established [9,10]. A CO2 based renewable plant, named in the honor of Prof. George Olah, was opened in April 2012 in Grindavik, Reykjanes, Iceland [9]. Geothermal energy was applied for hydrogen generation. This plant is expected to reclaim about 4500 t/year of CO2 from the Earth’s atmosphere and to produce about five million liters of methanol per year [9]. It should be enough to meet about 2.5% of the total gasoline market in Iceland [9]. Many catalysts have been investigated but the most studied one is Cu/ZnO based catalyst [10,11] for CO2 hydrogenation to methanol. However, the activity and methanol selectivity of developed catalysts still need to be improved. A significant effort has been made towards the development of new catalysts. Recently, during the density functional theory (DFT) studies of CO2 activation over Al2O3 [12,13], Ga2O3 [14,15] and In2O3 [16,17], we found that In2O3 theoretically possesses high activity for CO2 hydrogenation to methanol. Before that, no reported work can be found in the literature with high activity towards CO2 hydrogenation to methanol over a single oxide only. From the DFT studies [16,17], the perfect In2O3 crystal possesses high activity towards adsorption and activation of CO2 [16]. The defective In2O3 surface with oxygen
2
K. Sun et al. / Journal of CO2 Utilization 12 (2015) 1–6
vacancies, however, shows even higher activity for methanol synthesis from CO2 hydrogenation [17]. Hydrogenation of CO2 to HCOO on the defective In2O3 surface is both thermodynamically and kinetically favorable, although there are other routes for CO2 hydrogenation. The step for HCOO hydrogenation to H2CO is slightly endothermic with a barrier of 0.57 eV. Subsequently, H2CO hydrogenation to CH3O is more favorable than protonation to H2COH for methanol formation. The DFT studies [17] demonstrate that the oxygen vacancy on the calculated In2O3 surface assists CO2 activation and hydrogenation. It also stabilizes the key intermediates involved in methanol formation. In addition, methanol formation replenishes the oxygen vacancy sites whereas H2 helps to generate the vacancies. The cycle between the perfect and defective states of the surface catalyzes the formation of methanol from CO2 hydrogenation. Such unusual chemistry is not found over Ga2O3 [14,15]. The experimental study reported by Collins et al. [18] has showed that pure Ga2O3 has no activity for hydrogenation of CO2 to methanol. This confirms the results of our DFT studies [14,15]. To demonstrate the results of previous DFT studies, CO2 hydrogenation to CO (reverse water gas shift, RWGS) on In2O3 [19] and In2O3 based catalysts [20,21] at atmospheric pressure was firstly conducted with 1:1 feed ratio of CO2 and H2. These experimental studies have confirmed that In2O3 and related catalysts are indeed superior catalysts for CO2 activation [19–21]. In the present work, we aim to further experimentally confirm the excellent activity of In2O3 for hydrogenation of CO2 to methanol at elevated pressures. The previous DFT studies [16,17] suggested that In2O3 inhibits the RGWS reaction at the condition for methanol synthesis. This significantly helps methanol synthesis. We will also confirm it in this work. 2. Experimental 2.1. Catalysts preparation The In2O3 catalyst was prepared by calcination of commercial indium (III) oxide powders (Guangfu, 99.9%, Tianjin, China) at 500 C for 5 h under air. 2.2. Activity test The test for CO2 hydrogenation over the In2O3 catalyst was conducted in a tubular micro-reactor. 0.2 g of In2O3 catalyst (40–60 mesh), mixed with 1.0 g of SiC (40–60 mesh) as diluent, was loaded into the reactor. Prior to the catalytic test, the catalyst was firstly purged by N2 (30 mL/min) at room temperature for 0.5 h under atmospheric pressure. The reactant (H2/CO2/N2 = 3/1/1, molar ratio) was then fed into the reactor with a flow rate of 150 mL/min. After the pressure reached the targeted pressure, the flow rate of reactant gas was decreased to 50 mL/min. The gaseous hourly space velocity (GHSV) was 15 000 cm3 h1 gcat1. After that, the temperature was increased to 250 C at a heating rate of 10 C/min. The catalytic activities started to be recorded from 250 C to 350 C. The effluent was analyzed by an online gas chromatograph (Agilent 4890D) equipped with a two-column system connected to a flame ionized detector (FID) and a thermal conductivity detector (TCD), respectively. All the post-reactor lines and valves were heated to 140 C to prevent the condensation of the products. The CO2 conversion (X(CO2)), CH3OH yield (Y(CH3OH)) and formation rate of CH3OH (R(CH3OH)) were calculated according to the following equations: XðCO2 Þ ¼
nðCOÞout þ nðCH3 OHÞout þ nðCH4 Þout 100 nðCOÞout þ nðCH3 OHÞout þnðCH4 Þout þ nðCO2 Þout
Y ðCH3 OHÞ ¼
XðCO2 Þ SðCH3 OHÞ 100
RðCH3 OHÞ ¼
Y ðCH3 OHÞ GHSV 2 22:4
n represents the molar amount of the molecule in the effluent. S means the selectivity of methanol. 2.3. Catalyst characterization 2.3.1. Thermal gravimetric analysis (TGA) The TGA analysis was carried out under a mixed-gas atmosphere (total flow: 25 mL/min, O2/N2 = 1/4, molar ratio) at a constant rate of 10 C/min, using a Netzsch STA 449 F3 system. The sample (10 mg) was loaded into an alumina crucible and heated from room temperature to 1000 C. 2.3.2. X-ray diffraction (XRD) The phase structure of the catalyst was analyzed by X-ray diffraction, using a Rigaku D/max 2500 v/pc diffractometer with Cu Ka radiation (40 kV, 200 mA) at a scanning rate of 4 /min within the 2u range of 10–90 . The phase identification was made by comparison with the Joint Committee on Powder Diffraction Standards (JCPDSs). 2.3.3. High resolution transmission electron microscope (HR-TEM) TEM analysis was performed on a Philips Tecnai G2 F20 system operated at 200 kV. The sample was firstly suspended into ethanol and then dispersed ultrasonically for 10 min. A drop of the suspension was deposited on a copper grid coated with carbon to prepare the sample for HR-TEM analysis. 3. Results and discussion 3.1. Catalytic performance Fig. 1 shows the conversion of CO2 versus the temperature under different pressures. Obviously, the conversion of CO2 over In2O3 increases with the increase of reaction temperature and pressure. As shown in Fig. 2, the formation rate and yield of methanol have similar changing patterns. This indicates that CO2 hydrogenation on In2O3 has a similar trend of methanol yield and formation rate with the temperature at different pressures. The best yields and formation rates of methanol all appear at 330 C. With the increasing reaction pressure, the activities of all samples for methanol synthesis are enhanced. For example, the yield and formation rate of methanol at 4 MPa are higher than those at lower pressures at every temperature point. Because of the limitation of the reactor applied, reactions at higher pressures (higher than 4 MPa) have to be conducted in the future. We expect the methanol yield will be further improved with further increasing reaction pressures. Only CO, methanol and trace methane were detected. No other hydrocarbons can be identified. When the reaction temperature is above 330 C, the conversion of CO2 keeps increasing but the formation rate and the yield of methanol decrease. This is because high temperature is favored for RWGS, an endothermic reaction. CO2 hydrogenation to methanol is an exothermic reaction. With the rising temperature, more and more CO is generated by RWGS on In2O3 whereas In2O3 surface does not adsorb CO strongly according to our previous works [17]. Herein, CO2 tends to transform into CO rather than methanol at high temperature. The generated CO cannot be further converted to intermediate on In2O3
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Fig. 1. CO2 conversion over In2O3.
so that most of methanol should be produced by CO2. Consequently, the methanol yield and formation rate decrease beyond 350 C is due to the production of CO by RWGS. In order to clarify the activity of In2O3 for methanol synthesis via CO2 hydrogenation, a comparison of catalytic activity has been made with those reported studies in the literature. The comparative results have been shown in Table 1. As shown in Table 1, the methanol formation rate of In2O3 catalyst is 0.78 mol h1 kgcat1 at 270 C but reaches 3.69 mol h1 kgcat1 at 330 C. The methanol formation rate at 330 C is higher than that over most of developed catalysts at lower reaction temperatures [18,22–34]. Only Cu/ZnO/ ZrO2/Al2O3 [25], Cu/ZnO/Ga2O3 [24] and Ga2O3-Pd/SiO2 [33,34] shows higher methanol formation rate. The methanol formation rate at 270 C on In2O3 is sufficiently high because it is achieved with the pure oxide. All those reported catalysts require
combinations of catalyst with co-catalyst, or with promoter and supporting material. As mentioned before, no pure oxide except In2O3 shows high activity for methanol synthesis from CO2 hydrogenation. For the copper based catalysts [22–29], co-catalyst, promoter(s) and support have to be applied in order to possess a good enough activity with high CO2 conversion, good CH3OH selectivity and high CH3OH formation rate. To prepare these catalysts, wet impregnation and co-precipitation are common methods. After extensive studies, further improvement in the copper based catalysts has become difficult. Since the methanol formation reaction has to compete with the endothermic RGWS reaction at higher temperatures, the reported works on CO2 hydrogenation to methanol over Cu/ZnO based and Pd based catalysts were normally performed at temperatures below 300 C. It is very difficult for those reported catalysts to
Fig. 2. CH3OH formation rate (a) and yield (b) at different pressures.
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Table 1 The comparison of In2O3 and other catalysts for CO2 hydrogenation to methanol. Catalyst
Temperature ( C)
Pressure (MPa)
CO2 conversion (%)
CH3OH selectivity (%)
CH3OH formation rateb (mol h1 kgcat1
Reference
CuO/ZnO Cu/ZnO/ZrO2 Cu/ZnO/Al2O3 Cu/ZnO/ZrO2/Al2O3 CuO/ZrO2 Cu/Ga2O3/ZrO2 Cu/B2O3/ZrO2 Cu-Zn-Ga/SiO2 Cu-Zn/SiO2 Cu/ZnO/Ga2O3 Ga2O3–Pd/SiO2 Pd/Al2O3 PdMgAl PdZnAl PdMgGa Ni5Ga3 Pd(0.34)–Cu In2O3 In2O3 b-Ga2O3
250 230 230 230 240 250 250 270 270 250 250 250 250 250 250 200 250 270 330 250
5 3 3 3 2 2 2 2 2 8 3 5 3 3 3 0.1 4.1 4 4 3
11.7 19.3 18.7 23.2 6.3 13.71 15.83 2.1 1.8 / / 3.4 0.3 0.6 1 / 6.6 1.1 7.1 /
36.1 48.6 43 60.3 48.8 75.59 67.26 96.6 99.1 83 70 29.9 4 60 47 60a 34 54.9 39.7 /
/ 2.51 2.15 3.75 / 1.93 1.8 / 3.58 5.94 7.9 / 0.018 0.546 0.63 2.5 1.116 0.78 3.69 /
[22] [25] [25] [25] [28] [23] [23] [29] [26,27] [24] [33,34] [22] [30] [30] [30] [31] [32] This work This work [18]
a b
The CH3OH selectivity of Ni5Ga3 includes DME and CH3OH. The formation rate of CH3OH was calculated by the catalytic data in the references if not given directly.
maintain sufficiently high selectivity of methanol at temperatures higher than 300 C. The reported works in Table 1 were all conducted at temperatures below 270 C [18,22–34]. As abovementioned, the DFT studies indicated that In2O3 can inhibit the RGWS reaction [16,17]. This is the reason that In2O3 shows an unusual high selectivity of methanol (39.7%) at 330 C with a very high methanol formation rate. Catalytic activities of some alloyed catalysts [30–32] are also given in Table 1. Moreover, compared with pure Ga2O3 [18], In2O3 shows a much higher activity towards CO2 hydrogenation to methanol. Pure Ga2O3 does not show any activity for CO2 hydrogenation to methanol as above-indicated [18]. It has to combine with catalyst (like Cu/ZnO [24] or Pd [33,34]) to present a nice activity. These results indicate that In2O3 possesses a great potential as catalyst, promoter or supporting material. More important, the preparation of In2O3 catalyst is easier than other catalysts in Table 1. The products over In2O3 catalyst are simple. Only
methanol, CO and trace methane are produced. These products can be easily separated by condensation. CO can be recycled into the reaction for further methanol synthesis with modification of the catalyst. 3.2. Characterization To decide the temperature of thermal treatment, the commercial In2O3 powder was analyzed by TGA. As shown in Fig. 3, the weight loss (1%) of sample does not change when the temperature increases beyond 500 C. This weight loss is caused by the impurity (like moisture) of commercial In2O3 powders. Therefore, we chose 500 C to treat the commercial powder. From 300 C to 600 C, there is an endothermic peak in the DSC curve, which is induced from the endothermic decomposition of the impurity. XRD patterns of the samples are illustrated in Fig. 4. The peaks for these four used samples (after reactions at 350 C as shown in (Figs. 1 and 2) and pure In2O3 (fresh) appear at 2u = 21.498 , 30.580 ,
Fig. 3. TGA curves of commercial In2O3 sample.
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Fig. 4. XRD patterns of fresh and used catalysts.
35.466 , 45.691, 51.037 and 60.676 , which are assigned to the diffractions from the (2 11), (2 2 2), (4 0 0), (4 3 1), (4 4 0) and (6 2 2) planes, belonging to Ia3(2 0 6) space group, according to PDF#653170. As shown in the diffraction patterns, the In2O3 peaks for all samples are similar, indicating that the reaction at various pressures and temperatures has no influence on the crystal structure of In2O3. This suggests In2O3 remains stable after the reaction within the temperature range tested. The average size of fresh In2O3 particles and the used samples is 25.8 nm and 28.1 nm, according to the Scherrer equations [20]. There is a minor increase in the particle size after reaction. To understand the morphological
and the structural changes of all samples, HR-TEM analyses were performed. Fig. 5 shows HR-TEM images of the fresh catalyst and the used samples. For fresh In2O3 particles, some rectangular and hexagonal holes can be observed. No lattice fringes of pure indium can be found over the used samples, suggesting that the catalyst exists in the form of In2O3 and has good crystallinity of In2O3. Meanwhile, (2 11), (2 2 2) and (4 11) lattice planes can be easily found on the fresh In2O3 and all the used samples. In particular, the particle sizes of all the used samples are close to that of the fresh In2O3. In2O3 particles possess good structural stability in the reaction within the
Fig. 5. HR-TEM of fresh and used catalysts.
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conditions tested. All these results are consistent with the XRD results. 4. Conclusions This work confirms the previous theoretical studies that In2O3 is an excellent catalyst for CO2 hydrogenation to methanol. It also demonstrates the micro-kinetic modeling that the increasing reaction pressure can increase the formation rate of methanol [35,36]. The catalyst characterization using TGA, XRD and HR-TEM indicates that In2O3 has good thermal and structural stability for the reactions below 500 C. Higher pressure favors the methanol synthesis. For the temperature effect, there exist a maximum for methanol yield. Beyond it, the yield of methanol will be reduced. With the reaction condition tested, the highest methanol yield (2.82%) is achieved at 330 C and 4 MPa with a CO2 conversion of 7.13%. The highest formation rate of methanol is 3.69 mol h1 kgcat1, higher than most of the other reported catalysts in Table 1. Compared with other catalysts, In2O3 catalyst can be easily prepared and the products of reaction are mainly methanol and CO with trace methane. For future possible applications, CO can be easily recycled into the reaction system for methanol synthesis together with CO2 hydrogenation (after some modification of the catalyst). The present investigation is very helpful for the future development of In2O3 based new catalysts for CO2 hydrogenation to methanol. Especially, In2O3 based catalyst is a promising photocatalyst [37–40]. One can expect a rapid progress in the CO2 activation and conversion over In2O3 based catalysts. Acknowledgments This work was supported by SINOPEC (with contract No. 413109) and the National Natural Science Foundation of China (with contract No. 91334206). References [1] http://CO2forum.cpe.fr. [2] M.B. Ansari, S.E. Park, Energy Environ. Sci. 5 (2012) 9419–9437. [3] R.X. Yuan, Y. Li, H.B. Yan, H. Wang, J. Song, Z.S. Zhang, W.B. Fan, J.G. Chen, Z.W. Liu, Z.T. Liu, Chin. J. Catal. 35 (2014) 1329–1336. [4] B. Yu, Z.F. Diao, C.X. Guo, L.N. He, J. CO2 Utilization 1 (2013) 60–68. [5] S. Saeidi, N.A.S. Amin, M.R. Rahimpour, J. CO2 Utilization 5 (2014) 66–81.
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