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Pt nanoparticles/TiO2 for photocatalytic degradation of phenols in wastewater M. A. Barakat
a b
a
c
a
, R. I. Al-Hutailah , E. Qayyum , J. Rashid & J. N. Kuhn
c
a
Department of Environmental Sciences, Faculty of Meteorology and Environment , King Abdulaziz University , Jeddah , Saudi Arabia b
Central Metallurgical Research and Development Institute , Helwan , 11421 , Cairo , Egypt
c
Department of Chemical & Biomedical Engineering , University of South Florida , Tampa , FL , USA Accepted author version posted online: 01 Jul 2013.Published online: 20 Aug 2013.
To cite this article: M. A. Barakat , R. I. Al-Hutailah , E. Qayyum , J. Rashid & J. N. Kuhn , Environmental Technology (2013): Pt nanoparticles/TiO2 for photocatalytic degradation of phenols in wastewater, Environmental Technology, DOI: 10.1080/09593330.2013.820796 To link to this article: http://dx.doi.org/10.1080/09593330.2013.820796
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.820796
Pt nanoparticles/TiO2 for photocatalytic degradation of phenols in wastewater M.A. Barakata,b∗ , R.I. Al-Hutailaha , E. Qayyumc , J. Rashida and J.N. Kuhnc a Department b Central
of Environmental Sciences, Faculty of Meteorology and Environment, King Abdulaziz University, Jeddah, Saudi Arabia; Metallurgical Research and Development Institute, Helwan 11421, Cairo, Egypt; c Department of Chemical & Biomedical Engineering, University of South Florida, Tampa, FL, USA
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(Received 23 October 2012; final version received 13 May 2013 ) Pt nanoparticles/TiO2 catalysts were prepared and evaluated for UV-photocatalytic degradation of phenol and 2-chlorophenol (2-CP) in synthetic wastewater solutions. The catalysts were synthesized by immobilizing colloidal Pt nanoparticles onto titanium dioxide (rutile TiO2 ). Analytical techniques, such as standard Brunauer–Emmett–Teller isotherms, X-ray diffraction, transmission electron microscope, were utillized for investigating the specific surface area, structure, and particle size distribution of the catalysts and its components. The photocatalytic activities of both phenol and 2-CP solutions were studied in a 1 L batch photoreactor independently, under 450 W UV irradiation. Samples were drawn at regular intervals and residual concentration of phenol and 2-CP in the samples was analysed using an UV-visible spectrophotometer. Parameters controlling the photocatalytic process, including catalyst concentration, solution pH, and initial phenol (2-CP) concentration, were investigated. The obtained results revealed that Pt/TiO2 showed higher photocatalytic degradation for both phenol and 2-CP pollutants in solution (as compared to the rutile TiO2 ). The degradation efficiencies of 87.7% and 100% were obtained for phenol and 2-CP, respectively, under optimized conditions (0.5 g/L catalyst with a pollutant concentration of 50 mg/L after irradiation time of 180 min). Keywords: wastewater; phenolic pollutants; photocatalytic degradation; Pt/TiO2
1. Introduction During the recent decades, photocatalytic applications involving semiconductors have received much attention to solve certain environmental problems.[1–4] Photocatalysis is a promising technique for the treatment of contaminated waters and ground waters, which has been widely studied in the recent years due to its ability to oxidize organic molecules completely without the accumulation of hazardous by-products. Among many candidates for photocatalysts, TiO2 is the most suitable semiconductor material for industrial applications; recognized for its high efficiency, low cost, high physical and chemical stability, widespread availability, and non-corrosive property.[5,6] Upon the absorption of light energy that is equal to or greater than band-gap energy, the electrons in the valence band of the semiconductors such as titanium dioxide (TiO2 ) can be excited to the conduction band, forming a positive hole (h+ ) in valence band.[7,8] The positive hole is a strong oxidant, which can oxidize a compound directly or react with the electron donors in the environment such as water or hydroxide ions to form free hydroxyls radicals (OH· ) that are also potent oxidants. The photocatalytic activity of TiO2 is due to its wide band gap and long lifetime of photo-generated
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
holes and electrons compared to other semi conductors, but high degree of recombination of photo-generated electrons and holes is a major limiting factor in controlling its photocatalytic efficiency and impedes the practical application of these techniques in the degradation of contaminants in water and air. Thus, a major challenge in heterogeneous photocatalysis is the need to increase charge separation efficiency of the photocatalyst.[9] Coupled semiconductor photocatalysts exhibit a very high photocatalytic activity for both gas- and liquid-phase reactions by increasing the charge separation and extending photo-excitation energy range. Recently, many researchers had shown a lot of interest in coupling two semiconductor particles with different band-gap widths. Research groups have carried out photocatalytic activity experiments using various coupled semiconductor particle systems such as TiO2 –CdS,[10] TiO2 –WO3 ,[11] TiO2 – SnO2 ,[12] TiO2 –MoO3 ,[13] TiO2 – Fe2 O3 .[14] Research on the photocatalytic activity of TiO2 – ZnO coupled oxides was also carried out.[15] Surface modification by doping with metal ions and organic polymers has been proven to be an efficient route in improving the photocatalytic activity of TiO2 . Special attention has been focused on doping TiO2 -based materials with noble
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elements.[16–21] Study revealed that Ag dopant accelerates the transformation of anatase TiO2 to its rutile form, and relatively low Ag concentration (2–5%), results in increase in specific surface areas of the TiO2 powders.[22] The presence of Ag in crystalline TiO2 was found to strongly enhance the photocatalytic activity of TiO2 in various degradation processes.[23–25] In recent studies deposition of Pt on TiO2 has showed marked improvements in photocatalysed destruction of organic pollutants.[26–28] This is due to that Pt may act as a holes scavenger that decreases the e· /h+ recombination process.[28] Phenol and chlorophenols are widely used in the production of wood preservers, pesticides, and biocides, and constitute an important class of recalcitrant pollutants.[29] They are also found in the wastewater of petrochemical industries, plastics industry, and can be found in pulp and insulation materials.[30,31] Wastewater emanating from oil refineries is often contaminated with aliphatic and aromatic petroleum hydrocarbons and organochlorinated compounds [32,33] and the current conventional processes for treatment of such effluents have caused environmental concerns and resulted in accumulation of hazardous sludge.[34,35] These compounds cause serious environmental problems, due to their high toxicity, recalcitrance, bioaccumulation, strong odour emission, persistence in the environment and suspected carcinogenity, and mutagenity.[36] Phenols concentration in a groundwater sample from a contaminated aquifer was reported as high as 25–55 mg L−1 . In wastewater, the concentration can reach higher than 200 mg L−1 .[37] These findings illustrate the seriousness of the phenolic pollutants and the importance of finding an effective method for treating such hazardous wastes. The objective of this research work is to evaluate Pt nanoparticles/TiO2 sample for the UV-photocatalytic degradation of some phenolic pollutants (phenol and 2-chlorophenol (2-CP)) in synthetic wastewater.
2.
Experimental
2.1. Materials Titanium dioxide (rutile TiO2 ) from Alfa-Aesar was used as the photocatalyst support. Chloroplatinic acid (H2 PtCl6 6H2 O, 99.9% pure; Sigma-Aldrich) was used as a precursor for Pt. Standard grades of phenol and 2-CP solutions (Merck) were utilized as pollutants in synthetic wastewater for the photocatalytic degradation experiments. Compressed gases, used for Brunauer–Emmett–Teller (BET) analysis, were UHP from Airgas. All other chemicals utilized were reagent-grade. A 450 W water-cooled highpressure mercury lamp (Hanovia 608A36, ACE Glass, NJ, USA), with a spectral irradiance of 260 W/m2 with spectral ranges from 228 to 420 nm at a distance of 1 m from the light source, according to information provided by the manufacturer, was used as an irradiation source.
2.2.
Synthesis and characterization
2.2.1. Nanoparticle synthesis and characterization Pt nanoparticles capped by polyvinlypyrrolidone (PVP) were prepared by colloidal routes established in the literature.[38–41] First, chloroplatinic acid (H2 PtCl6 6H2 O) was dissolved into DI water to achieve a solution of 20 mL and 6.0 mM. This solution was then diluted with 180 mL of methanol. PVP (molecular weight of 40,000, 133 mg) was added to this mixture. The combined solution was heated to 110◦ C and allowed to reflux for 3 h to achieve the metal nanoparticles. After cooling, the suspension was triple washed (cycles of hexane and ethanol with intermediate centrifugation) to remove excess PVP. The cleaned particles were re-dispersed in an ethanol suspension. The size of particles was measured by transmission electron microscopy (TEM). TEM images were acquired at acceleration voltages of 60 kV using an FEI Morgagni 268D microscope. For each synthesis, the average particle size and a particle size histogram were measured by counting the particle size of 100 particles. 2.2.2. Supported catalyst synthesis and characterization Samples of Pt/TiO2 catalysts were synthesized by immobilizing Pt nanoparticles onto titanium dioxide (rutile TiO2 from Alfa-Aesar). The particles were supported by mild sonication for 3 h at a metal:titanium dioxide mass ratio of 0.3%:99.7%. Following sonication, the composite materials were dried with mild heating on a hot plate (T ∼ 60◦ C) and then in a drying over (T ∼ 90◦ C). A standard BET analysis was performed to measure the specific surface area from the nitrogen adsorption isotherm using an Autosorb IQ (Quantachrome Instruments). Catalysts were degassed at 200◦ C prior to analysis at a temperature of 77 K. X-ray diffraction (XRD) was performed by a Philips PANalytical X-Pert Pro x-ray diffractometer with a Cu Kα x-ray source.
2.3. Catalytic activity The activity of the synthesized catalyst samples, Pt nanoparticles/TiO2 , was evaluated by UV-photodegradation experiments. A batch-scale reactor consisting of a cylindrical 1 L capacity pyrex-glass cell was used for the photocatalytic experiments. A 450-W mercury lamp as a UV source was placed vertically in a quartz tube fitted with a water cooling jacket to maintain a steady temperature of 25◦ C, and was then inserted into the photoreactor. To maintain aerobic conditions, compressed air was purged into the reactor by means of a bubbler at a constant air flow. The solution contents were kept uniformly distributed by using a magnetic stirrer and the solution pH values were adjusted when needed by using 1 M HCl and NaOH. All the experiments were conducted under 450 W of UV irradiation for a period of 180 min (after keeping in dark for 30 min in each run to reach equilibrium state). A 20-mL
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Figure 2.
3
XRD pattern for Pt nanoparticles/TiO2 catalyst.
Table 1. Porosimetry of Pt nanoparticles/TiO2 as determined by nitrogen physisorption. BET surface area (m2 /g) 2.7
Figure 1. TEM image and particle size distribution of synthesized and washed Pt nanoparticles. (a) TEM image. (b) Particle size distribution.
sample was drawn every 30 min and filtered for analysis through 0.45 μm nylon membrane syringe filters (Gelman Acrodic, Michigan) and analysed for the initial and residual concentrations of both phenol and 2-CP by UV-visible spectrophotometer (UV mini 1240 – Shimadzu) at 271 nm. 3. Results and discussions 3.1. Structure of Pt nanoparticles During the syntheses of the Pt nanoparticles, the precursor solution turned black indicating the formation of metal nanoparticles. A representative TEM image and particles size distribution of the synthesized and washed Pt nanoparticles is presented in Figure 1(a) and 1(b). From this image, the average particle size and a distribution were determined to be 2.84 ± 0.61 nm by measuring the size of 100 particles and statistically analysing the resulting histograms. This particle size and deviation agreed with the literature results [38–41] for this synthesis approach. 3.2.
Characterization of Pt/TiO2 nanoparticles Following incorporation of the synthesized and washed Pt nanoparticles onto TiO2 , the resulting structure was
Pore diameter (nm)
Pore volume (cm3 /g)
2.8
8.1 × 10−3
characterized by XRD and nitrogen physisorption. XRD (Figure 2) showed the rutile structure as indicated by the Miller indices. Due to the low loading and the small particle size of the Pt nanoparticles, the Pt phase is not observed by this technique. Based on nitrogen physisorption data (Table 1), the doping of Pt onto TiO2 did not impact the porosity or the surface area. That is, similar results were obtained for the titania substrate as for the Pt/titania sample.
3.3.
Photocatalytic activity It is known that the photocatalytic activity of TiO2 anatase for organic degradation in water is considered to be much more active than rutile. However, regarding the difference in their sorption capacities towards O2 in water, which might be critical to the activity determination, Pt has been used as an electron scavenger.[28] Evidence clearly shows that with the same amount of electron scavenger on the catalyst surfaces, anatase and rutile actually have a similar photocatalytic activity for organic degradation in water.[42] Moreover, from economical point of view, rutile is much cheaper than anatase. Accordingly, the photocatalytic degradation of both phenol and 2-CP solutions over the rutile sample was evaluated with the Pt supported rutile titania samples). All photo experimental runs were performed in dark with the catalyst samples for 30 min before UV illumination to reach adsorption equilibrium state. The solution pH is an important factor that affects the photocatalytic reaction by controlling the adsorption of pollutants on the photocatalyst surface.
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Figure 3(a) and 3(b) shows the effect of change in solution pH with time on the photocatalytic degradation of both phenol and 2-CP solutions, respectively, over Pt/TiO2 with UV light. It is clear that the efficiency of both phenol and 2CP degradation was inversely proportional to the pH values, optimum pH value was 3. However, the efficiency values and rate of photo degradation of phenol were lower than that of 2-CP for all pH values. The degradation efficiency of phenol was gradually increased with time up to 180 min for a wide range of pH values. The maximum percentage degradation values of phenol were found to be 87.7%, 38.14%, 23.78%, and 10.4% with solution pH values of 3, 5, 7, and 9, respectively. On another hand, the degradation efficiency of 2-CP increased rapidly from the beginning. The optimum pH value was 3 with 2-CP complete degradation efficiency after 180 min. The efficiency values of 2-CP degradation were 93.8, 81.6, and 65.5 with solution pH of 5, 7, and 9, respectively. In a previous work,[43] the photocatalytic degradation of both phenol and 2-CP solutions over the TiO2 rutile sample was evaluated (for comparison with the Pt/TiO2 samples). The efficiency of photo degradation was found to be very low with phenol, while it
was comparatively significant with 2-CP for all pH values. However, the degradation of both phenol and 2-CP in solutions was enhanced in the acid medium rather than neutral or alkali medium, the optimum pH value was 3. The maximum percentage degradation values for both phenol and 2-CP were 34.4% and 66.5%, respectively, after 180 min at solution pH 3. The lowering of the degradation percentage with increasing pH can be described by the amphoteric nature of TiO2 in aqueous solution. The zero point charge (pHpzc) of TiO2 is 6.8, below which TiO2 surface would be positively charged while above this pH, it is negatively charged. At pH above 6.8, phenol occurs as negatively charged phenolate ions which experience repulsion from the TiO2 surface. Also the higher concentration of OH− ions hinders the penetration of UV light to the catalyst surface.[43] Moreover, higher pH favours the carbonate ions formation which are very effective scavengers of OH− ions and may reduce the degradation rate.[44,45] Figure 4(a) shows the effect of initial phenols concentration with time on its photocatalytic degradation over Pt/TiO2 in the range of 50–200 ppm. As mentioned above, the degradation efficiency of phenol was gradually increased with time up to 180 min for the studied range
Figure 3. (a) Effect of pH with time on the photocatalytic degradation of phenol over Pt/TiO2 catalyst. Phenol concentration = 50 ppm, Pt/TiO2 dose = 1 g/L. (b) Effect of pH with time photocatalytic degradation of 2-CP over Pt/TiO2 . 2-CP concentration = 50 ppm, Pt/TiO2 dose = 1 g/L.
Figure 4. (a) Effect of initial concentration of phenol with time on the photocatalytic degradation of phenol over Pt/TiO2 . pH = 3, Pt/TiO2 dose = 1 g/L. (b) Effect of initial concentration of 2-CP with time on its photocatalytic degradation over Pt/TiO2 . pH = 3, Pt/TiO2 dose = 1 g/L.
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Environmental Technology of phenolic concentration. On another hand, the degradation efficiency was inversely proportional to the initial phenolic concentration. The initial phenolic concentration value was 50 ppm with 87.7% degradation after 180 min, while the efficiency was drastically decreased to 71.4 ppm and 7.5% with increase in the phenolic concentration to 100 and 200 ppm, respectively. This might be related to the insufficient catalytic sites for phenol direct oxidation. Figure 4(b) illustrates the effect of 2-CP initial concentration with time on its photocatalytic degradation over Pt/TiO2 with UV light. It can be seen that the 2-CP degradation rate and efficiency were higher than that of phenol for a wide range 2-CP concentration values. With a 2-CP concentration value of 50 ppm, a complete degradation was achieved after 180 min. The efficiency values were 84.2% and 55.8% with initial 2-CP concentrations of 100 and 200 ppm, respectively, within 180 min. The decline in the degradation rate of phenols (phenol and 2CP) at higher concentrations can be due to more light being absorbed by the phenolate species than that by TiO2 which is not very effective for carrying out photocatalytic degradation. Furthermore, at the equilibrium adsorption the catalyst active sites are loaded with phenolic species; therefore, competitive adsorption of OH− on the same sites is less likely resulting in decrease in the amount of OH− and O− 2 on the catalyst surface.[45] The effect of the Pt/TiO2 catalyst concentration on photocatalytic degradation of 2-CP and phenol was investigated (Figure 5(a) and 5(b)). Different Pt/TiO2 dosings ranging from 0.25 to 2 g/L were used. In Figure 5(a), it is clear that a high degradation rate of phenol was achieved after 180 min with a catalyst dose of 0.5 or 1 g/L with degradation efficiencies of 87.7 and 86.7%, respectively. However, increasing the catalyst dose to 2 g/L resulted in decreasing the efficiency value to 66.1%. In Figure 5(b), the degradation rate of 2-CP increased rapidly at early time for all dosages of Pt/TiO2 . A complete degradation of phenol was achieved after 180 min with catalyst dosages of 0.5 or 1 g/L. However, increasing the catalyst dose to 2 g/L resulted in decreasing the catalytic efficiency to 93.9%. The inverse effect of the catalyst concentration on the degradation efficiency can be due to the catalyst aggregation/agglomeration that can result in lowering the total number of active sites on the surface.[45,46] Also the excessive opacity imparted by TiO2 particles acts as a shield against incident light and consequently makes hindrance in light penetration. Lowering of active surface area for harvesting light induces resulted in a decrease in the photocatalytic activity.[47,48] Figure 6 summarizes the efficiency of the photodegradation processes of phenol and 2-CP over both Pt/TiO2 and TiO2 rutile supports. The photodegradation processes were optimized by using 0.5 g/L catalyst with pollutant concentration of 50 mg/L within irradiation time of 180 min at different solution pH values for all the samples. The highest degradation efficiencies for both phenol and 2-CP were
5
Figure 5. (a) Effect of Pt/TiO2 dosing with time on the photocatalytic degradation of phenol. pH = 3, phenol concentration = 50 ppm. (b) Effect of Pt/TiO2 dosing with time on the photocatalytic degradation of 2-CP. pH = 3, 2-CP concentration = 50 ppm.
achieved at solution pH3 with the two catalyst samples. An increase in the pH values above 6.8 resulted in a gradual decrease in the degradation rate. However, a very significant increase in the phenol and 2-CP degradation efficiency was achieved with the Pt/TiO2 catalyst compared with the TiO2 rutile (from 34.4% to 87.7% for phenol, and from 94.5% to 100% for 2-CP), at solution pH3 after 180 min. This phenomenon can be explained as follows: the most promising method to increase the photocatalytic activity of TiO2 catalyst is surface modification, this can be achieved by metal immobilization or doping into the catalyst. Pt can act as good electrons scavenger from the conduction band of the semiconductor. Thus, it can hinder the recombination of photo-generated electrons and holes by increasing the charge separation.[49,50] On the other hand, noble metal incorporation into TiO2 dielectric provides an absorption feature due to the surface plasmon resonance (SPR) occurring over the visible range of the spectrum. In particular, Ag, Pt, and Au metals are the most popular materials due to the strong SPR characteristic.[51] Table 2 shows a comparison with results of both phenol and 2-CP photodegradation from relevant works (Pt/TiO2 = 0.5 g/L, pH = 3, phenol/2-CP concentration = 50 ppm, time = 180 min). The results of the current work are very comparative with that of the bimetallic Ag-Pt/TiO2 nanoparticles particularly with the degradation of 2-CP, at the same experimental conditions.
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Figure 6. Efficiency of the photodegradation of both phenol and 2-CP over Pt/TiO2 and rutile TiO2 supports. Phenol (2-CP) concentration = 50 ppm, catalyst dose = 1 g/L, time = 180 min.
as measured through TEM analysis. The catalytic activity of the samples was evaluated by UV-photocatalytic degradation of 2-CP and phenol in synthetic wastewater solution. Pt/TiO2 showed a higher activity for UVphotocatalytic degradation of both phenol and 2-CP pollutants in solution (as compared with the TiO2 rutile). The photodegradation processes were optimized by using 0.5 g/L catalyst with a pollutant concentration of 50 mg/L and a solution pH value of 3 after irradiation time of 180 min. The degradation efficiency values were 87.7% and 100% for both of phenol and 2-Cp, respectively.
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Acknowledgements Table 2. A comparison with results of both phenol and 2-CP photodegradation from relevant works (Pt/TiO2 = 0.5 g/L, pH = 3, phenol/2-CP concentration = 50 ppm, time = 180 min). Catalyst Plain TiO2 (rutile) Ag-Pt/TiO2 (rutile) Pt/TiO2 (rutile)
Phenol degradation 2-CP degradation efficiency (wt%) efficiency (wt%) Reference 34.4
66.5
99.8
100
87.7
100
The authors gratefully acknowledge King Abdulaziz University (KAU, Saudi Arabia) for funding this work through the cooperation agreement with University of South Florida, USA (KAU-USF).
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References
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[1] Fujishima A, Rao TN, Tryk DA. Titanium dioxide photocatalysis. J Photochem Photobiol C: Photochem Rev. 2000;1:1–21. [2] Barakat MA, Chen YT, Huang CP. Removal of toxic cyanide and Cu (II) ions from water by illuminated TiO2 catalyst. Appl Catal B: Environ. 2004;53:13–20. [3] Gaya UI, Abdullah AH. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems. J Photochem Photobiol C: Photochem Rev. 2008;9:1–12. [4] Malato S, Fernández-Ibáñez EzP, Maldonado MI, Blanco J, Gernjak W. Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal Today. 2009;147:1–59. [5] Paola AD, Cufalo G, Addamo M, Bellardita M, Compostrini R, Ischia M, Ceccato R, Palmisano L. Photocatalytic activity of nanocrystalline TiO2 (brookite, rutile and brookite-based) powders prepared by thermo-hydrolysis of TiCl4 in aqueous chloride solutions. Colloids Surf A: Physicochem Eng Aspects. 2008;317:366–376. [6] Barakat MA. Adsorption behavior of copper and cyanide ions at TiO2 – solution interface. J Colloid Interface Sci. 2005;291:345–352. [7] Hermann JM. Heterogenous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catal Today. 1999;53:115–129. [8] Hu C, Wang YZ, Tang HX. Destruction of phenol aqueous solution by photocatalysis or direct photolysis. Chemosphere. 2000;41:1205–1209. [9] Braun IE, Pelizzetti MS, editors. Photochemical conversion and storage of solar energy. Dordrecht: Kluwer; 1991. [10] Gao P, Liu J, Zhang T, Sun DD, Ng W. Hierarchical TiO2 /CdS ‘spindle-like’ composite with high photodegradation and antibacterial capability under visible light irradiation. J Hazard Mater 2012;229–230: 209–216. [11] Lin CF, Wu CH, Onn ZN. Degradation of 4-chlorophenol in TiO2 , WO3 , SnO2 , TiO2 /WO3 and TiO2 /SnO2 systems. J Hazard Mater. 2008;154:1033–1039. [12] Song KY, Park MK, Kwon YT, Lee HW, Chung WJ, Lee WI. Preparation of Transparent particulate MoO3 /TiO2 and WO3 /TiO2 film and their photocatalytic properties. Chem Mater. 2001;13:2349–2355.
Thiswork
Figure 7. Reproducibility of the photodegradation of both phenol and 2-CP over Pt/TiO2 .
The reproducibility of the photodegradation of both phenol and 2-CP over Pt/TiO2 at the optimum conditions is shown in Figure 7. The experiments were performed for four runs. It can be seen that the photodegradation efficiency values slowly decreased with the runs. The efficiency values decreased from 87.1% and 100% in the first run to 72.1% and 82.5%, for both phenol and 2-Cp, respectively. This can be attributed to the fractional losses of some of the catalysts during filtration process. 4. Conclusions Pt nanoparticles/TiO2 catalysts were synthesized by immobilizing Pt nanoparticles onto titanium dioxide (rutile TiO2 ). The average particle size of the catalyst was 2.84 ± 0.6
Downloaded by [University of Birmingham] at 05:28 21 September 2013
Environmental Technology [13] Ma BJ, Kim JS, Choi CH, Woo SI. Enhanced hydrogen generation from methanol aqueous solutions over Pt/MoO3 /TiO2 under ultraviolet light. Int J Hydrog Energy. 2013;38:3582–3587. [14] Akhavan O, Azimirad R. Photocatalytic property of Fe2 O3 nanograin chains coated by TiO2 nanolayer in visible light irradiation. Appl Catal A: Gen. 2009;369:77–82. [15] Aal AA, Barakat MA, Mohamed RM. Electrophoreted Zn–TiO2 –ZnO nanocomposite coating films for photocatalytic degradation of 2-chlorophenol. Appl Surf Sci. 2008;254:4577–4583. [16] Elsalamony RA, Mahmoud SA. Preparation of nanostructured ruthenium doped titania for the photocatalytic degradation of 2-chlorophenol under visible light. Arab J Chem. 2012;http://dx.doi.org/10.1016/j.arabjc.2012.06.008 [17] Korologos CA, Nikolaki MD, Zerva CN, Philippopoulos CJ, Poulopoulos SG. Photocatalytic oxidation of benzene, toluene, ethylbenzene and m-xylene in the gas-phase over TiO2 -based catalysts. J Photochem Photobiol A: Chem. 2012;244:24–31. [18] Fan JW, Liu XH, Zhang J. The synthesis of TiO2 and TiO2 – Pt and their application in the removal of Cr (VI). Environ Technol. 2011;32:427–437. [19] Kuo YL, Su TL, Chuang KJ, Chen HW, Kung FC. Preparation of platinum- and silver-incorporated TiO2 coatings in thin-film photoreactor for the photocatalytic decomposition of o-cresol. Environ Technol. 2011;32:1799–1806. [20] Shokri M, Jodat A, Modirshahla N, Behnajady MA. Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles. Environ Technol. 2013;34:1161–1166. [21] Suri RPS, Thornton HM, Muruganandham M. Disinfection of water using Pt- and Ag-doped TiO2 photocatalysts. Environ Technol. 2012;33:1651–1659. [22] Lin X, Rong F, Fu D, Yuan C. Enhanced photocatalytic activity of fluorine doped TiO2 by loaded with Ag for degradation of organic pollutants. Powder Technol. 2012;219: 173–178. [23] Cao Y, Tan H, Shi T, Tang T, Li J. Preparation of Ag-doped TiO2 nanoparticles for photocatalytic degradation of acetamiprid in water. J Chem Technol Biotechnol. 2008;83:546–552. [24] Behnajady MM, Modirshahla N, Rad B. Enhancement of photocatalytic activity of TiO2 nanoparticles by silver doping: photodeposition versus liquid impregnation methods. Global NEST J. 2008;10:1–7. [25] Barakat AM, Kanjwal MA, Al-Deyab SS, Chronakis IS, Kim HY. Influences of silver-doping on the crystal structure, morphology and photocatalytic activity of TiO2 nanofibers. Mater Sci Appl. 2011;2:1188–1193. [26] Feng C, Wang Y, Zhang J, Yu L, Li D, Yang J, Zhang Z. The effect of infrared light on visible light photocatalytic activity: an intensive contrast between Pt-doped TiO2 and N-doped TiO2 . Appl Catal B: Environ. 2012;113–114: 61–71. [27] Neppolian B, Bruno A, Bianchi CL, Kumar MA. Graphene oxide based Pt–TiO2 photocatalyst: ultrasound assisted synthesis, characterization and catalytic efficiency. Ultrason Sonochem. 2012;19:9–15. [28] El-Bahy ZM, Ismail AA, Mohamed RM. Enhancement of titania by doping rare earth for photodegradation of organic dye (Direct Blue). J Hazard Mater. 2009;166: 138–143. [29] Xiangchun Q, Hanchang S, Jainlong W, Yi Q. Biodegradation of 2,4-dichlorophenol in sequencing batch reactors augmented with immobilized mixed culture. Chemosphere. 2003;50:1069–1074.
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[30] Heidi MR, Chien MW, Tao Y, Jun-Kyoung K, William DM. Catalytic hyrodechlorination of chlorophenols in aqueous solution under mild conditions. Appl Catal A: Gen. 2004;271:137–143. [31] Liu D, Maguire RJ, Pacepavicius G, Dutka BJ. Biodegradation of recalcitrant chlorophenols by cometabolism. Environ Toxicol Water Qual. 1991;6:85–95. [32] Han DH, Cha SY, Yang HY. Improvement of oxidative decomposition of aqueous phenol by microwave irradiation in UV/H2 O2 process and kinetic study. Water Res. 2004;38:2782–2790. [33] Alinsafi A, Evenou F, Abdulkarim EM, Zahraa MNP, Benhammou A, Yaacoubi A, Nejmeddine A. Treatment of textile industry wastewater by supported photo catalysis. Dyes Pigments. 2007;74:439–445. [34] Tiburtius ERL, Peralta-Zamora P, Emmel A. Treatment of gasolinecontaminated waters by advanced oxidation processes. J Hazard Mater. 2005;126:86–90. [35] Cho L-H, Kim Y-G, Yang J-K, Lee N-H, Lee S-M. Solar-chemical treatment of groundwater contaminated with petroleum at gas station sites: ex situ remediation using solar/TiO2 photocatalysis and solar photo-Fenton. J Environ Sci Health A. 2006;41:457–473. [36] Contreras S, Rodriguez M, Al Momani F, Sans C, Esplugas S. Contribution of the ozonation pretreatment to the biodegradation of aqueous solutions of 2,4 dichlorophenol. Water Res. 2003;37:3164–3171. [37] Wang Y, Ren J, Deng K, Gui L, Tang Y. Preparation of tractable platinum, rhodium, and ruthenium nanoclusters with small particle size in organic media. Chem Mater. 2000;12:1622–1627. [38] Rioux RM, Song H, Hoefelmeyer JD, Yang P, Somorjai GA. High-surface-area catalyst design: synthesis, characterization, and reaction studies of platinum nanoparticles in mesoporous SBA-15 silica. J Phys Chem B. 2005;109: 2192–2202. [39] Song H, Rioux RM, Hoefelmeyer JD, Komor R, Niesz K, Grass M, Yang P, Somorjai GA. Hydrothermal growth of mesporous SBA-15 silica in the presence of PVP-stabilized Pt nanoparticles: synthesis, characterization, and catalytic properties. J Am Chem Soc. 2006;128:3027–3037. [40] Kuhn JN, Huang W, Tsung C-K, Zhang Y, Somorjai GA. Structure sensitivity of carbon-nitrogen ring opening: impact of platinum particle size from below 1 to 5 nm upon pyrrole hydrogenation product selectivity over monodisperse platinum nanoparticles loaded onto mesoporous silica. J Am Chem Soc. 2008;130:14026–14027. [41] Sun Q, Xu Y. Evaluating intrinsic photocatalytic activities of anatase and rutile TiO2 for organic degradation in water. J Phys Chem C. 2010;114(44):18911–18918. [42] Barakat MA, Al-Hutailah RI, Hashim MH, Qayyum E, Kuhn JN. Titania-supported silver-based bimetallic nanoparticles as photocatalysts. Environ Sci Pollut Res. 2013;20: 3751–3759. [43] Qamar M, Muneer M, Bahnemann D. Heterogeneous photocatalysed degradation of two selected pesticide derivatives, triclopyr and daminozid in aqueous suspensions of titanium dioxide. J Environ Manage. 2006;80: 99–106. [44] Akbal F, Onar AN. Photocatalytic degradation of phenol. Environ Monit Assess. 2003;83:295–302. [45] Naeem K, Feng O. Parameters effect on heterogeneous photocatalysed degradation of phenol in aqueous dispersion of TiO2 . J Environ Sci. 2009;21:527–533. [46] Chen SF, Liu YZ. Study on the photocatalytic degradation of glyphosate by TiO2 photocatalyst. Chemosphere. 2007;67(5):1010–1017.
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Downloaded by [University of Birmingham] at 05:28 21 September 2013
[47] Barakat MA, Schaeffer H, Hayes G, Shah SI. Photocatalytic degradation of 2-chlorophenol by Co-doped TiO2 nanoparticles. Appl Catal B: Environ. 2004;57:23–30. [48] Barakat MA, Tseng JM, Huang CP. Hydrogen peroxideassisted photocatalytic oxidation of phenolic compounds. Appl Catal B: Environ. 2005;59:99–104. [49] Wang H, Wu Z, Liu Y, Wang W. Influences of various Pt dopants over surface platinized TiO2 on the photocatalytic oxidation of nitric oxide. Chemosphere. 2008;74:773–778.
[50] Barakat NAM, Kanjwal MA, Al-Deyab SS, Chronakis IS, Kim HY. Influences of silver-doping on the crystal structure, morphology and photocatalytic activity of TiO2 nanofibers. Mater Sci Appl. 2011;2:1188–1193. [51] Barakat NAM, Woo KD, Kanjwal MA, Choi KE, Khil MS, Kim HY. Surface plasmon resonances, optical properties and electrical conductivity thermal hystersis of silver nanofibers produced by electrospinning Ttechnique. Langmuir. 2008;24:11982–11987.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Pt nanoparticles/TiO2 for photocatalytic degradation of phenols in wastewater M. A. Barakat
a b
a
c
a
, R. I. Al-Hutailah , E. Qayyum , J. Rashid & J. N. Kuhn
c
a
Department of Environmental Sciences, Faculty of Meteorology and Environment , King Abdulaziz University , Jeddah , Saudi Arabia b
Central Metallurgical Research and Development Institute , Helwan , 11421 , Cairo , Egypt
c
Department of Chemical & Biomedical Engineering , University of South Florida , Tampa , FL , USA Accepted author version posted online: 01 Jul 2013.Published online: 20 Aug 2013.
To cite this article: M. A. Barakat , R. I. Al-Hutailah , E. Qayyum , J. Rashid & J. N. Kuhn , Environmental Technology (2013): Pt nanoparticles/TiO2 for photocatalytic degradation of phenols in wastewater, Environmental Technology, DOI: 10.1080/09593330.2013.820796 To link to this article: http://dx.doi.org/10.1080/09593330.2013.820796
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.820796
Pt nanoparticles/TiO2 for photocatalytic degradation of phenols in wastewater M.A. Barakata,b∗ , R.I. Al-Hutailaha , E. Qayyumc , J. Rashida and J.N. Kuhnc a Department b Central
of Environmental Sciences, Faculty of Meteorology and Environment, King Abdulaziz University, Jeddah, Saudi Arabia; Metallurgical Research and Development Institute, Helwan 11421, Cairo, Egypt; c Department of Chemical & Biomedical Engineering, University of South Florida, Tampa, FL, USA
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(Received 23 October 2012; final version received 13 May 2013 ) Pt nanoparticles/TiO2 catalysts were prepared and evaluated for UV-photocatalytic degradation of phenol and 2-chlorophenol (2-CP) in synthetic wastewater solutions. The catalysts were synthesized by immobilizing colloidal Pt nanoparticles onto titanium dioxide (rutile TiO2 ). Analytical techniques, such as standard Brunauer–Emmett–Teller isotherms, X-ray diffraction, transmission electron microscope, were utillized for investigating the specific surface area, structure, and particle size distribution of the catalysts and its components. The photocatalytic activities of both phenol and 2-CP solutions were studied in a 1 L batch photoreactor independently, under 450 W UV irradiation. Samples were drawn at regular intervals and residual concentration of phenol and 2-CP in the samples was analysed using an UV-visible spectrophotometer. Parameters controlling the photocatalytic process, including catalyst concentration, solution pH, and initial phenol (2-CP) concentration, were investigated. The obtained results revealed that Pt/TiO2 showed higher photocatalytic degradation for both phenol and 2-CP pollutants in solution (as compared to the rutile TiO2 ). The degradation efficiencies of 87.7% and 100% were obtained for phenol and 2-CP, respectively, under optimized conditions (0.5 g/L catalyst with a pollutant concentration of 50 mg/L after irradiation time of 180 min). Keywords: wastewater; phenolic pollutants; photocatalytic degradation; Pt/TiO2
1. Introduction During the recent decades, photocatalytic applications involving semiconductors have received much attention to solve certain environmental problems.[1–4] Photocatalysis is a promising technique for the treatment of contaminated waters and ground waters, which has been widely studied in the recent years due to its ability to oxidize organic molecules completely without the accumulation of hazardous by-products. Among many candidates for photocatalysts, TiO2 is the most suitable semiconductor material for industrial applications; recognized for its high efficiency, low cost, high physical and chemical stability, widespread availability, and non-corrosive property.[5,6] Upon the absorption of light energy that is equal to or greater than band-gap energy, the electrons in the valence band of the semiconductors such as titanium dioxide (TiO2 ) can be excited to the conduction band, forming a positive hole (h+ ) in valence band.[7,8] The positive hole is a strong oxidant, which can oxidize a compound directly or react with the electron donors in the environment such as water or hydroxide ions to form free hydroxyls radicals (OH· ) that are also potent oxidants. The photocatalytic activity of TiO2 is due to its wide band gap and long lifetime of photo-generated
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
holes and electrons compared to other semi conductors, but high degree of recombination of photo-generated electrons and holes is a major limiting factor in controlling its photocatalytic efficiency and impedes the practical application of these techniques in the degradation of contaminants in water and air. Thus, a major challenge in heterogeneous photocatalysis is the need to increase charge separation efficiency of the photocatalyst.[9] Coupled semiconductor photocatalysts exhibit a very high photocatalytic activity for both gas- and liquid-phase reactions by increasing the charge separation and extending photo-excitation energy range. Recently, many researchers had shown a lot of interest in coupling two semiconductor particles with different band-gap widths. Research groups have carried out photocatalytic activity experiments using various coupled semiconductor particle systems such as TiO2 –CdS,[10] TiO2 –WO3 ,[11] TiO2 – SnO2 ,[12] TiO2 –MoO3 ,[13] TiO2 – Fe2 O3 .[14] Research on the photocatalytic activity of TiO2 – ZnO coupled oxides was also carried out.[15] Surface modification by doping with metal ions and organic polymers has been proven to be an efficient route in improving the photocatalytic activity of TiO2 . Special attention has been focused on doping TiO2 -based materials with noble
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elements.[16–21] Study revealed that Ag dopant accelerates the transformation of anatase TiO2 to its rutile form, and relatively low Ag concentration (2–5%), results in increase in specific surface areas of the TiO2 powders.[22] The presence of Ag in crystalline TiO2 was found to strongly enhance the photocatalytic activity of TiO2 in various degradation processes.[23–25] In recent studies deposition of Pt on TiO2 has showed marked improvements in photocatalysed destruction of organic pollutants.[26–28] This is due to that Pt may act as a holes scavenger that decreases the e· /h+ recombination process.[28] Phenol and chlorophenols are widely used in the production of wood preservers, pesticides, and biocides, and constitute an important class of recalcitrant pollutants.[29] They are also found in the wastewater of petrochemical industries, plastics industry, and can be found in pulp and insulation materials.[30,31] Wastewater emanating from oil refineries is often contaminated with aliphatic and aromatic petroleum hydrocarbons and organochlorinated compounds [32,33] and the current conventional processes for treatment of such effluents have caused environmental concerns and resulted in accumulation of hazardous sludge.[34,35] These compounds cause serious environmental problems, due to their high toxicity, recalcitrance, bioaccumulation, strong odour emission, persistence in the environment and suspected carcinogenity, and mutagenity.[36] Phenols concentration in a groundwater sample from a contaminated aquifer was reported as high as 25–55 mg L−1 . In wastewater, the concentration can reach higher than 200 mg L−1 .[37] These findings illustrate the seriousness of the phenolic pollutants and the importance of finding an effective method for treating such hazardous wastes. The objective of this research work is to evaluate Pt nanoparticles/TiO2 sample for the UV-photocatalytic degradation of some phenolic pollutants (phenol and 2-chlorophenol (2-CP)) in synthetic wastewater.
2.
Experimental
2.1. Materials Titanium dioxide (rutile TiO2 ) from Alfa-Aesar was used as the photocatalyst support. Chloroplatinic acid (H2 PtCl6 6H2 O, 99.9% pure; Sigma-Aldrich) was used as a precursor for Pt. Standard grades of phenol and 2-CP solutions (Merck) were utilized as pollutants in synthetic wastewater for the photocatalytic degradation experiments. Compressed gases, used for Brunauer–Emmett–Teller (BET) analysis, were UHP from Airgas. All other chemicals utilized were reagent-grade. A 450 W water-cooled highpressure mercury lamp (Hanovia 608A36, ACE Glass, NJ, USA), with a spectral irradiance of 260 W/m2 with spectral ranges from 228 to 420 nm at a distance of 1 m from the light source, according to information provided by the manufacturer, was used as an irradiation source.
2.2.
Synthesis and characterization
2.2.1. Nanoparticle synthesis and characterization Pt nanoparticles capped by polyvinlypyrrolidone (PVP) were prepared by colloidal routes established in the literature.[38–41] First, chloroplatinic acid (H2 PtCl6 6H2 O) was dissolved into DI water to achieve a solution of 20 mL and 6.0 mM. This solution was then diluted with 180 mL of methanol. PVP (molecular weight of 40,000, 133 mg) was added to this mixture. The combined solution was heated to 110◦ C and allowed to reflux for 3 h to achieve the metal nanoparticles. After cooling, the suspension was triple washed (cycles of hexane and ethanol with intermediate centrifugation) to remove excess PVP. The cleaned particles were re-dispersed in an ethanol suspension. The size of particles was measured by transmission electron microscopy (TEM). TEM images were acquired at acceleration voltages of 60 kV using an FEI Morgagni 268D microscope. For each synthesis, the average particle size and a particle size histogram were measured by counting the particle size of 100 particles. 2.2.2. Supported catalyst synthesis and characterization Samples of Pt/TiO2 catalysts were synthesized by immobilizing Pt nanoparticles onto titanium dioxide (rutile TiO2 from Alfa-Aesar). The particles were supported by mild sonication for 3 h at a metal:titanium dioxide mass ratio of 0.3%:99.7%. Following sonication, the composite materials were dried with mild heating on a hot plate (T ∼ 60◦ C) and then in a drying over (T ∼ 90◦ C). A standard BET analysis was performed to measure the specific surface area from the nitrogen adsorption isotherm using an Autosorb IQ (Quantachrome Instruments). Catalysts were degassed at 200◦ C prior to analysis at a temperature of 77 K. X-ray diffraction (XRD) was performed by a Philips PANalytical X-Pert Pro x-ray diffractometer with a Cu Kα x-ray source.
2.3. Catalytic activity The activity of the synthesized catalyst samples, Pt nanoparticles/TiO2 , was evaluated by UV-photodegradation experiments. A batch-scale reactor consisting of a cylindrical 1 L capacity pyrex-glass cell was used for the photocatalytic experiments. A 450-W mercury lamp as a UV source was placed vertically in a quartz tube fitted with a water cooling jacket to maintain a steady temperature of 25◦ C, and was then inserted into the photoreactor. To maintain aerobic conditions, compressed air was purged into the reactor by means of a bubbler at a constant air flow. The solution contents were kept uniformly distributed by using a magnetic stirrer and the solution pH values were adjusted when needed by using 1 M HCl and NaOH. All the experiments were conducted under 450 W of UV irradiation for a period of 180 min (after keeping in dark for 30 min in each run to reach equilibrium state). A 20-mL
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Figure 2.
3
XRD pattern for Pt nanoparticles/TiO2 catalyst.
Table 1. Porosimetry of Pt nanoparticles/TiO2 as determined by nitrogen physisorption. BET surface area (m2 /g) 2.7
Figure 1. TEM image and particle size distribution of synthesized and washed Pt nanoparticles. (a) TEM image. (b) Particle size distribution.
sample was drawn every 30 min and filtered for analysis through 0.45 μm nylon membrane syringe filters (Gelman Acrodic, Michigan) and analysed for the initial and residual concentrations of both phenol and 2-CP by UV-visible spectrophotometer (UV mini 1240 – Shimadzu) at 271 nm. 3. Results and discussions 3.1. Structure of Pt nanoparticles During the syntheses of the Pt nanoparticles, the precursor solution turned black indicating the formation of metal nanoparticles. A representative TEM image and particles size distribution of the synthesized and washed Pt nanoparticles is presented in Figure 1(a) and 1(b). From this image, the average particle size and a distribution were determined to be 2.84 ± 0.61 nm by measuring the size of 100 particles and statistically analysing the resulting histograms. This particle size and deviation agreed with the literature results [38–41] for this synthesis approach. 3.2.
Characterization of Pt/TiO2 nanoparticles Following incorporation of the synthesized and washed Pt nanoparticles onto TiO2 , the resulting structure was
Pore diameter (nm)
Pore volume (cm3 /g)
2.8
8.1 × 10−3
characterized by XRD and nitrogen physisorption. XRD (Figure 2) showed the rutile structure as indicated by the Miller indices. Due to the low loading and the small particle size of the Pt nanoparticles, the Pt phase is not observed by this technique. Based on nitrogen physisorption data (Table 1), the doping of Pt onto TiO2 did not impact the porosity or the surface area. That is, similar results were obtained for the titania substrate as for the Pt/titania sample.
3.3.
Photocatalytic activity It is known that the photocatalytic activity of TiO2 anatase for organic degradation in water is considered to be much more active than rutile. However, regarding the difference in their sorption capacities towards O2 in water, which might be critical to the activity determination, Pt has been used as an electron scavenger.[28] Evidence clearly shows that with the same amount of electron scavenger on the catalyst surfaces, anatase and rutile actually have a similar photocatalytic activity for organic degradation in water.[42] Moreover, from economical point of view, rutile is much cheaper than anatase. Accordingly, the photocatalytic degradation of both phenol and 2-CP solutions over the rutile sample was evaluated with the Pt supported rutile titania samples). All photo experimental runs were performed in dark with the catalyst samples for 30 min before UV illumination to reach adsorption equilibrium state. The solution pH is an important factor that affects the photocatalytic reaction by controlling the adsorption of pollutants on the photocatalyst surface.
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Figure 3(a) and 3(b) shows the effect of change in solution pH with time on the photocatalytic degradation of both phenol and 2-CP solutions, respectively, over Pt/TiO2 with UV light. It is clear that the efficiency of both phenol and 2CP degradation was inversely proportional to the pH values, optimum pH value was 3. However, the efficiency values and rate of photo degradation of phenol were lower than that of 2-CP for all pH values. The degradation efficiency of phenol was gradually increased with time up to 180 min for a wide range of pH values. The maximum percentage degradation values of phenol were found to be 87.7%, 38.14%, 23.78%, and 10.4% with solution pH values of 3, 5, 7, and 9, respectively. On another hand, the degradation efficiency of 2-CP increased rapidly from the beginning. The optimum pH value was 3 with 2-CP complete degradation efficiency after 180 min. The efficiency values of 2-CP degradation were 93.8, 81.6, and 65.5 with solution pH of 5, 7, and 9, respectively. In a previous work,[43] the photocatalytic degradation of both phenol and 2-CP solutions over the TiO2 rutile sample was evaluated (for comparison with the Pt/TiO2 samples). The efficiency of photo degradation was found to be very low with phenol, while it
was comparatively significant with 2-CP for all pH values. However, the degradation of both phenol and 2-CP in solutions was enhanced in the acid medium rather than neutral or alkali medium, the optimum pH value was 3. The maximum percentage degradation values for both phenol and 2-CP were 34.4% and 66.5%, respectively, after 180 min at solution pH 3. The lowering of the degradation percentage with increasing pH can be described by the amphoteric nature of TiO2 in aqueous solution. The zero point charge (pHpzc) of TiO2 is 6.8, below which TiO2 surface would be positively charged while above this pH, it is negatively charged. At pH above 6.8, phenol occurs as negatively charged phenolate ions which experience repulsion from the TiO2 surface. Also the higher concentration of OH− ions hinders the penetration of UV light to the catalyst surface.[43] Moreover, higher pH favours the carbonate ions formation which are very effective scavengers of OH− ions and may reduce the degradation rate.[44,45] Figure 4(a) shows the effect of initial phenols concentration with time on its photocatalytic degradation over Pt/TiO2 in the range of 50–200 ppm. As mentioned above, the degradation efficiency of phenol was gradually increased with time up to 180 min for the studied range
Figure 3. (a) Effect of pH with time on the photocatalytic degradation of phenol over Pt/TiO2 catalyst. Phenol concentration = 50 ppm, Pt/TiO2 dose = 1 g/L. (b) Effect of pH with time photocatalytic degradation of 2-CP over Pt/TiO2 . 2-CP concentration = 50 ppm, Pt/TiO2 dose = 1 g/L.
Figure 4. (a) Effect of initial concentration of phenol with time on the photocatalytic degradation of phenol over Pt/TiO2 . pH = 3, Pt/TiO2 dose = 1 g/L. (b) Effect of initial concentration of 2-CP with time on its photocatalytic degradation over Pt/TiO2 . pH = 3, Pt/TiO2 dose = 1 g/L.
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Environmental Technology of phenolic concentration. On another hand, the degradation efficiency was inversely proportional to the initial phenolic concentration. The initial phenolic concentration value was 50 ppm with 87.7% degradation after 180 min, while the efficiency was drastically decreased to 71.4 ppm and 7.5% with increase in the phenolic concentration to 100 and 200 ppm, respectively. This might be related to the insufficient catalytic sites for phenol direct oxidation. Figure 4(b) illustrates the effect of 2-CP initial concentration with time on its photocatalytic degradation over Pt/TiO2 with UV light. It can be seen that the 2-CP degradation rate and efficiency were higher than that of phenol for a wide range 2-CP concentration values. With a 2-CP concentration value of 50 ppm, a complete degradation was achieved after 180 min. The efficiency values were 84.2% and 55.8% with initial 2-CP concentrations of 100 and 200 ppm, respectively, within 180 min. The decline in the degradation rate of phenols (phenol and 2CP) at higher concentrations can be due to more light being absorbed by the phenolate species than that by TiO2 which is not very effective for carrying out photocatalytic degradation. Furthermore, at the equilibrium adsorption the catalyst active sites are loaded with phenolic species; therefore, competitive adsorption of OH− on the same sites is less likely resulting in decrease in the amount of OH− and O− 2 on the catalyst surface.[45] The effect of the Pt/TiO2 catalyst concentration on photocatalytic degradation of 2-CP and phenol was investigated (Figure 5(a) and 5(b)). Different Pt/TiO2 dosings ranging from 0.25 to 2 g/L were used. In Figure 5(a), it is clear that a high degradation rate of phenol was achieved after 180 min with a catalyst dose of 0.5 or 1 g/L with degradation efficiencies of 87.7 and 86.7%, respectively. However, increasing the catalyst dose to 2 g/L resulted in decreasing the efficiency value to 66.1%. In Figure 5(b), the degradation rate of 2-CP increased rapidly at early time for all dosages of Pt/TiO2 . A complete degradation of phenol was achieved after 180 min with catalyst dosages of 0.5 or 1 g/L. However, increasing the catalyst dose to 2 g/L resulted in decreasing the catalytic efficiency to 93.9%. The inverse effect of the catalyst concentration on the degradation efficiency can be due to the catalyst aggregation/agglomeration that can result in lowering the total number of active sites on the surface.[45,46] Also the excessive opacity imparted by TiO2 particles acts as a shield against incident light and consequently makes hindrance in light penetration. Lowering of active surface area for harvesting light induces resulted in a decrease in the photocatalytic activity.[47,48] Figure 6 summarizes the efficiency of the photodegradation processes of phenol and 2-CP over both Pt/TiO2 and TiO2 rutile supports. The photodegradation processes were optimized by using 0.5 g/L catalyst with pollutant concentration of 50 mg/L within irradiation time of 180 min at different solution pH values for all the samples. The highest degradation efficiencies for both phenol and 2-CP were
5
Figure 5. (a) Effect of Pt/TiO2 dosing with time on the photocatalytic degradation of phenol. pH = 3, phenol concentration = 50 ppm. (b) Effect of Pt/TiO2 dosing with time on the photocatalytic degradation of 2-CP. pH = 3, 2-CP concentration = 50 ppm.
achieved at solution pH3 with the two catalyst samples. An increase in the pH values above 6.8 resulted in a gradual decrease in the degradation rate. However, a very significant increase in the phenol and 2-CP degradation efficiency was achieved with the Pt/TiO2 catalyst compared with the TiO2 rutile (from 34.4% to 87.7% for phenol, and from 94.5% to 100% for 2-CP), at solution pH3 after 180 min. This phenomenon can be explained as follows: the most promising method to increase the photocatalytic activity of TiO2 catalyst is surface modification, this can be achieved by metal immobilization or doping into the catalyst. Pt can act as good electrons scavenger from the conduction band of the semiconductor. Thus, it can hinder the recombination of photo-generated electrons and holes by increasing the charge separation.[49,50] On the other hand, noble metal incorporation into TiO2 dielectric provides an absorption feature due to the surface plasmon resonance (SPR) occurring over the visible range of the spectrum. In particular, Ag, Pt, and Au metals are the most popular materials due to the strong SPR characteristic.[51] Table 2 shows a comparison with results of both phenol and 2-CP photodegradation from relevant works (Pt/TiO2 = 0.5 g/L, pH = 3, phenol/2-CP concentration = 50 ppm, time = 180 min). The results of the current work are very comparative with that of the bimetallic Ag-Pt/TiO2 nanoparticles particularly with the degradation of 2-CP, at the same experimental conditions.
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Figure 6. Efficiency of the photodegradation of both phenol and 2-CP over Pt/TiO2 and rutile TiO2 supports. Phenol (2-CP) concentration = 50 ppm, catalyst dose = 1 g/L, time = 180 min.
as measured through TEM analysis. The catalytic activity of the samples was evaluated by UV-photocatalytic degradation of 2-CP and phenol in synthetic wastewater solution. Pt/TiO2 showed a higher activity for UVphotocatalytic degradation of both phenol and 2-CP pollutants in solution (as compared with the TiO2 rutile). The photodegradation processes were optimized by using 0.5 g/L catalyst with a pollutant concentration of 50 mg/L and a solution pH value of 3 after irradiation time of 180 min. The degradation efficiency values were 87.7% and 100% for both of phenol and 2-Cp, respectively.
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Acknowledgements Table 2. A comparison with results of both phenol and 2-CP photodegradation from relevant works (Pt/TiO2 = 0.5 g/L, pH = 3, phenol/2-CP concentration = 50 ppm, time = 180 min). Catalyst Plain TiO2 (rutile) Ag-Pt/TiO2 (rutile) Pt/TiO2 (rutile)
Phenol degradation 2-CP degradation efficiency (wt%) efficiency (wt%) Reference 34.4
66.5
99.8
100
87.7
100
The authors gratefully acknowledge King Abdulaziz University (KAU, Saudi Arabia) for funding this work through the cooperation agreement with University of South Florida, USA (KAU-USF).
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References
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[1] Fujishima A, Rao TN, Tryk DA. Titanium dioxide photocatalysis. J Photochem Photobiol C: Photochem Rev. 2000;1:1–21. [2] Barakat MA, Chen YT, Huang CP. Removal of toxic cyanide and Cu (II) ions from water by illuminated TiO2 catalyst. Appl Catal B: Environ. 2004;53:13–20. [3] Gaya UI, Abdullah AH. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems. J Photochem Photobiol C: Photochem Rev. 2008;9:1–12. [4] Malato S, Fernández-Ibáñez EzP, Maldonado MI, Blanco J, Gernjak W. Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal Today. 2009;147:1–59. [5] Paola AD, Cufalo G, Addamo M, Bellardita M, Compostrini R, Ischia M, Ceccato R, Palmisano L. Photocatalytic activity of nanocrystalline TiO2 (brookite, rutile and brookite-based) powders prepared by thermo-hydrolysis of TiCl4 in aqueous chloride solutions. Colloids Surf A: Physicochem Eng Aspects. 2008;317:366–376. [6] Barakat MA. Adsorption behavior of copper and cyanide ions at TiO2 – solution interface. J Colloid Interface Sci. 2005;291:345–352. [7] Hermann JM. Heterogenous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catal Today. 1999;53:115–129. [8] Hu C, Wang YZ, Tang HX. Destruction of phenol aqueous solution by photocatalysis or direct photolysis. Chemosphere. 2000;41:1205–1209. [9] Braun IE, Pelizzetti MS, editors. Photochemical conversion and storage of solar energy. Dordrecht: Kluwer; 1991. [10] Gao P, Liu J, Zhang T, Sun DD, Ng W. Hierarchical TiO2 /CdS ‘spindle-like’ composite with high photodegradation and antibacterial capability under visible light irradiation. J Hazard Mater 2012;229–230: 209–216. [11] Lin CF, Wu CH, Onn ZN. Degradation of 4-chlorophenol in TiO2 , WO3 , SnO2 , TiO2 /WO3 and TiO2 /SnO2 systems. J Hazard Mater. 2008;154:1033–1039. [12] Song KY, Park MK, Kwon YT, Lee HW, Chung WJ, Lee WI. Preparation of Transparent particulate MoO3 /TiO2 and WO3 /TiO2 film and their photocatalytic properties. Chem Mater. 2001;13:2349–2355.
Thiswork
Figure 7. Reproducibility of the photodegradation of both phenol and 2-CP over Pt/TiO2 .
The reproducibility of the photodegradation of both phenol and 2-CP over Pt/TiO2 at the optimum conditions is shown in Figure 7. The experiments were performed for four runs. It can be seen that the photodegradation efficiency values slowly decreased with the runs. The efficiency values decreased from 87.1% and 100% in the first run to 72.1% and 82.5%, for both phenol and 2-Cp, respectively. This can be attributed to the fractional losses of some of the catalysts during filtration process. 4. Conclusions Pt nanoparticles/TiO2 catalysts were synthesized by immobilizing Pt nanoparticles onto titanium dioxide (rutile TiO2 ). The average particle size of the catalyst was 2.84 ± 0.6
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Environmental Technology [13] Ma BJ, Kim JS, Choi CH, Woo SI. Enhanced hydrogen generation from methanol aqueous solutions over Pt/MoO3 /TiO2 under ultraviolet light. Int J Hydrog Energy. 2013;38:3582–3587. [14] Akhavan O, Azimirad R. Photocatalytic property of Fe2 O3 nanograin chains coated by TiO2 nanolayer in visible light irradiation. Appl Catal A: Gen. 2009;369:77–82. [15] Aal AA, Barakat MA, Mohamed RM. Electrophoreted Zn–TiO2 –ZnO nanocomposite coating films for photocatalytic degradation of 2-chlorophenol. Appl Surf Sci. 2008;254:4577–4583. [16] Elsalamony RA, Mahmoud SA. Preparation of nanostructured ruthenium doped titania for the photocatalytic degradation of 2-chlorophenol under visible light. Arab J Chem. 2012;http://dx.doi.org/10.1016/j.arabjc.2012.06.008 [17] Korologos CA, Nikolaki MD, Zerva CN, Philippopoulos CJ, Poulopoulos SG. Photocatalytic oxidation of benzene, toluene, ethylbenzene and m-xylene in the gas-phase over TiO2 -based catalysts. J Photochem Photobiol A: Chem. 2012;244:24–31. [18] Fan JW, Liu XH, Zhang J. The synthesis of TiO2 and TiO2 – Pt and their application in the removal of Cr (VI). Environ Technol. 2011;32:427–437. [19] Kuo YL, Su TL, Chuang KJ, Chen HW, Kung FC. Preparation of platinum- and silver-incorporated TiO2 coatings in thin-film photoreactor for the photocatalytic decomposition of o-cresol. Environ Technol. 2011;32:1799–1806. [20] Shokri M, Jodat A, Modirshahla N, Behnajady MA. Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles. Environ Technol. 2013;34:1161–1166. [21] Suri RPS, Thornton HM, Muruganandham M. Disinfection of water using Pt- and Ag-doped TiO2 photocatalysts. Environ Technol. 2012;33:1651–1659. [22] Lin X, Rong F, Fu D, Yuan C. Enhanced photocatalytic activity of fluorine doped TiO2 by loaded with Ag for degradation of organic pollutants. Powder Technol. 2012;219: 173–178. [23] Cao Y, Tan H, Shi T, Tang T, Li J. Preparation of Ag-doped TiO2 nanoparticles for photocatalytic degradation of acetamiprid in water. J Chem Technol Biotechnol. 2008;83:546–552. [24] Behnajady MM, Modirshahla N, Rad B. Enhancement of photocatalytic activity of TiO2 nanoparticles by silver doping: photodeposition versus liquid impregnation methods. Global NEST J. 2008;10:1–7. [25] Barakat AM, Kanjwal MA, Al-Deyab SS, Chronakis IS, Kim HY. Influences of silver-doping on the crystal structure, morphology and photocatalytic activity of TiO2 nanofibers. Mater Sci Appl. 2011;2:1188–1193. [26] Feng C, Wang Y, Zhang J, Yu L, Li D, Yang J, Zhang Z. The effect of infrared light on visible light photocatalytic activity: an intensive contrast between Pt-doped TiO2 and N-doped TiO2 . Appl Catal B: Environ. 2012;113–114: 61–71. [27] Neppolian B, Bruno A, Bianchi CL, Kumar MA. Graphene oxide based Pt–TiO2 photocatalyst: ultrasound assisted synthesis, characterization and catalytic efficiency. Ultrason Sonochem. 2012;19:9–15. [28] El-Bahy ZM, Ismail AA, Mohamed RM. Enhancement of titania by doping rare earth for photodegradation of organic dye (Direct Blue). J Hazard Mater. 2009;166: 138–143. [29] Xiangchun Q, Hanchang S, Jainlong W, Yi Q. Biodegradation of 2,4-dichlorophenol in sequencing batch reactors augmented with immobilized mixed culture. Chemosphere. 2003;50:1069–1074.
7
[30] Heidi MR, Chien MW, Tao Y, Jun-Kyoung K, William DM. Catalytic hyrodechlorination of chlorophenols in aqueous solution under mild conditions. Appl Catal A: Gen. 2004;271:137–143. [31] Liu D, Maguire RJ, Pacepavicius G, Dutka BJ. Biodegradation of recalcitrant chlorophenols by cometabolism. Environ Toxicol Water Qual. 1991;6:85–95. [32] Han DH, Cha SY, Yang HY. Improvement of oxidative decomposition of aqueous phenol by microwave irradiation in UV/H2 O2 process and kinetic study. Water Res. 2004;38:2782–2790. [33] Alinsafi A, Evenou F, Abdulkarim EM, Zahraa MNP, Benhammou A, Yaacoubi A, Nejmeddine A. Treatment of textile industry wastewater by supported photo catalysis. Dyes Pigments. 2007;74:439–445. [34] Tiburtius ERL, Peralta-Zamora P, Emmel A. Treatment of gasolinecontaminated waters by advanced oxidation processes. J Hazard Mater. 2005;126:86–90. [35] Cho L-H, Kim Y-G, Yang J-K, Lee N-H, Lee S-M. Solar-chemical treatment of groundwater contaminated with petroleum at gas station sites: ex situ remediation using solar/TiO2 photocatalysis and solar photo-Fenton. J Environ Sci Health A. 2006;41:457–473. [36] Contreras S, Rodriguez M, Al Momani F, Sans C, Esplugas S. Contribution of the ozonation pretreatment to the biodegradation of aqueous solutions of 2,4 dichlorophenol. Water Res. 2003;37:3164–3171. [37] Wang Y, Ren J, Deng K, Gui L, Tang Y. Preparation of tractable platinum, rhodium, and ruthenium nanoclusters with small particle size in organic media. Chem Mater. 2000;12:1622–1627. [38] Rioux RM, Song H, Hoefelmeyer JD, Yang P, Somorjai GA. High-surface-area catalyst design: synthesis, characterization, and reaction studies of platinum nanoparticles in mesoporous SBA-15 silica. J Phys Chem B. 2005;109: 2192–2202. [39] Song H, Rioux RM, Hoefelmeyer JD, Komor R, Niesz K, Grass M, Yang P, Somorjai GA. Hydrothermal growth of mesporous SBA-15 silica in the presence of PVP-stabilized Pt nanoparticles: synthesis, characterization, and catalytic properties. J Am Chem Soc. 2006;128:3027–3037. [40] Kuhn JN, Huang W, Tsung C-K, Zhang Y, Somorjai GA. Structure sensitivity of carbon-nitrogen ring opening: impact of platinum particle size from below 1 to 5 nm upon pyrrole hydrogenation product selectivity over monodisperse platinum nanoparticles loaded onto mesoporous silica. J Am Chem Soc. 2008;130:14026–14027. [41] Sun Q, Xu Y. Evaluating intrinsic photocatalytic activities of anatase and rutile TiO2 for organic degradation in water. J Phys Chem C. 2010;114(44):18911–18918. [42] Barakat MA, Al-Hutailah RI, Hashim MH, Qayyum E, Kuhn JN. Titania-supported silver-based bimetallic nanoparticles as photocatalysts. Environ Sci Pollut Res. 2013;20: 3751–3759. [43] Qamar M, Muneer M, Bahnemann D. Heterogeneous photocatalysed degradation of two selected pesticide derivatives, triclopyr and daminozid in aqueous suspensions of titanium dioxide. J Environ Manage. 2006;80: 99–106. [44] Akbal F, Onar AN. Photocatalytic degradation of phenol. Environ Monit Assess. 2003;83:295–302. [45] Naeem K, Feng O. Parameters effect on heterogeneous photocatalysed degradation of phenol in aqueous dispersion of TiO2 . J Environ Sci. 2009;21:527–533. [46] Chen SF, Liu YZ. Study on the photocatalytic degradation of glyphosate by TiO2 photocatalyst. Chemosphere. 2007;67(5):1010–1017.
8
M.A. Barakat et al.
Downloaded by [University of Birmingham] at 05:28 21 September 2013
[47] Barakat MA, Schaeffer H, Hayes G, Shah SI. Photocatalytic degradation of 2-chlorophenol by Co-doped TiO2 nanoparticles. Appl Catal B: Environ. 2004;57:23–30. [48] Barakat MA, Tseng JM, Huang CP. Hydrogen peroxideassisted photocatalytic oxidation of phenolic compounds. Appl Catal B: Environ. 2005;59:99–104. [49] Wang H, Wu Z, Liu Y, Wang W. Influences of various Pt dopants over surface platinized TiO2 on the photocatalytic oxidation of nitric oxide. Chemosphere. 2008;74:773–778.
[50] Barakat NAM, Kanjwal MA, Al-Deyab SS, Chronakis IS, Kim HY. Influences of silver-doping on the crystal structure, morphology and photocatalytic activity of TiO2 nanofibers. Mater Sci Appl. 2011;2:1188–1193. [51] Barakat NAM, Woo KD, Kanjwal MA, Choi KE, Khil MS, Kim HY. Surface plasmon resonances, optical properties and electrical conductivity thermal hystersis of silver nanofibers produced by electrospinning Ttechnique. Langmuir. 2008;24:11982–11987.
