The crystal facet effect of ZnAl2O4 in the CO2 hydrogenation to methanol

Zinc aluminate (ZnAl₂O₄) is one of the most widely used catalysts in the hydrogenation of carbon dioxide to methanol. During the CO₂ hydrogenation reaction, ZnAl₂O₄ undergoes surface reconstruction to form ZnO, creating a ZnO/ZnAl₂O₄ active interface that promotes methanol production. However, the active crystal facets on which this surface reconstruction occurs, as well as the intrinsic and extrinsic factors influencing the reconstruction process, remain unclear, posing challenges to understanding the structure of the real active sites and the structure-activity relationship. In this work, ZnAl₂O₄ with three morphologies—granular, rod-like and plate-like—were synthesized, primarily exposing the (222), (311) and (440) crystal facets, respectively. The granular ZnAl₂O₄ exhibited superior methanol synthesis performance compared to the rod-like and plate-like morphologies. This enhancement is attributed to the reaction-induced formation of highly active ZnO predominantly exposing the (002) facet on the surface of granular ZnAl₂O₄. Furthermore, the intrinsic and extrinsic factors affecting the surface reconstruction process were investigated. Increasing the reaction temperature, the gas hourly space velocity (GHSV) and the H₂/CO₂ ratio were found to promote the surface reconstruction rate and enhance the steady-state space-time yield (STY) of oxygenates (Oxy). The granular ZnAl₂O₄, with its (222) facet featuring a spatial hexagonal arrangement of adjacent Zn atoms, exhibits stronger H₂ activation capability, thereby promoting the surface reconstruction of active ZnO. These findings provide important guidance for the design and synthesis of highly efficient zinc-based oxide catalysts.

1 Introduction

In the current era, the world is facing severe challenges related to the greenhouse effect and energy shortages. (Mondal and Yadav, 2021; Jiutian et al., 2022; Ye et al., 2025; Gui et al., 2021). The efficient catalytic conversion of CO₂ into methanol, followed by its further transformation into high-value-added chemicals, has been regarded as a vital approach to reducing carbon emissions and promoting the efficient recycling of carbon resources. (Lim and Zeng, 2023; Ye et al., 2019; Brunetti et al., 2021; Sen et al., 2022; Zhou et al., 2024).

Zinc aluminate spinel oxide (ZnAl₂O₄) has attracted wide attention for its application in CO₂ catalytic hydrogenation to methanol. (Liu et al., 2020; Han et al., 2023; Song et al., 2023; 2025; Wang et al., 2023). Zhang et al. found that ZnAl₂O₄ undergoes surface reconstruction during CO₂ hydrogenation, with the formation of amorphous ZnO promoting H₂ activation and methanol formation. (Zhang et al., 2021). Wang et al. prepared two ZnAl₂O₄ catalysts using co-precipitation (ZnAl-C) and hydrothermal (ZnAl-H) methods. (Wang et al., 2025). They found that under similar conditions, the selectivity towards methanol and dimethyl ether for ZnAl-C was significantly higher than that for ZnAl-H. The high performance of the ZnAl-C catalyst was attributed to the abundant surface ZnO species and the interaction between these ZnO species and ZnAl₂O₄ promoted the formation of oxygen vacancies, facilitating CO₂ adsorption and activation. Surface ZnO favored the formation of highly active formate species, which were further hydrogenated to methanol and dimethyl ether. Wang et al. pointed out that the ZnO/ZnAl₂O₄ interface formed after reconstruction is the key site for methanol generation. (Wang et al., 2024). Although some foundational research exists on ZnAl₂O₄ for CO₂ hydrogenation to methanol, the specific active crystal facets involved in the surface reconstruction process of ZnAl₂O₄, as well as the key factors influencing this reconstruction process, remain unclear. This lack of clarity poses challenges for understanding the structure of the active sites and the structure-activity relationships.

In this work, three ZnAl₂O₄ samples with distinct morphologies, primarily exposing different crystal facets, were synthesized via different methods. The differences in their catalytic structure and performance in CO₂ hydrogenation to methanol were investigated to elucidate the crystal facet effect of ZnAl₂O₄ in this reaction. Furthermore, internal and external factors influencing surface reconstruction were investigated, identifying the reaction temperature, the feed gas space velocity (GHSV) and the H₂/CO₂ ratio as external factors and the atomic arrangement of the crystal facets as an internal factor, all of which affect the surface reconstruction rate and the catalytic performance. By revealing the differences in the catalytic performance and the process of surface reconstruction for different ZnAl₂O₄ crystal facets in CO₂ hydrogenation to methanol, this study clarifies the synergistic mechanism between the crystal facet effect and reaction conditions on the catalytic performances, which provides an important theoretical basis for the rational design of high-efficiency catalysts and the optimization of catalytic processes.

2 Experimental section

2.1 Synthesis of ZnAl₂O₄

ZnAl₂O₄ with three distinct morphologies (granular, plate-like and rod-like) was synthesized using different methods. (Zhu et al., 2013; Davis and Occelli, 2016; Zhang et al., 2021). The details are shown in the Supplementary Material.

2.2 Evaluation of the catalytic performance

The CO₂ hydrogenation to methanol was carried out in a fixed-bed reactor. The products were analyzed using an online gas chromatograph (GC) (Agilent 7890B). The details are shown in the Supplementary Material.

2.3 Catalyst characterization

The catalysts were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H₂-temperature program reduction (H₂-TPR), CO₂-temperature program desorption (CO₂-TPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. The operation details are shown in the Supplementary Material.

3 Results and discussion

The morphologies of ZnAl₂O₄ synthesized by the three methods were examined using transmission electron microscopy. As shown in Figure 1 and Table 1, plate-like, rod-like and granular ZnAl₂O₄ were successfully obtained. The plate-like ZnAl₂O₄ (denoted as p-ZA) exhibited lattice fringes with a spacing of 0.143 nm, corresponding to the (440) plane of ZnAl₂O₄. The fast Fourier transform (FFT) pattern further confirmed the presence of diffraction spots indexed to the (440) and (220) planes. The rod-like ZnAl₂O₄ (denoted as r-ZA) showed lattice fringes with a spacing of 0.243 nm, assigned to the (311) plane, which was also supported by its FFT pattern. The granular ZnAl₂O₄ (denoted as k-ZA) displayed lattice fringes with a spacing of 0.235 nm, corresponding to the (222) plane. This particular plane has reported in the literature as a polar facet prone to surface reconstruction. For instance, Wan et al. reported that when ZnAl₂O₄ supported Pd nanoparticles, the interaction between the ZnAl₂O₄ (222) plane and Pd promoted the formation of a Zn-rich surface and PdZn alloy. (Liu et al., 2021; Zhong et al., 2023). The exposure of distinct crystal facets—rod-like, plate-like and nanocrystalline—in ZnAl₂O₄ results in different atomic arrangements, which in turn lead to variations in their catalytic performance for CO₂ hydrogenation to methanol.

Figure 1

Figure 1. Transmission electron microscope images of ZnAl₂O₄ with different morphologies. (a1-a6) p-ZA, (b1-b6) r-ZA, (c1-c6) k-ZA, red is O atom, gray is Zn atom, and pink is Al atom.

Table 1

Table 1. Specific surface area and catalytic performance of ZnAl₂O₄ with different morphologies.

3.1 CO₂ hydrogenation performance of ZnAl₂O₄ with different morphologies

Three ZnAl₂O₄ samples with distinct morphologies were evaluated for CO₂ hydrogenation to methanol and their catalytic performances are shown in Figure 2. Significant differences in the catalytic performance were observed among the three morphologies. The granular ZnAl₂O₄ (k-ZA) exhibited a reaction induction period of approximately 10 h. During this period, the product distribution changed markedly: methanol selectivity increased from 0% to 45%, accompanied by the formation of 7% dimethyl ether (CH₃OCH₃). The dimethyl ether likely originates from the dehydration of methanol on either the ZnAl₂O₄ surface or a reconstructed Al₂O₃ surface. (Renfeng et al., 2011). As the reaction proceeded, CO selectivity decreased from 100% to 47%, while CO₂ conversion gradually increased to 6%. In contrast, neither the rod-like (r-ZA) nor the plate-like (p-ZA) ZnAl₂O₄ exhibited an induction period. Their CO₂ conversions remained low at around 1.7%, with CO selectivity being nearly 100%. A further comparison of methanol formation rates revealed that the space-time yield (STY) of methanol over granular ZnAl₂O₄ increased continuously during the reaction, rising from the initial value to 0.42 mmol g_cat^-1 h^-1 after 10 h. In comparison, the STY of methanol for the rod-like and plate-like samples remained nearly zero. It is inferred that the structure of granular ZnAl₂O₄ underwent changes during the reaction, whereas the rod-like and plate-like catalysts retained their initial structures after the reaction.

Figure 2

Figure 2. Hydrogenation performance of CO₂ catalyzed by ZnAl₂O₄ with different morphologies. (a) CO₂ conversion (b) CH₃OH selectivity (c) CO selectivity (d) CH₃OH STY, reaction condition: m = 1.0 g, H₂:CO₂:N₂ = 3:1:1, P = 3 MPa, WHSV = 1.8 L g^-1 h^-1, T = 280°C, heating up to reaction temperature in N₂ flow.

3.2 Structural characterization of ZnAl₂O₄ with different morphologies

Figures 3a,b present the X-ray diffraction (XRD) patterns of the three ZnAl₂O₄ morphologies. The results indicate that all three samples exist as pure phases without detectable ZnO diffraction peaks. The characteristic diffraction peaks observed at 31.2°, 36.8°, 44.7°, 55.6°, 59.3° and 65.2° can be indexed to the (220), (311), (400), (422), (511) and (440) crystal planes, respectively. (Zawadzki, 2006). The compositional differences among the three morphologies of ZnAl₂O₄ were compared using XRF characterization. The results show that the Al/Zn atomic molar ratios for all three are approximately 2. This indicates that all three are pure-phase ZnAl₂O₄ spinel structures. Compared with the fresh catalysts, the spent catalysts of all three morphologies show no significant changes in their XRD patterns. This suggests that the reconstruction of granular ZnAl₂O₄ is likely confined to the surface and does not alter the bulk structure. Combined with the catalytic performance of the rod-like and plate-like samples, it is inferred that both their bulk and surface structures remain unchanged. Figure 3c shows that the N₂ adsorption-desorption isotherms of all catalysts can be classified as Type IV, indicating the presence of mesopores formed by the stacking of particles.

Figure 3

Figure 3. X-ray diffraction patterns of ZnAl₂O₄ with different morphologies. (a) fresh catalyst (b) used catalyst and (c) isothermal adsorption curve.

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to compare the surface structures of the three ZnAl₂O₄ morphologies before and after the reaction. A gas mixture of CO₂ and N₂ (CO₂:N₂ = 1:19 mL/min) was introduced at 300 °C to investigate the changes in the surface structure. Characteristic absorption peaks near 1,339 cm^-1, corresponding to carbonate species adsorbed on ZnO, were monitored to detect the possible formation of surface ZnO. (Rethwisch and Dumesic, 1986; Zhang et al., 2021). As shown in Figure 4a, none of the three fresh catalysts exhibited the absorption peak near 1,339 cm^-1, indicating the absence of surface ZnO. In contrast, Figure 4b reveals that the spent granular ZnAl₂O₄ sample showed a characteristic absorption band at approximately 1,330 cm^-1, which is attributed to carbonates adsorbed on ZnO. This confirms the formation of ZnO on its surface. No such absorption peak was observed for the spent rod-like or plate-like ZnAl₂O₄, indicating that their spinel surface structures remained intact.

Figure 4

Figure 4. In situ DRIFTS of ZnAl₂O₄ with different morphologies (a) fresh catalyst (b) used catalyst, test condition:T = 300°C, CO₂:N₂ = 1:19 mL/min.

In situ DRIFTS was used to monitor the evolution of surface species on granular ZnAl₂O₄ under reaction conditions, as shown in Figure 5. After introducing the reaction gases, the fresh catalyst exhibited the gradual emergence of absorption peaks at 1,370 cm^-1, 2,878 cm^-1, 2,900 cm^-1, 2,960 cm^-1 and 2,993 cm^-1, which are attributed to surface formate species. (Tada et al., 2022; Feng et al., 2023). Peaks associated with adsorbed CO were also observed at 2,110 cm^-1 and 2,170 cm^-1. (Yang et al., 2023). However, no absorption peaks corresponding to methoxy species were detected. In contrast, the spent granular ZnAl₂O₄ catalyst displayed characteristic absorption peaks at 1,093 cm^-1 and 2,943 cm^-1, which are assigned to surface methoxy species. (Wang et al., 2017; Sun et al., 2024). According to literature results, CO₂ hydrogenation to methanol generally follows the formate pathway: CO₂ is first hydrogenated to formate (HCOO*), then successively to dioxymethylene (H₂COO*), formaldehyde (H₂CO*) and methoxy (H₃CO*), with methoxy finally hydrogenated to produce methanol. (Bowker et al., 1988; Hu et al., 2024). Among these steps, the hydrogenation of the HCOO* and H₃CO* intermediates is relatively slow and is generally considered the rate-determining step(s) for methanol synthesis, leading to their accumulation on the catalyst surface. (Meunier et al., 2023; Lin and Bhan, 2024). Based on these experimental results and literature reports, it can be concluded that the fresh granular ZnAl₂O₄ catalyst possesses a weak ability to activate H₂, preventing the further conversion of surface formate species and resulting in no methanol formation. After reaction, the formation of ZnO on the surface of granular ZnAl₂O₄ enhances its H₂ activation capability, enabling the conversion of CO₂ to methanol. These DRIFTS findings are consistent with the observed reaction performance, where the methanol formation rate was initially zero but increased steadily over time. Identical in situ DRIFTS experiments were conducted on the plate-like and rod-like ZnAl₂O₄ samples. The results in Figure 6 show that no methoxy absorption peaks were detected on these catalysts after reaction, indicating that no surface reconstruction to form ZnO occurred.

Figure 5

Figure 5. In situ DRIFTS of granular ZnAl₂O₄ (a) fresh catalyst, (b) used catalyst and (c) partial magnification of figure (b).

Figure 6

Figure 6. In situ DRIFTS of used ZnAl₂O₄ (a) p-ZA (b) r-ZA.

3.3 The comparison between granular ZnAl₂O₄ and plate-like ZnO

Zhou et al. investigated the effect of ZnO morphology on its performance in CO₂ hydrogenation to methanol. (Zhou et al., 2023). They found that plate-like ZnO exhibited CO₂ conversion and CH₃OH formation rates 3.8 and 2.3 times higher than those of rod-like ZnO, respectively. The plate-like ZnO demonstrated superior H₂ activation capability and CO₂ adsorption capacity compared to the rod-like morphology. The predominantly exposed (002) facet of the plate-like ZnO was identified as more favorable for CH₃OH formation. A comparison was made between the reaction performance of granular ZnAl₂O₄ and plate-like ZnO. As shown in Figures 7a,b,d, under identical reaction temperatures, the granular ZnAl₂O₄ achieved both higher CO₂ conversion and higher space-time yield of oxygenates (methanol + dimethyl ether, Oxy) than the plate-like ZnO. Specifically, the CO₂ conversion over granular ZnAl₂O₄ was approximately 1.7 times that over plate-like ZnO, while the oxygenate yield was 3 times higher. Furthermore, at similar CO₂ conversion levels, the selectivity toward oxygenates was significantly greater on granular ZnAl₂O₄ than on plate-like ZnO. Figure 7c shows that the apparent activation energy for total oxygenate formation on granular ZnAl₂O₄ was 38 kJ/mol, which is considerably lower than that for rod-like ZnO (54 kJ/mol) and close to the activation energy for methanol formation on plate-like ZnO (32 kJ/mol). Based on these results, it is concluded that the reaction induces the formation of highly active ZnO on the surface of granular ZnAl₂O₄. This generated ZnO likely exposes a high proportion of the (002) facet, thereby promoting the CO₂ hydrogenation to methanol.

Figure 7

Figure 7. Comparison of hydrogenation of CO₂ catalyzed by granular ZnAl₂O₄ and plate ZnO. (a) CO₂ conversion (b) oxygen-containing products yield (c) formation activation energy of oxygen-containing products (d) selectivity of oxygen-containing products under different CO₂ conversion, reaction condition: m = 1.0 g, H₂: CO₂:N₂ = 3:1:1, P = 3 MPa, WHSV = 1.8 L g_cat^-1 h^-1, granular ZnAl₂O₄ data are taken from steady state.

3.4 Effect of reaction temperature on the surface reconstruction

The influence of the reaction temperature on the surface reconstruction process was investigated. As shown in Figures 8a–c, increasing the reaction temperature led to higher CO₂ conversion, lower oxygenate selectivity and higher CO selectivity. Furthermore, the time required for the CO and oxygenate formation to reach a steady state decreased with increasing temperature (Figure 8d).

Figure 8

Figure 8. Hydrogenation of CO₂ catalyzed by granular ZnAl₂O₄ at different temperatures. (a) CO₂ conversion (b) oxygen-containing products selectivity (c) CO selectivity (d) formation rate of oxygen-containing products, reaction condition:m = 1.0 g, H₂:CO₂:N₂ = 3:1:1, P = 3 MPa, WHSV = 1.8 L g_cat^-1 h^-1, T = 280 °C–310 °C, heating up to reaction temperature in N₂ flow.

Figure 9a shows that the duration of the reaction induction period shortened from approximately 10 h at 280 °C to about 2 h at 310 °C. These observations collectively indicate that higher temperatures promote the surface reconstruction process. The slope of the tangent line at the initial point of the oxygenate formation rate curve was determined. As shown in Figure 9b, the slope of this tangent increased continuously with temperature, further confirming that the reconstruction rate accelerates at higher temperatures. The apparent activation energy for the surface reconstruction of granular ZnAl₂O₄ was determined from an Arrhenius plot of the logarithmic tangent slopes versus the inverse temperature. The result, presented in Figure 9c, yields an activation energy of 171 kJ/mol. This high value explains why the reconstruction is difficult to initiate and support the conclusion that the process is confined to the surface, while the bulk retains the spinel structure. Thermodynamic analysis of the phase separation of ZnAl₂O₄ into ZnO and Al₂O₃ over a temperature range of 100 °C–1,000 °C is shown in Figure 9d. The calculated standard Gibbs free energy change (ΔG) is approximately 40 kJ/mol, indicating that bulk phase separation is thermodynamically unfavorable.

Figure 9

Figure 9. Effect of reaction temperature on the surface reconstruction of granular ZnAl₂O₄. (a) Reaction induction time (b) slope of formation rate of oxygen-containing products. (c) Activation energy of surface reconstruction of the granular ZnAl₂O₄ at different temperatures, (d) the Δ_rG^Θ value of ZnAl₂O₄ decomposition at different temperatures.

The reaction temperature also affected the steady-state catalytic performance of ZnAl₂O₄ in CO₂ hydrogenation. As shown in Figure 10, both the CO₂ conversion and the yield of oxygenates increased with rising temperatures. However, the product distribution showed different trends: CO selectivity increased continuously, while oxygenate selectivity decreased. This is because the reverse water-gas shift (RWGS) reaction is endothermic and thus favored at higher temperatures. The increase in overall CO₂ conversion and oxygenate yield at elevated temperatures is attributed to the general promotion of the reaction kinetics.

Figure 10

Figure 10. Catalytic performance of granular ZnAl₂O₄ for CO₂ hydrogenation at different temperatures. (a) CO₂ conversion (b) oxygen-containing products selectivity (c) CO selectivity (d) formation rate of oxygen-containing products, reaction condition: m = 1.0 g, H₂:CO₂:N₂ = 3:1:1, P = 3 MPa, WHSV = 1.8 L g_cat^-1 h^-1, T = 280 °C–310 °C, data acquisition in steady state.

3.5 Effect of gas hourly space velocity on surface reconstruction

The influence of gas hourly space velocity (GHSV) on the surface reconstruction of ZnAl₂O₄ was subsequently investigated. As shown in Figure 11, when the GHSV was increased from 1.8 to 18 L g_cat^-1 h^-1, the CO₂ conversion, product selectivity and the time required for the oxygenate formation rate to reach steady state were significantly shortened—from an initial 10 h–2 h. This indicates that a higher GHSV promotes the surface reconstruction process. The initial slope of the tangent to the oxygenate formation rate curve increased with rising GHSV, demonstrating that the surface reconstruction rate is accelerated at higher space velocities. The nature of the reconstruction is a chemical reaction between ZnAl₂O₄, H₂ and CO₂. Changes in reactant concentration inevitably affect the reconstruction rate. The increase in the GHSV raises the local concentration of CO₂ and H₂ per unit surface area, thereby enhancing the reaction rate and consequently accelerating the surface reconstruction.

Figure 11

Figure 11. Hydrogenation of CO₂ catalyzed by particle ZnAl₂O₄ at different gas space velocity. (a) CO₂ conversion (b) oxygen-containing products selectivity (c) CO selectivity (d) formation rate of oxygen-containing products (e) reaction induction time (f) slope of formation rate of oxygen-containing products, reaction condition: m = 1.0 g, H₂:CO₂:N₂ = 3:1:1, P = 3 MPa, WHSV = 1.8–18 L g_cat^-1 h^-1, T = 280 °C, heating up to reaction temperature in N₂ flow.

Variations in GHSV also influence the steady-state performance of ZnAl₂O₄ in CO₂ hydrogenation. As shown in Figure 12, an increase in GHSV led to decreased CO₂ conversion and CO selectivity, while both the selectivity and yield of oxygenates increased. This is attributed to the reduced residence time of the reactant gases on the catalyst surface at higher space velocities, which lowers the proportion of CO₂ molecules undergoing reaction and thus decreases CO₂ conversion. Simultaneously, a higher GHSV can suppress methanol decomposition, resulting in lower CO selectivity.

Figure 12

Figure 12. Catalytic performance of granular ZnAl₂O₄ for CO₂ hydrogenation at different gas space velocity. (a) CO₂ conversion (b) oxygen-containing products selectivity (c) CO selectivity (d) formation rate of oxygen-containing products, reaction condition: m = 1.0 g, H₂:CO₂:N₂ = 3:1:1, P = 3 MPa, WHSV = 1.8–18 L g_cat^-1 h^-1, T = 280 °C, data acquisition in steady state.

3.6 Effect of H₂/CO₂ feed ratio on the surface reconstruction

The influence of the H₂/CO₂ feed ratio on the surface reconstruction was investigated by maintaining a fixed CO₂ flow rate while varying the H₂/CO₂ ratio. As shown in Figure 13, when the H₂/CO₂ volumetric flow ratio was increased from 1:1 to 9:1, the time required for CO₂ conversion, oxygenate selectivity and oxygenate formation rate to reach steady state gradually decreased from 10 h to approximately 5 h. Concurrently, the initial slope of the tangent to the oxygenate formation rate curve increased with a higher H₂/CO₂ ratio, indicating that the surface reconstruction rate accelerates as the ratio rises, indicating that the surface reconstruction is induced by H₂, and increasing the H₂ concentration in the reaction gas promotes the reconstruction process.

Figure 13

Figure 13. Hydrogenation of CO₂ catalyzed by granular ZnAl₂O₄ at different H/C feed ratio. (a) CO₂ conversion (b) oxygen-containing products selectivity (c) CO selectivity (d) formation rate of oxygen-containing products (e) reaction induction time (f) slope of formation rate of oxygen-containing products, reaction condition: m = 1.0 g, H₂:CO₂:N₂ = ^1:1-9:1, P = 3 MPa, WHSV = 1.8 g_cat^-1 h^-1, T = 280 °C, data acquisition in steady state.

Variations in the H₂/CO₂ feed ratio also affected the steady-state reaction performance. As shown in Figure 14, as the H₂/CO₂ volumetric flow ratio increased from 1:1 to 9:1, the CO₂ conversion, oxygenate selectivity and oxygenate yield all exhibited an increasing trend, while CO selectivity gradually decreased. The increased H₂ feed shifts the equilibria of both CO₂ hydrogenation to methanol and the reverse water-gas shift reaction forward, thereby promoting CO₂ conversion. Since methanol formation requires more active hydrogen species, the increased H₂ availability favors methanol production, leading to reduced CO selectivity.

Figure 14

Figure 14. Catalytic performance of granular ZnAl₂O₄ for CO₂ hydrogenation at different H/C feed ratio. (a) CO₂ conversion (b) oxygen-containing products selectivity (c) CO selectivity (d) formation rate of oxygen-containing products, reaction condition: m = 1.0 g, H₂:CO₂:N₂ = ^1:1-9:1, P = 3 MPa, WHSV = 1.8 L g_cat^-1 h^-1, T = 280 °C, data acquisition in steady state.

3.7 Intrinsic factors underlying morphology-dependent surface reconstruction of ZnAl₂O₄

The surface structure of granular ZnAl₂O₄ underwent changes during the reaction, while the rod-like and plate-like ZnAl₂O₄ maintained their original surface structures. To investigate the cause of this differential reconstruction behavior, H₂-TPR experiments were subsequently conducted. As shown in Figure 15, the granular ZnAl₂O₄ exhibited two reduction peaks at 454 °C and 560 °C, whereas no reduction peaks were observed for the rod-like or plate-like ZnAl₂O₄, confirming the high stability of their crystal plane structures. Zhang et al. proposed a mechanism for the surface reconstruction: H₂ undergoes heterolytic cleavage on Zn-O-Al structural units, leading to the breaking of Zn-O bonds and the formation of Al-OH species during the reaction.² Adjacent Al-OH groups then dehydrate to form more stable Al₂O₃, accompanied by the formation of ZnO. (Zhang et al., 2021). It can be concluded that the surface of granular ZnAl₂O₄ is more readily reduced by H₂, allowing for reaction-induced formation of surface ZnO. In contrast, the highly stable crystal plane structures of the rod-like and plate-like ZnAl₂O₄ prevent surface structural changes under the reaction atmosphere.

Figure 15

Figure 15. H₂-TPR diagrams of ZnAl₂O₄ with different morphologies.

The crystal plane structures of ZnAl₂O₄ are shown in Figure 1. In the (311) plane, the Zn atoms are arranged in a triangular pattern; in the (440) plane, they form a rectangular arrangement; and in the (222) plane, the Zn atoms adopt a spatial hexagonal arrangement. By comparing the different crystal planes of ZnAl₂O₄, it is inferred that planes with a hexagonal arrangement of adjacent Zn atoms are more prone to surface reconstruction. Combined with the H₂-TPR results, it is hypothesized that Zn atoms in a spatial hexagonal arrangement possess a stronger ability to activate H₂. The heterolytic cleavage of H₂ between Zn-O bonds, coupled with the breaking of Zn-O bonds in the Zn-O-Al units, weakens the stability of Zn atoms within the lattice, facilitating their migration to the surface to form ZnO.

3.8 Analyzing from the perspectives of thermodynamics and kinetics

It is crucial to analyze the structural evolution of ZnAl₂O₄ from both thermodynamic and kinetic perspectives. As shown in Figure 9d, theoretical calculations indicate that in the temperature range of 100 °C–600 °C, the ΔG for the complete bulk decomposition of ZnAl₂O₄ into ZnO and Al₂O₃ is approximately +40 kJ/mol. This significantly positive value clearly confirms that bulk decomposition is thermodynamically unfeasible. Combined with the in situ DRIFTS results, we infer that the observed structural changes are limited to surface reconstruction, specifically the formation of ZnO on the surface of ZnAl₂O₄. From a kinetic standpoint, increasing the temperature accelerates the reconstruction process by enhancing molecular collision frequency and energy. An increase in gas hourly space velocity may promote this kinetic process by improving mass transfer process. More importantly, raising the H/C feed ratio and the specific crystal planes exposed by the catalyst itself jointly regulate the kinetics of hydrogen activation. Specifically, increasing the H/C ratio enhances the coverage of surface hydrogen species, while the predominantly exposed (222) crystal plane of granular ZnAl₂O₄, which exhibits optimal heterolytic H₂ activation capability, further promotes the cleavage of Zn–O bonds and the formation of ZnO. Based on this, it is easy to understand that Pd loaded on ZnAl₂O₄ promotes the kinetic process of ZnAl₂O₄ surface reconstruction by enhancing hydrogen activation and the spillover process. (Liu et al., 2022). Therefore, these factors primarily influence the rate of ZnAl₂O₄ structural evolution within the kinetic domain, without overcoming the thermodynamic limitation of bulk decomposition. Additionally, changes in these reaction conditions simultaneously exert complex effects on the intrinsic reaction kinetics of CO₂ hydrogenation to methanol, collectively determining the final catalytic performance.

4 Conclusion

In this study, granular, rod-like and plate-like ZnAl₂O₄ with distinct morphologies were synthesized and their differences in catalytic performance and structure for CO₂ hydrogenation to methanol were investigated. The granular, rod-like and plate-like ZnAl₂O₄ facets primarily exposed the (222), (311) and (440) planes, respectively. The reaction induced the formation of highly active ZnO on the surface of granular ZnAl₂O₄, thereby promoting CO₂ hydrogenation to methanol. In contrast, the rod-like and plate-like ZnAl₂O₄ showed no induction period, maintained their spinel surface structure and exhibited nearly 100% CO selectivity. Increasing the reaction temperature, GHSV or H₂/CO₂ ratio effectively shortened the induction period and enhanced the steady-state space-time yield of oxygenates. The spatial hexagonal arrangement of adjacent Zn atoms in the (222) plane of granular ZnAl₂O₄ conferred a stronger ability to activate H₂, facilitating surface reconstruction and the formation of ZnO. These findings provide important guidance for the rational design and synthesis of highly efficient zinc-based oxide catalysts.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

YH: Investigation, Visualization, Writing – original draft. QL: Formal Analysis, Investigation, Methodology, Writing – original draft. ZW: Formal Analysis, Investigation, Writing – original draft. XnZ: Investigation, Methodology, Writing – original draft. XaZ: Investigation, Methodology, Writing – original draft. GZ: Conceptualization, Formal Analysis, Funding acquisition, Writing – review and editing. XG: Conceptualization, Funding acquisition, Methodology, Supervision, Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was financially supported by the National Natural Science Foundation of China (22172013 and 22372022), Special Project for Key Research and Development Program of Xinjiang Autonomous Region (2022B01033-3), the Fundamental Research Funds for the Central Universities (DUT22LAB602), the Liaoning Revitalization Talent Program (XLYC2203126), the CUHK Research Startup Fund (No.#4930981), and the Excellence Co-innovation Program International Exchange Fund Project (Grant number: DUTIO-ZG-202505).

Conflict of interest

Authors YH and QL were employed by Shandong Electric Power Engineering Consulting Institute Co., Ltd.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fceng.2025.1748891/full#supplementary-material

References

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