Photochemistry and Photobiology, 20**, **: *–*
Rapid Communication The Effect of Loading Palladium on Zinc Oxide on the Photocatalytic Degradation of Methyl tert-Butyl Ether (MTBE) in Water Zaki S. Seddigi1, Saleh A. Ahmed*1, Shahid P. Ansari1, Naeema H. Yarkandi1, Ekram Danish2, Abdullah Abu Alkibash3, Mohammed D. Y. Oteef4 and Shakeel Ahmed5 1
Chemistry Department, College of Applied Sciences, Umm Al-Qura University, Makkah, Saudi Arabia Chemistry Department, College of Sciences, King Abdulaziz University, Jeddah, Saudi Arabia 3 Chemistry Department, College of Sciences, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia 4 Chemistry Department, College of Science, King Khalid University, Abha, Saudi Arabia 5 Center for Refining & Petrochemicals, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia 2
Received 26 July 2013, accepted 14 January 2014, DOI: 10.1111/php.12242
ABSTRACT
water contaminated with MTBE (7). The United States Environmental Protection Agency (USEPA) has considered the presence of MTBE in water as a potential human carcinogen and has issued a ban on its usage as an additive in the fuel (8). Many traditional techniques like aerobic/anaerobic degradation, airstripping, activated carbon treatments were not successful to remove MTBE from water. This can be attributed to the resistance of MTBE to biodegradation and to the Henry’s law constant (5.5 9 104 atm m3mol1 at 25°C) (9). Recently, heterogeneous photocatalysis have been used to degrade the environmental remediation where the toxic and nonbiodegradable organic molecules are subjected to degradation. Photocatalysis is initiated by irradiating a suitable catalyst with an electromagnetic light of suitable wavelength having an energy that is higher than or equal to that of the band gap of the catalyst material (10,11). This irradiation forces an electron from the valance band to conduction band, thus creating a hole (h+). These photogenerated electrons and holes will act as reducing and oxidizing agents during the photocatalytic reactions. ZnO as a photocatalyst has been reported to give good results when applied using various practical conditions (13–17). The increasing interest in ZnO as a photocatalyst can be attributed to its distinct photochemical and catalytic properties and the possible mechanism of the photocatalysis may be given as below: (12).
A series of heterogeneous catalysts was prepared by doping zinc oxide with different palladium loadings in the range of 0.5%–1.5%. The prepared catalysts were characterized by SEM, TEM and XRD. These catalysts were applied to study the degradation of Methyl tert-Butyl Ether (MTBE). An amount of 100 mg of each of these catalysts was added to an aqueous solution of 100 ppm of MTBE. The resulting mixtures were irradiated with UV light for a period of 5 h. A 99.7% removal of MTBE was achieved in the case of the zinc oxide photocatalyst particles doped with 1% Pd. The photoreaction was found to be a first-order one.
INTRODUCTION Fuel oxygenates are special types of organic compounds blended with gasoline to improve its quality (1). These compounds provide enough oxygen which enhances the combustion process to come to completion which will in turn increase the octane rating of the fuel. As a result, the harmful vehicular emissions are reduced (2). The compounds that are used as fuel oxygenates include methyl tert-butyl ether (MTBE), ethyl tertbutyl ether, tert-amyl methyl ether, ethanol and methanol (3). MTBE is generally used because of its unique properties such as its significant blending properties and the low cost of its production (4,5). However, as a result of accidental spills, the underground leakage of pipelines and the problems associated with the storage tanks, MTBE has started to appear in the water bodies. MTBE is stable chemically as well as biologically and its adsorption on soil is insignificant. It is also hydrophilic (~50 gL1), thus it dissolves in water/moisture and moves rapidly through soil and accumulates in water bodies (6). Increased concentrations of MTBE in drinking water render this water unsafe for human consumption. Skin eruption, diarrhea, respiratory problem are the general symptoms noted on exposure to
ZnO þ hv ! e þ hþ
ð1Þ
ðO2 Þads: þ e ! O2
ð2Þ
þ
ð3Þ
O2 þ Hþ ! HOO
ð4Þ
H2 O ! OH þ H
HOO þ e !HOO
HOO þ Hþ ! H2 O2
ð6Þ
ð7Þ
H2 O2 þ e ! 2 OH
*Corresponding author e-mail: [email protected] (Saleh A. Ahmed) © 2014 The American Society of Photobiology
1
ð5Þ
2
Zaki S. Seddigi et al.
H2 O2 þ hþ ! Hþ þ OH
ð8Þ
Moreover, ZnO has wide-ranging morphology and offers many possibilities for the photocatalytic degradation of the organic contaminants (13). It is important to mention here that the Pd doping is crucial in controlling the recombination of photoexcited species (electrons and holes) and, therefore, help to enhance photoactivity of the catalyst (14,15). In this study, the photodegradation of MTBE using ZnO doped with palladium was investigated. The reaction conditions and the optimum percentage of Pd used in doping the ZnO particles were also investigated.
MATERIALS AND METHODS Methyl tert-Butyl Ether from Sigma–Aldrich of 99.9% purity, double distilled water, palladium (II) nitrate dihydrate from Merck and zinc oxide (J.T. Baker, USA) were used in this study. The Pd-doped ZnO photocatalyst was prepared by incipient wetness impregnation method (15). A typical synthesis procedure was as follows: For the preparation of the 1.0% Pd-supported ZnO photocatalyst (5.0 g), Pd(NO3)2. xH2O (40% Pd basis, 0.0122 g) was dissolved in deionized water to make a solution of 1.25 mL for each gram of solid ZnO powder. Then this solution (a volume of the calculated amount 6.25 mL) was added to solid ZnO powder (5.0 g) and the mixture was left to soak for a period of 1 h at room temperature. The sample was left overnight at 100°C to dry and then it was calcined for a period of 3 h at 450°C then cooled to room temperature. Three levels of Pd loadings (from 0.5 to 1.5 wt%) were supported on ZnO. Methyl tert-Butyl Ether solution (350 mL of 100 ppm MTBE) prepared in distilled water was transferred into the photoreactor and an amount of 100 mg of each of the photocatalysts: ZnO doped with 0.5%, 1.0% and 1.5% of Pd was added to each reaction mixture. The temperature of this mixture was controlled during the irradiation process by means of circulating cold water at a temperature of 16 1°C. The reaction mixture was stirred to achieve equilibrium and the oxygen was passed at a moderate flow rate for a period of 30 min. Then the oxygen flow was stopped and the UV lamp was turned on. The experimental setup was covered with an aluminum foil and left for a period of 5 h and a sample for analysis was collected at the end of every hour during that period.
RESULTS AND DISCUSSION Figure 1(a) shows the XRD pattern of the photocatalyst ZnO doped with 1% Pd. The X-ray diffraction spectrum was obtained
by means of X’pert PRO PANalytical using CuKa radiation (wavelength = 1.5406 Å) with a 2h value of 0.02 as a step size in the scan range of 2h = 10–80° at 25°C. The diffraction pattern represents the hexagonal wurtzite structure of ZnO (16). It was noted that that the Pd peaks could not be observed and at the same time there was no specific change in the diffraction pattern of ZnO particles. This can be attributed to the small amount of these particles that have small size and are homogeneously distributed (14). The crystallite size was calculated from peak broadening (in nm) using the Scherrer approximation, which is defined as (16): t ¼ ½0:9k=BCosh
ð9Þ
where k is the wavelength of the X-ray (15.418 nm), B is the full width at half maximum (FWHM, radian) and h is the Bragg angle (degree). The value of FWHM was obtained by performing profile fitting using an XRD pattern processing software. The average crystallite size of 1% Pd-doped ZnO photocatalyst was 420.4 nm. The size and the shape of the particles of the photocatalyst play an important role in its activity. Therefore, SEM and TEM both were applied to study the shape and the size of ZnO doped with 1% Pd. The morphology of these particles is shown in Fig. 1b and they are cubic/cuboid in shape having different sizes. TEM (Fig. 2) clearly shows nano-Pd particles dispersed on ZnO crystals. Figure 3 and Table 1 show the elemental analysis conducted by EDX. The results are normalized for the elements detected in each sample. The level of Pd detected is very close to the nominal values for each sample. This indicates that, the expected amount of Pd is in close agreement with the actual amount present in the catalyst. The heterogeneous photocatalytic degradation of the pollutants in water has been found to follow a kinetic model proposed by Langmuir–Hinshelwood (13,16). According to this model, the rate of the photocatalytic degradation of the pollutant is proportional to the surface coverage of the photocatalyst by the organic pollutant molecules: RateðRÞ ¼ ðdC=dTÞah ¼ kr h ¼ ðkr KC=1 þ KCÞ; If KC\\1 and negligible
Figure 1. a) XRD spectrum and b) SEM micrograph of 1%Pd doped ZnO.
ð10Þ ð11Þ
Photochemistry and Photobiology
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Figure 2. TEM images of 1% Pd-doped ZnO particles a) lower and b) higher magnification.
Table 2. Kinetic data of MTBE photodegradation on UV irradiation in presence of catalysts. Catalyst No Catalyst ZnO 0.5% Pd/ZnO 1.0% Pd/ZnO 1.5% Pd/ZnO
k
r2
0.021 0.337 0.814 1.142 0.918
0.917 0.996 0.986 0.980 0.985
MTBE = Methyl tert-Butyl Ether.
Figure 3. EDX spectra of 1% Pd/ZnO photocatalyst.
lnðC=Co Þ ¼ Kapp t
Table 1. Results of EDX analysis of Pd-supported ZnO catalysts. Weight % Element OK Zn K Pd L
0.5% Pd/ZnO
1.0% Pd/ZnO
1.5% Pd/ZnO
29.67 69.87 0.46
29.64 69.28 1.08
33.72 64.68 1.60
ð12Þ
where h is the surface coverage, kr is the rate constant of the reaction, Kapp is the apparent rate constant, K is the adsorption coefficient and C is the concentration of the reactant. If the concentration C becomes very low, KC would be negligible compared to 1. A plot of ln (C0/C) versus irradiation time gives a straight line which represents a first-order reaction. The rate constant of this reaction can be obtained from the slope of the plot in Fig. 3 (15,16). It was found that the rate constant of the photodegradation reaction increases with the percentage of the Pd that is used for doping. Fig. 4a shows that ZnO particles
Figure 4. a) Kinetic study and b) photodegradtion of Methyl tert-Butyl Ether (MTBE) in aqueous solution.
4
Zaki S. Seddigi et al.
Figure 5. Effect of (a) pH and (b) initial concentration of Methyl tert-Butyl Ether (MTBE) on degradation efficiency.
doped with 1% Pd gives the highest rate constant and the values of linear regression (r2) and first-order constant (k) are given in Table 2. The photodegradation of MTBE in water was studied by applying the ZnO particles doped with 0.5% Pd, 1% Pd and 1.5% Pd as photocatalysts. It can be seen that the ZnO particles doped with 1% Pd has the most effective photoactivity. Almost complete degradation of MTBE was achieved with 1% Pd/ZnO after 5 h. Therefore, ZnO particles doped with 1% Pd can be considered an effective photocatalyst in the degradation of MTBE in water. Initially, the rate of the photodegradation of MTBE was found to be very fast as can be seen from Fig. 4b, during the first hour of UV irradiation, the concentration of MTBE was found to be 21.7 ppm in case of ZnO particles doped with 1% Pd MTBE. The initial fast photodegradation of MTBE can be attributed to the presence of the hydroxyl radicals whose concentration increases with the UV irradiation. However, as the reaction proceeds, more by-products will be formed and they compete with MTBE in consuming the hydroxyl radicals, consequently, the rate of the photocatalytic reaction decreases. The increased efficiency of the ZnO particles doped with 1% Pd is due to the fast transfer of the photoexcited electrons from the surface of the semiconducting photocatalyst to the noble metal which will act as an electron reservoir (13,14). Consequently, the recombination of the photogenerated electrons and holes will be efficiently controlled (18). This will in turn increase the photocatalytic activity of the catalyst. The smaller the size of the particles of the photocatalyst and the metallic dopant, the larger is the surface area. As a result, the photocatalytic activity will increase (19). The effect of size of the particles on the catalytic activity has been reported previously (20). The supporting of noble metal such as Pd or Au on the surface of the semiconducting metal oxide photocatalyst decreases the work function at the interface region with the adsorbed oxygen. Accordingly, the electron transfer between the photocatalyst and the adsorbed oxygen will increase. This will in turn increase the number of peroxy/superoxy species which are highly oxidizing in nature and thus, increase the rate of the photocatalytic reaction (21). However, the concentration of the doped metallic particles was found to affect the photocatalyst. On doping ZnO particles with 1.5% Pd, the activity of this catalyst was decreased. This is due to the fact that an increase in the number of the doped metal on the surface of the ZnO particles resulted in less adsorption of MTBE. Consequently, the photocatalytic activity of this catalyst has decreased (19).
The pH of the reaction medium could affect the distribution of the surface charge, the size of the aggregates of the photocatalyst particles as well as the dispersion of these particles. As a result, the adsorption of the pollutant molecules on the surface of the photocatalyst will be affected (22). The photocatalytic degradations of MTBE were also performed in solutions of initial pH values in the range of 4.0–9.0 using 1.0% Pd/ZnO particles as a photocatalyst. The effects of changing the pH of the solution on the photocatalytic degradation of MTBE are illustrated in Fig. 5a. It was observed that if the pH ranged from 4.0 to 9.0 of the medium it had insignificant effect on the photocatalytic efficiency of the catalyst employed. Kaur et al. (23) in their recent investigation have supported that there is good relation between the Pd doping, band gap, the oxygen vacancies and the photoproperties of the Pd-doped ZnO. Eslami et al. (24,25) found that a pH of 7 was optimum for the photocatalytic degradation of MTBE using either ZnO or TiO2 as photocatalysts. Hu et al. (11) found that a pH of 3 was optimum for the degradation of MTBE using UV/TiO2 system. Fu et al. (26) reported slight increase in the degradation efficiency of methylene blue when pH increased from acidic to neutral. However, this efficiency has increased by several folds in an alkaline medium due to the presence of large numbers of hydroxyl ions and hydroxide-free radicals. Moreover, at higher pH, the extent of recombination between photogenerated electrons and holes will be reduced; as a result, the photodegradation efficiency will be improved. However, in case of the photocatalyst TiO2, a decrease in the photocatalytic efficiency has been observed in an alkaline medium. This has been attributed to the repulsion between the large number of the hydroxyl anions and the negatively charged TiO2 surface (27–29). The effect of the concentration of MTBE on its degradation was studied for solutions of 100, 200, 300 and 400 ppm MTBE. The efficiency of the degradation of MTBE in each of these solutions was studied by keeping the amount of the catalyst and the irradiation time constant. A solution of 100 ppm MTBE was found to give the best photocatalytic degradation efficiency. This efficiency was found to decrease on increasing the concentration of MTBE beyond 100 ppm as can be seen from Fig. 5b. This drop in the degradation efficiency is probably due to the effect of the large number of MTBE molecules where some of them get adsorbed on the surface of the catalyst and act to block the active sites of that catalyst. Moreover, the presence of a large number of molecules of MTBE will shield the catalyst and thus reduce the penetration of the UV radiation that reaches the
Photochemistry and Photobiology catalyst. This will in turn reduce the number of the photoexcited species responsible for the degradation of MTBE.
CONCLUSIONS Photocatalytic degradation of MTBE in water was studied using both ZnO doped with Pd as a photocatalyst. Approximately complete removal of MTBE was achieved within 5 h by using ZnO particles doped with 1% Pd. The efficient degradation of MTBE is due to the higher concentration of the hydroxyl radicals and to the presence of Pd that controls the recombination of photogenerated electron hole pair. The photocatalytic degradation reaction of MTBE was observed to follow first-order kinetics. Acknowledgement—The authors acknowledge the support by King Abdul Aziz City for Science and Technology (KACST) through the Science & Technology Unit at Umm Al-Qura University for funding through project No. 10-wat1240-10 as part of the National Science, Technology and Innovation Plan.
REFERENCES 1. Day, M. J., R. F. Reinke and J. A. M. Thomson (2001) Fate and transport of fuel components below slightly leaking underground storage tanks technical note. Environ Forensic 2, 21–28. 2. Selli, E., C. L. Bianchi, C. Pirola and M. Bertelli (2005) Degradation of methyl tert-butyl ether in water: Effect of the combined use of sonolysis and photocatalysis. Ultrasonics Sonochem. 12, 395–400. 3. Kanai, H., V. Inouye, R. Goo, R. Chow, L. Yazawa and J. Maka (1994) GC/MS Analysis of MTBE, ETBE, and TAME in Gasolines. Anal. Chem. 66, 924–927. 4. Cassada, D. A., Y. Zhang, D. D. Snow and R. F. Spalding (2000) Trace analysis of ethanol, MTBE, and related oxygenate compounds in water using solid-phase microextraction and gas chromatography/ mass spectrometry. Anal. Chem. 72, 4654–4658. 5. Guillard, C., N. Charton and P. Pichat (2003) Degradation mechanism of t-butyl methyl ether (MTBE) in atmospheric droplets. Chemosphere 53, 469–477. 6. Asadi, A. and M. Mehrvar, (2006) Degradation of aqueous methyl tert-butyl ether by photochemical, biological, and their combined processes. Int. J. Photoenergy, 2006, 1–7. 7. Arana, J., A. P. Alonso, J. M. D. Rodriguez, J. A. Herrera, O. Gonzalez and J. P. Pena (2008) Comparative study of MTBE photocatalytic degradation with TiO2 and Cu-TiO2. Appl. Catal. B 78, 355–363. 8. Xu, X.-R., H.-B. Li and J.-D. Gu (2006) Simultaneous decontamination of hexavalent chromium and methyl tert-butyl ether by UV/ TiO2 process. Chemosphere 63, 254–260. 9. Fischer, A., M. M€uller and J. Klasmeier (2003) Determination of henry’s law constant for methyl tert-butyl ether (MTBE) at groundwater temperatures. Chemosphere 54, 689–694. 10. Chan, M. S. M. and R. J. Lynch (2003) Photocatalytic degradation of aqueous methyl tert-butyl ether (MTBE) in a supported-catalyst reactor. Environ. Chem. Lett. 1, 157–160. 11. Hu, Q., C. Zhang, Z. Wang, Y. Chen, K. Mao, X. Zhang, Y. Xiong and M. Zhu (2008) Photodegradation of methyl tert-butyl ether (MTBE) by UV/H2O2 and UV/TiO2. J. Hazard. Mater. 154, 795–803. 12. Shinde, S. S., P. S. Shinde, C. H. Bhosale and K. Y. Rajpure (2011) Zinc oxide mediated heterogeneous photocatalytic degradation of
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organic species under solar radiation. J. Photochem. Photobiol., B 104, 425–433. Pal, B. and M. Sharon (2002) Enhanced photocatalytic activity of highly porous ZnO thin films prepared by sol-gel process. Mater. Chem. Phys. 76, 82–87. Xiaohong, W., D. Xianbo, Q. Wei, H. Weidong and J. Zhaohua (2006) Enhanced photo-catalytic activity of TiO2 films with doped La prepared by micro-plasma oxidation method. J. Hazard Mater B 137, 192–197. Seddigi, Z. S., A. Bumajdad, S. P. Ansari, S. A. Ahmed, E. Danish, N. H. Yarkandi and S. Ahmed (2013) Preparation and characterization of Pd doped ceria-ZnO nanocomposite catalyst for methyl tertbutyl ether (MTBE) photodegradation (2014). J. Hazard. Mater. 264, 71–80. Hayat, K., M. A. Gondal, M. M. Khaled, S. Ahmed and A. M. Shemsi (2011) Nano ZnO synthesis by modified sol gel method and its application in heterogeneous photocatalytic removal of phenol from water. Appl. Catal. A 393, 122–129. Jakob, M. and H. Levanon (2003) Charge distribution between UVirradiated TiO2 and gold nanoparticles: Determination of shift in the fermi level. Nano Lett. 3, 353–358. Neppolian, B., A. Bruno, C. L. Bianchi and M. Ashokkumar (2012) Graphene oxide based Pt-TiO2 photocatalyst: Ultrasound assisted synthesis, characterization and catalytic efficiency. Ultrasonics Sonochem. 19, 9–15. Orlov, A., D. A. Jefferson, M. Tikhov and R. M. Lambert (2007) Enhancement of MTBE photocatalytic degradation by modification of TiO2 with gold nanoparticles. Catal. Commun. 8, 821–824. Zhu, Y., Y. Fu and Q. Q. Ni (2011) Preparation and performance of photocatalytic TiO2 immobilized on palladium- doped carbon fibers. Appl. Surf. Sci. 257, 2275–2280. Sykes, E. C. H., F. J. Williams, M. S. Tikhov and R. M. Lambert (2002) Nucleation, growth, sintering, mobility, and adsorption properties of small gold particles on polycrystalline titania. J. Phys. Chem. B 106, 5390–5394. Gogate, P. R. and A. B. Pandit (2004) A review of imperative technologies for wastewater treatment I: Oxidation technologies at ambient conditions. Adv. Environm. Res. 8, 501–551. Kaur, J., P. Kumar, T. S. Sathiaraj and R. Thangaraj (2013) Structural, optical and fluorescence properties of wet chemically synthesized ZnO:Pd2+ nanocrystals. Int. Nano Lett. 3, 1–7. Eslami, A., S. Nasseri, B. Yadollahi, A. Mesdaghinia, F. Vaezi and R. Nabizadeh (2009) Removal of methyl tert-butyl ether (MTBE) from contaminated water by photocatalytic process. Iran. J. Pub. Health 38, 18–26. Eslami, A., S. Nasseri, B. Yadollahi, A. Mesdaghinia, F. Vaezi and R. Nabizadeh (2008) Photocatalytic degradation of methyl tert-butyl ether (MTBE) in contaminated water by ZnO nanoparticles. J. Chem. Technol. Biotechnol. 83, 1447–1453. Fu, D., G. Han, Y. Chang and J. Dong (2012) The synthesis and properties of ZnO-graphene nano hybrid for photodegradation of organic pollutant in water. Mater. Chem. Phys. 132, 673–681. Bekkouche, S., M. Bouhelassa, N. H. Salah and F. Z. Meghlaoui (2004) Study of adsorption of phenol on titanium oxide (TiO2). Desalination 166, 355–362. Yashimita, H., Harada M., Mi saka J., Takeuchi M., Ikeue K. and M. Anpo (2002) Degradation of propanol diluted in water under visible light irradiation using metal ion-implanted titanium dioxide photocatalysts. J. Photochem. Photobiol., A 148, 257–261. Zang, Y. and R. Farnood (2005) Photocatalytic decomposition of methyl tert-butyl ether in aqueous slurry of titanium dioxide. Appl. Catal. B 57, 275–282.
Rapid Communication The Effect of Loading Palladium on Zinc Oxide on the Photocatalytic Degradation of Methyl tert-Butyl Ether (MTBE) in Water Zaki S. Seddigi1, Saleh A. Ahmed*1, Shahid P. Ansari1, Naeema H. Yarkandi1, Ekram Danish2, Abdullah Abu Alkibash3, Mohammed D. Y. Oteef4 and Shakeel Ahmed5 1
Chemistry Department, College of Applied Sciences, Umm Al-Qura University, Makkah, Saudi Arabia Chemistry Department, College of Sciences, King Abdulaziz University, Jeddah, Saudi Arabia 3 Chemistry Department, College of Sciences, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia 4 Chemistry Department, College of Science, King Khalid University, Abha, Saudi Arabia 5 Center for Refining & Petrochemicals, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia 2
Received 26 July 2013, accepted 14 January 2014, DOI: 10.1111/php.12242
ABSTRACT
water contaminated with MTBE (7). The United States Environmental Protection Agency (USEPA) has considered the presence of MTBE in water as a potential human carcinogen and has issued a ban on its usage as an additive in the fuel (8). Many traditional techniques like aerobic/anaerobic degradation, airstripping, activated carbon treatments were not successful to remove MTBE from water. This can be attributed to the resistance of MTBE to biodegradation and to the Henry’s law constant (5.5 9 104 atm m3mol1 at 25°C) (9). Recently, heterogeneous photocatalysis have been used to degrade the environmental remediation where the toxic and nonbiodegradable organic molecules are subjected to degradation. Photocatalysis is initiated by irradiating a suitable catalyst with an electromagnetic light of suitable wavelength having an energy that is higher than or equal to that of the band gap of the catalyst material (10,11). This irradiation forces an electron from the valance band to conduction band, thus creating a hole (h+). These photogenerated electrons and holes will act as reducing and oxidizing agents during the photocatalytic reactions. ZnO as a photocatalyst has been reported to give good results when applied using various practical conditions (13–17). The increasing interest in ZnO as a photocatalyst can be attributed to its distinct photochemical and catalytic properties and the possible mechanism of the photocatalysis may be given as below: (12).
A series of heterogeneous catalysts was prepared by doping zinc oxide with different palladium loadings in the range of 0.5%–1.5%. The prepared catalysts were characterized by SEM, TEM and XRD. These catalysts were applied to study the degradation of Methyl tert-Butyl Ether (MTBE). An amount of 100 mg of each of these catalysts was added to an aqueous solution of 100 ppm of MTBE. The resulting mixtures were irradiated with UV light for a period of 5 h. A 99.7% removal of MTBE was achieved in the case of the zinc oxide photocatalyst particles doped with 1% Pd. The photoreaction was found to be a first-order one.
INTRODUCTION Fuel oxygenates are special types of organic compounds blended with gasoline to improve its quality (1). These compounds provide enough oxygen which enhances the combustion process to come to completion which will in turn increase the octane rating of the fuel. As a result, the harmful vehicular emissions are reduced (2). The compounds that are used as fuel oxygenates include methyl tert-butyl ether (MTBE), ethyl tertbutyl ether, tert-amyl methyl ether, ethanol and methanol (3). MTBE is generally used because of its unique properties such as its significant blending properties and the low cost of its production (4,5). However, as a result of accidental spills, the underground leakage of pipelines and the problems associated with the storage tanks, MTBE has started to appear in the water bodies. MTBE is stable chemically as well as biologically and its adsorption on soil is insignificant. It is also hydrophilic (~50 gL1), thus it dissolves in water/moisture and moves rapidly through soil and accumulates in water bodies (6). Increased concentrations of MTBE in drinking water render this water unsafe for human consumption. Skin eruption, diarrhea, respiratory problem are the general symptoms noted on exposure to
ZnO þ hv ! e þ hþ
ð1Þ
ðO2 Þads: þ e ! O2
ð2Þ
þ
ð3Þ
O2 þ Hþ ! HOO
ð4Þ
H2 O ! OH þ H
HOO þ e !HOO
HOO þ Hþ ! H2 O2
ð6Þ
ð7Þ
H2 O2 þ e ! 2 OH
*Corresponding author e-mail: [email protected] (Saleh A. Ahmed) © 2014 The American Society of Photobiology
1
ð5Þ
2
Zaki S. Seddigi et al.
H2 O2 þ hþ ! Hþ þ OH
ð8Þ
Moreover, ZnO has wide-ranging morphology and offers many possibilities for the photocatalytic degradation of the organic contaminants (13). It is important to mention here that the Pd doping is crucial in controlling the recombination of photoexcited species (electrons and holes) and, therefore, help to enhance photoactivity of the catalyst (14,15). In this study, the photodegradation of MTBE using ZnO doped with palladium was investigated. The reaction conditions and the optimum percentage of Pd used in doping the ZnO particles were also investigated.
MATERIALS AND METHODS Methyl tert-Butyl Ether from Sigma–Aldrich of 99.9% purity, double distilled water, palladium (II) nitrate dihydrate from Merck and zinc oxide (J.T. Baker, USA) were used in this study. The Pd-doped ZnO photocatalyst was prepared by incipient wetness impregnation method (15). A typical synthesis procedure was as follows: For the preparation of the 1.0% Pd-supported ZnO photocatalyst (5.0 g), Pd(NO3)2. xH2O (40% Pd basis, 0.0122 g) was dissolved in deionized water to make a solution of 1.25 mL for each gram of solid ZnO powder. Then this solution (a volume of the calculated amount 6.25 mL) was added to solid ZnO powder (5.0 g) and the mixture was left to soak for a period of 1 h at room temperature. The sample was left overnight at 100°C to dry and then it was calcined for a period of 3 h at 450°C then cooled to room temperature. Three levels of Pd loadings (from 0.5 to 1.5 wt%) were supported on ZnO. Methyl tert-Butyl Ether solution (350 mL of 100 ppm MTBE) prepared in distilled water was transferred into the photoreactor and an amount of 100 mg of each of the photocatalysts: ZnO doped with 0.5%, 1.0% and 1.5% of Pd was added to each reaction mixture. The temperature of this mixture was controlled during the irradiation process by means of circulating cold water at a temperature of 16 1°C. The reaction mixture was stirred to achieve equilibrium and the oxygen was passed at a moderate flow rate for a period of 30 min. Then the oxygen flow was stopped and the UV lamp was turned on. The experimental setup was covered with an aluminum foil and left for a period of 5 h and a sample for analysis was collected at the end of every hour during that period.
RESULTS AND DISCUSSION Figure 1(a) shows the XRD pattern of the photocatalyst ZnO doped with 1% Pd. The X-ray diffraction spectrum was obtained
by means of X’pert PRO PANalytical using CuKa radiation (wavelength = 1.5406 Å) with a 2h value of 0.02 as a step size in the scan range of 2h = 10–80° at 25°C. The diffraction pattern represents the hexagonal wurtzite structure of ZnO (16). It was noted that that the Pd peaks could not be observed and at the same time there was no specific change in the diffraction pattern of ZnO particles. This can be attributed to the small amount of these particles that have small size and are homogeneously distributed (14). The crystallite size was calculated from peak broadening (in nm) using the Scherrer approximation, which is defined as (16): t ¼ ½0:9k=BCosh
ð9Þ
where k is the wavelength of the X-ray (15.418 nm), B is the full width at half maximum (FWHM, radian) and h is the Bragg angle (degree). The value of FWHM was obtained by performing profile fitting using an XRD pattern processing software. The average crystallite size of 1% Pd-doped ZnO photocatalyst was 420.4 nm. The size and the shape of the particles of the photocatalyst play an important role in its activity. Therefore, SEM and TEM both were applied to study the shape and the size of ZnO doped with 1% Pd. The morphology of these particles is shown in Fig. 1b and they are cubic/cuboid in shape having different sizes. TEM (Fig. 2) clearly shows nano-Pd particles dispersed on ZnO crystals. Figure 3 and Table 1 show the elemental analysis conducted by EDX. The results are normalized for the elements detected in each sample. The level of Pd detected is very close to the nominal values for each sample. This indicates that, the expected amount of Pd is in close agreement with the actual amount present in the catalyst. The heterogeneous photocatalytic degradation of the pollutants in water has been found to follow a kinetic model proposed by Langmuir–Hinshelwood (13,16). According to this model, the rate of the photocatalytic degradation of the pollutant is proportional to the surface coverage of the photocatalyst by the organic pollutant molecules: RateðRÞ ¼ ðdC=dTÞah ¼ kr h ¼ ðkr KC=1 þ KCÞ; If KC\\1 and negligible
Figure 1. a) XRD spectrum and b) SEM micrograph of 1%Pd doped ZnO.
ð10Þ ð11Þ
Photochemistry and Photobiology
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Figure 2. TEM images of 1% Pd-doped ZnO particles a) lower and b) higher magnification.
Table 2. Kinetic data of MTBE photodegradation on UV irradiation in presence of catalysts. Catalyst No Catalyst ZnO 0.5% Pd/ZnO 1.0% Pd/ZnO 1.5% Pd/ZnO
k
r2
0.021 0.337 0.814 1.142 0.918
0.917 0.996 0.986 0.980 0.985
MTBE = Methyl tert-Butyl Ether.
Figure 3. EDX spectra of 1% Pd/ZnO photocatalyst.
lnðC=Co Þ ¼ Kapp t
Table 1. Results of EDX analysis of Pd-supported ZnO catalysts. Weight % Element OK Zn K Pd L
0.5% Pd/ZnO
1.0% Pd/ZnO
1.5% Pd/ZnO
29.67 69.87 0.46
29.64 69.28 1.08
33.72 64.68 1.60
ð12Þ
where h is the surface coverage, kr is the rate constant of the reaction, Kapp is the apparent rate constant, K is the adsorption coefficient and C is the concentration of the reactant. If the concentration C becomes very low, KC would be negligible compared to 1. A plot of ln (C0/C) versus irradiation time gives a straight line which represents a first-order reaction. The rate constant of this reaction can be obtained from the slope of the plot in Fig. 3 (15,16). It was found that the rate constant of the photodegradation reaction increases with the percentage of the Pd that is used for doping. Fig. 4a shows that ZnO particles
Figure 4. a) Kinetic study and b) photodegradtion of Methyl tert-Butyl Ether (MTBE) in aqueous solution.
4
Zaki S. Seddigi et al.
Figure 5. Effect of (a) pH and (b) initial concentration of Methyl tert-Butyl Ether (MTBE) on degradation efficiency.
doped with 1% Pd gives the highest rate constant and the values of linear regression (r2) and first-order constant (k) are given in Table 2. The photodegradation of MTBE in water was studied by applying the ZnO particles doped with 0.5% Pd, 1% Pd and 1.5% Pd as photocatalysts. It can be seen that the ZnO particles doped with 1% Pd has the most effective photoactivity. Almost complete degradation of MTBE was achieved with 1% Pd/ZnO after 5 h. Therefore, ZnO particles doped with 1% Pd can be considered an effective photocatalyst in the degradation of MTBE in water. Initially, the rate of the photodegradation of MTBE was found to be very fast as can be seen from Fig. 4b, during the first hour of UV irradiation, the concentration of MTBE was found to be 21.7 ppm in case of ZnO particles doped with 1% Pd MTBE. The initial fast photodegradation of MTBE can be attributed to the presence of the hydroxyl radicals whose concentration increases with the UV irradiation. However, as the reaction proceeds, more by-products will be formed and they compete with MTBE in consuming the hydroxyl radicals, consequently, the rate of the photocatalytic reaction decreases. The increased efficiency of the ZnO particles doped with 1% Pd is due to the fast transfer of the photoexcited electrons from the surface of the semiconducting photocatalyst to the noble metal which will act as an electron reservoir (13,14). Consequently, the recombination of the photogenerated electrons and holes will be efficiently controlled (18). This will in turn increase the photocatalytic activity of the catalyst. The smaller the size of the particles of the photocatalyst and the metallic dopant, the larger is the surface area. As a result, the photocatalytic activity will increase (19). The effect of size of the particles on the catalytic activity has been reported previously (20). The supporting of noble metal such as Pd or Au on the surface of the semiconducting metal oxide photocatalyst decreases the work function at the interface region with the adsorbed oxygen. Accordingly, the electron transfer between the photocatalyst and the adsorbed oxygen will increase. This will in turn increase the number of peroxy/superoxy species which are highly oxidizing in nature and thus, increase the rate of the photocatalytic reaction (21). However, the concentration of the doped metallic particles was found to affect the photocatalyst. On doping ZnO particles with 1.5% Pd, the activity of this catalyst was decreased. This is due to the fact that an increase in the number of the doped metal on the surface of the ZnO particles resulted in less adsorption of MTBE. Consequently, the photocatalytic activity of this catalyst has decreased (19).
The pH of the reaction medium could affect the distribution of the surface charge, the size of the aggregates of the photocatalyst particles as well as the dispersion of these particles. As a result, the adsorption of the pollutant molecules on the surface of the photocatalyst will be affected (22). The photocatalytic degradations of MTBE were also performed in solutions of initial pH values in the range of 4.0–9.0 using 1.0% Pd/ZnO particles as a photocatalyst. The effects of changing the pH of the solution on the photocatalytic degradation of MTBE are illustrated in Fig. 5a. It was observed that if the pH ranged from 4.0 to 9.0 of the medium it had insignificant effect on the photocatalytic efficiency of the catalyst employed. Kaur et al. (23) in their recent investigation have supported that there is good relation between the Pd doping, band gap, the oxygen vacancies and the photoproperties of the Pd-doped ZnO. Eslami et al. (24,25) found that a pH of 7 was optimum for the photocatalytic degradation of MTBE using either ZnO or TiO2 as photocatalysts. Hu et al. (11) found that a pH of 3 was optimum for the degradation of MTBE using UV/TiO2 system. Fu et al. (26) reported slight increase in the degradation efficiency of methylene blue when pH increased from acidic to neutral. However, this efficiency has increased by several folds in an alkaline medium due to the presence of large numbers of hydroxyl ions and hydroxide-free radicals. Moreover, at higher pH, the extent of recombination between photogenerated electrons and holes will be reduced; as a result, the photodegradation efficiency will be improved. However, in case of the photocatalyst TiO2, a decrease in the photocatalytic efficiency has been observed in an alkaline medium. This has been attributed to the repulsion between the large number of the hydroxyl anions and the negatively charged TiO2 surface (27–29). The effect of the concentration of MTBE on its degradation was studied for solutions of 100, 200, 300 and 400 ppm MTBE. The efficiency of the degradation of MTBE in each of these solutions was studied by keeping the amount of the catalyst and the irradiation time constant. A solution of 100 ppm MTBE was found to give the best photocatalytic degradation efficiency. This efficiency was found to decrease on increasing the concentration of MTBE beyond 100 ppm as can be seen from Fig. 5b. This drop in the degradation efficiency is probably due to the effect of the large number of MTBE molecules where some of them get adsorbed on the surface of the catalyst and act to block the active sites of that catalyst. Moreover, the presence of a large number of molecules of MTBE will shield the catalyst and thus reduce the penetration of the UV radiation that reaches the
Photochemistry and Photobiology catalyst. This will in turn reduce the number of the photoexcited species responsible for the degradation of MTBE.
CONCLUSIONS Photocatalytic degradation of MTBE in water was studied using both ZnO doped with Pd as a photocatalyst. Approximately complete removal of MTBE was achieved within 5 h by using ZnO particles doped with 1% Pd. The efficient degradation of MTBE is due to the higher concentration of the hydroxyl radicals and to the presence of Pd that controls the recombination of photogenerated electron hole pair. The photocatalytic degradation reaction of MTBE was observed to follow first-order kinetics. Acknowledgement—The authors acknowledge the support by King Abdul Aziz City for Science and Technology (KACST) through the Science & Technology Unit at Umm Al-Qura University for funding through project No. 10-wat1240-10 as part of the National Science, Technology and Innovation Plan.
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