Hong Lei,
Ruheng Zheng,
Yeping Liu,
Jiacheng Gao,
Xiang Chen and
Xiaoliang Feng*
College of Chemical and Material Engineering, Quzhou University, Quzhou 324000, China. E-mail: 28826589@qq.com; Fax: +86-570-8015112; Tel: +86-570-8026546
First published on 3rd May 2019
Hydrogenation of CO2 to chemicals is of great importance in the reduction of greenhouse gas emission. And the interaction and/or the boundary between Cu and ZnO played a crucial role in the performance of the Cu–ZnO catalyst for CO2 hydrogenation to methanol. In this work, cylindrical shaped ZnO was first synthesized via controlled hydrothermal precipitation of Zn(CO2CH3)2·2H2O, and Cu was further deposited on ZnO via in situ reduction in aqueous solution. Characterizations indicated that the crystallization degree of ZnO decreased with the increasing content of Cu, while the exposed surface area of Cu exhibited a volcano shaped curve. It was found that the cylindrical shaped ZnO combined Cu catalysts were active for the hydrogenation of CO2, and the space time yield of methanol reached 0.50 g-MeOH (g-cat h)−1 at H2/CO2 = 3, 240 °C, 3.0 MPa, and 0.54 mol (g-cat h)−1, but the methanol selectivity decreases with the reduction of the (002) polar plane of ZnO. The conversion of CO2 and methanol selectivity were discussed with the detected exposed Cu surface area and the number of oxygen vacancies.
In published works, it was popularly accepted that Cu/ZnO-based catalysts were active for the hydrogenation of CO2 to methanol,6–11 and the activity of Cu-based catalysts depended mainly on the dispersion of Cu. However, the latest achievements disclosed that the interfacial area between Cu and ZnO played a crucial role for the activity and stability of the Cu/ZnO catalyst in methanol synthesis.12–14 Cu/ZnO catalysts are usually prepared by co-precipitation using precipitation agents containing sodium,8–11,15–18 while Kondrat et al. pointed out that residual sodium ions were a potential catalyst poison.19 Therefore, the development of novel and effective techniques for preparing Cu/ZnO-based catalysts is of great importance.
Karelovic et al. found that Cu/ZnO catalysts prepared by combustion exhibited higher activity than those prepared via conventional coprecipitation methods, and the activity related to the contact between Cu and ZnO.20 Valant et al.21 reported that the activity of Cu/ZnO for methanol production changed with the content of Zn showed a volcano-like profile. Similar studies also confirmed that synergetic effect between Cu and ZnO could be responsible for the catalytic activity of Cu/ZnO catalysts in the synthesis of methanol from CO2 hydrogenation.7,9,22–28 Tsang et al.7 disclosed that morphology of ZnO had a significant effect on its interaction with Cu in the hydrogenation of CO2 to methanol. Plate-like ZnO showed a strong interaction with Cu, which leaded to high selectivity of methanol. Previous work in our laboratory also found that filament-like ZnO supported Cu catalyst was highly active for CO2 hydrogenation,11 which might be attributed to that filament-like ZnO possessed large polar plane, and the interaction between Cu and ZnO was strong. At the same time, Khanna et al.29 developed a wet chemical reduction method to prepare nanosized Cu particles, in which a reduction reaction occured between Cu2+ and reducing agent. This method is simple, easy in control and can prepare metallic Cu without further reduction.
In this work, cylindrical shaped ZnO was synthesized via controlled hydrothermal precipitation of Zn(CO2CH3)2·2H2O, and Cu was further deposited on ZnO via the in situ reduction of Cu2+ with L-ascorbic acid in aqueous solution. The physicochemical properties of prepared Cu/ZnO were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), N2 adsorption, N2O chemisorption techniques, Raman spectra, X-ray photoelectron spectroscopy (XPS), H2 and CO2 chemisorption, H2-TPD, and CO2-TPD techniques. The relationship between the physiochemical properties of Cu/ZnO and its performance in hydrogenation of CO2 was discussed.
Catalysts | Cu/(Cu + Zn)a (mol%) | SBET (m2 g-cat−1) | dCu (nm) | SCub (m2 g-cat−1) |
---|---|---|---|---|
a Determined by ICP method.b Determined by N2O chemisorption method. | ||||
ZnO | 0 | 7.8 | — | — |
CZ-1 | 49.2 | 23.7 | 9.6 | 8.2 |
CZ-2 | 66.3 | 35.5 | 7.3 | 12.3 |
CZ-3 | 74.4 | 26.3 | 13.2 | 9.6 |
CZ-4 | 79.6 | 21.2 | 14.1 | 7.4 |
CZ-CC | 66.9 | 38.8 | 10.5 | 10.9 |
Cu | 100.0 | 3.3 | 67.1 | 3.1 |
XRD analysis was carried out on a Rigaku D/MAX-2500 diffractometer with a scanning angle of 15–85° by Cu Kα radiation. SEM images of these catalysts were obtained using Leo Series VP 1430 microscope operated at 10 kV. After samples were pretreated at 250 °C for 4 h in vacuum, N2 adsorption was measured at −196 °C using an ASAP 2010 analyzer. Surface area was calculated according to the BET method, which used 0.164 nm2 as the cross-sectional area of the nitrogen molecule. The metallic copper surface areas (SCu) were measured using a N2O chemisorption method as described elsewhere.30 The surface area of copper was calculated by assuming that the surface density of atomic copper was 1.46 × 1019 Cu atoms per m2 and molar stoichiometry of N2O/Cu was 0.5. Raman analysis was performed on a Jobin Yvon Labram HR800 spectrometer. XPS measurement was performed using a Mg Kα ray as excitation source (hν = 1253.6 eV) on a PerkinElmer PHI 5000C system. Surface contamination of C 1s peak (284.6 eV) was used as internal standard calibration binding energy.
H2 (or CO2) chemisorption measurements were performed at 30 °C using a Micromeritics Autochem 2920 II apparatus. 0.30 g of the catalyst was loaded between two quartz wool balls into a quartz U-shaped tube reactor. The measurements were carried out by pulses injection (0.551 mL) of 10% H2/Ar (or 10% CO2/He) until saturation. Chemisorbed and physisorbed H2 (or CO2) consumption was monitored by a thermal conductivity detector (TCD), which was denoted Atotal. The sample was then degassed with Ar (or He) for 10 min to evacuate physisorbed H2 (or CO2). Physisorbed H2 (or CO2) consumption was measured by another series of H2 (or CO2) pulses under the same conditions, which was denoted Aphys. The chemisorbed H2 (or CO2) was calculated as follows:
Achem = Atotal − Aphys |
H2 or CO2 temperature-programmed desorption (H2-TPD or CO2-TPD) of catalysts were performed on Micromeritics Autochem 2920 II apparatus. H2 or CO2 stream was introduced for adsorption (30 min) at room temperature. After adsorption, the samples were flushed with Ar (H2-TPD) or He (CO2-TPD) stream (30 mL min−1) for 30 min to remove weakly adsorbed H2 or CO2, and then they were heated from 20 to 700 °C at a rate of 10 °C min−1. The desorbed H2 or CO2 was detected by TCD.
Fig. 1a shows the XRD patterns of cylindrical shaped ZnO prepared by the hydrothermal synthesis method and Cu obtained with chemical reduction method. All diffraction peaks of ZnO could be ascribed to wurtzite ZnO, and no other impurity peaks was observed. These results indicated that mainly well crystallized ZnO was formed in the hydrothermal reaction. It can be found that the diffraction lines of Cu appeared at 2θ = 43.1° and 50.2°. These data confirmed that the Cu2+ was reduced to metallic Cu by L-ascorbic acid in the aqueous solution.
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Fig. 1 XRD patterns of cylindrical shaped ZnO, Cu and CZ catalysts. (a) Cylindrical shaped ZnO and Cu; (b) CZ catalysts; (c) Cu (111). |
Fig. 1b and c show the XRD patterns of CZ catalysts with different Cu/ZnO molar ratios obtained by in situ reduction reaction and carbonate coprecipitation. The diffraction peak of Cu (111) was weakened and broadened when Cu/Zn molar ratio was less than 2, which implied that crystallite size of Cu was small. In contrast, the diffraction peaks of Cu enhanced when the molar ratio Cu/Zn increased to 4, which meant that the crystallite size of Cu would be large. Similar phenomena were also reported by Lee et al.31 The average crystallite size of Cu was calculated using Scherrer's equation and summarized in Table 1. It can be found that the crystallite size of Cu was 67.1, 9.6, 7.3, 13.2, 14.1 and 10.5 nm in Cu, CZ-1, CZ-2, CZ-3, CZ-4 and CZ-CC, respectively, which indicated that ZnO could promote the dispersion of Cu. Another interesting observation was that the diffraction intensity of ZnO decreased gradually with the increase of Cu content. The diffraction peak of polar (002) plane of ZnO disappeared and only wide and weak diffraction peaks of nonpolar (001) and (101) planes were observed over CZ-4, which suggested that the polarity and crystallite size of ZnO decreased with the increase of Cu content. The diffraction peak of (002) plane of ZnO over CZ-CC was also weak, which suggested that the polarity of ZnO was also weak. Zhu et al.32 found that plate-like ZnO possessed mostly polar (002) faces contained more oxygen vacancies, while rod-like ZnO had mostly nonpolar (101) faces contained less oxygen vacancies.
Fig. 2a shows the SEM image of ZnO sample prepared by controlled hydrothermal precipitation of Zn(CO2CH3)2·2H2O. The diameter of cylindrical shaped ZnO is about 7 μm. Fig. 2b is the image of Cu prepared by chemical reduction method. It reveals that the shape of Cu is sphere-like with diameter of 3 μm. Interestingly, when ZnO was used as carrier for heterogeneous nucleation and growth of Cu, different shapes of Cu were observed, ranging from sphere to irregular shape (Fig. 2c–e). The result demonstrates that there is strong interaction between Cu and ZnO. The particle sizes of Cu over CZ catalysts are also smaller than pure Cu, implying that ZnO can promote the dispersion of Cu.
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Fig. 2 SEM images of cylindrical shaped ZnO, Cu and CZ catalysts. (a) Cylindrical shaped ZnO; (b) Cu; (c) CZ-1; (d) CZ-2; (e) CZ-4. |
Fig. 2c–e are the images of CZ-1, CZ-2 and CZ-4 prepared by a in situ chemical reduction reaction. An increase in Cu content leaded to a change in particle size of Cu. The minimum particle size was obtained for CZ-2, and then an increase in particle size of Cu appeared for CZ-4. This result was in accordance with the result of XRD. Furthermore, it can be seen from Fig. 2c–e that the particle size of cylindrical shaped ZnO decreased with the increase in the Cu content and ZnO was coated with large amount of Cu over CZ-4. The result also correlated well with the result of XRD.
Fig. 3 shows Raman spectra of catalysts with different copper loading. The Raman shift at around 323 cm−1 belonged to the E2H–E2L phonon mode of ZnO, and another peak at around 428 cm−1 was attributed to the high frequency optical phonon mode (E2H) of ZnO.33 The characteristic peaks of Raman spectra indicated that ZnO was in the form of wurtzite structure. Fig. 3 also confirmed that the position, width and intensity of E2H–E2L and E2H peaks changed obviously with the increasing content of Cu. The position of peak shifted to low wave number, width of peak broadened and its intensity became small, which indicated that Cu contacted strongly with ZnO.33,34
The BET surface area and exposed Cu surface area in pure Cu, ZnO and CZ catalysts were summarized in Table 1. The exposed Cu surface area of CZ catalysts was higher than that of pure Cu, which indicated that ZnO could dispersed Cu. It can be also found that surface area of Cu/ZnO catalyst prepared by in situ chemical reduction reaction passed its maximum (35.5 m2 g-cat−1, in CZ-2) and then decreased slightly with the increasing content of Cu, and the detected surface area of exposed Cu exhibited in a similar tendency. However, both BET surface area and exposed Cu surface area reduced progressively with high Cu loadings, which could be due to copper agglomeration at Cu/Zn molar ratio > 2. CZ-2 had the largest BET surface area (35.5 m2 g-cat−1) and the largest exposed Cu surface area (12.3 m2 g-cat−1). These results showed that the Cu/Zn ratio could influence the BET surface area and exposed Cu surface area. It can be also found from Table 1 that CZ-2 prepared by in situ chemical reduction reaction has larger exposed Cu surface areas than that obtained by carbonate coprecipitation, which correlated well with Cu crystallite size.
Fig. 4 shows the XPS date of CZ-1, CZ-2 and CZ-4 catalysts obtained by in situ chemical reduction reaction. Fig. 4a reveals that the binding energy value of O 1s for CZ-1 (528.9 eV) is lower than that of CZ-2 (529.7 eV) and CZ-4 (530.2 eV), implying that electrons are easier to be excited from ZnO over CZ-1 and ZnO over CZ-1 has more oxygen vacancies.11 From Fig. 4a, it is found that O 1s XPS band can be decomposed into low binding energy peak and high binding energy peak, which are attributed to lattice oxygen (Olatt) and oxygen vacancies (Ovac), respectively.35,36 With the increase of copper content, the Ovac/Olatt molar ratio decreases gradually. The result indicates that CZ-1 has more oxygen vacancies. The binding energy value of Cu 2p3/2 of CZ catalysts is shown in Fig. 4b. It can be found that the binding energy value of Cu 2p3/2 of CZ-1 is lower than that of CZ-2 and CZ-4 catalysts. The [O] terminated polar (002) facet is known to be electron richer than those of nonpolar facets (i.e. 100) that contain equal number of cations and anions.7 The XRD result has demonstrated that reflection of polar (002) facet of ZnO reduces with the increase of Cu content, implying that electrons are easier to transfer from ZnO over CZ-1 to Cu.
Fig. 5a shows that when the Cu content increases, the amount of chemisorbed CO2 decreases, and bare Cu does not adsorb CO2. The observation indicates that adsorbed CO2 amount is inversely proportional to the Cu content since the addition of Cu decreases the basicity of the catalyst and therefore is not in favor of the adsorption of acidic CO2. However, this decrease is slower than what can be expected from a linear trend (Fig. 5a, dotted lines). This indicates that the basicity of ZnO in catalysts has been increased with increase of Cu content, which is in agreement with the amount of oxygen vacancies (the Lewis basicity is increased in the absence of vacancies). Hydrogen chemisorption experiments are shown in Fig. 5b. It can be found that hydrogen is weakly chemisorbed on pure Cu since Cu is known to quickly decompose and recombine hydrogen.37 However, ZnO can adsorb atomic H through hydrogen spillover,21,38 which leads a strong adsorption. When the ZnO content increases, a volcano-type profile is observed showing a maximum of chemisorbed hydrogen for CZ-2. This shape is characteristic of a synergetic effect where the combination of Cu and Zn permits a significant increase of adsorbed hydrogen. This evolution matches exposed Cu surface area. The investigation of CO2 and H2 chemisorption suggests that both Cu and ZnO are active sites.
Fig. 6 shows the H2-TPD patterns of CZ-1, CZ-2 and CZ-4 catalysts obtained by in situ chemical reduction reaction. For comparison, the CZ-CC catalyst obtained by carbonate coprecipitation was also investigated. It is found that all samples display a H2 desorption peak in the range of 120–300 °C, which is assigned to the desorption of atomic hydrogen adsorbed on the surface of metallic Cu sites.39 Another strong H2 desorption peak located in the range of 450–600 °C is also discovered for all samples. It represents the desorption of strongly-adsorbed hydrogen on the ZnO surface through spillover from Cu to ZnO.39 From Fig. 6, it is found that the maximum H2 desorption amount is obtained for CZ-2 with increase in Cu content. Under the same Cu/Zn molar ratio, the H2 desorption amount CZ-2 is larger than that over CZ-CC. This result matches exposed Cu surface area.
Fig. 7 shows the CO2 desorption profiles of CZ-1, CZ-2, CZ-4 and CZ-CC catalysts. From CO2-TPD results, it can be observed that all profiles can be deconvoluted into two Gaussian peaks, which can be assigned to the weakly-adsorbed CO2 on the weak basic sites and strongly-adsorbed CO2 on the strong basic sites, respectively. The CO2 adsorption capacity order of CZ catalysts as follows: CZ-1 > CZ-CC > CZ-2 > CZ-4. The result indicates that the addition of Cu decreases the basicity of the catalyst. CZ-CC has stronger basicity than CZ-2 under the same ZnO content, which can be attributed to fewer oxygen vacancies.
Catalysts | CO2 conversion (%) | CH3OH selectivity (%) | STY (g-MeOH (g-cat h)−1) | TOF of CH3OH ×103 (s−1) |
---|---|---|---|---|
a Reaction conditions: H2/CO2 = 3, T = 240 °C, P = 3.0 MPa, GHSV = 0.54 mol (g-cat h)−1. | ||||
ZnO | 0.2 | 91.2 | 0.01 | — |
CZ-1 | 11.9 | 71.5 | 0.37 | 17.0 |
CZ-2 | 17.8 | 64.7 | 0.50 | 15.4 |
CZ-3 | 12.6 | 54.2 | 0.30 | 11.7 |
CZ-4 | 8.7 | 48.3 | 0.18 | 9.4 |
CZ-CC | 13.2 | 49.9 | 0.28 | 9.5 |
Cu | 0.0 | 0.0 | 0.00 | 0.0 |
Furthermore, it is noteworthy that a remarkable decrease in methanol selectivity can be observed with the increase in Cu content. The value of methanol selectivity over CZ-4 was 48.3%, which decreased by about 47% relative to ZnO (91.2%). This result was consistent with the reduction of oxygen vacancies. Hydrogen was spilled on ZnO and ZnOx (oxygen vacancies) by hydrogen spillover phenomenon9 where CO2 was adsorbed. We supposed that atomic H adsorbed on ZnO reacted with CO2 to produce CO, while atomic H adsorbed on ZnOx reacted with CO2 to synthesize methanol.
Table 2 shows the effect of Cu loading on TOF value of methanol over CZ catalyst. The TOF value of methanol decreased with the increase of Cu/Zn molar ratio. These results indicated that the CZ catalyst was a structural sensitive catalyst for methanol synthesis from CO2 hydrogenation. Tsang et al.7 proposed that Cu/ZnO catalyst had double active centers: one was Cu, and another was the oxygen vacancy on the interface of the Cu–Zn. In our experiment, the increase of Cu loading led to the decrease of TOF value of methanol, which was attributed to reduction of oxygen vacancy between Cu–Zn interface leading to the decrease of methanol selectivity. In view of the above, we presented that the mechanism of CO2 hydrogenation over Cu/ZnO catalysts could be described as follows: firstly, the hydrogen was adsorbed on the Cu surface and dissociated into hydrogen atoms. Secondly, hydrogen atoms arrived at the ZnO and ZnOx surface on the interface of Cu–Zn by hydrogen spillover effect to react with CO2 adsorbed on ZnO and ZnOx. Finally, methanol was generated on ZnOx through a series of intermediate, and CO was produced on ZnO.
The most efficient CZ-2 catalyst was selected for a stability test, and the result was presented in Fig. 8. The conversion of CO2 and the selectivity of methanol decreased slightly during the continuous 100 h, indicating that the CZ catalysts prepared by in situ chemical reduction reaction involved impregnation of cylindrical shaped ZnO with Cu2+ aqueous solution had a stable catalytic performance for CO2 hydrogenation to methanol.
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Fig. 8 Stability of CZ-2 for the CO2 hydrogenation process. (Reaction conditions: H2/CO2 = 3, T = 240 °C, P = 3.0 MPa, GHSV = 0.54 mol (g-cat h)−1). |
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