J. Phys. Chem. 1994,98.6797-6803
6791
Electrochemically Assisted Photocatalysis. 2. The Role of Oxygen and Reaction Intermediates in the Degradation of 4-Chlorophenol on Immobilized Ti02 Particulate Films K. Vinodgnpa1,'J Ulick Stafford,* Kimberly A. Gray,"$ and Prsshant V. Kamat'.* Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408, Department of Chemical Engineering, Department of Civil Engineering and Geological Sciences, and Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Receiued: December 27. 1993; In Final Form: May 6, 1994.
The electrochemically-assisted photocatalytic degradation of 4-chlorophenol (4-CP) using immobilized Ti02 particulate films has been investigated hy analyzing reaction intermediates under a variety of experimental conditions. The degradations were carried out in both nitrogen- and oxygen-saturated solutions to explore the role of reduced oxygen species and molecular oxygen in the formation of reaction intermediates and in the mineralization of 4-CP. The degradation rate can be greatly improved even in a nitrogen-saturated atmosphere by applyingananodicbias to theTi02filmelectrode. 4-Chlorocatechol(4-CC) is the predominant intermediate observed in oxygen-saturated solutions, whereas hydroquinone (HQ) is the primary intermediate in nitrogensaturated solutions. Molecular oxygen plays an important role in the enhancement of the electrochemically assisted photocatalytic decay rate of 4-CP and the subsequent degradation of reaction intermediates, viz., 4-CC and HQ.
Introduction A great deal of attention has been focused in recent years on the development of nanocrystalline semiconductor thin films because of both their interesting properties and their ease of preparation.14 In a recent paper,' we presented preliminary results of a photocatalytic degradation experiment involving particulate Ti02 semiconductor thin films immobilized on an optically transparent electrode (OTE). We demonstrated that these Ti02/OTE systems exhibit photocatalytic activity similar to TiOz slurries in water. Similar effort has also been made recently by Anderson and his co-workers in the degradation of formic acid." This concept of achieving charge separation in a semiconductor system with an electrochemical bias was first introduced by Honda and Fujishima.6 Using a single crystal Ti02, they were able to carry out the photoelectrolysis of water under the influence of an anodic bias. The recent developments in designing microporous semiconductor thin films presented elsewherelJ.' and in our laboratoryz have made it possible to use an anodic bias for achieving charge separation in immobilized semiconductor nanocrystallites which are in contact with an aquwusor nonaquwuselectrolyte. Theprinciplebehindutilizing a porous nanocrystalline semiconductor thin film for the electrochemically assisted photocatalytic degradation of organic contaminants is shown in Scheme 1. The externally applied anodic bias greatly improves the efficiency of charge separation by driving the photogenerated electrons via the external circuit to the counter electrode compartment. The ability of such a photoelectrochemicalcell to rectify the flow of charge carriers makes it possible to separate the anodic and cathodic processes. This arrangement then provides a unique opportunity to isolate the various reactions occurring in photocatalytic systems and also demonstrates the abilityofparticulatefilms tocontrol chemical reaction pathways. Photocatalytic degradation of several organic contaminants using large bandgapsemiconductorparticlessuspendedin aquwus solutions as well as immobilzed semiconductor films has been studied extensively [see for example refs 7-25]. The major Univmity Nonhwcst. Chemical Engineering. 8 Department of Civil Engineering and Geological Scicnccs. A Notn Dame Radiation Laboratory. *Abstract published in Advance ACS Abslracfs, June I, 1994. 7 Indiana
1 Department of
0022-3654/94/2098-6797$04SO/O
SCHEME 1: Schematic Diagram of the Two-Compartment Cell Employed in the Electrochemically Assisted Photocatalytic Degrndation of 4-Chlorophenol Using a Porous Nanocrystalline T i 4 Working Electrode (WE),a Pt Gauze Counter Electrode (CE), and a Saturated Calomel Reference Electrode (RE)
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oxidation step is believed to be a secondary oxidation initiated by hydroxyl radicals which are generated followingthe oxidation of hydroxyl ions by photogenerated holes trapped at the semiconductorsurfaa."lf Inslurry-based photocatalyticreactor systems, the rate determining step in the degradation process is believed to be the reduction of oxygen by the trapped electrons on the semiconductor surface to produce reduced oxygen species such as thesuperoxide radical ion 02.-or H z O ~ . ~ .For " example, Hoffmann and his cc-workers have demonstrated the formation of HZOzby 2-electron transfer to O2in colloidal TiOz and ZnO suspensions.26.2' It is not clear from previous studies if the participationof oxygen islimited tosuch ascavengingof electrons to prevent charge recombination or whether activated oxygen 0 1994 American Chemical Society
Vinodgopal et al.
6798 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994
species (02+, HOZ’,H202, et~.)~*.29 influencethe rate or pathway of degradation. Some studies also suggest the involvement of molecular oxygen as an intrinsic rea~tant.”J5~ The photocatalytic degradation of chlorophenols in Ti02 slurries has been investigated extensiveIy.ls2l While these investigators report complete mineralizationis possible in the presence of oxygen, the route the reactions follow is dependent upon the reaction conditions, and different observations have been reoprted. It is not well understood how differences in reaction conditions account for differencesin product yield, reaction rates, etc. In order to obtain further insight into the photocatalyticdegradation, we have scrutinized the role of oxygen in the overall degradation reactions. By employinga two-compartment electrochemicalcell, we have now investigated the influence of oxygen on the reaction pathways of 4-CP degradation. The primary intermediates formed during the electrochemicallyassisted photocatalyticdegradation of 4-CP under nitrogen- and oxygen-saturated conditions have been analyzed, and their effect on the overall photocatalytic degradation rate has been evaluated.
Experimental Section Materials and Electrode Preparation. 4-Chlorophenol (4-CP), hydroquinone (HQ, Aldrich), and 4-chlorocatechol(4-CC,Kasei Chemical Co., Japan) were used as supplied. Optically transparent electrodes (OTE) were cut from a conducting glass plate obtained from Donelley Corp., Holland, MI. Ti02 powder (product name P-25, particle size 30 nm, surface area 50 m2/g) was a gift sample from Degussa Corp. TheTiO2 particulate film was prepared by applying 0.25 mL of Ti02 slurry (1.64 g TiOz/L water) to half the area of the OTE plate (5 cm X 0.9 cm) and drying in the oven at 673 K. The Ti02 loading corresponds to 6.6 mg of Ti02 spread on an area of approximately 2.2 cmz of the OTE plate. The uncovered area of the OTE plate was used tomake theelectrical contact. Theseelectrodeswith immobilized Ti02 film (referred to as OTE/Ti02) were directly employed as a working electrode in the electrochemical cell. Unless otherwise specified all the electrochemical and photoelectrochemical measurements were carried out in a standard two-compartmentcell in which working and counter electrodes were separated by a fine glass frit. The electrode assembly consisted of an OTE/Ti02 working electrode, a platinum wire gauze counter electrode, and a saturated calomel reference electrode (SCE). The evaluation of the OTE/Ti02 electrode was performed in an alkaline medium (0.05 M NaOH) while all the photocatalyticexperimentswere performed in an unbuffered aqueous medium. The initial pH of a 1 mM chlorophenolsolution employed in the photocatalysis experiment was around 6 and dropped to approximately4 following the photocatalytic reaction due to the production of HCl. The N2 and 0 2 atmospheres in individual compartments were maintained by purging individual gases with a slow stream separately in the two compartments during the operation of the cell. A Princeton Applied Research (PAR) Model 173 potentiostat and Model 175 universal programmer and Bioanalytical Systems (BAS) Model 100 ElectrochemicalAnalyzer were used in all the electrochemicalmeasurements. Photocurrent measurements were carried out with a Keithley Model 617 programmable electrometer. All measurements were carried out at room temperature (-296 K). A collimated light beam from a 250 W xenon lamp was usedfor excitation of the electrode in the front face (electrolyte side) configuration. To ensure selective excitation of the Ti02 film (X > 300 nm) and to avoid direct photolysis of the 4-CP, a 10 cm long CuSO4 solution filter was placed in the optical path. The intermediates formed from the degradation of 4-CP were determined by HPLC using a Waters 600E HPLC instrument, equipped with a Supelco Supelcosil25 cm long LC-18 column. The detection with UV absorption was performed at 280 nm for 4-CP, HQ, and 4-CC, and 254 nm for benzoquinone. The eluent
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Figure 1. (a) Diffuse reflectance absorption spectrum and (b) photocurrent action spectrum of the OTE/Ti02 electrode in deaerated 0.05 MNaOH. (Excitationsource: monochromaticlight from thexenonlamp.)
consisted of a mixture of water/methanol/acetic acid in a ratio of 600:400:1. For the HPLC analyses, 200 pL samples were withdrawn at appropriate time intervals (usually 5, 15, 35, 75, and 155 min) during the irradiation from the working electrode compartment of the electrochemical cell.
Results and Discussion Pbotoelectrockmical Properties of the OTE/TiOz Electrode. Thin Ti02 films prepared from Degussa P-25 (primarily anatase) particles are photoactive and exhibit photoelectrochemical effects under UV irradiation. The immobilized particles on the OTE surface collectively yield anodic photocurrent similar to that of an n-type semiconductor material.I2 Thus, OTE/Ti02 electrodes can conveniently be used as a photoanode in a photoelectrochemical cell. An open-circuit. voltage of 800 mV and shortcircuit current of 8 pA/cm2 were observed when this immobilized Ti02 film was irradiated (3 15 nm) in a solution of deaerated 0.05 M NaOH. As shown in our earlier study, the charge separation in these immobilized particles is controlled by the differing rates of electron and hole injection into the electrolyte and can be directly controlled with the application of an anodic bias. Figure 1 shows the photoelectrochemicalresponseof the anatase Ti02 particulate film immobilized on a conductingglass surface. The incident photon-to-photocurrent efficiency (IPCE) was determined by measuring the photocurrent of the OTE/TiOz electrode at various excitation wavelengths and using expression 1, IPCE (%) = 100( 1240is)/(~inc) where iscis the short-circuit current photocurrent (A/cm2), Zlw is the incident light intensity (W/cm2), and X is the excitation wavelength (nm). The onset of photocurrent is seen at a wavelength of -370 nm. The increase in the photocurrent at excitation wavelengthsbelow 370 nm is sharp and closely matches the absorption characteristics of anatase Ti02 (E, = 3.2 eV). This indicates that the observed photocurrent is initiated by the excitation of Ti02 particles in the semiconductor film. A maximum IPCE of 5.5% was obtained at 3 15 nm. The observed decrease in the photocurrent at wavelengths less than 3 15 nm can be attributed to the high absorption coefficient of the Ti02 particulate film at shorter wavelengths which leads to the inhomogeneous absorption of the light within the film. The generation of anodic current is also indicative of the fact that the direction of flow of electronsis toward the OTE surface. The low value of IPCE (5.5% at 315 nm) observed under steady-state illumination conditionsshows that a majority of the charge carriers are lost in the recombination process. However, it is possible to improve the efficiency of charge separation by a factor of 2 with the application of an anodic bias of 0.6 V vs SCE. The performance of an OTE/Ti02 electrode in a photoelectrochemicalcell was evaluated in the photovoltaic mode following its excitation with UV light (>300 nm). The photocurrent and photovoltages which were recorded at various load resistances
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6199
Electrochemically Assisted Photocatalysis
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Figure 2. Power characteristics of a photoelectrochemical cell employing an OTE/Ti02 electrode in contact with the deaerated 0.05 M NaOH. (Pt counter electrode in air-saturated 0.05 M NaOH. Illumination source: UV light (>300 nm from a xenon lamp.)
areshown in Figure 2. Theelectrodecharacterization was carried out at a higher pH (0.05 M NaOH) to ensure the operational stability of the photoelectrochemical cell during these measurements. (In unbuffered solutionsthe photocurrent was not stable. A nearly 50% drop in the photocurrent was observed when the electrode was illuminated for a period of 2 h.) A fill factor (rf = P&( VwiE),where Pmsxis the maximum power output of the cell),determinedfrom theresultsin Figure2, was0.35. Although this value of the fill factor is small, it is comparable to those of other semiconductorparticulate film based photoelectrochemical cells.14 Direct Electrochemical Oxidation of 4-CP. The oxidation potential of 4-CP in water at a glassy carbon electrode measured by square wavevoltammetry was +0.80V vs SCE. Theoxidation potential was obtained primarily to ensure that in our electrochemically assisted photocatalysis experiments,the applied anodic bias potential was lower than the oxidation potential of the 4-CP, so that oxidative degradation does not interfere with the photocatalysis. Thus, while there was a utilitarian logicin studying the electrochemistryof these systems,the goal was also to analyze the intermediates formed during electrochemical decomposition and compare them with those obtained from photocatalysis. Electrochemical oxidation of the 4-CP was carried out at controlled potentials, in aqueous solution (initial pH - 6 ) both with and without oxygen. The electrode was kept in the dark. Changes in the absorption spectra of the 4-CP were observed when the applied potential was increased to 1.0 V, suggesting direct oxidation of 4-CP at the OTE/Ti02 electrode. However, aromatic intermediates such as 4-CC and HQ were not detected by our HPLC method. A similar observation has also been made in the azide radical oxidation of 4-CP in a y-radiolysis experiment." These results suggest that direct oxidation of 4-CP leads to the formation of phenoxy1 radical which further undergoes ring cleavage30 and/or formation of olig~mers/polymers.~~ PhotocatalyticProperties of OTE/TiOl Electrodes. As shown in our preliminary study, the immobilized Ti02 particles are very effective in the degradation of 4-CP when used as a photoanode in a photoelectrochemical cell. The advantages of these semiconductor particulate film are that (a) oxygen is not necessary to scavenge the photogenerated electrons since efficient charge separation of the photogenerated electrons and holes can be achieved by applying an anodic bias potential to the Ti02/0TE, and (b) the rate of degradation is enhanced by a factor of 2 as the applied anodic potential is increased from 0 to +0.6 V. The reaction intermediates of this photocatalytic process on immobilized Ti02 particulate films have now been analyzed to probe the reaction pathways in nitrogen-saturated and oxygen-saturated solutions. The major photoelectrochemicalreactions that initiate redox processes in the electrodecompartments can be summarized
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Figure 3. Kinetics of the photodegradation of 4-CP (m) in a nitrogensaturated aqueous solution (initial pH -6) using theOTE/TiOl electrode at an applied potential of +0.6 V. Also shown are the formation and decay kinetics of the intermediates HQ ( 0 )and 4CC (A). (The Nzsaturated solution in the working electrode compartment of the twocompartment cell was continuously bubbled with N2.)
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(a) Nitrogen-SaturatedSolutions. Figure 3 shows the kinetics of the disappearance of 4-CP in the presence of nitrogen along with the formation and subsequent degradation of the intermediates HQ, BQ, and 4-CC in the working electrode compartment of a two-compartment cell. The experiments were carried out by continuously bubbling nitrogen in the working electrode and 02 in the counter electrode compartments. The initial pH (unbuffered) of the solution was around 6 . A rapid degradation of 4-CP is seen at the OTE/Ti02 electrode which was held at a constant anodic potential of +0.6 V vs SCE and irradiated with UV light. The initial photocurrent density was of the order of 20 NA/cm2 and decayed slowly during the operation of the cell due to the pH drop to -4. Nearly 90% of 4-CP disappeared in 2.5 h. The intermediates detected by HPLC during the course of the degradation were hydroquinone (HQ) (major component), 4-chlorocatechol (4-CC), and benzoquinone (BQ). When the applied potential was reduced to 0.0 V vs SCE, the degradation rate decreased, but the nature and relative amounts of intermediates observed were similar to those observed at higher potential (+0.6 V). This decreased degradation rate further verifies the fact that charge separation is more efficient when the electrode is maintained at higher anodic potentials. (It should be noted that the photocurrent at 0.0 V was approximately half the value observed at 0.6 V. See ref 5 for the i-Vcharacteristics of nanocrystalline Ti02 film.) When no external potential was applied (Le. under open-circuit conditions), an insignificant amount (
