Comparative Study in Liquid-Phase Heterogeneous Photocatalysis

Ind. Eng. Chem. Res. 2010, 49, 8397–8405

8397

Comparative Study in Liquid-Phase Heterogeneous Photocatalysis: Model for Photoreactor Scale-Up Dong Li,† Kui Xiong,† Wei Li,‡ Zhuhong Yang,† Chang Liu,† Xin Feng,† and Xiaohua Lu*,† State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, Jiangsu, China, and Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, China

A scale-up model for photoreactors based on a comparative study of the photocatalytic efficiency of suspended and immobilized systems was developed. The model is independent of reactor size and configurations, and it assumes that photocatalytic efficiency is the same when normalized per unit of illuminated catalyst area in both systems. In all cases, phenol/TiO2 (Degussa P25) was selected as the photodegradation system. First, a kinetic model was built in an immobilized system based on the corresponding experimental data, and then predicted rates of phenol degradation in the suspended system were calculated using the above kinetic model combined with a simplified radiation model, which was expressed as an apparent form of the Lambert law. Second, to obtain experimental rates, experiments conducted in the suspended system were carried out under the same conditions used in the immobilized system. Ratios between experimental rates and predicted rates were obtained, revealing the differences in efficiency between the suspended and immobilized systems. The typical value of the ratio was 2.5-9.2, suggesting that the efficiency of the suspended system was 2.5-9.2 times higher than that of the immobilized system. The ratio decreased with increasing concentrations of both phenol and catalyst. When the catalyst concentration and initial concentration of phenol were set, the ratio became constant within the range of the light intensity of 1.71-3.60 mW cm-2. Finally, for photoreactor scale-up, the proposed model was validated in a larger photoreactor operated in the suspended system, and good agreements were obtained with errors less than 5%. This methodology provides an alternative to the scale-up of photoreactors, which allows for easier engineering applications. Introduction 1

Since Fujishima and Honda discovered the photocatalytic splitting of water on TiO2 photocatalytic electrodes in 1972, scientific and engineering interests in heterogeneous photocatalytic applications, such as those related to water and air purification, disinfection, chemical synthesis, hydrogen evolution, and photovoltaic cells, have grown rapidly.2-8 However, to date, industrial applications remain limited by low photocatalytic process efficiency and by the lack of knowledge on engineering scale-up, more especially in liquid than in gas-phase photocatalysis.9-11 The design and scale-up of photoreactors are more complex than conventional reactors because radiation transfer must be considered along with mass, momentum, and heat transfers. In liquid-phase photocatalytic systems, catalysts can be either suspended or immobilized in reactors. Generally, suspended systems are more efficient than immobilized systems.12-14 Thus, much research has been focused on suspended systems.15 On the other hand, due to the attenuation of light intensity by the absorption and scattering effects of suspended particles, researchers have resorted to the solution of radiation transfer equation (RTE) to determine the light intensity illuminated on every particle. This brings difficulties in kinetic model research and subsequently in the scale-up of reactors because the kinetic model in suspended systems cannot be extrapolated to another when the reactor size and the configurations are changed. In contrast, radiation transfer is less critical in immobilized systems than in suspended systems because absorption and scattering * To whom correspondence should be addressed. Tel.: +86-2583588063. Fax: +86-25-83588063. E-mail: [email protected]. † Nanjing University of Technology. ‡ The University of Hong Kong.

effects do not exist in immobilized systems. Hence, the kinetic model is simpler in immobilized systems, and it is easier to scale-up based on the illuminated surface area of catalysts. Actually, some researchers have found that the photocatalytic efficiencies of immobilized films may be similar when quoted as per unit area of the catalyst16-18 in liquid-phase photocatalytic systems, and, even more, this method has been validated successfully in gas-phase photocatalytic systems for scale-up study.19 However, probably because of the uncertainty of the mass transfer effect, there are still no reports on achieving photoreactor scale-up based on the illuminated surface area of catalysts in liquid-phase photocatalysis. To develop a methodology for the scale-up of photoreactors based on the illuminated surface area of catalysts in liquid-phase photocatalysis, the unclear photocatalytic efficiency relationship between suspended and immobilized systems must be investigated. From the viewpoint of the mechanism of heterogeneous photocatalysis, the process is initially activated by light illumination with a photon energy that is larger than the bandgap, and then the generated electrons and holes transfer to the catalyst surface and react with the absorbed reactants. Theoretically, photocatalytic efficiency in both suspended and immobilized systems is the same as long as the incident light intensity and the area of the illuminated catalysts are the same. However, the efficiencies are different because of the existence of mass transfer limitations in both systems.20 The photocatalytic efficiency of various pollutants in suspended catalysts and immobilized systems has been compared, and different results were obtained.17,18,21-24 In the literature, although the same mass of photocatalyst has been applied to compare the efficiency of two different systems, the illuminated area of the photocatalyst in the two different systems can be quite different, hence resulting in varied findings. On the other hand, a uniform

10.1021/ie100277g  2010 American Chemical Society Published on Web 07/28/2010

8398

Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010

incident light intensity in suspended systems is difficult to achieve because of light attenuation. Keeping the same incident light intensity in experimental conditions for both suspended and immobilized systems is difficult. Therefore, a rational model focused on the light intensity and area of the illuminated catalyst must be established for purposes of comparing the efficiency of both systems. The results of the comparison indicate that a photocatalytic efficiency relationship can be established between suspended and immobilized systems. This article aims to establish a rational model to compare the photocatalytic efficiency of immobilized and suspended systems and to apply such a model in the scale-up of photoreactors that operate in suspended systems. In all cases, a phenol/ P25 reaction system is selected as a typical system.11,25,26 However, in this article, we are not compelled by the requirement to obtain the absolute values of photocatalytic efficiency in both systems. What is really important is the provision of an alternative to the design and scale-up of photoreactors using a simple method, which allows for easier engineering applications.

where β is the apparent extinction coefficient, which can be obtained from conventional spectrophotometric transmittance measurements; I is the light intensity; I0 is the incident light intensity; and d is the depth of suspension. Light intensity varies only along the radiation direction. On the basis of these definitions and assumptions, a simple approach in calculating the predictive reaction rate (rp) in suspended systems is proposed as follows. If the illuminated reactor surface has an area Sr, the number (np) of agglomerates in a layer is given by:

Model Description

and the illuminated catalyst area in each layer is18

This proposed model is based on the following definitions and assumptions. (1) The reaction rate is defined as:27 V dC ) kaImCn r)S dt

duced to describe the radiation profiles, which are expressed in terms of a simple exponential decay Lambert law: I ) I0 exp(-βd)

Sr

np )

1 Si ) np · πD2 ) 2

( )

6cp Sr πFs

h2

2/

D2

(1)

Sr )

( ) 6cp πFs

(4)

2/

3

(5)

D2

3

( )

2 1 π 6cp /3 · πD2 ) Sr , 2 2 πFs i ) 1,2, ...j j ) H/h

(6)

The incident light intensity in each layer is where ka is the apparent rate constant, I is the incident light intensity, C is the initial concentration of the reactant, V is the volume of the reaction solution, S is the geometrical surface area of the illuminated catalyst, and m and n are the orders of the reaction on light intensity and concentration, respectively. Photocatalytic efficiency is expressed as the initial reaction rate. (2) The photocatalytic efficiency is the same for catalyst with both immobilized and suspended configurations when expressed as unit area of illuminated catalyst. (3) The catalyst suspension consists of parallel layers of a rectangular space, where the incident radiation impinges perpendicularly from the collimated beam. All of the agglomerates of the catalysts are spherical and of the same size. The thickness of the layer is given by:28 h)

( ) πFs 6cp

Ii ) I0 exp[-β(i - 1)h] The total illuminated catalyst area (S) is S ) Si ·

( ) ( )

D

(2)

3

·

H 6cp D πFs

1/

3

)

3HSrcp FsD

(8)

( )

(9)

The mass of degraded phenol in each layer is π 6cp Mi ) kImi CnSi ) kCn · (I0e-β(i-1)h)m · Sr 2 πFs

2/

3

The total mass of degraded phenol in the whole reactor is M)

∑

( )

i)j

π 6cp kCn · (I0e-β(i-1)h)m · Sr 2 πFs i)1

∑

Mi )

i)1

where Fs is the catalyst density, cp is the catalyst concentration, and D is the diameter of the catalyst agglomerates. (4) The size of the catalyst agglomerates satisfies the following inequality:27

2/

π 6cp H ) Sr h 2 πFs

i)j

1/ 3

(7)

2/ 3

(10)

If the total mass of degraded phenol is divided by the total illuminated catalyst area (S), the predictive rate of phenol degradation (rp) in the suspended system is calculated as follows: i)j

πDnλ >5 λ

(3)

where D is the average diameter of the catalyst agglomerates, λ is the radiation wavelength, and nλ is the refractive index of the particle (for titanium dioxide, nλ ) 2.4-2.7). The fulfillment of this condition allows us to model light scattering by geometrical optics. The wavelength of the incident light is 365 nm; hence, we can conclude that the model holds for agglomerate sizes larger than about 0.25 µm. Actually, this condition is fulfilled by some commercial catalysts.29 (5) The apparent extinction coefficient (the sum of the absorption coefficient and the scattering coefficient)30 is intro-

M ) rp ) S

∑M i)1

S

i

( )

πFs ) 6cp

1/

3

D · H

i)j

∑ kC · (I e

-β(i-1)h m

n

0

)

i)1

(11)

However, from eq 11, we know that the rate of phenol degradation in the suspended system is an average value. To compare the efficiency of the suspended and immobilized systems, we define R as the ratio of initial reaction rate between the suspended system (rs) and the immobilized system (ri): R)

rs re ) ri rp

(12)


8399

mance of larger scale photoreactors in suspension systems can then be predicted. Experimental Details

Figure 1. Schematic methodology of photoreactor scale-up based on the comparative efficiency ratio.

where re is the experimental rate in the suspended system under the same reaction conditions as that of the immobilized system. Therefore, from eq 12, the ratio of the experimental rate and predictive rate represents the efficiency comparison between the suspended and immobilized systems. For purposes of the design and scale-up of photoreactors, eq 12 can be transformed as follows: re ) rpR

(13)

The methodology of the proposed model for the efficiency comparison of suspended and immobilized systems and for the scale-up of reactors is summarized in Figure 1. The model assumes that the photocatalytic efficiencies of immobilized catalysts and suspended configurations are the same when expressed per unit of the illuminated catalyst area. On the basis of this assumption, we can study photocatalytic kinetics in catalyst immobilized systems; meanwhile, radiation profiles in suspended systems can be approximated by the form of the Beer-Lambert law using an apparent extinction coefficient.31,32 Next, by combining photocatalytic kinetics in immobilized catalyst systems and radiation profiles in suspended systems, the photocatalytic efficiency of suspended systems can be calculated. For purposes of comparison, the photocatalytic efficiency of a suspended system was obtained through experiments under the similar conditions used for an immobilized system. Finally, the comparison between the immobilized catalyst and the suspended system was made through the ratio of experimental efficiency to calculated efficiency. The perfor-

1. Materials. Phenol and other chemicals needed for quantifying phenol were of AR grade (Sinopharm Chemical Reagent Co., Ltd., China). Deionized water (resistivity >18 MΩ cm) was used to prepare the solution. A Degussa P25 catalyst (Degussa Co., Germany) was used without further modification throughout this work. Its main physical data were as follows: Brunauer-Emmett-Teller surface area was 55 ( 15 m2 g-1; average primary particle size was around 30 nm; purity was above 97%; and anatase/rutile was 80:20. 2. Experimental Setup. The experimental setup used for phenol degradation is shown in Figure 2. The reactor was a rectangular cell that consisted of five plates. The two sides exposed to the radiation direction were made of quartz, and the other three sides were made of glass. The reactor was irradiated with a 40 W tubular black light lamp with maximum emission at 365 nm. Two different systems were used for the experiments: catalyst immobilized in thin layer on one-side ground quartz plate (immobilized system) and catalyst suspended in solution (suspended system). The volume of the reaction solution or suspension was 16 mL. In the immobilized system, the area of the ground quartz plate was 2 × 1 cm2. In the suspended system, the exposed side of the reactor was covered with aluminum foil, leaving an exposed area of 2 × 1 cm2. The adopted catalyst immobilization method is a successful method adopted by many researchers.16,33 The ground quartz plate was cleaned with alkaline solution followed by acid solution overnight to remove impurities, washed with deionized water, and finally dried at 393 K for 2 h before coating. The ground quartz plate was then placed on an electric heating plate maintained at 393 K and was coated by spraying 5 wt % Degussa P25/water suspension over the plate using a laboratory spray gun (NEWSTA F-470). Subsequently, the coated glass plate was calcined in a furnace by gradually raising the temperature at a rate of 5 K/min to a final temperature of 573 K, was held there for 3 h, and was finally cooled using the same ramping rate until it reached room temperature. This method is advantageous because it maintained the morphology of the catalyst even after immobilization. The experimental setup of the radiation profile measurement in the suspended system is shown in Figure 3. The vessel was a cylindrical glass (ID ) 50 mm); the annulus was covered with aluminum foil, and the lower side was sealed with a quartz plate. A UV radiometer was placed beneath the quartz plate. The vessel was placed under the exact center of the lamp; the vertical distance (L) was 2 cm. 3. Experimental Procedure and Analysis. Four series of experiments were carried out in this study. The first series of

Figure 2. Schematic diagram of the photoreactor: (A) immobilized system and (B) suspended system.

8400


replicated experiments was within 7%. Least squares regression analysis was carried out to determine the best fit for the data. Results and Discussion

Figure 3. Schematic diagram of the vessel used for determining the radiation profile.

experiments was performed in the immobilized system to obtain the intrinsic kinetic equation. A required amount of phenol stock solution was added to deionized water to get the initial concentration (1, 2, 4, 6, 8 mg L-1) of the pollutant. The pH of the solution was left as it was; no buffer or other agents were added to control the pH. The coated ground quartz plate was placed at the center of the reactor. Various light intensities (0.51, 1.18, 1.71, 2.35, 3.60 mW cm-2) were obtained by adjusting the distance between the lamp and the reactor. Subsequently, phenol solution (16 mL) was added to the reactor. The solution was stirred in the dark for 30 min to attain adsorption equilibrium before the reaction. Oxygen was continuously introduced into the solution to maintain a constant concentration throughout the reaction. After the reaction, samples were stored in the dark for further analysis. Phenol concentration was analyzed through a standard colorimetric method27,34 using a UV-vis spectrophotometer (Unico UV-2802S). The second series of experiments was performed to assess phenol degradation efficiency in the suspended system for comparison with model prediction. The total volume of the suspension used was 16 mL. Experimental conditions were the same as those used in the immobilized system. In each experiment, the transmitted light intensity through reactor was greater than zero. After the reaction, samples were filtered through a 0.22 µm syringe filter and stored in the dark for further analysis. The third series of experiments was designed to describe radiation profiles in the suspended system. A catalyst suspension of known P25 concentration was prepared using deionized water. This suspension was sonicated for 15 min in an ultrasonic bath to ensure uniform catalyst dispersion. The suspension was then immediately charged into the vessel to obtain different heights, and the corresponding light intensities were measured using a UV radiometer (Photoelectric Instrument Factory of Beijing Normal University UV-A). In each measurement, the distance between the lamp and the upper surface of the suspension was kept constant. The particle agglomerate size (D) was about 1.3 µm, which was determined using a particle size laser analyzer (Malvern Zetasizer 3000HSA). This value is similar to that reported in the literature.32 The fourth series of experiments was designed to validate the proposed model for the scale-up of the reactor with totally different sizes and configurations in the suspended system. The experimental setup consisted of a reactor, a buffer tank, a pump, and connecting pipes. The reactor is a rectangular vessel, of which two sides exposed to radiation direction are made of quartz. The illuminated area was 25 cm2 (12.5 × 2 cm), and the reactor thickness was 1 cm. Oxygen was supplied continuously, keeping the concentration of oxygen constant throughout the reaction. All above experiments were performed at room temperature and repeated at least three times. The standard deviation of the

1. Phenol Degradation Kinetics in Immobilized System. The kinetics of the photocatalytic degradation of organic compounds usually follows the Langmuir-Hinshelwood (LH) model.35-37 r)-

kKC dC ) dt 1 + KC

(14)

where r represents the initial rate of photodegradation, C is the concentration of the reactant, t is the irradiation time, k is the rate constant of the reaction, and K is the adsorption coefficient of the reactant. However, in recent studies, researchers have realized that the parameters in the LH model are varied with light intensity and they do not reflect their original physical meanings, so there exist kinetic disguises and misunderstanding in heterogeneous photocatalysis.10,38,39 Usually, the initial concentration of the reactant is at the millimolar level, and the equation can be simplified to an apparent first-order equation.37,40 Meanwhile, in the photocatalytic process, the incident light intensity is of importance to determine the reaction rate and should be the key parameter in kinetic equation. Based on the foregoing point of view, the initial reaction rate can be defined as eq 1.39,41 Also, all rates are reported as mass of phenol degraded per minute per cm2 of illuminated catalyst.16 To gain relatively accurate kinetic parameters, some preliminary experiments are needed in our study. In immobilized systems, the reaction rate is affected by film thickness and will be constant when film thickness reaches a saturation value.33,42,43 In our study, this critical value was found to be about 2 mg P25/cm2 coating, which is consistent with that reported in other studies.16,33 Therefore, experiments were performed at this film thickness. Blank tests showed no significant degradation in the absence of light, TiO2, or in the presence of uncoated substrates, which had been treated with the same procedure as the coated samples. The external mass transfer limitation is considered to easily occur in immobilized catalyst systems,44,45 so some experimental conditions must be met to ensure the absence of mass transfer limitation. In this study, results indicated that at a magnetic stirrer speed more than 1000 rpm and with additional agitation provided by continuous O2 purging (0.2 L min-1), the rate of reaction was independent of stirrer speed for all studied phenol concentration and incident light intensity. Thus, under all of these reaction conditions, the observed kinetics was independent of mass transfer limitation. Experimental results showed that in all cases zero-order reaction rates were obtained in the initial stages with reaction time less than 20 min, so the initial rate of phenol degradation can be calculated with the average rate within first 20 min.46,47 Parallel tests showed good reproducibility. The rate of phenol degradation was investigated as a function of the initial concentration of phenol (C) and incident light intensity (I) in the immobilized system (Figure 4). The rates always followed a first-order reaction on the phenol concentration from 1 to 8 mg L-1 in the incident light intensity range 0.51-3.60 mW cm-2 as shown in Figure 4A. This apparent first-order reaction rate has been reported by many researchers,48,49 and it accounts for low initial concentrations in most photocatalytic processes that adopted the LH kinetic model. Figure 4B shows that the rates increased with incident light intensity following linear dependency with I in the range studied at


8401

Figure 4. Dependence of the rate of phenol degradation on (A) phenol concentration, C; and (B) incident light intensity, I. Table 1. Apparent Extinction Coefficients in the P25 Suspension catalyst concentration (g L-1)

apparent extinction coefficient (β)

R2

0.05 0.1 0.2 0.4 0.6 0.8 1.0

0.574 0.960 1.595 2.196 3.530 4.614 6.985

0.99258 0.99736 0.99447 0.99865 0.99397 0.96448 0.99495

different phenol concentrations. However, this linear dependency means a relatively low light intensity was used and that the maximum value was 3.60 mW cm-2, which was within the critical values range reported by many studies.50-52 From the view of the practical application and cost, the optimal incident light intensity should be in the domain where rate is linearly dependent on incident light intensity.41,52 On the other hand, this low incident light intensity avoids the transition points of the dependence of the reaction rate with the light intensity from 1.0 to 0.5 and is helpful to the calculation of proposed models. 2. Radiation Profiles in Suspended System. Incident light intensities in different suspension depths along the radiation direction were obtained by measuring the light intensity transmitted from different heights of the catalyst suspension. The characteristics of the radiation behavior of the suspension under study remained unchanged during the experiments. The obtained data were fitted with the exponential decay form, which was similar to Lambert’s law (eq 4). The regressive β values for different concentrations of catalyst suspension are reported in Table 1. A good correlation coefficient (R2) was obtained in each catalyst concentration. As can be seen, when the catalyst concentration was doubled, the β value was always less than the multiples. This indicates that the radiation attenuation did not follow the Lambert-Beer law. Rigorously speaking, the Lambert-Beer law is an integrated form that combines the Lambert law and the Beer law. It is correct only in dilute solutions. The Lambert law represents the relationship between absorption and medium thickness, whereas the Beer law represents the relationship between absorption and solute concentration. However, the LambertBeer law does not exist in heterogeneous systems.53 Radiation transfer in heterogeneous suspensions is a complex problem, and exact distribution needs to be resolved using RTE. The solution of RTE has been carried out using numerical methods based on Monte Carlo simulations54 and also by means of discrete ordinate methods.55 A simple model that allows one to get an immediate grasp of the way main physical parameters affect the performance of photoreactors is needed for engineering purposes. In most circumstances, researchers report that the experimental results can be viewed as an apparent form of the Lambert-Beer

equation.29-31,53 Therefore, as a phenomenological parameter, the apparent extinction coefficient can be confidently assumed to be a reliable parameter that characterizes the radiation behavior of the solid suspension of large particles.32,56,57 In this study, experimental results with good correlation and stable repeatability demonstrated that the Lambert law was suitable in describing radiation profiles in catalyst suspensions. 3. Comparison between Suspended System and Immobilized System. By combining phenol degradation kinetics in the immobilized catalyst system with radiation profiles in the suspended system into the model, predicted values can be calculated. The predicted and experimental rates of phenol degradation for different initial phenol concentrations and incident light intensities are shown in Figures 5-7. It was found that rates of phenol degradation monotonously decreased with increasing catalyst concentration in both predicted and experimental conditions, and the decreased ratios of experimental values were much larger than those of the predicted values. Meanwhile, the experimental rates were much larger than the predicted values. As can be seen from Figure 5, with increasing catalyst concentrations from 0.05 to 0.4 g L-1 at incident light intensity of 3.60 mW cm-2, the predicted rate of phenol degradation decreased by 38% from 4.2 × 10-5 to 2.6 × 10-5 mg cm-2 min-1, while the experimental rate of phenol degradation decreased by 79% from 37.6 × 10-5 to 8.0 × 10-5 mg cm-2 min-1. On the other hand, from Figures 5A-7A, we can see that the decreased ratios were reduced with lower incident light intensity. This suggests that the high incident light intensity and high catalyst loading should be applied carefully, because they may decrease the utilization ratio of both photons and catalysts. However, this phenomenon did not appear in Figures 5B-7B, which were conducted in experiments. Therefore, there may be a distinct difference between predicted and experimental results. The ratios of experimental and predicted rates, which represent the comparison of efficiency between the suspended and immobilized systems, are shown in Figure 8. In the studied range, the ratios varied from 2.5 to 9.2. In a certain range of incident light intensity, the ratios almost remained constant in each catalyst concentration, and this constant decreased with increasing catalyst concentration and initial phenol concentration. In other words, the comparison of efficiencies between the suspended and immobilized systems is a complex problem, because it is not constant and varied with reaction conditions, such as catalyst concentration and substrate concentration. Furthermore, the ratios decreased dramatically in the low phenol initial concentration as the catalyst concentration increased, while it decreased gently in the high phenol initial concentration. When the catalyst concentration increased to 0.4 g L-1, initial

8402


Figure 5. Dependence of the rate of phenol degradation (C ) 1 mg L-1) on catalyst loading. (A) Predicted values with models and (B) experimental values.



Figure 8. Dependence of the ratio on light intensity, catalyst concentration, and initial phenol concentration.

phenol concentrations did not have a large difference for the ratios, and the ratios decreased to around 3. As is known, the concentration gradient is considered as the driving force of mass transfer, while it also represents the existence of mass transfer limitation in a steady state. However,

there are two kinds of concentration gradients in the liquidphase heterogeneous photocatalytic process. One occurs at the interface of the reactant solution and the catalyst solid, which is similar to the heterogeneous catalytic system. The direction of this concentration gradient is from the liquid-solid interface to the bulk solution. The other is present in the whole reaction space along the radiation direction. It is particular in photocatalytic processes with suspended catalysts in the system. The direction of this concentration gradient is the same as the radiation direction. For the first kind of concentration gradient, it is the main reason that results in this remarkable difference of defined rate between the two systems. When the catalyst immobilized on substrate to form thin film, the increasing diffusion length of the reactant from the bulk solution to catalyst surface and the lower catalyst surface to illumination result in the easier occurrence of external mass transfer limitation. At the same time, with increasing catalyst film thickness, the internal mass transfer limitation may be the main reason to reduce the reaction rate. However, external mass transfer limitation can be reduced to a


8403

Table 2. Predicted Values in the Photoreactor Scale-Up residual concentration of phenol after 20 min (mg L-1) initial phenol concentration (mg L-1)

incident light intensity (mW cm-2)

illuminated area (cm2)

catalyst concentration (g L-1)

ratio

suspension volume (mL)

predicted value

experimental value

error

1 1 4 4 8 8

1.71 2.35 3.60 3.60 3.60 3.60

25 25 25 25 25 25

0.05 0.2 0.1 0.4 0.2 0.4

9.1 5.0 6.3 3.0 4.0 2.6

200 200 200 200 200 200

0.88 0.66 2.84 2.39 5.83 5.62

0.87 0.69 2.93 2.46 5.75 5.60

+1.1% -4.3% -3.1% -2.8% +1.4% +0.4%

negligible value by increasing the circulating flow rate or by increasing the stirring speed. While internal mass transfer is an intrinsic property of the catalyst films, it is determined by the nature of the catalyst, reaction system, and the coating techniques, which are difficult to alter.33 On the other hand, due to the aggregation of catalyst particles, the internal mass transfer limitation also exists in suspended systems.58 Furthermore, it is not easy to find an easy method to calculate these differences of mass transfer precisely in both systems. The ratios obtained in Figure 8, to some extent, revealed the degree of effect that mass transfer limitation imposes on both systems. Also, the obtained varied ratios are consistent with values reported by many researchers.3,59,60 Also, this explained well the different comparison results of efficiency between these two systems. For the second kind of concentration gradient, it may be used to explain the dependence of ratios on catalyst concentration and initial phenol concentration. From the proposed model, we know that the variational ratios virtually show that the defined initial reaction rate in suspended system is affected by this concentration gradient. In suspended system, light will attenuate due to the strong absorption and reflection by catalyst particles.61,62 However, the light can be looked at as the fourth phase in photocatalytic process.63 Because the speed of light transmit is much faster than macro movement, the particular characteristic of light is that it can not be mixed by convective motions of the fluid. In other words, there always exists a “concentration gradient of light” in suspended system. From the kinetic equation, we know that the reaction rate is proportional to the incident light intensity. With increasing catalyst concentration, light intensity falls dramatically and leads to a sharp phenol concentration gradient along the radiation direction. Meanwhile, in liquid-phase heterogeneous systems, due to the low diffusion coefficient of reactants, the concentration gradient cannot be eliminated through molecular diffusion and convective diffusion provided by general mixing operations. Actually, the concentration gradient in the whole reactor space does exist along the radiation direction.64 Also, the sharper is the phenol concentration gradient, the more serious is the mass transfer limitation. On the other hand, the ratios also decrease with increasing initial phenol concentration. In the heterogeneous photocatalytic process, only when the reactant diffuses from the bulk solution to the catalyst surface region can reaction occur. Because the fast radical reaction and reaction rate is proportional to initial reactant concentration, the higher is the initial reactant concentration, the sharper is the concentration gradient between bulk solution and catalyst surface, and subsequently the sharper is the concentration gradient along the radiation direction. Therefore, both the high catalyst concentration and the high initial reactant concentration can increase the concentration gradient in suspended systems, thus contributing to the occurrence of mass transfer limitation. However, the contributions of catalyst concentration and initial reactant concentration on mass transfer limitation have

their own dominant regions. When the catalyst concentration is low, the attenuation of light intensity is not serious, and thus the concentration gradient caused by catalyst concentration was light, whereas the concentration gradient caused by initial reactant concentration was relatively remarkable. With increasing catalyst concentration, the attenuation of light intensity became steeper, and the concentration gradient caused by catalyst concentration became serious and played a dominant role. Hence, the ratios decreased with an increase in initial phenol concentrations when the catalyst concentration was low, and there was not much difference when the catalyst concentration increased to 0.4 g L-1. On the other hand, the phenomenon of the ratios decreasing with increasing initial phenol concentration suggests that, at high phenol concentrations, the immobilized system is more favorable as compared to the suspended system, while the suspended system becomes relatively more effective at low phenol concentrations. The same findings are also reported by other researchers.59 Experiment results showed that with further reducing the incident light intensity in each catalyst concentration, the ratio decreased and it could not remain constant. The reactor space far away from the radiation source is almost dark when incident light intensity is low. Also, this illumination is so weak that it could hardly initiate the photocatalytic reaction. So, this dark region does not participate in the reaction. This effect is not involved in the rate calculated with models, and thus results in the decreasing ratio. 4. Validation in Photoreactor Scale-Up. The degradation of phenol in photoreactor with totally different size and configurations operated in suspended recirculation batch mode was investigated. The experimental values and predicted values with model are listed in Table 2. Good agreements were obtained with errors less than 5%. This validated the feasibility of the proposed model. Furthermore, it validated an alternative method for heterogeneous photocatalytic reactor scale-up, which allows for easier engineering applications. Meanwhile, during the experiments, we found that the 2.7 L min-1 (Re ) 2982) circulating flow rate was high enough to ensure a good mass transfer. Yet mass transfer limitation visibly occurred when the flow rate was lower than 1 L min-1 (Re ) 1104), and in this circumstance the residual concentration of phenol after 20 min was higher than the predicted value. This can be explained by that mass transfer limitation results in the mass of phenol degraded in the whole reaction system in 20 min being lower than the predicted value. So, this model can also be applied to evaluate and optimize the operation conditions. Conclusion A model associating the photocatalytic efficiency of suspended and immobilized systems is established and successfully applied in the scale-up of a photoreactor, which operated in a suspended system. The main conclusions are as follows:

8404


(1) In catalyst immobilized system, the phenol degradation rate presents a first-order reaction rate on the phenol concentration and a linear dependency with incident light intensity in range studied. (2) The phenol degradation rate in suspended system is 2.5-9.2 times higher than that in immobilized system. Under the same initial phenol concentration and catalyst concentration, the ratio (R) is a constant. It decreases with an increase in both catalyst and initial phenol concentration. (3) Good agreements were obtained with errors less than 5% when using this model for larger photoreactor scale-up. Acknowledgment This work was supported by the Chinese National Key Technology Research and Development Program (Grant No. 2006AA03Z455), the National Basic Research Program of China (No. 2009CB226103), the National Natural Science Foundation of China (Grant Nos. 20676062, 20736002, 20976080), the NSFC-RGC Joint Research Award (No. 20731160614 and HKU 735/07), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (08KJB530003), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0732), and the Social Development Program of Jiangsu Province Science and Technology Department (BS2007097). We also appreciate the many detailed comments and suggestions from one of the referees. Appendix The unit of light intensity adopted by einstein s-1 cm-2 or mW cm-2 is equivalent when the wavelength of UV light is determined. The unit of einstein cm-2 s-1 is more convenient and easier to compare to units of mole for substrate concentrations or calculation of quantum yield. So, we provide the unit conversion as follows. For λ ) 365 nm: 1 einstein s-1 cm-2 ) 6.02 × 1023 · 6.63 × 10-34 · 3.0 × 108 J s-1 cm-2 -9 365 × 10 ) 3.29 × 105 J s-1 cm-2 ) 3.29 × 108 mW cm-2 So, in this Article: 1 mW cm-2 ) 3.05 × 10-9 einstein s-1 cm-2

(1)

On the basis of eq 1, we can obtain the unit conversion table as below: mW cm-2

einstein s-1 cm-2

0.51 1.18 1.71 2.35 3.60

1.55 × 10-9 3.60 × 10-9 5.21 × 10-9 7.16 × 10-9 1.10 × 10-8

Literature Cited (1) Fujishima, A. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37–38. (2) Ollis, D. F.; Pelizzetti, E.; Serpone, N. Photocatalyzed destruction of water contaminants. EnViron. Sci. Technol. 1991, 25, 1522–1529. (3) Kabra, K.; Chaudhary, R.; Sawhney, R. L. Treatment of hazardous organic and inorganic compounds through aqueous-phase photocatalysis: A review. Ind. Eng. Chem. Res. 2004, 43, 7683–7696.

(4) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. ReV. 1995, 95, 69–96. (5) Cho, M.; Chung, H.; Choi, W.; Yoon, J. Linear correlation between inactivation of E-coli and OH radical concentration in TiO2 photocatalytic disinfection. Water Res. 2004, 38, 1069–1077. (6) Gonzalez, M. A.; Howell, S. G.; Sikdar, S. K. Photocatalytic selective oxidation of hydrocarbons in the aqueous phase. J. Catal. 1999, 183, 159– 162. (7) Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renewable Sustainable Energy ReV. 2007, 11, 401– 425. (8) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Gratzel, M. Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature 1998, 395, 583–585. (9) Bickley, R. I.; Slater, M. J.; Wang, W. J. Engineering development of a photocatalytic reactor for waste water treatment. Process Saf. EnViron. Prot. 2005, 83, 205–216. (10) Ollis, D. F. Kinetic disguises in heterogeneous photocatalysis. Top. Catal. 2005, 35, 217–223. (11) Carp, O.; Huisman, C. L.; Reller, A. Photoinduced reactivity of titanium dioxide. Prog. Solid State Chem. 2004, 32, 33–177. (12) Pozzo, R. L.; Baltanas, M. A.; Cassano, A. E. Supported titanium oxide as photocatalyst in water decontamination: State of the art. Catal. Today 1997, 39, 219–231. (13) Matthews, R. W. Photocatalytic oxidation of organic contaminants in water - an aid to environmental preservation. Pure Appl. Chem. 1992, 64, 1285–1290. (14) Bideau, M.; Claudel, B.; Dubien, C.; Faure, L.; Kazouan, H. On the immobilization of titanium-dioxide in the photocatalytic oxidation of spent waters. J. Photochem. Photobiol., A 1995, 91, 137–144. (15) Brandi, R. J.; Alfano, O. M.; Cassano, A. E. Evaluation of radiation absorption in slurry photocatalytic reactors. 1. Assessment of methods in use and new proposal. EnViron. Sci. Technol. 2000, 34, 2623–2630. (16) Byrne, J. A.; Eggins, B. R.; Brown, N. M. D.; McKinney, B.; Rouse, M. Immobilisation of TiO2 powder for the treatment of polluted water. Appl. Catal., B 1998, 17, 25–36. (17) Dumitriu, D.; Bally, A. R.; Ballif, C.; Hones, P.; Schmid, P. E.; Sanjines, R.; Levy, F.; Parvulescu, V. I. Photocatalytic degradation of phenol by TiO2 thin films prepared by sputtering. Appl. Catal., B 2000, 25, 83– 92. (18) Dijkstra, M. F. J.; Michorius, A.; Buwalda, H.; Panneman, H. J.; Winkelman, J. G. M.; Beenackers, A. Comparison of the efficiency of immobilized and suspended systems in photocatalytic degradation. Catal. Today 2001, 66, 487–494. (19) Kako, T.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Comparison of photocatalytic properties of a batch reactor with those of a flow reactor in a nearly controlled mass transport region. Res. Chem. Intermed. 2005, 31, 371–378. (20) Turchi, C. S.; Ollis, D. F. Photocatalytic degradation of organicwater contaminants - mechanisms involving hydroxyl radical attack. J. Catal. 1990, 122, 178–192. (21) Bajt, O.; Zita, J.; Novotna, P.; Krysa, J.; Jirkovsky, J. Photocatalytic degradation of dibutyl phthalate: Effect of catalyst immobilization. J. Sol. Energy Eng. 2008, 130, 0410051-0410055. (22) Pozzo, R. L.; Baltanas, M. A.; Cassano, A. E. Towards a precise assessment of the performance of supported photocatalysts for water detoxification processes. Catal. Today 1999, 54, 143–157. (23) Ochuma, I. J.; Osibo, O. O.; Fishwick, R. P.; Pollington, S.; Wagland, A.; Wood, J.; Winterbottorn, J. M. Three-phase photocatalysis using suspended titania and titania supported on a reticulated foam monolith for water purification. Catal. Today 2007, 128, 100–107. (24) Ray, A. K. A new photocatalytic reactor for destruction of toxic water pollutants by advanced oxidation process. Catal. Today 1998, 44, 357–368. (25) Serpone, N. Relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. J. Photochem. Photobiol., A 1997, 104, 1– 12. (26) Serpone, N.; Sauve, G.; Koch, R.; Tahiri, H.; Pichat, P.; Piccinini, P.; Pelizzetti, E.; Hidaka, H. Standardization protocol of process efficiencies and activation parameters in heterogeneous photocatalysis: Relative photonic efficiencies zeta(r). J. Photochem. Photobiol., A 1996, 94, 191–203. (27) Loddo, V.; Addamo, M.; Augugliaro, V.; Palmisano, L.; Schiavello, A.; Garrone, E. Optical properties and quantum yield determination in photocatalytic suspensions. AIChE J. 2006, 52, 2565–2574.

Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010 (28) Curco, D.; Gimenez, J.; Addardak, A.; Cervera-March, S.; Esplugas, S. Effects of radiation absorption and catalyst concentration on the photocatalytic degradation of pollutants. Catal. Today 2002, 76, 177–188. (29) Cabrera, M. I.; Alfano, O. M.; Cassano, A. E. Absorption and scattering coefficients of titanium dioxide particulate suspensions in water. J. Phys. Chem. 1996, 100, 20043–20050. (30) Marugan, J.; Van Grieken, R.; Alfano, O. M.; Cassano, A. E. Optical and physicochemical properties of silica-supported TiO2 photocatalysts. AIChE J. 2006, 52, 2832–2843. (31) Augugliaro, V.; Palmisano, L.; Schiavello, M. Photon-absorption by aqueous tio2 dispersion contained in a stirred photoreactor. AIChE J. 1991, 37, 1096–1100. (32) Salaices, M.; Serrano, B.; de Lasa, H. I. Photocatalytic conversion of organic pollutants extinction coefficients and quantum efficiencies. Ind. Eng. Chem. Res. 2001, 40, 5455–5464. (33) Chen, D. W.; Li, F. M.; Ray, A. K. Effect of mass transfer and catalyst layer thickness on photocatalytic reaction. AIChE J. 2000, 46, 1034– 1045. (34) Standard Methods for the Examination of Water and Wastewater, 21st ed.; American Public Health Association/American Water Works Association/Water Environment Federation: Washington, DC, 2005. (35) Peral, J.; Ollis, D. F. Heterogeneous photocatalytic oxidation of gas-phase organics for air purification - acetone, 1-butanol, butyraldehyde, formaldehyde, and meta-xylene oxidation. J. Catal. 1992, 136, 554–565. (36) Gaya, U. I.; Abdullah, A. H. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problems. J. Photochem. Photobiol., C 2008, 9, 1– 12. (37) Konstantinou, I. K.; Albanis, T. A. TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations - A review. Appl. Catal., B 2004, 49, 1–14. (38) Mills, A.; Wang, J. S.; Ollis, D. F. Kinetics of liquid phase semiconductor photoassisted reactions: Supporting observations for a pseudo-steady-state model. J. Phys. Chem. B 2006, 110, 14386–14390. (39) Emeline, A. V.; Ryabchuk, V.; Serpone, N. Factors affecting the efficiency of a photocatalyzed process in aqueous metal-oxide dispersions - Prospect of distinguishing between two kinetic models. J. Photochem. Photobiol., A 2000, 133, 89–97. (40) Chiou, C. H.; Wu, C. Y.; Juang, R. S. Influence of operating parameters on photocatalytic degradation of phenol in UV/TiO2 process. Chem. Eng. J. 2008, 139, 322–329. (41) Chen, D. W. S. M.; Ray, A. K. Heterogeneous photocatalysis in environmental remediation. DeV. Chem. Eng. Miner. Process. 2000, 8, 505– 550. (42) Chen, D. W.; Li, F. M.; Ray, A. K. External and internal mass transfer effect on photocatalytic degradation. Catal. Today 2001, 66, 475– 485. (43) Camera-Roda, G.; Santarelli, F. Optimization of the thickness of a photocatalytic film on the basis of the effectiveness factor. Catal. Today 2007, 129, 161–168. (44) Subramanian, V.; Kamat, P. V.; Wolf, E. E. Mass-transfer and kinetic studies during the photocatalytic degradation of an azo dye on optically transparent electrode thin film. Ind. Eng. Chem. Res. 2003, 42, 2131–2138. (45) Ray, A. K.; Beenackers, A. Novel swirl-flow reactor for kinetic studies of semiconductor photocatalysis. AIChE J. 1997, 43, 2571–2578. (46) Ilisz, I.; Laszlo, Z.; Dombi, A. Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. I: Effect of charge-trapping species on the degradation kinetics. Appl. Catal., A 1999, 180, 25–33.

8405

(47) Stafford, U.; Gray, K. A.; Kamat, P. V. Photocatalytic degradation of 4-chlorophenol: The effects of varying TiO2 concentration and light wavelength. J. Catal. 1997, 167, 25–32. (48) Sobczynski, A.; Duczmal, L.; Zmudzinski, W. Phenol destruction by photocatalysis on TiO2: an attempt to solve the reaction mechanism. J. Mol. Catal. A: Chem. 2004, 213, 225–230. (49) Hosseini, S. N.; Borghei, S. M.; Vossoughi, M.; Taghavinia, N. Immobilization of TiO2 on perlite granules for photocatalytic degradation of phenol. Appl. Catal., B 2007, 74, 53–62. (50) Mills, A.; Wang, J. S.; Ollis, D. F. Dependence of the kinetics of liquid-phase photocatalyzed reactions on oxygen concentration and light intensity. J. Catal. 2006, 243, 1–6. (51) Chen, D. W.; Ray, A. K. Photodegradation kinetics of 4-nitrophenol in TiO2 suspension. Water Res. 1998, 32, 3223–3234. (52) Herrmann, J. M. Heterogeneous photocatalysis: State of the art and present applications. Top. Catal. 2005, 34, 49–65. (53) Martin, C. A.; Baltanas, M. A.; Cassano, A. E. Photocatalytic reactors. 1. Optical behavior of titanium-oxide particulate suspensions. J. Photochem. Photobiol., A 1993, 76, 199–208. (54) Pasquali, M.; Santarelli, F.; Porter, J. F.; Yue, P. L. Radiative transfer in photocatalytic systems. AIChE J. 1996, 42, 532–537. (55) Romero, R. L.; Alfano, O. M.; Cassano, A. E. Cylindrical photocatalytic reactors. Radiation absorption and scattering effects produced by suspended fine particles in an annular space. Ind. Eng. Chem. Res. 1997, 36, 3094–3109. (56) Augugliaro, V.; Loddo, V.; Palmisano, L.; Schiavello, M. Performance of heterogeneous photocatalytic systems - influence of operational variables on photoactivity of aqueous suspension of TiO2. J. Catal. 1995, 153, 32–40. (57) Brucato, A.; Rizzuti, L. Simplified modeling of radiant fields in heterogeneous photoreactors. 1. Case of zero reflectance. Ind. Eng. Chem. Res. 1997, 36, 4740–4747. (58) Ballari, M. D.; Brandi, R.; Alfano, O.; Cassano, A. Mass transfer limitations in photocatalytic reactors employing titanium dioxide suspensions - II. External and internal particle constrains for the reaction. Chem. Eng. J. 2008, 136, 242–255. (59) Matthews, R. W.; McEvoy, S. R. Photocatalytic degradation of phenol in the presence of near-uv illuminated titanium-dioxide. J. Photochem. Photobiol., A 1992, 64, 231–246. (60) Thiruvenkatachari, R.; Vigneswaran, S.; Moon, I. S. A review on UV/TiO2 photocatalytic oxidation process. Korean J. Chem. Eng. 2008, 25, 64–72. (61) Mehrotra, K.; Yablonsky, G. S.; Ray, A. K. Macro kinetic studies for photocatalytic degradation of benzoic acid in immobilized systems. Chemosphere 2005, 60, 1427–1436. (62) Mukherjee, P. S.; Ray, A. K. Major challenges in the design of a large-scale photocatalytic reactor for water treatment. Chem. Eng. Technol. 1999, 22, 253–260. (63) Mehrotra, K.; Yablonsky, G. S.; Ray, A. K. Kinetic studies of photocatalytic degradation in a TiO2 slurry system: Distinguishing working regimes and determining rate dependences. Ind. Eng. Chem. Res. 2003, 42, 2273–2281. (64) Ballari, M. D.; Brandi, R.; Alfano, O.; Cassano, A. Mass transfer limitations in photocatalytic reactors employing titanium dioxide suspensions - I. Concentration profiles in the bulk. Chem. Eng. J. 2008, 136, 50–65.

ReceiVed for reView February 4, 2010 ReVised manuscript receiVed July 8, 2010 Accepted July 19, 2010 IE100277G

Comparative Study in Liquid-Phase Heterogeneous Photocatalysis

Recommend Documents