PHOTOCATALYTIC ACTIVITY ASSESSMENT OF MICRO-SIZED TIO2 USED AS POWDERS AND AS STARTING MATERIAL FOR PORCELAIN GRES TILES PRODUCTION

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 pe

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