Oxygen vacancies on nanosized ceria govern the NOX storage capacity of NSR catalysts

Article abstract of DOI:10.1039/c5cy01660f

Pt/BaO/CeO₂ catalysts derived from CeO₂ nanomaterials with shapes of rods, cubes, and particles were investigated for NO_x storage/reduction. Catalytic tests were performed in a transient flow reactor system. A series of characterization techniques including XRD, TEM, XPS, EXAFS, NO_x-TPD, H₂-TPR and in situ DRIFTS were conducted to investigate the electrical, chemical, and structural properties. The NO_x storage-reduction performance ranked by the CeO₂ support was nanorods > nanoparticles > nanocubes. Amazingly, the CeO₂-nanorod based NSR catalyst possessed a superior NO_x storage capacity (NSC) of 913.8 μmol NO_x g_cat^-1 at 350 °C in the absence of H₂O and CO₂, which almost reached the theoretical value. Even under harsh lean-rich cycling conditions (90 s vs. 6 s) and a high GHSV of 360000 h^-1, the nanorod-based catalyst also showed the best reduction efficiency, affording ~ 99% NO_x conversion levels from 200 °C to 400 °C under the conditions without H₂O and CO₂. The morphology of ceria has significant influences on the selectivity of ammonia, and on the H₂O and CO₂ tolerance during the NSR process. For the first time, a close linear correlation was drawn between the NO_x storage capacity and the amount of oxygen vacancies of NSR catalysts. Over the NSR catalysts, oxygen vacancies play a crucial role in anchoring Pt. Meanwhile, H₂-TPR results showed that the number of active surface oxygen species trapped in oxygen vacancies was closely related to the NSC value. This suggests that the oxygen vacancies on the NSR surface govern the NO_x storage capacity by creating efficient sites or channels for the formation of nitrate and its further transformation to Ba-based storage sites. These findings may be fundamental for designing ceria-based NSR catalysts with better performance.

Full text of DOI:10.1039/c5cy01660f

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Oxygen vacancies on nanosized ceria govern the
NOx storage capacity of NSR catalysts†
Cite this: DOI: 10.1039/c5cy01660f
*
Yan Zhang,‡ Yunbo Yu‡ and Hong He
Pt/BaO/CeO2 catalysts derived from CeO2 nanomaterials with shapes of rods, cubes, and particles were in-
vestigated for NOx storage/reduction. Catalytic tests were performed in a transient flow reactor system. A
series of characterization techniques including XRD, TEM, XPS, EXAFS, NOx-TPD, H2-TPR and in situ DRIFTS
were conducted to investigate the electrical, chemical, and structural properties. The NOx storage-
reduction performance ranked by the CeO2 support was nanorods > nanoparticles > nanocubes. Amaz-
ingly, the CeO2-nanorod based NSR catalyst possessed a superior NOx storage capacity (NSC) of 913.8
−1
μmol NOx gcat at 350 °C in the absence of H2O and CO2, which almost reached the theoretical value.
Even under harsh lean–rich cycling conditions (90 s vs. 6 s) and a high GHSV of 360 000 h−1, the nanorod-
based catalyst also showed the best reduction efficiency, affording ~ 99% NOx conversion levels from 200
°C to 400 °C under the conditions without H2O and CO2. The morphology of ceria has significant influ-
ences on the selectivity of ammonia, and on the H2O and CO2 tolerance during the NSR process. For the
first time, a close linear correlation was drawn between the NOx storage capacity and the amount of oxy-
gen vacancies of NSR catalysts. Over the NSR catalysts, oxygen vacancies play a crucial role in anchoring
Pt. Meanwhile, H2-TPR results showed that the number of active surface oxygen species trapped in oxygen
vacancies was closely related to the NSC value. This suggests that the oxygen vacancies on the NSR sur-
face govern the NOx storage capacity by creating efficient sites or channels for the formation of nitrate
and its further transformation to Ba-based storage sites. These findings may be fundamental for designing
ceria-based NSR catalysts with better performance.
Received 30th September 2015,
Accepted 5th January 2016
DOI: 10.1039/c5cy01660f
www.rsc.org/catalysis
catalysts are periodically switched between fuel-lean and fuel-
rich streams. Under lean conditions, NO is oxidized to more
Introduction
Vehicular engines operating under lean burn conditions are
effective for fuel cost improvement and environmental protec-
tion. Under these excess oxygen conditions, however, the NOx
components cannot be efficiently removed by a traditional
three-way catalyst, which is effective only under stoichiomet-
ric conditions. Developing new technologies to eliminate NOx
emission from lean-burn exhaust has, therefore, attracted
much attention in environmental catalysis. One of the most
promising technologies is NOx storage reduction (NSR, also
known as lean NOx trap, LNT), which was first developed by
Toyota Corporation in the 1990s.1–4 Generally, NSR is an in-
herently transient operation in which the gases fed to the
active NO2 followed by storage on the catalyst; under a subse-
quent fuel-rich atmosphere, the stored NOx is reduced and
thus the active sites are released for the next cycle.4,5 Storage
periods of 1–2 min followed by rich phases of several seconds
are typically used in real engines;3 however, many studies on
NSR catalysts have used long rich regeneration periods com-
pared to those of real systems. Consequently, minimizing the
duration of the rich phase without decreasing NOx conversion
is highly desired for avoiding excessive fuel consumption. In
principle, a NSR catalyst should consist of three major com-
ponents: (1) noble metals (Pt, Pd), for NO oxidation/reduc-
tion; (2) alkali metal or alkaline earth metal compounds (e.g.
Ba), for NOx sorption; (3) a high-surface-area support (e.g.
Al2O3).3,6
Ceria (CeO2) has been regarded as one of the most signifi-
cant components in many catalytic systems due to its unique
redox properties and high oxygen storage capacity (OSC), re-
leasing/taking up oxygen through redox processes involving
the Ce4+/Ce3+ couple.7 Moreover, several features of CeO2 ap-
pear to be beneficial for NSR reactions: sustaining high metal
dispersion, promoting the water-gas shift reaction and steam
State Key Joint Laboratory of Environment Simulation and Pollution Control,
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,
Beijing, 100085, China. E-mail: honghe@rcees.ac.cn
† Electronic supplementary information (ESI) available: TEM and HRTEM im-
ages of NSR catalysts, the effect of CO2 and H2O on the catalytic performance,
NH3 selectivity, XPS analyses of Pt and Ce, Pt–LIII EXAFS, methanol adsorption
and H2-TPR fitting results. See DOI: 10.1039/c5cy01660f
‡ These authors contributed equally to this work.
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reforming reaction, and providing surface basicity to benefit
the formation of carbonates.7–9 With these in mind, CeO2
has been used as either an additive or support, and its role
varies according to research focus.10–21 As for NSR catalysts
containing BaO, Casapu et al.22 found that ceria serving as a
support significantly improved the NOx storage capacity
(NSC), due to the high instability of BaCeO3 in the presence
of NO2/H2O or CO2 at 300–500 °C. Compared with conven-
tional Pt/BaO/Al2O3 catalysts, a higher NOx storage capacity
and better regeneration behaviour were also observed for Pt/
BaO/CeO2 (or Pt/BaO/CeO2/Al2O3) at low temperatures by Shi
et al.,13 which was attributed to enhanced Pt dispersion and
improved redox properties due to ceria introduction (or mod-
ification). It has also been found23 that the addition of CeO2
to Pt/Al2O3 helped maintain metallic Pt dispersion on Al2O3
and introduce more NOx adsorption sites, finally relating to
the enhanced NSC. Indeed, the strong metal–support interac-
tion (SMSI) between noble metals and ceria has been recog-
nized as an important factor that affects both the microstruc-
ture and redox properties of ceria-based catalysts.7,24,25
The design and fabrication of catalytic materials with de-
sired performance are key issues in heterogeneous catalysis.
Recently, the structure-sensitive behaviour of nanosized ceria
systems has attracted significant attention owing to their
unique catalytic properties, and fruitful results have been
achieved in catalytic applications such as CO oxidation,26–28
water-gas shift reaction,29 CO2 reforming,30 NO reduction31,32
and ethanol reforming.33 Si et al.29 have found that gold
supported on CeO2 nanorods and nanopolyhedra exhibited
higher activity for the water-gas shift reaction than that
supported on nanocubes, due to the higher levels of defects
in the former two. As for the preferential CO oxidation in H2-
rich gas, Yi et al.34 confirmed that the catalytic activity of Au/
CeO2 catalysts varied as a function of the CeO2 nanomaterial
support as follows: nanorods > nanopolyhedra > nanocubes.
More recently, higher activity was achieved for MnOx/
ZrO2–CeO2 in the selective catalytic reduction of NO by using
ZrO2–CeO2 nanorods as a support compared with that using
nanocubes and nanopolyhedra.31 Further results revealed by
XPS, Raman, HRTEM, and kinetic measurements demon-
strated that the excellent catalytic activity of the nanorod cat-
alyst could be attributed to adsorbed surface oxygen and oxy-
gen vacancies associated with ZrO2–CeO2 nanorods' exposed
(110) and (100) planes. The achievements mentioned above
confirmed that the activity of CeO2-based catalysts is often
shape-dependent, creating a guideline for designing catalysts
with high efficiency, and also for revealing the structure–ac-
tivity relationship.
techniques including XRD (powder X-ray diffraction), TEM
(transmission electron microscopy), XPS (X-ray photoelectron
spectra), EXAFS (extended X-ray absorption fine structure),
NOx–TPD (NOx temperature programmed desorption), H2-
TPR (H2 temperature programmed reduction) and in situ
DRIFTS (Diffuse Reflectance Infrared Fourier Transform
Spectroscopy) were conducted to investigate the electrical,
chemical, and structural properties. Even under harsh lean–
rich cycling operation (90 s vs. 6 s) and a high GHSV of
360 000 h−1, it was found that superior NOx storage–reduction
performance was achieved on the CeO2-nanorod supported
NSR catalyst, which was attributed to the higher numbers of
oxygen vacancies in the exposed (110) and (100) planes of
ceria.
Experimental
Catalyst preparation
Preparation of different shaped CeO2 nanomaterials. The
CeO2 nanorods and nanocubes were prepared by a hydrother-
mal method modified on the basis of previous studies.26,30,35
Specifically, 6.9 mmol of cerium nitrate (AR grade, Tianjin
Fuchen Chemical Reagent Factory, China) and specific
amounts of NaOH (AR grade, Sinopharm Chemical Reagent
Beijing Co., Ltd., China, 0.375 mol for nanorods, 0.75 mol for
nanocubes) were dissolved in 30 ml and 50 ml of deionized
water, respectively. Then the mixture was stirred for 60 min
to obtain a purple slurry and subsequently transferred into a
Teflon-lined stainless autoclave at a temperature of 100 °C
and held for 12 h for nanorods (for nanocubes, this process
was performed at 180 °C for 24 h). The fresh precipitates
were thoroughly washed with deionized water and anhydrous
ethanol (AR grade, Sinopharm Chemical Reagent Beijing Co.,
Ltd., China) to remove any possible ionic remnants. The solid
obtained was dried at 60 °C in air for 24 h and calcined at
550 °C for 4 h in air.
The CeO2 nanoparticles were prepared via a traditional
precipitation method: 6.9 mmol of CeIJNO3)3·6H2O was
dissolved in 30 ml of deionized water, and then 50 ml of 0.01
mol L−1 NaOH solution was dropped into the above solution
with vigorous stirring. After 60 min of stirring, the mixture
was transferred into a water bath kettle at 30 °C for 12 h. The
fresh precipitates were washed with deionized water and an-
hydrous ethanol, dried at 60 °C in air for 24 h and calcined
at 550 °C for 4 h in air.
Catalyst preparation. The NSR catalysts were synthesized
by an impregnation method using nanosized CeO2 materials
as supports. First, the CeO2 (nanoparticles, nanocubes, or
nanorods) was impregnated in a BaIJCH3COO)2 (AR grade,
Sinopharm Chemical Reagent Beijing Co., Ltd., China) solu-
tion. After stirring for 60 min, the excess water was removed
in a rotary evaporator at 50 °C. The samples were then dried
at 120 °C overnight and calcined at 500 °C for 3 h in air. Sub-
sequently, Pt was loaded on the surface of Ba/CeO2 by the
same procedure using PtCl4 as a precursor. The catalysts
To the best of our knowledge, however, there has been lit-
tle information concerning the morphology effect of CeO2
nanomaterials on the activity of CeO2-supported NSR cata-
lysts under cyclic lean–rich conditions. To highlight this is-
sue, herein, nanosized ceria with the shapes of rods, cubes,
and particles was synthesized and used as a support for Pt/
BaO/CeO2 catalysts. Catalytic tests were performed in a tran-
sient flow reactor system and a series of characterization
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prepared with CeO2 nanoparticles, nanocubes, and nanorods
are hereafter denoted as Pt/BaO/CeO2-NP, Pt/BaO/CeO2-NC,
and Pt/BaO/CeO2-NR, respectively, with a nominal Ba loading
of 8 wt% and a value of 1 wt% for Pt loading.
avoid the influence of H2O, a cold trap was set before the MS
detector.
H2 temperature programmed reduction (H2-TPR) was
performed in the same instrument as the NOx-TPD. The 100
mg samples were pretreated at 450 °C in a flow of air (50 mL
min−1) for 30 min and cooled down to room temperature.
Then reduction profiles were obtained by passing a flow of
10% H2/Ar through the sample at a rate of 50 mL min−1, dur-
ing which the temperature was increased from 50 to 950 °C
at a ramp rate of 10 °C min−1.
Characterization
Powder X-ray diffraction (XRD) patterns were measured on a
PANalytical X'Pert PRO X-ray diffractometer (Netherlands, Cu
Kα as radiation source, λ = 0.154 nm).
The specific surface areas of the catalysts were obtained at
−196 °C over the whole range of relative pressures, using a
Quantachrome Quadrasorb SI-MP. Prior to N2 physisorption,
the catalysts were degassed at 300 °C for 5 h. Specific surface
areas were calculated from these isotherms by applying the
BET equation in the 0.05–0.3 partial pressure range. The
component contents of catalysts were analyzed using an in-
ductively coupled plasma instrument (OPTMIA 2000DV) with
a radial view of the plasma. All samples were dissolved using
a strong acid solution before testing. Transmission electron
microscopy (TEM) images with low magnification were
obtained on a Hitachi H-7500 transmission electron micro-
scope (Hitachi) with an acceleration voltage of 80 kV. High-
resolution transmission electron microscopy (HR-TEM) im-
ages were obtained on a JEOL JEM 2010 TEM with 200 kV ac-
celeration voltage. High-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) and energy
dispersive X-ray spectroscopy (EDS) elemental mapping were
performed on a probe Cs-corrected JEOL JEM-ARM 200 F op-
erated at 200 kV.
X-ray photoelectron spectra (XPS) measurements were
recorded in a scanning X-ray microprobe (PHI Quantera,
ULVAC-PHI, Inc.) using Al Kα radiation. Binding energies
were calibrated using the C 1 s peak (BE = 284.8 eV) as standard.
Extended X-ray absorption fine structure (EXAFS) measure-
ments were carried out on the XAFS station in 1W1B
beamline of Beijing Synchrotron Radiation Facility (BSRF) op-
erating at about 150 mA and 2.2 GeV. Data were analyzed
using the program REX2000 (Rigaku Co.). The EXAFS oscilla-
tion χ(k) was extracted using spline smoothing with a Cook–
Sayers criterion,36 and the filtered k3-weighted χ(k) was Fou-
rier transformed into R space (k range: 3–12 Å−1 for Pt-LIII
EXAFS). In the curve fitting step of Pt-LIII EXAFS, the back-
scattering amplitude and phase shift were calculated using
FEFF8.4.37
In situ DRIFTS experiments were performed on an FTIR
spectrometer (Nicolet Nexus 670) equipped with a smart col-
lector and an MCT/A detector cooled with liquid nitrogen.
Prior to each experiment, the sample was pretreated with 3%
H2/N2 at 450 °C for 1 h, and then cooled down to the speci-
fied temperature. A mixture containing 500 ppm NO and 8%
O2 at a total flow rate of 300 mL min−1 was employed to in-
vestigate the behavior of the NSR catalyst for NOx storage,
while a stream of 3% H2 was used to measure the activity of
ad-NOx over the Pt/Ba/CeO2 catalyst. All spectra presented
here were recorded by accumulating 30 scans with a resolu-
tion of 4 cm−1.
Catalytic activity measurements
NOx storage capacity experiment. NOx uptake experiments
were carried out under fuel-lean conditions as a function of
temperature. Before each experiment, the catalyst was
pretreated in 3% H2/N2 for 1 h at 450 °C, and then cooled to
the desired temperature. A mixture of 500 ppm NO, 8% O2,
1% CO2 (if used), and 2% H2O (if used) at a total flow rate of
300 mL min−1 was used to simulate the fuel-lean exhaust,
using N2 as a balance gas. The outlet NOx (NO + NO2) con-
centration was monitored by a chemiluminescence detector
(ECO Physics CLD 62). The outlet NH3 was analyzed using a
chemiluminescence detector (ECO Physics CLD 822).
Cyclic NOx storage reduction tests. NSR cyclic measure-
ments were conducted with 100 mg of catalyst using a fixed-
bed quartz reactor. The reactor was connected to a pneumati-
cally actuated four-way valve, which provides quick switching
between the lean and rich atmospheres. Constant flows (300
mL min−1) of 500 ppm NO + 8% O2 + 1% CO2 (if used) + 2%
H2O (if used) and 500 ppm NO + 3% H2 + 1% CO2 (if used) +
2% H2O (if used) were introduced alternately, during which a
lean period of 90 s and a rich period of 6 s were performed
between 200 °C and 400 °C. The NOx conversion was aver-
aged over 20 lean/rich cycles to give a mean value according
to the following formula:
NOx temperature programmed desorption (NOx-TPD) ex-
periments were performed in a Micromeritics AutoChem II
2920 apparatus, equipped with
a
computer-controlled
CryoCooler, a thermal conductivity detector (TCD), and a
quadrupole mass spectrometer (MKS Cirrus). The samples
were first pre-reduced with 10% H2/Ar at 450 °C for 30 min,
and cooled down to room temperature. After purging with He
for 30 min, the gas was switched to NO2 adsorption for 60
min. After He flow for another 30 min, the temperature was
increased to 900 °C at a heating rate of 10 °C min−1, and the
signal of NO (m/z = 30) was recorded simultaneously. To
The NH3 selectivity was calculated by the following formula:
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Results and discussion
Structural features of nanoceria and Pt/BaO/CeO2 catalysts
After the deposition of Ba and Pt, it can be clearly seen
from Fig. S1d–f† that the three different CeO2 nanomaterials
maintained their original crystal shapes, while no structural
features of Pt and Ba can be observed. With this in mind, ele-
mental maps were measured by high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM)
(Fig. 1e–g), revealing that the active components of Pt and Ba
were highly dispersed on all three NSR catalysts.
The specific surface areas of CeO2 nanomaterials and cor-
responding NSR catalysts are listed in Table 1. Pure CeO2
nanoparticles had the largest surface area (134.9 m2 g−1),
while the CeO2 nanocubes gave the lowest value of 26.5
m2 g−1. After deposition of Pt and BaO, the surface areas of
Pt/BaO/CeO2-NP, Pt/BaO/CeO2-NR, and Pt/BaO/CeO2-NC de-
creased to 115.7, 84.2, and 20.0 m2 g−1, respectively. The ICP-
OES data (Table 1) showed that the contents of the active
components had no significant difference among the NSR
catalysts, all of which were close to the nominal values (1%
Pt and 8% BaO).
The crystal structures of the different shaped samples
were investigated by XRD and shown in Fig. 2. The diffrac-
tion peaks at 28.5, 33.0, 47.5, 56.3, 59.1, 69.4, 76.7, 79.1 and
88.4° could be ascribed to the fluorite structure of ceria
(JCPDS no. 43-1002), while no peaks from Pt species were ob-
served in any of the samples, indicating high dispersion and
further confirming the results of the elemental maps. Using
the Scherrer equation for the peak due to the (111) plane, the
CeO2 sizes of CeO2 nanomaterials and corresponding NSR
catalysts were calculated (Table 1), giving values in good
agreement with the results of the HRTEM. In every Ba-
containing sample, XRD analysis confirmed the decomposi-
tion of BaIJCH3COO)2 into crystalline BaCO3 (JCPDS no. 44-
1487) during catalyst calcination, in agreement with previous
results.13
The morphology of CeO2 nanomaterials and corresponding
NSR catalysts was characterized by TEM and HRTEM (Fig. 1
and S1†). As shown in Fig. S1a,† the CeO2 nanoparticles (de-
noted as CeO2-NP hereafter) displayed a mean diameter of
around 9 nm. HRTEM images (Fig. 1a) further revealed that
the CeO2-NP were dominated by (111) planes together with
small amounts of (100) planes, exhibiting interplanar spac-
ings of 0.31 and 0.27 nm, respectively. This feature indicates
that CeO2 nanoparticles were in the form of a truncated octa-
hedron enclosed by eight (111) and six (100) planes, with the
structural model presented in the insert of Fig. 1a.38,39 As
shown in Fig. S1b,† the CeO2 nanocubes (CeO2-NC), with an
inhomogeneous grain size ranging from 10–50 nm, were
mostly well-developed. Calculating for 200 nanocubes from
the HRTEM images, the average size of ca. 0.27 nm was
obtained. An interplanar spacing of 0.27 nm implies that the
CeO2 nanocubes are enclosed by six polar (100) planes, show-
ing a cubic structure (Fig. 1b). The CeO2 nanorods (CeO2-NR)
had a mean diameter of 8.1 nm and a length from 40–110
nm (Fig. S1c†). Fig. 1c–d depicts the HRTEM image of CeO2-
NR combined with a fast Fourier transform (FFT) analysis.
According to the FFT analysis, three kinds of lattice fringe di-
rections attributed to (111), (002), and (220) were observed
for the nanorods, which have respective interplanar spacings
of 0.31, 0.27, and 0.19 nm in the HRTEM image. The nano-
rods show a 1D growth structure with a preferred growth di-
rection along [110], and are enclosed by (110) and (100)
planes.
Catalytic performance
NOx storage performance. The NOx breakthrough curves
versus time of Pt/BaO/CeO2-NP, Pt/BaO/CeO2-NC and Pt/BaO/
CeO2-NR at different temperatures are recorded as shown in
Fig. S2.† As soon as the lean feed was switched from the by-
pass loop to the catalysts, the NOx concentration in the outlet
dropped sharply from 500 ppm to 0 ppm, followed by a com-
plete uptake process. With increasing time, the outlet NOx
concentration gradually increased and a saturation state was
achieved slowly by the end of 3600 s. The areas between the
inlet feed and the breakthrough curves represent the NOx
storage capacities (NSC), which were calculated and summa-
rized in Fig. 3. It was apparent that Pt/BaO/CeO2-NR showed
much larger NSC than Pt/BaO/CeO2-NP and Pt/BaO/CeO2-NC
at all temperatures tested. Specifically, Pt/BaO/CeO2-NR
presented the highest NOx storage capacity at 350 °C,
affording about 913.8 μmol gcat−1, which approached the the-
oretical NOx storage capacity (937.4 μmol gcat−1, see the ESI†).
The oxidation of NO to NO2 is regarded as a key step in
the NO trapping process, during which platinum plays a
Fig. 1 HRTEM images of CeO2-NP (a), CeO2-NC (b), and CeO2-NR (c),
FFT analysis of (c) (d); HAADF-STEM EDS elemental mapping images of
Pt/BaO/CeO2-NP (e), Pt/BaO/CeO2-NC (f), and Pt/BaO/CeO2-NR (g).
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Table 1 Specific surface area, active component loading, and particle size (D) of CeO2 nanomaterials and Pt/BaO/CeO2 samples
Catalyst
Surface area (m2 g−1
)
Pt loading (%)
BaO loading (%)
Da (nm)
CeO2 sizeb (nm)
CeO2-NP
Pt/BaO/CeO2-NP
CeO2-NC
Pt/BaO/CeO2-NC
CeO2-NR
Pt/BaO/CeO2-NR
134.9
115.7
26.5
20.0
103.6
84.2
—
1.07
—
0.92
—
1.01
—
7.50
—
7.33
—
7.82
—
8.8 2.4
—
26.9 13.7
—
(8.1 1.4) × (40–110)
11.2
11.3
33.3
33.1
12.3
12.2
a
Caculated for 200 CeO2 nanomaterials from the HRTEM images. b Caculated based on the CeO2 diffraction peak at 2θ = 28.5° by applying the
Scherrer equation.
significant role.40–42 Fig. S3† compares the molecular ratio of
NO2/(NO + NO2) over the series of Pt/BaO/CeO2 catalysts at
the end of the storage process, where the NO2/NOx ratio
reached a steady state. For a given NSR catalyst, a gradually
increased NO2/(NO + NO2) always relates to an increase in
the NSC value within the temperature range of 200–350 °C,
confirming the key role of NO oxidation.
rich regeneration time of several seconds is typically used in
real systems.33 However, many studies use a longer rich pe-
riod compared to those of real engines. In this study, the
NOx removal efficiency was investigated under harsh cycling
conditions, with a lean period of 90 s and a rich period of 6 s
at a quite high gas hourly space velocity (GHSV) of 360 000
h−1.
The effects of CO2 and H2O on the NOx storage process
were investigated over all the NSR catalysts, with the results
shown in Fig. S4.† Introducing of 1% CO2 and 2% H2O into
the feed gas decreased the NOx storage capacity, while the ex-
tent of NSC reduction was quite different among the tested
NSR samples. Over Pt/BaO/CeO2-NP and Pt/BaO/CeO2-NC,
drastic decreases in NSC values were observed within the
whole temperature range if compared with those in the ab-
sence of H2O and CO2. In the presence of H2O and CO2, in
contrast, Pt/BaO/CeO2-NR still presented a relatively high
NOx storage capacity. At the temperature of 350 °C, Pt/BaO/
CeO2-NR afforded about 713.6 μmol NO gcat−1, which is nearly
80% of that without H2O and CO2.
Fig. 4a–c depicts the evolution of NOx in the outlet feed
over twenty lean/rich cycles for all NSR catalysts at tempera-
tures between 200 and 400 °C. The averaged twenty-cycle NOx
conversion values over Pt/BaO/CeO2-NP, Pt/BaO/CeO2-NC and
Pt/BaO/CeO2-NR are compared in Fig. 4d. On Pt/BaO/CeO2-
NR, it was easy to see that the outlet NOx breakthrough was
close to 0 ppm during all the cycles performed at 200 °C and
400 °C. At other temperatures, NOx spilled out still
maintained relatively low values (10–60 ppm) at the end of
the lean durations. This indicates that Pt/BaO/CeO2-NR pos-
sessed the highest catalytic performance, obtaining nearly
100% NOx conversion from 200 °C to 400 °C. Similarly, the
NOx removal efficiency of Pt/BaO/CeO2-NP was quite high,
reaching 93.5% at all temperatures. For the Pt/BaO/CeO2-NC
catalyst, however, the outlet NOx breakthrough increased sig-
nificantly (to 100 ppm at 200 °C) during the first cycle. And
even worse, much more NOx spilled out and reached around
400 ppm after 5 cycles, presenting extremely low NOx removal.
Dynamic NOx storage and reduction performance. The
NOx removal efficiency is not only concerned with the storage
capacity in the lean phase, but also related to the reduction
of trapped NOx during the rich phase. Minimizing the dura-
tion of the rich phase can improve the fuel efficiency, so a
Fig.
2
XRD patterns of CeO2 nanomaterials and Pt/BaO/CeO2
Fig. 3 NOx storage capacities (NSC) tested at different temperatures
samples.
over Pt/BaO/CeO2 catalysts.
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Ammonia, as the main by-product of the NSR process, was
also measured under cyclic lean–rich conditions, with the re-
sults shown in Fig. S5 and Table S1.† Obviously, the three
NSR catalysts show quite different features of NH3 release.
Over Pt/BaO/CeO2-NP, the selectivity to NH3 decreased mono-
tonically with a rise in temperature, changing from 52.8% at
200 °C to 0.88% at 400 °C. In the whole temperature range,
Pt/BaO/CeO2-NC exhibited much higher production of NH3
than Pt/BaO/CeO2-NP, particularly at temperatures above 250
°C. Among the tested NSR catalysts, interestingly, Pt/BaO/
CeO2-NR shows the lowest NH3 selectivity, exhibiting the low-
est value of 0.74% at 300 °C and the highest one of 11.54%
at 200 °C.
In the initial 2 lean/rich cycles (Fig. 4b), it is clear that the
NOx conversion over Pt/BaO/CeO2-NC was well related to the
NSC value. As cycles continued, however, the NOx was not
fully trapped under lean conditions, and was not further re-
duced during the rich phase. Such gradual deactivation with
time on stream was more pronounced within the tempera-
ture range of 300–400 °C (Fig. 4b), which was also indicated
by the average NOx conversion over 20 lean/rich cycles
(Fig. 4d). This result indicates that, in our case, the NOx re-
moval efficiency under a large scale is not only concerned
with the storage capacity under lean conditions, but also re-
lated to the reduction efficiency for the trapped NOx during
the rich phase. Under rich conditions, the consumption of
H2 was involved in the reduction of released NOx and resid-
ual oxygen species on the catalyst surface. Because of a long
lean period (90 s) and a short rich period (6 s), the residual
oxygen on the surface would increase gradually with an in-
crease in cyclic operation, reducing of which needs a greater
amount of H2 if compared with the initial several cycles.43 As
a result, the amount of H2 for the reduction of trapped NOx
decreased with time going on, leading to the partial regenera-
tion of the storage sites in the rich phase and a decrease in
NOx storage under lean conditions. A slightly decrease in
NOx conversion was also observed on Pt/BaO/CeO2-NR and
Pt/BaO/CeO2-NP during the lean–rich cycling operation, par-
ticularly at the temperature of 300 °C (Fig. 4). A further inten-
sive study will be performed to reveal the temperature and
sample shape dependency of the NOx conversion over Pt/
BaO/CeO2.
The formation of ammonia and its reaction pathway for
the NSR process have been intensively investigated by
Courtois and co-workers.16,44 Over Pt/BaO/Al2O3, it was pro-
posed that the NH3 formation rate via NOx reduction by H2,
is higher that the NH3 reaction rate with NOx to form N2,
leading to NH3 emission during the NSR process when H2 is
employed as a reductant.44 When Pt/BaO/Al2O3 is modified
with an optimal amount of Ce, interestingly, the ammonia se-
lectivity significantly decreased, attributed to the promotion
of ammonia oxidation into N2 via the available oxygen at the
catalyst surface.16 Among the three NSR catalysts, in our case,
Ce XPS and H2-TPR results (in the following section) indi-
cated that the greatest amount of surface active oxygen
trapped by oxygen vacancies was available from Pt/BaO/CeO2-
Fig.
4 Evolutions of NOx concentrations under cyclic lean–rich
conditions at different temperatures on Pt/BaO/CeO2-NP (a), Pt/BaO/
CeO2-NC (b), and Pt/BaO/CeO2-NR (c); average NOx conversion over
all NSR catalysts under cyclic lean–rich conditions at different
temperatures (d).
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NR. Such active oxygen from ceria possibly enhances the am-
monia oxidation to produce N2. As a consequence, a low NH3
selectivity was achieved on Pt/BaO/CeO2-NR due to the high
amount of active oxygen trapped by oxygen vacancies, in ac-
cordance with the results of previous studies.16,44
The influences of CO2 and H2O on the NSR performance
under lean–rich cycling operation were also evaluated, with
the results shown in Fig. S6.† Similar to the NOx storage pro-
cess, an inhibition effect of H2O and CO2 on NOx conversion
was also observed over all the samples. Among the tested
NSR catalysts, Pt/BaO/CeO2-NR showed the highest NOx re-
moval efficiency, exceeding 80% in the temperature range of
200–350 °C.
Chemical states and oxygen vacancies
XPS analysis was performed to verify the state of Pt and Ce
on the surface of the NSR catalysts. Fig. S7† shows the Pt 4f
XPS spectra of fresh Pt/BaO/CeO2 catalysts, and those re-
duced by H2 (hereafter denoted as Pt/BaO/CeO2-R), consider-
ing that the samples were pre-treated by H2 before activity
measurement. The binding energies of the individual peaks
of the Pt 4f spectra are summarized in Table S2.† According
to previous studies,45,46 the peaks at 72.5 (4f7/2) and 75.8 eV
(4f5/2) should belong to PtO while those at 74.1 (4f7/2) and
77.5 eV (4f5/2) can be attributed to PtO2. It could be con-
cluded that platinum on all the three NSR catalysts was pres-
ent in the form of PtO and PtO2. In addition to the peaks be-
longing to PtO and PtO2, the H2-treated NSR catalysts also
showed peaks at 71.0 (4f7/2) and 74.2 eV (4f5/2) correspond-
ing to Pt0. Thus the reduced NSR catalysts contained metallic
Pt and Pt oxides.
Thermal stabilities of the stored NOx
The NOx storage features and thermal stabilities of the NOx
stored on Pt/BaO/CeO2-NP, Pt/BaO/CeO2-NC and Pt/BaO/
CeO2-NR were compared by TPD experiments after NO2 ad-
sorption at room temperature (Fig. 5). Because NO2 is easily
dissociated to the NO ionization fragment under our MS con-
ditions, the signal of NO2 (m/z = 46) in our experiment was
very weak and the main signal was NO (m/z = 30).43 Clearly,
Pt/BaO/CeO2-NR showed the largest amount of desorbed NO,
while the amount of desorbed NO of Pt/BaO/CeO2-NP was ap-
proximately the same as that of Pt/BaO/CeO2-NC. This was
consistent with the NOx storage capacity as presented in
Fig. 3. It is noteworthy that Pt/BaO/CeO2-NC exhibits the low-
est amount of desorbed NO at temperatures below 350 °C,
while the strongest peak is centered at much higher tempera-
ture than for the other two catalysts. This result clearly shows
that the ad-NOx on Pt/BaO/CeO2-NC possesses high stability,
benefiting for NO storage at high temperatures. As a result, a
larger NSC value was obtained over this catalyst at a tempera-
ture of 400 °C (Fig. 3). On the other hand, the ad-NOx with
high stability makes against its release under fuel-rich condi-
tions, thus lowering NOx conversion over Pt/BaO/CeO2-NC
(Fig. 4).
The EXAFS of Pt-LIII edges were also measured over all the
samples (Fig. S8†). The results of curve-fitting analysis of the
EXAFS are summarized in Table S3.† In the Fourier transform
spectrum of Pt foil, the peak at 2.77 Å is assigned to the Pt–
Pt bond. As for the reference sample of PtO2, the peaks at
2.02 and 3.10 Å are assigned to the Pt–O and Pt–O–Pt bonds,
respectively. Interestingly, only the intense peak at 2.00 Å cor-
responding to the Pt–O bond was observed over the fresh
NSR catalysts. The formation of defined Pt–O–Ce bonds, as
reported in the references,47,48 which may inhibit the
sintering of Pt could not be observed in our Pt/BaO/CeO2 cat-
alysts, in agreement with previous studies.49,50 Moreover, Pt–
Pt or Pt–O–Pt peaks could not be observed in all the NSR cat-
alysts, suggesting that there are no large Pt metal or oxide
particles. In other words, highly dispersed Pt oxides were
present on the surface of the CeO2 supports, in agreement
with the elemental mapping images. Generally, Pt0 is consid-
ered to be the active site for the catalytic reaction. Therefore,
the EXAFS of Pt-LIII edges over the reduced NSR catalysts
were also presented. After H2 reduction at 450 °C, the Pt–O
peak decreased significantly compared with the fresh cata-
lysts. In addition, the Pt–Pt peak could be observed in the
reduced catalysts. These results suggested that all of the Pt
species were reduced from Pt4+ to Pt0 and Pt2+, further
confirming the XPS results.
Fig. 6a–c shows the Ce 3d XPS spectra of the pure CeO2
nanomaterials and the NSR catalysts, together with the corre-
sponding peak fitting results. The v′, and u′ peaks correspond
to Ce3+; while v, v″, v‴, u, u″, and u‴ are contributed by
Ce4+.51–53 The peak positions for all the samples are listed in
Table S4.† The Ce3+ concentration in the CeO2 nanomaterials
and corresponding NSR catalysts was calculated by analysis
of the integrated peak area, with the equation shown as fol-
lows:
Fig. 5 NOx-TPD profiles of the NSR catalysts.
where Ai is the integrated area of peak “i”.
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vacancies are formed. Fig. 6 summarizes the concentrations
of Ce3+ present in the CeO2 nanomaterials and corresponding
NSR catalysts. The results showed that the concentration of
oxygen vacancies in the CeO2 nanorods was much higher
than that in the other two CeO2 nanomaterials. The addition
of Ba and Pt components increased the Ce3+ contribution on
the NSR catalysts, except for Pt/BaO/CeO2-NC. A similar result
was also observed over Ag/CeO2 by Chang et al., in which the
Ag–CeO2 interaction affected the concentration and structure
of oxygen vacancies in the CeO2 support.55
As reported in the literature,56 the presence of Ce4+ can be
estimated by the high binding energy band u‴. The propor-
tions of the relative intensity of the u‴ peak over all the sam-
ples are shown in Table S4.† The H2 treatment decreased the
proportion of Ce4+, indicating the decrease in the surface
content of Ce4+. In other words, the reduced NSR catalysts
had a higher amount of Ce3+. Moreover, the proportions of
the relative intensity of the u‴ peak on the reduced Pt/BaO/
CeO2-NP, Pt/BaO/CeO2-NC, and Pt/BaO/CeO2-NR were 13.4%,
14.3%, and 12.7%, respectively, indicating that the surface
content of Ce4+ on Pt/BaO/CeO2-NR was the lowest, in agree-
ment with the concentrations of Ce3+.
Among all the NSR catalysts, the amount of Ce3+ in the
nanorod-based sample was the highest, which indicates a
strong interaction between active components and CeO2-NR.
XPS measurements were also carried out to identify the sur-
face concentrations of Ce3+ on the reduced catalysts. As
shown in Fig. 6, the surface concentrations of Ce3+ as percent
of the total Ce on the reduced Pt/BaO/CeO2-NP, Pt/BaO/CeO2-
NC, and Pt/BaO/CeO2-NR were 15.2, 14.9, and 16.8%, respec-
tively, indicating that higher concentrations of oxygen vacan-
cies were created by H2 pre-treatment. Theoretical calculation
predicted that57 the formation of oxygen vacancies on the
CeO2 (110) plane is much more favourable than those on the
(100) and (111) planes. As shown in Fig. 1, the CeO2 nano-
rods were enclosed by the (110) and (100) facets, while the
nanoparticles and nanocubes mainly exposed {(100) + (111)}
and (100), respectively. As a result, it is reasonable that the
Pt/BaO/CeO2-NR catalyst exhibits a higher concentration of
oxygen vacancies than the other two.
As reported by Lavalley et al.,58 methanol adsorbed on the
surface of CeO2 prefers to dissociate into methoxy species
and H+. In that case, the methoxy species adsorbed on the
potential redox sites of CeO2 exhibits the feature band
around 970–1085 cm−1. As a result, the methanol adsorption
technique provides a useful method for revealing the amount
or states of oxygen vacancies.59 With this in mind, the IR
spectra of Pt/BaO/CeO2-R (H2 pretreatment) were recorded
during flowing methanol (Fig. S9†).
Fig. 6 Ce 3d XPS spectra of three different shaped CeO2 (a), Pt/BaO/
CeO2 (b), and Pt/BaO/CeO2-R (c).
Within the region of vIJC–O), four bands grow conjointly at
1006, 1032, 1052, and 1114 cm−1 during the introduction of
methanol, assignable to on-top methoxy (1006 cm−1), bridg-
ing methoxy (1032 and 1052 cm−1), and three-coordinate
methoxy (1114 cm−1), respectively.60 After exposure of metha-
nol for 30 min, the intensity of methoxy species was stable
for all the samples. Based on the spectra at 30 min, the
As we all know, once Ce3+ appears in fluorite-structure
ceria, oxygen vacancies will be produced by the transforma-
tion of Ce4+ to Ce3+, 4Ce4+ + O2− → 2Ce4+ + 2Ce3+ + □ + 0.5O2
(where □ presents an empty position).54 The higher the Ce3+
concentration in these materials is, the more oxygen
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amount of redox sites on Pt/BaO/CeO2-R was calculated from
the integral area of 970–1085 cm−1 per gram of catalyst, and
the results are shown in Table S5.† Obviously, the integral
area relating to the redox sites on Pt/BaO/CeO2-NR was much
higher than the other two, indicating a higher concentration
of oxygen vacancies, and further confirming the results of Ce
3d XPS.
Redox properties
Fig. S10† shows the H2-TPR profiles of pure nano-CeO2 with
different shapes. All the CeO2 samples exhibited a broad re-
duction peak at 200–500 °C and a high-temperature reduc-
tion peak above 600 °C, attributed to surface and bulk ceria
reduction, respectively.7 Fig. 7 illustrates the H2-TPR profiles
and corresponding fitting results of the NSR catalysts with
different shaped CeO2 serving as supports. The peak at tem-
peratures below 300 °C can be assigned to the combined re-
duction of PtOx and its neighbouring surface ceria. This sur-
face ceria (denoted as promoted surface ceria hereafter) was
intimately in contact with PtOx, the reduction of which was
attributable to the abstraction of O2− by hydrogen and facili-
tated by the presence of platinum.61,62 Another peak was ob-
served at an intermediate temperature (ca. 570 °C), resulting
from the reduction of surface ceria that was more remote
from the platinum. In other words, the peak at the intermedi-
ate temperature originates from O2− abstraction from surface
ceria that is not in direct contact with Pt. Finally, a high-
temperature peak was observed at ca. 750 °C, due to the re-
duction of bulk ceria to Ce3+ 61,62
If compared with the pure
.
nano-CeO2, the reduction of surface ceria occurs at a much
lower temperature after loading of Pt and BaO, but the peak
intensity significantly changed depending on the shape of
CeO2. This indicates that the morphology of nanoscale ceria
has a great influence on the synergistic interaction between
platinum and ceria nanostructures (possibly via H2 spill-
over).
During the H2-TPR experiments, the total H2 consumption
was measured, with a value of 110.8 μmol for Pt/BaO/CeO2-
NP, 123.6 μmol for Pt/BaO/CeO2-NC, and 158.8 μmol for Pt/
BaO/CeO2-NR. Based on the fitting results of the H2-TPR, the
H2 consumption due to the reduction of the total surface
ceria for all three NSR catalysts is given in Table S6 (ESI†).
The proportions of promoted surface ceria were calculated
(the area under the relevant TPR curve divided by the area
under all the surface-related curves), giving 13% for Pt/BaO/
CeO2-NP, 3.3% for Pt/BaO/CeO2-NC and 14.5% for Pt/BaO/
CeO2-NR. This demonstrates that the interaction between
platinum and CeO2-NR is stronger than that with CeO2-NP
and CeO2-NC. The morphology of nanoscale ceria, therefore,
influences the synergistic interaction between platinum and
ceria nanostructures. It is worth noting that a sharp peak due
to PtOx reduction was observed for all the NSR catalysts,
showing the good dispersion of PtOx and further confirming
the results of elemental maps. The proportions of PtOx reduc-
tion were also calculated, with a value of 13.7% for Pt/BaO/
Fig. 7 H2-TPR profiles of the NSR catalysts based on the different
shaped CeO2 (a), and fitting results for nanoparticle (b), nanocube (c),
and nanorod (d) based catalysts, respectively.
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CeO2-NP, 8.2% for Pt/BaO/CeO2-NC, and 20.0% for Pt/BaO/
CeO2-NR (Table S6†). Over the nanorod-supported sample,
meanwhile, the reduction of PtOx occurred at 137 °C, 76 °C
lower than that of Pt/BaO/CeO2-NP. This result reveals that
more Pt species are available on the surface of Pt/BaO/CeO2-
NR, which also exhibits a higher reducibility. As a result, it is
reasonable that Pt/BaO/CeO2-NR is more active for NO oxida-
tion than the other two NSR catalysts (Fig. S3†).
As suggested by Fridell and co-workers,63 the adsorption
of NO2 and its further transformation into surface nitrates in-
volve the oxidation of the surface or oxygen species by NO2 to
produce NO. It means that, in turn, the reducibility of the
NSR catalysts or the availability of active oxygen species plays
a crucial role in NO2 adsorption and storage. Over the Pt/
BaO/CeO2 catalysts presented here, these properties were
closely related to the oxygen vacancies on the ceria surface as
confirmed by Fig. 6 and 7. Pt/BaO/CeO2-NP and Pt/BaO/CeO2-
NC exhibit a similar concentration of oxygen vacancies
(15.2% for Pt/BaO/CeO2-NP and 14.9% for Pt/BaO/CeO2-NC),
thus giving a similar amount of desorbed NO in TPD (Fig. 5).
Reaction sites and reaction testing by in situ DRIFTS
In Fig. 8, in situ DRIFTS show the evolution of surface species
during the NOx storage and reduction process at 350 °C. After
exposing the Pt/BaO/CeO2-NP catalyst to a NO/O2/N2 mixture
for 40 s (Fig. 8a), two bands were observed at 1020 and 1294
cm−1, which were assigned to the symmetric and asymmetric
stretching modes of –NO2 due to bidentate nitrate species
adsorbed on Ba sites.10 Meanwhile, another significant band
attributed to monodentate nitrate on ceria was observed at
1538 cm−1.23 With increasing exposure time, bands at 816
and 1770 cm−1 appeared and their intensities gradually in-
creased, indicating the formation of bulk nitrate associated
with Ba.64,65 Along with the formation of nitrates, two nega-
tive bands appeared at 863 and 1750 cm−1, originating from
the decomposition of surface barium carbonate.23 As time
passed, a shoulder was observed at 1349 cm−1 which was
assigned to free nitrate ions.66 When the gas feed was
switched to a rich atmosphere (Fig. 8b), the bands at 1538,
1349, 1294, and 1020 cm−1 disappeared in the first 20 sec-
onds, indicating the high activity of surface nitrates and ni-
trate ions. In contrast, the bands at 1770 and 816 cm−1 de-
creased gradually, disappearing after 5 min of reduction,
suggesting that the bulk nitrates are not as active as those
adsorbed on the surface.
The same experiments were carried out on Pt/BaO/CeO2-
NC, with the corresponding DRIFT spectra shown in
Fig. 8c and d. The NOx storage process on Pt/BaO/CeO2-NC
was similar to that on Pt/BaO/CeO2-NP, except that the inten-
sity of the peak assignable to nitrate ions (1349 cm−1) was
much stronger than that on the nanoparticle supported cata-
lyst. Under rich conditions, the nitrates adsorbed on Pt/BaO/
CeO2-NC displayed a significantly lower activity for reaction
with H2, compared with that on Pt/BaO/CeO2-NP. Notably, in
the first 20 s of H2 flow, the intensities of the IR bands due
Fig. 8 In situ DRIFTS studies of storage (a, c, e) and reduction (b, d, f,)
process at 350 °C for Pt/BaO/CeO2-NP, Pt/BaO/CeO2-NC and Pt/BaO/
CeO2-NR.
to nitrates adsorbed on the surface of Pt/BaO/CeO2-NC (1538,
1294, and 1024 cm−1) and free nitrate (1349 cm−1) only de-
creased slightly, disappearing after introduction of H2 for 1
min. As a result, it is reasonable that Pt/BaO/CeO2-NC ex-
hibits low activity for NOx conversion under lean–rich cyclic
conditions as shown in Fig. 4.
Obviously, bidentate nitrates on Ba sites (1020 and 1294
cm−1) and monodentate nitrate on Ce sites (1538 cm−1) were
also formed on the surface of the Pt/BaO/CeO2-NR catalyst
(Fig. 8e). By comparing the intensities of the bands due to
the two nitrate species, we can easily conclude that the Pt/
BaO/CeO2-NR catalyst possesses more adsorption sites avail-
able for NOx storage than Pt/BaO/CeO2-NP and Pt/BaO/CeO2-
NC. This conclusion was further confirmed by the appear-
ance of a shoulder peak at 1230 cm−1 assigned to bridged
bidentate nitrites on Ba or Ce sites, while it was not observed
in Fig. 8a and c.67 Also, bulk nitrates (816 and 1770 cm−1)
and free ions (1349 cm−1) were observed after feeding of NO
+ O2 for 10 min.
When Pt/BaO/CeO2-NR was exposed to a reducing atmo-
sphere (Fig. 8f), the bands at 1538, 1294, 1230, and 1020
cm−1 decreased sharply, indicating the high activity of surface
nitrates and nitrate ions. After 40 s of reduction, few
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adsorbed species remained on Pt/BaO/CeO2-NR, further dem-
onstrating its high NOx removal efficiency as shown in
Fig. 4c.
Looking back at the NOx storage behavior (Fig. 8a, c, and e),
one can easily find that, within the initial 2 min over all
the NSR catalysts, the nitrates adsorbed on the surface sites
of Ba (1020 and 1294 cm−1) and Ce (1538 cm−1) were predom-
inant. This means that, in our lean (90 s)–rich (6 s) cyclic
conditions, the availability of surface Ba and Ce sites is cru-
cial for NOx storage.
Relationship between the properties of the catalysts and
catalytic performance
Even though all Pt/BaO/CeO2 samples had almost the same
Ba and Pt loadings, their catalytic performance for the NSR
process was strongly dependent on the shape of the ceria
support. The nanoparticle-based NSR catalyst exhibits a
much larger BET surface area than the nanorod-related one
(Table 1), while the former shows a significantly lower NSC
value than the latter (Fig. 3). Also, under lean–rich condi-
tions, the activity of Pt/BaO/CeO2-NP for NOx conversion is
poorer than Pt/BaO/CeO2-NR. These results show that the sur-
face area of Pt/BaO/CeO2 is not a determining factor for the
NSR process. It also suggested that ceria can serve as a com-
ponent for NOx storage, which was identified by our in situ
DRIFTS measurements. As shown in Fig. S9,† however, the
NOx storage capacity of ceria is marginal.
Previous studies showed that the oxidation of NO to NO2
was crucial for the NOx storage capacity, the occurrence of
which was closely related to the presence of a noble metal.
Combining the result of H2-TPR (Fig. 7 and Table S6†) with
that of NSC (Fig. 3), the larger amount of the available Pt is,
the higher the NOx storage capacity is. Over CeO2 supported
catalysts, experimental measurement and theoretical predic-
tion confirmed that the oxygen vacancies are anchoring sites
for the noble metal, creating a strong metal–support bond,
thus enhancing the catalytic activity.9,68,69 If the results
presented in Fig. 6 and 7 are taken into account, it is easy to
find that the higher the concentration of Ce3+ is, the higher
the availability of Pt is. These results mentioned above sug-
gest that the oxygen vacancies play a key role in NOx storage
over Pt/BaO/CeO2. Furthermore, it is interesting to note that
a good linear correlation between the Ce3+ concentrations of
reduced NSR catalysts and their NOx storage capacities was
observed in the temperature range of 200–350 °C (Fig. 9a), in-
dicating that oxygen vacancies may govern the process of
NOx storage. At temperatures above 350 °C, however, such a
linear relationship was not observed between the Ce3+ con-
tent and NSC value. Glancing at Fig. 5, one can easily find
that the stored NOx begins to decompose at 350 °C, the oc-
currence of which gives a negative contribution to the NSC
value of Pt/BaO/CeO2. As a result, a linear relationship was
not obtained at the high temperature of 400 °C.
Fig. 9 The relationship between the concentration of Ce3+ measured
by XPS and the NOx storage capacity of all the Pt/BaO/CeO2 catalysts
(a); the relationship between the H2 consumption of surface CeO2
measured by H2-TPR and the NOx storage capacity of all the Pt/BaO/
CeO2 catalysts (b).
the surface of ceria,54 on which active oxygen (O*) is cre-
ated.70 A linear relationship between the Ce3+ concentration
on the reduced NSR catalysts and the H2 consumption of to-
tal surface CeO2 (Fig. S12†) further suggests that the oxygen
vacancies both neighbouring the Pt and far away from the Pt
are active for oxygen activation.
Interestingly, there is a close linear correlation between
the H2 consumption of total surface CeO2 and the NOx stor-
age capacity of different shaped ceria-based catalysts at 200–
350 °C (Fig. 9b), revealing that the surface active oxygen
trapped by oxygen vacancies plays a crucial role in the NOx
storage process.
As presented in Fig. S11,† however, ceria only exhibits a
marginal NOx storage capacity even though it contains large
amounts of oxygen vacancies. With this in mind, a question
may arise: how does the oxygen species activated by oxygen
vacancies on ceria govern the NOx storage, which is mainly
trapped on Ba sites?
As described in the previous literature, the presence of
Ce3+ is accompanied by the formation of oxygen vacancies on
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Among the three NSR catalysts, Pt/BaO/CeO2-NR has the
National High Technology Research and Development Pro-
gram of China (863 Program, 2013AA065301).
highest concentration of Ce3+, and the highest number of oxy-
gen vacancies both neighbouring the Pt and far away from the
Pt, creating the largest amount of active oxygen species (eqn
(1)). The O* species are active for NO2 formation on the Pt
sites (eqn (2))71 and also for nitrate production (eqn (3)).72
Considering that the Ba sites exhibit higher basicity than the
Ce sites,73 the produced nitrates are thus preferably stored on
and/or transfer to the former, which has been established by
Fig. S11 and 8.† This feature of nitrate adsorption also guaran-
tees the availability of oxygen vacancies for further activation
of O2, nitrate formation, and further transformation of nitrate.
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2Pt ⋯ Ce3+ ‐ □ + O2 → 2Pt ⋯ Ce4+ − O *
NO + Pt ⋯ Ce4+ ‐ O * → NO2 + Pt + Ce3+ ‐ □
BaO + 2NO2 + Ce4+ − O * → Ba(NO3)2 + Ce3+ ‐ □
BaO + 2NO + 3Ce4+ − O * → Ba(NO3)2 + 3Ce3+ ‐ □
(1)
(2)
(3)
(4)
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where, Pt⋯Ce3+-□ represents the Pt sites neighbouring oxy-
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Conclusions
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The CeO2-nanorod based NSR catalyst displayed an excellent
NOx storage capacity that can reach as high as 913.8 μmol
NOx gcat−1, as well as high NOx removal efficiency in periodi-
cally alternating lean–rich atmospheres (90 s vs. 6 s). During
20 lean–rich cycles of operation, an extraordinarily high NOx
conversion (>96.6%) was achieved on Pt/BaO/CeO2-NR in the
temperature range 200–400 °C. During the cyclic operation,
this catalyst also exhibits much lower selectivity to ammonia
and higher tolerance of H2O and CO2. XPS measurements
confirm the formation of oxygen vacancies on the surface of
ceria and Pt/BaO/CeO2. A higher concentration of oxygen va-
cancies guarantees that large amounts of Pt species are avail-
able for NO oxidation to NO2 over Pt/BaO/CeO2, indicating
the key role of oxygen vacancies in anchoring and dispersion
of Pt. Over all the NSR catalysts, the concentration of Ce3+ is
linearly correlated with the NOx storage capacity. This rela-
tionship reveals that the oxygen vacancies on the ceria sur-
face govern the formation and/or transformation of nitrates
via the reaction between NO (and/or NO2) and active O spe-
cies created on oxygen vacancies. The superior activity of the
CeO2-nanorod-supported NSR catalyst for NOx storage thus
originates from the higher numbers of oxygen vacancies,
located both neighbouring and far away from the Pt on the
ceria surface.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51221892 and 21373261) and the
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2016

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