Synthesis and photocatalytic properties of dense and porous TiO 2-anatase thin films prepared by sol–gel

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Synthesis and photocatalytic properties of dense and porous TiO 2-anatase thin films prepared by sol–gel

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  Review Synthesis and photocatalytic properties of dense and porous TiO 2 -anatase thinfilms prepared by sol–gel N. Arconada a , A. Dura´n a , S. Sua´rez b , R. Portela b , J.M. Coronado b , B. Sa´nchez b , Y. Castro a, * a Instituto de Cera´ mica y Vidrio (CSIC), Campus de Cantoblanco, Kelsen 5, Cantoblanco, 28049 Madrid, Spain b CIEMAT-PSA, Avda Complutense 22, 28040 Madrid, Spain Contents 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.1. Synthesis and characterisation of sols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.1.1. Synthesis of sols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.1.2. Viscosity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2. Deposition and characterisation of coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2.1. Coatings deposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2.2. Optical and field-emission scanning electron microscopy (FE-SEM) characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2.3. Fourier transform infrared spectroscopy (FTIR) and grazing X-ray diffraction (GXRD) characterisation. . . . . . . . . . . . . . . . . . 22.2.4. Specific surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2.5. Photocatalytic activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33. Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.1. TiO 2  sols and films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.2. Effect of sintering temperature and time in the crystallisation of TiO 2 -anatase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.3. Textural and morphological properties of TiO 2  thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.4. Photocatalytic activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction Volatile organic compounds (VOCs) are recognized asdangerous pollutant compounds. Besides being carcinogenagents and contribute to ozone production in the troposphere Applied Catalysis B: Environmental 86 (2009) 1–7 A R T I C L E I N F O  Article history: Received 17 April 2008 Received in revised form 17 July 2008 Accepted 24 July 2008 Available online 31 July 2008 Keywords: Sol–gel TiO 2  coatingsTemplate (PEG)PhotocatalysisTrichloroethylene A B S T R A C T Porous TiO 2 -anatase films were prepared by sol–gel route showing higher photocatalytic activity indegradationoftrichloroethylene(TCE)inaircomparedtodensetitaniafilms.Titaniasolsweresynthesizedwithandwithoutaporegeneratingagent,polyethyleneglycol(PEG),toevaluatetheeffectofporosityinthephotocatalytic activity of the coatings. The films were deposited by dipping and sintered at differenttemperature and time. The characterisation was performed by profilometry, Fourier transform infraredspectroscopy(FTIR),grazingX-raydiffraction(GXRD)andfieldemisionscanelectronmicroscopy(FE-SEM),observing that anatase phase is obtained at temperatures as low as 350 8 C. The maximum specific surfacearea( S  s  = 43 m 2 /g)wasobtainedforcoatingspreparedfromTiO 2 solwithPEGandsinteredat400  8 C.PorousTiO 2 -anatase films present TCE conversion around 20% higher than that of dense films. Porous volume,surface area and thickness of the coating play a key role in the photocatalytic activity. On the other side,variation in particle size seems not to be a critical parameter in the studied range.   2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +34 91 7355840; fax: +34 91 7355843. E-mail address:  castro@icv.csic.es (Y. Castro). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter    2008 Elsevier B.V. All rights reserved.doi:10.1016/j.apcatb.2008.07.021  [1,2], the major problem arises from their high resistance tophysical, chemical or biological treatments. The paint andadhesive industry, as well as combustion processes are amongthe most important anthropogenic sources of VOCs emissionsinto the atmosphere [3].Photocatalysis is an efficient, attractive and clean technologyfor pollutant abatement either in aqueous media or in gas phase[4,5]. TiO 2  is the archetypical photocatalytic material since it isendowed with an inherent photocatalytic activity [6,7]. Moreover,it is inexpensive, very stable and available in large amounts.SpeciallyforairtreatmenttheusageofTiO 2 particlesisnotfeasibledue to thehigh costs of theconcomitant filtrationfacilities neededtorecoverthecatalystandtherisktoreleaseTiO 2 particlesintotheatmosphere. Therefore, processes focused on the development of supported catalysts with high photocatalytic performance arereceivinggreatattentioninthelastyearseitherinair[8,9]orwatertreatments [10–12].Different methods have been used to prepare TiO 2  films:reactive method [13], chemical vapour deposition, sputtering,pulsed laser deposition (PLD) [14] or hydrothermal method [15]. But sol–gel process is considered as one of the most promisingalternatives because it presents a number of advantages such aslow sintering temperature, versatility of processing and homo-geneity at molecular level. This method allows obtaining TiO 2 -anatase at low temperature. This phase has been extensivelyinvestigated because its high activity in photocatalytic applica-tions [16,17].TiO 2  powders and gels with porous structure and highphotocatalytic performance have been reported [18,19]. However,thepreparationofporousTiO 2 filmswithhighspecificsurfaceareais attracting more and more attention [20–22]. Photocatalyticprocesses are chemical reactions on the surface. Thus, the increaseof surface area should improve the efficiency of the processbecause it implicates larger contact surfaces exposed to thepollutants [23,24].Porous inorganic TiO 2 -anatase films can be obtained usingtemplating membranes [25] or conventional alkoxide sol–gelroute with the addition of surfactants [26]. The templates permitto retain the initial polymer morphology up to final porousstructure. Polyethylene glycol is especially suitable for modify-ing the porous structure of coatings [27,28] due to its completedecomposition at relatively low temperature [29,30]. However,the control of synthesis and deposition processes is crucial forobtaining thick, crack-free and homogeneous coatings.Crystalline phase, specific surface area, surface OH groups,morphology and aggregation of particles are some of theparameters playing a decisive role in the photocatalyticefficiency of titania [31,32]. However, diverse results can befound in literature. While some authors highlight the highsurface area as determining factor to increase efficiency [33–35]others point out the influence of crystallinity and particle size[36].The aim of this work was to investigate the photocatalyticactivity of porous TiO 2 -anatase coatings as a function of surfacearea, porosity, crystallinity and particle size using trichloroethy-lene (TCE) as VOC molecule. This compound has been widely usedas a model pollutant in semiconductor photocatalysis because itcan be easily photodegraded.Porouscoatingswerepreparedusingasimpleandeasytoscale-up alkoxide route by adding polyethylene glycol (PEG) as poregenerating agent. Dense nanocrystalline TiO 2 -anatase thin filmsused as reference were also synthesized. The synthesis and heattreatment conditions were optimised for obtaining coatings withhigh crystallinity and surface area. An innovative method todetermine surface area of coatings is also described. 2. Experimental  2.1. Synthesis and characterisation of sols 2.1.1. Synthesis of sols Two titania sols (sol A and sol B) were prepared usingtitanium isopropoxide (TISP) as precursor via acid catalysis withthe following route. First, TISP was chemically modified byadding acetyl-acetone (AcAc) to control the hydrolysis andcondensation reactions. This solution was maintained for 1 hunder stirring up to obtain the complex chelate. Ethanol mixedwith acidified water (0.1N HCl) was added drop by drop to thissolution to start hydrolysis and condensation reactions. The finalmolar ratio of sol A was 1 TISP:1 AcAc:40 EtOH:1 H 2 O, the oxideconcentration being fixed to 30 g/L. Sol A was aged for 1 daybefore coating deposition.A similar process was used to prepare titania sol B but addingPEG (average molecular weight of 400) to the ethanol–acidifiedwater. Different PEG amounts (1, 3, 5 and 10 mol%), wereincorporated maintaining the previous molar ratios of precursors.Sol B was heated up to 80  8 C for 1 h to ensure the reactivitybetween titania oligomers and PEG [21].  2.1.2. Viscosity The stability of the sols was studied through the evolutionof viscosity with time, using a rheometer (Haake, RS50,Germany) under controlled rate conditions. The shearrate was increased from 0 to 1000 s  1 in 5 min, with 1 minat the maximum rate and decreasing again to 0 in 5 min, at25  8 C.  2.2. Deposition and characterisation of coatings 2.2.1. Coatings deposition AfirstlayerofdenseSiO 2 [37](200 nm)wasdepositedontotheglass substrates to avoid the degradation of the photocatalyticactivity [38]. This coating, with a 95% of theoretical density, hasdemonstrated to inhibit the diffusion of Na + cations from the glasssubstrate during firing [39,40].TiO 2  films were deposited by dip-coating from TiO 2  sol A andTiO 2  sol B onto SiO 2  coated glass-slides and silicon wafers. Thewithdrawal rate varied from 5 to 60 cm/min. The coatings weretreated in air at 350, 400, 450 and 500  8 C for 1, 3 and 10 h usingheating and cooling rates of 10  8 C/min. Finally, multilayercoatingsof each sol were prepared using an intermediate treatment of 200  8 C/30 min between depositions.  2.2.2. Optical and field-emission scanning electron microscopy (FE-SEM) characterisation Optical and FE-SEM (Hitachi-S4700, Japan) were used to studythe homogeneity and microstructure of the coatings.  2.2.3. Fourier transform infrared spectroscopy (FTIR) and grazing X-ray diffraction (GXRD) characterisation Coatings deposited onto silicon wafers were analysed by FTIR to study the structural evolution. FTIR spectra were recorded intransmission mode in the frequency range 4000–400 cm  1 witha resolution of 2 cm  1 using a PerkinElmer FTIR (Spectrum 100equipment). The crystallisation of TiO 2 -anatase was followed bygrazing angle X-ray diffraction using CuK a  radiation in a Brukerdiffractometer (Siemens-D5000). The diffractrogram wasrecorded in 2 u   ranges of 20–70 8  and 23–27 8 , using a fixedcounting time of 20 s/step and 2 u   increment of 0.03 8 . Thecrystallite size ( D ) of the coatings sintered at 350, 400, 450 and500  8 C for 1, 3 and 10 h was estimated using the Scherrer’s N. Arconada et al./Applied Catalysis B: Environmental 86 (2009) 1–7  2  equation: D ¼ 0 : 9 lb cos u  B (1)where  l  is the wavelength of K a  (0.15409 nm),  b  the quartzstandard-corrected full width at half-maximum (FWHM) of theBragg peak (rad) and  u  B  is the Bragg angle ( 8 ). The peak 2 u  B =  25.2 8 was chosen as the main peak for determining anatase phase. Thecrystal fraction evolution was followed through the area of thispeak.  2.2.4. Specific surface area The specific surface area was analysed as a function of temperature and time following a method described by Dura´nand Nieto [41]. Glass-microspheres with size between 40 and70 m mwereplacedinaglasscolumnclosedbyafilter.SolAandsolB were dripped through the microspheres in the column andshaken in order to homogenize the system. Then, a flow of N 2  waspassed through the column to dry the product. The coatedmicrosphereswereseparatedinfourpartsandheat-treatedat350,400,450and500  8 Cfor1,3and10 h.TheamountofTiO 2 depositedonto the microspheres was determined by chemical analysis (XRF,MagiX PW2424, Philip, Holland) and the surface area wasmeasuredbyN 2 -adsorptionBETmethodusingaMonosorbSurfaceAreaAnalyserMS-13(QuantachromeCo.,USA).Thesurfaceareaof the coatings was obtained through the equation: d TiO 2  ¼ d measured  d microspheres TiO 2  amount (2)where  d TiO 2  (m 2 /g) is the surface are of TiO 2  coating,  d measured  (m 2 /g) the value obtained through BET method with the coatedmicrospheres,  d microspheres  (m 2 /g) the surface area of the uncoatedmicrospheresandTiO 2 amount,istheweightofTiO 2 obtainedfromchemical analysis.  2.2.5. Photocatalytic activity The photocatalytic properties of the supported photo-catalyserswere evaluated by studying the photocatalyst oxidation of TCE as amodel VOCs molecule in a continuous plug flow gas-phase flatphotoreactor. This photoreactor, with external dimensions120 mm  50 mm  10 mm (length  wide  depth) was con-structed in stainless steel except one window of 27 cm 2 , closedwith borosilicate glass with low iron content. Illumination isprovided by two UVA Philips TL-8W/05 fluorescent lamps with amaximum emission at 365nm wavelength and light intensity of 4.4 mW/cm 2 . A gas mixture of TCE/ air was prepared using a gascylinderofTCE/N 2 (AirLiquide,500 ppm)andcompressedairfreeof waterandCO 2 .Theflowratewascontrolledbyusingelectronicmassflow controllers. The TCE concentration was maintained at 90ppm,andthetotalgasflowwasvariedfrom50to200 mlmin  1 operatingat residence time between 3.0 and 0.8 s. The TCE evolution withreactiontimewasanalysedbygaschromatographyusingaHewlettPackard HP6890 series with a flame injector detector (FID). 3. Results and discussion  3.1. TiO  2  sols and films Homogeneous and transparent sols were obtained for bothcompositions.ViscosityofTiO 2 solAandBwith1,3,5and10 mol%of PEG was measured at 25  8 C. In all the cases, a Newtonianbehaviour was observed with viscosities between 1.7 and 2 mPa srevealing a good stability for more than 1 month.TiO 2  films were deposited by dip-coating from sol A and sol Busing different withdrawal rates and heat treated at differenttemperatures and times. The coatings obtained from sol A presentgood optical quality being homogeneous, transparent and crackfree,withacriticalthicknessof80 nmaftertreatedat500  8 C/1 h.Inthe case of sol B, the coatings obtained with 5 and 10 mol% of PEGpresented some defects and roughness, making difficult thethickness measurement. A molar ratio of 3% was thus selectedas the maximum amount of PEG compatible with homogeneouscoatings. Under this condition, critical thickness of coatings with3% molar of PEG was found near of 100 nm after treatment at500  8 C/1 h. This indicates that the addition of the surfactantgenerates thicker coatings compared to dense film, a phenomenonrecently explained in porous coatings as the result of stressrelaxation in pores [42].  3.2. Effect of sintering temperature and time in the crystallisation of TiO  2 -anatase The coatings prepared on silicon wafers from both sols wereanalysed by FTIR and GXRD to follow the crystallisation of TiO 2 -anatase. FTIR spectra confirm that the coatings are free of organicresidues. Fig. 1 shows the FTIR spectra of (a) TiO 2 -sol A and (b)TiO 2 -sol B coatings treated at 350  8 C for different times. The bandat 435 cm  1 assigned to TiO 2  in anatase phase [43] is clearlyobserved. For coatings obtained from sol A the band is onlydetected for the longest treatment of 10 h at 350  8 C. However, Fig.1. FTIR spectra of (a) TiO 2 -solAand(b) TiO 2 -solB treatedat 350  8 C for differenttimes. N. Arconada et al./Applied Catalysis B: Environmental 86 (2009) 1–7   3  coatings from sol B show this band after only 3 h of thermaltreatment at the same temperature, showing that the incorpora-tion of PEG permits to reduce the sintering time. For highertemperatures, anatase band appears clearly after 1 h of treatment.Thus, FTIR appears as a suitable and rapid method with highsensitivity to detect small amounts of anatase, though it is notstrictly quantitative.GXRD was further used to quantify the crystal size and area of peak (25.2 8 ) as a function of temperature and sintering time. Theareawascalculatedtoevaluatethedevelopmentofcrystalfractionwhich is proportional to the area of the peak. The diffractionpatterns of coatings of TiO 2  sol A and sol B with 3% PEG treated atdifferent temperatures and times were obtained in the 2 u   range of 20–70 8 (notshown).Themainbandat2 u   25.2 8 ,associatedtothe(1 0 1) lattice plane of the tetragonal TiO 2 -anatase phase (JCPDS-89-4921) was identified in both types of coatings. A narrowerinterval of 2 u   (23–27 8 ) was further used to follow the evolution of thewidthandareaoftheanatasepeakasafunctionoftemperatureand time of heat treatment. Fig. 2 shows the GXRD spectra in thisrangeforcoatingsfromsolBtreatedat500  8 Cfor1,3and10 h.Thepeak corresponding to anatase increases slowly and becomesnarrowerandmoresymmetricwiththesinteringtime,indicatingabetter crystallisation and/or higher crystal fraction. Even usingsevereheattreatmentconditions,i.e.500  8 C/10 hTiO 2 -rutilephasewas not observed.Fig. 3 shows the evolution of crystalline fraction, consideredproportional to the area of (1 0 1) peak obtained from GXRD, as afunction of temperature and time of the heat treatment. Animportant increment is observed for short treatments, associatedwiththerapidformationand/orgrowingofanatasecrystals.Above3 h of heat treatment, the crystalline fraction maintains quiteconstant for  T   400  8 C. Coatings treated at 350  8 C need a longertime (10 h) for total crystallisation.The evolution of anatase crystal size ( D ) was estimated usingthe Scherrer’s equation. The FWHM values were extracted fromthe XRD patterns fitted using the Peakfit software. The thicknessand the calculated particle size of the coatings are representedin Fig. 4 as a function of temperature and sintering time. Thecoating thickness decreases with temperature and time of thethermal treatment due to contraction of the network associatedwith the sintering process. On the other hand, and in spite of theerror is quite high (around 5%), an increasing trend in crystallitesize with sintering temperature and time is apparent for 450 and500  8 C. At 400  8 C the growing is much slower not beingappreciable in the used time range. This behaviour is probablyrelated with the activation energy necessary for an atom to leavethe matrix and attach to the crystal that strongly depends ontemperature [44]. A similar effect was observed in densecoatings obtained from sol A.  3.3. Textural and morphological properties of TiO  2  thin films Other relevant property related with photocatalytic activity isthe specific surface area ( S  s ). Since no experimental techniquesareavailable to measure this property in coatings an alternativeprocedure was used. Glass micro-spheres were coated with TiO 2 sols A and B with 3% of PEG and sintered at different temperaturesand times. The specific surface area was measured by N 2 adsorption BET method and  S  s  of the coatings were calculatedusing Eq. (2). Fig. 5 shows the specific surface area of the TiO 2 coatings sintered for 3 h at different temperatures. At 350  8 C lowvalues of   S  s  are observed probably due to the incomplete removalof organic residues, still present in the structure. Above thistemperature,  S  s  increases up to a maximum situated around 400–450  8 C.ForsolAthismaximum(25 m 2 /g)maintainsquiteconstant Fig.2. GXRDspectraofcoatingsfromTiO 2 -solBtreatedat500  8 Cfordifferenttimes. Fig.3.  Area of GXRD peaks on coatings fromTiO 2  sol B fordifferent heat treatments. Fig. 4.  Crystal size and thickness of coatings obtained from TiO 2 -sol B for differenttreatments. N. Arconada et al./Applied Catalysis B: Environmental 86 (2009) 1–7  4  when temperature increases, indicating that maximum densityhas been reached around 400  8 C. On the other hand, for TiO 2  sol B,the maximum specific surface area is near 43 m 2 /g, suggestingcoatingswithmoreporousstructure.Whentemperatureincreases S  s  decreases and a gradual closing and collapse of the structuretakes places, down to reach the  S  s  value of dense coatings fromTiO 2 -sol A. For these conditions (500  8 C, 3 h), similar results forboth preparation routes were obtained.The evolution of   S  s  with sintering time (1, 3 and 10 h) was alsostudied observing a decrease likely associated with the partialcollapse or densification of the coating structure.FE-SEMwasusedtoanalysethemorphology(homogeneityandporosity) of the coated microspheres (Fig. 6). Micrograph 6(a) shows the total crystallisation of TiO 2  coating, with a narrowcrystal size distribution centred around 27 nm, in good agreementwith the values obtained by GXRD. Coatings produced by TiO 2 -solB show a more porous structure compared with TiO 2 -sol A,micrographs Fig. 6(c) and (b), confirmed by  S  s  measurements.  3.4. Photocatalytic activity The effect of sintering time in the photocatalytic activity wasevaluated for TiO 2  thin films prepared using sol A and sol B routestreated at 500  8 C. The TCE conversion curves for TiO 2  samplesprepared with a 3% PEG treated during 1, 3 and 10 h are shown inFig. 7. Photocatalytic curves present similar trends, decreasing theTCE conversion with the sintering time and gas flow. A 90% TCEconversion was obtained for 1 h sintering time and 50 mL min  1 gas flow. When catalysts prepared from sol A route were analysedsimilar results were obtained. As described in Fig. 5, the specificsurface area of coatings prepared from both sols and treated at500  8 C/3 h was similar and thus, a comparable photoactivity isexpected. Longer sintering times lead to a decrease in thephotocatalytic activity independent of the sol type. This effect isespecially relevant when sintering time increases from 3 to 10 h.According to previous data, TiO 2 -anatase was the only phasedetected, and crystal fraction and particle size do not significantlychange with sintering time from 3 to 10 h. Thus, the reduction of photoactivity with sintering time should mainly be related to thedensification of the coatings and the subsequent decrease of thesurface area and porosity. Photoactivity data were collected in Fig. 5.  Specific surface area ( S  s ) of TiO 2  coatings from sol A and B as a function of sintering temperature. Fig. 6.  FE-SEM microphotographs of microspheres coated and treated at 450  8 C for 10 h from (a) and (b) TiO 2 -sol A and (c) TiO 2 -sol B. N. Arconada et al./Applied Catalysis B: Environmental 86 (2009) 1–7   5
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