Thermal stability of MnO_x–CeO₂ mixed oxide for soot combustion: influence of Al₂O₃, TiO₂, and ZrO₂ carriers

Abstract

MnO_x–CeO₂ (Mn/Ce = 0.15/0.85) mixed oxide materials supported on ZrO₂, Al₂O₃, and TiO₂ were prepared by citrate sol–gel method to prevent the CeO₂-based oxide catalyst from sintering during soot combustion reaction. The materials were characterized using XRD, BET method, HRTEM, H₂-TPR, and XPS. Results indicate that the MnO_x–CeO₂/TiO₂ catalyst is rapidly sintered at 800 °C because of the TiO₂ phase change from anatase to rutile. By contrast, MnO_x–CeO₂ catalysts supported on Al₂O₃ and ZrO₂ show high thermal stability. After aging at 800 °C for 10 h, the MnO_x–CeO₂ crystalline structure remained highly dispersed on the carrier. Particularly with the ZrO₂ carrier, the amount of surface oxygen species and redox properties of MnO_x–CeO₂ can be well-maintained for the synergistic effect of CeO₂ and ZrO₂ in high temperature environments. Good agreement is found between the low-temperature redox property (200 °C to 400 °C) and the soot combustion activity of the catalysts.

---

1. Introduction

During the last decade, diesel engines have increased in popularity compared with gasoline engines owing to their superior fuel economy, reliability, and durability. However, emission from diesel engines includes important contaminants, among which particulate matter (PM) is of major concern. PM is mostly composed of carbonaceous soot and a soluble organic fraction of hydrocarbons condensed on the soot. This fraction consists of known harmful compounds that affect human health, as well as produce acid rain and photochemical smog, causing severe environmental problems. Therefore, elimination of PM from the exhaust gases of diesel engine is an important issue for environmental and chemical engineering.^1,2 Among various treatments, the combination of a filter and oxidation catalysts is the most widely studied approach to eliminate soot particles. In this combination, soot is initially trapped by diesel particulate filters (DPFs) and then catalytically oxidized to CO₂ by catalysts coated on DPFs.^3,4

Considerable attention has been focused on transition metal-doped ceria mixed oxides because of their high activity for soot combustion, which promotes the mobility of active oxygen at low temperature either by generating solid solutions with more oxygen vacancies or a synergistic effect between transition metals and ceria.^5–9 In particular, the incorporation of manganese cations into the ceria lattice markedly enhances the redox property, oxygen storage capacity (OSC), and surface oxygen mobility of ceria, resulting in excellent catalytic performance.^9–12 However, a major drawback of the MnO_x–CeO₂ catalyst is that significant deactivation occurs at above 650 °C, at which the catalyst undergoes dramatic particle sintering and severe phase separation.^7,11,13 Thermal stability is a requirement for soot oxidation catalysts, because the temperature inside a DPF can sharply increase as a consequence of the highly exothermal soot combustion process, with temperature gradients of 100 °C cm⁻¹ along both radial and longitudinal directions of the DPF.^14–16 Indeed, for promising industrial DPF catalysts, the CeO₂-based mixed oxides should maintain the catalytic activity even for aging at 800 °C.^7,8

Combination with suitable supports (such as Al₂O₃,^11,17,18 SBA-15,^9,19 ZrO₂,^13,20 TiO₂,^21–23etc.) is an effective approach to improve the activity and thermal stability of MnO_x–CeO₂, thereby maintaining the integrity of the MnO_x–CeO₂ solid solution structure. Strong metal–support interactions are a key factor that determines the catalytic performance of heterogeneous catalysts. Understanding how various support oxides affect the physicochemical properties of CeO₂-based mixed oxides is important in selecting appropriate carrier to improve thermal stability. ZrO₂, Al₂O₃, and TiO₂, which exhibit a significant impact on activity and stability, are introduced to MnO_x–CeO₂ mixed oxides as supports with reasonably good chemical stability for soot combustion. This work aims to develop an appropriate support for MnO_x–CeO₂ mixed oxide systems to attain high thermal stability for soot oxidation, thereby enabling their application in catalytic purification of atmospheric pollutants.

2. Experimental

2.1. Catalyst preparation

ZrO₂ and Al₂O₃ supports were synthesized by precipitation of the corresponding hydroxides (at pH 9 to pH 10) from aqueous solution of Zr(NO₃)₄·5H₂O and Al(NO₃)₃·9H₂O, respectively, by adding diluted ammonia water (drop wise). Precipitates were collected by filtration and washed with deionized water for three times. Then, the precipitates were dried at 110 °C overnight and calcined at 500 °C for 4 h. TiO₂ support (S_BET = 180 m² g⁻¹) was supplied by Nanjing Haitai Nano-materials Co., Ltd.

MnCe/MO_x(MO_x = TiO₂, Al₂O₃, and ZrO₂) catalysts were prepared by citric acid-assisted sol–gel method with a molar ratio of Mn/Ce = 0.15/0.85 and a weight ratio of (MnO_x + CeO₂) : MO_x = 2 : 1, with the corresponding nitrates as precursors. Ce(NO₃)₃·6H₂O and Mn(NO₃)₂ were dissolved in deionized water and mixed with MO_x carrier powders. Two molar equivalents of citric acid were added as a complexing agent. Polyglycol (10 wt% to that of citric acid) was then added. Each solution was stirred and heated until a porous gel is formed. This gel was then dried at 110 °C overnight, decomposed at 300 °C for 1 h, and then calcined at 500 °C for 3 h. For comparison, a MnO_x–CeO₂ sample was prepared through a similar method with the same Mn/Ce molar ratio. Further thermal treatment was performed on both MnCe/MO_x and MnCe samples at 800 °C for 10 h to obtain the aged catalysts with a suffix of “-A”.

2.2. Catalytic activity measurement

The oxidation of soot (Printex-U provided by Degussa) by the catalysts was studied in a thermogravimetric (TG) analyzer (Netzsch STA 409PC, Germany).²⁴ Activity measurements were obtained under“tight contact” conditions, under which soot–catalyst mixtures with a weight ratio of 1 : 10 were ground in an agate mortar for 10 min. Such conditions allow the activity of the catalysts under optimum conditions to be investigated. Oxidation experiments involved heating each mixture (about 10 mg) at a rate of 10 °C min⁻¹ from 25 °C to 650 °C under a flow of 0.1% NO/N₂ (10 mL min⁻¹) and 10% O₂/N₂ (50 mL min⁻¹). Catalytic activity was evaluated in the following three parameters: (a) the ignition temperature of soot (T_i, the extrapolated starting point of a TG curve); (b) the temperature at which soot oxidation proceeded at the highest rate (T_m, the maximum of a DTG curve); and (c) the burnout temperature of soot (T_f, the extrapolated end point of a TG curve).

2.3. Catalyst characterization

Powder X-ray diffraction (XRD) patterns were obtained by a diffractometer (ARLS CINTAG X'TRA) using nickel-filtered Cu Kα radiation and recorded at 0.02° intervals in the range of 10° ≤ 2θ ≤ 80° at a voltage of 45 kV and current of 30 mA. The specific surface areas of samples were determined from N₂ adsorption–desorption isotherms, which were measured at −196 °C using the Brunauer–Emmett–Teller (BET) method and a micropore system (ASAP 2020 M, Micromeritics, USA). Samples were pretreated under vacuum at 300 °C for 2 h prior to isotherm measurement.

High-resolution transmission electron microscopy (HRTEM, G2 F30 S-Twin, Philips-FEI, the Netherlands) was employed at an accelerating voltage of 300 kV. H₂-TPR experiments were conducted using a chemisorption analyzer (FINESORB-3010E, Fine Technology, China). Each sample (200 mg of fresh catalyst) was pretreated at 200 °C for 1 h and subsequently cooled to 80 °C under Ar flow (30 mL min⁻¹). Then, a 5% H₂/Ar flow (30 mL min⁻¹) was flowed over the catalyst bed at 80 °C for 20 min, followed by heating at a rate of 10 °C min⁻¹ up to 750 °C. Finally, the catalyst was cooled under an Ar flow (30 mL min⁻¹). H₂ consumption was monitored by a thermal conductivity detector operating at 60 °C and 60 mA. X-ray photoelectron spectroscopy (XPS) data were obtained using an Escalab 220i-XL electron spectrometer from VG scientific with 300 W Al Ka. The base pressure was about 3 × 10⁻⁹ mbar. The binding energies were referenced to the C 1s line at 284.6 eV from adventitious carbon.

3. Results and discussion

3.1. XRD and BET

The powder XRD patterns of fresh and aged MnCe and MnCe/MO_x samples are shown in Fig. 1(a and b), and the lattice constants and mean crystallite sizes of ceria in the samples were calculated by Scherrer equation (Table 1). All patterns of fresh samples are characteristic of fluorite-like cubic structure of ceria with a smaller lattice constant in comparison with that of pure ceria (d lattice ≈ 0.541 nm),²⁵ indicating that Mn–Ce solid solutions are formed by partial replacement of Ce^x+ (Ce³⁺ = 0.103 nm, Ce⁴⁺ = 0.092 nm) with smaller Mn^x+ (Mn²⁺ = 0.080 nm, Mn³⁺ = 0.066 nm, Mn⁴⁺ = 0.060 nm). In particular, diffraction peaks of monoclinic ZrO₂ and anatase TiO₂ are detected on MnCe/ZrO₂ and MnCe/TiO₂, whereas the Al₂O₃ phase is not found because of its smaller particle size. Notably, the major diffraction peak of CeO₂ shifts to a lower angle (2θ = 28.235°) with the introduction of ZrO₂.

Fig. 1 XRD patterns of (a) fresh MnCe and MnCe/MO_x, and (b) aged MnCe and MnCe/MO_x catalysts.

Table 1 Textural properties of fresh and aged MnCe and MnCe/MO_x catalysts

Sample	2θ (°)	Lattice constant (Å)	Crystallite size (nm)	A BET (m^2 g^−1)
MnCe	28.6712	5.388	7.8	78.4
MnCe-A	28.5840	5.404	49.9	<1
MnCe/ZrO2	28.2350	5.471	7.6	100.8
MnCe/ZrO2-A	28.6471	5.393	23.9	31.6
MnCe/Al2O3	28.6759	5.387	9.0	94.3
MnCe/Al2O3-A	28.6336	5.401	31.2	19.5
MnCe/TiO2	28.6635	5.404	7.8	96.7
MnCe/TiO2-A	28.5552	5.410	41.6	5.1

---

In general, the literature data for ZrO₂ prepared by ammonia co-precipitation reveals that a diffraction peak of monoclinic ZrO₂ appeared at 2θ ≈ 28°.²⁶ Thus, we suggest that a portion of CeO₂ particles are cladded by the ZrO₂ support, because the special diffraction peak belongs to the overlapping peak of cubic CeO₂ and monoclinic ZrO₂. The solubility limitation of Mn cations in CeO₂ as reported by Kang et al. was 5 mol% to 10 mol%, which depended highly on the preparation procedure.²⁷ This limit is lower than the Mn content in our study (15 mol%). Thus, we can infer the existence of a minimum of two kinds of Mn species in the fresh catalysts, that is, Mn cations in ceria lattice and highly dispersed MnO_x clusters on the surface of CeO₂ and/or ZrO₂, although only typical diffraction peaks of CeO₂ are observed in the XRD patterns.

After thermal aging at 800 °C for 10 h, a new diffraction peak of MnCe-A is detected at 2θ = 36.116°. This peak is attributed to tetragonal Mn₃O₄. Meanwhile, the crystallite size of the MnCe-A sample increases from 8.8 nm to 49.9 nm, accompanied by the reduction of SBET from 78.4 m² g⁻¹ to about less than 1.0 m² g⁻¹. This finding implies the severe sintering of the particles and phase separation of Mn–Ce solid solutions. An evident phase transformation of TiO₂ from anatase to rutile, as well as crystallite growth and dramatic reduction of surface area, is observed on the MnCe/TiO₂-A catalyst. This finding is opposite of the results with the introduction of ZrO₂ or Al₂O₃, which is effective in retarding the phase separation of solid solutions after aging. In particular, a significant increase in lattice constant is noted for MnCe/ZrO₂-A as compared with MnCe/ZrO₂ and appears to approximate that of the fresh MnCe sample. This finding indicates the separation of the overlapping peak ascribed to cubic CeO₂ and monoclinic ZrO₂. With respect to Cu–Mn–Ce/ZrO₂ mixed oxides, Zr–O–Ce interactions are supposedly involved in the stabilization of the dispersed CeO₂ solid solution, which leads to high activity and thermal stability in the total oxidation of toluene.²⁸ Thus, the introduction of the support ZrO₂ markedly maintains the high dispersity of the CeO₂ phase and itself, as well as prevents the phase separation of MnCe solid solutions. In addition, all supports result in the improvement of surface areas for the corresponding fresh and aged samples as compared with the MnCe catalyst.

3.2. TEM

The TEM and fast Fourier transform (FFT) images of fresh and aged MnCe and MnCe/MO_x samples are shown in Fig. 2. The reciprocal of the distance (nm⁻¹) from a certain point to the center in a FFT image has been proven by theoretical studies to be the relevant crystal spacing (nm). This fact is consistent with the result from direct measurement on the TEM image of lattice fringe spacing, but higher precision for large amounts of statistical results for similar crystal facets can be obtained by using the FFT image.²⁹ All FFT images of fresh samples show a quaternary mixed crystal ring, which, from the outside in, corresponds to (311) (d = 0.16 nm), (220) (d = 0.19 nm), and (111) (d = 0.31 nm) surface planes of ceria, respectively. However, high temperature treatment under 800 °C of MnCe-A and MnCe/TiO₂-A is shown to induce a rearrangement of crystals, with the dominant lattice plane belonging to the {111}⁹ families associated with a sharp decrease of the exposure of other facets, such as {311} and {220} planes, which can be still clearly visible in the FFT images of MnCe/ZrO₂-A and MnCe/Al₂O₃-A. Strong evidence provided shows that the surface structure performs a crucial function in the enhancement of OSC and catalytic activity over well-defined ceria nanocrystals. Theoretical results have indicated that the compact {111} surfaces are the most stable,³⁰ and the activity of ceria-based catalysts for numerous reactions, such as ethanol reforming reaction and gas shift reaction, mainly depend on the proportion of non-{111} faces. Following aging and heating of ceria polycrystalline powders, the {111} ceria surface have been shown to undergo structural evolution, and the amount of {111} faces progressively decrease. In comparison, exposure of other faces gradually increases, leading to a higher specific activity for CO oxidation with respect to the fresh samples.³¹ In general, NO_x-assisted soot catalytic oxidation reaction is considered to follow oxygen spillover mechanism,^32,33 in which the active surface oxygen spill/transfer over soot in the presence of air, leading to the formation of CO/CO₂. In this case, the formation of more oxygen vacancies on the surface of catalysts facilitates OSC enhancement, as lower energy needed to form an oxygen vacancy indicates the higher reactivity of the surface plane toward the reductant molecule. Hence, the ability to form surface vacancy is highly correlated to the type of exposed plane, as shown on MnCe/ZrO₂-A and MnCe/Al₂O₃-A, which maintain a certain number of non-{111} faces.

Fig. 2 HRTEM and FFT images for fresh and aged MnCe and MnCe/MO_x catalysts.

3.3. H₂-TPR

Soot combustion in air and on a catalyst proceeds via the “gas (O2)–solid (soot)–solid (catalyst)” three-phase reaction. Several researchers indicated that the effectiveness of the catalyst can be closely related to its ability to better deliver active oxygen to the solid carbon reactant within an appropriate temperature range.^24,34 During TPR process, the metallic cations with high valence state are reduced to ions with low valence state or metallic atoms by H₂; in addition, lattice oxygen ions are involved in the process because of H₂O formation.

The images in Fig. 3(a) and (b) exhibit the H₂-TPR profiles of fresh and aged samples, respectively. The corresponding H₂ consumption of reduction peaks for each catalyst is calculated by integrating the corresponding peak areas with the CuO standard sample (Table 2). For the MnCe mixed oxide, the two peaks at 229 and 331 °C, which are lower than H₂-TPR reduction temperature of pure CeO₂ and MnO_x indicate that the redox property is significantly improved because of the generation of the MnCe solid solution structure. By introducing ZrO₂, Al₂O₃, and TiO₂ supports, the reduction peaks of the mixed oxide shift slightly toward higher temperature range. However, the larger areas of the first peak imply an increase in surface adsorbed oxygen species that is attributed to the high dispersion or a synergistic effect between MnO_x and CeO₂ crystals on the support. In combination with the XRD analysis, the result indicated that the low-temperature redox properties of both MnCe-A and MnCe/TiO₂-A samples largely decrease because of particle sintering and phase separation of solid solutions after aging, especially for MnCe/TiO₂-A, which undergoes severe phase transformation and encapsulation of MnO_x–CeO₂ nanoparticles by TiO₂. However, high oxygen delivery capacity, which maintains the integrity of Mn–Ce solid solutions, is still observed on MnCe/ZrO₂-A and MnCe/Al₂O₃-A, resulting in relatively stable reducibility.

Fig. 3 H₂-TPR patterns of (a) fresh MnCe and MnCe/MO_x, and (b) aged MnCe and MnCe/MO_x catalysts.

Table 2 TPR characterization and catalytic performance for soot combustion

Sample	Temperature of peak and H2 consumption (H2-TPR)				T i/°C	T m/°C	T f/°C
	Peak 1		Peak 2
	T/°C	n(H2)/(mmol g^−1)	T/°C	n(H2)/(mmol g^−1)
MnCe	229	1.021	331	2.481	320	368	398
MnCe-A	—	—	408	2.234	418	491	542
MnCe/ZrO2	254	2.312	379	1.578	338	390	422
MnCe/ZrO2-A	278	2.035	431	0.936	366	423	468
MnCe/Al2O3	263	1.059	368	2.384	361	412	447
MnCe/Al2O3-A	305	1.934	445	0.757	400	472	516
MnCe/TiO2	258	1.771	377	1.727	350	397	431
MnCe/TiO2-A	—	—	574	1.881	432	525	552

---

3.4. XPS characterization

The amount and type of surface oxygen species have been shown to be closely related to the activity of the catalyst. Hence, to investigate the influence of ZrO₂ and Al₂O₃ carriers on the distribution of surface oxygen on the MnCe catalyst, the corresponding fresh and aged samples were characterized by X-ray photoelectron spectroscopy (XPS). As can be noted from Fig. 4, the broad O 1s spectra in all samples indicate the existence of several kinds of surface oxygen species. As is evidently shown by the fitting figures, three different types of oxygen species exist on the surface of all catalysts at the binding energies of ca. 529.2, 531.3, and 532.7 eV, respectively.³⁵ According to the related literature, the oxygen species at the lowest binding energy (ca. 529.2 eV) can be attributed to lattice oxygen, whereas the two species at higher (ca. 532.7 and 531.3 eV) are generally ascribed to superoxide and peroxide, respectively,^13,36 which are probably formed via the oxygen adsorption of oxygen vacancies on the reduced surface of catalysts. In an interesting observation noted by comparing Fig. 4(a) with Fig. 4(b), the amount of superoxide and peroxide species of MnCe and MnCe/Al₂O₃ samples decrease markedly and become substantially less than lattice oxygen under high-temperature treatment. This finding particularly implies a significant reduction of oxygen vacancies that is due to the separation of the solid solution in the MnCe-A sample, whereas the most surface oxygen species can be maintained for the MnCe/ZrO₂-A sample. Moreover, instead of reducing, the peroxide species of MnCe/ZrO₂-A were increased, indicating transformation from the original superoxide species. We can conclude that the introduction of ZrO₂ maintains the integrity of Mn–Ce solid solution structure, as well as the amount of surface oxygen vacancies, which can considerably promote the adsorption of superoxide and peroxide species.

Fig. 4 XPS spectra of (a) fresh MnCe and MnCe/MO_x, and (b) aged MnCe and MnCe/MO_x catalysts.

3.5. Catalytic activity and overall discussion

The TG and DTG curves of soot combustion over fresh and aged MnCe, MnCe/ZrO₂, MnCe/Al₂O₃, and MnCe/TiO₂ catalysts are shown Fig. 5(a–d). The values of T_i, T_m, and T_f, as determined from the curves are summarized in Table 2. Soot oxidation of MnCe sample starts at a low temperature of 320 °C and is completed at 398 °C. However, this catalyst experiences a striking deactivation at high calcination temperature (800 °C) for 10 h. During soot combustion on the MnCe-A sample, the T_i, T_m and T_f shift upwards by more than 100 °C. All fresh supported catalysts exhibit high activities for soot oxidation, with fine dispersion of each support, especially for MnCe/ZrO₂ and MnCe/Al₂O₃ catalysts, as the closeness of two TG/DTG curves for corresponding fresh and aged catalysts indicates higher thermal stability of the catalyst for soot combustion. The lowest thermal stability is observed on TiO₂-supported MnCe sample, which is mainly due to the severe sintering of particles, phase transformation of TiO₂ and encapsulation of active components. On the contrary, MnCe/ZrO₂ and MnCe/Al₂O₃ catalysts are more resistant toward thermal deactivation. In particular, the T_i, T_m, and T_f of the ZrO₂-supported MnCe sample are only increased by 28, 33, and 46 °C, respectively, after extended high-temperature calcination.

Fig. 5 TG and DTG curves of soot/catalyst mixture with fresh and aged (a) MnCe, (b) MnCe/ZrO₂, (c) MnCe/Al₂O₃, and (d) MnCe/TiO₂ catalysts.

Our previous study showed that the active oxygen delivery capacity of CeO₂ modified by a transition metal at low temperature (200 °C to 400 °C) performs a crucial function in improving catalytic performance by H₂-TPR measurement of ceria-based mixed oxides.³⁷ In these oxides, the incorporation of Mn greatly enriches the active oxygen species and bestows high mobility at low temperature, thus contributing to superior catalytic activity. Accordingly, the H₂ consumption at 200 °C to 400 °C of fresh and aged MnCe and MnCe/MO_x catalysts, which are calculated by integrating the corresponding peak areas with the CuO standard sample, are shown in Fig. 6. A significantly positive correlation is found between soot combustion activity and the released active oxygen (200 °C to 400 °C) of the samples, as the value of T_m gradually increase with the reduction of H₂ consumption (200 °C to 400 °C). Among these catalysts, MnCe-A shows almost half of the oxygen delivery capacity (200 °C to 400 °C) as MnCe. In addition, a marked decrease in redox property and activity is exhibited by MnCe/TiO₂-A, indicating that TiO₂ is not a suitable support for MnCe catalysts for thermal stability improvement. Moreover, Fig. 6 highlights that the H₂ consumption variation (200 °C to 400 °C) between MnCe/ZrO₂ and MnCe/ZrO₂-A is less than 30%, implying that MnCe/ZrO₂-A can still provide both larger surface area and richer lattice oxygen in this temperature range.

Fig. 6 H₂ consumption at 200 °C to 400 °C and T_m for soot combustion of (a) fresh MnCe and MnCe/MO_x, and (b) aged MnCe and MnCe/MO_x catalysts.

Al₂O₃ and ZrO₂ have different mechanisms of action for Mn–Ce oxides. Al₂O₃ could prevent Mn–Ce migration of nanoparticles by creating an isolation or confinement environment for the active components. However, significant changes in physicochemical properties (such as redox and crystallinity) were still found after aging, although the size did not obviously increase during the aging process. Therefore, the amount of superoxide and peroxide species of MnCe/Al₂O₃-A samples decreased markedly. However, for ZrO₂ carrier, our previous study¹³ showed that heat treatment leads to the interaction of ZrO₂ and CeO₂-based oxides via the formation of a new phase, Zr_0.88Ce_0.12O₂, in the interface. Zr_0.88Ce_0.12O₂ appears to function as the actual carrier of the active phase of Cu–Mn–Ce (CMC), which can not only stabilize the surface active structure of CMC, but also enhance the mobility of active oxygen and improve the catalytic performance of total oxidation. Therefore, the higher thermal stability of MnCe/ZrO₂ than that of MnCe/TiO₂ and MnCe/Al₂O₃ can be attributed to the synergistic effect of the abundance of the two active oxygen species at low temperature.

4. Conclusion

MnCe/MO_x (M = Zr, Al, Ti) catalysts with different supports are compared both in terms of activity and thermal stability for soot combustion. Loading Mn–Ce mixed oxides on ZrO₂ was found to be the most effective approach in promoting the thermal stability of oxides. This improvement should be mainly ascribed to restraining the sintering of MnO_x and CeO₂ crystallites, as well as preventing the phase separation of MnCe solid solutions to a certain extent. Moreover, the thermal stability of MnCe/ZrO₂ depends on the proportion of reactive crystal planes (non-{111} planes), the amount and type of surface oxygen species, and the redox capacity after high-temperature treatment at 800 °C for 10 h. In particular, the low-temperature redox property (200 °C to 400 °C) of fresh and aged catalysts is consistent with their soot oxidation activity.

Acknowledgements

We would like to acknowledge the financial support from the Natural Science Foundation of China (No. 21107096, 21506194), Zhejiang Provincial Natural Science Foundation of China (No. Y14E080008, Y16B070011), the Commission of Science and Technology of Zhejiang province (No. 2013C03021) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20133317110004).

Notes and references

1. B. R. Stanmore, J. F. Brilhac and P. Gilot, Carbon, 2001, 39, 2247–2268.
2. R. D. Zhang, N. Luo, B. H. Chen and S. Kaliaguine, Energy Fuels, 2010, 24, 3719–3726.
3. K. Yamamoto and T. Sakai, Catal. Today, 2015, 242, 357–362.
4. S. Quiles-Diaz, J. Gimenez-Manogil and A. Garcia-Garcia, RSC Adv., 2015, 5, 17018–17029.
5. M. Piumetti, S. Bensaid, N. Russo and D. Fino, Appl. Catal., B, 2015, 165, 742–751.
6. P. Venkataswamy, D. Jampaiah, K. N. Rao and B. M. Reddy, Appl. Catal., A, 2014, 488, 1–10.
7. S. Liu, X. Wu, D. Weng and R. Ran, J. Rare Earths, 2015, 33, 567–590.
8. A. Bueno-Lopez, Appl. Catal., B, 2014, 146, 1–11.
9. H. Zhang, F. Gu, Q. Liu, J. Gao, L. Jia, T. Zhu, Y. Chen, Z. Zhong and F. Su, RSC Adv., 2014, 4, 14879–14889.
10. X. D. Wu, F. Lin, H. B. Xu and D. Weng, Appl. Catal., B, 2010, 96, 101–109.
11. X. Wu, S. Liu, D. Weng, F. Lin and R. Ran, J. Hazard. Mater., 2011, 187, 283–290.
12. X. Wu, S. Liu, D. Weng and F. Lin, Catal. Commun., 2011, 12, 345–348.
13. H. F. Lu, Y. Zhou, W. F. Han, H. F. Huang and Y. F. Chen, Appl. Catal., A, 2013, 464–465, 101–108.
14. G. Zhang, Z. Zhao, J. Xu, J. Zheng, J. Liu, G. Jiang, A. Duan and H. He, Appl. Catal., B, 2011, 107, 302–315.
15. I. Atribak, A. Bueno-Lopez and A. Garcia-Garcia, Catal. Commun., 2008, 9, 250–255.
16. K. Shimizu, M. Katagiri, S. Satokawa and A. Satsuma, Appl. Catal., A, 2011, 108, 39–46.
17. I. Spassova, T. Tsontcheva, N. Velichkova, M. Khristova and D. Nihtianova, J. Colloid Interface Sci., 2012, 374, 267–277.
18. X. Wu, H.-R. Lee, S. Liu and D. Weng, J. Rare Earths, 2012, 30, 659–664.
19. F. Z. Zhao, S. F. Ji, P. Y. Wu, Z. F. Li and C. Y. Li, Catal. Today, 2009, 147, S215–S219.
20. H. Li, N. Ta, X. Zhang, X. Huang and W. Shen, Catal. Commun., 2011, 12, 1361–1365.
21. D. Q. Yu, Y. Liu and Z. B. A. Wu, Catal. Commun., 2010, 11, 788–791.
22. B. Murugan and A. V. Ramaswamy, J. Phys. Chem. C, 2008, 112, 20429–20442.
23. Z. Y. Sheng, Y. F. Hu, J. M. Xue, X. M. Wang and W. P. Liao, Environ. Technol., 2012, 33, 2421–2428.
24. L. Katta, P. Sudarsanam, G. Thrimurthulu and B. M. Reddy, Appl. Catal., B, 2010, 101, 101–108.
25. E. Aneggi, M. Boaro, C. de Leitenburg, G. Dolcetti and A. Trovarelli, J. Alloys Compd., 2006, 408, 1096–1102.
26. T. Sreethawong, S. Ngamsinlapasathian and S. Yoshikawa, Chem. Eng. J., 2013, 228, 256–262.
27. C. Y. Kang, H. Kusaba, H. Yahiro, K. Sasaki and Y. Teraoka, Solid State Ionics, 2006, 177, 1799–1802.
28. H. F. Lu, Y. Zhou, W. F. Han, H. F. Huang and Y. F. Chen, Catal. Sci. Technol., 2013, 3, 1480–1484.
29. N. T. Huu, A. Lamacz, P. Beaunier, S. Czajkowska, M. Domanski, A. Krzton, L. Tiep Van and G. Djega-Mariadassou, Appl. Catal., B, 2014, 152, 360–369.
30. M. Nolan, S. Grigoleit, D. C. Sayle, S. C. Parker and G. W. Watson, Surf. Sci., 2005, 576, 217–229.
31. K. Zhou, X. Wang, X. Sun, Q. Peng and Y. Li, J. Catal., 2005, 229, 206–212.
32. P. G. Harrison, I. K. Ball, W. Daniell, P. Lukinskas, M. Cespedes, E. E. Miro and M. A. Ulla, Chem. Eng. J., 2003, 95, 47–55.
33. Z. L. Zhang, Y. X. Zhang, Z. P. Wang and X. Y. Gao, J. Catal., 2010, 271, 12–21.
34. B. van Setten, M. Makkee and J. A. Moulijn, Catal. Rev.: Sci. Eng., 2001, 43, 489–564.
35. C. Lee, J.-I. Park, Y.-G. Shul, H. Einaga and Y. Teraoka, Appl. Catal., B, 2015, 174, 185–192.
36. E. Aneggi, N. J. Divins, C. de Leitenburg, J. Llorca and A. Trovarelli, J. Catal., 2014, 312, 191–194.
37. Y. Q. Sheng, Y. Zhou, H. F. Lu, Z. K. Zhang and Y. F. Chen, Chin. J. Catal., 2013, 34, 25–33.

---