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Mesoporous TiO2-Y2O3-ZrO2 oxides with crystalline framework obtained by surfactant template synthesis
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
309
Mesoporous TiO2-Y2O3-ZrO2 oxides with crystalline framework obtained by surfactant template synthesis Viorica Pârvulescu,a Simona omcescu,a Petre Osiceanu,a Speranta Tanasescu,a Bao-Lian Su,b a
Institute of Physical Chemistry, Spl. Independenei 202, 060021 Bucharest, Romania The University of Namur (FUNDP), Laboratoire de Chimie des Matériaux Inorganiques,61 rue de Bruxelles, B-5000 Namur, Belgium
b
Abstract Mesostructured ZrO2- 8%Y2O3-x% TiO2 (were x = 10, 15, 30) were synthesized by hydrothermal treatment using ionic and non-ionic surfactants as structure directing agents. The obtained solids were calcined in air at 873K and characterized by XRD, TEM, SEM, N2 adsorption-desorption and XP spectroscopy. These obtained materials show a small particle size, high surface area and mesopores. The specific surface area, microstructure, porosity, surface chemical composition and thermodynamic properties of TYZ mixed oxides are dependent on composition and nature of surfactant.
Keywords: mesoporous oxides, YSZ, titanium incorporation, hydrothermal treatment, SOFC anode materials
1. Introduction Since the discovery of mesoporous silicates based on amphiphilic supramolecular templates many efforts were directed toward the design of catalyst materials based on mixed transition metal oxides as a replacement for noble metal catalysts [1]. A variety of mesoporous transition metal (TM) oxides have been synthesized using the selfassemble of surfactant molecules as a template [2] and used for various applications, such as catalysis and SOFC [3]. However, the synthesis of mesoporous transition metal (TM) oxides is more difficult than that of silica based materials, in view of the complexity of TM chemistry. Among these non-silica oxides, zirconium oxide has drawn much attention in the areas of ceramics and catalysis. In this paper mesostructured ZrO2-TiO2-Y2O3 complex oxides are synthesized by surfactant-assisted preparative chemistry. The effects of several synthesis parameters as surfactant, molar composition, mixing and aging temperature on structure, morphology, thermodynamic properties, surface composition and stoichiometry are reported. Our studies are focused upon the investigation of the correlation existing between the thermodynamic properties and different compositional variables: the grain size, the nature and the concentration of the dopants, the oxygen stoichiometry change.
2. Experimental Mesostructured TiO2-Y2O3-ZrO2 (TxYZ samples) with 8 wt. % yttria and various titania content (x=5-40%), were prepared using various ionic (cetyltrimethylammonium bromide-CT) and non-ionic (Brij 30-Bj, polyoxiethylene tridecylether-Po and block
310
V. Pârvulescu et al.
copolymers pluronic F68-Pl and P123-Bk) surfactants as structure directing agents. The precursors were zirconium chloride, titanium isopropoxide and yttrium nitrate. The hydrolyzing agent was urea and temperature 353K. The obtained gels with 1: 0, 36: 7 oxide/surfactant/urea molar ratio were loaded into Teflon link steel autoclave and heated 3 days at 373 K. The solids were filtered, washed, dried and calcined in air at 873K. For comparing the thermodynamic data, the oxide samples were treated at two temperatures: 873K and 1273 K. The solid electrolyte electrochemical cells method for investigating thermodynamic properties of transition metal oxides was used [4]. The obtained materials were characterized by X-ray diffraction, SEM and TEM microscopy, N2 adsorption-desorption, IR and UV-Vis and X-ray Photoelectron Spectroscopy 3. Results and Discussion The XRD patterns of all the calcined samples correspond to the fluorite phase of the oxide systems (Fig. 1). The small-angle XRD patterns (2 in the range 0.5-5) for the samples synthesized using various surfactants have shown a relatively intensive peak in this region and a small shift of them to higher angle in case of the nonionic surfactants (Fig 2). These results and ordered structure were confirmed by TEM microscopy (Fig. 3), N2 adsorption-desorption isotherms and pore size distributions. A B (111)
(111)
T30YZCT
(200)
Intensity, a.u
Intensity, a.u
(220) (200)
(311)
T10YZCT T5YZCT 10
20
30
T10YZPl
(220) (311)
T10YZPo T10YZBk T10YZBj
40
2θ
50
60
70
10
20
30
40
50
60
70
2θ
Figure 1. XRD patterns of TiO2-Y2O3-ZrO2 samples with different TiO2 content (A) and various nonionic surfactants (B) The microstructure of TxYZ ceramic material was influenced by the concentration of surfactant used in the synthesis. TEM images show the formation of aggregates with inorganic polymers and confirm textural porosity (Fig. 3). Spherical morphology of the obtained particles is typically for mesoporous materials (Fig. 4). The isotherm of mesoporous oxides calcined in air exhibits distinct hysteresis loop in the p/p0 range of 0.4-0.9 (Fig. 5).
Mesoporous TiO2-Y2O3-ZrO2 oxides with crystalline framework obtained by surfactant template synthesis
311
T10YZCT T10YZPo I, a.u
T10YZPl T20YZCT
1
2
2θ
3
4
5
Figure 2. The small-angle XRD patterns
Figure 3. TEM image of T10YZCT oxide
Characterization revealed that the mesoporous powder obtained by hydrothermal method with various surfactants as structure directing agents, calcined at 873 K was weakly agglomerated and presented a high surface area (around 150-200 m2/g). Textural porosity, pore diameter and surface area were influenced by surfactant.
Adsorbed volume, a.u
T10YZBj T10YZBk T10YZPo
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, p/p0
Figure 4. SEM image of T10YZCT oxide
Figure 5. N2 adsorption-desorption isotherms of T10YZ oxides synthesized with different surfactants
The BET surface area decreases and the pore radius increases with the increasing of titania content of the sample. This is probably due to the appearance and growth of a new phase as titania. The high surface area was obtained for the powders synthesized using cationic surfactant (CT samples). By varying the surfactants, was observed that area surface decreases in order PO>Pk >Pl>Bj, for the samples obtained with non-ionic surfactants. A high porosity and large pore diameter was obtained for the samples synthesized with block copolymers (Pl and Bk samples).
312
V. Pârvulescu et al.
The XPS analysis was used to determine the chemical states of the elements present on surface and their relative concentrations, as well. The increasing of titanium content above 15% decreases the relative concentration of yttrium suggesting that takes place a diffusion process of the yttrium in the subsurface region and o segregation of zirconium towards the surface of the sample takes place. It is expected that these surface findings should be found in the catalytic behaviour which is very sensitive to the surface chemical state. In nanocrystalline ceramics, where defect formation energies are likely to be reduced, significant changes in overall defect concentration are expected. Considering the partial pressure of oxygen as a key parameter for the thermodynamic characterisation of the materials, we investigated the variation of the log p O data 2 obtained with two samples of Y0.08Zr0.87Ti0.05O1.96CT sintered at two different temperatures: 873 K and 1273 K (Fig.6). Enhanced log p O values were systematically 2
found for the sample sintered at 873 K. This trend infers that the variations of the thermodynamic data can be explained as a consequence of truly grain-size dependent properties. -12
Y0.08Zr0.87Ti0.05O1.96 (sintered at 1273K) Y0.08Zr0.87Ti0.05O1.96 (sintered at 873K)
-13
-15
Figure 6. Variation of log p O for 2 Y0.08Zr0.87Ti0.05O1.96CT calcined at 873 and 1273 K
-16
2
log(pO /atm)
-14
-17 -18 -19 -20 8.8
9.0
9.2
9.4
9.6 4
9.8
10.0
10.2
10.4
-1
10 (T/K)
The results suggest the decreasing of the oxygen vacancies concentration which, at its turn could be related to the ionic conductivity decreasing and the electronic conductivity increasing found in these materials.
4. Conclusion The characterization of the mesoporous TiO2-Y2O3-ZrO2 oxides reveals crystalline structure, high surface area, tailored particle and pore diameter, surface stoichiometry and electronic conductivity.
References [1] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stuky, Nature, 396 (1998) 152. [2] M. Han, X. Tang, H. Yin, S. Peng, J. Power Sources, 165 (2007) 757. [3] C. Lu, S. An, W.L. Worrell, J.M. Vohs, R.J. Gorte, Solid State Ionics 175 (2004) ,47. [4] S. Tanasescu, F. Maxim, F. Teodorescu, L. Giurgiu, J. Nanosci. Nanotechnol. 8 (2008) 914.
309
Mesoporous TiO2-Y2O3-ZrO2 oxides with crystalline framework obtained by surfactant template synthesis Viorica Pârvulescu,a Simona omcescu,a Petre Osiceanu,a Speranta Tanasescu,a Bao-Lian Su,b a
Institute of Physical Chemistry, Spl. Independenei 202, 060021 Bucharest, Romania The University of Namur (FUNDP), Laboratoire de Chimie des Matériaux Inorganiques,61 rue de Bruxelles, B-5000 Namur, Belgium
b
Abstract Mesostructured ZrO2- 8%Y2O3-x% TiO2 (were x = 10, 15, 30) were synthesized by hydrothermal treatment using ionic and non-ionic surfactants as structure directing agents. The obtained solids were calcined in air at 873K and characterized by XRD, TEM, SEM, N2 adsorption-desorption and XP spectroscopy. These obtained materials show a small particle size, high surface area and mesopores. The specific surface area, microstructure, porosity, surface chemical composition and thermodynamic properties of TYZ mixed oxides are dependent on composition and nature of surfactant.
Keywords: mesoporous oxides, YSZ, titanium incorporation, hydrothermal treatment, SOFC anode materials
1. Introduction Since the discovery of mesoporous silicates based on amphiphilic supramolecular templates many efforts were directed toward the design of catalyst materials based on mixed transition metal oxides as a replacement for noble metal catalysts [1]. A variety of mesoporous transition metal (TM) oxides have been synthesized using the selfassemble of surfactant molecules as a template [2] and used for various applications, such as catalysis and SOFC [3]. However, the synthesis of mesoporous transition metal (TM) oxides is more difficult than that of silica based materials, in view of the complexity of TM chemistry. Among these non-silica oxides, zirconium oxide has drawn much attention in the areas of ceramics and catalysis. In this paper mesostructured ZrO2-TiO2-Y2O3 complex oxides are synthesized by surfactant-assisted preparative chemistry. The effects of several synthesis parameters as surfactant, molar composition, mixing and aging temperature on structure, morphology, thermodynamic properties, surface composition and stoichiometry are reported. Our studies are focused upon the investigation of the correlation existing between the thermodynamic properties and different compositional variables: the grain size, the nature and the concentration of the dopants, the oxygen stoichiometry change.
2. Experimental Mesostructured TiO2-Y2O3-ZrO2 (TxYZ samples) with 8 wt. % yttria and various titania content (x=5-40%), were prepared using various ionic (cetyltrimethylammonium bromide-CT) and non-ionic (Brij 30-Bj, polyoxiethylene tridecylether-Po and block
310
V. Pârvulescu et al.
copolymers pluronic F68-Pl and P123-Bk) surfactants as structure directing agents. The precursors were zirconium chloride, titanium isopropoxide and yttrium nitrate. The hydrolyzing agent was urea and temperature 353K. The obtained gels with 1: 0, 36: 7 oxide/surfactant/urea molar ratio were loaded into Teflon link steel autoclave and heated 3 days at 373 K. The solids were filtered, washed, dried and calcined in air at 873K. For comparing the thermodynamic data, the oxide samples were treated at two temperatures: 873K and 1273 K. The solid electrolyte electrochemical cells method for investigating thermodynamic properties of transition metal oxides was used [4]. The obtained materials were characterized by X-ray diffraction, SEM and TEM microscopy, N2 adsorption-desorption, IR and UV-Vis and X-ray Photoelectron Spectroscopy 3. Results and Discussion The XRD patterns of all the calcined samples correspond to the fluorite phase of the oxide systems (Fig. 1). The small-angle XRD patterns (2 in the range 0.5-5) for the samples synthesized using various surfactants have shown a relatively intensive peak in this region and a small shift of them to higher angle in case of the nonionic surfactants (Fig 2). These results and ordered structure were confirmed by TEM microscopy (Fig. 3), N2 adsorption-desorption isotherms and pore size distributions. A B (111)
(111)
T30YZCT
(200)
Intensity, a.u
Intensity, a.u
(220) (200)
(311)
T10YZCT T5YZCT 10
20
30
T10YZPl
(220) (311)
T10YZPo T10YZBk T10YZBj
40
2θ
50
60
70
10
20
30
40
50
60
70
2θ
Figure 1. XRD patterns of TiO2-Y2O3-ZrO2 samples with different TiO2 content (A) and various nonionic surfactants (B) The microstructure of TxYZ ceramic material was influenced by the concentration of surfactant used in the synthesis. TEM images show the formation of aggregates with inorganic polymers and confirm textural porosity (Fig. 3). Spherical morphology of the obtained particles is typically for mesoporous materials (Fig. 4). The isotherm of mesoporous oxides calcined in air exhibits distinct hysteresis loop in the p/p0 range of 0.4-0.9 (Fig. 5).
Mesoporous TiO2-Y2O3-ZrO2 oxides with crystalline framework obtained by surfactant template synthesis
311
T10YZCT T10YZPo I, a.u
T10YZPl T20YZCT
1
2
2θ
3
4
5
Figure 2. The small-angle XRD patterns
Figure 3. TEM image of T10YZCT oxide
Characterization revealed that the mesoporous powder obtained by hydrothermal method with various surfactants as structure directing agents, calcined at 873 K was weakly agglomerated and presented a high surface area (around 150-200 m2/g). Textural porosity, pore diameter and surface area were influenced by surfactant.
Adsorbed volume, a.u
T10YZBj T10YZBk T10YZPo
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, p/p0
Figure 4. SEM image of T10YZCT oxide
Figure 5. N2 adsorption-desorption isotherms of T10YZ oxides synthesized with different surfactants
The BET surface area decreases and the pore radius increases with the increasing of titania content of the sample. This is probably due to the appearance and growth of a new phase as titania. The high surface area was obtained for the powders synthesized using cationic surfactant (CT samples). By varying the surfactants, was observed that area surface decreases in order PO>Pk >Pl>Bj, for the samples obtained with non-ionic surfactants. A high porosity and large pore diameter was obtained for the samples synthesized with block copolymers (Pl and Bk samples).
312
V. Pârvulescu et al.
The XPS analysis was used to determine the chemical states of the elements present on surface and their relative concentrations, as well. The increasing of titanium content above 15% decreases the relative concentration of yttrium suggesting that takes place a diffusion process of the yttrium in the subsurface region and o segregation of zirconium towards the surface of the sample takes place. It is expected that these surface findings should be found in the catalytic behaviour which is very sensitive to the surface chemical state. In nanocrystalline ceramics, where defect formation energies are likely to be reduced, significant changes in overall defect concentration are expected. Considering the partial pressure of oxygen as a key parameter for the thermodynamic characterisation of the materials, we investigated the variation of the log p O data 2 obtained with two samples of Y0.08Zr0.87Ti0.05O1.96CT sintered at two different temperatures: 873 K and 1273 K (Fig.6). Enhanced log p O values were systematically 2
found for the sample sintered at 873 K. This trend infers that the variations of the thermodynamic data can be explained as a consequence of truly grain-size dependent properties. -12
Y0.08Zr0.87Ti0.05O1.96 (sintered at 1273K) Y0.08Zr0.87Ti0.05O1.96 (sintered at 873K)
-13
-15
Figure 6. Variation of log p O for 2 Y0.08Zr0.87Ti0.05O1.96CT calcined at 873 and 1273 K
-16
2
log(pO /atm)
-14
-17 -18 -19 -20 8.8
9.0
9.2
9.4
9.6 4
9.8
10.0
10.2
10.4
-1
10 (T/K)
The results suggest the decreasing of the oxygen vacancies concentration which, at its turn could be related to the ionic conductivity decreasing and the electronic conductivity increasing found in these materials.
4. Conclusion The characterization of the mesoporous TiO2-Y2O3-ZrO2 oxides reveals crystalline structure, high surface area, tailored particle and pore diameter, surface stoichiometry and electronic conductivity.
References [1] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stuky, Nature, 396 (1998) 152. [2] M. Han, X. Tang, H. Yin, S. Peng, J. Power Sources, 165 (2007) 757. [3] C. Lu, S. An, W.L. Worrell, J.M. Vohs, R.J. Gorte, Solid State Ionics 175 (2004) ,47. [4] S. Tanasescu, F. Maxim, F. Teodorescu, L. Giurgiu, J. Nanosci. Nanotechnol. 8 (2008) 914.