Morphology effects on surface chemical properties and lattice defects of Cu/CeO2 catalysts applied for low-temperature CO oxidation

Here, we synthesized a series of Cu/CeO2 catalysts with different morphology and size, including Cu/CeO2 nanospheres (Cu/CeO2-S), Cu/CeO2 nanoparticles (Cu/CeO2-P), Cu/CeO2 nanorods (Cu/CeO2-R) and flower-like Cu/CeO2 microspheres (Cu/CeO2-F) to systematically explore the structure-activity relationship in CO oxidation. Crucially, the effect of morphology, crystal size, Ce4+/Ce3+ species, oxygen vacancies derived from the removal of lattice oxygen (Olatt) species in CeO2 and lattice defect sites on CO activity was revealed through various characterizations. It was clearly discovered that the activity of these catalysts was as follows: Cu/CeO2-R > Cu/CeO2-P > Cu/CeO2-S > Cu/CeO2-F, and the Cu/CeO2-R catalyst preferentially showed the best catalytic performance with a 90% conversion of CO even at 58 °C, owned the smaller particles size of CeO2 and CuO, and exhibited the higher concentration of Olatt species and oxygen vacancies. Besides, it is also verified that the Cu/CeO2-F sample exhibited the larger CeO2 crystal size (17.14 nm), which led to the lower Cu dispersion and CO conversion, even at 121 °C (T90). Most importantly, we discovered that the amount of surface lattice defect sites was positively related to the reaction rate of CO. Simultaneously, DFT calculation also demonstrated that the introduced oxygen vacancies in CeO2 could accelerate the oxidation of CO by the alteration of CO adsorption energy. Therefore, the morphology, the crystal size, the content of oxygen vacancies, as well as lattice defects of Cu/CeO2 catalyst might work together for CO oxidation reaction.

greatly affect its catalytic performance, thus we take CO oxidation as an example to evaluate the catalytic activity and stability of these Cu/CeO 2 catalysts. According to Fig. 1a, the Cu/CeO 2 -R catalyst exhibited better activity than other three catalysts, with 90% CO conversion at 58 °C, which was substantially lower than the corresponding temperature for the Cu/CeO 2 -F catalyst (T 90 = 121 °C). In addition, it was also discovered that the complete conversion temperature of CO over Cu/CeO 2 -P catalyst at 91 °C was close to that of Cu/CeO 2 -S catalyst at 103 °C. Figure 1b showed Arrhenius plots of CO conversion with respect to the inverse reaction temperature. The apparent reaction barriers (Supporting Information) are derived as 37.2 kJ mol −1 , 74.5 kJ mol −1 , 82.4 kJ mol −1 and 153.6 kJ·mol −1 for the Cu/CeO 2 -R, Cu/CeO 2 -P, Cu/CeO 2 -S and Cu/CeO 2 -F catalysts, respectively. All the reaction results indicated that the CeO 2 nanorods covered with Cu NPs exhibited the best CO oxidation activity. The introduction of CeO 2 nanorods greatly decreased the energy barrier of CO oxidation reaction, while the using of flower-like CeO 2 microspheres resulted in the higher energy barrier of CO oxidation.
The thermal stability is very important for the practical application of catalyst, and thus the cycle stability of Cu/CeO 2 -R catalyst is tested at different temperature. According to Fig. 1d, the Cu/CeO 2 -R catalyst was tested through three runs, and it was discovered that CO conversion at the third run still kept almost equivalent with the first run. These results verified that the Cu/CeO 2 -R catalyst owned the superior thermal stability.
For practical circumstance, there is generally a small amount of H 2 O vapor in the feed gas. Thus, the H 2 O resistance was performed at 58 °C on the best Cu/CeO 2 -R catalyst (Fig. 2a). Interestingly, it could be seen that the introduction of 3% H 2 O at 58 °C resulted in an obvious increase of CO conversion from 90% to 100% over Cu/ CeO 2 -R catalyst. Simultaneously, a 100% conversion of CO could keep 9 h and then exhibited a decline. When the CO conversion decreased to 75% in the present of H 2 O, we turned off H 2 O and it resulted in a gradual increase of CO conversion, indicating that H 2 O played the significant role in promoting the conversion of CO. However, the excessive present of H 2 O would lead to the deactivation of Cu/CeO 2 -R catalyst, and the reasons might be related to the comparative adsorption of H 2 O and CO on the surface of catalysts 41 . A. Martínez-Arias et al. 42 reported that the deactivation of a CuO/CeO 2 catalyst under humidity conditions CO oxidation are mainly related to the modifications of interfacial sites due to the formation of specific carbonates and a blocking effect induced by the presence of adsorbed molecular water, respectively. The decrease of active sites could limit the catalytic activity of CO oxidation. Accordingly, the dramatic fast drop of CO conversion after 9 hours in this work might be related to a blocking effect induced by the presence of excessively adsorbed water molecules. It was also easy to understand about a gradual increase of CO conversion after stopping water, and the excessively adsorbed molecular water was slowly consumed, thereby weakening the competitive adsorption capacity of the active sites to expose more active sites for adsorption and activation of CO and O 2 molecules. www.nature.com/scientificreports www.nature.com/scientificreports/ To define the specific promotion degree of H 2 O, in further we investigated the effect of H 2 O at the different reaction temperature and compared CO conversion with respect to temperature. According to Fig. 2b, we discovered that the CO conversion increased from 18% to 54% at 40 °C under humidity, while the further increase of reaction temperature led to the obvious improvement of CO conversion. For example, the reaction temperature increased from 40 °C to 45 °C, which resulted in the extremely significant increase of CO conversion from 24% to 100%. For Fig. 2b, it was discovered that the promotion of H 2 O became more and more critical with the increase of reaction temperature.
Texture properties and Morphology of these Cu/CeO 2 catalysts. The textural properties of CeO 2 and Cu/CeO 2 samples were explored from their corresponding N 2 adsorption-desorption isotherms at −196 °C. The BET specific surface area and cumulative pore volume were summarized in Tables 1 and S1. As shown in Figure 1. CO conversion (a), Arrhenius plots of CO reaction under CO conversion below 10% (b), CO reaction rate (r CO ) derived from Arrhenius plots (c) of these Cu/CeO 2 catalysts with different morphology, the cyclic stability of Cu/CeO 2 -R catalyst (d).   Fig. S1b, which was concluded from the sharp increase of the pore volume at low p/p 0 . Besides, BET specific surface area of these Cu/CeO 2 catalysts is ranging between 88.6 m 2 g −1 and 28.5 m 2 g −1 , and the reason is most probably attributable to the size difference of CeO 2 and a certain degree of pore blockage caused by the presence of copper species on the surface of CeO 2 .
The morphology of CeO 2 not only affected the exposed crystal plane, but also modulated the size of supported CuO NPs as it was treated as support. Herein, to explore the relation of morphology and the exposed crystal plane, SEM, TEM and HRTEM analysis were conducted. According to Fig. 3, we discovered that these Cu/CeO 2 catalysts formed their unique morphology, such as a Cu/CeO 2 -R with nanorods shape, a Cu/CeO 2 -S with nanospheres shape, a Cu/CeO 2 -P with nanoparticles shape and a Cu/CeO 2 -F with flower-like microspheres shape. The Cu/CeO 2 -P catalyst was constructed by a large number of small nanoparticles with average sizes of around 10-20 nm (Figs 3a and S2), and the exposed crystal planes were related to (200) (Figs 3h and S9). Therefore, the above results suggested these Cu/CeO 2 catalysts with different morphology exposed the different crystal planes: the {111} and {100} crystal planes for Cu/CeO 2 -P and Cu/CeO 2 -S catalysts, the {110} crystal planes for Cu/CeO 2 -R catalyst, and the {100} crystal planes for Cu/CeO 2 -F catalyst. Combined with their activity, it was considered that the exposed (110) crystal planes of CeO 2 nanorods might be beneficial to promote the conversion of CO. Figure 4 shows the XRD patterns of Cu/CeO 2 catalysts with different morphology. According to Fig. 4, these Cu/CeO 2 samples exhibited the diffraction peaks at 28.5°, 33.1°, 47.5° and 56.3°, indicating that the main diffraction peaks of these Cu/CeO 2 samples matched with the fluorite-type cubic CeO 2 phase (JCPDS 89-8436). The crystal size was calculated according to Scherrer equation, and half width of CeO 2 (111) plane at 2θ = 28.5° was selected. The half width at 28.5° follows the order of Cu/ CeO 2 -S < Cu/CeO 2 -R < Cu/CeO 2 -P < Cu/CeO 2 -F ( Table 1), indicating that the morphology and texture properties of CeO 2 support greatly altered its crystal size. Notably, in Fig. 4 the Cu/CeO 2 -F sample displayed two weak diffraction peaks of CuO (JCPDS 80-1268) at 2θ of 35.6° and 38.7°, and the Cu/CeO 2 -R sample also displayed two extremely weak diffraction peaks of CuO at 35.6° and 38.7°, indicating that copper oxide crystal size of Cu/ CeO 2 -R was further smaller than that of Cu/CeO 2 -F. However, it is no obvious CuO diffraction peak for Cu/ CeO 2 -S and Cu/CeO 2 -P, which was possibly related to the contribution of the ultra-small CuO grain on these two catalysts. Therefore, it was concluded that the difference of CeO 2 morphology and crystal size might also lead to the growth difference of CuO grain (Fig. S10). Combined with the reaction rate of CO (r co ) on these Cu/CeO 2 catalysts, it was discovered that the lower r co of Cu/CeO 2 -F catalyst might be due to the larger CeO 2 and CuO crystal size, which decreased the dispersion of CuO NPs and inhibited the conversion of CO.

Size effect of CeO 2 and Cu nanoparticles (NPs).
In order to verify the reducibility of these Cu/CeO 2 catalysts, H 2 -TPR analysis was conducted, and the results shown in Fig. 5. It was discovered that Cu/CeO 2 -R catalyst exhibited an asymmetric reduction peak at 226 °C, and Cu/CeO 2 -P catalyst also showed an asymmetric reduction peak at 232 °C. Besides, the Cu/CeO 2 -S and Cu/ CeO 2 -F catalysts respectively appeared asymmetric reduction peak at 236 °C and 249 °C. These asymmetric reduction peaks might be related to the different particles size of CuO NPs and the interaction of Cu species and CeO 2 support. The former works 43,44 reported that the small CuO particles are more easily reduced than large CuO particles. Therefore, it was verified that the Cu/CeO 2 -F catalyst owned the larger CuO particles. Similarly, the particles size of CuO on these catalysts is in the order of Cu/CeO 2 -F > Cu/CeO 2 -S > Cu/CeO 2 -P > Cu/ CeO 2 -R catalysts on basis of H 2 -TPR analysis. The smaller CuO NPs can accelerate the oxidation of CO 45 , which may be one of the reasons that Cu/CeO 2 -R catalyst exhibited the best activity in CO oxidation reaction. Besides, the strong interaction of Cu species and CeO 2 support (Ce 4+ + Cu + → Ce 3+ + Cu 2+ ) was advantageous to promoting the dispersion of copper species, and then it might be considered that the Cu/CeO 2 -R catalyst had a stronger www.nature.com/scientificreports www.nature.com/scientificreports/ interaction between Cu species and CeO 2 nanorods. The H 2 -TPR results are consistent with CO-TPR (Fig. S11). Besides, the reduction peak above 300 °C was not experimentally included, and then we assumed that the surface oxygen originated from the complete reduction of Cu 2+ to Cu°. In order to quantitatively determine the quality hydrogen consumption, a series of pure CuO samples are treated as the reference, and the H 2 uptake (µmol H 2 / g cat ) of these Cu/CeO 2 catalysts is calculated by the external standard method in Table 1. On basis of standard curve for hydrogen consumption of different masses of CuO, the amount of H 2 assumption over these Cu/CeO 2 samples follows the order of Cu/CeO 2 -P > Cu/CeO 2 -R > Cu/CeO 2 -S > Cu/CeO 2 -F, which is attributed to the different amounts of oxygen derived from the bulk and surface Cu x O (x ≤ 2).
The surface chemical properties and lattice defects. In order to obtain the chemical state and the surface composition information of these Cu/CeO 2 catalysts, the XPS characterization was performed. Figure 6a showed the Ce 3d XPS spectra of Cu/CeO 2 catalysts. Accordingly, it was divided into the ten peaks corresponding www.nature.com/scientificreports www.nature.com/scientificreports/   www.nature.com/scientificreports www.nature.com/scientificreports/ to five pairs of Ce 3d spin-orbit doublets. There are two types spin-orbit components, including the Ce 3d 3/2 (marked as u 0 -u"') and the Ce 3d 5/2 (marked as v 0 -v"'). Generally, oxygen vacancies were formed in the CeO 2 fluorite lattice to maintain charge balance, and the number of oxygen vacancies affected the adsorption properties of Ce-based catalysts. For Fig. 6a, the peaks of u°, u' , v° and v' were related to the 3d 3/2 and 3d 5/2 of Ce 3+ species, and other peaks were ascribed to the Ce 4+ species [46][47][48][49] . The relative content of Ce 3+ species can be calculated on basis of peak area ratio of u°, u' , v° and v' to the total peak area of Ce 3d ( Table 2). The production of surface oxygen vacancy derived from the removal of lattice oxygen (O latt ) species in CeO 2 and resulted in the formation of Ce 3+ species (Ce 4+ + e − → Ce 3+ + ◇), since the content of Ce 3+ species can semi-quantitatively determine the surface oxygen vacancy. According to Table 2, the surface content of Ce 3+ species is in the order of Cu/CeO 2 -F > Cu/ CeO 2 -R > Cu/CeO 2 -S > Cu/CeO 2 -P catalysts, indicating that the Cu/CeO 2 -F and Cu/CeO 2 -R catalysts owned the relative more surface oxygen vacancies. Combined with Figs 1a and 6a, it was discovered that the CO oxidation activity might be also related to the amount of surface oxygen vacancies (Ce 3+ species), indicating that surface oxygen vacancies played the key role in determining the conversion of CO. However, due to the lower dispersion of CuO species over the Cu/CeO 2 -F catalyst exhibited the lower CO oxidation activity, indicating the Ce 3+ species and the dispersion of CuO species both affected the activity of Cu/CeO 2 catalyst.
The XPS spectra in the O 1 s region can also be deconvoluted into two peaks in Fig. 6b. According to former reports 50,51 , the peak with high binding energy was related to the surface adsorbed oxygen species (denoted as O ads ), and the peak with low binding energy was assigned to surface lattice oxygen species (denoted as O latt ). The binding energy of the lattice oxygen (O latt ) in Ce (IV) oxide was about 529.6 eV. The binding energy of O ads on the surface of ceria oxide, such as hydroxyl groups on the surface, oxygen chemisorbed on the surface, grain-boundary impurities and oxide ions in the defective CeO x (x < 2) was about 531-533 eV 52-54 . Yao. K et al. [55][56][57] also reported that the binding energy of lattice oxygen (O latt ) in Cu 2 O phase shifted to 530.3 eV, the binding energy of the lattice oxygen (O latt ) in CuO phase was related to 529.3-529.6 eV, and the binding energy of the oxygen (O ads ) adsorbed on the surface of CuO and -Cu 2 O phase was ascribed to at 531.5 eV. Accordingly, it was concluded that in this work the binging energy of peaks was close to that of lattice oxygen in CuO and Ce (IV) oxide, indicating that CuO and Ce (IV) oxide phases were formed in these Cu/CeO 2 catalysts. Generally, for CO oxidation reaction, the O latt species were considered as the active oxygen 58 . Herein, we calculated the content of O latt species according to the peak area, and the results in Table 2 showed that the content of O latt follows in order of Cu/CeO 2 -R > Cu/CeO 2 -P > Cu/CeO 2 -S > Cu/CeO 2 -F. Generally, the removal of O latt species in CeO 2 would form a large number of oxygen vacancies, which is advantageous to promoting the conversion of CO.
In order to theoretically explore the role of oxygen vacancy on CeO 2 surface, a density functional theory (DFT) calculation was also conducted. The calculation was done by density functional theory (DFT) method employing the VASP package with PBE + U (Ueff = 5.0 eV) approximation [59][60][61] . First, CO adsorption energy on the CeO 2 (110), (111) and (100) surface without oxygen vacancy was systematically investigated, and the results shown in Fig. 7. The adsorption of CO on the clean CeO 2 (110) surface existed two models, including the adsorption on the O-O bridge site and the top of Ce. The adsorption energy on these two models of the clean CeO 2 (110) surface was calculated, and the E ads° was respectively −3.28 eV and −0.21 eV in Table 3. Besides, the adsorption energy of CO on the clean CeO 2 (111) surface is −0.18 eV, which was considered on the top of Ce atom. Similarly, it was also found that CO adsorption on the clean CeO 2 (100) surface was greatly unstable. When the oxygen termination is selected, CO can immediately react with the oxygen to form CO 2 .
Comparatively, the adsorption energy of CO on the CeO 2 (110), (100) and (111) surface with oxygen vacancy is also investigated using DFT method. Before this, the formation energy on the CeO 2 (110), (111) and (100) surface with oxygen vacancy is necessarily explored, and the results shown in Fig. 8a and Table 3. The adsorption of CO on the CeO 2 (110) surface containing oxygen vacancy also existed two models, including the adsorption on the O-O bridge site and the top of Ce atom. The adsorption energy of CO at the O-O bridge site of CeO 2 (110) surface is −3.78 eV, which is higher than that of the clean CeO 2 (110) surface. The adsorption energy of CO at the top of Ce atom is −0.21 eV, which is the same to that of the clean CeO 2 (110) surface. In addition, the adsorption energy of CO on the CeO 2 (111) surface is −0.67 eV, which is lower than that of the clean CeO 2 (111) surface. Besides, it is found that CO adsorption energy on the CeO 2 (100) surface is −0.33 eV, indicating that the existence of oxygen vacancy decrease CO adsorption energy of CeO 2 (100) surface. Interestingly, the formation of oxygen vacancy on the CeO 2 (110) surface could enhance the adsorption of CO due to the decrease of adsorption energy, which is vitally significant for promoting the conversion of CO. Combined with Figs 1, 3 and 8, it is verified that the CeO 2 (110) crystal plane is advantageous to promote CO oxidation, which is also one of the reasons that Cu/ CeO 2 -R sample exhibited the superior performance in CO oxidation reaction.  www.nature.com/scientificreports www.nature.com/scientificreports/ Raman characterization was performed on these CeO 2 and Cu/CeO 2 catalysts to further investigate their surface information. As shown in Figs 9 and S13, the peak centered at 458 cm −1 in Raman spectra was ascribed to the F 2g symmetric vibration (Ce-O-Ce stretching). According to work 62 , the peak centered at near 464 cm −1 was related to the pure cubic fluorite CeO 2 . The obvious red shift of F 2g peak in these Cu/CeO 2 samples indicated the change of surface crystal lattice parameter in CeO 2 63 . Besides, a broad peak of the Raman spectra centered at 600 cm −1 is due to the existence of oxygen vacancy on the surface of these Cu/CeO 2 catalysts 64 . The relative amount of oxygen vacancy can be calculated as intensity ratio of bands centered at 600 cm −1 and 458 cm −1 (I D / I F2g ) 27 , also indicating the relative amount of surface lattice defect site, and the results were summarized in Table 2. It was discovered that the I D /I F2g ratio of the Cu/CeO 2 -R catalyst was higher, indicating the relative amount of surface lattice defect site was more. Besides, the I D /I F2g ratio of Cu/CeO 2 -P catalyst is very close to that of Cu/CeO 2 -R catalyst. The I D /I F2g ratio indicated that the morphology of Cu/CeO 2 catalysts can greatly modulate the surface lattice defect sites due to the strong interface interaction between CuO NPs and CeO 2 . The above results suggested that the CeO 2 nanorods as support can notably promote the production of more surface lattice defect sites.
The relation of surface lattice defect sites and CO activity on these Cu/CeO 2 catalysts was associated in Fig. 10. Through comparing the relation of surface lattice defect sites derived from Raman analysis and r CO in Fig. 10a, we discovered that the surface lattice defect sites was also a factor in promoting the conversion of CO, and the amount of surface lattice defect sites was positively related to r CO . Besides, in Fig. 10b, the relation of lattice oxygen species (O latt ) derived from O1s XPS was also positively related to the activity of Cu/CeO 2 catalyst in CO oxidation reaction. Therefore, the increase of surface lattice defect sites would be beneficial to improving the catalytic performance of Cu/CeO 2 catalysts, which can modulate the adsorption properties of reactant molecular.
Combined with a series of characterizations and DFT calculation, it was verified that morphology of CeO 2 played the key role in determining the dispersion of supported Cu NPs, its exposed crystal plane, the interaction of Cu species and CeO 2 support, the number of O latt species and oxygen vacancies, which were vitally important for improving the catalytic performance of Cu/CeO 2 catalysts. For example, former works 38,65,66 considered that the Ce-based catalysts with exposed CeO 2 (111)/(100) planes owned much higher activity in comparison to the Ce-based catalysts exposed by CeO 2 (110)/(100) planes, indicating that the exposed plane was vitally important for determining the catalytic activity of Ce-based catalyst. Simultaneously, we discovered that the lattice oxygen species (O latt ) exhibited the effect on catalytic performance of Cu/CeO 2 catalysts in CO oxidation reaction, and then a reaction pathway was proposed over Cu/CeO 2 catalysts in Fig. 11. Combined with the O 1s XPS characterization, it was verified the Cu/CeO 2 -R catalyst exhibited the more O latt species, which might be the main reason with the better activity in CO oxidation reaction. The oxidation of CO over the Cu/CeO 2 -R catalyst is as follows: CO molecular first adsorbed the metal active sites, and thus reacted with the adjacent lattice oxygen (O latt ) to form CO 2 , H 2 O and oxygen vacancy. Afterwards, O 2 molecular was absorbed and replenished into this oxygen vacancy,  www.nature.com/scientificreports www.nature.com/scientificreports/ and reacted with another CO molecular. So far, a redox cycle was completed. A large number of works 67,68 also reported that oxygen vacancies are very significant for promoting the conversion of CO. Therefore, the morphology and size of CeO 2 could not only modulate the dispersion of supported metal, but also alter the interaction of Cu species and CeO 2 support (Ce 4+ + Cu + → Ce 3+ + Cu 2+ ), the amount of lattice oxygen (O latt ) species and lattice defect sites, which played the important role in determining the reaction rate of CO.  www.nature.com/scientificreports www.nature.com/scientificreports/ The promoting role of H 2 O on the catalytic performance of Cu/CeO 2 -R catalyst. It has been verified that Cu/CeO 2 -R catalyst owned the best catalytic performance in CO oxidation reaction, and the excellent H 2 O resistance was also discovered. To explore the promoting role of H 2 O, the used Cu/CeO 2 -R catalyst was systematically analyzed on basis of a series of characterizations, and the results were listed in Fig. 12. It was discovered that the XRD of used Cu/CeO 2 -R catalyst is the same to that of fresh Cu/CeO-R catalyst, indicating that its phase composition remained unchanged before and after H 2 O resistance reaction. Interestingly, the XPS results verified that the Ce 3+ and O latt species of used Cu/CeO 2 -R catalyst are obviously higher than that of fresh Cu/CeO 2 -R catalyst, suggesting that the adsorption of H 2 O in the surface of Cu/CeO 2 -R catalyst can promote the formation of surface Ce 3+ species. More importantly, the results of Raman also verified that the obvious increase of surface lattice defect sites was discovered on used Cu/CeO 2 -R catalyst in Fig. S14. Besides, the H 2 -TPR characterization of Cu/CeO 2 -R catalyst after H 2 O resistance reaction was also investigated (Fig. S15), and the results is consistent with XRD results. The above results confirmed that the formation of Ce 3+ species and the increase of surface lattice defect sites in humidity conditions were the main factors to strengthen the H 2 O resistance of Cu/ CeO 2 -R catalyst.

Conclusion
In summary, Cu/CeO 2 catalysts with different morphology and size have been successfully synthesized by hydrothermal and solvothermal methods, and followed by deposition precipitation process. Notably, these catalysts were studied for the catalytic oxidation of CO under dry and humid conditions to explore the shape effect on CO oxidation performance. The results verified that the complete conversion temperature of CO was 60 °C on Cu/ CeO 2 -R catalyst, 90 °C on Cu/CeO 2 -P catalyst, 105 °C on Cu/CeO 2 -S catalyst and 125 °C on Cu/CeO 2 -F catalyst, respectively. Based on a series of characterizations, it was concluded that the Cu/CeO 2 -R catalyst exposed the highly active CeO 2 (110) crystal plane, owned the smaller particle size of CeO 2 and CuO, formed the stronger interaction between Cu species and CeO 2 nanorods (Ce 4+ + Cu + → Ce 3+ + Cu 2+ ), and formed a large number of oxygen vacancies derived from the removal of lattice oxygen (O latt ) species in CeO 2 and the lattice defect sites, which jointly promoted the conversion of CO at low temperature (T 90 = 58 °C). In addition, the presence of humidity greatly improved the activity of Cu/CeO 2 -R catalyst, exhibited an obvious increase of CO conversion at the same conditions and required for the reaction to reach 100% CO conversion at the lower temperature.  For the synthesis of the Cu/CeO 2 catalysts, the CeO 2 powders (1.0 g) were suspended in 50 mL of deionized water under vigorous stirring. 1.4 mmol Cu(NO 3 ) 2 ·3H 2 O were added in the suspension of CeO 2 . After that, the Na 2 CO 3 aqueous solution (0.50 M) was treated as the precipitant to obtain the Cu/CeO 2 precursor. Finally, the powder was calcined at 400 °C for 4 h in air.
Catalytic oxidation of CO. To evaluate the activity in CO oxidation reaction, the experiments were carried out in a continuous-flow fixed-bed glass tube reactor (6.0 mm inner diameter). In a typical run, a continuous flow of the reactant mixture containing 1 vol % CO, 15 vol % O 2 , and N 2 balance was passed through the reactor with a total flow rate of 36 mL min −1 . A definite amount of catalyst (300 mg) was added to the isothermal region of the reactor tube. A series of Cu/CeO 2 catalysts were tested to explore the effect of morphology, the exposed crystal plane of CeO 2 and crystal sizes on CO oxidation reaction. The CO oxidation reaction under humid condition was also conducted by passing the N 2 stream for adding water vapor to the carrier gas (3 vol %). After each round of reaction, the composition of the gas was detected with an online GC-7890 II gas chromatograph equipped with a thermal conductivity detector and a molecular sieve 5A column. The CO conversion rate (X CO ) was calculated:  Catalyst characterization. The specific surface area and the pore diameter of the Cu/CeO 2 samples were determined by the N 2 adsorption-desorption isotherms with a Micromeritics ASAP 2010 instrument in accordance with the BET and BJH mehod, respectively. The BET surface area was related to six measurements at relative pressures of N 2 in the range of 0.05-1.00. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was performed on Agilent 725-ES apparatus to determine the metal loadings. The morphology of these Cu/ CeO 2 samples was confirmed by Field emission scanning electron microscopy (FE-SEM, JSM-6701F) at 30 kV. Besides, the nanostructures of the samples were also characterized through a JEOL JEM-2010 transmission electron microscope operating at 200 kV, and a suspension of the Cu/CeO 2 samples in ethanol was drop-casted onto carbon-coated copper grids and naturally dried under ambient conditions. Powder X-ray diffraction (XRD) patters was recorded on a Rigaku D/MAX-RB X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the range of 10-90°. H 2 -TPR and CO-TPR measurements were performed on the chemical adsorption instrument. The reducing gas was respectively 5 vol% H 2 and 10% CO balanced by N 2 , and a flow rate of 40 ml min −1 was used, and the test was carried out from room temperature to 800 °C at a heating rate of 10 °C min −1 . Before each measurement, the sample was purged with N 2 at 300 °C for 2 h. X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB 210 Electron Spectrometer with a Mg Kα(1253.6 eV) radiation, and the spectra were corrected and treated the C1s binding energy of 284.6 eV as the standard. Raman spectroscopy was performed on a RM 2000 microscope confocal Raman spectrometer with 532 nm laser (Renishaw PLC).