Hydrogenation of CO2 to Methanol at Atmospheric ... - ACS Publications

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Hydrogenation of CO2 to Methanol at Atmospheric Pressure over Cu/ ZnO Catalysts: Influence of the Calcination, Reduction, and Metal Loading Javier Díez-Ramírez,* Fernando Dorado, Ana Raquel de la Osa, José Luis Valverde, and Paula Sánchez Departamento de Ingeniería Química, Facultad de Ciencias y Tecnologías Químicas, Avenida Camilo José Cela 12, 13071 Ciudad Real, Spain S Supporting Information *

ABSTRACT: Cu/ZnO catalysts have been widely studied for the hydrogenation of carbon dioxide to methanol at atmospheric pressure. In the work described here, several interesting issues are highlighted that have rarely been considered previously. An extensive study of the influence of the calcination and reduction temperatures and the metal loading was carried out. The best conditions found for catalyst preparation were calcination at 350 °C and reduction at 200 °C. The role of the different oxidation states of copper (Cu2+, Cu1+, and Cu0) was proven in the methane and methanol formation. CuZn alloy formation was observed when a reduction temperature of 400 °C was used. The use of this alloy led to higher methanol selectivity at higher temperatures (>200 °C). Finally, the metal loading study confirm the dual-site nature of the methanol synthesis mechanism.

1. INTRODUCTION Cu/ZnO catalysts for the hydrogenation of carbon dioxide to give methanol (eq 1) have been widely studied.1−24 This reaction is a route for the valorization of CO2,25,26 which is well-known for its influence on the greenhouse effect and therefore on global warming. Cu/ZnO catalysts play an important role in methanol production because of their high activity and selectivity toward this valuable product. Moreover, compared to catalysts based on noble metals, such as Pd/ ZnO,27,28 Cu/ZnO catalysts are more cost-effective, and this allows the transfer of the process from the laboratory to the industrial scale. In this sense, the first pilot plant for the production of methanol from CO2 and H2 was built in Japan, and a SiO2-modified Cu/ZnO catalyst was used in the hydrogenation process.29 SiO2 was incorporated in this catalyst in order to improve the Cu/ZnO catalytic performance. Other compounds, such as Al2O3,30,31 Ga2O3, or ZrO2,32−35 have also been incorporated in an effort to increase the methanol production (eq 1) while simultaneously decreasing the selectivity to the undesired byproduct carbon monoxide (eq 2).

the same that appear in the reverse process of steam reforming of methanol,36−38 and the role played by each of these. However, definitive conclusions remain elusive due to the large number of variables involved in the synthesis of methanol, and a general consensus has not been reached to date. It has to be taken into account that the main findings about CO 2 hydrogenation have been found in catalytic runs performed under pressure. In general, some authors explain the results by considering the area of copper metal obtained or the interaction with the support.11,23,35 The oxidation state is also a controversial issue, as the nature of the active phase is not clear: metallic copper, a ratio of metallic Cu and Cu+, Cu−Zn sites, or CuZn alloys are some of the theories published to date.4−6,12,24,39 In addition, the catalytic behavior of the d ifferent sur faces, e.g., Cu(1 11), Cu(100), a nd Cu(110),16,17,21 the surface structural changes during the methanol synthesis,18,19 the influence of the oxygen vacancies,15 the presence of lattice defects40 and the different precursor structures have also been studied.9,10,41 Based on all of the variables mentioned above, some authors have studied the different pathways for the CO and CH3OH reactions (eqs 1 and 2) along with the different intermediate species formed and how they influence the methanol synthesis.3,8,15,20,22,42 Finally,

CO2 + 3H 2 ⇆ CH3OH + H 2O ΔH25 ° C = −49.5 kJ/mol CO2 + H 2 ⇆ CO + H 2O

(1)

ΔH25 ° C = 41 kJ/mol

Received: Revised: Accepted: Published:

(2)

Numerous studies have been published on the Cu/ZnO characteristics involved in the synthesis of methanol, which are © XXXX American Chemical Society

A

December 1, 2016 February 10, 2017 February 10, 2017 February 10, 2017 DOI: 10.1021/acs.iecr.6b04662 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

temperature (∼25 °C) to 900 °C (10 °C min−1), with a reducing gas mixture of 17.5% v/v H2/Ar (60 cm3 min−1). The XRD experiments were conducted with a Philips X’Pert instrument using nickel-filtered Cu Kα radiation. Samples were scanned at a rate of 0.02° step−1 over the range 5° ≤ 2θ ≤ 90° (scan time = 2 s step−1). Transmission electron microscopy (TEM) analyses were carried out on a JEOL JEM-4000EX unit with an accelerating voltage of 400 kV. Samples were prepared by ultrasonic dispersion in acetone with a drop of the resulting suspension evaporated onto a holey carbon-supported grid. 2.3. Equation section. In this paper, the lattice parameters of the ZnO hexagonal crystalline structure and copper cubic structure were calculated from XRD diffractograms using Bragg’s Law (eq 3), the formula for the hexagonal crystal structure (eq 4), and the formula for the cubic crystal structure (eq 5):

models of the kinetics for methanol synthesis and deactivation have also been developed.43−46 The work described here concerns the influence that the Cu/ ZnO catalyst has on the hydrogenation of carbon dioxide to methanol at atmospheric pressure. As stated by Fujita et al.,1 very few studies have been published in this area, probably due to commercial interests. Fujita et al.1,7 studied the influence of the heating rate in the calcination step and different reduction methods, but they did not study the influence of the calcination and reduction temperatures. The different conditions used for the reduction/calcination steps in a range of papers are summarized in Table S1. However, the influence of the calcination or reduction temperature on the Cu/ZnO catalyst has not been studied previously in any depth. For this reason, the work described here is intended to clarify these points and to contribute to a better understanding of the Cu/ZnO catalyst.

2. EXPERIMENTAL SECTION 2.1. Catalyst preparation. Catalysts were prepared by the impregnation method using ZnO (Panreac, 99% of minimum purity) as the support and copper(II) nitrate trihydrate [Cu(NO3)2·3H2O, Panreac, 99.95% purity] as the precursor. First, the support was placed in a glass vessel and kept under vacuum at room temperature (∼25 °C) for 2 h to remove water and other impurities adsorbed on the structure. Second, a solution of copper nitrate in distilled water was then poured over the sample, with the appropriate quantities to obtain catalysts with Cu loadings of 5, 10, 20, and 40 wt %. Third, the solvent was removed under vacuum at 90 °C for 2 h. After impregnation, the catalysts were dried at 120 °C overnight. The calcination was carried out in a furnace Nabertherm HTC 03/15, which is open to atmospheric air, at different temperatures: 300, 350, 400, and 600 °C. Each temperature was kept constant for 4 h, and the heating rate was 3.5 °C min−1 in all cases. This heating rate was selected based on the paper by Fujita et al.,7 which confirmed that a slow heating rate is necessary in order to obtain a lower CuO crystal size and a higher methanol synthesis activity. Prior to the reaction, the catalysts were reduced in situ for 2 h in a hydrogen stream (10 vol %) diluted with nitrogen at a flow rate of 25 cm3 min−1 at different temperatures (150, 200, 250, 300, and 400 °C) at a heating rate of 5 °C min−1. The catalysts were denoted as XCuZnO-Y-Z, where X indicates the metal loading, Y the calcination temperature, and Z the reduction temperature. In an effort to facilitate identification, the preparation conditions and the nomenclature of the samples are summarized in Table S2. 2.2. Support/catalyst characterization. The Cu metal loading was determined by atomic absorption (AA) spectrophotometry on a SPECTRA 220FS analyzer. Samples (ca. 0.5 g) were treated with 2 mL of HCl, 3 mL of HF, and 2 mL of H2O2 followed by microwave digestion (250 °C). Surface area/ porosity measurements were carried out using a QUADRASORB 3SI sorptometer apparatus with N2 as the sorbate at −196 °C. The samples were outgassed at 250 °C under vacuum (5 × 10−3 Torr) for 12 h prior to analysis. Specific surface areas were determined by the multipoint BET method. Specific total pore volume was evaluated from N2 uptake at a relative pressure of P/P0 = 0.99. Temperature-programmed reduction (TPR) experiments were conducted in a commercial Micromeritics AutoChem 2950 HP unit with TCD detection. Samples (ca. 0.15 g for the catalyst with 10 wt % of copper) were loaded into a U-shaped tube and ramped from room

n·λ = 2·d ·sin(θ )

(3)

1 4 ⎛ h2 + hk + k 2 ⎞ l2 = ⎜ ⎟+ 2 2 2 3⎝ ⎠ c d a

(4)

1 h2 + k 2 + l 2 = d2 a2

(5)

Particle size from TEM images was also calculated. Mean copper particle size evaluated as the surface-area weighted diameter (ds) was computed according to ds̅ =

∑i nidi3 ∑i nidi2

(6)

where ni represents the number of particles with diameter di (∑ini ≥ 200). 2.4. Catalyst activity. Catalytic performance tests were carried out in a tubular quartz reactor (45 cm length and 1 cm diameter). The catalyst, which had a particle size in the range 250−500 μm and was not diluted, was placed on a fritted quartz plate located at the end of the reactor. The amount of catalyst used in the experiments was 0.8 g. The temperature of the catalyst was measured with a K-type thermocouple (Thermocoax) placed inside the inner quartz tube. The entire reactor was placed in a furnace (Lenton) equipped with a temperature-programed system. Reaction gases were Praxair certified standards of CO2 (99.999% purity), H2 (99.999% purity), and N2 (99.999% purity). The gas flows were controlled by a set of calibrated mass flow meters (Brooks 5850 E and 5850 S). The hydrogenation of CO2 was carried out at atmospheric pressure in the temperature range 150−300 °C. The total flow rate used in the experiments, which involved a CO2/H2 mixture (CO2/H2 = 1/9), was 100 cm3 min−1. Gas effluents were monitored with a micro gas chromatograph (Varian CP-4900) fitted with a PoraPLOT Q column and a molecular sieve column, each of which was connected to a thermal conductivity detector (TCD). All the catalytic tests were performed twice, with the relative error being lower than 5%.

3. RESULTS AND DISCUSSION 3.1. Influence of the calcination temperature. Four different calcination temperatures were tested for the catalysts with copper loadings of 10 wt %: namely 300, 350, 400, and 600 °C. The XRD patterns of the catalysts 10CuZnO-300 and B

DOI: 10.1021/acs.iecr.6b04662 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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samples is represented in Figures 2b and 2d, respectively, where more than 200 particles were measured. Both catalysts showed a Gaussian particle distribution. The particle distribution shows that the 10CuZnO-600-200 has a higher proportion of larger particles. Finally, the intensity ratio I(111)/I(200) was also calculated (Table S3). It has been suggested1,7 that the morphology of copper crystallites depends on the reduction method, and this in turn affects the selectivity for methanol formation. It was pointed out7 that the methanol selectivity increases with the I(111)/I(200) ratio. The highest ratio was found for the sample 10CuZnO-350-200. TPR profiles are shown in Figure 3. The reduction of CuO to metallic Cu takes place from 130 to 220 °C in two steps, as is shown by α and β. The first peak (α) is related to the reduction of Cu2+ to Cu+, and the second (β) to the reduction of Cu+ to Cu0.47 For all samples, these processes overlapped and the second peak appeared as a shoulder on the first one. This overlap was more pronounced at higher calcination temperatures. This finding indicates that a higher calcination temperature makes it more difficult to reduce the catalyst. This behavior is due to the larger particles found at higher calcination temperatures, which are more difficult to reduce because hydrogen cannot access the bulk easily and mainly interacts with the surface (following a classical unreacted core model).48 The slower reduction kinetic for the larger particles explains the overlap of the peaks. The catalytic results for these samples are represented in Figure 4. The 10CuZnO-600-200 catalyst showed the lowest CO2 conversion. 10CuZnO-350-200 and 10CuZnO-300-200 showed similar behavior, with higher carbon dioxide conversion than the sample calcined at 600 °C. Regarding the activity and selectivity toward methanol, these results are related to the intensity ratio I(111)/I(200), which in turn is linked to the calcination temperature, as commented above. Thus, the highest methanol formation rates were found for the samples 10CuZnO-350-200 and 10CuZnO-300-200 at temperatures of 225 and 240 °C, respectively. For reaction temperatures from 150 to 225 °C, the former sample showed a higher selectivity to methanol. In contrast, lower reaction temperatures minimize CO formation. Therefore, the calcination temperature selected as the most appropriate was 350 °C. It should be noted that, even though the catalytic experiments were carried out under atmospheric pressure, the results are far away from the thermodynamic equilibrium values, showing that there are not thermodynamic limitations (see Table S4). The thermodynamic equilibrium values were calculated using a flowsheet simulator (Aspen HYSYS V8.4 licensed by Aspen Technology, Inc.). Peng−Robinson was used as the equation of state, and the reactor modeling was based on a Gibbs reactor. The conditions used for the simulation (flow rate, CO2/H2 ratio) were the same as in the experimental reactor. 3.2. Influence of the reduction temperature. Five different reduction temperatures (150, 200, 250, 300, and 400 °C) were tested for the catalyst 10CuZnO calcined at the best temperature (350 °C). An enlargement of the XRD pattern is shown in Figure 5 in order to focus attention on the changes produced by the reduction step. The diffractograms of the catalyst before reduction (10CuZnO-350) and after reduction at 150 °C are shown in Figure 5a. These diffractograms are fairly similar. The main peaks correspond to zinc oxide (JCPDS 80-0075) and copper oxide (JCPDS 80-1917). However, the 10CuZnO-350 sample showed a small peak at around 43.3°, which

10CuZnO-600 after the calcination step are shown in Figure 1a. The main peaks correspond to the hexagonal crystal structure

Figure 1. XRD profiles (a) before reduction and (b) after reduction of the catalyst calcined at different temperatures.

of zinc oxide (JCPDS 80-0075) and the monoclinic system of copper oxide (JCPDS 80-1917). The diffractograms of the catalysts overlap each other, and they appear to be very similar. However, it can be seen from the enlargement in Figure 1a that there is a slight displacement to higher 2θ angles (deg) of the ZnO peaks for the 10CuZnO-600 sample. This indicates that a contraction between lattices of the ZnO occurs and that a change in the lattice parameters of the ZnO hexagonal system (JCPDS 80-0075) has taken place. These lattice parameters were calculated based on Bragg’s Law (eq 3) and the formula for the hexagonal crystal structure (eq 4). The lattice parameters changed from a = 3.249 Å and c = 5.209 Å for the 10CuZnO-300 catalyst to a = 3.245 Å and c = 5.182 Å for the 10CuZnO-600 catalyst. After the reduction step (Figure 1b), the copper oxide peaks disappeared and the diffraction peaks of the metallic copper cubic crystal structure appeared (JCPDS 85-1326). As shown in the enlargement in Figure 1b, after the reduction step a restructuring of the ZnO crystal structure had occurred and, at the end, both catalysts had the same lattice parameters, a = 3.249 Å and c = 5.184 Å. Therefore, at first glance the crystal structures could seem to be the same. However, the main physical properties of the catalysts (Table S3) show that there are numerous differences between them. The surface area and the total pore volume are higher for the samples calcined at lower temperatures. The trend in the copper oxide particle size is the opposite, and this is due to the water produced during the decomposition of the precursor (copper nitrate), which accelerates the growth of CuO crystallites.1 During the reduction, sintering of the Cu0 crystals occurs due to an increase in the local temperature of the CuO crystallites due to the exothermic reaction between hydrogen and CuO.7 On the other hand, the decrease in the concomitant surface area favors the metal sintering. Therefore, as shown by the results in Table S3, large Cu0 particle sizes were also observed for the higher calcination temperature. This situation is consistent with the TEM analysis carried out on the 10CuZnO-350-200 (Figure 2a) and 10CuZnO-600-200 (Figure 2c) samples. The particle size distribution for both C


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Figure 2. TEM images and metal particle distribution of (a, b) 10CuZnO-350-200, (c, d) 10CuZnO-600-200, (e, f) 10CuZnO-350-150, and (g, h) 10CuZnO-350-400.

corresponds to metallic copper (JCPDS 85-1326). Thus, the reduction carried out at 150 °C was not sufficient to reduce completely the CuO to Cu0 and, as a result, only a few particles of metallic copper were formed. The TEM image (Figure 2e) also shows this behavior; particles seem not to be completely

formed and a high amount of small particles were found, as shown by the particle distribution (Figure 2f). The XRD diffractograms for the other samples showed peaks corresponding to zinc oxide (JCPDS 80-0075) and metallic copper (JCPDS 85-1326). As for the zinc oxide peaks observed during D


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Figure 3. TPR profiles. Comparison between the catalysts calcined at different temperatures.

Figure 5. XRD profiles of (a) the 10CuZnO-350 sample before and after reduction at 150 °C in an enlargement to show the CuO (38.8°) and metallic Cu (43.3°) peaks; (b) the rest of the samples reduced at different temperatures in an enlargement of the ZnO peak (around 31.8°); and (c) in an enlargement of the metallic Cu (around 43.3°).

both lattice parameters have changed. These lattice parameters (Table S5) were calculated based on Bragg’s Law (eq 3), the formula for the hexagonal crystal structure for ZnO (eq 4), and the formula of the cubic crystal structure for metallic copper (eq 5). The physical properties of the catalysts are provided in Table S5. The copper particle size increased with the reduction temperature, with the exception of the 10CuZnO-350-400 sample. TEM results (Figure 2g) are in agreement with XRD analysis. The big particles detected in the 10CuZnO-350-200 sample seem to have been broken into smaller ones in the 10CuZnO-350-400 sample, as also confirmed by the particle distribution for this sample (Figure 2h). These data support the theory that a new structure had been formed. The c/a ratio was calculated to compare the hexagonal structures formed for the ZnO compound with the ideal structure: i.e., the hexagonal wurtzite (Wz) structure, where each anion is surrounded by four cations at the corners of a tetrahedron and vice versa. This ideal structure has a c/a ratio of 1.633.49 Therefore, on increasing the reduction temperature, the hexagonal crystal system became more ideal. According to different authors,4,12,18 the new structure formed when the catalyst was reduced at 400

Figure 4. Catalytic activity for the catalysts calcined at different temperatures. Reaction conditions: CO2/H2 = 1/9 and W/F = 0.008 g min cm−3.

the calcination step, a displacement to higher 2θ angles (deg) of the ZnO (Figure 5b) and Cu0 peaks (Figure 5c) was found when the reduction temperature was increased. The 10CuZnO350-200 and 10CuZnO-350-250 samples were similar, whereas the 10CuZnO-350-400 catalyst showed the most marked change. For this latter sample, taking into account the high displacement observed, it is reasonable to consider that a new structure had been formed. As explained in the previous section, this displacement indicates that a contraction between lattices of the ZnO and Cu occurs, which in turn means that E


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Industrial & Engineering Chemistry Research °C was a CuZn alloy. In other solid mixtures, such as Ptalloys,50,51 the peaks due to the platinum are shifted to higher 2θ angles when the other metals in the alloy are incorporated into the platinum crystal structure. In our case, the displacement was produced in both structures (ZnO and Cu crystal systems), which means that particles of copper moved into the hexagonal structure of ZnO and particles of Zn were incorporated into the cubic structure of the metallic copper. For the 10CuZnO-350-400 sample, preliminary calculations of the percentage of zinc incorporated into the copper structure were carried out by applying Vergard’s Law, which has been used for other solid mixtures such as Pt-metal alloys50,51 and PdZn alloys.28 It should be noted that, taking into account the limitations of this law,52 the value obtained is only a rough approximation. According to these calculations, the new CuZn alloy formed would be Cu95.7Zn4.3. The catalytic results for the samples reduced at different temperatures are shown in Figure 6. The 10CuZnO-350-150 catalyst was the most active for methane production (Figure

6b). This result could be due to two different factors: (i) 10CuZnO-350-150 has the lowest Cu particle size, which increases the carbon dioxide conversion; and (ii), as shown by the XRD pattern, the reduction carried out at 150 °C was not sufficient to completely reduce the CuO to Cu. The XRD pattern after reaction of this sample (not shown) only contained peaks corresponding to metallic copper (with a copper particle size of 57.84 nm), whereas those corresponding to CuO had disappeared. This finding indicates that these particles continued to be reduced during the reaction. Thus, there was a combination of different active sites (Cu2+, Cu1+ and Cu0), and this led to higher amounts of methane. It has to be taken into account that the reduction during the reaction only is produced in the 10CuZnO-350-150 catalyst. The other samples showed XRD diffractograms before and after reaction (not shown) with no difference, so that metal reduction during reaction can be discarded. The samples 10CuZnO-350-200 and 10CuZnO-350-250 seem to show similar catalytic behavior, although the selectivity toward methanol is higher for the former. This behavior is consistent with the intensity ratio I(111)/I(200) (Table S5), for which this sample has the highest value. The 10CuZnO-350-400 catalyst showed the lowest CO2 conversion despite having a small particle size. This result is due to the presence of CuZn alloy active sites. This catalyst showed the highest values for methanol selectivity above 185 °C (Figure 6d). This result is consistent with the literature data18,39 and suggests that surface alloys may yield structures with high activity toward methanol. It is remarkable that the lowest intensity ratio I(111)/I(200) was determined for this catalyst, so that for this sample there is no link between methanol selectivity and the intensity ratio. The low intensity ratio is clearly not a drawback for performance. This parameter is mentioned in the literature1,7 when the catalyst has been reduced at lower temperature and, therefore, copper does not form an alloy with zinc. If the alloy has been formed, the relationship between the intensity ratio I(111)/I(200) and the methanol selectivity cannot be applied. The lower CO2 conversion and higher CH3OH selectivity for the 10CuZnO350-400 catalyst appear to indicate a different mechanism over CuZn active sites than over the Cu surface is produced. To demonstrate this assumption, a further study using DFT comparison or IR analysis in situ would be required. Finally, the temperature at which the highest methanol formation rate is obtained was shifted to higher values as the reduction temperature increased (Figure 6c). The best reduction temperature was 200 °C. On applying this reduction temperature, the maximum methanol yield was obtained at lower reaction temperatures, where carbon monoxide formation does not take place. 3.3. Influence of the metal loading. The XRD patterns for the four samples with different Cu loadings (5, 10, 20, and 40 wt %) are shown in Figure 7. The intensity of the copper peaks increased with the metal loading, as expected, but other differences were not observed. The main physical properties for these catalysts are listed in Table S6. The surface area and the total pore volume decreased with the Cu loading. This decrease occurs for two reasons: (i) the support is degraded by the metal precursor, which can dissolve the ZnO (due to the acidity of the aqueous copper nitrate solution53), and this degradation is more intense with higher metal loadings; and (ii) when the metal content increased, more particles can block the support and reduce the surface area. The particle size values are quite

Figure 6. Catalytic activity for the catalysts reduced at different temperatures. Reaction conditions: CO2/H2 = 1/9 and W/F = 0.008 g min cm−3. F


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Figure 7. XRD profiles for the samples with different metal loadings after reduction.

similar, which means that an increase in the metallic copper content does not affect the size of the particles formed. The TPR profiles are shown in Figure 8. TPR measurements have been performed with a weight of sample depending on the

Figure 9. Catalytic activity for the catalysts with different metal loadings. Reaction conditions: CO2/H2 = 1/9 and W/F = 0.008 g min cm−3.

together. Hence, an increase in the metal loading favors the mechanism of the methanol synthesis reaction. However, if the increment in the metal loading is very high, as happened for the 40CuZnO-350-200 sample, doubling the amount of copper does not duplicate the activity toward methanol. Thus, an optimum in the metal loading can be reached, as also can be seen in the selectivity (Figure 9c). The 20CuZnO-350-200 sample showed the highest selectivity to methanol, although it does not have the highest I(111)/I(200) ratio. Due to the different amounts of active sites, this parameter does not have influence in this study compared with the influence which is provided by the mechanism.

Figure 8. TPR profiles. Comparison between the catalysts with different metal loadings.

metal loading, so that the metal content in every measurement was always the same. The 5CuZnO-350-200 sample showed a predominant peak of reduction (called α in the figure) at 168 °C. This indicates the faster kinetic of reduction of copper particles from Cu2+ to Cu0 when the particles are better dispersed (as shown by the XRD particle size). At higher metal loading a third peak appears (γ) at higher temperatures (around 213 °C). It can be related to the more difficult reduction of a bulk of CuO particles,11 which are more connected between them. It would also be in agreement with the surface area decrease. According to the catalytic results (Figure 9), a higher metal loading gives rise to a higher methanol formation rate and higher CO2 conversion. Due to the different numbers of active sites, the turnover frequency (TOF) was calculated to normalize the results (Figure 9b). TOF results of the samples with 5, 10, and 20 wt % of copper, show that an increase in the metal loading favors the synthesis of methanol and the selectivity toward this compound. This could be explained following the theory of the dual site mechanism for the methanol production,33,43 where the dissociative adsorption of the hydrogen molecule is produced on the ZnO sites while the carbon dioxide is adsorbed on copper active sites. Therefore, the mechanism is favored when there are more active sites

4. CONCLUSIONS The following conclusions can be drawn from this study: • A high activity and selectivity toward methanol at low temperatures (

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