Enhancement of Photocatalytic Overall Water Splitting

sweetening of Photocatalytic Overall Water SplittingEnhancement of photocatalytic boilers suit pissing separate on H1.9K0.3La0.5Bi0.1Ta2O7 by lode Pt, Rh(oxide) as co-catalystsWei Chen, Hui Wang, Liqun Mao, Xiaoping Chen, Wenfeng ShangguanAbstractPt and Rh were cockeyed on protonated layered pervoskite H1.9K0.3La0.5Bi0.1Ta2O7 by an in suit photodeposition method. The Rh loading facilitates the O2 evolution and the H2/O2 is tightlipped to stoichiometric ratio ( 2). Yet the Pt loading is un equal to(p) to improve the O2 evolution, although the photocatalytic operation for H2 evolution was fired app arently. The antithetical effect of Pt and Rh on phtocatalytic natural action for H2 or O2 evolution is due to the chemical states of Pt and Rh on HKLBT. The Pt particles loaded on HKLBT by photodepositon are Pt0, execution as action at law sites for H2 evolution. However, the Rh particles loaded on HKLBT are not all Rh0, but also RhO2, which provide the activity sites for O2 e volution by means of reducing the all overpotential of O2 evolution.1 IntroductionPhotocatalytic weewee system splitting has attracted some(prenominal) attention in the past decades for producing clean and renewable hydrogen naught1-3. The photocatalytic water splitting contains both water reduction process(for H2) and water oxidisation(for O2) process. Currently, the water oxidation process is consider as a key bottleneck in photocatalytic reaction4. The difficulty of water oxidation is due to the high overpotential of O2 evolution, which is attributed to charge lodging by emerge states and slow oxygen evolution kinetics5-7. This is also explaining why some photocatalytic reactions in pure water only produce H2 without O2 (or H2/O22). Therefore, providing the activity sites for oxygen evolution to reduce the overprotential is a key task on overall water splitting. It has reported that co-catalysts such as NiO8, RuO29, Mn2O310 loading on photocatalysts play an very important role on achieving overall water splitting by mean of providing the activity sites or inhibiting the occurrence of retracted reaction. However, almost all of admixturelic element oxides loading on photocatalytic materials bespeak heat-treatment, which is insalubrious to some materials with less heat-resistant, such as protonated layered materials11, metal (oxy)sulfide12 and metal (oxy)nitride13, etc. Recently, Kudo et al14 loaded PbO2 from Pb2+ by a photodeposition method without heat-treatment, indicating that oxides also are loaded by oxidation of photogenerated holes, which will provide us with a feasible way to load metal oxides on photocataytic materials by a photodeposition method.In our previous study, we found that, although protonated layered perovskite H1.9K0.3La0.5Bi0.1Ta2O7 showed high activity for overall water splitting, the ratio of H2/O2 was stoichiometric15,16. In addition, H1.9K0.3La0.5Bi0.1Ta2O7 (HKLBT) possesses puny heat endurance and the heat-treatment wou ld yield the collapse of layer structure. Herein, we report Rh oxide loading on HKLBT by in suit photodepositon to provide the activity sites for Oxygen evolution and enhance photocatalytic water splitting. The chemical sates of Pt, Rh loading on HKLBT are investigated. The effect of co-catalysts on water splitting, especially for oxygen evolution, are also discussed.2 Experimental2.1 Preparation of materialsThe protonated layered perovskite oxides H1.9K0.3La0.5Bi0.1Ta2O7was watchful according to the method presented in our previous papers15. The detail processes are as follows(1)The layered compound K0.5La0.5Bi2Ta2O9 were prepared by the polymerized complex method as follows. 60 mL wood spirit was used as a solvent to dissolve 0.006 mol of TaCl5. A large excess of citric acid (CA, 0.09 mol) was added into the methanol solution with constant stirring. After achieving complete dissolution, 0.006 mol Bi(NO3)35H2O, 0.0015 mol La(NO3)3nH2O, and 0.00825 mol K2CO3 were added to the s olution. The mixture was because magnetically stirred for 1 h to afford a out-and-out(a) solution and 0.36 mol of ethylene glycol (EG) was added to this solution. Then, the solution was heated at 130 C to promote esterification between EG and CA, yielding brown rosin. The resin was then calcined at 350 oC for 1 h to form black full-blooded mass. The resulting black powder was calcined on an Al2O3 plate at 650 C for 2 h in air, then calcined at 900 C for 6 h to obtain K0.5La0.5Bi2Ta2O9 (denoted as KLBT).(2) Proton exchange reaction KLBT (0.8g) was performed in 250 mL of 3 M HCl solution for 72 h at room temperature with stand-in of the acid every 24 h, and then the product was washed with deionized water and dried at 60 C for 3 h to deal the water on the surface of potocatalysts to obtain HKLBT.2.2 CharacterizationsThe UV-vis overspread reflection spectra (DRS) were persistent by a UV-vis spectrophotometer UV-2450 (Shimadzu, Japan) and were converted to absorbance by the Kulb elka-Munk method. The transmission electron microscopy (TEM) measurements were conducted using a JEM-2100F (Japan). The surface electronic state was analyzed by roentgen ray photoelectron spectroscopy (XPS, Shimadzu-Kratos, Axis Ultra DLD, Japan). All the binding energy (BE) determine were calibrated by using the standard BE value of taint carbon (C1s =284.6 eV) as a reference.2.3 In suit photodeposition and photocataltyic reactionThe in suit photodeposition and photocatalytic reactions were carried out in a 350 ml top shot reaction Quartz cell at room temperature and a 250 W high-press Hg lamp was used as the light source. The catalyst powder (50 mg) was hang up in 60 ml aqueous solution containing a original bar of (NH3)4PtCl2 and Rh(NO3)2 solution by magnetic stirring. At initial stage (about 2 h), Pt or Rh species were gradually deposited on the surface of HKLBT. After 2 h, generated gases were analyzed by an on-line GC with TCD detector (Huaai, GC9160, China, MS-5A, ar gon Argon as carrier gas).3 Results and discussion3.1 UV-vis DRSThe UV-vis diffuse reflection spectra (DRS) of bare-assed HKLBT and Pt (Rh)/HKLBT photocatalyts are shown in form 1. The immersion edges of HKLBT loading Pt or Rh have no far-famed change. However, after in suit photodeposition, the spectrum longer than 320nm exhibit new weak concentrations, which are ascribe to the adsorption of Pt or Rh(oxides) on the surface of HKLBT. The result indicates co-catalysts loaded on photocatalyst successfully as well.3.2 TEM and XPS analysisThe vocalisation HRTEM images of Rh/HKLBT and Pt/HKLBT are shown in chassis 2. Fig 2a shows that the size of it of the Rh on HKLBT is about 5-10 nm. The hoop fringes of Rh and HKLBT can be observed clearly, suggesting the well-defined crystal structure. The lattice fringes with d spacing of ca. 0.251 nm and 0.184 nm can be appoint to the (101) lattice plane of RhO2 and (002) lattice plane of Rh0, respectively. The result is indicatinged that Rh particles photodeposited on HKLBT contain two different Rh species. The fig 2b shows the lattice fringe with d value of ca. 0.197 nm corresponds to the (200) lattice plane of Pt0, suggesting that Pt2+ was photo-reduced to Pt metal by photogenerated electrons instead of universe oxideted by holes.The chemical states of Rh and Pt species on HKLBT photocatalysts were investigated by the XPS. Fig 3 shows the Rh 3d (a) and Pt 4f (b) binding energy spectra of Pt/HKLBT and Rh/HKLBT. As shown in Fig 3a, the Rh 3d5/2 XPS spectrum displays resistant peaks and has been fitted to two overlapped peaks at ca 307.2 eV and 309.2 eV. The low binding energy at 307.2 eV is attributed to the Rh017, and the high binding energy at 309.2 eV is ascribed to RhO218, demonstrating further Rh photodeposited on HKLBT possesses two chemical states. The result is severe agreement with the HRTEM result. The spectrum for the Pt7/2 (Fig 3b) exhibits two major peaks with binding energies of ca. 70.8 and 72.3 e V, which are assigned to Pt0 and Pt0 with adsorbed oxygen(Pt0-Oads) based on the previous report by kimi et al19.Photocatalytic water splitting activities of Pt/HKLB and Rh/HKLBT() hold over 1 lists the rates of H2 and O2 on HKLBT loaded with different amounts of Pt-cocatalyst and Rh-catalyst by photodepositon method. As shown in table1, the photocataytic activity of HKLBT was improved by Pt or Rh loading. The highest hydrogen evolution rate was obtained when the amount of cocatalytsts was 0.3wt%, which reached the 2.0 times (Pt) and 1.2 times(Rh) of naked photocatalyst, indicating cocatalysts provide catalytic active sites for water splitting. redundance loading of cocatalysts caused a fall down of photocatalytic activitybecause the excessive Pt or Rh loaded on HKLBT would not only block the absorption to light photon but become the recombination centers of carriers as well20. Moreover, it is noteworthy that Pt loading enhanced remarkably the photocatalytic activity of H2 evoluti on instead of O2 evolution, and the H2/O2 is also deviating from the stoichiometric ratio. Instead, the photocataytic O2 production is considerably promoted by Rh loading and the ratio of H2 to O2 is close to 2, when the amount of Rh loading is 0.3 wt%.If photocatalytic reaction occurred, the photocatalytic materials not only need to meet requirement of energy, but also have enough activity sites for H2 and O2 evolution. Generally, during an overall water splitting reaction, active sites of O2 evolution are much important than that of hydrogen evolution, due to high overpotential of O2 evolution of photocatalyst4. It has been reported, some metal oxides cocatalysts, such as RuO2, IrO2, are good candidates as cocatalysts for enhancing O2 evolution by reducing the overpotential of O2 evolution9,21. Valds er al22also reported that the overpotentials of O2 evolution on RuO2, IrO2 and RhO2 were congeneric low (2 evolution from the surface of the photocatalysts. In our study, Pt0 partic les photocdeposited on HKLBT only depart as activity sites for H2 evolution, while the activity sites for O2 evolution are inadequacy as well. In contrast, Rh species loaded on HKLBT was not only reduced by electrons to Rh0, but oxidated by holes to RhO2, which is benefit to O2 evolution.To further licence the effect of RhO2 on O2 evolution, the photocatalytic O2 evolution of HKLBT, Pt/HKLBT and Rh/HKLBT in FeCl3 aqueous solution was carried out(Fig 4). As shown in Fig4, It can be clearly observed that the photocatalytic O2 evolution of Rh/HKLBT is higher than that of both Pt/HKLBT and naked HKLBT, indicting that RhO2 functions as an O2 evolution promoter indeed23.ConclusionThe Rh oxide (RhO2) was loaded on HKBT by in suit photodeposition method without heat-treatment. Compared to Pt, the Rh oxide on the surface of the HKLBT is able to enhance the photocatalytic O2 evolution, and the ratio of H2 to O2 is stoichiometric ratio (H2/O2=2). The enhancement of O2 is due to the decrease of overpotential of O2 evolution by means of RhO2. The present work is also providing a possibility without heat-treatment to load metal oxides on photocatalyst with low heat perceptual constancy for overall water splitting.AcknowledgementsThis work was supported by the National spicy Technology Research and Development Program of China (2012AA051501), the National list Basic Research and Development Program (2009CB220000)the National Natural knowledge Foundation of China (51072116) and the International Cooperation Project of Shanghai Municipal scholarship and Technology Commission (12160705700).Reference1 A Kudo , Y Miseki. 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Journal of Physical Chemistry C 116 (2012) 3161-317022 Valds, J Brillet, M Grtzel, H Gudmundsdttir, H Hansen, H Jnsson, P Klpfel, G Kroes, F Formal, I Man, R Martins, J Nrskov, J Rossmeisl, K Sivula, A Vojvodic, M Zch, Physical Chemistry Chemical Physics 14 (2012) 49-7023 A Iwase, H Kato, A Kudo, utilise Catalyisi B Environmental 136-137 (2013) 89-93Fig 1 UV-vis diffuse reflectance spectra of naked H KLBT, Pt/HKLBT and Rh/HKLBTFig 2 TEM images of Rh/HKLBT (a) and Pt/HKLBT (b)Fig 3 XPS spectra for Rh 3d and Pt 4f of 0.3wt%Rh/HKLBT and 0.2wt%Pt/HKLBTTable 1 Photocatalytic overall water splitting on Pt/HKLBT and Rh/HKLBTCo-catalystLoading content(wt%)Gas evolution (molh-1)aH2O2H2/O2 rationone122.833.73.6Pt0.1163.4051.93.2Pt0.3242.772.53.3Pt0.5210.872.92.9Rh0.1133.539.03.4Rh0.3146.167.92.1Rh0.565.331.22.1Photocatlaytic reaction conditions 50 mg catalyst, 250 W high hug Hg lamp (200 nm), 60 ml pure watera Average rate of H2 and O2 in 4 h Fig 5 Photocatalytic O2 evolution from FeCl3 aqueous solution over naked HKLBT, Pt/HKLBT and Rh/HKLBT (30 mg catalytst, 250 W high pressure Hg lamp, 60 ml 0.05 mol/L FeCl3 )