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The role of copper valence states in CuZnAl catalysts for CO2-to-methanol conversion
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Zhengpu QIU1, Yunzhao XU2, Peng WANG2, 3, *, Xiaoxia TAO2, Huimin ZHANG1, Yang CHEN2, Yi LIU1, Hua YANG4, Fenghai CAO2, Yajie FU2, Lizhi WU2, Yu TANG2, Xiaoying XU1, *, Li TAN2, *
Journal of Fuel Chemistry and Technology | 2026, 54(4) : 20250210-598 - 20250210-607
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Journal of Fuel Chemistry and Technology | 2026, 54(4): 20250210-598-20250210-607
• RESEARCH PAPER •
The role of copper valence states in CuZnAl catalysts for CO2-to-methanol conversion
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Zhengpu QIU1, Yunzhao XU2, Peng WANG2, 3, *, Xiaoxia TAO2, Huimin ZHANG1, Yang CHEN2, Yi LIU1, Hua YANG4, Fenghai CAO2, Yajie FU2, Lizhi WU2, Yu TANG2, Xiaoying XU1, *, Li TAN2, *
Affiliations
  • 1National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, China
  • 2College of Chemistry, Fuzhou University, Fuzhou 350108, China
  • 3School of Future Membrane Technology, Fuzhou University, Fuzhou 350108, China
  • 4Chongqing Vocational Institute of Safety Technology, Chongqing 404020, China
Published: 2026-04-10 doi: 10.1016/S1872-5813(25)60616-5
Outline
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CuZnAl (CZA) is a classic industrial catalyst widely used for the synthesis of methanol from syngas, but its catalytic performance is not optimal for the hydrogenation of CO2 to methanol. Meanwhile, understanding the catalytic mechanism of Cu species in the CZA catalyst remains a great challenge. In this study, we systematically investigated the valence state change of active Cu species in CZA catalyst and their influence on catalytic performance by modifying the catalysts with varying amounts of electron donor K, thus identifying the catalytic function of Cu species with different valence states. H2-TPR, XPS and HR-TEM characterizations reveal that the highly dispersed K species supported on CZA catalysts will inhibit the reduction of CuO, resulting in a small amount of Cu2O active species being produced under reaction conditions thus causing a decrease in catalytic activity. Furthermore, XRD and Cu LMM spectra show that the proportion of Cu0 in K-modified CZA catalysts increases with K loading, but a higher proportion of Cu0 species on the surface obviously promotes the reverse water gas shift (RWGS) reaction. According to the results of in situ infrared spectroscopy, CZA catalyst follows the reaction pathway mediated by HCOO* in the hydrogenation of CO2 to methanol.

CO2 hydrogenation  /  CuZnAl catalysts  /  methanol  /  active species  /  electronic promoter
邱正璞, 许运钊, 王鹏, 陶小霞, 张惠民, 陈阳, 刘意, 杨桦, 曹逢海, 傅亚杰, 吴立志, 汤禹, 徐晓颖, 谭理. CuZnAl催化剂中Cu价态对CO2加氢合成甲醇反应的影响. 燃料化学学报（中英文）, 2026 , 54 (4) : 20250210-598 -20250210-607 . DOI: 10.1016/S1872-5813(25)60616-5
Zhengpu QIU, Yunzhao XU, Peng WANG, Xiaoxia TAO, Huimin ZHANG, Yang CHEN, Yi LIU, Hua YANG, Fenghai CAO, Yajie FU, Lizhi WU, Yu TANG, Xiaoying XU, Li TAN. The role of copper valence states in CuZnAl catalysts for CO2-to-methanol conversion[J]. Journal of Fuel Chemistry and Technology, 2026 , 54 (4) : 20250210-598 -20250210-607 . DOI: 10.1016/S1872-5813(25)60616-5
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The growing carbon dioxide (CO2) emissions and increasing reliance on fossil fuels highlight critical challenges in today's energy landscape, necessitating a shift toward decarbonization technologies and the development of renewable energy[1]. The catalytic conversion of CO2 into methanol represents a cutting-edge sustainable technology with significant implications for reducing carbon emissions[2]. This catalytic process utilizes catalysts to facilitate the transformation of CO2 and green hydrogen into methanol. As a crucial chemical building block, methanol serves as a fundamental feedstock for numerous fine chemicals, fuel additives, and chemical raw materials[3]. Additionally, methanol is regarded as a potential energy carrier due to its high hydrogen storage density and transportation safety. Therefore, catalytic processes that utilize CO2 to produce methanol demonstrate the substantial potential in the energy and environmental sectors, offering a viable pathway toward achieving carbon neutrality.
Cu-based catalysts have been extensively studied for methanol synthesis in recent decades[3−4]. Among the reported non-noble metal catalysts for CO2 hydrogenation to methanol, they are highly favored by researchers due to their high activity, good selectivity, and low cost[5−9]. In these catalysts, Cu serves as the main active component, while oxides such as ZnO, ZrO2, Al2O3, CeO2, SiO2 and TiO2 are used as supports to disperse and stabilize the active species[10−14]. Currently, industrial methanol synthesis primarily employs the classic CuZnAl (CZA) catalyst, typically composed of 60% Cu, 30% ZnO and 10% Al2O3[15]. Although CZA catalysts are used in industrial methanol production from syngas, their catalytic performance in CO2 hydrogenation to methanol is not satisfactory. Owing to the high stability and chemical inertness of CO2, CO2 conversion remains low at reaction temperatures of 220−280 °C. While increasing the temperature can enhance conversion, it also promotes the reverse water-gas shift (RWGS) side reaction, leading to substantial CO formation and reduced methanol selectivity. Moreover, Cu particles tend to sinter at high temperatures, and the H2O by-product generated during the reaction can promote the sintering of ZnO and oxidation of active Cu species to CuO, resulting in severe deactivation of the CZA catalyst.
Therefore, considerable research has been devoted to unravel the individual roles of Cu, Zn and Al components and the underlying catalytic mechanisms on CZA catalysts [16], aiming to design and develop highly efficient Cu-based catalysts with improved performance, including activity, selectivity, water resistance, and stability. Li et al.[17] prepared Cu-ZnO/Al2O3 catalysts via the co-precipitation method, followed by hydrothermal treatment at varying durations and temperatures. They found that hydrothermal treatment caused the agglomeration of Cu and ZnO particles, particularly metallic Cu, leading to a consequent reduction in Cu surface area and catalytic activity. Zhang et al.[18] synthesized Cu/ZnO/Al2O3 catalysts with different Cu-ZnO interface densities by tuning the chemical composition of zinc-malachite precursors. The results showed that when zinc substitution reached 27.13%, the obtained catalyst exhibited uniformly dispersed Cu and ZnO species with a small Cu crystallite size, which promoted the formation of abundant Cu-ZnO interfaces and significantly enhanced catalytic performance in methanol synthesis. Huang et al.[19] found that doping Er into the ZnO lattice reduced its particle size, increased the Cu-ZnO interface, and induced strong electronic metal-support interaction between Cu and ZnO. Zhang et al.[20] synthesized a series of ZnO1−x/Cu catalysts via the ball milling method to achieve the strong interfacial interaction between ZnO1−x and Cu, thereby allowing the controllable generation of numerous oxygen vacancies. The ZnO1−x/Cu-100 catalyst prepared under optimal conditions with more oxygen vacancies, exhibited the best performance in CO2 hydrogenation. Zhang et al.[21] introduced highly dispersed La species through a one-pot synthesis method, achieving a methanol space-time yield as high as 814.9 g/(kg∙h), significantly outperforming the CZA catalyst. The incorporated LaOx helped disperse and stabilize Cu+/Cu0 active sites, strengthened the interaction between Cu active sites and the support, and effectively improved the sintering resistance of Cu nanoparticles. These studies collectively indicate that the active structures of Cu-based catalysts have not yet been fully elucidated.
Herein, we systematically investigate the structure-activity relationship of Cu species with different valence states in CZA catalysts for CO2 hydrogenation to methanol. By modifying the catalyst with varying amounts of the alkali metal K, the valence states of Cu species in CZA were effectively tuned. It was revealed that highly dispersed K species on the CZA surface suppress the reducibility of CuO, resulting in fewer Cu+ active species under reaction conditions and consequently a decline in catalytic activity. Meanwhile, the incorporation of K promotes the dispersion of Cu0 species, indicating a strengthened interaction between Cu0 and ZnO. However, the increased abundance of Cu0 primarily promotes the RWGS side reaction, thereby suppressing methanol formation. In situ DRIFTS results further confirm that methanol synthesis proceed via a HCOO*-mediated pathway over CZA catalyst.
1　Experimental
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1.1　Chemicals and materials
The chemicals used in this work included: copper(II) nitrate trihydrate (Cu(NO3)2·3H2O, AR), aluminum nitrate nonahydrate (Al(NO3)3·9H2O, AR), and sodium carbonate anhydrous (Na2CO3, AR), all purchased from Macklin Reagent Co.; zinc nitrate hexahydrate (Zn(NO3)2·6H2O, AR) and potassium carbonate (K2CO3, AR), which were obtained from Aladdin Reagent Co.
1.2　Catalyst preparation
Synthesis of the CZA catalyst. First, 5.03 g of Cu(NO3)2·3H2O, 2.53 g of Zn(NO3)2·6H2O and 0.26 g of Al(NO3)3·9H2O were weighed and dissolved in 100 mL of deionized water to form an aqueous nitrate solution. Subsequently, 4.77 g of anhydrous sodium carbonate was weighed and dissolved in 100 mL of deionized water to prepare a 0.45 mol/L sodium carbonate solution. Both solutions were simultaneously added dropwise via peristaltic pumps with the same flow rate into a beaker containing 100 mL of deionized water. The coprecipitation was carried out at 80 °C under constant stirring in a water bath. Upon completion of the titration, the mixture was aged for 2 h. After cooling to room temperature, the product was subjected to suction filtration, followed by washing with deionized water until the filtrate was neutral. The solid was then dried overnight in an oven at 80 °C, and finally calcined in a muffle furnace at 350 °C for 4 h to obtain the CZA catalyst. ICP-OES analysis of the synthesized CZA sample determined the actual mass percentages of Cu, Zn, and Al to be 70.5%, 28.6% and 0.9%, respectively, closely matching the theoretically calculated values.
A series of K-modified CZA catalysts were synthesized via the incipient wetness impregnation method. The synthesis procedure was as follows: first, K2CO3 was dissolved in 1.3 mL of deionized water and the resulting solution was then added dropwise using a pipette onto 1 g of the CZA catalyst support under constant stirring with a glass rod. After the addition of the solution, the mixture was aged at room temperature for 2 h. Subsequently, the sample was dried in an oven at 80 °C, yielding the xK/CZA (x = 0.1%, 0.5%, 1%) catalysts where x represents the nominal mass percentage of K loaded on the CZA support.
1.3　Catalyst characterization
X-ray diffraction (XRD) measurements were performed on a Rigaku Smart Lab SE diffractometer using Cu Kα radiation (λ = 0.15406 nm). The data were collected in the 2θ range of 5° to 90° with a scanning rate of 5(°)/min, operating at 40 kV and 40 mA.
N2 physisorption analyses were conducted on a Micromeritics ASAP 2020PLUS HD88 instrument at −196 °C to determine the specific surface area and pore volume. Before analysis, the samples were degassed under vacuum at 300 °C for 6 h.
Inductively coupled plasma optical emission spectrometry (ICP-OES) was carried out on a Thermo Scientific iCAP 7000 spectrometer to determine the elemental composition.
H2 temperature-programmed reduction (H2-TPR) experiments were performed on a Micromeritics AutoChem II chemisorption analyzer. Approximately 0.1 g of catalyst was pretreated in a He stream at 300 °C for 2 h. After cooling to room temperature, the TPR profile was recorded under a 10% H2/Ar flow.
CO2 temperature-programmed desorption (CO2-TPD) was conducted using the same Micromeritics AutoChem II analyzer. About 0.1 g of catalyst was first pretreated in a 10% H2/Ar atmosphere at 230 °C for 2 h. Subsequently, the CO2-TPD profile was obtained using a 10% CO2/Ar stream.
Transmission electron microscopy (TEM) images were acquired on a Thermo Scientific Talos F200S G2 microscope. A small amount of catalyst was ultrasonically dispersed in ethanol, and a drop of the suspension was deposited onto a copper grid for observation.
In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) studies were performed on a Thermo Scientific Nicolet iS50 spectrometer equipped with an MCT detector and a Harrick reaction cell. The catalyst sample was first pretreated in pure H2 at 230 °C for 2 h. After switching to Ar and cooling to room temperature, a background spectrum was collected. The sample was then exposed to a gas mixture of 5% CO2/15% H2/80% Ar at 3 MPa and a flow rate of 20 mL/min at 230 °C, and the spectra were recorded.
X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific ESCALAB 250 Xi spectrometer using Al Kα monochromatic radiation (hv = 1486.6 eV). All binding energies were calibrated relative to the C 1s peak at 284.8 eV.
1.4　Catalytic performance evaluation
The catalytic performance was evaluated through online analysis in a fixed-bed reactor coupled with a Shimadzu GC-2014 gas chromatograph. A mixture of 0.2 g of catalyst particles (40−60 mesh) and 0.8 g of quartz sand (30−40 mesh) was loaded into a quartz tube and placed in the isothermal zone of the reactor. The catalyst was first pretreated in pure H2 at 230 °C for 2 h. Subsequently, a feed gas mixture of 24% CO2, 72% H2 and 4% Ar was introduced, and the reaction was carried out at 230 °C and 4 MPa.
Product analysis was performed using the gas chromatograph equipped with both a flame ionization detector (FID) and a thermal conductivity detector (TCD). The analysis utilized a combination of four chromatographic columns—MC‑1 (PONA 0.25 mm × 50 m × 0.5 µm), PC‑1 (P‑N 80/100 mesh 3.2 × 2.1 mm × 1.0 m), MC‑2 (P‑N 80/100 mesh 3.2 × 2.1 mm × 1.0 m), and MC‑3 (MS‑13X 80/100 mesh 3.2 × 2.1 mm × 3.0 m)—configured in series and/or parallel.
The catalytic activity was assessed in terms of CO2 conversion, product selectivity, and methanol space-time yield (STY). The CO2 conversion was calculated using the following equation:
$ \begin{array}{c}{C}_{{{\mathrm{CO}}}_{2}}\left(\%\right)=\dfrac{{X}_{{{\mathrm{CO}}}_{{\mathrm{2,in}}}}-{X}_{{{\mathrm{CO}}}_{2,{\mathrm{out}}}}}{{X}_{{{\mathrm{CO}}}_{{\mathrm{2,in}}}}}\times 100\%\end{array} $
In the equation, $X_{{\mathrm{CO_{2,in}}}} $ represents the mole fraction of CO2 in the feed gas, and $X_{{\mathrm{CO_{2,out}}}} $ denotes the mole fraction of CO2 in the outlet gas mixture.
The product selectivity is calculated as follows:
$ \begin{array}{c}{s}\left(\%\right)=\dfrac{{R}_{i}\times {f}_{i,m}}{{\displaystyle\sum }_{i}{R}_{i}\times {f}_{i,m}}\times 100\%\end{array} $
where $ {R}_{i} $ is the peak area of the product in the chromatogram, and $ {f}_{i,m} $ is the mass correction factor. The formula for calculating the methanol space-time yield (STY) is:
$ \begin{split} & {\mathrm{STY}}\left({{\mathrm{g}}}_{{\mathrm{MeOH}}}\cdot {{\mathrm{kg}}}_{{\mathrm{cat}}}^{-1}\cdot {{\mathrm{h}}}^{-1}\right)=\\&\qquad \dfrac{{F}_{{{\mathrm{CO}}}_{2,{\mathrm{in}}}}\times {C}_{{{\mathrm{CO}}}_{2}}\times {s}_{{\mathrm{MeOH}}}\times {M}_{{\mathrm{MeOH}}}}{{W}_{{\mathrm{cat}}}}\end{split} $
where $F_{{\mathrm{CO_{2,in}}}} $ is the molar flow rate of CO2 in the feed gas (mol/h), $M_{\mathrm{MeOH}} $ is the molecular weight of methanol (32 g/mol), and $W_{\mathrm{cat}} $ is the mass of the catalyst (kg).
2　Results and discussion
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2.1　Analysis of the active structure of the catalysts
The XRD patterns of the unmodified and K-modified CZA catalysts before and after reaction are presented in Figure 1. The fresh and K-modified CZA catalysts both exhibit diffraction peaks of CuO at 35.5° and 38.7°, as well as a diffraction peak of the ZnO(100) crystal plane at 31.8°, indicating a mixed oxide phase structure. However, diffraction peaks corresponding to an Al2O3 phase were not detected, suggesting that Al2O3 is likely present in a highly dispersed or amorphous state within the synthesized CZA catalysts. Furthermore, after the incorporation of alkali metal K, diffraction peaks that are characteristic of a hydrotalcite structure appear at 11.8° and 23.6°[22−24]. This is attributed to localized structural reconstruction during the preparation process induced by the relatively high pH of the solution, where the modified K species exist in the form of hydrotalcite-like particles. As shown in Figure 1(b), the hydrotalcite structure disappears in the spent K-modified CZA catalyst, indicating a transformation of the surface hydrotalcite phase under the reaction conditions at 230 °C. Notably, the intensity of the diffraction peak at 38.7° (corresponding to the CuO crystalline phase) in the spent catalysts (both CZA and K-modified CZA), increases with increasing K content. Correspondingly, the spent unmodified CZA catalyst shows the strongest diffraction peaks at 43.3° and 50.3°, which are assigned to the metallic Cu0 species formed by the reduction of CuO. Moreover, the intensity of the diffraction peaks corresponding to the Cu0 phase in the series of K-modified CZA catalysts gradually decreases as the K loading increases and completely disappears when the K content up to 1%. The crystal sizes of different phases, calculated from the XRD peaks using the Scherrer equation, are listed in Table 1. For the fresh catalysts, the introduction of the K modifier did not significantly alter the crystal sizes of ZnO and CuO. However, for the spent catalysts, the crystal sizes of both ZnO and CuO increased noticeably, suggesting particle agglomeration occurred during the reaction. Concurrently, relatively large Cu0 crystallites (approximately 16 nm) appeared in the spent catalysts. Overall, the crystal sizes in the K-modified CZA catalysts are almost unchanged compared to the parent CZA, indicating that this series of catalysts possesses comparable particle dimensions. This allows for a subsequent comparative analysis of their electronic structures using H2-TPR and XPS techniques.
The reduction properties of the parent CZA and K-modified CZA catalysts were investigated by H2-TPR, and the results are shown in Figure 2 and Table 2. The reduction peaks observed in the temperature range of 100−500 °C are attributed to the reduction of CuO species. The CZA catalyst exhibits three main reduction peaks (α, β and γ). They are assigned to the reduction of highly dispersed surface CuO, bulk CuO particles, and bulk-phase CuO, respectively. Notably, these three reduction peaks shift to higher temperatures in the K-modified CZA catalysts, indicating that the introduction of K inhibits the reduction of CuO species. This observation is consistent with the results reported by Xu et al.[25] for a Cs-modified CuFeZn catalyst. Although the overall reduction degree is similar for all four catalysts as shown in Table 2, the main reduction peak of the 1K/CZA catalyst shifts most significantly to a higher temperature. Considering the reduction temperature and the actual reaction temperature (230 °C), it can be inferred that the amount of CuO reduced during the reaction is the lowest for this catalyst. Therefore, the H2-TPR results indicate that the extent of reduction of CuO species under reaction conditions is lower in the K-modified CZA catalysts compared to the parent CZA catalyst.
The spent catalysts were characterized by XPS to further investigate the electronic states of the surface metal species with the results shown in Figure 3 and Table 3. Figure 3(a) displays the Cu 2p spectra of the spent catalysts. All catalysts exhibit satellite peaks in the ranges of 940−945 and 960−965 eV, which are characteristic of Cu2+ species, indicating that a portion of CuO remained unreduced. For the CZA sample, the characteristic peaks of Cu 2p3/2 and Cu 2p1/2 appear at 933.1 and 952.9 eV, respectively, which are attributed to Cu+/Cu0 species[26], suggesting the coexistence of multiple copper valence states in the catalyst. Compared with CZA, the Cu 2p characteristic peaks of the 0.1K/CZA and 1K/CZA catalysts shift toward the lower binding energies, indicating alterations in their electronic structures. Concurrently, the proportion of the fitted peaks corresponding to Cu+/Cu0 species progressively decreases with increasing K content, implying that the deposited K species reduces the exposure of reduced Cu species. Given the difficulty in distinguishing Cu+ from Cu0 species based solely on Cu 2p spectra, we therefore employed Cu LMM Auger spectra for further analysis (Figure 3(b)). The Auger peaks of the spent CZA sample at kinetic energies of 916.7, 917.5 and 918.8 eV are assigned to Cu+, Cu2+ and Cu0 species, respectively[27]. Semi-quantitative analysis reveals that the surface Cu species of the spent CZA catalyst consist of 42.4% Cu+, 24.9% Cu2+（=100%−42.4%−32.7%） and 32.7% Cu0.
For the 0.1K/CZA catalyst, the surface Cu species also appear in a mixed state, comprising 31.2% Cu+, 32.6% Cu2+ and 36.2% Cu0. When the K content reaches 1%, the surface Cu species of the sample consist of 11.2% Cu+, 42.9% Cu2+ and 45.9% Cu0. Evidently, the proportion of Cu2+ species rises with the increase of K content, confirming that K inhibits the reduction of CuO, which is consistent with the H2-TPR results. Although the incorporation of K helps increase the proportion of Cu0 and modulates the Cu+/Cu0 ratio, it simultaneously reduces the overall content of the reduced Cu species (Cu0 and Cu+), thereby weakening the hydrogenation capability of the catalyst and ultimately leading to a reduced catalytic activity.
Figure 3(c) shows the Zn 2p spectra of the spent catalysts. The characteristic peaks for Zn 2p3/2 and 2p1/2 of the CZA catalyst are located at 1022.3 and 1045.4 eV, respectively[28]. Notably, the Zn 2p peaks of the 0.1K/CZA and 1K/CZA samples shift toward the lower binding energies with K incorporation, following a trend similar to that observed for the Cu 2p peaks. Since the Zn 2p spectra can not distinguish between Zn2+ and Znδ+ (0 < δ < 2) species, the Zn LMM Auger spectra were used to identify changes in the zinc valence states. As shown in Figure 3(d), the CZA catalyst exhibits the characteristic peaks at kinetic energies of 987.7 and 990.9 eV that are assigned to Zn2+ and Znδ+ species, respectively[29]. Moreover, the Auger peaks of the K-modified CZA catalysts shift toward the higher kinetic energies. The presence of Znδ+ species indicates a strong interaction between Cu and Zn species, which drives the partial reduction and migration of ZnO, resulting in the formation of defective ZnOx species[30]. Consequently, the addition of 1% K decreases the proportion of Znδ+ species from 28.5% to 25.7%, suggesting that the K modifier influences the formation of the active structure.
2.2　Assessment of catalytic performance
The catalytic performance of the CZA catalyst and CZA catalysts with different K loadings was evaluated under identical reaction conditions, with the results presented in Figure 4. The unmodified CZA catalyst exhibits the CO2 conversion of 16.5% and the methanol selectivity of 61.7%. For the K-modified CZA catalysts, both the CO2 conversion and methanol selectivity significantly decrease with increasing K contents, accompanied by an enhanced RWGS side reaction. When the K loading reaches 1%, the CO2 conversion and methanol selectivity drop to 5.6% and 21.4%, respectively. These results demonstrate that the K modification significantly alters the active structure of the CZA catalyst.
2.3　Analysis of pore structure and electron imaging of the catalysts
Figure 5 shows the N2 adsorption-desorption isotherms of the catalysts. All catalysts exhibit type IV isotherms, indicating a mesoporous structure. The presence of an H3-type hysteresis loop in the isotherms suggests the existence of slit-like, irregular pores within the catalysts. Furthermore, the introduction of K species leads to a noticeable reduction in the hysteresis loop, which may be attributed to the partial pore blockage caused by the structural changes of the catalysts. The specific surface areas of the CZA, 0.1K/CZA, 0.5K/CZA and 1K/CZA catalysts were estimated to be 54, 48, 47 and 43 m2/g, respectively.
These results indicate that the addition of K has only a marginal effect on the specific surface area of the catalysts. Therefore, the primary role of K modifier lies in its influence on the active structure rather than on altering the textural properties.
The morphological structures of the spent catalysts were analyzed by TEM imaging. As shown in Figure 6(a), the spent CZA catalyst exhibits lattice fringes with spacings of 2.5, 2.4, 2.3 and 2.1 Å, corresponding to the (101) plane of ZnO, the (111) plane of Cu2O, the (200) plane of CuO, and the (111) plane of metallic Cu, respectively. This suggests that the Cu species in the catalyst were reduced by H2 during the reaction, resulting in the coexistence of multiple valence states. Notably, Cu0 species are mainly dispersed adjacent to ZnO, forming Cu-ZnO interfaces[18]. This, combined with the elemental mapping showing a uniform distribution of Cu, Zn and Al, promotes the formation of the abundant Cu-ZnOx structures[21,30]. This phenomenon can be attributed to the strong interaction between Cu and Zn species. With the incorporation of 0.1% K, distinct Cu species in different valence states and their interfaces with ZnO can still be clearly observed in Figure 6(b). The elemental mapping confirms the highly dispersed distribution of the K species. For the 1K/CZA catalyst, only Cu2O, CuO and ZnO species are observed on the surface (Figure 6(c)). Combined with the aforementioned XRD, H2-TPR and XPS analyses, it can be inferred that the surface Cu0 species are present in a highly dispersed or amorphous state. Additionally, the relatively high amount of K species likely covers some active sites, leading to the lower catalytic activity. While no distinct Al2O3 crystallites are detected in any of the three samples, the uniform distribution of Al in the elemental mapping indicates that Al2O3 exists in a highly dispersed or amorphous form.
2.4　Analysis of catalyst surface adsorption and reactivity
The influence of the modification of K on the CO2 adsorption behavior of the CZA catalyst was investigated by CO2-TPD (Figure 7). All catalysts exhibit similar CO2 desorption peak with a broad desorption peak centered at around 100 °C, corresponding to the weak basic sites[31]. The intensity of the desorption signal decreases significantly with increasing K content, which indicates that the modification of K species masks the original basic sites of the CZA catalyst, thereby weakening the CO2 adsorption and inhibiting the catalytic reaction activity.
To elucidate the reaction mechanism of CO2 hydrogenation to methanol, the formation and evolution of surface species during the reaction were monitored using in situ DRIFTS, as shown in Figure 8.
Figure 8 displays the time-dependent evolution of surface-adsorbed species for the CZA and 1K/CZA samples under 3 MPa and 230 °C. The vibrational band appearing at 1617 cm−1 is assigned to HCO3* species, while the characteristic peaks appearing at approximately 1586, 1390 and 1372 cm−1 correspond to HCOO* species. The intensities of these vibrational bands associated with the active intermediates gradually increase with reaction time[29,32−34]. It is generally accepted that HCO3* transforms into HCOO, which is subsequently hydrogenated to CH3O and finally to methanol[35−36]. As reflected in Figure 9, the intensity of the HCOO* species (~1586 cm−1) increases rapidly during the initial stage of the reaction (0−20 min) and then approaches a steady-state value in the subsequent stage (20−100 min). This behavior may be attributed to either a reduction in the number of available active sites due to the accumulation of adsorbed intermediates or a balance between the formation and consumption rates of the active intermediates, enabling the reaction to proceed steadily. The variation trends in the formation rate of HCOO* are similar for the CZA and 1K/CZA catalysts, suggesting that the difference in their catalytic activities likely stems from the subsequent hydrogenation steps rather than the formation of HCOO*. Based on the above IR results, it can be concluded that both catalysts favor the HCOO-mediated reaction pathway.
3　Conclusions
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This study systematically investigated how structural modifications affect the catalytic performance of CZA catalysts by incorporating varying amounts of the alkali metal K, in an effort to more accurately identify the catalytic functions associated with different valence states of Cu species. By combining multiple characterization techniques, we elucidated the influence of the K promoter on the active structure of the catalyst for CO2 hydrogenation to methanol. CO2-TPD results indicate that K modification covers a significant number of basic sites on the catalyst surface, thereby suppressing the CO2 adsorption capacity. H2-TPR, XPS and HR-TEM analyses reveal that highly dispersed K species on the CZA catalyst surface inhibit the reduction of CuO species, consequently fewer Cu2O active species are generated under reaction conditions in the K-modified CZA catalysts, leading to decreased catalytic activity. However, the K modifier effectively promotes the dispersion of Cu0 species and strengthen their interaction with ZnO, which facilitates the formation of more Cu0-ZnO interfaces. Nevertheless, the increased population of Cu0 species primarily promotes the RWGS side reaction, thereby suppressing methanol formation. According to in situ IR spectroscopy, the CO2 hydrogenation to methanol over the CZA catalyst follows a HCOO*-mediated pathway. This study enhances the understanding of the true active sites of Cu species in methanol synthesis and may provide valuable insights for designing more efficient industrial CZA catalysts.
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doi: 10.1016/S1872-5813(25)60616-5
  • Receive Date：2025-08-29
  • Online Date：2026-09-23
  • Published：2026-04-10
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  • Received：2025-08-29
  • Revised：2025-09-27
  • Accepted：2025-10-27
Affiliations
    1National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, China
    2College of Chemistry, Fuzhou University, Fuzhou 350108, China
    3School of Future Membrane Technology, Fuzhou University, Fuzhou 350108, China
    4Chongqing Vocational Institute of Safety Technology, Chongqing 404020, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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