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Sep 26, 2013 - ammonium persulfate (APS) was refined by recrystallization. All other .... shrinkage phenomena are caused by the decomposition of PS.

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Comparative study of 3D ordered macroporous Ce0.75Zr0.2M0.05O2−δ (M = Fe, Cu, Mn, Co) for selective catalytic reduction of NO with NH3† Sixiang Cai,ab Dengsong Zhang,*a Lei Zhang,a Lei Huang,a Hongrui Li,a Ruihua Gao,a Liyi Shiab and Jianping Zhanga The three dimensional ordered macroporous (3DOM) Ce0.75Zr0.2M0.05O2−δ (M = Fe, Cu, Mn, Co) is synthesized by a colloidal crystal template method for comparative study on selective catalytic reduction (SCR) of NO with NH3. The obtained catalysts are mainly investigated by the measurements of X-ray diffraction (XRD), N2 sorption, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by hydrogen (H2-TPR), as well as temperature-programmed desorption of ammonia (NH3-TPD). The XRD, XPS and TPR analysis clarified the dopants are effectively doped into the Ce–Zr oxide solid solution, which contribute to the strong synergistic effect between the dopants and Ce–Zr oxides. Moreover, higher Oα/(Oα + Oβ) ratio, improved surface reducibility and enhanced surface acidity are observed after the in situ doping. These facts lead to a better low temperature

Received 9th June 2013, Accepted 26th September 2013

catalytic performance for the Ce0.75Zr0.2M0.05O2−δ catalysts. Particularly, the Co doped catalysts exhibit the highest active oxygen species, the highest reducibility as well as the strongest NH3 absorption ability, which corresponds to the significant increase of low temperature activity. Meanwhile, the specific surface area

DOI: 10.1039/c3cy00398a

and pore volume are increased efficiently by Fe and Mn doping, which broadens the operating temperature window. Moreover, the strong interaction between the dopants and the Ce–Zr oxide solid solution could

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be ascribed to the good stability of those catalysts doped with Fe, Mn and Co.

1. Introduction Nitrogen oxides (NOx) can cause great environmental problems including acid rain, ozone depletion and global warming.1,2 Recently, the selective catalytic reduction (SCR) was proved to be one of the most cost-effective ways to reduce the emission of NOx from the mobile or stationary sources.3–5 For decades, vanadium based catalysts have been commercially used to eliminate the emission of NOx.6,7 Nevertheless, this kind of catalysts only exhibit high efficiency within a narrow temperature window between 350–450 °C and the volatilization of vanadium is hazardous to the environment and human health.7,8 In this regard, it is urgent to develop novel de-NOx catalysts with lower reaction temperature, broader operating temperature window as well as minimal environmental impacts.

a

Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China. E-mail: [email protected]; Fax: +86 21 66136079 b School of Material Science and Engineering, Shanghai University, Shanghai 200072, China † Electronic supplementary information (ESI) available: Scheme for the catalysts preparation, SEM image of PS arrays and Ce0.8Zr0.2O2, and EDS analysis for all the catalysts. See DOI: 10.1039/c3cy00398a

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Attracted by its unique oxygen storage capacity and redox properties, ceria (CeO2) has been widely explored in the catalysis field.9–12 Moreover, CeO2 is considered as an environmental friendly and inexpensive rare earth oxide in catalysys.13 However, the major drawbacks of cerium oxide are the poor thermal stability as well as the sintering at high temperature. Fortunately, it has been found that the thermal resistance, textural stability as well as the surface acid sites of pure CeO2 can be greatly enhanced by forming the solid solution with the introduction of zirconium oxide.14–16 Therefore, the CexZr1−xO2 (x = 0.1–0.5) solid solutions have been widely used as catalysts or catalyst supports towards NH3-SCR of NO with outstanding catalytic performance.13,17–19 Furthermore, recent studies show that the doped ceria with transition metals such as Mn, Fe, Cu and Co could result in the formation of structural defects and solid solution, thereafter enhancing the redox and catalytic properties.20–23 Recently, three dimensional ordered macroporous (3DOM) metal oxides have been widely used in the field of catalysis due to their characteristic properties.24–27 The well-defined structure contains large amounts of macropores and interconnected through pores, which can provide mass transfer channels for the gaseous reactant to the inner active sites. Moreover, 3DOM materials have higher specific surface area

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than the nonporous granules.19,21 It is generally accepted that the enlarged surface area could provide more reactive centers, which is beneficial to homogeneous catalysis.27,28 Zhao et al. developed a series of 3DOM CexZr1−xO2 supported catalysts for the catalytic oxidation of diesel soot and demonstrated elevated catalytic performance and thermal stability.29 Petkovich et al. showed that the 3DOM structure of CexZr1−xO2 catalysts could be preserved under heat-treated condition.30 These reports convincingly prove that agglomeration of active species can be suppressed in the 3DOM structure. In addition, the synthesis of the 3DOM structure most commonly uses the metal salt solution as precursor, and thus the corresponding 3DOM homogeneous solid solution could be easily obtained after the template removal process.19–22,26–30 This feature indicates the possibility for in situ doping and good diffusion of active components into the CexZr1−xO2 solid solution, which may result in the strong interaction between the CexZr1−xO2 host and dopants. However, few reports about both 3DOM CexZr1−xO2 solid solution and their in situ doping by transition metals for NH3-SCR of NO were involved in previous studies. Herein, through a colloidal crystal template method, we synthesized a series of 3DOM CexZr1−xO2 catalysts in situ doped with some variant valence metals such as Mn, Co, Fe, and Cu for the comparative study. The corresponding catalysts are systematically investigated by XRD, TEM, N2 sorption, XPS, H2-TPR and NH3-TPD measurements. The study is mainly focused on understanding the influence of the dopants on the structure properties, surface components, redox behavior, NH3 adsorption properties, catalytic performance as well as the SO2 tolerance capability.

2. Experimental 2.1 Catalyst preparation The reagents were supplied by Sinopharm Chemical Reagent Co. Ltd (China). The CP grade styrene (ST) and methacrylic acid (MAA) was purified by reduced pressure distillation. The ammonium persulfate (APS) was refined by recrystallization. All other reagents were of analytical grade and used without further purification. Deionized water (DI) was also used in the experiments. The 180 nm electronegative polystyrene (PS) nanospheres were prepared by an emulsifier-free emulsion polymerization method reported by Zhang et al.31 At the typical synthesis, 0.24 g of ammonium hydrogen carbonate was first dissolved in 85 ml DI. Afterwards, the aqueous solution was transferred to a 250 ml three-necked bottom flask equipped with a mechanical stirrer, a nitrogen gas inlet as well as a condenser. Then 10 g of ST and 1 g of MAA were added into the flask under a constant stirring rate as 350 r min−1. After 20 minutes, 0.37 g of APS dispersed in 5 ml DI were added into the mixture as initiator. The whole reaction process was operated under nitrogen protection and a stationary temperature as 78 °C for 10 h. The obtained PS nanospheres were filtered and then washed by several rinse–centrifugation cycles with ethanol and water. The emulsion was transferred

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to an evaporating dish for gravitational deposition at 40 °C. After that, the dried pieces were pretreated under 120 °C for 8 minutes to stabilize the connection between each sphere. Finally, the well-ordered PS arrays (Fig. S2, ESI†) were used as template for the synthesis of the 3DOM catalysts. The 3DOM Ce0.8Zr0.2O2 (thereafter denoted as CeZrO) and Ce0.75Zr0.2M0.05O2−δ (thereafter denoted as CeZrMO, M = Fe, Cu, Mn, Co) samples were prepared by the route as indicated in Fig. S1 (ESI†), and the doping content of transition metals was fixed to 5% mole fraction. The Ce(NO3)3·6H2O, ZrOCl2·8H2O and M(NO3)x·nH2O (M = Fe, Cu, Mn, Co) were used as metal precursors. Firstly, the appropriate amounts of those metal precursors were dispersed in the intermixture of deionized water and EG under 30 min vigorous stirring. Subsequently, citric acid was added into the mixture to achieve the precursor solution. Afterwards, the PS template was immersed into the precursor solution for 6 h for complete permeation. Then, vacuum filtration was proceeded to remove the excess liquid. The product was dried in a vacuum desiccator overnight, followed by calcination at 450 °C for 2 h (heating ramp 1 °C min−1) in air to remove the PS template. Afterwards, the desired 3DOM catalysts were obtained.

2.2 Catalyst characterization Powder X-ray diffraction (XRD) was measured by a Rigaku D/MAX-2200 X-ray diffractometer with Cu–Ka (40 kV, 40 mA) radiation and a secondary beam graphite monochromator. The diffraction data were collected over a 2θ range of 10–90° with 0.02° intervals. The structure, pore size was observed by a transmission electron microscope (TEM) on a JEOL JEM-200CX system. Samples for TEM were prepared by drying the ethanol dispersion of catalyst powders on the carboncoated copper grid. The nitrogen adsorption–desorption isotherms of the samples were performed at 77 K using an ASAP 2020 volumetric adsorption analyzer. The specific surface area and the pore volume were calculated via the Brunauer–Emmett–Teller (BET) method and Barrett–Joyner– Halenda (BJH) model, respectively. X-ray photoelectron spectroscopy (XPS) analysis was performed on a RBD upgraded PHI-5000C ESCA system with a dual X-ray source, using an Mg–Ka (1253.6 eV) anode and a hemispherical energy analyzer. The background pressure in the analyzer chamber during data collection was kept below 10−6 Pa. All the binding energies were standardized according to contaminant carbon (C 1s = 284.6 eV) and the peak fitting was carried out using the AugerScan (version 3.21) software. Temperature-programmed reduction by hydrogen (H2-TPR) was carried out on a FineSorb3010D apparatus. In a typical measurement, 50 mg of the calcined catalyst was firstly outgassed at 300 °C under Ar flow. Then the outgassed sample was cooled down to room temperature under Ar flow, too. Afterwards, the flowing gas was switched to 10% H2/Ar and the reaction temperature was increased to 650 °C at a heating rate of 10 °C min−1. The H2 consumption was monitored by a thermal conductivity detector

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(TCD). Temperature-programmed desorption experiments of ammonia (NH3-TPD) were conducted on a Tianjin XQ TP5080 auto-adsorption apparatus. Prior to TPD, each sample was pretreated with high-purity (99.999%) Ar (flow rate = 30 ml min−1) flow at 300 °C for 0.5 h and cooled to 100 °C with the protection by Ar flow. Subsequently, the catalysts sample was saturated with high-purity anhydrous ammonia at 100 °C for 1 h and then physical absorbed ammonium was removed by 1 h flushing with argon flow under the same temperature. Finally, the TPD measurement was carried out from 100 to 650 °C at a ramping rate of 10 °C min−1. The amount of NH3 desorbed from the catalysts was monitored by TCD.

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2.3 Catalytic performance tests The NH3-SCR activity measurement was performed in a fixedbed quartz reactor (inner diameter = 8 mm) in a steady state flow mode. The typical feed gas composition is listed as follows: [NO] = [NH3] = 500 ppm, [O2] = 3 vol%, [SO2] = 200 ppm (when used), N2 as balance gas,32 and gas hourly space velocity (GHSV) of 20 000 h−1. Prior to each SCR activity test, 0.4 g of catalysts were crushed and sieved with 20–60 mesh and then used. In order to avoid the axial diffusion of reactant gas, 50 mg quartz wool was inserted into the quartz reactor. The SCR reaction was carried out between the chosen temperatures from 150 to 450 °C and the data was recorded while the reaction reached a steady state. The concentrations of the feed gases and the effluent streams are analyzed continuously by a KM9106 flue gas analyzer. The NO conversion was calculated according to the following expression:

NO Conversion (%) 

[NO]in  [NO]out  100% [NO]in

where the [NO]in and [NO]out are assigned to the inlet and outlet concentration of NO at a steady-state, respectively.

3. Results and discussion 3.1 XRD and N2 sorption analysis The XRD measurement was carried out to analyze the composition and phase structures of the catalysts. As shown in Fig. 1, the distinct diffraction peaks in the XRD pattern of CeZrO can be indexed to the CeO2 (111), (200), (220), (311) and (331) lattice planes, which can be attributed to the cubic structure of CeO2 (JCPDS card no. 43-1002).33,34 No characterization peaks belonging to ZrO2 can be detected in the XRD profiles of CeZrO and the diffraction peaks shift slightly to higher angle as compared to the pure CeO2, indicating the formation of CeZrOx solid solution.24 The CeZrMO samples also show the typical diffraction peaks of CeO2, suggesting the in situ doping does not change the cubic structure. In addition, the diffraction peaks derived from Fe, Mn, Cu, Co species are absent in the XRD observation, which may be related to the small amounts of addition features and their well dispersion on the catalyst surface. However, it is worth noticing that characteristic diffraction peaks of Ce–Zr oxide

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Fig. 1 XRD patterns of the catalysts: (a) CeZrO, (b) CeZrFeO, (c) CeZrCuO, (d) CeZrMnO, and (e) CeZrCoO.

solid solution become weaker and broader after the in situ doping, indicating decreased crystal size and crystallinity degree. The grain size calculated by the Scherrer equation with the most prominent peak (111) is listed in Table 1. The result suggests the doping of Mn and Fe can efficiently inhibit the growth of the cubic phase, which agrees well with the previous reports.21,22 As the decrease of grain size is conductive to the contact between the catalysts and the reactant gas flow,34 the Mn and Fe dopants should have the promotion effect on the catalytic performance. Meanwhile, the slight excursion of diffraction peaks to the CeZrO can be detected in the XRD patterns of the doped samples. This behavior indicates that some of the Fe, Cu, Mn and Co ions are inserted into the ceria lattice to form the solid solution. In summary, the XRD results reveal those elements are effectively doped into the CeZrO, which could contribute to the strong interaction between the dopants and the CeZrO solid solution. The N2 sorption was performed in order to detect the surface area and porous structure of catalysts, and the obtained nitrogen adsorption–desorption isotherms are shown in Fig. 2. All the samples show a type II isotherm and a significant increase of nitrogen adsorption can be observed at the relative pressure beyond 0.8. The result is in good agreement to many macroporous materials, indicative of large amount of macropores in the catalysts.26,35 The specific surface area and pore volume of CeZrO and CeZrMO samples are also listed in Table 1. The CeZrO sample exhibits a specific surface area as 72.6 m2 g−1. After the introduction of Cu and Co, both the surface areas and pore volume are slightly decreased. On contrast,

Table 1 Textural and structural properties of different samples

Sample

Specific surface (m2 g−1)

Pore volume (cm2 g−1)

Grain size (nm)

CeZrO CeZrFeO CeZrCuO CeZrMnO CeZrCoO

72.6 81.5 59.5 98.7 58.9

0.281 0.300 0.250 0.294 0.275

6.1 5.1 6.0 4.5 5.8

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Fig. 2 N2 adsorption–desorption isotherms of the catalysts: (a) CeZrO, (b) CeZrFeO, (c) CeZrCuO, (d) CeZrMnO, and (e) CeZrCoO.

the surface areas and pore volume of CeZrMnO and CeZrFeO are increased. As the enlarged surface area can do promotional effects to the catalytic reaction, these changes indicate the incorporation of Fe and Mn can effectively improve the texture property of CeZrO. 3.2 TEM analysis The TEM observation was carried out to investigate the morphology and structure of all catalysts. Fig. 3 shows TEM images of CeZrO and CeZrMO catalysts obtained by the colloidal crystal template method. From the images, we can clearly observe the 3DOM structure of CeZrO and CeZrMO. The average pore diameter of CeZrO sample is around

Fig. 3 TEM images of the catalysts: (a) CeZrO, (b) CeZrFeO, (c) CeZrCuO, (d) CeZrMnO, and (e) CeZrCoO.

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100 nm, which indicates a 44% shrinkage rate compared with the initial size of PS nanospheres (Fig. S2, ESI†). The shrinkage phenomena are caused by the decomposition of PS nanospheres and the sintering during the calcination process.35 In addition, some interconnected holes could also be observed in the TEM image. The small holes among the macropores are formed at the contact point between PS spheres that could not be immersed by metal salt precursor solution.36 Those interconnected networks and well-ordered macropores can provide great mass transfer ability and more inner surface area for the catalytic reaction, which is in favor of the NH3-SCR of NO.27,28,37 The uniform structure can also be observed in the SEM images (Fig. S3, ESI†), indicating the long ordered macroporous structure is the predominant constituent part in the catalyst. The morphology and structure properties of all CeZrMO catalysts (Fig. 3b–e) are similar to those of the CeZrO sample, suggesting the 3DOM structure is maintained after the in situ doping, consistent with the N2 sorption analysis. To confirm the existence of doping elements in the 3DOM samples, The EDS spectrum of all 3DOM catalysts was conducted (Fig. S4, ESI†). The peaks related to Fe, Mn, Cu, Co species can be found in their corresponding EDS spectrum, respectively. The result is also supported by the XPS analysis as discussed below. Those measurements can verify the doping elements are mainly diffused into the 3DOM structure.

3.3 XPS analysis The XPS measurement was carried out to analyze the near surface component as well as the valence of O and Ce species after the in situ doping. The surface atom ratios of the transition metals on the catalysts are listed in Table 2. It is noticeable that the Cu content on the catalysts surface is significantly higher than other dopants among the CeZrMO samples, which indicates some of Cu species are not well inserted into the Ce lattice or the 3DOM skeleton under the same conditions. The XPS spectra of Ce 3d could split into ten peaks denoted as v0 (880.5 eV), v (899.0 eV), v′ (884.9 eV), v′′ (888.8 eV), v′′′ (898.3 eV), u0 (899.0 eV), u (901.1 eV), u′ (903.5 eV), u′′ (907.5 eV), u′′′ (916.6 eV).38 As is well known, the signals v, v′′, v′′′, u, u′′, u′′′ are assigned to Ce4+ species while the labeled v0, v′, u0, u′ are related to Ce3+ species. The fitted Ce 3d spectra of CeZrO and CeZrMO are shown in Fig. 4. From these peaks, it is clear that the Ce4+ is the primary valance state of Ce, while the peak of Ce3+ can be observed distinctly, indicating a certain amount of Ce3+ exists in the catalysts. Mamontov and coworkers suggested that the incorporation of ZrO2 in the CeO2 could lead to the formation of Frenkel-type defects and the displacement of lattice oxygen, so the occurrence of Ce3+ mainly corresponds to the interaction between Ce and Zr atoms.39 The Ce3+/(Ce4+ + Ce3+) ratios of all catalysts concluded from the peak fitting results are listed in Table 2. For the CeZrO sample, the Ce3+/(Ce4+ + Ce3+) ratio is 37.7% and the ratio of Ce3+/(Ce4+ + Ce3+) for the CeZrMO samples

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Table 2 The quantitative results of the mole ratio of different atoms by XPS and peak-fitting results of O1s and Ce 3d spectra of different samples

Surface composition (at.%) Sample

Ce

Zr

O

Ma

Oα/(Oα + Oβ) (%)

Ce3+/(Ce3+ + Ce4+) (%)

CeZrO CeZrFeO CeZrCuO CeZrMnO CeZrCoO

25.2 18.0 17.1 19.8 17.0

4.6 4.8 5.2 5.4 4.9

70.3 76.4 76.0 74.6 77.1

— 0.7 1.8 0.2 0.9

46.7 53.7 47.8 50.9 56.1

37.7 44.0 41.0 45.0 46.9

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a

M referred to Fe, Cu, Mn and Co.

Fig. 4 Ce 3d core level XPS spectra of catalysts: (a) CeZrO, (b) CeZrFeO, (c) CeZrCuO, (d) CeZrMnO, and (e) CeZrCoO.

Fig. 5 O 1s core level XPS spectra of catalysts: (a) CeZrO, (b) CeZrFeO, (c) CeZrCuO, (d) CeZrMnO, and (e) CeZrCoO.

can be sequenced as CeZrCoO (46.9%) > CeZrMnO (45.0%) > CeZrFeO (44.0%) > CeZrCuO (41.0%). Obviously, the content of Ce3+ of all CeZrMO samples is higher than that of the CeZrO catalyst, indicating part of those doped elements are moved into the lattice of CeZrO.40,41 Furthermore, the transformation from Ce4+ to Ce3+ is possibly derived by the charge imbalance, which can reflect the different intensity of interaction between the doping elements and CeZrO.25,35,40 As reported in the previous literatures, oxygen vacancies can be generated through the transformation between Ce4+ and Ce3+ (4Ce4+ + O2− → 4Ce4+ + 2 e−/□ + 0.5O2 → 2Ce4+ + 2Ce3+ + □ + 0.5O2, □ represents an empty position) and the higher Ce3+ concentration could lead to the production of more oxygen vacancies and relatively high mobility of bulk oxygen species.9,10,42 It was suggested that the oxygen vacancies should be increased by the introduction of those transition metals. The O 1s XPS spectra of CeZrO sample are compared with all CeZrMO samples in Fig. 5. By deconvolution, the O 1s spectra can be fitted into two Gaussian peaks. The peak centered at 529.6–530.1 eV stands for lattice oxygen (thereafter denoted as Oβ) and the one located at 531.3–532.0 eV is related to surface-absorbed oxygen from the oxide defects or hydroxyl groups (thereafter denoted as Oα).12,43 Due to its high mobility, Oα is more active than Oβ in oxidization reaction. As a result, higher content of Oα can promote the oxidation of NO to NO2 at low temperature and thereafter facilitate

the “fast SCR” reaction.37,44,45 Thus, the variation of Oα/(Oα + Oβ) ratio could lead to the diversity in performance for NH3-SCR of NO in the low temperature range.43 As listed in Table 2, the highest atomic ratio of Oα/(Oα + Oβ) can be found on the surface of CeZrCoO (56.1%), followed by CeZrFeO (53.7%), CeZrMnO (50.9%), CeZrCuO (47.8%) and CeZrO (46.7%). Obviously, the Oα/(Oα + Oβ) ration increased after the introduction of foreign cations, especially for CeZrCoO. This result indicates there are more oxide defects or hydroxyl groups in the in situ doped catalysts than that in CeZrO. The abundant oxide defects could effectively absorb the O2 and form active oxygen species, which is beneficial for the low-temperature SCR performance.43 Moreover, the surface hydroxyl groups could act as Brønsted sites. As is well known, the Brønsted sites could absorb NH3 and form NH4+ species. Therefore, it can be deduced that the NH3 absorption ability could be enhanced by the introduction of foreign cations owing to the enriched hydroxyl groups.

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3.4 H2-TPR analysis The H2-TPR analysis was conducted to evaluate the influence of the in situ doping on the redox property of CeZrO. Fig. 6 illustrates the TPR results of CeZrO and CeZrMO. Only one reduction peak located at 567 °C can be observed in the H2-TPR curve of CeZrO. According to the literatures, the

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Fig. 6 H2-TPR profiles of the catalysts: (a) CeZrO, (b) CeZrFeO, (c) CeZrCuO, (d) CeZrMnO, and (e) CeZrCoO. Fig. 7 NH3-TPD profiles of the catalysts: (a) CeZrO, (b) CeZrFeO, (c) CeZrCuO, (d) CeZrMnO, and (e) CeZrCoO.

single peak could be attributed to the reduction of surface capping oxygen of CeZrO solid solution.46,47 After the introduction of the foreign cations, the reduction peak moved to a lower temperature range in the sequence of CeZrCoO > CeZrFeO > CeZrCuO > CeZrMnO, indicating the mobility of oxygen species is generally enhanced by the in situ doping. The enhancement is mainly due to the synergistic effect between the dopants and the Ce–Zr oxide solid solution, which can create oxygen defects and structural distortion.23,37 It is worth noticing that the area of the main reduction peak for CeZrMnO sample is larger than that of other samples. As the area of the reduction peak is related to H2 consumption, this result indicates that CeZrMnO have more active oxygen species.48 According to the N2 sorption results mentioned above, the enlarged specific surface area of CeZrMnO may relate to the abundant surface absorbed oxygen, which could facilitate the catalytic activity. Since the reduction peak of the Fe, Mn, Co species are absent in the TPR curves, we could draw the conclusion that those dopants are well incorporated into the 3DOM CeZrO skeleton and play a synergistic role with the Ce–Zr oxide solid solution. In particular, two small peaks located at 228 °C and 256 °C can be detected in the H2-TPR profile of CeZrCuO. The two extra peaks may relate to the surface Cu species in amorphous state or crystalline form.20,41 This result denotes that some of the Cu2+ ions are not well inserted into the Ce crystalline, in conformity with the XPS results.

NH3-TPD profiles, which are related to NH3 molecules absorbed on the stronger acid sites belonging to the catalysts. Particularly, as compared with other catalysts, a new peak located at relatively high temperature can be observed in the NH3-TPD profiles of CeZrCoO, indicating the quantity of strong acid sites has been increased by the Co doping. As is well known, the ammonia adsorbed on Brønsted acid sites is less thermally stable than the Lewis-coordinated ammonia during the TPD process.2 Therefore, the low temperature desorption peak may be attributed to the Brønsted acid sites while desorption peaks located at higher temperature could be assigned to the Lewis acid sites. Obviously, most of absorbed NH3 are linked to the weaker acid sites, suggesting Brønsted acid sites play a predominant role in the reaction. Previous studies have shown that the peak location is closely related to the adsorption strength and that the peak area corresponds to the adsorption amount of NH3.11 Interestingly, after the introduction of foreign cations, the strongest desorption peak associated with Brønsted acid sites moves to higher temperature and becomes more insensitive in varying degrees. In this regard, the in situ doping not only enhances the acid strength but also promotes the acid amount on the surface of catalysts, which agrees well with the deduction in the XPS section. As a result, more ammonia species can participate in the SCR reaction, which could lead to the promotion of the activity of NH3-SCR of NO.

3.5 NH3-TPD analysis It is generally accepted that the adsorption and activation of NH3 on the catalysts surface is the key factor in the NH3-SCR reaction.12 In order to understand the surface acid strength and amount of the catalysts, the NH3-TPD test was performed. The NH3-TPD profiles of catalysts are presented in Fig. 7. As shown in Fig. 7, a strong peak located below 200 °C can be found in the NH3-TPD profiles of all catalysts, which can be assigned to the NH3 desorbed by weak and medium acid sites on the catalysts surface. Moreover, with the increasing temperature, several small peaks occurred in the

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3.6 Catalytic performance In this work, the catalytic activities of all samples were estimated by NH3-SCR for NO in a temperature range from 150 °C to 450 °C and their catalytic performances are demonstrated in Fig. 8. As shown clearly, the NO conversion rate over all samples is firstly increased then decreased with the temperature rising. In this research, the CeZrO sample exhibits a wide operation window ranging from 270 °C to 450 °C, illustrating the CeZrO solid solution exhibits good performance for NH3-SCR of NO. Moreover, the doped

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3.7 Stability and SO2 tolerance

Fig. 8 NH3-SCR performance of CeZrO and CeZrMO catalysts as function of temperatures. Reaction conditions: NO=NH3 = 500 ppm, O2 = 3 vol%, N2 as balance gas, GHSV: 20 000 h−1.

samples show higher NO conversion than the CeZrO catalysts in the temperature range of 180–240 °C. The enhancement in the low temperature catalytic activity of the CeZrMO catalysts is not only due to their enhanced surface reduction ability and acidity as indicated by H2-TPR and NH3-TPD results, but also results from the enriched chemisorbed oxygen as clarified by XPS spectra. Among the catalysts, the CeZrCoO sample exhibits the strongest surface mobility and the most acid sites. As a result, the operation temperature of the CeZrCoO sample shifts towards the low temperature range by about 60 °C. However, the catalytic activity of CeZrCoO dropped dramatically at 360 °C. Co based catalysts generally exhibit low N2 selectivity at high temperature range, so the sharp decrease of NO conversion of the Co containing catalyst may result from the non-selective NH3 oxidation.49,50 After introducing Fe and Mn additives, the NO conversion rate at 240 °C increased from 71% to 93% and 91%, respectively. In the meantime, the catalytic performance in the high temperature range maintained a relatively high level when compared to other catalysts. The promotional effect of Fe and Mn to the catalytic performance could be attributed to the strong interaction with the CeZrO as well as the improved textural properties of the catalysts. Due to the decreased surface area and the weak interaction between Cu and the CeZrO support, only a slight up shift of catalytic activities of CeZrCuO sample in the low temperature range can be observed. Meanwhile, the catalytic performance of CeZrCuO decreased sharply in the temperature region of 390 °C. Kwak et al. reports the bulk Cu oxide species could react with NH3 to produce NO2.51 Considering a higher content of Cu than other doped elements on the surface of catalysts observed by XPS spectra, the agglomeration and sintering of surface Cu species might be the main reason for the rapid drop of catalytic activity. In summary, the Mn, Fe, Co doping effectively improved the catalysis activity of CZO. When compared to the other high-activity catalysts reported by other researchers,52–54 the CZMnO, CZFeO, CZCoO catalysts show a comparable activity in the low temperature region.

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In order to estimate the catalytic performance of the catalysts, the stability test was measured in this study. In addition, even after desulphurization, there is still low concentration of SO2 in the exhaust fumes. The residual SO2 gas could cause a serious poisoning effect on the catalytic activity.17,55,56 Therefore, the SO2 tolerance test of the catalysts was also conducted after the 16 h stability test. Fig. 9 shows the test results under a constant temperature (270 °C) and a high SO2 concentration (200 ppm). In the first 16 hours, the catalysts worked without the SO2 flow. The catalytic activities for CeZrO, CeZrCoO, CeZrMnO and CeZrFeO samples maintained high activity and stability, suggesting the 3DOM structure could prevent the sintering of active components during the test period. However, the NO conversion of the CeZrCuO sample slightly dropped from 92% to 89%. This result verified that some of the surface Cu species are agglomerated in the catalytic process, which is consistent with the catalytic performance tests described above. After switching SO2 on, the CeZrMO catalysts start to show different anti-sulfur abilities. The CeZrFeO sample exhibited the best sulfur resistance performance. In the first 2 hours, the NO conversion rate only slightly decreased from 95% to 87% and then stabilized. While turning SO2 off, the NO conversion rate restored to 90% in 1 h and then reached a steady state. In the presence of SO2, the NO conversion of CeZrCoO and CeZrMnO catalysts dropped to 83% and 79%, respectively. After the removal of SO2 from the feed gas, the conversion of NO over the two catalysts was restored to 85% and 83%. In contrast, when adding SO2 in the feed gas, the NO conversion of CeZrO and CeZrCuO dropped significantly by 34% and 39%, respectively. Their catalytic performance were both restored after the elimination of SO2 gas. According to the previous reports, the decreased catalytic activity is mainly due to the generation of sulfate species on the surface of the catalysts, which subsequently occupy the surface active sites.57,58 So the deactivation of catalytic performance can be

Fig. 9 Stability and SO2 tolerance test of CeZrO and CeZrMO catalysts at 270 °C. Reaction conditions: NO=NH3 = 500 ppm, SO2 = 200 ppm (when used), O2 = 3 vol%, N2 as balance gas, GHSV: 20 000 h−1.

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explained by the following two reasons: (1) formation of ammonia sulfate and their deposition on the surface of catalysts. It is reported that ammonia sulfate could start to decompose at the temperature above 250 °C.59 Therefore, the regain of de-NOx activity after removal of SO2 gas could be attributed to the decomposition of surface ammonia sulfate species as the reaction temperature (270 °C) was higher than 250 °C. (2) Sulfation of the metal oxide and their inactivation for the NH3-SCR ability. Moreover, the metal sulfates are difficult to decompose in the low temperature range, which could lead to the irreversible poisoning effect.32 This effect is related to the physical chemistry of the components on the catalysts’ surface.12 From the TPR and XPS result, the majority of Mn, Fe and Co species are possibly moved into the Ce crystal lattice or highly dispersed on the surface of the catalysts, which could lead to strong interaction between the active components. By contrast, the agglomerated Cu species on the surface of the catalysts could be easily inactivated by sulfation. Thus, we can draw the conclusion that the robust anti-sulfur capabilities of CeZrFeO, CeZrCoO as well as CeZrMnO are mainly due to the strong interaction between the in situ doped transition elements and the CeZrO solid solution, which is clarified by the XPS, TPR and TPD results. Nevertheless, the separation of the Cu species from CeZrO can be observed in the XPS and TPR analysis, which indicates that the poor SO2-tolerance ability of CeZrCuO catalysts is not only caused by the sulfation of the exposed Cu species on the surface of CeZrMO, but also by the weak interaction between the dopant and the CeZrO solid solution.

4. Conclusions In this study, we successfully fabricated 3DOM CeZrO catalysts via the colloidal template method and in situ doping with Mn, Fe, Cu, Co. The XRD, XPS and TPR analysis indicated those additives were well diffused into the Ce lattice, except for the Cu species. The high dispersion could lead to a strong synergistic effect between the dopants and the CeZrO host. Moreover, the active oxygen species, surface reducibility and acidity are enhanced by the in situ doping. Based on these favorable properties, the CeZrMO catalysts display better low temperature catalytic activity than CeZrO. Among them, the Co doped catalyst exhibited the best low temperature catalytic activity, which is contributed to the highest Oα/(Oα + Oβ) ratio, the highest reducibility as well as the strongest NH3 absorption ability. Meanwhile, the Fe and Mn dopants effectively broadened the operating temperature window, as a result of the larger surface area and pore volumes. In addition, the catalysts doped by Fe, Mn and Co demonstrated satisfactory stability and SO2-tolerance, which could attributed to the strong interaction between the dopants and CeZrO solid solutions.

Acknowledgements The authors acknowledge the support of the National Natural Science Foundation of China (51108258), the Shanghai

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Municipal Education Commission (14ZZ097), the Science and Technology Commission of Shanghai Municipality (11NM0502200 &13NM1401200) and the Doctoral Fund of Ministry of Education of China (20123108120018). The authors would like to thank Prof. W. J. Yu for help with the TEM measurements and Miss Cheng Fang for productive discussions.

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