Considerations to improve adsorption and photocatalysis of low concentration air pollutants on TiO2

Catalysis Today 225 (2014) 24–33

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Considerations to improve adsorption and photocatalysis of low concentration air pollutants on TiO2 Jinze Lyu a,b , Lizhong Zhu b,∗ , Clemens Burda a,∗ a b

Department of Chemistry, Center for Chemical Dynamics and Nanomaterials Research, Case Western Reserve University, Cleveland, OH 44106, USA Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310058, China

a r t i c l e

i n f o

Article history: Received 12 July 2013 Received in revised form 25 October 2013 Accepted 31 October 2013 Available online 5 December 2013 Keywords: Low concentrations Realistic conditions Adsorption Residence time Pore Adsorbent

a b s t r a c t Rapid development of nanoscience and nanotechnology has greatly supported the industrialization of titanium dioxide for environmental pollution control during the past decade. Nowadays, low concentration air purification seems to be one of the most promising directions of environmental TiO2 applications. However, much more effort is needed to perfect this technology and make it broadly applicable. Understanding the nature of the adsorption and photooxidation under realistic and practical conditions would give clear guidance for the development of novel catalytic materials and technologies. This paper describes the significant effects of the adsorption of low concentration gas-phase pollutants in practical conditions on the photocatalytic oxidation efficiency and mechanism. We also review the influences of several important conditions, such as pollutant concentration, contact time, co-existing pollutants, water vapor, and light exposure, on the nature of the adsorption process and thereby the photooxidation. Finally, catalytic materials which might enhance the adsorption of low-concentration pollutants are summarized. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The continued aggravation of the environment keeps on impelling scientists to develop novel “green” technologies to overcome the increasing pollution crisis. Photocatalysis has been considered one of the most promising technologies for environmental cleanup [22,25,47,61]. Titanium dioxide with its comprehensive advantage, such as nontoxicity, large band gap, and low cost, has drawn the most attention from the science and industry communities during the past decades [11,12,19,29,42,47,57,82]. Hundreds of scientific papers are published every year for improving the performance of this wide-band gap semiconductor for water and gas purification. However, the development of industrial technology lags far behind scientific interests. As far as we know, the photocatalytic technology is still not a matured process for industrial waste water or gas treatment. This is largely due to the fact that the low incident-photon-to-charge-carrier-efficiency (IPCE) of photocatalysts hardly bears the load of industrial productivity [57,94]. As a matter of fact, the photocatalysis technology is probably best applied to indoor air purification because of extremely low concentrations (sub-ppb to ppb) of the indoor air pollutants [39,58,89,93].

∗ Corresponding authors. E-mail addresses: [email protected] (L. Zhu), [email protected], [email protected] (C. Burda). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.10.089

Several air purifier products based on this technology have been brought to market in recent years. However, much more effort is needed to perfect this technology and make it broadly applicable. To solve a specific issue like indoor air pollution, practical conditions must be taken into full account: (i) the concentrations of the pollutants range from sub-ppb to ppb level, (ii) the composition of the pollutants is very complex, and the characters of the pollutants, such as polarity, molecule size, and chemical reactivity, are different, (iii) as high flow rate is usually used for air purification systems, the contact time of the pollutants over the catalysts surface is as short as tens of millisecond, (iv) thousands of ppm of water vapor co-exist with the pollutants. Ignoring any practical conditions mentioned above would significantly change the outcome and conclusions. Actually, only few scientific studies fully considered all the above-mentioned conditions in their laboratory experiments [15,24,48,94]. This may partially explain why most of the catalysts having good performance in a laboratory study show low efficiency in practical conditions. It is also interesting to note that the industry considers adsorbing air pollutants onto the catalysts surface as the major challenge for indoor air purification [61], while the photocatalysts adsorption capacity is usually unvalued in scientific papers for the same purpose. Scheme 1 shows the main process of the adsorption and photooxidation of gas-phase pollutants. The mineralization efficiency (M) is given as follows: M = ˛ˇ

(1)

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Fig. 1. Photographs of water droplets on an N-doped TiO2 film with various surface wettabilities [86].

catalytic photooxidation of air pollutants. Furthermore, studies using adsorption-enhanced catalysts are described. 2. Effect of practical conditions on the adsorption Scheme 1. Main process of the adsorption and photo-oxidation of gas-phase pollutants.

Scheme 2. Simple oxidation mechanisms of the pollutants (R) over TiO2 (adapted from Ref. [7]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

where ˛ is the adsorption efficiency; ˇ is the mineralization efficiency of the adsorbed pollutants which is depended on the photoactivity of the photocatalysts. With the pollutant concentrations in the ppm range, the adsorption efficiency ˛ is more likely to be a constant [12]. Therefore, the total mineralization efficiency is largely determined by the photoactivity of the catalysts. However, in practical conditions, with the pollutants in the ppb range or with relatively short contact times, the adsorption efficiency ˛ would decrease and be significantly varied with the concentration and contact time of the pollutants. Therefore, the total mineralization efficiency would be controlled by both the adsorption capacity and photoactivity of the photocatalysts. On the other hand, the pollutant’s adsorption also influences the oxidation mechanisms of the pollutants (shown in Scheme 2) [7]. With more amount of adsorbed pollutants on the catalyst surface, the reaction probability between photoexcited holes and adsorbed pollutants increases (shown as the blue arrows), which benefits from the light-induced separation of electrons and holes [53]. Besides, as the band-edge potential of the TiO2 valence band (3.06 eV) is higher than the oxidation potential of H2 O (2.8 V) [70], photoexcited holes on TiO2 have stronger oxidation power than the HO• radical and thereby may increase the mineralization efficiency. Above all, the adsorption significantly affects the oxidation efficiency of low concentration pollutants. With the rapid development of nanoscience and nanotechnology, understanding the nature of the adsorption and photo-oxidation under practical conditions could directly guide the synthesis of novel photocatalysts and promote their industrialization. In the following, we provide a concise literature overview of the effects of several practical parameters, such as pollutant concentration, contact time, co-existing pollutants, water vapor, and light exposure, on the adsorption and thereby the

2.1. Light irradiation Considering the adsorption of the pollutants, it must be clear that the nature of the photocatalysts surface under light irradiation is different from that in the dark. In 1996, Jacoby et al. [27] compared the amounts of adsorbed benzene on TiO2 in dark and under light illumination. They measured the amount of adsorbed benzene under UV irradiation in N2 atmosphere to keep the benzene from being decomposed. The results interestingly showed that the amount of adsorbed benzene under UV irradiation was much higher than that in dark. They described that UV irradiation of the catalyst can transfer the excitation energy to the catalyst’s surface, which influences ultimately the adsorption. The adsorption isothermal curves over the photocatalysts in dark could be well fitted by Langmuir model, while the photo-oxidation rate of the pollutants could be described by Langmuir–Hinshelwood model. The two models are expressed by Eqs. (2) and (3), respectively. Langmuir isotherm :

qe =

qm bCe 1 + bCe

Langmuir–Hinshelwood rate law :

(2)

r0 =

kKC0 1 + KC0

(3)

where qe is the amount of adsorbed pollutant; qm is the maximal adsorption capacity of the medium; b is equilibrium adsorption constant; Ce is equilibrium concentration; r0 is the pollutant degradation rate; k is the reaction kinetic constant; K is the equilibrium adsorption constant; C0 is the initial pollutant concentration. Several studies stated that the adsorption constants obtained from the Langmuir–Hinshelwood kinetic (K) model are higher than those obtained from the adsorption isotherms (b) [6,13,43,46,105], which is consistent with the observation of Jacoby’s work [27]. Most studies believe that the increase in adsorption capacity under UV irradiation is due to the redistribution of the electrons on the surface [44]. Many studies have focused on the unique property of TiO2 , amphiphilic effect [52,72,86,100]. Under UV irradiation, the surface of TiO2 could turn to both hydrophilic and oleophilic, as shown in Fig. 1. This phenomenon may have direct relationship with adsorption of pollutants under UV irradiation and share the same mechanism. However, the detailed mechanism of both the light-induced enhancement of adsorption capacity and amphiphilic effect still needs to be confirmed in upcoming future studies. 2.2. Concentration Indoor and outdoor air pollutants are generally in the ppb range, some of them even at sub-ppb levels [18,21,39,58,69,84,92,93,95,109]. However, because of the detection limit of the available analysis techniques and the difficulty

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in the generation of gas phase pollutants at the ppb level, a large amount of research was carried out on indoor air purification with pollutants in the ppm range. Many papers agreed that the removal of pollutants in the ppb and ppm concentration range was quite different. Thus an increasing number of studies focused on the treatment of air pollutants in the lower ppb concentration range [76]. Although, in many papers, adsorption was mentioned as a crucial factor that affected the photo-degradation of low-level pollutants, it still seems that the importance of the adsorption was not sufficiently considered because of the lack of quantitative detection of the adsorption capacity of the catalysts. Several papers reported the adsorption of pollutants at ppm levels on photocatalysts, as shown below. Jacoby et al. [27] reported that, with concentrations of benzene ranging from 30 to 120 ppm, the maximum and steady-state reaction rates are not strongly dependent on the gas-phase concentration of benzene. Zhang et al. [50,103] stated that the photocatalytic oxidation (PCO) rates of gaseous toluene follow the L-H rate form, and as the concentrations of toluene are higher than 10 ppm, the PCO rate of toluene increased only very slowly with the toluene concentration. This is indicative of a process in which the rate of adsorption is faster than the rate of surface reaction. Thus, surface reaction is the rate-limiting step in this concentration range. However, Obee [55] reported that, as the concentrations of toluene becomes lower than 4 ppm, the oxidation rate of toluene decreased quickly with concentration. Lee et al. [98] detected the oxidation rate and CO2 yields of six organic pollutants at concentrations lower than nine ppm, and similar trends were observed. The body of work above clearly indicates the difference of the adsorption–photodegradation mechanisms of the pollutants in the ppb versus ppm ranges. At a low concentration, the adsorption rate of the pollutants over the surface of catalysts is obviously slower than the oxidation reaction rate. Therefore, the adsorption of the gas-phase pollutants is the major factor that determines the removal rate. As the concentration of the pollutants increase, the adsorption and reaction rate get closer, and both of them are important for the overall removal rate. When the concentrations of the pollutants increase to a high level (ppm), the adsorption is much quicker than the photooxidation, and the photoactivity should be the rate controlling factor. For pollutants on the sub-ppb to ppb levels, the effect of the adsorption is therefore, in general, not negligible. Several papers reported the adsorption isothermal curves of gas phase organic pollutants over a range of photocatalysts. Coronado et al. [13] modeled the adsorption isotherms of acetone and methyl isobutyl ketone in the dark under different concentrations of water vapor considering a two-site Langmuir model. Raillard et al. [66] found out that the adsorption of acetone and 2-butanone fitted well with the single-site Langmuir isotherm equation. Maudhuit and his co-workers [43] fitted the obtained adsorption isothermal curves of toluene, acetone and heptane properly with a new model called the “Langmuir-multi”. The differences of fitting models in these works may be due to different substrates used to support the catalysts, and the Langmuir model could fit all the data as long as the concentration of these pollutants was lower than about 1000 ppm. However, there is still a lack of detailed knowledge about the adsorption of the pollutants in the ppb and sub-ppb range. The surface of the catalysts is not uniform and can be simply divided into strong adsorption sites and weak adsorption sites based on the interaction strength between the pollutants and these sites. As the concentration of the pollutants increases, the pollutants would be firstly adsorbed onto strong adsorption sites, followed by weak adsorption sites. When the concentration increases to a relatively high level, further adsorption leads to interactions between the gas-phase pollutants and adsorbed

Table 1 Lists the works using contact time lower than one second. Number of pollutants

Concentration (ppb)

Contact time (s)

Reference

1 37 1 7 1 1

500a 0.64–134 2500 0.4–2.2a 500–1000 90–800

0.023b 0.029–0.1 0.285b 0.027–0.159 0.036–0.072 0.2

[73] [24] [49] [15] [41] [48]

a b

Calculated from the original data in units ␮g/m3 . Calculated from the data of reactor size and flow rate.

Fig. 2. Effect of residence time (contact time) on removal efficiency [24].

molecules on the catalysts surface. Therefore, the adsorption of low concentration pollutants is much more sensitive to the nature of the surface than adsorption at relatively high concentrations (> ppm). 2.3. Contact time (residence time) The contact time, also called residence time, is defined slightly different in different research fields. Here, we simply define contact time as the time that gas molecules are in the space near the surface of the photocatalysts. It is a factor that is extremely important in the practical application of gas purification, yet often neglected in scientific research. As high flow rates are necessary for a purification system, the contact time of pollutants are usually in the range of tens to hundreds of millisecond, while, in most published scientific work on indoor air purification, the contact time is in the range of seconds to minutes. This is one of the most important reasons why the photocatalysts performing well in lab show low removal efficiencies under practical conditions (see Table 1). Ao and Lee [2] reported that the removal rate of NO and BETX decreased quickly as the contact time decrease from 3.7 to 0.6 min. At a contact time of 3.7 min, the difference in the removal rate between TiO2 and TiO2 /active carbon (AC) is not as significant as that at a contact time of 0.6 min. They assigned this observation to the fact that the pollutant diffusion rate from the gaseous phase to TiO2 is similar to the pollutant diffusion rate from AC to TiO2 , given such a long contact time. The amount of the by-product of NO was increased as the contact time decreased. Similar trends of removal efficiency with different contact times of >1 s was also reported elsewhere [3,17]. The effect of contact time on removal efficiencies can be seen in Fig. 2.

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Hodgson and Destaillats [24] studied the photo-oxidation of complex VOC mixtures, containing tens of VOCs all at the ppb level, at the contact time ranging from 0.029 s to 0.1 s with a relative humidity (RH) in the range of 42–65%. The decrease of residence time significantly reduced the removal efficiencies of almost all VOCs; the removal efficiencies of organic pollutants were lower than 44% at the residence time of 0.029 s. After that, Destaillats et al. also investigated the effect of contact time on removal efficiency in a single-pass bench-scale reactor [65] and room-size environmental chamber under practical indoor conditions [15], respectively. The results showed that also the single-pass removal efficiency decreased with residence time. Lv and Zhu [41] compared the removal efficiencies of cyclohexanone using P25 and microporous TiO2 at different residence time. At the residence time of 0.072 s, the removal efficiencies for P25 and microporous TiO2 were similar, while the removal efficiency for microporous TiO2 became significantly higher than P25 at the residence time of 0.036 s. This clearly shows that the performance of the catalysts at short residence time may be very different from the performance at long residence times. Ao and Lee [2] observed that the amount of adsorbed pollutants decreased with the contact time. As the adsorption is the crucial step before photo-oxidation in the removal process, this may well explain why the removal efficiencies of the pollutants decrease with the contact time. In summary, it seems like the adsorption capability of a photocatalysts for pollutants at ppb levels and contact times of tens of millisecond should be carefully considered during the development of a novel photocatalyst for air pollutants. 2.4. Interaction between co-existing pollutants The composition of indoor gas phase pollutants is very complex, such as VOCs [58,92,108], PAHs [39,40,109], and NO [23,34] etc. The characteristics of these compounds, such as polarity, molecule size, and chemical activity can be quite different. This brings a challenge to the purification process of low concentration pollutant mixtures. Therefore, it is important to understand the interaction mechanisms of different pollutants during adsorption and photooxidation. Chen et al. [10] observed the inhibition effect of SO2 on NOx and VOCs during the photodegradation of synchronous indoor air pollutants at ppb levels on TiO2 . They concluded that the major effect of the presence of sulfate ions is the competitive adsorption on the catalyst between the reactant and the sulfate anion. Zorn et al. [110] studied relative photocatalytic degradation using multi-component experiments with propanone + propene, propanal + propanone, and ethanol + propanone. In each case it was observed that compounds with stronger binding energy to the photocatalyst surface displaced compounds with weaker binding energies and inhibited their further reaction until the stronger binding species was oxidized to sufficiently low levels. However, several publications report that multi-component pollutants can be oxidized simultaneously in spite of the inhibition effect between the compounds [4,10,24,85,103]. This apparent contradiction is largely based on the different initial concentrations of the pollutants in Zorn’s work, where the pollutant concentrations were hundreds of ppm, much higher than the indoor levels used in other studies. Ao and Lee [4] showed that the presence of SO2 on TiO2 inhibits the conversion of NO and increases the generation of NO2 by 7% and more than 10%, respectively. The conversions of benzene, toluene, ethylbenzene, and o-xylene (BTEX) also decreased by 18, 15.6, 6.4, and 3.9%, respectively, with the presence of SO2 compared with the photodegradation of BTEX only. They derive that the inhibition effect is due to the sulfate ion competing with the pollutants for adsorption sites on TiO2 , which conforms with Chen’s conclusions [10]. The presence of BTEX reduced NO

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conversion by more than 10%. However, they also observed that the presence of NO improved the removal of BTEX, and they ascribed the promotion effect of NO to the generation of hydroxyl radicals from the photodegradation of NO. Zhang et al. [103] studied the competition effect between two aromatic compounds, toluene and benzene, in the photodegradation. The results show that the reaction-coefficient for the photocatalytic oxidation of the individual contaminants differs from that in the competitive adsorption by the L-H rate law. The impact factor of toluene on benzene was high, and the impact factor of benzene on toluene was low. They explained the results with the higher chemical activity of toluene compared to benzene. D. Vildozo [85] carried out experiments of continuous-flow photocatalytic oxidation of 2-propanol/toluene mixtures with different concentration ratios. The results showed that the oxidation of both 2-propanol and toluene in the ppb range can proceed simultaneously. The removal rate was largely determined by the affinity between the pollutants and the catalyst surface as well as the solubility of the pollutants in water. Furthermore, the results of Jan’s research revealed that photocatalytic decomposition of NO was dramatically enhanced in the presence of acetone. NO also promoted the acetone oxidation under humid conditions [28]. The research presented above gives a brief overview of the interactions between the co-existing pollutants: (i) the photo-oxidation of the pollutants in multi-component gas mixtures can occur simultaneously with all the components in the ppb range; (ii) the presence of some compounds, such as SO2 , may inhibit the removal rate of other co-existing pollutants. This is largely because pollutants or their by-products with better affinity to the surface of the catalysts tend to occupy the adsorption and oxidation sites more easily and thereby inhibit the adsorption and photo-oxidation of other pollutants; (iii) in some cases additional compounds could enhance the removal of co-existing pollutants. These points underline that it is difficult to treat low concentration air pollutants with a mix of components, such as typical indoor air pollution, with a single photocatalyst. It may be more advantages to develop several novel photocatalysts, each suiting to different kinds of pollutants, and assemble them all together in an air purification array system. 2.5. Water vapor Water vapor in air is of several thousand ppm, much higher than the concentrations of gas-phase pollutants. As the surface of photocatalysts is hydrophilic under light irradiation [86–88], water vapor forms monolayers to multilayers on the surface and significantly influences the adsorption and photo-oxidation of the pollutants. There is a rather large body of work that discusses the effect of water vapor or relative humidity on photocatalytic air purification [5,13,54,56,59,62,66,67,99,106]. In this review we focus on the influence of water vapor during the adsorption process of the pollutants and their byproducts on the surface of the catalysts. Firstly, the competition between water vapor and gas-phase pollutants inhibits the adsorption of the pollutants on the surface sites thereby reducing the removal efficiency. The degree of the inhibition effect is determined by the affinity of the pollutants with the surface of the catalysts relative to water. Raillard [66] reported that the adsorption of acetone and 2-butanone on the TiO2 -containing paper was obviously inhibited by water vapor. However, the effect of water vapor on the oxidation of acetone was much greater than on 2-butanone.

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Similar inhibition effects can be found in most of publications on the photo-oxidation of low-concentration pollutants. Secondly, the competition between water vapor and the by-products influences the mineralization of the pollutants [51,74]. Generally, the adsorbed compounds would be firstly decomposed to different intermediates. If the intermediates have better affinity with the catalysts surface than water molecules, they would be further decomposed to the species with smaller molecular size or degraded to CO2 and H2 O. On the contrary, if the affinity between the intermediates and the catalysts surface is low than the water molecules, the adsorbed intermediates on the surface would be replaced by water and easily released to the gas phase. This may explain why some pollutants had high removal efficiencies but low mineralization rate and why the pollutants obtain higher mineralization efficiencies in dry air than in humid air. The results of the following studies will further support this hypothesis. Coronado [13] found that water vapor slightly inhibited the mineralization of methyl isobutyl ketone, while acetone could completely convert to CO2 at different relative humidity (RH). Vildozo et al. [85] reported that, in the gas phase, the concentration of by-products was higher during irradiation in humid conditions than in dry conditions, due to the displacement of the intermediates from the surface before the complete mineralization was achieved. On the other hand, Zhao et al. [106] reported that RH enhanced the formation rate of hydroxide radicals, leading to more intermediates with higher oxidation states in the gas phase. With increasing RH, more intermediates are released into the gas phase. The mineralization efficiency of N-TiO2 is increased with RH. The byproducts generated in relatively high RH are “greener” than that in low RH.

3. Adsorption enhanced catalysts 3.1. Sorbent-supported catalysts As mentioned above, for low concentration pollutants, the removal efficiency is quite sensitive to surface characteristics of the photocatalysts and practical conditions. Therefore, the main idea of materials synthesis is to introduce the surface structures which could enhance the adsorption of pollutants under realistic conditions. Up to now, coating photocatalysts onto sorbents is the most common method to enhance adsorption. There are many materials reported as supports of photocatalysts, such as activated carbon [2,3,75], zeolites [28,49,83], alumina [77,80], silica [14,68,77–81,101], carbon nanotubes [30,96], graphene [90,102], clay [31,33,71], glass fibers [67,97], and paper [66] as shown in Fig. 3. In 1995, Takeda et al. [77] investigated the effects of using inert supports for TiO2 loading on photocatalysis decomposition of propionaldehyde in the gas phase for several kinds of supports such as zeolites, alumina, silica, and activated carbon (AC). In cases where the adsorption constant is low, the decomposition rate is determined by the amount of adsorbed substrate, while if the adsorption constant of the support is high, as in the case of femerite and AC, the adsorbed propionaldehyde on the support cannot move easily to the TiO2 particles due to their high adsorption strength, even if a large concentration gradient exists between the TiO2 particles and their vicinities. They concluded that the use of an inert support having a medium adsorption constant is necessary to obtain the highest activity, where a high amount of adsorbed pollutant can be supplied to the TiO2 particles. Ao and Lee [2–4] reported that the use of TiO2 on AC reduced both the competition effect of the pollutant and water vapor on TiO2 . The inhibition effect of BTEX and SO2 on NO conversion was significantly reduced when TiO2 immobilized on AC

compared to TiO2 only. Mo et al. [49] mixed 12 commercial adsorbents with P25, respectively, at specific weight ratios. Only two samples using mordenite and silica as substrates showed better decomposition efficiencies than pure P25. Kibanova et al. [32] synthesized two novel composite materials, hectorite–TiO2 and kaolinite–TiO2 . However, both photocatalysts showed lower removal efficiencies of toluene than P25. In some studies, sorbent-supported TiO2 materials showed higher mineralization efficiencies compared to bare TiO2 . This may be ascribed to two effects: (i) concentrated pollutants could enhance the photoinduced electron–hole pair separation step by the increased use of the photoinduced charge carriers on the TiO2 surface; (ii) the sorbent may adsorb some or all of the catalysis products and retard the release of those products, which can allow for a more efficient reaction. Summarizing the presented research above, we can obtain three main points for sorbent-supported photocatalysts: (i) the gasphase pollutants are first adsorbed on the surface of the sorbents (adsorption sites) and diffuse to the oxidation sites on the photocatalysts; (ii) the sorbents with medium adsorption constant for a certain pollutant may have the highest photo decomposition efficiency. Either too low or too high of an adsorption constants lead to a decrease in removal rate; (iii) a certain sorbent has different adsorption constants for different pollutants. These results suggest that one type of sorbent may only be a good fit for treating one kind of pollutant. This is the major limitation of sorbent-supported catalysts for removing indoor air pollution. 3.2. Mesoporous/microporous TiO2 Another method to improve the adsorption capability of photocatalysts is to directly alter the structure of photocatalysts to the forms that are fit for attracting pollutants in low concentrations. This method can also avoid the significant limitation of sorbentsupported catalysts via directly adsorbing the pollutants to the reaction sites. Typically, there are two strategies to achieve this goal. One is to increase the number of adsorption sites on the catalysts surface; the other is to enhance the adsorption potential of the catalysts surface. Enlarging the surface area of the photocatalysts is the most common way to increase the number of adsorption sites. This can be achieved via synthesizing photocatalysts with either regular mesopores/micropores or nanosize crystals. Actually, the later also could be considered as porous materials, where the pores are formed by the aggregation of nanocrystals. According to IUPAC classification, these nanomaterials can be divided into mesoporous (2–50 nm) and microporous (<2 nm) photocatalysts. When the pore size is narrowed to below 2 nm, the adsorption potential of adjacent pore walls overlaps and gas molecules can experience much enhanced attractive forces than in mesoporous materials of equal composition [20,91]. Mesoporous TiO2 has drawn increasing an attention since Antonelli and Ying synthesized ordered mesoporous TiO2 by gel–sol method as shown Fig. 4 [1]. The major challenge of synthesizing mesoporous TiO2 is how to obtain both ordered porous structures and simultaneously high crystallinity. Jinwoo Lee and his coworkers presented the combined assembly via the soft and hard (CASH) method to synthesize ordered mesoporous TiO2 with high crystallinity and thermal stability, as shown in Fig. 5 [37]. Yuming Zhou and his coworkers fabricated mesoporous anatase-phase TiO2 hollow shells by the solvothermal and calcination process using SiO2 and carbon as templates [104]. The schematic illustration of the synthesis procedure is shown in Fig. 6. Compared to mesoporous TiO2 , the microporous TiO2 is much more difficult to obtain at high crystallinity and ordered pore

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Fig. 3. SEM images of TiO2 doped on different substrates (substrates in A: mordenite [50]; B: SiO2 [50]; C: carbon [96]; D: nanotube [30]; E and F: grapheme [90]; G and H: glass fiber [97]).

structure. Asim Bhaumik fabricated super-microporous TiO2 with surface area as high as 634 m2 /g by using newly designed chelating, structure-directing agents [8]. However, in this case, the TiO2 showed amorphous pattern and the pores were formed by disordered aggregation of nanoparticles. So far as we understand it, there has still no paper reported the successful case of microporous TiO2 with high crystallinity as well as ordered micropore structure. Earlier research has used porous photocatalysts with large surface area to treat waste water or air pollution at the ppm level [9,26,29,36,45,60,104,107]. However, one rarely finds investigations of mesoporous/microporous photocatalysts for ppb level gas-phase pollutant purification. Puddu et al. [64] reported that

the conversion efficiency of 610 ppm trichloroethylene decreased as the specific surface area of TiO2 increased. Lv and Zhu [41] investigated the adsorption and photo-oxidation of cyclohexanone on the ppb level over microporous TiO2 , as shown in Fig. 7. The results showed that the amount of adsorbed pollutant increased with the micropore area, while the removal efficiency of cyclohexanone increased much slower. Increasing surface area could result in the increase of bulk defects and bad crystallinity thereby reducing the effective separation efficiency of photoexited charge carriers [35,63]. If the issue between large surface area and high crystallinity can be solved, then microporous photocatalysts with larger surface area will be highly promising for the degradation of low concentration pollutants.

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Fig. 4. TEM image of Ti-TMSI prepared from tetradecylphosphite and calcined at 500 ◦ C under O2 for ten hours showing the retained 32 A˚ pores ordered in a hexagonal array [1].

Fig. 5. Schematic representation of the combined assembly via the soft and hard CASH method [37].

3.3. Porous composites Another approach to avoid the low crystallinity of porous TiO2 is to modify TiO2 nanocrystal into porous SiO2 framework. Zhao et al. [16] successfully synthesized a series of highly ordered hexagonal mesoporous TiO2 –SiO2 with variable Ti/Si ratios, using TIPO as a titania source, TEOS as a silica source, and P123 as a template. The scheme of the synthesis procedure is reviewed in Fig. 8. TiO2 nanoparticles are the component of pore walls, and the ordered porous structure is much easier to be controlled by adjusting the SiO2 fabrication. The composite could be heated to a relatively high temperature thereby reaching good crystallinity and thermal stability. Li and Kim [38] have prepared TiO2 –xSiO2 composites by the sol–gel method, which exhibit a core/shell structure of a nano titania/Ti O Si species modified TiO2 embedded in mesoporous silica. The as-synthesized TiO2 –xSiO2 composites exhibit both much higher absorption capability of organic pollutants and better photocatalytic activity for the photooxidation of benzene than pure titania. The authors believe that this improvement can be attributed to the high surface area, higher UV absorption, and easy diffusion of adsorbed pollutants on the adsorption sites to photogenerated oxidizing radicals on the photoactive sites. Although there are several synthesis strategies for TiO2 –SiO2 , their composites have been studied in the field of nano material science, and their photoactivity has been evaluated by degradation of pollutants in liquid phase, their performance in indoor air purification is still largely unknown. Since the low concentrations of the indoor air pollutants are sensitive to the pore size, the pore size of the TiO2 –SiO2 composites may need to be further narrowed to enhance the adsorption capacity. Nevertheless, the strategies of these synthesis methods are valuable starting points for designing novel catalysts for indoor air purification.

Fig. 6. Left: schematic illustration of carbon coating and silica-protected calcination procedure for the fabrication of mesoporous TiO2 hollow shells. Right: corresponding TEM images of the core–shell nanoparticles after sequential treatments: (a) original SiO2 /TiO2 particles, (b) SiO2 /TiO2 /C, (c) SiO2 /TiO2 /SiO2 after calcination, and (d) final hollow TiO2 after the removal of the silica [104].

Fig. 7. TEM image of amorphous microporous TiO2 [41].

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Fig. 8. Scheme for the synchronous assembly of titanate oligomers from TIPO and silicate species from TEOS molecules with triblock copolymer P123 template to form highly ordered mesoporous anatase nanocrystalline TiO2 –SiO2 composites [16].

4. Summary Low concentration air purification has been one of the most progressive environmental applications for titanium dioxide. To diminish the gap between scientific research and industry needs, it is necessary to better understand the effects of practical conditions on the adsorption and photo-oxidation of the pollutants. The low concentrations (sub-ppb to ppb) and short contact times (tens of milliseconds) significantly increase the difficulty in adsorbing the pollutants from gas phase to the photocatalysts surface thereby reducing their efficiency. In a multi-compound gas mixture, the pollutants’ adsorption and byproduct generation and release are mainly determined by the affinity of the pollutants with the catalyst’s surface. Light exposure dramatically changes the adsorption nature of the catalysts. Porous TiO2 with both a large micropore area and high crystallinity may be promising catalyst for future indoor air purification. To further promote photocatalytic air purification technology, it is important to further study the nature of the adsorption and photo-oxidation under realistic conditions and to integrate the obtained findings into the existing material science to develop novel and more specific photocatalysts. Acknowledgements Financial support by the National High Technology Research and Development Program (“863” Program No. 2010AA064902) and China Scholarship Council is greatly appreciated. References [1] D.M. Antonelli, J.Y. Ying, Synthesis of hexagonally packed mesoporous TiO2 by a modified sol–gel method, Angewandte Chemie International Edition in English 34 (1995) 2014–2017. [2] C.H. Ao, S.C. Lee, Enhancement effect of TiO2 immobilized on activated carbon filter for the photodegradation of pollutants at typical indoor air level, Applied Catalysis B: Environmental 44 (2003) 191–205. [3] C.H. Ao, S.C. Lee, Combination effect of activated carbon with TiO2 for the photodegradation of binary pollutants at typical indoor air level, Journal of Photochemistry and Photobiology A: Chemistry 161 (2004) 131–140. [4] C.H. Ao, S.C. Lee, S.C. Zou, C.L. Mak, Inhibition effect of SO2 on NOx and VOCs during the photodegradation of synchronous indoor air pollutants at parts per billion (ppb) level by TiO2 , Applied Catalysis B: Environmental 49 (2004) 187–193. [5] N. Bouazza, M.A. Lillo-Rodenas, A. Linares-Solano, Photocatalytic activity of TiO(2)-based materials for the oxidation of propene and benzene at low concentration in presence of humidity, Applied Catalysis B: Environmental 84 (2008) 691–698. [6] A. Bouzaza, A. Laplanche, Photocatalytic degradation of toluene in the gas phase: comparative study of some TiO2 supports, Journal of Photochemistry and Photobiology A: Chemistry 150 (2002) 207–212.

[7] T.D. Bui, A. Kimura, S. Ikeda, M. Matsumura, Determination of oxygen sources for oxidation of benzene on TiO2 photocatalysts in aqueous solutions containing molecular oxygen, Journal of the American Chemical Society 132 (2010) 8453–8458. [8] D. Chandra, A. Bhaumik, Super-microporous TiO2 synthesized by using new designed chelating structure directing agents, Microporous and Mesoporous Materials 112 (2008) 533–541. [9] S.M. Chang, C.Y. Lee, A salt-assisted approach for the pore-size-tailoring of the ionic-liquid-templated TiO2 photocatalysts exhibiting high activity, Applied Catalysis B: Environmental 132–133 (2013) 219–228. [10] H.Y. Chen, O. Zahraa, M. Bouchy, Inhibition of the adsorption and photocatalytic degradation of an organic contaminant in an aqueous suspension of TiO2 by inorganic ions, Journal of Photochemistry and Photobiology A: Chemistry 108 (1997) 37–44. [11] X. Chen, C. Burda, The electronic origin of the visible-light absorption properties of C-, N- and S-doped TiO2 nanomaterials, Journal of the American Chemical Society 130 (2008) 5018. [12] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications and applications, Chemical Reviews 107 (2007) 2891–2959. [13] J.M. Coronado, M.E. Zorn, I. Tejedor-Tejedor, M.A. Anderson, Photocatalytic oxidation of ketones in the gas phase over TiO2 thin films: a kinetic study on the influence of water vapor, Applied Catalysis B: Environmental 43 (2003) 329–344. [14] K. De Witte, V. Meynen, M. Mertens, O.I. Lebedev, G. Van Tendeloo, A. Sepulveda-Escribano, F. Rodriguez-Reinoso, E.F. Vansant, P. Cool, Multi-step loading of titania on mesoporous silica: influence of the morphology and the porosity on the catalytic degradation of aqueous pollutants and VOCs, Applied Catalysis B: Environmental 84 (2008) 125–132. [15] H. Destaillats, M. Sleiman, D.P. Sullivan, C. Jacquiod, J. Sablayrolles, L. Molins, Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions, Applied Catalysis B: Environmental 128 (2012) 159–170. [16] W. Dong, Y. Sun, W.L. Chul, W. Hua, X. Lu, Y. Shi, S. Zhang, J. Chen, D. Zhao, Controllable and repeatable synthesis of thermally stable anatase nanocrystal-silica composites with highly ordered hexagonal mesostructures, Journal of the American Chemical Society 129 (2007) 13894–13904. [17] Y. Dong, Z. Bai, R. Liu, T. Zhu, Decomposition of indoor ammonia with TiO2 loaded cotton woven fabrics prepared by different textile finishing methods, Atmospheric Environment 41 (2007) 3182–3192. [18] R. Golden, Identifying an indoor air exposure limit for formaldehyde considering both irritation and cancer hazards, Critical Reviews in Toxicology 41 (2011) 672–721. [19] J.L. Gole, S.M. Prokes, O.J. Glembocki, J. Wang, X. Qiu, C. Burda, Study of concentration-dependent cobalt ion doping of TiO2 and TiO2 -xNx at the nanoscale, Nanoscale 2 (2010) 1134–1140. [20] Y. Hanzawa, T. Suzuki, K. Kaneko, Entrance-enriched micropore filling of nnonane, Langmuir 10 (1994) 2857–2859. [21] S. Harrad, C.A. De Wit, M.A.E. Abdallah, C. Bergh, J.A. Björklund, A. Covaci, P.O. Darnerud, J. De Boer, M. Diamond, S. Huber, P. Leonards, M. Mandalakis, C. Östman, L.S. Haug, C. Thomsen, T.F. Webster, Indoor contamination with hexabromocyclododecanes, polybrominated diphenyl ethers, and perfluoroalkyl compounds: an important exposure pathway for people? Environmental Science and Technology 44 (2010) 3221–3231. [22] J.-M. Herrmann, Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants, Catalysis Today 53 (1999) 115–129. [23] L.G. Hersoug, L.L.N. Husemoen, S.F. Thomsen, T. Sigsgaard, B.H. Thuesen, A. Linneberg, Association of indoor air pollution with rhinitis symptoms, atopy and nitric oxide levels in exhaled air, International Archives of Allergy and Immunology 153 (2010) 403–412.

32

J. Lyu et al. / Catalysis Today 225 (2014) 24–33

[24] A.T. Hodgson, H. Destaillats, D.P. Sullivan, W.J. Fisk, Performance of ultraviolet photocatalytic oxidation for indoor air cleaning applications, Indoor Air 17 (2007) 305–316. [25] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chemical Reviews 95 (1995) 69–96. [26] A.A. Ismail, D.W. Bahnemann, Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms, Journal of Materials Chemistry 21 (2011) 11686–11707. [27] W.A. Jacoby, D.M. Blake, J.A. Fennell, J.E. Boulter, L.M. Vargo, M.C. George, S.K. Dolberg, Heterogeneous photocatalysis for control of volatile organic compounds in indoor air, Journal of the Air and Waste Management Association 46 (1996) 891–898. [28] Y.H. Jan, L.Y. Lin, M. Karthik, H. Bai, Titanium dioxide/zeolite catalytic adsorbent for the removal of NO and acetone vapors, Journal of the Air and Waste Management Association 59 (2009) 1186–1193. [29] Y. Ji, K.-C. Lin, H. Zheng, C.-C. Liu, L. Dudik, J. Zhu, C. Burda, Solarlight photoamperometric and photocatalytic properties of quasi-transparent TiO2 nanoporous thin films, ACS Applied Materials & Interfaces 2 (2010) 3075–3082. [30] W.-K. Jo, K.-H. Kim, Feasibility of carbonaceous nanomaterial-assisted photocatalysts calcined at different temperatures for indoor air applications, International Journal of Photoenergy (2012). [31] D. Kibanova, J. Cervini-Silva, H. Destaillats, Efficiency of clay-TiO2 nanocomposites on the photocatalytic elimination of a model hydrophobic air pollutant, Environmental Science and Technology 2012 (2009) 1500–1506, http://dx.doi.org/10.1155/2012/594214. [32] D. Kibanova, J. Cervini-Silva, H. Destaillats, Efficiency of clay – TiO2 nanocomposites on the photocatalytic elimination of a model hydrophobic air pollutant, Environmental Science and Technology 43 (2009) 1500–1506. [33] D. Kibanova, M. Trejo, H. Destaillats, J. Cervini-Silva, Synthesis of hectoriteTiO2 and kaolinite-TiO2 nanocomposites with photocatalytic activity for the degradation of model air pollutants, Applied Clay Science 42 (2009) 563–568. [34] B. Kolarik, L. Lagercrantz, J. Sundell, Nitric oxide in exhaled and aspirated nasal air as an objective measure of human response to indoor air pollution, Indoor Air 19 (2009) 145–152. [35] M. Kong, Y. Li, X. Chen, T. Tian, P. Fang, F. Zheng, X. Zhao, Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency, Journal of the American Chemical Society 133 (2011) 16414–16417. [36] R. Kun, S. Tarjan, A. Oszko, T. Seemann, V. Zoellmer, M. Busse, I. Dekany, Preparation and characterization of mesoporous N-doped and sulfuric acid treated anatase TiO2 catalysts and their photocatalytic activity under UV and Vis illumination, Journal of Solid State Chemistry 182 (2009) 3076–3084. [37] J. Lee, M.C. Orilall, S.C. Warren, M. Kamperman, F.J. Disalvo, U. Wiesner, Direct access to thermally stable and highly crystalline mesoporous transition-metal oxides with uniform pores, Nature Materials 7 (2008) 222–228. [38] Y. Li, S.J. Kim, Synthesis and characterization of nano titania particles embedded in mesoporous silica with both high photocatalytic activity and adsorption capability, Journal of Physical Chemistry B 109 (2005) 12309–12315. [39] Y.J. Liu, L.Z. Zhu, X.Y. Shen, Polycyclic aromatic hydrocarbons (PAHs) in indoor and outdoor air of Hangzhou, China, Environmental Science and Technology 35 (2001) 840–844. [40] H. Lu, L. Zhu, S. Chen, Pollution level, phase distribution and health risk of polycyclic aromatic hydrocarbons in indoor air at public places of Hangzhou, China, Environmental Pollution 152 (2008) 569–575. [41] J. Lv, L. Zhu, Highly efficient indoor air purification using adsorptionenhanced-photocatalysis-based microporous TiO2 at short residence time, Environmental Technology (United Kingdom) 34 (11) (2013) 1447–1454. [42] A. Markowska-Szczupak, K. Ulfig, A.W. Morawski, The application of titanium dioxide for deactivation of bioparticulates: an overview, Catalysis Today 169 (2011) 249–257. [43] A. Maudhuit, C. Raillard, V. Héquet, L. Le Coq, J. Sablayrolles, L. Molins, Adsorption phenomena in photocatalytic reactions: the case of toluene, acetone and heptane, Chemical Engineering Journal 170 (2011) 464–470. ˜ J.M.D. Rodríguez, E.T. Rendón, J.A.H. Melián, [44] E.P. Melián, O.G. Díaz, J. Arana, Kinetics and adsorption comparative study on the photocatalytic degradation of o-, m- and p-cresol, Catalysis Today 129 (2007) 256–262. [45] G. Miao, L. Chen, Z. Qi, Facile synthesis and active photocatalysis of mesoporous and microporous TiO2 nanoparticles, European Journal of Inorganic Chemistry 2012 (35) (2012) 5864–5871. [46] A. Mills, S. Le Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol. A. 108 (1) (1997) 1–35. [47] A. Mills, S.K. Lee, A web-based overview of semiconductor photochemistrybased current commercial applications, Journal of Photochemistry and Photobiology A: Chemistry 152 (2002) 233–247. [48] J. Mo, Y. Zhang, Q. Xu, Effect of water vapor on the by-products and decomposition rate of ppb-level toluene by photocatalytic oxidation, Applied Catalysis B: Environmental 132-133 (2013) 212–218. [49] J. Mo, Y. Zhang, Q. Xu, J.J. Lamson, R. Zhao, Photocatalytic purification of volatile organic compounds in indoor air: a literature review, Atmospheric Environment 43 (2009) 2229–2246.

[50] J. Mo, Y. Zhang, Q. Xu, R. Yang, Effect of TiO2 /adsorbent hybrid photocatalysts for toluene decomposition in gas phase, Journal of Hazardous Materials 168 (2009) 276–281. [51] J. Mo, Y. Zhang, Q. Xu, Y. Zhu, J.J. Lamson, R. Zhao, Determination and risk assessment of by-products resulting from photocatalytic oxidation of toluene, Applied Catalysis B: Environmental 89 (2009) 570–576. [52] A. Nakajima, S.I. Koizumi, T. Watanabe, K. Hashimoto, Photoinduced amphiphilic surface on polycrystalline anatase TiO2 thin films, Langmuir 16 (2000) 7048–7050. [53] R. Nakamura, T. Tanaka, Y. Nakato, Mechanism for visible light responses in anodic photocurrents at N-doped TiO2 film electrodes, Journal of Physical Chemistry B 108 (2004) 10617–10620. [54] A.Y. Nosaka, T. Fujiwara, H. Yagi, H. Akutsu, Y. Nosaka, Characteristics of water adsorbed on TiO2 photocatalytic systems with increasing temperature as studied by solid-state1H NMR Spectroscopy, Journal of Physical Chemistry B 108 (2004) 9121–9125. [55] T.N. Obee, TiO2 photocatalysis for indoor air applications: effects of humidity and trace contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3-butadiene, Environmental Science and Technology 29 (1995) 1223–1231. [56] T.N. Obee, R.T. Brown, TiO2 photocatalysis for indoor air applications – effects of humidity and trace contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3-butadiene, Environmental Science and Technology 29 (1995) 1223–1231. [57] T. Ochiai, A. Fujishima, Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification, Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 247–262. [58] T. Ohura, T. Amagai, X. Shen, S. Li, P. Zhang, L. Zhu, Comparative study on indoor air quality in Japan and China: characteristics of residential indoor and outdoor VOCs, Atmospheric Environment 43 (2009) 6352–6359. [59] H.-H. Ou, S.-L. Lo, Photocatalysis of gaseous trichloroethylene (TCE) over TiO2 : the effect of oxygen and relative humidity on the generation of dichloroacetyl chloride (DCAC) and phosgene, Journal of Hazardous Materials 146 (2007) 302–308. [60] J.H. Pan, X. Zhang, A.J. Du, D.D. Sun, J.O. Leckie, Self-etching reconstruction of hierarchically mesoporous F-TiO2 hollow microspherical photocatalyst for concurrent membrane water purifications, Journal of the American Chemical Society 130 (2008) 11256–11257. [61] Y. Paz, Application of TiO2 photocatalysis for air treatment: patents’ overview, Applied Catalysis B: Environmental 99 (2010) 448–460. [62] P. Pichat, Some views about indoor air photocatalytic treatment using TiO2 : conceptualization of humidity effects, active oxygen species, problem of C1–C3 carbonyl pollutants, Applied Catalysis B: Environmental 99 (2010) 428–434. [63] P. Pichat, H. Courbon, R. Enriquez, T.T.Y. Tan, R. Amal, Light-induced isotopic exchange between O2 and semiconductor oxides, a characterization method that deserves not to be overlooked, Research on Chemical Intermediates 33 (2007) 239–250. [64] V. Puddu, H. Choi, D.D. Dionysiou, G.L. Puma, TiO2 photocatalyst for indoor air remediation: influence of crystallinity, crystal phase, and UV radiation intensity on trichloroethylene degradation, Applied Catalysis B: Environmental 94 (2010) 211–218. [65] N. Quici, M.L. Vera, H. Choi, G.L. Puma, D.D. Dionysiou, M.I. Litter, H. Destaillats, Effect of key parameters on the photocatalytic oxidation of toluene at low concentrations in air under 254 + 185 nm UV irradiation, Applied Catalysis B: Environmental 95 (2010) 312–319. [66] C. Raillard, V. Héquet, P. Le Cloirec, J. Legrand, Kinetic study of ketones photocatalytic oxidation in gas phase using TiO2 -containing paper: effect of water vapor, Journal of Photochemistry and Photobiology A: Chemistry 163 (2004) 425–431. [67] C. Raillard, V. Hequet, P. Le Cloirec, J. Legrand, TiO2 coating types influencing the role of water vapor on the photocatalytic oxidation of methyl ethyl ketone in the gas phase, Applied Catalysis B: Environmental 59 (2005) 213–220. [68] S. Rodrigues, K.T. Ranjit, S. Uma, I.N. Martyanov, K.J. Klabunde, Single-step synthesis of a highly active visible-light photocatalyst for oxidation of a common indoor air pollutant: acetaldehyde, Advanced Materials 17 (2005) 2467–2471. [69] D.A. Sarigiannis, S.P. Karakitsios, A. Gotti, I.L. Liakos, A. Katsoyiannis, Exposure to major volatile organic compounds and carbonyls in European indoor environments and associated health risk, Environment International 37 (2011) 743–765. [70] J.J. Sene, W.A. Zeltner, M.A. Anderson, Fundamental photoelectrocatalytic and electrophoretic mobility studies of TiO2 and v-doped TiO2 thin-film electrode materials, Journal of Physical Chemistry B 107 (2003) 1597–1603. [71] R. Sharmin, M.B. Ray, Application of ultraviolet light-emitting diode photocatalysis to remove volatile organic compounds from indoor air, Journal of the Air and Waste Management Association 62 (2012). [72] T. Shibata, H. Irie, K. Hashimoto, Photoinduced hardness change on TiO2 single crystal surfaces, Journal of the Air & Waste Management 62 (9) (2009) 1032–1039, http://dx.doi.org/10.1080/10962247.2012.695760. [73] F. Shiraishi, S. Yamaguchi, Y. Ohbuchi, A rapid treatment of formaldehyde in a highly tight room using a photocatalytic reactor combined with a continuous adsorption and desorption apparatus, Chemical Engineering Science 58 (2003) 929–934.

J. Lyu et al. / Catalysis Today 225 (2014) 24–33 [74] M. Sleiman, P. Conchon, C. Ferronato, J.M. Chovelon, Photocatalytic oxidation of toluene at indoor air levels (ppbv): towards a better assessment of conversion, reaction intermediates and mineralization, Applied Catalysis B: Environmental 86 (2009) 159–165. [75] M. Sleiman, C. Ferronato, J.M. Chovelon, Photocatalytic removal of pesticide dichlorvos from indoor air: a study of reaction parameters, intermediates and mineralization, Environmental Science and Technology 42 (2008) 3018–3024. [76] L. Stevens, J.A. Lanning, L.G. Andersen, W.A. Jacoby, N. Chornet, Investigation of the photocatalytic oxidation of low-level carbonyl compounds, Journal of the Air and Waste Management Association 48 (1998) 979–984. [77] N. Takeda, T. Torimoto, S. Sampath, S. Kuwabata, H. Yoneyama, Effect of inert supports for titanium dioxide loading on enhancement of photodecomposition rate of gaseous propionaldehyde, Journal of Physical Chemistry 99 (1995) 9986–9991. [78] J. Taranto, D. Frochot, P. Pichat, Photocatalytic treatment of air: comparison of various TiO2 , coating methods, and supports using methanol or n-octane as test pollutant, Industrial & Engineering Chemistry Research 48 (2009) 6229–6236. [79] M. Tasbihi, M. Kete, A.M. Raichur, N.N. Tusar, U.L. Stangar, Photocatalytic degradation of gaseous toluene by using immobilized titania/silica on aluminum sheets, Environmental Science and Pollution Research 19 (2012) 3735–3742. ˇ [80] M. Tasbihi, M. Kete, A.M. Raichur, N.N. Tuˇsar, U.L. Stangar, Photocatalytic degradation of gaseous toluene by using immobilized titania/silica on aluminum sheets, Environmental Science and Pollution Research 19 (2012) 3735–3742. [81] F. Thevenet, O. Guaitella, J.M. Herrmann, A. Rousseau, C. Guillard, Photocatalytic degradation of acetylene over various titanium dioxide-based photocatalysts, Applied Catalysis B: Environmental 61 (2005) 58–68. [82] T.L. Thompson, J.T. Yates Jr., Surface science studies of the photoactivation of TiO2 – new photochemical processes, Chemical Reviews 106 (2006) 4428–4453. [83] C.W. Tsai, C.T. Chang, C.S. Chiou, J.L. Shie, Y.M. Chang, Study on the indoor volatile organic compound treatment and performance assessment with TiO2 /MCM-41 and TiO2 /quartz photoreactor under ultraviolet irradiation, Journal of the Air and Waste Management Association 58 (2008) 1266–1273. [84] I. van der Veen, J. de Boer, Phosphorus flame retardants: properties, production, environmental occurrence, toxicity and analysis, Chemosphere 88 (2012) 1119–1153. [85] D. Vildozo, R. Portela, C. Ferronato, J.M. Chovelon, Photocatalytic oxidation of 2-propanol/toluene binary mixtures at indoor air concentration levels, Applied Catalysis B: Environmental 107 (2011) 347–354. [86] J. Wang, B. Mao, J.L. Gole, C. Burda, Visible-light-driven reversible and switchable hydrophobic to hydrophilic nitrogen-doped titania surfaces: correlation with photocatalysis, Nanoscale 2 (2010) 2257–2261. [87] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature 388 (1997) 431–432. [88] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Photogeneration of highly amphiphilic TiO2 surfaces, Advanced Materials 10 (1998) 135. [89] S. Wang, H.M. Ang, M.O. Tade, Volatile organic compounds in indoor environment and photocatalytic oxidation: state of the art, Environment International 33 (2007) 694–705. [90] W. Wang, J. Yu, Q. Xiang, B. Cheng, Enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2–graphene composites for photodegradation of acetone in air, Applied Catalysis B: Environmental 119–120 (2012) 109–116. [91] Z.M. Wang, K. Kaneko, Effect of pore width on micropore filling mechanism of SO2 in carbon micropores, Journal of Physical Chemistry B 102 (1998) 2863–2868.

33

[92] M. Weng, L. Zhu, K. Yang, S. Chen, Levels and health risks of carbonyl compounds in selected public places in Hangzhou, China, Journal of Hazardous Materials 164 (2009) 700–706. [93] M. Weng, L. Zhu, K. Yang, S. Chen, Levels, sources, and health risks of carbonyls in residential indoor air in Hangzhou, China, Environmental Monitoring and Assessment 163 (2010) 573–581. [94] A. Wisthaler, P. Strøm-Tejsen, L. Fang, T.J. Arnaud, A. Hansel, T.D. Mark, D.P. Wyon, PTR-MS assessment of photocatalytic and sorption-based purification of recirculated cabin air during simulated 7-h flights with high passenger density, Environmental Science and Technology 41 (2007) 229–234. [95] P. Wolkoff, C.K. Wilkins, P.A. Clausen, G.D. Nielsen, Organic compounds in office environments – sensory irritation, odor, measurements and the role of reactive chemistry, Indoor Air 16 (2006) 7–19. [96] Y. Yang, K. Chiang, N. Burke, Porous carbon-supported catalysts for energy and environmental applications: a short review, Catalysis Today 178 (2011) 197–205. [97] H.G. Yu, S.C. Lee, J.G. Yu, C.H. Ao, Photocatalytic activity of dispersed TiO2 particles deposited on glass fibers, Journal of Molecular Catalysis A: Chemical 246 (2006) 206–211. [98] K.P. Yu, G.W.M. Lee, W.M. Huang, C. Wu, S. Yang, The correlation between photocatalytic oxidation performance and chemical/physical properties of indoor volatile organic compounds, Atmospheric Environment 40 (2006) 375–385. [99] L. Zhang, W.A. Anderson, S. Sawell, C. Moralejo, Mechanistic analysis on the influence of humidity on photocatalytic decomposition of gas-phase chlorobenzene, Chemosphere 68 (2007) 546–553. [100] L. Zhang, R. Dillert, D. Bahnemann, M. Vormoor, Photo-induced hydrophilicity and self-cleaning: models and reality, Energy and Environmental Science 5 (2012) 7491–7507. [101] M. Zhang, T. An, J. Fu, G. Sheng, X. Wang, X. Hu, X. Ding, Photocatalytic degradation of mixed gaseous carbonyl compounds at low level on adsorptive TiO2 /SiO2 photocatalyst using a fluidized bed reactor, Chemosphere 64 (2006) 423–431. [102] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, TiO2 –graphene nanocomposites for gas-phase photocatalytic degradation of volatile aromatic pollutant: is TiO2 –graphene truly different from other TiO2 –carbon composite materials? ACS Nano 4 (2010) 7303–7314. [103] Y.P. Zhang, R. Yang, Q.J. Xu, J.H. Mo, Characteristics of photocatalytic oxidation of toluene, benzene, and their mixture, Journal of the Air and Waste Management Association 57 (2007) 94–101. [104] Z. Zhang, Y. Zhou, Y. Zhang, S. Zhou, J. Shi, J. Kong, S. Zhang, Well-crystallized mesoporous TiO2 shells for enhanced photocatalytic activity: prepared by carbon coating and silica-protected calcination, Dalton Transactions 42 (2013) 5004–5012. [105] J. Zhao, X. Yang, Photocatalytic oxidation for indoor air purification: a literature review, Building and Environment 38 (2003) 645–654. [106] W. Zhao, J. Dai, F. Liu, J. Bao, Y. Wang, Y. Yang, D. Zhao, Photocatalytic oxidation of indoor toluene: process risk analysis and influence of relative humidity, photocatalysts, and VUV irradiation, Science of the Total Environment 438 (2012) 201–209. [107] Y. Zhao, X. Qiu, C. Burda, The effects of sintering on the photocatalytic activity of N-doped TiO2 nanoparticles, Chemistry of Materials 20 (2008) 2629–2636. [108] J. Zhu, R. Newhook, L. Marro, C.C. Chan, Selected volatile organic compounds in residential air in the city of Ottawa, Canada, Environmental Science and Technology 39 (2005) 3964–3971. [109] L. Zhu, H. Lu, S. Chen, T. Amagai, Pollution level, phase distribution and source analysis of polycyclic aromatic hydrocarbons in residential air in Hangzhou, China, Journal of Hazardous Materials 162 (2009) 1165–1170. [110] M.E. Zorn, S.O. Hay, M.A. Anderson, Effect of molecular functionality on the photocatalytic oxidation of gas-phase mixtures, Applied Catalysis B: Environmental 99 (2010) 420–427.