Large-scale synthesis of ceria-based nano-oxides with high CO oxidation activity

Abstract

We report large-scale synthesis of high surface area nano-oxides of Ce_0.8M_0.2O₂/Al₂O₃ (M = Tb and Hf) possessing cuboctahedral shape of 3–5 nm size, which exhibit excellent oxygen storage capacity (OSC) and superior CO oxidation activity in comparison to the most advanced ceria–zirconia/alumina catalyst supports employed in the existing three-way-catalytic converters (TWCs).

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Nanostructured oxide materials have attracted great interest in recent years owing to their unusual mechanical, electrical, optical, and catalytic properties endowed by confining the dimensions of such materials, and because of the combination of bulk and surface properties to the overall behavior.^1–3 CeO₂ is one of the most fascinating oxides industrially because oxygen vacancy defects can be rapidly formed and eliminated, giving it a high oxygen storage capacity (OSC).^4–11 The present study brings light to new opportunities in the development of high-performance OSC materials. The ceria-based materials are mainly employed in three-way automotive catalytic converters (TWCs). Other significant catalytic applications include water-gas shift reaction, steam reforming of oxygenates, and preferential oxidation of CO (PROX), all of which hold promise for enabling a hydrogen economy.^12–17 The TWC technology is essential for de-pollution processes in auto-exhaust purification systems of ubiquitous automobiles which can effectively oxidize the harmful CO and hydrocarbons, and reduce NO_x to the safer gases. Recently, CeO₂ nanoparticles have also been demonstrated to protect biological tissues against radiation induced damage and scavenging of superoxide anions, prevent laser induced retinal damage, reduce spinal injury, prevent cardiovascular myopathy, and as a tool for immunoassays and other inflammatory diseases.^18–20 In the typical design of modern TWCs, alumina is employed as a carrier because of its high specific surface area and relatively better thermal stability under the hydrothermal conditions of the exhausts. Till date Al₂O₃ supported Ce_1−xZr_xO₂ catalyst has been extensively used and proven to be the most advanced material for TWCs.^21–32 However, one of the major deactivation pathways of this material is sintering at higher temperatures leading to loss of crucial OSC. In this context, design of most efficient and selective materials with high OSC is of critical importance for modern TWCs.^21–27

Herein, we report synthesis of two new nano-oxides namely Ce_0.8Tb_0.2O_2−δ/Al₂O₃ and Ce_0.8Hf_0.2O₂/Al₂O₃ (hereafter CTA and CHA, respectively) by a deposition coprecipitation method using ultrahigh dilute aqueous solutions. A nanosized Ce_0.75Zr_0.25O₂/Al₂O₃ (hereafter CZA) was also synthesized for comparison. Calcination under air at 773 K was applied as the final step of synthesis in any case. A multitechnique analysis detailing structural and electronic characteristics was undertaken (ESI†).

The crystallographic structure of the synthesized materials is inspected by XRD (Fig. 1). All the diffraction lines could be indexed to cubic CeO₂ with slight shift in the 2θ values compared to pristine CeO₂, which evidenced the formation of solid solutions of the corresponding oxides.^{12–17,21–27} The broadening effect of the peaks could be attributed to the fine nature of the particles in the respective oxides, which are 3–4 nm as calculated using Scherrer's formula. The estimated lattice parameter values for CTA, CHA, and CZA are 5.38, 5.36, and 5.29 Å, respectively. The observed decrease in the lattice parameter in comparison to pristine CeO₂ (5.41 Å) is a strong evidence of the penetration of the doped cations (Tb⁴⁺, Hf⁴⁺, and Zr⁴⁺) into the ceria lattice.^21–27 The specific surface areas determined by N₂ physisorption are reasonably high, exhibiting 148, 156, and 146 m² g⁻¹ for CTA, CHA, and CZA samples, respectively. The morphology and crystallite growth of the mixed oxides were examined by TEM. Small and well faceted mixed oxides of 3–5 nm in size are clearly visible on the surface of an amorphous Al₂O₃ matrix. The shapes of the particles are nearly cuboctahedral (Fig. 1 and Fig. S1, ESI†). It is observed that there are some overlapping regions of the mixed oxide particles, especially in the case of CTA, which might be responsible for better catalytic activity.

Fig. 1 Powder XRD patterns of Al₂O₃ supported Ce_0.8Tb_0.2O_2−δ (CTA), Ce_0.8Hf_0.2O₂ (CHA) and Ce_0.75Zr_0.25O₂ (CZA), and TEM image of CTA.

The presence of surface oxygen vacancies in the materials is monitored by UV-Raman spectroscopy (Fig. S2, ESI†). The phonon mode at ∼600 cm⁻¹ is characteristic of oxygen vacancies in the CeO₂ lattice, as is clearly observed in the figure. The relatively strong intensity of the band in the case of CTA signifies the larger amount of oxygen vacancies, which means the presence of more exposed Ce³⁺ ions. These groups of exposed Ce³⁺ ions are the potentially effective surface sites for catalysis, because adsorbed gases could interact exclusively with Ce³⁺ ions facilitating higher activity.^{4–17,21–27} The band at 454 cm⁻¹ could be assigned to the symmetric breathing mode of oxygen atoms around the cerium ions. This mode is ca. 10 cm⁻¹ lower than that observed for pristine CeO₂, which can be explained by the enlarged Ce–O bond lengths resulting from lattice distortions. The weak band observed at ∼320 cm⁻¹ is attributed to the displacement of oxygen atoms from their ideal fluorite lattice positions.^21–27 UV–vis DRS results (Fig. S3, ESI†) show that the materials exhibit three absorption maxima with slight shifts from the values ∼255, 285, and 340 nm, which are normally observed for pristine and doped CeO₂ corresponding to various charge transfer and electronic transitions.^23–25

The valence states of Ce and Al ions in the solid solutions are determined by XPS analysis. Although most of the cerium ions are in the Ce⁴⁺ oxidation state, the presence of Ce³⁺ is clearly observed in all the samples (Fig. S4, ESI†).^21–27,31 In addition, the presence of oxygen vacancies and Ce³⁺ ← O²⁻ charge transfer transitions as revealed by Raman and DRS analyses provide more evidence for the existence of Ce³⁺ in these materials.^21–27 It is well-known that once Ce³⁺ appears in the fluorite lattice, oxygen vacancies are generated to maintain electrostatic balance. Alumina is exclusively present in the Al³⁺ oxidation state (Fig. S5(a), ESI†). The O 1s peak (Fig. S5(b), ESI†) is observed at 530–531 eV. However, the flat nature of the peaks may mask any probable shoulder towards the high binding energy side which is more likely to be present due to the adsorbed oxygen or surface hydroxyl species and/or adsorbed water present as contaminant at the material's surface. The OSC results determined by a thermogravimetry method follow an order of CTA > CHA > CZA, CTA exhibiting the highest value of 562 μmoles O₂/g CT.^21–27,31

Fig. 2(A) presents the activity curves of CO oxidation for CTA, CHA, and CZA samples which shows that the former two are more active. The light-off temperatures for CTA, CHA, and CZA are 604, 610, and 658 K, respectively. This order of activity is directly correlated to the OSC of the oxides, which demonstrates that oxide with higher OSC confers superior activity for CO oxidation. These results also corroborate well with the conclusions drawn from XRD, TEM, Raman, and XPS studies. To support the superior activity, the samples were further characterized by TPR. The TPR profiles (Fig. 2B and C) of the most active CTA sample show two reduction maxima at 473 and 773–873 K, which are assigned to the reduction of surface and bulk Ce–Tb–O, respectively. The broad nature of the low temperature TPR peak (Fig. 2B) in the 1st cycle reveals a high H₂ consumption which is due to high specific surface area. No characteristic peaks are observed for Tb⁴⁺ to Tb³⁺ transformation at temperatures of ∼576, 940, and 993 K. After the 1st reduction cycle the sample is reoxidized with O₂/Ar, and it is observed that the reduction temperatures in the 2nd cycle (Fig. 2C) are shifted towards higher temperatures due to decrease in the surface area and increase in the particle size after high temperature exposure in the 1st cycle.²³ The IS spectra of CTA (Fig. 2D and E) after 2nd and 30th scans show the main Ce peak accompanied by a small signal of Tb. Though the two signals cannot be completely resolved, it is seen that Tb is exposed on the surface of the sample. It shows (Fig. S6, ESI†) that there is a linear increase in the Tb/Ce intensity ratio during the sputter series, which means that Ce is enriched at the external surface of the mixed oxide. This slight surface enrichment of Ce in CTA facilitates easy reducibility of the material leading to better catalytic activity.²³

Fig. 2 (A) CO oxidation activity curves of Al₂O₃ supported Ce_0.8Tb_0.2O_2−δ (CTA), Ce_0.8Hf_0.2O₂ (CHA), and Ce_0.75Zr_0.25O₂ (CZA); TPR plots of CTA after definite cycles: (B) cycle 1 and (C) cycle 2; IS spectra of CTA after definite scans: (D) scan 2 and (E) scan 30.

In conclusion, we synthesized new OSC materials based on nanosized CeO₂ exhibiting high CO oxidation activity, which could act as highly active catalyst supports in the design of superior TWCs. Interestingly, the catalytic materials prepared by adopting the present synthesis method in small (<10 g batch) and large quantities (∼500 g batch) exhibited same physicochemical characteristics and CO oxidation activity. The present results might bring light to new opportunities in the development of high-performance OSC materials for TWCs. Further work is underway to explore the catalytic mechanism in detail and to expand the knowledge for designing better OSC materials.

Acknowledgements

The authors wish to thank Prof. Dr. W. Grünert, Ruhr University of Bochum, Germany for the research facilities to carry out ISS and CO oxidation measurements. Thanks are due to DST, New Delhi and DAAD, Germany for financial support under a bilateral collaboration program (DST-DAAD-PPP-2010).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details concerning the syntheses of the materials; their characterization by powder XRD, TPR, IS spectra, TEM, Raman spectra, XP spectra, and UV-vis DR spectra; OSC and catalytic carbon monoxide oxidation experiment. See DOI: 10.1039/c2cy20024d

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