YCeZrO Ternary Oxide Solid Solution Supported Nonplatinic Lean

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YCeZrO Ternary Oxide Solid Solution Supported Non-Platinic LeanBurn NOx Trap Catalysts Using LaCoO Perovskite as Active Phase 3

Rui You, Yuxia Zhang, Dongsheng Liu, Ming Meng, Li Rong Zheng, Jing Zhang, and Tiandou Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp505601x • Publication Date (Web): 10 Oct 2014 Downloaded from http://pubs.acs.org on October 14, 2014

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YCeZrO Ternary Oxide Solid Solution Supported Non-Platinic Lean-Burn NOx Trap Catalysts Using LaCoO3 Perovskite as Active Phase Rui You 1, Yuxia Zhang 1, Dongsheng Liu 1, Ming Meng *1, Lirong Zheng 2, Jing Zhang 2, Tiandou Hu 2 1

Collaborative Innovation Center for Chemical Science & Engineering (Tianjin), Tianjin Key

Laboratory of Applied Catalysis Science & Engineering, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072 (P. R. China) 2

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, Beijing 100049 (P. R. China) * Corresponding author TEL/FAX: +86-(0)22-2789-2275 E-mail: [email protected] ABSTRACT: A series of non-platinic ceria-based supported catalysts LaCoO3/K2CO3/S (S=CeO2, Ce0.75Zr0.25O2 or 5%Y/Ce0.75Zr0.25O2) were prepared by successive impregnation and employed for lean-burn NOx trapping. It is found that the doping of Zr or YZr into CeO2 facilitates the formation of CeZrO binary or YCeZrO ternary solid solutions, increasing the specific surface area and improving the redox property of ceria. The results of EXAFS and H2-

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TPR reveal that the LaCoO3 in

the

solid

solution

supported catalysts possesses higher dispersion and better reducibility than that in CeO2 supported one. The result of O2-TPD

shows

that

the

surface active oxygen species are remarkably increased after loading LaCoO3 on the supports. The YCeZrO ternary solid solution supported catalyst containing 5 wt. % K2CO3 exhibits the best performance for NO oxidation and reduction at 350 oC, showing a high NO-to-NO2 conversion (66.5%) at lean condition, and a very high NOx reduction percentage (98.2%) and an extremely high NOx-to-N2 selectivity (98.8%) at rich condition in the absence of CO2. After addition of 5 vol. % CO2 in the atmosphere, the NOx reduction percentage during NOx storage and reduction tests can still be maintained at high level (above 90%) in the temperature region of 350-400 oC. The results of FT-IR, CO2-TPD and in situ DRIFTS indicate that the potassium in catalysts exist as –OK groups, K2O, bulk or bulk-like K2CO3. At low K2CO3 loading ( 5 wt. %), NOx is stored as nitrates with diverse coordination structures, while at higher K2CO3 loading (8 wt. %) it is mainly stored as bulk nitrates without forming nitrites. Based upon all characterizations, a carbonate-involved NOx storage and reduction mechanism is revealed in molecular level. Keywords: Lean-burn NOx Trap; Supported perovskites; Ceria; Solid solution; Potassium carbonate

1. INTRODUCTION

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Noble metals are widely used now in aftertreatment systems, including the diesel oxidation catalysts (DOC),1 catalyzed diesel particulate filter (CDPF),2 selective catalytic reduction of NOx by hydrocarbon (HC-SCR),3 three-way catalysts (TWC)4 and lean-burn NOx trap (LNT) catalysts.5-7 The employment of noble metals undoubtedly increases the catalyst cost, especially the LNT catalysts which often contain 1% or more Pt or Rh as active component for NO oxidation and reduction.5-7 To decrease the cost of LNT catalysts, researchers have been endeavoring to explore noble metal-free LNT catalysts in recent years. Perovskites with a chemical formula of ABO3 possess not only excellent redox property but also high chemical/structural stability, displaying good catalytic performance for the purification of vehicle emissions such as diesel soot oxidation and selective catalytic reduction (SCR) of NOx.8-10 For NOx storage, perovskite oxides can also exhibit high performance if they have high oxidation performance, since the oxidation of NO to NO2 is the key step for lean-burn NOx storage.5-7 It is reported that La-based perovskite oxides such as LaCoO3 and LaMnO3 are highly active for NO-to-NO2 oxidation at the temperature of 300-350 oC,7, 11 especially the LaCoO3.7 After partial substitution of A-site ions (i.e., La3+) by other low-valence cations such as Sr and Ce, the oxidation property of LaCoO3 would be further enhanced due to the formation of Co4+ ions and/or oxygen vacancies.11-13 The strong basicity of Sr oxide makes the substituted perovskites La1-xSrxCoO3 can act as LNT catalysts directly. Our previous work has demonstrated that the La0.7Sr0.3CoO3 perovskite possesses good performance for NO oxidation and reduction at the cyclic lean/rich condition, showing a NO-to-NO2 conversion of 74.1% and a NOx reduction percentage (NRP) of 71.4% at 300 oC.11 However, for the maintaining of the perovskite structure, only limited amounts of Sr can be introduced into the lattice of LaCoO3, which restricts the NOx storage capacity of the substituted perovskites La1-xSrxCoO3. Besides, the bulk perovskites usually possess low specific surface areas (< 15 m2/g) after calcination at high temperature

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(above 700 oC). To increase the specific surface area, we ever synthesized a mesoporous perovskite of LaCoO3 by using a nano-casting method, which exhibited a specific surface area as high as 75 m2/g.14 After loading 5 wt. % K on it, the mesoporous non-platinic K/LaCoO3 LNT catalyst displayed a surprising NOx reduction percentage of 97.0% and a high NOx to N2 selectivity of 97.3%. Nevertheless, such catalyst can hardly be used extensively since both the employment of large amounts of organic template during preparation and the high content of Co in bulk perovskite make it very expensive. If the perovskite is loaded on appropriate supports with large specific surface area and no organic templates are used during preparation, the catalyst cost can be lowered remarkably. Considering the excellent redox property of LaCoO3 perovskite and its good performance for NOx-SCR, the dispersed LaCoO3 perovskite may be a potential replacement for platinum in supported LNT catalysts. Ceria is widely recognized as an important component in three-way catalysts and soot oxidation catalysts due to its high capability for oxygen storage/release; in the presence of ceria the oxygen activation and transferring can be significantly promoted.15-19 In addition, ceria can not only improve the dispersion but also increase the thermal stability of other oxides through the metal-support interaction. In order to avoid the sintering of ceria itself, Zr or/and Y is often doped in it to get the thermo-stable solid solutions. Compared with the single oxide CeO2, the binary CeO2-ZrO2 or ternary Y2O3-CeO2-ZrO2 solid solution usually has larger specific surface area, more oxygen vacancies and higher oxygen storage capacity (OSC),20-23 so, in this work, these solid solutions were prepared and used as supports of LNT catalysts. As to the NOx storage component, many works have demonstrated that potassium is an appropriate NOx storage material since it has strong basicity and better regeneration performance after sulfur poison as compared with barium.24-28

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Based upon above elucidation, a series of ceria-based oxides supported non-platinic leanburn NOx trap catalysts LaCoO3/K2CO3/S (S=CeO2, Ce0.75Zr0.25O2 or 5%Y/Ce0.75Zr0.25O2) were designed and prepared, using LaCoO3 as active component and K2CO3 as NOx storage medium. It is surprisingly found that this series of catalysts are very active for lean-burn NOx storage and reduction, especially the 5%Y/Ce0.75Zr0.25O2 supported one. By using multiple techniques such as XRD, XPS, EXAFS, FT-IR and CO2-TPD, the catalyst structure was carefully characterized; the effect of Zr or YZr addition on catalyst structure and catalytic performance was also investigated; meanwhile, based upon in situ DRIFTS characterization, the NOx storage routes and reduction mechanism were revealed in molecular level. 2. EXPERIMENTAL 2.1 Support and Catalyst Preparation 2.1.1 Support Preparation The support Ce0.75Zr0.25O2 and Y-doped 5%Y/Ce0.75Zr0.25O2 were prepared by coprecipitation. Requisite amounts of Ce(NO3)3·6H2O, Zr(NO3)3·5H2O and Y(NO3)3·6H2O were simultaneously dissolved in deionized water with a total concentration of 0.1 mol/L for all the cations (Ce + Zr, or Y + Ce + Zr). The molar ratios of Ce/Zr and Y/(Ce + Zr) were set at 3 and 0.05, respectively. An ammonia (25 wt. %) solution was added dropwise to the above solution at room temperature under vigorous stirring until pH = 11. After continuous stirring for 3 h and aging for 1 h, the samples were filtered. The obtained wet precursors were dried at 120 oC overnight and calcined in static air at 700 oC for 4 h to get the final supports Ce0.75Zr0.25O2 and 5%Y/Ce0.75Zr0.25O2, which are denoted as CZ and Y5CZ, respectively. Pure CeO2 support was also prepared by the same method. Before use, all the supports were crushed and sieved to fine powder (< 200 mesh). 2.1.2 Catalyst Preparation

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La(NO3)3·6H2O, Co(NO3)2·6H2O and citric acid monohydrate (molar ratio: citric acid/La/Co was 2.2/1/1) were used as raw materials to prepare the precursor solution. The amounts of La and Co salts were calculated according to the loading of 10 wt. % for LaCoO3 in the final catalysts. Firstly, the fine support powder was impregnated into the above solution to obtain a slurry mixture, which was stirred for 5 h at room temperature; then the slurry was evaporated at 40 oC in a rotary evaporator to remove the excess water. Subsequently, the obtained powder was further dried at 120 oC overnight and calcined at 350 oC for 2 h to ensure the decomposition of nitrates and the removal of organic materials. After this, the samples were calcined in flow air at 700 oC for 4 h so as to get the supported perovskite LaCoO3. The loading of potassium was performed by incipient-wetness impregnation using an aqueous solution of K2CO3 with desired concentration. After dried at 120 oC overnight, the precursor materials were calcined at 500 oC for 2 h in static air to get the final NOx trap catalysts LaCoO3/K2CO3/S (S=CeO2, CZ or Y5CZ). The contents of K2CO3 on the supports were kept at 5 wt. %; however, to optimize the loading of K2CO3 on Y5CZ support, other two different loadings of 2 wt. % and 8 wt. % were also employed. These catalysts are denoted as LC/xK/S (S=CeO2, CZ or Y5CZ) with the x = 2, 5 or 8. 2.2. Catalyst Characterization The measurements of the specific surface areas (SBET) were carried out at 77 K on a Quantachrome QuadraSorb SI instrument using nitrogen adsorption method. Prior to measurements, the samples were degassed in vacuum at 300 °C for 4 h to remove the adsorbed species. The X-ray diffraction (XRD) patterns of the catalysts were recorded on an X’pert Pro rotatory diffractometer (PANAlytical Company) operating at 40 mA and 40 kV using Cu Kα as

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radiation source (λ = 0.15418 nm). The diffraction data of 2θ from 10 to 90o were collected at a stepsize of 0.02o. The X-ray photoelectron spectra (XPS) measurements were performed on a PHI-1600 ESCA spectrometer using Mg Kα as radiation source (1253.6 eV). The base pressure was about 5 × 10-8 Pa. The binding energies were calibrated using C 1s peak of contaminant carbon (B.E. = 284.6 eV) as standard, and quoted with a precision of ± 0.2 eV. A standard Gaussian–Lorentzian and Shirley background were applied for peak fitting and calculating. The Raman spectra were recorded on the Raman spectrometer (Thermo, DXR Model) using an excitation laser wavelength of 532 nm at room temperature. The laser power is 5.0 mW. The spectra of extended X-ray absorption fine structure (EXAFS) of Co K-edge were recorded on the XAFS station in 1W1B beam line of Beijing Synchrotron Radiation Facility (BSRF) operating at about 150 mA and 2.2 GeV. The absorption spectra of Co K-edge for the samples were collected at room temperature in fluorescence mode with an energy resolution of 0.3 eV, and that of the reference LaCoO3 (home-made) was collected in transmission mode. A Si (1 1 1) double-crystal monochromator was used to reduce the harmonic content of the monochrome beam. In the experiments, a cobalt foil was employed for energy calibration. The Fourier transforming of the k3-weighted EXAFS data was performed in the range of k = 3–14 Å−1 using a Hanning function window to get the radial distribution function (RDF). The temperature-programmed tests including temperature-programmed reduction and desorption (TPR/TPD) were performed on a Thermo-Finnigan TPDRO 1100 instrument equipped with a thermal conductivity detector (TCD). Before detection, the gas was purified by a solid trap containing CaO + NaOH materials in order to remove the H2O and CO2, or containing Mg(ClO4)2 to remove H2O only.

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For temperature-programmed reduction by H2 (H2-TPR), each time, 30 mg powder sample was heated from room temperature to 900 oC at a slope of 10 oC/min; a flow gaseous mixture of 5 vol. % H2/N2 with a flow rate of 20 mL/min was used as reductant. For temperature-programmed desorption of oxygen (O2-TPD), 200 mg sample was preheated in pure O2 from room temperature to 500 oC and held for 30 min. After cooling to room temperature under pure O2 and a stable baseline was achieved, the sample was heated from room temperature to 900 oC in highly pure helium (20 mL/min). The temperature-programmed desorption of CO2 (CO2-TPD) derived from carbonates decomposition was performed similar to those for O2-TPD but without the step of pretreatment in O2. Before detection, the gas was purified by a solid trap containing Mg(ClO4)2 to remove H2O only. The temperature-programmed desorption of NO (NO-TPD) was performed on a HIDEN HPR20 mass spectrometer, monitoring the m/z ratios of 4 (He), 30 (NO), and 46 (NO2). 100 mg of samples were pretreated in situ at 500 oC for 30 min under pure O2 (30 mL/min). After the sample was cooled to room temperature (~30 oC) in this gas flow, it was purged by pure helium for 30 min and then exposed to 8 vol. % NO/N2 for 30 min at the same temperature. Afterward, the sample was purged again in pure helium (30 mL/min) for 90 min at 30 oC to remove the gaseous NO and physisorbed NO; finally, the sample was heated in the same atmosphere at a slope of 10 oC/min. The Fourier-transform infrared spectra (FT-IR) and in situ diffuse reflectance Fouriertransform infrared spectra (in situ DRIFT) were recorded on a Nicolet Nexus spectrometer equipped with a DTGS detector and a MCT detector cooled by liquid nitrogen, respectively. For FT-IR measurements, the fresh catalysts were pressed into a pellet with KBr before recording. For in situ DRIFTS experiments pure powder samples were loaded in an in situ chamber which

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can be heated up to 600 oC. The NOx sorption was performed in the atmosphere of 400 ppm NO + 5 vol. % O2 in N2 at 350 oC. Before adsorption, the samples were in situ pretreated by 5 vol. % O2 in N2 at 350 oC for 1 h; then the background spectrum was collected. Time-dependent in situ DRIFT spectra based on 32 scans were recorded in 650–4000 cm-1 at a spectral resolution of 4 cm-1. 2.3. Catalytic activity measurements NOx storage capacity (NSC) of the catalysts were measured in a quartz-tubular continuous flow reactor (i.d. = 8 mm) loaded with 200 mg of each catalyst (40–60 mesh). The samples were first pretreated at 350 oC in pure N2 flow for 30 min; then a mixture gas consisting of 400 ppm NO, 5 vol. % O2 and balance N2 was introduced to the reactor at a flow rate of 150 mL/min in normal conditions, corresponding to a space velocity of ca. 45,000 h-1. The concentrations of NO, NO2, and total NOx at the reactor outlet were monitored online by a Chemiluminescence NO−NO2−NOx Analyzer (Model 42i-HL, Thermo Scientific). The NOx reduction performance of the samples was evaluated in alternative lean/rich cyclic atmosphere. The experiments were conducted in the same reactor as above, using a lean period of 3 min and a rich period of 1 min. In lean and rich period, the mixture gas of 400 ppm NO + 5 vol. % O2 + 5 vol. % CO2 (when need) + balance N2, and 1000 ppm C3H6/N2 with a flow rate of 150 mL/min were introduced to the sample, respectively. Meanwhile, the possible by-product of N2O in all cycles was monitored constantly by a N2O Modular Gas Analyzer (S710, SICK MAIHAK). In each case, the lean/rich cyclic tests were operated for 10 or 20 times to achieve relatively stable performance of the catalysts. 3. RESULTS AND DISCUSSSION 3.1. Structural and Texture properties 3.1.1 XRD

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The powder XRD patterns of the catalysts are shown in Fig. 1. For all catalysts, a typical cubic fluorite structure (JCPDS 43-1002 space group, Fm3m) of ceria is identified, showing four main diffraction peaks at 28.6o, 33.1o, 47.6o and 56.4o, which correspond to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes,22 respectively. For pure CeO2 supported catalyst, the XRD peaks are highly symmetric, without peak shift observed, which suggests an ideal cubic phase. However, after Zr or YZr doping, the main diffraction peaks of CeO2 are remarkably lowered in intensity and broadened to some extent, indicating the decrease of its crystallization degree and particle size. By enlarging the main diffraction peaks at 2θ = 28.6o, slight peak shifts of ~0.1o to larger angle direction (not shown) could be found for the samples doped with Zr or YZr. Such unconspicuous shifts suggest the formation of small amounts of CeZrO or YCeZrO solid solutions by referring to CexZr1-xO2 system.28 In addition to the supports, the perovskite LaCoO3 (JPCDS 48-0123) phase is also detected, which exhibits two individual diffraction peaks at 23.2o and 40.7o, and several overlapped peaks at 33.0o, 47.6o and 59.0o. For all the catalysts, no diffraction peaks of single oxides like Y2O3, La2O3 and Co3O4 are found in the XRD patterns. Meanwhile, no K-related phases are detected, either, suggesting that the K species are highly dispersed or existing in amorphous state in these catalysts. 3.1.2 FT-Raman FT-Raman spectroscopy is a powerful tool for the detection of impurities or small crystalline domains due to its high sensitivity, so, it is often used to distinguish the cubic and tetragonal forms of zirconia.29 Fig. 2a shows the Raman spectra of the supports. For pure ceria, only one sharp peak at 464 cm-1 is detected, which corresponds to the typical F2g symmetric vibration (O-Ce-O stretching) of cubic fluorite structure. After Zr or YZr doping, although the Raman spectra are still dominated by the F2g vibration of the cubic fluorite lattice, some differences are observable. Besides the broadening of the F2g peak, a considerable decrease in

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intensity and an obvious shift toward higher frequency (from 464 to 470 and 473 for CZ and Y5CZ, respectively) are clearly found, which could be connected with the decrease of vibrational symmetry caused by the increase of disorder in the fluorite structure. The cell contraction induced by partial substitution of Ce4+ (0.093 nm, ionic radius) by smaller Zr4+ ions (0.079 nm, ionic radius) or by Zr and Y3+ ions (0.089 nm, ionic radius) accounts well for the deformation of fluorite lattice structure.20, 23, 29, 30 Furthermore, for the support Y5CZ containing Y, the characteristic Raman peak of Y2O3 at 376 cm-1 is not detected, which suggests that the doped Y3+ ions may have dissolved into the lattice of CeZrO, forming a ternary YCeZrO solid solution. In the Raman spectra of CZ and Y5CZ, two new weak bands at ca. 304 and 613 cm-1 are ascribed to a tetragonal displacement of oxygen atoms from their ideal fluorite sites, which are caused by the insertion of Zr4+ ions into ceria lattice.29 Fig. 2b shows the Raman spectra of the corresponding catalysts. Obviously, the band of main F2g symmetric vibration of the supports is still the dominant one, but the change in frequency and intensity can be observed. After loading perovskite LaCoO3 and K2CO3 on the supports, the F2g peaks for CeO2, CZ and Y5CZ shift from 464, 470 and 473 cm-1 to 463, 458 and 461 cm-1, respectively, which may be resulted from the interaction of surface Ce4+/Ce3+ ions with the supported LaCoO3 or K-species. Such interaction at the interface would generate new bond linkages such as M-O-Ce (M = La, Co or K), leading to the distortion of the original O-CeO. As a result, the F2g peak shifts toward lower frequencies to some extent. Similar phenomena were also found in CeO2-ZrO2 supported cobalt and copper oxides.19 For CeO2 and its supported catalyst, the peak shift is nearly neglectable, implying the very weak interaction between the support and loaded components in this catalyst. However, for the supports CZ and Y5CZ as well as the corresponding catalysts, the peak shifts are considerable, especially for Y5CZ supported one. Generally, the amounts of the formed M-O-Ce linkages are largely determined by the

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number of surface binding sites and the dispersion of supported crystallites such as LaCoO3. The data listed in Table 1 indicate that the supports CZ and Y5CZ have larger specific area than CeO2, which is favorable to the dispersion of the supported LaCoO3 or K-species; so, it is believed that more M-O-Ce linkages are formed in CZ and Y5CZ supported catalysts than in CeO2 supported one. It is known that the Raman active modes of A1g + 4 Eg of LaCoO3 often show vibrational signal in the frequency region of 500-800 cm-1.31, 32 Thus, as compared with those for the corresponding supports (Fig. 2a), the broadening and rising of the Raman signals for the catalysts in the same frequency region (Fig. 2b) can be assigned to the contribution of LaCoO3. 3.1.3 XAFS To further confirm the formation of LaCoO3 perovskite and get more information about the states of Co species, EXAFS characterization was performed on the catalysts. Fig. 3a shows the radial structure functions (RSFs) of Co K-edge for the fresh catalysts and the reference LaCoO3. It is clear that the RSFs of the catalysts are similar to that of reference LaCoO3, showing two distinct coordination peaks around 0.149 nm and 0.315 nm. The first one is assigned to the octahedrally coordinated Co–O shell in LaCoO3 perovskite, and the second one should contain the contributions from not only the single scattering of Co–La or Co–Co and the multiple scattering of Co–O–Co or Co–O–Co–O.33 The RSFs similarity for the catalysts and reference LaCoO3 suggests that the cobalt in the catalysts should mainly exist in the form of perovskite LaCoO3. However, the crystallite size of LaCoO3 in catalysts is much smaller, as reflected by the much lower intensity of the main coordination peaks. Generally, the intensity of the peaks for high coordination shells is more sensitive to the crystallization degree of the corresponding metal or metal oxide phases; larger crystallites correspond to stronger coordination peak.15 Thus, it could be deduced that the LaCoO3 on Y5CZ support possesses smaller crystallite size as

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compared with those on CZ and CeO2. Fig. 3b presents the Co K-edge RSFs of the spent catalysts after used in NOx storage or used in cyclic NOx storage and reduction by C3H6 (10 cycles). It is found that the perovskite structure of LaCoO3 was well maintained after these tests, suggesting its high stability. 3.1.4 XPS The above XRD and EXAFS characterizations mainly focus on the bulk structure of the catalysts or active phases. To reveal the chemical states of surface Co species, XPS characterization was carried out on the catalysts. Fig. 4 displays the XPS results of Co 2p for CeO2 and Y5CZ supported catalysts. Due to the tiny difference of the binding energy, Co2+ and Co3+ ions are hard to be distinguished just from the Co 2p spectra; however, the spin-orbit splitting of the Co 2p peak (ΔE) can reflect the oxidation state of Co. Generally, cobaltous compounds exhibit larger ΔE value (~16.0 eV), while cobaltic compounds display smaller ΔE value (~15.0 eV). For Co3O4 containing multiple-valence Co, the spin-orbit splitting value is about 15.2 eV.9,

15, 34

From Fig. 4 and Table 1, it is found that both of the two samples

LC/5K/CeO2 and LC/5K/Y5CZ exhibit a spin-orbit splitting value of 15.1 eV, close to that for LaCoO3 system.8 This result implies that most cobalt ions in these two samples possess the valence of +3, only a very small amount of them possess the valence of +2. This deduction seems in contradiction with the result of EXAFS, in which LaCoO3 pervoskite has been identified as the sole cobalt-containing phase. The imperfectness of crystal structure may be a plausible interpretation. It is known that the LaCoO3 pervoskite with small crystallite size often has lattice defects on its surface such as oxygen vacancies, which means that some surface Co atoms may be in unsaturated coordination state; according to electrovalence equilibrium, the average valence of cobalt in LaCoO3 should be a little lower than +3. Actually, in most cases,

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such pervoskite is expressed as LaCoO3-. In a summary, the XPS results of Co 2p are consistent with those of XRD and EXAFS. 3.1.5 BET The BET specific surface areas (SBET) of the samples are listed in Table 1. It can be seen that pure CeO2 exhibits the lowest specific surface area (23.4 m2/g), demonstrating its lowest thermal stability during calcination. After introduction of certain amounts of Zr or YZr into CeO2, the specific surface areas of the supports CZ and Y5CZ significantly increase to 42.9 and 55.3 m2/g, respectively; the formation of solid solution may have inhibited the sintering of CeO2 during calcination. As a consequence, the smaller particle size or higher dispersion of the supported LaCoO3 on Y5CZ is achieved, as verified by EXAFS results. After loading of the perovsktie LaCoO3 and K2CO3 on the supports, the specific surface areas of the catalysts decrease to some extent, which may be caused by pore blocking and the decrease of weight percentages of supports in the final catalysts. Even so, they still exhibit much larger specific surface areas than bulk perovskites. 3.2. Reducibility of the samples (H2-TPR) The H2-TPR profiles of the supports, the corresponding catalysts and the bulk LaCoO3 (prepared by sol-gel method and calcined at 700 oC for 8 h) are shown in Fig. 5a. For pure CeO2, it is generally accepted that the H2 consumption above 650 oC is ascribed to the reduction of bulk Ce4+ (referred as γ), and that below 650 oC is attributed to the reduction of the Ce4+ in the uppermost layers.20, 35 While the two successive reduction peaks appearing at 468 and 559 oC are ever specifically attributed to the removal of surface and subsurface (or near-surface36) oxygen (referred as α and β) by H2, respectively.18 After Zr or YZr doping, these three peaks shift to lower temperatures, with the peaks α and β remarkably increasing and peak γ decreasing dramatically. This result suggests that the formation of solid solution can improve the

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reducibility of the surface and subsurface oxygen species in the supports. As compared with CeO2, the larger specific surface areas of CZ and YCZ increase the amounts of surface and subsurface reducible species. The smaller crystallite size and the structural distortion caused by the replacement of Ce4+ by Zr4+ or/and Y3+ account well for the decrease of reduction temperatures of peaks α, β and γ. The decline of peak γ means the decrease of the amount of reducible bulk lattice oxygen, which should be mainly resulted from the decreased crystallite size of the supports; the migration of bulk lattice oxygen to subsurface or surface through doping-induced oxygen vacancies may be also a potential reason. Among the three supports, Y5CZ exhibits the lowest reduction temperature, suggesting that this support possesses the smallest crystallite size and the most obvious lattice distortion, as confirmed by the FT-Raman characterization. To investigate the reducibility of the samples after loading LaCoO3 on the supports, H2TPR tests were also carried out on LC/CeO2, LC/CZ and LC/Y5CZ (LaCoO3 loading: 10.5 wt. %, corresponding to 10 wt. % loading in the final catalysts), the results of which are shown in Fig. 5b (solid lines). As seen in Fig. 5a, the bulk LaCoO3 exhibits two main reduction peaks which is attributed to the stepwise reduction of LaCoO3 perovskites; the one at lower temperature (300–500 oC) corresponds to the reduction of Co3+ to Co2+, forming oxygendeficient perovskite (La2CoO4), while the other at higher temperature (500–700 oC) is attributed to the further reduction of La2CoO4 to Co0 and La2O3.8,

9

The H2 consumption ratio for the

second step to the first step is very close to the theoretical value of 2. For the LaCoO3 supported on CeO2, CZ and Y5CZ, its first step reduction of LaCoO3 overlaps with the reduction of Ce4+ on support surface. To get the exact reduction temperatures of the perovskite LaCoO3 on the supports, peak-fitting was performed to obtain the individual peaks with Gauss distribution, as shown in Fig. 5b (dot lines). Six peaks are derived, among which the former five ones are

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displayed in Fig. 5b. The sixth peak corresponding to the reduction of bulk lattice oxygen in the supports at high temperature is not shown due to the incompleteness of the reduction peak. Comparing with the H2-TPR profiles of pure LaCoO3 and the supports shown in Fig. 5a, the deconvoluted peak V with similar reduction region is attributed to the second-step reduction of the LaCoO3 on the supports. The peak I is assigned to the reduction of surface oxygen species mainly contributed by supports. The attributions of the peaks II and III are complicated due to the potential peak shift and small difference in the reduction temperature. However, according to the theoretical H2 consumption ratio for the second step to the first step, the peak III is ascribed to the first step reduction of LaCoO3. The temperatures of the reduction peaks and the total quantity of H2 consumption for LaCoO3 are summarized and listed in Table 2. Based on the temperature, peak I and peak II are assigned to the successive reduction of surface and subsurface oxygen relating to the Ce4+ in the uppermost layers of ceria, respectively. Compared with those of the pure supports, the peaks I and II of LC/CeO2, LC/CZ and LC/Y5CZ exhibit much lower temperatures. The metal-support interaction between the supports (CeO2/CZ/Y5CZ) and LaCoO315, 16, 18 as confirmed by Raman results may have weakened the strength of Ce-O and Co-O bonds, leading to the decrease of reduction temperature for the surface Ce4+ and Co3+ ions which locate at the interface between ceria and LaCoO3 in the form of Ce-O-Co or Ce-O-La bond linkages. As to the peak IV, due to the similar reduction temperature region to that of pure supports, it should be related to the reduction of the Ce4+ in the uppermost layers of the isolated ceria not contacting with LaCoO3. The reduction of Ce4+ ions in the bulk structure of CeO2, CZ and Y5CZ is also promoted, probably due to the hydrogen spillover effect of the metallic cobalt derived from the total reduction of Co oxides. Similar promotional effect of cobalt on the reduction of ceria was also found elsewhere.16

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The H2-TPR profiles of the final LNT catalysts displayed in Fig. 5a are very similar to each other. Compared with LC/CeO2, LC/CZ and LC/Y5CZ, the corresponding catalysts show higher reduction temperature. The presence of K2CO3 may have inhibited the diffusion of H2 from surface to the covered ceria and cobalt oxides, retarding the reduction of Ce4+ and Co3+. Interestingly, the peak γ corresponding to the reduction of bulk lattice oxygen in the supports is very sensitive to the loading amount of K2CO3. Covering effect can hardly account for this opposite phenomenon. The increase of H2 consumption signal may be contributed by the dilution effect of CO2 produced from the decomposition of K2CO3 in the same temperature range (650900 oC). To confirm this point, similar H2-TPR experiment was performed on pure K2CO3 (not shown), and similar phenomenon was observed. So, it is thought that the gaseous CO2 from the decomposition of K2CO3 should have diluted the stream, decreasing the concentration of H2 in it, increasing the H2 consumption signal. In a summary, the reducibility of the catalysts is really improved by the interaction between the loaded LaCoO3 and the supports. 3.3. O2 temperature programmed desorption (O2-TPD) The temperature programmed desorption of O2 was performed on the supports CeO2 and Y5CZ as well as the supported samples LC/CeO2 and LC/Y5CZ. As seen from the O2-TPD profiles shown in Fig. 6, for pure CeO2, nearly no O2 is desorbed during the whole temperature increment; however, after doping with YZr, tremendous amounts of desorbed oxygen species are detected in the region of 100-650 oC. The desorption of different oxygen species in Y5CZ undergoes a successive process, showing a very broad desorption peak, which is similar to the case in H2-TPR. From the temperature and width of the desorption peak, it is speculated that the main peak probably includes the desorption of adsorbed oxygen, surface and subsurface lattice oxygen. The larger specific surface area and the smaller crystallite size of Y5CZ increase the amounts of surface and subsurface lattice oxygen. After LaCoO3 loading on CeO2 and Y5CZ,

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both the two catalysts display three individual desorption peaks labeled as α, β and γ. The peak γ above 650 oC corresponds to the desorption of bulk lattice oxygen in LaCoO3. The peak α below 300 oC is ascribed to the desorption of surface adsorbed oxygen species such as O2- or O-.16 On the basis of XPS results, it is thought that the surface adsorbed oxygen species are mainly contributed by the perovskite LaCoO3 due to the existence of surface defects and oxygen vacancies on it. The peak β in middle temperature region is attributed to the desorption of surface lattice oxygen in LaCoO3 and ceria. After a careful comparison between the desorption profiles of LC/CeO2 and LC/Y5CZ, it is found that the temperatures of β and γ desorption peaks for LC/Y5CZ are a little lower and the amounts of desorbed oxygen are a little larger, which may be related to the higher dispersion or smaller crystallites of LaCoO3 on LC/Y5CZ. 3.4. NO temperature-programmed desorption (NO-TPD) In order to investigate the property of the catalysts for NO adsorption/desorption, the NOTPD experiments were carried out on the supports CeO2, CZ and Y5CZ as well as the samples LC/CeO2, LC/CZ and LC/Y5CZ. The desorbed species were detected by an online spectrometer as displayed in Fig. 7. In each TPD test, the sample was pretreated in pure O2 at 500 oC for 0.5 h, and cooled to room temperature in the same atmosphere to achieve a saturation state for oxygen adsorption. The purpose of this step is to ensure that the formation of NO2 species in following NO adsorption originates from the reaction between the surface active oxygen species and NO. It is found that the temperatures for NO2 desorption from LC/CeO2, LC/CZ and LC/Y5CZ are obviously lower as compared with those from the supports CeO2, CZ and Y5CZ, respectively. Based upon the peak areas, it is found that the amounts of desorbed NO2 for the samples LC/CeO2, LC/CZ and LC/Y5CZ are also larger than those for the supports, which indirectly demonstrates that there are more surface oxygen species on them than on the corresponding supports, as confirmed by the previous H2-TPR and O2-TPD results. From the curves of NO

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desorption, it can be seen that the loading of LaCoO3 can also enhance the desorption of NO, increasing the desorbed amount and decreasing the desorption temperature, especially for the desorption peaks below 300 oC. The desorbed NO and NO2 species may come from diverse pathways. As reported,29 the NO/NO2 species detected in the region of 100~300 oC mainly corresponds to the less stable chemisorbed species, while those detected in the region of 300~500 oC may originate from the decomposition of surface stored nitrates/nitrites species formed during NO adsorption at elevated temperature. 3.5. The analysis of K species in the catalysts Since no information about K species is obtained from XRD, the techniques of FT-IR and CO2-TPD were employed to characterize the K2CO3 species. Fig. 8 shows the FT-IR spectra of the samples and the bulk K2CO3 which was used as the precursor salt of potassium during catalyst preparation. It is revealed that all the IR spectra of the catalysts are similar to that of bulk K2CO3, exhibiting the characteristic bands of carbonate species in the region of 1800-1000 cm-1. The bands at 1116 and 1659 cm-1 correspond to the νs(OCO) and ν(C=O) modes of bridging bidentate carbonates, while the other two at 1049 and 1382 cm-1 are attributed to the νs(OCO) and νas(OCO) modes of chelating bidentate carbonates, respectively.37,

38

The

monodentate carbonates are also characterized by the band appearing at 1400 cm-1.39 The bands in the region of 1660-1630 cm-1 and 1490-1450 cm-1 are attributed to the characteristic vibrations of ν(C=O) and νas(OCO) modes of bicarbonates, respectively.37-41 As the K2CO3 loading in Y5CZ supported catalysts increases from 2% to 8%, the bands in the region of 1500-1420 cm-1 shift from 1480 to 1477 and 1452 cm-1, approaching 1434 cm-1 gradually, which means the increase of bulk K2CO3. Similarly, the band at 1460 cm-1 for the sample LC/5K/CeO2, which is closer to 1434 cm-1 as compared with those for LC/5K/CZ or LC/5K/Y5CZ, implies a larger proportion of bulk K2CO3 in LC/5K/CeO2 due to the small specific surface area of CeO2. The

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couple bands at 1594 and 1330 cm-1 are assigned to the νas(OCO) and νs(OCO) modes of carboxylate ion (CO2-) which is attached on the potassium ion (K+).38 The intensity of this couple bands is determined by the amount of surface K+ sites. During the impregnation process of catalyst preparation, the aqueous K+ ions can interact with the surface hydroxyl groups on the supports, producing the support-O-K+ (aq) surface species. The subsequent drying and calcination treatments strengthen the solid/solid interfacial interaction, leading to lowtemperature decomposition of the surface carbonates to form −OK groups.42 This phenomenon is ever demonstrated by TPD method in numerous reports for K2CO3 supported on Al2O3,40 ZrO2,27 TiO2-Al2O3-ZrO243 and ceria-based materials in this work (as seen in the following part). In situ DRIFT spectra also proved that the low-temperature decomposition of surface K2CO3 on Al2O3 could occur at 200 oC with the simultaneous decrease of –OH stretching intensity in the region of 2500-3700 cm-1.38 So, it is convincing that the surfaces of the ceria-based samples are covered by highly dispersed K+ species. The interaction between surface K2CO3 and the hydroxyl groups on supports can be described by the following reaction equation. K2CO3 + 2(−OH) → CO2 + H2O + 2(−OK) In Fig.8, the couple bands at 1594 and 1330 cm-1 for CeO2 supported sample are obviously weaker than those for other samples, which is probably due to that the CeO2 has a much smaller specific surface area than other supports. Fig. 9 shows the temperature-programmed decomposition of K2CO3 in the catalysts. Several peaks of CO2 desorption are found at different temperatures, implying a high surface heterogeneity of K species on the supported samples. The CO2 desorption at much lower temperature (< 200 oC) is related to the surface carbonates strongly interacted with the supports, similar phenomenon of CO2 desorption at low temperature is also found in K/Al2O3 system.40 Generally, the pure bulk K2CO3 decomposes at very high temperature (~ 900 oC). The CO2

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desorption peak between 200 and 761 oC may come from the decomposition of surface smallsize K2CO3 species which are indirectly affected by the supports. The CO2 desorption peak at the temperature above 761 oC should correspond to the decomposition of bulk or bulk-like K2CO3 species which barely interact with the support. In the sample LC/2K/Y5CZ with lower K loading, less amounts of bulk or bulk-like K2CO3 species are formed, showing much weaker desorption signal at the temperature above 761 oC. Since the decomposition of both the surface K2CO3 and bulk or bulk-like K2CO3 species can produce K2O or analogous KxOy species, a variety of K species including the highly dispersed surface –OK groups, the K2O or KxOy oxide species, and the bulk or bulk-like K2CO3 species, should co-exist in the final calcined catalysts. 3.6. NOx storage and reduction performance of the catalysts Isothermal NOx storage over the fresh catalysts at 350 oC was carried out, the results of which are presented in Fig. 10a and Table 1. From the NOx storage curves, it is seen that at the beginning of NOx uptake the catalysts can quickly capture and store most NOx in the stream, corresponding to a fast NOx storage period, during which the lowest level of NOx concentration is achieved and maintained for several to several ten minutes. Subsequently the NOx uptake undergoes a slow storage period, making the concentration of NOx at the reactor outlet gradually increase until to a steady-state value approaching its inlet concentration (400 ppm). Since the oxidation of NO to NO2 is a crucial step for NOx storage, it is believed that the fast NOx storage mainly takes place on the surface storage medium surrounding the oxidation components such as LaCoO3 perovskite; while the slow NOx uptake should correspond to the NOx storage on the remote NOx storage sites including the subsurface and bulk storage sites, due to the lack of active oxygen species and the diffusion resistance from surface to subsurface or bulk structure.6 In general, the highly dispersed surface K species near the oxidation sites are favorable to fast and complete NOx storage with little NOx leak.27

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From the storage curves displayed in Fig. 10a, it can still be seen that different catalysts exhibit rather different NOx storage performance. Among the catalysts LC/5K/CeO2, LC/5K/CZ and LC/5K/Y5CZ, LC/5K/Y5CZ exhibits the best NOx storage performance, showing not only the lowest NOx concentration (6 ppm) at the bottom of the storage curve but also the longest maintaining time at this value. Meanwhile, this catalyst also displays the highest conversion of NO to NO2 (66.5%) after reaching the steady-state as listed in Table 1, which reflects that this catalyst has the strongest ability for NO oxidation. For both the oxidation of NO and the subsequent NOx storage as nitrites or nitrates during NOx storage, the O2 activation and transferring on catalyst surface are key steps.6, 44 The strongest oxidation capability and the best NOx storage performance of LC/5K/Y5CZ can be elucidated from the following several aspects: (1) the previous H2-TPR, O2-TPD and NO-TPD results have indicated that the Y5CZ supported sample has better reducibility and more reducible oxygen species than CeO2 supported one; (2) the EXAFS results have revealed a higher dispersion or smaller crystallites of LaCoO3 on LC/5K/Y5CZ sample, which can offer more sites for NO and O2 adsorption; (3) the larger specific surface area of Y5CZ provides enhances the dispersion of K species. However, as the K2CO3 loading increases to 8%, the conversion of NO to NO2 decreases, showing a larger NOx leak concentration (16 ppm) at the bottom of storage curve. This is mainly caused by the decrease of K2CO3 dispersion or the increase of K2CO3 crystallites size, both of which are unfavorable to the diffusion of NO and O2 from surface to subsurface or bulk structure. The sample LC/5K/CeO2 exhibits the highest NOx concentration at the bottom due to its lowest oxidation ability and its smallest specific surface area. Comparing the two concentration curves of NO and NO2 for LC/5K/CeO2 in Fig. 10b, different changing trends for NO and NO2 concentrations are found, namely, the NO concentration is always increasing after reaching the lowest point, while the NO2 concentration keeps stable at the lowest level for about 20 minutes,

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which suggests the occurrence of potential disproportionation reactions (3-4-1/3-4-2) during this period, leading to NO leak.6, 29, 45 3NO2 + K2O → 2KNO3 + NO 3NO2 + K2CO3 → 2KNO3 + NO + CO2

(3-4-1) (3-4-2)

Differently, for LC/5K/Y5CZ both NO and NO2 concentrations keep at the lowest values for about 25 minutes, as shown in Fig. 10c. During this period, the NO is oxidized to NO2 and quickly stored in the form of nitrates as described in the reactions (3-4-3/3-4-4). 2NO2 + K2O + O* → 2KNO3 2NO2 + K2CO3 + O* → 2KNO3 + CO2

(3-4-3) (3-4-4)

To investigate the performance of the catalysts for lean NOx reduction, successive NOx storage and reduction at alternative lean/rich cyclic atmosphere (without CO2) were performed at 350 °C over all the as-prepared samples, the results of which are shown in Fig. 11 (a-e). It is found that these catalysts display excellent and stable performance for NOx storage and reduction after 10 lean/rich cyclic tests, showing very low NOx concentration at the reactor outlet. By careful comparison, the NOx slip amounts in the lean period (3 min) and rich period (1 min) are both showing the lowest value on the sample LC/5K/Y5CZ. Based on the last two lean/rich cycles the average NOx reduction percentages (NRP) for different catalysts are calculated and listed in Table 2. It is seen that all of these catalysts exhibit high NRP above 95%, and the largest NRP (98.2%) is achieved on LC/5K/Y5CZ. It is worth noting that during the whole storage and reduction period the concentration of the by-product N2O is always very low. As an example, the curve of N2O concentration of LC/5K/Y5CZ is selected and displayed in Fig. 11f. It is found that on this sample the N2O concentration is less than 4 ppm, which indirectly demonstrates that this series of catalysts possess weak capability for the dissociation of NO molecules since the formation of N2O is through the combination of adsorbed NO and N species.

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Since no other by-products are detected, the NOx to N2 selectivity for LC/5K/Y5CZ is obtained as 98.8% through the N balance. To further investigate the effect of CO2 on the performance of this perovskite-based LNT catalyst, the NSR tests in cyclic lean/rich atmosphere containing 5 vol. % CO2 were conducted on the sample LC/5K/Y5CZ at various temperatures (250-450 oC), the results of which are shown in Fig. 12 (a-e). As seen in Fig. 12c, at 350 oC, when 5 vol. % CO2 is introduced into the atmosphere in lean period, the NOx slip in the lean period gets increased, resulting in decreased NOx storage capacity as compared with the situation without CO2 (Fig. 11c), which may be due to the competitive adsorption of CO2 and NOx on the basic storage sites. At lower temperatures (250 oC or 300 oC), the NOx slip in lean period is more obvious, especially at 250 oC. The lower temperature is kinetically unfavorable to the activation of oxygen and the oxidation of NO to NO2, thus decreasing the NOx trapping capability of the catalyst. As temperature is elevated to 400 oC, the NSR performance of the catalyst gets a little better, showing less NOx leak. However, at the higher temperature of 450 oC, the NOx leak increases again. According to the thermodynamic equilibrium of NO oxidation to NO2, high temperature is favorable to the decomposition of NO2 to NO and O2, unfavorable to NOx storage and reduction. So, for NSR the appropriate temperature is often between 350-400 oC. Based upon 20 lean/rich cycles, the average NOx reduction percentages at different temperature are calculated, as listed in Fig. 12f. It is obvious that the highest NOx reduction percentage is achieved at the temperature between 350-400 oC. 3.7. In situ DRIFTS characterization on NOx sorption and storage. 3.7.1 NOx sorption on pure supports (CeO2, CZ and Y5CZ) Fig. 13 shows the in situ DRIFT spectra of NO/O2 co-adsorption upon pure supports (CeO2, CZ and Y5CZ). For each set of spectra (Fig. 13a, b and c), all IR bands are growing with the

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exposure time increasing. It is ever reported that29 for CexZr1-xO2 catalysts with the increase of Zr amount doped into CeO2, the mass of adsorbed NOx is decreased as proved by thermogravimetric measurements. Thus it could be deduced that NOx adsorption and release mainly take place on Ce sites. In Fig. 13a, b and c, the bands in the range of 1620-1470 and 1320-1170 cm-1 are reported as the ν3 mode of nitrates (NO3-) vibration which splits into νs(NO2) and νas(NO2), respectively, in light of the fact that the surface nitrates usually has a C2v symmetry. The bands in 1035-970 cm-1 are related to the ν1 mode of nitrates vibration as νs(NO3-).46 More specifically, the bands at ca. 1595, 1220-1210 and around 1000 cm-1 are ascribed to bridging bidentate nitrates;29,

46

those at ca. 1500, 1290-1250 and 1035-970 cm-1 are attributed to

monodentate nitrates;46 while the bands at ca. 1581, 1573, 1560, 1530, 1250-1220 and 10301000 cm-1 are assigned to the multiple sets of ν(NO3-) bands for chelating bidentate nitrates. The formation of chelating bidentate nitrates is via two possible pathways, namely the oxidation of bidentate nitrites and the oxidation of adsorbed NO2, similar to the reported work.29 It is noted that the bands of 1581 and 1573 cm-1 are only detected on CZ and Y5CZ, suggesting that apart from the Ce-O-Ce sites bidentate nitrates may also be formed on Ce-O-Zr sites, showing different vibration frequencies due to the presence of Zr and YZr addition. The bands at ca. 1450-1360 (νas(NO2) mode) and 1360-1320 (νs(NO2) mode) cm-1 are similar to the vibration of nitro compounds, where the NO2- is coordinated to metal cations via its N atom.46 The 1627 cm-1 band is ascribed to δ(HOH) of H2O which is formed by the interaction of basic/neutral hydroxyls and NO2, involving the formation of nitrates in the meantime. This description is also proven by the continuous decrease of 3800-3630 cm-1 (OH) band all along the adsorption process in Fig. 13d (for Y5CZ) and by the synchronous growth of absorptions near 3630-3450 cm-1 due to the increase of H-bonded H2O.29

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The interaction of NOx with ceria-based materials is thought to mainly occur via oxygen or electron transfer as clarified by DRIFTS study reported in literature.29 In their work surface NO2or NO- species were found on CexZr1-xO2 at room temperature (30 oC) while these species are not detected at the much higher temperature (350 oC) in this work. As reported, the formation of nitrates is usually through two pathways: “nitrites” and “NO2” routes.6 The absence of nitrites species in DRIFTS at 350 oC may be resulted from the rapid oxidation of nitrites to nitrates. The surface defects on the ceria-based oxides should be responsible for the activation of oxygen via one-electron or two-electron transferring, giving O2- or O22-, respectively. These surface activated oxygen species are mainly responsible for the formation of NO2 and nitrates via oxidation pathway. As temperature increases to 350 oC, the surface lattice oxygen O2- can also act as oxidative species. However, due to the high concentration of oxygen in gaseous mixture, the gas oxygen which can be adsorbed and activated by ceria-based oxides via surface electron transferring is still believed to the dominant oxidative species. 3.7.2 NOx sorption on LNT catalysts samples After loading more and more K2CO3 on the supports, the behaviors of NOx sorption and the co-adsorption of NO/O2 at 350 oC are greatly affected by the K-related basic sites, as revealed by the time-dependent in-situ DRIFT spectra (1, 3, 5, 10, 30 and 60 min, respectively) shown in Fig. 14. The simultaneous appearance of positive and negative bands for all spectra is induced by the formation of nitrates/nitrites and the consumption of carbonates, respectively. The reaction between NOx and carbonates leads to the transformation of carbonates to nitrates/nitrites.25, 27, 45 The bands at ca. 2360 and 2330 cm-1 attributed to νas(OCO) mode of gaseous CO2 are clearly observed, which demonstrates the decomposition of the surface carbonates during NOx sorption. However, nearly no CO2 vibration bands for LC/2K/Y5CZ are detected due to the low percentage of surface K2CO3 species. Unfortunately, due to the overlapping of some vibration

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regions for carbonates and nitrates/nitrites, it is hard to distinguish all the bands of carbonates and nitrates/nitrites, and only the marked bands can be taken into assignment. Based upon the fingerprint spectra of nitrates/nitrites in the range of 1500-1200 cm-1, diverse nitrates species including bidentate, monodentate and free ionic nitrates are revealed. Specifically, the bands appear at 1395, 1380 and 1370-1365 cm-1 are ascribed to free ionic nitrates (νas(NO3-));37, 38 the other two at 1598 and 1277 cm-1 are ascribed to the νs(NO2) and νas(NO2) of bridging bidentate nitrates,37, 47, 48 respectively; and the bands at 1440-1432 and 1340 cm-1 are attributed to νas(NO2) and νs(NO2) of monodentate nitrates,11, 48-50 respectively. While the bands at 14201413 and 1360 cm-1 are recognized as “nitrates on perovskite”, similar to those ever observed on other perovskite-type catalysts (La1-xSrxCoO311, BaFeO3-x50 and BaSnO351). Their formation is ever explained as the direct NO2 adsorption on the oxygen vacancies of perovskites.11, 51 The bands at 1460 and 1160 cm-1 are ever reported as the ν(N=O) and ν(N-O) vibrations of monodentate nitrites on CexZr1-xO2 mixed oxides,29 respectively. The visible positive bands appearing in the region of 1565-1530 cm-1 are recognized as chelating bidentate nitrates with νs(NO2).29, 37 The region of negative bands which represent the transformation of carbonates is consistent with the results of FT-IR for carbonates. Obviously, with K2CO3 loading increasing to 8 wt%, free ionic nitrates with the band at 1380 cm-1 become the main NOx sorption species. For comparison with the spectra of LC/5K/Y5CZ (Fig. 14f), in situ DRIFT spectra of the sample 5K/Y5CZ after co-adsorption in NO + O2 at 350 oC were also recorded so as to investigate the influence of LaCoO3, as shown in Fig. 14f. It is found that the appearance of nitrite and its evolution with time are obviously different from the case of LC/5K/Y5CZ. The bands at 1230 cm-1 which is assigned to νs(NO2) of bridging bidentate nitrites48, 49 appear at the beginning of exposure to NO + O2 and reach the strongest intensity at 5 min. With the gradual decrease of the band at 1230 cm-1, new bands at 1630-1590 and 1275 cm-1 appear, which are

27


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assigned to the νs(NO2) and νas(NO2) of bridging bidentate nitrates with νas(NO3-) split,37, 47, 48

respectively. This change could be attributed to the transformation of nitrites to bridging

bidentate nitrates through oxidation. It should be noted that the band at 1594 cm-1 appearing at the beginning also corresponds to the bridging bidentate nitrates derived from the NO2 adsorption on K species. Due to the oxidation of nitrites to nitrates, the vibration frequency in this region has a gradual blue shift till to 1627 cm-1, resulting in the difference for the two kinds of bridging bidentate nitrates formed through different pathways.29 Meanwhile, the band at 1230 cm-1 gradually decreases and reaches a stable intensity after 30 min, showing a red shift till to 1214 cm-1. In the end of the transformation of nitrites to nitrates the remaining of the band at 1214 cm-1 indicates the presence of bidentate nitrites (ν(N-O)) species arising from NO adsorption on Y5CZ.29 In addition to the formation of bridging bidentate nitrates, free ionic nitrates characterized by the bands at 1375 cm-1-νas(NO3-) and 1030 cm-1-νs(NO3-) and chelating bidentate nitrates characterized by 1541 cm-1-νs(NO2) are always present during the whole adsorption process.37 The negative bands are also assigned to the NOx-storage induced consumption of carbonates as proved by the release of gaseous CO2 with its characteristic bands appearing at 2366 and 2338 cm-1. Different from the case of the sample 5K/Y5CZ, no obvious vibration bands for nitrites formed on K species are found for the sample LC/5K/Y5CZ, which is related to the high oxidation capability of LaCoO3. The formation of numerous NO2 and the following rapid oxidation of nitrites to nitrates make the K species transform to nitrates species. The results of the in situ DRIFTS study for the supports and corresponding catalysts are totally consistent with the characterization results of H2-TPR and O2-TPD. 3.8. Mechanism discussion.

28



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During NOx storage (Fig. 10b and c), it is seen that there are always some gaseous NO2 in the feed stream since at the mixing stage the gaseous NO and O2 can readily react with each other to form some NO2 species. While nearly no gaseous NO2 are detected before the breakthrough, which means that the gaseous NO2 is easily captured by the catalyst. Due to the small proportion of NO2 to the total NOx in the feed gas, it is believed that the adsorbed NO2 is the main storage species, which comes from the NO adsorption and oxidation. By now many studies thought that the oxidation of NO(ad) to NO2(ad) is a key step for NOx storage.6 To confirm the importance of NO oxidation to NOx storage, the NOx storage performance of the catalyst LC/5K/Y5CZ in the atmosphere without oxygen (NO + N2) at 350 oC was conducted and shown in Fig. 15. Notably, in the absence of O2, although NO can also be trapped, the NOx storage is more difficult, showing much slower storage rate and larger NOx slip, as compared with those measured in the presence of oxygen (Fig. 10c in the manuscript). It is obvious that in the presence of O2 the NOx storage efficiency is much higher. Both the oxidation of NO to NO2 and the transformation of nitrites to nitrates need active oxygen species. The O2 adsorption and activation on catalyst surface are necessary for NOx storage. In this work, the “fast” NOx storage stage should correspond to the NOx trapping by the storage components surrounding the oxidation sites (LaCoO3); in this “fast” storage process, NO oxidation may be the rate limiting step. The “slow” NOx storage stage perhaps corresponds to the NOx storage on the storage sites far from the oxidation sites; during this stage the resistance of NO2(ad) migration or diffusion (from surface to bulk) decreases the NOx storage rate and efficiency, making the gradual increase of effluent NOx concentration. Although in this “slow” storage process, NO oxidation is also important, it may be not the rate limiting step, since the whole rate is highly related to the surface migration or surface-to-bulk diffusion of NO2(ad) as well as O species which are needed for the transformation of nitrite to nitrate.

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On the basis of all characterization results and the NOx storage and reduction performance of the catalysts, potential NOx storage and reduction pathways are proposed, as listed below: Lean period: LaCoO3-δ + O2 → LaCoO3-λ(O2-)

(3-6-1)

LaCoO3-λ(O2-) + LaCoO3-δ → 2LaCoO3-λ(O-)

(3-6-2)

LaCoO3-λ(O-) + NO → LaCoO3-λ(NO2-)

(3-6-3)

Ce3+□ + O2 → Ce4+(O2-)

(3-6-4)

Ce4+(O2-) + Ce3+□ → 2Ce4+(O-)

(3-6-5)

Ce4+(O-) + NO → Ce4+(NO2-)

(3-6-6)

(NO2)ads + S-OK + O* → S-O + KNO3

(3-6-7)

2(NO2)ads + K2O + O* → 2KNO3

(3-6-8)

2(NO2)ads + K2CO3 + O* → 2KNO3 + CO2

(3-6-9)

NO + S-OK + O* → S-O + KNO2

(3-6-10)

2NO + K2O + O* → 2KNO2

(3-6-11)

2NO + K2CO3 + O* → 2KNO2 + CO2

(3-6-12)

KNO2 + O* → KNO3

(3-6-13)

Rich period: 4C3H6 + 18(NO2)ads → 9N2 + 12CO2 + 12H2O

(3-6-14)

(NO2)ads → NO2

(3-6-15)

C3H6 + 18KNO3 → 9K2O + 18NO2 + 3CO2 + 3H2O

(3-6-16)

C3H6 + 6KNO3 → 3K2O + 6NO + 3CO2 + 3H2O

(3-6-17)

4C3H6 + 18NO2 → 9N2 + 12CO2 + 12H2O

(3-6-18)

2C3H6 + 18NO → 9N2 + 6CO2 + 6H2O

(3-6-19)

30



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The S in S-OK or S-O (3-6-7/3-6-10) represents Ce4+, La3+ or Co3+. The (NO2)ads represents LaCoO3-λ(NO2-) (3-6-3) and Ce4+(NO2-) (3-6-6). The O* represents LaCoO3-λ(O-) (3-6-2) and Ce4+(O-) (3-6-5). The steps from 3-6-1 to 3-6-6 are describing the catalytic NO oxidations on metal oxides as LaCoO3 and CeO2 in terms of a Mars-van Krevelen mechanism in which vacancies in the oxide lattice facilitate the adsorption and dissociation of O2.52, 53 4. CONCLUSIONS With comprehensive and in-depth investigation upon the series of ceria-based solid solution supported pervoskite LaCoO3 for lean NOx elimination, we find that the LNT catalysts LaCoO3/K2CO3/S (S=CeO2, Ce0.75Zr0.25O2 or 5%Y/Ce0.75Zr0.25O2) exhibit superior NOx oxidization and NOx storage performance by forming diverse nitrates species at relatively low K2CO3 loading (2-5 wt. %); the formed nitrates could be reversibly reduced by C3H6 under richburn atmosphere to realize an efficient NOx storage-reduction cycles. The sample of LC/5K/Y5CZ shows not only the highest oxidizability for NO to NO2 (conversion: 66.5%), but also the best NOx reduction efficiency (98.2%) and the highest selectivity of NOx to N2 (98.8%) at 350 oC in the absence of CO2. The CO2 addition makes more NOx leak in the lean period and thus decreases the NOx reduction efficiency from 98.2% to 91.1% at 350 oC. In the temperature region of 350-400 oC this sample still keeps high NRP above 90% and stable NOx removal efficiency even after 20 lean/rich cycles. Both of the surface oxygen vacancies on catalyst surface and the interaction between LaCoO3 and the supports enhance the oxygen adsorption, activation and transferring. The strong oxidizability for NO to NO2 oxidation and the high dispersion of K species facilitate the rapid NOx trapping, showing little NOx leak. In addition, this series of catalysts are highly stable during cyclic NOx storage and reduction for 10 times at 350 oC. The outstanding advantages of as-prepared supported pervoskite catalyst LC/5K/Y5CZ including high NOx storage/reduction efficiency, high NOx to N2 selectivity, high catalytic

31


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stability and low cost make them promising in the replacement of noble metals-based LNT catalyst in the future.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Nos. 21276184, U1332102, 21476160), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No.20120032110014) and the Program of Introducing Talents of Discipline to University of China (No. B06006). REFERENCES (1) Herreros, J. M.; Gill, S. S.; Lefort, I.; Tsolakis, A.; Millington, P.; Moss, E. Enhancing the Low Temperature Oxidation Performance over a Pt and a Pt–Pd Diesel Oxidation Catalyst. Appl. Catal., B 2014, 147, 835–841. (2) Lim, C. B.; Kusaba, H.; Einaga, H.; Teraoka, Y. Catalytic Performance of Supported Precious Metal Catalysts for the Combustion of Diesel Particulate Matter. Catal. Today 2011, 175, 106-111. (3) Yan, Y.; Yu, Y.; He, H.; Zhao, J. Intimate Contact of Enolic Species with Silver Sites Benefits the SCR of NOx by Ethanol over Ag/Al2O3. J. Catal. 2012, 293, 13-26. (4) Lan, L.; Chen, S.; Zhao, M.; Gong, M.; Chen, Y. The Effect of Synthesis Method on the Properties and Catalytic Performance of Pd/Ce0.5Zr0.5O2-Al2O3 Three-way Catalyst. J. Mol. Catal., A 2014, 394, 10-21

32



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 63

(5) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S. i.; Tanizawa, T.; et al. The New Concept 3-way Catalyst for Automotive Lean-burn Engine: NOx Storage and Reduction Catalyst. Catal. Today 1996, 27, 6369. (6) Roy, S.; Baiker, A. NOx Storage-Reduction Catalysis: From Mechanism and Materials Properties to Storage-Reduction Performance. Chem. Rev. 2009, 109, 4054-4091. (7) Kim, C. H.; Qi, G.; Dahlberg, K.; Li, W. Strontium-Doped Perovskites Rival Platinum Catalysts for Treating NOx in Simulated Diesel Exhaust. Science 2010, 327, 1624-1627. (8) Xu, J. F.; Liu, J.; Zhao, Z.; Xu, C. M.; Zheng, J. X.; Duan, A. J.; Jiang, G. Y. Easy Synthesis of Three-dimensionally Ordered Macroporous La1-xKxCoO3 Catalysts and Their High Activities for the Catalytic Combustion of Soot. J. Catal. 2011, 282, 1-12. (9) Li, Z. Q.; Meng, M.; Zha, Y. Q.; Dai, F. F.; Hu, T. D.; Xie, Y. N.; Zhang, J. Highly Efficient Multifunctional Dually-substituted Perovskite Catalysts La1-xKxCo1-yCuyO3-δ Used for Soot combustion, NOx Storage and Simultaneous NOx-soot Removal. Appl. Catal., B 2012, 121, 65–74. (10) Zhang, R. D.; Villanueva, A.; Alamdari, H.; Kaliaguine, S. Catalytic Reduction of NO by Propene over LaCo1-xCuxO3 Perovskites Synthesized by Reactive Grinding. Appl. Catal., B 2006, 64, 220-233. (11) Li, X. G.; Dong, Y. H.; Xian, H.; Hernández, W. Y.; Meng, M.; Zou, H. H.; Ma, A. J.; Zhang, T. Y.; Jiang, Z.; Tsubaki, N.; et al. De-NOx in Alternative Lean/Rich Atmospheres on La1-xSrxCoO3 Perovskites. Energ. Environ. Sci. 2011, 4, 3351.

33


Page 35 of 63

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Wen, Y. X.; Zhang, C. B.; He, H.; Yu, Y. B.; Teraoka, Y. Catalytic Oxidation of Nitrogen Monoxide over La1-xCexCoO3 Perovskites. Catal. Today 2007, 126, 400-405. (13) Białobok, B.; Trawczyński, J.; Miśta, W.; Zawadzki, M. Ethanol Combustion over Strontium- and Cerium-doped LaCoO3 Catalysts. Appl. Catal., B 2007, 72, 395-403. (14) Ye, J. S.; Yu, Y. F.; Meng, M.; Jiang, Z.; Ding, T.; Zhang, S.; Huang, Y. Y. Highly Efficient NOx Purification in Alternating Lean/Rich Atmospheres over Non-platinic Mesoporous Perovskite-based Catalyst K/LaCoO3. Catal. Sci. Technol. 2013, 3, 1915. (15) Luo, J. Y.; Meng, M.; Li, X.; Zha, Y. Q.; Hu, T. D.; Xie, Y. N.; Zhang, J. Mesoporous Co3O4-CeO2 and Pd/Co3O4-CeO2 Catalysts: Synthesis, Characterization and Mechanistic Study of Their Catalytic Properties for Low-temperature CO Oxidation. J. Catal. 2008, 254, 310-324. (16) Xue, L.; Zhang, C. B.; He, H.; Teraoka, Y. Catalytic Decomposition of N2O over CeO2 Promoted Co3O4 Spinel Catalyst. Appl. Catal., B 2007, 75, 167-174. (17) Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Venezia, A. M.; Deganello, G. Co3O4/CeO2 Composite Oxides for Methane Emissions Abatement: Relationship between Co3O4-CeO2 Interaction and Catalytic Activity. Appl. Catal., B 2006, 66, 217-227. (18) Li, G. F.; Wang, Q. Y.; Zhao, B.; Zhou, R. X. Modification of Ce0.67Zr0.33O2 Mixed Oxides by Coprecipitated/Impregnated Co: Effect on the Surface and Catalytic Behavior of Pd only Three-way Catalyst. J. Mol. Catal., A 2010, 326, 69-74. (19) Liu, L. J.; Chen, Y.; Dong, L. H.; Zhu, J.; Wan, H. Q.; Liu, B.; Zhao, B.; Zhu, H. Y.; Sun, K. Q.; Dong, L.; et al. Investigation of the NO Removal by CO on CuO-CoOx Binary Metal Oxides Supported on Ce0.67Zr0.33O2. Appl. Catal., B 2009, 90, 105-114.

34



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 63

(20) Markaryan, G. L.; Ikryannikova, L. N.; Muravieva, G. P.; Turakulova, A. O.; Kostyuk, B. G.; Lunina, E. V.; Lunin, V. V.; Zhilinskaya, E.; Aboukais, A. Red–ox Properties and Phase Composition of CeO2-ZrO2 and Y2O3-CeO2-ZrO2 Solid Solutions. Colloids. Surf., A 1999, 151, 435-447. (21) Rohart, E.; Larcher, O.; Deutsch, S.; Hédouin, C.; Aïmin, H.; Fajardie, F.; Allain, M.; Macaudière, P. From Zr-rich to Ce-rich: Thermal Stability of OSC Materials on the Whole Range of Composition. Top. Catal. 2004, 30/31, 417-423. (22) Fuentes, R. O.; Baker, R. T. Synthesis of Nanocrystalline CeO2-ZrO2 Solid Solutions by a Citrate Complexation Route: A Thermochemical and Structural Study. J. Phys. Chem., C 2008, 113, 914-924. (23) Vidmar, P.; Fornasiero, P.; Kašpar, J.; Gubitosa, G.; Graziani, M. Effects of Trivalent Dopants on the Redox Properties of Ce0.6Zr0.4O2 Mixed Oxide. J. Catal. 1997, 171, 160-168. (24) Kim, D. H.; Mudiyanselage, K.; Szányi, J.; Zhu, H.; Kwak, J. H.; Peden, C. H. F. Characteristics of Pt-K/MgAl2O4 Lean NOx Trap Catalysts. Catal. Today 2012, 184, 2-7. (25) Liu, Y.; Meng, M.; Li, X. G.; Guo, L. H.; Zha, Y. Q. NOx Storage Behavior and Sulfurresisting Performance of the Third-generation NSR Catalysts Pt/K/TiO2-ZrO2. Chem. Eng. Res. Des. 2008, 86, 932-940. (26) Castoldi, L.; Lietti, L.; Bonzi, R.; Artioli, N.; Forzatti, P.; Morandi, S. The NOx Reduction by CO on a Pt-K/Al2O3 Lean NOx Trap Catalyst. J. Phys. Chem., C 2010, 115, 12771286.

35


Page 37 of 63

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Hou, N. N.; Zhang, Y. X.; Meng, M. Carbonate-Based Lean-Burn NOx Trap Catalysts PtK2CO3/ZrO2 with Large NOx Storage Capacity and High Reduction Efficiency. J. Phys. Chem., C 2013, 117, 4089-4097. (28) He, X. X.; He, J. J.; Meng, M. Perovskite-Based Lean-Burn NOx Trap Catalysts Without Using Platinum Group Metals: K/LaCoO3/Ce1-xZrxO2. Catal. Lett. 2011, 141, 1364-1370. (29) Azambre, B.; Zenboury, L.; Koch, A.; Weber, J. V. Adsorption and Desorption of NOx on Commercial Ceria-Zirconia (CexZr1-xO2) Mixed Oxides: A Combined TGA, TPD-MS, and DRIFTS study. J. Phys. Chem., C 2009, 113, 13287-13299. (30) Vidal, H.; Kašpar, J.; Pijolat, M.; Colon, G.; Bernal, S.; Cordón, A.; Perrichon, V.; Fally, F. Redox Behavior of CeO2–ZrO2 Mixed Oxides I. Influence of Redox Treatments on High Surface Area Catalysts. Appl. Catal., B 2000, 27, 49-63. (31) Motin Seikh, M.; Sudheendra, L.; Narayana, C.; Rao, C. N. R. A Raman study of the Temperature-induced Low-to-intermediate-spin State Transition in LaCoO3. J. Mol. Struct. 2004, 706, 121-126. (32) Ishikawa, A.; Nohara, J.; Sugai, S. Raman Study of the Orbital-Phonon Coupling in LaCoO3. Phys. Rev. Lett. 2004, 93, 1-4. (33) Haas, O.; Struis, R. P. W. J.; McBreen, J. M. Synchrotron X-ray Absorption of LaCoO3 Perovskite. J. Solid State Chem. 2004, 177, 1000-1010. (34) Liu, Z. M.; Hao, J. M.; Fu, L. X.; Zhu, T. L. Study of Ag/La0.6Ce0.4CoO3 Catalysts for Direct Decomposition and Reduction of Nitrogen Oxides with Propene in the Presence of Oxygen. Appl. Catal., B 2003, 44, 355-370.

36



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 63

(35) Atribak, I.; Bueno-López, A.; García-García, A. Combined Removal of Diesel Soot Particulates and NOx over CeO2–ZrO2 mixed oxides. J. Catal. 2008, 259, 123-132. (36) Zhao, M.; Shen, M.; Wang, J. Effect of Surface Area and Bulk Structure on Oxygen Storage Capacity of Ce0.67Zr0.33O2. J. Catal. 2007, 248, 258-267. (37) Prinetto, F.; Manzoli, M.; Morandi, S.; Frola, F.; Ghiotti, G.; Castoldi, L.; Lietti, L.; Forzatti, P. Pt-K/Al2O3 NSR Catalysts: Characterization of Morphological, Structural and Surface Properties. J. Phys. Chem., C 2009, 114, 1127-1138. (38) Toops, T. J.; Smith, D. B.; Partridge, W. P. Quantification of the in situ DRIFT Spectra of Pt/K/γ-A12O3 NOx Adsorber Catalysts. Appl. Catal., B 2005, 58, 245-254. (39) Epling, W. S.; Peden, C. H. F.; Szanyi, J. Carbonate Formation and Stability on a Pt/BaO/γ-Al2O3 NOx Storage/Reduction Catalyst. J. Phys. Chem., C 2008, 112, 10952-10959. (40) Montanari, T.; Castoldi, L.; Lietti, L.; Busca, G. Basic Catalysis and Catalysis Assisted by Basicity: FT-IR and TPD Characterization of Potassium-doped Alumina. Appl. Catal., A 2011, 400, 61-69. (41) Stork, W. H. J.; Pott, G. T. Studies of Compound Formation on Alkali/γ-Aluminum Oxide Catalyst Systems Using Chromium, Iron, and Manganese Luminescence. J. Phys. Chem. 1974, 78, 2496-2506. (42) Iordan, A.; Zaki, M. I.; Kappenstein, C. Interfacial Chemistry in the Preparation of Catalytic Potassium-modified Aluminas. J. Chem. Soc., Faraday Trans. 1993, 89, 2527-2536.

37


Page 39 of 63

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Zou, Z. Q.; Meng, M.; He, J. J. Surface Distribution State and Storage Performance of the Potassium Species in the Lean-burn NOx Trap Catalyst Pt/K/Al2O3-TiO2-ZrO2. Mater. Chem. Phys. 2010, 124, 987-993. (44) Weiss, B. M.; Artioli, N.; Iglesia, E. Catalytic NO Oxidation Pathways and Redox Cycles on Dispersed Oxides of Rhodium and Cobalt. Chemcatchem 2012, 4, 1397-1404. (45) Liu, Y.; Meng, M.; Zou, Z. Q.; Li, X. G.; Zha, Y. Q. In situ DRIFTS Investigation on the NOx Storage Mechanisms over Pt/K/TiO2-ZrO2 Catalyst. Catal. Commun. 2008, 10, 173-177. (46) Hadjiivanov, K. I. Identification of Neutral and Charged NxOy Surface Species by IR Spectroscopy. Cat. Rev., Sci. Eng. 2000, 42, 71-144. (47) Luo, J. Y.; Epling, W. S.; Qi, G.; Li, W. Low Temperature Ceria-Based Lean NOx Traps. Catal. Lett. 2012, 142, 946-958. (48) Su, Y.; Amiridis, M. D. In situ FTIR Studies of the Mechanism of NOx Storage and Reduction on Pt/Ba/Al2O3 Catalysts. Catal. Today 2004, 96, 31-41. (49) Sedlmair, C.; Seshan, K., Jentys, A., Lercher, J. A. Elementary Steps of NOx Adsorption and Surface Reaction on a Commercial Storage-Reduction Catalyst. J. Catal. 2003, 214, 308316. (50) Xian, H.; Zhang, X. W.; Li, X. G.; Li, L. Y.; Zou, H. H.; Meng, M.; Li, Q.; Tan, Y. S.; Tsubaki, N. BaFeO3-x Perovskite: An Efficient NOx Absorber with a High Sulfur Tolerance. J. Phys. Chem., C 2010, 114, 11844-11852.

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(51) Hodjati, S.; Vaezzadeh, K.; Petit, C.; Pitchon, V.; Kiennemann, A. NOx SorptionDesorption Study: Application to Diesel and Lean-burn Exhaust Gas. Catal. Today 2000, 59, 323-334. (52) Choi, S. O.; Penninger, M.; Kim, C. H.; Schneider, W. F.; Thompson, L. T. Experimental and Computational Investigation of Effect of Sr on NO Oxidation and Oxygen Exchange for La1xSrxCoO3

Perovskite Catalysts. ACS Catal., 2013, 3, 2719-2728.

(53) Mars, P.; Van Krevelen, D. W. Oxidations Carried Out by Means of Vanadium Oxide Catalysts. Chem. Eng. Sci., 1954, 3, 41-59. Table 1. Specific surface area (SBET) of fresh catalysts, NOx storage capacity (NSC) and NO-toNO2 conversion (%) at the final steady-state for NOx storage; the NOx reduction percentage (NRP) of the catalysts after 10 lean/rich cyclic NOx storage-reduction tests; Co 2p binding energies (B.E.) from XPS characterization.

Sample

BET (m2/g)

NSC (μmol/g)

NO-toNO2 (%)

NRP (%)

Co 2p3/2 (eV)

Co 2p1/2 (eV)

ΔE (eV)

LC/5K/CeO2a

12.9

672.0

60.5

97.4

780.0

795.1

15.1

LC/5K/CZb

22.1

595.3

66.0

98.1

-

-

-

LC/5K/Y5CZc

26.2

639.0

66.5

98.2

780.0

795.1

15.1

LC/2K/Y5CZc

29.7

296.6

66.2

95.8

-

-

-

LC/8K/Y5CZc

18.0

1028.2

65.5

98.1

-

-

-

a: SBET of the pure CeO2 is 23.4 m2/g. b: SBET of the pure CZ is 42.9 m2/g. c: SBET of the pure Y5CZ is 55.3 m2/g.

39


Page 41 of 63

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Peak-fitting results of H2-TPR profiles.

Sample

LC/CeO2

Peak

I

II

III

IV

V

T/ oC

357

-

409

492

572

Areaa

1850

-

9427

705

19240

T/ oC

297

377

398

494

580

LC/CZ Area

LC/Y5CZ

a

6790

16152

9730

2004

18472

T/ oC

295

363

395

486

580

Areaa

8756

16421

9629

1705

18684

T/ oC

-

-

433

-

590

LaCoO3 Area

a

-

-

95239

-

a: arbitrary unit b: ratio of the peak V to III c: summation of peak III and V

40


191383

ratiob

Σc

2.04

28668

1.93

28473

1.94

28312

2.01

286622


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 63

Table 3. IR assignments of surface NOx species formed upon co-adsorption of NO/O2 at 350 oC. Sample

Band positions (cm-1)

NOx species

Ref.

1595

νs(NO2)

29, 46

1220-1210

νas(NO2)

29, 46

around 1000

νs(NO3-)

29, 46

1500

νs(NO2)

46

1290-1250

νas(NO2)

46

1035-970

νs(NO3-)

46

1581, 1573, 1560 and 1530

νs(NO2)

29

1250-1220

νas(NO2)

29

1030-1000

νs(NO3-)

29

1450-1360

νas(NO2)

46

1360-1320

νs(NO2)

46

H2O (ads)

1627

δ(HOH)

29

LC/xK/S (S=CeO2, CZ or Y5CZ)

ionic nitrates

1395, 1380 and 1370-1365

νas(NO3-)

37-38

1030

νs(NO3-)

37

5K/Y5CZ

bridging bidentate nitrates

1630-1590 and 1598

νs(NO2)

37, 47-48

1277

νas(NO2)

37, 47-48

1440-1432

νas(NO2)

11, 48-51

1340

νs(NO2)

11, 48-51

chelating bidentate nitrates

1565-1530 and 1541

νs(NO2)

29, 37

monodentate nitrites

1460

ν(N=O)

29

1160

ν(N-O)

29

bridging bidentate nitrites

1230

νs(NO2)

48-49

bidentate nitrites

1214

ν(N-O)

29

CeO2, CZ and Y5CZ

bridging bidentate nitrates

Vibration

monodentate nitrates

chelating bidentate nitrates

nitro compounds

monodentate nitrates

41


Page 43 of 63

FIGURES

*

*: support (CeO 2 /CZ/Y5CZ) &: LaCoO 3 & * & *

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


&

&

(a)

* & *

*

60

70

*

*

*

80

90

(b) (c) (d) (e)

10

20

30

40

50 

2

Figure 1. XRD patterns of the fresh samples: (a) LC/5K/CeO2, (b) LC/5K/CZ, (c) LC/5K/Y5CZ, (d) LC/2K/Y5CZ , (e) LC/8K/Y5CZ

42



(CZ) 470

(a)

464 (CeO2)

Intensity / cps

(Y5CZ) 473

500

475

450

CeO2 *0.5 613

304

612

302

CZ *1 Y5CZ *1

1000

800

600

400

200

Raman Shift / cm-1

(b)

463 (LC/5K/CeO2)

458 (LC/5K/CZ)

Intensity / cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 63

461 (LC/5K/Y5CZ)

500

475

450

425

LC/5K/CeO2 *0.5 LC/5K/CZ *0.7 LC/5K/Y5CZ *1

1000

800

301 614

600

Raman Shift / cm

400

200

-1

Figure 2. FT-Raman spectra of the fresh samples: (a) the supports of CeO2, CZ and Y5CZ, (b) the catalysts LC/5K/CeO2, LC/5K/CZ and LC/5K/Y5CZ.

43


Page 45 of 63

(a)

LaCoO 3

Co-Co Co-La

Co-O

LC/5K/CeO2

FT-magnitude / a. u.

LC/5K/CZ LC/5K/Y5CZ

0

1

2

3

4

5

6

R/Å (b)

FT-magnitude / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LaCoO3 LC/5K/Y5CZ-f LC/5K/Y5CZ-s LC/5K/Y5CZ-r LC/5K/CZ-r LC/5K/CeO2-r

0

1

2

3

4

5

6

7

8

R/Å Figure 3. Co K-edge RSFs of the reference compounds and the catalysts: (a) fresh catalysts, (b) spent catalysts (f: fresh catalysts; s: spent catalysts after used in NOx storage; r: spent catalysts after used in NOx storage and reduction).

44



Co 2p1/2

Co 2p3/2

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 63

LC/5K/Y5CZ LC/5K/CeO 2 800

795

790

785

780

775

Binding energy / eV Figure 4. XPS spectra for Co 2p of the fresh catalysts.

45


770

765

Page 47 of 63

(a)   396431



597

336

H2 Consumption / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LC/8K/Y5CZ LC/2K/Y5CZ LC/5K/Y5CZ 350

LC/5K/CZ LC/5K/CeO2

355

406 586 437

417

539

775

403

Y5CZ

546

419

809

CZ 468

CeO2 LaCoO3

100

433

200 300 400

559

858

590

500 600

700 800 900

o

Temperature / C

46



(b)

LC/CeO 2

H2 Consumption / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 63

LC/CZ

LC/Y5CZ

100

200

300

400

500

600

700

800

900

o

Temperature / C Figure 5. (a) H2-TPR profiles of the supports and fresh catalysts; (b) H2-TPR profiles of the perovskite LaCoO3 supported on CeO2, CZ and Y5CZ without K.

47


Page 49 of 63







LC/Y5CZ

O2 desorption / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Y5CZ

LC/CeO2 CeO2

0

100

200

300

400

500

600 o

Temperature / C Figure 6. O2-TPD profiles of the fresh catalysts.

48


700

800

900


(a)

NO for CeO 2 NO 2 for CeO 2

o

360 C

NO for LC/CeO 2 NO 2 for LC/CeO 2 o

MS Signal

5x10

268 C

-9

o

450 C

o

210 C

o

101 C

0

500

1000

1500

2000

2500

3000

Time / s

(b)

NO for CZ NO 2 for CZ

o

380 C

NO for LC/CZ NO 2 for LC/CZ 2.5x10

MS signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 63

o

453 C

-8

o

368 C o

220 C o

105 C

o

368 C o

313 C

0

500

1000

1500

2000

Time / s

49


2500

3000

Page 51 of 63

NO for Y5CZ NO2 for Y5CZ

o

o

380 C

NO for LC/Y5CZ NO2 for LC/Y5CZ 2.5x10

MS signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


464 C

(c)

o

358 C

-8

o

184 C o

o

310 C

o

103 C

0

500

358 C

1000

1500

2000

2500

3000

Time / s Figure 7. NO-TPD profiles of the fresh catalysts: (a) CeO2 and LC/CeO2, (b) CZ and LC/CZ, (c) Y5CZ and LC/Y5CZ

50



1382

0.05

Absorbance / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 63

1400 1330 1629 1452 1659 1594

LC/8K/Y5CZ

1116 1049

1480

LC/2K/Y5CZ

1477

LC/5K/Y5CZ

1474

LC/5K/CZ

1460

LC/5K/CeO 2

2200

2000

1037 1061

1434

K 2 CO3 1800

1600

1400

1200

1000

-1

Wavenumber / cm

Figure 8. FT-IR spectra of the fresh catalysts LC/xK/S (S=CeO2, CZ or Y5CZ) and reference compounds.

51


Page 53 of 63

(1) LC/5K/CeO 2

(4) LC/2K/Y5CZ

(2) LC/5K/CZ (3) LC/5K/Y5CZ

(5) LC/8K/Y5CZ

CO 2 evolution signal / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

761 C

(5)

o

200 C

(4) (3)

(2) (1) 0

100

200

300

400

500

600

700

800

900

o

Temprature / C Figure 9. CO2-TPD profiles of the catalysts LC/xK/S (S=CeO2, CZ or Y5CZ).

52



400

(a) (3)

NOx Concentration / ppm

(2) (4)

(1)

300

(5)

200

(1): LC/5K/CeO2

100

(2): LC/5K/CZ (3): LC/5K/Y5CZ (4): LC/2K/Y5CZ (5): LC/8K/Y5CZ

6 ppm

0 0

20

40

60

80

100

120

T / min 400

(b) NOx


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 63

300

NO2

200

NO

100 26 ppm 0 0

20

40

60

80

T / min

53


100

120

Page 55 of 63

400

(c) NOx


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

NO 2

200

NO

100

0

6 ppm 0

20

40

60

80

100

120

T / min Figure 10. Isothermal NOx storage curves of the fresh catalysts (a), the samples LC/5K/CeO2 (b) and LC/5K/Y5CZ (c).

54



400

400

(b) NOx Concentration / ppm


(a) 300

200

100

0 0

5

10

15

20

25

30

35

300

200

100

0

40

0

5

10

15

T / min



200

100

5

10

15

20

30

35

40

(d)

300

0

25

400

(c)

0

20

T / min

400

25

30

35

300

200

100

0

40

0

5

10

15

T / min

20

25

30

35

40

T/ min 40

400

(e)

(f)

35

N2O Concentration / ppm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 56 of 63

300

200

100

30 25 20 15 10 5

0 0

5

10

15

20

25

30

35

40

0 0

5

10

15

20

25

30

35

40

T / min

T / min

Figure 11. NOx concentration curves over the catalysts: (a) LC/5K/CeO2, (b) LC/5K/CZ, (c) LC/5K/Y5CZ, (d) LC/2K/Y5CZ, (e) LC/8K/Y5CZ; and (f) N2O concentration curve over LC/5K/Y5CZ during lean/rich cycles in the absence of CO2.

55


Page 57 of 63

500

500

(b)

400



(a)

300

200

100

0

400

300

200

100

0 0

20

40

60

80

0

20

40

T / min

60

500

(c)

(d)

400



80

T / min

500

300

200

100

0

400

300

200

100

0 0

20

40

60

80

0

20

40

T / min

60

80

T / min 100

500

(e)

(f) NOx Reduction Percentage / %


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

300

200

100

0 0

20

40

60

80

90

80

70

60

50 250

300

350

400

450

o

T / min

Temperature / C

Figure 12. NOx concentration curves over the catalyst LC/5K/Y5CZ during lean/rich cycles in the presence of 5 vol. % CO2 at various temperatures: (a) 250 oC, (b) 300 oC, (c) 350 oC, (d) 400 o

C, (e) 450 oC; and (f) NOx reduction percentages at different temperatures.

56



(a) 1558 1530

1004 985

1262 1235 1211

1 min 3 min 5 min 10 min 15 min

1354

1552 1526

1504

1627 1595

Kubelka-Munk unit

0.005

1800 1700 1600 1500 1400 1300 1200 1100 1000

Wavenumber / cm

900

-1

1581 1560 1534

(b) 1 min 3 min 5 min 10 min 15 min

1024 1006

1213

1273

1345

1499

1626

1237

1596

0.01

Kubelka-Munk unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 58 of 63

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 -1

Wavenumber / cm

57


Page 59 of 63

1218 1273

3605 3640

1 min 3 min 5 min 10 min 15 min 3696

1021 1005

1344

1500

(d) 3554

1239

1530 1595 1553

1549 1523

1627

Kubelka-Munk unit

1 min 3 min 5 min 10 min 15 min

3729

1558

(c)

1573

0.02

Kubelka-Munk unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1800 1700 1600 1500 1400 1300 1200 1100 1000

900

-1

3800 3700 3600 3500 3400 -1

Wavenumber / cm

Wavenumber / cm

Figure 13. Time-dependent in-situ DRIFTS spectra of NO/O2 co-adsorption at 350 oC on the catalysts: (a) CeO2, (b) CZ, and (c-d) Y5CZ

58



2360 2325

1458 1436 1421 1387

(a)

1413

Kubelka-Munk unit

60 min 30 min 20 min 10 min 5 min 3 min

1600

1400

Wavenumber / cm 1436 1420 1380 1361 1338

1273

1126 1487

1661

1520

1600

1063

1 min 3 min 5 min 10 min 20 min 30 min 60 min

1573 1540

2400 2300

1000

(b)

1457

1561

2366 2335

1153

1200 -1

1056

2200

1156

2400

1317

1517

1 min

Kubelka-Munk unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 60 of 63

1400

1200 -1

Wavenumber / cm

59


1000

1067

1132

1160

1277

1420 1385 1371 1342

1520

1464 1440

1598

(c)

1661

Kubelka-Munk unit

1502


1559 1541

2400 2300

1600

1400

1200

1000

-1

30 min

1432 1415 1399 1380 1362 1342

1598

(d)

1565 1548 1532

60 min

1504

Wavenumber / cm

20 min 10 min 5 min 3 min 1 min 1571 1556 1545 1524

Kubelka-Munk unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2363 2333

Page 61 of 63

2400 2200 1700

1600

1500

1400

Wavenumber / cm

-1

60


1300

1200


1381

(e)

1555

1305

1458

2366 2333

Kubelka-Munk unit

60 min 30 min 20 min 10 min

1537 1517

5 min 3 min

1454

1 min

1400

Wavenumber / cm

1000

(f) 5 min

20 min

1214

1030

1375

1627 1594 1541

1230


1557 1528

1450

1659

2366 2338

1200 -1

1060

1600

1117

2200

1275

2400

Kubelka-Munk unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 62 of 63

2500

2400

1600

1400

1200

1000

-1

Wavenumber / cm

Figure 14. Time-dependent in-situ DRIFTS spectra of NO/O2 co-adsorption at 350 oC on the catalysts: (a) LC/5K/CeO2, (b) LC/5K/CZ, (c) LC/5K/Y5CZ, (d) LC/2K/Y5CZ, (e) LC/8K/Y5CZ, and (f) 5K/Y5CZ

61


Page 63 of 63

400


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NO NOx NO2

300

200

100

0 0

20

40

60

80

100

120

T / min Figure 15. NOx concentration curves over the catalyst LC/5K/Y5CZ during NOx storage at 350 o

C in the atmosphere of 400 ppm NO balanced by N2.

62


YCeZrO Ternary Oxide Solid Solution Supported Nonplatinic Lean

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