Enhanced oxygen storage capacity of CeO2 with doping-induced unstable crystal structure

Accepted Manuscript Full Length Article Enhanced oxygen storage capacity of CeO2 with doping-induced unstable crystal structure Pan Min, Shizheng Zhang, Yaohui Xu, Ruixing Li PII: DOI: Reference:

S0169-4332(18)31063-8 https://doi.org/10.1016/j.apsusc.2018.04.103 APSUSC 39102

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

12 November 2017 5 April 2018 10 April 2018

Please cite this article as: P. Min, S. Zhang, Y. Xu, R. Li, Enhanced oxygen storage capacity of CeO2 with dopinginduced unstable crystal structure, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc. 2018.04.103

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced oxygen storage capacity of CeO2 with doping-induced unstable crystal structure Pan Min, Shizheng Zhang, Yaohui Xu, Ruixing Li* Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China. *

Corresponding author.

E-mail: [email protected] (R. Li). ABSTRACT Doping CeO2 with certain metallic ions has been shown to be an effective route to improving its oxygen storage capacity (OSC). We aimed to study the effects of dopants on the OSC of CeO2 from the perspective of crystallography. In the present study, we improved the OSC by construction of an extremely unstable CeO2 crystal structure based on crystallographic principles. By doping CeO2 with smaller Hf4+ and Sn4+ cations, the incorporated cations produced a lower cation to anion radius ratio in the crystal. The relative oxygen vacancy concentrations were 0.452 and 0.514, respectively, for 3 mol.% doping of Hf4+ and Sn4+, respectively. Our results showed that smaller dopant cations (radius of Sn4+ < Hf4+) led to more vacancies. The low temperature OSC of a CeO2 sample doped with a saturated amount of Hf4+ was 2.2 times as high as that of undoped CeO2 with a similar BET specific surface area. The partial least squares method was used to construct two linear functions for the Hf4+and Sn4+-doping concentration vs. lattice parameters, and the relative oxygen vacancy concentration vs. low temperature OSC per BET surface area. Structure-performance relationships were developed to enable the design of CeO2 three-way catalysts.

1

Key words: Ceria; Doping; Oxygen storage capacity; Solvothermal; Crystal structure; Structure-performance relationship

2

1. Introduction Ceria is the most abundant rare earth oxide and an important material[1], which has been extensively studied and is widely used in many areas, such as three-way catalysts (TWCs) for controlling emissions of automobile exhaust gases[2, 3], water-gas-shift reactions[4, 5], hydrocarbon reforming catalysts[6, 7], and photocatalysts[8–13]. Ceria is effective in these applications owing to its excellent oxygen storage capacity (OSC), which is related to the facile conversion of cerium ions between reduced and oxidized states (Ce3+  Ce4+) through rapid formation and elimination of oxygen vacancies in CeO2[14]. The redox activity of CeO2 is related to the oxygen equilibrium, as shown in equation (1) (V: oxygen vacancy). CeO2 is able to provide oxygen under oxygen-lean conditions, forming a nonstoichiometric oxide CeO2−x/2. Conversely, reduced ceria is able to store oxygen in an oxygen-rich atmosphere[15, 16]. Exhaust gases from fuel efficient vehicles are of a low temperature during the cold-start period; thus, from an environmental perspective, it is necessary to improve the low-temperature OSC of ceria catalysts to handle such vehicle emissions[17]. Hence, considerable efforts have been made to design and synthesize CeO2 catalysts. Doping of CeO2 with appropriate metallic ions has been proven to be an effective route to improving the OSC and reductivity of CeO2[18–23]. From a chemical perspective, the OSC is affected by strengthening or weakening of M-O bonds induced by doping. These effects are related to the tendency for the dopant to achieve a normal coordination environment in its host oxide[24–27]. CeO2  Ce14xCex3O2 x /2Vx / 2  x / 4O2

(1)

From a crystallographic perspective, CeO2 adopts a primitive fluorite structure, where cerium cations have eight-fold coordination with their nearest-neighbor oxygen anions, while each oxygen

3

anion is coordinated by four nearest-neighbor cerium cations. An ideal radius ratio for cations to anions in an eight-coordination oxide with a two-atom basis should be greater than 0.732 and less than 1.0. However, this ratio is only 0.703 for CeO2, and just below the ideal ratio. Hence, the size of the cation Ce4+ is not large enough to fully stabilize the fluorite structure. To make the eight-coordination structure of CeO2 more stable, some Ce4+ ions have a tendency to transform into Ce3+ (r = 0.1143 nm), which has a larger radius than that of Ce4+ (r = 0.097 nm). As a result, oxygen vacancies form through self-doping of nonstoichiometric oxides of the form CeO2−x/2[28]. Hence, oxygen vacancies and the OSC of ceria can be tuned by partial substitution of smaller cations into Ce4+ sites. Form the perspective of catalytic performance, it is desirable for CeO2 to have an unstable crystal structure to promote the disproportionation reaction between Ce3+ and Ce4+, which leads to the formation of more oxygen vacancies and the release of fresh oxygen atoms. Thus, it is possible to design an unstable CeO2 crystal structure to obtain a high OSC. The coordination and optimization of the structural stability and catalytic performance are core issues for improving OSC. On the basis of the above principles, a lower ratio of rcation/ranion leads to more unstable CeO2 structural forms. The crystal structure can maintain equilibrium by generating more Ce3+, and more oxygen vacancies to compensate for the lattice deformation caused by the dopant on the basis of crystallography and defect chemistry, as illustrated in Fig. 1. The performance of the OSC might be enhanced by the introduction of cations smaller than Ce4+ into CeO2 to coordinate and optimize the structural stability, and catalytic performance.

4

Fig. 1. 3×2×1 supercell solid-sphere models of ceria projected along the <100> direction (a) stoichiometric oxide CeO2 in a perfect fluorite structure, this structure is unstable and transforms into (b) a nonstoichiometric oxide CeO2−x/2 through the formation of Ce3+ and oxygen vacancies; and (c) doping of smaller cations (M4+) into CeO2 decreases rcation/ranion and produces Ce3+ together with more oxygen vacancies, finally forming Ce1−mMmO2−x/2−y.

Hf4+-, Ti4+-, Zr4+-, etc. doped CeO2 particles have drawn considerable interest in recent years[29–33]. However, these studies have focused on the influence of the dopants on OSC directly, and not on the mechanism between crystallography and OSC. In terms of synthesis methods, in most of the above studies, CeO2 was prepared by a co-precipitation method with alkaline precipitants. Compared with co-precipitation, solvothermal synthesis techniques are a more effective route to nanomaterials with the advantages of good homogeneity, uniformity of dopants, and good dispersion of products at the nanoscale. In this work, we examined various geometric, steric, electronic, valance, and OSC properties of CeO2 to determine structure-performance relationships. We synthesized samples with a lower ratio of rcation/ranion via dopant size selection to achieve a higher concentration of tervalence cerium and oxygen vacancies, leading to modified chemical reactivity of CeO2. We introduced smaller cations, 5

namely Hf4+ (r = 0.083 nm) or Sn4+ (r = 0.081 nm), into CeO2 with a multilayered structure by a solvothermal route at 200 °C followed by calcination at 500 °C. The concentrations of saturation doping and relative oxygen vacancies in CeO2 were determined. Finally, the corresponding OSC values of the samples was investigated by hydrogen temperature-programmed reduction (H2-TPR). Simulations of the doping concentration versus lattice parameters and relative oxygen vacancy concentration versus low temperature OSC per BET were calculated by partial least squares methods on the basis of our systematic experimental data. This study provides insight into the structure-performance relationships that will be useful for designing three-way catalysts. 2. Experimental 2.1. Starting materials Cerium nitrate hexahydrate (Ce(NO3)3∙6H2O, > 99.95%) was supplied by Aladdin Co. Ltd. Hafnium chloride (HfCl4, > 99.9%) was purchased from Alfa Aesar. Tin chloride pentahydrate (SnCl4∙5H2O, > 99.0%) was supplied by Xilong chemical Co. Ltd and ethylene glycol was obtained from Beijing Chemical Works. All reagents were commercially obtained and used as received without further purification. 2.2. Sample preparation A typical synthesis procedure was as follows. For the synthesis of a sample with Hf/(Hf + Ce) (mol.%) = 10%, 0.142 g HfCl4, and 1.736 g Ce(NO3)3∙6H2O was dissolved in 5 mL of deionized water and 30 mL of ethylene glycol with vigorous stirring at room temperature. The solution was carefully decanted into a 50-mL Teflon-lined stainless steel autoclave and sealed. The solution was maintained in the autoclave at 200°C for 24 h. After the autoclave cooled to room temperature naturally the precipitates formed were washed with distilled water and ethanol and dried under

6

vacuum at 60 °C for 12 h. Finally, the sample was calcined in air at 500 °C for 2 h and cooled to room temperature in the furnace. A series of samples with Hf/(Hf + Ce) (mol.%) = 0, 3%, 5%, 10%, and 20% were prepared, where the total amount of Ce(NO3)3∙6H2O was unchanged. Samples with Sn/(Sn + Ce) (mol.%) = 0, 1%, 3%, 5%, and 10% were prepared by the same method with SnCl4∙5H2O as the tin source. 2.3.Characterization The crystallographic phase of the samples was examined by X-ray diffraction (XRD; Rigaku D/MAX 2200 PC) with Cu Kα radiation. The N2 adsorption-desorption isotherms were measured on a QuadraSorb SI at 77 K. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. X-ray photoelectron spectroscopy was measured with an ESCALab 250 Xi electron spectrometer. The morphology was examined by a field-emission scanning electron microscope (JEOL JSM-7500F). High-resolution transmission electron microscope (HRTEM) images were recorded on a JEOL JEM-2100 transmission electron microscope to further examine the microstructure of the samples. Raman spectroscopy was performed on a Horiba Jobin Yvon LabRAM Aramis. The excitation wavelength was 325 nm from a He-Cd laser. The scanning range was 200 – 1000 cm–1. Lattice parameters were calculated by the formulae: 2d sin   

a  d h2  k 2  l 2

(2)

where a referred to the CeO2 lattice parameter, h, k, l are the indexes of crystallographic planes, d is the interplanar spacing, θ is the diffraction angle, and λ is the X-ray wavelength corresponding to Cu Kα radiation (1.5406Å in the present experiment). The (111) peak position of a high purity NaCl internal standard was used for corrections of the spectra.

7

The oxygen storage capacity was estimated by H2-TPR, which was performed on a TP-5080 conventional setup equipped with a thermal conductivity detector (TCD) and gas chromatograph. Typically, 50 mg of sample was used and pretreated in a 5%-O2/N2 stream (100 mL/min) at 500 °C for 1 h. Then, the oxidized sample was cooled to room temperature in the N2 atmosphere. Afterwards the H2-TPR experiment was run in a 5%-H2/N2 stream; the reduction process was performed from room temperature to 960 °C with a heating rate of 10 °C/min. The amount of H2 consumed during the reduction was measured by the TCD. 3. Results and discussion XRD analysis was used to identify the phase composition and crystallographic structure of the samples. Figure 2 shows XRD patterns of the samples synthesized at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h with various Hf/(Hf + Ce) and Sn/(Sn + Ce) (mol.%). All the identified peaks were assigned to face-centered cubic structure of CeO2 (JCPDS card no. 43-1002), no additional phases from impurities, such as Hf or Sn oxides (e.g., HfO2 or SnO2) were detected. Compared with the JCPDS card, the relative intensity of the peaks showed no clear differences, suggesting that there was no preferential orientation or preferential crystal growth. Moreover, in Fig. 1 we observed a peak shift towards higher diffraction angles for the Hf4+- or Sn4+-doped CeO2 compared with the undoped counterpart. These findings indicate that the ceria maintained a cubic fluorite structure and the small Hf4+ or Sn4+ cations partially substituted the cerium ions to form a solid solution based on the Bragg’s equation.

8

Fig. 2. XRD patterns of (a) Hf4+- and (b) Sn4+-doped samples synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

The lattice parameter is extremely sensitive to dopants with radii different from the host and it is widely used to determine the solubility limit of alloys. Therefore, the lattice parameter was studied based on the XRD patterns. Figure 3 shows plots of the lattice parameter as a function of doping concentration, i.e., M/(M+Ce) (mol.%), where M = Hf or Sn. As shown in Fig. 3, for the Hf4+-doped samples, the lattice parameters of the cubic CeO2 decreased as the hafnium content increased up to Hf/(Hf + Ce) = 5 mol.%. The radius of Hf4+ is smaller than that of the host cation Ce4+; hence, lattice contraction is expected from the incorporation of Hf4+ into the CeO2 lattice. The above results suggested that Hf/(Hf + Ce) = 5 mol.% might be close to the saturation doping concentration, or the so-called solid solubility limit, for Hf4+-doped CeO2. However, no peaks from any impurities could be seen in the XRD [see Fig. 2 (a)] for Hf/(Hf + Ce) = 10 mol.% and 20 mol.%. One reason for this result might be that the amounts of impurities were too low to be detected by XRD. The Raman spectra were measured to give further insight. For the Sn4+-doped samples, as shown in Fig. 3, Sn/(Sn + Ce) = 3 mol.% might be close to the saturation doping concentration. As expected, the 9

saturation doping concentration of Sn4+-doped CeO2 was lower than that of Hf4+ because the size of Sn4+ (r = 0.081 nm) deviated more from that of Ce4+ (r = 0.097 nm) compared with the deviation of Hf4+ (r = 0.083 nm). A greater size mismatch leads to greater lattice distortion and a lower saturation doping concentration.

Fig. 3. Lattice parameters as a function of doping concentration (M = Hf or Sn) for CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

The deformation of the lattice was reflected by the doping concentration and the self-equilibrium of the crystal structure could be quantified from the lattice parameters of CeO2. Figure 3 shows that the lattice parameter of CeO2 decreased linearly as the concentration of Hf4+ or Sn4+ increased until the saturated doping concentration. At this point an equilibrium was reached with no further changes to the lattice parameters with increasing Hf4+ or Sn4+ content. In the literature, the partial least squares (PLS) method is preferred for analyzing this type of data because it applies feature extraction and induction in one step. To better understand the relationship between the lattice parameters and the content of small cations, we applied a PLS fitting algorithm to the data 10

from Hf4+- and Sn4+-doped CeO2 solid solutions with dopant concentrations below the saturation doping concentration. We then compared the results with calculations based on Vegard’s law[34], as shown in Fig. 4. These results suggested that the lattice parameters for both Hf4+- and Sn4+-doped CeO2 were in good agreement with the predictions of Vegard’s law when the dopant content was below the saturation doping concentration. However, the experimental values were inconsistent with the calculated values based on Vegard’s law when the doping concentration was higher than the saturation doping concentration. Thus, the lattice parameter a could be expressed as a function of doping concentration m, as follows: ma  (1  m)ah a(m)   d as

0  m  ms (Vegard ’s law) m  ms

(3)

where ad and ah are the lattice parameters of the dopant oxide and host oxide, respectively, and ms is the saturation doping concentration. The term as represents the lattice parameter of the solid solution at the saturation doping concentration. Hence, both Hf4+ and Sn4+ follow Vegard’s law in the concentration range of 0 < m ≤ ms, as shown in Fig. 4 (dashed lines). This result provides a strategy for optimizing the doping concentration to achieve the desired lattice parameters or for determining the lattice parameters based on the doping concentration.

11

Fig. 4. A comparison of lattice parameters calculated by experimental data and Vegard’s law for (a) Hf4+- and (b) Sn4+-doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h (as: lattice parameter at saturation doping concentration).

HRTEM analysis was conducted to investigate the structural features at the atomic level. Figure 5 shows HRTEM images of the undoped and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 (i.e., saturated doping samples). Here, the (111) plane was dominant in agreement with the XRD results (see Fig. 2). For a face centered cubic structure, (111) is a close-packed plane, which possesses the lowest surface energy[35]. The interplanar spacing of the (111) plane estimated from the HRTEM images for the undoped CeO2 and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 were 0.3124, 0.3115, and 0.3119 nm, respectively. The lattice contraction could be attributed to doping by Hf4+ or Sn4+ and was consistent with the lattice parameter results shown in Fig. 3.

12

Fig. 5. HRTEM images of undoped CeO2 and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

To clarify the effects of the dopants on the morphology of the CeO2 samples, SEM analysis was conducted. Figure 6 shows SEM images of the undoped CeO2 and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2. As observed, the morphology of the undoped CeO2 was a multilayered structure consisting of nanoflakes approximately 6 nm thick. After doping, the morphology of the products did not change remarkably, i.e., the multilayered structure was maintained. However, a shift in the thickness of the nanoflakes and the average diameter of the multilayered structure was observed. In particular, for the case of 3 mol.% Sn4+-doped CeO2, adhesion was clearly observed between the nanoflakes.

Fig. 6. SEM images of undoped CeO2 and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2

13

synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

X-ray photoelectron spectroscopy (XPS) was used to obtain information about the elemental composition from a survey scan XPS spectrum and the valence states of cerium by inspecting the spectral line shape and intensities of the Ce3d core-level electrons. The survey scan XPS spectra of the undoped CeO2 and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 are illustrated in Fig. 7. For the undoped CeO2, the elements Ce, O, and C could be detected from peaks assigned to Ce 3d, O 1s and C 1s binding energies, respectively. The weak C 1s peak was caused by contamination of the sample and was used as a reference to correct the peaks of other elements. In addition, the presence of Hf and Sn in doped CeO2 could be detected in the spectra, as shown in Figs. 7(b) and (c). These features were assigned to Hf 4f and Sn 3d binding energies, confirming that doping was achieved. Figures 8(a), (b) and (c) show the Ce 3d core-level spectra of the undoped CeO2 and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2. The electron binding energy fitted well with results reported for cerium oxide[36]. The curves of the Ce 3d spectra could be fitted by ten peaks corresponding to five pairs of spin-orbit doublets[37–39]. Six peaks labeled v were collectively attributed to Ce4+ and the other four peaks labeled u were attributed to Ce3+. Peaks v0, v1, v2, u0, and u1 represent the Ce3d5/2 spin-orbit split doublet peaks, while v0ʹ, v1ʹ, v2ʹ, u0ʹ, and u1ʹ characterize the corresponding Ce3d3/2 spin-orbit split doublet peaks.

14

Fig. 7. XPS survey spectra of (a) undoped CeO2 and (b) 5 mol.% Hf4+- and (c) 3 mol.% Sn4+-doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

Fig. 8. Ce 3d core-level spectra of (a) undoped CeO2 and (b) 5 mol.% Hf4+- and (c) 3 mol.% Sn4+-doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

To investigate the chemical states of oxygen in the synthesized samples, the O 1s core level XPS spectra of the undoped CeO2 and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 were measured

15

and the results are shown in Fig. 9. The O 1s XPS spectra could be fitted by two peaks, which confirmed the existence of two kinds of oxygen species in the ceria. The lower binding energy peak (Oβ) observed at 529.2 eV corresponded to lattice oxygen and the smaller peak at higher binding energy (Oα, 531.5 eV) was attributed to surface chemisorbed oxygen[40, 41]. The surface chemisorbed oxygen ratio [R(Oα)] could be estimated as follows[42, 43]: R(Oα) = area(Oα)/[area(Oα) + area(Oβ)]

(4)

The estimated values R(Oα) of undoped CeO2 and 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 were 30.9%, 51.2%, and 39.7%, respectively. The R(Oα) values after doping were higher than those of the undoped sample. This result indicates that the dopants Hf and Sn promoted the chemisorption of oxygen. Chemisorbed oxygen is typically considered to be more active than lattice oxygen, thus, the higher R(Oα) value might be beneficial for the oxygen storage capacity of doped CeO 2.

Fig. 9. O 1s core level XPS spectra of (a) undoped CeO2 and (b) 5 mol.% Hf4+- and (c) 3 mol.% Sn4+-doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

16

Raman analysis was used to investigate the structural characteristics of the ceria-based solid solutions because Raman scattering is sensitive to the Ce-O bond arrangement and lattice defects, as well as providing nondestructive and rapid analysis of materials[44]. Figure 10 shows the Raman spectra of the as-synthesized samples. The Raman spectrum of each sample showed a prominent high intensity peak of ca. 465 cm–1 and a less prominent peak of ca. 600 cm–1. The peak of ca. 465 cm–1 could be attributed to the triply degenerate F2g mode of the fluorite structure, which can be viewed as a symmetric stretching mode of oxygen atoms around cerium ions. Because only the O atoms stretch, the mode frequency should be nearly independent of the cation mass. For example, this frequency is 465 cm–1 for CeO2, whereas for ThO2, with a cation mass 65% larger that of Ce4+ the frequency is only 1 cm–1 higher[45]. According to a study of McBride et al.[46] for Eu-, Gd-, and Tb-doped CeO2, XRD analyses showed two distinct phases; however, in the Raman spectra no other peaks were observed. This line became asymmetric and the line shape changed for doping concentrations greater than 20%. The results for our as-synthesized samples of doped CeO2 were similar to this case. Figure 10 shows that for the supersaturated dopant concentrations, i.e., 10, 20 mol.% Hf4+- and 5, 10 mol.% Sn4+-doped CeO2, the F2g mode became slightly asymmetric and the peak of 600 cm–1 underwent a change of its line shape, which could be evidence for the formation of a minor second phase. Thus, the Raman analysis suggests the formation of a second phase and complements the XRD results shown in Fig. 2. The peak of ca. 600 cm–1 in Fig.10 was attributed to the nondegenerate longitudinal optical (LO) mode of ceria, arising from relaxation of symmetry rules, which could be linked to oxygen vacancies in the ceria lattice. As noted from Fig. 10, this peak was less prominent in the case of the undoped CeO2, while it became stronger in the case of Hf4+- and Sn4+-doped CeO2. This result indicates that more oxygen vacancies formed in the Hf4+- and

17

Sn4+-doped samples.

Fig. 10. Raman spectra of (a) Hf4+- and (b) Sn4+-doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

Furthermore, the relative oxygen vacancy concentration could also be determined from the Raman spectra, i.e., from the ratio of the peak area of 600 to 465 cm–1 (A600/A465)[47]. The relative oxygen vacancy concentrations were calculated and are summarized in Table 1. Here, it should be noted that the value of A600/A465 is meaningless when the doping concentration (Hf4+ or Sn4+) is higher than the saturation concentration. The relative oxygen vacancy concentration for all the Hf4+or Sn4+-doped CeO2 samples increased considerably compared with that of the undoped counterpart. For the Hf4+-doped CeO2, we found that the relative oxygen vacancy concentration gradually increased to 0.716 at a saturation doping concentration of Hf/(Hf + Ce) = 5 mol.%. This same tendency was found for the Sn4+-doped samples, although both Hf4+ and Sn4+ are tetravalent ions and do not change the crystal electroneutrality to give rise to vacancies directly. Furthermore, in the case of the 3 mol.% doping concentration, the relative oxygen vacancy concentrations were 0.452 and 18

0.514 for Hf4+- and Sn4+-doped CeO2, respectively. These results provide strong evidence that smaller dopants could create more vacancies. The BET specific surface area is an important property affecting the OSC. By calculating the N2 adsorption-desorption isotherms, BET surface area data for CeO2 are summarized in Table 1. We found that Sn4+-doping caused a large decrease in the BET surface area; however, Hf4+-doping did not affect in the range of saturation concentration greatly. In other words, the introduction of Sn4+ into CeO2 caused a decrease of the originally high BET surface area. A low surface area is a drawback for a catalyst. The results in Fig. 6, suggest that the CeO2 particles are multilayered assembly structures. The Sn4+-doping of CeO2 disrupted this structure and the layer adhesion resulted in a decrease of BET surface area. Hydrogen temperature-programmed reduction (H2-TPR) was used to estimate the OSC and determine the reducibility of as-synthesized CeO2. The H2-TPR profiles of CeO2 synthesized at 200 °C after calcination with increasing Hf4+- and Sn4+-contents are shown in Fig. 11. The undoped and Hf4+- and Sn4+-doped CeO2 exhibited two reduction peaks, i.e., one low temperature peak located at ca. 500 and 300 °C for the Hf4+- and Sn4+-doped CeO2, respectively. Another high temperature peak was found at 780 °C for all the CeO2 samples. The low temperature peaks could be attributed to reduction of the surface oxygen of ceria; the high temperature peak was related to the reduction of lattice oxygen of ceria[48–50]. As illustrated in Fig. 11 the peak area of the low temperature reduction increased after doping, relative to the profile of the undoped ceria. This result implies that the low temperature OSC of the samples was enhanced. However, the high temperature reduction peak area did not show any major differences before and after doping, indicating that Hf4+or Sn4+-doping mainly affects the low temperature OSC rather than the high temperature OSCs.

19

Notably, the low temperature peak for 5 mol.% Hf4+-doped CeO2 was much more intense and sharp than those of the other samples, as shown in Fig. 11(a). The results above confirm that 5 mol.% Hf4+ is the saturated concentration in CeO2. This concentration leads to the greatest amount of lattice distortion and the strongest tendency for a shift to Ce3+ from Ce4+ with the formation of more oxygen vacancies, as shown in Table 1. Thus, this sample was more active during the reaction with H2. In the case of the Sn4+-doped CeO2, Fig. 11(b) shows that the low temperature peak shifted to lower temperature compared with that of undoped CeO2. This result implies that the reducibility of this sample was considerably enhanced. According to the literature[33], Ce4+ partly substituted by Sn4+ forms a Ce-O-Sn bond, in which oxygen is bonded more weakly than in Ce-O-Ce. As a result, the reduction temperature on the surface of the doped samples decreased. Therefore, Sn4+-doped CeO2 presents a good example of the OSC being affected by weakening of M-O bonds. When Sn/(Sn + Ce) > 3 mol.%, a new peak is observed at ca. 440 °C, which might be attributed to the reduction of a second phase that contains Sn4+[51]. This result further suggests that 3 mol.% Sn4+ is close to the saturation doping concentration, as supported by the Raman analysis that indicated the existence of a second phase. However, the reduction in the bulk is dominated by diffusion processes, making it difficult for the high temperature peaks to shift to lower temperatures[52].

20

Fig. 11. Hydrogen temperature-programmed reduction (H2-TPR) profiles of (a) Hf4+- and (b) Sn4+-doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

In addition, to the quantitatively analysis of the low temperature OSC of the as-synthesized samples, the hydrogen consumption was calculated on the basis of the low temperature peak area in the H2-TPR profiles and the results are shown in Fig. 12 and Table 1, respectively. There was a considerable increase in the low temperature OSC for the doped CeO2 compared with that of the undoped counterpart. The ratio rcation/ranion decreased and more Ce4+ transformed into Ce3+ after Hf4+ and Sn4+ were introduced into CeO2. Thus, doping led to the release of oxygen molecules and the formation of oxygen vacancies in the CeO2 to maintain the electroneutrality of the crystal [see disproportionation reaction (1)]. During the oxidation process before hydrogen reduction, Ce3+ is oxidized to Ce4+ accompanied by oxygen vacancies becoming filled by oxygen atoms. In the following H2-TPR process, H2 reacts with the samples by reducing Ce4+ to Ce3+. Therefore, the increases in the concentrations of Ce3+ and oxygen vacancies caused by the process of doping of smaller cations leads to an increase in the OSC of ceria. A similar mechanism explains the increase 21

in OSC for the Sn4+-doped CeO2 samples. However, the H2 consumption of the 10 mol.% Sn4+-doped CeO2 was higher than that of the 3 mol.% doped sample, despite the 3 mol.% Sn4+ being the saturation doping concentration. This result might be explained by the appearance of a reducible second phase of Sn oxide. These results are consistent with the Raman analysis (as shown in Fig. 10). The H2 consumption of the 5 mol.% Sn4+-doped CeO2 was lower than that of the 3 and 10 mol.% doped samples. This result might be attributed to the decrease of the BET surface area for the 5 mol.% Sn4+-doped CeO2, as shown in Table 1. Figure 12 shows that at the same Hf4+ or Sn4+ doping level, the OSC values of Hf4+-doped CeO2 were higher than those of Sn4+-doped CeO2, although the ionic radius of Sn4+ is smaller than that of Hf4+. This finding might be attributed to the higher BET surface area of the Hf4+-doped CeO2, as shown in Table 1. Catalysts with a large BET surface area are advantageous for OSC. Here, a new term is introduced to describe the OSC: OSC per BET 

Low temperature oxygen storage capacity specific surface area

(5)

For example, the OSC per BET surface area values of 3 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 were 0.00297 and 0.00324 mmol O2 m–2, respectively. Thus, the OSC per BET surface area of 3 mol.% Sn4+-doped CeO2 was higher than that of Hf4+. This finding reveals the real contribution of the oxygen vacancies to the OSC, by considering the relative BET surface area. Low temperature OSC of CeO2 doped with Sn4+ at saturation concentration was 1.4 times as high as that of undoped CeO2; however, that of Hf4+ was 2.2 times as high as that of undoped CeO2. This difference is attributed to the Sn4+-doping causing a decrease of the BET surface area, which offsets the increase in the OSC achieved by the increase in oxygen vacancies. Thus, together with the oxygen vacancy content, the BET surface area also has a major influence on the OSC.

22

Fig. 12. Hydrogen consumption as a function of doping concentration (M = Hf or Sn) for CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

Table 1. Summary of the BET specific surface area, relative oxygen vacancy concentration, and low temperature OSC of Hf4+- and Sn4+-doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h.

Sample

Relative oxygen vacancy

Low temperature

concentration (A600/A465)

OSC (mmol O2 g–1)

2

BET (m /g)

Undoped

92.354

0.288

0.184

3 mol.% Hf4+

97.709

0.452

0.290

5 mol.% Hf4+

86.376

0.716

0.403

10 mol.% Hf4+

73.506

Invalid

0.343

20 mol.% Hf4+

71.641

Invalid

0.277

1 mol.% Sn4+

71.282

0.427

0.210

3 mol.% Sn4+

79.653

0.514

0.258

23

5 mol.% Sn4+

57.853

Invalid

0.246

10 mol.% Sn4+

80.413

Invalid

0.267

[Note] The A600/A465 for Raman spectrum results are not valid if the dopant concentration is higher than the saturation concentration. The OSCs of Hf4+- and Sn4+-doped CeO2 studied by Vinodkumar et al. and Ayastuy et al. were 0.248 and 0.207 mmol O2 g–1, respectively[29, 33].

Table 1 shows that the low temperature OSC and relative oxygen vacancy concentration showed a similar relationship. For example, the ratio of relative oxygen vacancy concentrations was 2.49 for the 5 mol.% Hf4+-doped CeO2 and undoped CeO2. Alternatively, the ratio of the low temperature OSC was 2.20 for these two samples. The two ratios were similar because the BET surface area of Hf4+-doped samples changed little. Therefore, we developed a function for the relative oxygen vacancy concentration versus low temperature OSC per BET surface area. The results are shown in Fig. 13, based on the partial least squares method. We observed a linear dependence, i.e., a higher relative oxygen vacancy concentration produced a greater low temperature OSC per BET surface area for both Hf4+- and Sn4+-doped CeO2. In other words, the low temperature OSC per BET surface area was linearly affected by the amount of oxygen vacancies in CeO 2. In addition, the slopes of the two lines were both approximately 0.00616. Thus, the OSC per BET surface area could be predicted as a function of the relative oxygen vacancy concentration (ROVC) for CeO2: OSC per BET surface area = 0.00616ROVC + 0.000211.

(6)

Equation (6) describes the linear dependence of the OSC performance on the crystalline structure of CeO2. The amount of oxygen vacancies can be tailored by decreasing the radius ratio of rcation/ranion in

24

CeO2.

Fig. 13. OSC per BET as a function of relative oxygen vacancy concentration for Hf4+- and Sn4+(mol.%, ≤ saturation doping concentration) doped CeO2 synthesized solvothermally at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h. [Note] OSC: Low temperature oxygen storage capacity, BET: Specific surface area

In summary, the low temperature OSC values for the saturation doping concentration of 5 mol.% Hf4+-and 3 mol.% Sn4+-doped CeO2 in this study are 0.403 and 0.258 mmol O2 g–1, respectively. These data are higher than those of previous reports. The OSC of Hf4+-doped CeO2 studied by Vinodkumar et al. was 0.248 mmol O2 g–1 and the maximum OSC of Sn4+-doped CeO2 studied by Ayastuy et al. was 0.207 mmol O2 g–1[29, 33]. We believe that the application of our approach might provide new ways to synthesize and understand the composition and structure and properties of CeO2. 4. Conclusions We obtained a high OSC in CeO2 by designing an extremely unstable crystal structure. Hf4+- or 25

Sn4+-doped CeO2 nanoparticles with a multilayered structure were prepared by a solvothermal method at 200 °C for 24 h followed by calcination in air at 500 °C for 2 h. The results from XRD, lattice parameter measurements, and XPS suggested that Hf4+ and Sn4+ were doped into the CeO2 lattice and the saturation doping concentrations of Hf4+ and Sn4+ were 5 mol.% and 3 mol.%, respectively. The relative oxygen vacancy concentrations for 3 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 were 0.452 and 0.514, respectively, indicating that the smaller the size of the dopant, the more oxygen vacancies were produced. Notably, the vacancies here were not formed to maintain electroneutrality of the CeO2 molecule because Hf and Sn are both tetravalent cations. The doped CeO2 samples achieved high OSC by manipulation of the crystal structural parameter of rcation/ranion in CeO2. The low temperature OSC values of 5 mol.% Hf4+- and 3 mol.% Sn4+-doped CeO2 were 0.403 and 0.258 mmol O2 g–1, respectively, and much higher than the 0.184 mmol O2 g–1 of undoped CeO2. The lattice parameters, relative oxygen vacancy concentration and low temperature OSC per BET surface area of the as-synthesized CeO2 were calculated and simulated by PLS methods to determine structure-performance relationships. Both the relative oxygen vacancy concentration and BET surface area of the catalyst affected the low temperature OSC of CeO2. The term OSC per BET surface area was used to account for the effects of surface area and determine clear structure-performance relationships. A good linear relationship between the OSC per BET surface area and relative oxygen vacancy concentration for CeO2 was found. Our systematic study provides a new strategy for the design of three-way catalysts.

Acknowledgments The authors appreciate the financial support from the National Science Foundation of China (NSFC51372006 and NSFC 51772013). We thank Andrew Jackson, PhD, from Liwen Bianji, Edanz 26

Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript. References [1] T. Montini, M. Melchionna, M. Monai, P. Fornasiero, Fundamentals and Catalytic Applications of CeO2-Based Materials, Chem Rev, 116 (2016) 5987–6041. [2] S.Y. Christou, A.M. Efstathiou, Efficient in-situ regeneration method of the catalytic activity of aged TWC, Top Catal, 42–43 (2007) 415–419. [3] A. Bueno-Lopez, Diesel soot combustion ceria catalysts, Appl Catal B-environ, 146 (2014) 1–11. [4] R. Si, M. Flytzani-Stephanopoulos, Shape and crystal-plane effects of nanoscale ceria on the activity of Au-CeO2 catalysts for the water-gas shift reaction, Angew Chem Int Ed, 47 (2008) 2884–2887. [5] C. Ratnasamy, J.P. Wagner, Water Gas Shift Catalysis, Catal Rev, 51 (2009) 325–440. [6] D.H. Prasad, H.I. Ji, H.R. Kim, J.W. Son, B.K. Kim, H.W. Lee, J.H. Lee, Effect of nickel nano-particle sintering on methane reforming activity of Ni-CGO cermet anodes for internal steam reforming SOFCs, Appl Catal B-environ, 101 (2011) 531–539. [7] H.-S. Roh, I.-H. Eum, D.-W. Jeong, Low temperature steam reforming of methane over Ni–Ce(1−x)Zr(x)O2 catalysts under severe conditions, Renew Energ, 42 (2012) 212–216. [8] D. Sun, M. Gu, R. Li, S. Yin, X. Song, B. Zhao, C. Li, J. Li, Z. Feng, T. Sato, Effects of nitrogen content in monocrystalline nano-CeO2 on the degradation of dye in indoor lighting, Appl Surf Sci, 280 (2013) 693–697. [9] A.D. Liyanage, S.D. Perera, K. Tan, Y. Chabal, K.J. Balkus, Synthesis, Characterization, and Photocatalytic Activity of Y-Doped CeO2 Nanorods, ACS Catalysis, 4 (2014) 577–584. [10] H. Miao, G.-F. Huang, J.-H. Liu, B.-X. Zhou, A. Pan, W.-Q. Huang, G.-F. Huang, Origin of enhanced photocatalytic activity of F-doped CeO2 nanocubes, Appl Surf Sci, 370 (2016) 427–432. [11] X. Zhang, F. Hou, Y. Yang, Y. Wang, N. Liu, D. Chen, Y. Yang, A facile synthesis for cauliflower like CeO2 catalysts from Ce-BTC precursor and their catalytic performance for CO oxidation, Appl Surf Sci, 423 (2017) 771–779. [12] X. Zhang, F. Hou, H. Li, Y. Yang, Y. Wang, N. Liu, Y. Yang, A strawsheave-like metal organic framework Ce-BTC derivative containing high specific surface area for improving the catalytic activity of CO oxidation reaction, Micropor Mesopor Mater, 259 (2018) 211–219. [13] Z. Qu, X. Zhang, D. Chen, X. Li, M. Wen, Y. Wang, D. Ma, J. Wu, Effect of CeO 2 on the Silver Species and Its Catalytic Activity for CO Oxidation, Chem J Chin U, 32 (2011) 1605–1609. [14] L.C.D. Trovarelli A, Dolcetti G, Design Better Cerium-Based Oxidation Catalysts, Chemtech, 27 (1997) 32–37. [15] N.V. Skorodumova, S.I. Simak, B.I. Lundqvist, I.A. Abrikosov, B. Johansson, Quantum origin of the oxygen storage capability of ceria, Phys Rev Lett, 89 (2002). [16] D.C. Sayle, S.A. Maicaneanu, G.W. Watson, Atomistic models for CeO 2 (111), (110), and (100) nanoparticles, supported on yttrium-stabilized zirconia, J Am Chem Soc, 124 (2002) 11429–11439. [17] B.H.-Y. Chen, H.-L. Chang, Development of Low Temperature Three-Way Catalysts for Future Fuel Efficient Vehicles, Johnson Matthey Technology Review, 59 (2015) 64–67. [18] A. Gupta, U.V. Waghmare, M.S. Hegde, Correlation of Oxygen Storage Capacity and 27

Structural Distortion in Transition-Metal-, Noble-Metal-, and Rare-Earth-Ion-Substituted CeO2 from First Principles Calculation, Chem Mater, 22 (2010) 5184–5198. [19] Z.X. Song, W. Liu, H. Nishiguchi, A. Takami, K. Nagaoka, Y. Takita, The Pr promotion effect on oxygen storage capacity of Ce-Pr oxides studied using a TAP reactor, Appl Catal A-Gen, 329 (2007) 86–92. [20] X. Liu, J. Ding, X. Lin, R. Gao, Z. Li, W.-L. Dai, Zr-doped CeO2 nanorods as versatile catalyst in the epoxidation of styrene with tert-butyl hydroperoxide as the oxidant, Appl Catal A-Gen, 503 (2015) 117–123. [21] K. Kenevey, F. Valdivieso, M. Soustelle, M. Pijolat, Thermal stability of Pd or Pt loaded Ce0.68Zr0.32O2 and Ce0.50Zr0.50O2 catalyst materials under oxidising conditions, Appl Catal B-environ, 29 (2001) 93–101. [22] M.F. Luo, J. Chen, L.S. Chen, J.Q. Lu, Z. Feng, C. Li, Structure and redox properties of CexTi1-xO2 solid solution, Chem Mater, 13 (2001) 197–202. [23] H. Bao, K. Qian, J. Fang, W. Huang, Fe-doped CeO2 solid solutions: Substituting-site doping versus interstitial-site doping, bulk doping versus surface doping, Appl Surf Sci, 414 (2017) 131–139. [24] R.J. Gorte, Ceria in Catalysis: From Automotive Applications to the Water Gas Shift Reaction, Aiche J, 56 (2010) 1126–1135. [25] G.Vlaic, P. Fornasiero, S.Geremia, J.Kaspar, M.Graziani, Relationship between the Zirconia-Promoted Reduction in the Rh-Loaded Ce0.5Zr0.5O2 Mixed Oxide and the Zr-O Local Structure, J Catal, 168 (1997) 386–392. [26] A. Uzunoglu, H. Zhang, S. Andreescu, L.A. Stanciu, CeO2-MOx (M: Zr, Ti, Cu) mixed metal oxides with enhanced oxygen storage capacity, J Mater Sci, 50 (2015) 3750–3762. [27] J.Q. Wang, M.Q. Shen, J. Wang, J.D. Gao, J. Ma, S.X. Liu, CeO 2-CoOx mixed oxides: Structural characteristics and dynamic storage/release capacity, Catal Today, 175 (2011) 65–71. [28] R.X. Li, S. Yabe, M. Yamashita, S. Momose, S. Yoshida, S. Yin, T. Sato, Synthesis and UV-shielding properties of ZnO- and CaO-doped CeO2 via soft solution chemical process, Solid State Ionics, 151 (2002) 235–241. [29] T. Vinodkumar, D. Naga Durgasri, B.M. Reddy, Design of transition and rare earth metal doped ceria nanocomposite oxides for CO oxidation, International Journal of Advances in Engineering Sciences and Applied Mathematics, 5 (2013) 224–231. [30] B.M. Reddy, L. Katta, G. Thrimurthulu, Novel nanosized Ce xZr1–xO2, CexHf1–xO2 and CexTb1–xO2-delta solid solutions: Structural characteristics and catalytic performance, Catal Today, 175 (2011) 585–592. [31] R. Sasikala, N.M. Gupta, S.K. Kulshreshtha, Temperature-programmed reduction and CO oxidation studies over Ce-Sn mixed oxides, Catal Lett, 71 (2001) 69–73. [32] Y.Z. Chen, B.J. Liaw, C.W. Huang, Selective oxidation of CO in excess hydrogen over CuO/CexSn1–x O2 catalysts, Appl Catal A-Gen, 302 (2006) 168–176. [33] J.L. Ayastuy, A. Iglesias-González, M.A. Gutiérrez-Ortiz, Synthesis and characterization of low amount tin-doped ceria (CexSn1–xO2−δ) for catalytic CO oxidation, Chem Eng J, 244 (2014) 372–381. [34] L. Vegard, H. Dale, Untersuchungen über Mischkristalle und Legierungen, Zeitschrift für Kristallographie - Crystalline Materials, 67 (1928) 148. [35] R.X. Li, C. Fu, C.Q. Li, S. Yin, Y. Zhang, T. Sato, Synthesis of bundle-like CeO2 28

nanofibers and their sub-microstructures, J Ceram Soc Jpn, 118 (2010) 555–557. [36] F. Larachi, J. Pierre, A. Adnot, A. Bernis, Ce 3d XPS study of composite Ce xMn1–xO2–y wet oxidation catalysts, Appl Surf Sci, 195 (2002) 236–250. [37] T.A. Trudeau M L, Ying J Y, XPS investigation of surface oxidation and reduction in nanocrystalline CexLa1–xO2–y, Surf Interface Anal, 23 (1995) 219–226. [38] T. Naganuma, E. Traversa, Stability of the Ce3+ valence state in cerium oxide nanoparticle layers, Nanoscale, 4 (2012) 4950–4953. [39] F. Zhang, P. Wang, J. Koberstein, S. Khalid, S.W. Chan, Cerium oxidation state in ceria nanoparticles studied with X-ray photoelectron spectroscopy and absorption near edge spectroscopy, Surf Sci, 563 (2004) 74–82. [40] L. Ma, D. Wang, J. Li, B. Bai, L. Fu, Y. Li, Ag/CeO2 nanospheres: Efficient catalysts for formaldehyde oxidation, Applied Catalysis B: Environmental, 148-149 (2014) 36–43. [41] G. Gao, J.-W. Shi, C. Liu, C. Gao, Z. Fan, C. Niu, Mn/CeO2 catalysts for SCR of NOx with NH3: comparative study on the effect of supports on low-temperature catalytic activity, Appl Surf Sci, 411 (2017) 338–346. [42] Z. Qu, F. Yu, X. Zhang, Y. Wang, J. Gao, Support effects on the structure and catalytic activity of mesoporous Ag/CeO2 catalysts for CO oxidation, Chem Eng J, 229 (2013) 522–532. [43] B. Xu, Q. Zhang, S. Yuan, M. Zhang, T. Ohno, Synthesis and photocatalytic performance of yttrium-doped CeO2 with a porous broom-like hierarchical structure, Applied Catalysis B: Environmental, 183 (2016) 361–370. [44] M. Fernandez-Garcia, A. Martinez-Arias, A. Guerrero-Ruiz, J.C. Conesa, J. Soria, Ce-Zr-Ca ternary mixed oxides: Structural characteristics and oxygen handling properties, J Catal, 211 (2002) 326–334. [45] V.G. Keramidas, W.B. White, Raman spectra of oxides with the fluorite structure, The Journal of Chemical Physics, 59 (1973) 1561–1562. [46] J.R. McBride, K.C. Hass, B.D. Poindexter, W.H. Weber, Raman and x-ray studies of Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb, J Appl Phys, 76 (1994) 2435–2441. [47] Z.Y. Pu, J.Q. Lu, M.F. Luo, Y.L. Me, Study of oxygen vacancies in Ce0.9Pr0.1O2-delta solid solution by in situ x-ray diffraction and in situ Raman spectroscopy, J Phys Chem C, 111 (2007) 18695–18702. [48] X.L. Tang, B.C. Zhang, Y. Li, Y.D. Xu, Q. Xin, W.J. Shen, Carbon monoxide oxidation over CuO/CeCO2 catalysts, Catal Today, 93-5 (2004) 191–198. [49] Tana, M.L. Zhang, J. Li, H.J. Li, Y. Li, W.J. Shen, Morphology-dependent redox and catalytic properties of CeO2 nanostructures: Nanowires, nanorods and nanoparticles, Catal Today, 148 (2009) 179–183. [50] K. Sohlberg, S.T. Pantelides, S.J. Pennycook, Interactions of hydrogen with CeO 2, J Am Chem Soc, 123 (2001) 6609–6611. [51] T. Baidya, A. Gupta, P.A. Deshpandey, G. Madras, M.S. Hegde, High Oxygen Storage Capacity and High Rates of CO Oxidation and NO Reduction Catalytic Properties of Ce1–xSnxO2 and Ce0.78Sn0.2Pd0.02O2-delta, J Phys Chem C, 113 (2009) 4059–4068. [52] J. Vecchietti, S. Collins, W.Q. Xu, L. Barrio, D. Stacchiola, M. Calatayud, F. Tielens, J.J. Delgada, A. Bonivardi, Surface Reduction Mechanism of Cerium-Gallium Mixed Oxides with Enhanced Redox Properties, J Phys Chem C, 117 (2013) 8822–8831.

29

Graphical Abstract

30

Highlights 

OSC of CeO2 was efficiently enhanced by doping-induced unstable crystal structure.



Oxygen vacancy content was adjusted by manipulation of crystal structure parameter.



Low temperature OSC of Hf-doped CeO2 was 2.2 times as high as that of undoped CeO2.



Linear functions of composition vs. lattice parameter, defect vs. OSC were gotten.



Quantitative structure-performance relationships of CeO2 catalyst were constructed.

31