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Influence of the oxygen partial pressure on the reduction of CeO2 and CeO2ZrO2 ceramics
Solid State Sciences 7 (2005) 539–544 www.elsevier.com/locate/ssscie
Influence of the oxygen partial pressure on the reduction of CeO2 and CeO2–ZrO2 ceramics S. Huang a,b , L. Li a , O. Van der Biest b , J. Vleugels b,∗ a School of Material Science and Engineering, Shanghai University, Yanchang Road 149, Shanghai 200072, China b Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium
Received 9 December 2004; received in revised form 14 February 2005; accepted 28 February 2005 Available online 12 April 2005
Abstract Based on recent thermodynamic estimations on the CeO2 –CeO1.5 , CeO2 –ZrO2 and CeO1.5 –ZrO2 systems, isothermal sections of the ternary CeO2 –ZrO2 –CeO1.5 system are calculated in the 1300–1700 ◦ C region. Additionally, the complex relation between the nonstoichiometry, y, in CeO2−y , the composition of the CeO2 –ZrO2 solid solution and the oxygen partial pressure (P O2 ) for different ZrO2 containing solid solutions Cez Zr1−z O2−x (with z = 0.2, 0.5, 0.8) are evaluated from 600 to 900 ◦ C. The relation between the degree of Ce+4 to Ce+3 reduction under different P O2 in the fluorite CeO2−y and Cez Zr1−z O2−x solid solutions at different temperatures can be used as a guide in the development of functional ceramics or assist in explaining their performance as function of the operating atmosphere and temperature. 2005 Elsevier SAS. All rights reserved. Keywords: CeO2 –ZrO2 –CeO1.5 ; Thermodynamics; Cez Zr1−z O2−x ; Ceria reduction; Catalyst
1. Introduction CeO2 -containing ceramics have received increasing attention because of their great importance in science and technology [1,2]. CeO2 is used as an oxygen storing component in so-called three-way catalysts for automotive exhaust treatment. The oxygen storage capacity (OSC) of CeO2 is due to its ability to undergo rapid redox cycles. A major problem is the loss of oxygen storage capacity due to the sintering of CeO2 particles and grain growth at higher temperature. Special attention has been focused on the preparation of CeO2 , structurally doped with ZrO2 in recent years. Better catalytic activity, redox properties and thermal stability in comparison with single-phase CeO2 have been reported [3]. CeO2 doped with lower valence cations like Gd3+ , Sm3+ and Y3+ are known to have a higher ionic conductivity and lower activation energy than the commonly used 8 mol % * Corresponding author. Tel.: +32-16-321244; Fax: +32-16-321992.
E-mail address: [email protected] (J. Vleugels). 1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.02.005
Y2 O3 -doped ZrO2 and are potential candidate electrolyte materials for solid oxide fuel cells (SOFCs) for intermediate temperature below 800 ◦ C. The prominent drawback of CeO2 is the degradation of the mechanical integrity due to lattice expansion in the low oxygen partial pressure environment arising from the transition of Ce4+ to Ce3+ . This may result in the formation of cracks at the electrode-electrolyte interface and subsequent delamination of the electrode from the electrolyte [4]. The success of CeO2 and CeO2 -based materials is mainly due to the unique combination of an elevated oxygen transport capacity coupled with the ability to easily shift between reduced and oxidized states, i.e., the sesquioxide CeO1.5 and the dioxide CeO2 , incorporating Ce+3 and Ce+4 ions, respectively. Under reducing atmosphere, the removal of oxygen from CeO2 at elevated temperatures leads to the formation of an oxygen-deficient nonstoichiometric CeO2−y phase with 0 < y < 0.5. CeO2−y retains the same fluorite crystal structure as CeO2 , facilitating rapid and complete refilling of oxygen vacancies upon exposure of CeO2−y to oxygen, with recovery of CeO2 . The same phenomenon exists in
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S. Huang et al. / Solid State Sciences 7 (2005) 539–544
CeO2 -stabilized ZrO2 ceramics for structural [5] or functional [4] applications when exposed to an inert atmosphere or low oxygen partial pressure at elevated temperature. Consequently, the fluorite Cez Zr1−z O2 phase in the CeO2 –ZrO2 system converts to a nonstoichiometric Cez Zr1−z O2−x phase in the CeO2 –ZrO2 –CeO1.5 system. In their work on CeO2 – ZrO2 mixed oxide as support for noble metal catalysts for the reduction of NO by CO, Fornasiero et al. [6] found that the redox behaviour strongly depends both on the ZrO2 content in the CeO2 solid solution and on the nature of the phase present. Therefore, it is necessary to understand the relation between the oxygen partial pressure (P O2 ), temperature and the phase diagram of the CeO2 –ZrO2 –CeO1.5 system. It is clear that the P O2 has a strong influence on the performance of CeO2 -based systems. In this context, special attention is given to the calculation of the isothermal sections of the CeO2 –ZrO2 –CeO1.5 system, the prediction of the phase constitution in the CeO2 –CeO1.5 system and the cubic Cez Zr1−z O2−x phase as function of the P O2 and temperature. The relation between the degree of Ce+4 reduction and the P O2 in the fluorite CeO2−y and Cez Zr1−z O2−x solid solutions at different temperatures can be used as a guide in the development of functional ceramics or assist in explaining their performance as function of the operating atmosphere.
phases and the stoichiometric Zr2 Ce2 O7 (P) compound were considered. In the CeO2 –CeO1.5 system, the miscibility gap was evaluated with the experimental data from Campserveux et al. [16], where the highest temperature of the miscibility gap is 673 ◦ C. Only the CeO2−y phase with 2 > 2 − y > 1.818 was taken into account to evaluate the thermodynamic coefficients. In the ionic characteristics model of the Css, a twosublattice model was used in the CeO2 –CeO1.5 –ZrO2 system. The phase Css in the CeO1.5 –ZrO2 system is defined as (Zr+4 ,Ce+3 )1 (O−2 ,Va0 )2 , whereas the nonstoichiometric CeO2−y phase in the CeO2 –CeO1.5 system is described as (Ce+4 ,Ce+3 )1 (O−2 ,Va0 )2 and the Css phase in the CeO2 – ZrO2 system was as (Zr+4 ,Ce+4 )1 (O−2 )2 . Therefore, the Css (Cez Zr1−z O2−x ) in the CeO2 –ZrO2 –CeO1.5 system is defined as (Zr+4 ,Ce+3 ,Ce+4 )1 (O−2 ,Va0 )2 . The formalism of the Gibbs free energy expression and the thermodynamic optimization are reported elsewhere [17]. Using the Thermo-Calc program [18], the isothermal sections of the CeO2 –ZrO2 –CeO1.5 system at 1300 to 1700 ◦ C were extrapolated, as presented in Fig. 1. There is a large composition range for the Css phase in these diagrams, which extends with increasing temperature. The commonly investigated compositions in the CeO2 –ZrO2 –CeO1.5 system, Ce0.5 Zr0.5 O2−x and Ce0.8 Zr0.2 O2−x , are located in the Css area, whereas Ce0.2 Zr0.8 O2−x is located in the Tss + Css phase region.
2. Results and discussion 2.1. Estimating phase diagrams in the CeO2 –ZrO2 –CeO1.5 system Although there are a lot of experimental data on the phase equilibria in the binary CeO2 –ZrO2 , CeO1.5 –ZrO2 , CeO2 – CeO1.5 systems, it is difficult to describe the phase diagram of the higher order systems through experimental analysis. Using the CALPHAD technique, the isothermal sections of ternary systems can be extrapolated directly by combining the thermodynamic properties of the binary systems with suitable thermodynamic models. As a pioneer, Kaufman and Nesor [7] contributed a substitutional model, which considered the oxides as pure components in the formalism of the Gibbs free energy of phases. This model has been successfully applied to many ZrO2 containing oxide systems [8–13]. Hillert and Staffansson [14] developed a two-sublattice model suggesting vacancies located at one of the sublattices. Guillermet et al. [15] introduced vacancies at both sublattices for fitting the requirements of structural and electron neutrality. To obtain the phase diagram of the CeO2 –ZrO2 –CeO1.5 system at high temperature, a thermodynamic evaluation was initially performed on the CeO2 –ZrO2 , CeO1.5 –ZrO2 and CeO2 –CeO1.5 systems. The CeO2 –ZrO2 system includes liquid (L) and cubic (Css), tetragonal (Tss) and monoclinic (Mss) solid solution phases. In the CeO1.5 –ZrO2 system, the liquid, Tss, Css, Hss (hexagonal solid solution)
2.2. Redox behaviour of CeO2 and CeO2 –ZrO2 solid solutions For CeO2 -based applications, the atmosphere and especially the influence of the oxygen partial pressure on the reduction behaviour of CeO2 must be taken into account. The reduction behaviour of CeO2 and CeO2 –ZrO2 solutions can be thermodynamically calculated by means of the substitutional and sublattice model. In order to do so, the y coefficient in CeO2−y is calculated as function of the PO2 and temperature (T ). Following the suggestion of Lindemer [19], the chemical activity of oxygen in CeO2−y can be represented by the thermodynamic properties of the two components, CeO2 and CeO1.5 . In the CeO1.5 –CeO2 system, 4CeO1.5 + O2 ⇔ 4CeO2 .
(1)
To obtain the y-T -P O2 relation, the thermodynamic parameters of the Css are adopted. A nonstoichiometric phase Cez Zr1−z O2−x (Css) is assumed to be composed of z mole of CeO2−y and (1 − z) mole of ZrO2 , whereas the CeO2−y consists of m1 mole CeO1.5 , m2 mole CeO2 and m3 mole ZrO2 (m3 = 0). The number of moles, mi , of each species can be calculated from the mass-balance for cerium and oxygen as: z = m1 + m2
and z(2 − y) = 2m2 + 1.5m1
(2)
S. Huang et al. / Solid State Sciences 7 (2005) 539–544
(a)
541
(d)
(b) (e) Fig. 1. Continued.
can be calculated as: 2 − y = 2 − x − 2(1 − z) /z.
(4)
Consequently, the relation between x, y and z is that x = zy. In a system without ZrO2 [z = 1 or m3 = 0], the mole fraction of CeO1.5 is N1 = m1 /(m1 + m2 ) and the mole fraction of CeO2 is N2 = m2 /(m1 + m2 ). Moreover, y = 0.5 − 0.5N2 . Since the free energy difference for Eq. (1) is (4GCeO2 − 4GCeO1.5 ), the oxygen partial pressure is given by: RT ln(P O2 ) = 4GCeO2 − 4GCeO1.5 . (c) Fig. 1. Calculated isothermal section of the ZrO2 –CeO2 –CeO1.5 system at 1700 ◦ C (a), 1600 ◦ C (b), 1500 ◦ C (c), 1400 ◦ C (d) and 1300 ◦ C (e).
resulting in: m1 = 2zy
and m2 = z(1 − 2y).
(3)
For nonstoichiometric phase Zr1−z Cez O2−x , the O/(Ce + Zr) ratio is 2 − x and the O/Ce ratio in CeO2−y
(5)
Where ln(P O2 ) is the natural logarithm of P O2 . When z = 0, m3 = 0 and N3 = 0, the relation between y and the P O2 for phase CeO2−y can be obtained from Eq. (5). The general equation for the partial molar Gibbs free energy of each mass parameter is needed to calculate the free energy change for Eq. (1). The partial molar free energy for mass parameter i equals to: ∂[G Zr1−z Cez O2−x ]/∂mi = Gi .
(6)
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Therefore, the composition-temperature-oxygen pressure relationship in the CeO2 –ZrO2 –CeO1.5 system can be evaluated from Eqs. (1) to (6) and the molar Gibbs free energy of the nonstoichiometric phase, G Zr1−z Cez O2−x , which is described as function of m1 , m2 , m3 and the standard Gibbs free energy of formation of the ZrO2 , CeO2 and CeO1.5 Css phases in Ref. [20]. Although the thermodynamic properties are obtained with the ionic characteristics model [15], which is totally different from that of Lindemer [19], the calculated ln(y) − ln(P O2 ) relationship in the CeO2 –CeO1.5 system at 1400, 1200 and 1100 ◦ C is found to be in good agreement with the experimental data [19], as shown in Fig. 2. The calculated non-stoichiometry, y, and the corresponding CeO1.5 mole fraction as function of the oxygen partial pressure at 600, 700, 800 and 900 ◦ C are presented in Figs. 2 and 3. For a given P O2 , more and more CeO2 is reduced to CeO1.5 with increasing temperature. As mentioned above,
Fig. 2. Calculated relationship between ln(y) and ln(P O2 ) in the CeO2 –CeO1.5 system at different temperatures, together with the experimental data of [19].
the oxygen storage capacity (OSC), a parameter reflecting the ability of a three-way catalyst to store and release oxygen under different conditions, is related to the oxygen partial pressure, P O2 . The OSC of CeO2 increases with the working temperature for a given composition, as seen in Fig. 3. However, a too high working temperature leads to sintering of the CeO2 particles and a reduced surface area, resulting in an overall loss of OSC. When z < 1, part of the CeO2 transforms to CeO1.5 and the transformed portion can also be evaluated as graphically presented in Fig. 4 at 800 ◦ C. CeO2 reduction becomes relatively easier at higher ZrO2 contents, e.g., the ln(P O2 ) increases from around −42 to −12 for pure CeO2 and Ce0.4 Zr0.6 O2−x respectively, for a given reduction value x = 0.05. Therefore, the total OSC increases with increasing ZrO2 content in the Cez Zr1−z O2−x mixed oxides. It has been reported that the defect crystal structure and the concomitant oxygen vacancies play a fundamental role in the catalytic activity of CeO2 -based materials [21,22]. The use of Zr4+ as a dopant promotes the kinetics of Ce4+ reduction and enhances oxygen storage of ceria, due to an increased oxygen vacancy content and enhanced oxygen mobility in the Css solid solution compared to pure CeO2 [23,24]. The optimum composition is reported to be around z = 0.5 [25]. At high ZrO2 -contents however, a tetragonal solid solution (Tss) is formed in agreement with the calculated phase diagram of the CeO2 –ZrO2 –CeO1.5 system (Fig. 1), reducing the catalytic activity of the Cez Zr1−z O2−x system due to a reduced Css content. In the 0.5 < z < 0.8 region, depending on the CeO2 /Ce2 O3 ratio, a maximum in pyrochlore (P) phase content in combination with a Css phase is to be expected based on Fig. 1(e). This pyrochloretype Ce2 Zr2 O7+2δ phase can be converted into metastable κ-CeZrO4 or tetragonal metastable t -(Ce0.5 Zr0.5 )O2 upon oxidation, and into Ce2 Zr2 O7 upon reduction [26]. The influence of these phase transitions on the oxygen storage capacity however has not been revealed yet.
Fig. 3. Calculated ln(P O2 ) versus the corresponding CeO1.5 mole fraction in the CeO2 –CeO1.5 system at different temperatures.
Fig. 4. Composition of the Cez Zr1−z O2−x phase as function of the oxygen partial pressure and the ZrO2 content at 800 ◦ C.
S. Huang et al. / Solid State Sciences 7 (2005) 539–544
(a)
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Fig. 5 shows the influence of temperature on the reduction of CeO2 at different ln(P O2 ) for the Ce0.5 Zr0.5 O2−x , Ce0.8 Zr0.2 O2−x , and Ce0.2 Zr0.8 O2−x solid solutions. It is clear that a lower oxygen potential and higher temperature lead to a stronger reduction of CeO2 . Moreover, the effect of temperature on the relative fraction of CeO2 reduction decreases with decreasing overall CeO2 content (see Fig. 5(a)– (c)). From a thermodynamic point of view, this can be explained by the formation of an increasing amount of Tss with increasing overall ZrO2 content, with poorer oxygen storage properties than the Css phase [27]. These calculations are in agreement with the reported experimental results revealing that the extent of reduction at z > 0.2 increases with increasing temperature, showing a temperature related maximum at 0.5 < z < 0.8 [24,25,28]. Moreover, the effect of temperature on the relative extent of reduction indeed decreases with increasing ZrO2 content for 0.1 < z < 0.8 [24,25,28]. Interestingly, this maximum might be directly related to a maximum pyrochlore (P) phase content in the system, as suggested by the calculated equilibrium phase diagram in Fig. 1(e). The presented thermodynamic calculations allow to evaluate the total oxygen storage capacity for different compositions of the Cez Zr1−z O2−x solid solution as function of temperature. However, the influence of the sample preparation method, the processing conditions as well as the reduction kinetics and oxygen diffusivity on the OSC also play an important role in the development of CeO2 –ZrO2 based materials for functional applications.
3. Conclusions
(b)
The oxygen partial pressure has a strong influence on the performance of CeO2 -based systems. In this context, special attention was given to the calculation of the isothermal sections of the CeO2 –ZrO2 –CeO1.5 system at 1300–1700 ◦ C, the prediction of the phase constitution in the CeO2 –CeO1.5 system as well as the cubic Cez Zr1−z O2−x solid solution as function of the P O2 in the 600–900 ◦ C range.
Acknowledgements This work is financially supported by the Research Fund of K.U. Leuven in the framework of the Flanders–China bilateral projects BIL 02/04, BIL 04/13 and BIL 04/14, and the National Natural Science Foundation of China under grant No. 50471101.
(c) Fig. 5. Calculated ln(P O2 ) versus x in the temperature range from 600 to 900 ◦ C for Ce0.8 Zr0.2 O2−x (a), Ce0.5 Zr0.5 O2−x (b) and Ce0.2 Zr0.8 O2−x (c).
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Influence of the oxygen partial pressure on the reduction of CeO2 and CeO2–ZrO2 ceramics S. Huang a,b , L. Li a , O. Van der Biest b , J. Vleugels b,∗ a School of Material Science and Engineering, Shanghai University, Yanchang Road 149, Shanghai 200072, China b Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium
Received 9 December 2004; received in revised form 14 February 2005; accepted 28 February 2005 Available online 12 April 2005
Abstract Based on recent thermodynamic estimations on the CeO2 –CeO1.5 , CeO2 –ZrO2 and CeO1.5 –ZrO2 systems, isothermal sections of the ternary CeO2 –ZrO2 –CeO1.5 system are calculated in the 1300–1700 ◦ C region. Additionally, the complex relation between the nonstoichiometry, y, in CeO2−y , the composition of the CeO2 –ZrO2 solid solution and the oxygen partial pressure (P O2 ) for different ZrO2 containing solid solutions Cez Zr1−z O2−x (with z = 0.2, 0.5, 0.8) are evaluated from 600 to 900 ◦ C. The relation between the degree of Ce+4 to Ce+3 reduction under different P O2 in the fluorite CeO2−y and Cez Zr1−z O2−x solid solutions at different temperatures can be used as a guide in the development of functional ceramics or assist in explaining their performance as function of the operating atmosphere and temperature. 2005 Elsevier SAS. All rights reserved. Keywords: CeO2 –ZrO2 –CeO1.5 ; Thermodynamics; Cez Zr1−z O2−x ; Ceria reduction; Catalyst
1. Introduction CeO2 -containing ceramics have received increasing attention because of their great importance in science and technology [1,2]. CeO2 is used as an oxygen storing component in so-called three-way catalysts for automotive exhaust treatment. The oxygen storage capacity (OSC) of CeO2 is due to its ability to undergo rapid redox cycles. A major problem is the loss of oxygen storage capacity due to the sintering of CeO2 particles and grain growth at higher temperature. Special attention has been focused on the preparation of CeO2 , structurally doped with ZrO2 in recent years. Better catalytic activity, redox properties and thermal stability in comparison with single-phase CeO2 have been reported [3]. CeO2 doped with lower valence cations like Gd3+ , Sm3+ and Y3+ are known to have a higher ionic conductivity and lower activation energy than the commonly used 8 mol % * Corresponding author. Tel.: +32-16-321244; Fax: +32-16-321992.
E-mail address: [email protected] (J. Vleugels). 1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.02.005
Y2 O3 -doped ZrO2 and are potential candidate electrolyte materials for solid oxide fuel cells (SOFCs) for intermediate temperature below 800 ◦ C. The prominent drawback of CeO2 is the degradation of the mechanical integrity due to lattice expansion in the low oxygen partial pressure environment arising from the transition of Ce4+ to Ce3+ . This may result in the formation of cracks at the electrode-electrolyte interface and subsequent delamination of the electrode from the electrolyte [4]. The success of CeO2 and CeO2 -based materials is mainly due to the unique combination of an elevated oxygen transport capacity coupled with the ability to easily shift between reduced and oxidized states, i.e., the sesquioxide CeO1.5 and the dioxide CeO2 , incorporating Ce+3 and Ce+4 ions, respectively. Under reducing atmosphere, the removal of oxygen from CeO2 at elevated temperatures leads to the formation of an oxygen-deficient nonstoichiometric CeO2−y phase with 0 < y < 0.5. CeO2−y retains the same fluorite crystal structure as CeO2 , facilitating rapid and complete refilling of oxygen vacancies upon exposure of CeO2−y to oxygen, with recovery of CeO2 . The same phenomenon exists in
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S. Huang et al. / Solid State Sciences 7 (2005) 539–544
CeO2 -stabilized ZrO2 ceramics for structural [5] or functional [4] applications when exposed to an inert atmosphere or low oxygen partial pressure at elevated temperature. Consequently, the fluorite Cez Zr1−z O2 phase in the CeO2 –ZrO2 system converts to a nonstoichiometric Cez Zr1−z O2−x phase in the CeO2 –ZrO2 –CeO1.5 system. In their work on CeO2 – ZrO2 mixed oxide as support for noble metal catalysts for the reduction of NO by CO, Fornasiero et al. [6] found that the redox behaviour strongly depends both on the ZrO2 content in the CeO2 solid solution and on the nature of the phase present. Therefore, it is necessary to understand the relation between the oxygen partial pressure (P O2 ), temperature and the phase diagram of the CeO2 –ZrO2 –CeO1.5 system. It is clear that the P O2 has a strong influence on the performance of CeO2 -based systems. In this context, special attention is given to the calculation of the isothermal sections of the CeO2 –ZrO2 –CeO1.5 system, the prediction of the phase constitution in the CeO2 –CeO1.5 system and the cubic Cez Zr1−z O2−x phase as function of the P O2 and temperature. The relation between the degree of Ce+4 reduction and the P O2 in the fluorite CeO2−y and Cez Zr1−z O2−x solid solutions at different temperatures can be used as a guide in the development of functional ceramics or assist in explaining their performance as function of the operating atmosphere.
phases and the stoichiometric Zr2 Ce2 O7 (P) compound were considered. In the CeO2 –CeO1.5 system, the miscibility gap was evaluated with the experimental data from Campserveux et al. [16], where the highest temperature of the miscibility gap is 673 ◦ C. Only the CeO2−y phase with 2 > 2 − y > 1.818 was taken into account to evaluate the thermodynamic coefficients. In the ionic characteristics model of the Css, a twosublattice model was used in the CeO2 –CeO1.5 –ZrO2 system. The phase Css in the CeO1.5 –ZrO2 system is defined as (Zr+4 ,Ce+3 )1 (O−2 ,Va0 )2 , whereas the nonstoichiometric CeO2−y phase in the CeO2 –CeO1.5 system is described as (Ce+4 ,Ce+3 )1 (O−2 ,Va0 )2 and the Css phase in the CeO2 – ZrO2 system was as (Zr+4 ,Ce+4 )1 (O−2 )2 . Therefore, the Css (Cez Zr1−z O2−x ) in the CeO2 –ZrO2 –CeO1.5 system is defined as (Zr+4 ,Ce+3 ,Ce+4 )1 (O−2 ,Va0 )2 . The formalism of the Gibbs free energy expression and the thermodynamic optimization are reported elsewhere [17]. Using the Thermo-Calc program [18], the isothermal sections of the CeO2 –ZrO2 –CeO1.5 system at 1300 to 1700 ◦ C were extrapolated, as presented in Fig. 1. There is a large composition range for the Css phase in these diagrams, which extends with increasing temperature. The commonly investigated compositions in the CeO2 –ZrO2 –CeO1.5 system, Ce0.5 Zr0.5 O2−x and Ce0.8 Zr0.2 O2−x , are located in the Css area, whereas Ce0.2 Zr0.8 O2−x is located in the Tss + Css phase region.
2. Results and discussion 2.1. Estimating phase diagrams in the CeO2 –ZrO2 –CeO1.5 system Although there are a lot of experimental data on the phase equilibria in the binary CeO2 –ZrO2 , CeO1.5 –ZrO2 , CeO2 – CeO1.5 systems, it is difficult to describe the phase diagram of the higher order systems through experimental analysis. Using the CALPHAD technique, the isothermal sections of ternary systems can be extrapolated directly by combining the thermodynamic properties of the binary systems with suitable thermodynamic models. As a pioneer, Kaufman and Nesor [7] contributed a substitutional model, which considered the oxides as pure components in the formalism of the Gibbs free energy of phases. This model has been successfully applied to many ZrO2 containing oxide systems [8–13]. Hillert and Staffansson [14] developed a two-sublattice model suggesting vacancies located at one of the sublattices. Guillermet et al. [15] introduced vacancies at both sublattices for fitting the requirements of structural and electron neutrality. To obtain the phase diagram of the CeO2 –ZrO2 –CeO1.5 system at high temperature, a thermodynamic evaluation was initially performed on the CeO2 –ZrO2 , CeO1.5 –ZrO2 and CeO2 –CeO1.5 systems. The CeO2 –ZrO2 system includes liquid (L) and cubic (Css), tetragonal (Tss) and monoclinic (Mss) solid solution phases. In the CeO1.5 –ZrO2 system, the liquid, Tss, Css, Hss (hexagonal solid solution)
2.2. Redox behaviour of CeO2 and CeO2 –ZrO2 solid solutions For CeO2 -based applications, the atmosphere and especially the influence of the oxygen partial pressure on the reduction behaviour of CeO2 must be taken into account. The reduction behaviour of CeO2 and CeO2 –ZrO2 solutions can be thermodynamically calculated by means of the substitutional and sublattice model. In order to do so, the y coefficient in CeO2−y is calculated as function of the PO2 and temperature (T ). Following the suggestion of Lindemer [19], the chemical activity of oxygen in CeO2−y can be represented by the thermodynamic properties of the two components, CeO2 and CeO1.5 . In the CeO1.5 –CeO2 system, 4CeO1.5 + O2 ⇔ 4CeO2 .
(1)
To obtain the y-T -P O2 relation, the thermodynamic parameters of the Css are adopted. A nonstoichiometric phase Cez Zr1−z O2−x (Css) is assumed to be composed of z mole of CeO2−y and (1 − z) mole of ZrO2 , whereas the CeO2−y consists of m1 mole CeO1.5 , m2 mole CeO2 and m3 mole ZrO2 (m3 = 0). The number of moles, mi , of each species can be calculated from the mass-balance for cerium and oxygen as: z = m1 + m2
and z(2 − y) = 2m2 + 1.5m1
(2)
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(d)
(b) (e) Fig. 1. Continued.
can be calculated as: 2 − y = 2 − x − 2(1 − z) /z.
(4)
Consequently, the relation between x, y and z is that x = zy. In a system without ZrO2 [z = 1 or m3 = 0], the mole fraction of CeO1.5 is N1 = m1 /(m1 + m2 ) and the mole fraction of CeO2 is N2 = m2 /(m1 + m2 ). Moreover, y = 0.5 − 0.5N2 . Since the free energy difference for Eq. (1) is (4GCeO2 − 4GCeO1.5 ), the oxygen partial pressure is given by: RT ln(P O2 ) = 4GCeO2 − 4GCeO1.5 . (c) Fig. 1. Calculated isothermal section of the ZrO2 –CeO2 –CeO1.5 system at 1700 ◦ C (a), 1600 ◦ C (b), 1500 ◦ C (c), 1400 ◦ C (d) and 1300 ◦ C (e).
resulting in: m1 = 2zy
and m2 = z(1 − 2y).
(3)
For nonstoichiometric phase Zr1−z Cez O2−x , the O/(Ce + Zr) ratio is 2 − x and the O/Ce ratio in CeO2−y
(5)
Where ln(P O2 ) is the natural logarithm of P O2 . When z = 0, m3 = 0 and N3 = 0, the relation between y and the P O2 for phase CeO2−y can be obtained from Eq. (5). The general equation for the partial molar Gibbs free energy of each mass parameter is needed to calculate the free energy change for Eq. (1). The partial molar free energy for mass parameter i equals to: ∂[G Zr1−z Cez O2−x ]/∂mi = Gi .
(6)
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Therefore, the composition-temperature-oxygen pressure relationship in the CeO2 –ZrO2 –CeO1.5 system can be evaluated from Eqs. (1) to (6) and the molar Gibbs free energy of the nonstoichiometric phase, G Zr1−z Cez O2−x , which is described as function of m1 , m2 , m3 and the standard Gibbs free energy of formation of the ZrO2 , CeO2 and CeO1.5 Css phases in Ref. [20]. Although the thermodynamic properties are obtained with the ionic characteristics model [15], which is totally different from that of Lindemer [19], the calculated ln(y) − ln(P O2 ) relationship in the CeO2 –CeO1.5 system at 1400, 1200 and 1100 ◦ C is found to be in good agreement with the experimental data [19], as shown in Fig. 2. The calculated non-stoichiometry, y, and the corresponding CeO1.5 mole fraction as function of the oxygen partial pressure at 600, 700, 800 and 900 ◦ C are presented in Figs. 2 and 3. For a given P O2 , more and more CeO2 is reduced to CeO1.5 with increasing temperature. As mentioned above,
Fig. 2. Calculated relationship between ln(y) and ln(P O2 ) in the CeO2 –CeO1.5 system at different temperatures, together with the experimental data of [19].
the oxygen storage capacity (OSC), a parameter reflecting the ability of a three-way catalyst to store and release oxygen under different conditions, is related to the oxygen partial pressure, P O2 . The OSC of CeO2 increases with the working temperature for a given composition, as seen in Fig. 3. However, a too high working temperature leads to sintering of the CeO2 particles and a reduced surface area, resulting in an overall loss of OSC. When z < 1, part of the CeO2 transforms to CeO1.5 and the transformed portion can also be evaluated as graphically presented in Fig. 4 at 800 ◦ C. CeO2 reduction becomes relatively easier at higher ZrO2 contents, e.g., the ln(P O2 ) increases from around −42 to −12 for pure CeO2 and Ce0.4 Zr0.6 O2−x respectively, for a given reduction value x = 0.05. Therefore, the total OSC increases with increasing ZrO2 content in the Cez Zr1−z O2−x mixed oxides. It has been reported that the defect crystal structure and the concomitant oxygen vacancies play a fundamental role in the catalytic activity of CeO2 -based materials [21,22]. The use of Zr4+ as a dopant promotes the kinetics of Ce4+ reduction and enhances oxygen storage of ceria, due to an increased oxygen vacancy content and enhanced oxygen mobility in the Css solid solution compared to pure CeO2 [23,24]. The optimum composition is reported to be around z = 0.5 [25]. At high ZrO2 -contents however, a tetragonal solid solution (Tss) is formed in agreement with the calculated phase diagram of the CeO2 –ZrO2 –CeO1.5 system (Fig. 1), reducing the catalytic activity of the Cez Zr1−z O2−x system due to a reduced Css content. In the 0.5 < z < 0.8 region, depending on the CeO2 /Ce2 O3 ratio, a maximum in pyrochlore (P) phase content in combination with a Css phase is to be expected based on Fig. 1(e). This pyrochloretype Ce2 Zr2 O7+2δ phase can be converted into metastable κ-CeZrO4 or tetragonal metastable t -(Ce0.5 Zr0.5 )O2 upon oxidation, and into Ce2 Zr2 O7 upon reduction [26]. The influence of these phase transitions on the oxygen storage capacity however has not been revealed yet.
Fig. 3. Calculated ln(P O2 ) versus the corresponding CeO1.5 mole fraction in the CeO2 –CeO1.5 system at different temperatures.
Fig. 4. Composition of the Cez Zr1−z O2−x phase as function of the oxygen partial pressure and the ZrO2 content at 800 ◦ C.
S. Huang et al. / Solid State Sciences 7 (2005) 539–544
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Fig. 5 shows the influence of temperature on the reduction of CeO2 at different ln(P O2 ) for the Ce0.5 Zr0.5 O2−x , Ce0.8 Zr0.2 O2−x , and Ce0.2 Zr0.8 O2−x solid solutions. It is clear that a lower oxygen potential and higher temperature lead to a stronger reduction of CeO2 . Moreover, the effect of temperature on the relative fraction of CeO2 reduction decreases with decreasing overall CeO2 content (see Fig. 5(a)– (c)). From a thermodynamic point of view, this can be explained by the formation of an increasing amount of Tss with increasing overall ZrO2 content, with poorer oxygen storage properties than the Css phase [27]. These calculations are in agreement with the reported experimental results revealing that the extent of reduction at z > 0.2 increases with increasing temperature, showing a temperature related maximum at 0.5 < z < 0.8 [24,25,28]. Moreover, the effect of temperature on the relative extent of reduction indeed decreases with increasing ZrO2 content for 0.1 < z < 0.8 [24,25,28]. Interestingly, this maximum might be directly related to a maximum pyrochlore (P) phase content in the system, as suggested by the calculated equilibrium phase diagram in Fig. 1(e). The presented thermodynamic calculations allow to evaluate the total oxygen storage capacity for different compositions of the Cez Zr1−z O2−x solid solution as function of temperature. However, the influence of the sample preparation method, the processing conditions as well as the reduction kinetics and oxygen diffusivity on the OSC also play an important role in the development of CeO2 –ZrO2 based materials for functional applications.
3. Conclusions
(b)
The oxygen partial pressure has a strong influence on the performance of CeO2 -based systems. In this context, special attention was given to the calculation of the isothermal sections of the CeO2 –ZrO2 –CeO1.5 system at 1300–1700 ◦ C, the prediction of the phase constitution in the CeO2 –CeO1.5 system as well as the cubic Cez Zr1−z O2−x solid solution as function of the P O2 in the 600–900 ◦ C range.
Acknowledgements This work is financially supported by the Research Fund of K.U. Leuven in the framework of the Flanders–China bilateral projects BIL 02/04, BIL 04/13 and BIL 04/14, and the National Natural Science Foundation of China under grant No. 50471101.
(c) Fig. 5. Calculated ln(P O2 ) versus x in the temperature range from 600 to 900 ◦ C for Ce0.8 Zr0.2 O2−x (a), Ce0.5 Zr0.5 O2−x (b) and Ce0.2 Zr0.8 O2−x (c).
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