8495
2008, 112, 8495–8498 Published on Web 04/03/2008
In Situ Time-Resolved Energy-Dispersive XAFS Study on Photodeposition of Rh Particles on a TiO2 Photocatalyst Kentaro Teramura,*,† Shin-ichi Okuoka,‡ Seiji Yamazoe,‡ Kazuo Kato,§ Tetsuya Shishido,‡ and Tsunehiro Tanaka*,‡ Kyoto UniVersity Pioneering Research Unit for Next Generation, Kyoto UniVersity, Kyoto 615-8510, Japan, Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan, and Japan Synchrotron Radiation Research Institute, 1-1 Kouto, Sayo-cyo, Sayo-gun, Hyogo 679-5198, Japan ReceiVed: January 31, 2008; ReVised Manuscript ReceiVed: March 12, 2008
It is the first report to observe the real-time behavior of photodeposition and the continuous structural change of Rh metal particles anchored on a TiO2 photocatalyst from an aqueous solution of Rh trivalent ions in the liquidssolid suspension by an in situ time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) analysis. The DXAFS analysis uncovered that the Rh trivalent ions are adsorbed on the surface of the TiO2 photocatalyst, followed by reducing to the Rh metal particles by acceptance of electrons because the coordination number of a RhsRh bond evaluated by the Fourier transforms of EXAFS spectra and the amount of the Rh trivalent ions determined by the ICP analysis linearly increased and decreased with elongation of photoirradiation time, respectively. The generation rates of the Rh metal particles and the diminution rates of the Rh trivalent ions depended on the type of sacrificial reagents and tends to become slower in the order corresponding to methanol > ethanol . 1-propanol >2-propanol. The coordination number of a RhsRh bond stopped increasing after 90 min of photoirradiation in the case of methanol. The coordination number was evaluated to 10, although that of Rh foil is 12, suggesting the generation of Ultrafine Rh particles on TiO2. Introduction Photodeposition as well as impregnation is one of the most popular methods to modify various metal species as a cocatalyst on photocatalysts.1–5 Since 1970s, noble metal (for example, Pt, Rh, Pd, etc.) was often loaded on a TiO2 photocatalyst as a modifier to enhance photocatalytic activity. The photodeposition method is easy to load metal on a photocatalyst because of doing nothing other than introducing a photocatalyst and a metal precursor into an aqueous solution involving alcohol as a sacrificial reagent and illuminating this suspension with a light source. It is hard to find any articles approaching systematic kinetics, thermal dynamics, and reaction mechanism of the photodeposition, although a few of research groups reported behavior of the photodeposition of various kinds of metal on the photocatalysts.6–12 Time-resolved energy-dispersive XAFS (DXAFS)13,14 analysis is an up-to-date method to quickly observe the structural changes of various materials in situ.15–17 Recently, some groups * To whom correspondence should be addressed. E-mail: kentaro.
[email protected]. Tel: +81-75-585-6095. Fax: +81-75585-6096. E-mail:
[email protected]. Tel: +81-75-383-2558. Fax: +81-75-383-2561. † Kyoto University Pioneering Research Unit for Next Generation, Kyoto University. ‡ Department of Molecular Engineering, Graduate School of Engineering, Kyoto University. § Japan Synchrotron Radiation Research Institute.
10.1021/jp8010537 CCC: $40.75
reported the DXAFS studies on kinetics and dynamics of catalysis in several synchrotron radiation sources in the world.18 In Japan, the pioneering works about the DXAFS analysis have been carried out with the use of the synchrotron radiation source in the Photon Factory, KEK in Tsukuba.19,20 Iwasawa and his co-workers reported the kinetics and the dynamics studies by DXAFS analysis on the reduction processes of Cu cations in ZSM-5,21–23 the structural changes of different kinds of metal clusters,23–26 the oxygen storage/release in a Pt/ordered CeO2-ZrO2 catalyst,27 and the surface changes on the Pt/C cathode in a fuel cell.28 On the other hand, a few of beamlines in the SPring-8, Hyogo, which can supply highly brilliant X-rays, are also equipped with the DXAFS facilities.29 Okumura et al.30,31 clarified the structural changes of Pd species in the Heck reaction over a zeolite-supported Pd cluster catalyst by the DXAFS analysis. In these two synchrotron radiation sources in Japan, the Photon Factory, and the SPring-8, the DXAFS analysis currently applies to various catalytic reactions to clarify the kinetics and the dynamics of these processes. However, it has not been seen that the DXAFS analysis applies to photocatalytic reactions containing the photodeposition, although several ex situ and in situ XAFS studies on the photocatalytic reactions were reported.32,33 In this study, we investigated the appearance process of Rh particles on a TiO2 photocatalyst under photoirradiation by the in situ DXAFS analysis in the SPring-8 BL28B2 beamline. 2008 American Chemical Society
8496 J. Phys. Chem. C, Vol. 112, No. 23, 2008
Letters
Figure 1. Series of the Fourier transforms of k3-weighted Rh-K edge EXAFS spectra of the Rh species in the presence of TiO2 as a photocatalyst and methanol as a sacrificial reagent under photoirradiation.
Experimental Section The photocatalyst used in this study was anatase phase of TiO2 (JRC-TIO-8 supplied from the Japan Catalysis Society) calcined in a furnace in air at 673 K for 3 h before the photodeposition. The photodeposition of the Rh metal particles onto a TiO2 photocatalyst was carried out in the closed batch system 0.5 g of JRC-TIO-8 as a photocatalyst, 0.8 mL of RhCl3 · 3H2O solution (0.095 mol/L) as a precursor, and 3.2 mL of alcohol as a sacrificial reagent were introduced into the reactor made of Pyrex glass with a flat glass ceiling window for illumination. The suspension liquid was irradiated with a 200-W Hg-Xe lamp equipped with fiber optics, collective lens, and a mirror (San-Ei Electric Co., Ltd., UVF-204S type C) after Ar bubbling for 10 min. The suspension liquid was filtered and washed with purified water after the photodeposition. The amount of residual Rh ions in the filtrate for each instant of photoirradiation time was determined by the sequential inductively coupled plasma (ICP) spectrometer. The Rh-K edge (∼23.2 keV) X-ray absorption fine structure (XAFS) measurements were made at the BL28B2 beamline of the SPring-8 synchrotron radiation facility. The main equipment of the DXAFS spectroscopy system at the BL28B2 beamline in this study consists of a polychromator that is set to a Laue configuration with net plane Si(422) and a position-sensitive detector (PSD) mounted on a θ-2θ diffractometer. The photon energy of the X-ray was calibrated with Rh foil as a reference. Data reduction was performed using the REX2000 program Ver. 2.5.9 (Rigaku Corp.). Fourier transform of the extended X-ray absorption fine structure (EXAFS) spectra was performed in the 2.64-10.5 Å-1 regions. Results and Discussion It was confirmed that the photodeposition of Rh metal particles was able to proceed on the TiO2 photocatalyst under photoirradiation in the presence of alcohol as a sacrificial reagent. The Rh-K edge XANES and EXAFS spectrum after reaction for 30 min in the absence of a TiO2 photocatalyst, light illumination, or a sacrificial reagent was consistent with those before the reaction. We confirmed that Rh ion is not reduced
under X-ray irradiation. In addition, no Rh metal particles were deposited on TiO2 under photoirradiation of light (λ > 400 nm) with a cutoff filter (L-42, Hoya), corresponding to the band gap energy of anatase phase of the TiO2 photocatalyst (3.39 eV). An inert material such as SiO2 cannot deposit the Rh metal particles for 2 h photoirradiation. Therefore, the photodeposition of the Rh metal particles proceeded via band gap transition of a TiO2 photocatalyst from the valence band comprising O2p to the conduction band comprising Ti3d. In this study, the photodeposition of the Rh metal particles was carried out in the absence of oxygen because the Rh metal particles were not deposited in the presence of oxygen. The X-ray absorption near edge structure (XANES) spectra before and after the photoirradiation were consistent with those of RhCl3 solution and Rh foil, respectively. The energy position of absorption edge consecutively shifted to low-energy side with elongation of time for the photoirradiation. Accordingly, the evolved electron under photoirradiation got trapped, the Rh trivalent ions adsorbed on the surface, and the Rh metal particles deposited on the TiO2 photocatalyst. In addition, a series of XANES spectra exhibited isosbestic points showing that the transient state between Rh metal particle and Rh3+ ion cannot be captured in the measurement, if any, and we can regard the spectrum as a simple overlapping one consisting of those for Rh0 and Rh3+. Figure 1 shows a series of the Fourier transforms (FT) of k3-weighted Rh-K edge EXAFS spectra of the Rh species in the presence of TiO2 as a photocatalyst and methanol as a sacrificial reagent under photoirradiation. The exposure time of the PSD was 267 ms. Fifty snapshot spectra were accumulated to obtain one spectrum; therefore, we could observe a variation of a spectrum every 13.35 s in this study. The Fourier transformation was performed without phase shift correction. The first shell peak located at 1.73 Å disappeared after the photoirradiation and an alternative peak located at 2.45 Å appeared. This peak is assigned to a Rh-Rh bond of Rh metal, in comparison with the FT spectrum of Rh foil as a reference. The peak height linearly increased with photoirradiation time and was saturated after 90 min. The structure parameters of the
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J. Phys. Chem. C, Vol. 112, No. 23, 2008 8497
TABLE 1: Curve Fitting Results of a Rh-Rh Bond of the Rh Metal Particles in the Presence of TiO2 as a Photocatalyst and Methanol as a Sacrificial Reagent under Photoirradiationa photoirradiation time/min
C. N.b
rc/Å
σd/Å
Re (%)
26.9 53.8 80.7 107.6
3.1(6) 5.9(12) 8.6(18) 10.1(21)
2.720(9) 2.719(9) 2.718(9) 2.730(8)
0.060(22) 0.066(22) 0.068(21) 0.070(22)
6.8 6.1 7.9 9.7
a
R fitting range is 2.07-2.92 Å, and k fitting range is 2.64-10.5 . b C. N., coordination number. c r: interatomic distance. d σ, Debye-Waller factor. e So-called R factor.
Å-1
Figure 2. Photoirradiation time dependence of the coordination number evaluated by the curve fitting of the RhsRh bond at 2.45 Å in the FT of EXAFS spectra for the photodeposition of the Rh metal particles on the TiO2 photocatalyst in the presence of various sacrificial reagents: (a) methanol (black), (b) ethanol (red), (c) 1-propanol (blue), and (d) 2-propanol (green).
Rh metal particles generated after 26.9, 53.8, 80.7, and 107.6 min of photoirradiation (the photodeposition is over after 90 min) were obtained by a general curve fitting analysis attached in the REX2000 program as shown in Table 1. For a Rh-Rh
pair, the phase and amplitude functions extracted from Rh foil as a reference was used. The coordination number was evaluated to 10, although that of Rh foil is 12, suggesting the generation of fine Rh metal particles on TiO2. The diameter is estimated to be 3 nm, assuming that the structure of the Rh metal particles is a cuboctahedron. Figure 2 shows the photoirradiation time dependence of the coordination number evaluated by the curve fitting of the Rh-Rh bond at 2.45 Å in the FT of EXAFS spectra for the photodeposition of the Rh metal particles on the TiO2 photocatalyst in the presence of various sacrificial reagents. It is known that the XAFS spectrum reflects an average structure of all kinds of target species in a system. Therefore, the increase in the coordination number of a Rh-Rh bond means the reduction of Rh ions to form Rh metal particles because the amount of Rh species is constant in our case. At present, we suppose that the Rh particles do not grow up in incremental steps but the Rh particles with a uniform size appear one after another on the surface because the coordination number of a Rh-Rh bond increases in a linear fashion and the Debye-Waller factor does not undergo a lot of changes. As described above, the increase in coordination number of a Rh-Rh bond stopped after 90 min of photoirradiation in the presence of methanol. This is due to whether all Rh ions in the aqueous solution have been consumed or the appearance of the Rh metal particles is inhibited. On the other hand, the photodeposition in the presence of ethanol, 1-propanol, and 2-propanol did not finish after 90 min of photoirradiation as one can see that the coordination number of a Rh-Rh bond was not saturated. The photoirradiation time dependence of the residual Rh trivalent ions in the filtrate determined by ICP analysis is shown in Figure 3. The amount of residual Rh ions at t ) 0 is substracted from the amount of adsorbed Rh ions on TiO2 from 76 µmol as an initial budget. The residual Rh trivalent ions could be detected in the presence of ethanol, 1-propanol, and 2-propanol after 90 min of photoirradiation. On the contrary, there were no ions in the solution in the case of methanol above 90 min of photoirradiation. The amounts of the residual Rh trivalent
Figure 3. Photoirradiation time dependence of the residual Rh ions in the filtrate estimated by ICP: methanol (black), ethanol (red), 1-propanol (blue), and 2-propanol (green).
8498 J. Phys. Chem. C, Vol. 112, No. 23, 2008 ions linearly decreased with elongation of photoirradiation time in all cases. The rate of the photodeposition estimated by the ICP analysis tends to become slower in the order corresponding to methanol > ethanol . 1-propanol >2-propanol as well as that estimated by the XAFS analysis as shown in Figure 2. Therefore, the rate of a photodeposition depends on the bond strength of O-H bond of a sacrificial alcohol reagent. It is speculated that pKa value of alcohol influences the rate of the photodeposition. The order of the diminution rates of Rh trivalent ions determined by the ICP analysis gave close agreement with that of the generation rates of the coordination number of a Rh-Rh bond evaluated by the FT of EXAFS spectra. Accordingly, the appearance rate of the Rh metal particles on irradiated TiO2 would be compatible with the diminution rate of the Rh trivalent ions. The kinetics and dynamics of the photodeposition of the Rh metal particles on the TiO2 photocatalyst is now under discussion. Conclusion We observed the behavior of the photodeposition of Rh metal particles onto a TiO2 photocatalyst from an aqueous solution involving RhCl3 as a precursor and alcohol as a sacrificial reagent by means of the in situ time-resolved energy-dispersive XAFS (DXAFS) analysis. The Rh trivalent ions are adsorbed on the surface of the TiO2 photocatalyst, followed by reducing to the Rh metal particles by acceptance of electrons. The appearance rate of the Rh metal particles on irradiated TiO2 was closely related to the diminution rate of the Rh trivalent ions. The coordination number of a Rh-Rh bond stopped after 90 min of photoirradiation in the presence of methanol as a sacrificial reagent, suggesting the generation of fine Rh metal particles on TiO2. The rate of the photodeposition depended on the type of sacrificial reagents and tends to become slower in the order corresponding to methanol > ethanol . 1-propanol >2-propanol. Acknowledgment. This study was supported by Program for Improvement of Research Environment for Young Researchers from Special Coordination Funds for Promoting Science and Technology (SCF) commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The XAFS measurements at the Spring-8 were carried out with the approval (proposal 2007A1921, 2007B1094) of Japan Synchrotron Radiation Research Institute (JASRI). Supporting Information Available: A series of Rh-K edge XANES spectra of the Rh species, k3-weighted Rh-K edge EXAFS spectra of the Rh species, and inverse-Fourier-filtered spectra of Rh-Rh peak. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 7729. (2) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4317.
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JP8010537
