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Controlled Deposition of Iron Oxide on the Surface of Zirconia by the Molecular Designed Dispersion of Fe(acac)3: A Spectroscopic Study P. Van Der Voort,*,† R. van Welzenis,‡ M. de Ridder,‡ H. H. Brongersma,‡ M. Baltes,† M. Mathieu,† P. C. van de Ven,‡ and E. F. Vansant† University of Antwerp (UIA), Department of Chemistry, Universiteitsplein 1, B-2610 Wilrijk, Belgium, and Eindhoven University of Technology, Department of Applied Physics, P.O. Box 513, NL 5600 MB Eindhoven, The Netherlands Received February 28, 2002. In Final Form: March 27, 2002
The reaction of Fe(acac)3 with the surface of zirconia has been studied for the first time using in situ infrared diffuse reflectance spectroscopy, photoacoustic spectroscopy, and Fourier transform Raman spectroscopy. The unstable Fe(acac)3 reacts readily with the surface of zirconia at room temperature in the liquid phase or at 110 °C in the gas phase, yielding grafted Fe-OH species and Zr-acac surface groups. We present evidence that the reaction occurs both with coordinatively unsaturated Zr sites and with the surface hydroxyls. The grafted Zr-acac groups are thermally unstable and form Zr-acetate groups after thermal treatment at 110 °C in ambient air. After removal of the organic ligands, noncrystalline iron oxide species are formed on the zirconia surface. The grafting of iron oxide on zirconia is a relevant procedure to form either redox catalysts or solid-state fuel cells.
Introduction The surface of zirconia and its chemical modification by grafting heteroelements have been studied less extensively than the surfaces of silica and alumina. One of the reasons might be the difficulty in synthesizing zirconia supports with a sufficiently high surface area to be interesting as a catalytic support. Nonetheless, applications as supports are promising since zirconia has a high thermal stability and acid, base, and redox properties. Since the development of the so-called SBA-15 type of materials1-3 and the successes that have been reported in the literature to synthesize mesoporous zirconia with surface area exceeding 250 m2/g, the interest in zirconia as a catalytic support has grown exponentially.4 Zirconia, and especially yttria-stabilized zirconia (YSZ), also attracts a lot of interest because of its potential electrochemical applications in solid oxide fuel cells (SOFCs).5 Iron is a strong candidate for improvement of the surface oxygen kinetics of SOFCs, since it has multiple valences needed for the conversion of gaseous oxygen molecules to oxygen ions, which will diffuse through the electrolyte. Moreover, Sasaki6 showed that iron is partially present in the +2 valence state, even in the highly oxidizing conditions that are used in the operation of SOFCs. Zirconia has also been considered as a candidate for catalytic supports. Okamoto7 prepared Fe/ZrO2 systems * Corresponding author. E-mail:
[email protected]. † University of Antwerp. ‡ Eindhoven University of Technology. (1) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G.; Chmelka, B.; Stucky, G. D. Nature 1998, 396, 152. (2) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G.; Chmelka, B.; Stucky, G. D. Science 1998, 279, 548. (3) Yang, P.; Zhao, D.; Margolese, I.; Chmelka, B.; Stucky, G. D. Nature 1998, 396, 152. (4) Chen, F.; Liu, M. J. Mater. Chem. 2000, 10, 2603. (5) Guillodo, M.; Vernoux, P.; Fouletier, J. Solid State Ionics 2000, 127, 99. (6) Sasaki, K.; Maier, J. Solid State Ionics 2000, 134, 303. (7) Okamoto, Y.; Kubota, T.; Ohto, Y.; Nasu, S. J. Catal. 2000, 192, 412.
using a simple impregnation with the nitrate salt, tested these catalysts for the NO-CO reaction (reduction of NO and oxidation of CO), and found a much higher activity than for the Fe/Al2O3 or Fe/SiO2 systems. The authors also concluded that the amount of Fe3+ species that could be adsorbed in a truly dispersed way by impregnation is very limited. Suo8 has compared Fe/ZrO2, Fe/TiO2, and Fe/Al2O3, also prepared by impregnation, for the CO2 hydrogenation and concluded that both titania- and zirconia-supported catalysts performed much better than the alumina counterpart. Laperdrix9 concluded that zirconia-supported Fe catalysts are much more durable than the conventional alumina catalysts for the reduction of sulfates by H2S, as the latter one is deactivated very rapidly. The authors argued that the use of zirconia systems is industrially interesting, despite its higher costs. These few examples clearly show a renewed interest in the use of Fe/ZrO2 as a catalytic support. However, little to nothing is known about the exact interactions between the supported metal oxides and the zirconia surface. During the past decade, several sophisticated surface modification techniques have been developed, aiming at a very controlled and precise generation of surface species with known geometry. In many cases, one of the main objectives of these surface modification techniques is the generation of isolated and active surface species. Molecular designed dispersion (MDD)10 and atomic layer deposition (ALD)11 are some of the techniques to achieve this goal. Although many reports have appeared on the activation of silica and alumina with metal-acetylacetonate com(8) Suo, Z. H.; Kou, Y.; Niu, J. Z.; Zhang, W. Z.; Wang, H. L. Appl. Catal., A 1997, 148, 301. (9) Laperdrix, E.; Sahibed-dine, A.; Costentin, G.; Saur, O.; Bensitel, M.; Nedez, C.; Mohammed Saad, A. B.; Lavalley, J. C. Appl. Catal., B 2000, 26, 71. (10) Van Der Voort, P.; Mitchell, M. B.; White, M. G.; Vansant, E. F. Interface Sci. 1997, 5, 169. (11) Haukka, S.; Kyto¨kivi, A.; Lakomaa, E. L.; Lehtovirta, U.; Lindbladt, M.; Lujala, V.; Suntola, T. Preparation of Catalysts VI, Scientific bases for the preparation of heterogeneous catalysts; Poncelet, G., et al., Eds.; Elsevier: Amsterdam, 1995; p 957.
10.1021/la025679u CCC: $22.00 © 2002 American Chemical Society Published on Web 05/21/2002
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plexes of V,12-15 Cr,16,17 Mo,18 Ti,19 and so forth, the activation of zirconia has never been studied. Here, we will present a detailed spectroscopic study of the reaction of Fe(acac)3 with the surface of zirconia. Experimental Section The zirconia support (ECO 100 E1/8, MEL Chemicals) has a specific surface area (SBET) of 64 m2/g, after pretreatment in air at 400 °C for 16 h. Its chemical composition is ZrO2 (93.4 wt %), HfO2 (5.7 wt %), and TiO2 (0.9 wt %). The sample contains no silica impurities. Modification of the support with Fe(acac)3 (Acros) is performed both in the liquid phase and in the gas phase. The zirconia support is stored and handled in dry conditions (water-free nitrogen) before and after the reaction with Fe(acac)3. In the liquid-phase reaction,13,14 2 g of the thermally pretreated support was stirred at room temperature for 1 h in a zeolitedried toluene solution containing 0.5 g of iron acetylacetonate. Meanwhile, the reaction vessel was purged with a stream of dry nitrogen. After the reaction, the modified support was filtered and washed five times with fresh solvent inside a dry nitrogen glovebox and dried under vacuum at room temperature. A washing cycle consists of washing 2 g of the reacted substrate with an aliquot of 25 mL of solvent, before drying. The liquid-phase reaction with 2,4-pentanedione (acetylacetone, Hacac) was performed using the same modus operandi. In this case, the 2,4-pentanedione was dissolved in zeolite-dried toluene. All other reaction conditions are the same as for the Fe(acac)3 modification. The gas-phase modification was carried out in a vacuum deposition reactor. Experimental details can be found in a previous publication.20 About 1 g of ZrO2 and an appropriate amount of the Fe(acac)3 complex were placed in a special sample holder. The system is brought under vacuum, and the temperature is set at 110 °C, at which the Fe(acac)3 sublimes and subsequently reacts with the support substrate. The formation of iron acetylacetonate crystals near the outlet of the reactor indicates the end of the reaction, as the support is saturated. Subsequently, the holder with the excess of Fe(acac)3 is removed and the modified support sample is further evacuated at reaction temperature for 2 h. The samples obtained from the liquid- and the gas-phase modifications were stored in a nitrogen glovebox to avoid hydration until analysis was completed. Finally, the samples were calcined in air at 550 °C for 16 h. Fe loadings were determined by electron microprobe analysis. Photoacoustic infrared spectra (FTIR-PAS) were measured on a Nicolet 20 SX spectrometer, equipped with a MTEC photoacoustic detector. The in situ DRIFT measurements were performed on a Nicolet Nexus 670 bench equipped with an in situ Spectra Tech High-Temperature Vacuum Chamber, controlled by a Spectra Tech Time Proportional Temperature Controller and a MCT detector. Samples were mixed with KBr (95% KBr, 5% sample). Spectra were collected in a vacuum at various temperatures starting from room temperature up to 400 °C. FT-Raman spectra were recorded on a Nicolet Nexus 670 bench equipped with a Raman module, a 1064 nm Nd:YAG excitation laser, and a Ge (12) Van Der Voort, P.; Possemiers, K.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1996, 92, 843. (13) Van Der Voort, P.; Babitch, I. V.; Grobet, P. J.; Verberckmoes, A. A.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1992, 92, 3635. (14) Baltes, M.; Collart, O.; Van Der Voort, P.; Vansant, E. F. Langmuir 1999, 15, 5841. (15) Baltes, M.; Van Der Voort, P.; Weckhuysen, B. M.; Rao, R. R.; Catana, G.; Schoonheydt, R. A.; Vansant, E. F. Phys. Chem. Chem. Phys. 2000, 2, 2673. (16) Babitch, I. V.; Plyuto, Y. V.; Van Der Voort, P.; Vansant, E. F. J. Colloid Interface Sci. 1997, 189, 144. (17) Babitch, I. V.; Plyuto, Y. V.; Van Der Voort, P.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1997, 93, 3191. (18) Collart, O.; Van Der Voort, P.; Vansant, E. F.; Gustin, E.; Bouwen, A.; Schoemaker, D.; Rao, R. R.; Weckhuysen, B. M.; Schoonheydt, R. A. Phys. Chem. Chem. Phys. 1999, 1, 4099. (19) Schrijnemakers, K.; Vansant, E. F. J. Porous Mater. 2001, 8, 83. (20) Baltes, M.; Van Der Voort, P.; Collart, O.; Vansant, E. F. J. Porous Mater. 1998, 5, 317.
Figure 1. Diffuse reflectance infrared spectra of (a) Zr(acac)4, (b) Fe(acac)3, (c) ZrO2/Fe(acac)3 (liquid phase), and (d) ZrO2/ Hacac (liquid phase). Spectra of the pure compounds (a) and (b) are downscaled by a factor of 10. detector. Spectra were measured at room temperature in a 180° reflective powder sampling configuration.
Results and Discussion Spectroscopic Study of the Interaction of Fe(acac)3 with the Surface of Zirconia. Figure 1 shows the DRIFT spectra of the zirconia sample, before and after liquid-phase reaction with Fe(acac)3, and the spectra of “pure” Fe(acac)3 and “pure” Zr(acac)4. Comparison of spectra a and b in Figure 1 reveals the diagnostic power of the C-O stretching vibrations of the acac ligand. It is known for quite some time that the C-O and C-C infrared absorption bands of the acac ligand are influenced by the central metal ion of the complex, as would be expected from the ligand field theory.21 The •s(C-O)ring stretching vibration is situated at 1570 cm-1 for the Fe(acac)3 complex and at 1590 cm-1 for the Zr(acac)4 complex. There is no significant difference in the position of the •as(C-C-C)ring stretching vibration; it is situated at 1530 cm-1 for both complexes. Also, the •s(CH3) deformation vibrations are positioned at the same frequency (1275 cm-1) for both complexes. Spectrum c shows the ZrO2 after liquid-phase deposition of Fe(acac)3. It does not show a spectral feature at 1570 cm-1, but it shows a clear peak around 1590 cm-1, suggesting that the surface species that are formed are Zr-acac surface groups rather than Fe-acac groups. For comparison, spectrum d is the measurement of the ZrO2 support, reacted with Hacac (acetylacetone, 2,4pentanedione). In the absence of Fe, the only surface sites that can be formed are Zr-acac sites. Spectrum d is remarkably similar to spectrum c, corroborating the suggestion that Zr-acac species are formed, when Fe(acac)3 reacts under very mild conditions with the ZrO2 surface. Fe(acac)3 is known to be a fairly unstable complex,22,23 which loses its ligands readily, whereas Zr(acac)4 is much more stable. Figure 2 shows the photoacoustic spectra of pure ZrO2, pretreated at 400 °C, and the spectra of this support after the liquid-phase reaction with Fe(acac)3 and Hacac, respectively. Spectrum a of pure zirconia shows two characteristic OH vibrations: a strong band at 3670 cm-1, assigned to a polydentate hydroxyl function, and a weaker band at 3745 cm-1, assigned to a monodentate OH (21) Lawson, K. L. Spectrochim. Acta 1961, 17, 245. (22) Van Hoene, J.; Charles, R. G.; Hickam, W. H. J. Phys. Chem. 1958, 62, 1098. (23) Ismail, H. M. J. Anal. Appl. Pyrolysis 1991, 21, 315.
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Van Der Voort et al. Table 1. Physical and Chemical Analysis of the Zirconia Support, before and after Reaction with Fe(acac)3
surface area Fe loading acac loading R-value
Figure 2. Infrared photoacoustic spectra of (a) parent zirconia, pretreated at 400 °C, (b) after reaction with Fe(acac)3, and (c) after reaction with Hacac. All samples were prepared by the liquid-phase procedure.
Figure 3. FT-Raman spectra of (a) parent zirconia, (b) zirconia reacted with Hacac, and (c) zirconia reacted with Fe(acac)3.
vibration (Zr-OH).24,25 After reaction with Fe(acac)3 in the liquid phase, followed by drying in vacuo, one can observe in spectrum b that most of the isolated hydroxyls have disappeared. However, the broad feature centered around 3400 cm-1 suggests the presence of residual or newly created hydroxyls that are in hydrogen bond interaction. The strong bands in the 1600 cm-1 region have been discussed previously and clearly indicate the presence of Zr-acac surface species. A similar reaction of the zirconia support with Hacac in the liquid phase yields spectrum c in Figure 2. Hacac (2,4-pentanedione or acetylacetone) is known to react selectively with coordinatively unsaturated sites (cus) (Lewis sites), forming M-acac surface groups. Spectrum c shows that the reaction between zirconia and Hacac occurs readily, again producing strong Zr-acac bonds. The broad band in the 3600-3200 cm-1 region, typical for hydroxyls in hydrogen bond interactions, should also be noted at this point. Figure 3 presents the FT-Raman spectra of the parent zirconia (a) and zirconia modified with Hacac (b) and Fe(acac)3 (c). The bands at 638, 615, 476, 381, 346, 334, 190, and 178 cm-1 are characteristic for the monoclinic phase.26 These results are confirmed by X-ray diffraction (not shown). The modification with Fe(acac)3 or Hacac has no influence on the number and relative intensity of these bands. There are no additional bands observable in the (24) Yamaguchi, T. Catal. Today 1994, 20, 199. (25) Cerrato, G.; Bordiga, S.; Barbera, S.; Morterra, C. Appl. Surf. Sci. 1997, 115, 53. (26) Sanati, M.; Anderson, A.; Wallenberg, L. R.; Rebenstorf, B. Appl. Catal., A 1993, 106, 51.
ZrO2 support
ZrO2 + Fe(acac)3 liquid phase
ZrO2 + Fe(acac)3 gas phase
60 m2/g
60 m2/g 0.12 mmol/g 1.2 #/nm2 0.24 mmol/g 2.4 #/nm2 2.0
60 m2/g 0.23 mmol/g 2.3 #/nm2 0.47 mmol/g 4.7 #/nm2 2.1
1200-800 cm-1 region, which is a very sensitive region for either MdO vibrations or for clustering of the surface species (M-O-M species). Finally, there are no bands that would indicate the formation of crystalline surface species or mixed phases. Analytical Study. The physical and chemical analysis data of the parent support material and of the Fe(acac)3activated ZrO2 are presented in Table 1. A mild liquidphase reaction of Fe(acac)3 with the support yields 1.2 Fe/nm2 on the surface and 2.4 acac groups/nm2. Exactly twice as many surface acac groups as Fe groups are created. The gas-phase reaction produces a higher surface loading of 2.3 Fe/nm2 and 4.7 acac/nm2. Taking into account the molecular size of the acac ligand,13 this reaction is obviously limited by sterical effects, or, in other words, the gas-phase reaction of Fe(acac)3 with zirconia is a surface saturation reaction. An R-value is defined as the ratio of the amount of acac species grafted on the surface and the amount of Fe species on the surface. For instance, if Fe(acac)3 would simply physisorb on the surface this R-value would be 3. It can be seen from Table 1 that for the reaction of Fe(acac)3 with ZrO2, the R-value is 2, for both the gas-phase and the liquid-phase reactions. This suggests that 1 acac group of the Fe(acac)3 does not produce a surface group on the support and must leave the system as a volatile compound. We will explain later that this volatile compound is probably Hacac, as the acac ligand picks up a proton of either a hydroxyl group or a water molecule during the hydrolysis step. It also suggests that the reaction mechanism is the same for liquid-phase and gas-phase reaction conditions. Table 2 summarizes the elemental analysis for the samples that have been pretreated with Hacac. The direct reaction of Hacac with the zirconia support yields a number of surface acac groups that is very similar to the number of surface acac groups created by the direct reaction with Fe(acac)3. When the Hacac-prereacted ZrO2 is subsequently reacted with Fe(acac)3, the increase in acac surface groups is negligible (0.03 mmol/g). It is remarkable that in the case of Hacac pretreatment, the incremental R-value (number of acac groups additionally created on the Hacacpretreated zirconia over the number of Fe groups (additionally) created) is only 0.33; this indicates that when Fe(acac)3 is reacted with a Hacac-pretreated zirconia sample, most of the acac ligands do not adsorb on the surface but are lost as volatile compounds. This will be explained in the next section. Reaction Mechanisms. The combination of the spectroscopic and analytical observations allows us to propose the following reaction mechanisms. The reaction of Hacac with ZrO2 occurs readily, and typical bands of Zr-acac surface species occur. Moreover, additional hydroxyls are created that are in strong hydrogen bond interaction. This leads to the reaction mechanism shown in Scheme 1.
Controlled Deposition of Iron Oxide on Zirconia
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Table 2. Chemical Analysis of the Zirconia Support, Reacted with Hacac and Fe(acac)3 in the Liquid Phase ZrO2 + Fe(acac)3 Fe loading acac loading R-value
0.12 mmol/g 1.2 #/nm2 0.24 mmol/g 2.4 #/nm2 2.0
ZrO2 + Hacac
0.27 mmol/g 2.7 #/nm2
ZrO2 + Hacac + Fe(acac)3
incremental acac and Fe (column 4 - column 3)
0.09 mmol/g 0.9 #/nm2 0.30 mmol/g 3.0 #/nm2 3.33
0.09 mmol/g 0.9 #/nm2 0.03 mmol/g 0.3 #/nm2 0.33a
Incremental R-value calculated as (incremental acac/incremental Fe) ) (0.30 mmol/g acac - 0.27 mmol/g acac)/(0.09 mmol/g Fe -0 mmol/g Fe) ) 0.33. a
Scheme 1
The hydrogen bonding occurs between the hydrogen of the surface hydroxyl and the pseudo π-system of the acac ligand.12 The reaction of Fe(acac)3 with zirconia is more complex. The following observations should be taken into account: (1) The Fe(acac)3 complex reacts both with cus Zr and with the surface hydroxyls. (2) Two out of three acac ligands remain on the surface, but they are not bonded anymore to the Fe central atom; they form Zr-acac surface species. (3) The structure of the zirconia remains intact; no microcrystals or mixed phases are formed. The conclusion that Fe(acac)3 reacts with the surface hydroxyls follows directly from the spectroscopic evidence, presented in Figure 2. However, the exclusive reaction of Fe(acac)3 with hydroxyls would never lead to the high surface concentration that is found in Table 1. In fact, Table 1 clearly shows that the gas-phase reaction of Fe(acac)3 with the zirconia support is a surface saturation reaction, that is, the reaction continues until all the available surface is covered with acac species. These observations lead to the reaction mechanisms in Schemes 2 and 3. Scheme 2 presents only the reaction of Fe(acac)3 with the cus Zr sites; the reaction with the hydroxyls proceeds very similarly, according to Scheme 3. The dashed lines in both schemes indicate the additional coordination of the central Fe ion with oxygens. The presented reaction schemes are schematic, as the central Fe ion will strive for a coordination of 6 with surrounding oxygens, either from the zirconia support or from the acac ligands. Also, the two-dimensional representations should be visualized as three-dimensional; therefore, the actual bond angles might look a bit distorted in this representation. Furthermore, traces of water are included in the reaction scheme. We were never able to observe the remaining Fe-acac surface group. This might be due to the sample manipulation, as the sample is necessarily in contact with air for brief moments when, for example, the infrared analysis cups are loaded. This hypothetical remaining Feacac species is extremely unstable and will be subjected to a partial olation process using traces of water to form an Fe-OH surface species and the release of Hacac as a volatile compound. Reaction schemes 1-3 also clearly explain the behavior of the Hacac-pretreated zirconia sample, when subjected to Fe(acac)3. It can be inferred from Table 2 that the acac
Figure 4. (A) In situ DRIFT specta of ZrO2/Fe(acac)3 after heating in air at (a) 25 °C, (b) 110 °C, and (c) 300 °C. (B) In situ DRIFT spectra of ZrO2/Fe(acac)3 after heating in vacuo at (a) 25 °C, (b) 100 °C, (c) 200 °C, (d) 300 °C, and (e) 400 °C. The parent samples were prepared by the liquid-phase procedure.
concentration of the Hacac-pretreated sample does not increase significantly after reaction with Fe(acac)3, whereas still a fairly high concentration of Fe ions is grafted on the surface. This means that the Fe ions in this case are grafted on the surface as “naked” ions. This is due to the fact that the cus Zr are no longer available for bonding with the acac ligand, since they have been already consumed by the reaction with Hacac. The unstable Fe(acac)3 will therefore still decompose upon contacting the surface, but the released acac species will leave the surface as volatile compounds. Thermal Conversion of the Surface Species. The thermal stability and the conversion of the grafted species were studied by in situ diffuse reflectance Fourier transform (DRIFT) spectroscopy and thermogravimetric analysis. Figure 4A shows the thermal behavior of the ZrO2/Fe(acac)3 sample in ambient air. This sample was prepared by the liquid-phase procedure. Spectrum a shows the infrared spectrum at room temperature, spectrum b after a treatment at 110 °C, and spectrum c after thermal treatment at 300 °C. These spectra clearly indicate a significant change in surface groups after a thermal
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Van Der Voort et al. Scheme 2
Scheme 3
Scheme 4
treatment at 110 °C. At this temperature, the typical Zracac bands disappear and three bands appear at 1560, 1440, and 1345 cm-1, which are characteristic for Zracetate surface species. Similar observations were made previously for Al-acac surface species,27 and the conversion mechanism presented in Scheme 4 is suggested. Also in this case, TGA-GC-FTIR confirmed that acetone is the main decomposition product in this temperature range. The stoichiometry of the reaction shows that water is needed for this conversion. Above 300 °C, the surface Zr(OAc) species are completely oxidized, and no traces of organic ligands are visible in the infrared spectra. In the absence of air and traces of water, the decomposition of the acac ligands is slightly different, as shown in Figure 4B. The conversion of the acac ligands toward the acetate ligands is retarded to about 200 °C. We were unable to measure the decomposition products in this case. The Zr-acetate species are remarkably stable in vacuum conditions and can withstand easily thermal treatments of 400 °C. Figure 5 shows the infrared spectrum of a calcined ZrO2/ Fe(acac)3 sample, which was prepared by the liquid-phase procedure. The infrared spectrum shows a very prominent band at 3755 cm-1. This band cannot be assigned to ZrOH modes, which appear at 3670 and 3745 cm-1, nor can it be assigned to the Hf-OH impurity, as this stretching vibration is situated at 3735 cm-1.7 However, an Fe-OH vibration is known to occur at 3755 cm-1.28 It can therefore be concluded that the spectrum clearly shows the presence of the Fe-OH surface group and the absence of any organic ligands. Inspection of the hydroxyl region shows that the zirconium hydroxyls are restored to a great extent. The (27) Van Der Voort, P.; White, M. G.; Vansant, E. F. Langmuir 1998, 14, 106. (28) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley and Sons: New York, 1986.
Zr-acetate species are combusted, and with the aid of ambient water or water produced by the decomposition of the organic ligands, a fraction of the surface hydroxyl groups are recreated. The Raman spectra of this sample (not shown) showed no peaks of crystalline iron oxide species, and only bands of the zirconia support could be observed. Conclusions Fe(acac)3 reacts readily with the surface of ZrO2, both in the liquid and the gas phase. Upon reaction with ZrO2, Zr-acac surface species are formed. The Fe(acac)3 decomposes upon contacting the zirconia surface; two of the three ligands bond to the zirconia surface, and the third one presumably remains on the Fe atom as an extremely unstable species that hydrolyzes very quickly with the release of 2,4-pentanedione. The reaction occurs partially via the surface hydroxyls but predominantly by the cus Zr. The Zr-acac sites are thermally unstable and transform into Zr-acetate species after a very mild thermal treatment in air (110 °C) or a more severe thermal treatment in a vacuum (>200 °C). After calcination, the
Figure 5. Infrared photoacoustic spectra of (a) zirconia pretreated at 400 °C and (b) ZrO2/Fe(acac)3 after calcination at 500 °C. The parent sample was prepared by the liquid-phase procedure.
Controlled Deposition of Iron Oxide on Zirconia
organic ligands are removed and the zirconium hydroxyl groups are restored on the surface. Also, a clear band at 3755 cm-1 appears, assigned to surface Fe-OH groups. No crystalline iron oxide clusters could be observed by Raman spectroscopy. Acknowledgment. This research is sponsored by a grant of the F.W.O.-Vlaanderen, Grant Number G.0449.99.
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P.V.D.V. acknowledges the F.W.O.-Vlaanderen for financial support; M.M. acknowledges the I.W.T.-Vlaanderen for a Ph.D. grant. The authors thank Mrs. F. Quiroz for her aid in the experimental work. The European Science Foundation (ALE network) is acknowledged for financial support. LA025679U
