Photocatalytic activity of Ag-substituted and impregnated nano-TiO2

Applied Catalysis A: General 366 (2009) 130–140

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Photocatalytic activity of Ag-substituted and impregnated nano-TiO2 R. Vinu, Giridhar Madras * Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 April 2009 Received in revised form 26 June 2009 Accepted 29 June 2009 Available online 15 July 2009

Ag-substituted (Ag sub) and Ag-impregnated (Ag imp), anatase phase nano-TiO2 have been synthesized by solution combustion technique and reduction technique, respectively. The catalysts were characterized extensively by powder XRD, TEM, XPS, FT-Raman, UV absorption, FT-IR, TGA, photoluminescence, BET surface area and isoelectric pH measurements. These catalysts were used for the photodegradation of dyes and for the selective photooxidation of cyclohexane to cyclohexanone. The photoactivities of the combustion-synthesized catalysts were compared with those of commercial Degussa P 25 (DP 25) TiO2, and Ag-impregnated DP 25 (Ag DP). For the photocatalytic degradation of dyes, unsubstituted combustion-synthesized TiO2 (CS TiO2) exhibited the highest activity, followed by 1% Ag imp and 1% Ag sub. For the photoconversion of cyclohexane, the total conversion of cyclohexane and the selectivity of cyclohexanone followed the order: 1% Ag sub > DP 25 > CS TiO2 > 1% Ag imp > 1% Ag DP. The kinetics of the photodegradation of dyes and of the photooxidation of cyclohexane were modeled using Langmuir–Hinshelwood rate equation and a free radical mechanism, respectively, and the rate coefficients were determined. The difference in activity values of the catalysts observed for these two reactions and the detailed characterization of these catalysts are described in this study. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Ag substitution Ag impregnation Combustion synthesis Cyclohexane Photooxidation Rate coefficients

1. Introduction Strict environmental regulations have led to the development of clean processes to selectively synthesize chemical compounds and degrade pollutants. In this regard, heterogeneous photocatalysis has emerged to be an effective route by which toxic organics can be degraded and organic compounds can be synthesized with high selectivity [1]. Among the different photocatalysts, TiO2 is one of the most widely used semiconductor photocatalysts; its unique characteristics are well documented [2]. TiO2 is also used in antifogging and anti-corrosion surfaces, photocatalytic lithography and photochromic materials [3]. Different synthesis procedures have been adopted to enhance the photoactivity of TiO2 by altering its pore size [4], crystallinity [5] or particle size [6]. Numerous attempts have also been carried out to synthesize anion- or cation-doped TiO2 to reduce the bandgap, thereby enhancing the photonic efficiencies. Serpone [7] has addressed the red-shift in the absorption edge of the doped TiO2, and has shown that this is due to the oxygen vacancy that gives rise to color centers, rather than the mixing of dopant and oxygen states. Many different transition metals, viz., lower valent (+1, +2,

* Corresponding author. Tel.: +91 80 22932321; fax: +91 80 23601310. E-mail address: [email protected] (G. Madras). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.06.048

+3), isovalent (+4) and higher valent (+5, +6) cations have been doped in TiO2 by different techniques [8–12]. The photoactivity of the doped TiO2 was tested for a variety of reactions like water splitting, oxidation and reduction of organic compounds, and for gas phase reactions. As the photoactivity of the doped TiO2 strongly depends on the dopant concentration, the energy level of the dopant within the TiO2 lattice, the d-electronic configuration, the distribution of dopant, the interfacial charge transfer and the light intensity [8], an assessment of the effectiveness of the doped TiO2 compared to the undoped TiO2 is not straightforward. Recently, we have reported that Pd doped (substituted) combustion-synthesized TiO2 (CS TiO2) is more photoactive compared to unsubstituted TiO2 for gas phase CO oxidation, NO reduction and NO decomposition [11] due to the creation of redox adsorption sites by Pd2+ ion substitution. However, Pd substituted TiO2 is less active than unsubstituted TiO2 for aqueous phase photodegradation of organics due to the reduced photoluminescence intensity [13]. Ag possesses an inherent anti-microbial activity; the disinfection capability of Ag-loaded TiO2 for the inactivation of E. coli has already been studied [14]. Previous studies elucidated the enhancement in photocatalytic activity of Ag-loaded TiO2 compared to undoped TiO2 for the degradation of monoazo and diazo dyes [15,16], Rhodamine B [17] and humic acid [18]. In all the above studies, Ag was impregnated onto the surface of commercial TiO2 by photoreduction of Ag+ to Ag0. The higher activity was

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attributed to the trapping of the conduction band electrons by Ag particles, which prevents the electron-hole recombination, thereby enhancing the redox reactions on the TiO2 surface. However, the photoactivity of Ag-substituted TiO2 has not been extensively investigated. The selective conversion of cyclohexane to cyclohexanone is an industrially important reaction, because the latter is a starting material for the synthesis of e-caprolactum, which is a raw material for nylon-6. Many catalysts like TiO2 [19], Fe2O3/TiO2 [20], V2O5/Al2O3 [21], Ce-doped MCM-41 [22], Mn aluminium phosphates (MnAPO-5) [23], and LnVO4 and LnMo0.15V0.85O4 (Ln = Ce, Nd, Pr) [24] have been tested for the selective oxidation of this hydrocarbon. In the current work, we report, (a) the synthesis of Agsubstituted TiO2 by solution combustion technique; and the synthesis of Ag-impregnated CS TiO2 and Ag-impregnated commercial Degussa P-25 TiO2 by reduction technique; (b) their characterization by various methods; the photoactivity of these catalysts for (c) the degradation of dyes and (d) the conversion of cyclohexane to cyclohexanol and cyclohexanone. We have studied the kinetics of both the reactions and evaluated the rate coefficients, and rationalized the activity of these catalysts with their structure. 2. Experimental 2.1. Materials Titanium (IV) isopropoxide (TTIP, Alfa Aesar), glycine (NH2– CH2–COOH), silver nitrate (AgNO3) (Merck, India) and hydrazine hydrate (S. D. Fine Chem., India) were used in the preparation of the catalysts. Degussa P-25 TiO2 (DP 25) was received as a gift from Degussa Inc. The dyes chosen in this study belong to different groups by functionality: viz., azoic (Orange G, OG, C16H10N2Na2S2O7), anthraquinonic (Alizarin cyanine green, AG, C28H20N2Na2S2O8), triphenyl methane (Methyl green, MG, C27H35Cl2N3xZnCl2) and heteropolyaromatic (Azure B, AZ, C15H16ClN3S) (see Figure S1 in supplementary data). The dyes: OG, AZ (S. D. Fine Chem., India), MG (Loba Chemie, India) and AG (Rolex Lab, India) were used without prior purification. Cyclohexane, cyclohexanol, cyclohexanone, phenol and chloroform (Merck, India) used were of AR grade. n-hexane (Merck, India) was of HPLC grade. Double distilled Millipore filtered water was used for all purposes. 2.2. Catalyst preparation Solution combustion synthesis methodology was adopted to prepare the catalysts. Ag-substituted TiO2 (Ag sub) was prepared by the combustion of a stoichiometric mixture of titanyl nitrate (which was prepared by the hydrolysis and subsequent nitration of TTIP), glycine and AgNO3 in a muffle furnace at 350 8C. For example, 1% Ag sub was synthesized by combusting 30 mL of the above mixture in the molar ratio 0.99:1.1:0.01. Further details of the synthesis are reported elsewhere [9,11,25]. Ag-impregnated TiO2 (Ag imp) was prepared from CS TiO2 by the reduction technique. In a typical synthesis, CS TiO2 was dispersed in water and a calculated amount of AgNO3 was added. Hydrazine hydrate was then added dropwise with vigorous stirring. The reduction of Ag from Ag+ to Ag0 was confirmed by the darkening of the solution. 1% Ag imp was greyish in color, while higher concentrations of Ag impregnation led to darker appearance. For example, 5% Ag imp was blackish in appearance. Finally, the particles were separated by centrifugation and dried at 120 8C for 5 h. Similarly, 1% Ag-impregnated DP 25 (1% Ag DP) was also prepared from DP 25.

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2.3. Catalyst characterization The synthesized catalysts were characterized for structure, phase, particle size, photoluminescence, surface area, hydroxyl content and band-gap by various techniques. Powder X-ray diffraction (XRD) patterns of the catalysts were collected on a Philips X’pert PRO diffractometer with a Cu Ka source; the instrument was fitted with an X’celerator detector (PW 3050/60). An absolute scan was performed in the 2u range of 15–908 with a step size of 0.017 and an effective time per step of 300 s. Raman spectra were collected in a NXR FT-Raman module with a Ge detector in the range 3500–10 cm1, with an incident laser (Nd:YVO4, operating at 1064 nm) power of 1.5 W. Transmission electron microscopy (TEM) of 1% Ag sub and 1% Ag imp was carried out in JEOL (Model – JEM 2000 FX II) TEM, with an acceleration voltage of 200 kV. Field emission scanning electron microscopy (FE SEM) was carried out in a Carl Zeiss ULTRA 55 FE SEM, with a filament voltage of 10 kV. X-ray photoelectron spectra (XPS) of the catalysts were recorded in an ESCA-3 Mark II spectrometer (VG scientific Ltd., England) using Al Ka (1486.6 eV) radiation as the source. The spectra were referenced to the binding energy of C(1 s), which is 285 eV. UV–vis absorption spectra of the catalysts were recorded in a Perkin Elmer Lambda 35 UV/vis spectrometer, with BaSO4 as the reference. Photoluminescence (PL) spectra of CS TiO2, 1% Ag sub and 1% Ag imp were recorded in a Perkin Elmer LS 55 luminescence spectrometer at an excitation wavelength of 285 nm. 60 mg of the samples were dry pressed and analyzed to compare the spectra of the catalysts. Fourier transform infrared (FT-IR) spectra were collected in a Nicolet 6700 FTIR with a diffuse reflectance accessory. The samples were finely ground with KBr and analyzed in the range 4000–400 cm1. Thermal analysis of the catalysts was conducted in a Perkin ElmerPyris Diamond thermogravimetric-differential thermal analyzer (TG/DTA) at a heating rate of 10 8C min1, in the presence of inert nitrogen at a flow rate of 150 mL min1. Brunauer-EmmettTeller (BET) surface areas of the catalysts were determined in a Smart Sorb 92/93 surface area analyzer by the N2 adsorption– desorption method. 2.4. Photocatalytic reactor The photoreactor consisted of a jacketed quartz tube of 3.8 cm i.d., 4.5 cm o.d. and 21 cm length and an outer pyrex reactor of 5.7 cm i.d. and 16 cm length. A high pressure mercury vapor lamp of 80 W average power was placed inside the quartz tube. The lamp radiated predominantly at 365 nm with an incident intensity of 5.8  106 Einstein L1 s1 and a photon flux of 12.8 W m2, measured using o-nitrobenzaldehyde actinometry [26]. No emission peaks in the visible region of the spectrum were observed, corresponding to the absorption of the dyes. Hence, the degradation of the dye is effected only by direct band-gap excitation of TiO2. Water was circulated in the inner quartz tube to quench the heat generated by ultraviolet (UV) radiation and to maintain the solution temperature below 35 8C. 100 mL of the aqueous dye solution with 1 g L1 of the catalyst was taken in the outer container and degraded by dipping the quartz tube just touching the solution. For the oxidation of cyclohexane, the outer reactor was modified with gas inlet and outlet connections for the bubbling of O2. O2 was admitted at a flow rate of 10 mL min1. 100 mL of 925 mmol L1 of cyclohexane in chloroform was exposed to UV radiation with an optimal catalyst loading of 1 g L1. A detailed description of the setup is available elsewhere [24]. Both dye and cyclohexane solutions were continuously stirred during the reaction for efficient mass transfer of the reactants and products. Samples were withdrawn at regular intervals for the analyses of concentration.

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2.5. Sample analysis Prior to each analysis, all the samples were centrifuged to isolate the catalyst particles. The degradation of the dyes was monitored in a Shimadzu UV-1700 UV–vis spectrophotometer. The concentrations of OG, AG, MG and AZ were calibrated for absorbance at characteristic wavelengths (lmax) of 480, 640, 626 and 647 nm, respectively, according to Beer–Lambert’s law. The pH of the solutions was measured using a pH meter (Eutech Instruments, Singapore). Cyclohexane and its oxidation products, viz., cyclohexanol and cyclohexanone, were monitored in a gas chromatograph (Varian CP-3800) with a flame ionization detector. A VF-5 ms (5% phenyl, 95% dimethyl polysiloxane, 30 m  0.25 mm  1 mm) capillary column was used with helium (UHP grade) as the carrier gas at 0.5 mL min1, and a split ratio of 50. The injector and detector temperatures were maintained at 250 8C. The column oven temperature was initially maintained at 70 8C for 2 min, then ramped at a rate of 2 8C min1 to 90 8C, and finally increased at 15 8C min1 to 120 8C. The aliquots were diluted 30 times in nhexane and then 5 mL of phenol was added as the internal standard before injection. The retention time of cyclohexane, cyclohexanol, cyclohexanone and phenol were 5.1, 9.9, 10.3 and 13.2 min, respectively. The concentration of the analytes was calibrated against the peak area of standard samples. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Powder XRD Fig. 1 shows the powder XRD pattern of 1% Ag sub, 1% Ag imp, 5% Ag imp and 1% Ag DP. The XRD pattern of 1% Ag sub (Fig. 1(a)) shows all the characteristic peaks of anatase TiO2, without any impurity peak corresponding to Ag or to rutile phase. This shows that Ag is substituted for Ti in the lattice. The impregnation of 1% Ag on CS TiO2 results in the formation of Bragg peaks at 38.28, 44.38,

Fig. 1. Powder X-ray diffraction pattern of the catalysts (j-Ag peaks).

64.58 and 77.58, corresponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) plane reflections (Fig. 1(b)). It was also found that impregnation of 5% Ag resulted in an increase in intensity of these peaks (Fig. 1(c)). 1% Ag DP also exhibits Ag peaks at the same position, but with a lower intensity (Fig. 1(d)). These results collectively show that Ag+ is reduced to Ag0, and is well dispersed on the surfaces of CS TiO2 and DP 25. The crystallite sizes of CS TiO2, 1% Ag sub, 1% Ag imp and 1% Ag DP were calculated using the Scherrer formula with CeO2 as the reference; they were found to be 8  2, 10  2, 13  1 and 64  3, respectively. The crystallite size of anatase grains of DP 25 was previously reported to be 37 nm [6]. Full profile Rietveld refinement of 1% Ag sub was carried out using GSAS [27]. Initially, the cell parameters of CS TiO2, viz., a = 3.7865 A˚ and c = 9.5091 A˚ [25], in the tetragonal space group I41/amd were taken, and 4a and 8e Wycoff sites were assigned to Ti and O, respectively. Full occupancy was assigned to Ti, O and Ag, and the isotropic thermal parameters, unit cell and background parameters were refined. Figure S2 (see supplementary data) shows the experimental, calculated and difference XRD profiles of 1% Ag sub, with residual factors of Rp = 0.069, wRp = 0.092 and DWd = 1.21. The cell parameters of 1% Ag sub were found to be: a = 3.785(1) A˚ and c = 9.527(1) A˚, with a cell volume of 136.488(1) A˚3 and a cell density of 3.917 g cm3. As the radius of Ag+ ion (c.a. 1.26 A˚) is larger than that of Ti4+ ion (c.a. 0.68 A˚) [28], the increase in cell parameter indicates the substitution of Ag in the TiO2 lattice. 3.1.2. FT-Raman spectra FT-Raman spectra of CS TiO2, 1% Ag sub and 1% Ag imp are provided in Figure S3 (see supplementary data). Raman active modes corresponding to Ti–O bond stretching type vibrations were found at 636.7 cm1 (Eg) and 515.7 cm1 (A1g), while O–Ti–O bending type vibrations were observed at 400 cm1 (B1g) and 152 cm1 (Eg) [29]. These results collectively show that the catalysts synthesized are in the pure anatase phase without any rutile impurity. 3.1.3. TEM From the bright field TEM image and the FE SEM image of 1% Ag sub and 1% Ag imp in Fig. 2(a) and Figure S4 (see supplementary data), respectively, it is evident that the particle size agrees well with the crystallite size determined from powder XRD measurements. The dark field TEM image of 1% Ag imp clearly shows the Ag clusters deposited on the surface of CS TiO2 (Fig. 2(b)). The ring-type electron diffraction patterns of 1% Ag sub and 1% Ag imp are shown in Figure S5 (see supplementary data). A diffraction ring corresponding to the plane reflection from Ag on the surface of TiO2 is present for 1% Ag imp, while it is absent for 1% Ag sub. This suggests the incorporation of Ag in the lattice in the case of 1% Ag sub. 3.1.4. XPS The electronic state of Ag in the catalysts was verified by XPS. Fig. 3 shows the Ag(3d) core level spectra of the synthesized catalysts. For 1% Ag sub, Ag(3d5/2,3/2) doublet was observed at binding energies of 367.9 and 374 eV, while for 1% Ag imp, it was observed at 368.2 and 274.3 eV. Earlier studies on the XPS of Ag, Ag2O and AgO indicated that Ag(3d5/2) binding energies are in the order: Ag (368.2 eV) > Ag2O (367.8 eV) > AgO (367.4 eV) [30]. Such a negative shift in binding energy from the metallic state to higher ionic states is peculiar for Ag, and it is attributed to shifts in initial-state potential of ionic charge and lattice potential. Hence, it is clear that Ag is in +1 state and in zero valent state in 1% Ag sub and 1% Ag imp, respectively. This is consistent with Bera et al. [31] for 1% Ag substituted in CeO2, and these results show that solution combustion technique results in the doping of Ag in +1 state in the lattice. The peak binding energy of Ag(3d5/2,3/2) for 1% Ag DP shows the presence of Ag in zero valent state (Fig. 2(c)). However, the

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energies of 2p3/2 and 2p1/2 peaks at 459 and 465 eV, respectively, indicate that Ti is indeed in +4 state in the substituted and impregnated catalysts. CS TiO2 also exhibited Ti(2p) peaks at the same binding energy, confirming the state of Ti. Quantification of Ag incorporation onto TiO2 was performed using the following equation [11,31]: X Ag IAg s Ti lTi DE ðTiÞ ¼ X Ti ITi s Ag lAg DE ðAgÞ

(1)

where X, I, s, l and DE denote the surface concentration, area under the intensity peaks, photoionization cross section, mean escape depth and geometric factor, respectively. The values of s (sum of both the spin contributions) and l were taken from Scofield [32] and Penn [33], respectively. Therefore, the surface concentration of Ag in 1% Ag sub, 1% Ag imp and 1% Ag DP were calculated to be 3.7%, 13% and 3.3%, respectively. This proves that Ag+ is ionically dispersed in the TiO2 lattice in 1% Ag sub, and that Ag0 is properly loaded onto CS TiO2. An increase in electron density near the bandgap of TiO2 (3 eV), upon 1% Ag substitution is clear from the valence band spectra shown in Figure S7 (see supplementary data). Therefore, from the XRD and XPS of 1% Ag sub, it is clear that electron rich Ag+ occupies the Ti4+ site. This results in an oxide ion vacancy, due to lower valent ion substitution in TiO2. Hence, the structure of 1% Ag sub can henceforth be written as Ti0.99Ag0.01O2d, where d > x (=0.01), as Ag is mostly in +1 state.

Fig. 2. TEM image of (a) Ti0.99Ag0.01O2d and (b) 1% Ag imp.

3.1.5. UV–vis absorption spectra Fig. 4 shows the UV–vis absorption spectra of the catalysts. DP 25, Ti0.99Ag0.01O2d and 1% Ag imp exhibit absorption edges at 400, 439 and 490 nm, corresponding to band-gap energies of 3.1, 2.82 and 2.53 eV, respectively. CS TiO2 exhibits absorption edges at 467 and 570 nm, corresponding to band-gap energies of 2.66 and 2.17 eV, respectively. The reduction in band-gap can be attributed to carbide ion substitution for oxide ion in TiO2 of the form TiO22xCxVx, where V is the vacancy site [34]. In the case of Ti0.99Ag0.01O2d, a new band in the visible region, with a redshifted absorption threshold at 800 nm, can be observed. This can be attributed to two important factors, viz., the charge transfer transitions between the d electrons of Ag+ and the TiO2 conduction or valence band, and the creation of oxygen vacancies in the TiO2 lattice due to the incorporation of Ag. It is found that Ag impregnation reduces the band-gap compared to that for CS

Fig. 3. Ag (3d) core level spectra of the catalysts.

peaks 3d5/2 and 3d3/2 are significantly widened, suggesting that Ag2+ and Ag+ might also be present on the surface, centered at 367.2 and 367.7 eV (indicated by arrows), respectively, albeit at low concentrations. Figure S6 (see supplementary data) shows the Ti(2p3/2,1/2) core level spectra of 1% Ag sub, 1% Ag imp and 1% Ag DP. The binding

Fig. 4. UV–vis absorption spectra of the catalysts.

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Fig. 5. Photoluminescence spectra of the catalysts.

Fig. 6. Diffuse reflectance FT-IR spectra of the catalysts.

TiO2, with the evolution of a shoulder absorption band in the region 400–520 nm. This is characteristic of the surface plasmon resonance of Ag0 species deposited on the surface of CS TiO2. The observed red-shift in the plasmon absorption (compared to 400 nm for Ag sol) [28] may be due to the changes in the refractive index at the Ag–TiO2 interface.

3.1.8. BET surface area The BET surface area of the catalysts followed the order, CS TiO2 (150 m2 g1) [25] > 1% Ag imp (126 m2 g1) > 1% Ag sub (113 m2 g1) > DP 25 (50 m2 g1) [34] > 1% Ag DP (45 m2 g1) [38]. This order is in correspondence with the crystallite sizes calculated by the Scherrer formula.

3.1.6. Photoluminescence spectra CS TiO2, Ti0.99Ag0.01O2d and 1% Ag imp exhibit emission peaks at 425 and 489 nm, corresponding to the excitation at 285 nm, which denotes the O2 ! Ti4+ charge–transfer transition (Fig. 5). The emission at 425 nm of 2.9 eV can be attributed to the free exciton and Wannier–Mott free exciton [35], and the emission at 489 nm corresponding to 2.53 eV, which is less than the band-gap of CS TiO2 and Ti0.99Ag0.01O2d, and equal to the band-gap of 1% Ag imp, can be due to the transition from defect sites like oxygen vacancies to the Ti(3d) state. It is also observed that the intensity of the luminescence peaks reduces with Ag substitution and impregnation. This can be ascribed to the non-radiative recombination of charge-carriers, which are trapped in the Ag(3d) energy level below the conduction band [9] in the case of Ti0.99Ag0.01O2d, and in the Schottky barrier formed in the Ag–TiO2 interface in the case of 1% Ag imp. The quenching of fluorescence also occurs when electrons have enhanced lifetimes in the Ag(3d) level and in the Schottky barrier, thereby favoring charge-carrier separation [36].

3.1.9. pHpzc The isoelectric point or the pH of the point of zero charge (pHpzc) of the catalysts was determined by the pH drift method, described elsewhere [39]. The pHpzc of CS TiO2, Ti0.99Ag0.01O2d, 1% Ag imp, DP 25 and 1% Ag DP were 2.4, 2.7, 4.0, 6.25 [2] and 6.2, respectively. This shows that the surfaces of combustionsynthesized catalysts are highly acidic; such acidity mainly originates from the TiO(NO3)2 precursor. However, Ag impregnation onto CS TiO2 increases the pHpzc to 4.0. In the case of 1% Ag DP the change in pHpzc compared to DP 25 is not significant, due to the low surface incorporation of Ag onto DP 25.

3.1.7. FT-IR spectra Characteristic absorption of the different hydroxyl moieties present in the combustion-synthesized samples was investigated using diffuse reflectance FT-IR spectroscopy. In Fig. 6, a broad band in the region 3600–3000 cm1 and centered at 3300 cm1 can be assigned to stretching vibrations of surface hydroxyl (–OH) groups. Another strong band centered at 1640 cm1 signifies the bending mode of chemisorbed H2O at the TiO2 surface. Ti–H stretching vibration of the hydrated HTiO- Ti–H species is also observed in the region 1450–1280 cm1, centered at 1380 cm1 [37]. Hence, the intensity of these bands, which denote the surface hydroxyl content, follow the order: Ti0.99Ag0.01O2d  CS TiO2 > 1% Ag imp > 1% Ag DP. A similar trend was also observed in the weight loss of the catalysts at the end of 200 and 700 8C in TGA, due to the loss of physisorbed and chemisorbed water (see Figure S8 in supplementary data).

3.2. Photocatalytic degradation of dyes The photocatalytic degradation of OG, AG, MG and AZ was carried out with CS TiO2, Ti0.99Ag0.01O2d, 1% Ag imp, DP 25 and 1% Ag DP. No detectable degradation of the dyes was observed in the absence of the catalysts or without UV irradiation. The degradation was carried out in natural atmosphere, without any external source of aeration, i.e., only the dissolved O2 in the dye solution aids in the formation of superoxide radicals. Hence, due to the absence of an equilibrium O2 concentration [40], one can conclude that the degradation of the dye occurs only due to the direct bandgap excitation of TiO2 and not due to electron injection from excited dye states. The initial concentration of the dyes was chosen according to Beer Lambert’s law. Prior to irradiation, the mixture of dye solution and catalyst particles was stirred in the dark for 30 min for the establishment of adsorption–desorption equilibrium. The concentration of the dye after equilibration was taken as the initial concentration for all the experiments. No significant adsorption of the dyes was observed with the combustionsynthesized catalysts, while DP 25 and 1% Ag DP showed nearly 50% adsorption of the cationic dyes (MG and AZ), while the anionic dyes were not adsorbed. This observation is validated by the pHpzc of DP 25 (6.25) and 1% Ag DP (6.2), which signifies that the surfaces

R. Vinu, G. Madras / Applied Catalysis A: General 366 (2009) 130–140 Table 1 Variation of solution pH and characteristic wavelength for the photocatalytic degradation of dyes with DP 25 and 1% Ag DP. Dye

Initial pH

Final pH

Initial lmax (nm)

Final lmax (nm)

OG AG MG AZ

7.9 7.3 6.5 7.0

4.8 5.2 5.2 5.0

480 640 626 647

493 590 615 595

of these catalysts are negatively charged for pH > 6.2. As the solution pH values of the cationic dyes were greater than 6.2 (Table 1), significant adsorption of these dyes was observed. Initial experiments performed to assess the optimal concentration of Ag in TiO2 by substituting and impregnating 0.5%, 1% and 2% of Ag indicated that 1% Ag is the most photoactive for the degradation of OG. This is because higher surface concentrations of Ag might screen the incident UV radiation before it reaches the TiO2 surface, thereby hindering the generation of charge-carriers. Moreover, Ag might also act as electron-hole recombination centers at higher loadings. Furthermore, 2% and higher concentrations of Ag substitution resulted in the formation of Ag impurity peaks in the powder XRD, indicating that Ag was not properly substituted in the TiO2 lattice. Hence, further experiments were carried out with 1% Ag substitution and impregnation. This is consistent with Bansal et al. [18], where 1% Ag loading exhibited maximum degradation and mineralization of humic acid. Photocatalytic degradation reactions were carried out at the natural pH of the dye-catalyst solution. The solution pH with CS TiO2 and Ti0.99Ag0.01O2d was 3.5–3.7 for all the dyes and the pH was invariant throughout the reaction. In the case of 1% Ag imp, the initial solution pH was 6.5 and it reduced to 6.0 at the end of 2 h period for all the dyes. Although the solution pH of the dyes with

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1% Ag imp is greater than that with CS TiO2 and Ti0.99Ag0.01O2d, the nearly identical reaction condition with these catalysts is exemplified by the pHpzc. A careful comparison of the solution pH with pHpzc of the catalysts signifies that the concentration of hydrogen ions is comparable for the combustion-synthesized catalysts. The observed pH values of the dye solutions with DP 25 and 1% Ag DP are listed in Table 1. The reasons for the observed trends in pH during the course of reaction are discussed in Section 3.2.3. 3.2.1. Degradation of dyes with CS TiO2, Ti0.99Ag0.01O2d and 1% Ag imp Fig. 7 shows the concentration profiles of the dyes, OG, AG, MG and AZ, with all the catalysts. Among the combustion-synthesized catalysts, CS TiO2 exhibits higher rate of degradation and complete decolorization of the dyes at the end of the reaction time. The efficiency of the catalysts for the degradation of AG, MG and AZ followed the order: CS TiO2 > 1% Ag imp > Ti0.99Ag0.01O2d. For the degradation of OG, Ti0.99Ag0.01O2d showed a higher rate of degradation compared to 1% Ag imp. Kinetic analysis of the degradation of the dyes with Ti0.99Ag0.01O2d and 1% Ag imp was carried out using the Langmuir–Hinshelwood (LH) rate equation derived earlier for photocatalytic degradation of dyes [41]. The rate of degradation of the dye is given by r D ¼ k0 ½D=1 þ K 0 ½D; where K0 is the adsorption equilibrium constant, k0 is the effective rate constant for the degradation of the dye by the hydroxyl radicals formed by the electron and hole pathways, and [D] is the concentration of the dye. Inverting the rate equation results in the following expression: 1 1 K0 ¼ þ r D;0 k0 ½D0 k0

Fig. 7. Comparison of activity of the different catalysts for the photocatalytic degradation of (a) OG, (b) AG, (c) MG and (d) AZ.

(2)

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Table 2 Langmuir–Hinshelwood rate constants for the degradation of dyes with Ti0.99Ag0.01O2d and 1% Ag imp. Dye

Catalyst

k0  102 mg L1 min1

K0  102 L mg1

OG

Ti0.99Ag0.01O2d 1% Ag imp

5.28 2.45

8.87 7.15

AG

Ti0.99Ag0.01O2d 1% Ag imp

5.62 2.68

15.75 4.73

MG

Ti0.99Ag0.01O2d

3.22

3.77

AZ

Ti0.99Ag0.01O2d 1% Ag imp

3.05 2.82

25.92 15.24

Hence, by plotting the inverse of initial rate with the inverse of initial concentration, the rate coefficients can be determined from the slope and intercept of the linearly regressed rate curve. Figures S9, S10, S11 and S12 (see supplementary data) show the concentration profiles of the dyes OG, AG, MG and AZ of different initial concentrations, with Ti0.99Ag0.01O2d and 1% Ag imp. The inset figures show the initial rate plot for the corresponding dyecatalyst systems, with a regression coefficient R2 = 0.98–0.99. Table 2 lists the rate coefficients for the dyes photodegraded in the presence of Ti0.99Ag0.01O2d and 1% Ag imp. The rate coefficient, k0 reflects the trends in degradation of the dyes with these two catalysts. From the relative values of k0 for Ti0.99Ag0.01O2d and 1% Ag imp, it can be observed that Ti0.99Ag0.01O2d exhibits nearly 50% higher initial rate of degradation compared to 1% Ag imp for OG, while the latter exhibits 50% higher degradation for AG. Since the concentration of MG with 1% Ag imp was invariant in the initial 10 min of UV irradiation, the rate coefficients were not evaluated, because the initial rate method of analysis was not possible. For AZ, 1% Ag imp exhibited 8% higher degradation rate compared to Ti0.99Ag0.01O2d. The reasons for the observed trends are as follows. It was earlier established that in aqueous media, there are primarily two different kinds of hydroxyl (Ti–OH) groups [3]. The first one is the bridging hydroxyl groups (Ti–OH+–Ti), which are formed when water is molecularly adsorbed onto the (1 0 1) face of anatase TiO2. These are acidic in nature and are used to trap electrons. Dissolved O2 reacts with the bridging hydroxyls, and is used to scavenge the trapped electrons, thereby forming superoxide radicals. The second one is the terminal Ti–OH- groups, which are formed by the dissociative adsorption of H2O at the 5coordinate Ti4+ sites. These are basic in nature and are considered to trap holes. Finally, the superoxide radicals, trapped holes and hydroxyl radicals initiate the oxidation of the organic substrate. Therefore, the trapping of electrons and holes by the hydroxyl groups in hydrated TiO2 results in the effective separation of charge-carriers, thereby preventing the undesirable recombination reaction. As the hydroxyl radicals are formed to a higher extent in aqueous medium, the relative contribution of surface hydroxyl species in the catalyst is insignificant. Therefore, in aqueous medium, the specific surface area of the catalyst is the primary influential factor, which determines the photoactivity towards the oxidation of organic substrates. This is because higher surface area results in higher amounts of surface monolayer coverage of H2O on the TiO2 surface, which leads to higher rate of formation of surface hydroxyl groups. Previously, it was proven that substitution of a metal into the TiO2 lattice results in a reduction in surface area [9]. Accordingly, the rates of degradation of the dyes with the combustionsynthesized catalysts follow the same trend in the order of decreasing surface area, viz., CS TiO2 > 1% Ag imp > Ti0.99Ag0.01O2d. This is also in accordance with the bandgap of the synthesized catalysts, where Ti0.99Ag0.01O2d shows a higher band-gap compared to the other two catalysts, thereby

requiring higher energy for photoexcitation. The higher activity of 1% Ag imp compared to Ti0.99Ag0.01O2d arises from the surface plasmon resonance of Ag particles on the surface of TiO2, which might induce some electron transfer reactions involving adsorbed O2, which would result in the oxidation of the substrate. Finally, the photoactivity of the catalysts is also in good agreement with the intensity of the emission peaks in the PL spectra, suggesting that non-radiative recombination of trapped electrons and holes in the Ag(3d) level of Ti0.99Ag0.01O2d, and in the Schottky barrier of 1% Ag imp dominates the electron and hole scavenging processes for the oxidation reactions. The stability of 1% Ag imp was examined by carrying out powder XRD analysis of the dried sample, after isolating it from the reaction mixture of MG after 2 h of UV irradiation. From Fig. 1(e) it is clear that there is no significant reduction in intensity of the Ag peaks, confirming that there is no leaching of the Ag particles from the TiO2 surface. Furthermore, the absence of any new peaks in the powder XRD pattern indicates that there is no adsorption of either the dye or the intermediates during the degradation of the dye on the catalyst surface. 3.2.2. Degradation of dyes with DP 25 and 1% Ag DP Owing to the high pHpzc of DP 25 and 1% Ag DP compared to those of combustion-synthesized catalysts, a direct comparison of the photoactivity of the catalysts for the degradation of dyes was difficult, as the surface state of the catalysts is an important factor for the generation of hydroxyl radicals, which are the precursors of degradation. Although the anionic dyes were not significantly adsorbed onto DP 25 and 1% Ag DP, the cationic dyes showed significant adsorption in the initial equilibration period. For an initial concentration of 40 mg L1 of MG, 57% and 75% adsorption was observed with 1% Ag DP and DP 25, respectively. Similarly, for 10 mg L1 of AZ, 35% and 45% adsorption was observed with 1% Ag DP and DP 25, respectively. This shows that impregnation of Ag results in the blocking of adsorption sites, which are shallow traps on the surface of DP 25, thereby resulting in the lower adsorption of cationic dyes. However, 1% Ag DP exhibited a higher rate of degradation of all the dyes compared to that of DP 25. In the case of anionic dyes, the initial rates of degradation of OG and AG were 0.61 and 0.48 mg L1 min1, and 1.31 and 0.78 mg L1 min1 with DP 25 and 1% Ag DP, respectively (Fig. 7(a) and (b)). This shows that 1% Ag DP exhibits 54% and 38% higher rates of degradation compared to that of DP 25 for anionic dyes. For the cationic dyes, though MG and AZ were adsorbed to a higher extent on DP 25, 1% Ag DP degraded the dyes to near completion within a shorter time compared to DP 25 (Fig. 7(c) and (d)). This shows that impregnation of Ag on DP 25 is beneficial for the degradation of dyes. This is primarily due to the formation of a space charge layer or an accumulation layer, owing to the contact of metal (Ag) and n-type semiconductor (TiO2) [42]. The work function of TiO2 is 4.2 eV, while that of Ag is 4.6 eV [36]. Hence, the photoexcited electrons migrate from the TiO2 surface to Ag, until the Fermi levels of TiO2 and Ag are aligned. This results in the formation of a Schottky barrier in the TiO2–Ag interface, which aids in the separation of charge-carriers. It is to be noted that the time scale of this charge-trapping process is of the order of 100 femto seconds (fs), while the time scale for electron-hole recombination is of the order of ms [3]. Therefore, the electrons in the space charge layer can be effectively scavenged by electron acceptors like O2 molecules, which leads to the formation of hydroxyl radicals by the electron pathway of photocatalysis. 3.2.3. Mechanism of degradation of dyes During the degradation of dyes with DP 25 and 1% Ag DP, it was observed that the lmax of the dyes shifted to different wavelengths,

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accompanied by a significant reduction in pH of the solution. These led us to investigate the mechanism of degradation and attribute possible reasons for the observed phenomena. Table 1 lists the wavelength shifts and the associated change in pH of the dye solutions during the degradation with DP 25 and 1% Ag DP. OG exhibited a red-shift, while blue shift in lmax was observed for the other dyes. It was earlier reported that the photodegradation of the azo dye, OG, involves the formation of naphthol intermediates, which further degrade to benzaldehyde, benzyl alcohol and ring opened fragments [43,44]. OG consists of acidic auxochromes, viz., –OH and –SO3H groups, which modify the ability to absorb light. The red-shift in wavelength during the degradation of this dye can be due to the binding of the auxochromes to the dye molecule when they are in direct conjugation with the p-state, thereby resulting in the formation of hydroxylated OG intermediates. The mechanism of photodegradation of AG involves the cleavage of benzene sulfonic acid moiety, which results in the formation of 1,4-amino substituted anthraquinonic derivatives [45]. These intermediates absorb in the wavelength range 550–620 nm, closely matching with that of AG [46]. Therefore, the reduction in lmax of AG during degradation can be attributed to the slower rate of consumption of the intermediates compared to their rate of formation. It was reported in a previous study that the photocatalytic degradation of the cationic dye, Rhodamine B, with Ag–TiO2 nanosol involves a blue shift in lmax, accompanied by a simultaneous reduction in concentration of the dye [17]. This was due to the competition between the de-ethylation step, a surface-controlled process and degradation of the dye, a bulk solution-controlled process. The mechanism of degradation of MG also involves N-de-methylation and N-de-alkylation reactions [47], which result in the formation of intermediates that absorb at shorter wavelengths compared to those for MG. Mechanistic studies on the degradation of methylene blue [48], the parent compound of AZ, have proved that the initial step in the photocleavage of the dye involves the opening of the central aromatic ring, followed by the formation of metabolites by N-de-methylation and photo-Kolbe decarboxylation reaction. We have clearly shown the structure of each of the dyes and the possible site of cleavage during the photocatalytic degradation in Figure S1 (see supplementary data). The shifts in lmax during the course of degradation of the dyes are accompanied by a reduction in solution pH from neutral to acidic regime with DP 25 and 1% Ag DP. This shows the role of pHpzc of the catalyst and pH of the solution in facilitating the formation of such intermediates during the degradation of the dyes. Furthermore, the absence of such shifts in the presence of combustion-synthesized catalysts suggests that the solution pH, which is acidic throughout the reaction, is not favorable for the intermediates to persist in solution that they can undergo further transformations to form intermediates that absorb in the UV region of the spectrum. On further exposure to UV these intermediates form ring opened fragments like aliphatic alcohols,

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aldehydes and acids, and finally undergo mineralization to yield CO2 and H2O. 3.3. Photocatalytic oxidation of cyclohexane The selective photooxidation of cyclohexane is a classic example of a mild-oxidation reaction, which is sensitive to the solvent medium and to the O2 flow rate in which the reaction is carried out. These two parameters affect both the overall conversion of cyclohexane achieved and the product distribution yields. We have recently evaluated the effects of various reaction parameters for the selective photoconversion of cyclohexane to the chief oxidation products, viz., cyclohexanol and cyclohexanone, in the presence of lanthanide orthovanadates and lanthanide molybdovanadates [24]. Accordingly, the photooxidation reactions in the present work were carried out in the presence of chloroform, with an O2 flow rate of 10 mL min1 and a catalyst loading of 1 g L1. Initial photooxidation experiments performed with/without any catalyst indicated that the products of oxidation were cyclohexanol and cyclohexanone. This was confirmed in the GC chromatogram, where no new peaks were observed other than those for these two products. This shows that the reaction condition is optimum and mild, which prevents the formation of other oxidation byproducts like 2-cyclohexenone [1], cyclohexyl hexanoate and 1,10 -oxybis-cyclohexane [19]. An initial photolysis experiment performed in the absence of any catalyst indicated that the total conversion of cyclohexane at the end of 5 h was 7%, with 30% selectivity for cyclohexanone. This can be compared with the thermal oxidation of cyclohexane in presence of catalysts like Fe2O3/TiO2 [20], Ce-doped MCM-41 [22] and Mn APO-5 [23], where cyclohexanol was the major product. Photocatalyic oxidation of cyclohexane was performed with CS TiO2, Ti0.99Ag0.01O2d, 1% Ag imp, DP 25 and 1% Ag DP. Before the cyclohexane solution was irradiated with UV, the solution was equilibrated by stirring in the presence of catalyst and O2. No significant adsorption of cyclohexane was observed on the catalysts. During the course of UV irradiation, it was observed that the catalyst particles were not fully dispersed in chloroform so that they agglomerated and got immobilized on the quartz tube. Table 3 shows the total conversion of cyclohexane and the selectivity of cyclohexanone achieved with each of the catalysts. 3.3.1. Kinetics of photocatalytic oxidation During the course of the reaction it was found that the concentration of cyclohexanol increased and reached a saturation concentration at a long time (4–5 h), while the concentration of cyclohexanone continuously increased. These observations are depicted in Fig. 8 and Figure S13 (see supplementary data) for all the catalysts. This suggests that both cyclohexanol and cyclohexanone are formed during oxidation. Then, with further UV exposure, cyclohexanol oxidizes to cyclohexanone. Based on the above observations, we have proposed a comprehensive

Table 3 Rate constants for the photocatalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone with various catalysts. 0

0

0

Catalyst

Total conversion of cyclohexane at the end of 5 h

Selectivity of cyclohexanone at the end of 5 h

k0  104 s1

k13  106 s1

k23  104 s1

k43  106 s1

CS TiO2 Ti0.99Ag0.01O2d 1% Ag imp DP 25 1% Ag DP

8.0% 9.0% 7.3% 8.5% 6.7%

50% 63% 48% 53% 35%

3.43 4.42 3.32 4.36 2.77

8.90 9.50 8.50 10.50 9.10

1.95 2.60 1.85 2.30 1.70

3.75 4.80 3.40 4.40 2.40

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Fig. 8. Experimental and model predicted concentration profiles of cyclohexane (RH), cyclohexanol (ROH) and cyclohexanone (RO) in presence of (a) CS TiO2, (b) Ti0.99Ag0.01O2d, (c) 1% Ag imp (lines are model fit).

mechanism of photooxidation [24] based on the earlier works of Palmisano et al. [1] and Pohorecki et al. [49]. The mechanism comprises the following radical reactions: k1

RH þ O2 !R þ HO2  k2

R þ O2 !RO2  k3

RH þ RO2  !ROOH þ R hv;k4

R þ OH ! ROH

pseudo steady state on the intermediate (ROOH) and radical species. The detailed derivation of the rate expression is available elsewhere [24]. The final rate expressions for RH, ROH and RO are given by:

(I) (II) (III)



d½RH 0 ¼ k ½RH dt

2 d½ROH 0 ½RH 0 ¼ k13  k23 ½RH dt ½ROH

(3)

(4)

(IV) 0

k5

ROOH!RO þ OH k6

RO þ RH!ROH þ R e ;k7

ROOH ! RO þ OH k8

ROH þ OH !RO þ H2 O

(V) (VI)

(VII) (VIII)

where, RH, ROOH, ROH and RO denote cyclohexane, cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone, respectively. The different radical species involved are R, RO, RO2 and OH. The rate coefficients of the individual steps are denoted by k1–k8. The kinetics of the consumption of cyclohexane and the formation of cyclohexanol and cyclohexanone was modeled by assuming

d½RO k43 ½RH2 ¼ dt ½ROH

(5)

Eq. (3) signifies an exponential decrease in concentration of RH ([RH]). The rate constant k0 can be determined by plotting ln([RH]0/ [RH]) vs time, where [RH]0 is the initial concentration of cyclohexane. Eqs. (4) and (5) indicate that there are three 0 0 0 adjustable parameters: namely, k13 , k23 and k43 . Based on the observation that [ROH] becomes constant at long time (5 h), i.e., (d[ROH]/dt)5h = 0, Eq. (4) can be rewritten as: 0

0

k23 ¼ k13

½RH5h ½ROH5h

(6)

Furthermore, the experimental data show that the initial rate (up to 2 h) of increase of [ROH] and [RO] is linear. Thus Eqs. (4)

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and (5) can be written as:  d½ROH ½RH2init 0 0 ¼ S1 ¼ k13  k23 ½RHinit dt ½ROHinit init

(7)

 0 d½RO k ½RH2init ¼ S2 ¼ 43 dt ½ROHinit init

(8)

The initial slopes S1 and S2 were obtained from the experimental data. Thus, Eqs. (6)–(8) provide initial estimates for the three adjustable parameters in Eqs. (4) and (5). Finally, Eqs. (4) and (5) were solved in Mathematica 5.1, and the parameters were further adjusted to obtain the best fit for both [ROH] and [RO] with a regression coefficient >0.98. From Fig. 8 and S13 (see supplementary data), it is evident that the model fits the experimental data well and captures all the essential features of the photooxidation process for all the catalysts. It is also clear from Table 3 that the catalysts exhibit total conversion of cyclohexane and selectivity of cyclohexanone in the order: Ti0.99Ag0.01O2d > DP 25 > CS TiO2 > 1% Ag imp > 1% 0 0 Ag DP. This is also reflected in the rate coefficients k0 , k13 , k23 and 0 k43 (Table 3), which signify the conversion of cyclohexane, formation of cyclohexanol, consumption of cyclohexanol in the formation of cyclohexanone, and the formation of cyclohexanone, respectively, for all the catalysts. The rate coefficients were evaluated for duplicate experimental measurements and the error was within 3%. The rate coefficients also serve to quantify the 0 mutual effectiveness of the catalysts. From the value of k43 , it can be concluded that Ti0.99Ag0.01O2d is 8%, 22%, 30% and 50% more effective than DP 25, CS TiO2, 1% Ag imp and 1% Ag DP, respectively, for the selective yield of cyclohexanone. Similarly, for the conversion of cyclohexane (k’), Ti0.99Ag0.01O2d is 2%, 22%, 25% and 35% more effective than DP 25, CS TiO2, 1% Ag imp and 1% Ag DP, respectively. These collectively show that substitution of Ag into the TiO2 lattice is beneficial for achieving high conversions of cyclohexane and high selectivity of cyclohexanone, while impregnation of Ag on the TiO2 surface, both on CS TiO2 and on DP 25, reduces the overall conversion of cyclohexane and shifts the selectivity to cyclohexanol. The reasons for the trends observed among the catalysts are as follows. The primary difference in the activity of the catalysts for the photocatalytic degradation of dyes and the photooxidation of cyclohexane arises from the solvent medium in which both the reactions are carried out. While the photocatalytic degradation of dyes are highly non-selective, which is due to the over-oxidation induced by the aqueous medium, selective photooxidation of cyclohexane occurs under mild conditions in the presence of chloroform and O2 atmosphere, even though the incident UV light intensity is the same for both the reactions. Hence, the dominant physical property of the catalyst which contributes to the photoactivity is expected to vary for the reactions carried out in aqueous and in organic media. In a recent study on the electronic structure of anatase TiO2, Thomas et al. [50], by using resonant photoemission and X-ray absorption spectroscopy, have correlated the structure and photocatalytic activity in terms of the surface oxygen vacancy and the mixing of the Ti 3d and 4sp states at the valence band maximum of TiO2. Oxygen vacancy refers to the defect states, which are mainly 3d in character. This vacancy introduces two localized Ti3+ states at about 1 eV below the conduction band edge at concentrations of 15% for the (1 0 1) surface of anatase TiO2. When the reactions are carried out in O2 atmosphere, O2 can adsorb in these vacancy sites as O2, at a ratio of up to 3 molecules per oxygen vacancy [3]. These vacancies act as effective electron trapping sites, after the conduction band electrons relax to these positions. Moreover, the trapped electrons react more rapidly with

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O2 compared to free electrons, forming superoxide radicals leading to higher photocatalytic activity. This is suggestive of the fact that higher oxygen vacancy in the catalyst can lead to higher photoactivity. In our study, the introduction of oxide ion vacancies in the TiO2 lattice by substitution of Ag for Ti sites was established by XRD and XPS studies. Similarly, vacancy sites in CS TiO2 are due to the carbide ion substitution for oxide ion [34]. The impregnation of Ag onto the TiO2 surface might not induce any defect sites, because the metal is incorporated more into the surface than in the lattice. Another factor that influences the photoactivity of a catalyst in the presence of an organic reaction medium is the inherent presence of the surface bound hydroxyl groups in the catalyst. Two different types of surface Ti–OH groups are found to be active in charge-carrier trapping, which is an important step in preventing the electron-hole recombination process [3]. The first type is the Ti4+–OH radicals, which are used to trap holes, and the Ti3+–OH groups, which are used to trap electrons. Hence, any increase in activity of the catalyst in an organic solvent should arise due to the higher hydroxyl content on its surface. For the photooxidation of cyclohexane, hydroxyl radicals are the key species, which react with the cyclohexyl radicals (R) and cyclohexanol to form cyclohexanol and cyclohexanone, according to reactions (IV) and (VIII). From the FT-IR (Fig. 6) and TGA results (see Figure S8 in supplementary data) of the catalysts, it is clear that the surface hydroxyl content among the combustion-synthesized catalysts follow the order: Ti0.99Ag0.01O2d > CS TiO2 > 1% Ag imp. Therefore, the photoconversion of cyclohexane and the selectivity of cyclohexanone also follow the same trend. Though CS TiO2 possesses higher surface area than any other catalyst, the conversion and selectivity for the oxidation of cyclohexane is lesser than that of Ti0.99Ag0.01O2d. This is because of the agglomeration and immobilization of the catalyst particles onto the quartz tube during the reaction. This shows that the surface area of the catalysts, which was a dominant property that determined the activity in aqueous phase degradation reaction, is not significant for organic phase photooxidation of cyclohexane. Between DP 25 and 1% Ag DP, though the hydroxyl contents in both the catalysts are nearly the same, the former exhibits a higher conversion and selectivity. This can be attributed to the presence of surface Ag centers in 1% Ag DP, which possess lower concentrations of defect sites to effectively separate the charge-carriers. The higher selectivity of cyclohexanol obtained with 1% Ag imp and 1% Ag DP may be due to the slower rate of reaction (VII), owing to the trapping of electrons in deep traps, which makes them inaccessible for reaction with ROOH intermediate. It is also interesting to note that DP 25 exhibits a higher activity compared to that for CS TiO2, though the defect density in the latter is higher. This can be due to the rutile (1 1 0) surface in DP 25, which aids in the better mixing of the Ti 3d and 4sp states, resulting in the increased p-bonding interaction between the O 2p and Ti 3d t2g states. These factors collectively enhance electron transfer to vacancy sites, thereby resulting in higher photoactivity. 4. Conclusions In the current study we have synthesized Ti0.99Ag0.01O2d by solution combustion synthesis, and 1% Ag imp and 1% Ag DP by reducing AgNO3 on CS TiO2 and DP 25, respectively. The powder XRD pattern of Ti0.99Ag0.01O2d indicated that Ag was substituted in the TiO2 lattice and XPS confirmed that Ag exists mostly in +1 state in Ti0.99Ag0.01O2d, and in zero valent state in 1% Ag imp. FTRaman spectra indicated that the synthesized catalysts were indeed in the anatase phase without any rutile impurity. TEM of the catalysts indicated that the particle size is consistent with the crystallite size determined using the Scherrer formula. UV–vis

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absorption spectra showed that Ag substitution and impregnation increased and decreased the band-gap, respectively, compared to unsubstituted CS TiO2. FT-IR and TGA of the catalysts confirmed that Ti0.99Ag0.01O2d possesses higher surface hydroxyl content compared to CS TiO2 and 1% Ag imp. pHpzc measurements showed that combustion-synthesized catalysts were acidic in nature, while DP 25 and 1% Ag DP were nearly neutral. The photoactivity of these catalysts was tested for the degradation of anionic and cationic dyes, and the selective conversion of cyclohexane to cyclohexanone. The order of photoactivity among the combustion-synthesized catalysts for the degradation of dyes was CS TiO2 > 1% Ag imp > Ti0.99Ag0.01O2d. 1% Ag DP exhibited a higher photoactivity compared to DP 25, due to the charge-carrier trapping by the Schottky barrier formed on the TiO2 surface. A shift in lmax of the dyes during photodegradation with DP 25 and 1% Ag DP was observed. This was attributed to the surface charge of these catalysts, which induces the formation of long-lived intermediates. For the photooxidation of cyclohexane, Ti0.99Ag0.01O2d exhibited the highest activity for the total conversion of cyclohexane and the selective formation of cyclohexanone, followed by DP 25, CS TiO2, Ti0.99Ag0.01O2d and 1% Ag DP. The kinetic rate coefficients for both these two reactions were evaluated by following the LH rate equation for the degradation of dyes, and a free radical mechanism for the oxidation of cyclohexane. The differences in activity among the catalysts employed in this study were correlated with the properties of the catalyst, which controls the reaction in aqueous and organic media. While the surface area of the catalyst is important for aqueous phase reactions, the oxide ion vacancy is an important parameter for organic phase reactions. Therefore, the present study proves that the photoactivity of a catalyst is not solely determined by a single physical property, but rather depends on a number of variables including the surface area, band gap, surface hydroxyl content, photoluminescence intensity, oxide ion vacancy and pHpzc of the catalyst. Acknowledgments The corresponding author thanks Professor M.S. Hegde for helpful discussions and the department for science and technology, India for the Swarnajayanthi fellowship.

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