SiO2 as a Hydrogen-Evolution Catalyst in a

Article pubs.acs.org/JPCC

Robustness of Ru/SiO2 as a Hydrogen-Evolution Catalyst in a Photocatalytic System Using an Organic Photocatalyst Yusuke Yamada,† Shinya Shikano,† and Shunichi Fukuzumi*,†,‡ †

Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ‡ Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea S Supporting Information *

ABSTRACT: Effects of various metal oxide supports (SiO2, SiO2−Al2O3, TiO2, CeO2, and MgO) on the catalytic reactivity of ruthenium nanoparticles (RuNPs) used as a hydrogenevolution catalyst have been evaluated in photocatalytic hydrogen evolution using 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+−NA) and dihydronicotinamide adenine dinucleotide (NADH) as a photocatalyst and an electron donor, respectively. The 3 wt % Ru/SiO2 catalyst freshly prepared by an impregnation method exhibited the highest catalytic reactivity among RuNPs supported on various metal oxides, which was nearly the same as that of commercially available Pt nanoparticles (PtNPs) with the same metal weight. However, the initial catalytic reactivity of 3 wt % Ru/SiO2 was lost after repetitive use, whereas the catalytic reactivity of PtNPs was maintained under the same experimental conditions. The recyclability of the 3 wt % Ru/SiO2 was significantly improved by employing the CVD method for preparation. The initial catalytic reactivity of 0.97 wt % Ru/SiO2 prepared by the CVD method was higher than that of 2 wt % Ru/SiO2 prepared by the impregnation method despite the smaller Ru content. The total amount of evolved hydrogen normalized by the weight of Ru in 0.97 wt % Ru/SiO2 was 1.7 mol gRu−1, which is now close to that normalized by the weight of Pt in PtNPs (2.0 mol gPt−1). Not only the preparation method but also the morphology of SiO2 supports affected significantly the catalytic activity of Ru/SiO2. The Ru/SiO2 catalyst using nanosized SiO2 with undefined shape exhibited higher catalytic activity than Ru/SiO2 catalysts using mesoporous SiO2 or spherical SiO2. The kinetic study and TEM observation of the Ru/SiO2 catalysts suggest that the microenvironment of RuNPs on SiO2 surfaces plays an important role to exhibit the high catalytic performance in the photocatalytic hydrogen production.

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studied extensively.26−62 In such homogeneous systems, however, the combination of water oxidation catalysts with water reduction catalysts results in the reversed flow of electrons from the reduced water reduction catalysts to the oxidized water oxidation catalysts. Thus, sacrificial electron donors have been required for the homogeneous photocatalytic H2 production from water.26−62 Nevertheless, the homogeneous photocatalytic systems have provided valuable insights into the photocatalytic mechanisms.26−62 In addition, the use of precious metals such as ruthenium complexes can be avoided by using organic electron donor−acceptor linked molecules as homogeneous photocatalysts for H2 production.63−66 With regard to H2 production catalysts, which are combined with homogeneous organic photocatalysts, either homogeneous or heterogeneous catalysts have been employed.26−66 Although platinum nanoparticles (PtNPs) exhibit the highest catalytic activity because of the lowest overpotential for proton reduction to evolve H2,63−66 precious Pt is desired to be

INTRODUCTION Hydrogen (H2) has been regarded as a highly potential candidate for energy carrier of the next generation because of its high energy density per weight and environmentally benign properties of nonemission of green house gas and other harmful chemicals after burning.1−11 Currently, H2 supplied to the industries is produced by the steam reforming of nonrenewable hydrocarbons such as methane.12 A major byproduct of the steam reforming is carbon dioxide, which is a typical green house gas. To be a really sustainable and green energy carrier, H2 should be produced from carbon neutral chemicals or water by utilizing natural energy.13−16 Thus, there have been extensive studies on the photocatalytic H 2 production from water using semiconductor photocatalysts, which can split water into H2 and dioxygen under UV irradtiation.17−25 The fine control of the band gap of semiconductor photocatalysts has enabled one to use visible light for the photocatalytic H2 production from water.18−25 However, the quantum efficiency has remained too low for any practical application.18−25 Photocatalytic H2 production from water using metal complexes as homogeneous photocatalysts has also been © 2013 American Chemical Society

Received: April 20, 2013 Revised: May 31, 2013 Published: June 5, 2013 13143

dx.doi.org/10.1021/jp403925v | J. Phys. Chem. C 2013, 117, 13143−13152

The Journal of Physical Chemistry C

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transmission electron microscope have revealed how the morphology of SiO2 supports affects the catalytic reactivity of Ru/SiO2.

replaced by nonprecious and more earth-abundant metals. In this context, nanoparticles of ruthenium (RuNPs), which is more available than Pt,67 have been shown to exhibit catalytic activity comparable to PtNPs in the photocatalytic H2 production from water using NADH as an electron source and an organic photocatalyst.68,69 NiNPs have also been reported to act as a good H2-evolution catalyst in the homogeneous H2 production from water with NADH and an organic photocatalyst.70 The main drawback of those metal nanoparticles is lack of stability,68−70 partly because of agglomerates formation of the metal nanoparticles capped with organic molecules during the photocatalytic H 2 production. Such an agglomeration problem of metal nanoparticles may be improved by the application of size-controlled nanoparticles to catalysts on a solid support, because controlled and precise growth of nanoparticles with the desired shape and size can be tuned by altering the morphology of the support as reported in the field of heterogeneous catalysis.71−76 However, there have been no systematic studies on the effects of supports on photocatalytic H2 production with metal nanoparticles used as H2-evolution catalysts. We report herein an efficient photocatalytic H2-evolution system composed of a donor−acceptor linked dyad, 2-phenyl4-(1-naphthyl)quinolinium ion (QuPh+−NA),77 as a photocatalyst and dihydronicotinamide adenine dinucleotide (NADH) as a sacrificial electron donor using RuNPs supported on various metal oxides (SiO2, Al2O3−SiO2, CeO2, TiO2, and MgO) as H2-evolution catalysts. The chemical structure of the organic photocatalyst used in this study and the overall reaction scheme are depicted in Scheme 1.68,69 Upon photoexcitation of

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EXPERIMENTAL METHOD Materials. All chemicals were obtained from chemical companies and used without further purification. Ruthenium trichloride, bis(ethylcyclopentadienyl)ruthenium, silicon dioxide (10−20 nm), mesoporous silica (MSU-H), titanium isopropoxide, cerium nitrate, and benzyl alcohol were purchased from Sigma−Aldrich. Platinum(II) dichloride, magnesium chloride hexahydrate, sodium aluminate, acetic acid, hydrochloric acid, and sodium hydroxide were obtained from Wako Pure Chemical Industries. Cetyltrimethylammonium bromide and sodium borohydride were purchased from Kanto Chemical. An aqueous solution of ammonia (28%) and β-nicotinamide adenine dinucleotide disodium salt (reduced form) (NADH) were obtained from Tokyo Chemical Industry. Tetraethyl orthosilicate was delivered by Shin-Etsu Chemical. Acetonitrile and dehydrated acetone were obtained from Nakalai tesque. An aqueous solution of colloidal Pt nanoparticles (∼2 nm, 4 wt %) capped with polyvinylpyrrolidone (PVP) was purchased from Tanaka Kikinzoku Kogyo K.K. 2Phenyl-4-(1-naphthyl)quinolinium perchlorate (QuPh+−NA) was synthesized by reported methods.77 Metal oxide supports of alumina−silica, cerium oxide, titania, magnesium oxide, and spherical silica were prepared by reported methods.85−82 Preparation of Ru/MOx by an Impregnation Method. A typical procedure for the preparation of metal oxide catalysts with 3 wt % Ru loading by an impregnation method is as follows: metal oxide supports were suspended in ethanol (145 mg, 100 mL). An ethanol solution (30 mL) of ruthenium(III) chloride (10 mg, 44 μmol) was added to the suspension of metal oxides. Ethanol was slowly evaporated under reduced pressure by using a rotary evaporator at room temperature. The obtained dried metal oxides were calcined at 350 °C for 4 h in air. The RuO2 formed on metal−oxide supports were reduced by sodium borohydride (50 mg, 1.3 mmol) for 24 h in ethanol. The reduced catalysts were collected by centrifugation and washed with pure water for three times. No chloride contamination was confirmed by the addition of silver nitrate to the filtrate. Ru/SiO2 catalysts with different Ru loading amounts were prepared by a similar procedure. When bis(ethylcyclopentadienyl)ruthenium was used as a precursor, preparation was performed in dehydrated acetone instead of ethanol. Preparation of Ru/SiO 2 by a Chemical Vapor Deposition Method.83 An SiO2 support dried under vacuum a t 1 2 0 ° C w a s e x p o s e d t o t h e va p o r o f bi s (ethylcyclopentadienyl)ruthenium under reduced pressure at 120 °C. After removal of the unreacted precursor by evacuation, the SiO2 adsorbing the ruthenium complex was calcined at 243 or 300 °C for 4 h in air. The calcined catalysts were reduced by sodium borohydride in ethanol and washed with purified water. The obtained catalysts were kept under reduced pressure. The loading amount of Ru was determined by X-ray fluorescence. Preparation of Pt/SiO2 by an Impregnation Method. An SiO2 support was dispersed in ethanol (145 mg, 100 mL). An ethanol solution (30 mL) of platinum(II) chloride (6.3 mg, 24 μmol) was added to the suspension. Ethanol was slowly evaporated under reduced pressure by using a rotary evaporator. The obtained dried SiO2 was calcined at 350 °C

Scheme 1. (a) Structure of QuPh+−NA and (b) the Overall Photocatalytic Cycle for H2 Evolution Using QuPh+−NA and Ru/MOx Catalysts

QuPh+−NA, electron transfer from the NA moiety to the singlet excited state of the QuPh+ moiety occurs to produce the electron-transfer state (QuPh•−NA•+). Electron transfer from NADH to QuPh•−NA•+ then occurs to produce NADH•+ and QuPh•−NA. NADH•+ undergoes deprotonation releasing one proton to afford NAD• that can transfer an electron to QuPh+− NA to produce NAD+ and QuPh•−NA. Two equivalents of QuPh•−NA thus produced can inject two electrons to catalysts to produce H2 from two protons. It should be noted that no electron mediator, which is frequently used in homogeneous photocatalytic H2 production, is required in the present photocatalytic system, because the electron-transfer state of QuPh+−NA has a sufficient lifetime for the oxidation of NADH and also for electron injection to RuNPs.68,69 Among RuNPs supported on various metal oxides, Ru/SiO2 exhibited the highest catalytic reactivity. The preparation methods of Ru/ SiO2 and morphology of the SiO2 support were optimized for further improvement in the catalytic reactivity. Kinetic study of various Ru/SiO2 catalysts and their nanostructures observed by 13144



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Figure 1. (a) Time courses of H2 evolution under photoirradiation (λ > 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+−NA (0.22 mM) and NADH (1.0 mM) with metal nanoparticles [1.0 mg L−1, RuNPs (■) and PtNPs (red ●)]. (b,c) TEM images of Ru nanoparticles (b) before and (c) after the reaction.

μL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing NADH (20 mM) to the solution after reaction, repeatedly. Kinetic Measurements. A mixed solution (2.0 mL) of a deaerated aqueous buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+−NA (0.88 mM) and NADH (1.0 mM) was photoirradiated for several minutes with a xenon lamp through a color filter glass transmitting λ > 340 nm. A deaerated aqueous solution containing Ru/SiO2 then was added to the photoirradiated solution by using a microsyringe with stirring. Electron-transfer rates from the one-electron reduced species of QuPh+−NA (QuPh•−NA) to Ru/SiO2 were determined from the decay of absorption at 420 nm due to QuPh•−NA monitored by using a Hewlett-Packard 8453 diode array spectrophotometer with a quartz cuvette (path length = 10 mm) at 298 K.

for 4 h in air. The platinum species on silica was reduced by sodium borohydride (50 mg, 1.3 mmol) for 24 h in ethanol. The reduced catalyst was collected by centrifugation and washed with pure water three times. No chloride contamination was confirmed by the addition of silver nitrate to the filtrate. Pt/SiO2 catalysts with different Pt loading amounts were prepared by a similar procedure. Transmission Electron Microscopy (TEM). The sizes and shapes of Ru/SiO2 catalysts were determined from bright field images using a JEOL JEM-2100 that has a cold field emission gun with an accelerating voltage of 200 keV. The observed samples were prepared by dropping a dispersion of catalysts and allowing the solvent to evaporate and then scooped up with an amorphous carbon supporting film. X-ray Diffraction. X-ray diffraction patterns were recorded by a Rigaku Ultima IV. Incident X-ray radiation was produced by Cu X-ray tube, operating at 40 kV and 40 mA with Cu Kα radiation of 1.54 Å. The scanning rate was 2° min−1 from 5° to 80° in 2θ. N2 Adsorption for BET Surface Area Determination. Nitrogen adsorption−desorption at 77 K was performed with a Belsorp-mini (BEL Japan, Inc.) within a relative pressure range from 0.01 to 101.3 kPa. A sample mass of ∼100 mg was used for adsorption analysis after pretreatment at 120 °C for 1 h under vacuum conditions and kept in N2 atmosphere until N2adsorption measurements. The samples were exposed to a mixed gas of He and N2 with a programmed ratio, and the adsorbed amount of N2 was calculated from the change of pressure in a cell after reaching the equilibrium (at least 5 min). The surface area of each catalyst was determined by the Brunauer−Emmett−Teller (BET) method for multiple N2 adsorption amounts under the conditions of partial pressure less than 0.3. Photocatalytic H2 Evolution. A mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1(v/v)] containing QuPh+−NA (0.88 mM), NADH (1.0 mM), and an H2-evolution catalyst was flushed with N2 gas. The solution was then irradiated for a certain time with a xenon lamp (Ushio Optical, model X SX-UID 500X AMQ) through a color filter glass (Asahi Techno Glass L39) transmitting λ > 340 nm at room temperature. After 1 min stirring in the dark, gas in a headspace was analyzed by Shimadzu GC-14B gas chromatography (detector, TCD; column temperature, 50 °C; column, active carbon with the particle size 60−80 mesh; carrier gas, N2 gas) to determine the amount of evolved H2. Repetitive examination was performed by adding the mixed solution (100

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RESULTS AND DISCUSSION Photocatalytic H2 Evolution with PtNPs and RuNPs. Figure 1a shows the time courses of H2 evolution under photoirradiation (λ > 340 nm) of a mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing NADH (1.0 mM), QuPh+−NA (0.88 mM), and PtNPs or RuNPs (1.0 mg L−1). QuPh+−NA efficiently absorbs the light at wavelengths shorter than 340 nm as reported previously,66,77 and NADH was used as a sacrificial electron donor, which can afford the stoichiometric amount of H2.84 The H2-evolution rates obtained with PtNPs and RuNPs were quite similar to 5 min photoirradiation; however, the H2-evolution rate from the reaction solution with RuNPs became slower than that with PtNPs at a prolonged photoirradiation time. The deceleration with RuNPs resulted in low H2 yield of 48%, which is calculated from the amount of NADH used in the reaction, whereas the stoichiometric amount (2.0 μmol) of H2 was produced in the reaction with PtNPs (Figure 1a, red). Thus, improvement in durability of RuNPs is necessary to use RuNPs as an alternative catalyst to PtNPs. Transmission electron microscopy (TEM) images of RuNPs before and after reaction are displayed in Figure 1b and c, respectively. The TEM image taken for the sample after the reaction (Figure 1c) manifested agglomeration of RuNPs, which may result in deactivation. The RuNPs are capped with PVP; however, more effective methods are necessary to avoid the agglomeration. Photocatalytic H2 Evolution with Ru/MOx Catalysts Prepared by an Impregnation Method. To suppress the agglomeration of RuNPs, RuNPs were supported on metal 13145



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oxides by the impregnation method using RuCl3 as a precursor. The loading amount of RuNPs was 3.0 wt % for all of the metal oxides. The BET surface areas of the metal oxides used for supporting RuNPs were higher than 29 m2 g−1, which seems to be large enough to support small RuNPs on the surfaces. The RuO2 formed on the metal oxides after calcination at 350 °C in air was reduced by sodium borohydride in ethanol to form RuNPs. X-ray diffraction patterns of Ru species supported on metal oxides (Ru/MOx, MOx = SiO2, Al2O3−SiO2, CeO2, TiO2, and MgO) indicate that RuO2 was formed on SiO2, SiO2−Al2O3, and TiO2 under ambient conditions (Figure S1). Because the XRD measurements were performed in air, the surfaces of reduced RuNPs were oxidized to RuO2. These results suggested that the size of RuNPs on these metal oxides should be larger than nanometer size, although RuNPs were not clearly observed at low Ru-loading by TEM observations (vide infra). No peaks assignable to Ru metal or RuO2 suggest that Ru species are highly dispersed on CeO2 and MgO, and few nanoparticles grew up to a sufficient size to afford diffraction peaks. Figure 2a shows the time courses of photocatalytic H2 evolution under photoirradiation (λ > 340 nm) of a mixed

without reductive treatment prior to use was also examined as an H2-evolution catalyst. As indicated in Figure 2a (red ○, dashed line), the H2 evolution with RuO2/SiO2 was much slower than that in the reaction system using Ru/SiO2, suggesting that Ru/SiO2 is more active than RuO2/SiO2. The effect of loading amount of RuNPs on SiO2 was examined by changing the loading amount from 1 to 4 wt %, when the weight concentration of RuNPs was varied from 1 to 4 mg L−1 by keeping the weight concentration of Ru/SiO2 catalysts 100 mg L−1. The H2-evolution rate with the Ru/SiO2 catalysts increased to 16 μmol h−1 in proportion to the loading amount of RuNPs up to 3 wt %; however, further increase in RuNPs loading to 4 wt % resulted in no increase in the H2evolution rate (Figure 2b). This indicates that the optimal loading amount of RuNPs is 3 wt %. As previously reported, RuNPs capped with PVP in the size of 4.1 nm exhibited higher catalytic activity than larger or smaller RuNPs capped with PVP.68 Such optimum size observed in the case of RuNPs capped with PVP may also exist in the case of RuNPs supported on SiO2. The 3 wt % of Ru-loading provided the RuNPs with an optimum size on the SiO2 surfaces when Ru/ SiO2 was prepared by the impregnation method. On the contrary to RuNPs supported on SiO2, the catalytic activity of PtNPs supported on SiO2 was significantly reduced as compared to that of PtNPs capped with PVP. All Pt/SiO2 catalysts prepared by the impregnation method with different Pt loadings of 1−4 wt % exhibited catalytic activity far below Pt nanoparticles (Figure S2). Only 25% of the stoichiometric amount of H2 evolved from the reaction solutions using Pt/ SiO2 as H2-evolution catalysts. The lower catalytic activity observed for Pt/SiO2 may result from a strong metal−support interaction between Pt and SiO2 to form Pt3Si-alloy as reported previously.90,91 The robustness of the 3 wt % Ru/SiO2 in photocatalytic H2 evolution was scrutinized by the repeated use in the photocatalytic H2 evolution. The photocatalytic H2 evolution was repetitively performed by photoirradiation (λ > 340 nm) of a deaerated mixed solution (2.0 mL) of a phosphate buffer (pH 4.5) and MeCN containing NADH (1.0 mM) and RuNPs capped with PVP or the 3 wt % Ru/SiO2 (12.5 mgRu L−1). After each catalytic reaction, a concentrated solution of NADH was added to the solution (two times). Time courses of H2 evolution are depicted in Figure 3. In the case of the photocatalytic reaction with RuNPs, the large H2-evolution rate of 29 μmol h−1 was observed for the first run; however, the H2-evolution rate decreased to 13 μmol h−1 in the third run (Figure 3, black). In contrast to this, the H2-evolution rate of 16 μmol h−1 for the first run was maintained for the third run by using the 3 wt % Ru/SiO2 as the H2-evolution catalyst (Figure 3, red). These results demonstrate that the 3 wt % Ru/SiO2 is more stable than RuNPs, although the catalytic activity of Ru/ SiO2 should be further improved. Preparation of Ru/SiO2 by Chemical Vapor Deposition. Generally, the catalytic activity of metal nanoparticles (MNPs) supported on a metal oxide can be improved by using a different preparation method, because structures of MNPs on metal oxide surfaces are highly sensitive to the preparation methods. Chemical vapor deposition (CVD) is a preparation method utilizing chemical interaction with a precursor molecule and the surfaces of a metal oxide support.92 Usually ligand exchange reaction between a precursor ligand and hydroxy group of the metal oxide surfaces assures the atomically dispersed structure of metal ions on the surfaces.83 The

Figure 2. (a) Time courses of H2 evolution performed by photoirradiation (λ > 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+−NA (0.88 mM), NADH (1.0 mM), and RuNPs supported on metal oxides (3.0 wt %, 100 mg L−1, Ru/SiO2, red ●; Ru/Al2O3−SiO2, blue ■; Ru/CeO2, green ▲; Ru/TiO2, purple ▼; and Ru/MgO, ◆) or RuO2 supported on silica (red ○). (b) Time courses of H2 evolution using Ru/SiO2 with different Ru loading amounts (100 mg L−1; 1.0 wt %, ●; 2.0 wt %, blue ■; 3.0 wt %, red ▲; and 4.0 wt %, green ▼).

solution (2.0 mL) of a phosphate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing NADH (1.0 mM), QuPh+−NA (0.88 mM), and Ru/MOx (100 mg mL−1, 3.0 mgRu mL−1) as a sacrificial electron donor, a photocatalyst, and H2-evolution catalysts, respectively. No H2 evolution was observed without photoirradiation. Among the Ru/MOx, Ru/SiO2 (Figure 2, red ●) and Ru/Al2O3−SiO2 (blue ■) provided the evolution of nearly stoichiometric amount of H2 (2.0 μmol) in a certain reaction time. The initial H2-evolution rates ( 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+−NA (0.88 mM), NADH (1.0 mM), and 3 wt % Ru/SiO2 prepared by the impregnation method (12.5 mgRu L−1, red) or Ru nanoparticles (12.5 mg L−1, black). The mixed solution of a phthalate buffer and MeCN [1:1 (v/ v)] containing NADH was added to the reaction solution three times after each run.

atomically dispersed structure may be beneficial to reduce the size of MNPs and increase the effective surface areas of MNPs. Ru/SiO2 catalysts with 0.97 and 3.4 wt % Ru loadings were prepared with a CVD method by using bis(ethylcyclopentadienyl)ruthenium(II) as a precursor. The exact loading amounts of Ru on the Ru/SiO2 were determined by X-ray fluorescence spectroscopy. Figure 4a shows

Figure 5. (a) Time courses of H2 evolution performed by photoirradiation (λ > 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing NADH (1.0 mM), 3 wt % Ru/SiO2 (100 mg L−1), and QuPh+−NA (0.22 mM, ●; 0.44 mM, blue ■; 0.88 mM, red ▲; 1.1 mM, green ▼; and 1.3 mM, purple ◆). (b) Plots of H2-evolution rates as a function of the concentration of QuPh+−NA.

carried out under photoirradiation (λ > 340 nm) of a mixed solution of a phthalate buffer (pH 4.5) and MeCN containing NADH (1.0 mM), Ru/SiO2 prepared by the impregnation method (3.0 mgRu L−1), and QuPh+−NA. The H2-evolution rate increased in proportion to the concentration of QuPh+− NA up to the concentration of 0.88 mM with the H2-evolution rate of 16 μmol h−1 as shown in Figure 5b. No further increase in the H2-evolution rate was observed at higher concentration of QuPh+−NA (>1.3 mM). Thus, the optimum concentration of QuPh+−NA is around 1.0 mM. The concentration effect of 0.97 wt % Ru/SiO2 prepared by the CVD method in the reaction solution was investigated by changing the concentration from 12.5 to 125 mg L−1 corresponding to the RuNPs concentration from 0.12 to 1.2 mgRu L−1. Time courses of H2 evolution by photoirradiation of the mixed solution containing NADH, QuPh+−NA, and 0.97 wt % Ru/SiO2 with various concentrations are shown in Figure 6a. H2-evolution rates determined from the initial slopes ( 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+−NA (0.88 mM), NADH (1.0 mM), and 1.0 wt % Ru/SiO2 catalysts prepared by an impregnation method (100 mg L−1, blue ■) and a chemical vapor deposition method (100 mg L−1; 0.97 wt %, red ●; and 3.4 wt %, green ▲) using bis(ethylcyclopentadienyl)ruthenium(II) as a precursor. (b,c) TEM images of (b) 0.97 wt % Ru/SiO2 and (c) 3.4 wt % Ru/SiO2 prepared by the CVD method.

comparison of the time profiles of H2 evolution in the photocatalytic systems employing 0.97 wt % Ru/SiO2 prepared by the CVD method and 1.0 wt % Ru/SiO2 prepared by the impregnation method using the same precursor for CVD as H2evolution catalysts with the weight concentration of 100 mg L−1, in which the weight concentration of RuNPs was 1.0 mgRu L−1. The initial H2-evolution rate ( 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN containing QuPh+−NA (0.88 mM), 0.97 wt % Ru/u-SiO2 (100 mg), and NADH (1.0 mM). An aliquot of a concentrated NADH solution was added to the solution after

Figure 6. (a) Time courses of H2 evolution under photoirradiation (λ > 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+−NA (0.88 mM), NADH (1.0 mM), and 0.97 wt % Ru/SiO2 with different catalyst concentrations (12.5 mg L−1, red ●; 25 mg L−1, blue ■; 50 mg L−1, green ▲; 100 mg L−1, pink ▼; and 125 mg L−1, ◆). (b) Plots of initial rates (15 min) of H2 evolution against the weight concentration of RuNPs in the form of 0.97 wt % Ru/SiO2.

influences the catalysis of Ru/SiO2 for H2 evolution, because of the change in the size of RuNPs and fine structure of RuNPs on SiO2 surfaces. For example, an SiO2 support with larger surface area is beneficial for high dispersion of RuNPs on the surface. Also, the location of RuNPs inside and outside of pores may affect the catalysis. Thus, mesoporous SiO2 with high surface area and nonporous SiO2 with spherical shape were examined as supports of RuNPs. The morphology of SiO2 was confirmed by transmission electron microscopy as indicated in Figure 7: SiO2 with undefined shape (u-SiO2), mesoporous SiO2 with hexagonally packed array (m-SiO2), and SiO2 with spherical shape (s-SiO2). u-SiO2 was used as the support in preceding experiments. The BET surface areas of the SiO2 supports

Figure 7. TEM images of silica in the shape of (a) undefined, (b) hexagonally packed mesoporous, and (c) nonporous sphere. (d) Time courses of H2 evolution performed by photoirradiation (λ > 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+−NA (0.88 mM), NADH (1.0 mM), and RuNPs supported on SiO2 prepared by the CVD method (1 wt % Ru/SiO2 in the shape of undefined shape, red ●; mesoporous SiO2, green ▲; and spherical SiO2, blue ■). 13148



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Figure 9. (a) Time courses of H2 evolution examined by photoirradiation (λ > 340 nm) of a deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+− NA (0.88 mM), NADH (1.0 mM), and 1 wt % Ru/SiO2 prepared by the CVD method (100 mg, black) or Pt nanoparticles (1.0 mg, gray) for the first run. (b) Amount of H2 evolved in the repetitive photocatalytic H2 evolution using Ru/SiO2 (black) and PtNPs (gray). A mixed solution containing NADH was added to the reaction solution after each run.

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CONCLUSIONS The durability of Ru nanoparticles (RuNPs) in the photocatalytic H2 evolution was improved by being supported on SiO2. RuNPs capped with polyvinylpyrrolidone (PVP) have been reported as an active catalyst comparable to Pt nanoparticles (PtNPs) capped with PVP;68 however, durability of the RuNPs was inferior to that of the PtNPs because of easy agglomerates formation. Supporting RuNPs on SiO2 by the CVD method effectively suppressed the agglomeration of RuNPs to elongate the catalyst life. The morphology of SiO2 used as a support affected the effective surface area of RuNPs on Ru/SiO2. The durability of RuNPs in the photocatalytic H2 evolution was dramatically improved by employing the suitable preparation method and the morphology of SiO2 support without losing high catalytic activity. Thus, this study has demonstrated that suitable choices of a catalyst preparation method and morphology of metal oxide supports provide a promising strategy for the improvement of the catalytic reactivity and durability of metal nanoparticles.

Figure 8. (a) Decay time profile of absorption at 420 nm due to QuPh•−NA (black) in electron transfer from QuPh•−NA to 0.97 wt % Ru/u-SiO2 (100 mg L−1) and time profile of H2 evolution (red) in a mixed solution of a phthalate buffer (pH 4.5) and MeCN [1:1 (v/v)]. QuPh•−NA solution (2.0 mL) was produced by photoirradiation (λ > 340 nm) of QuPh+−NA (0.88 mM) in the presence of NADH (1.0 mM). (b) Time courses of H2-evolution with Ru/u-SiO2 (red ●), Ru/ m-SiO2 (green ▲) and Ru/s-SiO2 (blue ■) (Ru content is about 1 wt %). (c) Plots of the pseudo-first-order rate constants (kobs) of electron transfer from QuPh•−NA to RuNPs vs weight concentrations of RuNPs in the form of Ru/SiO2.

each reaction four times. When the same amount of RuNPs (1.0 mg L−1) capped with PVP was used as the H2-evolution catalyst, H2 only less than the stoichiometric amount was evolved even for the first run as indicated in Figure 1. In contrast to this, the H2 evolution was kept four times with 0.97 wt % Ru/u-SiO2 (∼1 mgRu L−1) prepared by the CVD method. The total amount of evolved H2 normalized by Ru weight was 1.7 mol gRu−1 for 0.97 wt % Ru/u-SiO2 (CVD), which is more than 7 times larger than the amount with RuNPs (0.22 mol gRu−1). The catalytic activity and robustness of 0.97 wt % Ru/u-SiO2 were compared to that of PtNPs capped with PVP. Figure 9a shows the time courses of H2 evolution with 0.97 wt % Ru/uSiO2 (black) and the PtNPs (gray) in the reaction system for the first runs. From both reaction systems, the quantitative amount of H2 was evolved with similar H2-evolution rates. Thus, 0.97 wt % Ru/u-SiO2 exhibits the catalytic activity as high as PtNPs. When these catalysts were repeatedly used in the photocatalytic H2 evolution, 0.97 wt % Ru/u-SiO2 and PtNPs kept the activity after the fourth and fifth cycles, respectively, as shown in Figure 9b. PtNPs exhibited higher stability than RuNPs; however, aggregation of PtNPs after fifth cycles was observed by the DLS measurement (Figure S4). The total amount of evolved H2 normalized by Pt weight was 2.0 mol gPt−1, which is only slightly higher than the amount with 0.97 wt % Ru/u-SiO2. The durability of 0.97 wt % Ru/u-SiO2 now gets closer to that of PtNPs by optimizing the preparation method and morphology of an SiO2 support.

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ASSOCIATED CONTENT

* Supporting Information S

X-ray diffraction patterns of Ru/MOx (Figure S1), time courses of H2 evolution in the photocatalytic system using Pt/SiO2 (Figure S2), and dynamic laser scattering (DLS) of spherical SiO2, Ru/s-SiO2 (Figure S3), and PtNPs (Figure S4). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-6-6879-7368. Fax: +81-6-6879-7370. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid (nos. 20108010 to S.F., 24350069 and 25600025 to Y.Y.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and NRF/MEST of Korea through the WCU (R31-2008-00010010-0) and GRL (2010-00353) Programs. We sincerely 13149



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acknowledge Prof. Norimitsu Tohnai for the measurements of powder X-ray diffractions and the Research Centre for UltraPrecision Science & Technology, Osaka University, for TEM measurements.

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