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Applied Catalysis A: General 253 (2003) 389–396

Characterization and photocatalytic activity of noble-metal-supported surface TiO2 /SiO2 Chun Hu a,∗ , Yuchao Tang a,b , Zheng Jiang a , Zhengping Hao a , Hongxiao Tang a , Po Keung Wong c b

a Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China Department of Environmental Engineering, Anhui Institute of Architecture and Industry, Hefei 230022, PR China c Department of Biology, The Chinese University of Hong Kong, Shatin, N. T., Hong Kong SAR, PR China

Received 16 January 2003; received in revised form 13 May 2003; accepted 21 June 2003

Abstract M-TiO2 /SiO2 photocatalysts were prepared by the photodeposition method using noble-metal salts (M: Pt4+ , Pd2+ , and Ag+ ) as precursors and the surface bond-conjugated TiO2 /SiO2 as supporter in N2 atmosphere. The photocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflection spectra (DRS), photoluminescence (PL) spectra, and zeta-potential. Their photocatalytic activities were evaluated using reactive Brilliant Red K-2G (K-2G) and Cationic Blue X-GRL (CBX) that showed different types of adsorption behavior on the oxides. XPS analysis verified that the introduction of noble metal led to the changes of the electronic environmental of Ti cations and the zeta-potential of oxides. As a result, K-2G has higher adsorption on Pt-TiO2 /SiO2 than on TiO2 /SiO2 , while the adsorption of CBX has little change on the modified TiO2 /SiO2 catalysts. At the same time, Pt-modified catalyst shows 2.8 times higher photoactivity than TiO2 /SiO2 for the photodegradation of K-2G, but has a decrease in activity for CBX degradation. These noble-metal-supported TiO2 /SiO2 can efficiently extend the light absorption to the visible region. The PL results demonstrated that the noble metal dopant acted as electron acceptor to hinder the recombination of the photoinduced electron–hole pairs. © 2003 Elsevier B.V. All rights reserved. Keywords: Azo dyes; Adsorption; Noble metal; Surface modification; Titania/silica

1. Introduction TiO2 is the most widely used photocatalyst, due to its optical and electronic properties, low cost, chemical stability and non-toxicity. Due to its band-gap energy, TiO2 utilizes only a very small fraction of the solar spectrum; thus, doping with transition metals has been employed to extend the light absorption to the visible region [1–5]. The presence of foreign ∗ Corresponding author. Tel.: +86-10-628-491-94; fax: +86-10-629-235-64. E-mail address: [email protected] (C. Hu).

metal species is generally detrimental for the degradation of organic species in aqueous system, but many controversial results are reported. The modified photocatalyst is more active for one compound, but it is unavailable for another one. The effect of the additive metal species on the photoactivity depends on the support (previous catalyst), the preparation methods, and the degraded pollutant. Some researches have dealt with the photocatalytic deposition of Pt and other metals on TiO2 powders whose reports have suggested the application of this technique to the preparation of supported catalysts [6]. The photodeposition technique yields more active

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00545-3

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photocatalyst [7–9]. The photodeposition process involves the reduction of metal ions by conduction band (CB) electrons, the anodic process being the oxidation of water by valence band (VB) holes. Oxidizable additives are generally added to improve the rate of deposition: for example, acetate, formaldehyde, methanol or 2-propanol. In a previous paper, we have shown that fixed surface bond-conjugated TiO2 /SiO2 photocatalyst has higher photoactivity than TiO2 powder in eliminating azodyes. However, the catalyst could not be excited by visible light. In this present work, noble-metal-modification of TiO2 /SiO2 by photodeposition technique was investigated. Pt and Pd have a suitable quantity of holes in the d-band and a very low overpotential. The addition of these noble metals can change the photocatalytic performance of TiO2 /SiO2 . Therefore, Pt, Pd, and Ag were loaded on the surface of the catalyst under UV illumination with N2 atmosphere and addition of acetic acid. These noble-metal-supported catalysts were evaluated by the photodegradation of reactive Brilliant Red K-2G and Cationic Blue X-GRL, which have different types of adsorption behavior. The relationship between the photoactivity of the catalyst and reactant is discussed. 2. Experimental 2.1. Materials and methods Tetrabutyl titanate (Chemical Purity) was from Changcheng Chemical Company, Beijing. Silica gel was purchased from Qingdao Chemical Factory, China. K-2G and CBX (Shanghai Chemical Company) were used without further purification. All other chemical reagents were of analytical grade. 2.1.1. Preparation of photocatalysts Silica gel was dried and impregnated with cyclohexane solution of Ti(OC4 H9 )4 for 15 h. After the cyclohexane solvent was completely vaporized at 313 K, the silica gel was dried at 393 K for 6 h, and then calcined in the air in three different steps: 473 K for 1 h, 623 K for 1 h, and then 723 K for 8 h. 2.1.2. Noble-metal-modified TiO2 /SiO2 catalysts A variety of 1% M-TiO2 /SiO2 photocatalysts (M: Pt, Pd, and Ag) were prepared by photodeposition in a

way similar to the method used by Kraeutler and Bard [10]. All preparation experiments were carried out in a fluidized bed photoreactor, which consists of a 400 ml cylindrical glass body with sampling port, gas outlet port, and gas inlet at the bottom of the glass body. Photodepositions of 1% Pt-TiO2 /SiO2 , 1% Pd-TiO2 /SiO2 and 1% Ag-TiO2 /SiO2 were achieved by 8 W blacklight lamp (365 nm)-illumination of 275 ml of suspensions containing 15 ml acetic acid, 5 g TiO2 /SiO2 , and the PdCl2 or H2 PtCl6 ·6H2 O solution required for 15 h in a deaerated system (N2 atmosphere). During the photodeposition of PdCl2 , the color of the suspensions gradually turned from dark brown to gray. During the illumination of H2 PtCl6 ·6H2 O, the suspension gradually turned from orange to gray. After the illumination, the three dark gray catalysts were washed with distilled water, and then were dried at ambient temperature. 2.2. Characterization of M-TiO2 /SiO2 The M-TiO2 /SiO2 particles were analyzed by X-ray diffraction (XRD) patterns using Cu K␣ radiation (λ = 1.54059 Å) on a D/Max-RC X-ray diffraction meter. Diffraction patterns were taken over the 2θ range of 20–80◦ . To confirm the morphology of metal loaded on the catalyst, X-ray photoelectron spectra were recorded with a VG AM-08-09-2000 XPS instrument using the Al K␣1,2 excitation source. The samples were pretreated before XPS measurement. Calibration of the spectra was conducted at the C 1s peak of surface contamination taken at 284.6 eV. Quantitative analysis was carried out using the sensitive factors supplied with the instrument. The zeta-potential of the catalyst was measured for suspensions containing 0.1 g l−1 of catalyst in 1 mM KNO3 with a Model JS94F micro-electrophoresis apparatus (Shanghai). The isoelectric point of the catalyst was obtained by the measurement of the zeta-potential. The UV-Vis diffuse reflectance spectra (DRS) of the samples in the wavelength range of 200–800 nm were determined using a spectrophotometer (Hatch UV 3010) with 150 mm␾ integral ball. 2.3. Photocatalytic activity measurement The tested photoreactor is the same as the one used in the preparation of M-TiO2 /SiO2 . A 300 W highpressure mercury lamp surrounded by a glass thimble

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(330–550 nm) was located in the center of the reactor. The photocatalytic activities of various catalysts were evaluated by measuring the loss of K-2G and CBX. In typical experiments, 1.5 g catalyst was suspended in 275 ml of 20 ppm K-2G or CBX aqueous solution by air bubbling at 340 ml min−1 . Prior to irradiation, the suspension aqueous solution was mixed continuously in the dark for 30 min to ensure adsorption/desorption equilibrium. The adsorption percentage of the dye onto the photocatalyst refers to the ratio of the difference between the initial concentration of dye (C0 ) and the concentration of dye at adsorption/desorption equilibrium point (C30 min ) to the initial concentration of dye in solution, that is, adsorption percentage = (C0 − C30 min )/C0 . The concentration of substrate in bulk solution at this point, C30 min , was used as the initial value for the further kinetic treatment of the photodegradation processes. Variations in the concentration of dyes in each degraded solution were monitored using a model 752 UV-Vis spectroscope (Shanghai).

3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD and XPS analysis The XRD patterns showed that only anatase was found in all samples of noble-metal-supported

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TiO2 /SiO2 . No peak from noble metal was observed. The result indicated that the supported metals were dispersed uniformly and packed crystal was not formed. We have shown the structure of TiO2 /SiO2 in detail in the previous paper [11]. In current paper, the noble-metal-supported TiO2 /SiO2 will be investigated. The samples of noble-metal-modified TiO2 /SiO2 and the bare catalyst were investigated by X-ray photoelectron spectroscopy (XPS). There are two types of Pt 4f in the photoelectron peaks of Pt-TiO2 /SiO2 shown in Fig. 1, corresponding to Pt0 and Pt2+ , respectively. The atomic number ratio of Pt0 to Pt2+ is 47.06:52.934 (Table 1). Some studies have argued the influence of different valences of platinum on the activity of photocatalyst [12]. Different valence states of metal play important roles in affecting charge trapping, releasing and migration, recombination, and interfacial charge transfer, and thus the photocatalytic behavior. The binding energy of Ti 2p in Pt-TiO2 /SiO2 (458.8 eV) is shifted to an energy higher than that of TiO2 /SiO2 (458.4 eV), as shown in Table 1. This result shows that the chemical environment of the lattice Ti is changed due to the introduction of Pt. Fig. 2 shows the XPS bands of O 1s core level from different catalysts. Clearly, the position and shape of O 1s peak of Pt-TiO2 /SiO2 differ from that of TiO2 /SiO2 (Fig. 2A). Such results indicated that the negativity of oxygen was changed due to the change of the central ions. Fig. 3 shows two types of Pd 3d photoelectronic

Fig. 1. Pt 4f photoelectron peak of Pt loaded on TiO2 /SiO2 as measured by XPS.

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Table 1 XPS binding energies (eV) for the selected photocatalysts Catalysts

Ti 2p

Si 2p

O 1s

M(0) 2p

M(II) 2p

M/Ti

TiO2 /SiO2 Pt-TiO2 /SiO2 Pd-TiO2 /SiO2

458.4 458.9 457.7

103.3 103.4 103.0

532.5, 530.0 532.9, 530.2 531.9

– 70.81 334.09

– 73.24 335.88

– 0.0409 0.0437

M: Pt, Pd.

peaks for Pd-TiO2 /SiO2 photocatalyst, corresponding to Pd0 and Pd2+ , respectively. The atomic number ratio of Pd0 to Pd2+ is 57.45:42.55 (see Table 1). The binding energy of Ti 2p of Pd-TiO2 /SiO2 (457.7 eV) is shifted to an energy lower than that of TiO2 /SiO2 . Moreover, the position and shape of O 1s peak of Pd-TiO2 /SiO2 differ from that of TiO2 /SiO2 (Fig. 2B). These results imply that the chemical environments

of the lattice Ti and O were changed by the introduction of palladium. The electronic environment of the Ti cation is affected by the number of bonds with oxygen and also by the coordination number of these oxygen ligands. As a result, several types of Ti cation can exist at different metal additions. 3.1.2. UV-Vis DRS and photoluminescence analysis Fig. 4 shows diffuse reflection spectra of these noble-metal-supported catalysts and of the unmodified catalyst. All modified catalysts showed absorption bands due to the formation of metal cluster ions. Thus, the charge transfer transition of a Mn+ → Ti4+ caused the visible light. Fig. 5 shows that these catalysts exhibit a photoluminescence spectrum at around 390 nm by excitation at 300 nm. In our previous work [11], we verified that the growth of titania (predominately anatase) on the silica substrate seems to occur by anchoring of the TiO2 phase through Ti–O–Si cross-linking bonds. When TiO2 /SiO2 samples were excited by around UV light at 300 nm, an electron transfer was brought about from the lattice 4+ oxygen (O2− l ) to the titanium ion (Til ) to form 3+ a charge-transfer excited state (Ti –O− )∗ . Therefore, the observed photoluminescence spectrum is attributed to the radiative decay process from the charge excited state formed in this way to the ground state of the highly dispersed titanium oxide as shown in the following scheme. h␯ [Ti4+ –O2− ]
[Ti3+ –O− ]∗ h ␯

Fig. 2. O 1s photoelectron peak of Pt-TiO2 /SiO2 (A) and Pd-TiO2 /SiO2 (B).

The result is similar to that of the highly dispersed tetrahedrally coordinated titanium oxides chemically anchored onto Vycor glass [13–16]. As shown in Fig. 5, the additions of Pt, Pd, and Ag onto the TiO2 /SiO2 catalyst lead to the efficient quenching of the photoluminescence with different efficiencies. Because the DRS and XPS results indicate that the local

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Fig. 3. Pd 3d photoelectron peak of Pd loaded on TiO2 /SiO2 as measured by XPS.

structures of the titanium oxide species on the silica gel were altered by noble-metal doping, which made an acceptor level in the forbidden band of TiO2 /SiO2 . The effective quenching of photoluminescence can be attributed to the electron transfer from the photoexcited titanium oxide species, electron–holes pair state of (Ti3+ –O− )∗ , to doped noble metal while the holes remain in the titanium oxide species, resulting in the charge separation of electrons and holes from the photoformed electron–hole pair states.

3.1.3. Catalyst zeta-potential Fig. 6 shows that the zeta-potential of the catalysts changes with increasing pH of solution. The iso-electric points of TiO2 /SiO2 is around 3.0 pH units, while those of noble-metal modified catalysts are around 2.00 pH units. The zeta-potential of the Pt modified catalyst is higher than that of the bare one. Such a difference indicates that the surface charge of the former is more positive than the latter, leading to different adsorption behavior as shown below. The

Fig. 4. The diffuse reflection UV-Vis absorption spectra of different catalysts.

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Fig. 5. Photoluminescence spectra of TiO2 /SiO2 and noble-metal catalysts at room temperature after the excitation at 300 nm.

zeta-potentials of Pd and Ag modified catalysts are almost the same as that of the bare one, resulting in similar adsorption behavior. This phenomena could be specified by the above XPS analysis of the catalysts. The introduction of noble-metal-resulted in the change of the electronic environment of the lattice titanium. Thus, the surface properties of the catalysts were changed to bring about different photoactivity. 3.2. Adsorption measurements The equilibrium adsorptions of K-2G and CBX on the surfaces of different catalysts were investigated. Suspensions were stirred for 30 min in the dark to en-

Fig. 7. Adsorptions of K-2G and CBX on the surfaces of different catalysts. (1) TiO2 /SiO2 , (2) Pt-TiO2 /SiO2 , (3) Pd-TiO2 /SiO2 , (4) Ag-TiO2 /SiO2 .

sure equilibration of the dyes over the oxide surface, and the amounts of adsorption of the dyes on the oxides were tested by comparing the concentration before and after stirring. From Fig. 7, it is observed that the amounts of K-2G adsorbed on Pt-supported TiO2 /SiO2 show higher values than that on the bare one, and show lower values for CBX adsorption. Pd or Ag-supported ones have almost the same adsorption amounts as the bare one for two selected dyes. Our previous research has verified that K-2G is adsorbed on the surface of TiO2 /SiO2 by sulfonate group. Based on the XPS and the zeta-potential results, the positive electronic environment of Ti cations of Pt modified

Fig. 6. Plot of the zeta-potential as a function of pH for different catalyst suspensions (0.1 g l−1 ) in the presence of KNO3 (10−3 M).

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catalysts became higher than that of the bare one. The attractive role between sulfonate group and Ti cation increased and caused more adsorption of K-2G on the oxides surface, while CBX was adsorbed on the surface of the catalyst by a positive penta-heterocycle N group. The mode of CBX causes lower adsorption on the positively charged oxide surface due to the repulsive electrostatic power between the oxide surface and adsorbed group. Therefore, the adsorption reduces slightly on the Pt-modified catalyst. The different adsorption behavior for K-2G and for CBX will result in various values of photocatalytic efficiency. The other modified catalysts have similar zeta-potential values to that of the bare one, so no significant adsorption change was observed. 3.3. Photocatalytic activity Fig. 8 shows the results of the photocatalytic degradation of K-2G and CBX over the different noble-metal-supported samples. It is obvious that all noble metal-doped samples show different reactivity values for the both various dyes. For K-2G decolorization, the catalyst modified with Pt exhibits 2.8 times higher photocatalytic activity than the bare one, while the photoactivity is only slightly higher with the addition of Pd and Ag. In contrast, only Pd-TiO2 /SiO2 shows a slightly increased photoactivity over that of the bare one for CBX photodegradation, while the other ones have a decrease in the photoactivity.

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The photo-efficiency depends on light absorption of catalyst, adsorption of catalyst to dyes, charge transfer, and electron–hole recombination. Although the noble-metal-supported catalysts have significant red shift, most of them decrease the photoactivity for CBX photodegradation. The Pt-modified catalyst decreased the adsorption amount of CBX, leading to the reduction of photoactivity, while the catalyst increased the adsorption of K-2G, showing a positive influence. The introductions of Pd and Ag have slight influence on the adsorption for two dyes, and play no significant role in the photo-efficiency for their degradation. The results indicate that the amount of substrate adsorbed on the catalyst is an important factor affecting the photocatalytic activity in photodegradation of dyes. As is well known, the photocatalytic reaction occurs on the surface or near to the catalysts, and recombination of photogenerated electron and hole is very fast, so interfacial charge carrier transfer is possible only when the donor or acceptor is pre-adsorbed before the photocatalytic reaction. The preliminary adsorption of the substrates and the amount of adsorption are very important pre-requisites for highly efficient degradation. It was found that Pd-modified catalysts have slightly higher photoactivity values than the bare one, although their adsorptions have no significant changes for the two dyes. This small increase in activity is due to the inhibition of recombination. Based on the photoluminescence analysis of the samples, these doped noble metals could trap photoinduced electrons to prevent electron–hole recombination. At the same time, the supported metallic Pt, Pd, and Ag could liberate holes to participate in degradation reaction. The XPS and DRS results demonstrated that the doped noble metals affected the structure of TiO2 /SiO2 , most likely leading to modification of the photocatalytic activity.

4. Conclusions

Fig. 8. Photocatalytic degradation rate constants of K-2G and CBX under various illuminated photocatalysts. (1) TiO2 /SiO2 , (2) Pt-TiO2 /SiO2 , (3) Pd-TiO2 /SiO2 , (4) Ag-TiO2 /SiO2 .

The introduction of noble metals affects the structure of TiO2 /SiO2 , leading to different photocatalytic activities. Pt-TiO2 /SiO2 enhances photocatalytic activity in the degradation of K-2G by 2.8 times due to higher adsorption, but exhibits a decrease in activity for CBX due to lower adsorption. Activity and adsorption have parallel changes. The results indicate that the photoactivity could be increased in photodegradation

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of dyes by changing the performances of adsorption to dyes. The PL measurements demonstrated that these supported noble metals are efficient quenching agents of the photoluminescence. They prevent electron–hole recombination and liberate holes to participate in the degradation reaction.

Acknowledgements This research was supported by the National 863 program of China (2002AA649040) and the National Natural Science Foundation of China (contract no. 20007004). References [1] E. Borgarello, J. Kiwi, M. Grätzel, E. Peilizzetti, M. Visca, J. Am. Chem. Soc. 104 (1982) 2996. [2] J.M. Herrmann, J. Disdier, P. Pichat. Chem. Phys. Lett. 6 (1984) 61.

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