Design of Prussian blue analogue-derived double-cone structure Ce-Fe catalysts and their enhanced performance for the selective catalytic reduction of NOxwith NH3

Article abstract of DOI:10.1039/d0nj04419a

Prussian blue (PB) and its analogues (PBA) with different structures and adjustable compositions have been recognized as promising materials for catalysis, energy storage, and biological applications. Herein, a simple surface anchoring strategy is proposed to achieve the uniform deposition of CeO2 nanocrystals on the double-cone structure surface of PBA (denoted as Ce@Ce-Fe catalyst) for the selective catalytic reduction of NOx with NH3 (NH3-SCR). Compared with Ce-Fe and Ce@CeO2-Fe2O3, the Ce@Ce-Fe catalyst exhibited the best catalytic activity, widest working temperature window, and highest SO2 tolerance, implying its good application in the process of NH3-SCR. Moreover, the catalysts were characterized systematically to elucidate their surface properties and morphological structure. Taking advantage of the excellent redox performance, unique morphology, mesoporous structure with large surface area and pore diameter, and greater acid content, the Ce@Ce-Fe catalyst exhibited a higher NOx performance. In addition, the Ce@Ce-Fe catalyst also exhibited significant resistance to H2O and SO2 due to its higher content of Fe atoms, implying that Fe plays a critical role in this Ce-Fe based catalytic system. More importantly, the present study indicates that well-dispersed active components and unique architectures can effectively enhance the performance of catalysts. This journal is

Full text of DOI:10.1039/d0nj04419a

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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DOI: 10.1039/D0NJ04419A
Design of Prussian blue analogues derives double-cone
structure Ce-Fe catalysts and their enhanced performances
for the selective catalytic reduction of NOx with NH3
Jie Guoa,b, Guodong Zhanga, Zhicheng Tanga,c*, Jiyi Zhangb*
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(a. State Key Laboratory for Oxo Synthesis and Selective Oxidation, National Engineering
Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, PR China
b. School of petroleum and chemical, Lanzhou University of Technology, Lanzhou 730050,
China.
c. Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, 116023, China)
*Corresponding author. Tel.: +86-931-4968083, Fax: +86-931-4968019, E-mail address:
tangzhicheng@licp.cas.cn (Z.Tang); zhangjiyi@lut.cn (J.Zhang).
Abstract
Prussian blue (PB) and its analogues (PBA) with differences in structure and
adjustable composition have been recognized as promising materials for catalysis,
energy storage, and biological applications. Here, a simple surface anchoring strategy
is proposed to achieve the uniform deposition of CeO2 nanocrystals on the
double-cone structure PBA surface (denoted Ce@Ce-Fe catalyst) for the selective
catalytic reduction of NOx with NH3 (NH3-SCR). Compared with Ce-Fe and
Ce@CeO2-Fe2O3, Ce@Ce-Fe catalyst exhibited enhanced catalytic activity, more

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extensive working temperature window, improved SO2 tolerance, implying thDeOirI: g10o.1o03d9/D0NJ04419A
application in the process of NH3-SCR. Moreover, the catalysts have been
characterized systematically to elucidate their surface properties and morphological
structure. Taking advantages of the excellent redox performance, unique morphology,
mesoporous structure with big surface area and pore diameter, and more acid amount,
the Ce@Ce-Fe catalyst exhibits higher deNOx performance. In addition, the
Ce@Ce-Fe catalyst also manifests significant resistance of H2O and SO2 due to the
higher content of Fe atom ratio, implying that Fe acted a critical role in Ce-Fe based
catalytic system. More importantly, the present study indicates that well-dispersed
active components and unique architectures can effectively enhance the performance
of catalysts.
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Key words: NH3-SCR; Prussian blue analogues; Ceria anchoring; double-cone
structure; resistance of H2O and SO2
1. Introduction
Under the premise of the rapid development of urbanization and industrialization,
the proportion of air pollution in environmental pollution has gradually increased
[1-5]. Nitrogen oxides (NOx) is regarded as a common air pollutants, which is the
main cause of photochemical smog, destruction of the ozone layer, and acid rain [6, 7].
Currently, the SCR technology has been proven to be a very mature technology with
great promise for the removal of nitrogen oxides and has been successfully applied to
industrial application. Furthermore, vanadium-based catalyst has been developed an

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industrialized and commercial catalysts due to its high catalytic activity and
selectivity. Unfortunately, the drawbacks of V-based catalysts still existed, such as
poor thermal stability, narrow operating temperature window and toxicity of
vanadium species [8, 9]. Therefore, further seeking of high-performance,
cost-efficient and environment friendly catalysts with abundant resources for
replacement of previous catalysts for NH3-SCR were urgently needed.
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Ce-based catalysts among rare earth-based catalysts had non-toxicity, excellent
oxygen storage capacity and redox characteristics, which were considered as good
substitutes for V-based catalysts [10-12]. In the NH3-SCR catalytic reaction process, a
catalytic cycle could be completed when Ce4+ and Ce3+ were converted to each other
in the catalytic system. In the above process, the Ce3+ substance was essential to
increase the catalytic activity of the NH3-SCR reaction, and the formed oxygen
vacancies also promoted the NH3-SCR reaction [13]. However, Ce-based catalysts
were easily poisoned by SO2 in flue gas while Ce and SO2 undergo oxidation
reactions to produce Ce2(SO4)3, thereby destroying the mutual conversion of Ce4+ and
Ce3+ species [14]. Therefore, the modification research based on Ce-based catalysts
has been widely studied for NH3-SCR in recent years [15-19]. Interestingly, Fe-based
catalyst was a typical active ingredient or promoter in NH3-SCR system, which
exhibited good NH3-SCR activity, N2 selectivity and non-poison due to its inherently
environmentally friendly character, prominent thermal stability and outstanding
resistance of H2O/SO2 [20-22]. Based on above conclusions, Fe-based catalyst could
be a suitable candidate and promoter to improve the resistance of H2O/SO2.

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Therefore, further exploitation of Ce-Fe types of NH3-SCR catalyst is ofDOsIp: 1e0c.1i0a3l9/D0NJ04419A
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interest.
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Prussian blue analogue (PBA) is a type of crystalline metal organic framework
(MOF) composed of divalent and trivalent metal ions bridged by cyanide ligands [23,
24]. It could be made into compounds with uniform composition and size as well as
various forms and structures [25, 26]. Due to its special thermal properties and unique
reactivity, PBA could be used as a precursor for the synthesis of layered materials
with hollow or porous structures [27, 28]. Recently, PBA was able to create special
and enhanced structural and physicochemical properties. Furthermore, this materials
have great potential serving as building components for catalysts. More interestingly,
PBA also owed unique morphology and structure by adjusting methods of synthesis,
making it ideal precursors for fabrication of novel catalysts for NH3-SCR reaction.
In our work, we report the fabrication of Prussian blue analogues of Ce-Fe
catalysts derived from the potassium ferricyanide as the precursor framework and
cerium as the metal salt solution which was synthesized using a simple
co-precipitation method. In addition, the optimal catalyst was determined by changing
the synthesis conditions. On this basis, in order to further improve catalytic activity,
Ce@Ce-Fe and Ce@CeO2-Fe2O3 catalysts were synthesized by anchoring Ce on
surface of Ce-Fe precursors and CeO2-Fe2O3 oxide, respectively. Moreover, the
resulting Ce@Ce-Fe catalyst with unique morphology and structure exhibited a better
operating temperature window, excellent catalytic activity than other catalysts. Hence,
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it was critical to investigate the structure and morphology of the catalyst and the effect
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of Ce anchoring on the surface of catalyst.
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2 Experimental
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2.1 Preparation of samples
The specific synthesis steps were as follows:
(1) Synthesis of Ce-Fe Prussian blue precursor
0.17 g of cerium acetate (Ce(CH3COO)3) and 1.2 g of polyvinylpyrrolidone
(PVP, K-30) were dissolved in a mixed solution of 20 ml of ethanol and 10 ml of
deionized water, referred to as solution A; 0.192 g of potassium ferricyanide
(K3[Fe(CN)6]2) was dissolved in 25 ml of deionized water solution and recorded it as
solution B; solution B was added dropwise to solution A, stirred evenly under the
same temperature, and aged for 24 h at room temperature. Next, the suspension was
centrifuged, and the obtained product was washed with ethanol and deionized water
alternately at least five times, dried at 60 °C for 12 h, and the obtained product was
recorded as Ce-Fe catalyst.
Repeated the above experiment, only changed the amount of PVP to 0.6 g, the
concentration of metal salt solution was reduced by half and the molar ratio of Ce and
Fe became 1: 1, and the products obtained were respectively denoted Ce-Fe-PVP,
Ce-Fe-C and Ce-Fe-M catalyst.
(2) Synthesis of CeO2-Fe2O3 Prussian blue catalyst
The above-prepared Ce-Fe-based Prussian blue precursor was calcined at 400 °C
for 5 h at a temperature increase rate of 1 °C/min, and, and the obtained products were

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denoted CeO2-Fe2O3 catalyst.
(3) Synthesis of Ce@Ce-Fe catalyst
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0.1 g Ce-Fe Prussian blue precursor was ultrasonically dispersed in a mixed
solution of 40 ml water and 40 ml absolute ethanol, and then 0.2 g of
hexamethylenetetramine (C6H12N4) and 0.15 g cerium acetate hexahydrate
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(Ce(NO3)3 6H2O) were added sequentially with magnetic stirring. The suspension was
further heated to 70 °C and maintained for 2 h. After natural cooling to room
temperature, the product was collected by centrifugation and washed twice with
deionized water, then dried at 60 °C for 12 h, calcined at 400 °C for 3 h at a heating
rate of 1 °C/min, and the obtained product was Ce@Ce-Fe catalyst.
(4) Synthetic Ce@CeO2-Fe2O3 catalyst
Repeat (3) the experimental procedure, only 0.1 g Ce-Fe precursor is replaced
with 0.1 g CeO2-Fe2O3 oxides, and the resulting product was recorded as
Ce@CeO2-Fe2O3 catalyst.
2.2 Characterizations of catalysts
The morphology of samples was illustrated by using a scanning electron
microscope (SEM). In addition, the SEM image was recorded on the JSM-6701F
emission scanning electron microscope, and the profiles of SEM were recorded at 30
kV.
Transmission electron microscopy (TEM) experiments of catalysts were carried
out on a JEOL JEM-2010 transmission electron microscope with an operating
voltage of 200 kV.

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Brunauer-Emmett-Teller (BET) surface area, pore size and pore voDluOmI: 1e0.10o3f9/D0NJ04419A
sample was carried out by nitrogen adsorption and desorption at 76.2 K on the
Micromeritics ASAP 2020 instrument.
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Powder X-ray diffraction (XRD) patterns analysis of catalysts were exhibited
by a D8 discover X-ray diffractometer, which varied from 10° to 90° with a scanning
speed of 0.5° min-1.
Raman Spectroscopy (RS) was conducted by a confocal Raman spectrometer
(Horiba JobinYvon S. A. S, France) with an argon green ion laser (532 nm).
X-ray photoelectron spectroscopy (XPS) data was collected on a VG
ESCALAB 210 Electron Spectrometer (Mg Ka radiation; hv = 1253.6 eV), and XPS
data was calibrated as the standard by using the C 1s binding energy of 284.6 eV.
Temperature program reduction with hydrogen (H2-TPR) was carried out by
DAS-7000 automatic multi-function adsorption apparatus. In general, 50 mg catalyst
sample was performed on the quartz tube reactor, and the temperature ranged from
room temperature to 850 °C with a heating rate of 10 °C min-1. Furthermore, 5 vol.%
H2 balanced N2 was used with a flow rate of 40 mL min-1.
Temperature program desorption of NH3 (NH3-TPD) was carried out by
DAS-7000 automatic multi-function adsorption apparatus. Before each analysis, the
catalyst sample was purged at 300 °C with a He gas for 30 min. Furthermore, 50 mg
sample of catalyst was performed on the quartz tube reactor, and the temperature
varied from 100 °C to 650 °C with a heating rate of 10 °C min-1. Moreover, 5 vol. %
NH3 balanced He was used with a flow rate of 40 mL min-1.

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Fourier Transform Infrared Spectroscopy (FTIR) was scanned with a resolution
of 4 cm-1 by Nexus 870 under a range of 400-4000 cm-1 at room temperature.
2.3 NH3-SCR activity measurements
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The detailed measurement process is shown in the supplementary information.
3 Result and discussion
3.1 Morphology of sample
The scanning electron microscope (SEM) of the Ce-Fe-PVP, Ce-Fe-M, Ce-Fe
and Ce@Ce-Fe precursors is presented in Fig. 1. As could be observed from Fig. 1a,
b, Ce-Fe-PVP had a uniform double-cone dodecahedron structure with the same
particle size, and at the same time, the size of each face of the double-cone
dodecahedron was also basically the same. However, the particle size of the
Ce-Fe-PVP catalyst precursor was relatively large. According to reports in related
literature, the amount of PVP could control the size of the particle size, and the
particle size was too large would affect the catalytic activity of the catalyst. According
to Fig. 1c, d, the precursor of Ce-Fe-M catalyst also has a double-cone structure, and
the particle size was relatively uniform. Although the twelve faces on the precursor
were not very obvious, they could also be observed to faceted structure. The particle
size of the catalyst precursor of Ce-Fe-M was small, which was beneficial to the
improvement of the NO conversion in the catalytic reaction, but the molar ratio
between Ce and Fe might affect the catalytic activity of the catalyst, which would be
described during the characterization work. Specially, the Ce-Fe catalyst is shown in
Fig. 1e, f. The morphology of the catalyst precursor in the figure was also a

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double-cone structure, the size and structure were uniform, and the particle size was
basically the same. In summary, the precursors of Ce-Fe catalysts all exhibited a
double-cone structure, the PVP and molar ratio would affect the structure size of the
catalyst, and indirectly affect the activity of the catalyst. Meanwhile, Ce@Ce-Fe
catalyst is presented in Fig. 1g, h. It was clear that the morphology of precursor was
also a double-cone structure, the size and structure were uniform, and the particle size
was basically the same. According to these study, exploring the size of the catalytic
structure and the effect of morphology on the catalytic performance of the catalyst.
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Fig. 1 SEM micrographs of precursors: Ce-Fe-P (a, b), Ce-Fe-M (c, d), Ce-Fe (e, f)
and Ce@Ce-Fe (g, h).
Fig. 2 presents a typical low-magnification transmission electron microscopy
(TEM) image of the three samples, which further formed the unique morphology of
the catalyst. The morphology was consistent with the SEM images. The selected-area
electron-diffraction pattern (SAED) in Fig. 2c, there was three diffraction rings
indicating that the Ce@Ce-Fe was pleomorphic. The elemental analysis exerted a
significant role in the exploration of distribution of iron and cerium. Furthermore,

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from the high-resolution transmission electron microscope image (HTEM) inDFOiI:g1.0.210b3,9/D0NJ04419A
it can be clearly seen that the well-defined pores with a uniform diameter in
Ce@Ce-Fe, which further confirmed the mesoporous feature of the Ce@Ce-Fe. The
result of mapping spectra of the Ce@Ce-Fe was tested in Fig. 2i-g. It was proved that
Ce@Ce-Fe was consisted of Ce and Fe element, and Fig. 2i-g also shows the
existence of Ce and Fe, which were uniformly distributed. Meanwhile，according to
Fig. S1, the element atomic of Ce was 67.95% while the element atomic of Fe was
32.05%. Notably, the presence of Ce was more speared, indicating that the
successfully anchoring of Ce on the surface of Ce-Fe precursor, which could enhance
the catalytic performance, because the atomic of Ce has excellent redox. In addition,
the introduction of Ce and Fe increased the redox of materials and the tolerance of
SO2.
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Fig. 2 TEM images of Ce-Fe (a, d), Ce@Ce-Fe (b, c, e) and mapping (g, h, i),
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Ce@CeO2-Fe2O3 (f).
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3.2 Structure of sample
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As shown in Fig. 3a, the typical diffraction peaks of fluorite cubic structure
could be observed in the three types of Prussian blue synthetic materials Ce-Fe,
Ce@Ce-Fe and Ce@CeO2-Fe2O3. It could be observed that the diffraction peaks at
28.5°, 33.0°, 47.5°, 56.4°, 69.4°, and 76.9° could be attributed to (111), (200), (220),
(222), (400), and (420) planes of CeO2, respectively (PDF#78-0694, Fm3m). And it
was clearly observed that the diffraction peaks of ferric oxide were not detected in the
materials of Ce-Fe, Ce@Ce-Fe and Ce@CeO2-Fe2O3 catalysts, which indicated that
ferric oxide of these three catalysts were evenly dispersed on the catalyst. In addition,
it was worth noting that the peak intensity of the Ce@Ce-Fe catalyst was the strongest,
indicating that the crystallinity of the catalyst was higher.
Fig. 3b presents the Raman spectra of Ce-Fe, Ce@Ce-Fe and Ce@CeO2-Fe2O3
catalyst. For mixed oxides, the main energy band was also observed around 460 cm-1,
and as Ce anchors from Ce-Fe precursors to Ce-Fe oxides, the energy band
sequentially shifted to lower frequencies [29]. In the Raman spectrum of fluorite-type
oxides, it had been reported that the red shifted of the ceria band was related to lattice
shrinkage due to cation doping [30]. It should be emphasized that through XRD
measurement, the lattice shrinkage of CeO2 after Ce anchoring was very limited, but
the Raman shift of the cerium oxide band was obvious. This phenomenon indicated
that the content of Ce-Fe catalyst was inconsistent, and the content of Ce anchoring

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on the surface of the solid solution was higher than the content of Ce in theDcOaI:t1a0l.1y0s3t9/D0NJ04419A
[31]. The high Ce content caused the full width at half maximum (FWHM) of the
main band of ceria to widen, indicating that the crystallite size of ceria was small ，
which was beneficial to enhance the catalytic performance. [32]. According to the
findings of XRD and Raman, it was clear that after Ce was anchored, it would have a
significant effect on the structure of Ce-Fe catalyst.
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Fig. 3 XRD spectrum of catalysts (a); Raman spectrum of catalysts (b).
The specific surface area, pore size and pore volume of the three catalysts and
precursors are shown in Table 1. According to Table 1, the BET specific surface areas
of Ce-Fe-P, Ce@Ce-Fe-P and Ce@CeO2-Fe2O3-P precursors were 61.1, 44.6 and 89.2
m2g-1, respectively. Meanwhile, the pore diameters of Ce-Fe-P, Ce@Ce-Fe-P and
Ce@CeO2-Fe2O3-P precursors are 14.0, 16.7 and 10.7 nm, respectively; the pore
volumes of different precursors are 0.2, 0.2 and 0.2 m2g-1, respectively. In the alkaline
environment, the Ce anchored the precursor, the pores were blocked or Ce was

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successfully anchored above the surface of the precursors, so that the BETDsOpI:e1c0.i1f0i3c9/D0NJ04419A
surface areas of the Ce anchor precursor was reduced; but for the Ce anchors the
oxide, it was happened to erosion, increasing the BET specific surface areas of the Ce
anchored oxide. Meanwhile, the BET specific surface areas of Ce-Fe, Ce@Ce-Feand
Ce@CeO2-Fe2O3 catalysts were 40.3, 26.6 and 81.2 m2g-1, respectively. Moreover,
the pore diameters of Ce-Fe, Ce@Ce-Fe and Ce@CeO2-Fe2O3 catalysts were 5.9, 6.9
and 10.5 nm, respectively; the pore volumes of different precursors were 0.1, 0.1 and
0.2 m2g-1, respectively. Compared with the precursor, the BET specific surface area of
the catalyst was reduced. Because during calcination, the internal pores would sinter
and collapse, resulting in a decrease in the specific surface area of the catalyst. The
nitrogen adsorption and desorption curves of precursors and catalysts are shown in
Fig. 4a, b. In addition, in the Brunauer classification, the N2 adsorption and desorption
curves of these three catalysts corresponded to type IV isotherms in Fig. 4b.
According to the IUPAC classification, the hysteresis loop of the three catalysts was
H3 type, indicating that all three catalysts exhibited a typical mesoporous structure.
After Ce anchored the Ce-Fe precursor, the BET surface area of the Ce@Ce-Fe-P
precursor and Ce@Ce-Fecatalyst were both reduced to some extent. This was because
the anchored Ce could partially cover the accumulation pores. Therefore, after Ce was
anchored, the average pore size of Ce@Ce-Fe-P precursor and Ce@Ce-Fe catalyst
decreased, which was similar to previous reports. In addition, Fig. 5 also shows the
pore size distribution curve of the BJH about precursors and catalysts. It was worth
noting that the pore sizes of the three catalysts were mainly distributed between 2 and
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50 nm, which mean that hree catalysts all exhibited typical mesoporous structures,
which was consistent with the results in Fig. 4. At the same time, the nitrogen
adsorption and desorption curves, and BJH pore size distribution curves of three
catalysts showed typical mesoporous structures. Based on the above conclusions, it
could be concluded that the Ce-Fe-based Prussian blue catalyst had a typical
mesoporous structure, which was beneficial to the dispersion of active ingredients,
exposure of more active sites and adsorption of reactants. In general, these factors
were critical to improve the catalytic performance of the catalyst.
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Table 1 Surface area, pore characteristics of different catalysts and precursors.
Crystal size
(nm)
Surface area
(m2 g-1)
61.1
Pore size
(nm)
14.0
16.7
10.7
5.9
Pore volume
Sample
(m3 g-1)
0.2
Ce-Fe-P
Ce@Ce-Fe-P
Ce@CeO2-Fe2O3-P
Ce-Fe
-
44.6
0.2
-
89.2
0.2
-
40.3
0.1
9.508
8.984
9.096
Ce@Ce-Fe
26.6
6.9
0.1
Ce@CeO2-Fe2O3
81.2
10.5
0.2
P represented precursors.

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Fig. 4 N2 absorbed-desorption isotherms of precursors (a) and catalysts (b).
Fig. 5 Pore size distribution curves of catalysts. (Inner picture was a comparison
about precursors)
3.3 Catalytic performance
The catalytic activity of these related Ce-Fe catalyst is shown in Fig. 6a.
According to Fig. 6a, the conversion of Ce-Fe catalyst could reach 80 % at 300 °C
and the conversion rate above 80 % could be kept up to 450 °C. Meanwhile, it was
clear that the operating temperature window was relatively wide. Based on Ce-Fe
catalyst, the amount of PVP was adjusted in the synthesis process, the concentration
of metal salt Ce as well as the molar ratio between Ce and Fe to obtain catalyst
Ce-Fe-PVP, Ce-Fe-C and Ce-Fe-M catalysts, respectively. For Ce-Fe-C catalyst, the
NO conversion could reach 80 % at 260 °C, and arrive at the maximum at 320 °C,

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which could reach 90 %. However, the operating temperature window wasDnOaI:r1r0o.1w03,9/D0NJ04419A
only changing from 260 °C to 360 °C. Specially, for the Ce-Fe-PVP and Ce-Fe-M
catalysts, changing the amount of PVP in the Ce-Fe catalyst and the molar ratio
between Ce and Fe would reduce the NO conversion as well as decrease the operating
temperature window. Therefore, according to above summary, the synthesis condition
of Ce-Fe catalyst was optimal. After exploring the synthesis conditions of Ce-Fe
catalyst, the method of CeO2 anchoring catalyst was adopted to promote the catalytic
activity and board the temperature window. As shown in Fig. 6b, CeO2 was anchored
on the Ce-Fe catalyst precursor and the calcined CeO2-Fe2O3 catalyst oxide to obtain
Ce@Ce-Fe and Ce@CeO2-Fe2O3 catalysts, respectively. In addition, three catalysts
were tested at a temperature of 120 °C to 500 °C. For Ce@Ce-Fe catalyst, it had a
higher NO conversion and a wider operating temperature window than the other two
catalysts in the NH3-SCR reaction. Specifically, the conversion rate of NO exceeded
80 % in the temperature range of 295 °C to 480 °C. In contrast, when the conversion
rate of NO exceeded 80 %, the working temperature window of Ce-Fe and
Ce@CeO2-Fe2O3 catalysts would become narrower. At the same time, the Ce@Ce-Fe
catalyst exhibited the best catalytic activity. When the temperature was 320 °C, the
NO conversion near arrived at 92 %. However, for Ce@CeO2-Fe2O3 catalyst, when
the temperature was 260 °C, the conversion rate of NO reached 80 %. Although the
low-temperature activity was excellent, the operating temperature window became
very narrow. In detail, when the temperature was higher than 280 °C, the conversion
rate of NO was less than 80 %. Therefore, among these three catalysts, the
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Ce@CeO2-Fe2O3 catalyst had the worst catalytic activity and the narrowest oDpOeIr: a10t.i1n03g9/D0NJ04419A
temperature window. Furthermore, Fig. S2 illustrates the N2 selectivity and NH3
conversion of the catalysts. As shown in Fig. S2, the N2 selectivity almost kept slight
decline during the whole test, indicating that all of the catalysts showed good N2
selectivity for NH3-SCR of NO. In terms of NH3 conversion, when temperature was
below 200°C for three catalysts, the NH3 oxidation was very low and maintained a
steady state. However, the temperature further increased to 240 °C, which leaded to a
significantly higher conversion of NH3 over Ce@CeO2-Fe2O3. Meanwhile, a similar
result was observed over Ce-Fe and Ce@CeO2-Fe2O3 catalysts in inner picture of
Fig.S2, which could be demonstrated that NH3 was mainly oxidized to NO at high
temperature. Furthermore, by combining the NO conversion results in the NH3-SCR
procedure with NH3 oxidation, the decline of catalytic activity at high temperature
was caused by the obvious increasing of oxidation of NH3 with different catalysts. In
summary, Ce@Ce-Fe catalyst had the best catalytic activity, board operating
temperature window, and higher N2 selectivity. Hence, it could be proved that Ce
anchored Ce-Fe catalyst precursor was the best experimental modification scheme.
Schematic S1 explains the fabrication of catalysts in supplementary information.
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(a)
(b)

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Fig. 6 NO conversion of different reaction temperature. Reaction conditions:D[ONI:O10].10=39/D0NJ04419A
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[NH3] =500 ppm, [O2] =5 vol. %, balance N2 and GHSV= 30,000 h-1.
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As we all know, air pollutants contained a part of water, so the resistance of
water for catalysts was very significant in industry. As could be observed from Fig.
7a, the NO conversion of three catalysts did not change significantly during the whole
resistance test of entire water. It was worth noting that the Ce-Fe catalyst could slowly
recover its NO conversion after H2O was turned off for two hours. For the
Ce@CeO2-Fe2O3 catalyst, the NO conversion fluctuated only slightly during the
entire process, and there was no obvious change. And two hours after stopping the
water flow, the conversion of NO could slowly recover to fresh condition. In addition,
Ce@Ce-Fe catalyst had almost negligible effect on NO conversion during the
resistance test of entire water, which indicated that Ce@Ce-Fe catalyst had good
resistance of H2O.
Fig. 7 NOx conversion under H2O conditions (a) and SO2 conditions (b).

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Generally, the sulfur dioxide in the flue gas had a fatal effect on the conversion
of NO in the NH3-SCR reaction, so it was necessary to explore the resistance of SO2
of catalyst. Among them, the test conditions of Ce-Fe, Ce@Ce-Fe and
Ce@CeO2-Fe2O3 catalysts were: 100 ppm SO2, 290 °C, GHSV = 30000 h-1.
According to Fig. 7b, it could be observed that the NO conversion of the Ce-Fe
catalyst decreased from 75 % to 68 % within two hours after the SO2 was introduced.
At the same time, the NO conversion of the Ce@Ce-Fe catalyst was reduced from
78 % to 72 % within two hours after the SO2 was introduced. Compared with the other
two catalysts, the Ce@Ce-Fe catalyst had good resistance of SO2 performance during
the whole test. Before introducing SO2, the NO conversion of Ce@Ce-Fe catalyst at
290 °C was 81 %, indicating good catalytic activity. However, once SO2 was
introduced, the NO conversion would drop sharply to 74 %. At the same time, in the
next 6 hours of the resistance of SO2 test, the NO conversion always fluctuated in the
range of 73 % to 75 %. It should be emphasized that during the last two hours of the
resistance of SO2 test, the conversion of NO hardly changed, which might be the
equilibrium state in which the catalyst was deactivated by the resistance of SO2
reaction. In addition, when we stopped passing SO2, the conversion of Ce@Ce-Fe
catalyst could gradually recover to 79 % in the next two hours, which indicated that
the catalyst had undergone competitive absorption between SO2 and NH3, resulting in
the temporary deactivation of the catalyst. However, a stability test of 24 h was
performed to examine the tolerance of SO2 for our best catalyst in Fig. S3a.
Furthermore, after long test of 24 h, the catalytic activity would basically keep stable.
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Compared with previous test, the catalytic activity would not recover at initial
standard. It might be concluded that the surface of catalyst was formed cerium sulfate
species, which was hard to decompose on the surface of catalyst, resulting in the low
conversion of NOx. According to Fig. S3b, when SO2 and H2O simultaneously added
into catalytic system, the NO conversion would be an obvious drop at once, indicating
that wet environment of SO2 played a negative role in catalytic activity. Meanwhile,
after four hours of test, the deNOx would keep stable until the end of test. Although,
after two hours of the end of test, the catalytic activity would be slowly rise, it would
not totally recover, indicating that the process was irreversible, which might be
explained that the surface of catalyst was formed cerium sulfate species, and
catalyst’s active sites were blocked by those sulfates and the specific area was
decreased. Accordingly, the NH4HSO4 would decompose more easily after SO2 and
H2O were removed, which resulted in the recovery of the catalysts’ activities to some
extent. Based on the above conclusions, it could be concluded that the Ce@Ce-Fe
catalyst had relatively satisfactory performance about the resistance of SO2 in the
NH3-SCR reaction.
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3.4 Catalytic reducibility

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Fig. 8 H2-TPR profiles of different catalysts (a), XPS spectra of O 1s (b), Ce 3d (c),
Fe 2p (d).
It was generally believed that the redox activity of the catalyst was critical to the
catalytic cycle in the NH3-SCR process. H2-TPR was an effective technique for
evaluating the reducibility of cerium and iron species on these three catalysts.
According to Fig. 8a, the three catalysts showed peaks at 440 and 650 °C, indicating
that the catalysts gradually reduced Fe2O3 to Fe metal [33]. The TPR curve of the
Fe-Ce sample attributed the low-temperature peak (about 400 °C) to the first-step
reduction of Fe3+ inside the Ce-Fe-O solid solution and the simultaneous reduction of
Ce4+ on the solid solution surface [34]. Meanwhile, the further reduction process
(reduction peaks at about 600 and 800 °C) was considered to be related to the

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intermediate reduction step of iron oxide and the overall reduction of Ce4+ [35]. In
addition, for Ce@CeO2-Fe2O3 catalyst, because its reduction temperature was the
lowest, so its catalytic activity of low temperature was the best. This conclusion was
consistent with the conversion of NO, as shown in Fig. 6b. Moreover, for Ce-Fe
catalyst and Ce@Ce-Fe catalyst, the peak intensity of Ce@Ce-Fe was stronger, and
the peak appearing at 600 °C was more obvious, indicating that there was mutual
synergy between Ce and Fe, which could be explained that Ce@Ce-Fe catalyst had
better catalytic performance and wider operating temperature window than Ce-Fe
catalyst [36].
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Table 2 XPS results for surface atomic concentration (%)
Oα /
(Oα +Oβ)
46.33
Ce3+/
(Ce3++Ce4+)
7.81
Sample
Ce
Fe
O
Fe/Ce Fe3+/Fe2+
Ce-Fe
13.35 11.91 74.65
14.90 11.94 73.11
0.89
0.80
1.49
2.49
1.83
1.83
Ce@Ce-Fe
58.33
19.47
Ce@CeO2-Fe2O3 17.97 11.00 71.02
Ce@Ce-Fe-U 4.32 4.32 78.87
35.20
13.28
0.61
12.48
61.90
18.27
U represented the tolerance of H2O and SO2.
XPS analysis was performed on Ce-Fe, Ce@Ce-Fe and Ce@CeO2-Fe2O3
catalysts to clarify the surface atomic concentration and valence state of all elements
in these three catalysts. The surface atomic concentrations of Fe, Ce and O are
measured and summarized in Table 2. The corresponding XPS spectra of Fe, Ce and
O are shown in Fig. 8b-d. As shown in Fig. 8b, the XPS spectrum of O 1s was fitted

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to two peaks, one of which had a lower binding energy (529.5 eV) correspoDnOdIi:n10g.10t3o9/D0NJ04419A
lattice oxygen (expressed as Oβ) and the other was located at a higher binding energy
(531.7 eV) corresponded to chemisorbed oxygen (denoted as Oα) [37]. Chemically
adsorbed oxygen (Oα) was a better active oxygen because it had a higher mobility and
acted a significant role in the SCR reaction. It could be observed from Table 2 that the
percentage of Oα species on the surface of Ce@Ce-Fe (58.33 %) was much higher
than that of Ce-Fe (46.33 %), which was attributed to the structural difference of the
Ce anchoring catalyst. In addition, the percentage of Oα species on the surface of
Ce@Ce-Fe was also higher than the percentage of Ce@CeO2-Fe2O3 (35.20 %).
Hence, Ce@Ce-Fe had the best catalytic activity among three catalysts. In summary,
chemically adsorbed oxygen (Oα) played a key role in the NH3-SCR reaction.
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Fig. 8c shows Ce 3d XPS spectra of three catalysts. The XPS spectrum of Ce 3d
usually was consisted of eight peaks, namely v (882.4 eV), v' (885.9 eV), v'' (889.8
eV), v‴ (898.7 eV), u (901.3 eV), u' (904.5 eV), u'' (907.8 eV) and u‴ (917.1 eV).
Among them, the peaks of v, v'', v‴, u, u” and u‴corresponded to the initial electronic
state of 3d104f0 of Ce4+ species, while the peaks of v′ and u′ were due to the initial
electronic state of 3d104f1 corresponding to Ce3+ species [38]. Generally, a higher
Ce3+ ratio indicated that there were more oxygen vacancies on the surface of the
catalyst, which might facilitate the conversion of active oxygen species, increase the
oxidation of NO to NO2, and promote the adsorption and activation of reactants.
Table 2 summarizes the surface concentration of Ce3+ on the surface of Ce-Fe,
Ce@Ce-Fe and Ce@CeO2-Fe2O3. It could be observed that the percentage of Ce3+ on

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the surface of Ce@Ce-Fe (19.47 %) was much higher than the percentage of Ce-Fe
(7.81 %). After Ce was anchored on the Ce-Fe catalyst, the Ce3+ surface concentration
was higher than the Ce-Fe surface concentration, indicating that Ce successfully
anchored the surface of Ce-Fe catalyst and the catalytic activity got improved after Ce
anchoring. At the same time, oxygen vacancies on the catalyst could be generated by
the electron conversion between Ce3+ and Ce4+, and higher Ce3+ concentration could
form more oxygen vacancies, indicating that the oxygen vacancy of Ce@Ce-Fe was
much more than that of Ce-Fe.
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As displayed in the high-resolution XPS spectrum of Fe 2p (Fig. 8d), the peaks
located at 711.4 and 725.2 eV, along with two shake-up satellite peaks at 717.9 and
732.5 eV, could be attributed to Fe3+, and another two peaks centered at 710.5 and
723.8 eV could be ascribed to Fe2+ [39]. Apparently, the Fe element was present in the
chemical state of Fe3+ and Fe2+. For the Ce@Ce-Fe catalyst, the peak shifted towards
a higher binding energy than Ce-Fe, which indicated that Ce was anchored on the
Ce-Fe catalyst and the interaction between Ce and Fe occurred on the surface of
catalysts [31]. Meanwhile, Ce@Ce-Fe catalyst had highest the ratio of Fe3+/Fe2+
(2.49) among three catalysts. According to previous reports, the available Fe3+ could
participate in the reversible redox cycle, which was beneficial to the SCR catalytic
activity. In addition, a more appropriate Fe3+ ratio was conducive to the reduction of
NO, thereby improving the catalytic activity. In summary, Ce@Ce-Fe catalyst with
higher surface atomic concentration of Fe (11.94 %) and Fe3+/Fe2+ (2.49) had better
NH3-SCR performance.

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3.5 Surface acidities
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Fig. 9 NH3-TPD profiles of different catalysts.
Fig. 10 The relationship of surface acid and surface area among three catalysts.
Surface acidities were beneficial to the SCR catalysts because of efficient
absorption of NH3. Therefore, NH3-TPD was evaluated to investigate the effect of
surface acidities on the performance of catalysts for NH3-SCR reaction. In addition,
Ce-Fe, Ce@Ce-Fe and Ce@CeO2-Fe2O3 catalyst profiles are presented in Fig. 9,
which could be divided into four or five peaks ranged from 100 °C to 700 °C.

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Notably, for Ce@Ce-Fe catalyst, the peaks at 240 °C was assigned to middlDeOsI:t1r0o.1n03g9/D0NJ04419A
acid, which was considered to be significant for NH3-SCR reaction. Meanwhile, for
Ce@CeO2-Fe2O3 catalyst, the peak at 200°C was ascribed to weak acid. As a
comparison, for Ce-Fe catalyst, it only appeared strong acid, owing weak surfaced
acidities. Notably, the amount of ammonia desorbed had been determined and
normalized with respect to the specific surface area, which are shown in Table 3 and
Fig. 10. On the one hand, it was clear that Ce@Ce-Fe had the biggest surface acid and
average acid by calculation from surface area, indicating that surface acid sites acted a
significant role in the process of NH3-SCR reaction. On the other hand, the NH3-TPD
results demonstrate that the Ce@Ce-Fe catalyst present not only the larger acid
amount but also stronger acid strength, which could facilitate the chemisorption and
activation of NH3, resulting in the better performance in the NH3-SCR of NO.
Therefore, it was worth noting that the amount of surface acidities with weak and
middle strong acid obviously increased after the anchoring of Ce on Ce-Fe catalyst. In
summary, abundant surface acidities mainly including weak and middle strong acid
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sites might be essential for a better catalytic activity in NH3-SCR system.
Table 3 The amount of surface acid sites.
Weak acid
Middle strong acid
Sum area
(a.u.)
Ta
Area
Ta
Area
(a.u.)
54
sample
(°C)
(a.u.)
(°C)
266
237
320
285
Ce-Fe
-
-
-
-
54
118
62
Ce@Ce-Fe
118
26
Ce@CeO2-Fe2O3
Ce@Ce-Fe-U
200
-
36
-
21
21

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a The peak at temperature (°C), U represented the tolerance of H2O and SO2.
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3.6 The physical and chemical properties of catalysts after enduring SO2 test
The FTIR test is conducted to explore the effect of H2O and SO2 on catalytic
performance in Fig. 10. Firstly, the peak appearing at 3430 cm-1 might be caused by
the surface hydroxyl material (O-H), and the vibration of N-H might appear at 3184
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cm-1 through the vibration area [40]. In addition, the band vibration of NH4 might
occur at 1725 cm-1 belonging to the Bronsted acid site [41]. Meanwhile, the peak
between 1000 and 1300 cm-1 corresponded to the tensile vibration of the C-O bond,
which might be likely related to the presence of carbonates. Furthermore, the bands
appearing at 1515 cm-1 and 1329 cm-1 were attributed to the Lewis acid site and the
coordination NH3 on the Lewis acid site, respectively [42]. Meanwhile, the symmetric
bending vibration of NH4+ species might appear at 2957 cm-1, which indicated that the
adsorption of NH3 was formed on the acidic sites of the surface [43]. After the
reaction, a small band at 619 cm-1 could be assigned to the new Fe-O phase formed on
the surface of the Fe-Ce catalyst [44]. After the reaction of H2O and SO2, a new peak
2–
at 1122 cm-1, which corresponds to the characteristic band of SO4 [45]. Therefore,
the introduction of SO2 leaded to the formation of by-products such as (NH4)2SO3 and
NH4HSO4. Because the sulfate formed occupied the active site on the surface of the
Ce@Ce-Fe catalyst, the NO conversion might decrease, which was mainly related to
the deposition of ammonium sulfate or ammonium sulfite.

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Fig. 11 FTIR spectra of catalysts：fresh catalysts and after SO2 test.
After H2O and SO2 enduringtest, the chemical state of element is also explored to
compare Ce@Ce-Fe and Ce@Ce-Fe-U catalyst by XPS methods in Table 2 and Fig.
12. Apparently, compared with fresh catalyst, Oα ratio of Ce@Ce-Fe catalyst
decreased after enduring SO2 test at 290 °C, which might be an important reason for
the decline of catalytic activity. Another reason was that SO2 could react with H2O
and NH3 to form NH4HSO4, which could block the active sites. Meanwhile, on the
one hand, the surface atomic ratio of Ce3+/ (Ce3+ + Ce4+) was slightly lower than fresh
catalyst, illustrating that the oxide of Ce3+ species was restrained. Furthermore, SO2
with Ce3+ could easily formed cerium sulfate species, which was hard to decompose
on the surface of catalyst, which might result in the low conversion of NOx. On the
other hand, the influence of SO2 on acidic properties is explored in Table 3 and Fig.
S4. Obviously, the effective acid amount would be declined, indicating that SO2
exposure led to significant changes on the acidic properties. According to above
analysis, the declination of Oα ratio, the formation of NH4HSO4 and cerium sulfate

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species might lead to low catalytic activity.
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Fig.12 XPS spectra of O 1s and Ce 3d after H2O as well as SO2 test.
4 Conclusions
In summary, we had successfully fabricated Ce@Ce-Fe Prussian blue analogues
with unique morphology and structure as good performance deNOx catalysts via
anchoring of Ce on the surface of Ce-Fe, which were derived from Ce-Fe precursors
synthesized via a simple co-precipitation method. The double-cone structure
Ce@Ce-Fe catalyst displayed enhanced catalytic activity, wider working temperature
window and improved tolerance of H2O and SO2. It has been demonstrated that the
characteristic of unique double-cone structure, the uniform distribution of active sites,
as well as the strong redox performance were attributed to the enhanced catalytic
performance of Ce@Ce-Fe catalyst in NH3-SCR. The feature of unique morphology
and mesoporous structures supplies a larger surface area and more active sites to
adsorb and activate reagents, resulting in the higher catalytic activity. Moreover,
excellent redox performance and high content of Fe atoms resulted in high catalytic
cycle stability and improved SO2 tolerance. Meanwhile, acidic properties also acted a