Rational Precipitant Selection Enables Efficient CO2 Hydrogenation Toward Tailored CH4 and CO Product Selectivity

He, Jiebing; Zhang, Pan; Han, Lupeng

doi:10.3390/catal16020187

Abstract

Direct conversion of CO₂ into fuels or chemicals using solar energy has long been a topic of interest among researchers. However, the severe recombination of photogenerated carriers in photocatalysts often results in low conversion efficiency. To synergistically utilize both the light and thermal energy components of sunlight, this study designed a structurally simple Zn-Co bimetallic catalyst to enhance photocatalytic efficiency while lowering the temperature required for the thermal catalytic reaction. Through screening different precipitants, it was found that the Zn-Co catalyst prepared with NH₃ as the precipitant exhibits outstanding activity for CO₂ hydrogenation to methane at 240 °C, with CH₄ selectivity exceeding 93%. Under the same conditions, the photothermal synergistic effect leads to an approximately 10% increase in turnover frequency (TOF). Structural characterization revealed that the NH₃ precipitant effectively modulates the crystal phase, surface active sites, and electron transfer efficiency of the catalyst, thereby influencing the formation and desorption rates of key intermediates such as CO*, HCOO*, and *OCH₃ during CO₂ hydrogenation. The optimal photothermal synergy observed in this system further highlights the potential of catalyst design for achieving targeted product control.

Keywords:

intermediate regulation; CO₂ hydrogenation; precipitant engineering; directional selectivity; Zn-Co bimetallic catalyst

1. Introduction

The escalating emission of carbon dioxide (CO₂) from the combustion of fossil fuels has become a paramount global challenge, exacerbating the greenhouse effect and threatening ecological balance and energy sustainability [1,2]. Amidst various carbon capture, utilization, and storage (CCUS) technologies, the hydrogenation of CO₂ to value-added fuels (e.g., methane (CH₄) and carbon monoxide (CO)) has emerged as a promising strategy to mitigate CO₂ accumulation while addressing the growing demand for renewable energy [3]. CH₄, as a high-energy-density fuel, can be directly integrated into existing natural gas infrastructure, while CO serves as a critical feedstock for Fischer-Tropsch synthesis to produce liquid hydrocarbons [4,5]. However, the inherent chemical inertness of CO₂, attributed to its stable C=O double bond (bond energy ≈ 750 kJ/mol), necessitates efficient catalytic systems to lower the activation energy barrier and achieve selective conversion under mild conditions [6].

Catalytic CO₂ hydrogenation has been extensively investigated under thermal, photocatalytic, and photothermal catalytic conditions [7]. Traditional thermal catalysis typically operates at high temperatures (300–500 °C) and pressures (4–30 bar), resulting in substantial energy consumption and limited applicability for distributed energy system [7]. Photocatalysis, leveraging solar energy to drive CO₂ reduction at room temperature, is regarded as an eco-friendly alternative [7]. Nevertheless, its practical implementation is hindered by rapid recombination of photo-generated electron-hole (e⁻/h⁺) pairs, low quantum yields, and poor light absorption in the visible and near-infrared (NIR) region [8]. In recent years, photothermal catalysis has emerged as a breakthrough technology that synergistically integrates the advantages of photocatalysis and thermal catalysis [9]. By converting light energy into thermal energy via photothermal materials, photothermal catalysis not only enhances light utilization efficiency but also provides moderate thermal energy to accelerate reactant activation, charge transfer, and mass transport, thereby boosting catalytic activity and product selectivity. This hybrid approach has demonstrated great potential in overcoming the limitations of single catalytic modes, enabling efficient CO₂ hydrogenation under relatively mild conditions.

Transition metal-based catalysts, particularly bimetallic systems, have attracted significant attention for CO₂ hydrogenation due to their adjustable electronic structures, synergistic metal interactions, and cost-effectiveness compared to noble metal catalysts [10]. Among these, zinc-cobalt (Zn-Co) bimetallic catalysts have garnered increasing interest owing to their unique physicochemical properties [4]. Zn, as a relatively inert metal, can modulate the electronic state and dispersion of Co active sites, while Co exhibits excellent catalytic activity for CO₂ activation and H₂ dissociation [4]. The synergistic effect between Zn and Co not only enhances the adsorption and activation of CO₂ but also regulates the reaction pathway, thereby influencing the selectivity toward CH₄ or CO. Previous studies have shown that Zn-Co catalysts can be tailored to favor either the methanation reaction (CO₂ + 4H₂ → CH₄ + 2H₂O) or the reverse water–gas shift (RWGS) reaction (CO₂ + H₂ → CO + H₂O) by adjusting structural parameters such as metal ratio, crystal phase, and surface properties [11]. However, the performance of Zn-Co catalysts is highly dependent on their synthesis method and preparation conditions, which directly affect their structural characteristics and catalytic behavior.

Several studies have demonstrated the potential of bimetallic catalysts and photothermal materials in CO₂ hydrogenation. For example, MXene-derived heterojunctions (e.g., Bi₂WO₆/Ti₃C₂T_x@Ag) have shown enhanced CO₂ reduction activity and tunable CH₄ selectivity due to their excellent photothermal conversion efficiency and efficient charge separation [12]. Similarly, Ni-based catalysts supported on V₂C MXene (Ni@NiO/VC) have exhibited high CO₂ conversion and CH₄ selectivity under photothermal conditions, attributed to the synergistic effect of Ni/NiO and the superior photothermal properties of MXene [13]. These studies highlight the importance of catalyst structure engineering and photothermal material integration in improving CO₂ hydrogenation performance. However, the application of Zn-Co bimetallic catalysts in photothermal CO₂ hydrogenation, especially the effect of precipitants on their photothermal catalytic behavior, has not been fully explored.

In this work, Zn-Co bimetallic catalysts were synthesized via a co-precipitation method using Na₂CO₃, NaOH, and NH₄OH as precipitants, denoted as Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH), respectively. The structural properties of the catalysts were systematically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, hydrogen temperature-programmed reduction (H₂-TPR), CO₂ temperature-programmed desorption (CO₂-TPD), and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). The catalytic performance for CO₂ hydrogenation to CH₄/CO was evaluated under both thermal and photothermal conditions. The objectives of this study are (1) to investigate the effect of precipitants on the microstructure, crystal phase, surface chemical state, and electronic properties of Zn-Co catalysts; (2) to explore the catalytic activity, selectivity, and stability of Zn-Co catalysts under thermal and photothermal conditions; (3) to reveal the structure-performance relationship and the mechanism of CO₂ hydrogenation over Zn-Co catalysts; and (4) to elucidate the synergistic effect of light and heat in photothermal catalytic CO₂ hydrogenation. The findings of this work are expected to provide valuable insights into the rational design of efficient bimetallic catalysts for selective CO₂ hydrogenation and promote the development of photothermal catalysis for sustainable energy production.

2. Results and Discussions

2.1. Catalyst Characterizations

Figure 1 presents the XRD patterns of Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts. All three samples exhibit similar crystalline phases. The characteristic peaks observed at 2θ = 31.77°, 34.42°, 36.25°, 47.54°, 56.60°, 62.86°, 66.38°, 67.96°, and 69.10° correspond to the hexagonal wurtzite structure of ZnO (JCPDS File No. 36-1451). Meanwhile, peaks at 41.68°, 44.76°, 47.57°, and 76.38° can be indexed to the (100), (002), (101), and (110) planes of Co species (JCPDS File No. 05-0727), respectively. Notably, the crystallinity of the catalysts decreases in the order of Zn-Co(Na₂CO3) > Zn-Co(NH₄OH) > Zn-Co(NaOH), which may be attributed to the different basicity of the precipitating agents and could also affect the CO₂ hydrogenation performance [14]. These results indicate that the co-precipitation method successfully yields Zn-Co composite catalysts with well-defined ZnO and Co crystalline features.

Figure 1. XRD Patterns of Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) Catalysts.

To further investigate the microstructure of the catalysts, Figure 2 displays the SEM images of the Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts. As shown in Figure 2a,b, Zn-Co(Na₂CO₃) exhibits a densely packed, fragmented sheet-like structure. Figure 2c,d reveal that Zn-Co(NaOH) displays a densely packed granular morphology composed of fine particles. Meanwhile, Figure 2e,f illustrate that Zn-Co(NH₄OH) adopts a loosely stacked, well-defined sheet-like structure. These observations indicate that the choice of precipitating agent significantly influences the micromorphology of the Zn-Co catalysts.

Figure 2. SEM images of (a,b) Zn-Co(Na₂CO₃), (c,d) Zn-Co(NaOH), and (e,f) Zn-Co(NH₄OH).

Figure 3a illustrates the uniform distribution of Zn, Co, and O elements in Zn-Co(Na₂CO₃), with corresponding weight percentages of 42.4%, 38.7%, and 18.9%, respectively (Figure 3b, Table 1). Figure 3c,d show that the elemental composition of Zn-Co(NaOH) comprises 38.3% Zn, 32.9% Co, and 28.8% O (Table 1). Similarly, Figure 3e,f indicate that Zn-Co(NH₄OH) contains 22.1% Zn, 20.0% Co, and 57.9% O (Table 1). These results demonstrate a progressive increase in oxygen content as the precipitating agent changes from Na₂CO₃ to NaOH, and then to NH₄OH. The variation in active oxygen species associated with this trend may play a significant role in modulating the catalytic activity.

Figure 3. (a,c,e) EDS elemental mapping and (b,d,f) content analysis of (a,b) Zn-Co(Na₂CO₃), (c,d) Zn-Co(NaOH), and (e,f) Zn-Co(NH₄OH).

Table 1. Elemental composition (wt %) of Zn, Co, and O in Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts obtained by EDS mapping analysis.

To further investigate the effect of varying oxygen contents on the bonding characteristics of Zn-Co catalysts, Figure 4 presents their Raman spectra. Characteristic peaks are observed for all three catalysts at 448, 520, 712 and 1051 cm⁻¹, with the peaks at 448 and 1051 cm⁻¹ assigned to the E₂ high vibration mode characteristic and the E₁(LO) mode of ZnO [15,16], corresponding to the Zn-O bond, respectively. In contrast, the peaks at 520 and 712 cm⁻¹ are attributed to the F_2g and A_1g vibration modes of the Co-O bond in cobalt-based oxides [17]. Notably, the characteristic peak corresponding to the F_2g vibration mode undergoes a red shift for Zn-Co(Na₂CO₃) (520 cm⁻¹), Zn-Co(NaOH) (517 cm⁻¹) and Zn-Co(NH₄OH) (516 cm⁻¹). In accordance with Hooke’s law [18], a lower Raman shift for the F_2g vibration mode of the Co-O bond indicates a weaker bond strength, which is likely to induce an elongation of the Co-O bond and thus enhance the catalytic reaction activity.

Figure 4. Raman patterns of Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts.

To further elucidate the effects of variations in oxygen content and Co-O bonding on the valence states of the Zn-Co catalysts, Figure 5 and Table 2 present the Co 2p, O 1s and Zn 2p XPS spectra of the three catalysts. Figure 5a reveals the appearance of characteristic peaks assigned to Co³⁺, Co²⁺ and Co sat. at 780.00, 781.40 and 788.95 eV, respectively. Specifically, the Zn-Co(Na₂CO₃) catalyst has a Co³⁺ content of 29.87% with Co²⁺ accounting for 70.13%, while Zn-Co(NaOH) and Zn-Co(NH₄OH) show Co³⁺ fractions of 51.46% and 50.74%, corresponding to Co²⁺ contents of 48.54% and 49.26%, respectively [19]. These results demonstrate that the use of NaOH and NH₄OH as precipitants favors the formation of a higher abundance of Co²⁺ species. Figure 5b and Table 2 show the characteristic peaks of O_latt, O_def and -OH at 529.60, 531.14 and 532.41 eV, respectively [20]. For Zn-Co(Na₂CO₃), the relative contents of O_latt, O_def and -OH are 31.09%, 53.28% and 15.63%, respectively, whereas Zn-Co(NaOH) shows 21.86% O_latt, 54.83% O_def and 23.31% -OH, and Zn-Co(NH₄OH) displays 30.19% O_latt, 68.02% O_def and only 1.79% -OH. All characteristic peaks of Zn 2p in Figure 5c are attributable to ZnO species [21]. Collectively, these findings indicate that the employment of NaOH and NH₄OH as precipitants yields more Co²⁺ sites with lower valence states and more O_def species, which is more conducive to facilitating the catalytic reaction [22,23].

Figure 5. XPS spectra of (a) Co 2p, (b) O 1s and (c) Zn 2p for the Zn-Co(Na₂CO₃) (blue line in (c)), Zn-Co(NaOH) (red line in (c)) and Zn-Co(NH₄OH) (orange line in (c)) catalysts.

Table 2. Contents of Co²⁺, Co³⁺, O_latt, O_def and -OH over the Zn-Co(Na₂CO₃), Zn-Co(NaOH) and Zn-Co(NH₄OH) catalysts.

2.2. Catalyst Performance

To illustrate the differences in the thermal catalytic and photothermal catalytic performance of Zn-Co catalysts synthesized with different precipitants, Figure 6 presents the catalytic performance for CO₂ hydrogenation to CH₄ and CO tested under dark and light conditions, respectively. Figure 6a shows the TOF of CH₄ formation over the Zn-Co(Na₂CO₃) catalyst: under dark conditions, the catalyst became active at 210 °C with a TOF of 3.35 h⁻¹, reaching 15.61 h⁻¹ at 240 °C; under light conditions, the TOF was 4.88 h⁻¹ at 210 °C and increased to 23.34 h⁻¹ at 240 °C. Figure 6b displays the TOF of CO formation over the Zn-Co(Na₂CO₃) catalyst: under dark conditions, the TOF values were 2.04, 4.91 and 5.55 h⁻¹ at 180, 210 and 240 °C, respectively; under light conditions, the corresponding TOF values were 3.94, 5.19 and 4.59 h⁻¹ at the same temperatures. It can be seen that the TOF of CH₄ over the Zn-Co(Na₂CO₃) catalyst increased with rising temperature, with the photothermal catalytic performance being superior to the thermal catalytic performance. In contrast, the TOF of CO indicated a lower activation temperature for the reaction, yet showed little variation with increasing temperature, and there was no significant difference between the photothermal and thermal catalytic performance for CO formation. Meanwhile, Table 3 and Table 4 show that the total TOF of CO and CH₄ formation via photothermal catalysis (27.93 h⁻¹) and photocatalysis (21.16 h⁻¹) at 240 °C was relatively low, demonstrating that this catalyst lacked efficient CO₂ hydrogenation activity. This phenomenon can be attributed to its low contents of Co²⁺ species and active oxygen species.

Figure 6. TOF values of (a,c,e) CH₄ and (b,d,f) CO over the (a,b) Zn-Co(Na₂CO₃), (c,d) Zn-Co(NaOH) and (e,f) Zn-Co(NH₄OH) catalysts under thermal catalytic and photothermal catalytic conditions.

Table 3. Thermal catalytic TOF of CO and CH₄, total TOF, and CO/CH₄ selectivity over the Zn-Co(Na₂CO₃), Zn-Co(NaOH) and Zn-Co(NH₄OH) catalysts at 240 °C.

Table 4. Photothermal catalytic TOF of CO and CH₄, total TOF, and CO/CH₄ selectivity over the Zn-Co(Na₂CO₃), Zn-Co(NaOH) and Zn-Co(NH₄OH) catalysts at 240 °C.

Figure 6c depicts the TOF for CH₄ formation over the Zn-Co(NaOH) catalyst: under dark conditions, the catalyst-initiated activity at 210 °C with a TOF of 8.41 h⁻¹, which rose to 16.30 h⁻¹ at 240 °C; under light irradiation, the TOF was 6.52 h⁻¹ at 210 °C, peaking at 11.51 h⁻¹ at 240 °C. For CO formation over the same catalyst (Figure 6d), the TOF values under dark conditions were 25.88 h⁻¹ and 84.35 h⁻¹ at 210 °C and 240 °C, respectively; under light conditions, the catalyst exhibited TOF values of 6.38 h⁻¹, 43.49 h⁻¹ and 94.74 h⁻¹ at 180 °C, 210 °C and 240 °C, respectively. These results reveal that the TOF for CH₄ over Zn-Co(NaOH) increased slightly with increasing temperature, and its photothermal catalytic performance for CH₄ production was inferior to the thermal catalytic performance. In contrast, the CO formation reaction showed a reduced activation temperature over this catalyst, with the TOF rising sharply as the temperature increased; notably, the photothermal catalytic performance for CO production was significantly superior to the thermal catalytic performance. Additionally, as shown in Table 3 and Table 4, the total TOF for CO and CH₄ formation over Zn-Co(NaOH) at 240 °C reached 106.26 h⁻¹ under photothermal conditions and 100.65 h⁻¹ under thermal catalytic conditions, which is markedly higher than that over the Zn-Co(Na₂CO₃) catalyst. This confirms the excellent CO₂ hydrogenation performance of Zn-Co(NaOH), with the selectivity for the CO product being far higher than that for CH₄.

Figure 6e illustrates the TOF for CH₄ formation over the Zn-Co(NH₄OH) catalyst: under dark conditions, catalytic activity was initiated at 210 °C with a TOF of 57.02 h⁻¹, increasing substantially to 89.28 h⁻¹ at 240 °C; under light conditions, the TOF stood at 61.19 h⁻¹ at 210 °C and climbed to 101.47 h⁻¹ at 240 °C. For CO formation over this catalyst (Figure 6f), detectable TOF values were only observed at 240 °C, with values of 5.79 h⁻¹ and 5.54 h⁻¹ recorded under dark and light conditions, respectively. These findings indicate that the TOF for CH₄ production over Zn-Co(NH₄OH) increased remarkably with elevating temperature, and its photothermal catalytic performance for CH₄ formation outperformed the thermal catalytic performance. In sharp contrast, the CO formation reaction exhibited a notably high activation temperature of 240 °C, with negligible differences between the photothermal and thermal catalytic performance for CO production. Furthermore, Table 3 and Table 4 reveal that the total TOF for CO and CH₄ formation at 240 °C reached 95.07 h⁻¹ under photothermal conditions and 107.71 h⁻¹ under thermal catalytic conditions over Zn-Co(NH₄OH), which is considerably higher than that over the Zn-Co(Na₂CO₃) catalyst. This verifies the excellent CO₂ hydrogenation performance of the Zn-Co(NH₄OH) catalyst, which exhibited a far higher selectivity for CH₄ over CO, which shows an observation that stands in stark contrast to the catalytic behavior of the Zn-Co(NaOH) catalyst.

Figure 6 illustrates the distinct CO₂ hydrogenation performance toward CH₄/CO over Zn-Co catalysts synthesized with different precipitating agents, along with notable variations in selectivity for CH₄ and CO. As shown in Figure 7, the Zn-Co(Na₂CO₃) catalyst exhibits high CO selectivity at low temperatures, while CH₄ selectivity gradually increases with rising temperature. At 240 °C, the selectivity toward CH₄ under thermal catalytic and photothermal catalytic conditions reaches 73.8% and 83.6%, respectively, whereas CO selectivity is 26.2% and 16.4%, albeit with overall low activity (Table 3 and Table 4). In contrast, Zn-Co(NaOH) demonstrates low CH₄ selectivity under both thermal and photothermal conditions, with CO being the dominant product. At 240 °C, the CH₄ selectivity for thermal and photothermal catalysis is only 16.2% and 10.8%, respectively, while CO selectivity reaches 83.9% and 89.2%. This catalyst exhibits high overall yield, favoring the selective production of CO (Table 3 and Table 4). On the other hand, Zn-Co(NH₄OH) primarily generates CH₄ under both thermal and photothermal environments. At 240 °C, the selectivity toward CH₄ reaches 93.9% and 94.8% for thermal and photothermal catalysis, respectively, with CO selectivity as low as 6.1% and 5.2%. This catalyst shows exceptionally high overall activity, promoting the targeted formation of CH₄ (Table 3 and Table 4).

Figure 7. Selectivity of (a,c,e) CH₄ and (b,d,f) CO over the (a,b) Zn-Co(Na₂CO₃), (c,d) Zn-Co(NaOH) and (e,f) Zn-Co(NH₄OH) catalysts under thermal catalytic and photothermal catalytic conditions.

The Zn-Co(NH₄OH) and Zn-Co(NaOH) catalysts exhibit both high yield and high selectivity for CH₄ and CO under photothermal catalysis at 240 °C. To further investigate their stability, we performed a 10 h stability test. As shown in Figure 8, the TOF of CH₄ for the Zn-Co(NH₄OH) catalyst shows a slight decline within the initial 3 h, after which it stabilizes around 80 h⁻¹. Similarly, the TOF of CO for the Zn-Co(NaOH) catalyst experiences a minor decrease in the first 3 h, then remains steady at approximately 70 h⁻¹. These results indicate that both catalysts possess excellent stability under photothermal catalytic conditions.

Figure 8. Photothermal experiments stability test of TOF of CH₄ for Zn-Co (NH₄OH) and TOF of CO for Zn-Co (NaOH) catalysts.

2.3. Electronic Properties of Catalysts

To investigate the electronic properties of the catalysts, temperature-programmed reduction with hydrogen (H₂-TPR) measurements were conducted on Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts. As illustrated in Figure 9, the Zn-Co(Na₂CO₃) catalyst exhibits only two characteristic peaks at 322 °C and 374 °C, which are assigned to the reduction of Co³⁺ to Co²⁺ [24,25]. For the Zn-Co(NaOH) catalyst, two low-temperature reduction peaks appear at 223 °C and 265 °C (corresponding to the Co³⁺→Co²⁺ reduction), accompanied by a more intense peak at 395 °C attributed to the reduction of Co²⁺ to Co⁰. Meanwhile, the Zn-Co(NH₄OH) catalyst shows two low-temperature reduction peaks at 275 °C and 338 °C (assigned to Co³⁺→Co²⁺) and a prominent peak at 413 °C corresponding to Co²⁺→Co⁰ reduction.

Figure 9. H₂-TPR of Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts.

Comparison of the reduction peak intensities reveals the order: Zn-Co(Na₂CO₃) < Zn-Co(NaOH) < Zn-Co(NH₄OH). Furthermore, the reduction temperatures of Zn-Co(NaOH) and Zn-Co(NH₄OH) are significantly lower than that of Zn-Co(Na₂CO₃). These findings indicate that Zn-Co(NaOH) and Zn-Co(NH₄OH) possess superior reducibility, which contributes to their enhanced catalytic performance in CO₂ hydrogenation.

To investigate the CO₂ adsorption properties of the catalysts, CO₂-TPD measurements were performed on Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts. As shown in Figure 10, Zn-Co(Na₂CO₃) exhibits only two characteristic peaks at 186 and 243 °C, attributed to physiosorbed CO₂ and weakly bound carbonate species, respectively, along with a weak desorption peak at 382 °C corresponding to the decomposition of carbonate species [26]. In contrast, Zn-Co(NaOH) and Zn-Co(NH₄OH) display two low-temperature desorption peaks at 108 and 152 °C, ascribed to physisorbed CO₂, and two further high-temperature desorption peaks at 324 and 365 °C, originating from the decomposition of surface carbonates. Comparison of the desorption peak intensities reveals that the number of basic sites follows the order: Zn-Co(NH₄OH) > Zn-Co(NaOH) > Zn-Co(Na₂CO₃), with the peak intensities of the former two being significantly higher than that of Zn-Co(Na₂CO₃). These results indicate that Zn-Co(NaOH) and Zn-Co(NH4OH) possess superior CO₂ adsorption and activation capabilities.

Figure 10. CO₂-TPD of Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts.

To systematically characterize the electronic properties of the catalysts, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) measurements were performed on Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts. As shown in Figure 11, the absorption peak at 261 nm for all three catalysts is attributed to the oxygen-to-metal (O→M) charge transfer; the absorption peak at 485 nm corresponds to the characteristic transition of octahedral Co^II species; while the absorption peaks at 533 nm, 581 nm, and 635 nm originate from isolated tetrahedral Co^II species, which are assigned to the ⁴A₂→⁴T₁(⁴P), ⁴A₂→⁴T₁(⁴F), and ⁴A₂→⁴T₂ electronic transition processes, respectively [27,28,29]. The DRS data were transformed and analyzed using the Kubelka-Munk function, and the results indicate that the Zn-Co(Na₂CO₃) catalyst exhibits a single direct band gap at 1.76 eV. This feature usually reflects a relatively simple electronic structure that has not been effectively optimized, which is directly associated with its low overall catalytic activity. It is speculated that the underlying reason lies in the low generation efficiency and fast recombination rate of charge carriers under light irradiation, making it difficult to efficiently drive the CO₂ reduction reaction. In contrast, the Zn-Co(NaOH) catalyst shows dual direct band gaps at 1.88 eV and 2.71 eV, while the Zn-Co(NH₄OH) catalyst forms dual direct band gaps at 1.66 eV and 2.46 eV. These results demonstrate the successful construction of tightly bonded heterojunction structures inside both catalysts. Such engineered interfacial structures play a crucial role in the spatial separation of photogenerated electron-hole pairs, thereby explaining the significantly enhanced and stable catalytic activity of the two catalysts in long-term catalytic tests. Notably, the higher-energy band gap of the Zn-Co(NH₄OH) catalyst (~2.46 eV) is narrower than that of the Zn-Co(NaOH) catalyst (~2.71 eV), implying a more negative conduction band position. This electronic structure characteristic endows photogenerated electrons with stronger reducing ability, which is thermodynamically more favorable for driving the multi-electron deep reduction pathway required for CH₄ formation rather than promoting the desorption of CO intermediates, ultimately resulting in superior CH₄ selectivity.

Figure 11. UV-VIS of Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts.

2.4. Study on the Catalytic Mechanism

To gain profound insights into the reaction mechanism of CO₂ hydrogenation over the Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts, temperature-programmed in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) measurements were performed under both dark (thermal catalysis) and light (photothermal catalysis) environments. As illustrated in Figure 12a, under dark conditions, the Zn-Co(Na₂CO₃) catalyst exhibits weak characteristic peaks of -CH₂O, HCO₃²⁻, HCOO⁻, and bidentate carbonate (b-CO₃^2-) species in the wavenumber range of 1200–1800 cm⁻¹. A strong characteristic peak corresponding to CO₂ species is observed in the range of 2000–2500 cm⁻¹, while characteristic peaks of -OH, CH₄, and C-H species are also detected in the range of 2600–4000 cm⁻¹ [30,31,32]. Figure 12b shows that under light irradiation, the intensities of the characteristic peaks for -CH₂O, HCO₃⁻, HCOO⁻, b-CO₃²⁻, and CO₂ over this catalyst are significantly enhanced, with an additional characteristic peak of -OCH₃ species appearing. This indicates that photothermal catalysis is more favorable for CH₄ formation compared to thermal catalysis, which is consistent with the catalytic performance results presented in Figure 6a. However, the overall intensities of the characteristic peaks for all intermediate species over this catalyst are relatively weak in both dark and light environments, which is in line with its low CO₂ hydrogenation activity.

Figure 12. In situ DRIFT spectra of Zn-Co(Na₂CO₃) catalyst for CO₂ hydrogenation under (a) thermal and (b) photothermal conditions during temperature-programmed reaction.

Figure 13a reveals that under dark conditions, the Zn-Co(NaOH) catalyst not only exhibits characteristic peaks of -CH₂O, HCO₃⁻, HCOO⁻, b-CO₃²⁻, -OH, and C-H species but also a distinct desorption peak of CO, with no detectable characteristic peaks of -OCH₃ or CH₄. This suggests that the catalyst prefers to produce CO rather than CH₄ during thermal catalytic CO₂ hydrogenation, which is consistent with the experimental results in Figure 6d. Under light irradiation (Figure 13b), the types of intermediate species on the catalyst surface are the same as those under dark conditions, but the intensity of the CO characteristic peak is further enhanced, which is also consistent with the higher CO selectivity under photothermal catalytic conditions.

Figure 13. In situ DRIFT spectra of Zn-Co(NaOH) catalyst for CO₂ hydrogenation under (a) thermal and (b) photothermal conditions during temperature-programmed reaction.

Figure 14 demonstrates that the Zn-Co(NH₄OH) catalyst exhibits significantly stronger characteristic peaks of -CH₂O, -OCH₃, HCO₃²⁻, HCOO⁻, b-CO₃²⁻, -OH, CH₄, and C-H species in both dark and light environments, while the characteristic peak of CO is extremely weak. This indicates that the catalyst can effectively promote the reaction between CO₂ and H₂ under both thermal and photothermal catalytic conditions. From the distribution of intermediate species, the reaction is more inclined to generate CH₄ rather than CO, which is perfectly consistent with the experimental phenomena in Figure 6e.

Figure 14. In situ DRIFT spectra of Zn-Co(NH₄OH) catalyst for CO₂ hydrogenation under (a) thermal and (b) photothermal conditions during temperature-programmed reaction.

Combining the in situ DRIFTS results from Figure 12, Figure 13 and Figure 14, it can be concluded that: compared with the Zn-Co(NaOH) and Zn-Co(NH₄OH) catalysts, the characteristic peaks of various intermediate species on the Zn-Co(Na₂CO₃) catalyst are significantly weaker. This is mutually corroborated by the low intensities of the desorption peaks for CO₂ and carbonate species in the CO₂-TPD test, which is also the core reason for its low CH₄/CO yields. In contrast, the characteristic peaks of intermediate species on both Zn-Co(NaOH) and Zn-Co(NH₄OH) catalysts are significantly enhanced, but there are differences in product selectivity between the two. For the Zn-Co(NaOH) catalyst, the characteristic peaks of -OCH₃ and CH₄ are weak while the CO desorption peak is stronger, which is the key to its high CO yield and selectivity. For the Zn-Co(NH₄OH) catalyst, no obvious CO characteristic peak is detected, and the intensities of the C-H, -OCH₃, and CH₄ characteristic peaks are higher, thus facilitating the selective formation of CH₄.

The Zn-Co(NaOH) and Zn-Co(NH₄OH) catalysts exhibit distinctly different selectivity in the CO₂ hydrogenation reaction. Zn-Co(NaOH) predominantly produces CO with a selectivity of 89.2%, whereas Zn-Co(NH₄OH) mainly generates CH₄ with a selectivity of 94.8%. This difference stems primarily from the profound influence of precipitants on the catalyst structure, electronic properties, and the regulatory mechanism of reaction intermediates. XPS analysis reveals that both precipitants can increase the contents of Co²⁺ species and oxygen vacancies, which is conducive to the adsorption and activation of CO₂. Among them, Zn-Co(NH₄OH) possesses the highest oxygen vacancy content. The CO₂-TPD results also confirm the excellent CO₂ adsorption capacities of the two catalysts. The key lies in the difference in electronic structure revealed by UV-Vis DRS analysis, the narrower band gap and more negative conduction band position of the Zn-Co(NH₄OH) catalyst endow the photogenerated electrons with stronger reducibility, which thermodynamically favors the multi-electron deep reduction pathway required for CH₄ formation. In situ DRIFTS results further verify that the Zn-Co(NaOH) catalyst inhibits the formation of -OCH₃ and CH₄ by promoting the desorption of CO intermediates, leading to high CO selectivity. In contrast, the Zn-Co(NH₄OH) catalyst enhances the signals of intermediates such as -CH₂O, -OCH₃, CH₄, and C-H bonds, while the characteristic peak of CO is extremely weak, thereby facilitating the selective generation of CH₄.

Therefore, the selection of precipitants ultimately determines the selectivity of CH₄ and CO products in the CO₂ hydrogenation reaction by regulating the electronic structure of the catalyst and influencing the generation and desorption rates of key reaction intermediates.

3. Experimental Section

3.1. Catalyst Synthesis

All catalysts were prepared via a co-precipitation method. In a typical synthesis, 1 g of Zn(NO₃)₂ and 1 g of Co(NO₃)₂ were dispersed in 20 mL of deionized water under ultrasonication to obtain solution A. Separately, 1.23 g of Na₂CO₃, 1 g of NaOH, or 5 mL of NH₄OH were dissolved in 20 mL of deionized water to prepare three different precipitating solutions (solution B). Each solution B was then added dropwise into three parallel batches of solution A under continuous stirring. The resulting solid products were collected by centrifugation, washed thoroughly with deionized water and absolute ethanol, and dried overnight under vacuum at room temperature. Finally, the samples were calcined at 400 °C for 3 h with a heating rate of 2 °C/min to obtain ZnO/Co catalysts, designated as Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH), respectively.

3.2. Catalyst Characterization

X-ray diffraction (XRD) patterns were collected on a D/MAX-RB diffractometer (Rigaku, Tokyo, Japan) with Cu-Kα radiation (40 kV, 40 mA), scanning a 2θ range from 5 ° to 80 ° at a rate of 8 °/min. UV-Vis spectra were recorded using an Agilent Cary 5000 spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). H₂ temperature-programmed reduction (H₂-TPR) experiments were conducted on a Micromeritics AutoChem 2920 II analyzer (Norcross, GA, USA) equipped with a thermal conductivity detector (TCD). CO₂ temperature-programmed desorption (CO₂-TPD) was performed on the same apparatus, with the signal monitored in real-time. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a R3000 spectrometer (VG SCIENTA, Uppsala, Sweden) with Al Kα radiation, and binding energies were calibrated using the C 1s peak of adventitious carbon at 284.8 eV. Scanning electron microscopy (SEM) images were acquired on a ZEISS SIGMA-300 field-emission scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany). Raman spectra were collected using a LabRAM HR Evolution spectrometer (Horiba, Paris, France). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies were performed on a Thermo Fisher Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA).

3.3. Catalytic Performance Evaluation

In a typical batch-mode test, 100 mg of catalyst was uniformly dispersed on a quartz fiber filter to form a thin film with an irradiation area of approximately 3.14 cm². The catalyst-loaded film was placed at the center of the reactor bottom. After sealing, the reactor was purged with the reactant gas mixture (CO₂/H₂/N₂ = 3:9:1). A 300 W xenon lamp with an emission wavelength of 300–800 nm was used as the photocatalytic light source for the reaction. The reaction was conducted under 2.8 W cm⁻² light irradiation at 0.2 MPa for 2 h. The temperature of the catalyst bed was monitored using thermocouples during the reaction. After completion, the reactor was heated to 220 °C to vaporize liquid products, which were then transferred via a 200 °C heated line to a gas chromatograph (Panna 91 Plus) equipped with TCD and FID detectors for analysis. Reference dark-reaction experiments were performed under identical conditions but without light irradiation.

According to the XPS elemental analysis, the Co contents of Zn-Co(Na₂CO₃), Zn-Co(NaOH), and Zn-Co(NH₄OH) catalysts are 25.12%, 24.43%, and 25.10%, respectively. It can be seen that there is almost no difference in Co content among the three catalysts. Therefore, the number of Co active sites N(Co) can be calculated based on the catalyst dosage and Co content. Turnover frequency (TOF) and selectivity were calculated using the following equations:

TOF = \frac{Reaction rate per unit time}{Number of active sites}

(1)

S e l e c t i v i t y = \frac{T O F o f a s p e c i f i c p r o d u c t}{S u m o f T O F s f o r a l l p r o d u c t s}

(2)

4. Conclusions

This study systematically investigates the thermal and photothermal catalytic CO₂ hydrogenation performance of Zn-Co bimetallic catalysts synthesized via co-precipitation with Na₂CO₃, NaOH, and NH₄OH as precipitants. The results demonstrate that precipitant engineering is a pivotal strategy to tailor the catalytic behavior, with product yield and selectivity strongly dependent on the precipitant type. Zn-Co(Na₂CO₃) exhibits inferior activity in both catalytic modes, attributed to its low Co²⁺ and defect oxygen contents, weak CO₂ adsorption/activation capability, and inefficient charge separation. In contrast, Zn-Co(NaOH) achieves excellent CO yield and ultra-high selectivity (up to 89.2% under photothermal conditions), benefiting from enhanced CO desorption peaks and weak -OCH₃/CH₄ reaction intermediate species, which favor the reverse water–gas shift pathway. Zn-Co(NH₄OH) stands out with outstanding CH₄ turnover frequency and selectivity exceeding 93%, enabled by strengthened C-H/-OCH₃/CH₄ intermediate signals, negligible CO formation, and favorable photothermal conversion efficiency. Structural characterizations confirm that precipitants regulate the catalyst’s crystal phase, surface active sites, and electron transfer efficiency, thereby modulating the generation and desorption of key intermediates (e.g., CO*, HCOO⁻, -OCH₃) and directing product formation. This work highlights the critical role of precipitant selection in optimizing Zn-Co catalysts for selective CO₂ hydrogenation, offering a feasible and cost-effective approach to design efficient catalysts tailored for targeted CH₄ or CO production, which holds significant implications for advancing sustainable energy conversion technologies.

Author Contributions

Conceptualization, J.H., P.Z. and L.H.; methodology, J.H.; software, J.H.; validation, J.H. and L.H.; formal analysis, J.H.; data curation, J.H.; writing—original draft preparation, J.H.; writing—review and editing, J.H. and L.H.; supervision, J.H. and L.H.; project administration, J.H.; funding acquisition, J.H., P.Z. and L.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the China Postdoctoral Science Foundation (2024M752806), the “Tianchi Talents” Introduction Plan and the Natural Science Foundation of Xinjiang Uygur Autonomous Region (2023D01C170).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors acknowledge the support provided by the Carbon Neutrality Innovation Research Center of Shanghai University.

Conflicts of Interest

The authors declare no conflicts of interest.

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