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TiO2/SiO2 Films for Removal of Volatile Organic Compounds (VOCs) from Indoor Air

  • Nataša Novak Tušar
  • Andraž Šuligoj
  • Urška Lavrenčič ŠtangarEmail author
Living reference work entry

Abstract

Volatile organic compounds (VOCs) are the major pollutants in indoor air, which significantly impact indoor air quality and thus influence human health. A long-term exposure to VOCs will be detrimental to human health causing sick building syndrome (SBS). Photocatalytic decomposition of VOCs using TiO2 as a photocatalyst is a key technology for air cleaning devices because it can totally convert most VOC pollutants at low concentrations to harmless inorganic products at ambient temperature. UVA light required in the air cleaning device is nowadays a very cheap light source. A common approach to enhance the photocatalytic activity of TiO2 is also to increase its surface area (100–200 m2/g to 400–1000 m2/g). This can be achieved by immobilization of TiO2 on the porous supports such as porous silica and the preparation of such a catalyst in the form of a thin layer using an appropriate carrier. Porous silica is superior support for accommodating photocatalyst nanoparticles because it is chemically inert, possesses high surface area, is transparent to UV radiation, has great physical stability, and has hydrophobic character.

An overview of the design and development of TiO2/SiO2 composite photocatalyst in the form of films with superior activity for removal of VOCs from the polluted air is presented.

Introduction

Indoor Air and Volatile Organic Compounds (VOCs)

Indoor air quality has become an important community concern due to the increased amount of personal time spent (80%) in the indoor environment (home, office, car, shopping center). Main indoor air pollutants are nitrogen oxides (NOx), carbon oxides (CO and CO2), particulates, and volatile organic compounds (VOCs). It is found that indoor air typically contains a greater number of VOCs at higher concentrations than outdoor air. The most occurring VOCs in the indoor environment include ethanol, limonene, acetone, toluene, and methylene chloride. The concentration of individual VOC is usually between 5 and 50 μg/m3; however, total VOCs are much higher [25]. Main sources of VOCs are transportation, consumer products, and industry. Three methods are suggested to improve the indoor air quality: source control, increased ventilation, and air cleaning. Source control is often ungovernable. Increased ventilation might even transport more pollutants from the outdoor environment. Thus, air cleaning remains to be the most feasible option to improve indoor air quality.

Conventionally, VOCs are removed by air cleaning devices that employ built-in filters to remove particulate matter and may use sorption materials (e.g., activated carbon) to adsorb gases or odors. However, these technologies only transfer the pollutants from gas to solid phase rather than eliminate them. Improper maintenance of these air cleaning devices may even become a source of VOCs. The photocatalytic process (photocatalytic built-in filter) is emerging as a promising alternative technology for the degradation of VOCs.

Photocatalytic Built-in Filters in Air Cleaning Devices

Air cleaning devices operate using a variety of technologies that claim to remove VOCs. But how effective are these devices really? A study that has been recently concluded at Syracuse University in the USA evaluated the effectiveness of 12 commercially available air cleaning devices for removing VOCs from indoor air [4]. These technology types include simple air filters specially treated with activated carbon or zeolite, UV photocatalytic oxidation (UVPCO), ozone oxidation, air ionization (plasma decomposition), and botanical air cleaning. During the test, 16 different VOCs commonly found in a home or an office were tested. Each air cleaner operated for a 12-h period. The results showed that no single air cleaning device removed all VOCs from indoor air, but some technologies worked better than the others.

However, photocatalytic decomposition of VOCs is a key technology for air cleaning devices (Fig. 1), because it can totally convert most VOC pollutants at low concentrations to harmless inorganic products at ambient temperature. UVA light required in the air cleaning device is nowadays a very cheap light source.
Fig. 1

TiO2 built-in filter in air cleaning device (Andatech Company, Australia)

In the photocatalytic process, a semiconductor activated by ultraviolet (UV) radiation is used as a catalyst to destroy most VOCs. The process has many advantages over conventional processes. The most important is that the photocatalytic reaction is not specific and it is able to destroy most VOCs to harmless inorganic products at room temperature.

Titania Photocatalysis

TiO2 is superior to other photocatalysts due to its interesting characteristics: it is low cost, safe, and very stable, shows high photocatalytic activity, and can promote ambient temperature oxidation of the major class of indoor air pollutants [41]. For the photocatalytic oxidation, an important step of photoreaction is the formation of hole-electron pairs which needs the energy to overcome the band gap between the valence band (VB) and conduction band (CB). When the energy provided by means of photons is larger than the band gap, the pairs of electron-holes are created in the semiconductor, and the charge will transfer between electron-hole pairs and adsorbed species (reactants) on the semiconductor surface, and then photooxidation happens (Fig. 2). Based on the value of band gap of 3.2 eV for anatase TiO2, the required wavelength of light excitation is 388 nm, which is in the UV range. The properties of TiO2, including its light absorption, charge transfer, and surface adsorption, are closely related to its defect disorder, which plays a significant role in the photocatalytic performance of TiO2 [26]. Among all the defects identified in TiO2, an oxygen vacancy is one of the most important and is supposed to be the prevalent defect in many metal oxides, which has been widely investigated both by theoretical calculations and experimental characterization [26].
Fig. 2

Mechanism of photocatalytic reaction

The metal doping method (Cr, Mn, Fe, Ni, Cu, etc.) [2, 10] or doping with nonmetals (N, C, S, F, etc.) [21] has been widely used to modify the electronic properties of bulk titania and make TiO2 effective also under visible light. Other approaches are (i) the combination of TiO2 and graphene, where sufficient interfacial contact and, in some cases, chemical bonding between semiconductor and graphene enable the extension of the light absorption edge [46, 47], and (ii) one-dimensional structure, where high surface area and short radial distances make possible the absorption of light and low recombination of photogenerated electrons and holes, respectively [17]. The energy band levels can be engineered also by the combination of TiO2 with other semiconductor oxides facilitating the formation of mixed oxides [20]. These can act as a sink for the photogenerated electrons by the formation of different types of heterojunctions, preventing the recombination with the excited holes and thus enhancing the photocatalytic efficiency. At the same time, the combination with semiconductor oxides with visible light photoresponsive properties may shift the photocatalytic applications toward the visible (solar) light. The novel one-dimensional nanostructure of self-ordered TiO2 nanotube arrays (NTs) has attracted scientific community worldwide [51]. Compared to TiO2 nanoparticles, TiO2 NTs are more efficient for solar energy conversion and photodegradation with their precisely oriented nature and high surface area. They can be fabricated by sol–gel synthesis, chemical treatments, and electrodeposition techniques; among them in situ anodization method has been verified as the best way to construct self-organized and highly ordered TiO2 NTs [30]. Well-defined combination of TiO2 NTs with ferrites in the position of spinel structure MFe2O4 (M = Ni, Co, Zn, Fe, Sr) could form multiple hetero-structures followed by enhanced solar light-induced photoelectrochemical and photoelectrocatalytic activities [13].

Commercial photocatalytic TiO2 is available in the form of powder (e.g., Evonik Degussa Aeroxide® P25, Aeroxide® P90, Hombikat UV100, Millennium PC500 and PC50, Kronos KRONOClean 7000, Tipe VPC-10) or in the form of colloidal solution (e.g., Ishihara STS 21, Cinkarna CCA 100 AS) with excellent activity. Colloidal TiO2 nanoparticles possess compelling benefits of low-cost, large-scale solution processing and tunable optoelectronic properties through controlled synthesis and surface chemistry engineering [3]. These merits make them promising candidates for a variety of applications. TiO2 colloidal solutions can be prepared by different chemical precipitation-peptization processes at low temperatures (<100 °C), in which titanium butoxide (Ti(OBu)4), titanium ethoxide (Ti(OEt)4), and titanium tetraisopropoxide (Ti(OiPr)4) are used as precursors. To avoid the contamination of TiO2 solutions with organic impurities, titanium tetrachloride (TiCl4), titanyl sulfate (TiOSO4), and metatitanic acid (H2TiO3) precursor can be used [18, 42].

To use TiO2 in the built-in filter units for air cleaning devices [33], it has to be deposited on a suitable carrier which is usually honeycomb monoliths. There are several methods available to coat TiO2 onto material’s surface (carriers) as a thin film such as dip-coating, spin-coating, spraying, chemical vapor deposition (CVD), and others. A dip-coating sol–gel method and thermal treatment method are commonly recognized as convenient and practical methods for immobilization of the photocatalytic materials to the carriers due to their simplicity, low cost of operation, and tunable output of the final properties of the materials. However, due to the high consumption of the sols, alternatives have also been used, such as brush deposition. Ceramics, glass, metals, and other materials have been employed for the manufacture of carriers. Soda lime glass represents a commonly used TiO2 film carrier even though it contains sodium which has a detrimental effect on photoactivity if deposited film is treated at higher temperatures. The negative effect of sodium could be eliminated by introducing amorphous barrier SiO2 layer between TiO2 film and carrier or using mixed TiO2/SiO2 films [24, 33]. The addition of SiO2 to TiO2 also promotes the well-known synergetic effect of both phases: the formation of Si–O–Ti cross-linking bonds and oxygen vacancies in titania [9, 31]. The addition of TiO2 to porous supports also enhances the photocatalytic activity of TiO2 via increasing its surface area (100–200 m2 g−1 to 400–1000 m2 g−1). The increase of surface area can be achieved by immobilization of TiO2 on porous supports like porous silica [29, 36, 37] or porous carbon [45, 48] and the preparation of such a catalyst in the form of a thin layer using an appropriate carrier. Facet-dependent photocatalytic properties of TiO2-based composites for environmental remediation have been first described by pioneering work of Yang et al. [44] and recently reviewed by Ong et al. [16]. Development of TiO2-based crystals with small particle sizes and a high percentage of exposed {001} facets remarkably improved photocatalytic performances.

Porous Silicates

Porous silicates with pore sizes ranging from a few nanometers up to several micrometers and with specific surface areas up to 1500 m2/g have found numerous commercial applications as adsorbents, molecular sieves, and particularly as shape-selective solid catalysts and catalyst supports [23]. The uniform arrangement of pores in porous silicates, their structural and textural diversity, as well as the possibility to modify their chemical (surface) properties, i.e., by functionalization of the pore surfaces with specific organic ligands, metals, or organometallic complexes, offer a wide variety of applications in molecular separations, metal ion trapping, controlled drug release, low dielectric films, and heterogeneous catalysis. To focus on heterogeneous catalysis, the advantage of using silica (or silicates) due to their hydrophobic surface properties is an important benefit for oxidation reactions. Typically, hydrophobic reactants (which need to be adsorbed) are converted into hydrophilic products (which need to be expelled from the surface). One class of suitable inert supports for TiO2 immobilization is mesoporous silicates. Engineering the pore size of ordered mesoporous silicates is of great importance for their application especially in the preparation of supported nanoparticle catalysts. The pore size of 2D or 3D mesoporous (MCM, SBA) silicates can be tuned from 2 nm to 16 nm (Fig. 3), of foamlike 3D mesoporous silicates (TUD) from 3 nm to 25 nm, of mesocellular silica foams (MCF) up to 50 nm, and of mesostructured wormhole silicates with textural porosity (HMS, MSU, NBS) and interparticle porosity (KIL) even above 50 nm. In addition, to designing the pore system, strategies for selective immobilization of TiO2 nanoparticles inside the channels of mesoporous silicas are crucial. Conventional post-synthesis methodologies such as impregnation, ion exchange, template ion exchange, or direct synthesis methodologies such as hydrothermal crystallization are usually a good choice.
Fig. 3

Porous silicates: microporous (pore size <2 nm) and mesoporous silicates (pore openings 2–50 nm for ordered and above 50 nm for disordered)

Porous silicates (Fig. 3) such as zeolites and mesoporous silicates are superior supports for accommodating photocatalyst nanoparticles because they are chemically inert, possess high surface areas, are transparent to UV radiation, have great physical stability, and have hydrophobic character [14, 19, 28]. Mesoporous silica SBA-15 [50] with highly ordered hexagonal straight pore arrangement, thick pore walls, and high surface area has a lot of advantages if compared to other porous silica supports. It can be prepared over a wide range of pore sizes (5–15 nm) and pore wall thicknesses (3–6 nm) at low temperature (35–100 °C) and possesses excellent adsorption properties. As already mentioned, in addition to the adjustable silica pore system, strategies for selective immobilization of TiO2 nanoparticles inside the channels of the porous support are crucial.

TiO2/SiO2 Composites as Powders and Films

The incorporation of photocatalytic active components (TiO2) in mesoporous silica [28] (e.g., SBA-15, MCM-41) can be achieved by applying different synthesis methods such as wet impregnation, inner-pore hydrolysis/nonhydrolysis, co-hydrolysis and co-condensation, sol–gel processes, and sol–gel/hydrothermal methods. Among them, the simplest and industrial-friendly are conventional post-synthesis methodologies such as grafting, precipitation, or impregnation followed by solvent evaporation [40]. Highly cited studies have been published on TiO2/SBA-15 composites between years 2002 and 2006 with TiO2 loadings up to 80% for decomposition of organic compounds in the liquid phase (photodegradation of organic compounds in water) [1, 27, 39, 43]. Our studies suggest that the best performance is achieved with a TiO2/SiO2 molar ratio equal to 1:1 in the form of powders in gas [37] and liquid phase [22] as well as in the form of films in both phases [11, 33, 34]. Our studies also suggest that the reaction rate in TiO2/SiO2 solid thin films significantly depends on the porosity/surface area together with the interfacial area of the tested compound with titania layer [6, 7, 8, 38].

Design and Development of Highly Efficient TiO2/SiO2 Photocatalysts as Films

The highly efficient low-cost TiO2/SiO2 composite photocatalyst as a film on glass carrier was designed and developed under the simple procedure, low temperature and at ambient pressure. Moreover, the procedure involved no washing steps, hence practically zero waste was generated. The inertness of the material and its immobilization to a solid carrier further increase the sustainability of the synthesis method [33]. Our previous studies for the decomposition of VOCs in the gas phase (photodegradation of VOCs in air) with TiO2/SiO2 powders [22, 36] or TiO2/SiO2 films deposited on the aluminum carriers [32, 35] suggest the best performance of the TiO2/SiO2 composites with TiO2/SiO2 = 1:1 (100% loading). Increasing the titania loading caused an increase in the number of active sites at appropriate places, whereas a further increase of Ti/Si molar ratio led to decrease of the surface area and random dispersion of titania nanoparticles inside the channels of SBA-15 thus narrowing parts of the mesopores of SBA-15. In this respect we developed the low-cost process of immobilization of active TiO2 anatase nanoparticles 5–10 nm in size (colloidal TiO2 from Cinkarna Company denoted as AS) into porous silicate support (SBA-15) with high surface area (855 m2/g) and with the molar ratio TiO2:SiO2 = 1:1 (100% loading). AS/SBA-15 composite with high surface area (460 m2/g) was deposited via brush deposition method in the form of a thin layer on glass carriers.

Commercial pure titania P25 (powder produced by Evonik Degussa, surface area 47 m2/g) is a well-known benchmark material, used for photocatalytic decomposition of organic compounds in water and air, mainly due to its specific anatase to rutile ratio (78% anatase, 14% rutile and 8% amorphous phase), high water dispersion and stability. However, since the size of P25 nanoparticles is around 25 nm, they are too large for their incorporation into the pores of SBA-15 (pore size 9 nm). Hence, the pure P25 catalyst was used as a reference. On the other hand, commercial PC500 (powder produced by Cristal Global, France, crystal size 9 nm) is a pure anatase-phase titania with high surface area (276 m2/g), making it an ideal commercial reference to AS (pure anatase-phase titania produced by Cinkarna Company, Slovenia, surface area 291 m2/g, nanoparticles 5–10 nm in size) which has even higher surface area but similar nanoparticle sizes. Hence, it was used as a second reference photocatalyst.

Synthesis

TiO2/SiO2 composite films made from pure anatase TiO2 (commercial AS – colloidal TiO2 produced by Cinkarna Company, Slovenia) and mesoporous SiO2 (SBA-15) with 100% loading (TiO2:SiO2 molar ratio 1:1) were prepared under simple procedure, which involved wet chemistry and low-temperature immobilization under ambient pressure (Fig. 4).
Fig. 4

(1) The preparation of pure TiO2/SiO2 composite and TiO2 samples (commercial AS, colloidal TiO2 produced by Cinkarna; commercial P25, TiO2 powder produced by Evonik Degussa; commercial PC500, TiO2 powder produced by Cristal Global) as films via wet chemistry, (2) brush deposition of prepared samples, (3) low-temperature treatment of deposited samples

Ordered mesoporous silica SBA-15 powder was synthesized according to the slightly modified well-known procedure [33, 50]. SBA-15 silica was added to pure TiO2 sample AS and the resulting mixture was diluted with 1-propanol. The suspension was afterward sonicated for 10 min at room temperature. The TiO2/SiO2 composite sample was additionally stirred during pre-deposition step at 300 rpm at room temperature for half an hour. Then the sample was sonicated for 10 min at room temperature. It was deposited on glass slides (240 mm × 12 mm × 2 mm) using brush technique. After drying the glass slides with deposited catalysts were heat-treated at 150 °C for 1 h. The procedure was repeated until the final amount of 1.0 mg cm−2 of the catalyst was obtained. This could take from two to five layers. The samples were then ready for characterization and photocatalytic testing. Pure TiO2 samples (AS, P25, PC500) were prepared in the same way but without the addition of silica.

Characterization

Photocatalytic characteristics of prepared materials depend on their structure and photocatalytic activity of TiO2. Prepared synthetic products are continuously characterized in detail with suitable characterization methods, and regarding the results, further synthetic approaches for the achievement of desired properties are planned.

Characterization Techniques

Basic structural characterization of obtained products (in the form of powders or films) is usually performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), elemental analysis (EDAX – energy dispersion analysis by X-ray), scanning electron microscopy (SEM), thermogravimetric analysis (TG/DTG, DSC), N2 physisorption, UV-Vis spectroscopy (UV-Vis), and atomic force microscopy (AFM). XRD provides the identification of the TiO2 structure and some SiO2 mesoporous structures, phase purity, and information about the degree of crystallinity. The size and morphology of the TiO2 crystals on SiO2 supports are evaluated by SEM and TEM. High-temperature powder XRD and TG/DTG yield the information on structural stability of the TiO2/SiO2 composites. With N2 adsorption isotherms, we analyze the porous structure of the composites. The optical properties of the composites in the form of films are investigated by UV-Vis, while their surface morphology by AFM.

Suitable spectroscopic techniques are used for the study of the local environment of titanium: UV-Vis spectroscopy, IR (infrared) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy methods (extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES)). A combined use of XAS and XPS is an excellent characterization tool to elucidate the local environment of a titanium atom. XAS became available with the development of synchrotron radiation sources. Powerful experimental methods of XAS (XANES and EXAFS) for the investigation of atomic and molecular structures of materials enable the identification of the local structure around the selected type of atoms in the sample. The analysis can be applied to crystalline, nanostructural, and amorphous materials, liquids, and molecular gases. In XANES analysis the valence state of the selected type of the atom (e.g., titanium atom) in the sample and its unoccupied orbitals can be deduced from the information hidden in the shape and energy shift of the X-ray absorption edge. In EXAFS analysis the number and species of neighbor atoms, their distance from the selected atom, and the thermal or structural disorder of their positions can be determined. The XPS spectroscopy is a surface-sensitive method with the analytical depth of up to 10 nm. This method is based on irradiation of sample surface with soft monochromatic X-ray beam in ultra-high vacuum and subsequent energy analysis of photoemitted core-level electrons from the surface atoms. The energy distribution of emitted electrons presents the XPS spectrum, from which one can deduce the surface composition, and in addition from the chemical shift of the XPS spectra, one can conclude about chemical bonds, i.e., valence states of selected atoms (e.g., titanium atoms). In combination with Ar ion sputtering, it is possible to obtain information on the depth distribution of elements up to a few hundreds of nm in depth.

Structural Parameters of Prepared TiO2/SiO2 Films Using some Characterization Techniques Described above

XRD patterns of commercial TiO2 materials P25, PC500, and AS show that PC500 and AS consist of pure anatase crystalline phase (not shown). The XRD analysis of P25 indicated the presence of anatase and rutile phase. The final quantitative analysis resulted in the composition of 82% of anatase and 18% of rutile with crystallite sizes of 10 nm and 3 nm, respectively. The higher content of rutile found (18%) if compared to commercial data (14%) is not new as it is well known that fluctuation in P25 crystal phase content is attributable to inhomogeneity of the crystal composition of P25. Structural parameters of prepared TiO2/SiO2 composite and commercial TiO2 samples (AS, P25, PC500) such as specific surface area, pore volume, pore size, particle size, and band gap are shown in Table 1.
Table 1

Structural parameters of prepared photocatalysts using X-ray diffraction (XRD, crystal size), nitrogen physisorption (specific surface area SBET, total pore volume Vtot, pore size), and UV-Vis spectroscopy (band gap)

Sample

SBET (m2/g)a

Vtot (cm3/g)a

Pore size (nm)a

Crystal size (nm)b

Band gap (eV)c

SBA-15

855

1.227

9.2

/

As/SBA-15

460

0.711

6.3d, 9.3, 18.2

8.6

3.16

AS

291

0.368

2.2

11.4

3.02

P25

47

/

2.5

25.8

3.03

PC500

276

0.276

2.1

12.9

3.14

aSBET, Vtot, and pore sizes determined by N2-sorption isotherms

bCrystal size determined by Scherrer equation from the XRD data for the largest anatase (101) peak at 25.1°

cBand gaps determined from UV-Vis diffuse reflectance spectra

dShoulder

TEM image in Fig. 5a shows that AS comprises a single-phase anatase (previously confirmed by XRD) and consists of crystals with their size ranging from 6 to 10 nm. In Fig. 5b AS/SBA-15 composite clearly shows two phases – TiO2 anatase and mesoporous silicate SBA-15.
Fig. 5

High-resolution TEM images of (a) AS titania nanoparticle with visible lattice fringes (0.34 nm in size) and (b) AS/SBA-15 sample with clearly seen TiO2 nanoparticles inside and outside the SBA-15 mesopores

Infrared spectroscopy (IR) of AS/SBA-15 sample showed asymmetric stretching of Si–O–Si bonds, stretching mode of the O–H bond of the surface-adsorbed water and hydroxyl groups, and Ti–O–Ti vibration. The results lead to the conclusion that adopted preparation method did not result in covalent bonds between titania and silica, i.e., TiO2/SiO2 mixed oxide, but rather in van der Waals attraction forces binding the TiO2 nanoparticles into the framework of SBA-15 . These results further support the conclusion that TiO2 was impregnated not only in SBA-15 mesopores, as this would result in higher Ti–Si interaction, but also outside the mesoporous silica as a separate phase, which was also evident in the TEM images (Fig. 5b).

Catalytic Performance

For air treatment , the high surface area of the catalyst is very important in order to enhance adsorption of VOCs on the catalyst surface before their degradation. The synthesized TiO2/SiO2 (AS/SBA-15) composite possesses 460 m2/g. The catalytic activity of synthesized AS/SBA-15 composite was compared with AS (colloidal TiO2) and also different commercially available pure powder TiO2 (P25, PC500), used for benchmarking. Synthesized AS/SBA-15 composite in the form of film was evaluated for the decomposition of formaldehyde (Fig. 6) and toluene (Fig. 7) as model VOCs.
Fig. 6

The highest (91.7%) decomposition of formaldehyde in case of AS/SBA-15: the trend of photocatalytic efficiency for decomposition of formaldehyde was AS/SBA-15 > P25 > PC500 > AS; P25 showed two times faster kinetics than colloidal titania AS, while both exhibited very low adsorption; addition of silica SBA-15 increased adsorption and kinetics of AS (AS/SBA-15)

Fig. 7

Total decomposition of toluene for all samples: colloidal titania AS showed faster kinetics than P25; the adsorption capability and kinetics of the AS/SBA-15 were higher in comparison to the pure TiO2 analogues (AS, P25, PC500)

The AS/SBA-15 photocatalyst, immobilized by brush deposition on glass carriers, showed total decomposition of toluene and 91.7% decomposition of formaldehyde as model VOCs in the gas phase, at room temperature under UVA irradiation in the lab-made batch photoreactor [33]. Other researchers [15, 49] have shown that TiO2 photocatalysts were able to decompose formaldehyde to a certain degree, whereas the 10 wt.% addition of other metal oxides, i.e., CrO2, WO3, MnO3, and ZnO, generally decreased the decomposition efficiency, with the exception of SiO2 where the efficiency was increased up to 94%, which is in accordance with our results. The trend of photocatalytic efficiency for decomposition of formaldehyde was AS/SBA-15 > P25 > PC500 > AS (Fig. 6). The adsorption capability of the AS/SBA-15 was higher in comparison to its pure TiO2 analogues (AS, P25, PC500) in case of toluene (Fig. 7). Turnover frequency (TOF) of the AS/SBA-15 was approximately six times higher in comparison to its pure TiO2 analogue AS for toluene and formaldehyde (Fig. 8).
Fig. 8

Turnover frequency (TOF) for toluene decomposition: AS/SBA-15 the highest TOF, six times higher TOF of AS/SBA-15 if compared to AS (improved adsorption). Decomposition of formaldehyde: P25 the highest TOF, 5.5 times higher TOF of AS/SBA-15 if compared to AS (improved adsorption). TOF calculated following the literature [5, 12]

Our results indicate that the increase in photocatalytic activity of the AS/SBA-15 sample is a combination of quantum size effect, due to the reduced size of the nanocrystals, decreased number of crystal defects, and enhanced adsorption of the pollutants due to the mesoporous nature of the SBA-15 support [33]. Since real indoor air is a mixture of many pollutants, it is important to design a photocatalyst that addresses many of them in the highest decomposition efficiency possible. The two contaminants used in this study (toluene and formaldehyde) are being considered as surrogates for two of the six major classes (aromatic, aldehyde, alkane, ketone, alcohol, and chlorocarbon) of indoor air contaminants and therefore represent a good choice for testing the efficiency of newly synthesized photocatalysts.

Conclusion and Further Outlook

On account of environment-friendly synthesis and its prospective application in sustainable technologies, TiO2/SiO2 photocatalyst presented can be certainly classified as an ecomaterial. The photocatalytic experiments were performed in a photocatalytic batch reactor with model VOC circulation and glass as a carrier. Further work and efforts are therefore to be oriented toward continuous one-pass gas flow reactor with glass fiber filter based on quartz textile as a carrier (to mimic the airflow in the built-in filter in air cleaning devices). The advantage of the quartz fiberglass cloth as a carrier when compared to glass carrier is in the low content of sodium, lightweight, and shape flexibility. The long-term stability of the material is to be studied to evaluate the feasibility and sustainability of its use on a larger scale as a built-in filter in air cleaning devices for public buildings.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Nataša Novak Tušar
    • 1
    • 2
  • Andraž Šuligoj
    • 1
    • 3
  • Urška Lavrenčič Štangar
    • 3
    • 2
    Email author
  1. 1.National Institute of ChemistryLjubljanaSlovenia
  2. 2.University of Nova GoricaNova GoricaSlovenia
  3. 3.Faculty of Chemistry and Chemical TechnologyUniversity of LjubljanaLjubljanaSlovenia

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