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Surface nature of nanoparticle zinc-titanium oxide aerogel catalysts
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Applied Surface Science 254 (2008) 4500–4507 www.elsevier.com/locate/apsusc
Surface nature of nanoparticle zinc-titanium oxide aerogel catalysts Chien-Tsung Wang *, Jen-Chieh Lin Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin 640, Taiwan, ROC Received 22 November 2007; received in revised form 2 January 2008; accepted 7 January 2008 Available online 15 January 2008
Abstract Nanoparticle zinc-titanium oxide materials were prepared by the aerogel approach. Their structure, surface state and reactivity were investigated. Zinc titanate powders formed at higher zinc loadings possessed a higher surface area and smaller particle size. X-ray photoelectron spectroscopy (XPS) revealed a stronger electronic interaction between Zn and Ti atoms in the mixed oxide structure and showed the formation of oxygen vacancy due to zinc doping into titania or zinc titanate matrices. The 8–45 nm aerogel particles were evaluated as catalysts for methanol oxidation in an ambient flow reactor. Carbon dioxide was favorably produced on the oxides with anion defects. Titanium based oxides exhibited a high selectivity to dimethyl ether, so that a strong Lewis acidic character suggested for the catalysts was associated primarily with the Ti4+ center. Both methanol conversion and dimethyl ether formation rates increased with increasing the zinc content added to the oxide support. Results demonstrate that cubic zinc titanate phases produce new Lewis acid sites having also a higher reactivity and that the nature of the catalytic surface transforms from Lewis acidic to basic characters due to the presence of reactive oxygen vacancies. # 2008 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Aerogels; Zinc titanate; Catalysts; Methanol oxidation
1. Introduction Transition metal oxides are extensively employed as catalysts because they possess featured active centers to adsorb reaction molecules. Chemical properties of the active sites can be adjusted by mixing an oxide catalyst with an oxide support, so the interaction between two metal cations is highly critical to catalytic performance. For example, iron oxide possesses a Lewis acidic character to favor dehydration of methanol to dimethyl ether [1], whereas the ferric oxide species dispersed on irreducible silica support present redox activity for partial oxidation of methanol to formaldehyde and methyl formate [2]. Mixing iron oxide with titanium oxide to form Fe2TiO5 solid solution causes the generation of new acid sites having a stronger Lewis acidity [3]. Moreover, oxygen vacancies formed in an oxide catalyst can act as the active centers in catalytic reactions, for instance, the oxidative coupling of methane over Ni-doped Nd2O3 [4]. Therefore, the surface nature of catalytic
* Corresponding author. Tel.: +886 5 5342601x4623; fax: +886 5 5312071. E-mail address: [email protected] (C.-T. Wang). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.024
materials is of great importance for the determination of product distribution in a chemical reaction. Aerogels, one of the known nanostructured materials, are normally prepared by the sol-gel technique and a supercritical drying process. They have great potentials for use as catalysts [1–3] and sensors [5] because of some attractive structural properties. For example, high surface area favors adsorption of reaction molecules, and small particle size is advantageous for minimal internal diffusion resistances of molecules. The aerogel approach has been demonstrated highly effective for preparing nanoparticles of iron-titanium oxide solid solution [3] and gold/iron oxide [6]. Zinc-titanium oxide materials are widely applied to isobutene dehydrogenation [7] and coal-derived fuel gas adsorption [8]. Zinc titanate phases with a cubic crystalline structure exhibit a good performance in the two processes. However, attempts to prepare zinc titanate crystallites are not so successful because the oxide compounds are easy to decompose into two crystalline phases. For example, ZnTiO3 decomposes into Zn2TiO4 and TiO2 (rutile) at 965 8C [9]. Consequently, there is a need to find a new approach appropriate for use to synthesize the solid solution material.
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In the present study, we have prepared a series of zinctitanium oxide materials using the aerogel approach. They were characterized for oxidation states of surfaces with X-ray photoelectron spectroscopy and evaluated as catalysts in a fixed-bed reactor for the catalytic oxidation of methanol. Results are quite encouraging as the work below will show. 2. Experimental 2.1. Aerogel preparation Zinc-titanium oxide aerogels prepared for this work are summarized in Table 1. In a typical preparation of the Zn-Ti binary oxide, zinc acetate (99.6%, Sigma) and 1-titanium butoxide (97%, Aldrich) were used as precursors and dissolved in ethanol (99.9%, J. T. Baker) under agitation. An enough amount of water triply deionized was added to ensure complete hydrolysis (20 mol% excess with respect to the stoichiometric amount required for hydrolyzing both precursors to their corresponding hydroxides). A clear sol solution was obtained followed by kept under stirring for another 6 h. The Pyrex glass liner containing the solution was then placed into a 316 stainless steel autoclave, and heating commenced till the temperature and pressure reached supercritical points (normally peak condition at 265 8C and 110 bar) with respect to ethanol. Then the reactor began to depressurize to ambient, the heating stopped, and a purge flow of nitrogen gas through the vessel and pipeline followed. One of the most difficult steps was to control releasing the high-pressure hot liquid solution from the autoclave through volume expansion. The resultant aerogel powders were collected in jars for analysis. 2.2. Structural characterization The aerogel materials were characterized by various techniques. Surface areas were determined by 77 K nitrogen gas adsorption measurements (Micromeritics, Model ASAP 2010) and the BET (Brunauer–Emmett–Teller) method. Particle morphology was examined on a JEOL field emission scanning electron microscope (FESEM, Model JSM-7600F). Fourier transform infrared spectra were obtained on a Perkin
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Elmer spectrometer (FT-IR, Model Spectrum One), and the sample tablets for analysis were prepared at a mass ratio of oxide:KBr = 1:14. X-ray diffraction (XRD) patterns for the aerogel powders were obtained with a Mac Science diffractometer (Model MXP18) equipped with Ni-filtered Cu Ka ˚ ), carried out in the 2u radiation (20 mA, 30 kV and l = 1.54 A range of 5–908 at a scan rate of 48 min1. Elemental compositions and oxidation states of surfaces were determined with an X-ray photoelectron spectrometer (Thermo VG Scientific, Model Sigma Probe) with an Al Ka radiation source (1486.6 eV) to excite photoelectrons in an ultra vacuum atmosphere around 109 Torr. The binding energy scale was precisely calibrated by taking the adventitious C 1s peak at 285.0 eV as a reference. Experimental data points were resolved by a curve-fitting procedure, and the fitting function was chosen to be the superposition of two doublets with Gaussian–Lorentzian line shapes. 2.3. Catalytic activity evaluation The oxide aerogels were evaluated in a fixed-bed flow reactor (i.d. 10 mm) for the catalytic oxidation of methanol. The gas feed consisted of 2.1% methanol, 12.7% oxygen and 85.2% nitrogen, and the total flow rate was regulated at 6.9 L/h at normal conditions. The catalyst powders (0.05 g) were packed into a Pyrex glass tube with a ceramic frit that held the small clusters in position. The catalyst pretreatment was performed in a 50 vol.% O2/N2 gas stream for 1 h at a selected temperature. The hydrocarbon species in the reactor streams were nicely separated in a Cowax 10 capillary column (Supelco, 60 m 0.53 mm 2.0 mm film thickness) and analyzed on a gas chromatograph (China Chromatography, Model 9800) equipped with a flame ionization detector (FID, Varian) in series with a thermal conductivity detector (TCD, Varian). Carbon oxides were analyzed with a Carboxen 1000 packed column (Supelco, 15 ft 1/8 in.) and by a TCD. Conversion (%) is defined as: 100 mole of methanol consumed/mole of methanol fed. Selectivity (%) is defined as: 100 mole of product/mole of methanol converted SR, where SR is the stoichiometric carbon ratio of product to methanol.
Table 1 Structural properties of ZnO–TiO2 aerogels Sample (at. ratio)
Zn/[Zn + Ti] (at.%)
BET surfacea (m2 g1)
Packing densitya (g cm3)
XRD measurements a Phase detected
TiO2 Zn/Ti = 0.1 Zn/Ti = 0.3 Zn/Ti = 0.5 Zn/Ti = 0.7
0 9 23 33 41
75.9 68.5 73.7 130.9 210.4
0.175 0.131 0.125 0.115 0.080
Anatase Anatase Anatase, Zn2Ti3O8 Anatase, Zn2Ti3O8 Zn2Ti3O8
Zn/Ti = 1
50
179.4 (42.6)b
0.095 (0.185)b
Zn2Ti3O8 (ZnTiO3)b
0.375
Zincite
ZnO a b c
100
17.2
Sample calcined at 500 8C for 2 h. Sample calcined at 700 8C for 2 h. Calculated by the full width at half maximum of XRD peaks and the Scherrer equation.
Crystallite sizec (nm) 9–13 12–21 7–28 15–19 7–12 5–6 (19–24)b 9–14
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3. Results and discussion 3.1. Structural properties Zinc-titanium oxide aerogel materials prepared for this work and their structural properties are listed in Table 1. The BET surface area is a function of Zn/Ti mole ratio. Among the samples, the Zn/Ti = 0.7 aerogel has the largest surface area around 210.4 m2 g1 and the lowest packing density of 0.080 g cm1. In comparison, the obtained aerogels are greater in surface area than the oxide homologues (57–62 m2 g1) reported in literature [8,10]. Fig. 1 shows FESEM micrographs taken for the oxide aerogels calcined at 500 8C. Pure TiO2 powders are in the form of large clusters composed of 15–25 nm round particles (Fig.1a). The Zn-doped titania aerogel (Zn/Ti = 0.1) is composed of 15–32 nm round particles and some 50 nm 30 nm ellipse-shaped grains (Fig.1b). The Zn/Ti = 1 mixed oxide aerogel has primary particles as extremely small as about 8 nm in size, based on 200 random counts (Fig.1c). Pure ZnO aerogel has 27–45 nm particles of both sphere and rod shapes (Fig.1d). The crystal structure of the aerogel powders was determined by the XRD analysis, as listed in Table 1. The pure zinc oxide aerogel was characterized by the hexagonal zincite structure (JCPDS No. 05-0664). The pure titania aerogel was characterized only by the anatase phase (JCPDS No. 21-1272). The same diffraction patterns were recorded for the Zn-doped titania aerogel (Zn/Ti = 0.1), indicating a substitutional solid solution
of Zn2+ in the TiO2 lattice, due to similar ionic radii of the two ˚ for Zn2+ and 0.61 A ˚ for Ti4+ [11]). Marcı` cations (i.e., 0.60 A 2+ et al. [10] reported that Zn ions completely dissolved into TiO2 (anatase) only at atomic ratios Zn/Ti < 0.052 for the ZnO/ TiO2 powders prepared by a wet impregnation method. Further, with adding more zinc species to the titanium oxide matrix (Zn/ Ti = 0.3 and 0.5), the cubic Zn2Ti3O8 phase (JCPDS File No. 13-0471) appeared together with the anatase phase. As the Zn/ Ti ratio was raised to 0.7 and 1, the crystal structure found was only the Zn2Ti3O8 phase, and neither TiO2 nor ZnO phase was detected. Fig. 2 shows XRD patterns of the Zn/Ti = 1 aerogel as a function of calcination temperature. The XRD peaks of the compound corresponding to Zn2Ti3O8 appeared at 500 and 600 8C (curves a and b), and the peak intensity increased rapidly up to 700 8C (curve c). We assigned the zinc titanate phase found at 700 8C as cubic ZnTiO3 (JCPDS File No. 390190), on the basis of the suggestions by some authors [12]. In fact, it was difficult to identify whether the zinc titanate phase was ZnTiO3 or Zn2Ti3O8 using the XRD reflections obtained. As reported by Yamaguchi et al. [9], the compound Zn2Ti3O8 (cubic) is a low-temperature form of cubic ZnTiO3 and the crystallization occurs at 600–765 8C. For the Zn-Ti oxide aerogels, neither the phase transformation of TiO2 from anatase to rutile nor the precipitation of bare ZnO crystallites was found. Probably, zinc oxide is stabilized in the titania (anatase) matrix in the aerogel processing. However, the two phenomena have been reported for the Zn-Ti oxide materials prepared by solid state reaction [13], precipitation [8] and impregnation [10].
Fig. 1. FESEM micrographs of aerogels after calcination at 500 8C (a) TiO2, (b) Zn/Ti = 0.1, (c) Zn/Ti = 1 and (d) ZnO.
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Fig. 2. XRD patterns of ZnO–TiO2 (Zn/Ti = 1) aerogel after calcination at (a) 500 8C, (b) 600 8C and (c) 700 8C.
FT-IR spectra of the Zn-Ti oxide aerogels calcined at 500 8C are shown in Fig. 3. The pure titania aerogel was characterized by a broad absorption region composed of vibration bands at 790, 712, 628, 548 and 435 cm1 (curve a). They were attributed to the Ti–O stretching vibration in octahedral TiO6 groups. The pure zinc oxide aerogel was characterized by a sharp band at 447 cm1 and a shoulder at 550 cm1 (curve f), assigned to the Zn–O stretching vibration. For the Zn-doped titania sample (Zn/Ti = 0.1, curve b), the absorbance of the strong band at 790 cm1 (in curve a) decreased remarkably, due to the substitution of Ti by Zn in the Ti–O bond. As the Zn/Ti ratio was raised to 0.7 (curve c), the 790 cm1 band completely disappeared, and the peak at 712 cm1 (in curve a) shifted to 694 cm1. The band shift is associated with the phase transformation from anatase to zinc titanate (Zn2Ti3O8), according to XRD (Table 1). The Zn/Ti = 1 mixed oxide, after calcined at 500 8C, was characterized by a new absorption band at 735 cm1 (symbol *, curve d), and the absorbance of the peak increased rapidly up to 700 8C (curve e). Therefore, the 735 cm1 band can be assigned to the Zn–O–Ti bond structure in zinc titanate.
Fig. 3. FT-IR spectra of aerogels after calcination at 500 8C (a) TiO2, (b) Zn/ Ti = 0.1, (c) Zn/Ti = 0.7, (d) Zn/Ti = 1 and (f) ZnO, and at 700 8C (e) Zn/Ti = 1.
3.2. Surface analysis by XPS Table 2 lists element compositions measured for the ZnOTiO2 aerogels by X-ray photoelectron spectroscopy (XPS). For the aerogel powders, the XPS-derived Zn/Ti ratios are slightly higher than the corresponding nominal values. The same observation has ever been reported for zinc titanate films [12]. This zinc enrichment on the surface was seen more pronounced in a study by heating the Zn/Ti = 1 oxide from 500 to 700 8C. We consider that the zinc species may migrate to the nearsurface region during the crystallization of the Zn/Ti = 1 oxide; that is, in the phase transformation from Zn2Ti3O8 to ZnTiO3 (Fig. 2). Binding energy (BE) measurements corresponding to the core electrons in atomic levels Ti 2p3/2 and Zn 2p3/2 are listed in Table 2. The obtained BE values are similar to those reported in literature [10,13]. All ZnO–TiO2 samples showed a lower BE for Ti 2p3/2 than the TiO2, but a higher BE for Zn 2p3/2 than the
Table 2 Surface analysis by XPS spectra for ZnO–TiO2 aerogels Samplea
TiO2, 500 8C, fresh Zn/Ti = 0.1, 500 8C, fresh Zn/Ti = 0.7, 500 8C, fresh Zn/Ti = 1, 500 8C, fresh Zn/Ti = 1, 700 8C, fresh Zn/Ti = 1, 500 8C, spentb ZnO, 500 8C, fresh a b
Composition (%)
Binding energy (eV)
Atomic ratio
Zn
Ti
O
Zn 2p3/2
Ti 2p3/2
O 1s
Ti3+/Ti4+
Odef2/Olat2
OH/Olat2
– 8.1 14.5 19.1 21.9 19.8 43.8
31.8 24.1 19.3 16.7 15.8 15.6 –
68.2 67.7 66.2 64.2 62.3 64.6 51.7
– 1022.30 1021.90 1021.92 1021.67 1021.87 1021.85
459.23 458.85 458.27 458.76 458.60 458.70 –
530.27 530.10 530.05 530.30 530.55 530.30 530.35
0 0 0.06 0.03 0.40 0.05 –
0 0.30 0 0.11 0 0.05 0.35
0.20 0.13 0.12 0.15 0.06 0.25 0.21
Calcined for 2 h in air. Collected from the reactor after two 12-h methanol oxidation runs.
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ZnO. This indicates a stronger electronic interaction between Zn and Ti atoms in the mixed oxide structure. Due to asymmetry, the Ti 2p3/2 peak can be deconvoluted into two components, and they are assigned to Ti3+ (Ti2O3) and Ti4+ (TiO2) ions [3]. As listed in Table 2, the analysis on Ti3+/Ti4+ ratio reveals that a majority of the Ti species in the titanium oxide based aerogels are present in the Ti4+ oxidation state. On the other hand, the obtained Zn 2p peaks are quite symmetric for the zinc based oxides, so that the Zn species exist only in the formal valence state of Zn2+, in good agreement with the findings of some authors [7,14]. As regards the XPS O 1s analysis, the TiO2 and ZnO–TiO2 aerogels exhibited lower O 1s BE values than the ZnO (Table 2). The result seems to indicate a higher mobility of the O2 ions placed near to Ti sites. Fig. 4 shows O 1s spectra for
the TiO2, Zn-doped TiO2 (Zn/Ti = 0.1) and ZnO aerogels calcined at 500 8C. The asymmetry of these O 1s broad bands is indicative of the presence of at least two types of oxygen species on the surface. For the TiO2 sample, deconvolution of the O 1s peak gave rise to two symmetric components (Fig. 4a). One at 530.3 eV was assigned to lattice oxygen ions (Olat2), and the other at 531.6 eV was associated with hydroxyl species (OH). For the Zn/Ti = 0.1 sample, three components at 530.1, 531.1 and 532.0 eV were resolved from the O 1s peak (Fig. 4b). The one at 531.1 eV, not found in the TiO2, was characteristic of oxygen ions in the oxygen-deficient regions (Odef2) [7]. The Odef2 component can also be derived from the O 1s spectrum of the ZnO sample (Fig. 4c). Therefore, the oxygen vacancies not only existed within the ZnO matrix, similarly reported in Ref. [15], but also were created in the oxides doped with Zn2+ (e.g., Zn/Ti = 0.1 and 1). Table 2 lists atomic ratios of Odef2/ Olat2 and OH/Olat2. Lee et al. [4] reported the formation of oxygen vacancy due to Ni2+-doping into Nd2O3. Fig. 5 presents XPS Ti 2p and O 1s spectra of the ZnO-TiO2 (Zn/Ti = 1) samples calcined at 500 and 700 8C in air. The heating caused not only a reduction in the intensity of the main Ti 2p3/2 peak at 458.76 eV but also the growth of a new peak at 457.30 eV. Also, the intensity of the main O 1s peak at 530.30 eV and higher-BE side was weakened remarkably, and a new O 1s peak at 529.30 eV appeared. These results suggest the transformation from Ti4+ to Ti3+ cations and the formation of a Ti3+–O bond structure due to calcination. We consider that the
Fig. 4. XPS O 1s spectra of aerogels after calcination at 500 8C (a) TiO2, (b) Zn/ Ti = 0.1 and (c) ZnO. (symbols: 1: Olat2; 2: OH; 3: Odef2).
Fig. 5. XPS Ti 2p and O 1s spectra of the ZnO–TiO2 (Zn/Ti = 1) aerogels calcined at 500 and 700 8C.
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formation of the Ti3+ ion and Ti3+–O bond is related to the thermal dissociation of H from OH groups (i.e., OH/Olat2 decrease from 0.15 to 0.06 in Table 2) and the disappearance of oxygen vacancy in the oxide (i.e., Odef2/Olat2 = 0 at 700 8C). Oxygen vacancies formed in an oxide are known to be adsorption sites for oxygen molecules [16]. When an adsorbed O2 reacts with two electrons trapped at an oxygen vacancy site, two oxygen ions (Oads) are produced (Eq. (1)) [4]. Then, one Oads ion reduces an adjacent or neighboring Ti4+ cation to generate an O–Ti3+ bond (Eq. (2)). It is proposed that the O formation becomes more probable as the annealing temperature is raised [18]. Onishi and coworkers [17] studied the O2 adsorption on H2-reduced cerium oxide using infrared spectroscopy and found that superoxide species (O2) are formed immediately after introduction of gaseous oxygen, and successively converted into O22, O and finally Olat2. Table 2 indicates that the O 1s binding energy increases from 530.30 eV at 500 8C to 530.55 eV at 700 8C. It is known that the basic strength of metal oxide increases with decreasing the O 1s binding energy [19]. For the Zn/Ti = 1 mixed oxide, the thermal treatment has reduced the surface density of oxygen vacancy, and consequently the Lewis acidity increases. O2ðgÞ þ 2e ? 2OðadsÞ
(1)
OðadsÞ þ Ti4þ ! OTi3þ
(2)
3.3. Catalytic properties The zinc-titanium oxide aerogels were evaluated as catalysts for the oxidation of methanol in an ambient fixed-bed flow reactor. Pure TiO2 and ZnO were included for reference. Table 3 lists methanol conversion and product selectivity measured at the reaction temperature of 350 8C and the Arrhenius activation energy (Ea;CH3 OH ) and pre-exponential factor (ko;CH3 OH ). The catalytic activity, expressed in terms of methanol conversion, increased remarkably with higher zinc loadings in the oxide catalysts. An approximate 3-fold increase in the conversion was noted for the oxides doped heavily with zinc (i.e., Zn/Ti = 0.3 to 1). In other words, the cubic Zn2Ti3O8 phase promoted the oxidative conversion of methanol, and the catalysts containing this phase exhibited the lower activation
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energy (Table 3). Similarly, Chen et al. [12] reported that zinc titanate phases with a cubic crystal structure are active in isobutane dehydrogenation, but the other phases are not. We consider that the reducibility of the more labile bridging Zn–O– Ti bond in the zinc titanate structure, due to a stronger electronic interaction between Zn and Ti atoms, is responsible for an increase in the catalytic activity for methanol oxidation. A higher mobility of the O2 ions placed near to Ti sites helps the chemisorption of methanol molecules onto the Ti centers. Yang et al. [20] investigated the catalytic wet air oxidation over CeO2–TiO2 catalysts and concluded that the interaction of Ce and Ti affects the surface and structural properties of the oxide catalysts and their activity. Wachs and coworkers [21] studied methanol oxidation over supported metal oxide catalysts using quantitative methanol chemisorption and in situ infrared techniques and proposed that bridging metal–O–support bonds are the critical active sites for an oxidation reaction. Products obtained from the methanol oxidation reactor at 350 8C were dimethyl ether (CH3OCH3), methyl formate (HCOOCH3) and carbon dioxide. It is known that the dimethyl ether selectivity describes the pure dehydration ability of a catalyst, which is generally related to its Lewis acidity [2,22]. In contrast, the basic sites of a catalyst promote the oxidation of methanol to formate and CO2 [22]. In this work, all titanium oxide based aerogels exhibited a high selectivity to dimethyl ether, in the range of 44.8–99.3%, so that their catalytic surfaces had a strong Lewis acidic character, which is associated primarily with the Ti4+ center. In addition, a decrease in the dimethyl ether selectivity was found for the zinc-doped oxide catalysts. This indicates that the acidic strength of the pure titania is decreased by Zn2+-doping. The role of the titanium oxide phase is to serve as a reducible support to maintain the catalytic surface at a high Lewis acidity. On the other hand, carbon dioxide was favorably produced on the ZnO and ZnO–TiO2 (Zn/Ti = 0.1 and 1). A strong basic character suggested for the catalysts can be accounted by the presence of oxygen vacancies on their surfaces, according to XPS (Table 2). Fig. 6 illustrates a dependence of the dimethyl ether yield on the catalyst composition, obtained at reaction temperatures of 300, 325 and 350 8C. The CH3OCH3 formation rate increased abruptly in the Zn/(Zn + Ti) range between 23 and 50 at.% (i.e.,
Table 3 Catalytic properties of ZnO–TiO2 aerogels for methanol oxidation and Arrhenius parameters Catalyst
TiO2 Zn/Ti = 0.1 Zn/Ti = 0.3 Zn/Ti = 0.5 Zn/Ti = 0.7 Zn/Ti = 1 ZnO
Conversion and selectivities at 350 8C (%) a CH3OH
CH3OCH3
HCOOCH3
CO2
18.4 47.4 63.0 66.9 70.4 69.6 53.1
99.3 44.8 87.5 90.7 94.1 54.6 3.0
0.3 9.2 1.0 0.4 0.1 0.1 0.6
0.2 46.0 11.5 8.8 7.2 45.3 96.4
ko;CH3 OH b (m3 h1 kg1cat.)
Ea;CH3 OH b (kJ mol1)
7.37 1010 7.56 1014 1.99 1012 1.07 1010 1.39 108 1.89 109 8.19 1013
110.3 9.0 155.4 25.5 118.0 9.4 90.5 8.6 66.5 5.0 80.8 8.2 139.5 16.9
Reactor conditions: catalyst mass of 0.05 g; pretreatment at 500 8C; gas feed of 6.9 L h1; feed composition of CH3OH/O2/N2 = 2.1 mol%/12.7 mol%/ 85.2 mol%. b Estimated by a first-order rate model RCH3 OH ¼ ko expðEa =RTÞCCH3 OH based on methanol conversions at 250–400 8C. a
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the basic active centers that favor the oxidation of methanol to CO2. It is postulated that Lewis-chemisorbed methanol is oxidized into formate species and CO2 possibly via reaction with electrons trapped at oxygen vacancy sites or with oxygen ions (O) formed in the near-surface region. A previous study [23] reported that oxygen ions produced on oxygen vacancy sites or basic sites may abstract hydrogen from methane during the oxidative coupling of methane over rare earth metal oxide catalysts. 4. Conclusions
Fig. 6. Dimethyl ether formation rate as a function of catalyst composition at reaction temperatures (a) 300 8C, (b) 325 8C and (c) 350 8C.
Zn/Ti = 0.3 to 1). The result demonstrates an increase in the number of Lewis acid sites, due to the cubic Zn2Ti3O8 phase. A previous study [3] reported that the Fe2TiO5 phase produces new acid sites having a stronger Lewis acidity. Moreover, the surface area effect is also profound on the dimethyl ether formation, suggesting that more Ti–O surface atoms are closely situated to promote the Lewis chemisorption of two neighboring methanol groups as initially methoxy groups (i.e., Ti4+– OCH3) that subsequently combine to give dimethyl ether. Fig. 7 shows an effect of the catalyst pretreatment temperature on the dehydration of methanol to dimethyl ether, evaluated at reaction temperatures from 250 to 375 8C over the Zn/Ti = 1 mixed oxide. The catalyst pretreated at 700 8C showed a remarkably higher selectivity to dimethyl ether than the others treated at 500 and 600 8C. As indicated by the XPS analysis (Table 2), for the Zn/Ti = 1 oxide surface, the calcination to 700 8C caused the disappearance of oxygen defects and the transformation from Ti4+ to Ti3+ cations. These results demonstrate that oxygen vacancy sites play the role of
The surface nature of the zinc-titanium oxide aerogel catalysts has been elucidated by the XPS analysis and the selectivity pattern and formation rates of the reaction products in methanol oxidation. The titanium oxide based catalysts exhibit a high selectivity to dimethyl ether, so that their catalytic surfaces possess a strong Lewis acidic character, primarily related to the Ti4+ center. Carbon dioxide is favorably produced on the oxide catalysts with anion defects. Thus, the oxygen vacancies, formed by doping zinc species into titania or zinc titanate matrices, are the basic sites active towards the total combustion. The catalysts with cubic zinc titanate phases are more active for the conversion of methanol and also promote the production rate of dimethyl ether. It is suggested that the reducibility of the more labile Zn–O–Ti bond, due to a stronger electronic interaction between Zn and Ti atoms, controls the catalytic activity in the oxidation of methanol. Results demonstrate that the zinc titanate solid solutions produce new Lewis acid sites having also a higher reactivity and that the nature of the catalytic surface transforms from Lewis acidic to basic characters due to the presence of reactive oxygen vacancies. Acknowledgements This work is financially supported by The National Science Council in Taiwan under grant number NSC 94-2214-E-224006. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Fig. 7. Influence of pretreatment temperature of ZnO–TiO2 (Zn/Ti = 1) aerogel on dehydration of methanol to dimethyl ether (a) 500 8C, (b) 600 8C and (c) 700 8C.
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[17] C. Li, K. Domen, K. Maruya, T. Onishi, J. Am. Chem. Soc. 111 (1989) 7683. [18] G.U. Kulkarni, C.N.R. Rao, M.W. Roberts, J. Phys. Chem. 99 (1995) 3310. [19] K. Tanabe, in: J.R. Anderson, M. Boudart (Eds.), Catalysis: Science and Technology, vol. 2, Springer, New York, 1981. [20] S. Yang, W. Zhu, Z. Jiang, Z. Chen, J. Wang, Appl. Surf. Sci. 252 (2006) 8499. [21] L.J. Burcham, L.E. Briand, I.E. Wachs, Langmuir 17 (2001) 6164. [22] J.M. Tatiboue¨t, Appl. Catal. A 148 (1997) 213. [23] K. Otsuka, K. Jinno, A. Morikawa, J. Catal. 100 (1986) 353.
Applied Surface Science 254 (2008) 4500–4507 www.elsevier.com/locate/apsusc
Surface nature of nanoparticle zinc-titanium oxide aerogel catalysts Chien-Tsung Wang *, Jen-Chieh Lin Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin 640, Taiwan, ROC Received 22 November 2007; received in revised form 2 January 2008; accepted 7 January 2008 Available online 15 January 2008
Abstract Nanoparticle zinc-titanium oxide materials were prepared by the aerogel approach. Their structure, surface state and reactivity were investigated. Zinc titanate powders formed at higher zinc loadings possessed a higher surface area and smaller particle size. X-ray photoelectron spectroscopy (XPS) revealed a stronger electronic interaction between Zn and Ti atoms in the mixed oxide structure and showed the formation of oxygen vacancy due to zinc doping into titania or zinc titanate matrices. The 8–45 nm aerogel particles were evaluated as catalysts for methanol oxidation in an ambient flow reactor. Carbon dioxide was favorably produced on the oxides with anion defects. Titanium based oxides exhibited a high selectivity to dimethyl ether, so that a strong Lewis acidic character suggested for the catalysts was associated primarily with the Ti4+ center. Both methanol conversion and dimethyl ether formation rates increased with increasing the zinc content added to the oxide support. Results demonstrate that cubic zinc titanate phases produce new Lewis acid sites having also a higher reactivity and that the nature of the catalytic surface transforms from Lewis acidic to basic characters due to the presence of reactive oxygen vacancies. # 2008 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Aerogels; Zinc titanate; Catalysts; Methanol oxidation
1. Introduction Transition metal oxides are extensively employed as catalysts because they possess featured active centers to adsorb reaction molecules. Chemical properties of the active sites can be adjusted by mixing an oxide catalyst with an oxide support, so the interaction between two metal cations is highly critical to catalytic performance. For example, iron oxide possesses a Lewis acidic character to favor dehydration of methanol to dimethyl ether [1], whereas the ferric oxide species dispersed on irreducible silica support present redox activity for partial oxidation of methanol to formaldehyde and methyl formate [2]. Mixing iron oxide with titanium oxide to form Fe2TiO5 solid solution causes the generation of new acid sites having a stronger Lewis acidity [3]. Moreover, oxygen vacancies formed in an oxide catalyst can act as the active centers in catalytic reactions, for instance, the oxidative coupling of methane over Ni-doped Nd2O3 [4]. Therefore, the surface nature of catalytic
* Corresponding author. Tel.: +886 5 5342601x4623; fax: +886 5 5312071. E-mail address: [email protected] (C.-T. Wang). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.024
materials is of great importance for the determination of product distribution in a chemical reaction. Aerogels, one of the known nanostructured materials, are normally prepared by the sol-gel technique and a supercritical drying process. They have great potentials for use as catalysts [1–3] and sensors [5] because of some attractive structural properties. For example, high surface area favors adsorption of reaction molecules, and small particle size is advantageous for minimal internal diffusion resistances of molecules. The aerogel approach has been demonstrated highly effective for preparing nanoparticles of iron-titanium oxide solid solution [3] and gold/iron oxide [6]. Zinc-titanium oxide materials are widely applied to isobutene dehydrogenation [7] and coal-derived fuel gas adsorption [8]. Zinc titanate phases with a cubic crystalline structure exhibit a good performance in the two processes. However, attempts to prepare zinc titanate crystallites are not so successful because the oxide compounds are easy to decompose into two crystalline phases. For example, ZnTiO3 decomposes into Zn2TiO4 and TiO2 (rutile) at 965 8C [9]. Consequently, there is a need to find a new approach appropriate for use to synthesize the solid solution material.
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In the present study, we have prepared a series of zinctitanium oxide materials using the aerogel approach. They were characterized for oxidation states of surfaces with X-ray photoelectron spectroscopy and evaluated as catalysts in a fixed-bed reactor for the catalytic oxidation of methanol. Results are quite encouraging as the work below will show. 2. Experimental 2.1. Aerogel preparation Zinc-titanium oxide aerogels prepared for this work are summarized in Table 1. In a typical preparation of the Zn-Ti binary oxide, zinc acetate (99.6%, Sigma) and 1-titanium butoxide (97%, Aldrich) were used as precursors and dissolved in ethanol (99.9%, J. T. Baker) under agitation. An enough amount of water triply deionized was added to ensure complete hydrolysis (20 mol% excess with respect to the stoichiometric amount required for hydrolyzing both precursors to their corresponding hydroxides). A clear sol solution was obtained followed by kept under stirring for another 6 h. The Pyrex glass liner containing the solution was then placed into a 316 stainless steel autoclave, and heating commenced till the temperature and pressure reached supercritical points (normally peak condition at 265 8C and 110 bar) with respect to ethanol. Then the reactor began to depressurize to ambient, the heating stopped, and a purge flow of nitrogen gas through the vessel and pipeline followed. One of the most difficult steps was to control releasing the high-pressure hot liquid solution from the autoclave through volume expansion. The resultant aerogel powders were collected in jars for analysis. 2.2. Structural characterization The aerogel materials were characterized by various techniques. Surface areas were determined by 77 K nitrogen gas adsorption measurements (Micromeritics, Model ASAP 2010) and the BET (Brunauer–Emmett–Teller) method. Particle morphology was examined on a JEOL field emission scanning electron microscope (FESEM, Model JSM-7600F). Fourier transform infrared spectra were obtained on a Perkin
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Elmer spectrometer (FT-IR, Model Spectrum One), and the sample tablets for analysis were prepared at a mass ratio of oxide:KBr = 1:14. X-ray diffraction (XRD) patterns for the aerogel powders were obtained with a Mac Science diffractometer (Model MXP18) equipped with Ni-filtered Cu Ka ˚ ), carried out in the 2u radiation (20 mA, 30 kV and l = 1.54 A range of 5–908 at a scan rate of 48 min1. Elemental compositions and oxidation states of surfaces were determined with an X-ray photoelectron spectrometer (Thermo VG Scientific, Model Sigma Probe) with an Al Ka radiation source (1486.6 eV) to excite photoelectrons in an ultra vacuum atmosphere around 109 Torr. The binding energy scale was precisely calibrated by taking the adventitious C 1s peak at 285.0 eV as a reference. Experimental data points were resolved by a curve-fitting procedure, and the fitting function was chosen to be the superposition of two doublets with Gaussian–Lorentzian line shapes. 2.3. Catalytic activity evaluation The oxide aerogels were evaluated in a fixed-bed flow reactor (i.d. 10 mm) for the catalytic oxidation of methanol. The gas feed consisted of 2.1% methanol, 12.7% oxygen and 85.2% nitrogen, and the total flow rate was regulated at 6.9 L/h at normal conditions. The catalyst powders (0.05 g) were packed into a Pyrex glass tube with a ceramic frit that held the small clusters in position. The catalyst pretreatment was performed in a 50 vol.% O2/N2 gas stream for 1 h at a selected temperature. The hydrocarbon species in the reactor streams were nicely separated in a Cowax 10 capillary column (Supelco, 60 m 0.53 mm 2.0 mm film thickness) and analyzed on a gas chromatograph (China Chromatography, Model 9800) equipped with a flame ionization detector (FID, Varian) in series with a thermal conductivity detector (TCD, Varian). Carbon oxides were analyzed with a Carboxen 1000 packed column (Supelco, 15 ft 1/8 in.) and by a TCD. Conversion (%) is defined as: 100 mole of methanol consumed/mole of methanol fed. Selectivity (%) is defined as: 100 mole of product/mole of methanol converted SR, where SR is the stoichiometric carbon ratio of product to methanol.
Table 1 Structural properties of ZnO–TiO2 aerogels Sample (at. ratio)
Zn/[Zn + Ti] (at.%)
BET surfacea (m2 g1)
Packing densitya (g cm3)
XRD measurements a Phase detected
TiO2 Zn/Ti = 0.1 Zn/Ti = 0.3 Zn/Ti = 0.5 Zn/Ti = 0.7
0 9 23 33 41
75.9 68.5 73.7 130.9 210.4
0.175 0.131 0.125 0.115 0.080
Anatase Anatase Anatase, Zn2Ti3O8 Anatase, Zn2Ti3O8 Zn2Ti3O8
Zn/Ti = 1
50
179.4 (42.6)b
0.095 (0.185)b
Zn2Ti3O8 (ZnTiO3)b
0.375
Zincite
ZnO a b c
100
17.2
Sample calcined at 500 8C for 2 h. Sample calcined at 700 8C for 2 h. Calculated by the full width at half maximum of XRD peaks and the Scherrer equation.
Crystallite sizec (nm) 9–13 12–21 7–28 15–19 7–12 5–6 (19–24)b 9–14
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3. Results and discussion 3.1. Structural properties Zinc-titanium oxide aerogel materials prepared for this work and their structural properties are listed in Table 1. The BET surface area is a function of Zn/Ti mole ratio. Among the samples, the Zn/Ti = 0.7 aerogel has the largest surface area around 210.4 m2 g1 and the lowest packing density of 0.080 g cm1. In comparison, the obtained aerogels are greater in surface area than the oxide homologues (57–62 m2 g1) reported in literature [8,10]. Fig. 1 shows FESEM micrographs taken for the oxide aerogels calcined at 500 8C. Pure TiO2 powders are in the form of large clusters composed of 15–25 nm round particles (Fig.1a). The Zn-doped titania aerogel (Zn/Ti = 0.1) is composed of 15–32 nm round particles and some 50 nm 30 nm ellipse-shaped grains (Fig.1b). The Zn/Ti = 1 mixed oxide aerogel has primary particles as extremely small as about 8 nm in size, based on 200 random counts (Fig.1c). Pure ZnO aerogel has 27–45 nm particles of both sphere and rod shapes (Fig.1d). The crystal structure of the aerogel powders was determined by the XRD analysis, as listed in Table 1. The pure zinc oxide aerogel was characterized by the hexagonal zincite structure (JCPDS No. 05-0664). The pure titania aerogel was characterized only by the anatase phase (JCPDS No. 21-1272). The same diffraction patterns were recorded for the Zn-doped titania aerogel (Zn/Ti = 0.1), indicating a substitutional solid solution
of Zn2+ in the TiO2 lattice, due to similar ionic radii of the two ˚ for Zn2+ and 0.61 A ˚ for Ti4+ [11]). Marcı` cations (i.e., 0.60 A 2+ et al. [10] reported that Zn ions completely dissolved into TiO2 (anatase) only at atomic ratios Zn/Ti < 0.052 for the ZnO/ TiO2 powders prepared by a wet impregnation method. Further, with adding more zinc species to the titanium oxide matrix (Zn/ Ti = 0.3 and 0.5), the cubic Zn2Ti3O8 phase (JCPDS File No. 13-0471) appeared together with the anatase phase. As the Zn/ Ti ratio was raised to 0.7 and 1, the crystal structure found was only the Zn2Ti3O8 phase, and neither TiO2 nor ZnO phase was detected. Fig. 2 shows XRD patterns of the Zn/Ti = 1 aerogel as a function of calcination temperature. The XRD peaks of the compound corresponding to Zn2Ti3O8 appeared at 500 and 600 8C (curves a and b), and the peak intensity increased rapidly up to 700 8C (curve c). We assigned the zinc titanate phase found at 700 8C as cubic ZnTiO3 (JCPDS File No. 390190), on the basis of the suggestions by some authors [12]. In fact, it was difficult to identify whether the zinc titanate phase was ZnTiO3 or Zn2Ti3O8 using the XRD reflections obtained. As reported by Yamaguchi et al. [9], the compound Zn2Ti3O8 (cubic) is a low-temperature form of cubic ZnTiO3 and the crystallization occurs at 600–765 8C. For the Zn-Ti oxide aerogels, neither the phase transformation of TiO2 from anatase to rutile nor the precipitation of bare ZnO crystallites was found. Probably, zinc oxide is stabilized in the titania (anatase) matrix in the aerogel processing. However, the two phenomena have been reported for the Zn-Ti oxide materials prepared by solid state reaction [13], precipitation [8] and impregnation [10].
Fig. 1. FESEM micrographs of aerogels after calcination at 500 8C (a) TiO2, (b) Zn/Ti = 0.1, (c) Zn/Ti = 1 and (d) ZnO.
C.-T. Wang, J.-C. Lin / Applied Surface Science 254 (2008) 4500–4507
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Fig. 2. XRD patterns of ZnO–TiO2 (Zn/Ti = 1) aerogel after calcination at (a) 500 8C, (b) 600 8C and (c) 700 8C.
FT-IR spectra of the Zn-Ti oxide aerogels calcined at 500 8C are shown in Fig. 3. The pure titania aerogel was characterized by a broad absorption region composed of vibration bands at 790, 712, 628, 548 and 435 cm1 (curve a). They were attributed to the Ti–O stretching vibration in octahedral TiO6 groups. The pure zinc oxide aerogel was characterized by a sharp band at 447 cm1 and a shoulder at 550 cm1 (curve f), assigned to the Zn–O stretching vibration. For the Zn-doped titania sample (Zn/Ti = 0.1, curve b), the absorbance of the strong band at 790 cm1 (in curve a) decreased remarkably, due to the substitution of Ti by Zn in the Ti–O bond. As the Zn/Ti ratio was raised to 0.7 (curve c), the 790 cm1 band completely disappeared, and the peak at 712 cm1 (in curve a) shifted to 694 cm1. The band shift is associated with the phase transformation from anatase to zinc titanate (Zn2Ti3O8), according to XRD (Table 1). The Zn/Ti = 1 mixed oxide, after calcined at 500 8C, was characterized by a new absorption band at 735 cm1 (symbol *, curve d), and the absorbance of the peak increased rapidly up to 700 8C (curve e). Therefore, the 735 cm1 band can be assigned to the Zn–O–Ti bond structure in zinc titanate.
Fig. 3. FT-IR spectra of aerogels after calcination at 500 8C (a) TiO2, (b) Zn/ Ti = 0.1, (c) Zn/Ti = 0.7, (d) Zn/Ti = 1 and (f) ZnO, and at 700 8C (e) Zn/Ti = 1.
3.2. Surface analysis by XPS Table 2 lists element compositions measured for the ZnOTiO2 aerogels by X-ray photoelectron spectroscopy (XPS). For the aerogel powders, the XPS-derived Zn/Ti ratios are slightly higher than the corresponding nominal values. The same observation has ever been reported for zinc titanate films [12]. This zinc enrichment on the surface was seen more pronounced in a study by heating the Zn/Ti = 1 oxide from 500 to 700 8C. We consider that the zinc species may migrate to the nearsurface region during the crystallization of the Zn/Ti = 1 oxide; that is, in the phase transformation from Zn2Ti3O8 to ZnTiO3 (Fig. 2). Binding energy (BE) measurements corresponding to the core electrons in atomic levels Ti 2p3/2 and Zn 2p3/2 are listed in Table 2. The obtained BE values are similar to those reported in literature [10,13]. All ZnO–TiO2 samples showed a lower BE for Ti 2p3/2 than the TiO2, but a higher BE for Zn 2p3/2 than the
Table 2 Surface analysis by XPS spectra for ZnO–TiO2 aerogels Samplea
TiO2, 500 8C, fresh Zn/Ti = 0.1, 500 8C, fresh Zn/Ti = 0.7, 500 8C, fresh Zn/Ti = 1, 500 8C, fresh Zn/Ti = 1, 700 8C, fresh Zn/Ti = 1, 500 8C, spentb ZnO, 500 8C, fresh a b
Composition (%)
Binding energy (eV)
Atomic ratio
Zn
Ti
O
Zn 2p3/2
Ti 2p3/2
O 1s
Ti3+/Ti4+
Odef2/Olat2
OH/Olat2
– 8.1 14.5 19.1 21.9 19.8 43.8
31.8 24.1 19.3 16.7 15.8 15.6 –
68.2 67.7 66.2 64.2 62.3 64.6 51.7
– 1022.30 1021.90 1021.92 1021.67 1021.87 1021.85
459.23 458.85 458.27 458.76 458.60 458.70 –
530.27 530.10 530.05 530.30 530.55 530.30 530.35
0 0 0.06 0.03 0.40 0.05 –
0 0.30 0 0.11 0 0.05 0.35
0.20 0.13 0.12 0.15 0.06 0.25 0.21
Calcined for 2 h in air. Collected from the reactor after two 12-h methanol oxidation runs.
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C.-T. Wang, J.-C. Lin / Applied Surface Science 254 (2008) 4500–4507
ZnO. This indicates a stronger electronic interaction between Zn and Ti atoms in the mixed oxide structure. Due to asymmetry, the Ti 2p3/2 peak can be deconvoluted into two components, and they are assigned to Ti3+ (Ti2O3) and Ti4+ (TiO2) ions [3]. As listed in Table 2, the analysis on Ti3+/Ti4+ ratio reveals that a majority of the Ti species in the titanium oxide based aerogels are present in the Ti4+ oxidation state. On the other hand, the obtained Zn 2p peaks are quite symmetric for the zinc based oxides, so that the Zn species exist only in the formal valence state of Zn2+, in good agreement with the findings of some authors [7,14]. As regards the XPS O 1s analysis, the TiO2 and ZnO–TiO2 aerogels exhibited lower O 1s BE values than the ZnO (Table 2). The result seems to indicate a higher mobility of the O2 ions placed near to Ti sites. Fig. 4 shows O 1s spectra for
the TiO2, Zn-doped TiO2 (Zn/Ti = 0.1) and ZnO aerogels calcined at 500 8C. The asymmetry of these O 1s broad bands is indicative of the presence of at least two types of oxygen species on the surface. For the TiO2 sample, deconvolution of the O 1s peak gave rise to two symmetric components (Fig. 4a). One at 530.3 eV was assigned to lattice oxygen ions (Olat2), and the other at 531.6 eV was associated with hydroxyl species (OH). For the Zn/Ti = 0.1 sample, three components at 530.1, 531.1 and 532.0 eV were resolved from the O 1s peak (Fig. 4b). The one at 531.1 eV, not found in the TiO2, was characteristic of oxygen ions in the oxygen-deficient regions (Odef2) [7]. The Odef2 component can also be derived from the O 1s spectrum of the ZnO sample (Fig. 4c). Therefore, the oxygen vacancies not only existed within the ZnO matrix, similarly reported in Ref. [15], but also were created in the oxides doped with Zn2+ (e.g., Zn/Ti = 0.1 and 1). Table 2 lists atomic ratios of Odef2/ Olat2 and OH/Olat2. Lee et al. [4] reported the formation of oxygen vacancy due to Ni2+-doping into Nd2O3. Fig. 5 presents XPS Ti 2p and O 1s spectra of the ZnO-TiO2 (Zn/Ti = 1) samples calcined at 500 and 700 8C in air. The heating caused not only a reduction in the intensity of the main Ti 2p3/2 peak at 458.76 eV but also the growth of a new peak at 457.30 eV. Also, the intensity of the main O 1s peak at 530.30 eV and higher-BE side was weakened remarkably, and a new O 1s peak at 529.30 eV appeared. These results suggest the transformation from Ti4+ to Ti3+ cations and the formation of a Ti3+–O bond structure due to calcination. We consider that the
Fig. 4. XPS O 1s spectra of aerogels after calcination at 500 8C (a) TiO2, (b) Zn/ Ti = 0.1 and (c) ZnO. (symbols: 1: Olat2; 2: OH; 3: Odef2).
Fig. 5. XPS Ti 2p and O 1s spectra of the ZnO–TiO2 (Zn/Ti = 1) aerogels calcined at 500 and 700 8C.
C.-T. Wang, J.-C. Lin / Applied Surface Science 254 (2008) 4500–4507
formation of the Ti3+ ion and Ti3+–O bond is related to the thermal dissociation of H from OH groups (i.e., OH/Olat2 decrease from 0.15 to 0.06 in Table 2) and the disappearance of oxygen vacancy in the oxide (i.e., Odef2/Olat2 = 0 at 700 8C). Oxygen vacancies formed in an oxide are known to be adsorption sites for oxygen molecules [16]. When an adsorbed O2 reacts with two electrons trapped at an oxygen vacancy site, two oxygen ions (Oads) are produced (Eq. (1)) [4]. Then, one Oads ion reduces an adjacent or neighboring Ti4+ cation to generate an O–Ti3+ bond (Eq. (2)). It is proposed that the O formation becomes more probable as the annealing temperature is raised [18]. Onishi and coworkers [17] studied the O2 adsorption on H2-reduced cerium oxide using infrared spectroscopy and found that superoxide species (O2) are formed immediately after introduction of gaseous oxygen, and successively converted into O22, O and finally Olat2. Table 2 indicates that the O 1s binding energy increases from 530.30 eV at 500 8C to 530.55 eV at 700 8C. It is known that the basic strength of metal oxide increases with decreasing the O 1s binding energy [19]. For the Zn/Ti = 1 mixed oxide, the thermal treatment has reduced the surface density of oxygen vacancy, and consequently the Lewis acidity increases. O2ðgÞ þ 2e ? 2OðadsÞ
(1)
OðadsÞ þ Ti4þ ! OTi3þ
(2)
3.3. Catalytic properties The zinc-titanium oxide aerogels were evaluated as catalysts for the oxidation of methanol in an ambient fixed-bed flow reactor. Pure TiO2 and ZnO were included for reference. Table 3 lists methanol conversion and product selectivity measured at the reaction temperature of 350 8C and the Arrhenius activation energy (Ea;CH3 OH ) and pre-exponential factor (ko;CH3 OH ). The catalytic activity, expressed in terms of methanol conversion, increased remarkably with higher zinc loadings in the oxide catalysts. An approximate 3-fold increase in the conversion was noted for the oxides doped heavily with zinc (i.e., Zn/Ti = 0.3 to 1). In other words, the cubic Zn2Ti3O8 phase promoted the oxidative conversion of methanol, and the catalysts containing this phase exhibited the lower activation
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energy (Table 3). Similarly, Chen et al. [12] reported that zinc titanate phases with a cubic crystal structure are active in isobutane dehydrogenation, but the other phases are not. We consider that the reducibility of the more labile bridging Zn–O– Ti bond in the zinc titanate structure, due to a stronger electronic interaction between Zn and Ti atoms, is responsible for an increase in the catalytic activity for methanol oxidation. A higher mobility of the O2 ions placed near to Ti sites helps the chemisorption of methanol molecules onto the Ti centers. Yang et al. [20] investigated the catalytic wet air oxidation over CeO2–TiO2 catalysts and concluded that the interaction of Ce and Ti affects the surface and structural properties of the oxide catalysts and their activity. Wachs and coworkers [21] studied methanol oxidation over supported metal oxide catalysts using quantitative methanol chemisorption and in situ infrared techniques and proposed that bridging metal–O–support bonds are the critical active sites for an oxidation reaction. Products obtained from the methanol oxidation reactor at 350 8C were dimethyl ether (CH3OCH3), methyl formate (HCOOCH3) and carbon dioxide. It is known that the dimethyl ether selectivity describes the pure dehydration ability of a catalyst, which is generally related to its Lewis acidity [2,22]. In contrast, the basic sites of a catalyst promote the oxidation of methanol to formate and CO2 [22]. In this work, all titanium oxide based aerogels exhibited a high selectivity to dimethyl ether, in the range of 44.8–99.3%, so that their catalytic surfaces had a strong Lewis acidic character, which is associated primarily with the Ti4+ center. In addition, a decrease in the dimethyl ether selectivity was found for the zinc-doped oxide catalysts. This indicates that the acidic strength of the pure titania is decreased by Zn2+-doping. The role of the titanium oxide phase is to serve as a reducible support to maintain the catalytic surface at a high Lewis acidity. On the other hand, carbon dioxide was favorably produced on the ZnO and ZnO–TiO2 (Zn/Ti = 0.1 and 1). A strong basic character suggested for the catalysts can be accounted by the presence of oxygen vacancies on their surfaces, according to XPS (Table 2). Fig. 6 illustrates a dependence of the dimethyl ether yield on the catalyst composition, obtained at reaction temperatures of 300, 325 and 350 8C. The CH3OCH3 formation rate increased abruptly in the Zn/(Zn + Ti) range between 23 and 50 at.% (i.e.,
Table 3 Catalytic properties of ZnO–TiO2 aerogels for methanol oxidation and Arrhenius parameters Catalyst
TiO2 Zn/Ti = 0.1 Zn/Ti = 0.3 Zn/Ti = 0.5 Zn/Ti = 0.7 Zn/Ti = 1 ZnO
Conversion and selectivities at 350 8C (%) a CH3OH
CH3OCH3
HCOOCH3
CO2
18.4 47.4 63.0 66.9 70.4 69.6 53.1
99.3 44.8 87.5 90.7 94.1 54.6 3.0
0.3 9.2 1.0 0.4 0.1 0.1 0.6
0.2 46.0 11.5 8.8 7.2 45.3 96.4
ko;CH3 OH b (m3 h1 kg1cat.)
Ea;CH3 OH b (kJ mol1)
7.37 1010 7.56 1014 1.99 1012 1.07 1010 1.39 108 1.89 109 8.19 1013
110.3 9.0 155.4 25.5 118.0 9.4 90.5 8.6 66.5 5.0 80.8 8.2 139.5 16.9
Reactor conditions: catalyst mass of 0.05 g; pretreatment at 500 8C; gas feed of 6.9 L h1; feed composition of CH3OH/O2/N2 = 2.1 mol%/12.7 mol%/ 85.2 mol%. b Estimated by a first-order rate model RCH3 OH ¼ ko expðEa =RTÞCCH3 OH based on methanol conversions at 250–400 8C. a
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the basic active centers that favor the oxidation of methanol to CO2. It is postulated that Lewis-chemisorbed methanol is oxidized into formate species and CO2 possibly via reaction with electrons trapped at oxygen vacancy sites or with oxygen ions (O) formed in the near-surface region. A previous study [23] reported that oxygen ions produced on oxygen vacancy sites or basic sites may abstract hydrogen from methane during the oxidative coupling of methane over rare earth metal oxide catalysts. 4. Conclusions
Fig. 6. Dimethyl ether formation rate as a function of catalyst composition at reaction temperatures (a) 300 8C, (b) 325 8C and (c) 350 8C.
Zn/Ti = 0.3 to 1). The result demonstrates an increase in the number of Lewis acid sites, due to the cubic Zn2Ti3O8 phase. A previous study [3] reported that the Fe2TiO5 phase produces new acid sites having a stronger Lewis acidity. Moreover, the surface area effect is also profound on the dimethyl ether formation, suggesting that more Ti–O surface atoms are closely situated to promote the Lewis chemisorption of two neighboring methanol groups as initially methoxy groups (i.e., Ti4+– OCH3) that subsequently combine to give dimethyl ether. Fig. 7 shows an effect of the catalyst pretreatment temperature on the dehydration of methanol to dimethyl ether, evaluated at reaction temperatures from 250 to 375 8C over the Zn/Ti = 1 mixed oxide. The catalyst pretreated at 700 8C showed a remarkably higher selectivity to dimethyl ether than the others treated at 500 and 600 8C. As indicated by the XPS analysis (Table 2), for the Zn/Ti = 1 oxide surface, the calcination to 700 8C caused the disappearance of oxygen defects and the transformation from Ti4+ to Ti3+ cations. These results demonstrate that oxygen vacancy sites play the role of
The surface nature of the zinc-titanium oxide aerogel catalysts has been elucidated by the XPS analysis and the selectivity pattern and formation rates of the reaction products in methanol oxidation. The titanium oxide based catalysts exhibit a high selectivity to dimethyl ether, so that their catalytic surfaces possess a strong Lewis acidic character, primarily related to the Ti4+ center. Carbon dioxide is favorably produced on the oxide catalysts with anion defects. Thus, the oxygen vacancies, formed by doping zinc species into titania or zinc titanate matrices, are the basic sites active towards the total combustion. The catalysts with cubic zinc titanate phases are more active for the conversion of methanol and also promote the production rate of dimethyl ether. It is suggested that the reducibility of the more labile Zn–O–Ti bond, due to a stronger electronic interaction between Zn and Ti atoms, controls the catalytic activity in the oxidation of methanol. Results demonstrate that the zinc titanate solid solutions produce new Lewis acid sites having also a higher reactivity and that the nature of the catalytic surface transforms from Lewis acidic to basic characters due to the presence of reactive oxygen vacancies. Acknowledgements This work is financially supported by The National Science Council in Taiwan under grant number NSC 94-2214-E-224006. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Fig. 7. Influence of pretreatment temperature of ZnO–TiO2 (Zn/Ti = 1) aerogel on dehydration of methanol to dimethyl ether (a) 500 8C, (b) 600 8C and (c) 700 8C.
[10]
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