Effect of Pt Loading Order on Photocatalytic Activity ... - ACS Publications

Jul 8, 2009 - ... Changhua 500, Taiwan, and Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu, 300 Taiwan...
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J. Phys. Chem. C 2009, 113, 13832–13840

Effect of Pt Loading Order on Photocatalytic Activity of Pt/TiO2 Nanofiber in Generation of H2 from Neat Ethanol Feng-Chieh Wang,† Chun-Hsuan Liu,† Chih-Wei Liu,† Jiunn-Hsing Chao,‡ and Chiu-Hsun Lin*,† Department of Chemistry, National Changhua UniVersity of Education, Changhua 500, Taiwan, and Nuclear Science and Technology DeVelopment Center, National Tsing Hua UniVersity, Hsinchu, 300 Taiwan ReceiVed: April 11, 2009; ReVised Manuscript ReceiVed: June 3, 2009

TiO2 nanofibers that are loaded with 1 wt % Pt metal that is introduced at different stages of the wet impregnation procedure have very different photocatalytic activities. Two photocatalysts prepared using different procedures were denoted as PtHTN and PtB catalysts. The former was prepared by impregnating hydrogen trititanate nanofibers (H2Ti3O7, abbreviated as HTN) directly with H2PtCl6, before calcining between 573 and 873 K and then reducing at 423 K in flowing hydrogen. The latter was prepared by initially calcining HTN nanofiber between 573 and 873 K to yield TiO2 nanofiber, and then impregnating this TiO2 support with H2PtCl6; the support was then calcined and reduced as PtHTN to produce PtB catalysts. Although most of their physical properties such as surface area, pore volume, crystalline phase composition and crystallinity, capability to absorb UV light, and band gap energy are quite similar, the optimized H2 yield over PtB catalyst in the photocatalytic dehydrogenation of neat ethanol was 1.86 times that over PtHTN catalyst. XPS and subambient temperature temperature-programmed reduction indicated that the stronger photocatalytic activity of PtB was associated with its higher surface Pt concentration and better reducibility and electron conductivity. The specific Pt impregnation order generated in the PtHTN catalyst a strong interaction between the Pt nanoparticles and the TiO2 nanofiber surface that was not present in the PtB catalyst. This interaction was revealed by the particular microstructure at the Pt-nanofiber interface, as observed by HRTEM, which was responsible for the marked difference between the electronic properties and the photocatalytic activities of the two catalysts. 1. Introduction Since global oil reserves will be depleted in a few decades, alternative energy sources must be sought to replace fossil fuels. Hydrogen gas will have a vital role in the future energy production because it is a renewable fuel that burns cleanly. Current large-scale production of hydrogen gas involves the catalytic reforming of fossil fuels such as methane and coal. These catalytic processes are energy-intensive and generate a large amount of CO2. The photochemical transformation of solar energy into hydrogen gas over a semiconductor surface is an attractive alternative since it is environmentally friendly. Therefore, many researchers have studied the photocatalytic generation of H2 from neat or aqueous aliphatic alcohol solution over a platinized titanium oxide photocatalyst. Under deaerated conditions, both primary1-5 and secondary alcohols6,7 can be effectively dehydrogenated to aldehydes or ketones with the evolution of an equal molar amount of H2 gas, when the TiO2 photocatalyst suspended in an alcohol solution is used. However, pure TiO2 is not active in the photocatalytic dehydrogenation of alcohol, and must be combined with a precious transition metal to yield an active photocatalyst. Pt metal has been demonstrated to be an effective promoter.8-10 Promoting TiO2 with Pt metal generates a Schottky barrier at the interface between Pt and TiO2, which effectively captures photogenerated electrons and reduces the rate of electron-hole * Corresponding author. E-mail: [email protected]. Phone: 886-47232105 ext. 3541. Fax: 886-4-7292361. † National Changhua University of Education. ‡ National Tsing Hua University.

SCHEME 1: Mechanism for Photocatalytic Generation of Hydrogen from Ethanol over Pt/TiO2 Catalyst

recombination to enhance the photocatalytic activity.11 Furthermore, these captured electrons combine with protons, which are generated by interaction between h+ and the ethanol, adsorbed at these Pt sites producing H2 (see Scheme 1). Therefore, controlling the deposition of such Pt decoration on the TiO2 surface and obtaining accurate knowledge of its action are important to improve the photocatalytic efficiency of Pt/TiO2. Hence, this work explores the effects of impregnation order in which Pt metal is introduced onto the TiO2 nanofiber surface on efficiency of the resulting Pt/TiO2 photocatalyst in the generation of H2 gas from neat ethanol. One of the obstacles that must be overcome in effective TiO2 photocatalysis is the low efficiency of the utilization of photons. Therefore, a new photocatalyst material must be found. TiO2 (B) is a lesser known crystallographic form of TiO2 and was first synthesized by Marchand.12 It is a monoclinic TiO2 with a band gap energy comparable to that of its anatase counterpart.13 Although nanotube structure is very attractive due to its high

10.1021/jp9033535 CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2009

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Figure 1. FE-SEM micrographs of (a) HTN nanofibers after drying at 383 K, and the inset is a higher-magnification micrograph showing the fiber-bundles, (b) TEM micrograph of the fiber-bundles after drying at 383 K, (c) HRTEM micrograph shows the lattice fringe of HTN nanofiber, (d) EDX spectrum of HTN nanofiber.

surface area, thermal stability of a nanotube against high temperature calcination, which usually is a necessary step to prepare a supported metal catalyst, is lower than that of a solid nanofiber. Recently, TiO2 (B) nanofiber was synthesized using a hydrothermal method in aqueous NaOH.5,14 However, few works have addressed the deposition of Pt metal on a TiO2 (B) nanofiber support.5,15 Therefore, a platinized TiO2 (B) nanofiber was prepared herein to investigate the effects of the Pt promoter on its photocatalytic efficiency. 2. Experimental Section 2.1. Preparation of Photocatalysts. To prepare the photocatalysts, 2.0 g of anatase TiO2 powder (Aldrich) was mixed with 600 mL of 10 M NaOH in a 1.0 L perfluoroalkoxy container, in which the mixture was maintained at 403 K for seven days. The resulting slurry was filtered to a dry paste by vacuum filtration. The paste was washed a few times in deionized water and then in 0.10 M HCl. The washing process was very thorough: 1.0 g of paste was dispersed in 1000 mL of deionized water or 0.10 M HCl, and the suspension solution was stirred at 300 rpm for 1.0 h using a mechanical stirrer. Thereafter, the paste was again washed in deionized water, as described above, until no Cl- ion was present in the filtrate. The final paste was dried at 383 K for 24 h to yield a hydrogen trititanate fiber. The hydrogen trititanate nanofiber was calcined between 573 and 873 K at a heating rate of 1 K min-1 for 3 h to yield TiO2 nanofiber. Two Pt introduction sequences were adopted to prepare Pt/ TiO2 nanofiber photocatalysts. In the first, Pt was introduced

directly onto the H2Ti3O7 nanofiber by wet impregnation, and the product was calcined at a high temperature to yield Pt/TiO2 (PtHTN catalysts). In this approach, H2Ti3O7 nanofiber was initially mixed with 60 mL of deionized water in a roundbottomed flask to form a suspension. The calculated volume of 1.0 mM of H2PtCl6 (Showa) solution was added dropwise to the suspension and stirred for 2 h at room temperature. A rotary evaporator with a water bath at 323 K was employed to remove the water. The resulting paste was dried at 383 K for 12 h to yield a yellow powder. This powder was calcined between 573 and 873 K in air for 3 h to yield the PtOx/TiO2 catalyst precursor, which was reduced in flowing hydrogen at a rate of 30 mL min-1 at 423 K for 3 h, to yield a dark-gray PtHTN photocatalysts. The reduction temperature of Pt was set based on results of the temperature-programmed reduction (TPR) experiments (see section 2.2, below). In the second approach, H2Ti3O7 nanofiber was first converted to TiO2 nanofiber by calcining it between 573 and 873 K, and then Pt was introduced into the TiO2 (B) nanofiber using the same impregnation method as was used above. The resulting H2PtCl6/TiO2 (B) powder was calcined again between 573 and 873 K in air for 3 h to yield PtOx/TiO2 catalyst precursor. This catalyst precursor was reduced at 423 K in the same manner as PtHTN, to yield a dark-gray Pt/TiO2 nanofiber (PtB catalysts). Pt/TiO2 (P-25) catalyst was prepared using exactly the same procedure as that used by PtB catalysts, and the sample used to obtain the H2 yield was calcined at 673 K. 2.2. Characterization of Photocatalysts. The BET surface areas of the photocatalysts were measured using a Micromeritics

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ASAP 2010 with N2 gas at the liquid nitrogen temperature. The distributions of pore sizes were determined by the BJH method. X-ray powder diffraction patterns of calcined nanofibers were obtained using a Shimadzu Lab-X XRD-6000 spectrometer and Fe K R irradiation (λ ) 1.93604 Å). The diffused-reflectance UV-vis spectra of these photocatalysts were recorded using a Hitachi U-3010 UV-vis spectrometer that had been equipped with an integrating sphere, to estimate the band gap energies of the photocatalysts. XPS spectra were obtained using an ULVAC-PHI Quantera SXM X-ray photoelectron spectrometer with an Al anode. A charge-compensating electron gun was employed during the recording of the XPS spectra to prevent charging of the catalyst. The vacuum in the analysis chamber was at a pressure of lower than 3 × 10-8 torr. The C (1s) signal at 284.5 eV of the residual carbon in catalyst was used as the calibration standard for the binding energy of Pt signal in XPS spectra. A JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM) was adopted to observe the morphology of the photocatalyst at various calcination temperatures. The FESEM samples were placed onto a 4 mm × 5 mm silicon wafer by adding a drop of aqueous solution that contained the nanofiber before drying at 383 K. Before the FE-SEM experiments were performed, the dried wafer was coated with a thin layer of gold by sputtering. JEOL TEM-3010 analytical scanning transmission electron microscope and Phillips Tecani 20 transmission electron microscope coupled with an EDX device (energy-dispersive X-ray) were adopted to determine the fine structure of the prepared photocatalysts and to analyze their elemental compositions. A bar chart of the particle size distribution was obtained using 50 Pt particles. Since Pt oxide was reduced easily, a subambient temperature TPR (SAT-TPR) technique was adopted to characterize the Pt species that were supported on the titania nanofiber surface. These experiments were performed using a Micromeritics Autochem 2910, which was equipped with a Cryocooler device to reduce the reactor temperature using cold N2 gas that was generated from liquid N2. The SAT-TPR procedure was as follows. After 100 mg of catalyst was placed in a U-shape quartz reactor, the reactor was purged with 30 mL min-1 of Ar (99.999%) for 30 min at room temperature. It was then cooled to 223 K in the flowing Ar. Once the baseline had been stabilized, the SAT-TPR experiments were begun by switching the gas to 10% H2/Ar and simultaneously ramping the temperature from 223 to 673 K at a heating rate of 5 K min-1. The Pt contents in the catalysts were determined by ICPAES using a Jarrell-Ash ICAP-9000. The catalysts were digested in aqua regia and standard solutions of known concentrations were adopted as calibration standards. 2.3. Photocatalytic Activity Tests. An 80 mL quartz tube that had been sealed with a rubber septum was adopted as the reactor to study the production of H2 gas from neat ethanol. The ethanol was dried over a Molecular Sieve 5A before use. A 10 mg sample of photocatalyst and 15 mL of neat ethanol were placed in the reactor in an ice bath and purged with Ar for 20 min to remove all of the O2 that was dissolved in the ethanol. In these experiments, the reactor was carefully placed at a fixed distance in front of the UV lamps. The suspension was stirred using a magnetic stirrer and irradiated under two 15 W UV lamps (λmax ) 352 nm, Sankyo Denki) for various periods at the ambient temperature. A volume of 10 µL of gaseous products was sampled using a gastight syringe and analyzed using a Varian 3300 GC that was equipped with a TCD detector and a CP-Carbon PLOT P7 capillary column with

Wang et al. Ar as the carrier gas. A volume of 0.4 µL of the liquid products were analyzed using a HP 6890 GC that was equipped with an FID detector and a HP-5, 5% phenyl methyl siloxane capillary column with N2 as the carrier gas. 3. Results and Discussion 3.1. Characterization of Morphology and Microstructure by Electron Microscopy. Figure 1a presents the FE-SEM micrograph of the hydrothermal product after it was washed in deionized water and HCl and dried at 383 K. The lower magnification micrograph in Figure 1a indicates that the hydrothermal product has numerous fiber-like materials with diameters of 20-200 nm and lengths of several micrometers. The higher magnification micrograph in the inset indicates that these large-diameter fibers are not single fibers but bundles of fiber. The TEM micrograph in Figure 1b verifies that the fiberbundle in fact consists of many smaller fibers with external diameters of 10-20 nm, bound tightly to each other. The lack of bright and dark contrast in the fibrous material in the micrograph confirms that the fibers are solid and not hollow tubes. The HRTEM micrograph in Figure 1c indicates that the HCl-washed nanofiber that is dried at 383 K has a lattice fringe of 0.90 nm, which value is close to the (001) interplanar distance in a hydrogen trititanate (H2Ti3O7).16 The EDX spectrum in Figure 1d shows that the material comprises only Ti and O elements and contains no Na ion. Figure 2 presents the HRTEM micrographs of PtHTN and PtB nanofibers that were calcined at 673 and 873 K. The lattice fringes in these HRTEM micrographs indicate that H2Ti3O7 nanofiber transforms into TiO2 (B) phase and into the anatase phase when calcined at 673 and 873 K, respectively. In Figure 2a,c, the lattice fringes of 0.60 and 0.63 nm are associated with the (001) crystal plane of the TiO2 (B) phase (PDF #350088), and those of 0.35 and 0.36 nm in Figure 2b,d are associated with the (101) crystal plane of the anatase phase (PDF #841286). These TEM micrographs also show that the crystallinity of the nanofiber improved and that the Pt particles grew larger as calcination temperature increased. FE-SEM was adopted to study the effect of the calcination temperature on the morphology of PtB and PtHTN catalysts. Figures 3 and 4 present these results. Increasing calcination temperature from 383 to 673 K did not cause any noticeable change in the morphology of the PtB catalysts in Figure 3 (compare Figure 1a and Figure 3a,b.). However, when the calcination temperature was increased to above 773 K, the surface of the nanofiber seemed to become smoother, which change was most obvious at the closed end of the nanofiber. This smoothing effect became so significant for some of the shorter nanofibers that they appeared as round particles. This phenomenon was observed more clearly at 873 K in Figure 3d. The variation of the morphology of PtHTN catalysts with calcination temperature was similar to that exhibited by PtB catalysts. However, the smoothing of the PtHTN nanofiber surface at 873 K was not as severe as that observed in PtB. (compare Figure 3d and Figure 4d.) 3.2. Surface Area and Characterization of Pore Structure. Figure 5 plots the pore size distributions of hydrogen trititanate nanofibers that had been calcined between 383 and 873 K and those of PtB and PtHTN that had been calcined at 573 K. The figure reveals a wide range of pore diameters, from a few nanometers (d < 10 nm) to a peak maximum between 30 and 60 nm. Since these materials are solid fibers, they contain no pore. Therefore, the smaller pores (d < 10 nm) in Figure 5 are the secondary pores that are formed by the spaces between the

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Figure 2. HRTEM micrographs for PtB calcined at (a) 673 K and (b) 873 K, and for PtHTN calcined at (c) 673 K and (d) 873 K, showing the lattice fringe of TiO2 support and Pt particle size at two calcination temperatures.

Figure 3. FE-SEM micrographs for 1% PtB calcined at (a) 573 K, (b) 673 K, (c) 773 K, and (d) 873 K.

nanofibers in the fiber-bundle, and the larger pores (30 nm < d < 60 nm) are the spaces between the randomly arranged fiberbundles. When these nanofibers were calcined at 873 K, the small pores shrunk significantly, but the large pores shrunk only

slightly. The surface area and pore size of the nanofiber that was dried at 383 K were 99 m2 g-1 and 0.39 cm3 g-1, respectively, and were reduced to 43 m2 g-1 and 0.26 cm3 g-1 upon calcining at 873 K.

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Figure 4. FE-SEM micrographs for 1% PtHTN calcined at (a) 573 K, (b) 673 K, (c) 773 K, and (d) 873 K.

TABLE 1: Physical Properties of Bare and 1% Pt-Promoted TiO2 Nanofibers that Were Calcined at Different Temperatures catalysta HTN-383 HTN-573 HTN-673 HTN-773 HTN-873 PtB-573 (0.87)d PtB-673 (0.90)d PtB-773 (0.91)d PtB-873 (0.94)d PtHTN-573 PtHTN-673 (0.91)d PtHTN-773 (0.91)d PtHTN-873 (0.94)d

Figure 5. Pore size distribution curves for HTN nanofibers calcined at (a) 383 K, (b) 573 K, (c) 673 K, (d) 773 K, and (e) 873 K, and for (f) 1% PtB and (g) 1% PtHTN calcined at 573 K.

Loading these nanofibers with Pt metal by impregnation did not reduce their surface areas but did slightly increase their pore volumes (Table 1). As presented in Figure 5, the increase in pore volume was caused by the change in the large pores (d ) 30-60 nm) as fiber-bundles were redispersed during the impregnation. The results are reasonable because loading small Pt particles onto the surface of nonporous nanofibers should not significantly influence their physical properties. Changing the Pt loading order also did not significantly change physical properties of PtB or PtHTN. Table 1 presents the physical properties of these nanofibers after calcination at various temperatures, including BET surface area, total pore volume, particle size of TiO2 and Pt, and band gap energy. 3.3. Determination of Phase Composition by XRD. Figure 6a,b presents the XRD spectra of PtB and PtHTN catalysts. These XRD data show that Pt particles were well-dispersed in

surface area pore volume (m2/g) (cm3/g) 99 84 74 59 43 84 78 59 45 88 77 67 42

0.39 0.34 0.34 0.31 0.26 0.46 0.36 0.37 0.35 0.47 0.34 0.38 0.31

particle sizeb (nm)

band gap energy (eV)

6.8 7.3 12.2 22.8 5.8 8.2 12.0 27.8 (45.4)c 5.5 8.3 10.7 21.9 (28.7)c

3.213 3.180 3.206 3.219 3.214 3.127 3.116 3.133 3.157 3.121 3.116 3.112 3.105

a The number after the catalyst abbreviation is the calcination temperature and that in parentheses is the Pt wt % in the catalyst. b Estimation of TiO2 particle size using (110) peak of TiO2 (B) phase and (101) peak of the anatase phase. c The number in the parentheses is the estimated Pt particle size. d The number in the parentheses is the Pt loading in wt %.

both catalysts so that no Pt signal was observed in the XRD spectra until the calcination temperature reached 873 K. At this temperature, the diffraction peak of the Pt (111) plane appeared at 2θ ) 51° in both catalysts, and the Pt peak intensity in PtB exceeded that in PtHTN. The Pt particle sizes estimated by the Scherrer equation for PtB and PtHTN at 873 K are 45.4 and 28.7 nm. Additionally, these XRD spectra also indicated that the variation of the crystalline phase of TiO2 nanofibers in PtB and PtHTN with the calcination temperature was similar to that of the pure nanofibers. As reported previously,5 calcination of the hydrogen trititanate nanofiber at 573 K resulted in the formation of TiO2 (B) nanofiber, and calcination at 673 K improved its crystallinity. At 773 K, the anatase phase began to appear in the calcined product, forming a TiO2 (B)/anatase

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Figure 6. XRD spectra for (a) 1% PtB and (b) 1% PtHTN calcined at between 573 K and 873 K.

bicrystalline mixture. At 873 K, the anatase phase became the main crystalline phase. The crystallinity of the TiO2 nanofibers in both catalysts was improved by calcination, as evidenced by the increase in the particle sizes with calcination temperature (Table 1). However, as stated above, at 873 K the Pt diffraction peak intensity in PtB exceeded that in PtHTN. The sluggish growth of Pt particle size in PtHTN suggests a strong interaction between the Pt particle and the TiO2 nanofiber surface in PtHTN (but not in PtB), such that the crystallization of Pt is retarded in PtHTN as the calcination temperature increases. The occurrence of such an interaction in PtHTN is further supported by the morphological change that was revealed by FE-SEM at 873 K, at which temperature the smoothing of the nanofiber surface was less than that of PtB, and also by the appearance of a particular structure at Pt-nanofiber interface in PtHTN that was observed by HRTEM at 673 K (see Figure 2c and description in section 3.5.3). 3.4. Characterization of Band Gap Energy by UV-vis Spectroscopy. Since the materials prepared herein were to be employed as photocatalysts, their diffused-reflectance UV-vis spectra were obtained to estimate their bang gap energies (Eg). Because no significant difference was observed among them, UV-vis spectra of the pure and Pt-loaded TiO2 nanofibers that

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13837 had been calcined between 383 and 873 K were moved to Supporting Information (see Figures S1 to S3). Because of Pt metal particles on the surface, they exhibited a certain degree of absorption over the entire range of visible wavelengths, and therefore, both PtB and PtHTN catalysts were dark gray. The absorption band edges of pure and Pt-promoted titania nanofibers were red-shifted slightly as the calcination temperature increased. Their Eg values were estimated by extrapolating the linear part of the absorption band edges to zero absorbance (see Figure S1b and Figure S3, Supporting Information, for the extrapolation.),17,18 Table 1 lists these values. The variation in the Eg values of these photocatalysts with calcination temperature was small, from 3.1 to 3.2 eV. Therefore, Eg of TiO2 (B) almost equaled that of anatase nanofibers and was close to that of anatase powder.19 3.5. Characterization of Pt Species by TPR, XPS and TEM. 3.5.1. TPR Characterization. As explained in Introduction, impregnating TiO2 with Pt metal forms a Schottky barrier that captures photogenerated electrons and, therefore, increases photocatalytic activity. Hence, the chemical characteristics of Pt particles such as reducibility and oxidation state are expected to affect the photocatalytic activity. Temperature-programmed reduction is an effective tool for characterizing the chemical environment of transition metals on a solid surface.20 Figure 7 presents the SAT-TPR spectra of the catalyst precursors of PtB and PtHTN. The spectra show that both materials have one positive peak and one inverted peak: the former is the H2 consumption peak, which is associated with the reduction of PtOx, and the latter is associated with H2 desorption by the reverse H2 spillover.5 As reported elsewhere,5,21 the PtOx reduction peaks were shifted to a lower temperature as the calcination temperature of the catalyst increased. Additionally, PtOx on TiO2 begins to decompose into Pt metal and oxygen molecules upon calcinations over 673 K.21 This fact explains the smaller reduction peak of catalysts that were calcined at 873 K, at which temperature most of the PtOx is already decomposed into Pt metal. Please note that the Pt contents in the PtB and PtHTN catalysts were approximately equal (around 0.90 wt %, see Table 1), according to ICP-AES analysis. Most interestingly, the results in Figure 7 show that the chemical natures of PtB and PtHTN catalysts differ substantially. The TPR peaks of PtHTN (Figure 7b) are clearly much broader than those of PtB (Figure 7a), and its peak temperatures were also higher. Integrating these TPR peaks indicates that PtHTN consumed more H2 to reduce PtOx than did PtB (except at 873 K, at which temperature the peaks were too broad to enable their areas to be determined exactly). These results clearly show that the Pt species in PtHTN interacts more strongly with the nanofiber support than dose that in PtB, and therefore, PtHTN is more difficult to reduce than is PtB. Additionally, although the surface areas of PtB and PtHTN at different calcination temperatures are similar, the peak areas that are associated with reverse hydrogen spillover for PtHTN catalysts are consistently higher than those of PtB catalysts. This fact also supports the claim that the Pt species in PtHTN and PtB differ markedly, resulting in different adsorbed H atoms and in different degrees of H2 desorption when heated. 3.5.2. XPS Characterization. Figure 8 presents the Pt signals in XPS spectra of PtB and PtHTN catalysts that were calcined at various temperatures. The binding energies (BE) 70.9 and 74.3 eV are 4f7/2 and 4f5/2 peaks of Pt° metallic state, respectively.22,23 The reversed 4f7/2 to 4f5/2 peaks intensity ratio indicates that both catalysts may contain PtO species.24 Although the Pt loading in both catalyst systems are about the same,

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Figure 7. SAT-TPR spectra for (a) 1% PtB and (b) 1% PtHTN calcined at 573 K to 873 K. The numbers beside TPR peaks are integrated peak areas.

around 0.90 wt %, the Pt/Ti peak area ratios of PtHTN are obviously less than those of PtB. The maximum Pt/Ti ratio was obtained from PtHTN catalyst that was calcined at 773 K and that for PtB that was calcined at 673 K; the ratio of their Pt/Ti (PtHTN to PtB) was 0.57. Therefore, some of the Pt metal in the PtHTN catalyst seems not to be detectable by XPS. As the calcination temperature was increased from 673 K to 873 K, the BE of Pt in the PtB catalysts gradually shifted downward (Figure 8a): the BE of the 4f7/2 peak of PtB calcined at 873 K was shifted to 69.8 eV, which was 1.1 eV lower than that of the Pt° state, indicating that this Pt metal was in a negative oxidation state, Ptδ-. In contrast, the BE shift was not as pronounced in PtHTN catalysts (Figure 8b). This result demonstrates that electron transfer in PtB may be easier than in PtHTN, and therefore, enable its PtOx reduction to occur at a lower temperature (as shown in SAT-TPR); the resulting Pt metal is rich in electrons and, so, is in a negative oxidation state. On the basis of the FE-SEM results, the nanofiber surface of PtB melted after it was calcined at >773 K, improving the contact between the Pt particles and the nanofiber surfaces as well as promoting electron transfer and reducing binding energy

Wang et al.

Figure 8. Pt signals in XPS spectra for (a) 1% PtB and (b) 1% PtHTN calcined at 573 K to 873 K. The numbers beside the XPS peaks are peak area ratios of Pt and Ti.

of Pt. However, the strong interaction between Pt metal and the nanofiber support described in the preceding sections stabilized the surface of the PtHTN catalysts, such that the melting of the nanofiber was not as extensive as that in PtB and so the shift in BE was not as significant. 3.5.3. TEM Characterization. Figure 9 plots the Pt particle size distributions of PtB and PtHTN catalysts that were calcined at 673 and 873 K. These HRTEM data show that most of the Pt particles in PtB and PtHTN that were calcined at 673 K had sizes 1-5 nm and 2-3 nm, respectively. This result was consistent with the fact that Pt particles in the two catalysts calcined at below 773 K were too small to be detected by XRD. However, when the calcination temperature was increased to 873 K, thermal sintering dominated the strong interaction between the Pt and the TiO2 nanofiber, and Pt particles in both catalysts grew very large, to a few tenths of nanometers, as presented in Figure 9b,d, and Pt diffraction peaks were also observed in the XRD spectra. Comparing HRTEM micrographs in Figure 2a,c also reveals the effect of the strong interaction between the Pt particles and the TiO2 nanofiber on the microstructure at the Pt-nanofiber interface in the PtHTN catalyst. The micrograph in Figure 2c

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Figure 9. Pt particle size distribution bar charts for 3% PtB calcined at (a) 673 K and (b) 873 K, and for 3% PtHTN calcined at (c) 673 K and (d) 873 K.

presents a magnified PtHTN catalyst surface that is decorated with Pt particles, whose bottom section appears to adhere tightly to the nanofiber surface (these particles are indicated by the red arrows in Figure 2c). This observation may be the evidence for the strong interaction between the Pt particle and the nanofiber surface in PtHTN. In contrast, the Pt particles in the PtB catalyst in Figure 2a are almost spherical and only sit atop the nanofiber surface. 3.6. Photocatalytic Activities of PtB and PtHTN Catalysts. Figure 10 plots the H2 yields following 2 h of reaction over 1% PtB and 1% PtHTN catalysts. The H2 yields over 1% Pt/P-25 and pure TiO2 nanofiber are also shown for comparison. The most interesting result in Figure 10 is that H2 yields over PtB markedly exceed those of PtHTN, except when the catalysts are calcined at 873 K: the optimized H2 yield over PtB calcined 673 K (477 µmol) is 1.86 times that obtained over PtHTN calcined at 773 K (257 µmol). The variations of the physical properties of the TiO2 nanofiber such as surface area, pore volume, crystalline phase composition, and crystallinity, as well as its ability to absorb UV and band gap energy, with the calcination temperature of the two catalysts, do not fully account for the significant difference between their photocatalytic activities. However, the SAT-TPR and XPS results show that the significant difference between the photocatalytic activities of the two catalysts may be related to the surface concentration of Pt metal, the reducibility and electron conductivity of the catalysts, or a combination of these factors. Since Pt particles act as a Schottky barrier that traps photoelectrons and as active sites for the production of H2, the number of surface Pt atoms should clearly influence the

Figure 10. H2 yields over 10 mg of 1% Pt-promoted catalysts of PtB and PtHTN calcined between 573 K and 873 K, and those of 1% Pt/P-25 and pure TiO2 (B) nanofiber calcined at 673 K after reaction for 2 h.

photocatalytic efficiency of the catalysts. XPS data in Figure 8 showed that PtB contained more surface Pt metal than PtHTN. Additionally, according to SAT-TPR, PtB could be reduced at a lower temperature than PtHTN, and following reduction, the Pt metal in PtB had a negative oxidation state (Ptδ-), as indicated by XPS. These results showed that the PtB catalyst had a higher electron density and better electron conductivity than PtHTN. These two electronic factors are also basic prerequisites for the generation of H2 gas over a Pt/TiO2 photocatalyst, as suggested by the reaction mechanism in Scheme 1. Considering these

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findings, that the H2 yield of PtB markedly exceeds that of PtHTN, is unsurprising. The different Pt impregnation order, that leads to the significant difference in the interaction between Pt particles and the nanofiber surface, as indicated by the HRTEM micrographs in Figure 2a,c, may be the origin of the very large difference between the electronic and the photocatalytic properties of the PtHTN and PtB catalysts. The H2 yields of PtB catalysts were very sensitive to calcination temperature, while those of PtHTN were not. The maximum H2 yield was obtained over PtB catalyst that was calcined at 673 K (477 µmol) and slightly exceeded that obtained over Pt/P-25 (441 µmol). The Pt XPS signal intensity was the highest for PtB calcined at 673 K, suggesting that the dispersion of Pt metal was optimized following calcinations at this temperature. Beyond Pt dispersion, the increase in H2 yield over PtB (from 294 µmol to 477 µmol) as the calcination temperature increased (from 573 to 673 K) was partly caused by the improvement in crystallinity as indicated by the XRD results in Figure 6. A further increase in the calcination temperature to 773 K reduced the surface area of TiO2 nanofiber and the Pt dispersion, but the H2 yield remained almost constant (at 450 µmol). At 773 K, the catalyst comprised a TiO2(B)/ anatase bicrystalline mixture. The two crystalline phases promote the separation of photogenerated holes and electrons and enhance photocatalytic activity, in which the effect is referred to as the bicrystalline synergetic effect.25-29 Increasing the calcination temperature to 873 K markedly reduced the H2 yield to 233 µmol. Such dramatic drop in photocatalytic activity may be related not only to a reduction in the surface area of nanofibers but also to severe sintering of the Pt particles as shown by data in Figure 2 and Figure 9, which prevents the Pt particles from functioning as Schottky barrier or active sites for the generation of H2 gas. Calcination is well-known to strongly influence the phase composition, the crystallinity, and the surface area of TiO2, changing its photocatalytic activities.11,30,31 The change in band gap energy of the photocatalysts with the calcination temperature was rather small, and it does not seem to have an important role in this photocatalytic reaction. The variation of H2 yields with calcination temperature in PtHTN catalysts was weaker than that in PtB catalysts, but the H2 yield was still maximal over the catalyst that was calcined at 773 K, which yielded the strongest Pt signal in the XPS spectra. This small variation in photocatalytic activity is explained by the presence of strong interaction between the Pt and the nanofiber surface in PtHTN, which reduces the surface Pt concentration, the reducibility, and the electron conductivity. The thermal energy at 873 K overcame this strong interaction, resulting in the crystallization of Pt particles (lower Pt surface area) in the PtHTN catalyst. In addition, calcination at this high temperature further reduced the surface area of the anatase TiO2 nanofiber. A combination of both factors led to a lower H2 yield for PtHTN calcined at 873 K. 4. Conclusions This study investigated the effects of introducing Pt metal onto a nanometer-size TiO2 support at different stages of an impregnation process on the structures and photocatalytic activities of Pt/TiO2 nanofiber catalysts. Although many of their physical properties including surface area, pore volume, phase composition and crystallinity, ability to absorb UV light, and band gap energy are quite similar, the two-stage PtB catalysts exhibited a significantly better photocatalytic activity than their time-saving one-stage counterparts, PtHTN catalysts. Both SAT-

Wang et al. TPR and XPS results indicated that PtHTN catalysts contained less surface Pt metal and were less reducible and electronconducting than PtB catalysts, and so were poorer photocatalysts. The differences in electronic and photocatalytic properties between PtB and PtHTN catalysts may arise from the presence of a strong interaction between the Pt nanoparticles and the TiO2 nanofiber surfaces in PtHTN catalysts, caused by the introduction of Pt in different order during preparation of catalysts. Acknowledgment. C.-H. Lin is grateful for a grant from Nation Science Council of Taiwan (NSC-96-2113-M-018-004MY2) to support this research. Supporting Information Available: Diffuse-reflectance UV-vis spectra of pure TiO2 nanofiber, 1% PtB and 1% PtHTN that were calcined between 573 K and 873 K. This material is available free of charge via the Internet at http://pubs.acs.org References and Notes (1) Sakata, T.; Kawai, T. Chem. Phys. Lett. 1981, 80, 341. (2) Bamwenda, G. R.; Tsubota, S.; Nakamura, T.; Haruta, M. J. Photochem. Photobiol.: A Chem. 1995, 89, 177. (3) Lin, C. H.; Lee, C. H.; Chao, J. H.; Kuo, C. Y.; Cheng, Y. C.; Huang, W. N.; Chang, H. W.; Huang, Y. M.; Shih, M. K. Catal. Lett. 2004, 98, 61. (4) Kuo, H. L.; Kuo, C. Y.; Liu, C. H.; Chao, J. H.; Lin, C. H. Catal. Lett. 2007, 113, 7. (5) Lin, C. H.; Chao, J. H.; Liu, C. H.; Chang, J. C.; Wang, F. C. Langmuir 2008, 24, 9907. (6) Meisel, D. J. Am. Chem. Soc. 1979, 101, 6133. (7) Yang, Y. Z.; Chang, C. H.; Idriss, H. Appl. Catal. B: EnViron. 2006, 67, 217. (8) Mizukoshi, Y.; Makise, Y.; Shuto, T.; Hu, J.; Tominaga, A.; Shironita, S.; Tanabe, S. Ultrason. Sonochem. 2007, 14, 387. (9) Zou, J. J.; He, H.; Cui, L.; Du, H. Y. J. Hydrogen Ener. 2007, 32, 1762. (10) Zou, J. J.; Liu, C. J.; Yu, K. L.; Cheng, D. G.; Zhang, Y. P.; He, F.; Du, H. Y.; Cui, L. Chem. Phys. Lett. 2004, 400, 520. (11) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. ReV. 1995, 735. (12) Marchand, R.; Brohan, L.; Tournoux, M. Mater. Res. Bull. 1980, 15, 1129. (13) Feist, T. P.; Mocarski, S. J.; Davies, P. K.; Jacobson, A. J.; Lewandowski, A. J. Solid State Ionics 1988, 28-30, 1338. (14) Pavasupree, S.; Suzuki, Y.; Yoshikawa, S.; Kawahata, R. J. Solid State Chem. 2005, 178, 3110. (15) Jitputti, J.; Suzuki, Y.; Yoshikawa, S. Catal. Commun. 2008, 9, 1265. (16) Chen, Q.; Du, G. H.; Zhang, S.; Peng, L. M. Acta Crystallogr. 2002, B58, 587. (17) Korosi, L.; Dekany, I. Colloids Surf., A. 2006, 280, 146. (18) Chen, H. W.; Ku, Y.; Kuo, Y. L. Water Res. 2007, 41, 2069. (19) Debeila, M. A.; Raphulu, M. C.; Mokoena, E.; Avalos, M.; Peranovskii, V.; Coville, N. J.; Scurrell, M. S. Mater. Sci. Eng., A 2005, 396, 70. (20) Mcnicol, B. D. Catal. ReV. Sci. Eng. 1982, 24, 233. (21) Huizinga, T.; van Grondelle, J.; Prins, R. Appl. Catal. 1984, 10, 199. (22) Silvestre- Albero, J.; Sepulveda-Escribano, A.; Rodriguez-Reinoso, F.; Anderson, J. A. J. Catal. 2004, 223, 179. (23) Kim, K. S.; Winograd, N.; Davvis, R. E. J. Am. Chem. Soc. 1971, 93, 6296. (24) Wang, X.; Yu, J. C.; Yip, H. Y.; Wu, L.; Wong, P. K.; Lai, S. Y. Chem. Eur. J. 2005, 11, 2997. (25) Ohno, T.; Tokieda, K.; Higashida, S.; Matsumura, M. Appl. Catal., A 2003, 244, 383. (26) Yu, J. C.; Zhang, L.; Yu, J. Chem. Mater. 2002, 14, 4647. (27) Wu, C.; Yue, Y.; Deng, X.; Hua, W.; Gao, Z. Catal. Today 2004, 93-95, 863. (28) Yu, J.; Zhang, L.; Cheng, B.; Su, Y. J. Phys. Chem. C 2007, 111, 10582. (29) Yu, J.; Xiong, J.; Cheng, B.; Liu, S. Appl. Catal. B: EnViron. 2005, 60, 211. (30) Ohtani, B.; Ogawa, Y.; Nishimoto, S.-I. J. Phys. Chem. B 1997, 101, 3746. (31) Zhang, Q.; Gao, L.; Guo, J. Appl. Catal. B: EnViron. 2000, 26, 207.

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