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Apr 22, 2010 - Fax: +86-532-84022757. E-mail: [email protected]., †. Qingdao University of Science and Technology. , ‡. Weifang University. Cite th...
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Ind. Eng. Chem. Res. 2010, 49, 4644–4652

Adsorption and Photocatalytic Degradation Kinetics of Gaseous Cyclohexane in an Annular Fluidized Bed Photocatalytic Reactor Qijin Geng,†,‡ Qingjie Guo,*,† and Xuehai Yue† College of Chemical Engineering, Qingdao UniVersity of Science and Technology, Key Laboratory of Clean Chemical Process, Shandong ProVince, 266042, People’s Republic of China, and Department of Chemistry and Chemical Engineering, Weifang UniVersity, Shandong ProVince, 261061, People’s Republic of China

The adsorption and photocatalytic degradation kinetics of gaseous cyclohexane using nano-titania agglomerates were investigated in an annular fluidized bed photocatalytic reactor (AFBPR). A series of adsorption and photocatalytic degradation kinetic equations were developed to explore the relationship of adsorption/ degradation efficiency and operating variables based on Langmuir adsorption law and photocatalytic elementary reactions. The adsorption equilibrium constant, adsorption active sites, and apparent reaction rate coefficient of cyclohexane were determined by linear regression analysis with variation of gas velocity and relative humidity (RH). It has been demonstrated that the initial concentration, RH, and gas velocity have obviously influenced the adsorption/photocatalytic degradation efficiency and corresponding kinetic parameters. In the adsorption process, the variation of adsorption sites and adsorption efficiency with gas velocity indicated that the adsorption controlling step was related to gas velocity. In the photocatalytic degradation process, the relationship of photocatalytic degradation efficiency and RH indicates that the water molecule played a promotion role in photocatalytic degradation of cyclohexane below a humidity inflection point, while it played an inhibition role in photocatalytic degradation of cyclohexane after this point. In addition, the optimal operating conditions were determined according to the maximum degradation efficiency with respect to RH at 20% and the fluidization number at 1.62. 1. Introduction The decomposition of trace contaminants from polluted air using a photocatalytic reactor system has been investigated extensively in the last three decades.1-10 It has been demonstrated that photocatalytic oxidation is a complete and efficient remediation technology in a broad range of pollutants under certain operating conditions at ambient temperature and pressure without any chemical additives. Furthermore, the nanosemiconductor TiO2, employed extensively as a photocatalyst, is inexpensive, safe, and very stable with a high photocatalytic activity. Therefore, these photocatalytic degradation systems are actively used as an economical remediation method for various purification applications. Different types of photoreactors have been developed to accommodate the TiO2 photocatalyst. Generally, the photoreactors can be classified as fluidized bed photoreactors and fixed bed photoreactors. As compared to fixed bed photoreactors, the fluidized bed photoreactors can offer superior mass transfer efficiency and light transmission. The idea of using a fluidized bed reactor as both uniform light distribution and an immobilizing support for photocatalysts was originally proposed and theoretically evaluated by Yue and Khan.1 Experimental application of this idea has been demonstrated by Dibble and Raupp,2 who designed a bench-scale flat plate fluidized bed photoreactor for photocatalytic oxidation of trichloroethylene (TCE). Recently, Lim et al.3 have developed a modified twodimensional fluidized bed photocatalytic reactor system and determined the effects of various operating variables on the decomposition of NO. Nelson4 reported the photocatalytic oxidation of methanol using a titania-based fluidized bed reactor. * To whom correspondence should be addressed. Tel: +86-53284022757. Fax: +86-532-84022757. E-mail: [email protected]. † Qingdao University of Science and Technology. ‡ Weifang University.

The photocatalytic degradation of TCE in a fluidized bed reactor was investigated by Lim and Kim.5 In particular, Lim and Kim6 have systematically investigated the performance of supported TiO2 photocatalysts for the degradation of gaseous organics in an annular type fluidized bed photoreactor. This unique reactor configuration provides both good penetration of the UV light into the photocatalyst bed and sufficient contact of the photocatalyst and reactant gas. Therefore, photocatalytic oxidation of airborne contaminants in the annular fluidized bed photoreactor appears to be a promising process for the remediation of volatile pollutants in an indoor environment. In recent years, many papers have been published on the treatment of gaseous pollutants, such as formaldehyde,7 benzene,8 TCE,2,5,6 ammonia,9 and acetone,10 in a fluidized bed reactor or other reactors. It should be noticed that the C-H bond activation leading to photocatalytic oxidation of hydrocarbons was one of the most challenging chemical problems. Up to now, some progress has been made in the area of photocatalytic oxidation of cyclohexane under mild conditions.11 However, there is little information in the open literature concerning the adsorption and photocatalytic oxidation of gaseous cyclohexane in an annular fluidized bed photocatalytic reactor (AFBPR). So far, the kinetic analyses for these processes have been not presented in the published literature. In the present work, the photocatalytic degradation of cyclohexane was chosen to test for the photocatalytic activity of nano-titania agglomerates in the AFBPR, focused on an investigation of the adsorption/photocatalytic degradation kinetics of gaseous cyclohexane and the influences of the corresponding operating variables. The effects of relative humidity (RH) and initial concentration (C0) on adsorption/degradation efficiency have been investigated based on the Langmuir model. The influences of RH and initial concentration on adsorption equilibrium constant (Ka), adsorption active site (NT), and the apparent reaction rate constant (kapp) were determined. The main

10.1021/ie100114e  2010 American Chemical Society Published on Web 04/22/2010

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The adsorption efficiency (ηA) and degradation efficiency (ηD) of cyclohexane were calculated by eqs 1 and 2 based on the corresponding peak areas in GC curves, respectively.

Figure 1. Schematic diagram of AFBPR. Key: 1, gas inlet; 2, air cleaner; 3, gas pump; 4, valve; 5, rotameter; 6, humidity adjustor with temperature and humidity detector, 608-H1; 7, cyclohexane concentration adjustor; 8, AFBPR; 9, gas outlet; 10, cyclohexane concentration detector, GC; 11, pressure transducer; 12, signal sensor; 13, particulate velocity detector, PV6A; 14, multifunction data processor, LD-185B; 15, PC data system; and 16, UV light lamp.

roles of gas velocity in this AFBPR have been explored based on adsorption and degradation efficiencies. 2. Experimental Section 2.1. AFBPR Design. In the present work, a schematic diagram of AFBPR is presented in Figure 1 and is composed of a concentric double-pipe structure fluidized bed reactor, equipped with an ultraviolet lamp (OD, 18.5 mm; length, 640 mm; 25 w; and the maximum emission intensity at a wavelength of 254 nm; Shanghai Yaming Lighting Co., Ltd., China) at the center of the inner and outer sleeves of the annular fluidized bed reactor. The distance between the lamp outside surface and the inner sleeve (Pyrex glass tubes with a thickness of 1.8 mm) is 100 mm. The annular distance and height of the annular photocatalytic fluidized bed reactor are 20 and 640 mm, respectively. Nanosized TiO2 (Degussa P25, Shanghai, China), as a photocatalyst, was fed into the annular reaction region. SEM (field emission scanning electron microscopy, JSM-6700F, JEOL, Japan) micrographs of nanosized TiO2 agglomeration before and after fluidization are presented in Figure 2. Air mixed with gaseous cylcohexane and water vapors was used as a fluidized gas media. The experimental unit permitted us to generate polluted air with a specific cyclohexane concentration and RH. 2.2. Analytical Methods. Several operating variables in this photocatalysis have been tested, including the initial concentration of cyclohexane (C0), gas velocity (ug), and RH. The gas velocity is controlled by a rotor flow meter. The RH is determined using a temperature and humidity detector (Type 608-H1, Shanghai, China). The gaseous cyclohexane samples were collected from the sampling pore by a syringe (10 µL), measured by GC-FID (gas chromatograph-flame ionization detector, Clarus-500, PerkinElmer Inc. America). This GC equipped with a FID was used to determine the cyclohexane concentration during the adsorption and photocatalytic oxidation process. The GC operating parameters were as follows. The analytical column was a capillary column with length × ID × OD as 30 m × 0.32 mm × 0.4 µm and a column temperature at 150 °C. The carrier gas and flame gas were hydrogen. The injected volume was 10 µL, and the FID detector temperature was 300 °C.

ηA )

C0 - Ct A0 - At × 100% ) × 100% C0 A0

(1)

ηD )

ce - ct A e - At × 100% ) × 100% ce Ae

(2)

where Ct, C0, and Ce are the t minute and original and equilibrium concentrations of cyclohexane, respectively; A0, Ae, and At were the corresponding peak areas of cyclohexane at the original and equilibrium and t times, respectively. 2.3. Absorption and Photocatalytic Degradation. A typical adsorption experiment in the catalyst in AFBPR was conducted to study the isotherm adsorption of cyclohexane based on the following steps. (1) The valves of the reactor and air pump were opened, and the gaseous pollutant in reactor was circulated from the reservoir (volume, 150 L) to the reaction region at the given speed for 60 min. This time was sufficient for achieving steadystate conditions. (2) At the adsorption-desorption equilibrium, the cyclohexane samples were collected from the sampling pore by syringe and measured using GC-FID, as mentioned before. The photocatalytic activity of the photocatalyst agglomerates was evaluated by the variation of the cyclohexane concentration as a function of irradiation time. A typical experiment in the AFBPR was performed to study the photocatalytic degradation of cyclohexane by the following processes. First, the valve of the reactor and air pump was run, and the gaseous pollutant in the reactor was circulated from the reservoir to the reaction region at the given speed for 60 min to reach the adsorptiondesorption equilibrium; then, a UV light lamp was switched on to illuminate the nano-TiO2 agglomerates under the gas circulation at the required speed ranging from 0.5umf to 2.5umf. Ten microliters of cyclohexane sample was taken as a measurement every 10 min, as mentioned before. 3. Results and Discussions 3.1. Characterization of TiO2 Catalyst Agglomerates. The fluidization behavior of particles is strongly dependent on their physical properties such as the size and density. In the early 1970s, Geldart divided particles into four groups (Geldart C, A, B, and D) according to variations of density and average size. Ultrafine particles, nano-titania, which belong to Geldart C, are extremely difficult to fluidize. However, the formation of agglomerates is a common phenomenon for ultrafine particles because of the strong cohesive forces between primary particles caused by their high surface-to-volume ratios and the small distances between them. The formation of agglomerates strongly affects many properties of bulk powders such as flowability, chemical reactivity, etc., and agglomerates, rather than fine powders, are essential for many processing industries. Ultrafine powders are seldom fluidized discretely but only as agglomerates, and agglomeration fluidization of ultrafine particles has been extensively studied.12 Typical SEM photographs of the unfluidized particles and fluidized agglomerates are shown in Figure 2A-D. From the SEM micrograph of unfluidized particles shown in Figure 2A, it can be seen that the tridimensional netlike structures coalesce into larger conglomerations, and this kind of conglomeration is called a simple agglomeration or nature agglomeration. The results of Figure 2A indicate that these simple agglomerates are loose as coralline and have little interspaces between

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Figure 2. SEM micrographs of nano-titania agglomerates before and after fluidization (A, nature agglomerates; B-D, fluidized agglomerates). Note the fluidization number from 0.5 to 2.5 and the cyclohexane concentration from 7.5 to 187.5 µg L-1 for 1 h. RH ) 20%, temperature ) 20 °C, and nano-titania amount ) 100 g.

agglomerates. The nature agglomerates formed by van der Waals force have enough connecting points between primary particulates; it can be proven by the slugging phenomenon that is yielded at the initial phase of fluidization. In contrast, the typical SEM photographs of the fluidized agglomerates are shown in Figure 2B-D; as observed, the fluidized agglomerates are elliptical with sizes of 1-100 µm, presented in Figure 2D. Therefore, the surface area of fluidized agglomerates becomes smaller than that of nature agglomerates, and the distance between particles is enlarged by the abundant interspaces and tridimensional netlike structures between fluidization agglomerates. These fluidized agglomerates are very light and have limited connecting points between each other, so that when gases pass through the channel (void space between fluidized agglomerates), the agglomeration fluidization, adsorption, and diffusion can occur in AFBPR. Comparing Figure 2A to 2B, the interspaces between particles are obviously different, and the surface area of particles becomes smaller. That is, fluidization agglomeration only changed the surface area and interspaces between particles, not the photocatalytic activity, which is just required to carry out the photocatalytic degradation of cyclohexane in a fluidized bed reactor. 3.2. Isotherm Adsorption of Cyclohexane on the Fluidized Agglomerates. 3.2.1. Influence of Initial Concentration. Isotherm adsorption of pollutants onto photocatalyst is the first step for the photocatalytic reaction. In the present work, the isotherm adsorption of cyclohexane has been investigated in the presence of the nano-TiO2 photocatalyst agglomerates in AFBPR with the variation of cyclohexane concentration. The adsorption of cyclohexane, evaluated by adsorption efficiency (eq 1), is shown in Figure 3. It indicated that the adsorption efficiency of

Figure 3. Influence of cyclohexane concentration and fluidization number on the adsorption efficiency in AFBPR. Note the fluidization number from 0.5 to 2.5 and the cyclohexane concentration from 7.5 to 187.5 µg L-1 for 1 h. RH ) 20%, temperature ) 20 °C, and nano-titania amount ) 100 g.

cyclohexane is a function of cyclohexane concentration, decreased from 40 to 20% with increasing initial concentrations ranging from 7.5 to 187.5 µg L-1 at the given gas velocity and RH (20%). This result can be explained as follows: For the fixed amount of catalyst (100 g), the ratio of adsorption sites to cyclohexane concentration decreased with the growing concentration of cylohexane. Subsequently, the adsorption efficiency decreased with the increasing cyclohexane concentration. 3.2.2. Influence of Gas Velocity. To explore the influence of gas velocity on the adsorption process, the isotherm adsorption of cyclohexane in the dark on the catalyst agglomerations during 60 min under circulation at fluidization number (where

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Figure 4. Regression analysis by eq 4 for the adsorption of cyclohexane on nano-TiO2 photocatalyst agglomerates with variation of gas velocity. At the 0.05 level, the population mean is significantly different than the test mean (0); the confidence intervals of intercept and slope are (0.1078, 1.20171) and (1.52595, 1.78447), respectively.

the fluidization number is NF ) ug/umf and umf ) 36.2 mm s-1) ranging from 0.5 to 2.5 in the AFBPR has been studied. It is shown in Figure 3 that the adsorption efficiency decreased dramatically at a certain high gas velocity or fluidization number, NF ) 1.0-2, while it decreased little at a fluidization number of NF < 1.0 and NF > 2. These results may be attributed to the fact that the adsorption affinity of gaseous cyclohexane molecules onto the surface of the TiO2 catalyst agglomerates is a Van der Waals force. As a result, cyclohexane adsorption onto nano-titania agglomerates is a physical adsorption. As compared to the fixed bed (NF ) 0.5-1), the adsorption efficiency decreased with an increase in the fluidization number ranging from 1 to 1.5 since the enhancement of desorption was yielded by the formation of bubbles in the fluidization bed. However, at the higher gas velocity (NF ) 1.5-2.5), the adsorption efficiency at the given concentrations increased and was approximated to the values at the fixed bed (NF ) 0.5). This interesting finding can be explained as follows: Gaseous cyclohexane molecules penetrate through the boundary layer surrounding the agglomerates and diffuse into the inner channels of agglomerates. Many researchers13 reported that the adsorption of gas molecules followed the Langmuir isotherm adsorption law. The conventional Langmuir isotherm model with a surface coverage θ can be expressed by eq 3. θ)

Nads KaCe ) NT 1 + KaCe

(3)

which can be modified to eq 4 to determine the total number of adsorption sites available for the gas molecules NT and the adsorption equilibrium constant Ka from the data of Figure 3. 1 1 1 1 ) + Nads NT NTKa Ce

(4)

where the Nads is the number of adsorbed gaseous cyclohexane molecules and Ce is the concentration of gaseous cyclohexane at adsorption equilibrium. The isotherm parameters obtained, NT and Ka, were determined using linear regression analysis, and the corresponding values are shown in Figures 4 and 5 with various fluidization numbers ranging from 0.5 to 2.5.

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Figure 5. Regression results for the adsorption sites and adsorption equilibrium constants of cyclohexane on nano-TiO2 agglomerates with variation of gas velocity.

As shown in Figure 5, the number of adsorption sites is a function of gas velocity. In particular, the maximum value of adsorption sites was approximated to 2.71 × 10-4 mmol g-1 at a fluidization number of 2.11, and the minimum value of adsorption sites was approximated to 0.81 × 10-4 mmol g-1 at fluidization number of 1.37. These results may be related to the fluidized bed characteristic and agglomerate structure. For a fixed bed (NF < 1), nano-titania nature agglomerates with the loose tridimensional netlike structures coalesce into the larger conglomerations presented in Figure 2A and are immobilized in the bed. As a result, gas molecules can efficiently come into contact with the adsorption sites on the particle surface when they pass through these loose tridimensional netlike interspaces. When the gas velocity increased and exceeded the minimum fluidization velocity, nanoparticles began to congregate to form fluidized agglomerates, shown in Figure 2B-D. The total surface areas of agglomerates became obviously smaller than those of nanoparticles in a fixed bed due to the augmentation of the particle diameter, and the distance between particles was enlarged. As a result, the molecule adsorption active sites decreased with increasing gas velocity ranging from umf to 1.5umf. Moreover, at a gas velocity above 1.5umf, the cause for the increase of adsorption active sites may be not only involved with the surface adsorption of agglomerates but related to penetration through the boundary layer surrounding the agglomerates and diffusion or migration into the pores or inner channels of these fluidized agglomerates. Generally, an adsorption process can be described by the following four consecutive stages:14 (I) transport of adsorbate in bulk, (II) diffusion across the boundary layer surrounding the adsorbent particles (subsurface region diffusion), (III) migration of adsorbate within the pores of the adsorbent (intraparticle diffusion), and (IV) adsorption/desorption on the solid surface. All of these processes may be involved in controlling the adsorption rate. However, it is usually assumed that one of these steps is the slowest and controls the rate of sorption. On the basis of the values of adsorption efficiency and adsorption sites in the present work, the conclusions can be obtained that the controlling step in adsorption process is up to the gas velocity. From the results presented in Figures 3-5, at a fluidization number exceeding 2.11, where the adsorption and adsorption active sites decreased with increasing gas velocity, this process may be controlled by adsorption/ desorption on the agglomerate surface. While the fluidization number varies from 1.37 to 2.11, the controlling step may be

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Figure 6. Influence of RH and initial concentration on the adsorption of cyclohexane. Note the RH from 5 to 40% and the cyclohexane concentration from 0.75 to 187.5 µg L-1 at a gas velocity of 2umf.

the migration or diffusion of cyclohexane molecules into the pores or inner channels of the agglomerates. Finally, at a fluidization number less than 1.37, this adsorption process may be related to diffusion across the boundary layer surrounding the agglomerates (subsurface region diffusion) or transport of cyclohexane in the bulk. In addition, the calculated values of the adsorption equilibrium constant range from 0.23 × 10-4 to 0.61 × 10-4 mmol-1 g-1 with variation of gas velocity, shown in Figure 5. From the curve in Figure 5, the maximum value of adsorption equilibrium constant was approximated to 0.61 × 10-4 mmol-1 g-1 at a fluidization number of 1.42. It can be explained by the molecule adsorption equilibrium constant definition. Equation 5 shows that the adsorption equilibrium constant is a function of resident time and adsorption active site (NT) at the given temperature. Ka )

τ A (2πMRT)1/2 NT

(5)

where A is Avogadro’s constant (6.02 × 1023 mol-1), M is the molecular weight of cyclohexane (kg mol-1), R is the gas constant (8.314 J K mol), T is the temperature (K), and τ is the resident time (τ ) Hexp/ug, s). It is obvious that the adsorption equilibrium constant is directly proportional to the resident time but inversely proportional to adsorption active sites, which are related to gas velocity. Therefore, the adsorption equilibrium constant is approximated to the maximum value only at the optimal gas velocity or the fluidization number at 1.42. 3.2.3. Influence of RH. For the photocatalytic oxidation of a series of organic compounds, RH has a significant influence on photocatalytic oxidation rates and reaction paths.15,16 In the present investigation, the influence of RH on cyclohexane adsorption was investigated with respect to RH from 5 to 40%. The results presented in Figure 6 indicated that the adsorption efficiency of cyclohexane was a function of RH and cyclohexane concentration, decreasing with an increase in RH and/or concentration. In particular, the adsorption efficiency decreased dramatically at a high concentration and RH. This can be explained by competition adsorption between water and cyclohexane molecules on nano-titania agglomerates. This significant decrease in cyclohexane adsorption with a variation of RH indicated that cyclohexane molecules were precluded from contacting the TiO2 agglomerates surface or absorbing onto the adsorbed water layer due to the hydrophobic nature of cyclohexane. It was similar to the result reported by Goss.17

Figure 7. Regression analysis by eq for the adsorption of cyclohexane on nano-TiO2 photocatalyst agglomerates with variation of RH. At the 0.05 level, the population mean is significantly different than the test mean (0); the confidence intervals of intercept and slope are (0.01328, 1.63202) and (1.58469, 2.0407), respectively.

Figure 8. Regression results for the adsorption sites and adsorption constant of cyclohexane on TiO2 photocatalyst agglomerations with variations of RH.

The Langmuir isotherm model and the linear regression equation can be used again to determine the total number of adsorption sites available for the gas molecules NT and the adsorption equilibrium constant Ka based on the data of Figure 6. Isotherm parameters, NT and Ka, were determined using linear regression analysis, shown in Figure 7, and the corresponding values are presented in Figure 8 at a RH from 5 to 40%. As observed from Figure 8, the number of adsorption active sites decreased dramatically with increasing RHs ranging from 5 to 20%, while keeping constant after a RH over 20%. However, the adsorption equilibrium constants increased with a variation of RH below 20% and varied very little with respect to the RH above 20%. This result indicated that a high RH could inhibit cyclohexane molecules from adsorbing onto the surface of agglomerates. This result was similar to the report of Zhang18 that both adsorption onto the water surface and penetration through the water film were limited due to the hydrophobic nature of chlorobenzene (CB). 3.3. Photocatalytic Degradation of Cyclohexane on Catalyst Agglomerations. A traditional simplified mechanism of heterogeneous photocatalysis typically involves the photoexcitation of a semiconductor catalyst, leading to the formation of free charge carriers (electrons, e-; and holes, h+). A portion of these photogenerated pairs recombines in the bulk of the semiconductor, while the rest migrate to the surface of particles,

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Table 1. Mechanism of Photocatalytic Oxidation step

elementary reaction

R1

TiO2 + hν 98 e- + h+

R2

e- + h+ 98 heat

R3

h+ + H2O 98 •OH + H+

R4

•

OH + organic 98 product

R5

•

OH + •OH 98 H2O2

k1

k2

k3

k4

Figure 9. Influence of cyclohexane concentration on the degradation efficiency.

k5

where the holes are subsequently trapped on the surface and poised to react with adsorbed water and oxygen to produce active species, that is, hydroxyl radicals. These species initiate a wide range of chemical redox reactions, leading to a complete mineralization of pollutants. Generally, the following elementary reactions presented in Table 1 have been taken into account: R1 refers to the photonic activation step; R2 depicts the recombination step between e- and h+; R3 represents the formation of hydroxyl radicals; R4 characterizes the transformation of the organic compound (Org) into product (P) by OH• attack; and in R5, the recombination between two hydroxyl radicals is suggested.19 The photocatalytic degradation rate (r) of the organic compound is represented by the following expression according to step R4. r ) k4[•OH][θ]M

d[•OH] ) k3[h+][H2O]-k4[•OH][C]-k5[•OH]2 = 0 (11) dt where the recombination of hydroxyl radicals and oxidation of target compound is competitive. The excessive OH• radical generation at high intensity led to their self-recombination. At low concentrations of target compound, the recombination of hydroxyl radicals can be assumed as the predominant reaction (k4[•OH][C] , k5[•OH]2). Subsequently, the concentration of holes can be obtained as follows.

[•OH] )

(6)

where the target compound adsorption coverage, [θ]M, can be given using the Langmuir-Hinshelwood (L-H) model, considering the competition adsorption between water and target molecules. The target compound and water molecule adsorption coverage can be given by eqs 7 and 8. [θ]M )

Considering the formation, oxidation, and the recombination of hydroxyl radicals based on the elementary reactions R3-R5 in Table 1, the concentration of •OH can be obtained according to the steady-state assumption.

KA[C] 1 + KH[H2O] + KA[C]

[( (( k3

r ) k4[•OH][θ]M ) k4

k3

where

(8)

The concentration of photoinduced holes, [h ], can be obtained based on the mentioned elementary reactions R1-R3 and the steady-state assumption. d[h+] ) k1I[CTiO2] - k2[h+]2 - k3[h+][H2O] = 0 dt

(9)

Under high irradiation intensities, the recombination of electron-hole is predominant, that is, k2[h+ ]2 . k3[h+ ][H2O]. As a result, the concentration of holes can be given by [h+] )

(

k1I[CTiO2] k2

)

1/2

(10)

k5

k1I[CTiO2]

(( k3

+

k2

)

1/2

] )

1/2

[θ]H2O

(12)

Therefore, the corresponding reaction rate can be expressed as

(7)

KH[H2O] [θ]H ) 1 + KH[H2O] + KA[C]

k1I[CTiO2]

k' ) k4

k2 k5

)

1/2

1/2

[θ]H2O

[θ]M

) k'[θ]H2O1/2[θ]M (13)

k1I[CTiO2] k2 k5

)

)

1/2 1/2

3.3.1. Influence of Cyclohexane Concentration. The effect of the initial concentration on the photocatalytic degradation efficiency was investigated ranging from 7.5 to 187.5 µg L-1. The experimental results presented in Figure 9 indicate that the photocatalytic degradation efficiency in AFBPR was a function of illumination time and cyclohexane concentration and decreased with an increase in the initial concentrations of cyclohexane at the given gas velocity and RH. Theoretically, at low concentrations of cyclohexane, the reaction rate equation can be given by the following expression at the fixed RH and gas velocity based on Langmuir adsorption law without considering competition adsorption.

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r ) k'

KH1/2[H2O]1/2KA[C]

1 . KA[C]

(1 + KA[C])(1 + KH[H2O]

1/2

) 98 kapp[C]

(14) where the apparent reaction rate coefficient is kapp ) k'KA

KH1/2[H2O]1/2 1 + KH[H2O]1/2

According to eq 14, the reaction rate, dependent on the cyclohexane concentration, is an apparent first-order kinetic expression at the given conditions. This can be attributed to the fact that the colliding probability between hydroxyl radicals and cyclohexane molecules increases with increasing cyclohexane concentration. This behavior is in agreement with the results proposed by Kim and Hong.20 The corresponding integral equations can be yielded as follows based on eq 14.

( )

ln

C0 ) kappt C

ηD ) 1 - exp(-kappt)

(15) (16)

At the given experimental conditions, that is, illumination, humidity, amount of catalyst, and gas velocity, a plot of ln(C0/ C) vs t should be linear, and the values of apparent reaction rate coefficient can be obtained. The fitted results presented in Figure 10 indicate that the values of the apparent reaction rate coefficient decreased with an increase in the cyclohexane concentration. It may be attributed to the following reasons. (I) At high concentrations, the fixed surface adsorption active sites of catalyst are not enough for the amount of cyclohexane molecules. (II) The amount of products or intermediates formed in photocatalysis can occupy part of the active sites of catalyst to inhibit the oxidation progression. The half-life is an important kinetic parameter to explore the influence of concentration on the photocatalytic degradation progression of cyclohexane in AFBPR. The half-life of photocatalytic degradation reaction was introduced based on the halflife definition of the first-order reaction rate equation (t1/2 ) ln 2/kapp), and the results are presented in Figure 11. From the results listed in Figure 11, the photocatalytic degradation halflife was a monotone increasing function of cyclohexane concentration, expressed as t1/2 ) 50.238 + 0.793C0

Figure 10. Relationship of apparent reaction rate coefficient and concentration of cyclohexane. At the 0.05 level, the population mean is significantly different than the test mean (0); the confidence interval for apparent reaction rate coefficient is (0.00223, 0.01229).

(17)

From eq 16, we can make the conclusion that the photocatalytic degradation efficiency increases with increasing illumination time, while it decreases with thr growing concentration of cyclohexane. In sum, the influence of concentration on the degradation efficiency and extension of half-life may be related to the following reasons. (1) In the present experiment, byproducts or intermediates were extracted from the catalyst surface using methanol solvent after the finishing experiment. Then, the extracted solution was detected using a UV-visible spectrophotometer (TU-1901, Beijing Purkinje General Instrument Co., Ltd., China). As a result, we have identified cyclohexanol, cyclohexanone, and 2-cyclohexen-1-one as the main intermediate products in the catalyst surface based on the following information. These byproducts adsorbed on the surface of the catalyst can inhibit the cyclohexane molecules from adsorbing into the reactive sites of the catalyst. (2) The degraded products,

Figure 11. Relationship of half-life and cyclohexane concentration.

as scavengers of holes generated in the photocatalytic process, were detrimental to the photocatalytic action. (3) The process of photocatalytic degradation of cyclohexane may be retarded by the degradation path in which cyclohexane was decomposed into cyclohexanol, cyclohexanone, and 2-cyclohexen-1-one. 3.3.2. Influence of Gas Velocity. For AFBPR, the photocatalytic degradation of cyclohexane was carried out with variation of the gas velocity for a fluidization number ranging from 0.5 to 2.5, and the relationship of photocatalytic degradation efficiencies and gas velocity is shown in Figure 12. From Figure 12, it was found that the photocatalytic degradation efficiencies increased gently with increasing fluidization numbers ranging from 0.5 to 1. However, the photocatalytic degradation efficiencies increased dramatically with increasing fluidization numbers ranging from 1 to 1.62. At high gas velocities for fluidization numbers from 1.62 to 2.5, the photocatalytic degradation efficiencies decreased slowly. These experimental results indicated that the photocatalytic degradation efficiencies of cyclohexane in AFBPR exhibited the maximum values at a certain operating gas velocity. As observed from Figure 12, the fluidization number at 1.62 may be the optimal gas velocity for the efficient contact of photocatalyst, UV light, and reactant molecules. In this case, the mechanisms of gas velocity on the photocatalytic degradation of cyclohexane in AFBPR are very complex since the bed expansion, axial distribution of bed

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Figure 12. Influence of gas velocity on the degradation efficiency of cyclohexane.

voidage, radial distribution of incident light irradiation, residence time of reactants, and mass transfer between reactant and particulates vary with gas velocity. The comparison to adsorption efficiency was approximated to the minimum value at a fluidization number of 1.62 in Figure 3, while the degradation efficiency was approximated to the maximum value in Figure 12, and we can infer that mass transfer resistance does not exist, but the photocatalytic degradation process is obviously involved in the illumination transmission resistance. For the fixed bed, gas velocity below the minimum fluidization velocity, the whole bed is approximated to an immobilized bed, and gas molecules can break through the interspaces among particles out of the reaction region. From the adsorption curve in Figure 3, we can infer that mass transfer resistance does not exist, but the illumination transmission is inhibited by immobilized agglomerates. 3.3.3. Influence of RH. RH is known to significantly influence the photocatalytic oxidation process. The water molecules can be cleaved and transformed into hydroxyl radicals (OH•) by reaction with the photogenerated holes (h+) at the photocatalyst surface based on step R3. In the absence of water vapor, the photocatalytic degradation of some chemical compounds is seriously retarded, and the total mineralization to CO2 does not occur. However, excessive water vapor on the catalyst surface will lead to the decrease of the reaction rate since water molecules can occupy the active sites of the reactants on the surface. In this case, the influence of RH on photocatalytic degradation of cyclohexane was investigated. The results presented in Figure 13 indicated that the photocatalytic degradation efficiency of cyclohexane was approximated to the maximum value at RH inflection points with variation of cyclohexane concentrations (RH inflection points at 22.13, 24.28, and 26.80% with variations of concentration at 7.5, 37.5, and 112.5 µg L-1, respectively). In addition, the influence of RH on the photocatalytic degradation kinetics of cyclohexane in AFBPR was discussed. The apparent reaction rate coefficient of photocatalytic degradation reaction was calculated based on eq 15 and presented in Figure 14. From the result in Figure 14, the photocatalytic degradation reaction rate coefficient was a nonlinear function of RH and cyclohexane concentration and approximated to the maximum value at RH ranging from 10 to 20%. The RH inflection point with variation of cyclohexane concentrations and photocatalytic degradation reaction rate coefficients as a nonlinear function of RH and cyclohexane

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Figure 13. Influence of humidity on the degradation efficiency of cyclohexane.

Figure 14. Influence of RH and cyclohexane concentration on the apparent photocatalyic degradation reaction rate coefficient. Note the RH from 5 to 40% and the cyclohexane concentration from 7.5 to 112.5 µg L-1 at a gas velocity of 2umf.

concentration can be explained by photocatalytic oxidation kinetics based on the elementary reaction, R3, which represents the formation of hydroxyl radicals. The amount of hydroxyl radicals increases with an increase in RH before the RH inflection point. However, the photocatalyitc degradation efficiency and reaction rate coefficient decrease with an increase in RH above these points due to competition adsorption of water and cyclohexane molecules. This result was in agreement with a review of photocatalytic decomposition characteristics of some VOCs reported by Wang et al.21 The results showed that the conversions of toluene and hexane initially increased with an increase of RH up to 40% and then show a decrease. Thereafter, we can conclude that the competition adsorption between water and cyclohexane molecules must be considered at high humidity conditions. According to the binary-molecule L-H model, the target compound adsorption coverage can be expressed as eqs 7 and 8 without considering the intermediates and byproducts adsorbed. Theoretically, the reaction rate equation can be modified as eq 18. r ) k'

KH1/2[H2O]1/2KA[C] (1 + KA[C] + KH[H2O])3/2

(18)

As a result, the derivation equation for the reaction rate over RH can be obtained as follows.

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r' ) k'

0.5KH1/2[H2O]-1/2KA[C](1 + KA[C] - 2KH[H2O]) (1 + KA[C] + KH[H2O])

5/2

(19) As the derivation value of reaction rate equals to zero, the reaction was approximated to the maximum value. Therefore, we can obtain the optimal RH condition for the maximum reaction rate. [H2O] )

1 + KA[C] 2KH

(20)

The desired effect by variation of humidity is to determine the optimal humidity condition for the promotion of the oxidation efficiency in a photocatalytic process. Equation 20 indicated that the optimal RH is not only inversely proportional to the adsorption constant of water molecule (KH) but is dependent on the concentration and adsorption characteristics of cyclohexane (Ka) as well. It is in agreement with the experimental results of Figure 13, where the value of the RH inflection point increases with an increase in the cyclohexane concentration. It is attributed to two reasons: (i) Photocatalytic degradation of cyclohexane with increasing concentration needs the more hydroxyl radicals produced by water molecules adsorbed on the surface of catalyst at a lower RH; and (ii) the competition adsorption between water and cyclohexane molecules may be enhanced with an increasing concentration of target molecules at higher RH, which may retard the photocatalytic oxidation. 4. Conclusions The adsorption and photocatalytic degradation kinetics of gaseous cyclohexane using nano-titania agglomerates have been investigated in AFBPR, and the following conclusions can be derived from the present study. (1) A high adsorption efficiency of cyclohexane was achieved by nano-TiO2 catalyst agglomerates in AFBPR. The mechanism of adsorption between gaseous cyclohexane molecules and adsorption active sites of agglomerates is explored using linear fitted analysis results of adsorption active sites and adsorption equilibrium constants with variations of gas velocity and RH. On the basis of the values of adsorption sites and adsorption equilibrium constants, we can infer that the controlling step in adsorption process is related to gas velocity. (2) A high photocatalytic degradation efficiency of cyclohexane was achieved by nano-TiO2 catalyst agglomerates in AFBPR under UV light. The photocatalytic degradation kinetic parameter, half-life is determined as a monotone increasing function of cyclohexane concentration. The optimal operating gas velocity is determined as 1.62umf. The degradation controlling step may be up to the illumination transmission in the agglomeration fluidization processes. (3) There are some RH inflection points with variations of cyclohexane concentrations, where the reaction kinetic rate equation was presented as [H2O] ) 1 + KA[C]/2KH. The optimal RH conditions at various cyclohexane concentrations for photocatalytic degradation were determined as RH at 22.13, 24.28, and 26.80% with variations of concentrations at 7.5, 37.5, and 112.5 µg L-1, respectively. Acknowledgment This investigation was supported by the Doctorate Programs Foundation of Ministry of Education of China (Contract No.

200804260002), Construction Project of Taishan Scholar of Shandong Province (JS200510036), and Program for New Century Excellent Talents in University (NCET-07-0473). Literature Cited (1) Yue, P. L.; Khan, F. Photocatalytic ammonia synthesis in a fluidized bed reactor. Chem. Sci. Eng. 1983, 38, 1893–1900. (2) Dibble, L. A.; Raupp, G. B. Fluidized-bed photocatalytic oxidation of trichloroethylene in contaminated air streams. EnViron. Sci. Technol. 1992, 26, 492–495. (3) Lim, T. H.; Jeong, S. M.; Kim, S. D.; Gyenis, J. Photocatalytic decomposition of NO with TiO2 particles. J. Photochem. Photobiol., A 2000, 134, 209–217. (4) Nelson, R. J.; Flakker, C. L.; Muggli, D. S. Photocatalytic oxidation of methanol using titania-based fluidized beds. Appl. Catal., B 2007, 69, 189–195. (5) Lim, T. H.; Kim, S. D. Photocatalytic degradation of trichloroethylene (TCE) over TiO2/silica gel in a circulating fluidized bed (CFB) photoreactor. Chem. Eng. Process. 2005, 44, 327–334. (6) Lim, T. H.; Kim, S. D. Trichloroethylene (TCE) degradation by photocatalysis in annular flow and annulus fluidized bed photoreactors. Chemosphere 2004, 54, 305–312. (7) Zhang, M.; An, T.; Fu, J.; Sheng, G.; Wang, X.; Hu, X.; Ding, X. Photocatalytic degradation of mixed gaseous carbonyl compounds at low level on adsorptive TiO2/SiO2 photocatalyst using a fluidized bed reactor. Chemosphere 2006, 64, 423–431. (8) Geng, Q.; Guo, Q.; Cao, C.; Wang, H. Investigation into photocatalytic degradation of gaseous benzene in CPCR. Chem. Eng. Technol. 2008, 31, 1023–1030. (9) Geng, Q.; Guo, Q.; Cao, C.; Wang, L. Investigation into photocatalytic degradation of gaseous ammonia in CPCR. Ind. Eng. Chem. Res. 2008, 47, 4363. (10) Vincent, G.; Marquaire, P. M.; Zahraa, O. Abatement of volatile organic compounds using an annular photocatalytic reactor: Study of gaseous acetone. J. Photochem. Photobiol., A 2008, 197, 177–189. (11) Li, X.; Chen, G.; Po-Lock, Y.; Kutal, C. Photocatalytic oxidation of cyclohexane over TiO2 nanoparticles by molecular oxygen under mild conditions. J. Chem. Technol. Biotechnol. 2003, 78, 1246–1251. (12) Jin, Y.; Wei, F. Multi-Phase Chemical Reaction Engineering and Technology (Part II); Tsinghua University Press: Beijing, 2006; pp 841852. (13) Ibhadon, A. O.; Arabatzis, I. M.; Falaras, P. The design and photoreaction kinetic modeling of a gas-phase titania foam packed bed reactor. Chem. Eng. J. 2007, 133, 317. (14) Plazinski, W.; Rudzinski, W. Kinetics of adsorption at solid/solution interfaces controlled by intraparticle diffusion: A theoretical analysis. J. Phys. Chem. C 2009, 113, 12495–12501. (15) Wu, J. F.; Hung, C. H.; Yuan, C. S. Kinetic modeling of promotion and inhibition of temperature on photocatalytic degradation of benzene vapour. J. Photochem. Photobiol., A 2005, 170, 299–306. (16) Obee, T. N.; Brown, R. T. TiO2 photocatalysis for indoor air applications: effects of humidity and trace contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3-butadiene. EnViron. Sci. Technol. 1995, 29, 1223–1231. (17) Goss, K. U.; Schwarzenbach, R. P. Quantification of the effect of humidity on the gas-mineral oxide and gas-salt adsorption of organic compounds. EnViron. Sci. Technol. 1999, 33, 4073–4078. (18) Zhang, L.; Anderson, W. A.; Sawell, S.; Moralejo, C. Mechanistic analysis on the influence of humidity on photocatalytic decomposition of gas-phase chlorobenzene. Chemosphere 2007, 68, 546–553. (19) Vincent, G.; Marquaire, P. M.; Zahraa, O. Abatement of volatile organic compounds using an annular photocatalytic reactor: Study of gaseous acetone. J. Photochem. Photobiol., A 2008, 197, 177–189. (20) Kim, S. B.; Hong, S. C. Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst. Appl. Catal., B 2002, 35, 305–315. (21) Wang, S.; Ang, H. M.; Tade, M. O. Volatile organic compounds in indoor environment and photocatalytic oxidation: State of the art. EnViron. Int. 2007, 33, 694–705.

ReceiVed for reView January 18, 2010 ReVised manuscript receiVed April 3, 2010 Accepted April 12, 2010 IE100114E