See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/236844382 A simple route for the preparation of Pmodified TiO2: Effect of phosphorus on thermal stability and photocatalytic activity Article in Journal of the Taiwan Institute of Chemical Engineers · January 2012 DOI: 10.1016/j.jtice.2011.06.011 CITATIONS READS 24 35 4 authors, including: Kais Elghniji Sylvie Rossignol Faculty of Science of Gafsa Center european ceramic 23 PUBLICATIONS 290 CITATIONS 227 PUBLICATIONS 2,991 CITATIONS SEE PROFILE SEE PROFILE Mohamed Ksibi University of Sfax/High Institute of Biotechnology 109 PUBLICATIONS 4,005 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Wet air oxidation of acetic acid over platinum and ruthenium catalysts supported on cerium based materials View project Geopolymers View project All content following this page was uploaded by Kais Elghniji on 24 February 2019. 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Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139 Contents lists available at ScienceDirect Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice A simple route for the preparation of P-modified TiO2: Effect of phosphorus on thermal stability and photocatalytic activity Kais Elghniji a,*, Julien Soro b, Sylvie Rossignol b, Mohamed Ksibi a a b Laboratoire Eau, Energie et Environnement (LR3E), Ecole Nationale d’Ingénieurs de Sfax, BP W 3038, Sfax, Tunisia Groupe d’Etude des Matériaux Hétérogènes (GEMH-ENSCI) Ecole Nationale Supérieure de Céramiques Industrielles de Limoges, 47-73 Avenue Albert Thomas, 87065 Limoges, France A R T I C L E I N F O A B S T R A C T Article history: Received 28 February 2011 Received in revised form 30 April 2011 Accepted 5 June 2011 Available online 15 August 2011 Phosphorus-modified dioxide nanoparticles were prepared by sol–gel method. The effect of phosphorus precursor and calcination temperatures on phase transformation, grain growth and surface area were investigated using various spectroscopic and basic techniques (ICP-AES, XRD, BET, 31P MAS NMR, FT-IR, and UV–vis methods). It was found that the phosphorus existed as amorphous titanium phosphate in TiO2 framework after calcination at temperature of 500–700 8C. As results, slows down the particle growth of anatase and increases the anatase-to-rutile phase transformation. The average crystallite size of P-modified TiO2 increased dramatically from 8 to 59 nm when the temperature increased from 500 to 900 8C. This change was associated with the formation of pyrophosphate TiP2O7 species through condensation of the concentrated phosphate species. The BET surface area of modified samples was 3.4fold higher than that of unmodified TiO2 and was 70% higher than that of commercial Degussa P-25. The photocatalytic activity of P-modified TiO2 was 1.5-fold higher than that of commercial Degussa P-25 and was 49% higher than that of unmodified TiO2 under UV irradiation. Phytotoxicity was assessed before and after irradiation against seed germination of tomato (Lycopersicon esculentum). ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: TiO2 Phosphorus content Particle growth TiP2O7 Thermal stability 1. Introduction TiO2 is considered to be the most promising heterogeneous photocatalyst, because of its high photocatalytic activity, nontoxicity, inexpensiveness, chemical stability and favourable optoelectronic properties. Titania exists in three different forms: anatase, rutile, and brookite, of which anatase generally show the best photocatalytic activity performance [1,2]. Without any modification, transformation of anatase to rutile usually occurs at 500–700 8C, which may result in the decrease in photocatalytic ability of TiO2 [3–5]. In addition to the crystallite transformation obtained by hightemperature calcination, crystallite growing and serious sintering are observed with increasing the temperature of calcinations leading to the drastic decrease in surface area and photocatalytic activity. Hence, the inhibition of the growth of the anatase crystals could maintain the anatase metastability at high temperature and control crystallite size. To improve the thermal stability and photocatalytic activity, titania has been modified with La2O3 [6], ZrO2, SiO2 [7], or doped by inorganic nonmetal such as sulphur [8] and fluor [9,10]. Lv et al. [10] developed a simple and novel synthetic method for the fabrication of TiOF2 via a microwave-assisted hydrothermal route using tetrabutyl titanate and hydrofluoric acid as raw materials. The * Corresponding author. Tel.: +216 25 511 432; fax: +216 74 665 190. E-mail address: [email protected] (K. Elghniji). prepared anatase TiO2 from TiOF2 shows very high thermal stability and the phase transformation temperature from anatase to rutile is up to 1000 8C. The high thermal stability and the photocatalytic enhancement of catalysts were attributed to the adsorbed Fluoride ion on the surface of anatase after calcination at 700 8C. Among the various methods, modification and doping of TiO2 by phosphorus seems to be a another promising approach [11–16]. Kõrösi and Dekany [13] prepared a series of phosphate modified-TiO2 samples by the sol–gel method. They argued that the surface bound phosphate have delayed the formation of the anatase phase, crystallite growth and inhibited the anatase–rutile phase transformation. Lin et al. [14] demonstrated that the doping of phosphorus could efficiently inhibit the grain growth and enhance the surface area of TiO2 nanoparticles. The above researches in literature seem discrepant and complicated. This is probably due to the variety of synthetic methods adopted to prepare the solid and the different phosphorus contents. The present work aims at complementing such investigations with P-modified TiO2 nanoparticles which are synthesized through an easy procedure. The thermal stability, the control of crystalline structure and the effect of phosphorous content on the photocatalytic degradation of 4-chlorophenol were extensively investigated. To the best of our knowledge, there is no report on the impact of the 4-CP solution before and after photocatalytic treatment to environment using P-modified TiO2. In fact, chlorophenols constitute a group of serious environmental pollutants that must be eliminated [17–21]. As a result of their widespread use in mothproofing, miticides, pesticides, herbicides, germicides and 1876-1070/$ – see front matter ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2011.06.011 K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139 133 atomic emission (ICP-AES) spectrometry. The intensity of the spectral lines of 213.6 and 336.1 nm were measured. fungicides, chlorophenols pose a serious threat to the environment. The US Environmental Protection Agency [22] and the European Union directive [23] have labelled chlorophenols as ‘‘priority pollutants’’, which means that they need to be constantly monitored in the aquatic environment. Considering the potential effect of this compound on the ecosystem, a phytotoxicity test using tomato (Lycopersicon esculentum) seeds was successfully applied in ecological risk assessment of 4-CP phototreated solutions. 2.3.2. Second method The concentration of phosphate in the above samples was measured using UV-Vis Double Beam PC, Scanning spectrophotometer (UVD-2950) following the ascorbic acid reduction method [24]. 2. Experimental 2.4. Surface acidity measurement 2.1. Chemical and catalyst preparation Surface acidity was determined volumetrically by the adsorption of sodium hydroxide NaOH from solutions of different concentrations. 0.2 g of the solid oxide was mixed with 30 ml of the base solutions for 10 h, with continuous stirring then left 24 h. For each sample, a blank run was carried out first on the base alone and the difference between the blank run and that with the sample gave the amount adsorbed. Titanium (IV) isopropoxide (Ti(OC2H5)4, 97%, Aldrich) and isopropylic alcohol were obtained from Fluka ((CH3)2CHOH, 99.8%, Riedel de Haei #n), phosphoric acid (H3PO4, 85%, Aldrich,), the commercially available TiO2 (Degussa P25) was obtained from Degussa Chemical and UHQ (Milli-Q 18.2 MV) water. The catalyst P-modified TiO2, were prepared by the sol–gel method. In a typical preparation procedure, titanium (IV) isopropoxide was diluted in isopropylic alcohol (molar ratio 1:4). Deionized water was added dropwise to Ti(OPri)4 solution during stirring (molar ratio between water and alcohol 1:5). White precipitate starts appearing indicating the hydrolysis process. After being aged for an hour, 100 ml of a phosphoric acid aqueous solution with concentration 34.5 mM was added to the resulting titania suspension to prepare TiO2 sample of phosphorus content. For comparison, unmodified TiO2 was also prepared by the same procedure without the addition of H3PO4. The dried materials were calcined at 500, 700, 900 and 1000 8C in air for 3 h and with heating rate 108/min and the calcined powders are labelled according to its phosphorus content and calcination temperature. TPt, and Tt, where t means the calcination temperature. 2.2. Structural characterization The powder X-ray diffraction patterns were recorded at room temperature on a (advanced D8, Bruker, Germany). The experimental conditions for refinement of XRD data of TP1000 are the following: Xray tube operating at 40 kV and 40 mA, 0.6 mm fixed divergence slits, diffracted beam curved graphite monochromator (Cu Ka+1 radiation, l1 = 1.540600 Å, l2 = 1.544390 Å) and 0.1 mm fixed slit in front of the scintillation detector. The data were collected in the 2u range 2–708 with a step size of 0.028 and a counting time of 5 s/step. Infrared absorption spectra were measured on a (Nicolet 380 ATR/FT-IR, International Equipment Trading Ltd., USA) spectrometer by the transmission method using the KBr pellet technique with 4 cm1 resolution. The Brunauer–Emmett–Teller (BET) surface area was measured by a fully automated surface area analyzer (ASAP 2020 Accelerated Surface Area and Porosimetry, Micromeritics, USA). The samples were degassed in vacuum overnight at 180 8C prior to adsorption measurements. The UV–Vis diffuse reflectance (DR UV– Vis) spectra were recorded by a Varian Cary 5/UV–Vis-N.I.R.spectrometer. The 31P and (coupling polarization) 1H–31P CP NMR/MAS spectra of the prepared TiO2 solids were recorded on a (300 ultra shield, Bruker, USA) spectrometer at 300 MHz, resonance frequency operating at 121.5 MHz and the external magnetic field was 9.4 T. The pulse repetition is 10 s, pulse width of 7 ms and a spinning speed of 8.0 kHz. Chemical shifts were indicated using an external H3PO4 (85%) reference (0 ppm). 2.3. Determination of phosphorus content 2.3.1. First method The Ti and P content of the samples was determined by all argon sequential (Thermo Jarrell ASH, USA) inductively coupled plasma 2.5. Photocatalytic reaction experiment Photodegradation of 4-chlorophenol (4-CP) (98%) was conducted in a laboratory-scale photoreactor. It is a 120 cm3 cylindrical photoreactor, operating in a closed recirculating circuit driven by a centrifugal pump and with a stirred reservoir tank equipped with a device for withdrawal of samples. Illumination was carried out using a lamp (11 W low-pressure mercury lamp, Philips, Holland) with a wavelength (lmax 254 nm). At given time intervals, about 4 ml aliquots were sampled, centrifuged and filtered with a cellulose acetate membrane filter membrane (0.45 mm pore size, 25 mm diameter) to remove all solid particles. The 4-CP concentration was estimated by measuring their absorbance at 225 and 280 nm using a UV-Vis Double Beam PC, Scanning spectrophotometer UVD-2950. Free chloride ions are quantified directly after each irradiation period with an ion chromatograph (HIC-6A Shimadzu, Japan) equipped with a conductivity detector and a Shim-pack column. The separation was achieved using an isocratic elution at a flow rate of 1.5 ml/min. A mobile phase of 1 mM of tris(hydroxymethyl)aminomethane and 1 g/l of sodium chloride was used as standards solution. Chemical oxygen demand (COD) was measured according to standard methods described in the Japanese International Standard handbook [25]. 2.6. Phytotoxicity (germination tests) 4-Chlorphenol phytotoxicity was assessed before and after irradiation against seed germination of tomato (L. esculentum). Phytotoxicity was determined using a modified Zucconi test [26] by measuring seed germination. Twenty seeds were placed on filter papers in 9 cm Petri dishes and 6 ml of treated solution was then uniformly added to each dish. Dishes were incubated in the dark at 26 2 8C for 5 days. Distilled water was used as control. All samples, including controls, were triplicated. A germination index (GI) was calculated by counting the number of germinated seeds and the average root length observed in each sample compared to control treatments [27]. Results finally expressed according to the following formula: GI ¼ number of germinated seeds in sample number of germinated seeds in control average of root lengths in sample 100 average of root lengths in control A seed was considered germinated when the root length exceeded 5 mm. For root lengths below 5 mm, it was considered 3.3. Lattice parameters From Rietveld analysis of P-modified and unmodified TiO2, lattice parameters and anatase molar fraction were obtained and summarized in Table 2. As clearly shown, calcination at temperature in the range from 500 to 900 8C produces a progressive increasing on the tetragonality (c/a) of anatase structure of Pmodified TiO2 catalyst (TP). This is similar to the value reported in the (JPCDS 84-1286) for well-crystallized anatase (c/a = 2.510). Hence, the observed expansion is not likely related to a crystal size effect. Even though small changes are expected in the lattice parameters (c/a) upon changing the titania anatase crystal size. Regarding the unmodified TiO2 (T), the increase of particle size is in inverse proportion to the tetragonality. That means 47 nm is critical size to undergo anatase-to-rutile transformation at 700 8C with (c/a = 0.644) for well-crystallized rutile. Fig. 2(a) shows a typical XRD data of TP1000 analyzed using the program FULLPROF [30]. The quality of the agreement between observed and calculated patterns for each phase is measured by a set of factors given by the FULLPROF program. These results confirmed Table 1 Phosphorus content in P-modified TiO2 samples obtained from ICP-AES and ascorbic acid reduction method. Sample Weight calculated (ppm) ICP-AES weight (ppm) A.R. method weight (ppm) TP500 3.97 3.84 4.04 r [101] r [211] r [220] r [111] a [004] r [200] a [101] 3.2. XRD studies T700 TP700 a [105] a [211] T500 a [200] a [103] a [004] a [112] a [101] XRD was used to investigate the changes of phase structure of the as-prepared catalysts after heat treatment at different temperatures. Fig. 1 shows the XRD patterns of unmodified and P-modified TiO2 calcined at different temperatures. With increasing calcination temperature, the peak intensity of anatase increases, and the width of the (1 0 1) plane diffraction peak of anatase becomes narrower, indicating the enhancement of crystallization. For unmodified TiO2, the rutile phase starts to appear at 700 8C, and it becomes the only phase at 900 8C (JPCDS 83-2242). In contrast, P-modified TiO2 retains a pure anatase phase (JPCDS 84-1286) in the calcination temperature range of 500– 900 8C, and no obvious XRD peaks corresponded to titanium phosphate are observed after phosphorus modification. The titanium phosphate should be amorphous also in nature, mixed to anatase TiO2. When calcined at 900 8C, two peaks at 2u = 22.55 and 27.708 appeared in spectrum of TP900 correspond to the (6 0 0) and (7 2 1) XRD diffraction peak of crystalline TiP2O7 (JPCDS 381468) [28,29]. This phase with cubic lattice phase comes into being at the same time with anatase phase. At temperature extremely higher, the anatase TP900 transformed to the rutile TP1000 structure and the TiP2O7 phase still remained in the solid composition. r [210] p [ 721] r [211] p [ 600] r [110] TP900 a [211] r [220] TP1000 a [105] Table 1 summarizes the P content of P-modified TiO2 determined by ICP-AES and the ascorbic acid reduction method. As shown, the Phosphorus content is identical with this of calculated one, indicating the added phosphoric acid reacts completely with titania at the applied preparation conditions. p [ 630] a [200] T900 r [111] 3.1. Determination of phosphate content r [210] 3. Results and discussion r [200] a [101] equal to 0 and the seed was not considered germinated. The average sum of root lengths comprised the sum of the lengths of all germinated seeds in a Petri dish. r [101] r [110] K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139 134 TP500 10 20 30 40 50 60 2θ Fig. 1. The XRD patterns of T and TP calcined at various temperatures. a, r and p denote anatase, rutile and TiP2O7 respectively. the presence of rutile TiO2 and TiP2O7 phases at 1000 8C. Fig. 2(b) shows the 31P MAS NMR spectrum of the TP1000 spun at 8 kHz and at room temperature. The existence of lines at 38.6, 40.5, 42.9, 44.5, 46.15, 49.48 and 52.35 ppm should be associated with the presence of several crystallographic sites for P atoms in a 3 3 3 crystal structure of TiP2O7 with space group (Pa-3) [31,32]. 3.4. Average crystalline sizes and surface area The average crystalline size anatase and rutile can be calculated from the broadening of the (1 0 1) peak and the (1 1 0) peak, as shown in Fig. 3. For the T500, the average crystalline size of anatase Table 2 Crystallographic Parameters and phase contents of unmodified and P-modified TiO2. Sample Phase contentsa Lattice parameters V (Å3) A (%) R (%) a (Å) b (Å) c (Å) T500 T700 T900 TP500 TP700 TP900 TP1000 100 26 – 100 100 100 + P – – 74 100 – – – 100 + P 3.783 4.589 4.598 3.788 3.789 3.784 4.594 3.783 4.589 4.598 3.788 3.789 3.784 4.594 9.472 2.960 2.963 9.480 9.507 9.531 2.958 135.77 62.38 62.37 135.98 136.30 136.48 62.44 a The phase content of TiO2 was obtained from the following formulas: WR = 1/ [1 + 0.884(AA/AR)], WA = 1 WR, where WA and WR are the content of anatase and rutile, respectively, AA and AR are the diffraction intensity of anatase (1 0 1) and rutile (1 1 0). A, R and P denote anatase, rutile and TiP2O7 respectively. The cell volume and lattice parameters obtained from the Rietveld refinement of X-ray diffraction data. K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139 135 140 200 120 TP T TP T Crystallite size (nm) 2 SBET(m /g) 160 100 80 60 120 80 40 0 500 600 700 800 900 1000 Temperature(°C) 40 20 0 500 600 700 800 900 Temperature (°C) Fig. 3. Average crystallite sizes and specific surface areas (inserted) of the samples calcined at various temperatures. The average crystallite sizes are calculated from the broadening degree of the (1 0 1) XRD peak of anatase phase. Fig. 2. (a) Rietveld plot for the TP calcined 1000 8C, r and p are denote rutile TiO2 and TiP2O7, respectively. (b) 31P MAS-NMR spectra of TiP2O7 obtained at 1000 8C. Chemical shifts are referenced to that of phosphoric acid. (21 nm) quickly increases at temperatures above 500 8C, indicating the sintering of TiO2 particles and the rutile phase becomes the major phase at 700 8C. At this temperature, the mass fraction of rutile is 74%, and its crystallite size is 45 nm. At 900 8C, anatase completely turns into rutile phase and the crystallite size of rutile rapidly increases to more than 100 nm. The similar result is also reported by Yu and Wang [33]. They prepared TiO2 nanotube arrays by electrochemical anodization of titanium foil. With the aid of XRD and SEM morphology of the 600–700 8C-calcined samples, they found a stable tubular structure with anatase/rutile particles size of 35–40 nm. At 800 8C, the nanotube arrays are destroyed and only dense rutile crystallites with size of over 180 nm are observed. They suggested that the high temperature and phase-transformation heat cause the growth of rutile crystallites, on the other hand the diffusion of oxygen in air into the nanotube–support interface region oxidize the titanium in that region and directly transform titanium into rutile phase. However, for TP500, the crystallite size of anatase is 8.5 nm at 500 8C. That means the P species markedly slows down the crystalgrowth rate of anatase and maintains the anatase metastability at high temperature. Beyond this temperature an increase in anatase particle size is observed, becoming significant at 900 8C, reflecting increased sintering. Reidy et al. [34] reported that the critical size for 25% anatase-to-rutile conversion was 45.1 nm, respectively. In the current work, the particle size of TP800 catalyst is 34 nm, i.e., it is small to undergo anatase-to-rutile transformation. The BET surface areas of the samples calcined at various temperatures are shown in Fig. 3 inset. With increase of calcination temperature, the measured specific surface areas oppose to the calculated crystallite size, indicating the crystallite growth of anatase during calcination. Upon calcination at 500–700 8C, the surface area of T700 is quickly decreased to 5 m2/g, reflecting the loss of coordinated water (as revealed by FT-IR analysis) and sintering of rutile T700 particles [35,36]. In contrast, the surface area of TP500 is larger than that of the anatase T500 one. This finding indicates that P-modified TiO2 surface has more hydroxyl groups than the pure TiO2 which may be attributed to the less loss of water during calcination temperature. Above 700 8C, a decrease in surface area is observed for modified TiO2, becoming significant at 900 8C. This is a reflection of sintering of anatase particles at higher temperatures (as revealed by FT-IR and XRD analysis). 3.5. FT-IR studies FT-IR spectra of the unmodified and P-modified TiO2 samples calcined at various temperatures are shown in Fig. 4. For all samples, the broad peak at 1630 cm1 and the peak at 3420 cm1 corresponds to bending and asymmetric stretching modes of molecular water, respectively. Obviously, the P-modified TiO2 has more surface-adsorbed water and hydroxyl groups than the unmodified TiO2, and these absorption bands gradually decrease as the temperature increases. In addition, three absorption bands at 1100, 1135 and 1035 cm1 are observed in the IR spectra of TP500 but absent for T500 (Fig. 4(a) inset). The shifts of these bands suggest that different chemical environments existed around the phosphorus. The former band is possibly the 1082 cm1 band, characteristic of the n3 vibration of the phosphate ions coordinated to TiO2, shifted to lower wavelength as a result of hydrogen bond interactions in the host metal oxide lattice (hydrogen phosphate states) [37]. The shoulder peak at 1135 cm1 appears gradually, characteristic of the n2 vibration of the phosphate in a bidentate state (bridging bidentate surface) [38]. The band at 1036 cm1 is assigned peak to the vibrations of Ti–O–P bond [39–42]. Therefore, the phosphorus may exist as the surface (1100 and 1135 cm1) and the form Ti–O– P (1036 cm1) bulk of TiO2 anatase, with P replacing part of Ti4+ ions. Furthermore, the TP700 catalyst shows a gradual appearance of bands at 1047 cm1, 968 cm1 and a shoulder broad band at 1184 cm1 (Fig. 4(b) inset). These bands can be assigned to the phosphate probably in a bidentate state [43]. K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139 136 (a) (a) Absorbance (a.u) TP 500 T 500 103 5 1082 1135 2 1 1400 130 0 120 0 1100 cm TP 500 100 0 90 0 80 0 -1 -100 -50 0 50 100 ppm T 500 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) (b) x 10 (b) TP 700 Absorbance (a.u) 460 1047 968 1184 -100 Fig. 5. 1300 120 0 11 00 100 0 -1 cm T P 700 90 0 800 3.6. T 700 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Absorbance (a.u) (c) T P 1000 T P 900 1130 467 1107 1 0 50 9 7 8 523 1184 1214 TP 1000 1 4 00 1300 1 2 00 1100 cm 1 0 00 900 80 0 -1 TP 900 T 900 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Fig. 4. The FT-IR spectra of TP and T calcined at (a) 500 8C, (b) 700 8C, and (c) 900 8C and 1000 8C. -50 0 50 100 31 P MAS-NMR (a) and 1H–31P CP NMR (b) spectra of TP calcined at 500 8C. 31 P MAS-NMR studies MAS-NMR measurements were carried out to study the phosphorus microenvironments. As shown in Fig. 5(a), the broadening of 31P NMR spectra for TP500 stems from the wide distribution of phosphorus among two sites with slightly different environments. Two chemical shifts are observed after computer simulation. The first one at 4.49 ppm (deconvolution-peak 1), which in good argument with chemical shift values for phosphates coordinated via one oxygen to one titanium atoms 1P–O–Ti bound, i.e. Ti(H2PO4)4. The second signal at 9.5 ppm (deconvolutionpeak 2) and may connect to phosphate coordinated via two oxygens to two titanium atoms P–O–Ti bounds in bridging bidentate structure, i.e. Ti(HPO4)2 [44,45]. Similar results have been reported by Fan et al. [46]. They argued that the use of H3PO4 as phosphorus source leads to three possible chemical states of amorphous titanium phosphate, including Ti3(PO4)4, Ti(HPO4)2 and Ti(H2PO4)4 after calcination temperature (<600 8C). As mentioned in XRD section, the titanium phosphate should be amorphous in nature, mixed to anatase TiO2. Evidently, the 1H–31P (coupling polarization) NMR spectrum, Fig. 5(b) shows the detection of remaining P–OH groups on the TP500 catalyst. On the basis of the results of XRD and 31P NMR, phosphorus exists as amorphous titanium phosphate that embedded in the TiO2 crystalline grain at 500–700 8C. While the FT-IR spectra support a bulk/surface Ti–O–P structure in the titanium phosphate samples. 3.7. Surface acidity Evidently, the TP900 displays sharp peaks at 978, 1110 and 1214 cm1 attributable to frequencies of PO43 co-ordinated to metals such as Ti4+ on the solid surface. These bands somewhat shifted to lower wave number and we can observe a significant change in the trend of the FT-IR spectrum (at 1107, 1184, 1050 and 523 cm1) at 1000 8C (Fig. 4(c) inset). Considering the formation of the bands at 523 cm1 (formation of rutile TiO2), it is possible to infer that all of these changes at 1000 8C consequently caused the anatase–TiO2 to completely transform to the rutile structure as revealed by DRX analysis. Surface acidity values, expressed in mmol/g for catalysts T and TP calcined at different temperatures, are presented in Fig. 6. As calcination process is carried out, the acidity of the T sample is seen to decrease fastly. This was believed to be due to a reflection of the fast loss of surface-adsorbed water and hydroxyl groups and the increases of sintering due to the formation of rutile. Moreover, it has been established that the amount of the surface-adsorbed water and hydroxyl groups is related to the crystallite form and surface area. As a result, anatase is more active than rutile in K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139 4.0 3.5 shows that the UV–vis spectrum of TP500 is blue-shifted by several nanometers in comparison with that of T500. This indicates that the band gap of the TP500 catalyst is larger than that of the T500. To determine the band gap of the powders, the Kubelka–Munk method based on the diffuse reflectance spectra was employed [50]. The (F(R)hn)1/2 versus hn plots of the samples calcined at various temperature are presented in Fig. 7 (inset). The red shift observed during the calcination at higher temperature may be due TP T 3.0 2.5 3 2.0 1.5 1.0 TP 500 (a) 250 T P25 Photolysis -0.5 500 600 700 800 900 Temperature (°C) Fig. 6. Evolution of surface acidity of TP and T as function of calcination temperature. adsorbing water and hydroxy groups and this is probably the main reason of higher photocatalytic efficiency of anatase compared to rutile phase [47]. Regarding the P-modified TiO2 calcined at 500 8C, the surface acidity is higher than that of T500 because of the additional hydroxyl groups resulting from phosphorus modification. This is corroborated by the infrared spectra, where different chemical environments can exist around the phosphorus (bidentate structure) as revealed by 31P NMR analysis. Similar results reported by Ramadan et al. [48]. Who argued that the observed acidity between 400 and 500 8C would be arising from the acidic hydrogen in the structure (bidentate ligation). After calcination at 800 8C, who observed a drastic decrease in acidity associated with the phosphate groups devoid of the acidic hydrogen. However, the decrease in the surface acidity at high temperature up to 900 and 1000 8C is probably due to formation of polyphosphate TiP2O7 and rutile phase, which decreases the number of Brönsted acid sites and consequently the total number of acid sites. 140 120 100 150 80 60 100 40 20 50 0 -20 0 0 50 100 150 200 (b) TP P25 T 70 60 50 40 30 20 10 0 Because of the quantum-size effect, the absorption band edge is a strong function of titania size for diameters <10 nm [49]. Fig. 7 150 210 8 T TP P25 7 3.15 6 3.10 kapp(10-3min-1) Eg (eV) 270 Irradiation Time (min) TP T 3.20 90 0 (c) 3.25 Absorbance (a.u) 250 Irradiation Time (min) 3.8. UV–vis diffuse reflectance spectroscopy 3.05 3.00 2.95 2.90 5 4 3 2 2.85 800 900 1000 1100 1200 1 Temperature (°c) T TP 0 500 300 180 160 200 COD (mg/L) 0.0 Concentration of 4-CP (uM) 0.5 Concentration of CL-(uM) Surface acidity (10 umol/L) 137 400 500 600 700 800 λ (nm) Fig. 7. The diffuse reflectance spectra of catalysts calcined at 500 8C, the inset shows the band gaps energy of TP and T as function of calcination temperature. 600 700 800 900 Calcination Temperature (°C) Fig. 8. (a) Typical curves of the degradation of 4-CP (white symbols), release concentration of chloride ions (black symbols) under UV irradiation. 4-CP; (b) chemical oxygen demand (COD); (c) the dependence of the apparent rate constants kapp at various temperatures under UV: 0.23 mM, TiO2: 1.5 g, natural pHs. 138 K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139 partly to increasing particle size of anatase and partly to the presence of rutile phase at temperature extremely higher. 3.9. Photocatalytic degradation of 4-CP Fig. 8(a) shows the photocatalytic properties of 4-chlorophenol (4-CP) on the TiO2 photocatalysts under UV-irradiation times, using lmax 254 nm ultraviolet light (11 W). A control experiment was performed by an identical aqueous solution without catalyst. Control experiment showed that degradation of 4-CP was less than 8% within 270 min in the direct photolysis. The photocatalytic degradation, dechloration rates were enhanced after the addition of phosphorus species. Nearly 85, 43 and 72% of 4-CP was degraded and about 74, 38 and 55% of chlorine was removed by using respectively TP500, T500 catalysts and Degussa P25 within the same irradiation time (270 min). The release of chloride ions is easily understood because the breaking of C–Cl bond is energetically more favourable than that of C–OH bond (the bond dissociated energy for a C–Cl bond is approximately 10 kcal/mol less than that for a C–OH bond) [51]. The temporal changes of chemical oxygen demand (COD) in the degradation process of 4-CP are shown in Fig. 8(b). It can be observed that the removal rates of COD became a faster in the Pmodified TiO2 systems than that in P25 and unmodified-TiO2 system. For the P-modified TiO2 case, the COD reduction is slower up to 87% and 13% COD remained in solution within 270 min. This could be related to organic intermediates generated during the oxidation of 4-CP, as reported in previous work both from our laboratory and from other research group [52,53]. Hence, the complete mineralization of 4-chlorophenol by photocatalytic degradation is rather difficult. As shown in Fig. 8(c), the apparent rate constant kapp value of TP500 under UV irradiation is 7 103 min1, which is 3.2-fold higher than that of T500 (2.0 103 min1) and is 1.6-fold higher than that of commercial Degussa P-25 (4.3 103 min1). The high activity of P-modified TiO2 after calcination at different temperatures may be attributed to the following reasons. First, according to FT-IR spectra, the P-modified TiO2 has more surface-adsorbed water and hydroxyl groups than the unmodified TiO2. These species are considered to play a key role in the photocatalytic reaction because the photoinduced holes can attack those surface hydroxyl groups and yield surface hydroxyl radicals with high oxidation capability [54,55]. Second, the P-modified TiO2 calcined at 500 8C has a higher surface area and size-quantized particle (8.5 nm) than that of unmodified titania. It is well known that the photocatalytic activity of UV illuminated semiconductors is due to the production of excited electrons in the conduction band, along with corresponding positive holes in the valence band. Apparently, the small size of P-modified TiO2 expedites the surface charge carrier transfer, consequently decreasing the chance of recombination of photoinduced electron– hole pairs, hence increasing the photocatalytic activity [56]. Third, according to the Scherrer equation from the (1 0 1) plane, the average crystallite size of TP700 is 15 nm. Indeed, it has been established the optimal size of anatase crystal for the excellent photocatalytically active is 15 nm [57]. Therefore, the reason why TP700 have the highest photocatalytic activity could be related to their good crystallization (anatase crystal) and appropriate particle size. In contrast, the T700 shows poor activity under UV irradiation, which can be attributed to its smaller surface area and large crystalline size. Concerning the P25 Degussa, it is also known that its optimal photocatalytic efficiency is due to the mixture of anatase and rutile (70:30%) which meant a low density of recombination centers. The decrease in photocatalytic abilities of the TP900 should be mainly ascribed to the formation of nonphotocatalytic phase TiP2O7 and rutile. Fig. 9. Germination index as a function as a function of reaction time under UV illumination, using TP500 catalyst. 3.10. Phytotoxicity Phytotoxicity tests were conducted to assess the impact of the release of the irradiated solution to the environment as well as to evaluate the possible use of the pre-treated aqueous solution in the irrigation field. Indeed, this practice can alleviate the burden on underground water overexploitation and promote the practice of using treated water to irrigate golf courses, parks and gardens. Phytotoxicity of raw and treated 4-chlorophenol solution was tested using a seed germination assay and are depicted in Fig. 9. As clearly shown the seed germination in the tomato (L. esculentum) was strongly inhibited by raw solution or with a photocatalysis duration (45 min) reflecting the recalcitrant nature of 4-CP. However, as photocatalysis progressed, a fast increase of germination index was observed. At 270 min; the germination index reaches a maximum (75%). the toxicity of the treated solutions fell within the non-toxic range [58]. Hence, our data indicated that the photocatalytic oxidation technique could be useful as a pretreatment technique for reducing toxicity of toxic/hazardous wastewaters. 4. Conclusions On the basis of the described results, a number of conclusions can be proposed: - Preparation method adopted in this work leads to P-modified TiO2 showing a stability of anatase structure at relatively high temperatures and new crystalline TiP2O7 phase comes into being at the same time with anatase TiO2 for TP900. - The BET surface area of P-modified samples largely exceeds which can offer more active sites and surface acidity (hydroxyl groups) for photocatalytic reaction. - The quantum efficiency of P-modified TiO2 photocatalysts is higher than that of unmodified TiO2, as a result of the inhibition of charge carrier recombination. - The photocatalytic activity of 4-chlorophenol was significantly faster on P-modified than it is on unmodified TiO2 and commercial Degussa P-25. Phytotoxicity to tomato (L. esculentum) seeds was successfully applied to assess the feasibility of the treated solution in the environment. Acknowledgement The author would like to thank Miss. 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