Ce–Ti Amorphous Oxides for Selective Catalytic Reduction of NO with

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Ce−Ti Amorphous Oxides for Selective Catalytic Reduction of NO with NH3: Confirmation of Ce−O−Ti Active Sites Ping Li,† Ying Xin,† Qian Li,† Zhongpeng Wang,† Zhaoliang Zhang,*,† and Lirong Zheng‡ †

School of Chemistry and Chemical Engineering, Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, University of Jinan, 106 Jiwei Road, Jinan 250022, China ‡ Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, PR China S Supporting Information *

ABSTRACT: The amorphous Ce−Ti mixed oxides were reported to be catalysts for selective catalytic reduction of NOx with NH3, in which Ce and not Ti acts as their solvent in spite of the fact that Ce is low in content. The amorphous catalysts were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) equipped with selective area electron diffraction (SAED). The Ce−Ti amorphous oxide shows higher activity than its crystalline counterpart at lower temperatures. Moreover, the presence of small CeO2 crystallites as for the impregnated sample is deleterious to activity. The Ce−O−Ti short-range order species with the interaction between Ce and Ti in atomic scale was confirmed for the first time to be the active site using temperature programmed reduction with H2 (H2−TPR), in situ FTIR spectra of NO adsorption, X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine-structure (XAFS). Lastly, the Ce−O−Ti structure was directly observed by fieldemission TEM (FETEM). great decrease in activity.9 In the latest work of He and coauthors,15 the active Ce−W mixed oxide was suggested to show exclusively the Ce−O−W phase; however, no direct evidence was provided. Furthermore, the synergistic effect between Ce and Ti10 or W15 was suggested. These declarations seem to be contradictory to the above-mentioned assignment of the active phase. In order to exclude the possibility of the formation of small CeO2 crystallites and thus the corresponding contribution to the SCR activity, the amorphous Ce−Ti oxide, which is thought to be homogeneous in compositions not only on the surface but also in the bulk, was studied in this paper for the SCR reaction of NOx with NH3. Importantly, the amorphous sample shows higher activity than its crystalline counterpart at lower temperatures. Moreover, the presence of small CeO2 crystallites as for the impregnated sample is deleterious to activity. The amorphous materials as catalysts are well-known for a long time. For the SCR reaction, the amorphous Cr2O3 catalyst is more active than the crystalline Cr2O3.17,18 Mn-based amorphous oxides were also reported to have excellent low temperature activity.19,20 However, no information on active phases or active sites is deeply analyzed for the amorphous

1. INTRODUCTION Nitrogen oxides (NOx) contribute much to acid rain, photochemical smog, and the depletion of tropospheric ozone. Selective catalytic reduction (SCR) of NOx with NH3 on commercial V2O5−WO3(MoO3)/TiO2 (VWTiO2) is now the most widely used technology for NOx control.1 However, for diesel engines, industrial boilers, and kilns as well as power plants where the SCR catalyst is placed downstream of the desulfurizer and electrostatic precipitator, the exhaust temperatures lie beneath the working temperature window of VWTiO2. Thus, an urgent demand for high activity at low temperatures is put forward, and a lot of works have been performed to find new SCR catalyst systems. Therein, CeO2based oxides attract increasing attention due to the well-known redox property of CeO2.2−7 Significantly, He and co-workers first found the active and selective Ce−Ti mixed oxides at medium temperatures.8 Afterward, Gao et al.9,10 and Shan et al.11 reported that the catalysts prepared by the single step sol− gel and homogeneous precipitation methods possess the superior SCR activity, respectively. The promotion effects by doping of Cu,12 Mn,13 W,14,15 and Mn (Fe) 16 were also proposed. Some of these authors agree that the highly dispersed CeO2 nanocrystallite is the active phase in Ce−Ti mixed oxides.8−11 However, only the TiO2 crystalline was observed in the most active catalysts.10,11 CeO2 was suggested to be in the amorphous phase. Increasing calcination temperature results in the crystallization of the amorphous CeO2, which also caused a © 2012 American Chemical Society

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April 27, 2012 July 11, 2012 August 13, 2012 August 13, 2012 dx.doi.org/10.1021/es301661r | Environ. Sci. Technol. 2012, 46, 9600−9605

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structure was finally characterized using a JEM-2100F FETEM. XPS data were obtained on an AXIS-Ultra instrument from Kratos Analytical using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) and low-energy electron flooding for charge compensation. To compensate for surface charge effects, the binding energies were calibrated using the C 1s hydrocarbon peak at 284.80 eV. H2−TPR experiments were performed in a quartz reactor with a thermal conductivity detector (TCD) to monitor the H2 consumed. A 50 mg sample was pretreated in situ at 500 °C for 1 h in a flow of O2 and cooled to room temperature in the presence of O2. TPR was conducted at 10 °C/min up to 900 °C in a 30 mL/min flow of 5 vol.% H2 in N2. To quantify the total amount of H2 consumed, CuO was used as a calibration reference. The in situ FTIR spectra of NO adsorption were recorded on a Bruker Tensor 27 spectrometer over 400−4000 cm−1 after 16 scans at a resolution of 4 cm−1. Self-supporting wafers were pretreated in the IR cell at 500 °C in a flow of He for 30 min to remove any adsorbed species. After cooling to 100 °C, the background spectrum was recorded. The IR spectra were recorded at 100 °C in the flow of 500 ppm NO + He (120 mL/ min) for 30 min followed by purging with He. XAFS measurements for the Ti K-edge (4850−5700 eV) and the Ce LIII-edge (5600−6100 eV) were performed in the transmission mode at room temperature on the XAFS station of the 1W1B beamline of Beijing synchrotron radiation facility (BSRF, Beijing, China). XAFS data were analyzed using IFEFFIT software package. In the least-squares-fitting procedure, the possible scattering paths were also calculated using this software package.28

structure. Often, a lot of high-activity catalysts present as mostly amorphous phases. For instance, Ce−Ti oxide systems were reported to contain a certain amount of amorphous CeO2 that are responsible for the activity.8−10 Similar amorphous components were also found in other catalyst systems, including Mn−Ti oxides21−23 and Fe−Ti oxides.24,25 However, for these two systems, different viewpoints are proposed. The iron titanate crystallites (FeTiO3 and Fe2TiO5) were claimed to be the active phase notwithstanding only some broad bumps were observed on the X-ray powder diffraction (XRD) patterns of Fe−Ti oxide systems.24,25 Unfortunately, the corresponding selective area electron diffraction (SAED) patterns were not performed in the given transmission electron microscopy (TEM) images. The Mn−Ti system is the same case.23 This strongly suggests that the elucidation of the catalyst structure and active sites of these oxide systems is necessary and crucial. In this work, a concept of the Ce−Ti amorphous oxide being the SCR catalyst is reported.26 The Ce−O−Ti short-range order species with the interaction between Ce and Ti in atomic scale is proposed for the first time to be the active site, which is confirmed by temperature programmed reduction with H2 (H2−TPR), in situ FTIR spectra of NO adsorption, X-ray photoelectron spectroscopy (XPS), and X-ray absorption finestructure (XAFS) and is directly observed by field-emission TEM (FETEM).

2. EXPERIMENTAL SECTION A series of Ce−Ti mixed oxides with different atomic ratios of Ce and Ti were prepared by a coprecipitation method. Typically, NH3•H2O solution (25%) was dropped into a stoichiometric solution (100 mL) of Ce(NO3)3•6H2O and Ti(SO4)2 under vigorous agitation until pH = 10. The suspension was aged in air for 48 h at room temperature and atmospheric pressure. The resultant precipitates were dried at 100 °C overnight and calcined at 500 °C for 5 h in air flow. Hereafter, they are denoted as CeaTiOx, where a represents the Ce/Ti atomic ratios and equals to 0.2, 0.3, and 0.5. For comparison, CeO2, TiO2, Ce/TiO2 (the impregnated sample corresponding to Ce0.3TiOx after calcination at 500 °C for 5 h), Ce0.3TiOx650 (Ce0.3TiOx after calcination at 650 °C for 5 h), and 1%V2O5−10%WO3/TiO227 were also prepared. A fixed-bed U-shaped quartz reactor (I. D. = 6 mm) with a thermocouple placed inside catalysts was used for reaction tests under atmospheric conditions. The model flue gas consisting of 500 ppm NO, 500 ppm NH3, and 5.3 vol.% O2 in He in 120 mL/min was employed. About 0.288 and 0.072 mL catalysts (40−60 mesh) were used, which correspond to gas volume hourly space velocity (GHSV) of 25 000 and 100 000 h−1, respectively. Concentrations of NO and NO2 were monitored by a chemiluminiscence NOx analyzer (42i-HL, Thermo). N2O and NH3 were detected by a FTIR spectrometer (Tensor 27, Bruker) with a 2.4 m gas cell. The data for steady-state activity of catalysts were collected after about 1 h testing. XRD patterns were recorded on a Rigaku D/max-2500/PC diffractometer employing Cu Kα radiation (λ = 1.5418 Å) operating at 50 kV and 200 mA. The Brunauer−Emmett− Teller (BET) surface area and pore structure were measured by N2 adsorption/desorption using a Micromeritics 2020 M instrument. Before N2 physisorption, the sample was outgassed at 300 °C for 5 h. High-resolution TEM equipped with SAED was conducted on a JEOL JEM-2010 microscope at an accelerating voltage of 200 kV. FESEM equipped with EDS was performed on a Hitachi SU-70 microscope. The micro-

3. RESULTS AND DISCUSSION Figure 1a shows XRD patterns of TiO2, CeO2, CeaTiOx, and Ce/TiO2 after calcination at 500 °C for 5 h. Ce0.3TiOx650 was also included. Pure CeO2 and TiO2 are present as cerianite (JCPDS 34-0394) and anatase (JCPDS 21-1272), respectively. Although Ce0.2TiOx shows the TiO2 phase, no obvious peak was observed for Ce0.3TiOx and Ce0.5TiOx, suggesting a completely amorphous structure,29 and thereby the maximized interaction between Ce and Ti.4 Increase in calcination temperature results in the emergence of a new phase CeTi2O6 (JCPDS 12-0477)30,31 as for Ce0.3TiOx650. The CeTi2O6 compound has a brannerite structure (TiU2O6), which consists of TiO6-octahedra sharing edges to form a zigzag layer similar to the anatase structure.32 However, Ce/ TiO2 is composed of TiO2 and CeO2 phases, which indicates that no strong interaction as in Ce0.3TiOx and Ce0.5TiOx exists between Ti and Ce for the impregnated sample, and thus leading to the segregation of TiO2 and CeO2 crystallites. As shown in Table S1, the BET surface area of CeaTiOx is much larger than that of TiO2, CeO2, Ce/TiO2, and VWTiO2. It decreases in the sequence of Ce0.3TiOx ≈ Ce0.2TiOx > Ce0.5TiOx > Ce/TiO2 > Ce0.3TiOx650. As usual, the surface area decreased with increasing calcination temperature. The resulting high surface area of CeaTiOx is attributed to the inhibition of the individual crystallization during coprecipitation,33−36 as shown by XRD. Figure 1b shows the TEM image and the corresponding SAED pattern (inset) of Ce0.3TiOx. Neither circles nor dots were observed, which is the direct evidence of the amorphous structure of Ce0.3TiOx. Furthermore, the FESEM images and the corresponding distribution maps of Ce, Ti, and O elements 9601

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Figure 2. NOx conversions in the SCR reactions as a function of reaction temperatures from 150 °C−450 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 5.3 vol.% O2 + He; total flow rate: 120 mL/min; GHSV: 25 000 h−1.

Figure 1. a) XRD patterns of the Ce−Ti mixed oxides and b) TEM image and SAED pattern (inset) of Ce0.3TiOx.

for Ce0.3TiOx (Figure S1) show that the atomic ratio between Ce and Ti analyzed by EDS was the same value as that in the starting materials. This suggests the formation of the homogeneous solution of CeO2 and TiO2, consistent with XRD and TEM results. Moreover, the amorphous Ce0.3TiOx shows a broad pore distribution, which is different from that of crystallized samples (Figure S2). Figure 2 shows NOx conversions for Ce−Ti mixed oxides and VWTiO2 as a function of reaction temperatures. TiO2 and CeO2 show much low activity. Surprisingly, CeaTiOx is more active than VWTiO2 at the low-temperature range. Especially, Ce0.3TiOx shows NOx conversion above 90% from 175 to 400 °C, and the N2 selectivity was always above 95% (Figure S3a). However, both Ce/TiO2 and Ce0.3TiOx650 are less active than VWTiO2. Ce0.3TiOx is still the most active at 100 000 h−1 (Figure S3b and c), though the temperature at which 90% NOx conversion is achieved increased to 250 °C. Since SO2 and H2O are present in the exhausts and it is known that they influence the performances of SCR systems, the durability of SO2 and H2O was studied on Ce0.3TiOx. As shown in Figure S4a, the NOx retained about 95% conversion at least for 65 h in the presence of 200 ppm SO2 and 10% H2O at 350 °C. After reactions, the amorphous characteristics are reserved (Figure S4b). The above XRD and activity results suggest that the SCR properties are not associated with the crystallinity but possibly with the short-range order of oxides. The low-temperature activity is generally determined by the reducibility of catalysts.37 Figure 3 shows H2−TPR profiles of Ce−Ti mixed oxides. TiO2 has no obvious H2 consumption peaks during the whole temperature range. In contrast, CeO2

Figure 3. H2−TPR profiles of the Ce−Ti mixed oxides.

shows two peaks centered at about 500 and 800 °C, which can be assigned to the reduction of surface oxygen and lattice oxygen, respectively.38 However, CeaTiOx shows only one peak. Furthermore, the H2 consumed is considerably higher than that of CeO2 (Table S1). The two facts suggest that an interaction does exist between Ce and Ti and that the presence of Ti ions weakens the Ce−O bond in CeaTiOx, which makes the Ce−O component be more easily reduced. The reduction of Ce/TiO2 is similar to that of CeO2, but the low-temperature H2 consumption is much less than that of Ce0.3TiOx, which is an indicator of the weak interaction between Ce and Ti on Ce/ TiO2. Ce0.3TiOx650 shows the highest peak temperature; however, its H2 consumption is the same as that of Ce0.3TiOx, implying the possible existence of a Ce−O−Ti species in the amorphous Ce0.3TiOx sample, due to the presence of CeTi2O6 in Ce0.3TiOx650 from XRD results. Figure 4 shows the in situ FTIR spectra of NO adsorption on Ce−Ti mixed oxides at 100 °C after pretreatment by He at 500 °C for 30 min. The assignments of the present IR bands were collected in Table S2. Different from TiO2, CeO2, and Ce/ TiO2, CeaTiOx and Ce0.3TiOx650 show a new band at 1235 cm−1, which might be assigned to a bridge-bound nitrite4 or nitrate23,39 on Ce and Ti sites, suggesting the existence of the Ce−O−Ti structure. 9602

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Figure 4. In situ FTIR spectra of Ce−Ti mixed oxides treated with 500 ppm NO in He at 100 °C for 30 min and then purged by He.

Figure 5. Ce LIII edge radial structure functions for Ce−Ti mixed oxides.

As shown in the XPS analysis (Figure S5 and Table S3), the higher concentration of Ce on Ce/TiO2 than that on CeaTiOx coincides with the fact that the former was prepared by the impregnation method. However, the percentage of Ce3+ on the latter is much more than that on the former. The partial reduction of Ce4+ can be attributed to the incorporation of Ti into the Ce−O−Ce species to form the Ce−O−Ti structure.33 Furthermore, the higher Ce 3d binding energy in Ce0.3TiOx than that in Ce/TiO2 suggests that the Ce species in Ce0.3TiOx shows more severe deviation of electron cloud by interacting with the Ti species, leading to an enhancement in oxidative ability,25 in agreement with the H2−TPR result. The Ti 2p3/2 binding energies for Ce/TiO2 and Ce0.3TiOx are 458.5 and 458.2 eV, respectively, which are characteristic of Ti4+ in TiO2 (458.6 eV).40 This indicates that Ti is in a +4 valence state and Ce atoms do not enter the Ti−O−Ti lattice, which is reasonable considering the much larger size of Ce4+ (0.97 Å) or Ce3+ (1.23 Å) than that of Ti4+ (0.61 Å).33−35 However, a binding energy increase of Ce as for Ce0.3TiOx compared with Ce/TiO2 should be accompanied by a little binding energy decrease of Ti in an isolated system. The O 1s peaks for Ce0.3TiOx are between those of TiO2 and Ce/TiO2 (CeO2), again suggesting the chemical interaction between Ce and Ti and thus the possible formation of the above-mentioned Ce− O−Ti structure in CeaTiOx. XAFS is a powerful method to investigate the local structure around a specific component and can be used to obtain the structure information around Ce and Ti atoms in amorphous Ce−Ti mixed oxides. The Ti K-edge X-ray absorption near edge structure (XANES) (Figure S6a) and extended X-ray absorption fine structure (EXAFS) (Figure S6b) spectra for TiO2, Ce/TiO2, and CeaTiOx are similar. The best-fit EXAFS data (Figure S6c and Table S4) definitely show the first and second shells of Ti−O and Ti−Ti, respectively. The presence of Ce3+ is clearly perceptible in Figure S7a. The sequence of Ce3+ content (Figure S7b and Table S5) is consistent with that from XPS (Figure S7c). In the radial structure function (RSF) curves around Ce in CeO2 and Ce/ TiO2 (Figure 5), two similar peaks of A and B are recognized, corresponding to the interatomic distances of Ce−O (first shell) and Ce−Ce (second shell), respectively. Although the position of the Ce−O peak shows a slight shift to lower R value with the increasing Ce concentration for CeaTiOx, the peak B (Ce−Ce) seems to be vanished. The presence of a new peak C

at ∼3 Å demonstrates that the local structure of Ce in CeaTiOx is rather different from that in CeO2 and Ce/TiO2. The observed change in the local environment of Ce atoms is consistent with literature,36,41 and the peak C was assigned to the Ce−Ti bond. This can be attributed to the fact that Ti atoms substitute for Ce atoms in the lattice of CeO2 to form octahedral Ti sites, as discussed in the following fitting results. Quantitative curve fitting in R-space of the Ce LIII-edge EXAFS spectra was made based on the structure of cubic CeO2, assuming that the dopant Ti cations are on the ideal Ce cation positions (Figure S7d and Table S6). The interatomic distances of pure CeO2 approximately agree with those (Ce−O: 2.343 Å; Ce−Ce: 3.826 Å) calculated from the Inorganic Crystal Structure Database (ICSD) database. The coordination number and the average bond length of the first and the second shells for CeaTiOx are lower than those of CeO2, which can be explained by the slight lattice contraction caused by Ti doping. This result also confirms that the bond distance at about 3 Å (peak C) can be attributed to the Ce−Ti correlation because of the substitution of Ce ions by the smaller Ti ions (Figure S8). As the findings in metal glasses,26 it is reasonable that Ce is the solvent in spite of the fact that it is low in content. Moreover, the decrease in coordination number in CeaTiOx indicates the smaller particle size (meaning larger surface area, Table S1) and higher dispersion of Ce−O−Ti on the surface.42 In order to find the possible Ce−O−Ti phase, the FETEM with higher resolution than that of high-resolution TEM were performed on Ce0.3TiOx (Figure 6). Through careful searching, the trace CeTi2O6 (JCPDS 12-0477) and TiO2 (JCPDS 211272) clusters, which cannot be detected by XRD and TEM, were distinguished. This suggests that Ce in the amorphous structure bands with Ti through O (Ce−O−Ti), while the remaining Ti forms the Ti−O−Ti structure (CeTi2O6 and TiO2 are the crystallized phases of Ce−O−Ti and Ti−O−Ti, respectively). This is reasonable because the atomic ratio of Ce and Ti equals to 0.3. After calcinations at 650 °C, for instance, the two local structures in Ce0.3TiOx will crystallize and CeTi2O6 and TiO2 phases are formed, respectively, as shown by the XRD pattern of Ce0.3TiOx650. For Ce0.3TiOx, H2−TPR suggests the strong interaction between Ce and Ti and thus the promoted reducibility. In situ FTIR spectra of NO adsorption definitely propose the Ce−O− Ti local structure, which is confirmed by XPS (on surface) and XAFS (in bulk)36 and directly observed by FETEM. As XAFS 9603

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Figure 6. FETEM image of Ce0.3TiOx.

shows the absence of the Ce−O−Ce structure in CeaTiOx, the assignment of the activity of Ce0.3TiOx to the crystalline and amorphous CeO2 can be excluded. However, the detected Ti− O−Ti local structure contributes less to the SCR reaction (see analysis in the SI based nn Figure S9). The last possibility for the high activity of Ce0.3TiOx is the presence of the Ce−O−Ti short-range order structure rather than the crystallized CeTi2O6 due to the lower activity of Ce0.3TiOx650 (Figure 2).

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ASSOCIATED CONTENT

S Supporting Information *

Tables of S1−S6 and Figures of S1−S9 were shown. This material is available free of charge via the Internet at http:// pubs.acs.org.s

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AUTHOR INFORMATION

Corresponding Author

*Phone: + 86 531 89736032. Fax: + 86 531 89736032. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21077043, 21007019 and 21107030) and the Development Program of the Science and Technology of Shandong Province (No. 2011GSF11702). We are also grateful to Prof. Tiandou Hu and Prof. Jing Zhang from Beijing Synchrotron Radiation Facility (BSRF) for their help on the experiments of XAFS.

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REFERENCES

(1) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review. Appl. Catal., B 1998, 18, 1− 36. (2) Qi, G.; Yang, R. T. A superior catalyst for low-temperature NO reduction with NH3. Chem. Commun. 2003, 848−849. (3) Qi, G.; Yang, R. T. Performance and kinetics study for lowtemperature SCR of NO with NH3 over MnOx−CeO2 catalyst. J. Catal. 2003, 217, 434−441. 9604

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dx.doi.org/10.1021/es301661r | Environ. Sci. Technol. 2012, 46, 9600−9605