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Rapid synthesis of titania–silica nanoparticles photocatalyst by a modified sol–gel method for cyanide degradation and heavy metals removal
Journal of Alloys and Compounds 551 (2013) 1–7
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Rapid synthesis of titania–silica nanoparticles photocatalyst by a modified sol–gel method for cyanide degradation and heavy metals removal Farid A. Harraz a,⇑, Omar E. Abdel-Salam b, Ahlam A. Mostafa c, Reda M. Mohamed a,d, M. Hanafy b a
Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87 Helwan, Cairo 11421, Egypt Faculty of Engineering, Cairo University, Giza, Egypt c Aircraft Factory, Helwan, Egypt d Faculty of Science, King Abdulaziz University, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 17 June 2012 Received in revised form 27 September 2012 Accepted 3 October 2012 Available online 12 October 2012 Keywords: Titania–silica Modified sol–gel TEM Photocatalysis
a b s t r a c t Titania–silica (TiO2–SiO2) photocatalyst was prepared by a modified sol–gel technique. Titania sol was firstly synthesized by acid hydrolysis of a TiCl4 precursor instead of titanium alkoxides. The titania sol was further modified with SiO2 to obtain a modified catalyst. The as-prepared TiO2–SiO2 catalyst demonstrated a remarkable photocatalytic activity toward degradation of cyanide and heavy metals removal (Cr(III), Co(II) and Pb(II)). The influence of the preparation parameters; the reaction time, the calcination temperature and time, the [H+]/[Ti] ratio, the pH value and the acid concentration on the structural and chemical properties of the catalyst was investigated in details. The catalytic performance was found to depend essentially on the catalyst and target concentrations and the reaction time. The as-synthesized catalyst was characterized by a variety of techniques including surface area measurement, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) spectroscopy measurements. Results of the synthesis and characterization of TiO2–SiO2 catalyst and its photocatalytic performance are presented and thoroughly discussed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Heterogeneous photocatalysis is a discipline that includes different possible reactions: mild or total oxidation, dehydrogenation, hydrogen transfer, metal deposition, water detoxification, gaseous pollutant removal, bactericidal action etc. [1]. It is considered as one of the recent ‘‘Advanced Oxidation Technologies’’ for air and water purification treatments. Heterogeneous photocatalysis could be conducted in gas phase, pure organic liquid phases or aqueous solutions. Photocatalysis is a technique of choice that could be used efficiently for treatment of polluted water. Cyanide and heavy metals are poisonous substances which often pollute water and accordingly need recovery, removal or destruction. Cyanide is originated from metal finishing, ore extraction and hydrometallurgical industries. It is used in the production of organic chemicals such as nitrile, nylon, and acrylic plastics. Other industrial applications of cyanide include electroplating, metal processing, steel hardening and synthetic rubber production. The traditional treatment method is based on the chlorination of simple cyanide and hence cyanogens gas (CNCl) which is known as carcinogenic is usually produced. The produced gas is poisonous, harmful and essentially causes environmental pollution and conse⇑ Corresponding author. Tel.: +20 2 25010 640; fax: +20 2 25010 639. E-mail address: [email protected] (F.A. Harraz). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.004
quently needs further treatment. Heavy metals, on the other hand, commonly exist in process waste streams from mining operations, metal plating, tanneries, electronic device manufacturing units and power generation facilities [2]. The adsorption process is one of the popular methods for removal of heavy-metal ions because of its simplicity, convenience, and high removal efficiency [3,4]. Traces of metals such as Hg, Cr, and Pb in addition to other metals are highly health hazardous, and hence removing of these toxic metals are particularly important for human health and water quality. The environmental applications of heterogeneous photocatalysis for removing various heavy metals such as Hg, Cr, Pb, Cd, As, Ni and Cu have been reported [5]. The prime objective of this work is to treat the cyanide and heavy metals ions (Cr(III), Co(II) and Pb(II)) in aqueous solutions by the photocatalytic degradation (oxidation of cyanide and reduction of heavy metals) over titania–silica (TiO2–SiO2) catalyst prepared by a modified sol–gel technique. The sol–gel method is a well-known, versatile process that could be applied successfully for the synthesis of various nanomaterials that did find a wide room for catalysis applications [6]. The high porosity and the large specific surface area of materials prepared by the sol–gel method are characteristic properties that made them very attractive from a catalytic point of view [7]. Physical methods are also known to synthesize TiO2 based photocatalysts; for instance, TiO2 ultra-thin films have been prepared by pulsed dc reactive magnetron sputtering [8] and TiO2
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mesoporous microspheres were fabricated via chemical vapor deposition [9]. The application of photocatalysts for the treatment of polluted or waste water has attracted significant attention from the scientific community as a promising environmental purification technique [10–14]. Titanium dioxide (TiO2) has been known as the most widely used photocatalyst with its high quantum yield, extended chemical stability, low hazard to human body and easy processibility [15–17]. In addition to the photocatalysis, enormous efforts have been devoted to use TiO2 for other applications including photovoltaics, electrochromics and sensors [18–20]. However, the use of TiO2 alone as a catalyst usually forms a milky dispersion in aqueous solutions, and thus mixing it with SiO2 is a proper way to facilitate the separation and recovery of the catalyst. SiO2 is essentially used as a support to increase the specific surface area. Accordingly, synthesis of a mixture of TiO2–SiO2 often leads to an increase in the surface area and could improve the photocatalytic activity of the final produced catalyst [21–24]. It should be mentioned also that doping TiO2 (nanoparticles or thin films) based photocatalysts with noble metals could effectively enhance the catalytic activity [25–28]. Titania based catalysts prepared by the sol–gel process are commonly obtained by the controlled hydrolysis of titanium alkoxides. However, much less attention has been given to utilizing simple salts such as chloride, sulfate or nitrate. This is probably due to the rapid hydrolysis of the inorganic salt, making the formation of precipitates more easy and uncontrolled. In a previous work, it was found that the fresh precipitate produced from the hydrolysis of either alkoxide or inorganic salt could be peptized by acid to yield a stable sol under appropriate conditions [29]. The use of inorganic salt precursor to prepare titania based catalysts rather than organic alkoxide has the virtue of simplicity in operation and lower cost, no need for supercritical drying of the final product and would avoid the use of organic solvent to decrease the pollution. To better understand the conditions necessary for generating TiO2–SiO2 catalyst via acid hydrolysis of a TiCl4, a detailed investigation was conducted in which the catalyst formation was correlated to systematic variation in operating parameters. The effects of operating reaction time, the calcination temperature and time, the [H+]/[Ti] ratio, the pH value and the acid concentration have been investigated in detail in this work. The as-synthesized photocatalyst was consequently examined and evaluated for the degradation of cyanide ions and removal of heavy metals (Cr(III), Co(II), Pb(II)). Detailed investigation concerning to the catalyst synthesis and characterization and performance evaluation is thoroughly addressed and discussed in this work. 2. Experimental 2.1. Preparation of photocatalyst All chemicals are analytical grade reagents and used without further purification. The titania sol was prepared via acid peptizing the precipitate of titanium tetrachloride solution (TiCl4) with ammonia solution. In a typical synthesis procedure, a 10% NH4OH was dropped into 20 ml TiCl4 (0.18 mol) solution until a white precipitate is obtained at a pH = 7. The precipitate was washed with deionized water to remove the excess Cl and NH4+ ions. Afterward, about 25 ml of deionized water was added to form a suspension. By adding a calculated amount of 1.6 M HNO3 ([H+]/[Ti] = 0.5 with strong stirring for 24 h at 70 °C, the precipitate was peptized to form a highly dispersed and stable titania sol. Appropriate amount (6 ml) of tetraethyl orthosilicate (TEOS) 98% Si(OC2H5)4 solution was dropped into the above titania sol. The modified sol was dried and calcined at 400 °C for 3 h to obtain SiO2–TiO2 catalyst. The content of SiO2 in the modified catalyst was 10 wt.%. A flow chart describing the main steps employed during the modified sol–gel method is depicted in Fig. 1.
2.2. Characterization of photocatalyst The as-produced catalysts were evaluated and characterized using different techniques. The phase and crystalline structures were analyzed by powder X-ray
TiCl4 solution NH4OH White precipitate at pH 7 DI H2O Suspension HNO3 TiO2 sol TEOS Modified sol
Drying, calcination at 400 °C, 3 h TiO2-SiO2 catalyst Fig. 1. Flow chart of the applied modified sol–gel technique.
diffraction (XRD) using Bruker, Axs D8 advance, Germany, with Cu Ka radiation, k = 1.5406 Å Measurements were taken with a tube powder of 40 kV and 40 mA, from 10° to 80° 2h, with a 0.02° 2h step size and 0.4 s count time. Surface area of the samples was measured by nitrogen adsorption using Nova 2000 Series Quanta chrome (USA) surface area analyzer. For detailed morphological and structural analysis, an JEOL JEM-1230 transmission electron microscope (TEM) operating at 200 kV was used. Selected-area electron diffraction (SAED) patterns for some prepared catalysts were also recorded. Surface morphology of the catalyst was examined using SEM JEOL 5410 (Japan) and the chemical composition of as-prepared catalyst was measured by EDS OXFORD (England). UV–vis spectroscopy measurement was also performed with a model JASCO V-570 UV–vis system equipped with a Labsphere integrating sphere diffuse reflectance accessory. 2.3. Photocatalytic activity evaluation All the experiments were carried out using a vertical cylinder annular batch reactor. A backlight-blue florescent bulb (F18W-BLB) was positioned at the axis of the reactor to supply UV illumination. The wavelength of the used UV lamp was 365 nm. The experiments were performed by suspending a certain weight of catalyst into the reactor with 300 ml KCN (100 ppm CN). The effect of catalyst loading on the photodegradation of 150 ppm CN was also performed. The reaction was carried out isothermally at 25 °C, at a solution pH 10.5 to avoid the evolution of HCN gas. The CN content in the solution after a certain reaction time was analyzed by volumetric titration with AgNO3 [30]. For heavy metals removal experiments, a specific weight of catalyst was suspended into the reactor with 300 ml of nitrate salts mixture (100 ppm of each metal ions: Cr(III), Co(II) and Pb(II)). The concentration of the metal ions in the solution after a certain reaction time was analyzed by inductive coupled plasma (ICP) analysis. The photocatalytic activity of the catalyst is evaluated by measuring the removal of cyanide and metal ions. The removal percent was calculated by applying the following equation:
ðRÞ% ¼ ½ðC 0 C t Þ=C 0 100
ð1Þ
where R is the removal efficiency, C0 is the initial concentration of cyanide or metal ions, Ct is the concentration at any time.
3. Results and discussion 3.1. Preparation and characterization of TiO2–SiO2 photocatalyst In the preparation process of titania based catalyst, several factors are crucial for improving the surface area and photoactivity of the final product. Thus, to better understand the conditions necessary for the formation of titania–silica catalyst, series of experiments were conducted in which the phase formation, particle size and morphology were correlated to systematic variation in
F.A. Harraz et al. / Journal of Alloys and Compounds 551 (2013) 1–7
operating parameters, including reaction time, calcination temperature, calcination time, [H+]/[Ti] ratio, content of NH4OH (solution pH) and HNO3 acid concentration. To study the effect of different reaction times (1, 3, 6, 18, 24, and 30 h) on surface area and cyanide removal efficiency; a series of experiments has been carried out under the following conditions: pH = 7, concentration of acid = 1.6 M HNO3, [H+]/[Ti] ratio = 0.5, calcination temperature = 400 °C and calcination time = 3 h. The results shown in Fig. 2 reveal that at reaction times of 1 or 3 h, low surface areas and cyanide removal efficiencies were obtained with a small change in their values. This is because short reaction time leads to un-reacted matters with incomplete reaction and poor productivity which means that the hydrolysis and condensation reactions are not completed. With increasing reaction time from 3 to 6 h, the surface area increased from 177.6 to 286.7 m2/ g and the cyanide removal efficiency increased from 61.5% to 80.3% as a result of increase of surface area. With a further increase in reaction time from 6 to 30 h, a decrease in surface area from 286.7 to 165 m2/g and also a decrease in cyanide removal efficiency from 80.3% to 64.3% were observed. This may be attributed to that prolonged reaction time enhanced the dissolution of the tiny crystal particles in solution and large crystal particles continue the growth and accordingly, the particle size was increased. The above results showed that the cyanide removal efficiency and surface area have their maximum values at 80.3% and 286.7 m2/g, respectively after 6 h reaction time. XRD measurement is essential in determination of the crystal structure and the crystallinity and to estimate the crystal grain size according to the Scherrer equation:
D ¼ kk=b cos h
ð2Þ
where k is a dimensionless constant, 2h is the diffraction angle, k is the wavelength of the X-ray radiation, and b is the full width at half-maximum (fwhm) of the diffraction peak [31]. XRD patterns of as-synthesized TiO2–SiO2 catalyst prepared at different reaction times are shown in Fig. 3. Phase identification indicates that the product is pure, single phase of anatase with no other peaks for impurities. The intensity and shape of the peaks indicate the good crystalline state. The broadening of XRD peaks is essentially attributed to the small diameter size of as synthesized TiO2 catalyst. From the main diffraction peak and applying Scherrer equation, the crystallite size was calculated to be 6, 7, 12.4 and 9 nm at reaction times 6, 18, 24 and 30 h, respectively. With an increase in reaction time, the (1 0 1) diffraction peak became stronger and sharper, whereas other peaks remained similar in shape but
300 280 260
80 240 70
220
60
200
2
90
Surface area / m /g
Cyanide removal efficiency / %
100
180
50
160 40 0
5
10
15
20
25
30
Reaction time / h Fig. 2. Effect of reaction time on surface area and photocatalytic degradation of cyanide.
3
Fig. 3. X-ray diffraction patterns of TiO2–SiO2 catalyst prepared at different reaction times.
slightly increased in intensity. The minimum crystallite size was 6 nm at a reaction time 6 h. This corresponds to the highest surface area obtained at 6 h as shown in Fig. 2. SEM micrographs taken for TiO2–SiO2 catalyst synthesized at 6 and 30 h reaction times are shown in Fig. 4 along with the corresponding EDS chemical analysis profile depicted in the inset of image (b). As can be seen, the morphology of the catalyst prepared at 6 h exhibits spherical particles with relatively homogenous feature without apparent agglomeration, while the catalyst prepared at 30 h consisted of large aggregates. The SEM morphologies are consistent with the result depicted in Fig. 2, as the increase in particle size with reaction time led to a decrease in surface area and cyanide removal efficiency. The EDS analysis shown in inset of Fig. 4b reveals that the as-formed structure consists mainly of Ti, Si and O elements, confirming the high purity of final product with no impurities. The morphology of as-synthesized catalyst was further observed with TEM. The recorded image is shown in Fig. 5 with the selectedarea electron diffraction (SAED) pattern depicted in the inset. The TEM image clearly shows spherical nanoparticles with almost uniform diameter with a narrow range distribution. The average diameter was found to be around 8 nm. A composite microstructure for the catalysts is likely formed, with no definite morphology ordering, since the TiO2 is randomly distributed in SiO2 support. The SAED pattern shown in the inset exhibits the (1 0 1), (0 0 4), (2 0 0) and (1 0 5) directions confirming again the crystalline nature of the anatase phase of catalyst, in consistent with XRD result of Fig. 3. In order to understand the optical absorption property and the UV light utilization efficiency of as-prepared TiO2–SiO2 photocatalyst, the UV–vis absorption data in suspended aqueous solution were measured and the profile is shown in Fig. 6. The spectral data recorded showed a cut off at around 367 nm; where the absorbance value is minimum. According to the absorption edge, the optical band gap of the catalyst was calculated and found to be 3.37 eV, which is slightly larger than the reported value for bulk TiO2 anatase phase (3.2 eV) [32]. It is generally accepted that the values of band gap are influenced by the synthesis method, the existence of impurities doping the crystalline network, and the average crystal size of the semiconductor. Although the presence of SiO2 as a support led to widening the band gap, but it essentially enhanced the specific surface area of the mixed catalyst. Calcination is an effective treatment to optimize the photocatalytic activity of TiO2 which results from the improvement of crystallinity. The effect of calcination temperature and calcination time on phase formation, crystallite size, surface area and cyanide removal efficiency were investigated under different calcination
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Fig. 4. SEM images of TiO2–SiO2 catalyst prepared at; (a) 6 h and (b) 30 h. EDS analysis is depicted in the inset.
Fig. 5. TEM image of TiO2–SiO2 catalyst synthesized at 6 h. The SAED pattern is shown in the inset.
Fig. 6. UV–vis. absorption spectrum obtained for as-synthesized TiO2–SiO2 catalyst.
temperatures (400–700 °C) and different calcination times (1–5 h). The results are summarized in Table 1. As one can notice, increasing calcination temperature from 400 to 700 °C led to a decrease in surface area from 286.7 to 146.8 m2/g with an increase in
crystallite size from 6 to 12 nm and a decrease in cyanide removal efficiency from 80.3% to 61.2%. One could also observe that when calcination time increased from 1 to 3 h, the surface area increased from 222 to 286.7 m2/g and cyanide removal efficiency increased consequently from 60% to 80.3%. By increasing calcination time from 3 to 4 h, a decrease in surface area from 286.7 to 213 m2/ g occurred and also cyanide removal efficiency decreased from 80.3% to 58.8%. This is likely related to the sintering and growth of TiO2 crystallites resulting in the decrease of surface area. At calcination temperature of 400 °C for 3 h, TiO2 based catalyst showed the highest photocatalytic activity due to better crystallization, highest surface area and lowest crystallite size. [H+]/[Ti] ratio and solution pH are also key operating parameters during the catalyst synthesis using the present sol–gel technique. The effect of [H+]/[Ti] ratio and solution pH value on surface area and cyanide removal efficiency were investigated under the previously optimal conditions. Various ratios of [H+]/ [Ti] (0.5, 1, 1.5 and 2) were used to elucidate its influence on asformed catalyst. The results are collected in Table 2. When [H+]/ [Ti] ratio increased from 0.5 to 1.0, the surface area decreased from 286.7 to 264.5 m2/g, which led to a decrease in cyanide removal efficiency from 80.3% to 65.0%. When [H+]/[Ti] ratio increased to 1.5, the surface area and cyanide removal efficiency decreased again to 255.7 m2/g and 62.8%, respectively. Further increase of [H+]/[Ti] ratio to 2.0 led to a decrease in surface area from 255.7 to 251.9 m2/g and also a decrease in cyanide removal efficiency from 62.8% to 61.2%. This behavior may be attributed to poor condensation and high crystals growth rate which led finally to larger particles size. Based on these results, the optimum condition of [H+]/[Ti] ratio is taken at 0.5 at which 80.3% cyanide removal efficiency and 286.7 m2/g surface area could be achieved. The effect of variation of solution pH is also given in Table 2. When the pH value increased from 6 to 7, this led to an increase in surface area to 286.7 m2/g which in turn resulted in an increase in cyanide removal efficiency (80.3%). A drop in both surface area and cyanide removal efficiency was observed when the solution pH increased above 7. In general, when the pH value of the precipitation medium was low, the small quantity of hydroxyl groups would limit the hydrolysis of unhydrolyzed alkyls and Cl ions that remained in the precursors, and finally prevented the crystallization of the samples and the growth of the TiO2 crystallites. We also examined the effect of using different nitric acid concentrations on the catalyst synthesis. The finding revealed that when nitric acid concentration increased from 1.0 to 1.6 M, the cyanide removal efficiency increased from 74.2% to 80.3%, when the concentration increased to 2, the cyanide removal efficiency decreased from 80.3% to 77%. This may be attributed to poor con-
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F.A. Harraz et al. / Journal of Alloys and Compounds 551 (2013) 1–7 Table 1 Effect of calcination temperature and time on surface area, crystallite size and cyanide removal efficiency.
Table 3 Effect of catalyst weight, reaction time and cyanide ions concentration on cyanide removal efficiency.
Calcination temperature, (°C)/calcinations time, (h)
Surface area (m2/g)
Crystallite size (nm)
Cyanide removal efficiency (%)
Entry
Cyanide ions concentration (ppm)
Catalyst weight (mg)
Reaction time (min)
Cyanide removal efficiency (%)
400/3 500/3 600/3 700/3 400/1 400/2 400/4 400/5
286.7 199.1 179.7 146.8 222.0 235 213.0 207.0
6 9.2 14.7 12.4 9.2 7.5 9.5 11.6
80.3 70.0 67.0 61.2 60.0 62.0 58.8 56.5
1 2 3 4 5 6 7 8 9 10
100 100 100 100 100 100 100 75 50 25
100 200 300 400 500 500 500 500 500 500
60 60 60 60 60 120 180 180 180 180
65.7 72.5 80.3 83.7 88.2 90.7 93.3 94.6 96.1 99.4
Table 2 Effect of [H+]/[Ti] ratio and solution pH on surface area and cyanide removal efficiency. [H+]/[Ti] ratio/pH value
Surface area (m2/g)
Crystallite size (nm)
Cyanide removal efficiency (%)
0.5/7 1.0/7 1.5/7 2.0/7 0.5/6 0.5/8 0.5/9
286.7 264.5 255.7 251.9 268.4 176.26 233.0
6 8.5 8.9 9.9 8.2 12.5 14.4
80.3 65.0 62.8 61.2 74.0 60.7 51.0
taken for analysis after 3 h. The results reveal that decreasing CN content from 100 to 25 ppm led to 5% increase in cyanide removal efficiency (93.3% increased to 99.4%). Few control experiments were carried out to gain a better understanding of the photolytic and photocatalytic degradation process. Control experiments in the dark or in absence of catalyst under illumination failed to give a significant activity; 2% for cyanide removal was detected. This is clear proof that the photodegradation of cyanide is mainly catalyzed by TiO2–SiO2 mixed catalyst. The process of removal of CN ions over TiO2–SiO2 catalyst can be represented by the following equations [33]: þ
densation and high crystals growth which led to high particles size and hence lower removal efficiency. Therefore, the optimum condition of nitric acid concentration is taken at 1.6 M at which 80.3% cyanide removal efficiency could be achieved. Based on the above results, one can conclude that the relation between the physical properties of catalyst and its photocatalytic activities is complicated, and judging from the current experimental observations, the optimal conditions for catalyst synthesis could be determined only by taking into consideration several factors that also may vary from case to case. 3.2. Photocatalytic performance 3.2.1. Photodegradation of cyanide ions The catalytic activity of as-synthesized TiO2–SiO2 nanoparticles was firstly investigated using cyanide degradation as a model catalytic reaction. The catalytic reactions were conducted by suspending a certain weight of catalyst into the reactor containing 300 ml KCN (100 ppm CN). Determination of minimum concentration of catalyst necessary for complete degradation of cyanide is indispensable step in optimizing the catalytic reaction. Different weights of TiO2–SiO2 catalyst, mainly 100–500 mg in 100 ppm CN solutions were accordingly examined at room temperature for 60 min. The results of cyanide degradation % are summarized in Table 3. At smaller weight of catalyst 100 mg, only 65.7% removal of cyanide was obtained after 60 min, in comparison with 72.5% in case of using 200 mg catalyst for the same time. With a moderate catalyst weight of 300 mg, the photodegradation reaction was further enhanced affording 83.7% cyanide removal. The maximum removal efficiency of 88.2% was achieved using 500 mg catalyst. The effect of degradation time on cyanide removal efficiency was further investigated using the 500 mg catalyst in 100 ppm CN and a sample of the reaction mixture was taken for analysis at different reaction times (1–3 h). The results are collected in Table 3. The cyanide removal efficiency was found to increase from 88.2% to 93.3% when degradation time increased from 1 to 3 h. Different CN contents (100, 75, 50 and 25 ppm CN) were consequently utilized in 500 mg catalyst weight and a sample of the reaction mixture was
TiO2 þ 2hc ¼ TiO2 ð2h þ 2e Þ
ð3Þ
1=2O2 þ 2e þ H2 O ¼ 2OH
ð4Þ
þ
2OH þ 2h ¼ 2OH
ð5Þ
CN þ 2OH ¼ OCN þ H2 O
ð6Þ
2OCN þ O2 ¼ 2CO2 þ N2
ð7Þ
Overall reaction: TiO2 =H2 O
2CN þ 2O2 ! 2CO2 þ N2 UV light
ð8Þ
It is worth mentioning here that the CN could be converted to N2 and CO2, which are environmental friendly. The kinetics of cyanide photodegradation was further investigated using different loadings 100, 300, and 500 mg of TiO2–SiO2 catalyst in a higher concentration of cyanide (150 ppm) for different illumination times. The effect of UV illumination with no catalyst on the cyanide removal was also tested. The results of photodegradation are shown in Fig. 7. The loss of the cyanide ions could be fitted to the expression [34]:
log½ct ¼ kt þ log½c0
ð9Þ
where [c]0 and [c]t represent the concentration in ppm of cyanide ions at zero time and time t of illumination, respectively, and k represents the apparent rate constant (min1). Plotting ln(ct/c0) as a function of time (t) gives linear relation. This indicated that the photodegradation of cyanide ions using TiO2–SiO2 photocatalyst is a first order reaction. We obtained for each run the k (min1) constant from the slopes of the simulated straight lines. The rate constant k was found to increase with increasing the loading of TiO2–SiO2 catalyst. The values of k obtained at 100, 300, and 500 mg catalyst are respectively, 5.8 103, 9.8 103 and 13.3 103 min1. It should be noted that, negligible cyanide degradation was detected under UV illumination, in absence of TiO2–SiO2 photocatalyst. 3.2.2. Removal of heavy metals To extend the application of TiO2–SiO2 photocatalyst to the removal of heavy metals, we examined its catalytic performance for the removal of Cr, Co and Pb. The catalytic reaction was evaluated
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Fig. 7. Reaction kinetics of cyanide photocatalytic degradation at different loadings of TiO2–SiO2 catalyst. The concentration of cyanide was 150 ppm.
using 500 mg catalyst in 300 ml of nitrate salts mixture (100 ppm of each metal ions (Cr(III), Co(II) and Pb(II)). The concentration of metal ions in solution after a certain reaction time was analyzed by inductive coupled plasma (ICP) analysis. The photoactivity of optimized sample for reduction of heavy metals can be identified
Table 4 Effect of degradation time and catalyst weight on heavy metals removal efficiency. Degradation time (h)/catalyst weight (mg)
Heavy metal removal efficiency (%) Cr
Co
Pb
1/500 2/500 3/500 3/700 3/1000
18.7 32.2 46 67.8 90.0
19.7 35.5 50.2 71.0 91.9
25.4 40.7 54.3 75.2 98.6
by changing illumination time and catalyst weight; therefore the best heavy metals removal efficiency can be determined. The effect of illumination time and catalyst weight on heavy metals removal efficiency is shown in Table 4. As a general trend, the removal efficiency was increased for all heavy metals as the contact time increases from 1 to 3 h. A maximum removal efficiency of 54.3% was obtained for Pb as a target metal ion after 3 h contact time. Longer exposure between heavy metal ions and the catalyst led to a higher diffusion rate toward the catalyst which in turn increases the adsorption capacity. We then examined the effect of using different catalyst weights (500, 700 and 1000 mg) on the removal efficiency keeping the reaction time constant at 3 h. The results shown in Table 4 indicated that as the catalyst weight was doubled from 500 to 1000 mg, the removal efficiency was increased consequently to 90.0%, 91.9% and 98.6% for Cr, Co and Pb, respectively. As the catalyst weight increased, there is a greater availability of the adsorbent for higher adsorption capacity and consequently the amount of metals adsorbed on the surface increased. The higher adsorption capacity of metals was in the order (Pb > Co > Cr). The adsorption capacity reached a maximum value at 1000 mg TiO2–SiO2 catalyst and the maximum removal % was 98.6% for Pb metal. It has been reported earlier that the TiO2–SiO2 catalyst has a high relatively adsorption capacity for such heavy metals [35]. Control experiments were performed without addition of TiO2– SiO2 catalyst in order to determine whether photochemical reactions could occur in absence of catalyst. No photolytic removal of the current heavy metals was detected after 3 h UV illumination under the present experimental conditions. This indicates the photocatalytic nature of the above results in presence of TiO2– SiO2 catalyst. Further, the adsorption of heavy metals on catalyst surface in the dark was analyzed after mixing for 3 h. The results indicated a sluggish dark adsorption (7%) on catalyst surface. The metal ions in aqueous solution may undergo salvation and hydrolysis according to the following expression [2,36].
M2þ þ nH2 O ¼ MðH2 OÞ2þ n
ð10Þ
þ þ MðH2 OÞ2þ n ¼ ½MðH2 OÞn1 ðOHÞ þ H
ð11Þ
or totally
M2þ þ nH2 O ¼ ½MðH2 OÞn1 ðOHÞþ þ Hþ
Fig. 8. Proposed photocatalytic reaction mechanism for water pollutant in a semiconductor TiO2 particle.
ð12Þ
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The relationship between the amount of substance adsorbed per unit mass of adsorbent at constant temperature and its concentration in the equilibrium solution is called the adsorption isotherm. Adsorption isotherm is often applied to describe the way the metal ions interact with the catalyst, which is being underway to elucidate reaction mechanism and the rate of photodegradation in this TiO2–SiO2 catalyzed removal of heavy metals. Based on the above finding, it can be concluded that 1000 mg of our TiO2–SiO2 catalyst is most appropriate for removal of 100 ppm of each metal ions in its nitrate solution during 3 h reaction time to obtain 90.0%, 91.9% and 98.6% photoactivity efficiency for Cr, Co and Pb, respectively. 3.2.3. Photocatalytic reaction mechanism The photocatalytic reaction mechanisms utilizing the semiconductor TiO2 have been intensively reported in the literatures [20,37–41]. When photon energy (hc) of larger than or equal to the band gap energy of TiO2 is illuminated onto the surface, usually 3.2 eV for anatase or 3.0 eV for rutile, electrons are excited from the valence band to the conduction band, creating electron–hole pairs (e–h+). Such charge carriers migrate to the surface, react with the pollutants adsorbed on the surface and decompose them via inducing a series of reduction–oxidation reactions. This photodecomposition process usually involves one or more radicals or intermediate species such as OH, O 2 , H2O2, or O2, which play crucial roles in the photocatalytic reaction mechanisms. A simplified, photo-induced formation mechanism of (e–h+) pair in a semiconductor TiO2 particle in the presence of water pollutant (P) is presented in Fig. 8. The valence band hole has oxidizing behavior, while the conduction band electron has the capability for reduction process. Oxidation of water or OH by the hole produces the hydroxyl radical (OH), an extremely powerful oxidant, which in turn could rapidly attack pollutants adsorbed on the surface or even in solution. Beside, the reduction of adsorbed O2 to O 2 is an important reaction of the conduction band electron. Both reactions help to prevent the (e–h+) recombination which results in an accumulation of oxygen radical species that can also participate in attacking water contaminants [14]. It is worthy to note that the catalytic activity of TiO2 photocatalyst illustrated in Fig. 8 is essentially dependent on (i) the light absorption properties, (ii) the rate of reduction–oxidation reactions, (iii) and the recombination rate of (e–h+). Constant surface density of adsorbents with large surface area often leads to faster photocatalytic reaction rates [9]. In this context, the higher photocatalytic activity is related to the larger specific surface area and higher crystallinity. 4. Conclusion TiO2–SiO2 photocatalyst was successively synthesized via rapid, efficient sol–gel approach using TiCl4 precursor instead of titanium alkoxides. The XRD and TEM analysis confirmed the presence of crystalline anatase phase with almost uniform diameter and a narrow range distribution of spherical nanoparticles. The as-synthesized catalyst exhibited a good photocatalytic activity for cyanide degradation and heavy metals (Pb(II), Cr(III), Co(II)) removal under mild conditions in aqueous solutions. The photocatalytic performance in both cases was markedly dependent on catalyst and target concentrations and reaction time. The present photocatalyzed
7
degradation of cyanide and heavy metals over TiO2–SiO2 is environmentally friendly; the process does not add any toxic pollutant. The photocatalyst is expected to demonstrate a broad range of utility for other heterogeneous catalytic reactions which are under investigation in our labs. References [1] J.-M. Herrmann, Top. Catal. 34 (1–4) (2005) 49–65. [2] A. Afkhami, M. Saber-Tehrani, H. Bagheri, J. Hazard. Mater. 181 (1–3) (2010) 836–844. [3] A. Stafiej, K. Pyrzynska, Sep. Purif. Technol. 58 (2007) 49–52. [4] A. Afkhami, T. Madrakian, Z. Karimi, A. Amini, Colloids Surf. A 304 (2007) 36– 40. [5] D.F. Ollis, E. Pelizzetti, N. Serpone, Environ. Sci. Technol. 25 (1991) 1522–1529. [6] J.C. Brinker, W.G. Scherer, Sol–Gel Sci., Academic Press, New York, 1990. [7] F.A. Harraz, R.M. Mohamed, A. Shawky, I.A. Ibrahim, J. Alloys Comp. 508 (2010) 133–140. [8] M. Horprathum, P. Eiamchai, P. Limnonthakul, N. Nuntawong, P. Chindaudom, A. Pokaipisit, P. Limsuwan, J. Alloys Comp. 509 (2011) 4520–4524. [9] J. Hou, X. Yang, X. Lv, M. Huang, Q. Wang, J. Wang, J. Alloys Comp. 511 (2012) 202–208. [10] P.V. Kamat, Chem. Rev. 93 (1993) 267–300. [11] J.-X. Zhao, B.-P. Zhang, Y. Li, L.-P. Yan, S.-J. Wang, J. Alloys Comp. 535 (2012) 21–26. [12] H.L. Meng, C. Cui, H.L. Shen, D.Y. Liang, Y.Z. Xue, P.G. Li, W.H. Tang, J. Alloys Comp. 527 (2012) 30–35. [13] T.K. Ghorai, M. Chakraborty, P. Pramanik, J. Alloys Comp. 509 (2011) 8158– 8164. [14] H.L. Shen, H.H. Hu, D.Y. Liang, H.L. Meng, P.G. Li, W.H. Tang, C. Cui, J. Alloys Comp. 542 (2012) 32–36. [15] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [16] T. Ohno, S. Tagawa, H. Itoh, H. Suzuki, T. Matsuda, Mater. Chem. Phys. 113 (1) (2009) 119–123. [17] H.S. Jahromi, H. Taghdisian, S. Tasharrofi, Mater. Chem. Phys. 122 (1) (2010) 205–210. [18] M. Grätzel, Nature 414 (2001) 338–344. [19] A.L. Linsebigler, G. Lu, J.T. Jr, Yates, Chem. Rev. 95 (1995) 735–758. [20] A. Millis, S. Le Hunte, J. Photochem. Photobiol. A 108 (1997) 1–35. [21] Z. Zhang, C. Wang, R. Zakaria, J. Ying, J. Phys. Chem. B 102 (52) (1998) 10871– 10878. [22] M. Strauss, C.M. Maroneze, J.M. de Souza e Silva, F.A. Sigoli, Y. Gushikem, I.O. Mazali, Mater. Chem. Phys. 126 (1-2) (2011) 188–194. [23] B. Mahltig, E. Gutmann, D.C. Meyer, Mater. Chem. Phys. 127 (1–2) (2011) 285– 291. [24] C.M. Wang, A. Heller, H. Gerischer, J. Am. Chem. Soc. 114 (1992) 5230–5234. [25] S.J. Wang, B.P. Zhang, L.P. Yan, W. Deng, J. Alloys Comp. 509 (2011) 5731–5735. [26] P.R. Ren, H.Q. Fan, X. Wang, J. Shi, J. Alloys Comp. 509 (2011) 6423–6426. [27] H.-Fu Yu, S.-Te Yang, J. Alloys Comp. 492 (2010) 695–700. [28] K.V. Baiju, P. Periyat, P. Shajesh, W. Wunderlich, K.A. Manjumol, V.S. Smitha, K.B. Jaimy, K.G.K. Warrier, J. Alloys Comp. 505 (2010) 194–200. [29] X. Fu, L.A. Clark, Q. Yang, M.A. Anderson, Environ. Sci. Technol. 30 (1996) 647– 653. [30] A.I. Vogel, Quantitative Inorganic Analysis, Longmans, London, 1978. [31] E.L. Crepaldi, G.J.D.A. Soler-Illia, D. Grosso, F. Cagnol, F. Ribot, C. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770–9786. [32] G. Ricchiardi, A. Damin, S. Bordiga, C. Lamberti, G. Spano, F. Rivetti, A. Zecchina, J. Am. Chem. Soc. 123 (2001) 11409–11419. [33] X.Z. Ding, X.H. Liu, Y.Z. He, J. Mater. Sci. Lett. 15 (1996) 1789–1796. [34] H. Tada, M. Yamamoto, S. Ito, Langmuir 15 (1999) 3699–3705. [35] S.K.R. Yadanaparthi, D. Grabill, R.V. Wandruszka, J. Hazard. Mater. 171 (2009) 1–15. [36] G. Wulfsberg, Principles of Descriptive Chemistry, Brookes Cole Publishing, Montery, 1987. [37] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1–21. [38] D. Beydoun, R. Amal, G. Low, S. McEvoy, J. Nanopart. Res. 1 (1999) 439–458. [39] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [40] M. Anpo, T. Shima, S. Kodama, Y. Kubokawa, J. Phys. Chem. 91 (1987) 4305– 4310. [41] M. Alvaro, B. Cojocaru, A.A. Ismail, N. Petrea, B. Ferrer, F.A. Harraz, V.I. Parvulescu, H. Garcia, Appl. Catal. B 99 (2010) 191–197.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Rapid synthesis of titania–silica nanoparticles photocatalyst by a modified sol–gel method for cyanide degradation and heavy metals removal Farid A. Harraz a,⇑, Omar E. Abdel-Salam b, Ahlam A. Mostafa c, Reda M. Mohamed a,d, M. Hanafy b a
Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87 Helwan, Cairo 11421, Egypt Faculty of Engineering, Cairo University, Giza, Egypt c Aircraft Factory, Helwan, Egypt d Faculty of Science, King Abdulaziz University, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 17 June 2012 Received in revised form 27 September 2012 Accepted 3 October 2012 Available online 12 October 2012 Keywords: Titania–silica Modified sol–gel TEM Photocatalysis
a b s t r a c t Titania–silica (TiO2–SiO2) photocatalyst was prepared by a modified sol–gel technique. Titania sol was firstly synthesized by acid hydrolysis of a TiCl4 precursor instead of titanium alkoxides. The titania sol was further modified with SiO2 to obtain a modified catalyst. The as-prepared TiO2–SiO2 catalyst demonstrated a remarkable photocatalytic activity toward degradation of cyanide and heavy metals removal (Cr(III), Co(II) and Pb(II)). The influence of the preparation parameters; the reaction time, the calcination temperature and time, the [H+]/[Ti] ratio, the pH value and the acid concentration on the structural and chemical properties of the catalyst was investigated in details. The catalytic performance was found to depend essentially on the catalyst and target concentrations and the reaction time. The as-synthesized catalyst was characterized by a variety of techniques including surface area measurement, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) spectroscopy measurements. Results of the synthesis and characterization of TiO2–SiO2 catalyst and its photocatalytic performance are presented and thoroughly discussed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Heterogeneous photocatalysis is a discipline that includes different possible reactions: mild or total oxidation, dehydrogenation, hydrogen transfer, metal deposition, water detoxification, gaseous pollutant removal, bactericidal action etc. [1]. It is considered as one of the recent ‘‘Advanced Oxidation Technologies’’ for air and water purification treatments. Heterogeneous photocatalysis could be conducted in gas phase, pure organic liquid phases or aqueous solutions. Photocatalysis is a technique of choice that could be used efficiently for treatment of polluted water. Cyanide and heavy metals are poisonous substances which often pollute water and accordingly need recovery, removal or destruction. Cyanide is originated from metal finishing, ore extraction and hydrometallurgical industries. It is used in the production of organic chemicals such as nitrile, nylon, and acrylic plastics. Other industrial applications of cyanide include electroplating, metal processing, steel hardening and synthetic rubber production. The traditional treatment method is based on the chlorination of simple cyanide and hence cyanogens gas (CNCl) which is known as carcinogenic is usually produced. The produced gas is poisonous, harmful and essentially causes environmental pollution and conse⇑ Corresponding author. Tel.: +20 2 25010 640; fax: +20 2 25010 639. E-mail address: [email protected] (F.A. Harraz). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.004
quently needs further treatment. Heavy metals, on the other hand, commonly exist in process waste streams from mining operations, metal plating, tanneries, electronic device manufacturing units and power generation facilities [2]. The adsorption process is one of the popular methods for removal of heavy-metal ions because of its simplicity, convenience, and high removal efficiency [3,4]. Traces of metals such as Hg, Cr, and Pb in addition to other metals are highly health hazardous, and hence removing of these toxic metals are particularly important for human health and water quality. The environmental applications of heterogeneous photocatalysis for removing various heavy metals such as Hg, Cr, Pb, Cd, As, Ni and Cu have been reported [5]. The prime objective of this work is to treat the cyanide and heavy metals ions (Cr(III), Co(II) and Pb(II)) in aqueous solutions by the photocatalytic degradation (oxidation of cyanide and reduction of heavy metals) over titania–silica (TiO2–SiO2) catalyst prepared by a modified sol–gel technique. The sol–gel method is a well-known, versatile process that could be applied successfully for the synthesis of various nanomaterials that did find a wide room for catalysis applications [6]. The high porosity and the large specific surface area of materials prepared by the sol–gel method are characteristic properties that made them very attractive from a catalytic point of view [7]. Physical methods are also known to synthesize TiO2 based photocatalysts; for instance, TiO2 ultra-thin films have been prepared by pulsed dc reactive magnetron sputtering [8] and TiO2
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mesoporous microspheres were fabricated via chemical vapor deposition [9]. The application of photocatalysts for the treatment of polluted or waste water has attracted significant attention from the scientific community as a promising environmental purification technique [10–14]. Titanium dioxide (TiO2) has been known as the most widely used photocatalyst with its high quantum yield, extended chemical stability, low hazard to human body and easy processibility [15–17]. In addition to the photocatalysis, enormous efforts have been devoted to use TiO2 for other applications including photovoltaics, electrochromics and sensors [18–20]. However, the use of TiO2 alone as a catalyst usually forms a milky dispersion in aqueous solutions, and thus mixing it with SiO2 is a proper way to facilitate the separation and recovery of the catalyst. SiO2 is essentially used as a support to increase the specific surface area. Accordingly, synthesis of a mixture of TiO2–SiO2 often leads to an increase in the surface area and could improve the photocatalytic activity of the final produced catalyst [21–24]. It should be mentioned also that doping TiO2 (nanoparticles or thin films) based photocatalysts with noble metals could effectively enhance the catalytic activity [25–28]. Titania based catalysts prepared by the sol–gel process are commonly obtained by the controlled hydrolysis of titanium alkoxides. However, much less attention has been given to utilizing simple salts such as chloride, sulfate or nitrate. This is probably due to the rapid hydrolysis of the inorganic salt, making the formation of precipitates more easy and uncontrolled. In a previous work, it was found that the fresh precipitate produced from the hydrolysis of either alkoxide or inorganic salt could be peptized by acid to yield a stable sol under appropriate conditions [29]. The use of inorganic salt precursor to prepare titania based catalysts rather than organic alkoxide has the virtue of simplicity in operation and lower cost, no need for supercritical drying of the final product and would avoid the use of organic solvent to decrease the pollution. To better understand the conditions necessary for generating TiO2–SiO2 catalyst via acid hydrolysis of a TiCl4, a detailed investigation was conducted in which the catalyst formation was correlated to systematic variation in operating parameters. The effects of operating reaction time, the calcination temperature and time, the [H+]/[Ti] ratio, the pH value and the acid concentration have been investigated in detail in this work. The as-synthesized photocatalyst was consequently examined and evaluated for the degradation of cyanide ions and removal of heavy metals (Cr(III), Co(II), Pb(II)). Detailed investigation concerning to the catalyst synthesis and characterization and performance evaluation is thoroughly addressed and discussed in this work. 2. Experimental 2.1. Preparation of photocatalyst All chemicals are analytical grade reagents and used without further purification. The titania sol was prepared via acid peptizing the precipitate of titanium tetrachloride solution (TiCl4) with ammonia solution. In a typical synthesis procedure, a 10% NH4OH was dropped into 20 ml TiCl4 (0.18 mol) solution until a white precipitate is obtained at a pH = 7. The precipitate was washed with deionized water to remove the excess Cl and NH4+ ions. Afterward, about 25 ml of deionized water was added to form a suspension. By adding a calculated amount of 1.6 M HNO3 ([H+]/[Ti] = 0.5 with strong stirring for 24 h at 70 °C, the precipitate was peptized to form a highly dispersed and stable titania sol. Appropriate amount (6 ml) of tetraethyl orthosilicate (TEOS) 98% Si(OC2H5)4 solution was dropped into the above titania sol. The modified sol was dried and calcined at 400 °C for 3 h to obtain SiO2–TiO2 catalyst. The content of SiO2 in the modified catalyst was 10 wt.%. A flow chart describing the main steps employed during the modified sol–gel method is depicted in Fig. 1.
2.2. Characterization of photocatalyst The as-produced catalysts were evaluated and characterized using different techniques. The phase and crystalline structures were analyzed by powder X-ray
TiCl4 solution NH4OH White precipitate at pH 7 DI H2O Suspension HNO3 TiO2 sol TEOS Modified sol
Drying, calcination at 400 °C, 3 h TiO2-SiO2 catalyst Fig. 1. Flow chart of the applied modified sol–gel technique.
diffraction (XRD) using Bruker, Axs D8 advance, Germany, with Cu Ka radiation, k = 1.5406 Å Measurements were taken with a tube powder of 40 kV and 40 mA, from 10° to 80° 2h, with a 0.02° 2h step size and 0.4 s count time. Surface area of the samples was measured by nitrogen adsorption using Nova 2000 Series Quanta chrome (USA) surface area analyzer. For detailed morphological and structural analysis, an JEOL JEM-1230 transmission electron microscope (TEM) operating at 200 kV was used. Selected-area electron diffraction (SAED) patterns for some prepared catalysts were also recorded. Surface morphology of the catalyst was examined using SEM JEOL 5410 (Japan) and the chemical composition of as-prepared catalyst was measured by EDS OXFORD (England). UV–vis spectroscopy measurement was also performed with a model JASCO V-570 UV–vis system equipped with a Labsphere integrating sphere diffuse reflectance accessory. 2.3. Photocatalytic activity evaluation All the experiments were carried out using a vertical cylinder annular batch reactor. A backlight-blue florescent bulb (F18W-BLB) was positioned at the axis of the reactor to supply UV illumination. The wavelength of the used UV lamp was 365 nm. The experiments were performed by suspending a certain weight of catalyst into the reactor with 300 ml KCN (100 ppm CN). The effect of catalyst loading on the photodegradation of 150 ppm CN was also performed. The reaction was carried out isothermally at 25 °C, at a solution pH 10.5 to avoid the evolution of HCN gas. The CN content in the solution after a certain reaction time was analyzed by volumetric titration with AgNO3 [30]. For heavy metals removal experiments, a specific weight of catalyst was suspended into the reactor with 300 ml of nitrate salts mixture (100 ppm of each metal ions: Cr(III), Co(II) and Pb(II)). The concentration of the metal ions in the solution after a certain reaction time was analyzed by inductive coupled plasma (ICP) analysis. The photocatalytic activity of the catalyst is evaluated by measuring the removal of cyanide and metal ions. The removal percent was calculated by applying the following equation:
ðRÞ% ¼ ½ðC 0 C t Þ=C 0 100
ð1Þ
where R is the removal efficiency, C0 is the initial concentration of cyanide or metal ions, Ct is the concentration at any time.
3. Results and discussion 3.1. Preparation and characterization of TiO2–SiO2 photocatalyst In the preparation process of titania based catalyst, several factors are crucial for improving the surface area and photoactivity of the final product. Thus, to better understand the conditions necessary for the formation of titania–silica catalyst, series of experiments were conducted in which the phase formation, particle size and morphology were correlated to systematic variation in
F.A. Harraz et al. / Journal of Alloys and Compounds 551 (2013) 1–7
operating parameters, including reaction time, calcination temperature, calcination time, [H+]/[Ti] ratio, content of NH4OH (solution pH) and HNO3 acid concentration. To study the effect of different reaction times (1, 3, 6, 18, 24, and 30 h) on surface area and cyanide removal efficiency; a series of experiments has been carried out under the following conditions: pH = 7, concentration of acid = 1.6 M HNO3, [H+]/[Ti] ratio = 0.5, calcination temperature = 400 °C and calcination time = 3 h. The results shown in Fig. 2 reveal that at reaction times of 1 or 3 h, low surface areas and cyanide removal efficiencies were obtained with a small change in their values. This is because short reaction time leads to un-reacted matters with incomplete reaction and poor productivity which means that the hydrolysis and condensation reactions are not completed. With increasing reaction time from 3 to 6 h, the surface area increased from 177.6 to 286.7 m2/ g and the cyanide removal efficiency increased from 61.5% to 80.3% as a result of increase of surface area. With a further increase in reaction time from 6 to 30 h, a decrease in surface area from 286.7 to 165 m2/g and also a decrease in cyanide removal efficiency from 80.3% to 64.3% were observed. This may be attributed to that prolonged reaction time enhanced the dissolution of the tiny crystal particles in solution and large crystal particles continue the growth and accordingly, the particle size was increased. The above results showed that the cyanide removal efficiency and surface area have their maximum values at 80.3% and 286.7 m2/g, respectively after 6 h reaction time. XRD measurement is essential in determination of the crystal structure and the crystallinity and to estimate the crystal grain size according to the Scherrer equation:
D ¼ kk=b cos h
ð2Þ
where k is a dimensionless constant, 2h is the diffraction angle, k is the wavelength of the X-ray radiation, and b is the full width at half-maximum (fwhm) of the diffraction peak [31]. XRD patterns of as-synthesized TiO2–SiO2 catalyst prepared at different reaction times are shown in Fig. 3. Phase identification indicates that the product is pure, single phase of anatase with no other peaks for impurities. The intensity and shape of the peaks indicate the good crystalline state. The broadening of XRD peaks is essentially attributed to the small diameter size of as synthesized TiO2 catalyst. From the main diffraction peak and applying Scherrer equation, the crystallite size was calculated to be 6, 7, 12.4 and 9 nm at reaction times 6, 18, 24 and 30 h, respectively. With an increase in reaction time, the (1 0 1) diffraction peak became stronger and sharper, whereas other peaks remained similar in shape but
300 280 260
80 240 70
220
60
200
2
90
Surface area / m /g
Cyanide removal efficiency / %
100
180
50
160 40 0
5
10
15
20
25
30
Reaction time / h Fig. 2. Effect of reaction time on surface area and photocatalytic degradation of cyanide.
3
Fig. 3. X-ray diffraction patterns of TiO2–SiO2 catalyst prepared at different reaction times.
slightly increased in intensity. The minimum crystallite size was 6 nm at a reaction time 6 h. This corresponds to the highest surface area obtained at 6 h as shown in Fig. 2. SEM micrographs taken for TiO2–SiO2 catalyst synthesized at 6 and 30 h reaction times are shown in Fig. 4 along with the corresponding EDS chemical analysis profile depicted in the inset of image (b). As can be seen, the morphology of the catalyst prepared at 6 h exhibits spherical particles with relatively homogenous feature without apparent agglomeration, while the catalyst prepared at 30 h consisted of large aggregates. The SEM morphologies are consistent with the result depicted in Fig. 2, as the increase in particle size with reaction time led to a decrease in surface area and cyanide removal efficiency. The EDS analysis shown in inset of Fig. 4b reveals that the as-formed structure consists mainly of Ti, Si and O elements, confirming the high purity of final product with no impurities. The morphology of as-synthesized catalyst was further observed with TEM. The recorded image is shown in Fig. 5 with the selectedarea electron diffraction (SAED) pattern depicted in the inset. The TEM image clearly shows spherical nanoparticles with almost uniform diameter with a narrow range distribution. The average diameter was found to be around 8 nm. A composite microstructure for the catalysts is likely formed, with no definite morphology ordering, since the TiO2 is randomly distributed in SiO2 support. The SAED pattern shown in the inset exhibits the (1 0 1), (0 0 4), (2 0 0) and (1 0 5) directions confirming again the crystalline nature of the anatase phase of catalyst, in consistent with XRD result of Fig. 3. In order to understand the optical absorption property and the UV light utilization efficiency of as-prepared TiO2–SiO2 photocatalyst, the UV–vis absorption data in suspended aqueous solution were measured and the profile is shown in Fig. 6. The spectral data recorded showed a cut off at around 367 nm; where the absorbance value is minimum. According to the absorption edge, the optical band gap of the catalyst was calculated and found to be 3.37 eV, which is slightly larger than the reported value for bulk TiO2 anatase phase (3.2 eV) [32]. It is generally accepted that the values of band gap are influenced by the synthesis method, the existence of impurities doping the crystalline network, and the average crystal size of the semiconductor. Although the presence of SiO2 as a support led to widening the band gap, but it essentially enhanced the specific surface area of the mixed catalyst. Calcination is an effective treatment to optimize the photocatalytic activity of TiO2 which results from the improvement of crystallinity. The effect of calcination temperature and calcination time on phase formation, crystallite size, surface area and cyanide removal efficiency were investigated under different calcination
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Fig. 4. SEM images of TiO2–SiO2 catalyst prepared at; (a) 6 h and (b) 30 h. EDS analysis is depicted in the inset.
Fig. 5. TEM image of TiO2–SiO2 catalyst synthesized at 6 h. The SAED pattern is shown in the inset.
Fig. 6. UV–vis. absorption spectrum obtained for as-synthesized TiO2–SiO2 catalyst.
temperatures (400–700 °C) and different calcination times (1–5 h). The results are summarized in Table 1. As one can notice, increasing calcination temperature from 400 to 700 °C led to a decrease in surface area from 286.7 to 146.8 m2/g with an increase in
crystallite size from 6 to 12 nm and a decrease in cyanide removal efficiency from 80.3% to 61.2%. One could also observe that when calcination time increased from 1 to 3 h, the surface area increased from 222 to 286.7 m2/g and cyanide removal efficiency increased consequently from 60% to 80.3%. By increasing calcination time from 3 to 4 h, a decrease in surface area from 286.7 to 213 m2/ g occurred and also cyanide removal efficiency decreased from 80.3% to 58.8%. This is likely related to the sintering and growth of TiO2 crystallites resulting in the decrease of surface area. At calcination temperature of 400 °C for 3 h, TiO2 based catalyst showed the highest photocatalytic activity due to better crystallization, highest surface area and lowest crystallite size. [H+]/[Ti] ratio and solution pH are also key operating parameters during the catalyst synthesis using the present sol–gel technique. The effect of [H+]/[Ti] ratio and solution pH value on surface area and cyanide removal efficiency were investigated under the previously optimal conditions. Various ratios of [H+]/ [Ti] (0.5, 1, 1.5 and 2) were used to elucidate its influence on asformed catalyst. The results are collected in Table 2. When [H+]/ [Ti] ratio increased from 0.5 to 1.0, the surface area decreased from 286.7 to 264.5 m2/g, which led to a decrease in cyanide removal efficiency from 80.3% to 65.0%. When [H+]/[Ti] ratio increased to 1.5, the surface area and cyanide removal efficiency decreased again to 255.7 m2/g and 62.8%, respectively. Further increase of [H+]/[Ti] ratio to 2.0 led to a decrease in surface area from 255.7 to 251.9 m2/g and also a decrease in cyanide removal efficiency from 62.8% to 61.2%. This behavior may be attributed to poor condensation and high crystals growth rate which led finally to larger particles size. Based on these results, the optimum condition of [H+]/[Ti] ratio is taken at 0.5 at which 80.3% cyanide removal efficiency and 286.7 m2/g surface area could be achieved. The effect of variation of solution pH is also given in Table 2. When the pH value increased from 6 to 7, this led to an increase in surface area to 286.7 m2/g which in turn resulted in an increase in cyanide removal efficiency (80.3%). A drop in both surface area and cyanide removal efficiency was observed when the solution pH increased above 7. In general, when the pH value of the precipitation medium was low, the small quantity of hydroxyl groups would limit the hydrolysis of unhydrolyzed alkyls and Cl ions that remained in the precursors, and finally prevented the crystallization of the samples and the growth of the TiO2 crystallites. We also examined the effect of using different nitric acid concentrations on the catalyst synthesis. The finding revealed that when nitric acid concentration increased from 1.0 to 1.6 M, the cyanide removal efficiency increased from 74.2% to 80.3%, when the concentration increased to 2, the cyanide removal efficiency decreased from 80.3% to 77%. This may be attributed to poor con-
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F.A. Harraz et al. / Journal of Alloys and Compounds 551 (2013) 1–7 Table 1 Effect of calcination temperature and time on surface area, crystallite size and cyanide removal efficiency.
Table 3 Effect of catalyst weight, reaction time and cyanide ions concentration on cyanide removal efficiency.
Calcination temperature, (°C)/calcinations time, (h)
Surface area (m2/g)
Crystallite size (nm)
Cyanide removal efficiency (%)
Entry
Cyanide ions concentration (ppm)
Catalyst weight (mg)
Reaction time (min)
Cyanide removal efficiency (%)
400/3 500/3 600/3 700/3 400/1 400/2 400/4 400/5
286.7 199.1 179.7 146.8 222.0 235 213.0 207.0
6 9.2 14.7 12.4 9.2 7.5 9.5 11.6
80.3 70.0 67.0 61.2 60.0 62.0 58.8 56.5
1 2 3 4 5 6 7 8 9 10
100 100 100 100 100 100 100 75 50 25
100 200 300 400 500 500 500 500 500 500
60 60 60 60 60 120 180 180 180 180
65.7 72.5 80.3 83.7 88.2 90.7 93.3 94.6 96.1 99.4
Table 2 Effect of [H+]/[Ti] ratio and solution pH on surface area and cyanide removal efficiency. [H+]/[Ti] ratio/pH value
Surface area (m2/g)
Crystallite size (nm)
Cyanide removal efficiency (%)
0.5/7 1.0/7 1.5/7 2.0/7 0.5/6 0.5/8 0.5/9
286.7 264.5 255.7 251.9 268.4 176.26 233.0
6 8.5 8.9 9.9 8.2 12.5 14.4
80.3 65.0 62.8 61.2 74.0 60.7 51.0
taken for analysis after 3 h. The results reveal that decreasing CN content from 100 to 25 ppm led to 5% increase in cyanide removal efficiency (93.3% increased to 99.4%). Few control experiments were carried out to gain a better understanding of the photolytic and photocatalytic degradation process. Control experiments in the dark or in absence of catalyst under illumination failed to give a significant activity; 2% for cyanide removal was detected. This is clear proof that the photodegradation of cyanide is mainly catalyzed by TiO2–SiO2 mixed catalyst. The process of removal of CN ions over TiO2–SiO2 catalyst can be represented by the following equations [33]: þ
densation and high crystals growth which led to high particles size and hence lower removal efficiency. Therefore, the optimum condition of nitric acid concentration is taken at 1.6 M at which 80.3% cyanide removal efficiency could be achieved. Based on the above results, one can conclude that the relation between the physical properties of catalyst and its photocatalytic activities is complicated, and judging from the current experimental observations, the optimal conditions for catalyst synthesis could be determined only by taking into consideration several factors that also may vary from case to case. 3.2. Photocatalytic performance 3.2.1. Photodegradation of cyanide ions The catalytic activity of as-synthesized TiO2–SiO2 nanoparticles was firstly investigated using cyanide degradation as a model catalytic reaction. The catalytic reactions were conducted by suspending a certain weight of catalyst into the reactor containing 300 ml KCN (100 ppm CN). Determination of minimum concentration of catalyst necessary for complete degradation of cyanide is indispensable step in optimizing the catalytic reaction. Different weights of TiO2–SiO2 catalyst, mainly 100–500 mg in 100 ppm CN solutions were accordingly examined at room temperature for 60 min. The results of cyanide degradation % are summarized in Table 3. At smaller weight of catalyst 100 mg, only 65.7% removal of cyanide was obtained after 60 min, in comparison with 72.5% in case of using 200 mg catalyst for the same time. With a moderate catalyst weight of 300 mg, the photodegradation reaction was further enhanced affording 83.7% cyanide removal. The maximum removal efficiency of 88.2% was achieved using 500 mg catalyst. The effect of degradation time on cyanide removal efficiency was further investigated using the 500 mg catalyst in 100 ppm CN and a sample of the reaction mixture was taken for analysis at different reaction times (1–3 h). The results are collected in Table 3. The cyanide removal efficiency was found to increase from 88.2% to 93.3% when degradation time increased from 1 to 3 h. Different CN contents (100, 75, 50 and 25 ppm CN) were consequently utilized in 500 mg catalyst weight and a sample of the reaction mixture was
TiO2 þ 2hc ¼ TiO2 ð2h þ 2e Þ
ð3Þ
1=2O2 þ 2e þ H2 O ¼ 2OH
ð4Þ
þ
2OH þ 2h ¼ 2OH
ð5Þ
CN þ 2OH ¼ OCN þ H2 O
ð6Þ
2OCN þ O2 ¼ 2CO2 þ N2
ð7Þ
Overall reaction: TiO2 =H2 O
2CN þ 2O2 ! 2CO2 þ N2 UV light
ð8Þ
It is worth mentioning here that the CN could be converted to N2 and CO2, which are environmental friendly. The kinetics of cyanide photodegradation was further investigated using different loadings 100, 300, and 500 mg of TiO2–SiO2 catalyst in a higher concentration of cyanide (150 ppm) for different illumination times. The effect of UV illumination with no catalyst on the cyanide removal was also tested. The results of photodegradation are shown in Fig. 7. The loss of the cyanide ions could be fitted to the expression [34]:
log½ct ¼ kt þ log½c0
ð9Þ
where [c]0 and [c]t represent the concentration in ppm of cyanide ions at zero time and time t of illumination, respectively, and k represents the apparent rate constant (min1). Plotting ln(ct/c0) as a function of time (t) gives linear relation. This indicated that the photodegradation of cyanide ions using TiO2–SiO2 photocatalyst is a first order reaction. We obtained for each run the k (min1) constant from the slopes of the simulated straight lines. The rate constant k was found to increase with increasing the loading of TiO2–SiO2 catalyst. The values of k obtained at 100, 300, and 500 mg catalyst are respectively, 5.8 103, 9.8 103 and 13.3 103 min1. It should be noted that, negligible cyanide degradation was detected under UV illumination, in absence of TiO2–SiO2 photocatalyst. 3.2.2. Removal of heavy metals To extend the application of TiO2–SiO2 photocatalyst to the removal of heavy metals, we examined its catalytic performance for the removal of Cr, Co and Pb. The catalytic reaction was evaluated
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F.A. Harraz et al. / Journal of Alloys and Compounds 551 (2013) 1–7
Fig. 7. Reaction kinetics of cyanide photocatalytic degradation at different loadings of TiO2–SiO2 catalyst. The concentration of cyanide was 150 ppm.
using 500 mg catalyst in 300 ml of nitrate salts mixture (100 ppm of each metal ions (Cr(III), Co(II) and Pb(II)). The concentration of metal ions in solution after a certain reaction time was analyzed by inductive coupled plasma (ICP) analysis. The photoactivity of optimized sample for reduction of heavy metals can be identified
Table 4 Effect of degradation time and catalyst weight on heavy metals removal efficiency. Degradation time (h)/catalyst weight (mg)
Heavy metal removal efficiency (%) Cr
Co
Pb
1/500 2/500 3/500 3/700 3/1000
18.7 32.2 46 67.8 90.0
19.7 35.5 50.2 71.0 91.9
25.4 40.7 54.3 75.2 98.6
by changing illumination time and catalyst weight; therefore the best heavy metals removal efficiency can be determined. The effect of illumination time and catalyst weight on heavy metals removal efficiency is shown in Table 4. As a general trend, the removal efficiency was increased for all heavy metals as the contact time increases from 1 to 3 h. A maximum removal efficiency of 54.3% was obtained for Pb as a target metal ion after 3 h contact time. Longer exposure between heavy metal ions and the catalyst led to a higher diffusion rate toward the catalyst which in turn increases the adsorption capacity. We then examined the effect of using different catalyst weights (500, 700 and 1000 mg) on the removal efficiency keeping the reaction time constant at 3 h. The results shown in Table 4 indicated that as the catalyst weight was doubled from 500 to 1000 mg, the removal efficiency was increased consequently to 90.0%, 91.9% and 98.6% for Cr, Co and Pb, respectively. As the catalyst weight increased, there is a greater availability of the adsorbent for higher adsorption capacity and consequently the amount of metals adsorbed on the surface increased. The higher adsorption capacity of metals was in the order (Pb > Co > Cr). The adsorption capacity reached a maximum value at 1000 mg TiO2–SiO2 catalyst and the maximum removal % was 98.6% for Pb metal. It has been reported earlier that the TiO2–SiO2 catalyst has a high relatively adsorption capacity for such heavy metals [35]. Control experiments were performed without addition of TiO2– SiO2 catalyst in order to determine whether photochemical reactions could occur in absence of catalyst. No photolytic removal of the current heavy metals was detected after 3 h UV illumination under the present experimental conditions. This indicates the photocatalytic nature of the above results in presence of TiO2– SiO2 catalyst. Further, the adsorption of heavy metals on catalyst surface in the dark was analyzed after mixing for 3 h. The results indicated a sluggish dark adsorption (7%) on catalyst surface. The metal ions in aqueous solution may undergo salvation and hydrolysis according to the following expression [2,36].
M2þ þ nH2 O ¼ MðH2 OÞ2þ n
ð10Þ
þ þ MðH2 OÞ2þ n ¼ ½MðH2 OÞn1 ðOHÞ þ H
ð11Þ
or totally
M2þ þ nH2 O ¼ ½MðH2 OÞn1 ðOHÞþ þ Hþ
Fig. 8. Proposed photocatalytic reaction mechanism for water pollutant in a semiconductor TiO2 particle.
ð12Þ
F.A. Harraz et al. / Journal of Alloys and Compounds 551 (2013) 1–7
The relationship between the amount of substance adsorbed per unit mass of adsorbent at constant temperature and its concentration in the equilibrium solution is called the adsorption isotherm. Adsorption isotherm is often applied to describe the way the metal ions interact with the catalyst, which is being underway to elucidate reaction mechanism and the rate of photodegradation in this TiO2–SiO2 catalyzed removal of heavy metals. Based on the above finding, it can be concluded that 1000 mg of our TiO2–SiO2 catalyst is most appropriate for removal of 100 ppm of each metal ions in its nitrate solution during 3 h reaction time to obtain 90.0%, 91.9% and 98.6% photoactivity efficiency for Cr, Co and Pb, respectively. 3.2.3. Photocatalytic reaction mechanism The photocatalytic reaction mechanisms utilizing the semiconductor TiO2 have been intensively reported in the literatures [20,37–41]. When photon energy (hc) of larger than or equal to the band gap energy of TiO2 is illuminated onto the surface, usually 3.2 eV for anatase or 3.0 eV for rutile, electrons are excited from the valence band to the conduction band, creating electron–hole pairs (e–h+). Such charge carriers migrate to the surface, react with the pollutants adsorbed on the surface and decompose them via inducing a series of reduction–oxidation reactions. This photodecomposition process usually involves one or more radicals or intermediate species such as OH, O 2 , H2O2, or O2, which play crucial roles in the photocatalytic reaction mechanisms. A simplified, photo-induced formation mechanism of (e–h+) pair in a semiconductor TiO2 particle in the presence of water pollutant (P) is presented in Fig. 8. The valence band hole has oxidizing behavior, while the conduction band electron has the capability for reduction process. Oxidation of water or OH by the hole produces the hydroxyl radical (OH), an extremely powerful oxidant, which in turn could rapidly attack pollutants adsorbed on the surface or even in solution. Beside, the reduction of adsorbed O2 to O 2 is an important reaction of the conduction band electron. Both reactions help to prevent the (e–h+) recombination which results in an accumulation of oxygen radical species that can also participate in attacking water contaminants [14]. It is worthy to note that the catalytic activity of TiO2 photocatalyst illustrated in Fig. 8 is essentially dependent on (i) the light absorption properties, (ii) the rate of reduction–oxidation reactions, (iii) and the recombination rate of (e–h+). Constant surface density of adsorbents with large surface area often leads to faster photocatalytic reaction rates [9]. In this context, the higher photocatalytic activity is related to the larger specific surface area and higher crystallinity. 4. Conclusion TiO2–SiO2 photocatalyst was successively synthesized via rapid, efficient sol–gel approach using TiCl4 precursor instead of titanium alkoxides. The XRD and TEM analysis confirmed the presence of crystalline anatase phase with almost uniform diameter and a narrow range distribution of spherical nanoparticles. The as-synthesized catalyst exhibited a good photocatalytic activity for cyanide degradation and heavy metals (Pb(II), Cr(III), Co(II)) removal under mild conditions in aqueous solutions. The photocatalytic performance in both cases was markedly dependent on catalyst and target concentrations and reaction time. The present photocatalyzed
7
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