Palladium Doped Porous Titania Photocatalysts: Impact of

Dec 16, 2009 - The photocatalytic oxidation of CH3OH using either highly ordered hexagonal networks in samples calcined at 350 °C, catalysts with ran...
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108 Chem. Mater. 2010, 22, 108–116 DOI:10.1021/cm902500e

Palladium Doped Porous Titania Photocatalysts: Impact of Mesoporous Order and Crystallinity Adel A. Ismail,*,† Detlef W. Bahnemann,† Lars Robben,‡ Viktor Yarovyi,§ and Michael Wark§ †

Institut f€ ur Technische Chemie, Center of Solid State Chemistry and New Materials, Leibniz Universit€ at Hannover, Callinstrasse 3, 30167 Hannover, Germany, ‡Institut f€ ur Mineralogie, Center of Solid State Chemistry and New Materials, Leibniz Universit€ at Hannover, Callinstrasse 3, 30167 Hannover, Germany, ur Physikalische Chemie and Elektrochemie, Center of Solid State Chemistry and and §Institut f€ New Materials, Leibniz Universit€ at Hannover, Callinstrasse 3-3a, 30167 Hannover, Germany Received August 14, 2009. Revised Manuscript Received November 26, 2009

Herein, we report a facile synthesis method for highly ordered hexagonal P6m mesoporous palladium doped titania nanoarchitectures using the F127 triblock copolymer as a template. The mesoporous Pd/TiO2 nanoarchitectures possess high surface areas of 223 m2g-1 and large pore volumes of 0.42 cm3g-1 at 300 °C that are reduced to 162 m2g-1 and 0.29 cm3g-1, respectively, as a result of calcination at 450 °C with their tunable mesopore diameter ranging from 5.7 to 8.3 nm. Transmission electron microscopy (TEM) measurements evince that the framework of the highly crystalline mesoporous Pd/TiO2 is composed of aligned anatase phase grown along the [101] direction. The Pd nanoparticles are well dispersed and exhibit sizes of about 20 nm and they are separated by 1.95 A˚, which agrees with the (200) lattice spacing of face-centered cubic Pd. The newly prepared photocatalysts have been compared with Pd photodeposited onto the commercial photocatalyst Sachtleben Hombikat UV-100 by the determination of the rate of HCHO formation generated by the photocatalytic oxidation of CH3OH. This study revealed that hexagonal P6m mesoporous Pd/TiO2 nanoarchitectures possess a 2.5 times higher activity for the photooxidation of CH3OH than Pd/Hombikat UV-100. The photocatalytic oxidation of CH3OH using either highly ordered hexagonal networks in samples calcined at 350 °C, catalysts with randomly ordered mesoporous channels after calcinations at 450 °C or disordered mesostructures prepared at 550 °C are comparable although the crystallinity of the TiO2 nanoparticles increases strongly only with calcination temperatures exceeding 500 °C. From the economic point of view, ordered mesochannels prepared upon calcination at 450 °C are considered to be the optimum for saving energy in the photocatalyst preparation without much loss of photocatalytic performance. 1. Introduction TiO2 as a photocatalyst has been applied for a variety of environmentally interesting operations regarding water and air purification.1,2 An enhancement of the activity of TiO2 for photocatalytic applications can be achieved, for example, by synthesizing highly structured mesoporous TiO23 or by doping with transition and noble metals.4 At *Author to whom correspondence should be addressed. E-mail: a-ismail@ iftc.uni-hannover.de.

(1) (a) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. Chem. Rev 1995, 95, 69. (b) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341–357. (2) (a) Legirini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 1993, 93, 671. (b) Maldotti, A.; Molinari, A.; Amadelli, R. Chem. Rev. 2002, 102, 3811–3836. (3) (a) Soni, S. S.; Henderson, M. J.; Bardeau, J.-F.; Gibaud, A. Adv. Mater. 2008, 9999, 1–6. (b) Allain, E.; Besson, S.; Durand, C.; Moreau, M.; Gacoin, T.; Boilot, J.-P. Adv. Funct. Mater. 2007, 17, 549–554. (c) Shiraishi, Y.; Saito, N.; Hirai, T. J. Am. Chem. Soc. 2005, 127, 12820–12822. (4) (a) Bannat, I.; Wessels, K.; Oekermann, T.; Rathousky, J.; Bahnemann, D.; Wark, M. Chem. Mater. 2009, 21, 1645–1653. (b) Tang, J.; Wu, Y.; McFarland, E. W.; Stucky, G. D. Chem. Commun. 2004, 14, 1670. (c) Pan, J. H.; Lee, W. I. Chem. Mater. 2006, 18, 847– 853.

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the mesoscale (2-50 nm) the photocatalytic characteristics are greatly enhanced by increased surface areas,4 a proper spatial arrangement can further assist the electron/energy transfer within the mesoporous framework.5 Pd-based catalysts are widely used for the oxidation of higher alcohols to aldehydes at low temperatures,6,7 but they have rarely been used to oxidize smaller and less reactive alcohols ( 320 nm (a cutoff filter was used to remove light with wavelengths below 320 nm) and the reactor was cooled by circulation of H2O. The temperature of the cooling water was stabilized to perform the reactions at 25 °C. Photooxidation reactions were carried out suspending 1 g/L of either mesoporous Pd/TiO2 or Pd/UV-100 with oxygen being purged through the reaction vessel continuously. The suspensions were sonicated at the desired aqueous solution of methanol [30 mM] before the experiment was started and they were stirred in the dark for 30 min to reach the adsorption equilibrium prior to irradiation. HCHO samples were withdrawn at regular intervals 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). The photooxidation rate was determined by measuring the HCHO generated as a result of methanol oxidation during the first 60 min of illumination employing the Nash method.19 The detection limit for HCHO determined by the Nash method is 1.66 μM. The relative error of the measured HCHO concentration was ( 5% as judged from repeated runs under identical conditions. This method is based on the reaction of formaldehyde with acetylacetone and ammonium acetate to form a yellow colored product with a maximum of absorbance at 412 nm. Measurements were carried out using a Varian Cary 100 Scan UV-vis spectrophotometer, following an incubation time of 15 min at 60 °C. H2O2 was analyzed using an enzymatic method.12a 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. The photonic efficiency was calculated for each experiment as the ratio of the HCHO (18) Tauc, J.; Grigorovici, R.; Vanuc, A. Phys. Status Solidi 1966, 15627. (19) Nash, T. Biochem. J. 1953, 55, 416.

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Figure 1. SAXS patterns of as made Pd/TiO2(a) (Inset) and calcined at 350 (b), 450 (c), and 550 °C (d). Inset, WXRD of as made Pd/TiO2(a) and calcined at 350 (b), 450 (c), and 550 °C (d) for 4 h. Shifted for sake of clarity.

formation rate and the incident light intensity as given in the following equation.20 ξ ¼

r  100 I

where ξ is the photonic efficiency (%), r the photooxidation rate of methanol (mol L-1s-1), and I the incident photon flux (4.94  10-6 Ein L-1s-1). The UV-A incident photon flow was determined by ferrioxalate actinometry.21

3. Results and Discussions Structural Investigations. The small-angle X-ray scattering (SAXS) patterns of highly ordered mesoporous Pd/ TiO2 as-made and of samples calcined at 350, 450, and 550 °C (labeled P-350, P-450, and P-550) 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.15 The observed high intensities and the sharpness of the peaks prove that a long-range order exists in the Pd/TiO2 nanoarchitectures. With increasing calcination temperature the diffraction peaks are becoming weaker with the (10) and (20) reflections indicating that the long-range ordering of the mesopores is already disappearing for the samples calcined at 450 °C. After template removal, the structural regularity declines but the lattice parameters calculated from the d10 value decrease only from 12.96 to 11.34 nm indicating an approximately 12.5% contraction of the structure. The (10) reflection indicating the presence of worm-like distributed mesopores is completely lost only after calcinations at 500 °C and above (Table 1). It is evident that after the collapses of the hexagonal ordering even the pore channels themselves start to collapse and disordered mesostructures of crystalline TiO2 are (20) Serpone, N.; Terzian, R.; Lawless, D.; Kennepohl, P.; Sauve, G. J. Photochem. Photobiol. A 1993, 73, 11. (21) Salinaro, A.; Emeline, A.; Hidaka, H.; Ryabchuk, V. K; Serpone, N. Pure Appl. Chem. 1999, 71, 321–335.

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obtained. Wide angle XRD (Figure 1, Inset) showed that the as-made sample is amorphous since no distinct reflections can be observed. With increasing calcination temperature the Pd/TiO2 nanoarchitectures begin to show reflections from anatase phases with peaks characteristic for the (101), (004), (200), (211), and (213) lattice planes evincing that TiO2 phase easily nucleates during heating and already upon calcination at 350 °C transforms into nanocrystals (Figure 1, Inset curve b). Even upon calcination at 450 °C the WAXS reflections are still of low intensity and quite broad. Analyzing the width at halfmaximum of the reflections employing Scherrer0 s equation22 results in TiO2 nanocrystal sizes with a maximum of 10 nm (Table 1). Interestingly, it is clearly seen that no crystalline pd phase is formed before the calcination temperature exceeds 450 °C. Thus, at low Pd content a high dispersion of smaller nanoparticles of Pd within the pores has been obviously achieved. For P-550 the width of all the reflections narrows and their intensity increases indicating that larger TiO2 anatase particles and Pd crystals have been formed at this higher calcination temperature (Table 1). The crystallinity of the TiO2 phase obtained upon calcinations at different temperatures (i.e., samples P-300, P-350, P-450, and P-550) was also followed by Raman spectroscopy (Figure 2). The Raman spectrum of P-300 sample evinces that this sample entirely consists of amorphous phase while with increasing calcination temperature from 350 to 450 °C, the peak intensity of TiO2 anatase phase23 at 148 cm-1 first raises slowly but then above 450 °C strongly proofing increasing crystallinity. The Raman spectra are consistent with the result obtained from the XRD analysis (Figure 1 inset). Nitrogen adsorption isotherms of the mesoporous Pd/TiO2 nanoarchitectures are shown in Figure 3. Typical reversible type IV adsorption isotherms are found for P-350 and P-450. The sharpness of the inflection resulting from capillary condensation at relative pressures p/p0 between 0.45 and 0.7 is characteristic for mesopores ordered in two-dimensional hexagonal symmetry. The mesoporous Pd/TiO2 nanoarchitectures possess high surface areas of 223 m2g-1 and large pore volumes of 0.42 cm3g-1 at 300 °C; they are reduced to 162 m2g-1 and 0.29 cm3g-1, respectively, as a result of calcination at 450 °C (Table 1). The slight decrease in pore size with increasing calcination temperature up to 450 °C reveals that the thickness of the pore walls increases concomitantly from 5.7 to 7.3 nm. Compared with the size of the TiO2 nanocrystallites between 6 and 10 nm (Table 1), the wall thickness is found to be slightly smaller, implying that some of the TiO2 nanocrystals could partially pierce even into the channel space,24 which is particularly evident from the HRTEM images (Figure 4d and e). For P-550 the hysteresis loop is broader and shifts to higher (22) Azaroff, L. V.; Buerger, M. J. The Powder Method in X-ray Crystallography; McGraw-Hill: New York, 1958, 255. (23) Ohsaka, T.; Izumi, F.; Fujiki, Y. J. Raman Spectrosc. 1978, 7, 321– 324. (24) Liu, R.; Ren, Y.; Shi, Y.; Zhang, F.; Zhang, L.; Tu, B.; Zhao, D. Chem. Mater. 2008, 20, 1140–1146.

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Table 1. Textural Properties of Pd/TiO2 as Made and Calcined at 300, 350, 400, 450, 500, and 550 °C and Pure TiO2 Calcined at 350 and 450 °C and Commercial Hombikat UV100 and Pd Photodeposited on Hombikat UV100 and Their Photocatalytic Propertiesa photocatalysts SBET/m2g-1 PTiO2 (nm) PPd (nm) r  107 (molL-1s-1) d100 (nm) unit cell size/nm pore wall/nm ξ, % Vp (cm3/g) Dp (nm) As made P-300 P-350 P-400 P-450 P-500 P-550 T-350b T-450b Pd/UV-100 pure UV-100

277 223 194 163 116 78 268 174 225 230

6 8 9 10

20 22 23 25

13 3.5 8 10 10

30 5

0.86 5.18 6.20 7.16 7.50 8.10 2.83 4.28 3.33 2.30

12.96 12.19 11.78 11.31 11.34

14.98 14.08 13.60 13.06 13.10

5.77 5.95 6.41 7.33

11.87 11.45

13.71 13.23

6.50 6.70

1.74 10.5 12.5 14.7 15.3 16.3 5.71 8.63 6.72 4.62

0.42 0.33 0.32 0.29 0.22 0.19 0.30 0.30 0.31

8.31 7.64 6.65 5.77 7.96 12.6 7.21 6.53 3.68

a SBET, surface area; PSTiO2, average particle size of TiO2 nanoparticle; PSPd, average particle size of Pd nanoparticle; r HCHO production rate; ξ photonic efficiency; Vp, pore volume; Dp, pore diameter. b Pure TiO2 calcined at 350 and 450 °C.

Figure 2. Raman spectra of mesoporous Pd/TiO2 calcined at 300 (a), 350 (b), 450 (c), and 550 °C (d) for 4 h. Shifted for sake of clarity.

Figure 3. N2 sorption isotherms and pore size distributions (inset) of the mesoporous of Pd/TiO2 calcined at 350, 450, and 550 °C for 4 h and Pd/commercial Hombikat UV-100.

relative pressure (Figure 3). This broadening and shifting indicates the loss in long-range ordering of the mesopores and the hysteresis loop can instead be interpreted as resulting from the voids between nonordered particles. The main pore sizes increased to 12.6 nm upon calcination of the Pd/TiO2 nanoarchitectures to 550 °C. To allow

correct interpretation of the photocatalytic data nitrogen adsorption isotherms were also measured for Pd photodeposited onto the commercial photocatalyst Sachtleben, Hombikat UV-100 used as reference (Figure 3). The absence of a hysteresis loop shows that the mesoporosity this sample exhibits is even less than ordered in P-550; that is, all pores can be regarded as irregular voids between TiO2 particles. The TEM images of the mesoporous Pd/TiO2 nanoarchitectures for P-350 show a well-defined 2D hexagonal mesostructure, evincing the formation of a highly ordered mesostructure,25 which is consistent with the analysis of the SAXS pattern (Figure 1a). Already upon calcination at 350 °C the TiO2 nanocrystals are randomly oriented within the amorphous walls as indicated by the characteristic lattice fringes (Figure 4a). Selected area electron diffraction (SAED) pattern (Figure 4b and c inset) further confirm that anatase nanocrystallites are progressively formed with increasing calcination temperatures. The particle size of these TiO2 nanocrystals has been measured to be between 6 and 10 nm. As seen in the HRTEM images (Figure 4d) the atomic planes of the Pd particles are separated by 1.95 A˚, which agrees with the (200) lattice spacing of face-centered cubic Pd.26a Furthermore, TEM images (Figure 4c-e) of mesoporous Pd/TiO2 clearly show Pd nanoparticles are well dispersed and exhibit diameters of about 20-30 nm. Although being that large there are no indications that the Pd nanoparticles are not located on the outer surface of the mesoporous TiO2 network. In such case an increased concentrations at the rims of the particles should be observable on the TEM pictures; this is, however, not the case. Instead the growing Pd particles seem to create new pores in the TiO2 network through destruction of parts of the channel walls. Such creation of new pores is well-known, for example, from the growth of Pt particles in zeolites.26b The Pd nanoparticles are found to be much larger than the TiO2 nanocrystals. Both sorts of nanoparticles, TiO2 and Pd, are partly in close contact as seen most (25) Zhao, D. Y.; Feng, J. L.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (26) (a) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Yin, Y.; Li, Z.-Y. Nano Lett. 2005, 5, 1237–1242. (b) Tonscheidt, A.; Ryder, P. L.; Jaeger, N. I.; Schulz-Ekloff, G. Surf. Sci. 1993, 281, 51–61.

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Figure 4. TEM images of two-dimensional hexagonal mesoporous Pd/TiO2 nanocomposites calcined at 350 (a), 400 (b), and 450 °C (c). The insets show the SAED patterns for the anatase phase at 400 (b) and 450 °C (c). HRTEM image of Pd cuboctahedron using the (200) reflection beams and TiO2 anatase phase using (101) (d), the bright-field TEM images of Pd/TiO2 at 350 °C (e), and the dark-field TEM image of Pd/commercial Hombikat UV-100 (f).

impressively in Figure 3e; the lattice fringes exhibit the typical distances, that is, Pd (200) (1.95 A˚), Pd (111) (2.24 A˚), and TiO2 (101) (3.54 A˚). It is important to note that the incorporation of Pd nanoparticles into the TiO2 framework does not completely destroy the latter’s mesostructure. In contrast, TEM of Pd photodeposited onto the TiO2 Hombikat UV-100 (Figure 4f) show that Pd particles are highly dispersed onto the TiO2 surface with an average particle size of 5 nm. FTIR spectra for pure TiO2 and of Pd/TiO2 calcined at 350 °C (See Supporting Information (SI) S1) demonstrate a broad absorbance peak in the range from 3100 to 3450 cm-1 assigned to hydroxyls vibration and a strong absorbance peak around 1640 cm-1 attributed to the vibrations of the surface-adsorbed H2O and Ti;OH bonds.27 These FTIR spectra show no distinct absorption peaks that can be attributed to C;H stretching and bending modes, suggesting that the bulk of the templates have been removed by calcination at 350 °C. To evaluate the temperatures at which the template removal is compeleted and at which anatase begins to crystallize, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on the as-synthesized hexagonal mesostructured PdO/TiO2 nanocomposites. (See SI S2). Diffuse reflectance UV-visible spectra of Pd/TiO2 nanoparticles calcined at different temperatures and Hombikat UV-100 are shown in Figure 5. It is evident that for Hombikat UV-100 and Pd/TiO2 samples calcined at different temperatures, there is a broadband absorption from 250 to 350 nm, due to the transition from the O2- antibonding orbital to the lowest empty orbital of Ti4þ.28 The band gaps of the Hombikat UV-100 as a standard and P-300, P-350, P-450, and P-550 photocatalysts estimated from the tangent lines are 3.38, 3.33, 3.35, (27) Li, H. X.; Li, J. X.; Huo, Y. N. J. Phys. Chem. B 2006, 110, 1559. (28) Xu, Z.; Shang, J.; Liu, C.; Kang, C.; Guo, H.; Du, Y. Mater. Sci. Eng., B 1999, 63, 211.

Figure 5. Plot of transferred Kubelka-Munk versus energy of the light absorbed of the mesoporous of Pd/TiO2 calcined at 300 (a), 350 (b), 450 (c), and 550 °C (d) for 4 h and Hombikat UV-100 as a standard. Inset, diffuse reflectance spectra for the samples.

3.4, and 3.22 eV, respectively. These values are all in the range of experimental error ((0.2 eV) and thus the observed slight increase in particle size of TiO2 crystallites (Table 1) with increasing calcination temperature has no influence on the band gap energy. Photocatalytic Oxidation of Methanol. 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 (Scheme 1). The photoinduced holes migrate to the surfaces of the particles where they react with adsorbed hydroxide ions yielding surface adsorbed •OH radicals. At the same time, the Pd particles, in contact with the TiO2 network, are acting as electron sinks promoting the reduction of O2 onto their surfaces (eq 3). We have employed methanol to determine the amount of •OH radicals produced in the photocatalytic process. In the presence of molecular oxygen, HCHO is

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Scheme 1. Proposed Antenna and Reaction Mechanisms for Methanol Photooxidation to Illustrate the Enhanced Photonic Efficiency of Mesostructured Pd/TiO2 Photocatalyst, Absorption of UV light by the Semiconducting Nanoparticle Promotes an Electron from the Valence Band to the Conduction Band. a

a The lines in the scheme show cut perpendicular to the c axis of the hexagonal pore system which extends infinitely in this direction.

formed as intermediate oxidation product in a quantitative reaction as follows in eqs 1 and 2. The photonic efficiency of •OH radical formation can, thus, can be determined as the ratio of the production rate of HCHO and the incident light intensity. •

OH þ CH3 OH f •CH2 OH þ H2 O

ð1Þ

•

CH2 OH þ O2 f HCHO þ HO• 2

ð2Þ

TiO2 -Pdðe - Þ þ O2 f TiO2 -Pd þ O2 -•

ð3Þ

O2 -• þ 2Hþ eq f H2 O2

ð4Þ

2HO• 2 þ 2Hþ eq f H2 O2 þ 2• OH

ð5Þ

H2 O2 þ e - f •OH þ - OH

ð6Þ

The photocatalytic efficiencies of the newly synthesized 3D Pd/TiO2 nanoarchitectures calcined at temperatures between 300 and 550 °C for the photooxidation of aqueous of CH3OH [30 mM] to HCHO were compared with that of Pd photodeposited onto Hombikat UV-100 (Table 1 and Figure 6). Figure 6 shows the change of the HCHO concentration as a function of the irradiation time for the different Pd/TiO2 photocatalysts. A linear relation of the HCHO concentration with irradiation time was obtained during the first 60 min of illumination. From this figure the rate of HCHO formation was found to increase from 0.86 to 8.1  10-7 mol L-1s-1 with increasing calcination temperature from 300 to 550 °C whereas the rate using Pd/ Hombikat UV-100 is 3.33  10-7 mol L-1s-1. For calcination temperatures of at least 350 °C the photocatalytic activities of the hexagonally meso-

Figure 6. Photooxidation of methanol over Pd/TiO2 mesostructured calcined at 300, 350, 400 and 450, 500, and 550 °C and Pd/ Hombikat UV-100 photocatalysts for HCHO formation as a function of illumination time. Photocatalyst loading, 1 g/L; 30 mM aqueous solution of CH3OH (O2- saturated, natural pH; T = 20 °C); reaction volume, 75 mL; Io = 4.49  10-6 Einstein L-1 s-1 (ca. >320 nm).

structured Pd/TiO2 nanoarchitectures exceed that of Pd/Hombikat UV-100, although the Hombikat TiO2 material was calcined at 450 °C (100% anatase phase, SI S3) and is, thus, much more crystalline than most of newly prepared mesostructured Pd/TiO2. This difference cannot be explained by different surface areas, because this is even higher for the Hombikat material (Table 1). Such high photonic efficiencies of the mesoporous Pd/TiO2 as compared with Pd/Hombikat UV-100 can be attributed to several effects, such as a lower light scattering effect of the ordered mesopores, an accumulated local concentration of •OH,29 or a fast transport of the target molecule CH3OH to the active sites due to the facile diffusion of the CH3OH through the ordered porous network, which for the Hombikat UV-100 reference sample is hindered by the heterogeneities existing in the bulk sample. Interesting, however, is also the comparison of the activities of the new nanoarchitectures prepared at different calcination temperatures. As shown in Figure 7, the crystallinity increases by a factor of 4 when the samples are calcined at 550 °C rather than at 350 °C whereas the increase in photonic efficiencies is smaller than 50%. Considering the larger internal surface area (163 m2/g) of P-450 as compared with the surface area of P-550 (78 m2/g), in P-450 most of the anatase phase exists as nanoparticulate bricks forming the pore walls. Hence, the photocatalytic •OH production must occur mainly on an internal surface. Furthermore, CH3OH adsorption onto P-450 should take place mainly within the pores of this high surface area material. Therefore, it can be expected that the concentration of CH3OH inside the pores will be higher in the latter samples as compared with those achievable for P-550. The slightly higher HCHO (29) (a) Lawless, D.; Serpone, N.; Meisel, D. J. Phys. Chem. 1991, 95, 5166–5170. (b) Tojo, S.; Tachikawa, T.; Fujitsuka, M.; Majim, T. Chem. Phys. Lett. 2004, 384, 312–316.

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Figure 7. Correlation between calcination temperature, crystallinity and photonic efficiency of mesoporous Pd/TiO2 nanocomposites calcined at 300, 350, 400 and 450, 500, and 550 °C.

formation rate found on P-550 can be explained by its much higher crystallinity as compared with P-450 (Figure 7). These crystalline anatase particles are believed to be responsible for the production of reactive •OH radicals, the formation probability of which should thus be much higher for P-550. Moreover, the high photocatalytic activity of P-350 is surprising since its crystallinity is by a factor of more than 2 weaker than that of P-450. The higher BET surface area of P-350, which exceeds that of P-450 by 30%, cannot explain the good photocatalytic performance alone because Pd-doped Hombikat 100 has a similar surface area. The latter is, however, by a factor of 1.5 photocatalytically less active. Therefore, we suggest that the well-ordered mesostructure of P-350 supports the transport properties of all reactants involved in the photocatalytic process and, thus, enhances the overall activity. The good photocatalytic performance of P-550, on the other hand, indicates that a highly ordered mesoporous system is not a prerequisite for high photocatalytic activity. However, both P-350 and P-450 must be considered as economically more viable photocatalysts as compared to P-550 since for their preparation energy can be saved in the calcination step. A second explanation for the higher photocatalytic activity of the newly prepared mesoporous Pd/TiO2 in comparison with Pd/Hombikat UV-100 might be the size of the Pd particles. Smaller Pd (5 nm) particles like in Pd/ Hombikat UV-100 have been reported to be more oxophilic thus exhibiting an increased OH- adsorption.30 This might block the active sites for the CH3OH oxidation and/or reduce the number of active sites on which the oxygen reduction can proceed. Considering that the surface coverage by OH- is particle size dependent, one may expect that the kinetics of the oxygen reduction will also be determined by the oxophilicity of the Pd (30) (a) Mayrhofer, K. J.; Blizanac, B. B.; Arenz, M.; Stamenkovic, V. R.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2005, 109, 14433. (b) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 117. (c) Norskov, J. K.; Rossmeisel, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108, 17886.

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nanoparticles.30a The difference in activity found for pure mesoporous TiO2 (T-350 and T-450) and doped ones (P-350 and P-450, See table 1) thus suggests that the rate of electron transfer from mesoporous TiO2 nanocrystals to adsorbed oxygen is increased when Pd is incorporated into mesoporous TiO2 (Table 1 and figure 6). Furthermore, Pd nanoparticles embedded within the TiO2 framework may serve as sink of electron thus facilitating photoelectron transfer to the pore surface and reducing the probability of charge recombination (Scheme 1). A positive correlation was observed between the photonic efficiency and the H2O2 generation ability. •OH generated in the mesopore scarcely has the chance to diffuse out of the pores, and hence the equilibrium concentration of •OH in the mesopore under UV irradiation should depend on the local concentration of the H2O2 molecules in the pores resulting from the O2 reduction on the surfaces of both TiO2 and Pd (See eqs 3-5 and scheme 1); Thus, the photocatalytic activity of the mesoporous TiO2 nanoarchitectures is suggested to be closely related to the pore size of its mesoporous structures that determine the accumulated concentration of H2O2.31 The increased H2O2 concentration from 0.016 to 0.037 mM, detected with increasing illumination time from 10 to 60 min on the P-450 photocatalysts, can hence be well explained by H2O2 produced on both the TiO2 and the Pd surfaces because TiO2 was confirmed to be active in reducing oxygen to H2O2 (two-electron transfer).12a,32 In general, the rate of photooxidation of CH3OH is clearly proportional to the rate of H2O2 formation. The aqueous methanol solution was purged with Ar for 30 min to remove dissolved gas and the experiments were carried out under the same condition in presence of O2. No H2O2 is detected in the absence of oxygen. It is clearly seen that oxygen is present to act as the primary electron acceptor. As a consequence of the two-electron reduction of oxygen,32 H2O2 is formed (see eqs 3-5 and scheme 1). According to the above data analysis the following mechanism is suggested. While in this three-dimensional solid/surface state framework the excited TiO2 nanoparticle can transfer the absorbed energy through the mesoporous hexagonal TiO2 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 CH3OH molecule is considered to be high. Consequently, the probability of electron transfer to the Pd particle is increased by an increased CH3OH diffusion through the pores of the nanostructures. It is therefore suggested that the so-called antenna mechanism33 together with an increased CH3OH diffusion can be employed to explain (31) Naito, K.; Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Am. Chem. Soc. 2009, 131, 934–936. (32) Kormann, C.; Bahnemann, D.; Hoffmann, M. R. Environ. Sci. Technol. 1988, 22, 798–806. (33) (a) Wang, C.-Y.; Pagel, R.; Dohrmann, J. K.; Bahnemann, D. C. R. Chim. 2006, 9, 761–773. (b) Wang, C.; Pagel, R.; Bahnemann, D.; Dohrmann, J. K. J. Phys. Chem. B 2004, 108, 14082–14092. (c) Lakshminarasimhan, N.; Bae, E.; Choi, W. J. Phys. Chem. C 2007, 111, 15244–15250.

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the efficient photooxidation of methanol using the newly prepared Pd/TiO2 mesopores. Within this antenna model, it can be imagined that the overlap of the energy bands of the TiO2 nanoparticles forming this network will result in unified energy bands for the entire system enabling a quasi-free movement of the photogenerated charge carriers throughout.10a Consequently, an electron generated by light absorption within one of the nanoparticles forming the network will subsequently be available to promote redox processes anywhere within the structure (Scheme 1). Therefore, it should be possible to apply smart molecular engineering approaches to design photocatalytic systems exhibiting considerably higher photoactivity than existing systems through an improvement of the photonic efficiency using mesostructured photocatalysts. This could lead to an important breakthrough in photocatalytic research since the photonic efficiency still represents the greatest bottleneck for photocatalytic systems. Conclusions Hexagonal P6m mesoporous Pd/TiO2 nanoarchitectures showed 2.5 times higher activity for the photooxidation of CH3OH than Pd photodeposited on commercial Sachtleben Hombikat UV-100. The increased HCHO formation rate revealed that the photocatalytic oxidation efficiencies within the mesoporous Pd/TiO2 system is (in spite of its lower surface area) superior to that of Pd/UV-100. The key to this success is the preparation of Pd/TiO2 networks with ordered mesopores

Ismail et al.

which at the same time render the methanol diffusion into the bulk of the photocatalysts facile and hence provide fast transport channels for the methanol molecules. The photocatalytic oxidation of methanol using either highly ordered hexagonal networks in samples calcined at 350 °C, catalysts with randomly ordered mesoporous channels after calcinations at 450 °C or disordered mesostructures prepared at 550 °C are comparable although the crystallinity of the TiO2 nanoparticles increases strongly only with calcination temperatures exceeding 500 °C. This indicates that good charge carrier transport properties are as well important as high crystallinity of the anatase phase. From the economic point of view, ordered mesochannels prepared upon calcination at 450 °C are considered to be the optimum for saving energy in the photocatalyst preparation without much loss of photocatalytic performance. Acknowledgment. A.A.I. acknowledges the Alexander von Humboldt (AvH) Foundation for granting him a research fellowship. We thank Mr. M. Sharifi, Institut f€ ur Physikalische Chemie and Elektrochemie, Leibniz Universit€ at Hannover for N2 adsorption measurements. Supporting Information Available: FTIR spectra for Pure TiO2 and Pd/TiO2, thermogravimetric analysis (TGA) and differential thermal analysis for as made sample PdO/TiO2 and X-ray diffraction of Pd photodeposited onto the commercial photocatalyst Sachtleben Hombikat UV-100. This material is available free of charge via the Internet at http:// pubs.acs.org.