Enhanced Oxygen Storage Capacity of Porous CeO2 by Rare Earth Doping

CeO2 is an important rare earth (RE) oxide and has served as a typical oxygen storage material in practical applications. In the present study, the oxygen storage capacity (OSC) of CeO2 was enhanced by doping with other rare earth ions (RE, RE = Yb, Y, Sm and La). A series of Undoped and RE–doped CeO2 with different doping levels were synthesized using a solvothermal method following a subsequent calcination process, in which just Ce(NO3)3∙6H2O, RE(NO3)3∙nH2O, ethylene glycol and water were used as raw materials. Surprisingly, the Undoped CeO2 was proved to be a porous material with a multilayered special morphology without any additional templates in this work. The lattice parameters of CeO2 were refined by the least–squares method with highly pure NaCl as the internal standard for peak position calibrations, and the solubility limits of RE ions into CeO2 were determined; the amounts of reducible–reoxidizable Cen+ ions were estimated by fitting the Ce 3d core–levels XPS spectra; the non–stoichiometric oxygen vacancy (VO) defects of CeO2 were analyzed qualitatively and quantitatively by O 1s XPS fitting and Raman scattering; and the OSC was quantified by the amount of H2 consumption per gram of CeO2 based on hydrogen temperature programmed reduction (H2–TPR) measurements. The maximum [OSC] of CeO2 appeared at 5 mol.% Yb–, 4 mol.% Y–, 4 mol.% Sm– and 7 mol.% La–doping with the values of 0.444, 0.387, 0.352 and 0.380 mmol H2/g by an increase of 93.04, 68.26, 53.04 and 65.22%. Moreover, the dominant factor for promoting the OSC of RE–doped CeO2 was analyzed.


Introduction
Rare earth (RE), known as "Industrial vitamin", "Industrial monosodium glutamate" and "Mother of new material", has irreplaceable excellent magnetic, optical, and electrical properties, playing a huge role in improving product performance, increasing product variety and improving production efficiency. Although the amount is small, it can greatly optimize the properties of materials. In view of its large effect and low dosage, RE has become an important national strategic resource in improving product structure, increasing technological content, and promoting industry technological progress, and is broadly utilized in many fields, such as metallurgy, military, petrochemical, glass ceramics, agriculture and new materials, and so on [1][2][3]. Cerium (Ce) is the most abundant RE element in the crust of Earth, which has good redox performance, so that its oxide (cerium oxide, CeO 2 ) where "V O " represents the oxygen vacancy defects produced via the vacancy compensation mechanism. Interestingly, CeO 2 can exhibit a large deviation from stoichiometry at low oxygen partial pressure, forming nonstoichiometric oxide CeO 2−x . Even after the loss of oxygen from the lattice and the consequent formation of numerous V O , CeO 2−x still retains a fluorite crystal structure [19,20] and captures oxygen by filling the V O upon exposure to oxygen, accompanied by the recovery of CeO 2 [21]. Moreover, the doping of other metallic elements into the CeO 2 lattice could control their structure and physical properties [22][23][24], such as rare-earth elements [25][26][27], transition elements [28][29][30] and alkaline earth elements [31][32][33]. In spite of the successful synthesis of CeO 2 -based composite oxides, most of the previous reports have focused on the investigation of catalytic performances [34,35], transport properties [36,37] and the origin of room-temperature ferromagnetism [38,39], the theoretical data about OSC were usually quite scattered, and only a few fundamental studies on the OSC of doped CeO 2 have been reported. For example, Singh [40] et al. synthesized a series of Ce 1−x M x O 2−σ (M = Zr, Ti, Pr, Y and Fe) nanocrystallites using the hydrothermal method using melamine and diethylenetriamine as complexing agents; up to 50% Zr and Y, 40% Ti, 25% Pr and 15% Fe were substituted for Ce 4+ in CeO 2 , and Ce 0.85 Fe 0.15 O 1.85 showed a higher OSC and higher CO conversion at a lower temperature than Ce 1−x Zr x O 2 . Ansari et al. [41] reported the redox properties of Fe-doped CeO 2 nanoparticles obtained by a polyol-assisted co-precipitation process, and the 10 mol.% Fe doped CeO 2 nanoparticles exhibited excellent reduction performance. Si et al. [42] prepared Ce 1−x Zr x O 2 (x = 0~0.8) powders via a mild urea hydrolysis based on the hydrothermal method, and validated a linear relationship between the lattice strain and the OSC value of CeO 2 -ZrO 2 solid solutions. Therefore, the microstructure and OSC of doped CeO 2 have to be understood at a fundamental level through a series of dopants to design advanced materials. For that, four rare earth elements (RE = Yb, Y, Sm and La) were selected as dopants to improve the OSC of CeO 2 based on the similarity-intermiscibility theory. In order to avoid the influence of other ions on the doping effect, we only used Ce(NO 3 ) 3 ·6H 2 O, RE(NO 3 ) 3 ·nH 2 O, ethylene glycol and water as raw materials. Moreover, all experimental conditions and the purity of raw materials were the same, so, the comparison of structure and properties of RE-doped CeO 2 was reliable and effective. Based on this, the influence of the dopant elements and their amounts on the non-stoichiometric V O and OSC were investigated and discussed. Surprisingly, the undoped CeO 2 was proved to be a porous material with a multilayered morphology without any additional templates, and the effect of RE-doping on morphology of CeO 2 also was investigated.

Synthesis of Undoped and RE-Doped CeO 2
Firstly, the desired amounts of Ce(NO 3 ) 3 ·6H 2 O and RE(NO 3 ) 3 ·nH 2 O (RE = Yb, Y, Sm and La) with different RE/(RE + Ce) (mol.%) were dissolved in a mixed solution of 25 mL ethylene glycol and 5 mL distilled water, the total amount of Ce 3+ and RE 3+ ions was 4.0 mmol. Then, the mixed solution was decanted into a 50 mL Teflon-lined stainless steel autoclave and sealed. Subsequently, the solvothermal process lasted for 24 h at 200 • C. After the reaction, the resulting precipitates were collected by centrifugation, and washed thrice alternately with distilled water and ethanol. At this point, the precursors synthesized by the hydrothermal process were obtained after drying in air at 80 • C for 12 h. Finally, a series of RE-doped CeO 2 powders were obtained by following calcination in air at 500 • C for 2 h. For comparison, the Undoped CeO 2 was synthesized using the same procedure, albeit in the absence of dopants RE(NO 3 ) 3 ·nH 2 O.

Characterization
The actual doping amounts of RE elements in CeO 2 were determined using an inductively coupled plasma-atomic emission spectrometer (ICP-AES, SPECTRO ARCOS EOP, Kleve, Germany). The crystallographic phases of samples were characterized by X-ray diffraction (XRD, Rigaku D/MAX 2200 PC, Rigaku, Japan) analysis using graphite monochromatized Cu Karadiation with 40 kV tube voltages and a 40 mA current. The morphologies of CeO 2 were observed by field-emission scanning electron microscopy (SEM, JEOL-7500F, Tokyo, Japan). The surface composition and binding energy of CeO 2 were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Scientific, Waltham, MA, USA). The natures of surface V O defects were identified using Raman spectroscopy (LabRAM Aramis, Horiba Jobin Yvon, Paris, France) with a He-Cd laser of 325 nm. N 2 adsorption-desorption isotherms were measured on a QuadraSorb SI (Quantachrome, Boynton Beach, FL, USA), and the specific surface areas were determined using the Brunauer-Emmett-Teller method.

Evaluation of OSC
For the Undoped and RE-doped CeO 2 samples synthesized using the hydrothermal process at 200 • C for 24 h, followed by calcination in air at 500 • C for 2 h, the hydrogen temperature programmed reduction (H 2 -TPR) measurements were employed to evaluate their OSC, which was carried out on a TP-5080 instrument with a thermal conductivity detector of gas chromatography. Typically, 50 mg CeO 2 powder was pre-treated in a 5% O 2 /N 2 stream at 500 • C for 1 h. After cooling down, the sample was purged with N 2 to remove the excess O 2 . Then, a flow of 5% H 2 /N 2 was introduced into the reactor with a flow rate of 30 mL/min, and the temperature was raised to~650 • C with a heating rate of 10 • C/min.

Results and Discussion
XRD analyses were employed to identify the phase composition and crystallographic structure of the as-obtained precursors and samples. Figure 1a showed the XRD patterns of the precursors synthesized using the solvothermal process at 200 • C for 24 h before calcination. For the precursor synthesized without RE, its major phase component was CeCO 3 OH (JCPDS no. 52-0352), and similar profiles were observed for these precursors synthesized with the introduction of 10 mol.% RE in the solvothermal process. Figure 1b showed the XRD patterns of Undoped and 10 mol.% RE-doped samples synthesized at 200 • C for 24 h after calcination in air at 500 • C for 2 h. All identified peaks had a good match with the standard CeO 2 pattern (cubic fluorite structure, JCPDS no. 34-0349), and the intensities of the corresponding diffraction peaks were comparable. Moreover, no impurity phases were detected, such as Yb 3 O 4 , Y 3 O 4 , Sm 3 O 4 and La 3 O 4 . The absence of RE impurity phases could be explained as follows. The RE impurity phases in the sample might exist as highly dispersed amorphous species. Another possibility was that the RE impurity ions partially substituted the host Ce ions to form a solid solution. Compared with Undoped CeO 2 , the relative diffraction intensities of 10 mol.% RE-doped CeO 2 showed no clear differences, suggesting that there was no preferential orientation or preferential crystal growth upon the incorporation of RE. In addition, compared with Undoped CeO 2 , a recognizable peak shift towards lower diffraction angles for 10 mol.% RE-doped CeO 2 was observed. These findings indicate that the larger RE impurity ions partially substituted the host Ce ions to form the RE-based solid solution based on Bragg's equation, and the cubic fluorite crystal structure of CeO 2 was maintained.

Results and Discussion
XRD analyses were employed to identify the phase composition and crystallographic structure of the as-obtained precursors and samples. Figure 1a showed the XRD patterns of the precursors synthesized using the solvothermal process at 200 °C for 24 h before calcination. For the precursor synthesized without RE, its major phase component was CeCO3OH (JCPDS no.52-0352), and similar profiles were observed for these precursors synthesized with the introduction of 10 mol.% RE in the solvothermal process. Figure 1b showed the XRD patterns of Undoped and 10 mol.% RE-doped samples synthesized at 200 °C for 24 h after calcination in air at 500 °C for 2 h. All identified peaks had a good match with the standard CeO2 pattern (cubic fluorite structure, JCPDS no. 34-0349), and the intensities of the corresponding diffraction peaks were comparable. Moreover, no impurity phases were detected, such as Yb3O4, Y3O4, Sm3O4 and La3O4. The absence of RE impurity phases could be explained as follows. The RE impurity phases in the sample might exist as highly dispersed amorphous species. Another possibility was that the RE impurity ions partially substituted the host Ce ions to form a solid solution. Compared with Undoped CeO2, the relative diffraction intensities of 10 mol.% RE-doped CeO2 showed no clear differences, suggesting that there was no preferential orientation or preferential crystal growth upon the incorporation of RE. In addition, compared with Undoped CeO2, a recognizable peak shift towards lower diffraction angles for 10 mol.% RE-doped CeO2 was observed. These findings indicate that the larger RE impurity ions partially substituted the host Ce ions to form the RE-based solid solution based on Bragg's equation, and the cubic fluorite crystal structure of CeO2 was maintained. When the impurity ions were introduced into the lattice of the matrix, its lattice parameter (a) would change. So, the change in the a value could be used to determine the solubility limit of these dopants in the matrix. In this work, the a value of CeO2 was refined by the least-squares method, in which the highly pure NaCl (≥99.999%) was selected as an internal standard to calibrate the peak position of CeO2. Figure 2a showed the XRD patterns of Undoped CeO2 and 10 mol.% RE-doped CeO2 with the internal standard of NaCl. It could be found that the diffraction intensities of (111) peak from CeO2 and (200) peak from NaCl were comparable, suggesting the feasibility of this internal standard method. Moreover, the a values of Undoped and 1~10 mol.% RE-doped CeO2 were calculated, and the calculated a as a function of RE contents in CeO2 were summarized in Figure 2b. From Figure 2b, the a values of all RE-doped CeO2 were greater than that of the Undoped one (5.4117 Å). Under the same doping concentration, the variation trend of a values was as follows: aYb < aY < aSm < aLa, which was consistent with the sequence of their ionic radii for CN8: RCe (0.97 Å) < RYb (0.98 Å) < RY (1.02 Å) < RSm (1.08 Å) < RLa (1.16 Å) according to Shannon's compilation [43]. The increased a values When the impurity ions were introduced into the lattice of the matrix, its lattice parameter (a) would change. So, the change in the a value could be used to determine the solubility limit of these dopants in the matrix. In this work, the a value of CeO 2 was refined by the least-squares method, in which the highly pure NaCl (≥99.999%) was selected as an internal standard to calibrate the peak position of CeO 2 . Figure 2a showed the XRD patterns of Undoped CeO 2 and 10 mol.% RE-doped CeO 2 with the internal standard of NaCl. It could be found that the diffraction intensities of (111) peak from CeO 2 and (200) peak from NaCl were comparable, suggesting the feasibility of this internal standard method. Moreover, the a values of Undoped and 1~10 mol.% RE-doped CeO 2 were calculated, and the calculated a as a function of RE contents in CeO 2 were summarized in Figure 2b. From Figure 2b, the a values of all RE-doped CeO 2 were greater than that of the Undoped one (5.4117 Å). Under the same doping concentration, the variation trend of a values was as follows: a Yb < a Y < a Sm < a La , which was consistent with the sequence of their ionic radii for CN8: R Ce (0.97 Å) < R Yb (0.98 Å) < R Y (1.02 Å) < R Sm (1.08 Å) < R La (1.16 Å) according to Shannon's compilation [43]. The increased a values after the introduction of RE indicated that the partial host Ce 4+ (0.97 Å) ions substituted by the larger RE ions and the local lattice expansion of CeO 2 crystal occurred as a result. Moreover, the a values linearly increased with increasing RE contents, reached a maximum at 5, 4, 4 and 7 mol.% for Yb, Y, Sm and La, before decreasing and maintaining a certain level for higher RE contents. This would indicated that the solubility limits of Yb, Y, Sm and La ions in CeO 2 were 5, 4, 4 and 7 mol.%.
after the introduction of RE indicated that the partial host Ce 4+ (0.97 Å) ions substituted by the larger RE ions and the local lattice expansion of CeO2 crystal occurred as a result. Moreover, the a values linearly increased with increasing RE contents, reached a maximum at 5, 4, 4 and 7 mol.% for Yb, Y, Sm and La, before decreasing and maintaining a certain level for higher RE contents. This would indicated that the solubility limits of Yb, Y, Sm and La ions in CeO2 were 5, 4, 4 and 7 mol.%. In order to further confirm the incorporation of RE ions and their effect on the CeO2 lattice, high-resolution electron microscopy (HR-TEM) was performed and the corresponding HR-TEM images of Undoped and 10 mol.% RE-doped CeO2 were synthesized using the hydrothermal process at 200 °C for 24 h, followed by calcination in air at 500 °C for 2 h, as shown in Figure 3. From the HR-TEM image of Undoped CeO2 in Figure 3a, the interplanar spacing was measured with a value of 0.3110 nm, which fitted well with the (111) plane of cubic CeO2, proving the generation of the CeO2 phase. After the incorporation of 10 mol.% RE (RE = Yb, Y, Sm and La), the interplanar spacings of CeO2 in Figure 3b-e had increased to 0.3178, 0.3202, 0.3209 and 0.3231 nm, respectively. Combined with XRD analysis results in Figure 2, both the local lattice expansion and the increased interplanar spacing indicated that these large RE (RYb = 0.98 Å; RY = 1.02 Å; RSm = 1.08 Å; RLa = 1.16 Å) impurity ions partially substituted the host Ce ions (RCe = 0.97 Å), and a solid solution was formed. Importantly, the size of the RE impurity ions was consistent with the trends of interplanar spacing. In other words, the larger the size of the doped RE ion, the greater the interplanar spacing of the as-obtained RE-doped CeO2. In addition, the practical RE contents in CeO2 were measured by ICP-AES, and the results are shown in Table 1. As observed in Table 1, it could be found that the practical RE contents in CeO2 were close to the corresponding nominal doped one. In order to further confirm the incorporation of RE ions and their effect on the CeO 2 lattice, high-resolution electron microscopy (HR-TEM) was performed and the corresponding HR-TEM images of Undoped and 10 mol.% RE-doped CeO 2 were synthesized using the hydrothermal process at 200 • C for 24 h, followed by calcination in air at 500 • C for 2 h, as shown in Figure 3. From the HR-TEM image of Undoped CeO 2 in Figure 3a, the interplanar spacing was measured with a value of 0.3110 nm, which fitted well with the (111) plane of cubic CeO 2 , proving the generation of the CeO 2 phase. After the incorporation of 10 mol.% RE (RE = Yb, Y, Sm and La), the interplanar spacings of CeO 2 in Figure 3b-e had increased to 0.3178, 0.3202, 0.3209 and 0.3231 nm, respectively. Combined with XRD analysis results in Figure 2, both the local lattice expansion and the increased interplanar spacing indicated that these large RE (R Yb = 0.98 Å; R Y = 1.02 Å; R Sm = 1.08 Å; R La = 1.16 Å) impurity ions partially substituted the host Ce ions (R Ce = 0.97 Å), and a solid solution was formed. Importantly, the size of the RE impurity ions was consistent with the trends of interplanar spacing. In other words, the larger the size of the doped RE ion, the greater the interplanar spacing of the as-obtained RE-doped CeO 2 . In addition, the practical RE contents in CeO 2 were measured by ICP-AES, and the results are shown in Table 1. As observed in Table 1, it could be found that the practical RE contents in CeO 2 were close to the corresponding nominal doped one.
XPS analysis was employed to probe the surface chemical composition and various oxidation states before and after RE-doping. Figure 4a-e shows the wide-scan XPS spectra of Undoped and 4 mol.% RE-doped CeO 2 synthesized using the hydrothermal process at 200 • C for 24 h and followed by calcination in air at 500 • C for 2 h, respectively. As observed, all wide-scan XPS spectra showed the clear CeO 2 features by the signals of Ce 3d, Ce 4d and O 1s, in good agreement with those XPS patterns of Gd- [44], Y- [45] and Dy- [46] doped CeO 2 . It is worth noting that the obvious C 1s peaks located at~284.8 eV were derived from adventitious carbon to calibrate the tested samples. Moreover, the faint RE 3d or RE 4d signals can be seen in the red dotted box in Figure 4b-e, and the corresponding Yb 4d, Y 3d, Sm 3d and La 3d XPS regions were recorded, as shown in Figure 4f-i, respectively. The characteristic peaks in Figure 4f-i implied that the Yb, Y, Sm and La elements were in +3 states. It indicated that the Yb, Y, Sm and La elements had been successfully incorporated into the CeO 2 lattice with positive trivalent states (RE 3+ ). Molecules 2023, 28, x FOR PEER REVIEW 6 of 17  XPS analysis was employed to probe the surface chemical composition and various oxidation states before and after RE-doping. Figure 4a-e shows the wide-scan XPS spectra of Undoped and 4 mol.% RE-doped CeO2 synthesized using the hydrothermal process at 200 °C for 24 h and followed by calcination in air at 500 °C for 2 h, respectively. As observed, all wide-scan XPS spectra showed the clear CeO2 features by the signals of Ce 3d, Ce 4d and O 1s, in good agreement with those XPS patterns of Gd- [44], Y- [45] and Dy- [46] doped CeO2. It is worth noting that the obvious C 1s peaks located at ~284.8 eV were derived from adventitious carbon to calibrate the tested samples. Moreover, the faint RE 3d or RE 4d signals can be seen in the red dotted box in Figure 4b-e, and the corresponding Yb 4d, Y 3d, Sm 3d and La 3d XPS regions were recorded, as shown in Figure 4f-i, respectively. The characteristic peaks in Figure 4f-i implied that the Yb, Y, Sm and La elements were in +3 states. It indicated that the Yb, Y, Sm and La elements had been successfully incorporated into the CeO2 lattice with positive trivalent states (RE 3+ ).   In order to understand the effect of RE-doping on Ce ions in the CeO2 crystal, the Ce 3d XPS regions of Undoped and 4 mol.% RE-doped CeO2 were recorded and fitted, as shown in Figure 5a-e. The Ce 3d XPS core-levels of all CeO2 samples were fitted into eight peaks, corresponding to four pairs of spin-orbit doublets (u1-4 and v1-4) of Ce ions, in which the ui and vi bands corresponded to the contributions of Ce 3d3/2 and Ce 3d5/2. Moreover, the bands of u4, u3 and u1 (and those for v4, v3, v1) were attributed to the Ce 4+ state, while u2 and v2 were due to the Ce 3+ state [47]. Meanwhile, the relative concentration of Ce 3+ ions in CeO2, labeled as [Ce 3+ ]XPS, could be calculated by the ratio of integrated peak areas of the peak related to the Ce 3+ species (u2 and v2 peaks) to that of all peaks (u1-4 and v1-4 peaks) in Figure 5, and the results were summarized in Table 2. As observed, the [Ce 3+ ]XPS values of 4 mol.% Yb, Y, Sm and La-doped CeO2 were 13.78, 12.60, 10.94 and 9.78%, respectively, higher than that of Undoped CeO2 (6.54%), which indicates that Undoped CeO2 itself contained a certain number of Ce 3+ ions, and RE-doping could In order to understand the effect of RE-doping on Ce ions in the CeO 2 crystal, the Ce 3d XPS regions of Undoped and 4 mol.% RE-doped CeO 2 were recorded and fitted, as shown in Figure 5a-e. The Ce 3d XPS core-levels of all CeO 2 samples were fitted into eight peaks, corresponding to four pairs of spin-orbit doublets (u 1-4 and v 1-4 ) of Ce ions, in which the u i and v i bands corresponded to the contributions of Ce 3d 3/2 and Ce 3d 5/2 . Moreover, the bands of u 4 , u 3 and u 1 (and those for v 4 , v 3 , v 1 ) were attributed to the Ce 4+ state, while u 2 and v 2 were due to the Ce 3+ state [47]. Meanwhile, the relative concentration of Ce 3+ ions in CeO 2 , labeled as [Ce 3+ ] XPS , could be calculated by the ratio of integrated peak areas of the peak related to the Ce 3+ species (u 2 and v 2 peaks) to that of all peaks (u 1-4 and v 1-4 peaks) in Figure 5, and the results were summarized in Table 2. As observed, the [Ce 3+ ] XPS values of 4 mol.% Yb, Y, Sm and La-doped CeO 2 were 13.78, 12.60, 10.94 and 9.78%, respectively, higher than that of Undoped CeO 2 (6.54%), which indicates that Undoped CeO 2 itself contained a certain number of Ce 3+ ions, and RE-doping could promote the formation of Ce 3+ species. In other words, the amount of reducible-reoxidizable Ce n+ (namely, Ce 3+ ⇔ Ce 4+ ) ions increased with the introduction of RE ions into CeO 2 lattice, indicating that RE-doping was conducive to improving the redox capacity of CeO 2 .  To investigate the chemical states of O in CeO2, the O 1s core-level XPS spectra of Undoped and 4 mol.% RE-doped CeO2 were recorded and fitted, as shown in Figure 6ae. For Undoped CeO2 in Figure 6a, its O 1s XPS spectrum could be curve-fitted into three peaks, indicating the presence of three kinds of oxygen species in CeO2. The peaks with a binding energy of ~529.8 and ~528.4 eV could be assigned to lattice oxygen of O-Ce(Ⅳ) species and O-Ce(Ⅲ) species, respectively, whereas that of ~531.6 eV (yellow region peak) could be assigned to the chemisorption of oxygen or/and weakly bonded oxygen species related to VO defects. For the O 1s spectra of RE-doped CeO2 in Figure 6b-e, besides the above three peaks, a new curve fitting could be observed, which might be attributed to the corresponding O-RE species, namely, the O-Yb species at ~527.6 eV, O-Y species at ~528.2 eV, O-Sm species at ~528.2 eV and O-La species at ~532.9 eV. Furthermore, the relative VO content could be estimated by the ratio of the integrated area of the peak related to the VO defect (yellow region peak in Figure 6a-e) to that of all peaks, labeled as [VO]XPS, and the results were summarized in Table 2. As observed in Table 2, the  To investigate the chemical states of O in CeO 2 , the O 1s core-level XPS spectra of Undoped and 4 mol.% RE-doped CeO 2 were recorded and fitted, as shown in Figure 6a-e. For Undoped CeO 2 in Figure 6a, its O 1s XPS spectrum could be curve-fitted into three peaks, indicating the presence of three kinds of oxygen species in CeO 2 . The peaks with a binding energy of~529.8 and~528.4 eV could be assigned to lattice oxygen of O-Ce(IV) species and O-Ce(III) species, respectively, whereas that of~531.6 eV (yellow region peak) could be assigned to the chemisorption of oxygen or/and weakly bonded oxygen species related to V O defects. For the O 1s spectra of RE-doped CeO 2 in Figure 6b-e, besides the above three peaks, a new curve fitting could be observed, which might be attributed to the corresponding O-RE species, namely, the O-Yb species at~527.6 eV, O-Y species at 528.2 eV, O-Sm species at~528.2 eV and O-La species at~532.9 eV. Furthermore, the relative V O content could be estimated by the ratio of the integrated area of the peak related to the V O defect (yellow region peak in Figure 6a-e) to that of all peaks, labeled as [V O ] XPS , and the results were summarized in Table 2. As observed in Table 2 From the results of XPS analyses in Figures 4-6 and Table 2, it could be concluded that RE elements were successfully incorporated into the CeO2 lattice with positive trivalent states, and RE-doping could increase the amount of redox Ce n+ (Ce 3+ /Ce 4+ ) of CeO2, as well as the VO defects. Due to its sensitivity to the VO defect, Raman scattering was employed to investigate the structure of Undoped and RE-doped CeO2 synthesized using the hydrothermal process at 200 °C for 24 h and followed by calcination in air at 500 °C for 2 h [48,49]. For the Undoped CeO2 in Figure 7a, the peak at ~458 cm −1 was attributed to the triply degenerate F2g mode from the symmetric O-Ce-O stretching mode [50], while the weak peak at ~592 cm −1 was assigned to the optical LO mode related to VO defects [51][52][53]. Upon the incorporation of RE 3+ ions into the CeO2 lattice, the band intensity of the F2g mode decreased, while that of the LO mode related to the VO defect increased (Figure 7b-e). It indicated that Undoped CeO2 itself had a certain number of VO defects and RE-doping could favor the presence of substoichiometric CeO2-x underscoring an increase in VO defects, as consistent with the analysis results of O 1s core-level XPS spectra in Figure 6. Due to its sensitivity to the V O defect, Raman scattering was employed to investigate the structure of Undoped and RE-doped CeO 2 synthesized using the hydrothermal process at 200 • C for 24 h and followed by calcination in air at 500 • C for 2 h [48,49]. For the Undoped CeO 2 in Figure 7a, the peak at~458 cm −1 was attributed to the triply degenerate F 2g mode from the symmetric O-Ce-O stretching mode [50], while the weak peak at 592 cm −1 was assigned to the optical LO mode related to V O defects [51][52][53]. Upon the incorporation of RE 3+ ions into the CeO 2 lattice, the band intensity of the F 2g mode decreased, while that of the LO mode related to the V O defect increased (Figure 7b The band at ~590 cm −1 in Raman spectra was known to be associate defect and has been widely observed in substoichiometric CeO2−x [54]. F the band intensity of both the F2g and LO modes obviously changed upon tion of RE 3+ ions into the CeO2 lattice, which was attributed to the increa tortion caused by RE-doping and hence interfered with the vibrations of C the quantitative analysis of VO defects difficult. For this, an alternativ quantitatively estimate the relative contents of VO defects was adopted by integrated area of the LO mode to that of the F2g mode from the Raman sp showed the calculated relative VO concentrations of Undoped and 1~9 mo CeO2 synthesized by the hydrothermal process at 200 °C for 24 h and fol nation in air at 500 °C for 2 h. As observed, there existed a certain amount o Undoped CeO2, and the calculated value was 0.67, consistent with the ana the O 1s core-level XPS spectra in Figure 6. These intrinsic VO defec evolved from the redox cycle of Ce n+ in CeO2 (Ce 3+ ⇔ Ce 4+ ). The relative VO increased almost linearly with increasing RE contents, and reached maxim RE contents were 5, 4, 4 and 7 mol.% for Yb, Y, Sm and La-doped CeO2, decreased above this doping level. Before this turning point, the variation tive VO concentration under the same doping concentration was as follow La, which was consistent with their electronegativity: χYb (1.26) > χY (1.22 χCe (1.12) > χLa (1.11). After the RE 3+ ions substituted the host Ce ions into th the bigger its electronegativity, the stronger its ability to attract the surroun to itself, and the surrounding O 2− anions lost electrons more easily, thus r trinsic VO defects. The band at~590 cm −1 in Raman spectra was known to be associated with the V O defect and has been widely observed in substoichiometric CeO 2−x [54]. From Figure 7a, the band intensity of both the F 2g and LO modes obviously changed upon the incorporation of RE 3+ ions into the CeO 2 lattice, which was attributed to the increased lattice distortion caused by RE-doping and hence interfered with the vibrations of CeO 2−x . It made the quantitative analysis of V O defects difficult. For this, an alternative approach to quantitatively estimate the relative contents of V O defects was adopted by the ratio of the integrated area of the LO mode to that of the F 2g mode from the Raman spectra. Figure 8 showed the calculated relative V O concentrations of Undoped and 1~9 mol.% RE-doped CeO 2 synthesized by the hydrothermal process at 200 • C for 24 h and followed by calcination in air at 500 • C for 2 h. As observed, there existed a certain amount of V O defects in Undoped CeO 2 , and the calculated value was 0.67, consistent with the analysis results of the O 1s core-level XPS spectra in Figure 6. These intrinsic V O defects might have evolved from the redox cycle of Ce n+ in CeO 2 (Ce 3+ ⇔ Ce 4+ ). The relative V O concentrations increased almost linearly with increasing RE contents, and reached maximum when the RE contents were 5, 4, 4 and 7 mol.% for Yb, Y, Sm and La-doped CeO 2 , and gradually decreased above this doping level. Before this turning point, the variation trend of relative V O concentration under the same doping concentration was as follows: Yb > Y > Sm > La, which was consistent with their electronegativity: χ Yb (1.26) > χ Y (1.22) > χ Sm (1.17) > χ Ce (1.12) > χ La (1.11). After the RE 3+ ions substituted the host Ce ions into the CeO 2 lattice, the bigger its electronegativity, the stronger its ability to attract the surrounding electrons to itself, and the surrounding O 2− anions lost electrons more easily, thus resulting in extrinsic V O defects. H2-TPR measurements were employed to evaluate the OSC of CeO2. Figure 9a-e illustrated the H2-TPR profiles of Undoped and 4 mol.% RE-doped CeO2 (RE = Yb, Y, Sm and La) synthesized using the hydrothermal process at 200 °C for 24 h and followed by calcination in air at 500 °C for 2 h. For all CeO2 samples in Figure 9, one can clearly find a distinct H2 reduction band from 30 to 610 °C, with the strongest H2 reduction peak at ~510 °C; the maximum H2 consumption occurred at 510 °C and then decreased until ~600 °C, and after that it tended to rise. The reduction band from 30 °C to ~600 °C could be attributed to the reduction in surface/subsurface lattice oxygen, which was consistent with these reported results [55,56]. Before 200 °C, the RE-doped CeO2 in Figure 9b-e exhibited more H2 consumption than that of the Undoped CeO2; especially for 4 mol.% Y, Sm and La-doped CeO2, a minima at 170 °C occurred. This indicated that the specific surface area of CeO2 played a dominant role in its OSC at low temperatures. To prove this conjecture, we tested the specific surface areas of 4 mol.% Yb, Y, Sm and La-doped CeO2, and the results were summarized in Table 2. The specific surface areas of 4 mol.% Y, Sm and La-doped CeO2 were 98.1, 112.6 and 104.6 m 2 /g, respectively, higher than that of Undoped CeO2 (96.0 m 2 /g); however, these decreased after 4 mol.% Yb-doping (89.7 m 2 /g). Moreover, compared to Undoped CeO2 in Figure 9a, there appeared to be a visible shoulder from ~350 °C in the H2-TPR profiles of RE-doped CeO2 in Figure 9b-e, and the reduction bands of RE-doped CeO2 at ~600 °C were far higher than the baseline. These phenomena suggested that RE-doping optimized the surface states of CeO2, thereby enhancing its OSC.  Figure 9, one can clearly find a distinct H 2 reduction band from 30 to 610 • C, with the strongest H 2 reduction peak at~510 • C; the maximum H 2 consumption occurred at 510 • C and then decreased until~600 • C, and after that it tended to rise. The reduction band from 30 • C to~600 • C could be attributed to the reduction in surface/subsurface lattice oxygen, which was consistent with these reported results [55,56]. Before 200 • C, the RE-doped CeO 2 in Figure 9b-e exhibited more H 2 consumption than that of the Undoped CeO 2 ; especially for 4 mol.% Y, Sm and La-doped CeO 2 , a minima at 170 • C occurred. This indicated that the specific surface area of CeO 2 played a dominant role in its OSC at low temperatures. To prove this conjecture, we tested the specific surface areas of 4 mol.% Yb, Y, Sm and La-doped CeO 2 , and the results were summarized in Table 2. The specific surface areas of 4 mol.% Y, Sm and La-doped CeO 2 were 98.1, 112.6 and 104.6 m 2 /g, respectively, higher than that of Undoped CeO 2 (96.0 m 2 /g); however, these decreased after 4 mol.% Yb-doping (89.7 m 2 /g). Moreover, compared to Undoped CeO 2 in Figure 9a, there appeared to be a visible shoulder from~350 • C in the H 2 -TPR profiles of RE-doped CeO 2 in Figure 9b-e, and the reduction bands of RE-doped CeO 2 at~600 • C were far higher than the baseline. These phenomena suggested that RE-doping optimized the surface states of CeO 2 , thereby enhancing its OSC. H2-TPR measurements were employed to evaluate the OSC of CeO2. Figure 9a-e illustrated the H2-TPR profiles of Undoped and 4 mol.% RE-doped CeO2 (RE = Yb, Y, Sm and La) synthesized using the hydrothermal process at 200 °C for 24 h and followed by calcination in air at 500 °C for 2 h. For all CeO2 samples in Figure 9, one can clearly find a distinct H2 reduction band from 30 to 610 °C, with the strongest H2 reduction peak at ~510 °C; the maximum H2 consumption occurred at 510 °C and then decreased until ~600 °C, and after that it tended to rise. The reduction band from 30 °C to ~600 °C could be attributed to the reduction in surface/subsurface lattice oxygen, which was consistent with these reported results [55,56]. Before 200 °C, the RE-doped CeO2 in Figure 9b-e exhibited more H2 consumption than that of the Undoped CeO2; especially for 4 mol.% Y, Sm and La-doped CeO2, a minima at 170 °C occurred. This indicated that the specific surface area of CeO2 played a dominant role in its OSC at low temperatures. To prove this conjecture, we tested the specific surface areas of 4 mol.% Yb, Y, Sm and La-doped CeO2, and the results were summarized in Table 2. The specific surface areas of 4 mol.% Y, Sm and La-doped CeO2 were 98.1, 112.6 and 104.6 m 2 /g, respectively, higher than that of Undoped CeO2 (96.0 m 2 /g); however, these decreased after 4 mol.% Yb-doping (89.7 m 2 /g). Moreover, compared to Undoped CeO2 in Figure 9a, there appeared to be a visible shoulder from ~350 °C in the H2-TPR profiles of RE-doped CeO2 in Figure 9b-e, and the reduction bands of RE-doped CeO2 at ~600 °C were far higher than the baseline. These phenomena suggested that RE-doping optimized the surface states of CeO2, thereby enhancing its OSC. OSC was the fundamental performance of CeO 2 and CeO 2 -based oxygen storage materials; so, the quantification of OSC was the key to evaluate their oxygen storage/release property. For that, the OSC was quantified using the amount of H 2 consumption per gram of CeO 2 powders by measuring the corresponding peak areas of H 2 -TPR profiles in this work. The quantified OSC (labeled as [OSC], mmol H 2 /g CeO 2 ) from 30 • C to~600 • C, which was the value of H 2 consumption per gram of CeO 2 powders, is shown in Figure 10. The [OSC] of Undoped CeO 2 was 0.23 mmol H 2 /g, indicating that pure CeO 2 itself possessed a certain OSC, which was attributed to the unique structure of its intrinsic V O defect or the redox cycle of Ce 3+ ⇔ Ce 4+ , supported by the XPS analyses in Figures 5 and 6 and Raman analyses in Figures 7 and 8 .22% compared with that of the Undoped one (0.230 mmol H 2 /g). These findings indicate that RE-doping could effectively improve the OSC of CeO 2 , combined with the H 2 -TPR curves. This enhanced OSC of RE-doped CeO 2 could be explained as follows. When RE 3+ ions were doped into the CeO 2 lattice to substitute host Ce 4+ ions, more V O defects would be generated to keep the electric neutrality of the fluorite structure, and a substoichiometric solid solution Ce 1-x RE x O 2−σ (RE = Yb, Y, Sm and La) was formed based on RE-doping. During the H 2 reduction of H 2 -TPR, H 2 reacted with a chemisorbed oxygen from the CeO 2 surface, which was fixed by intrinsic and extrinsic V O defects on the CeO 2 surface. As the surface chemisorbed oxygen was gradually consumed, the intrinsic and extrinsic V O defects were exposed, and the bulk lattice oxygen began to move to the CeO 2 surface for replenishment by the V O defects. The oxygen in the bulk RE-doped CeO 2 diffused more easily to the surface to fill the V O defects than that in the Undoped CeO 2 due to the activation effect of RE 3+ dopants which induced oxygen mobility [57].
OSC was the fundamental performance of CeO2 and CeO2-based oxygen materials; so, the quantification of OSC was the key to evaluate their oxyg age/release property. For that, the OSC was quantified using the amount of sumption per gram of CeO2 powders by measuring the corresponding peak area TPR profiles in this work. The quantified OSC (labeled as [OSC], mmol H2/g CeO 30 °C to ~600 °C, which was the value of H2 consumption per gram of CeO2 pow shown in Figure 10. The [OSC] of Undoped CeO2 was 0.23 mmol H2/g, indicat pure CeO2 itself possessed a certain OSC, which was attributed to the unique s of its intrinsic VO defect or the redox cycle of Ce 3+ ⇔ Ce 4+ , supported by the XPS in Figures 5 and 6 and Raman analyses in Figures 7 and 8 65.22% compared with that of the Undoped one (0.230 mmol H2/g). These findin cate that RE-doping could effectively improve the OSC of CeO2, combined with TPR curves. This enhanced OSC of RE-doped CeO2 could be explained as follow RE 3+ ions were doped into the CeO2 lattice to substitute host Ce 4+ ions, more VO would be generated to keep the electric neutrality of the fluorite structure, an stoichiometric solid solution Ce1-xRExO2−σ (RE = Yb, Y, Sm and La) was formed b RE-doping. During the H2 reduction of H2-TPR, H2 reacted with a chemisorbed from the CeO2 surface, which was fixed by intrinsic and extrinsic VO defects on t surface. As the surface chemisorbed oxygen was gradually consumed, the intri extrinsic VO defects were exposed, and the bulk lattice oxygen began to move to t surface for replenishment by the VO defects. The oxygen in the bulk RE-dop diffused more easily to the surface to fill the VO defects than that in the Undop due to the activation effect of RE 3+ dopants which induced oxygen mobility [57].  In order to investigate the effect of RE-doping on the morphology of CeO 2 , SEM was employed. Figure 11a-e showed the SEM images of Undoped and 10 mol.% Yb, Y, Sm and La-doped CeO 2 particles synthesized using the hydrothermal process at 200 • C for 24 h and followed by calcination in air at 500 • C for 2 h, respectively. From Figure 11a, it could be seen that the morphology of the Undoped CeO 2 particle was a multilayered structure consisting of flakes, and these flakes intertwined to form an open porous structure. After the incorporation of 10 mol.% RE (RE = Yb, Y, Sm and La) into CeO 2 , the multilayered morphology was still maintained, as seen in Figure 11b-e. This finding indicates that the low concentration of RE-doping had little effect on the morphology of CeO 2 . Generally, CeO 2 with a porous structure or special morphology was usually synthesized by a templatebased method, in which either surfactants as soft templates or other porous inorganic material as hard templates were used. Surprisingly, the porous CeO 2 with a multilayered morphology was obtained without any additional templates in this work. The abundant porous structure and highly specific surface area would undoubtedly enhance the OSC of CeO 2 . Further analysis of the porous structures was conducted using an N 2 adsorptiondesorption isotherm, as discussed later.
Molecules 2023, 28, x FOR PEER REVIEW 13 of 17 In order to investigate the effect of RE-doping on the morphology of CeO2, SEM was employed. Figure 11a-e showed the SEM images of Undoped and 10 mol.% Yb, Y, Sm and La-doped CeO2 particles synthesized using the hydrothermal process at 200 °C for 24 h and followed by calcination in air at 500 °C for 2 h, respectively. From Figure 11a, it could be seen that the morphology of the Undoped CeO2 particle was a multilayered structure consisting of flakes, and these flakes intertwined to form an open porous structure. After the incorporation of 10 mol.% RE (RE = Yb, Y, Sm and La) into CeO2, the multilayered morphology was still maintained, as seen in Figure 11be. This finding indicates that the low concentration of RE-doping had little effect on the morphology of CeO2. Generally, CeO2 with a porous structure or special morphology was usually synthesized by a template-based method, in which either surfactants as soft templates or other porous inorganic material as hard templates were used. Surprisingly, the porous CeO2 with a multilayered morphology was obtained without any additional templates in this work. The abundant porous structure and highly specific surface area would undoubtedly enhance the OSC of CeO2. Further analysis of the porous structures was conducted using an N2 adsorption-desorption isotherm, as discussed later. In order to further demonstrate the porous structure of CeO2, an N2 adsorptiondesorption experiment was performed, and the N2 adsorption-desorption isotherm of Undoped CeO2 is shown in Figure 12a. As observed in Figure 12a, the isotherm was similar to the Langmuir IV(a) type according to the IUPAC classification, and an obvious hysteresis loop was observed in the relative pressure range of 0.4~1.0, attributable to the type H3. It suggests that Undoped CeO2 was a mesoporous material with a disordered mesoporous structures [58], and the isotherm was consistent with that of other reported porous CeO2 [59][60][61]. Moreover, the specific surface areas of Undoped CeO2 and REdoped CeO2 with solubility limits were estimated based on the N2 adsorption-desorption experiment using a Brunauer-Emmett-Teller method, and the results are shown in Figure 12b as a histogram. Combined with the specific surface areas of 4 mol.% REdoped CeO2 in Table 2, it can be found that RE-doping had a certain influence on the specific surface area of CeO2. However, the specific surface area was not the dominant factor for promoting the OSC of RE-doped CeO2. Among the CeO2 samples with 4 mol.% RE-doping, 4 mol.% Sm-doped CeO2 displayed the minimum [OSC] value of 0.352 mmol H2/g in Figure 10; however, it possessed the maximum specific surface area of 112.6 m 2 /g in Table 2. Among RE-doped CeO2 with saturation doping concentration, 5 mol.% Yb-doped CeO2 exhibited the minimum specific surface area of 93.1 m 2 /g in Fig-Figure 11. SEM images of (a) Undoped, 10 mol.% (b) Yb, (c) Y, (d) Sm and (e) La-doped CeO 2 synthesized using the hydrothermal process at 200 • C for 24 h and followed by calcination in air at 500 • C for 2 h.
In order to further demonstrate the porous structure of CeO 2 , an N 2 adsorptiondesorption experiment was performed, and the N 2 adsorption-desorption isotherm of Undoped CeO 2 is shown in Figure 12a. As observed in Figure 12a, the isotherm was similar to the Langmuir IV(a) type according to the IUPAC classification, and an obvious hysteresis loop was observed in the relative pressure range of 0.4~1.0, attributable to the type H3. It suggests that Undoped CeO 2 was a mesoporous material with a disordered mesoporous structures [58], and the isotherm was consistent with that of other reported porous CeO 2 [59][60][61]. Moreover, the specific surface areas of Undoped CeO 2 and RE-doped CeO 2 with solubility limits were estimated based on the N 2 adsorption-desorption experiment using a Brunauer-Emmett-Teller method, and the results are shown in Figure 12b as a histogram. Combined with the specific surface areas of 4 mol.% RE-doped CeO 2 in Table 2, it can be found that RE-doping had a certain influence on the specific surface area of CeO 2 . However, the specific surface area was not the dominant factor for promoting the OSC of RE-doped CeO 2 . Among the CeO 2 samples with 4 mol.% RE-doping, 4 mol.% Sm-doped CeO 2 displayed the minimum [OSC] value of 0.352 mmol H 2 /g in Figure 10; however, it possessed the maximum specific surface area of 112.6 m 2 /g in Table 2. Among RE-doped CeO 2 with saturation doping concentration, 5 mol.% Yb-doped CeO 2 exhibited the minimum specific surface area of 93.1 m 2 /g in Figure 12b; however, it possessed the maximum [OSC] value of 0.444 mmol H 2 /g. Alternatively, the morphology was also not a major factor influencing the OSC of RE-doped CeO 2 , which is supported by the similar multilayered morphology in Figure 11a-e. Combined with the analyses of morphology and specific surface area of Undoped and RE-doped CeO 2 , one conclusion could be drawn that the enhanced OSC might be attributed to the incorporation of positive trivalent RE 3+ ions into the CeO 2 lattice, and partially substituted the host Ce 4+ ions, promoting the formation of more V O defects and the oxidation/reduction cycle of Ce 3+ ⇔ Ce 4+ . This result could be supported by the lattice parameter analysis in Figure 2, the O 1s XPS analysis in Figure 6 and the Raman spectra analysis in Figure 7.
Molecules 2023, 28, x FOR PEER REVIEW 14 of 17 ure 12b; however, it possessed the maximum [OSC] value of 0.444 mmol H2/g. Alternatively, the morphology was also not a major factor influencing the OSC of RE-doped CeO2, which is supported by the similar multilayered morphology in Figure 11a-e. Combined with the analyses of morphology and specific surface area of Undoped and RE-doped CeO2, one conclusion could be drawn that the enhanced OSC might be attributed to the incorporation of positive trivalent RE 3+ ions into the CeO2 lattice, and partially substituted the host Ce 4+ ions, promoting the formation of more VO defects and the oxidation/reduction cycle of Ce 3+ ⇔ Ce 4+ . This result could be supported by the lattice parameter analysis in Figure 2, the O 1s XPS analysis in Figure 6 and the Raman spectra analysis in Figure 7.

Conclusions
In summary, a series of RE-substituted CeO2 was synthesized just using Ce(NO3)3•6H2O, RE(NO3)3•nH2O (RE = Yb, Y, Sm and La), ethylene glycol and water as raw materials. The Undoped CeO2 was proved to be a mesoporous material with a multilayered morphology; both its multilayered morphology and cubic fluorite structure could be maintained even after 10 mol.% RE introduction. The RE elements were successfully incorporated into the CeO2 lattice with positive trivalent states. REdoping was beneficial for the oxidation/reduction cycle of Ce 3+ ⇔ Ce 4+ , as well as the creation of extrinsic VO defects. The solubility limits of Yb, Y, Sm and La ions in CeO2 were determined as 5, 4, 4 and 7 mol.%. After the incorporation of larger RE 3+ , the lattice expansion of the CeO2 crystal occurred, and more VO defects appeared, which could induce the oxygen mobility from bulk to surface, and promote its OSC. The [OSC] values were 0.444, 0.387, 0.352 and 0.380 mmol/g, much higher than that of the Undoped one (0.230 mmol/g), with an increase of 93.04, 68.26, 53.04 and 65.22%, respectively. The enhanced OSC of RE-doped CeO2 should be attributed to the impurity-induced defects by the substitution of host Ce 4+ with RE 3+ into CeO2, rather than the effects of its specific surface area and morphology.

Conclusions
In summary, a series of RE-substituted CeO 2 was synthesized just using Ce(NO 3 ) 3 ·6H 2 O, RE(NO 3 ) 3 ·nH 2 O (RE = Yb, Y, Sm and La), ethylene glycol and water as raw materials. The Undoped CeO 2 was proved to be a mesoporous material with a multilayered morphology; both its multilayered morphology and cubic fluorite structure could be maintained even after 10 mol.% RE introduction. The RE elements were successfully incorporated into the CeO 2 lattice with positive trivalent states. RE-doping was beneficial for the oxidation/reduction cycle of Ce 3+ ⇔ Ce 4+ , as well as the creation of extrinsic V O defects. The solubility limits of Yb, Y, Sm and La ions in CeO 2 were determined as 5, 4, 4 and 7 mol.%. After the incorporation of larger RE 3+ , the lattice expansion of the CeO 2 crystal occurred, and more V O defects appeared, which could induce the oxygen mobility from bulk to surface, and promote its OSC. The [OSC] values were 0.444, 0.387, 0.352 and 0.380 mmol/g, much higher than that of the Undoped one (0.230 mmol/g), with an increase of 93.04, 68.26, 53.04 and 65.22%, respectively. The enhanced OSC of RE-doped CeO 2 should be attributed to the impurity-induced defects by the substitution of host Ce 4+ with RE 3+ into CeO 2, rather than the effects of its specific surface area and morphology.