TiO2 Nanocomposites as Highly Active

ARTICLE pubs.acs.org/JPCC

Mesostructured Pt/TiO2 Nanocomposites as Highly Active Photocatalysts for the Photooxidation of Dichloroacetic Acid Adel A. Ismail*,† and Detlef W. Bahnemann‡ †

Nanostructured & Nanotechnology Division, Advanced Materials Department, Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87 Helwan, Cairo 11421, Egypt ‡ Photocatalysis and Nanotechnology Unit, Institut f€ur Technische Chemie, Leibniz Universit€at Hannover, Callinstrasse 3, 30167 Hannover, Germany ABSTRACT: Mesoporous Pt/TiO2 nanocomposites have been synthesized by using two pathways: (1) the in-situ preparation of Pt/TiO2 nanocomposites was carried out using a one-step synthesis by dissolving the Pt and TiO2 precursors in the presence of a triblock copolymer as the structure-directing agent followed by drying, calcinations, and reduction under H2 gas. (2) Platinum particles have been photochemically deposited onto mesoporous TiO2. The TEM images of the mesoporous Pt/TiO2 nanocomposites calcined at 450 °C demonstrate that the TiO2 nanoparticles with an average diameter of about 10 nm are not agglomerated and are quite uniform in size and shape. Following the photodeposition process, the Pt nanoparticles are well-dispersed and highly uniform, exhibiting diameters of ∼3 nm; however, following the in situ preparation, the Pt particles are reaching diameters of approximately 15 nm, most likely as a result of the calcination and reduction at high temperatures. The photocatalytic activity of the newly synthesized mesoporous photocatalysts was measured and compared with that of nonporous commercial Aeroxide TiO2 P 25 and Pt/TiO2 P 25 by measuring the rate of Hþ ion release during the photodegradation of dichloroacetic acid (DCA) and confirmed by measuring the concomitant total organic carbon removal. For both preparation routes the photonic efficiency of the mesoporous TiO2 photocatalysts is found to be increased by the addition of the Pt islands and to correlate with the size of the Pt particles. The mesoporous Pt/TiO2 nanocomposites showed 2 times higher activity for the photocatalytic DCA photodegradation than Aeroxide TiO2 P 25. The larger photoactivity of the mesoporous Pt/TiO2 nanocomposites prepared by the photodeposition process is attributed to the higher dispersity and the small size of the Pt particles (3 nm). To the best of our knowledge, the measured photonic efficiency of ξ = 7.95% for the photodeposited Pt/TiO2 nanocomposites is among the highest ξ value reported up to now.

1. INTRODUCTION The design of TiO2 with a well-defined mesoporous structure is a promising way to achieve high photocatalytic activity, since the ordered mesoporous channels facilitate fast intraparticle molecular transfer of the substrate molecules.1 On the other hand, a high degree of crystallization of the photocatalysts is favorable for the rapid transfer of photogenerated charge carriers from the bulk of the TiO2 particles to their surface, thus inhibiting their recombination and leading to enhanced photonic efficiencies.2 However, the preparation of semiconducting oxides exhibiting both an ordered mesoporous structure and highly crystalline pore walls is usually a challenging task.3 Although mesoporous TiO2 has already been synthesized by Antonelli and co-workers in 1995,4 only a limited number of reports concerning the synthesis of ordered mesoporous TiO2 with crystalline walls are found in the literature.1,3-5 Recently, nanosized noble metal particles and clusters have attracted significant attention due to their unique properties and their potential applications in r 2011 American Chemical Society

the fields of photochemistry, electrochemistry, optics, electronics, and catalysis.6-8 For example, the presence of noble metal deposits on the TiO2 surface can help to efficiently separate the electron hole pairs by attracting the conduction band electrons. This process has been shown to improve the overall efficiency for a number of photocatalytic reactions.9 There are several factors strongly effecting the activity of TiO2 photocatalysts: the active surface area and the concentration of reaction centers thereon, the adsorption of substrate molecules and their type of coordination, the value of the Fermi level and the redox properties of the oxidizing centers (e.g., the trapped holes) formed on the surface. Hence, an increase of the efficiency of TiO2 photocatalysts can, in principle, be realized by different methods of surface modification: adsorption of noble metal ions Received: November 16, 2010 Revised: February 9, 2011 Published: March 04, 2011 5784

dx.doi.org/10.1021/jp110959b | J. Phys. Chem. C 2011, 115, 5784–5791

The Journal of Physical Chemistry C and/or nanoparticles, anchoring of redox-active molecules, etc. Depending on the preparation conditions, photodeposition procedures typically yield small metal deposits ranging from a few to around 20 nm in diameter.10 The size and dispersion of the metal deposits on the TiO2 particles will, however, be critical for the control of their photocatalytic activity. Given the strong effect of deposit size and dispersion, the development of preparation methods providing a close control over the deposit size is essential.11 In addition, a high uniformity of both Pt and TiO2 in size and shape also appears to be essential for achieving enhanced photocatalytic activities.12 In the present study, dichloroacetate (DCA) was chosen as the model compound to test the newly prepared photocatalysts, because it is a relevant industrial pollutant and the mechanism of its photocatalytic degradation has been completely elucidated. The photocatalytic degradation of DCA has been extensively studied using Aeroxide TiO2 P 25, colloidal TiO2, Fe2O3-TiO2, Ag/TiO2, Pt/TiO2, and Hombikat UV100, respectively, under UV(A) illumination13-17 as well as mesoporous iron oxide-layered titanate nanohybrids under visible-light irradiation.18 The present study focuses on the preparation and the properties of nanocrystalline Pt/TiO2 networks assembled through a novel in situ preparation method starting from suitable precursors in the presence of a triblock copolymer employed as the structure-directing agent. The use of surfactants enabled the controlled synthesis of uniform TiO2 nanocrystals with very small particle size (about 10 ( 2 nm) in a one-step process. Reaching this size domain is important because the thermodynamic stability of TiO2 nanoparticles is size-dependent, i.e., at particle diameters 2. The photocatalytic oxidation of one DCA anion results in the formation of Hþ, two CO2 molecules, and two Cl- ions. The reaction pH was 5785

dx.doi.org/10.1021/jp110959b |J. Phys. Chem. C 2011, 115, 5784–5791

The Journal of Physical Chemistry C

Figure 1. SAXS patterns of mesoporous TiO2 (a), PtTiO2-1 (b), and PtTiO2-2 (c) nanocomposites calcined at 450 °C. Inset, SAXS pattern of as-made Pt/TiO2.

maintained constant using a pH-stat setup with the addition of NaOH as needed. The employed pH-stat technique allows the in situ measurement of the amount of protons formed during the photolysis experiments. The rate of the photocatalytic degradation of DCA was followed by measuring the amount of OHadded to keep the pH constant, which is equivalent to the amount of Hþ formed.23 Samples were taken at the beginning and at the end of each run from the upper part of the reactor with the catalyst being removed from the liquid phase by filtration through nylon syringe filters (pore size 0.45 μm) for total organic carbon (TOC) analysis. The reaction time was 6 h for each run. H2O2 was analyzed using an enzymatic method.24 Analytical reagents (4-aminoantipyrine and phenol in presence of peroxidase) were added to an aqueous solution containing H2O2 to obtain the red quinoneimine dye. Consequently, the H2O2 concentration is selectively determined by measuring the absorbance of the colored solution at 505 nm. If the solution becomes colored, therefore, the presence of H2O2 can be identified.

3. RESULTS AND DISCUSSION Structural Investigations. Mesoporous TiO2 and Pt/TiO2 nanocrystals were synthesized through a simple one-step sol-gel process in the presence of the F127 triblock copolymer as structure-directing agent. Our strategy for the synthesis of mesoporous Pt/TiO2 nanocomposites thus employs a surfactant-templated self-assembly with the ordered micelles formed by the surfactant molecules acting as templates for the condensation of the inorganic agents. For crystallization of amorphous TiO2, template removal, and Pt reduction, we carried out the calcinations under atmospheric pressure at 450 °C and reduction process using H2 gas at 300 °C. This photocatalyst is denoted as PtTiO2-1. Another way to keep the mesostructure order of TiO2, the crystallization of TiO2 and Pt reduction were first completed under H2 gas at 450 °C. Then the template was removed under atmospheric calcinations at 350 °C. This photocatalyst is denoted as PtTiO2-2. In the third method, Pt was photochemically deposited onto mesoporous TiO2, and this photocatalyst is denoted as PtTiO2-3. The small-angle X-ray scattering (SAXS) patterns of as-made highly ordered mesoporous Pt/TiO2 (Figure 1, Inset) and of the PtTiO2-1, PtTiO2-2, and PtTiO2-3

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Figure 2. WAXRD patterns of mesoporous TiO2 (a), mesoporous PtTiO2-1 (b), PtTiO2-2 (c), and PtTiO2-3 (d) nanocomposites calcined at 450 °C. Shifted for the sake of clarity.

samples are shown in Figure 1. The as-made sample shows two well-resolved peaks, which can be indexed to the (10) and (20) Bragg reflections, confirming an ordered 2D-hexagonal mesostructure of the P6m space group (Figure 1, inset).20 The observed high intensities and the sharpness of the peaks prove that a long-range order exists in the Pt/TiO2 nanocomposites. After calcination at 450 °C, ordered structure of the sample PtTiO2-3 is collapsed; however, in case of the in situ preparations, a shoulder peak is observed at 2θ = 0.75 and the diffraction peak is becoming weaker with the (10) reflections, indicating that the long-range ordering of the mesopores is decreasing. This is explained by the presence of Pt enhancing the TiO2 wall formation during the crystallization process. In addition, the SAXS results show that the degree of the mesostructural order of PtTiO2-2 is higher than that of PtTiO2-1, because the preparation procedure of mesoporous Pt/TiO2 nanocomposites is critical in determining the nature of the mesoporous. After template removal, the structural regularity declines but the lattice parameter calculated from the d10 value decreases only from 14.98 to 13.23 nm, indicating an approximately 11.68% contraction of the structure. Figure 2 shows the wide-angle XRD results obtained for mesoporous TiO2, PtTiO21, PtTiO2-2, and PtTiO2-3 nanocomposites calcined at 450 °C, respectively. In all cases, the reflections from the anatase phase with peaks characteristic for the (101), (004), (200), (211), and (213) lattice planes have been observed, evidencing that the TiO2 phase easily nucleates during heating and remains intact upon calcination. Analyzing the width of the reflections at halfmaximum by employing Scherrers equation25 results in TiO2 nanocrystal sizes with a maximum diameter of 10 ( 2 nm (Table 1). Interestingly, the XRD results do not indicate the formation of any crystalline Pt phase. Thus, at low Pt content a high dispersion of smaller Pt nanoparticles within the pores has obviously been achieved. The nitrogen adsorption isotherms of the mesoporous PtTiO2-1, PtTiO2-2, and PtTiO2-3 nanocomposites calcined at 450 °C are shown in Figure 3. The respective pore size distributions were obtained from these adsorption isotherms using the BJH model (shown as an inset in Figure 3). A typical reversible type IV adsorption isotherm is observed for all samples. Sharp 5786



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Table 1. Textural Properties of Mesoporous TiO2 and Pt/TiO2 Nanocomposites Calcined at 450 °C and of Commercial Aeroxide TiO2 P 25 and Pt/TiO2 P 25 and Photonic Efficiencies Obtained for the Photocatalytical Degradation of DCA as Well As the Respective Initial Hþ Formation Ratesa photocatalysts

unit cell size (nm)

pore wall (nm) -

r 107 (mol L-1 s-1)

ξ, %

Vp (cm3/g)

Dp (nm)

-

14.98

-

-

-

-

174

10 ( 2

13.23

6.7

2.65

4.2

0.3

6.53

Pt/TiO2-1

155

10 ( 2

13.16

6.76

3.02

4.77

0.239

6.2

Pt/TiO2-2

152

10 ( 2

13.11

6.46

4.64

6.76

0.268

6.65

Pt/TiO2-3 TiO2 P 25

150 50

10 ( 2 26 ( 2

13.17 -

6.28 -

5.03 2.24

7.95 3.54

0.241 -

6.89 -

45

27 ( 2

-

-

3.72

5.71

-

-

Pt/TiO2 P 25

-

PSTiO2 (nm)

pure TiO2

as-made

a

SBET (m2 g-1)

SBET, surface area; PSTiO2, average particle size of TiO2 nanoparticles; r, Hþ formation rate; ξ, photonic efficiency; Vp, pore volume; Dp, pore diameter.

Figure 3. N2 sorption isotherms and pore size distributions (inset) of the mesoporous PtTiO2-1, PtTiO2-2, and PtTiO2-3 nanocomposites calcined at 450 °C.

inflections at relative pressures P/P0 between 0.45 and 0.75 due to capillary condensation within the mesopores are observed, which are characteristic of a two-dimensional hexagonal symmetry.21 The addition of Pt decreases the surface area from 170 to 150 m2/g. The thickness of the pore walls of the Pt/TiO2 nanocomposites is also slightly decreased from 6.7 to 6.38 nm as compared with mesoporous TiO2, suggesting the presence of Pt nanoparticles anchored to the pore walls of TiO2. Compared with the size of the TiO2 nanocrystallites between 10 ( 2 nm (Table 1), the wall thickness is found to be slightly smaller, implying that some of the TiO2 nanocrystals will partially pierce into the channel space,26 which is particularly evident from the HRTEM images (Figure 4d,e). The compositions and the pore parameters of mesoporous TiO2 and of the Pt/TiO2 nanocomposites are summarized in Table 1. The TEM images of the mesoporous 0.5 Pt wt %/TiO2 nanocomposites calcined at 450 °C for 4 h demonstrate that the TiO2 nanoparticles are not agglomerated and are quite uniform in size and shape, with an average diameter of about 10 nm (Figure 4a,b). As seen in the HRTEM images (Figure 4c) the lattice fringes exhibit the typical distances for the anatase

phase, i.e., TiO2(101) (3.54 Å). The selected area electron diffraction (SAED) patterns (insets of Figure 4a,c) further confirm that anatase nanocrystallites are indeed formed. From the inset image in Figure 4c, it is evident that the angle between (1, 0, -1), zero point, and (0, 1, -1) is 81.7°. This further confirms that the newly prepared photocatalyst is anatase. The dark-field TEM images of PtTiO2-1 (Figure 4d) and PtTiO2-3 (Figure 4e) clearly show that the Pt nanoparticles are well-dispersed and exhibit diameters of about 15 and 3 nm, respectively. In situ prepared Pt/TiO2 nanocomposites samples PtTiO2-1 and PtTiO2-2 indicated that, even though relatively large Pt nanoparticles (15 nm) are obviously formed, there are no indications that the Pt nanoparticles are located on the outer surface of the mesoporous TiO2 network (Figures 4d). In such a case, an increased concentration at the rims of the network should be observable in the TEM pictures; this is, however, not the case. Instead, the growing Pt particles seem to create new pores in the TiO2 network through a partial destruction of the channel walls, thus creating new pores in the walls. Such creation of new pores is well-known, e.g., from the growth of Pt particles in zeolites.9b,27 Even though the Pt (15 nm) nanoparticles are found to be considerably larger than the TiO2 (10 nm) nanocrystals, it is important to note that the incorporation of the Pt nanoparticles into the TiO2 framework does not completely destroy the latter’s mesostructure. Energy dispersive X-ray (EDX) analysis reveals the presence of Pt, Ti, and O and confirms that the final Pt and Ti content in the composite materials is consistent with the Pt:Ti ratio used in the starting sol mixtures, and the quantitative results of EDX of the PtTiO2-2 sample clearly show that the weight percents of Pt, Ti, and O are 0.49, 40, and 59.5, respectively. Photocatalytic Oxidation of Dichloroacetate (DCA). It is well-known that DCA, which was used as the model compound in all experiments assessing the photocatalytic activity of the newly prepared photocatalysts (employing a pH-stat technique at pH 3), can be completely mineralized photocatalytically upon UV illumination, obeying the following stoichiometry.28 UV

CHCl2 COO- þ O2 sf 2CO2 þ Hþ þ 2ClPt=TiO2

ð1Þ

The rate of the photodegradation of DCA was followed by measuring the amount of OH- added to keep the pH constant, thus measuring the amount of Hþ formed, which is equivalent to the amount of DCA degraded. The photonic efficiencies (ζ) of the photocatalytic degradation of DCA were calculated as the ratio of the initial degradation rate of DCA (the initial degradation rate calculated from the initial slope of the individual concentration versus time profiles) and the incident photon flux according to eqs 2 and 3. The incident photon flux per volumetric unit has been calculated to 5787



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Figure 4. TEM images of mesoporous Pt/TiO2 nanocomposites calcined at 450 °C for 4 h: PtTiO2-3 (a) and in situ prepared PtTiO2-1 (b). HRTEM image of PtTiO2-3 (c). Dark-field TEM image of mesoporous PtTiO2-1 nanocomposites (d) and PtTiO2-3 nanocomposites (e).

Figure 5. Time course of DCA photocatalytic degradation (shown as release of Hþ) using mesoporous TiO2, Pt/TiO2 nanocomposites, TiO2 P 25, and Pt/TiO2 P 25 as photocatalysts. Slope produced from the initial rate used for the determination of the photonic efficiency of each run [photocatalyst loading, 0.5 g/L; 1 mM aqueous solution of DCA (O2- saturated, pH 3; T = 25 °C); reaction volume, 60 mL; Io = 6.32 10-6 einstein L-1 s-1 (ca. >320 nm)].

be 6.32 10-6 einstein L-1 s-1 based upon the UV-A light meter measurements and assuming an average illumination wavelength λ = 350 nm; the irradiated surface area was 3.14 cm2, and the volume of the suspension was 0.06 L. Io ¼ Iλ=NA hc

ð2Þ

%ξ ¼ ðkco V =Io AÞ 100

ð3Þ

with Io being the photon flux, I the light intensity, NA Avogadro’s number, h the Planck constant, c the light velocity, k the rate

Figure 6. Comparison of photocatalytic activities of TiO2, PtTiO2-1, PtTiO2-2, and PtTiO2-3 nanocomposites and commercial TiO2 P 25 and Pt/TiO2 P 25.

constant, A the illuminated area, co the initial DCA concentration, λ the illumination wavelength, and V the reactor volume. Figure 5 shows the time profiles of the DCA degradation observed for the investigated mesoporous TiO2 and for the Pt/ TiO2 nanocomposites prepared by different processes compared with that of the commercial photocatalyst Aeroxide TiO2 P 25 (Evonik Industries AG) and Pt photodeposited onto TiO2 P 25 (Pt/TiO2 P 25). The mineralization of DCA was confirmed by TOC measurements at the end of the experimental runs. In all runs, more than 90% DCA removal was observed. A linear relation between the DCA degradation and the irradiation time was obtained during the first 60 min of these experiments. The initial Hþ formation rates calculated from these slopes show that 5788


The Journal of Physical Chemistry C the photocatalytic activity for the degradation of DCA can indeed be considerably increased by this platinization of TiO2, i.e., when Pt islands are present on its surface or within the TiO2 framework (Table 1 and Figure 5). In general, for both preparation routes the photonic efficiency of the mesoporous TiO2 photocatalysts is found to be increased by the addition of the Pt islands and to correlate with the size of the Pt particles. Figure 6 shows the photonic efficiencies of the mesoporous Pt/TiO2 nanocomposites compared with that of Aeroxide TiO2 P 25 and Pt/TiO2 P 25. The results indicate that the photonic efficiencies ξ of TiO2 P 25 and of mesoporous TiO2 are 3.92 and 4.2%, respectively, while the photonic efficiency of the mesoporous TiO2 photocatalyst is drastically increased to 7.85% by Pt addition (PtTiO2-3). Also, photonic efficiency of nonporous TiO2 P 25 was increased from 3.92 to 5.71% after Pt photodeposition (Table 1). Moreover, the results indicated that the initial rate of Hþ formation and thus the photonic efficiency ξ of mesoporous TiO2 are higher than that of the nonporous material TiO2 P 25 (Figure 6 and Table 1). This can be attributed to a higher intrinsic activity of the mesoporous photocatalyst enabling rapid DCA diffusion, thus increasing the photonic efficiency. Second, the increased photocatalytic activity of this mesoporous TiO2 can be explained by its high surface area, which should be beneficial for the adsorption of the DCA molecules (Table 1). On the other hand, it is well-known that Pt deposits on semiconductor surfaces act as efficient catalysts for electron transfer reactions.29 And also, platinization decreases the recombination probability of photoholes with their counterparts (e-cb or e-tr) and increases the fraction of photoholes available for the oxidizing interfacial charge-transfer reactions.31 It is therefore proposed that the reaction of the electrons formed upon the illumination of the photocatalyst with molecular O2 is rate-limiting when DCA is reacting with the holes (eq 5). Photodeposition of Pt on the TiO2 (PtTiO2-3) surface seem to catalyze this reaction effectively, and therefore, a considerable increase in ξ is observed. Also, the Hþ rate formation and thus the ξ value of PtTiO2-3 is higher than that of either PtTiO2-1, PtTiO2-2, or Pt/TiO2 P 25. This difference cannot be explained by different surface areas, because all of those samples exhibit almost similar BET values ∼ 150 m2/g (Table 1). The higher photonic efficiency of the mesoporous PtTiO2-2 as compared with PtTiO2-1 can be attributed to a faster transport of the target molecule DCA to the active sites due to the facile diffusion of the DCA through the ordered porous network, as concluded from the SAXS results (Figure 1). The even larger photoactivity of the mesoporous Pt/ TiO2 nanocomposites prepared using the photodeposition method (PtTiO2-3) is attributed to the high dispersity and the small size of Pt (3 nm). When photons with energies larger than the bandgap are absorbed by TiO2 particles, electrons are promoted from the valence band to the conduction band, leaving holes behind in the valence band (eq 4 and Scheme 1). Pt particles, in contact with the TiO2 network, are acting as electron sinks promoting the reduction of O2 onto their surfaces (eq 5). At the same time, the photoinduced holes migrate to the surfaces of the particles, where they react with adsorbed CHCl2COO-, yielding surface-adsorbed CHCl2COO• radicals that were oxidized to Hþ, 2CO2, and 2Cl- (eqs 10-12). Also, H2O2 molecules could be formed in the pores resulting from the O2 reduction on the surfaces of both TiO2 and Pt (eqs 6-9). Therefore H2O2 has been detected during the photocatalytic degradation of DCA. The results revealed that H2O2 concentration was gradually increased from 0.011 to 0.032 mM with increasing illumination time from 10 to 60 min on the PtTiO2-3 photocatalysts. It can be well-explained by H2O2 produced on both the TiO2 and the Pt surfaces, because TiO2 was confirmed to be active in reducing

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Scheme 1. Schematic Illustration of the Proposed Mechanism To Illustrate the Enhanced Photonic Efficiency of Pt/ TiO2 Nanocomposites as Photocatalyst for DCA Photooxidation, Where Absorption of UV Light by the Semiconducting Nanoparticle Promotes an Electron from the Valence Band to the Conduction Band

oxygen to H2O2 (two-electron transfer).24,30 TiO2 þ hν f TiO2 ðecb - þ hvb þ Þ

ð4Þ

TiO2 ðeÞ þ Pt f TiO2 - PtðeÞ

ð5Þ

TiO2 - PtðeÞ þ O2 f TiO2 - Pt þ O2 -•

ð6Þ

O2 -• f Hþ aq f HO2 •

ð7Þ

2HO2 • f H2 O2 þ O2

ð8Þ

H2 O2 þ ecb - f • OH þ

-

ð9Þ

OH

hvb þ =• OH þ CHCl2 COO- f CHCl2 COO• þ

-

OH ð10Þ

CHCl2 COO• f CHCl2 • þ 2CO2

ð11Þ

CHCl2 • þ O2 -• f 2CO2 þ Hþ þ 2Cl-

ð12Þ

In general, the presence of O2 is considered as a prerequisite for efficient TiO2-photocatalyzed oxidation of organic pollutants in wet systems. Contrary to expectation, Wang et al. have shown that removal of O2 by purging the suspensions with N2 resulted in only a small decrease of the yield of oxidation products when they studied the photocatalytic oxidation of methanol in the presence of various photocatalysts.29 Similar observations have been made here (data not shown). The second reason for the higher photocatalytic activity of PtTiO2-3 in comparison with either PtTiO2-1 or PtTiO2-2 might be the size of the Pt particles. The particle size of Pt (3 nm) in PtTiO2-3 is smaller than that in either PtTiO2-1 or PtTiO2-2 (15 nm). Assuming a spherical geometry of the particles, the volume of one Pt nanoparticle prepared by the photodeposition method (particle diameter of 3 nm for 0.5 wt % Pt/TiO2) is calculated to be 14.1 nm3. Taking the density of Pt (21.4 g cm-3), the average weight of one Pt nanoparticle is calculated to be 3.02 5789


The Journal of Physical Chemistry C 10-19 g. Likewise, the average weight of one 10 nm TiO2 nanoparticle (3.894 g cm-3) is 2.04 10-18 g. Therefore, the molar ratio between Pt and TiO2 nanoparticles for the 0.5 wt % Pt/TiO2 preparation is calculated to be about 1:30. For the in situ prepared Pt/TiO2 samples (with larger Pt particles of 15 nm diameter in average) the molar ratio of Pt to TiO2 is calculated to be about 1:3700. According to the above data analysis the following mechanism is suggested. While in the three-dimensional mesoporous TiO2 network one excited TiO2 nanoparticle can transfer the absorbed energy through the network to other ground-state TiO2 particles, the probability of the hole trapping at an hydroxyl surface group forming an adsorbed hydroxyl radical that is subsequently transferred to an adsorbed DCA molecule is considered to be rather high. Consequently, the probability of electron transfer to the Pt particle is increased by an increased DCA diffusion through the pores of the nanostructures. It is therefore proposed that the so-called antenna mechanism together with an increased DCA diffusion can be employed to explain the efficient photooxidation of DCA using mesoporous Pt/TiO2 nanocomposites. Through this mesoporous antenna system the initially generated electrons can be readily transported from the location of light absorption to a suitable interface with the noble metal catalyst, where the actual electron transfer reaction will take place.32 Previous studies concerning the photocatalytic degradation of DCA yielded maximum photonic efficiencies of 4 and 15% for Aeroxide TiO2 P 25 and Sachtleben Hombikat UV-100, respectively. While the photonic efficiency was found to increase by a factor of 4 when the loading of Hombikat UV-100 was raised from 0.5 to 5 g/L, a further increase of the photocatalyst to 10 g/L resulted in an additional increase of the DCA degradation efficiency of 50%. Therefore, considering the increasing experimental problems resulting from very high catalyst loadings, 0.5 g/L for P 25 and 5 g/L for Hombikat UV-100, respectively, have been used in these previous studies.17 In fact, with ξ = 7.95%, the newly prepared mesoporous photocatalysts still do not reach the ξ value reported for Hombikat UV-100, however, only 0.5 g/L catalyst loading has been used here in all experimental work to avoid filtration problems at high photocatalysts loading and excessive materials consumption. Moreover, we have previously9b compared mesoporous Pd/TiO2 and Pt/TiO2 photocatalysts with Hombikat UV-100 concerning their activity for the HCHO formation during the photocatalytic oxidation of CH3OH. This study revealed that mesoporous Pt/TiO2 or Pd/ TiO2 possess a 2.5 times higher activity for the photooxidation of CH3OH than Hombikat UV-100. Upon the basis of these observations, it is difficult right now to attain a clear judgment as to which photocatalyst is more photoactive when only a single model pollutant is tested and only one catalyst loading is employed. Moreover, it is even more difficult to unambiguously compare the results from one laboratory to the next.

4. CONCLUSIONS The choice of the preparation procedure for mesoporous Pt/ TiO2 photocatalysts is shown to be of significant importance for the observed changes in their photocatalytic activity. The preparation procedure of mesoporous Pt/TiO2 nanocomposites is critical in determining the nature of the mesoporous order and the Pt particle size. The addition of Pt into the mesoporous TiO2 network employing either an in situ preparation or a photodeposition method has been found to enhance the photocatalytic activity of TiO2. Platinization of mesoporous TiO2 by

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photodeposition increases the photonic efficiency by a factor of 2 as compared with pure mesoporous TiO2. It is concluded that the network structure has to be taken into account to understand the strong enhancement of photonic efficiency by such a small amount of Pt. Transfer of excitation energy or of photogenerated charge carriers through the network, i.e., an antenna mechanism, is suggested to explain the increased photonic efficiency of DCA oxidation observed for the platinized TiO2 particles.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT A.A.I. acknowledges the Alexander von Humboldt (AvH) Foundation for granting him a research fellowship. ’ REFERENCES (1) (a) Li, H. X.; Bian, Z. F.; Zhu, J.; Huo, Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 4538. (b) Yu, J. C.; Li, G. S.; Wang, X. C.; Hu, X. L.; Leung, C. W.; Zhang, Z. D. Chem. Commun. 2006, 2717. (c) Yu, J. C.; Wang, X. C.; Fu, X. Z. Chem. Mater. 2004, 16, 1523. (2) (a) Hoffmann, M. R.; Martin, S. T.; Chen, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (b) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (c) Ismail, A. A.; Bahnemann, D. W. ChemSusChem 2010, 3, 1057–1062. (3) (a) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (b) Bartl, M. H.; Puls, S. P.; Tang, J.; Lichtenegger, H. C.; Stucky, G. D. Angew. Chem., Int. Ed. 2004, 43, 3037. (c) Dong, W. Y.; Sun, Y. J.; Lee, C. W.; Hua, W. M.; Lu, X. C.; Shi, Y. F.; Zhang, S. C.; Chen, J. M.; Zhao, D. Y. J. Am. Chem. Soc. 2007, 129, 13894. (4) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. 1995, 34, 2014. (5) Crepaldi, E. L.; Soler-Illia, G. J.; de, A. A.; Grosso, D.; Cagnol, F.; Ribot, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125, 9770. (6) (a) Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. Nat. Mater. 2006, 5, 782. (b) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (c) Ishida, T.; Haruta, M. Angew. Chem., Int. Ed. 2007, 46, 7154. (7) (a) Bennett, R. D.; Xiong, G. Y.; Ren, Z. F.; Cohen, R. E. Chem. Mater. 2004, 16, 5589. (b) Rao, C. N. R.; Thomas, P. J.; Edwards, P. P. Chem. Soc. Rev. 2000, 29, 27. (8) (a) Chan, K. Y.; Ren, J.; Cheng, S.; Tsang, K. Y. J. Mater. Chem. 2004, 14, 505. (b) Ciebien, J. F.; Clay, R. T.; Sohn, B. H.; Cohen, R. E. New J. Chem. 1998, 22, 685. (9) (a) Ismail, A. A.; Bahnemann, D. W.; Bannat, I.; Wark, M. J. Phys. Chem. C 2009, 113, 7429–7435. (b) Ismail, A. A.; Bahnemann, D. W.; Robben, L.; Yarovyi, V.; Wark, M. Chem. Mater. 2010, 22, 108–116. (c) Ismail, A. A.; Kandiel, T. A.; Bahnemann, D. W. J. Photochem. Photobiol., A 2010, 216, 183–193. (10) Tran, H.; Scott, J.; Chiang, K.; Amal, R. J. Photochem. Photobiol. A 2006, 183, 41–52. (11) (a) Tada, H.; Kiyonaga, T.; Naya, S.-i. Chem. Soc. Rev. 2009, 38, 1849–1858. (b) Zhang, F.; Guan, N.; Li, Y.; Zhang, X.; Chen, J.; Zeng, H. Langmuir 2003, 19, 8230–8234. (c) Teoh, W. Y.; Madler, L.; Beydoun, D.; Pratsinis, S. E.; Amal, R. Chem. Eng. Sci. 2005, 60, 5852–5861. (12) (a) Shao, Y.; Yin, G.; Gao, Y.; Shi, P. J. Electrochem. Soc. 2006, 153, A1093. (b) Watanabe, S.; Fujiwara, R.; Hada, M.; Okazaki, Y.; Iyoda, T. Angew. Chem. Int. Ed. 2007, 46, 1120. (c) Maldotti, A.; Molinari, A.; Amadelli, R.; Carbonell, E.; Garcia, H. Photochem. Photobiol. Sci. 2008, 7, 819–825. (d) Alvaro, M.; Aprile, C.; Benitez, M.; Carbonell, E.; Garcia, H. J. Phys. Chem. B 2006, 110, 6661–6665. 5790



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