A new heterogeneous photocatalyst based on Wells–Dawson polyoxometalate and nickel coordination compounds: synthesis, structure and property

Abstract

Polyoxometalate (POM) as a promising photocatalyst has gained much attention for the removal of organic pollutants from water. However, the two main drawbacks of low visible-light photocatalytic activity due to wide oxygen-to-metal charge transfer band of POM and high solubility in solution make them difficult to recover and recycle impeding potential applications. We have prepared a new heterogeneous POM photocatalyst, namely [Ni(2,2′-bpy)₃]₃[P₂W₁₈O₆₂] (2,2′-bpy = 2,2′-bipyridine, NiPW), characterized by X-ray single crystal diffraction, elemental analyses, FTIR, XRPD, and UV-DRS. The photocatalyst NiPW exhibits two broad band centered at 513 nm and 770 nm, extending induced light to visible region. As a photocatalyst, NiPW composite not only shows excellent visible light photocatalytic activity for the photodegradation of methyl orange in the presence of H₂O₂, but also can be easily separated by simple centrifugation for its insoluble property and recycled.

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1. Introduction

Polyoxometalate (POM), as a large variety of oxygen-bridged metal clusters with unique structural characteristics and rich photocatalytic activity,^1–4 has been extensively studied for the removal of organic pollutants or transition metal ions from water.^5–9 Many of those POM share similar photochemical characteristics as semiconductor photocatalysts in terms of the overall mechanism of photodecomposition of organic compounds, the intermediate species and the final photodegradation products.^10–12 The near-visible and ultraviolet light induce POM to produce oxygen-to-metal charge transfer (OMCT) with promoting electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).^12,13 The charge-transfer excited state (POM*) with strong oxidising properties can direct oxidize the target pollutant, or react with water or other electron donor to generate ˙OH radical.^14,15 However, the drawbacks of wide band between the well-defined HOMO–LUMO leading to low visible-light photocatalytic activity and high solubility in solution making them difficult recovering and recycling impede their potential applications. Many researchers have made efforts in order to resolve the problems. Incorporation of homogeneous POMs with support materials, such as TiO₂, ZrO₂, or SiO₂, and some organic polymers with UV light resistance has been extensively investigated.^16–18 Those hybrids overcome some difficulties in recovering and recycling, but these methods have other shortcomings such as limited activity or stability during the reaction.¹⁹ On the other hand, dye sensitization is an established means to extend absorbance to the visible region.²⁰ Ruthenium polypyridyl complex was ever used as sensitizer to improve the visible-light photocatalytic activity,⁴ but rare ruthenium is very expensive. Although another two POM photocatalysts (CuPW, CB[6]–POMs) with a broad band centered at 690 nm and ranged from 600 to 900 nm, respectively, exhibited visible-light activity, they couldn't efficiently utilize the solar spectrum.^12,20 As is well known, the wavelength of visible light of irradiated to the surface of the earth is mainly near 500 nm, being about 43% of the total energy in the visible region of the sun.²¹ This inspired us to synthesize a heterogeneous POM photocatalyst with the absorption near 500 nm to maximumly utilize the solar light.

Herein, we will report a new heterogeneous composite constructed from Wells–Dawson POMs and nickel coordination compounds, namely [Ni(2,2′-bpy)₃]₃[P₂W₁₈O₆₂] (2,2′-bpy = 2,2′-bipyridine, NiPW), exhibiting two broad band centered at 513 nm and 770 nm in the region of visible light and improved visible light photocatalytic activity. To our knowledge, this type of POM photocatalyst has not been reported yet.

2. Experimental section

2.1. General procedures

All chemicals purchased from Shanghai Chemical Reagents Co., China were all analytical grade and used without further purification. The hydrothermal reactions were carried out in 25 ml Teflon-lined stainless steel autoclaves under autogenously pressure with a fill factor of approximately 80%. Elemental analyses (C, H, and N) were carried on an Elementar Vario EL III analyzer. Ni, P and W were determined by a Jobin Yvon Ultima2 ICP atomic emission Spectrometer. Fourier transform infrared (FTIR) spectra were recorded in the range of 400–4000 cm⁻¹ on a Perkin Elmer Spectrum by transmission mode using KBr pellets. The UV-light diffuse reflectance spectrum (UV-DRS) was measured on a Lambda-900 using a BaSO₄ plate. X-ray powder diffraction (XRPD) was performed with a Rigaku DMAX 2500 diffractometer.

2.2. Synthesis of NiPW

A mixture of Na₂WO₄·2H₂O (0.429 g, 1.30 mmol), NiCl₂·6H₂O (0.049 g, 0.21 mmol), 2,2′-bipyridine (0.075 g, 0.48 mmol), 0.5 ml H₃PO₄ and H₂O (20 ml) was stirred for 30 minutes or so, and the pH value was adjusted to 2.5–3 with 2 mol l⁻¹ NaOH solution. Finally, the resulting suspension was transferred to a Teflon-lined autoclave (25 ml) and kept at 160 °C for 5 days. After slow cooling to room temperature for 2 days, purple crystals of NiPW (yield about 52% based on Ni) were obtained by filtration, washed with distilled water and dried in desiccators at ambient temperature. Anal. calcd for C₉₀H₇₂N₁₈Ni₃O₆₂P₂W₁₈ (M_w: 5945.03): C, 18.19%; H, 1.22%; N, 1.97%; Ni, 2.96%; P, 1.04%; W, 55.65%. Found: C, 18.31%; H, 1.30%; N, 2.12%; Ni, 2.74%; P, 1.15%; W, 55.43%.

2.3. X-Ray crystallography

Structural measurements for NiPW was performed on a Mercury CCD (2 × 2 bin mode) diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 20 °C. Empirical from equivalents were made from ψ-scan data using the program SHELXTL 97 at the data reduction stage along with the correction for Lorentz and polarization effects. The structure analysis was performed by using the Crystal Structure crystallographic program package. The structure of NiPW was solved by the direct methods (Wingx32, SIR-92), and successive Fourier difference syntheses. The final structure was examined with the program PLATON and no additional symmetry element was detected. All the heavy atoms were refined with anisotropic thermal parameters except part of oxygen, carbon, and nitrogen atoms and all hydrogen atoms. Crystal parameters and other experimental details of the data collection for NiPW are summarized in Table S1. Selected bond lengths and bond angles for NiPW are listed in Table S2. CCDC reference number 1026655 for complex NiPW.†

2.4. Photocatalytic reaction

In the photodegradation experiments, continuous irradiation was performed with a 300 W xenon lamp (LSXS-300, Zolix company, China) as a parallel light source equipped with a 420 nm cutoff filter to ensure visible light. The photocatalytic reaction was carried out in a cylindrical pyrex reactor of 250 ml capacity, located at 25 cm distance from the xenon lamp. Visible light dose measured by radiometer (CEL-VIS400, China) is about 100 mW cm⁻². The reactor was cooled by a cooling jacket to keep the chamber at room temperature. NiPW photocatalyst were ground by agate mortar and pestle two hours or so. Then, powder of NiPW (0.5 g l⁻¹) was dispersed in methyl orange (MO) solution (15 mg l⁻¹, pH = 2.5) containing H₂O₂ (from 0.8 to 1.6 mmol l⁻¹) by simultaneous shaking (600 rpm). Prior to irradiation, the system was magnetically stirred in the dark for 30 min to ensure the establishment of absorption equilibrium. The reactor was kept under constant air-equilibrated conditions at room temperature before and during the irradiation. Every 20 min interval, 3 ml aliquot was sampled and then centrifuged to remove the particles of catalyst. The MO concentration (C) was determined by measuring the maximum absorbance at 505 nm as a function of irradiation time using a Lambda35 spectrophotometer (Perkin Elmer, USA). The photocatalytic activities of the samples were evaluated by measuring the decolorization efficiency (%) of MO solution, which was calculated by the following formula:where D is decolorization efficiency, C₀, A₀ and C₁, A₁ are the concentration and absorbency of methyl orange solution, respectively.

For comparison, the experiments for MO with O₂ but in the absence of H₂O₂ were done, also. A MO solution with high O₂ concentration (6.75 mg l⁻¹) was obtained by purging with O₂ gas at room temperature. Another MO solution with low O₂ concentration (1.62 mg l⁻¹) was kept under air-equilibrated condition at room temperature. The oxygen concentration in the MO solution was measured by Dissolved Oxygen Analyzer (OXY5401S-S, China).

The reproducibility of the photocatalytic degradation activity on NiPW composite was studied at constant MO concentration (15 mg l⁻¹) containing 1.3 mmol l⁻¹ H₂O₂ with a pH of 2.5 and a catalyst dosage of 0.5 g l⁻¹ in each cycle. After the photocatalytic degradation of the MO solution, the NiPW photocatalyst can be separated by simple centrifugation for its insoluble property in common solvents, and was dried at 80 °C in vacuum for overnight. Then the recovered photocatalyst was reused in the next cycle.

3. Results and discussion

3.1. Crystal structure of NiPW

Single X-ray diffraction analysis reveals that NiPW photocatalyst crystallizes in the tetragonal system, chiral space group P4₃, and consists of a classical Wells–Dawson polyoxoanion [α-P₂W₁₈O₆₂]⁶⁻, three six-coordinate [Ni(2,2′-bpy)₃]²⁺ cations (Fig. 1). The [α-P₂W₁₈O₆₂]⁶⁻ polyoxoanion behaves D_3h point symmetry and contains two [α-A-PW₉O₃₄]⁹⁻ units which are linked together through corner-sharing with the elimination of six oxygen atoms. In the Wells–Dawson polyoxoanion, oxygen atoms can be divided into two groups: (I) 54 surface oxygen including 18 terminal oxygen atoms (O_t) only connecting with one W atom with W–O bond distances of 1.673(11)–1.746(13) Å, and 36 μ₂-O atoms coordinated to two W atoms with W–O bond distances of 1.841(12)–1.975(11) Å; (II) 8 interior oxygen atoms consisting of 6 μ₃-O atoms with W–O bond distances of 2.350(10)–2.398(11) Å, and 2 μ₄-O atoms bonding to three W atoms and one P atom with W–O bond distances of 2.342(10)–2.390(11) Å. The 54 surface oxygen atom usually can be as receptors to connect other organic or inorganic donors via hydrogen bonding interaction. It is interesting that one Wells–Dawson polyoxoanion connects twelve [Ni(2,2′-bpy)₃]²⁺ cations via C–H⋯O hydrogen bonding (Fig. S1a†). In the asymmetric unit the three crystallographically independent nickel ions behave similar octahedron geometry and are coordinated with six nitrogen atom from 2,2′-bpy ligands with Ni–N bond distances from 2.023(17)–2.155(15) Å. It should be noted that each of three [Ni(2,2′-bpy)₃]²⁺ cations connects four Wells–Dawson polyoxoanions via C–H⋯O hydrogen bonding (Fig. S1b–d†). The C⋯O distance is average 3.126 Å (Table S3†). Thus, discrete of NiPW are extended into a three-dimensional supramolecular array in the solid via C–H⋯O hydrogen bonding interactions (Fig. 2).

Fig. 1 Drawing of the asymmetric unit of NiPW.

Fig. 2 A packing of NiPW along a axes. Green, purple, and pink polyhedrons show the [WO₆], [NiN₆], and [PO₄] units, respectively.

3.2. Characterizations of NiPW

The FTIR spectrum of NiPW was also investigated to further confirm the as-prepared composite (Fig. 3). The 3083–3111 cm⁻¹ region should be ascribed to the O⋯H hydrogen bonding vibrations. The 1200–1700 cm⁻¹ region is indicative of the 2,2′-bpy organic ligand. The P–O vibrations are observed only at 1093 cm⁻¹, indicating that the polyoxoanion in NiPW is the [P₂W₁₈O₆₂]⁶⁻ anion.²² The bands between 940–980 cm⁻¹ are assigned to the M = O_t (M = W) stretching vibrations, peaks between 700–900 cm⁻¹ are attributed to M–O_b–M stretching modes, and peaks between 550–700 cm⁻¹ represent the M–O_c–M vibrations. The phase purity of the NiPW photocatalyst is confirmed by X-ray powder diffraction (XRPD). The experimental and simulated XRPD patterns of NiPW were also displayed in Fig. S2.† Their peak positions are in good agreement with each other, showing the phase purity of the bulk products. The differences in intensity may be due to the preferred orientation of the power.

Fig. 3 The FTIR curves of the NiPW photocatalyst before (a) and after (b) photocatalytic degradation for 5 cycles.

The UV-vis diffuse reflectance spectrum (DRS) of NiPW was measured at room temperature. The absorption (α/S) data were calculated from the reflectance using the Kubelka–Munk function:²³

where α is the absorption, S is the scattering coefficient and R is the reflectance. As shown in Fig. 4a, the DRS of NiPW exhibited two broad bands centered at 513 nm and 770 nm in the region of visible light, which is very close to the absorbance band of [Ni(2,2′-bpy)₃]₃⁶⁺. It was reported that the [Ni(2,2′-bpy)₃]₃⁶⁺ has two absorbance band centered at 516 nm and 788 nm.²⁴ While the [P₂W₁₈O₆₂]⁶⁻ doesn't show absorbance band in the visible light.²⁵ Therefore, two broad bands for the [Ni(2,2′-bpy)₃]₃[P₂W₁₈O₆₂] (NiPW) arises from [Ni(2,2′-bpy)₃]₃⁶⁺, not [P₂W₁₈O₆₂]. For the NiPW photocatalyst, the [Ni(2,2′-bpy)₃]₃⁶⁺ acting as sensitizer (S) can be induced by visible light, then the electrons transmit into the LUMO of [P₂W₁₈O₆₂]⁶⁻ being as POM unit. The POM core is like a reservoir of electrons which can undergo electron-reduction processes without significantly deforming its framework.²⁶ Fig. 3b shows the relationship between the sensitizer and POM unit. Thus, the visible light can induce the organic–inorganic hybrid NiPW, making it possible to efficiently utilize solar light.

Fig. 4 (a) UV-vis diffuse reflectance spectra of the photocatalyst NiPW (inset shows the photo of NiPW photocatalyst). (b) Relationships between the nickel coordination compound as sensitizer and the Wells–Dawson polyoxoanion as POM unit.

3.3. Photocatalytic activity

The photodegradation results of MO over NiPW as photocatalysts under visible light irradiation were illustrated in Fig. 5. Curve (A) in Fig. 5 is the initial time (30 min for all samples) during which the whole system was stirred in the dark, and curve (B) (from 30 min to 150 min) denotes the time for the mixture to be exposed to visible light irradiation. In the dark, there is negligible degradation of MO solution even in the presence of catalyst and H₂O₂. Only after the visible light irradiation started, the degradation of MO was initiated. The absorbance of MO in an aqueous solution was decreased with increasing irradiation time under visible light irradiation (Fig. 6). After 2 hours, the decolorization efficiency of MO in NiPW/H₂O₂ system can achieve 99.58% (curve (a)), which is much higher than that in the absence of H₂O₂. This result indicates that H₂O₂ is an efficient electron-acceptor in the heterogeneous NiPW system.¹² The photodegradation of the MO in the NiPW/O₂ system (curve (b)) is higher than that of the low O₂ concentration system (curve (c)), showing that O₂ is a electron-acceptor in current system. Meanwhile the decolorization efficiency at the presence of only H₂O₂ (curve (d)) was very low, implying that NiPW is an effective photocatalyst. Compared with other known photocatalysts, NiPW indicated much better catalytic activity towards photodegrading MO dyes in terms of the reaction time and efficiency.^27–34 The known photocatalysts Fe-doped ZnO and BiOBr–Bi₂WO₆ could degrade 72% and 90% MO within 120 min, respectively, which were much poorer than NiPW. It may be because that the NiPW photocatalyst behaves a broad brand centered at 513 nm, being about 43% of the total energy in the visible region of the sun.

Fig. 5 Decolorization efficiencies of MO in different reaction systems. MO concentration: 15 mg l⁻¹; pH: 2.5. (a) NiPW/H₂O₂; (b) NiPW/O₂ (O₂ concentration: 6.75 mg l⁻¹); (c) NiPW/low O₂ (O₂ concentration: 1.62 mg l⁻¹); (d) H₂O₂.

Fig. 6 UV-vis spectral changes of MO solution in the presence of NiPW under visible light irradiation. MO concentration: 15 mg l⁻¹; pH: 2.5; H₂O₂: 1.3 mmol l⁻¹; NiPW dosage: 0.5 g l⁻¹.

3.4. Effect of H₂O₂

Fig. 7 showed the initial decolorization efficiency of methyl orange (15 mg l⁻¹, pH = 2.5) containing different H₂O₂ concentration after two hour irradiation. It can be seen that the decolorization efficiency increases with increase of H₂O₂ concentration from 0.8 to 1.3 mmol l⁻¹, and then decreases with increase of H₂O₂ concentration from 1.3 mmol l⁻¹ to 1.6 mmol l⁻¹. The optimum concentration of adding H₂O₂ is about 1.3 mmol l⁻¹. In photocatalytic degradation process, addition of adequate H₂O₂ is beneficial to the formation of hydroxyl radicals.^35,36 H₂O₂ will facilitate the generation of ˙OH and promote the decolorization efficiency (eqn (2) and (3)). However, under the condition of exorbitant H₂O₂ concentration, H₂O₂ acts as a scavenger of ˙OH and exhaust ˙OH in solution (eqn (4) and (5)),^35,36 and the decolorization reaction will be retarded. Therefore, moderate added H₂O₂ is beneficial to achieve higher degradation efficiency.

H₂O₂ + e⁻ → OH⁻ + ˙OH (3)

H₂O₂ + ˙O₂⁻ → O₂ + OH⁻ + ˙OH (4)

H₂O₂ + ˙OH → OH⁻ + HO₂˙ (5)

HO₂˙ + ˙OH → O₂ + OH⁻ + ˙OH (6)

Fig. 7 The decolorization efficiencies of methyl orange (MO) solution with the different H₂O₂ concentration in the presence of NiPW. MO concentration: 15 mg l⁻¹; pH: 2.5; H₂O₂: 1.3 mmol l⁻¹; NiPW dosage: 0.5 g l⁻¹; irradiation time: 60 min.

3.5 Photocatalytic kinetic

The kinetics of MO dye degradation by NiPW photocatalyst in aqueous solution under visible light irradiation was also investigated, as depicted in Fig. 8. It can be seen that MO dye degradation follows an apparent first-order in good agreement with a generally observed Langmuir–Hinshelwood kinetics model:³⁷where r is the degradation efficiency of the reactant (g l⁻¹ min⁻¹), C is the concentration of the reactant (mg l⁻¹), t is the illumination time (min), k is the kinetic constant (g l⁻¹ min⁻¹), and K is the adsorption coefficient of the reactant (l g⁻¹). When the initial concentration C₀ is micromolar solution, the eqn (6) can be simplified to an apparent first-order equation:

Fig. 8 First-order linear ln(A₀/A) = f(t). MO concentration: 15 mg l⁻¹; pH: 2.5; H₂O₂: 1.3 mmol l⁻¹; NiPW dosage: 0.5 g l⁻¹.

The plot of ln(A₀/A) versus time represents a straight line, the slope of which upon linear regression gives rate constant K_app (0.0458 min⁻¹).

3.6. Proposed mechanism

S* → S⁺ + e⁻ (10)

H₂O₂ + POM⁻ → POM + OH⁻ + ˙OH (12)

O₂ + POM⁻ → POM + ˙O₂⁻ (13)

˙OH + MO(dye) → degradated product (14)

In current system, the nickel coordination compound acts as sensitizer (S) and can be induced by visible light (eqn (9) and (10)),¹² meanwhile Wells–Dawson [P₂W₁₈O₆₂]⁶⁻ polyoxoanion acts as POM unit to accept the electrons and deposit them in its LUMO (eqn (11)).² The adsorbed H₂O₂ and O₂ can easily trap an electron in LUMO of the POM anion to yield the oxidizing species ˙OH (eqn (12) and (13)). Then, radicals attack organic substrates and degrade dye molecules (eqn (14)). Proposed photodegradation mechanism of dyes is shown in Scheme 1.

Scheme 1 Proposed photodegradation mechanism.

3.7. Repeatability of the photocatalyst

In photocatalytic degradation reaction, the repeatability of the photocatalytic activity for the photocatalyst is a very important parameter to assess the photocatalyst practicability. The reproducibility of the photocatalytic degradation activity on NiPW was also studied, as presented in Fig. 9. It was observed that the decolorization efficiency decreased from 99.58% to 99.01% after five cycles, which shows that the photocatalytic activity has a good repeatability and the considerable stability of the photocatalyst under the present conditions. The FTIR spectra of the photocatalyst after photocatalysis degradation for 5 cycles have been further recorded (Fig. 2). Curve (b) is very similar to (a), indicating that the POM structure of NiPW remains intact after photocatalysis degradation. It is because that the Wells–Dawson [P₂W₁₈O₆₂]⁶⁻ polyoxoanions have extensive hydrogen bond interactions with [Ni(2,2′-bpy)₃]₃⁶⁺ sensitizer, and the POM units don't easily leak from the system. In a word, the photocatalyst not only has a good photocatalytic activity under visible light, but also has good reproducibility of photocatalytic degradation by a simple recycled procedure, which are of great significance for practical use of the photocatalyst.

Fig. 9 Photocatalytic decolorization efficiency for MO with NiPW composite in different recycling times. MO concentration: 15 mg l⁻¹; pH: 2.5; H₂O₂: 1.3 mmol l⁻¹; irradiation time: 120 min.

4. Conclusions

In summary, a heterogeneous POM photocatalyst based on Wells–Dawson polyoxoanions as POM unit and nickel coordination compounds as sensitizer has been successfully synthesized. The photocatalyst exhibits excellent visible-light photocatalytic activity and a simple recovered and recycled procedure. This research may supply a new insight into POM-based photocatalysis materials, and promotes us to further develop the POM systems sensitized by other transition metal coordination compounds. This work, to some extent, provides some effective strategies in the design and construction of POM-based hybrid materials.

Acknowledgements

This work was financially supported by the Natural Science Foundation of the Jiangsu Higher Education Institution of China (14KJB150007), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS, SEM), and the State Key Laboratory of Structural Chemistry of Fujian Institute of Research on the Structure of Matter (20150020).

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic files for NiPW in CIF format, the tables about the bond distances and angles, and XRPD. CCDC 1026655. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra15932b

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