Ruthenium supported on zirconia–carbon nanocomposites derived by using UiO-66 for efficient photothermal catalytic CO₂ reduction

Abstract

Resource utilization of carbon dioxide (CO₂) is an effective strategy to mitigate global warming and achieve carbon neutrality and peak carbon goals. It is well known that different preparation methods affect the catalytic performance of catalysts. Herein, we designed an efficient Ru–ZrO₂/C catalyst for photothermal catalytic CO₂ reduction by pretreating UiO-66 in a N₂ atmosphere and then loading Ru species. Compared to the reference sample of Ru/ZrO₂ obtained by calcining UiO-66 in an air atmosphere, Ru–ZrO₂/C exhibits much superior catalytic activity under full-spectrum light irradiation with a methane yield of 504.1 mmol g⁻¹ h⁻¹ and selectivity of 98.9%, respectively. In addition, the catalytic performance of Ru–ZrO₂/C for photothermal CO₂ methanation remains stable without obvious reduction in a 24 hour continuous test. The physicochemical characterization studies of Ru–ZrO₂/C determine that the remarkable heat resistance, effective light-heat conversion ability, abundant oxygen vacancies, low-valence Ru, and good CO₂ adsorption properties are responsible for the enhanced performance of photothermal CO₂ hydrogenation. This work expands the application of MOFs as precursors and provides an effective guide for designing highly efficient photothermal catalysts for solar fuel production.

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1 Introduction

In the face of increasingly serious environmental and energy problems, carbon neutrality has become the consensus of the international community.¹ Resource utilization of carbon dioxide (CO₂) could mitigate the adverse effects of global warming and provide potential solutions to the problems of the energy crisis. However, the chemical stability of the CO₂ molecule necessitates high reaction temperatures to break the carbon–oxygen double bond, even with a suitable catalyst. Utilizing renewable solar energy for photothermal CO₂ hydrogenation provides an environment-friendly solution to mitigate climate degradation, produce alternative fuels, and promote CO₂ recycling. In particular, the excellent absorption properties of the photothermal catalyst in the solar spectrum are utilized in the photothermal CO₂ methanation reaction, which can efficiently convert light into heat and provide the same amount of energy as conventional thermal catalysis. The thermal energy can be rapidly transferred to the surface interface and active sites of the photothermal catalyst, promoting charge transfer and the photothermal catalytic reaction rate. The photothermal CO₂ hydrogenation reaction is a complex chemical process due to differences in catalyst properties, reaction temperatures, and pressures, resulting in the hydrogenation of CO₂ to formate (HCOOH),² ethanol,³ methanol,^4,5 methane,^6–8 and lower olefins⁹ in the actual reaction. Among these products, CH₄ is a clean and nontoxic ideal energy fuel with significant practical potential. Therefore, the research of CO₂ methanation has currently become one of the hot spots in catalytic reaction research.^10,11

In recent years, metal–organic frameworks (MOFs) have garnered significant attention due to their unique properties, including controllable chemical structures, uniform pores, and tunable electronic structures.^12–15 In previous literature studies, MOFs were frequently employed in the photocatalytic reduction of CO₂ by exploiting their exceptional light adsorption properties and excellent adsorption of CO₂.^16–18 Besides, carbon material is also an excellent photothermal material support due to its chemical inertness, structural tunability, and excellent full-spectrum absorption characteristics.^19–21 Recent research has explored novel methods for synthesizing metal oxide/porous carbon nanocomposites using MOFs as precursors, opening new avenues for catalyst development.^22,23 Therefore, studies to expand the application of MOFs as a precursor and template for the preparation of metal oxide carbon nanocomposites are carried out. In turn, it is essential to study the heat treatment process of MOFs under an inert atmosphere.²⁴ Apart from this, pyrolysis of MOFs is an effective and simple strategy for dispersing reactive metals. For instance, Ru as an active site can be well atomically dispersed on the surface of zirconia/porous carbon due to the generation of oxygen vacancies in ZrO_x.²⁴ The catalytic performance of a catalyst is influenced not only by the carrier but also by the active metal loaded on the catalyst. Ye et al. report that group VIII metals (Ru, Rh, Ni, Co, Pd, Pt, Ir, and Fe) have excellent photothermal capabilities.²⁵ Moreover, numerous studies have shown that the less expensive noble metal Ru has excellent CO₂ methanation properties.^26–28 Precious metal (e.g., Ru and Rh) nanoparticles significantly increase the local temperature and activate H₂ in photothermal catalysis, which triggers CO₂ (or CO₃²⁻) hydrogenation.^6,25,29,30

Herein, a metallic zirconium metal–organic framework (UiO-66) was used as a precursor for the preparation of zirconia–carbon nanocomposites (ZrO₂/C) by heat treatment under a N₂ atmosphere, while a reference support was prepared under an air atmosphere.²⁴ Then a series of Ru-based catalysts were prepared via the impregnation method. The optimal Ru–ZrO₂/C catalysts exhibit good photothermal performance during the light-driven CO₂ hydrogenation reaction. The r_CH4 of Ru–ZrO₂/C under full-spectrum simulated sunlight conditions reaches 504.1 mmol g⁻¹ h⁻¹ with high selectivity (98.9%). By contrast, under the same reaction condition, the Ru/ZrO₂ catalyst as a reference sample is sluggish. In addition, the activity and selectivity of Ru–ZrO₂/C for photothermal CO₂ methanation remain stable in a long-term test. The physicochemical characterization tests of Ru–ZrO₂/C determine that the remarkable heat resistance, effective light-heat conversion ability, low-valence Ru, and good CO₂ adsorption properties are responsible for the enhanced performance of photothermal CO₂ hydrogenation. This demonstrates that ZrO₂/C as a photothermal support has an important contribution to the catalytic performance of the catalysts. This not only expands the application of MOFs as precursors but also provides new ideas for the construction of efficiently loaded photothermal catalysts.

2 Experimental section

2.1. Materials

All chemicals were purchased from commercial sources without further processing. Zirconium chloride (ZrCl₄) was obtained from Aladdin Reagent. Terephthalic acid (H₂BDC, C₈H₆O₄), N,N-dimethylformamide (DMF, C₃H₇NO), acetic acid (CH₃COOH), ruthenium chloride trihydrate (RuCl₃·3H₂O), and methanol (CH₃OH) were bought from Sinopharm Chemical Reagent. Ultrapure water (Millipore Milli-Q grade) with a resistivity of 18.2 MΩ cm was used for all the experiments.

2.2. Sample preparation

UiO-66 was prepared by the hydrothermal method with slight modifications based on previous studies.^31,32 First, 233.0 mg of ZrCl₄ and 166.1 mg of H₂BDC were dispersed in a Teflon-lined autoclave (100 mL) with 50.0 mL DMF. After stirring for 10 min, 3.6 mL of CH₃COOH was added dropwise and stirring was continued for 30 min to form a homogeneous solution. Next, the autoclave was sealed and placed in an oven at 120 °C for 24 hours. After cooling down to ambient temperature, the precipitate was centrifuged and rinsed with methanol several times and finally dried at 100 °C for 12 hours.

The two supports of zirconia–carbon nanocomposites (ZrO₂/C) and zirconia (ZrO₂) were prepared as follows. UiO-66 powder was annealed in a tube furnace and heated to 400 °C at a rate of 5 °C min⁻¹ for 30 min and then to 600 °C at a rate of 2 °C min⁻¹ for 2 hours in an N₂ atmosphere (100 mL min⁻¹). After cooling to room temperature, the black ZrO₂/C was obtained. The reference support ZrO₂ was obtained via annealing UiO-66 in air.

The Ru-based catalysts were fabricated via the wet impregnation method. The Ru–ZrO₂/C samples were prepared as follows: first, 200 mg of ZrO₂/C and a quantitative RuCl₃·3H₂O solution were added to 200 mL of deionized water under ultrasonic treatment for 10 min to form a suspension. Then, the suspension was stirred at 350 rpm for 6 hours at room temperature. Finally, the resulting solution was stirred in a 90 °C water bath until dry. The solids were reduced at 300 °C for 3 hours in H₂ (30 mL min⁻¹). The reference sample Ru/ZrO₂ was synthesized similarly except that ZrO₂ was used directly to replace ZrO₂/C as the support.

2.3. Characterization

The X-ray diffraction (XRD) patterns were obtained via an X′ Pert Pro automatic powder diffractometer with Cu Kα monochromatized radiation (40 kV, 40 mA). A CNS elemental analyzer (Vario MAX) was used to measure the carbon content of the samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded using a JEM 2100 F electronic microscope. The elemental maps of the samples were obtained using an energy dispersive spectrometer (EDS), which was equipped with a TEM. Nitrogen adsorption–desorption isotherms were obtained on a Quantachrome Autosorb IQ instrument, and the mean diameter (D_BJH) and specific surface area (S_BET) of the samples were analyzed by Barrett–Joyner–Halenda (BJH) and Brunauer–Emmet–Teller (BET) methods, respectively. The chemical properties of the catalysts were measured via a Kratos/Shimadzu X-ray photoelectron spectrometer (XPS) equipped with AlKα radiation (1486.6 eV). All binding energies were calibrated to C 1s at 284.6 eV. Diffuse reflectance spectra (DRS) in the range of 200–800 nm were collected at room temperature using a Varian Cary 5000 ultraviolet-visible spectrophotometer with a white standard BaSO₄. Photocurrent and electrochemical impedance tests both were performed in a three-electrode system with an electrochemical workstation model CHI 660E. 5 mg of the sample was ultrasonically dispersed in 1.5 mL of ethanol, and 15 μL of the sample was dropped onto FTO conductive glass with an area of π × 0.05 × 0.05 cm² as the working electrode. Pt was used as the counter electrode, Ag/AgCl as the reference electrode, and KCl/K₃Fe(CN)₆/K₄Fe(CN)₆ as the electrolyte. Electron paramagnetic resonance (EPR) spectroscopy was carried out on a Bruker A300 spectrometer at the temperature of liquid nitrogen. The CO₂ temperature-programmed desorption (CO₂-TPD) experiments were executed on a Quantachrome chemisorption instrument with online mass spectroscopy (Tilon, LC-200M). About 50 mg of the sample powder was first pretreated in He flow (30 mL min⁻¹) at 400 °C for 30 min. Second, after the sample was cooled down to 40 °C, CO₂ flow (30 mL min⁻¹) was introduced for adsorption for 1 hour, and then the physically adsorbed CO₂ was removed by using He flow (30 mL min⁻¹) for 1 hour. Finally, CO₂-TPD was recorded online from 40 to 700 °C in He flow. CH₄-TPD followed the same procedure, except that it used CH₄ as the adsorbed gas.

2.4. In situ DRIFTS analysis

In situ DRIFTS was performed to study the intermediates occurring on the catalysts on a ThermoFisher iS50 spectrometer during catalytic reduction reactions. First, the sample was pretreated in a 5% H₂/Ar atmosphere flow for 30 min at 300 °C, and then cooled to ambient temperature by using a N₂ stream. Next, the spectra at 25 °C were collected as the background in a N₂ atmosphere. Subsequently, the gas mixture (10% CO₂, 40% H₂, and 50% He) was injected into the reaction cell, followed by heating to a certain temperature (light or heating conditions) and reaction for 15 min. Finally, the reacted mixed gas to detect the intermediates by TCD.

2.5. Catalytic activity evaluation

The catalytic performance of the catalysts for photothermal CO₂ methanation was evaluated in a vertical stainless-steel reactor with a quartz window. A 300 W xenon lamp (PLS-SXE300+, Beijing Perfect Light Technology Co. Ltd) was used as a light source for CO₂ methanation. The light intensity was measured at the same position of the catalyst layer using an optical power meter (CELNP2000-2, Perfect Light). The surface temperature of the catalyst layer was measured via a thermocouple. The catalyst powder (10 mg) was placed at the center of the reactor to evaluate the catalytic performance of the catalyst in the CO₂ methanation reaction. The gas mixture of H₂/CO₂/He with a volume ratio of 4 : 1 : 5 was introduced into the reactor and kept for 15 min to remove air. Next, the gas mixture was continuously introduced into the stainless-steel reactor with a rate of 25 mL min⁻¹ for CO₂ methanation. UV-visible and visible-IR region were obtained through a cutoff filter to verify the different light functions in the photothermal CO₂ methanation process. Meanwhile, the thermal catalytic activity evaluation was performed in a tube furnace as a comparison. Specifically, a mixture containing 10 mg of catalyst and 1 g of silica sand (size 40–60 mesh) was placed in the furnace. Besides this, the long-term stability test for CO₂ methanation was conducted under the same light conditions. The low-temperature reaction was controlled by the cooling circulation tank. Concentrations of CO₂, CO, and CH₄ were determined by using an online gas chromatograph equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The yield and selectivity for CO₂ methanation were calculated by using the following equations.where

Here, F represents the gas flow rate (mL min⁻¹); [CH₄] and [CO] are the respective concentrations (vol%) of CH₄ and CO detected by the online GC.

3 Results and discussion

3.1. Structural and morphological characterization

We utilized the supports of ZrO₂/C and ZrO₂ to load Ru, respectively, and the obtained samples are shown in Fig. S1.† ²¹ The total content of C determined by using the CNS elemental analyzer in Ru–ZrO₂/C and Ru/ZrO₂ are 6.0 wt% and 0.6 wt%, respectively. However, no characteristic peaks of carbon are observed in XRD patterns (Fig. 1a). As shown in Table 1, the actual loading Ru contents of Ru–ZrO₂/C (3.3 wt%) and Ru/ZrO₂ (3.2 wt%) determined by the XRF technique are similar to each other. The XRD patterns for the samples are shown in Fig. 1a. Pure ZrO₂/C, Ru–ZrO₂/C, and Ru/ZrO₂ clearly show diffraction peaks at 30.3°, 35.4°, 50.4°, 59.5°, and 73.2° corresponding to the typical ZrO₂ crystal phase (JCPDS PDF# 96-210-0389). A weak Ru characteristic peak (44.0°, JCPDS PDF# 96-900-8514) is observed only at a Ru content of 4.6 wt% (Fig. S2†). It is noteworthy that the peaks of metal Ru are not obvious in Ru–ZrO₂/C and Ru/ZrO₂, suggesting that the metal Ru NPs in the Ru-based samples retain a small-size and highly dispersed state.^21,33 Generally, highly dispersed and small-size Ru NPs are essential for CO₂ methanation to produce high activity.^34–36

Fig. 1 (a) XRD patterns, ZrO₂ represents JCPDS PDF# 96-210-0389; (b) N₂ adsorption–desorption isotherms; (c) pore distribution of the samples.

Table 1 Element composition and properties of the samples

Samples	Ru loading (wt%)	S BET (m^2 g^−1)	D BJH (nm)	Total pore volume (cm^3 g^−1)
ZrO2/C	—	16.6	3.7	0.03
Ru–ZrO2/C	3.3	35.5	3.7	0.05
Ru/ZrO2	3.2	41.3	3.9	0.06

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The N₂ static adsorption–desorption isotherms were obtained to examine the adsorption properties, specific surface area, and pore size of the samples (Fig. 1b and c). Specific surface area (S_BET) and total porosity were calculated using BET and BJH methods. The relationship between the structural characteristics and support of the samples is shown in Table 1. The S_BET of pure ZrO₂/C is 16.6 m² g⁻¹ while adding Ru increases the S_BET value of Ru–ZrO₂/C to 35.5 m² g⁻¹. A similar trend is also reflected in the variation of pore volume for Ru–ZrO₂/C. It can be reasonably assumed that a small amount of Ru species in the precursor matrix of ZrO₂/C maintains its intrinsic structure and avoids excessive shrinkage during the pyrolysis process.³⁷ It is noteworthy that the S_BET and total pore volume of the reference sample (Ru/ZrO₂) are larger than that of Ru–ZrO₂/C. This suggests that surface area might not be the primary factor influencing catalytic activity.

The morphology and structure of the samples were investigated using TEM and HRTEM. The TEM images of ZrO₂/C and ZrO₂ (Fig. S3a and b†) show a typical quasi-cubic shape. The TEM images of Ru–ZrO₂/C, Ru–ZrO₂/C-3h (Ru–ZrO₂/C reacted for 3 hours of the photothermal catalytic reaction), and Ru/ZrO₂ are shown in Fig. 2a–c. There is no obvious change in the morphology between the fresh and used Ru–ZrO₂/C. HRTEM images of ZrO₂/C and ZrO₂ show a distance of 0.297 nm corresponding to the (111) plane of ZrO₂ (Fig. S3c and d†). HRTEM patterns of Ru–ZrO₂/C, Ru–ZrO₂/C-3h, and Ru/ZrO₂ clearly show the microstructure of ZrO₂ and tiny Ru metal nanoparticles in Fig. 2d–f, where the Ru nanoparticles have a well-defined lattice with the same distance of 0.205 nm, corresponding to the (011) plane of Ru. The size of Ru nanoparticles over Ru–ZrO₂/C and Ru/ZrO₂ is 3.2 nm and 5.5 nm, respectively. The Ru nanoparticles are dispersed in the microstructure of ZrO₂/C, and the corresponding elemental mapping diagrams are shown in Fig. 2g. However, no carbon is observed in the HRTEM image of Ru–ZrO₂/C and Ru/ZrO₂. According to the above analysis, no carbon is observed in the XRD pattern and HRTEM image of Ru–ZrO₂/C probably because the amorphous structure of carbon is formed during the calcination process. In addition, carbon is a typical photothermal material and may provide a facile electron transport pathway for CO₂ methanation.²¹ Moreover, the energy dispersive spectroscopy (EDS) elemental mapping shows that the elements of Ru, Zr, and O are uniformly dispersed throughout the microstructure of the cubic particles, which roughly confirms that Ru–ZrO₂/C is mainly composed of C, O, Zr, and Ru combined with the HRTEM image (Fig. 2d and g).

Fig. 2 (a–c) TEM images of Ru–ZrO₂/C, Ru–ZrO₂/C-3h, and Ru/ZrO₂; (d–f) HRTEM images of Ru–ZrO₂/C, Ru–ZrO₂/C-3h and Ru/ZrO₂; (g–i) energy dispersive spectroscopy (EDS) mapping of Ru–ZrO₂/C, Ru–ZrO₂/C-3h, and Ru/ZrO₂, respectively.

3.2. Photothermal catalytic performance of CO₂ methanation

The light-driven photothermal CO₂ methanation reaction was studied in a stainless-steel reactor with a quartz window (filled with a mixture gas of 10% CO₂, 40% H₂, and 50% He with a rate of 25 mL min⁻¹). No reaction products are detected over pristine ZrO₂/C during the photothermal CO₂ reduction reaction (Fig. S4a†) under full-spectrum irradiation with a light intensity of 2614 mW cm⁻², indicating neither photocatalytic nor photothermal catalytic activity on the pure ZrO₂/C support. The performance of Ru–ZrO₂/C is also evaluated under a 5% H₂/Ar atmosphere to eliminate the interference of carbon substances. As shown in Fig. S4b,† no product is detected over Ru–ZrO₂/C, indicating that the carbon-containing products during the CO₂ methanation process are all derived from CO₂ in the feedstock gas (the mixture gas of 10% CO₂, 40% H₂, and 50% He). Besides, we performed the thermogravimetric analysis of Ru–ZrO₂/C in N₂ gas with a temperature increase of 40–600 °C (Fig. S5†). The mass loss was 0.78% between 40 and 200 °C, which might be caused by the loss of water in the sample, 0.81% between 200 and 400 °C, and 1.39% between 400 and 600 °C. Among them, there is no significant loss of mass in the temperature range of 200 to 400 °C, and the result indicates that the Ru–ZrO₂/C sample has good thermal stability during the CO₂ catalytic reduction reaction. The stable thermal catalytic performance also confirms that the CO and CH₄ products originate from CO₂ and not from any organic pollutants.³⁸ It's worth noting that the CH₄ yield was enhanced with the Ru loading on ZrO₂/C increasing from 0.4% to 4.6% (Fig. S6†), indicating the key role of Ru nanoparticles as catalytic active sites for CO₂ methanation.

As displayed in Fig. 3a, the rate of CH₄ production (r_CH4) of Ru/ZrO₂ at the ninth hour is 104.5 mmol g⁻¹ h⁻¹. Nevertheless, the r_CH4 of Ru–ZrO₂/C is 504.1 mmol g⁻¹ h⁻¹ with nearly 98.9% selectivity of CH₄ under the same reaction conditions, which is much higher than that of Ru/ZrO₂. Furthermore, the production rate of Ru–ZrO₂/C remains stable, with high S_CH4 (≈98.0%) during 24 hours of CO₂ methanation at a light intensity of 2858 mW cm⁻² (Fig. S7†). In addition, the catalytic durability for multiple cycle tests over Ru–ZrO₂/C at a light intensity of 2858 m W cm⁻², lasts for at least 24 h (each cycle for 4 h, 6 cycles). During the 6 cycles, the CH₄ production rate did not change significantly, indicating that the catalyst had stable reactivity. The HRTEM image and XRD spectra of Ru–ZrO₂/C-3h (Fig. 2e, h, and S9†) show no significant changes in the catalyst and as well as no agglomeration and sintering during the catalytic reaction, confirming the structural stability of Ru–ZrO₂/C during the reaction process. In addition, Ru–ZrO₂/C exhibits good catalytic performance compared with the recently reported photothermal CO₂ hydrogenation catalysts (Table S1†). Therefore, Ru–ZrO₂/C prepared by pyrolysis of UiO-66 in an N₂ atmosphere is an excellent choice for photothermal CO₂ methanation reactions with high performance.

Fig. 3 (a) Production rate in the ninth hour of Ru–ZrO₂/C and Ru/ZrO₂ at a light intensity of 2614 mW cm⁻²; (b) CH₄ formation rate at different light intensities of Ru–ZrO₂/C; (c) CH₄ formation rate and equilibrium temperature of Ru–ZrO₂/C at different light intensities; (d) production rate of Ru–ZrO₂/C under light and electric hearting conditions, respectively. The catalytic reaction was performed under the focus light via a Fresnel lens in a continuous flow of 10 vol% CO₂, 40 vol% H₂, and 50 vol% He (25 mL min⁻¹). Vis–IR and UV–vis light were obtained by using different cutoff filters. ^a The surface temperature of the catalyst layer was controlled by using an ice-water bath.

To investigate the role of light in the CO₂ methanation reaction process, the catalytic performance of the Ru–ZrO₂/C catalysts was evaluated in different optical regions (Fig. 3b). When under the full solar spectrum with a light intensity of 2614 mW cm⁻², the r_CH4 of Ru–ZrO₂/C reached 504.1 mmol g⁻¹ h⁻¹ and the surface equilibrium temperature of the catalyst (T_eq) detected by the thermocouple was 370 °C. By introducing circulating cooling water around the reactor, the surface temperature of the catalyst reduces to 338 °C from 370 °C, and simultaneously the r_CH4 drops to 461.5 mmol g⁻¹ h⁻¹. Moreover, by cutting off the infrared light at the above full-spectrum incident light intensity (2614 mW cm⁻²), the T_eq of the catalyst reduced to 324 °C and the r_CH4 only reaches 262.1 mmol g⁻¹ h⁻¹ under irradiation of UV-vis light. However, after increasing the incident light intensity to 3146 mW cm⁻², the T_eq and r_CH4 significantly increased to 370 °C and 507.5 mmol g⁻¹ h⁻¹, respectively, which was comparable to the r_CH4 at full-spectrum temperature (370 °C). Besides this, using the ice-water bath method under vis-IR incident light conditions to control the surface temperature at 337 °C, the r_CH4 of the CO₂ methanation reaction was similar to that under the full-spectrum ice-water bath conditions (338 °C, 461.5 mmol g⁻¹ h⁻¹). These indicate that Ru–ZrO₂/C exhibits similar catalytic activity at different light intensities when the temperature reaches the same value. That is to say, the Ru–ZrO₂/C photothermal catalytic material may convert concentrated solar energy into thermal energy and then drive the reaction, which is similar to conventional thermocatalysis. Subsequently, we further investigate the photothermal CO₂ catalytic reactions at different light intensities over Ru–ZrO₂/C catalysts. As shown in Fig. 3c, the surface temperature and r_CH4 of Ru–ZrO₂/C increased proportionally with increasing light intensity. This indicates that the reaction temperature plays an important role in the photothermal catalytic process, while conventional photocatalysis involving photoinduced electrons is not effective in the system.¹¹ At similar surface temperatures, Ru–ZrO₂/C exhibits similar r_CH4 in both photo- and thermally driven catalytic processes at a light intensity of 2614 mW cm⁻² (Fig. 3d), suggesting consistent thermocatalytic performance with CO₂ methanation. In summary, the photothermal CO₂ methanation over Ru–ZrO₂/C is essentially a light-driven thermocatalysis, where light mainly acts as a heat source to supply thermal energy and then triggers the CO₂ methanation reaction over the catalyst. Furthermore, it can be seen from the above that the temperature is an important factor in the CO₂ methanation reaction. However, as shown in Fig. 3a and c, even at a similar T_eq, the r_CH4 of the catalyst Ru/ZrO₂ (290 °C, 104.5 mmol g⁻¹ h⁻¹) is lower than that of Ru–ZrO₂/C (286 °C, 162.8 mmol g⁻¹ h⁻¹). This suggests that the catalytic performance of Ru–ZrO₂/C is also affected by other factors.

3.3. The origin of enhanced photothermal catalytic performance

To find out why Ru–ZrO₂/C exhibits superior CO₂ methanation performance to Ru/ZrO₂, as displayed in Fig. 4a, we first obtain the photon absorption spectra of the catalysts. Black ZrO₂/C exhibits an excellent spectral response compared with ZrO₂ in the 200–800 nm range, indicating that ZrO₂/C is more favorable for light absorption. Additionally, black ZrO₂/C has a high photothermal conversion capacity because of its inherent black color, which can convert solar energy into heat energy and promote photothermal catalytic reduction reactions. When the Ru species is introduced, the Ru-based catalysts exhibit higher light absorption. Ru–ZrO₂/C exhibits a stronger signal response than Ru/ZrO₂ in this region, revealing that Ru–ZrO₂/C has a higher light absorption capacity. The stronger light absorption capacity can realize a strong photothermal effect, which can be confirmed from the monitoring of the catalyst surface temperature during the reaction process (Fig. 4b). The central temperatures of the Ru–ZrO₂/C and Ru/ZrO₂ catalysts reach 370 °C and 290 °C, which are much higher than that of supports of ZrO₂/C (314 °C) and ZrO₂ (181 °C), respectively. The results show that the prepared Ru-based samples exhibit good absorption in the spectral range. Efficient light-to-heat conversion increases the temperature of the catalyst layer to the desired level, thus potentially driving the photothermal catalytic reaction. In addition, we have provided the spectra of the photocurrent test and EIS Nyquist plots of Ru/ZrO₂ and Ru–ZrO₂/C, as shown in Fig. S8.† Both Ru/ZrO₂ and Ru–ZrO₂/C exhibited significant reversible photocurrent responses (Fig. S8a†), and the magnitude of photocurrent density follows Ru–ZrO₂/C > Ru/ZrO₂. It is generally believed that higher photocurrent responses represent higher carrier separation efficiencies. As shown in Fig. S8b,† the semicircle in the Nyquist plot of Ru–ZrO₂/C is smaller compared with that of Ru/ZrO₂, which indicates its better carrier migration ability. Taken together the photocurrent and electrochemical impedance tests indicate the existence of a conventional photocatalytic reaction process in this photothermally catalyzed CO₂ methanation. As shown in Fig. 3d, it was found in the controlled experiments under photoirradiation heating and electrically heated conditions that the photo enhancement ratio (the ratio of the incremental yield under photoirradiation to the thermocatalytic yield) on Ru–ZrO₂/C increased and then decreased with the increase in the reaction temperature, and when the reaction temperature reaches 370 °C (2614 mW cm⁻²), Ru–ZrO₂/C exhibits a similar methane yield in the catalytic reaction process.^39,40 This reveals that the catalytic reduction reaction is essentially a photo-driven thermocatalysis under the reaction conditions (light intensity: 2614 mW cm⁻², reaction temperature: 370 °C). According to the above analysis, the high light-heat effect of Ru–ZrO₂/C promotes the CO₂ methanation reaction.

Fig. 4 (a) DRS spectra of the samples; (b) the surface temperature of the samples at a light intensity of 2614 mW cm⁻²; (c and d) EPR spectra of Ru–ZrO₂/C and Ru/ZrO₂.

EPR is further executed at −173 °C to discuss oxygen vacancies (OVs). As shown in Fig. 4c and d, ZrO₂/C exhibits a characteristic oxygen vacancy signal at g = 2.003, much stronger than that of ZrO₂. Notably, after loading Ru nanoparticles on ZrO₂/C, the signal intensity of OVs on Ru–ZrO₂/C is significantly lower than that on ZrO₂/C (Fig. 4c), which could be attributed to the occupation of OVs by Ru.²¹ The oxygen defects in ZrO₂/C are likely to affect metal adhesion and anchoring and limit the growth of Ru nanoparticles.^41,42 This suggests that more OVs formed by calcination under N₂ conditions promote the anchoring of Ru species, which may be one of the reasons for the elevated methanation reaction activity of Ru–ZrO₂/C. In contrast, the signal intensity of OVs on Ru/ZrO₂ is higher than that on ZrO₂ (Fig. 4d) due to the Ru species promoting the formation of OVs during the reduction process in H₂.⁴³ However, due to the lower catalytic activity of Ru/ZrO₂ with abundant oxygen vacancies compared with Ru–ZrO₂/C, it is reasonable to think that oxygen vacancies are not the main factor for catalytic activity.

X-ray photoelectron spectroscopy (XPS) was performed to better understand the elemental composition and surface chemical states of catalysts. The Ru 3d photoelectron line is usually used to detect the chemical state of ruthenium, but since this line overlaps with the C 1s line, we replace the Ru 3d spectra with Ru 3p spectra to avoid interference from C 1s.^44,45 The spectra for the Ru 3p line for Ru–ZrO₂/C and Ru/ZrO₂ catalysts are depicted in Fig. 5a and b. Meanwhile, Table 2 summarizes the ratios of different chemical states of all catalysts. The high-resolution Ru 3p spectra indicate that Ru consisted of mainly metallic states (Ru⁰) and a certain amount of Ru⁴⁺. The binding energies of 461.6 eV and 463.9 eV are assigned to the Ru 3p_3/2 of Ru⁰ and Ru 3p_1/2 of Ru⁴⁺ of Ru–ZrO₂/C, respectively.^46,47 The Ru 3p XPS spectrum of Ru/ZrO₂ is shown in Fig. 5b. We can obtain the value of Ru⁰/Ru⁴⁺ following Ru–ZrO₂/C > Ru/ZrO₂ (Table 2). The result agrees with EPR results, where OVs promote anchoring of Ru. Remarkably, the Ru 3p binding energy of Ru–ZrO₂/C-3h moves toward higher binding energy regions, suggesting the migration of electrons from Ru to the support during the photothermal CO₂ hydrogenation process (Fig. S10a†). This observation indicates a strong interaction between Ru nanoparticles and the support, which is usually considered an indicator of high activity for metal-based heterogeneous catalysts.^34,45 Also, the ratio of Ru⁰/Ru⁴⁺ increases from 1.62 to 2.46 after CO₂ methanation for 3 hours. This proves that there is more production of Ru⁰ species during the CO₂ hydrogenation reaction. It has been reported in the literature that under suitable temperature conditions, metallic Ru as an active form of the metallic phase can activate H₂ molecular to initiate CO₂ hydrogenation.^6,13 The high value of Ru⁰/Ru⁴⁺ is one of the reasons for the highest activity on Ru–ZrO₂/C exceeding that on Ru/ZrO₂. Moreover, it further explains the increase in activity at the beginning of the stability experiment. In the O 1s spectra, the spectra of Ru–ZrO₂/C and Ru/ZrO₂ can be divided into three peaks (Fig. 5c and d). Namely, the peaks can be attributed to the surface adsorbed oxygen species (533.3 eV),^48–50 oxygen vacancies (532.1 eV),⁴¹ and lattice oxygen (530.3 eV),^51,52 respectively. The surface adsorbed oxygen species may come from the surface hydroxyl groups, transformation of the surface adsorbed O₂ molecules, adsorption of water molecules, and CO₃²⁻.⁵³ The O 1s spectra determine the presence and concentration of OVs in catalyst samples. We note that the values of OVs/O_latt of Ru–ZrO₂/C, Ru/ZrO_2, and Ru–ZrO₂/C-3h are similar (Fig. 5c, d, S10b,† and Table 2). According to previous reports, oxygen vacancies contribute to the CO₂ methanation reduction reaction.^54–56 The photoreduction of CO₂ is enhanced by the photoreduction of g-C₃N₄ with nitrogen-rich vacancies and the oxidation of BiOCl with oxygen-rich vacancies in BiOCl/g-C₃N₄ Z-type heterojunctions. The DFT calculations revealed that BiOCl/g-C₃N₄ Z-type heterojunctions engineered with double defects can reduce the energy barrier, and are thermodynamically more inclined to release CO.⁵⁷ In the construction of a TiO₂/g-C₃N₄ scheme heterojunction, abundant oxygen vacancies enhance photocatalytic CH₄ production and selectivity. In addition, DFT calculations showed that oxygen vacancies could effectively modulate the energy barrier, leading to a decrease in the production of the two-electron product CO, while improving the selectivity of the eight-electron reaction. Meanwhile, the oxygen vacancies promoted the formation of ultrafast charge carrier transport channels and facilitated the transfer of photoexcited charges to the surface, thus enhancing the high redox capacity in the catalytic reaction.⁵⁸ In this study, we analyzed EPR signals before and after loading Ru on the support and found that ZrO₂/C obtained by calcination under inert gas conditions had more abundant oxygen vacancies than ZrO₂ obtained by calcination in air. ZrO₂/C has significantly more oxygen vacancies than ZrO₂, and the introduction of Ru decreases the oxygen vacancies, probably because the Ru atoms are tilted and anchored to the oxygen defects of ZrO_2−x.⁵⁹ In XPS characterization, Ru⁰/Ru⁴⁺ of Ru–ZrO₂/C is significantly higher than that of Ru/ZrO₂, which just proves the anchoring of Ru by oxygen vacancies. Also, it's worth noting that the CH₄ yield increased with the Ru loading on ZrO₂/C increasing from 0.4% to 4.6% (Fig. S6†). Fig. S6† and Table 1 indicate the key role of Ru nanoparticles as catalytic active sites for CO₂ methanation. The above analysis shows that Ru is the active site of CO₂ methanation and is the main reason for the enhancement of catalytic activity. In contrast, the oxygen vacancy concentration increased after the introduction of Ru into ZrO₂. However, the catalytic activity of Ru/ZrO₂ did not increase due to the increase in oxygen vacancies, so it is reasonable to assume that oxygen vacancies are not the main reason for the difference in the activity of Ru–ZrO₂/C and Ru/ZrO₂. Based on the EPR and XPS analysis, we conclude that N₂ calcination may make ZrO₂/C have more OVs, enhance the anchoring of Ru, obtain a lower Ru state, and thus enhance the catalytic performance.³⁴

Fig. 5 (a–d) Ru 3p and O 1s of Ru–ZrO₂/C and Ru/ZrO₂, respectively; (e and f) CO₂-TPD and CH₄-TPD profiles of Ru–ZrO₂/C and Ru/ZrO₂.

Table 2 Binding energies of elemental components calculated by using the XPS spectrum

Samples	The binding energy of Ru		Binding energy of O			Molar ratios by XPS
	Ru^0	Ru^4+	Oads	OVs	Olatt	Ru^0/Ru^4+	OVs/Olatt
Ru–ZrO2/C	461.6	463.9	533.3	532.1	530.3	1.62	0.36
Ru/ZrO2	462.0	463.9	532.9	531.8	530.6	1.54	0.35
Ru–ZrO2/C-3h	462.1	464.8	533.5	532.1	530.4	2.46	0.37

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The better adsorption capacity of the reactants facilitates more reactant molecules to participate in the reaction, while the better desorption capacity of the products accelerates the departure of the product molecules, and leaves more adsorption sites for the subsequent CO₂ hydrogenation reaction. In this study, the CO₂-TPD profile is obtained as shown in Fig. 5e. Ru–ZrO₂/C shows a strong signal at the center of 460 °C and 663 °C, which is attributed to the medium-strength and strong basic sites, respectively. The peaks at around 663 °C can be attributed to the CO₂ adsorption on the strong basic sites and surface oxygen vacancies.⁶⁰ Beyond this, the CO₂ desorption signal extends up to 700 °C due to the presence of strongly basic sites. The intensity of Ru–ZrO₂/C is significantly higher than that of Ru/ZrO₂, suggesting the presence of more CO₂ adsorption active sites on Ru–ZrO₂/C. More CO₂ adsorption active sites can adsorb and activate more CO₂ molecules and promote electron–CO₂ interactions, thus favoring the formation of CH₄.⁴⁷ In addition, the CH₄-TPD spectra of Ru–ZrO₂/C show that the desorption of CH₄ is lower than that of Ru/ZrO₂ (Fig. 5f), which indicate that CH₄ is less likely to be adsorbed on the surface of Ru–ZrO₂/C than Ru/ZrO₂. Therefore, CH₄ as a product can be rapidly desorbed from the Ru–ZrO₂/C surface and leave more active sites for adsorption and activation of CO₂ molecules.⁴⁷ This ultimately improves the catalytic performance of light-driven CO₂ methanation.

In situ DRIFTS is performed under a continuous flow of the mixture gas (10% CO₂, 40% H₂, and 50% He) to identify the intermediates involved and to determine the CO₂ reaction pathways. The intermediates appear in the infrared fingerprint region at 1200–1700, 1900–2500, and 3000–3050 cm⁻¹, and the band assignments of the surface-activated species are shown in Table S2.† The in situ DRIFTS spectra of Ru–ZrO₂/C at different temperatures are recorded after reaching reaction equilibrium under the same continuous flow reaction conditions as photothermal catalysis (Fig. 6a). The signals of the characteristic peaks increase dramatically with the increase in reaction temperature. After the introduction of the mixture gas at 250 °C, bands of bidentate carbonates (b-CO₃²⁻, 1581, 1542, and 1513 cm⁻¹) are observed.⁶¹ With the temperature increasing from 250 to 400 °C, the signals of the important intermediates are enhanced in the bands at 1230, 1641, and 1900–2100 cm⁻¹ belonging to COOH*, CH₃O⁻, and linear CO, respectively. The characteristic bands at 1305 and 3016 cm⁻¹ correspond to CH₄, which gradually increases as the temperature increases from 250 °C to 400 °C. Meanwhile, similar peaks are observed on Ru–ZrO₂/C without light, as shown in Fig. 6b. This indicates that photothermal and thermal CO₂ hydrogenation follow a similar reaction pathway, which again suggests that the photo-generated electrons play a minor role in CO₂ methanation and the reaction system is a light-driven thermocatalytic reduction reaction.⁶²

Fig. 6 In situ DRIFTS spectra of Ru–ZrO₂/C were measured under light (a) and without light (b) radiation with external heating. During the temperature-programmed reaction: a gas mixture (20 mL min⁻¹) containing 10% CO₂/40% H₂/50% He was employed.

Based on the above discussion, a photothermal catalytic reaction mechanism for CO₂ reduction on Ru–ZrO₂/C is proposed. The light-absorbing properties of ZrO₂/C and the “nano-heater” effect of ruthenium nanoparticles convert light energy into heat energy through the photothermal effect under irradiation, thus triggering the catalytic CO₂ methanation reaction. CO₂ and H₂ are adsorbed on the catalyst surface, H₂ is dissociated into H atoms, and H atoms reduce the adsorbed CO₂ and intermediate products on the catalyst surface. The adsorbed CO₂ molecules are dissociated to produce the intermediate HCOO* (CO₂ + O* → OCO₂* + OH* → HCOO*),⁶³ which generates CO intermediates and a small amount of H₂O by the reverse water–gas reaction, and a small amount of CO signal escapes to be detected by the detector (CO₂(g) + H₂(g) → CO(g) + H₂O(g)). Most of the intermediates such as CO*, H*, HCOO*, and hydroxyl groups are adsorbed on the surface of the catalyst and further react to produce CH₄ (CO* + H* → HCOO* + H* → H₃CO* + OH* → *CH₄).⁶⁴ This is confirmed by the in situ DRIFTS spectroscopic results, and it is finally converted to CH₄ by the methanation reaction, which can be expressed as follows: CO₂–CO–CH₃O⁻–CH₄.⁶⁵

4 Conclusion

In summary, this work reports the successful construction of an efficient Ru–ZrO₂/C catalyst via inert gas calcination treatment for photothermal CO₂ methanation. The Ru–ZrO₂/C catalysts simultaneously achieve a r_CH4 of 504.1 mmol g⁻¹ h⁻¹ in a flow reaction with a selectivity of 98.9% under full-spectrum simulated illumination. Moreover, the catalytic performance of Ru–ZrO₂/C remains essentially constant in the 24 hour stability test. The comprehensive characterization of the physical and chemical properties of the catalyst reveal the remarkable heat resistance, metal Ru, metal–support interaction and the good adsorption performance for CO₂ synergistic promotion of the photothermal CO₂ methanation reaction. Besides, better photothermal catalytic performance is obtained due to the light-absorbing properties of ZrO₂/C and the “nano-heater” effect of the ruthenium nanoparticles. The above suggests that the ZrO₂/C support has an important influence on the catalytic performance of the catalysts. This not only expands the application field of MOFs as precursors, but also provides a new idea for the construction of highly efficient loaded photothermal catalysts, and provides an effective guide for the design and synthesis of highly efficient photocatalysts for solar fuel production.

Author contributions

Huiling Wang: conceptualization, data curation, formal analysis, investigation, methodology, resources, writing – original draft. Qiang Li: methodology, writing – review & editing. Jing Chen: conceptualization, writing – review & editing. Hongpeng Jia: conceptualization, supervision, writing – review & editing, project administration, funding acquisition.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Nature Science Foundation of China [No. 22376193 and 22176187]; the Science and Technology Planning Project of Xiamen [No. 3502Z20226022].

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta01821d

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