CeO₂ decorated CuO hierarchical composites as inverse catalyst for enhanced CO oxidation

Abstract

CuO microspheres were constructed with self-assembled CuO nanosheets through a hydrothermal method. Then, CeO₂ nanoplatelets were decorated on the surface of CuO nanosheets to obtain inverse CeO₂/CuO catalysts. The catalysts were characterized by using XRD, SEM, HRTEM, XPS, Raman spectroscopy, and H₂-TPR techniques. It was found that the CeO₂/CuO inverse catalysts had a hierarchical structure and a much lower 100% CO conversion temperature than that of CuO microspheres. A model of the structure and catalytic effect based on the contact interface between CeO₂ and CuO is discussed. The two-dimensional CuO nanosheets and CeO₂ nanoplatelets were most favorable for the formation of long periphery at the CeO₂/CuO interface, therefore, enhanced catalytic activity was achieved.

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1. Introduction

The catalyzed oxidation of carbon monoxide is important in several technological areas such as automotive exhausts catalysis that seeks to limit pollutant emissions; proton exchange membrane fuel cell applications where CO tends to poison the catalysts; and synthesis gas conversion where the CO preferential oxidation reaction was used to remove any residual CO. It is generally recognized that noble metals, such as Pd, Pt, and Rh, are the most effective catalysts to eliminate CO.^1–3 However, their application might be limited by their high price and limited resources. So far, various binary oxides, especially a series of bimetal oxides of copper and cerium, have been extensively investigated due to their low cost and high activity.^4–8 However, to our knowledge, most of the reports have focused on the composites of CuO supported on CeO₂.

In fact, the inverse CeO₂/CuO catalysts with a configuration of CeO₂ nanoparticles supported on CuO exhibit a more promising alternative from an economical point of view. Martínez-Arias's group first reported the inverse CeO₂/CuO catalyst for CO oxidation, and found that the inverse CeO₂/CuO catalyst has wider CO conversion window than that of classical CuO/CeO₂ catalyst.⁹ Then, Su's group prepared the inverse CeO₂/CuO catalysts using a precipitation–impregnation method.^10–12 Chen et al. prepared inverse CeO₂/CuO catalysts using heterobimetallic metal–organic frameworks as the precursor.¹³ Cámara et al. explored the catalytic properties of inverse CeO₂/CuO catalysts doping of the CuO component with Zn.¹⁴ An inverse catalysts composed of CeO₂/Cu₂O/Cu(111) have also been reported.¹⁵ Yin et al. investigated the influence of the structure and morphology of CuO supports on the amount and properties of copper–cerium interfacial sites in inverse CeO₂/CuO catalysts.¹⁶ However, the control of the interfacial and surface structures of this type of system remains as a challenge because of the complexity of the structures that can be generated for both of the CuO supports and the CeO₂ species. In addition, the role of the ceria nanoparticles and the CuO support in this inverse configuration was challenging to understand. Herein, we report the synthesis of inverse CeO₂/CuO catalysts with 3D hierarchical nanostructures. It has been accepted that complex 3D architecture has highly stabilized and easily operated features for its micro scale properties and highly activated properties for its nanoscale building blocks.¹⁷ The dispersed CeO₂ nanoplatelets were decorated on the surface of the assembled CuO ultrathin nanosheets. The two-dimensional CuO nanosheets and CeO₂ nanoplatelets were most favorable for the formation of long periphery at the CeO₂/CuO interface, therefore, enhanced catalytic activity was achieved.

2. Experimental

2.1 Synthesis of the samples

Synthesis of CuO microspheres with hierarchical structure. All the chemicals were of analytical grade and used as received without further purification. CuO microspheres were synthesized using a modified solution method.¹⁸ In a typical experimental process, 1.2 mmol of Cu(NO₃)₂·3H₂O was dissolved in 10 mL of ethanol, 29 mmol of NaNO₃ was added to 25 mL of ammonium hydroxide, 5 mmol of NaOH was dissolved in 5 mL of deionized water, and then the above solutions were mixed under constant stirring. Afterwards the mixture solution was transferred to a stoppered flask and kept at 90 °C for 6 h. After the flask being cooled to room temperature, the as-obtained black precipitates were centrifuged and washed with deionized water and ethanol three times.

Synthesis of CeO₂/CuO composite microspheres with hierarchical architecture. 5.0 mmol of as-prepared CuO powders were ultrasonically dispersed in 50 mL of deionized water, and then 0.018 mmol of Ce(NH₄)₂(NO₃)₆ and 1.6 mmol of urea were added in turn with continuous stirring for about 30 min at room temperature. The mixture solution was transferred to a stoppered flask and kept at 90 °C for 12 h. After the mixture solution was cooled to room temperature, the as-obtained precipitates were washed with deionized water for three times. The above procedure was repeated one more times. Finally, the as-obtained precipitates were dried at 60 °C for 6 h, and then, calcined at 300 °C for 0.5 h at a ramping rate of 2 °C min⁻¹ (0.8% CeO₂/CuO). For comparison, a series of CeO₂ and CeO₂/CuO catalysts with different amount of CeO₂ were also prepared following the same procedure.

2.2 Characterization

The X-ray powder diffraction (XRD) patterns of the products were recorded on a D/max-2500/PC X-ray diffractometer with Cu Kα radiation. The morphologies and microstructures of the products were characterized using a JSM-6700F field emission scanning electron microscope (SEM) and a JEM-2010 high-resolution transmission electron microscope (HRTEM).

Raman analysis was performed on a LabRam HR 800 laser confocal micro-Raman spectrometer (Jobin Yvon-Horiba) with a 532 nm YAG laser as the excitation source. X-ray photoelectron spectra were recorded using a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer with Al Kα radiation (hν = 1486.6 eV) at 150 W. The charging shifts of the spectra were calibrated using the C 1s peak at 284.8 eV.

H₂ temperature-programmed reduction (H₂-TPR) was conducted on a Micromeritics Apparatus (AutoChem II 2920). The reactions were operated in a quartz reactor with 50 mg of the as-prepared catalyst and the amount of H₂ consumption was analyzed by a thermal conductivity detector (TCD). The reduction profiles were collected in the 10% H₂/Ar gas calibration from room temperature to 800 °C. The flow rate of gas was 50 mL min⁻¹ and the heating rate was 5 °C min⁻¹.

2.3 Catalytic testing

The activities of the catalysts for CO oxidation were carried out in a continuous flow fixed-bed microreactor using catalytic evaluation device under atmospheric pressure. In a typical experiment, 50 mg catalyst and 40 mg quartz sand were mixed and were placed in the reactor. The gas stream was switched to the reaction atmosphere (i.e., CO oxidation in excess O₂; 1% CO and 10% O₂ balanced with N₂) at a flow rate of 60 mL min⁻¹. The operation temperature was controlled using a K-type thermocouple inserted directly into the quartz tubular to touch catalyst bed. The operation temperature was ramped with a rate of 2 °C min⁻¹ to the final temperature. The composition of the gas exiting the reactor was analyzed with an online infrared gas analyzer (Gasboard-3121, China Wuhan Cubic Co.), which simultaneously detects CO and CO₂ with a resolution of 10 ppm.

3. Results and discussion

3.1 Sample characterization

The composition and phase purity of the samples were identified using XRD. Fig. 1 shows the XRD patterns of the CuO and the CeO₂/CuO composites. For both of the patterns, the major reflections were labeled and can be indexed to the planes of the monoclinic structured CuO (JCPDS card no. 48-1548). It can be seen from Fig. 1b that no obvious reflection peaks related to cubic phase CeO₂ can be identified, suggesting that CeO₂ species are highly dispersed on the surface of CuO carriers⁷ or the CeO₂ content was below the detection limit of XRD. The lattice constants were calculated to be a = 0.468 nm, b = 0.344 nm, and c = 0.514 nm for the sample, which are consistent with the standard value.

Fig. 1 XRD patterns of (a) CuO microspheres and (b) CeO₂/CuO composite microspheres.

Fig. 2a and d show the low-magnification SEM images of CuO and CeO₂/CuO nanostructures. It can be seen that both CuO and CeO₂/CuO samples are composed of microspheres with diameters of about 3.0 µm. The SEM images in Fig. 2b and e reveal that the microspheres have pompon-like structures and are built up with a lot of attached nanosheets and that the textures of CuO nanostructures maintain well after loading CeO₂ and being calcined. Fig. 2c and f show magnified SEM images of an individual microsphere of CuO and CeO₂/CuO respectively, exhibiting that the attached nanosheets have a thickness of about 10 nm. Fig. 2g shows HRTEM image of the CeO₂/CuO nanosheet. It is observed that some CeO₂ nanoplatelets with a size of about 5 nm were highly dispersed and decorated on the surface of CuO nanosheets. The d-spacing of 0.23 nm corresponds to (−111) planes of the CuO component and the d-spacing of 0.31 nm corresponds to (111) planes of the CeO₂ component. The lattice fringes of CuO are continuous as a whole but with interruption in the coarse surfaces of the nanosheets. And this structure defects favors the adsorption of Ce⁴⁺ ions and the deposition of dispersed CeO₂ nanoplatelets onto the CuO surface.

Fig. 2 SEM images of (a–c) CuO microspheres and (d–f) CeO₂/CuO composite microspheres, and HRTEM image of CeO₂/CuO composite microspheres (g).

3.2 Raman analysis

Fig. 3 shows the Raman spectra of the CeO₂/CuO composites. It has been reported that bulk CuO crystals exhibit three modes of 296 cm⁻¹ (Ag), 346 cm⁻¹ (Bg (1)) and 636 cm⁻¹ (Bg (2)).¹⁹ For the classical CeO₂/CuO catalyst, it has been reported that a broad band with a relatively high intensity at 462 cm⁻¹ corresponds to cubic CeO₂ and the bands at about 584 and 1176 cm⁻¹ are related to oxygen vacancies in the CeO₂ lattice and can be attributed to the presence of CeO₂ defects.^20–22 However, the Raman pattern of the inverse CeO₂/CuO catalysts of this work is somewhat different. As shown in Fig. 3, three bands at 290 cm⁻¹, 330 cm⁻¹, and 625 cm⁻¹ can be assigned to the modes of CuO in CeO₂/CuO composite, which shift slightly to a lower frequency and the peaks broaden because of the quantum confinement effects produced by the relatively smaller crystallite sizes of CuO.²³ It is noticeable that the intensity of the Ag peak is stronger than that of the Bg (1) and Bg (2) peaks, which demonstrates that the crystallization degree of CuO in the catalysts was good with some defects.²⁴ The broad, but relatively strong band at about 1110 cm⁻¹ can also be ascribed to CuO.^25,26 The Raman scattering signals of CeO₂ became weak due to the presence of the CuO support crystals. The weak peak at 465 cm⁻¹ is ascribed to F2g model vibration of cubic phase CeO₂ in the CeO₂/CuO catalysts,²⁷ and the bands related to the oxygen vacancies become broad and weak due to the smaller size of CeO₂ nanoplatelets.

Fig. 3 Raman spectra of the CeO₂/CuO catalysts. Inset is the enlarged spectrum.

3.3 XPS analysis

The surface composite and element valence states of the as-prepared CeO₂/CuO sample were further determined by XPS analysis. The core level Ce 3d peaks are displayed in Fig. 4a. It is evident that the XPS curve of Ce 3d is somewhat rough, and this might be due to relatively low amount of CeO₂ nanoparticles decorated on the surface of the composites.²⁸ As indicated by the fitted peaks, the two groups of spin–orbital multiplets, corresponding to 3 d^3/2 and 3 d^5/2, are denoted as u and v in the binding energy range of 880–920 eV. The peaks labeled u‴ and v‴, u″ and v″, u and v are related to Ce⁴⁺, while the other peaks labeled u₀, u′, v₀, and v′ can be attributed to Ce³⁺.^29,30 This result indicates that the chemical valence of cerium on the surface of the CeO₂/CuO sample is mixed. The peak intensity features of several typical binding energy peaks (such as v, v‴, and u‴) in the Ce 3d spectrum confirm that Ce⁴⁺ is the primary valance state in the sample together with a small amount of Ce³⁺. As a result, oxygen vacancies are formed to keep charge neutrality in the sample, and the interface is the most preferential site for oxygen vacancies in oxide composites.^31–33 For CeO₂/CuO sample, the stability of Ce³⁺ states and the concominant presence of oxygen vacancies is expected to favor the catalytic reactions.

Fig. 4 XPS of CeO₂/CuO: (a) O 1s, (b) Ce 3d, and (c) Cu 2p, and (d) Cu LMM Auger spectrum.

Fig. 4b shows the O 1s electron core level XPS spectrum. The fitted symmetric peak around 529.8 eV can be assigned to lattice oxygen O²⁻ in CeO₂/CuO, while the shoulder peak at about 531.8 eV is possible of absorbed oxygen or hydroxide.³⁴ The adsorption of oxygen generates an oxygen adsorption bond where oxygen vacancies are located. Owing to their higher mobility, a higher content of surface absorbed oxygen can promote the transferring to lattice oxygen and therefore facilitating the oxidation reactions.³⁵

Fig. 4c shows the XPS spectrum of Cu 2p. It can be seen that the main peaks of Cu 2p^3/2 and Cu 2p^1/2 are located at 933 eV and 953 eV with a spin–orbit coupling energy gap of 20 eV. It can also be noticed that both 2p^3/2 and 2p^1/2 peaks are accompanied by intense satellite peaks at 942 and 962 eV, which are about 9 eV greater than the corresponding main peaks. These results are in good agreement with the widely reported XPS spectra of CuO.³⁶ Because Cu 2p banding energy can not be used to identify Cu(I) and Cu(II), the Cu LMM Auger spectrum was recorded. As presented in Fig. 4d, the characteristic feature of Cu LMM binding energy at about 568.7 eV suggests that the oxidation state of copper in the sample is mainly Cu(II). Cu(0) and Cu(I) do not seem to be present in the as-prepared sample.

3.4 TPR-H₂ analysis

It is well known that the temperature programmed reduction by H₂ (H₂-TPR) has been used extensively to characterize the oxygen reducibility of metal oxides. Previous report revealed that CeO₂ has two reduction peaks centered at around 450 and 850 °C, while pure CuO is characterized by a peak located approximately at 300 °C.^37–39 Fig. 5 shows the H₂-TPR profiles of the pure CuO and CeO₂/CuO catalysts. It is evident that the CuO sample is characterized by a wide single H₂-TPR reduction peak at ∼264 °C, while the as-prepared CeO₂/CuO nanostructures exhibit a strong reduction peak at ∼197 °C. However, the reduction peak of CeO₂ cannot be detected for CeO₂/CuO sample in the present study due to its low content in the sample and the sensitivity of the TPR instrument. The peak shifts to lower temperature comparing to pure CuO nanostructures after being decorated with CeO₂ nanoplatelets. Obviously, CeO₂ has modified and improved the reduction properties of CuO due to the high oxygen-storage capacity of CeO₂, suggesting that the existence of an interaction between CeO₂ and CuO at the interface.^40,41

Fig. 5 H₂-TPR of (a) CuO and (b) CeO₂/CuO catalysts.

3.5 Catalytic activity

The catalytic activity of the CuO and CeO₂/CuO catalysts was further investigated by performing a CO conversion experiment, and the result is shown in Fig. 6. It can be clearly observed that the CO conversion rate increases with increasing reaction temperature and a 100% CO conversion is achieved at 136 °C for CeO₂/CuO catalysts (curve a). And the pure CuO catalyst shows 100% conversion of CO at about 170 °C (curve d). The catalytic performance of CuO nanostructures for CO oxidation is enhanced obviously after being decorated with CeO₂. The 100% CO conversion temperature of CeO₂/CuO catalysts is lower than that of CeO₂/CuO nanoparticles,⁵ porous CeO₂/CuO catalysts,¹⁰ and CeO₂/CuO flower-like microspheres.¹² The effect of amount of CeO₂ on the catalytic activity of CeO₂/CuO composite has been investigated. As shown in Fig. 6 (curve b and c), the catalytic activity of the CeO₂/CuO catalysts became lower when more or less CeO₂ was loaded on the CuO microspheres. While pure CeO₂ catalyst showed no obvious catalytic activity for CO oxidation below 200 °C (curve e).

Fig. 6 CO conversion rate with different catalysts: (a) 0.8% CeO₂/CuO, (b) 1.6% CeO₂/CuO, (c) 0.4% CeO₂/CuO, (d) CuO, and (e) CeO₂.

Previous studies have shown that bulk CuO does not show activity in CO oxidation below 200 °C. In the current study, CuO nanostructures showed obvious catalytic activity. It is expected that the two-dimensional ultrathin CuO nanosheets expose more unsaturated coordination sites to CO molecules, which favors the efficient oxidation of CO at low temperature.⁴² The CO oxidation over Cu–Ce–O catalysts has been extensively investigated in the literature. Martínez-Arias et al.⁹ found that the active sites of the CO oxidation are closely related to the interfacial Cu⁺ species generated through a reduction process. Su's group also reported that Cu⁺ species existed on the interface of CuO and CeO₂ are the key active component for the CO oxidation.²¹ Auger Cu LMM spectrum of the catalyst has been recorded after reaction and is presented in Fig. 7. The fitted shoulder peak at 569.6 eV confirms the formation of lower valence state Cu⁺ species after reaction,⁴³ indicating that Cu⁺ was involved in the CO oxidation reaction. Hočevar et al. proposed that the formation of Cu⁺ might be induced by substitution at the interface of the two oxide phases because of the analogous Ce⁴⁺ and Cu⁺ ionic radii.^44,45 It is believed that the reducibility of Ce⁴⁺ to Ce³⁺ forces the copper ions to adapt to a different oxidation state (Ce³⁺ + Cu²⁺ → Ce⁴⁺ + Cu⁺), which maintains the charge balance of the lattice. This is very consistent with the variation of Cu species in CeO₂/CuO catalyst before and after reaction. Ce 3d core level XPS spectrum in Fig. 4 indicated the coexistence of the typical bands of both Ce³⁺ and Ce⁴⁺. It is expected that Ce³⁺ and Cu²⁺ species suffer a facile electron transfer between CuO and CeO₂, which favors the oxidation of CO to CO₂.⁴⁶

Fig. 7 Cu LMM Auger spectrum of CeO₂/CuO catalysts after reaction.

As mentioned above, CeO₂ has a high oxygen-storage capacity and can generate oxygen vacancies. It is expected that the interaction and synergism between ceria nanoparticles and CuO nanostructure support, as well as the redox cycles between Ce⁴⁺/Ce³⁺ and Cu²⁺/Cu⁺ pairs play very important roles to CO oxidation.³⁵ Initially, CO molecules chemisorb on the interface of the CeO₂/CuO catalyst, forming Cu⁺ carbonyl species, and O₂ chemisorb on the oxygen vacancies in CeO₂, generating active oxygen (O₂ + O_vacan → O₂⁻) and/or lattice oxygen (O₂ + 2O_vacan → 2O_lattice). Then, the carbonyl species are oxidized by the interface lattice oxygen to generate the CO₂ final product, leaving oxygen vacancies on the surface (CO + O_lattice → CO₂ + O_vacan) (Scheme 1). Finally, the oxygen vacancies are replenished by adsorbed oxygen, returning the CeO₂/CuO catalyst to its original state.^35,47–49 The two-dimensional CuO nanosheets and CeO₂ nanoplatelets favor the unique interactions that occur in a mixed metal oxide at nanometer level with long periphery at the CeO₂/CuO interface, therefore, enhanced catalytic activity was achieved.

Scheme 1 Reaction mechanism of CO oxidation over CeO₂/CuO.

The catalytic tests were performed over six cycles, and clearly, the catalytic performance remains nearly constant. The 100% CO conversion temperature remains the same; therefore, the CeO₂/CuO sample shows excellent stability and recycling performance.

4. Conclusion

In conclusion, the decoration of CeO₂ nanoplatelets on CuO ultrathin nanosheets proved to be an efficient way to improve the catalytic properties of both metal oxides. The ultrathin CuO nanosheets and CeO₂ nanoplatelets were most favorable for the formation of long periphery at the CeO₂/CuO interface. The surface distortion and defect structures enhanced the oxygen storage capacity and the CO absorption, therefore, enhanced CO oxidation catalytic activity was achieved. The findings open up the way to design catalysts taking advantages of their low cost and high activity.

Acknowledgements

This work is supported by National Natural Science Foundation of China (51272118, 51342005) and Natural Science Foundation of Shandong Province (ZR2013BM002).

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