An Overview of Photocatalysis Phenomena Applied to Nox Abatement Joana Ângelo, Luísa Andrade, Luís M

Journal of Environmental Management 129 (2013) 522e539

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Journal of Environmental Management

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Review

An overview of photocatalysis phenomena applied to NOx abatement Joana Ângelo, Luísa Andrade, Luís M. Madeira, Adélio Mendes*

LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, rua Dr. Roberto Frias, 4200-465 Porto, Portugal article info abstract

Article history: This review provides a short introduction to photocatalysis technology in terms of the present envi- Received 21 March 2013 ronmental remediation paradigm and, in particular, NOx photoabatement. The fundamentals of photo- Received in revised form electrochemical devices and the photocatalysis phenomena are reviewed, highlighting the main reaction 2 July 2013 mechanisms. The critical historical developments on heterogeneous photocatalysis are brieﬂy discussed, Accepted 1 August 2013 giving particular emphasis to the pioneer works in this ﬁeld. The third part of this work focus mainly on Available online 7 September 2013 NOx removal technology considering topics such as: TiO2 photochemistry; effect of the operating conditions on the photocatalysis process; LangmuireHinshelwood modeling; TiO photocatalytic immobi- Keywords: 2 lization approaches; and their applications. The last section of the paper presents the main conclusions NOx removal technology Photocatalysis and perspectives on the opportunities related to this technology. Titanium dioxide Ó 2013 Elsevier Ltd. All rights reserved. Immobilization Applications

1. Introduction organic matter by microbial and solar action. However, a very significant fraction of emissions of these pollutants results from human Presently, urban environments are facing serious problems activities. Among the anthropogenic sources of NOx emissions, the concerning air pollution, mainly due to exhaust gases from traffic combustion processes in stationary and mobile units are very and burning fuels in industries, leading to a major environmental relevant. The most common nitrogen oxides are nitrogen monoxide issue on a global scale. This has a great impact on human health, (NO) and nitrogen dioxide (NO2). There are three basic mechanisms animal life, plants and climate. There are two main air pollutants leading to the formation of nitrogen oxides during combustion and sources: natural sources, such as volcanic eruption, soil erosion, thus three types of NOx are identified: prompt NOx, fuel NOx and forest fires; and anthropogenic sources, in particular emissions thermal NOx (Charles, 2005; Roy et al., 2009). Prompt NOx is formed resulting from human actions. The latter is actually the most active from hydrocarbon radicalar reactions with N2 species. The corre- cause of air quality degradation. In particular, urban areas present a sponding reactions are summarized by the Fenimore mechanism, as high level of air pollution mainly due to the intensification of in- shown in equations (1)e(3) (Charles, 2005; Mahmoudi et al., 2010). dustrial activity and the increasing number of motor vehicles in circulation (Colls, 2002). CH þ N24HCN þ N (1) Air pollutants can be classified according to their origin as primary or secondary pollutants. Primary pollutants are those that are C þ N24CN þ N (2) emitted directly by the sources to the atmosphere, like carbon monoxide (CO), sulfur dioxide (SO ), nitrogen oxides (NO ) and 2 x þ 4 þ particles (PM10 and PM2.5). Secondary pollutants are formed from N O2 NO O (3) chemical reactions occurring in atmosphere, such as tropospheric Formation of prompt NOx occurs during the rich combustion, ozone (O3), nitric acid (HNO3) and sulfuric acid (H2SO4)(Colls, 2002; where high concentrations of hydrocarbons are present and, Nevers, 2000). Nitrogen oxides will be the focus of this review; they therefore, more likely to form the corresponding radical. Activation are derived naturally from volcanic activity, transported by diffusion energies of these reactions are relatively low, occurring very quickly from high to low atmosphere, and from the decomposition of during combustion. Fuel NOx is formed by oxidation of nitrogen compounds present in the fuel. The fuel contains nitrogen compounds with NeH and CeN bonds. During combustion, the ring * Corresponding author. Tel.: þ351 225081695. structures are broken and species such as HCN, NH3 or NH2 and CN E-mail address: [email protected] (A. Mendes). are formed, which by reaction with O2 originate NOx; fuel NOx only

0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.08.006 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 523 depends on the nitrogen content in the fuel (Charles, 2005; overviewed and related to the NOx photoabatement mechanisms to Mahmoudi et al., 2010). Thermal NOx is formed by high- provide a better understanding of the phenomena behind those temperature (normally above 1600 C) reactions between nitro- photocatalytic systems. Then, titanium dioxide photochemistry will gen and oxygen. The reactions can be summarized by the Zeldovich be discussed since TiO2 is the most used semiconductor with mechanism, as shown in equations (3) and (4) (Charles, 2005; photocatalysis for environmental applications and, in particular, for Mahmoudi et al., 2010). NOx abatement. The effect of the operational conditions on the photocatalytic activity with titanium dioxide systems will be also fl N2 þ O4NO þ N (4) brie y analyzed, as well as experimental details about the most used photoreactor designs. Finally, different methods for TiO2 Thermal NOx formation increases with temperature in the flame immobilization will be described to bring to discussion this topic, zone and thus is strongly dependent on the combustion which strongly limits the photocatalytic process but that is often temperature. forgotten. In a combustion process, the three types of NOx and the radicals involved in their formation may interact with each other. The 2. Fundamentals of photoelectrochemical processes importance of each type of NOx in the total amount of pollutants emitted depends on the combustion temperature, residence time, Photocatalysis is a photoelectrochemical phenomenon that ratio air/fuel and type of fuel used. given its complexity has been interpreted essentially based on NOx pollution is a serious environmental concern. This kind of empirical knowledge. However, the fundamental understanding pollution is responsible for well-known effects that have been gained from the study of photoelectrochemical phenomena in widely discussed as production of tropospheric ozone, acid rains, areas such as photoelectrochemical cells for solar fuels production global warming and human diseases, particularly respiratory and in showed to be of great importance for exploring and improving the the immune systems e Fig. 1. This brings specific regulations to photocatalytic effect. There are various ways of harvesting solar control NOx emissions, established by the US Environmental Pro- radiation to produce useful products: devices converting directly tection Agency (EPA) and by the European Environmental Agency solar energy to electrical energy (e.g. photovoltaic-solar cells) or to (EEA). EPA and EEA established the hourly NOx air concentration chemical energy (e.g. hydrogen, synthesis gas). Those systems are limit at 0.1 ppm and 0.2 ppm, respectively (EEA; EPA). based on photoelectrochemical cells where one or both electrodes Some methods have been developed and implemented in order are photoactives. A photoelectrochemical (PEC) cell is constituted to control NOx emissions, which are divided into primary and by a semiconductor (anode) and a counter electrode (cathode), secondary methods (Chen and Chu, 2011). Primary methods refer immersed in an aqueous electrolyte solution. When exposed to fi to preventing the formation of NOx, while the secondary methods sunlight, the semiconductor absorbs photons with suf cient energy are applied to reducing the already formed NOx. Some prevention to inject electrons from the valence band to its conduction band, methods include the replacement of common fuel by a low-level creating electronehole pairs (Bard et al., 2002; Rajeshwar, 2007). nitrogen fuel, the usage of fuel additives and the fuel pretreat- Semiconductor-based photoelectrochemical cells are regarded as ment. However, combustion control techniques are not enough to an important class of solar energy conversion systems since they ensure the low emissions required by the present legislation and, offer a simple and efficient way of converting light into electricity, furthermore, they cannot be used in all situations. If the combus- into a fuel or simply for photoabatement of pollutants present in air tion waste is heterogeneous, secondary treatment methods are or in water. Solar energy conversion by semiconductors gained required. Selective non-catalytic or selective catalytic reductions special attention from researchers as a very promising technology are examples of processes used to reduce the NOx formed (Chen since the work by Fujishima and Honda (1972). and Chu, 2011; Gómez-García et al., 2005; Skalska et al., 2010). An alternative method to decompose this type of pollutants is 2.1. Semiconductors physics photocatalysis with titanium dioxide (TiO2). Thus, the purpose of this review is to summarize the current approaches of TiO2 pho- 2.1.1. Energy bands tocatalysis applied to NOx abatement according to the following Semiconductors have intermediate electrical conductivities, structure. The basic photoelectrochemistry fundamentals will be between those of conductors and insulators, roughly in the range of 10 2e104 Scm 1. The energy band diagrams of semiconductors are rather complex. Nevertheless, it is possible to simplify the energy band diagram since only the electrons in the highest almost-filled band (valence band e VB) and the lowest almost-empty band (conduction band e CB) dominate the behavior of the semiconductor. This leads to a simplified energy band diagram for semiconductors as shown in Fig. 2. Semiconductors differ from conductors and insulators since they contain an “almost-empty” conduction band and an “almost- full” valence band. This also means that the transport of charge in both bands has to be considered. The energy difference between conduction and valence edges is the bandgap energy of the semiconductor (EBG) e Fig. 2. The semiconductor energy bandgap is usually in the range of 1e4 eV and it is a forbidden energy region or gap in which energy states do not exist e Table 1. When semiconductors are irradiated with photons with energy equal or higher than its bandgap energy, electrons (e ) are excited from the valence to the conduction band, leaving positively charged vacancies in the þ valence band, called holes (h ). Both conduction band electrons

Fig. 1. Problems and sources associated to NOx air pollution. and valence band holes contribute to electrical conductivity. To be 524 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539

Fig. 2. Schematic representation of semiconductor energy band. precise, holes actually do not move. It is a neighboring electron that moves to fill the hole, leaving a hole at the place it has just come from; in this way holes appear to move and behave as if they were actual positively charged particles. There are two types of bandgaps: direct and indirect bandgaps. The bandgap is called direct if the momentum of electrons and holes is the same in both the conduction band and the valence band e Fig. 3(a). In case of indirect transitions an electron cannot shift from the lowest-energy state in the conduction band to the highest-energy state in the valence band without a change in momentum. The needed momentum is obtained when the inject- ing electron absorbs a phonon e Fig. 3(b). One of the simplest methods for probing whether a bandgap is direct or indirect is to measure the absorption spectrum. For a direct transition it is obtained a parabolic energy structure e equation (5): pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a 0 Direct transition : ¼ A hv EBG (5) where a is the light absorption coefficient, A0 is a constant of pro- Fig. 3. Schematic representation of direct (a) and indirect (b) energy band. portionality, h is the Planck constant and n is the frequency. Plotting 2 (ahn) vs. hn should yield a straight line and the EBG can be deter- Dirac distribution function, also called Fermi function, provides the mined from the intercept with the abscissa axis. For an indirect probability of occupancy by an electron at energy level E and in transition the relation between a and hn is given by equation (6): thermal equilibrium (at a constant temperature with no external injection or generation of carriers). The Fermi function is given by : a ¼ 0ð v Þ Indirect transition A h EBG (6) equation (7). A straight line will be obtained by plotting (ahn)1/2 vs. hn 1 (Memming, 2001). f ðEÞ¼ (7) 1 þ exp E EF kBT 2.1.2. Electron distribution in semiconductors The number of electrons, ne , per unit of volume occupying The system is characterized by its temperature, T, and Fermi levels in the conduction band is the integral of the product between energy, EF; kB is the Boltzmann constant. However, for energies well the density-of-states in the conduction band and the FermieDirac above EF, the exponential term of equation (7) is quite higher than [ e distribution over all energies in the conduction band. The Fermie unity and it reduces to the Boltzmann equation (E EF kBT) equation (8). Table 1 ð Þz E EF Examples of semiconductors used for photoelectrochemical process. f E exp (8) kBT Semiconductor Conductivity type Optical band gap energy/eV Since the conduction band energy (EC) is normally well above the Fermi level the corresponding electron density is given by Si n, p 1.11 equation (9). GaAs n, p 1.42 GaP n, p 2.26 InP n, p 1.35 EC EF ne ¼ NC exp (9) CdS n 2.42 kBT CdSe n 1.70 CdTe n, p 1.50 A similar approach is held for holes, according to equation (10). TiO2 (rutile) n 3.00 TiO (anatase) n 3.20 2 ¼ EF EV ZnO n 3.35 nh NV exp (10) kBT J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 525

NC and NV are the density-of-states at the conduction and For an intrinsic semiconductor, the Fermi Level lies at the mid- valence bands, respectively, and they are given by equations (11) point of the bandgap (Fig. 4(a)) and the energy of its Fermi Level

and (12) (Memming, 2001); can be calculated as presented in equation (14) (Luque and " #3 = Hegedus, 2003). 2 2pm*k T N ¼ 2 e B (11) C h2 1 1 NV

EF ¼ ðEVB þ ECBÞþ kT ln (14) " #3 = 2 2 2 NC 2pm* k T ¼ h B NV 2 (12) On the other hand, doping semiconductors means the intro- h2 duction of deliberate impurity atoms in their structure e extrinsic or impurity doped semiconductor. They can be either n-type or where h is the Plank’s constant and m* and m* are the effective h e p-type depending if donors (D) or acceptors (A) states are intro- masses of holes and electrons, respectively. duced, respectively. The relative position of the band edges, as well In an undoped (intrinsic) semiconductor in thermal equilibrium, as the Fermi level in the bulk of the semiconductor, depends on the the number of electrons in the conduction band equals the number doping level. When a semiconductor is doped with donors or of holes in the valence band: n ¼ n ¼ n (n is the intrinsic e h i i acceptor atoms, then the corresponding energy levels are intro- carrier concentration). The number of charge carriers is therefore duced within the called forbidden zone. In case of n-type semi- determined by the properties of the material itself instead of the conductors the Fermi level is below the conduction band e Fig. 4(b) amount of impurities. Considering equations (9) and (10), the e and in case of p-type semiconductors the Fermi level is above the intrinsic carrier concentration is given by equation (13). valence band e Fig. 4(c). The majority charge carriers for p-type and pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffi n-type semiconductors are holes and electrons, respectively. ¼ ¼ EV EC fi ni ne nh NCNV exp If the energy level, ED, introduced by a donor atom is suf ciently 2kBT close to the conduction band, there will be sufficient thermal en- pffiffiffiffiffiffiffiffiffiffiffiffi EBG ergy to allow the extra electron to occupy a state in the conduction ¼ NCNV exp (13) 2kBT band. The donor state will be then positively charged (ionized). On

Fig. 4. Schematic representation of semiconductor energy band e intrinsic (a) and extrinsic semiconductors (b and c). 526 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 the other hand, if an acceptor state is introduced in the lattice, depleted of majority charge carriers e electrons e a depletion layer it will become negatively charged (ionized). Assuming that all is formed in this region. In the case of a p-type semiconductor, the donors and acceptors are completely ionized, in n-type Fermi level is lower than the redox potential of the electrolyte so- materials and in p-type materials one can write respectively lution, so electrons move from the electrolyte solution to the ð Þ= ð Þ= z ¼ EC EF kBT z ¼ EF EV kBT ne nD NC e and nh nA NV e . Thus, semiconductor leading to accumulation of negative charge in the the Fermi energy is given by equation (15) for an n-type semi- space-charge region; this causes downward band bending e conductor and by equation (16) for a p-type semiconductor (Luque Fig. 5(b). As the space-charge region in the semiconductor is and Hegedus, 2003; Würfel, 2008). depleted of majority charge carriers, in this case holes, a depletion layer is also formed. ne Other types of space charges can also be formed in semi- EF ¼ EC þ kBT ln (15) NC conductors: inversion layer, deep depletion layer and accumulation layer. An inversion layer occurs when the number of adsorbed surface charges increases beyond a certain number, the Fermi level ¼ þ NV EF EV kBT ln (16) crosses the middle of the bandgap and the n-type surface region nh becomes a p-type or the p-type surface region becomes n-type. A deep depletion layer is usual for materials with large bandgaps, 2.2. Semiconductoreelectrolyte interface larger than 2 eV, because they have less minority carriers; this means that minority carriers are consumed faster than they are When a semiconductor comes into contact with an electrolyte generated. An accumulation layer is formed to compensate the free containing a redox couple, both with different initial chemical po- majority carriers accumulated near the surface. This layer is formed tentials, charges are transferred between them until equilibrium is when an excess of positive or negative charges are adsorbed at the established. The balance of the chemical potentials occurs by surface of n-type or p-type semiconductors, respectively. transferring charges from the semiconductor to the electrolyte. This When no net excess charge in the interface of a semiconductor is produces a space-charge layer in the semiconductor, also called a observed e Fig. 2 e the bands are flat reflecting the potential of zero depletion layer since the region is depleted of the respective ma- charge. This potential is called the flat band potential Vfb and it is jority charge carriers (electrons in the case of n-type semi- the potential that is necessary to apply to the semiconductor to conductors and holes for p-type). Nevertheless, a charged layer also reduce the band bending to zero. This potential can be determined exists in the electrolyte adjacent to the interface with the solid by several techniques, e.g. measurement of the photocurrent onset electrode e Helmholtz layer. This layer consists of charged ions potential, electrolyte electroreflectance (EER) or MotteSchottky from the electrolyte adsorbed on the solid electrode surface. Fig. 5 analysis. Actually, this last technique is the most used. Motte shows a diagram for a semiconductoreelectrolyte junction (Nozik, Schottky is a graphical analysis of the inverse square of space- = 2 1978). The electrochemical potential of the liquid phase is deter- charge capacitance 1 CSC plotted as a function of the applied po- mined by its redox potential; on the other hand, the electro- tential V, yielding a straight line. The space-charge layer capaci- chemical potential of the semiconductor material is determined by tance is inversely proportional to the width of the depletion layer. the corresponding Fermi level. As the depletion layer approaches zero (no band bending, flat band Considering an n-type semiconductor, the Fermi level is typically potential conditions), the capacitance approaches infinity. Hence, = 2 fl higher than the redox potential of the electrolyte solution, so elec- 1 CSC will be zero at the at band potential; its value is given by the trons move from the semiconductor to the electrolyte solution intercept of the straight line on the potential axis, according to leaving a positive charge in the space-charge region. In the process, equation (17): the Fermi level in the semiconductor moves “down” and the process 1 2 kBT stops when it equals the redox potential of the electrolyte side. This ¼ V Vb (17) 2 3 3 eN A2 q movement is also followed by the conduction band edge, giving rise CSC 0 r D to an upward band bending e Fig. 5(a). As the space-charge region is where 3 is the dielectric constant of the semiconductor, 30 is the permittivity of free space, ND is the donor density (electron donor concentration for an n-type semiconductor and hole acceptor concentration for p-type semiconductor), q is the charge of electron, kB is the Boltzmann constant, T is the absolute temperature and A is the electrode surface. The donor density ND can be calcu- = 2 lated from the slope of 1 CSC vs. applied potential V.

2.3. Photocatalysis

Photocatalysis is one example of a photoelectrochemical process. Actually, all the extensive knowledge obtained during the development of semiconductor photoelectrochemistry during the 1970s and 1980s has greatly biased the development of photocatalysis. For environmental applications, usually TiO2 anatase is used as semiconductor and water present in air humidity is the electrolyte. Titanium dioxide is an n-type semiconductor with 3.2 eV of bandgap energy e Table 1. In this semiconductor, the energy of incident photons have to be greater or equal to the bandgap energy to excite the electrons, corresponding this value Fig. 5. Depletion layer for n-type (a) and p-type (b) semiconductors. mainly to the UV light region. J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 527

Photocatalysis principle is the activation of a semiconductor 2. Holes react with water molecules adsorbed on the semi- with sun or artificial light. When exposed to sunlight, the semiconductor surface, resulting in the formation of hydroxyl conductor absorbs photons with sufficient energy to inject elec- radicals: trons from the valence band to its conduction band, creating electronehole pairs. Those holes have a potential sufficiently pos- þ þ þ / þ þ itive to generate OH radicals from water molecules adsorbed onto SC hVB H2Oads SC HOads H (19) the semiconductor surface, which can oxidize then organic contaminants. The electrons react with oxygen molecules to form the 3. Electrons in the conduction band reduce the adsorbed oxygen superoxide anion, O e Fig. 6. The photocatalytic efficiency de- 2 to superoxide: pends on the competition between the process in which the electron reacts with a chemical species on the semiconductor surface and the electronehole recombination process, which results in heat þ / þ SC eCB O2ads SC O2 (20) or radiation release. One aspect of particular interest in this type of catalytic process is the usage of the sun as the energy source. For a semiconductor to be photochemically active, the potential Actually, the sun is our primary source of clean and abundant en- of the photogenerated valence band hole must be sufficiently ergy, striking 120 PW of radiation onto the surface of Earth positive to generate OH radicals, which can subsequently oxidize (Fujishima et al., 2000; Kuhn et al., 2003; Mahmoodi et al., 2007; the organic pollutant. The redox potential of the photogenerated Markowska-Szczupak et al., 2011; Muneer et al., 2005; Yu et al., conduction band electrons must be sufficiently negative to be able e 2005). Nevertheless, the possibility of occurring electron hole to reduce adsorbed oxygen to superoxide. The surface adsorbed recombination greatly limits the photocatalytic activity and thus water on the photocatalyst particles is essential to photocatalysis fi several efforts are envisaged to allow a more ef cient charge carrier since it provides the electrolyte support needed for the redox re- þ separation (Hager and Bauer, 1999; Teh and Mohamed, 2011). action promoted by the e /h pairs generated by the excited Photocatalysis is governed by the kinetics of charge carriers and semiconductor. redox reactions that take place at the surface of the photocatalyst. Therefore, the understanding of the electronic processes occurring at the semiconductor nanoparticles level, as well as the dynamics of 3. NOx photoabatement charge separation/transport and reactive mechanisms in the different interfaces is of great importance. In the semiconductor As mentioned throughout this work, a promising approach for context, the photocatalysis process is usually interpreted with a decreasing the high levels of nitrogenous oxides NOx in urban en- band model, where at least two reactions may occur simulta- vironments is their ability to be photochemically converted to ni- neously: i) oxidation from photogenerated holes and ii) reduction trates by heterogeneous photocatalytic oxidation. Even though from photogenerated electrons. These processes should occur at earlier works focused mainly on the treatment of waste water, the same rate to maintain the photocatalyst mostly electrically heterogeneous photocatalytic oxidation has received particular neutral. Fig. 6 shows the following sequence of charge transfer attention regarding removal of air pollutants in the last years. TiO2 processes responsible for the photocatalytic process: material has strongly contributed for that due to its excellent photocatalytic properties and, therefore, these characteristics will 1. Photons with energy higher than the bandgap of the semi- be discussed below. The effect of the operational conditions on the conductor (SC) generate electronehole pairs in the conduction photocatalysis process for photodegradation of NOx compounds; and valence bands, respectively: experimental details concerning the main photoreactors used for that purpose; as well as an overview of the LangmuireHinshel- wood model will be also discussed. Besides, it is well known that a hn þ variety of experimental materials for NO photoabatement were SC!e þ h (18) x CB VB already developed in several research laboratories; nevertheless, they still not being currently used in real world applications. This is mainly due to difficulties on finding effective ways of immobilizing the developed photocatalysts and thus the immobilization issues will be also discussed. Some real applications will be presented to illustrate that photocatalysis is indeed a promising way for pollutants degradation.

3.1. TiO2 photochemistry

In 1791, titanium dioxide was isolated by the first time by Rev- erend William Gregor, but the industrial production of this sub- stance only started in the 20th century. Over the years, several studies were developed to improve titanium dioxide properties as a pigment for paints, plastics, paper and synthetic fibers (Carp et al., 2004; Meinhold, 2010). Titanium dioxide exists in three crystalline forms: anatase, rutile and brookite. However, due to the difficulty of producing brookite crystals in laboratory, only anatase and rutile are usually considered. For photocatalysis, anatase has been widely used because it presents higher photocatalytic activity and it is easier to prepare. However, rutile is the most thermodynamically Fig. 6. Mechanism of photocatalysis with main reaction: electronehole pair generation (1), charge separation and migration to surface reaction sites (2a) and to recom- stable crystal structure of titanium dioxide, whereas anatase and bination sites (2b) and surface chemical reaction on active sites (3). brookite are considered meta-stable. In fact, anatase and brookite 528 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 phases convert to rutile upon heating, while the reverse process is the backward reaction. The use of highly porous materials of not possible (Alemany et al., 2000; Bellardita et al., 2011; Birnie nanosized particles and nanostructures increases the surface area et al., 2006; Hassan et al., 2010; Hu et al., 2003; Teh and available to promote the forward reaction and the adsorption of Mohamed, 2011; Zhang et al., 2011). reactants and indeed facilitate transfer of reactants to the active As mentioned before, when TiO2 is subjected to irradiation with sites (Alemany et al., 2000). photons of energy higher than its bandgap (3.2 eV for anatase and In summary, there are three main approaches to enhance the 3.0 eV for rutile e Table 1), the generated electronehole pairs can TiO2 photocatalytic activity: i) bandgap tuning to visible light har- induce the formation of reactive oxygen and hydroxyl species vesting; ii) minimizing electron/hole recombination; and iii) pro- during the photocatalytic process. Indeed, titanium dioxide is a mote the forward reaction at the surface of the particles and the light-sensitive oxide which absorbs electromagnetic radiation in adsorption of reactants. the ultraviolet region (Markowska-Szczupak et al., 2011; Teh and There are already a number of commercial photocatalytic TiO2 Mohamed, 2011). From the available semiconductors that can be products available on the market. The most used commercial used as photocatalysts (Table 1), TiO2 is generally considered to be photocatalyst is P25, from Evonik (Germany), composed by anatase the best material (Águia, 2011; Fujishima et al., 2000; Teh and and rutile (ca. 75:25 wt.:wt.). This semiconductor is used as a Mohamed, 2011). In fact, titanium dioxide is close to being an standard reference in several photocatalytic research works ideal photocatalyst, exhibiting almost all of the required properties (Abramovic et al., 2011; Águia et al., 2011; Hussain et al., 2010; for an efficient photocatalytic process, only with the important Lydakis-Simantiris et al., 2010; Markowska-Szczupak et al., 2011). drawback of not absorbing visible light. Furthermore, TiO2 is non- toxic, thermally stable, chemically inert, photostable (i.e. not prone to photocorrosion), readily available and relatively cheap. It 3.2. Photocatalytic performance shows band edges well positioned, exhibiting strong oxidizing power at ambient temperature and pressure (3.0 V) and the pho- Photo-oxidation of NO forms nitrite (NO2 ), nitrate (NO3 ) and togenerated electrons are able to reduce oxygen to superoxide nitrogen dioxide (NO2). Since NO2 is even more harmful than NO to (0.2 V). human health, the desired reaction products are the ionic species. Going back to Fig. 6, there are three key processes for an efficient In this way, two parameters are used to characterize the photocatalyst performance: conversion of NO (equation (21)) and selec- photocatalytic process using TiO2 semiconductor. The first key process (i) is the ability of absorbing photons to generate electrone tivity for the formation of ionic species (equation (22)). hole pairs. This is directly determined by the bandgap of the ! Cin Cout semiconductor since the energy of the incident light has to be X ¼ NO NO 100 (21) NO in greater than the bandgap energy. Since TiO2 anatase form presents CNO a bandgap of 3.2 eV this constitutes a primary reason for the pre- fi ! sent low solar photoconversion ef ciency; TiO2 with its bandgap of Cout 3.2 eV absorbs mainly UV light at wavelengths lower than 386 nm, S ¼ 1 NO2 100 (22) in out which actually covers only about 5% of the solar spectrum reaching CNO CNO Earth. Thus, the development of a photocatalyst with a bandgap XNO is the conversion of NO, S is the selectivity for the formation lower than 3.2 eV is of great interest to red-shift light harvesting of ionic species, CNO and CNO stand for the concentration of NO and into de visible range. Photocatalytic activity of titanium dioxide in 2 NO2, respectively, and the superscripts (in and out) refer to the the visible light region is being improved by different techniques, reactor’s inlet and outlet streams. such as: i) semiconductor sensitization; ii) doping and co-doping; or iii) using spatially structured titanium dioxide (Rehman et al., 2009). 3.3. Operating conditions effect The second key process (ii) is the effective charge separation and transport of photogenerated electron and holes. They should travel The photocatalytic activity of titanium dioxide semiconductor is through the nanoparticle to the surface reactive sites where the strongly dependent on several operating and process conditions, oxidation and reduction reactions will occur. However, simulta- namely: i) inlet concentration of the pollutant; ii) air humidity; iii) neously a recombination process may happen, through which light intensity and light spectrum; iv) flow rate/residence time; v) active photogenerated charges are lost. Time resolved spectro- photocatalyst amount per active area; vi) substrates (Hassan et al., scopic studies revealed that electronehole trapping and recombi- 2010). These aspects will be reviewed in the following sub-sections. nation rates are extremely fast, lowering significantly the photocatalytic activity (Hoffmann et al., 1995). At this point, 3.3.1. Inlet concentration nanosize engineering plays a crucial role in controlling the The inlet concentration of NO has an important effect on pho- recombination. Actually, this backward reaction occur mainly in the tocatalytic activity. Numerous authors demonstrated that the boundaries and defects of the particles and thus reducing their size, highest values of degradation rate of pollutants are obtained for the the photogenerated charges have to travel a smaller distance until lowest feed concentrations, independently of the substrates used e they reach the surface reaction sites, making recombination less Fig. 7 illustrates this effect for the case of NO (Chin et al., 2011; probable. Important to mention that this approach may be not Devahasdin et al., 2003; Hüsken et al., 2009; Jeong et al., 2005; straightforward since the decreasing of the nanoparticles size may Martinez et al., 2011; Wang et al., 2007; Yu and Brouwers, 2009). induce additional surface defects. Therefore, it is of utmost Assuming that the photocatalytic reaction is controlled by the importance to develop synthesis routes where the quality of the adsorption of the pollutant, for an inlet concentration high enough, particle and its size are balanced (Yu et al., 2001). saturation of the adsorption sites on titanium dioxide surface oc- The third key process concerns the surface chemical reactions, curs and the photocatalytic degradation rate levels out. Accord- for which engineering surface features (active surface sites) and ingly, Herrmann (2010) and Hüsken et al. (2009) observed that in size (surface area available) is very important. Co-catalysts may the low range of inlet concentrations small changes on this improve the catalytic activity by introducing active sites in the parameter have a deeper effect in the photocatalytic conversion of surface. An appropriate selection of a co-catalyst may also prevent the pollutants than in the high range of inlet concentrations. J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 529

oxidation/reduction steps during the photocatalytic phenomenon and thus increasing the degradation of pollutants (Devahasdin et al., 2003; Hüsken et al., 2009; Melo and Trichês, 2012; Yu and Brouwers, 2009). The electron generation term is described by the BeereLambert law, which relates the absorption of light to the properties of the material through which light is traveling: aðlÞ ¼ h aðlÞ x h fi a l Ge inj I0 e . inj is the injection ef ciency, ( ) is the wavelength-dependent absorption coefficient of the material, I0 is the incident photon flux and L is the semiconductor thickness. Considering a material being irradiated with energy higher than its bandgap, electrons will be excited from the valence band to the conduction band. This absorption process is wavelength- dependent and is described by the absorption coefficient a of the material, which increases with the decrease of the wavelength of light incident on it. Several studies were developed to better understand the light Fig. 7. Effect of inlet NO concentration on NOx degradation rate. Adapted from Hüsken et al. (2009). intensity effect on the photocatalytic phenomenon for different inlet concentrations. Yu and Brouwers (2009) observed a loga- rithmic relation between light intensity and NOx (NO þ NO2) con- 3.3.2. Relative humidity version for low values of inlet concentration (0.5 and 1.0 ppm), in The relative humidity (RH) effect on photocatalytic reaction was the range of light intensities of 0.1 up to 1.3 mW cm 2 e Fig. 9. studied and reported by several authors. Water is essential for Devahasdin et al. (2003) performed the same type of studies but for photocatalysis because it is responsible for the formation of OH and higher inlet concentrations of NO e 5 and 40 ppm e under close 2 O2 radicals, crucial species in the photoabatement; for this reason light intensities, from 0.2 to 0.8 mW cm . Contrary to what was it is expectable that increasing the relative humidity the pollutant observed by Yu and Brouwers, for low inlet concentrations (ca. conversion increases (Wang et al., 2007; Yu and Brouwers, 2009). 5 ppm), the degradation rate does not depend on light intensity, Nevertheless, many authors have shown that the pollutant con- whereas for values of inlet concentration of ca. 40 ppm an increase version increases with relative humidity only up to 40e50% of RH, of 20% in the degradation rate was observed e Fig. 10. It is impor- becoming the conversion constant afterward e Fig. 8 (Chin et al., tant to mention that these discrepancies may be due to the use of 2011; Devahasdin et al., 2003; Jeong et al., 2005); this effect was different systems and thus different photocatalytic behaviors are assigned to limitations on the UV light availability to form hy- observed. Moreover, reactors were also different: in the study by Yu droxyl/superoxide radicals (Chin et al., 2011; Jeong et al., 2008) and and Brouwers a wall paper was used while in the study by Deva- to competition between water and pollutant for the adsorption hasdin et al. a cylindrical photocatalytic reactor was considered. sites on titanium dioxide. Maggos et al. (2007) concluded that The role of light intensity on the degradation rate is still scarcely relative humidity effect on the conversion of pollutants is also understood and should be further studied. influenced by their concentration: if pollutants concentration is at ppb level the competition for adsorption sites is much higher than 3.3.4. Flow rate/residence time if they are at ppm concentrations. This happens because for feed Another parameter that influences the photocatalytic activity is concentrations at ppb level it is needed only a small amount of the flow rate of polluted air, which is directly related to the resi- water to produce enough hydroxyl/superoxide radicals. dence time on the photoreactor. If, during the photocatalytic reaction, a high flow rates are considered, the polluted air will have a 3.3.3. Light intensity and light spectrum small residence times meaning short contact times with the pho- In photocatalytic reactions the light source is responsible for tocatalyst; those short contact times yield smaller pollutant con- providing enough energy to generate electronehole pairs on the versions. Most works that studied the flow rate effect have reported photocatalyst surface, leading to the photocatalytic oxidation of flow rates between 1.0 and 5.0 L min 1 e Fig. 11 (Chin et al., 2011; pollutants. Therefore, a high light intensity is able to produce a high Hüsken et al., 2009; Jeong et al., 2005; Martinez et al., 2011; Melo amount of electronehole pairs that will participate in the

Fig. 9. Effect of light intensity on the NOx (NO þ NO2) conversion (Yu and Brouwers, Fig. 8. Effect of the relative humidity on the NO conversion (Devahasdin et al., 2003). 2009). 530 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539

Fig. 12. Inﬂuence of the mass of catalyst (m) on reaction rate, r (Herrmann, 2010). Fig. 10. Effect of light intensity on the NO conversion (Devahasdin et al., 2003).

immobilized on a glass fiber filter seems to be higher and conse- and Trichês, 2012; Yu and Brouwers, 2009). Devahasdin et al. quently the generation of NO2 is smaller (Ao and Lee, 2003). This (2003) and Wang et al. (2007) describe an increase of the photo- situation may be explained by the improved adsorption capacity of catalytic conversion with the residence time but only up to 12e15 s; the activated carbon. Other comparisons were also performed. A above that value the degradation rate of the pollutant becomes porous building material as mortar was compared with a non- constant. This happens because the reaction equilibrium is reached adsorptive substrate as glass (Martinez et al., 2011). A decrease on (Devahasdin et al., 2003; Wang et al., 2007). conversion of NOx over time for the TiO2 immobilized on glass was observed, while NOx oxidation on a mortar substrate remained 3.3.5. Photocatalyst amount per active area constant (Martinez et al., 2011). This behavior was justified by the The amount of photocatalyst present in the substrate used for adsorption of nitrate ions produced from NO2 oxidation on the photocatalytic processes influences the pollutant degradation rate surface of substrates, occupying the active sites and thus leading to (Herrmann, 2010; Hüsken et al., 2009; Martinez et al., 2011; Toma deactivation. For mortars, ions are also formed but this deactivation et al., 2004b; Yu and Brouwers, 2009). As expected, the pollutant does not happen or it is retarded due to the large adsorption ca- degradation increases with the TiO2 amount (Hüsken et al., 2009; Yu pacity of the substrate (Martinez et al., 2011). and Brouwers, 2009) up to a certain level, which is reached when the e upper TiO2 layers hide the light to reach the lower layers e Fig. 12 3.4. Langmuir Hinshelwood model (Herrmann, 2010; Martinez et al., 2011; Toma et al., 2004b) e optical thickness. The degradation rate increase is related to the larger As mentioned before, photocatalysis is a surface phenomenon ð Þ illuminated catalyst surface area available to promote the reaction. that originates hydroxyl radicals (OH ) and superoxide anions O2 , which are the oxidation mediators of the organic contaminants. 3.3.6. Substrate According to Yu et al. (2010), the mechanism of photocatalytic The photocatalytic activity is greatly influenced by the type of degradation of NOx can be divided in three stages: i) adsorption of substrate and by the technique used for depositing the photo- gas reactants in the photocatalyst surface (equations (23)e(25)); ii) catalytic material. These two elements should be carefully com- generation/recombination of electron/hole pairs (equations (26) bined in order to maximize the photocatalytic activity and its long- and (27)); and iii) oxidation of NO and water reduction (equa- term stability. Literature describes different substrates such as op- tions (28)e(32)). tical fiber, fiberglass, quartz, borosilicate glass and stainless steel. K The substrate shape also can affect performance, depending on if H2O TiO þ H O % H O (23) they are cylinders, tubes, sheets or plate, beads, etc. 2 2 2 ads Several studies comparing different substrates have been per- KO formed. For instance, photocatalytic degradation of NO by immo- þ %2 TiO2 O2 O2ads (24) bilized-TiO2 on an activated carbon filter compared with TiO2

K þ %NO TiO2 NO NOads (25)

n !h þ þ TiO2 eCB hVB (26) þ þ / TiO2 eCB hVB TiO2 (27)

þ þ / þ þ hVB H2Oads HO H (28)

þ / eCB O2;ads O2 (29)

þ kNO NOads OH !HNO2;ads (30)

kHNO Fig. 11. Effect of the volumetric feed ﬂow rate on the NOx conversion rate (Yu and 2 HNO ; þ OH ! NO ; þ H O (31) Brouwers, 2009). 2 ads 2 ads 2 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 531

kNO þ K C C NO þ OH !2 NO þ H (32) ¼ NO TiO2 NO 2;ads 3 CNO;ads þ þ þ (40) 1 KNOCNO KNO2 CNO2 KH2OCH2O

When TiO2 is irradiated by sunlight it absorbs photons with n energy (h ) higher than the bandgap of the semiconductor, gener- KNO2 CTiO2 CNO2 C ; ¼ (41) ating free electrons ðe Þ in the conduction band and free holes NO2 ads þ þ þ CB 1 KNOCNO KNO2 CNO2 KH2OCH2O ð þ Þ e hVB in the valence band equation (26). Then, electrons and holes can react accordingly to equations (28) and (29), originating hy- KH2OCTiO2 CH2O droxyl radicals (OH ) and superoxide anions ðO Þ. However, at the C ; ¼ (42) 2 H2O ads 1 þ K C þ K C þ K C photocatalyst surface, free electrons and free holes instead of NO NO NO2 NO2 H2O H2O generating radicals can recombine (surface recombination) as The photocatalytic process is governed by the kinetics of the shown in equation (27), which strongly limits the photocatalytic different reactions occurring during it usually described by a activity (Demeestere et al., 2004; Valencia et al., 2011). Additionally, LangmuireHinshelwood (LeH) model, which allows formulating another back reaction process can occur: generated hydroxyl rad- the photocatalytic oxidation rate equation. Thus, considering a icals and superoxide anions can be consumed by inactive species generic reaction of the type A þ B / C þ D (assumed irreversible), (Barka et al., 2010). the reaction rate r is given by equation (43) (Herrmann, 2010). The photocatalytic degradation of NO should result in the pro- ¼ q q duction of HNO3; though, two other intermediate products can also r k A B (43) be generated in this process: HNO2 and NO2. HNO2 is totally q q consumed as reaction (31) is very fast; on the other hand, not all A and B are given by equations (44) and (45), respectively: NO is converted into HNO and ends desorbing e equation (33). 2 3 K X q ¼ A A (44) A 1 þ K X KNO A A þ %2 TiO2 NO2 NO2;ads (33) KBXB Finally, to describe the photocatalytic phenomenon reactants qB ¼ (45) 1 þ KBXB adsorption on the active sites of TiO2 has to be also analyzed. The adsorption equilibrium is reached when NO adsorption (r ) and ads where K and K are the adsorption equilibrium constants in the desorption (r ) rates are equal e equation (34) (Yu et al., 2010; Yu A B des dark, X and X are the concentrations in the liquid phase or partial and Brouwers, 2009). A B pressures in the gas phase and k is the reaction rate constant. Ac- cording to Arrhenius’ law, k depends only on temperature e r r ¼ k C ; C k C ; ¼ 0 (34) ads des ads TiO2 free i des i ads equation (46); where i is the species NO, NO2 or H2O; kads and kdes are the Ea adsorption and desorption kinetic constants, respectively; and k ¼ k exp (46) 0 RT is the concentration of free active sites on TiO surface. CTiO2;free 2 Therefore, manipulating equation (34), the concentration of where Ea is the activation energy, T the absolute temperature and R adsorbed species comes as described in equations (35) and (36). is the universal gas constant. In the same way, equilibrium constants KA and KB depend on the C ; ¼ K C ; C (35) i ads i TiO2 free i temperature according to the van’t Hoff’s law (Equation (47)), where DHi is the enthalpy of the reaction: k K ¼ ads (36) i DHi kdes K ¼ðK Þ exp (47) i i 0 RT The total concentration of active sites in the titanium dioxide e surface is given by the sum of the concentration of free active sites Applying now the L H model as described above to the photo- oxidation of NO, the correspondent reaction rate can be written. For and the concentration of occupied sites by NO, NO2 and H2O species e equation (37). that, it is important to consider the reactional mechanism at steady state proposed by Devahasdin and illustrated in Fig. 13; HNO3 and ¼ þ þ þ NO2 species are in equilibrium on the catalyst surface (Devahasdin CTiO CTiO ;free CNO;ads CNO ;ads CH O;ads (37) 2 2 2 2 et al., 2003). Substituting equation (35), applied to the different adsorbed This mechanism considers that the reaction that produces HNO3 species, into equation (37), the concentration of total active sites in occurs very fast; when the catalyst becomes saturated with HNO3 TiO2 becomes: the reaction stops at NO2 production, which starts desorbing according to equation (33). Indeed, the reaction rate of NOx degra- C ¼ C þ K C C þ K C C dation becomes as described by equation (48). TiO2 TiO2;free NO TiO2;free NO NO2 TiO2;free NO2

þ K C ; C (38) 0 0 H2O TiO2 free H2O k C C k C A NO H2O B NO2 rNO ¼ (48) Solving to C ; as follows: 0 2 TiO2 free 1 þ K C þ K C K C H2O H2O NO NO NO2 NO2 C C ¼ TiO2 (39) 0 0 TiO2;free rNO is the reaction rate of NO, k A and k B are the reaction rate 1 þ KNOCNO þ KNO CNO þ KH OCH O 2 2 2 2 constants, K0 ; K0 ; K0 are the adsorption equilibrium con- H2O NO NO2 Finally, the concentration of each adsorbed species can be ob- stants of water, nitrogen oxide and nitrogen dioxide, respectively, ; ; tained combining equation (35), applied to the different adsorbed CH2O CNO CNO2 are concentration of water, nitrogen oxide and species, and equation (39): nitrogen dioxide in the gas phase. 532 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539

used tubular reactors mainly to study the photocatalytic oxidation of organic compounds targeting air purification (Lewandowski and Ollis, 2003; Peral and Ollis, 1992; Tsoukleris et al., 2007). Typical annular reactors have two concentric tubes and pollutant stream flows between them; the light source is placed inside of the inner tube (Fig. 14). This configuration ensures that the

Fig. 13. Reaction scheme for the photocatalytic oxidation of nitric oxide on TiO2. active material absorbs all emitted photons. Several studies have been reporting photocatalytic degradation of NOx using annular flow reactors (Komazaki et al., 1999; Lim et al., 2000; Matsuda and This LangmuireHinshelwood approach has been used by Hatano, 2005). Removal efficiencies of NOx pollutants higher than several authors to describe the photocatalytic oxidation of nitrogen 90% under UV light were reported (Komazaki et al., 1999; Matsuda oxides in different semiconductor materials. For instance, Wang and Hatano, 2005). On the other hand, Lim et al. (2000) compared et al. (2007) modeled the photocatalytic behavior of TiO2 loaded the typical annular photoreactor with a modified two-dimensional on a woven glass fabric and Folli et al. (2011) used TiO2 coated on fluidized-bed photoreactor and concluded that the NO removal is e glass beads. In both studies the L H model was applied and similar higher in this type of reactor. results were obtained for the kinetic constant k and for the Flat plate reactors accommodate the photocatalyst support, also adsorption equilibrium constant in the dark K (Folli et al., 2011; flat and usually made of glass or metal, and the polluted stream Wang et al., 2007). However, other authors report more complex flows parallel and on the top of the photocatalyst support e Fig. 15. e fl studies where the L H model takes also into account the in uence The light source can be placed inside the reactor chamber or fl of other experimental conditions such as: volumetric ow rate, otherwise can be located outside the reactor that then has to have a irradiance, relative humidity, amount of photocatalyst and reactor glass window. TiO2 immobilized on materials such as paints or size (Hunger et al., 2010; Yu et al., 2010). For all these different mortars is usually characterized on flat plate reactors (Águia et al., e cases, the Langmuir Hinshelwood model was applied to describe 2010; Allen et al., 2008; Folli et al., 2012; Maggos et al., 2007; Yu the photocatalytic reaction rate of nitrogen oxide and a good and Brouwers, 2009). agreement was achieved between experimental data and the modeling results (Devahasdin et al., 2003; Folli et al., 2011; Hunger 3.6. Immobilization of TiO et al., 2010; Wang et al., 2007; Yu et al., 2010). 2 As discussed above titanium dioxide exhibits several advantages 3.5. Photocatalytic reactors as a photocatalyst for environmental applications. Usually this material is used in suspension and mainly for water treatment; how- Several photocatalytic reactors can be used for studying air ever, several technical difficulties arise for TiO2 removal, as it brings fi treatment, so it is important to choose the right con guration of the additional costs to the process. Thus, this material can be immobi- fi reactor to improve the photocatalytic ef ciency, allowing compa- lized in specific supports, eliminating the removal extra step that rable and repeatable measurements. Even if the experimental setup brings complexity to the final photocatalysis process. This is espe- includes other elements such as the light source, NOx analyzer or cially important in water and wastewater treatment. Nevertheless, the gas supplier, the core of the test setup is the photoreactor, the photocatalytic activity strongly depends on the type of immo- which is responsible for an effective contact among photocatalyst, bilization and on the supports used, and so this will be discussed pollutants, water and light (Birnie et al., 2006; Paz, 2009, 2010). hereafter (Kaneko and Okura, 2002; Rao et al., 2004). TiO2 semi- Reactors are usually categorized by their mode of operation or conductor is a nanoparticulated powder that may cause some risks by their geometry that determines the reactor design; still they can to the human health when inhaled. Consequently, the immobiliza- be also categorized by the light source used or by the way the tion process may also contribute to diminish this risk (Baan et al., photocatalyst is introduced in the reactor (Paz, 2009, 2010). Ac- 2006). The choice of the right support and methods to immobilize cording to the mode of operation, the photoreactors can be classi- the titanium dioxide has some specific requirements, as follows: fied as batch reactors or continuous flow reactors. Batch reactors are very interesting for lab-scale applications since they are usually a) the photocatalytic activity should not be inhibited during the used for determining the reaction kinetics and for characterizing immobilization process; newly developed photocatalysts (Paz, 2009; Spadavecchia et al., b) there must be good adhesion of the particles to the support; fl 2010). On other hand, continuous ow reactors are usually used c) a support has to be chemically inert; in applications with a continuous source of pollutants. Moreover, d) the support should have a large specific surface area (this is an they are especially useful for industrial applications that handle important factor because the photocatalytic reaction is a sur- with process gases, thus reducing harmful gases emissions into the face reaction); atmosphere. This type of reactors is also used at laboratory scale, e) the support should be compatible with the environmental use. where the initial concentration of pollutants is known and the output concentration is continuously measured to calculate the Materials that satisfy all the above requirements are difficult to fi pollutant removal ef ciency (Águia et al., 2010; Ao et al., 2003; obtain. Up to now, several approaches have been proposed for Martinez et al., 2011; Toma et al., 2006, 2004a). Reactors can be tubular, annular or flat plate, among others. Tubular reactors are quite common since they are easy to assemble and to handle. They can be composed of one e single tubular reactor e or more glass tubes e multitubular reactor e where the photocatalyst is applied under several forms, e.g. a coated film, fluidized particles, free powder on a support, etc. The single or multiple tubes are then placed in the glass reactor vessel and the light sources are located externally to the tubes and parallel to them; the gas flow is fed along the axis of the tubes. Several authors Fig. 14. Annular flow reactor. J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 533

Fig. 15. Schematic representation of the flat plate reactor, in this case, authors used a UV lamp external to the reactor and parallel to sample (top view) (Águia et al., 2011). immobilization on various supports, e.g. solegel, dip coating, during the immobilization process. The dip-coating process needs a physical and chemical vapor deposition, electrophoretic methods, step of heat treatment to assure good adhesion of the material to among others. the support. The heat treatment was made at a temperature close to the melting point of the glass, which induces the ions diffusion 3.6.1. Dip-coating technique from glass to the titanium dioxide layer. This ions transfer was Dip coating is a process to create thin films; the substrates are verified by XPS analysis and sodium was identified on the titanium immersed in the solution of the coating material and a layer is dioxide surface. The transition of ions to the deposited layer led to formed on the substrate. Then, this substrate is removed from the electronehole recombination that conducted to photocatalytic ac- solution and the solvent is allowed to evaporate e Fig. 16. tivity inhibition. Quartz showed stability under heating, proved by Fernández et al. (1995) applied this technique on two supports, XPS analysis. Therefore, this support presents a higher degree of glass and quartz, which were immersed in a solution of titanium purity leading to better results of photocatalytic activity. Thereby, tetra-isopropoxide up to three times to increase the films thickness. these authors used this support to immobilize titanium dioxide and At the end, samples were calcined to eliminate solvent and impu- assess the photocatalytic activity of these films on another study rities. These supports were characterized by several techniques and (Herrmann et al., 1997). their photocatalytic ability tested and compared. The photo- More recently, Kwon et al. (2003) investigated the deposition of catalytic activity was analyzed based on the degradation of malic TiO2 and TiO2eSiO2 on glass by dip coating. Similarly to Fernández acid. For this reaction, quartz was the better support because et al. (1995), a high-temperature step was also considered to pro- showed higher photocatalytic activity comparatively with glass: mote a good adhesion of titania to the support. However, these the degradation of malic acid occurred in 1 h in the quartz support authors also added silica to titanium dioxide to improve this and in case of glass this reaction took 3 h, under the employed adhesion; they observed that a molar ratio of 1:1 presented the best conditions. These two supports were chosen by their characteris- compromise between adhesion and photocatalytic activity. The tics; they are chemically and photocatalytically inert. The differ- photocatalytic activity of these materials was measured based on ences observed in the photocatalytic activity were associated to the the degradation of methylene blue. interaction between titanium dioxide particles and the support 3.6.2. Solegel technique Solegel is a method that involves a colloidal suspension, sol, that after gelation and by means of precursors allows the formation of a gel (Lim et al., 2009). Different precursors can be used, e.g. titanium alkoxide, titanium tetrachloride or titanium halogenide. The solegel method e Fig. 17 e includes as a final step spread coating or dip coating but the latter offers the possibility to coat both surfaces of the substrate (Shan et al., 2010). Lim et al. (2009) studied solegel with dip-coating over fiberglass, woven fiberglass, aluminum plate and glass slide. The efficiency of titanium dioxide immobilization to the support and the photocatalytic

Fig. 16. Schematic diagram of the dip-coating process. Fig. 17. Schematic diagram of solegel method associated to dip-coating. 534 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 activity were evaluated for all supports; the photoactivity was better performance. However, the film obtained by CVD presents assessed based on the degradation of methylene blue. Woven optical clarity and produces a more robust film. These two advan- fiberglass and fiberglass showed the best results in terms of tages can make viable the commercialization of films prepared by immobilization, while glass slide showed the lowest photocatalytic CVD. This technique can also be used to deposit titanium dioxide on activity. As a support material, fiberglass has the advantage of other substrates as window glasses; TiO2-coated windows are self- providing a larger surface area; on the other hand, glass slides cleaning and fog proof (Byun et al., 2000). present a flat surface and so the adhesion of titanium dioxide to the support did not present the expected results. The aluminum plate 3.6.4. PVD technique was not a good substrate because it has also a flat surface and ex- Physical vapor deposition (PVD) technique, also known as hibits a high thermal expansion that leads to cracking problems on sputtering, allows to form thin films wherein atoms or molecules deposition. are ejected from a target material to the substrate e Fig. 19 (Rachel Páez and Matousek (2004) also studied the deposition of TiO2 et al., 2002b; W. Zhang et al., 2004; C. Zhang et al., 2009). Among layers on a glass support by solegel with dip-coating technique. the techniques for coating thin films onto supports, PVD is one of Contrary to what was stated before, these authors reported a good the most versatile and environmental-friendly. In particular, a adhesion to a soda-lime glass of the titania layer. This good adhe- magnetron sputtering system consists of a vacuum chamber con- sion may be related to the use of different precursors for the taining a target of the material to be deposited and a substrate. This preparation of titanium dioxide and to the thermal treatments method allows precise control of film thickness, grain size and layer performed between layers deposition. Chen and Dionysiou (2006) composition and microstructure. Zhang et al. (2004) used PVD to reported a study of titanium dioxide deposition on stainless steel deposit TiO2 thin films on microscope glass and fused quartz slides, by solegel technique and the adhesion of this titanium dioxide using a precursor of titania with a very high purity (99.9%). The layer to the support was assessed. These authors incorporated decomposition of aqueous methyl orange was determined in this different amounts of titanium dioxide (P25, Evonik) to prepare the work to evaluate the photocatalytic activity of the films produced; solegel and concluded that to have a good adhesion of titania to the the authors concluded that the photocatalytic activity increases stainless steel this amount has to be less than 50 g L 1. with thickness from 300 to 900 nm. However, the increase of thicknesses above a certain limit did not have any significant effect 3.6.3. CVD technique on the photocatalytic activity; this saturation occurs because films Another technique used to immobilize titanium dioxide is the exhibit more opacity, so it is more difficult the light to pass. chemical vapor deposition (CVD). CVD uses a reactor, a source of gas Zhang et al. (2009) also studied the PVD deposition of titanium and a precursor e Fig. 18. For TiO2 deposition, organic, inorganic or dioxide on glass substrates. They studied the influence of the vac- mixed precursors can be used, such as titanium tetra-isopropoxide, uum pressure of PVD chamber on the titania film photocatalytic titanium tetrachloride or titanium tetra-isopropoxide with tita- activity toward methyl orange degradation. In this study, these nium tetrachloride, respectively (Byun et al., 2000; Ding et al., authors concluded that deposition of titania at high pressure 2001). Ding et al. (2001) described TiO2 immobilization by CVD (1.4 Pa) exhibits higher photocatalytic activity. The film obtained at on porous solids, e.g. carbon, g-alumina and silica gel as supports, this higher pressure allowed to completely degrade the methyl and titanium tetra-isopropoxide as precursor. Deposition of TiO2 on orange after 5 h of UV irradiation (W. Zhang et al., 2004; C. Zhang silica gel produced the best immobilized system, demonstrated by et al., 2009). the highest level of photocatalytic degradation of phenol in water. The authors justified this observation since silica gel has more 3.6.5. Electrophoretic technique surface hydroxyl groups and higher macropore surface area. In fact, Electrophoretic technique is used for deposition of particles on high surface area is crucial to get high photocatalytic activity. metallic substrates. Powder material is dispersed in a solvent Another important characteristic is the anatase particles size that where two electrodes are immersed, the metallic substrate and the should be small; this allows to a better distribution of titanium counter electrode, both arranged in parallel. Then, an electric po- dioxide on supports with more active sites for the photocatalytic tential is applied to the system and particles move toward the reaction. metallic substrate to make the film e Fig. 20. Fernández et al. (1995) Mills et al. (2002) compared a thin film of TiO2 deposited by CVD used electrophoretic technique to deposit titania on stainless steel technique on a glass support with another thin film of P25 particles and they compared the correspondent photocatalytic activity with immobilized by spin-coating. These authors concluded that both other films prepared by dip coating and supported on glass and types of films have photocatalytic activity, but P25 film presents a

Fig. 18. Schematic diagram of chemical vapor deposition method. Adapted from Herrmann (2010). Fig. 19. Schematic diagram of physical vapor deposition. J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 535

results on degradation of NO (concentration between 0.05 and 10 ppm) from air, however, paint films and cement plates also demonstrated ability to efficiently abate this pollutant. Paint coatings, in opposition to the previous deposition methods, allow creating a 3D layer where TiO2 nanoparticles are available for photocatalysis up to the optic thickness, which is around 100 mm. A photocatalytic paint coat has a very large inter- facial area illuminated and available for photocatalysis, originating then very photoactive surfaces. Águia et al. (2011) studied the immobilization of different commercial titanium dioxide samples on a water-based vinyl paint. Photocatalytic titanium dioxide from different manufactures were tested, e.g. from Kemira, Millenium, Evonik, Sachtleben, Kronos and Tayca. It was measured the conversion of NO for all the paints. TiO2 from Millenium, Sachtleben and Tayca showed the highest values of conversion, about 40e50%. On the other hand, TiO2 from Kemira showed the lowest level of conversion, about 15%. Maggos et al. (2007) studied two commercial paints containing TiO2 to degrade NO and NO2. In this study mineral silicate paint and a water-based styrene acrylic paint were used, allowing to obtain very efficient TiO -immobilized photocatalysts: in the case of the Fig. 20. Schematic diagram of electrophoretic deposition. 2 silicate paint it was observed a photodegradation of 74% for NO and 27% for NO2; for acrylic paint these values are significantly higher, quartz. These authors concluded that the films deposited on 91% for NO and 71% for NO2, under the tested conditions. stainless steel and glass showed lower photocatalytic activity and adhesion problems when compared to films deposited on quartz. In 3.7. Applications of TiO2 photocatalyst the deposition process the adhesion was prompted by heating the samples, and some cations migrated from the support to the pho- As described before, when the photocatalyst is irradiated in tocatalytic layer. The presence of these cations in the lattice of presence of water hydroxyl radicals are formed and the photo- titania enable the electronehole recombination which inhibits the catalytic degradation occurs. Actually, in recent years interest has photocatalytic activity of the titania. Byrne et al. (1998) studied the been focused on the use of semiconductor materials as photo- deposition of titanium dioxide on stainless steel substrates, on ti- catalysts for the removal of organic and inorganic species from tanium alloys substrates, on tin oxide coated glass substrates and aqueous phase. This approach has been suggested for environ- on titanium metal substrates and the correspondent photocatalytic mental protection due to TiO2 ability to oxidize organic and inor- activity was evaluated by degradation of phenol. Among all sup- ganic substrates, leading in some cases to the total mineralization ports, stainless steel and titanium alloys seem to present the best of these water pollutants. Photodegradation in water presents performance on phenol degradation. It was expected tin oxide lower efficiency than photodegradation in air because it faces some coated glass to show good rate of degradation because it has the limitations such as: a) in water the pollutant concentration is advantage of being satisfactorily transparent to UV-A light giving higher; b) pollutants have less contact with the photocatalyst; c) thus the possibility of irradiation above and below the coated when used dispersed, recovery of the semiconductor from water surface. However, glass supports are breakable and expensive. The causes engineering difficulties in automatic operation (Hashimoto use of cheaper materials as stainless steel is envisaged in order to et al., 2005; Kaneko and Okura, 2002; Rao et al., 2004). turn TiO2 immobilization a cost-effective process; actually elec- For photoabatement of atmospheric pollutants, like in air puri- trophoretic technique allows to deposit titanium dioxide on fication and deodorizing, TiO2 is used to treat air outdoor (under cheapest supports unlike other techniques as for instance solegel. sunlight illumination) and air streams in reactors illuminated with A support with a large surface area is one of the main re- UV lamps. The former approach is used for odors elimination, such quirements mentioned above for an efficient immobilization pro- as cigarette smoke and for degradation of air pollutants, such as cess. Besides glass, quartz and stainless steel, other types of nitrogen oxides (Hashimoto et al., 2005; Kaneko and Okura, 2002; supports should be used to enhance this property. Therefore, Rao et al., 2004). Rachel et al. (2002b) analyzed the viability of using brick, cement Until the present some photocatalytic commercial products and ceramic tiles as substrates by PVD. However, the photocatalytic were already developed and tested. Examples are hereafter activity exhibited by immobilizing TiO2 on these substrates was described. In the 2000 year, Masakazu (2000) described applica- very low. The immobilization of titanium dioxide on pumice stone tions of TiO2 photocatalyst in green chemistry. Table 2 presents by dip-coating technique was also investigated by Rachel et al. some commercial products, made available recently, and the (2002a). Pumice stone was immersed in a sonicated slurry of TiO2 correspondent producers. to impregnate this semiconductor on the substrate and it was In 2007 was estimated that in Japan more than 50 000 m2 of observed that calcination steps between layers deposition allow to different surfaces, e.g. houses, monuments, buildings, roads, etc., obtain an improved deposition (Fernández et al., 1995; Rachel et al., have been coated with photocatalytic titanium dioxide (Osburn, 2002b). 2008). Kaneko and Okura (Kaneko and Okura, 2002) described a In what NOx photoabatement is concerned, Kaneko and Okura case study of a highway in Tokyo with photocatalytic panels to (2002) described three supports with a porous structure to pro- reduce the level of pollutants in that area. The photocatalytic panels vide a large surface area for TiO2 immobilization: i) PTFE-based are made of a polyvinyl chloride resin and placed alongside a six Ò sheets (fluorocarbon polymer); ii) hardened cement past; and iii) lane highway. These panels had a Pyrex glass window that allows an alkoxysilane-based paint. The titanium dioxide was added to the solar UV light to pass and polluted air was pumped between the supports by mechanical mixture. PTFE sheets exhibited the best panel and glass. Nitrogen oxides in air were in consequence 536 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539

Table 2 urban smog. Consequently, the performance of these products was Some producers of photocatalytic materials. characterized in terms of self-cleaning properties and ability to Product Companies decompose pollutants. One of the most famous buildings protected with this material is the Dives in Misericordia Church in Rome e Air cleaner containing TiO2 photocatalysts Sharp Co. Ltd; Daikin Ind. Ltd; Fig. 22. This church was designed by the architect Richard Meier Toyota Home Ltd and it was concluded in 2003 (Cassar, 2004; Italcementi, 2009; White paper containing TiO2 photocatalysts Mitsubishi Paper Mills, Inc. Osburn, 2008; Pacheco-Torgal and Jalali, 2011). Borgo Palazzo fi Anti-bacterial textile bers Kurare, Inc. Street in Bergamo (Italy) was also protected by this material. This containing TiO photocatalysts 2 fi Systems for the purification of Furukawa Kikai-Kinzoku project consisted in the requali cation of 500 m of the mentioned polluted air, e.g., the elimination of NOx Ind., Inc. street with gray paving stones for the road and red ones for the Super-hydrophilic, self-cleaning Toto, Inc. sidewalks, both coated with TX Active. The pollution level was systems, and coating materials for cars measured during two weeks in two different occasions, November Soundproof walls using TiO2 photocatalysts Furukawa Kikai-Kinzoku Ind., Inc. 2006 and January 2007. The results were compared to the asphalt Photocatalyst coated lamps (or lamp covers) Toshiba Light. & Tech., Inc. reference existing in the same street. This new pavement showed a Cement containing TiO2 photocatalysts Taiheiyou Cement, Inc. NOx pollution decrease between 30% and 40% (Italcementi, 2009). Self-cleaning tent Taiyo Ind., Inc. Another well-known application of TX Active is on Umberto I Tunnel in Rome (Italy), outfitted with an artificial UV lighting system in order to enhance the photocatalytic activity. The pollution oxidized to nitrates. The panels showed a fraction of pollutants level was measured before and after this rehabilitee and yielded a e e removal between 31 and 69% of NOx;67 79% of SO2 and 17 20% of NOx reduction of about 25% (Italcementi, 2009). In France, the ef- nonmethane hydrocarbons (NMHC). Photocatalytic titanium diox- ficiency of degradation of NOx by TiO2-mortar panels with TX ide products used on outdoor surfaces to air treatment are usually Active was also tested. Three artificial canyon streets were con- also used for self-cleaning purposes. The self-cleaning effect con- structed and the level of NOx pollution was monitored. Differences cerns preventing the accumulation of dirt in the surfaces, keeping on the NOx concentration level were observed between the TiO2- surfaces cleaned. However, the products can only be optimized for protected area and the reference canyon. NOx values in TiO2 canyon one of these applications, since the best materials for one appli- were 37e82% lower than the ones observed for the reference cation may not be the best material for the other. Therefore, TOTO (Maggos et al., 2008). Ò Inc. in Japan is producing another interesting photocatalytic prod- Boysen KNoxOUTÔ is a photocatalytic commercial paint used uct, the TiO2-based self-cleaning tiles (Fig. 21), already applied on for NOx photoabatement, able to convert pollutants and harmless several buildings in Japan. Buildings covered with the common compounds. This paint was developed for interior and exterior type of tiles have to be cleaned every 5 years, while with the new applications; for interior an acrylic-based paint is used, while for technology it is predicted that the tiles could stay clean for more exterior is a silicone-based one. These paints also present self- than 20 years. Besides the self-cleaning characteristic, these tiles cleaning, anti-bacterial and de-odorizing properties. To test the can also reduce air pollutants, such as NOx (Fujishima and Zhang, efficiency of this new product, the Vinci Car Park in Paris was coated 2006). with this paint and an impressive 90% reduction of NO levels was Ò x The Italian group Italcementi developed the TX Active (Cassar, observed. Due to the great first results obtained with this product, 2004; Italcementi, 2009; Pacheco-Torgal and Jalali, 2011), a prod- other studies are ongoing as project EDSA e “Everyone Deserves Ò uct with photocatalytic properties to apply on different substrates, Safe Air” (Boysen; EDSA). STOphotosan NOx is another photo- such as mortars, paints or pavements. TX Active gives self-cleaning catalytic paint for nitrogen oxides degradation, composed by VLP properties to materials but it has also showed to be able to oxidize 7000 from Kronos as photocatalytic TiO2 and an organic binder nitrogen oxides (NOx), sulfur oxides (SOx), volatile organic com- (Laufs et al., 2010). In Portugal, paint company CIN is also devel- fi pounds (VOCs), ammonia, carbon monoxide, hydrogen sul de and oping photocatalytic paints for NOx photoabatement, self-cleaning and inactivation of microorganisms e project NOxOUT. At the moment, the developed coatings showed very promising results.

Fig. 21. Comparison of self-cleaning effect on treated tiles (A) and common tiles (B); Ó this ﬁgure was published in Fujishima and Zhang (2006). Copyright 2005 Académie Fig. 22. Dives in Misericordia Church in Rome designed by Richard Meier. Adapted des sciences. Published by Elsevier Masson SAS. All rights reserved. from Italcementi. J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539 537

4. Conclusions “Programa Operacional Factor de Competitividade”. The authors also acknowledge financing from FCT through the project PTDC/ The main fundamental aspects and technological issues of EQU-EQU/115614/2009. Joana Ângelo is grateful to the Portuguese photocatalysis as a green technology for NOx abatement were Foundation for Science and Technology (FCT) for her Ph.D Grant reviewed. The increasing levels of air pollution in the last decades (Reference: SFRH/BD/79974/2011). Luísa Andrade also acknowl- and their consequences on human health and well-being are edges FCT for her post-doctoral grant (Reference: SFRH/BPD/74944/ motivating the development of new regulations to control the 2010). pollutants emissions. This air pollution is related to the production of tropospheric ozone, acid rains, global warming and human dis- References eases, particularly of respiratory and immune systems. 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Photocatalytic actors, namely: i) inlet concentration of the pollutant; ii) air hu- titania based surfaces: environmental benefits. Polymer Degradation and Sta- midity; iii) light intensity and light spectrum; iv) flow rate/ bility 93, 1632e1646. residence time; v) photocatalyst concentration per active area; vi) Ao, C.H., Lee, S.C., 2003. Enhancement effect of TiO2 immobilized on activated carbon filter for the photodegradation of pollutants at typical indoor air level. substrates where TiO2 is deposited. A choose of the right configu- Applied Catalysis B: Environmental 44, 191e205. ration of the photocatalytic reactor is also important to improve the Ao, C.H., Lee, S.C., Mak, C.L., Chan, L.Y., 2003. Photodegradation of volatile organic fi photocatalytic efficiency. On other hand, the TiO photocatalytic compounds (VOCs) and NO for indoor air puri cation using TiO2: promotion 2 versus inhibition effect of NO. Applied Catalysis B: Environmental 42, 119e129. activity can be enhanced and three main approaches can be used to Baan, R., Straif, K., Grosse, Y., Secretan, B., El Ghissassi, F., Cogliano, V., 2006. Car- achieve this goal: i) bandgap tuning for visible light harvesting; ii) cinogenicity of carbon black, titanium dioxide, and talc. The Lancet Oncology 7, minimizing electron/hole recombination; and iii) promoting the 295e296. Bard, Allen J., Stratmann, M., Licht, Stuart, 2002. Fundamentals of Semiconductor forward reaction at the surface of the photocatalyst. Nevertheless, Electrochemistry and Photoelectrochemistry. In: Encyclopedia of Electro- several studies refer that a higher surface area should be an chemistry e Semiconductor Electrodes and Photoelectrochemistry. Wiley-VCH. Barka, N., Qourzal, S., Assabbane, A., Ait-Ichou, Y., 2010. Kinetic modeling of the important feature to enhance the photocatalytic activity and TiO2 photocatalytic degradation of methyl orange by supported TiO2. Journal of immobilization should be the way of get it. Different methods are Environmental Science & Engineering 4, 1e5. being used for titanium dioxide immobilization, e.g. solegel, dip Bellardita, M., Di Paola, A., Palmisano, L., Parrino, F., Buscarino, G., Amadelli, R., 2011. coating, physical vapor deposition, chemical vapor deposition, Preparation and photoactivity of samarium loaded anatase, brookite and rutile catalysts. Applied Catalysis B: Environmental 104, 291e299. electrophoretic, among others. These methods of immobilization Birnie, M., Riffat, S., Gillott, M., 2006. Photocatalytic reactors: design for effective air can be used on several substrates, as described along this review: purification. International Journal of Low-carbon Technologies 1, 47e58. glass, porous solids, metal, cement, ceramic tiles and paints. Actu- Boysen. http://www.knoxoutpaints.com/ (accessed February 2013.). ally, it has been found that paint coatings are an efficient way of Byrne, J.A., Eggins, B.R., Brown, N.M.D., McKinney, B., Rouse, M., 1998. Immobilisa- tion of TiO2 powder for the treatment of polluted water. Applied Catalysis B: immobilizing photocatalysts. Indeed, they present a 3D layer where Environmental 17, 25e36. TiO2 nanoparticles are available for photocatalysis up to the optical Byun, D., Jin, Y., Kim, B., Kee Lee, J., Park, D., 2000. Photocatalytic TiO2 deposition by e thickness, which is around 100 mm. chemical vapor deposition. Journal of Hazardous Materials 73, 199 206. Carp, O., Huisman, C.L., Reller, A., 2004. Photoinduced reactivity of titanium dioxide. Until now, only few photocatalytic products have been devel- Progress in Solid State Chemistry 32, 33e177. oped and made commercially available. In the literature it is Cassar, L., 2004. Photocatalysis of cementitious materials: clean buildings and clean possible to find examples of panels, tiles, tents, stones, mortar air. MRS Bulletin 29, 328e331. Charles, B., 2005. Everything you need to know about NOx: controlling and mini- panels and paints with photocatalytic properties. One major chal- mizing pollutant emissions is critical for meeting air quality regulations. Metal lenge to work with TiO2 material is to find a way of immobilizing Finishing 103, 18e24. this semiconductor in a photodegradation-resistant substrate. Chen, M., Chu, J., 2011. NOx photocatalytic degradation on active concrete road surface e from experiment to real-scale application. Journal of Cleaner Pro- Another concern in using this semiconductor is related to the fact duction 19, 1266e1272. that it is a nanoparticulated powder, which may bring some risks to Chen, Y., Dionysiou, D.D., 2006. TiO2 photocatalytic films on stainless steel: the role human health. However, the immobilization processes may also of Degussa P-25 in modified solegel methods. Applied Catalysis B: Environ- mental 62, 255e264. contribute to diminish this risk. Chin, S., Park, E., Kim, M., Jeong, J., Bae, G.-N., Jurng, J., 2011. Preparation of TiO2 Photocatalysis appears as a green process to answer to a serious ultrafine nanopowder with large surface area and its photocatalytic activity for e environmental issue: NOx pollution. Several efforts have been made gaseous nitrogen oxides. Powder Technology 206, 306 311. to produce new photocatalytic products for NO photoabatement; Colls, J., 2002. Air Pollution. Spon Press. x Demeestere, K., Visscher, A.D., Dewulf, J., Leeuwen, M.V., Langenhove, H.V., 2004. these products should be efficient, resistant, easy to apply and A new kinetic model for titanium dioxide mediated heterogeneous photo- cheap. catalytic degradation of trichloroethylene in gas-phase. Applied Catalysis B: Environmental 54, 261e274. Devahasdin, S., Fan Jr., C., Li, K., Chen, D.H., 2003. TiO2 photocatalytic oxidation of Acknowledgments nitric oxide: transient behavior and reaction kinetics. Journal of Photochemistry and Photobiology A: Chemistry 156, 161e170. fi This work is co-founded by FEDER (Fundo Europeu de Desen- Ding, Z., Hu, X., Yue, P.L., Lu, G.Q., Green eld, P.F., 2001. Synthesis of anatase TiO2 supported on porous solids by chemical vapor deposition. Catalysis Today 68, volvimento Regional)/QREN (NOxOUT project with reference 173e182. FCOMP 01-0102-FEDER 005365) under the framework of EDSA, B.P. http://boysenknoxoutproject.com/ (accessed February 2013.). 538 J. Ângelo et al. / Journal of Environmental Management 129 (2013) 522e539

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Greek symbols List of abbreviations and symbols a: light absorption coefﬁcient, m 1 1 DHi: enthalpy of the reaction, J mol A: electrode area, m2 3: dielectric constant of the semiconductor 0 A : constant of proportionality hinj: injection efﬁciency, % CB: conduction band qA: fractional coverage of the surface by A -2 q Ci: concentration of species i, mol m B: fractional coverage of the surface by B 3 l: wavelength, nm CH2 O: H2O concentration, mol m 3 n CNO: NO concentration, mol m : frequency, Hz 3 CNO2 : NO2 concentration, mol m 2 CH O; ads: concentration of H2O adsorbed, mol m Physical constants 2 2 34 CNO,ads: concentration of NO adsorbed, mol m h: Planck constant, 6.63 10 Js C : concentration of NO adsorbed, mol m 2 k : Boltzmann constant, 1.38 10 23 JK 1 NO2 ;ads 2 B 2 19 Csc: differential space-charge capacity, F m q: charge of electron, 1.60 10 C 2 1 1 CTiO : total concentration of active sites on TiO2, mol m R: universal gas constant, 8.31 JK mol 2 2 3 : permittivity of free space, 8.85 10 12 Fm 1 CTiO2 ;free: concentration of free active sites on TiO2 surface, mol m 0