Designing Highly Efficient Cu2O-CuO Heterojunction CO Oxidation Catalysts: The Roles of the Support Type and Cu2O-CuO Interface Effect

In this work, a series of Cu2O/S (S = α-MnO2, CeO2, ZSM-5, and Fe2O3) supported catalysts with a Cu2O loading amount of 15% were prepared by the facile liquid-phase reduction deposition–precipitation strategy and investigated as CO oxidation catalysts. It was found that the Cu2O/α-MnO2 catalyst exhibits the best catalytic activity for CO oxidation. Additionally, a series of Cu2O-CuO/α-MnO2 heterojunctions with varied proportion of Cu+/Cu2+ were synthesized by further calcining the pristine Cu2O/α-MnO2 catalyst. The ratio of the Cu+/Cu2+ could be facilely regulated by controlling the calcination temperature. It is worth noting that the Cu2O-CuO/α-MnO2-260 catalyst displays the best catalytic performance. Moreover, the kinetic studies manifest that the apparent activation energy could be greatly reduced owing to the excellent redox property and the Cu2O-CuO interface effect. Therefore, the Cu2O-CuO heterojunction catalysts supported on α-MnO2 nanotubes are believed to be the potential catalyst candidates for CO oxidation with advanced performance.


Introduction
With the acceleration of economic globalization, power plants, cement plants, automobile exhaust emissions [1,2], biomass combustion [3], and other sources of fuel produce large quantities of CO due to the incomplete combustion [4][5][6][7][8]. It is reported that when the CO content in the air is larger than 0.1%, it will cause poisoning in humans [9], which further results in nausea, dizziness, loss of consciousness, headache, and even fatal accidents [10]. As well known, CO is a colorless, odorless, and asphyxiating toxic gas, and a flammable and explosive air pollutant, which greatly threatens the health of humans and the safety of the living environment. There are various methods of CO removal reported in the literature, such as the physisorption, CO methanation [11], and catalytic oxidation [12]. Among these strategies, catalytic oxidation is regarded as one of the most efficient techniques for the elimination of CO [13,14]. Additionally, CO oxidation is widely investigated as an interesting probe reaction for other oxidation processes.
Currently, CO oxidation catalysts based on precious metals and transition metal oxides have attracted extensive research interest. Although the noble metal-based catalysts exhibit excellent low-temperature catalytic activity, their high cost and rarity limit their industrial

Preparation of the Supports
The detailed preparation process of the α-MnO 2 , CeO 2 , and ZSM-5 supports are given in Supplemental Materials S1.

Preparation of the Catalysts 2.2.1. Preparation of Cu 2 O/S Supported Catalysts
A series of the Cu 2 O/S (S = α-MnO 2 , CeO 2 , ZSM-5, and Fe 2 O 3 ) supported catalyst was prepared by the liquid-phase-reduction deposition-precipitation synthesis strategy. The specific process for Cu 2 O/S supported catalysts preparation is described in detail in Supplemental Materials S1.

Preparation of Cu 2 O-CuO Heterojunction Catalysts
The Cu 2 O-CuO/α-MnO 2 heterojunction catalysts with various Cu + /Cu 2+ ratios were synthesized. The Cu 2 O-CuO/α-MnO 2 heterojunction catalyst obtained was designated as Cu 2 O-CuO/α-MnO 2 -T, in which "T" denotes the targeted calcination temperature. Further details related to preparation are shown in Supplemental Materials S1.

Catalyst Characterizations
A series of characterization analyses were carried out on the supports and corresponding catalysts. Further details related to the equipment information, the operational details, and the determination parameters are summarized in Supplemental Materials S2.

Catalytic Activity Measurements
The catalytic performance of catalyst for CO oxidation was evaluated in a fixed-bed reactor. The productions were detected online by gas chromatography. Further information on the specific reactor and evaluation of catalyst is summarized Supplemental Materials S3.

Results and Discussions
3.1. Catalytic Property toward CO Oxidation 3.1.1. Effect of the Support Type on the Catalytic Activity of CO Oxidation The catalytic activity of CO oxidation on Cu 2 O/S (S = α-MnO 2 , CeO 2 , ZSM-5, Fe 2 O 3 ) catalysts with different supports was evaluated to investigate the influence of the support type on the catalytic activity of the CO oxidation. Figure 1 shows that the trend of the CO conversions was increasing with the increase in reaction temperature until 100% CO conversion was finally reached. Additionally, it was also interesting to observe that the catalytic activities of the Cu 2 O-based catalysts supported on ZSM-5 and Fe 2 O 3 were much lower than those of Cu 2 O/α-MnO 2 and Cu 2 O/CeO 2 catalysts. The presence of the α-MnO 2 and CeO 2 supports could greatly improve the catalytic activities of the Cu 2 O-based catalyst, which might be due to the excellent oxygen storage and release capacities of the α-MnO 2 and CeO 2 supports. This indicates that the properties of the supports had great effect on the catalytic performance of the Cu 2 O-based catalysts. It is worth noting that the Cu 2 O/α-MnO 2 catalyst exhibited the best CO oxidation activity among the Cu 2 O-based catalysts investigated. Therefore, α-MnO 2 was considered as the promising candidate among the investigated supports.

Effect of Cu 2 O-CuO Heterojunction on the Catalytic Activity of CO Oxidation
As shown in Figure 2, detailed evaluations of the catalytic performance for CO oxidation on the Cu 2 O/α-MnO 2 , Cu 2 O-CuO/α-MnO 2 -T, and CuO/α-MnO 2 -500 catalysts were also conducted. Figure 2 indicates that the CO conversion on these catalysts increased with the increase in the reaction temperature until reaching 100%. In addition, all the investigated catalysts performed CO oxidation well, and 100% CO conversion could be achieved below 120 • C. However, their reaction temperatures of 10% (T 10 ), 50% (T 50 ) and 100% (T 100 ) CO conversion were quite different. This might be due to the difference in Cu 2 O-CuO heterojunction composition between the CuO and Cu 2 O. Thus, the presence of the Cu 2 O-CuO heterojunction in Cu 2 O-CuO/α-MnO 2 -T catalyst could greatly decrease the ignition temperature (T 10 ) of the catalyst. Specifically, the Cu 2 O-CuO/α-MnO 2 -T catalysts performed at lower ignition temperatures (even to the room temperature) than the pure Cu 2 O/α-MnO 2 (73 • C) and CuO/α-MnO 2 -500 (48 • C). Furthermore, the T 50 and T 100 of the Cu 2 O-CuO/α-MnO 2 -T heterojunction catalysts were lower than those of the pure Cu 2 O/α-MnO 2 and CuO/α-MnO 2 -500 catalysts. The phenomenon indicated that the Cu 2 O-CuO heterojunction of Cu 2 O-CuO/α-MnO 2 -T catalysts greatly contributed to enhanced CO oxidation activity at low temperature, which was on account of the synergetic effect of the Cu 2 O-CuO heterojunction [7,34]. The synergistic effect of Cu + and Cu 2+ was mainly derived from the atomic scale distance between Cu 2 O and CuO, which was conducive to the rapid migration of adsorbed oxygen on the Cu 2 O-CuO surface. In addition, it is noteworthy that the Cu 2 O-CuO/α-MnO 2 -260 catalyst with the lowest T 10 , T 50 , and T 100 showed the highest reactivity among the Cu 2 O-CuO/α-MnO 2 -T catalysts investigated. Therefore, the reaction temperature required for the CO catalytic oxidation was greatly reduced and the efficient removal of CO could be realized at low temperature when assisted with the combined effect of the Cu 2 O-CuO heterojunction.

Kinetic Study
The kinetic study of CO oxidation was carried out over the Cu 2 O/α-MnO 2 , Cu 2 O-CuO/α-MnO 2 -T, and CuO/α-MnO 2 -500 catalysts to investigate the Cu 2 O-CuO heterojunction on the catalytic performance. The Arrhenius curves are shown in Figure 3, and the specific value of the apparent activation energies are listed in Table 1. It is noteworthy that the apparent activation energies of the Cu 2 O-CuO/α-MnO 2 -T heterojunction catalysts were in the range from 41.9 to 62.2 kJ·mol −1 , which were greatly lower compared with the pure Cu 2 O/α-MnO 2 (83.4 kJ·mol −1 ) and CuO/α-MnO 2 -500 (64.0 kJ·mol −1 ) catalysts. These results indicate that the synergy effect of the Cu 2 O-CuO heterojunction can greatly increase the speed of the activation process of O 2 . Specifically, the Cu 2 O-CuO/α-MnO 2 -T catalyst reduced the activation energy of the CO oxidation process. At the same time, the apparent activation energy of Cu 2 O-CuO/α-MnO 2 -260 was the lowest among the Cu 2 O-CuO/α-MnO 2 -T catalysts. These results suggest that the Cu 2 O-CuO-T catalyst with the appropriate Cu + /Cu 2+ ratio dramatically reduced the activation energy owing to the improvement of the O 2 activation ability. The two rate-determining steps of the Cu 2 O-CuO/α-MnO 2 -260 • C catalysts were close to the optimal dynamic equilibrium ratio. Therefore, the Cu 2 O-CuO heterojunction structure displays great advantages in improving the CO oxidation activity at low temperature by reducing the apparent activation energy. In addition, it is noteworthy that the apparent activation energy of the CuO/α-MnO 2 -500 (64.0 kJ·mol −1 ) catalyst was also much lower than that of the Cu 2 O/α-MnO 2 (83.4 kJ·mol −1 ) catalyst. This proved that the CuO-MnO 2 interface provided the new reactive site for the CO catalytic oxidation, which was consistent with the study reported in [34]. The Cu 2 O-CuO/α-MnO 2 -T heterojunction catalysts performed excellent CO oxidation activity at low temperature through the comprehensive effects of the Cu 2 O-CuO heterojunction.

Long-Term Stability Test
The Cu 2 O/α-MnO 2 , Cu 2 O-CuO/α-MnO 2 -260 and CuO/α-MnO 2 -500 catalysts were selected as representative catalysts for the CO oxidation stability test at 90 • C for 12 h under certain conditions. Their stability test results are displayed in Figure 4. It can be observed that these three catalysts showed excellent stability throughout the whole 12 h, with no signs of deactivation. Additionally, the Cu 2 O-CuO/α-MnO 2 -260 catalyst showed a higher conversion rate than the Cu 2 O/α-MnO 2 and CuO/α-MnO 2 -500 catalysts in the catalytic stability test due to the presence of the Cu 2 O-CuO heterojunction. Similarly, the CuO/α-MnO 2 -500 catalyst showed higher conversion than the Cu 2 O/α-MnO 2 catalyst. These results indicate that the Cu 2 O-CuO/α-MnO 2 -T heterojunction catalyst exhibited not only excellent low-temperature activity, but good stability owing to the synergistic interaction of Cu 2 O-CuO heterojunction.

Effect of Cu2O-CuO Heterojunction on the Catalytic Activity of CO Oxidation
As shown in Figure 2, detailed evaluations of the catalytic performance for CO oxidation on the Cu2O/α-MnO2, Cu2O-CuO/α-MnO2-T, and CuO/α-MnO2-500 catalysts were also conducted. Figure 2 indicates that the CO conversion on these catalysts increased with the increase in the reaction temperature until reaching 100%. In addition, all the investigated catalysts performed CO oxidation well, and 100% CO conversion could be achieved below 120 °C. However, their reaction temperatures of 10% (T10), 50% (T50) and 100% (T100) CO conversion were quite different. This might be due to the difference in Cu2O-CuO heterojunction composition between the CuO and Cu2O. Thus, the presence of the Cu2O-CuO heterojunction in Cu2O-CuO/α-MnO2-T catalyst could greatly decrease the ignition temperature (T10) of the catalyst. Specifically, the Cu2O-CuO/α-MnO2-T catalysts performed at lower ignition temperatures (even to the room temperature) than the pure Cu2O/α-MnO2 (73 °C) and CuO/α-MnO2-500 (48 °C). Furthermore, the T50 and T100 of the Cu2O-CuO/α-MnO2-T heterojunction catalysts were lower than those of the pure Cu2O/α-MnO2 and CuO/α-MnO2-500 catalysts. The phenomenon indicated that the Cu2O-CuO heterojunction of Cu2O-CuO/α-MnO2-T catalysts greatly contributed to enhanced CO oxidation activity at low temperature, which was on account of the synergetic effect of the Cu2O-CuO heterojunction [7,34]. The synergistic effect of Cu + and Cu 2+ was mainly derived from the atomic scale distance between Cu2O and CuO, which was conducive to the rapid migration of adsorbed oxygen on the Cu2O-CuO surface. In addition it is noteworthy that the Cu2O-CuO/α-MnO2-260 catalyst with the lowest T10, T50, and T10 showed the highest reactivity among the Cu2O-CuO/α-MnO2-T catalysts investigated Therefore, the reaction temperature required for the CO catalytic oxidation was greatly reduced and the efficient removal of CO could be realized at low temperature when assisted with the combined effect of the Cu2O-CuO heterojunction.

Kinetic Study
The kinetic study of CO oxidation was carried out over the Cu2O/α-MnO2, Cu2O-CuO/α-MnO2-T, and CuO/α-MnO2-500 catalysts to investigate the Cu2O-CuO het-  The Cu2O/α-MnO2, Cu2O-CuO/α-MnO2-260 and CuO/α-MnO2-500 catalysts were selected as representative catalysts for the CO oxidation stability test at 90 °C for 12 h under certain conditions. Their stability test results are displayed in Figure 4. It can be observed that these three catalysts showed excellent stability throughout the whole 12 h, with no signs of deactivation. Additionally, the Cu2O-CuO/α-MnO2-260 catalyst showed a higher conversion rate than the Cu2O/α-MnO2 and CuO/α-MnO2-500 catalysts in the catalytic stability test due to the presence of the Cu2O-CuO heterojunction. Similarly, the CuO/α-MnO2-500 catalyst showed higher conversion than the Cu2O/α-MnO2 catalyst. These results indicate that the Cu2O-CuO/α-MnO2-T heterojunction catalyst exhibited not only excellent low-temperature activity, but good stability owing to the synergistic interaction of Cu2O-CuO heterojunction.   Figure S1 displays the XRD patterns of the as-prepared Cu 2 O/S (S = α-MnO 2 , CeO 2 , ZSM-5, Fe 2 O 3 ) catalysts with 15% Cu 2 O loading amount. As shown in Figure  phase, was eventually oxidized into the CuO/α-MnO 2 -500 black powder when the calcination temperature further increased to 500 • C. As a result, the XRD patterns of the CuO/α-MnO 2 -500 catalyst only displayed the characteristic diffraction peaks of CuO and α-MnO 2 phases. This indicates that Cu 2 O was completely transformed into CuO after the calcination at 500 • C for 1 h. Based on this analysis, it can be concluded that the Cu 2 O-CuO/α-MnO 2 heterojunction catalysts with different Cu + /Cu 2+ ratios can be obtained by adjusting the calcination temperature. The appropriate Cu + /Cu 2+ ratio heterojunction could greatly improve the catalytic activity of catalysts based on the catalytic results given in Figure 2. Cu2O-CuO/α-MnO2 heterojunction catalysts with different Cu + /Cu 2+ ratios can be obtained by adjusting the calcination temperature. The appropriate Cu + /Cu 2+ ratio heterojunction could greatly improve the catalytic activity of catalysts based on the catalytic results given in Figure 2. Figure 5b shows the XRD patterns of the spent (SP-) Cu2O/α-MnO2, Cu2O-CuO/α-MnO2-260, and CuO/α-MnO2-500 catalysts after 12 h long-term stability tests. Meanwhile, the XRD patterns of their corresponding fresh catalysts are also presented for comparison. As shown in Figure 5b, this pattern represents the fresh catalyst, and the pattern below represents the spent catalyst (SP-) after the stability test. The characteristic diffraction peaks of Cu2O (PDF#-90-0041), CuO (PDF#-72-1982), and α-MnO2 (PDF#-78-0428) can still be obviously observed on the spent catalysts, especially on the SP-Cu2O-CuO/α-MnO2-260 heterojunction catalyst. Therefore, the XRD results confirmed that the Cu2O-CuO heterojunction existed in the Cu2O-CuO/α-MnO2-260 catalyst after the 12 h long-term stability test of CO oxidation. This implies that the oxygen activation cycle during the CO oxidation on the Cu2O-CuO/α-MnO2-260 heterojunction catalyst was sustainable owing to the thermal stability of the Cu2O-CuO heterojunction. At the same time, the characteristic peaks of α-MnO2 were also perfectly retained. These results illustrate the important roles of the Cu2O-CuO heterojunction interface when constructing stable and efficient CO oxidation catalysts.

TG Analysis
The thermal stability and the phase transformation process of the Cu2O/α-MnO2 catalyst were studied by thermogravimetric analysis (TG) in air atmosphere. The weight decreased slowly in the range 30-200 °C, as shown in Figure 6. This might be caused by

TG Analysis
The thermal stability and the phase transformation process of the Cu 2 O/α-MnO 2 catalyst were studied by thermogravimetric analysis (TG) in air atmosphere. The weight decreased slowly in the range 30-200 • C, as shown in Figure 6. This might be caused by the removal of the water of physisorption and water of crystallization, together with the removal of trace organic reagent. However, when the temperature further rose to 511 • C, the weight began to drop sharply. The weight loss was equivalent to the loss of oxygen in the lattice of MnO 2 , leading to the formation of Mn 2 O 3 . Furthermore, when the temperature further increased to 769 • C, the weight again dropped sharply. This indicates that the Mn 2 O 3 once again lost part of the lattice oxygen, leading to the formation of Mn 3 O 4 . These results are consistent with precursory literature reports [35][36][37]. It is worth noting that the mass loss of the catalyst (10.61%) was less than the theoretical value (12.26%) during the conversion of α-MnO 2 to Mn 3 O 4 . The reason for the actual mass reduction being less than the theoretical mass reduction was that the transformation of the loaded Cu 2 O to CuO increased the weight.

N2 Physisorption Analysis
The structural characteristics of the Cu2O/α-MnO2, Cu2O-CuO/α-MnO2-T, and CuO/α-MnO2-500 catalysts were investigated by N2 physisorption characterization. Their N2 adsorption-desorption isotherms and pore size distributions of catalysts are shown in Figure 7. It can be observed in Figure 7a that all the catalysts were characterized by the type IV isotherm and H4 hysteresis loops. This also indicates that the Cu2O-CuO/α-MnO2-T and CuO/α-MnO2-500 catalysts still had mesoporous structures

N 2 Physisorption Analysis
The structural characteristics of the Cu 2 O/α-MnO 2 , Cu 2 O-CuO/α-MnO 2 -T, and CuO/α-MnO 2 -500 catalysts were investigated by N 2 physisorption characterization. Their N 2 adsorption-desorption isotherms and pore size distributions of catalysts are shown in Figure 7. It can be observed in Figure 7a that all the catalysts were characterized by the type IV isotherm and H 4 hysteresis loops. This also indicates that the Cu 2 O-CuO/α-MnO 2 -T and CuO/α-MnO 2 -500 catalysts still had mesoporous structures similar to Cu 2 O/α-MnO 2 after calcination at different temperatures, thus exhibiting the excellent thermal stability. Typically, the H 4 -shaped hysteresis loops indicate the existence of narrow wedge-shaped mesopores. Additionally, the mesopores might originate from the hollow α-MnO 2 nanotube in the catalyst support, the sintering of catalysts in calcination process, and the rupture of internal mesopores. As shown in Figure 7b, the average pore size of the Cu 2 O-CuO/α-MnO 2 -T and CuO/α-MnO 2 -500 catalysts was similar or larger than the original Cu 2 O/α-MnO 2 catalyst. This suggested that their mesoporous structures were not severely damaged by the thermal aggregation and phase transformation during the calcination process at different temperature. The nanotube hollow microsphere catalyst exhibited excellent thermal stability. In addition, the specific data of the structural properties of these catalysts are listed in Table 2. The results show that the specific surface areas, average pore diameters, and pore volumes of the Cu 2 O-CuO/α-MnO 2 -T catalysts were very similar to those of the pristine Cu 2 O/α-MnO 2 catalyst. This again confirms the excellent thermal performance stability of these catalysts. The slight reduction in the specific surface area might be due to the sintering of Cu 2 O in the process of the calcination. It would further affect the surface morphology of the catalyst-supported nanotube hollow spheres by creating internal pores and surface defects. In contrast, the specific surface area of the CuO/α-MnO 2 -500 catalyst was relatively smaller, which might be caused by the complete oxidation of the surface Cu 2 O specifies and the closure and/or blockage of the hollow pores due to the long-term high temperature calcination.

SEM and TEM Analyses
The TEM and SEM photos of the support α-MnO2 nanotube are shown in Figure  S2a,b. Figure 8 shows the SEM images of the as-prepared Cu2O/S (S = α-MnO2, CeO2, ZSM-5, Fe2O3) catalysts with 15% Cu2O loading amount. As shown in Figure 8a,b, the as-synthesized Cu2O/ZSM-5 catalyst exhibits regular cuboid shape with the a-axis (205 nm), b-axis (100 nm), and c-axis (1054 nm). In contrast, Cu2O/Fe2O3 catalyst supported on the commercial Fe2O3, shown in Figure 8c,d, resulted in an irregular morphology in the

SEM and TEM Analyses
The TEM and SEM photos of the support α-MnO 2 nanotube are shown in Figure S2a,b. Figure 8 shows the SEM images of the as-prepared Cu 2 O/S (S = α-MnO 2 , CeO 2 , ZSM-5, Fe 2 O 3 ) catalysts with 15% Cu 2 O loading amount. As shown in Figure 8a,b, the assynthesized Cu 2 O/ZSM-5 catalyst exhibits regular cuboid shape with the a-axis (205 nm), b-axis (100 nm), and c-axis (1054 nm). In contrast, Cu 2 O/Fe 2 O 3 catalyst supported on the commercial Fe 2 O 3 , shown in Figure 8c,d, resulted in an irregular morphology in the particle state, which might not be conducive to the dispersion of the Cu 2 O active sites. In addition, it is interesting to find in Figure 8e,f that the as-prepared Cu 2 O/CeO 2 catalyst exhibited spherical nanoparticles with uniform size distribution about 130 nm. At the same time, it can be observed in Figure 8g,h that the Cu 2 O/α-MnO 2 catalyst presented the morphology of hollow spheres, which were assembled by hollow nanotubes.    Nanomaterials 2022, 12, x FOR PEER REVIEW 12 process of the calcination, and the morphology of hollow microspheres had been cessfully maintained.

FTIR Analysis
To investigate the phase transition process from Cu2O to CuO in the calcina process, FTIR analysis of Cu2O/α-MnO2, Cu2O-CuO/α-MnO2-T, and CuO/α-MnO2 catalysts in the range 400-4000 cm −1 was performed. As shown in Figure 10, it is n worthy that the samples show infrared transmittance peaks at 598 and 468 cm −1 , w

FTIR Analysis
To investigate the phase transition process from Cu 2 O to CuO in the calcination process, FTIR analysis of Cu 2 O/α-MnO 2 , Cu 2 O-CuO/α-MnO 2 -T, and CuO/α-MnO 2 -500 catalysts in the range 400-4000 cm −1 was performed. As shown in Figure 10, it is noteworthy that the samples show infrared transmittance peaks at 598 and 468 cm −1 , which are attributed to the stretching vibration of the Cu(+1)-O and Cu(+2)-O bond, respectively [38]. In addition, the coexistence of stretching vibration peaks at 598 and 468 cm −1 indicate the existence of the Cu 2 O-CuO heterojunction. When the calcination temperature increased to 240 • C or other higher temperature, the characteristic stretching vibration peak of Cu(+2)-O bond gradually appeared at 468 cm −1 , while the Cu(+1)-O stretching vibration at 598 cm −1 progressively weakened due to the complete oxidation of Cu 2 O to CuO until it finally disappeared at 500 • C. This further verifies the change in crystal structure and valence state of Cu species in the calcination process in air atmosphere. Meanwhile, it can be found that the catalyst also provided strong infrared transmission peaks at 720 and 523 cm −1 , which are attributed to the stretching vibration of O-Mn-O and layered manganese oxide, respectively [39][40][41][42]. As for the transmittance peaks around 3423 and 1637 cm −1 , they are attributed to the stretching and flexural oscillations of the O-H groups caused by the physiosorbed water [43,44]. Therefore, the results of FTIR characterization further confirm the oxidation process of the Cu 2 O supported on the surface of α-MnO 2 nanotube and the formation of Cu 2 O-CuO heterojunction in the process of the calcination.   Figure 11. Specifically, the pristine Cu2O/α-MnO2, Cu2O/Fe2O3, Cu2O/CeO2, and Cu2O/ZSM-5 catalysts showed the 2p3/2 peak of Cu + at 931.38 eV (Figure 11a). This indicates the presence of the Cu2O species on the support surface. As shown in Figure 11b (Figure 11a). This indicates the presence of the Cu 2 O species on the support surface. As shown in Figure 11b, the O 1s main peak of the catalyst was around 529.36-529.46 eV, and the shoulder peak was around 530.76-530.96 eV. To be specific, the peaks of 529.36-529.46 eV and 530.76-530.96 eV could be attributed to surface adsorbed oxygen (O ads ) and lattice oxygen (O latt ), respectively [45]. However, the location of the main peak and shoulder peak of Cu 2 O/ZSM-5 catalyst differed greatly from the catalysts above. The shoulder peak at 530.58 eV should be attributed to the O species of Si-OH on the surface of SiO 2 support, rather than the surface-adsorbed oxygen (O latt ) [46,47]. In general, the number of the O ads mainly depends on the number of oxygen vacancies at the surface. The reason is that the O ads could only be absorbed on the oxygen vacancies. Thus, the O ads /(O latt + O ads ) ratio became the valid parameter to analyze the content of the surface oxygen vacancy. Table 3  The XPS results of Cu 2p, O 1s, and Mn 2p over Cu2O/α-MnO2 Cu2O-CuO/α-MnO2-T, and CuO/α-MnO2-500 catalysts are shown in Figure 12. Generally there are two main peaks around 933.08-933.82 eV and 952.68-952.88 eV observed ove these catalysts (Figure 12a), which can be attributed to the Cu 2p3/2 and Cu 2p1/2 peaks respectively [48]. Interestingly, the Cu 2p3/2 peak was accompanied by an oscillating sat ellite peak from 944.78 to 945.28 eV. It was previously reported that the satellite peaks ar generated by the transfer of electrons from the ligand orbital to the 3d orbital of Cu [49,50]. This indicates that the Cu 2+ exists in the divalent form with the 3d 9 structure, ra ther than the Cu + or Cu 0 species with the 3d 10 -filled energy level. It was reported that th CO oxidation activity of the Cu2O-CuO heterojunction catalyst is largely dependent on the surface of Cu + /Cu 2+ ratio [26]. The Cu 2+ /Cu 1+ relative percentages can be estimated by the peak-fitted areas of their corresponding XPS peaks. As shown in Table 4, the relativ percentages of different oxidation states in the Cu2O/α-MnO2, Cu2O-CuO/α-MnO2-240 Cu2O-CuO/α-MnO2-260, Cu2O-CuO/α-MnO2-280, and CuO/α-MnO2-500 catalysts were 0 6.30, 9.64, 9.99, and ∞, respectively. These results indicate that the calcination tempera ture greatly affects the relative percentages of different oxidation states over the catalys surface. Meanwhile, the XPS results also reveal the formation of the Cu2O-CuO hetero junction structure on the surface of the Cu2O-CuO-T catalyst, which is consistent with th   Figure 12. Generally, there are two main peaks around 933.08-933.82 eV and 952.68-952.88 eV observed over these catalysts (Figure 12a), which can be attributed to the Cu 2p 3/2 and Cu 2p 1/2 peaks, respectively [48]. Interestingly, the Cu 2p 3/2 peak was accompanied by an oscillating satellite peak from 944.78 to 945.28 eV. It was previously reported that the satellite peaks are generated by the transfer of electrons from the ligand orbital to the 3d orbital of Cu [49,50]. This indicates that the Cu 2+ exists in the divalent form with the 3d 9 structure, rather than the Cu + or Cu 0 species with the 3d 10 -filled energy level. It was reported that the CO oxidation activity of the Cu 2 O-CuO heterojunction catalyst is largely dependent on the surface of Cu + /Cu 2+ ratio [26]. The Cu 2+ /Cu 1+ relative percentages can be estimated by the peak-fitted areas of their corresponding XPS peaks. As shown in Table 4 Figure 12a indicates that in the O 1s spectra of the catalysts studied, each catalyst showed a main peak centered at 529.40 eV and a shoulder peak around 531.12 eV. The results in 3 show that the O ads /(O latt + O ads ) ratio of the Cu 2 O-CuO/α-MnO 2 -T heterojunction catalyst was higher than that of Cu 2 O/α-MnO 2 . CuO/α-MnO 2 -500 was also found to have a relatively high acromion area ratio O ads /(O latt + O ads ), possibly due to the large amount of additional oxygen species provided by the CuO-MnO 2 interface effect [34]. Among Cu 2 O-CuO-T heterojunction catalysts, Cu 2 O-CuO-260 catalyst possessed the highest proportion of shoulder peak area. According to previous report [51], the surface oxygen vacancies could enhance the redox properties of catalysts, which would be beneficial to the improvement of the performance of catalysts. The two binding energy peaks around 642.7 and 654.0 eV belong to Mn 2p 3/2 and Mn 2p 1/2 spin orbits, respectively, from the Mn 2p spectra shown in Figure 12c. It should be noted that these two peaks are the characteristic signals of Mn 4+ (IV). These phenomena indicate the occurrence of interfacial reactions and the existence of α-MnO 2 [52,53] As can be observed, ZSM-5 zeolite had no obvious reduction peak, which might be due to the inert support. Additionally, it could be interesting to find that the Fe 2 O 3 support exhibited a small reduction peak at 532 • C and a wide non-termination peak at 745 • C. According to the literature [54], the Fe 2 O 3 support experiences the reduction from Fe 2 O 3 to Fe 3 O 4 and from Fe 3 O 4 to Fe. The reduction of CeO 2 support might be due to the existence of Ce 4+ /Ce 3+ redox pairs. In addition, α-MnO 2 showed two reduction peaks at 432 and 579 • C. The reduction process of α-MnO 2 samples could be speculated to be a two-step reduction process with MnO as the final state, namely MnO 2 →Mn 3 O 4 →MnO [34,44,55].
As for the H 2 -TPR curves of Cu 2 O/S catalysts supported on different oxides, shown in Figure 13a, they reveal two or three reduction peaks in the range 30-800 • C. This indicates that the reduction of Cu 2 O species at different temperatures was related to the different metal-support interaction. Specifically, the Cu 2 O/ZSM-5 catalyst showed a large reduction peak and a shoulder peak near 296 and 420 • C, respectively, which could correspond to the interaction strength of the Cu 2 O and ZSM-5 with different intensities. The Cu 2 O/Fe 2 O 3 catalyst showed three reduction peaks, which might correspond to the interaction strength of Cu 2 O and Fe 2 O 3 with different intensities, and the reduction peak of Fe 2 O 3 support. The Cu 2 O/CeO 2 catalyst showed two reduction peaks, and the peak at 258 • C might be the common reduction peak of Cu 2 O and CeO 2 species. It is worth noting that the reduction peak intensity of the Cu 2 O/α-MnO 2 catalyst was the largest among these four catalysts. Therefore, the nature of the support had an important influence on the reduction property of the catalyst and the metal-support interaction. The H2 reduction profiles of the Cu2O/α-MnO2, Cu2O-CuO/α-MnO2-T, and CuO/α-MnO2-500 catalysts are shown in Figure 13b. These catalysts had three reduction peaks: 156-290, 337-385, and 442-487 °C. Generally, their H2-TPR curves are analogous in the shape, with a large reduction peak at 442-487 °C (γ-type), a big shoulder peak at 337-385 °C (β-type), and a small shoulder peak at 156-290 °C (α-type). Additionally, it is noteworthy that the reduction peak area of the CuO/α-MnO2-500 catalyst was somewhat larger than that of the Cu2O/α-MnO2 catalyst. The main reason for this was that the reduction process of the CuO consisted of two reduction stages, which contained the reduction processes from Cu 2+ to Cu + and then from Cu + to Cu 0 . Therefore, compared with the reduction of Cu2O/α-MnO2, the reduction of the Cu2O-CuO/α-MnO2 heterojunction and CuO/α-MnO2 catalysts required more H2 reductant, resulting in the larger H2 consumption. Furthermore, it is interesting to find that the initial reduction temperature of Cu2O-CuO/α-MnO2-T heterojunction catalysts gradually shifted to the lower temperature with the increase in the calcination temperature. This suggests that the presence of the Cu2O-CuO heterojunction had an important influence on the reduction property of the catalysts.

Conclusions
In summary, a series of Cu2O/S (S = α-MnO2, CeO2, ZSM-5, Fe2O3) catalysts were prepared by the liquid-phase reduction deposition-precipitation strategy and used for CO oxidation. It was found that the Cu2O/α-MnO2 catalyst performed the optimum low-temperature activity of CO oxidation. Furthermore, the influence of the Cu2O-CuO heterojunction structure on the catalytic activity of CO oxidation was also carefully investigated. It was found that Cu2O-CuO/α-MnO2-260 with the Cu2O/total Cu proportion of 9.4% exhibited the highest catalytic activity. The presence of the Cu2O-CuO heterojunction greatly increased the content of the surface oxygen vacancy. This further enhanced the activation ability of oxygen, and finally improved the low temperature CO oxidation property. Kinetic study showed that the Cu + /Cu 2+ proportion of the Cu2O-CuO heterojunction and redox property of the Cu2O-CuO/α-MnO2-T catalyst significantly reduced the apparent activation energy of CO oxidation. As a result, the catalytic activity of CO oxidation at low temperature was greatly improved. In addition, there are few reports on Cu2O-CuO heterojunction catalysts supported on MnO2 for CO oxidation.  Figure 13b. These catalysts had three reduction peaks: 156-290, 337-385, and 442-487 • C. Generally, their H 2 -TPR curves are analogous in the shape, with a large reduction peak at 442-487 • C (γ-type), a big shoulder peak at 337-385 • C (β-type), and a small shoulder peak at 156-290 • C (α-type). Additionally, it is noteworthy that the reduction peak area of the CuO/α-MnO 2 -500 catalyst was somewhat larger than that of the Cu 2 O/α-MnO 2 catalyst. The main reason for this was that the reduction process of the CuO consisted of two reduction stages, which contained the reduction processes from Cu 2+ to Cu + and then from Cu + to Cu 0 . Therefore, compared with the reduction of Cu 2 O/α-MnO 2 , the reduction of the Cu 2 O-CuO/α-MnO 2 heterojunction and CuO/α-MnO 2 catalysts required more H 2 reductant, resulting in the larger H 2 consumption. Furthermore, it is interesting to find that the initial reduction temperature of Cu 2 O-CuO/α-MnO 2 -T heterojunction catalysts gradually shifted to the lower temperature with the increase in the calcination temperature. This suggests that the presence of the Cu 2 O-CuO heterojunction had an important influence on the reduction property of the catalysts.

Conclusions
In summary, a series of Cu 2 O/S (S = α-MnO 2 , CeO 2 , ZSM-5, Fe 2 O 3 ) catalysts were prepared by the liquid-phase reduction deposition-precipitation strategy and used for CO oxidation. It was found that the Cu 2 O/α-MnO 2 catalyst performed the optimum low-temperature activity of CO oxidation. Furthermore, the influence of the Cu 2 O-CuO heterojunction structure on the catalytic activity of CO oxidation was also carefully investigated. It was found that Cu 2 O-CuO/α-MnO 2 -260 with the Cu 2 O/total Cu proportion of 9.4% exhibited the highest catalytic activity. The presence of the Cu 2 O-CuO heterojunction greatly increased the content of the surface oxygen vacancy. This further enhanced the activation ability of oxygen, and finally improved the low temperature CO oxidation property. Kinetic study showed that the Cu + /Cu 2+ proportion of the Cu 2 O-CuO heterojunction and redox property of the Cu 2 O-CuO/α-MnO 2 -T catalyst significantly reduced the apparent activation energy of CO oxidation. As a result, the catalytic activity of CO oxidation at low temperature was greatly improved. In addition, there are few reports on Cu 2 O-CuO heterojunction catalysts supported on MnO 2 for CO oxidation. Although researchers have prepared CuOx-type catalysts with MnO 2 as the support by some methods, its catalytic CO oxidation performance is not ideal. In conclusion, the Cu 2 O-CuO/α-MnO 2 -T hetero-junction catalysts with adjustable Cu + /Cu 2+ ratios are expected to be promising catalyst candidates for CO oxidation in future applications.