Environ. Sci. Technol. 2010, 44, 4747–4752
Hydrotalcite-Derived MnxMg3-xAlO Catalysts Used for Soot Combustion, NOx Storage and Simultaneous Soot-NOx Removal Q I A N L I , † M I N G M E N G , * ,† H U I X I A N , † NORITATSU TSUBAKI,‡ XINGANG LI,† YANING XIE,§ TIANDOU HU,§ AND JING ZHANG§ Tianjin Key Laboratory of Applied Catalysis Science and Engineering, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P. R. China, Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama city, Toyama 930-8555, Japan, and Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China
Received November 9, 2009. Revised manuscript received March 13, 2010. Accepted May 11, 2010.
The hydrotalcite-based MnxMg3-xAlO catalysts with different Mn: Mg atomic ratios were synthesized by coprecipitation, and employed for soot combustion, NOx storage and simultaneous soot-NOx removal. It is shown that with the increase of Mn content in the hydrotalcite-based MnxMg3-xAlO catalysts the major Mn-related species vary from MnAl2O4 and Mg2MnO4 to Mn3O4 and Mn2O3. The catalyst Mn1.5Mg1.5AlO displays the highest soot combustion activity with the temperature for maximal soot combustion rate decreased by 210 °C, as compared with the Mn-free catalyst. The highly reducible Mn4+ ions in Mg2MnO4 are identified as the most active species for soot combustion. For NOx storage, introduction of Mn greatly influences bulk NOx storage, with the adsorbed NOx species varying from linear nitrites to ionic and chelating bidentate nitrates gradually. The coexistence of highly oxidative Mn4+ and highly reductive Mn2+ in Mn1.0Mg2.0AlO is favorable to the simultaneoussoot-NOx removal,givingaNOx reductionpercentage of 24%. In situ DRIFTS reveals that the ionic nitrate species are more reactive with soot than nitrites and chelating bidentate nitrates, showing higher NOx reduction efficiency.
1. Introduction Concerns have grown on the negative effects of soot and nitrogen oxides (NOx) on human health and environment (1, 2). Engine modifications can only lower the amounts of NOx and soot to a limited extent, so advanced catalytic techniques for exhaust after-treatment should be developed to remove these hazardous materials. Simultaneous soot-NOx removal was first proposed by Yoshida et al. over CuO-based catalysts (3). Afterward, many other catalysts (4, 5) were also applied to this reaction. Recently, Mn-based oxides have been reported to be extremely active for both soot combustion and NOx storage/ * Corresponding author phone/fax: +86-(0)22-2789-2275; e-mail:
[email protected]. † Tianjin University. ‡ University of Toyama. § Chinese Academy of Sciences. 10.1021/es9033638
2010 American Chemical Society
Published on Web 05/21/2010
reduction (6-8). Besides, calcined hydrotalcite-like compounds (HTlcs) have also received more and more attention (9-11) and have been successfully applied to many reactions such as NOx and SOx removal (12, 13). Among them, the Mg3AlO oxide seems to be a promising support for both soot combustion and NOx storage/reduction. It is found that KNO3 or K2CO3 supported Mg-Al hydrotalcite-derived mixed oxides possess high catalytic activity for soot combustion (14). Moreover, the hydrotalcite-based Mg-Al-O catalysts promoted by 1 wt % Pt show much better NOx storage properties in comparison with Toyota-type Pt-Ba/Al2O3 catalyst at low temperatures (e250 °C) (15). As a combination, what about the catalytic performance of Mn-incorporated hydrotalcitebased Mg3AlO catalysts for simultaneous soot-NOx removal? Since the redox performance of calcined hydrotalcitelike compounds strongly depends on the metal content, a series of MnxMg3-xAlO catalysts with different contents of Mn were prepared and used for soot combustion, NOx storage and simultaneous soot-NOx removal. The purpose of this study is to investigate that how Mn-related species with variable oxidation states (+2, +3, +4) affect the catalytic performance of the Mn-doped hydrotalcite-based catalysts. Based on characterization results, the reaction mechanisms are revealed or discussed.
2. Experimental Section 2.1. Catalyst Preparation. The hydrotalcite-like compounds MnxMg3-xAl were prepared using coprecipitation method by adding mixed salt solution and mixed basic solution dropwise into distilled water simultaneously at constant pH (10 ( 0.5) under vigorous mechanical stirring. The mixed salt solution consists of Mn(CH3COO)2 · 4H2O, Mg(NO3)2 · 6H2O and Al(NO3)3 · 9H2O with the designed molar ratio. The mixed basic solution contains NaOH and Na2CO3 with [OH-] ) 2.0 M and [OH-]/[CO32-] ) 16. All the reagents are analytical grade, supplied by Kewei Chemicals Company of China. The formed precipitates were kept in suspension at 60 °C for 4 h, then filtered and thoroughly washed with distilled water. After the cake was dried at 70 °C for 12 h and at 120 °C overnight, the precursor of hydrotalcite-like compound was calcined at 800 °C for 4 h to get the desired catalysts MnxMg3-xAlO (x ) 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0), denoted as Mnx, where x represents the atomic content of Mn in the samples. It should be noted that the catalysts are simply named according to the theoretical contents of metallic elements in the starting materials. 2.2. Catalyst Characterization. A Perkin-Elmer Diamond TG/DTA Instruments was used to obtain the differential TG curves (DTG). Each time, about 10 mg of the sample were heated at a rate of 10 °C/min. The X-ray diffraction measurement was performed on an X’pert Pro rotatory diffractometer (PANAlytical) operating at 30 mA and 45 kV using Co KR as radiation source (λ ) 0.1790 nm). The data of 2θ from 10 to 100° were collected at a stepsize of 0.033°. Extended X-ray absorption fine structure (EXAFS) was determined on the 1W1B beamline of Beijing Synchrotron Radiation Facility (BSRF) operating at ∼120 mA and 2.5 GeV. The Mn K-edge spectra of the samples and reference compounds were recorded at room temperature. A Si(111) double crystal monochromator was used to reduce the harmonic content in monochrome beam. The back-subtracted EXAFS function was converted into k space and weighted by k3 in order to compensate for the diminishing amplitude due to the decay of the photoelectron wave. The Fourier transforming of the k3-weighted EXAFS data was VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Chemical Composition, Texture Data, NOx Storage Capacities and NOx Reduction Percentages of the MnxMg3-xAlO Catalysts sample
Mn:Mg:Al atomic ratioa
specific surface area (m2/g)
NOx uptake (µmol/g · catalyst)
NOx reduction percentage (%)
Mn-free Mn0.5 Mn1.0 Mn1.5 Mn2.0 Mn2.5 Mn3.0
0.0:2.8:1.0 0.5:2.3:1.0 0.9:2.0:1.0 1.3:1.6:1.0 1.5:1.3:1.0 1.7:0.5:1.0 1.8:0.0:1.0
174 92 48 32 32 39 58
373 (100-272 °C) 657 (100-404 °C) 502 (100-386 °C) 271 (100-336 °C) 233 (100-355 °C) 85 (100-202 °C) 108 (100-213 °C)
7.2 (278-700 °C) 20.4 (327-614 °C) 24.0 (295-554 °C) 12.6 (308-700 °C) 6.9 (253-460 °C) 6.5 (263-618 °C) 10.4 (212-648 °C)
a
Mn:Mg:Al atomic ratios are calculated from ICP analysis.
performed in the range from k ) 3-14 Å-1 using a Hanning window function. X-ray photoelectron spectra (XPS) were recorded on a PHI-1600 ESCA spectrometer using Mg KR radiation (1253.6 eV). The base pressure was 5 × 10-8 Pa. The binding energies were calibrated using C1s peak (BE ) 284.6 eV) as standard and quoted with a precision of (0.2 eV. Surface area was measured by nitrogen adsorption/ desorption at 77 K using a Quadrasorb SI instrument. The samples were degassed at 350 °C for 10 h before adsorption experiments. The surface area (SBET) was determined by BET method. Chemical composition of the samples was determined by inductively coupled plasma (ICP) method using Vista MPX instrument. Temperature-programmed measurements were performed on a Thermo-Finnigan TPDRO 1100 instrument equipped with a thermal conductivity detector. A heating rate of 10 °C/min and a gas flow rate of 20 mL/min were used. In situ DRIFTS measurement was performed on a Nicolet Nexus spectrometer, equipped with a MCT detector cooled by liquid nitrogen. The spectra were recorded against a background spectrum of the sample purified just before introducing the adsorbates. Each time, 15 mg of the sample in powder form were used. The NOx adsorption was performed to reveal NOx storage mechanism. The sample was pretreated under 5 vol% O2/He at 100 °C for 30 min, then exposed to a flow of 750 ppm NO, 2.5 vol% O2 and balance He. The spectra of NOx adsorption from 100 to 600 °C at intervals of 50 °C were recorded with a spectral resolution of 4 cm-1. 2.3. Activity Measurement. For soot combustion, the catalytic activity of the samples was evaluated by TG/DTA technique using Degussa Printex-U as the model soot, which possesses a surface area of 100 m2/g and a average particle size of 25 ( 3 nm (C: 92.2 wt %; H: 0.6 wt %; volatiles: 6 wt %). The soot was mixed with the catalyst to obtain a tight contact in a weight ratio of 1:20. Then, the mixture (8 mg) was loaded to the sample chamber and heated from room temperature to 800 °C at a rate of 10 °C/min in the atmosphere of 750 ppm NO and 10% O2 balanced by N2. The catalytic activity of the samples was evaluated by the characteristic temperatures in DTG profiles. Besides, TG/DTA experiments in pure N2 were also carried out. For NOx storage and reduction, experiments were carried out in a continuous fixed-bed reactor under the atmosphere of gaseous mixture containing 750 ppm NO, 10 vol% O2 and balance N2. The granular sample (40-60 mesh, 0.3 mL) of soot/catalyst mixture (1:20 w/w) was fixed in a quartz tube (i.d. ) 8 mm) by packing quartz wool at the end of the bed. Gaseous mixture was fed to the sample at 240 mL/min and the sample was heated from 100 to 700 °C at a rate of 10 °C/min. An NO-NO2-NOx analyzer (Thermo Scientific) was used to measure the NOx concentration. 4748
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3. Results and Discussion 3.1. Characterization of the Precursors. The MnxMg3-xAlO precursors just after drying at 120 °C were characterized by DTG technique, as shown in Figure S1 in the Supporting Information (SI). Three weight-loss peaks are identified for the samples with x e 1.5, which correspond to the thermal decomposition of the hydrotalcite-like compounds (16). For the precursor of Mn-free catalyst, the first peak (100-200 °C) is attributed to the loss of interlayer and adsorbed water molecules whereas the layered structure still remained. Removal of hydroxyl and interlayer nitrate anions takes place from 250 to 400 °C. At this stage, part of the layered structure is collapsed. Upon heating to 600 °C, complete pyrolysis occurs and the layered structure is destroyed thoroughly. When small amounts of Mn are introduced (x e 1.5), the corresponding temperatures of these three peaks are lowered to some degree possibly due to the decrease in crystallinity of hydrotalcite-like compounds. For the samples with high Mn content (x > 2.0), only two peaks appear with one at 210 °C and the other at 334 °C, which can be attributed to the decomposition of Mn(OH)2 and MnCO3 (17). A single phase corresponding to hydrotalcite-like compound is obtained for the precursor of Mn-free sample, characterized by a group of X-ray diffraction peaks at 13.2°, 26.7°, 40.3°, 45.4°, 53.9°, 71.8°, and 73.1° (see SI Figure S2) (18). With the increase of Mn content (x g 1.5), the peaks corresponding to Mn(OH)2 and MnCO3 appear, suggesting that only 37.5% of the metal ions (Mg, Al) can be isomorphously substituted by Mn in the MgAl hydrotalcite-like compound. For the sample without Mg (Mn3.0-precursor), only phases of Mn(OH)2 and MnCO3 can be detected. 3.2. Characterization of the Catalysts. The Mn:Mg:Al atomic ratios in the calcined catalysts are calculated from ICP results, as listed in Table 1. For the samples with low Mn content (x e 1.5), no obvious differences was found between the Mn:Mg:Al atomic ratios in the calcined catalysts and that in the starting mixed solution, suggesting that the hydrotalcite-like MnxMg3-xAl compounds can be successfully prepared when small amounts of Mn are introduced (x e 1.5). This is in good accordance with the XRD and TG-DTA results of the corresponding precursors (see SI Figures S1 and S2). However, for the samples with high Mn content (x > 1.5), the content of Mn in the calcined catalysts was much lower than that in the starting mixed solution, indicating that hydrotalcite-like precursor was not formed when so much Mn was added. This result can be very well correlated with the XRD and TG-DTA analysis of the corresponding precursor. Figure 1 displays the XRD patterns of the hydrotalcitebased catalysts. For Mn-free sample, the peaks at 43.3°, 50.4°, 74.0°, and 95.0° can be assigned to MgO phase. A small amount of MgAl2O4-like spinel is also detected, characterized by the peaks at 43.1°, 52.6°, and 77.5°. After addition of Mn, MnAl2O4 is formed with the peaks appearing at 35.6°, 42.0°,
FIGURE 1. The XRD patterns of MnxMg3-xAlO catalysts.
FIGURE 2. Mn K-edge radial structure functions of the reference compounds (a) and the catalysts (b). 51.3°, 68.5°, and 75.6°. When more Mn are introduced (x ) 2.0), Mn3O4 and Mn2O3 phases can be clearly identified. Further increase of dopant Mn leads to the formation of more Mn2O3. Due to the broadness and complexity of the XRD peaks, it seems difficult to clearly distinguish the existing Mn phases in the Mn-containing catalysts. Thus, EXAFS measurement was performed for a more precise determination of the Mn phases. The radial structure functions (RSFs) of Mn K-edge of the reference compounds (MnO2, Mn2O3, Mn3O4, MnAl2O4, and Mg2MnO4) and the catalysts are presented in Figure 2 (a) and (b), respectively. By comparing Figure 2 (b) with Figure 2 (a), it is found that when a small amount of Mn exists (x
e 1.5), the RSFs of the samples are imilar to that of Mg2MnO4 with the coordination peaks appearing at 0.155, 0.208, 0.250, and 0.295 nm. These four peaks can be ascribed to MnsO bond (the first shell), MnsMn bonds (the second and third shells), and a mixture of MnsO and MnsMn bonds (the fourth shell), respectively (19). As the dopant Mn increases, the coordination distance for the third peak (around 0.24 nm) increases, indicating that the existence of MnAl2O4 is also potential. This is because the coordination distance of MnsAl bond at this position in MnAl2O4 (0.271 nm) is larger than that of MnsMn bond in Mg2MnO4 (0.250 nm) (20, 21). As the amount of Mn is further increased (x > 1.5), the RSFs become more similar to that of Mn3O4. However, the coordination peak appearing at 0.282 nm is hard to be assigned to any bond of Mn3O4. After careful analysis, this peak is most possibly contributed by Mn2O3, which has been identified in the XRD patterns. Based on EXAFS results, it is concluded that Mg2MnO4 and MnAl2O4 are the dominant Mn-related species in the samples with low Mn content, while Mn3O4 and Mn2O3 are the prevailing Mn-related species in the samples with high Mn content. To gain more information on the oxidation state of the relevant Mn cations, the XPS spectra of Mn2p were recorded and the deconvolution of the Mn2p3/2 peaks was performed, as displayed in SI Figure S3. It can be observed that all Mn2p3/2 peaks can be deconvoluted into two peaks, suggesting that two different oxidation states of Mn species exist for each Mn-containing sample. For the samples with low Mn content (x e 1.5), the Mn2p3/2 peak appears at the position of 642.2 eV. The corresponding deconvoluted peaks at 642.5 and 640.9 eV can be attributed to Mn4+ and Mn2+ (22), respectively. This correlates well with the EXAFS results, showing that Mg2MnO4 and MnAl2O4 are the dominant Mn-related species in the samples with low Mn content. As the dopant Mn increases, the ratio of Mn4+ and Mn2+ increases, indicating that Mg2MnO4 was formed gradually. For the samples with high Mn content (x > 1.5), the Mn2p3/2 peak appears at the position of 641.3 eV. The corresponding deconvoluted peaks at 641.8 and 640.9 eV can be attributed to Mn3+ and Mn2+, respectively. This is also consistent with the EXAFS results, suggesting that further increase of dopant Mn leads to the formation of Mn3O4 and Mn2O3. The texture data of MnxMg3-xAlO catalysts are listed in Table 1. The sample without Mn possesses much larger specific surface area. After introduction of Mn, an obvious decrease in the specific surface area can be observed due to the destroyed structure of hydrotalcite-like precursor (see SI Figure S2). When the dopant Mn is increased to a certain degree (x g 2.0), slight recovery of the surface area is observed, implying that the formed Mn3O4 and Mn2O3 should possess smaller crystallite size. To investigate the redox properties of the catalysts, H2TPR experiment was performed. As presented in Figure 3, no any reduction peaks is detected for the Mn-free sample. The profiles of Mn0.5 and Mn1.0 display two reduction regions, one between 450 and 550 °C, the other at ∼648 °C. According to the results of XRD (see Figure 1) and EXAFS (see Figure 2), the former can be attributed to the reduction of Mn4+ species in the Mg2MnO4-like phase, while the latter can be assigned to the reduction of Mn-related species in MnAl2O4 (23, 24). For Mn1.5, a peak centered at 332 °C is observed, which is also attributed to the reduction of Mn4+ in Mg2MnO4. Compared with Mn0.5 and Mn1.0, the reduction peak shifts to much lower temperature. In the samples Mn0.5 and Mn1.0, the interaction between Mg2MnO4 and aluminum-rich spinel possibly decreases the reducibility of surface Mn4+ in Mg2MnO4. As the dopant Mn increases to x ) 2.0, a weak reduction peak appears at 271 °C, which can be ascribed to the reduction of Mn3+ in small Mn3O4 crystallites. For Mn2.5 and Mn3.0, the peak around 400 °C can be attributed to the VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. H2-TPR profiles of MnxMg3-xAlO catalysts.
FIGURE 4. DTG profiles of soot combustion over MnxMg3-xAlO catalysts in the atmosphere of 750 ppm NO and 10 vol% O2 balanced by N2. reduction of Mn2O3 to Mn3O4 and the other one at 529 °C can be assigned to the reduction of Mn3O4 to MnO. 3.3. Catalytic Soot Combustion over MnxMg3-xAlO Catalysts. The performance of MnxMg3-xAlO catalysts for catalytic soot combustion, as well as the uncatalyzed soot combustion, is shown in Figure 4. It is obvious that without catalysts the oxidation of soot starts from ∼550 °C with the center at 640 °C. When the Mg3AlO (Mn-free) catalyst is loaded, the temperature for the maximal rate of soot combustion (Tm) is reduced by 65 °C. After further introduction of manganese, the Tm values display a volcano-type dependence on the Mn content in catalysts, following the order Mn1.5 > Mn1.0 > Mn0.5 ≈ Mn2.0 > Mn2.5 ≈ Mn3.0 > Mnfree. The sample Mn1.5 shows the best performance with the Tm at least 15 °C lower than that over other samples. Such sequence cannot be correlated with the specific surface areas of the samples or the redox properties derived from H2-TPR results, either. So other aspects and/or comprehensive factors should be considered. It is worth noting that from the XRD, EXAFS and XPS results, it has been concluded that Mg2MnO4 and MnAl2O4 are the main Mn-related species in the low Mn-content samples (x e 1.5). Therefore, the Mn4+ in Mg2MnO4 with high reducibility is proposed as the main active phase for soot combustion. However, the coexistence of Al3+ partially prevents the oxidation of Mn2+, resulting in the formation of stable MnAl2O4 spinel. So, increase of Mn content should be beneficial to the formation of Mg2MnO4. However, further increase of Mn content (x > 1.5) leads to the growth of Mn3O4 and Mn2O3 crystallites, which are regarded as the major Mn-related species in the high Mncontent samples, as a result, decreasing the soot combustion 4750
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FIGURE 5. NOx storage behaviors of the catalysts in the atmosphere of 750 ppm NO and 10 vol% O2 balanced by N2. activity. According to above analysis, it is natural that the sample Mn1.5 shows the best performance for soot combustion. To directly measure the reducibility of the samples, soot oxidation activity over them in pure N2 was determined (see SI Figure S4). It is found that Mn-free sample can hardly evoke the soot combustion. With the elevation of Mn content, the soot combustion activity over the catalysts increases at first and then decreases, which is in good agreement with that in the atmosphere of NO and O2 balanced by N2. So, it is deduced that the reducibility of the samples is also an important factor for soot combustion. The highly reducible manganese with +4 oxidation state in Mg2MnO4, as well as small Mn3O4 and Mn2O3 crystallite can greatly facilitate soot combustion. 3.4. NOx Storage and Reduction over MnxMg3-xAlO Catalysts. Figure 5 displays the NOx storage performance of MnxMg3-xAlO catalysts. Two major storage periods are observed, corresponding to the surface storage and bulk storage, respectively. The calculated amounts of NOx uptake based on peak area are listed in Table 1. It can be found that with the introduction of a certain amount of Mn (x e 1.0), NOx storage capacity (NSC) is increased obviously. However, with the further increase of Mn content, the NOx uptake decreases gradually, except for Mn3.0. This is conformable with the order for the specific surface areas of the catalysts, suggesting that the surface storage is important in the whole storage process. It should be noted that Mn0.5 and Mn1.0 show larger NOx storage capacity even though they possess smaller specific surface areas as compared with Mn-free catalyst. This implies that Mn species must have played a key role in the storage period, possibly enhancing the bulk storage via their oxidation ability. In a summary, the NOx storage performance of MnxMg3-xAlO catalysts is determined by both the specific surface area and the oxidation ability of the catalysts. For a better understanding on NOx storage, in situ DRIFTS experiments were carried out to figure out the storage pathways. On Mn-free catalyst, as shown in SI Figure S5(a), NOx is stored as bidentate or chelating nitrite species below 250 °C, characterized by the peak at 1220 cm-1 (25-27). Above 250 °C, a peak at 1460 cm-1 is observed which is attributed to linear nitrite species. The adsorption behaviors of Mn0.5, Mn1.0, and Mn1.5 are similar to each other, so only the results of Mn1.0 are chosen as the representative, as illustrated in SI Figure S5(b). It can be seen that chelating bidentate nitrates (1275 cm-1) are the major species in the low temperature range (100-150 °C). As the adsorption temperature increases (200-550 °C), the band at 1370 cm-1 appears, due to the formation of ionic nitrates. For the samples with high Mn content, Mn3.0 is taken as an example, as shown in SI Figure S5(c). Two adsorption periods are also observed, but the
adsorption only happens below 300 °C. In the first step (100-150 °C), NOx is stored as nitrite species, characterized by the peak at 1220 cm-1; in the second step (150-250 °C), NOx is stored as the chelating bidentate nitrate species, characterized by the peak at 1285 cm-1. As described above, in situ DRIFTS results correlate well with the NOx storage behaviors illustrated in Figure 5. The storage period below 200 °C is considered as the surface storage, which is in good agreement with the specific surface area of the catalysts, while the storage period at higher temperature is attributed to the bulk storage. Introduction of Mn shows profound effects on the bulk NOx storage species. As the dopant Mn increases, the adsorbed NOx in the bulk storage period varies from linear nitrites to ionic nitrates and chelating bidentate nitrates gradually. To evaluate the catalytic performance for the simultaneous NOx-soot removal over MnxMg3-xAlO catalysts, the NOx reduction percentages are calculated by comparing the amounts of desorbed NOx from the catalysts mixed with soot or not, as described elsewhere (10). The results are listed in Table 1. When a small amount of Mn is doped (x e 1.5), much better NOx reduction activity is achieved than Mnfree sample. It is hardly explained only on the basis of BET surface area or the oxidation ability of catalysts from H2-TPR results. In situ DRIFTS results of the samples with low Mn content have revealed that ionic nitrates are generated during the bulk storage period. Compared with nitrite and chelating bidentate nitrates, ionic nitrate may have stronger reactivity with soot, thus leading to higher NOx reduction efficiency over these samples. Additionally, it has been drawn that Mg2MnO4 and MnAl2O4 are the major Mn-related phases when x e 1.5. The existence of Mn4+ in Mg2MnO4 may enhance the formation of ionic nitrate; meanwhile, the Mn2+ in MnAl2O4 is proposed as the active site for the reduction of NOx by soot. Both the two aspects contribute to the NOx removal process. High Mn4+/ Mn2+ ratio in Mn1.5 or high Mn2+/Mn4+ ratio in Mn0.5 can only facilitate one part of the overall removal of soot and NOx. Medium content of Mn2+ and Mn4+ in Mn1.0 should be beneficial to both the formation of ionic nitrate and the reduction of NOx, resulting in the highest NOx reduction percentage of this catalyst. With further increase of dopant Mn (x > 1.5), NOx removal efficiency decreases, possibly due to the disappearance of highly oxidative Mg2MnO4 species. However, the Mn3.0 is an exception, which shows higher NOx reduction percentage than Mn2.0 and Mn2.5. This is related to the recovery of its specific surface area, as reflected in Table 1. This makes the Mn2O3 in Mn3.0 possess higher dispersion than that in Mn2.0 and Mn2.5, thus leading to higher simultaneous soot-NOx removal efficiency. In a summary, the soot combustion activity of MnxMg3-xAlO catalysts follows the order: Mn1.5 > Mn1.0 > Mn0.5 ≈ Mn2.0 > Mn2.5 ≈ Mn3.0 > Mn-free; the major Mn-related species varies from MnAl2O4 and Mg2MnO4 to Mn3O4 and Mn2O3 as the dopant Mn increases; the highly reducible Mg2MnO4 is the most active species for soot combustion; introduction of Mn to Mg3AlO catalysts greatly influences bulk NOx storage; with the increase of Mn, the adsorbed NOx in bulk storage period varies from linear nitrites to ionic and chelating bidentate nitrates gradually. Simultaneous sootNOx removal is achieved over the hydrotalcite-derived MnxMg3-xAlO catalysts; among them the sample Mn1.0 shows the best performance. The Mn4+ in Mg2MnO4 enhances the formation of ionic nitrate which possesses stronger reactivity with soot; meanwhile, the Mn2+ in MnAl2O4 facilitates the reduction of NOx by soot. Medium contents of Mn2+ and Mn4+ are favorable to both the formation of ionic nitrate and the reduction of NOx.
Acknowledgments This work is financially supported by the National Natural Science Foundation of China (No.20876110), the “863” Programs of the Ministry of Science and Technology of China (No. 2008AA06Z323), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20090032110013), and the Program of New Century Excellent Talents in University of China (NCET-07-0599). We are also grateful to the support from the Cheung Kong Scholar Program for Innovative Teams of the Ministry of Education (No. IRT0641) and the Program for Introducing Talents of Discipline to Universities of China (No. B06006).
Supporting Information Available Figure S1, S2, S3, S4 and S5. This material is available free of charge via the Internet at http://pubs.acs.org.
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