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Preparation and Physicochemical Properties of ZrO2 and Fe/ZrO2 Prepared by a Sol-Gel Technique J.A. Navı´o,*,† M.C. Hidalgo,† G. Colo´n,† S. G. Botta,‡ and M. I. Litter‡ Instituto de Ciencia de Materiales de Sevilla, Centro de Investigaciones Cientı´ficas “Isla de la Cartuja”, Av. Ame´ rico Vespucio, s/n, Isla de la Cartuja, 41092 Sevilla, Spain, and Unidad de Actividad Quı´mica, Comisio´ n Nacional de Energı´a Ato´ mica, Centro Ato´ mico Constituyentes, Av. Gral. Paz 1499, 1650 San Martı´n, Prov. de Buenos Aires, Argentina Received June 27, 2000. In Final Form: September 12, 2000 Pure ZrO2 and 0.5-5 wt % iron-containing samples have been prepared by a sol-gel technique and thoroughly characterized by several techniques, X-ray diffraction (XRD), differential thermal analyses (DTA), diffuse reflectance spectroscopy (DRS), scanning and transmission electron microscopies (SEM and TEM), X-ray photoelectron spectroscopy (XPS), and other complementary techniques. Introduction of iron in zirconia produced a phase transition from monoclinic to tetragonal. In contrast to generally reported values of zirconia band gap (5 eV), the values of the present samples ranged between 2.3 and 3.8 eV, determined from DRS, depending on the preparation conditions and on the iron content. Preliminary experiments of adsorption of different species (nitrite, ethylenediaminetetraacetic acid (EDTA), and Cr(VI)) are also presented and the results explained by a correlation with the physicochemical properties of the characterized oxides. Oxygen photoadsorption and oxygen isotopic exchange (OIE) results can be corelated to the photocatalytic activity recently found, showing the potentiality of this technique for the characterization of photocatalysts in order to account for its photoactivity.

Introduction Zirconium dioxide (ZrO2) is an important material widely used in ceramics technology1 and heterogeneous catalysis.2,3 Acid-base properties make this oxide suitable for important applications in catalytic processes such as paraffin isomerization,4 hydrogenation of olefins,5 alcohol dehydrogenation,6,7 and other technological uses. Due to its nature as an n-type semiconductor, it has been considered recently as a photocatalyst in photochemical heterogeneous reactions.8-21 The most common method of preparation of zirconium oxides is by pyrolysis of very pure chlorides or alkoxides. * Corresponding author. E-mail: [email protected]. † Centro de Investigaciones Cientificas “Isla de la Cartuja”. ‡ Centro Ato ´ mica Constituyentes. (1) Ward, D. A.; Ko, E. I. Chem. Mater. 1993, 5, 956. (2) Yamaguchi, T. Catal. Today 1994, 20, 199. (3) Corma, A. Chem. Rev. 1995, 95, 559. (4) Hino, M.; Kobayashi, S.; Arata, K. J. Am. Chem. Soc. 1979, 101, 101. (5) Yamaguchi, T.; Hightower, J. J. Am. Chem. Soc. 1977, 99, 4201. (6) Winfield, M. E., Emmett, P. H., Eds. Catalysis; Reinhold: New York, 1960; Vol. 7, p 93. (7) Yamaguchi, T.; Sasaki, H.; Tanabe, K. Chem. Lett. 1973, 1017. (8) Navı´o, J. A.; Colo´n, G. Stud. Surf. Sci. Catal. 1994, 82, 721. (9) Navı´o, J. A.; Colo´n, G.; Herrmann, J. M. J. Photochem. Photobiol. A: Chem. 1997, 108, 179. (10) Pichat, P.; Herrmann, J. M.; Disdier, J.; Mozzanega, M. N. J. Phys. Chem. 1979, 83, 3122. (11) Al-Sayyed, G.; D’Oliveira, J. C.; Pichat, P. J. Photochem. Photobiol. A: Chem. 1991, 58, 99. (12) Navı´o, J. A.; Colo´n, G.; Macı´as, M.; Sa´nchez-Soto, P. J.; Augugliaro, V.; Palmisano, L. J. Mol. Catal. A: Chem. 1996, 109, 239. (13) Sayama, K.; Arakawa, H. J. Phys. Chem. 1993, 97, 531. (14) Juillet, F.; Lecomte, F.; Mozzanega, H.; Teichner, S. J.; Thevenet, A.; Vergnon, P. Faraday Symp. Chem. Soc. 1973, 7, 57. (15) Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S. Chem. Commun. 1997, 841. (16) Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1998, 94, 1875. (17) Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S. Chem. Lett. 1997, 993. (18) Fox, M. A.; Ogawa, H.; Pichat, P. J. Org. Chem. 1989, 54, 3847. (19) Sayama, K.; Arakawa, H. J. Photochem. Photobiol. A: Chem. 1996, 94, 67. (20) Courbon, H.; Pichat, P. C. R. Acad. Sci. 1977, C285, 171.

The stable crystalline form at room temperature is the monoclinic one, and the reversible transformation to the tetragonal form is produced at 1100-1200 °C. The cubic phase appears only at 2373 °C. ZrO2 obtained by pyrolysis of zirconium salts crystallizes in the metastable tetragonal or cubic phases that by heating revert to the monoclinic phase.22 Regarding the semiconductor properties of zirconia, there is no agreement in the literature with respect to the band gap (Eg) value. Table 1 summarizes Eg values found for different materials. As it can be seen, almost all values are close to 5 eV (corresponding to an absorption edge close to 250 nm). A value of -1.0 V vs NHE for the redox potential of the conduction band at pH 0 is reported.23 The high value of Eg and the highly negative value of the conduction band allowed the use of zirconia as a photocatalyst for H2 production.14 Some other samples present, in addition to the maximum around 250 nm, a nonnegligible absorption in the near-UV range (290-390 nm).9,13,24,25 Iron-containing specimens show lower Eg values.26 In this paper, zirconia samples, pure and doped with Fe(III), have been prepared by a sol-gel technique and thoroughly characterized. Results on oxygen photoadsorption and oxygen isotopic exchange (OIE) and preliminary results on adsorption of nitrite, ethylenediaminetetraacetic acid (EDTA), and Cr(VI) onto selected samples are also presented. Together with the physicochemical properties, these features are important pa(21) Nielsen, R. H.; Schelwitz, J. H.; Nielsen, H. In Kirk-Othmer Encyclopedia of Chemical Technology, 30th ed.; Grayson, M., Ekcroth, D., Eds.; Wiley: New York, 1984; Vol. 24, p 863. (22) Clechet, P.; Martin, J. R.; Ollier, R.; Valloy, C. C. R. Acad. Sci. 1976, C282, 887. (23) Sato, S.; Kadowaki, T. J. Catal. 1987, 106, 295. (24) Gauguly, K.; Sarkar, S.; Bhattacharyya, S. J. Chem Soc., Chem. Commun. 1993, 683. (25) Newmark, A. R.; Stimming, U. Langmuir 1987, 3, 905. (26) Maksimov, Y. V.; Suzdalev, I. P.; Tsodikov, M. V.; Kugel, V. Y.; Buktenko, O. V.; Slivinky, E. V.; Navı´o, J. A. J. Mol. Catal. A: Chem. 1996, 105, 167.

10.1021/la000897d CCC: $20.00 © 2001 American Chemical Society Published on Web 12/12/2000

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Table 1. Eg Values of ZrO2 Found in the Literature sample

preparation method

Eg (eV)

ref

ZrO2 powder ZrO2 film ZrO2 passivating film ZrO2 passivating film ZrO2 ZrO2 ZrO2 powder ZrO2 film ZrO2 passivating film Fe/ZrO2 passivating film

thermal ZrOCl2 decomposition “sputtering” anodic oxidation of Zr0 anodic oxidation of Zr0 commercial sample commercial sample precipitation of ZrOCl2 anodic oxidation of Zr0 anodic oxidation of Zr0 anodic oxidation of Zr/Fe alloys

3.25 4.1 4.8 4.9 4.99 5.0 5.0 4.9-5.1 5.1 3.3

57 58 25 22 59 23 24 58 49 25

Scheme 1. Diagram of the Preparation of the ZrO2 and Fe/ZrO2 Samples

ZrO2 and Fe/ZrO2 samples (named hereafter “X-Y”, where X is the weight percentage of iron in the ZrO2 matrix and Y is the calcination temperature in °C) were prepared by a sol-gel technique according to the procedure described in Scheme 1. To aqueous suspensions of commercial ZrOCl2‚8H2O (Merck) containing different amounts of Fe(NO3)3‚8H2O (Panreac), aqueous ammonia (Merck, 25 wt %) was added dropwise to the mixture with continuous stirring at pH 9-10. After gelation, the solids were filtered and repeatedly washed until a negative AgNO3 test was acheived, to ensure the absence of chloride ions in the samples, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. Pure ZrO2 samples were prepared by the same procedure but in the absence of iron. The samples were dried at 100 °C for 24 h, and the gel precursors (referred to in the text as “original samples”) were annealed at different temperatures (500 or 600 °C) and times (3 or 24 h.); the furnace was preheated to 500 or 600 °C prior to annealing. TiO2 (Degussa P-25) was a commercial sample, kindly supplied by the manufacturer (Degussa A. G.) and used as provided.

The iron content in the Fe/ZrO2 samples was checked by atomic absorption using a Perkin-Elmer spectrophotometer 2380, and it was very close to the nominal value for all the samples. Specific surface areas (SBET) were obtained with an automatic system (Micromeritics 2200 A) with nitrogen gas as adsorbate at liquid nitrogen temperature. X-ray diffraction (XRD) patterns of the samples were obtained at room temperature with a Siemens D-501 diffractometer, using filtered Cu KR radiation. Scanning electron microscopy (SEM) was performed with a JEOL apparatus, model JSM-5400, equipped with a Link analyzer, model ISIS, for X-ray energy-dispersive analyses (EDAX). All SEM measurements were carried out on gold-coated samples. Transmission electron spectroscopy (TEM) pictures were taken with a Philips CM200 electronic microscope (200 kV) working with a tungsten filament. Diffuse reflectance spectra (DRS) were obtained either on a Shimadzu UV-2021 or a Shimadzu UV210A apparatus, both equipped with an integrating sphere. MgO and BaSO4 were used as references. Characterization of the surfaces was carried out using a combination of differential thermal analysis (DTA) and infrared (IR) and X-ray photoelectron (XPS) spectroscopies. DTAs were obtained simultaneously using a high-temperature analyzer Setaram 92, model 16.18, in the presence of static air at a heating rate of 10 °C min-1. Finely powdered R-alumina was used as the reference material. The IR spectra were taken on a Perkin-Elmer model 883 spectrophotometer, using KBr pellets and working in the transmittance mode. XPS measurements were performed with a LeyboldHeraeus LHS-10 spectrometer, working with a constant pass energy of 50 eV; Mg KR radiation was used for excitation (hν ) 1253.6 eV). A final pressure of 10-9 Torr was always attained before XPS recording. NaNO2 (Merck), Na4EDTA (Carlo Erba), and K2Cr2O7 (Carlo Erba) used for adsorption experiments were of high-quality grade and used as provided. Diluted HClO4 and NaOH were used for pH adjustments. In all cases, a fresh solution (10 cm3) of the substrate at a known concentration was adjusted to the desired pH, and the oxide was suspended in the solution at a 4.0 g dm-3 concentration. Comparative tests with 1.0 g dm-3 Degussa P-25 were carried out. Suspensions were kept in the dark and stirred at 25 °C for a time enough to ensure substrate-surface equilibrium. The extent of adsorption of the substrate on the catalyst was determined by measuring concentrations before and after stirring. Nitrite experiments were performed at 1.0 mM and different pH conditions. The suspensions were stirred for 45 min until equilibration. The final nitrite concentration was determined in the filtered solution by spectrophotometry at 520 nm using the sulfanilic acid method.30 Experiments with EDTA were performed using 1.0 mM suspensions at pH 2. Suspensions were stirred in the dark for 30 min, and EDTA concentration was evaluated by spectrophotometric analysis of the Co(III) complex.31 In the case of Cr(VI) experiments, 0.4 mM K2Cr2O7 solutions at pH 2 were used, and suspensions were stirred for 30 min. Changes in Cr(VI) concentration were followed by UV spectrophotometry at 349 nm.32 These experiments were repeated in the presence of 1.0 mM EDTA.

(27) Navı´o, J. A.; Macias, M.; Sa´nchez-Soto, P. J.; Hidalgo, M. C.; Restrepo, G. M.; Botta, S. G.; Litter, M. I.; Tsodikov, M. V. In Proc. Actas del XVI Simposio Iberoamericano de Cata´ lisis; Centeno, A., Giraldo, S. A., Pa´ez Mozo, E. A., Eds.; Cartagena de Indias 1998; Vol. III, p 1829. (28) Botta, S. G. Master Thesis, Universidad de General San Martı´n, Buenos Aires, 1998.

(29) Botta, S. G.; Navı´o, J. A.; Hidalgo, M. C.; Restrepo, G. M.; Litter, M. I. J. Photochem. Photobiol. A: Chem. 1999, 129, 89. (30) Boltz, D. F., Ed. Colorimetric Determination of Non Metals; Interscience: New York 1958; p 124. (31) Kaiser, K. L. E. Water Res. 1973, 7, 1465. (32) Wei, C.; German, S.; Basak, S.; Rajeshwar, K. J. Electrochem. Soc. 1993, 140, L60.

rameters for evaluating the photocatalytic properties, presented by us elsewhere.27-29 Experimental Section

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Table 2. Properties of ZrO2 and Fe/ZrO2 Prepared by the Sol-Gel Methoda sample

wt % Fe

calcination temp (°C)

calcination time (h)

SBET (m2 g-1)

crystalline phases

IEP

particle size (µm)

0-500 0-600 0.5-500 1-500 3-500 5-500 5-600

0 0 0.5 1 3 5 5

500 600 500 500 500 500 600

24 3 24 24 24 24 3

30 8 15 15 15 15 22

monoclinic tetragonal (tr) monoclinic monoclinic (0.7), tetragonal (0.3) monoclinic, tetragonal (tr) tetragonal tetragonal tetragonal

5.3 ND ND ND ND 5.3 ND

5 to 50 5 to 80 5 to 50 5 to 50 5 to 80 5 to 50 10 to 200

a

tr ) traces. ND ) not determined.

Figure 1. DTA profiles for the original Fe/ZrO2 samples: (a) 0.5, (b) 1, (c) 3, and (d) 5 wt %. In all cases, aqueous solutions and suspensions were prepared with double-distilled water treated in a quartz apparatus. Oxygen isotopic exchange and photoadsorption experiments were carried out in a static cylindrical cell, 1 cm high, closed by two parallel optical Pyrex windows of 6 cm diameter, transmitting wavelengths higher than 290 nm. A thin layer of sample was deposited onto the horizontal lower optical window. This cell was connected to a vacuum system (residual pressure, 10-5 to 10-6 Pa) equipped with a Datametric Dresser barocell pressure sensor and a Riber QMM17 quadrupole analyzer. UV light was provided by a Philips HPK-125W mercury lamp. A water filter was used to minimize IR radiation. The radiant flux reaching the sample in these experiments was measured by use of a radiometer (United Detector Technology, model 21A) and maintained constant by appropriate adjustments.

Results Table 2 summarizes the main characteristics of the oxide samples together with the different treatments to which original samples were submitted. DTA diagrams of selected Fe/ZrO2 samples are presented in Figure 1. All samples exhibit endothermic effects below 300 °C, together with an exothermic peak centered at 500 °C whose position is independent of the iron content; the most concentrated samples show a slight exothermic effect around 320 °C. XRD results indicate that Fe/ZrO2 samples calcined below 500 °C are amorphous. Crystallization occurred after heating in air at 500 °C for 24 h. The following effects were observed: (a) pure ZrO2 samples present the monoclinic phase with only traces of the tetragonal one; (b) samples with nominal values of Fe g3 wt % exhibit only the tetragonal phase of zirconia; (c) samples with lower

Fe content present a mixture of the monoclinic and the tetragonal phases. No separated phases of Fe2O3 and/or Fe2ZrO5 could be detected in the spectra, even for samples with a 5 wt % iron content. All these features are summarized in Table 2. The morphological properties of Fe/ZrO2 samples have been studied by SEM. All samples are nonuniform powders consisting of particles with a large distribution of shape and dimensions. It was noted that as the iron content increases, the particles develop morphologies with a high degree of wrinkled texture and surface deposits; this fact was observed independently of the calcination temperature. EDX analyses showed iron uniformly distributed between particles, and within one particle; the obtained values were in agreement with the corresponding nominal ones. The calcination process does not seem to affect the distribution of iron determined by EDX analysis. At the same time, results obtained by using the TEM technique indicate no significant differences on the morphology of the particles either caused by the iron content or the calcination procedures (500 °C for 24 h or 600 °C for 3 h). All the samples are formed by grains of different sizes, constituted by agglomerates of very small and round particles; in several cases, the shape of the grains is enlarged. The IR spectra of original samples (Figure 2A) show bands in the range 1300-1700 cm-1, which can be assigned to hydroxyl groups of molecular water (δOH ) 1630 cm-1), to CO32- species (ν2 ) 1560 and 1650 cm-1) and to NH4+ (δas ) 1380 cm-1).33 Ammonium peaks are enhanced in the samples with higher iron contents and disappear totally after calcination above 400 °C (data not shown). After calcination, all IR spectra show only bands corresponding to HO (ν ≈ 3430 cm-1) and to adsorbed molecular water (δOH ) 1630 cm-1) (Figure 2B). These bands are more intense in the samples calcined at 600 °C for 3 h, but less intense than those of the samples dried at 100 °C (see Figure 3 as a selected example). There are no significant differences between the IR spectra of samples calcined at 500 °C for 24 h with different Fe contents (Figure 2B). Figure 4A shows the O(1s), Zr(3d), and Fe(2p) photoelectron transition diagrams for selected original Fe/ZrO2 samples. Figure 4B shows the XPS results for the same samples after calcination at 500 °C for 24 h. The corresponding binding energies obtained are reported in Table 3. It can be observed that the original samples present nonsymmetric O(1s) peaks centered at 530.5 eV with a shoulder around 532.5 eV. This shoulder seems to be more pronounced as the iron content increases. A value of 530.5 eV has been reported for O(1s) assigned to hydroxyl groups,34while values close to 531 eV have been assigned to the O(1s) peak in pure zirconia samples.35-37 (33) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1985. (34) Barr, T. L. J. Phys. Chem. 1978, 82, 1801. (35) Himpsel, F. J.; Mcteely, F. R.; Taleb-Ibrahimi, A.; Yarnof, J. A.; Hollinger, G. Phys. Rev. 1988, B38, 6084.

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Figure 2. IR spectra of the original ZrO2 and Fe/ZrO2 samples: (A) original samples; (B) after calcination at 500 oC.

Figure 3. IR spectra of the Fe/ZrO2 sample (5 wt % Fe) calcined at the indicated temperatures and times.

Usually O(1s) for metal oxides is e531 eV, and O(1s) for hydroxide is g531 eV. On the other hand, if comparing Figure 4A with Figure 4B, the peak with higher binding energy disappeared after calcination at 500 °C for 24 h. This indicates that the binding energy at 530.5 eV must be assigned to the metal-oxygen-metal bonding and the (36) Naito, S.; Tnimoto, M.; Soma, M.; Udagawa, Y. In New Frontiers in Catalysis; Guczi, L., et al., Eds.; Proceedings of the 10th International Congress on Catalysis; Elsevier: Amsterdam, 1993; Vol. C, p. 2043. (37) Ingo, G.; Dire´, S.; Babonneau, F. Appl. Surf. Sci. 1993, 70/71, 230.

Figure 4. XPS results of the Fe/ZrO2 samples: (A) original samples; (B) samples calcined at 500 °C 24 h.

value at 532.5 eV must be related with hydroxyl groups. Thus, from the XRD information and XPS results we can conclude that our original Fe/ZrO2 samples exhibit the

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Table 3. XPS Binding Energies (eV) of O(1s), Zr(3d), and Fe(2p) of the Indicated Fe/ZrO2 Samples binding energiesa (eV) sample

O(1s)

Zr(3d)

Fe(2p)

original 1 wt % original 5 wt % 1-500 5-500

530.5 530.5 530.5 530.4

182.5 182.5 182.5 182.5

710.8 711.9 711.9 711.6

a

BE reference: 182.5.

Figure 6. Diffuse reflectance spectra of 0-600 and 5-600.

Figure 5. Diffuse reflectance spectra of 0-500 and 0-600. Table 4. Comparison between Iron Nominal Composition (wt %) and the Fe Content Estimated via XPS Analysis for Two Selected Fe/ZrO2 Samples (in Parentheses, the Corresponding Values Obtained from EDX Analysis) wt % Fe from XPS and EDAX Fe nominal compsn (wt %)

original

500 °C for 24 h

600 °C for 3 h

1 5

3.8 (1) 5.9 (4-5)

2.4 (1) 4.6 (4-5)

2.7 (1) 7.0 (4-5)

typical amorphous oxo-hydroxo halo pattern. It is worth noting that after calcination at 500 °C for 24 h, the O(1s) peaks of the three indicated samples lose the broad character showed in Figure 4A, leading to weak component peaks centered at 530.5 eV with a small shoulder around 532.5 eV (Figure 4B). The shoulder can be associated with either residual hydroxyl groups or oxygen linked to iron species. At the same time, no significant differences in the Zr(3d) and Fe(2p) peaks are found between the original and the calcined Fe/ZrO2 samples. In Table 4, the atomic surface composition for two selected Fe/ZrO2 samples, estimated by XPS and EDX analyses, are presented. XPS results show that, for the more diluted in iron samples (e.g. 1 wt %), the amount of iron on the surface is higher than the corresponding nominal value, whereas for all samples there is good agreement between the nominal iron composition values and those found by EDAX. Taking into account that these two techniques supply information corresponding to different degrees of penetration in the particle (very superficial from XPS), these results could indicate that only low-loaded Fe/ZrO2 particles (