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ZnCr catalyst
Fuel 256 (2019) 115975
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Full Length Article
Hydrogenation of CO2 to methanol over Cu/ZnCr catalyst a
a
b
Shuhao Xiong , Yun Lian , Hong Xie , Bing Liu
a,⁎
T
a
Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan 430074, China Beijing Zhongzihuayu Environmental Technology Limited Company, China
b
G R A P H I C A L A B S T R A C T
The Cu/ZnCr catalysts were studied for the CO2 hydrogenation in a fixed-bed reactor. It was found that the ratio of Zn/Cr had a great effect on the methanol selectivity, while it did not influence the catalyst activity in terms of CO2 conversion. XRD results confirmed the existence of some dissociative ZnO in the 10Cu/ZnCr3.5 catalyst. The complex of ZnO and ZnCr2O4 in the 10Cu/ZnCr-3.5 catalyst inhibited the reverse water gas shift reaction, and promoted the formation of methanol. The formation of CO could be inhibited drastically by the addition of Co to the 10Cu/ZnCr-3.5 catalyst, but Co accelerated the hydrogenation of CO2 to methane. 10Cu/ZnCr-3.5 was still the best catalyst for the hydrogenation of CO2 into methanol. At 300 °C and 2 MPa, the STY of methanol reached 4.7 mmol/gcat/h (XCO2 = 25.1%, SMeOH = 31.1%) over the 10Cu/ZnCr-3.5 catalyst.
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 hydrogenation Methanol synthesis Cu/ZnCr catalyst
In this study, Cu/ZnCr catalysts were prepared by the impregnation method, and were characterize by BET, XRD, H2-TPR technologies. The as-prepared Cu/ZnCr catalysts were studied for the CO2 hydrogenation in a fixed-bed reactor. Both the metal sites and the support showed a great influence on the efficiency of the CO2 hydrogenation. It was found that the ratio of Zn/Cr had a great effect on the methanol selectivity, while it did not influence the catalyst activity in terms of CO2 conversion. XRD results confirmed the existence of some dissociative ZnO in the 10Cu/ZnCr-3.5 catalyst. The complex of ZnO and ZnCr2O4 in the 10Cu/ZnCr-3.5 catalyst inhibited the reverse water gas shift reaction, and promoted the formation of methanol. Interestingly, the content of Co was also found to have a great influence on the product selectivity. The formation of CO could be inhibited drastically by the addition of Co to the 10Cu/ZnCr-3.5 catalyst, but Co accelerated the hydrogenation of CO2 to methane. 10Cu/ZnCr-3.5 was still the best catalyst for the hydrogenation of CO2 into methanol. At 300 °C and 2 MPa, the STY of methanol reached 4.7 mmol/gcat/h (XCO2 = 25.1%, SMeOH = 31.1%) over the 10Cu/ZnCr-3.5 catalyst.
⁎
Corresponding author. E-mail address: [email protected] (B. Liu).
https://doi.org/10.1016/j.fuel.2019.115975 Received 17 March 2019; Received in revised form 28 May 2019; Accepted 5 August 2019 Available online 17 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 256 (2019) 115975
S. Xiong, et al.
1. Introduction
support. The product was dried at 100 °C overnight and calcined in air at 400 °C for 4 h. The obtained catalyst was denoted as 10Cu/ZnCr. For the Co-Cu/ZnCr catalysts, 0.15 or 0.29 g Co(NO3)2·6H2O mixed with 0.18 g Cu(NO3)2·3H2O were dissolved in 2 ml deionized water, and then the solution was added in 1 g of ZnCr support. The other step was the same as the 10Cu/ZnCr catalyst. The prepared catalysts were denoted as 5Co-10Cu/ZnCr and 10Co-10Cu/ZnCr. The weight content of Cu was 10% in theory, the weight content of Co was 5% or 10% in theory.
In recent years, CO2 capture and conversion has attracted great interest by researchers due to the growing deterioration of climate and diminishing of fossil resources [1]. Therefore, much effort has been put forth to the capture and the transformation of CO2 into fuels and valueadded chemicals [2,3]. Among the transformation of CO2, one attractive route is the hydrogenation of CO2 process, which can produce many kinds of fine chemicals such as methanol [4], hydrocarbon [5] and higher alcohols [6]. Although the CO2 hydrogenation has been extensively studied in the last decades, it still exists a large room to develop new catalytic routes for the transformation with high active and high selectivity [7,8]. Among these CO2 hydrogenated products, methanol contains high hydrogen weight content (12.5 wt%) and it is also a basic commodity chemical for the synthesis of aromatics, olefins (ethylene and propylene) and other chemicals [9]. The industrial methanol is produced from synthesis gas (CO + H2) with a small amount (< 5%) of CO2 [10]. Catalysts that devoted to the methanol synthesis from CO2 hydrogenation mainly bases on the modified catalysts for the synthesis of methanol from syngas including low pressure Cu/ZnO or high pressure ZnCrOx catalyst [11], noble-metal based catalysts [12,13], and others [14]. The hydrogenation of CO2 into methanol is exothermic reaction, but the competitive reverse water gas shift (RWGS) reaction is endothermic process. Therefore, the hydrogenation of CO2 at low temperature and high pressure favors the formation of methanol from the thermodynamic viewpoint [15]. The widely studied zinc chromite catalyst (ZnCrOx) for the hydrogenation of CO2 into methanol was generally performed at a relative high reaction temperature in the range from 350 to 410 °C [16] To improve the catalytic activity of the ZnCrOx catalysts for the hydrogenation of CO2 into methanol, a second metal such as Cu and Co were used as the co-active site low temperature [17]. In this bifunctional system, the metal sites facilitate the dissociation od H2 and the oxide supports primarily facilitate C]O bond cleavage [18]. Cobalt based catalysts, which are well-known as an efficient catalyst for CeC coupling-based reactions (e.g., Fischer-Tropsch synthesis) would enhance the sustainability of CO2 hydrogenation [19]. In our precious previous study, partly reduced Co3O4 promotes the production of alcohol, however, metallic Co improves the production of methane in CO2 hydrogenation reaction [6]. In this paper, we prepare the Cu/ZnCr and Co-Cu/ZnCr catalysts for the hydrogenation of CO2 to methanol species in a fix-bed reactor. The effect of chemical composition of Zn/ Cr, the content of Cu and Co are investigated. From the above works and systematic comparisons, we aim to produce a more comprehensive understanding of different component function for the reaction of CO2 hydrogenation to methanol.
2.2. Catalyst characterization The powder X-ray diffraction (XRD) patterns were recorded on an automated powder X-ray diffractometer (40 kV, 40 mA, Bruker-D8) using a Cu Ka radiation source (λ = 1.54056 Å) in the range of 10–80 with step size of 2°/min. The surface area and pore volume were measured by low-temperature nitrogen adsorption/desorption (77 K, quantachrome Autosorb-1-C-MS). The specific surface area was determined from the linear portion of the using standard Brunauer-Emmett-Teller (BET) plot. The total pore volumes and the average pore sizes were calculated from the desorption branch of the adsorption isotherm using the Barrett–Joyner–Halenda (BJH) formula. Before the adsorption measurements, the sample was degassed at 350 °C under vacuum for 3 h. H2-temperature program reduction (H2-TPR) was experiments were carried out using AMI-200 from Zeton Altamira Company. For each measurement, about 100 mg of the catalyst was placed in a quartz tube reactor and outgassed at 150 °C at a ramp rate of 10°/min under Ar stream with 30 ml/min for 1 h to remove traces of water. After cooling to 50 °C, the sample was heated from 50 °C to 800 °C at a rate of 10 °C/ min in a flow of 10 vol% H2/Ar (30 ml/min). 2.3. Typical procedure of CO2 hydrogenation All catalytic tests were performed in a fixed-bed stainless steel minireactor (length of 53 cm and inside diameter of 8 mm). The mixture gas (22.5% CO2, 67.5% H2 and 10%N2) were used without further purification. The pressure of the reactor was controlled with a backpressure regulator. For each experiment, the catalyst (0.2 g) was mixed with 0.2 g of silica, and then reduced at 300 °C (2 °C/min of ramping rate) for 2 h with H2 flow rate of 20 ml/min at atmosphere pressure. After the reduction, the temperature was cooled down to room temperature and the system was pressurized to 2 MPa by feeding mixture gas at 20 ml/ min. The catalyst was kept for 50 h at 250 °C or 300 °C and GHSV = 6 L h−1gcat−1. The gas products were analyzed on line by Agilent GC3000 gas chromatography (thermal conductivity and flame ionization detectors). The liquid product was collected in cold trap and analyzed by 4890 gas chromatography (flame ionization detectors) off line. The reaction parameters of CO2 conversion, selectivity and space time yield (STY) are defined as (where MeOH refers to methanol).
2. Experimental section 2.1. Catalyst preparation
XCO2 = ZnCr mixed oxide was prepared by the co-precipitation method. Briefly, 14.56 g of Zn(NO3)2·6H2O, 5.6 g of Cr(NO3)3·9H2O, 5.25 g of Al (NO3)3·9H2O were first dissolved in 150 ml distilled water. Then 100 ml (NH4)2CO3 aqueous solution with a concentration of 0.1 M was added dropwise into the above solution at 70 °C with a rate of 2 ml/min, and then the mixture aged for 3 h at the same temperature. Then the precipitate was filtrated and washed by distilled water. The solid powder was dried at 110 °C overnight and then calcined at 500 °C for 1 h in the air. The as-prepared support was denoted as ZnCr-3.5, in which 3.5 represents the molar ratio of Zn to Cr. Similarly, the other ZnCr-X supports were also prepared by the use of different molar ratio of Zn (NO3)2·6H2O to Cr(NO3)3·9H2O. Cu/ZnCr and Co-Cu/ZnCr catalysts were prepared via the impregnation method. 0.18 g Cu(NO3)2·3H2O was dissolved in 1 ml deionized water, and then the solution of Cu(NO3)2 was added in 1 g of ZnCr
nproduct , out × carbon number nCO2, in − nCO2, out selectivity = , nCO2, out nCO2, in
STYMeOH =
FCO2, in × XCO2 × 60 × SMeOH 22.4 × Mcat
3. Results and discussion The X-ray diffraction patterns of the ZnCr supports and the 10Cu/ ZnCr catalysts are shown in Figs. 1 and 2 respectively. The XRD patterns of ZnCr-0.875 (Fig. 1) show the peaks with 2θ at 30.3°, 35.7°, 57.5°, and 63.1°, which were assigned to the (2 2 0), (3 1 1), (5 1 1), and (4 4 0) crystalline planes of ZnCr2O4 (JCPDS Card No. 22-1107). With the increase of Zn content, the main diffraction peak gradually shifted from 35.7 to 36.2°, which should be attributed to the appearance of the ZnO crystalline. It was reported that the peaks with 2θ around 36.2° and 2
Fuel 256 (2019) 115975
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Table 1 The physical structure of ZnCr and 10Cu/ZnCr. sample
Surface area (m2/g)
Pore diameter (nm)
Total pore volume (ml/g)
ZnCr-0.875 ZnCr-1.75 ZnCr-3.5 10Cu/ZnCr-0.875 10Cu/ZnCr-1.75 10Cu/ZnCr-3.5
113.7 89.5 104.7 65.5 54.6 63.2
13.8 9.5 13.2 9.2 7.4 11.7
0.51 0.34 0.48 0.22 0.17 0.23
30
265 ZnCr-0.785 280
20
Fig. 1. XRD patterns of ZnCr support.
ZnCr-1.75 10Cu/CrZn-0.875
289
10
ZnCr-3.5
10Cu/ZnCr-1.75 in t e s it y ( a .u .)
0
10Cu/ZnCr-3.5
100
200
300
400
500
600
700
800
o
Temperature ( C) ZnO
Fig. 3. H2-TPR profile of ZnCr support.
CuO
220 200
ZnCr2O4
10
20
30
40
50
60
70
10Cu/ZnCr-0.875
80
2theta(degree)
220
150
Fig. 2. XRD patterns of 10Cu/ZnCr catalyst.
10Cu/ZnCr-1.75
100
56.6° were assigned to the (1 0 1) and (1 1 0) crystalline planes for ZnO (JCPDS Card No. 36-1451). As shown in Fig. 2, the diffraction peaks of ZnO become more obvious after the impregnation of Cu (Fig. 2). This indicates that some ZnO dissociated from ZnCr2O4 with the assist of Cu. Compared with the supports, two additional peaks with the 2θ value centering at 35.5° and 38.7° were correspond to the (0 0 2) and (1 1 1) crystalline planes of CuO (JCPDS Card No. 45-0937). The size of CuO was calculated from the XRD peak (38.7°) widths using the Scherrer equation, and the calculated crystallite sizes of CuO were 27.5 nm, 30.6 nm, 35.2 nm for the three 10Cu/ZnCr-0.875, 10Cu/ZnCr-1.75 and 10Cu/ZnCr-3.5 catalysts, respectively. The N2 absorption isotherms of both the ZnCr supports and the 10Cu/ZnCr catalysts are shown in Fig. S1. All of them have Type-IV reversible isotherms, which were the characteristics of the mesoporous structure. The surface area, pore diameter and total pore volume of all samples were observed to decrease with the increase of Zn content, and then increased by the further increase of Zn content. Compared with ZnCr supports, the surface area and pore volume of the 10Cu/ZnCr catalysts greatly decreased, which should be due to the loading of CuO nanoparticles in the pores of the ZnCr supports (Table 1). The reduction ability of ZnCr and 10Cu/ZnCr were then studied by H2-TPR method (Figs. 3 and 4). The main reduction peak at 289 °C and the small reduction peak at about 350 °C were observed in H2-TPR profile of the ZnCr-3.5 catalyst, which resemble to the reduction of
230
50
10Cu/ZnCr-3.5
0 100
200
300
400
500
600
700
800
o
Temperature ( C) Fig. 4. H2-TPR profile of 10Cu/ZnCr catalyst.
ZnCr2O4 and ZnO particles on the surface [20]. Interestingly, the reduction peak shifted to lower temperatures with the decrease of Zn content. As far as the reduction peaks of the 10Cu/ZnCr catalysts (Fig. 4), all of them demonstrates a new reduction peak at low temperature, and this peak was attributed to the reduction of CuO phase along with the support in the 10Cu/ZnCr catalysts [21]. This reduction peak shift from 230 °C for the 10Cu/ZnCr-3.5 catalyst to 220 °C for the 10Cu/ZnCr-1.75 and 10Cu/ZnCr-0.875 catalysts, which suggested that the interaction between Cu and the ZnCr-3.5 support in the 10Cu/ZnCr3.5 catalyst was stronger than the others. The catalytic performance of the 10Cu/ZnCr catalysts were 3
Fuel 256 (2019) 115975
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20
5Co-10Cu/ZnCr-3.5
Intensity (a.u.)
15
X CO2 (%)
10Co-10Cu/ZnCr-3.5
10Cu/ZnCr-3.5 10Cu/ZnCr-1.75 10Cu/ZnCr-0.875
10
10Cu/ZnCr-3.5 CuO
5
0
Co3O4 ZnO ZnCr2O4 0
10
20
30
40
50
10
20
30
40
TOS (h) Fig. 5. Effect of temperature on CO2 conversion over the 10Cu/ZnCr catalysts. Reaction conditions: reduction temperature = 250 °C, 2 h, P = 2 MPa, GHSV = 6 L h−1 gcat−1, H2/CO2 = 3.
10Cu/ZnCr-3.5 10Cu/ZnCr-1.75 10Cu/ZnCr-0.875
XCO2 (%)
10.9 11.7 11.1
60
70
80
Fig. 6. XRD of Co-Cu/ZnCr-3.5 catalysts.
Table 2 The activity and selectivity of 10Cu/ZnCr catalysts. catalyst
50
2Theta (degree)
100
Selectivity (%)
STY (mmol/gcat/h)
CO
MeOH
MeOH
52 60 72
48 40 28
3.2 2.7 1.9
10Co-10Cu/ZnCr-3.5
5Co-10Cu/ZnCr-3.5
50
Reaction temperature 250 °C, MeOH: methanol.
10Cu/ZnCr-3.5
evaluated by the hydrogenation of CO2. Fig. 5 shows CO2 conversion during the 50 h test, which reveals that CO2 conversions over the 10Cu/ ZnCr catalysts were almost the same around 11% at the 250 °C. Only methanol and CO were detected as the products with the hydrogenation of CO2. No other hydrocarbons were identified. The results indicated that the 10Cu/ZnCr catalysts demonstrated the catalytic activity towards both reverse water gas shift reaction (RWGS) and the hydrogenation of CO2 into methanol. The selectivity of MeOH decreased from 48% over the 10Cu/ZnCr-3.5 catalyst to 28% over the 10Cu/ZnCr0.875 catalyst (Table 2). The highest space time yield (STY) of MeOH was obtained in 3.2 mmol gcat−1h−1 over the 10Cu/ZnCr-3.5 catalyst. The Cu/ZnCr catalysts had similar physical structure and Cu dispersion, which indicated that the production selectivity might be related to the chemical properties of the supports. It seems that some dissociated ZnO in 10Cu/ZnCr-3.5 increased the interaction between Cu and the ZnCr support, which inhibited the reverse water gas shift reaction, and thus improved the production of methanol. Tisseraud and co-workers have also found that the increase of the surface Zn concentration enhanced the methanol selectivity during the hydrogenation of CO2 [22]. It should be pointed out that our prepared 10Cu/ZnCr-3.5 catalyst demonstrated better catalytic performance towards the hydrogenation of CO2 into methanol than the reported Cu based catalysts (Table 3), which achieved a much high value of STY of methanol under low
0 100
200
300
400
500
600
700
800
o
Temperature( C) Fig. 7. H2-TPR of Co-Cu/ZnCr-3.5catalysts. Table 4 The activity and selectivity of Co-Cu/ZnCr catalysts. Catalyst
10Cu/ZnCr-3.5 5Co-10Cu/ ZnCr-3.5 10Co-10Cu/ ZnCr-3.5
XCO2 (%)
Selectivity (%)
STY (mmol/ gcat/h)
CO
CH4
C2+H
MeOH
C2+OH
MeOH
25.1 21.0
68.9 12.9
0 50.3
0 8.2
31.1 22.6
0 2.0
4.7 3.4
25.5
6.3
73.8
6.6
12.3
1.0
1.9
Reaction temperature 300 °C, C2+H: higher hydrocarbon, C2+OH: higher alcohol.
Table 3 Catalytic activity comparison of different Cu based catalysts. Catalyst
STY of methanol (mmol/gcat/h)
T (°C)
P(MPa)
H2: CO2
Ref.
10Cu-/ZnCr Cu/Zr Cu/CeTi
3.2 0.6 0.5
250 240 225
2 8 3
3:1 3:1 3:1
This work [23] [24]
4
Fuel 256 (2019) 115975
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4. Conclusion
pressure. The low reaction pressure was more suitable for the industrial application. To further improve the selectivity of methanol, the addition of Co into the 10Cu/ZnCr-3.5 catalyst was also prepared. The morphology of the 5Co-10Cu/ZnCr-3.5 and 10Co-10Cu/ZnCr-3.5 catalysts and the particle size distribution were characterized by TEM technology (Fig. S2). TEM images reveal that most of Cu and Co nanoparticles dispersed homogeneously on the ZnCr-3.5 support. The weight percentage of Cu and Co were detected by EDS method (Table S1), which were similar as the content in theory. The molar ratio of Zn to Cr was nearly to 1:2, which indicated that most of Zn and Cr existed in the form of ZnCr2O4. The XRD patterns of 10Co-10Cu/ZnCr-3.5 display three new peaks with 2θ at 31.3°, 36.8° and 59.3°, which were assigned to the (2 2 0), (3 1 1) and (5 1 1) crystalline planes of Co3O4 (JCPDS Card No. 43-1003) (Fig. 6). In addition, it was observed that the XRD peak intensity of the dissociated ZnO decreased with the increase of Co content. The H2-TPR results of three catalysts are shown in Fig. 7. The main peak at 220 °C and an accompanied peak at 360 °C are noted for the 5Co-10Cu/ZnCr-3.5 catalyst. The main peak shifted to 225 °C with the increase of Co content, and the peak at 360 °C became more obvious in 10Co-10Cu/ZnCr-3.5. Similar H2-TPR results were also reported by Pierro and co-workers. [25]. The reduction peak at the low temperature was corresponded to the simultaneous reduction of Co3O4, CuO and Co1−xCuxCo2O4 phases to CoO and Cu. The high temperature peak was attributed to the reduction of CoO to metallic Co. Our early results indicated that metallic Co improved the production of methane in CO2 hydrogenation reaction, but partly reduced Co3O4 promoted the production of alcohols [6]. Melaet et al. also found that TiO2 supported Co catalysts with an oxidized state, was superior to the metallic state of Co for the hydrogenation of CO2 [26]. Based on the above analysis, the temperature of the hydrogenation of CO2 into methanol was set at 300 °C for the Co-Cu/ZnCr catalysts in the following experiments. The CO2 conversion and the product selectivity over the three representative catalysts are shown in Table 4. Compared with results listed in Table 2, CO2 conversion increased and the selectivity of methanol decreased with the increase of reaction temperature. It should due to the fact that CO2 hydrogenation into methanol is exothermic reaction, and thus low temperature favored the formation of methanol from the thermodynamic viewpoints. After the addition of Co with the percentage of 5%, CO2 conversion dropped slightly, but the product selectivity changed drastically. The selectivity of CO decreased greatly from 68.9% to 12.9% over the 5Co-10Cu/ZnCr-3.5 catalyst. On the meanwhile, the selectivity of methane increased from zero to 50.3%. Besides the formation of C1 product, higher hydrocarbons (ethane, propane and butane) and higher alcohols (ethanol, propyl alcohol and butanol) were produced due to the CeC coupling activity of Co. The CO selectivity decreased and CH4 selectivity increased by the further with the increase of Co content to 10%. The methanol selectivity decreased with the addition of Co. The 10Cu/ZnCr-3.5 catalyst gave the highest STY of methanol among three catalysts (4.7 mmol/gcat/h), followed by the 5Co-10Cu/ZnCr-3.5 catalyst with 3.4 mmol/gcat/h, the 10Co-10Cu/ ZnCr-3.5 catalyst with 1.9 mmol/gcat/h. This is due to the high activity of Co towards the hydrogenation of CO2 into methane. According to the above results, we can conclude that the addition of Co promoted the subsequent hydrogenation of CO to hydrocarbon and alcohols, but metallic Co increased the selectivity of methane at the expense of the decrease of the methanol selectivity.
In summary, series of Cu/ZnCr catalysts were prepared by the impregnation method, and the 10Cu/ZnCr-3.5 demonstrated the best catalytic performance towards the CO2 hydrogenation into methanol. The methanol could be produced in a high yield of 4.7 mmol/gcat/h over the 10Cu/ZnCr-3.5 catalyst. The ratio of Zn/Cr was found to have a great effect on products selectivity without the change of CO2 conversion. XRD results confirmed the existence of some dissociative ZnO in the 10Cu/ZnCr-3.5 catalyst. The dissociative ZnO increased the interaction between Cu and ZnCr2O4, and improved the generation of methanol. Furthermore, the addition of Co decreased the selectivity of CO drastically, but greatly increased the selectivity of methane at the expense of the decrease of the methanol selectivity. Acknowledgment This project was supported by the Fundamental Research Funds for the Central University (CZT19007). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.115975. References [1] Ouyang B, Xiong SH, Zhang YH, Liu B, Li JL. Appl Catal A 2017;543:189–95. [2] Schemmea S, Breuera JL, Samsuna RC, Petersa R, Stolten D. J CO₂ Util 2018;27:223–37. [3] Posada-Pérez S, Ramírez PJ, Evans J, Viñes F, Liu P, Illas F, et al. J Am Chem Soc 2016;138:8269–78. [4] Ouyang B, Tan WL, Liu B. Catal Commun 2017;95:36–9. [5] Gao P, Li SG, Bu XN, Dang SS, Liu ZY, Wang H, et al. Nature Chem 2017;9:1019–24. [6] Liu B, Ouyang B, Zhang YH, Lv KL, Li Q, Ding YB, et al. J Catal 2018;366:91–7. [7] Chen Y, Choi S, Thompson LT. J Catal 2016;343:147–56. [8] Dasireddy VDBC, Štefančič NS, Likozar B. J CO₂ Util 2018;28:189–99. [9] Kothandaraman J, Dagle RA, Dagle VL, Davidson SD, Walter ED, Burton SD, et al. Catal Sci Technol 2018;8(5098):5103. [10] Dong X, Li F, Zhao N, Xiao F, Wang J, Tan Y. Appl Catal B 2016;191:8–17. [11] Bonuraa G, Cordarob M, Cannillaa C, Arenab F, Frusteria F. Appl Catal B 2014;152–153:152–61. [12] Kattel S, Yu WT, Yang XF, Yan BH, Huang YQ, Wan WM, et al. Angew Chem Int Ed 2016;55:7968–73. [13] Bahruji H, Bowker M, Hutchings G, Dimitratos N, Wells P, Gibson E, et al. J Catal 2016;343:133–46. [14] Duyar MS, Tsai C, Snider JL, Singh JA, Gallo A, Yoo JS, et al. Angew Chem Int Ed 2018;57:15045–50. [15] Jia C, Gao JJ, Dai YH, Zhang J, Yang YH. J Energy Chem 2016;25:1027–37. [16] Fujiwara M, Souma Y. J Chem Soc Chem Commun 1992;10:767–8. [17] Marcosa FCF, Assafb JM, Assafa EM. J Mol Catal 2018;458:297–306. [18] Yang Y, Evans J, Rodriguez JA, White MG, Liu P. Phys Chem Chem Phys 2010;12:9909–17. [19] Wang LX, Wang L, Zhang J, Liu XL, Wang H, Zhang W, et al. Angew Chem Int Ed 2018;57:6104–8. [20] Liu N, Yuan ZS, Wang CW, Wang SD, Zhang CX, Wang SJ. Fuel Process Technol 2008;89:574–81. [21] Dong F, Zhua YL, Zheng HY, Zhua YF, Li XQ, Li YW. J Mol Catal A: Chem 2015;398:140–8. [22] Tisseraud C, Comminges C, Pronier S, Pouilloux Y, Valant AL. J Catal 2016;343:106–14. [23] Samson K, Sliwa M, Socha RP, Cora-Marek K, Mucha D, Rutkowska-Zbik D, et al. ACS Catal 2014;4:3730–41. [24] Chang K, Wang TF, Chen JGG. Appl Catal B 2017;206:704–11. [25] Fierro G, Jacono FL, Inversi M, Gradone R, Porta P. Top Catal 2010;10:39–48. [26] Melaet G, Ralston WT, Li CS, Alayoglu S, An K, Musselwhite N, et al. J Am Chem Soc 2014;136:2260–3.
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Full Length Article
Hydrogenation of CO2 to methanol over Cu/ZnCr catalyst a
a
b
Shuhao Xiong , Yun Lian , Hong Xie , Bing Liu
a,⁎
T
a
Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan 430074, China Beijing Zhongzihuayu Environmental Technology Limited Company, China
b
G R A P H I C A L A B S T R A C T
The Cu/ZnCr catalysts were studied for the CO2 hydrogenation in a fixed-bed reactor. It was found that the ratio of Zn/Cr had a great effect on the methanol selectivity, while it did not influence the catalyst activity in terms of CO2 conversion. XRD results confirmed the existence of some dissociative ZnO in the 10Cu/ZnCr3.5 catalyst. The complex of ZnO and ZnCr2O4 in the 10Cu/ZnCr-3.5 catalyst inhibited the reverse water gas shift reaction, and promoted the formation of methanol. The formation of CO could be inhibited drastically by the addition of Co to the 10Cu/ZnCr-3.5 catalyst, but Co accelerated the hydrogenation of CO2 to methane. 10Cu/ZnCr-3.5 was still the best catalyst for the hydrogenation of CO2 into methanol. At 300 °C and 2 MPa, the STY of methanol reached 4.7 mmol/gcat/h (XCO2 = 25.1%, SMeOH = 31.1%) over the 10Cu/ZnCr-3.5 catalyst.
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 hydrogenation Methanol synthesis Cu/ZnCr catalyst
In this study, Cu/ZnCr catalysts were prepared by the impregnation method, and were characterize by BET, XRD, H2-TPR technologies. The as-prepared Cu/ZnCr catalysts were studied for the CO2 hydrogenation in a fixed-bed reactor. Both the metal sites and the support showed a great influence on the efficiency of the CO2 hydrogenation. It was found that the ratio of Zn/Cr had a great effect on the methanol selectivity, while it did not influence the catalyst activity in terms of CO2 conversion. XRD results confirmed the existence of some dissociative ZnO in the 10Cu/ZnCr-3.5 catalyst. The complex of ZnO and ZnCr2O4 in the 10Cu/ZnCr-3.5 catalyst inhibited the reverse water gas shift reaction, and promoted the formation of methanol. Interestingly, the content of Co was also found to have a great influence on the product selectivity. The formation of CO could be inhibited drastically by the addition of Co to the 10Cu/ZnCr-3.5 catalyst, but Co accelerated the hydrogenation of CO2 to methane. 10Cu/ZnCr-3.5 was still the best catalyst for the hydrogenation of CO2 into methanol. At 300 °C and 2 MPa, the STY of methanol reached 4.7 mmol/gcat/h (XCO2 = 25.1%, SMeOH = 31.1%) over the 10Cu/ZnCr-3.5 catalyst.
⁎
Corresponding author. E-mail address: [email protected] (B. Liu).
https://doi.org/10.1016/j.fuel.2019.115975 Received 17 March 2019; Received in revised form 28 May 2019; Accepted 5 August 2019 Available online 17 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 256 (2019) 115975
S. Xiong, et al.
1. Introduction
support. The product was dried at 100 °C overnight and calcined in air at 400 °C for 4 h. The obtained catalyst was denoted as 10Cu/ZnCr. For the Co-Cu/ZnCr catalysts, 0.15 or 0.29 g Co(NO3)2·6H2O mixed with 0.18 g Cu(NO3)2·3H2O were dissolved in 2 ml deionized water, and then the solution was added in 1 g of ZnCr support. The other step was the same as the 10Cu/ZnCr catalyst. The prepared catalysts were denoted as 5Co-10Cu/ZnCr and 10Co-10Cu/ZnCr. The weight content of Cu was 10% in theory, the weight content of Co was 5% or 10% in theory.
In recent years, CO2 capture and conversion has attracted great interest by researchers due to the growing deterioration of climate and diminishing of fossil resources [1]. Therefore, much effort has been put forth to the capture and the transformation of CO2 into fuels and valueadded chemicals [2,3]. Among the transformation of CO2, one attractive route is the hydrogenation of CO2 process, which can produce many kinds of fine chemicals such as methanol [4], hydrocarbon [5] and higher alcohols [6]. Although the CO2 hydrogenation has been extensively studied in the last decades, it still exists a large room to develop new catalytic routes for the transformation with high active and high selectivity [7,8]. Among these CO2 hydrogenated products, methanol contains high hydrogen weight content (12.5 wt%) and it is also a basic commodity chemical for the synthesis of aromatics, olefins (ethylene and propylene) and other chemicals [9]. The industrial methanol is produced from synthesis gas (CO + H2) with a small amount (< 5%) of CO2 [10]. Catalysts that devoted to the methanol synthesis from CO2 hydrogenation mainly bases on the modified catalysts for the synthesis of methanol from syngas including low pressure Cu/ZnO or high pressure ZnCrOx catalyst [11], noble-metal based catalysts [12,13], and others [14]. The hydrogenation of CO2 into methanol is exothermic reaction, but the competitive reverse water gas shift (RWGS) reaction is endothermic process. Therefore, the hydrogenation of CO2 at low temperature and high pressure favors the formation of methanol from the thermodynamic viewpoint [15]. The widely studied zinc chromite catalyst (ZnCrOx) for the hydrogenation of CO2 into methanol was generally performed at a relative high reaction temperature in the range from 350 to 410 °C [16] To improve the catalytic activity of the ZnCrOx catalysts for the hydrogenation of CO2 into methanol, a second metal such as Cu and Co were used as the co-active site low temperature [17]. In this bifunctional system, the metal sites facilitate the dissociation od H2 and the oxide supports primarily facilitate C]O bond cleavage [18]. Cobalt based catalysts, which are well-known as an efficient catalyst for CeC coupling-based reactions (e.g., Fischer-Tropsch synthesis) would enhance the sustainability of CO2 hydrogenation [19]. In our precious previous study, partly reduced Co3O4 promotes the production of alcohol, however, metallic Co improves the production of methane in CO2 hydrogenation reaction [6]. In this paper, we prepare the Cu/ZnCr and Co-Cu/ZnCr catalysts for the hydrogenation of CO2 to methanol species in a fix-bed reactor. The effect of chemical composition of Zn/ Cr, the content of Cu and Co are investigated. From the above works and systematic comparisons, we aim to produce a more comprehensive understanding of different component function for the reaction of CO2 hydrogenation to methanol.
2.2. Catalyst characterization The powder X-ray diffraction (XRD) patterns were recorded on an automated powder X-ray diffractometer (40 kV, 40 mA, Bruker-D8) using a Cu Ka radiation source (λ = 1.54056 Å) in the range of 10–80 with step size of 2°/min. The surface area and pore volume were measured by low-temperature nitrogen adsorption/desorption (77 K, quantachrome Autosorb-1-C-MS). The specific surface area was determined from the linear portion of the using standard Brunauer-Emmett-Teller (BET) plot. The total pore volumes and the average pore sizes were calculated from the desorption branch of the adsorption isotherm using the Barrett–Joyner–Halenda (BJH) formula. Before the adsorption measurements, the sample was degassed at 350 °C under vacuum for 3 h. H2-temperature program reduction (H2-TPR) was experiments were carried out using AMI-200 from Zeton Altamira Company. For each measurement, about 100 mg of the catalyst was placed in a quartz tube reactor and outgassed at 150 °C at a ramp rate of 10°/min under Ar stream with 30 ml/min for 1 h to remove traces of water. After cooling to 50 °C, the sample was heated from 50 °C to 800 °C at a rate of 10 °C/ min in a flow of 10 vol% H2/Ar (30 ml/min). 2.3. Typical procedure of CO2 hydrogenation All catalytic tests were performed in a fixed-bed stainless steel minireactor (length of 53 cm and inside diameter of 8 mm). The mixture gas (22.5% CO2, 67.5% H2 and 10%N2) were used without further purification. The pressure of the reactor was controlled with a backpressure regulator. For each experiment, the catalyst (0.2 g) was mixed with 0.2 g of silica, and then reduced at 300 °C (2 °C/min of ramping rate) for 2 h with H2 flow rate of 20 ml/min at atmosphere pressure. After the reduction, the temperature was cooled down to room temperature and the system was pressurized to 2 MPa by feeding mixture gas at 20 ml/ min. The catalyst was kept for 50 h at 250 °C or 300 °C and GHSV = 6 L h−1gcat−1. The gas products were analyzed on line by Agilent GC3000 gas chromatography (thermal conductivity and flame ionization detectors). The liquid product was collected in cold trap and analyzed by 4890 gas chromatography (flame ionization detectors) off line. The reaction parameters of CO2 conversion, selectivity and space time yield (STY) are defined as (where MeOH refers to methanol).
2. Experimental section 2.1. Catalyst preparation
XCO2 = ZnCr mixed oxide was prepared by the co-precipitation method. Briefly, 14.56 g of Zn(NO3)2·6H2O, 5.6 g of Cr(NO3)3·9H2O, 5.25 g of Al (NO3)3·9H2O were first dissolved in 150 ml distilled water. Then 100 ml (NH4)2CO3 aqueous solution with a concentration of 0.1 M was added dropwise into the above solution at 70 °C with a rate of 2 ml/min, and then the mixture aged for 3 h at the same temperature. Then the precipitate was filtrated and washed by distilled water. The solid powder was dried at 110 °C overnight and then calcined at 500 °C for 1 h in the air. The as-prepared support was denoted as ZnCr-3.5, in which 3.5 represents the molar ratio of Zn to Cr. Similarly, the other ZnCr-X supports were also prepared by the use of different molar ratio of Zn (NO3)2·6H2O to Cr(NO3)3·9H2O. Cu/ZnCr and Co-Cu/ZnCr catalysts were prepared via the impregnation method. 0.18 g Cu(NO3)2·3H2O was dissolved in 1 ml deionized water, and then the solution of Cu(NO3)2 was added in 1 g of ZnCr
nproduct , out × carbon number nCO2, in − nCO2, out selectivity = , nCO2, out nCO2, in
STYMeOH =
FCO2, in × XCO2 × 60 × SMeOH 22.4 × Mcat
3. Results and discussion The X-ray diffraction patterns of the ZnCr supports and the 10Cu/ ZnCr catalysts are shown in Figs. 1 and 2 respectively. The XRD patterns of ZnCr-0.875 (Fig. 1) show the peaks with 2θ at 30.3°, 35.7°, 57.5°, and 63.1°, which were assigned to the (2 2 0), (3 1 1), (5 1 1), and (4 4 0) crystalline planes of ZnCr2O4 (JCPDS Card No. 22-1107). With the increase of Zn content, the main diffraction peak gradually shifted from 35.7 to 36.2°, which should be attributed to the appearance of the ZnO crystalline. It was reported that the peaks with 2θ around 36.2° and 2
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Table 1 The physical structure of ZnCr and 10Cu/ZnCr. sample
Surface area (m2/g)
Pore diameter (nm)
Total pore volume (ml/g)
ZnCr-0.875 ZnCr-1.75 ZnCr-3.5 10Cu/ZnCr-0.875 10Cu/ZnCr-1.75 10Cu/ZnCr-3.5
113.7 89.5 104.7 65.5 54.6 63.2
13.8 9.5 13.2 9.2 7.4 11.7
0.51 0.34 0.48 0.22 0.17 0.23
30
265 ZnCr-0.785 280
20
Fig. 1. XRD patterns of ZnCr support.
ZnCr-1.75 10Cu/CrZn-0.875
289
10
ZnCr-3.5
10Cu/ZnCr-1.75 in t e s it y ( a .u .)
0
10Cu/ZnCr-3.5
100
200
300
400
500
600
700
800
o
Temperature ( C) ZnO
Fig. 3. H2-TPR profile of ZnCr support.
CuO
220 200
ZnCr2O4
10
20
30
40
50
60
70
10Cu/ZnCr-0.875
80
2theta(degree)
220
150
Fig. 2. XRD patterns of 10Cu/ZnCr catalyst.
10Cu/ZnCr-1.75
100
56.6° were assigned to the (1 0 1) and (1 1 0) crystalline planes for ZnO (JCPDS Card No. 36-1451). As shown in Fig. 2, the diffraction peaks of ZnO become more obvious after the impregnation of Cu (Fig. 2). This indicates that some ZnO dissociated from ZnCr2O4 with the assist of Cu. Compared with the supports, two additional peaks with the 2θ value centering at 35.5° and 38.7° were correspond to the (0 0 2) and (1 1 1) crystalline planes of CuO (JCPDS Card No. 45-0937). The size of CuO was calculated from the XRD peak (38.7°) widths using the Scherrer equation, and the calculated crystallite sizes of CuO were 27.5 nm, 30.6 nm, 35.2 nm for the three 10Cu/ZnCr-0.875, 10Cu/ZnCr-1.75 and 10Cu/ZnCr-3.5 catalysts, respectively. The N2 absorption isotherms of both the ZnCr supports and the 10Cu/ZnCr catalysts are shown in Fig. S1. All of them have Type-IV reversible isotherms, which were the characteristics of the mesoporous structure. The surface area, pore diameter and total pore volume of all samples were observed to decrease with the increase of Zn content, and then increased by the further increase of Zn content. Compared with ZnCr supports, the surface area and pore volume of the 10Cu/ZnCr catalysts greatly decreased, which should be due to the loading of CuO nanoparticles in the pores of the ZnCr supports (Table 1). The reduction ability of ZnCr and 10Cu/ZnCr were then studied by H2-TPR method (Figs. 3 and 4). The main reduction peak at 289 °C and the small reduction peak at about 350 °C were observed in H2-TPR profile of the ZnCr-3.5 catalyst, which resemble to the reduction of
230
50
10Cu/ZnCr-3.5
0 100
200
300
400
500
600
700
800
o
Temperature ( C) Fig. 4. H2-TPR profile of 10Cu/ZnCr catalyst.
ZnCr2O4 and ZnO particles on the surface [20]. Interestingly, the reduction peak shifted to lower temperatures with the decrease of Zn content. As far as the reduction peaks of the 10Cu/ZnCr catalysts (Fig. 4), all of them demonstrates a new reduction peak at low temperature, and this peak was attributed to the reduction of CuO phase along with the support in the 10Cu/ZnCr catalysts [21]. This reduction peak shift from 230 °C for the 10Cu/ZnCr-3.5 catalyst to 220 °C for the 10Cu/ZnCr-1.75 and 10Cu/ZnCr-0.875 catalysts, which suggested that the interaction between Cu and the ZnCr-3.5 support in the 10Cu/ZnCr3.5 catalyst was stronger than the others. The catalytic performance of the 10Cu/ZnCr catalysts were 3
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20
5Co-10Cu/ZnCr-3.5
Intensity (a.u.)
15
X CO2 (%)
10Co-10Cu/ZnCr-3.5
10Cu/ZnCr-3.5 10Cu/ZnCr-1.75 10Cu/ZnCr-0.875
10
10Cu/ZnCr-3.5 CuO
5
0
Co3O4 ZnO ZnCr2O4 0
10
20
30
40
50
10
20
30
40
TOS (h) Fig. 5. Effect of temperature on CO2 conversion over the 10Cu/ZnCr catalysts. Reaction conditions: reduction temperature = 250 °C, 2 h, P = 2 MPa, GHSV = 6 L h−1 gcat−1, H2/CO2 = 3.
10Cu/ZnCr-3.5 10Cu/ZnCr-1.75 10Cu/ZnCr-0.875
XCO2 (%)
10.9 11.7 11.1
60
70
80
Fig. 6. XRD of Co-Cu/ZnCr-3.5 catalysts.
Table 2 The activity and selectivity of 10Cu/ZnCr catalysts. catalyst
50
2Theta (degree)
100
Selectivity (%)
STY (mmol/gcat/h)
CO
MeOH
MeOH
52 60 72
48 40 28
3.2 2.7 1.9
10Co-10Cu/ZnCr-3.5
5Co-10Cu/ZnCr-3.5
50
Reaction temperature 250 °C, MeOH: methanol.
10Cu/ZnCr-3.5
evaluated by the hydrogenation of CO2. Fig. 5 shows CO2 conversion during the 50 h test, which reveals that CO2 conversions over the 10Cu/ ZnCr catalysts were almost the same around 11% at the 250 °C. Only methanol and CO were detected as the products with the hydrogenation of CO2. No other hydrocarbons were identified. The results indicated that the 10Cu/ZnCr catalysts demonstrated the catalytic activity towards both reverse water gas shift reaction (RWGS) and the hydrogenation of CO2 into methanol. The selectivity of MeOH decreased from 48% over the 10Cu/ZnCr-3.5 catalyst to 28% over the 10Cu/ZnCr0.875 catalyst (Table 2). The highest space time yield (STY) of MeOH was obtained in 3.2 mmol gcat−1h−1 over the 10Cu/ZnCr-3.5 catalyst. The Cu/ZnCr catalysts had similar physical structure and Cu dispersion, which indicated that the production selectivity might be related to the chemical properties of the supports. It seems that some dissociated ZnO in 10Cu/ZnCr-3.5 increased the interaction between Cu and the ZnCr support, which inhibited the reverse water gas shift reaction, and thus improved the production of methanol. Tisseraud and co-workers have also found that the increase of the surface Zn concentration enhanced the methanol selectivity during the hydrogenation of CO2 [22]. It should be pointed out that our prepared 10Cu/ZnCr-3.5 catalyst demonstrated better catalytic performance towards the hydrogenation of CO2 into methanol than the reported Cu based catalysts (Table 3), which achieved a much high value of STY of methanol under low
0 100
200
300
400
500
600
700
800
o
Temperature( C) Fig. 7. H2-TPR of Co-Cu/ZnCr-3.5catalysts. Table 4 The activity and selectivity of Co-Cu/ZnCr catalysts. Catalyst
10Cu/ZnCr-3.5 5Co-10Cu/ ZnCr-3.5 10Co-10Cu/ ZnCr-3.5
XCO2 (%)
Selectivity (%)
STY (mmol/ gcat/h)
CO
CH4
C2+H
MeOH
C2+OH
MeOH
25.1 21.0
68.9 12.9
0 50.3
0 8.2
31.1 22.6
0 2.0
4.7 3.4
25.5
6.3
73.8
6.6
12.3
1.0
1.9
Reaction temperature 300 °C, C2+H: higher hydrocarbon, C2+OH: higher alcohol.
Table 3 Catalytic activity comparison of different Cu based catalysts. Catalyst
STY of methanol (mmol/gcat/h)
T (°C)
P(MPa)
H2: CO2
Ref.
10Cu-/ZnCr Cu/Zr Cu/CeTi
3.2 0.6 0.5
250 240 225
2 8 3
3:1 3:1 3:1
This work [23] [24]
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4. Conclusion
pressure. The low reaction pressure was more suitable for the industrial application. To further improve the selectivity of methanol, the addition of Co into the 10Cu/ZnCr-3.5 catalyst was also prepared. The morphology of the 5Co-10Cu/ZnCr-3.5 and 10Co-10Cu/ZnCr-3.5 catalysts and the particle size distribution were characterized by TEM technology (Fig. S2). TEM images reveal that most of Cu and Co nanoparticles dispersed homogeneously on the ZnCr-3.5 support. The weight percentage of Cu and Co were detected by EDS method (Table S1), which were similar as the content in theory. The molar ratio of Zn to Cr was nearly to 1:2, which indicated that most of Zn and Cr existed in the form of ZnCr2O4. The XRD patterns of 10Co-10Cu/ZnCr-3.5 display three new peaks with 2θ at 31.3°, 36.8° and 59.3°, which were assigned to the (2 2 0), (3 1 1) and (5 1 1) crystalline planes of Co3O4 (JCPDS Card No. 43-1003) (Fig. 6). In addition, it was observed that the XRD peak intensity of the dissociated ZnO decreased with the increase of Co content. The H2-TPR results of three catalysts are shown in Fig. 7. The main peak at 220 °C and an accompanied peak at 360 °C are noted for the 5Co-10Cu/ZnCr-3.5 catalyst. The main peak shifted to 225 °C with the increase of Co content, and the peak at 360 °C became more obvious in 10Co-10Cu/ZnCr-3.5. Similar H2-TPR results were also reported by Pierro and co-workers. [25]. The reduction peak at the low temperature was corresponded to the simultaneous reduction of Co3O4, CuO and Co1−xCuxCo2O4 phases to CoO and Cu. The high temperature peak was attributed to the reduction of CoO to metallic Co. Our early results indicated that metallic Co improved the production of methane in CO2 hydrogenation reaction, but partly reduced Co3O4 promoted the production of alcohols [6]. Melaet et al. also found that TiO2 supported Co catalysts with an oxidized state, was superior to the metallic state of Co for the hydrogenation of CO2 [26]. Based on the above analysis, the temperature of the hydrogenation of CO2 into methanol was set at 300 °C for the Co-Cu/ZnCr catalysts in the following experiments. The CO2 conversion and the product selectivity over the three representative catalysts are shown in Table 4. Compared with results listed in Table 2, CO2 conversion increased and the selectivity of methanol decreased with the increase of reaction temperature. It should due to the fact that CO2 hydrogenation into methanol is exothermic reaction, and thus low temperature favored the formation of methanol from the thermodynamic viewpoints. After the addition of Co with the percentage of 5%, CO2 conversion dropped slightly, but the product selectivity changed drastically. The selectivity of CO decreased greatly from 68.9% to 12.9% over the 5Co-10Cu/ZnCr-3.5 catalyst. On the meanwhile, the selectivity of methane increased from zero to 50.3%. Besides the formation of C1 product, higher hydrocarbons (ethane, propane and butane) and higher alcohols (ethanol, propyl alcohol and butanol) were produced due to the CeC coupling activity of Co. The CO selectivity decreased and CH4 selectivity increased by the further with the increase of Co content to 10%. The methanol selectivity decreased with the addition of Co. The 10Cu/ZnCr-3.5 catalyst gave the highest STY of methanol among three catalysts (4.7 mmol/gcat/h), followed by the 5Co-10Cu/ZnCr-3.5 catalyst with 3.4 mmol/gcat/h, the 10Co-10Cu/ ZnCr-3.5 catalyst with 1.9 mmol/gcat/h. This is due to the high activity of Co towards the hydrogenation of CO2 into methane. According to the above results, we can conclude that the addition of Co promoted the subsequent hydrogenation of CO to hydrocarbon and alcohols, but metallic Co increased the selectivity of methane at the expense of the decrease of the methanol selectivity.
In summary, series of Cu/ZnCr catalysts were prepared by the impregnation method, and the 10Cu/ZnCr-3.5 demonstrated the best catalytic performance towards the CO2 hydrogenation into methanol. The methanol could be produced in a high yield of 4.7 mmol/gcat/h over the 10Cu/ZnCr-3.5 catalyst. The ratio of Zn/Cr was found to have a great effect on products selectivity without the change of CO2 conversion. XRD results confirmed the existence of some dissociative ZnO in the 10Cu/ZnCr-3.5 catalyst. The dissociative ZnO increased the interaction between Cu and ZnCr2O4, and improved the generation of methanol. Furthermore, the addition of Co decreased the selectivity of CO drastically, but greatly increased the selectivity of methane at the expense of the decrease of the methanol selectivity. Acknowledgment This project was supported by the Fundamental Research Funds for the Central University (CZT19007). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.115975. References [1] Ouyang B, Xiong SH, Zhang YH, Liu B, Li JL. Appl Catal A 2017;543:189–95. [2] Schemmea S, Breuera JL, Samsuna RC, Petersa R, Stolten D. J CO₂ Util 2018;27:223–37. [3] Posada-Pérez S, Ramírez PJ, Evans J, Viñes F, Liu P, Illas F, et al. J Am Chem Soc 2016;138:8269–78. [4] Ouyang B, Tan WL, Liu B. Catal Commun 2017;95:36–9. [5] Gao P, Li SG, Bu XN, Dang SS, Liu ZY, Wang H, et al. Nature Chem 2017;9:1019–24. [6] Liu B, Ouyang B, Zhang YH, Lv KL, Li Q, Ding YB, et al. J Catal 2018;366:91–7. [7] Chen Y, Choi S, Thompson LT. J Catal 2016;343:147–56. [8] Dasireddy VDBC, Štefančič NS, Likozar B. J CO₂ Util 2018;28:189–99. [9] Kothandaraman J, Dagle RA, Dagle VL, Davidson SD, Walter ED, Burton SD, et al. Catal Sci Technol 2018;8(5098):5103. [10] Dong X, Li F, Zhao N, Xiao F, Wang J, Tan Y. Appl Catal B 2016;191:8–17. [11] Bonuraa G, Cordarob M, Cannillaa C, Arenab F, Frusteria F. Appl Catal B 2014;152–153:152–61. [12] Kattel S, Yu WT, Yang XF, Yan BH, Huang YQ, Wan WM, et al. Angew Chem Int Ed 2016;55:7968–73. [13] Bahruji H, Bowker M, Hutchings G, Dimitratos N, Wells P, Gibson E, et al. J Catal 2016;343:133–46. [14] Duyar MS, Tsai C, Snider JL, Singh JA, Gallo A, Yoo JS, et al. Angew Chem Int Ed 2018;57:15045–50. [15] Jia C, Gao JJ, Dai YH, Zhang J, Yang YH. J Energy Chem 2016;25:1027–37. [16] Fujiwara M, Souma Y. J Chem Soc Chem Commun 1992;10:767–8. [17] Marcosa FCF, Assafb JM, Assafa EM. J Mol Catal 2018;458:297–306. [18] Yang Y, Evans J, Rodriguez JA, White MG, Liu P. Phys Chem Chem Phys 2010;12:9909–17. [19] Wang LX, Wang L, Zhang J, Liu XL, Wang H, Zhang W, et al. Angew Chem Int Ed 2018;57:6104–8. [20] Liu N, Yuan ZS, Wang CW, Wang SD, Zhang CX, Wang SJ. Fuel Process Technol 2008;89:574–81. [21] Dong F, Zhua YL, Zheng HY, Zhua YF, Li XQ, Li YW. J Mol Catal A: Chem 2015;398:140–8. [22] Tisseraud C, Comminges C, Pronier S, Pouilloux Y, Valant AL. J Catal 2016;343:106–14. [23] Samson K, Sliwa M, Socha RP, Cora-Marek K, Mucha D, Rutkowska-Zbik D, et al. ACS Catal 2014;4:3730–41. [24] Chang K, Wang TF, Chen JGG. Appl Catal B 2017;206:704–11. [25] Fierro G, Jacono FL, Inversi M, Gradone R, Porta P. Top Catal 2010;10:39–48. [26] Melaet G, Ralston WT, Li CS, Alayoglu S, An K, Musselwhite N, et al. J Am Chem Soc 2014;136:2260–3.
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