Dual functionality of TiO2-flyash nanocomposites: Water vapor adsorption and photocatalysis

Catalysis Today 230 (2014) 205–213

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Dual functionality of TiO2 -flyash nanocomposites: Water vapor adsorption and photocatalysis A.N. Ökte a,∗ , D. Karamanis b , D. Tuncel a a b

Department of Chemistry, Bo˘gazic¸i University, Bebek 34342, Istanbul, Turkey Department of Environmental & Natural Resources Management, University of Ioannina, 30100 Agrinio, Greece

a r t i c l e

i n f o

Article history: Received 21 June 2013 Received in revised form 7 January 2014 Accepted 26 January 2014 Available online 20 February 2014 Keywords: Supported catalysts Photocatalysis Methyl orange Water vapor adsorption

a b s t r a c t TiO2 nanoparticles were in situ supported on lignite fly ash (TiO2 -FA) and investigated by several techniques. X-ray diffraction (XRD) analysis supplied information about the generation of anatase phase. Scanning electron microscopy (SEM) images with energy dispersive X-ray analysis (EDX) revealed variations in the surface morphology of raw FA after TiO2 loading. Nitrogen adsorption–desorption isotherms (BET) isotherms indicated the formation of a mesoporous structure. X-ray photoelectron spectroscopy (XPS) confirmed the buildup of TiO2 nanoparticles on the FA matrix with the form of Ti4+ oxidation state. The optimum FA content of the composite was found as 88%. The supported nanocatalysts were tested for water vapor adsorption towards evaporative cooling of hydrophilic surfaces and for decolorization and degradation of methyl orange (MO) under UV irradiation. The mesoporous TiO2 -FA was found to be hydrophilic with capillary condensation in the water vapor adsorption isotherm. Dark adsorption experiments followed pseudo-second order kinetics and Langmuir type of isotherms. Kinetics was discussed in terms of Langmuir–Hinshelwood model. The repeatability of photocatalytic activity was also tested. H2 O2 used as an electron scavenger accelerated decolorization and degradation of MO. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Environmental pollution is an increasing problem of recent decades due to rapid industrialization and population growth throughout the world [1]. Among many technologies to deal with this problem, heterogeneous photocatalysis is regarded as a promising technology to degrade harmful pollutants to final nontoxic products [2]. TiO2 is the most investigated catalyst for the treatment of contaminated aqueous and gaseous streams because of its high stability, non-toxicity and inexpensiveness [3]. However, poor adsorption capacity, formation of rapid aggregates in a suspension and also recycling difficulties restricted the utilization of bare TiO2 . Therefore, in practical applications, attempts have been made to immobilize TiO2 on some adsorbent materials without loss of activity. Among these materials, silica, alumina, activated carbon, zeolites, clays, MCM, layered hydroxides and glass are commonly used ones [4–6 and references therein]. Obviously, cost reduction with the selection of an abundant and inexpensive matrix with complementary adsorption properties is of primary importance. Fly ash (FA) is one of the solid wastes largely produced from power generation. Currently, its applications are only limited to civil engineering including cement and brick production [7,8].

∗ Corresponding author. Tel.: +90 212 359 7390; fax: +90 212 287 2467. E-mail address: [email protected] (A.N. Ökte). 0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2014.01.031

There are growing concerns about FA disposal problems. Therefore, research is needed to develop new alternative environmental friendly applications. Immobilizing TiO2 on the surface of FA, and then using this supported catalyst system for the treatment processes, may be a good attempt to solve the problems described above. Furthermore, chemical pollution in the urban environment is strongly connected with thermal pollution or the so called “heat island effect” (temperature increase within cities) [9]. This results in detrimental conditions to human health and increased energy consumption due to higher demand for cooling. Among the effective measures are the increase of the area of green tract land (rooftop plants) or the surface area of water (artificial lakes) [10]. However, the high real estate value of urban space limits the wide applicability of these methods. The application of the composite is not restricted in buildings. In fact, the composite can be easily applied in pavements as well in order to fight the thermal and chemical pollution. In this case, a storm event will produce dilute solutions containing an array of dissolved organics deposited on concrete surfaces [11]. Therefore, the medium for the photocatalytic processes can be aqueous. Recently, there has been a renewal interest in the application of the passive and building integrated evaporative cooling (BIEC) as an alternative and sustainable way to cool the surfaces of a building or the pavement of outdoor areas. In the BIEC application of porous materials, stored water or night sorbed moisture inside the

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pores are evaporated during the hot day and the porous surface temperature is reduced due to the release of the latent heat [12]. Accordingly, TiO2 supported catalysts could produce a multifunctional composite with building application prospects of enhanced water vapor adsorption and surface solar evaporative cooling in addition to the degradation of pollutants. Several researchers have investigated the degradation ability of the TiO2 loaded FA catalysts under certain preparation and reaction conditions [13,14]. Very recently, Visa et al. showed that the TiO2 /fly ash composite can be used for the combined methylorange and cadmium simultaneous removal and also removal of heavy metals and surfactants [15,16]. However and to the best of our knowledge, there are no articles in the scientific literature that present combined processes under sun irradiation with different and simultaneous utilization of the solar spectrum. In the line of these studies, we have also examined ZnO loaded FA systems [17]. The objective of this work is to extend the usage of TiO2 supported FA catalysts in bifunctional applications for environmental and energy prospects, which to the best of our knowledge has not been reported before. In this context, TiO2 nanoparticles were in situ synthesized on the FA matrix and characterized by several techniques. The materials were evaluated for the moisture sorption on FA with or without TiO2 buildup. The sorption isothermic process and kinetics were phenomenologically described. Decolorization and degradation of methyl orange (MO) was followed under UV illumination in the absence and presence of H2 O2 . 2. Experimental 2.1. Materials Fresh lignite by-product of fly ash, coded FA, was obtained from the major lignite power plant of Agios Dimitrios, Greece (1595 MW). It was collected in a dry state from the electrostatic precipitators of the power station, ground by hand, sieved to a fragment size less than 200 ␮m and used without any pretreatment in the experiments. The major constituents were SiO2 (42.82%), Al2 O3 (20.82%), Fe2 O3 (4.57%), CaO (23.45%), MgO (1.74%), K2 O (1.31%), TiO2 (1.47%), Na2 O (0.32%), ZnO (0.01%). According to ASTM C 618 standard, this FA was C type due to the quantity of CaO (more than 10% with 23.45%) and the sum of SiO2 , Al2 O3 , Fe2 O3 (more than 50% with a total of 70.48%). Degussa P-25 (TiO2 ) was a generous gift from Degussa Ltd. Titanium tetraisopropoxide (98%, Aldrich), acetic acid (96%, Merck), methyl orange (Merck), H2 O2 (30%, Merck), were used as provided by the suppliers without further purification. 2.2. Preparation of the photocatalysts TiO2 was prepared using a sol–gel method [18]. Briefly, 20 mL of titanium tetraisopropoxide was added gradually to 80 wt% acetic acid solution under continuous stirring for 2 h at 50 ◦ C to produce a transparent sol. Depending on the loading of TiO2 on the support, requisite amount of titania-sol was added to the aqueous suspension of the FA (initially 2 g in water for 2 h). After agitation and extensive washings, the catalysts were dried, calcinated at 500 ◦ C and ground into fine powder with 10% TiO2 FA, 25% TiO2 -FA and 40% TiO2 -FA labels. 25% TiO2 was also prepared by using the above procedure in the absence of the support. 2.3. Characterization techniques The X-ray powder diffraction (XRD) analysis were recorded on a Rigaku-D/MAX-Ultima diffractometer using Cu K␣ radiation

˚ operating at 40 kV and 40 mA and scanning rate ( = 1.54 A) of 2 min−1 . The nitrogen adsorption/desorption isotherms were obtained at 77 K by using Quantachrome Nova 2200e automated gas adsorption system. The specific surface areas were determined by using multi-point BET analysis and the pores sizes were measured by the BJH method of adsorption. The surface morphologies were determined by using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis on an ESEM-FEG/EDAX Philips XL-30 instrument operating at 20 kV using catalyst powders supported on carbon tape. Si and Ti concentrations in the composites were measured by the proton induced gamma ray emission (PIGE) technique at the Tandem accelerator of the NCSR “Demokritos”. In X-ray photoelectron spectroscopy (XPS) tests, Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer equipped with a hemispherical electron analyzer and Al K␣ micro-focused monochromator was used. The areas of peaks were estimated by calculating the integral of each peak after subtracting a Shirley background and fitting the experimental curve to a combination of Lorentzian/Gaussian lines. The UV–vis diffuse reflectance spectra (UV–vis DRS) of the FA, 0.25 M ZnO and all supported catalysts obtained by using UV–vis spectrophotometer (UV-2450, Shimadzu) equipped with an integrating sphere reflectance accessory. The baseline correction was done by BaSO4 . The spectra were recorded in the range 200–600 nm for the catalysts and the FA using BaSO4 as reference. Moisture sorption isotherms were determined by applying a modified version of the discontinuous method ASTM E96-80. According to the method, samples were placed in sealed desiccators with saturated salt solutions for controlling relative humidity while temperature was air-conditionally controlled at 25 ◦ C and were periodically weighed. The moisture content was calculated as the difference of mass measurements in different time periods and the initial dry state. 2.4. Photocatalytic experiments MO, selected as the probe molecule, is produced as a carcinogenic compound in the concentration range of 32.7–0.327 mg L−1 [19,20]. Thus, the 3.27 mg L−1 was used in our experiments unless concentration effect of MO was tested. Reaction systems were setup by adding 0.2 g of catalysts into 200 mL of MO in a pyrex flask, with an inlet for the air circulation and an outlet for the collection of aliquots, was used for all experiments. The flask was located in an “irradiation box”, equipped with eight black light lamps (Philips TL 15 W/5BLB) of 320–440 nm and cooled by a fan. The lamps were positioned to surround the flask with an incident photon flux of 4.7 × 1015 photons/s. Prior to illumination, to ensure the equilibrium of adsorption process, suspensions were magnetically stirred in the dark for 30 min. UV–vis spectrophotometer (UV-2450, Shimadzu) was used to monitor the absorbance spectra of MO as a function of illumination time. The decrease of the band at 274 nm indicated degradation of MO’s aromatic moiety while the one at 464 nm was followed for the decolorization of MO solution. The natural pH of FA in water was 11 due to its alkaline nature. All experiments were performed at room temperature and at pH 8.5 (3.27 mg L−1 MO in the presence of 25% TiO2 -FA) without concerning the degradation intermediates. Also, measurements were conducted at least twice and the average value was recorded. The degradation and decolorization rate percentages of MO were calculated by the following equation: Degradation (or decolorization)% =

C0 − C × 100 C0

(1)

where C0 is the initial concentration of MO and C is the concentration of MO after “t” minutes UV irradiation. For the experiments conducted in the presence of H2 O2 (used as an electron scavenger),

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Fig. 1. (A) XRD patterns of FA and supported catalysts (A: Anatase). (B) SEM and mapping images of FA and 40% TiO2 -FA. (C) Nitrogen adsorption/desorption isotherms and (D) pores size distribution plots of FA, 10% TiO2 -FA, 25% TiO2 -FA and 40% TiO2 -FA.

10 mM H2 O2 concentration was added to the supported catalystMO solution before UV irradiation. 3. Results and discussion 3.1. Characterization of catalysts 3.1.1. XRD analysis XRD patterns of raw FA and the supported catalysts are shown at both low-angle (2 < 10◦ ) and high-angle (20◦ < 2 < 60◦ ) ranges (Fig. 1A). The crystallinity of TiO2 nanoparticles is very low in the presence of 10% TiO2 -FA. However, anatase diffractions of d 101, d 200, d 105 and d 211 at 25.4◦ , 48.2◦ , 54.2◦ and 55.3◦ (2) are detected in the patterns of 25% TiO2 -FA and 40% TiO2 -FA with wider and lower intensities. The signals are not intensified for the 40% TiO2 -FA, indicating the existence of a saturation effect at the 25% TiO2 loading. Rutile phase does not form in the synthesis of the supported catalysts, as evidenced by the peak absence at 27.4◦ (2). The crystalline sizes (DTiO2 ) calculated by using Scherrer’s equation for the broadening of (1 0 1) anatase peak reflection, was found as 15.1 nm (for 25% TiO2 -FA) and 18.1 nm (40% TiO2 -FA) (Table 1). Although TiO2 nanoparticles do not show a significant difference in dimensions, the decrements in the reflections of the raw support suggest their uniform distribution throughout the surface and bulk.

3.1.2. SEM (EDX-mapping) analysis In SEM images, the FA shows spherical and non-shaped particles (Fig. 1B). The EDX analysis demonstrates Si, Al and Ca as major, Mg, K, Ti and Fe as the minor constituents of the support (not shown). The Ti percentages are found as 0.2% and 0.4% in the bulk and on the sphere of the FA, respectively. For 40% TiO2 -FA, the appearance differs with the formation of smooth layers on top of the aggregates (40% TiO2 -FA-a and 40% TiO2 -FA-b). The EDX-spot analysis displays significant increments in the Ti percentages; 21.3% on the sphere ‘1 and 53.3% on the aggregate ‘2 . This is also illustrated in Ti mappings where Ti signal dominated on top of the sphere and the bright-edged agglomerate. The optimum FA content of the TiO2 -FA composite was evaluated as the average of EDX spot measurements and proton induced gamma ray emission (PIGE) bulk measurements and was found 88%. 3.1.3. Nitrogen adsorption isotherms Nitrogen adsorption–desorption isotherms and pore size distributions are shown in Fig. 1C and D, respectively. The FA reveals Type II isotherms, specific of non-porous materials. Isotherms of the supported nanocatalysts follow Type IV sorption behavior [21]. It is associated with capillary condensation taking place in the mesopores at P/P0 [22,23]. The pore size distribution curves display that the pores of the supported catalysts are in typical

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Table 1 Binding energy (eV) values and atomic number ratios. Catalysts TiO2 (Degussa P25) 25% TiO2 25% TiO2 -FA FA

Ti Ti 2p3/2

Ti 2p1/2

Ti 2p1/2–2p3/2 splitting (eV)

Bulk O2−

O 1s OH

Si–O–Ti

Ti 2p3/2 to 2p1/2

Ti/O

458.59 458.59 458.59 –

464.27 464.21 464.29 –

5.68 5.62 5.70 –

529.86 529.80 529.76 529.11

– – 531.35 531.34

– – 532.94 532.78

2 2.1 2.1 –

0.44 0.43 0.15 –

mesoporous region of 20–40 A˚ (Fig. 1D). The supported nanocatalysts indicate higher pore volumes (0.019 cm3 g−1 for 10% TiO2 -FA, 0.048 cm3 g−1 for 25% TiO2 -FA, 0.061 cm3 g−1 for 40% TiO2 -FA) than that of the FA (0.009 cm3 g−1 ). This results in bigger surface areas (15.97 m2 g−1 for 10% TiO2 -FA, 29.64 m2 g−1 for 25% TiO2 FA, 37.84 m2 g−1 for 40% TiO2 -FA) in comparison to the area of the FA (5.61 m2 g−1 ). The transformation of FA structure into a mesoporous adsorber with in situ growth of TiO2 nanoparticles is expected to extend the energy and environmental application prospects of the coal residues. The increased adsorbent surface area and thus, the availability of more adsorption sites are expected to induce an improved-and combined- moisture sorption character and decomposition ability of the supported catalysts. These properties are discussed in the following sections thorough enhanced capillary condensation in the water vapor adsorption isotherms and decolorization-degradation of MO pollutant.

3.1.4. XPS analysis XPS analysis was performed on the raw FA, TiO2 (Degussa P25), 25% TiO2 and 25% TiO2 -FA (Fig. 2). Binding energy shifts are observed in all samples and hence, the instrument is calibrated using the carbon peak (C-1s) at 285 eV as in the other studies [24,25]. Survey spectrum of 25% TiO2 -FA contains Ti 2p and O 1s peaks (Fig. 2a). TiO2 (Degussa P-25) was used as a reference for the 25% TiO2 and 25% TiO2 -FA catalysts. Fig. 2b displays a doublet corresponding to Ti 2p3/2 and 2p1/2 core levels for TiO2 (Degussa P-25), 25% TiO2 and 25% TiO2 -FA. The peak positions and also the peak separations of Ti 2p3/2 and 2p1/2 for 25% TiO2 and 25% TiO2 -FA are quite similar to that of TiO2 (Degussa P-25) (Table 1), indicating the presence of a single oxidation state of Ti (Ti4+ ), typical of Ti in the TiO2 lattice. The values are also in good agreement with that of TiO2 (anatase) [26–28]. The O 1s signal for TiO2 (Degussa P-25) and 25% TiO2 shows a well resolved bulk oxide (O2− ) peak at 529.86 and 529.80 eV, respectively (Fig. 2c). Similar binding energy values of the bulk oxide were reported in the studies of Riakar et al. [29] and Simmons et al. [30]. The O 1s spectra of the FA and 25% TiO2 -FA appear with wider and asymmetric peaks (Fig. 2c). For the raw FA, the O 1s signal is deconvoluted by three subspectral components of MgO and TiO2 (529.11 eV, 17.8% spectral area), CaO and Al2 O3 (531.34 eV, 46.1% spectral area) and SiO2 (532.78 eV, 36.1% spectral area) components of the FA [31,32]. Although the O 1s signal for 25% TiO2 -FA also posseses three components, they differ in their constituents in comparison to those noticed in the FA. The peak at 529.76 eV with 33% of total oxygen is attributed to oxide (O2 − ) in the TiO2 lattice. The secondary peak at 531.35 eV with 40% of total oxygen is assigned to hydroxyl (OH) species on the surface. The difference between the binding energies of the OH and O2 − species is found as 1.59 eV, which is close to the reported differences of 1.24–1.8 eV (Table 1) [33 and references therein]. The third component at 532.94 eV with 27% of total oxygen is attributed to the formation of Ti–O–Si bond. The electron density around the Ti atoms decreases due to the greater electronegativity of Si via O bonded to Ti. Thus, an increase in the binding energy (about 0.5 eV) is expected in comparison to SiO2 (532.5 eV) [33–35]. Moreover, the shift in the Ti 2p1/2 signal to 464.29 eV in the presence of the supported nanocatalyst relative to 464.21 eV (25% TiO2 ) and 464.27 eV

(TiO2 -Degussa P25) corroborate the indication for the creation of Ti–O–Si bond. The atomic number ratio of Ti 2p3/2 to 2p1/2 is found to be 2 for TiO2 (Degussa P25). The similar ratios obtained in the presence of 25% TiO2 (2.1) and 25% TiO2 -FA (2.1) demonstrate successful creation of TiO2 nanoparticles in the synthesized materials. The ratios of Ti to O (based on Scofield photoionization cross sections of the core level photoelectrons) are found as 0.44 (TiO2 (Degussa P25)) and 0.43 (25% TiO2) , which are close to that expected from the stoichiometry of TiO2 [36]. However, TiO2 loading on the surface of the FA decreases the ratio of Ti to O (0.15), due to the higher oxygen content of the whole matrix in the presence of FA. 3.1.5. UV–vis DRS analysis UV–vis absorption spectroscopies of the raw FA, 25% TiO2 and the supported are presented in Fig. 3. The support shows an absorption tail in between 200 and 600 nm. The characteristic absorption edge of the TiO2 is not observable in the supported catalysts even with 40% TiO2 loading. Instead, they resemble mostly to the absorption spectra of the FA and the profiles extend to the longer wavelength regions. The masking role of the support, however, does not affect the performances of the supported catalysts under UV irradiation, i.e., efficient decolorization and degradation rates are noticeable due the higher surface areas and larger pore volumes. Meanwhile, visible light activity is not expected to be high for the supported catalysts. Modifications in the profiles seem to be little higher than the sum of the absorbances of FA and TiO2 , indicating minor alteration in the crystal structure of the photocatalytically active TiO2 in order to justify a high absorbance in the visible range. 3.2. Adsorption and photocatalytic applications 3.2.1. Kinetics and Isotherms of water vapor sorption A typical water vapor sorption kinetics at 62% RH of the TiO2 -FA sample is shown in Fig. 4a. The pseudo-first-order rate equation was used to describe the water vapor sorption kinetics on the prepared catalysts. The fitting parameters of the rate constant k1 (min) and the equilibrium capacity wo (g/g) 25% TiO2 -FA for 62% RH were determined as 7 h and 0.14 g g−1 , respectively. The hydrophilicity of the supported nanocatalysts was investigated by the water vapor adsorption isotherms. From very low relative humidity, the water vapor adsorption of 25% TiO2 -FA is higher than the raw FA and the differences increase with increasing relative humidity (Fig. 4b). The equilibrium adsorption of water vapor on the FA exhibits a Type III isotherm indicating the hydrophobicity of the material with chemisorption rather than physisorption and monolayer sorption even at high relative pressure. In contrast, water vapor isotherms of the 25% TiO2 -FA show a Type V behavior due to the transformation of the hydrophobic fly ash to a hydrophilic mesoporous form with capillary condensation at high relative humidity. Similar to activated carbon, water vapor adsorption on TiO2 supported FA preferentially occurs in low hydrophilic sites at low relative pressures. The monolayer formation is followed by multilayer sorption (at relatively higher pressures) and finally by capillary condensation (at the more

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Fig. 3. UV–vis DRS spectra of FA, 25% TiO2 , 10% TiO2 -FA, 25% TiO2 -FA and 40% TiO2 FA.

porous active sites that are created upon TiO2 build up on the FA cenospheres). Furthermore, a logistic function of the form w=

wo 1 + exp[k(b − (p/po ))]

(2)

was fitted to the experimental results. Eq. (2) exhibits a S-shape behavior with an inflection point b around 0.6, 0.22 g g−1 for wo and k equals to 8.85. Therefore, 25% TiO2 -FA catalyst exhibits high water vapor adsorption at RH higher than 50% and can be also used as a thermoresponsive and solar cooling material for the utilization of the water vapor absorption resonances and continuum in the visible and infrared part of the solar radiation spectrum in addition to the photodegradation applications.

Fig. 2. (a) XPS survey analysis of 25% TiO2 -FA, (b) Ti 2p XPS spectra of TiO2 (Degussa P25), 25% TiO2 and 25% TiO2 -FA, (c) O 1s XPS spectra of FA, 25% TiO2 -FA, 25% TiO2 TiO2 (Degussa P25).

3.2.2. Dark adsorption experiments Results of experiments performed in the dark are shown in Fig. S1a. MO percentages remaining in the solution are found as 85.5% at 464 nm and 91.2% at 274 nm in the presence of only FA. However, 25% TiO2 -FA exhibits lower percentages (77% at 464 nm and 80% at 274 nm), demonstrating a better adsorption capacity in comparison to the support. After 30 min mixing in the dark, no significant change in the MO percentages is noticed. The color of MO is yellow and its natural pH in water is 5.85. Accordingly, MO is negatively charged in water solution due to its pKa value of 3.4 [37]. The alkaline character of FA neutralizes the acidic pH of the MO (pHFA+MO solution 11). Thus, repulsive forces seem to be dominant among the FA surface and MO molecules (in the anionic form). However, heterogeneity on the surface of the FA, the existence of minor amounts of Fe2 O3 , TiO2 , ZnO within the structure and also the pores (though they have very restricted volumes) may create a kind of weak interaction with the MO molecules. This results in decolorization, however, due to the dominant grayish color of the FA cannot be visualized, instead can be followed by the decrease in the absorption band at 464 nm. Moreover, the interaction may also vary the conjugation in the benzenic rings through the chromophore ( N N ) and resulting in the limited decrement of the 274 nm band. For 25% TiO2 -FA, the pH of solution with 3.27 mg L−1 MO is 8.5, indicating a reduction in the alkalinity of the FA solution. As a consequence, alteration in the charge nature of the supported catalyst is expected with a stronger adsorption effect of TiO2 nanoparticles. This may again induces damage within the localized conjugation of the MO skeleton being responsible from the decrease in the 274 nm peak during

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Fig. 4. Water vapor adsorption (a) kinetics of 25% TiO2 -FA and (b) isotherms of FA and 25% TiO2 -FA at 25 ◦ C.

the dark period. Additionally, the higher surface area and the larger pore volume of the supported catalyst increase adsorption of MO and results in lower decolorization percentages in comparison to that of FA alone. For the adsorption details, the pseudo-second order equation in the integrated form is applied for 3.27 mg L−1 (0.01 mmol L−1 ) MO on the 25% TiO2 -FA (Eq. (3)) t = qt



1 k2 q2e



+

t qe

(3)

where qt (mmol g−1 ) is the amount of adsorbed MO per gram of adsorbent at time t (min), qe (mmol g−1 ) the amount of adsorbed MO per gram of adsorbent at equilibrium and k2 is the pseudosecond order rate constant (min) [38,39]. The linearity obtained in the plot of t/qt versus t results in 0.9674 (for degradation) and 0.9763 (for decolorization) correlation coefficients (Fig. S1b). Thus, it is suggested that the adsorption is well consistent with the applied kinetics. Adsorption isotherms are analysed in order to investigate the interactions of adsorbate molecules with the adsorbent surface. Due to their simplicity and their validity in a wide range of concentration, the Langmuir and Freundlich models are correlated to our experimental results to investigate the effect of surface heterogeneity and sites of different affinity for MO adsorption on the supported catalyst. The isotherm models are employed at five different initial concentrations of MO (16.3, 8.17, 4.91, 3.27 and 1.62 mg L−1 ). The linearized form of the Freundlich isotherm is given by Eq. (4). ln qe = ln KF + bF ln Ce

(4)

where KF is the Freundlich constant, bF the adsorption intensity constant and Ce is the concentration of MO at equilibrium. The line fittings of ln qe versus ln Ce plot give 0.9395 (for degradation) and 0.8149 (for decolorization) correlation coefficients (Fig S1c and d). The form of Langmuir isotherm is as follows Ce Ce 1 = + qe qmax qmax KL

(5)

where qmax (mmol g−1 ) is the maximum capacity of the adsorbent and KL (L mmol−1 ) is the Langmuir adsorption constant. The linearity obtained in the plots of Ce /qe versus Ce for both degradation and decolorization processes results in 0.9653 and 0.9981 correlation coefficients, respectively (Fig. S1e and f). From the slope of the lines, Langmuir adsorption capacities are calculated as 0.015 and 1.24 mg g−1 for degradation and decolorization processes, respectively. 25% TiO2 -FA catalyst. Since higher coefficients are obtained with the Langmuir model, it can be suggested that the adsorption sites are identical, energetically equivalent and the adsorption process occurs through the same mechanism for a monolayer adsorption at the maximum amount of adsorption. The adsorption of several dye molecules reported in literature previously obey the Langmuir sorption isotherm model [40] and our results also seem to be in agreement with this trend. The Langmuir model suggests that dye molecules are adsorbed uniformly and that the TiO2 -FA materials are fairly homogenous with the titania phase well dispersed in the FA matrix [40].

3.2.3. Photocatalytic activity under UV irradiation, kinetics and reuse properties Photodegradation experiments were performed with the catalysts of different TiO2 contents (Fig. 5a). The percentages of MO remaining in solution following the decolorization processes are found to be much lower than that of the degradation processes since it is more difficult to destroy the aromatic moiety of the substrate. The best activity is noticed in the presence of 40% TiO2 -FA catalyst with the lowest MO remaining percentages (30.5% at 464 nm and 63.3% at 274 nm) in solution. Decrements in the TiO2 contents lower the performances of the catalysts. The 25% TiO2 -FA catalyst was further used to analyze the rate of reactions in the initial concentration range of MO from 16.3 to 3.27 mg L−1 . Pseudo-first-order kinetics is confirmed by the linearity in the plot of ln(C0 /C) versus t, where C0 is taken as the equilibrium concentration of MO (mg L−1 ) after dark adsorption and C is the concentration of MO at time t. The rate-constants

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Fig. 5. (a) Photocatalytic activity of the supported catalysts under UV irradiation, (b) kinetic analysis and (c) reuse properties of 25% TiO2 -FA.

of decolorization (or degradation) processes (k, min−1 ) calculated from the slopes of the lines in the same plot decrease with increasing concentration of MO (from 0.0026 (3.27 mg L−1 ) to 0.0009 (16.3 mg L−1 ) for decolorization and from 0.0016 (3.27 mg L−1 ) to 0.0008 min−1 (16.3 mg L−1 ) for degradation). This can be a result of blocking of the photocatalytically active sites on the supported catalyst and reducing the interaction of photons with these sites. Langmuir–Hinshelwood model was also used to express the kinetics by the following rearranged form [41]. 1 1 1 + = R kKC k

(6)

where R is the rate of decolorization (or degradation), K the adsorption coefficient of MO onto the 25% TiO2 -FA catalyst (L mg−1 ) and k is the reaction rate constant (mg L−1 min−1 ). The linearity in the plot of reciprocal of rate (1/R) against reciprocal of initial MO concentration (1/C0 ) (Fig. 5b) proves the validity of the model for our supported catalyst system. The values of K and k are found to be 0.788 L mg−1 and 0.0237 mg L−1 min−1 (for decolorization) and 0.215 L mg−1 and 0.0163 mg L−1 min−1 (for degradation). For the each run of the recycling experiments the 25% TiO2 -FA is filtrated, washed and calcined at 500 ◦ C for 2 h. After four cycles, the percentage of MO remaining in solution is found to increase only approximately 2.5% (from 50.4% to 53% for decolorization process) and 4.68% (from 72.3% to 77% for degradation process) (Fig. 5c). These results indicate that the photocatalytic activity of 25% TiO2 FA has repeatability. The insignificant loss of photoactivity can be explained either by the presence of reaction by-products adsorbed on the photocatalyst after the first run or by the loss of the photocatalysts during each collection and rinsing steps. 3.2.4. Effect of H2 O2 addition under UV irradiation Dye pollutants were also referred as antenna molecules [42–45]. After adsorption on the surface of semiconductors (like TiO2 ), they are able to absorb visible light and inject electrons into the conduction band of TiO2 . Thus, the oxidized antenna molecules induce charge separation, widen the photoresponse of TiO2 from UV to the visible region and create a unique route to use visible light from the sun [46]. Since MO shows absorption in the emission spectrum of the lamps, a question arises; whether it behaves as an antenna molecule or not? In order to provide more insight to the mechanism

underlying the photocatalytic reaction route, as a first step, photolysis of MO is controlled and a negligible decolorization (about 4%) is found (Fig. 6a). This is consistent with the results of Yu et al. [47] and Dvininov et al. [48]. Their comments for the stability of MO toward light lies in the fact that in the absence of TiO2 catalyst, all the excited electrons return to the ground state in nanoseconds. A series of comparative tests are then, conducted by employing an electron scavenger, H2 O2 , under UV irradiation in the absence and in the presence of the supported catalysts (Fig. 6a and b). The concentration of H2 O2 in all experiments is kept constant as 10 mM since higher concentrations are known to consume photogenerated holes and hydroxyl radicals [49,50]. H2 O2 shows a stronger electron trapping property than O2 and it improves the system efficiency mainly by the formation of hydroxyl radicals (Eq. (7)) [49–51]. H2 O2 + e− → • OH + OH−

(7)

The existence of H2 O2 in the solution decreases the remaining MO percentage to 79% in 30 min irradiation time (Fig. 6a). Extension of the irradiation period does not induce further decrements, indicating that the excitation of MO is limited under the UV light. However, in the presence of the supported catalysts, MO percentages decrease rapidly within one hour irradiation time (Fig. 6b). Thus, the pronounced activity in the TiO2 -FA and H2 O2 systems mostly depends on the activation of TiO2 nanoparticles under UV irradiation (Eq. (8)). TiO2 + h(UV) → eCB − + hVB

+

(8)

For the efficiency of the process, inhibition of electron–hole pair recombination is required. At this stage, H2 O2 traps the electrons mostly from the conduction band of the TiO2 , increasing the number of hydroxyl radicals in the reaction media (Eq. (7)—where e− should be changed to eCB − ). In the meantime, the limited number of electrons produced with the direct excitation of MO molecules can also be inactivated by H2 O2 molecules. This enhances the direct (Eq. (9)) and/or indirect (Eqs. (10) and (11)) oxidation of MO with the valence band holes (hVB + ) via hydroxyl radicals (Eq. (12)). h+ + MO → MO

•+

→ MO (decolorization and degradation)

h+ + H2 O → • OH + H+

(9) (10)

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The efficiency of H2 O2 can also be ascribed for the formation of hydroxyl radicals by homolytic cleavage of the bonds into two hydroxyl groups (Eq. (13)) [49,50]. H2 O2 + h → • OH + • OH

(13)

However, since MO decolorization is not accelerated only in the presence of H2 O2 after 30 min irradiation (Fig. 6a), this route of hydroxyl radical generation and their subsequent attack cannot be considered for the destruction of the MO moiety. 4. Conclusion A supported nano-catalyst has been prepared by the buildup of TiO2 nanoparticles on the cenospheres and bulk surfaces of the FA (with an optimum value of 88%). This material shows bifunctional properties with adsorption capability in addition to photocatalytic activity. Enhanced adsorption of MO and water vapor on the supported catalysts are detected. By extensive characterization techniques, the pronounced performance was attributed to the transformation of FA to a mesoporous structure. Improved photocatalytic efficiencies were obtained by the application of the supported nano-catalysts. Combined with the water vapor adsorption and the utilization of the IR part of the solar spectrum open up new opportunities and widens the application prospects of the prepared nanocatalysts. Acknowledgements This study was supported by the framework of the Joint Research and Technology Programmes 2011–2013 between Greece (General Secretariat for Research and Technology and European Regional Development Fund, GSRT 10 TUR/1-25-1) and Turkey (TUBITAK, 209T109) and also Bo˘gazic¸i University Research Foundation (Project No. 11B05P4). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2014.01.031. References

Fig. 6. (a) Photolysis of MO with and without H2 O2 under UV irradiation, (b) effect of H2 O2 on the photocatalytic activities of the supported catalysts.

h+ + OH− → • OH

(11)

• OH + MO

(12)

→ MO (decolorization and degradation)

Similar to the previous experiments performed in the absence of H2 O2 , MO remaining percentages are found to be lower in the decolorization processes and the efficiencies of the supported catalysts depend on the TiO2 content.

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