Drastic improvement in the photocatalytic activity of Ga₂O₃ modified with Mg–Al layered double hydroxide for the conversion of CO₂ in water

Abstract

Photocatalytic conversion of CO₂ to useful chemicals is a promising countermeasure against increasing concentrations of CO₂ in the atmosphere as photocatalytic reactions can drive a thermodynamically uphill reaction (so-called “artificial photosynthesis”). Our group has earlier reported that Ga₂O₃ photocatalysts loaded with a Ag cocatalyst exhibit high activity for the conversion of CO₂ to CO in water, and further modification of the catalysts with ZnGa₂O₄ improves the selectivity for CO over the other reduction products by suppressing H₂. In this study, Ga₂O₃ modified with a Mg–Al layered double hydroxide (LDH) considerably enhanced not only the amount of CO evolved but also the selectivity toward CO evolution. By using 1.0 g of a Mg–Al LDH/Ga₂O₃ composite photocatalyst loaded with 0.25 wt% of a Ag cocatalyst, the CO formation rate and selectivity for CO were 211.7 μmol h⁻¹ and 61.7%, respectively. Moreover, the turnover number (TON) of electrons utilized to reduce CO₂ to CO was 75.4 per Ag atom during 5 h of photoirradiation.

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Introduction

Photocatalysis has been increasingly attracting attention because it can be used to convert light energy to chemical energy, which can be stored. This conversion has implications for the current critical environmental issues. The photocatalytic splitting of H₂O to H₂ and O₂ under solar light illumination has been already reported.^1–3 H₂ can be used as an energy source for various applications, such as fuel cells and hydrogen internal combustion engines. Photocatalytic artificial photosynthesis involves photoabsorption (light harvesting), charge separation, and H₂O oxidation, as well as the consumption of the photogenerated electrons for reduction.^4,5 Because it is important to remove CO₂ (a greenhouse gas) from the atmosphere, a photocatalytic reaction that can reduce CO₂via an uphill process (ΔG > 0) can be a promising technique.^5–7 Previously, our group has reported the photocatalytic activity of Ga₂O₃, Zn-modified Ga₂O₃, and ZnGa₂O₄ for the conversion of CO₂ to CO in an aqueous NaHCO₃ solution in the presence of a Ag cocatalyst.^8–10 Although the Ag-loaded Ga₂O₃ photocatalyst exhibited high performance, the selectivity toward CO evolution among the reduction products was extremely low, and H₂ was predominantly formed.⁸ As the process for H₂ formation competes for the electrons required for CO₂ reduction, further optimization is crucial for suppressing this process. In this regard, a Ga₂O₃ photocatalyst modified with a Zn species has been reported to drastically increase the selectivity toward CO evolution by suppressing H₂ evolution.¹⁰ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analyses have revealed that after calcination at 1223 K, ZnGa₂O₄ is formed on the surface of Ga₂O₃ modified with Zn species.^8,10 The ZnGa₂O₄ thus formed, which itself can act as a photocatalyst,¹⁰ clearly suppresses H₂ production and improves the selectivity toward CO evolution among the reduction products. The CO evolution rate (108 μmol h⁻¹ for 1.0 g of Ag/ZnGa₂O₄/Ga₂O₃) was deemed insufficient, while the selectivity toward CO evolution was improved by modification with ZnGa₂O₄.

Modification of a photocatalyst with a solid base material, which can provide adsorption sites for CO₂ molecules, is promising to regulate the selectivity for the photocatalytic conversion of CO₂ in water. For this purpose, MgO-modified TiO₂,^11,12 magnesium/aluminum layered double oxide (LDO)-grafted TiO₂,¹³ and self-assembled carbon nitride (C₃N₄), as well as different types of LDHs (LDH-C₃N₄),¹⁴ have been reported to exhibit photocatalytic activity for CO₂ conversion. Moreover, our group has reported that a Ta₂O₅ photocatalyst modified with alkaline earth metal oxides, particularly SrO, drastically improves the selectivity toward CO evolution for CO₂ conversion in an aqueous NaHCO₃ solution.¹⁵ Layered double hydroxides (LDHs) are known as anionic clays, consisting of highly ordered two-dimensional hydroxide sheets and charge-compensating interlayer anions.^16,17 The general formula of LDHs is [M_1−α²⁺M_α³⁺(OH)₂]^α+(A_α/nⁿ⁻)·mH₂O, where M²⁺ and M³⁺ represent the divalent and trivalent cations, respectively; α represents the molar ratio of M³⁺/(M²⁺ + M³⁺), and Aⁿ⁻ is the interlayer anion with a valence of n.¹⁸ Mg–Al LDH (Mg₆Al₂(OH)₁₆(CO₃)·4H₂O), the most well-known LDH, is widely used as a solid base material for the removal of various anions from wastewater,^19–21 for base-catalyzed reactions,^22–24 and for CO₂ capture and storage.²⁵ Moreover, Ebitani et al. have reported that Mg–Al LDH catalyzes the aldol condensation of carbonyl compounds in an aqueous solution,²⁶ indicating that the Mg–Al LDH and its derivatives can be used as solid base catalysts in the presence of water because their surface basic sites exhibit high water tolerance.

Meanwhile, the photocatalytic conversion of CO₂ in water using a series of LDH photocatalysts, whose surface properties can drive the selective reduction of CO₂ to CO by suppressing H₂ evolution, has also been reported by our group.^27–29 In this study, drastic changes in the amount of CO evolved and the selectivity toward CO by the modification of a Ga₂O₃ photocatalyst with Mg–Al LDH were investigated.

Experimental section

Preparation of Mg–Al LDH-modified Ga₂O₃

Gallium oxide (Ga₂O₃, 99.99%, Kojundo Chemical Laboratory Co., Ltd.) was used as received. First, an aqueous solution of metal components (5.80 g of MgCl₂·6H₂O (Kojundo Chemical Laboratory Co., Ltd.) and 1.27 g of AlCl₃ (Wako Pure Chemical Industries, Ltd.), total volume 333 mL) was slowly dropped into 0.06 M aqueous Na₂CO₃ (Wako Pure Chemical Industries, Ltd.) solution containing the required amount of Ga₂O₃ powder while simultaneously maintaining the pH between 9.9 and 10.1 by the addition of aqueous NaOH solution. The Mg/Al ratio in Mg–Al LDH was maintained constant at 3. The resulting suspension was aged at 333 K for 22 h at ambient pressure with vigorous stirring. The solid precipitate was washed with ultrapure water and collected by suction filtration. The cake was dried at 333 K under vacuum. The concentration of modified Mg–Al LDH was varied from 30 to 95 mol% in accordance with eqn (1).

x (mol%) = 100 × (A_Mg + A_Al)/(A_Mg + A_Al + A_Ga). (1)

Here, A_Mg, A_Al, and A_Ga represent the amount of Mg atoms, Al atoms, and Ga₂O₃ in the prepared photocatalyst. For example, x = 50 implies that the molar ratio of (Mg + Al)/Ga₂O₃ = 1.0. Hereafter, x-MgAl/Ga₂O₃ represents Mg–Al LDH-modified Ga₂O₃ with a molar ratio of x. Reference composite samples of x-Mg/Ga₂O₃ and x-Al/Ga₂O₃, corresponding to Mg(OH)₂- and Al(OH)₃-modified Ga₂O₃, respectively, were prepared by the same procedure as that used to prepare x-MgAl/Ga₂O₃.

Loading of the Ag cocatalyst on the photocatalysts

The Ag cocatalyst was loaded on the photocatalysts by impregnation. The required amount of the as-synthesized x-MgAl/Ga₂O₃ powder was dispersed in an aqueous solution containing the required amount of AgNO₃ (Wako Pure Chemical Industries, Ltd.) and evaporated at 353 K. The resulting powder was calcined at 723 K for 2 h under air. In this study, yAg/x-MgAl/Ga₂O₃ represents a Ag cocatalyst (y wt%) loaded on a x-MgAl/Ga₂O₃ photocatalyst.

Preparation of the reference photocatalysts

Ag cocatalyst-loaded Mg–Al LDH (Ag/MgAl). Mg–Al LDH was synthesized by co-precipitation as described above with the exception of using Ga₂O₃ powder in aqueous Na₂CO₃. The Ag cocatalyst (1.0 wt%) was loaded on the prepared Mg–Al LDH by impregnation as described above.

Ag cocatalyst-loaded bare Ga₂O₃ (yAg/Ga₂O₃). The Ag cocatalyst (y wt%) was loaded on Ga₂O₃ by the impregnation method as described above.

Mg–Al LDH-modified yAg/Ga₂O₃ (x-MgAl/yAg/Ga₂O₃). An aqueous solution of precursors (MgCl₂·6H₂O and AlCl₃) was slowly dropped into aqueous Na₂CO₃, which contains the yAg/Ga₂O₃ powder, by the same procedure as that utilized for x-MgAl/Ga₂O₃. A Ag cocatalyst was primarily loaded on a Ga₂O₃ photocatalyst for this reference material, and then Mg–Al LDH was prepared on it, while the Ag cocatalyst was deposited on Mg–Al LDH-modified Ga₂O₃ finally using the normal procedure.

Physical mixture of Mg–Al LDH and yAg/Ga₂O₃ (x-MgAl + yAg/Ga₂O₃). Calculated amounts of the prepared Mg–Al LDH and yAg/Ga₂O₃ powders were sufficiently mixed in an agate mortar.

Ag cocatalyst-loaded SiO₂-modified Ga₂O₃ (yAg/x-Si/Ga₂O₃). The required amounts of Ga₂O₃ powder, tetraethyl orthosilicate (TEOS, Wako Pure Chemical Industries, Ltd.), and urea (Wako Pure Chemical Industries, Ltd.) were mixed in ultrapure water, and the resulting suspension was aged at 373 K for 24 h under hydrothermal conditions in a stainless-steel autoclave equipped with an inner Teflon vessel. The impregnation of the Ag cocatalyst and calcination at 723 K afforded the yAg/x-Si/Ga₂O₃ composite.

Catalyst characterization

Powder X-ray diffraction (XRD) patterns of the prepared samples were recorded on an XRD instrument (Ultima IV, Rigaku) using Cu K_α radiation (λ = 0.154 nm). The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were estimated from their N₂ adsorption isotherms at 77 K (liquid N₂ temperature) using a volumetric adsorption measurement instrument (BELSORP-miniII, MicrotracBEL). Prior to the measurement, the powder sample was evacuated at 373 K for 1 h using a pretreatment instrument (BELPREP-vacII, MicrotracBEL). Scanning electron microscopy (SEM) images of the synthesized samples were recorded on a field-emission scanning electron microscopy instrument (FE-SEM, SU-8220, Hitachi High-Technologies) at an accelerating voltage of 3.0 kV. Transmission electron microscopy (TEM) images were recorded on a JEM-2100F TEM system (Japan Electron Optics Laboratory; JEOL) using an accelerating voltage of 200 kV. Further characterizations such as UV/Vis diffuse reflectance spectra (DRS), surface atomic ratios of metal components based on X-ray photoelectron spectroscopy (XPS) measurements, Ag 3d XPS measurements, TEM images of the photocatalysts, and transient photocurrent measurements are available in the ESI.†

Photocatalytic conversion of CO₂ in an aqueous solution

The photocatalytic activity for the conversion of CO₂ in an aqueous solution was evaluated using an inner-irradiation type reaction vessel in a quasi-flowing batch system. First, photocatalyst powder was dispersed in 1.0 L of an aqueous NaHCO₃ solution, and the dissolved air in this suspension was degassed by a flow of high-purity CO₂ (99.999%). CO₂ was continuously bubbled into the reaction solution at a flow rate of 30 mL min⁻¹. The suspension was irradiated using a 400 W high-pressure Hg lamp with a quartz filter equipped with a cooling water system. The gaseous products thus obtained, e.g., CO, H₂, and O₂, were analyzed by gas chromatography (GC-2010 Tracera, Shimadzu Corp.) using a MICROPACKED ST column equipped with an on-line barrier discharge ionization detector. The formation rates (μmol h⁻¹) of these products were calculated using a 1.0 mL volumetric pipe. The selectivity toward CO evolution in the reduction products and the balance between the consumed electrons (e⁻) and holes (h⁺) were expressed by eqn (2) and (3), respectively.

Selectivity toward CO evolution (%) = 100 × R_CO/(R_CO + R_H2). (2)

Consumed e⁻/h⁺ = (R_CO + R_H2) × 2/(R_O2 × 4). (3)

Here, R_CO, R_H2, and R_O2 represent the formation rates of CO, H₂, and O₂, respectively.

Results and discussion

Fig. 1 shows the XRD patterns of the synthesized composite photocatalysts (A and B) and reference samples (C and D). Bare Ga₂O₃ (D) exhibited a typical reflection pattern corresponding to β-phase Ga₂O₃.³⁰ In the XRD pattern of bare Mg–Al LDH (C), sharp diffraction peaks, corresponding to the reflections of the (003), (006), and (009) planes, were observed at 11.5°, 23.2°, and 34.8°, respectively.¹⁶ The positions of these peaks should be determined by the interlayer distance between each hydroxide sheet; a previous study has reported that carbonate ions (CO₃²⁻) are mainly intercalated in the interlayer space.³¹ Moreover, two diffraction peaks were observed at around 60°, corresponding to the reflections of the (110) and (113) planes, respectively. As compared to bare Ga₂O₃ (D) and bare Mg–Al LDH (C), the XRD pattern of 95-MgAl/Ga₂O₃ (B) showed a mixed phase comprising Ga₂O₃ and Mg–Al LDH. The peak position corresponding to the reflection of the (003) plane was the same as that observed for bare Mg–Al LDH, indicating that CO₃²⁻ intercalated Mg–Al LDH was formed in the 95-MgAl/Ga₂O₃ composite. With the incorporation of Ga³⁺ into the hydroxide sheets within the LDH structure, the peak position corresponding to the reflection of the (110) plane is expected to be shifted to a low angle, because the ionic radii of six-fold coordinated Al³⁺ and Ga³⁺ are 53.5 and 62.0 pm, respectively.³² Based on this consideration, the peak corresponding to the reflection of the (110) plane slightly shifted from 60.7° to 60.4°, suggesting that the LDH structure contains marginal amounts of Ga³⁺ derived from Ga₂O₃. In other words, the results from XRD analysis indicated that Mg–Al LDH primarily consists of Mg²⁺, Al³⁺, and a small amount of Ga³⁺ within the hydroxide sheets, and a chemical connection is formed between Ga₂O₃ and Mg–Al LDH through these hydroxide sheets. The absorption edge of the 95-MgAl/Ga₂O₃ composite photocatalyst in the UV/Vis diffuse reflectance spectra (Fig. S1†) was almost the same as that of bare Ga₂O₃, indicating that the modification of Ga₂O₃ with Mg–Al LDH does not affect its absorption properties. With the Ag cocatalyst loading of 0.25 wt% on the 95-MgAl/Ga₂O₃ composite, a broad absorption shoulder was observed at approximately 280–330 nm, corresponding to the plasmonic absorption of Ag metal particles or the d–d transitions of Ag₂O. Fig. 2 shows the SEM and TEM images of the composite photocatalysts. Several micrometer-sized pillar-shaped particles with several cracks and pore structures were observed in the SEM images of Ga₂O₃ (A). In the SEM images of the composite photocatalyst (B), almost all of the Ga₂O₃ particles were covered by platelet-like particles, which is characteristic of Mg–Al LDHs. Moreover, a core–shell structure (Ga₂O₃ core and Mg–Al LDH shell) was clearly observed in the TEM image (C). Small black circles observed in the magnified view of the TEM image (D) were expected to be Ag-cocatalyst particles, with a diameter of several nanometers. Fig. 3 shows the formation rate of the gas evolved, the selectivity toward CO evolution, and the consumed e⁻/h⁺ ratio in the photocatalytic conversion of CO₂ in water under UV light irradiation. 0.25Ag/95-MgAl/Ga₂O₃ (A) exhibited excellent activity for the photocatalytic conversion of CO₂ to CO with a high selectivity of greater than 60%. CO₂ is thought to be reduced by the photogenerated electrons in preference to proton (H₂O). On the other hand, 0.25Ag/Ga₂O₃ (D) mainly produced H₂ as the reduction product. The modification with Mg–Al LDH significantly improved the activity and selectivity for the photocatalytic conversion of CO₂; nevertheless, Ag-loaded bare Mg–Al LDH (C) did not exhibit activity. The consumed e⁻/h⁺ ratio was approximately 1.0 for (A), (B), and (D), indicating that stoichiometric “reduction” and “oxidation” successfully occur.

Fig. 1 XRD patterns of (A) 0.25Ag/95-MgAl/Ga₂O₃, (B) 95-MgAl/Ga₂O₃, (C) Mg–Al LDH, and (D) Ga₂O₃.

Fig. 2 SEM images of (A) Ga₂O₃ and (B) 95-MgAl/Ga₂O₃, and TEM images of (C) 95-MgAl/Ga₂O₃ and (D) 1.0Ag/95-MgAl/Ga₂O₃.

Fig. 3 Formation rate of the products evolved (bar graphs), selectivity toward CO evolution (black circle), and the consumed e⁻/h⁺ ratio (red diamond) in the photocatalytic conversion of CO₂ in water using (A) 0.25Ag/95-MgAl/Ga₂O₃, (B) 95-MgAl/Ga₂O₃, (C) 1.0Ag/Mg–Al LDH, and (D) 0.25Ag/Ga₂O₃. Red: CO, green: O₂ and blue: H₂. Weight of Ga₂O₃ in the photocatalysts (A, B, and D) 0.19 g; weight of the photocatalyst (C) 0.5 g; reaction solution: 1.0 L of an aqueous NaHCO₃ solution (0.1 M); CO₂ supply: 30 mL min⁻¹; light source: a 400 W high-pressure Hg lamp (through a quartz glass jacket); photoirradiation time: 1 h.

Fig. 4 shows the XRD patterns of 1.0Ag/x-MgAl/Ga₂O₃ (where x = 0, 50, 70, 85, and 95). The (003) plane reflection was not observed for 1.0Ag/0-MgAl/Ga₂O₃ (G) and 1.0Ag/50-MgAl/Ga₂O₃ (F), whereas an XRD pattern was observed for 1.0Ag/x-MgAl/Ga₂O₃ (x = 70, 85, and 95), with diffraction peaks corresponding to the LDH layer structure despite performing calcination at 723 K during the loading of the Ag cocatalyst. However, when bare Mg–Al LDH is calcined at 723 K, it is known to decompose to an amorphous mixed oxide, accompanied with the collapse of the layered structure, as shown in (A).¹⁶ This result indicates that the thermal stability of Mg–Al LDH in the 1.0Ag/x-MgAl/Ga₂O₃ composite photocatalyst is better than that of bare Mg–Al LDH, probably because of the chemical interaction between LDH and Ga₂O₃. Table 1 summarizes the differences in the specific surface areas and photocatalytic activities with different molar ratios of Mg–Al LDH. Only Ga₂O₃ was considered to serve as a photocatalyst in this composite material because Ag-loaded bare Mg–Al LDH did not exhibit any photocatalytic activity. Therefore, the amount of photocatalyst that can generate photoexcited electrons and holes should vary with the x value; for example, 500 mg of the 50-MgAl/Ga₂O₃ composite contains 420 mg of the Ga₂O₃ photocatalyst and 80 mg of Mg–Al LDH. Ag-Loaded bare Ga₂O₃ (x = 0) produced H₂, O₂, and CO in the stoichiometric balance of e⁻ to h⁺ (e⁻/h⁺ = 1.03). The selectivity for CO over the other reduction products was 29.9% for bare Ga₂O₃, indicating that the photogenerated electrons are considerably consumed for the reduction of protons (H⁺) to H₂. Meanwhile, with the increase of x (x = 30 and 50), the total photocatalytic activity (determined from the amount of evolved O₂) improved despite the decrease in the amount of Ga₂O₃. The balance of e⁻/h⁺ was maintained approximately stoichiometric by varying the x value. With respect to the reduction products, the modification of Ga₂O₃ with Mg–Al LDH led to the increase in the formation rate of CO evolved from the reduction of CO₂. In particular, the formation rate of CO was 149.7 μmol h⁻¹ for x = 50; this value is more than twice as high as that observed for bare Ga₂O₃. With the increase of x to greater than 50, the photocatalytic activity, and hence, the formation rate of CO, gradually decreased with the increase in the Mg–Al LDH content (i.e., x). For example, the formation rate of O₂ for x = 95, reflecting the total photocatalytic activity, was less than half of that for bare Ga₂O₃. In contrast, the selectivity for CO increased with increasing x. The suppression of H₂ is considered to be particularly important because the reduction of H⁺ to H₂ easily occurs for the photocatalytic conversion of CO₂ in water. The 1.0Ag/95-MgAl/Ga₂O₃ composite photocatalyst exhibited 71.2% selectivity for CO. In other words, the Mg–Al LDH modification of Ga₂O₃ effectively suppresses H₂ evolution. Based on the result obtained for bare Mg–Al LDH, which itself was not active for this reaction, the composite photocatalyst comprising Ga₂O₃ and Mg–Al LDH is crucial for the selective reduction of CO₂ to CO. As shown in Fig. S2(a–c),† the particle size of the Ag cocatalyst loaded on the photocatalyst was dispersed by the modification of Ga₂O₃ with Mg–Al LDH. The CO evolution with high selectivity in the case of the 1.0Ag/95-MgAl/Ga₂O₃ composite photocatalyst might be caused by not only the alteration of surface properties but also the existence of highly-dispersed Ag cocatalyst particles. Moreover, a small shift to high binding energy in the Ag 3d X-ray photoelectron spectrum (Fig. S3(d)†) indicated that the as-synthesized Ag cocatalyst particle, which was mainly composed of Ag metal, was slightly oxidized by the use in the photocatalytic conversion of CO₂ in water. The atomic ratio of Ga on the surface of the composite photocatalysts clearly decreased with the increasing molar ratio of Mg–Al LDH (Fig. S4†), reflecting that the shielding effect of Mg–Al LDH also drastically decreases the total photocatalytic activity. Accordingly, the anodic photocurrent of the Ga₂O₃ photocatalyst in the presence of methanol as a sacrificial reagent remarkably decreased by the modification with Mg–Al LDH (Fig. S5†). SEM and TEM images (Fig. 2), photocatalytic activities (Table 1), the surface atomic ratio (Fig. S4†), and photocurrent values (Fig. S5†) clearly revealed that the modification of Ga₂O₃ with Mg–Al LDH improves the selectivity for CO, while the total photocatalytic activity is suppressed by inactive Mg–Al LDH.

Fig. 4 XRD patterns of (A) Mg–Al LDH calcined at 723 K, (B) Mg–Al LDH, (C) 1.0Ag/95-MgAl/Ga₂O₃, (D) 1.0Ag/85-MgAl/Ga₂O₃, (E) 1.0Ag/75-MgAl/Ga₂O₃, (F) 1.0Ag/50-MgAl/Ga₂O₃, and (G) 1.0Ag/Ga₂O₃.

Table 1 Specific surface area (S_BET) and the results from the conversion of CO₂ in water for the x-MgAl/Ga₂O₃ composite photocatalyst (x = 30, 50, 70, 75, and 95), bare Ga₂O₃, and bare Mg–Al LDH. The weight of the Ag cocatalyst: 1.0 wt%. The total weight of the photocatalyst was fixed at 0.5 g. Reaction solution: 1.0 L of an aqueous NaHCO₃ solution (0.1 M). CO₂ supply: 30 mL min⁻¹. Light source: a 400 W high-pressure Hg lamp (through a quartz glass jacket). Photoirradiation time: 1 h

Photocatalyst	S BET/m^2 g^−1	Weight/g		Formation rate of products evolved/μmol h^−1			Selectivity toward CO evolution (%)	Consumed e^−/h^+ ratio
		Total	Ga2O3	H2	O2	CO
Bare Ga2O3	13	0.50	0.50	147.3	101.6	62.9	29.9	1.03
30-MgAl/Ga2O3	16	0.50	0.46	167.5	137.5	116.9	41.1	1.03
50-MgAl/Ga2O3	22	0.50	0.42	200.7	168.7	149.7	42.7	1.04
70-MgAl/Ga2O3	35	0.50	0.35	169.7	149.9	145.5	46.2	1.05
75-MgAl/Ga2O3	47	0.50	0.30	107.6	125.0	145.3	57.5	1.01
95-MgAl/Ga2O3	92	0.50	0.09	21.7	36.9	53.7	71.2	1.02
Bare Mg–Al LDH	89	0.50	0.00	Trace	Trace	Trace	—	—

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Fig. 5 shows the result obtained from control experiments for the photocatalytic conversion of CO₂ in an aqueous NaHCO₃ solution using the 95-MgAl/Ga₂O₃ composite photocatalyst. The effects of the presence of the photocatalyst, photoirradiation, addition of NaHCO₃ into the reaction solution, flow of CO₂ gas, and loading of the Ag cocatalyst on 95-MgAl/Ga₂O₃ were investigated. A photocatalytic reaction did not occur in the absence of the photocatalyst (A) and in the absence of photoirradiation (B). With the irradiation of UV light, stoichiometric amounts of H₂ and O₂ were formed from the photocatalytic splitting of pure water (C). In this experiment, H₂ was mainly produced as the reduction product despite the supply of CO₂ to the system and the stable reaction solution pH of 6.0, which is probably not suitable for the reduction of CO₂ to CO. Under He gas flow (D), water splitting to H₂ and O₂ was accompanied with marginal amounts of formed CO, presumably owing to the presence of the NaHCO₃ additive. Photocatalytic activity for CO₂ conversion was not observed in the absence of the Ag cocatalyst (E). These results indicated that all of the above-discussed factors are crucial to the conversion of CO₂ to CO using the 95-MgAl/Ga₂O₃ composite photocatalyst. We previously reported that bidentate HCO₃–Ga derived by reacting the dissolved CO₂ molecule in water with the hydroxyl group anchored on the Ga atoms is the intermediate species for the photocatalytic conversion of CO₂ in water over the Ga₂O₃-based photocatalyst.³³ In this sense, the selectivity toward CO evolution was changed with the concentration of dissolved CO₂ molecules in the reaction solution, which should be altered by the above listed factors and the pH value.

Fig. 5 Amount of products evolved from various control experiments for the conversion of CO₂ in water using the 0.25Ag/95-MgAl/Ga₂O₃ composite photocatalyst. Red circle: CO, green square: O₂, blue triangle: H₂. (A) Without photocatalyst powder, (B) without irradiation (dark conditions), (C) without the NaHCO₃ additive, (D) without the CO₂ gas flow (He gas flow), (E) without the Ag cocatalyst, and (F) normal conditions. Photocatalyst weight: 1.0 g, reaction solution: 1.0 L of an aqueous NaHCO₃ solution (0.1 M), CO₂ supply: 30 mL min⁻¹, light source: a 400 W high-pressure Hg lamp (through a quartz glass jacket), and photoirradiation time: 5 h.

Fig. 6 shows the formation rates of the products evolved from the conversion of CO₂ in an aqueous NaHCO₃ solution using the 95-MgAl/Ga₂O₃ composite photocatalyst with various amounts of the Ag cocatalyst. The 95-MgAl/Ga₂O₃ photocatalyst did not exhibit activity for the conversion of CO₂ to CO in the absence of the loaded Ag cocatalyst. However, the introduction of 0.1 wt% Ag led to considerable enhancement of CO evolution. The formation rate of CO was 213.9 μmol h⁻¹ for the 95-MgAl/Ga₂O₃ photocatalyst with the Ag cocatalyst loading of 0.1 wt%, whereas it was only 1.4 μmol h⁻¹ for the 95-MgAl/Ga₂O₃ photocatalyst without any Ag cocatalyst. From this result, it can be concluded that the presence of the Ag cocatalyst is crucial for the conversion of CO₂ to CO in this photocatalytic system, as described above. Furthermore, with increasing Ag cocatalyst loading on the 95-MgAl/Ga₂O₃ photocatalyst, the selectivity toward CO evolution was improved. However, the actual amount of the evolved products drastically decreased, and the formation rates of H₂, O₂, and CO were 26.5, 45.1, and 53.3 μmol h⁻¹, respectively, with a Ag cocatalyst loading of 1.0 wt%. The sintered Ag particles, which were observed in the TEM images of 1.0Ag/95-MgAl/Ga₂O₃ and 3.0Ag/95-MgAl/Ga₂O₃ (Fig. S2(e) and (f)†), might induce a recombination of the photogenerated charges. In contrast, the conspicuous Ag particles were not found in the TEM image of the 0.25Ag photocatalyst (Fig. S2(d)†), indicating that the highly-dispersed Ag cocatalyst, which exhibits supreme activity for the selective conversion of CO₂, was loaded on it. By considering the selectivity and formation rate of the product, the 0.25Ag/95-MgAl/Ga₂O₃ composite photocatalyst is the most suitable catalyst for the conversion of CO₂ to CO under UV light irradiation. The formation rate of CO using the catalyst loaded with 0.25 wt% Ag was 211.7 μmol h⁻¹. This value reflects remarkably high activity as the actual amount of the reduction products of CO₂ (211.7 μmol h⁻¹ per 0.19 g of Ga₂O₃) is greater than that reported previously.⁶ Typically, the amount of the products evolved in the photocatalytic reaction is often expressed as unreal amounts, which is normalized by the photocatalyst weight, photoirradiation area, or light source intensity (for example, the normalized activity with the photocatalyst weight for 0.25Ag/95-MgAl/Ga₂O₃ was 1114 μmol h⁻¹ g_Ga2O3⁻¹, g_Ga2O3: weight of Ga₂O₃ in the composite photocatalyst). In this case, the turnover number (TON) of the consumed electrons per Ag atom for CO evolution was 75.4 during 5 h of photoirradiation. Moreover, the ratio of the consumed electrons to holes, e⁻/h⁺, was 1.01 after 5 h of photoirradiation, indicating that this photocatalytic system reduces CO₂ into CO, accompanied with the stoichiometric oxidation of H₂O into O₂.

Fig. 6 Effect of the amount of the Ag cocatalyst on the formation rates of products evolved and the selectivity toward CO evolution in the photocatalytic conversion of CO₂ in water. Red: CO, green: O₂, and blue: H₂. Black circle: selectivity toward CO evolution (right axis). Photocatalyst weight: 1.0 g, reaction solution: 1.0 L of an aqueous NaHCO₃ solution (0.1 M), CO₂ supply: 30 mL min⁻¹, light source: a 400 W high-pressure Hg lamp (through a quartz glass jacket), and photoirradiation time: 1 h.

Fig. 7 shows the formation rates of products and the selectivity for CO over bare Ga₂O₃ (A), 95-MgAl/Ga₂O₃ (B), 95-Al/Ga₂O₃ (C), 95-Mg/Ga₂O₃ (D), and 95-Si/Ga₂O₃ (E) with a Ag cocatalyst loading of 0.25 wt%. The photocatalytic activity of 95-Si/Ga₂O₃, which has a sufficiently large specific surface area, was significantly less than that of bare Ga₂O₃, and CO evolution was completely suppressed by SiO₂ modification. Hence, the increase in the specific surface area is not the only reason for the improved selectivity toward CO evolution by the modification of Ga₂O₃ with Mg–Al LDH. Modification with Al(OH)₃ led to the increased specific surface area and improved selectivity toward CO evolution as compared with those observed for bare Ga₂O₃. Al₂O₃ (or Al(OH)₃) has been reported to serve as an adsorbate for capturing CO₂, resulting in the desorption of CO₂ at 473–673 K in CO₂-temperature programmed desorption (TPD) measurements; in other words, i.e., medium-strength basic sites are mainly present.³⁴ In contrast, the specific surface area of the 95-Mg/Ga₂O₃ composite was not altered by modification with Mg(OH)₂ because of its low specific surface area; hence, it does not exhibit improved selectivity toward CO evolution despite the high ability of MgO or Mg(OH)₂ as a CO₂-capturing material.^35,36 Hence, the high activity as well as selectivity toward CO evolution for the 95-MgAl/Ga₂O₃ composite photocatalyst are related to the high specific surface area, resulting from Mg–Al LDH modification, and the excellent ability of Mg–Al LDH as an adsorbent for CO₂,^37,38 which helps in preserving the CO₂ molecules near the Ag cocatalyst. As shown above, the 95-MgAl/Ga₂O₃ composite exhibited high activity for the stoichiometric splitting of water into H₂ and O₂ in the absence of the Ag cocatalyst, and a Ag cocatalyst loading of 0.25 wt% was beneficial for the formation of a considerable amount of CO as the reduction product of CO₂, with high selectivity toward CO evolution. Interestingly, 95-MgAl/0.25Ag/Ga₂O₃, which was prepared by a procedure slightly different from that used to prepare 0.25Ag/95-MgAl/Ga₂O₃, exhibited a lower CO formation rate and selectivity for CO as compared with those observed for 0.25Ag/95-MgAl/Ga₂O₃ (Table S1†). For the 95-MgAl/0.25Ag/Ga₂O₃ composite, all Ag particles were considered to be present on the Ga₂O₃ surface, i.e., Ag/Ga₂O₃ should be covered with Mg–Al LDH, whereas almost the entire Ag cocatalyst is impregnated on Mg–Al LDH for 0.25Ag/95-MgAl/Ga₂O₃, which covers Ga₂O₃. Hence, the transfer of the photogenerated electrons from the conduction band of the Ga₂O₃ photocatalyst to the Ag particles via Mg–Al LDH permits the selective reduction of CO₂ into CO. Moreover, the direct transfer of the photogenerated electrons from Ga₂O₃ to the Ag cocatalyst may preferentially cause the reduction of H⁺ into H₂. Furthermore, the physical mixture of Mg–Al LDH and Ag/Ga₂O₃ did not exhibit any remarkable improvement in the amount of CO evolved as compared with that observed for Ag/Ga₂O₃, but only exhibited enhanced water splitting to H₂ and O₂, indicating that the selectivity toward CO evolution decreases. The chemical connection between Ga₂O₃ and Mg–Al LDH, as described in the part for XRD patterns, was estimated to play a crucial role in the improved selectivity toward CO evolution (see Fig. S6† for the schematic of these photocatalysts).

Fig. 7 Formation rates of the products evolved and the selectivity toward CO evolution in the photocatalytic conversion of CO₂ in water using (A) 0.25Ag/Ga₂O₃ (without Mg–Al LDH modification), (B) 0.25Ag/95-MgAl/Ga₂O₃, (C) 0.25Ag/95-Al/Ga₂O₃ (Al modification), (D) 0.25Ag/95-Mg/Ga₂O₃ (Mg modification), and (E) 0.25Ag/95-Si/Ga₂O₃ (Si modification). Red: CO, green: O₂, and blue: H₂. Black circle: selectivity toward CO evolution (right axis). Photocatalyst weight: 0.19 g (A), 1.0 g (B–E), reaction solution: 1.0 L of an aqueous NaHCO₃ solution (0.1 M), CO₂ supply: 30 mL min⁻¹, light source: a 400 W high-pressure Hg lamp (through a quartz glass jacket), and photoirradiation time: 1 h.

Fig. 8 shows the results obtained from isotopic experiments, which were conducted to confirm the carbon source in the CO evolved from the conversion of ¹³C-labeled CO₂ (¹³CO₂) in an aqueous NaHCO₃ solution using the 0.25Ag/95-MgAl/Ga₂O₃ photocatalyst. The GC-MS profile was obtained by monitoring ion currents of m/z = 28 and 29 for the gas-phase products, which were initially separated using the GC system, accompanied by chromatograms recorded using a GC system equipped with a thermal conductivity detector (TCD). Three peaks were observed at approximately 100, 150, and 500 s in the GC/TCD chromatogram, corresponding to H₂, O₂, and CO, respectively. Another peak was observed at m/z = 29 at the same position as the peak corresponding to CO in the GC chromatogram, indicating that this photocatalytic system produces ¹³C-labeled CO (¹³CO, m/z = 29) as the reduction product of ¹³CO₂. Meanwhile, a peak was not observed at m/z = 28 (unlabeled CO was not formed in the gas phase). Moreover, the amount of ¹³CO evolved, as evaluated from the peak area in the m/z = 29 profile, was in agreement with that quantified using the flame ionization detection GC equipped with a methanizer. This result clearly indicated that the CO evolved in this reaction originates from CO₂ substrates and that the decomposition of the NaHCO₃ additive or contaminated organic residues does not affect the photocatalytic activity for CO₂ conversion. Moreover, the selectivity toward CO evolution was gradually decreased with repeating the photocatalytic reaction, as shown in Fig. S7.† Ag 3d XPS analysis (Fig. S3†) revealed the formation of Ag⁺ during the photocatalytic reaction, and TEM images (Fig. S8†) clearly elucidated that the aggregation of Ag cocatalyst particles accompanied with a slight change in the oxidation state was a fundamental factor of the deactivation of photocatalytic activity for the conversion of CO₂ in water. The enhancement in the durability of the photocatalyst should be promoted in the near future for making a contribution to our sustainable society.

Fig. 8 GC-MS profiles for the isotopic experiments for the conversion of CO₂ in water using the 0.25Ag/95-MgAl/Ga₂O₃ composite photocatalyst. (A) GC/TCD chromatogram, (B) Q-mass profile of m/z = 28, and (C) Q-mass profile of m/z = 29. Photocatalyst weight: 1.0 g, reaction solution: 1.0 L of an aqueous NaHCO₃ solution (0.1 M), ¹³CO₂ supply: 30 mL min⁻¹, and light source: a 400 W high-pressure Hg lamp (through a quartz glass jacket).

Conclusions

In this study, a Mg–Al LDH/Ga₂O₃ composite photocatalyst modified with a Ag cocatalyst exhibited high activity for CO₂ conversion in an aqueous NaHCO₃ solution under UV light irradiation. The CO formation rate for the 95-MgAl/Ga₂O₃ photocatalyst loaded with 0.25 wt% Ag was 211.7 μmol h⁻¹. In addition, the selectivity toward CO evolution among the reduction products was improved by the modification of the Ga₂O₃ photocatalyst with Mg–Al LDH, indicating that the transfer of the photogenerated electrons from the Ga₂O₃ photocatalyst to the Ag particles via Mg–Al LDH facilitates the selective reduction of CO₂ into CO. Control experiments and isotopic labeling studies revealed that CO₂ molecules dissolved in the reaction solution form CO by the conversion of CO₂ using the Ag-loaded composite photocatalyst. In addition, the stoichiometric formation of O₂ as the oxidation product of H₂O suggested that H₂O serves as the electron donor and a proton source. Hence, the increase in the specific surface area by the modification with Mg–Al LDH and the high ability of Mg–Al LDH as an adsorbent for CO₂, which concentrates CO₂ molecules near the Ag cocatalyst, improve the selective conversion of CO₂ into CO using H₂O as the reductant under UV light irradiation. The strategic improvement in the photocatalytic activity for the CO₂ conversion, which is based on providing the CO₂ adsorption sites, was reasonably demonstrated in this study.

Acknowledgements

This study was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas “All Nippon Artificial Photosynthesis Project for Living Earth” (No. 2406) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan; the Precursory Research for Embryonic Science and Technology (PRESTO), supported by the Japan Science and Technology Agency (JST); and the Program for Elements Strategy Initiative for Catalysts & Batteries (ESICB), commissioned by the MEXT of Japan. Shoji Iguchi is grateful for the JSPS Research Fellowships for Young Scientists.

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Footnote

† Electronic supplementary information (ESI) available: UV/Vis DR spectra, TEM images, Ag 3d XPS, surface atomic ratios of the photocatalysts, photoelectrochemical measurements, photocatalytic activities of reference photocatalysts, durability of the photocatalytic activity, and schematic illustrations of photocatalysts. See DOI: 10.1039/c7se00204a

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