Mass-Transfer and Kinetic Studies during the Photocatalytic

flat substrate. A detailed study of physical parameters including light intensity, flow rate, stirring, ... kinetics independent of mass-transport lim...
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Ind. Eng. Chem. Res. 2003, 42, 2131-2138

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Mass-Transfer and Kinetic Studies during the Photocatalytic Degradation of an Azo Dye on Optically Transparent Electrode Thin Film Vaidyanathan Subramanian,†,‡ Prashant V. Kamat,† and Eduardo E. Wolf*,‡ Notre Dame Radiation Laboratory and Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana 46556-0579

Photocatalysis is an efficient method to eliminate azo dyes. We have studied the various parameters that affect the kinetics of the degradation process for an azo dye in an aquatic flow system using a novel batch continuous-loop flow reactor with TiO2 colloids immobilized on a flat substrate. A detailed study of physical parameters including light intensity, flow rate, stirring, catalyst loading, dye concentration, and temperature has been performed to obtain the reaction kinetics independent of mass-transport limitations. The kinetic parameters of the degradation process, viz., rate constant, order of reaction, and activation energy, have been obtained first using a power-law rate expression and then using an expression based on a LangmuirHinshelwood mechanism. The kinetic parameters for a model consistent with the reaction pathway have been estimated. Metal nanoparticles have been reported to be useful for enhancing the photocatalytic activity of semiconductors. We have studied the effect of metal nanoparticles on the photocatalytic activity of TiO2 under the optimized conditions of a flow reactor to determine the role of metal nanoparticles in the photocatalytic degradation of AO7. Introduction Azo dyes, such as Acid Orange 7 (chemical name AO7), are waste products of the dyeing process and are frequently discharged from textile mills into rivers and lakes, causing environmental problems. These nonbiodegradable dyes are hazardous for aquatic species and marine life, and to degrade them, advanced oxidation processes such as photocatalysis using TiO2 powders (Degussa P25) have been studied both by using slurry reactors1-5 and by immobilizing them on glass substrates.4,6-10 Immobilization of TiO2 on fixed substrates avoids the posttreatment and recovery of the catalyst, providing a more economically feasible alternative to slurry reactors. Immobilized films made of Degussa P25 are opaque in nature; however, preparations made by many groups using a sol-gel process to produce a TiO2 colloidal solution have been shown to give stable and transparent films unlike those prepared with Degussa P25 powders.11-14 Such films have a higher catalytic activity and better light adsorption characteristics, and they have shown promise in photocatalytic degradation studies.8,12,14 Most of the previous photocatalytic studies carried out with azo dyes, such as AO7,15-18 involved the use of commercial P25 TiO2 alone or TiO2 doped with metal ions. The types of reactors commonly used have been either slurry-type reactors or reactors with TiO2 immobilized on substrates such as glass beads and plates. Of the studies carried out with TiO2 immobilized on plates, no systematic study of the role of mass transfer has been performed. Cassano and co-workers19 * To whom correspondence should be addressed. Tel.: (574) 631-5897. Fax: (574) 631 8366. E-mail: [email protected]. † Notre Dame Radiation Laboratory. ‡ Department of Chemical Engineering.

have reviewed the importance of the role of mass transfer in photocatalytic processes and the need to address this. This issue is particularly important in kinetic studies on immobilized photocatalysts because of the existence of a boundary layer along the glass plate. A correct evaluation of the true reaction kinetics is important in understanding the reaction mechanism and the role of added promoters such as noble metals in facilitating photocatalytic reactions. Consequently, in this work, we studied the various parameters that affect the kinetics of the degradation process for an azo dye in an immobilized TiO2 film supported on an optically transparent electrode (OTE). The OTE not only provided a substrate to immobilize the TiO2 particles but also functioned as an electrically conductive medium in photoelectrochemical studies. Previous work20 on catalytic films deposited on OTEs was conducted on electrochemical cells designed to evaluate photocurrent efficiencies. In our previous work, we used such a cell and demonstrated an increase in the photocurrent when metal Au and Pt nanoparticles deposited on TiO2 films were used. Photoelectrochemical cells, however, although appropriate for electron-transfer measurements, have small volumes and lack stirring, and thus, they are likely to be affected by mass-transfer effects. To account for parameters affecting mass transfer, we designed a novel photocatalytic recycle reactor that permits one to study the effect of parameters such as flow rate, stirring, catalyst loading, dye concentration, light intensity, and temperature on the degradation rate. In this work, we describe a critical study of the roles of these parameters in the photocatalytic degradation of AO7 on TiO2 films that elucidates the role of metal nanoparticles under the optimized conditions of a flow reactor.

10.1021/ie020636u CCC: $25.00 © 2003 American Chemical Society Published on Web 04/11/2003

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Experimental Section The TiO2 colloids for the film catalyst were prepared using 2-propanol, glacial acetic acid, and titanium isopropoxide. Au and Pt nanoparticles were prepared from the corresponding metal salts (hydrogen tetrachloroaureate and hydrogen hexachloroplatinate, respectively). All chemicals were obtained from Aldrich Chemical Co. and used without further purification. Preparation of TiO2 Films Supported on OTE. A detailed description of the preparation of the film substrates and nanoparticles was reported previously;20 hence, only a brief description is presented here. Titanium dioxide colloids were prepared by hydrolysis of titanium isopropoxide in aqueous solution containing glacial acetic acid. A white precipitate is obtained which is then aged in a water bath until a bluish-white milky gel forms. This gel is then transferred into a thickwalled glass bulb and heated in an oven for 12 h at 500 K. The bulb is then gradually cooled for 24 h, and then the contents are removed. The colloids of TiO2 thus formed are sonicated and deposited as thin films on optically transparent electrodes. The use of a transparent OTE for immobilizing the catalyst is advantageous because thin films can be deposited on both sides of the substrate without significant loss of light transmission. Films were also prepared from commercially available TiO2 powder (Degussa P25). The powder was suspended in deionized water, and the resulting slurry was used to make thin films. Colloidal TiO2 and P25 slurry samples are removed by syringe and deposited onto an OTE coated with a thin conducting layer of SnO2 obtained from Pilkington (Libbey Owens Ford). The solutions are deposited stepwise and spread out using intermittent injections of the TiO2 to obtain a uniform coating on the OTE. The resulting deposits are air-dried for about 2 h and heated in an oven at 673 K for 1 h. This leads to the formation of a thin, transparent, stable, and porous film from the TiO2 colloids and of an opaque film from Degussa P25. Metal nanoparticles were prepared as colloids suspended in toluene using tetraoctyl ammonium bromide (TOAB) as a phase-transfer catalyst. The metal ion complexes thus transferred from water to the organic phase (toluene) were reduced using aqueous sodium borohydride. The colloids were washed with H2SO4 and water and dried with Na2SO4. Further details such as the characterization of these colloids are described in detail elsewhere.21 Acid Orange 7 was used as a representative azo dye. The dye (87% purity) was obtained from Aldrich Chemical Co. and further purified using column chromatography.22 Aqueous solutions of the dye were degraded using different catalyst films and conditions using a 5-mL flow cell in the presence of air. Novel Photocatalytic Reactor. The experimental setup is shown in Scheme 1. It has the standard collimated beam of light from a 250-W xenon lamp (whose intensity can be controlled using a voltage varying device) as the UV source that can illuminate the OTE. A convex lens is used to concentrate the incoming light into a collimated beam, which is focused onto the TiO2 film for maximum utilization of the light. The reactor has a volume of 5 mL, and it was designed to accommodate the OTE and allow for its easy replacement (for specifications, refer Table 1). The reactor is connected to a jacketed reservoir (25-cm3) via a peri-

Scheme 1. Schematic Representation of the Flow Reactor Setup Used for the Photocatalytic Degradation of AO7

Table 1. Reactor Specifications parameter cell shape dimensions electrode shape dimensions catalyst area thickness

specification rectangular; input at bottom, output at top 1 cm × 1 cm × 4 cm rectangular thin slide 1 cm × 0.2 cm × 4 cm 1 cm × 2 cm 0.3-1 µm

static pump (Masterflex) that controls the flow rate of the solution containing the dye past the OTE plate. Water is circulated through the reservoir jacket to maintain a constant temperature of the AO7 solution in the reservoir. The reservoir permits sampling without affecting the solution concentration. The reactor operates in batch mode, and the UV absorption of the dye is monitored periodically by withdrawing a small aliquot from the reservoir and by measuring the dye absorption using a Shimadzu UV 3101 PC spectrophotometer. Sampling was carried out at different intervals of UV illumination. AO7 has a distinctive absorption peak at 480 nm. Figure 1 shows typical absorption spectra of the AO7 dye recorded after different illumination times during photocatalytic degradation. The degradation of the dye was monitored from the decrease in the peak at 480 nm. We performed blank experiments with UV and visible light illumination of AO7 on the OTE with no TiO2 catalyst. We found just a marginal decrease (625 mW/ cm2, the light intensity was found to be independent of the TiO2 thickness on either side of the OTE. The photonic absorption model26 was used to evaluate the film thickness using the data presented in Figure 3. The absorption coefficient was evaluated using the conventional exponential relationship

I ) Ioe-Rβ where R is the absorption coefficient of the film and β is the film thickness.25 R was found to be 0.14 × 104 cm-1, which is similar to values reported in the literature.26 Kinetic Studies. The first half of this study involved an evaluation of the physical parameter settings that provide optimum conversions beyond which the conversion becomes independent of the effects of the physical parameters, i.e., the reaction becomes kinetically controlled. In the case of light intensity, our studies indicate that, at light intensities of >625 mW, the rate of photocatalytic degradation is independent of the light intensity. Although, in general, rate expressions for photocatalytic processes depend on the light intensity, because subsequent experiments were performed at intensities above 625 mW, the observed rate was not affected by the light intensity. Consequently, light intensity was not explicitly included as a variable in the rate expression. To study the effect of concentration on the rate of degradation, the AO7 concentration was varied from 10 to 60 µM. Physical parameters such as the flow rate and light intensity were set on the basis of the previously optimized mass-transfer studies so that rate measurements should reflect the true reaction kinetics. Figure 6 shows that the fractional conversion of the dye decreases as the initial concentration of AO7 increases. Under these experimental conditions, an inverse relation between the AO7 concentration and the degradation rate is observed, indicating that the degradation of AO7 is not a first-order process,27 as is the case for many typical dyes.16,28,29

To estimate the kinetic parameters, an nth-order rate equation of the form

-rAO7 )

-d[AO7] ) k[AO7]n dt

(1)

was used first. The rate and the order were obtained from the data presented in Figure 6. Two approaches27,30 were used to estimate the values of k and n,viz., a halflife method and a more rigorous differential method. In the half-life method, the following relationship holds for an (n * 1) irreversible reaction

t1/2 )

2n-1 - 1 [AO7]01-n k(n - 1)

(2)

where t1/2 is the time required for the concentration of AO7 to decrease to one-half of its initial value. In the differential method, the logarithm of the rate (eq 1) is plotted as a function of the logarithm of the conversion. Figure 7 displays a log-log plot of the d[AO7]/dt vs [AO7] data obtained using the two methods. It can be seen that the two methods yield straight lines and give similar values for the overall rate constant (k ) 0.02) and the order (n ) 0.73) of the reaction. A similar value of n was obtained for slurry systems using Degussa P25.4 The data for the rate constants for parameters optimized earlier are given in Table 3. The effect of temperature on the degradation of AO7 was also studied keeping the light intensity at 150 mW to minimize heating of the reactor due to irradiation. As expected, the photocatalytic degradation rate increased as the temperature was increased. The change in rate with temperature was plotted in an Arrhenius plot to estimate the activation energy of the dye degradation reaction. The activation energy thus obtained is 16 kcal/mol. The observed activation energy is an apparent value that collectively represents photoinduced processes within the solid as well as on the surface of the TiO2 particles. Increasing temperature affects the electron-hole separation and recombination lifetimes and thus plays an important role in altering the degradation kinetics. Work is underway to elucidate the role of light-induced effects and chemical effects in the reaction. It is well-known that photocatalytic degradation rates are much higher in slurry systems because of the availability of higher active catalyst surface areas per gram of solution.31 TiO2 colloid samples with a weight (∼4 mg) equivalent to that deposited on the OTE, when

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Table 3. Rate Constants under Different Reactor Operating Conditions

experimental group

value of parameter controlled

overall rate constant k (min-1)

catalyst loading (mg)a

1.2 2.4 3.5 4.7

0.0056 0.0078 0.0097 0.0101

flow rate (mL/s)b

0.25 1 1.6 2.6 3 4 4.5

0.0021 0.0056 0.0065 0.0082 0.0095 0.0108 0.0107

intensity (mW)c

150 245 365 625 860 1175

0.004 0.0056 0.0086 0.0118 0.0123 0.0123

temperature (°C)d

22 30 36 40

0.0038 0.0054 0.0071 0.0091

effect of metalse

TiO2 TiO2/Au TiO2/Pt TiO2/Ir

0.0119 0.0129 0.0125 0.0120

Figure 8. Langmuir-Hinshelwood plot for the estimation of K and kr.

where A represents AO7, S represents a TiO2 active site, and B represents a reaction intermediate. If the rate of reaction is assumed to be rate-limiting, the concentrations of the oxidizing species (OH radicals and holes) are assumed to be in large excess, the reaction rate is assumed to be proportional to the concentration of AO7 adsorbed, and the adsorbed species are assumed to be in equilibrium, then the kinetics is described by a Langmuir-Hinshelwood (LH) type of rate expression. The linearized form of an such expression is

a TiO on both sides of OTE; flow rate ) 3 mL/s; intensity ) 2 550 mW. b TiO2 on both sides of OTE; intensity ) 550 mW. c TiO2 on both sides of OTE; flow rate ) 4 mL/s. d TiO2 on both sides of OTE; flow rate ) 4 mL/s; intensity ) 620 mW. e TiO2 on both sides of OTE; flow rate ) 4 mL/s; intensity ) 620 mW.

injected into an AO7 solution, gave almost 2 times higher degradation rates. An analysis of the results of previous works showed that the rates of degradation of AO7 vary widely.10,32-35 In some cases, they are lower than the values obtained here, and in some cases, they are higher. The higher degradation rates could be attributed to either higher available surface area of the catalyst and/or the nature of the pollutant being degraded. The previous works also used different reactor configurations, such as an electrochemical cell and an annular reactor, or a different light source and a different concentration of the pollutant. An exhaustive analysis of the mass-transfer parameters affecting the photocatalytic degradation in film reactors was performed by Chen et al.36 The reaction order is fractional, indicating that the reaction mechanism is not a single elementary step process. Rather, AO7 degradation is a complex process involving adsorption followed by the sequential decomposition of several intermediates. Consequently, it is often assumed that the rate of adsorption of AO7 on the active sites of TiO2 is the rate-limiting step. The general reaction pathway can be represented as follows (eqs a-e)

A + S f A-S (adsorption)

(a)

A-S f A + S (desorption)

(b)

A-S + TiO2(h+) f B-S (hole-mediated oxidation of A) (c) A-S + OH• f B-S (OH•-mediated oxidation of A) (d) B-S f B + S (desorption of reaction product)

(e)

1 1 1 + ) kr -d[AO7] krK[AO7] dt

(3)

where K is the adsorption equilibrium constant and kr is the adsorption rate constant. Equation 3 is plotted at different initial concentrations of AO7 in Figure 8. It can be seen that the values of K and the actual reaction rate constant estimated from the LH plot are similar to the overall rate constant (k ) Kkr) calculated using the differential and half-life methods. This indicates that the rate of AO7 degradation on TiO2 films is consistent with a single-site-adsorption pathway. LH plots were also made using the data of AO7 degradation at different temperatures to determine the k values (Table 3) and estimate the value of the activation energy E using an Arrhenius plot. It can be seen that the two methods give similar values of the energy of activation, thus further supporting the proposed LH reaction pathway. To show that this is indeed the reaction mechanism, it is necessary to provide spectroscopic evidence of the adsorbed species, as well as measurements of intermediates in the liquid phase. It was not possible, however, to differentiate in the UV spectra between AO7 species adsorbed and those in solution, and therefore, we can only conclude that there is consistency between the kinetic results and the LH adsorption reaction pathway proposed. In Figure 4, for flow rates above 4.5 mL/s, there is no change in conversion with increasing flow rate. This led us to believe that there is a uniform gradient of the dye across the illumination path in the cell and that mass transfer of the dye to the surface is not the rate-limiting process. To further confirm this hypothesis, the masstransfer parameters were evaluated by using the theory for boundary layer flow along a flat plate. Following the calculation of the Reynolds number, the mass-transfer coefficient was estimated to be 2.14 × 10-3 cm/s applying the Colburn analogy (f/2 ) 0.332Nre-1/2).37 A comparison of the mass flux with reaction rate for flow rates greater than 4.5 mL/s showed that the mass flux was about 10 times higher than the reaction rate.

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Metal nanoparticles exhibit interesting photochemical and photocatalytic properties.38 Numerous researchers have investigated the role of metal in photocatalysis. Whereas some of them have found the deposition of metals to be useful, other reports suggest a detrimental effect of metal loading.15,39-45 We reported the effect of using metal nanoparticles for the photocatalytic degradation of AO7 using a standard electrochemical cell.20 Metal nanoparticles prepared in toluene21 were deposited by an electrophoretic deposition technique onto a TiO2 film at 400 V dc. These films were used for photocatalytic studies to ascertain the role of metal nanoparticles in photocatalysis under the kinetically controlled conditions used for the recirculating film reactor. The results presented in Table 3 show that the rate constant in the presence of metals is similar to that of TiO2 film alone. Although metal nanoparticles have been observed to enhance photoelectrochemical properties initially upon UV illumination,21 they undergo alterations in their charged states, leading to their irreversible deactivation.20 The experiments carried out in the new flow reactor setup under optimized masstransfer conditions show that metal nanoparticles do not have a substantial effect in altering the degradation rate. Efforts are underway to study the transformation at the semiconductor-metal interface using EXAFS in an attempt to understand the deactivation process. Conclusion The mass transfer and kinetics of the degradation of the azo dye AO7 have been investigated in a flow reactor setup using an immobilized TiO2 film. The benefits of using transparent TiO2 films were fully realized in this new reactor configuration, and the limits of operation of various physical parameters were determined. The kinetic parameters for AO7 degradation were estimated using the differential and half-life methods. The order of reaction of AO7 degradation was found to be 0.73, and the overall rate constant was estimated as 0.02 min-1. A single-site LH mechanism was also proposed for the degradation of AO7. The kinetic parameters estimated on the basis of the LH mechanism are similar to those observed from the differential and half-life methods. The role of metal nanoparticles was analyzed critically using the optimized reactor configuration. This study has highlighted the importance of considering mass-transfer effects in evaluation of the properties of photocatalytic films. The fact that metal nanoparticles do not enhance the catalytic activity of TiO2 was demonstrated in the absence of mass-transfer limitations. It was also conclusively shown that electrochemical studies, which are normally carried out under such limitations, might provide varying results under non-mass-transfer-limited conditions. Acknowledgment We thank Mr. Ian B. Duncanson for fabricating the flow cell for the photocatalytic experiments. The work described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. This is Contribution No. 4398 from Notre Dame Radiation Laboratory. Literature Cited (1) Tang, W.; An, H. UV/TiO2 Photocatalytic oxidation of commercial dyes in aqueous solutions. Chemosphere 1995, 31, 4157.

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Received for review August 16, 2002 Revised manuscript received January 3, 2003 Accepted February 20, 2003 IE020636U