Enhanced Photocatalytic Activity of TiO2 Rutile by Coupling with FlyAshes for the Removal of NO GasesE. Luevano-Hipolito*,†,‡ and A. Martínez-de la Cruz†
†CIIDIT, Facultad de Ingeniería Mecanica y Electrica, Universidad Autonoma de Nuevo Leon, Ciudad Universitaria, C.P. 66451, SanNicolas de los Garza, N. L., Mexico‡Departamento de Ecomateriales y Energía, Facultad de Ingeniería Civil, Universidad Autonoma de Nuevo Leon, CiudadUniversitaria, C.P. 66451, San Nicolas de los Garza, N. L., Mexico
*S Supporting Information
ABSTRACT: A composite of TiO2 rutile with fly ash was prepared by acoupling method assisted with ultrasound irradiation in order to enhancethe photocatalytic activity of the semiconductor oxide. The effect of thetype of the acid added (acetic, oxalic, citric, and nitric) in the course ofthe coupling of TiO2 with the fly ash was investigated. The use of acidswith carboxylic groups promoted a better integration of the fly ashes intothe TiO2 surface, as was revealed from the photocatalytic experiments.Among the different acids tested, the acetic acid promoted the formationof the photocatalyst with the highest photocatalytic activity for the photo-oxidation reaction of nitric oxide (NO) in gaseousphase. For this reaction, the optimal load of fly ash on TiO2 rutile was determined in 1.0% of weight. The selectivity of the photo-oxidation reaction of nitric oxide until the formation of nitrate ions (NO3
−) was also investigated. The addition of fly ash overTiO2 rutile had a positive effect in increasing the formation of nitrate ions in comparison with the bare TiO2 rutile. Thecomposite made with affordable raw material represents a potential photocatalyst with low production cost.
1. INTRODUCTION
Titanium dioxide (TiO2) is the semiconductor oxide mostwidely used as photocatalyst due to its high photocatalyticactivity, photostability, high inertia to chemical corrosion, andlow toxicity. Titanium dioxide is a solid of a marked ioniccharacter due to the presence of Ti4+ and O2− ions in itscrystalline structure. It has four polymorphs: anatase(tetragonal), rutile (tetragonal), brookite (orthorhombic), andTiO2 (B) (monoclinic).1 According to calorimetric data, rutileis the TiO2 polymorph thermodynamically stable in a widetemperature range and at pressures over 60 kbar.2 In nature, therutile polymorph is abundantly available. Synthetic rutile isproduced employing as raw materials natural rutile, ilmenitemineral (FeTiO3), slags, and beach sands.3 The TiO2 rutileproduced is mainly used as pigment in paints (56%), plastics(25%), paper (9%), titanium sponge (4%) and for welding(6%).4
The commercial oxide used in most of the scientific papers inthe field of heterogeneous photocatalysis is the Aeroxide TiO2P-25 (Evonik), which is a mixture of the anatase (80% wt) andrutile (20% wt) polymorphs. There are limited reports thatstudied the photocatalytic activity of the rutile polymorph inliquid and gaseous phase. For example, Macphee et al. studiedthe photocatalytic activity of different TiO2 oxides including therutile and brookite polymorphs.5 They found that thecommercial rutile had the lowest photocatalytic activity forNOx conversion. The high photocatalytic activity of the anatasecompared with the rutile polymorph can be explained due tothe higher electron mobility, low dielectric constant, higher
Fermi level, lower ability to adsorb oxygen, and higher degreeof hydroxylation of the anatase.6,7 In addition, anotherexplanation is based on the type of electronic transition ofanatase (indirect) and rutile (direct).8 In general, semi-conductors with indirect transitions produce charge carriersspecies with lifetimes greater than the related with directtransitions.Different strategies have been proposed to increase the
photocatalytic activity of TiO2. For NOx photo-oxidationreaction, several modifications of the photocatalyst have beenproposed in order to increase the specific surface area, decreasethe particle size, and reduce the recombination process of thecharge carrier species generated in the photocatalyst.9−11
Additionally the modification of the TiO2 by adding lowamounts of other oxides or metals has been proposed in orderto reduce the recombination rate of the charge carrierspecies.12−14 On the other hand, different authors haveproposed the addition of fly ashes (FA) into the synthesisprocess of TiO2 anatase to increase its photocatalytic activity indifferent reactions. For example, Hak-Yong Kim et al. studiedthe addition of FA into the sol−gel synthesis of TiO2 anataseand its potential use as photocatalyst to remove organicpollutants from water and for antibacterial tests.15 On the otherhand, Yeon-Tae Yu used TiCl4, HCl, and fly ash to develop a
Received: August 28, 2016Revised: October 6, 2016Accepted: October 12, 2016Published: October 12, 2016
TiO2 anatase photocatalyst with an ability to remove 67% ofnitric oxide from air. They found that the transformation ofFe2O3 present in the fly ash from magnetite to hematite byheating the sample to 700 °C plays an important role inincreasing the photocatalytic activity.16 Nevertheless, to reachthese values of NO conversion degree, it was necessary to add10% wt of TiO2 anatase into the fly ash.Titanium dioxide can be incorporated as raw material in large
applications such as cement, asphalt, ceramic tiles, glass, andpaints. Because these materials are usually applied over surfacesexposed to sun light irradiation, this technology can beconsidered for the development of new sustainable materialsfor the industry of construction. For this reason, an importantfactor to choosing the commercial oxide is the cost involved.For example, the commercial cost of a rutile oxide (DuPont Ti-Pure R-706 $2.95 USD/kg) is considerably lower than thetraditional TiO2 used in photocatalytic applications (EvonikAeroxide P-25, $45.00 USD/kg). On the basis of this parameterand of a possible incorporation of the TiO2 in a commercialmaterial for the industry of the construction, the enhancementof the photocatalytic activity of the cheap rutile oxide is aninteresting expectation.In the present work, the photocatalytic activity of the
commercial TiO2 rutile (R-706) was enhanced by the additionof different amounts of fly ash into the matrix of thesemiconductor oxide. The composite prepared incorporatesthe use of low cost raw materials by taking advantage of anindustrial waste, i.e., the fly ash. The re-collection of this type ofindustrial waste represents an additional step for airpurification, since its disposal requires large quantity of land,water, and energy. In addition, the fine particles present in thefly ash can volatilize in air and generate different issues in theenvironment. The photocatalytic activity of the materialsprepared was tested in the photo-oxidation reaction of NO.
2. EXPERIMENTAL SECTION
2.1. Synthesis. The titanium dioxide rutile used as pristinematerial was the oxide R-706 commercialized by DuPont,which according with the technical data has a chemicalcomposition mainly of 93% wt of TiO2, and the rest is
Al2O3, SiO2, and ZnO oxides. The oxide R-706 is widely usedas a pigment with an excellent dispersibility due to its aluminacontent. Three fly ashes (FA1, FA2, and FA3) with differentcompositions were supplied from the local steel industry. Thecontent of free carbon in each one was 6.5%, 4.5%, and 7.5% forFA1, FA2, and FA3, respectively. The chemical composition ofthe fly ashes was supplied by the provider, and the data areshown in Figure 1.The preparation of the composite was carried out by a
coupling method assisted with ultrasound irradiation. In atypical procedure, 1 g of TiO2 R-706 was added to a solutionwith 10% v/v of acetic acid (CH3CO2H, Aldrich, 99%) undervigorous stirring for 30 min. Afterward, different amounts of flyash (0.5, 1.0, 5.0 and 10% wt) were added into the TiO2dispersion under vigorous stirring for 30 min. Later, thedispersion of TiO2 and fly ash was placed in an ultrasonic bathfor 1 h to promote the coupling of the ash in the particles ofrutile. The dispersion obtained was kept in repose during 1 day,and then it was washed three times with deionized water andethanol. Finally, the powder was dried at 70 °C for 12 h.Furthermore, the composites were prepared under different
acidic conditions in order to carry out the activation of R-706surface. In addition to the acetic acid (C2H4O2), the oxalic(C2H2O4), citric (C6H8O7), and one inorganic acid (HNO3)were chosen as surface activators. One additional experimentwas performed using NaOH as activator due to its knownproperties to dissolve the silicates that can be present in the flyashes.
2.2. Characterization. The structural characterization wascarried out by X-ray powder diffraction using a Bruker D8Advance diffractometer with Cu Kα radiation (40 kV, 30 mA).A typical run was made with a 0.05° of step size and a dwelltime of 0.5 s. The energy band gap of the samples wasestimated from the UV−vis diffuse reflectance absorptionspectra obtained using an Agilent Technologies UV−vis−NIRspectrophotometer model Cary 5000 series equipped with anintegrating sphere. The reflectance spectra were transformed tothe Kubelka−Munk remission function considering the directtransition of TiO2 (rutile). The BET surface area measurementswere carried out by N2 adsorption−desorption isotherms by
Figure 1. Chemical composition of the fly ashes used.
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means of a Bel-Japan Minisorp II surface area and pore sizeanalyzer. The N2 adsorption−desorption isotherms wereevaluated at −196 °C after a pretreatment of the samples at150 °C for 24 h.2.3. Photocatalytic Experiments. The removal of nitric
oxide from air was determined at NTP conditions (T = 293 Kand P = 1 atm) in a continuous flow reactor designed accordingto the ISO 22197-1 as was previously described.17,18 The massof the photocatalyst was 0.1 g, and it was brush coated over anarea of 0.08 m2 of a glass substrate using ethanol as agentdispersant. The concentration of inlet gas was 1 ppm of nitricoxide in air and was introduced into the reactor at 1 L·min−1.The irradiance in the center of the photocatalytic reactor was8.2 W·m−2, and it was provided by two fluorescent black lamps(TecnoLite) of 20 W. The concentration of NO wascontinuously measured with a chemiluminescent NO analyzer(EcoPhysics CLD88p).In addition, the final products of the NO photo-oxidation
reaction were also investigated. For this purpose, a dispersion ofthe photocatalyst used was sonicated for 30 min and then it wascentrifuged to obtain a crystalline solution. The concentrationof nitrates ions in the obtained solution was measured in a DR/890 Hach colorimeter trough by the reduction of nitrate tonitrite using cadmium as catalyst. In the same way, a similarcolorimetric method was used for the determination of nitrites.It was estimated from the reddish color of the solution,proportional to the concentration of nitrites, which wasdeveloped by a diazotization reaction in the presence ofsulfanilic and chromotropic acid.
3. RESULTS AND DISCUSSION3.1. Characterization. The commercial oxide R-706 and
the composites obtained were characterized structurally by X-ray powder diffraction technique, and their diffractograms areshown in Figures 2 and 3. In the first instance, the main
reflections of the diffractogram of the commercial TiO2correspond with the crystalline structure of rutile accordingto the card JCPDS 01-1292. However, additional diffractionlines corresponding with the hexagonal ZnO phase (JCPDS 36-1451) were detected. Other compounds present in R-706 atlow content (<5%) were not detected due to the level ofresolution of the X-ray diffraction technique, i.e., Al2O3 andSiO2. The diffractogram of the three fly ashes used are shown as
Supporting Information Figure S1. The main compoundsfound in the fly ashes were SiO2, CaO, MgO, Al2O3, Na2O, andFe2O3. Figure 3a shows the diffractograms obtained whendifferent fly ashes were added into the rutile in a concentrationof 1% wt. As was expected, it was not observed significantchanges in their diffractograms due to the low concentration ofthe fly ash. Nevertheless in all cases the crystallinity of thesamples, related to the intensity of the (110) plane of TiO2rutile, was increased. For comparative purposes, a blankexperiment was performed where R-706 was exposed to thesame experimental conditions without any addition of the flyash (see Supporting Information Figure S2). The oppositeeffect observed in the samples prepared will be furtherdiscussed.The fly ash with the highest photocatalytic activity (as will be
shown later) FA3 was selected to be added in different amountsin R-706 in order to investigate its effect in the physical andphotocatalytic properties of TiO2. In all cases, the addition ofthe fly ash FA3 had a positive effect in increasing thecrystallinity of rutile regarding the intensity of the (110)plane (Figure 3b).The diffuse reflectance spectra of TiO2 samples were
obtained by UV−vis spectroscopy in the range of 200−800nm. There were not observed significant changes in the energyband gap values of the samples. The energy band gap valueswere similar between them and minor than 3.1 eV, Table 1.The effect of the addition of fly ashes over the specific surface
area of TiO2 was investigated by means of N2 adsorption−desorption isotherms, which are shown in SupportingInformation Figure S3. The specific surface area values ofTiO2 rutile samples are shown in Table 1. First, the fly ashesused had lower surface values (<0.7 m2·g−1) due to its origin incombustion process at high temperatures (>1000 °C).However, when the fly ashes were added into the oxide R-706 (10.3 m2·g−1), the surface area increased 55% by theincorporation of only 1% wt of FA3. At higher loads of fly ash,the surface area does not increase significantly. When FA1 andFA2 where added into R-706 oxide, the increase in surface areawas only 24% and 28%, respectively. The addition of nitric andcitric acid during the coupling of R-706 and FA3 had adetrimental effect in the surface area. Only the oxalic acidshows a similar effect in the surface area obtained whencompared with acetic acid. This can be attributed to thecarboxylic group present in the acetic and oxalic acids, whichcan act as steric stabilizers that promote the formation ofparticles with low size and thus high surface area values whenthe ashes are present in the medium. In this sense, the citricacid is rich in carboxylic groups; however its chain is larger thanacetic and oxalic acids and can be adsorbed in the TiO2 pores.Thus, these functional groups can further create obstacles forphysical adsorption, which prevents that more adsorbatemolecules can be further adsorbed. In addition, due to itsrelative larger molecular weight, its removal will be moredifficult in the washing process in comparison with the otheroxides.On the other hand, when the reaction medium was basic by
the presence of NaOH, the surface area value was 6.4 m2·g−1. Inthe literature, it is well-known that a basic medium (NaOH)promotes the dissolution of Si and Al atoms present in the flyash producing a gel, which is product of the polymerization ofthese atoms with the OH− present in the medium. The alkalineattack breaks the cover of the fly ash particles, promoting atheoretical higher number of particles expelled, which can be
Figure 2. XRD patterns of the commercial TiO2 photocatalyst.
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available to attach to the TiO2 surface. However, in this case theavailable particles in the medium can block the active sites ofTiO2, which had a detrimental effect in the adsorption of N2 toincrease the specific surface area.In addition, the morphology of the samples FA2 and FA3
was analyzed by scanning electron miscroscopy (SEM). Figure4a,b shows a sphere morphology of the fly ashes with at leasttwo differences between them. The first one is related to theirparticle sizes, which were on average 260 and 345 μm for FA2and FA3, respectively. The second difference between thestudied ashes was the exposure of their surfaces. The FA3shows damage in its surface. The high content of carbonreported (7.5%) for this fly ash is undesirable because it canblock the light absorption on TiO2 and act as recombinationcenter of the photogenerated electron and hole. For this reason,a higher amount of carbon on FA surface is not recommendedwhen only FA is used as photocatalyst but can promote a betterintegration of the FA in the surface of TiO2. For example, theaddition of 1% of FA3 in R-706 decreases slightly the averageparticle size of R-706 sample from 272 to 236 nm in thesamples (Figura 4d), which can be beneficial for the removal ofnitric oxide from air.3.2. Photocatalytic Experiments. In the first instance, the
photocatalytic activity of the fly ashes and TiO2 R-706 wasinvestigated separately (Figure 5). The oxide R-706 by itself
decreases 17% the NO concentration after being irradiated for1 h. In this sense, Macphee et al. studied the photocatalyticactivity of TiO2 rutile and found a very low conversion of NO(<5%) when used as photocatalyst.5 In this case, the sample R-706 is composed of a mixture of phases where the TiO2 rutile isthe main phase. The presence of ZnO as wurtzite in R-706could improve the photocatalytic activity due to its higheractivity with respect to the TiO2 rutile.Regarding with the activity of the fly ashes, the FA2
promoted a higher decrease in NO conversion in 1 h (22%).This can be related to its chemical composition. In this context,the fly ashes used have a similar amount of SiO2 (>30% wt) buta slightly different content of CaO. The FA2 has the highestCaO content followed by FA3, which was previously reportedto promote the adsorption of NOx gases.
19,20 This adsorptionaffinity is attributed to the base-type sites present on the CaOsurface. The basicity of the FA2, which is commonly known asthe ratio of the content of CaO/SiO2 in ceramic mixtures, hadthe highest value (1.0) followed by FA3 with 0.9 and FA1 with0.7 (Figure 1). Furthermore, the chemical composition of theflash ashes has a variety of oxides that can participate in thephoto-oxidation of NO, in spite of the low photocatalyticactivity of the oxides TiO2 rutile, Fe2O3, MgO, and K2O. Forexample, Kasem prepared a composite with Fe2O3 and MgOthat promotes an efficient separation of the electron and hole
Figure 3. XRD patterns of the TiO2 rutile photocatalyst integrated with fly ashes.
Table 1. Physical Properties of the Samples
sample fly ash used % wt fly ash acid or base used band gap (eV) BET surface area (m2·g‑1)
pair to carry out the reduction of H2O to H2.21 In the NO
photo-oxidation, both processes (adsorption and efficient pairelectron−hole generation) can promote a decrease in the nitricoxide concentration. In order to corroborate that the decreasein the NO concentration using FA2 as photocatalyst was bymeans of the combination of the photocatalyst oxides (TiO2,Fe2O3, MgO, and K2O) present in its composition, and notonly for an adsorption process, the irradiance in the reactor wasincreased at 10 W·m−2. The obtained results indicated theimportant role of photocatalysis heterogeneous in the oxidationof NO since the efficiency of the reaction increased 13.2% athigher value of irradiance.
However, using FA2 in combination with R-706 onlydecreased the NO concentration 32%. The better result wasthrough the coupling of FA3 with R-706, resulting in 42% ofdecrease of NO concentration after 1 h (Figure 6). Theseresults can be related to the highest surface area developed bythe sample R-706-FA3-1.0%.
Additionally, some experiments were performed in order toinvestigate the effect of the amount of FA3 in R-706. Figure 7shows that between the intervals of % FA studied, the bestresults were obtained with the addition of 1.0% wt of FA3 intoR-706 oxide. From this point, increasing the % FA3 in R-706had a negative effect on the ability of the photocatalyst tooxidize NO. The origin of the low conversion of nitric oxide at
Figure 4. SEM images of (a) FA2, (b) FA3, (c) R-706, and (d) R-706-FA3-1.0%.
Figure 5. Evolution of nitric oxide concentration when the rawmaterials were employed as photocatalyst (Q = 1 L·min−1, <0.4 ppmof H2O, ±2% error in the measurements of NO).
Figure 6. Evolution of nitric oxide concentration when TiO2 rutile wascoupled with different fly ashes (Q = 1 L·min−1, I = 8.2 W·m−2, <0.4ppm of H2O, ±2% error in the measurements of NO).
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high loads of FA in R-706 can be related to the recombinationof the pair electron and hole in the ashes when it reaches thesurface of R-706. In the photocatalysis literature, the use of anadditional material as cocatalyst has a positive effect as long asits load was less than 1.4 wt %.22 For example, Riaz studied theoptimal concentration of Fe2O3 in TiO2 anatase and reported abrief review of different techniques for carrying out an effectivedoping. According to this analysis, the optimal loadings ofFe2O3 in TiO2 employing different synthesis routes were alsoless than 1.4 wt %, which is in line with the obtained results. Onthe contrary, low amounts of FA3 in R-706 (1.0 wt %) promotea rapid decrease in nitric oxide concentration of about 75% inthe first 25 min and a 40% of removal of pollutant in steadystate. This fact can be associated with several factors, such ashigh surface area and an increase in the lifetime of thephotogenerated electron and holes.The effect of the type of acid added during the coupling of
FA3 in R-706 was investigated to carry out the oxidation of NO(Figure 8). The obtained results indicated that the acetic and
oxalic acid showed the best result when used as TiO2 surfaceactivator during the coupling with ashes, a situation that can berelated to their relative high surface areas values. In addition, ascan be seen in the Figure 8, the activation of the oxalic acid wasslower in comparison with acetic acid. This situation can beattributed to the carbon residual that can be present in thesamples, which can act as recombination center of the electronand hole produced during the photocatalytic process. Never-theless, after 1 h of being irradiated the conversion of nitricoxide was almost the same. The good results obtained withacetic and oxalic acid can be attributed to their carboxylicgroups (−COOH) in solution that promotes a betterintegration of FA over the surface of R-706. In this sense, ithas been reported the ability of carboxylic group to beefficiently attached into the TiO2 nanoparticles (anatase andrutile).23 Once the acid is adsorbed on TiO2 surface, it canpromote the coupling of the FA with the R-706 activatedsurface by different mechanism, which depends on the availablefunctional groups.Figure 9 shows a scheme of the adsorption of the different
acids in TiO2 surface. As can be seen, the acetic, oxalic, and
citric acid provide the TiO2 surface with carboxylic groups,which promote the integration of the ashes into the TiO2surface. However, its removal is difficult during the washprocess, limiting active sites to carry out the oxidation of NO,especially citric acid due to its relative higher molecular weight(192) in comparison with the acetic (60) and oxalic (90) acids.In the same way, the larger carbon chain of citric acid is difficultto remove during the washing process. On the contrary, aceticand nitric acids have a small chain, which in theory facilitatestheir removal of the final product. Nevertheless, the nitric acidpromotes the formation of monodentade ligands highly stablein the TiO2 surface. This fact can also make difficult theremoval of residual nitrates of surface by a washing process.Thus, according to the obtained results, the acetic acid seems tobe the ideal activator molecule to carry out an efficientintegration of the fly ash in TiO2 surface. When NaOH wasused during the coupling of FA3 with R-706, the obtainedphotocatalyst did not have a remarkably ability to oxidize NO(18%), which can be related to a poor integration of the asheswith the TiO2 surface even if they are in hydroxide form.The products of the photo-oxidation of nitric oxide (NO3
−/NO2
−) were analyzed by a colorimetric method, as wasdescribed in the experimental procedure. For this purpose, thephotocatalyst (R-706-FA3-1.0%) was washed several times(prior its use) to remove the residual nitrate ions from therutile and ashes (Figure 10). The wash process was repeatedseveral times until the mass of the ions was constant. Once thesample was washed and dried, it was used as photocatalyst inthe oxidation of nitric oxide under UV irradiation for 1 h. At the
Figure 7. Effect of the adding different amounts of FA3 to TiO2 rutilein the evolution of nitric oxide concentration (Q = 1 L·min−1, I = 8.2W·m−2, <0.4 ppm of H2O, ±2% error in the measurements of NO).
Figure 8. Effect of the acid added during the coupling of TiO2 rutilewith fly ashes in the evolution of nitric oxide concentration (Q = 1 L·min−1, I = 8.2 W·m−2, <0.4 ppm of H2O, ±2% error in themeasurements of NO).
Figure 9. Activation of TiO2 surface by different acids.
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end of the photocatalytic reaction, the sample R-706-FA3-1.0%was washed in order to determine the nitrate and nitrite ions,which were associated with the photo-oxidation of NO.According to the obtained results, the selectivity for NO3
−
ion formation from NO gas was 12% when the sample R-706-FA3-1.0% was employed as photocatalyst at 8 W·m−2. Forcomparative purposes, the selectivity for NO3
− ion formationemploying R-706 as photocatalyst was calculated, which wasless than 1%.The obtained results suggest that coupling the commercial
TiO2 R-706 with an industrial waste (fly ash) allows theproduction of a low cost photocatalyst with the ability to carryout a partial oxidation of NO to NO3
− ions, which areinnocuous and can be absorbed by plants and used as fertilizer.
4. CONCLUSIONSThe ability of a commercial oxide composed of rutile structureto oxidize nitric oxide was improved by adding fly ash in itscomposition. When 1.0% in weight was added to thecommercial TiO2 rutile, the photocatalytic activity wassignificant improved which can be related to a higher numberof active sites to carry out the oxidation of NO. In the sameway, the increase of the surface area of the material can play animportant role in the photocatalytic process. In addition, theacetic acid was efficient in the impregnation of the fly ash in theTiO2 surface due to the presence of carboxylic groups in itschemical composition and a relative small carbon chain. Thephotocatalyst proposed had the ability to oxidize 12% the NOmolecule to innocuous products, such as NO3
− ions. Thepreparation of the photocatalyst proposed represents a low costprocess to produce efficient photocatalysts to carry out theremoval of NOx gases and purify the quality of the air.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.iecr.6b03302.
Figures S1 and S2 showing the X-ray diffraction patternsof references employed in this work (fly ashes FA1, FA2,FA3, and TiO2 R-706) and Figure S3 showing the N2adsorption−desorption isotherms (PDF)
■ ACKNOWLEDGMENTSWe thank the CONACYT for its invaluable support throughthe Project 167018. In addition, we thank Arturo Gonzalez forproviding us with the fly ashes.
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Figure 10. Accumulative mass of nitrates generated in the photo-catalytic experiment using R-706 (rutile) and R-706 with 1.0% of FA3(Q = 1 L·min−1, I = 8.2 W·m−2, <0.4 ppm of H2O).
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(18) Luevano-Hipolito, E.; Martínez de la Cruz, A. Enhancement ofphotocatalytic properties of TiO2 for NO photo-oxidation byoptimized sol-gel synthesis. Res. Chem. Intermed. 2016, 42, 7065−7084.(19) Huang, H. Y.; Long, R. Q.; Yang, R. T. The Promoting Role ofNoble Metals on NOx Storage Catalyst and Mechanistic Study of NOxStorage under Lean-Burn Conditions. Energy Fuels 2001, 15, 205−213.(20) Verrier, C.; Kwak, J. H.; Kim, D. H.; Peden, C. H. F.; Szanyi, J.NOx uptake on alkaline earth oxides (BaO, MgO, CaO and SrO)supported on γ-Al2O3. Catal. Today 2008, 136, 121−127.(21) Kasem, K. Hydrogen Production by Selective Photo-dissociationof Water in Aqueous Colloidal Nano-particles of Doped Iron (III)Oxides Semiconductors. J. Mater. Sci. Technol. 2010, 26, 619−624.(22) Riaz, N.; Bustam, M. A. B. Iron doped TiO2 photocatalysts forenvironmental applications: fundamentals and progress. Proceedings ofthe 2013 International Conference on Biology and Biomedicine; WSFASPress: Greece, 2013; pp 72−77.(23) Jiang, B.; Yin, H.; Jiang, T.; Jiang, Y.; Feng, H.; Chen, K.; Zhou,W.; Wada, Y. Hydrothermal synthesis of rutile TiO2 nanoparticlesusing hydroxyl and carboxyl group-containing organics as modifiers.Mater. Chem. Phys. 2006, 98, 231−235.
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