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Article

EDTA-Reduction of Water to Molecular Hydrogen Catalyzed by Visible-Light-Response TiO2-Based Materials Sensitized by Dawson- and Keggin-Type Rhenium(V)-Containing Polyoxotungstates

1
Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
2
Department of Chemistry, Faculty of Science, Kanagawa University, Tsuchiya 2946, Hiratsuka, Kanagawa 259-1293, Japan
*
Author to whom correspondence should be addressed.
Materials 2010, 3(2), 897-917; https://doi.org/10.3390/ma3020897
Submission received: 27 November 2009 / Revised: 12 January 2010 / Accepted: 2 February 2010 / Published: 2 February 2010
(This article belongs to the Special Issue Polyoxometalate Compounds)

Abstract

:
The synthesis and characterization of a Keggin-type mono-rhenium(V)- substituted polyoxotungstate are described. The dimethylammonium salt [Me2NH2]4[PW11ReVO40] was obtained as analytically pure homogeneous black-purple crystals by reacting mono-lacunary Keggin polyoxotungstate with [ReIVCl6]2- in water, followed by crystallization from acetone at ca. 5 °C. Single-crystal X-ray structural analysis of [PW11ReVO40]4- revealed a monomeric structure with overall Td symmetry. Characterization of [Me2NH2]4[PW11ReVO40] was also accomplished by elemental analysis, magnetic susceptibility, TG/DTA, FTIR, UV-vis, diffuse reflectance (DR) UV-vis, and solution 31P-NMR spectroscopy. Furthermore, [PW11ReVO40]4- and the Dawson-type dirhenium(V)-oxido-bridged polyoxotungstate [O{ReV(OH)(α2-P2W17O61)}2]14- were supported onto anatase TiO2 surface by the precipitation methods using CsCl and Pt(NH3)4Cl2. With these materials, hydrogen evolution from water in the presence of EDTA·2Na (ethylenediamine tetraacetic acid disodium salt) under visible light irradiation (≥400 nm) was achieved.

Graphical Abstract

1. Introduction

Molecular hydrogen is known to be a clean-burning fuel free of CO2 emissions; it is considered a promising candidate to mitigate the current energy problems [1,2]. The development of efficient photocatalysts for hydrogen production from water has attracted much attention in the fields of solar light energy utilization and storage; Honda and Fujishima were the first to demonstrate a photo-electrochemical cell consisting of a TiO2 photo-anode and Pt cathode to decompose water into hydrogen and oxygen under ultraviolet (UV) irradiation with an external bias [3]. The design and preparation of new photocatalysts that are responsive in a similar manner to visible light are a key target for utilizing solar energy for hydrogen production. There have been several attempts to develop efficient photocatalysts that work under visible light irradiation: e.g., a chemically modified n-type TiO2 by controlled combustion of Ti metal in a neutral gas flame (this material absorbs light at wavelengths below 535 nm) [4]; visible-light-responsive TiO2 thin films prepared by a radio-frequency magnetron sputtering deposition method [5]; NiOx-promoted (partly oxidized nickel) In0.9Ni0.1TaO4 [6]; (AgIn)xZn2(1-x)S2 [7]; (Ga1-xZnx)(N1-xOx) with RuO2, transition metal mixed-oxides consisting of Cr, and noble-metal/Cr2O3 (core/shell) nanoparticles as co-catalysts [8]. These materials are powerful photocatalysts for water splitting under visible light irradiation; however, research on using other materials as visible-light-responsive photocatalysts still holds considerable interest.
Polyoxometalates (POMs) have attracted considerable attention because of their extreme versatility and unique range properties; these include catalytic and biological activities and/or photochemical, electrochromic, and magnetic properties [9,10,11,12]. Recently, it is known that the POMs-supported TiO2 and/or zeolite materials show higher activities for various photoreactions under visible light irradiation [13,14,15,16,17]. We also succeeded in developing a TiO2-based visible-light-responsive photocatalyst: a Dawson-type dirhenium(V)-oxido-bridged POM [O{ReV(OH)(α2-P2W17O61)}2]14- (1) was grafted onto TiO2 through electrostatic interaction using a silane coupling reagent with cationic quaternary ammonium groups. The material showed activity for hydrogen evolution from water vapor under visible light irradiation (≥400 and ≥420 nm); however, the surface silane coupling reagent was decomposed by the visible light irradiation [18].
In this study, we focused on using the Keggin-type mono-rhenium(V)-substituted POM [PW11ReVO40]4- (2) as a sensitizer to investigate the influence of molecular structures of rhenium(V)-containing polyoxometalates for hydrogen evolution from water under visible light irradiation. The potassium and tetra-n-butylammonium salts of 2 have already been reported; however, the X-ray crystal structure of 2 has never been clarified [19,20]. In this paper, we synthesized the dimethylammonium salt of 2, [Me2NH2]4[PW11ReVO40] (Me2NH2-2), by a different method, and characterized it using X-ray crystallography, elemental analysis, magnetic susceptibility, TG/DTA, FTIR, UV-vis, diffuse reflectance (DR) UV-vis, and solution 31P-NMR spectroscopy. The polyoxoanions 1 and 2 were then supported onto anatase TiO2 surface by the precipitation methods using CsCl and Pt(NH3)4Cl2 with some loadings. The photocatalytic activities of these materials were demonstrated for hydrogen evolution from water in the presence of EDTA·2Na (ethylenediamine tetraacetic acid disodium salt) under visible light irradiation (≥400 nm). The polyhedral representations of polyoxoanions 1 and 2 are shown in Figure 1.
Figure 1. Polyhedral representation of (a) [O{ReV(OH)(α2-P2W17O61)}2]14- (1) and (b) [PW11ReVO40]4- (2). The one and two rhenium groups are represented by the purple octahedra. The WO6 and internal PO4 groups are represented by white octahedra and yellow tetrahedra, respectively.
Figure 1. Polyhedral representation of (a) [O{ReV(OH)(α2-P2W17O61)}2]14- (1) and (b) [PW11ReVO40]4- (2). The one and two rhenium groups are represented by the purple octahedra. The WO6 and internal PO4 groups are represented by white octahedra and yellow tetrahedra, respectively.
Materials 03 00897 g001

2. Experimental Section

2.1. Materials

The potassium salt of 1, K14[O{ReV(OH)(α2-P2W17O61)}2]·21H2O (K-1), was synthesized by the published method [18]. The mono-lacunary Keggin POM K7[PW11O39]·11H2O was synthesized by the published method, and was characterized by 31P-NMR, TG/DTA, and FTIR measurements [21]. The number of solvated water molecules for K7[PW11O39]·11H2O was determined by TG/DTA analysis. K2[ReIVCl6] was purified by the reprecipitation from water/ethanol. Other reagents and solvents were obtained and used as received from commercial sources.

2.2. Instrumentation/Analytical Procedures

Elemental analyses were carried out by Mikroanalytisches Labor Pascher (Remagen, Germany). The samples were dried at room temperature under 10-3–10-4 torr overnight before analysis. Microanalyses for Re and P were specially ordered for POMs-supported TiO2 samples. Infrared spectra were recorded with a Jasco 4100 FTIR spectrometer on KBr disks at room temperature. Thermogravimetric (TG) and differential thermal analysis (DTA) data were acquired using a Rigaku Thermo Plus 2 series TG/DTA TG 8120. TG/DTA measurements were performed in air with the temperature increasing at a rate of 4 °C/min between R. T. and 500 °C. The 31P-{1H} NMR (161.70 MHz) spectra in solution were recorded in 5-mm outer diameter tubes with a JEOL ECA-600 NMR spectrometer. The 31P-{1H}-NMR spectra were referenced to an external standard of 85% H3PO4 in a sealed capillary. Chemical shifts were reported as negative on the δ scale with resonances upfield of H3PO4 (δ 0). Solution and diffuse reflectance (DR) UV-vis spectra were recorded on a Jasco V-570 spectrophotometer. For the DR UV-vis measurement, a Jasco diffuse-reflectance attachment was equipped. The positions of sharp bands were automatically determined by software of UV-vis spectrometer, and those of broad bands were picked up at the highest values in the ASCII files. Magnetic susceptibility of Me2NH2-2 was evaluated using the Gouy method at Kanagawa University. The measurement was carried out at 25 °C.

2.3. Synthesis and Characterization of Me2NH2-2

A mixture of K2ReCl6 (0.157 g, 0.33 mmol) and K7[PW11O39]·11H2O (1.0 g, 0.33 mmol) in 20 mL water was stirred for 1 h at 25 °C. The resulting dark purple-black solution was filtered through a folded filter paper (Whatman #5). Solid Me2NH2Cl (1.0 g, 12.26 mmol) was added to the dark purple-black filtrate at 25 °C. After stirring for 15 min, a purple-black precipitate was formed. This precipitate was collected on a membrane filter (JG 0.2 μm) and washed with ethanol (90 mL). At this stage, the product dimethylammonium salt was obtained in 0.14 g yield. The product (0.080 g) was dissolved in 50 mL acetone at room temperature, which was followed by filtering through a folded filter paper (Whatman #5). The purple-black solution was then allowed to evaporate slowly in a refrigerator at ca. 5 °C. After a few weeks, purple-black granular crystals were formed. The crystals were obtained in 36.1% yield (0.029 g scale) based on [(CH3)2NH2]4[PW11ReO40]. The obtained product was soluble in dimethylsulfoxide and slightly soluble in acetone, water, methanol, and ethanol. Elemental analysis: Found (calcd.) for [(CH3)2NH2]4[PW11ReO40] = H32C8N4O40P1Re1W40: H, 0.99% (1.05%); C, 3.40% (3.14%); N, 1.60% (1.83%); P, 0.96% (1.01%); Re, 5.80% (6.08%); W, 65.8% (66.01%). TG/DTA under atmospheric conditions: a weight loss of 6.27% with an exothermic point at 421.5 ºC was observed in the temperature range 64–500 ºC; the weight loss was due to the decomposition of Me2NH2+ ions (calculated value for 4Me2NH2+ ions was 6.02%). No weight loss due to the solvated water molecules was observed. Infrared spectrum (cm-1): 1076, 1020 (νas(P-Oa)), 972 (νas(W-Od)), 885 (νas(W-Ob-W)), 800, 770 (νas(W-Oc-W)). 31P-NMR (in DMSO-d6, at 25 °C, referenced to 85% H3PO4): −14.93 ppm. UV-vis absorption (in DMSO, 1.00 × 10-6 M and 1.00 × 10-4 M): λ 264 nm (ε 49,950 M-1cm-1), λ 512 nm (ε 1,625 M-1cm-1), 741 nm (ε 786 M-1cm-1). DR UV-vis spectrum in the visible-light region: λ 526 nm, 688 nm. Magnetic moment: 1.28 B. M.

2.4. Preparation of 1- and 2-Supported TiO2 Materials by the Precipitation Method Using CsCl

For the preparation of 1-supported TiO2 materials, anatase TiO2 support (2.0 g) was dispersed in 50 mL of water at 25 ºC. K-1 (42.9 and 97.5 mg; 4.4 and 10.0 μmol of Re/g) was dissolved in 30 mL of water. The POM solution was then added to the TiO2 suspension. CsCl (414.9 and 942.5 mg; 1,232 and 2,800 μmol/g) was dissolved in 20 mL water and added to the mixtures. At this stage, 560 eq. of CsCl was required for the precipitation of polyoxoanion 1. The elemental analysis results of Cs for the precipitate prepared by adding 560 eq. of CsCl to the aqueous solution of 1 was 16.9%; this showed that the formula of the obtained precipitate was Cs14[O{Re(OH)(α2-P2W17O61)}2]·22H2O (the calculated value was 16.91%). TG/DTA showed a weight loss of 3.93% with an endothermic point at 41.4 °C in the temperature range of 15.1–246 °C; the weight loss was due to the hydrated water molecules (calculated value for 22 H2O was 3.60%). After stirring for 2.5 h at 25 °C, the obtained purple-white products were collected by a Büchner funnel (Whatman #5), washed with water (30 mL × 3), and then dried in an oven at 50 °C overnight. Elemental analysis results for Re were 0.038% and 0.062%, respectively, which corresponded to 2.0 and 3.3 μmol of Re/g. These samples are abbreviated as 1-Cs-TiO2(2.0) and 1-Cs-TiO2(3.3), respectively. The DR UV-vis spectra in the visible light region of these materials showed sharp bands at 499 and 501 nm, respectively, which was assigned to the ReV ➔ WVI intervalence charge transfer (IVCT) band. In addition, a broad band at around 750 nm assigned to the d-d band of ReV was observed for both samples (Figure S1).
For the 2-supported TiO2 materials, Me2NH2-2 (27.0 and 61.0 mg; 4.4 and 10.0 μmol of Re/g) was added to anatase TiO2 support (2.0 g) suspended in 300–550 mL of water at 25 ºC. CsCl (356 and 808 mg; 1056 and 2400 μmol/g) was dissolved in 20 mL water and added to the mixture. At this stage, 240 eq. of CsCl was required for the complete precipitation of 2. The elemental analysis results of Cs for the precipitates prepared by adding 240 eq. of CsCl to the aqueous solution of 2 was 13.9%; this showed that the formula of the obtained precipitate was Cs3.5H0.5[PW11ReO40]·4H2O (the calculated value of Cs was 13.61%). TG/DTA showed a weight loss of 2.18% with an endothermic point at 59.2 °C in the temperature range of 17.4–168 °C; the weight loss was due to the hydrated water molecules (calculated value for 4 H2O was 2.11%), and no weight loss due to the decomposition of Me2NH2+ ions was observed. After stirring for 2 h at 25 °C, the obtained purple-white products were collected by a Büchner funnel (Whatman #5), washed with water (300 mL), and then dried in an oven at 50 °C overnight. Elemental analysis results of Re were 0.0069% and 0.042%, which corresponded to 0.37 and 2.3 μmol of Re/g, respectively. These samples are abbreviated as 2-Cs-TiO2(0.37) and 2-Cs-TiO2(2.3), respectively. The DR UV-vis spectrum in the visible light region of 2-Cs-TiO2(2.3) showed a sharp band at 541 nm (ReV➔WVI IVCT band) and a broad band at around 750 nm (d-d band of ReV), as shown in Figure S2a.

2.5. Preparation of 1- and 2-Supported TiO2 Materials by the Precipitation Method Using Pt(NH3)4Cl2

For the preparation of 1-supported TiO2 materials, anatase TiO2 support (2.0 g) was dispersed in 50 mL of water at 25 ºC. Compound K-1 (42.9, 97.5, and 146.2 mg; 4.4, 10.0, and 15.0 μmol of Re/g) was dissolved in 30 mL of water. The POM solution was then added to the TiO2 suspension. The mixture was stirred at 25 ºC overnight in the dark. Pt(NH3)4Cl2·H2O (15.6, 35.2, and 52.8 mg; 44, 100, and 150 μmol/g) was dissolved in 20 mL of water and added to the mixture. At this stage, 10 eq. of Pt(NH3)4Cl2·H2O was required for the precipitation of polyoxoanion 1. The elemental analysis result of Pt for the precipitate prepared by adding 10 eq. of Pt(NH3)4Cl2·H2O to the aqueous solution of 1 was 12.2%; this showed that the formula of the obtained precipitate was [Pt(NH3)4]6.5K[O{Re(OH) (α2-P2W17O61)}2]·15H2O (11.77%). TG/DTA showed a weight loss of 2.57% with endothermic points at 30.9 and 64.0 °C in the temperature range of 15.9–180 °C; the weight loss was due to the hydrated water molecules (calculated value for 15 H2O was 2.51%). After stirring overnight at 25 °C, the obtained purple-white product was collected by a Büchner funnel (Whatman #5) and washed with water (30 mL × 3). The elemental analysis results for Re were 0.030%, 0.073%, and 0.105%, which corresponded to 1.6, 3.9, and 5.6 μmol of Re/g, respectively. These samples were abbreviated as 1-Pt-TiO2(1.6), 1-Pt-TiO2(3.9), and 1-Pt-TiO2(5.6), respectively. The DR UV-vis spectra in the visible light region of these materials showed a sharp band at 505, 523, and 525 nm (ReV ➔ WVI IVCT band), respectively. In addition, a broad band was observed at around 750 nm (d-d band of ReV) for all samples (Figure S3).
For the 2-supported TiO2 materials, Me2NH2-2 (27.0 and 61.3 mg; 4.4 and 10.0 μmol of Re/g) dissolved in water (200 mL) was added to anatase TiO2 support (2.0 g) suspended in 50 mL of water at 25 ºC. Pt(NH3)4Cl2·H2O (62 and 141 mg; 176 and 400 μmol/g) was dissolved in 20 mL water and added to the mixture. At this stage, 20 eq. of [Pt(NH3)4]Cl2·H2O was required for the complete precipitation of 2. The elemental analysis results of Pt for the precipitates prepared by adding 20 eq. of [Pt(NH3)4]Cl2·H2O to the aqueous solution of 2 was 12.0%; this showed that the formula of the obtained precipitate was [Pt(NH3)4]2[PW11ReO40] (the calculated value of Pt was 11.46%). TG/DTA observed no weight loss in the temperature range of 17.1–200 °C, showing that no hydrated water was contained. After stirring overnight at 25 °C, the obtained purple-white products were collected by a Büchner funnel (Whatman #5), washed with water (30 × 3 mL), and then dried in an oven at 50 °C overnight. Elemental analysis results of Re were 0.0011% and 0.0035%, which corresponded to 0.059 and 0.19 μmol of Re/g, respectively. These samples are abbreviated as 2-Pt-TiO2(0.059) and 2-Pt-TiO2(0.19), respectively. No clear bands were observed for 2-Pt-TiO2(0.059) and 2-Pt-TiO2(0.19) for their DR UV-vis spectra due to the low loadings.

2.6. Catalytic Reaction Experiments

The H2 evolution from water was carried out at 25 ºC. A mixture of catalyst (200 mg), water (10 mL), and EDTA·2Na (30 mM) was placed into a glass reaction vessel; this was connected to a Pyrex conventional closed gas circulation system (238.8 cm3). The photoreaction was started by light irradiation with a 500 W Xe lamp equipped with a cut-off filter (λ ≥400 nm). H2, O2, CO, and CH4 were analyzed by GC (TCD, Molecular Sieve 5A stainless columns), and water and CO2 were analyzed by GC (TCD, Porapak Q stainless columns): the samples were assigned after they were compared with authentic samples analyzed under the same conditions. Turnover number (TON) was calculated as 2[hydrogen evolved (mol/g of catalyst)]/[Re atoms (mol/g of catalyst)]. Turnover frequency (TOF) was calculated as [TON] / [reaction time (h)].

3. Results and Discussion

3.1. Synthesis and Characterization of [Me2NH2]4[PW11ReVO40] (Me2NH2-2)

The dimethylammonium salt of 2, [Me2NH2]4[PW11ReVO40] (Me2NH2-2), was synthesized with a slight modification of the published method for the potassium salt of the Dawson-type dirhenium(V)-oxido-bridged POM K14[O{ReV(OH)(α2-P2W17O61)}2]·21H2O (K-1) [18]. The compound Me2NH2-2 was formed by stirring a mixture of K2ReIVCl6 and mono-lacunary Keggin POM [PW11O39]7- in an aqueous solution under air at 25 °C; this was followed by the addition of excess Me2NH2Cl to form the dark purple-black precipitate. The unreacted Me2NH2Cl was completely removed by washing with ethanol.
The elemental analysis for compound Me2NH2-2 had to be performed after drying at room temperature at 10-3–10-4 torr overnight. The result was consistent with the composition [(CH3)2NH2]4[PW11ReVO40]. The weight loss observed during the course of drying before analysis was 0.3% for Me2NH2-2; this suggested the absence of solvated or adsorbed water molecule. The TG/DTA measurements performed under atmospheric conditions showed a weight loss of 6.27% with an exothermic point; this value corresponds to four Me2NH2+ ions. No weight loss due to the solvated water molecules was observed.
Figure 2. FTIR spectrum (as KBr disks) of Me2NH2-2.
Figure 2. FTIR spectrum (as KBr disks) of Me2NH2-2.
Materials 03 00897 g002
The FTIR spectrum of compound Me2NH2-2 that was measured on a KBr disk is shown in Figure 2. The positions of all bands (1076, 1020, 972, 885, 800, and 770 cm-1) in the polyoxoanion region of this compound are characteristic of polyoxoanion; however, they were different from those of [PW12O40]4- (1080, 984, 893, and 808 cm-1) and [PW11O39]7- (1086, 1043, 953, 903, 862, 810, and 734 cm-1). This suggests the coordination of rhenium atoms into the monovacant site of [PW11O39]7-.
The magnetic moment of Me2NH2-2 was 1.28 B. M., which is in the range for Cs2[ReVOCl5] (1.0–2.0 B. M.) [22]. This value was smaller than the theoretical value (2.83 B. M.), suggesting that the rhenium(V) site in Me2NH2-2 is weakly paramagnetic.
The 31P-NMR spectrum in DMSO-d6 of Me2NH2-2 before crystallization from acetone showed a clear single-line spectrum at δ = −14.93, as shown in Figure 3. The signal exhibited a shift from that of [PW11O39]7- (δ = −10.12) and [PW12O40]3- (δ = −14.67), indicating the complete coordination of rhenium atom into the monovacant site of [PW11O39]7- and the high purity of Me2NH2-2. The 31P NMR spectrum in D2O of the potassium salt of 2 (δ = −15.1) was first reported by Pope et al. [20]. The 31P NMR spectrum of the crystalline sample for Me2NH2-2 as crystallized from the acetone solution also showed the same chemical shifts (δ = −14.92) as those of the powder sample.
Figure 3. 31P-NMR spectra in DMSO-d6 of Me2NH2-2. The resonance at 0.0 ppm is due to the external reference: 85% H3PO4.
Figure 3. 31P-NMR spectra in DMSO-d6 of Me2NH2-2. The resonance at 0.0 ppm is due to the external reference: 85% H3PO4.
Materials 03 00897 g003
The UV-vis spectrum of Me2NH2-2 in DMSO showed three absorption bands at 264 (ε 49,950 M-1cm-1), 512 (ε 1625 M-1cm-1), and 741 nm (ε 786 M-1cm-1), as shown in Figure 4a. A large band at 264 nm was assigned to the charge transfer (CT) band of W-O, and two small bands at 512 and 741 nm were assigned to the ReV➔WVI intervalence charge transfer (IVCT) band and d-d band of the rhenium(V) atom, respectively [19,20]. The positions of the two bands at 512 and 741 nm were similar to those of K-1 (496 and 737 nm). The DR UV-vis spectrum of Me2NH2-2 in the visible-light region also showed two bands at 526 and 688 nm due to the ReV➔WVI IVCT band and d-d band of the rhenium(V) atom, respectively (Figure 4b). The positions of these bands were quite similar to those of compound K-1 (532 and 686 nm).
Figure 4. (a) UV-vis spectrum in DMSO of Me2NH2-2 at 200–800 nm (1.0 × 10-6 M). Inset: 400–800 nm (1.0 × 10-4 M). (b) Diffuse reflectance UV-vis spectrum of Me2NH2-2 at 400–800 nm.
Figure 4. (a) UV-vis spectrum in DMSO of Me2NH2-2 at 200–800 nm (1.0 × 10-6 M). Inset: 400–800 nm (1.0 × 10-4 M). (b) Diffuse reflectance UV-vis spectrum of Me2NH2-2 at 400–800 nm.
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The crystallization of Me2NH2-2 for X-ray crystal analysis was performed by slow evaporation from acetone in the dark. X-ray crystallography revealed that 2 was a monomeric α-Keggin POM with Td symmetry, but the molecular structure of 2 was not determined because of the disorder of eleven tungsten(VI) atoms due to a highly symmetric space group; this is similar to the earlier cases of [W9ReO32]5- [23] and [PW11(TiO2)O39]5- [24].

3.2. Hydrogen Evolution from an Aqueous Solution Containing EDTA·2Na Catalyzed by 1-Supported TiO2 Materials under Visible Light Irradiation (≥400 nm)

We examined the hydrogen evolution from water in the presence of EDTA·2Na under light irradiation (≥400 nm), which was catalyzed by 1-Cs-TiO2(2.0), 1-Cs-TiO2(3.3), 1-Pt-TiO2(1.6), 1-Pt-TiO2(3.9), and 1-Pt-TiO2(5.6) at 25ºC in a heterogeneous system; the results are summarized in Table 1. For the time course of H2 evolution in the first run, an induction period was observed for 1-Cs-TiO2(2.0), 1-Cs-TiO2(3.3), and 1-Pt-TiO2(5.6), as shown in Figure 5 and Figure 6c. In particular, inactivation was observed for 1-Pt-TiO2(5.6). In contrast, a linear increase in H2 with time was observed for 1-Pt-TiO2(1.6) and 1-Pt-TiO2(3.9), as shown in Figure 6a and Figure 6b. O2, CO2, CO, and CH4 were not detected under the present reaction conditions. The colors of these materials changed from white-purple to blue during the reactions; however, the blue-color disappeared and the photoreactions stopped when the visible light irradiation stopped. The pH of the solution changed from ca. 4.6 to ca. 6.2 after 6 h for all samples, suggesting that OH- may have formed [25,26].
For 1-Cs-TiO2(2.0) and 1-Cs-TiO2(3.3), the amount of H2 after 1 h evolved was 1.24 and 1.48 μmol/g of catalyst, respectively. After 6 h, the amount of H2 evolved increased to 47.5 and 44.4 μmol/g, respectively (TON reached 48 and 27); however, the activities did not increase with the loading of 1. In the control experiments, polyoxoanion 1 dissolved in aqueous EDTA solution showed no reaction, and hydrogen was slightly detected when catalyzed by TiO2; the sample was washed with a large amount of water and dried at 200 °C overnight. On contrary, polyoxoanion 1 (4.4 μmol of Re/g) dissolved in aqueous EDTA solution showed 221.4 μmol/g (TON = 101) of hydrogen evolution in the presence of TiO2. Thus, the polyoxoanion 1 leached into the aqueous solution is not negligible in the presence of TiO2.
To determine whether the surface polyoxoanion 1 leached into the solution during the photoreactions, 200 mg of 1-Cs-TiO2(2.0) and 1-Cs-TiO2(3.3) were irradiated in a 30 mM EDTA·2Na solution (10 mL) for 6 h; these mixtures were then filtered. The elemental analysis results for Re revealed that the loadings of the obtained solids after the first visible-light irradiation were 0.32 and 0.15 μmol/g, respectively; this suggested that 4.5%–16% of the surface polyoxoanion 1 remained on the TiO2 surface. In the recycle experiments, where the obtained solids after the first irradiation were used as catalysts for the second run, the amount of hydrogen after 6 h evolved was 138.0 and 103.5 μmol/g, respectively, which was 2–3 times larger than results for the first run. TON reached 863 and 1380, which were 18–51 times higher than in the results for the first run. As a control experiment, PW12O403--supported TiO2 material was precipitated using CsCl, where the loading of PW12O403- was 9.2 μmol/g; 89.9 μmol/g·h of hydrogen evolution resulted after 6 h (TON = 20). This result indicated that the hydrogen evolution occurred without the rhenium(V) site in polyoxoanion under the present conditions. However, the recycle experiment of the PW12O403--supported TiO2 material in the second run showed 87.3 μmol/g of hydrogen evolution, and TON as calculated on the basis of the elemental analysis results for P after the first irradiation was 17 after 6 h; this was 51–81 times lower than the results for 1-Cs-TiO2(2.0) and 1-Cs-TiO2(3.3) in the second run.
Table 1. Hydrogen evolution from water catalyzed by 1-supported TiO2 materials under visible light irradiation. [a]
Table 1. Hydrogen evolution from water catalyzed by 1-supported TiO2 materials under visible light irradiation. [a]
EntryCatalystReaction time [h]RecycleH2 [μmol/g]TON[b]
11-Cs-TiO2(2.0)11st run1.24
6 47.548
12nd run9.17
6 138.0863[c]
21-Cs-TiO2(3.3)11st run1.48
6 44.427
12nd run13.0
6 103.51380[c]
31-Pt-TiO2(1.6)11st run62.6
6 426.1533
12nd run37.3
6 412.39062[c]
41-Pt-TiO2(3.9)11st run80.3
6 473.1243
12nd run73.7
6 545.73307[c]
51-Pt-TiO2(5.6)11st run19.1
6 339.7121
12nd run118.9
6 558.01313[c]
[a] Reaction conditions: water (10 mL), catalyst (200 mg), EDTA·2Na (30 mM), light (≥400 nm), 25 ºC.
[b] Turnover number (TON) was calculated as 2[H2 evolved (mol/g)] per [Re atoms (mol/g)].
[c] The concentration of Re atoms for 1-supported TiO2 materials after the first visible-light irradiation (≥400 nm) was 0.32, 0.15, 0.091, 0.33, and 0.85 μmol/g, respectively.
To investigate the stability of polyoxoanion 1 during the photoreactions, polyoxoanion 1 dissolved in an aqueous solution containing EDTA·2Na was irradiated by visible light (≥400 nm) in the presence of TiO2 for 6 h and characterized by 31P-NMR spectroscopy. The 31P-NMR spectrum in D2O of 1 after visible light irradiation showed a set of signals at −11.87 and −12.86 ppm, which was similar to the values of the as-prepared polyoxoanion 1 (δ −12.06 and −13.05) [18]. This result suggested that the molecular structure of 1 dissolved in aqueous solution still remained after visible light irradiation in the presence of TiO2. The DR UV-vis spectra of 1-Cs-TiO2(2.0) and 1-Cs-TiO2(3.3) after the photoreactions are shown in Figure S4. The spectra for the two showed the ReV ➔ WVI IVCT band at 505 and 504 nm, respectively; these are the same values as those for the as-prepared materials, suggesting that the rhenium(V) species still remained after the photoreactions. Thus, the conditions of 1 in solution and solid state, e.g., concentration, dispersion, and interaction of 1 with TiO2 surface, might influence the activities in the second run.
Figure 5. Time course for H2 evolution catalyzed by (a) 1-Cs-TiO2(2.0) and (b) 1-Cs-TiO2(3.3) under light irradiation (≥400 nm). The first and second runs are represented by circles (○) and squares (□), respectively. Reaction conditions: see Table 1.
Figure 5. Time course for H2 evolution catalyzed by (a) 1-Cs-TiO2(2.0) and (b) 1-Cs-TiO2(3.3) under light irradiation (≥400 nm). The first and second runs are represented by circles (○) and squares (□), respectively. Reaction conditions: see Table 1.
Materials 03 00897 g005
Figure 6. Time course for H2 evolution catalyzed by (a) 1-Pt-TiO2(1.6), (b) 1-Pt-TiO2(3.9), and (c) 1-Pt-TiO2(5.6) under light irradiation (≥400 nm). The first and second runs are represented by circles (○) and squares (□), respectively. Reaction conditions: see Table 1.
Figure 6. Time course for H2 evolution catalyzed by (a) 1-Pt-TiO2(1.6), (b) 1-Pt-TiO2(3.9), and (c) 1-Pt-TiO2(5.6) under light irradiation (≥400 nm). The first and second runs are represented by circles (○) and squares (□), respectively. Reaction conditions: see Table 1.
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For 1-Pt-TiO2(1.6), 1-Pt-TiO2(3.9), and 1-Pt-TiO2(5.6), the amount of H2 evolved after 1 h was 62.6, 80.3, and 19.1 μmol/g of catalyst, respectively. After 6 h, the amount of H2 evolved reached 426.1, 473.1, and 339.7 μmol/g, respectively. TON was 533, 243, and 121; these values were 2.5–19.7 times higher than those of 1-Cs-TiO2(2.0) and 1-Cs-TiO2(3.3), showing that Pt enhanced the catalytic activities remarkably as reported for various types of photocatalysts [3,27]. The TOF of 1-Pt-TiO2(1.6), 1-Pt-TiO2(3.9), and 1-Pt-TiO2(5.6) were 20–89 h-1, which were higher than those of the reported dye-sensitized TiO2 materials used in heterogeneous systems containing EDTA; e.g., platinum-loaded Langmuir-Blodgett film of viologen-linked porphyrin (720 > λ > 390; TOF = 0.8 h-1) [28], acid-restacked calcium niobate nanosheets sensitized by Ru(bpy)2 (4,4’-(PO3H2)2bpy)2+ (λ > 420 nm; TOF = 5.4 h-1) [29], and zinc porphyrin/Pt/TiO2 system (λ >520 nm; TOF = 20 h-1) [30]. However, the TON decreased with the loading of 1, as observed for 1-Cs-TiO2(2.2) and 1-Cs-TiO2(3.3).
The leaching of the surface polyoxoanion 1 into the solution was also determined for 1-Pt-TiO2(1.6), 1-Pt-TiO2(3.9), and 1-Pt-TiO2(5.6) under the same reaction conditions as those for 1-Cs-TiO2(2.0) and 1-Cs-TiO2(3.3). The elemental analysis results for Re revealed that the loadings of the obtained solids after the first visible-light irradiation were 0.091, 0.33, and 0.85 μmol/g, respectively; this suggested that 5.7%–15% of the surface polyoxoanion 1 remained on the TiO2 surface. The recycle experiments for the obtained solids showed hydrogen evolved after 6 h was 412.3, 545.7, and 558.0 μmol/g, respectively. TON reached 9062, 3307, and 1313, which were 10.9–17.0 times higher than the values in the first run. Although the total amounts of H2 evolved after 6 h somewhat increased with the loading of 1, TON was observed to decrease in the second run. The DR UV-vis spectra of 1-Pt-TiO2(1.6), 1-Pt-TiO2(3.9), and 1-Pt-TiO2(5.6) after the photoreactions are shown in Figure S5. The ReV➔WVI IVCT bands for these materials were observed at 488, 500, and 500 nm; these bands were somewhat blue-shifted from those of the as-prepared materials, and the band shapes were broadened after the photoreactions.

3.3. Hydrogen Evolution from an Aqueous Solution Containing EDTA·2Na Catalyzed by 2-Supported TiO2 Materials under Visible Light Irradiation (≥400 nm)

We examined the hydrogen evolution from an aqueous solution containing EDTA·2Na under light irradiation (≥400 nm) that was catalyzed by 2-Cs-TiO2 (0.37), 2-Cs-TiO2 (2.3), 2-Pt-TiO2 (0.059), and 2-Pt-TiO2(0.19) at 25 ºC in a heterogeneous system; the results are summarized in Table 2. For the time course of H2 evolution in the first run, a linear increase in H2 with time was observed for 2-Cs-TiO2 (0.37), 2-Cs-TiO2 (2.3), and 2-Pt-TiO2 (0.059) at the initial step; the increases are shown in Figure 7 and Figure 8. In contrast, significant inactivation was observed for 2-Pt-TiO2 (0.19). The colors of these materials also changed from white-purple to blue during the reactions, and the pH of the solutions changed from ca. 4.7 to (5.9–6.1) after 6 h for all samples; this suggested that OH- may have formed [25,26]. As a control experiment, 2 dissolved in aqueous solution showed no reaction under the present conditions.
For 2-Cs-TiO2 (0.37) and 2-Cs-TiO2 (2.3), the amount of H2 evolved after 1 h was 15.1 and 16.5 μmol/g of catalyst, respectively. After 6 h, the amounts of H2 evolved increased to 163.7 and 139.1 μmol/g, respectively. TON reached 885 and 121, which were 2.5–33 times higher than the values for 1-Cs-TiO2 (2.0) and 1-Cs-TiO2 (3.3). For the recycle experiments, 200 mg of 2-Cs-TiO2 (0.37) and 2-Cs-TiO2 (2.3) were irradiated in a 30 mM EDTA·2Na solution (10 mL) for 6 h; these mixtures were then filtered. The elemental analysis results for Re revealed that the loadings of the obtained solids after the first visible-light irradiation were 0.016 and 0.11 μmol/g, respectively; this suggested that only 4.3%–4.8% of the surface polyoxoanion 2 remained after the first reactions. The obtained solids after the first light irradiation showed values of 500.2 and 169.6 μmol/g after 6 h, respectively, which were 1.2–3 times larger than the values for the first run. TON also reached 62525 and 3084, which were 25–71 times higher than those for the first run, and 2.2–72 times higher than the values obtained for 1-Cs-TiO2(2.0) and 1-Cs-TiO2(3.3) in the second run. The 31P NMR spectrum in D2O of 2 dissolved in aqueous EDTA solution containing TiO2 after light irradiation for 6 h showed two signals at −10.61 and −15.36 ppm with ca. 1:2 intensities, which were assigned to [PW11O39]7- and 2, respectively. The DR UV-vis spectrum of 2-Cs-TiO2(3.3) after the photoreaction showed significant reduction of the ReV ➔ WVI IVCT and d-d bands (Figure S2b), suggesting that the molecular structure and/or oxidation state of polyoxoanion 2 were changed under the present reaction conditions. Thus, the catalytic activities of 2-supported TiO2 materials in the second run might be influenced by the conditions of the species formed by the light irradiation and those of 2 in solution and solid state.
Table 2. Hydrogen evolution from water catalyzed by 2-supported TiO2 materials under visible light irradiation. [a]
Table 2. Hydrogen evolution from water catalyzed by 2-supported TiO2 materials under visible light irradiation. [a]
EntryCatalystReaction time [h]RecycleH2 [μmol/g]TON[b]
12-Cs-TiO2(0.37)11st run15.1
6 163.7885
12nd run85.5
6 500.262525[c]
22-Cs-TiO2(2.3)11st run16.5
6 139.1121
12nd run25.0
6 169.63084[c]
32-Pt-TiO2(0.059)11st run64.3
6 402.513644
12nd run55.1
6 414.385423[c]
42-Pt-TiO2(0.19)11st run59.7
6 141.01484
12nd run76.0
6 352.626119[c]
[a] Reaction conditions: water (10 mL), catalyst (200 mg), EDTA·2Na (30 mM), light (≥400 nm), 25 ºC.
[b] Turnover number (TON) was calculated as 2[H2 evolved (mol/g)] per [Re atoms (mol/g)].
[c] The concentration of Re atoms for 1-supported TiO2 materials after the first visible-light irradiation (≥400 nm) was 0.016, 0.11, 9.7 × 10-3, and 0.027 μmol/g, respectively.
Figure 7. Time course for H2 evolution catalyzed by (a) 2-Cs-TiO2(0.37) and (b) 2-Cs-TiO2(2.3) under light irradiation (≥400 nm). The first and second runs are represented by circles (○) and squares (□), respectively. Reaction conditions: see Table 2.
Figure 7. Time course for H2 evolution catalyzed by (a) 2-Cs-TiO2(0.37) and (b) 2-Cs-TiO2(2.3) under light irradiation (≥400 nm). The first and second runs are represented by circles (○) and squares (□), respectively. Reaction conditions: see Table 2.
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Figure 8. Time course for H2 evolution catalyzed by (a) 2-Pt-TiO2(0.059) and (b) 2-Pt-TiO2(0.19) under light irradiation (≥400 nm). The first and second runs are represented by circles (○) and squares (□), respectively. Reaction conditions: see Table 2.
Figure 8. Time course for H2 evolution catalyzed by (a) 2-Pt-TiO2(0.059) and (b) 2-Pt-TiO2(0.19) under light irradiation (≥400 nm). The first and second runs are represented by circles (○) and squares (□), respectively. Reaction conditions: see Table 2.
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For 2-Pt-TiO2(0.059), the amount of H2 evolved after 1 and 6 h were 64.3 and 402.5 μmol/g of catalyst, respectively. TON after 6 h was 13644, which was 15–113 times higher than the values for 2-Cs-TiO2(0.37) and 2-Cs-TiO2(2.3). In contrast, 2-Pt-TiO2(0.19) showed significant inactivation; thus, H2 evolved 141.0 μmol/g of catalyst (TON = 1484) after 6 h. Only a trace amount of Pt was needed to enhance the catalytic activities; however, inactivation was also caused at higher loadings of Pt species, as reported for the Ru(bpy)32+/methyl viologen/EDTA system [31].
The leaching of the surface polyoxoanion 2 into the solution was also determined for 2-Pt-TiO2(0.059) and 2-Pt-TiO2(0.19) under the same reaction conditions. The elemental analysis results of Re revealed that the loadings of the obtained solids after the first visible-light irradiation were 9.7 × 10−3 and 0.027 μmol/g, respectively; this suggested that 14%–16% of the surface polyoxoanion 2 remained on the TiO2 surface. The recycle experiments of the obtained solids after the first light irradiation showed values of 414.3 and 352.6 μmol/g after 6 h, respectively. TON was 85423 and 26119, which were 6.3–17.6 times higher than the values obtained in the first run.

4. Conclusions

The synthesis and full characterization of a Keggin-type mono-rhenium(V)-substituted polyoxoanion are presented. We successfully obtained black-purple crystals of the dimethylammonium salt [Me2NH2]4[PW11ReVO40] (Me2NH2-2) by treating [ReIVCl6]2- with a mono-lacunary Keggin polyoxoanion. Compound Me2NH2-2 was characterized by X-ray structure analysis, elemental analysis, TG/DTA, UV-vis absorption, FTIR, and solution 31P NMR spectroscopy. The crystal structure of 2 revealed a monomeric structure with overall Td symmetry; however, the rhenium(V) site was not determined due to the high symmetry.
The Dawson- and Keggin-type rhenium(V)-containing polyoxoanions [O{ReV(OH) (α2-P2W17O61)}2]14- (1) and 2 were supported onto a TiO2 surface by precipitation methods using CsCl and Pt(NH3)4Cl2. With these 1- and 2-supported TiO2 materials, hydrogen evolution from water in the presence of EDTA·2Na under visible light irradiation (≥400 nm) was achieved. The results for the photoreactions showed the following. (1) The catalytic activities of 2-supported TiO2 materials were higher than those of 1-supported materials at similar loadings. (2) Significant leaching of the surface polyoxoanions 1 and 2 was observed after the first light irradiation, and the polyoxoanion leached into the solution showed hydrogen evolution from water in the presence of TiO2. (3) The stability of 1 in both solution and solid state was higher than that of 2 under the light irradiation. (4) Surface Pt species enhanced the catalytic activities; however, inactivation was also observed at higher loadings. (5) The catalytic activities in the second run were higher than those in the first run for both 1- and 2-supported TiO2 materials, regardless of the grafting methods used for CsCl and [Pt(NH3)4]Cl2.

Supporting Information

Figures S1–S5 can be downloaded online at can be downloaded online at https://www.mdpi.com/1996-1944/3/2/897/s1.

Acknowledgements

C. N. K. is grateful for the support of the ESPEC Foundation for Global Environment Research and Technology (Charitable Trust), Hayashi Memorial Foundation for Female Natural Scientists, and the Grant-in Aid for Young Scientists (B) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. W. M. acknowledges the support of the High-Tech Research Center Project from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The research was partially carried out using an instrument at the Center for Instrumental Analysis, Shizuoka University.

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Kato, C.N.; Hara, K.; Kato, M.; Amano, H.; Sato, K.; Kataoka, Y.; Mori, W. EDTA-Reduction of Water to Molecular Hydrogen Catalyzed by Visible-Light-Response TiO2-Based Materials Sensitized by Dawson- and Keggin-Type Rhenium(V)-Containing Polyoxotungstates. Materials 2010, 3, 897-917. https://doi.org/10.3390/ma3020897

AMA Style

Kato CN, Hara K, Kato M, Amano H, Sato K, Kataoka Y, Mori W. EDTA-Reduction of Water to Molecular Hydrogen Catalyzed by Visible-Light-Response TiO2-Based Materials Sensitized by Dawson- and Keggin-Type Rhenium(V)-Containing Polyoxotungstates. Materials. 2010; 3(2):897-917. https://doi.org/10.3390/ma3020897

Chicago/Turabian Style

Kato, Chika Nozaki, Kazunobu Hara, Masao Kato, Hidekuni Amano, Konomi Sato, Yusuke Kataoka, and Wasuke Mori. 2010. "EDTA-Reduction of Water to Molecular Hydrogen Catalyzed by Visible-Light-Response TiO2-Based Materials Sensitized by Dawson- and Keggin-Type Rhenium(V)-Containing Polyoxotungstates" Materials 3, no. 2: 897-917. https://doi.org/10.3390/ma3020897

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