New insights into the effect of morphology on catalytic properties of MnO_x–CeO₂ mixed oxides for chlorobenzene degradation

Abstract

We synthesized four CeO₂–MnO_x mixed oxides with different morphologies using simple hydrothermal methods. The catalytic activity for chlorobenzene (CB) degradation decreases in the following order: rod-CeO₂–MnO_x > plate-CeO₂–MnO_x > polyhedra-CeO₂–MnO_x > cube-CeO₂–MnO_x. CeO₂ and MnO_x in the mixed oxides are highly dispersed and two new phases of both todorokite (S.G.: P2/m:b) and vernadite (S.G.: I4/m) with a special tunnel-like structure are found. Both rod-CeO₂–MnO_x and plate-CeO₂–MnO_x exhibit increased lattice microstrains generated from lattice distortion and defects; further, there are more oxygen vacancies and more MnO_x (Mn⁴⁺ and Mn²⁺) species on the surface, particularly when compared to cube-CeO₂–MnO_x. Therefore, this promotes deeper oxidation activity for CB. Moreover, the strong interaction between CeO₂ and MnO_x also promotes the redox ability of CeO₂–MnO_x mixed oxides, while their oxygen storage capacity (OSC) properties are not only intrinsic to their structures but also limited to their surfaces and by their particle sizes.

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1. Introduction

Chlorinated volatile organic compounds (Cl-VOCs) as the main air pollutants are known to be dangerous to the health of humans as well as the environment.^1,2 Catalytic combustion technology is considered to be an efficient way to control Cl-VOC emissions and is extensively used.³ So far, efforts have been made to investigate all kinds of catalysts including noble metals, zeolites, transition metal oxides, and their mixtures. Although the catalytic activity of noble metals is higher, their industrial applications are very difficult because of high cost and easy deactivation. In order to find cheaper substitutes to noble metal catalysts, the use of various mixed metal oxides has attracted much attention. Mixed oxides such as CeO₂–MnO_x,^4,5 CeO₂–CuO,⁶ CeO₂–ZrO₂,⁷ CeO₂–CrO_x,⁸ and CeO₂–TiO₂ (ref. 9) exhibit excellent catalytic properties for the deep oxidation of Cl-VOCs. Recently, metal oxide crystals with different exposed facets have attracted a lot of attention due to their unique electronic properties and higher reactivity when compared to the bulk phase. For example, Shen et al.¹⁰ found that Co₃O₄ nanorods still exhibit exemplary catalytic performances for CO oxidation even at −77 °C. Mai et al.¹¹ also found that the exposed crystalline planes of CeO₂ have an important effect on the ability of a catalyst to release and uptake oxygen as well as its catalytic performance. CeO₂ nanorods with exposed {110} and {100} facets show higher catalytic activity than CeO₂ nanoparticles with primarily exposed {111} facets toward CO oxidation, because the {110} and {100} facets form oxygen vacancies more easily than the stable {111} facet. Similarly, for other oxidation reactions, the structure and morphology of CeO₂ also displays major effects on the catalytic performance. For instance, Dai et al.¹² found that in the dichloroethane oxidation reaction, CeO₂ nanorods have higher catalytic activity than CeO₂ nanocubes and CeO₂ nanopolyhedra, and the catalytic activity decreases in the sequence of rod > cube > polyhedron. Chen et al.¹³ also found that Ce_1−xM_xO₂ (M = Ti, Zr, and Hf) nanomaterials with exposed {110}, {100}, and {111} facets also display significant morphological effects in ethanol steam reforming reaction and the catalytic performance decreases in the sequence of rod > pipe > cube. The explanation for the better catalytic performance of the metal-doped Ce_1−xM_xO₂ nanorods is not only related to their exposure to the {110} and {100} facets, but the doping of metals also significantly increases the specific surface area as well as oxygen storage capacity (OSC) of nanomaterials. Recently, CeO₂–MnO_x mixed oxides with high OSC and multiple valences have also attracted much attention, out of which mixed oxides exhibit superior catalytic activity in different reactions like the catalytic reduction of NO_x with NH₃, catalytic decomposition of NO_x, and catalytic oxidation of CO/VOCs at lower temperatures.^14–17 However, their characteristics are still under debate and meaningful to investigate.

In the present study, four CeO₂–MnO_x mixed oxides with different morphologies (rods, plates, polyhedra, and cubes) were synthesized using simple hydrothermal methods and evaluated for the deep oxidation performance of chlorobenzene (CB) as typical Cl-VOCs. The mixed oxides were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), N₂ adsorption–desorption, and temperature-programmed reduction (H₂-TPR) techniques. The structural properties of CeO₂–MnO_x catalysts were investigated by XRD Rietveld refinement in order to obtain new information about the morphological effect.

2. Experimental

2.1 Catalysts' preparation

Nanorod and nanocube CeO₂–MnO_x mixed oxides were synthesized by simple hydrothermal methods.¹⁸ Here, 1.16 g Ce(NO₃)₃·6H₂O and 0.24 g Mn(NO₃)₂ were added into 20 mL deionized water. In a Teflon-lined autoclave, the mixture was slowly added into 60 mL NaOH solution with different concentrations (rod: 4 mol L⁻¹; cube: 6 mol L⁻¹) under vigorous stirring for 30 min and then heated to different temperatures (rod: 100 °C; cube: 180 °C) for 24 h (rod) or 12 h (cube).

The nanoplate CeO₂–MnO_x catalyst was synthesized by a CTAB-assisted hydrothermal method.¹⁹ Here, 2.61 g Ce(NO₃)₃·6H₂O, 0.54 g Mn(NO₃)₂, and 0.73 g CTAB were added into 70 mL distilled water. Afterwards, 10 mL NH₃·H₂O was slowly added dropwise into the mixed solution under vigorous stirring for 30 min. Then, the mixture was transferred to a Teflon-lined autoclave and heated to 100 °C for 24 h.

The nanopolyhedra CeO₂–MnO_x catalyst was synthesized by the method reported in the literature.²⁰ Here, 3.47 g Ce(NO₃)₃·6H₂O, 0.72 g Mn(NO₃)₂, and 1.0 g polyvinyl pyrrolidone (PVP; molecular weight = 3000 g mol⁻¹) were added into an autoclave with 40 mL deionized water under vigorous stirring for 30 min. Then, 10 mL N₂H₄·H₂O was slowly added dropwise into the mixed solution under vigorous stirring for 30 min and heated to 180 °C for 12 h.

The above precipitates were obtained after centrifuging and washing in turn by distilled water and ethanol. The precipitated solids were dried at 60 °C overnight and then calcined in air at 500 °C for 2 h. Afterwards, the samples were sieved to 40–60 meshes (0.3–0.45 mm) and labeled as rod-CeO₂–MnO_x, cube-CeO₂–MnO_x, plate-CeO₂–MnO_x, and polyhedral-CeO₂–MnO_x. The molar ratio of Ce/Mn is 2 : 1 in all the samples.

2.2 Catalysts' characterization

XRD was performed on ARLX’TRA apparatus (Cu Kα radiation, 250 mA, and 40 kV) with 2θ = 10–100°. The XRD Rietveld refinements of the catalysts were performed with Maud software to obtain the microstructure data and the pseudo-Voigt profile function was used to qualitatively and quantitatively analyze the structure.

UV-Raman spectra were recorded on a UV-HR Raman spectrograph apparatus equipped with a laser at 325 nm. The range of the Raman spectra was 100–1000 cm⁻¹ and the spectral resolution was 4 cm⁻¹.

The XPS spectra were recorded on a Thermo K-Alpha apparatus equipped with 84 W Al Kα radiation. The binding energies (BEs) of various elements were calibrated using the C 1s peak (284.6 eV).

HR-TEM images were obtained using a TECNAI G220 apparatus at 200 kV. In order to identify the chemical composition, energy dispersive spectrometry (EDS) analysis was performed.

H₂-TPR experiment was carried out on a quartz fixed-bed microreactor equipped with a TCD detector. Before testing, 50 mg of the sample was pretreated in N₂ flow (30 mL min⁻¹) at 200 °C for 30 min and then cooled down to 100 °C. After stabilization, TPR experiments were performed from 100 to 600 °C at a heating rate of 10 °C min⁻¹ under 5 vol% H₂/Ar flow (40 mL min⁻¹).

The OSC experiment was carried out on a CHEMBET-3000 apparatus (Quantachrome Co.) equipped with a TCD detector using the pulse injection of CO. Before testing, the samples were pretreated under H₂ flow (30 mL min⁻¹) at 500 °C for 2 h and then cooled down to 400 °C. After stabilization, the gas was switched to He flow (40 mL min⁻¹) for 30 min.

The specific surface areas (S_BET) were determined by N₂ adsorption/desorption isotherms at 77 K with Brunauer–Emmett–Teller (BET) theory operating on a Micrometrics TriStar II 2020 analyzer.

2.3 Catalytic activity tests

The catalytic performances of the samples employed in the oxidation of CB were measured in a fixed-bed quartz reactor with 300 mg of catalyst under atmospheric pressure. The reactant gas mixture consisted of CB (1000 ppm) and dry air with a GHSV of 15 000 h⁻¹. The outlet gas was monitored online by a gas chromatograph equipped with TCD and FID detectors. The durability test of the catalysts for DCE degradation was also evaluated. It was exposed to dry air or in the existence of water (2.3 v/v%) or benzene (500 ppm) continuously for a long time.

3. Results and discussion

3.1 Catalytic performance tests

3.1.1 Catalytic activity. The catalytic activity of the CeO₂–MnO_x mixed oxides with different morphologies for CB degradation is shown in Fig. 1A. It is obvious that the catalytic activity is significantly influenced by the morphology of CeO₂–MnO_x catalysts, and rod-CeO₂–MnO_x shows superior catalytic activity for CB destruction than the other three catalysts. It can be seen that the T_90% (temperature with CB conversion of 90%) is the highest for cube-CeO₂–MnO_x (365 °C) and is decreased to different degrees as the morphology of CeO₂–MnO_x catalysts changes from cube to polyhedral, plate, and rod, which is greatly reduced to 303 °C for rod-CeO₂–MnO_x catalyst. Moreover, in the process of CB degradation, no carbon-containing byproducts and polychloride benzene or polychlorobenzene were detected over the samples, which is different from the catalytic behavior of noble metal catalysts.^21,22

Fig. 1 (A) Catalytic activity of CeO₂–MnO_x catalysts with different morphologies and (B) rod-CeO₂–MnO_x with different CB concentrations and GHSV for CB degradation.

Fig. 1B shows the catalytic performances of rod-CeO₂–MnO_x with different CB concentrations and GHSVs. On one hand, it can be seen that when the GHSV is fixed at 15 000 h⁻¹, the catalytic activity of the catalyst gradually decreases with higher CB concentration; here, T_90% increases by about 100 °C (from 244 °C to 342 °C) as the CB concentration increases from 500 to 2000 ppm. On the other hand, the CB concentration is equal to 1000 ppm and the catalytic performance is evaluated with different GHSVs. The results indicate that higher GHSV leads to poor catalytic performance of rod-CeO₂–MnO_x for CB destruction, which is due to the shorter retention time of CB in the catalyst bed with high GHSV.

3.1.2 Durability of catalysts. In industrial applications, the deactivation of catalysts for Cl-VOCs degradation is an important evaluation index. As shown in Fig. 2, as compared to rod-CeO₂–MnO_x and plate-CeO₂–MnO_x catalysts, CB conversions over the other catalysts decrease in an obvious manner at lower temperatures for the first 5 h at the beginning of the continuous reaction and then tend toward stability. After the reaction system is injected with water steam (2.3 v/v%), CB conversion over rod-CeO₂–MnO_x catalyst decreases slightly and it increases slightly over the other catalysts; however, their catalytic activities return to the original level after cutting off the water steam. Generally, during the degradation of Cl-VOCs, the catalysts may be partially or even completely deactivated because the carbon deposits and/or the chlorine species are strongly adsorbed on the surface of the catalysts at lower temperatures. Therefore, after being treated in a stream of dry air at 400 °C for 0.5 h, the decreased activities of the CeO₂–MnO_x mixed oxides are completely recovered. According to the literature,²³ on one hand, the presence of H₂O can decrease the chloride and carbon contents on the used samples, which is beneficial to improve the catalytic performances, and on the other hand, the competitive adsorption between H₂O and CB would impair the catalytic activity for CB destruction. For rod-CeO₂–MnO_x, the reduced CB conversion indicates that the presence of H₂O severely inhibits CB oxidation because of the easy competitive adsorption on the active sites than the reactant molecules (i.e., CB). Further, for the other three catalysts, the addition of H₂O mainly promotes the movement of Cl species and carbonaceous deposits from the surface of the catalysts, yielding improved catalytic performances. Moreover, the complete recovery of the catalytic activity with the treatment of dry air at 400 °C is mainly due to the fact that dry air could remove the carbon deposits and/or chlorine species that get strongly adsorbed on the surface of the catalysts at lower temperatures during the degradation of Cl-VOCs.

Fig. 2 Durability test of CeO₂–MnO_x catalysts with different morphologies for CB degradation in dry air, H₂O (2.3 v/v%), or C₆H₆ (500 ppm) at 310 °C.

In addition, the effect of other VOCs on the catalytic activities of CeO₂–MnO_x for CB degradation is also evaluated. As shown in Fig. 2, after a certain concentration of benzene (500 ppm) was injected into the reaction system, CB conversions over the CeO₂–MnO_x catalysts decrease in an obvious manner, and the catalytic activity of all the four catalysts could not recover after removing C₆H₆. However, after being treated in dry air at 400 °C for 30 min, the decreased activities of CeO₂–MnO_x catalysts are completely recovered. According to the literature,²⁴ the presence of C₆H₆ could result in strong competitive adsorption and oxidation of C₆H₆ on the active sites of catalysts for CB destruction and increase the carbon content on the used samples, which dramatically decreases the catalytic activity of CeO₂–MnO_x catalysts.

3.2 Structural properties of catalysts

3.2.1 XRD and XRD Rietveld analyses. Efforts have been made on the XRD Rietveld refinement by using Maud software to obtain the microstructural information of CeO₂–MnO_x mixed oxides with different morphologies. XRD and Rietveld XRD patterns of the CeO₂–MnO_x mixed oxides are shown in Fig. 3, and the related results are listed in Table 1. The characteristic diffraction peaks of CeO₂ (S.G.: Fm3̄m) with a fluorite structure are observed in all the samples, but the rod-MnO_x–CeO₂ shows the broadest peak as compared to the other three catalysts, indicating that the particle size of the rod-MnO_x–CeO₂ is smaller. Except for the characteristic peaks of CeO₂, the diffraction peaks of Mn₃O₄ (S.G.: I4₁/amd) appeared at 2θ = ∼36.3° and 59.9° in all the catalysts, and the diffraction peak of MnO₂ (S.G.: I4/m) appeared at 2θ = 12.5° in rod-MnO_x–CeO₂ and plate-MnO_x–CeO₂ are also observed.^25,26 In addition, it is noteworthy that two new phases (todorokite and vernadite) with a special tunnel-like structure, which are usually ignored, are found in the MnO_x–CeO₂ mixed oxides. The diffraction peak assigned to the new phase appears at lower than 20°. The todorokite phase (S.G.: P2/m:b) exists in all the MnO_x–CeO₂ mixed oxides, while the vernadite phase (S.G.: I4/m) only exists in rod-MnO_x–CeO₂. According to the intensity of the diffraction peak at lower than 20°, this means that the content of the new phases is the highest in rod-MnO_x–CeO₂ and the lowest in cube-CeO₂–MnO_x, which is consistent with the activity of the catalysts. Therefore, the result probably implies that the existence of both todorokite and vernadite phases with a special tunnel-like structure can promote the catalytic performance for CB oxidation. On the other hand, the results of Rietveld refinements also suggest that for CeO₂–MnO_x mixed oxides, CeO₂ and MnO_x crystallites are highly dispersed within each other, indicating that Mn³⁺ or Mn²⁺ cannot enter into the CeO₂ lattice to form CeMnO_x solid solution, which is considered in the literature.^4,5 As shown in Table 1, the lattice microstrains of CeO₂ on different facets in rod-CeO₂–MnO_x are bigger than those in the other three catalysts. In particular, for the {110} and {100} facets, the lattice microstrain of rod-CeO₂–MnO_x is the highest and apparently higher than those of both polyhedra-CeO₂–MnO_x and cube-CeO₂–MnO_x. The lattice microstrain decreases in the following sequence: rod-CeO₂–MnO_x > plate-CeO₂–MnO_x > polyhedra-CeO₂–MnO_x > cube-CeO₂–MnO_x. The microstrain is generated due to lattice distortion and defects, which is favorable to forming more oxygen vacancies. Thus, the increased oxygen vacancy concentration would facilitate the process in which the active oxygen species existing on the subsurface migrate toward the surface of the catalysts, resulting in increasing catalytic activity. Moreover, the size of the CeO₂ crystallites (Table 1) can also explain the growth characteristics of the exposed crystal planes; for example, the exposed {100} (78.49 Å) and {110} (84.77 Å) planes of rod-CeO₂–MnO_x grow on the orientated attachment with a [110] growth direction. Further, this is in agreement with the results of the HRTEM (discussed below).

Fig. 3 XRD and Rietveld XRD patterns of CeO₂–MnO_x catalysts with different morphologies: (A) rod-CeO₂–MnO_x, (B) plate-CeO₂–MnO_x, (C) polyhedra-CeO₂–MnO_x, and (D) cube-CeO₂–MnO_x.

Table 1 Structural parameters deduced from Rietveld refinement of the XRD patterns for CeO₂–MnO_x catalysts with different morphologies

Samples	Component	S.G.	a/Å	b/Å	c/Å	CeO2 crystallite (Å)	R.M.S. microstrain/CeO2
Rod-CeO2–MnOx	CeO2	Fm3̄m	5.376	—	—
	MnO2	I4/m	10.049	—	2.371	{111}:79.10	{111}:0.005
	Mn3O4	I41/amd	5.765	—	9.450	{100}:78.49	{100}:0.013
	Todorokite	P2/m:b	7.905	2.237	10.535	{110}:84.77	{110}:0.008
	Vernadite	I4/m	9.445	—	2.855
Plate-CeO2–MnOx	CeO2	Fm3̄m	5.381	—	—	{111}:86.10	{111}:0.003
	MnO2	I4/m	10.049	—	2.371	{100}:80.49	{100}:0.009
	Mn3O4	I41/amd	5.765	—	9.450	{110}:82.75	{110}:0.006
	Todorokite	P2/m:b	7.916	2.248	10.533
Polyhedra-CeO2–MnOx	CeO2	Fm3̄m	5.389	—	—	{111}:105.62	{111}:0.004
	Mn3O4	I41/amd	5.765	—	9.450	{100}:103.79	{100}:0.005
	Todorokite	P2/m:b	7.989	2.253	10.563	{110}:113.74	{110}:0.003
Cube-CeO2–MnOx	CeO2	Fm3̄m	5.398	—	—	{111}:116.62	{111}:0.004
	Mn3O4	I41/amd	5.765	—	9.450	{100}:107.73	{100}:0.004
	Todorokite	P2/m:b	7.934	2.247	10.559	{110}:110.29	{110}:0.003

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3.2.2 UV-Raman results. The UV-Raman spectra provided information about the vibration of oxygen lattices. The UV-Raman profiles and T_90% values versus A_γ/A_β ratios of CeO₂–MnO_x mixed oxides with different morphologies are shown in Fig. 4A and B, respectively. In Fig. 4A, three main peaks appearing at 342 cm⁻¹ (α), 471 cm⁻¹ (β), and 595 cm⁻¹ (γ) can be observed, which is the same as that in the literature.^27,28 The weak peak α is assigned to an intermediate phase named as t′′, between t′ and cubic CeO₂, which is metastable. Peak β is related to the F_2g vibration mode of the cubic fluorite-type structure. The shift in the broad F_2g band implies that the ceria species apparently possess a distorted structure, which can cause the generation of oxygen vacancies in ceria.²⁹ Peak γ is assigned to the oxygen vacancies in CeO₂, which is favorable to the defective structure in CeO₂. Usually, the intensity ratio (A_γ/A_β) of the peaks between 595 cm⁻¹ and 471 cm⁻¹ is calculated to show the relative oxygen vacancies concentration. From Fig. 4B, rod-CeO₂–MnO_x shows a much higher A_γ/A_β ratio (2.49), while the A_γ/A_β ratio for cube-CeO₂–MnO_x is only 69% of that for rod-CeO₂–MnO_x. The A_γ/A_β ratio decreases in the following order: rod-CeO₂–MnO_x (2.49) > plate-CeO₂–MnO_x (2.22) > polyhedra-CeO₂–MnO_x (2.07) > cube-CeO₂–MnO_x (1.72). This suggests that the oxygen vacancies concentration in the rod-CeO₂–MnO_x catalyst is more than that of the other three catalysts, which is in good agreement with the XRD Rietveld refinement results. The oxygen vacancies with high concentration would promote the diffusion of active oxygen species from bulk phase to the surface of the catalysts, which promotes the catalytic activity of CeO₂–MnO_x mixed oxides for CB oxidation.

Fig. 4 UV-Raman spectra (A) and T_90% values versus A_γ/A_β ratios (B) of CeO₂–MnO_x catalysts with different morphologies.

3.2.3 XPS results. The XPS data of the CeO₂–MnO_x mixed oxides with different morphologies are given in Table 2. As shown in Table 2, rod-CeO₂–MnO_x has a higher relative concentration of Ce³⁺ in Ce than the other catalysts and the ratio of Ce³⁺/Ce⁴⁺ is in the sequence of rod-CeO₂–MnO_x (0.48) > plate-CeO₂–MnO_x (0.42) > polyhedra-CeO₂–MnO_x (0.36) > cube-CeO₂–MnO_x (0.28). Generally, there are some direct relationships between the presence of Ce³⁺ and the generation of oxygen vacancies. Thus, the above result is also in good agreement with the UV-Raman results. As for the Mn element, the ratio of Mn/Ce in rod-CeO₂–MnO_x is much higher than its theoretical value (0.50) and the content of MnO₂ is also higher than that in the other catalysts. The value of Mn/Ce decreases in the order of rod-CeO₂–MnO_x (0.70) > plate-CeO₂–MnO_x (0.52) > polyhedra-CeO₂–MnO_x (0.37) > cube-CeO₂–MnO_x (0.29). This result indicates that Mn enrichment and the presence of more Mn⁴⁺ species on the surface of the catalysts would increase the catalytic performances of CeO₂–MnO_x mixed oxides for Cl-VOC oxidation. Moreover, there are more Mn²⁺ species in rod-CeO₂–MnO_x and plate-CeO₂–MnO_x catalysts, which is related to the generation of oxygen vacancies owing to retaining the charge neutrality in the oxides.³⁰ Thus, the reduction of Ce⁴⁺ to Ce³⁺ may also be promoted due to the presence of the redox cycle of Mn²⁺–Mn⁴⁺, which would promote the migration of lattice oxygen as well as the catalytic performance. On the other hand, it suggests that there is a stronger interaction between MnO_x and rod/plate-CeO₂ species in rod-CeO₂–MnO_x and plate-CeO₂–MnO_x, which is beneficial toward the formation of more Mn⁴⁺ and Ce³⁺ species, further promoting the deeper oxidation of Cl-VOCs. In addition, with respect to the O element, from Table 2, there are two different oxygen species:^31,32 lattice oxygen (O_latt) and surface oxygen species (O_sur) such as hydroxyl, carbonate species, adsorbed oxygen (O⁻/O₂²⁻), and adsorbed water on the surface. It is noteworthy that the value of O_sur/O_latt decreases in the sequence of rod-CeO₂–MnO_x (0.67) > plate-CeO₂–MnO_x (0.59) > polyhedra-CeO₂–MnO_x (0.45) > cube-CeO₂–MnO_x (0.35), indicating that high O_sur species content on the surface of CeO₂–MnO_x mixed oxides would promote the deeper oxidation of CB.

Table 2 Surface elemental composition and the oxidation state of Ce and Mn measured by XPS

Samples	Surface composition (at%)			Mn distribution (at%)			O distribution (at%)		Mn/Ce	Ce^3+/Ce^4+
	Ce 3d	Mn 2p	O 1s	Mn^4+	Mn^3+	Mn^2+	Olatt	Osur
Rod-CeO2–MnOx	16.12	11.25	72.63	6.04	1.49	3.56	43.47	29.16	0.70	0.48
Plate-CeO2–MnOx	19.09	9.85	71.06	4.89	1.40	3.45	44.71	26.35	0.52	0.42
Polyhedra-CeO2–MnOx	23.45	8.66	67.89	4.21	2.93	1.42	46.75	21.14	0.37	0.36
Cube-CeO2–MnOx	26.79	7.72	65.49	4.74	2.08	0.84	48.37	17.12	0.29	0.28

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3.3 Morphologies of catalysts

As shown in Fig. 5, CeO₂–MnO_x mixed oxides with different morphologies were successfully synthesized. Fig. 5A and B show that rod-CeO₂–MnO_x catalyst is a long rod-like nanoparticle with dimensions of (10.5 ± 1.6) nm × (50–200) nm, clearly showing three different lattice plane spacings ascribed to the {111} (0.31 nm), {002} (0.28 nm), and {110} (0.19 nm) facets. Interestingly, when observed and calculated along the long attitudinal axis, the plane-intersecting angle of 45° further proves that rod-CeO₂–MnO_x mainly exposes the {100} (0.26 nm) facet and preferentially grows along the direction of the {110} facet.³³ Fig. 5C and D reveal that plate-CeO₂–MnO_x is a rhombic plate with a diameter of 18 nm and thickness of ∼4 nm, exposing two facets, namely, {111} (0.31 nm) and {200} (0.27 nm). From Fig. 5E, polyhedra-CeO₂–MnO_x is an irregular hexagonal nanoparticle, which comprises truncated octahedra with an average size of about 20 ± 1.5 nm. From Fig. 5F, the polyhedra-CeO₂–MnO_x shows {200}, {111}, and {220} facets corresponding to the interplanar spacings of 0.26, 0.33, and 0.19 nm, respectively, indicating that it exposes the {111} and {100} facets.³⁴ From Fig. 5G and H, it can be seen that cube-CeO₂–MnO_x is a more uniform cube block with a size of 20–25 nm and has a bigger particle size than the others. Further, it is only enclosed by the {100} facet. Generally, the exposed crystal face plays an important role in the catalytic performance of the catalysts. The {111} facet is much more stable than the {110} and {100} facets with a higher surface energy; the former is inactive as compared to the latter facets. Therefore, rod-CeO₂–MnO_x that mainly exposed the {100} facet exhibits higher catalytic activity for CB degradation. Although cube-CeO₂–MnO_x is enclosed by the {100} facets, it has a bigger particle size and lower concentration of oxygen vacancies, resulting in a dramatic decrease in the catalytic activity for CB oxidation in the low-temperature range. Moreover, the chemical composition is also identified by high-resolution EDS. As shown in Fig. 5A, manganese and cerium species are detected in rod-CeO₂–MnO₂, implying the existence of Mn species on the surface of CeO₂.

Fig. 5 TEM and HRTEM images of CeO₂–MnO_x catalysts with rod (A and B), plate (C and D), polyhedral (E and F) and cube (G and H) morphologies.

3.4 Redox properties of catalysts

The redox properties of CeO₂–MnO_x mixed oxides with different morphologies were tested by H₂-TPR, and the related profiles are shown in Fig. 6. As shown in Fig. 6, for pure rod-CeO₂, the reduction peak below 600 °C is rather weak, which corresponds to the reduction of two kinds of oxygen species that are present on the surface and subsurface. It implies that CeO₂ represents poor oxidation performance at lower temperatures. For CeO₂–MnO_x mixed oxides, except for polyhedra-CeO₂–MnO_x, two obvious reduction peaks (α and β) are observed, which belong to the reduction of MnO_x species and oxygen species on the surface/subsurface of CeO₂. Generally, at lower temperatures, peak α corresponds to the reduction of MnO_x species that are highly dispersed and interact with ceria. As for peak β, it may be attributed to the combined reduction of Mn₂O₃ to MnO or MnO_x species with larger particles and surface/subsurface oxygen species of ceria.^35–37 Further, for polyhedra-CeO₂–MnO_x, the profile of H₂-TPR shows multiple peaks, which may be related to more exposed planes and heterogeneous polyhedral particles.³⁸ Among them, rod-CeO₂–MnO_x catalyst has the best reducibility, while cube-CeO₂–MnO_x catalyst has the worst. The reducibility of mixed oxides decreases in the following order: rod-CeO₂–MnO_x > plate-CeO₂–MnO_x > polyhedra-CeO₂–MnO_x > cube-CeO₂–MnO_x. However, as reported in the literature,³⁹ the temperatures for the reduction of pure MnO_x are much higher than those for MnO_x species in CeO₂–MnO_x mixed oxides. This implies that there is a strong interaction between CeO₂ and MnO_x, which promotes the mobility of active oxygen species and further enhances the catalytic activity for CB oxidation.

Fig. 6 H₂-TPR profiles of CeO₂–MnO_x catalysts with different morphologies.

3.5 OSC properties of catalysts

The OSC value has a substantial impact on the catalytic performance for deep oxidation. Thus, the OSC measurement of CeO₂–MnO_x mixed oxides with different morphologies was carried out at 400 °C, and the data is listed in Table 3. The OSC value of rod-CeO₂–MnO_x (574 μmol O₂ g⁻¹) is the highest and much larger than that of cube-CeO₂–MnO_x (106 μmol O₂ g⁻¹). The OSC value decreases in the following order: rod-CeO₂–MnO_x > plate-CeO₂–MnO_x > polyhedra-CeO₂–MnO_x > cube-CeO₂–MnO_x, which is consistent with the results of the catalytic activity test. According to the literature,⁴⁰ the OSC values of CeO₂-based mixed oxides are intrinsic to their structures. A more homogeneous structure would create more oxygen that would result in an increase in the OSC value. Moreover, it is worth noting that the variation of the OSC value is consistent with those of their BET surface area (S_BET) and OSC/S_BET values, suggesting that the OSC value of the catalysts is also limited to its surface and particle size. As the BET surface area dramatically decreases, the available oxygen species on the surface also decline, which leads to a decrease in the OSC value. In addition, for rod-CeO₂–MnO_x and plate-CeO₂–MnO_x, their OSC/S_BET values are also higher than the theoretical ones (calculated OSCs). This result indicates that the mobility of active oxygen species in rod-CeO₂–MnO_x and plate-CeO₂–MnO_x is better, which increases the migration rate of bulk oxygen toward the surface, thereby improving the catalytic activity for CB oxidation.

Table 3 Exposed facets, OSCs, and surface areas of CeO₂–MnO_x catalysts with different morphologies

Samples	Exposed facets	OSC (μmol O2 g^−1)	SBET (m^2 g^−1)	OSC/SBET (μmol O2 m^−2)	Calculated OSC (μmol O2 m^−2)^a
a Calculated according to the theoretical OSC of the {100}, {110}, and {111} facets for CeO2.^41b Assumption: balanced distribution of oxygen species on different facets.^42
Rod-CeO2–MnOx	{100} + {110}	574	75.6	7.5	4.9^b
Plate-CeO2–MnOx	{200} + {111}	479	70.8	6.8	6.6
Polyhedra-CeO2–MnOx	{100} + {111}	256	48.1	5.3	6.6^b
Cube-CeO2–MnOx	{100}	106	33.7	3.1	5.7

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4. Conclusion

Four CeO₂–MnO_x mixed oxides with different morphologies were prepared by simple hydrothermal methods and characterized using XRD, XPS, HRTEM, N₂ adsorption–desorption, and H₂-TPR techniques. Rod-CeO₂–MnO_x mainly exposes the {100} facet and preferentially grows along the {110} facet direction. Plate-CeO₂–MnO_x is dominated by the {111} facet and polyhedra-CeO₂–MnO_x exposes the {111} and {100} facets, while cube-CeO₂–MnO_x with a bigger particle size is only enclosed by the {100} facet. The XRD Rietveld refinement results show that CeO₂ and MnO_x in the CeO₂–MnO_x mixed oxides are highly dispersed with respect to each other. Two new phases of both todorokite and vernadite with a special tunnel-like structure are found. The todorokite phase (S.G.: P2/m:b) exists in all the MnO_x–CeO₂ mixed oxides, while the vernadite phase (S.G.: I4/m) only exists in rod-MnO_x–CeO₂. Moreover, the lattice microstrain generated from the lattice distortion and defects decreases in the following order: rod-CeO₂–MnO_x > plate-CeO₂–MnO_x > polyhedra-CeO₂–MnO_x > cube-CeO₂–MnO_x, which is consistent with their OSC values. The results of UV-Raman and XPS spectra reveal that rod-CeO₂–MnO_x and plate-CeO₂–MnO_x have higher concentrations of Ce³⁺ and Mn²⁺ as compared to the other catalysts, particularly when compared to cube-CeO₂–MnO_x, which is favorable for the promotion of lattice oxygen mobility and further enhancing the catalytic activity for Cl-VOCs oxidation. Moreover, Mn enrichment and more Mn⁴⁺ species on the CeO₂–MnO_x surface are also beneficial toward improved catalytic activity. The H₂-TPR and OSC results show that the catalytic performance of the catalysts can be enhanced because CeO₂ and MnO_x strongly interact with each other, while their OSC properties are not only intrinsic to their structures but also limited to their surfaces and by their particle sizes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial supports from the National Key Research and Development Program of China (2016YFC0204300) and the Natural Science Foundation of China (No. 21477109) are gratefully acknowledged.

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Footnote

† These authors contributed equally to this work.

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