Exploring the Effect of the Solvothermal Time on the Structural Properties and Catalytic Activity of Cu-ZnO-ZrO₂ Catalysts Synthesized by the Solvothermal Method for CO₂ Hydrogenation to Methanol

Abstract

A series of Cu-ZnO-ZrO₂ (CCZ) catalysts were prepared by the solvothermal method with different solvothermal times (1 h, 3 h, 6 h, and 12 h). The physicochemical properties of these catalysts and the catalytic performance for CO₂ hydrogenation to methanol were studied. The highest methanol yield was achieved when the solvothermal time was 6 h (CCZ-6). Furthermore, we found that the copper surface area (S_Cu) increases and then decreases with an increase in the solvothermal time and that there is a strong correlation between the methanol yield and the S_Cu. This research highlights the crucial influence of the solvothermal time on the structure and catalytic behavior of Cu-ZnO-ZrO₂ catalysts, providing a valuable reference for the development of efficient catalysts.

1. Introduction

With the continuous growth of global energy demand and the increasing severity of environmental problems, the exploration of renewable energy sources and the reduction of greenhouse gas emissions have become important issues that need to be urgently addressed [1]. Among the greenhouse gases, the conversion and utilization of carbon dioxide (CO₂) has received much attention [2,3]. Methanol (CH₃OH) has gradually gained recognition as a clean and renewable fuel source by global industry due to its widespread availability, significant economic volume, and sustainable development of the whole industrial chain [4,5]. Therefore, among the many ways to convert CO₂, the conversion of CO₂ to methanol by the hydrogenation reaction is a prospective solution [6,7,8].

Copper-based catalysts have become a popular research topic due to their low cost and high catalytic activity for the CO₂ hydrogenation to methanol reaction [9,10]. Among them, Cu-ZnO-ZrO₂ ternary catalysts have attracted much attention due to their exceptional catalytic performance [11,12,13]. Witoon et al. [13] claimed that the doping of graphene oxide significantly improved the catalytic performance of CuO-ZnO-ZrO₂ catalysts. Chang et al. [14] demonstrated that the ZnO/ZrO₂ composition in Cu/ZnO/ZrO₂ catalyst impacted CuZn alloy formation and surface basic sites, and the suitable ZnO/ZrO₂ composition significantly enhanced the catalytic activity and the methanol yield produced by CO₂ hydrogenation.

It has been shown that the preparation methods and conditions of catalysts have a significant effect on their structures and properties [15]. As we all know, the most commonly used method for the preparation of methanol synthesis catalysts is co-precipitation. The co-precipitation method involves the simultaneous precipitation of multiple components from a solution, leading to well-mixed and homogeneously distributed precursors [7,11]. However, the co-precipitation process is very sensitive to the conditions of the pH, temperature, and stirring speed of the solution, and a little carelessness may lead to the non-uniform particle size distribution of the precipitates, which may in turn affect the performance of the catalyst. The solvothermal method, as an alternative effective synthesis method, can prepare nanomaterials with high dispersibility and excellent catalytic properties under relatively mild conditions [16,17]. Specifically, the time of the solvothermal treatment, as a key parameter in the solvothermal method, plays a crucial role in regulating the physicochemical properties of the catalyst [18,19]. A previous study [16] has shown that the solvothermal temperature has a significant effect on the structure of Cu-ZnO-ZrO₂ catalysts, and CCZ-180 with a solvothermal temperature of 180 °C had the best catalytic activity. However, the effect of the solvothermal time on the catalyst performance is equally important and needs to be further studied.

In this paper, a series of Cu-ZnO-ZrO₂ catalysts were prepared by the solvothermal method under different solvothermal times (1 h, 3 h, 6 h, and 12 h). The catalysts were analyzed via characterizations such as XRD, SEM, EDX mapping, N₂ adsorption–desorption, N₂O chemisorption, XPS, H₂-TPR, and CO₂/H₂-TPD. Furthermore, the relationships between the catalytic performance and the physicochemical properties were studied in detail.

2. Results and Discussion

2.1. Characterization

2.1.1. XRD

Figure 1 shows the XRD patterns of the calcined and reduced Cu-ZnO-ZrO₂ catalysts. From Figure 1a, the diffraction peaks at 2θ of 35.5°, 38.7°, 48.8°, 61.6°, 66.3°, 68.1°, and 75.0°, respectively, can be attributed to the presence of the CuO phase (PDF #48-1548), whereas the diffraction peaks appearing at 2θ = 30.3° correspond to tetragonal ZrO₂ (t-ZrO₂) (PDF #050-1089). No diffraction peak of ZnO was found in any of the four catalysts, indicating that ZnO was highly dispersed or existed in an amorphous state in all four catalysts [20,21].

As shown in Figure 1b, after reduction at 300 °C, several peaks were observed at 2θ of 43.3°, 50.4°, and 74.1°, representing the crystalline surfaces of Cu(111), Cu(200), and Cu(220), respectively [22]. However, no peaks of CuO were found, suggesting that the CuO in these samples had been reduced [23]. The sizes of the metallic Cu and CuO crystals were determined by Scherer’s formula and the results are summarized in

Table 1. Physicochemical properties of different Cu-ZnO-ZrO₂ catalysts.

Catalyst	SBET (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)	dCuO1 (nm)	dCu1 (nm)	SCu2 (m2/g)
CZZ-1	20.7	0.055	10.6	26.8	30.2	4.6
CZZ-3	32.7	0.069	8.1	22.2	27.2	12.0
CZZ-6	40.5	0.087	8.6	19.1	25.7	15.7
CZZ-12	16.4	0.054	13.3	27.7	29.1	4.5

¹ Determined by XRD. ² Determined by the N₂O chemisorption method.

. It can be observed that the sizes of both CuO and metallic Cu first decrease and then increase with the extension of the solvothermal time. Especially, CZZ-6 exhibited the smallest size of copper species, which improved the dispersion and increased the surface area, as evidenced by the maximum S_BET and S_Cu of CZZ-6. On the other hand, the sizes of metallic Cu are noticeably larger than those of CuO for the same catalysts, and the most plausible reason for this phenomenon is that the reduction of CuO to metallic Cu with H₂ is an exothermic process (CuO + H₂ → Cu + H₂O, ΔH = −129.2 kJ/mol), and the heat released makes the metallic Cu particles aggregate and become larger. This phenomenon has also been reported in previous studies [17,24].

2.1.2. SEM and EDX Mapping

Figure 2 shows SEM images of the representative CZZ-6 sample. It can be seen that CZZ-6 is a built-up structure consisting of many particles that form a loose microstructure. An EDX mapping analysis was performed to investigate the distribution of the elements. It was confirmed that all the Cu, Zn, and Zr atoms in CCZ-6 are well-dispersed on the catalyst surface.

2.1.3. N₂ Adsorption–Desorption

The N₂ adsorption–desorption isotherms and pore size distribution curves for the different catalysts are shown in Figure 3. Clearly, all the samples showed type IV isotherms with H3-type hysteresis loops (Figure 3a), indicating that all the catalysts were mesoporous materials [25,26]. Furthermore, the hysteresis return line of CZZ-6 was larger than that of the other catalysts, indicating that it had the largest mesopore volume. As can be seen in Figure 3b, all four samples exhibited wide pore distributions, except for the concentration of pores at 1.9 nm, where the pore sizes gradually increased in the order of CZZ-3 < CZZ-6 < CZZ-1 < CZZ-12. These results indicate that the CZZ-3 catalyst had the narrowest pore distribution among the four catalysts.

The effects of the solvothermal time on the pore structure and specific surface area (S_BET) of the Cu-ZnO-ZrO₂ catalysts are listed in Table 1. As the solvothermal time increases, the S_BET and pore volume of the catalyst initially increase and then decrease. This result is consistent with the order of CuO particle size measured by XRD (Table 1). The CZZ-6 catalyst, with a solvothermal time of 6 h, possesses the largest S_BET and pore volume. A larger pore volume and S_BET could enhance the adsorption and desorption of the reaction gases, thereby improving the catalytic performance [27].

2.1.4. S_Cu

The S_Cu of Cu-based catalysts has an important effect on the catalytic performance [12,27,28], so the N₂O chemisorption method was employed to determine the S_Cu of the different Cu-ZnO-ZrO₂ catalysts. As shown in Table 1, with the extension of the solvothermal time, the S_Cu first increases and then decreases. This trend is consistent with the change in the S_BET of the catalyst, with both reaching their maximum values in the CZZ-6 catalyst prepared with a solvothermal time of 6 h. The subsequent activity test results (Section 2.2) revealed that the larger the S_Cu, the better the catalytic activity that could be achieved. Especially, the CZZ-6 had the largest S_Cu, implying more highly dispersed copper particles [27,29], which was confirmed by its smallest copper particle size (d_Cu) in Table 1.

2.1.5. XPS

Figure 4 presents the XPS spectra of the copper species after calcination and in situ reduction. As depicted in Figure 4a, the peaks of the copper species appeared around 933 eV and 953 eV, with broad vibrational peaks appearing within 940–945 eV, indicating the presence of copper species in the form of Cu²⁺ [30]. Following in situ reduction at 300 °C (Figure 4b), the vibrational peaks disappeared and the signal peak of the copper species became narrower and shifted to lower binding energies at around 932 eV, suggesting the reduction of Cu²⁺ to Cu⁰ and/or Cu⁺ [31].

The Cu LMM XAES spectra of the Cu species were used to further distinguish between Cu⁰ and Cu⁺. From Figure 5, it can be observed that the CZZ-1 and CZZ-12 catalysts exhibited two overlapping peaks, indicating the coexistence of Cu⁺ and Cu⁰ species on the catalyst surface [32]. In contrast, only Cu⁰ species were observed at the binding energy of 919.0 eV for CZZ-3 and CZZ-6. On the other hand, the characteristic peaks of Cu⁺ were not observed in CZZ-1 and CZZ-12 according to the XRD analysis (Figure 1), possibly due to the amorphous nature of Cu₂O or its high dispersion.

Figure 6 shows the O 1s XPS spectra of the Cu-ZnO-ZrO₂ catalysts after in situ reduction. Three peaks were present on the spectra of all the catalysts: the α peak at 529 eV represented the lattice oxygen of the metal oxides, the β peak was the chemisorbed oxygen species, and the γ peak was the hydroxyl-like species [33]. Among them, the β and γ peaks were considered to be active sites and played an important role in CO₂ hydrogenation to methanol [34,35]. From Table S1, the largest A_β + A_γ value was found in CZZ-6, which provided the foundation for the high activity.

The binding energy values and surface contents of the catalyst surfaces before and after in situ reduction are illustrated in Table S2. From Table S2, it can be seen that the binding energy values of Zn 2p_3/2 were in the range of 1021.3–1021.5 eV, and those of Zr 3d_5/2 were in the range of 181.7–181.9 eV, and there was no significant change in the binding energy values of Zn 2p_3/2 and Zr 3d_5/2 before and after in situ reduction, which indicated that Zn and Zr existed in the catalyst in the form of oxides and would not be reduced at 300 °C under a hydrogen atmosphere. Moreover, with the extension of the solvothermal time, the copper content on the catalyst surface followed the order: CZZ-1 > CZZ-3 > CZZ-6 > CZZ-12, while the S_Cu order was: CZZ-12 < CZZ-1 < CZZ-3 < CZZ-6. Generally, the larger the copper content on the catalyst surface, the smaller the S_Cu. Clearly, in our study, there was no direct correlation between the copper content on the catalyst surface and the S_Cu. This discrepancy may be due to the aggregation of copper particles on the surface of the CZZ-12 with a prolonged solvothermal time, resulting in the smallest S_Cu.

2.1.6. H₂-TPR

The reduction properties of the Cu-ZnO-ZrO₂ catalysts were studied by H₂-TPR. As shown in Figure 7, all the catalysts showed a broad reduction peak between 150 and 300 °C. As mentioned earlier, ZnO and ZrO₂ were not reduced at 300 °C, and therefore, the peak was attributed to the reduction of the CuO_x species [7].

The small shoulder peak (α peak) on the low-temperature side can be clearly observed in the CZZ-1, and this result indicated that there were two different sizes of CuO particles in the CZZ-1. Unlike CZZ-1, the reduction peaks of the CZZ-3, CZZ-6, and CZZ-12 catalysts were relatively symmetrical, indicating that the CuO crystal sizes on these three catalysts were more uniform than those on CZZ-1. Furthermore, CZZ-6 was observed to have the lowest reduction temperature (202 °C) for CuO_x species. Low-temperature reduction usually indicates strong interactions between the metal and the oxide in the catalyst, which can enhance the dispersion of the metal species and make the reduction process easier to carry out [36,37,38].

Table 2. The temperature and H₂ consumption in the H₂-TPR patterns of the various catalysts.

Catalyst	Temperature of Peaks (°C)		H2 Consumption of Peaks (μmol/g)			Degree of Reduction 1
	Tα	Tβ	α	β	Total
CZZ-1	211	262	138	2676	2814	0.81
CZZ-3	210	-	2802	-	2802	0.80
CZZ-6	201	-	3306	-	3306	0.94
CZZ-12	242	-	3378	-	3378	0.96

¹ Determined by the ratio of the actual H₂ consumption of the catalysts to the theoretical H₂ consumption.

displays the quantitative data of the hydrogen consumption of the CuO-ZnO-ZrO₂ catalysts. It is observed that CZZ-1 and CZZ-3 have similar H₂ consumption, while CZZ-6 and CZZ-12 have more H₂ consumption, which suggests the presence of more reducible CuO_x species in CZZ-6 and CZZ-12. Furthermore, the H₂ consumption of all the catalysts was lower than the theoretical value required for the complete reduction of copper oxide to the metal Cu (the degree of the reduction was less than 1), further confirming that ZnO and ZrO₂ were not reduced. It has been well-documented that Cu-based catalysts with high activity for CO₂ hydrogenation to methanol have high reducibility [39,40]. CZZ-6 was expected to exhibit the highest activity due to its optimal reducibility among the four catalysts, as confirmed by the results of the activity tests (Section 2.2).

2.1.7. CO₂-TPD

The adsorption capacity of the catalysts for CO₂ was determined through CO₂-TPD. Figure 8 illustrates the CO₂-TPD profiles of the pre-reduced Cu-ZnO-ZrO₂ catalysts. It is observed that all the catalysts exhibited three CO₂ desorption peaks, which were deconvolved Gaussian peaks designated as α, β, and γ peaks, respectively. The low-temperature α peak corresponds to the desorption of CO₂ from weak basic sites associated with hydroxyl functional groups. The β peak represents the desorption of CO₂ from medium basic sites formed by the combination of metal and oxygen. The γ peak corresponds to the desorption of CO₂ from strong basic sites associated with low-coordination oxygen anions [16]. The CO₂ desorption data for different Cu-ZnO-ZrO₂ catalysts are presented in

Table 3. The data of the CO₂-TPD profiles of the different Cu-ZnO-ZrO₂ catalysts.

Catalyst	α Peak		β Peak		γ Peak		(Aα + Aβ + Aγ)/(a.u.)
	Tα/(°C)	Aα/(a.u.)	Tβ/(°C)	Aβ/(a.u.)	Tγ/(°C)	Aγ/(a.u.)
CZZ-1	105	47	150	72	387	42	161
CZZ-3	114	116	165	207	389	46	369
CZZ-6	105	70	151	144	380	101	315
CZZ-12	111	46	160	68	390	42	156

. It can be observed that the CZZ-3 and CZZ-6 catalysts exhibit larger CO₂ desorption (A_α + A_β + A_γ values), but the strength of the basic sites of the CZZ-6 is relatively weak compared to CZZ-3. The total desorption of CO₂ is crucial for the production of methanol by CO₂ hydrogenation [25], which will be discussed in Section 2.3.

2.1.8. H₂-TPD

Figure 9 displays the H₂-TPD profiles of the pre-reduced Cu-ZnO-ZrO₂ catalysts. The desorption peaks on the catalysts represent the H species adsorbed on the surface of the active component Cu [15,27]. It is generally observed that the low-temperature α peak represents the desorption of undissociated molecular hydrogen (H₂) weakly adsorbed on the catalyst surface, while the high-temperature β peak represents the desorption of dissociated atomic hydrogen (H) [41]. The quantitative data for the temperature and peak area of the H₂ desorption peaks are listed in

Table 4. The data of the H₂-TPD profiles of the different Cu-ZnO-ZrO₂ catalysts.

Catalyst	α Peak		β Peak		Aα + Aβ/(a.u.)
	Tα/(°C)	Aα/(a.u.)	Tβ/(°C)	Aβ/(a.u.)
CZZ-1	137	20	285	80	100
CZZ-3	148	25	324	81	106
CZZ-6	106	30	282	172	202
CZZ-12	161	25	327	82	107

. Clearly, the temperatures of the α and β desorption peaks show irregular changes with the prolongation of the solvothermal time. However, the lowest desorption peak temperature of CZZ-6 indicates the weakest adsorption strength of H₂. On the other hand, with the extension of the solvothermal time, the peak areas of the α and β peaks first increased and then decreased, with the largest peak area observed at 6 h of solvothermal time. The maximum peak area of CZZ-6 indicates its highest H₂ adsorption capacity.

2.2. Catalytic Activity

Figure 10 presents the activity evaluation results of the Cu-ZnO-ZrO₂ catalysts prepared with different solvothermal times in the CO₂ hydrogenation to methanol. Under the current reaction conditions, CO and methanol were detected as carbon-containing reaction products. However, with the extension of the solvothermal time, the CO₂ conversion and methanol yield first increased and then decreased. CZZ-1 and CZZ-12 had low CO₂ conversion despite the high methanol selectivity, resulting in low methanol yields. The CO₂ conversion of CZZ-6 was 15.6%, with methanol selectivity of 46.1%, and the methanol yield reached a maximum of 7.2%. This indicates that the catalyst prepared with 6 h of solvothermal time is most suitable for methanol production.

To provide a comprehensive context for our catalytic activity results, we compared our findings with similar Cu-Zn-Zr catalysts reported in the literature for CO₂ hydrogenation to methanol. The comparison is summarized in Table S3 in the Supplementary Materials. It is well known that the performance of Cu-Zn-Zr catalysts for CO₂ hydrogenation heavily depends on factors such as the composition, pressure, temperature, H₂/CO₂ ratio, and feed gas space velocity [27,31]. Nevertheless, under similar reaction conditions, our CCZ-6 catalyst demonstrated comparable or even superior methanol yields. Our study indicates that optimizing the reaction time during solvothermal synthesis can effectively enhance the catalytic performance for CO₂ hydrogenation to methanol.

2.3. Structure–Activity Relationship Analysis

As mentioned in relation to CO₂-TPD, the amount of CO₂ adsorbed by the different catalysts was observed to be clearly different. The adsorption of CO₂ is a pre-condition for conducting CO₂ hydrogenation [25,27]. The total CO₂ adsorption is an important index in the evaluation of high-performance catalysts. Therefore, the effect of the total amount of desorbed CO₂ on the methanol yield was investigated (Figure 11a). However, the relationship was not completely linear (R² = 0.79), which indicated that the amount of CO₂ absorbed was not directly responsible for the methanol yield.

On the other hand, the S_Cu stands as a crucial parameter for copper-based catalysts in methanol synthesis. Researchers have extensively explored the relationship between the catalytic activity of copper-based catalysts and the S_Cu [12,42,43]. However, there are conflicting views about their relationship. Natesakhawa et al. [42] reported a linear correlation between the methanol yield and the S_Cu. Our previous study [27] also demonstrated a strong link between the S_Cu and CO₂ conversion over Cu-Ce_1-xZr_xO₂ catalysts in CO₂ hydrogenation to methanol. Conversely, Zhang et al. [43] and Hou et al. [44] found no connection between the methanol yield and the S_Cu. In this study, the solvothermal time significantly impacted the S_Cu of the Cu-ZnO-ZrO₂ catalysts (Table 1). Figure 11b shows the relationship between the S_Cu and the methanol yield. It can be seen that there is a strong linear correlation between the methanol yield and the S_Cu (R² = 0.95), indicating that the S_Cu of the Cu-ZnO-ZrO₂ catalyst is a determining factor in methanol production. From the above analyses, it is clear that a suitable solvothermal time can promote the uniform growth and high dispersion of Cu particles, and thus increasing the S_Cu. This can be confirmed by the results of the XRD, SEM, EDX mapping, N₂O chemisorption, and N₂ adsorption–desorption, as described above.

It is well known that the solvothermal time greatly affects the properties of materials synthesized by the solvothermal method [18,19,45]. During solvothermal synthesis, the reaction time determines the growth rate of the catalyst particles and the interaction between different components. An optimal reaction time can lead to the formation of highly dispersed and well-crystallized catalysts with a large specific surface area, enhancing their catalytic performance. However, an insufficient or excessive reaction time can result in catalysts with poor activity due to incomplete crystallization or excessive particle growth. In our case, as the solvothermal time increased from 1 h to 6 h, the size of the formed CuO particles decreased, as observed from the XRD findings (Figure 1). As a result, the catalysts exhibited a larger S_BET, as indicated by the N₂ adsorption–desorption (Figure 3 and Table 1), and the S_Cu increased noticeably (Table 1). As the solvothermal time further increased to 12 h, nevertheless, small particles of CuO began to aggregate together and larger particles were formed, as evidenced by the data in Table 1. Consequently, the S_Cu was distinctly decreased. Evidently, the CZZ-6 catalyst possesses the largest S_Cu, which exhibits a maximal capacity for dissociative adsorption of H₂ (Table 4) and thus the highest CO₂ conversion and methanol yield (Figure 10) among the investigated CZZ-X catalysts. Figure 12 illustrates the effect of the solvothermal time for CO₂ hydrogenation to methanol.

3. Materials and Methods

3.1. Catalyst Preparation

In this paper, Cu-ZnO-ZrO₂ catalysts with the same molar ratio of Cu/Zn/Zr = 5:2:3 were prepared by the solvothermal method. The choice of this specific molar ratio was made to ensure comparability with our prior studies [16,17] and to build upon the successful results achieved earlier. Firstly, Cu(NO₃)₂·3H₂O (AR), Zn(NO₃)₂·6H₂O (AR), and Zr(NO₃)₄·5H₂O (AR) were dissolved in 50 mL of ethylene glycol to prepare a solution with a total ion concentration of 1 mol/L. After stirring and dissolving, the solution was transferred to a reaction kettle and placed in an oven at 180 °C for a certain period (1 h, 3 h, 6 h, and 12 h). After the reaction, the kettle was cooled to room temperature, and the obtained precipitate was filtered, washed thoroughly with anhydrous ethanol and deionized water, and dried overnight in an oven at 120 °C. The yields of the solid products obtained at different solvothermal times were not noticeably different. Finally, the dried product was calcined at 450 °C for 3 h in a muffle furnace. Both the heating rate during the solvothermal process and the calcination process were set to 5 °C/min. The obtained catalysts were named as CZZ-X, where X represents the solvothermal synthesis time.

3.2. Characterization

X-ray diffraction analysis (XRD) of the catalysts was conducted using a Bruker D8 ADVANCE diffractometer (Karlsruhe, Germany) with Cu Kα radiation. The scanning range was set from 10° to 80° at a scan rate of 10°/min.

Scanning Electron Microscopy (SEM) and (Energy-dispersive X-ray spectroscopy) (EDX) mapping data of samples were gained on Hitachi SU8010 (Tokyo, Japan).

N₂ adsorption–desorption experiments were performed using an ASAP 2020 HD88 instrument (Norcross, GA, USA). The specific surface area and pore volume of the samples were calculated by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) models, respectively.

The copper surface area (S_Cu) was determined using two-step H₂-TPR. Initially, 0.1 g of catalyst was purged with N₂ at 300 °C for 60 min. The first reduction was then performed in a 10 vol% H₂/N₂ mixture to 300 °C. After the reduction, the sample was cooled to 60 °C, purged with N₂ for 30 min, oxidized in 2 vol% N₂O/He for 1 h to ensure the complete oxidation of surface metallic Cu to Cu₂O, and then purged with N₂ for another 30 min. Finally, the catalyst was again reduced to 500 °C in the 10 vol% H₂/N₂ mixture. The S_Cu was calculated using the formula:

$$S_{\mathrm{C}\mathrm{u}}=\frac{\left(2X\times N\right)}{\left(1.46\times 10^{19}\times W\right)}$$

(1)

where X is the amount of H₂ consumed during the second TPR process, N is Avogadro’s number (6.02 × 10²³ atoms/mol), and W is the weight of the catalyst (g).

X-ray photoelectron spectroscopy (XPS) analysis was conducted using an ESCALAB 250Xi spectrometer from Thermo Scientific (Waltham, MA, USA), with Al Kα as the excitation source, and the binding energy of the measured elements was calibrated to C 1 s (284.6 eV).

The H₂ temperature-programmed reduction (H₂-TPR) was used to evaluate the reducibility of the catalysts. The catalyst (30 mg) was pretreated in N₂ at 300 °C for 1 h to remove the surface-adsorbed water and impurities, then cooled to 50 °C. Subsequently, the sample was exposed to the 10 vol% H₂/N₂ mixture until the chromatographic baseline stabilized, followed by heating to 600 °C while monitoring the consumption of H₂ with a thermal conductivity detector (TCD).

The temperature-programmed desorption of H₂ and CO₂ (H₂/CO₂-TPD) was used to characterize the adsorption property of the catalysts. Prior to CO₂ adsorption, the catalyst was reduced at 300 °C for 1 h in a 10 vol% H₂/N₂ mixture. The sample was then cooled to 50 °C, saturated with 10 vol% CO₂/N₂ for 30 min, purged with He for 30 min to remove physically adsorbed CO₂ molecules, and subjected to CO₂-TPD under He flow from 50 °C to 600 °C, with CO₂ signal detection using a mass spectrometer (Pfeiffer Vacuum Quadstar, 32-bit, Aßlar, Germany). The procedures for the H₂-TPD and CO₂-TPD were similar, with the only difference being the replacement of the adsorbed 10 vol% CO₂/N₂ gas to 10 vol% H₂/N₂, and the signal detection was performed using TCD. It should be noted that the TPD experiments for CO₂ and H₂ were conducted on different charges of the sample to ensure independent and accurate analysis of each gas’s adsorption and desorption behavior. Fresh samples were used for each TPD experiment, allowing for precise monitoring of the desorbed species without interference from previous gas exposures.

3.3. Activity Tests

A schematic diagram of the reaction apparatus for CO₂ hydrogenation to methanol is illustrated in Figure S1. Different catalysts for CO₂ hydrogenation were evaluated for their activity and selectivity using a continuous-flow fixed-bed reactor. Here, 0.3 g of catalyst with a particle size of 40–60 mesh was loaded into a stainless-steel reactor with an inner diameter of 5 mm. The catalyst was reduced in a 10 vol% H₂/N₂ stream at 300 °C for 3 h. Subsequently, the reaction was conducted at 240 °C, a pressure of 3 MPa, and a space velocity of 2400 mL/(g_cat·h). After a stable reaction period, the products were analyzed using gas chromatography. Under the experimental conditions, the products of the CO₂ hydrogenation reaction included CO and methanol, along with unreacted CO₂ and N₂. No other by-products were detected. The effluent from the reaction was analyzed online using an Agilent 6820 gas chromatograph. Methanol was detected using a flame ionization detector (FID) with a Porapak Q capillary column, while CO, CO₂, and N₂ were detected using a TCD equipped with a carbon molecular sieve column. The CO₂ conversion, product selectivity, and methanol yield were calculated using the following formulas:

$${\mathrm{C}\mathrm{O}}_2 \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} (\%)=\frac{A_{CO}\cdot f_{CO}+A_{C{H}_{3}OH}\cdot f_{C{H}_{3}OH}}{A_{CO}\cdot f_{CO}+A_{C{H}_{3}OH}\cdot f_{C{H}_{3}OH}+A_{C{O}_2}\cdot f_{C{O}_2}}$$

(2)

$$\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l} \mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\left(\%\right)=\frac{A_{C{H}_{3}OH}\cdot f_{C{H}_{3}OH}}{A_{CO}\cdot f_{CO}+A_{C{H}_{3}OH}\cdot f_{C{H}_{3}OH}}$$

(3)

$$\mathrm{C}\mathrm{O} \mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\left(\%\right)=\frac{A_{CO}\cdot f_{CO}}{A_{CO}\cdot f_{CO}+A_{C{H}_{3}OH}\cdot f_{C{H}_{3}OH}}$$

(4)

$$\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\%\right)={\mathrm{C}\mathrm{O}}_2 \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\times \mathrm{M}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l} \mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\times 100$$

(5)

In the above formulas, A_i denotes the integrated peak area of each substance in the gas chromatographic analysis at the reactor outlet and f_i denotes the correction factor of each substance with respect to N₂.

4. Conclusions

The Cu-ZnO-ZrO₂ catalysts were prepared by the solvothermal method. The effects of different solvothermal times (1 h, 3 h, 6 h, and 12 h) on the physicochemical properties of the Cu-ZnO-ZrO₂ catalysts were investigated. The results of the catalytic activity test revealed that the CZZ-6 exhibited the highest catalytic activity and methanol yield at a solvothermal time of 6 h. It can be observed that the solvothermal treatment time directly affected the size of the catalyst particles and the surface area, which in turn affected the surface properties and the distribution of active sites. The high activity of the CZZ-6 is ascribed to a suitable solvothermal time that promotes the uniform growth and high dispersion of Cu particles and increases the S_Cu. This work provides an in-depth study of the regulation of the solvothermal time on Cu-ZnO-ZrO₂ catalysts prepared by the solvothermal method, which provides valuable guidance for high-performance catalysts.