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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

CuO-CuO Hollow Nanospheres as Heterogeneous Catalyst for Synergetic Oxidation of CO Bo Wei, Nating Yang, Fei Pang, and Jianping Ge J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04690 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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The Journal of Physical Chemistry

Cu2O-CuO Hollow Nanospheres as Heterogeneous Catalyst for Synergetic Oxidation of CO Bo Wei, Nating Yang, Fei Pang and Jianping Ge* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200062, China. E-mail: [email protected].

Abstract: Copper oxides have attracted great attention in oxidation of CO due to its high activity at low temperature. In this work, uniform Cu2O nanospheres were synthesized through a hightemperature polyol process, and they were further transformed into a series of Cu2O-CuO nanospheres catalysts with tunable Cu2+/Cu+ ratio by controlled calcination in air. Based on the study of surface composition and the corresponding catalytic activities, a synergetic catalytic mechanism was proposed where CO adsorbed on CuO is oxidized into CO2 by lattice oxygen of CuO, and O2 molecules adsorbed on Cu2O splits into oxygen adatoms and transfer to fill the oxygen vacancies on CuO. The synergetic mechanism well explains the high activity and good stability of Cu2O-CuO catalyst as well as the self-activation effect of uncalcined catalysts.

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Introduction CO oxidation has attracted great interest due to the potential applications including exhaust purification in vehicles,1 protection of Pt electrode from CO poisoning in fuel cell2-3 and mechanism investigation in a catalytic reaction4-6. In general, the low-temperature oxidation of CO (LTOX) and preferential oxidation of CO (PROX) in the presence of H2 can only be accomplished in the existence of highly efficient catalysts. Among all the reported catalysts, supported noble metals have been used as stable and high-efficient catalysts for a long time due to the good dispersion and strong interaction between the support and the metal.7-9 For instance, Jan-Dierk Grunwaldt et al.10-11 synthesized TiO2 supported gold nanoparticles with mean particle size of 2 nm, which exhibits high activity for the low-temperature oxidation of CO. Zheng et al. demonstrated that Pt nanoparticles supported by FeNi(OH)x nanoparticles showed high activity for catalytic oxidation of carbon monoxide (CO) at room temperature.12 Considering the high cost and confined availability of noble metal, transition metal oxides, such as α-MnO2, CeO2, Co3O4 and CuOx, are good substitutional material for the catalytic oxidation of CO.13-17 Among these metal oxides, CuO and Cu2O have attracted much attention because of its high activity at low temperature, but the accurate understanding to the reaction mechanism still requires more experimental evidences.18 Some researchers believe that the active sites are located on Cu2O because of the effective adsorption of CO on Cu+ and thereafter oxidation on the catalyst. It has been proved by Stephen et al.19 through the highly active oxidation of CO (99.5 %) below 250 °C using uniform Cu2O nanocrystals as catalyst, although both Cu2+ and Cu+ are present on its surface. While, other researchers propose that the major active sites are located on CuO because it offers the lattice oxygen for the oxidation of CO. For example, Huang et al.20 have prepared octahedral and cubic Cu2O nanocrystals via solution-


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phase methods, and investigated the catalytic mechanism of oxide through CO. It demonstrates that the in-situ formed CuO on the Cu2O nanocrystals is responsible for the catalytic activity. In this work, a series of Cu2O-CuO hollow nanospheres with similar structures but increasing Cu2+/Cu+ ratio have been prepared by calcination of Cu2O nanoparticle clusters in air under temperature set from 265 to 300 °C. Accordingly, the solid nanosphere becomes hollow along with the oxidation, which is proved by structural characterizations with TEM, XRD, H2-TPR and XPS. The as-prepared catalysts were applied to the catalytic oxidation of CO and it showed the highest activity when the ratio of Cu2+/Cu+ on the surface of catalysts is about 7.2. According to the catalytic activities of copper oxide nanospheres and their structures, we proposed a synergetic reaction mechanism based on the composite structure of Cu2O and CuO for low temperature oxidation of CO. Here, CuO provides lattice oxygen for oxidation of CO, and the generated vacancies were refilled by the oxygen adatoms formed on the surface of Cu2O, which brings high activity and long life to the catalyst.

Experimental Materials: Diethylene glycol (DEG, 99%) was purchased from Sigma Aldrich. Ethanol (99.7%), polyvinyl pyrrolidone (PVP, K30) and copper (II) acetate monohydrate (Cu(Ac)2·H2O, AR) were obtained from the Sinopharm Chemical Reagent Co. Ltd. All chemicals were used without further purification. In a typical synthesis, PVP (K30, 20 mmol) dissolved in DEG (60 mL) was first heated to 180 °C under N2 protection. Cu(Ac)2·H2O (2 mmol) dissolved in DEG (4 mL) was then injected into


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the hot solution to initiate the generation of Cu2O. The transparent bluish green solution rapidly turned to an orange slurry in 5 min after the injection, and the mixture was further stirred at 180 °C for another 2 h to accomplish the reaction. After cooling the reaction solution down to room temperature, the Cu2O nanoparticles were diluted with ethanol, separated from the solution by centrifugation, washed with ethanol for 3 times and eventually dried in vacuum to generate orange powders. The as-prepared Cu2O nanospheres (0.4 g, Sample 1) were transformed into Cu2O-CuO (Sample 2), Cu2O-CuO (Sample 3) and CuO (Sample 4) nanospheres through calcination at 265 °C for 0.5 h, 275 °C for 1 h and 300 °C for 2 h, respectively. Catalytic oxidation of CO: The catalytic activity of the copper oxide nanospheres was evaluated by CO oxidation at atmospheric pressure. In a typical reaction, the catalysts (100 mg, 40-60 mesh) were first loaded into a U-shaped quartz microreactor with internal diameter of 3.8 mm. After introducing the feed gas (1% CO and 3% O2 in N2) with a total flow rate of 40 mL·min-1, the reaction temperature was continuously raised from 20 to 180 °C with a speed of 10 °C /min. The reaction was maintained at a specific temperature within the above temperature range for 5 min. Then the gaseous products were sampled and analyzed by gas chromatography equipped with TDX-01 column and TCD detector. In the stability test, the oxidation of CO was performed at 110 °C, 130 °C and 150 °C for 24 h and the products were sampled and analyzed every 1 h. The conversions of CO were calculated according to the following equations: Conv. = [n (COin) – n (COout)] / n (COin). Characterization: The morphology of samples was characterized by a Hitachi S4800 scanning electron microscope (SEM) at 3 kV and a FEI Tecnai G2 F30 transmission electron microscope (TEM) operated at 300 kV. Nitrogen adsorption-desorption isotherms, Brunauer-Emmett-Teller (BET) surface areas and BJH pore diameters were measured at 77 K with a Quanta Quadrasorb


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analyzer. All the samples were degassed in N2 flow at 200 °C for 1 h before the absorption test. A Rigaku Ultima VI X-ray diffractometer operated at 35 kV and 40 mA with Cu-Kα radiation was employed to record the X-ray diffraction (XRD) patterns with a scanning speed of 60 °/min between 10 ° and 90 °. The FT-IR spectra of the samples were measured by Nicolet FT-IR 560 spectrometer. Thermal gravimetric analysis (TGA) operated by NETZSCH STA 449F3 was used to monitor the mass loss of products at a heating rate of 10 °C/min ranged from 25-800 °C under air flow. X-ray photoelectron spectroscopy (XPS) analyses for the surface of the samples were performed with AXIS Ultra DLD. H2 temperature-programmed reduction (H2-TPR) was carried out on a full automatic adsorption analyzer (TP-5080). Before the chemical adsorption test, the samples (0.05 g, 40-60 mesh) were preheated to 150 °C and maintained for 1 h in He flow with a flow rate of 30 mL·min-1. After cooling down to room temperature, the samples were gradually heated to 450 °C at 5 °C·min-1 in the mixture of H2/N2 (H2, 5 vol %) with a flow rate of 30 mL·min-1, during which the consumption of H2 were recorded as peaks at specific temperatures. O2 temperature-programmed desorption (O2-TPD) was also measured on adsorption analyzer TP-5080. CO absorption was measured on Micromeritics AutoChem II chemisorption Analyzer.

Results and Discussions Uniform Cu2O nanospheres were synthesized through a high-temperature polyol process, and they were further transformed into Cu2O-CuO nanospheres by controlled calcination in air. (Figure 1a) First of all, the Cu2O nanospheres were synthesized through decomposition and reduction of copper acetate in diethylene glycol (DEG) at high temperature. In this polyol reaction, PVP was used as a surfactant to control the formation of seeds and growth of


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nanospheres. After separation and drying, the Cu2O nanospheres were calcined at 265-300 °C in air. Along with the calcination, the solid Cu2O nanospheres gradually transformed into hollow Cu2O-CuO composite nanospheres. The final chemical composition and crystal structure of copper oxide can be adjusted by tuning the calcination temperature and reaction time, which were described in the experimental section.

According to the statistical analysis of Cu2O

nanospheres and hollow Cu2O-CuO nanospheres in TEM images, the average diameter of Sample 1 to 4 were determined to be 186.5 nm, 190.8 nm, 200.3 nm and 201.9 nm. (Figure S1) It suggested that the calcination below 275 °C would lead to the oxidation of cuprous oxide along with the increasing of particle size and the gradual turning to hollow structures. However, further calcination to 300 °C brought no difference to the morphologies of particles, but the oxidation continued during the calcination. The formation of hollow structure might be explained by the Kirkendall effect, where the interior Cu+ prefer to transfers to the surface of the nanosphere and reacts with O2 in the calcination to form copper oxides shell.21-22 As shown in the TEM images (Figure 1b-e) and SEM images (Figure S2), the Cu2O nanospheres with diameter of about 180 nm are generally composed of a large number of primary Cu2O nanocrystals and mesopores between them, which is similar to the structure of many oxide nanoparticles prepared in the polyol reaction.23 When the nanospheres were calcined in air, the surface Cu2O was quickly oxidized to a thin layer of CuO-Cu2O which drove the migration of interior Cu+ to the surface to continue the oxidation. On the other side, oxygen can diffuse into the interior part of the Cu2O nanospheres through the mesopores between the primary particles and directly oxidize the interior Cu2O into CuO. However, this oxidation will be much slower than the surface oxidation.24-25 Therefore, the Cu2O-CuO hollow structures form eventually due to the faster diffusion speed of interior Cu+ to


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the surface than that of the O2 into the inner part of the nanospheres, which could be categorized as an example of Kirkendall effect. EDS linear scanning of Cu2O nanosphere and CuO hollow sphere also confirm the formation of hollow structures by calcination. (Figure S3) The hollow structure does not collapse during high-temperature treatments and shows adequate mechanic strength, good mass transportation, which is favorable for the catalytic applications. The precise calcination temperature for the controlled synthesis of Cu2O-CuO hollow nanospheres are determined based on the thermal gravity analysis (TGA) of Cu2O nanospheres. As shown in Figure 2, the weight of Cu2O nanospheres decreases about 4.6% as the temperature increases from 42 to 215 °C, which could be attributed to the desorption of water and organic species from the synthesis. Then, its mass increases about 7.2% as the temperature further being raised to 365 °C, which suggests that the oxidation of Cu2O might be initiated around 215 °C and fully completed at 365 °C. It should be noted that the weight increment (7.2 %) is less than the theoretical value (11 %) for the conversion from Cu2O to CuO, because the burning of carbon residuals from PVP on Cu2O nanospheres would lead to an extra decreases of mass during the heating process. Based on the TGA results, we designed a controlled calcination, which can transform Cu2O into Cu2O-CuO nanospheres with increasing Cu2+/Cu+ ratios through calcination at 265 °C for 0.5 h, 275 °C for 1 h and 300 °C for 2 h, respectively. They were marked as Sample 2, 3 and 4, while the Cu2O nanoparticles without calcination treatment were noted as Sample 1. The gradual oxidation of Cu2O and the chemical composition of the as-prepared copper oxides are also confirmed by XRD and IR analysis. The XRD patterns (Figure 3a) shows that all the diffraction peaks of the as-prepared Cu2O are consistent with those of cuprous oxide (Cu2O, JCPDF #05-0667). When the calcination temperature raises from 265 to 275 °C, the Cu2O


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nanosphere turns to Cu2O-CuO composites, whose XRD patterns are composed of the diffraction peaks from both cuprous oxide and cupric oxide (CuO, JCPDF #45-0937). According to the positive relationship between the diffraction peak intensity and the mass ratio in composites, the CuO contents in Sample 2 and Sample 3 are estimated to be about 30 and 60 wt%, respectively. When the calcination temperature increases to 300 °C, the sample is further oxidized into black powders, whose XRD pattern only shows the diffraction peaks of CuO. No apparent diffraction peak of Cu2O is observed, which suggests that Cu2O is either completely oxidized to CuO or kept as a component with very small amount after calcination. According to the Scherrer formula, the grain sizes were calculated to be 5-7 nm for Cu2O nanosphere and 8-12 nm for CuO hollow nanospheres (Table 1), which suggested that the crystal domain grew along with the calcination and oxidation. Furthermore, the conversion of Cu2O and a series of oxidation products can also be proved by the change of FTIR spectra in the fingerprint region. As shown in IR spectra (Figure 3b), the IR peaks from 629 cm-1 to 588 cm-1 are attributed to the Cu(I)-O stretching vibration, and those from 503 cm-1 to 534 cm-1 are attributed to the Cu(II)-O stretching vibration.26 The original Cu2O nanospheres have typical Cu(I)-O stretching vibration peaks at 629 cm-1. As the calcination temperature is raised, the IR peak at 629 cm-1 shifts towards 588 cm-1 and decreases in intensity, while new IR peak appears at 503 cm-1 and shifts towards 534 cm-1 with increasing intensity, both of which confirm the change in crystal structure and valence state during the oxidation process. A broad peak between 503 cm-1 and 629 cm-1 usually indicates the Cu2O-CuO composite nanospheres. As the calcination temperature further raised to 300 °C, a strong peak at 534 cm-1 with a weak shoulder at 588 cm-1 can be observed, which indicates the existence of small amount of Cu+ in the hollow CuO nanospheres.


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The hollow Cu2O-CuO nanospheres have tunable Cu2+/Cu+ ratio on the surface and interior cavity for gas transportation, which is favorable for heterogeneous catalytic oxidation of CO. In the following discussion, oxidation of CO will be used to investigate the influence of chemical composition and nanostructures upon catalytic activity. In a typical reaction, Cu2O and Cu2OCuO nanospheres with similar size (~180 nm) and closed surface area (11 m2·g-1, Figure S4 and Table S1) are used as catalysts for CO oxidation. The experimental results show that all samples are active for the oxidation reaction of CO below 170 °C in the first cycle, and the reaction catalyzed by Sample 3 has the highest activity at the same temperature. (Figure 4) Here, the light-off temperature (T50) is usually used to evaluate the catalytic activity quantitatively, which is defined as the temperature where half of the CO is oxidized into CO2. When the Cu(II)/Cu(I) ratio of the catalyst increases with raising of calcination temperature from 265 to 300 °C, the catalytic activity shows a volcanic trend. (Inset of Figure 4) For the Cu2O nanosphere without calcination (Sample 1), its T50 value (158 °C) is the highest among all four samples’, which means Sample 1 has the lowest activity for CO oxidation. After calcining at 265 and 275 °C in air, the Cu2O nanosphere turns to Cu2O-CuO composites and the corresponding T50 decreases to 149 and 117 °C, respectively. However, when the calcination temperature increases further to 300 °C, T50 of Cu2O-CuO increases to 129 °C and the activity start to decrease. The superior catalytic activity of Cu2O-CuO composite nanosphere might be attributed to the synergetic effect of neighboring Cu2O and CuO in the oxidation reaction of CO, which can be confirmed through the comparison of activity between the Cu2O-CuO nanospheres and the milling mixtures of Cu2O and CuO nanospheres. (Figure 5) Here, the ratio of Cu2O/CuO in the milling mixture are set to be 1:2, 1:1 and 2:1, so that the mixture and the Cu2O-CuO composite nanospheres have the identical nanostructure and similar chemical compositions but different


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chemical environment for specific Cu2O or CuO domains. The dependence of CO conversion rate upon the temperature show that the light-off temperature (T50) of the mixture catalysts is 166, 150 and 146 °C, respectively. While for Cu2O-CuO composite nanosphere (Sample 3), its T50 is only about 117 °C, which is much lower than that of the three mixtures. It indicates that the fusion of adjacent Cu2O and CuO domain in the composite nanospheres, that is the formation of Cu2O-CuO composite structure, leads to a much faster conversion of CO than the simply mixing of Cu2O and CuO nanospheres. The volcanic trend of catalytic activity suggests that the activity of Cu2O-CuO catalyst is determined by the surface Cu2+/Cu+ ratio of nanosphere, which can be studied via the X-ray photoelectron spectra (XPS) of Cu 2p3/2 and O 1s. (Figure 6, Figure S5) Generally, XPS provide abundant information of the valence state and interaction between the surface chemical species. The XPS spectra of O 1s show the transformation of peaks towards the low binding energies, which is consistent with the elimination of surface OH-, carbonyl group and H2O during the calcination. For pure Cu2O nanosphere, its XPS spectra of Cu 2p3/2 only shows one peak (1) for Cu+ with binding energy of 932.4 eV. However, the XPS spectra of calcined Cu2O-CuO nanosphere will present some new peaks for Cu2+ in addition to peak-1, which include a major peak (2) at 934.2 eV and triple satellite peaks (3, 4, 5) located at 940.7, 942.2 and 943.8 eV respectively.27 The amount of surface Cu+ and Cu2+ can be approximately estimated by the area of their XPS peaks, which suggests that the Cu2+/Cu+ ratio increased in the sequence of 5.6, 7.2 and 8.6 for Sample 2, Sample 3 and Sample 4, where Cu2O is gradually oxidized into CuO. (Tabel S2) Combined with the results of catalysis, it can be concluded that the Cu2O-CuO nanosphere with ratio of 7.2 has the highest catalytic activity, where a small amount of Cu2O remains in the surface of nanospheres, which forms composite structure with CuO.


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The synergistic effect of Cu2O and CuO in the composite structure is also investigated by temperature programmed reduction (TPR) in H2. (Figure 7) For pure Cu2O nanosphere (Sample 1), its H2-TPR profile shows two peaks at 216 °C and 306 °C, which can be assigned to reduction of amorphous Cu2O and crystalline Cu2O respectively.28-30 For Cu2O-CuO nanosphere (Sample 2) prepared by calcination at 265 °C, its H2-TPR profiles shows two peaks at 227 °C and 306 °C, which can be attributed to the reduction of CuO (β peak) and the unreacted crystalline Cu2O in calcination. Since the H2 consumption is proportional to the peak area, the weight percentage of CuO is estimated to be 28%, which is close to the result from XRD pattern. Along with the calcination at higher temperature, the Cu2O-CuO nanosphere (Sample 3) will give H2-TPR profiles at 260 °C and 295 °C, which corresponds to the reduction of CuO (γ peak) and Cu2O. These two peaks reveals that more crystalline CuO are produced by oxidation and the left crystalline Cu2O has higher reaction activity, as indicated by a decreased reduction temperature in H2 from 306 °C to 295 °C. Combined with the catalytic activity of Sample 1, 2 and 3, their H2-TPR profiles prove again the importance of Cu2O-CuO composite structure with appropriate ratio in catalysis. Furthermore, the higher activity of Sample 3 might also benefit from the high surface activity of crystalline Cu2O for absorption and reaction. The H2-TPR profile of Sample 4 shows a broad H2 consumption peak at 260 °C, which indicates most of the Cu2O have been converted into CuO. Therefore, an over high Cu2+/Cu+ ratio will decrease its activity. According to the catalytic activities of copper oxide nanospheres and characterizations of their structure via XRD, FTIR, XPS and H2-TPR, we shall propose a synergetic reaction mechanism based on the composite structure of Cu2O and CuO for low temperature oxidation of CO.19-20, 31 (Figure 8) At the beginning, the O2 and CO molecules can be preferentially adsorbed by the


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Cu2O and CuO when they are introduced to the reaction system. The CO molecules will react with the lattice oxygen of CuO to produce CO2, which leaves oxygen vacancies on the surface of CuO. On the other hand, the adsorbed O2 molecules splits into oxygen adatoms with the help of Cu2O domain. These oxygen adatoms then fill the oxygen vacancies and restore the surface state of CuO to be ready for the following catalysis. In a short word, the Cu2O and CuO domains provide the reaction sites for the dissociation of O2 and the oxidation of CO respectively, which synergistically accomplishes the oxidation through continuous creation and filling of oxygen vacancies on catalyst. The proposed reaction mechanism can be supported by temperature programmed desorption of O2 and chemical absorption of CO. First of all, O2-TPD experiments are performed on Cu2O nanosphere (Sample 1) and CuO hollow sphere (Sample 4), where the desorption peak at lower temperature usually corresponds to the adsorption oxygen (O2- or O- species) and the desorption peak at higher temperature corresponds to the lattice oxygen.17, 32-33 Apparently, there is a large amount of adsorption oxygen on the surface of Cu2O, and only few of them can be found on the surface of CuO. (Figure S6) Therefore, O2 molecules will be selectively adsorbed on Cu2O. Secondly, chemical absorption of CO on Cu2O nanosphere and CuO hollow sphere are measured to be 0.097 µL/g and 0.156 µL/g, which suggest that more CO can be adsorbed on the surface of CuO. (Table S3) Although there is competitive adsorption for CO and O2 over Cu2O, it will be difficult for Cu2O to adsorb CO when it is covered by O2. Therefore, there is great chance for CO to be selectively adsorbed on CuO. Thirdly, the literature works have proved through calculation that the adsorbed CO is easy to react with the lattice oxygen but not the adsorbed oxygen to generate CO2.19, 34 Since CO molecules are adsorbed on CuO and the lattice oxygen is dominant on the surface of CuO, it is believed that the adsorbed CO can be oxidized by lattice oxygen in


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the reaction, where the oxygen vacancies can be filled by oxygen adatom from the adsorbed O2 on Cu2O. This mechanism well explains the much higher activity of Cu2O-CuO composite nanospheres compared to the milling mixture of Cu2O and CuO, because the composite structure with atomic scale distance between Cu2O and CuO domain is the premise for fast transportation of oxygen adatom on Cu2O to fill the oxygen vacancies on CuO, Furthermore, this mechanism also well explains the volcanic trend of catalytic activity of Cu2O-CuO composite nanospheres along with the increasing of their Cu2+/Cu+ ratios, where an appropriate Cu2+/Cu+ ratio helps to realize oxygen dissociation and CO oxidation in a matched speed and thereby improve the catalytic activity. In the study of copper oxide catalysts, it is interesting to find that the activity of all the Cu2O and Cu2O-CuO nanospheres are improved when they are collected from the 1st round of catalytic reaction and reused for the 2nd run. (Figure S7) This self-activation effect during the continuous catalysis is especially obvious for Cu2O nanospheres (Sample 1) without calcination. (Figure 9a) Its light-off temperature (T50) sharply decreased from 158 °C to 124 °C when it is used for CO oxidation again. XRD patterns and TEM images of Sample 1 before and after the 1st round reaction showed identical results. (Figure 9b-c) However, the XPS spectra after reaction shows the appearance of new peaks around 934 eV and 940 eV to 945 eV, which indicates part of the surface Cu2O has been oxidized to CuO in the CO oxidation. (Figure 9d) The characteristic XPS peaks for CuO are much weaker than those from Sample 2 to 4, which explains why XRD peaks of CuO are not observed since the oxidation of Cu2O only take place on the surface to a small extent. Now, it is clear that the oxidation of Cu2O catalyst is inevitable in the catalytic oxidation of CO with rich oxygen atmosphere. This oxidation preferentially takes place at the surface of


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Cu2O nanospheres, which makes major contributions to the conversion of CO. With the formation of Cu2O-CuO composite structure by oxidation, the catalytic activity is improved due to the synergetic effect, which behaves as the self-activation of CuO-Cu2O catalysts. Although the self-activated Cu2O nanospheres show gradually improved activity in CO oxidation, the pre-calcination of Cu2O to generate Cu2O-CuO composite catalyst is still required because of the higher activity compared to the untreated one. In order to confirm the necessity of calcination, we have compared the CO oxidation catalyzed by Cu2O (Sample 1) and Cu2O-CuO (Sample 3) nanospheres for 24 hours at 110 °C, 130 °C and 150 °C. (Figure 10) The experiment results of long-term thermostatic reaction show that all the catalysts have an increase in catalytic activity with the progress of the reaction, which means these catalysts are self-activated during the CO reduction reaction. At higher temperature, the self-activation is more apparent for pure Cu2O catalysts as the conversion of CO grows much faster than that of pre-calcinated catalysts. For instance, the conversion of CO at 130 °C increases by 42% when it is catalyzed by pure Cu2O catalysts, and it increases by 13% when it is catalyzed by Cu2O-CuO catalysts. At lower temperature, the self-activation is limited for both two catalysts. Although the increase of CO conversion is smaller for Cu2O-CuO catalysts, it always shows higher activity than that of pure Cu2O, which means the pre-calcination of Cu2O to generate Cu2O-CuO composite is still a necessary treatment for a mild oxidation at low temperatures.

Conclusions Uniform Cu2O nanospheres are synthesized through a high-temperature polyol process, and they are further converted into Cu2O-CuO composite hollow nanospheres through calcination at


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265-300 °C. These nanoparticles possess similar size, close specific surface area and tunable Cu2+/Cu+ ratio on the surface, which provide ideal material platform for the investigation of active component and reaction mechanism for copper oxide nanocatalysts. The XRD, FTIR, XPS and H2-TPR results show that Cu2O-CuO composite with an optimal ratio of Cu2+/Cu+ exhibits best activity, which indicates a synergetic mechanism for low temperature oxidation of CO using the composite structure of Cu2O and CuO as catalysts. Here, Cu2O and CuO offer the active sites for oxygen dissociation and CO oxidation respectively, and an atomic scale distance between Cu2O and CuO domain is favorable to the fast transportation of oxygen adatom on Cu2O to fill the oxygen vacancies on CuO. An appropriate Cu2+/Cu+ ratio offers a matched reaction speed on Cu2O and CuO domain and thereby improves the catalytic activity. The synergetic mechanism well explains the high activity and good stability of Cu2O-CuO catalyst as well as the selfactivation effect of uncalcined catalysts. Although the self-activated Cu2O nanospheres show gradually improved activity in CO oxidation, the pre-calcination is still required for higher activity at low temperature.

Supporting Information. Particle size distribution, SEM images, EDS mapping, N2 adsorption-desorption isotherms, BET surface areas, the Cu 2p3/2 and O 1s XPS spectra, chemical absorption of CO, O2 -TPD curves and the catalytic activity of the 2nd run reaction for the 4 samples were supplied as supporting information.

Acknowledgement


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This work is supported by the National Key Research and Development Program of China (2016YFB0701103), National Natural Science Foundation of China (21471058, 21671067) and ShuGuang Program supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (15SG21).

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Figures and Captions

Figure 1. (a) Schematic illustration to the synthesis of Cu2O nanospheres (Sample 1) by polyol reaction and Cu2O-CuO hollow nanospheres (Sample 2-4) through a controlled calcination, and (b-e) their corresponding TEM images.


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Figure 2. Thermal gravity analysis (TGA) of Cu2O nanospheres (Sample 1) in air.

Figure 3. XRD patterns and FTIR spectra of Cu2O (Sample 1) and Cu2O-CuO (Sample 2-4) nanospheres.


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Figure 4. (a) CO conversions under different temperatures catalyzed by Cu2O and Cu2O-CuO nanospheres in the first run. The corresponding T50 values are presented as inset.

Figure 5. CO conversions under different temperatures catalyzed by Cu2O-CuO nanospheres (Sample 3) and three milling mixtures of Cu2O and CuO nanospheres with ratio of 1:2, 1:1 and 2:1.


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Figure 6. XPS spectra of Cu 2p3/2 for Cu2O nanosphere (Sample 1) and Cu2O-CuO nanospheres (Sample 2-4).

Figure 7. Temperature programmed reduction profiles of Cu2O (Sample 1) and Cu2O-CuO (Sample 2-4) in H2.


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Figure 8. The synergetic mechanism scheme between Cu2O and CuO domains in Cu2O-CuO composite nanospheres.

Figure 9. a) CO conversions under different temperatures catalyzed by Cu2O for two runs, b) XRD patterns and d) the Cu 2p3/2 XPS spectra of Cu2O before and after reaction, c) TEM image of Cu2O nanospheres after reaction.


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Figure 10. Kinetic curves of CO conversions for Sample 1 (Cu2O) and Sample 3 (Cu2O-CuO) at 110 °C, 130 °C and 150 °C

Tables

Table 1. Comparison of the particle diameter, grain size, Cu2+/Cu+ ratio and light-off temperature for 4 copper oxide catalysts. Particle Diameter (nm)

Grain Size (nm) / Cu2O

Grain Size (nm) / CuO

Cu2+/Cu+ Ratio

T50 (°C)

Sample 1

186.5

6.5

/

/

158

Sample 2

190.8

7.1

8.0

5.6

149

Sample 3

200.3

5.2

9.7

7.2

117

Sample 4

201.9

/

12.5

8.6

129


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TOC Graphic


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