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Article

CO Oxidation over Metal Oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2) Doped CuO-Based Catalysts Supported on Mesoporous Ce0.8Zr0.2O2 with Intensified Low-Temperature Activity

by
Yan Cui
1,†,
Leilei Xu
1,*,†,
Mindong Chen
1,*,
Chufei Lv
1,
Xinbo Lian
1,
Cai-e Wu
2,
Bo Yang
1,
Zhichao Miao
3,
Fagen Wang
4 and
Xun Hu
5,*
1
Collaborative Innovation Center of the Atmospheric Environment and Equipment Technology, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing 210044, China
2
College of Light Industry and Food Engineering, Nanjing Forestry University, Nanjing 210037, China
3
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo 255049, China
4
School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China
5
School of Material Science and Engineering, University of Jinan, Jinan 250022, China
*
Authors to whom correspondence should be addressed.
†
These authors equally contributed to this work.
Catalysts 2019, 9(9), 724; https://doi.org/10.3390/catal9090724
Submission received: 7 August 2019 / Revised: 22 August 2019 / Accepted: 26 August 2019 / Published: 28 August 2019
(This article belongs to the Section Environmental Catalysis)
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Abstract

:
CuO-based catalysts are usually used for CO oxidation owing to their low cost and excellent catalytic activities. In this study, a series of metal oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2)-doped CuO-based catalysts with mesoporous Ce0.8Zr0.2O2 support were simply prepared by the incipient impregnation method and used directly as catalysts for CO catalytic oxidation. These mesoporous catalysts were systematically characterized by X-ray powder diffraction (XRD), N2 physisorption, transmission electron microscopy (TEM), energy-dispersed spectroscopy (EDS) mapping, X-ray photoelectron spectroscopy (XPS), and H2 temperature programmed reduction (H2-TPR). It was found that the CuO and the dopants were highly dispersed among the mesoporous framework via the incipient impregnation method, and the strong metal framework interaction had been formed. The effects of the types of the dopants and the loading amounts of the dopants on the low-temperature catalytic performances were carefully studied. It was concluded that doped transition metal oxides could regulate the oxygen mobility and reduction ability of catalysts, further improving the catalytic activity. It was also found that the high dispersion of rare earth metal oxides (PrO2, Sm2O3) was able to prevent the thermal sintering and aggregation of CuO-based catalysts during the process of calcination. In addition, their presence also evidently improved the reducibility and significantly reduced the particle size of the CuO active sites for CO oxidation. The results demonstrated that the 15CuO-3Fe2O3/M-Ce80Zr20 catalyst with 3 wt. % of Fe2O3 showed the best low-temperature catalytic activity toward CO oxidation. Overall, the present Fe2O3-doped CuO-based catalysts with mesoporous nanocrystalline Ce0.8Zr0.2O2 solid solution as support were considered a promising series of catalysts for low-temperature CO oxidation.

Graphical Abstract

1. Introduction

CO emitted from industrial processes and automobile exhaust has been one of the most serious environmental issues for a long time, and is greatly harmful to human health and the living environment [1,2]. To control and reduce the emission of carbon monoxide, its catalytic oxidation has been widely considered to be the most efficient solution from the standpoint of environmental considerations. To date, the research related to catalyst systems for the CO oxidation process have mainly been focused on the noble metal- and transition metal oxide-based supported catalysts. Precious metal-based catalysts (such as Pt, Au, etc.) have attracted extensive research interest due to their outstanding low-temperature catalytic activity [3,4,5]. For example, the Au-based catalyst in the CO oxidation process is able to completely convert CO into CO2 at −70 °C [6]. As a comparison, traditional metal oxide-based catalysts usually require relatively higher temperatures to obtain superior activity. Although noble metal (such as Pt, Au)-based catalysts usually show high activity for the CO catalytic oxidation reaction at low reaction temperatures, their wide application has been restricted by their high cost and limited availability. Therefore, supported catalysts based on transitional metal oxides with high catalytic activity (such as CuO, Co3O4, etc.) have attracted increasing attention due to their excellent catalytic activity and economical properties [7,8,9].
Co3O4-based catalysts have been widely studied for their exceptional low-temperature catalytic activities, which are comparable to those of noble metal-based catalysts [9,10]. However, the Co3O4-based catalysts usually suffer quick deactivation under high humidity. As a comparison, CuO-based catalysts have a strong tolerance to the moisture, although its low-temperature activity is relatively poor [11]. Therefore, the development and fabrication of CuO-based catalysts with excellent low-temperature activity toward CO oxidation is commonly considered a research hotspot and an ongoing challenge in this field.
It is widely believed that the activity of CuO-based catalysts is greatly affected by the types and the preparation strategies of the catalysts. Among the CuO-based catalysts, CuO-CeO2-based metal oxide catalysts have become the most promising candidates for CO oxidation process due to their outstanding redox performance and low cost [12], and because their catalytic performance is excellent, being comparable even to the noble metal-based catalysts [13]. The reason for their high catalytic activity might be that the Cu species is highly active, and exhibits exceptional co-chemical adsorption by Cu2+-Cu+ couple with CeO2 support. Meanwhile, CeO2 support has the capacity for storage and release of oxygen due to the presence of oxygen vacancy, which could promote the activation of the oxygen. Specifically, it has oxidation-coupled Ce4+/Ce3+, which could activate oxygen molecules during CO oxidation [14]. However, pure CeO2 usually exhibits poor thermal stability. Therefore, the incorporation of transition or rare earth metal oxides (such as ZrO2, PrOx, etc.) into the lattice of the CeO2 structure can significantly improve the thermal stability of CeO2 during high-temperature calcination [15,16]. For instance, although the incorporated ZrO2 does not participate in the catalytic reaction, it can modify the structure and the site of CeO2 lattice by forming Ce-Zr solid solution. This greatly improves the oxygen storage capacity (OSC), thermal stability, and the dispersion of mixed oxides [17,18]. Additionally, previous research has suggested that the synergistic effect between CuO and Ce-Zr solid solutions could enhance the catalytic oxidation activity toward CO [19].
Compared with traditional metal oxide catalytic supports, mesoporous metal oxides usually provide higher specific surface areas, bigger pore volumes, and more uniform pore channels [20]. Therefore, they can be used as efficient catalytic supports or catalysts. If the mesoporous Ce-Zr solid solution can be used as the support of CuO-based catalyst, a high dispersion of CuO over the catalysts can be obtained due to the advantageous structural properties of the supports [21]. Therefore, it has great potential to further improve low-temperature catalytic performance toward CO oxidation by providing sufficient accessible active sites for gaseous reactants.
In recent years, great efforts have been devoted to improving the low-temperature catalytic activity of CuO-based catalysts, especially by doping various catalytic promoters, including transition metal oxides (such as MnO2, Fe2O3, etc.) [22,23,24] and rare earth metal oxides (such as Sm2O3, PrO2, La2O3, etc.) [25,26,27,28]. Among them, the incorporation of transition metal oxides into CuO-based catalysts can adjust the oxygen mobility and reducing ability of catalyst, which could further improve the catalytic activity. It has also been found that the presence of rare earth metal oxides could achieve high dispersion of the CuO active sites. As a result, the agglomeration of CuO-based catalysts could effectively be prevented. Furthermore, the reductivity and particle size of the CuO could be promoted, which is beneficial to the formation of the active sites for CO oxidation [29].
Herein, Ce0.8Zr0.2O2 solid solution with a mesoporous nanocrystalline structure was successfully synthesized by evaporation-induced self-assembly (EISA) [30]. A series of CuO-based catalysts doped with different transition metal oxides and rare earth metal oxides was prepared with mesoporous Ce0.8Zr0.2O2 solid solution support. Various characterizations, such as XRD, N2 physisorption, TEM, EDS-mapping, XPS, and H2-TPR, were used to systematically characterize both the supports and catalysts. The effects of the types of different metal oxides dopants and their loading amounts on low-temperature catalytic activities toward CO oxidation were thoroughly studied.

2. Results and Discussion

2.1. Characterizations of the Support and Catalysts

2.1.1. XRD Analysis

To investigate the crystalline phase states of the support and catalysts, the XRD analyses were carried out. Figure 1(1) displayed the XRD patterns of the M-Ce80Zr20 support and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts, respectively. It is worth noting that both the support and catalysts exhibited four pronounced diffraction peaks around 2θ = 28.6°, 33.04°, 47.4° and 56.36°, suggesting the presence of the cubic crystalline structure similar to the cubic CeO2 (PDF#34-0394). Furthermore, the absence of ZrO2 diffraction peaks indicates that the ZrO2 had been successfully incorporated into the CeO2 lattice to form a solid solution while maintaining the fluorite crystalline structure [31]. It was found that the intensities of the CuO diffraction peaks (PDF#45-0937) over 15CuO-3MOx/M-Ce80Zr20 catalysts were highly similar with each other due to the identical CuO loading (15 wt. %). However, pristine 15CuO/M-Ce80Zr20 catalyst without any modification displayed a slightly stronger CuO diffraction peak intensity than that of the modified catalysts, which could be attributed to the poor dispersion of CuO species over M-Ce80Zr20 support due to the thermal agglomeration. This also implied that the incorporation of the MOx metallic oxide was helpful for improving the dispersion of CuO species over the surface of the catalysts.
The XRD patterns of 15CuO-zFe2O3/M-Ce80Zr20 catalysts with different CuO loading amounts are exhibited in Figure 1(2). It can be observed that the CuO diffraction peaks are very similar in intensity when the Fe2O3 loading amount is less than 5 wt. %. However, the intensity of the CuO diffraction peak undergoes an intense increase when the Fe2O3 content is further increased from 5 wt. % to 7 wt. %. This indicates that the dispersion of CuO species over the catalyst surface gradually becomes worse with increasing amounts of Fe2O3 loading. As a result, the growth of the crystalline size of the CuO nanoparticles would take place, which might be attributable to the limited surface area of the M-Ce80Zr20 support.

2.1.2. N2 Physisorption Analysis

N2 physisorption analyses were conducted to investigate the textural properties of the M-Ce80Zr20 support and the catalysts. The N2 adsorption–desorption isotherms (panel A) and pore size distributions (panel B) of the M-Ce80Zr20 support, as well as the investigated catalysts in this study, are shown in Figure 2. As displayed in Figure 2A(1 and 2), almost all of the 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) and 15CuO-zFe2O3/M-Ce80Zr20 catalysts exhibited type IV isotherms with H2-shaped hysteresis loops, which were highly similar to the isotherm of M-Ce80Zr20 support and 15CuO/M-Ce80Zr20 pristine catalyst. This indicates that the mesoporous structure of the catalysts had been successfully retained after doping the metallic oxide. The corresponding pore size distribution curves of these mesoporous catalysts are shown in Figure 2B(1 and 2). It can be observed that the pore size distribution curves of these catalysts display narrow pore size distribution curves centered in the range of 9.6–17.5 nm. Furthermore, it is worth noting that the catalysts display slightly larger pore diameters than those of M-Ce80Zr20, suggesting that the thermal shrinkage of the mesoporous frameworks or channels did not take place, and that the M-Ce80Zr20 support demonstrates excellent thermal stability. Furthermore, the size of the pore diameter further confirms that the pore diameters of these catalysts are located in the mesopore range (2.0–50.0 nm) [32].
The specific structural properties of the M-Ce80Zr20 support and catalysts are summarized in Table 1. It can be found that the introduction of metal oxides generally led to the decrease of specific surface areas and the increase of mesopore diameters. For the specific surface areas and pore volumes, the 15CuO-3MOx/M-Ce80Zr20 and 15CuO-zFe2O3/M-Ce80Zr20 catalysts doped with metal oxides displayed relatively smaller values due to the slight blockage of the mesopore. Additionally, the 15CuO/M-Ce80Zr20, 15CuO-3MOx/Ce80Zr20, and 15CuO-zFe2O3/M-Ce80Zr80 catalysts exhibited slightly bigger average pore diameters than the M-Ce80Zr20 support. Specifically, the pore diameter of the catalysts doped with La2O3, PrO2, MnO2, and Sm2O3 were about 12.7 nm, and the pore diameters of the samples doped with Fe2O3 with various loading amounts were about 17.5 nm, which is slightly higher than those of the M-Ce80Zr20 support (7.9 nm) and the 15CuO/M-Ce80Zr20 catalyst (9.6 nm).

2.1.3. TEM, SAED, STEM, and EDS-Mapping Analyses

Figure 3 shows the TEM and SAED images of the M-Ce80Zr20, 15CuO/M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts. For the M-Ce80Zr20 support, the mesoporous structure with a worm-like porous network formed by the stacking of Ce-Zr solid solution nanoparticles around the P123 template can be clearly observed in Figure 3a. Based on its HRTEM image in Figure 3b, it can be observed that the lattice fringe spacing of the M-Ce80Zr20 support is in the range of 0.299–0.309 nm, which may correspond to the (1 1 1) crystal plane of mesoporous Ce0.8Zr0.2O2 solid solution. From SAED (inset of Figure 3a), the clear diffraction rings could be observed, indicating high crystallinity of mesoporous walls. This is well consistent with the conclusion based on the above XRD analysis in Figure 1.
The TEM images of the 15CuO-3MOx/M-Ce80Zr20 catalysts in Figure 3c,e,g,i,k,m exhibit the same mesoporous structure as their corresponding M-Ce80Zr20 support with worm-like porous structure. However, no obvious large CuO cluster is observed, even when the CuO loading amount is as high as 15 wt. %, indicating that CuO nanoparticles are well dispersed on the surface of mesoporous M-Ce80Zr20 support. However, compared with the mesoporous Ce-Zr solid solution support, the HRTEM images of the catalysts exposed to the new crystal faces with different lattice stripe spacings about 0.230 nm and 0.250 nm, which could be attributed to the (1 1 1) and (0 0 2) crystal faces of CuO, respectively. Additionally, clear SAED diffraction rings can be observed in Figure 3c,e,g,i,k,m, indicating that these CuO-based mesoporous catalysts possess good crystalline properties.
Scanning transmission electron microscope (STEM) and EDS-mapping were carried out to investigate the spatial distribution of the elements among the mesoporous structure of the catalysts. The STEM and EDS-mapping images of 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts are shown in Figure 4. It can be seen that Ce, Cu, Zr, and corresponding doped metal elements were uniformly distributed among the catalysts, suggesting that the high dispersion of the Cu and metal dopants over the surface of M-Ce80Zr20 had been achieved by the incipient impregnation method.
The TEM and SAED analyses of the mesoporous catalysts with different Fe2O3 loading amounts were carried out and their images are displayed in Figure 5. It can also be seen from Figure 5 that the (1 1 1) crystal plane of mesoporous Ce-Zr solid solution, as well as the (1 1 1) and (0 0 2) crystal planes of CuO, can be clearly observed on catalysts with various Fe2O3 contents. However, the crystal plane of the iron oxides (FeOx) was absent, regardless of its loading amount. This indicates that the Fe species were highly dispersed over the surface of the catalysts, which had been proved by the EDS-mapping analysis. Additionally, the SAED diagram in the insets of Figure 5 showed clear diffraction rings, indicating that their mesoporous framework structure was in the form of a good crystalline structure.

2.1.4. H2-TPR Analysis

The H2-TPR profiles of M-Ce80Zr20, 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2), and 15CuO-zFe2O3/M-Ce80Zr20 catalysts are displayed in Figure 6. As can be seen from Figure 6(1), almost all of the 15CuO/M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 catalysts showed similar hydrogen consumption profiles in the intensities and shapes. Specifically, there were two groups of peaks in the range of 197–323 °C and 354–472 °C, which could be attributed to the hydrogen consumption caused by the reduction of CuO and MOx with various interactions with the M-Ce80Zr20 support. It is also worth noting that the reduction peak intensity in the range of 354–472 °C was larger than the shoulder peak in the range of 197–323 °C. The reduction peak in the range of 354–472 °C should be the reduction peak of the CuO species, because its reduction peak is usually centered around 380 °C, according to the pioneer report [33]. In addition, it can be found from the figure that the M-Ce80Zr20 support showed a broad peak at a higher temperature (620 °C), which can be attributed to the reduction of the surface Ce4+. It has been reported that the reduction temperature of CuO species should be lower than that of the free CuO when supported on fluorite-type oxides [34]. Therefore, the reduction peak in the range of 197–323 °C could be ascribed to the reduction of the CuO species, which had a closely synergistic effect on the M-Ce80Zr20 supports. As a result, the CuO species could be reduced at relatively lower temperature. The M-Ce80Zr20 carriers greatly promoted the reduction of CuO to a certain degree. Additionally, the lower peak temperature of the 15CuO/M-Ce80Zr20 catalysts might be also derived from the reduction of finely dispersed CuO species weakly interacting with the supports, which has commonly been considered to be the active site for CO oxidation with high efficiency [35,36]. However, the ascription of the wide reduction peaks with strong intensity in the high temperature region (354–472 °C) becomes a bit complicated. On the one hand, they might be partially attributed to the reduction of CuO species strongly interacting with the supports. On the other hand, they might also be attributed to the reduction of surface Ce4+ in the mesoporous Ce-Zr solid solution supports. Compared with the 15CuO/M-Ce80Zr20 reference catalyst, the reduction peaks of 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts doped with different metal oxides shifted to the higher reduction temperature region. This indicates that the doping of these metal oxides was helpful for enhancing the metal–support interaction. Therefore, the severe thermal agglomeration the CuO active sites had been effectively avoided during the process of the calcination, as reflected in the above XRD analyses of the 15CuO-MOx/M-Ce80Zr20.
As for the 15CuO-zFe2O3/M-Ce80Zr20 catalysts with different loading amounts of Fe2O3, their H2-TPR profiles were displayed in Figure 6(2). It is noticeable that the 15CuO-zFe2O3/M-Ce80Zr20 catalysts display much higher reduction temperatures than the 15CuO/M-Ce80Zr20 reference catalyst. However, with the increase of the Fe2O3 loading amount from 1 wt. % to 7 wt. %, both the intensities and positions of the reduction peak suffer no obvious changes. Therefore, the introduction of the Fe2O3 dopant could influence the reductivity of the CuO active sites.

2.1.5. XPS Analysis

The surface compositions and chemical states of the elements of the catalysts were analyzed by X-ray photoelectron spectroscopy (XPS) and detailed information related to the surface elements was obtained. The XPS profiles of Cu 2p, Ce 3d, Zr 3d, and O 1s of the 15CuO/M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts are exhibited in Figure 7, and the corresponding binding energies of these surface elements are summarized in Table 2.
It can be observed in Figure 7(1) that the XPS peaks of Cu 2p are centered at 954.0 eV and 934.0 eV, which could correspond to Cu 2p1/2 and Cu 2p3/2, respectively. Avgouropoulos and Ioannides reported that the existence of oscillating peaks in the range of 939.0–944.0 eV and high binding energy of Cu 2p3/2 in the range of 933.0 eV–934.0 eV were the two main XPS characteristics of CuO, where the state of copper species is in the Cu2+ state [37]. The presence of Cu 2p3/2 peak around about 934.0 eV, the shake-up peak in the range of 939.0–944.0 eV, and the satellite W peak around 962.5 eV indicates the presence of CuO species on the surface of 15CuO/M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts. Additionally, it is worth noting in Table 2 that the introduction of the transitional metal oxides and rare earth metal oxides did not obviously influence the binding energy as well as the coordination environment of Cu2+ species.
For the state of the cerium species, it is noticeable in Figure 7(2) that the spectra of Ce 3d can be split into eight peaks centered around 882.0 eV, 885.0 eV, 889.0 eV, 899.0 eV, 901.0 eV, 904.0 eV, 908.0 eV, and 917.0 eV binding energies, which can be categorized into the groups of V and U, according to the pioneer reports [38]. Generally, the peaks categorized into the groups V and U can be attributed to Ce 3d5/2 and Ce 3d3/2, respectively. The peaks referred to U‴ and V‴ can be attributed to the Ce cation in the state of Ce4+. The U, V, U″ and V″ groups are produced by one or two electrons moving from a filled O 2p orbit to an empty Ce 4f orbit. The presence of U′ and V′ are due to the presence of Ce3+ species [38,39,40,41]. The Ce cations in most of the catalysts were mainly present in the form of Ce4+, and at the same time, the presence of Ce3+ was also found. In the case of the present 15CuO/M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 catalysts, the U′ and V′ shoulder peaks can be clearly observed, suggesting the presence of Ce3+ species. This would facilitate the activation of oxygen and redox recycling of the catalysts, which would promote the low-temperature catalytic performance toward CO oxidation.
Figure 7(3) shows the Zr 3d XPS spectra of the investigated catalysts. It can be clearly seen in the figure that the binding energy of Zr 3d5/2 is around 184.5 eV for almost all the catalysts. This is a typical feature with the presence of Zr4+ on the surface of these mesoporous catalysts [41,42].
As for the O 1s profiles of the studied catalysts, it can be seen from Figure 7(4) that the profile of each sample exhibits a main peak centered at 530.0 eV and a shoulder peak at around 533.0 eV. It has been reported that the shoulder peak near 533.0 eV should correspond to the O2− species associated with the Ce3+, usually generating oxygen pockets or vacancies around them in order to maintain the neutrality of the charge, based on the previous literature [38,43]. Therefore, the shoulder peak of the O 1s at high binding energy was finally formed.
The XPS profiles of La 3d, Sm 3d, Pr 3d, Fe 2p and Mn 2p of the 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts are shown in Figure 8. As shown in Figure 8(1), the position of the La 3d5/2 peak is around 834.0 eV and there is a satellite peak around 838.0 eV. The △E between them is at about 4 eV, which is probably the characteristic peak of La3+ in La2(CO3)3. When catalyst 15CuO-3La2O3/M-Ce80Zr20 was exposed to the atmosphere, its surface La2O3 interacted with CO2 to form La2(CO3)3 [44]. As can be seen for the XPS profile of Sm in Figure 8(2), it can be observed that the peaks of Sm 3d3/2 and Sm 3d5/2 were centered on 1110.0 eV and 1083.0 eV, respectively. Therefore, Sm was mainly present in the form of Sm3+ in catalyst 15CuO-3Sm2O3/M-Ce80Zr20 [45]. As for the profile of the Pr 3d5/2 in Figure 8(3), its profile overlapped a bit with that of the Ce species. However, compared with 15CuO/M-Ce80Zr20 catalyst without Pr modification, it was noticeable that the Pr 3d5/2 XPS profile of catalyst 15CuO-3PrO2/M-Ce80Zr20 showed an additional peak around 930.0 eV, which was assumed to be the peak of Pr 3d5/2 [45,46]. The XPS profiles of Fe 2p of the 15CuO-3Fe2O3/M-Ce80Zr20 are shown in Figure 8(4). There are two major peaks Fe 2p3/2 and Fe 2p1/2 located at around 710.4 eV and 724.3 eV. In addition, a satellite peak was observed at about 718.3 eV. This indicates that Fe was present in the form of Fe3+ over the surface of 15CuO-3Fe2O3/M-Ce80Zr20 [47,48]. Figure 8(5) shows the Mn 2p XPS spectrum over 15CuO-3MnO2/M-Ce80Zr20 with Mn 2p3/2 peak range at 642.0 eV and of the Mn 2p1/2 range at 652.7 eV. This indicates that the Mn species in the 15CuO-3MnO2/M-Ce80Zr20 catalyst are in the form of Mn4+ [49,50,51].

2.2. Catalytic Performance toward CO Oxidation

2.2.1. The Evaluation of the Catalytic Activity during the Process of Screening Catalysts

The catalytic activities toward CO oxidation were carried out over the M-Ce80Zr20 support, 15CuO-3MOx/M-Ce80Zr20 and 15CuO-zFe2O3/M-Ce80Zr20 catalysts and their catalytic results are summarized in Figure 9 and Figure 10. The effects of both the types of the metal oxides and the loading amounts of the dopants on the catalytic activities toward CO oxidation were carefully evaluated during the process of screening high-efficiency catalysts.
It can be observed in Figure 9 that the catalytic activities of CuO-based mesoporous catalysts (15CuO/M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20) were significantly higher than that of the corresponding M-Ce80Zr20 catalytic supports, demonstrating the critical role of CuO active site in catalyzing the CO oxidation. Additionally, it is noticeable that the CO conversion gradually increases with the increase in reaction temperature until reaching 100%, which can be attributed to the dynamic control feature of CO oxidation at low temperature. In addition, the 15CuO-3MOx/M-Ce80Zr20 catalysts also display different catalytic activities from the pristine 15CuO/M-Ce80Zr20 catalyst, although they had identical CuO loading amounts. Among these catalysts, the Fe2O3-doped 15CuO-3Fe2O3/M-Ce80Zr20 catalyst exhibited much higher catalytic activity than the corresponding counterparts with other dopants (La2O3, PrO2, Sm2O3, MnO2), demonstrating the superiority of Fe2O3 as the catalytic modifier. The reason for this can be attributed to the synergistic effect between CuO and Fe2O3, which has a strong influence on the catalytic activity of CO low-temperature oxidation. Typically, Fe2O3 promotes the reduction of CuO, according to the pioneer report [52]. As a result, the activity of the Fe2O3-doped CuO-based catalysts is higher than that of the reference catalyst without modification. Due to the dual function of Fe2O3, it can carry out the CO oxidation reaction in the presence and absence of oxygen by losing its lattice oxygen [53]. Therefore, Fe2O3 was considered the most promising catalytic promoter toward the CO low-temperature catalytic oxidation process.
The effect of the Fe2O3 loading amount on the catalytic activities over 15CuO-zFe2O3/M-Ce80Zr20 catalysts is reflected in Figure 10. As can be seen from Figure 10(1), the loading amounts of the Fe2O3 greatly influenced the catalytic CO oxidation activity. To better demonstrate the effect of the Fe2O3 modification, the CO conversion versus Fe2O3 loading amount over 15CuO-zFe2O3/M-Ce80Zr20 catalysts at 60 °C is displayed in Figure 10(2), which would more obviously reflect the effect of Fe2O3 loading amount (z) on the catalytic activity. As observed in Figure 10(1) and (2), the catalytic activity of 15CuO-zFe2O3/M-Ce80Zr20 catalysts could be gradually enhanced with the increase of Fe2O3 content up to 3 wt. %, and the 15CuO-3Fe2O3/M-Ce80Zr20 catalyst exhibited the highest catalytic activity, especially in the low reaction temperature region. However, when the Fe2O3 was further increased from 3 wt. % to 7 wt. %, the catalytic activity suffered a great decrease. The reason for this phenomenon might be derived from the poor dispersion of the high loading amounts of Fe2O3. The 15CuO-3Fe2O3/M-Ce80Zr20 catalyst, which had a high surface area of 43.8 m2/g, exhibited the highest catalytic activity. The high specific surface area of the nanostructured catalyst led to more unsaturated surface coordination sites exposed to the reaction gas, which improved the activity of the catalyst. High specific surface area and narrow mesoporous size distribution could promote the high dispersion of metal or oxide catalyst nanoparticles, which had a certain beneficial effect on catalytic performance [52].

2.2.2. Long-Term Stability Test

Catalytic stability was also an important concern for CuO-based catalysts toward CO oxidation. Therefore, the long-term stability test of the 15CuO-zFe2O3/M-Ce80Zr20 catalyst was investigated under specific reaction conditions (CO/O2 = 0.05, GHSV = 12000 mL/(g·h), 70 °C, 1 atm), and the 15CuO-3Fe2O3/M-Ce80Zr20 was selected as a representative of the catalyst. It can be observed in Figure 11 that the 15CuO-3Fe2O3/M-Ce80Zr20 catalyst not only exhibited high catalytic activity, but also exhibited superior catalytic stability during the entire 50 h stability test. There was no obvious deactivation during the whole 50 h reaction time on stream, demonstrating excellent catalytic stability.

3. Experimental

3.1. Catalyst Preparation

The mesoporous Ce0.8Zr0.2O2 solid solution was synthesized by the evaporation-induced self-assembly (EISA) strategy [30]. In a specific synthesis process, 1.0 g of P123 (Mav = 5800, EO20PO70EO20, Aladdin) was dissolved in 20.0 mL anhydrous ethanol (C2H5OH, Sinopharm Chemical Reagent Co., Ltd., China) and stirred vigorously at room temperature to dissolve. 8 mmol Ce(NO3)3·6H2O (Shanghai Macklin Bio-Chem Co., Ltd., China) and 2 mmol ZrOCl2·8H2O (Shanghai Aladdin Bio-Chem Co., Ltd., China) were added to the ethanol solution with a 4:1 molar ratio and stirred at room temperature for at least 4 hours until the solution became transparent. The transparent homogeneous sol was transferred to a Petri dish, which was covered with a PE film with small holes. The petri dish was transferred to an oven with the desired temperature and humidity (temperature: 60 °C, relative humidity: ≤ 50%) for solvent evaporation and template assembly for more than 48 hours. A yellow xerogel was obtained after the EISA process, which was further calcined at 500 °C with a ramping rate of 1 °C/min. The obtained mesoporous material was designated as M-Ce80Zr20.
The Cu(NO3)2·6H2O, La(NO3)3·6H2O, Sm(NO3)3·6H2O, Pr(NO3)3·6H2O, Fe(NO3)3·9H2O, Mn(NO3)2·4H2O (Shanghai Aladdin Bio-Chem Co., Ltd., China) were employed as the precursors of metal oxides. The metal oxide-modified CuO-based with M-Ce80Zr20 support was prepared by an incipient impregnation method, in which the weight percentage of CuO and MOx (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2) were controlled at 15 wt. % and 3 wt. %, respectively, which were calculated by the following formulas: CuO wt. % = mCuO / (mCuO + mMOx + msupport) × 100%; MOx wt. % = mMOx/(mCuO + mMOx + msupport) × 100%. After impregnation, the catalyst precursor was dried in a 60 °C oven for 24 hours and then calcined at 600 °C for 5 h with a ramping rate of 1 °C/min. The obtained catalyst with 15 wt. % CuO and 3 wt. % MOx loading was designated as 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2). The catalyst with 15 wt. % CuO and z % Fe2O3 loading amount was defined as 15CuO-zFe2O3/M-Ce80Zr20.

3.2. Catalyst Characterizations

X-ray powder diffraction (XRD) patterns were collected on a XRD-6100 diffractometer (Shimadzu, Japan) using Cu Kα radiation at 40 kV and 100 mA in the range of 20°–80° (2θ) with a step length of 0.02° at a scanning rate of 3°/min.
N2 physisorption analysis were collected using an Autosorb-iQ-AG-MP instrument (Quantachrome, US) at −196 °C. Before the measurement, samples were degassed at 300 °C for at least 3 h. The specific surface areas of the catalysts were calculated by the multipoint Brunauer–Emmet–Teller (BET) method. The pore size distributions and pore volumes were calculated from the adsorption branches of the isotherms by using the Barett–Joyner–Halenda (BJH) method.
Transmission electron microscopy (TEM) images, scanning transmission electron microscopy (STEM) images, selected area electron diffraction (SAED), and energy-dispersed spectroscopy (EDS) mapping measurements were carried out on a high-resolution transmission electron microscopy (FEI TECNAI G2 F20) with accelerating voltage of 200 kV. For the sample preparation, they were firstly dispersed in absolute ethanol together with 30 min ultrasonic and then deposited on a carbon-coated copper grid.
X-ray photoelectron spectroscopy (XPS) measurements were performed on an Escalab 250Xi (Thermo Fisher Scientific, US) spectrometer. The powder of the sample was spread and coated on the conductive tape on the sample holder. The binding energies were calibrated using the C 1s line at 284.5 eV as the reference.
H2 temperature programmed reduction (H2-TPR) experiments were performed on homemade fixed bed reactor and the consumption of H2 was recorded by a LC-D200 mass spectrometer (TILON, US) by using 200 mg catalyst under the mixture of 5 vol % H2-95 vol % Ar stream (50 mL/min). Prior to the H2-TPR measurements, the samples were purged with the Ar stream at 300 °C for 30 min and cooled down to room temperature. Then, 5 vol % H2-95 vol % Ar mixture was introduced, and the programmed temperature was conducted with a ramping heating rate of 20 °C/min to 800 °C. The hydrogen consumption was measured using a LC-D200 mass spectrometer.

3.3. Catalytic Activity Evaluation

Catalytic activity tests for CO oxidation were carried out in a vertical fixed-bed continuous flow quartz reactor. The entire reaction and analysis system consists of three parts: (1) the mass flow controlling unit (Brooks Instruments), (2) the reactor unit (homemade furnace), and (3) the analysis unit (PerkinElmer Gas Chromatography Clarus 680). Typically, 100 mg of catalyst was placed over the quartz wool in the quartz tube of the fixed-bed reactor. The feed gas consisting of 1 vol % CO, 20 vol % O2 and balanced N2 with a total flow rate of 20 mL/min, corresponding to the gas hourly space velocity (GHSV) of 12000 mL/(g·h) gas, was introduced into the reactor and investigated with designated temperature range. Finally, the effluent gas was analyzed online using the GC-680 Perkin Elmer gas chromatography equipped with a thermal conductivity detector (TCD). The catalytic activity was reflected and expressed in the form of CO conversion. Therefore, the CO conversion (abbreviated as CCO) can be calculated based on the carbon balance according to the following formula, where the FCO, inlet and FCO, outlet represented the flow rate of CO species into and out of the reactor.
CCO = (FCO, inlet − FCO, outlet)/ FCO, inlet × 100%

4. Conclusions

A series of metal oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2)-doped CuO-based catalysts supported on mesoporous Ce0.8Zr0.2O2 were prepared by incipient impregnation method and used directly as catalysts for CO catalytic oxidation. This study demonstrated that doped transition metal oxides could regulate the oxygen mobility and reduction ability of catalysts and the high dispersion of rare earth metal oxides (PrO2, Sm2O3) could prevent the thermal sintering and aggregation of CuO-based catalysts during the process of calcination. It was found that the CuO-based catalysts with 3 wt. % Fe2O3 loading amount exhibited the highest catalytic activity. Compared with other catalysts, the 15CuO-3Fe2O3/M-Ce80Zr20 catalyst exhibited much higher catalytic activity at low temperature. This was due to the synergistic effect between CuO and Fe2O3 and the dual function of Fe2O3 by losing its lattice oxygen. Moreover, these mesoporous catalysts had no deactivation phenomenon after 50 h stability test, demonstrating outstanding catalytic stability. In conclusion, the present Fe2O3 doped CuO-based catalysts supported on mesoporous Ce0.8Zr0.2O2 solid solution can be used as a series of promising catalysts with intensified low-temperature activity toward CO oxidation.

Author Contributions

Conceptualization, Y.C. and L.X.; methodology, Y.C., L.X., M.C. and X.H.; software, Y.C., C.L. and X.L.; validation, C.W., B.Y., Z.M. and F.W.; formal analysis, Y.C. and L.X.; investigation, M.C.; resources, M.C.; data curation, C.W., B.Y., Z.M. and F.W.; writing—original draft preparation, Y.C.; writing—review and editing, Y.C. and L.X.; visualization, L.X., M.C. and X.H.; supervision, L.X., M.C., Z.M. and X.H.; project administration, M.C.; funding acquisition, M.C.

Funding

The authors sincerely acknowledge the financial support from National Natural Science Foundation of China (Grant No. 21503113, 21577065, 21503142, and 91543115), National Key Research and Development Program of China (2018YFC0213802), International ST Cooperation Program of China (2014DFA90780), Environmental Protection Projects of Jiangsu province (2017022) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. This study is also supported by Startup Foundation of Nanjing University of Information Science & Technology.

Conflicts of Interest

There are no conflict to declare.

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Figure 1. (1) XRD patterns of M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2): (a) M-Ce80Zr20, (b) 15CuO/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-3Sm2O3/M-Ce80Zr20, (e) 15CuO-3PrO2/M-Ce80Zr20, (f) 15CuO-3La2O3/M-Ce80Zr20, (g) 15CuO-3MnO2/M-Ce80Zr20. (2) XRD patterns of 15CuO-zFe2O3/M-Ce80Zr20: (a) 15CuO/M-Ce80Zr20, (b) 15CuO-1Fe2O3/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-5Fe2O3/M-Ce80Zr20, (e) 15CuO-7Fe2O3/M-Ce80Zr20.
Figure 1. (1) XRD patterns of M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2): (a) M-Ce80Zr20, (b) 15CuO/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-3Sm2O3/M-Ce80Zr20, (e) 15CuO-3PrO2/M-Ce80Zr20, (f) 15CuO-3La2O3/M-Ce80Zr20, (g) 15CuO-3MnO2/M-Ce80Zr20. (2) XRD patterns of 15CuO-zFe2O3/M-Ce80Zr20: (a) 15CuO/M-Ce80Zr20, (b) 15CuO-1Fe2O3/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-5Fe2O3/M-Ce80Zr20, (e) 15CuO-7Fe2O3/M-Ce80Zr20.
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Figure 2. N2 adsorption-desorption isotherms (A) and pore size distributions (B) of the catalytic support and the catalysts: (1) M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2), (2) 15CuO-zFe2O3/M-Ce80Zr20.
Figure 2. N2 adsorption-desorption isotherms (A) and pore size distributions (B) of the catalytic support and the catalysts: (1) M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2), (2) 15CuO-zFe2O3/M-Ce80Zr20.
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Figure 3. TEM and SAED inset of the M-Ce80Zr20 support and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts: (a,b) M-Ce80Zr20, (c,d) 15CuO/M-Ce80Zr20, (e,f) 15CuO-3Fe2O3/M-Ce80Zr20, (g,h) 15CuO-3MnO2/M-Ce80Zr20, (i,j) 15CuO-3La2O3/M-Ce80Zr20, (k,l) 15CuO-3Sm2O3/M-Ce80Zr20, (m,n) 15CuO3-3PrO2/M-Ce80Zr20.
Figure 3. TEM and SAED inset of the M-Ce80Zr20 support and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts: (a,b) M-Ce80Zr20, (c,d) 15CuO/M-Ce80Zr20, (e,f) 15CuO-3Fe2O3/M-Ce80Zr20, (g,h) 15CuO-3MnO2/M-Ce80Zr20, (i,j) 15CuO-3La2O3/M-Ce80Zr20, (k,l) 15CuO-3Sm2O3/M-Ce80Zr20, (m,n) 15CuO3-3PrO2/M-Ce80Zr20.
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Figure 4. STEM and EDS element mapping images showing the spatial distribution of Ce, Zr, Cu and M (M = Fe, La, Sm, Mn, Pr) elements: (a) 15CuO-3Fe2O3/M-Ce80Zr20, (b) 15CuO-3La2O3/M-Ce80Zr20, (c) 15CuO-3Sm2O3/M-Ce80Zr20, (d) 15CuO-3MnO2/M-Ce80Zr20, (e) 15CuO-3PrO2/M-Ce80Zr20, (f) M-Ce80Zr20.
Figure 4. STEM and EDS element mapping images showing the spatial distribution of Ce, Zr, Cu and M (M = Fe, La, Sm, Mn, Pr) elements: (a) 15CuO-3Fe2O3/M-Ce80Zr20, (b) 15CuO-3La2O3/M-Ce80Zr20, (c) 15CuO-3Sm2O3/M-Ce80Zr20, (d) 15CuO-3MnO2/M-Ce80Zr20, (e) 15CuO-3PrO2/M-Ce80Zr20, (f) M-Ce80Zr20.
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Figure 5. TEM and SAED images of the 15CuO-zFe2O3/M-Ce80Zr20: (a,b) 15CuO-1Fe2O3/M-Ce80Zr20, (c,d) 15CuO-3Fe2O3/M-Ce80Zr20, (e,f) 15CuO-5Fe2O3/M-Ce80Zr20, (g,h)15CuO-7Fe2O3/M-Ce80Zr20.
Figure 5. TEM and SAED images of the 15CuO-zFe2O3/M-Ce80Zr20: (a,b) 15CuO-1Fe2O3/M-Ce80Zr20, (c,d) 15CuO-3Fe2O3/M-Ce80Zr20, (e,f) 15CuO-5Fe2O3/M-Ce80Zr20, (g,h)15CuO-7Fe2O3/M-Ce80Zr20.
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Figure 6. (1) H2-TPR profiles of M-Ce80Zr20 support and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts: (a) M-Ce80Zr20, (b) 15CuO/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-3La2O3/M-Ce80Zr20, (e) 15CuO-3MnO2/M-Ce80Zr20, (f) 15CuO-3Sm2O3/M-Ce80Zr20, (g) 15CuO-3PrO2/M-Ce80Zr20. (2) H2-TPR profiles of 15CuO-zFe2O3/M-Ce80Zr20 catalysts: (a) 15CuO/M-Ce80Zr20, (b) 15CuO-1Fe2O3/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-5Fe2O3/M-Ce80Zr20, (e) 15CuO-7Fe2O3/M-Ce80Zr20.
Figure 6. (1) H2-TPR profiles of M-Ce80Zr20 support and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts: (a) M-Ce80Zr20, (b) 15CuO/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-3La2O3/M-Ce80Zr20, (e) 15CuO-3MnO2/M-Ce80Zr20, (f) 15CuO-3Sm2O3/M-Ce80Zr20, (g) 15CuO-3PrO2/M-Ce80Zr20. (2) H2-TPR profiles of 15CuO-zFe2O3/M-Ce80Zr20 catalysts: (a) 15CuO/M-Ce80Zr20, (b) 15CuO-1Fe2O3/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-5Fe2O3/M-Ce80Zr20, (e) 15CuO-7Fe2O3/M-Ce80Zr20.
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Figure 7. XPS spectra of Cu 2p (1), Ce 3d (2), Zr 3d (3), and O 1s (4) for 15CuO/M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts: (a) 15CuO/M-Ce80Zr20, (b) 15CuO-3PrO2/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-3MnO2/M-Ce80Zr20, (e) 15CuO-3La2O3/M-Ce80Zr20, (f) 15CuO-3Sm2O3/M-Ce80Zr20.
Figure 7. XPS spectra of Cu 2p (1), Ce 3d (2), Zr 3d (3), and O 1s (4) for 15CuO/M-Ce80Zr20 and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts: (a) 15CuO/M-Ce80Zr20, (b) 15CuO-3PrO2/M-Ce80Zr20, (c) 15CuO-3Fe2O3/M-Ce80Zr20, (d) 15CuO-3MnO2/M-Ce80Zr20, (e) 15CuO-3La2O3/M-Ce80Zr20, (f) 15CuO-3Sm2O3/M-Ce80Zr20.
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Figure 8. XPS profiles of La 3d (1), Sm 3d (2), Pr 3d (3), Fe 2p (4) and Mn 2p (5) over the 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts.
Figure 8. XPS profiles of La 3d (1), Sm 3d (2), Pr 3d (3), Fe 2p (4) and Mn 2p (5) over the 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts.
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Figure 9. The CO conversion versus reaction temperature over M-Ce80Zr20 support and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts; reaction conditions: 1%CO-20%O2-N2 balance, GHSV = 12000 mL/(g·h), 1 atm.
Figure 9. The CO conversion versus reaction temperature over M-Ce80Zr20 support and 15CuO-3MOx/M-Ce80Zr20 (MOx = La2O3, Fe2O3, PrO2, Sm2O3, MnO2) catalysts; reaction conditions: 1%CO-20%O2-N2 balance, GHSV = 12000 mL/(g·h), 1 atm.
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Figure 10. The CO conversion versus reaction temperature over (1) 15CuO-zFe2O3/M-Ce80Zr20 catalysts with different Fe2O3 loading amounts, (2) the CO conversion versus the loading amounts of Fe2O3 (z%)) over 15CuO-zFe2O3/M-Ce80Zr20 catalysts; reaction conditions: 1%CO-20%O2-N2 balance, GHSV = 12000 mL/(g·h), 1 atm.
Figure 10. The CO conversion versus reaction temperature over (1) 15CuO-zFe2O3/M-Ce80Zr20 catalysts with different Fe2O3 loading amounts, (2) the CO conversion versus the loading amounts of Fe2O3 (z%)) over 15CuO-zFe2O3/M-Ce80Zr20 catalysts; reaction conditions: 1%CO-20%O2-N2 balance, GHSV = 12000 mL/(g·h), 1 atm.
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Figure 11. Long-term stability test over the 15CuO-3Fe2O3/M-Ce80Zr20 catalyst; reaction conditions: 1%CO-20%O2-N2 balance, GHSV = 12000 mL/(g·h), 70 °C, 1 atm.
Figure 11. Long-term stability test over the 15CuO-3Fe2O3/M-Ce80Zr20 catalyst; reaction conditions: 1%CO-20%O2-N2 balance, GHSV = 12000 mL/(g·h), 70 °C, 1 atm.
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Table
Table 1. Textual properties of M-Ce80Zr20 catalytic support and catalysts.
Table 1. Textual properties of M-Ce80Zr20 catalytic support and catalysts.
SamplesSpecific Surface Area (m2/g)Pore Volume (cm3/g)Average Pore Diameter (nm)Isotherm Type
M-Ce80Zr2068.90.127.9IV H2
15CuO/M-Ce80Zr2055.20.139.6IV H2
15CuO-3Mn2O3/M-Ce80Zr2031.10.1313.0IV H2
15CuO-3PrO2/M-Ce80Zr2048.40.1412.3IV H2
15CuO-3Sm2O3/M-Ce80Zr2048.90.1412.3IV H2
15CuO-3La2O3/M-Ce80Zr2054.80.1412.4IV H2
15CuO-3Fe2O3/M-Ce80Zr2043.80.1617.5IV H2
15CuO-1Fe2O3/M-Ce80Zr2044.50.1717.4IV H2
15CuO-5Fe2O3/M-Ce80Zr2039.50.1417.4IV H2
15CuO-7Fe2O3/M-Ce80Zr2037.30.1317.5IV H2
Table
Table 2. Binding energies (eV) of the surface elements of the catalysts.
Table 2. Binding energies (eV) of the surface elements of the catalysts.
SamplesCu 2p3/2O 1sCe 3d5/2Zr 3d5/2
15CuO/M-Ce80Zr20933.3529.8882.5181.3
15CuO-3PrO2/M-Ce80Zr20933.7529.5882.4181.4
15CuO-3Fe2O3/M-Ce80Zr20933.1529.6882.5181.5
15CuO-3MnO2/M-Ce80Zr20933.4529.8882.6181.4
15CuO-3La2O3/M-Ce80Zr20933.2529.8882.7181.4
15CuO-3Sm2O3/M-Ce80Zr20933.2529.8882.5181.3

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MDPI and ACS Style

Cui, Y.; Xu, L.; Chen, M.; Lv, C.; Lian, X.; Wu, C.-e.; Yang, B.; Miao, Z.; Wang, F.; Hu, X. CO Oxidation over Metal Oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2) Doped CuO-Based Catalysts Supported on Mesoporous Ce0.8Zr0.2O2 with Intensified Low-Temperature Activity. Catalysts 2019, 9, 724. https://doi.org/10.3390/catal9090724

AMA Style

Cui Y, Xu L, Chen M, Lv C, Lian X, Wu C-e, Yang B, Miao Z, Wang F, Hu X. CO Oxidation over Metal Oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2) Doped CuO-Based Catalysts Supported on Mesoporous Ce0.8Zr0.2O2 with Intensified Low-Temperature Activity. Catalysts. 2019; 9(9):724. https://doi.org/10.3390/catal9090724

Chicago/Turabian Style

Cui, Yan, Leilei Xu, Mindong Chen, Chufei Lv, Xinbo Lian, Cai-e Wu, Bo Yang, Zhichao Miao, Fagen Wang, and Xun Hu. 2019. "CO Oxidation over Metal Oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2) Doped CuO-Based Catalysts Supported on Mesoporous Ce0.8Zr0.2O2 with Intensified Low-Temperature Activity" Catalysts 9, no. 9: 724. https://doi.org/10.3390/catal9090724

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