Template and interfacial reaction engaged synthesis of CeMnO_x hollow nanospheres and their performance for toluene oxidation

Abstract

A series of well-dispersed CeMnO_x hollow nanospheres with uniform diameter and thickness were synthesized by a novel approach combining the template method and interfacial reaction. A SiO₂ template was used as a hard template for preparation of SiO₂@CeO₂ nanospheres by solvothermal reaction. SiO₂@CeMnO_x could be formed after KMnO₄ was reacted with SiO₂@CeO₂ by interfacial reaction between MnO₄⁻ and Ce³⁺. Among all the prepared catalysts, CeMnO_x-3 with a moderate content of Mn (15 wt%) exhibited the lowest temperature for complete combustion of toluene (280 °C). Moreover, it showed high stability for 36 h with toluene conversion above 97.7% and good water tolerance with 5 vol% H₂O. With characterization, we found that the reaction between Ce and Mn in the Ce–Mn binary oxides gave rise to increased Ce³⁺ and oxygen vacancies, which led to the formation of enhanced reducibility and more surface-absorbed oxygens (O₂²⁻, O²⁻ and O⁻), and improved the catalytic performance further.

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1 Introduction

As one of the primary sources of air pollutants, volatile organic compounds (VOCs) are not only significant contributors to smog formation, but also can cause environmental pollution and bring great negative effects to human health.^1,2 Some VOCs have been classified as carcinogens, such as benzene, toluene, dichloromethane, and dichloroethane.³ Therefore, the effective removal of VOCs has been an important research topic in environmental governance.

Numerous methods have been used to eliminate VOCs, including incineration, absorption, adsorption, condensation, and catalytic oxidation.^4–6 Among them, catalytic oxidation has been recognized as one of the most effective and economical ways for abating VOCs, as it has the advantages of low degradation temperature, high removal efficiency, and no secondary pollution.^4,7 To realize the industrial application of catalytic oxidation, the catalysts need to be developed extensively. So far, two types of traditional catalysts were researched deeply, including supported noble metals (such as Au, Pd, and Pt)^8,9 and transition metal oxides.^10,11 Although the former exhibits high activity and CO_x selectivity at low temperatures, the high cost and high sensitivity to water vapor, sulfur and chlorine compounds limit their applications. Transition metal oxides, such as Co₃O₄,^11,12 NiO,¹³ MnO_x,¹⁴ CuO,¹⁵ and their composite oxides,^16–18 possess the advantages of low cost and high toxicity resistance. However, the reaction temperature for these catalysts is relatively high, which will affect their stability and increase energy costs.

In addition to these two types of catalysts, CeO₂ is also reported as an effective catalyst for the catalytic oxidation of VOCs as it has high oxygen storage capacity caused by its excellent redox properties. In recent years, CeO₂ with different morphologies and structures was researched for VOCs oxidation.^19,20 The catalytic activity of CeO₂ in the VOCs oxidation process can be improved by adding other elements to form composites, such as Mn, Cu, Ni and Co. The increment in catalytic performance may originate from the synergistic effect between CeO₂ and other metal oxides. The oxygen vacancy due to different electric states between Ce⁴⁺ and M^x+ is another essential factor for the enhanced activity.^16,21 Thereinto, Ce–Mn composite oxides with tunable structure and size have been often used for VOCs oxidation because of the low costs, improved catalytic performances, and high thermal stability. For example, Wang et al. synthesized CeO₂–MnO_x and proved that the synergistic effect between Mn and Ce is the decisive factor on its excellent activity for benzene oxidation.²² Zhao et al. prepared a series of Ce_aMnO_x microspheres with arbutus-like hierarchical structure by coprecipitation. The Ce_0.03MnO_x showed best catalytic activity for toluene oxidation with high stability and water resistance, due to the Mn⁴⁺ species and abundant surface oxygen.²³

As 3D nano/micro-structured materials (including hollow nanospheres) possess high surface area and low density, they exhibit some advantages superior to corresponding bulk materials, which are beneficial for their application in catalysis, drug delivery, supercapacitors and so on.^23–27 Although little progress has been made in terms of synthesis and catalytic performance of CeO₂–MnO_x hollow nanospheres, the as-obtained materials could not possess homogeneous structure, including uniform morphology, diameter, and interior architecture.^28,29 In this paper, we prepared well-defined CeMnO_x hollow nanospheres with different amounts of MnO_x by the combination of template method and interfacial reaction. Toluene was chosen as the target contaminant for catalytic oxidation. By tuning the content of MnO_x in the catalysts, the effect of Ce/Mn ratio on catalytic activity was evaluated. Our results demonstrated that CeMnO_x-3 with moderate content of Mn (15 wt%) showed low temperature for complete combustion of toluene (280 °C) and high stability for 36 h with toluene conversion above 97.7%. It also exhibited good water tolerance and recyclability under the condition of 5 vol% H₂O. Based on the study of catalysts, including structure, morphology, element states, and catalytic performance, the synergistic effect between Ce and Mn was proposed to play a critical role in the improvement of catalytic performance.

2 Experimental

The detailed Experimental section is described in the ESI.†

3 Results and discussion

3.1 Composition and structure

XRD was conducted to characterize the crystalline structures of as-prepared catalysts. As displayed in Fig. 1, the peaks for all the samples are similar and could be indexed to fluorite CeO₂ (JCPDS no. 34-0394) and no diffraction peak of MnO_x species is observed. Meanwhile, compared with the prominent reflection peaks (111 and 220) of CeO₂, a slight shift to higher Bragg angles is observed for the CeMnO_x binary oxides. For the radius of Mn³⁺ (0.064 nm) and Mn⁴⁺ (0.060 nm) are smaller than those of Ce³⁺ (0.134 nm) and Ce⁴⁺ (0.114 nm), the spacing between crystal planes (d) of CeO₂ decreased when the manganese ions substituted cerium ions. Therefore, the value of θ for CeMnO_x is calculated to be higher than that of CeO₂ from Bragg's law: 2d sin θ = kλ.³⁰ This shift also illustrates that Mn has successfully migrated into the CeO₂ lattice through interfacial reaction due to the close contact between Ce^δ+ and Mn^δ+ during the calcination process. The introduction of Mn species could modify the crystallization of CeO₂, and form Mn–Ce solid solution. The difference in ionic radius means that the defects in the CeO₂ lattice are easily formed, including oxygen vacancies that can provide active centers for the creation of reactive oxygen species.³¹

Fig. 1 XRD patterns of CeO₂ and the as-prepared CeMnO_x hollow sphere catalysts.

The synthesis process of CeMnO_x composite included three main steps and was illustrated in Scheme 1. Firstly, homogeneous SiO₂@CeO₂ was synthesized by the solvothermal method.^32–34 In this step, uniform SiO₂ spheres with a diameter of about 250 nm (Fig. S1 in the ESI†) were used as templates, and CeO₂ nanoparticles were covered on the surface of SiO₂ spheres by the self-assembly method. From XRD in Fig. S2,† we can see that the nanoparticles on SiO₂ were CeO₂ even though the broad diffraction peaks illustrate its low crystallinity, indicating CeO₂ could be obtained just after the solvothermal treatment of Ce(NO₃)₃ directly. In the second step, SiO₂@CeO₂ was dispersed in KMnO₄ solution to coat the MnO_x layer. Although most of Ce in SiO₂@CeO₂ existed in the form of Ce⁴⁺ with high valence, there was still a small amount of Ce³⁺ coming from the transformation of Ce⁴⁺ due to the excellent redox behavior of CeO₂. KMnO₄ with strong oxidation ability would oxidize this tiny amount of Ce³⁺ to CeO₂ and be reduced to MnO_x itself. This reaction can also be indirectly observed by the color change of the reaction system from purple to dark brown, corresponding to the color of MnO₄⁻ and MnO_x, respectively. In addition, the rough surface formed by the accumulation of CeO₂ nanoparticles in the first step could increase the contact between KMnO₄ and Ce(III) and accelerate the surface interaction. At last, the SiO₂ core was etched with NaOH solution and CeMnO_x hollow spheres were obtained.

Scheme 1 Schematic illustration for the preparation of CeMnO_x hollow spheres.

ICP-AES was used to ascertain the amount of Mn in the CeMnO_x catalysts. The actual contents of Mn in CeMnO_x catalysts increase with the concentration of KMnO₄ solution. It should be noticed that the preparation method involves the oxidation of Ce³⁺ on the surface of CeO₂ with KMnO₄ to obtain MnO_x. As the amount of Ce³⁺ in CeO₂ is fixed, the content of MnO_x reaches saturation for CeMnO_x-4 (19.0%), which is close to sample 5, 18.90%.

Raman spectra of catalysts CeO₂ and the CeMnO_x are shown in Fig. 2. The band at 460 cm⁻¹ is assigned to the triply degenerated F_2g active mode of CeO₂, which reflects the symmetric breathing pattern of oxygen atoms around cerium ions in the fluorite CeO₂.³⁵ With the increase of Mn contents, the F_2g band shifts to lower wavenumbers (red shift) gradually, suggesting the variations of Ce–O bonding symmetry. The red shifts may attribute to the lattice contraction^36,37 and consequent oxygen vacancies,³⁸ which both originated from the partial incorporation of Mn ions with smaller radius into fluorite CeO₂. No Raman peaks for MnO_x species (e.g. Mn₃O₄) could be observed in the CeMnO_x catalysts, which reveals that the MnO_x in the binary oxides is highly dispersed, in accordance with XRD results.^39–41

Fig. 2 Raman spectra of CeO₂ and the as-prepared CeMnO_x catalysts.

N₂ adsorption–desorption isotherms of CeO₂ and CeMnO_x are displayed in Fig. 3. All the catalysts present type II isotherms with H3-type hysteresis loops. According to IUPAC classification, these isotherms suggest the presence of slit-shaped micropores in the samples. The Brunauer–Emmett–Teller (BET) surface areas of the catalysts in Table 1 show an obvious downtrend (from 107.2 to 23.8 m² g⁻¹) opposite to that of Mn content, indicating the addition of Mn leads to a more compact structure, which could be confirmed by SEM images in the following section.

Fig. 3 N₂ adsorption/desorption isotherms curves of CeO₂ and the as-prepared CeMnO_x catalysts.

Table 1 Textural parameters of CeO₂ and CeMnO_x catalysts

	CeO2	CeMnOx-1	CeMnOx-2	CeMnOx-3	CeMnOx-4	CeMnOx-5
a Based on ICP results.
Mn ratio^a (%)	—	1.7	8.6	14.9	19.0	18.9
SBET (m^2 g^−1)	107.2	108.8	60.0	44.4	35.7	23.8

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As CeMnO_x-3 exhibited the best catalytic activity among all the CeMnO_x catalysts, it was chosen as an example to study the morphologies by SEM and the images of other samples were given in Fig. S3.† The SiO₂@CeMnO_x-3 before NaOH treatment are composed of large-scale uniform and well-dispersed microspheres with 250–400 nm in diameter (Fig. 4a). The surfaces of the microspheres are rough and consists of small particles, indicating that CeO₂ nanoparticles has deposited on the SiO₂ templates successfully. After removing of SiO₂ templates with NaOH treatment, CeMnO_x-3 hollow spheres can be obtained (Fig. 4b), which inherit the morphologies of their precursors well. A few cracked spheres in Fig. 4b and c reveal their hollow features clearly. A high magnified SEM image of a ruptured CeMnO_x-3 microsphere in Fig. 4d reveals that the thickness of the hollow sphere is about 50 nm and the sphere is composed of abundant nanoparticles, contributing to a high specific surface area. For SiO₂@CeO₂ before NaOH treatment, the surfaces were composed of nanoparticles with 20 nm in diameter (Fig. S3a†). With careful observation, we can see that the SiO₂@CeO₂ nanospheres are loosely structured, with some areas showing a tendency to delaminate, indicating the weak interaction between the nanoparticles. For this reason, the shells of CeO₂ hollow sphere are fragile and easily to be broken after NaOH treatment, which is indicated by many broken spheres in Fig. S3b.† With the increase of Mn content, the ratio of broken spheres decreases and the surface of the spheres gets smoother and more compact, indicating the interaction between nanoparticles is enhanced, which is also the reason for the downward trend in the specific surface area (Fig. S3c–f†).

Fig. 4 SEM images of SiO₂@CeMnO_x-3 (a) and CeMnO_x-3 (b–d).

TEM images in Fig. 5a, e and i confirm the hollow character of the CeMnO_x catalysts with uniform diameter and thickness, in consistent with the SEM images. From the TEM images of CeMnO_x-1 in Fig. 5b and c, we can see some large nanocrystals with high crystallinity on the surface. For CeMnO_x-3, large nanoparticles on the surface disappear and the structure of the sphere gets puffy (Fig. 5f and g). The thickness of the hollow sphere increases obviously from CeMnO_x-3 to CeMnO_x-5, and the structure becomes more compact (Fig. 5j). For the HRTEM images of the three catalysts, there is apparent lattice fringe with a spacing of about 0.31 nm, corresponding to the (111) plane of face-centered cubic CeO₂. No lattice fringe indexed to MnO_x species can be observed, but there are some areas with amorphous structure (marked with red circles in Fig. 5c, g and k), which attributes to part of Mn species that have not entered the CeO₂ lattice. We can see that there are more amorphous MnO_x particles that well dispersed on the surface of CeMnO_x-3 than those in CeMnO_x-1. But with a further increase of Mn content, CeMnO_x-5 is nearly covered by a layer of amorphous MnO_x particles, illustrating the high amount of Mn (Fig. 5k). The diffraction points and rings in SAED in Fig. 5d prove the coexistence of pure CeO₂ and few CeMnO_x with low crystallinity. For CeMnO_x-3, the diffraction points that indexed to CeO₂ was hard to be observed, further confirming the formation of a large amount of amorphous MnO_x on the surface of CeMnO_x solid solution (Fig. 5h). The diffraction points disappeared totally for CeMnO_x-5, caused by the coverage of the MnO_x layer on CeO₂. The structural differences would affect the catalytic activity (Fig. 5l).

Fig. 5 TEM images and SAED patterns of CeMnO_x-1 (a–d), CeMnO_x-3 (e–h) and CeMnO_x-5 (i–l).

3.2 Chemical states of the catalysts

The chemical compositions of CeO₂ and CeMnO_x were investigated by XPS (Fig. 6). Ce 3d spectra (Fig. 6a) are fitted to eight peaks by deconvolution and labelled as U and V, contributing to 3d_5/2 and 3d_3/2 spin–orbit components of Ce, respectively.^42,43 These peaks marked with V, V′′, V′′′, U, U′′ and U′′′ are attributed to the Ce⁴⁺ species while V′ and U′ are assigned to the Ce³⁺ species. The relative content of Ce³⁺ can be calculated by Ce³⁺/(Ce³⁺ + Ce⁴⁺) based on the peak area and the results are shown in Table 2. The relative surface Ce³⁺ content increases slightly, reaches a maximum for CeMnO_x-3. According to literatures, toluene catalytic oxidation on CeO₂ and MnO_x usually conforms to Mars–van-Krevelen (MvK) mechanism, in which oxygen atoms on the surface of the catalysts are key factors as they participate in the reaction.³⁸ So, the concentrations of oxygen vacancies and surface adsorbed oxygen species on the catalysts should be investigated deeply. Generally, nonstoichiometric Ce³⁺ species in CeO₂ leads to the generation of crystal defects and oxygen vacancies to maintain electrostatic balance following the reaction: 4Ce⁴⁺ + O²⁻ → 4Ce⁴⁺ + 0.5O₂ + 2e⁻/□ → 2Ce⁴⁺ + 2Ce³⁺ + 0.5O₂ + □ (□ represents oxygen vacancy).^38,44–47 Therefore, higher Ce³⁺ content indicates more oxygen vacancies, contributing to superior redox properties and excellent catalytic activity. The corresponding XPS spectra of Mn 2p for CeMnO_x are deconvoluted into three main peaks with binding energies at 640.5, 641.7 and 643.0 eV (Fig. 6b), which are assigned to the spin–orbital of Mn²⁺, Mn³⁺ and Mn⁴⁺ species, respectively. The results in Table 2 show that CeMnO_x-3 has the largest concentration of Mn with a lower valence state (52.5%). The existence of more Mn²⁺ and Mn³⁺ also imply the formation of more oxygen vacancies, similar to the effect of Ce³⁺.

Fig. 6 XPS spectra of Ce 3d (a), Mn 2p (b) and O 1s (c) for CeO₂ and CeMnO_x-1, CeMnO_x-3 and CeMnO_x-5.

Table 2 The XPS fitting results of the catalysts

Catalysts	Surface element ratio (%)
CeO2	14.6	—	48.8
CeMnOx-1	16.9	41.0	49.3
CeMnOx-3	20.9	52.5	88.5
CeMnOx-5	19.5	49.5	61.4

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All the O 1s spectra in Fig. 6c can be deconvoluted into three peaks located at about 529.6, 530.5, and 533.1 eV, attributing to the lattice oxygen (O_I), adsorbed oxygen with low coordination (such as O₂²⁻, O²⁻ and O⁻, denoted as O_II), and carbonates (O_III), respectively.^40,48 The percentages of adsorbed oxygen for all the catalysts exhibit the same trend with Ce³⁺ and Mn^δ+ with lower valence states, consistent with the conclusion that Ce and Mn species with lower valence states are benefit for increasing the amount of surface adsorbed oxygen.⁴⁹ Strong interaction between Ce and Mn weakens the Ce–O bonds located at the interface of MnO_x and CeO₂, and increases the mobility of lattice oxygen. As a result, oxygen vacancies can be easily formed and surface adsorbed oxygen species are enhanced consequently, which are benefit for toluene oxidation according to the MvK mechanism.

3.3 Reducibility of the catalysts

Redox properties of the catalysts were investigated by H₂-TPR and the curves were exhibited in Fig. 7. CeO₂ exhibits a reduction peak at about 520 °C, which can be ascribed to the consumption of H₂ by CeO₂.⁵⁰ After the introduction of Mn, the reduction peaks of CeMnO_x-1 and CeMnO_x-2 shift to the lower temperature continually, implying that the introduction of Mn ions can improve the reduction reducibility of CeO₂ significantly. It should be noted that a broad band of H₂ consumption for CeMnO_x-2 began to occur at about 300 °C, which can be assigned to the reduction of MnO_x species.³⁶ With the increase of Mn content, CeMnO_x-3 exhibit three peaks at about 230, 285 and 354 °C, corresponding to the reduction of Mn⁴⁺ to Mn³⁺, Mn³⁺ to Mn²⁺ and partial reduction of surface ceria, respectively.^50–52 For CeMnO_x-4 and CeMnO_x-5, even the peaks shift to high temperature again, they are still much lower than that of pure CeO₂. It is reported that –Mn–O–Ce– bond of CeMnO_x hollow spheres can reduce the oxygen vacancy formation energy remarkably, and then strengthen the mobility of oxygen species from the bulk to the surface when compared with pure ceria.⁵³ These results indicate that the addition of an appropriate amount of Mn into the CeO₂ lattice can lead to high mobility and XPS results. Combined with all these results above, it is reasonable to speculate that the CeMnO_x can exhibit better promote the release of the lattice oxygen, inconsistent with the catalytic activity than pure CeO₂, and CeMnO_x-3 should be the best one.

Fig. 7 H₂-TPR profiles of CeO₂ and the as-prepared CeMnO_x catalysts.

3.4 Catalytic activity

The catalytic oxidation of toluene over the hollow structured CeMnO_x catalyst is present in Fig. 8a. In general, the activities of all catalysts reveal a similar positive correlation with the reaction temperature. CeO₂ shows the lowest catalytic activity, whose T₅₀ and T₉₀ are as high as 310 and 369 °C, respectively. After the introduction of Mn, all the CeMnO_x exhibit better performance than CeO₂, with the following order: CeMnO_x-3 > CeMnO_x-4 ≈ CeMnO_x-5 > CeMnO_x-2 > CeMnO_x-1 (deducing by T₅₀ and T₉₀, marked with red lines in Fig. 8a). This result demonstrates that the introduction of Mn can improve the catalytic activities due to the synergistic effect between Ce and Mn. Especially for CeMnO_x-3, the complete conversion temperature of toluene is only 280 °C, while the conversion is only 8.9% for CeO₂ at the same temperature.

Fig. 8 (a) Toluene conversions as a function of reaction temperature and (b) Arrhenius linear fit plots for toluene oxidation over the catalysts.

It is reported that with excess oxygen, the catalytic oxidation of toluene follows first-order kinetics with the toluene concentration.^34,54 When the toluene conversion is less than 20%, the ln r dependence of 1/T can be used to calculate apparent activation energies (Fig. 8b). For clarification, the reaction rates and activation energies calculated from the slope of the linear fit of the Arrhenius scatter plots were listed in Table 3. The reaction rate over CeMnO_x-3 is 0.141 μmol g⁻¹ s⁻¹, the highest among these catalysts. E_a for CeMnO_x-3 is the lowest and increased with the order: CeMnO_x-3 < CeMnO_x-4 < CeMnO_x-2 < CeMnO_x-5 < CeMnO_x-1 < CeO₂. The highest reaction rate and lowest E_a value for CeMnO_x-3 both confirm its excellent performance on toluene oxidation.

Table 3 r and E_a for toluene combustion over CeO₂ and CeMnO_x catalysts

Catalysts	r^a (μmol g^−1 s^−1)	Ea (kJ mol^−1)
a The values of r for CeO2 and CeMnOx-1 were obtained at 250 °C and the others were obtained at 200 °C.
CeO2	0.025	84
CeMnOx-1	0.055	79
CeMnOx-2	0.087	66
CeMnOx-3	0.141	51
CeMnOx-4	0.097	59
CeMnOx-5	0.070	71

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3.5 Structure–activity relationships

The activation ability of oxygen molecules for catalysts is an essential factor in evaluating their catalytic activity. It has been proved that the surface oxygen vacancies are favourable for the adsorption and activation of O₂.³⁷ Therefore, the catalysts with high oxygen vacancies may exhibit good catalytic performance. XPS spectra (Fig. 6) confirm that after the addition of Mn, the content of Ce³⁺ species increase and give rise to more oxygen vacancies, favourable for the activation of O₂. Accordingly, O 1s spectra of CeMnO_x confirm the existence of more surface-absorbed oxygens compared with CeO₂, which would increase the redox property of CeMnO_x catalysts significantly. According to the MvK mechanism, O_latt (O²⁻) on the surface of the catalyst are the active species. After being consumed of O²⁻ in the oxidation, they can be regenerated by migration of electrophilic surface-absorbed oxygens (O₂²⁻, O²⁻ and O⁻) into oxygen vacancies with the dissociation of O₂ in the feed continually (Scheme 2). Therefore, CeMnO_x-3 with the most Ce³⁺ species and surface-absorbed oxygens showed the best catalytic activity. However, when Mn continues to increase, the surface MnO_x species would increase while the Ce³⁺ content decreases gradually (Table 2). From Fig. 5k, there was even a layer of amorphous MnO_x that covered the solid solution CeMnO_x, inhibiting the contact between –Mn–O–Ce– and O₂. Therefore, the surface-absorbed oxygens and catalytic activity decreased correspondingly.

Scheme 2 Schematic illustration for the synergy effect of Ce and Mn in the CeMnO_x catalyst.

3.6 The stability test and water resistance

The stability of the catalysts is also a critical factor for practical application. Fresh CeMnO_x-3 was tested for the toluene conversion (inlet concentration 1000 ppm) as a function of time on stream at 280 °C. As shown in Fig. 9a, the catalyst was relatively stable for 10 h, with 100% toluene conversion. In the next 10 h, the conversion only exhibits a slight decrease, and is stable at 99%. After reacting for 20 h, the reaction system was cooled to room temperature and reheated to 200 °C for reusability test. It can be seen that in the second cycle, there is no apparent change in toluene conversion. Even after the third test, the toluene conversion can maintain 97.7%. Obviously, the catalyst has excellent catalytic stability and reusability for the oxidation of toluene, endowing it the possibility of future industrialization.

Fig. 9 Catalytic stability without (a) and with (b) water (10 vol%) of CeMnO_x-3 for toluene oxidation at 280 °C.

In practical application, moisture tolerance of the catalyst is another important factor to be considered as VOCs gas in the industry often contains water. Therefore, the influence of vapor on the long-term catalytic activity of CeMnO_x-3 under the feed gas stream including 5 vol% of water at 280 °C was investigated and the result was given in Fig. 9b. In the absence of water vapor, the toluene conversion maintained 100%, consistent with the results in Fig. 9a. After introducing water vapor into the reaction stream, the toluene conversion decreased to 95% directly and remained stable afterward. Once the water vapor was stopped, the conversion recovered to 97%. For the subsequent two cycles, the catalytic activity showed a similar trend and the toluene conversion was above 90% throughout the test. Finally, the toluene conversion could recover to 97% after the water vapor was out. Therefore, the catalyst owned excellent water vapor tolerance, which is a favorable factor for achieving industrial application.

4 Conclusions

In this work, we combined the template method and interfacial reaction to prepare a series of CeMnO_x binary oxide hollow spheres with uniform morphology, diameter and thickness. Mn was incorporated by an interfacial redox reaction between KMnO₄ and Ce elements with low valence on the surface of SiO₂@CeO₂. The as-prepared binary oxides exhibit better catalytic activities than CeO₂ hollow spheres. CeMnO_x-3 exhibits the lowest T₉₀ (nearly 100 °C lower than that of CeO₂), excellent stability and H₂O resistance even with WHSV of 120 000 h⁻¹. Redox properties and surface oxygen vacancies of the binary oxide catalysts are improved as the addition of Mn would cause strong interaction between Ce and Mn, increasing of surface-adsorbed oxygen species, which are critical to the enhanced catalytic performance.

Author contributions

Yuhua Zheng: methodlogy, analysis, writing and editing. Jing Zhou: synthesis. Xi Zeng and Dandan Hu, investigation and characterization. Fang Wang: data curation. Yanbin Cui: supervision, funding acquisition and review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the financial support from National Natural Science Foundation of China (No. 22076189) and the Open Research Fund Program of Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry (No. CP2021YB06).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, XRD of SiO₂@CeO₂, SEM images of SiO₂ and CeMnO_x. See https://doi.org/10.1039/d2ra04678d

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