Photocatalytic hydrogen production from water using porous material [Ru₂(p-BDC)₂]_n

Abstract

We have detected the first example of open porous metal–organic frameworks (MOFs) that functions as an activity site for the reduction of water into hydrogen molecules in the presence of Ru(bpy)₃²⁺, MV²⁺, and EDTA–2Na under visible light irradiation. This activity is highly efficient: the turnover number based on MOFs and the apparent quantum yield are 8.16 and 4.82%, respectively. Furthermore, this material enables the adsorption of various gases into its pores; its hydrogen uptake capacity is 1.2 wt% at 77.4 K.

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Broader context

Efficient light-harvesting systems for converting solar energy into chemical energy have been actively studied from the view of achieving artificial photosynthesis and storage of the energy source. In particular, the photochemical reduction of water into hydrogen molecules using a photocatalyst is an interesting objective. We have detected the first example of the open porous material [Ru₂(p-BDC)₂]_n that functions as an activity site for the photochemical reduction of water into hydrogen molecules in the presence of Ru(bpy)₃²⁺, MV²⁺ and EDTA–2Na. Activity is highly efficient: the turnover number is 8.16 based on [Ru₂(p-BDC)₂]_n and 81.6 based on Ru(bpy)₃²⁺ after 4 h of light irradiation. Furthermore, this material enables the adsorption of various gases into its pores; its hydrogen uptake capacity is 1.2 wt% at 77.4 K (760 mmHg).

In the past several decades, efficient light-harvesting systems for converting solar energy into chemical energy have been actively studied from the view of achieving artificial photosynthesis and storage of the energy source. In particular, the photochemical reduction of water into hydrogen molecules using a photocatalyst is not dependent on fossil fuels, and is therefore an ideal method for producing clean energy.¹ Several different photocatalysts have been proposed; some have achieved a high quantum yield.² For example, a NiO-modified La/NaTaO₃ photocatalyst showed extremely high activity, and the quantum yield amounted to 56% at 270 nm.³ However, most photocatalysts proposed to date consist of semiconductor or noble metal-based (Pt, Pd, Fe, etc.) complexes and only function in the ultraviolet region under the absence of any promoter or photosensitizer. Thus, research in the light-harvesting system design for the reduction of water into hydrogen molecules is still an interesting objective.

Construction of open porous metal coordination polymers, called metal–organic frameworks (MOFs), by the copolymerization of organic ligands with metal ions and clusters are of great interest because of their structural topology and their potential functions for gas adsorption, catalytic action, selective adsorption, and so on.^4–6 Thus far, we have synthesized various MOFs constructed with paddlewheel metal dimer cores including [Cu₂(p-BDC)₂]_n (p-BDC = 1,4-benzenedicarboxylate),⁷ identified their gas-adsorption properties, and observed its catalytic performance.⁸ Of all the others, Ru–MOFs constructed with paddlewheel diruthenium cores possess intriguing functions such as magnetic behavior or hydrogenation catalytic performance.⁹ In an extension of the study of Ru–MOFs catalytic performance, we incidentally detected that our Ru–MOFs function as activity sites for the photochemical reduction of water into hydrogen molecules in the presence of Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine), MV²⁺ (N,N′-dimethyl-4,4′-bipyridinium), and EDTA–2Na. Until now, there have been no reports regarding the use of MOFs as photocatalysts for the reduction of water into hydrogen molecules.

In this study, we describe our observations of the first example of photochemical reduction of water into hydrogen molecules driven by a MOF photocatalyst in the presence of Ru(bpy)₃²⁺, MV²⁺, and EDTA–2Na. In addition, H₂ or N₂ adsorption behavior and their calculated data are studied and discussed.

The photocatalyst, [Ru₂(p-BDC)₂]_n (R1), was prepared by a previously reported method^{9d, 19} (Scheme 1). R1 has a 2-dimensional square grid sheet assembly structure having 1-dimensional channel pores, it is unusually stable in air, and has a strong degree of hydroscopicity.

Scheme 1 Ru₂ paddlewheel motif structure (left) and self-assembled molecular structure of [Ru₂(p-BDC)₂]_n (right; Ru: blue, O: red, C: gray)

To determine the permanent porosity and capacity of this material for the uptake of gases, we measured the gaseous N₂ sorption isotherm on samples of dehydrated R1 at 77.4 K. These sorption isotherms are shown in Fig. 1. The N₂ sorption isotherm shows type I behavior, and fitting the data with the Langmuir and Brunauer–Emmett–Teller (BET) equations gave apparent surface areas of 803.9 and 516.5 m² g⁻¹, respectively. The Langmuir surface area was greater than that of the same motif-type MOFs ([M₂(p-BDC)₂]_n; M = Cu,¹⁰Rh,¹¹Mo,¹²Zn¹³). Therefore, it is effective to apply the ruthenium ion to the building blocks of the MOFs. After micropore filling, the slope of the volume adsorbed at the isotherm gently rose to saturated pressure. The pore size distribution was obtained by the Horvath–Kawazoe method¹⁴ and the pore volume of this material calculated using the Dubinin–Radushkevich (DR) equation from the isotherm were 4.8Å and 0.26 cm³ g⁻¹, respectively.

Fig. 1 Adsorption isotherm plot of R1 at 77.4 K (, N₂ adsorption; , H₂ adsorption; , H₂ desorption; STP = Standard Temperature Pressure).

The shape of the low-pressure H₂ adsorption and desorption isotherm at 77.4 K, shown in Fig. 1, is typical for their kind. R1 could take up H₂ equivalent to 133.8 cm³/g STP or 1.2 wt%¹⁵ at 760 mmHg. This amount of H₂ adsorption was a tempting value as a two-dimensional grid sheet structure. The observed hysteresis in the H₂ adsorption and desorption curves indicated that the desorption isotherm is lower than that of the adsorption. Therefore, H₂ was physically adsorbed on R1.

To confirm that our synthesized Ru–MOFs function as activity sites for the photochemical reduction of H₂O into H₂, we initially tested the reduction of aqueous solution (10 ml) in the presence of EDTA–2Na (150 mM) as a sacrificial donor with R1 (0.4 mmol) under UV irradiation (longer than 320 nm) at 300 K in a heterogeneous reaction system. However, under these conditions, no H₂ gas was detected even after 24 h of irradiation.

Next, to harvest light efficiently, Ru(bpy)₃²⁺ as a photosensitizer (ps) and MV²⁺ (N,N′-dimethyl-4,4′-bipyridinium) as an electron relay were added to the aqueous solution of EDTA–2Na. The photocatalyst powders used were R1, Ru₂(CH₃COO)₄BF₄ (R2; formed Ru₂ paddlewheel motif structure, as in Scheme 1),¹⁶ and [Ru^II,III₂(p-BDC)₂BF₄]_n (R3; similar formed structure of R1).^{9d, 20} See Notes for the measurement method we used. As soon as visible light (longer than 420 nm) was irradiated, H₂ evolution was observed on R1–R3. Fig. 2 shows the time course of H₂ evolution on the R1–R3 and anatase-type TiO₂ semiconductor (BET surface area: 40.0 m² g⁻¹) in the presence of Ru(bpy)₃²⁺, MV²⁺, and EDTA–2Na. The TON (turnover numbers), as one of several measurement conditions 1 and 4 h after the photoirradiation, are listed in Table 1.

Table 1 Photocatalytic hydrogen production in solution using 150 mM EDTA–2Na as a sacrificial electron donor

Run	Cat.^a	Ru(bpy)3^2+ (mM)	MV^2+ (mM)	λ ^b (nm <)	Time (h)	TON based on catalyst	TON based on Ru(bpy)3^2+
a Cat. = catalyst; all catalyst was used 0.01 mmol. b irradiation wavelength. c anatase-type TiO2.
1	R1	0.1	5.0	420	1	3.22	32.2
2	R1	0.1	5.0	420	4	8.16	81.6
3	R1	0.1	0	420	4	0	0
4	R1	0	5.0	420	4	0	0
5	R1	0.1	5.0	—	4	0	0
6	R2	0.1	5.0	420	1	0.32	3.16
7	R2	0.1	5.0	420	4	2.52	25.2
8	R3	0.1	5.0	420	1	1.47	14.7
9	R3	0.1	5.0	420	4	4.7	47.0
10	TiO2 ^c	0.1	5.0	420	4	0.57	5.66
11	—	0.1	5.0	420	1	0	0

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Fig. 2 Time course of H₂ evolution from 10 ml water solution after irradiation started under 0.1 mM Ru(bpy)₃Cl₂/ 5.0 mM MV²⁺/ 150 mM EDTA–2Na/ 0.01 mmol photocatalyst condition (, R1; , R2; , R3; , anatase-type TiO₂).

The initial slopes of H₂ evolution for the three photocatalysts (R1–R3) were measured to be 16.1 µmol h⁻¹ for R1, 1.58 µmol h⁻¹ for R2, and 7.33 µmol h⁻¹ for R3.

The evolution rate of H₂ increased as follows: R2 < R3 < R1. Prolonged irradiation led to increased H₂ production, up to 41 µmol H₂ evolution with 8.16 TON based on R1 and 81.6 based on Ru(bpy)₃²⁺ achieved after 4 h irradiation. The apparent quantum yield of R1 was 4.82% at 450 nm. Thus, the catalytic reaction showed evidence of a catalytic cycle with good efficiency. For example, the amount of hydrogen molecules produced after 4 h irradiation by 1.0 g of R1 was 0.14 mol; the activity was 14.4 times greater than that of equal molar anatase-type TiO₂ semiconductor photocatalyst in the same system (Run 10) ²¹

The activities of R1 and R3 (porous material) were far superior to that of R2 (non-porous material). Therefore, forming a porous coordination structure was valuable for improving the activity of the photochemical water reduction. The Ru^II₂ paddlewheel motif more effectively influenced the catalytic activity than did that of Ru^II,III₂ because of the activity of R1 was 1.7 times greater (after 4 h of photolysis) than was that of R3.

Several control experiments were performed in these heterogeneous systems. As expected, no H₂ generation was detected in the dark (Run 4). In addition, if any one of Ru(bpy)₃²⁺, MV²⁺, Ru-MOFs (Runs 3,4 and 11), and EDTA–2Na was omitted from the reaction system, no H₂ evolution was detected. Therefore, their promoters are essential for the reduction of water using R1–R3. When 1 equiv. of pyridine was added to the systems, no potential effect on photocatalytic activity was observed. The results of the control experiments indicate Scheme 2 that the H₂ evolution in the present system is indeed photoinduced via the excited state of Ru(bpy)₃²⁺, the electron relay for MV²⁺, catalyzed by the Ru–MOFs, and the reduction of Ru(bpy)₃³⁺ by EDTA–2Na.

Scheme 2 The reaction scheme of photochemical hydrogen production from water using Ru–MOFs in the presence of Ru(bpy)₃²⁺, MV²⁺, and EDTA–2Na.

In conclusion, we demonstrated that our porous Ru–MOFs were catalyzed at the activity site of the reduction of water into hydrogen molecules in the presence of Ru(bpy)₃²⁺, MV²⁺, and EDTA–2Na. The turnover number was 8.16 based on Ru–MOFs and 81.6 based on Ru(bpy)₃²⁺ after 4 h of light irradiation. This is the first example in which MOFs function as activity sites for the photochemical reduction of water into hydrogen molecules. Extensive studies are now in progress in our laboratory.

Notes

Photocatalytic reaction

All photocatalytic reactions were carried out in a closed gas circulation system with a Pyrex reaction cell. The reaction solution was irradiated at 300 K using a Xe lamp equipped with an optical cut-off filter (λ > 420 nm). The amount of H₂ evolution was determined by using a gas chromatograph (Shimadzu; GC-14B, with a TCD and FID detector, a molecular sieves 5A column, ultra-pure Ar carrier gas). The reaction solution (10 ml) containing EDTA–2Na (150 mM), Ru(bpy)₃Cl₂ (0.1 mM), MV²⁺ (5.0 mM), and Ru–photocatalyst (0.01 mmol)¹⁷ was irradiated at 300 K. Measurements were performed under non-oxygen conditions for all photocatalytic reactions. The apparent quantum yield (AQY)¹⁸ defined by the following equation was measured using filters combined with a bandpass and cut-off filter (Kenko; 450 nm) and a photodiode (PM3; Coherent).

AQY = (number of reacted electrons) / (number of incident photons) × 100 = (number of evolved H₂ molecules) × 2 / (number of incident photons) × 100.

(The number of incident photons was 8.81 × 10⁻⁵).

Measurement of gas adsorption

Adsorption was measured with an ASAP2010 volumetric adsorption analyzer. As before treatment, a sample was dried overnight under high vacuum at 300 K and for 1 h at 373 K to remove any solvated water. In the measurements and before treatment, ultra-high-purity grades (99.9999%) of H₂, N₂ (analysis gas; adsorbent gas), and He (analysis gas for free space) were used.

Acknowledgements

This work was supported by a Grant-in-Aid for Specially Promoted Research (No. 19350077) and the High-Tech Research Center Project from the Ministry of Education, Culture, Sports, Science and Technology of Japan. We acknowledge T. Miyao (Yamanashi University) for many valuable conversations and K. Sakai, S. Masaoka, and co-workers (Kyusyu University) for their advice on the instruments used in photocatalytic reactions.

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