CeO 2 /CuO/3DOM SiO 2 Catalysts with Very High Efficiency and Stability for CO Oxidation

2 ABSTRACT: Based on the urgent need for high active and stabile catalyst for CO conversion at low temperature, a CeO 2 /CuO/SiO 2 ordered macroporous composite catalyst with 3DOM structure was prepared. The 3DOM matrix possessed a fairly high specific surface area (598 m 2 /g) and a large pore size (250-300 nm), which stably assembled CeO 2 and CuO nanoparticles but without clogging the pores. The 0.5CeO 2 /0.25CuO/3DOM SiO 2 catalyst exhibits excellent catalytic activity to CO, due to the synergistic effect of CeO 2, CuO and the metal-support interactions. The conversion of CO can achieve 100% at 160 °C and maintain excellent stability up to 12h, due to the abundant lattice oxygen vacancies in CeO 2 . The adsorption behavior of CO molecules on 0.5CeO 2 /0.25CuO/3DOM catalyst was further confirmed by in-situ Fourier transform infrared spectroscopy, and the possible mechanism of CO oxidation was explained.


Introduction
Nowadays, air pollution caused by CO, such as in the cases of environmental emergency in core mining, metallurgy exhausts, and diesel submarines atmosphere, is extremely harmful to environment and human health [1,2]. The influences of CO pollution, that is one of the most poisonous pollutions, need to be reduced or even eliminated imminently. The oxidation reaction of CO is a key step and also an effective method to reduce its negative affections [3,4].
The CO catalytic oxidation has a series of advantages such as low energy consumption, high processing capacity and low secondary pollution, which show its superiority from many CO removal technologies [5][6][7]. Moreover, CO catalytic oxidation can be used as a typical model reaction to study gassolid interface reaction in multiphase catalysis [8][9][10]. The traditional noble metal-based CO oxidation catalysts show excellent CO oxidation activity, but it is difficult to realize large-scale application due to the scarcity and high price of noble metals [8,9,11]. In recent years, catalysts composed of transition metal and rare earth metal oxide composite materials have been widely studied in CO catalytic oxidation [2][3][4]12]. Extensive experiments have demonstrated that the reducible metal oxides such as CeO 2 /CuO and their composite catalysts possess high CO oxidation properties, which can be appropriate candidates for high-activity noble metal oxidation catalysts, due to its low price, it is expected to replace the noble metal-based catalyst [13,14].
The uniqueness of applying CeO 2 in catalyst lies in the high density of oxygen vacancies, which makes it have high oxygen storage and release capabilities, and effectively provides oxygen to the catalytic reaction [8,15,16]. In addition, there are two main valence states of Ce including Ce 3+ and Ce 4+ in cerium oxide. The interaction between Ce 3+ and Ce 4+ can provide very high catalytic activity for surface 4 attention due to their high activity, selectivity and persistence for CO catalytic oxidation. The substantial literatures have proved that the interaction between CeO 2 and CuO could improve the catalytic oxidation performance of CO [20][21][22]. Since the radius of Ce atom are larger than that of Cu atom, Cu atom can be doped into lattice of CeO 2 [23]. The doping and modification of CuO can form Cu x+ -Ce 3+ -Ce 4+ configuration and simultaneously create an unsaturated second phase CuO, which will increase the defects concentration in the material (such as oxygen vacancies), generate extra electrons, and make the gaseous oxygen molecular around the material be adsorbed and converted into active oxygen atoms to improve the efficiency of CO catalytic oxidation [19,24,25]. In addition, the catalyst support will affect the catalytic performance. The three-dimensional ordered macroporous material (3DOM), with an open, uniform and interconnected three-dimensional void structure, efficient mass transfer efficiency and high surface accessibility, is a promising support in heterogeneous catalytic reactions [26][27][28]. Assembling other functional components inside the 3DOM support and designing functional 3DOM materials with special applications can ensure that the guest molecules can easil1y enter the internal pores of the material and release the active sites, hence the resistance to molecular diffusion will be greatly decreased.
Here, we report a 3DOM SiO 2 prepared by colloidal crystal template method using PS microspheres as template and tetraethyl orthosilicate (TEOS) as organic silicon source. The CeO 2 /CuO was loaded by a simple grinding calcination method for efficient catalytic oxidation of CO. We studied the phase structure, microstructure and surface chemical state of the composite catalysts, and focused on the effect of different proportions of Ce and Cu loading on the physicochemical properties and catalytic activity of CeO 2/ CuO/3DOM SiO 2 nanocomposites, aiming to design an effective and stable non-noble metal catalyst for low temperature CO oxidation catalyst.

Materials preparation and methods
Polystyrene (PS) microspheres were prepared by soap-free emulsion polymerization. Firstly, the monodisperse PS microspheres were synthesized by surfactant free emulsion polymerization, and then the colloidal crystal template was formed by centrifugal assembly, similar to the method of Tian et al [29].
The 3DOM SiO 2 material used TEOS as the precursor. In a typical synthesis procedure, 6.24 g TEOS, 5.40 g ethanol, 0.97 g H 2 O, and 0.89 g 37 wt% HCl were mixed and stirred for 15 min. Then 0.8 g preprepared PS microsphere template was added and allowed to stand at room temperature for 24 h. The obtained sol-state liquid was subjected to procedures such as suction filtration, water washing, drying, and calcination to obtain 3DOM SiO 2 material. And the calcination procedure was 350 ℃ calcination for 2 h, then heated to 600 ℃ and calcined for 2 h with a heating ramp of 5 ℃/min. The CeO 2 /CuO/3DOM SiO 2 composite catalyst was prepared by a simple grinding and calcination method. Firstly, a certain amount of Ce(NO 3 ) 3 ·6H 2 O and Cu(NO 3 ) 2 ·3H 2 O were weighed in proportion and subjected to grind into powder, 3DOM SiO 2 was added, and continue grinded and mixed well. And then, the above mixture was calcined at 450 ℃ for 4 h, and the heating rate was 5 ℃/min. The ratio of the two nitrates was adjusted to obtain CeO 2 /CuO/3DOM SiO 2 composites with different loading contents. The preparation procedure was schematically shown in Fig. 1, the content and proportion of cerium and copper of CeO 2 /CuO/3DOM SiO 2 series catalysts were shown in Table 1.

Property characterization of catalysts
In order to determine the conditions of the preparation process of the catalysts, the thermal behaviors of PS microspheres and the Nitrates/3DOM SiO 2 samples were analyzed under air atmosphere (Fig. 2).
The PS microspheres begin to lose weight at about 250 ℃ and almost keep constant after 600 ℃ (Fig. 2a).
The total weight loss rate of PS microspheres is about 93%, which mainly includes two processes. The weight loss in the first stage (250-400 ℃) is about 83.1%, and two significant endothermic peaks are observed in this range. The first endothermic peak can be attributed to the fracture of the polymer, and the second endothermic peak is mainly caused by the carbonization, decomposition and volatilization of polystyrene [30,31]. The weight loss of the second stage (400-600 ℃) is about 8.9%, which is caused by the low degree graphitization of carbon combustion generated by polystyrene during the carbonization process [32]. The thermo-gravimetric analysis of the PS microspheres shows that the PS microspheres template can be completely burned at about 600 ℃ in air atmosphere. It is reasonable to determine that the removal temperature is 350 ℃ and 600 ℃. The polymer can be fully broken and volatilized at 350 ℃, and then calcined at 600 ℃ to ensure the complete removal of the remaining hard-to-decompose substances. The weight loss of the Nitrate/3DOM SiO 2 in the temperature range is 36% (Fig. 2b), which can be attributed to the thermal decomposition of cerium and copper nitrates to produce metal oxides. The weight loss occurring at about 30-227 ℃ is mainly due to the evaporation and decomposition of hydroxyls in the raw sources when being heated. The weight loss rate at this stage is about 20%. The second stage 7 of weight loss in the range 220-450 ℃ is accompanied with a weight loss of about 16%, which is mainly ascribed to the decomposition of nitrates into oxides. It indicates that 450 ℃ is the appropriate calcination temperature for the preparation of the catalysts. The XRD spectra of 3DOM SiO 2 and those loaded with different contents of cerium and copper is shown in Fig. 3. There is no presence of crystal structural characteristic peaks in 3DOM SiO 2 (Fig. 3a), except for a wide peak packet observed at 2θ = 22°, belonging to the amorphous SiO 2 structure. In addition, the XRD patterns of the other four 3DOM SiO 2 with different CeO 2 loading (Fig. 3a) indicate that the CeO 2 in the four samples is cubic fluorite structure (PDF JCPDS#34-0394) [24]. The diffraction peaks of 1CuO/3DOM SiO 2 shown in Fig. 3c can be designated as the monoclinic CuO structure (PDF JCPDS#48-1548) [33,34]. Comparing the positions of the (110) characteristic peaks of CeO 2 in XRD patterns of different samples, a slight shift of about 0.16° can be detected on the three mixed oxide samples (inset in Fig. 3a). A similar phenomenon also appears in Fig. 3c, where the CuO (11-1) diffraction peaks of the three mixed oxide samples shifted to a certain extent (0.058°), compared to the 1CuO/3DOM SiO 2 (inset in Fig. 3b). This may be originated from the smaller radius of Cu 2+ than that of Ce 4+ (0.073 nm for the former and 0.101 nm for the latter), which leads to the Cu 2+ partial doping into the lattice of CeO 2 and causes the lattice expansion of CeO 2 [35,36]. In addition, the relative intensities of XRD peaks of CeO 2 and CuO in all samples are proportional to their loading contents, which is in consistent with the actual 8 situation.   Table 2. The specific surface area of the 3DOM SiO 2 is 598 m 2 /g, and the pore volume is 0.591 cm 3 /g. The larger specific surface area and pore volume will provide various active sites for the catalytic reaction, and can serve as an excellent support for nanoparticles [3]. Obviously, the CeO 2 /CuO/3DOM SiO 2 catalysts show significant hysteresis loops in relative pressure range 0.5~1.0, but there is no type IV isotherm platform at high P/P 0 , indicating that the samples contain a certain amount of mesopores (responsible for the hysteresis loops) and numerous macropores (resulting in the lack of platform for type IV isotherms) [2,37]. The existence of H3-type hysteresis rings indicates that a certain number of small voids exist in the 3DOM SiO 2 , which should be the mesoporous produced by the tangential parts among PS microspheres.
Hence, the loop is in consistent with the morphology of the prepared porous materials. At P/P 0 =0.45, the desorption is forced to lock-in due to the "tensile strength effect". This indicates that the CeO 2 /CuO/3DOM SiO 2 series of catalysts have a certain number of apparent mesopores (~5 nm) [38]. The volume absorbance near P/P 0 =1.0 indicates that the total porosity of the catalyst has reached the 9 macropore size range (about 300 nm), which is in line with the reality [39,40]. For 3DOM SiO 2 loaded with different contents of Ce and Cu (Fig. 4a, b), the pore volume decreased with the increase of the loading amount, which may be caused by the accumulation of CeO 2 and CuO nanoparticles on the pore wall of the original 3DOM SiO 2 . As a result, the macroporous-mesoporous structure of CeO 2 /CuO/3DOM SiO 2 series catalysts have been determined. According to the desorption branch of the isotherms, the corresponding pore size distribution curves were measured by BJH method, which displays a

Surface morphology analysis
The SEM images of PS microspheres template confirms that the PS microspheres prepared in this experiment are highly ordered arranged (Fig. 5a). And the prepared PS microspheres are complete and full spheres with a relatively uniform particle size, the diameter are about 350-400 nm (Fig. 5b, inset is the enlarged view). The SEM image of 3DOM SiO 2 shows that the catalyst has a three-dimensional ordered macroporous structure (Fig. 5c-e), and the overall structure is highly ordered face-centered cubic (FCC). The inner structure of the macroporous 3DOM SiO 2 is opposite to the structure of PS microsphere template and shows an inverted opal structure. And the thickness of the pore wall is moderate, and the pore channels are neatly arranged, regular, and interconnected. Combined with the pore size distribution  Fig. 4c and Fig. 4d, the pore size of the openings on the 3DOM SiO 2 surface is uniform, with average size of about 250-300 nm, which is in consistent with the SEM results. Compared with the PS microsphere template, the pore size decreased by about 25%, which is mainly caused by the shrinkage of Si skeleton during the high temperature calcination. Fig. 5f is the SEM image of the cross section of the 3DOM SiO 2 , and Fig. 5g is the enlarged image.
Obviously, the internal structure of 3DOM SiO 2 is also highly ordered, and the pore and window among the macropores are interconnected. To observe the microstructure of the 3DOM SiO 2 in more detail, a transmission electron microscope (TEM) is used to characterize the catalyst (Fig. 5h-i). It can be seen that the catalyst presented an ordered macroporous structure with a three-dimensional connected macroporous network, consistent with the results observed by SEM. catalyst.
The morphology, structure and element distribution of 0.5CeO 2 /0.25CuO/3DOM SiO 2 catalyst were observed by SEM, HAADF elemental mapping and EDS energy spectrum ( Fig. 6a-b). It can be observed that the catalyst loaded with CeO 2 and CuO still maintains a 3DOM structure, similar to that of 3DOM SiO 2 , and the channel distribution is still uniform. Furthermore, it is also found that the pore wall thickness of the 3DOM was significantly increased compared to the unloaded matrix. This is originated from the uniform loading of the cerium copper oxide on the pore walls. The element distribution diagram of the catalyst can be seen that Ce and Cu elements are evenly dispersed on the surface of the catalyst (Fig. 6d-g), indicating that the distribution of CeO 2 and CuO nanoparticles is relatively uniform on the framework of 3DOM structure. Moreover, the peaks of Si, O, Ce and Cu can be observed in EDS spectrum (Fig. h), which also demonstrates that the CeO 2 and CuO nanoparticles are successful loaded on the 3DOM SiO 2 matrix.

Surface XPS analysis
The surface composition and chemical bonds of the 0.5CeO 2 /0.25CuO/3DOM SiO 2 catalyst and that after stability testing were studied by XPS spectroscopy. The spectra of Ce 3d, Cu 2p, O 1s and Si 2p electrons for the catalyst were corrected by the C 1s peak at 284.8 eV. The XPS structure of Ce 3d is consisted of four pairs of 3d 5/3 and 3d 3/2 peaks, and Ce 3+ and Ce 4+ coexisted in the 0.5CeO 2 /0.25CuO/3DOM SiO 2 catalyst according to the Ce 3d spectrum in Fig. 8a [41] . Except for the peaks at 884.1 eV and 903.1 eV, which belonging to the Ce 3+ state, the other six peaks are all come from the Ce 4+ state [24,34] . Ce 3+ is most likely formed by oxygen vacancies in the catalyst. Besides, it may also be caused by insufficient coordination of Ce 3+ on the surface of the nano-sized Ce 2 O 3 crystals, or the doping of Cu 2+ [24,42,43] . There is an obvious difference between the used and the fresh sample. The peak of Ce 3+ is obviously enhanced in the former, which can be considered as the result of oxygen vacancies generated during the reaction. As shown in Fig. 8b, there are two asymmetric broad peaks in the Cu 2p XPS spectrum, Cu 2p 1/2 (929 ~ 938 eV) and Cu 2p 3/2 (948 ~ 959 eV), which can fit two Cu 2p 3/2 peaks centered at 933.2 eV and 934.7 eV, respectively. The main peak at 934.7 eV is typical Cu 2+ state [44] , and the presence of the other two satellite peaks at 942.6 eV and 962.5 eV also indicates that it is a typical feature of Cu 2+ [45] . It should be noted that it is difficult to accurately distinguish the oxidation state of copper by the location of XPS spectra due to the close binding energy of Cu + and Cu 2+ states [22,46].
Therefore, the copper in the 0.5CeO 2 /0.25CuO/3DOM SiO 2 may have both ions.
The information about the factors influencing catalytic performance can also be obtained from the O 1s XPS spectrum of the fresh and the used catalysts, where two O 1s peaks can be fitted to the obtained spectra. The peak near the binding energy of 529.8 eV can be assigned to the lattice oxygen [24], while the peak near 532.8 eV are the Si-O-Si bond on the 3DOM SiO 2 support [47]. Obviously, the lattice oxygen of the used catalyst is significantly consumed compared to the fresh catalyst, indicating that a large oxygen vacancies will be generated during the reaction, and these vacancies is supplemented by the migration of the bulk lattice oxygen of the metal oxide [48]. This is crucial for the adsorption/dissociation of oxygen in CO oxidation reaction and should play a key role in understanding the reaction mechanism.

Catalytic performance evaluation
The catalytic activity of the CeO 2 /CuO/3DOM SiO 2 series catalyst was investigated using CO oxidation reaction as a model. To simulate the exhaust gas of a lean fuel engine and obtain the best catalytic performance, an excessive amount of O 2 (CO/O 2 =1/10) was used in the reaction. Meanwhile, to match the standard exhaust conditions of the vehicle, the GHSV was maintained at 120,000 mL/ g·h. The results show that 100% complete catalytic oxidation of CO could be achieved at 160 ℃ on the 0.5CeO 2 /0.25CuO/3DOM SiO 2 and 0.5CeO 2 /0.5CuO/3DOM SiO 2 . Meanwhile, the former is better as its activity is significantly higher than that of latter when the temperature is below 160 ℃. The catalytic activity of CeO 2 /CuO/3DOM SiO 2 series products are not specifically linear associated with the CeO 2 /CuO ratios, and an appropriate CeO 2 /CuO ratio can stimulate their catalytic activity. In addition, the appropriate proportion of CuO will have a favorable effect on the interaction between CeO 2 and CuO.
At low CuO ratio range, the Cu + (Cu + is the active site for CO adsorption) produced by the CeO 2 -CuO interaction showed gradual increment when the CuO content increased, thus promoted the catalytic activity. However, when the proportion of CuO reaches a critical value, the CuO nanoparticles will agglomerate to generate larger particles, which will cover part of the surface adsorption active sites, leading to a sharp decrease of Cu + content and a weakened interaction between Ce and Cu oxides, resulting in a decreased activity of the catalyst [49,50].
The CO catalytic oxidation stability of the 0.5CeO 2 /0.25CuO/3DOM SiO 2 catalyst at 160 ℃ and 240 ℃ for 13 hours were studied (Fig. 9b). The results show that the catalyst maintains excellent stability at both temperatures and the CO conversion rate always remains at about 100%. At 160 ℃, the unstable conversion rate of the catalyst at the initial stage can be attributed to the insufficient activation of the catalyst. In addition, the performance of CeO 2 /CuO/3DOM SiO 2 and other nanostructured materials were compared (as shown in Fig. 9c and d).

In-situ FTIR analysis
To determine the adsorption sites and the oxidation process of CO during oxidation process on the 0.5CeO 2 /0.25CuO/3DOM SiO 2 catalyst, in-situ FTIR technology was selected to monitor the possible real time radicals at different reaction temperatures. When comparing the CO adsorption at room temperature and 160 ℃ (Fig. 10a and c) the peak emerged in the range 2000~2250 cm -1 belonging to the Cu δ+ -CO species [58]. In detail, the infrared vibration band at ~2115 cm -1 corresponds to the linear CO on the Cu + , and the signal at ~2173 cm -1 can be attributed to the gas phase CO [59]. Compared with the IR spectra of CO adsorption at room temperature, weaker infrared signals at 2065 cm -1 and 2090 cm -1 can be observed during CO adsorption at 160 ℃, which can be attributed to the Cu δ+ -multicarbonyl species formed by the binding of two or three CO ligands to a Cu δ+ site ( Fig. 10c and d) [60,61]. Besides, an obvious difference is the longer time for a CO adsorption-desorption recycle on the 0.5CeO 2 /0.25CuO/3DOM SiO 2 catalyst at room temperature, more than 10 minutes was required for a complete adsorption saturation/desorption process. When the reaction was conducted at 160 ℃, the adsorption saturation/desorption is completed in only about 5 minutes. This indicates that the increased temperature can promote the reaction ability to CO on the catalyst surface. The in-situ IR spectra of CO oxidation process from 50 to 300 ℃ is shown in Fig. 10e, the intensity of the CO related peaks shows a continuous downward trend. The gaseous CO 2 related band appears at 2300-2400 cm -1 with the decrease in the intensity of the CO correlation peaks [61]. The gaseous CO 2 peaks starts to appear at 50 ℃. And they were enhanced significantly when the temperature increase, which is in consistent with the increased CO conversion rate. The appearance of CO 2 peaks at near room temperature also confirms that the 0.5CeO 2 /0.25CuO/3DOM SiO 2 catalyst has an excellent CO oxidation activity at near RT circumstance. Fig. 10. In-situ FTIR spectra of CO catalytic oxidation on 0.5CeO 2 /0.25CuO/3DOM SiO 2 at conditions: Combining the results of in-situ FTIR and other characterizations, Fig. 11 concluded the possible mechanisms of CO oxidation over CeO 2 /CuO/3DOM SiO 2 . Doping a small amount of Cu 2+ in CeO 2 crystal will increase its lattice defects while forming oxygen vacancies and releasing electrons [59], and the oxygen exchange between adsorbed oxygen and lattice oxygen can enhance this process [19]. The O 2 molecules and CO molecules first diffuse rapidly in the macropores of 3DOM SiO 2 . The synergistic effect of copper and cerium follows the avenue: Cu 2+ + Ce 3+ →Cu + + Ce 4+ , this often occurs at the doped site, and will lead to an increase in the active oxygen content of the catalyst. CO is adsorbed on Cu-doped CeO 2 active sites on the surface and will react with the adjacent oxygen anion to generate CO 2 . Then the CO 2 desorption process will create oxygen vacancies, which will be refilled rapidly by O 2 molecules from the feed flow and is ready to react in the next step [61,62]. The possible reaction processes are as follows: → (5)

Conclusions
The CeO 2 /CuO/3DOM SiO 2 composite catalysts were prepared by a colloidal crystal template method and simple grinding calcination method. The products showed very high-efficiency conversion of CO due to the high mass transport efficiency of 3DOM structure and synergistic effects in the composite catalysis. Among all of as-prepared samples, the 0.5CeO 2 /0.25CuO/3DOM SiO 2 showed the highest catalytic activity for CO oxidation, which achieved complete CO conversion at 160 ℃ and showed excellent stability. In-situ FTIR spectroscopy indicated that the Cu + was the main adsorption site for CO.
Combined the results of in-situ FTIR and other characterizations, the possible mechanism for the CO oxidation on CeO 2 /CuO/3DOM SiO 2 was supposed to be the reaction between the adsorbed CO on the Cu + active sites and the adjacent oxygen anions to form CO 2 , and rapidly diffuses and separates in