Copper-doped ZnO-ZrO2 solid solution catalysts for promoting methanol synthesis from CO2 hydrogenation

Copper-doped ZnO-ZrO2 solid solution catalysts were synthesized via co-precipitation for promoting CH3OH synthesis via hydrogenation of CO2. Various testing methods were applied to investigate the effect of various copper contents on the catalysts. The catalytic performance was evaluated by a fixed bed reactor. XRD, HRTEM and Raman spectra collectively indicated that a ZnO-ZrO2 solid solution catalyst with 3% Cu had a higher Cu dispersion, while the H2-TPR results confirmed that a catalyst with 3% Cu had more Cu active sites under low temperature H2 pretreatment. When the copper content increased to 5% and 10%, the catalyst showed a better Cu crystallinity and a worse Cu dispersion, which could have a negative effect. Therefore, the CO2 conversion and methanol yield with a 3% CuZnO-ZrO2 catalyst at 5 MPa, 250°C and 12 000 ml/(g h) increased by 8.6% and 7.6%, respectively. Moreover, the CH3OH selectivity and catalytic stability of the solid solution catalyst were better than those of the traditional CZA catalyst.


Introduction
With the progress and development of industrial society, increasing amounts of carbon dioxide have been released into the atmosphere [1,2]. CO 2 is the main greenhouse gas, so it is necessary to reduce carbon dioxide emissions [3]. CO 2 hydrogenation to methanol could provide a solution [4]. Methanol is one of the basic organic raw materials. It can be used to produce formic acid [5], olefins [6] and DME [7], and can also be used as a fuel product [8].
Among the reported catalysts, the Cu/ZnO/Al 2 O 3 (CZA), Pd/Zn/CeO 2 and Cu/Ni/CeO 2 catalyst showed better performance. Pd/Zn/CeO 2 is a noble metal catalyst and its large-scale application is limited by the higher cost, while the CZA catalysts with ultra-low cost have been extensively applied to CH 3 OH synthesis from hydrogenation [9]. Besides, CuO/ZnO/ZrO 2 catalysts are also applied to methanol synthesis. Compared with alumina, zirconia has many excellent properties to provide copper dispersion sites and surface alkalinity, so the CuO/ZnO/ZrO 2 catalysts could be more effective on the CO 2 conversion to methanol [10]. In order to achieve better catalytic activity, coprecipitation [11], immersion method [12] and sol-gel [13] have been investigated extensively and coprecipitation is the most frequently used method.
Recently, Li [14] et al. developed a ZnO-ZrO 2 solid solution with a CH 3 OH selectivity of 91% and CO 2 conversion of 10%. The catalyst also showed excellent stability and provided new ideas. As we all know, the solid solution catalysts that have been reported at present are CeO 2 -based (e.g. ZrO 2 -CeO 2 , CuO-CeO 2 , MnO-CeO 2 ) and ZrO 2 -CeO 2 solid solution catalysts have excellent CO catalytic oxidation activity [15]. For Ce x Zr 1-x O 2 -based catalysts, noble metals usually act as active components [16], and meanwhile, transition metal oxides are widely used as surrogates [17]. Among these transition metal oxides, copper oxide modified Ce x Zr 1-x O 2 showed a higher catalytic ability of CO oxidation and NO removal. For the copper oxide modified Ce x Zr 1-x O 2 and CuO-CeO 2 solid solution, the copper oxide species generally consist of three types and it has been widely accepted that highly dispersed CuO is more active than lattice doped and bulk phase CuO [18,19]. There are some limitations on catalytic CO 2 conversion into methanol, while RWGS is the most likely secondary reaction. On the other hand, CO 2 has strong chemical stability, so it is very important to study catalysts with fabulous catalytic activity and stability [20]. Apart from ZnO-ZrO 2 solid solution, Li [21] et al. also reported a class of metal oxide solid solution catalysts: MaZrOx (Ma = Cd, Ga) for CO 2 hydrogenation to methanol. Other solid solution catalysts, such as ZnO-ZrO 2 -MOx, were subsequently reported for the same application [22].
In this paper, when the Zn content was 20% (Zn/Zr = 1/4, molar ratio), ZnO and ZrO 2 formed a solid solution structure. On this basis, several x% CuOZnO-ZrO 2 were prepared by the co-precipitation (x% represents the molar percentage of copper). Generally, Cu 2+ ions could embed into the lattice of CeO 2 and form a solid solution at samll Cu/(Ce + Cu) proportions (less than 0.1). The ranges of copper doping content were less than 10% (x = 0, 0.5, 1, 3, 5, 10).
Cu-Zn binary catalysts have been widely used for CO 2 hydrogenation to methanol, and there are many studies on introducing other supports or promoters into Cu-Zn catalysts. In our work, CuZnO-ZrO 2 solid solution catalyst is the focus of research, and CZA is prepared as the comparison catalyst. Although there are a lot of catalysts that appear to be the same or similar in composition in literatures, the solid solution structure and catalytic performance of the CuZnO-ZrO 2 catalyst in our work is significantly different from the other catalysts. After the ZnOZrO 2 solid solution catalyst is doped with copper, a ternary solid solution is formed, which can improve its CO 2 conversion while maintaining relatively high methanol selectivity. This is also the novelty of this study.
The representative physical and chemical properties of the catalysts were tested by various techniques. The facilitation of Cu components and the effects of Cu content on CO 2 hydrogenation to CH 3 OH of the catalysts were studied and discussed.

Preparation of catalysts
Bimetallic oxide catalysts with various molar ratios of Zn-Zr, pure ZnO and ZrO 2 were synthesized by a coprecipitation method, then the copper-doped ZnO-ZrO 2 catalysts (the Cu/Zn/Zr molar ratio is 0-0.1/ 1/4) were also prepared by co-precipitation. The specific synthesis procedures are as follows: 1.49 g Zn(NO 3 ) 2 ·6H 2 O, 8.59 g Zr(NO 3 ) 4 ·5H 2 O and 0.18 g Cu(NO 3 ) 2 ·3H 2 O (taking the 3% CuZnO-ZrO 2 catalyst as an example) were dissolved in distilled water, then 0.5 mol l −1 of Na 2 CO 3 was dispersed in the aforementioned solution under stirring at 70°C to form a precipitate. The pH during precipitation was kept at 7.5 under continuous stirring for 1 h at 70°C, and then it was incubated for 3 h at 70°C. When cooled, the precipitate was centrifuged, washed with deionized water and dried at 105°C overnight. Finally, the product was calcined at 500°C in the air for 3 h. Other catalysts with different copper contents were prepared following the same method. The CZA catalysts (with molar ratios of royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221213 Cu/Zn/Al = 6/3/1) were also synthesized by co-precipitation and their preparation process is consistent with the above statement. The amount of reagent is 7.2 g Cu(NO 3 ) 2 ·3H 2 O, 4.5 g Zn(NO 3 ) 2 ·6H 2 O, and 1.9 g Al(NO 3 ) 3 ·9H 2 O. For comparison, all reaction conditions, such as reaction temperature, reaction time and pH value, are completely consistent with the above experiments.

Characterization methods
The crystalline phase of the catalysts was investigated by a Panalytical X'Pert Pro X-ray diffractometer with Cu Kα radiation (λ = 0.15416 nm), operating at 40 kv and 40 mA. N 2 adsorption-desorption isotherms were obtained at −196°C by a Beckman Coulter SA 3100 type specific surface area and aperture measurement instrument (Beckman Coulter, Inc. Brea, USA) and the surface areas of the catalysts were calculated by the BET method. TG curves of the catalyst precursors was collected by an STA 449 TG-DSC synchronous thermal analyzer (50-750°C, in a steady air flow, heating rate = 5°C min −1 ). Raman spectra were recorded by a Raman microscope with a laser beam (λ = 325 nm). XPS spectra were used to measure the element composition and valence distribution of the catalysts (C1s peak at 284.8 eV).
H 2 -TPR was used to detect the reducibility. First, the catalyst in the tube was heated to 130°C in argon (50 ml min −1 ) keeping the samples at pretreatment for 2 h. Next, they were cooled to 50°C, then the samples were heated to 800°C in a 10% H 2 /Ar mixture and hydrogen consumption was measured by the AutoChemII2920 with a TCD detector. TEM images were observed on a TEM instrument at 200 kV accelerating voltage.

Catalytic activity measurements
CO 2 hydrogenation to CH 3 OH was tested in a tubular reactor. The catalyst (0.5 g catalyst and 0.5 g quartz sand were mixed together and passed through a 40-80 mesh) was pretreated for 4 h in a 50% H 2 /N 2 stream (20 ml min −1 and 0.2 MPa). The temperature was raised to 280°C, maintained for 4 h, and then cooled down. Next, the reaction was conducted at 1-5 MPa and 12 000 ml/(g h) GHSV, the reaction temperature was 190°C to 310°C and the volume ratio of H 2 : CO 2 : N 2 was 72 : 24 : 4. The reaction products were collected after 3 h and then the reaction gas was analysed. Conversion of CO 2 , CH 3 OH selectivity and yield were calculated by the following equations (2.1)-(2.4). A i means the peak area of the corresponding product and f i means its correction factor.   Figure 1b shows the XRD of Cu-doped catalysts. Four sharp diffraction peaks are assigned to the crystal planes of t-ZrO 2 , respectively (JCPDS Card No. 50-1089) [23]. The diffraction peak of CuO could not be observed when the copper content was less than 5%, indicating the dispersity of Cu species is too high to detect by XRD. As the copper content increased, the peak of CuO was observed at 2θ of 36.1°(JCPDS Card No. 03-0879) in the 5% CuZnO-ZrO 2 catalyst and in the 10% CuZnO-ZrO 2 catalyst, which reveals that the Cu species existed in large particles of CuO [24]. Figure 1c shows the XRD of the t-ZrO 2 (011) peak in all of the catalysts. It was found that the characteristic peak of ZrO 2 (011) became broader, while the peak position offset to a smaller angle. This migration phenomenon may be due to the lattice distortion caused by copper ions entering the ZnO-ZrO 2 tetragonal lattice. In table 1, the average crystalline sizes of the 0% CuZnO-ZrO 2 , 3% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts that were calculated by the Scherrer equation were 11.3 nm, 11.8 nm and 8.6 nm, respectively. The Raman spectra of the 0% CuZnO-ZrO 2 , 3%

Results and discussion
royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221213 CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 are shown in figure 1d. The peak of 0% CuZnO-ZrO 2 at 564 cm −1 proves that there is a solid solution structure. Similarly, the peak at approximately 564 cm −1 was observed and CuO and Cu 2 O weren't observed, which proved that the Cu-Zn-Zr-O ternary solid solution was generated in the 3% CuZnO-ZrO 2 catalyst. However, the peaks at 295, 331 and 625 cm −1 are assigned to the Raman-active Ag, 2Bg symmetry of CuO that existed in the 10% CuZnO-ZrO 2 catalyst. The scanning results of the Raman spectra fit those of XRD.

TG analysis
We investigated the thermal decomposition behaviour of the solid solution catalyst precursor with different copper doping amounts. The precursors of the 0% CuZnO-ZrO 2 , 3% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 are shown in figure 2a. The TG curve of the catalyst precursor consists of four stages: before 100°C, the weight loss was recognized as the loss of physically absorbed water, corresponding to the endothermic peaks in figure 2b-d. The decomposition of zinc carbonate 100-250°C corresponds to the endothermic peaks in figure 2b-d. Copper carbonate precipitates decompose at 250-450°C without an obvious endothermic peak in figure 2c, while significant endothermic peaks appear in figure 2d, which implies that there is copper in precursor of 10% CuZnO-ZrO 2 . A significant weight loss between 500 and 550°C reveals the formation of a solid solution structure in the precursors of the 0% CuZnO-ZrO 2 and 3% CuZnO-ZrO 2 catalysts, corresponding to the exothermic peak in figure 2b,c,    while there was no obvious exothermic peak in the 10% CuZnO-ZrO 2 precursor in figure 2d [25]. The total weight loss from the 0% CuZnO-ZrO 2 , 3% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 precursors were 19.6%, 21.4% and 23.4%, respectively (table 1). TG analysis showed that the 3% CuZnO-ZrO 2 catalysts have a structure similar to the 0% CuZnO-ZrO 2 catalyst, and proper copper doping is beneficial to form a solid solution structure, tallying with the XRD and Raman spectra analysis. Figure 3 shows the N 2 adsorption-desorption isotherms of the 0% CuZnO-ZrO 2 , 3% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts. All of the isotherms belong to the classical type IV due to their mesoporous structure. Remarkably, a steep increase of adsorption at the P/P 0 of 0.4-0.8 was observed due to capillary condensation of the adsorbate. The desorption isotherm is above the adsorption isotherms, and the hysteresis loop belongs to the H 2 -type [26]. The pore size distribution of the catalysts is shown in the inset of figure 3. Pores with a smaller radius can be detected by the DFT method, which is beyond the ability of the BJH methods to detect them [27]. It could be observed that peaks of the catalysts all appear at 3-15 nm, which is in the range of mesopore [28]. In table 1, the surface area of the 3% CuZnO-ZrO 2 catalyst was 38.41 m 2 g −1 , higher than those of the 0% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts. Furthermore, the pore volume of the 3% CuZnO-ZrO 2 catalyst was 0.078 cm 3 g −1 , which is also higher than those of the 0% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts. These results indicate that suitable copper doping in the ZnO-ZrO 2 is critical to improve the catalysts' textural properties.

TEM/HRTEM analysis
TEM morphology and size distribution of the catalysts are depicted in figure 4a-c. It can be observed that there are individual separated oxide nanoparticles in all of the catalysts. Sintering areas of three catalysts were marked by white circles, which increased with the copper content in the catalysts. Compared with the 0% CuZnO-ZrO 2 and 3% CuZnO-ZrO 2 catalysts, it is not easy to distinguish individual nanoparticles in the 10% CuZnO-ZrO 2 catalyst, which indicates serious sintering. The particle size distribution showed that 66% of the granules were between 8 nm and 13 nm in 0% CuZnO-ZrO 2 , whose average particle size was 11.46 nm, while 74% granules are distributed in the same range in 3% CuZnO-ZrO 2 , whose average particle size was 11.05 nm, and 69% granules in 10% CuZnO-ZrO 2 with an average particle size of 10.14  royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221213 nm. When comparing with table 1, the average particle sizes of 0% CuZnO-ZrO 2 and 3% CuZnO-ZrO 2 were close to the crystalline size calculated by the Scherrer formula from the ZrO 2 (011) peak, while the average size of the 10% CuZnO-ZrO 2 catalyst was bigger than the crystalline size calculated by the Scherrer equation.
It can be concluded that the nanoparticles of the 3% CuZnO-ZrO 2 catalyst are distributed uniformly. The HRTEM images of the 0% CuZnO-ZrO 2 , 3% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts are collected in figure 4d-f. The distance between two adjacent lattice stripes was approximately 0.294 nm, which is equal to the spacing of the (011) plane of t-ZrO 2 [29]. The lattice fringes of CuO, Cu 2 O and ZnO were not found, which corresponds to the XRD results: no diffraction peaks of the ZnO and CuO or Cu 2 O can be observed in the patterns. Figure 5a displays XPS spectra of the 0% CuZnO-ZrO 2 , 3% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts, which confirm the coexistence of Zr, C, O, Cu, Zn and Na species. The copper content on the surface was higher than the experimental addition amount in 3% CuZnO-ZrO 2 , while the copper content of the surface was less than the experimental addition amount in 10% CuZnO-ZrO 2 . The Zn content was less than the theoretical value in the 0% CuZnO-ZrO 2 and 3% CuZnO-ZrO 2 catalysts, indicating that the ZnO surface was partly covered in CuO and ZrO 2 . Conversely, enriched Zn on the surface was observed in the 10% CuZnO-ZrO 2 catalysts, and analogous results were reported for other Cu-Zn catalysts [30].

XPS analysis
The XPS data are listed in table 2. The binding energy of Zn 2p3/2 at 1021 eV and Zr 3d 5/2 at 182 eV can be assigned to ZnO and ZrO 2 , respectively [31]. The Cu 2p 3/2 XPS spectra are exhibited in figure 5b. The binding energy of 10% CuZnO-ZrO 2 is greater than 3% CuZnO-ZrO 2 , and the peak at 932.1 eV corresponds to Cu 0 or Cu + , which indicates a strong effect between Cu and the solid solution. The peak at 933.8 eV of 3% CuZnO-ZrO 2 and the peak at 933.4 eV and 935.7 eV of 10% CuZnO-ZrO 2 correspond to Cu 2+ [32].  figure 6. Obviously, the reduction temperatures were much lower for the 3% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts than for the 0% CuZnO-ZrO 2 catalyst, which indicates the solid solutions containing copper are more easily reduced than a copper free system. Both the 3% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts exhibited a broad reduction peak at 250-350°C as shown in the inset of figure 6; the temperature is higher than for traditional Cu-based catalysts [33]. In the document of CuO-CeO 2 , the first peak, the second peak and the third peak are designated as α, β and γ peak, respectively, where the α peak represents the CuO interacting with CeO 2 , the β peak originates from the deoxidation of highly dispersed CuO granules, and the γ peak is related to the deoxidation of bulk CuO [34]. Compared with the 10% CuZnO-ZrO 2 catalyst, the α and β peaks of the 3% CuZnO-ZrO 2 catalyst both moved to a lower temperature and the γ peak only existed in 10% CuZnO-ZrO 2 , which corresponds to the results of XRD, Raman and other characterizations.

.1. Effect of copper content
The performance of CO 2 hydrogenation on the CuZnO-ZrO 2 solid solution is shown in figure 7. With the increase of Cu percentage in the solid solution, an obvious rise in CO 2 conversion and CH 3 OH yield occurred when the copper addition amount was lower than 3%, while a decrease appeared when the copper amount was between 3% and 10%. However, the methanol selectivity did not change sharply with the copper content. A maximum catalytic activity was observed at a copper content of 3% in the solid solution catalyst. It could be concluded that the CO 2 conversion increased significantly when copper was doped into the solid solution catalyst, and the optimal addition amount was 3%.

Effect of reaction temperature
The catalytic performance of the temperature dependence (210-310°C) is shown in figure 8. As is apparent in figure 8a-c, CO 2 conversion increased with temperature, attended by a decline of CH 3 OH selectivity and an increase of CO selectivity, which is due to the competition between CH 3 OH synthesis and RWGS. The CH 3 OH synthesis is an exothermic reaction, but RWGS is the opposite. As seen in figure 8a, CO 2 conversion of the traditional CZA catalyst was greater than the copper-doped ZnO-ZrO 2 solid solution, while the CH 3 OH selectivity was the opposite (figure 8b). Compared with the solid solution catalysts, the CH 3 OH selectivity of the CZA catalyst decreased significantly with temperature. The solid solution catalyst in a high temperature reaction zone still maintains high methanol selectivity, which provides a theoretical basis C-C coupling reactions on the modified catalyst [23]. As shown in figure 8d, the CH 3 OH yield has a maximum value for all of the catalysts. However, the maximum CH 3 OH yield for each of the catalysts corresponds to a different critical temperature, which is 250°C for 3% CuZnO-ZrO 2 , 270°C for 10% CuZnO-ZrO 2 , and 230°C for CZA catalyst. Clearly, the maximum CH 3 OH yield reveals the critical point of the transformation from dynamics to thermodynamics [35]. When the methanol yield reaches the maximum value, the reaction  temperature of the solid solution is greater than the CZA catalyst, which also indirectly indicates that the solid solution catalyst is not easily deactivated at high temperatures.

Effect of reaction pressure
CO 2 hydrogenation on ZnO-ZrO 2 solid solution catalysts with various copper contents and CZA catalysts was studied in the reaction pressure range of 1-5 Mpa, as shown in figure 9. CO 2 conversion, CH 3 OH selectivity and yield increased with reaction pressure ( figure 9a,b,d), attended by a decline of CO selectivity (figure 9c). The number of molecules is reduced in CO 2 hydrogenation to       royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221213 CH 3 OH, so the catalytic reaction will move into a positive direction with an increasing reaction pressure, and catalytic activity will increase significantly in the meantime. At 5 MPa, the CO 2 conversion is 17.7%, the methanol selectivity is 84.1%, and then the highest methanol yield of 14.9% was obtained using 3% CuZnO-ZrO 2 . Compared with the ZnO-ZrO 2 solid solution without copper addition, the CO 2 conversion and CH 3 OH yield were increased by 8.6% and 7.6%, respectively. The highest CO 2 conversion of 19.7% was found with the CZA catalyst, while the methanol selectivity was relatively low, and thus the methanol yield was not high.

Stability analysis
The stability of the catalyst is also a key performance in CH 3 OH synthesis. The effect of the reaction time on the catalytic stability was investigated for 100 h. In figure 10,   royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221213 phenomenon of the CZA catalyst can be attributed to sintering of Cu particles. Howcver, the 3% CuZnO-ZrO 2 catalyst had the highest stability for CO 2 methanation to CH 3 OH and could run for 100 h almost without any inactivation.

Catalytic performance comparisons
Although there are a lot of catalysts that appear to be the same or similar in composition in literature, the solid solution structure and catalytic performance of the CuZnO-ZrO 2 catalyst in our work is significantly different from the other catalysts. Here, the catalysts closely related to this paper, such as CuZnZr, CuZnAl catalysts prepared by different methods and other solid solution catalysts, were listed in table 3 for catalytic performance comparisons with our work. As shown in table 3, the CO 2 conversion and CH 3 OH selectivity of the reported solid solution catalysts ZnZr, Cd/GaZr and MgZnZr are 10-12.9% and 80-86% respectively at 3-5 MPa, 320°C and H 2 : CO 2 molar ratio of =3 : 1 [14,21,22]. The CO 2 conversion of CuZnZr catalysts prepared by different methods under different conditions varies from 12.0% to 18.7%, and the CH 3 OH selectivity varies from 45% to 71.1% [36][37][38][39][40]. The CO 2 conversion of CuZnAl catalysts prepared by different methods under different conditions varies from 11% to 25%, and the CH 3 OH selectivity varies from 26% to 75% [41][42][43][44][45]. In a benchmark study of the catalytic performance of CuZnAl catalysts, the CH 3 OH selectivity varies from 28% to 54% at the pressure range of 3-5 MPa, temperature range of 210-250°C, H 2 : CO 2 molar ratio range of 5 : 3-3 : 1 and flow rate range of 500-1000 NmL g cat −1 min −1 [46].
In our study, the CO 2 conversion and CH 3 OH selectivity of CZA catalyst are 16.27% and 36.1% respectively at 2 MPa, 250°C, 12 000 ml/(g h) and H 2 : CO 2 molar ratio of = 3 : 1, which is roughly in line with the range reported in the above literature. The CO 2 conversion and CH 3 OH selectivity of CuZnO-ZrO 2 solid solution catalyst is 12.4-19.7% and 70.2-81.4% respectively at 2-5 MPa, 250°C, 12 000 ml/(g h) and H 2 : CO 2 molar ratio of = 3 : 1, whose catalytic activity level appears to be above average. Due to the different reaction conditions used in various studies, the comparison of catalytic performance has some reference value but cannot be completely trusted.

Conclusion
ZnO-ZrO 2 solid solution catalysts with various copper contents were synthesized for CO 2 hydrogenation to CH 3 OH. The XRD, HRTEM and Raman spectra analysis results showed that there is a solid solution structure in the 3% CuZnO-ZrO 2 catalyst, and its specific surface area is 38.408 m 2 g −1 , higher than those of the 0% CuZnO-ZrO 2 and 10% CuZnO-ZrO 2 catalysts. The H 2 -TPR analysis proved that the 3% CuZnO-ZrO 2 catalyst exhibited higher catalytic performance at a relatively lower reaction temperature. As against 10% CuZnO-ZrO 2 , 3% CuZnO-ZrO 2 contains more dispersed copper species, whose catalytic activity is higher than that of the bulk CuO species. The XPS analysis indicates a strong interaction between Cu and the ZnO-ZrO 2 solid solution. The CO 2 conversion and CH 3 OH yield of the 3% CuZnO-ZrO 2 reached 17.7% and 14.9%, respectively (5 MPa, 250°C and 12 000 ml/(g h)), while those of the 0% CuZnO-ZrO 2 were 9.1% and 7.3%, respectively. Compared with the traditional CZA catalyst, the Cu-doped solid solution had higher methanol selectivity and better high temperature resistance.
Ethics. This article does not present research with ethical considerations. Data accessibility. The data have been uploaded as electronic supplementary material [47].