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Photocatalytic degradation of cyanide using titanium dioxide modified with copper oxide
Advances in Environmental Research 6 Ž2002. 471᎐485
Photocatalytic degradation of cyanide using titanium dioxide modified with copper oxide 夽 K. Chiang, R. AmalU , T. Tran Centre for Particle and Catalyst Technologies, School of Chemical Engineering & Industrial Chemistry, Uni¨ ersity of New South Wales, Sydney NSW 2052, Australia
Abstract Copper ŽII. oxide was loaded onto the surface of Degussa P25 TiO 2 particles by photodeposition. The doped material was subsequently utilized as the photocatalyst for cyanide oxidation. The copper content on the TiO 2 surface was varied from 0.05 to 10.0 at.% of Cu. It was found that nanosized CuO deposits were present on the surface of TiO 2 . Modifying TiO 2 with CuO changed the optical properties of TiO 2 and the onset of absorption was red shifted. The photocatalytic activity of the CuO loaded TiO 2 catalysts was measured to determine their ability to oxidize cyanide. It was found that the rate of photooxidation of cyanide assisted with the doped catalyst was improved slightly at 0.10 at.% Cu. Any further increase of the copper dopant concentration decreased the oxidation rate markedly. The presence of Cu 2q ions Ž0.002᎐0.5 mM. in the solution also decreased the photocatalytic degradation of CNy. The decrease in the activity was explained in terms of the competition reaction of CuŽI. cyanide complex ions for surface hydroxyl radicals. In all cases cyanide was being oxidized to cyanate, the end product of cyanide photooxidation. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Photocatalysis; Cyanide; Photodeposition; Oxidation; Copper-doped titanium dioxide
1. Introduction Cyanide is the universal leaching agent for gold because of its strong complexation ability with many metals. The presence of free and complex cyanides in industrial effluent therefore imposes many environmental problems on the public domain on account of their toxicity even at very low concentrations. Yet there is no satisfactory chemical to replace cyanide in mineral processing. Ions such as iron, zinc, copper and
夽
http:rrwww.ceic.unsw.edu.aurcentersrPartcatr Corresponding author. Tel.: q61-29385-4361; fax: q6129385-5966. E-mail address: [email protected] ŽR. Amal.. U
cobalt present in the ore also form stable complexes with cyanide, which create significant problems in downstream treatment of tailings. Various oxidants can be used to oxidize free cyanide and metal cyanide complexes with different degrees of effectiveness. The most common methods are by oxidation, such as chlorination, the INCO process using sulfur dioxide and oxygen, and the Degussa process using hydrogen peroxide. However, these oxidation processes do suffer from several limitations: Ž1. oxidants such as chlorine can form chloro-cyanogen which is equally toxic; Ž2. oxidants being used in cyanide remediation technologies are generally expensive; and Ž3. thermodynamically stable metal cyanide complexes, such as ferro-cyanide and ferri-cyanide, remain undestroyed after the oxidation.
1093-0191r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 1 . 0 0 0 7 4 - 0
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K. Chiang et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 471᎐485
Over the past several decades, numerous studies revealed that many organics and inorganics can be photocatalytically oxidized into less harmful substances using aqueous suspensions of TiO 2 Žband gap energy of 3.2 eV for anatase . ŽSerpone and Pelizzetti, 1989; Hagfeldt and Gratzel, 1995.. The detailed mechanisms involved in semiconductor photocatalysis can be found in several excellent reviews ŽFox and Dulay, 1993; Hoffmann et al., 1995; Mills and Le Hunte, 1997; Howe, 1998.. To become active the photocatalyst must be first excited by energetic photons Ž - 385 nm., which induces the transition of electrons from the valence band to the conduction band of the semiconductor and generation of electronrhole pair for subsequent redox reactions ŽHoffmann et al., 1995.. Photo generation of electron-hole pair: q TiO 2 q h ª ey cbq h vb
Ž1.
Trapping of photo generated electron and hole: III Ti IVOH q ey cbª Ti OH
⭈. Ž IV Ti IVOH q hq vbª Ti OH
Ž2. q
Ž3.
Interfacial electron and hole trapping by adsorbed species: Ti III OH q O 2 ª Ti IVOH q O⭈2y
Ž4.
q Ž Ti IVOH ⭈ . q Redª Ti IVOH q Red⭈q
Ž5.
The adsorbed surface hydroxyl group on the semiconductor surface traps the hole to form the hydroxyl radical, which is a very strong oxidant and capable of oxidizing many organics and inorganics present in the wastewater. The electron reduces surface adsorbed species such as oxygen and could form superoxide radicals. However, the present drawbacks for TiO 2 photocatalysis in the treatment wastewater include: Ž1. the difficulty of separating the fine semiconductor photocatalyst from treated water; Ž2. the rapid unfavourable charge carrier recombination reaction in TiO 2 compared with the redox reactions resulting in low quantum yield; and Ž3. the high band gap energy which limits its application from using solar energy. Many transition metal ion dopants have been demonstrated to influence the rate of TiO 2 photocatalytic oxidation to different extents by changing the dynamics of electron-hole recombination and interfacial charge transfer. The observed change in reaction rates has been well documented and explained in terms of elec-
tron and hole trapping by metal ions, M nq, within the semiconductor photocatalyst ŽChoi et al., 1994.. Ž ny1.q M nqq ey cbª M
Ž6.
Ž nq1.q M nqq hq cbª M
Ž7.
The transition metal ion dopants most frequently studied and reported in literature include Zn2q, Ni 2q, Mn 2q, Co 2q, Cu2q, Cr 3q, Fe 3q, V 4q, Mo 5q, Pb 2q and Tlq ŽTanaka et al., 1986; Choi et al., 1994; Fujitsu and Hamada, 1994; Cordoba et al., 1998; Wilke and Breuer, 1999.. Reaction 6 is thermodynamically feasible if the reduction potential of M nq is more positive than the conduction band edge. However, there is no correlation between the reduction potential of M nq and the photocatalytic activity of the doped MrTiO2 photocatalyst. The mechanism of photocatalytic oxidation of cyanide using TiO 2 has been well documented in the literature ŽAugugliaro et al., 1997; Frank and Brad, 1977; Pollema et al., 1992.. The oxidation of cyanide is possible via the reaction of CNy with surface hydroxyls or holes. According to Domenech and Peral Ž1988. the initial step of cyanide photocatalytic oxidation is the formation of cyanide radical, which subsequently dimerize to form cyanogen. Finally, the cyanogen molecule undergoes dismutation under alkali conditions to give cyanide and cyanate. The cyanate produced is further oxidized to give NOy 3 and CO 2 . The corresponding reactions are presented as follows wEqs. Ž8. ᎐ Ž11.x. hqrO H ⭈ y
CN
›
CN ⭈
2CN ⭈ª Ž CN. 2
Ž8. Ž9.
Ž CN. 2 q 2OHyª CNyq CNOyq H 2 O
Ž 10 .
CNOyq 8OHyq 8 hqª NOy 3 q CO 2 q 4H 2 O
Ž 11.
In this paper, we attempt to examine a number of variables, which affect the photocatalytic activity of copper doped TiO 2 during the oxidation of cyanide. There are two reasons why copper is used as the dopant in the current study. Firstly, the profound influence of Cu2qrCuq couple in aqueous solution has been reported in heterogeneous photocatalysis ŽSykora, 1997.. Secondly, the rate of oxidation of cyanide by ozone, which enters a cyclic oxidation᎐reduction pathway, was found to increase in the presence of copper ions ŽGurol and Holden, 1988..
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2. Experimental 2.1. Chemical reagents All chemicals were analytical grade and used without further purification. The titanium dioxide ŽP25. purchased from Degussa Ž30 nm primary crystal size. has a specific surface area of 51 m2rg, and contains predominantly anatase Ž79% anatase and 21% rutile as determined from X-ray diffraction.. Technical grade sodium cyanide ŽNaCN. was used as the source of cyanide. Copper nitrate, CuŽNO 3 . 2 ⭈ 2.5H 2 O was used as the source of Cu2q ion both for studying the effect of Cu2q ion in cyanide photooxidation and for loading copper onto the surface of TiO 2 . Nitric acid and sodium hydroxide were used to adjust the pH of solution. Ultra pure gas Žeither air or N2 . was used in all experiments. All solutions were prepared using freshly prepared Milli-Q water.
2.2. Apparatus All experiments were carried out using a 1-l flat bed reactor to assess the activity of the developed catalysts for cyanide degradation ŽFig. 1.. The irradiated area of the solution was 15 cm = 30 cm with a depth of 2.2 cm. The top of the reactor is covered with a quartz plate that serves to isolate the system from the surroundings while allowing UV light Ž) 99%. to pass through. The bottom part is made of four gas chambers topped with sintered glass to allow an even distribution of the purging gas used during the experiment. The liquid in the reactor was circulated through a glass coil and mixed well by pumping at a flow rate of 4 lrmin. The temperature of the solution inside the reactor was kept
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constant by immersing the glass coil in a warm water bath. Purified air or nitrogen was continuously bubbled through which aerated and de-aerated the system and further promoted mixing in the reactor. A 20-W NEC UVA lamp provided radiation in the near visible region with a peak at max s 355 nm. A parabolic reflector was used to reflect irradiation from the UV fluorescent tube to the reactor. The gas leaving the reactor was passed into a solution of 1 M NaOH solution, which served as a hydrogen cyanide trap and therefore prevented the potential release of toxic hydrogen cyanide gas. The pH of the solution was monitored using an Orion pH meter throughout the experiment. The temperature of the solution was also monitored and recorded periodically in all runs.
2.3. Preparation of copper doped catalysts Copper was loaded onto the surface of TiO 2 ŽDegussa P25. by reducing the Cu2q ions photocatalytically using CuŽNO 3 . 2 ⭈ 2.5H 2 O along with sodium formate which acted as a hole scavenger. Firstly, 2 g of TiO 2 was suspended in 500 ml of water and sonicated for 30 min to break up any loosely attached aggregates. The copper dopant levels used in this work were 0.05, 0.10, 0.25. 0.5, 1.0, 2.0, 5.0 and 10.0 at.% of Cu to Ti. Pre-determined amounts of copper nitrate and sodium formate were then added to the suspension and made up to a final volume of 1 l. The photoreduction of aqueous Cu 2q ions was carried out under slightly acidic conditions at pH 3.8 adjusted by the addition of concentrated HNO3. The solution was de-aerated for 15 min before commencing the experiment and nitrogen was continuously purged into the solution at a rate of 500 mlrmin so that no photoreduced copper species
Fig. 1. Set-up of the experiment. Ža. Metal parabolic reflector, Žb. UVA lamp located at focus, Žc. quartz plate cover, Žd. reactor, Že. and Žf. gases inlet and outlet, Žg. pump, and Žh. glass coil inside a water bath.
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could be re-oxidized by dissolved oxygen and returned to the solution phase. The temperature of solution was maintained at 30⬚C by circulating the solution through a coil which was immersed in a water bath. The copper loaded TiO 2 particles were then filtered, dried at 110⬚C for 12 h and stored under vacuum. Unmodified TiO 2 powder was also prepared in this manner except that no Cu2q ions were added. The concentration of Cu2q in the aqueous solution dropped as the experiment proceeded and finally its concentration became undetectable at the end. This indicated that all Cu2q in the solution had been reduced and deposited on the surface of TiO 2 .
2.4. Photoreacti¨ ity It is well known that the p K a of hydrogen cyanide is equal to 9.21 at 25⬚C ŽSharpe, 1976. and therefore it is necessary to work in a strongly alkaline condition in order to avoid the liberation of toxic hydrogen cyanide gas during the experiment. Any hydrogen cyanide formed during the experiment would be trapped by the 1-M NaOH solution. To assess the activity of the copper loaded TiO 2 , 1 g of sample was suspended in 500 ml of Milli-Q water and sonicated for 30 min. A pre-weighed amount of NaOH was first added to the suspension followed by addition of NaCN. The volume was then made up to 1 l by the further addition of water to reach a final pH of 12.0 and cyanide concentration of 100 mgrl Ž3.85 mM.. The resulting suspension was saturated with air at a flow rate of 500 mlrmin for 15 min before the reaction started and kept aerated throughout the experiments. The mixture was irradiated for 3 h to investigate the removal rate of cyanide at a temperature of 30⬚C. In order to study the photooxidation of cyanide and formation of cyanate intermediate, the irradiation period was lengthened until the concentration of cyanide fell to below the undetectable level. Samples were withdrawn at regular intervals, filtered through a 0.45-m syringe filter and finally analysed for free cyanide and cyanate.
2.5. Characterization techniques A Hitachi 4500II field emission scanning electron microscope ŽFE-SEM. was used for microstructural analysis on the surface of TiO 2 . In addition, an Oxford Isis energy dispersive X-ray ŽEDX. analyser was interfaced to the column to perform semi-quantitative chemical analysis and to obtain information on the copper distribution on the TiO 2 surface. The crystal sizes of the samples were also determined using a Philips CM200 FEGTEM. The specific surface areas of all the samples were measured using the single point dynamic surface area analyser, which allowed the accu-
rate determination of surface area from 0.5 to 1000 m2rg. The crystalline phase was determined using powder X-ray diffraction ŽXRD.. The XRD patterns were obtained at room temperature with a Siemens Diffraktometer D5000 using Ni-filtered CuK ␣ radiation. A Cary 5 UVrvis-NIR spectrophotometer equipped with an integrated sphere was used to record the diffuse reflectance spectra ŽDRS. and absorbance data of the solution samples. The base line correction was performed using a calibrated reference sample of barium sulfate. The reflectance spectra of the copper loaded samples were analysed under ambient conditions in the wavelength range of 250᎐700 nm. The electron paramagnetic resonance ŽEPR. spectra of the copper doped TiO 2 samples were recorded by using Bruker EMX X-band Žwith helium cryostat. at 25⬚C. The g-values of the copper doped TiO 2 samples were obtained from a software developed by Bruker using the measurement of the magnetic field and the microwave frequency. The concentration of Cu2q ions in the aqueous solution was measured by atomic absorption spectroscopy ŽAAS. at 324 nm using a Varian SpectrAA-20 spectrophotometer. The concentration of free cyanide was determined by potentiometric titration using silver nitrate standard. An automated system consisting of a Metrohm 665 Dosimat, a silver Titrode in conjunction with a 682 Titroprocessor was used to detect the end point of titration. Samples were first filtered through a 0.45-m syringe filter and 1-ml aliquots were titrated against standardized AgNO3 solution. This method is suitable to analyse cyanide concentrations between 1 and 1000 mgrl CNy with an accuracy of "1 mgrl. The concentration of cyanate was determined spectrophotometrically employing a modified method of Guilloton and Karst Ž1985. based on the reaction between cyanate ions and 2-aminobenzoic acid under buffered conditions. The concentration of cyanate could be precisely determined independently of the pH of the system, the presence of Cu 2q ions and cyanide ions, by measuring the absorbance of the complex formed at 310 nm. It was found that this analysis was most accurate at cyanate concentrations from 0.5 to 80.0 mgrl.
3. Results and discussion 3.1. Electron microscopy and energy dispersi¨ e X-ray spectroscopy studies The surface morphology of all copper loaded TiO 2 samples was studied using scanning and transmission electron microscopy. The colour of the TiO 2 particles changed from white to greyish as the concentration of
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Fig. 2. FESEM micrographs of Ža. pure undoped P25, and Žb. P25 doped with 10 at.% Cu.
copper increased. Selected SEM images of pure unmodified P25 TiO 2 and P25 TiO 2 loaded with 10 at.% Cu are shown in Fig. 2a,b, respectively, for comparison. It can be seen from the SEM images that the shape and crystal size of all copper loaded samples resembled that of unmodified P25 TiO 2 . The average primary crystal sizes of the samples loaded with various copper dopant levels were determined to be 30 nm from SEM and TEM studies ŽTable 1., which are comparable with those found by Wang and Wan Ž1994.. In their nucleation study of photoelectrochemical reduction of aqueous Cu 2q ions, copper nuclei were rarely found on the TiO 2 particles. From a study by Cunningham et al. Ž1993. who used copper malonate as the copper precursor, the formation of aggregates consisting of smaller copper metal islands Žapprox. 10 nm. on the surface of 13% CurTiO 2 samples was observed. In the present study, no similar discrete deposits could be observed for our copper doped TiO 2 samples. However, from the qualitative information obtained from the EDX analysis on different particles of the same sample or in different
locations on the same particle, copper was found to exist on the surface of all loaded TiO 2 samples. As shown in Fig. 3, by scanning the doped catalyst surface utilizing the EDX dot mapping technique, it was unambiguously revealed that even at a very low copper dopant concentration Ž0.05 at.% Cu., the copper had covered the surface of the catalyst uniformly. As the copper dopant concentration increased, the copper dopant became more and more densely packed and finally covered the surface entirely at ) 5 at.% Cu.
3.2. Specific surface area of copper-loaded TiO2 Measurements of specific surface area of all copperloaded TiO 2 samples are listed in Table 1. Considering P25 TiO 2 , a non-porous substance having a density of 3.96 grcm3, the theoretical surface area was calculated to be 50.5 m2rg which agrees very well with the experimentally obtained value of 51 m2rg. As it can be seen in Table 1, loading copper within the range being studied onto the surface of P25 by the photo-reductive
Table 1 Physical properties of doped TiO 2 samples: atomic percentage of Cu relative to Ti, surface concentration of copper on TiO 2 , crystallite sizes as observed under SEM and TEM, and specific surface area Sample ID
Copper Žat.%.
Loading ŽCurnm2 .
Crystallite size obtained from TEM Žnm.
BET surface area Žm2 rg.
P25 CuP005 CuP010 CuP025 CuP050 CuP100 CuP200 CuP500 CuP1000
᎐ 0.05 0.10 0.25 0.50 1.00 2.00 5.00 10.0
0 0.07 0.15 0.37 0.74 1.49 2.97 7.43 14.87
30᎐40 30᎐40 30᎐40 30᎐40 30᎐40 30᎐40 30᎐40 30᎐40 30᎐40
51 52 51 52 51 51 51 51 50
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Fig. 3. FESEM micrographs of P25 doped with 0.05 at.% Cu. Ža. Image observed under SEM. Žb. SEM-EDX dot mapping of Ti Žwhite dot. on the catalyst surface as shown in Ža.. Žc. SEM-EDX dot mapping of Cu on the catalyst surface as shown in Ža..
route did not significantly change the surface area and therefore the role of copper is unbiased by the difference in surface area. Based on the surface area of P25 TiO 2 Ž51 m2rg. used in this work, the concentrations of copper could also be represented as the number of Cu atomsrnm2 of TiO 2 support as included in Table 1.
3.3. XRD studies The XRD diffraction patterns of the copper-loaded P25 TiO 2 are shown in Fig. 4. The diffraction pattern of undoped P25 is also included in the figure for comparison. The angles 2 s 25.28⬚ and 2 s 27.42⬚ correspond to the Ž101. plane of anatase and Ž110. plane of rutile, respectively. It can be seen that for all copper modified TiO 2 samples, only the characteristic peaks corresponding to P25 were found Žwhich consisted of 79% anatase and 21% rutile.. No other peaks from any copper species could be detected even for the heaviest copper-doped TiO 2 sample. In addition, XRD did not reveal any formation of new crystalline species during catalyst preparation. Based on the results obtained from XRD patterns, scanning electron micro-
scopy and EDX studies, we conclude that a homogeneous surface dispersion of Cu was achieved for all doped samples. As demonstrated by Wang and Wan Ž1994. who performed a systematic study of the product formation during the photocatalytic reduction of aqueous Cu2q ions in the presence of methanol and TiO 2 , it was proved that copper metal was not present but rather copper deposited on the surface of TiO 2 as Cu 2 O. Although thermodynamically copper metal could feasibly be formed, the freshly formed copper metal could be readily oxidized and converted into Cu 2 O according to Wang and Wan Ž1994.. It is possible that Cu 2 O was first found during the initial stage of our catalyst preparation. However, as the doped catalysts were heated to 110⬚C in air, it is also likely that the Cu 2 O was converted to CuO. To confirm this, the electron paramagnetic resonance spectra of dried samples of copper deposited TiO 2 were also recorded ŽFig. 5. and used to identify the form of copper oxide present on the doped TiO 2 . Typical EPR parameters g < < and g H of CuO on TiO 2 Žanatase, rutile and amorphous. were found to vary between 2.20 and 2.46 and 2.04 and 2.11, respectively ŽCarrington and McLachlan, 1979; Komova et al.,
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Fig. 4. XRD diffraction patterns of Ža. P25, Žb. 0.05 at.% CurTiO 2 , Žc. 0.10 at.% CurTiO 2 , Žd. 0.25 at.% CurTiO 2 , Že. 0.50 at.% CurTiO 2 , Žf. 1.0 at.% CurTiO 2 , Žg. 2.0 at.% CurTiO 2 , Žh. 5.0 at.% CurTiO 2 and Ži. 10.0 at.% CurTiO 2 .
1994; Cordoba et al., 1998; Pruvost et al., 1999.. It was found that the EPR parameters obtained in our work agree very well with the values from the literature and
the signal increased as the CuO content was increased. It is concluded that CuO is the major constituent present on the TiO 2 surface.
Fig. 5. Electron paramagnetic resonance spectra recorded at 298 K of CurTiO 2 sample at three different Cu concentrations. Ža. 0.05 at.% Cu, Žb. 0.50 at.% Cu and Žc. 5.0 at.% Cu.
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Fig. 6. DRS patterns of Ža. P25, Žb. 0.05 at.% CurTiO 2 , Žc. 0.10 at.% CurTiO 2 , Žd. 0.25 at.% CurTiO 2 , Že. 0.50 at.% CurTiO 2 , Žf. 1.0 at.% CurTiO 2 , Žg. 2.0 at.% CurTiO 2 , Žh. 5.0 at.% CurTiO 2 and Ži. 10.0 at.% CurTiO 2 .
Further evidence was also shown in the work of Herrmann et al. Ž1986.. According to their study, an aqueous suspension of TiO 2 containing Cu2q ions under UV irradiation converted the colour of TiO 2 particles from white to a purple grey. As soon as the photoreactor was opened to air, however, the photocatalyst was restored to its original white colour. This observation signifies that Cu2q ions are photocatalytically reduced to Cuq ions which re-oxidize spontaneously when exposed to oxygen. In their experiment metallic copper could not be obtained even by using acetic acid as the hole scavenger in the solution or by performing the reaction under reducing atmosphere. Our present results are also consistent with the work of Jacobs et al. Ž1989., who found that at the initial stage of photodeposition of copper from its salts solution, nanosized Ž- 3 nm. Cu 2 O particles are formed and copper metal could only be observed after relatively long illumination and intense irradiation.
3.4. Diffuse reflectance spectroscopic analysis The DRS patterns of P25 TiO 2 and all CuO-loaded samples are shown in Fig. 6. The bulk phase bandgap for anatase is approximately 3.2 eV which corresponds to the absorption of wavelength - 400 nm. Surface modification of TiO 2 with CuO significantly affects the absorption properties as shown in Fig. 6. It can be seen that the diffuse reflectance spectra of the CuO-loaded P25 TiO 2 are different from unmodified P25. It is
noticeable that the absorbance of the CuO-loaded samples increases with increasing copper content. The absorption spectra ŽFig. 7. were obtained by analysing the reflectance measurement with Schuster ᎐ Kubelka᎐Munk emission function, f Ž R 8 ., given by f Ž R 8 . s Ž1 y R 8 . 2r2 R 8 , where R 8 is the reflectance of the sample ŽWendlandt and Hecht, 1966.. Since f Ž R 8 . is proportional to the absorption constant of the material, it is indicative of the absorptivity of the sample at a particular wavelength. For clarity only the selected absorption spectra of P25 and 0.10 at.% Cu, with 10 at.% Cu shown in the inset figure, are displayed. All CuO modified samples show an increase in onset absorption from 395 nm into the visible region 405᎐450 nm with increasing CuO content Žas shown in the inset of Fig. 7. compared with the onset absorption wavelength for P25 TiO 2 . The study by Sakata et al. Ž1998. showed increasing photocatalytic activity under the irradiation of visible light Ž) 460 nm. after copper Ž1 wt.% in terms of CuO. was loaded onto the TiO 2 . Moreover, the modified absorption property was evidenced in the CuOrZnO photocatalyst and the shift in the absorption was interpreted as an indirect charge transition from CuO valence band to ZnO conduction band ŽChiorino et al., 1987.. The diffuse reflectance spectra of copper-loaded TiO 2 also exhibit a progressive increase in absorbance with an increase in copper content and the measured values were always higher than unmodified P25 TiO 2 . In the present study, this phenomenon can be at-
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Fig. 7. Schuster᎐Kubelka᎐Munk plot of P25 TiO 2 and P25 TiO 2 doped with 0.10 at.% Cu. Only the selected range of wavelength 300᎐420 nm is displayed. The inset of the figure shows the onset of absorption of P25 TiO 2 doped with 10 at.% Cu.
tributed to the increased absorption of irradiation by the metal oxide layer, i.e. CuO, on the TiO 2 surface. Many studies in the literature use the DRS technique to prove the increase in absorption in the visible region of 500᎐700 nm due to the increase in concentration of transition metal dopants ŽMoser et al., 1987; Palmisano et al., 1988; Navio et al., 1999.. In accordance with the crystal field approach, this weak absorption band arises from the d᎐d transitions of the transition metal, i.e. the slightly distorted octahedral Cu2q system, which is present on the TiO 2 surface ŽCotton et al., 1987.. Two kinds of d orbitals, namely the t 2g and e g orbitals, are present in transition metal ions with e g orbitals having higher energy than t 2g orbitals. On irradiation with suitable frequency of light, the d-electron occupying the t 2g orbital could be excited to the e g orbitals. Absorption results from this process could be observed in the visible spectrum.
3.5. Photocatalytic oxidation of cyanide 3.5.1. Reproducibility of results The photocatalytic activities of the copper loaded P25 TiO 2 catalysts were evaluated by measuring their ability to degrade cyanide. The initial pH of the system was adjusted to 12.0 to prevent the volatilization of
cyanide as toxic hydrogen cyanide. In all cases, the pH decreased from 12.0 to 11.7 after 3 h of UV illumination. By monitoring the cyanide concentration in the reactor and the alkali trap, it was confirmed that an insignificant amount of cyanide was lost due to the drop in pH. To estimate the data error and reproducibility, each experimental run was repeated three times under identical conditions and the agreement of the cyanide degradation rates was found to be within "3%.
3.5.2. Photocatalytic degradation of cyanide in the presence of Cu 2 q ions After mixing TiO 2 in the cyanide solution, no adsorption of cyanide onto or complexation formation with the TiO 2 surface was found as no change in concentration of CNy ions was detected in this period. According to Vrachnou et al. Ž1989., at pH) 7 no complex formation has been found and the adsorption of cyanide is insignificant. This is due to the fact that at high pH the surface of TiO 2 is negatively charged and the formation of surface-cyanide complexes is inhibited. To investigate the role of CuO on P25 TiO 2 in cyanide degradation, control experiments were carried out by irradiating the unmodified P25 catalysts in solutions containing different concentrations of aqueous
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Fig. 8. The influence of dissolved Cu2q ions on the photocatalytic degradation rate of cyanide by P25 TiO 2 . Initial cyanide concentration of 100 mgrl, 1 grl catalyst and initial pH at 12.
Cu2q ions in the form of CuŽNO 3 . 2 . The concentration of Cu2q ions was varied from 0.002 to 0.5 mM so that the copper content in solution was identical to that of the doped TiO 2 catalysts. The oxidation of CNy follows zero order kinetics and the CNy concentration decreased linearly with time over 90% of the reaction. The linear cyanide destruction rate was plotted as a function of Cu2q ions concentration and it can be seen that the presence of dissolved Cu2q ions significantly hindered the cyanide degradation rate as illustrated in Fig. 8. The initial and final concentrations of Cu2q ions in the solution were also determined by AAS analysis and it was found that in all cases the total concentration of Cu2q in the solution, including complexed and un-complexed Cu2q ions, remained constant throughout the experiment. This proves that the copper in solution was not reduced and deposited onto the surface of TiO 2 at the end of all runs. It has been shown in other studies that in the same cases the presence of small amounts of Cu2q ions in fact enhances the photocatalytic oxidation of acetic acid ŽBideau et al., 1991., propionic acid ŽBideau et al., 1992., formic acid ŽBideau et al., 1990. and toluene ŽBulter and Davis, 1993.. The observed acceleration in the reaction rate in these studies was explained in terms of the effects of Cu2qrCuq redox couple which inhibits the electron-hole recombination and the inner sphere mechanism of Cu 2q ions with the organic compounds, forming organo-metallic intermediates. However, in another study by Brezova et al. Ž1995., it was
found that the presence of aqueous Cu2q ions Ž0.28᎐1.1 mM. significantly retarded the degradation of phenol. The inhibitory effect of Cu2q ions in phenol degradation was interpreted as the reduction of Cu 2q ions from the solution and the subsequent deposition of Cu0 and Cu 2 O on the surface of TiO 2 . However, in our study the decrease in the cyanide oxidation rate could not be interpreted as the hindrance effect by the deposition of copper species on the TiO 2 surface. In the presence of free CNy all copper should exist as CuŽI. ᎐CNy complexes in different speciations depending on the CNy:Cu ratio ŽSharpe, 1976; Cotton et al., 1987; Gurol and Holden, 1988; Vrachnou et al., 1989.. This is the reason why there was no CuŽOH. 2 precipitate formation even at alkaline pH in the experiments. In our system, the CNy:Cu ratio remained much higher than four throughout the experiment and it is therefore likely that the predominant species was CuŽCN. 32y. All Cu᎐CNy complexes display weak absorption above 250 nm with peak maxima at 238᎐250 nm ŽSharpe, 1976. which is well below the operating wavelength of 350᎐385 nm for TiO 2 photooxidation. Therefore the presence of these CuŽI. ᎐CNy complexes would not decrease the transmission of UV light to the reactor during the photooxidation. The negative effect of CNy photooxidation due to the presence of CuŽI. ᎐CNy species can be explained via the competition reaction for hydroxyl radicals produced from the TiO 2 surface under UV illumination. Studies from pulse radiolysis have shown that the reac-
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Fig. 9. Cyanide destruction plotted against irradiation time for Ža. P25 and Žb. 0.10 at.% CurTiO 2 . Initial cyanide concentration of 100 mgrl, 1 grl catalyst and initial pH at 12.
tion rate constants between CuŽI. complexes and OH ⭈ radicals ŽSukhov et al., 1986; Goldstein et al., 1992. have the same magnitude as the one between CNy and OH ⭈ radical ŽBehar, 1974; Buchler et al., 1976; Bielski and Allen, 1977.. As a result of the competition reaction from CuŽI., CNy was oxidized less efficiently.
3.5.3. Photocatalytic degradation of cyanide using copper doped TiO2 At the beginning of the experiment the CuO doped TiO 2 catalyst was suspended for 15 min in the test solution. During this period, up to 30% of copper was found to dissolve into the CNy solution, decreasing its initial concentration from 3.85 to 3.45 mM, even before the UV light was switched on. The concentration of dissolved copper did not change after the mixing in the dark indicating that the dissolution of copper from the doped photocatalyst was completed. The photocatalytic effect was measured after the light was switched on. The photocatalytic degradation of cyanide using unmodified P25 TiO 2 and the P25 TiO 2 loaded with 0.10 at.% Cu is shown in Fig. 9. The plot shows that loading 0.10 at.% Cu of copper oxide onto the surface of TiO 2 enhances slightly the photodegradation of cyanide. The slightly higher photooxidation rate of CNy ions from the solution found in the 0.10 at.% Cu-loaded TiO 2 catalyst is attributed to the higher photoactivity of the CuO loaded on the TiO 2 surface. Similar to many other studies on transition metal ions doped TiO 2 systems, there exists an optimum concentration of do-
pant beyond which the observed photocatalytic activity decreases. From this study, the optimum dopant level was found to be 0.10 at.% Cu and only at this copper oxide level the cyanide photooxidation rate was slightly faster than that of the undoped P25 TiO 2 . Further increase in the copper oxide loading beyond this level had a detrimental effect on the cyanide destruction rate ŽFig. 10.. In several studies on photocatalytic oxidation of cyanide solution, cyanate was the main product of the oxidation reaction ŽAugugliaro et al., 1997; Frank and Brad, 1977; Domenech and Peral, 1988; Pollema et al., 1992.. Fig. 11 shows the concentration profiles of cyanide and cyanate at different irradiation times using TiO 2 doped with 0.10 at.% Cu. Similar results were obtained from experiments using TiO 2 doped with different Cu dopant concentrations except for the time required for completing the cyanide oxidation. In all cases, cyanate was the main product formed from the photooxidation of cyanide similar to the results obtained from the experiment using the undoped P25. It can be seen from Fig. 11 that during the oxidation of cyanide, the concentration of cyanate increased as the reaction proceeded. The carbon balance was close to 100% throughout the experiment. After 6 h, over 99% of the cyanide ions were oxidized and converted into less toxic cyanate ions wEqs. Ž8. ᎐ Ž10.x. No cyanate oxidation was detected during the experimental run which is consistent with other works ŽAugugliaro et al., 1997, 1999; Frank and Brad, 1977; Pollema et al., 1992..
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Fig. 10. The dependence of initial cyanide degradation rate on the Cu dopant level in the irradiated TiO 2 system. Initial cyanide concentration of 100 mgrl, 1 grl catalyst, and initial pH at 12.
Many reported results show that the rate of reaction of CNy with OH ⭈ radicals is two orders of magnitude higher than the reaction between CNOy and OH ⭈ radicals ŽBehar, 1974; Buchler et al., 1976; Bielski and Allen, 1977; Leopold and Faraggi, 1977.. Therefore in heterogeneous photocatalysis, cyanide would be oxi-
dized preferentially by the surface adsorbed hydroxyl group. The band edge potentials Žmeasured vs. NHE. for the valence and conduction band of anatase are roughly q3.2 V and y0.2 V ŽFox and Dulay, 1993.. Considering the potentials of Cu2qrCu0 and Cu2qrCuq lie
Fig. 11. Concentration profile of cyanide and cyanate plotted against irradiation time. ŽB. CNy concentration; Ž䢇. CNOy concentration; Ž'. Total carbon concentration CNyq CNOy. Initial cyanide concentration of 3.85 mM, 1 grl catalyst, and initial pH at 12.
K. Chiang et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 471᎐485
below the conduction band edge of TiO 2 , i.e. q0.34 V and q0.17 V respectively, the electron could be trapped by CuII O present on the TiO 2 surface. Choi et al. Ž1994. suggested that a closed shell configuration of the dopant generally is unfavourable for the electron or hole trapping. However, since a Cu2q ion has an unfilled 3d shell Žt 62g e g3 configuration. and the reduction of Cu2q is thermodynamically feasible, it is valid to assume that electron can be trapped by Cu II O on the surface of TiO 2 . As a result of the electron trapping by CuII O, the rate of the electron-hole recombination reaction is slowed down and more holes are available for the redox reactions. The possible charge transfer reactions can be represented by ⭈. Ž IV Ti IVOH q hq vbª Ti OH
q
I CuII O q ey cbª Cu O
Ž 12. Ž 13.
The role of CuII O is therefore to act simply as an electron trap and the electron has to be consumed in someway otherwise there will be an accumulation of charge on the surface. One pathway which accounts for the disappearance of a trapped electron is by the recombination reaction with the photogenerated hole from the TiO 2 . Trapped electrons could also be consumed via the reduction of adsorbed oxygen molecules ŽOkamoto et al., 1985; Gerischer and Heller, 1991. which is believed to be one of the rate determining steps in TiO 2-based photocatalytic reactions. Since the solution in our system is continuously saturated with air, the majority of electrons trapped on the surface are dissipated through the oxygen reduction to form super oxide radical wEq. Ž4.x. Foster et al. Ž1993. also reported that if oxygen is present in the system, the Cuq could be oxidized back to Cu2q. As a result of the ey
O2
Cu II O ª Cu I O ª Cu II O sequential reactions, the electron hole recombination rate could be reduced. At high loading of copper oxide Ž) 0.10 at.% Cu., since the copper oxide was confined on the surface of TiO 2 , there is a high possibility for the trapped electrons to recombine with the holes. The oxidation of CuI O to CuII O by the photogenerated holes is expected to be faster than the oxidation of CNy ion, since the former is simply a direct transfer of the trapped electron from CuI O to the valence band hole. In that case the Cu I O might act as a recombination centre and promote the recombination reaction as shown in the following reaction, q
Ž Ti IVOH ⭈ . q Cu I O ª Ti IVOH q CuII O
Ž 14.
Okamoto et al. Ž1985. found that the copper deposits ŽCu0 and Cu I O. on the surface of anatase could be oxidized by the photogenerated holes during phenol
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degradation. Other studies carried out by Hirano et al. Ž1992, 1997. using a mixed suspension of copper metal powder and TiO 2 found that the copper metal could be oxidized by the holes forming Cu 2 O and Cu2q ions in the solution. Therefore the reaction described by Eq. Ž14. is likely to happen in the CuO-doped TiO 2 system. As shown in Figs. 5 and 6, no direct correlation could be found between the absorbance of copper oxide loaded TiO 2 and cyanide photooxidation rate. Undoubtedly, the light harvesting properties of the photocatalyst is a crucial parameter to control the overall efficiency of the photocatalytic reaction. Although the absorbance of the Cu doped TiO 2 catalysts at the visible region progressively increased as Cu dopant level increased, cyanide removal rate was significantly retarded at high Cu dopant levels. The detrimental effect observed as the copper loading increased beyond 0.10 at.% could also be explained by the coverage of the CuO on the TiO 2 . This would block the TiO 2 from absorbing the incoming photons. The CuO present on the surface could also bind with the hydroxyl groups and this would reduce the hydroxylated surface of TiO 2 , which is the main source of OH ⭈ radicals. Consequently, a decrease in the photocatalytic activity was observed. These negative effects are responsible for the low enhancement Ž13%. observed for the CuO-loaded TiO 2 systems.
4. Conclusion The presence of Cu2q ions Ž0.002᎐0.5 mM. in the CNyrTiO 2rUV system significantly retarded the photooxidation of cyanide. The decrease in the cyanide oxidation rate could be explained in terms of the competition reaction of copperŽI. cyanide complexes for the photogenerated hydroxyl group. It has been demonstrated that copper could be loaded onto the surface of TiO 2 by the photodeposition method. The concentration of dopant was varied from 0.05 to 10 at.% Cu. This method of deposition results in nanosized copper particles dispersed uniformly on the TiO 2 surface as indicated by the SEM-EDX study. The EPR results also indicated that CuO was present in the doped TiO 2 samples. There was only a slight increase in the cyanide oxidation rate at a copper dopant concentration of 0.10 at.% Cu which reflects the possibility of electron trapping by CuO dopant present on the surface of TiO 2 . Further increase in the CuO dopant content beyond 0.10 at.% Cu considerably reduced the photocatalytic activity of TiO 2 . Firstly, the well dispersed nanosized CuO dopant would cover the surface of TiO 2 leading to a drastic decrease in photon absorption. Secondly, the high concentration of dopant promotes the recombination of photogenerated holes with
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the trapped electrons resulting in a decrease in availability of holes for redox reactions. For both the undoped and CuO doped TiO 2 , cyanate is the main product of the photooxidation of cyanide. References Augugliaro, V., Loddo, V., Marci, G., Palmisano, L., LopezMunoz, M.J., 1997. Photocatalytic oxidation of cyanides in aqueous titanium dioxide suspensions. J. Catal. 166, 272᎐283. Augugliaro, V., Lopez, E.G., Loddo, V. et al., 1999. Photodegradation of free and complex cyanides in irradiated homogeneous and heterogeneous systems. Fres. Environ. Bull. 8, 350᎐357. Behar, D., 1974. Pulse radiolysis study of aqueous hydrogen cyanide and cyanide solutions. J. Phys. Chem. 78 Ž6., 2660᎐2663. Bideau, M., Claudel, B., Faure, L., Rachimoellah, M., 1990. Photo-oxidation of formic acid by oxygen in the presence of titanium dioxide and dissolved copper ions: oxygen transfer and reaction kinetics. Chem. Eng. Commun. 93, 167᎐179. Bideau, M., Claudel, B., Faure, L., Kazouan, H., 1991. The photo oxidation of acetic acid by oxygen in the presence of titanium dioxide and dissolved copper ions. J. Photochem. Photobiol. A: Chem. 61, 269᎐280. Bideau, M., Claudel, B., Faure, L., Kazouan, H., 1992. The photo oxidation of propionic acid by oxygen in the presence of TiO 2 and dissolved copper ions. J. Photochem. Photobiol. A: Chem. 67, 337᎐348. Bielski, B.H.J., Allen, A.O., 1977. Radiation chemistry of aqueous cyanide ion. J. Am. Chem. Soc. 99 Ž18., 5931᎐5934. Brezova, V., Blazkova, A., Borosova, E., Ceppan, M., Fiala, R., 1995. The influence of dissolved metal ions on the photocatalytic degradation of phenol in aqueous TiO 2 suspensions. J. Mol. Catal. A 98, 109᎐116. Buchler, H., Buhler, R.E., Copper, R., 1976. Pulse radiolysis of aqueous cyanide solutions. Kinetics of the transient OH and H adducts and subsequent rearrangement. J. Phys. Chem. 80 Ž14., 1549᎐1553. Bulter, E.C., Davis, A.P., 1993. Photocatalytic oxidation in aqueous titanium dioxide suspensions: the influence of dissolved transition metals. J. Photochem. Photobiol. A: Chem. 70, 273᎐283. Carrington, A., McLachlan, A.D., 1979. Introduction to Magnetic Resonance. Wiley, New York. Chiorino, A., Boccuzzi, F., Ghiotti, G., 1987. Optical studies of phase interaction in CurZnO catalysts. Surf. Sci. 189r190, 894᎐902. Choi, W., Termin, A., Hoffmann, M.R., 1994. The role of metal ion dopants in quantum sized TiO 2 : correlation between photoactivity and charge carrier recombination dynamics. J. Phys. Chem. 98, 13669᎐13679. Cordoba, G., Viniegra, M., Fierro, J.L.G., Padilla, J., Arroyo, R., 1998. TPR, ESR, and XPS study of Cu2q ions in sol gel derived TiO 2 . J. Solid State Chem. 138, 1᎐6. Cotton, F.A., Wilkinson, G., Gaus, P.L., 1987. Basic Inorganic Chemistry, second ed Wiley, New York, p. 429. Cunningham, J., Hickey, J.N., Brown, N.M.D., Meenan, B., 1993. Catalytic and physicochemical characterization of novel oxide supported copper catalysts, Part I: hydrosol
prepared CurTiO 2 and effects of pre-reduction on hydrogenation and oligomerization of acetone. J. Mater. Chem. 3 Ž7., 743᎐750. Domenech, J., Peral, J., 1988. Removal of toxic cyanide from water by heterogeneous photocatalytic oxidation over ZnO. Solar Energy 41, 55᎐59. Foster, N.S., Noble, R., Koval, C.A., 1993. Reversible photoreductive deposition and oxidative dissolution of copper ions in titanium dioxide aqueous suspensions. Environ. Sci. Technol. 27, 350᎐356. Fox, M.A., Dulay, M.T., 1993. Heterogeneous photocatalysis. Chem. Rev. 93, 341᎐357. Frank, S.N., Brad, A.J., 1977. Heterogeneous photocatalytic oxidation of cyanide ion in aqueous solutions at TiO 2 powder. J. Am. Chem. Soc. 99 Ž1., 303᎐304. Fujitsu, S., Hamada, T., 1994. Electrical properties of manganese doped titanium dioxide. J. Am. Ceram. Soc. 77 Ž12., 3281᎐3283. Gerischer, H., Heller, A., 1991. The role of oxygen in photooxidation of organic molecules on semiconductor particles. J. Phys. Chem. 95, 5261᎐5267. Goldstein, S., Czapski, G., Cohen, H., Meyerstein, D., 1992. Deamination of -alaine induced by hydroxyl radicals and monovalent copper ions. A pulse radiolysis study, Inorg. Chim. Acta 192, 87᎐93. Guilloton, M., Karst, F., 1985. A spectrophotometric determination of cyanate using reaction with 2-aminobenzoic acid. Anal. Biochem. 149, 291᎐295. Gurol, M.D., Holden, T.E., 1988. The effect of copper and iron complexation on removal of cyanide by ozone. Ind. Eng. Chem. Res. 27, 1157᎐1162. Hagfeldt, A., Gratzel, M., 1995. Light induced redox reactions in nanocrystalline systems. Chem. Rev. 95, 49᎐68. Herrmann, J.M., Disdier, J., Pichat, P., 1986. Photo-assisted platinum deposition on TiO 2 powder using various platinum complexes. J. Phys. Chem. 90, 6028᎐6034. Hirano, K., Inoue, K., Yatsu, T., 1992. Photocatalysed reduction of CO 2 in aqueous TiO 2 suspension mixed with copper powder. J. Photochem. Photobiol. A: Chem. 64, 255᎐258. Hirano, K., Asayama, H., Hoshino, A., Wakatsuki, H., 1997. Metal powder addition effect on the photocatalytic reactions and the photogenerated electric charge collected at an inert electrode in aqueous TiO 2 suspensions. J. Photochem. Photobiol. A: Chem. 110, 307᎐311. Hoffmann, M.R., Martin, S.T., Choi, W., Bahnemann, D.W., 1995. Environmental applications of semiconductor photocatalysis. Chem. Rev. 95, 69᎐96. Howe, R.F., 1998. Recent development in photocatalysis. Dev. Chem. Eng. Miner. Process. 6 Ž1r2., 55᎐84. Jacobs, J.W., Kampers, F.W., Rikken, J.W.G., Bulle-Lieuwma, C.W.T., Koningsberger, D.C., 1989. Copper photodeposition on TiO 2 studied with HREM and EXAFS. J. Electrochem. Soc. 136 Ž10., 2914᎐2923. Komova, O.V., Tsykoza, L.T., Simakov, A.V. et al., 1994. Effect of parent titanium oxide on the physico-chemical properties of Cu᎐Ti oxide catalysts. React. Kinet. Catal. Lett. 52 Ž1., 129᎐137. Leopold, J.G., Faraggi, M., 1977. Pulse radiolysis on the cyanide anion in aqueous solution. J. Phys. Chem. 81 Ž8., 803᎐806.
K. Chiang et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 471᎐485 Mills, A., Le Hunte, S., 1997. An overview of semiconductor photocatalyst. J. Photochem. Photobiol. A: Chem. 108, 1᎐35. Moser, J., Gratzel, M., Gallay, R., 1987. Inhibition of electron hole recombination in substantially doped colloidal semiconductor crystallites. Helvetica Chim. Acta 70, 1596᎐1604. Navio, J.A., Colon, G., Macias, M., Real, C., Litter, M.I., 1999. Iron doped titania semiconductor powders prepared by sol gel method. Part I: synthesis and characterization. Appl. Catal. A: Gen. 177, 111᎐120. Okamoto, K., Yamamoto, Y., Tanaka, H., Tanaka, M., Itaya, A., 1985. Heterogeneous photocatalytic decomposition of phenol over TiO 2 powder. Bull. Chem. Soc. Jpn. 58, 2015᎐2022. Palmisano, L., Augugliaro, V., Sclafani, A., Schiavello, M., 1988. Activity of chromium ion doped titania for the dinitrogen photoreduction to ammonia and for the phenol photodegradation. J. Phys. Chem. 92, 6710᎐6713. Pollema, C.H., Hendrix, J.L., Milosavljevic, E.B., Solujic, L., Nelson, J.H., 1992. Photocatalytic oxidation of cyanide to nitrate at TiO 2 particles. J. Photochem. Photobiol. A: Chem. 66, 235᎐244. Pruvost, C., Courcot, D., Abi Aad, E., Zhilinskaya, E.A., Aboukais, A., 1999. Identification of different Cu2q sites in titania supported copper systems by EPR spectroscopy. J. Chim. Phys. 96, 1527᎐1535. Sakata, Y., Yamamoto, T., Okazaki, T., Imamura, H., Tsuchiya, S., 1998. Generation of visible light response on the photo-
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catalyst of a copper ion containing TiO 2 , Chem. Lett. 1253᎐1254. Serpone, N., Pelizzetti, E., 1989. Photocatalysis ᎏ Fundamentals and Application. Wiley Interscience, New York. Sharpe, A.G., 1976. The Chemistry of Cyano Complexes of the Transition Metals. Academic Press, London. Sukhov, N.L., Akinshin, M.A., Ershov, B.G., 1986. Pulse radiolysis of aqueous solutions of monovalent copper. High Energy Chem. 20 Ž5., 303᎐306. Sykora, J., 1997. Photochemistry of copper complexes and their environmental aspects. Coor. Chem. Rev. 159, 95᎐108. Tanaka, K., Harada, K., Murata, S., 1986. Photocatalytic deposition of metal ions onto TiO 2 powder. Solar Energy 36 Ž2., 159᎐161. Vrachnou, E., Gratzel, M., McEnvoy, A.J., 1989. Efficient visible light photoresponse following surface complexation of titanium dioxide with transition metal cyanides. J. Electroanal. Chem. 258, 193᎐205. Wang, Y.Y., Wan, C.C., 1994. Products and nucleation analysis of photoelectrochemical reduction of cupric ions in the CuSO4 ᎐methanol᎐TiO 2 suspension system. J. Photochem. Photobiol. A: Chem. 79, 203᎐211. Wendlandt, W.W., Hecht, H.G., 1966. Reflectance Spectroscopy. Wiley Interscience, New York, p. 60. Wilke, K., Breuer, H.D., 1999. The influence of transition metal doping on the physical and photocatalytic properties of titania. J. Photochem. Photobiol. A: Chem. 121, 49᎐53.
Photocatalytic degradation of cyanide using titanium dioxide modified with copper oxide 夽 K. Chiang, R. AmalU , T. Tran Centre for Particle and Catalyst Technologies, School of Chemical Engineering & Industrial Chemistry, Uni¨ ersity of New South Wales, Sydney NSW 2052, Australia
Abstract Copper ŽII. oxide was loaded onto the surface of Degussa P25 TiO 2 particles by photodeposition. The doped material was subsequently utilized as the photocatalyst for cyanide oxidation. The copper content on the TiO 2 surface was varied from 0.05 to 10.0 at.% of Cu. It was found that nanosized CuO deposits were present on the surface of TiO 2 . Modifying TiO 2 with CuO changed the optical properties of TiO 2 and the onset of absorption was red shifted. The photocatalytic activity of the CuO loaded TiO 2 catalysts was measured to determine their ability to oxidize cyanide. It was found that the rate of photooxidation of cyanide assisted with the doped catalyst was improved slightly at 0.10 at.% Cu. Any further increase of the copper dopant concentration decreased the oxidation rate markedly. The presence of Cu 2q ions Ž0.002᎐0.5 mM. in the solution also decreased the photocatalytic degradation of CNy. The decrease in the activity was explained in terms of the competition reaction of CuŽI. cyanide complex ions for surface hydroxyl radicals. In all cases cyanide was being oxidized to cyanate, the end product of cyanide photooxidation. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Photocatalysis; Cyanide; Photodeposition; Oxidation; Copper-doped titanium dioxide
1. Introduction Cyanide is the universal leaching agent for gold because of its strong complexation ability with many metals. The presence of free and complex cyanides in industrial effluent therefore imposes many environmental problems on the public domain on account of their toxicity even at very low concentrations. Yet there is no satisfactory chemical to replace cyanide in mineral processing. Ions such as iron, zinc, copper and
夽
http:rrwww.ceic.unsw.edu.aurcentersrPartcatr Corresponding author. Tel.: q61-29385-4361; fax: q6129385-5966. E-mail address: [email protected] ŽR. Amal.. U
cobalt present in the ore also form stable complexes with cyanide, which create significant problems in downstream treatment of tailings. Various oxidants can be used to oxidize free cyanide and metal cyanide complexes with different degrees of effectiveness. The most common methods are by oxidation, such as chlorination, the INCO process using sulfur dioxide and oxygen, and the Degussa process using hydrogen peroxide. However, these oxidation processes do suffer from several limitations: Ž1. oxidants such as chlorine can form chloro-cyanogen which is equally toxic; Ž2. oxidants being used in cyanide remediation technologies are generally expensive; and Ž3. thermodynamically stable metal cyanide complexes, such as ferro-cyanide and ferri-cyanide, remain undestroyed after the oxidation.
1093-0191r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 1 . 0 0 0 7 4 - 0
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Over the past several decades, numerous studies revealed that many organics and inorganics can be photocatalytically oxidized into less harmful substances using aqueous suspensions of TiO 2 Žband gap energy of 3.2 eV for anatase . ŽSerpone and Pelizzetti, 1989; Hagfeldt and Gratzel, 1995.. The detailed mechanisms involved in semiconductor photocatalysis can be found in several excellent reviews ŽFox and Dulay, 1993; Hoffmann et al., 1995; Mills and Le Hunte, 1997; Howe, 1998.. To become active the photocatalyst must be first excited by energetic photons Ž - 385 nm., which induces the transition of electrons from the valence band to the conduction band of the semiconductor and generation of electronrhole pair for subsequent redox reactions ŽHoffmann et al., 1995.. Photo generation of electron-hole pair: q TiO 2 q h ª ey cbq h vb
Ž1.
Trapping of photo generated electron and hole: III Ti IVOH q ey cbª Ti OH
⭈. Ž IV Ti IVOH q hq vbª Ti OH
Ž2. q
Ž3.
Interfacial electron and hole trapping by adsorbed species: Ti III OH q O 2 ª Ti IVOH q O⭈2y
Ž4.
q Ž Ti IVOH ⭈ . q Redª Ti IVOH q Red⭈q
Ž5.
The adsorbed surface hydroxyl group on the semiconductor surface traps the hole to form the hydroxyl radical, which is a very strong oxidant and capable of oxidizing many organics and inorganics present in the wastewater. The electron reduces surface adsorbed species such as oxygen and could form superoxide radicals. However, the present drawbacks for TiO 2 photocatalysis in the treatment wastewater include: Ž1. the difficulty of separating the fine semiconductor photocatalyst from treated water; Ž2. the rapid unfavourable charge carrier recombination reaction in TiO 2 compared with the redox reactions resulting in low quantum yield; and Ž3. the high band gap energy which limits its application from using solar energy. Many transition metal ion dopants have been demonstrated to influence the rate of TiO 2 photocatalytic oxidation to different extents by changing the dynamics of electron-hole recombination and interfacial charge transfer. The observed change in reaction rates has been well documented and explained in terms of elec-
tron and hole trapping by metal ions, M nq, within the semiconductor photocatalyst ŽChoi et al., 1994.. Ž ny1.q M nqq ey cbª M
Ž6.
Ž nq1.q M nqq hq cbª M
Ž7.
The transition metal ion dopants most frequently studied and reported in literature include Zn2q, Ni 2q, Mn 2q, Co 2q, Cu2q, Cr 3q, Fe 3q, V 4q, Mo 5q, Pb 2q and Tlq ŽTanaka et al., 1986; Choi et al., 1994; Fujitsu and Hamada, 1994; Cordoba et al., 1998; Wilke and Breuer, 1999.. Reaction 6 is thermodynamically feasible if the reduction potential of M nq is more positive than the conduction band edge. However, there is no correlation between the reduction potential of M nq and the photocatalytic activity of the doped MrTiO2 photocatalyst. The mechanism of photocatalytic oxidation of cyanide using TiO 2 has been well documented in the literature ŽAugugliaro et al., 1997; Frank and Brad, 1977; Pollema et al., 1992.. The oxidation of cyanide is possible via the reaction of CNy with surface hydroxyls or holes. According to Domenech and Peral Ž1988. the initial step of cyanide photocatalytic oxidation is the formation of cyanide radical, which subsequently dimerize to form cyanogen. Finally, the cyanogen molecule undergoes dismutation under alkali conditions to give cyanide and cyanate. The cyanate produced is further oxidized to give NOy 3 and CO 2 . The corresponding reactions are presented as follows wEqs. Ž8. ᎐ Ž11.x. hqrO H ⭈ y
CN
›
CN ⭈
2CN ⭈ª Ž CN. 2
Ž8. Ž9.
Ž CN. 2 q 2OHyª CNyq CNOyq H 2 O
Ž 10 .
CNOyq 8OHyq 8 hqª NOy 3 q CO 2 q 4H 2 O
Ž 11.
In this paper, we attempt to examine a number of variables, which affect the photocatalytic activity of copper doped TiO 2 during the oxidation of cyanide. There are two reasons why copper is used as the dopant in the current study. Firstly, the profound influence of Cu2qrCuq couple in aqueous solution has been reported in heterogeneous photocatalysis ŽSykora, 1997.. Secondly, the rate of oxidation of cyanide by ozone, which enters a cyclic oxidation᎐reduction pathway, was found to increase in the presence of copper ions ŽGurol and Holden, 1988..
K. Chiang et al. r Ad¨ ances in En¨ ironmental Research 6 (2002) 471᎐485
2. Experimental 2.1. Chemical reagents All chemicals were analytical grade and used without further purification. The titanium dioxide ŽP25. purchased from Degussa Ž30 nm primary crystal size. has a specific surface area of 51 m2rg, and contains predominantly anatase Ž79% anatase and 21% rutile as determined from X-ray diffraction.. Technical grade sodium cyanide ŽNaCN. was used as the source of cyanide. Copper nitrate, CuŽNO 3 . 2 ⭈ 2.5H 2 O was used as the source of Cu2q ion both for studying the effect of Cu2q ion in cyanide photooxidation and for loading copper onto the surface of TiO 2 . Nitric acid and sodium hydroxide were used to adjust the pH of solution. Ultra pure gas Žeither air or N2 . was used in all experiments. All solutions were prepared using freshly prepared Milli-Q water.
2.2. Apparatus All experiments were carried out using a 1-l flat bed reactor to assess the activity of the developed catalysts for cyanide degradation ŽFig. 1.. The irradiated area of the solution was 15 cm = 30 cm with a depth of 2.2 cm. The top of the reactor is covered with a quartz plate that serves to isolate the system from the surroundings while allowing UV light Ž) 99%. to pass through. The bottom part is made of four gas chambers topped with sintered glass to allow an even distribution of the purging gas used during the experiment. The liquid in the reactor was circulated through a glass coil and mixed well by pumping at a flow rate of 4 lrmin. The temperature of the solution inside the reactor was kept
473
constant by immersing the glass coil in a warm water bath. Purified air or nitrogen was continuously bubbled through which aerated and de-aerated the system and further promoted mixing in the reactor. A 20-W NEC UVA lamp provided radiation in the near visible region with a peak at max s 355 nm. A parabolic reflector was used to reflect irradiation from the UV fluorescent tube to the reactor. The gas leaving the reactor was passed into a solution of 1 M NaOH solution, which served as a hydrogen cyanide trap and therefore prevented the potential release of toxic hydrogen cyanide gas. The pH of the solution was monitored using an Orion pH meter throughout the experiment. The temperature of the solution was also monitored and recorded periodically in all runs.
2.3. Preparation of copper doped catalysts Copper was loaded onto the surface of TiO 2 ŽDegussa P25. by reducing the Cu2q ions photocatalytically using CuŽNO 3 . 2 ⭈ 2.5H 2 O along with sodium formate which acted as a hole scavenger. Firstly, 2 g of TiO 2 was suspended in 500 ml of water and sonicated for 30 min to break up any loosely attached aggregates. The copper dopant levels used in this work were 0.05, 0.10, 0.25. 0.5, 1.0, 2.0, 5.0 and 10.0 at.% of Cu to Ti. Pre-determined amounts of copper nitrate and sodium formate were then added to the suspension and made up to a final volume of 1 l. The photoreduction of aqueous Cu 2q ions was carried out under slightly acidic conditions at pH 3.8 adjusted by the addition of concentrated HNO3. The solution was de-aerated for 15 min before commencing the experiment and nitrogen was continuously purged into the solution at a rate of 500 mlrmin so that no photoreduced copper species
Fig. 1. Set-up of the experiment. Ža. Metal parabolic reflector, Žb. UVA lamp located at focus, Žc. quartz plate cover, Žd. reactor, Že. and Žf. gases inlet and outlet, Žg. pump, and Žh. glass coil inside a water bath.
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could be re-oxidized by dissolved oxygen and returned to the solution phase. The temperature of solution was maintained at 30⬚C by circulating the solution through a coil which was immersed in a water bath. The copper loaded TiO 2 particles were then filtered, dried at 110⬚C for 12 h and stored under vacuum. Unmodified TiO 2 powder was also prepared in this manner except that no Cu2q ions were added. The concentration of Cu2q in the aqueous solution dropped as the experiment proceeded and finally its concentration became undetectable at the end. This indicated that all Cu2q in the solution had been reduced and deposited on the surface of TiO 2 .
2.4. Photoreacti¨ ity It is well known that the p K a of hydrogen cyanide is equal to 9.21 at 25⬚C ŽSharpe, 1976. and therefore it is necessary to work in a strongly alkaline condition in order to avoid the liberation of toxic hydrogen cyanide gas during the experiment. Any hydrogen cyanide formed during the experiment would be trapped by the 1-M NaOH solution. To assess the activity of the copper loaded TiO 2 , 1 g of sample was suspended in 500 ml of Milli-Q water and sonicated for 30 min. A pre-weighed amount of NaOH was first added to the suspension followed by addition of NaCN. The volume was then made up to 1 l by the further addition of water to reach a final pH of 12.0 and cyanide concentration of 100 mgrl Ž3.85 mM.. The resulting suspension was saturated with air at a flow rate of 500 mlrmin for 15 min before the reaction started and kept aerated throughout the experiments. The mixture was irradiated for 3 h to investigate the removal rate of cyanide at a temperature of 30⬚C. In order to study the photooxidation of cyanide and formation of cyanate intermediate, the irradiation period was lengthened until the concentration of cyanide fell to below the undetectable level. Samples were withdrawn at regular intervals, filtered through a 0.45-m syringe filter and finally analysed for free cyanide and cyanate.
2.5. Characterization techniques A Hitachi 4500II field emission scanning electron microscope ŽFE-SEM. was used for microstructural analysis on the surface of TiO 2 . In addition, an Oxford Isis energy dispersive X-ray ŽEDX. analyser was interfaced to the column to perform semi-quantitative chemical analysis and to obtain information on the copper distribution on the TiO 2 surface. The crystal sizes of the samples were also determined using a Philips CM200 FEGTEM. The specific surface areas of all the samples were measured using the single point dynamic surface area analyser, which allowed the accu-
rate determination of surface area from 0.5 to 1000 m2rg. The crystalline phase was determined using powder X-ray diffraction ŽXRD.. The XRD patterns were obtained at room temperature with a Siemens Diffraktometer D5000 using Ni-filtered CuK ␣ radiation. A Cary 5 UVrvis-NIR spectrophotometer equipped with an integrated sphere was used to record the diffuse reflectance spectra ŽDRS. and absorbance data of the solution samples. The base line correction was performed using a calibrated reference sample of barium sulfate. The reflectance spectra of the copper loaded samples were analysed under ambient conditions in the wavelength range of 250᎐700 nm. The electron paramagnetic resonance ŽEPR. spectra of the copper doped TiO 2 samples were recorded by using Bruker EMX X-band Žwith helium cryostat. at 25⬚C. The g-values of the copper doped TiO 2 samples were obtained from a software developed by Bruker using the measurement of the magnetic field and the microwave frequency. The concentration of Cu2q ions in the aqueous solution was measured by atomic absorption spectroscopy ŽAAS. at 324 nm using a Varian SpectrAA-20 spectrophotometer. The concentration of free cyanide was determined by potentiometric titration using silver nitrate standard. An automated system consisting of a Metrohm 665 Dosimat, a silver Titrode in conjunction with a 682 Titroprocessor was used to detect the end point of titration. Samples were first filtered through a 0.45-m syringe filter and 1-ml aliquots were titrated against standardized AgNO3 solution. This method is suitable to analyse cyanide concentrations between 1 and 1000 mgrl CNy with an accuracy of "1 mgrl. The concentration of cyanate was determined spectrophotometrically employing a modified method of Guilloton and Karst Ž1985. based on the reaction between cyanate ions and 2-aminobenzoic acid under buffered conditions. The concentration of cyanate could be precisely determined independently of the pH of the system, the presence of Cu 2q ions and cyanide ions, by measuring the absorbance of the complex formed at 310 nm. It was found that this analysis was most accurate at cyanate concentrations from 0.5 to 80.0 mgrl.
3. Results and discussion 3.1. Electron microscopy and energy dispersi¨ e X-ray spectroscopy studies The surface morphology of all copper loaded TiO 2 samples was studied using scanning and transmission electron microscopy. The colour of the TiO 2 particles changed from white to greyish as the concentration of
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Fig. 2. FESEM micrographs of Ža. pure undoped P25, and Žb. P25 doped with 10 at.% Cu.
copper increased. Selected SEM images of pure unmodified P25 TiO 2 and P25 TiO 2 loaded with 10 at.% Cu are shown in Fig. 2a,b, respectively, for comparison. It can be seen from the SEM images that the shape and crystal size of all copper loaded samples resembled that of unmodified P25 TiO 2 . The average primary crystal sizes of the samples loaded with various copper dopant levels were determined to be 30 nm from SEM and TEM studies ŽTable 1., which are comparable with those found by Wang and Wan Ž1994.. In their nucleation study of photoelectrochemical reduction of aqueous Cu 2q ions, copper nuclei were rarely found on the TiO 2 particles. From a study by Cunningham et al. Ž1993. who used copper malonate as the copper precursor, the formation of aggregates consisting of smaller copper metal islands Žapprox. 10 nm. on the surface of 13% CurTiO 2 samples was observed. In the present study, no similar discrete deposits could be observed for our copper doped TiO 2 samples. However, from the qualitative information obtained from the EDX analysis on different particles of the same sample or in different
locations on the same particle, copper was found to exist on the surface of all loaded TiO 2 samples. As shown in Fig. 3, by scanning the doped catalyst surface utilizing the EDX dot mapping technique, it was unambiguously revealed that even at a very low copper dopant concentration Ž0.05 at.% Cu., the copper had covered the surface of the catalyst uniformly. As the copper dopant concentration increased, the copper dopant became more and more densely packed and finally covered the surface entirely at ) 5 at.% Cu.
3.2. Specific surface area of copper-loaded TiO2 Measurements of specific surface area of all copperloaded TiO 2 samples are listed in Table 1. Considering P25 TiO 2 , a non-porous substance having a density of 3.96 grcm3, the theoretical surface area was calculated to be 50.5 m2rg which agrees very well with the experimentally obtained value of 51 m2rg. As it can be seen in Table 1, loading copper within the range being studied onto the surface of P25 by the photo-reductive
Table 1 Physical properties of doped TiO 2 samples: atomic percentage of Cu relative to Ti, surface concentration of copper on TiO 2 , crystallite sizes as observed under SEM and TEM, and specific surface area Sample ID
Copper Žat.%.
Loading ŽCurnm2 .
Crystallite size obtained from TEM Žnm.
BET surface area Žm2 rg.
P25 CuP005 CuP010 CuP025 CuP050 CuP100 CuP200 CuP500 CuP1000
᎐ 0.05 0.10 0.25 0.50 1.00 2.00 5.00 10.0
0 0.07 0.15 0.37 0.74 1.49 2.97 7.43 14.87
30᎐40 30᎐40 30᎐40 30᎐40 30᎐40 30᎐40 30᎐40 30᎐40 30᎐40
51 52 51 52 51 51 51 51 50
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Fig. 3. FESEM micrographs of P25 doped with 0.05 at.% Cu. Ža. Image observed under SEM. Žb. SEM-EDX dot mapping of Ti Žwhite dot. on the catalyst surface as shown in Ža.. Žc. SEM-EDX dot mapping of Cu on the catalyst surface as shown in Ža..
route did not significantly change the surface area and therefore the role of copper is unbiased by the difference in surface area. Based on the surface area of P25 TiO 2 Ž51 m2rg. used in this work, the concentrations of copper could also be represented as the number of Cu atomsrnm2 of TiO 2 support as included in Table 1.
3.3. XRD studies The XRD diffraction patterns of the copper-loaded P25 TiO 2 are shown in Fig. 4. The diffraction pattern of undoped P25 is also included in the figure for comparison. The angles 2 s 25.28⬚ and 2 s 27.42⬚ correspond to the Ž101. plane of anatase and Ž110. plane of rutile, respectively. It can be seen that for all copper modified TiO 2 samples, only the characteristic peaks corresponding to P25 were found Žwhich consisted of 79% anatase and 21% rutile.. No other peaks from any copper species could be detected even for the heaviest copper-doped TiO 2 sample. In addition, XRD did not reveal any formation of new crystalline species during catalyst preparation. Based on the results obtained from XRD patterns, scanning electron micro-
scopy and EDX studies, we conclude that a homogeneous surface dispersion of Cu was achieved for all doped samples. As demonstrated by Wang and Wan Ž1994. who performed a systematic study of the product formation during the photocatalytic reduction of aqueous Cu2q ions in the presence of methanol and TiO 2 , it was proved that copper metal was not present but rather copper deposited on the surface of TiO 2 as Cu 2 O. Although thermodynamically copper metal could feasibly be formed, the freshly formed copper metal could be readily oxidized and converted into Cu 2 O according to Wang and Wan Ž1994.. It is possible that Cu 2 O was first found during the initial stage of our catalyst preparation. However, as the doped catalysts were heated to 110⬚C in air, it is also likely that the Cu 2 O was converted to CuO. To confirm this, the electron paramagnetic resonance spectra of dried samples of copper deposited TiO 2 were also recorded ŽFig. 5. and used to identify the form of copper oxide present on the doped TiO 2 . Typical EPR parameters g < < and g H of CuO on TiO 2 Žanatase, rutile and amorphous. were found to vary between 2.20 and 2.46 and 2.04 and 2.11, respectively ŽCarrington and McLachlan, 1979; Komova et al.,
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Fig. 4. XRD diffraction patterns of Ža. P25, Žb. 0.05 at.% CurTiO 2 , Žc. 0.10 at.% CurTiO 2 , Žd. 0.25 at.% CurTiO 2 , Že. 0.50 at.% CurTiO 2 , Žf. 1.0 at.% CurTiO 2 , Žg. 2.0 at.% CurTiO 2 , Žh. 5.0 at.% CurTiO 2 and Ži. 10.0 at.% CurTiO 2 .
1994; Cordoba et al., 1998; Pruvost et al., 1999.. It was found that the EPR parameters obtained in our work agree very well with the values from the literature and
the signal increased as the CuO content was increased. It is concluded that CuO is the major constituent present on the TiO 2 surface.
Fig. 5. Electron paramagnetic resonance spectra recorded at 298 K of CurTiO 2 sample at three different Cu concentrations. Ža. 0.05 at.% Cu, Žb. 0.50 at.% Cu and Žc. 5.0 at.% Cu.
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Fig. 6. DRS patterns of Ža. P25, Žb. 0.05 at.% CurTiO 2 , Žc. 0.10 at.% CurTiO 2 , Žd. 0.25 at.% CurTiO 2 , Že. 0.50 at.% CurTiO 2 , Žf. 1.0 at.% CurTiO 2 , Žg. 2.0 at.% CurTiO 2 , Žh. 5.0 at.% CurTiO 2 and Ži. 10.0 at.% CurTiO 2 .
Further evidence was also shown in the work of Herrmann et al. Ž1986.. According to their study, an aqueous suspension of TiO 2 containing Cu2q ions under UV irradiation converted the colour of TiO 2 particles from white to a purple grey. As soon as the photoreactor was opened to air, however, the photocatalyst was restored to its original white colour. This observation signifies that Cu2q ions are photocatalytically reduced to Cuq ions which re-oxidize spontaneously when exposed to oxygen. In their experiment metallic copper could not be obtained even by using acetic acid as the hole scavenger in the solution or by performing the reaction under reducing atmosphere. Our present results are also consistent with the work of Jacobs et al. Ž1989., who found that at the initial stage of photodeposition of copper from its salts solution, nanosized Ž- 3 nm. Cu 2 O particles are formed and copper metal could only be observed after relatively long illumination and intense irradiation.
3.4. Diffuse reflectance spectroscopic analysis The DRS patterns of P25 TiO 2 and all CuO-loaded samples are shown in Fig. 6. The bulk phase bandgap for anatase is approximately 3.2 eV which corresponds to the absorption of wavelength - 400 nm. Surface modification of TiO 2 with CuO significantly affects the absorption properties as shown in Fig. 6. It can be seen that the diffuse reflectance spectra of the CuO-loaded P25 TiO 2 are different from unmodified P25. It is
noticeable that the absorbance of the CuO-loaded samples increases with increasing copper content. The absorption spectra ŽFig. 7. were obtained by analysing the reflectance measurement with Schuster ᎐ Kubelka᎐Munk emission function, f Ž R 8 ., given by f Ž R 8 . s Ž1 y R 8 . 2r2 R 8 , where R 8 is the reflectance of the sample ŽWendlandt and Hecht, 1966.. Since f Ž R 8 . is proportional to the absorption constant of the material, it is indicative of the absorptivity of the sample at a particular wavelength. For clarity only the selected absorption spectra of P25 and 0.10 at.% Cu, with 10 at.% Cu shown in the inset figure, are displayed. All CuO modified samples show an increase in onset absorption from 395 nm into the visible region 405᎐450 nm with increasing CuO content Žas shown in the inset of Fig. 7. compared with the onset absorption wavelength for P25 TiO 2 . The study by Sakata et al. Ž1998. showed increasing photocatalytic activity under the irradiation of visible light Ž) 460 nm. after copper Ž1 wt.% in terms of CuO. was loaded onto the TiO 2 . Moreover, the modified absorption property was evidenced in the CuOrZnO photocatalyst and the shift in the absorption was interpreted as an indirect charge transition from CuO valence band to ZnO conduction band ŽChiorino et al., 1987.. The diffuse reflectance spectra of copper-loaded TiO 2 also exhibit a progressive increase in absorbance with an increase in copper content and the measured values were always higher than unmodified P25 TiO 2 . In the present study, this phenomenon can be at-
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Fig. 7. Schuster᎐Kubelka᎐Munk plot of P25 TiO 2 and P25 TiO 2 doped with 0.10 at.% Cu. Only the selected range of wavelength 300᎐420 nm is displayed. The inset of the figure shows the onset of absorption of P25 TiO 2 doped with 10 at.% Cu.
tributed to the increased absorption of irradiation by the metal oxide layer, i.e. CuO, on the TiO 2 surface. Many studies in the literature use the DRS technique to prove the increase in absorption in the visible region of 500᎐700 nm due to the increase in concentration of transition metal dopants ŽMoser et al., 1987; Palmisano et al., 1988; Navio et al., 1999.. In accordance with the crystal field approach, this weak absorption band arises from the d᎐d transitions of the transition metal, i.e. the slightly distorted octahedral Cu2q system, which is present on the TiO 2 surface ŽCotton et al., 1987.. Two kinds of d orbitals, namely the t 2g and e g orbitals, are present in transition metal ions with e g orbitals having higher energy than t 2g orbitals. On irradiation with suitable frequency of light, the d-electron occupying the t 2g orbital could be excited to the e g orbitals. Absorption results from this process could be observed in the visible spectrum.
3.5. Photocatalytic oxidation of cyanide 3.5.1. Reproducibility of results The photocatalytic activities of the copper loaded P25 TiO 2 catalysts were evaluated by measuring their ability to degrade cyanide. The initial pH of the system was adjusted to 12.0 to prevent the volatilization of
cyanide as toxic hydrogen cyanide. In all cases, the pH decreased from 12.0 to 11.7 after 3 h of UV illumination. By monitoring the cyanide concentration in the reactor and the alkali trap, it was confirmed that an insignificant amount of cyanide was lost due to the drop in pH. To estimate the data error and reproducibility, each experimental run was repeated three times under identical conditions and the agreement of the cyanide degradation rates was found to be within "3%.
3.5.2. Photocatalytic degradation of cyanide in the presence of Cu 2 q ions After mixing TiO 2 in the cyanide solution, no adsorption of cyanide onto or complexation formation with the TiO 2 surface was found as no change in concentration of CNy ions was detected in this period. According to Vrachnou et al. Ž1989., at pH) 7 no complex formation has been found and the adsorption of cyanide is insignificant. This is due to the fact that at high pH the surface of TiO 2 is negatively charged and the formation of surface-cyanide complexes is inhibited. To investigate the role of CuO on P25 TiO 2 in cyanide degradation, control experiments were carried out by irradiating the unmodified P25 catalysts in solutions containing different concentrations of aqueous
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Fig. 8. The influence of dissolved Cu2q ions on the photocatalytic degradation rate of cyanide by P25 TiO 2 . Initial cyanide concentration of 100 mgrl, 1 grl catalyst and initial pH at 12.
Cu2q ions in the form of CuŽNO 3 . 2 . The concentration of Cu2q ions was varied from 0.002 to 0.5 mM so that the copper content in solution was identical to that of the doped TiO 2 catalysts. The oxidation of CNy follows zero order kinetics and the CNy concentration decreased linearly with time over 90% of the reaction. The linear cyanide destruction rate was plotted as a function of Cu2q ions concentration and it can be seen that the presence of dissolved Cu2q ions significantly hindered the cyanide degradation rate as illustrated in Fig. 8. The initial and final concentrations of Cu2q ions in the solution were also determined by AAS analysis and it was found that in all cases the total concentration of Cu2q in the solution, including complexed and un-complexed Cu2q ions, remained constant throughout the experiment. This proves that the copper in solution was not reduced and deposited onto the surface of TiO 2 at the end of all runs. It has been shown in other studies that in the same cases the presence of small amounts of Cu2q ions in fact enhances the photocatalytic oxidation of acetic acid ŽBideau et al., 1991., propionic acid ŽBideau et al., 1992., formic acid ŽBideau et al., 1990. and toluene ŽBulter and Davis, 1993.. The observed acceleration in the reaction rate in these studies was explained in terms of the effects of Cu2qrCuq redox couple which inhibits the electron-hole recombination and the inner sphere mechanism of Cu 2q ions with the organic compounds, forming organo-metallic intermediates. However, in another study by Brezova et al. Ž1995., it was
found that the presence of aqueous Cu2q ions Ž0.28᎐1.1 mM. significantly retarded the degradation of phenol. The inhibitory effect of Cu2q ions in phenol degradation was interpreted as the reduction of Cu 2q ions from the solution and the subsequent deposition of Cu0 and Cu 2 O on the surface of TiO 2 . However, in our study the decrease in the cyanide oxidation rate could not be interpreted as the hindrance effect by the deposition of copper species on the TiO 2 surface. In the presence of free CNy all copper should exist as CuŽI. ᎐CNy complexes in different speciations depending on the CNy:Cu ratio ŽSharpe, 1976; Cotton et al., 1987; Gurol and Holden, 1988; Vrachnou et al., 1989.. This is the reason why there was no CuŽOH. 2 precipitate formation even at alkaline pH in the experiments. In our system, the CNy:Cu ratio remained much higher than four throughout the experiment and it is therefore likely that the predominant species was CuŽCN. 32y. All Cu᎐CNy complexes display weak absorption above 250 nm with peak maxima at 238᎐250 nm ŽSharpe, 1976. which is well below the operating wavelength of 350᎐385 nm for TiO 2 photooxidation. Therefore the presence of these CuŽI. ᎐CNy complexes would not decrease the transmission of UV light to the reactor during the photooxidation. The negative effect of CNy photooxidation due to the presence of CuŽI. ᎐CNy species can be explained via the competition reaction for hydroxyl radicals produced from the TiO 2 surface under UV illumination. Studies from pulse radiolysis have shown that the reac-
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Fig. 9. Cyanide destruction plotted against irradiation time for Ža. P25 and Žb. 0.10 at.% CurTiO 2 . Initial cyanide concentration of 100 mgrl, 1 grl catalyst and initial pH at 12.
tion rate constants between CuŽI. complexes and OH ⭈ radicals ŽSukhov et al., 1986; Goldstein et al., 1992. have the same magnitude as the one between CNy and OH ⭈ radical ŽBehar, 1974; Buchler et al., 1976; Bielski and Allen, 1977.. As a result of the competition reaction from CuŽI., CNy was oxidized less efficiently.
3.5.3. Photocatalytic degradation of cyanide using copper doped TiO2 At the beginning of the experiment the CuO doped TiO 2 catalyst was suspended for 15 min in the test solution. During this period, up to 30% of copper was found to dissolve into the CNy solution, decreasing its initial concentration from 3.85 to 3.45 mM, even before the UV light was switched on. The concentration of dissolved copper did not change after the mixing in the dark indicating that the dissolution of copper from the doped photocatalyst was completed. The photocatalytic effect was measured after the light was switched on. The photocatalytic degradation of cyanide using unmodified P25 TiO 2 and the P25 TiO 2 loaded with 0.10 at.% Cu is shown in Fig. 9. The plot shows that loading 0.10 at.% Cu of copper oxide onto the surface of TiO 2 enhances slightly the photodegradation of cyanide. The slightly higher photooxidation rate of CNy ions from the solution found in the 0.10 at.% Cu-loaded TiO 2 catalyst is attributed to the higher photoactivity of the CuO loaded on the TiO 2 surface. Similar to many other studies on transition metal ions doped TiO 2 systems, there exists an optimum concentration of do-
pant beyond which the observed photocatalytic activity decreases. From this study, the optimum dopant level was found to be 0.10 at.% Cu and only at this copper oxide level the cyanide photooxidation rate was slightly faster than that of the undoped P25 TiO 2 . Further increase in the copper oxide loading beyond this level had a detrimental effect on the cyanide destruction rate ŽFig. 10.. In several studies on photocatalytic oxidation of cyanide solution, cyanate was the main product of the oxidation reaction ŽAugugliaro et al., 1997; Frank and Brad, 1977; Domenech and Peral, 1988; Pollema et al., 1992.. Fig. 11 shows the concentration profiles of cyanide and cyanate at different irradiation times using TiO 2 doped with 0.10 at.% Cu. Similar results were obtained from experiments using TiO 2 doped with different Cu dopant concentrations except for the time required for completing the cyanide oxidation. In all cases, cyanate was the main product formed from the photooxidation of cyanide similar to the results obtained from the experiment using the undoped P25. It can be seen from Fig. 11 that during the oxidation of cyanide, the concentration of cyanate increased as the reaction proceeded. The carbon balance was close to 100% throughout the experiment. After 6 h, over 99% of the cyanide ions were oxidized and converted into less toxic cyanate ions wEqs. Ž8. ᎐ Ž10.x. No cyanate oxidation was detected during the experimental run which is consistent with other works ŽAugugliaro et al., 1997, 1999; Frank and Brad, 1977; Pollema et al., 1992..
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Fig. 10. The dependence of initial cyanide degradation rate on the Cu dopant level in the irradiated TiO 2 system. Initial cyanide concentration of 100 mgrl, 1 grl catalyst, and initial pH at 12.
Many reported results show that the rate of reaction of CNy with OH ⭈ radicals is two orders of magnitude higher than the reaction between CNOy and OH ⭈ radicals ŽBehar, 1974; Buchler et al., 1976; Bielski and Allen, 1977; Leopold and Faraggi, 1977.. Therefore in heterogeneous photocatalysis, cyanide would be oxi-
dized preferentially by the surface adsorbed hydroxyl group. The band edge potentials Žmeasured vs. NHE. for the valence and conduction band of anatase are roughly q3.2 V and y0.2 V ŽFox and Dulay, 1993.. Considering the potentials of Cu2qrCu0 and Cu2qrCuq lie
Fig. 11. Concentration profile of cyanide and cyanate plotted against irradiation time. ŽB. CNy concentration; Ž䢇. CNOy concentration; Ž'. Total carbon concentration CNyq CNOy. Initial cyanide concentration of 3.85 mM, 1 grl catalyst, and initial pH at 12.
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below the conduction band edge of TiO 2 , i.e. q0.34 V and q0.17 V respectively, the electron could be trapped by CuII O present on the TiO 2 surface. Choi et al. Ž1994. suggested that a closed shell configuration of the dopant generally is unfavourable for the electron or hole trapping. However, since a Cu2q ion has an unfilled 3d shell Žt 62g e g3 configuration. and the reduction of Cu2q is thermodynamically feasible, it is valid to assume that electron can be trapped by Cu II O on the surface of TiO 2 . As a result of the electron trapping by CuII O, the rate of the electron-hole recombination reaction is slowed down and more holes are available for the redox reactions. The possible charge transfer reactions can be represented by ⭈. Ž IV Ti IVOH q hq vbª Ti OH
q
I CuII O q ey cbª Cu O
Ž 12. Ž 13.
The role of CuII O is therefore to act simply as an electron trap and the electron has to be consumed in someway otherwise there will be an accumulation of charge on the surface. One pathway which accounts for the disappearance of a trapped electron is by the recombination reaction with the photogenerated hole from the TiO 2 . Trapped electrons could also be consumed via the reduction of adsorbed oxygen molecules ŽOkamoto et al., 1985; Gerischer and Heller, 1991. which is believed to be one of the rate determining steps in TiO 2-based photocatalytic reactions. Since the solution in our system is continuously saturated with air, the majority of electrons trapped on the surface are dissipated through the oxygen reduction to form super oxide radical wEq. Ž4.x. Foster et al. Ž1993. also reported that if oxygen is present in the system, the Cuq could be oxidized back to Cu2q. As a result of the ey
O2
Cu II O ª Cu I O ª Cu II O sequential reactions, the electron hole recombination rate could be reduced. At high loading of copper oxide Ž) 0.10 at.% Cu., since the copper oxide was confined on the surface of TiO 2 , there is a high possibility for the trapped electrons to recombine with the holes. The oxidation of CuI O to CuII O by the photogenerated holes is expected to be faster than the oxidation of CNy ion, since the former is simply a direct transfer of the trapped electron from CuI O to the valence band hole. In that case the Cu I O might act as a recombination centre and promote the recombination reaction as shown in the following reaction, q
Ž Ti IVOH ⭈ . q Cu I O ª Ti IVOH q CuII O
Ž 14.
Okamoto et al. Ž1985. found that the copper deposits ŽCu0 and Cu I O. on the surface of anatase could be oxidized by the photogenerated holes during phenol
483
degradation. Other studies carried out by Hirano et al. Ž1992, 1997. using a mixed suspension of copper metal powder and TiO 2 found that the copper metal could be oxidized by the holes forming Cu 2 O and Cu2q ions in the solution. Therefore the reaction described by Eq. Ž14. is likely to happen in the CuO-doped TiO 2 system. As shown in Figs. 5 and 6, no direct correlation could be found between the absorbance of copper oxide loaded TiO 2 and cyanide photooxidation rate. Undoubtedly, the light harvesting properties of the photocatalyst is a crucial parameter to control the overall efficiency of the photocatalytic reaction. Although the absorbance of the Cu doped TiO 2 catalysts at the visible region progressively increased as Cu dopant level increased, cyanide removal rate was significantly retarded at high Cu dopant levels. The detrimental effect observed as the copper loading increased beyond 0.10 at.% could also be explained by the coverage of the CuO on the TiO 2 . This would block the TiO 2 from absorbing the incoming photons. The CuO present on the surface could also bind with the hydroxyl groups and this would reduce the hydroxylated surface of TiO 2 , which is the main source of OH ⭈ radicals. Consequently, a decrease in the photocatalytic activity was observed. These negative effects are responsible for the low enhancement Ž13%. observed for the CuO-loaded TiO 2 systems.
4. Conclusion The presence of Cu2q ions Ž0.002᎐0.5 mM. in the CNyrTiO 2rUV system significantly retarded the photooxidation of cyanide. The decrease in the cyanide oxidation rate could be explained in terms of the competition reaction of copperŽI. cyanide complexes for the photogenerated hydroxyl group. It has been demonstrated that copper could be loaded onto the surface of TiO 2 by the photodeposition method. The concentration of dopant was varied from 0.05 to 10 at.% Cu. This method of deposition results in nanosized copper particles dispersed uniformly on the TiO 2 surface as indicated by the SEM-EDX study. The EPR results also indicated that CuO was present in the doped TiO 2 samples. There was only a slight increase in the cyanide oxidation rate at a copper dopant concentration of 0.10 at.% Cu which reflects the possibility of electron trapping by CuO dopant present on the surface of TiO 2 . Further increase in the CuO dopant content beyond 0.10 at.% Cu considerably reduced the photocatalytic activity of TiO 2 . Firstly, the well dispersed nanosized CuO dopant would cover the surface of TiO 2 leading to a drastic decrease in photon absorption. Secondly, the high concentration of dopant promotes the recombination of photogenerated holes with
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