Multifunctional photocatalytic coatings for construction materials

Multifunctional photocatalytic coatings for construction materials

23

Marisol Faraldos, Ana Bahamonde Institute of Catalysis and PetrochemistrydCSIC, Madrid, Spain

23.1

Introduction

Application of multifunctional coatings to urban infrastructures responds to some additional demanded functionalities such as mechanical resistance, anticorrosion, aesthetic appearance, self-cleaning, biocide behavior, or depolluting. Coating formulation must be chemically compatible with the pH of the substrate (alkaline in the case of concrete) and provide the required physical, chemical, and thermal properties. Good adhesion to substrate, efficiency, and durability are conditions studied to select the optimal coating. Specifically, photocatalytic coatings are applied to urban infrastructures to mitigate the negative effects of urban pollution, favoring its abatement. At the same time, selfcleaning and biocide behavior give added-value to these coatings. Urban air quality has been continuously worsened during last decades, fundamentally in the worldwide greatest cities. Some pollutants have reached elevated concentrations that became a risk for human health, environmental balance, and climate exchange. Urban air pollution is mainly contributed by harmful inorganic and organic molecules (NOx, SOx, CO2, and COVx) and particulate matter (Cros et al., 2015a, 2015b; Guo et al., 2015a; Zhong and Haghighat, 2015), with nitrogen oxides (NOx) representing some of the largest air polluting agents, which are produced mostly by vehicle engines (Table 23.1). However, indoor pollution involves both chemical agents and pathogenic microorganisms (Chen and Poon, 2009; Nath et al., 2016; Saeli et al., 2017; Zhong et al., 2017) as illustrated in Fig. 23.1. Furthermore, the action of these agents on the exposed surfaces of buildings and urban infrastructures provokes the damage of construction materials and aesthetic appearance, due to dirt accumulation (Carmona-Quiroga et al., 2018). Both detrimental effects require attention and economic resources, even more when cultural heritage constructions are involved. Many studies have been carried out to reduce urban pollution, but one of the most innovative, energetically friendly, and nondisturbing the mobility of urban life and citizens is the application of photocatalytic technology over infrastructure surfaces to create the concept of photocatalytic isle where the contact between pollutants and photocatalyst is favored by the canyon street effect. The photocatalytic oxidation (PCO) process transforms contaminants into nonharmful compounds that will be further Nanotechnology in Eco-efficient Construction. https://doi.org/10.1016/B978-0-08-102641-0.00023-2 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Table 23.1 Urban population exposed to concentrations of air pollutants above selected air quality standards of the Air Quality Directive, 2015. European Environment Agency (EEA). https://www.eea.europa.eu/airs/2017/environment-and-health/outdoor-air-quality-urban-areas Countries

PM 10 (daily limit value)

Austria Belgium Bulgaria Croatia Cyprus Czech Republic Denmark Estonia Finland France Germany Greece Hungary Ireland Italy Latvia Lithuania Luxembourg Malta Netherlands Poland Portugal Romania Slovakia Slovenia Spain Sweden United Kingdom EU-28

0 0 78 81 6 19 0 0 0 1 <1 4 27 0 60 4 2 0 100 0 81 1 54 6 100 5 0 0 19

O3 (target value) 98 0 0 94 0 89 0 0 0 17 37 97 100 0 80 0 0 0 NA 0 38 0 12 60 100 34 0 0 30

NO2 (annual limit value) 5 3 <1 3 0 1 2 0 1 4 5 3 2 0 35 4 0 9 0 2 1 2 1 5 0 16 <1 11 9

The color coding of exposure estimates refers to the fraction of urban population exposed to concentrations above the reference level:

0%

<5%

5–50%

50–75%

>75%

washed by rain and conducted to the sewage treatment plant. To obtain that functionality, building materials have to be improved by incorporating the photocatalytic agents in the bulk during their manufacture or by coating. In this case, the advantage is that the photocatalyst could be applied in situ and it presents a higher photoexposed surface/photocatalyst loading ratio. The photocatalytic process is based on the utilization of a photocatalyst, the most widely used being titanium dioxide (TiO2), a semiconductor material that creates an electronehole pair when irradiated with solar or similar light. Then, very reactive

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1 2 4

2 5

2 3

3

5 4

3

6

Figure 23.1 Indoor air pollution. We spend a large part of our time indoors, in our homes, workplaces, schools, or shops. Certain air pollutants can exist in high concentrations in indoor spaces and can trigger health problems. (1) Tobacco smoke: exposure can exacerbate respiratory problems (e.g., asthma), irritate eyes, and cause lung cancer, headaches, coughs, and sore throats. (2) Allergens (including pollens): can exacerbate respiratory problems and cause coughing, chest tightness, breathing problems, eye irritation, and skin rashes. (3) Carbon monoxide (CO) and nitrogen dioxide (NO2): CO can be fatal in high doses and causes headaches, dizziness, and nausea. NO2 can cause eye and throat irritation, shortness of breath, and respiratory infection. (4) Moisture: hundreds of species of bacteria, fungi, and moulds can grow indoors when sufficient moisture is available. Exposure can cause respiratory problems, allergies, and asthma, and affect the immune system. (5) Chemicals: some harmful and synthetic chemicals used in cleaning products, carpets, and furnishings can damage the liver, kidneys, and nervous system, cause cancer, headaches, and nausea, and irritate the eyes, nose, and throat. (6) Radon: inhalation of this radioactive gas can damage the lungs and cause lung cancer. European Environment Agency (EEA).

and unselective radicals ( OH, O 2 , OOH, among others) are generated, able to degrade most of the chemical and biological contaminants. The photocatalytic technology could be applied to new building construction and urban infrastructures or implemented in restoration works, and presents multifunctionalities such as urban pollution abatement, cleaning indoor air, sterilization, sanitation, and remediation applications, self-cleaning, antifogging, antireflecting, being energy safe, and eco-friendly. One of the most intrinsic limiting handicaps of TiO2 is the narrow band-gap in the UVA region (3.2 eV for the anatase phase) that reduces the harvesting of solar radiation (sunlight shows 60% of visible photons in comparison with a mere 4.5% of UV photons). For that reason, many efforts trying to increase the efficiency of catalysts








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under visible radiation have been performed. Different strategies have been used: (1) use of alternative semiconductor oxides (ZnO, SnO2, ZrO2, WO3, etc.), (2) metaldoped titanium oxide (Fe, V, Cu, Au, Ag, etc.), (3) nonmetal-doped TiO2 (C, N, S, etc.), (4) deposition of semiconductor oxides on high surface area materials (zeolite, carbon, fullerenes, graphene, MOFs, etc.), or (5) mixtures of these materials. Photocatalytic materials constitute a solution to mitigate the harmful effect of air pollution in human health that is being already commercialized and in expansive application in Japan and more recently in Europe. Previous studies have reported that construction materials containing TiO2 exposed to UV rays trigger photocatalytic oxidation of organic and inorganic substances (NOx, CO, VOCs, formaldehyde, industrial emissions, and others) deposited on the surface. In addition to imparting pollution abatement and self-cleaning properties, a few studies have shown that nano-TiO2 can accelerate the early-age hydration of Portland cement, improve compressive and flexural strengths, and enhance the abrasion resistance of concrete. However, there are some aspects that still need attention and recent researches are focused on them: development of more efficient photocatalysts, improving the compatibility with building materials, enlarging the durability of photocatalytic activity under weathering conditions, and reducing costs increment. In this sense, the application of a photocatalytic material over a large surface, such as roads or buildings, requires a large amount of photocatalyst and will be important to optimize the costs. Considering that photocatalysis is a surface process, only the outer exposed photoactive material could be working in the depollution photocatalytic reaction (Yang et al., 2018; Fig. 23.2). However, additional variables should be evaluated like porosity and photo-access of exposed surface, durability, efficiency, and application procedure, to proceed with the economic analysis of costs. A review of published research showed that some of these studies have employed concrete blocks prepared by mixing TiO2 particles with cementitious materials or by applying a “double layer” where only the upper layer contains the photocatalyst. The amount of TiO2 in these samples is very high and most of the catalyst is in the internal structure unavailable to light. Bulk photocatalytic materials will be analyzed in another chapter of this book.

NO NO NO2

NO NO

NO2

NO

NO

NO2 NO2

NO3-

Mixed

NO

NO NO

NO2

NO

NO

NO2 NO

NO2

NO2

NO

NO

NO3-

NO

NO2

NO2

NO2

NO

NO3-

NO3-

Supported

Figure 23.2 Differences in the exposed surface of photocatalyst mixed with substrate and supported on substrate (Yang et al., 2018).

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The option of photocatalytic coating is cost effective (only 5%e10% by weight of cement is required for such application) in the field applications and it allows maintaining mechanical, physical, and chemical characteristics of the concrete. The challenge could be the preparation of an effective, sprayable photocatalytic coating material. Frequently, photocatalytic coatings for cementitious materials are hydrophilic, but when heritage and archeological stone-built constructions are studied, where water infiltration prevention is needed, hydrophobic coatings could be recommended. In both cases, compatibility with the substrate, reversibility, aging, and preservation of the aesthetic aspect of the building or infrastructure are mandatory conditions for their applicability.

23.2

Composition of photocatalytic coatings

The use of photocatalysts together with building materials started from the early 1990s. The versatile function of a semiconductor, such as e.g., TiO2, ZnO, Fe2O3, WO3, and CdSe (Pacheco-Torgal and Jalali, 2011) which can both serve as photocatalytic materials and structural materials, has facilitated its application in exterior construction and interior furnishing materials, such as cement mortar, exterior tiles, paving blocks, glass, and PVC fabric. In this field, the most used methodology to introduce photocatalytic properties to construction industry is adding a photoactive coating. Previous research reported the presence of voids in concrete, which is about 1%e2% that leads to decrement of compressive strength. These voids between cement particles would be filled by photocatalyst nanoparticles, the pores become smaller which contributes to increase the strength, modify the properties, and improve durability, thermal, mechanical, and electrical properties of cementitious materials (Zailan et al., 2017). It is known that nanosilica (n-SiO2) and nanotitania (n-TiO2) can improve the mechanical and durability properties of concrete causing a more withstanding concrete structure. SEM (Scanning Electron Microscopy) observation showed the reduction of Ca(OH)2 between the hydrate, uniformly spread nanoparticles which act as activator than filler in order to improve the microstructure of the cement paste. The pozzolanic activity of added nanoparticles in hydrating cement contributes to increase the compressive and flexural strength with the amount of TiO2 nanoparticles added up to 1% due to the fast reaction between n-TiO2 and calcium hydroxide.

23.2.1 Titanium dioxide In the field of photocatalytic construction and building materials, TiO2 is the most widely used photocatalyst, having been traditionally used as a white pigment in paints, cosmetics, and food stuffs in ancient times (Fujishima et al., 1999). Over the years, titanium dioxide has been considered one of the most appropriate photoactive materials (Chen and Poon, 2009), given that it exhibits almost all the required properties for an efficient photocatalytic process, only with the important drawback of not absorbing visible light. The anatase crystal form of titanium dioxide

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is more widely used because it has a higher photoactivity than the other types of TiO2 (brookite and rutile). The widespread use of TiO2 in photocatalytic building materials is attributed to the following characteristics: (1) it is relatively inexpensive, safe, and chemically stable; (2) it has a high photocatalytic activity compared with other metal oxide photocatalysts; (3) it is compatible with traditional construction materials, such as cement, without changing any original performance; and (4) it is effective under weak solar irradiation in an ambient atmospheric environment. One classification of the major TiO2 based construction and building materials is presented in Table 23.2 (Chen and Poon, 2009). Nowadays, the coating application of photocatalysts onto building materials is gaining a growing interest owing to several advantages (Pérez-Nicol
as et al., 2018). (1) It allows applying active agents in preexisting construction structures. (2) Taking into account that the photocatalytic process needs pollutant adsorption onto active sites, these photoactive sites have to be located at the material surface, avoiding the insertion of the photocatalysts in the inner side, inaccessible to contaminants, thus yielding better performance of pollutants removal. (3) Coating application requires a lower consumption of photocatalytic additive than bulk addition. From a comparison study between titanium dioxide coated mortars and mortars with TiO2 in bulk, similar photocatalytic performances were found, although coating was loaded with 20 times less amount of TiO2. Recently, photocatalytic titanium dioxide has been addressed for coating of building stones (Pinho et al., 2013, Quagliarini et al., 2013) and it has been getting an increasing attention for the conservation of the stone Cultural Heritage. The photocatalytic stone coatings by TiO2 nanoparticles are very attractive for conservation of buildings because they can preserve the integrity of the stone surface by protecting against the deposition of many organic urban particulates that feed on surface deposits, thus diminishing the maintenance of the building façades by cleaning works. Natural stones with self-cleaning and depolluting abilities are appealing to preserve building façades in polluted urban sites and simultaneously to provide air-purification. Some different strategies have been conducted trying to improve the photocatalytic activity of titanium dioxide in the visible region: (1) semiconductor sensitization; (2) doping and codoping, or (3) using spatially structured titanium dioxide.

Table 23.2 Classification of TiO2-based photocatalytic construction and building materials (Chen and Poon, 2009) Categories

Products

Function

Exterior construction materials

Tiles, glass, tents, plastic films, panels

Self-cleaning

Interior furnishing materials

Tiles, wall papers, window blinds, paints, finishing coatings

Self-cleaning, antibacterial

Road construction materials

Soundproof walls, tunnel walls, road-blocks, concrete pavements

Air-cleaning, self-cleaning

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In this context, to extend the photocatalytic activity of nanocrystalline titania into the visible spectral range, doping of titania by nonmetal elements such as N, C, F, and S has been reported (Bergamonti et al., 2017). There, two tests on carbonatic Travertine stones coated with two self-cleaning coatings based on N-doped TiO2 nanoparticles have been described. Both treatments exhibit good photocatalytic activity under UVeVIS irradiation in the discoloration of organic dyes such as methyl orange and rhodamine B used as contaminants of the Travertine, even in the presence of potential dye photosensitization effects. However, coating with photocatalytic titanium dioxide is also promising to preserve building facades with self-cleaning properties; nonetheless, stone coating issues need better insights to support large-scale applications. The role of roughness and porosities has been investigated on two different limestone photocatalytic surfaces (Calia et al., 2017). In this study, coatings obtained from either water or alcohol based colloidal suspensions of TiO2 nanoparticles, which were synthetized by a solegel and hydrothermal process and sprayed with different loads on the stone surface, have been compared. The overall results showed that all the obtained coatings were able to deliver photocatalytic surface of both limestones, which have a potential to be implemented as eco-efficient materials on buildings, with high selfcleaning efficiency, through the photodegradation test of rhodamine B. Conversely, the efficiency in a NOx abatement test was dependent on the porosity and roughness of the stones. Coatings of different visible light absorption TiO2 commercially available catalysts: one doped with carbon and the other doped with nitrogen, were presented as improved photocatalysts (Ballari et al., 2016). Two representative air pollutants, nitrogen oxide and acetaldehyde, were successfully degraded for two of the four catalysts tested, the carbon and nitrogen doped TiO2 powders. On the other hand, commercial glazed ceramic tiles were functionalized with a micrometric TiO2 layer adopting an industrial-like process (Tobaldi et al., 2017). The authors confirmed that the developed functionalized glazed ceramic tile possessed excellent self-cleaning and photocatalytic properties. Cement-based materials are widely used for buildings and infrastructures. Many studies (Shen, 2015; Yousefi, 2013) indicated that the addition of photocatalysts to cement-based materials by an incorporation method provides photocatalytic properties and improves their mechanical properties. In this context, the photocatalytic and hydrophobic activity of cement-based materials from benzyl-terminated-TiO2 spheres with coreeshell structures were studied (Wang et al., 2017), where hydrophobic titanium dioxide samples, obtained via a solvothermal process, were designed by employing organic small molecules for coating the cement based materials. After irradiating for 15 h, the color fading rates of cement paste coated TiO2 are more than 80%, which are about 3.6 times than the untreated cement paste. Although the use of titanium dioxide photocatalyst on cementitious materials for air purification has been developing rapidly in the last decades, the photocatalytic effect of TiO2 is considerably reduced as a consequence of low specific surface areas, poor gas diffusion, and light transmission performance of cementitious substrates. A novel hierarchical porous structure magnesium cementitious/TiO2 photocatalyst (FMOC/ TiO2) has been presented to improve the photocatalytic effect and photodegradation

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rate of titanium dioxide in cementitious materials (Wang et al., 2014a). The obtained results revealed that the hierarchical porous structure of the substrate was beneficial to the dispersion of TiO2, which provided more active TiO2 sites and large catalytic areas. The good gas diffusion and light transmittance performance of the substrate improved the photocatalytic efficiency of titanium dioxide particles. TiO2 nanocrystals with a rod-like geometry were synthetized by capping oleic acid molecules, in anatase phase, which is reported to be the most photoactive phase of TiO2. The nanosize and anisotropic shape originate high surface area TiO2 NRs. The nanorod geometry facilitates the charge separation, limiting e/hþ recombination. Additionally, due to oleic acid capping the coated surface resulted to be hydrophobic (Petronella et al., 2017).

23.2.2

Other semiconductors, composites, and mixtures

Recently, the combination of titanium dioxide nanomaterials with other semiconductors has been demonstrated to be very promising in enhancing the photocatalytic activity. Mostly, coupling TiO2 with semiconductors of different band gaps to extend the absorption wavelength range to the visible range and to avoid e/hþ recombination rate has already been proposed (Wang et al., 2014b; Truppi et al., 2017). The TiO2/semiconductors hybrid heterostructures can be divided into (Truppi et al., 2017): pen semiconductor heterojunction and non-pen heterojunction systems. When p- and n-type semiconductors come into contact, the so-called “space-charge region” is generated at the interface, thus forming a pen junction. These systems, including Cu2O/TiO2 (Wang et al., 2014b), CuBi2O4/TiO2, NiS/TiO2, and graphene oxide/ TiO2 (Chen et al., 2010), show several advantages: (1) improved charge separation; (2) improved charge transfer to the catalyst; and (3) longer charge carrier lifetime. On the other hand, in non-pen heterojunctions, the two semiconductors are tightly bound to construct an efficient heterostructure in which the internal field is able to promote the separation and migration of photogenerated carriers (Truppi et al., 2017). For such non-pen heterojunction systems, like CdS/TiO2, InO3/TiO2, WO3/TiO2, Fe2O3/ TiO2, and ZnO/TiO2 (Wang et al., 2015c) the staggered band gap type structure is the most suitable for photocatalytic applications. Doping TiO2 with rare earth elements is an effective way to improve light utilization of photocatalysts, the scope of light absorption can be broadened and the quantum conversion efficiency of TiO2 improved by introducing defects into the crystal or changing the crystal structure of the photocatalyst. La-doped TiO2 nanoparticles were prepared by the ultrasonic assisted solegel method, well dispersed in silica sol and then loaded on the surface of asphalt concrete specimens. The characteristic diffraction peaks of lanthanum oxide were not observed in spheroid small size Lae TiO2 particles. The LaeTiO2 dispersion solution was sprayed on the surface of specimens, which then were dried at 60 C. 0.5% dosage of La presented the optimal photocatalytic activity for NO removal on asphalt concrete. The results demonstrate that the specimens present considerable photocatalytic activity under different conditions of light sources, and NO photodegradation rates diminish gradually with the increase of NO concentration in the mixed gas and the flow velocity. Under ultraviolet light

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and visible light irradiation, the NO degradation rates are 39.37% and 19.7%, respectively (Tang et al., 2017). A catalyst prepared with a-Fe2O3 supported on rice husk ash (RHA) showed enhanced photochemical NO oxidation. The photooxidation of NO was related to the catalyst surface area (Balbuena et al., 2018). In the best case, a NO molar conversion value of 24% was arisen, the highest reported for hematite photocatalysts. Moreover, the selectivity of NO to nitrates reached values as high as 72% with the presence of large hexagonal plate hematite crystals. Recently, a highly active photocatalyst, ruthenium-modified zinc oxide, which was able to utilize the red region of the visible light spectrum for photocatalytic reactions, has been reported (Bloh et al., 2014). The ruthenium dioxide particles acted as catalysts increasing the efficiency of the reaction by improving the oxygen reduction properties of the material. The visible light photoactivity was observed for the gas-phase degradation of acetaldehyde and by the oxidation of nitric oxide in the gas phase. The incorporation of a SiO2 interlayer between the substrate and photocatalytic layer was studied (Mendoza et al., 2015) with the aim to improve adhesion and therefore durability. Sequentially sprayed layers of hydrophilic nano-SiO2 sol and different TiO2 homemade and commercial sols were coated on mortars. The effect of pH of suspensions and the presence of SiO2 interlayer were evaluated in the photocatalytic degradation of NOx and bleaching of rhodamine B. The presence of SiO2 had a detrimental effect in combination with commercial sols, however with n-TiO2 homemade sol the coating becomes more stable without loss of photocatalytic activity. The higher half reaction times observed suggest the necessity of an induction period to activate the coating. A new nanostructured BiOBr@SiO2 photocatalyst has been innovatively used for surface-treatment of cement-based materials (Wang et al., 2018) with the aim to wide radiation harvesting in the visible-light region. The SiO2 layer on the flower-like BiOBr@SiO2 contributes to continuous and homogeneous distribution of the photocatalyst, and promotes chemical bonding between the coating and cement matrix. BiOBr@SiO2-cem reveals a denser and smoother surface after curing, free of cracks or peeling out, and with a pore-filling effect. Additional CeSeH gel can be formed due to the pozzolanic reaction between BiOBr@SiO2 and the hardened cement matrix, with the SiO2 acting as the bonding agent between the photocatalysts and cement matrix. The increasing dosage of SiO2 in the system narrows the pore content of BiOBr@ SiO2-cem gradually. The BiOBr material showed excellent photocatalytic efficiency under xenon lamp irradiation, and therefore, BiOBr@SiO2 exhibits great potential as a photocatalyst. A novel composite based on TiO2 has been formulated using purified diatomite as a carrier of vanadium doped TiO2 (Wang et al., 2015a). A V-doped TiO2/diatomite composite was synthesized by the solegel method. The photocatalytic activity on rhodamine B degradation using solar light significantly improved than that of undoped and unsupported TiO2. Substituent V4þ ions in Ti4þ sites of the TiO2 lattice are responsible for increased visible-light absorption. Meanwhile, V5þ species (in the form of V2O5) on the surface of TiO2 particles are responsible for efficient charges separation and

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enhanced charge-transfer to the oxygen molecules adsorbed TiO2 surface producing O 2.


23.2.3

Additives

A multifunctional coating with photocatalytic and hydrophobic properties has been reported for the preservation of the stone building heritage (Colangiuli et al., 2015). These authors combined the advantageous photocatalytic properties of the TiO2 nanoparticles and the hydrophobic properties of a fluorinated polymer, by adding different amounts of a 3% TiO2 water solution to a commercial fluorinated polymer (10% in water solution). The addition of TiO2 to the polymer was found to decrease the color impact of the coatings on the stone surface. The formulation of TiO2 coatings in siloxane hydrophobic matrixes promoted superhydrophobic surfaces (Faraldos et al., 2016). The loading of TiO2 has been optimized in the photodegradation of NOx, where a threshold of 5% was determined, higher than found for a hydrophilic matrix. The incorporation of superplasticizers to photocatalytically active coatings for cement and air lime mortars improved their photoactivity (Pérez-Nicol
as et al., 2018). Coatings made with water dispersions of different nanoparticles of photocatalytic additives (titania and titania doped with iron and vanadium) were prepared with diverse superplasticizers to optimize the atmospheric NO removal efficiency when applied onto cement and air-lime mortars.

23.2.4

Substrates

Different factors related to the characteristics of the substrate have strong influence on the performance of the photoactive coatings (Pérez-Nicol
as et al., 2018): roughness, pore size distribution, and chemical and mineralogical composition, which can affect the adhesion of the coating. This last point is strongly related to the loss of photocatalytic and photoefficiency properties of these active coatings after weathering or abrasion phenomena. Substrates with high porosity and roughness have shown better retention of particles of the photocatalysts, thus enhancing the resistance of the materials to different degradation mechanisms. Literature shows successful coating applications onto limestone, bricks, cement, and lime mortars (Pérez-Nicol
as et al., 2018; Martinez et al., 2014).

23.3

Coatings: application procedures and characteristics

Once defined the composition of the coating to assure photocatalytic activity, good adherence to the substrate, additional properties such as (super)hydrophobicity, (super)hydrophilicity, etc., the stability of the suspension in the application conditions and drying procedure could condition the characteristics and performance of the applied layer.

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23.3.1 Coating techniques Before proceeding with the coating stage, nanoparticles of photocatalysts must be dispersed in a solvent (ethanol, methanol, silane, lime-saturated water, or modified polycarboxylate polymer) with the help of stirring and ultrasonication treatment to ease the spreadability of the deposition on the surface of building materials (Zhong and Haghighat, 2015). The function of these dispersing agents is to prevent the tendency of agglomeration of nanoparticles suspended in deionized water. Stable suspensions of photocatalyst nanoparticles could be deposited by spray, dipcoating, spin-coating, electrodeposition (EDP), chemical or physical vapor deposition (CVP or PVP), etc. (Angelo et al., 2013). The procedure will be carefully chosen according to the substrate to coat, thickness to apply, photocatalyst sol properties, and economic costs. The thickness of the coating is an important parameter, considering that usually, the higher the photocatalysts loading ratio the weaker the surface adhesion force to the coated building materials. Dip-coating is a highly used method for deposition of very thin films on flat substrates such as ceramic, glass, or metals. The surface tension of sol, contact angle between the sol and substrate, porosity of substrate, and withdraw speed, are key parameters which should be optimized to customize the thickness, porosity, and adhesion of the coated layer. The thickness (h) of the deposited layer could be modulated according to the relationship derived by Landau and Levich for Newtonian liquids, that is the usual behavior of solegel suspensions: h ¼

0:94ðh$UoÞ2=3 1=6

gLV $ðr$gÞ1=2

(23.1)

where h is the liquid viscosity; Uo is the substrate speed; gLV is the liquidevapor surface tension; r is the density, and g is the gravity acceleration (Brinker et al., 1991). Some construction materials, with high porosity, such as stones, could require previous conditioning treatment of washing and drying (Petronella et al., 2017) to further apply the photocatalysts (TiO2 suspended in chloroform) by casting. The practical application consists in brushing the sol to introduce the photocatalyst on internal and external stones surface in a polymeric matrix and let dry. Alternatively, the dipcoating method could be applied to small pieces by immersion of the stones in the photocatalyst suspension. Guo et al. (2015a) also applied the anatase aqueous dispersion onto the architectural mortars by brushing in three layers (dried at room temperature and humidity for approximately 15 min between successive layers). Spin-coating is another conventional reported method to coat thin films. The final film thickness, h (m), could be determined by:

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1 h¼ u

sffiffiffiffiffiffiffi 3h 4rt

(23.2)

which considers the material parameters: density, r (kg/m3), and viscosity, h (Pa$s), of the fluid; upon the process parameters: angular velocity, u (s1), and spin time, t (s); and is affected by solvent evaporation. The spin-coating process consists of a drop of the sol placed on the upper surface of the substrate, and then rotated to spread forming a thin film under the centrifugal force. Obtaining very thin films is frequently desired because their fabrication is more economical, consumes less material and all without leading to reduced photocatalytic performance. Considering that the depth of UV light penetration into TiO2 films is approximately 0.3 mm, the use of thinner films will result in a higher photocatalytic efficiency (Wang et al., 2013). Spraying is another technique in which the suspension in the form of an aerosol of small drops dispersed in the impulsion gas is thrown onto the surface of substrate. Under optimal conditions the impulsion pressure would control the quantity of dry particles deposited on the surface, where most of the solvent should be evaporated along the walk between the nozzle and surface. The optimal gas pressure will depend on sol evaporation, volume rate sprayed, distance, porosity of the substrate, etc. The spraying deposition could be assisted by application of potential which helps the generation of porosity layers (Chen et al., 1999). The thickness of dry films can be parametrically calculated by: h ¼ k$

m$cos q r$p$Z2 $tana$tanb

(23.3)

where k is the coefficient of the spray coating process; m is the weight of coating material in liquid solution; r is the coating material density; Z is the spray gun standoff-distance; a, b, and q are span angle with respect to the major axis, span angle with respect to the minor axis, and the inclination angle of spray gun, respectively (Luangkularb et al., 2014). The deposition by industrial jet spray technique of TiO2 photocatalytic layer on glazed ceramic tiles required some additives (sodium tripolyphosphate, Na5P3O10, and carboxymethyl cellulose, CMC) to the aqueous suspensions of commercial titania to adjust the viscosity of the sol (Tobaldi et al., 2017). Obtaining of a uniform layer was possible after optimization of the deposition conditions. The jet sprayed layers were dried in an oven at z200 C, and then subjected to fast firing for 60 min at a maximum temperature of 950 C. Aqueous and hydrophilic sols can be easily applied by spray, but hydrophobic dense suspension could not be applied by spray and require dip-coating techniques (Faraldos et al., 2016). Other authors reported the need to mechanical compaction to help the percolation of photocatalysts suspension and increase the loading of active layer (Guo et al., 2017). A suspension of TiO2 P25 (30 g/L) in ethanol was sprayed 20 times, followed by

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mechanical compaction, and another 20 times after the compaction, estimating approximately 6 mg/cm2 of TiO2 was added to each sample. This procedure performs better in pollutants abatement and robust abrading resistance. Considering the most relevant studies in the field of photocatalytic coatings applied to construction material surfaces (Munaf o et al., 2015) only two application methods are massively used: spray coating and brushing, mostly because of their simplicity, cost-efficiency, and their compatibility with most of the construction surfaces, shapes, extension, and roughness (Enríquez et al., 2016). In general, independently of the coating method used, control of the drying stage contributes to the optimal constitution of coating. Fast drying and low humidity conditions may result in flake formation and cracking layer.

23.3.2 Thickness and roughness The thickness of the photocatalyst layer could be firstly estimated by geometrical calculation knowing the coated surface area. In general, very variable values for thickness are found in bibliography, being from tens of nanometers to hundreds of micrometers the most common. Photocatalytic processes take place in the few nanometers of the photoexposed surface and thicker coating has no effect on the photocatalytic activity although it could be important for durability due to abrasion and loss of photocatalytic material. An average roughness (Tobaldi et al., 2017) could be measured by the difference between specimens without and with the photocatalyst layer; however these measurements are not always possible due to different construction surfaces and thickness. The thickness parameter has been shown to directly influence both photocatalytic power and other photo-properties of coatings. The titania layer gave also the functionalized material a higher surface roughness, if compared to that untreated. This peculiarity showed itself to be the key-point in the microalgal growth over the treated specimens and has a positive impact on pollutant abatement due to improvement of adsorption (Yang et al., 2017b). The application of thin film coatings on rough substrates promotes retention of photocatalyst sol on the surface and limits its absorption, thus a thick film was formed on the surface (Graziani et al., 2014). The increase of TiO2 coating thickness induces the increase of cracks in the coating layer. This tendency is mainly due to the large shrinkage stress of the thick coating layer in the process of drying (Li et al., 2016). The precise measurement of thickness could be performed by profilometer, Scanning Electron Microscopy (SEM), and Atomic Forces Microscopy (AFM). The observation on SEM of cross-section cuts of the layer at different points of the coating provides the most accurate way to know the thickness, composition, morphology, and uniformity of the photocatalytic layer (Pérez-Nicol
as et al., 2018). However this analysis is not always possible due to in situ applications. In these cases, a coating in similar application conditions could be done on glass slides and the resulting layer observed by SEM. Although some considerations should be considered such as porosity, wettability, roughness, chemical affinity, etc. between both substrates, a real estimation of thickness could be measured (Tryba et al., 2015).

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The performance of photocatalytic degradation for high pollutants (NOx) concentration resulted to be clearly more relevant when thicker coatings were applied on concrete surfaces (Faraldos et al., 2016). This behavior was even more demanding when hydrophobic coatings were applied, where a loading threshold of 5% was necessary to reach NOx conversion higher than 90%. The thickness reduction associated to natural aging when exposed outdoor causes particles release of the nanocoating that provokes loss of TiO2 efficiency. Surface roughness for different photocatalytic layers was related to agglomeration of nanoparticles and the particle size distribution of photocatalytic suspension has a decisive influence (Table 23.3 and Fig. 23.3). The bigger size of agglomerates, aggregates and/or particles, and the higher surface roughness (Kete et al., 2014) could contribute to enhance the specific surface area of the coating and therefore to improve the photocatalytic performance on pollutants abatement (Kamaruddin and Stephan, 2013).

23.3.3

Adhesion and durability

The durability of the coated photocatalytic layer is directly related to the coating method and adhesion forces. However, the processing of photocatalytic films by soft chemistry deposition procedures (e.g., solegel methods) is increasingly demanded due to their simplicity, scalability, and flexibility. Therefore a balance among adhesion, durability, and photocatalytic activity should be found. The durability of coating involves long run performance of the photocatalytic surface and nowadays, main concerns about the release of TiO2 nanoparticles into the environment and associated bioaccumulation, biotoxicity, and biodegradability potential problems that will be treated in a different section of this book. An evaluation of the anchorage of the films on surface could be made through the peeling “Scotch Tape test”. Approximately 6 cm length strips were cut and weighed on the analytical balance Table 23.3 Roughness of photocatalytic layers (2  2 mm) (AFM measurements) and agglomerate/particle sizes of titania nanopowders dispersed in water (DLS measurements) (Kete et al., 2014) Sample

Agglomerate/particle size (nm)

P25

44.4

52  4

P90

23.2

90  7

VPC-10

70.0

168  3

122.0

302  5

Hombitan LO-CR-S-M PC500 KRONOClean 7000 a

Rmsa (nm)

Rms (root mean square deviation).

87.1

392  5

152.0

338  3

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Hombitan LO-CR-S-M

P90 0.36 μm

0.79 μm

0.00 μm x:

4.0

μm

4.

y:

0.00 μm y:

m 0μ

4.0

μm

P25

x:

4.0

μm

PC500 0.91 μm

1.00 μm

0.00 μm x:

4.0

μm

m 0μ

4.

y:

0.00 μm y:

4.0

μm

x:

4.0

μm

VPC-10

KRONOClean 7000

0.95 μm

0.88 μm

0.00 μm

0.00 μm y:

2.0

μm

x

.0 :2

μm

y:

4.0

μm

x:

4.0

μm

Figure 23.3 AFM micrograph images (4  4 mm) of photocatalytic layers. KRONOClean 7000 coating was too rough to allow scan at these dimensions (Kete et al., 2014).

(0.1 mg precision). Then, it was stuck to the surface and finger pressure applied; after 15 s, it was removed and weighed to determine the amount of detached material by difference. A second test of water impact could be performed, to study the resistance of the coatings under a stronger mechanical action. A water jet was sprayed on the specimen surface at a pressure of 0.2 bars from a distance of 50 cm. The specimen was placed horizontally and impacted by a perpendicular jet. The test was carried out for 10 min; then it was stopped, and the erosion of the surface observed (Calia et al., 2016). The presence of cracks in the coatings due to excessive amounts of deposited photocatalyst has a detrimental effect on film durability. However, higher surface roughness accounts for the better adhesion of both fissured and crack free films to the substrate. Other types of test are those that simulate weathering conditions to know the performance of coating during long time. Accelerated aging tests help to obtain information in short time periods, but they cannot accurately reproduce all environmental factors and can conduce to anomalous effects. And testing under natural environmental conditions could provide the most interesting results, but they take many years.

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Consequently, to run both accelerated and natural aging tests in parallel would be desirable to evaluate materials durability when it is possible (Maury-Ramirez et al., 2012). Many studies are being carried out trying to understand the main cause reducing the photoactivity of thin films coated roads. In this sense, the behavior of two different TiO2 applied on an Italian highway over bituminous emulsion and concrete mortar (Bocci et al., 2016) has been analyzed. The effectiveness of the photocatalytic treatments was evaluated by continuous flow tests NO degradation on pieces taken off 1, 17, 46, 88, 218, and 527 days after the application. Although first samples exhibited a high NOx abatement, the decay was very acute for concrete mortar which was related to traffic abrasion. Bituminous emulsion maintained longer the high photodegradation and the decay was associated to weathering conditions (Fig. 23.4). The study concluded the suitability of photocatalytic bituminous emulsions for air pollution control in tunnels, where climatologic conditions are softer. The durability of coating could be assessed in multiple ways: through evaluating the photocatalytic activity of the surfaces, as explained, determining the presence and thickness of the coating on the surfaces with SEM/EDX and microRaman analysis or monitoring changes in the aesthetic properties of the surfaces. When artistic heritage is involved, the aesthetic is essential, and color alterations (DE*) should be avoided. Color changes are evaluated using the following equation expressed in the CIELAB system: a uniform color space mimicking the human vision:

Bituminous emulsion Concrete mortar

η NOmean (%)

30

20

10

0 1

10

100 t (days)

Figure 23.4 Degradation of nitrogen oxides on photocatalytic bituminous emulsion and concrete mortar. Adapted from Bocci, E., Riderelli, L., Fava, G., Bocci, M., 2016. Durability of NO oxidation effectiveness of pavement surfaces treated with photocatalytic titanium dioxide. Arabian Journal for Science and Engineering 41, 4827e4833.

Multifunctional photocatalytic coatings for construction materials

 1=2 DE ¼ DL2 þ Da2 þ Db2

573

(23.4)

where L* is the lightness; a* the red component, and b* the yellow (Carmona-Quiroga et al., 2018). Values of DE* < 5 are considered acceptable and DE* ¼ 1 is imperceptible by naked human eye (Munaf o et al., 2015). The interest to improve durability of photocatalytic thin films drives to develop new coating formulation with the incorporation of additives helping the dispersion of nanoparticles, increase the adhesion to substrates, and present longer durability (PérezNicol
as et al., 2018; Rudic et al., 2015; Wang et al., 2018; Yang et al., 2018).

23.4

Techniques for physico-chemical characterization

Physico-chemical characterization of photocatalytic coated construction materials could be faced in different stages: Firstly, the substrate properties could be evaluated to select the optimal composition of photocatalyst suspension, facilitate the adhesion, and avoid detrimental consequences in mechanical hardness; secondly, the photocatalyst suspension is analyzed to assure the optical, structural, and morphological properties to provide best photocatalytic performance, and thirdly, the thin film coated surface is analyzed to determine any modification of photocatalyst properties once deposited on the substrate surface (Faraldos et al., 2011).

23.4.1 Determination of main photocatalyst characteristics The analysis of photocatalyst properties could be carried out in suspension or aerogel powder. The Z-potential determines the surface charge of the particles as a function of pH and could influence the interaction between substrate and particles in the coating. The analysis of particle size distribution in suspension will influence the thickness and roughness of coated layer, at the same time it could affect the interaction with the substrate. This distribution can be measured directly in the diluted suspension by Dynamic Light Scattering (DLS) for sizes between 0.5 nm and 10 mm or laser diffraction (20 nme2 mm). Transmission microscopy (TEM) observation could corroborate the measured particle size and shape, density of aggregates, and crystalline structure (HR-TEM). If the instrument has an X-ray Energy Dispersive Analyzer (EDX) coupled to it, it could afford local chemical composition analysis. The crystalline phases will be identified by X-ray Diffraction (XRD) of powder aerogels. When nanoparticles are formed in the sols, the quality of diffractogram could be poor with wide and low intensity peaks due to the small crystallite size. The primary particle size (D) could be calculated using Scherrer equation without considering stress contribution (Patterson, 1939): D¼

K$l b$cos q

(23.5)

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where K is the crystallite shape factor, l the X-ray source wavelength, b the pure diffraction broadening, and q the diffraction angle. In the literature different equations could be found to determine crystallite size and structural stress from XRD data. The optical properties of photocatalysts will be addressed by UV-Vis molecular absorption spectroscopy. This technique permits the determination of band-gap (energy gap between valence band and conduction band in semiconductors), the radiation absorption to know the light harvesting of the system that permits to calculate the photonic efficiency. The absorption spectra of doped and nondoped photocatalysts give information of the different sensitization of the system when irradiated at different wavelengths. Derived from UV-Vis measurements the absorption, scattering, and extinction constants could be calculated to model the photocatalytic performance (Tolosana-Moranchel et al., 2017). Fluorescence spectroscopy (PL) provides information of e/hþ recombination processes and permits to follow the evolution of the characteristic band when synergistic mixtures of photocatalyst favor the reduction of this extinction process. Infrared and Raman spectroscopy complement the definition of molecular species in the photocatalyst, substrate, and modifications that could originate the interaction between them. The superhydrophilicity associated to irradiated photocatalytic surfaces (mainly TiO2-coated) could be characterized by Diffuse Reflectance Infrared Spectroscopy (DRIFTS) following the increment of OH bands during irradiation. Textural properties could be determined by nitrogen adsorption isotherm and mercury intrusion porosimetry to complete the micro-meso and macroporosity, respectively. Specific surface area is a key parameter in gaseous photocatalysis related to adsorption of contaminants as previous stage in the photodegradation procedure. BET surface area could be calculated from the data of volume adsorbed of nitrogen until a relative pressure of 0.3 (Faraldos et al., 2011). Textural characteristics of substrate will be important to avoid excessive percolation of suspension, because deep diffusion of the photoactive layer far from the surface will be unattainable by light resulting in reduced photoexposed surface and decrease of photocatalytic performance.

23.4.2

Analysis of coating surface properties

Color variation, wettability, water transfer properties, and stability of the coating are important parameters for the applicability of photocatalytic thin films and should be monitored as a function of time and to optimize the application method. Color changes have always been expressed in the CIELAB system (Munafo et al., 2015). As previously noted, tolerance values should fulfill DE* < 5. Water contact angle (WCA) measurements determine the wettability of the surface giving information of hydrophobicity (WCA > 90 )/hydrophilicity (WCA < 90 ) associated to the photocatalytic coating application and the effect of irradiation (Yang et al., 2017a). Highly porous or very rough samples present usually low values of WCA difficult to measure, however water absorption and permeability are frequently high (Petronella et al., 2017).

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Scanning electron microscopy (SEM) gives information of the microstructure of coated surface, distribution of photocatalyst on the substrate surface, homogeneity, and morphology of particles in the coating. A cross-section observation provides a valuable determination of thickness and uniformity of coating thickness. The mapping image of elemental composition obtained from back-scattering detection provides the real distribution of species, offering a view of segregation, dispersion, and percolation processes between the photocatalytic coating and substrate (Mendoza et al., 2015). X-ray photoelectric spectroscopy is a surface characterization technique sensible to different energy bonding between atoms, giving information of surface molecular species. The calculation of atomic ratios permits to analyze bulk and surface differences of interest. The comparative analysis of bare and coated substrates, or doped and undoped materials, or composites, mixtures, etc. provides valuable data to understand the interactions occurred during photocatalysts preparation and deposition (Lv et al., 2017; Nu~ no et al., 2015; Wang et al., 2015b; Appavu et al., 2016; Kim et al., 2016; Li et al., 2017; Karapati et al., 2017).

23.5

Photocatalytic performance

In the beginning of the 21st century, mankind must face the problem of air pollution, especially in urban areas as a consequence of exhaust gases from traffic and burning fuels in industries given that it is one of the most environmental issues on a global scale. Taking into account the adverse effects of nitrogen oxides (NOx) and volatile organic compounds (VOCs) on human and environmental health, it is absolutely necessary to obtain a technological solution in the immediate future to avoid degrading air quality in urban areas over the world. Actually, there are many different technological applications developed for the last two decades, among those the application of titanium dioxide photocatalysis to infrastructure and construction materials can be emphasized. This technology of photocatalysis has advanced rapidly, and became very attractive in the development of construction materials like ceramic tiles, blocks, glass, paints, etc. with self-cleaning, air or water purification, and antibacterial functions.

23.5.1 NOx abatement Nowadays, photoactive coatings on architectural materials such as glass, tiles, concrete, cement infrastructures, roads, paints, facades, and fabrics are resulting proposed way to reducing NOx in the atmosphere, via their ability to facilitate the photooxidation of NOx by ambient oxygen to nitric acid, where the overall process can be summarized as follows (Mills et al., 2016): TiO2 =UVA

2 NO þ 1:5O2 þ H2 O ! 2HNO3

(23.6)

Even though the end product is nitric acid, largely surface-bound species can then be washed away from the surface by rain water. Therefore, one proposed way of

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reducing NOx concentration in the atmosphere is the use of photocatalyst in construction materials, near to mobile sources such as photocatalytic cement infrastructures or concrete roads to improve quality of air. Usually, the photocatalytic oxidation reactions take place in the presence of water, oxygen, and under UV and near-UV light where photoactive materials, such as nano-TiO2, are able to oxidize or decompose nitrogen oxides, as well as organic and inorganic compounds (Lee et al., 2014). In this section a brief summary of nitrogen oxides removal by the application of titanium dioxide photocatalysis to infrastructure and construction materials is given. First of all, to assess the photocatalytic efficiency, there are a few standards to test the activity and efficiency of developed photocatalytic materials, such as the ISO standard (International Organization for Standardization, 2007), JIS standard (Japanese Industrial Standards, 2010), or UNI (Ente Nazionale Italiano di Unificazione, 2010) that have been established for advanced ceramics, where a photocatalyst is added by coating, impregnation, or mixing. Nevertheless, these standards utilize only NO gas in their experimental procedures. But since NO gas being emitted by combustion sources is typically converted to NO2 gas relatively quickly (i.e., within hours), it is important to also determine the extent to which NO2 is oxidized by photocatalytic surfaces.  Through the NO photooxidation process, nitrite (NO 2 ), nitrate (NO3 ), and nitrogen dioxide (NO2) can be formed, in the following three-step reaction (Bloh et al., 2014): HO•

HO•

HO•

NO ! HONO ! NO2 ! HNO3

(23.7)

Considering that NO2 is even more harmful than NO, more emphasis on nitrate selectivity, a strict measure of the nitrogen oxides conversion to nitrate, and how to maximize it, should be given in engineering photocatalytic systems for improved urban air quality. So, the following two parameters are usually used to characterize the photocatalytic performance in these studied standard tests: XNO ¼

S¼

! out Cin NO  CNO $100 Cin NO

1

Cout NO2 out Cin NO  CNO

(23.8)

! $100

(23.9)

where XNO is the nitric oxide molar conversion, S is the selectivity for ionic species formation, CNO and CNO2 are the concentrations of NO and NO2 respectively, and the superscripts (in and outlet) are referred to reactor’s inlet and outlet. Moreover, it is also very important to analyze the influence of the different operating conditions on photocatalytic efficiency. In general, the photocatalytic activity of any studied semiconductor, or in particular in the case of titanium dioxide which is one of the most known and studied photocatalysts, is strongly dependent on

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577

numerous operating and process conditions, among which those worth mentioning at lab scale are: (1) inlet concentration of pollutants; (2) percentage of relative air humidity; (3) type of light intensity and light spectrum; (4) total flow rate or residence time; (5) photocatalytic loading, and (6) support or substrates where there is coating, impregnation or mixing of the corresponding photocatalyst (Angelo et al., 2013). It is known that the inlet NO concentration has a significant role on photocatalytic activity and it has been demonstrated by several authors that the highest photooxidation rates have been obtained for the lowest NO concentrations (1 ppbv) (Angelo et al., 2013; Balbuena et al., 2018). Moreover, small changes in the low range of inlet concentrations led to more important effect in photocatalytic NO conversions than in the case of high range of inlet NO concentrations (1e20 ppmv) (Angelo et al., 2013; Ballari et al., 2016; Bengtsson and Castellote, 2014; Faraldos et al., 2016). The presence of water is essential for the photocatalytic process since it is responsible for HO and O 2 generation, crucial hydroxyl species in NOx photooxidation; for that higher NO conversions are expectable to increase the relative humidity (RH) of the air to be treated (Yu and Brouwers, 2009). However many authors have observed decreases in NO conversions for RH higher than 50% (Chin et al., 2011) or constant NO conversion afterward (Devahasdin, 2003). This effect observed to increase RH can be taking place as a consequence of limitations in hydroxyl radicals generation and/or by saturation and competition between the water molecules and nitrogen oxide on the active sites of the catalyst surface (Angelo et al., 2013). Moreover, the influence of RH percentage on NO conversion can be also connected with the inlet NO concentration. Thus, in the case of feed concentrations at ppbv level there is much higher competition between water and NO for adsorption active sites, than in the case of inlet ppmv concentrations, given that only a small amount of water is necessary at ppbv to generate enough hydroxyl radical species (Angelo et al., 2013). Therefore, a certain level of relative humidity is indispensable for hydroxyl radicals production to get good NOx photodegradation. When a semiconductor catalyst such as n-type TiO2 is illuminated with photons whose energy is equal to or greater than their band-gap energy, there is absorption of these photons and formation within the bulk of electronehole pairs, which dissociate into free photoelectrons in the conduction band and photoholes in the valence band, corresponding this value generally to the UV light region. So, the light source is responsible to provide enough energy to generate e/hþ pairs on the catalyst surface, leading to NO photooxidation. However, in the analysis of light intensity and light spectrum there are so many discrepancies in the literature given that it is so difficult to try to compare different studies where not always the same operating conditions were studied. So, the real role of light intensity on NOx photocatalytic performance is still scarcely understood and should be further studied in detail. Other important parameter that significantly affects the photocatalytic process is the total flow rate of polluted air, because it is directly related to the residence time inside the photoreactor. Whereas high flow rates can provoke short contact times, very low flow rates can give place to photodegradation rates limited by mass transfer phenomena, being detrimental in both cases for NO photooxidation. However, most of the authors have studied flow rate ranges among 1e5 L/min (Chin et al., 2011; de Melo and





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Trichês, 2012) revealed that with increases in the flow rate, for test simulated at the lab scale, two conclusions arise: with an increase in the flow rate, the photodegradation rate is reduced in terms of NO volume (ppmv or mg/m3) as a consequence of lower NO residence times. However, when the photoefficiency of the catalytic pieces is considered over a certain time interval (e.g., 1 h), with an increase in the flow rate from 1 to 3 or even 5 L/min, the NOx photodegradation, in mgNO/h m2, was greater. All of that could be explained because to increase the flow rate from 1 to 5 L/min, the pollutant volume in contact to catalytic piece should be three to five times greater, and then the total amount of NO photodegraded within this time interval (1 h), compared to a flow of 1 L/min, should be higher. Another important restriction in all photocatalytic processes is the fact that the photodegradation rate increases to increase the amount of TiO2 loadings until a value where a significant shield effect could begin to take place because not all titania particles would be perfectly illuminated given that the upper TiO2 layers hide the light to reach the lower layers (Angelo et al., 2013). Finally, the NO photocatalytic performance in this type of process is seriously influenced by the type of support or substrate used and the technique or preparation method used to add the photoactive material, where the working principle is the implementation of a semiconductor, typically titanium dioxide (TiO2) as the photocatalyst inside or on the surface of a suitable support. The different substrates employed in infrastructures or construction applications can be classified as follows (RILEM, 2011): 1. Horizontal applications: concrete pavements, paving blocks, and paving plates, coating systems for pavements and roads (white toppings, self-leveling mortars, etc.), roofing tiles, roofing panels, and cement-based tiles; 2. Vertical applications: indoor and outdoor paints, finishing coatings, plasters and other final rendering cement-based materials, permanent formworks, masonry blocks, sound-absorbing elements for buildings and roof applications, traffic divider elements, street furniture, and retaining fair-faced elements; and 3. Tunnels: paints and renderings, concrete panels, concrete pavements, and ultrathin whitetoppings.

On the other side, among the different procedures recently discussed to include photoactive catalysts in substrates such as construction materials (Angelo et al., 2013; Chen and Poon, 2009; Pacheco-Torgal and Jalali, 2011; Pérez-Nicol
as et al., 2018) can be highlighted e.g., in the form of, paints, mortars, pavement stones, and other concrete-based materials. In addition, since photocatalytic materials can be used in both indoor and outdoor applications, it is essential to know the environmental and weather conditions to which the catalyst will be subjected to determine its correct formulation and thus define the best application in real conditions. In this scenario, it is complicated to carry out a comparison study of NOx photocatalytic activity from literature papers, because even if some of the standard tests have been used, many other different operating parameters have been changed in any case. Maybe the lack of consensus is due to the fact there are not yet extended standard tests that include the main operating conditions close to real conditions in air urban environments. For example, analysis of the effect of the presence of different pollutants at the

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same time in the presence of relative humidity percentages, or with or without particles in the inlet gases, should be crucial to know the real durability or end-of-life of the studied photocatalysts before defining a real application. So, although there are a great variety of experimental materials for NOx photo-abatement developed in several laboratories, they are still not currently used in real word applications. Nevertheless a brief summary of the last developments of photocatalytic urban structures used in outdoor conditions is presented following in this section. The potential of a novel photocatalytic technology has been studied dealing with the immobilization of TiO2 on the surface of asphalt pavement, where two bituminous emulsions and a cement mortar were applied on a highway section in Italy (Fig. 23.5). NO photodegradation studied along different months and one year and a half showed interesting results, in particular the bituminous emulsion-based products proved to have a good effectiveness in depolluting air, even if a decay of performance was noted, depending on traffic and weather conditions (Bocci et al., 2016). Long-term efficiencies of photocatalytic materials in real environments have been comparatively studied (Cros et al., 2015a) in the laboratory prior to exposure and then placed at field sites in Austin and Houston, Texas (USA). The effects of weathering and traffic exposure on removal of nitrogen oxides by three commercially available photocatalytic coatings (a stucco, a white paint, and a clear paint) applied to roadside concrete structures for a 20-month period were analyzed. The stucco coating was the most effective for NOx removal, but the efficacy even so diminished over time. The long term use on road side structures could also be hindered by the deactivation of the catalyst through adsorption of non-water-soluble pollutants and requires specific cleaning methods to overcome this issue.

Figure 23.5 Location of trial action on a highway section in Italy (Bocci et al., 2016).

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The effect of environmental parameters that mostly affected NOx removal efficiency of photocatalytic coatings in hot and humid climate conditions was analyzed on concrete samples coated with a commercially available photocatalytic coating (a stucco) and an uncoated sample. Samples were tested simulating reasonable summertime outdoor sunlight, relative humidity, and temperature conditions in southeast Texas. It was found that contact time, relative humidity, and temperature significantly influenced both NO and NO2 removal (Cros et al., 2015b). K. Dewi et al. have concluded that the development of TiO2-coated paving blocks is a promising green technology for reduction of ambient NOx concentrations. Approximately 200 g of TiO2 per 1 m2 square paving block (Fig. 23.6) was capable of changing ambient NOx to nitrate at an average rate of 0.0046 mg/m2 min, with an average temperature 29.89 C, humidity 45.31%, and wind speed 0.84 m/s (Dewi et al., 2016). A. Folli et al. have reported a field study of air purifying paving elements containing TiO2 in Copenhagen (Denmark) to remove nitrogen oxides pollution (Folli et al., 2015). The close relationship between performance data and solar UV irradiance revealed that adequate NOx conversions (monthly average) were arisen for total UV irradiance (600 kJ/m2 day). Under ideal weather and irradiation conditions, i.e., summer months, data relative to the solar noon showed instantaneous NO abatement in some cases higher than 45%, corresponding to a total NOx abatement higher than 30%. A real application for photocatalytic degradation of nitrogen oxides (NOx) was analyzed in the Leopold II tunnel in Brussels, Belgium (Fig. 23.7), using photocatalytic cementitious coating materials and an artificial UV lighting system (Gallus et al., 2015) during the course of the European Life project: PhotoPAQ, although no significant reduction of NOx was observed. From laboratory experiments a serious deactivation of the photocatalytic material under the heavily polluted tunnel conditions was showed. In addition, the UVA irradiance (0.6 and 1.6 W/m) was below the targeted values (above 4 W/m), which

Figure 23.6 Placement of paving blocks coated with TiO2 on Juanda Street in Bandung City (Dewi et al., 2016).

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Monitored parameters on both sites: Site 1 (upwind) Wind direction and wind speed Sampling lines

- Gases: NO/NO2, ozone, formaldehyde, carbonyts, HONO, VOC’s, CO2, CO,etc.;

Site 2 (downwind) Sampling lines

- Particles: Size,composition, (aerosol mass spectrometry (AMS), impaction); - Physical parameters: Wind speed, wind direction, temperature, car counting, etc.

Wind direction and wind speed

Photocatalytic coating (walls/ceiling) UV lamps Leopold ll tunnel

Technical room

70 m (september 2011), 160 m (january 2013)

Figure 23.7 Experimental set-up in the Leopold II tunnel in Brussels, Belgium (Gallus et al., 2015).

contributed to deactivation phenomena and further reduced the photocatalytic activity. At the same time, the typical wind speed (w3 m/s during daytime) and the cold and humid (70%e90% RH) conditions during the campaign in January 2013 also resulted in a reduction of the activity of the photocatalytic material. More recently, M. Halilovic et al. have addressed a potential solution for improving the air quality by removing NOx emitted from the cars in the particular area of urban core of Marijin Dvor in Sarajevo using an innovative photocatalytic architecture approach, with a total area covered by TiO2 of 15,778 m2, reducing the amount of NOx emitted by cars per year by 52.39% (Halilovic and Alibegovic, 2017). Therefore, this proposed application of titanium dioxide photocatalysis to construction materials was very effective and suggests that this study can contribute to further analyses and lead to possible implementation in the future. The use of TiO2 in combination with construction materials has shown a favorable synergistic effect in the reduction of nitrogen oxides in urban areas. Actually, the capability of photocatalysis to reduce the levels of urban pollutants such as NOx has been demonstrated at the laboratory scale but some advances have already been made in real applications of urban environments. As research working continues to get these materials or developing new ones better, the use of the technology will advance. So, the potential for widespread use of photocatalysts in construction materials looks promising as a new way to reduce air pollutants and improve the quality of urban air.

23.5.2 VOCs abatement Nowadays, the idea of more eco-compatible use of light with photoactive materials is becoming to be developed for cleaner environment and a better quality of life. This

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methodology is beginning to be an important strategy to reduce indoor air pollutants through the combination of photocatalysts with construction materials. This technology has demonstrated a high removal rate toward numerous pollutants, such as hydrocarbons, chlorinated hydrocarbons, volatile organic compounds (VOCs), sulfur dioxide, carbon monoxide, carbon dioxide, nitrogen oxides, and other compounds (Nath et al., 2016). The quality of indoor air can be deteriorated by several pollutants, such as microbial contaminants, volatile and semivolatile organics, particulate matter, and other sources that can affect the comfort level of air and induce health hazards. VOCs are not only hazardous compounds, but also participate in undesirable mechanisms with harmful by-products, including organic compounds, ozone, and secondary organic aerosols (Shayegan et al., 2018). Their photocatalytic oxidation frequently employs nanosemiconductor catalysts and ultraviolet (UV) light to totally transform organic compounds in indoor air into water vapor (H2O) and carbon dioxide (CO2) (Dianatdar and Jamshidi, 2017). The efficiency of this technology depends on numerous environmental factors, including the size of the surface exposed, the concentration and types of pollutants, the relative humidity, UV light intensity, air velocity, the residence time, etc., as it has been analyzed in the previous section of NOx abatement. However, many environmental factors can affect the photoefficiency, and for that it often varies by photocatalytic material, thus making comparisons difficult. One of the most practical applications of this technology in the last two decades consists in the incorporation of a nanocatalyst into building materials, such as paints, concretes, or cementitious materials or as coatings in order to remediate indoor environments. Monteiro et al. incorporated 9% of nano-TiO2 into exterior water-based vinyl paint to photoreduce n-decane (Monteiro et al., 2014). Depending on the experimental conditions, up to 90% conversion was reported. Xiao et al. used acrylic-silicon films with 1% of nano-TiO2 and investigated conversion of formaldehyde and NO2 (Xiao et al., 2013). An effective method to obtain high photocatalytic efficiency with multiple photocatalytic functions has been studied by analyzing the strategy of directly applying a TiO2 containing paint on the surface of self-compacting architectural mortars (SCAM). Best performance was given by TiO2 coated SCAM sample displaying high photocatalytic NOx and xylene removal ability under UV-A and sunlight irradiation conditions (Guo et al., 2015b). A novel photocatalytic cement based material was reported (Peng, 2017) in the photocatalytic degradation of formaldehyde and benzene. A 15 wt% TiO2/cement presented the highest degradation efficiency and capability, where formaldehyde and benzene can be photodegraded within 4 and 9 h, respectively. Besides, inorganic ions can induce TiO2 agglomeration. The photocatalytic cement based materials were fabricated and the degradation efficiency of formaldehyde was measured on a building roof under sunlight illumination. New data on intensive weathering effects on TiO2 coated cementitious materials were studied by A. Maury-Ramirez et al. Autoclaved aerated concrete was coated by TiO2 nanoparticles through a dip-coating (DC) and a novel vacuum saturation

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(VS) method to investigate the weathering resistance and gaseous toluene removal potential of both coating types. Both types of coatings show high air purification potential toward toluene removal, with removal efficiencies higher than 95% and photodegradation rates up to 75 mg/m2 h. Accelerated and intensive weathering, simulating a period of 25 years at central Europe weather conditions, clearly affected the physical characteristics of the coatings, observing a significant decrease in both the TiO2 mass and the coating thickness by 93%e99% and more than 98%, respectively. Nevertheless, and quite surprisingly, their photocatalytic activity toward toluene removal was maintained (Maury-Ramirez et al., 2012). From the studies carried out to date, it can be postulated that photocatalytic construction materials are beginning to represent a new frontier in air quality improvement related to VOCs removal because photocatalysis can accelerate the natural oxidation process, thereby promoting faster decomposition of pollutants, preventing them from accumulating, and favoring their removal (Nath et al., 2016).

23.6

Challenges and future perspectives

Most of the studies present photocatalytic coatings based on TiO2, ZnO or similar semiconductors, modified with metal-doping (such as iron, copper, nickel, or vanadium) or nonmetals (nitrogen, sulfur, phosphorus, carbon, or boron), composites (with carbonaceous materials, graphene or zeolites, among others), metallic nanoparticles deposition (such as silver, gold, or platinum), etc. with the aim to enhance the photocatalyst efficiency by widening the range of usable wavelength, increasing the incident radiation harvesting, favoring the absorption, and increasing the interaction between pollutants and the photocatalytic surface. Another research target is focused on the durability of photocatalytic activity. Outdoor surfaces become fouled with dust, grease, and dirt that impede pollutants adsorption on photocatalyst surfaces and subsequent degradation. The easier way to recover the surface photoactive functionality and increase the lifetime would be the possibility of feasible washing procedure. Considering that the photocatalytic process occurs on the accessible surface of the material, different routes to incorporate the photocatalyst (physical mixtures with substrate, percolation of photocatalytic emulsions, or application on thin surface layer: spraying, dip-coating, spin-coating, PVD-coating, CVD-coating, etc. photocatalytic sols) in order to obtain homogeneous, durable and photoefficient catalytic surfaces. The multitude of different types of supports used (roads, pavements, tiles, paints, roofing, fabrics, polymers, glass, metals, conditioning air filter and ducts, etc.) indicates that the best solution is not unique and may depend on application. Finally, the development of theoretical models allows to predict the distribution of pollutants in urban air according to the urban architecture of buildings and their height, dimensions of streets and sidewalks; density of traffic and pedestrians; winds, rainfall, and temperatures regimen, and sources of pollutants emission; in order to analyze, prior to their installation, the ability of the photocatalytic material to reduce the

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concentration of contaminating species in urban environments. The information provided by these models could facilitate the optimization of strategies for urban air quality management in a more realistic manner. Some of the more recent research fields deal with scaling of laboratory results to real applications; study of the different behavior of a photocatalytic material applied on different substrates, its durability, maintenance, weathering, and modeling; in situ studies of depollution efficiency by monitoring quality air using sensors distributed in zones with and without photocatalytic infrastructure. In regions with lower hours of solar light, the studies are focused on artificial radiation. This knowledge has an important application on tunnels; concretely, it would be recommendable the design of tunnel with high surface/volume ratio profiles, and essential the reduction of roughness to diminish fouling on photocatalytic surfaces where natural washing is not possible, use of intense (>10 W/m2) UVA radiation, control of humidity (60%), and low air velocity inside the tunnel (<2 m/s) to increase pollutants residence time. The focus on multifunctional coatings with a combination of depollution, biocide behavior, and self-cleaning properties is increasingly demanded. The application of these coatings on urban infrastructures reduces the maintenance time and cost, presents a highly aesthetic standard, and is associated to elevated quality and high added-value. These multiple properties and growing social conscience have contributed to actualize government rules to foster the implement of photocatalytic infrastructures in future urban projects.

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