Ce_0.5Zr_0.4Sn_0.1O₂/Al₂O₃ catalysts with enhanced oxygen storage capacity and high CO oxidation activity

Abstract

Ce_0.5Zr_0.4Sn_0.1O₂/Al₂O₃ catalysts were prepared by a coprecipitation route using Ce(NH₄)₂(NO₃)₆, Zr(NO₃)₃·2H₂O, SnCl₄·5H₂O and Al₂O₃ as the starting materials, and were evaluated as a promoter in automotive catalysis by X-ray diffraction, transmission electron microscopy and the Brunauer–Emmet–Teller (BET) technique. The oxygen storage capacity (OSC) values of the samples were measured using thermogravimetric-differential thermal analysis at 600 °C with a continuous flow of CO–N₂ gas and air alternately. The CO oxidation activity of the synthesized sample was evaluated via a gas FTIR spectrometer for analysis of gas compositions. The Ce_0.5Zr_0.4Sn_0.1O₂/γ-Al₂O₃ composite possessed excellent thermal stability, enhanced OSC values, and high CO oxidation activity at the low temperatures even after calcination at 1000 °C for 20 h.

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1. Introduction

Ceria (CeO₂)-based materials have attracted considerable interest for more than half a century for its far-ranging applications in catalysts, fuel cell, cosmetics, gas sensors, solid-state electrolytes, and especially its crucial application as a promoter of three-way catalysts (TWCs, this derives from its ability to simultaneously remove the three categories of pollutants, i.e. CO, NO_x, and hydrocarbons (HC)), because of their excellent oxygen storage capacity (OSC).^1–9 The OSC of CeO₂ based on the reversible redox reaction between Ce⁴⁺ and Ce³⁺, is of relevant technological importance in automotive TWCs. The highest simultaneous conversions of the three pollutants CO, NO_x, and HC are indeed attained over TWCs at air/fuel (A/F) ratios close to the stoichiometric value, while fuel-lean (net oxidizing) and fuel-rich (net reducing) conditions severely decrease, respectively, CO, NO_x, and HC conversions. The OSC of added CeO₂ acts as an oxygen partial pressure regulator, minimising these undesirable effects.¹⁰ In the last two decades it has been established that the partial incorporation of Zr⁴⁺ into the CeO₂ lattice results in the formation of Ce_xZr_1−xO₂ mixed oxides, which show enhanced structural/textural features and redox properties with improved thermal stability at elevated temperatures.^10–12 For example, Fornasiero et al. have reported that an optimum composition, like Ce_0.5Zr_0.5O₂ can exist as a cubic phase, which can have excellent redox properties.¹³ Hence, Ce_xZr_1−xO₂ mixed oxides gradually replaced the pure CeO₂ as the OSC promoters in TWCs.

In the typical design of modern TWCs, γ-alumina (γ-Al₂O₃) is employed as a carrier because of its high specific surface area and relatively excellent thermal stability.^14,15 At present, γ-Al₂O₃ supported ceria-based materials have been extensively used and proven to be the most advanced materials for TWCs.^16–20 Roberta Di Monte et al.^21,22 found that the homogeneous CeO₂–ZrO₂ structure and strong interaction between Al₂O₃ carriers and CeO₂–ZrO₂ are excellent for obtaining thermal stability. Morikawa et al.²³ testified that the introduction of Al₂O₃ into Ce_1−xZr_xO₂ primary particles can improve durability at high temperatures and the oxygen release rate of the materials. Besides such conventional materials, recently, Imanaka et al. described the preparation of a CeO₂–ZrO₂–Bi₂O₃ (CZB) solid solution supported on La-stabilized γ-Al₂O₃ (CZB/Al₂O₃), led to low-temperature redox activities.²⁴ Bharali et al. synthesized two new OSC materials namely Ce_0.8Tb_0.2O_2−x/γ-Al₂O₃ and Ce_0.8Hf_0.2O₂/γ-Al₂O₃, which exhibit excellent OSC and superior CO oxidation activity in comparison with the most advanced CeO₂–ZrO₂/Al₂O₃ catalyst supports employed in the existing TWCs.²⁵ However, after high-temperature treatment for a long time (for example: 10 h), retention of reduction behavior for the TWCs remains elusive because of a lowering of OSC values and phase separation.^15,26 Consequently, in terms of the practical applications as an automotive exhaust catalyst, the new material with high thermal stability and enhanced OSC is a requisite for developing advanced TWCs.

On the other hand, although the reduction temperature can be decreased obviously by adding noble metal particles on the surface of the mixed oxides, the amount of such precious metals employed in the automotive catalysts has increased drastically over the last decade, reflecting the energy crisis and the growing strength of worldwide emission regulations.^27,28 Therefore, it is a greatly urgent task to develop new materials which can reduce or substitute the utilization amount of noble metals in automotive exhaust catalysts.

In the present work, for the first time, we describe the preparation of Ce_0.5Zr_0.4Sn_0.1O₂/γ-Al₂O₃ (hereafter CZS/A) catalysts via a coprecipitation route without adding any noble metals. CeO₂/γ-Al₂O₃ (hereafter C/A) and Ce_0.5Zr_0.5O₂/γ-Al₂O₃ (hereafter CZ/A) catalysts were also prepared via the same method as the reference materials. All samples were finally calcined at 1000 °C for 20 h in air to evaluate the thermal stability.

2. Experimental

Ce(NH₄)₂(NO₃)₆, ZrO(NO₃)₂, SnCl₄·5H₂O and NH₄OH were of analytical grade, and were purchased from Kanto Chemical Co. Inc., Japan (purity 99.999%). γ-Al₂O₃ powder (G025) was provided by Sumitomo Chemical Co., Ltd. The chemicals were used without further purification.

2.1 Catalyst preparation

In a typical procedure, 1.8 g γ-Al₂O₃ (specific surface area of 407 m² g⁻¹) is firstly dispersed in 60 mL distilled water and stirred for 1 h. Separately, the stoichiometric amounts of Ce(NH₄)₂(NO₃)₆ (6 mmol), ZrO(NO₃)₂ (4.8 mmol) and SnCl₄·5H₂O (1.2 mmol) were dissolved in 60 mL distilled water and stirred for 30 min. Subsequently, the above obtained solutions were added to the alumina dispersed solution under continuous stirring for 1 h. Then, NH₄OH solution was slowly dropped into the above mixed solution, and the pH value was maintained at 9, under vigorous stirring. The chemical reactions for the formation of Ce_0.5Zr_0.4Sn_0.1O₂ can be written as follows:

NH₄OH → NH₄⁺¹ + OH⁻¹ (a)

Ce⁴⁺ + Zr⁴⁺ + Sn⁴⁺ + OH⁻¹ → Ce_0.5Zr_0.4Sn_0.1O₂ + H₂O (b)

The obtained precipitate was filtered and washed with distilled water and dried in air at 100 °C for 12 h, and subsequently calcined at 1000 °C for 20 h in air atmosphere. The same synthesis route was employed for the preparation of the CeO₂/γ-Al₂O₃ and Ce_0.5Zr_0.5O₂/γ-Al₂O₃.

2.2 OSC analysis

The OSC of catalysts was determined using a thermogravimetric differential thermal analysis (TG-DTA, Rigaku TAS-200). Before the measurements, the samples were held in flowing air at 600 °C for 30 min to remove residual water and other volatile gases. The mixed gas of CO–N₂ (100 cm³ min⁻¹) and air (100 cm³ min⁻¹) was flowed alternatively at 600 °C. Finally the OSC was analyzed after getting the TGA profile.

2.3 CO oxidation measurements

The CO oxidation activity of the synthesized sample was evaluated in a fixed-bed flow reactor by passing gas mixtures of 2 vol% CO in N₂ and 2 vol% O₂ in N₂ at a rate of 500 cm³ min⁻¹ over 0.15 g sample. The sample was pressed, crashed and sieved to obtain granules of 212–355 μm in diameter and mounted into a quartz tube reactor 6 mm in diameter. The samples were heated at a heating rate of 10 °C min⁻¹ from room temperature to 600 °C. A gas FTIR spectrometer (MIDAC Co., IGA-4000) was employed for in situ analysis of gas composition. The conversion of CO was obtained by using the formula XCO = (1 − [CO]/[CO]₀) × 100%, where [CO]₀ and [CO] are initial and transient concentration of CO, respectively.

2.4 Characterization

The phase composition of the catalyst was determined by X-ray diffraction analysis (XRD, Shimadzu XD-D1) using graphite-monochromized CuKα radiation. The morphology and size of the samples were determined by a transmission electron microscopy (TEM, JEOL JEM-2010). High resolution transmission electron microscopy (HRTEM) images were obtained on a ZEISS LEO 922 with an accelerating voltage of 200 kV. The specific surface area and pore size were measured using BET analysis (NOVA 4200e).

3. Results and discussion

All samples of (a) C/A, (b) CZ/A and (c) CZS/A showed the characteristic XRD peaks of a fluorite structure of CeO₂ (Fig. 1). No phase separation was observed even after calcination at such high temperatures as 1000 °C for 20 h. The shift of the diffraction peaks of CZ/A and CZS/A to higher angles, as compared to those of C/A, suggests that the Zr⁴⁺ and Sn⁴⁺ ions dissolve into the CeO₂ lattice to form a solid solution. The calculated lattice parameters of CZ/A (a = 0.5345 nm) and CZS/A (a = 0.5317 nm) are smaller than that of C/A (a = 0.5413 nm). The observed decrease in the lattice parameter in comparison with C/A is a strong evidence of the penetration of the doped cations (Zr⁴⁺ and Sn⁴⁺) into the ceria lattice. The broadening effect of the peaks could be attributed to the nanocrystalline materials, such as 11 and 9 nm calculated by Scherer's formula for CZ/A and CZS/A, respectively.

Fig. 1 XRD patterns of the samples. (a) C/A (CeO₂/γ-Al₂O₃), (b) CZ/A (Ce_0.5Zr_0.5O₂/γ-Al₂O₃), (c) CZS/A (Ce_0.5Zr_0.4Sn_0.1O₂/γ-Al₂O₃).

The morphology and crystalline growth of samples were confirmed by TEM (Fig. 2). The crystalline sizes of CZ and CZS on γ-Al₂O₃ were about 15 and 12 nm, respectively, and agreed with the calculated values using the XRD patterns and Scherer’s formula (Fig. 2(b) and (c)). However, the C/A exhibited large particle sizes of 50–60 nm (Fig. 2(a)). It has been reported that there is a synergic stabilization effect between CZ and γ-Al₂O₃. This effect inhibits both the transition of Al₂O₃ from the γ- to the α-phase and the growth of the CZ particles.^13,14 The high-resolution TEM (HRTEM) images of C/A, CZ/A and CZS/A exhibited clear lattice fringes with spacings of 0.311, 0.305 and 0.302 nm (Fig. 3), respectively, which can be corresponded to the (111) planes of CeO₂. The BET nitrogen adsorption–desorption analysis was undertaken to measure the specific surface area of as-prepared samples. As a result, the specific surface area of CZS/A (56 m² g⁻¹) was higher than C/A (28 m² g⁻¹) and CZ/A (41 m² g⁻¹), shown as in Table 1.

Table 1 OSC (at 600 °C) and BET specific surface areas of (a) C/A, (b) CZ/A and (c) CZS/A

Chemical composition	OSC of samples (μmol-O g^−1)	Surface areas of samples (m^2 g^−1)
C/A	111	28
CZ/A	596	41
CZS/A	774	56

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Fig. 2 TEM images of (a) C/A, (b) CZ/A and (c) CZS/A samples.

Fig. 3 HRTEM images of (a) C/A, (b) CZ/A and (c) CZS/A samples.

The OSC values of the samples were determined at 600 °C with a continuous flow of CO–N₂ gas and air alternatively. Fig. 4 shows how the typical oxygen release/storage profiles of the C/A, CZ/A, and CZS/A samples at 600 °C change with time. CZS/A exhibited the highest OSC of 774 μmol-O g⁻¹, when compared with those of C/A (111 μmol-O g⁻¹) and CZ/A (596 μmol-O g⁻¹) samples (Table 1). It is accepted that the OSC is strongly depended on the specific surface area. It is obvious that CZS/A exhibited the highest specific surface area and highest OSC values. Moreover, CZS/A exhibits higher OSC than those of Ce_0.5Zr_0.5O₂/Al₂O₃ (697 μmol-O g⁻¹) and Pd/Ce_0.67Zr_0.33Sr_0.01O_2.01/Al₂O₃ (152 μmol-O g⁻¹).^14,29 In order to examine OSC performance stability, oxygen release/storage cycle measurements were taken, and CZS/A retained the same OSC even after 128 cycles (Fig. S1, ESI†). Comparing with the OSC of Ce_0.7Zr_0.3O₂/Al₂O₃,³⁰ the result indicates that CZS/A has good OSC performance stability.

Fig. 4 Oxygen release/storage properties (TG profiles) of (a) C/A, (b) CZ/A and (c) CZS/A at 600 °C.

In order to test the role of Al₂O₃ in enhancing the OSC by increasing the surface area, supplementary experiments for the OSC analysis were carried out for the unsupported CeO₂ (C), Ce_0.5Zr_0.5O₂ (CZ) and Ce_0.5Zr_0.4Sn_0.1O₂ (CZS) samples. All the unsupported samples showed much smaller specific surface areas (C (3 m² g⁻¹), CZ (8 m² g⁻¹) and CZS (24 m² g⁻¹)) as shown in Fig. S2 (ESI†). As a result, the OSC was enhanced greatly by coupling with γ-Al₂O₃ (Fig. S3 and S4 in ESI†). The increased surface area and high OSC are thought to be related to the Al₂O₃ support and the involvement of Ce⁴⁺/Ce³⁺ and Sn⁴⁺/Sn²⁺ redox couples, in which the CZS lattice oxygen is thought to be utilized for CO oxidation.³¹ It is proposed that SnO₂, being easily reducible, gives out its lattice oxygen for the oxidation reaction, which is possibly rejuvenated by subtracting oxygen from the adjacent CeO₂ molecules.³² Based on these results, the improvement of the OSC can be explained by the combination of two effects: the dispersion of the nanometer-sized C, CZ and CZS particles on the Al₂O₃ surface, thereby increasing the surface area, and the incorporation of Sn⁴⁺ into the CZ lattice, which enhances the number of redox couples.

Moreover, the high OSC may contribute to the oxidation of CO, which is one of the important reactions of interest in the catalysis process of the automotive exhaust cleaning process. The activity curves of CO oxidation for all samples are plotted in Fig. 5. Comparison with the light-off temperatures of C/A (329 °C) and CZ/A (301 °C) CZS/A showed the lowest light-off temperature to be 275 °C, indicating that CZS/A was the most active catalyst. Furthermore, the light-off temperature of CZS/A is significantly lower than those of Ce_0.5Zr_0.5O₂/Al₂O₃ (384 °C), Ce_0.8Tb_0.2O_2−x/Al₂O₃ (331 °C), Ce_0.8Hf_0.2O₂/Al₂O₃ (337 °C) catalysts,^14,25 in particular, the light-off temperature is also lower than that of Pd/Ce_0.67Zr_0.33Sr_0.01O_2.01/Al₂O₃ (286 °C) catalysts,²⁹ where Pd is used as a noble metal. It is apparently recognized that the OSC values of the catalysts are correlated to their CO oxidation activities. Based on these results, it can be suggested that the high OSC activities play an important role in the oxidation of CO at low temperatures, in other words, the order of activity is directly related to the OSC of the catalysts, which demonstrates that catalysts with higher OSC possess superior CO oxidation activity.

Fig. 5 CO oxidation activity curves of (a) C/A, (b) CZ/A and (c) CZS/A.

4. Conclusions

A new OSC material of Ce_0.5Zr_0.4Sn_0.1O₂/Al₂O₃ is prepared via a coprecipitation method. Ce_0.5Zr_0.4Sn_0.1O₂/Al₂O₃ showed the high OSC even after calcination at 1000 °C for 20 h. Studies using CO oxidation, as a model reaction, showed that the high oxidation activity of the sample at low temperatures correlates with their enhanced OSC. The prepared Ce_0.5Zr_0.4Sn_0.1O₂/Al₂O₃ has the potential to be a key material in advanced catalytic converters without the help of noble metals in the design of superior three way catalysts (TWCs).

Acknowledgments

This work was partly supported by the Rare Metal Substitute Materials Development Project of New Energy and the Industrial Technology Development Organization (NEDO), Japan and the Management Expenses Grants for National Universities Corporations from the Ministry of Education, Culture, Sports and Science for Technology of Japan (MEXT).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20425h

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