Constructing Efficient CuO-Based CO Oxidation Catalysts with Large Specific Surface Area Mesoporous CeO2 Nanosphere Support

CeO2 is an outstanding support commonly used for the CuO-based CO oxidation catalysts due to its excellent redox property and oxygen storage–release property. However, the inherently small specific surface area of CeO2 support restricts the further enhancement of its catalytic performance. In this work, the novel mesoporous CeO2 nanosphere with a large specific surface area (~190.4 m2/g) was facilely synthesized by the improved hydrothermal method. The large specific surface area of mesoporous CeO2 nanosphere could be successfully maintained even at high temperatures up to 500 °C, exhibiting excellent thermal stability. Then, a series of CuO-based CO oxidation catalysts were prepared with the mesoporous CeO2 nanosphere as the support. The large surface area of the mesoporous CeO2 nanosphere support could greatly promote the dispersion of CuO active sites. The effects of the CuO loading amount, the calcination temperature, mesostructure, and redox property on the performances of CO oxidation were systematically investigated. It was found that high Cu+ concentration and lattice oxygen content in mesoporous CuO/CeO2 nanosphere catalysts greatly contributed to enhancing the performances of CO oxidation. Therefore, the present mesoporous CeO2 nanosphere with its large specific surface area was considered a promising support for advanced CO oxidation and even other industrial catalysts.


Introduction
Carbon monoxide (CO) is a kind of toxic pollutant that widely exists in vehicle exhaust and industrial waste gas [1,2].CO can also cause extreme damage to the human health because CO has a high affinity for hemoglobin, which is well known to cause poisoning at high concentrations.Furthermore, CO acts as a precursor to ground-level ozone, leading to the potential for severe respiratory irritation [3][4][5].Therefore, the removal of CO has received a lot of public attention.Nowadays, there are many ways to remove CO, such selective oxidation of CO.The results showed that the CuO/CeO 2 catalyst with 6 wt.% CuO loading amount exhibited the best performance (T 100 = 125 • C).The large specific surface area of the CeO 2 support not only could reduce the particle size of CeO 2 , but also could facilitate the dispersion of CuO.As a result, the CuO/CeO 2 catalysts exhibited a strong synergy between the small CeO 2 particles and the highly dispersed CuO.Luo et al. [32] prepared CuO/CeO 2 supported catalysts with different loading amounts of CuO using the modified citrate sol-gel method.The heating and calcination were performed in an N 2 atmosphere during the process of catalyst preparation.The results showed that the citric acid was decomposed into ultrafine carbon powders, which encapsulated the CuO or separated Cu-Ce oxides.The high dispersion of CuO could be obtained on the surface of CeO 2, and this catalyst exhibited optimum CO oxidation activity.The CeO 2 catalyst with 10 mol% CuO content presented the best activity (T 90 = 100 • C, S BET = 131 m 2 /g).
Although these preparation methods mentioned above could successfully fabricate CeO 2 supports with large specific surface areas, yet the preparation processes involved were usually very complicated, which required more energy and time consumption [17].Hence, it is necessary to design and invent a simpler method to prepare structurally stable CeO 2 supports with large specific surface areas for the CO oxidation reaction with excellent low-temperature performances.The innovation of this work is to use the solvothermal method to prepare CeO 2 nanospheres with large specific surface areas by adding reactants that are different from other works and controlling the reaction time.Based on the simplicity of the preparation method, the oxidation temperature of CO on the catalyst should be reduced as much as possible.
In this work, a sort of novel mesoporous CeO 2 nanosphere with a large specific surface area and excellent thermal stability was facilely synthesized by the improved solvothermal method reported elsewhere [33,34].Then, a series of CuO-based supported catalysts were prepared using the incipient wet impregnation method with the CeO 2 nanosphere as the support for low-temperature CO oxidation.The effects of the CuO loading amount, the calcination temperature, mesostructure, and redox property on the performances of CO oxidation were systematically investigated.It was found that high Cu + concentration and lattice oxygen content in mesoporous CuO/CeO 2 nanosphere catalysts greatly contributed to enhancing the performance of CO oxidation.The obtained result demonstrated the advantages of the mesoporous CuO/CeO 2 nanosphere catalyst with a large specific surface area and excellent redox properties.Furthermore, various characterization techniques, such as the XRD, N 2 physisorption, SEM, XPS, TEM, H 2 -TPR, in situ DRIFTS, etc., were used to investigate the CuO-based catalysts.The relationship between the catalyst structure and catalytic performance was established.

The Synthesis of the Mesoporous CeO 2 Nanosphere
The mesoporous CeO 2 nanosphere was synthesized using the improved hydrothermal method according to the procedures reported in the previous literature [33,34].The detailed preparation procedure of the mesoporous CeO 2 nanosphere was summarized in Supplementary Materials S1.The reactants in the preparation method used in this work are different from those in the literature [33], and the solvothermal time is different from that in the literature [34].The finally obtained mesoporous CeO 2 nanosphere was labeled as the NS-CeO 2 -T, where the NS represented the nanosphere and T represented the calcination temperature.The mesoporous CeO 2 nanosphere without calcination was labeled as NS-CeO 2 -P, where P represents the precursor.

The Preparation of the CuO/NS-CeO 2 Supported Catalysts
The CuO-based catalyst with x wt.% CuO (x wt.% = m CuO /(m CuO + m support ) × 100%) loading amount was synthesized using the incipient impregnation method.The preparation details of the CuO/CeO 2 catalyst are found in Supplementary Materials S2.The as-prepared catalysts were denoted as xCuO/NS-CeO 2 -T, where x represented the mass percentage of the CuO and T represented the calcination temperature.

Catalyst Characterizations
The catalytic supports and catalysts were systematically characterized using X-ray powder diffraction (XRD), N 2 physisorption, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and H 2 temperature-programmed reduction (H 2 -TPR).The calcination processes of the CuO-based catalyst precursors were further studied using in situ DRIFTS analyses.Detailed information about the characterizations is listed in Supplementary Materials S3.

The Measurements of the Catalytic Performances
The evaluation of the catalytic activity of CO oxidation was conducted on a fixed-bed reactor.The reaction products were detected using the online gas chromatography (GC-7900, Techcomp, Hong Kong, China).The detailed information for these are summarized in Supplementary Materials S4.The normalized reaction rate of the catalyst was also calculated, and the specific calculation process is summarized in Supplementary Materials S5.

Characterizations of the Supports
The XRD characterization was accomplished to determine the crystalline phase state of the mesoporous CeO 2 nanosphere calcined at different temperatures and the commercial supports.Their XRD patterns are recorded in Figure 1.As presented in Figure 1a, diffraction peaks at 2θ = 28.5 • , 33.0 • , 47.5 • , 56.5 • , 69.6 • , and 76.7 • were observed over all NS-CeO 2 -T supports, which were attributed to the CeO 2 crystalline phase with a face-centered cubic fluorite structure (PDF #34-0394).Furthermore, as the calcination temperature increased, the intensities of the CeO 2 diffraction peak were gradually enhanced over the NS-CeO 2 -T supports.The observed phenomenon was attributed to the progressive increase in the grain size of CeO 2 with the increasing calcination temperature.Similarly, the XRD patterns of the NS-CeO 2 -P commercial supports (CeO 2 , Al 2 O 3 , and SiO 2 ) are shown in Figure 1b.Among them, the diffraction peak intensity of the commercial CeO 2 (PDF #43-1002) support was stronger than that of the mesoporous CeO 2 nanosphere, which might be attributed to the larger crystalline grain size of commercial CeO 2 .As for the commercial SiO 2 and Al 2 O 3 supports, the diffraction peaks of amorphous SiO 2 (PDF #29-0085) at 2θ = 22.7 • and the γ-Al 2 O 3 (PDF #29-0063) at 37.6 • , 39.5 • , 45.8 • , 66.8 • could be observed.
In order to analyze the structural properties of the catalytic supports, nitrogen adsorptiondesorption analyses of the catalytic supports were carried out.It was found from Figure 2a,c that all samples showed IV-type isotherms.This indicated that the mesoporous structures existed in these investigated supports based on the IUPAC classification [35].Furthermore, it was worth observing that the NS-CeO 2 -T supports exhibited H3-shaped hysteresis loops, suggesting the existence of narrow-slit-like mesopores [35].In Figure 2b,d, it was observed that the pore sizes of NS-CeO 2 -T and commercial supports were below 10 nm.The calcination temperature had no effect on the pore size distribution of NS-CeO 2 -T supports.The detailed data regarding the structural properties of these supports are presented in Table 1.Combining the results from Table 1, the specific surface areas of NS-CeO 2 -T supports decreased as the calcination temperature increased.The mesoporous CeO 2 nanosphere successfully maintained a large specific surface area of up to 181 m 2 /g even after being calcined at 500 • C.This indicated that the current mesoporous CeO 2 nanosphere exhibited excellent thermal stability at high temperatures.The pore volumes and average pore sizes of NS-CeO 2 -T in Table 1 did not change significantly.Furthermore, the C-CeO 2 and C-SiO 2 supports exhibited H3-shaped hysteresis loops, indicating the existence of narrow-slit-like mesopores [35].The H4-shaped hysteresis loop of commercial Al 2 O 3 might be due to the existence of slotted pores within the intergranular locations [36].In order to analyze the structural properties of the catalytic supports, nitrogen adsorption-desorption analyses of the catalytic supports were carried out.It was found from Figure 2a,c that all samples showed IV-type isotherms.This indicated that the mesoporous structures existed in these investigated supports based on the IUPAC classification [35].Furthermore, it was worth observing that the NS-CeO2-T supports exhibited H3-shaped hysteresis loops, suggesting the existence of narrow-slit-like mesopores [35].In Figure 2b,d, it was observed that the pore sizes of NS-CeO2-T and commercial supports were below 10 nm.The calcination temperature had no effect on the pore size distribution of NS-CeO2-T supports.The detailed data regarding the structural properties of these supports are presented in Table 1.Combining the results from Table 1, the specific surface areas of NS-CeO2-T supports decreased as the calcination temperature increased.The mesoporous CeO2 nanosphere successfully maintained a large specific surface area of up to 181 m 2 /g even after being calcined at 500 °C.This indicated that the current mesoporous CeO2 nanosphere exhibited excellent thermal stability at high temperatures.The pore volumes and average pore sizes of NS-CeO2-T in Table 1 did not change significantly.Furthermore, the C-CeO2 and C-SiO2 supports exhibited H3-shaped hysteresis loops, indicating the existence of narrow-slit-like mesopores [35].The H4-shaped hysteresis loop of commercial Al2O3 might be due to the existence of slotted pores within the intergranular locations [36].The NS-CeO 2 -T supports calcined at different temperatures and the commercial supports were characterized by SEM to investigate their morphologies and sizes.As shown in Figure 3a,b, the as-prepared CeO 2 nanosphere without calcination was in perfect spherical shape, with uniform size distribution around 130 nm.In addition, it was found in Figure 3c-e that the uniform spherical morphologies of the NS-CeO 2 -T supports were successfully maintained even after calcination up to 500 • C.This once again illustrated the good thermal stability of these mesoporous CeO 2 nanosphere materials.In comparison, the reference commercial CeO 2 support was composed of small CeO 2 nanoparticles, as shown in Figure 3f.As for the commercial Al 2 O 3 and SiO 2 supports, it could be observed from Figure 3g,h that all of them showed irregular morphologies, and their particle sizes were in the micron level.The NS-CeO2-T supports calcined at different temperatures and the commercial supports were characterized by SEM to investigate their morphologies and sizes.As shown in Figure 3a,b, the as-prepared CeO2 nanosphere without calcination was in perfect spherical shape, with uniform size distribution around 130 nm.In addition, it was found in Figure 3c-e that the uniform spherical morphologies of the NS-CeO2-T supports were successfully maintained even after calcination up to 500 °C.This once again illustrated the good thermal stability of these mesoporous CeO2 nanosphere materials.In comparison, the reference commercial CeO2 support was composed of small CeO2 nanoparticles, as shown in Figure 3f.As for the commercial Al2O3 and SiO2 supports, it could be observed from Figure 3g,h that all of them showed irregular morphologies, and their particle sizes were in the micron level.The in situ DRIFTS analyses were carried out to identify the possible decomposition products of copper nitrate catalyst precursor.It was found in Figure 4 that all catalysts had CO 2 absorbance peaks (1256 cm −1 , 2350 cm −1 ) [37,38], which might be due to the adsorption of CO 2 from the atmosphere on the catalyst surface.Additionally, the infrared absorbance peaks of all catalyst precursors were observed in the range of 1380 cm −1 and 760-715 cm −1 , which were attributed to NO 3 − asymmetric stretching vibration and NO 3 − in-plane bending vibration [37].Interestingly, the intensity of the NO 3 − vibration peak gradually decreased with the increase in the calcination temperature, suggesting that NO 3 − in the nitrate ligand was gradually decomposed.The calcination of all catalyst precursors revealed the presence of an N=O stretching vibration (1650-1500 cm −1 ) and O-N-O asymmetric stretching vibration absorbance peak (1200-1350 cm −1 ), indicating that NO 3 − had decomposed into NO 2 gas [39].Moreover, Cu-O infrared absorption peaks were observed at 632 cm −1 [40] for all catalysts, indicating the transformation of Cu 2+ cation into CuO.The oxide supports themselves also exhibited characteristic absorbance peaks in all catalyst precursors.To be specific, the precursor of the 10CuO/NS-CeO 2 catalyst (Figure 4a) showed Ce-O peaks at 467 cm −1 and 569 cm −1 (Ce-O stretching vibration) [41][42][43].The precursor of the 10CuO/C-CeO 2 catalyst (Figure 4b) showed an absorbance peak of Ce-O-Ce at 1385 cm −1 [44].The characteristic absorbance peaks of the SiO 2 support, Si-O-Si symmetric stretching vibration at 805 cm −1 and Si-OH stretching at 970 cm −1 , were observed in the precursor of the 10CuO/C-SiO 2 catalyst (Figure 4c) [45].Finally, the precursor of the 10CuO/C-Al 2 O 3 catalyst (Figure 4d) exhibited a characteristic absorbance peak of Al-O at 765 cm −1 (Al-O vibrations in AlO 4 unit) [46].The in situ DRIFTS spectra during the calcination process of the catalyst precursors revealed the successful decomposition of the copper nitrate precursor into copper oxide in the presence of air.

XRD Analysis
The XRD patterns of the as-prepared xCuO/NS-CeO 2 -300 catalysts are displayed in Figure 5a.It was apparent that all these catalysts exhibited diffraction peaks of CeO 2 (PDF # 34-0394) similar to those of the CeO 2 support.Meanwhile, the featured diffraction peaks of CuO (PDF # 01-1117) appeared at 2θ = 35.7 • and 39.0 • .As the content of CuO increased, the intensity of the CuO diffraction peaks gradually increased.Furthermore, the diffraction peaks of CuO could not be detected when the CuO content was below 10 wt.%.The reason for this might be due to the high dispersion of CuO active sites over the NS-CeO 2 support [47].

XRD Analysis
The XRD patterns of the as-prepared xCuO/NS-CeO2-300 catalysts are displayed in Figure 5a.It was apparent that all these catalysts exhibited diffraction peaks of CeO2 (PDF # 34-0394) similar to those of the CeO2 support.Meanwhile, the featured diffraction peaks of CuO (PDF # 01-1117) appeared at 2θ = 35.7°and 39.0°.As the content of CuO increased, the intensity of the CuO diffraction peaks gradually increased.Furthermore, the diffraction peaks of CuO could not be detected when the CuO content was below 10 wt.%.The reason for this might be due to the high dispersion of CuO active sites over the NS-CeO2 support [47]. Figure 5b exhibits the XRD patterns of 10CuO/NS-CeO 2 catalysts calcined at different temperatures.The CuO diffraction peaks were observed over these catalysts calcined at three temperatures.Furthermore, it was found that the characteristic diffraction peak of CuO also did not visibly increase, demonstrating the good dispersion of CuO particles on the NS-CeO 2 support.This suggested that the thermal sintering of CuO was effectively inhibited owing to a powerful metal-support interaction.Similarly, the intensity of the characteristic diffraction peaks of CeO 2 also did not visibly increase at high calcination temperatures.This indicated no significant increase in CeO 2 crystalline particles, showing the outstanding thermal stability of the NS-CeO 2 support.
Figure 5c shows the XRD patterns of the CuO-based catalysts that were prepared using various supports.It was noticeable that all the catalysts exhibited the CuO active site (PDF #01-1117) and their corresponding supports' (commercial CeO 2 (PDF #43-1002), NS-CeO 2 (PDF #34-0394), SiO 2 (PDF #29-0085), Al 2 O 3 (PDF #29-0063)) diffraction peaks.However, the diffraction peak intensity of CuO was significantly different over different catalysts with the same CuO loading amount (10 wt.%).It was noteworthy that the CuO diffraction peak intensity over the 10CuO/NS-CeO 2 -500 catalyst was much weaker than that of the 10CuO/C-CeO 2 -500, 10CuO/C-SiO 2 -500, and 10CuO/C-Al 2 O 3 -500 catalysts with commercial catalysts.This indicated that the CeO 2 nanosphere support was more conducive to the dispersion of CuO.

N 2 Physisorption Analysis
Figure 6a depicts the N 2 adsorption-desorption isotherms of the xCuO/NS-CeO 2 -300 catalysts.The type IV isotherms with H3-shaped hysteresis loops are observed in Figure 6a over these catalysts.This indicated that the mesoporous structure of the NS-CeO 2 -300 support was maintained even after loading 15wt.%CuO. Figure 6b displays the pore size distribution curves of the xCuO/NS-CeO 2 -300 catalysts.The pore size distribution curves of xCuO/NS-CeO 2 -300 catalysts were similar to that of the NS-CeO 2 -300 catalyst, showing that the loading amount of CuO did not affect the structure of the NS-CeO 2 catalyst.Table 1 displays detailed data regarding the structural characteristics of all the catalysts.As noticed in Figure 6b and Table 1, all these catalysts displayed narrow pore size distribution around 3.4 nm.It was noteworthy that the specific surface area and pore volume of the xCuO/NS-CeO 2 -300 catalyst gradually decreased as the CuO content increased, which resulted from a blockage of the mesoporous channels of the NS-CeO 2 support [47].
Figure 6c shows the N 2 adsorption-desorption isotherms of the 10CuO/NS-CeO 2 -T catalysts calcined at different temperatures.The IV-type isotherms and H3-type hysteresis loops of the catalysts were maintained after high-temperature calcination.This indicated that neither the calcination temperatures nor the loading of CuO affected the mesoporous structure of the catalysts.This also indicated that the mesoporous structure of NS-CeO 2 supports was successfully kept after the calcination of catalyst precursors at 500 • C. Figure 6d presents the pore size distribution curves of the 10CuO/NS-CeO 2 -T catalysts.The pore size distribution curves of the 10CuO/NS-CeO 2 -T catalysts were largely similar.Furthermore, the specific surface areas and pore volumes of the as-prepared 10CuO/NS-CeO 2 -T catalysts decreased slightly after calcination at high temperatures.This was mainly attributed to the partial blockage of the mesopores by the nano-sized CuO active sites.However, the 10CuO/NS-CeO 2 -500 catalyst calcined at 500 • C still possessed a high specific surface area up to 176.0 m 2 /g.This demonstrated the excellent structural stability of the 10CuO/NS-CeO 2 catalysts.In regards to the average pore sizes of these catalysts, they did not change significantly when the calcination temperature of the catalysts increased.This suggested that the mesoporous structure of these catalysts was successfully maintained at high calcination temperatures.
Figure 6e illustrates the N 2 adsorption-desorption isotherms of the 10CuO/C-CeO 2 , 10CuO/C-Al 2 O 3 , and 10CuO/C-SiO 2 reference catalysts with commercial supports.The IV-type with an H3-shaped hysteresis loop could be observed in the 10CuO/C-CeO 2 and 10CuO/C-SiO 2 catalysts.The IV-type isotherms with an H4-shaped hysteresis loop could be observed in the 10CuO/C-Al 2 O 3 catalyst.This indicated that the isotherm shapes of C-CeO 2 , C-SiO 2 , and C-Al 2 O 3 catalyst supports did not undergo significant changes after being loaded with CuO.It was shown in Figure 6f that the pore size distribution curves of these reference catalysts were consistent with their respective supports.The loading of CuO did not change the porous structure of the supports.Similarly, the specific surface areas of the 10CuO/C-CeO 2 and 10CuO/C-Al 2 O 3 catalysts in Table 1 were also lower than their respective supports.Figure 6c shows the N2 adsorption-desorption isotherms of the 10CuO/NS-CeO2-T catalysts calcined at different temperatures.The IV-type isotherms and H3-type hysteresis loops of the catalysts were maintained after high-temperature calcination.This indicated that neither the calcination temperatures nor the loading of CuO affected the mesoporous structure of the catalysts.This also indicated that the mesoporous structure of NS-CeO2 supports was successfully kept after the calcination of catalyst precursors at 500 °C.Figure 6d presents the pore size distribution curves of the 10CuO/NS-CeO2-T catalysts.The pore size distribution curves of the 10CuO/NS-CeO2-T catalysts were largely similar.Furthermore, the specific surface areas and pore volumes of the as-prepared 10CuO/NS-CeO2-T

SEM, EDS Mapping, and TEM Analyses
The morphologies and spatial distributions of elements of the as-prepared catalysts were studied using SEM, EDS-mapping, and TEM techniques.The as-prepared 10CuO/NS-CeO 2 -500, 10CuO/C-CeO 2 -500, 10CuO/C-Al 2 O 3 -500, and 10CuO/C-SiO 2 -500 catalysts were selected as the representatives.The SEM and EDS-mapping images are presented in Figure 7.As could be observed, the as-prepared catalysts successfully kept the morphologies of the supports after loading the CuO active sites.Specifically, the mesoporous CeO 2 nanosphere (NS-CeO 2 ) supported catalyst remained with spherical morphology and had a uniform size for the NS-CeO 2 support after loading the CuO active sites.After loading 10 wt.% CuO and calcining at 500 • C, the 10CuO/CeO 2 -500 catalyst maintained the morphology of nanospheres.The average diameter of the nanospheres was 129.4 nm.It showed that neither loading of CuO nor high-temperature calcination changed the morphology of CeO 2 nanospheres.This also had been confirmed by the above N 2 physisorption analyses.Furthermore, the EDS-mapping photos confirmed that the CuO active sites had been successfully loaded on the support with uniform distribution.[51,52].It was noteworthy that weak shoulder Cu 2p3/2 peaks at 932.0-932.1 eV were also observed.This suggested the existence of low-valence copper oxide species [53].The previous studies had confirmed that the low-valence copper species mainly existed in the form of Cu + in CuO/CeO2 nanocomposites [54].It was believed that the Cu + species on the catalysts served as the chemisorption and activation sites for the CO molecules during the process of the CO oxidation reaction [55].Furthermore, the Cu + contents in different catalysts calculated based on the XPS characterization are presented in Table 2.It was discovered that the Cu + content gradually increased with the increase in the calcination temperature.The as-prepared 10CuO/NS-CeO2-500 catalyst exhibited the highest Cu + content at 500 °C calcination temperature.Meanwhile, Figure 9b shows the Cu 2p spectra of the as-prepared 10CuO/C-CeO2-500, 10CuO/C-SiO2-500, and 10CuO/C-Al2O3-500 reference catalysts.It was found that the peak positions of the Cu 2p spectra over these commercial catalysts were basically consistent with the as-prepared 10CuO/NS-CeO2-T catalysts.However, the peak intensities of the Cu 2p XPS spectra over these four catalysts were quite different, although their CuO loading amount was identical (10 wt.%).Therefore, the type of catalytic support had a great influence on the state of the  [51,52].It was noteworthy that weak shoulder Cu 2p 3/2 peaks at 932.0-932.1 eV were also observed.This suggested the existence of lowvalence copper oxide species [53].The previous studies had confirmed that the low-valence copper species mainly existed in the form of Cu + in CuO/CeO 2 nanocomposites [54].It was believed that the Cu + species on the catalysts served as the chemisorption and activation sites for the CO molecules during the process of the CO oxidation reaction [55].Furthermore, the Cu + contents in different catalysts calculated based on the XPS characterization are presented in Table 2.It was discovered that the Cu + content gradually increased with the increase in the calcination temperature.The as-prepared 10CuO/NS-CeO 2 -500 catalyst exhibited the highest Cu + content at 500 • C calcination temperature.Meanwhile, Figure 9b shows the Cu 2p spectra of the as-prepared 10CuO/C-CeO 2 -500, 10CuO/C-SiO 2 -500, and 10CuO/C-Al 2 O 3 -500 reference catalysts.It was found that the peak positions of the Cu 2p spectra over these commercial catalysts were basically consistent with the as-prepared 10CuO/NS-CeO 2 -T catalysts.However, the peak intensities of the Cu 2p XPS spectra over these four catalysts were quite different, although their CuO loading amount was identical (10 wt.%).Therefore, the type of catalytic support had a great influence on the state of the surface CuO.Specifically, the 10CuO/NS-CeO 2 -500 catalyst exhibited the highest surface Cu + content among these catalysts.This suggested that the type of support significantly influenced the Cu + content of the catalyst.a The content of Cu + can be calculated from the area ratio of Cu + /(Cu 2+ + Cu + ) × 100%.Figure 9c,d depicts the Ce 3d spectra of all the as-prepared catalysts.Generally, the Ce 3d spectra were decomposed into two groups of spin-orbit coupling peaks of Ce 3d 5/2 (marked as v) and Ce3d 3/2 (marked as u).Based on the pioneering literature, the v, v", and v"' peaks belonged to Ce 4+ 3d 5/2 , while the u, u", and u"' peaks belonged to Ce 4+ 3d 3/2 [56,57].Additionally, the v' and u' peaks represented the characteristic peaks of Ce 3+ 3d 5/2 and Ce 3+ 3d 3/2 , respectively [56,57].It was noticed in Figure 9c,d that Ce 3+ and Ce 4+ coexisted over the surface of all the as-prepared CeO 2 supported catalysts.As for the as-prepared 10CuO/NS-CeO 2 -T catalysts in Figure 9c, the shapes and positions of their Ce 3d spectra were essentially the same as each other.This indicated that the calcination temperature had minimal impact on the valence state of the Ce species over the catalyst surface.It was widely believed that the formation of Ce 3+ was caused by the removal of lattice oxygen and the creation of oxygen vacancies [58].The oxygen vacancy generated by Ce 3+ would positively promote the reaction (Cu 2+ + Ce 3+ ↔ Cu + + Ce 4+ ) to produce more Cu + , thereby enhancing the catalytic performance of CO oxidation [59].Furthermore, the specific proportions of Ce 3+ cations in these catalysts are displayed in Table S1.The as-prepared 10CuO/NS-CeO 2 -500 catalyst had the highest Ce 3+ proportion.The previous study indicated that the higher Ce 3+ content promised a greater reduction ability for CuO, which would lead to the formation of more active Cu + sites [60].The substitution of Cu + cation in the CeO 2 lattice would result in the generation of additional oxygen vacancies, which greatly enhanced the mobility of lattice oxygen in CeO 2 [61].
The O 1s spectra of these as-prepared catalysts are shown in Figure 9e,f.It was noticeable from Figure 9e that the shape and position of O 1s spectra of 10CuO/NS-CeO 2 -T catalysts were roughly the same.Generally, O 1s spectra were divided into the main peaks and shoulder peaks.As for the O 1s of the as-prepared 10CuO/NS-CeO 2 -T catalysts in Figure 9e, the main peaks at 529.0-529.2eV were ascribed to lattice oxygen (O latt ) [62], and the shoulder peaks at 531.0-531.3eV were attributed to the surface adsorbed oxygen (O ads ) species [48].The content of the surface adsorbed oxygen was proportional to the quantity of surface oxygen vacancy.The lattice oxygen peak area ratios of the as-prepared 10CuO/NS-CeO 2 -T catalysts are listed in Table 3.It was displayed in Table 3 that the content of the adsorbed oxygen decreased from 47.64% to 33.71% when the calcination temperature increased.This denoted that the concentration of the oxygen vacancy in the as-prepared 10CuO/NS-CeO 2 -T catalysts was closely related to the calcination temperature.In addition, it was observed in Table 3 that the lattice oxygen content of the 10CuO/NS-CeO 2 -T catalyst followed the sequence: 10CuO/NS-CeO 2 -500 > 10CuO/NS-CeO 2 -400 > 10CuO/NS-CeO 2 -300.Therefore, the 10CuO/NS-CeO 2 -500 catalyst possessed the largest lattice oxygen content.It was reported that the lattice oxygen of CeO 2 could react with CO and the O 2 in gaseous feed gases which subsequently replenished the lattice oxygen consumed [63].As a comparison, it was found in Figure 9f that the lattice oxygen content of the O 1s spectra of the 10CuO/C-CeO 2 -500 reference catalyst with commercial supports was totally different.These phenomena suggested that the nature of the catalytic support had a significant impact on the amount of lattice oxygen as well.The specific binding energies of surface elements for all catalysts are presented in Table S2.It was found that the binding energies of Cu 2p 3/2 were almost similar over almost all these catalysts.Based on the previous analysis, their primary oxidation states or valence states were identified as Cu 2+ , Cu + , Ce 4+ , Ce 3+ , and O 2− .Overall, XPS analysis provided valuable insights into the surface composition and chemical state of catalyst elements, which were used to optimize catalyst design and improve catalytic performance.

H 2 -TPR Analysis
To examine the interaction between the support and the active metal CuO, H 2 -TPR analysis was conducted over the as-prepared catalysts, and the results in the form of H 2 -TPR profiles are shown in Figure 10. Figure 10a reveals that the NS-CeO 2 -300 support did not show H 2 consumption between 35 and 600 • C, indicating that CeO 2 was not easily reduced by H 2 at lower temperatures.The as-prepared xCuO/NS-CeO 2 -300 catalysts displayed two or three reduction peaks.The H 2 consumption peak of the xCuO/NS-CeO 2 -300 catalysts all came from CuO reduction.With the increase in the loading amount of CuO, the reduction peak of the catalyst shifted slightly towards a lower temperature range.The reduction temperature of the catalyst reached the minimum when the loading of CuO was 10 wt.%.Simultaneously, the consumption of H 2 also increased with the increased loading amount of CuO.The CuO/CeO 2 supported catalysts typically presented two or three reduction peaks of Cu species at temperatures ranging from 150 • C to 250 • C, namely α, β, and γ reduction peaks according to the previous literature [54,64,65].Typically, the α reduction peak was ascribed to the reduction in CuO exhibiting strong interactions with CeO 2 or to the solid solution formed with CuO and CeO 2 ; the β reduction peak was assigned to the reduction in highly dispersed CuO species on the surface of CeO 2 ; and the γ reduction peak was derived from the reduction in massive CuO species with weak interaction with CeO 2 .This suggested the presence of CuO with strong interactions with CeO 2 and bulk CuO in the xCuO/NS-CeO 2 -300 catalysts.Furthermore, the β reduction peak observed in the 10CuO/NS-CeO 2 -300 catalyst was attributed to the presence of a highly dispersed CuO species after loading 10 wt.% of CuO.Notably, as the loading of CuO increased from 3 wt.% to 15 wt.%, the intensity of the γ reduction peak gradually increased.This denoted the formation of more bulk CuO species.The quantitative ratios of H 2 consumption are shown in Table 4.  Figure 10b displays the H2-TPR profiles of the NS-CeO2-T and 10CuO/NS-CeO2catalysts calcined at varying temperatures.The NS-CeO2-T supports did not exhibit no ticeable H2 consumption.It was noticed that the 10CuO/NS-CeO2-T catalysts exhibite similar hydrogen consumption profiles with three reduction peaks from 100 °C to 300 °C The 10CuO/NS-CeO2-400 catalyst exhibited the lowest reduction temperature (137 °C However, the proportion of the β reduction peak of 10CuO/NS-CeO2-500 catalysts wa substantially higher than those of the 10CuO/NS-CeO2-300 and 10CuO/NS-CeO2-40  However, the proportion of the β reduction peak of 10CuO/NS-CeO 2 -500 catalysts was substantially higher than those of the 10CuO/NS-CeO 2 -300 and 10CuO/NS-CeO 2 -400 catalysts, as shown in Table 4.This suggested that the CuO active sites were still highly dispersed in catalysts calcined at high temperatures, which had been confirmed using the XRD analyses.Interestingly, despite the higher H 2 reduction temperature of the 10CuO/NS-CeO 2 -500 catalyst compared to the 10CuO/NS-CeO 2 -300 and 10CuO/NS-CeO 2 -400 catalysts, it still exhibited the best CO oxidation activity.This indicated that the contents of Cu + and lattice oxygen were the decisive factors determining the catalyst activity.
Figure 10c displays the H 2 -TPR profiles of the CuO-based catalysts with various supports.As could be observed, these four catalysts exhibited two or three reduction peaks.The H 2 consumption peaks of the 10CuO/SiO 2 -500 catalyst at 291 • C and 342 • C were ascribed to the reduction in highly dispersed CuO on the SiO 2 support and the reduction in agglomerated bulk CuO distributed on the SiO 2 [66].The H 2 consumption peak of the 10CuO/Al 2 O 3 -500 catalyst at 243 • C was because of the reduction in highly dispersed CuO on the surface of Al 2 O 3 , and the reduction peak at 333 • C was because of the reduction in bulk CuO [67].Notably, the reduction peak temperature of the 10CuO/NS-CeO 2 -500 catalyst was notably lower than those of the other three reference CuO-based catalysts with commercial supports.The mesoporous CeO 2 nanosphere, as a reducible support, more easily participated in CO oxidation.

Effect of the CuO Loading Amount on the Catalytic Activity of CO Oxidation
The activities of the xCuO/NS-CeO 2 -300 catalysts toward CO oxidation were systematically evaluated at different reaction temperatures.It was found in Figure 11 that the loading amount of the CuO active site greatly influenced the activities of the xCuO/NS-CeO 2 -300 catalysts.The pure mesoporous CeO 2 nanosphere support did not exhibit CO oxidation activity.The CO oxidation activity of the catalyst was usually evaluated and expressed in the form of the temperature (denoted as T 90 ) at which the CO conversion reached 90% CO oxidation.Generally, the activities of the xCuO/NS-CeO 2 -300 catalysts gradually increased when the loading amount of the CuO increased from 3 wt.% to 10 wt.%.The reason for this was that the increase in the CuO loading amount increased the number of accessible CuO active sites for the gaseous reactants, accounting for the enhanced CO conversion.However, the catalytic activity decreased a bit when the content of CuO further increased to 15 wt.%, whereas extensively increasing the CuO loading amount up to 15 wt.% would cause the agglomeration of the active sites, causing a decrease in CO conversion.Specifically, the activities of the xCuO/NS-CeO 2 -300 catalysts expressed in the form of T 90 observed the following order: 10CuO/NS-CeO 2 -300 (89 Furthermore, the temperature, at which the CO conversion reached 10%, was defined as the catalyst activation temperature (denoted as T 10 ).Slightly different from the order of T 90 , the activation temperature (T 10 ) of the 15CuO/NS-CeO 2 -300 (42 • C) catalyst was lower than 10CuO/NS-CeO 2 -300 (48 • C).The activity of the 10CuO/NS-CeO 2 -300 catalyst was gradually higher than that of the 15CuO/NS-CeO 2 -300 catalyst when the conversion of CO increased from 10% to 90%.Therefore, the 10CuO/NS-CeO 2 -300 catalyst performed the best catalytic activity among these catalysts.Combined with the results from H 2 -TPR, this was due to the presence of highly dispersed CuO on the surface of the 10CuO/NS-CeO 2 -300 catalyst.

Effect of the Calcination Temperature on the Catalytic Activities of CO Oxidation
The influence of the calcination temperature of the 10CuO/NS-CeO2-T catalysts for CO oxidation activities was studied.Figure 12 shows the CO conversions over the 10CuO/NS-CeO2-T catalysts.It was noticed that the catalytic activity increased with the increase in the calcination temperature.Specifically, the T10 of the 10CuO/NS-CeO2-500 catalyst (45 °C) was lower than those of the 10CuO/NS-CeO2-300 (49 °C) and 10CuO/NS-CeO2-400 catalysts (50 °C), demonstrating a lower activation temperature of the CO oxidation.Similarly, the T90 of the 10CuO/NS-CeO2-500 catalyst was also the smallest among these investigated catalysts.Specially, the sequence of T90 of these three 10CuO/NS-CeO2-T catalysts were listed as below: 10CuO/NS-CeO2-300 (89 °C) > 10CuO/NS-CeO2-400 (87 °C) > 10CuO/NS-CeO2-500 (80 °C).The 10CuO/NS-CeO2-500 catalyst displayed the highest activity.The possible reason was that the 10CuO/NS-CeO2-500 catalyst still possessed the large specific surface area, which would facilitate the effective dispersion of CuO active sites and further provide enough accessible CuO active sites for the gaseous reactants.Furthermore, it was found that the activities of the 10CuO/NS-CeO2-T catalysts were closely related to the Cu + content and the lattice oxygen content of 10CuO/NS-CeO2-T catalysts, which also had a close relationship with the calcination temperature.Regarding the roles of oxygen species, such as lattice oxygen and the adsorbed oxygen, in the CO oxidation reaction over CuO/CeO2 catalysts, there has been no consensus so far in the previous reports.Liu et al. [68] proposed that surface-adsorbed oxygen played a major role in CO oxidation.However, Tang et al. [69] found that the lattice oxygen of the catalyst was involved in the oxidation of CO.Additionally, Zou et al. [70] reported that the reduction in both adsorbed oxygen and lattice oxygen might lead to a decrease in the activity of the catalyst.Their results indicated that both the adsorbed oxygen and lattice oxygen species could simultaneously participate in the CO catalytic oxidation reaction.In this work, it was found that the lattice oxygen was the main active oxygen species reacting with the CO reactant.Meanwhile, the 10CuO/NS-CeO2-500 catalyst also exhibited the highest Ce 3+

Effect of the Calcination Temperature on the Catalytic Activities of CO Oxidation
The influence of the calcination temperature of the 10CuO/NS-CeO 2 -T catalysts for CO oxidation activities was studied.Figure 12 shows the CO conversions over the 10CuO/NS-CeO 2 -T catalysts.It was noticed that the catalytic activity increased with the increase in the calcination temperature.Specifically, the T 10 of the 10CuO/NS-CeO 2 -500 catalyst (45 • C) was lower than those of the 10CuO/NS-CeO 2 -300 (49 • C) and 10CuO/NS-CeO 2 -400 catalysts (50 • C), demonstrating a lower activation temperature of the CO oxidation.Similarly, the T 90 of the 10CuO/NS-CeO 2 -500 catalyst was also the smallest among these investigated catalysts.Specially, the sequence of T 90 of these three 10CuO/NS-CeO 2 -T catalysts were listed as below: 10CuO/NS-CeO 2 -300 (89 • C) > 10CuO/NS-CeO 2 -400 (87 • C) > 10CuO/NS-CeO 2 -500 (80 • C).The 10CuO/NS-CeO 2 -500 catalyst displayed the highest activity.The possible reason was that the 10CuO/NS-CeO 2 -500 catalyst still possessed the large specific surface area, which would facilitate the effective dispersion of CuO active sites and further provide enough accessible CuO active sites for the gaseous reactants.Furthermore, it was found that the activities of the 10CuO/NS-CeO 2 -T catalysts were closely related to the Cu + content and the lattice oxygen content of 10CuO/NS-CeO 2 -T catalysts, which also had a close relationship with the calcination temperature.Regarding the roles of oxygen species, such as lattice oxygen and the adsorbed oxygen, in the CO oxidation reaction over CuO/CeO 2 catalysts, there has been no consensus so far in the previous reports.Liu et al. [68] proposed that surface-adsorbed oxygen played a major role in CO oxidation.However, Tang et al. [69] found that the lattice oxygen of the catalyst was involved in the oxidation of CO.Additionally, Zou et al. [70] reported that the reduction in both adsorbed oxygen and lattice oxygen might lead to a decrease in the activity of the catalyst.Their results indicated that both the adsorbed oxygen and lattice oxygen species could simultaneously participate in the CO catalytic oxidation reaction.In this work, it was found that the lattice oxygen was the main active oxygen species reacting with the CO reactant.Meanwhile, the 10CuO/NS-CeO 2 -500 catalyst also exhibited the highest Ce 3+ content based on the XPS analysis.It was believed that the high concentration of Ce 3+ facilitated the reduction in CuO and the generation of the Cu + species, which served as the adsorption and active sites for the reactants during the CO oxidation [60,71].The essential activity of the catalyst is typically evaluated using the specific surface area normalization [72].The result of the specific surface area normalization reaction rate is summarized in Table 3.It can be observed that the results of reaction rates were consistent with the activity order of the catalyst.This indicated that the effect of the specific surface area on catalytic activity can be eliminated.The results of the normalized reaction rate further illustrated this, as well as confirming the positive effects of lattice oxygen content and Cu + content on catalytic activity.content based on the XPS analysis.It was believed that the high concentration of Ce 3+ facilitated the reduction in CuO and the generation of the Cu + species, which served as the adsorption and active sites for the reactants during the CO oxidation [60,71].The essential activity of the catalyst is typically evaluated using the specific surface area normalization [72].The result of the specific surface area normalization reaction rate is summarized in Table 3.It can be observed that the results of reaction rates were consistent with the activity order of the catalyst.This indicated that the effect of the specific surface area on catalytic activity can be eliminated.The results of the normalized reaction rate further illustrated this, as well as confirming the positive effects of lattice oxygen content and Cu + content on catalytic activity.The proposed reaction mechanism of CO oxidation over the CuO/NS-CeO2 catalyst is illustrated in Figure 13.It was widely believed that the active sites for CO oxidation were located at the interface between CuO and CeO2 [61].The redox electron pairs (Cu 2+ + Ce 3+ ↔ Cu + + Ce 4+ ) would be formed between CuO and CeO2, activating lattice oxygen in the vicinity of CuO in CeO2.The CO chemisorbed on Cu + reacted with the activated oxygen species and/or lattice oxygen in CeO2.The gaseous oxygen species subsequently supplemented the reacted oxygen species and/or lattice oxygen on the oxygen vacancies [69,73].It was proposed that the O2 was activated on the oxygen vacancies in CeO2 to regenerate the reactive oxygen species and/or lattice oxygen [73].The proposed reaction mechanism of CO oxidation over the CuO/NS-CeO 2 catalyst is illustrated in Figure 13.It was widely believed that the active sites for CO oxidation were located at the interface between CuO and CeO 2 [61].The redox electron pairs (Cu 2+ + Ce 3+ ↔ Cu + + Ce 4+ ) would be formed between CuO and CeO 2 , activating lattice oxygen in the vicinity of CuO in CeO 2 .The CO chemisorbed on Cu + reacted with the activated oxygen species and/or lattice oxygen in CeO 2 .The gaseous oxygen species subsequently supplemented the reacted oxygen species and/or lattice oxygen on the oxygen vacancies [69,73].It was proposed that the O 2 was activated on the oxygen vacancies in CeO 2 to regenerate the reactive oxygen species and/or lattice oxygen [73].It was believed that the Cu + /Cu 2+ and Ce 4+ /Ce 3+ electron pairs were formed after loading the CuO active sites on the mesoporous CeO 2 nanosphere.It was reported that the formation of Cu/Ce electron pairs (Cu 2+ + Ce 3+ ↔ Cu + + Ce 4+ ) could greatly increase the surface oxygen vacancies of the catalyst and the presence of more Ce 3+ cations in the catalyst could evidently promote the formation of Cu + in the catalyst [59,74].The activity of the 10CuO/NS-CeO 2 -500 catalyst was also higher than the activity of the 10CuO/C-CeO 2 -500 catalyst.The morphology and specific surface area of the two CeO 2 supports were different.The large specific surface area of NS-CeO 2 support was conducive to the dispersion of CuO and further affected the valence and existence state of Cu species.As a result, the Cu + content as well as the highly dispersible CuO content of the 10CuO/NS-CeO 2 -500 catalyst was much greater than that of the 10CuO/C-CeO 2 -500 catalyst.It was believed that the Cu + in the CuO/CeO 2 catalyst was beneficial to the adsorption of CO.Therefore, the CeO 2 nanosphere with a large specific surface area was of great significance for increasing Cu + active sites.Meanwhile, the 10CuO/NS-CeO 2 -500 catalyst also had the highest oxygen vacancy content.The oxygen vacancies in the 10CuO/NS-CeO 2 -500 catalyst increased the mobility of lattice oxygen in CeO 2 .This demonstrated the superiority of mesoporous CeO 2 nanospheres with a high specific surface area.that the Cu + in the CuO/CeO2 catalyst was beneficial to the adsorption of CO.Therefore, the CeO2 nanosphere with a large specific surface area was of great significance for increasing Cu + active sites.Meanwhile, the 10CuO/NS-CeO2-500 catalyst also had the highest oxygen vacancy content.The oxygen vacancies in the 10CuO/NS-CeO2-500 catalyst increased the mobility of lattice oxygen in CeO2.This demonstrated the superiority of mesoporous CeO2 nanospheres with a high specific surface area.Furthermore, the activities of CO oxidation over different CuO/CeO2 catalysts between this work and the literature were compared.The results are summarized in Table 5.It could be observed that the 10CuO/NS-CeO2 catalyst reported in work performed the lowest T90 temperature (80 °C) among the reported results, suggesting the highest catalytic activity.In this work, a large specific surface area CuO/NS-CeO2 catalyst with high CO oxidation activity was prepared using simple materials and methods.Compared with CuO/CeO2 catalysts obtained using different preparation methods, the preparation method of NS-CeO2 support is simpler.Compared with the CuO/CeO2 catalyst obtained using the impregnation method, the CO oxidation activity of the CuO/NS-CeO2 catalyst Furthermore, the activities of CO oxidation over different CuO/CeO 2 catalysts between this work and the literature were compared.The results are summarized in Table 5.It could be observed that the 10CuO/NS-CeO 2 catalyst reported in work performed the lowest T 90 temperature (80 • C) among the reported results, suggesting the highest catalytic activity.In this work, a large specific surface area CuO/NS-CeO 2 catalyst with high CO oxidation activity was prepared using simple materials and methods.Compared with CuO/CeO 2 catalysts obtained using different preparation methods, the preparation method of NS-CeO 2 support is simpler.Compared with the CuO/CeO 2 catalyst obtained using the impregnation method, the CO oxidation activity of the CuO/NS-CeO 2 catalyst was higher.It was supposed that the large surface area of the NS-CeO 2 support made a great contribution to the high catalytic activity of the 10CuO/NS-CeO 2 -500 catalyst.As the catalyst with outstanding activity, the 10CuO/NS-CeO 2 -500 catalyst was selected as the presentative for the 12 h catalyst stability test at 90 • C. The curve of the CO conversion versus time on the stream is shown in Figure 15a.It was observed that the 10CuO/NS-CeO 2 -500 catalyst displayed about 95% CO conversion and also did not show obvious deactivation during the 12 h stability test, demonstrating excellent catalytic stability.
The 10CuO/NS-CeO 2 -500 catalyst after the 12 h stability test of the CO oxidation reaction was characterized using the XRD to evaluate the effect of the stability test on the catalyst.Their XRD patterns are exhibited in Figure 15b.Compared with the XRD pattern of the 10CuO/NS-CeO 2 -500 catalyst before the stability test, the XRD pattern of the 10CuO/NS-CeO 2 -500 catalyst after the stability test did not exhibit any noticeable sintering or change in the crystalline phase.The results of the XRD characterization demonstrated that the 10CuO/NS-CeO 2 -500 catalyst was provided with excellent thermal sintering resistance.
The 10CuO/NS-CeO2-500 catalyst after the 12 h stability test of the CO oxidation reaction was characterized using the XRD to evaluate the effect of the stability test on the catalyst.Their XRD patterns are exhibited in Figure 15b.Compared with the XRD pattern of the 10CuO/NS-CeO2-500 catalyst before the stability test, the XRD pattern of the 10CuO/NS-CeO2-500 catalyst after the stability test did not exhibit any noticeable sintering or change in the crystalline phase.The results of the XRD characterization demonstrated that the 10CuO/NS-CeO2-500 catalyst was provided with excellent thermal sintering resistance.

Conclusions
In this work, the large specific surface area mesoporous CeO2 nanosphere with outstanding thermal stability was facilely synthesized using the facile hydrothermal method.The mesoporous CeO2 nanosphere was used as the support of the CuO-based catalysts for a CO oxidation reaction.The catalytic supports and the CuO-based catalysts were systematically characterized using various techniques, such as XRD, N2 physisorption, SEM,

Conclusions
In this work, the large specific surface area mesoporous CeO 2 nanosphere with outstanding thermal stability was facilely synthesized using the facile hydrothermal method.The mesoporous CeO 2 nanosphere was used as the support of the CuO-based catalysts for a CO oxidation reaction.The catalytic supports and the CuO-based catalysts were systematically characterized using various techniques, such as XRD, N 2 physisorption, SEM, EDS-mapping, H 2 -TPR, XPS, in situ DRIFTS, etc.The effects of the CuO loading amount, catalyst calcination temperature, mesostructured support, and the redox property of the support on the catalytic activity of CO oxidation were also analyzed.It was observed that the catalyst with a mesoporous CeO 2 nanosphere, 10 wt.% CuO loading amount, and 500 • C calcination temperature demonstrated the optimum CO oxidation activity with a T 90 of 80 • C.This work also demonstrated that the mesoporous CeO 2 nanosphere with a large specific surface area and redox property displayed much higher CO oxidation activity than the reference catalysts with commercial CeO 2 , SiO 2 , and Al 2 O 3 supports.This was attributed to the large surface area of the mesoporous CeO 2 nanosphere support, which was effective in highly dispersing CuO active sites even at high calcination temperatures.The result of the XPS analysis demonstrated that the 10CuO/NS-CeO 2 -500 catalyst with excellent redox properties performed the highest Cu + and Ce 3+ concentrations, which was advantageous for the chemisorption and activation of the gaseous reactants.Additionally, the 10CuO/NS-CeO 2 -500 catalyst also exhibited a high lattice oxygen content.These advantages accounted for the excellent performance of the 10CuO/NS-CeO 2 -500 catalyst toward CO oxidation.Therefore, the mesoporous CeO 2 nanosphere with a large specific surface area was considered as an excellent catalyst to support the CuO-based catalysts for a CO oxidation reaction with enhanced performance.

Supplementary Materials:
The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/nano14060485/s1.Table S1.Ce 3d peak area of the catalysts based on XPS analysis; Table S2.Binding energies (eV) of the surface Cu 1s, O 1s, and Ce 3d elements of the as-prepared catalysts.Reference [72] is cited in the Supplementary Materials.

Figure 8 .
Figure 8. TEM images of the as-prepared catalysts: (a,b) NS-CeO 2 -P and (c,d) 10CuO/NS-CeO 2 -500.3.2.5.XPS AnalysisX-ray photoelectron spectroscopy (XPS) was utilized to analyze the surface compositions and chemical states of the elements over the as-prepared catalysts.Figure9adisplays the Cu 2p spectra of the 10CuO/NS-CeO 2 -T catalysts.It was found that these catalysts exhibited two main peaks near 933.2-933.7 eV and 953.3-953.6 eV, which were attributed to Cu 2p 3/2 and Cu 2p 1/2 of the Cu 2+ , respectively[48,49].Furthermore, satellite peaks associated with Cu 2p 3/2 and Cu 2p 1/2 were detected in the range of 940.5-943.6 eV and 961.7-962.0eV, respectively[50].The satellite peaks were mainly caused by the charge transference based on the pioneer literature[51,52].It was noteworthy that weak shoulder Cu 2p 3/2 peaks at 932.0-932.1 eV were also observed.This suggested the existence of lowvalence copper oxide species[53].The previous studies had confirmed that the low-valence copper species mainly existed in the form of Cu + in CuO/CeO 2 nanocomposites[54].It was believed that the Cu + species on the catalysts served as the chemisorption and activation sites for the CO molecules during the process of the CO oxidation reaction[55].Furthermore, the Cu + contents in different catalysts calculated based on the XPS characterization are presented in Table2.It was discovered that the Cu + content gradually increased with the increase in the calcination temperature.The as-prepared 10CuO/NS-CeO 2 -500 catalyst exhibited the highest Cu + content at 500 • C calcination temperature.Meanwhile, Figure9bshows the Cu 2p spectra of the as-prepared 10CuO/C-CeO 2 -500, 10CuO/C-SiO 2 -500, and 10CuO/C-Al 2 O 3 -500 reference catalysts.It was found that the peak positions of the Cu

Figure
Figure 10b displays the H 2 -TPR profiles of the NS-CeO 2 -T and 10CuO/NS-CeO 2 -T catalysts calcined at varying temperatures.The NS-CeO 2 -T supports did not exhibit noticeable H 2 consumption.It was noticed that the 10CuO/NS-CeO 2 -T catalysts exhibited

Nanomaterials 2024 , 29 Figure 13 .
Figure 13.The reaction mechanism of the CO oxidation on the CuO supported catalyst supported on the mesoporous CeO2 nanosphere.3.3.3.Effects of Mesostructure and Redox Property of the Support on the Catalytic Activity of CO OxidationTo investigate the effects of the mesoporous structure and redox property of the support on the catalytic activity of CO oxidation; the 10CuO/C-SiO2-500, 10CuO/C-Al2O3-500 and 10CuO/C-CeO2-500 catalysts with commercial supports were used as the reference catalysts.The curves of the CO conversion versus reaction temperature over these catalysts are shown in Figure14.As could be noticed, the 10CuO/C-SiO2-500 catalyst was basically not active toward CO oxidation even when the reaction temperature was up to 150 °C.The 10CuO/C-Al2O3-500 catalyst did not show any activity before 90 °C, and the CO

3. 3 . 3 .
Effects of Mesostructure and Redox Property of the Support on the Catalytic Activity of CO Oxidation To investigate the effects of the mesoporous structure and redox property of the support on the catalytic activity of CO oxidation; the 10CuO/C-SiO 2 -500, 10CuO/C-Al 2 O 3 -500 and 10CuO/C-CeO 2 -500 catalysts with commercial supports were used as the reference catalysts.The curves of the CO conversion versus reaction temperature over these catalysts are shown in Figure 14.As could be noticed, the 10CuO/C-SiO 2 -500 catalyst was basically not active toward CO oxidation even when the reaction temperature was up to 150 • C. The 10CuO/C-Al 2 O 3 -500 catalyst did not show any activity before 90 • C, and the CO conversion only reached 30.60% at 150 • C. Similar to this, the 10CuO/C-CeO 2 -500 catalyst was activated at 120 • C, and the CO conversion was achieved at 3.67% at 150 • C. In comparison, the CO conversion over the 10CuO/NS-CeO 2 -500 catalyst rapidly increased with the increase in temperature in the investigated temperature range (30-150 • C), and the CO conversion reached 100% at 120 • C. The activity of the 10CuO/NS-CeO 2 -500 catalyst was greatly enhanced compared to the activities of 10CuO/C-SiO 2 -500 and 10CuO/C-Al 2 O 3 -500.

Table 1 .
Structural properties of the supports and catalysts based on the N2 physisorption analyses.

Table 1 .
Structural properties of the supports and catalysts based on the N 2 physisorption analyses.

Table 2 .
Cu 2p peak areas and ratios of Cu + over different catalysts derived from XPS analyses.

Table 2 .
Cu 2p peak areas and ratios of Cu + over different catalysts derived from XPS analyses.The content of Cu + can be calculated from the area ratio of Cu + /(Cu 2+ + Cu + ) × 100%. a

Table 3 .
O 1 s peak areas, ratios of the lattice oxygen and surface adsorbed oxygen over the different catalysts, and the normalized reaction rate of 10CuO/NS-CeO 2 -T catalysts.

Table 4 .
The quantitative data of the H 2 -TPR profiles of the as-prepared catalysts.

Table 5 .
The activities of CO oxidation over different CuO/CeO 2 catalysts.