Photothermal Catalytic Reduction of CO₂ by Cobalt Silicate Heterojunction Constructed from Clay Minerals

Abstract

The coupled utilization of solar and thermal energy is considered an efficient way to improve the efficiency of CO₂ reduction. Herein, palygorskite (Pal) clay is as a silicon source, while Co²⁺ is introduced to prepare two-dimensional Co₂SiO₄ nanosheets, and the excess of Co²⁺ leads to the growth of Co₃O₄ on the surface of Co₂SiO₄ to obtain an S-scheme Co₂SiO₄/Co₃O₄−x heterojunction, which facilitates the charge transfer and maintains higher redox potentials. Benefiting from black color and a narrow band gap, the cobalt oxide on the surface can increase the light absorption and produce a local photothermal effect. Under proper thermal activation conditions, the photoelectrons captured by the abundant oxygen vacancies can obtain a secondary leap to the semiconductor conduction band (CB), suppressing the recombination of electron-hole pairs, thus favoring the electron transfer on Co₂SiO₄/Co₃O₄−x. The composites not only have abundant oxygen vacancies, but also have a large specific surface area for the adsorption and activation of CO₂. The yields of CH₃OH on Co₂SiO₄/Co₃O₄−5% reach as high as 48.9 μmol·g⁻¹·h⁻¹ under simulated sunlight irradiation. In situ DRIFTS is used to explore the photocatalytic reduction CO₂ mechanism. It is found that the thermal effect facilitates the generation of the key intermediate COOH* species. This work provides a new strategy for photothermal catalytic CO₂ reduction by taking advantage of natural clay and solar energy.

1. Introduction

With the rapid development of the fossil energy economy, CO₂ emissions are enormous, which disrupts the balance of nature and causes climate change and serious ecological damage in human society [1]. Therefore, research on CO₂ reduction using renewable fuel technology has received widespread attention [2,3].

Inspired by photosynthetic organisms in nature, visible light reduction is used to convert CO₂ into chemicals with high added value and solar fuels (e.g., CO, HCOOH, CH₃OH, CH₄, etc.) [4]. For example, Lin [5] et al. decomposed CO₂ into CO with an apparent quantum yield of 0.25% at 420 nm by using graphitic carbon nitride (g-CN) semiconductors. However, the yield of conventional photocatalytic reduction of CO₂ is generally low, and the difficulties are mainly related to the limited utilization of solar energy and the low efficiency of photogenerated electron/hole separation [6,7]. Recent studies have shown that the introduction of heat in some photocatalytic reactions can effectively enhance reaction performance [8].

Cobalt-based catalysts with full-spectrum absorption performance and multiple valence states are widely used in CO₂ reduction [9]. The modification of Pal clay can significantly change its physical and chemical properties, such as acidification, alkalization, and ion exchange [10]. In our previous work, a unique solar driven catalysis system was developed to convert cellulose into lactic acid using Cu-modified natural palygorskite (Pal) catalyst [11]. Liu [12] et al. reported on one-dimensional Pal treated with acid to remove most of the metal ions from the silica framework, and bismuth ions were incorporated to grow a two-dimensional (2D) bismuth silicate (Bi₁₂SiO₂₀) nanosheet under a microwave-hydrothermal process. Metal silicates can be prepared from silica and are often used in cement and glass processing, as well as in manufacturing [13].

Herein, acid modification of 1D Pal was performed to remove most of the metal ions in the Pal skeleton and retain its unique silicon-oxygen tetrahedral structure. A 2D cobalt silicate Co₂SiO₄ was synthesized by the microwave hydrothermal method, in which one-dimensional rod silicate was converted to two-dimensional flake silicate. Cobalt oxide particles were grown on the surface of Co₂SiO₄ by adjusting the amount of Co²⁺ in Co₂SiO₄. Co₂SiO₄/Co₃O₄ catalysts not only have a large specific surface area, but they also have abundant oxygen vacancies for the adsorption and activation of CO₂, which effectively achieves full-spectrum absorption and light-induced heat under solar light.

2. Result and Discussion

2.1. XRD

Figure 1a shows the XRD spectra of the Pal, SiO₂ Co₃O₄, Co₂SiO₄, and Co₂SiO₄/Co₃O₄ composites. The peaks located at 8.5°, 20.1°, 28.0°, and 35.8°correspond to the (110), (040), (400), and (161) crystal planes of the original Pal. Compared with the original Pal, the characteristic peaks of acid-treated Pal basically disappeared, indicating that the metal ions in Pal were largely dissolved after a long treatment with high concentrations of hydrochloric acid, corresponding to the SiO₂. The diffraction peaks appearing at 18.8°, 31.0°, and 36.5°, are ascribed to (111), (220), and (311), crystal planes of Co₂SiO₄ (PDF#29-0508). The characteristic peaks at 31.3°, 36.8°, 44.8°, and 65.2° are consistent with the characteristic peaks of cubic spinel Co₃O₄ (PDF#42-1467). It is noteworthy that the diffraction peaks of Co₃O₄ are very close to Co₂SiO₄. Figure 1b shows the magnified view of Figure 1a from 36.5° to 39.5°. The peaks of Co₂SiO₄/Co₃O₄ composites are obviously shifted to a higher angle compared with Co₃O₄ and Co₂SiO₄, indicating a stronger interaction between Co₂SiO₄ and Co₃O₄ [12,14].

2.2. TEM

Figure 2 shows the TEM picture of the Pal, Co₃O₄, Co₂SiO₄, and Co₂SiO₄/Co₃O₄ composites. As shown in Figure 2a, the morphology of Pal is a one-dimensional nanofiber structure with a diameter of about 30–50 nm. After a high concentration of acid treatment, the structure of Pal is destroyed, and the rod-like structure tends to break and assemble into sheets. Figure 2b,f indicates TEM and HRTEM of Co₂SiO₄. The microstructure of Co₂SiO₄ is a two-dimensional flake, which may be due to the preservation of the silicon-oxygen tetrahedron structure in the skeleton after the destruction of Pal [12]. After the addition of Co²⁺, the chain-like structure of the silicon-oxygen tetrahedron regularly grows into two-dimensional cobalt silicate flakes under alkaline hydrothermal conditions. The lattice spacing of the Co₂SiO₄ is 0.09 nm, corresponding to the (400) crystal plane of Co₂SiO₄. Figure 2c,g displays TEM and HRTEM of Co₂SiO₄/Co₃O₄−1%, respectively. It can be seen that, on the surface of Co₂SiO₄, the Co₃O₄ particles have a diameter of about 10 nm. Figure 2d,h shows TEM and HRTEM of Co₂SiO₄/Co₃O₄−5%; Co₃O₄ particles on the surface of Co₂SiO₄ become more stacked. However, when the amount of Co reaches 10%, it is difficult to distinguish Co₂SiO₄ and Co₃O₄, as shown in Figure 2e. The HRTEM images (Figure 2g,h) show that the Co₃O₄ nanoparticles are uniformly loaded on the surface of the Co₂SiO₄ 2D nanosheets. The lattice spacing of Co₃O₄ on the surface is 0.32 nm and 0.33 nm, corresponding to the (311) and (400) planes of Co₃O₄, suggesting the existence of a heterogeneous structure between Co₂SiO₄ and Co₃O₄.

2.3. UV-Vis-NIR

Figure 3a shows the UV-Vis-NIR absorption spectra of the Pal, Co₃O₄, Co₂SiO₄, and Co₂SiO₄/Co₃O₄ composites. Pal has a weak UV-Vis-NIR absorption edge of about 380 nm, while Co₂SiO₄ and Co₂SiO₄/Co₃O₄ composites obtained by modifying Pal have full-spectrum absorption to achieve the conversion of UV-responsive natural silicate minerals to visible-responsive silicates. Most of the cobalt-based compounds appear as black or dark green, which can absorb the full range of the spectrum and are ideal photothermal materials [8,15]. The band gaps of Co₂SiO₄ and Co₃O₄ in Figure 3b are estimated by the formula of (ahv)^n/2 = A (hv − Eg); where a, h, v, A, and Eg represent the optical absorption coefficient, Planck constant, optical frequency, a constant, and band gap. The values n = 1 and n = 4 represent the indirect and direct band gap semiconductors, respectively [16,17]. As shown in Figure 3b, the corresponding band gap energies of Co₂SiO₄ and Co₃O₄ are estimated to be 3.39 and 1.53 eV, respectively. The narrow band gap is more favorable for capturing the energy of sunlight, thus promoting the generation of e⁻/h⁺ pairs [18].

2.4. XPS

Figure 4a shows the XPS survey of Co₃O₄, Co₂SiO₄, and Co₂SiO₄/Co₃O₄−x, where the presence of Co and O elements can be determined; Si elements are present in the Co₂SiO₄ and Co₂SiO₄/Co₃O₄−x. Figure 4b shows the Co 2p spectra of Co₃O₄, Co₂SiO₄, and Co₂SiO₄/Co₃O₄−x. Co can be divided into four peaks, the peaks with binding energies at 794.59 eV and 795.96 eV correspond to Co 2p_1/2. The peaks with binding energies at 779.77 eV and 781.68 eV correspond to Co 2p_3/2, suggesting the presence of +2 valence and +3 valence of Co₃O₄, and also appear in the Co₂SiO₄ and Co₂SiO₄/Co₃O₄−x [19]. The peaks near the binding energy of 781.06 eV and 797.23 eV in (Figure 4b) correspond to Co 2p_1/2 and Co 2p_3/2, respectively. Nevertheless, the binding energy of Co₂SiO₄/Co₃O₄−x is higher than Co₂SiO₄, which indicates that Co₂SiO₄ has a strong interaction with the additional generated Co₃O₄, increasing the electron cloud density around Co [20]. The studies show that the binding energy of Co 2p in the Co₂SiO₄ structure is 781.3 eV, while the binding energy of Co 2p in Co₃O₄ is 779.6 eV. Obviously, the binding energy of cobalt is closer to the former, indicating that the product is Co₂SiO₄, rather than Co₃O₄, which is consistent with the results of the XRD analysis. In addition, the energy separation between Co 2p_1/2 and Co 2p_3/2 of the product is 15.96 eV, which is close to Co₂SiO₄ (ΔCo 2p = 15.5 eV) [21]. Figure 4c shows the O 1s spectra of Co₃O₄, Co₂SiO₄, and Co₂SiO₄/Co₃O₄−x. The peaks around the binding energy of 529.85 eV and 532.82 eV correspond to the lattice oxygen (O_L) and adsorbed oxygen (O_A), respectively. The ratio of O_A to O_L in Co₂SiO₄/Co₃O₄−x is higher than that of Co₃O₄, indicating the composite Co₂SiO₄/Co₃O₄−x has higher content of oxygen vacancy than that of Co₃O₄ [22].

Figure 4d shows the Si 2p spectra of Co₂SiO₄ and Co₂SiO₄/Co₃O₄−x. The Si 2p of Co₂SiO₄ is located at the binding energy of 102.75. The Si 2p characteristic peaks of Co₂SiO₄/Co₃O₄ composites exhibit lower binding energy and weaker peaks than Co₂SiO₄, which may be due to the formation of Si-O-Co bonds on the surface [23].

2.5. EIS, Mott–Schottky Plot, and VB-XPS

Figure 5a indicates that the radius of the electrochemical impedance spectroscopy (EIS) spectrum of the Co₂SiO₄/Co₃O₄−5% sample is far less than other counterparts, indicating that the value of resistance is lowest, and the transfer of surface charges is fastest [24,25]. Figure 5b–d exhibits the Mott–Schottky patterns of Co₂SiO₄, Co₃O₄, and Co₂SiO₄/Co₃O₄−x. The flat band potentials (E_fb) are 0.79, −0.32, and 0.51 V (vs. Ag/AgCl, pH = 7), respectively, suggesting that their Fermi energy levels are 0.99, −0.12, and 0.71 V (vs. normal hydrogen electrode (NHE), pH = 7) [26]. Generally, valence band (VB) XPS represents the distance between the valence band and the Fermi level [27]. The VB-XPS of Co₂SiO₄ and Co₃O₄ are shown in Figure 5e,f, which are 1.34 and 0.43 eV. Therefore, the valence bands (E_VB) of Co₂SiO₄ and Co₃O₄ are 2.33 and 0.31 V, respectively. According to E_CB = E_VB − Eg, the conduction bands (CBs) of Co₂SiO₄ and Co₃O₄ are calculated to be −1.06 and −1.22 V, respectively.

2.6. In-Situ DRIFTS

The reaction pathway of Co₂SiO₄/Co₃O₄−x was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The samples are exposed to simulated solar light irradiation (300 W Xe lamp) for 45 minutes, and exporting spectrograms every 5 min. The peak at 2360 cm⁻¹ can be attributed to the symmetric stretching vibration modes of CO₂ (Figure 6a) [28]. Figure 6b shows the magnified view of Figure 6a from 1750 cm⁻¹ to 1250 cm⁻¹. The peaks from 1600 to 1750 cm⁻¹ correspond to the stretching vibration of C–O. The peaks at 1560 and 1540 cm⁻¹ can be ascribed to COOH* species. The peaks at 1455 and 1340 cm⁻¹ relate to the O–H bending and C–O stretching vibration of COOH* groups, respectively [29,30]. It is observed that the peak of COOH* species becomes stronger with increasing light-irradiation time, indicating that the COOH* is one of the key intermediate species for CO₂-to-CH₃OH conversion [31]. Moreover, the signal of CH₂* is detected at 1385 cm⁻¹, confirming the hydrogenation of HCHO* [12]. The IR results show that the photocatalytic reduction of CO₂ on the Co₂SiO₄/Co₃O₄−x surface goes through a sequential process in which COO− and HCHO act as reaction intermediates to generate methanol groups [32]. According to the DRIFTS analysis, a possible CO₂ reduction pathway is summarized in the following reactions [33]:

Co₂SiO₄/Co₃O₄−x → e⁻ + h⁺

(1)

H₂O + h⁺ → H⁺ + OH⁻

(2)

CO₂ + e⁻ → CO₂*

(3)

CO₂* + H⁺ + e⁻ → COOH*

(4)

COOH* + H⁺ + e⁻ → HCOOH* or CO + H₂O

(5)

HCOOH* + 2H⁺ + 2 e⁻ → HCHO* + H₂O

(6)

HCHO + 2H⁺ + 2 e⁻→ CH₃OH

(7)

2.7. CO₂ Reduction Performance

The photothermal catalytic reduction of CO₂ performance was performed under solar light. The photothermal catalytic CH₃OH generation rate of Co₃O₄, Co₂SiO₄, and Co₂SiO₄/Co₃O₄−x is shown in Figure 7a. For Co₂SiO₄/Co₃O₄−x photocatalysts, the CH₃OH generation rate increases with the increase in Co₃O₄, but excessive Co₃O₄ may inhibit the photocatalytic reduction activity. Co₂SiO₄/Co₃O₄−5% expresses the best CO₂ reduction rates of CH₃OH and CH₄, corresponding to 48.9 and 14.1 μmol·g⁻¹·h⁻¹, which is better than what has been recently reported. After 4 h of sunlight exposure each time, the samples were washed with distilled water for the next test. The samples could maintain considerable photoactivity without significant reduction in activity after five cycling tests (Figure 7b).

2.8. Schematic Mechanism of Photothermal Catalytic Conversion

Figure 8 demonstrates the photothermal catalytic CO₂ reduction mechanism of Co₂SiO₄/Co₃O₄−x composites. Figure 8a shows the before-contact configuration of the band structures of Co₂SiO₄ and Co₃O₄−x. Co₂SiO₄ has a lower Fermi level than Co₃O₄−x (E_f = 0.99), and the free electrons of Co₃O₄−x can be transferred to Co₂SiO₄ until the Fermi reaches equilibrium (E_f = −0.12). After contact, the band edge of Co₂SiO₄ with a higher Fermi level bends downward. In comparison, the band edge of Co₃O₄−x with a higher Fermi level bends upward, thus forming a built-in electric field, facilitating the recombination of electrons in Co₂SiO₄ and holes in Co₃O₄−x. The Co₂SiO₄/Co₃O₄−x catalyst has an ultrathin two-dimensional structure with abundant sites for physisorption of CO₂, and abundant oxygen vacancies for chemisorption and activation of CO₂, which facilitates the reduction reaction. Benefiting from a black color and narrow band gap, Co₃O₄−x can increase light absorption and produce a local photothermal effect. Under proper heat activation, the photoelectrons captured by the abundant oxygen vacancies can obtain a secondary leap to the semiconductor conduction band (CB), suppressing the recombination of electron-hole pairs to a certain extent; thus, the energy-rich environment favors the electron transfer on Co₂SiO₄/Co₃O₄−x. Under solar light, Co₂SiO₄/Co₃O₄−x has full-spectrum absorption and can be stimulated to produce e⁻ and h⁺ pairs. The photogenerated e⁻ in the CB of Co₂SiO₄ and the photogenerated h⁺ in the VB of Co₃O₄−x recombines, facilitated by the built-in electric field, thus establishing an S-scheme process. Since E_CB of Co₃O₄ is more negative than CO₂/CH₃OH (−0.38 eV), the e⁻ in the CB position of Co₃O₄−x is kept converting CO₂ to methanol. Moreover, Co₂SiO₄ has a more positive VB potential energy than H₂O/H⁺ (1.23 eV). The photogenerated holes in the VB of Co₂SiO₄ oxidize H₂O to O₂ and H⁺ ions (Figure 8b).

3. Experimental Section

3.1. Materials and Chemicals

Raw Pal was obtained from Xuyi, China. Cobalt nitrate (Co(NO₃)₂·6H₂O), ammonium sulfate ((NH₄)₂SO₄), ammonium chloride (NH₄Cl), ammonia (NH₃·H₂O), and hydrochloric acid (HCl) were provided by Shanghai Lingfeng chemical company.

3.2. Preparation of SiO₂

An amount of 10 g of Pal was mixed with 30 g of ammonium sulfate and placed in a ceramic crucible. The mixtures were calcined in a muffle furnace at 500 °C for 2 h. An amount of 200 mL 0.5 M HCl was added with refluxing and stirring in a water bath at 80 °C for 6 h. After washing and drying, the SiO₂ powders were obtained.

3.3. Preparation of Co₂SiO₄ and Co₂SiO₄/Co₃O₄

An amount of 0.3 g modified Pal was mixed with 10 mmol Co(NO₃)₂·6H₂O in a beaker; 20 mmol NH₄Cl was added after 2 h, and then 1 mL ammonia was added drop by drop to adjust the pH. The solution was transferred into a 100 mL microwave hydrothermal reactor for hydrothermal treatment at 160 °C for 90 min. After the reaction, the samples were centrifuged, washed, dried overnight, and the Co₂SiO₄ was obtained.

The preparation method of Co₂SiO₄/Co₃O₄−x composite material was the same as Co₂SiO₄, except the quantity of Co(NO₃)₂·6H₂O was adjusted to deposit Co₃O₄. The different mass ratios in the composite material were denoted as Co₂SiO₄ /Co₃O₄−x; where x represents the theoretical mass fraction of Co₃O₄ in the composite from 1% to 10%. The preparation of pure Co₃O₄ was the same as this method without the addition of SiO₂.

3.4. Catalyst Characterizations

The XRD was used to measure the phase structure of materials on a Rigaku D/max 2500PC (Rigaku Corporation, Tokyo, Japan) diffractometer using Cu-Kα radiation (λ = 1.5406 Å) with a scanning range of 2θ from 5° to 80°. The TEM was operated on a JEOL-2100 (Japan Electronics Co., Ltd., Tokyo, Japan) with 200 kV working voltage. The UV-vis-NIR was carried out to measure the optical properties of materials with an integrating sphere (UV-3600, DRS, SHIMADZU, Kyoto, Japan). The XPS was performed on a Quantum 2000 Scanning ESCA Microprobe instrument (Thermo Nicolet Evolution, Thermo Fisher, Waltham, MA, USA). The photocurrent response and Mott-Schottky measured the optical properties of materials using an electrochemical workstation equipped with a 300 W Xe lamp and standard three electrodes (Auto Lab 302N, CH Instruments, Shanghai, China). In situ DRIFTS was conducted by in situ FT-IR spectrometer (Nicolet iS20 FT-IR, ThermoFisher, USA), with a specific designed reaction cell and MCT detector with 120 scans and a resolution of 4 cm⁻¹.

3.5. Photothermal Catalytic CO₂ Reduction Experiments

The photothermocatalytic activity of the samples was carried out in a closed photochemical reactor coupled with a condensation system and gas chromatography. First, 0.1 g of catalysts were added into 100 mL of water in the bottle in the reactor. After sealing, the reactor was infiltrated with CO₂ gas (99.99%) for 45 min to ensure that the reactor was filled with pure CO₂ and to remove the impurity gasses. Then, at room temperature, a xenon lamp was irradiated through the transparent window (diameter 30~40 mm) on the top of the visible reactor. During the reaction process, the temperature in the reactor was monitored, the reaction sampling interval was 30 min, while the mixture product was obtained from the photochemical reactor with a syringe, and the gaseous product was taken out with a gastight needle. The reaction product was detected by a gas chromatograph (GC-7860 Plus, Shanghai Xinuo Instrument Co., Ltd., Shanghai, China) with a flame ionization detector (FID) and a thermal conductivity detector (TCD).

4. Conclusions

In summary, a novel cobalt silicate nanosheet composite (Co₂SiO₄/Co₃O₄−x) was prepared using Pal clay as a silicon source. The acid-modified Pal removed most of the metal ions and retained its unique silicon-oxygen tetrahedral structure, then Co²⁺ were incorporated to acquire Co₂SiO₄ nanosheet and Co₂SiO₄/Co₃O₄−x heterojunction, which realized the conversion of one-dimensional rod silicate to two-dimensional flake silicate. Co₂SiO₄/Co₃O₄−x not only have abundant oxygen vacancies, but also have large specific surface areas for the adsorption and activation of CO₂. The yields of CH₃OH on Co₂SiO₄/Co₃O₄−5% were as high as 48.9 μmol·g⁻¹·h⁻¹ under simulated sunlight irradiation. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to explore the photocatalytic reduction CO₂ mechanism. Co₂SiO₄/Co₃O₄−x effectively and stably achieved full-spectrum absorption and light-induced heat under solar light. The photothermal synergistic effect facilitated the generation of key intermediate COOH* species to enhance methanol production. This work provides a new approach for photothermal catalytic CO₂ reduction by taking advantage of natural clay and solar energy.