Oxygen vacancy clusters essential for the catalytic activity of CeO2 nanocubes for o-xylene oxidation

Catalytic oxidation of o-xylene was investigated on CeO2 nanocubes calcined at 350, 450, 550, and 650 °C, among which the samples calcined at 550 °C exhibited the highest activity and long durability. Positron annihilation spectroscopy measurements revealed that the size and distribution of oxygen vacancies for CeO2 nanocubes could be tuned by carefully controlling the calcination temperature. An excellent linear correlation between a factor related to size and density of oxygen vacancy clusters and reaction rate of o-xylene oxidation was revealed on ceria nanocubes. This means that oxygen vacancy clusters with suitable size and distribution are responsible for catalytic reaction via simultaneous adsorption and activation of oxygen and o-xylene. Electron spin resonance spectra revealed that over the CeO2 cubes, water vapor significantly promoted the formation of ∙OH radicals with a sharp decrease in the signals relating to oxygen vacancies, accelerating the transformation of o-xylene to the intermediate benzoate species, resulting in an enhancement of catalytic activity. Water thus serves as a “smart” molecule; its introduction into the feed mixture further confirmed the key role of oxygen vacancies in the catalytic performance of CeO2 nanocubes. A possible mechanism of oxygen vacancy formation during the calcination process was also proposed.

for o-xylene oxidation which is similar to that of noble metal catalysts, while being much higher than that of noble-metal-free catalysts reported elsewhere 23 . Over these nanoceria materials with well-defined facets, it is interesting that oxygen vacancy clusters (VCs) play a key role in o-xylene oxidation. It was also found that the activity of CeO 2 cubes was enhanced by the presence of water vapor. The apparent water-tolerance of these nanocubes is highly desirable for their application, since water vapor is inevitable in various exhausts.
To further reveal the intrinsic properties of nanoceria for o-xylene oxidation, herein, cubic CeO 2 was prepared by a hydrothermal method, while the nature of oxygen vacancies was tuned by carefully controlling the calcination temperature. It was found that oxygen vacancy clusters with suitable size and/or structure are essential for catalytic reaction via simultaneous adsorption and activation of oxygen and o-xylene molecules. Also, oxygen vacancies play a crucial role in the activation and decomposition of water molecules to produce •OH radicals, thus relating to the enhancement of o-xylene oxidation over CeO 2 cubes by water vapor.

Results and Discussion
Structural features of CeO 2 nanocubes. As shown by XRD measurements (Figure S1), the typical diffraction peaks of the pure fluorite cubic structure were observed for all CeO 2 nanocubes calcined at different temperatures (JCPDS 34-0394). The fluorite phase of CeO 2 was also observed by Raman spectroscopy (Supporting Information, Figure S2), exhibiting a strong band at 460-463 cm −1 24,25 . The sample calcined at 350 °C exhibited a surface area of 78 m 2 ·g −1 ( Table 1). Calcination of the sample at 450 °C resulted in a slight increase in the surface area if compared with that calcined at 350 °C. As listed in Table 1, however, further increasing the calcination temperature lowered the surface area. TEM images (Fig. 1a-d) revealed that the calcined samples exhibit cubic morphology with a uniform size. The HRTEM images in Fig. 1e (and Figure S3) combined with FTT analysis (Fig. 1f) display the clear (200) and (220) lattice fringes with the interplanar spacings of 0.274 and 0.189 nm, respectively, implying that the CeO 2 nanocubes are enclosed by {100} planes 26 . Catalytic activity of CeO 2 nanocubes for o-xylene oxidation. Figure 2 shows the catalytic activities of CeO 2 calcined at different temperatures for o-xylene oxidation. For the sample calcined at 350 °C, 10% conversion of o-xylene was obtained at 200 °C (Fig. 2a). With increasing reaction temperature, the catalytic oxidation of o-xylene was promoted sharply, giving 50% conversion at 234 °C. After this, the o-xylene conversion increased gradually as temperature increased, showing 90% conversion and complete oxidation at 250 and 270 °C, respectively. Compared with the sample calcined at 350 °C, the CeO 2 calcined at 450 °C always showed higher activity for o-xylene oxidation at a given reaction temperature. The sample calcined at 550 °C exhibited much higher activity for o-xylene oxidation than those calcined at other temperatures ( Fig. 2a and Table 1), for which the specific rate at 230 °C was 0.004 μmol•s −1 m −2 , 1.7 times higher than that calcined at 350 °C. Catalytic oxidation of o-xylene was also carried out continuously at 230 °C over the CeO 2 nanocubes calcined at 550 °C, during which ca 50% conversion was maintained over the whole time range of 150 h, demonstrating a high level of durability (Fig. 2b).
The change in catalytic activity of CeO 2 nanocubes induced by water vapor addition was evaluated at the reaction temperature of 230 °C. In the absence of water vapor, o-xylene conversion remained at 32% over the CeO 2 calcined at 350 °C (Fig. 3a). When 2 vol% water vapor was introduced into the feed gas, an increase in the o-xylene conversion of 10% was observed. As shown in Fig. 3b, simultaneously, the CO 2 yield showed almost the same tendency for increase (growth of 11%), indicating that water vapor enhances the complete oxidation of o-xylene. When water vapor was removed from the gas mixture, the o-xylene conversion recovered to its initial level as that without water vapor. Such reversible effects of water vapor on o-xylene conversion and CO 2 yield were also clearly observed over the other samples.
Oxygen species and vacancies on the surface of CeO 2 nanocubes. XPS results demonstrated that there was Ce 3+ existed in all the CeO 2 nanocubes calcined at different temperatures (Supporting Information, Figure S4a,b and corresponding description), indicating the generation of oxygen vacancies 27 . Using XPS measurements performed under ultra-high vacuum (UHV), Xu and co-workers found that the Ce 3+ concentration was almost the same for CeO 2 samples with particle sizes ranging from 4.4-9.9 nm, which was not in agreement with the amount of surface oxygen species determined by a TPR experiment 28 . Ambient-pressure XPS measurements, however, have shown that the fraction of Ce 3+ in ceria-based samples is often related to the density of oxygen vacancies [29][30][31] . In our case, the XPS experiment was performed under UHV, thus the Ce 3d XPS results were only used to confirm the presence of Ce 3+ and oxygen vacancies, not to quantitatively analyze their concentrations in CeO 2 samples calcined at different temperatures. The existence of oxygen vacancies was also confirmed by the   Table 1. Physicochemical properties and o-xylene oxidation rates and orders over CeO 2 nanocubes calcined at different temperatures. a Calcination temperature; b o-xylene conversion kept below 15% at 230 °C by varying the space velocity to realize the differential-reactor assumption.
appearance of defect-induced mode bands in the Raman spectra at around 602 cm −1 25 , which means that active sites are available for oxygen adsorption and activation in the process of catalytic oxidation. Pfau and Schierbaum concluded that the formation of Ce 3+ -related surface defects leads to an O1s core level peak with a shift of 2.4 eV to higher binding energies as compared with the low energy O1s core level 32 . Wang et al. proposed that the peak at binding energy 529.01 eV can be attributed to lattice oxygen ions in CeO 2 , the peak at 530.13 eV to absorbed oxygen, and the peak at 531.33 eV to lattice oxygen ions in Ce 2 O 3 33 . Thus, the peak in the O1s spectra at 531.5 eV, which was attributed to surface oxygen, should include a certain content of Ce 3+ -related surface defects, i.e.  oxygen vacancies. The higher ratio of surface oxygen, the higher oxygen vacancy level is. Therefore, the CeO 2 samples calcined at 450 and 550 °C possess larger amounts of oxygen vacancies ( Figure S4c and Table 2).
Positron annihilation spectroscopy (PAS) is a well-established technique to detect atomic defects in solid materials. In this case, the lifetime of positrons depends on the electron density at the annihilation site: the lower the electron density, the longer the lifetime of positrons is. As a result, the size and intensity of defects in a solid can be revealed 34 . Three lifetime values (τ 1 , τ 2 , and τ 3 ) were clearly observed for all the CeO 2 nanocubes calcined at different temperatures, the relative intensities of which were denoted as I 1 , I 2 , and I 3 , respectively (Table 3 and Figure S5). The longest value, τ 3 , the so-called bulk lifetime, was assigned to the annihilation of ortho-positronium atoms in the large voids. For all the samples, the intensity of the bulk lifetime peak (I 3 ) was the lowest and hardly changed with calcination temperature, suggesting that few large voids were present in any of the samples. Based on the previous studies [35][36][37][38] , the shortest one (τ 1 ) was due to free annihilation of positrons (denoted as FA hereafter) or positrons trapped by single oxygen vacancies (denoted as SV hereafter). The middle one (τ 2 ) could be assigned to positrons captured by surface oxygen vacancy clusters (denoted as VCs hereafter, i.e. dimmers, trimers, or larger clusters) [35][36][37][38] . It is interesting to note that increasing the calcination temperature from 350 to 450 °C significantly increased the value of τ 2 , indicating the formation of larger size VCs. Further increase in calcination temperature to 550 °C hardly changed the size of VCs while increasing their concentration. The sample calcined at 650 °C exhibited the largest VCs at the expense of lowering their concentration. To further describe the feature of the overall defects status, the average lifetime (τ av ) was calculated as follows 23 : Table 3, the value of τ av increases with increasing calcination temperature, also indicating that calcinations of samples at high temperature enhances the formation of defects in nanoceria.   Table 3. Position lifetime parameters of CeO 2 nanocubes calcined at different temperatures. a Calcination temperature; b τ av = τ 1 I 1 + τ 2 I 2 + τ 3 I 3 .

As shown in
SCiENTifiC RepORtS | 7: 12845 | DOI:10.1038/s41598-017-13178-6 Relationship between structure of CeO 2 nanocubes and activity for o-xylene oxidation. Figure 4a and Table 1 Table 1 and Fig. 4b, the lowest reaction order of o-xylene (0.33) was achieved for CeO 2 calcined at 550 °C, which also indicates that the largest amount of o-xylene was adsorbed on the surfaces of this sample. On the CeO 2 calcined at 350 °C, however, the order of o-xylene was rose to 0.94, indicative of a weak interaction. Generally, the steric effect is a crucial factor governing the kinetics of chemical reactions. For a given molecule, its cross-sectional area or volume can be used as a metric for the steric effect. Using a space-filling model (CPK model) 39,40 , the cross-sectional areas of O 2 and o-xylene were calculated to be 0.167 and 0.626 nm 2 , respectively. As a result, it is reasonable that, on a given sample, the reaction order of o-xylene is always higher than that of O 2 . In other words, the adsorption and activation of o-xylene on the surface was more difficulty than that of O 2 . On the sample calcined at 350 °C, the VCs exhibit the smallest size. This possibly indicates that the active sites are not easily accessible for o-xylene adsorption, then relating to a high reaction order for o-xylene (0.94). The CeO 2 calcined at 650 °C possessed VCs with the biggest size, resulting in the strongest ability for O 2 adsorption (giving the lowest O 2 order of 0.19), thus suppressing the adsorption of o-xylene (giving the o-xylene order of 0.53). Such adsorption behavior of reactants may lower the catalytic performance of samples calcined at 650 °C since highly catalytic activity requires proper adsorption ability for both o-xylene and O 2 on the catalyst surface according to the Sabatier principle 41 . As for the sample calcined at 550 °C, it should be noted that, the reaction order of o-xylene was lower than for those calcined at other temperatures, indicating an advantage in adsorption and activation of o-xylene.
As mentioned above, oxygen vacancies dominate the electronic and chemical properties of ceria, playing an important role in catalysis. Recently, it has been reported that CeO 2 nanorods with predominantly exposed (111) and (100) planes exhibited higher activity for CO oxidation than those enclosed by (110) and (100) planes, originating from a larger amounts of oxygen vacancy clusters in the former 35 . Recently, Lawrence et al. found that nanosized CeO 2 rods and particles subjected to a low-pressure thermal pretreatment possessed a high density of oxygen vacancy defects, promoting CO oxidation at low temperatures 31 . If oxygen vacancies actually play a role in the catalytic oxidation of o-xylene, a relationship between the vacancies and the reaction rate of o-xylene should exist. To further identify which kinds of oxygen vacancies (single oxygen vacancy and aggregates) are essential for the catalytic oxidation of o-xylene, a variable (hereafter denoted as the Vac factor, in which the size and relative density of oxygen vacancies both contribute to the catalytic reaction) was defined as follows: i i av where, τ i and I i are lifetimes of positrons in SV or FA (or VCs) and the corresponding intensity, respectively; τ av is the average lifetime of the positron. Surprisingly, there is a negative linear correlation between the Vac factor of SV or FA and reaction rate of o-xylene (Fig. 5a), while a positive linear correlation between the Vac factor of VCs and reaction rate can be drawn for ceria calcined at different temperatures (Fig. 5b). Qualitatively, this result identifies that it is the VCs that determine the activity of ceria for the catalytic oxidation of o-xylene, during which the size and relative density of VCs both contribute to the catalytic reaction.
Driven by a gain in energy during agglomeration, oxygen vacancies on the ceria surface are prone form clusters [42][43][44] . As a result, VCs exhibit higher relative intensity ( Table 3). The DFT+U calculation performed by Jiang and Dai indicated that two neighboring single oxygen vacancies working together would be more efficient for O 2 activation than those working alone on the Co 3 O 4 (110) surface 7 . Such a theoretical result possibly predicts that the oxygen vacancy clusters in metal oxides possess higher catalytic activity than single oxygen vacancies. By using quantitative temporal analysis of products (TAP) techniques, it was found that the activity of Au/CeO 2 for CO oxidation was significantly enhanced by the removal of about 7% of the surface oxygen, while over-reduction led to lower activity 45 . By using a pulse CO analysis, a volcano-like relationship between the amount of oxygen vacancies and the activity of Ir-in-CeO 2 for CO oxidation was established 46 . In that case, the concentration of oxygen vacancies was calculated based on the released amount of CO 2 and correspondingly the accumulation of carbonates with pretreatment by pulses of CO, thus it was difficult to reveal the structural features of oxygen vacancies such as the size and distribution. Our results confirm that oxygen vacancy clusters were predominant for the CeO 2 cubes. A linear relationship between the value of τ 2 •I 2 /τ av and the reaction rate of o-xylene indicates that both the size and relative density of VCs contributed to the activity of the CeO 2 cubes. The sample calcined at the temperature of 550 °C exhibited the highest activity, indicating that oxygen vacancy clusters with suitable size and distribution are essential for catalytic reaction via simultaneous adsorption and activation of oxygen and o-xylene molecules.

Water vapor effect on the pathway of o-xylene oxidation over CeO 2 nanocubes. As o-xylene
oxidation occurred on the surface of CeO 2 cubes, another interesting finding is that the presence of water vapor promoted the complete conversion of o-xylene to CO 2 . To reveal how such enhancement is triggered by water vapor, ESR analysis was performed without or with water vapor (Fig. 6a). In the absence of water vapor, Ce 3+ , O 2 − species, and Ce 3+ -O-− Ce 4+ -type defect sites were distinctly observed on the sample calcined at 550 °C 47-49 , further confirming the results of XPS and PAS. Introduction of water vapor resulted in the disappearance of the characteristic signals due to Ce 3+ , oxygen vacancies, and active oxygen species, suggesting that some reactions occurred among these species/sites and water molecules. Based on previous studies 50, 51 , one possibility that could be speculated is the formation of •OH radicals. To test this hypothesis, ESR spectra of the DMPO-OH • spin adduct were measured after mixing the CeO 2 powders with DMPO solution at room temperature since •OH radicals are very unstable in water (Fig. 6b). The four characteristic peaks of the DMPO-OH • species, a 1:2:2:1 quartet pattern, were clearly observed, confirming the production of •OH radicals after adding water to the surface of CeO 2 50,51 . Numerous studies have been performed on the process of water adsorption on ceria, and it is well accepted that H 2 O molecules strongly and dissociatively bind on oxygen vacancy sites 11 . Thus, it is possible that the formation of •OH radicals is dependent on the activation of H 2 O molecules on the oxygen vacancy sites through Ce 3+ /Ce 4+ redox cycle 11,50,51 . Generally, •OH radicals possess high oxidation ability, and thus relating to the promoting effect of water vapor on complete oxidation of o-xylene to CO 2 . After removal of water vapor from the sample by heating the sample at 150 °C (Fig. 6a), it should be noted that the signals assignable to Ce 3+ , O 2 − , and Ce 3+ -O-− Ce 4+ -type defect sites appeared again and the sample recovered its initial reactivity level in the absence of water vapor. Such a reversible effect is in good agreement with the results of activity measurement presented in Fig. 3.
To investigate the pathway of o-xylene oxidation over the CeO 2 nanocubes, in situ DRIFTS experiments were performed. Figure 7a shows the DRIFT spectra of CeO 2 calcined at 550 °C during exposure to a flow of 500 ppm o-xylene in 20 vol% O 2 . At temperatures below 235 °C, four peaks attributed to the stretching vibrations of C-H bonds were observed. Among them, the bands at 3067 and 3031 cm −1 were assigned to the phenylic C-H stretching vibration (Table S2) 52,53 . The bands at 2948 and 2878 cm −1 are typically due to the symmetric and antisymmetric stretching vibrations of C-H bonds of benzyl species. The intensity of bands at 3031, 2948, and 2878 cm −1 decreased with increasing reaction temperature, and almost disappeared at 300 °C. Simultaneously, however, the intensity of the high-frequency band at 3067 cm −1 increased, indicating the transformation of the methyl group of the o-xylene molecule 52,53 . Within the range of 1800-1000 cm −1 , the bands related to ring vibrations of aromatic systems were clearly observed at 1604 and 1468 cm −1 , the intensity of which exhibited a gradual decrease as temperature increased from 120 °C to 235 °C, indicating a change in the electronic environment of the aromatic ring. Within this temperature range, meanwhile, an increased intensity of the characteristic bands attributed to the carboxylate group appeared at 1546 and 1396 cm −1 , suggesting the formation of benzoate species 52,53 . Further heating of the sample to 300 °C resulted in a disappearance of the two bands due to aromatic rings, the occurrence of which was companied by a sharply increased intensity of peaks for benzoate species. In addition, peaks at 1266 and 1047 cm −1 could be assigned to o-xylene, the intensity of which decreased with increasing temperature, and disappeared above 235 °C 54 . Obviously (Fig. 7b), the introduction of water vapor distinctly decreased the intensity of bands at 1604 and 1468 cm −1 while significantly increasing the intensity of bands at 1546 and 1396 cm −1 . These results strongly suggested that the presence of water vapor promoted the formation of benzoate at the expense of o-xylene.
Previous studies carried out within the temperature range of 450-1000 K showed that the gas-phase reaction of o-xylene with •OH radicals exhibited an Ea value of 0.29 kJ/mol 55,56 , which is also indicative of a low reaction energy barrier if this reaction occurs on the surface of catalysts. As indicated by Fig. 6, the introduction of water vapor triggered the formation of •OH radicals via dissociation of H 2 O on oxygen vacancy sites of CeO 2 . As a result, it is reasonable that the conversion of o-xylene over CeO 2 nanocubes was enhanced by water vapor (Figs 3  and 7). Using in situ DRIFTS measurement, Zhao et al. 53 revealed that benzoate species are the main intermediates in the catalytic oxidation of toluene, which should be further oxidized by O 2 to the final products of CO 2 and H 2 O. Figure 7 shows that adsorbed o-xylene predominated on the surface of CeO 2 nanocubes at temperatures below 235 °C, while at temperatures above 235 °C, only benzoate was observed during the catalytic oxidation of o-xylene. With this in mind, it is possible that the pathway of o-xylene oxidation over CeO 2 nanocubes can be regarded as follows: Introduction of water vapor into the feed of o-xylene + O 2 promoted the formation of benzoate and its further oxidation towards CO 2 . As a result, increased o-xylene conversion and CO 2 yield were obtained in the presence of water vapor (Fig. 3).
There have been seminal papers on ceria shapes and ceria oxygen vacancies, particularly on the atomic surface structures of CeO 2 nanocubes 57-60 . By using HRTEM and three-dimensional electron tomography, Kaneko et al. 57 found that CeO 2 nanocubes exposed predominantly {200} facets, with truncation of the corners exposing {111} facets. This structural feature of CeO 2 nanocubes was further revealed by clearly imaging the O atoms 60 . More importantly, it was found that, although {111} corners are present on the CeO 2 nanocubes, the contribution of these sites to the total adsorption of methoxy species should be negligible. A previous study confirmed that methanol can act as a "smart" molecule for probing the nature of the surface sites of ceria catalysts 59 . It dissociates on the surface to form methoxy species whose structure is dependent on the nature of the surface sites, i.e., the coordination status of surface Ce cations and the presence of oxygen vacancies. As a result, Lin and co-workers concluded that these mixed surface terminations described above should be intrinsic to the pristine {100} surfaces of the nanocubes 60 .
As shown in Fig. 1 and Figure S3, the CeO 2 nanocubes calcined at low temperatures of 350 and 450 °C were clearly enclosed by {100} facets, while truncated corners appeared on the sample calcined at 550 °C. This means that new facets composed of (111) surfaces were present at the corners of the sample calcined at 550 °C. More  performance. As shown in Fig. 3, the presence of water vapor always enhanced the catalytic activity of CeO 2 samples, regardless of the calcination temperature. As for the CeO 2 calcined at 450 °C and 550 °C, indeed, water vapor addition resulted in increases of 13% and 6% in o-xylene conversion, respectively. These slightly different levels of increase indicate that the contribution of new facets appearing in the sample calcined at 550 °C to o-xylene oxidation was not decisive. It should be noted that observation of the signals assignable to Ce 3+ , O 2 − , and Ce 3+ -O − Ce 4+ -type defect sites in CeO 2 is closely related to the introduction and removal of water vapor, which in turn indicates that oxygen vacancies play a key role in the catalytic performance.
Considering a reversible effect of water vapor on the oxygen vacancies, a possible mechanism of oxygen vacancy formation during the calcination process was also proposed. Electron spin resonance measurement revealed that water vapor sharply decreased the intensity of signals relating to oxygen vacancies, while removal of water by heating the sample at the low temperature of 150 °C resulted in the defect sites appearing again. Such reversible effect of water vapor on the oxygen vacancies may provide an important clue to understanding how the calcination process changes the properties of oxygen vacancies on CeO 2 nanocubes. To test this hypothesis, TG and DSC analyses were performed on an uncalcined sample ( Figure S6). This result clearly showed that water desorption and condensation of hydroxyl groups occurred 61 , leading to the formation of oxygen vacancies on the CeO 2 . In agreement with the result of Natile and co-workers, a continuous weight loss due to water desorption and condensation of hydroxyl groups was observed at temperatures below 700 °C, indicating that the higher the calcination temperature, the higher the concentration of oxygen vacancies.
Driven by a gain in energy during agglomeration of single oxygen vacancies or oxygen vacancy clusters with small size, previous studies revealed that oxygen vacancies on the ceria surface are prone to form clusters [42][43][44] . Using high-resolution scanning tunneling microscopy (STM), Esch et al. confirmed that the direct diffusion of oxygen vacancies (i.e. hopping of lattice oxygen) on the CeO 2 surface requires temperatures higher than 400 °C. With this in mind, we can deduce that the direct diffusion of oxygen vacancies can hardly occur if the samples are calcined at the temperature of 350 °C, thus giving a low possibility for gathering single oxygen vacancies together into a cluster. As a result, the oxygen vacancies on this sample are mainly present as single oxygen vacancies. Calcination the samples at 450, 550, and 650 °C triggered the diffusion of oxygen vacancies, thus oxygen vacancies clusters were predominant on these three samples. Such diffusion possibly became more pronounced with increasing temperature, particularly during calcination at higher temperatures such as 650 °C, leading to oxygen vacancy clusters with bigger size ( Table 3). As a result, it is reasonable that the CeO 2 nanocubes calcined at different temperatures exhibit oxygen vacancies with different sizes and size distributions.
In summary, CeO 2 nanocubes calcined at 550 °C exhibited the highest activity for catalytic oxidation of o-xylene among the CeO 2 samples calcined at 350, 450, 550, and 650 °C. The reason was that CeO 2 samples calcined at 550 °C had oxygen vacancy clusters with suitable size and distribution, which are essential for catalytic reaction via simultaneous adsorption and activation of O 2 and o-xylene molecules. Water vapor enhanced the complete oxidation of o-xylene through the formation of ·OH radicals, which were helpful for increasing the formation of the intermediate species benzoate, and then promoted the further oxidation of benzoate species into CO 2 , thus increasing the o-xylene conversion and CO 2 yield.

Methods
Catalyst preparation. CeO 2 nanocubes were synthesized by a hydrothermal method as described in our previous studies 23,62 . Briefly, Ce(NO 3 ) 3 ·6H 2 O was dissolved in deionized water, and then a suitable amount of 1 mol/L NaOH solution was added dropwise into the above solution. After 20 min of stirring, the mixture was then hydrothermally treated at 100 °C for 12 h. The fresh white precipitates was separated by centrifugation, and thoroughly washed with deionized water. The solid obtained was dried at 60 °C in air for 24 h and calcined at 350, 450, 550 and 650 °C for 4 h in air, respectively.
Catalytic activity test and kinetic measurements. The catalytic activity was evaluated in a fixed-bed quartz reactor with 100 mg of CeO 2 nanocubes (40-60 mesh) by passing a reaction gas of 500 ppm o-xylene and 20 vol% O 2 in N 2 at a rate of 100 mL/min. The o-xylene gas was supplied by bubbling liquid o-xylene with N 2 , the concentration of which was controlled by the flow rate of N 2 . Analysis of the concentrations of reactants and the products was carried out on-line using a GC-MS (Agilent 6890-5973N) with a HP-5MS capillary column and another GC (GC112A, Shangfen, China). The conversion of o-xylene (X o-xylene , %) and CO 2 yield (%) were calculated as the following: where C o-xylene (in) (ppm) and C o-xylene (out) (ppm) are the concentrations of o-xylene in inlet and outlet gas streams, respectively. [CO 2 ] is the concentration of CO 2 (ppm). Kinetic measurements for o-xylene oxidation (reaction rates and orders) were performed at 230 °C by using 100 mg catalyst (40-60 mesh). To realize a differential-reactor assumption, o-xylene conversion was kept below 15% by changing the gas hourly space velocity in the range of 2.3 × 10 5 -1.3 × 10 6 mL·h −1 ·g −1 .
Catalyst characterization. The BET surface areas of CeO 2 materials were analyzed by N 2 adsorption at 77 K on a Quantasorb-18 automatic system. XRD patterns were measured on a PANalytical X′Pert PRO X-ray diffractometer (Japan) with a CuKα radiation. TEM observations were carried out on a Hitachi H-7500. The HR-TEM images and SAED patterns were obtained on JEOL JEM 2011 TEM. Raman spectra were measured on a UV Resonance Raman Spectrometer (UVR DLPC-DL-03) equipped with a CCD detector. XPS spectra were measured with a scanning X-ray microprobe (PHI Quantera, ULVAC-PHI, Inc) using Al Kα radiation. Electron Spin Resonance (ESR) was recorded using a Bruker A300-10/12 ESR spectrometer at room temperature. In order to determine the formation of •OH radicals in the presence of water vapor, the sample for ESR measurement was also prepared by adding the CeO 2 to 5,5-dimethyl-1-pirroline-N-oxide (DMPO) solution to form an aqueous dispersion to test for the presence of DMPO-•OH. PAS measurements were carried out with a magnetically guided variable-energy (0-20 KeV) positron beam and the corresponding results were analyzed with the POSITRONFIT-88 program.
In situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTS). In situ DRIFTS experiments were carried out on a Nicolet Nexus 670 FTIR equipped with a mercury cadmium telluride (MCT) detector, which was cooled by liquid nitrogen. All spectra were measured with a resolution of 4 cm −1 and with an accumulation of 100 scans.