Metal-doped polyoxometalates with dual ligands for efficient CO₂ photoreduction

Abstract

The design of highly active photocatalysts for carbon dioxide reduction is of far-reaching significance for the mitigation of industrial exhaust pollution and the promotion of sustainable development. In this work, three catalysts containing dual ligands and doped with metal V, [Co(DAPSC)(H₂O)₂]₂[Co(H₂O)₅]_0.5[PW₁₁CoO₃₉(Hdatrz)]·3.5H₂O (1), (H₂bim)[Co(DAPSC)(bim)(H₃PW₁₀V^IV₂O₄₀)]·4H₂O (2), and [Co(DAPSC)(bbi)(H₂O)]₂(PW_11.5V^IV_0.5O₄₀)·4H₂O (3) [DAPSC = 2,6-diacetylpyridine bis-(semicarbazone), Hdatrz = 3,5-diamino-1,2,4-triazole, bim = bis(1-imidazolyl)methane, and bbi = 1,1′-(1,4-butanediyl)bis(imidazole)], have been successfully synthesized under hydrothermal conditions. A systematic exploration was undertaken to characterise the structures and photocatalytic transformations. The results of both the IR and EDS tests indicated the successful doping of V atoms. The photocatalytic CO₂ reduction exploration demonstrated that all catalysts modified with dual ligands exhibited high photocatalytic activity. The results therein revealed that catalyst 2 exhibited the most efficient photocatalytic performance in the reduction of carbon dioxide using visible light. Under optimal conditions, the CO yield reached 11 003.3 μmol g⁻¹ h⁻¹ and maintained good catalytic activity after five cycles, outperforming the majority of the reported heterogeneous POM-based photocatalysts. The present study proves that the additional doping of the transition metal vanadium into catalysts based on dual ligands downstream contributes to the development of highly active photocatalysts for CO₂ reduction.

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Introduction

The utilisation of substantial quantities of fossil fuels in industrial contexts has the potential to gradually erode the natural environment, and the process of conversion of the emitted carbon dioxide into high-value-added chemicals (CH₄, HCOOH, CH₃OH, etc.) can prove efficacious in the mitigation of carbon emissions.^1–4 Recent research has revealed the potential of visible-light-catalysed CO₂ reduction to hydrocarbon fuels and chemicals as a promising methodology. Nevertheless, the chemical inertness of CO₂ presents a significant challenge in terms of its activation and conversion, thus necessitating the development of highly active photocatalysts to facilitate the critical proton-assisted multiple electron transfer.^5–10

The field of photocatalytic carbon dioxide reduction has witnessed significant advancements since the pioneering work of Wang et al. The photocatalyst range has expanded to encompass a diverse array of materials, including metal–organic frameworks (MOFs), metal oxides, sulfides, nitrogen oxides and so on.^11–15 Among the most widely utilized photocatalysts are polyoxometalates (POMs), which are structurally precise clusters of metal oxides and exhibit the advantage of unique redox behaviour. In this regard, they have been demonstrated to enable the reversible storage and transfer of multiple electrons whilst maintaining their structural stability. Consequently, they are considered to be attractive candidates for photocatalytic CO₂ reduction.^16–19 In recent years, functional hybrid materials formed by combining POMs with transition metals and organic ligands have been widely recognized for their potential in photocatalytic applications.^20–22 Organic ligands not only assume the roles of supporting and connecting the metal sites, but also have the capacity to modulate the catalyst structure and performance due to their unique chemical properties and structures. In the course of the synthetic process, an attempt was made to incorporate two different ligands. In contrast to compounds with a single ligand, the incorporation of dual ligands not only serves to enhance the stability, light absorption capacity and number of active sites of the catalysts, but also facilitates the specific adsorption and activation of specific reactions through the design of ligand pairing.^23–26 It is anticipated that this can realize the design of multifunctional photocatalysts with a stable structure and excellent catalytic activity used in the CO₂RR.

Metal doping is a common material modification technique that can enhance a number of properties of a given material. These include electrical conductivity, catalytic activity, optical properties and thermal stability. The precise control that can be achieved over the properties of a material through doping allows for the tailoring of materials to meet a range of applications. This is achieved by adjusting the type and content of the doped metal. In the field of photocatalysis, the introduction of dopants can facilitate enhanced light absorption, promote electron transfer, and improve catalytic activity, thereby markedly enhancing the performance of photocatalysts.^27–30

Schiff bases offer a number of advantageous properties, including good light absorption, high catalytic activity, tunability, stability and environmental friendliness.^31–33 Accordingly, an effort was made to develop novel metal oxides through the introduction of diverse nitrogen-containing organic ligands and vanadium elements onto Schiff base DAPSC-modified POMs. The following compounds were successfully synthesized: [Co(DAPSC)(H₂O)₂]₂[Co(H₂O)₅]_0.5[PW₁₁CoO₃₉(Hdatrz)]·3.5H₂O (1), H₂(bim)[Co(DAPSC)(bim)(H₃PW₁₀V^IV₂O₄₀)]·4H₂O (2), and [Co(DAPSC)(bbi)(H₂O)]₂(PW_11.5V^IV_0.5O₄₀)·4H₂O (3), and these compounds feature two different ligands. Nevertheless, compound 1 demonstrated considerable catalytic activity and CO selectivity. In contrast, the metal-doped catalysts exhibited superior photocatalytic CO₂ reduction activity. It is noteworthy that catalyst 2 exhibited optimal catalytic performance, with a CO yield of 11 003.3 μmol g⁻¹ h⁻¹ and a selectivity of 91.7% at 7 h and 5 mg catalyst loading, with no significant decline in catalytic activity over five cycles.

Experimental section

Materials and physical property studies

2,6-Diacetylpyridine bis-(semicarbazone) (DAPSC), bis(1-imidazolyl)methane (bim) and 1,1′-(1,4-butanediyl)bis(imidazole) (bbi)^34–36 were synthesized according to known literature methods, and all other chemical reagents were commercially purchased and not further purified. The PerkinElmer 2400 elemental analyser was used to determine C, H, and N contents. Powder X-ray diffraction (PXRD) patterns were obtained using a Bruker D8X diffractometer equipped with a Cu-target tube and graphite monochromator, scanning from 5–50° at a rate of 20° min⁻¹. Fourier transform infrared (FTIR) spectra were recorded using KBr pellets (Nicolet Impact 410 FTIR spectrometer) in the range of 4000–400 cm⁻¹. The UV-vis absorption spectral (Shimadzu UV-2600 spectrophotometer) wavelength conversion range was 200–800 nm. The elemental composition of the catalysts was determined through morphological analysis using high-resolution transmission electron microscopy (HRTEM, Hitachi S4800) and energy dispersive X-ray spectroscopy (EDS) on selected areas. Thermogravimetric (TG) analysis was based on a diamond thermogravimetric analyser, which was operated under a N₂ atmosphere from room temperature to 1000 °C at a rate of 10 °C min⁻¹. Photoluminescent spectra were recorded using a Hitachi F-4600 FL spectrophotometer.

Synthesis of [Co(DAPSC)(H₂O)₂]₂[Co(H₂O)₅]_0.5[PW₁₁CoO₃₉(Hdatrz)]·3.5H₂O (1). A mixture of H₃PW₁₂O₄₀ (0.1000 g, 0.035 mmol), Co(OAc)₂·4H₂O (0.1250 g, 0.50 mmol), DAPSC (0.0200 g, 0.07 mmol), 3,5-diamino-1,2,4-triazole (Hdatrz) (0.0300 g, 0.30 mmol), and deionized water (10 mL) was stirred for 1 h at room temperature. Then, the pH of the mixture was modulated to about 2.0 using HCl (1 M). Finally, the mixture was put into a Teflon-lined stainless-steel autoclave (25 mL) and maintained at 120 °C for 4 days. After the autoclave was cooled to room temperature, the solution was washed with deionized water to obtain long purple crystals (Fig. S1a,† yield: 27% based on DAPSC). Elem anal. calcd (%): C: 7.75, H: 1.48, N: 7.16; found (%): C: 7.70, H: 1.48, N: 7.11. IR (KBr pellet, cm⁻¹): 3310 (w), 2970 (w), 1670 (s), 1210 (w), 1060 (m), 954 (s), 822 (s).

Synthesis of H₂(bim)[Co(DAPSC)(bim)(H₃PW₁₀V^IV₂O₄₀)]·4H₂O (2). A mixture of H₃PW₁₂O₄₀ (0.1000 g, 0.035 mmol), Co(OAc)₂·4H₂O (0.1250 g, 0.50 mmol), DAPSC (0.0200 g, 0.072 mmol), bim (0.0300 g, 0.20 mmol), NH₄VO₃ (0.0100 g, 0.085 mmol), and deionized water (10 mL) was stirred for 1 h at room temperature. Then, the pH of the mixture was adjusted to about 2.5 using HCl (1 M) and NaOH (1 M). Finally, the mixture was put into a Teflon-lined stainless-steel autoclave (25 mL) and then maintained at 120 °C for 4 days. After the autoclave was cooled to room temperature, the solution was washed with deionized water to obtain long black lamellar crystals (Fig. S1b,† yield: 35% based on H₃PW₁₂O₄₀). Elem anal. calcd (%): C: 9.03, H: 1.32, N: 6.32; found (%): C: 9.28, H: 1.34, N: 6.47. IR (KBr pellet, cm⁻¹): 3437 (m), 2924 (m), 1636 (w), 1533 (m), 1098 (s), 970 (s), 892 (w).

Synthesis of [Co(DAPSC)(bbi)(H₂O)]₂(PW_11.5V^IV_0.5O₄₀)·4H₂O (3). The synthesis of 3 was similar to that of 2, except that bim was replaced with bbi (0.0300 g, 0.16 mmol), and the pH value was adjusted to about 3.5. Thin black crystals were obtained (Fig. S1c,† yield: 37% based on H₃PW₁₂O₄₀). Elem anal. calcd (%): C: 12.68, H: 1.76, N: 7.75; found (%): C: 12.92, H: 1.80, N: 7.84. IR (KBr pellet, cm⁻¹): 3431 (m), 2930 (m), 1635 (w), 1532 (m), 1101 (s), 976 (s), 893 (w).

Results and discussion

Crystal structure

Based on the analysis of the single-crystal structure, it can be determined that compound 1 crystallizes in the triclinic P1̄ space group (Table S1†). The asymmetric unit comprises a single Co-substituted Keggin-type [PW₁₁CoO₃₉(Hdatrz)]⁵⁻ anion, two discrete Co–DAPSC complexes and cobalt complex cation fragments Co(H₂O)₅ (Fig. S2†). Among the previously reported phosphotungstates, the unique [PW₁₁CoO₃₉]⁵⁻ formed by the transition metal Co replacing W in the POM-based anion is very rare.^37,38 The Co1 atom is six-coordinate with five oxygen atoms in the mono-deficient POM {PW₁₁O₃₉} and the imidazole nitrogen atom on Hdatrz. Specifically, Co1 serves as a linking group, forming an O–Co1–N with the ligand Hdatrz, thereby establishing a link between {PW₁₁O₃₉} and the aforementioned ligand (Fig. S3†), and the lengths of the W–O bonds vary from 1.670(3) to 2.456(19) Å, which is consistent with the range observed in other W-containing compounds.^18,19 The Co4 atom is penta-coordinated with five oxygens in the water molecule. Co2 and Co3 are in the same hepta-coordinated environment, and each crystallographically independent Co²⁺ is tightly linked to two O and three N atoms from the DAPSC ligand and two axial O atoms from water molecules, displaying a pentagonal bipyramid geometry (Fig. S4†). In Co–DAPSC, the lengths of the Co–O(N) bonds vary from 2.120(4) to 2.380(4) Å, and the range of angles of O–Co–O is from 85.7(9) to 176.4(1)° (Table S2†). Moreover, the adjacent asymmetric units form a two-dimensional (2D) supramolecular layer through hydrogen bond interactions between the ligands (Fig. 1), and then adjacent 2D supramolecular layers further generate a three-dimensional (3D) supramolecular framework by packing and connecting regularly through hydrogen bonds between them (Fig. S5†).

Fig. 1 Schematic representation of the synthesis and ball-and-stick and polyhedral views of the 3D supramolecular framework in 1. The free water molecules and hydrogen atoms are omitted for clarity.

We further incorporated NH₄VO₃, hypothesizing that this would introduce vanadium atoms to facilitate the synthesis of bimetallic-modified catalysts, with enhanced photocatalytic activity. We are pleased to report the successful synthesis of the desired catalyst. It is noteworthy that the vanadium atoms have entered the anionic clusters. This may be attributed to the fact that vanadium is more oxidizable than cobalt. Single-crystal X-ray diffraction analysis demonstrated that 2 crystallizes in the orthorhombic Pbca space group. The basic unit of compound 2 comprises a {PW₁₀V^IV₂O₄₀} anion, a crystallographically independent Co²⁺, a DAPSC ligand, two bim ligands, and four lattice water molecules (Fig. 2a, S6†). Notably, different from reported V-doped phosphotungstates, W and V exhibit disorder and are evenly distributed in the {PW₁₀V^IV₂O₄₀} anion.^39,40 The hepta-coordinated Co atom presents a pentagonal bipyramidal geometry, which bridges three nitrogen atoms and two oxygen atoms from DAPSC, as well as one nitrogen atom from bim, forming the metal–dual ligand complex Co(DAPSC)(bim). Then, the complex coordinates to the terminal oxygen atom of the {PW₁₀V^IV₂O₄₀} anion. Notably, the free bim ligand acts as a “protective cap” for the cluster. Furthermore, 2D and 3D supramolecular structures were constructed through hydrogen bond interactions between neighboring asymmetric units (Fig. 2b, S7†).

Fig. 2 (a) Schematic representation of the synthesis of compound 2. (b) Ball-and-stick and polyhedral representation of the asymmetric unit in 2. (c) Ball-and-stick and polyhedral views of the 2D stacking diagram in 2.

Single-crystal diffraction analysis indicated that 3 crystallizes in the triclinic crystal system, P1̄ space group, with a = 12.25(15) Å, b = 12.51(15) Å, c = 14.84(18) Å, and β = 87.84(2)° (Table S1†). A zero-dimensional structure is constituted by a single [PW_11.5V^IV_0.5O₄₀]⁴⁻ anion unit and two cobalt metal–organic complexes (Co–DAPSC–bbi). One of the two cobalt metal complexes is positioned freely on either side of the POM-based anion, thereby forming a semi-enclosed structure (Fig. S8–S11†). The bond valence sum (BVS) calculations indicated that the valence of the element V is +4 in both compounds. It is noteworthy that only W5 and W6 of 3 exhibit both W and V elements, and the two compounds exhibit disparate W : V ratios. To the best of our knowledge, reported POM-based catalysts generally feature a single ligand. POM-based compounds co-modified with dual ligands are extremely rare.^41,42

Characterization

The infrared spectra of the compounds are presented in Fig. S12–S14,† and it can be observed that there are similarities in the vibrational modes present in the spectra of the compounds. The broad peak at about 3410 cm⁻¹ is assigned to the O–H stretching vibration for H₂O. The C–H and N–H stretching vibrations of the ligands are observed within the range of 2914–3218 cm⁻¹, and the peaks in the range of 1635–1670 cm⁻¹ correspond to the stretching vibration signals of C=N and C=O. The infrared spectra of compounds 2 and 3 exhibit a high degree of correlation in the short-wavelength region, which can be attributed to the presence of the same Keggin anion in their backbones. The bending peaks within the wave number range of 1098–1101 cm⁻¹ are attributed to the P–O of the POM unit, while the absorption peaks appearing near 892 cm⁻¹ are attributed to the V–O–V or W–O–W stretching vibration component. The peaks at 970–976 cm⁻¹ are attributed to the stretching vibration of W=O or V=O. This evidence substantiates the existence of V atoms.

The PXRD analyses of the three compounds are shown in Fig. S15–S17.† It can be observed that the main diffraction peaks in the PXRD patterns of these compounds are in good agreement with those of the corresponding simulated patterns, indicating that both compounds have high phase purity. To further confirm the successful loading of V atoms, elemental mapping (EDS) tests were conducted on both compounds. The results demonstrate that the V atoms are uniformly dispersed in the crystal structure, thereby corroborating the successful doping of V element (Fig. S18 and S19†).

Thermogravimetric analyses were conducted on the three compounds under a N₂ atmosphere, from room temperature to 1000 °C, with the objective of investigating their stability (Fig. S20–S22†). The three compounds exhibit comparable weight loss curves, as exemplified by 2 in the initial stage (below 150 °C). In this phase, the weight loss is 1.97% (calcd 2.17%), which is attributed to the pyrolysis of the 4 free water molecules. As the temperature increased, the ligands in the compounds underwent thermal decomposition and partial collapse of the skeleton, resulting in a decline in quality.

CO₂ photoreduction

In order to ascertain whether the three catalysts are photocatalytically active, a series of correlation characterization studies of their light-trapping ability were conducted. The capacity to absorb sunlight effectively is a prerequisite for photocatalytic reactions. Consequently, we initially evaluated the compounds for solid-state UV–vis diffuse reflectance spectra (Fig. 3a), and the compounds exhibit a broad absorption spectrum spanning 300–700 nm, extending from the ultraviolet to the visible region. Furthermore, it has been established that compounds 2 and 3 exhibit a broader light absorption region, which suggests the potential for enhanced production of carriers during photocatalytic reactions. The calculated band gap energy values of 1.88 eV, 1.76 eV, and 1.53 eV (Fig. 3b, S23 and S24†), respectively, exhibit characteristics reminiscent of those observed in semiconductors, based on the solid UV data of the compounds. Mott–Schottky tests were conducted on the catalysts at varying frequencies (Fig. 3c and S25 and S26†). The LUMO values for compounds 2 and 3 were found to be −0.82 and −0.85 V vs. NHE. Accordingly, the valence bands (VBs) are determined to be 0.71 and 0.91 V. It is notable that the CB and VB are capable of crossing the CO₂ reduction potential, which serves to substantiate the feasibility of CO₂ photoreduction (Fig. 3d). The aforementioned results demonstrate that the compounds possess the capacity for photocatalytic reduction of CO₂.⁴³

Fig. 3 Optical characterization of photocatalysts. (a) Solid-state UV–Vis DRS spectra of compounds. (b) Tauc plot of 2. (c) Mott–Schottky plot of 2. (d) Band-structure diagram of compounds 2 and 3.

On account of the above features of these compounds, visible light-driven photocatalytic conversion of CO₂ was carried out in an acetonitrile (CH₃CN) solvent system under a CO₂ atmosphere (1.0 MPa, 6 °C), with [Ru(bpy)₃]Cl₂·6H₂O as the photosensitizer and triethanolamine (TEOA) as the sacrificial agent. Gas chromatography was utilized for the determination of the gas-phase products formed during the catalytic process. It is notable that throughout the photocatalytic process, carbon dioxide was only reduced to CO without the presence of other carbon-containing products, and a small amount of H₂ production was detected. The impact of varying dosage on the photocatalytic performance of the three compounds was evaluated separately. An increase in catalyst dosage resulted in a linear increase in CO yield, while varying degrees of intensity decrease in the rate were observed.^44,45 This phenomenon may be attributed to the presence of excess photocatalyst, which has been shown to impede the photoelectron transfer from the PS to the catalyst. It was observed that compound 1 demonstrated comparatively diminished photocatalytic activity in comparison with the other compounds. However, its photocatalytic activity exhibited superior performance in relation to single-ligand catalysts, with a CO selectivity of 84.6% and a maximum CO yield of 4875.2 μmol g⁻¹ h⁻¹ at a dosage of 1 mg of catalyst (Fig. 4a).^46–48 This phenomenon is potentially attributable to interactions between different ligands present within the catalyst, which have been postulated to facilitate enhanced electron–hole separation efficiency within the catalyst.

Fig. 4 Contrast tests at different dosages of compounds (a) 1, (b) 2 and (c) 3. (d) TON comparison among the three compounds (5 mg). Reaction conditions: [Ru(bpy)₃]Cl₂·6H₂O (11.3 mg), solvent (MeCN = 40 mL and TEOA = 10 mL), CO₂ atmosphere (1.0 MPa, 6 °C), and 8 h.

When V is loaded onto the H₃PW₁₂O₄₀, it offers more active sites compared to monometallic loading. Compound 2 exhibits a better photocatalytic CO₂ reduction rate and selectivity compared to 3. Concretely, the amount of CO can only reach 66.0 μmol when a minor amount (1 mg) of 2 was added to the photoreaction system. The number of moles of CO increases to 428.0 μmol in nearly a linear manner as the catalyst dosage is increased. However, when the catalyst dosage is further increased to 10 mg, the formation rate of CO decreases sharply to 3891.1 μmol g⁻¹ h⁻¹. This is consistent with reported photocatalytic studies showing the phenomenon, the main attribution of which may be the limited generation of photoelectrons, resulting in the excess catalyst not being fully utilized. During an eight-hour photocatalysis period, when the dosage of catalysts was increased to 5 mg, the number of moles of CO reached 402.9 μmol, with a CO production rate of up to 10 073.1 μmol g⁻¹ h⁻¹, achieving 91.5% selectivity and a high total turnover number (TON) of 267.0.

Given the established superiority of 2 in terms of photocatalytic activity, the CO₂RR of compound 2 was investigated to ascertain the most favorable reaction conditions. The time evolution of CO₂ reduction gas products and the CO production rate with the preferred dosage (5 mg) of compound 2 as a photocatalyst are exhibited in Fig. 5a. The kinetic curve of photocatalytic CO₂ reduction by 2 at a dosage of 5 mg exhibited a consistent upward trajectory in the yield of CO₂ reduction products over the course of eight hours. From the perspective of generation rates, there appears a trend of increasing and then decreasing, and moreover, a remarkable reduction rate of CO₂ to CO was reached at 11 003.3 μmol g⁻¹ h⁻¹ at 7 h. Thereafter, the formation rate of CO decreased progressively as the response progressed, which may be attributed to the photo-bleaching or partial decomposition of [Ru(bpy)₃]Cl₂·6H₂O during the reaction process.⁴⁵ It is worth mentioning that the catalyst exhibits a higher reduction yield and rate for CO formation in this present study than the majority of reported photocatalysts based on MOFs and POMs (Table S5†).

Fig. 5 (a) Time-related CO, H₂, and CH₄ production of 2. (b) Research of various reaction conditions for compound 2. (c) Stability tests of 2 in five continuous runs. Reaction conditions: catalyst (5 mg), [Ru(bpy)₃]Cl₂·6H₂O (11.3 mg), solvent (MeCN = 40 mL and TEOA = 10 mL), CO₂ atmosphere (1.0 MPa, 6 °C), and 8 h. (d) PXRD pattern of 2 after five cycles.

To explore the main active center of 2, a series of comparative experiments were conducted using equimolar DAPSC + bim ligands, H₅PW₁₀V₂O₄₀ and Co(OAc)₂·4H₂O separately instead of 2 as the photocatalyst under the same reaction conditions (Table S6†). DAPSC + bim and H₅PW₁₀V₂O₄₀ only present weak catalytic activity, and Co(OAc)₂·4H₂O exhibits moderate catalytic activity, demonstrating that the metal sites Co^II in 2 might be the main active centers for CO₂ photoreduction. In comparison with the mechanical mixture of Co(OAc)₂·4H₂O and H₅PW₁₀V₂O₄₀, 2 exhibited outstanding catalytic activity, which represents the advantage of the covalent connection of various active components.

In addition, comparative experiments were conducted on compound 2 to carry out the crucial role of the individual components in the CO₂ photoreduction system. As shown in Fig. 5b, only trace moles of reduction products were observed in the reaction system lacking a photosensitizer, which indicates that compound 2 primarily acted as a cocatalyst in the CO₂ photoreduction system, and the PS complex was actually the photocatalyst. The CO yield dramatically decreased in the absence of TEOA, which presented that the sacrificial electron donor agent plays a significant role in the formation of CO. No reaction occurred in the absence of light or CO₂, indicating that the reaction requires light-driven or CO₂ atmosphere catalysis to proceed.

Photocatalytic stability is an important indicator of good photocatalysts. The photocatalytic durability of compound 2 was evaluated through the analysis of PXRD patterns and cycling tests (Fig. 5c and d). A comparative analysis of the three sets of PXRD data revealed that the positions of the characteristic catalyst peaks after the CO₂RR experiment remained unaltered. Furthermore, the cycling stability test indicated that there was minimal decline in the rate of CO production over five cycles. These findings collectively substantiate that compound 2 exhibits robust stability.

Photocatalytic CO₂RR mechanism

In order to gain further insight into the process of photo-excited electron migration, photoluminescence (PL) quenching experiments were carried out in acetonitrile solutions with a photosensitizer (PS) and catalysts. As illustrated in Fig. 6a, a gradual decline at 619 nm with an increasing amount of compound 2 was observed in the steady-state PL spectra of [Ru(bpy)₃]²⁺. This demonstrated that continuous electron transfer from the excited PS to 2 was attributed to phosphorescence quenching. It is worth noting that, in comparison with 1 and 3, the addition of 2 can significantly attenuate the PL intensity at 619 nm (Fig. 6b), as the photo-induced electron transfer from the PS towards 2 is more effective than towards 1 and 3. These phenomena were in accordance with the aforementioned results of photocatalytic CO₂ reduction.⁴⁹

Fig. 6 (a) Emission spectra of [Ru(bpy)₃]²⁺ in varying amounts of 2. (b) [Ru(bpy)₃]Cl₂·6H₂O in MeCN solutions with compounds 1, 2 and 3. (c) Presumed mechanism of the photocatalytic CO₂RR over compound 2.

The preceding findings regarding the photocatalyst 2 in the experiments have led to the formulation of a potential CO₂ photoreduction reaction mechanism, which is presented below for further discussion. First, under the irradiation of visible light, the photosensitive agent is converted into an excited state, which is rapidly quenched by the catalyst and converted into an oxidized state. At the same time, the excited photoelectrons are transferred to the catalyst due to the positional matching of the LUMO. Subsequently, the CO₂ molecules adsorbed on the catalyst surface are converted to CO* by the electrons and are released from the catalyst surface to form CO products. The oxidized Ru(bpy)₃³⁺ is converted back to Ru(bpy)₃²⁺ by the active reagent (TEOA). Finally, TEOA as a sacrificial electron donor combines with the photoinduced holes to form TEOA⁺, completing the catalytic cycle (Fig. 6c).

Conclusions

In summary, we reported three compounds on the basis of the combined action of dual ligands, of which compounds 2 and 3, doped with V, were synthesized by the addition of a vanadium source during the synthesis process. It is noteworthy that the V atoms are disordered and partially occupy the Keggin-type POM and that the percentage of V element differs between the two catalysts. Based on UV-vis, Mott–Schottky, and energy band analyses, these three compounds can be used as CO₂RR photocatalysts. Under visible light, compound 2 presents superior photocatalytic CO₂ reduction activity with the moles of CO reaching up to 428.0 μmol; at the same time, CO photocatalysis efficiency reaches 11 003.3 μmol g⁻¹ h⁻¹ with 91.5% selectivity over 7 h. This may be attributed to the greater proportion of element V in this catalyst. The successful doping of element V can effectively improve the photocatalytic performance of the catalyst. In addition, 2 can also retain a high catalytic activity and stability for photocatalytic CO₂ reduction even after five cycles. The success of this work enriches the structural types of POMs, while contributing to a promising avenue for the construction of catalysts with outstanding structural stability and remarkable activity for artificial photosynthesis.

Author contributions

Yu Lv: conceptualization, methodology, investigation, data curation and writing – original draft. Ji-Lei Wang and Ting Jin: methodology, formal analysis, data curation and writing – review and editing. Jiu-Lin Zhou: methodology, investigation, data curation and writing – review and editing. Zhi-Ming Dong: investigation and data curation. Hua Mei and Yan Xu: conceptualization, project administration, and supervision.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of China (92161109) and the Cultivation Program for the Excellent Doctoral Dissertation of Nanjing Tech University (2023-10).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, details of crystallographic data, structural figures, PXRD patterns, IR spectra, and TGA characterization. CCDC 2433946–2433948 for compounds 1–3. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi00851d

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