Accepted Manuscript

A series of ceria supported lean-burn NOx trap catalysts LaCoO3/K2CO3/CeO2 using perovskite as active component

Rui You, Yuxia Zhang, Dongsheng Liu, Ming Meng, Zheng Jiang, Shuo Zhang,Yuying Huang

PII: S1385-8947(14)01202-9DOI: http://dx.doi.org/10.1016/j.cej.2014.09.016Reference: CEJ 12634

To appear in: Chemical Engineering Journal

Received Date: 8 July 2014Revised Date: 2 September 2014Accepted Date: 4 September 2014

Please cite this article as: R. You, Y. Zhang, D. Liu, M. Meng, Z. Jiang, S. Zhang, Y. Huang, A series of ceriasupported lean-burn NOx trap catalysts LaCoO3/K2CO3/CeO2 using perovskite as active component, ChemicalEngineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.09.016

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A Series of Ceria Supported Lean-Burn NOx Trap Catalysts

LaCoO3/K2CO3/CeO2 Using Perovskite as Active Component

Rui You 1, Yuxia Zhang 1, Dongsheng Liu 1, Ming Meng * 1, Zheng Jiang 2,

Shuo Zhang 2, Yuying Huang 2

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

Laboratory of Applied Catalysis Science and Engineering, School of Chemical Engineering &

Technology, Tianjin University, Tianjin 300072, P. R. China;

2 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy

of Sciences, Shanghai 201204, P. R. China

* Corresponding author

TEL/FAX: +86-(0)22-2789-2275

E-mail: mengm@tju.edu.cn

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ABSTRACT

A series of ceria supported perovskite-based lean-burn NOx trap (LNT) catalysts

10wt%LaCoO3/xK2CO3/CeO2（denoted as L/xK/C, x=1, 3, 5, 8 wt%）were prepared by

successive impregnation. They display excellent performance for NOx storage and

reduction, especially the one with 3% K2CO3. The formation of supported pervoskite

LaCoO3 on CeO2 is confirmed by XAFS characterization. The results of XPS and

O2-TPD reveal that the supported perovskite LaCoO3 contains considerable amounts of

surface adsorbed oxygen, which are responsible for NO to NO2 oxidation during NOx

storage. The catalyst L/3K/C exhibits fast and complete NOx trapping in lean condition,

showing little NOx leak for long time, over which a particularly high NOx reduction

percentage (97.7%) and NOx to N2 selectivity (98.5%) are simultaneously achieved in

cyclic lean/rich atmospheres. FT-IR and CO2-TPD results demonstrate the diversity of

K species including –OK groups, dispersed surface K2CO3 species and bulk or bulk-like

K2CO3 species. The dispersion and states of K species show big impact on NOx storage

pathways. In-situ DRIFTS results indicate that on L/xK/C NOx is stored as diverse

nitrates without evident nitrite species detected during NOx adsorption and storage,

which further verifies their excellent oxidation ability.

Keywords: Lean-burn NOx Trap; Supported perovskite; Ceria; Potassium carbonate

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1. Introduction

Lean-burn NOx trap (LNT) technique is recognized as one of the most effective

and promising solutions to the NOx released from lean-burn engines. Conventional

LNT catalysts often use noble metals such as Pt or Rh as active components for NO

oxidation and NOx reduction [1-3]. To decrease the dependence on noble metals and

reduce the cost of LNT catalysts, noble metal-free catalysts are being explored.

In lean-burn period, NOx is expected to be quickly and completely stored on

storage sites, forming nitrates and/or nitrites. Since NO2 possesses higher reactivity with

storage components than NO, the oxidation of NO to NO2 is very crucial to the quick

and efficient NOx storage [3]. Perovskites like LaCoO3 and LaMnO3 are reported to be

highly active for NO oxidation to NO2, showing the highest NO-to-NO2 conversion at

the temperature between 300-350 oC, and the LaCoO3 shows higher activity than

LaMnO3 [4]. Our previous work has proved that the bulk substituted La1-xSrxCoO3

perovskite is efficient for lean-burn NOx storage and reduction [5] due to its multiple

intrinsic advantages: (1) partial substitution of A-site ions (i.e., La3+) by Sr2+ can induce

the formation of Co4+ ions and/or oxygen vacancies, increasing the oxidation capability

of LaCoO3; (2) the strong basic Sr oxide can quickly store the acidic NOx. However, to

maintain the perovskite structure, the amount of substituted Sr is limited, thus the NOx

storage capacity of perovskites La1-xSrxCoO3 is restricted. In addition, the low specific

surface area of bulk perovskite also limits its NOx storage performance. By using a

nano-casting method, a mesoporous perovskite LaCoO3 was successfully synthesized,

which exhibited a specific surface area as high as 75 m2/g [6]. After loading K on it, the

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as-formed LNT catalyst K/LaCoO3 displayed extremely good performance for cyclic

lean/rich NOx storage/reduction. Nevertheless, the complicated preparation procedure

and the very high synthesis cost of mesoporous LaCoO3 due to the employment of large

amounts of organic template and high content of Co make such mesoporous perovskite

catalyst inappropriate for wide industrial application. In such situation, the new notion

for the constitution of supported perovskite LNT catalyst is proposed in order to replace

the platinum in conventional LNT catalysts.

Ceria, as oxygen storage/release components, is often used in many catalysts such

as oxidation or three-way catalysts either as support [7-9] or as promoter [10-11]. It is

known that in the rich-burn period the LNT catalysts actually act as the three-way

catalysts for the simultaneous removal of NOx and reductants. The employment of ceria

as support of LNT catalysts should be an appropriate choice. The reported work has

indicated that the ceria supported noble metal-based LNT catalyst Pt-BaO/CeO2

possesses much better sulfur resistance as compared with the conventional alumina

supported catalyst Pt-BaO/Al2O3 [12]. Thus, in this work, ceria was selected to support

the perovskite LaCoO3 to achieve not only high dispersion of LaCoO3 but also high

oxidation/reduction performance of the LNT catalyst. Considering the strong basicity

and better regeneration of K sulfates than Ba sulfates [13-16], potassium was selected as

NOx storage component to constitute the series of ceria supported non-platinic LNT

catalysts LaCoO3/K2CO3/CeO2. The loading of K2CO3 in the catalysts was optimized,

and the K species were well characterized by using multiple techniques such as

CO2-TPD, FT-IR and in situ DRIFTS. The structure and physicochemical property of

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these catalysts were systematically investigated by XRD, XPS, EXAFS and H2-TPR,

and correlated with their catalytic performance.

2. Experimental

2.1 Catalysts Preparation

The support CeO2 was synthesized by a common ammonia-precipitation method

[17] using Ce(NO3)3·6H2O as precursor salt. After dried overnight at 120 oC, the

precipitate was calcined at 700 oC for 4 h in static air to form the final support CeO2;

before use the support was crushed and sieved to fine powder (< 200 mesh). The

loading of perovskite LaCoO3 on CeO2 was prepared by impregnation method using the

mixed solution of precursor salts La(NO3)3·6H2O, Co(NO3)2·6H2O and citric acid

monohydrate (molar ratio of citric acid/La/Co was 2.2/1/1). The weight ratio of the

solution to the support powder was 10. After impregnation and stirring for 5 h at room

temperature, the obtained slurry was slowly evaporated at 40 oC in a rotary evaporator.

In the next, the mixture was dried at 120 oC overnight, followed by a calcination at

350oC for 2 h to decompose the nitrates; finally it was further calcined at 700 oC for 4 h

in flowing air to give ceria supported LaCoO3. Subsequently, the K2CO3 was loaded on

LaCoO3/CeO2 by using incipient-wetness impregnation. After drying at the same

condition, the precursor was calcined at 500 oC for 2 h in static air to get the final LNT

catalyst LaCoO3/K2CO3/CeO2. The content of LaCoO3 in all final catalysts was fixed at

10% by weight, while that of K2CO3 was selected as 1%, 3%, 5% or 8%, respectively.

This series of catalysts are denoted as L/xK/C, where x = 1, 3, 5, 8wt. % stands for the

K2CO3 weight loading in the catalysts. For comparison, the sample of K2CO3/CeO2 with

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5wt% K2CO3 was also prepared by the same method, which is denoted as 5K/C.

2.2. Catalyst Characterization

The specific surface areas (SBET) were measured on a Quantachrome QuadraSorb

SI instrument. Prior to measurements, all samples were first degassed in vacuum at

300°C for 4 h to remove the adsorbed species, and then nitrogen physisorption at 77 K

was conducted on the samples.

Powder X-ray diffraction (XRD) analysis was performed on an X’pert Pro rotatory

diffractometer (PANAlytical Company) operating at 40 mA and 40 kV, using Cu Kα as

radiation source (λ = 0.15418 nm). The 2θ data were collected over the range of 10-90o

with a step size of 0.02o.

X-ray photoelectron spectra (XPS) measurements were carried out on a PHI-1600

ESCA spectrometer using Mg Kα radiation (1253.6 eV) with base pressure as 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.

Co K-edge X-ray absorption fine structure (XAFS) was conducted on the

14W1-XAFS beamline at Shanghai Synchrotron Radiation Facility (SSRF) operating at

250 mA and 3.5 GeV. The spectra of the samples were collected at room temperature in

fluorescence mode with an energy resolution of 0.3 eV, and that of reference LaCoO3

(home-made) was collected in transmission mode. In the experiment, a cobalt foil was

employed for energy calibration. The analysis of all spectra was performed by using

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Ifeffit software package. The Fourier transforming of the k3-weighted EXAFS data was

handled in the range of k = 3–14 Å−1 using a Hanning function window to get the radial

structural function (RSF).

Temperature-programmed measurements including temperature-programmed

reduction (H2-TPR) and temperature-programmed desorption (O2-TPD/CO2-TPD) were

all carried out on a Thermo-Finnigan TPDRO 1100 instrument equipped with a thermal

conductivity detector (TCD). For H2-TPR test, 30 mg of powder sample were heated

from room temperature to 900 oC at a rate of 10 oC/min with a gaseous mixture of 5

vol. % H2/N2 at a flow rate of 20 mL/min. Prior to O2-TPD test, 200 mg of sample were

pre-heated in pure O2 at 500 oC for 30 min. After cooling to room temperature under O2,

the sample was then heated again from room temperature to 900 oC in pure He. For

H2-TPR and O2-TPD measurements, before detection, the gas was purified by a solid

trap containing CaO + NaOH materials in order to remove the H2O and CO2. The

temperature-programmed desorption of CO2 (CO2-TPD) derived from carbonates

decomposition was conducted in pure He (20 mL/min) from room temperature to 900oC

using 200 mg of powder sample and a heating rate of 10 oC/min. Only H2O was

removed by a solid trap of Mg(ClO4)2 before detection.

The temperature-programmed desorption of NOx (NOx-TPD) was conducted in

the quartz-tubular continuous flow reactor (i.d. =8 mm) under constant N2 flow rate

(150 mL/min) from 50 to 700 °C with a temperature ramp of 10 °C/min. Prior to

measurements, samples were saturated with NOx under lean condition at 350 °C and

kept exposure to NO and O2 when cooling to 50 oC.

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The Fourier-transform infrared spectroscopy (FT-IR) and in situ diffuse reflectance

Fourier-transform infrared spectroscopy (in situ DRIFTS) characterization was recorded

on a Nicolet Nexus spectrometer. For FT-IR measurements, the fresh catalyst was

diluted with KBr and pressed into a pellet. For in situ experiments, the pure samples

were finely ground and then placed into an in situ chamber. The nature of NOx

ad-species generated upon reactive adsorption of 400 ppm NO + 5 vol. % O2 in N2 at

350 oC was investigated. Before admission of the adsorption gas, the samples were first

in situ pretreated with 5 vol. % O2 in N2 at 350 oC for 1 h and then the background

spectrum was collected with a MCT detector cooled by liquid nitrogen. Based upon 32

scans time-dependent difference spectra were recorded at a spectral resolution of 4 cm-1

in the range of 650–4000 cm-1.

2.3. Catalytic activity measurements

NOx storage capacity (NSC) of the catalysts was measured in a quartz-tubular

continuous flow reactor (i.d. =8 mm) using 200 mg of fresh catalyst (40–60 mesh). Each

time the sample was first pretreated at 350 oC in air flow for 30 min, and then a mixture

gas containing 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 outlet concentrations of NO, NO2, and total NOx were monitored

online by a Chemiluminescence NO−NO2−NOx Analyzer (Model 42i-HL, Thermo

Scientific).

Alternative lean/rich cyclic NOx storage and reduction tests (10 cycles) were

conducted at 350 oC in the same reactor as above to investigate the NOx reduction

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performance of the samples, using 3 and 1 min for lean and rich period, respectively. In

lean period, the mixture gas consists of 400 ppm NO, 5 vol. % O2 and balance N2, while

in rich period the mixture gas of 1000 ppm C3H6 and N2 was used. The flow rate was

kept at 150 mL/min. Meanwhile, the by-product N2O was monitored constantly by a

N2O Modular Gas Analyzer (S710, SICK MAIHAK), and the outlet C3H6 was detected

by a HIDEN HPR20 mass spectrometer (m/z = 42).

3. Results and discussion

3.1. Structural and Texture properties

3.1.1 XRD

Fig. 1 shows the XRD patterns of L/xK/C. All patterns show four main diffraction

peaks at about 2θ = 28.5o, 33.1o, 47.5o and 56.4o, attributed to the (1 1 1), (2 0 0), (2 2 0)

and (3 1 1) planes of cubic fluorite CeO2 (JCPDS 43-1002 space group, Fm3m) [18].

The highly symmetric shape suggests the formation of an ideal cubic phase for all

samples. Other two diffraction peaks at 2θ = 23.2o and 40.7o are identified as the phase

of perovskite LaCoO3 (JPCDS 48-0123), while the main diffraction peak of LaCoO3 at

about 33o is overlapped with the peak of CeO2 (2 0 0). The possible phases or oxides

like La2O3 or Co3O4 are not detected by XRD, and K-related species are not found,

either. The K phases may be highly dispersed or existing in amorphous state in the

catalysts. As storage components in LNT catalysts, the K species and their states are

particularly important which will be analyzed by IR and CO2-TPD in the following

sections.

3.1.2 XAFS

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To further reveal the states of Co species in the catalysts, XAFS characterization

was performed. Fig. 2 shows the radial structural functions (RSFs) of Co K-edge

derived from extended X-ray adsorption fine structure (EXAFS) of the reference

LaCoO3 (home-made), the fresh sample L/3K/C and the one after used in lean/rich

cyclic NSR tests. Two strong coordination peaks at the distance around 0.149 nm and

0.315 nm are detected for reference LaCoO3. The first one corresponds to octahedrally

coordinated Co–O shell in LaCoO3 perovskite, whereas the second one is

simultaneously contributed by single (Co–La, Co–Co) and multiple (Co–O–Co,

Co–O–La, Co–O–Co–O) scattering [19-20]. The RSFs of the samples are similar to that

of reference LaCoO3, confirming that the supported perovskite LaCoO3 with perfect

ABO3 structure has been successfully prepared, which is highly stable even after 10

cyclic NOx storage and reduction by C3H6 at 350 oC.

3.1.3 XPS

The XPS results of Co 2p and O 1s binding energies for CeO2 (denoted as C),

LaCoO3/CeO2 (denoted as L/C) and L/3K/C are shown in Fig. 3a, b, respectively.

Generally, the spin-orbit splitting of the Co 2p peak (∆E) can reflect the oxidation state

of Co, e.g., cobaltous compounds exhibit a larger ∆E value (~16.0 eV) and cobaltic

compounds display a smaller ∆E value (~15.0 eV). For Co3O4 with multiple-valence of

Co, the spin-orbit splitting value is about 15.2 eV [20-23]. From Fig. 3a and Table 1, it

is seen that the spin-orbit splitting of Co 2p peaks for L/C and L/3K/C exhibits a value

of ~15.1 eV, similar to that of LaCoO3 reported by Xu et al [21]. This result implies that

most cobalt ions in these two samples possess the valence of +3 and only a very small

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amount of cobalt ions possess the valence of +2. It is well known that the perovskite

LaCoO3 with small crystallite size usually has many crystal structure defects, such as

oxygen vacancies, especially on the surface, which means that some Co atoms are in

unsaturated coordination states, leading to the lower valence of Co than +3. It seems

that the XPS results are not consistent with the above EXAFS results in which

pervoskite LaCoO3 has been identified as the sole cobalt-containing phase; such

inconsistency is resulted from the difference of the principles of these two

characterization methods. The EXAFS technique reveals the characteristics of bulk

structure, while the XPS technique can only give the surface or subsurface structural

information. Concerning the O 1s spectra shown in Fig. 3b, there are two peaks

corresponding to two chemical states of oxygen. The one at lower binding energy

(~528.7 eV) is assigned to surface lattice oxygen (Olat), while the other one at higher

binding energy (~531.1 eV) is attributed to adsorbed oxygen species (Oads) which may

exist in surface oxygen vacancies. The ratios of adsorbed oxygen to lattice oxygen

(Oads/Olat) listed in Table 1 are calculated on the basis of peak areas. In pure CeO2, there

are always some oxygen vacancies due to the presence of some Ce3+ ions; the detected

small amount of surface adsorbed oxygen species should locate in the oxygen vacancies.

After LaCoO3 loading, the ratio of Oads/Olat is remarkably increased. Such obvious

increase of adsorbed oxygen species should be mainly contributed by the adsorbed

oxygen on LaCoO3 since the supported perovskite can provide more locations (oxygen

vacancies) for oxygen adsorption. Similar assignment could be found elsewhere [6,

20-21].

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3.1.4 BET

The BET specific surface area (SBET) of the pure CeO2 and catalysts are listed in

Table 2. It can be seen that after loading of perovsktie LaCoO3 and K2CO3, the specific

surface area decrease a little, and with the increase of K2CO3 content in L/xK/C samples,

the SBET decreases gradually, probably due to the pore blocking by K2CO3 and the

decreased weight percentage of CeO2 in the final catalysts. From L/3K/C (16.3 m2/g) to

L/5K/C (12.9 m2/g), the decrease extent is larger as compared with that from L/1K/C

(17.5 m2/g) to L/3K/C (16.3 m2/g). This larger difference may be caused by the changes

of kinds and distribution states of K species, which should have significant impact on

the NOx trapping performance of the catalysts, as talked later.

3.2. Reducible properties (H2-TPR)

Fig. 4 shows the H2-TPR profiles of the LNT catalysts L/xK/C as well as the pure

CeO2 and the bulk LaCoO3 (prepared by sol-gel method and calcined at 700 oC). For the

pure CeO2, three peaks with peak zenith at 468, 559 and 856 oC are identified, which

are ascribed to surface chemisorbed oxygen, surface lattice oxygen and bulk lattice

oxygen [9], respectively. After loading of K, the reduction profile of the sample 5K/C

shows little change. The bulk LaCoO3 presents two main reduction peaks in its H2-TPR

profile, which suggests the stepwise reduction of the perovskite LaCoO3. The one in

300-500 oC region is ascribed to the reduction of Co3+ to Co2+, forming

oxygen-deficient perovskite (La2CoO4), and the other at higher temperature (500–700

oC) is attributed to the further reduction of La2CoO4 to Co0 and La2O3 as reported in

literature [20-21]. For the samples L/xK/C, the stepwise reduction of LaCoO3 could be

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easily identified. The extra H2 consumption peak appearing around 350 oC is associated

with the reduction of surface oxygen species. In comparison with the reduction of

surface adsorbed oxygen species on CeO2, the reduction temperature for the adsorbed

oxygen species on L/xK/C is much lower. The metal-support interaction between CeO2

and LaCoO3 may have enhanced the reduction of adsorbed oxygen species by

weakening the strength of Ce-O bonds [9, 23]. It should be noted that reduction of bulk

Ce4+ ions is also promoted to some extent, probably due to the hydrogen spillover effect

of the metallic Co derived from the reduction of LaCoO3 before 650 oC [17, 24]. In

addition, it is interestingly found that the amount of H2 consumption above 650 oC for

bulk lattice oxygen in CeO2 exhibits an increasing tendency with the increase of K2CO3

loading. The apparent increased TCD signal may be partially contributed by the

decomposition of K2CO3 at the similar temperature (650-900 oC), since the gaseous

decomposition product (CO2) can dilute the stream and decrease the H2 concentration.

This assumption is proved by the H2-TPR profile of pure K2CO3 (not shown), in which

an obvious H2 consumption signal was detected even though K2CO3 is not reducible.

3.3. O2 temperature programmed desorption (O2-TPD)

XPS results have revealed that after loading LaCoO3 on CeO2 the amounts of

surface oxygen species are remarkably increased as compared with pure CeO2. To prove

this viewpoint, the tests of temperature programmed desorption of O2 were conducted

on CeO2 and LaCoO3/CeO2, the results of which are shown in Fig. 5. No O2 desorption

is found on pure CeO2 but several obvious O2 desorption peaks are detected on

LaCoO3/CeO2. The peaks below 300 oC correspond to the desorption of the surface

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adsorbed oxygen species such as O2- or O- [24], while the other two appearing at

300-650 oC and 650 oC-800 oC are attributed to the desorption of surface lattice oxygen

and bulk lattice oxygen of LaCoO3, respectively. The increased amounts of surface

oxygen species would improve the oxidative capability of the catalysts for NO to NO2

oxidation during lean NOx storage at 350 oC.

3.4. Analysis of K species

No information about K species was given by XRD results, thus IR technique was

employed to investigate the states of K2CO3 species, the results of which are displayed

in Fig. 6. The bulk K2CO3 used as the precursor of potassium salt shows the

characteristic IR bands in the region of 1800-1000 cm-1. For the samples L/xK/C, they

all exhibit similar IR bands to that of bulk K2CO3, which suggests that the potassium in

the catalysts should mainly exist as the carbonate species. The bands at 1107 (νs(OCO)),

1325 (νas(OCO)) and 1663/1690 cm-1 (ν(C=O)) are assigned to the bridging bidentate

carbonates, and those at 1051/1061 and 1370/1385 cm-1 are attributed to chelating

bidentate carbonates with νs(OCO) and νas(OCO) modes [25-26], respectively. The band

at 1400 cm-1 originates from monodentate carbonates [27]. The bands in the region of

1650-1620 cm-1 and 1485-1455 cm-1 are assigned to characteristic vibration of ν(C=O)

and νas(OCO) modes for bicarbonates [26-29], respectively. From the absorptivity of the

typical band for bulk K2CO3 at 1663 cm-1, it can be deduced that the samples L/5K/C

and L/8K/C possess more bulk K2CO3 than L/1K/C and L/3K/C. The biggish decrease

of the specific surface areas from L/3K/C (16.3 m2/g) to L/5K/C (12.9 m2/g) should be

related to the increased content of bulk K2CO3. Even though the samples L/xK/C show

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similar IR absorption to bulk K2CO3, other carbonate species such as surface amorphous

carbonates are hard to be excluded. Considering the difference of decomposition

temperatures for different carbonates, the tests of temperature-programmed

decomposition of carbonates (CO2-TPD) was performed on the samples L/xK/C.

Fig. 7 presents the CO2-TPD profiles of different samples. It is found that there are

several CO2 desorption regions at different temperature, further confirming the diversity

of surface K species on L/xK/C. The CO2 desorption peak at low temperature (< 220 oC)

should be related to the part of carbonates interacted with the supports, similar

phenomenon of CO2 desorption at so low temperature is also revealed on K/Al2O3

system [28]. The interaction between the surface K species and the hydroxyl groups on

the support may have enhanced the decomposition of K2CO3. Iordan et al [30] thought

that the interaction between K+ ions and surface hydroxyl groups on the support could

produce the support-O-K+(aq) surface species during impregnation; the following drying

and calcination could strengthen the solid/solid interfacial interaction, leading to

low-temperature decomposition of surface carbonates to form −OK groups as described

below:

K2CO3 + 2(−OH) → CO2 + H2O + 2(−OK)

At middle temperature between 220 and 800 oC, the CO2 desorption may be resulted

from the decomposition of surface K2CO3 species with small crystallite size or high

dispersion which have less or no chance to directly interact with the surface hydroxyl

groups. At the temperature above 800 oC the CO2 desorption should be related to the

decomposition of bulk or bulk-like K2CO3 species with large size which barely interact

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with the support. As a consequence of the decomposition for both surface K2CO3 and

bulk or bulk-like K2CO3 species, K2O or analogous KxOy species are generated via the

following reaction:

K2CO3 → CO2 + K2O

From the areas of the first peaks in the CO2-TPD profiles, we can see that the amounts

of the –OK species obviously increase from L/1K/C to L/3K/C, but no big change takes

place with the K2CO3 content increasing further, which suggests a nearly complete

covering of the support surface by the highly dispersed K+ species for the samples

L/xK/C (x=3, 5 and 8). While for 5K/C, it exhibits a much larger amount of CO2

desorption at low temperature (< 220 oC) than L/xK/C, which should be related to its

larger specific surface area and more hydroxyl groups of pure CeO2. It should be noted

that with the increase of K2CO3 loading, the contents of surface K2CO3 corresponding

to middle-temperature region show little change, but the bulk or bulk-like K2CO3

species get increased. In a summary, on the surfaces of L/xK/C a variety of K species

including the highly dispersed –OK groups, the surface K2CO3 species with small

particle sizes and the bulk or bulk-like K2CO3 species are simultaneously present, which

should be responsible for NOx sorption and storage.

3.5. NOx storage and reduction performance of the catalysts

Isothermal NOx storage at 350 oC was carried out over the fresh catalysts, the

results of which are presented in Fig. 8a and Table 2. It is known that the NOx storage is

a successive oxidation process including NO to NO2 and NO2 to nitrates, so the

oxidation ability is rather important to NOx storage [3]. The NOx species adsorbed on

17

the storage medium surrounding the oxidizing components like LaCoO3 perovskite can

be quickly oxidized and stored as nitrates or nitrites, since the active sites can provide

activated oxygen species to the storage sites through surface migration or spillover.

While the NOx adsorption and storage on the remote storage sites including the

subsurface and bulk storage sites take place slowly, due to the lack of active oxygen

species and the diffusion resistance from surface to subsurface or bulk structure of

storage medium, resulting in continuous NOx leak [3]. From Fig. 8a, it could be seen

that the sample L/3K/C can quickly capture the NOx at the beginning, reaching the

lowest point (~5 ppm) and keeping at this level for very long time, showing little NOx

leak. Although the sample L/1K/C with lower K content and higher K dispersion also

exhibits quick NOx uptake and saturation, it still shows a NOx leak of 22 ppm at the

lowest point. For the samples L/5K/C and L/8K/C with much higher amounts of K, the

reachable lowest NOx concentration is about 30 ppm, and the NOx storage is a rather

slow process; the presence of more bulk or bulk-like K2CO3 in these two samples makes

NOx storage more and more difficult due to the gradually increased surface to bulk

diffusion resistance. In addition, the increasing amount of K2CO3 can cover some of the

oxidizing components (LaCoO3), decreasing the NO oxidation ability of the samples

and the subsequent NOx storage. Table 2 lists the conversions of NO to NO2 acquired at

final steady-state for each sample. It can be seen that with the increase of K2CO3 loading

from 1% to 8%, the NO to NO2 conversion decreases from 67.8% to 55.0%. The

samples L/1K/C and L/3K/C show much stronger oxidation ability than L/5K/C and

L/8K/C. This changing tendency could be well correlated with that of BET. The best

18

performance of the sample L/3K/C for NOx storage should be mainly determined by the

appropriate amounts and good dispersion of K species as well as its strong capability for

NO to NO2 oxidation.

To better understand the NOx storage pathways, the change tendency for NO and

NO2 concentrations during NOx storage are monitored. Fig. 8b displays the two

concentration curves of NO and NO2 for the sample L/8K/C. It is found that after

reaching the lowest concentration the NO2 concentration keeps very stable for a rather

long time, while the NO concentration is always increasing. Based upon this

phenomenon, it is deduced that the NOx storage is probably through the

disproportionation reaction (3-5-1) in this period, thus leading to NO leak [3, 31-32].

3NO2 (g) + K2CO3 → 2KNO3 + NO (g) + CO2 (g) (3-5-1)

The corresponding NO and NO2 concentration curves for L/3K/C are shown in Fig. 8c.

It can be seen that both the NO and NO2 concentrations keep stable for some time after

reaching the lowest concentration, which means that no NO leak is resulted by NO2

storage, all the NO2 should have transformed to nitrate species as described by the

reaction (3-5-2).

2NO2 (g) + K2CO3 + O* → 2KNO3 + CO2 (g) (3-5-2)

In the absence of perovskite LaCoO3, the oxidation capability of the samples is

greatly decreased, e.g., the catalyst 5K/C presents a much lower conversion of NO to

NO2 (21.4%) at 350 oC, thus showing much higher NOx leak at the beginning and a

much slower NOx storage rate to saturation. As known, for both the oxidation of NO to

NO2 and the subsequent formation of nitrites or nitrates during NOx storage, the O2

19

activation and transferring on the catalyst surface are vital of importance [3, 33-34]; the

worse performance of 5K/C is highly related to its decreased capability for oxygen

adsorption and activation, which can be reflected by the previous results of XPS,

H2-TPR and O2-TPD. In a summary, the rapid and complete NOx storage over L/xK/C

depends on both the dispersed K species and the abundant surface active oxygen species

which can facilitate not only the oxidation of NO to NO2 but also the transformation of

adsorbed NOx species to nitrates or nitrites.

To investigate the performance of the samples for NOx reduction, NOx storage and

reduction by C3H6 in successive lean/rich (3 min/1 min) atmospheres were performed at

350 °C over the as-prepared catalysts. The outlet NOx concentration was continuously

recorded for 10 lean/rich cycles, as displayed in Fig. 9 (a-e). It is found that the outlet

NOx concentration for the sample L/1K/C shown in Fig. 9a gradually increases as time

going, especially at the moment switching from lean to rich atmosphere. Due to the very

low loading of K species on L/1K/C, some surface metal oxide sites such as LaCoO3

and CeO2 may also adsorb NOx as ad-NO, ad-NO2 species or nitrates formed via the

interaction between NO2 and surface basic/neutral hydroxyls [31] as the following

reaction:

3NO2 + 2OH- → 2NO3- + H2O + NO (3-5-3)

And these adsorbed species may be unstable as demonstrated in the following part of

NOx-TPD and thus release or decompose promptly at the temperature of 350 oC when

changing to rich atmosphere containing the reductant of C3H6. With the K loading

increasing, the corresponding NOx concentration obviously decreases at the switching

20

moment, as shown in Fig. 9b-d, especially for the sample L/3K/C. Based on the last two

lean/rich cycles the NOx reduction percentage (NRP) are calculated and listed in Table

2. It can be seen that the sample L/3K/C exhibits the highest NRP of 97.7%. The sample

5K/C without perovskite exhibits the highest NOx leak during the cyclic NOx storage

and reduction as shown in Fig. 9e. The absence of perovskite decreases not only the

ability for NO to NO2 oxidation at lean condition, but also the performance for NOx

reduction at rich condition. To know exactly the selectivity of NOx to by-product N2O,

its concentration was also monitored in the whole NOx storage and reduction process.

An an example, the N2O concentration curve for the best sample L/3K/C is presented in

Fig. 9f. It is found that the N2O concentration is always below 4 ppm. Generally, the

formation of N2O is through the combination of gaseous and/or adsorbed NO with the

adsorbed N(ad) produced from the dissociation of NO molecules. So, it is deduced that

the samples L/xK/C have weak capability for dissociating NO molecules. Based on the

N balance, the NOx to N2 selectivity over the sample L/3K/C is calculated as 98.5%. In

addition, concerning the C3H6 oxidation activity under rich conditions, the outlet C3H6

was detected by MS (m/z = 42) online over the sample L/3K/C in the meantime, as

shown in Fig. 9f. By calculation, there is almost 27% of C3H6 taking part in the

reduction of NOx or nitrates/nitrites.

3.6. Thermal stability of the stored NOx species

To investigate the thermal stability of stored NOx species after exposure to NO and

O2 in lean condition, temperature-programmed desorption of NOx (NOx-TPD) derived

from the decomposition of stored species was conducted, the results of which are shown

21

in Fig. 10. It is observed that the stored NOx species on the samples L/xK/C decompose

mainly in two temperature regions of 50-450 oC and 450-700 oC, respectively. The TPD

studies about the thermal stability of nitrate species on K2O/MgAl2O4 [13] after NO2

saturation (K loading: 10 and 20 wt. %) have revealed the high-temperature (above 640

oC) NOx evolution corresponds to the decomposition of bulk potassium nitrate species

form on bulk or bulk-like K2O; while the low-temperature (below 560 oC) NOx

evolution is resulted from the decomposition of the surface potassium nitrate species

formed on the dispersed surface K2O phase at low K loading (2 and 5 wt. %). Similar

phenomenon is also observed on BaO/Al2O3 during the TPD tests after NO2 saturation

[35]. The TPD results presented here for L/xK/C can be expounded in an analogous way.

At the K2CO3 loading of 3, 5 and 8 wt. %, the high-temperature NOx evolution above

600 oC is dominant, which is attributed to the decomposition of bulk KNO3 originated

from the NOx adsorption on bulk or bulk-like K2CO3. As K2CO3 loading increasing, the

high-temperature NOx evolution is getting more and more obvious, meaning the

increased fraction of bulk or bulk-like K2CO3 species in the samples L/xK/C, which is

supported by the CO2-TPD results. For the sample L/1K/C, nearly no high-temperature

(above 600 oC) NOx evolution is observed, probably due to the low amount and high

dispersion of K species. For the NOx adsorption on dispersed surface K2CO3 species

identified by CO2-TPD, the produced potassium phase should be dispersed surface

nitrates which present middle decomposition temperature around 510 oC as seen in

Fig.10 for all the samples L/xK/C. The evolution of NOx around 380 oC may come

from the decomposition of potassium nitrates formed on surface –OK groups which

22

have less basicity than surface or bulk K2CO3 species. It is interesting that the amounts

of evolved NOx around 380 oC are nearly equivalent for the samples L/3K/C, L/5K/C

and L/8K/C, but much larger than that for L/1K/C, which is consistent with the

CO2-TPD results below 220 oC. For the sample L/1K/C, an additional small peak

around 235 oC is observed, which could be attributed to the decomposition of the stored

NOx species on exposed LaCoO3 or CeO2 surface, originating from the interaction

between NO2 and basic/neutral hydroxyls as described in the reaction (3-5-3) above.

The NOx-TPD results indicate that the stored NOx species on different K species

possess different thermal stability. The good consistency between the results of

NOx-TPD and CO2-TPD demonstrates the diversities of K species on the samples

L/xK/C with different K2CO3 loadings.

3.7. In situ DRIFTS characterization on NOx sorption and storage

Time-dependent DRIFTS of NO + O2 co-adsorption upon the samples 5K/C and

L/xK/C were collected at 350 oC and shown in Fig. 11. During the co-adsorption of NO

+ O2 on the sample 5K/C (seen in Fig. 11a), nitrite species were clearly detected. The

vibration bands of bridging bidentate nitrites at 1230 cm-1 [36-37] with νs(NO2) mode

appear at the beginning of exposure to NO + O2, which reach the highest intensity at 10

min and then gradually decrease. The bands at 1585 and 1261 cm-1 which appear visibly

at 60 min could be assigned to ν(N=O) and νas(NO2) vibrations of bridging bidentate

nitrates [25, 36], respectively. Such nitrate species are produced from the oxidation

transformation of nitrites to nitrates. Besides, free ionic nitrates (1381 cm-1-νas(NO3-)

and 1038 cm-1-νs(NO3-)) [25-26] and monodentate nitrates (1434 cm-1-νas(NO2) and

23

1341 cm-1-νs(NO2)) [5, 36-38] are always present along with the adsorption process.

The bands at 1485 (νs(NO2)) and 1285 cm-1 (νas(NO2)) could be assigned to the

monodentate nitrates formed on the support CeO2 as reported [31]. The observed

negative peaks in the region 1560-1420 cm-1 are resulted from the transformation of

carbonates to nitrates/nitrites as proved by the released gaseous CO2 with its

characteristic bands at 2363 and 2329 cm-1 [14, 16, 32]. Here for the sample 5K/C, the

oxygen adsorption and following activation on CeO2 as O2- or O- species mainly

contribute to the formation of nitrates and NO2. The evolution of nitrites to nitrates can

be explained as NO adsorption on K species to form nitrites in absence of enough active

oxygen species from the CeO2 surface.

After LaCoO3 loading, the corresponding time-dependent DRIFT spectra of the

samples L/xK/C are shown in Fig. 11 (b-e). For all spectra, the negative peaks can also

be recognized as carbonates consumption or transformation to nitrates/nitrites, however,

the fingerprint spectra of nitrates/nitrites in the range of 1500-1200 cm-1 are overlapped

with those of carbonates, making the only marked bands identified. The positive bands

appear always in the process are related to the nitrates with diverse coordinated nitrates

like bidentate, monodentate and free ionic nitrate. Specifically, the bands appearing at

1395, 1380, 1370 cm-1 (νas(NO3-)) and 1027 cm-1 (νs(NO3

-)) are ascribed to free ionic

nitrates [25-26]; the bands at 1585-1520 cm-1 (νs(NO2)) and 1245-1220 cm-1 (νas(NO2))

are attributed to chelating bidentate nitrates [25, 31]; while those at 1644, 1598 cm-1

(ν(N=O)) and 1277 cm-1 (νas(NO2)) are ascribed to the bridging bidentate nitrates [25,

36]. Notably, the vibration frequency of the bands of the bridging bidentate nitrates for

24

the samples L/xK/C and 5K/C are different. As stated above, the formation of bridging

bidentate nitrates for 5K/C undergoes the oxidation route of nitrites to nitrates; while for

L/xK/C, these nitrates may come from the direct reaction between NO2 with K speices,

due to their powerful oxidization capability. Apart from the bands at 1440-1432 cm-1

(νas(NO2)) and 1343-1333 cm-1 (νs(NO2)) for monodentate nitrates formed on the K

species in L/xK/C and 5K/C [5, 36-38], some other bands at 1420-1413 cm-1 and

1363-1358 cm-1 are detected only for the samples L/xK/C, which is ever reported as

“nitrates on perovskite” with monodentate (νas(NO2) and νs(NO2), respectively) by Li et

al and Hodjati et al [5, 38-39]. It could distinguish the NO2 adsorption at different sites

as K-O-NO2 on K species and as LaCoO3-NO2 on the pervoskite. For L/xK/C, the

vibration bands of monodentate nitrates formed on CeO2 have shifted to 1473 cm-1

(νs(NO2)) and 1291 cm-1 (νas(NO2)), as compared with that for 5K/C. The metal-support

interaction between LaCoO3 and CeO2 may have changed the electronic states of CeO2,

and thus changing the vibration frequency of the monodentate nitrates formed on CeO2.

In addition, only for the samples L/1K/C and L/3K/C, the band of 1627 cm-1 is detected,

indicating the formation of H2O [31], which should be resultd from the NO2 adsorption

on support surface and its interaction with basic/neutral hydroxyls as described in the

reaction (3-5-3). Meanwhile, during the appearance of the 1627 cm-1, another band at

1598 cm-1 also occurs, especially for the sample L/3K/C (Fig. 11c), which suggests that

as the reaction (3-5-3) takes place the bridging bidentate nitrates are simultaneously

produced. For L/1K/C, the H2O could be detected at the beginning after exposure to NO

and O2, while for L/3K/C the bands of water are observed after 20 min exposure. These

25

results imply the incomplete surface covering by K+ for these two samples, and larger

fraction of exposed surface for L/1K/C. Although the K species have priority for NOx

adsorption, the NOx adsorption or interaction with the basic/neutral hydroxyls on

support surface is still possible. This deduction is consistent with the results and

description of cyclic lean/rich NSR and NOx-TPD for L/1K/C. The band at 1500 cm-1 is

attributed to monodentate nitrates as those reported in similar region [40-41]; moreover,

this band is also observed in the DRIFT spectra for pure CeO2 after exposure to NO +

O2 at 350 oC (not shown). Unlike the appearance of monodentate nitrates (1480 and

1295-1280 cm-1) on ceria after NO2 adsorption [31], the band at 1500 cm-1 observed on

CeO2 during NO + O2 co-adsorption may be resulted from the oxidation of monodentate

nitrites to monodentate nitrates.

Generally, the formation of nitrates undergoes two possible pathways: “nitrites”

and “NO2 reaction” [3]. Compared with the sample 5K/C which goes through an

obvious route of nitrite to nitrate during NO + O2 co-adsorption, the samples L/xK/C

always exhibit nitrate species during the co-adsorption, probably due to the formation of

numerous NO2 which can react with K species to produce nitrates directly; of course,

the rapid oxidation of nitrites to nitrates cannot be excluded though IR can hardly

capture the nitrite species. No matter the formation of numerous NO2 or the rapid nitrite

oxidation, they all demand surface activated oxygen species. The loaded perovskite

LaCoO3 is responsible for the supply of activated oxygen species, and the subsequent

nitrate formation or the rapid nitrite to nitrate transformation. The results of in situ

DRIFTS study can be well correlated with the characterization results including H2-TPR,

26

O2-TPD and XPS.

On the combination of all the characterization results and the performance of NOx

storage and reduction (NSR) for the samples L/xK/C, models describing the potential

NSR pathways on different K species are proposed, as displayed in Fig. 12. As shown in

Fig. 12(a-b), the K species are mainly surface –OK groups, K2O or surface K2CO3

species with small particle size. These K species are adjacent to active sites of LaCoO3

perovskite, which can provide active oxygen species (O*) for the formation of ad-NO2

and nitrate species. This rapid NOx adsorption model presented in Fig. 12(a-b) is

appropriate for the sample with low K2CO3 loadings like L/3K/C or L/1K/C. While for

the samples L/5K/C and L/8K/C, there are more percent of bulk or bulk-like K2CO3

species which are far away from the active sites. Due to the absence of enough active

oxygen species (O*), the nitrates formation on these K species mainly undergoes the

disproportionation reaction (3-5-1) with some NO leak, leading to very low NOx

storage capacities, as described in Fig. 12(c). Fig. 12(d) displays the reaction pathways

for NOx reduction in rich period using C3H6 as reductant. The C3H6 could be

catalytically oxidized over LaCoO3 perovskite by gaseous NO and NO2 coming from

the decomposition of nitrite/nitrate species, releasing the corresponding products of N2,

CO2 and H2O.

4. Conclusions

Ceria supported perovskite-based LNT catalysts LaCoO3/K2CO3/CeO2 possess

excellent oxidizability for NO to NO2 oxidation and superior NOx reduction

performance during lean/rich cyclic tests. The K2CO3 loading greatly influences the K

27

dispersion and the oxidation capability of the catalysts. The optimized sample L/3K/C

shows not only the best NOx storage performance with a perfect NOx trap, but also the

highest NOx reduction efficiency (97.7%) at 350 oC. Meanwhile, this series of

non-platinic LNT catalysts display a high N2 selectivity of 98.5% using C3H6 as

rich-period reductant. Diverse nitrate species as NOx storage species are identified by in

situ DRIFTS during the lean-burn period. In addition, the catalysts L/xK/C are highly

stable during cyclic NOx storage and reduction for 10 times. The as-prepared supported

perovskite-based LNT catalyst L/3K/C can be a hint to the exploration of noble

metal-free LNT catalysts with advantages of outstanding stability, rapid NOx storage

and high reduction efficiency.

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).

28

References

[1] N. Takahashi, H. Shinjoh, T. Iijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Suzuki,

N. Miyoshi, S.-i. Matsumoto, T. Tanizawa, T. Tanaka, S.-s. Tateishi, K. Kasahara, Catal.

Today 27 (1996) 63-69.

[2] S.i. Matsumoto, Catal. Today, 90 (2004) 183-190.

[3] S. Roy, A. Baiker, Chem. Rev. 109 (2009) 4054-4091.

[4] C.H. Kim, G. Qi, K. Dahlberg, W. Li, Science. 327 (2010) 1624-1627.

[5] X.-G. Li, Y.-H. Dong, H. Xian, W.Y. Hernández, M. Meng, H.-H. Zou, A.-J. Ma,

T.-Y. Zhang, Z. Jiang, N. Tsubaki, P. Vernoux, Energ. Environ. Sci. 4 (2011) 3351.

[6] J. Ye, Y. Yu, M. Meng, Z. Jiang, T. Ding, S. Zhang, Y. Huang, Catal. Sci. Technol. 3

(2013) 1915.

[7] J. Liu, Z. Zhao, J. Lan, C. Xu, A. Duan, G. Jiang, X. Wang, H. He, J. Phys. Chem. C.

113 (2009) 17114-17123.

[8] M. Alifanti, M. Florea, V.I. Pârvulescu, Appl. Catal. B: Environ. 70 (2007) 400-405.

[9] G. Li, Q. Wang, B. Zhao, R. Zhou, J. Mol. Catal. A: Chem. 326 (2010) 69-74.

[10] L.F. Liotta, G. Di Carlo, G. Pantaleo, A.M. Venezia, G. Deganello, Appl. Catal. B:

Environ. 66 (2006) 217-227.

[11] G. Kim, Ind. Eng. Chem. Prod. Res. Dev. 21 (1982) 267–274.

[12] J.H. Kwak, D.H. Kim, J. Szanyi, C.H.F. Peden, Appl. Catal. B: Environ. 84 (2008)

545-551.

[13] D.H. Kim, K. Mudiyanselage, J. Szányi, H. Zhu, J.H. Kwak, C.H.F. Peden, Catal.

Today. 184 (2012) 2-7.

29

[14] Y. Liu, M. Meng, X.-G. Li, L.-H. Guo, Y.-Q. Zha, Chem. Eng. Res. Des. 86 (2008)

932-940.

[15] L. Castoldi, L. Lietti, R. Bonzi, N. Artioli, P. Forzatti, S. Morandi, G. Ghiotti, J.

Phys. Chem. C. 115 (2010) 1277-1286.

[16] Y. Zhang, M. Meng, F. Dai, T. Ding, R. You, J. Phys. Chem. C. 117 (2013)

23691-23700.

[17] L. Liu, Y. Chen, L. Dong, J. Zhu, H. Wan, B. Liu, B. Zhao, H. Zhu, K. Sun, L.

Dong, Y. Chen, Appl. Catal. B: Environ. 90 (2009) 105-114.

[18] R.O. Fuentes, R.T. Baker, J. Phys. Chem. C. 113 (2009) 914-924.

[19] O. Haas, R.P.W.J. Struis, J.M. McBreen, J. Solid. State. Chem. 177 (2004)

1000-1010.

[20] Z. Li, M. Meng, Y. Zha, F. Dai, T. Hu, Y. Xie, J. Zhang, Appl. Catal. B: Environ.

121-122 (2012) 65-74.

[21] J. Xu, J. Liu, Z. Zhao, C. Xu, J. Zheng, A. Duan, G. Jiang, J. Catal. 282 (2011) 1-12.

[22] Z. Liu, J. Hao, L. Fu, T. Zhu, Appl. Catal. B: Environ. 44 (2003) 355-370.

[23] J. Luo, M. Meng, X. Li, Y. Zha, T. Hu, Y. Xie, J. Zhang, J. Catal. 254 (2008)

310-324.

[24] L. Xue, C. Zhang, H. He, Y. Teraoka, Appl. Catal. B: Environ. 75 (2007) 167-174.

[25] F. Prinetto, M. Manzoli, S. Morandi, F. Frola, G. Ghiotti, L. Castoldi, L. Lietti, P.

Forzatti, J. Phys. Chem. C. 114 (2010) 1127-1138.

[26] T.J. Toops, D.B. Smith, W.P. Partridge, Appl. Catal. B: Environ. 58 (2005) 245-254.

[27] W.S. Epling, C.H.F. Peden, J.n. Szanyi, J. Phys. Chem. C. 112 (2008) 10952-10959.

30

[28] T. Montanari, L. Castoldi, L. Lietti, G. Busca, Appl. Catal. A: Gen. 400 (2011)

61-69.

[29] W.H.J. Stork, G.T. Pott, J. Phys. Chem. 78 (1974) 2496-2506.

[30] A. Iordan, M.I. Zaki, C. Kappenstein, J. Chem. Soc., Faraday Trans. 89 (1993)

2527-2536.

[31] B. Azambre, L. Zenboury, A. Koch, J.V. Weber, J. Phys. Chem. C. 113 (2009)

13287-13299.

[32] Y. Liu, M. Meng, Z. Zou, X. Li, Y. Zha, Catal. Commun. 10 (2008) 173-177.

[33] B.M. Weiss, N. Artioli, E. Iglesia, Chemcatchem 4 (2012) 1397-1404.

[34] B.M. Weiss, E. Iglesia, J. Phys. Chem. C. 113 (2009) 13331-13340.

[35] J. Szanyi, J.H. Kwak, D.H. Kim, S.D. Burton, C.H.F. Peden, J. Phys. Chem. B. 109

(2005) 27-29.

[36] Y. Su, M.D. Amiridis, Catal. Today. 96 (2004) 31-41.

[37] C. Sedlmair, J. Catal. 214 (2003) 308-316.

[38] H. Xian, X. Zhang, X. Li, L. Li, H. Zou, M. Meng, Q. Li, Y. Tan, N. Tsubaki, J.

Phys. Chem. C. 114 (2010) 11844-11852.

[39] S. Hodjati, K. Vaezzadeh, C. Petit, V. Pitchon, A. Kiennemann, Catal. Today. 59

(2000) 323-334.

[40] K.I. Hadjiivanov, Cat. Rev. –Sci. Eng. 42 (2000) 71-144.

[41] K. Hadjiivanov, V. Bushev, M. Kantcheva, D. Klissurski, Langmuir 10 (1994)

464-471.

31

Table 1. Co and O binding energies (B.E., eV) from XPS characterization.

Sample Co 2p3/2 Co 2p1/2 ∆E Oads Olat Oads/Olat (%)

C - - - 531.2 528.7 28.0

L/C 780.0 795.1 15.1 531.1 528.6 60.5

L/3K/C 780.1 795.2 15.1 - - -

32

Table 2. Specific surface area (SBET) of fresh catalysts, NOx storage capacity (NSC)

and NO-to-NO2 conversion (%) after reaching steady-state during NOx storage; the

NOx reduction percentage (NRP) of the catalysts after 10 lean/rich cyclic NOx

storage-reduction tests.

Sample BET (m2/g) NSC (µmol/g) NO-to-NO2 (%) NRP (%)

5K/Ca 18.3 617.9 21.4 57.8

L/1K/Ca 17.5 187.5 67.8 64.4

L/3K/Ca 16.3 512.4 67.4 97.7

L/5K/Ca 12.9 672.0 60.5 97.4

L/8K/Ca 11.5 1199.1 55.0 96.2

a: SBET of the pure CeO2 (denoted as C) is 23.4 m2/g.

33

Figure captions

Figure 1 XRD patterns of the fresh catalysts L/xK/C.

Figure 2 Co K-edge RSFs of the catalysts in different states (f: fresh; r: used after NOx

storage and reduction).

Figure 3 XPS spectra for (a) Co 2p and (b) O 1s of the fresh catalysts.

Figure 4 H2-TPR profiles of the fresh catalysts.

Figure 5 O2-TPD profiles of the fresh catalysts.

Figure 6 FT-IR spectra of the fresh catalysts L/xK/C, 5K/C and pure K2CO3.

Figure 7 CO2-TPD profiles of the catalysts L/xK/C and 5K/C.

Figure 8 Isothermal NOx storage curves of (a) the fresh catalysts L/xK/C and 5K/C, (b)

L/8K/C and (c) L/3K/C.

Figure 9 NOx concentration curves over the catalysts (a) L/1K/C, (b) L/3K/C, (c)

L/5K/C, (d) L/8K/C, (e) 5K/C, and (f) N2O and C3H6 concentration curve over L/3K/C

during lean/rich cycles.

Figure 10 NOx-TPD profiles of the fresh catalysts L/xK/C after used in NOx storage.

Figure 11 Time-dependent in-situ DRIFTS spectra of NO/O2 co-adsorption collected at

350 oC over different catalysts. (a) 5K/C, (b) L/1K/C, (c) L/3K/C, (d) L/5K/C and (e)

L/8K/C

Figure 12 Models of K species and NOx storage and reduction pathways over L/xK/C

catalysts.

34

Figure 1

10 20 30 40 50 60 70 80 90

&

&

Inte

nsity

2θ / ο

(4)

(3)

(2)

&& *****

*

*

**&

*: CeO2

&: LaCoO3

(1)

(1) L/1K/C(2) L/3K/C(3) L/5K/C(4) L/8K/C

35

Figure 2

0 1 2 3 4 5 6 7 8

FT

-mag

nitu

de /

a. u

.

R / Å

Co-CoCo-LaCo-O

L/3K/C-r

L/3K/C-f

LaCoO3

36

Figure 3

765 770 775 780 785 790 795 800

Binding energy / eV

Inte

nsity

Co 2p1/2

Co 2p3/2

L/3K/C

L/C

(a)

524 526 528 530 532 534 536

L/C

C

(b)

Inte

nsity

Binding energy / eV

37

Figure 4

100 200 300 400 500 600 700 800 900

856

5K/C

559468

589437

LaCoO3

L/8K/C

L/5K/C

L/3K/C

L/1K/C

H2 C

onsu

mpt

ion

/ a. u

.

Temperature / oC

CeO2

353

38

Figure 5

0 200 400 600 800

O2 d

esor

ptio

n / a

. u.

Temperature / oC

L/C

C

39

Figure 6

2200 2000 1800 1600 1400 1200 1000

1690

1646

13705K/C

1624

1400

1472

Abs

orba

nce

/ a. u

.

Wavenumber / cm-1

1107

1061

10511325

1464

1385

1434

1663

K2CO3

L/1K/C

L/3K/C

L/8K/C

L/5K/C

40

Figure 7

0 200 400 600 800 1000

(5)

800 oC

(1)

(2)

(3)

(4)

Temprature / oC

CO

2 evo

lutio

n si

gnal

/ a.

u.

(1) L/1K/C (3) L/5K/C (5) 5K/C(2) L/3K/C (4) L/8K/C

220 oC

41

Figure 8

0 20 40 60 80 100 1200

100

200

300

400

(5) (4)

(3)(2)

NO

x C

once

ntra

tion

/ ppm

T / min

(1) L/1K/C(2) L/3K/C(3) L/5K/C(4) L/8K/C(5) 5K/C

(1)

(a)

5 ppm

0 20 40 60 80 100 1200

100

200

300

400

NO2

NO

NO

x C

once

ntra

tion

/ ppm

T / min

L/8K/C

NOx

30 ppm

(b)

42

0 20 40 60 80 100 1200

100

200

300

400

NO

NO2

NO

x C

once

ntra

tion

/ ppm

T / min

L/3K/C5 ppm

(c)NOx

43

Figure 9

0 5 10 15 20 25 30 35 400

100

200

300

400

500

600

NO

x C

once

ntra

tion

/ ppm

T / min

(a)

0 5 10 15 20 25 30 35 400

100

200

300

400

NO

x C

once

ntra

tion

/ ppm

T / min

(b)

0 5 10 15 20 25 30 35 400

100

200

300

400

NO

x C

once

ntra

tion

/ ppm

T / min

(c)

0 5 10 15 20 25 30 35 400

100

200

300

400

NO

x C

once

ntra

tion

/ ppm

T / min

(d)

0 5 10 15 20 25 30 35 400

100

200

300

400

500

600

700

800

900

NO

x C

once

ntra

tion

/ ppm

T / min

(e)

0

5

10

15

20

0 5 10 15 20 25 30 35 40

C3H

6 / a.

u.

T / min

N2O

/ pp

m

(f)

1000 ppm

44

Figure 10

0 20 40 60 80 100 120

0

200

400

600

800

700 oC

235 oC

510 oC

(4)

(3)

380 oC

(2)

Tem

pera

ture

/ o C680 oC

NO

x C

once

ntra

tion

/ ppm

T / min

(1) L/1K/C(2) L/3K/C(3) L/5K/C(4) L/8K/C

(1)

0

200

400

600

800

45

Figure 11

2400 1600 1400 1200 1000

1285

1485

1038

1230

1261

1341

138

1

143

41

462

153

2

1585

2329

Wavenumber / cm-1

2363

Kub

elka

-Mun

k un

it

60 min

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

30 min

(a)

2400 1600 1400 1200 1000

1644

1290

1376

1336

1024

1084

1271

1363

1396

1420

144314

73

150115

2215

4015

611597

Kub

elka

-Mun

k un

it

Wavenumber / cm-1

60 min

30 min

20 min

10 min

5 min

3 min

1 min

1627

(b)

46

2400

16001400

1200

1000

12751295

13341354

136113741395

14051419

14441436

14601474

15001525 1535

1545 15531571

1598

1627

1121

1050

23252361

Kubelka-Munk unit

Wavenum

ber / cm-1

60 min

30 min

20 m

in

10 min

5 min

3 min

1 min

(c)

2400

1600

1400

1200100

0

1278

1598

1421

Wavenum

ber / cm-1

Kubelka-Munk unit

1063

1153

1317

14131387

14361458

1517

2325

60 min

30 min

20 min

10 min

5 min

3 min

1 min

2360

(d)

47

24001600

14001200

1000

16411665

1276

13931414

1444

1150

1058

1027

13431358

12911323

1240

1473

15351560

15841601

23252361

Kubelka-Munk unit

Wavenum

ber / cm-1

60 min

30 min

20 min

10 min

5 min

3 min

1 min

(e)

48

Figure 12

49

Highlights:

► Non-platinic LNT catalysts LaCoO3/K2CO3/CeO2 are highly active for lean NOx

trap.

► NOx reduction percentage of 97.7% and NOx-to-N2 selectivity of 98.5% are

achieved.

► The catalysts also exhibit high stability in 10 NOx storage and reduction cycles.

► NOx trap performance is highly related to the oxidizability of LaCoO3 perovskite.