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Adsorption and Photocatalytic Oxidation of Methanol−Benzene Binary Mixture in an Annular Fluidized Bed Photocatalytic Reactor Geng Qijin,* Wang Qingming, and Zhang Bin Department of Chemistry-Chemical & Environment Engineering, Weifang University, Shandong Province, 261061, P. R. China ABSTRACT: Adsorption and photocatalytic degradation kinetics of a gaseous benzene−methanol binary mixture in an annular fluidized bed photocatalytic reactor (AFBPR) were investigated. On the basis of a series of adsorption and photocatalytic degradation kinetic equations developed, the influences of molar ratios of benzene−methanol and relative humidity (RH) on adsorption efficiency, degradation efficiency, and half-life were explored. The results indicated that the molar ratio of benzene− methanol and RH has obviously influenced the adsorption/photocatalytic degradation and corresponding kinetic parameters. In the adsorption process, the coadsorption mechanism of methanol−benzene not only was related to competition adsorption but also involved penetrating the multi- or mono-water layer formed on the surface of catalyst particles as well. On the basis of the photocatalytic degradation kinetics of the benzene−methanol binary component, a new mechanism occurred in the photocatalytic oxidation of the methanol−benzene binary mixture due to competitive adsorption of them and some new radicals produced on the same photocatalyst surface was deduced. The special complex relationship between the photocatalytic degradation efficiency and the molar ratio of benzene−methanol with various RH demonstrated that there was an obvious synergy effect between benzene, methanol, and water molecule in photocatalytic degradation processes. This investigation highlights the importance of controlling RH and molar ratio in binary mixture in order to obtain the desired synergy effect in PCO processes.

1. INTRODUCTION Daily and extensive use of large amounts of volatile organic compounds (VOCs) in various industrial and domestic activities leads to their presence in the environment, causing atmospheric pollution. In particular, VOCs emitted from a wide range of industrial facilities and activities, such as chemical, petrochemical, pharmaceutical, food processing, dyeing, and printing works, may pose a threat to human health and natural populations. On the other hand, VOCs are well-known indoor pollutants, i.e., benzene, formaldehyde, etc., emitted from various sources, such as combustion byproducts, cooking, construction materials, office equipment, and consumer products, etc. They are connected to sick building syndrome (SBS), where occupants experience symptoms of reduced human comfort or health.1 Over the last 30 years, health complaints related to poor air quality have dramatically increased. Therefore, air pollution has become a major public health concern. Photocatalytic oxidation (PCO) has been developed toward VOCs treatment for several years. Since 1995, the number of reports and patents per year on photocatalytic treatment of air pollutants has been continuously increasing. To date, some progress has been made in the area of oxidation of methanol and benzene under mild conditions. Chiarello et al. 2 investigated the effect of the CH3OH/H2O partial pressure ratio on the mechanism of the gas-phase photocatalytic reforming of methanol on Au-modified TiO2. The results indicated that methanol oxidation proceeds on the photocatalyst surface up to CO2 through formation of formaldehyde and formic acid as intermediate species. In addition, composite TiO2/carbon thin films were prepared on fused silica substrates to enhance the photocatalytic activity toward decomposition of methanol to CO2 and water, in which the observed enhance© 2012 American Chemical Society

ment of photocatalytic activity is due to synergy effects at the carbon/TiO2 interface.3 Sadale et al.4,5 also investigated into the PCO of gaseous methanol with Pt-loaded WO3−TiO2 composite film. In this case, PCO of gaseous methanol over this composite thin film proceeds with competitive direct and indirect hole-transfer reactions. By comparison, a glass-plate reactor coated with a commercial titanium dioxide was used to investigate the performance of PCO of benzene at an inlet concentration ranging from 0 to 3.5 ppm.6 In these cases, the concepts of the active region, deactivation region, inhibiting concentration, maximum reaction rate, and maximum required light intensity were applied to explain the deactivation model. The PCO of benzene, toluene, ethylbenzene, and m-xylene (BTEX) in the gas phase over TiO2-based catalysts was studied using P25 adding 0.25% (w/w) Pt, Fe, or Ce (P25, P25/Pt, P25/Fe, P25/ Ce) at inlet BTEX concentrations ranging from 0.5 to 21 ppmv.7 The corresponding kinetic data obtained confirmed the strong beneficial effect of P25/Ce on PCO of benzene and ethylbenzene. In summary, PCO concerning gaseous benzene or methanol was reported widely in the present published reports and explored the influence of initial concentration of target compound, relative humidity (RH), and ultraviolet (UV) photon flux on removal rate or photocatalytic degradation efficiency, even identifying or quantifying formation of reaction intermediates. However, PCO of the binary mixture has little information in recent reports. Adsorption and photocatalytic Received: Revised: Accepted: Published: 15360

August 16, 2012 October 13, 2012 October 23, 2012 October 23, 2012 dx.doi.org/10.1021/ie302207p | Ind. Eng. Chem. Res. 2012, 51, 15360−15373

Industrial & Engineering Chemistry Research

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Figure 1. Schematic diagram of AFBPR: (1) gas inlet; (2) air cleaner; (3) gas pump; (4) valve; (5) rotameter; (6) benzene concentration adjustor; (7) methanol concentration adjustor; (8) RH adjustor; (9) mixed air reservoir; (10) sample gas concentration detector, GC; (11) annular fluidized bed photocatalytic reactor; (12) pressure transducer and multifunction data processor; (13) particulate velocity detector; (14) UV light pump; (15) PC data system; (16) photocatalyst.

were carried out in a laboratory-scale version of AFBPR using nano-TiO2 (Degussa P25, Degussa Corp., Shanghai, China; primary particle size was from 5 to 20 nm, with a BET specific surface area of 50 m2 g−1; anatase and rutile percentages were 70% and 30%, respectively) as a photocatalyst at ambient conditions. A schematic diagram of AFBPR shown in Figure 1 was a concentric double-pipe structure fluidized bed reactor described previously,13 equipped with an ultraviolet lamp (25 W, maximum emission intensity at a wavelength of 253.7 nm, Shanghai Yaming Lighting Co., Ltd., China) at the center of the annular fluidized bed. AFBPR operates in batch mode with a given amount of gaseous benzene/methanol injected in a set volume of air. Benzene and methanol samples were vaporized almost instantaneously and mixed intimately with the treated air stream, respectively. The continuous gaseous stream of benzene/methanol was generated using two different steel cylinders filled with pure liquid benzene and methanol, respectively. Steel cylinders were placed inside two thermostatic water baths maintained at constant temperatures ranging from 50 to 80 °C depending on the desired inlet concentrations. The target benzene and methanol concentrations, ranging from 0.1 to 0.9 μmol L−1, were obtained by adjustment of the gas flow rates using mass flow controllers (Type LML-1, Changchun mobile fittings Co., Ltd., China). RH was determined using a temperature and humidity detector (Type 608-H1, Shanghai, China). Given that the available oxygen in air exceeded that consumed in stoichiometry, experiments were performed with organic model pollutant benzene and methanol as the limiting reagent in this case. As a result, oxygen effects were neglected in the kinetic modeling. Gaseous benzene and methanol samples, collected from the sampling pore by syringe (10 μL), were measured using a GCFID (gas chromatograph-flame ionization detector, Clarus-500, PerkinElmer Inc., United States) equipped with a flame ionization detector (FID). The corresponding operational parameters for GC-FID were described as follows. The analytical column is a capillary column with length × o.d. × i.d. as 30 m × 0.32 mm × 0.4 μm and column temperature fixed at 200 ◦C. The carrier gas and flame gas are hydrogen. The FID detector temperature is given at 300 °C. GC peak identification

degradation of 2-propanol and toluene binary mixture as model pollutants were investigated by Vildozo et al.8 over a commercial air cleaning TiO2 filter, at inlet concentrations from 80 to 400 ppbv, RH at 0% and 60%, respectively, and gas velocity at 300 mL min−1. The results indicated that the occurrence of different reaction pathways depends on RH. Chen et al.9 investigated the adsorption and degradation of VOCs mixture (ethyl acetate, ethanethiol, and toluene) using a combined titania−montmorillonite−silica (CTMS) photocatalyst in a fluidized bed photocatalytic reactor. In this case, competitive adsorptions were observed for the mixed pollutants on the catalysts, and almost 100% degradation efficiency was achieved within 120 min for each pollutant with about 500 ppb initial concentrations. These results indicated that the photocatalytic degradation rate constants of multicomponent systems were lower than those for single systems and increased with increasing adsorption capacities for various components. The influences of concentration proportion in formaldehyde− benzene binary mixture on the adsorption, degradation, kinetics, and selectivity in a circulated photocatalytic reactor (CPCR) were investigated.10 The results showed that the selectivity of photocatalytic oxidation of benzene and formaldehyde occurred in the binary mixture. PCO technology has already shown its advantages toward VOCs abatement;11,12 however, the complex interaction between the methanol−benzene binary mixture in the adsorption and photocatalytic reactions needed to be explored. In this work, the PCO of methanol−benzene binary mixture on commercial TiO2 agglomerations in an annular fluidized bed photocatalytic reactor (AFBPR) designed was investigated considering various RH at ambient conditions. Continuous experiments in the AFBPR, using presaturated polluted gas flows containing various molar ratio of methanol−benzene, were performed in an effort to obtain some information concerning the surface adsorption processes, photocatalytic reactions, and synergy effect between methanol−benzene binary mixtures during PCO processes.

2. EXPERIMENTAL SECTION 2.1. AFBPR Designed. Experiments of the photocatalytic degradation of gaseous benzene−methanol binary mixtures 15361

dx.doi.org/10.1021/ie302207p | Ind. Eng. Chem. Res. 2012, 51, 15360−15373


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of benzene/methanol was according to their different retention times. During the experiments, the samples were collected at regular time intervals at a total gas velocity of 40 mm s−1 (the minimum fluidization velocity for TiO2 agglomerate in AFBPR is 36.2 mm s−1). Attached, the repeatability of the whole analytical procedure (sampling and analysis) was tested with a series of three samples of standards, and the resulting relative standard deviation was less than 2%. 2.2. Absorption and PCO. A typical experiment was conducted to study the isotherm adsorption of benzene and methanol onto the catalyst agglomerates in AFBPR. Gaseous benzene and methanol mixed with air in the reactor was circulated from the reservoir (volume including pipelines, 150 L) to the reaction region at a given speed of 40 mm s−1 for 120 min. This time was sufficient for achieving the steady state conditions. At the adsorption−desorption equilibrium, the benzene and methanol samples were collected from the sampling pore by syringe and measured using GC-FID. The photocatalytic activity of photocatalyst agglomerate was evaluated by abatement of benzene and methanol with variation of molar ratio, RH, and illumination time. The gaseous mixture in the reactor was circulated from the reservoir to the reaction region for 120 min to reach the adsorption−desorption equilibrium and then switched on UV light lamp to illuminate the nano-TiO2 agglomerates under gas circulation. A 10 μL of benzene and methanol sample was taken as a measurement at a regular time interval, as mentioned before. The adsorption efficiency (ηA) and degradation efficiency (ηD) of benzene were calculated based on eqs 1 and 2 according to the peak areas in GC curves, respectively ηA =

C0 − C t A − At × 100% = 0 × 100% C0 A0

(1)

ηD =

Ce − C t A − At × 100% = e × 100% Ce Ae

(2)

Figure 2. SEM Micrographs of TiO2 agglomerates before and after fluidization: (A) nature agglomerates; (B) fluidized agglomerates. Note: gas velocity = 40 mm s−1, and the total target compounds concentration = 0.9 μmol L−1 for 1 h. RH = 20%, and nano-Titania amount = 100 g at ambient conditions.

100 μm. Generally, the fluidization concerning variation from nanosized particles to broad-distributed large agglomerates was named “agglomerate fluidization”.14 3.2. Isotherm Coadsorption of Benzene/Methanol. 3.2.1. Effect of Benzene−Methanol Molar Ratio in Binary Mixture. Isotherm adsorption of the pollutant onto the TiO2 agglomerate surface, as a critical step, determines photocatalysis rate and mechanism. The role of the adsorption capacities of binary pollutants is crucial when gas streams are treated due to competition for the active sites. In the present work, a series of adsorption experiments was conducted to evaluate the performance of coadsorption for benzene−methanol binary mixture in AFBPR. Gas velocity (40 mm s−1), TiO2 loading (100 g), and oxygen concentration (20%) were at fixed values; only individual inlet concentration of the binary mixture and RH were changed. The adsorption processes and results are presented in Figure 3. From Figure 3, it could be observed that the adsorption efficiency of benzene or methanol decreased with increasing initial concentrations from 0.1 to 0.8 μmol L−1 at the given conditions. It was attributed to the fact that for the fixed amount of photocatalyst in this case the adsorption active sites for benzene/methanol molecules decreased with growing concentration. This was similar to the adsorption efficiencies of cyclohexane13 in a single-component system, and toluene in a 2-propanol−toluene binary-component system8 decreased with increasing concentration. By comparison, the adsorption efficiency of methanol was obviously larger than that of benzene. This can be explained by the different adsorption affinity of methanol molecules and benzene molecules to the same surface of the TiO2 catalyst agglomerates. The later is a van der Waals force, while the former may be involved in the hydrogen bond affinity. Theoretically, adsorption affinity increased with increasing electron-donor ability of adsorbate and the proton-donor ability of adsorbent, which has often been attributed to hydrogenbond formation between two functional groups.15,16 The adsorption affinity of the methanol molecule was larger than that of the benzene molecule to the TiO2 agglomerate surface since the methanol molecule possesses a hydroxy group which is an electron-donor functional group. This was similar to the result of the 2-propanol−toluene binary component system reported by Vildozo et al.,8 where near 100% adsorption of 2propanol was quickly achieved and maintained for more than 4 h, while toluene adsorption was only around 70%. 3.2.2. Effect of RH in System. Generally, the influence of RH on adsorption is related to the solubility and concentration of

where Ct, C0, and Ce are the t minute, original, and equilibrium concentrations of benzene or methanol, respectively, and A0, Ae, and At were the corresponding peak area of target compounds at the original, equilibrium, and t time, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization of TiO2 Catalyst Agglomerates. Nanosized TiO2 (100 g used in all experiments) was fed into the annular reaction region. SEM (field emission scanning electron microscopy, JSM-6700F, JEOL, Japan) micrographs of TiO2 particles before and after fluidization are presented in Figure 2. Nanosized titania (primordial dimension, 5−20 nm, real density, 3800 kg m−3), which belong to Geldart-C, are extremely difficult to be fluidized, especially at high RH conditions. Formation of agglomerates in the fluidized bed reactor is a common phenomenon for these nanosized particles due to the strong cohesive forces among primary particles. From the SEM micrograph of the primary particles with an average diameter about 20 nm in Figure 2A it can be observed that the three-dimensional netlike structures coalesce into nature agglomeration, formed by van der Waals forces among the primary nanoparticles. In contrast, typical SEM photographs of the fluidized agglomerates are shown in Figure 2B, where the fluidized agglomerates were coarse in surface and near-spherical with a broad diameter distribution from 0.1 to 15362



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Figure 3. continued

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Figure 3. Coadsorption of benzene−methanol onto TiO2 in an AFBPR at various concentrations.

0.1 to 0.5 μmol L−1 in the benzene−methanol binarycomponent system (Figure 3D and 3F); in contrast, at a high concentration for methanol of 0.8 μmol L−1 there is a trough for methanol adsorption efficiency when RH is 20% (Figure 3A). These results indicated that the presence of water maybe changed the affinity of the TiO2 surface for the hydrophilic organic compounds, which can be explained by taking into

adsorbates.17 In this work, for benzene, as a hydrophobilic molecule, its adsorption efficiency increased with decreasing RH from 5% to 35% at benzene concentrations ranging from 0.1 to 0.8 μmol L−1 in benzene−methanol binary mixtures (Figure 3B, 3C, and 3E). While for methanol, as a hydrophilic molecule, the adsorption efficiency increased with increasing RH from 5% to 35% at methanol concentrations ranging from 15364



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account the relatively high amount of water vapor compared to the amount solubility of the adsorbates. Generally, the high values of methanol adsorption efficiency could not only be attributed to its strong affinity for TiO2 surfaces but to its high water solubility as well. At high levels of humidity, methanol is capable of penetrating the monolayers or multilayers of water formed at high level RH and adsorbed on the catalyst surface. On the contrary, benzene has little solubility and might have difficulty to penetrate this layer, consequently, the very low values of benzene adsorption at high levels of relative humidity as shown in Figure 3B, 3D, and 3E. This result is similar to the report concerning the 2-propanol− toluene binary-component system.8 3.2.3. Effect of Adsorption Time on Coadsorption. From Figure 3 it can be observed that the adsorption efficiencies of methanol and benzene were not only obviously influenced by the inlet concentrations ranging from 0.1 to 0.8 μmol L−1 but related to adsorption time as well. At lower concentrations of methanol (0.1 μmol L−1), about 60% of the adsorption efficiency of methanol was quickly achieved and maintained steadily after 60 min, while benzene adsorption efficiency fluctuated to the peak value and then decreased and approximated to 40%. Interestingly, during the start-up period, the adsorption efficiency of benzene grew sharply and approximated the adsorption peak value and then decreased slowly and tended to adsorption equilibrium. However, the adsorption efficiency of methanol increased with prolonging adsorption time and tended to 48%. This phenomenon indicated that at the start-up period of adsorption the concentration of benzene or methanol determined the adsorption process without selectivity and competition behavior, while benzene and methanol molecules maybe interacted only after a period of adsorption time, i.e., 40 min due to the fact that the adsorption efficiency of benzene decreased after this adsorption peak and approximated to a stable adsorption value. As a result, in the coadsorption process, benzene molecules adsorbed onto the special agglomerates may be displaced by methanol molecules until the dynamic equilibrium was reached. This result was similar to the multicomponent competitive adsorption of two and three model pollutants on CTMS.9 All model pollutants are adsorbed swiftly on the prepared photocatalyst at initial 15−20 min; competitive adsorption happens with an extended time. For binary-component adsorption between toluene and ethyl acetate, adsorption of toluene first decreases and then increases to reach equilibrium while adsorption of ethyl acetate increases constantly. The result can be ascribed to the different adsorption affinity of toluene and ethyl acetate to CTMS. Similar results were also observed in the binary-component adsorption of toluene with ethanethiol, ethyl acetate with ethanethiol, and in the ternary-component adsorption. The adsorption capabilities of toluene, ethyl acetate, and ethanethiol in single-component adsorption systems were all higher than those in the multicomponent adsorption systems, which is a confirmation of the competitive adsorption. 3.2.4. Adsorption Kinetic. Further, to explore this complex adsorption kinetics, the isotherm Langmuir model with surface coverage θ was used as follows θ=

NB,a NB,T

=

θ=

NM,T

=

KMCM,e 1 + KMCM,e

(4)

which can be modified as eqs 5 and 6 to determine the total number of adsorption sites available for the adsorbate molecules Ni,T (i = B and M) and the apparent adsorption equilibrium constant using linear regression analysis based on the data of Figure 3 1 1 1 1 = + NB,a NB,T NB,TKB C B,e

(5)

1 1 1 1 = + NM,a NM,T NM,TKM CM,e

(6)

where NB,a and NM,a are the number of adsorbed gaseous benzene and methanol molecules, CB,e and CM,e are the concentration of gaseous benzene and methanol at adsorption equilibrium, and KM and KB are the corresponding adsorption equilibrium constants of methanol and benzene molecules, respectively. Isotherm adsorption parameters were obtained and are shown in Figure 4.

Figure 4. Regression analysis (A) and adsorption equilibrium constant/adsorption active site (B) for adsorption of benzene and methanol molecules on nano-TiO2 photocatalyst agglomerates in AFBPR.

For adsorption of the benzene molecule, the water molecule has an obvious inhabitation since the active site value and value of the adsorption coefficient tended to decrease with increasing RH ranging from 5% to 35%. However, for the methanol molecule, the value of the adsorption coefficient increased with increasing RH ranging from 5% to 35% at initial concentrations from 0.1 to 0.5 μmol L−1. This result further validated that the adsorption mechanism of the methanol molecule was not only related to competition adsorption processes with other components but involved penetrating a multi- or monolayer formed on the surface of the catalyst particles as well. As observed from Figure 4B, the number of adsorption active sites and apparent adsorption equilibrium constants of methanol were larger than those of benzene. This result indicates that methanol can be strongly adsorbed on the TiO2 surface and is unlikely displaced by benzene molecules, while benzene shows a lower affinity for the TiO2 surface and is possibly displaced by methanol. Consequently, the affinity of adsorbate determined the adsorption mechanism, just as the

KBC B,e 1 + KBC B,e

NM,a

(3) 15365



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Figure 5. continued

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Figure 5. Influence of molar concentration and RH on the photocatalytic degradation efficiency of methanol−benzene binary mixture.

light with maximum emission intensity at a wavelength of 253.7 nm in photocatalytic oxidation processes previously. Only ≲5% of the pollutants is oxidized after 1.5 h irradiation, indicating that only UV light irradiation cannot effectively decompose organic pollutants. While the effect of pollutant structure on the photocatalytic degradation efficiency was investigated at the

conclusion reported that the hydrophilicity or hydrophobicity of VOCs may dominate the mass transport mechanisms.18 3.3. Photocatalytic Degradation of Benzene/Methanol on Catalyst Agglomerations. 3.3.1. Effect of Target Compound Structure on PCO. Photolysis of model pollutants without photocatalyst is introduced to explore the role of UV 15367



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inlet, the concentration of methanol and benzene was 0.9 μmol L−1, respectively, whereas RH varied from 5% to 35%. By comparison, the removal efficiency of methanol is higher than that of benzene in the presence of the photocatalyst at the same concentrations (0.9 μmol L−1), presented in Figure 5A−F. As discussed, the adsorption affinity of methanol to the photocatalyst P25 used in this work is stronger than that of benzene. Therefore, methanol is more strongly adsorbed on the photocatalyst and leads to better contact between photocatalyst and reactant molecules. As a result, a high photocatalytic oxidation efficiency of methanol could be obtained. 3.3.2. Effect of Molar Ratio of Target Compound on PCO. To explore the influence of mixed methanol with benzene on the photocatalytic degradation efficiency, a series of photocatalytic degradation experiments was conducted for a binary mixture of methanol and benzene at various molar ratios and RHs. First, from Figure 5D−F, we can observed that the degradation efficiency of benzene in a single system at an initial concentration of 0.9 μmol L−1 is approximated to that in the benzene−methanol binary system at an initial concentration of 0.8 μmol L−1, although the latter is slightly higher than the former. A similar conclusion was reported for the photocatalytic degradation of the mixtures of toluene and ethylbenzene, suggesting that this may be due to the small interference effect among tested VOCs within the selected concentration range.19 This interference effect was also found in the photocatalytic oxidation of mixed toluene and benzene.20 However, as observed from Figure 5A−C, the degradation efficiency of methanol in a single system at an initial concentration of 0.9 μmol L−1 is obviously higher than that in the benzene−methanol binary system at an initial concentration of 0.8 μmol L−1. Lichtin et al.21 also observed the inhibition effect on the removal efficiencies of a single compound by the other component after they tested 14 binary mixtures of VOCs. They explained that the inhibition may be a consequence of a competition reaction onto the photocatalyst. Therefore, the conclusion can be made here that weak mutual inhibition appears to occur in the photocatalytic oxidation of mixed methanol and benzene due to competitive adsorption of them on the photocatalyst surface, leading to a decrease of the degradation efficiencies of methanol for the binary system. Second, the experimental results presented in Figure 5A−C not only indicated that the photocatalytic degradation efficiency of methanol in AFBPR is a function of illumination time but also revealed that the degradation efficiencies of methanol decreased with increasing concentration. However, according to Figure 5D and 5E, the photocatalytic degradation efficiency of benzene is not similar to that of methanol and fluctuated with its various concentrations. Therefore, the conclusion can be made here that a new mechanism maybe occurs in the photocatalytic oxidation of mixed methanol and benzene due to competitive adsorption of them or new radicals produced on the same photocatalyst surface. 3.3.3. Effect of RH on PCO of Methanol−Benzene. The effect of water on the photocatalytic oxidation of methanol− benzene was studied at the following experimental conditions: the total inlet concentration of methanol and benzene was 0.9 μmol L−1, whereas RH varied from 5% to 35%. From Figure 5A−F, the effect of water vapor added to the reactor showed an obvious dependence on the type of pollutant and the concentration of water. Specifically, as shown in Figure 5A− C, the increase of RH from 5% to 20% enhanced the photo-

oxidation of methanol, while a further increase in RH to 35% led to a practically significant decrease of its photocatalytic degradation efficiency. Generally, the impact of water on the photocatalytic oxidation process seems to be the net effect of the generation of hydroxyl radicals from adsorbed water and the competition of water and target molecules for adsorption on the catalyst active sites. Thus, there is an optimum value of water concentration after which formation of hydroxyl radicals through water dissociation is not further enhanced and the water adsorbed on TiO2 surface amounts to more than a monolayer, thus inhibiting adsorption of the organic molecules.22−25 It seems to be the optimum RH at 20% for methanol in this case. This optimum RH was similar to the results reported that RH was an optimum water vapor concentration in the PCO of cyclohexane.13 For this case it may be attributed to the following reasons: (i) photocatalytic degradation of methanol 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 methanol molecules may be enhanced with an increasing concentration of target molecules at higher RH, which may retard the PCO of methanol. In contrast, based on Figure 5D−F, benzene exhibited an entirely different trend regarding the effect of water vapor, where the increase of RH ranging from 5% to 35% inhibited the PCO of benzene. For the benzene molecule, this inhibition resulting from water could be attributed to occupation of almost all of the surface adsorption sites of TiO2 by water molecules and consequently it was very difficult for its relatively hydrophobic organic compound to be adsorbed on the surface of the catalyst in the presence of water.26 The beneficial effect of water seemed to fade from decreasing water solubility or increasing octanol/water partition coefficient values of BTEX, namely, Kow 135, 490, 1413 and 1585, respectively.27 3.3.4. Kinetic and Mechanism of PCO of Benzene− Methanol Binary Mixture. Generally, the typical mechanism of heterogeneous PCO involves photoexcitation of semiconductor catalyst, leading to formation of free charge carriers (electrons, e−, and holes, h+). On the basis of the special photocatalyses of VOCs reported previously, we can put forward the following mechanism: a portion of these photogenerated pairs recombine in the bulk of the semiconductor, while the rest migrate to the surface of particles, where the holes are subsequently trapped on the surface and poised to react with adsorbed water and methanol to produce active species, i.e., HO• and MeO•.2,4 These species initiate a wide range of chemical redox reactions. Therefore, the corresponding elementary reactions and reaction rates are presented in Table 1. From Table 1, the photocatalytic degradation rate (r) of benzene and methanol are represented by the following expressions rB = k6[•OH][C6H6] + k 7[MeO•][C6H6]

(7)

rM = k4[h+][MeOH] + k5[HO•][MeOH]

(8)

The concentration of photoinduced holes, [h+], can be obtained based on the mentioned elementary reactions and the steady state assumption 15368



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Assumption of k10[MeO•]2 ≫ k7[MeO•][C6H6] + k8[•OH][MeO•], then

Table 1. Mechanism of Photocatalytic Oxidation of Benzene−Methanol Binary Mixturea elemental reaction k1

TiO2 + hν → e− + h+ + k2

−

reaction rates

r = k1I[C TiO2]

(R1)

k3

h+ + H 2O → •OH + H+ k4

k5

OH + MeOH → MeO• + H 2O

(R5)

k6

•

OH + C6H6 → C6H5• + H 2O k7

•

(R6)

MeO + C6H6 → C6H5OMe + H k8

•

OH + MeO• → MeOOH k9

•

OH + •OH → H 2O2

•

(R7)

(R8)

k10

MeO• + MeO• → MeOOeM

r = k5[•OH][MeOH]

(R10)

r = k 7[MeO•][C6H6]

r = k 8[•OH][MeO•]

⎞ ⎟ [MeOH]1/2 ⎟[C6H6] ⎟ ⎟ ⎠

r = k10[MeO•]2

Note: h is Planck’s constant, 6.62 × 10−34 J·s; r is the reaction rate, μmol L−1 min−1; I is the illumination intensity, mW cm−2; ki is the reaction rate constant, min−1; h+ is the hole generated in photocatalysis; e− is the electron generated in photocatalysis; ν is the frequency of photon. a

⎛ k [h+][H 2O] ⎞ ⎟[MeOH] rM = ⎜⎜k4[h+] + k5 3 ⎟ k9 ⎝ ⎠

+

d[h ] = k1I[C TiO2] − k 2[h+]2 − k 3[h+][H 2O] dt (9)

Under high irradiation intensities, recombination of electron− hole is predominant, that is

d[C6H6] dt ⎛ k [h+][H 2O] k4[h+] + k5 3 ⎜ + k9 k [h ][H 2O] = ⎜k 6 3 + k7 ⎜ k9 k10 ⎜ ⎝

As a result, the concentration of holes can be given by

(10)

⎞ ⎟ [MeOH]1/2 ⎟ ⎟ ⎟ ⎠

Considering formation, oxidation, and recombination of hydroxyl radicals based on the elementary reactions in Table 1, the concentrations of •OH and MeO• can be obtained according to the steady state assumption as follows d[•OH] = k 3[h+][H 2O] − k5[•OH][MeOH] dt

×

− k6[•OH][C6H6] − k 8[•OH][MeO•] − k 9[•OH]2 ≅ 0

− k 7[MeO•][C6H6] − k 8[•OH][MeO•] − k10[MeO•]2 (12)

×

Let k9[•OH]2 ≫ k5[•OH][MeOH] + k6[•OH][C6H6] + k8[•OH][MeO] hold then k 3[h+][H 2O] k9

(17)

rM =

d[MeO•] = k4[h+][MeOH] + k5[•OH][MeOH] dt

[•OH] =

KB[C6H6] 1 + KB[C6H6] + KH[H 2O] + KM[MeOH]

d[MeOH] dt ⎛ k [h+][H 2O] ⎞ ⎟ = ⎜⎜k4[h+] + k5 3 ⎟ k9 ⎝ ⎠

(11)

≅0

(16)

rB =

k 2[h+]2 ≫ k 3[h+][H 2O] + k4[h+][MeOH]

⎛ k1I[C TiO ] ⎞1/2 2 ⎟ [h+] = ⎜ k ⎝ ⎠ 2

(15)

Theoretically, at low concentrations, the reaction rate equation can be given by the following expression at a fixed RH and gas velocity based on the Langmuir adsorption equation considering competition adsorption. On the basis of the multicomponent Langmuir adsorption equation and reaction rate equation, the photocatalytic oxidation reaction rates of benzene and methanol can be obtained as follows

− k4[h+][MeOH] ≅0

k10

⎛ k [h+][H 2O] k4[h+] + k5 3 ⎜ + k9 k [h ][H 2O] rB = ⎜k6 3 + k7 ⎜ k9 k10 ⎜ ⎝

r = k6[•OH][C6H6]

r = k 9[•OH]2

(R9)

[MeOH]

On the basis of eqs 7 and 8, the photocatalytic degradation reaction rate expressions of benzene and methanol can be obtained, respectively, as follows

r = k4[h+][MeOH]

(R4)

k 3[h+][H 2O] k9

(14)

r = k 3[h+][H 2O]

(R3)

h+ + MeOH → MeO• + H+ •

[MeO ] =

r = k 2[h+]2

(R2)

e + h → heat

•

k4[h+][MeOH] + k5

KM[MeOH] 1 + KB[C6H6] + KH[H 2O] + KM[MeOH] (18)

According to eqs 17 and 18, the reaction rate, dependent on target compound concentration, is an apparent first-order kinetic expression at a given condition. This can be attributed to the fact that the colliding probability between hydroxyl

(13) 15369



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Figure 6. Relationship of apparent reaction rate coefficient and molar concentration of benzene−methanol. At the 0.05 level, the population mean of the apparent reaction rate coefficient is significantly different than the test mean (0) at the confidence interval.

Table 2. Values of Apparent Reaction Rate Coefficient (Ka, min−1) for Photocatalytic Degradation of Benzene−Methanol at Various RH and Concentrations (conc, mol L−1) Ka of benzene

Ka of methanol

conc

RH

5%

20%

35%

RH

5%

20%

35%

0.1 0.2 0.4 0.7 0.8 0.9

0.0093 0.00797 0.00659 0.00665 0.00605 0.00557

0.00882 0.00776 0.00633 0.00645 0.00545 0.00483

0.00589 0.00532 0.00484 0.00413 0.00358 0.00321

0.1 0.2 0.5 0.7 0.8 0.9

0.0083 0.0076 0.0059 0.0049 0.00449 0.00584

0.016 0.0127 0.00988 0.00821 0.00824 0.00859

0.01431 0.0116 0.00842 0.0074 0.00631 0.00724

onto photocatalyst P25, indicating that the PCO of a single component was up to its adsorption amount. In addition, the fitted results presented in Figure 6 also indicated that the values of the apparent reaction rate constant decreased with increasing initial concentration. It may be attributed to the following reasons. (I) At high concentrations, the fixed surface adsorption active sites of photocatalyst is not enough for the amount of target molecules. (II) The amount of products or intermediates formed in PCO can occupy part of the active sites of the catalyst to inhibit oxidation progression. To explore the influence of the concentration on the photocatalytic degradation, the corresponding integral equations can be yielded and the corresponding half-life can be obtained as follows

radicals and target molecules increases with increasing concentration. This behavior is in agreement with the results proposed by Kim et al.28 As known, photocatalytic oxidation of VOCs conformed to the L−H kinetics model29,30 and the linear plots of ln(C0)/C vs irradiation time revealed that the photocatalytic degradation kinetics of the binary components model also matched the L− H model. At the given experimental conditions, i.e., illumination, humidity, amount of catalyst, and gas velocity, a plot of ln(C0)/C vs t should be linear and the values of the apparent reaction rate constant can be obtained. The corresponding results are presented in Figure 6 and Table 2; from Table 2, the photocatalytic degradation rate constants for single model pollutants follow the order of methanol > benzene at RH ranging from 5% to 35%. Obviously, this reaction rate constant trend is consistent with the adsorption trend of them 15370



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Furthermore, to explore the special interaction between benzene and methanol in photocatalytic oxidation, we defined the synergy coefficient (Sc) as the dimensionless coefficient as the ratio of the total value of benzene and methanol degradation apparent rate constant for the binary mixture to that of the rate constants for them in a single system with various molar ratios. Similar to the equation developed by Madhavan,31 Cheng,32 and Torres-Palma et al.33 in photocatalysis mixed with sonolysis, the synergy coefficient between photocatalysis of methanol and benzene can be directly described as follows

t B,1/2 = (1 + KH[H 2O] + KM(0.9 − [C6H6]))ln 2 + [C6H6]/2 ⎛ ⎞ k [h+][H2O] k4[h+] + k5 3 ⎜ k9 k 3[h+][H 2O] 1/2 ⎟ + ‐ k k (0.9 [C H ]) 7 6 6 ⎜⎜ 6 ⎟⎟ k9 k10 ⎝ ⎠ (19)

tM,1/2 = (1 + KB(0.9‐[MeOH]) + KH[H 2O])ln 2 + [MeOH]/2 ⎛ k [h+][H O] + ⎜k4[h ] + k5 3 k 2 9 ⎝

)

Sc =

(20)

kB, i + kM,0.9 − i kM,0.9 + kB,0.9

(21)

where kM,0.9 and kB,0.9 are represented for photocatalytic degradation reaction apparent rate constants of methanol and benzene in a single component, respectively; kM,0.9−i and kB,i represent the photocatalytic degradation reaction apparent rate constants of methanol and benzene in the binary mixture, respectively. The synergy effect in PCO of methanol and benzene binary mixture is presented in Figure 8, showing that all values of Sc, more than 1 at RH ranging from 5% to 35%, varied with the molar ratio of benzene to methanol from 1:8 to 8:1.

The half-life is an important kinetic parameter to explore the influence of the concentration on the photocatalytic degradation progression of target compounds in AFBPR. The half-life of the photocatalytic degradation reaction was introduced based on the half-life definition of the first-order reaction rate equation (t1/2 = (ln 2)/kapp) and the results are presented in Figure 7. From Figure 7, except for the single component, the

Figure 7. Relationship of half-life and benzene or methanol concentration.

half-life was approximated to a monotone increasing function of methanol concentration in PCO of methanol−benzene in a binary mixture, while the half-life of benzene is a nonlinear function. This is consistent with the theoretical results presented in eqs 19 and 20. According to eq 20, the half-life for the PCO of methanol in a methanol−benzene binary mixture, dependent on methanol concentration, is approximated to a linear function of concentrations at a given condition theoretically, while based on eq 19 the half-life of benzene in PCO of methanol−benzene binary mixture, dependent on benzene concentrations, is approximated to a nonlinear function of concentration at the given conditions. In summary, 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 photocatalyst surface using methanol solvent after finishing the experiment. These byproducts adsorbed on the surface of catalyst can inhibit the target molecules from adsorbing into the reactive sites of catalyst. (2) The degraded products, as scavengers of holes generated in photocatalytic process, were detrimental to the photocatalytic action.

Figure 8. Synergy effect in the PCO of benzene−methanol binary mixture.

This result can be attributed to a new radical mechanism occurring in the photocatalytic oxidation of methanol and benzene binary mixture, where a new kink of radical directly or indirectly was produced on the photocatalyst surface exited by UV light illumination, expressed as eqs 22 and 23. k4

h+ + MeOH → MeO• + H+ •

k5

OH + MeOH → MeO• + H 2O

(22) (23)

This result can be validated by the special Sc value at a high molar ratio of benzene to methanol at higher RH (35%). In this case, it is very difficult to adsorb and photocatalytically degrade for the benzene molecule, but methanol, maybe as a sensitizing agent, can be directly or indirectly exited by UV light illumination to produce a new radical, MeO•, which initiates a new radical reaction with benzene as follows k7

MeO• + C6H6 → C6H5OMe + H• 15371

(24)



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In addition, at low RH (5−20%), there is a trough in the Sc vs molar ratio of benzene to methanol curve, indicating that the synergy effect was approximated to the minimum value when the molar ratio of benzene to methanol was about 1/2 and 3/4 at RH of 20% and 5%, respectively. Thereafter, we can conclude that little addition of methanol, especially at high RH, can efficiently enhance the PCO of benzene. As a result, based on the variation of the synergy coefficient, we can optimize the special operation parameters, i.e., RH and addition of sensitizing agent in practice.

REFERENCES

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4. CONCLUSIONS Adsorption and photocatalytic degradation of a gaseous benzene−methanol binary mixture using nano-titania agglomerates had been investigated in AFBPR, and the following conclusions can be derived from the present study. (1) The competition adsorption between benzene and methanol for reactive surface sites varied significantly at various RH. The hydrophilicity or hydrophobicity of adsorbate may dominate the special adsorption mechanisms, lower the solubility of absorbate, and increase the negative influence of RH on its adsorption on the TiO2 surface. The co-adsorption mechanism of methanol− benzene was not only related to competition adsorption but involved penetrating a multi- or mono-water layer formed on the surface of catalyst particles. (2) A high photocatalytic degradation efficiency of methanol−benzene binary mixture was achieved in AFBPR, obviously related to their molar ratio and RH. The trend of photocatalytic degradation efficiency of methanol is in agreement with the variation of adsorption efficiency. The photocatalytic degradation kinetic parameter, apparent reaction rate coefficient, and half-life are determined as an increasing function of their molar concentrations. (3) The effect of water vapor and pollutant molar concentration on PCO is different and depends on the VOCs nature. These experimental results highlight the importance of controlling RH and molar ratio in a binary mixture in order to obtain the desired synergy effect in PCO processes. (4) The special synergy effect of the methanol−benzene binary mixture in PCO related to the physicochemical effect of a sensitizing agent methanol added exhibits a high efficiency in PCO of benzene and could be a better alternative for the low concentration of sensitizing agent added in PCO.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-536-8785283; fax: +86-536-8785802; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This investigation was supported by the Doctorate Programs Foundation of Scientific Research Department of Weifang University (No. 2012BS07), Shandong Natural Science Foundation of China (No. ZR2011BM019), and University Teacher Programs Foundation of Shandong Education Department of China (No. J11LB55). 15372



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