Highly Reducible Nanostructured CeO 2 for CO Oxidation

: 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 N 2 adsorption-desorption. Ceria with different morphologies were evaluated as catalysts for CO oxidation. CeO 2 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 ( T 90 ) 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 CeO 2 nanorods. Another cause was their relatively large surface area.


Introduction
Ceria (CeO 2 ) 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, CeO 2 can be used as catalysts or non-inert support for heterogeneous catalysts [6][7][8], acting as an oxygen buffer through a fast Ce 3+ /Ce 4+ 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 CeO 2 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 CeO 2 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 CeO 2 particles. Gawade et al. [19] studied the water gas shift reactions catalyzed by Cu/CeO 2 and found that the CeO 2 nanoparticle supported catalyst achieved much higher CO conversion than a catalyst supported on CeO 2 nanorods. The results can be explained by the highly dispersed copper species over CeO 2 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]. CeO 2 is one of the CO oxidation catalysts that have attracted much attention in recent years [20]. Both size and morphology of CeO 2 particles has a significant impact on their catalytic performance in CO oxidation. Tana et al. [22] studied the morphology-dependent phenomenon of CeO 2 for CO oxidation. It was found that CeO 2 nanowires and nanorods, which mainly showed the reactive {110} and {100} planes, resulted in much higher activity for CO oxidation than CeO 2 particles. The CeO 2 nanowires that exposed more active planes exhibited the highest activity. González-Rovira et al. [23] prepared CeO 2 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 CeO 2 nanotubes was 400 times higher than CeO 2 particles for CO oxidation under certain conditions. Zhang et al. [24] synthesized cauliflower like CeO 2 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 CeO 2 nanorods that provided more active sites and oxygen vacancy for CO oxidation. Nowadays many papers have been published with the title 'CO oxidation on CeO 2 with different morphologies' [20]. However, different researchers usually got different activity results, even though the CeO 2 samples they prepared had similar morphologies. This may be attributed to the fact that the catalytic activity of CeO 2 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 CeO 2 .
In this paper, CeO 2 in nanoscale with different morphologies of rod, tube and cube were prepared through a hydrothermal process. The catalytic performance of these CeO 2 samples for CO oxidation was investigated. The results exhibited that the rod-like CeO 2 showed a far superior activity to the other two catalysts. The cause is discussed, and the highly reducible oxygen species on the surface of CeO 2 nanorods is considered to be the main reason. The patterns agree well with those of CeO 2 with fluorite structure (JCPDS Card No. 43-1002). There was no significant difference between the patterns of CeO 2 -R and CeO 2 -C, and the peaks were strong and sharp. However, the peaks in the pattern of CeO 2 -T were relatively weak and the full width at half maximum (FWHM) was larger than those of the other two samples. This indicates that CeO 2 -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 The morphologies of the three CeO 2 samples were characterized by SEM and TEM. As shown in Figure 2a1,a2, CeO 2 -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 CeO 2 -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. 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 c1 c2 d1 d2 CeO 2 -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 CeO 2 -T shows the side-opening tube structure. Zhao et al. [25] indicated that CeO 2 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 CeO 2 -T reveals that its tube wall was composed of small CeO 2 crystals, with diameters less than 20 nm, which are much smaller than those which formed the particles of CeO 2 -R or CeO 2 -C. This is consistent with the results of XRD. However, some ordered tube-like structures are also found in CeO 2 -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 CeO 2 -C. It can be clearly seen that CeO 2 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 CeO 2 -C, with length of the side about 40-280 nm, had smooth surfaces and ordered morphologies.

Catalyst Characterization
The textural properties of the three CeO 2 catalysts were investigated by N 2 adsorption-desorption at 196 • C, and the results are shown in Table 1 and Figure 3. The specific surface areas of CeO 2 -R (51.4 m 2 /g) and CeO 2 -T (55.6 m 2 /g) were much larger than that of CeO 2 -C (9.3 m 2 /g), which is due to the secondary pores that formed by the packing of CeO 2 particles with one-dimensional structures, both rod and tube. In addition, the surface area of CeO 2 -T was a little larger than that of CeO 2 -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 CeO 2 samples were in the mesoporous range, and the pore size distribution of CeO 2 -R was more uniform. The adsorption capacity of N 2 on CeO 2 -C was very small, which is accordance with its small surface area.
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.

CO Oxidation Catalyzed by CeO 2
The three CeO 2 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 CeO 2 catalysts and CeO 2 -R exhibited the highest activity. Moreover, the catalytic activity of CeO 2 -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. CeO 2 -T showed almost the same T 90 as CeO 2 -C, but its activity at low temperature was superior to the latter. Because of the relatively large specific surface area, CeO 2 with tube like morphologies usually exhibits higher activity in catalytic reaction than the others. Pan et al. [26] found that CeO 2 nanotubes gave the best activity for CO oxidation among CeO 2 with various morphologies, and they believed this could be due to its high surface area. Zhao et al. [27] also found CeO 2 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 CeO 2 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.  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  To find out the reasons for the difference between the catalytic performance of the three catalysts, H 2 -TPR measurements were carried out. The results are exhibited in Figure 5 and Table 2.  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.  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  There were two reduction peaks at 477 • C and 727 • C in TPR curve of CeO 2 -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 CeO 2 -T's, in the TPR curve of CeO 2 -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 H 2 consumption in relation to the reduction peaks, and it suggests that the bulk oxygen amounts in CeO 2 -T and CeO 2 -C were almost equal. However, the surface oxygen quantity in CeO 2 -C was much lower than that in CeO 2 -T. If the H 2 consumption of the peak at 303 • C is counted, the surface oxygen quantity of CeO 2 -C (401 µmol/g) approximately equaled that of CeO 2 -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 CeO 2 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 H 2 -TPR results, CeO 2 -C should have superior activity to CeO 2 -T for CO oxidation. However, the evaluation shows the opposite result and CeO 2 -T exhibited higher activity than CeO 2 -C. This may due to the large surface area of CeO 2 -T (55.6 m 2 /g) compared to CeO 2 -C (9.3 m 2 /g). Li et al. [29] prepared CeO 2 samples with different surface areas (67-205 m 2 /g) and found that the catalytic activity for CO oxidation increased with a specific surface area of CeO 2 . Guo et al. [30] also revealed similar results using CeO 2 catalysts with surface areas of 21-364 m 2 /g. Similar to CeO 2 -C, there were three reduction peaks in the TPR curve of CeO 2 -R. The bulk oxygen reduction temperature was 747 • C, and the surface oxygen reduction temperature decreased to 419 • C with a small H 2 consumption of 26 µmol/g. The third strongest reduction peak was at 217 • C with a H 2 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, CeO 2 -R had a relatively large specific surface area (51.4 m 2 /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 CeO 2 -R and CeO 2 -T particles. Figure 6a shows that the surface of CeO 2 -R was smooth and its growth direction was along the {110} orientation. While CeO 2 -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 CeO 2 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 CeO 2 -T for CO oxidation was inferior to that of CeO 2 -R, because the former was a combination of small particles and did not have the characteristics of CeO 2 nanotubes, though it had a tubular-like structure.
Catalysts 2018, 8, x FOR PEER REVIEW 7 of 11 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 m 2 /g) compared to CeO2-C (9.3 m 2 /g). Li et al. [29] prepared CeO2 samples with different surface areas (67-205 m 2 /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 m 2 /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 m 2 /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. 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 a b A comparison of the activity of CeO 2 -R with reported catalysts is illustrated in Table 3. 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 CeO 2 -R provided the lowest T 90 which means the best catalytic activity. However, it's worth noting that CeO 2 core-shell microspheres, reported by Zhang et al. [32], also exhibited excellent activity for CO oxidation with T 90 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 CeO 2 -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 CeO 2 samples for CO oxidation. Many factors, including morphology, size, texture properties, oxygen species reducibility, and/or something else, may influence the reaction synergistically. 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.

CeO 2 Nanorods
Preparation of CeO 2 nanorods was carried out via a hydrothermal process. Typically, 6 g of Ce(NO 3 ) 3 ·6H 2 O, 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 CeO 2 and denoted as CeO 2 -R.

CeO 2 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 CeCl 3 ·7H 2 O was added into the solution and stirred for 30 min. After that, NH 3 ·H 2 O 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 CeO 2 -T.

CeO 2 Nanocubes
Typically, 6 g of Ce(NO 3 ) 3 ·6H 2 O, 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 CeO 2 -C.

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 N 2 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 (H 2 -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 H 2 /Ar mixture (10%/90%, 50 mL/min) and the sample was heated to 1000 • C at an increasing rate of 10 • C/min. The H 2 consumption was tested using a thermal conductivity detector (TCD).

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:O 2 :N 2 = 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:

Conclusions
In summary, nanostructured CeO 2 samples with rod (CeO 2 -R), cubic (CeO 2 -C) and tube (CeO 2 -T) morphologies were prepared through a hydrothermal method. It was shown that CeO 2 -R exhibits the highest catalytic activity for CO oxidation (T 90 was 228 • C at space velocity of 12,000 mL·gcat −1 ·h −1 ) than the prepared samples with other morphologies, such as CeO 2 -T and CeO 2 -C. Characterization of the CeO 2 -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 CeO 2 -R sample possessed a large specific surface area (51.4 m 2 /g), that could be another reason of its superior activity.