O2

Subscriber access provided by ORTA DOGU TEKNIK UNIVERSITESI KUTUPHANESI

Article 2

2

Photocatalytic Z-Scheme Water Splitting for Independent H/O Production via a Stepwise Operation Employing a Vanadate Redox Mediator under Visible Light Yugo Miseki, Satoshi Fujiyoshi, Takahiro Gunji, and Kazuhiro Sayama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00905 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Photocatalytic Z-scheme Water Splitting for Independent H2/O2 Production via a Stepwise Operation Employing a Vanadate Redox Mediator under Visible Light Yugo Miseki,*,† Satoshi Fujiyoshi,†,‡ Takahiro Gunji,†,‡ Kazuhiro Sayama*,† †Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. ‡Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yama-zaki, Noda, Chiba 278-8510, Japan.

ABSTRACT: Photocatalytic Z-scheme water splitting is a promising technology for solar energy conversion and storage, as it allows the individual production of H2 and O2. However, this method is generally hindered by undesirable backward reactions. We herein report the use of a novel VO2+/VO2+ redox mediator to reveal that Fe-H-Cs-WO3 and Ru/SrTiO3:Rh photocatalyst surfaces exhibit selectivity toward O2 and H2 evolution reactions, respectively. The obtained hydrogen evolution rate of Ru/SrTiO3:Rh using VO2+ redox was 2-fold higher than those

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

obtained using conventional redox reagents (i.e., Fe2+ or I− ions). Z-scheme water splitting by a (Ru/SrTiO3:Rh)–(Fe-H-Cs-WO3)–(VO2+/VO2+) system was demonstrated both in the stepwise operation and in the conventional one-pot operation. The stepwise operation to produce H2 and O2 separately exhibited enhanced performance compared to the conventional one-pot operation. Finally, the solar to hydrogen energy conversion efficiency for the stepwise process using simulated solar irradiation was estimated to be 0.06%, which was demonstrated by the separated production of H2 and O2 for the first time.

Introduction Photocatalysis using powdered semiconductors has been extensively studied for application in solar energy conversion and storage.1–7 To harness solar energy efficiently, it is important to utilize a wide spectrum of visible light. The Z-scheme type water splitting reaction8 that mimics natural photosynthesis has attracted extensive attention because it allows restrictions on the conduction and valence band potentials of semiconductor photocatalysts to be widened (Figure 1). We previously reported the water splitting reaction via Z-scheme system under visible light9 using suspended particles of Pt-loaded SrTiO3:Cr/Ta10 as a H2 evolution photocatalyst (HEP), PtOx-loaded WO3 as an O2 evolution photocatalyst (OEP), and IO3−/I− as a redox mediator. Recently, various semiconductor photocatalysts have been reported, including oxides, (oxy)nitrides, and dye-sensitized oxides, possessing either water reduction or oxidation abilities.4,6 In contrast, variation of the redox mediators (IO3−/I−, Fe3+/2+, and [Co(bby)3]3+/2+) has been limited, and as such, the development of novel redox mediators is of particular interest to increase the number of potential combinations, and to improve activity.


2

Page 3 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In addition, the Z-scheme system has the unique advantage that H2 can be evolved separately from O2, and this is a particularly important feature for practical use. For example, Lee et al. reported the individual production of H2 and O2 via a stepwise process using Pt-loaded SrTiO3:Rh, WO3, and Fe3+/2+ as the HEP, OEP, and redox mediator, respectively.11 Furthermore, Kudo et al. separately obtained H2 and O2 via a dual bed operation using a two-compartment cell separated by a membrane filter.12 However, the performances of these systems were poor compared to the conventional one-pot mixed photocatalyst route. This low efficiency could be accounted for by the presence of undesirable backward reactions over the HEP and OEP surfaces (Figure S1(a)(b)). In the case of the HEP, it is necessary that the selective water reduction and oxidation of the redox reductant (Red) (shown as the solid line in Figure 1) are conducted. However, as the redox oxidant (Ox) concentration increases, the HEP performance decreases as Ox reduction (shown as the dashed line in Figure 1) proceeds preferentially over water reduction due to thermodynamic advantages. In terms of the OEP, Red oxidation, which is an undesirable reaction, proceeds easily upon increasing the Red concentration, resulting in an apparent reduction in OEP performance. Here, the equilibrium state of the Red/Ox ratio during the Z-scheme reaction is determined by the balance of the HEP and OEP performances, and is based on the influence of the backward reactions (shown as the intersection point in Figure S1). It is therefore important to develop an OEP/HEP combination that allows the efficient suppression of the undesirable reactions (Figure S1(c)). Moreover, as shown in Figures S1(a) and S1(b), the OEP and HEP performances are particularly sensitive to the Ox/Red ratio at equilibrium during the Z-scheme reaction. Therefore, the photocatalytic performances of spatially or temporally separated water splitting systems, such as stepwise and dual-bed operations, may be low compared to one-pot


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 32

operations, although it is advantageous for stepwise and dual bed operations that H2 and O2 can be produced separately. As such, it is desirable to construct an efficient Z-scheme water splitting system via not only a one-pot but also a stepwise or dual-bed operation, where the combination of the OEP and HEP can efficiently suppress the undesirable reactions. We recently reported that V5+ ions can function as an excellent electron acceptor for water oxidation over a surface modified WO3 photocatalyst (OEP).13 Thus, we herein chose to investigate the photocatalytic O2 evolution performances and the influence of the undesirable reaction (i.e., V4+ oxidation by photogenerated holes) over various OEPs using V5+ as an electron acceptor. In addition, we assess the H2 evolution performances and the influence of the backward reaction (i.e., V5+ reduction by photogenerated electrons) over various HEPs using V4+ as an electron donor. Moreover, we aim to demonstrate the highly efficient individual production of H2 and O2 under solar simulated light in a stepwise operation for the first time. Experimental Section Commercial rutile phase TiO2 (Kojundo Chemical Laboratory Co., Ltd.; 99.9%), anatase phase TiO2 (Kojundo Chemical Laboratory Co., Ltd.; 99.9%), and WO3 (Kojundo Chemical Laboratory Co., Ltd.; 99.99%) were employed as photocatalysts without any treatment. SrTiO3,14 Rh (1 mol%) doped SrTiO3,14 Cr (4 mol%) and Ta (4 mol%) co-doped SrTiO3,9 Fe-H-Cs-WO3,15 and BiVO416 powdered photocatalysts were prepared via either solid-state reactions or liquidsolid phase reactions as reported in the literature. The atomic ratios of Sr/Ti/dopant in the SrTiO3, Rh (1 mol%) doped SrTiO3, and Cr (4 mol%) and Ta (4 mol%) co-doped SrTiO3 were 1.03/1.00/0.00, 1.03/0.99/0.01, and 1.03/0.92/0.08, respectively. Pt (0.3 wt%) and Ru (0.2 wt%) co-catalysts for the H2 evolution photocatalysts were introduced via a photodeposition method in a 10 vol% aqueous MeOH solution (Wako pure Chemical Industries, Ltd.; 99.8%) containing


4

Page 5 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2PtCl6·6H2O (Wako pure Chemical Industries, Ltd.; >98.5%) or RuCl3·nH2O (Wako pure Chemical Industries, Ltd.; 99.9%), as reported previously.12 The resulting powders were filtered using membrane filter (pore size: 0.2 µm), washed with distilled water, and dried at 343 K for 2 h. Solutions of the V5+ (VO2+ and H2VO4−) and V4+ (VO2+ and HV2O5−) redox species were prepared by the dissolution of 1.09 g of V2O5 (Kanto Chemical Co. Inc.; 99.0%) in 100 mL of 0.9 M H2SO4 (Wako pure Chemical Industries, Ltd.; 95%) and 0.133 g of VOSO4·3.25H2O (Wako pure Chemical Industries, Ltd.; 99.9%) in 300 mL of distilled water, respectively. Finally, the pH values of these solutions were adjusted using 1M H2SO4 and KOH (Wako pure Chemical Industries, Ltd.; 85%). The phase purity of the resulting powder was confirmed by X-ray diffraction (PANalytical, EMPYREAN). Quantitative analyses of V4+ ions in the reactant solutions were conducted using a UV-Visible/NIR spectrophotometer (JASCO, V-730). The hydration number of VOSO4 was estimated via TG analysis (Rigaku, Thermo plus TG-8120). Photoelectrochemical measurements for estimation of the redox potential were performed using a potentiostat (BAS Inc., ALS660B) and a Pyrex glass cell. A Pt wire and Pt-loaded carbon felt electrodes were employed as the counter and working electrodes, respectively. Finally, a Ag/AgCl (3 M NaCl) electrode was used as the reference electrode, and the potential conversion from Ag/AgCl to the normal hydrogen electrode (NHE) (pH 0) was conducted according to Equation (1):

Potential NHE (pH 0) = Potential Ag/AgCl + 0.195 + 0.059 pH (1)

Photocatalytic reactions were performed in a Pyrex side-window cell connected to a gas-closed circulation system using a 300 W Xe illuminator (ILC Technology, Inc., CERMAX LX-300) as


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 32

shown in Figure S2. Incident light was controlled using an L42 cut-off filter (HOYA Corporation) where necessary, and it’s photon fluxes from 350 nm to 550 nm were estimated to be 71 mWcm−2 (without filter) and 60 mWcm−2 (with L 42 filter) using a spectroradiometer (SOMA Optics, Ltd.). Using a magnetic stirrer, the photocatalyst powder (50–150 mg) was dispersed in an aqueous solution (300 mL) of either the 2 mM V5+ or 2 mM V4+ redox solution. The quantity of evolved O2 was determined via on-line gas chromatography (Shimadzu Corporation), equipped with an MS-5A column and a thermal conductivity detector (TCD), with an Ar carrier. The quantity of V4+ ions produced was determined by examination of the absorption band at 766 nm. Errors on all photocatalytic data was less than 5%. The apparent quantum yield (AQY) was evaluated using a 420 nm monochromatic light source and calculated according to Equation (2):

AQY(%) =100 × [Number of reacted electrons] /[Number of incident photons] = 100 × [Number of products × A] /[Number of incident photons]

(2)

A = 2 (hydrogen), or 4 (oxygen).

The number of incident photons was determined using a Si photodiode that was proofread by the NMIJ (National Metrology Institute of Japan). The solar energy conversion efficiency was determined using Equation (3)12 and a solar simulator (San-Ei Electric Co., Ltd., XES-151S) calibrated to AM 1.5 (1 sun, 100 mWcm−2) using a spectroradiometer (SOMA Optics, Ltd.).


6

Page 7 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Solar-energy conversion efficiency (%) =100 × [Output energy as H2 / J s−1] /[Energy of incident solar light / W] =100 × [Gibbs free energy change, ∆G0298 / J mol−1] × [Rate of H2 evolution / mol s−1] /[Energy of incident solar light / W]

(3)

Results and discussion Photocatalytic water oxidation to O2 over various OEPs Table 1 presents the photocatalytic O2 evolution rates over TiO2 (rutile), SrTiO3, WO3, BiVO4, and Fe-H-Cs-WO3 in an aqueous 2 mM V5+ solution. As shown, O2 evolution was observed on all photocatalysts under light irradiation. Blank reactions conducted in the absence of either vanadium, photocatalyst powder, or light irradiation resulted in zero O2 evolution. In addition, the O2 evolution rates of TiO2 and Fe-H-Cs-WO3 decreased upon increasing the initial pH. Indeed, it has been reported that the structure of the V5+ complex is highly dependent on the solution pH (i.e., pH 0–3 = VO2+, pH 3–9 = H2VO4−, and pH 9–13 = HVO42−).17 Moreover, the standard V5+/V4+ redox potential has been reported to vary with changes in the V5+ complex structure (VO2+/VO2+ = 1.0 V and H2VO4−/HV2O5− = 0.7 V, vs. NHE, pH 0).18 Figure 2 shows the cyclic voltammetry curves obtained at the Pt/C-electrode in aqueous 2 mM V4+ solutions of pH 1.7 and 3.8, resulting in different redox potentials depending on the pH value. According to Equation (1), the redox potentials of the V5+/4+ ions at pH 1.7 and pH 3.8 were +1.0 V and +0.7 V vs. NHE (pH 0), respectively. Thus, the V5+ ions were therefore found to exist as VO2+ ions at low pH values, which were efficiently reduced to VO2+ ions by photocatalysis. Figure 3 shows


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 32

the O2 evolution with time over various OEPs. The pH value was unchanged during reaction test, because the quantity of proton consumption (2 mM) according to Equation (6) was ignorable under pH1.3 condition (50 mM). Photocatalytic O2 evolution proceeded steadily on both SrTiO3 (plot d) and Fe-H-Cs-WO3 (plot e) until the total quantity of evolved O2 reached 150 µmol, which matches the stoichiometric quantity expected from the presence of 600 µmol VO2+ in the reaction solution. Furthermore, following the reaction, the color of the reactant solution changed from yellow to blue, with a new absorption band at 766 nm correlating to the newly formed VO2+ ion (Figure 4). The stoichiometric quantity of VO2+ detected in the solution following the photoreaction confirms that VO2+ and O2 were photocatalytically produced in a 4:1 ratio according to Equations (4–6).

(Electrons)

4VO2+ + 8H+ + 4e− → 4VO2+ + 4H2O

(4)

(Holes)

2H2O + 4h+ → O2 + 4H+

(5)

(Total)

4VO2+ + 4H+ → 4VO2+ + O2 + 2H2O

(6)

The reaction rates of WO3 (plot a), BiVO4 (plot b), and TiO2 (plot c) decreased gradually over time (Figure 3). Table 2 shows the influence of the coexisting VO2+ ion in the VO2+ solution on the photocatalytic activity for water oxidation into O2 over WO3 and Fe-H-Cs-WO3. As indicated, the activity of pristine WO3 dropped significantly in the presence of VO2+, indicating that the VO2+ ions were oxidized preferentially over water. In contrast, the Fe-H-Cs-WO3 photocatalytic activity remained constant in the presence of VO2+ ions. This suggested that surface modification suppressed the undesirable reaction between the photogenerated holes and the VO2+ ions. These results are particularly important, as such photocatalysts exhibiting excellent selectivity are ideal


8

Page 9 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for Z-scheme photocatalysis using a redox mediator. Matsumura et al. have revealed that the iron(III) ions adsorbed preferentially on TiO2 over iron(II) ions, which enabled efficient oxidation of water, although this reaction was thermodynamically less favorable than oxidation of iron(II) ions.19 However, in the case of VO2+/VO2+ redox, both VO2+ and VO2+ ions were hardly adsorbed on the WO3 and Fe-H-Cs-WO3 surfaces, respectively. This result indicates that Fe-H-Cs-WO3 photocatalyst has another surface property to overturn a thermodynamic disadvantage. We previously reported that water oxidation reactions using Fe3+ or IO3- ions also proceed with excellent selectivities and experimentally concluded that a H3O+-incorporated surface site on the Fe-H-Cs-WO3 functions as an excellent water oxidation site.15 Although, the detail reaction mechanism has remained to be clarified, we have considered that water is selectively oxidized due to strongly incorporated as H3O+ on the surface of Fe-H-Cs-WO3 photocatalyst, and that this selective water oxidation may be much faster than the unfavorable reactions. Finally, the apparent quantum yield for water oxidation using VO2+ over Fe-H-CsWO3 was estimated to be 22% (at 420 nm). Photocatalytic water reduction to H2 over various HEPs Table 3 presents the H2 evolution rates obtained using various HEPs and an aqueous solution of V4+ ions. Pt/TiO2 (anatase), Pt/SrTiO3, and Pt/SrTiO3:Cr,Ta exhibited negligible activity under the conditions examined, although these materials showed the H2 evolution activity using methanol as a sacrificial reagent (Table S1). In contrast, Pt- or Ru-loaded SrTiO3:Rh photocatalysts yielded relatively high rates of H2 production under visible light irradiation. In addition, although H2 evolution was observed at all pH values examined (i.e., pH 1.3–11), the optimal performance was obtained under mildly acidic conditions (i.e., pH 3.8).


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 32

Figure 5 shows the variation in H2 evolution with time over Ru-loaded SrTiO3:Rh in the presence of redox reductants (i.e., VO2+/VO2+, IO3−/I−, and Fe3+/2+), where the highest activity was observed in the presence of VO2+ ions. The pH value was also unchanged during reaction test, despite the proton concentration have to be changed from pH2.4 condition (4 mM) to pH 2.2 condition (6 mM) because the quantity of proton generation (2 mM) according to Equation (9). It has been reported that the pH value is changed during Z-scheme reaction using Ru/SrTiO3:Rh.20 This behavior might be caused by partially Sr2+ dissolution from Ru/SrTiO3:Rh photocatalysts. In this case, H2 evolution was rapid until a stoichiometric quantity (i.e., 300 µmol) of H2 had been produced (calculated based on the presence of 600 µmol VO2+ in the solution). Furthermore, the absorption peak corresponding to the VO2+ ion at 766nm disappeared following this photoreaction, indicating that VO2+ and H2 were produced photocatalytically in a 2:1 ratio according to Equations (7–9).

(Electrons)

2H+ + 2e−

(Holes)

2VO2+ + 2H2O + 2h+ → 2VO2+ + 4H+

(8)

(Total)

2VO2+ + 2H2O → 2VO2+ + H2 + 2H+

(9)

→ H2

(7)

Thus, based on Equation (2), the apparent quantum yield of Ru/SrTiO3:Rh was calculated to reach 4% at 420 nm. Interestingly, the activity of Ru/SrTiO3:Rh was not affected by the concentration of VO2+ ions (Table 4), suggesting that the undesirable reaction between the photogenerated electrons and the VO2+ ions was suppressed over the Ru/SrTiO3:Rh surface. It should be noted that although the undoped SrTiO3 exhibited excellent performance as an OEP using VO2+ ions (Figure 3(d)), it did not exhibit activity as an HEP (Table 3). This indicates that


10

Page 11 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pristine SrTiO3 has an active site for VO2+ reduction. In contrast, the excellent performance of Ru/SrTiO3:Rh as an HEP in the presence of VO2+ ions indicates that this material contains active VO2+ oxidation sites, but no VO2+ reduction sites. As previously reported, Rh species doped in to the SrTiO3 contributed not only to the formation of the electron donor level in the forbidden band for visible light response but also the construction of the active site for oxidation of redox reductant ([Co(bby)3]2+ or [Co(phen)3]2+).12 Therefore, it was found that Rh species can also function as excellent VO2+ oxidation sites, resulting in ideal reaction selectivity towards H2 evolution in the presence of a VO2+/VO2+ redox mediator. This observation is particularly interesting in the context of HEP catalyst design. Z-scheme water splitting combining of HEP and OEP Figure 6 shows the progress of the Z-scheme water splitting operation over time in the presence of a (Ru/SrTiO3:Rh)–(Fe-H-Cs-WO3)–(VO2+/VO2+) system. This process was carried out via a one-pot operation under visible light irradiation, and the transformations taking place can be summarized in the following equations:

(HEP)

4VO2+ + 4H2O → 4VO2+ + 2H2 + 4H+

(9)×2

(OEP)

4VO2+ + 4H+ → 4VO2+ + O2 + 2H2O

(6)

(Total)

2H2O

→ 2H2 + O2

(10)

In the initial stages of the reaction, H2 production exceeded the predicted stoichiometric ratio (H2/O2 = 2) when the reaction was conducted in an aqueous VO2+ solution (Figure 6(a)), while excess O2 was produced using the VO2+ solution (Figure 6(b)). However, with time, the H2/O2 ratios for both situations gradually adjusted towards the stoichiometric ratio, and the overall


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

water splitting process was completed with a relatively high efficiency (H2: 50 µmol h−1 and O2: 25 µmol h−1) after 5 h in both cases. We could therefore conclude that the vanadium redox ion (VO2+/VO2+) had successfully been employed as an excellent redox mediator for Z-scheme type water splitting reactions under visible light for the first time. The reaction rate of the Z-scheme water splitting using Fe2+ ion as a conventional redox mediator is comparable rate with that using VO2+/VO2+ redox (Figure S3) despite the hydrogen evolution rate using VO2+ is higher than that using Fe2+ as shown in the Figure 5. There is therefore a possibility that negative interactions may be taking place in the only VO2+/VO2+ redox system through the physical mixture of the HEP and the OEP. An example of such negative interactions include the direct recombination of e− on the conduction band of Ru/SrTiO3:Rh and h+ on the valence band of Fe-H-Cs-WO3. As shown in Figures 3 and 5, Fe-H-Cs-WO3 and Ru/SrTiO3:Rh particle surfaces exhibited selectivity towards the O2 and H2 evolution reactions, respectively. We therefore decided to evaluate the overall water splitting process via a stepwise operation using an aqueous VO2+ solution to produce H2 and O2 separately (Figure 7). Initially, H2 was produced rapidly until the total H2 evolved reached 300 µmol, and all VO2+ ions had been completely oxidized to VO2+. Subsequently, the Ru/SrTiO3:Rh powder was removed from the reaction solution via suction filtration, and the recovered solution was re-used for the O2 evolution reaction over Fe-H-CsWO3. As a result, 150 µmol of O2 was produced, and all VO2+ ions were reduced to VO2+. Over a second operation, H2 and O2 were produced rapidly once again. The reaction rates for this stepwise operation were estimated as 97 µmol h−1 and 124 µmol h−1 for H2 and O2, respectively, with these values correlating with the results shown in Figures 3(e) and 5(c). Upon comparison of the gas evolution rates for the one-pot operation (H2: 50 µmol h−1 and O2: 25 µmol h−1) with


12

Page 13 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


those of the stepwise operation (H2: 97 µmol h−1 and O2: 124 µmol h−1), it was concluded that the stepwise operation was superior where the same reactor and lamp were employed, with the activity of the stepwise process being 1.4 times higher than that of the one- pot process (Figure S4). However, these results contradict the expectation that the photocatalytic performance of the stepwise operation would be lower than one pot process due to the inefficient use of irradiated light, even upon overlap of the HEP and OEP absorption spectra in the combination of Ru/SrTiO3:Rh and Fe-H-Cs-WO3 (Figure S5). This result also indicates that there is a possibility that negative interactions may be taking place in the VO2+/VO2+ system through the physical mixture of the HEP and the OEP. Examples of such negative interactions include the direct recombination of e− on the conduction band of Ru/SrTiO3:Rh and h+ on the valence band of FeH-Cs-WO3, or a strong shielding effect of irradiated light by Ru/SrTiO3:Rh due to the presence of the inactive absorption bands over a wide range of wavelengths.14 Finally, Figure 8 shows the splitting of water into H2 and O2 under simulated solar light onepot (a) and stepwise (b) operations. In the initial stages, H2 was produced rapidly in the one-pot system, and stoichiometric water splitting (H2/O2 = 2) was achieved after 20 h. In this case, the solar to hydrogen energy conversion efficiency was estimated to be 0.03% from the respective reaction rates for H2 (0.4 µmol h−1 cm−2) and O2 (0.2 µmol h−1 cm−2) production (Equation (3)). In contrast, for the stepwise operation, H2 and O2 were produced separately at rates of 0.9 and 2.4 µmol h−1 cm−2, respectively. This gave a superior estimated solar to hydrogen energy conversion efficiency of 0.06% for the stepwise operation This efficiency is relatively high compared to other typical one-pot Z-scheme systems (Ru/SrTiO3:Rh-Ir/CoOx/Ta3N5: 0.013%,21 Pt/BaZrO3BaTaO2N-PtOx/WO3: 0.0067%,22 Ru/SrTiO3:Rh-RuO2/TiO2:Ta/N: 0.02%,23 Ru/SrTiO3:RhBiVO4: 0.06%12, and 0.1%24) despite H2 can be evolved separately from O2. Consequently, this


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 32

stepwise operation employing VO2+/VO2+ redox ions not only allows the efficient individual production of H2 and O2, but also yields an improved photocatalytic performance for the (Ru/SrTiO3:Rh)–(Fe-H-Cs-WO3)–(VO2+/VO2+) system.

Conclusion We herein constructed a photocatalytic water splitting system capable of achieving the efficient independent production of H2 and O2. The primary requirement to achieve this efficient stepwise operation was to suppress the undesirable reactions taking place in the original H2 and O2 evolution photocatalysts. Among the various photocatalysts tested, Fe-H-Cs-WO3 and Ru/SrTiO3:Rh exhibited excellent reaction selectivity for water oxidation and water reduction, respectively, in the presence of a VO2+/VO2+ redox mediator. Initially, the rate limiting process of the Z-scheme type water splitting reaction was H2 evolution, and we successfully demonstrated that the H2 evolution rate from Ru/SrTiO3:Rh was superior to those of conventional redox reagents (Fe2+ or I−). Consequently, we demonstrated highly efficient solar water splitting to give H2 and O2 via not only the conventional one-pot means, but also via a stepwise operation. Indeed, a solar energy conversion efficiency of 0.06% was estimated for the stepwise operation, which was demonstrated for the first time in the context of separated H2 and O2 production. These findings could open new avenues towards the design of highly efficient separated gas production methods based on dual-bed photocatalyst systems with the assistance of a redox mediator. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.


14

Page 15 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The comparison of Z-scheme reaction combining various types of HEP and OEP, Photocatalytic H2 evolution in an aqueous methanol solution over various photocatalysts, Reaction setup for evaluation of photocatalytic performances, water splitting process using a Fe3+ solutions, comparison of (a) one pot and (b) stepwise operations, and diffuse reflection spectra of (a) Fe-HCs-WO3 and (b) SrTiO3:Rh(1%). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by The Funding Program for Next Generation World-Leading Researchers (NEXT Program).

REFERENCES 1.

Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water.

Chem. Mater. 2008, 20, 35-54. 2.

Inoue, Y. Photocatalytic Water Splitting by RuO2-Loaded Metal Oxides and Nitrides

with d(0)- and d(10)-Related Electronic Configurations. Energy Environ. Sci. 2009, 2, 364-386. 3.

Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem.

Soc. Rev. 2009, 38, 253-278.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4.

Page 16 of 32

Abe, R. Recent Progress on Photocatalytic and Photoelectrochemical Water Splitting

under Visible Light Irradiation. J. Photochem. Photobiol. C-Photochem. Rev. 2010, 11, 179-209. 5.

Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. Semiconductor-Based Photocatalytic

Hydrogen Generation. Chem. Rev. 2010, 110, 6503-6570. 6.

Maeda, K. Z-Scheme Water Splitting Using Two Different Semiconductor Photocatalysts.

Acs Catal. 2013, 3, 1486-1503. 7.

Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z. B.; Deutsch, T. G.;

James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H. L.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983-2002. 8.

Bard, A. J. Photoelectrochemistry and Heterogeneous Photocatalysis at Semiconductors.

J. Photochem. 1979, 10, 59-75. 9.

Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. Stoichiometric Water Splitting

into H2 and O2 Using a Mixture of Two Different Photocatalysts and an IO3-/I- Shuttle Redox Mediator under Visible Light Irradiation. Chem. Commun. 2001, 2416-2417. 10. Ishii, T.; Kato, H.; Kudo, A. H2 Evolution from an Aqueous Methanol Solution on SrTiO3 Photocatalysts Codoped with Chromium and Tantalum Ions under Visible Light Irradiation. J. Photochem. Photobiol. A-Chem. 2004, 163, 181-186. 11. Bae, S. W.; Ji, S. M.; Hong, S. J.; Jang, J. W.; Lee, J. S. Photocatalytic Overall Water Splitting with Dual-Bed System under Visible Light Irradiation. Int. J. Hydrogen. Energy 2009, 34, 3243-3249.


16

Page 17 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12. Sasaki, Y.; Kato, H.; Kudo, A. [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ Electron Mediators for Overall Water Splitting under Sunlight Irradiation Using Z-Scheme Photocatalyst System. J. Am. Chem. Soc. 2013, 135, 5441-5449. 13. Miseki, Y.; Kusama, H.; Sayama, K. Photocatalytic Energy Storage over SurfaceModified WO3 Using V5+/V4+ Redox Mediator. Chem. Lett. 2012, 41, 1489-1491. 14. Konta, R.; Ishii, T.; Kato, H.; Kudo, A. Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation. J. Phys. Chem. B 2004, 108, 8992-8995. 15. Miseki, Y.; Sayama, K. High-Efficiency Water Oxidation and Energy Storage Utilizing Various Reversible Redox Mediators under Visible Light over Surface-Modified WO3. RSC Adv. 2014, 4, 8308-8316. 16. Iwase, A.; Kato, H.; Kudo, A. A Simple Preparation Method of Visible-Light-Driven BiVO4 Photocatalysts from Oxide Starting Materials (Bi2O3 and V2O5) and Their Photocatalytic Activities. J. Sol. Energy Eng. 2010, 132 021106. 17. Larson, J. W. Thermochemistry of Vanadium(5+) in Aqueous-Solutions. J. Chem. Eng. Data 1995, 40, 1276-1280. 18. The Chemical Society of Japan. Kagaku Binran, 3rd ed.; Maruzen: Tokyo, 1984, II475. 19. Ohno, T.; Haga, D; Fujihara, K.; Kaizaki, K.; Matsumura, M. Unique Effects of Iron(III) Ions on Photocatalytic and Photoelectrochemical Properties of Titanium Dioxide. J. Phys. Chem. B 1997, 101, 6415-6419.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

20. Sasaki, Y.; Nemoto, K.; Saito, H.; Kudo, A. Solar Water Splitting Using Powdered Photocatalysts Driven by Z-Schematic Interparticle Electron Transfer without an Electron Mediator. J. Phys. Chem. C 2009, 113, 17536-17542. 21. Ma, S. S. K.; Maeda, K.; Hisatomi, T.; Tabata, M.; Kudo, A.; Domen, K. A RedoxMediator-Free Solar-Driven Z-Scheme Water-Splitting System Consisting of Modified Ta3N5 as an Oxygen-Evolution Photocatalyst. Chem. Eur. J. 2013, 19, 7480-7486. 22. Maeda, K.; Lu, D.; Domen, K. Solar-Driven Z-scheme Water Splitting Using Modified BaZrO3-BaTaO2N Solid Solutions as Photocatalysts. ACS catal., 2013, 3, 1206-1033. 23. Nakada, A.; Nishioka, S.; Vequizo, J. J. M.; Muraoka, K.; Kanazawa, T.; Yamakata, A.; Nozawa, S.: Kumagai, H.; Adachi, S.; Ishitani, O.; Maeda, K. Solar-Driven Z-scheme Water Splitting Using Tantalum/Nitrogen Co-doped Rutile Titania Nanorod as an Oxygen Evolution Photocatalyst. J. Mater. Chem. A, 2017, DOI: 10.1039/C6TA10541F. 24. Kato, H.; Sasaki, Y.; Shirakura, N.; Kudo, A. Synthesis of Highly Active RhodiumDoped SrTiO3 Powders in Z-scheme Systems for Visible-Light-Driven Photocatalytic Overall Water Splitting. J. Mater. Chem. A, 2013, 1, 12327-12333.


18

Page 19 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1 Desirable and undesirable reactions that proceed in the two-step watersplitting system.


19


5

2 pH 1.7

1.5

4

pH 3.8

3 Current (mA)

1 Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

0.5 0 -0.5 -1

2 1 0 -1 -2

-1.5 -2 0

0.2 0.4 0.6 0.8

1

1.2

Potential (V vs Ag/AgCl)

-3 -0.2 0 0.2 0.4 0.6 0.8 1 Potential (V vs Ag/AgCl)

Figure 2 Cyclic voltammetry curves obtained at the Pt/C-electrode in an aqueous 2 mM V4+ solution. Scan rate = 5 mV s−1.


20

Page 21 of 32

160 140 O2 evolution (µ µmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(e)

(c)

(d)

120 100 (b)

80 60

(a) 40 20 0 0

1

2

3 Time (h)

4

5

Figure 3 O2 evolution with time over various OEPs: (a) WO3, (b) BiVO4, (c) TiO2, (d) SrTiO3, and (e) Fe-H-Cs-WO3. Photocatalyst = 0.4 g, aqueous 2 mM V5+ solution = 300 mL (pH 1.3), incident light λ >300 nm (c and d) and >420 nm (a, b, and e). The broken line indicates the upper limit of O2 evolution expected stoichiometrically from 600 µmol VO2+.


21


0.05 0.04

(a)

(b)

0.03 Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

0.02 0.01 0 500

600

700

800

900

Wavlength (nm) Figure 4 Absorption bands of the 2 mM (a) VO2+ and (b) VO2+ solution (pH 1.7).


22

Page 23 of 32

300 (a) H2 evolution (µ µmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250 200 150

(b)

100 (c)

50 0 0

1

2

3

4 5 Time (h)

6

7

8

Figure 5 H2 evolution over time using Ru/SrTiO3:Rh in the presence of various reversible redox ions: (a) VO2+/VO2+ (pH 2.4), (b) Fe2+/Fe3+ (pH 2.4), and (c) IO3−/I− (pH 6.5). Photocatalyst = 0.1 g, reactant solution = 2 mM (300 mL), light source = 300 W Xe lamp (λ >420 nm). The broken line indicates the upper limit of H2 evolution expected stoichiometrically from 600 µmol VO2+.


23


Quantity of product (µ µmol)

700

4 (a)

3.5

600 H2

500

3 2.5

400

2 300 1.5 O2

200

H2/O2 ratio

1

100

0.5

0 0

4

8

12

16

20

24

0 28

Time (h)

Quantity of product (µ µmol)

600

4 (b)

500

3.5

H2

3 400

2.5

300

2 O2

200

1.5

H2/O2 ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

1 100

0.5

0 0

4

8

12

16

20

24

0 28

Time (h) Figure 6 Progress of the water splitting process using a one-pot (Ru/SrTiO3:Rh)–(Fe-HCs-WO3)–(VO2+/VO2+) system in aqueous (a) VO2+ and (b) VO2+ solutions under visible light irradiation. Catalyst = 50 mg (Ru/SrTiO3:Rh) and 100 mg (Fe-H-Cs-WO3), reactant solution = 2 mM VO2+ or VO2+ (300 mL), light source = 300 W Xe lamp (λ >420 nm), irradiated area = 21 cm2. The initial pH was adjusted to (a) 2.4 and (b) 1.7.


24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Quantity of product (µ µ mol)

Page 25 of 32

350 H2

300

H2

250 200

O2

O2

150 100 50 0 0

2

4

6 8 10 Time (h)

12

14

16

Figure 7 Progress of H2 and O2 production with time using a stepwise (Ru/SrTiO3:Rh)– (Fe-H-Cs-WO3)–(VO2+/VO2+) system in aqueous VO2+ solution under visible light irradiation. Catalyst = 100 mg (Ru/SrTiO3:Rh) and 200 mg (Fe-H-Cs-WO3), reactant solution = 2 mM VO2+ (300 mL), light source = 300 W Xe lamp (λ >420 nm), irradiated area = 21 cm2. The initial pH was adjusted to 2.4. Each HEP and OEP catalyst was replaced at the points indicated by the broken vertical lines.


25

-2

8

(a)

-2

Page 26 of 32

H2

7 H2

6 5 4

O2

3

O2

2 1 0 0

Quantity of product (µ µmol cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Quantity of product (µ µmol cm )


20

5

10 15 Time (h)

(b)

20

25

H2

15

10 O2 5

0 0

5

10 Time (h)

15

20

Figure 8 Progress of the overall water splitting reaction by a (Ru/SrTiO3:Rh)–(Fe-HCs-WO3)–(VO2+/VO2+) system via (a) a one-pot operation, and (b) a stepwise operation in aqueous VO2+ solution under simulated solar light irradiation. Catalyst = 100 mg (Ru/SrTiO3:Rh) and 200 mg (Fe-H-Cs-WO3), reactant solution = 2 mM VO2+ (300 mL), light source = solar simulator (AM 1.5), irradiated area = 9 cm2. The initial pH was adjusted to 2.4.


26

Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Photocatalytic O2 evolution reaction over various photocatalysts in the presence of V5+ ions Photocatalyst

Initial pH

Incident light O2 evolution rate (µmol h−1) λ (nm)

TiO2

1.7

>300

64

SrTiO3

1.7

>300

66

WO3

1.7

>420

14

Fe-H-Cs-WO3

1.7

>420

120

BiVO4

1.7

>420

29

TiO2

6.5

>300

2

TiO2

10.9

>300

420

110

Fe-H-Cs-WO3

1.3

>420

122

Fe-H-Cs-WO3

2.2

>420

118

Fe-H-Cs-WO3

3.0

>420

112

Fe-H-Cs-WO3

3.8

>420

3

Fe-H-Cs-WO3

7.0

>420

4

Photocatalyst = 0.4 g, aqueous 2 mM V5+ solution = 300 mL, light source = 300 W Xe lamp.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

Table 2 Photocatalytic O2 evolution in the presence of V5+ ions over various photocatalysts O2 evolution rate Photocatalyst

Initial reactant solution

Fe-H-Cs-WO3

2 mM VO2+

120

Fe-H-Cs-WO3

2 mM VO2+ + 1 mM VO2+

121

WO3

2 mM VO2+

15

WO3

2 mM VO2+ + 1 mM VO2+

3

(µmol h−1)

Photocatalyst = 0.4 g, reactant solution = 300 mL, pH = 1.7, light source = 300 W Xe lamp attached to a L42 cut-off filter.


28

Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3 Photocatalytic H2 evolution in the presence of V4+ ions over various photocatalysts Photocatalyst

Initial Incident light H2 evolution rate pH λ (nm) (µmol h−1)

Pt/SrTiO3:Rh

2.1

>420

40

Ru/SrTiO3:Rh

2.1

>420

56

Pt/SrTiO3:Cr,Ta

2.1

>420

300

300

420

15

Ru/SrTiO3:Rh

2.3

>420

107

Ru/SrTiO3:Rh

2.4

>420

103

Ru/SrTiO3:Rh

3

>420

97

Ru/SrTiO3:Rh

3.8

>420

143

Ru/SrTiO3:Rh

6

>420

34

Ru/SrTiO3:Rh

11

>420

30

Photocatalyst = 0.1 g, reactant solution = 2 mM V4+ (300 mL), light source = 300 W Xe lamp.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

Table 4 Effect of VO2+ on the photocatalytic performance of Ru/SrTiO3:Rh as an HEP Initial reactant solution

H2 evolution rate (µmol h−1)

2 mM VO2+

112

2 mM VO2+ + 1 mM VO2+ 115 Photocatalyst = 0.1 g, reactant solution = 300 mL, pH = 2.4, light source = 300 W Xe lamp attached to a L42 cut-off filter.


30

Page 31 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

75x47mm (150 x 150 DPI)


Page 32 of 32

O2

Recommend Documents