Chinese Chemical Letters  2019, Vol. 30 Issue (6): 1273-1276   PDF    
Polyoxometalate-based high-nuclear cobalt-vanadium-oxo cluster as efficient catalyst for visible light-driven CO2 reduction
Lizhen Qiaoa,b,1, Man Songa,b,1, Aifang Genga,*, Shuang Yaoa,b,*     
a School of Chemical and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, China;
b School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China
Abstract: A high-nuclear Co-V-O cluster was firstly isolated by lacunary polyoxoanion, resulting in the high-nuclear mixed metal-oxo cluster-containing polyoxometalate (POM), K4Na28[{Co4(O-H)3(VO4)}4(SiW9O34)4]·66H2O (1). In 1, the {Co4(O-H)3(VO4)}4 {Co16-V4} core, composed of a {Co4O4} cubane, four {Co4(OH)3} qusi-cubanes and four VO4 units, was stabilized by four lacunary A-α-{SiW9O34} units. Photocatalytic study reveals that 1 exhibits excellent photocatalytic activity for CO2-to-CO conversion with high selectivity under visible light irradiation. The turnover number (TON) and turnover frequency (TOF) reaches as high as 10492 and 0.29 s-1, respectively. Compound 1 represents the first high nuclear TM cluster-containing POM (TM=transition-metal) with efficient visible light catalytic activity for CO2 reduction, and its photocatalytic activity is much higher than those of most reported molecular catalysts. Photoluminescence spectroscopy study reveals that photoexcitation of Ru-photosensitizer is followed by an efficient electron transfer to POMs to reduce CO2.
Keywords: Polyoxometalate     High-nuclear cobalt cluster     Photocatalysis     CO2 reduction     Photosensitizer    

Visible-light-driven photocatalytic technology has been regarded as an efficient way to convert solar energy into chemical energy. In this filed, photocatalytic water splitting and CO2 reduction have attracted wide attention, as they are all regarded as promising processes for the storage and utilization of solar energy [1-6]. Global warming and energy demands require efficient utilization of carbon dioxide for energy recycle under mild conditions [7-13]. Up to date, great effort has been devoted to design and synthesis of catalysts with high activity for photocatalytic CO2 reduction [14-23]. However, most of reported molecular catalysts for photocatalytic CO2-to-CO conversion exhibit low performance, and their TON values are usually lower than 103 [16, 24, 25]. In this field, visible-light-driven CO2 reduction to CO is still a great challenge, which requires new and efficient strategy to explore cheap and highly efficient catalysts.

Polyoxomeataltes (POMs), a great subclass of anionic metal-oxo clusters composed of earth-abundant elements, possess of potential applications in various fields due to their structural and component diversity [26-36]. Especially, POMs could undergo fast, reversible, and stepwise multiple electron transfer without changing their structures, which endows them highly catalytic activity for various redox reactions, such as water splitting, oxidation desulfurization, photocatalytic degradation of organic pollutants [37-42]. For water splitting, various POMs containing different TM cations have been design and synthesized [43-49], which were used to construct photocatalytic systems for water splitting with the assistant of photosensitizers (PSs). Also, the POM molecules and PSs were integrated into metal-organic frameworks, resulting in POM@PSs molecular devices with much enhanced photocatalytic performance [50-53]. However, the POM clusters are rarely used as photocatalysts for CO2 reduction up to date [54-57], especially for high nuclear TM clustersubstituted POMs, their application for photocatalytic CO2 reduction has never been unexplored.

Herein, trivacant POMs were used as pure inorganic ligands to stabilize a {Co16-V4} core, resulting in the first high-nuclear cobalt-vanadium-oxo cluster-containing POM K4Na28[{Co4(OH)3(VO4)}4(SiW9O34)4]·66H2O (1). The {Co16-V4} core was composed of an {Co4O4} cubane surrounded by four qusi-cubanes {Co4(OH}3} and four VO4 units. Photocatalytic study reveals that 1 exhibits excellent photocatalytic activity for CO2-CO conversion under visible light irradiation with high CO selectivity of ca. 99.6%. The turnover number (TON) and turnover frequency (TOF) reach as high as 10492 and 0.29 s-1, respectively, with a quantum yield of 0.065%.

Complex 1 was synthesized by a conventional aqueous method. In this typical process, CoCl2·6H2O (0.25 g, 1.05 mmol) was dissolved in 20 mL of distilled water, followed by addition of Na10[α-SiW9O34]·18H2O (0.50 g, 0.17 mmol), and the mixture was stirred until a pink solution was obtained. Then, NaVO3 (0.25 g, 2.05 mmol) was added, and the pH of 9.5-10.0 was maintained with 2.0 mol/L NaOH (aq). The resulting turbid solution was stirred for 2 h at 100 ℃. After cooling to room temperature, the brown precipitate was removed by filtration. The filtrate was kept in a 25 mL beaker to allow slow evaporation at room temperature. After one week, pink block crystals suitable for X-ray crystallography diffraction analysis were isolated, washed with cold distilled water and air-dried with a yield of 16% (based on W). Anal. calcd. (%) for 1: W, 52.94; Co, 7.54; V, 1.63; K, 1.25; Na, 5.15 and Si 0.90. Found: W, 53.42; Co, 7.76; V, 1.75; K, 1.32; Na, 5.06 and Si 0.98. IR (KBr pellet, cm-1): 1634 (s), 1422 (m), 1129 (m), 1042 (w), 1015 (w), 886 (m), 775 (m), 691 (w), 622 (w), 562 (w) (Fig. S1 in Supporting information).

As well known, vacancy-directing has been widely observed in the reaction of TM cations and lacunary POMs [38, 58]. In this work, trivacant {SiW9O34}10-, obtained by removing three WO6 octahedra from saturated Keggin anion {SiW12O40}4-, was used as the precursor. Its vacant sites were filled with three cobalt cations in the assembly process, resulting in the Co3-substituted POM {SiW9O34Co3(OH)3}. Four {SiW9O34Co3(OH)3} units are fused together by four {VO4} units to construct high-nuclear cobaltvanadium-oxo cluster-containing POM of 1. During the synthesis of 1, the pH value play an important role, which was optimized in the range of 9.5-10.0. Also, relatively high reaction temperature of ca. 100 ℃ is an essential condition since no single crystal product can be prepared at a lower temperature under the similar conditions.

Single crystal X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic space group C2/c and contains a high-nuclear {Co16-V4} cluster stabilized by four {SiW9O34}10- units (Fig. 1, Table S1 in Supporting information). As well known, trivacant POM could be easily obtained by removing three WO6 octahedra from a saturated Keggin POM, which has been widely used to capture TM centers in their vacant sites to construct saturated Keggin structure. In the formation of 1, it could be observed that three CoO6 octahedra filled in the vacant sites of one trivacant POM, resulting in Co3-substituted {SiW9O34Co3(OH)3} unit. The vacant-directing effect has been widely observed in the lacunary POM chemistry, which leads to controllable synthesis of TM-substituted POMs. Further, in situ formed Co3(OH)3 unit could further capture another Co2+ via three OH groups, resulting in a qusi-cubane-like cluster {Co4(OH)3}, capping on trivacant {SiW9O34}10- unit. In this process, three O atoms in the OH groups can further coordinate with a TM center simultaneously to direct the assembly of quasi-cubane-like cluster. The OH-directing in the TM3 cluster plays an important role in the formation of cubane/qusi-cubane TM4 clusters. Further, four qusi-cubane Co4(OH)3-containing subunits {SiW9O34Co3(OH)3} are fused together by four VO4 units, resulting in the tetrameric polyoxotungstate [{Co4(OH)3VO4}4(SiW9O34)4]32-, where a highnuclear {Co16-V4} cluster is observed in the center of the tetrameric anion. In the high-nuclear {Co16-V4} cluster, four qusi-cubane {Co4(OH)3} units were fused together bysixteenoxygen atoms from four VO4 ligands, forminga{Co4O4}cubaneinthecenterof{Co16-V4} cluster. Thus, the high-nuclear {Co16-V4} cluster is composed of a {Co4O4} cubane, four {Co4(OH)3} qusi-cubanes and four VO4 units. The Co—O and V—O bond lengths fall into the range of 2.042(14)-2.359(15) Å… and 1.663(16)-1.765(14) Å…, respectively. In this work, the high-nuclear {Co16-V4} cluster was firstly encapsulated by four trivacant POM ligands {SiW9O34}10- forming a tetrameric polyoxoanion. As shown in Fig. 2A, the XRD pattern of 1 agrees well with that simulated from the single crystal data, indicating the bulk purity of as-synthesized sample of 1.

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Fig. 1. (A) Vacancy-directing effect of trivacant POM. (B) Ball-and-stick and (C) polyhedral representation of polyoxoanion 1. Color codes: WO6, green octahedron; SiO4, yellow tetrahedron; VO4, orange tetrahedron; W, green sphere; O, red sphere; Co, violet sphere).

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Fig. 2. (A) Simulated and experimental PXRD patterns of 1. (B) CO evolution catalyzed by 1 (0.065 μmol/L) and Co2+ (1.04 μmol/L) under visible light irradiation (450 nm LED light, light intensity of 100 mW/cm2) in the presence of [Ru(phen)3](PF6)2 (0.1 mmol/L) and TEOA (0.3 mol/L) in CH3CN/H2O (4:1, v/v) solution at 25 ℃ under 1 atm of CO2. (C) t = 1.26 min in the gas chromatogram corresponds to the retention time of 13CO. (D) Mass spectra, m/z = 29 corresponds to the formula weight of 13CO.

The photocatalytic performance of 1 for CO2 reduction was evaluated in a CO2-saturated CH3CN/H2O system with[Ru(phen)3]2+ as the PS, and triethanolamine (TEOA) as the sacrificial agent. Typically, a quartzose glass reactor containing a mixture of 5 mL CO2-saturated CH3CN/H2O (4:1, v/v), 1, [Ru(phen)3]2+ and TEOA, was irradiated bya 450 nm LED light with the light intensity of 100 mW/cm2 (Fig. S2 in Supporting information). The gas products were analyzed by a Shimadzu GC-2014 gas chromategrap-hy (GC), revealing that the CO is the major reduction product. As shown in Fig. 2B and Table S2 (Supporting information), 3.41 μmol of CO can be obtained in the photocatalytic system in 10 h, and trace amount of H2 could be detected in this water-bearing system. Accordingly, the TON and TOF values for CO can reach to 10492 and 0.29 s-1, respectively. The quantumyield was calculated to be 0.065% using a potassium ferrioxalate actinometer. Various control experiments reveal that no or trace amount of CO could be detected in the absence of 1, or the Ar was used to replace CO2 in the photocatalytic system. The 13CO2 isotope trace experiment was also performed. The peak with the m/z value of 29 confirmed the 13CO evolution during the photocatalytic process, revealing that CO dominantly comes from the CO2 reduction instead of the decomposition of other organic components (Figs. 2C and D). These results demonstrate that compound1 played important role in the CO2-to-CO conversion, and the CO comes from the CO2 reduction in this photocatalytic system. These TON and TOF values confirm that 1 is among the best visiblelight active molecular photocatalysts.

In the photocatalytic process, a slow increase of CO generation rate over 6 h was observed (Fig. 2B). This can be attributed to the photo-degradation of [Ru(phen)3](PF6)2 under the irradiation, which has been observed in the photocatalytic system containing [Ru(phen)3]2+ and [Ru(bpy)3]2+ as PSs (Supporting information), the UV-vis spectrum of 1 in the mixed solution of CH3CN/H2O remained unchanged within 10 h. Dynamic light scattering (DLS) measurements were also performed in CH3CN/H2O (4:1, v/v) solution of 0.065 μmol/L 1, no nanoparticles can be detected after photocatalytic reaction (Fig. S5 in Supporting information). Meanwhile, 0.065 μmol/L Co(NO3)2·6H2O with much lower concentration of Co2+ than the POM system, was used to perform the photocatlytic experiments. After the irradiation, nanoparticles with the diameter of ~145 nm were obviously observed (Fig. S6 in Supporting information). These results exclude the formation of cobalt hydroxide/oxide nanoparticles during the photocatalytic process with compound 1.

In the photocatalytic process, the phosphorescence titration of PSs with catalyst or electronic sacrificial agent was performed to study the electron transfer process. In this process, [Ru(phen)3]2+ can be easily excited by visible light to the excited state of 1MLCT, which can efficiently transfer to the excited state of 3MLCT through intersystem crossing. The luminescence spectroscopy of [Ru (phen)3]2+ was measured with addition of TEOA or 1 in the CH3CN solution. As shown in Fig. 3, the steady-state photoluminescence spectra of [Ru(phen)3]2+ at 594 nm gradually reduced with increasing the concentration of 1 (Figs. 3A and C) with a Ksv value of 1.09 × 105 L/mol-1. However, there is no obvious change in the photoluminescence spectra of [Ru(phen)3]2+ with increasing the concentration of TEOA (Fig. 3B). Transient absorption spectra confirm that 1 can accelerate the luminescence decay of [Ru (phen)3]2+ from 413 ns to 98.7 ns by addition of 1 into the [Ru (phen)3]2+ solution. It can be concluded that the phosphorescence quenching of [Ru(phen)3]2+ might be caused by electron transfer from PSs to 1. The generated [Ru(phen)3]3+ was subsequently reduced by TEOA. These results reveal an oxidative quenching mechanism in the photocatalytic reduction of CO2. The possible mechanism of CO2 reduction was proposed as shown in Fig. 4, where the excitation of Ru-PS IS followed by an efficient electron transfer to POMs to reduce CO2. Finally, the oxidative state of Ru-PS can be reduced by TEOA to complete the catalytic cycle under visible light irradiation.

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Fig. 3. Steady-state photoluminescence spectra of [Ru(phen)3]2+ (5 μmol/L) as a function of concentrations of (A) 1 and (B) TEOA with λex = 420 nm. (C) Stern-Volmer plot of 1 and TEOA. (D) Nanosecond transient absorption spectra of [Ru(phen)3]2+ in the presence of 1 followed at 450 nm.

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Fig. 4. Proposed photocatalytic mechanism for CO2 reduction with 1 under visible light irradiation.

In summary, a high-nuclear {Co16-V4} cluster composed of a {Co4O4} cubane, four {Co4(OH)3} quasi-cubanes and four VO4 units, was firstly isolated by four pure inorganic lacunary POM units. In the assembly process, the vacancy-directing and OH-directing effect were both observed, which play important role in constructing the cubane-like TM4O4-containing high-nuclear metal-oxo cluster. Photocatalytic study reveals that compound 1 exhibits excellent photocatalytic activity for CO2-to-CO conversion under visible light irradiation. The TON and TOF reach as high as 10492 and 0.29 s-1, respectively. The activity of 1 is much higher than those of most reported molecular photocatalysts for CO2 reduction, and 1 is among the best photocatalysts with visiblelight activity for CO2 reduction in the POM chemistry. These results may provide a new direction towards the development of efficient, low-cost and applicable CO2 reduction molecular photocatalysts.

Acknowledgement

We are grateful for the financial support from Science and Technology Research Foundation of the Thirteenth Five Years of Jilin Educational Committee (No. JJKH20170605KJ).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.01.024.

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