PHOTOCATALYTIC ACTIVITY ASSESSMENT OF MICRO-SIZED TIO2 USED AS POWDERS AND AS STARTING MATERIAL FOR PORCELAIN GRES TILES PRODUCTION
Creators: name: GATTO, SARA
Publisher: name: Universit� degli Studi di Milano
Year: 2014

description: General Abstract 1. Introduction Nowadays, it is essential to develop and find new ways to reduce the increasing pollution deriving from anthropogenic and environmental sources. Human activities are major responsible of climate changes and ecosystems alterations, because of the increasing release of CO2 and other harmful gases inside the atmosphere. In order to reduce the environmental impact of the human society, a great attention is now given to such processes able to reduce the pollutants concentration in both air and water systems. Advanced oxidation processes (AOPs), which involves the generation of highly reactive hydroxyl radicals (OH?), have emerged as promising air and water treatment technology for the degradation or mineralization of a wide range of pollutants. Titanium dioxide (TiO2) induced photocatalysis is an example of AOP processes and it has been demonstrated its efficiency in the decomposition of various organic contaminants. TiO2 is a very well known and well-researched material due to the stability of its chemical structure, biocompatibility, physical, optical, and electrical properties. TiO2-based photocatalysts are used for a variety of applications such as degradation of volatile organic compounds (VOCs) [1] and decomposition of nitrogen pollutants (NOx) [2] or also organic dyes, like Methylene Blue [3]. The crystalline forms of TiO2 are anatase, rutile and brookite. In general, TiO2 is preferred in anatase form because of its high photocatalytic activity, non-toxicity, chemically stability; moreover, it is relatively inexpensive. For a long time, new synthetic routes have been developed to prepare nano-TiO2 samples in order to enhance their photocatalytic efficiency [4-6]. In fact, since many years the attention has been focused on ultrasmall semiconductive particles, because they show peculiar and enhanced properties compared to the micrometric particles ones [7]. Nano-sized TiO2 is extremely efficient towards the photodegradation processes; in particular, photo-redox reactions are greatly enhanced thanks to the high numbers of active sites present on the extremely large surface area [8]. However, in recent years many papers published the possible health risks correlated with nano-sized materials [9,10]. The small size, shape, solubility and agglomeration degree of nano-sized materials, make them able to cross the cell boundaries or pass directly from the lungs into the blood stream and finally reach all the organs in the body [11]. On the other hand, larger particles are adsorbed by organs and cells with more difficulty. The main question is then if it is necessary to use the nano-sized particles in an exclusive way. Kwon et al. [12] stated that nanocatalysts having small particle size, high surface area, and a high density of surface coordination unsaturated sites offer improved catalytic performance over microscale catalysts but this does not imply the impossibility a priori to use these latter in selected conditions. The use in photocatalysis of TiO2 powders with larger-sized crystallites is a very interesting approach to reduce the possible health problems caused by nanoparticles. 2. Aims of work The aims of this PhD work is to evaluate the photoactivity of micro-TiO2 samples using as irradiation source both UV and LED lights. At first, commercial powdered micro- and nano-sized TiO2 catalysts, were tested and then improved for the degradation of pollutants in both gas and aqueous phase. The ultimate purpose of the PhD work is to test the possibility of using TiO2 for production of building materials; the photocatalytic activity of TiO2 can be then exploited for degrading air pollutants inside domestic environments or workplaces, thus making them healthier over time. Application of photocatalysis to construction buildings began towards the end of 1980s with the production of photocatalytic glasses, which provided self-cleaning and anti-fogging properties [13]. Afterward photocatalytic cementitious materials have been patented by Mitsubishi Corp. and Italcementi SpA [14,15]. In all these construction materials, the active photocatalyst is anatase TiO2. Although the use of photocatalytic cement is still restricted and limited, many buildings and city roads have been designed and constructed since 2000. Relevant examples are Church ?Dives in Misericordia?, Rome, Italy; Music and Arts City Hall, Chamber�ry, France [16]. In general, the mostly used powders of commercial TiO2 for photocatalytic applications are nanometric: this leads some advantages in terms of pollutants degradation efficiency, but many backwards too, like the difficulty to recover the catalyst or the possibility of inhalation with consequent health damage, even the high cost is not negligible. For this reasons, the optimization of the photocatalytic efficiency of micrometric compounds is desired, in order to replace definitely the nanometric catalysts. In this PhD work micro-sized TiO2 powder was used for the preparation of porcelain gres tiles, which are commercial manufactured products, opening a new generation of material intrinsically safer than the traditional photocatalytic products. All samples were fully characterized investigating textural, structural, morphological and surface properties. The photoefficiency was evaluated in different ways, which can be summarized as follows: ? Assessment of the photoactivity of commercial samples, both nanometric and micrometric, in gas and aqueous phases in the presence of typical indoor and outdoor pollutants (NOx and Volatile Organic Compounds (VOCs), textile dyes, surfactants); ? Assessment of the self-cleaning effect, evaluated by water contact angle measurements, during ultraviolet irradiation on micro-TiO2 tiles of building materials on whose surface the oleic acid is deposited (ISO/WD 27448-1); ? Assessment of the effects of the addition of anionic or cationic ions, like fluorine, tin, rhenium or tungsten, on the catalytic surface through the impregnation method. Doping is useful to lower the titanium band gap and accordingly to increase the photocatalytic activity of the material. 3. Experimental details 3.1 Catalytic materials a) Preparation of TiO2 powders Different commercially available micro- and nano-sized pigmentary-powdered TiO2 were chosen; the catalysts were characterized and used without further treatment. In the Table 3.1 the photocatalytic powders used in this PhD work are reported. For each powder, the different physico-chemical characteristics are specified: XRD for the crystalline nature, BET for the surface area, XPS for the atomic composition of elements, SEM and TEM for the particles morphology, FTIR for the chemical composition of samples supported with DRS (diffuse reflectance spectra) for the characterization of the light absorption features and band-gap determinations. Before starting the photooxidation process of pollutants, commercial TiO2 powders were deposited in two plains of glass sample (each plain of 7.5x2.5 cm2). TiO2 powders (0.050 g) were first suspended in 2-propanol (50 ml) so to obtain a homogeneous suspension and then deposited by drop casting onto one side of the laminas. The solvent was simply evaporated at room temperature without any further treatment. The samples consisted in a thicker layer, obtained by overlapping three TiO2 coatings (labelled as T, standing for triple layers, followed by the substrate abbreviation), as shown in previous works by Bianchi et al. [17,18]. Table 3.1. Main features of TiO2-based commercial powders, used as photocatalysts, with the corresponding crystalline phase: nanometric and micrometric samples. Powder Crystalline phase BET (m2/g) Micro/Nano XPS OH/Otot P25 (Evonik) 75% anatase; 25% rutile 52 NANO 0.14 PC105 (Crystal) anatase 80 NANO 0.85 1077 (Kronos) anatase 11 MICRO 0.32 AH-R (Hundsman) anatase 12 MICRO 0.19 AT-1 (Crystal) anatase 12 MICRO 0.24 1001 (Kronos) anatase 11 MIXED PHASE (micro+nano) 0.27 1002 (Kronos) anatase 9 MIXED PHASE (micro+nano) 0.35 1071 (Kronos) anatase 10 MIXED PHASE (micro+nano) 0.18 A-Z (Hombitam) 99% anatase 4 MICRO 0.25 AN (Hombitam) 98,5% anatase 12 MICRO 0.5 N.10 (HombiKat) 98% anatase; 2% rutile 13 MICRO 0.13 b) Preparation of vitrified tiles Among all building materials, commercially available white tiles by GranitiFiandre SpA (sample name White Ground Active� (WGA) or Orosei Active) were chosen and used for the preparation of photocatalytic tiles. Porcelain gres tiles are manufactured under high pressure by dry-pressing of fine processed ceramic raw materials, with large proportions of quartz, feldspar, and other fluxes. The body of these materials is then fired at very high temperatures (1200?1300?C) in kilns [19]. After impregnation with water, the tiles are subjected to temperature cycles between +5 and -5 �C, during a minimum of 100 freeze?thaw cycles, in order to verify their resistance to the frost and their durability. No evident cracks or damages were observed on the samples. The final material is thus characterized by lack of porosity, complete water-proofing, durability, hardness, wear resistance properties, and a complete frost resistance. The porcelain gres tiles were covered at the surface with a mixture of micro-TiO2 and a commercial SiO2-based compound prepared via ball?mill [20,21]. To achieve the desired product stability, at the end of the preparation procedure tiles were treated at high temperature (680 �C) for 80 min and then brushed to remove the powder present at the surface and not completely stuck. Temperature was precisely chosen to maintain the anatase form of the semiconductor and allow the vitrification of the tiles surface. Tiles were also prepared with the same procedure but without adding the photoactive oxide into the SiO2-based compound for the sake of comparison (sample name White Ground (WG) or Orosei)). The surface wettability of photoactive porcelain gres tiles was evaluated by static contact angle (CA) measurements performed with an OCA20 instrument (DataPhysics Co., Germany) equipped with a CCD camera and a 500 ?L-Hamilton syringe to dispense liquid droplets. [22,23]. c) Doping effect on TiO2 powders Micrometric TiO2 powders were doped with cations like tungsten (W), tin (Sn) and rhenium (Re), and fluoride anions (F-). This was done with the aim to improve the photoefficiency of the micro-sized TiO2 catalysts, which have lower activity than the traditional nanopowders. Ren at al. [24] demonstrated that the fluorination of TiO2 nanocrystals gave a photocatalytic enhancement due to the higher separation efficiency of photogenerated electrons and holes. Furthermore, it has been found that the surface fluorination favors the generation of free OH radicals, which are responsible of an enhanced oxidation [25]. Regarding the doping with metal cations, in the literature is reported that Re dopant could effectively inhibit the recombination of the photoinduced electrons and holes [26]. Re can act as electron trap and promote the interfacial charge transfer processes in the composite systems, which reduces the recombination of photoinduced electron-hole pairs, thus improving the photocatalytic activity of TiO2. Moreover, it was demonstrated that that metal particles doping can facilitate the electron excitation by creating a local electrical field, enhancing photoinduced surface redox reactions: it results in the extension of the wavelength of TiO2 response towards the visible region [27]. The band gap energy of the doped-TiO2 results less than that of naked TiO2, which induces the red shift of the adsorption edge to respond to visible light. This peculiar feature gets interesting for the use of LED (Light Emission Diode) as irradiation source for the photooxidation processes, because LED emissions are located only in the visible region of light. In fact, an important aspect is the use of irradiation by visible light, through LED lamps. Several cities, like Milano, Stockholm, Los Angeles, Copenhagen, have chosen to adopt the LED emission for the outdoor illumination: Milano will substitute the 80% of urban illumination with the LED light within May 2015 (Expo start date). Advantages, connected to this emerging technology (high durability, cheapness, low energy consume), adhere very well with the environmental safety. Thus, NOx and VOCs photodegradation was performed with LED lamp, using micrometric doped powder. The classical impregnation method was applied to dope the catalyst surface with fluoride anions, starting from inorganic fluoride salts (NaF, NH4F, CaF2 and F2). At the end of the impregnation procedure (24 h, room temperature), powders were calcined at 400�C for 4 h and rinsed in distilled water three times. The metal doping was performed in two different ways: it was used the same procedure of impregnation method for tin (Sn) surface doping, whereas a different surface deposition technique (decoration method) was performed for metals of tungsten (W) and rhenium (Re). Decoration of M- or MO-NPs is commonly implemented by means of ultra-sounds (US) in aqueous or organic solutions where ceramics or polymer substrate powders are dispersed [28]. In the latter case, the precursor of metal was sonicated at a costant temperature of 80�C for 3 h, with 33.0% amplitude and a 50 W cm-2 intensity. At the end, the solution was centrifugated many times to remove all the solvent; the final powders was washed with n-pentane and centrifugated again. The residual solvent was evaporated and the sample was finally calcined at 480�C for 40 h to completely remove the organic scents. 3.2 Testing procedure a) Photocatalytic set-up in gas-phase Photocatalyitc degradation of air pollutants, such as acetone, acetaldehyde, toluene (well known as VOCs) and NOx, were conducted in Pyrex glass cylindrical reactors having different volume depending on the type of analyzed pollutant: 5 L for VOCs and 20 L for NOx, respectively. In the case of VOCs analysis, the gaseous mixture in the reactor was obtained by mixing hot chromatographic air (f.i. 250 ?C for toluene), with relative humidity (RH) of 40%, and a fixed amount of volatilized pollutant, in order to avoid condensation. The initial concentration of VOCs in the reactor was 400 ppmv, monitored directly by micro-GC sampling. Photon sources were provided by a 500 W iron halogenide lamp (Jelosil, model HG 500) emitting in the 315?400 nm wavelength range (UV-A) at 30 Wm?2 or by a LED lamp, emitting into the visible region. Acetone and acetaldehyde degradation tests lasted for 2 h, whereas toluene tests for 6 h, due to the difficulty in degrading a molecule with an aromatic ring and with a complex degradation pathway [19]. For NOx photodegradation study, a first static experimental setup was obtained used the following conditions: RH: 50%, UV light of 10 Wm-2 (for TiO2 powders deposited on glass sheets) or 20 Wm-2 (for micro-sized TiO2 gres tiles), with a NOx starting value of 1000 ppb. The analytical procedure was reported by Bianchi et al. [21]. NOx degradation by TiO2 powders (always immobilized on a glass sheet) and photoactive tiles was conducted also in continuous conditions using a plug-flow reactor (with an effective volume of 0.025 L) built strictly following the ISO 22197-1 rule [29]. Experimental conditions were maintained as follows: RH: 40%, 20Wm?2, [NOx]inlet=500 ppb, and 180, 32.4, 9, and 4.2 L h?1 total flow, respectively. A chemiluminescent analyzer (Teledyne Instruments M200E) was used to check the conversion of the pollutant in both batch and plug-flow reactor setups. b) Photocatalytic set-up in aqueous-phase The photocatalytic apparatus was a 1 L glass stirred reactor equipped with an iron halogenide UV lamp (500 W, Jelosil� HG500) emitting light at wavelengths of 315?400 nm and able to irradiate the reactor with a specific power of 95 Wm-2, when TiO2 powder was used as catalyst. The UV lamp was placed beside the reactor, which was cooled with water at a temperature of 30 � 0.5?C, as reported previously by Gatto et al. [30]. TiO2 was introduced in the reactor at the beginning of each test (0.66 g/L for surfactant degradation and 0.1 g/L for textile dyes). The variation of the surfactant (PFOA) concentration in solution was monitored by total organic carbon (TOC) analysis and ionic chromatography. The PFOA initial concentration ([PFOA]0= 4 mM) was maintained lower than its critical micellar concentration (7.8 mM) in order to avoid the formation of emulsions during the kinetic tests. Samples (10 mL) of the reaction mixture were collected at different reaction times: typically at 0 min (before the start of the reaction), 30 min, 1 h, 2 h, 3 h, 4 h, 6 h and 9 h. Textile dyes, chosen for the photodegradation tests, were Rhodamine B (RhB), Methylene Blue (MB) and Crystal Violet (CV); dyes degradation was checked every 60 min by determining the dye concentration in the water solution by a UV?vis spectrophotometer analyzer (T60 UV?vis PG LTD instruments), using water as the reference. Pure CV has an absorbance maximum at 590 nm, RhB at 555 nm and MB around 670 nm. Textile dyes degradation was also performed using photoactive tiles, covered with the micrometric 1077 powder. For this aim, a cylindrical batch reactor of 1 L volume was used for dye degradation tests in presence of ten photoactive tiles (0.03 m2 total surface photoactive area) immersed into the liquid solution, as reported by Bianchi et al. [31]. Refrigeration was allowed by a cooling jacket. Two different lamps directly immersed into the dye solution were used with this setup: a typical germicidal 9 W UV-C lamp (Philips TUV BL-S, model AEPL-7913 mercury vapor low pressure), with a radiant power of 1 Wm-2 and a 125 W UV-A lamp (Jelosil, mercury vapor low pressure), with an illuminance of 65 Wm-2, in correspondence of the tiles surface. During photocatalytic tests, the TiO2 active faces of the tiles were turned towards the UV light. After each test, the tiles were simply washed using deionized water and acetone and then left in deionized water all night long. The same dyes solution (RhB, MB, CV) were used in the present setup at a concentration of 1 � 10?5 M. c) Self-cleaning effect The self-cleaning capability of TiO2 photoactive tiles was evaluated in two different ways: (1) through the measurement of the water contact angle (CA) (KRUSS GmbH) of a tile, after oleic acid deposition and UV irradiation (Jelosil, model HG 500) for 76 h and (2) through the monitoring, by a colorimeter, of the discoloration of dyes directly put on the tiles surfaces, after exposure to the sunlight (Milan ? Italy, May 2012). For water CA measurements, a test piece of porcelain gres tile of 100 � 2mm2 were pre-treated by ultraviolet irradiation of 20 Wm-2 for at least 24 hours. Then, the catalytic samples were dipped inside an oleic acid (Fluka, &gt;80%) solution (0.5 vol%) in order to simulate a polluting condition. The presence of oleic acids on the tile surface modify its wettability. After UV irradiation it was measured the CA at an appropriate time interval, observing a continuous decrease of the CA values related to a degradation of the polluting agent. The measurement can be considered concluded when the contact angle value of the clean photocatalytic tile is restored, as before the oleic acid deposition. For comparison, the measurement is repeated on a sample similarly polluted with oleic acid, but left in the dark for 76 hours. Furthermore, it was taken a sample of porcelain gres tile, not containing TiO2, and it was immersed into oleic acid solution and irradiated, with the aim to evaluate the pure contribute of UV irradiation. Dyes degradation instead was monitored by Vis-spectrometer equipped with an integrated sphere (OceanOptics, USB400-VIS-NIR-ES). 1 ?L of dyes, dissolved in water, was put on the tiles surface and left under the sunlight, whose power was continuously checked from 9 am to 5 pm every day by a radiometer DeltaOhm HD2012,2. A mean power irradiation value of 7.28 W/m2 was measured. The color analysis was performed using the CIEXYZ and CIELAB models [22]. 4. Results and discussion 4.1 Characterization results a) Powders characterization Anatase, evidenced by XRD patterns, is the unique polymorph present for all samples, except for P25 and N.10 (by Hombikat) powders, which exhibit even the rutile phase (25 and 2%, respectively). The crystallographic reflexes (1 0 1), (2 0 0) and (2 1 1) have been employed to calculate the average crystallites size of the various titania particles. P25 and PC105, commercial nanometric powders, have comparable crystallite size centered on 25 nm, while the other samples have values between 120 and 200 nm, confirming their micro-sized nature. These structural properties are reflected in their BET surface areas that are about 11-12 m2/g, which are much lower compared to the nano-sized ones (Table 3.1). For 1001, 1002, 1071 samples Sherrer calculation was not performed, as TEM analysis reveals the presence of both micro-sized and ultrafine fractions, as it is visible in Fig. 4.1, section d. HR-TEM and SEM images confirmed the average crystallites sizes extrapolated by XRD analysis; moreover, it was excluded the presence of ultrafine particles in 1007, AT-1, AH-R, A-Z, AN and N.10 powders. It can be evidenced that nano-sized materials perfectly fall within the ?nano? definition: in fact, both samples are characterized by average particles size of 15-30 nm (Fig. 4.1, section a), closely packed features and roundish contours [19]. As for what concerns the other powders (1077, AT-1, AH-R, A-Z, AN, N.10), they all exhibit well crystallized particles possessing smooth edge and average diameter size in the 120-200 nm range (see Fig. 4.1, section b and c), with fringes patterns belonging to the TiO2 anatase polymorph. On the contrary, for 1001, 1002 and 1071 powders TEM images again confirm that they are composed by a mixture of both micro-sized crystallites and some ultrafine particles (Fig. 4.1, section d). The surface state of the TiO2 particles was analyzed by XPS. No significant differences can be appreciated in the Ti 2p region among all the present samples concerning the binding energies (BE) and the full width at half-maximum (FWHM) values. The peak of Ti 2p3/2 is always regular and the BE at about 458.5 � 0.1 eV compares well with the data for Ti(IV) in TiO2 materials [32]. The analysis of the oxygen peaks exhibits the presence of more than one component, which can be attributed to lattice oxygen in TiO2 (529.9 eV) and to surface OH species (&gt;531.5 eV) respectively. A particular O1s shape was observed for PC105. In this case, the OH component is very intense probably due to a particular industrial synthesis in order to enhance the photocatalytic efficiency of the sample. The hydrophilicity/hydrophobicity character of photocatalysts surface plays a crucial role in determining the adsorption step and thus the photocatalytic activity, at least in the degradation of pollutants [33]. P105 exhibits the highest concentration of OH that represent the 85% of the oxygen at the surface, as it shown in Fig. 4.2. It is noteworthy that the micro-sized samples, with the exception of N.10 (by HombiKat) sample, present a higher atomic concentration of OH groups in comparison with P25, pointing out the higher hydrophilic character of their surface (see Table 3.1, fifth column). Fig. 4.1. TEM images of the various TiO2 powders. Section a: P25; section b: 1077; section c: AH-R; section d: 1071. FTIR spectra in the ?(OH) spectral range of the samples in air revealed two complex absorption bands, respectively located in the 3000?3450 cm-1 range and at ? ? 3600 cm-1. Based on the spectral behavior and of our previous data [19], the former envelope can be ascribed to the stretching mode of all H-bonded OH groups present at the surface of the various solids, whereas the latter corresponds to the stretching mode of all Ti?OH species free from hydrogen bonding interactions [34]. It is well-known that surface hydroxyl radicals play a fundamental role in the photocatalytic processes [35]. In particular, photo-generated holes react with water molecules adsorbed on TiO2 surface, leading to the formation of OH?: TiO2 + h? ? h+ + e- (3.1) h+ + H2O ? OH? + H+ (3.2) The pigmentary TiO2 powders showed appreciable amounts of OH groups and this validate their rather good performances in the photocatalyitc degradation, as reported in our previous study [19]. Fig. 4.2. O1s XPS spectra for (a) P25; (b) PC105; (c) 1077; (d) AT-1. b) Gres tiles characterization XPS measurement reveals the presence of only Ti(IV) and a Ti/Si ratio of 0.15 for the micro-TiO2+SiO2-based compound, which belongs to porcelain gr�s tiles. The preservation of the pure anatase form was verified by both XRPD and XPS measurements. As reported by Anderson and Bard [37] the presence of SiO2, together with TiO2, enhances the formation of hydroxyl radical OH?, which may be achieved via strong Br�nsted acid sites at the TiO2/SiO2 interface region. Such incorporation inhibits the crystal growth of TiO2 allowing the preservation of the anatase structure at high temperature. By the investigation of morphological features, the presence of SiO2-based compound is evident in gres tiles (Fig. 4.3), in the form of either small protruding particles or as amorphous coating which covers the TiO2 particles. Fig. 4.3. HR-TEM images of the TiO2 porcelain gres tiles materials. (a) refers to low magnification and (b) to high magnification. The very thin nature of these particles and/or coating allows to inspect the fringe patterns located below, confirming that the spacing among the fringes are still ascribable to the anatase TiO2 polymorph. 4.2 Photocatalytic tests 4.2.1 Photocatalytic activity in gas-phase a) NOx photoabatement with TiO2 powders In this section, several commercial pigmentary powders were tested for NOx degradation and were compared with the nanometric powders efficiency (P25 and PC105). At first, the tested concentration of NOx in the reactor was 1000 ppb, in order to follow the same pollutant concentration requested by the ISO 22197-1 rules [38]. All the samples showed good photocatalytic performances, because the abatement of NOx was early completed at the end of 3 hours, except the 1071 (by Kronos) sample, which showed lower photodegradation (61.5 %). The efficiency of the other samples was between 90 and 99%: this behavior leads to hypothesize a complete degradation of the pollutant within the chosen limited time of the run (3 h). In particular, it is interesting to observe the photodegradation trend of the only micro-sized samples (1077, AH-R, Hombitam A-Z, Hombitam AN and HombiKat N.10) at 15 min, 30 min, 60 min and 240 min, the most significantly times. In Fig. 4.4 we can observe the peculiar differences, which arise in the initial period of the degradation. 1077, Hombitam AZ and Hombitam AN seem to be the most active, showing the best efficiency in the first times of reaction (15, 30 min). This behavior can be explained through the physico-chemical features and the amount of hydroxyl radicals that initiate the oxidation of NO. The ratio of OH/Otot, obtained by XPS analysis, resulted to be, in fact, higher than the other micrometric ones (Table 3.1). In particular, after 2 h, the NOx conversion of these samples is higher than 90%, very close to that of P25, which reaches the complete pollutant degradation in the same time. Thus, even if the nano-sized materials (P25 and PC105) show the best performances, the photocatalytic activities of the pigmentary powders are comparable, in agreement with the presence of appreciable amount of surface hydroxyls, which are crucial species for the photooxidation processes [39]. From the trend in the Fig. 4.4 it is clear that the micrometric samples with the best photocatalytic performances are the ones showing the largest OH component, the following 1007, Hombitam AZ and Hombitam AN. Fig. 4.4. TiO2 commercial micro-sized powders (1077, AH-R, Hombitam AZ, Hombitam AN, HombiKat N.10) for NOx abatement at 15, 30, 60, 240 min under UV light irradiation. b) NOx photoabatement with photoactive tiles Another study concerns the application in photocatalysis of building materials. In this PhD work porcelain gres tiles, covered with micrometric TiO2 powder, were used for the NOx degradation, under UV light, in static experimental conditions in gas phase. Starting from 1000 ppb of NO2, i.e. the same amount required by the ISO 22197-1 specification, the 65% of degradation was measured after 6 h. A very interesting trend (Fig. 4.5) was observed also following the NO2 degradation by photocatalytic tiles. NO2 was chosen as specific reference pollutant instead of the more generic NOx, because of its higher hazardousness. The continued exposure to high NO2 levels, in fact, can contribute to the development of acute or chronic bronchitis [40]. More in detail, tests were carried out by using as starting pollutant concentration 106 ppb (value not to be exceeded more than 18 times in a calendar year), and 212 ppb (alert threshold), according to the Directive 2008/50/EC of the European Parliament, which states the guidelines for the protection of the human health. It is possible to observe (Fig. 4.5) that, as the amount of starting pollutant is decreased, the time necessary to bring its concentration under the limit required by the European Directive (21 ppb) also decreases. In the Fig. 4.5 inset the degradation trend can be observed in the case of an initial pollutant concentration close to the alert threshold. Fig. 4.5. Time necessary to degrade the pollutant and decrease its amount under the limit value required by the Directive 2008/50/EC of the European Parliament and of the council on ambient air quality and cleaner air for Europe (21 ppb); 20 W/m2, RH 50%, static conditions. Therefore under real pollution conditions, simulating a day in the absence of wind (static conditions) WGA is able to degrade NO2 in a very efficient way bringing the pollutant concentration down to the required limit (21 ppb) in a matter of hours [21]. Micro-sized TiO2 porcelain gres tiles were also tested in continuous conditions using a plug-flow reactor, whose the operating conditions have been softened cutting the inlet concentration by half (500 ppb, instead of 1000 ppb). It was investigated the role of the flow per hour on the final NO2 conversion. An interesting aspect revealed: the modification of the flow per hour leads to an evident change of the contact times that is the time the pollutant can stay ?in contact? with the catalyst surface. As expected, increasing the contact time, the final conversion proportionally increases. This result is very evident for Orosei Active sample that shows a conversion varying from 1.3% to 82.0% at 180 L h?1 and 4.2 L h?1, respectively. The obtained 82% conversion at 4.2 L h?1 flow can be consequently considered a very good value. c) VOCs photoabatement with TiO2 powders In order to study the photocatalytic activity of nano- and micro-sized samples, the degradation of three different VOCs, acetone, acetaldehyde and toluene, has been performed. As an illustrative example, it was reported the toluene photodegradation tests. For both nano-and micro-sized TiO2 powders, the pollutant was not completely degraded, even after 6 h of reaction. Moreover, it is noteworthy that the degradation percentages fell more or less in the same range (46?52%) with a slightly higher value for the nanometric P25 and PC105 catalysts, as it is shown in Fig. 4.6. Toluene degradation resulted very difficult due to the complexity of molecule, which presents the aromatic ring. The different catalysts show similar behavior toward the toluene degradation, irrespective of their physico-chemical characteristics. On the contrary, the pollutant mineralization is rather different for almost all samples. Furthermore, a low amount of CO2 formation confirmed the incompleteness of the degradation reaction. The possible by-products, which could take form during the degradation, were monitored by FTIR measurements. After the employment in toluene degradation, the spectra of the materials underwent deep changes. In particular, it was possible to recognize signals of unreacted toluene (T) and of several by-products deriving from its degradation, among which benzyl alcohol (BZOH), benzoic acid (BZAc) and benzaldehyde (BZH) [19]. In addition, the signals due to the stretching mode (?OH) of Ti-OH species free from hydrogen bonding interactions were disappeared with the parallel increase of the broad envelope generated by H-bonded OH groups [31]. Thus, it was possible to state that the catalysts surface underwent irreversible changes after the employment in the photodegradation reaction of toluene: the photo-active ?free? Ti-OH sites were completely absent, as a result of their participation to the reaction. Fig. 4.6. Toluene degradation histogram: photoefficiency achieved with commercial micro-sized TiO2 and compared to the P25 and PC105 ones (nanometric). Their disappearance was a clear evidence of why toluene degradation appeared incomplete even after 6 h of reaction for all the samples, regardless of the morphological features of the materials. Therefore, in the case of toluene and in general for all less hydrophilic VOCs, it was well evident that both micro-sized materials and nano-sized ones possess almost the same photocatalytic behavior. 4.2.2 Photocatalytic activity in aqueous-phase Parallel with photocatalytic tests in gas-phase, photodegradation of surfactants and textile dyes in aqueous phase were performed. In particular, the PFOA (perfluooroctanoic acid) was chosen as surfactant species. The abatement was conducted by using P25 nano-powder as catalyst. The photodegradation trend, monitored at different times, highlighted the incomplete PFOA mineralization. For the entire duration of the photo-abatement process, it was possible to observe a decrease in the PFOA content in solution. However, the mineralization after 4 h settled down: the fluoride content and the percentage mineralization after 6 and 9 h remained equal to 29% and 32%, respectively, as reported by Gatto et al. [29]. Through HPLC-MS analysis was confirmed the presence of the intermediates in the solution that took form through two possible degradation pathways: this surface modification might influence the catalyst reducing the photocatalytic efficiency of TiO2. Nevertheless, it is important to note that, as reported in the literature, no PFOA abatement was observed working in the presence of TiO2 as photocatalyst without UV irradiation as well as under UV irradiation in the absence of photocatalyst (photolysis) [31]. The other interesting study concerns the textile dyes photodegradation, using micro-sized TiO2 (1077) powders as catalysts. The textile dyes analyzed were Methylene Blue (MhB), Rhodamine B (RhB) and Crystal Violet (CV). Experimental dark tests showed a very low adsorption of all the dyes on both kinds of powders. The contribute of photolysis was almost negligible for MhB and CV, whereas 12% of dye degradation for simple photolysis (10% for P25) was achieved for RhB. Nano-sized powder showed the best results for all the considered dyes achieving the complete decolorizing of the water solution, but also micro-sized sample was able to degrade the pollutants with a good efficiency (ranging from 48 to 58% depending on the dye in six hours) (see Fig. 4.7), as reported by Bianchi et al. [30]. In addition, the micro-sized powder can be easily filtered and recovered in order to be immediately reused for further photodegradation reactions. In fact, 1077 was recovered by the simple centrifugation and reused in the same dye degradation test with no loss of photoactivity [30]. Fig. 4.7. Photocatalysis of dyes performed with powdered micro-TiO2 catalyst (1077): crystal violet ?; methylene blue ?; rhodamine B ?. Another application is relative to the photocatalytic efficiency of TiO2 porcelain gres tiles, evaluated through UV-vis measurements. This choice reflects the fact that photoactive porcelain gres tiles are covered with the micrometric 1077 powder. It was observed an increase of about 15% of dyes degradation in comparison to the simply photolysis. These porcelain gres tiles can be reused, just after insertion of the tiles in distilled water, and without affecting the photocatalytic activity. In fact, all the tests were done using the same batch of ten samples of industrial tiles, and no loss in their photoactivity was monitored. This indicates that the TiO2 deposited layers are not deactivated during the reaction either by loss or poisoning of the catalyst, and can be reutilized in subsequent runs. Thus, these new industrial ceramic materials are surely an interesting alternative to TiO2 suspensions in photocatalytic applications avoiding the removal of the particles at the end of the process. 4.2.3 Self-cleaning effect A different aspect for the evaluation of gres tiles photo-efficiency is the CA evaluation, measured on micro-sized TiO2 porcelain gres tiles, after the deposition of oleic acid and irradiation by UV lamp. At first, before the oleic acid (Fluka, &gt;80%) deposition, the pretreatment CA measurements were performed obtaining value of about 31�. The, the catalytic samples were dipped inside the oleic acid solution (0.5 vol%); the presence of oleic acids on the tile surface modify its wettability, the water contact angle in fact increases to about 65�. After UV irradiation it was measured the CA at an appropriate time interval, observing a continuous decrease of the CA values related to a degradation of the polluting agent. We observed that after 76 h of irradiation, the water CA reached the starting value before the oleic acid deposition (about 30�). This highlights the self-cleaning properties of TiO2 porcelain gres tile [22] and its photocatalytic efficiency for the degradation of organic contaminant deposited on the surface. On the contrary, the same kind of porcelain gres tile (Orosei Active), treated with oleic acid, but maintained in the dark, does not show modifications of CA in the range t0 and t76. The same procedure, consisting in the deposition of oleic acid solution and irradiation under UV light for 76 h, was performed for a porcelain gres tiles, not containing TiO2. Even in this case the CA measurement during the UV irradiation remained the same, i.e., the initial CA measured on the oleic acid film (65�). It is justified that the change in the value of the contact angle is due merely to the photodegradation of the oleic acid due to both the action of UV radiation and the photocatalytic efficiency of the used material and not by spontaneous degradation of oleic acid, induced by non photocatalytic factors. Thus, the photocatalytic process is necessary for the abatement of organic pollutants [17]. 4.3 Doping effect on TiO2 powders Micro-sized 1077 powder was even doped by the impregnation method. First of all the fluorination effect was investigated, making a comparison with the corresponding nanometric P25 powder: in both powders, after the fluorination, the photocatalytic activity of NOx and VOCs abatement resulted increased. The simply surface fluorination seems to be a good method to increase the photoactivity in commercial TiO2 samples, even with large crystallites [41]. In particular, the morphological features evidenced in the HR-TEM images and FT-IR spectral patterns, showed significant features. When the fluorination was carried out on the 1077 sample, there was an increasing of the OH groups interacting by H-bonding in F2 fluorination and new families of free OH groups involving Ca2+ and Na+ ions. The simple surface fluorination by fluorination resulted as an easy and good method to increase the photoactivity in commercial TiO2 samples, even with large crystallites, as reported in Fig. 4.8. Fig. 4.8. Toluene degradation for both micro- (1077) and nano-sized (P25) TiO2 samples, naked and fluorinated (NaF precursor). Physico-chemical characterization demonstrated that the surface fluorination influenced all the surface OH groups, leaving free only some particular OH ?families?, reasonably the more active in the photocatalytic process. Thus, the driving force of the process is both the presence of active OH population and the efficient adsorption of the pollutant molecules on the photocatalytic semiconductor surface. Parallel with this, the metal surface deposition with Sn, W and Re lead to an improved photoefficiency. In this case, micro-sized TiO2 powders exhibited a higher photoactivity compared with the naked TiO2 one. In particular, an interesting aspect was even the evaluation of photo-efficiency of doped 1077 using the LED light as irradiation source for the pollutant degradation. It has been observed that the photo-abatement efficiency of micro-sized catalysts for VOCs is improved by the presence of metals particles, in particular in the case of rhenium and tungsten. The degradation percentage of acetone was in fact, 37% for 1077_W and 33% for 1077_Re, compared with the 1077, which showed a negligible photoactivity (~2%), when the catalysts were irradiated by visible light. In Fig. 4.9 it is possible to see the improved photo-efficiency. In fact, the metal species like W and Re have the main properties of promote the charge transfer and the visible light absorption, which lead to enhanced photocatalytic degradation of pollutants than naked micro-sized TiO2, even under visible light irradiation [42]. Fig. 4.9. Acetone photodegradation in gas-phase under visible light (performed with a LED lamp). 5. Conclusions The photocatalytic activity of both nanometric and micrometric TiO2 powders was evaluated, revealing that nano-sized powders have the best photo-efficiency. However, commercial pigmentary micro-sized TiO2 powders have given good results proving that they could be good materials in photocatalysis and good alternative to nano-sized catalysts. In particular, 1077, Hombitam AZ and AN are the micro-sized TiO2 powders with the highest photoactivity for NOx abatement. The low surface area is not a discriminant factor if other features compensate it; the ratio of OH/O has a specific influence for the pollutants photodegradation together with the morphological features of particles. In fact, nanometric P25 is characterized by a significant higher amount of hydroxyl radicals, in agreement with the optimal efficiency in pollutants photodegradation. However, also pigmentary 1077, Hombitam AZ and AN samples show appreciable amount of OH? groups and this justifies their good catalytic performance. Furthermore, porcelain gres tiles, prepared entrapping micro-TiO2 at the SiO2 surface confirmed a stable and reproducible photocatalytic activity toward organic contaminants, such as dyes and NOx, in both liquid and gas phase. This indicates that these new industrial ceramic materials with micrometric TiO2 are surely an interesting application, which avoids the use of traditional nanomaterials in powder form for their preparation. In addition, the doping of micrometric TiO2 powders with anionic or cationic species highlighted the possibility to increase the catalytic performance obtaining comparable results with naked nanometric samples. And, as a consequence of the high demand of the use of LED lamps in the indoor and outdoor areas, the metal particles on the micrometric TiO2 surface confirmed their ability to adsorb visible light and to be considered sensitizers. To summarize, powders with large particles and low surface area can have good photoefficiency for the depollution abatement.