Highly Reducible Nanostructured CeO₂ for CO Oxidation

Abstract

Ceria in nanoscale with different morphologies, rod, tube and cube, were prepared through a hydrothermal process. The structure, morphology and textural properties were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and isothermal N₂ adsorption-desorption. Ceria with different morphologies were evaluated as catalysts for CO oxidation. CeO₂ nanorods showed superior activity to the others. When space velocity was 12,000 mL·gcat⁻¹·h⁻¹, the reaction temperature for 90% CO conversion (T₉₀) was 228 °C. The main reason for the high activity was the existence of large amounts of easily reducible oxygen species, with a reduction temperature of 217 °C on the surface of CeO₂ nanorods. Another cause was their relatively large surface area.

1. Introduction

Ceria (CeO₂) is a significant rare earth oxide and has widespread applications. It can be used in the fields of luminescent materials [1], gas sensors [2], electronic ceramics [3], biology, medical science [4,5] and so on. In the field of catalysis, CeO₂ can be used as catalysts or non-inert support for heterogeneous catalysts [6,7,8], acting as an oxygen buffer through a fast Ce³⁺/Ce⁴⁺ cycle involving the participation of lattice oxygen. Therefore, it has been extensively studied and applied in many fields, such as three-way catalysts [9,10], fuel cells [11], water–gas shifts [12], ethanol dehydrogenation [13], CO oxidation [14], alcohol steam reforming [15] and photocatalysis [16]. Usually, the catalytic reactivity of CeO₂ not only depends on the particle size but is also closely related to its morphology [17]. Yuan et al. [18] prepared a Pd-based catalyst using CeO₂ nanotubes as support for oxidative carbonylation of phenol to diphenyl carbonate (DPC) and obtained higher activity and DPC selectivity than those of Pd catalyst supported on the zero-dimensional CeO₂ particles. Gawade et al. [19] studied the water gas shift reactions catalyzed by Cu/CeO₂ and found that the CeO₂ nanoparticle supported catalyst achieved much higher CO conversion than a catalyst supported on CeO₂ nanorods. The results can be explained by the highly dispersed copper species over CeO₂ nanoparticles that constitute the active sites for the water gas shift reaction.

CO catalytic oxidation is an effective pollutant removal technology and is also a typical probe reaction that is widely used for studying the catalyst structure, adsorption/desorption and the reaction mechanism [20]. There are two groups of catalysts for CO oxidation: non-noble metal catalysts and supported noble metal catalysts [21]. CeO₂ is one of the CO oxidation catalysts that have attracted much attention in recent years [20]. Both size and morphology of CeO₂ particles has a significant impact on their catalytic performance in CO oxidation. Tana et al. [22] studied the morphology-dependent phenomenon of CeO₂ for CO oxidation. It was found that CeO₂ nanowires and nanorods, which mainly showed the reactive {110} and {100} planes, resulted in much higher activity for CO oxidation than CeO₂ particles. The CeO₂ nanowires that exposed more active planes exhibited the highest activity. González-Rovira et al. [23] prepared CeO₂ nanomaterials with a tubular structure by an electrochemical method. The outer diameter of the nanotubes was about 200 nm with lengths between 30 and 40 μm. The activity of the CeO₂ nanotubes was 400 times higher than CeO₂ particles for CO oxidation under certain conditions. Zhang et al. [24] synthesized cauliflower like CeO₂ through the decomposition of Ce-BTC (BTC: 1,3,5-benzenetricarboxylic acid) straw and found that it exhibited excellent catalytic activity and stability in CO oxidation. The superior catalytic performance could be ascribed to the cauliflower like structure, which was composed of porous CeO₂ nanorods that provided more active sites and oxygen vacancy for CO oxidation. Nowadays many papers have been published with the title ‘CO oxidation on CeO₂ with different morphologies’ [20]. However, different researchers usually got different activity results, even though the CeO₂ samples they prepared had similar morphologies. This may be attributed to the fact that the catalytic activity of CeO₂ for CO oxidation is influenced by many factors. Among them, the amount of surface oxygen species (representation for OSC, Oxygen Storage Capacity) and its reducibility are vital characteristics to determine the properties of CeO₂.

In this paper, CeO₂ in nanoscale with different morphologies of rod, tube and cube were prepared through a hydrothermal process. The catalytic performance of these CeO₂ samples for CO oxidation was investigated. The results exhibited that the rod-like CeO₂ showed a far superior activity to the other two catalysts. The cause is discussed, and the highly reducible oxygen species on the surface of CeO₂ nanorods is considered to be the main reason.

2. Results and Discussion

2.1. Catalyst Characterization

XRD patterns of the three CeO₂ samples with different morphologies are shown in Figure 1. There were diffraction peaks at 2θ = 28.5°, 33.1°, 47.5°, 56.3°, 59.1°, 69.4°, 76.7° and 79.1°, which can be ascribed to the {111}, {200}, {220}, {311}, {222}, {400}, {331} and {420} planes of CeO₂, respectively. The patterns agree well with those of CeO₂ with fluorite structure (JCPDS Card No. 43-1002). There was no significant difference between the patterns of CeO₂-R and CeO₂-C, and the peaks were strong and sharp. However, the peaks in the pattern of CeO₂-T were relatively weak and the full width at half maximum (FWHM) was larger than those of the other two samples. This indicates that CeO₂-T is composed of small crystal particles.

The morphologies of the three CeO₂ samples were characterized by SEM and TEM. As shown in Figure 2a1,a2, CeO₂-R are rod-like particles with lengths of 300 nm–1 μm and diameters of 20–40 nm. Moreover, it can be seen from the SEM image of CeO₂-R, there were some plate-like particles. However, in the TEM image, it is almost impossible to find the particles with the ‘two dimensional’ structure. Maybe the ultrasonic treatment during the sample preparation for TEM measurement would be the cause for the disappearance of the special structure.

CeO₂-T particles represent the straight tube morphology with lengths of 1–5 μm and external diameters of 30–70 nm (Figure 2a2,b2). Some near-spherical shape particles, with sizes smaller than 30 nm are also found adhered to the outside of the tube. In addition, from the SEM image, part of the CeO₂-T shows the side-opening tube structure. Zhao et al. [25] indicated that CeO₂ nanotubes might be formed through a process of dissolution-recrystallization, anisotropic growth and self-crimping of the Ce(OH)₃ crystal seed. Under the influence of P123, Ce(OH)₃ grew along the {110} direction and presented a sheet structure. Then the self-crimping of the sheet structure occurred to reduce the surface energy and it ended with a tube structure. The unfinished self-crimping would lead to a side-opening tube structure. As shown in Figure 2b2, the TEM image of CeO₂-T reveals that its tube wall was composed of small CeO₂ crystals, with diameters less than 20 nm, which are much smaller than those which formed the particles of CeO₂-R or CeO₂-C. This is consistent with the results of XRD. However, some ordered tube-like structures are also found in CeO₂-T (Figure 2d1). The SAED (Selected Area Electron Diffraction) patterns are shown in Figure 2d2, and the bright point reflections indicate the tube-like particles observed were monocrystals.

Figure 2c1 gives the SEM image of CeO₂-C. It can be clearly seen that CeO₂ particles of different sizes with cubic morphology were formed along with many small particles. The TEM image, Figure 2c2, shows that the small particles also had cube like morphologies. The particles of CeO₂-C, with length of the side about 40–280 nm, had smooth surfaces and ordered morphologies.

The textural properties of the three CeO₂ catalysts were investigated by N₂ adsorption- desorption at 196 °C, and the results are shown in

Table 1. Textural properties of CeO₂ samples and their catalytic performance for CO oxidation.

Entry	Catalyst	SBET (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)	Reaction Temperature (°C) 1
					T10	T50	T90
1	CeO2-R	51.4	0.218	14.2	158	216	228
2	CeO2-T	55.6	0.158	11.5	203	278	366
3	CeO2-C	9.3	0.043	29.8	253	325	375

¹ m(Cat) = 500 mg; GHSV = 12,000 mL·gcat⁻¹·h⁻¹; T₁₀, T₅₀ and T₉₀ represent temperatures that CO conversions are 10%, 50% and 90%, respectively.

and Figure 3. The specific surface areas of CeO₂-R (51.4 m²/g) and CeO₂-T (55.6 m²/g) were much larger than that of CeO₂-C (9.3 m²/g), which is due to the secondary pores that formed by the packing of CeO₂ particles with one-dimensional structures, both rod and tube. In addition, the surface area of CeO₂-T was a little larger than that of CeO₂-R, which might be attributed to its tubular structure that has two surfaces of inside and outside. As shown in Figure 3, all of the three samples exhibited type IV adsorption isotherms with hysteresis loops of an H3 type, which were due to capillary condensation in mesoporous pores. It can also be seen in Figure 3 that the pores of all CeO₂ samples were in the mesoporous range, and the pore size distribution of CeO₂-R was more uniform. The adsorption capacity of N₂ on CeO₂-C was very small, which is accordance with its small surface area.

2.2. CO Oxidation Catalyzed by CeO₂

The three CeO₂ samples with different morphologies were evaluated for their activity in CO oxidation. The results are shown in Figure 4 and Table 1. The CO conversion increased with temperature over all three CeO₂ catalysts and CeO₂-R exhibited the highest activity. Moreover, the catalytic activity of CeO₂-R for CO oxidation was sensitive to reaction temperature. The CO conversion was 28.8% at 200 °C, and it increased promptly to 90.0% at 228 °C. CeO₂-T showed almost the same T₉₀ as CeO₂-C, but its activity at low temperature was superior to the latter. Because of the relatively large specific surface area, CeO₂ with tube like morphologies usually exhibits higher activity in catalytic reaction than the others. Pan et al. [26] found that CeO₂ nanotubes gave the best activity for CO oxidation among CeO₂ with various morphologies, and they believed this could be due to its high surface area. Zhao et al. [27] also found CeO₂ nanotubes demonstrated better catalytic activity on methylene blue decolorization, which could be explained by the exposure of higher active surface {110} and considerable defects on the surface of the CeO₂ nanotubes.

To find out the reasons for the difference between the catalytic performance of the three catalysts, H₂-TPR measurements were carried out. The results are exhibited in Figure 5 and

Table 2. H₂-TPR results of CeO₂ with different morphologies.

Sample	Peak Temperature (°C)			H2 Uptake (μmol/g)
	Surface O-II	Surface O	Bulk O	Surface O-II	Surface O	Bulk O
CeO2-T	---	477	727	---	415	697
CeO2-C	303	483	762	193	208	668
CeO2-R	217	419	747	436	26	665

.

There were two reduction peaks at 477 °C and 727 °C in TPR curve of CeO₂-T. The low temperature peak was attributed to the reduction of surface oxygen species. The latter, high temperature peak, was attributed to the lattice oxygen in the bulk phase [18]. Unlike CeO₂-T’s, in the TPR curve of CeO₂-C, there was a third peak at 303 °C besides the surface and bulk phase oxygen peak at 483 °C and 762 °C. Table 2 gives the H₂ consumption in relation to the reduction peaks, and it suggests that the bulk oxygen amounts in CeO₂-T and CeO₂-C were almost equal. However, the surface oxygen quantity in CeO₂-C was much lower than that in CeO₂-T. If the H₂ consumption of the peak at 303 °C is counted, the surface oxygen quantity of CeO₂-C (401 μmol/g) approximately equaled that of CeO₂-T (415 μmol/g). Thus, the peak at 303 °C should be attributed to another kind of surface oxygen species that can be reduced at much lower temperature, which could be called Surface O-II. It is usually considered that the surface oxygen of CeO₂ shows higher activity and can easily react with the adsorbed CO [27]. Gurbani et al. [28] indicated that the higher redox capacity of the catalyst, the higher activity for CO oxidation. Therefore, according to the H₂-TPR results, CeO₂-C should have superior activity to CeO₂-T for CO oxidation. However, the evaluation shows the opposite result and CeO₂-T exhibited higher activity than CeO₂-C. This may due to the large surface area of CeO₂-T (55.6 m²/g) compared to CeO₂-C (9.3 m²/g). Li et al. [29] prepared CeO₂ samples with different surface areas (67–205 m²/g) and found that the catalytic activity for CO oxidation increased with a specific surface area of CeO₂. Guo et al. [30] also revealed similar results using CeO₂ catalysts with surface areas of 21–364 m²/g.

Similar to CeO₂-C, there were three reduction peaks in the TPR curve of CeO₂-R. The bulk oxygen reduction temperature was 747 °C, and the surface oxygen reduction temperature decreased to 419 °C with a small H₂ consumption of 26 μmol/g. The third strongest reduction peak was at 217 °C with a H₂ consumption of 436 μmol/g. All the data above-mentioned reveal that there were a large amount of surface oxygen species which could be reduced easily and were more active for CO oxidation. Moreover, CeO₂-R had a relatively large specific surface area (51.4 m²/g) which was also one of the causes for its high activity.

In addition, High Resolution Transmission Electron Microscope (HRTEM) was conducted to confirm the shape and size of CeO₂-R and CeO₂-T particles. Figure 6a shows that the surface of CeO₂-R was smooth and its growth direction was along the {110} orientation. While CeO₂-T (Figure 6b) was composed of small particles, smaller than 20 nm, which assembled the hollow tubular structure. Zhou et al. [31] has indicated that CeO₂ nanorods had unusually exposed {001} and {110} planes which were more reactive for CO oxidation than the irregular nanoparticles, that exposed more of the stable {111} planes. Therefore, in this study, the catalytic activity of CeO₂-T for CO oxidation was inferior to that of CeO₂-R, because the former was a combination of small particles and did not have the characteristics of CeO₂ nanotubes, though it had a tubular-like structure.

A comparison of the activity of CeO₂-R with reported catalysts is illustrated in

Table 3. Comparison of different CeO₂ catalysts for CO oxidation.

Catalyst	SBET (m2/g)	TSOR1 (°C)	Feed Gas (mL·gcat−1·h−1)	T90 (°C)	References
CeO2-R	51.4	217	CO:O2:N2 = 1%/9%/90% (12,000)	228	This work
CeO2-R	51.4	217	CO:O2:N2 = 1%/9%/90% (18,000)	242	This work
CeO2-R	51.4	217	CO:O2:N2 = 1%/9%/90% (24,000)	253	This work
cauliflower like CeO2	74.5	490	CO:O2:He = 1%/20%/79% (60,000)	370 2	[24]
CeO2 nanowires	130	~527 3	CO:O2:He = 1%/21%/78% (9000)	300	[22]
CeO2 nanorods	128	~520 3	CO:O2:He = 1%/21%/78% (9000)	310	[22]
CeO2 nanoparticles	364	---	CO:O2:N2 = 1%/1%/98% (4800)	282	[30]
CeO2 core-shell microspheres	87.8	487	CO:O2:N2 = 1%/20%/79% (60,000)	275	[32]

¹ Surface oxygen species reduction temperature. ² The temperature for 98% CO conversion. ³ The temperature was obtained from the TPR curve directly because the authors did not give the exact data.

. Because the feed gas composition and space velocity were different, the values here allow only a qualitative comparison. It can be seen from the data in Table 3 that CeO₂-R provided the lowest T₉₀ which means the best catalytic activity. However, it’s worth noting that CeO₂ core-shell microspheres, reported by Zhang et al. [32], also exhibited excellent activity for CO oxidation with T₉₀ of 275 °C when space velocity was 60,000 mL·gcat⁻¹·h⁻¹, though the reduction temperature of surface oxygen species over their catalyst was much higher than that of CeO₂-R. Combined with our experimental results and data from references, it could be concluded that it’s not easy to determine the catalytic activities of CeO₂ samples for CO oxidation. Many factors, including morphology, size, texture properties, oxygen species reducibility, and/or something else, may influence the reaction synergistically.

3. Materials and Methods

3.1. Materials

All of the reagents were analytical grade and used without further purification. Polyethylene oxide-polypropylene oxide-polyethyleneoxide (PEO-PPO-PEO) triblock co-polymers (P123) was purchased from Sigma-Aldrich (ST. Louis, MO, USA). Cerium (III) chloride heptahydrate, cerium (III) nitrate nexahydrate, aqueous ammonia (25%) and sodium hydroxide were purchased from Sinoreagent (Shanghai, China).

3.2. Preparation of CeO₂ with Different Morphologies

3.2.1. CeO₂ Nanorods

Preparation of CeO₂ nanorods was carried out via a hydrothermal process. Typically, 6 g of Ce(NO₃)₃·6H₂O, 84 g of NaOH and 150 mL of deionized water were mixed and stirred for 30 min. Then, the mixture was transferred into an autoclave with a Teflon liner and underwent the hydrothermal reaction process at 110 °C for 24 h. After that, the precipitate was filtered, repeatedly washed with deionized water (Jingchun, Tianjin, China) and anhydrous ethanol (Kermel, Tianjin, China) several times until the pH reached 7. Finally, it was dried at 80 °C and calcined at 500 °C for 4 h. The obtained sample was rod-like CeO₂ and denoted as CeO₂-R.

3.2.2. CeO₂ Nanotubes

Firstly, 17.4 g of P123 with a molecule of 5800 was dissolved in a mixture of 60 mL ethanol and 60 mL deionized water in an ultrasonic water bath for 1 h at room temperature. Then, 5.58 g of CeCl₃·7H₂O was added into the solution and stirred for 30 min. After that, NH₃·H₂O was added dropwise until the pH was adjusted to 10 and a red flocculent precipitate was formed. After stirring for another 1 h, the mixture was transferred into an autoclave with a Teflon liner for the hydrothermal reaction process at 160 °C for 72 h. Thirdly, the precipitate was filtered, repeatedly washed with deionized water and anhydrous ethanol several times until the pH reached 7. Finally, the solid was dried at 80 °C and calcined at 500 °C for 4 h. The obtained sample was denoted as CeO₂-T.

3.2.3. CeO₂ Nanocubes

Typically, 6 g of Ce(NO₃)₃·6H₂O, 84 g of NaOH and 150 mL of deionized water were mixed and stirred for 30 min. Then, the mixture was transferred into an autoclave with a Teflon liner and underwent the hydrothermal reaction process at 160 °C for 24 h. After that, the precipitate was filtered, repeatedly washed with deionized water and anhydrous ethanol several times until the pH reached 7. Finally, it was dried at 80 °C and calcined at 500 °C for 4 h. The sample was denoted as CeO₂-C.

3.3. Catalyst Characterization

X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max-2500 X-ray diffractometer (Tokyo, Japan) with Cu Kαradiation (40 kV, 100 mA) at the scanning range of 5°–85°. The scanning electron microscopy (SEM) images were recorded on a FEI Nova NanoSEM 450 (Hillsboro, OR, USA) and all samples were gold-coated. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) were obtained with a FEI Tecnai G2 F20 (Hillsboro, OR, USA) at 200 kV.

The textural properties of the samples were measured by a N₂ adsorption–desorption method using a Micromeritics ASAP 2020 M+C physisorption analyzer (Norcross, GA, USA). The specific surface area was calculated by a BET method. Hydrogen temperature-programmed reduction (H₂-TPR) was carried out using a Micromeritics AutoChem II-2920 automated catalyst characterization system (Norcross, GA, USA). The sample (0.1 g) was purged in Ar (50 mL/min) at ambient temperature. After that, the flowing gas was changed to a H₂/Ar mixture (10%/90%, 50 mL/min) and the sample was heated to 1000 °C at an increasing rate of 10 °C/min. The H₂ consumption was tested using a thermal conductivity detector (TCD).

3.4. Catalytic Activity Tests

The catalytic activity test for CO oxidation was performed in a fixed-bed reactor (a stainless steel tube reactor with an inner diameter of 10 mm and length of 30 cm) at atmospheric pressure in the temperature range of 150–400 °C. The catalyst (500 mg, 40–60 mesh) was mounted in the reactor, and the reaction gas mixture (CO:O₂:N₂ = 1%:9%:90%) was fed through the catalyst bed at a designed flow rate.

The composition of the gas mixture was analyzed using an online Agilent 7890B gas chromatograph (Agilent Technologies, Santa Clara, CA, USA) equipped with a TCD detector. The CO conversion (X_CO) was calculated as follows:

$${\mathrm{X}}_{\mathrm{CO}}(\%)=\frac{\left[\mathrm{CO}{]}_{\mathrm{in}}-\right[\mathrm{CO}{]}_{\mathrm{out}}}{{[\mathrm{CO}]}_{\mathrm{in}}}\times 100$$

4. Conclusions

In summary, nanostructured CeO₂ samples with rod (CeO₂-R), cubic (CeO₂-C) and tube (CeO₂-T) morphologies were prepared through a hydrothermal method. It was shown that CeO₂-R exhibits the highest catalytic activity for CO oxidation (T₉₀ was 228 °C at space velocity of 12,000 mL·gcat⁻¹·h⁻¹) than the prepared samples with other morphologies, such as CeO₂-T and CeO₂-C. Characterization of the CeO₂-R catalyst has shown that there are much more easily reducible oxygen species, with a reduction temperature of 217 °C on its surface, which was responsible for the high catalytic activity. It should be emphasized that CeO₂-R sample possessed a large specific surface area (51.4 m²/g), that could be another reason of its superior activity.