Highly Dispersed RuO2 Hydrates Prepared via Simple Adsorption as

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Highly dispersed RuO hydrates prepared via simple adsorption as efficient cocatalysts for visible-light-driven Z-scheme water splitting with an IO /I redox mediator 3–

–

Hajime Suzuki, Shinnosuke Nitta, Osamu Tomita, Masanobu Higashi, and Ryu Abe ACS Catal., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Highly dispersed RuO2 hydrates prepared via simple adsorption as efficient cocatalysts for visible-lightdriven Z-scheme water splitting with an IO3–/I– redox mediator Hajime Suzuki,† Shinnosuke Nitta,† Osamu Tomita,† Masanobu Higashi,† and Ryu Abe*,†,‡ †

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto

University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡

JST-CREST, Gobancho 7, Chiyoda-ku, Tokyo 102-0075, Japan

ABSTRACT: Loading a cocatalyst onto a photocatalyst is a well-known effective way to improve the efficiency of both one-step water splitting and Z-scheme water splitting with a redox mediator. In Z-scheme water splitting systems with an IO3–/I– redox couple, the reduction of IO3– on O2-evolving photocatalysts via a six-electron process often represents the rate-determining step of the overall process, and therefore necessitates effective cocatalysts such as PtOx and RuO2. However, these cocatalysts cannot be loaded onto thermally unstable materials via conventional impregnation processes involving calcination. In the present study, we introduce a

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Ru-based cocatalyst that can be loaded without calcination and effectively promotes the reduction of IO3– on various photocatalysts, including non-oxide materials. The results reveal that the Ru species adsorbed via simple stirring of photocatalyst particles such as WO3 in an aqueous RuCl3 solution effectively trigger O2 generation in the presence of IO3– as electron acceptor; moreover, the O2 evolution rate on the present Ru-loaded WO3 sample was much higher than that on WO3 loaded with conventional RuO2. The structural analysis indicates that the Ru species adsorbed on WO3 are highly dispersed and characterized by octahedral RuO6 environments similar to those found in RuO2·nH2O. It was confirmed that the loaded Ru species (most likely analogous to RuO2·nH2O) exhibit higher activity in the reduction of IO3– than anhydrous RuO2, whereas RuO2 exhibits much higher activity for the oxidation of water than RuO2·nH2O. The developed Ru-based cocatalyst was also applicable to thermally unstable materials such as H2WO4 and Ta3N5, thus enabling them to generate O2 in the presence of IO3–.

KEYWORDS: cocatalyst, water splitting, Z-scheme, heterogeneous photocatalysis, solar energy conversion

1. INTRODUCTION Photoinduced water splitting using semiconductor materials has attracted much attention due to its potential for clean H2 production from abundant solar light.1-7 To achieve practically high efficiency of H2 production under solar light irradiation, it is indispensable to utilize a wide range of the solar light spectrum up to the visible region, as well as to achieve a high quantum efficiency in the photocatalytic process. Introducing two-step photoexcitation (i.e., Z-scheme) in water splitting has been recently proven as one of the most promising strategies for harvesting a


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wider range of visible light.7,8 In such systems, the water-splitting reaction is divided in two stages, one leading to H2 evolution and the other leading to O2 evolution. Both trivalent/divalent iron (Fe3+/Fe2+) and iodate/iodide (IO3–/I–) pairs have previously been utilized as effective redox shuttles in various combinations of H2- and O2-evolving photocatalysts.9-12 Although the oneelectron redox behavior of Fe3+/Fe2+ facilitates the occurrence of redox reactions even on the surface of semiconductor particles, the rapid precipitation of Fe(OH)3 from Fe3+ cations (aqua ions) at pH higher than 2.5 limits the choice of photocatalyst materials to those with sufficient stability even under highly acidic conditions. On the other hand, the redox cycle of the IO3–/I– couple can stably proceed under a wider pH range (6–11), while the competitive generation of I3– takes place under acidic conditions, at pH below 5.13 Thus, various kinds of semiconductor materials, in addition to conventional metal oxides, oxynitrides, nitrides, and oxysulfides can be applied in this case.14-20 However, at variance with the Fe3+/Fe2+ redox couple, the multi-electron reduction of IO3– to I− (IO3– + 6e– + 3H2O → I– + 6OH–) often represents the rate-determining step, and thus necessitates the loading of an effective cocatalyst.6,14,21 We have previously reported that loading metal oxide cocatalysts such as PtOx or RuO2 (which have a lower overpotential for IO3− reduction) onto metal oxides such as WO3 or KCa2Nb3O10 is an effective way to facilitate IO3– reduction, whereas Pt(0) species loaded by convectional photodeposition rarely enhance O2 evolution from the IO3− electron acceptor.6,21 These cocatalysts are generally loaded on the photocatalyst via impregnation with calcination above ~300 °C in air. However such method is inapplicable to thermally unstable photocatalyst materials. We have recently demonstrated that tungstic acid (H2WO4) functions as an effective O2-evolving photocatalyst in the Z-scheme water splitting with an Fe3+/Fe2+ redox couple, by harvesting a wider range of visible light (~500 nm) than WO3 (~460 nm).22,23 However, a Z-scheme water splitting system


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based on H2WO4 and an IO3–/I– mediator has not been demonstrated to date, due to the difficulty of loading effective cocatalysts such as PtOx or RuO2 onto H2WO4. This difficulty is attributed to the facile dehydration of H2WO4 to WO3 or related compounds upon calcination above 180 °C. Mixed anion photocatalysts such as oxyhalides and oxynitrides,24-28 will also face the difficulty because they possess relatively low stability against calcination in air. Thus, a new method for loading effective cocatalysts without calcination processes is highly required in order to achieve efficient water splitting. In the present study, we show that hydrated RuO2·nH2O species loaded on a photocatalyst via a simple heating-free adsorption method can work as effective cocatalysts for IO3– reduction and therefore enhance the photocatalytic O2 evolution with an IO3– acceptor not only on conventional WO3, but also on thermally unstable H2WO4 and tantalum nitride (Ta3N5) materials.

2. EXPERIMENTAL SECTION 2.1. Preparation of the catalysts. Commercially available powdered samples of WO3 (Kojundo Chemical Laboratory Co., Ltd.) and H2WO4 (Wako Pure Chemical Industries, Ltd.) were used as photocatalysts. Ta3N5 particles were synthesized according to a previously reported method,29 by exposing Ta2O5 particles (Kojundo Chemical Laboratory Co., Ltd.) to NH3 at 850 °C and a flow rate of 500 mL/min for 15 h. The photocatalyst particles were stirred in an aqueous solution containing the required amount (0.5 wt%, as Ru metal to the photocatalyst) of RuCl3 at room temperature for 3 h. The suspended particles were then collected by centrifugation at 4800 rpm for 5 min, followed by thoroughly washing more than five times with pure water. The obtained particles will be denoted as Ru-WO3, Ru-H2WO4, and Ru-Ta3N5 in the following.


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The concentration of Ru cations remaining in the supernatant solution was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Scientific iCAP 7400 Duo), and then the amount of Ru species adsorbed on the photocatalyst was estimated. As for the WO3 photocatalyst, about 20% of the Ru precursor was found to remain in the collected supernatant, indicating that approximately 0.4 wt% of Ru had been loaded onto WO3. As for the H2WO4 photocatalyst, the amount of residual Ru precursor in the supernatant was negligibly low, indicating that ~0.5 wt% of Ru had been loaded. The Ru-WO3 sample was calcined at various temperatures (100–400 °C) in air for 17 h. The products will be denoted as Ru-WO3 (100–400). As H2-evolving photocatalysts for the Z-scheme process, particles of strontium titanate doped with rhodium (SrTiO3:Rh) were prepared by solid-state reaction.30 A mixture of SrCO3, TiO2, and Rh2O3 (Sr:Ti:Rh = 1.07:1:0.01) was calcined in air at 800 °C for 1 h and subsequently at 1000 °C for 10 h. A Pt cocatalyst was loaded onto SrTiO3:Rh by impregnation using H2PtCl6·6H2O and subsequent heating under H2 flow at 300 °C for 2 h. 2.2. Characterization. X-ray photoelectron spectroscopy (XPS) measurements of the catalysts were performed using an ULVAC-PHI 5500MT system. The binding energies were referenced to the Au 4f7/2 level of deposited Au metal. Commercially available RuO2·nH2O (Strem Chemicals, Inc.), RuCl3 (Strem Chemicals, Inc.), and RuO2 (Wako Pure Chemical Industries, Ltd.) were used as references. Home-made RuO2·nH2O was prepared by a simple precipitation method.31 Ru K-edge X-ray absorption fine structure (XAFS) measurements were performed at the BL01B1 beamline of SPring-8. The X-ray absorption spectra were measured in transmission or fluorescence mode at room temperature with a Si(311) two-crystal monochromator. Transmission electron microscopy (TEM) photographs were taken using a JEOL JEM-2100F microscope.


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2.3. Electrochemical measurements. The as-prepared Ru-loaded WO3 samples (100 mg) were added with 100 µL of water and grounded in a mortar to prepare viscous slurries. The slurries (15 µL) were then pasted onto a conductive fluorine-doped tin oxide (FTO) substrate and dried at room temperature. The electrochemical cell consisted of the prepared electrodes, a counter electrode (Pt wire), and a Ag/AgCl reference electrode. Current-voltage curves were measured in an aqueous Na2SO4 solution (0.5 M, 40 mL) as a supporting electrolyte. The potential of the working electrodes was controlled using a potentiostat (VersaSTAT3, Princeton Applied Research Co., Ltd.). A 300 W Xe lamp (Cermax LF-300F) was used as the light irradiation source. 2.4. Photocatalytic reactions. The photocatalytic reactions were carried out using a Pyrex glass reactor connected to a closed gas circulation system. For the photocatalytic water oxidation leading to O2 evolution in the presence of IO3– as electron acceptor, 0.1 g of photocatalyst powder was suspended in 250 mL of a 5 mM aqueous NaIO3 solution, under stirring with a magnetic stirrer and bar. For the two-step water-splitting reaction with the IO3–/I– redox couple, 0.1 g Pt-loaded SrTiO3:Rh and 0.5 g Ru-H2WO4, as H2-evolving and O2-evolving photocatalysts, respectively, were suspended in 250 mL of a 0.5 mM aqueous NaI solution. The suspensions were irradiated using a 300 W Xe lamp (Cermax LF-300F). The evolved gases were analyzed by on-line gas chromatography using a thermal conductivity detector (TCD), with a molecular sieve 5A as column packing and Ar carrier gas.

3. RESULTS AND DISCUSSION


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3.1. Photocatalytic O2 evolution from NaIO3 solution over WO3 loaded with Ru species. Figure 1 shows the time courses of O2 evolution from water under UV-visible light irradiation on two representative Ru-modified WO3 samples, along with the unmodified one, in the presence of IO3– as electron acceptor. Unmodified WO3 shows a negligibly low O2 evolution in the presence of IO3–, while it appears capable of evolving O2 in the presence of electron acceptors such as Ag+ and Fe3+,32-34 both of which are reduced via a one-electron process. The negligible activity of WO3 with the IO3– acceptor is undoubtedly due to the problematic IO3– reduction via the six-electron process indicated in eq. 1, as was previously suggested.6,21 IO3– + 3H2O + 6e– → I– + 6OH–

(1)

Figure 1. O2 evolution over bare WO3, Ru-adsorbed WO3 (Ru-WO3), and Ru-WO3 calcined at 300 °C (Ru-WO3 (300)) in aqueous NaIO3 solution (5 mM, 250 mL) under UV-visible light irradiation (λ > 300 nm).


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On the other hand, the Ru-WO3 (300) photocatalyst, which was loaded with RuO2 species (as shown by the XPS results discussed below), generates O2 under irradiation. This is due to the previously reported catalytic effect of RuO2 species for the multi-electron reduction of IO3–.21,35 Surprisingly, the Ru-WO3 sample, which was prepared by simple stirring of WO3 particles in aqueous RuCl3 solution, exhibited a higher rate of O2 generation. The Ru-WO3 photocatalyst also generated O2 under visible light (Figure S1). Figure 2 shows the influence of the calcination temperature on the initial rates of O2 evolution on the Ru-modified WO3 samples. The unheated Ru-WO3 sample clearly shows the highest O2 evolution rate, which then decreases considerably with increasing temperatures. These findings strongly suggest that the Ru species just adsorbed on WO3 catalyze IO3– reduction and/or water oxidation more efficiently compared to conventional RuO2.

Figure 2. Initial rates of O2 evolution over Ru-WO3 and Ru-WO3 calcined at various temperatures (100–400 °C) from aqueous NaIO3 solution (5 mM, 250 mL) under UV-visible light irradiation (λ > 300 nm).


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3.2. Characterizations of Ru species on WO3. Figure 3 shows the Ru 3d XP spectra obtained for the Ru-modified WO3 samples, along with the spectra of RuCl3, RuO2, commercial RuO2·nH2O, and home-made RuO2·nH2O. The home-made RuO2·nH2O sample was prepared from RuCl3 by a simple precipitation method using aqueous NaOH solution to adjust the pH, following a previously reported procedure.31 As representative Ru-WO3 samples, the XP spectra of Ru-WO3 and Ru-WO3 (300) are also shown in the figure. Due to the considerable overlap between the Ru 3d3/2 peak and the C 1s peak with binding energy of 284.7 eV, only the Ru 3d5/2

Figure 3. XP spectra of Ru-WO3 (300), Ru-WO3, and Ru-WO3 after the O2 evolution in Figure 1, along with the spectra of RuO2·nH2O, RuCl3, and RuO2.

peak region from 278 to 284 eV is shown in Figure 3. The Ru 3d5/2 peak of Ru-WO3 (300) at ~280.7 eV indicates that the Ru species loaded onto the catalyst predominantly consist of anhydrous RuO2. On the other hand, the Ru-WO3 sample exhibits a peak at higher energy


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compared with RuO2. The observed binding energy (281.7 eV) of this peak is closer to the energies measured for both commercial and home-made hydrous RuO2, rather than to that corresponding to the RuCl3 precursor. It has been reported that hydrous RuO2 species exhibit a Ru 3d5/2 peak at ~1 eV higher energies than anhydrous RuO2.36,37 In fact, both commercial and home-made RuO2·nH2O exhibit Ru 3d5/2 peaks at around 281.7 eV, i.e., 1 eV higher than RuO2 (280.7 eV). Additionally, peaks corresponding to residual Cl species were negligible for the RuWO3 sample. These results strongly suggest that Ru species similar to RuO2·nH2O were eventually loaded on WO3 by the simple stirring and washing procedure described in the experimental section. It has been reported that RuO2·nH2O can be prepared by increasing the pH of an aqueous solution containing ruthenium cations.31 Therefore, the deposition of RuO2·nH2O species on WO3 was probably induced by the pH changes that took place upon addition of the WO3 powder to the aqueous RuCl3 solution and/or during washing the Ru-WO3 particles with pure water. It should be noted that the position of the Ru 3d5/2 peak of Ru-WO3 did not change after the O2 evolution (Figure 1), indicating that the Ru species is not only chemically but also photochemically stable during the reaction. Figure 4 shows Ru K-edge X-ray absorption near-edge structure (XANES) spectra of the above samples and of Ru metal. The spectrum of Ru-WO3 is similar to those of RuO2·nH2O and RuO2, whereas it is significantly different from the spectra of Ru metal and RuCl3. Figure 5 shows the corresponding Fourier-transformed EXAFS spectra. The peaks at 1.5, 2.6, and 3.2 Å (no phase shift corrections were applied) in the Fourier-transformed spectrum of RuO2 can be assigned to the Ru-O bonds denoted as 1 and 2, to the Ru–Ru bond labeled 3, and to the Ru–Ru bond labeled 4 in Figure 6a, respectively, according to a previous report by McKeown et al.38 The Fourier-transformed spectra of both commercial and home-made RuO2·nH2O samples


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Figure 4. Ru K-edge XANES spectra of Ru-WO3, RuO2·nH2O, RuO2, Ru metal, and RuCl3.

Figure 5. Fourier-transformed EXAFS spectra of Ru-WO3, RuO2·nH2O, RuO2, and RuCl3.

exhibit no peak corresponding to the Ru–Ru shell signal at 3.2 Å, whereas the Ru-Ru signal at 2.6 Å can be clearly observed. These results support the previously suggested structure of


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Figure 6. (a) Structural models of anhydrous RuO2.38 (b) Hypothetical fragment of hydrous RuO2 (RuO2·2.32H2O).38

Figure 7. TEM images of Ru-WO3 and Ru-WO3 calcined at various temperatures (150–300 ˚C).

RuO2·nH2O, in which ruthenium cations are bound by two oxygen ions to form a onedimensional chainlike core structure (Figure 6b) without a three-dimensional network of linked


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RuO6 octahedral chains.38 It should be noted that the spectrum of Ru-WO3 is similar to that of RuO2·nH2O, but the peak at 2.6 Å is considerably weaker compared to the reference RuO2·nH2O samples. The present XAFS results, together with the XPS analysis, indicate that the Ru species adsorbed on WO3 are characterized by essentially the same RuO6 octahedral environments as RuO2·nH2O, even though they are probably present as highly dispersed cluster-like species. The occurrence of such high dispersion is supported by the TEM observations. Figure 7 shows the TEM images of Ru-WO3 and of the samples heated at different temperatures. Ru-WO3 exhibits highly dispersed bright particles with a diameter smaller than 2 nm. The comparison with unmodified WO3 clearly shows that these particles are Ru species (most likely RuO2·nH2O clusters). Although the high dispersion of the particles is mostly retained after heating at 150 °C, calcination at higher temperatures resulted in drastically increased grain sizes, up to ~30 nm.

Figure 8. XP spectra of Ru-WO3 and Ru-WO3 calcined at various temperatures (100–400 °C), along with the spectrum of RuO2.


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Figure 8 shows the XP spectra of the Ru-WO3 samples calcined at various temperatures (100– 400 °C), along with the spectrum of RuO2 as a reference. The Ru 3d5/2 peak position of Ru-WO3 approaches that of anhydrous RuO2 with increasing calcination temperature. The largest shift of this peak is observed from 150 to 200 °C, which is in good agreement with the changes highlighted by the TEM images (Figure 7). Calcination above 250 °C results in the Ru 3d5/2 peak shifting to almost the same binding energy as in anhydrous RuO2, strongly suggesting the complete dehydration to RuO2 on WO3. 3.3. Role of RuO2·nH2O cocatalyst in O2 evolution on WO3. As discussed above, the RuWO3 sample that was found to contain mainly RuO2·nH2O species showed higher activity for O2 evolution in the presence of IO3– than the RuO2-loaded WO3. To clarify the role of RuO2·nH2O species in the photocatalytic process, electrochemical measurements were carried out using electrodes based on Ru-WO3 and on the samples heated at different temperatures. The electrodes were prepared by coating Ru-WO3 particles pre-heated at various temperatures on a conductive FTO substrate. Figure 9a shows linear sweep voltammetry (LSV) profiles for the Ru-WO3 electrodes in an aqueous Na2SO4 solution containing NaIO3. An appreciable cathodic current for the reduction of IO3– was observed when the Ru-WO3 electrodes were employed; the current increased with a negative potential, whereas no cathodic current was detected in the case of the unmodified WO3 electrode. The cathodic current decreases with increasing calcination temperature, indicating that RuO2·nH2O is a more effective cocatalyst for the reduction of IO3– than anhydrous RuO2. The largest decrease is observed between 150 and 200 °C, which is in good agreement with the transformation of RuO2·nH2O to anhydrous RuO2 demonstrated by the TEM and XPS analyses (Figure 7, 8). This result clearly proves that RuO2·nH2O is a more effective catalyst for IO3– reduction compared to conventional RuO2. RuO2·nH2O is a


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Figure 9. (a) LSV profiles for Ru-WO3 electrodes in 0.5 M aqueous Na2SO4 solution containing 5 mM NaIO3. (b) I-V curves under chopped visible light irradiation on the same electrodes in 0.5 M aqueous Na2SO4 solution. (c) Current-potential relationships for the electrodes in Figure 9b.

well-known supercapacitor,31,38,39 which exhibits mixed electron-proton conductivity. The capacitance of non-heat-treated RuO2·nH2O was significantly higher than that of anhydrous RuO2,31,39 even when their different specific surface areas are taken into account. Therefore, one of the reasons for the higher catalytic activity of RuO2·nH2O in the multi-electron reduction of IO3– is its supercapacitance (i.e., its high electron-storage and electron-transport capabilities).


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Figure 9b shows current-voltage curves for the Ru-WO3 electrodes under intermittent irradiation with visible light in aqueous Na2SO4 solution, while Figure 9c shows the current-potential relationships for the WO3 electrodes in Figure 9b. Although simple adsorption of RuO2·nH2O on WO3 does not enhance the anodic photocurrent, the following calcination at higher temperature resulted in higher photocurrent. This result confirms the negligible activity of RuO2·nH2O for water oxidation, as well as the well-known high activity of anhydrous RuO2 for the same purpose.40 As shown above, RuO2·nH2O exhibits a higher activity for reduction of IO3– than anhydrous RuO2, whereas RuO2 exhibits a much higher activity for water oxidation than RuO2·nH2O. Thus, we can conclude that the loading of RuO2·nH2O on WO3 significantly enhances the reduction of IO3–, which is the rate-determining step of the O2 evolution process in the presence of IO3–, and therefore increases the rate of O2 generation. 3.4. Loading of RuO2·nH2O cocatalyst onto various photocatalysts. Recently, we have found that tungstic acid stably and efficiently generates O2 from aqueous solutions containing Fe3+ as electron acceptor, by harvesting a wider range of visible light (up to 500 nm) compared to conventional WO3 (460 nm).22,23 However, in spite of these favorable photoabsorption properties, the available redox couples that can be combined with H2WO4 are limited to Fe3+/Fe2+, which can efficiently operate even without cocatalyst due to the instability of H2WO4 upon calcination above 180 °C. For example, unmodified H2WO4 could not generate O2 in the presence of the IO3– electron acceptor, due to the difficult IO3– reduction through a six-electron process (see Figure 10a). Loading an effective cocatalyst such as PtOx or RuO2 onto H2WO4 by conventional impregnation failed, due to the facile dehydration into WO3 or related compounds


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Figure 10. (a) O2 evolution over bare H2WO4 and Ru-H2WO4 in aqueous NaIO3 solution (5 mM, 250 mL) under visible light irradiation (λ > 400 nm). (b) XP spectra of Ru-H2WO4 before and after reaction.

above 180 °C (see the thermogravimetric and differential thermal analysis (TG-DTA) of H2WO4 in Figure S2). Therefore, the loading of the present RuO2·nH2O cocatalyst, which can be added without any heat treatment and exhibits high activity for IO3– reduction, is expected to enable H2WO4 to generate O2 in the presence of the IO3– acceptor, by harvesting a wider range of visible light than conventional WO3. As shown in Figure 10a, loading the Ru species triggered O2 evolution on H2WO4. The XPS (Figure 10b) and XRD (Figure S3) analysis of Ru-H2WO4 confirmed the presence of RuO2·nH2O species similar to those found on Ru-WO3, without any change in the structure of H2WO4. Importantly, both the XRD pattern and XP spectrum were almost unchanged after the reaction. These findings indicate that RuO2·nH2O works as an effective cocatalyst of thermally unstable H2WO4 for the reduction of IO3–, steadily generating O2 under visible light. We then attempted to construct a Z-scheme water splitting system with an IO3–/I– redox mediator by combining Ru-H2WO4 as O2-evolving photocatalyst with a H2evolving photocatalyst. In the present study, SrTiO3:Rh was used as a H2-evolving


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photocatalyst.30 We also confirmed that SrTiO3:Rh loaded with Pt cocatalysts can generate H2 from aqueous solution containing I– as an electron donor under both UV and visible light irradiation.21 As shown in Figure 11, H2 and O2 were produced in a 2:1 stoichiometric ratio for a

Figure 11. Time courses of H2 and O2 evolution using a mixture of Pt-SrTiO3:Rh (0.1 g) and RuH2WO4 (0.5 g) in an aqueous NaI solution (0.5 mM, 250 mL) under visible light irradiation (λ > 400 nm, Xe lamp).

long irradiation time. The present results thus represent the first demonstration of a visible lightdriven Z-scheme water splitting system using tungstic acid as an O2-evolving photocatalyst in the presence of IO3–/I– redox mediators. The present Ru-based cocatalyst, which can be loaded onto the photocatalyst without any heat treatment, is expected to be applicable to other thermally unstable photocatalyst materials such as Ta3N5. Ta3N5 is an attractive material with a narrow band gap of 2.1 eV (absorption edge at 600 nm) and appropriate band levels for water reduction and oxidation: its conduction band minimum and valence band maximum are -0.4 and 1.7 V vs.


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NHE, respectively.17 However, loading of metal oxides such as PtOx and RuO2 onto Ta3N5 via conventional impregnation methods involving calcination in air resulted in the transformation of Ta3N5 into the corresponding oxynitride (or oxide). Thus, loading Ru species onto Ta3N5 was also attempted using the heating-free adsorption method. Figure 12a shows the time courses of O2 evolution over unmodified Ta3N5 and Ru-Ta3N5 samples. Ru-Ta3N5 generated appreciable amounts of O2, whereas no O2 was generated from unmodified Ta3N5. The Ru species loaded onto Ta3N5 were also confirmed to be RuO2·nH2O species, and remained stable during the reaction, as shown in Figure 12b. Although the loading of conventional RuO2 by air calcination at 300 °C also triggered O2 evolution, the rate of O2 evolution was much lower than that of RuTa3N5. This is due to the partial oxidation of the Ta3N5 surface and/or the lower activity for IO3– reduction resulting from the transformation of RuO2·nH2O into anhydrous RuO2. The partial oxidation was confirmed by the XPS analysis shown in Figure S4, which reveals that the intensity of the N 1s peak of Ta3N5 decreased significantly after calcination of Ru-Ta3N5.

Figure 12. (a) O2 evolution over bare Ta3N5, Ru-Ta3N5, and Ru-Ta3N5 calcined at 300 °C in NaIO3 aqueous solution (5 mM, 250 mL) under UV-visible irradiation (λ > 300 nm). (b) XP spectra of Ru-Ta3N5 before and after reaction.


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Although loading Ru species enabled Ta3N5 to generate O2, the rate of O2 evolution was still low, probably due to the slow water oxidation caused by a more negative valence band maximum compared to that of the oxides (which corresponds to a smaller driving force for water oxidation). Co-loading of cocatalysts for water oxidation such as Co species41,42 is expected to improve the rate of O2 evolution on Ru-Ta3N5.

4. CONCLUSION In summary, a novel RuO2·nH2O cocatalyst was developed as a stable and more effective surface modifier for IO3– reduction compared to conventional anhydrous RuO2. The RuO2·nH2O species were loaded on various photocatalyst materials such as WO3 via simple adsorption without heating. The WO3 sample loaded with the RuO2·nH2O species exhibited much higher rates of O2 evolution in the presence of IO3– than conventional RuO2, due to the enhanced IO3– reduction on the RuO2·nH2O species. Loading of the Ru species also enabled H2WO4 and Ta3N5 to generate O2 in the presence of IO3–. H2WO4 loaded with the RuO2·nH2O cocatalyst was, for the first time, demonstrated to function as an O2-evolving photocatalyst in Z-scheme system under visible light irradiation in the presence of IO3–/I– redox mediators. Although non-oxide and mixed-anion compounds such as (oxy)nitrides, (oxy)sulfides, and oxyhalides have recently been proposed as promising photocatalysts due to their more negative valence band maxima (i.e., narrower band gaps) than conventional oxides, they exhibit relatively low stability against calcination in air. Due to the associated difficulty of loading effective cocatalysts on such compounds without heating, there have been only a few reports describing Z-scheme water splitting systems with IO3–/I– redox mediators in which these materials are used as O2-evolving photocatalysts. Loading


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the RuO2·nH2O cocatalyst onto these photocatalysts will enable them to effectively trigger O2 generation in the presence of IO3–. Furthermore, co-loading cocatalysts for water oxidation is expected to improve the rate of O2 evolution on these photocatalysts, which have smaller driving forces for water oxidation compared to conventional oxides. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. O2 evolution over Ru-WO3 under visible light, TG-DTA profiles of H2WO4, XRD patterns of H2WO4 samples, XP spectra of Ta3N5 samples (PDF) AUTHOR INFORMATION Corresponding Author *R.A.: tel, +81-75-383-2478; fax, +81-75-383-2479; e-mail, [email protected] ACKNOWLEDGMENT This work was financially supported by JST-CREST project and JSPS Grant-in-Aid for Scientific Research (b) (Grant Number 15H03849) and for JSPS Research Fellow (Grant Number 16J11397). The authors are also indebted to the technical division of Institute for Catalysis, Hokkaido University for their help in building the experimental equipment.

REFERENCES (1) Domen, K.; Kondo, J. N.; Hara, M.; Takata, T. Bull. Chem. Soc. Jpn. 2000, 73, 1307-1331. (2) Osterloh, F. E. Chem. Mater. 2008, 20, 35-54.


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(3) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253-278. (4) Inoue, Y. Energy Environ. Sci. 2009, 2, 364-386. (5) Kitano, M.; Hara, M. J. Mater. Chem. 2010, 20, 627-641. (6) Abe, R. Bull. Chem. Soc. Jpn. 2011, 84, 1000-1030. (7) Maeda, K. J. Photochem. Photobiol. C 2011, 12, 237-268. (8) Abe, R. J. Photochem. Photobiol. C 2010, 11, 179-209. (9) Abe, R.; Sayama, K.; Domen, K.; Arakawa, H. Chem. Phys. Lett. 2001, 344, 339-344. (10) Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. Chem. Commun. 2001, 2416-2417. (11) Kato, H.; Hori, M.; Konta, R.; Shimodaira, Y.; Kudo, A. Chem. Lett. 2004, 33, 1348-1349. (12) Kato, H.; Sasaki, Y.; Iwase, A.; Kudo, A. Bull. Chem. Soc. Jpn. 2007, 80, 2457-2464. (13) Abe, R.; Sayama, K.; Sugihara, H. J. Phys. Chem. B 2005, 109, 16052-16061. (14) Abe, R.; Higashi, M.; Domen, K. ChemSuschem 2011, 4, 228-237. (15) Higashi, M.; Abe, R.; Ishikawa, A.; Takata, T.; Ohtani, B.; Domen, K. Chem. Lett. 2008, 37, 138-139. (16) Higashi, M.; Abe, R.; Takata, T.; Domen, K. Chem. Mater. 2009, 21, 1543-1549. (17) Tabata, M.; Maeda, K.; Higashi, M.; Lu, D. L.; Takata, T.; Abe, R.; Domen, K. Langmuir 2010, 26, 9161-9165.


22

Page 23 of 25

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(18) Maeda, K.; Lu, D. L.; Domen, K. ACS Catal. 2013, 3, 1026-1033. (19) Zhao, W.; Maeda, K.; Zhang, F. X.; Hisatomi, T.; Domen, K. Phys. Chem. Chem. Phys. 2014, 16, 12051-12056. (20) Martin, D. J.; Reardon, P. J. T.; Moniz, S. J. A.; Tang, J. W. J. Am. Chem. Soc. 2014, 136, 12568-12571. (21) Suzuki, H.; Tomita, O.; Higashi, M.; Abe, R. Catal. Sci. Technol. 2015, 5, 2640-2648. (22) Suzuki, H.; Tomita, O.; Higashi, M.; Abe, R. Chem. Lett. 2015, 44, 1134-1136. (23) Suzuki, H.; Tomita, O.; Higashi, M.; Abe, R. J. Mater. Chem. A 2017, DOI: 10.1039/C7TA02983G. (24) Fujito, H.; Kunioku, H.; Kato, D.; Suzuki, H.; Higashi, M.; Kageyama, H.; Abe, R. J. Am. Chem. Soc. 2016, 138, 2082-2085. (25) Zhang, F. X.; Yamakata, A.; Maeda, K.; Moriya, Y.; Takata, T.; Kubota, J.; Teshima, K.; Oishi, S.; Domen, K. J. Am. Chem. Soc. 2012, 134, 8348-8351. (26) Zhang, K. L.; Liu, C. M.; Huang, F. Q.; Zheng, C.; Wang, W. D. Appl. Catal., B 2006, 68, 125-129. (27) Hisatomi, T.; Katayama, C.; Moriya, Y.; Minegishi, T.; Katayama, M.; Nishiyama, H.; Yamada, T.; Domen, K. Energy Environ. Sci. 2013, 6, 3595-3599. (28) Higashi, M.; Abe, R.; Teramura, K.; Takata, T.; Ohtani, B.; Domen, K. Chem. Phys. Lett. 2008, 452, 120-123.


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Page 24 of 25

(29) Hitoki, G.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. Chem. Lett. 2002, 736-737. (30) Konta, R.; Ishii, T.; Kato, H.; Kudo, A. J. Phys. Chem. B 2004, 108, 8992-8995. (31) Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6-L8. (32) Darwent, J. R.; Mills, A. J. Chem. Soc., Faraday Trans. 2 1982, 78, 359-367. (33) Erbs, W.; Desilvestro, J.; Borgarello, E.; Gratzel, M. J. Phys. Chem. 1984, 88, 4001-4006. (34) Ohno, T.; Tanigawa, F.; Fujihara, K.; Izumi, S.; Matsumura, M. J. Photochem. Photobiol. A 1998, 118, 41-44. (35) Maeda, K.; Abe, R.; Domen, K. J. Phys. Chem. C 2011, 115, 3057-3064. (36) Arico, A. S.; Baglio, V.; Di Blasi, A.; Modica, E.; Antonucci, P. L.; Antonucci, V. J. Electroanal. Chem. 2003, 557, 167-176. (37) Rolison, D. R.; Hagans, P. L.; Swider, K. E.; Long, J. W. Langmuir 1999, 15, 774-779. (38) McKeown, D. A.; Hagans, P. L.; Carette, L. P. L.; Russell, A. E.; Swider, K. E.; Rolison, D. R. J. Phys. Chem. B 1999, 103, 4825-4832. (39) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699-2703. (40) Trasatti, S. J. Electroanal. Chem. 1980, 111, 125-131. (41) Kanan, M. W.; Nocera, D. G. Science 2008, 321, 1072-1075. (42) Zhang, G. G.; Zang, S. H.; Wang, X. C. ACS Catal. 2015, 5, 941-947.


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Highly Dispersed RuO2 Hydrates Prepared via Simple Adsorption as

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