Preparation Characterization and Photocatalytic Activity of

J. Phys. Chem. B 2001, 105, 1033-1040

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Preparation Characterization and Photocatalytic Activity of Polycrystalline ZnO/TiO2 Systems. 2. Surface, Bulk Characterization, and 4-Nitrophenol Photodegradation in Liquid-Solid Regime Giuseppe Marcı`,† Vincenzo Augugliaro,† Marıá J. Lo´ pez-Mun˜ oz,‡ Cristina Martıń,§ Leonardo Palmisano,*,† Vicente Rives,§ Mario Schiavello,† Richard J. D. Tilley,| and Anna Maria Venezia⊥ Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, UniVersita` degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy, Escuela Superior de Ciencias Experimentales y Tecnologıá, Campus de Mo´ stoles, UniVersidad Rey Juan Carlos, c/Tulipa´ n, 28933 Mo´ stoles, Madrid, Spain, Departamento de Quı´mica Inorga´ nica, UniVersidad de Salamanca, 37008 Salamanca, Spain, School of Engineering DiVision of Materials Engineering, UniVersity of Wales, Cardiff, Wales, CF2 1XH U.K., and ICTPN-CNR, Via Ugo La Malfa 153, 90100 Palermo, Italy ReceiVed: September 5, 2000; In Final Form: October 28, 2000

Electron spin resonance spectroscopy (ESR), Fourier transform infrared spectroscopy (FTIR), and monitoring of pyridine (py) and boric acid trimethyl ester (BATE) adsorption for determining surface acidity and basicity, respectively, were used to carry out further characterization of mixed ZnO/TiO2 polycrystalline solids prepared by different methods. Moreover, the powders were tested in a batch photoreactor for a probe reaction, i.e., 4-nitrophenol photodegradation in aqueous medium. ESR results indicated the presence of signals attributable to Zn+ species in ZnO/TiO2 (anatase) solids, while in ZnO/TiO2 (rutile) samples the presence of zinc induced only the formation of signals probably due to Ti3+ centers. FT-IR spectra showed no significant differences of the surface hydroxylation degree of the various photocatalysts, whereas the surface acidic properties generally decrease by increasing the amount of ZnO. Coupling of ZnO and TiO2 semiconducting powders was not so beneficial, as expected on the basis of the intrinsic electronic properties, to enhance the photoreactivity for the studied reaction, although some of the powders showed photoactivities slightly higher than those of bare TiO2 and ZnO. Nevertheless, some of the samples (the mixed particles ZnO/TiO2 (rutile)) appear promising from an application point of view because no filtration was needed after the occurrence of the photoreactivity tests to separate them from the solution because they decanted easily. The mineralization of 4-nitrophenol was checked by determining the total organic carbon (TOC) and a complete photooxidation occurred in few hours in the presence of some of the samples.

Introduction Further characterization of polycrystalline ZnO/TiO2 systems prepared with different methods, as described in part 1,1 is reported in this work. In particular, the solids have been characterized by means of electron spin resonance spectroscopy (ESR), Fourier transform infrared spectroscopy (FTIR), and monitoring pyridine (py) and boric acid trimethyl ester (BATE) adsorption for determining the surface acidity and basicity. Moreover, the powders were tested in a batch photoreactor for a probe reaction, i.e., 4-nitrophenol photodegradation in aqueous medium.2,3 The choice of ZnO/TiO2 systems for photocatalytic studies is based on several considerations. Polycrystalline TiO2 and ZnO have been widely used in past decades for carrying out various photoreactions both in gas-solid and in liquid-solid regimes.1,4,5 ZnO has been reported sometimes to be more efficient than TiO2 for the photooxidation of phenol and nitrophenols,6-8 * To whom correspondence should be addressed. Phone: ++39-0916567246. Fax: ++39-091-6567280. E-mail: [email protected]. † Universita ` degli Studi di Palermo. ‡ Universidad Rey Juan Carlos. § Universidad de Salamanca. | University of Wales. ⊥ ICTPN-CNR.

although some photocorrosion effects, due to anodic decomposition were observed especially in the liquid-solid phase.6,9 Other significant differences in the photocatalytic behavior of the two semiconductors have been reported: illuminated polycrystalline ZnO in an aqueous suspension, unlike TiO2, was found to act as an efficient electron scavenger in respect to adsorbed N2O,10,11 and Cunningham et al.12 showed that gaseous N2O dissociates to N2 and O2 at room temperature onto an illuminated surface of polycrystalline ZnO. It is worth noting that the separation of the photocatalyst from the reacting aqueous suspension is a major problem in view of application of the photocatalytic processes. To minimize the above problem, many authors have supported TiO2 on various rigid supports13-18 and the use of ZnO/TiO2 films for carrying out the photooxidation of salicylic acid in aqueous medium or of ZnO films in gas phase to reduce the concentration of NO2 under illumination have been also reported.19,20 Unfortunately, these systems are usually much less efficient than the corresponding powders, due to the low surface area available for the reacting species and to the existence of mass transfer resistance. An interesting alternative is to couple two different semiconductor photocatalyts modifying their electronic properties

10.1021/jp003173j CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001

1034 J. Phys. Chem. B, Vol. 105, No. 5, 2001

Figure 1. Schematic diagram representing the charge-transfer process in TiO2/ZnO systems (pH ) 0). Adapted from data and analogous diagrams at other pH values.11,24-27

(refs 10-26 in ref 1) or to support highly photoactive species on particles resistant to mixing in water, as for instance TiO2 (anatase) on Al2O3,21 TiO2 (anatase) on TiO2 (rutile),22 and ZnO on sand.23 In this work the choice of TiO2 (anatase) as support is due to the fact that ZnO/TiO2 coupling has been reported to produce an enhancement of the photocatalytic activity in comparison to pure ZnO and TiO2 both for polycrystalline systems used to degrade phenol, 2-chlorophenol, and pentachlorophenol11 and for films used to decompose salicylic acid.19 The modification of the electronic properties of the coupled materials with respect to the single ones is invoked to explain this behavior: the electron transfer from the conduction band of ZnO to the conduction band of TiO2 under illumination and, conversely, the hole transfer from the valence band of TiO2 to the valence band of ZnO11 give rise to a decrease of the pairs recombination rate, i.e., to an increase of their lifetime (see Figure 1). This phenomenon increases the availability of the pairs on the surface of the photocatalyst and consequently an improvement of the occurrence of redox processes can be expected. As far as TiO2 (rutile) is concerned, the choice of such a support, in addition to favorable electronic factors, is mainly related to its high resistance to disaggregation in water.

Marcı` et al. windows, which allows recording of the spectra in a vacuum or under a controlled atmosphere, was used. The procedure was as follows: the solid was calcined in situ at 673 K for 2 h in the cell in order to eliminate any organic impurity adsorbed during preparation and handling. The sample was then outgassed at 673 K for 2 h at a residual pressure of ca. 10-3 Pa, and after cooling to room temperature, it was equilibrated with pyridine or boric acid trimethyl ester (BATE) and then outgassed at increasing temperatures up to 673 K. The spectrum of the solid was subtracted using the software facilities provided by the spectrometer. Photoreactivity Experiments. A Pyrex batch photoreactor of cylindrical shape containing 0.5 L of aqueous suspension was used. The photoreactor was provided with ports in its upper section for the inlet and outlet of gases, for sampling, and for pH and temperature measurements. A 125 W medium-pressure Hg lamp (Helios Italquartz) was immersed within the photoreactor and the photon flux emitted by the lamp was Φi ) 13.5 mW‚cm-2. It was measured by using a radiometer “UVX Digital” leaned against the external wall of the photoreactor containing only pure water. Oxygen was continuously bubbled into the suspensions for ca. 0.5 h before switching on the lamp throughout the occurrence of the photoreactivity experiments. The amount of catalyst used for all of the experiments was 1 g‚L-1, and the initial 4-nitrophenol (BDH) concentration was 20 mg‚L-1. The initial pH of the suspension was adjusted to 4.5 by addition of H2SO4 (Carlo Erba RPE), and the temperature inside the reactor was ca. 300 K. The photoreactivity runs lasted 6.0 h including the first half hour during which the lamp was switched off. Samples of 5 mL volume were withdrawn from the suspensions every 30 or 60 min and the catalysts were separated from the solution by filtration through 0.45 µm cellulose acetate membranes (HA, Millipore) with the exception of the samples from the TRZnN and TRZnA sets because they decant easily in a few minutes. The quantitative determination of 4-nitrophenol was performed by measuring its absorption at 315 nm with a Varian DMS 90 spectrophotometer. To check for the presence of Zn2+ ions deriving from photocorrosion of ZnO, some qualitative analyses were performed by precipitating Zn2+ as Zn2[(Fe(CN)6] and K2Zn3[Fe(CN)6]2 by adding K4[Fe(CN)6]28 to the solution obtained after the separation of the catalyst and the reduction of its volume to about 20 mL. Finally, total organic carbon (TOC) determinations were carried out for all of the runs by using a TCM 480 (Carlo Erba) apparatus.

Experimental Section Catalyst Preparation. The preparation of the photocatalysts and their codes are reported in Part 1.1 Electron Spin Resonance Spectroscopy. ESR spectra were recorded at 77 K by using a Bruker ER200D X-band spectrometer, provided with a double rectangular cavity and a ER1600 data system. The g-values were measured relative to a 2,2diphenyl-1-picrylhydrazyl (DPPH) field marker (g ) 2.0036). Samples were placed in ESR quartz tubes with double greaseless stopcocks. Outgassing and oxygen adsorption treatments were performed in a greaseless vacuum glass manifold with residual pressures e10-4 Torr. UV irradiations were carried out on the sample tube placed in a Dewar vessel made of Pyrex, filled with liquid nitrogen. A 125 W medium-pressure Hg lamp (Sylvania GTE) was employed for irradiating the samples before recording the spectra. Fourier Transform Infrared Spectroscopy (FTIR) and Surface Acidity and Basicity. The FT-IR spectra were recorded by using a Perkin-Elmer 16-PC spectrometer (nominal resolution 2 cm-1, averaging 100 scans). A special Pyrex cell with CaF2

Results and Discussion ESR. Figure 2 shows the ESR spectra of TA and two selected samples of the TAZnN set. The spectrum of TA outgassed at room temperature mainly consists of a split structure centered at a g-value close to 2. A similar resonance has been previously described and assigned to Fe3+ ions in the anatase phase.29,30 The presence of such signals (which will be referred to as signals A) may be explained by considering that the presence of traces of iron is expected to be one of the major impurities in the starting materials used for the preparation of the samples.31 Incorporation of zinc into TA led to the appearance of a new signal (signal B) with a g-value of 1.972, whose intensity increased with the zinc content, as observed in Figure 2b,c. Figure 3 shows the ESR spectra recorded after outgassing the TR sample at room temperature and two selected samples of the TRZnN set. Prior to the zinc incorporation, the spectrum of the rutile sample exhibits an orthorhombic signal (C) with g-values of 1.966, 1.936, and 1.915 (Figure 3a). Signals A are absent in the spectrum of the TR sample, in agreement with its

Polycrystalline ZnO/TiO2 Systems. 2

Figure 2. ESR spectra of TA (a), TAZnN5 (b), and TAZnN10 (c) samples outgassed at room temperature.

Figure 3. ESR spectra of TR (a), TRZnN5 (b), and TRZnN10 (c) samples outgassed at room temperature.

assignment to Fe3+ in anatase. The disappearance of the signals could be due to the structural transformation of the TiO2 (anatase) crystal host to the rutile form. Moreover, ESR resonances associated with Fe3+ ions substituting for Ti4+ in the rutile lattice29,30 were detected at g-values of 7.987, 5.622, 3.411, and 2.604 (not shown in the figure). As the zinc content is increased in the TRZnN samples, a new signal with a g-value of 1.980 develops (Figure 3 b,c). The latter signal (which will be called signal D) seems to have an axial symmetry although its other g-component is not resolved. To study the evolution of the paramagnetic species during the photoreaction, ESR spectra were also recorded after UV irradiation of the samples at 77 K. As can be observed in Figures 4 and 5, different results were obtained depending on the TiO2 crystalline phase composition and zinc content. For TAZnN samples, the group of signals A was not substantially changed by the UV irradiation; in contrast, the intensity of signal B decreased with increasing irradiation time (Figure 4). For TRZnN samples, irradiation led to a progressive enhancement of the intensity of signal C, whereas no significant changes were observed in signal D (Figure 5).

J. Phys. Chem. B, Vol. 105, No. 5, 2001 1035

Figure 4. ESR spectra of TAZnN10 sample: outgassed at room temperature (a); outgassed at room temperature and successively irradiated at 77 K for 15 min (b); same as in (b) but for 45 min (c).

Figure 5. ESR spectra of TRZnN10 sample: outgassed at room temperature (a); outgassed at room temperature and successively irradiated at 77 K for 15 min (b); same as in (b) but for 45 min (c).

The assignment of these ESR signals is proposed as follows. The signals B-D have g-values < ge (free electron factor), which indicate that they are due to d electrons of a transition element. Signal B can be related to the introduction of zinc into TiO2 (anatase) because its intensity is progressively enhanced by increasing the zinc content. It appears a nearly symmetric signal with a g-value close to that of a signal previously assigned to Zn+ ions in ZnO.32,33 On this basis, signal B is tentatively ascribed to Zn+ ions in the small ZnO nuclei formed on the anatase surface. This assignment is in good agreement with the XRD and TEM results that show the presence of ZnO onto the surface of samples.1 Signal C, detected for the rutile samples, presents an orthorhombic symmetry that is in agreement with the change from D2d to D2h local symmetry at a Ti site produced when the anatase structure is irreversibly changed into the rutile structure.34 Moreover, the signal was not broadened nor did it disappear


Marcı` et al.

after exposure of the sample to molecular oxygen, thus indicating that the corresponding paramagnetic ions are located within the bulk and not at the TiO2 surface. The fact that signal C is detected prior to the introduction of zinc in the TR sample suggests that it might be due to either the above-reported impurities of Fe3+ ions substituting for Ti4+ in the rutile lattice or to Ti3+ centers. To the best of our knowledge, signals with g-values analogous to those of signal C have not been previously described for iron-doped rutile samples. Moreover, the ESR spectra of three different rutile samples doped with iron contents in the range 0-10% were recorded during this study and no evidence for signal C was found. On this basis, and according to the g-tensor components previously reported35 signal C may be ascribed to Ti3+ ions located in the TiO2-rutile lattice. As far as signal D is concerned, although its intensity is related to the amount of zinc introduced in the rutile samples, its g-value and its symmetry differs from the data described for signals ascribed to Zn+ ions in ZnO. In contrast, signal D presents parameters closer to a signal previously reported in rutile-containing samples and ascribed to Ti3+ ions.35 The formation of such Ti3+ centers in the TRZn samples is presumably induced by the increasing zinc content. In fact, doping of TiO2 with different metal ions has been previously proved to promote the formation of a Ti3+ ion lattice defect.36 It can be concluded that incorporation of zinc into TiO2 samples leads to different results depending on the crystalline composition of the matrix. Whereas in TAZn samples the presence of most likely Zn+ ions in small ZnO nuclei was detected, the formation of new paramagnetic titanium centers seems to be induced in TRZn ones. The presence of Zn+ paramagnetic centers in ZnO arises from the lack of stoichiometry of the oxide due to excess of metal.37 An allowed energy level, associated with the excess of zinc and usually ascribed to interstitial zinc ions (Zni), lies approximately 0.05 eV below the conduction band of ZnO. The interstitial ions are easily ionized, forming Zni+ ions according to

Zni f Zni+ + e-

(1)

UV irradiation of the TAZn samples results in a decrease of the intensity of the signal ascribed to Zn+, whereas the signals due to Fe3+ ions are not affected. It is well-known that photoexcitation of a semiconductor by photons with energy equal to or greater than its band gap promotes electrons from the valence to the conduction band, producing electron-hole pairs. The fading of the ESR signal assigned to Zn+ ions under UV irradiation might be due to hole trapping by those ions, according to

Zn+ + h+ f Zn2+

(2)

The injection of the promoted electrons from the conduction band of the ZnO oxide to the conduction band of the lightactivated TiO2 oxide, following a mechanism similar to that previously reported by Gopidas et al.38 and Serpone et al.,11 could account for eq 2. Indeed, neither a signal that could be ascribed to the localization of the photogenerated electrons nor the fading of the structure assigned to impurity Fe3+ ions was detected. A plausible explanation for the increase of the signals ascribed to Ti3+ ions when zinc is introduced into TiO2 (rutile) could be an electron-transfer mechanism between ZnO and rutile according to

Ti4+ + Zn+ f Ti3+ + Zn2+

(3)

Moreover, the UV irradiation induced in the TRZn samples the increase of the intensity of the signal ascribed to Ti3+ ions, which can be explained as the result of the stabilization of the UVpromoted electrons according to

Ti4+ + e- f Ti3+

(4)

FTIR and Surface Acidity and Basicity. The surface acidity was studied by monitoring pyridine (py) adsorption.39 The FTIR spectrum recorded after adsorption of py on TA support is very similar to that described previously40-42 with bands at 1606, 1576, 1493, and 1446 cm-1, due to modes 8a, 8b, 19a, and 19b, respectively, of py coordinated to surface Lewis acid sites (see Figure 6). Weak bands at 1638 and 1550 cm-1 could be ascribed to modes 8a and 19b, respectively, of pyridinium ions, thus suggesting the presence of surface Bro¨nsted acid sites. This kind of surface acid site has not been reported previously either for anatase or for rutile; however, both types of sites have been reported for TiO2 prepared by a sol-gel method.43 The rutile sample, TR, showed a spectrum with weaker bands at 1605, 1576, 1488, and 1442 cm-1, due to coordinated pyridine, and the bands caused by adsorbed pyridinium ions are also recorded at 1550-1520 cm-1, thus indicating that, as in the anatase support, both Lewis and Bro¨nsted acid sites are present. The bands are still observed after outgassing the samples at increasing temperatures. However, while for TA the bands are still present after outgassing at 573 K (Figure 6), they disappear for TR after outgassing above 473 K. These results indicate that the surface acid sites are stronger in the first support. The spectrum recorded after adsorption of py on ZnO ex N shows extremely weak bands at 1603, 1576, 1487, and 1443 cm-1, attributed to pyridine coordinated to surface Lewis acid sites. When ZnO was supported on both TiO2 supports, the solids obtained showed a lower transmittance, due to the decrease of specific surface areas, thus leading to weak absorption bands and ill-defined spectra. The spectrum corresponding to adsorption of py on TAZnN0.5 is also shown in Figure 6. The bands due to adsorbed py are recorded in the same positions as for the unloaded support; however, it should be noticed that bands due to pyridinium ion are stronger than those observed in the case of the original support. All of the bands are recorded even after outgassing the sample at 573 K. A general decrease of the intensities of all of the bands, was observed for TAZnN samples when the ZnO content was increased. Data indicate that only physisorbed pyridine exists for TAZnN50. Such a decrease in surface acidity as the ZnO content is increased was also observed for the series of samples prepared on the rutile support, TR. A different behavior is observed for the set of TRZnA samples, i.e., adsorption of py gives rise to FT-IR spectra where bands due to formation of pyridinium ions are not present (Figure 7), thus indicating the absence of surface Bro¨nsted acid sites. The general behavior of the spectra with respect to the persistence of the bands after increasing the outgassing temperature or the ZnO content runs parallel to that observed for samples prepared from Zn nitrate. So, it is concluded that the addition of ZnO to TiO2, contrary to the behavior usually observed upon addition of other oxides44-46 leads to a general decrease of the surface acidic properties.


J. Phys. Chem. B, Vol. 105, No. 5, 2001 1037

Figure 7. FT-IR spectra recorded after adsorption of pyridine on TRZnA0.5 sample and outgassing at room temperature (a) and 373 K (b).

Figure 6. FT-IR spectra recorded after adsorption of pyridine on support TA (left) and on TAZnN0.5 sample (right) and outgassing at room temperature (a), 373 K (b), and 573 K (c).

As far as the set of TAZnA samples is concerned, the FT-IR spectrum of the TAZnA10 sample, after adsorption of pyridine, is reported in Figure 8. It can be observed that it is slightly different from that observed for samples TAZnN and rather close to that of the support TA hp. Bands are stronger and better defined: they were recorded at 1603, 1575, 1492, and 1444 cm-1 and are due to modes 8a, 8b, 19a, and 19b, respectively, of pyridine coordinated to surface Lewis acid sites. Bro¨nsted sites were not detected (they were present in samples prepared from Zn nitrate), similar to results found for the TRZnA samples. The intensities of the bands decreased as the outgassing temperature was increased and shifted to higher wavenumbers,

Figure 8. FT-IR spectra recorded after adsorption of pyridine on TAZnA10 sample and outgassing at room temperature (a), 473 K (b), and 573 K (c).

i.e., from 1603 to 1612 and from 1444 to 1455 cm-1. Moreover, they were still recorded after outgassing at 573 K. Concerning the basic surface properties, boric acid trimethyl ester (BATE) is a probe molecule sensitive enough to detect and distinguish surface Lewis sites.47 This basic sites-BATE interaction may cause changes of the local structure of the [BO3] moiety (from D3h to C3V) or weaken the B-O bond, thus leading to the development of new bands or shifting of the old ones, recorded at 1485, 1360, and 1030 cm-1 for modes δ(CH3), ν(B-O), and ν(C-O), respectively. In Figure 9 the FTIR spectra of TA and TAZnN50 samples


Marcı` et al. TABLE 1: Specific Surface Areas (SSA), Zero Order Observed Rate Constants (kobs), and Zero-Order Observed Rate Constants Divided the Surface Areas Corresponding to 0.5 g of Catalyst Used in the Photoreactivity Runs (kobs/SA) samples

Figure 9. FT-IR spectra of BATE adsorbed at room temperature on TA (a) and TAZnN50 (b) samples.

are reported after adsorption of BATE at room temperature. The spectrum recorded on support TA shows absorption bands at 1485, 1390, 1362, 1307, 1098, and 1060 cm-1, indicating the presence of two types of surface basic sites (oxide anions) with different strengths.47 Bands are recorded at 1485, 1364, and 1113 cm-1, with shoulders at 1390, 1274, and 1060 cm-1 for samples with low ZnO loading and the spectra are rather similar to those recorded for TA. When the ZnO content was increased (see Figure 9, TAZnN50 sample), the bands were weaker and they easily disappeared after outgassing at temperatures lower than that required for samples with lower ZnO contents. With respect to TR and the sets of samples prepared by using this support, the spectra recorded after adsorption of BATE are closer to those of the TAZn samples with high ZnO loading. The above results indicate that two types of surface basic Lewis sites of medium strengths exist in all samples, and their surface concentration and strength decrease by increasing the ZnO content. With respect to the OH stretching mode region (4000-3000 cm-1), it can be observed (figures not shown for the sake of brevity) that the intensities of the OH bands at 3729 and 3677 cm-1 decrease as the ZnO loading is increased. Moreover, it is worth noting that only slight differences in the OH stretching mode region were found for samples ZnO ex A and ZnO ex N and these insights are in accord with the XPS investigation.1 Photoreactivity Experiments. Blank tests were carried out in the dark or in the absence of the photoactive catalysts, and they did not show an appreciable disappearance of the substrate. As far as the heterogeneous system is concerned, the photoreactivity results for all of the samples together with the specific surface areas are reported in Table 1, while in Figures10 and 11 the photoreactivity behavior of some selected samples is shown. Moreover, in Figure 12 TOC concentration versus irradiation time is reported for some runs. For all of the runs the data fit satisfactory zero-order kinetics that change to pseudo-first-order kinetics when the concentration of the substrate becomes low, as is well-known in photocataly-

TA TAZnN0.10 TAZnN0.50 TAZnN2.00 TAZnN5.00 TAZnN10.0 TAZnN50.0 TAZnN60.0 TAZnN67.0 TAZnA0.10 TAZnA10.0 TAZnA25.0 TAZnA25.0a TAZnA50.0 TR TRZnN0.10 TRZnN0.50 TRZnN2.00 TRZnN5.00 TRZnN10.0 TRZnN50.0 TRZnN60.0 TRZnN67.0 TRZnA0.50 TRZnA1.00 TRZnA10.0 TRZnA25.0 TRZnA25.0a TRZnA50.0 ZnO ex A ZnO ex N

SSA (m2‚g-1) 106kobs (M‚h-1) 106kobs/SA (M‚h-1 ‚m-2) 59 56 55 54 53 46 22 12 11 57 48 40 31 28 6 6 5 5 5 4 3 2 2 6 6 7 7 6.5 10 3 NDb

52 56 44 48 45 28 25 28 31 37 29 29 31 32 negligiblec,d negligiblec,d negligiblec,d negligiblec,d negligiblec,d negligiblec,d negligiblec,d 8d 9d negligiblec,d negligiblec,d negligiblec,d 15d 34 29 36 18d

1.8 2.0 1.6 1.8 1.7 1.2 2.3 4.7 5.6 1.3 1.2 1.4 2.0 2.3 negligiblec,d negligiblec,d negligiblec,d negligiblec,d negligiblec,d negligiblec,d negligiblec,d 8d 9d negligiblec,d negligiblec,d negligiblec,d 4.3d 10 5.8 24

a Samples prepared by performing the final heating treatment at 773 K. b Not determined because very small. c kobs values range between 1 × 10-6 and 2 × 10-6 M‚h-1, due to the occurrence of a slight homogeneous photooxidation. d TOC figures remain unchanged.

Figure 10. 4-Nitrophenol concentration versus irradiation time for some selected samples: TA ([); TR (b); ZnO ex A (9); ZnO ex N (2).

sis.24 The results are reported as zero-order observed rate constants (kobs) calculated by considering the first 2.5 h of the experiments. Moreover, also the kobs values divided by the surface areas corresponding to the amount of the catalysts used are reported, and it can be observed that the trend of the photoreactivity changes significantly, especially for the TRZnN set. Finally, it is worth noting that for the ZnO samples and all the mixed samples, some anodic decomposition was observed under irradiation, but it was not very significant when the


Figure 11. 4-Nitrophenol concentration versus irradiation time for some selected samples: TAZnN0.10 ([); TAZnN50 (b); TAZnA50 (9); TRZnA50 (2).

Figure 12. TOC versus irradiation time for some selected samples: TA ([); TAZnN50 (O); TAZnA50 (9); TRZnA50 (2); ZnO ex A (b).

amount of zinc in the mixed samples was lower than 10%, suggesting that the physical adhesion of ZnO on TiO2 particles is quite strong. Some general considerations can be drawn from the observation of Table 1 and Figures 10-12. (i) TA, ZnO ex A, and ZnO ex N appeared to be photoactive whereas TR hp did not show appreciable photoactivity; (ii) The mixed samples from the TAZnN set show photoactivities comparable with that of TA when both the kobs and the kobs/SA values are compared, although a beneficial effect, due to the presence of ZnO, could be noticed for some samples; (iii) The mixed samples from the TAZnA set show photoactivities reflecting that of TA and ZnO ex A, depending on the amount of zinc, although when the kobs/SA values are compared, the photoactivity of ZnO ex A appears to be 1 order of magnitude higher; (iv) The mixed samples from the TRZnN set show negligible photoactivities, with the exception of the TRZnN60 and TRZnN67 samples showing the highest photoactivities, when the kobs/SA values are compared with those of the TAZnN and TAZnA sets. Nevertheless, the photoactivity of the TRZnN set appears almost independent from the amount of zinc and TOC variation was found to be very small (about 1 mg‚L-1 after 3 h of irradiation), indicating that the mineralization of 4-nitrophenol was virtually negligible after that time; (v) It is worth noting that the samples TAZnN10.0TAZnA10 and TAZnN50-TAZnA50 with similar surface areas show a similar photoreactivity;

J. Phys. Chem. B, Vol. 105, No. 5, 2001 1039 (vi) The photoactivity of the mixed samples from the TRZnA set increased significantly when increasing the amount of zinc or the temperature of the final heating treatment (from 673 to 773 K). TAZnA25 and TRZnA25 samples (calcined at 773 K) show photoactivities not much lower than those of TA and ZnO ex A, when the kobs values are compared and higher than that of TA when the kobs/SA values are taken into consideration. Moreover the TOC decrease was higher compared to that of the corresponding samples of the TRZnN set, indicating a more efficient mineralization of the organic substrate. It should be remembered, finally, that all the mixed samples containing TR can be separated from the reacting system without filtration because they decant in a few minutes. An exhaustive explanation of these results is not easy because the photoreactivity depends on bulk, surface physicochemical, and intrinsic electronic properties of the catalysts; in principle, these last can be positively modified for the coupled TiO2/ZnO samples1,25-27,48 deriving from the various preparations (see Figure 1). The preparation methods dramatically influence the photoactivity of ZnO. As can be observed in Table 1, ZnO ex A is one of the more photoactive samples or the most photoactive one when the kobs and the kobs/SA values are considered, respectively. This behavior is also reflected in the mixed TRZnA samples with the highest amounts of zinc. As far as ZnO ex N is concerned, this sample is not suitable for photocatalytic studies, due to its low specific surface area, and its presence appeared beneficial only for a few samples from the TAZnN and TRZnN series. It is likely that the above differences mainly depend on surface physicochemical and electronic properties rather than on the bulk ones. Indeed, XRD measurements do not show significant differences between the two pure ZnO samples, with the exception of a higher crystallinity for ZnO ex N.1 Nevertheless, it has been reported that the surface chemistry of various polycrystalline ZnO samples strongly depends on their different morphology,49 which is responsible for different reactivity and electronic phenomena.50,51 The thermal treatments under different atmospheres and temperatures can influence the population of shallow defective levels and the conduction band.52 It appears reasonable that well-defined crystallites with sharp edges, as have been observed for the ZnO ex N sample,1 are less affected by the presence of defects and/or surface states, whereas the nature of smaller and less regular crystallites, as observed for ZnO ex A,1 is consistent with the presence of defective levels in the energy gap and some other transitions, in addition to the fundamental one, can occur. ESR results suggest the presence of ZnO with paramagnetic centers (Zn+) due to excess of metal in some mixed samples. Moreover, the growth of Ti3+ signals under irradiation suggests that zinc plays a beneficial role retarding the recombination of photoproduced electron-hole pairs in the TRZn samples. It is worth noticing that the recombination rate for TiO2 (rutile) has been reported higher than that of TiO2 (anatase).6 The presence of carbon in ZnO ex A sample and in the set of samples prepared by using Zn(CH3COO)2‚2H2O, as revealed by XPS measurements,1 could be beneficial, due to an improvement of the adsorption of the substrate followed by its transfer to the photoactive surface sites of the particles. A synergistic effect between activated carbon and TiO2 has been reported in the literature53 for the photocatalytic degradation of phenol carried out in aqueous suspensions, although the experimental conditions are different in that case because carbon and TiO2 are mixed as distinct powders.

1040 J. Phys. Chem. B, Vol. 105, No. 5, 2001 The higher porosity measured1 for the samples prepared by using Zn(CH3COO)2‚2H2O instead of Zn(NO3)2‚6H2O as the precursor for ZnO can also be responsible for an enhanced adsorption of the substrate, and consequently for the observed increase of photoactivity in the TRZnA set of samples (compare, for instance, TRZnA50 and TRZnN50 samples). Moreover, the surface acid-base properties can influence the photoactivity and, although the FT-IR and XPS measurements indicate that the surface hydroxylation degree of the catalysts is quite similar, a general decrease of the surface acidic properties by adding ZnO to TiO2 was observed by the FT-IR investigation. Bro¨nsted acid sites, indeed, were generally found weaker and weaker than those of the support by increasing the ZnO amount, and their complete absence was observed for the samples prepared by using Zn(CH3COO)2‚2H2O. ZnO surface enrichment, explaining the absence of such sites, was determined for these last samples by means of XPS measurements.1 Conclusions ESR measurements carried out on selected samples suggest that the presence of zinc contributes to the stabilization of the free charge carriers, at least in the supported rutile samples, and this is in principle beneficial for the occurrence of the photoreactivity. FTIR results indicate that the surface acidity of both TAZn and TRZn sets of samples decreases by increasing the amount of ZnO. The photoreactivity results indicate that coupling ZnO with TiO2 induces only a small beneficial effect, at least under the experimental conditions reported in this work. Nevertheless it is worth noting that the samples from the TRZnA set with the highest amounts of zinc showed a good photoactivity different from what is observed for TR and they easily decanted from the liquid phase. This last insight seems a useful property in view of application purposes. In conclusion, the photoreactivity behavior of many samples mainly reflects those of the two bare ZnO samples, depending on the amount of zinc, in accord with the characterization results suggesting the formation of a sort of ZnO layer on the particles surface. The thorough bulk and surface characterization carried out on these materials could be useful in view of further studies in the gassolid regime, and work is in progress to find different experimental conditions that could improve their photoactivity behavior. Acknowledgment. G.M., V.A., L.P., M.S., and A.M.V. thank the Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (Rome) for financially supporting this work. C.M. and V.R. acknowledge financial support from Consejerıá de Educacioń y Cultura (Junta de Castilla y Leoń, grant 71/99) and Ministerio de Educacioń y Ciencia (grant IN-96-0252). M.J.L.M. thanks the Ministerio de Cultura for financial support and Dr. J. Soria (Instituto de Cata´lisis, CSIC Madrid) for the use of the ESR spectrometer. References and Notes (1) Marcı`, G.; Augugliaro, V.; Lo´pez-Munõz, M. J.; Martin, C.; Palmisano, L.; Rives, V.; Schiavello, M.; Tilley, R. J. D.; Venezia, A. M. J. Phys. Chem. 2001, 105, 1026. (2) Augugliaro, V.; Palmisano, L.; Schiavello, M.; Sclafani, A.; Marchese, L.; Martra, G.; Miano, F. Appl. Catal. 1991, 69, 323. (3) Augugliaro, V.; Lo´pez-Munõz, M. J.; Palmisano, L.; Soria, J. Appl. Catal. A: General 1993, 101, 7. (4) Poulios, I.; Kositzi, M.; Kouras, A. J. Photochem. Photobiol. A: Chem. 1998, 115, 175. (5) Yeber, M. C.; Rodriguez, J.; Freer, J.; Baeza, J.; Duran, N.; Mansilla, H. D. Chemosphere 1999, 39, 1679. (6) Augugliaro, V.; Palmisano, L.; Sclafani, A.; Minero, C.; Pelizzetti, E. Toxicol. EnViron. Chem. 1988, 16, 89 and references therein.

Marcı` et al. (7) Kawaguchi, H.; Uejima, T. Kagaku Kogaku Ronbunshu 1983, 9, 107. (8) Kawaguchi, H. EnViron. Technol. Lett. 1984, 5, 471. (9) Ollis, D. F., Al-Ekabi, H., Eds.; Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (10) Kamat, P. V.; Patrick, B. J. Phys. Chem. 1992, 96, 6829. (11) Serpone, N.; Maruthamuthu, P.; Pichat, P.; Pelizzetti, E.; Hidaka, H. J. J. Photochem. Photobiol., A: Chem. 1995, 85, 247. (12) Cunningham, J.; Kelly, J. J.; Penny, A. L. J. Phys. Chem. 1971, 75, 617 and references therein. (13) Al-Ekabi, H.; Serpone, N. J. Phys. Chem. 1988, 92, 5726. (14) Anderson, M. A.; Gieselmann, M. J.; Xu, Q. J. Membr. Sci. 1988, 39, 243. (15) Matthews, R. W.; Abdullah, M.; Low, G. K. C. Anal. Chim. Acta 1990, 233, 171. (16) Al-Ekabi, H.; Safarzadey-Amiri, A.; Sifton, W.; Story, J. Int. J. EnViron. Pollut. 1991, 1, 125. (17) Tennakone, K.; Kottegoda, I. R. M. J. Photochem. Photobiol. A: Chem. 1996, 93, 79. (18) Fernańdez, A.; Lassaletta, G.; Jimeńez, V. M.; Justo, A.; Gonza´lezElipe, A. R.; Herrmann, J.-M.; Tahiri, H.; Ait-Ichou, Y. Appl. Catal. B: EnViron. 1995, 7, 49. (19) Sukharev, V.; Kershaw, R. J. Photochem. Photobiol., A: Chem. 1996, 98, 165. (20) Yumoto, H.; Inoue, T.; Li, S. J.; Sako, T.; Nishiyama, K. Thin Solid Films 1999, 345, 38. (21) Loddo, V.; Marcı`, G.; Palmisano, L.; Sclafani, A. Mater. Chem. Phys. 1998, 53, 217. (22) Loddo, V.; Marcı`, G.; Martin, C.; Palmisano, L.; Rives, V.; Sclafani, A. Appl. Catal. B: EnViron. 1999, 20, 29. (23) Reyes, J.; Dezotti, M.; Mansilla, H.; Villasenor, J.; Esposito, E.; Duran, N. Appl. Catal. B: EnViron. 1998, 15, 211. (24) Augugliaro, V.; Loddo, V.; Marcı`, G.; Palmisano, L.; Lo´pez-Munõz, M. J. J. Catal. 1997, 166, 272 and references therein. (25) Jaeger, C.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (26) Scaife, D. E. Solar Energy 1980, 25, 41. (27) Memming, R. Electrochim. Acta 1980, 25, 77. (28) Burriel Martı´, F.; Lucena Conde, F.; Arribas Jimeno, S.; Hernańdez Meńdez, J. Quimica Analitica CualitatiVa; Editorial Paraninfo: Madrid, 1994; p 694. (29) Cordischi, D.; Burriesci, N.; D’Alba, F.; Petrera, G.; Polizzotti, G.; Schiavello, M. J. Solid State Chem. 1985, 56, 182. (30) Amorelli, A.; Evans, J. C.; Rowlands, C. C.; Egerton, T. A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3541. (31) Soria, J.; Conesa, J. C.; Augugliaro, V.; Palmisano, L.; Schiavello, M.; Sclafani, A. J. Phys. Chem. 1991, 95, 274. (32) Volodin, A. M.; Cherkashin, A. E. React. Kinet. Caoal. Lett. 1982, 20, 335. (33) Codell, M.; Gisser, H.; Weisberg, J.; Iyengar, R. D. J. Phys. Chem. 1968, 72, 1853. (34) Che, M.; Fichelle, G.; Meriaudeau, P. Chem. Phys. Lett. 1972, 17, 66. (35) Meriaudeau, P.; Che, M.; Gravelle, P. C.; Teichner, S. J. Bull. Soc. Chim. Fran. 1971, 1, 13. (36) Boronicolos, A.; Vickerman, J. C. J. Catal. 1986, 100, 59. (37) Freund, T.; Gomes, W. P. Catal. ReV. 1969, 3, 37. (38) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990, 94, 6435. (39) Kno¨zinger, H. AdV. Catal. 1976, 25, 184. (40) Miyata, H.; Nakagawa, Y.; Ono, T.; Kubokawa, Y. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2343. (41) Rives, V. Opt. Pure Appl. 1983, 16, 61. (42) Martıń, C.; Martıń, I.; Rives, V. J. Catal. 1994, 145, 239. (43) Marcı´, G.; Palmisano, L.; Sclafani, A.; Venezia, A. M.; Campostrini, R.; Carturan, G.; Martin, C.; Rives, V.; Solana, G. J. Chem. Soc., Faraday Trans. 1996, 92, 819. (44) Tanabe, K.; Takeshita, T. AdV. Catal. 1965, 17, 315. (45) Kung, H. J. Solid State Chem. 1984, 52, 191. (46) Connell, G.; Dumesic, J. A. J. Catal. 1986, 52, 216. (47) Fu, Li, Sh.; Zhang, H.; Xin, Q. J. Chem. Soc. Chem. Commun. 1994, 17. (48) Schoonen, M. A. A.; Xu, Y.; Strongin, D. R. J. Geochem. Expl. 1998, 62, 201. (49) Bolis, V.; Fubini, B.; Giamello, E.; Reller, A. J. Chem. Soc., Faraday Trans. 1 1989, 85, 855. (50) Chiorino, A.; Ghiotti, G.; Boccuzzi, F. Vacuum 1990, 41, 16. (51) Ghiotti, G.; Chiorino, A.; Boccuzzi, F. Surf. Sci. 1993, 287, 228. (52) Boccuzzi, F.; Morterra, C.; Scala, R.; Zecchina, A. J. Chem. Soc., Faraday Trans. 2 1981, 77, 2059. (53) Matos, J.; Laine, J.; Herrmann, J.-M. Appl. Catal. B: EnViron. 1998, 18, 281.

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