R. Enriquez, B. Beaugiraud and P. Pichat Laboratoire “Photocatalyse, Catalyse et Environnement”, CNRS UMR “IFoS”, Ecole Centrale de Lyon, 69134 Ecully Cedex, France (E-mail:
[email protected]) Abstract The photocatalytic removal rates r of dichloroacetic acid (DCAA), 4-chlorobenzoic acid (4-CBA) and 4-chlorophenol (4-CP) in water were compared for TiO2 samples used either as a powder or as a coating on a fiber glass tissue, SiO2 being the binder. From SEM-EDX measurements it was deduced that SiO2 prevails over TiO2 in the coating top layers and 18O2–Ti16O2 isotopic exchange showed that the accessibility of O2 to TiO2 was markedly reduced when TiO2 was thus coated. The unfavorable effect of the restricted TiO2 accessibility on r was drastic for DCAA, much less pronounced for 4-CBA, and still smaller for 4-CP. It is inferred that DCAA can be attacked only when it directly interacts with TiO2, whereas 4-CP can also react within the near-TiO2 surface water layers. The 4-CBA intermediate behavior is in line with the structural similarities of 4-CBA with DCAA and 4-CP. Keywords Dichloroacetic acid; 4-chlorobenzoic acid; 4-chlorophenol; O2 isotope exchange; photocatalysis; TiO2
Introduction
One of us has previously shown that the removal rate of poorly adsorbed aromatic pollutants in UV-irradiated TiO2 aqueous suspensions can be correlated to the octan-1-ol/water partition coefficient (representing the adsorption) and the Hammett or Brown coefficient (representing the electron availability on the ring) (Pichat et al., 1993; Amalric et al., 1996). However, these relationships are limited to structurally similar pollutants which are assumed to react in the same way. For pollutants with different chemical functionalities, the role of the adsorbed amount is not as clear and has led us to investigate whether photocatalytic reactions might occur within the solution layers in the vicinity of the TiO2 surface (Cunningham et al., 1994). Our approach was to employ a fiber glass tissue onto which TiO2 had been coated using colloidal SiO2. Silica, which improves the anchoring of TiO2, restrains the TiO2 accessibility. The basic idea was to take advantage of this restricted accessibility to address the above mentioned question. It was assumed that the accessibility effect on the pollutant removal rate should differ depending on whether the primary reaction step required a close contact between TiO2 and the pollutant or could take place at some distance from the TiO2 surface. Two SiO2 samples were used to modify the accessibility to TiO2 and three TiO2 samples were employed to reach more reliable conclusions. As test compounds we chose chlorinated pollutants, viz. (i) dichloroacetic acid (DCAA) because it contains a carboxylic group and its H atom is poorly labile as a result of the two Cl atoms carried by the same C atom so that decarboxylation should be the dominant initial degradation pathway, (ii) 4-chlorophenol (4-CP) because its extent of adsorption on TiO2 is low, and (iii) 4-chlorobenzoic acid (4-CBA) because it has in common with DCAA and 4-CP either the carboxylic group or the benzenic ring, and because it adsorbs on TiO2 in greater amount than 4-CP.
Water Science and Technology Vol 49 No 4 pp 147–152© IWA Publishing 2004
Mechanistic implications of the effect of TiO2 accessibility in TiO2–SiO2 coatings upon chlorinated organics photocatalytic removal in water
147
Methods Materials
R. Enriquez et al.
The TiO2 samples whose characteristics are given in Table 1 were used either powdered or supported on a thin fiber glass tissue. The coatings were made of a mixture containing TiO2 and an aqueous colloidal suspension of SiO2 (particle size: 2–4 nm or 20–30 nm; TiO2/SiO2 mass ratio: 1) impregnated on the tissue by use of a “size press” (patent PCT/FR 99/00748). The impregnated tissue was allowed to dry at room temperature. The total mass supported was 80 g per m2 of tissue. Apparatuses and procedures
Photocatalytic degradations were performed in an open to air, cylindrical batch photoreactor (ca. 80 mL) with a bottom Pyrex optical window (ca. 11 cm2). One centimetre above this window a glass ring enabled one to place a TiO2-coated tissue disk. The photoreactor was installed above a Phillips HPK-125 W high pressure mercury lamp whose radiation was filtered through a circulating water cuvette (2.2 cm thick) equipped with a 340 nm cutoff filter (Corning 0-52). The radiant power, received by the optical window and measured with a UDT 21 A powermeter, was found to be ca. 33.3 mW cm–2 (corresponding number of photons per second potentially absorbable by TiO2 ca. 1 × 1017). A 20-mL volume of a stock solution containing about 20 ppm of 4-CBA (0.146 mmol L–1; pH = 4.2) or 4-CP (0.155 mmol L–1; pH = 6.6) or DCAA (0.166 mmol L–1; pH = 3.8) was poured into the reactor containing either powder TiO2 or a TiO2-coated tissue disk. The mass of TiO2 powder tested in suspension corresponded to the TiO2 quantity fixed on the disk. The suspension or solution was magnetically stirred and maintained in the dark for 1 h prior to UV irradiation. Water samples were passed through 0.45 µm Millipore filters before HPLC analyses. The analyses by scanning electron microscopy (SEM)-energy dispersive X-ray microanalysis (EDX) were carried out with a Philips XL 20 apparatus and 1 cm2 samples covered by an Au layer. The apparatus used for the 18O2–Ti16O2 isotopic exchange measurements comprised a sample cell with a 12 cm2 optical window. A 6.7 cm2 disk of TiO2-coated fiber glass tissue was placed perpendicularly to the irradiation beam into this cell. Alternatively, the same mass of TiO2 was spread as uniformly as possible onto a sample-holder of the same area. The irradiation system was the same as for the degradation studies in water. The sample, first evacuated under UV irradiation, was exposed to a 16O2 pressure of ca. 13.3 kPa under UV irradiation overnight. After evacuation to 10–4 Pa, 18O2 was admitted into the cell at the desired pressure in the dark. After equilibration, the sample was UV-irradiated. The cell atmosphere was periodically analyzed. Results and discussion Assessment of the accessibility of coated TiO2
TiO2 accessibility was assessed by SEM-EDX. The higher the electron beam energy, the greater the penetration depth of the electrons and hence the greater the coating thickness analyzed. Figure 1 shows, for TiO2 PC 50 as an example, that the atomic ratio Ti/Si – which Table 1 Characteristics of the powder TiO2 samples Samples
Millennium PC 50 Rhodia Degussa P25 148
Surface area
Average pore
Elementary
(m2 g–1)
diameter (nm)
crystallite size (nm)
54 150 55
20.1 8.6 nonporous
20–30 n.d. 30
Allotropic form
100% anatase 100% anatase 80% anatase/20% rutile
3 2.5 2 Ti/ Si 1.5 1
0 5
10
12
15
Electron beam accelerating voltage, kV
Figure 1 Variations in the Ti/Si ratio vs the electron accelerating voltage for TiO2 PC 50 and 20–30 nm SiO2
R. Enriquez et al.
0.5
was expected to be 0.75 in accordance with the TiO2/SiO2 1:1 mass ratio if the coating was perfectly homogeneous – increased with an increasing accelerating voltage of the electron beam, indicating that TiO2 was partially covered by SiO2. However, from these semi-quantitative results the accessibility of various chemical compounds to TiO2 cannot be directly deduced. Consequently, we have studied the UV-induced 18O2–Ti16O2 isotopic exchange because this reaction can take place only if O2 is on the TiO2 surface. Additionally, it is a reaction of interest as O2 is involved in any photocatalytic oxidation. Figure 2 shows that, for the same mass of TiO2 and various starting pressures of 18O2, the isotopic exchange initial rate expressed by the increase in 18O16O in the gas phase over the UV-irradiated solid sample was considerably lower when TiO2 was supported by use of SiO2 as a binder. The smaller the SiO2 particle size, the lower the increase in 18O16O. In line with the SEM-EDX results indicating that SiO2 predominates over TiO2 in the top layers of the coatings, the 18O –Ti16O isotopic exchange clearly demonstrates that the accessibility of O to TiO 2 2 2 2 was very much restricted by the silica binder. Removal rates of the pollutants over powdered or coated TiO2
To compare the photocatalytic removal rates we used the initial rate r0 rather that the rate constant since the kinetic order was not always the same (vide infra). r0 decreased for TiO2 supported by means of SiO2 particles with respect to the same mass of suspended TiO2. This decrease depended on the pollutant, the type of SiO2 and the type of TiO2. It was also affected by the TiO2/SiO2 ratio; here we only report results obtained for a 1:1 mass ratio as 10
8
-1
[dP34/dt]o (Pa h )
PC50
powder (22 mg)
6 SiO2 (20-30 nm)
1.0 0.5
SiO2 (4-6 nm)
0.0 0
20
40
60
80
100
P°36 (Pa)
Figure 2 Variations in the 18O16O initial pressure vs the initial 18O2 pressure for TiO2 PC 50 and the SiO2 indicated
149
R. Enriquez et al.
it corresponds to a good compromise between the photocatalytic activity and the adhesion strength of the coating to the fiber glass tissue. The histograms of Figure 3 sum up the comparisons. The coating effect on r0 was especially detrimental for DCAA removal. When 20–30 nm SiO2 was utilized, r0 for the coating amounted only to 1–3% of r0 for the powder both for TiO2 PC 50 and TiO2 Rhodia (Figure 3, right). The use of 4–6 nm SiO2 almost suppressed the photocatalytic activity for DCAA removal of the same TiO2 samples (Figure 3, left). Taking into account that SiO2 diminishes the accessibility of the reactants to TiO2 as shown by the SEM-EDX and 18O2–Ti16O2 isotopic exchange measurements, these results indicate that a free access of DCAA to TiO2 is a prerequisite for the removal of this carboxylic acid. The coating effect on r0 for 4-CBA removal was also marked, but considerably less than for DCAA removal. The influence of the SiO2 kind was again important, since r0 was lowered by a factor of 2.7 to 4.3 when 4–6 nm SiO2 instead of 20–30 nm SiO2 was used in the coating. The unfavorable effect of the silica with the smallest particle size was also shown by a change in the kinetic order of the removal rate from 1 to 0. The zero order means that the 4-CBA concentration in the solution did not limit r0, indicating that the TiO2 surface area exposed to the solution was drastically decreased when TiO2 was anchored by 4–6 nm SiO2. Moreover, the very restricted accessibility of TiO2 to 4-CBA was supported by adsorption measurements in the dark. Whereas for 4-CBA in a TiO2 suspension a relatively important dark adsorption was observed for the concentrations used, the adsorption was nil within experimental accuracy when TiO2 was supported. In order to have an idea of the amount of coated TiO2 participating to the 4-CBA removal, we carried out a set of experiments with different masses of TiO2 PC 50 suspended in 20 mL of solution. The mass of powder PC 50 necessary for having the same activity as that of the sample fixed with 20–30 nm SiO2 was about 2 mg. This means that only about 4% of the 44 mg of TiO2 PC 50 supported was effective for the removal of 4-CBA. For the removal of 4-CP the effect induced by fixing TiO2 was far less pronounced than for 4-CBA removal. Additionally, the decrease in r0 in the case of the SiO2 with the smallest particle size was only ca. 1.6 times greater than that corresponding to the SiO2 with the biggest particle size being for both TiO2 P25 and PC 50, and being insignificant for TiO2 Rhodia. Both observations clearly indicate that the restricted accessibility to TiO2 does not matter for 4-CP removal as much as for 4-CBA removal and, above all, for DCAA removal. Comparison of pollutant removal in water with 18O2–Ti16O2 isotopic exchange As already mentioned in this paper, the isotopic exchange 18O2–Ti16O2
requires O2 to be adsorbed on TiO2 and therefore can be used to estimate the accessibility of coated TiO2, at least to a gas like O2. A comparison of the rate decrease for the isotopic exchange, when TiO2 was coated with regard to powder TiO2, with the corresponding rate decreases for
sup / ro powder
0.4 0.07 0.06 0
0 P25
0.06 0
4-CP
0.43 0.6 0.4
0.19
0.17
0.26
0.2
4-CP 0.01
0 P25
RHODIA
0.70
0.8
4-CBA DCAA
PC 50
ro
0.44 0.27
0.2
150
0.69
0.67
0.8 0.6
20-30 nm SiO2
1
4-6 nm SiO2
ro
sup / ro powder
1
0.03
4-CBA DCAA
PC 50
RHODIA
Figure 3 Ratios of the initial removal rates for supported over powdered TiO2 for the pollutants and binders indicated
R. Enriquez et al.
4-CP and 4-CBA removal in water is shown in Figure 4 for TiO2 PC 50, the two SiO2 and an initial 18O2 pressure of 11 Pa (the plots in normalized a.u. were nearly identical within the 2–50 Pa range investigated). These decreases in rates were similar for the isotopic exchange and 4-CBA removal and markedly differed for 4-CP removal. This comparison supports the fact that 4-CBA removal occurs partially through a pathway for which accessibility to TiO2 is essential, whereas it is not the case for 4-CP removal. Much higher rate decreases produced by coating TiO2 for DCAA removal than for the isotopic exchange of O2 can be explained by a greater difficulty for DCAA, compared with O2, to access to TiO2. Note that DCAA being very soluble tends to partition to water. We are conscious that these comparisons have, however, only a semiquantitative significance because of the very different experimental conditions. Indeed, the correlation between the 4-CBA removal and the isotopic exchange was not as good for TiO2 P25 as for TiO2 PC 50. Nevertheless, the tendency was the same, that is for TiO2 P25 also the elimination rate of 4-CBA was much better correlated with the 18O2–Ti16O2 exchange rate than the elimination rate of 4-CP. Conclusions
Isotopic exchange activity (a.u)
Our results clearly show that photocatalytic decarboxylation necessitates a direct interaction between the carboxylate ion and TiO2, since DCAA and 4-CBA are eliminated much faster when the accessibility to TiO2 is not restricted, as in the case of the coating with SiO2 as a binder. The decrease in degradation rate is striking for DCAA for which decarboxylation is believed to be by far the main primary degradation pathway, whereas 4-CBA can also react via its aromatic ring. This conclusion is obviously in line with a direct electron transfer from the carboxylate ions to TiO2 (Bahnemann et al., 2002; Franch et al., 2002). The relatively limited effect of the diminished accessibility to TiO2 upon 4-CP removal rate corroborates the hypothesis that photocatalytic primary reaction events can occur for some compounds within the near-surface solution layers. This was proposed by Turchi and Ollis (1990) on the basis of kinetic considerations, and supported by a series of experiments reported and discussed by Cunningham et al. (1994). Also, from the interpretation of photoelectrochemical measurements, it was deduced that surface-formed OH radicals can escape into solution (Peterson et al., 1991), a possibility further considered for fluorinated TiO2 (Minero et al., 2000). In addition, the present methodology allows us to reject the alternative hypothesis according to which compounds poorly adsorbed in the dark might react at a priori surprisingly high 1 TiO2 powder
0.8
COOH
0.6
OH
0.4
Cl
20-30 nm SiO2
0.2
Cl
4-6 nm SiO2
0 0
0.2
0.4
0.6
0.8 1 Removal activity (a.u)
Figure 4 Comparative plots of the rates of 18O2–Ti16O2 isotopic exchange and 4-CBA or 4-CP removal for TiO2 PC 50 powdered or coated with the SiO2 indicated
151
R. Enriquez et al.
rates because UV irradiation induces an increase in the extent of adsorption (Cunningham et al., 1994). If this hypothesis was valid, the reduced accessibility to TiO2 should not have such distinct effects for the pollutants studied, since the accessible area of coated TiO2 is, in particular, roughly the same for 4-CBA and 4-CP which have similar molecular sizes. The importance of the aqueous microenvironment near the TiO2 surface has also been invoked to interpret the net positive effect of humic acid (HA) revealed at low HA concentration upon the photocatalytic removal rate of quinoline (Enriquez and Pichat, 2001). The adsorption of HA is supposed to create a pocket-like microenvironment around the TiO2 particles. Quinoline molecules can be sequestered in this microenvironment. This sequestration phenomenon would decrease the probability of the quinoline molecules thus trapped diffusing to the bulk solution, and hence would maintain them in the near-surface water layers without significantly changing the low amount of quinoline molecules directly in contact with TiO2. As in the case of the relatively small effect of the restricted accessibility to TiO2 upon r0 for 4-CP compared with r0 for 4-CBA, the increase in quinoline elimination rate owing to HA (displayed at low HA concentration only, as at high concentration UV absorption by HA dominates the HA influence) can be accounted for on assuming that quinoline as 4-CP reacts without being in direct contact with TiO2. Our results also show that if SiO2 is used as a binder to coat TiO2 on a solid support by a method analogous to that employed here, the SiO2 particle size must be carefully selected to obtain the best compromise between the coating durability and the photocatalytic activity. Acknowledgements
R.E. warmly thanks the CONACYT (Mexico) for her Ph. D. scholarship. The authors are grateful to Mr. H. Courbon (IFoS) for his helpful advice in carrying out the isotopic exchange measurements and to Mr. J. Dussaud (Ahlstrom) for the gift of the TiO2-coated materials. Financial support of the European INCO-DC program is acknowledged. References Amalric, L., Guillard, C., Blanc-Brude, E. and Pichat, P. (1996). Correlation between the photocatalytic degradability over TiO2 in water of meta and para substituted methoxybenzenes and their electron density, hydrophobicity and polarizability properties. Wat. Res., 30, 1137–1142. Bahnemann, D.W., Kholuiskaya, S.N., Dillert, R., Kulak, A.I. and Kokorin, A.I. (2002). Photodestruction of dichloroacetic acid catalyzed by nano-sized TiO2 particles. Appl. Catal. B, 36, 161–169. Cunningham, J., Al-Sayyed, G. and Srijaranai, S. (1994). Adsorption of model pollutants onto TiO2 particles in relation to photoremediation of contaminated water. In Aquatic and Surface Photochemistry, Helz, G.R., Zepp, R.G. and Crosby, D.G. (eds.), Lewis, Boca Raton, FL, pp. 317–348. Enriquez, R. and Pichat, P. (2001). Interaction of humic acid, quinoline, and TiO2 in water in relation to quinoline photocatalytic removal. Langmuir, 17, 6132–6137. Franch, M.I., Ayllon, J.A., Peral, J. and Domenech, X. (2002). Photocatalytic degradation of short-chain organic diacids. Catal. Today, 76, 221–223. Minero, C., Mariella, G., Maurino, V., Vione, D. and Pelizzetti, E. (2000). Photocatalytic transformation of organic compounds in the presence of inorganic ions. 2. Competitive reactions of phenol and alcohols on a titanium dioxide-fluoride system. Langmuir, 16, 8964–8972. Peterson, M.W., Turner, J.A. and Nozik, A.J (1991). Mechanistic studies of the photocatalytic behavior of TiO2. Particles in a photoelectrochemical slurry cell and the relevance to photodetoxification reactions. J. Phys. Chem., 95, 221–225. Pichat, P., Guillard, C., Amalric, E. and D’Oliveira, J.-C. (1993). TiO2 photocatalytic destruction of water aromatic pollutants: intermediates; properties-degradability correlation; effects of inorganic ions and TiO2 surface area; comparisons with H2O2 processes. In Photocatalytic Purification and Water Treatment of Water and Air. Ollis, D.F. and Al-Ekabi, H. (eds), Elsevier, Amsterdam, pp. 207–223. Turchi, C.S. and Ollis, D.F. (1990). Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack. J. Catal., 122, 178–192. 152
