3878
Ind. Eng. Chem. Res. 1999, 38, 3878-3885
MATERIALS AND INTERFACES Testing the Efficacy and the Potential Effect on Indoor Air Quality of a Transparent Self-Cleaning TiO2-Coated Glass through the Degradation of a Fluoranthene Layer Virginie Rome´ as,† Pierre Pichat,*,† Chantal Guillard,† Thierry Chopin,‡ and Corinne Lehaut‡ Laboratoire “Photocatalyse, Catalyse et Environnement”, CNRS UMR “IFoS”, Ecole Centrale de Lyon, BP 163, 69131 Ecully CEDEX, France, and Centre de Recherches, Rhodia, 93308 Aubervilliers CEDEX, France
Self-cleaning glass can be obtained by coating glass with a transparent, thin layer of TiO2 nanoparticles. To test the self-cleaning properties, fluoranthenesthe most abundant polycyclic aromatic hydrocarbon in the atmospheric particulate matterswas sprayed over the glass. Under solar-like UV light, not only was fluoranthene removed at a rate of ca. 0.73 nmol/h per cm2 of glass but also all fluoranthene degradation products were, and thus the coated-glass transparency was recovered, which did not occur with noncoated glass. The fluoranthene percentage converted to volatile carbonyl products released into ambient air was lower with than without TiO2 coating; i.e., the self-cleaning glass could have a positive influence on indoor air quality. Mechanisms are discussed to account for the main primary products among the 40 fluoranthene photocatalytic degradation intermediate products which we identified. Introduction When glass is coated with a transparent, thin TiO2 layer, it acquires self-cleaning properties under UV irradiation,1-3 since photoexcited TiO2 can mineralize nearly all organic compounds in the presence of dioxygen at room temperature,4 although obviously with differing rates. In addition, emission of intermediate products must be estimated. Methods to adequately create a thin TiO2 film onto glass have been patented.5,6 Results about the removal of stains formed from grease or tobacco smoke7,8 on TiO2-coated glass have been reported, as well as the removal rate of layers of stearic (octadecanoic)9 and palmitic (hexadecanoic)10 acids deposited on the same kind of material. One of our objectives was to study the removal of a still more stable compound representative of dirt which can be formed on window panes or glass items. Fluoranthene (Figure 1) was selected because it is one of the most abundant PAHs (polycyclic aromatic hydrocarbons) in the atmospheric aerosol. PAHs are produced during combustion processes.11,12 Vehicle exhaust is a major source of fluoranthene; the average emission amount of fluoranthene for a vehicle is on the order of 30 µg/km; this value corresponds to ca. 3.1 kg of fluoranthene emitted per hour in the Los Angeles area.13 Compared to other PAHs, fluoranthene is relatively slowly photolyzed.14,15 Moreover, its vapor pressure is only 0.254 Pa at 298 K;16 therefore, evaporation should not significantly contribute to fluoranthene removal from surfaces onto which this PAH is deposited. * Corresponding author. Fax: (+33)-4-78-33-03-37. Tel: (+33)-4-72-18-64-95. E-mail:
[email protected]. † Ecole Centrale de Lyon. ‡ Rhodia.
Figure 1. Fluoranthene structure.
Identification of degradation intermediate products of compounds deposited on TiO2-coated glass has been reported only for palmitic acid.10 This kind of research is deemed to be important not only to better understand the degradation mechanisms and pathways, but also to assess the potential effects on air composition if this type of glass was employed indoors. Identifying as many products as possible from deposited fluoranthene under simulated solar UV irradiation was, therefore, another objective of this study. Within the framework of research dealing with heterogeneous phase atmospheric photochemistry,17 degradation of fluoranthene supported on various insulating or semiconducting oxides particles, including TiO2, has been reported.17a The degradation percentages of fluoranthene were compared. Product analysis concerned only nitrofluoranthene isomers which were generated in the presence of NO2-polluted air. Eight aromatic products were found by two of us17b when another PAH, naphthalene, adsorbed on TiO2 Degussa P-25 powder, was photocatalytically degraded, and mechanisms were tentatively proposed to account for the formation of two primary products.
10.1021/ie990326k CCC: $18.00 © 1999 American Chemical Society Published on Web 09/10/1999
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3879 Table 1. Methods Used to Detect and Identify Intermediate Products on the Glass Plates method no.
product recovery
analysis
1
four or five glass plates in 200 mL of water 15 min ultrasonication (40 Hz) evaporation to ca. 2 mL
2
one glass plate in 200 mL of water 15 min ultrasonication (40 Hz) evaporation to 5 mL
HPLC-UV analysis, Sarasep Car-H column eluent, H2SO4 aqueous solution; pH 2 to 3 0.7 mL/min λ ) 210 nm quantitative HPLC-UV analysis Spherisorb ODS 2 column eluent, 50%/50% v/v water/methanol pH ) 2.5; 1 mL/min λ ) 270 nm GC-MS analysis injector, 493 K oven, from 348 K to 523 K (5 K/min), 10 min at 523 K detector, 533 K Addition of ca. 0.5 mL diazomethane in diethylether and GC-MS analysis in the conditions indicated above
3 four or five glass plates in 200 mL of water 15 min ultrasonication (40 Hz) complete evaporation and subsequent dissolving in 2 mL of methanol
4
Table 2. SPME-GC-MS Conditions Used for Analyzing the Confined Gaseous Phase method no.
SPME fiber
5
PDMS-DVB 65 µm
6
Carboxen-PDMS 75 µm
column CP-SiL 5CB 25 m, 0.25 mm, 1.2 µm Poraplot Q 25 m, 0.25 mm, 8 µm
In this work, we have studied the kinetics of disappearance of fluoranthene sprayed on TiO2-coated or TiO2-free glass. We have identified 40 aromatic and aliphatic degradation intermediate products recovered from the TiO2-coated glass or detected in the gaseous phase of the photoreactor containing the glass plates, and we have established the mass balance. Amounts of carbonyl products emitted from the glass with or without TiO2 coating during fluoranthene degradation have been measured. Mechanisms and pathways are discussed to interpret the formation of the polycyclic products. Experimental Section Fluoranthene (98%) was purchased from Aldrich, and its degradation intermediate products were purchased from Aldrich, Fluka, or Merck. Experimental details concerning the glass preparation, the photoreactor, and light sources have been published.10 Briefly, the dip-coating method and subsequent calcination above 623 K, depicted in ref 6a, were employed to cover the glass with TiO2 nanoparticles (>250 m2 g-1) mixed with a photocatalytically stable binder containing a titanium alkoxide and another organometallic precursor.6b The resulting transparent, hydrophilic layer was ca. 45 nm thick, and the TiO2 particles had the anatase structure. Scanning electron micrographs of this layer have been published.10 The photoreactor enabled homogeneous UV-A irradiation of five glass plates (10 cm long, 3 cm wide, 4 mm thick) simultaneously with a radiant power of 7-10 W m-2, i.e. on the same order of magnitude as that received from the sun on a horizontal plane at midlatitudes for wavelengths below 380 nm. The photoreactor was either filled with dioxygen and closed or a 50 mL min-1 dioxygen flow was circulated. Fluoranthene was deposited onto the glass plates by spraying (system from Ecospray, Roth) a 6 g/L solution of this compound in ethyl ether. When not otherwise specified, approximatively 1.0 mg of fluoranthene was
injector temp (K) 493 513
oven temp 5 min at 313 K, from 313 to 493 K (5 K/min) 5 min at 313 K, 10 K/min to 453 K, 5 K/min to 513 K
detector temp (K) 493 523
sprayed on each plate, i.e., 0.165 µmol/cm2, which corresponded to a layer of ca. 270 nm, as calculated from the density of fluoranthene at 273 K. The reproducibility of the deposited amount was evaluated to be (15%. The amount of fluoranthene present on each glass plate was determined by GC-FID analysis (Varian 3400, CP-SiL 5CB column, 25 m long, 0.32 mm i.d., 1.2 µm film thickness) of the solution obtained by washing the plate with 5 mL of cyclohexane containing 200 mg/L of naphthalene used as the internal standard. From subsequent dissolution experiments it was inferred that the fluoranthene recovery was total. To detect the watersoluble organic products remaining on the glass plates, the plates were immersed in 200 mL of ultrapure water. After 15 min of sonication (40 Hz), the solution was concentrated by use of a rotary evaporator. It was assumed that the highly water-soluble products were not evolved during this process. The subsequent analytical conditions are indicated in Table 1. Addition of diazomethane (method 4, Table 1) before some GC-MS analyses (HP 5890 series II GC, HP 5971A MS, CP-SiL 5CB column, 25 m long, 0.25 mm i.d., 1.2 µm film thickness) allowed the derivatization of carboxylic acids into their corresponding methyl ester for a better elution. Degradation intermediate products released in the atmosphere of the closed photoreactor were collected on poly(dimethylsiloxane)-divinylbenzene (PDMS-DVB) 65 µm or Carboxen-PDMS 75 µm SPME (solid-phase microextraction) fibers purchased from Supelco. Principle and main characteristics of the SPME technique are presented in refs 18 and 19. These fibers were exposed to the photoreactor atmosphere during 15 min in the dark after various irradiation durations. The collected analytes were desorbed and analyzed under the conditions listed in Table 2. Carbon dioxide was analyzed in the closed photoreactor by use of a catharometer gas chromatograph equipped with a Porapak Q column (80-100 mesh, 3 m × 1/4 in.). Other analyses were carried out when a 50 mL/min dioxygen flow was circulated through the photoreactor
3880
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999
Figure 2. UV absorption spectrum of fluoranthene in cyclohexane (0.2 g/L).
Figure 3. Disappearance, under simulated UV solar irradiation, of fluoranthene deposited on TiO2-coated or TiO2-free glass.
during the photocatalytic degradation. The objective was to determine the mass balance of the fluoranthene degradation under conditions which are close to the application for indoor glass panes or glass materials. To evaluate the amount of carbon dioxide, exiting dioxygen was bubbled into three successive bottles containing each 200 mL of barium dihydroxide aqueous solution (1 mol/L). Barium dihydroxide in excess was titrated with an aqueous solution of potassium hydrogenophthalate (1.5 mol/L). The devices used to trap the organic products were ORBO 32 activated charcoal or 2,4-DNPH-coated silica (LpDNPH10) cartridges, both from Supelco, through which the dioxygen exiting the photoreactor was passed. The subsequent analytical procedures have been described.10 Results and Discussion 1. Kinetics of Fluoranthene Disappearance. 1.1. Degradation of Fluoranthene Deposited on a TiO2-Free Glass. Fluoranthene absorbs in the 300400 nm range (Figure 2). Under UV irradiation, it becomes fluorescent and is photolyzed, although rather slowly compared to other PAHs.14 Fluoranthene was deposited onto TiO2-free glass and its disappearance under UV irradiation was measured: 0.8 mg of fluoranthene per 30 cm2 glass plate (0.132 µmol/cm2, ca. 220 nm thick layer) are degraded within ca. 80 h of irradiation (Figure 3) under our conditions. An apparent zero-order decay was observed for the beginning of the degradation with an apparent rate constant k ) 0.33 ( 0.12 nmol/h per cm2 of glass. Note that, in this case, the initial transparency of the glass is not recovered even after long irradiation durations. The glass plates were yellowish, whereas the initial fluoranthene layer was white. 1.2. Degradation of Fluoranthene Deposited on TiO2-Coated Glass. In this case, 1 mg of fluoranthene per plate (0.165 µmol/cm2, 270 nm thick layer) was degraded within ca. 45 h of irradiation (Figure 3) and
the visual transparency of the glass was recovered after roughly 90 h of irradiation. At the beginning of the degradation, an apparent zero-order decay was observed with a rate constant of k ) 0.73 ( 0.25 nmol/h per cm2 of glass. No difference was found when air was used in place of dioxygen. The degradation rate of fluoranthene was thus approximately doubled owing to the TiO2 coating. 2. Identification of the Degradation Intermediate Products of Fluoranthene Deposited on TiO2Coated Glass. 2.1. Intermediate Products Detected on the Glass. All the 14 products detected on the glass are shown in Figure 4. To dissolve them, polar solvents were more efficient than apolar solvents. Most products were carboxylic acids and, accordingly, had a strong affinity for TiO2. Oxalic (ethanedioic) acid, malonic (propanedioic) acid, succinic (butanedioic) acid, benzoic acid, phthalic (1,2-benzenedioic) acid, trimellitic (1, 2, 4-benzenetrioic) acid, and 9-fluorenone-1-carboxylic acid were identified by GC-MS after derivatization as methyl, dimethyl, or trimethyl esters (Table 1). 1,8Naphthalenedicarboxylic acid also is probably a degradation intermediate, as indicated by the computer match to the reference mass fragmentogram in the NBS 49K library (the standard was not available). Oxalic and phthalic acid were detected as well after dissolution and analysis by HPLC using a Car-H column (method 1, Table 1). The same method allowed the detection of maleic (cis-butenoic) acid. GC-MS analysis without previous derivatization (method 3, Table 1) revealed the presence of 1,8naphthalic anhydride, 9-fluorenone, and a hydroxy-9fluorenone isomer. For two minor chromatographic peaks, no matches with the computer library were satisfactory; on the basis of the interpretation of the mass fragmentograms, the origins of these peaks were thought to be a hydroxynaphthalic anhydride isomer and 2-(1-hydroxyethyl)-1,3-indenedione. 9-Fluorenone-1-carboxylic acid and 1,8-naphthalic anhydride were produced in sufficient amounts to be quantified (method 2, Table 1). The maximum amounts were reached when approximately 75% of fluoranthene was degraded; these maxima corresponded to ca. 10% and ca. 20% of converted fluoranthene for 9-fluorenone1-carboxylic acid and 1,8-naphthalic anhydride, respectively. (Figure 5). The temporal variations showed that the acid was formed less rapidly than the anhydride. Both products were unstable on the UV-irradiated TiO2 surface. 9-Fluorenone-1-carboxylic acid is orange and 1,8naphthalic anhydride is yellow. This explains that the glass, initially covered with a white fluoranthene layer, became yellowish during the degradation. 2.2. Intermediate Products Detected in the Gaseous Phase. When a 50 mL/min O2 flow was circulated through the photoreactor, carbonyl compounds were trapped and derivatized on two 2,4-DNPH-coated silica cartridges placed in series. Subsequent HPLC analysis revealed the presence of acetone and C1-C6 linear aldehydes. 9-Fluorenone was trapped in these cartridges without being derivatized and it was identified by GCMS. Under our analytical conditions, no intermediate products were detected after trapping in ORBO 32 cartridges. Most of the intermediate products were detected and identified by SPME-GC-MS. The best results were
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3881
Figure 4. Fluoranthene degradation intermediate products detected on the glass plates.
Figure 5. Temporal variations in the amounts of 1,8-naphthalic anhydride (diamonds) and 9-fluorenone-1-carboxylic acid (squares) in the closed photoreactor during fluoranthene (initial fluoranthene amount, 4.4 µmol per glass plate) degradation.
obtained with 75 µm Carboxen-PDMS SPME fibers and the Poraplot Q column (method 6, Table 2). Twenty-five products were identified by comparison with standards: methyl-2-propene, but-2-ene, pent-1-ene, hex-1-ene, hept1-ene, oct-1-ene, C5-C7 linear alkanes, cyclohexane, C1C7 linear aldehydes, propen-2-al, acetone, butanone, pentan-2-one, heptan-3-one, acetic acid, propanoic acid, ethyl formate, and ethyl acetate. Analyses performed after increasing irradiation durations showed the progressive formation and degradation of the volatile intermediate products. After 100 h of irradiation in the closed photoreactor, all organic products had almost disappeared. Analysis with 65 µm PDMS-DVB fibers and the CPSiL 5CB column (method 5, Table 2) confirmed the presence of hexanal, heptanal, hept-1-ene, and phthalic acid. The latter product was also detected on the glass. None of the intermediate products detected either on the glass plates or in the gaseous phase were found in the control experiments carried out without UV irradiation. 3. Mass Balance for the Degradation of Fluoranthene Deposited on TiO2-Coated Glass. The CO2 amount in the enclosed atmosphere regularly increased for the first 40 h of irradiation and reached a plateau within ca. 90 h (Figure 6). When the amount of fluoranthene initially deposited was 23 ( 4 µmol, which potentially corresponded to 360 ( 50 µmol CO2 for total mineralization, our measurements showed the formation of 438 ( 40 µmol of CO2. Considering these accuracies, the mineralization of fluoranthene can be
Figure 6. Production of CO2 during the photocatalytic degradation of fluoranthene in the closed photoreactor (initial fluoranthene amount, 23 µmol). Horizontal lines are maximum and minimum (considering the accuracy in the amount of deposited fluoranthene) of CO2 expected from complete mineralization.
regarded as being complete when the photocatalytic degradation took place in the closed photoreactor. The excess in CO2 could not entirely originate from TiO2 precontamination, since no excess was found in the case of a palmitic acid layer.10 When CO2 was quantified in the 50 mL/min O2 flow coming out of the photoreactor, the conversion rate of fluoranthene into CO2 was found to be 82 ( 15%. Carbonyl compounds emitted in this latter case were quantitatively collected in 2,4-DNPH-coated silica cartridges. To avoid saturation of the cartridge, the degradation was carried out on only one glass plate (instead of five) covered with 0.30 ( 0.04 mg fluoranthene (1.48 ( 0.22 µmol). The amounts of volatile carbonyl compound collected after 116 h of UV irradiation (complete fluoranthene degradation) are indicated in Table 3. The sum of the amounts of acetone and C1-C6 linear aldehydes emitted under these conditions represented 16 ( 3% of the amount of organic carbon initially present in the photoreactor. The amount of 9-fluorenone was ca. 2% of the carbon amount. Acrolein (propen-2-al) was detected in the closed photoreactor with a Carboxen-PDMS SPME fiber, but it was not detected in the exiting O2 flow by use of a LpDNPH10 cartridge. This means that less than 0.5 µg of acrolein was emitted from 0.30 mg of fluoranthene (i.e. 1 > 2 (Figure 1). This order is generally in agreement with that experimentally observed, except that positions 7 and 8 are inverted. When the radical-cation charge is located on the naphthalene moiety (Scheme 1a) a 4,5-(or 3,2-)oxirane would be formed because position 4 (or 3) is more reactive.25 Oxirane opening would then yield dialdehyde A. This product was not detected, presumably because it contains an unsaturated binding R-located with respect to one of the formyl groups and was therefore prone to be rapidly transformed into 9-fluorenone-1carbaldehyde B and then oxidized into 9-fluorenone-1carboxylic acid, which was detected. Decarboxylation, commonly observed in photocatalysis,26-28 would yield 9-fluorenone. When the radical-cation charge is located on the benzene moiety (Scheme 1b), the oxirane would be
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3883 Scheme 1. Fluoranthene Degradation Pathway Initiated by Direct Reaction of a Photogenerated Hole on the (a) Naphthalenic Structure and (b) Benzenic Structurea
Scheme 2. Fluoranthene Degradation Pathway Initiated by Hydroxyl Radical Attack on the (a) Naphthalenic Structure and (b) Benzenic Structurea
a
a
F
b
b
D D
C
C
E
a Framed products are those that were detected. See Scheme 1b for the transformation of naphthalene-1,8-dicarbaldehyde. a
Framed products are those that were detected.
formed on positions 7,8 (or 10,9) since these are the reactive sites according to ref 25. Oxirane opening would then yield dialdehyde C which has not been detected, presumably because it can undergo the same transformations as A (Scheme 1a) and lead to dialdehyde D and, after oxidation and decarboxylation, to acenaphthylene (E). Naphthalene-1,8-dicarboxylic acid and its anhydride are obtained when applying to E the same mechanisms as to fluoranthene, assuming that the radical-cation charge is located over the five-member ring, which is less stable than the naphthalene moiety. 5.2. Degradation Pathways Involving OH Radicals. We now consider that fluoranthene is initially attacked, predominantly at the reactive sites indicated in ref 25, by a hydroxyl radical formed from a photogenerated hole and an adsorbed water molecule or a negatively charged OH group. In the case of an attack on the naphthalene ring, a OH adduct such as that shown in Scheme 2a is formed and, by easy oxygen addition, leads to the polycyclic
peroxy radical RO2•. Elimination of HO2• from RO2• would produce an hydroxylated derivative. As no such derivatives were detected, this pathway is not included in Scheme 2a. Alternatively, RO2• could abstract a H atom29 from a fluoranthene molecule, FL (or possibly from another molecule depending on the stage the cleaning process has reached), to produce a hydroperoxide FL
RO2• 98 ROOH from which dialdehyde A could be formed through
ROOH f RO• + •OH RO• + O2 f A + HO2• Similar reactions are usually considered for hydroperoxides and alkoxy radicals. Formation of tetroxides via
2RO2• f RO4R and RO2• + HO2• f RO4H
3884
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999
has also been proposed30-32 and is believed to be predominant at least in some cases.9,33,34 Tetroxides could generate the radical labeled F in Scheme 2a
RO4R f 2F• + O2
RO4H f F• + O2 + •OH
if it is assumed that their decomposition causes the opening of the rings carrying the peroxide bonds. Formation of the aldehydic radical labeled F is equivalent to the formation of an aldehyde R1(or R2)CHO and an alkyl radical R1• or R2• from a (R1R2CH)2O4 tetroxide.30-32 Addition of dioxygen and removal of hydroperoxyl should readily transform the radical labeled F into the dialdehyde A. Because of the high number of fluoranthene molecules, the pathway through hydroperoxides may be thought to dominate the pathway involving tetroxides, at least initially. From dialdehyde A, 9-fluorenone-1-carboxylic acid and 9-fluorenone would be obtained through the same pathways as in Scheme 1a. Similar mechanisms in the case of a •OH radical attack on the benzene moiety of fluoranthene are presented in Scheme 2b. 5.3. Conclusion on the Photocatalytic Degradation Pathways. Formation of four out of seven of the polycyclic products found (Figure 4) is shown in the degradation schemes. The same mechanisms also allow one to easily explain the formation of two more polycyclic products, since they are monohydroxylated derivatives (hydroxynaphthalic anhydride and hydroxy-9fluorenone) of two of the products included in the degradation schemes. The mechanism through a radical-cation only involves reaction of this species with superoxide and dioxygen to transform a fluoranthene molecule into a primary product (Schemes 1a, 1b). The •OH radical initiated mechanism is less straightforward, since the transformation of the fluoranthene molecule initially attacked would require another fluoranthene molecule or radical except in the case where the formation of a RO4H tetroxide is assumed (Scheme 2a,b). Also, the absence of hydroxylated products that have kept the fluoranthene structure casts some doubt about the relative importance of the mechanism via •OH radicals. However, these products might have eluded our analyses because they are rapidly transformed. Note that the nucleophilic attack of a water molecule on the fluoranthene radical-cation and subsequent deprotonation would also produce hydroxylated fluoranthene,35,36 but results based on quinoline as a probe molecule suggested that, even in water, the nucleophilic attack of the corresponding radical-cation by superoxide (as shown in Schemes 1a,b for fluoranthene) is more probable than by water, especially at near to neutral pH.23 In short, although the fluoranthene structure, unlike that of quinoline with its two different rings, does not allow one to definitively discriminate between the two types of mechanisms commonly considered in heterogeneous photocatalysis, the above arguments seem to indicate that the mechanism via the radical-cation prevails for fluoranthene sprayed on the TiO2 coating. Conclusion The removal rate of a fluoranthene deposit over TiO2coated glass, although ca. 10 times lower than that of palmitic acid,10 which contains the same number of
carbon atoms, appears compatible with the expected application as is the time required to achieve complete cleaning. The identification of the degradation intermediate products shows that each aromatic ring of fluoranthene is attacked, which leads to tricyclic compounds as primary products. Direct transfer of a photogenerated hole to fluoranthene or hydroxyl radical attack can both explain the formation of these products. Nevertheless, the absence of hydroxylated fluoranthene suggests that the former mechanism dominates. Further degradation produces monocyclic aromatics and aliphatics pertaining to all chemical categories. However, most of these products are formed in amounts corresponding in all to
