Reac Kinet Mech Cat DOI 10.1007/s11144-013-0587-9
Methanol synthesis from CO2 hydrogenation over copper based catalysts Hania Ahouari • Ahce`ne Soualah • Anthony Le Valant • Ludovic Pinard • Patrick Magnoux • Yannick Pouilloux
Received: 27 March 2013 / Accepted: 11 May 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013
Abstract A series of Cu–ZnO/Al2O3 catalysts prepared by coprecipitation were used for methanol synthesis by CO2 hydrogenation in a fixed bed reactor system. The effect of the catalysts composition and the reaction temperature on the catalytic activity was investigated. The main results show that the highest CO2 conversion and the best yield of methanol are obtained with the catalyst containing 51 wt% Cu and 22 wt% Zn. This result is assigned to the highest metallic copper surface area and to the interaction between copper and zinc oxide. However, the reaction temperature increase is disadvantageous for the methanol synthesis reaction. Keywords
CO2 Methanol synthesis Cu–ZnO/Al2O3
Introduction Over the last few years, technological advances to improve the quality of life led to a gradual degradation of natural resources and increased the levels of pollutants in the atmosphere, thereby contributing to global warming (‘‘greenhouse effect’’) and inducing significant climate changes [1]. The main anthropogenic greenhouse gas is CO2 and many countries are heavily engaged in reducing its atmospheric concentration using several solutions such as the use of other energy sources like solar and nuclear energy [1, 2]. Nowadays, CO2 H. Ahouari A. Le Valant L. Pinard P. Magnoux Y. Pouilloux IC2MP, UMR 7285 CNRS, Universite´ de Poitiers, 86022 Poitiers Cedex, France e-mail:
[email protected] H. Ahouari (&) A. Soualah Laboratoire de Physico-chimie des Mate´riaux et Catalyse, Universite´ A. Mira, 06000 Bejaı¨a, Algeria e-mail:
[email protected] A. Soualah e-mail:
[email protected]
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capture and storage are widely used but scientists are more and more interested in transforming CO2 into a recoverable product [3, 4]. Indeed, using CO2 as a ‘‘reagent’’ for producing bulk chemicals like methanol, dimethylether (DME) and hydrocarbons [5, 6] is particularly interesting because these products are considered as potential substitutes for traditional fuels for automobiles [1, 7]. However, methanol is a very important chemical feedstock for the production of chemicals and is considered as a viable source for the storage and generation of hydrogen for fuel cell application [2, 8–10]. In the ICI industrial process, methanol is produced from synthesis gas (CO/CO2/ H2) under high temperature (220–300 °C) and high pressure (50–100 bar) using mainly copper/zinc based oxide catalysts [11–14]. Methanol synthesis is severely limited by thermodynamics because this reaction is extremely exothermic. The effect of temperature on methanol synthesis has already been reported in several studies [1, 9, 14]. Developing a low temperature process for methanol synthesis will greatly reduce the production cost, and high CO2 conversion becomes available at low-temperature [11]. This process requires the preparation of an efficient catalyst that must be highly active and selective for methanol [2]. Copper based catalysts (Cu–ZnO/Al2O3) were found to be effective towards methanol synthesis through CO or CO2 hydrogenation [10, 12, 13, 15–17]. These catalysts are generally prepared by coprecipitation of metal salts in a basic medium. The metallic copper is considered to be the active site [12, 14] whereas the functions of ZnO can be summarized as follows: (1) it promotes higher dispersion of Cu preventing the agglomeration of Cu particles, (2) it improves the resistance of Cu particles to poisoning by feed gas impurities, and (3) ZnO, as a basic oxide, partially neutralizes the acidity of the catalyst and enhances CO2 adsorption on the catalyst surface [8, 12]. Nakumura et al. [18] have reported that the role of ZnO is to create the active sites (Cu–Zn) for methanol synthesis by CO2 hydrogenation. ZnO also acts as a reservoir of atomic hydrogen and provides it for the accomplishment of methanol synthesis on the Cu surface. Al2O3, the widely used third component in Cu-based catalysts, stabilizes the highly dispersed Cu/ZnO structure [18]. That is why we examined the effect of the temperature and the catalysts composition on methanol synthesis from CO2/H2 in this work. For this purpose, a series of Cu–ZnO/ Al2O3 catalysts with different (Cu/Zn) weight ratio were prepared by coprecipitation.
Experimental procedure Catalysts preparation A series of Cu–ZnO/Al2O3 catalysts with final composition ranging from pure copper to pure zinc were prepared by coprecipitation of basic salts of copper and zinc from approximately 1 M nitrate solutions by the dropwise addition of 1 M sodium carbonate at 70 °C until the pH reached 7.0 ± 0.5. The third c-Al2O3 (10 wt%) component was added directly to the hot solution subsequently obtained from the coprecipitation of the copper and zinc components. The formed precipitate was washed with deionized water, dried overnight at 100 °C and further calcined in
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Reac Kinet Mech Cat Table 1 Physicochemical properties of various catalysts Catalysts
SBET (m2/g
Metal content (%wt) Cu
Zn
Al
CZA(x-y))
CO2 uptake (lmole/g
CZA(72–0)
72.0
–
4.3
14
6
CZA(51–22)
50.6
22.0
3.5
27
23
CZA(36–35)
35.6
35.2
4.1
21
73
CZA(21–51)
21.3
50.6
3.9
28
132
CZA(0–75)
–
75.3
3.9
33
106
CZA(x-y))
air at 350 °C for 4 h. After that, powdered catalysts were pressed, crushed and sieved to particle size fractions (0.2–0.4 mm) used for testing measurements. The prepared catalysts are summarized in Table 1, where they are named according to their composition CZA(x-y). x and y represent copper and zinc contents determined by ICP measurements. Characterization Inductively coupled plasma (ICP-AES) Inductively coupled plasma (ICP) optical emission spectroscopy was used for the determination of the metal content in each sample synthesized above. The measurements were performed with a Perkin Elmer Optima 2000DV spectrometer. The sample was previously dissolved in an acidic mixture of HNO3 and HCl. Nitrogen adsorption–desorption (BET) The surface areas of the calcined catalysts were determined by adsorption– desorption of nitrogen at liquid nitrogen temperature (-196 °C) using a Micromeritics Tristar instrument. Before analysis, all the samples were outgassed at 250 °C under vacuum for 4 h. The surface areas were calculated according to the method of Brunauer, Emmet and Teller (BET), using 0.162 nm2 as the cross-section area of the nitrogen molecule. In situ X-ray diffraction (XRD) In situ XRD measurements were carried out by means of Cu-Ka radiation (k = 0.154 nm) using an ADVANCE D8 diffractometer equipped with a high temperature reaction cell. This setup enabled in situ reduction of the catalysts with an increase of temperature. The catalysts were exposed to 3 % H2/He at 30 °C (1 h) and then the temperature was raised at a rate of 5 °C/min to 350 °C, at which the sample was kept for 4 h in the reducing atmosphere. The XRD patterns were then recorded in the 2h range from 20° to 90° with a step size of 0.024° and a time/step of 1 s.
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After background subtraction and correction for instrumental broadening, the peak width and diffraction angle 2h were used to calculate the average crystallite size dhkl [19]: dhkl ¼ 0:94k=bð2hÞ cos h
ð1Þ
Where, k is the X-ray wavelength, and b(2h) is the full width at half-maximum in radians. Temperature programmed reduction (TPR) A Micromeritics Autochem II 2920 apparatus was employed for the measurements. Approximately 100 mg of freshly calcined catalyst was introduced between two quartz wool balls into a quartz U-shaped tube. The TPR experiments were carried out in 10 % H2/Ar flowing at a rate of 30 mL/min with a temperature increase from 30 to 600 °C at 10 °C/min using a programmable temperature controller. The sample was kept at 600 °C for 20 min, and then was cooled in N2 until it reached the ambient temperature. Hydrogen consumption was monitored by a thermal conductivity detector (TCD). The effluent gas was passed through a cold trap (liquid nitrogen ? isopropanol) placed before TCD in order to remove water from the exit stream of the reactor. CO2 adsorption Carbon dioxide adsorption measurements were performed at 30 °C using a Micromeritics Autochem 2920II apparatus. The samples were reduced for 3 h at 350 °C in a flow of hydrogen (30 mL/min) and then evacuated with helium (30 mL/ min) at the same temperature. The reactor was cooled to the adsorption temperature using helium, and the measurements were carried out by pulses injection of 10 % CO2/He until saturation. The sample was then degassed with helium for 10 min to evacuate physisorbed CO2. This step was followed by another series of CO2 pulses in the same conditions.
Catalytic test Carbon dioxide hydrogenation was performed in a continuous flow fixed bed reactor (12.5-mm id) made of stainless-steel. 1 g of catalyst diluted with silicon carbide (SiC, d = 1.9 mm) at a weight ratio of 4:1 to avoid thermal effects (hot spots) was placed in the tubular reactor with a coaxially centered thermocouple in contact with the catalytic bed. Prior to reaction, the sample was reduced in situ at 350 °C at a heating rate of 5 °C/min with a flow of H2 (30 mL/min, 99.99 % H2) for 4 h at atmospheric pressure. After that, a gas mixture (H2/CO2 = 3) was introduced into the reactor at the reaction temperature (between 200 and 350 °C) and the pressure was then raised to 3.0 MPa. The liquid products were separated from the gas products in the gas liquid separator and condenser. The effluent products from the heated line were analyzed by an online gas chromatograph system, which was
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composed of three gas chromatographs: (1) Porapak Q, TCD for CO and CO2, (2) AT-Aquawax (FID) for methanol and DME (CH3OH, CH3OCH3), and (3) with a capillary column HP-Plot Q, FID for hydrocarbons (C1–C6). All analysis lines and valves were heated at 150 °C to prevent possible condensation of the products before entering the gas chromatograph.
Results and discussion Physical properties The metal content of the catalysts (wt%) as determined by ICP-AES, is given in Table 1. The composition values of the copper based catalysts (Cu/Zn) are in good agreement with the theoretical values indicating a well precipitated compound without any significant loss of metal components during the preparation. In the same table, it can be noted that the catalysts prepared by coprecipitation of copper oxide or zinc oxide and alumina, CZA(72-0) and CZA(0-75), show the lowest (14 m2/g) and the highest (33 m2/g) BET surface area, respectively. Therefore, in the case of the catalysts prepared with the three oxides (CuO, ZnO and Al2O3), the BET surface area values are comprised between 21 and 28 m2/g. These results indicate that the addition of zinc oxide to the catalyst prepared with copper oxide and alumina improves the surface areas. However, we can notice that the amount of CO2 uptake is increased with the zinc content in the catalysts. These results may be due to the increase of the basicity of the catalysts in the presence of ZnO. Our results are consistent with previous findings [20, 21]. In situ X-Ray diffraction XRD The XRD patterns of the calcined and reduced catalysts are shown in Figs. 1 and 2, respectively. The samples CZA(x-y) are characterized by the presence of two well-crystallized phases of CuO and ZnO. The mean peaks of CuO are observed at 2h = 35.6° and 38.8° and those assigned to ZnO could be seen at 2h = 31° and 36° [8, 22]. However, a peak indicative of alumina phase is detected at 2h = 44.3° (JCPDS 34-0493). This peak is characterized by a weak intensity, which implies that the alumina phase is present in a micro-crystallite state. Similar results were previously reported in the literature [23]. Moreover, another peak is observed at 2h = 29.5° for the catalysts containing a high content of zinc, which is ascribed to Zn(OH)2 phase (JCPDS 20-1437). After reduction in hydrogen, CuO is completely reduced to copper metal as it can be deduced from XRD analysis (Fig. 2). The XRD patterns of the reduced catalysts can be very well simulated by a mixture of ZnO and Cu. The main peaks of copper metal are observed at 2h = 43°, 50° and 73° (JCPDS 01-089-2838). Consequently, the reduction of the samples only changes the CuO phase, without having any effect on the ZnO phase. We also observed that the peak assigned to Zn(OH)2 disappears after reduction. This is due to the decomposition of zinc hydroxide with temperature
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Reac Kinet Mech Cat Fig. 1 XRD patterns of calcined catalysts
: CuO : ZnO
Intensity (a.u)
CZA(72-0)
CZA(51-22)
CZA(36-35)
CZA(21-51)
CZA(0-75) 20
30
40
50
60
70
80
90
2 (°) Fig. 2 XRD patterns of reduced catalysts with 3 % H2/He at 350 °C (4 h)
: Cu : ZnO
Intensity (a.u)
CZA(72-0)
CZA(51-22)
CZA(36-35)
CZA(21-51)
CZA(0-75)
20
30
40
50
60
2 (°)
123
70
80
90
Reac Kinet Mech Cat Table 2 Particles sizes (CuO, ZnO and Cu) determined by DRX analysis Catalysts
dhkl (nm)a
Cu/Znb
SCu (m2/g)c
11.4
Before reduction
After reduction
CuO
ZnO
Cu
CZA(72–0)
28.0
–
42.0
–
–
CZA(51–22)
28.0
34.0
34.0
41.0
2.3
9.9
CZA(36–35)
23.0
33.0
42.0
55.0
1.0
5.6
CZA(21–51)
18.0
37.0
42.0
41.0
0.4
CZA(0–75)
–
18.0
–
21.0
0
ZnO
3.4 –
a
Particle size of copper oxide, zinc oxide and metallic copper was calculated from FWHM values of XRD diffraction peaks at 2h = 38.8°, 36.3° and 43.3° using the Scherrer equation
b
Weight ratio
c
The surface area of metallic copper was calculated using the following equation [25]: 6000xm Scu ¼ 8:96 dhkl (2) SCu surface area of metallic copper (m2/g), xm fraction of copper in gram catalyst, dhkl particle size of metallic copper (nm)
(350 °C). However, Kanari et al. [24] have previously reported that the thermal decomposition of Zn(OH)2 starts at about 150 °C. The particle sizes of CuO, ZnO and Cu calculated using the Scherrer equation are reported in Table 2. Before reduction, the catalyst CZA(21-51) showed small CuO particles sizing around 18 nm. However, larger sizes of CuO particles are obtained by increasing the copper content in catalysts prepared with the three oxides. After reduction with hydrogen, the particles size of metallic copper was of 42 nm for all catalysts, except for CZA(51-22) which showed the smallest particle size of Cu (34 nm). The reduction at high temperature (350 °C) of small CuO particles produces relatively large Cu crystallites. This result is in agreement with that reported by Fujita et al. [10]. The reduction of CuO with H2 is an exothermic reaction that leads to a rise of the local temperature of CuO crystallites. This phenomenon would cause the coalescence of the crystallites of copper component resulting in the formation of relatively large Cu crystallites from the small CuO crystallites [10]. In Table 2, we also report the surface area of metallic copper. The CZA(51-22) displays the highest metallic copper surface area. It is expected that the copper based catalysts with a high copper surface area may show an excellent performance in CO2 hydrogenation to methanol. Temperature programmed reduction (TPR) In order to investigate the reduction behavior of the catalysts, TPR measurements were carried out. As shown in Fig. 3, the reduction profiles of all samples with different Cu/Zn ratios exhibit a broad band of H2 consumption in the range of 30–600 °C. The peak position and the amount of H2 consumption are summarized
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Reac Kinet Mech Cat Fig. 3 H2-TPR profiles of the copper based catalysts β
α CZA(72-0)
β α CZA(51-22) β
Signal (a.u)
α
γ
CZA(36-35) α
CZA(21-51)
β
γ
γ
CZA(0-75)
30
130
230
330
430
530
Temperature(°C)
in Table 3. The copper oxide reduction is generally described by the following reaction (a): CuO þ H2 ! Cu þ H2 O
ðaÞ
The TPR profile of CZA(0-75) displays one peak at 485 °C (c) due to the reduction of ZnO species. Liang et al. [26] have observed that ZnO is reduced at 550 °C in the catalysts prepared by the coprecipitaion of zinc nitrate salt with a solution of NH4OH. The shift of the peak towards a lower temperature observed in the CZA(0-75) catalyst is probably due to the presence of alumina, which enhances the reducibility of zinc oxide. TPR profile of CZA(72-0) shows two peaks (a, b) and the addition of zinc oxide induces the formation of a third peak at a higher temperature (c) (Fig. 3). This behavior was obtained with the catalysts containing higher zinc content [CZA(2151) and CZA(36-35)]. In the case of CZA(51-22), two peaks are formed. The first one (peak a) is assigned to the reduction of highly dispersed or isolated copper species, whereas the peak appearing at higher temperature (peak b) is due to the reduction of bulk CuO [9, 27–33]. The higher area of the peak at high temperature relative to that at low temperature in the catalysts CZA(72-0) and CZA(51-22) was indicative of well crystallized species according to literature [30]. The peak observed at high temperature (peak c) may be ascribed to the reduction of zinc oxide or to the reduction of copper interacting with zinc. Many researchers [34–38] have reported that a partial reduction of surface ZnO can be observed, which may lead to the formation of a-brass (i.e., a dilute alloy of
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Reac Kinet Mech Cat Table 3 TPR data of copper based catalysts, temperature of peak maximum and extent of hydrogen consumption
Catalysts
Temperature programmed reduction Tmax (°C)
Amount H2 (Cm3/gCZA(x-y))
H2/Cu (molar ratio)
CZA(72–0)
313
278
1.1
CZA(51–22)
357
204
1.1
CZA(36–35)
339
156
1.2
CZA(21–51)
310
109
1.4
CZA(0–75)
486
39
–
zinc in copper). This hypothesis is confirmed by thermodynamic calculations done by Spencer et al. [35–37]. These calculations show that the equilibrium zinc content in a surface a-brass is about 5 % during catalysts reduction at 537 K [34]. According to these results, we can conclude that the zinc oxide is partially reduced in the catalysts containing high content of zinc (50–75 wt%), but the formation of alloy Cu–Zn is not detected by DRX measurements. The physicochemical properties determined by TPR for the catalysts are presented in Table 3. The ratio between the amount of H2 consumed and the amount of reducible oxide species (H2/Cu, Table 3) was determined quantitatively. According to the stoichiometry of the CuO reduction, this ratio should be equal to 1 (reaction a). An experimental value higher than the theoretical one was obtained for the catalysts (H2/ Cu)exp = 1–1.4, which is too high to be ascribed to the experimental inherent error of this technique (up to 5 %). It is widely accepted that both ZnO and copper crystallites are capable of occluding large amounts of hydrogen both on the surface and in the subsurface regions [34, 39, 40]. This additional quantity of hydrogen, whether adsorbed in the surface or stored in the ZnO crystalline structure could explain the highest (H2/Cu)exp values found for the CZA(21-51) and CZA(36-35) samples.
Methanol synthesis Effect of the catalysts composition on the methanol synthesis The catalytic activity of the hydrogenation of CO2 to methanol is presented in Table 4. This table clearly shows that CO and methanol are the only carbon-containing products present under the reaction conditions. The conversion of CO2 increases with the increase of the copper content in the catalysts prepared with the three oxides (CuO, ZnO and Al2O3). The best conversion and the highest yield of methanol are obtained with the CZA(51-22) sample. We can note that, in the absence of copper, CO2 is not converted. This indicates that the presence of copper is necessary for the conversion of CO2 into methanol. The best catalytic performance obtained with the CZA(51-22) can be due to the highest copper metallic surface area.
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Reac Kinet Mech Cat Table 4 Catalytic performance for the hydrogenation of CO2 to methanol over various catalysts Catalysts
CO2 Conv. (%)
Yields (C-mol%)
Activity of CH3OH
TOF of CH3OH
CO
(mmol/g h)
(10-3 s-1)a
CH3OH
CZA(72–0)
6.0
0.6
5.4
1.7
1.7
CZA(51–22)
10.1
2.2
7.9
2.4
2.8
CZA(36–35)
6.7
0.6
6.2
1.9
3.9
CZA(21–51)
6.4
0.8
5.5
1.8
5.7
CZA(0–75)
0.0
0.0
0.0
–
–
Reaction conditions: t = 700 min, H2/CO2 = 3, T = 250 °C, P = 3.0 MPa, GHSV = 2730 h-1 a Formed molecular number of methanol ¼ AN a [41] TOFMeOH ðs1 Þ ¼ Time 3600SCu Na ðsÞnumber of metallic copper atom TOF turn over frequency, A methanol activity (mol/h g), N a number of Avogadro (6.023 9 1023), SCu metallic copper surface area (m2/g), Na number of Cu atoms in a monolayer (Na = 1.46 9 1019 atoms/m2)
9
Yield of MeOH (C-mol %)
8 7 6 5 4 3 2 1 0 0
2
4
6
8
10
12
Copper metal surface area(m2/gCZA(x-y)) Fig. 4 The relationship between the yield of methanol and copper metal surface area. Reaction conditions: t = 700 min, H2/CO2 = 3, T = 250 °C, P = 3.0 MPa, GHSV = 2730 h-1
Many researchers [8, 42–46] have reported that the activity of the copper based catalysts increases linearly with the increase of the metallic copper surface area. However, there are also conflicting reports [47–49] suggesting that the yield of methanol is not proportional to the copper surface area for Cu–ZnO and Cu–ZnO– Al2O3 and that other factors can be involved. In our experiment, the effect of the copper metallic surface area on the activity of methanol synthesis from CO2 hydrogenation over copper based catalysts is shown in Fig. 4.
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Reac Kinet Mech Cat 6
TOF of MeOH (10-3.s-1)
5
4
3
2
1
0 2
4
6
8
10
12
Copper metal surface area(m2/gCZA(x-y)) Fig. 5 The relationship between the TOF of methanol formation and copper metal surface area. Reaction conditions: t = 700 min, H2/CO2 = 3, T = 250 °C, P = 3.0 MPa, GHSV = 2730 h-1
We can see that the catalytic activity increases with the increase of the surface area of metallic copper, but not according to a linear relationship. This result indicates that the catalytic activity of the catalyst depends on both the metallic copper surface area and the interaction between copper and zinc oxide. In order to better understand the role of metallic copper in the process of hydrogenation of CO2, we calculated the TOF of methanol formation over various catalysts. The TOF results are listed in Table 4 and are also plotted versus metallic copper surface area (Fig. 5). According to Boudart’s theory [27, 41], if the catalytic activity of the catalyst only depends on the metallic copper surface area, the plot of TOF versus metallic copper surface area should be a horizontal line. However, as shown in Fig. 5, the TOF of methanol formation decreased with the increase of metallic copper surface area. This result reveals that the activity of the catalyst is not only related to the metallic copper surface area but also to the interaction between copper and zinc oxide. This means that the interaction between copper and zinc oxide promoted the catalytic properties of the catalysts for methanol synthesis. Effect of temperature The variation of the catalytic activity for methanol synthesis with different temperatures of CZA(51-22) is plotted on Fig. 6. We can see that the CO2 conversion and the yield of CO are increased with the elevation of reaction temperature whereas the yield of methanol (MeOH) reaches a
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CO2 conversion and yield( %)
35 30
XCO2 HC CO MeOH
25 20 15 10 5 0 200
250
300
350
Temperature /°C Fig. 6 Effect of reaction temperature on the conversion of CO2 and on the yield of methanol, CO and hydrocarbons (HC). Reaction conditions: t = 700 min, P = 3.0 MPa, H2/CO2 = 3, GHSV = 2730 h-1
maximum at 250 °C and then decreases. It is well known that the rate of reaction increases with the increase of temperature and more CO2 is converted to methanol [8]. However, the yield of methanol declines with the increase of temperature which can be due to the limitation of the thermodynamic equilibrium. It is noticed that only a fraction of CO2 was converted to methanol, while the rest was converted to CO. CO2 hydrogenation to methanol can be ascribed by a reaction network involving (b) the synthesis of methanol, (c) the reverse water gas shift (RWGS), and (d) the methanol decomposition equilibrium [8, 22] as follows: CO2 þ 3H2 CH3 OH þ H2 O DH298 = -49.47 KJ mol DG298 = 3.3 KJ mol-1
ðbÞ
-1
CO2 þ H2 CO þ H2 O
ðcÞ
-1
DH298 = 41.14 KJ mol DG298 = 28.64 KJ mol-1 and CH3 OH CO þ 2H2
ðdÞ
-1
DH298 = 90.64 KJ mol DG298 = 25.34 KJ mol-1 Obviously, raising the temperature is favorable for the RWGS reaction because of its endothermic character. Meanwhile, compared to methanol synthesis, the RWGS reaction has a higher apparent activation energy, which means that the increase of CO production is much faster than that of methanol with increasing
123
Reac Kinet Mech Cat
temperature. The methanol synthesis is an exothermic reversible reaction and its equation constant decreases with temperature increase. Therefore, the reaction temperature increase is disadvantageous to methanol synthesis. We also noted that the methanol synthesis was more sensitive than the RWGS according to the reaction temperature. At 350 °C, a small amount of hydrocarbons was formed (mainly CH4). The methane formation is attributed to CO hydrogenation according to Fischer– Tropsch reaction.
Conclusion Copper based catalysts (Cu–ZnO/Al2O3) containing various Cu/Zn weight ratios were prepared by coprecipitation. The XRD measurements of the catalysts reveal the presence of well dispersed CuO and ZnO phases in the calcined catalysts. However, under reduction conditions, CuO was completely reduced to Cu particles with an average size comprised between 34 and 42 nm. The catalytic activity shows that the best yield of methanol was obtained with the CZA(51-22) at 250 °C, 3.0 MPa and H2/CO2 = 3, which is due to the highest copper metallic surface area and the interaction between copper and zinc oxide. Acknowledgments Hania Ahouari thanks the Institute of Chemistry of Media and Materials in Poitiers (IC2MP) for the financial support that contributed to the achievement of this study.
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