A {Co₉}-Added Polyoxometalate for Efficient Visible-Light-Driven Hydrogen Evolution

Abstract

A polyanion cluster H₆Na₈Cs₃[Co₉(μ₃-OH)₃(H₂O)₆(HPO₄)₂(B-α-PW₉O₃₄)₃]Cl·40H₂O (1) was made with the guidance of the lacunary directing strategy under hydrothermal conditions. Compound 1 was characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and thermogravimetric analysis, respectively. Single-crystal X-ray diffraction analyses showed that 1 consists of three anions [B-α-PW₉O₃₄]⁹⁻ and a cyclic cationic [Co₉(μ₃-OH)₃(H₂O)₆]¹⁵⁺ and two anions HPO₄²⁻. Variable-magnetic properties indicate antiferromagnetic interactions in 1. Visible-light-driven hydrogen evolution tests demonstrated that 1 was an efficient water reduction catalyst with an H₂ evolution rate of 1217.6 μmol h⁻¹ g⁻¹.

1. Introduction

Polyoxometalates (POMs) have attracted increasing attention due to their intriguing structural diversity [1] and diverse applications in drugs [2,3], magnetism [4,5], and catalysis [4,5,6,7,8,9,10,11]. Lacunary POMs as precursors can induce transition metal (TM) cations’ aggregation to form TM oxo-clusters under hydrothermal conditions, producing a class of TM-added POMs (TMAPs) with interesting physicochemical properties [12,13,14,15]. Up to now, hydrothermal synthesis was successfully used in creating a large number of TMAPs. Since 2007, we have reported a large number of organic–inorganic hybrid Ni₆-added POMs under the guidance of the lacunary directing strategy, using the lacunary sites of tri-lacunary POMs fragments {XW₉O₃₄}ⁿ⁻ (X = P/Si/Ge, n = 9, 10) as structure-directing agents (SDAs) to induce the TM-oxo clusters aggregation to form novel TMAPs [16,17,18,19,20,21].

POMs have unique redox properties and modifiable Lewis acidity but their inherent poor stability and tendency to aggregate in solution have hindered further development [20]. TMAPs are widely used in photocatalysis [22,23] and electrocatalysis [24] fields due to their physicochemical properties and reversible multi-electron transfer ability. TMAPs have been widely studied due to their unique structural diversity, magnetic properties, and catalytic properties. Taking the above into account, TMAPs have demonstrated a massive potential for application for visible-light-driven Hydrogen evolution as heterogeneous water reduction catalysts (WRCs) [14,15,25].

Co-containing complexes were applied in photocatalytic water-splitting reactions with good reactivity [26] and Co²⁺ can be induced by tri-lacunary POMs fragments {XW₉O₃₄}ⁿ⁻, thus improving the performance of Co-added POMs (CoAPs) as the catalytic active center in visible-light-driven H₂ evolution. Herein, a polyanion (HPO₄)₂@-{Co₉(PW₉)₃} cluster was made with the guidance of the lacunary directing strategy under hydrothermal conditions. Interestingly, the HPO₄ groups of 1 originate from the central heteroatom of the tri-lacunary precursor [A-α-PW₉O₃₄]⁹⁻ and assemble with the cyclic (HPO₄)₂@{Co₉} cluster to form CoAP. Moreover, visible-light-driven H₂ evolution tests have demonstrated that CoAP is an efficient WRC with the H₂ evlution rate of 1217.6 μmol h⁻¹ g⁻¹, indicating that 1 is a potential heterogeneous WRC.

2. Experimental Section

2.1. General Procedure

All chemicals were acquired commercially and used without further purification. The synthetic method of the Na₁₀[A-α-PW₉O₃₄]·7H₂O ({PW₉}) was derived from previous literature methods [27]. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance XRD diffractometer (Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54056 Å). FT-IR spectra were measured by using a Thermo Scientific Nicolet iS10 FT-IR spectrometer (Waltham, MA, USA) in the range of 400–4000 cm⁻¹ with KBr pallets. Thermogravimetric analyses were conducted under air flowing on a Mettler−Toledo TGA/DSC 1000 (Zurich, Switzerland) with a heating rate of 10 °C min⁻¹ from 25 to 1000 °C. UV–Vis absorption spectra were obtained using a Shimadzu UV3600 spectrometer (Kyoto, Japan).

2.2. Synthesis of 1

Co(NO₃)·6H₂O (0.536 g, 1.84 mmol), {PW₉} (0.597 g, 0.233 mmol), and CsCl (0.308 g, 1.829 mmol) were mixed in 8 mL H₂O and 5 drops acetic acid. After stirring for 1 h, the mixed solution was transferred to a 25 mL Teflon-lined autoclave and kept at 80 °C for 24 h. When the reaction was complete and cooled to room temperature, the product was filtered and the hexagonal pink crystals were washed with distilled water in a yield of 0.052 g (0.058 mmol), a yield of 7.51% (based on {PW₉}). IR data (KBr pellet, cm⁻¹): 3430 (s), 1630 (m), 1030 (s), 933 (s), 883 (s), 806 (s), 721 (s).

2.3. X-ray Crystallography

A suitable crystal was selected and put on a Bruker APEX-II CCD diffractometer. This crystal was kept at 296.15 K during data collection. Using Olex2 [28], the structure was solved with the ShelXT [29] structure solution program using Intrinsic Phasing and refined with the ShelXL [30] refinement package using Least Squares minimization. The contribution of these disordered solvent molecules to the overall intensity data of the structure was treated using the SQUEEZE method [31] in PLATON (25 lattice water molecules in 1 are estimated by TGA). The crystallographic data structure refinement is listed in

Table 1. Crystal data and structure refinements for 1.

	1
Formula	H103Na8ClCs3Co9O158P5W27
Mr	8898.75
Crystal system	Hexagonal
Space group	P63/m
a [Å]	20.7840(8)
b [Å]	20.7840(8)
c [Å]	20.6385(9)
α [º]	90
β [º]	90
γ [º]	120
V [Å3]	7721.1
Z, Dc [g cm−3]	2
F(000)	7316.0
GOF on F2	1.036
Final R indices [I > 2σ(I)]	R1 = 0.0762 wR2 = 0.2026
R indices [all data] a	R1 = 0.01328 wR2 = 0.2464
CCDC number	2213769

^a R1 = Σ||F₀| − |F_c||/Σ|F₀|. wR₂ = [Σw(F²₀ − F²c)²/Σw(F²₀)²]^1/2.

. Detailed crystallographic data of the title crystal were deposited on the Cambridge Crystallographic Data Centre with CCDC reference number 2213769 for 1. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html accessed on 29 June 2022, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; Fax: +44-1223 336 033.

3. Result and Discussion

3.1. Structure of 1

The structure of 1 crystallized in the hexagonal space group P6₃/m. The structure contains one [Co₉(μ₃-OH)₃(H₂O)₆(HPO₄)₂(B-α-PW₉O₃₄)₃]¹⁶⁻ (abb. as (HPO₄)₂@{Co₉(PW₉)₃}, Figure 1a) cluster, three Cs⁺, eight Na⁺ and 15 water molecules. Bond valance sum (BVS) calculations show that the valence of W, Co, O, Cl and P atoms are +6, +2, −2, −1 and +5, respectively. The calculations reveal low values for O1 (1.299) and O3 (1.084) demonstrating that O1 and O3 further bond to H⁺ as OH groups (Table S1, Supplementary Materials). The polyanion (HPO₄)₂@{Co₉(PW₉)₃} cluster can be viewed as consisting of three [Co₃O₂(μ₃-OH)₂(H₂O)₂(B-α-PW₉O₃₄)]⁹⁻ (abb. as {Co₃PW₉}) units (Figure 1b) and two HPO₄ tetrahedra. Co²⁺ are six-coordinate octahedral configurations with Co–O bond lengths ranging from 1.998 to 2.182 Å. In addition, two Co²⁺ (Co2, Co2A) have coordinated water molecules (O1W, O1WA), and the other Co²⁺ (Co3) carries a μ₃-OH (O3, O3B) in the Co₃O₁₃ ({Co₃}) cluster (Figure 1b). Three Co²⁺ share edges to form a triangular {Co₃} cluster, and the {Co₃} cluster cap on the [B-α-PW₉O₃₄]⁹⁻ fragment to form the {Co₃PW₉} unit (Figure 1b,c). Three {Co₃PW₉} units are connected to each other through three μ₃-OH forming a cyclic {Co₉(PW₉)₃} unit. Two openings existed in the cyclic {Co₉(PW₉)₃} unit, in which each consists of three O atoms forming a triangle with a side length of 2.501 Å and matching the size of the HPO₄²⁻ tetrahedron (Figure 1d). The two HPO₄ units are capped on both sides of the ring to form a cluster (HPO₄)₂@{Co₉(PW₉)₃} (Figure 1a). Interestingly, [Cl@{Cs₃(H₂O)₆}] (Figure S1, Supplementary Materials) fills the middle of the anion clusters as can be seen from the stacking diagram (Figure S2, Supplementary Materials).

From a topological point of view, 1 can be simplified as a three-connected 2D network (Figure 2a), in which (HPO₄)₂@{Co₉(PW₉)₃} (red nodes) and [Cl@{Cs₃(H₂O)₆}] (green nodes) can be simplified as three-connected (3-c) nodes (Figure 2b), respectively. Interestingly, the 2D layers are arranged in an –ABAB– manner along the c-axis (Figure 2c).

3.2. IR Spectrum and Optical Band Gap

FT-IR spectra were measured in the range of 400–4000 cm⁻¹ with KBr pallets (Figure S3, Supplementary Materials). The strong peaks at around 3430 and 1630 cm⁻¹ are dominated by the stretching and bending modes of the water molecules. The characteristic bands derived from the Keggin POM fragments in the 721–1030 cm⁻¹ region. In detail, the peak at 1030 and 933 cm⁻¹ for 1 is attributable to ν(P-Oa) and ν(W-Ot). The peak at 806, 721, and 692 cm⁻¹ is attributable to ν(M-O-M) (M = W or Co), respectively. UV–Vis absorption and optical diffuse reflectance spectra of 1 were obtained in the wavelength range of 200–800 nm (Figure S4, Supplementary Materials). Absorption peaks of 260 nm and 535 nm correspond to the charge transfer between O→W and the d-d charge transfer between Co²⁺, respectively (illustration). The band-gap energy (Eg) is obtained by extrapolating the linear part of the rising curve to zero [32]. The Eg of 1 was estimated as 2.58 eV.

3.3. PXRD and Thermogravimetric Analysis

The pure phase was confirmed by powder X-ray diffraction (PXRD) patterns (Figure S5, Supplementary Materials). Thermogravimetric analysis was conducted under air flowing with the heating rate of 10 °C min⁻¹ from 25 to 1000 °C (Figure S6, Supplementary Materials). The TG curve indicates that the weight of 1 shows one step loss of 10.25% (calcd 10.3%) from 25 to 600 °C, corresponding to the release of 40 lattice water molecules. six coordination water molecules, three hydroxy groups (1.5 H₂O) and six protons (3 H₂O) for 1, respectively.

3.4. Photocatalytic H₂ Evolution Performance

Herein, the three-component system was examined: [Ir(ppy)₂(dtbbpy)][PF₆] (ppy = 2-phenylpyridine; dtbbpy = 5,5′-di-tert-butyl-2,2′-bipyridine) as a photosensitizer, triethanolamine (TEOA) as sacrificial electron donor, and 1 as water reduction catalyst (WRC). The device used for the photocatalytic reaction is a multichannel photocatalytic reactor (Beijing Perfectlight Technology, Beijing, China), which uses white LED lamp beads (400–800 nm, electric power: 10 W), and is continuously illuminated for 10 h at a temperature of 25 °C. The reaction system was deaerated for 20 min under an Ar/CH₄ (4:1) atmosphere. H₂ was analyzed using a GC979011 gas chromatograph (Fuli Instruments, Taizhou, China) with a TCD and a 5 Å molecular sieve column (3 m × 3 mm) with Ar as the carrier gas.

As shown in Figure 3a, the H₂ production increases with increasing radiation time. With the increasing concentrations of TEOA (5 mM, 10 mM, 15 mM, 20 mM) in the reaction system, the production of H₂ showed a trend of first increasing (11.88 μmol, 31.36 μmol, 36.53 μmol, respectively) and then decreasing (24.48 μmol) after 10 h of illumination. One possible explanation is that the addition of TEOA increases the pH value of the system, reducing the concentration of H⁺ in the system, and thereby reducing the catalytic effect. The H₂ production (9.9 μmol, 36.5 μmol and 40.7 μmol, respectively) increases with the increasing quantity of catalyst (1, 3, 6 mg), and the H₂ evolution rate was 745.6 μmol h⁻¹ g⁻¹, 1217.6 μmol h⁻¹ g⁻¹ and 745.6 μmol h⁻¹ g⁻¹, respectively (Figure 3b). In terms of catalytic efficiency, the maximum H₂ production rate of 3 mg can reach 1217.6 μmol h⁻¹ g⁻¹. Considering the catalytic system is heterogeneous catalysis, with the increase in the amount of catalyst, part of the light is scattered and the catalytic efficiency decreases. Hence, efficient catalysis may be achieved using the appropriate amount of catalyst. When the concentration of the photosensitizer (0.05 mM, 0.1 mM, 0.2 mM) was changed under the optimal conditions, the amount of the H₂ production was 20.6 μmol, 23.58 μmol and 36.53 μmol, respectively, and the amount of hydrogen production increased with the increase in the concentration of the photosensitizer (Figure 3c). The PXRD and FT-IR (Figure S7, Supplementary Materials) tests before and after the photocatalytic reaction proved that the structure of 1 was basically unchanged and had good catalytic stability.

For heterogeneous catalytic systems, the stability and recyclability of catalysts are always important factors. In the cycling experiments (Figure S8, Supplementary Materials), the yield of H₂ was detected by GC analysis, and the catalyst was isolated by centrifugation and washed with CH₃CN after two hours of reaction at a catalyst dosage of 9 mg. The experiments were repeated for three successive cycles, the amount of H₂ production was similar in the first two cycles but decreased the third time. Such a decrease in activity might be ascribed to the loss of the catalyst sample during the isolating operation. The PXRD pattern and FT-IR spectra (Figure S9, Supplementary Materials) of the catalyst before and after cycling experiments proved that 1 remains unchanged, which further demonstrated the recyclability and stability of complex 1.

In conclusion, the best reaction conditions explored were: 1 (3 mg), TEOA (15 mM) and photosensitizer (0.2 mM), and the best H₂ production efficiency can reach 1217.6 μmol h⁻¹ g⁻¹. This value is comparable to that of previously reported results for H₂ evolution (Table S3, Supplementary Materials). Considering the factors of catalyst efficiency, 3 mg 1 was used for in-depth studies. As shown in Figure 3d, H₂ was produced only when the photosensitizer [Ir(ppy)₂(dtbbpy)]-[PF₆] and TEOA electron sacrificial reagent were added with light irradiation, and a small amount of H₂ was produced when the equimolar amount of {PW₉} and CoCl₂·6H₂O was used as the control catalyst revealing the superiority of CoAPs in the visible-light-driven H₂ evolution. According to the reported literature [22,33,34], a possible photocatalytic mechanism for this visible-light-driven H₂ evolution process was proposed (Figure S10, Supplementary Materials). In visible-light-driven catalytic systems, the photosensitizer’s excited state can function as either an oxidant or reductant and thus can be quenched by an electron donor or an acceptor. A POMs-based catalyst and TEOA can oxidatively and reductively quench the excited state of [Ir(ppy)₂(dtbbpy)]⁺ * in the visible-light-driven H₂ evolution process catalyzed by 1.

3.5. Magnetic Properties

The variable-magnetic properties of 1 were measured in the temperature range of 2–300 K with a magnetic field of 5000 Oe. As shown in Figure 4a, the χ_mT value is 26.53 cm³ mol⁻¹ K at 300 K. The χ_mT value decreases continuously with decreasing temperature. When the temperature drops below 50 K, the χ_mT value decreases rapidly. The χ_mT drops to 4.64 cm³ mol⁻¹ K at 2 K. The above behaviors suggest antiferromagnetic interactions in 1. The magnetic susceptibility data of 1 are in accordance with the Curie–Weiss law χ = C/(T−θ) (Figure 4b) in the temperature range of 50–300 K with a constant of θ = −28.25 K, which is consistent with the overall antiferromagnetic interaction in 1.

4. Conclusions

In conclusion, a polyanion (HPO₄)₂@{Co₉(PW₉)₃} cluster was made with the guidance of a lacunary directing strategy under hydrothermal conditions. The HPO₄ of 1 originates from the central heteroatom of the tri-lacunary precursor [A-α-PW₉O₃₄]⁹⁻ and assembles with a cyclic (HPO₄)₂@{Co₉} cluster to form CoAP 1. Variable-magnetic properties indicate antiferromagnetic interactions in 1. Moreover, visible-light-driven H₂ evolution tests demonstrated that 1 is an eco-friendly, efficient and stable WRC with an H₂ evolution rate of 1217.6 μmol h⁻¹ g⁻¹. This work further proves that the lacunary directing strategy is an effective guideline for the synthesis of TMAPs under hydrothermal conditions. The photocatalytic performance showed that CoAP is an efficient WRC. In future work, the application of POMs in heterogeneous catalytic reactions is in progress.