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Building and Environment xxx (2015) 1e13

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Building and Environment

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Photocatalytic air cleaners and materials technologies e Abilitiesand limitations

Lexuan Zhong, Fariborz Haghighat*

Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Quebec H3G 1M8, Canada

a r t i c l e i n f o

Article history:Received 16 November 2014Received in revised form10 January 2015Accepted 12 January 2015Available online xxx

Keywords:Photocatalytic oxidation (PCO)Air cleanersBuilding materialsAir qualitySustainability

* Corresponding author. Tel.: þ1 514 848 2424 (319E-mail address: Fariborz.Haghighat@concordia.ca

http://dx.doi.org/10.1016/j.buildenv.2015.01.0330360-1323/© 2015 Elsevier Ltd. All rights reserved.

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a b s t r a c t

Heterogeneous photocatalytic oxidation (PCO) is one of the most active research areas in environmentalpurification. Recently, research on PCO technologies-based air cleaners and building materials has beenintensively conducted for indoor/outdoor air treatment. The innovative technology is dedicated todecompose gaseous pollutants using photocatalysts under illumination of UV or solar light without extraenergy added. Comprehensive knowledge on the properties and performance of PCO for treatment of airusing batch reactors or continuous reactors has been established. This critical review has been carriedout with aims to examine the state-of-the-art of PCO technologies in the field of air purification and theirapplication prospects. First, PCO fundamental principles, including mass transfer, adsorption, and surfacereaction, are reviewed, providing rational explanations for research results. Second, the experimentalwork in various scales and other related research, such as modeling methods, coating techniques, andtesting methods, are discussed and evaluated separately for PCO air cleaners and building materials.Based on these insights, the corresponding technological problems and future research directions forpromotion of commercial applications in two aspects are also described in detail.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Over hundreds of volatile organic compounds (VOCs) have beenqualitatively identified in indoor environment, and the maincompounds contain alkanes, aromatics, esters, alkenes, carboxylicacids and alcohols [1]. In addition, analysis of the available datademonstrates a statistical association between the IAQ conditionand occupants' health and performance [2]. Ventilation, as atraditional dilution method, is not strongly recommended incurrent practice due to its limitation in terms of outdoor air quality(OAQ) and energy cost. Therefore, in order to provide truly safe,healthy, productive, and comfortable environment, it is necessaryto develop technologies and/or effective strategies to improve IAQand reduce energy consumption.

Nano-technology is an emerging advanced technology that of-fers many opportunities for HVAC applications and aims to improveair quality. One of the most highly anticipated uses is photocatalyticoxidation (PCO) technology, which utilizes a nano-catalyst that isstimulated when illuminated by UV light to provide an alternative

2); fax: þ1 514 848 7965.(F. Haghighat).

, Haghighat F, Photocatalytic.doi.org/10.1016/j.buildenv.20

and energy efficient solution to air quality challenges. Photosensi-tive semiconductors, such as titanium dioxide (TiO2), absorbultraviolet (UV) light to form reactive hydroxyl radicals (�OH) in thepresence of oxygen and water vapor. These free radicals undertakea series of reactions with pollutants, such as containing bondcleavage, substitution, and electron transfer, to mineralize theminto CO2 and water. PCO represents the newest generation ofproactive air purification technology.

Over the years, comprehensive knowledge on the propertiesand working mechanisms of PCO for air treatment has beenestablished [3e10]. Fig. 1 presents the publications since 1985 onthe topic of the PCO technology for air purification, and it clearlyshows the number of articles steadily increasing in the past 10years. Extensive kinetic experiments and models associated withPCO reactions demonstrate that the reaction rate of a UV-PCOsystem depends on light intensity, types and concentrations ofreactants, flow rate, moisture levels, properties of catalyst used, andthe properties of the PCO reactor. It has been concluded that aheterogeneous PCO is suitable for the photo-degradation ofgaseous effluents at the low ppb concentrations due to its relativelymoderate photocatalytic removal rates and the occurrence ofcatalyst deactivation after the treatment of highly concentratedeffluents [6,11]. With photocatalysts integrated into construction

air cleaners and materials technologies e Abilities and limitations,15.01.033

Fig. 1. Number of articles published in Compendex from 1985 to 2013 through thesearching engine of Engineering Village (key words are photocatalytic oxidation andair).

L. Zhong, F. Haghighat / Building and Environment xxx (2015) 1e132

materials, photocatalysis has been demonstrated as a promisingtechnology for mitigation of urban air pollution (i.e. asphalt,pavement, and wall) [11e17].

TiO2, with favorable features of durability and stability, ensuresits wide applications in the chemical, medical, biological, environ-mental, and solar energy fields [18]. In the HVAC field, theoverwhelming majority of the catalysts are TiO2 and its derivatives.In this review, the principles of the photocatalysis and two majorapplications of PCO technologies in the field of the HVAC systemsand construction and building materials are summarized in detail.In addition, the technical challenges that hinder the UV-PCOtechnologies from being widely commercialized are discussed. Inthe end, future research priorities are proposed in order to breakthrough current bottlenecks. Fig. 2 is a flowchart depicting theframework in this review.

2. UV-PCO principles

Understanding the photo-catalysis fundamentals is of essentialimportance for interpreting the experimental data, analyzing theparameters influencing the PCO efficiency, and finally optimizing

Fig. 2. Review framework of PCO techno

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systems for building applications. In this section, the three basicprinciples have been fully reviewed, including mass transfer inporous media, electron excitation and photochemical reactionmechanisms, as well as adsorption on photocatalyst surfaces.

2.1. Mass transfer

Heterogeneous catalysis involves an interaction between solidfibers and reactant molecules in gas phase. The mass transferprocess is taking place at the interface between solidegas phases.There are two characters, UV lights participation in energyinitiation and occurrence of photochemical reactions at the catalystsurfaces, which differentiate mass transfer in PCO from the tradi-tional mass transfer occurring in the porous physical adsorbent likeactivated carbon. The surface morphology, such as surface area andcrystal structure, of a photocatalyst-coated supporting media, playsa critical role in its photocatalytic activities such as adsorption,reflectance, adhesion, and carrier transportation properties [19].Usually, the UV-PCO process can be divided into seven elementalmass transfer processes occurring in series, namely:

1) Advection (VOCs and precursor species are carried by airflows),2) Mass transfer of the reactants from themain flow to the exterior

surface of the catalyst particle,3) Molecular diffusion and/or Knudsen diffusion of the reactants

from the exterior surface of the catalyst particle into the interiorpore structure,

4) Adsorption onto the interior catalyst surface,5) Photochemical reaction on the catalyst surface,6) Desorption of the reaction product(s) from the surface of the

catalyst, and7) Mass transfer of the products from the interior catalyst pores to

the gross external surface of the catalyst by ordinary moleculardiffusion and/or Knudsen diffusion, and finally externaldiffusion to the main flow (Fig. 3).

Mass transfer of reactants in a heterogeneous catalyst is afunction of the molecule type, the dynamics of the flow conditions,and the geometric surface area of the catalyst [20]. To be specific,molecular diffusion, as illustrated in steps 3 and 7, depends on the

logies for air quality improvement.

air cleaners and materials technologies e Abilities and limitations,15.01.033

Fig. 3. Elemental mass transfer processes involved in the PCO of air pollutants withinTiO2.

L. Zhong, F. Haghighat / Building and Environment xxx (2015) 1e13 3

temperature, molecular weight, collision integral, pressure, andcharacteristic length of the diffusing reactants [21]. The interphasemass transfer during above-mentioned steps 2e3 and 6e7 can bedescribed by the corresponding empirical correlations relatingSherwood number to the Reynolds and Schmidt numbers [21,22].Photochemical interactions of reactants and products in Step 5make mass transfer more complicated. In theory, any of theabovementioned seven steps can be a rate-limiting step whichcontrols the overall PCO performance.

2.2. Electron excitation and photochemical reaction mechanisms

Unlikemetals containing free electrons, semiconductors possessa band gap which prevents an electron transferring from thevalence band (VB) to the conduction band (CB) when no additionalenergy adsorption occurs. Only when the energy hn of adsorbedphotons is equal to or greater than the band gap, the excitation andtransfers of electrons from the VB to the CB are initiated, and at thesame time positive holes in the VB are created. The minimumrequired excitation energy is associated with the lattice structuresof semiconductors. For example, the octahedron in anatase crystalsof TiO2 is significantly distorted than that in rutile crystals of TiO2,which results in different mass densities and electronic bandstructures (3.2 eV for the anatase structure; 3.0 eV for the rutilestructure) between the two forms [23]. After excitation, thesecreated highly reactive electron (e�CB)-hole (hþ

VB) pairs with a life-time of nanoseconds undergo a diversity of fates [24]. Fig. 4 showsseveral capture pathways of electrons and holes and the PCOreaction mechanism at the catalyst surface.

Migration of electrons and holes to the catalyst surface, wherethe target VOCs are adsorbed, is an important step for an effectiveheterogeneous photocatalysis. The positive holes move to thesurface and react with OH- dissociated from adsorbed water(pathway 3) to generate hydroxyl radicals (�OH), which are the keyreactive species to oxidize the adsorbed VOCs. Under optimalreaction conditions, VOCs can be completely oxidized to carbondioxide, water, etc., as the final products. Similarly, electrons on thecatalyst surface promptly reduce electron acceptors (adsorbedoxygen or other intermediate products, such as �OOH and H2O2,produced during a PCO chain reaction) (pathway 4) and �OH isformed finally as the major free radical to decompose VOCs.

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However, the PCO reactions could be interfered by recombinationof the separated electrons and holes with the release of heat, whichis an unfavorable process involved in the PCO process. Therecombination can happen on the surface (pathway 1) or within thesemiconductor particle volume (pathway 2) [23]. The process ofcharge transferring to adsorbed species competes with the elec-tronehole recombination. Recombination phenomenon results in aportion of adsorbed photons not being effectively utilized. Thepossible reaction pathways are described in the SupplementaryMaterial.

However, the clear and verified photochemical reaction mech-anism involved in photocatalysis cannot be provided due to the lackof suitable probe techniques. It is disputable that VOCs may bedirectly oxidized by photo-generated holes [25]; most researchersare prone to the possible mechanism of indirect oxidation via �OHradicals. For example, it has been verified that the condition ofindirect oxidation by �OH radicals as the PCO mechanism is whenwater exists as a dominant species, which was observed through anonlinear spectroscopic technique of sum frequency generation(SFG) [26].

2.3. Adsorption on photocatalyst surfaces

An understanding of the role of water molecule adsorption onthe photocatalyst surfaces from the gas phase is a prerequisite tostudy the adsorption properties of a photocatalyst. Normally, waterforms an adsorption layer close to the photocatalyst surfaces due toTiO2 nature of hydro-philicity [27,28]. As reported, the TiO2-waterbond length is 2.21 ± 0.02 Å, much longer than other chemisorptionbonds of formate (2.08 Å) and hydroxyl (2.02 Å) [29,30]. In addition,water dissociation in bridging oxygen vacancies (observed fromScanning Tunneling Microscopy (STM)) leads to the formation ofhydroxyl groups (Hammer et al., 2010), which can form hydrogenbonds with some functional groups, helping to adsorb those VOCssuch as alcohols and ketones. This can interpret the difference ofadsorption capacity for different chemical classes. Moreover, theadsorption performance for various types of single VOC decreaseddramatically as RH rose from 9% to 70% for three TiO2-basedsubstrates [28]. Therefore, water vapor plays a critical role on theadsorption behavior of VOCs on TiO2, and, particularly, its impact isassociated with its ratio in air.

Regarding the adsorption behavior of mixtures on the photo-catalyst surfaces, inhibition was observed as a common conse-quence of competition among VOCs for adsorption sites [31,32];adsorption enhancement was also detected for few mixtures andthis might result frommodification of the catalyst surface to a moreactive form through a rehydroxylation process induced by anadsorbate with high solubility [33]. Recently, Zhong and Haghighat[34] quantitatively identified that both the inhibitory and thepromotional effects occurred in the co-adsorption.

Therefore, adsorption, defined as one of the fundamentalprocesses occurring at a heterogeneous photocatalytic reaction,affects the UV-PCO performance in terms of selective adsorption,surface area and adsorption capacity. Development of adsorptivecatalysts could be a possible approach to improve the PCO perfor-mance, which will be discussed in Section 3.1.1.

3. PCO air purifier

3.1. Experimental work

Extensive PCO experiments have been performed in bench-topscales, pilot scales, and full scales for indoor air pollutants abate-ment. All prior work intends to theoretically provide technical

air cleaners and materials technologies e Abilities and limitations,15.01.033

Fig. 4. Electron excitation and PCO reaction mechanisms.

L. Zhong, F. Haghighat / Building and Environment xxx (2015) 1e134

support for the applicability of the PCO technology and thus boostthe commercialization of PCO air cleaners.

3.1.1. Photocatalyst characterizationPhysical and chemical properties, including surface area, pore

size, pore structure, chemical elements, morphology, and crystalstructure, of a photocatalyst are assumed to be closely responsiblefor the PCO performance. Much research on the characterization ofcommercially available or synthesized photocatalysts have beenconducted. They include X-ray diffraction (XRD) [35e39], X-rayphotoelectron spectroscopy (XPS) [35,37,39], energy-dispersiveX-ray spectra (EDX) [38], UVevis diffuse reflectance spectra (DRS)[35,37e39], BrunauereEmmetteTeller (BET) [10,28,36e39], Four-ier transform infrared spectroscopy (FTIR) [36,38], Transmissionelectron microscopy (TEM) [35e37] and Scanning electron micro-scopy (SEM) [28,36,38,39]. Through these observations photo-catalysts with various crystal structures as well as their unique PCOperformance have been developed and demonstrated. The detailedpreparationmethods for various photocatalysts have been criticallyreviewed by Mo et al. [40] and Ismail and Bahnemann [41].

It was concluded that the photo-degradation performance alsodepends on the characteristics of substrates. Lu et al. [42], Matoset al. [43] and Zhong et al. [28] observed an improved PCO removalefficiency using TiO2 films coated on activated carbon (AC) filtersthan other substrates, such as the glass plate and fiber glass filters.These results are attributed to the pollutant transfer cycle createddue to the presence of AC, that is, adsorbed pollutants constantlydiffuse from AC to TiO2 for photooxidation. Kibanova et al. [44]found that clay-supported TiO2 photocatalysts can potentiallyimprove the PCO performance because clay has a highly porousstructure which is in favor of both adsorption and prevention ofmacroscopic aggregation of nano-TiO2. But some clay-supportedTiO2 was subject to more negative RH effect on VOCs removalperformance than pure TiO2. This is due to the nature of high wateradsorption capacity for some clay. Similarly, Yamaguchi et al. [45]

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investigated the complete decomposition of 2-propanol(335 ppm) to CO2 when TiO2 was mechanically mixed withzeolite under photo-irradiation.

3.1.2. ExperimentalUsually, the UV-PCO air purifiers have been tested using bare

photocatalyst particles [46,47] or photocatalysts integrated withdifferent kinds of medias, such as gypsum plasterboard withnonwoven glass wool fabric [48], fiberglass [10,28,49], glass beads[50], dish [35,37], and carbon cloth [10,42,43]. One form of interestis TiO2 aerosol fluidized by an aerosol generator in a chamber [51].

Different measuring techniques have been employed to mea-sure the decomposition efficiency of UV-PCO air purifiers: sensoryassessments of air quality made by human subjects [52,53], ProtonTransfer Reaction-Mass Spectrometry (PTR-MS) [47,54,55], chro-matographic methods (Gas Chromatography/Mass Spectrometry orHigh Performance Liquid Chromatography with UV detection)[10,39,44,45,55e58], photoacoustic spectroscopy [10,28,57,59], andFourier Transform Infrared (FTIR) spectrometer [50,51].

The impacts of influential parameters such as initial concen-tration, flow rate, RH, and irradiance on the PCO performance havebeen thoroughly studied. It has been concluded that the lowerinitial concentration or the higher light intensity results in a higherconversion rate. However, no consistent conclusions have beenachieved on the impacts of flow rate and water vapor. In fact, theseconclusions are relevant in essence rather than contradictory [9].When the heterogeneous photocatalytic oxidation is controlled bythe gas-phase mass transfer, the oxidation rate increases with theincrease of the airflow rate [60]; when the PCO reaction iscontrolled by the surface reaction, the higher flow rate results inlower conversion rate [10,48,61]. Similarly, the optimal amount ofwater vapor required by the PCO reaction depends on the charac-teristic of challenge compounds, initial concentrations and thesubstrate for the photocatalyst. Besov et al. [51] reported thatincreasing the RH decreases the adsorption rate of acetone

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(304 ppm) and DMMP (102 ppm) due to the adsorption competi-tion of water vapor and gas. The increase of RH however increasesthe PCO reaction rates. The positive effect of RH in this case resultsfrom the high concentrations of challenge compounds which needmore hydroxyl radicals to be oxidized. Wang et al. [62] and Yu andBrouwers [48] found similar trend of an increase of the NOx con-version with an increase of the RH. These observations can beattributed to the hydrophilicity of HNO3 as one major by-product.For the scenario of low concentrations of reactants, mostresearchers found an increase of RH reduces the PCO efficiency dueto the interference of competitive adsorption by redundant watervapor [10,54,63e65].

Table S1 in Supplementary Material shows a summary of testmethods and conditions for PCO air cleaners in degradation ofvarious kinds of gaseous pollutants in the past five years. Allreported work was performed on a bench-top scale using a batchreactor or a continuous reactor: very few full-scale system experi-ments or field studies have been conducted to examine theperformance of PCO air cleaners. In addition, most reported ex-periments were carried out with low airflow rate (around L/minlevel), which is far from real applications. Therefore, results fromthese investigations may not sufficiently have direct significancefor the design/application of PCO air cleaners in HVAC system. Onlya few studies was conducted at high airflow rate and the resultsindicate the moderate PCO removal efficiency from 1% to 80%dependent on the experimental conditions [10,22,61,66e68].

3.1.3. By-productsThe main concern in applications of the PCO technology in a

building mechanical ventilation system is the formation of theundesired by-products. Most PCO reactions are stepwise, whichmeans they take many intermediate steps to form the final reactionproducts, and a by-product may be generated by one of the PCOmiddle steps. Therefore, by-products formation is a function of thePCO reactionmechanisms of contaminants types. It is a challenge toqualitatively and quantitatively predict the generation ofby-products in the context of complicated PCO mechanismsinvolved in as well as inconsistent experimental environmentalconditions. Limited work has been conducted to explore the im-pacts of operational conditions, including the presence and absenceof ozone, photocatalyst types, UV-lamps, initial concentration aswell as water vapor, on the distribution and production ofby-products [10,54,63,69]. It is concluded that formaldehyde andacetaldehyde are two common by-products regardless of challengecompounds. The concern of the formation of more toxic by-products during the process of UV-PCO makes it necessary toemploy a chemisorbent scrubber after a PCO air cleaner in order toreduce the potential health risks and indeed improve IAQ [70]. Inaddition, more by-products are produced when ozone-generatinglamps are employed in the PCO system due to the synthetic effectof photolysis and ozonation [10,71,72]. Therefore, the format andthe concentration of the by-products depend on the experimentalconditions, the PCO reactor configurations, the characteristics ofthe catalysts, as well as the different analytical methods [73,74].Shang et al. [75] and Vincent et al. [76] found that concentration ofthe by-products reduceswith the increase of the contact time of thecontaminant with UV illumination. Accumulation of by-productson the catalyst surfaces resulting in rapid catalyst deactivationwas observed by Hay et al. [59].

Mo et al. [54] found that water vapor has a significant effect notonlyon thePCOperformanceof the target compound,but alsoon theby-products generations. Usually, lowwater vapor is recommendedin the PCO reactors in order to obtain higher decomposition effi-ciency and lower by-product generations [9,10,54,63]. Due to natureof TiO2 hydrophilicity, water vapor has high affinity to the TiO2

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surface, especially for the case of high water vapor concentrations.The competitive adsorption of water molecules reduces the activesites of catalyst, obliging the generated by-products to be releasedinto the air phase, although some by-products may be chemicallyadsorbed. Hence, the common approach to regenerate catalyst sur-face is to expose it to high humid air under UV radiation [9].

3.2. Modeling

Besides experimental methods, modeling is another approach toexamine quality assurance, reliability, and durability of PCO tech-nology for the air purification. Most previous models were derivedfrom LangmuireHinshelwood isotherm since it has been widelyused to predict the PCO reaction rate with different pollutants[22,46,51,54,58,60,77]. The Langmuir expression describes that thePCO reaction rate is associated with the concentration of testedcompounds, adsorption constant (K), and reaction constant (k). Theextended Langmuir equations further consider the competitiveeffect of water vapor or other compounds at uni-molecular orbi-molecular adsorption sites. Two constants, k and K, can bedetermined from a regression analysis by conducting a series ofUV-PCO experiments with various inlet concentrations. Alterna-tively, two constants can be separately obtained from darkadsorption tests and UV-PCO tests. However, the adsorption con-stant and the reaction constant depend on the substrate type,relative humidity (RH), UV light irradiance, compound type, airvelocity, and the configuration of the PCO reactor: Making it diffi-cult to fairly compare the reported data with various units.Recently, some updated Langmuir models have been developedusing the lumped parameter method that considers both theadsorption and photocatalysis processes in one equation [78,79].The modified models take into account the impacts from otherfactors such as air RH, catalyst properties, UV irradiance, air ve-locity, and other operating parameters. Not only can these modelspredict the PCO reaction rate using k and K, but they also canestablish the relationships between the PCO reaction rate and themerged operating parameters.

Some kinetic models have been developed aiming to completelydescribe the PCO behavior on the basis of the PCO reaction mech-anism and the kinetic rate law. General hypotheses are made asfollows: 1) PCO reaction occurs at the catalyst surface; 2) UV lightirradiance is uniformly distributed at the surface; and 3) adsorptionbehavior and PCO reaction rate are based on the Langmuir mech-anism. Zhong et al. [21] developed and validated a two-phase basedtime-dependent UV-PCO model to describe the PCO behavior, inwhich all the influencing factors, including properties of lightsources and catalyst, reactor geometry, mass transfer parameters,kinetic parameters, and operational conditions, were taken intoaccount. The limitation of the developed model is that the modeldoes not consider the intermediates. Yu et al. [80] proposed a ki-netic reaction rate model to describe the PCO of NO and the un-desired intermediate NO2, which was expressed as a function of thelocal superficial rate of photon absorption and the primary quan-tum yield. The model considers the effect of pollutant concentra-tion, volumetric flow rate, RH, irradiance, photocatalyst dosage, andreactor size. The work provides a modeling method to predict thefate of by-products with time as long as the PCO reaction pathwaysare fully known. However, for most other gaseous contaminants,the PCO reaction pathways are not clear/known, which limits thismodeling method. This issue becomes more challenging in the caseof mixed compounds.

PCO reactors are expected to be designed to have an efficient UVlight irradiation. Consequently, light intensity scattering modelshave been developed to simulate radiation fields in a mechanicalventilation duct system [8,81e84]. The average irradiance received

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at the catalyst surface is associated with wall material reflectivity,emitted UV irradiance at UV surfaces, and distribution function ofspectral power. Sung et al. [67] proposed amethod of UVC radiationcalculation integrated into a computational fluid dynamics (CFD)simulation to evaluate the performance of PCO sheets for acetalde-hyde removal. It was showed that the predicted performances werehigher than the experimental results. Wang et al. [64] utilized a CFDsimulation todemonstrate that lowermoisture helps to enhance theeffectiveness of the PCO air cleaner on the VOCs removal.

3.3. Energy consumption analysis

Evaluation of energy consumption for UV-PCO air cleaners is ofinterest in order to provide reliable cost data for employing theseunits in buildings compare to other air cleaning techniques,although current research on UV-PCO energy consumption arelimited [22,85]. Tompkins et al. [85]found that a PCO device did notshow advantages compared to an adsorption-based one due to thehigh capital and operational costs.

The capital costs are associated with the configuration ofUV-PCO reactor designed, including catalyst type and property,substrate type and format, as well as the amount of UV lights.Table 1 lists the most popular PCO air filters available in the marketand compares their physical properties and quoted prices. It showsthat a carbon cloth-supported PCO filter has the highest BET surfaceand the lowest unit price, which may outstand as a good substratecandidate for building PCO air cleaners. However, the highestpressure drop by its texture leads to largely increased HVAC fanenergy consumption. If photocatalysts are coated on thehoneycomb-shape substrate, the pressure drop can be significantlyreduced, which in turn also sacrifice PCO performance due toportion of contaminated air in contact with catalyst surfaces.Therefore, the PCO air cleaners design has to consider variousfactors resulting in energy consumption when the PCO perfor-mance is acceptable.

It is reported that UV lights operation power and replacementcost are major contributions to the annual cost of PCO devices [85].Normally, low-pressure (LP) and medium-pressure (MP) mercurylamps are used. Although the LP lamps have lower UV intensity incomparison with MP ones, the significantly longer service life timeand lower electrical power input of LP lamps promote itself to beemployed in PCO system. It has to be pointed out that the walltemperature of MP lamps is 5e8 times higher than that of LP lamps,which will lead to the additional cost of cooling for the supply air.Besides, MP lamps have a wider spectrum and thus less effective tobe adsorbed by photocatalysts.

3.4. PCO versus other air cleaning techniques

Table 2 lists the comparisons of all air cleaning techniques forindoor air purification and/or sterilization, including working

Table 1Comparison of available PCO air filters.

PCO air filtersubstrate

Catalyst Pressure dropa

(Pa)@170 m3/h(100 CFM)

BET surfacearea (m2/g)

Costb ($/m2)

Aluminumhoneycomb mesh

V2O5/TiO2 1.8 ~5[86] 20e50

Nickel foam TiO2 10.7 ~34[22] 50e80Fiberglass TiO2 35.5 ~100[10] ~150Carbon cloth TiO2 240.7 ~800[10] 5e30

a The PCO reactor was set at one bank with two filters in a distance of 11 cm,which are perpendicular to the direction of air flow.

b The price can be quoted from www.alibaba.com.

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principles, treated pollutants, removal performance, advantages, aswell as disadvantages. It substantially shows that in theory UV-PCOor non-thermal plasma (NTP) alone can treat most air pollutantswidely existing in buildings. On the other hand, other air cleaningtechniques, such as adsorption, filtration, UV germicidal irradiation(UVGI), electrostatic precipitator (ESP), and ozonation, focus onremoving gaseous pollutants, solid-state pollutants, microbes, orcombination of some of them. Hence, UV-PCO or NTP can bepotentially applied in different scenarios to eliminate a variety ofindoor air pollutants. It also indicates that low energy inputs for UVlight benefit UV-PCO technology to be utilized as a sustainabletechnology, rather than high voltage inputs of NTP technology forions generation, which will boost the development of UV-PCOtechnology. However, immature UV-PCO technology displayssome shortcomings, which may be overcome by combination ofseveral air cleaning techniques to be designed as a hybrid aircleaner.

3.5. Problems and limitations

Despite an enormous research that have been performed in thelaboratory on UV-PCO technology-based air cleaners for removal ofVOCs and inorganic compounds limited work have been carried outconcerning the problems of catalyst deactivation, moderateremoval efficiency, and by-products prediction.

Usually, deactivation is a gradual irreversible process of losingactive sites, which could happen through different mechanisms,such as poisoning, sintering, fouling, and coking [99], and deacti-vation becomes worse under visible light [39]. Considering PCO airpurifiers operating at an ambient temperature and pressure, cata-lyst deactivation occurs mainly by a chemical process, which isassociated with number and concentration of compounds present,relative humidity level, and irradiance of UV sources. However, theresearch on the correlation of these influencing factors and deac-tivation is not fully exploited. For example, Hay et al. [59] reportedthe deactivation of PCO air purifiers and recommended the use ofan adsorbent filter before a PCO air purifier to capture largemolecular weight compounds. In addition, catalysts on the PCOfilters at high airflow rates may be subject to catalyst flake. More-over, models for predicting deactivation of PCO air purifiers arerarely found in the literature. Quantitative evaluation method fordeactivation needs to be developed in order to provide reliableknowledge for catalyst lifetime calculation. Catalyst regeneration isanother interesting topic following the catalyst deactivation, whichcovers catalyst regeneration technologies, catalyst replacementfrequency and catalyst regeneration cost [9].

Another issue that restricts wide applications of PCO air puri-fiers is their moderate efficiency [44,100], especially when they areoperated at airflow rates similar to the ones in real application.Experimental results show that the single-pass removal efficiencieswere 10%, 20%, 30%, and 50% for aromatics, alkanes, ketones, andalcohols when the concentration of challenge gases was in a sub-ppm level with an airflow rate of 170 m3/h [10]. Both experi-mental and modeling results demonstrate the surface reaction is arate-limiting process when the airflow rates are high [10,21].Hence, improvement of the photochemical reaction rate at thecatalyst surface is a crucial point to enhance the photocatalyticactivity. This problemmay be solved through several ways: the firstis to extend responsive spectrum into visible light regions so thatmore photons could be trapped; the second is to reduce the fastrecombination of electronehole pairs in order to improve thephoton quantum efficiency; and the third is to carefully design theconfiguration of PCO reactors by allowing sufficient photon energyto reach the full surface of the catalyst. All these methods intend tomaximize the hydroxyl radical production.

air cleaners and materials technologies e Abilities and limitations,15.01.033

Table 2Comparison of air cleaning technologies for indoor air treatment.

Air cleaning technology Purification mechanism Treated pollutants Purification performance Advantages Disadvantages

UV-PCO Photocatalysts underexposure of UV lightgenerate radicals inthe presence of waterand O2. Radicals arereactive species tooxidize pollutants.

A broad range of indoorpollutants: soots,inorganic compounds,VOCs, airborne microbes

Performance depends on thehumidity, light source, inletconcentration, photocatalyst,reactor

Degrade toxic compounds intonon-toxic final products; operationat room temperature and pressure;low pressure-drop; low energyconsumption

By-product generation; moderateremoval efficiency; catalystregeneration, deactivation andreplacement; UV light replacement

Filtration [87e89] The mechanical orphysical operationfor the separationof solids from gasesby trapping particlesas they pass through.

Particulate matter, airbornemicrobes (if a filter treatedwith antimicrobial agents)

Higher removal efficiencies forlarger particles; HEPA filter canremove 99.97% of 0.3 mmairborne particles.

No by-product concerns. No removal of most odors, chemicals,or gases; low efficiency for smallparticles; filter replacement

Adsorption (includingphysisorption andchemisorption) [90e93]

The adhesion of a gasto the surface of a solidby Van de Waals orforming new chemicalbonds.

Gases, odors, and fumes. Performance depends onadsorbent features. Normally,high efficiency is reached.

High adsorption capacity and highremoval efficiency for specificcompounds; operation at roomenvironment.

Adsorption is in a specific manner;desorption of light compounds byadsorption of heavy compounds; loseactivity with time; filter replacement;post-treatment of adsorbent is required;high pressure-drop; the adsorptioncapacity is low for light VOCs.

Non-thermal plasma(NTP) [94,95]

The use of plasma-generatedradicals and ions for airdepollution.

VOCs, aerosol particles,microbes

Performance depends onadsorption performance ofcatalyst, in-plasma or post-plasmaconfiguration, RH.

Production of ions and many highlyreactive oxidizing radicals.

Formation of more complicatedby-products than PCO; unsteadyoperation; low efficiency; high energyconsumption; plasma regeration.

UV germicidal irradiation(UVGI) [96]

Airborne microorganismsare killed by adsorption ofUVC light at 253.7 nm.

Airborne microbes Performance depends on the lampintensity, the distance of themicroorganism from the lamp,temperature, RH, and air exchangerate.

Effective in inactivating airbornemicroorganisms; low energy consumption.

UV light replacement; No removalof most odors, chemicals, gases orparticles.

Ozonation [72] Ozone, generated by UV orNTP discharge, takesreactions with pollutantsand oxidizes them.

Microorganisms, someVOCs, some smells andgases.

Performance depends on retentiontime, humidity level, and nature ofpollutants.

Ozone can produce hydroxyl radicals. Ozone output concentration needsto be strictly controlled.

Electrostatic precipitator(ESP) [97,98]

Ionization of stream isfollowed by changing,migration, and collectionof charged particles.

A wide range of particles Collection performance relies onparticle size and design parameterssuch as airflow rate, voltage, collectioncell area, and the electric field strengthand distribution.

Effective at removing particles largerthan 20 nm.

Production of ozone; high voltage;high energy consumption; solid wastegeneration.

L.Zhong,F.Haghighat

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cleanersand

materials

technologiese

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itations,Building

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Fig. 5. Schematic diagram of multi-functional photocatalytic building materials.

L. Zhong, F. Haghighat / Building and Environment xxx (2015) 1e138

It is inevitable to face the issue of by-product generation;especially some produced by-products, such as formaldehyde,acetaldehyde, and so on, are toxic and carcinogenic. At present, thekinetics and mechanisms of by-product generation during the PCOprocess is not fully understood, although some potential pathwaysfor a few challenge VOCs have been proposed [40,80]. At the sametime, the experiments attempting to examine relationshipsbetween experimental conditions and by-product generation arelimited. Hence, the reliable approaches to minimize by-productsgeneration are hardly found in the literature. And the field ofmodeling and simulation to predict the amounts of gaseousby-products needs further development.

3.6. Future prospects

Energy consumption determines the future prospects of PCO airpurifiers against traditional ventilation and other air purificationtechnologies. Presently, most of studies focus on the technicalperformance research, and only few researches aimed to analyzeenergy consumption of PCO air purifiers [22,85]. Although lowenergy consumption has been claimed as one of the advantages forthe PCO air cleaners, it has not been clearly proven. Furtherresearch in this area is needed.

Due to the issue of the moderate efficiency of the PCO aircleaners, the studies on the feasibility of combination employmentof other oxidation methods may accelerate the commercial utili-zation of PCO air cleaners. Besov et al. [101] examined the synergiceffect of PCO and plasma oxidation, and results showed that thedeep oxidation rate per unit of consumed electric power increasedby a factor of 1.2e3.6 for acetone (83 ppm), acetaldehyde(108 ppm), and toluene (12e46 ppm). Meanwhile, the deactivationof the photocatalyst can be decreased due to the presence of coronadischarge. Instead of immobilization of TiO2 on AC filters, Den andWang [102] proposed a method of utilizing a chemical filterincorporated with a TiO2-loaded filter in a touching way to over-come the weakness of adsorption sites decrease by direct loading.This novel method has the advantages of reducing the recombi-nation of electronehole pairs and accelerating molecular diffusionof VOCs from AC to TiO2. Other related research should be furtherexplored with the intention not only to improve the PCO removalefficiency, but also to minimize the by-products generation.

In order to gather more data to show the reliability and dura-bility of PCO technologies, long-term experiments are necessary tobe conducted. This long-term research aims to carry out anin-depth investigation and analysis of conditions for low/no by-products generation, energy costs, catalyst deactivation, UV life-time, and removal efficiency over time. The generated data couldhelp to develop a standard evaluation method, and further todevelop guidelines for designing PCO air cleaners for variouspurposes.

4. PCO building materials

Applying nano-catalysts to the building envelops, sidewalks(pavements), walls, windows, and along roads is another potentialpromising application, which can be used for the removal ofambient air gaseous pollutants. For the outdoor air applications,since sunlight is a natural source that provides photonic energy as adriving force to initiate the chemical reactions, PCO is a green andsustainable technology. Another key advantage of photocatalyticbuildingmaterials is the self-regeneration of TiO2 in the presence ofrainwater, which greatly reduces the maintenance costs [11,27].Besides air purification, photocatalytic building materials presentadditional functions, such as self-cleaning and air sterilization[12,27]. Fig. 5 illustrates three main functions of TiO2 modified

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building materials under sunlight illumination. For the indoor airapplications, similar functions could be demonstrated except forthe involvement of additional lights in a room.

Currently, a number of TiO2-coated buildingmaterials, includingsilicate, cement, glass, mortar, stone have been examined as asubstrate supporting photocatalysts in the laboratory/field envi-ronments [12,13,15,16,55,103e105]. These studies explain thephotochemical activity of new generation of building materialschallenged with a wide range of gaseous compounds.

4.1. Catalyst coating technology

Before the deposition of photocatalysts on the building mate-rials, nanoparticles are dispersed in a solvent, such as ethanol [106],methanol [103], silane [107], lime-saturated water [108], andmodified polycarboxylate polymer [12] followed by ultrasonicationtreatment. The function of these dispersing agents is to prevent theagglomeration of nanoparticles in deionized water so as to increasethe specific surface available for photocatalytic reactions [109].

For the flat substrates (i.e. ceramic, glass, and so on), dip-coatingis a highly popular method for deposition of very thin films fromthe above-mentioned suspension [48,54,55,103]. The variousamount of deposited photocatalyst can be achieved by varying thesuspension concentration and by controlling the dip-coating time.Spraying is another technique in which the suspension in the formof small drops is sprayed onto the surface of substrate [16,105].After a balance for a certain period, excess sols are removed by airspraying. Smits et al. [106] used a pipette to transport the coatingsolution to the top side of the substrates. Then, the coated samplesare dried under ambient conditions for several days or heated forseveral hours until the weight of samples remains constant. Themaximum drying temperature should be strictly controlled;otherwise phase transformation or reactions between photo-catalysts and substrates may be triggered to cause a decrease on thephotoactivity [105]. Fast drying may result in flake formation [106].The coating mass can be calculated according to the weight dif-ference before and after the deposition treatment. Aqueouscement-based paint, which is usually applied on exterior surfacesof the buildings or roads with a brushing method, is prepared bymixing TiO2with cement [14,107].

Loading ratio of the photocatalysts affects the surface adhesionforce to the coated building materials. For example, a 10% TiO2loading during solegel transition led to less strength onto the stonesurface [16]. Although three techniques of TiO2 immobilization(sedimentation, electrophoresis and dip-coating) did not demon-strate a significant effect on the photocatalytic activity, the impactsof particle sizes, crystalline phases, as well as TiO2 loadingpercentages were discussed [12,16,109].

air cleaners and materials technologies e Abilities and limitations,15.01.033

L. Zhong, F. Haghighat / Building and Environment xxx (2015) 1e13 9

4.2. Outdoor and indoor air

Outdoor air pollution is undesired consequences of urbaniza-tion. Numerous studies show that short-term or long-term expo-sure to air pollution may harm lung function, irritate therespiratory system and/or lead to carcinogenic adverse healtheffects [110e112]. In urban area with dense populations, automo-bile exhaust gas is the major air pollutant, resulting in highpollutant concentrations at street level, because the surroundingbuildings prevent these pollutants from spreading [12,113,114].External building surfaces and concrete pavement surfaces areideal places to be modified with photocatalysts to get an additionaldepollution function due to availability of sunlight. A number ofstudies focused on the application of TiO2 modified buildingmaterials and components, such as paints [14,53,115], wall paper[116], tile [35], and windows [117,118].

For the technology of air-cleaning building materials or concretepavement, various gaseous contaminants, including NO, NO2, SO2,CO, toluene, formaldehyde, and so on, as well as particulate matter,such as carbon soot [106], have been tested as challenge contami-nants to examine the photocatalytic degradation performance.Table S2 in Supplementary Material presents a summary of testmethods and operational conditions for PCO building materials indegradation of gaseous pollutants. Most of studies have beenimplemented on bench-top scales to obtain a theoretical basis ofheterogeneous photocatalysis, while a few studies have been car-ried out on the full-scale implementation and realistic environ-mental conditions. For example, Guerrini [14] observed 20% of NOxreduction 2 months after renovation of a cement-based coating onthe surface of a tunnel. Field measurement results of TiO2-con-taining paint brushed on road confirmed the effectiveness of thePCO technology for decomposition of car exhaust gases [107].Ballari and Brouwers [119] observed the NOx concentration was onaverage 19% lowered in a street coated with the photocatalyticmaterials. In addition, the impacts of a number of design andoperational parameters on the efficiency of photocatalytic mate-rials have been investigated. De Melo and Triches [114] found thatNOx removal rate increased 15%e25% by using TiO2-modifiedcement mortars when UVA irradiance increased from 10 W/m2 to40W/m2. They also found that when RH increased from 30% to 70%or flow rate increased from 1 L/min to 5 L/min, the photocatalyticactivity decreased 25% and 40%e55%, respectively. Demeestereet al. [11], Dylla et al. [120] and Ballari et al. [121] found similartrends about the effects of the airflow rate and air RH. The negativeimpact of common roadway contaminants (dirt, deicing salt, andmotor oil) on the photocatalytic NOx removal were evaluated, andamong them oil has the highest negative effect [120].

Table S2 also shows that NOx is a popular test compound and isfrequently used to examine the technology of PCO materials.Various TiO2-incorporating materials tested under differentenvironmental conditions indicate the positive impact of PCOtechnology on NOx decomposition with removal efficiencies from10% to 90%. Nevertheless, researchers made different conclusionsfor other gaseous pollutants. For example, Chen et al. [13] did notdetect the conversion of toluene using TiO2 modified white cementpaste: Demeestere et al. [11] observed up to 63% toluene removalefficiencies using TiO2-containing roofing tiles and corrugatedsheets. The different results for Demeestere et al.’s study may beattributed to the high TiO2 BET surface area, light intensity, and longexposure time.

Since the UV light region accounts for only 3e5% of sunlight,presently, considerable attentions are devoted to the design anddevelopment of new-generation of TiO2 photocatalysts or otherphotocatalysts with the aim of extending the absorption band tothe visible spectrum. Effective utilization of ambient light is an

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energy-saving method and meanwhile, enhances the photo-catalytic activity which owes to the high separation of photoin-duced electronehole pairs. The objective of using visible spectrumcan be achieved, for example, by implantation techniques of metal-ions such as Fe, Pt, Co, Ce, Cr, Mn, Mg, Ni and Ag, which introduceimpurity atoms into pure TiO2 so as to change electroneholeconcentrations in TiO2 [39,122e126]. This semiconductor dopingmethod leads to the transformation of an intrinsic TiO2 into acrystal phase of a P-type or N-type TiO2, which causes an enormousdecrease of the absorption band of TiO2 initiated from visible lightregions. For example, the optical band gap energy of the Fe-dopedanatase TiO2 is 0.51 eV [122], which is significantly lower than thatof pure anatase TiO2 of 3.2 eV. Thus, a much better utilization ofsolar photons is achieved. Other examples of second generation ofthe TiO2-based photocatalysts are metal oxide coupled with TiO2photocatalysts (Bi2WO6/TiO2, ZnO/TiO2, Al2O3/TiO2, Ca2SiO4/TiO2,SiMgOx/TiO2, BaO/TiO2) that exhibit enhanced photocatalyticactivity compared to the non-modified one [104,125,127e130]. Thefavorable results could be attributed to the fact that the migrationof electrons and/or holes between transition metal oxide and TiO2restrains the recombination of electrons and holes. Otherapproaches of modifying TiO2 photocatalysts responsive to visiblelight include doping of nonmetal elements intoTiO2. For example, itwas found that doping of N [37,39,131] or C [48] was efficient forTiO2 to be activated by visible lights. Other heterogeneous photo-catalysts of interest for indoor air purification under visible lightillumination are BiOBr, Zn2SnO4, a-Bi2O3, (BiO)2CO3, and ZnAl2O4,which were proven to be suitable candidates for the diffusion ofintermediates and final products due to their special hierarchicalstructure and electronic configuration [35,37,38,56,132].

4.3. Testing method for photocatalytic building materials

So far, the researchers developed their own testing methods,including the photoreactor systems and operational conditions, toassess the air-purification performance for a variety of photo-catalytic materials. The diversity of these proposed methods makesit difficult to properly compare the results from different experi-mental conditions such as the airflow rate, RH, light exposure timeand etc.

There are limited applicable standards for air-purificationperformance of semiconducting photocatalytic materials. To thebest of the authors’ knowledge, only five published photocatalystair-purification ISO methods are available, each dedicated to theremoval of different air-borne pollutants: NO [133], acetaldehyde[134], toluene [135], formaldehyde [136] and methyl mercaptan[137]. Limited studies have been carried out in accordance withthese standards [46,48,119]. Table 3 lists the values of the majorparameters for pre-conditioning tests and air purification standardtests. It shows that the preconditioning treatment and air purifi-cation test conditions are inconsistent for these five standards,making it impossible to speculate on the test conditions for otherunreported gases. It is required to establish a general standard inorder to improve the comparison of the photocatalytic products interms of the PCO removal performance.

4.4. Problems and limitations

Although the notion of improving air quality using photocatalystmodified building materials has been proposed by researchers,there are still some technical aspects that are needed to beimproved. The fundamental issues lie in photocatalyst deactivation,low performance of photocatalytic activity, by-product generation,and the lack of standard procedures and design tools [27,138].

air cleaners and materials technologies e Abilities and limitations,15.01.033

Table 3ISO test methods.

ISO 22197 Part1 Part2 Part3 Part4 Part5

Test gas NO Acetaldehyde Toluene Formaldehyde Methyl mercaptanSample pre-conditioning step conditionsUVA irradiance (W/m2) �10 �15 �15 �15 �15Duration (h) �5 16e24 16e24 16e24 16e24Air purification standard test conditionsConcentration, (ppm) 1.0 ± 0.05 5.0 ± 0.25 5.0 ± 0.25 1.0 ± 0.1 5.0 ± 0.25Flow rate(L/min)

3.0 1.0 0.5 3.0 1.0

Adsorption test time (h) �0.5 0.5e1.5 0.5e1.5 0.5e1.5 0.5e1.5Photoirradiation test

time (h)5 3 3 3 3

Irradiance (W/m2) 10 ± 0.5 10 ± 0.5 10 ± 0.5 10 ± 0.5 10 ± 0.5Temperature (�C) 25.0 ± 2.5 25.0 ± 2.5 25.0 ± 2.5 25.0 ± 2.5 25.0 ± 2.5RH (%) 50 50 50 50 50Intermediates NO2, HNO2, HNO3 Acetic acid, formic acid,

formaldehydeBenzaldehyde,benzoic acid

Other oxidationproducts

e

Analytical method Chemiluminescence: NO, NO2

Ion chromatography: NO3-GC-FID: AcetaldehydeIR: CO2

GC-FID DNPH-HPLC GC-FIDGC-FPD

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The photocatalytic reaction happens at the photocatalystsurfaces, which requires catalysts to be immobilized at the externalsurface of supporting media in order to get maximum exposure tothe contaminated air. The capability of resistance to mechanicalabrasion for the pavement application needs to be fully examined.In addition, environmental aging of the photocatalysts, particularlyfor outdoor applications, is another topic, on which there is lack ofsufficient information available. Zhang et al. [8] pointed out thatmetal sulfide and metal oxide semiconductors readily undergophotoanodic corrosion and photocathodic corrosion, respectively.To be practically feasible, the photocatalytic coatings need todemonstrate the stable photocatalytic activity for a long periodconsidering the long usage period of the construction materials.However, very limited studies [119] are available for the stabilityassessment of photocatalytic materials. For example, Pirola et al.[17] tested the photoactivity of two walls paint-based and twoplaster-based TiO2 materials in the NOx degradation for one yearand recorded the decrease of photoactivity with time for all prod-ucts. Environmental conditions, such as dust, fouling, and me-chanical abrasion, also have negative effect on the stablephotocatalytic efficiency, which are still not fully explored yet.

Most research results indicate moderate removal efficiencies fortested materials. The most possible technical limitation is lowreachability of gaseous contaminants to the photocatalyst surfaceunder the condition of low airflow rates. This drawback may beovercome by the natural wind across the streets or roads throughconveying pollutants to the photocatalyst surfaces. In addition,little research has been focused on the qualification and quantifi-cation of generated by-products which is a serious concern forhuman's health. Salthammer and Fuhrmann [115] reported thatTiO2-modified indoor wall paints on exposure to light producedundesired by-products, such as formaldehyde, acetaldehyde,ethylacrolein, pentanal, 1-hydroxy-butanone, and hexanal, whichare related to the composition of paints.

The available ISO standards are mainly applicable to fineceramics and five specific challenged gases and do not apply topowder or granular photocatalytic materials. As a result, designersonly have data from tests performed by manufacturers that areoften inappropriate to these applications. Since each manufacturerdesigns its own test procedures, the results of onemanufacturer arenot comparable to another. In addition, although simulationapproaches to investigate the impact of various strategies on OAQenhancement was developed using CFD technique, no PCO tech-nology was integrated into them [113, 139, 140]. Only a few kinetic

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reaction rate models have been proposed to describe the PCO of thespecific compounds on the photocatalytic materials [77,80]. Thelack of reliable modeling for prediction of the effect of differentfactors independently greatly limits the use of photocatalyticmaterials by designers. Therefore, the development of a universaltest procedure and a reliable design tool, valid to all forms ofphotocatalytic materials and applicable to the wide range of rele-vant airborne pollutants, would benefit the industry, the HVACdesign engineer, and all building occupants.

4.5. Future prospects

The development of photocatalytic building materials withsuperior performance presents a significant technological anddesign challenge. One of the promising opportunities for efficiencyimprovement would be an increase of the surface area of photo-catalysts by treating with some solutions during the catalystcoating preparation. Alternative way is expansion of active sitesfrom outer surfaces to inner surfaces by using the highly openporous supporting media. Effective utilization of photons fromsun-light is another key technological breakthrough if visible-light-responsive photocatalysts can be successfully developed andproven experimentally. For example, replacing conventional TiO2-based catalyst with Cu-modified WO3-based photocatalyst has tentimes higher reactivity under visible light conditions, which is inthe further verification stage [44]. Also, long-term experiments forthe evaluation of the decomposition performance of gaseous con-taminants and the durability of photocatalysts are of specialinterest. Last but not least, full investigation of by-products gen-eration and its adverse health consequences needs to be carefullyexamined. Although there is diverse photocatalytic finishing orcoatings available on the market, even patented for some products,unclear performance of these products and uncertain risk fromproduced by-products require the establishment of a standardtesting method, i.e. placing photocatalytic materials in an urbanneighborhood prototype, and the development of short-term andlong-term evaluation methodologies.

In addition, it has become apparent that development of amicro-climate simulation tool on the basis of knowledge of thephysical and photochemical processes is a viable solution to furtherpromotion of practical applications. This tool can be used to accu-rately predict the air-purification performance of photocatalyticbuilding materials for urban pollution island (UPI)/urban heatisland (UHI) mitigation. The scale-up factor also needs to be

air cleaners and materials technologies e Abilities and limitations,15.01.033

L. Zhong, F. Haghighat / Building and Environment xxx (2015) 1e13 11

determined when a model verified by laboratory results is appliedfor real outdoor applications. The validated design tool is expectedto solve the optimization problem by finding the dominating factorusing numerical methods and may also enable urban planners toselect appropriate building materials for a given local climate andthus to establish a sustainable city.

5. Conclusions

Recent developments of PCO air cleaners and PCO air-purification materials for indoor/outdoor air treatment have beenreviewed. Fundamental research on UV-PCO principles has beendiscussed in terms of mass transfer, surface adsorption, and surfacephotochemical reaction, which provides theoretical evidence todemonstrate the promising of PCO technologies. Extensivelaboratory research, field study, and modeling work have furtherconfirmed the capability of the PCO technologies in gaseous con-taminants decomposition. Especially, bench-top scale experimentsunder low airflow rates demonstrate the acceptable decompositionefficiencies of PCO-based air cleaners or materials. Major parame-ters affecting PCO oxidation efficiency, including RH, airflow rate,irradiance, gas concentration, and gas type, are discussed in detail.Other pertinent topics, such as PCO modeling, by-products gener-ation, catalyst preparation and characterization, coating technolo-gies, and PCO performance evaluation methods, have also beenreviewed. Although many advances have been achieved, the PCOair cleaners and PCO air-purification materials are far from widelycommercial applications due to the issues of unclear generationpathways of by-products, moderate PCO performance, no universalevaluation standards, limited simulation tools, unknown durabilityof photocatalysts, as well as photocatalysts regeneration.

To accelerate the progress of commercialization, there is ampleneed for conducting more basic research work to resolve thesetechnological challenges. In our opinion, priority research in thenear future should focus on the development of new photocatalystswhich can demonstrate properties of (i) high adsorption perfor-mance of various VOCs, (ii) being less influenced bymoisture levels,(iii) less or no generation of by-products, and (iv) an ability oftrapping photons from UV to sunlight spectrum. Aside fromchemical analysis of generated gaseous by-products, major obsta-cles in terms of minimization of by-products generation areexpected to be overcome by chemical quantification on PCO filtersurfaces, catalyst characterization, and by-product generationmodeling.

Acknowledgments

The authors would like to express their gratitude to the NaturalSciences and Engineering Research Council of Canada (NSERC) forsupporting this research project through a CREATE grant.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.buildenv.2015.01.033.

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