With the increased consumption of global fossil fuels and the growing need for sustainability, there are urgent demands for advanced technologies and green materials in the oil and gas industry [
1]. As an important part of the "Power-to-Gas (P2G)" system, a hydrogen energy storage technology, photothermal catalytic carbon dioxide (CO
2) methanation can achieve sustainable and clean energy conversion [
2,
3]. Photothermal catalysis harnesses non-radiative transition or plasmon excitation to concurrently drive both the thermochemical and photochemical processes of the catalyst [
4]. This approach not only overcomes the low efficiency in photocatalytic conversion but also decreases the required catalytic reaction temperature for thermal catalysis [
5], thus preventing high-temperature sintering and carbon deposition-induced deactivation of the catalyst [
6,
7]. In this process, regulating active sites and modifying photothermal conversion characteristics contribute to the fabrication of photothermal catalysts with high performance [
8]. On the one hand, the active sites required for photothermal catalytic CO
2 methanation are generally transition metals, such as Co, Ni, Rh, Pd, and Ru [
8–
10]. The catalytic selectivity and activity of the active sites of noble metals are superior to those of non-precious metals [
11]. Besides, non-precious metals Ni and Co can lead to deactivation in the reaction process under the action of carbon accumulation [
12,
13]. Nevertheless, the scarcity of precious metals has hindered their further development. Therefore, it is of great significance to improve the utilization of precious metals [
14]. On the other hand, metal nanomaterials can absorb light energy and convert it into heat energy through the local surface plasmon resonance (LSPR) effect, thereby raising the temperature and triggering catalytic reactions. However, the oxide supports are still important not only for dispersing and stabilizing the active metal nanoparticles but also for improving the light utilization efficiency to enhance the photothermal catalytic performance [
15,
16]. In particular, oxygen vacancies in the oxide supports also affect the adsorption and activation of CO
2 [
17].
Ultrafast synthesis technologies, such as Joule heating [
18–
20] and Laser synthesis [
21–
23], are beneficial to the synthesis of nanomaterials with defect structures. As a pollution-free energy source with high energy efficiency, lasers can interact with materials within an extremely short period to achieve heating, melting, vaporization, or phase changes. The high-temperature and high-pressure gas plume region at a small space region formed by laser treatment is prone to induce oxygen vacancy defects [
24]. In addition, the unique micro- and nano-integrated structures obtained
via laser treatment have also garnered significant attention in the catalysis field [
25,
26]. For example, some scholars performed the laser modulation of oxygen vacancies in TiO
2-x/Ti and then anchored single-atom-layer Pt clusters
via the confinement effect of oxygen vacancies [
24]. Besides, the ruthenium-modified catkin-like fluff defect-rich alumina synthesized using ultraviolet (UV) laser showed excellent performance in photothermal conversion and catalytic CO
2 methanation, which could be attributed to the efficient capture of scattered light [
27,
28] and enhanced light absorption as well as photothermal conversion efficiency [
29].
Meanwhile, the anchoring effect of oxygen vacancies can improve the dispersion of the metal active site to form ultra-small Ru nanoparticles and stimulate a strong interaction between the metal and support, thus promoting the catalytic reaction [
30,
31]. Therefore, the laser construction of defect-rich oxide supports and the anchoring of ultra-small noble-metal catalytic sites as the photothermal catalyst have exhibited certain advantages.
Tantalum oxide (Ta
2O
5) is renowned for its high refractive index and extensive applications in both optical and electronic devices. Owing to the exceptionally high Ta-O bond energy, Ta
2O
5 exhibits remarkable thermodynamic and chemical stability [
32,
33]. The laser construction of oxygen vacancy defects combined with the high Ta-O bond energy of Ta
2O
5 may play a unique role in the preparation of metal photothermal catalysts supported by defective oxide.
In this study, the defect-rich tantalum oxide on tantalum foil (Ta2O5-x/Ta) was prepared through laser ablation method. Besides, the Ru nanoparticles were loaded on the surface of Ta2O5-x/Ta to fabricate the Ru/Ta2O5-x/Ta catalyst under the thermal effect of lasers and the anchoring effect of oxygen vacancies. For the Ru/Ta2O5-x/Ta catalyst, the presence of oxygen vacancies enhanced the photothermal conversion efficiency. Meanwhile, the interfaces between Ru and Ta2O5-x enhanced the adsorption of CO2, and Ru nanoparticles served as efficient active sites for the hydrogenation of CO2 to methane (CH4). The Ru/Ta2O5-x/Ta catalyst exhibited excellent photothermal conversion efficiency (reaching 343.4 ℃ at 1.76 W/cm2) and excellent catalytic activity with a CH4 yield of 50.6 mol g(Ru)-1 h-1 (CH4 selectivity of 98%). Moreover, the application of the pulsed laser was extended to construct the photothermal catalyst with high photothermal temperature and efficient catalytic sites.
The preparation process of the laser-synthesized Ru/Ta
2O
5-x/Ta photothermal catalyst was illustrated in
Fig. 1a and Fig. S1 (Supporting information). Owing to the time-saving advantage of the laser processing technique, only 206 s was required for the as-prepared Ta
2O
5-x/Ta with a size of 10 mm × 10 mm. The defect-rich Ta
2O
5-x was first synthesized on Ta foil by the laser ablation process. The high-temperature and high-pressure vapor plume reacted with the ambient gas during laser treatment, which contributed to the generation and retention of oxygen vacancies. The formation of blue-black (rather than white) tantalum pentoxide in
Fig. 1b verified the presence of many oxygen vacancies, and the Ta foil was argenteous. The electron paramagnetic resonance (EPR) results (Fig. S2 in Supporting information) also confirmed the presence of oxygen vacancies in Ta
2O
5-x/Ta at
g = 2.004. The image of field emission scanning electron microscopy (FESEM) in
Figs. 1c and d showed that Ta
2O
5-x/Ta had a catkin-like fluff micro/nanostructure, which could efficiently trap the light and minimize light dissipation. The cross-section FESEM image in Fig. S3 (Supporting information) showed that the thickness of the laser-prepared Ta
2O
5-x was ~101 µm. As shown in
Fig. 1a, after the ruthenium carbonyl was dropped on the laser-prepared Ta
2O
5-x/Ta, the thermal effect of lasers in the under-focus mode was employed to decompose Ru
3(CO)
12 into Ru with the catalyst presenting black color (
Fig. 1b). The oxygen vacancies as anchoring sites, along with laser thermal decomposition, promoted the production of Ru nanoparticles. As illustrated by the FESEM images in
Figs. 1e and f, the addition of Ru did not destroy the primitive micro/nanostructure of Ta
2O
5-x/Ta. The X-ray powder diffraction (XRD) results in
Fig. 1g revealed that the miscible tantalum pentoxide composed of the hexagonal Ta
2O
5 (PDF #18–1304) with the diffraction peak at 22.9°, 28.3°, 36.8°, and 50.4° corresponding to (003), (200), (203), and (200) crystal faces and the triclinic Ta
2O
5 (PDF #21–1198) with the diffraction peak at 23.5°, 26.5°, 29.9°, and 36.5° corresponding to (101), (
105), (0012), and (1
16) crystal faces were obtained. Except for the detected main diffraction peak for Ta
2O
5, the other diffraction peaks at 38.5° (110) and 55.6° (200) of Ta (PDF #04–0788) were also detected. The results indicated that the miscible structure of tantalum pentoxide was successfully constructed by lasers on the Ta foil. Of note, the Ru content in Ru/Ta
2O
5-x/Ta was not detected by XRD due to the ultra-low loading but by inductively coupled plasma (ICP) spectroscopy (Table S1 in Supporting information), which was 0.015 wt%.
To verify the formation of Ru-decorated Ta
2O
5-x, the structure of the scraped Ru/Ta
2O
5-x samples was further analyzed using transmission electron microscopy (TEM). The flocculent morphology and Ru particles with higher contrast dispersed on Ta
2O
5-x were observed (
Fig. 1h). The Ru nanoparticles with a diameter of approximately 2 nm were ascribed to the oxygen vacancies within Ta
2O
5-x, which served as anchoring sites and prevented the nanoparticles from agglomeration. High-resolution transmission electron microscopy (HRTEM) of
Fig. 1i and Fig. S4 (Supporting information) showed the presence of Ta
2O
5 with lattice spacings of 0.313 nm and 0.377 nm, respectively. These two crystal planes correspond to the (200) crystal plane of hexagonal Ta
2O
5 and the (101) crystal plane of triclinic Ta
2O
5, respectively. Additionally, the lattice fringe of 0.208 nm was associated with the (101) crystal plane of Ru. Furthermore, the aggregation of Ru nanoparticles was not observed. The energy-dispersive spectroscopy (EDS) elements mapped in
Fig. 1j showed a uniform distribution of Ta, O, and Ru, which further validated the successful preparation of the Ru/Ta
2O
5-x composite.
The electronic properties of Ru/Ta
2O
5-x/Ta were investigated by X-ray photoelectron spectroscopy (XPS) (
Figs. 2a-c). The XPS survey spectra of Ru/Ta
2O
5-x/Ta were characterized by the presence of Ru, Ta, and O (Fig. S5 in Supporting information). The Ta 4f spectrum (
Fig. 2a) of Ta
2O
5-x/Ta prepared by laser ablation could be deconvolved into two component peaks, which were attributed to Ta
4+ at 27.34 and 25.07 eV and Ta
5+ at 28.07 and 26.22 eV, respectively. After the decoration using Ru nanoparticles, only the XPS peaks corresponding to Ta
5+ at 27.78 and 25.90 eV were observed. The disappearance of the Ta
4+ peak intensity in Ru/Ta
2O
5-x/Ta was ascribed to the anchoring of Ru, which then consumed the oxygen vacancies in the laser thermal decomposition process. Additionally, the Ta
5+ peaks were shifted to lower binding energy (27.78 and 25.90 eV) in Ru/Ta
2O
5-x/Ta compared with that of Ta
2O
5-x/Ta (28.07 and 26.22 eV), indicating the electron transfer from Ru to Ta
2O
5-x. In terms of the O 1s spectrum in
Fig. 2b, three peaks assigned to the adsorbed oxygen (O
ad), lattice oxygen (O
L), and oxygen vacancy (O
V) at 533.01, 529.72, and 531.25 eV were detected for Ta
2O
5-x [
35–
37]. After the decoration using Ru nanoparticles on Ta
2O
5-x/Ta, the vacancy oxygen was consumed, implying the anchoring effect of these oxygen vacancies. As shown in
Fig. 2c, the Ru 3p spectrum of Ru/Ta
2O
5-x/Ta revealed that the binding energy was 463.54 eV for the Ru 3p
3/2 peak and 485.87 eV for the Ru 3p
1/2 peak, respectively, suggesting the formation of metallic Ru by thermal decomposition and the anchoring effect of oxygen vacancies. Compared with Ru foil (462.10 and 484.40 eV), the XPS peak of Ru shifted to higher binding energy in Ru/Ta
2O
5-x/Ta, further implying the electron transfer from Ru to Ta
2O
5-x. This result was consistent with that of Ta
5+ 4f (
Fig. 2a). All the above results demonstrated the successful synthesis of Ru/Ta
2O
5-x/Ta and the establishment of a strong interaction between Ta
2O
5-x supports and Ru.
The X-ray absorption near-edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) spectra were observed to identify the valence states and local coordination structure of Ru/Ta
2O
5-x/Ta. As shown in
Fig. 2d, the Ru K-edge XANES spectrum revealed that Ru/Ta
2O
5-x/Ta possessed similar characteristics to Ru foil. Combined with the FT-EXAFS spectrum (
Fig. 2e), a significant peak corresponding to Ru-Ru scattering and a weak Ru-O bond could be observed, indicating that Ru
0 was the predominant species in the sample. The detailed structural parameters are summarized from the curve fitting in Table S2. Furthermore, when amplifying the edge energy of Ru/Ta
2O
5-x/Ta and Ru foil (inset in
Fig. 2d), the Ru K-edge in Ru/Ta
2O
5-x/Ta shifted to slightly higher energy compared with that of Ru foil. This suggested a higher average valence state of Ru in Ru/Ta
2O
5-x/Ta. This observation aligned with the XPS results (
Fig. 2c).
Fig. 2f displayed the
k3-weighted EXAFS signals. Ru/Ta
2O
5-x/Ta at low wavenumbers showed a slightly advanced phase relative to Ru foil, revealing different surrounding scatters of Ru atoms in Ru/Ta
2O
5-x/Ta. The Ru K-edge WT-EXAFS of Ru/Ta
2O
5-x/Ta and Ru foil displayed in
Figs. 2g and h illustrated that an evident WT signal corresponding to the Ru-Ru bond in the K (~10 Å
-1) and
R+
α (~2.5 Å) position was detected. This further validated the loading of Ru nanoparticles on Ta
2O
5-x/Ta. Additionally, Ru-O bond signals were also detected in Ru/Ta
2O
5-x/Ta due to the Ru-O interaction between Ru
0 and the Ta
2O
5-x/Ta support (
Fig. 2h, Figs. S6 and S7 in Supporting information). The differential charge density of Ru/Ta
2O
5-x/Ta supported this conclusion (
Fig. 2i and Fig. S8 in Supporting information). Due to the difference in electronegativity, the accumulation of electrons at Ta
2O
5-x/Ta was observed, indicating that electrons transferred from Ru to Ta
2O
5-x/Ta. This further revealed a strong interaction between Ru and Ta
2O
5-x/Ta, thereby enhancing the catalytic performance within the system.
The light absorption capacity was quantified using UV–vis absorption spectroscopy to investigate the photothermal properties of Ru/Ta
2O
5-x/Ta (
Fig. 3a). As a wide band gap semiconductor material (~4.0 eV), Ta
2O
5-x/Ta exhibited its intrinsic absorption edge at 310 nm. Meanwhile, due to the light trapping effect of the micro/nanostructures and the presence of oxygen vacancies, favorable light absorption of Ta
2O
5-x/Ta in the full spectral range was revealed. After Ru was loaded on Ta
2O
5-x/Ta, the catalyst exhibited slightly enhanced adsorption throughout the UV–vis wavelength range, thereby establishing a solid foundation for photothermal conversion, which was crucial for photothermal catalysis. Subsequently, the photothermal conversion efficiency of Ta
2O
5-x/Ta and Ru/Ta
2O
5-x/Ta was evaluated. The photothermal temperatures and infrared thermal images recorded at 1.76 W/cm
2 were presented in
Figs. 3b and c. The accuracy of the temperature measurements was confirmed by thermocouples and consistent results obtained from the infrared camera (Tables S3 in Supporting information). The surface temperature of the laser-synthesized Ta
2O
5-x/Ta catalyst increased sharply to 195.7 ℃ under irradiation. Notably, Ta
2O
5-x with different photothermal conversion temperatures can be obtained by controlling the processing power of the laser (Figs. S9 and S10 in Supporting information). After the decoration using Ru nanoparticles, the photothermal temperature of Ru/Ta
2O
5-x/Ta was further increased to 222.7 ℃. This further demonstrated that the incorporation of Ru improved the absorbance of Ru/Ta
2O
5-x/Ta (
Fig. 3a), thereby increasing the photothermal conversion of this catalyst. Of note, when subjected to identical irradiation conditions, the surface temperature of the Ru metal was only 76.2 ℃, implying that the primary photothermal conversion temperature of Ru/Ta
2O
5-x/Ta was provided by defective tantalum oxide (Fig. S11 in Supporting information). By comparing the photothermal properties of laser-prepared Ru/Al
2O
3-x, Ru/TiO
2-x and Ru/Ta
2O
5-x, among them, Ru/Ta
2O
5-x showed more excellent light-absorbing ability, which was conducive to obtaining better photothermal catalytic performance (Figs. S12-S14 in Supporting information).
To identify the catalytic sites of Ru/Ta
2O
5-x/Ta for CO
2 methanation, the photothermal catalytic CH
4 yields of Ta
2O
5-x/Ta and Ru/Ta
2O
5-x/Ta were calculated. As illustrated in Fig. S15 (Supporting information), the thermal insulation characteristic was enhanced by the incorporation of aluminosilicate fibers, resulting in a decreased thermal conductivity of Ru/Ta
2O
5-x/Ta. Ru/Ta
2O
5-x/Ta was placed in a 200 mL reactor for the catalytic reaction (Fig. S16 in Supporting information). As shown in
Fig. 3d, Ta
2O
5-x/Ta was hardly active for CO
2 hydrogenation toward CH
4. Only a small amount of CO (0.297 µmol cm
−2 h
−1) was produced. Although oxygen vacancies in Ta
2O
5-x/Ta facilitated the activation of CO
2, the insufficiency of effective H
2 dissociation sites limited the subsequent hydrogenation process. Conversely, after the introduction of Ru species into the Ta
2O
5-x/Ta system, a notable increase in the production of CH
4 was observed, suggesting that the Ru species acted as efficient active sites for the CO
2 methanation reaction. Notably, as shown in
Fig. 3e and Fig. S17 (Supporting information), the production of CH
4 per unit area of Ru/Ta
2O
5-x/Ta gradually increased with an increase in the loading amount of Ru. However, the mass activity of CH
4 production decreased with an increase in the content of Ru, which was attributed to the increasingly enhanced particle aggregation effect with an increase in the loading amount of Ru (Fig. S18 in Supporting information), which reduced the utilization of Ru [
34]. The optimal catalytic performance of Ru/Ta
2O
5-x/Ta reached 50.6 mol g
(Ru)-1 h
-1 at the Ru loading amount of 0.015 wt%. To verify the role of oxygen vacancies in Ru/Ta
2O
5-x/Ta during photothermal catalysis, these oxygen vacancies were reduced by calcining laser-synthesized Ru/Ta
2O
5-x/Ta in air. As shown in Fig. S19 (Supporting information), the XRD patterns showed that the phase component of the sample did not change after calcination. The EPR results (Fig. S20 in Supporting information) validated the consumed oxygen vacancies after calcination. However, the photothermal temperature of Ru/Ta
2O
5/Ta did not show a significant difference.
Fig. 3d showed that the corresponding CH
4 yield also decreased after the oxygen vacancies were reduced (7.03 mol g
(Ru)-1 h
-1 under 1.76 W/cm
2). This suggested that oxygen vacancies promoted photothermal catalysis by serving as anchoring sites and improved photothermal temperature. The cycle stability test results of Ru/Ta
2O
5-x/Ta in
Fig. 3f displayed that there was no noticeable performance degradation over the ten cycles, proving higher stability. Furthermore, the structural stability of Ru/Ta
2O
5-x/Ta was also verified by the morphology and structure characterization after the photothermal stability test (Figs. S21-S23 in Supporting information). The UV–vis-NIR absorption spectra (Fig. S24 in Supporting information) indicated that Ru/Ta
2O
5-x/Ta retained high optical absorption performance after the cycle stability test.
To further clarify the role of light and photothermal temperature, the temperature and CH
4 yield of Ru/Ta
2O
5-x/Ta at different optical power densities were explored in a batch reactor (Fig. S25 in Supporting information). The results showed that the photothermal temperature and CH
4 yield increased with an increase in the light intensity. To draw a comparison, the thermocatalytic reaction at corresponding temperatures was also carried out in the dark. The results showed that the CH
4 yield and selectivity of Ru/Ta
2O
5-x/Ta in the photothermal system were superior to those in the pure thermal system (Figs. S25 and S26 in Supporting information). The Arrhenius plots of the photothermal and thermal systems in
Fig. 3g showed that the activation energy of photothermal catalysis (48.77 kJ/mol) was lower than that of thermal catalysis (97.71 kJ/mol), which indicated that the introduction of light could significantly decrease the activation energy through a photochemical effect. The photogenerated charge transfer significantly diminished the reaction energy barrier for key intermediates in the CO
2 methanation process, thereby accelerating the reaction rate. In addition, when cooling water was used to exclude the thermal effect under light exposure, a sharp decrease in CH
4 yield was detected to only 0.24 mmol g
(Ru)-1 h
-1 (Fig. S27 in Supporting information). This demonstrated that the thermal effect induced by light exposure was also crucial in this photothermal catalytic reaction.
To assess the photothermal catalytic efficiency of Ru/Ta
2O
5-x/Ta, the CH
4 yields per unit of Ru from recently published photothermal catalysts were presented in
Fig. 3h and Table S4 (Supporting information). The CH
4 yield normalized to Ru for Ru/Ta
2O
5-x/Ta was superior to or comparable to such reported catalysts as Ru/HNT (56.8 mol g
(Ru)-1 h
-1) [
15], 0.5Ru-10Ni/CeZr(AcAc) (39.2 mol g
(Ru)-1 h
-1) [
38], Ru@Ni
2V
2O
7 (32.8 mol g
(Ru)-1 h
-1) [
39], Ru/ZrO
2@C(MIL) (20.45 mol g
(Ru)-1 h
-1) [
40], Ru@FL-LDHs (11.1 mol g
(Ru)-1 h
-1) [
11], 2%Ru-ZnO (5.4 mol g
(Ru)-1 h
-1) [
41], Ru/CeO
2 (4.64 mol g
(Ru)-1 h
-1) [
42], NiRu/CeZr (4.5 mol g
(Ru)-1 h
-1) [
43], RuO
2/SrTiO
3 (3.7 mol g
(Ru)-1 h
-1) [
44], and Ru/Al
2O
3 (3.4 mol g
(Ru)-1 h
-1) [
45]. Even in the comparison of the area activity of these catalysts, the production rate (2.943 mmol cm
-2 h
-1) of Ru/Ta
2O
5-x/Ta was still superior to or comparable to the reported catalysts (Table S5 in Supporting information). These results corroborated the superior intrinsic catalytic activity of Ru/Ta
2O
5-x/Ta. Finally, to demonstrate the practical application of Ru/Ta
2O
5-x/Ta, the photothermal catalytic system composed of a catalyst, a Fennel lens, aluminosilicate fiber wool, and a quartz reactor was constructed under the outdoor sunlight (Fig. S28 in Supporting information). After the reaction gas was injected into the reaction system, the yield of CO and CH
4 was recorded from 10:30 to 14:30. The results presented in
Fig. 3i demonstrate that the CH
4 production rate initially increases and subsequently decreases with time progression, reaching 51.4 mol g
(Ru)-1 h
-1 at 12:30. This observation confirms the sustained excellent catalytic activity of Ru/Ta
2O
5-x/Ta even under natural sunlight conditions.
The H
2 temperature-programmed reduction (H
2-TPR) and CO
2 temperature-programmed desorption (CO
2-TPD) were further employed to investigate the absorption and activation behavior of the Ru/Ta
2O
5-x/Ta catalyst for H
2 and CO
2, respectively. As shown in
Fig. 4a, Ta
2O
5-x/Ta possessed the weak H
2 reduction peak at 437 ℃, which can be attributed to the inert substrates. The TPR profile of Ru/Ta
2O
5-x/Ta displayed two unsymmetrical peaks centered at 134 ℃ and 282 ℃, respectively, which appeared at lower temperatures compared with that of Ta
2O
5-x/Ta due to efficient dissociation of H
2. In terms of CO
2-TPD (
Fig. 4b), Ta
2O
5-x/Ta and Ru/Ta
2O
5-x/Ta possessed similar peak positions and significant signals, indicating that Ta
2O
5-x/Ta provided the activation of CO
2 to a certain degree due to the presence of oxygen vacancies. Of note, the higher CO
2 adsorption peak areas after Ru loading for Ru/Ta
2O
5-x/Ta implied that the stronger absorption ability of CO
2 molecules might be attributed to the construction of the interface between Ru nanoparticles and Ta
2O
5-x. To identify whether the heterogeneous interfaces were active sites for CO
2 adsorption, the calculation based on the first-principles density functional theory (DFT) was conducted to investigate the adsorption and activation characteristics of CO
2. As shown in
Figs. 4c-f and Fig. S29 (Supporting information), the adsorption energy (
Ead) and O═C═O bond angles of CO
2 were calculated by the optimized CO
2 adsorption geometry on Ta
2O
5-x and Ru/Ta
2O
5-x. Distinctly, the
Ead of CO
2 at the interface between Ru and Ta
2O
5-x was more negative (−0.68 eV) than that on the Ta
2O
5-x surface (−0.32 eV) and Ru nanoparticles (−0.46 eV). This demonstrated that the heterogeneous interface between Ru and Ta
2O
5-x was a more efficient adsorption site for CO
2 molecules. Upon the catalytic adsorption of CO
2, the subsequent activation was initiated, which could be revealed by the changes in the O═C═O bond angle [
46]. As depicted, the changes in the O═C═O bond angle at the interface between Ru and Ta
2O
5-x were also more significant than those on the surface of Ru and Ta
2O
5-x in Ru/Ta
2O
5-x. This further validated the effect of the heterogeneous interface on accelerating CO
2 activation. The above theoretical calculation results were consistent with the CO
2-TPD results in
Fig. 4b, demonstrating the construction of interfaces between Ru and Ta
2O
5-x could further enhance the adsorption and activation of CO
2 based on the oxygen vacancies of Ta
2O
5-x.
To clarify the CO
2 to CH
4 reaction pathway and the effect of light on the reaction,
in-situ diffuse reflectance infrared Fourier transform spectroscopy (
in-situ DRIFTS) experiments were performed to detect the production of intermediates. As shown in
Figs. 4g and h, the peak at 1741 cm
-1 was attributed to HCOO* [
46,
47]. The peak observed at 2058 cm
-1 corresponded to the adsorption peak of CO* on Ru, indicating that chemically adsorptive CO species were formed during CO
2 methanation [
48,
49]. Notably, the adsorption peak of CO* on Ru gradually shifted with an increase in temperature, indicating the occurrence of CO* hydrogenation on Ru [
50,
51]. Through a comparative analysis between dark and illuminated conditions, it was found that there was no significant difference in the variety of surface intermediates detected, suggesting a similar reaction pathway for both photothermal and thermal CO
2 hydrogenation. Compared with thermal catalysis (
Fig. 4g), photothermal catalysis (
Fig. 4h) showed adsorption peaks of CO* on Ru as low as 50 ℃. This indicated that the introduction of light could effectively activate CO
2 [
49], which was consistent with the results in
Fig. 3g. When the temperature was above 250 ℃, the strong signals at 1305 cm
-1 and 3016 cm
-1 were ascribed to the C—H stretching vibrations of CH
4. Notably, under light irradiation conditions (
Fig. 4h), an increase in the CH
4 signal coincided with a decrease in the CO* absorption band, indicating that CO* species play a key role as intermediates in the production of CH
4. This finding also revealed that the introduction of light was beneficial to accelerating the conversion of CO*, thereby promoting the formation of CH
4. From the above analysis, a possible reaction pathway for the photothermal hydrogenation of CO
2 on Ru/Ta
2O
5-x/Ta was identified. Firstly, CO
2 molecules were adsorbed and activated on both Ta
2O
5-x and the interface between Ru and Ta
2O
5-x. Meanwhile, the H
2 molecule was chemically dissociated on Ru, thus producing atomic hydrogen species. The produced H* migrated and reacted with the activated CO
2* species to form HCOO*, which was then decomposed into CO* and H
2O (CO
2 → CO
2* → HCOO* → CO*). Following this, CO* species underwent further hydrogenation at the Ru sites. This process resulted in the formation of transient intermediates such as CHO* and CH
3O*, which were the pivotal intermediates in the production of CH
4 (CO* → CHO* → CH
2O* → CH
3O* → CH
4). Through the comparative analysis based on dark and illuminated conditions, light irradiation facilitated CO
2 activation, intermediate formation, and CO* transformation, eventually promoting the production of CH
4.
Overall, the Ru-decorated defective tantalum oxide on Ta foil (Ru/Ta2O5-x/Ta) was successfully prepared using the laser ablation method, and the catalyst was then applied to the photothermal catalytic CO2 methanation reaction. The high-temperature and high-pressure gas plume region within a small space formed by laser treatment facilitated the generation and preservation of oxygen defects, and the presence of oxygen defects successfully anchored the Ru nanoparticles on Ta2O5-x/Ta. In addition, the unique micro- and nano-integrated structures obtained via laser treatment also garnered significant attention in the catalysis field. On the one hand, the laser-prepared Ta2O5-x/Ta structure increased the photothermal conversion temperature under light illumination, which facilitated the photothermal catalytic reaction. On the other hand, the oxygen vacancies acted as anchoring sites to anchor and disperse Ru nanoparticles. With the addition of Ru nanoparticles as efficient active hydrogenation sites to CH4, the interface between Ru and Ta2O5-x served as the adsorption and activation sites of CO2 molecules. Therefore, Ru/Ta2O5-x/Ta exhibited higher photothermal conversion efficiency (reaching 343.4 ℃ at 1.76 W/cm2) and enhanced catalytic activity with a CH4 yield of 50.6 mol g(Ru)-1 h-1 (CH4 selectivity of 98%). The in-situ DRIFTS spectra indicated that light irradiation accelerated CO2 activation and promoted CH4 formation.
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