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Comparison of Aqueous Photoreactions with TiO2 in its Hydrosol Solution and Powdery Suspension for Light Utilization Tong-xu Liu,†,‡ Yuan Liu,† Zhao-ji Zhang,† Fang-bai Li,‡ and Xiang-zhong Li*,† † ‡
Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China Guangdong Key Laboratory of Agricultural Environment Pollution Integrated Control, Guangdong Institute of Eco-Environment and Soil Science, Guangzhou 510650, PR China
bS Supporting Information ABSTRACT: In aqueous photoreaction systems, the existence of gradient light intensity throughout the reaction solution is one of the important restrictions to the efficiency of the overall photoreaction, which highly relies on the optical properties of the catalyst such as light absorption and scattering effects, the particle size of the catalyst, and the degree of agglomeration in its slurry. In this study, both synthesized TiO2 hydrosol and commercial TiO2 powder, Degussa TiO2 P25, were used as catalysts for comparison, in which their particulate and optical properties in aqueous solution/suspension were investigated. The characteristic results showed that the TiO2 hydrosol had smaller crystal sizes and particle sizes, but a lower degree of crystallinity compared to the P25 powder. As TiO2 hydrosol solution had the much lower light extinction coefficient than P25 suspension, the attenuation of light intensity gradient in the reaction solution containing more transparent TiO2 hydrosol was greatly eliminated. However, both types of catalysts demonstrated similar effects of significant light scattering in their solution/suspension. The photocatalytic activities of two catalysts were then evaluated in two sets of experiments for methyl orange (MO) and methylsulfonic acid (MSA) degradations, respectively. The much higher reaction rates of MO and MSA degradations were found in the TiO2 hydrosol solution than in the P25 suspension, because TiO2 hydrosol catalyst with much finer particles and higher transparency allowed the photoreactor system to work at a higher catalyst loading than the conventional TiO2 powdery catalyst. A new kinetic model considering three main factors of (i) the volumetric rate of photon absorption, (ii) effective reaction volume, and (iii) the effective surface area of catalyst in a cylindrical photoreactor was also established. This new model summarizes that the catalyst with a higher surface area, a lower light extinction coefficient, and a lesser light scattering effect would achieve better light utilization within the photoreactor system and is beneficial to achieve a higher efficiency of overall photoreaction.
1. INTRODUCTION TiO2-based photocatalysis has attracted great attention due to the requirements of a mild reaction condition such as ambient temperature and normal pressure under UV illumination. Most studies indicated that the photocatalytic reactions with suspended catalysts in aqueous slurry demonstrated a higher degree of light utilization than those with immobilized catalysts due to the much higher total surface area of suspended catalyst available to absorb light energy in the reaction system.18 However, such aqueous slurry systems usually have an upper limit to their catalyst concentrations. The kinetics of photocatalytic reaction in aqueous slurry system demonstrated that the reaction rate increases with an increased catalyst loading in its low range, and beyond an optimum catalyst concentration it will gradually decrease when catalyst loading further increases.9,10 Actually, the gradient of light intensity throughout the reaction suspension can only allow a small fraction of catalyst to be effectively activated by the UV light irradiation, when the catalyst loading exceeds its optimum dosage.1113 In our previous study, a photocatalytic wet scrubbing process by combining the photocatalysis and water scrubbing technique was developed for treating odorous gases.14 It was found that a TiO2 (Degussa P25) dosage around 2 g L1 in aqueous solution achieved the best performance for methyl mercaptan (CH3SH) degradation, r 2011 American Chemical Society
and beyond such an optimum dosage, the light intensity inside the photoreactor declined sharply along with an increased distance from the UV lamp due to serious light scattering and agglomeration resulting from a high number of catalyst particles in aqueous suspension. Many other reports also indicated that most photocatalytic processes have large radiation flux gradients (i.e., very short radiation penetration depth inside the monolith channels) and a significant fraction of the available catalytic area cannot be well utilized.1517 However, TiO2 hydrosol catalyst with main particle size of about 510 nm has much larger surface area and is much more transparent than conventional TiO2 powder catalysts, which demonstrated better performance for the photocatalytic degradation of pollutants in a photoreactor system. Therefore, it is important to recognize the interaction between hydrodynamic attributes and the radiation characteristics. In photochemistry, the prediction of light distribution field in a homogeneous photoreactor has been well established.18 However, the major challenge in determining the radiation field in Received: June 13, 2010 Accepted: June 1, 2011 Revised: May 26, 2011 Published: June 01, 2011 7841
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Industrial & Engineering Chemistry Research heterogeneous photocatalysis is to account for the optical properties of the photocatalysts in water, as scattering and absorption occur simultaneously on the scattering centers in suspended catalyst solutions, and the scattering and absorption coefficients are highly associated.19,20 The traditional spectrophotometric measurements carried out on a heterogeneous sample can only determine the attenuation coefficient (i.e., the sum of the absorption and scattering coefficients). However, it is preferable to separate the scattering and absorption coefficients for evaluation of light utilization.21 Furthermore, a theoretical explanation on the effect of catalyst concentration and optical thickness on the radiation absorbed in a photoreactor has been recently presented by Li Puma’s group,22,23 in which they proposed to optimize the design of photoreactors based on a new concept of apparent optical thickness to remove the dependence of the optimum catalyst concentration on tube diameter and photocatalyst scattering albedo. Li Puma also studied the effect of hydrodynamic and catalyst loading on pollutant conversion in photoreactor.24 In principle, the point-local-values of the radiation intensity in an aqueous slurry system can be determined by solving the radiative transport equation and integrating the local volumetric rate of photon absorption (LVRPA) within the entire volume of the photoreactor.2125 The above-mentioned works mainly focused on the relationship between catalyst properties and photon absorption, but there is a lack of a clear relationship between catalyst concentration and the observed photoreaction rate of pollutant degradation. This study is an extension of our previous work on the development of a photocatalytic wet scrubbing process for gaseous odor treatment14 to compare two types of catalysts, TiO2 hydrosol and TiO2 powder, with different structural and optical properties, in terms of light utilization and photocatalytic reaction. The study is aimed at illustrating the influence of catalyst concentrations on the photoreaction rate of two model pollutants degradation in an annular photoreactor system by considering three main factors of (i) the number of photons absorbed by catalysts, (ii) the effective reaction volume/space within the photoreactor, and (iii) the effective surface area of catalysts in its suspension with establishment of a new mathematical model to describe the reaction kinetics in such a system.
2. EXPERIMENTAL SECTION 2.1. Materials. Metatitanic acid chemical (TiO2 51 wt %, H2O 29 wt %, H2SO4 20 wt %), a precursor of titania powder, was supplied from Panzhihua Iron & Steel Research Institute, China. NH4OH, HNO3, and other chemicals of analytical grade were obtained from Shanghai Reagent Ltd., China. Degussa P25 consisting of 80% anatase and 20% rutile was obtained from Degussa AG Company, Germany. Methyl orange (MO) and methylsulfonic acid (MSA, CH3SO3H) chemicals of analytical grade were provided by BDH and Aldrich Chemical Co., respectively, and used without further purification. Distilled and deionized water was used for preparation of all aqueous solutions/suspensions. 2.2. Preparation of TiO2 Hydrosols. TiO2 hydrosol sample was prepared by a hydrothermal process with the following procedure: 100 g of metatitanic acid as a precursor material was added into 2 L of deionized water and stirred continuously to form aqueous uniform suspension first; the diluted ammonia solution was then dropped very slowly until pH exceeded 9. The resulted suspension was further stirred continuously for 3 h, and
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then filtered; the filtered cake was washed with the deionized water for several times until no sulfate ion was present (determined by 0.5 M barium chloride solution) and pH was reduced to around 7 to ensure that most impurities could be substantively removed from the cake sample; finally, the cake sample was mixed with water again to form aqueous uniform suspension; nitric acid at 10% (v/v) was dropped into the suspension to reduce pH to 1.5; the resulted suspension was continuously stirred at room temperature for 4 h; after heating at 65 °C, the suspension was continuously peptized for 24 h and the TiO2 hydrosol sample was eventually obtained. 2.3. Characterization. To characterize the properties of the prepared TiO2 hydrosol sample, the titania xerogel powder was prepared by gelation treatment at 65 °C for 12 h. X-ray powder diffraction (XRD) patterns were recorded using a Rigaku D/ Max-III A diffract meter at room temperature, operating at 30 kV and 30 mA, using Cu KR radiation (λ = 0.15418 nm). The particle size distributions (PSDs) of the TiO2 hydrosols and P25 powder in aqueous suspension were determined using a lightscattering size analyzer (Beckman N5, USA). 2.4. Optical Measurement. The UV absorbance of the TiO2 hydrosol in aqueous solution and P25 powder in aqueous suspension after ultrasonic treatment for 30 min was first measured using a UVvisible spectrophotometer (TU-1801 Beijing, China) in the wavelength range of 300400 nm. According to the measurement method reported in the reference,21 the spectral diffuse reflectance and transmittance of the TiO2 hydrosol solution and P25 suspensions were further measured by an UVvisible spectrophotometer (TU-1901 Beijing, China) equipped with an IS19-1 integrating sphere reflectance attachment. The rectangular quartz cells with an optical path length of 0.8 cm were used for the transmittance and reflectance measurements in the wavelength range of 230450 nm. The measurements were conducted at different catalyst concentrations of 0.1, 0.5, 1, 2, 4, 8, and 16 g L1. 2.5. Experimental Procedures. MO and MSA chemicals were used as two model pollutants to evaluate their photocatalytic degradations in aqueous TiO2 solution/suspension. The experiments were conducted in a Pyrex cylindrical photoreactor (Figure S1 in the Supporting Information), in which an 8-W UV lamp (Philips TL 8W/08 F8 T5/BLB blacklight lamp) with a main emission at 365 nm (Figure S2) was positioned at the center (I0 = 12.8 W m2 at r0 = 0.025 m) and a Pyrex circulation water jacket was surrounded to control the solution temperature during reaction and covered with aluminum foil to avoid any indoor light invasion. Aqueous reaction solution/suspension was formed by adding TiO2 hydrosol or Degussa TiO2 P25 powder (as a reference catalyst for comparison) into 250 mL of aqueous MO or MSA solution. In all experiments, the initial concentrations of MO and MSA in solution/suspension were prepared at 20 and 40 mg L1, respectively, and the initial pH of the reaction solution/suspension was adjusted to around 6 using 0.1 M NaOH solution. Prior to the photoreaction, the reaction solution/suspension was magnetically stirred in the dark for 30 min to establish adsorption/desorption equilibrium. During the photoreaction, the reaction solution/suspension was irradiated by UV light with constant air blow and also magnetic stirring at around 600 r min1. At the given time intervals, water samples were taken from the reaction solution/suspension and stored in the dark prior to analysis. 2.6. Chemical Analyses. MO and MSA were used as two model pollutants to evaluate their photocatalytic activity. Whereas 7842
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Figure 3. Light absorbance at 343380 nm by TiO2 hydrosol and P25 powder vs optical thickness. Figure 1. X-ray diffraction patterns (XRD) of TiO2 hydrosol and P25 powder.
Figure 2. Particle size distributions (PSD) of TiO2 hydrosol and P25 powder.
the concentration of MO in aqueous solution/suspension was determined by a UVvis spectrophotometer (Libra S35, Biochrom) at the wavelength of 464 nm, the concentrations of MSA and its final product, sulfate ions, were determined by ion chromatography (DIONEX ICS-90) with an ion column of 4 250 mm (IonPac AS14A). The calibration curves of SO42- and CH3SO3 in the range of 050 mg L1 were obtained using standard H2SO4 and CH3SO3H solutions. The analytical samples taken from the solution/suspension containing catalyst particles were immediately centrifuged for 30 min at 4500 r min1 and filtered through a Millipore two-layer film (0.22 μm) before analysis.
3. RESULTS AND DISCUSSION 3.1. Characteristics of Two Catalysts. The crystal structures of TiO2 hydrosol xerogel powder and P25 powder were examined by XRD and the results are presented in Figure 1. The XRD patterns showed that TiO2 hydrosol had a pure anatase structure due to the presence of attributive peaks (2θ = 25.22°, 38.22°, 47.84°, 54.74°, and 62.64°), and P25 powder had a mixed crystal structure of anatase phase (about 80%) and rutile phase (about
20%). It was found that the crystalline degree of P25 powder was significantly higher than that of TiO2 hydrosol from the intensity of their peaks (101). The average crystal size of TiO2 hydrosol was calculated by the Scherrer equation to be 11.3 nm, much smaller than 35.1 nm of P25 powder. The PSD is an important parameter relevant to the stability and transparency of TiO2 hydrosol solution. As shown in Figure 2, TiO2 hydrosol had a single-modal distribution in the range of 12.855.7 nm with a peak at 24.2 nm, which is far smaller than that of P25 powder in the range of 148208 nm with a peak at 173 nm. The average particle size of TiO2 hydrosol (24.2 nm) was about double its crystal size (11.3 nm), due to the mild aggregation in aqueous colloidal solution,26 while that of P25 powder (173 nm) was about 5 times its crystal size (35.1 nm), due to the severe aggregation caused by calcination and also agglomeration in aqueous suspension.27 3.2. Optical Properties of Aqueous TiO2 Hydrosol Solution and TiO2 Powdery Suspension. The P25 powdery suspension and TiO2 hydrosol solution were first examined by traditional spectrophotometric measurements to determine their specific extinction coefficients. In the first set of measurements, the absorbance of UV light at 365 nm by TiO2 hydrosol solution with a solid content of 05 g L1 was determined in an 8-mmlong cell, while that by P25 suspension with a solid content of 00.5 g L1 was determined, respectively. The results showed while the light was fully blocked by the P25 powder suspension at 0.5 g L1, light could still penetrate the TiO2 hydrosol solution at a much higher solid content of >5 g L1 effectively, indicating that TiO2 hydrosol is much more transparent than P25 powder. The results in Figure S3 showed that light absorption by both suspensions was linearly increased with an increased cell length as an optical path. According to the BeerLambert law, the specific extinction coefficient (ελ) can be calculated by eq 1. ελ ¼
2:303 EXTλ d Ccat
ð1Þ
where d is the cell length and Ccat is the mass concentration of catalyst. According to the measured data, the specific extinction coefficients (ελ) at 343380 nm for both catalysts were determined to be 147 m2 kg1 for TiO2 hydrosol and 1447 m2 kg1 for P25 powder, which is close to the value of 1350 m2 kg1 for P25 reported by another study.30 These results indicate that the gradient light intensity throughout the reaction solution can be effectively eliminated by using more transparent catalysts such as 7843
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Figure 4. Dependence of the reaction rate constants (k, 103 min1) for MO degradation on the solid content of catalysts.
the TiO2 hydrosol. The new concept of optical thickness (τ) as a dimensionless parameter can be defined by eq 2.23 Optical thickness ðτÞ ¼ cell length ðd, mÞ specific extinction coefficient ðελ , m2 kg1 Þ catalyst concentration ðccat , kg m3 Þ
ð2Þ
The data obtained from the above optical measurements for two types of catalysts are compared in Figure 3 in terms of apparent light absorption vs optical thickness. The results showed that P25 powder suspension causes a much greater light absorption with an increased optical thickness (due to the increase of either cell length or catalyst concentration) than TiO2 hydrosol solution. Such an optical property of P25 powder would result in its optimum concentration in its slurry photoreaction system to a very low level. Light distribution in a photoreactor determines the LVRPA and therefore the rates of chemical reactions that involve absorbed photons. Thus, the assessment of the photon absorption rate is important for parameter estimation of reaction kinetics, quantum efficiency calculations,31 and photoreactor design.21 As the specific extinction coefficient (ε) is a sum of absorption coefficient (σa) and scattering coefficient (σs) (ε = σa þ σs), the optical properties of TiO2 hydrosol and TiO2 P25 powder in the range of 230450 nm were further measured by using an integrating sphere assembly to determine the values of σa and σs at different wavelengths, respectively. According to the measured data of transmittance (T%) and reflectance (R%), absorbance (A%) can be determined for TiO2 solutions at different concentrations and wavelengths. If the scattering albedo (ω) is defined as ω = (σs)/(σa þ σs), the four optical parameters of ε, σa, σs, and ω can be determined by integrating data in the defined wavelength range of 343380 nm. The scattering albedo (ω) was calculated according to the reflectance and transmittance measurements in the range of 0.1 to 2 g L1, as same as the method described in the reference.21 The results were determined to be ω = 0.696 for TiO2 hydrosol, and ω = 0.673 for P25 suspension, indicating both the catalysts have a significant effect of light scattering. 3.3. Kinetics of MO Degradation. To compare the photocatalytic degradation of organic pollutants in the photoreactors using TiO2 hydrosol and P25 powder catalysts, MO was used as the first model pollutant to conduct the photocatalytic reaction, because it is a well-known sulfonated azo dye indicator. Several recent studies have fully investigated the pathway of MO degradation in aqueous TiO2 photoreaction systems. Their
Figure 5. (a) Dependence of the reaction rate constants (k, 103 min1) for MSA degradation on the solid content of catalysts; (b) Dependence of the reaction rates of SO42- formation on the solid content of catalysts.
studies confirmed that hydroxyl radical (OH 3 ) can attack the aromatic rings of its primary intermediate products for further degradation.32 and also concluded that the photocatalytic disappearance of MO followed the pseudo-first-order kinetics satisfactorily.33,34 Two sets of experiments with TiO2 hydrosol (0.2545 g L1) and P25 powder (0.054 g L1) were conducted, respectively, under UVA illumination with the same initial MO concentration of 20 mg L1. The reaction kinetics of MO degradation in both sets of experiments demonstrated that all experimental data were well fitted by the first-order kinetic model. Both the first-order reaction rate constants (k) were calculated and their relationships with the catalyst loadings are presented in Figure 4. While a maximum k value (k = 0.048 min1, R2 = 0.9227) for the MO degradation with TiO2 powder was achieved at an optimum solid content of around 2 g L1, a nearly double value (k = 0.0834 min1, R2 = 0.9541) for that with TiO2 hydrosol was achieved at an optimum solid content of around 9 g L1. Beyond the optimum solid contents, the k values gradually declined with the further increased catalyst solid contents. 3.4. Kinetics of MSA Degradation and Sulfate Formation. In previous studies, it has been concluded that MSA (CH3SO3H) has been identified as a key intermediate product of many odorous sulfur-containing compounds such as methanethiol (CH3SH), alkyl sulfides, and disulfides through photocatalytic oxidation in aqueous solution, and also an important precursor to form aerosol in air.35 MSA can be photocatalytically oxidized to a final product of sulfate ion (SO42-) in aqueous solution.14,36 In this study, MSA was used as another model chemical and the photocatalytic activities of TiO2 hydrosol and P25 TiO2 powder 7844
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Industrial & Engineering Chemistry Research for MSA degradation and sulfate formation were investigated in aqueous MSA solution/suspension with an initial concentration of 40 mg L1 under UVA illumination, respectively. The results of MSA degradation at different solid contents are presented in Figure 5A. The experimental data were well fitted by the firstorder kinetic model, and the reaction rate constants (k) for the MSA degradation were determined accordingly. The results showed while the maximum reaction rate constants (k) for the MSA degradation with P25 powder was found to be 0.0079 min1 (R2 = 0.874) at its optimum solid content of 1 g L1, that with TiO2 hydrosol was found to be 0.043 min1 (R2 = 0.996) at an optimum solid content of 16 g L1. These results further confirmed that using TiO2 hydrosol as a catalyst would allow the photoreactor system to work at a much higher dosage (about 16 times) to achieve a much higher reaction rate (about 5 times) than using TiO2 powder. The results of sulfate formation at different catalyst contents are shown in Figure 5B. The data demonstrated that at a low catalyst dosage of below 0.5 g L1, the reaction rate of sulfate formation (r) in the TiO2 hydrosol solution (r = 0.14 mg L1 min1, R2 = 0.925) was lower than that in the P25 suspension (r = 0.20 mg L1 min1, R2 = 0.998), but a much higher rate of the reaction with TiO2 hydrosol was found at a higher dosage level of beyond 1.0 g L1. For example, the maximum rate of the sulfate formation reaction with TiO2 hydrosol was found to be 1.12 mg L1 min1 at an optimum solid content of 8 g L1, while that in the P25 suspension was only 0.23 mg L1 min1 at an optimum solid content of 1 g L1. These results can be explained as follows: (1) P25 powder has very good photoactivity due to a higher degree of crystallization than the hydrosol, when thermally treated at much higher temperature of around 400 °C, and (2) also P25 has a much higher extinction coefficient (1447 m2/kg) than the hydrosol (147 m2/kg) to absorb light energy more effectively at a low concentration. It is generally believed that the particle size can obviously influence the photocatalytic activity through the “quantum size effect”, but the apparent activity of photocatalyst relies on many other factors such as the crystallinity and crystal structure of catalysts, and also efficiency of light utilization in a photoreactor. The experimental results in this study demonstrated that the significant difference between two types of catalysts in terms of particle size and transparency can result in very different degrees of light utilization in a photoreactor with a same UV light source. 3.5. New Kinetic Model. In general, reactor geometry, photon distribution, and catalyst loading are interrelated concepts that require accurate modeling of the radiation field in a photoreactor. The spatial distribution of photons, i.e., LVRPA depends on the photon source, the optical properties of the catalyst, the distribution of the catalyst, and the reactor geometry. Several approaches have been proposed to calculate the LVRPA in photocatalytic reaction systems in which absorption and scattering of photons occur,21 but most kinetic models are quite complicated. In this study, we attempted to develop a simple kinetic model as a function of catalyst loading by considering three main factors of (i) volumetric rate of photon absorption (VRPA), (ii) effective reaction volume (ERV), and (iii) effective surface area of catalysts (ESAC). If the geometry of the photoreactor with a central UV lamp is illustrated as shown in Figure S1, we may assume that distribution of light intensity in such an annular photoreactor follows eq 3, and the light absorption by catalyst in the whole photoreactor as VRPA can be described by
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Figure 6. Comparison of P25 powder suspension and TiO2 hydrosol solution for (a) VRPA vs catalyst loading, (b) ERV vs catalyst loading, and (c) ESAC vs catalyst loading at a = 1 and b = 1, respectively.
integrating LVRPA in whole photoreactor by eq 4. Ir ¼ e2:303εcd I 0 r0 Z
R
VRPA ¼ r0
Z ðLVRPAÞdv ¼
R
ð3Þ
ðσa ccat IÞð2πrHÞdr
r0
¼ 0:87r0 πHI0 ð1 ωÞð1 e2:303ðR r0 Þεccat Þ
ð4Þ
For a particular photoreactor, the VRPA as a function of I0, ω, and ε increases with an increased catalyst loading (ccat) to gradually approach its maximum value. However, the maximum value can be significantly affected by the scattering albedo of catalyst (ω), which means the catalyst with less scattering fraction would benefit the light absorption with a higher value of VRPA in the whole photoreactor. Figure 6A shows the 7845
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variation of VRPA vs ccat for P25 powder and TiO2 hydrosol with different values of ε and ω. It can be seen that a slightly higher value of maximum VRPA can be achieved in the P25 powder suspension with a lower ω than that in the TiO2 hydrosol solution. However, due to the gradient of light intensity throughout the reaction solution in such a photoreactor system, the value of ERV can be estimated by eq 5. This equation indicates that ERV gradually decreases with an increased catalyst loading (ccat) in the photoreactor and its decreasing rate highly depends on the value of light extinction coefficient (ε) as shown in Figure 6B. Z R Z R I r0 2:303εcðr r0 Þ e Adr ¼ ð2πHrÞdr ERV ¼ I r0 0 r0 r ¼ 0:87πHr0
1 e2:303ðR r0 Þεccat εccat
ð5Þ
As this is a heterogeneous reaction in aqueous TiO2 slurry system and light emission only comes from one direction (a centralized vertical UV lamp), the photocatalytic reaction should occur on the ESAC in its slurry. The theoretical value of ESAC can be defined by eq 6. ESAC0 ¼ jccat
ð6Þ
where j is the total surface area of per gram of catalyst (m2 g1) (j = 49 m2 g1 for P25 and j = 379 m2 g1 for TiO2 hydrosol26). However, many studies confirmed that the agglomeration of TiO2 particles in its slurry system could be serious at high concentrations and result in a decrease in the number of surface active sites, and the effective surface area of catalyst is not always proportional to its concentration. Hence a more realistic value of ESAC may be expressed by eq 7 instead, indicating that at a very low catalyst loading range while ccat , a, ESAC increases proportionally with an increased ccat as ESAC ≈ bjccat. However, due to the agglomeration of free catalyst particles in TiO2 slurry at a very high catalyst loading range while ccat . a, ESAC will gradually approach to a constant value of bj as shown in Figure 6C. ccat ð7Þ ESAC ¼ bj a þ ccat where a and b are two constants. Furthermore, if we assume the photocatalytic reaction follows the first-order kinetic model ((dc)/(dt) = kappc) and the apparent rate constant, kapp, is proportional to the three main factors of VRPA, ERV, and ESAC, the kapp can be expressed by eq 8 as a function of I0, j, ω, ε, and ccat. kapp ¼ k0 VRPA ERV ESAC ¼ kI0 jð1 ωÞ
ð1 e2:303ðR r0 Þεccat Þ2 εða þ ccat Þ
ð8Þ
However, the above equation only considers the light absorption by catalyst alone and a real light distinction coefficient in the reaction solution/suspension could be greatly affected by coexistence of other substances, such as most organics and dyes. Therefore, it is necessary to introduce a factor (f) to correct the light extinction coefficient (ε) in real reaction solution due to coexistence of the substances other than catalyst and the eq 8 can be repressed as eq 9 with one more dimension to better fit the
Figure 7. Fitting of the apparent rate constant (kapp) for MO (a) and MSA (b) degradation vs catalyst loading by eq 9.
real experimental data: kapp ¼ k0 VRPA ERV ESAC ¼ kI0 jð1 ωÞ
ð1 eðR r0 Þf εccat Þ2 εða þ ccat Þ
ð9Þ
The above equation tells us that there is an optimum concentration of catalyst and its peak value depends on incident light intensity (I0), the optical properties of catalyst (ε and ω), and the surface area of catalyst (j). In other word, a good photocatalyst should have a large surface area, but a low fraction of scattering effect in its reaction suspension. According to the values of optical parameters of TiO2 hydrosol solution determined in this study and the data of P25 powder reported in literature, comparison of these two types of catalysts in terms of kapp vs Ccat in our photoreactor system are presented in Figure 7, in which the experimental data of MO and MSA degradations at different solid contents were well fitted by eq 9 with both correlation coefficients of R2 = 0.920.98, respectively. Actually, the higher solid content of catalysts in the aqueous solution/suspension would provide more active sites to absorb light and generate more excited electrons and active radicals. However, the TiO2 power could easily block the light penetration through the reaction solution/suspension due to a strong light extinction coefficient and a serious degree of agglomeration in its suspension. Therefore, the degree of light utilization is limited by its optimum solid content. Other research also reported that the reasons that high dosage of catalysts would cause negative effect in the photocatalytic reaction systems involve (i) the agglomeration of TiO2 particles at high concentrations causing a decrease in the number of surface active sites and (ii) the increase in opacity and light scattering of TiO2 7846
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Industrial & Engineering Chemistry Research particles at high concentration leading to decrease in the passage of irradiation through the sample.37 Therefore, the P25 suspension at its high concentration caused a rapid decline of light intensity in its bulk solution and resulted in a great reduction of its ERV in the photoreactor as shown in Figure 6B. In such a case, most P25 powder particles are in a “dark zone” and not able to work as a catalyst. As a result, the further increase of P25 powder solid content would not be in favor of further enhancing the photoreaction rate. Therefore, the TiO2 hydrosol demonstrated a few advantages over the TiO2 powder, including that (i) TiO2 hydrosol has much finer TiO2 particles than P25 powder; (ii) the aqueous TiO2 hydrosol solution has much better colloidal stability, better homogeneity, and better dispersion (less agglomeration); and (iii) the aqueous TiO2 hydrosol solution is more transparent26,28 (much lower ε) to eliminate the gradient light intensity throughout the reaction solution and allow the photoreactor system to work at a higher catalyst loading for achieving faster photoreaction due to better light utilization than using TiO2 powder.
4. CONCLUSIONS In this study, the characteristics of two types of photocatalysts were compared in detail and it can be concluded that TiO2 hydrosol had smaller crystal sizes and particle sizes in aqueous phase, but a lower degree of crystallinity than TiO2 powder. The TiO2 hydrosol demonstrated much better light penetration property than the TiO2 powder due to its higher transparency. Such an optical character can greatly eliminate the gradient light intensity throughout the reaction solution and allow the reaction system to work at a high catalyst dosage to maximize the efficiency of light utilization. Two sets of experiments for the MO and MSA degradations under UVA illumination confirmed that all degradation reactions with TiO2 hydrosol catalyst were much more efficient than those with TiO2 powder on the basis of same input of light energy. A new kinetic model established in this study can well describe the kinetics of MO and MSA degradation reactions with TiO2 catalysts in an annular photoreactor. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figures S1 through S3. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ852-27666016. Fax: þ852-23346389. E-mail: cexzli@ polyu.edu.hk.
’ ACKNOWLEDGMENT We thank the Research Grant Committee of Hong Kong Government for financial support to this work (RGC PolyU 5232/08E). ’ NOMENCLATURE ccat = catalyst mass concentration (kg m3) d = cell optical path (m) ERV = effective reaction volume (m3) ESAC = effective surface area of catalyst (m2)
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EXTλ = absorbance reading in the solution f = a factor to correct the light extinction coefficient (ε) in real reaction solution due to coexistence of the substances other than catalyst H = effective height of photoreactor (m) I = light intensity at r (W m2) I0 = incident light intensity at r0 (W m2) k and kapp = reaction rate coefficients LVRPA = local volumetric rate of photon absorption (W m3) r = radius inside photoreactor (m) r0 = radius of inner wall of photoreactor (m) R = radius of outer wall of photoreactor (m) VRPA = volumetric rate of photon absorption (W) ε = volumetric extinction coefficient (m1) λ = wavelength (nm) σa = volumetric absorption coefficient (m1) σs = volumetric scattering coefficient (m1) j = surface area of per gram of catalyst (m2 g1) ω = scattering albedo ελ = specific extinction coefficient (m2 kg1) τ = optical thickness
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