Investigation of Fe?Ni Mixed-Oxide Catalysts for the Reduction of NO by CO: Physicochemical Properties and Catalytic Performance

Article abstract of DOI:10.1002/asia.201900782

A series of Fe?Ni mixed-oxide catalysts were synthesized by using the sol–gel method for the reduction of NO by CO. These Fe?Ni mixed-oxide catalysts exhibited tremendously enhanced catalytic performance compared to monometallic catalysts that were prepared by using the same method. The effects of Fe/Ni molar ratio and calcination temperature on the catalytic activity were examined and the physicochemical properties of the catalysts were characterized by using XRD, Raman spectroscopy, N₂-adsorption/-desorption isotherms, temperature-programmed reduction with hydrogen (H₂-TPR), temperature-programmed desorption of nitric oxide (NO-TPD), and X-ray photoelectron spectroscopy (XPS). The results indicated that the reduction behavior, surface oxygen species, and surface chemical valence states of iron and nickel in the catalysts were the key factors in the NO elimination. Fe_0.5Ni_0.5O_x that was calcined at 250 °C exhibited excellent catalytic activity of 100 % NO conversion at 130 °C and a lifetime of more than 40 hours. A plausible mechanism for the reduction of NO by CO over the Fe?Ni mixed-oxide catalysts is proposed, based on XPS and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses.

Full text of DOI:10.1002/asia.201900782

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Investigation on Fe-Ni mixed oxide catalysts for NO reduction by
CO: Physicochemical properties and catalytic performance
Authors: Xuexun Du, Tian-Liang Yao, Qinglian Wei, Hao Zhang, and
Yongmin Huang
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Chemistry - An Asian Journal
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Investigation on Fe-Ni mixed oxide catalysts for NO reduction by CO: Physicochemical
properties and catalytic performance
a
b
a
a
a
Xuexun Du , Tian-Liang Yao , Qinglian Wei , Hao Zhang , Yongmin Huang *
Abstract: A series of Fe-Ni mixed oxide catalysts were synthesized
Iron catalysts have been reported to present a remarkable
catalytic performance in NO + CO reaction on account of the
tremendous redox capacity.[11] Robert et al. found that the redox
active Fe3+ ↔ Fe2+ centers took responsibility for the steady-state
activity.[12] Dong et al. reported the polymeric Fe3+ clusters were easier
to be reduced into Fe2+ than isolated Fe3+ ions under CO atmosphere,
leading to high activity of NO abatement.[13] Nevertheless, mono iron
catalyst always displays poor catalytic performance for NO elimination
at low temperatures. [11,14,15] Nickel catalysts also exhibit an excellent
activity in NO + CO reaction, especially for Ni-Ce catalysts, which can
eliminate NO completely below 200 °C. For instance, Wang et al.
prepared nickel catalysts by using different supports and the catalytic
through a sol-gel method for NO reduction by CO. Compared to
monometallic catalysts prepared using the same method, Fe-Ni mixed
oxide catalysts revealed a tremendous enhancement in catalytic
performance. The effects of Fe/Ni molar ratio and calcination
temperature on catalytic activity were examined. The
physicochemical properties of catalysts were characterized by XRD,
2 2
Raman, N adsorption-desorption, H -TPR, NO-TPD and XPS. The
results indicated that the reduction behavior, surface oxygen species
and surface chemical valence states of Fe and Ni in catalysts are the
x
key factors for NO elimination. The Fe0.5Ni0.5O sample calcined at
250 °C exhibited an excellent catalytic activity of 100% NO conversion
2 2 3 2
> NiO/γ-Al O > NiO/TiO
.[16]
at 130 °C and lifetime more than 40 h. A possible mechanism for NO
reduction by CO over Fe-Ni mixed oxide catalysts was proposed
based on the XPS and in-situ DRIFTS analyses.
activity decreased in order of NiO/CeO
Tang et al. synthesized NiO-CeO using a novel solid state method,
which displayed an entire NO elimination capacity at 175 °C.[17] Tang
et al studied the effect of exposed facets of CeO in NiO/CeO
catalysts and found that the strong interaction between NiO and CeO
2
2
2
2
Introduction
surface and the Ni2+ in the Ni-Ce-O structure are the crucial factors for
the low-temperature catalytic activity.[18] However, for these literature,
the lifetime of catalysts and the effect of water were seldom reported
Although renewable energy source, such as solar and wind
energy, have been developing rapidly, fossil fuels are still the
essential energy source for industrial application and daily life.
Nitric oxide (NO) derived from fossil fuel combustion process is
since the nickel catalysts always show poor stability and H
2
O
resistance.
one of the major air pollutions, which causes environmental
problems such as greenhouse effect and photochemical smog.[1]
In this work, we prepared Fe-Ni mixed oxide catalysts via a
sol-gel method. The effects of Fe/Ni molar ratio and calcination
temperature on catalytic performance for NO + CO reaction were
For decades, NO abatement has gained more attention owing to
the rigorous environmental regulations. NO reduction by different
x
examined. The 250 °C calcined Fe0.5Ni0.5O -250 catalyst ( Fe/Ni
reducing agents (NH
3 2
, hydrocarbon, CO and H ) is an effective
strategy to eliminate NO.[2-5] Among the various de-NO
molar ratio is 5/5) displayed a superb NO elimination performance,
the complete NO conversion temperature was as low as 130 °C,
and the 100% NO conversion could be maintained over 40 h. To
the best of our knowledge, this is the first study to report the Fe-
Ni mixed oxide catalysts for NO reduction by CO at such low
temperature. The influence of weight hour space velocity (WHSV),
CO concentration and water on catalytic performance over
x
technologies, NO reduction by CO is likely to be one of the most
feasible ways to solve the problem, since CO is always a
byproduct accompanying the generation of NO in vehicle engine,
as well as many stationary applications such as steel and coking
factories.[6]
Noble metals such as Au,[7] Rh,[8] Pt [9] and Pd [10] are
Fe0.5Ni0.5
O
x
-250 were further examined. In comparison of
frequently employed to prepare noble metal catalysts for NO +
CO reaction. However, high cost, low abundance and poor
thermal stability inhibit their large-scale application. Hence, many
studies have been focusing on searching for the alternatives of
noble metal. Under such backgrounds, transition metal oxides,
particularly iron, cobalt, nickel and copper oxides, have received
increasing attention, because they are cost-effective and easily
available.
previous studies, the catalytic performance of Fe0.5 -250 is
0.5 x
N O
higher than that of most reported non-precious metal catalysts,
even comparable to precious metal catalysts. Based on the
experiment results, the correlation between physiochemical
properties and catalytic performance were discussed and a
possible mechanism was proposed.
[
a] X.Du, Q.Wei, H.Zhang and Y.Huang
Key Laboratory of Specially Functional Polymeric Materials and Related
Technology
Results and Discussion
Catalytic activity
School of Chemistry and Molecular Engineering
East China University of Science and Technology
Shanghai 200237 （P. R. China）
Figure 1a displays the NO conversion of NiO
x
, FeO
x
and
-300 catalysts at the WHSV of 15000 ml•g •h . The NiO
catalyst presents a negligible NO elimination efficiency below
50 °C, the complete NO conversion temperature (T100NO) is
higher than 300 °C. The FeO sample exhibits better catalytic
performance than that of NiO , and the T100NO is 200 °C.
Compared to the Fe-only and Ni-only catalysts, all the Fe-Ni
E-mail: huangym@ecust.edu.cn
-1 -1
Fe
a
Ni
b
O
x
x
[
b] T Yao
Shanghai Institute of Space Propulsion,
1
Shanghai 201112（P. R. China）
x
Supporting information for this article is given via a link at the end of the
document.
x
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mixed oxide catalysts show higher catalytic activities. In detail, the
complete NO conversion temperature of Fe0.7Ni0.3
60 °C. With the decrease of Fe/Ni molar ratio, the T100NO exhibits
a volcano-type tendency, the T100NO of Fe0.5Ni0.5 -300 and
-300 are 140 and 170 °C, respectively. Figures 1b and
c show the N O selectivity and CO conversion of the catalysts
mentioned above. Since N yield (%)= NO conversion × (1- N
selectivity) ×100%, when the NO conversion is 100% and N
selectivity is 0%, the N yield is 100%. Table 1 shows the
temperatures of 100% N yield (T100N2), complete NO conversion
100NO) and CO conversion (T50CO) of each sample. As a result,
-300 possesses the highest catalytic performance,
x
O -300 is
1
O
x
Fe0.3Ni0.7
O
x
1
2
2
2
O
2
O
2
2
(T
Fe0.5Ni0.5
O
x
with the T100NO at 140 °C, T100N at 180 °C and T50CO at 180 °C,
2
which means that the NO can be eliminated completely at 140 °C
and converted to N
the significant effect of calcination temperature,[19] we further
examined the catalytic performance of Fe0.5Ni0.5 samples with
varied calcination temperatures. The results are shown in Figure
. As the calcination temperature rises from 200 °C to 350 °C, the
catalysts show a volcano-type tendency in
-300
-250 displays the optimal low-
temperature catalytic performance (T100NO = 130 °C, T100N2=
60 °C). Either increasing (300 and 350 °C) or decreasing
200 °C) the calcination temperature will lead to decreased
2
entirely at 180 °C. On this basis, considering
O
x
2
T100NO of Fe0.5Ni0.5O
x
the order of Fe0.5Ni0.5
x x x
O -200 > Fe0.5Ni0.5O -350 > Fe0.5Ni0.5O
>
Fe0.5Ni0.5 -250. The Fe0.5Ni0.5O
O
x
x
1
(
activity. In addition, since the Fe-Ni oxide catalysts were prepared
from metal nitrates, the lowest activity, which belongs to
Fe0.5Ni0.5
x
O -200, should be ascribed to the incomplete
decomposition of metal nitrates at such low temperature.
XRD
Figure 3a shows the XRD patterns of the NiO
Ni mixed metal oxide catalysts calcined at 300 °C. For the NiO
sample, both metallic Ni (JCPDS-04-0850) and NiO phases
JCPDS-78-0423) can be observed, indicating the product should
be a composite of metallic nickel and nickel oxide. The FeO
sample displays both hematite (-Fe , JCPDS-33-0664) and
maghemite (-Fe , JCPDS-39-1346). For the Fe Ni -300
-300, whose molar ratio of Fe/Ni is 7/3,
presents the mixed phases of NiFe (2θ = 30.3°, 35.7° and
7.4°, JCPDS-10-0325) and NiO (2θ = 43.3° and 62.9°, JCPDS-
7-1049). As the Fe/Ni molar ratio decreases to 5/5, the NiFe
x x
, FeO and Fe-
x
(
x
2
O
3
2
O
3
a
b x
O
samples, Fe0.7Ni0.3
O
x
2
O
4
5
4
2
O
4
diffraction peak becomes broad, which may cause by the strain.[20]
When the Fe/Ni molar ratio continues to decrease to 3/7, only NiO
phase can be observed (2θ = 37.3°, 43.3° and 62.9°), implying
the high dispersion of NiFe
patterns of Fe0.5Ni0.5
temperatures. All the samples present diffraction peaks of
NiFe and NiO phases.
2
O
4
phase. Figure 3b displays the XRD
O
x
catalysts calcined at different
2
O
4
a b x
Fig 1. Catalytic performance of (a-c) Fe Ni O -300 with different Fe/Ni molar
ratios. (Reaction conditions: [NO] =1000 ppm, [CO] =2000 ppm, [N
T = 100 - 300 °C; WHSV = 15000 ml•g-1•h-1)
2
] = balance;
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a b x
Fig 3. XRD patterns of (a) Fe Ni O -300 with different Fe/Ni molar ratios; (b)
Fe0.5Ni0.5
O
x
with different calcination temperatures.
Raman
Raman spectroscopy, an effective complement to acquire
additional information that cannot be obtained by XRD, was
employed to study the oxide phases of the as-prepared samples,
and the results are shown in Figure 4. From the Raman spectra
-1
of FeO
former one is associated with the A1g vibrational modes of Fe
and the latter origins from the red-shift for the Eg modes of
Fe sample, since nickel metal shows no
.[21] For the NiO
Raman signal, the band appears at 497 cm-1 is ascribed the Ni-O
vibration modes.[22] As the Fe
Ni -300 were synthesized, the
Raman spectra are different. For the Fe0.7Ni0.3 -300 sample, the
band at 497 cm is related to the NiO signal, and the two bands
x
(Figure 4a), two bands appear at 216 and 283 cm , the
2
3
O ,
2
O
3
x
a
b x
O
O
x
-1
−
1
appear at 315 and 688 cm are associated with the generation of
Fig 2. Catalytic performance of Fe0.5Ni0.5
temperatures. (Reaction conditions: [NO] =1000 ppm, [CO] =2000 ppm, [N
balance; T = 100 - 300 °C; WHSV = 15000 ml•g •h )
O
x
with different calcination
] =
[23]
NiFe
2
O
4
.
With the continuing decrease of Fe/Ni molar, the band
2
-
1
-1
-1
at 497 cm for the Fe0.7Ni0.3O -300 shifts to higher wavenumbers.
x
For the Fe-Ni mixed oxide, the addition of iron can cause the red-
shift of Ni-O band, which has been reported on bulk Ni3Fe
oxides.[24] Thus, the bands at 558 and 571 cm-1 for the
Fe0.5Ni0.5
O
x
-300 and Fe0.3Ni0.7
x
O -300 can be attributed to the Ni-
O band. For the Fe0.5Ni0.5
O
x
samples calcined at different
temperatures, the Raman spectra are similar (Figure 4b), all the
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-
Fig 4. Raman spectra of (a) Fe
a b x
Ni O -300 with different Fe/Ni molar ratios; (b)
Fe0.5Ni0.5
1
x
O samples exhibit two main bands, NiO at 545-555 cm
Fe0.5Ni0.5
O
x
with different calcination temperatures.
-1
andNiFe
2
O
4
at 685-690 cm , indicating the mixed phases of NiO
and NiFe O , which is in accordance with the XRD result.
2 4
2
N adsorption-desorption
Table 1. Catalytic performance and specific surface areas of catalysts.
2
The N adsorption-desorption isotherms of as-prepared
catalysts are shown in Figure S1, all the samples display the
distinct hysteresis loops in the P/P0 from 0.4 to 1.0, implying the
2
a
Sample
FeO
NiO
T
100NO (°C)
T
100N (°C)
2
T
50CO (°C)
S
BET (m /g)
x
200
> 300
150
140
170
180
130
150
> 200
> 300
200
250
> 300
200
95
existence of mesoporous structure. The specific surface areas
2
(
S
BET) are summarized in Table 1. The SBET of FeO
x
is 95 m /g,
x
35
2
while for NiO
x
, it is only 35 m /g. In comparison with monometallic
Fe0.7Ni0.3
Fe0.5Ni0.5
Fe0.3Ni0.7
Fe0.5Ni0.5
Fe0.5Ni0.5
Fe0.5Ni0.5
O
O
O
O
O
O
x
-300
x
-300
x
-300
x
-200
x
-250
x
-350
142
177
152
181
209
168
catalysts, the Fe-Ni mixed oxide catalysts have higher specific
180
180
surface areas. The SBET of Fe0.7Ni0.3
Fe0.3Ni0.7
O
x
-300, Fe0.5Ni0.5
O
x
-300 and
2
O
x
-300 are 142, 177 and 152 m /g, respectively. The
200
200
x
specific surface areas of Fe0.5Ni0.5O samples with varied
> 200
160
> 200
160
calcination temperatures are also displayed in Table 1. With the
increase of calcination temperature, the specific surface area of
Fe0.5Ni0.5
O
x
first increases, then decreases, exhibiting that the
sample calcined at 250 °C possesses the highest SBET (209 m2
g). A large SBET is beneficial for the adsorption of NO and CO,
which can elevate the catalytic performance.[25] Thus, the highest
BET is a reason for the optimal NO elimination capacity of
-250.
170
170
/
S
[
a] Determined by the BET method
Fe0.5Ni0.5
O
x
2
H -TPR
2
H -TPR was then tested for the reducibility of the obtained
catalysts. The results of pure FeO
x
, NiO
x
and Fe
a
Ni
b
O
x
calcined
exhibits
at 300 °C are shown in Figure 5a. The profile of FeO
x
three reduction peaks at 270 °C, 328 °C and 625 °C,
corresponding to the three steps of reduction procedure
(
Fe
composition of FeO
and Raman results. For the NiO
2
O
3
→Fe
3
O
4
→FeO→Fe0 ),[19,26] suggesting that the
is pure Fe , which is in line with the XRD
, two major reduction peaks
x
2 3
O
x
emerge at 265 °C and 300 °C, the former one is related to the
reduction of NiO to Niδ+ and the latter is assigned to the reduction
of Niδ+ to Ni0. [27, 28]
When nickel was introduced into the Fe-only catalyst, the H
TPR profiles are different from those of either FeO or NiO . For
-300 sample, three reduction peaks can be
2
-
x
x
the Fe0.7Ni0.3
O
x
observed at 300-600 °C. The low-temperature peak  is
0
associated with the reduction of NiO (NiO→Ni ), the peak  due
to the reduction of Ni2 to Ni and part of Fe to Fe2+ in NiFe
+
0
3+
O
4
2
0
phase (NiFe
2
O
4
→Ni + Fe
3
O
4
), the peak  at higher temperature is
(Fe
→FeO→Fe0).[29,30] As the
Fe/Ni molar ratio is 5/5, both the reduction peak  and  of
related to the reduction of Fe
3
O
4
3 4
O
Fe0.5Ni0.5
Fe0.7Ni0.3
O
x
-300 shift to a lower temperature than that of
-300, appearing at 320 °C and 371 °C, respectively.
O
x
Further reducing the Fe/Ni molar ratio to 3/7, the Fe0.3Ni0.7
sample shows the reduction peak  at 330 °C and the peak  at
80 °C. Apparently, with the decrease of Fe/Ni molar ratio, the
x
O -300
3
reduction peaks  and  firstly shift to low temperature, and then,
shift to high temperature. The reducing capacity is a crucial factor
to influence the catalytic performance in NO + CO reaction.[31, 32]
When metal oxide catalyst is exposed to the NO-CO mixture gas,
the high-state metal species of catalyst can be reduced by the CO
to form low-state metal species and surface oxygen vacancies
(SOVs), which provide NO dissociation site and further facilitate
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the NO conversion. Compared with Fe0.7Ni0.3
x
O -300 and
Fe0.3Ni0.7
x x
O -300 samples, Fe0.5Ni0.5O -300 shows a lower
temperature of reduction peak  (320 °C) and  (371 °C),
indicating that the Fe-O-Ni structure is easier to be reduced to
generate SOVs, lead to the higher efficiency of NO elimination.
2 x
The H -TPR profiles of Fe0.5Ni0.5O samples with varied
calcination temperatures are shown in Figure 5b. All the profiles
are similar, showing three reduction peaks between 250 °C and
550 °C. When calcination temperature is 200 °C, the peaks  and

x
on Fe0.5Ni0.5O -200 occur at 335 and 380 °C. As the calcination
temperature increases, the position of the peaks exhibits a
volcano-type tendency. For Fe0.5Ni0.5 -250, it shows the lowest
temperature of the peaks  and  among all the Fe0.5Ni0.5
O
x
O
x
catalysts, signifying the easiest reducibility and more SOVs can
be formed, resulting in the high catalytic activity at low
temperature.
NO-TPD
It is well known that the NO adsorption/desorption ability is a
crucial factor for the NO reduction by CO.[33] Figure 6 displays the
x x x x
NO-TPD profiles of FeO , NiO , Fe0.5Ni0.5O -300 and Fe0.5Ni0.5O -
250. Four peaks can be observed over the mentioned catalysts.
The stripping peaks N1 below 200 °C are related to desorption of
bridged nitrates.[34] The N2 peaks at about 275 °C are assigned
to desorption chelating nitrates and nitrites.[35] The N3 and N4
peaks above 300 °C are associated with the decomposition of
surface nitrite and nitrate.[36] For NiO
catalyst, an obvious N2
x
peak can be observed at 275 °C, indicating the presence of
chelating nitrates and nitrites on surface. The FeOx sample shows
a strong N3 peak at 420 °C and a weak N1 peak below 200 °C,
implying a small amount of bridged nitrates and a large number of
nitrite and nitrate species on the surface. For the bi-metal samples,
unlike the monometallic ones, only two apparent peaks can be
seen below 300 °C, suggesting the NO adsorption species are
mainly bridged nitrates, chelating nitrates and nitrites. Besides,
Fig 5. H
2
-TPR profiles of (a) Fe
a
Ni
b
O
x
-300 with different Fe/Ni molar ratios;
(
x
b) Fe0.5Ni0.5O with different calcination temperatures.
although the NO-TPD profiles of Fe0.5Ni0.5
O
x
-250 and Fe0.5Ni0.5
x
O -
300 are similar, the N1 and N2 peaks for Fe0.5Ni0.5
O
x
-250 are
more prominent than that for Fe0.5Ni0.5
x
O -300, revealing the
stronger chemisorption of NO at low temperature. Based on the
NO-TPD results, the intensity and area of N1 peak for these four
catalysts is arranged in the following order: Fe0.5Ni0.5
x
O -250 >
Fe0.5Ni0.5
O
x
-300 > FeO > NiO , in agreement with the NO
x
x
elimination performance in Figure 1. Therefore, we consider the
surface bridged nitrates plays an important role in the NO + CO
reaction, the introduction of nickel species into the Fe catalyst can
enhance the NO adsorption capacity at low temperature.
Compared with other catalysts, more bridged nitrates species on
the Fe0.5Ni0.5
x
O -250 sample’s surface can participate in the low-
temperature NO + CO reaction, resulting in enhanced catalytic
performance.
x x x x
Fig 6. NO-TPD profiles of FeO , NiO , Fe0.5Ni0.5O -250 and Fe0.5Ni0.5O -300.
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3
+
2+
2+
3+
XPS
x
type tendency, the Fe /Fe and Ni /Ni ratios of Fe0.5Ni0.5O -
2
50 are 1.35 and 1.22, higher than that of Fe0.5Ni0.5
-300 and Fe0.5Ni0.5 -350. From the XRD and Raman
samples present the mixed phases of
and NiO. Since NiFe is an inverse spinel compound,
shows the mixing valences of Fe and Ni. Therefore,
x
O -200,
XPS was utilized to investigate the surface elemental
composition and chemical valence states of the as-prepared
samples. Figure 7 shows the O 1s, Fe 2p and Ni 2p XPS spectra
Fe0.5Ni0.5
O
x
O
x
a b x
results, the Fe Ni O
NiFe
2
O
4
2 4
O
of FeO
s XPS spectra could be classified into two distinct peaks. The
peak at 529.3-529.7 eV stems from lattice oxygen O2- (O
); the
peak at around 530.9 eV is attributed to surface oxygen (O ),
including hydroxyl species and surface-adsorbed oxygen (O
2-/
x
, NiO
x
and Fe
a
Ni
b
O
x
catalysts calcined at 300 °C. The O
[48]
NiFe
2
O
4
1
Fe , Fe , Ni and Ni3+ can be observed on the Fe
2+
3+
2+
a
b x
Ni O surface
L
by XPS.
S
According to the result mentioned above, the Fe-Ni mixed
oxides with varied Fe/Ni molar ratios and calcination
temperatures display different surface chemical valence states.
2
O-).[37, 38] According to previous work,[13, 39-41] surface oxygen
species is a crucial factor for the NO reduction by CO, which can
oxidize CO and promote the adsorption of NO. The proportion of
3+
2+
Interestingly, the highest Fe , Ni and O
S
proportions belong to
Fe0.5Ni0.5
O
x
-250, which also shows the highest NO elimination
each oxygen species is shown in Table 2. The O
S
proportions of
-300 are
6.72%, 60.78% and 50.0%, obviously higher than that of FeO
27.17%). Particularly, the Fe0.5Ni0.5 -300 shows the highest O
capacity among all the as-prepared catalysts. Thus, we speculate
that the catalytic performance is not only correlated with the
Fe0.7Ni0.3
x x x
O -300, Fe0.5Ni0.5O -300 and Fe0.3Ni0.5O
4
x
abundant surface oxygen species but also related to the high
percentage of Fe3+ and Ni2+ on the surface of the catalyst. It is
(
O
x
S
proportion among the three Fe-Ni mixed oxide catalysts, and it
also shows the optimal NO elimination efficiency. On the other
reported that an interaction between different species in the metal
mixed oxide catalysts, namely, redox cycle, plays an essential
role for the NO reduction by CO. For instance, the redox cycle
hand, the O
S
proportions of NiO
x
(47.53%) and Fe0.7Ni0.3
x
O -300
(46.72%) are similar, but NiO
x
shows particularly poor NO
such as Cu + Mn3+ ↔ Cu + Mn [31], Ce3+ + Cu2+ ↔ Ce4+
2+
+
4+
+
2+
removal capacity below 200 °C. That is because, in the Fe-Ni
mixed oxide catalyst, the oxygen bonded by Fe and Ni ions,
namely surface synergetic oxygen, is more active than the oxygen
+ [34, 49]
3+
3+
4+
2+ [50]
3+
2+
Cu ,
Ce + Fe ↔ Ce + Fe ,
Fe3+[51] can promote the catalytic performance
and Co + Fe ↔ Co +
As we know, the
.
electronegativity of Ni (1.91) is larger than that of Fe (1.83), and
electrons may migrate from Fe2+ to Ni3+ to generate Fe3+ and Ni2+
only boned by Ni ions in NiO
spectra. For FeO , the peaks near 710 eV and 724 eV are
x
.[32] Figure 7b shows the Fe 2p XPS
x
in the catalysts. Hence, we propose a solid-state charge-transfer
redox couple of Fe2+ + Ni3+ ↔ Fe3+ + Ni2+ on Fe-Ni mixed oxides,
ascribed to the Fe 2p3/2 and Fe 2p1/2, respectively. The separation
of the two peaks is 13.6 eV, implying that the main composition is
and the redox cycle can shift to right through varying the Fe/Ni
molar ratio and calcination temperature, which can enhance the
catalytic activity for NO elimination.
Fe
Fe
2
O
3
,[42] consistent with the XRD and Raman results. For the
a
Ni O -300 catalysts, the peaks of Fe 2p3/2 and Fe 2p1/2 appear
b x
at about 710 and 724 eV, respectively, and they are further
2
+
deconvoluted into Fe (710.3-710.6 eV, 723.6-724.0 eV ) and
3+
[43-45]
Table 2. Surface compositions of as-prepared catalysts.
Fe ( 712.2-712.7 eV, 726.0-726.3 eV ).
18 and 732 eV are related to the satellite peaks. Besides, in
comparison with FeO , when nickel was added into Fe catalyst,
all the Fe Ni -300 catalysts show a kind of peak at 705.2-705.6
eV, which is assigned to the “pre-peak”, resulting from the
The peaks at about
7
Sample
FeO
Fe3+/Fe2+
-
Ni2+/Ni3+
-
O
S
(%)
L
O (%)
x
x
27.17
47.53
46.72
60.78
50.0
72.83
52.47
53.28
39.18
50.0
a
b x
O
NiO
x
-
0.49
0.90
1.11
0.78
0.81
1.22
0.98
[46]
generation of lattice defects on the catalyst surface.
The
Fe /Fe ratios were shown in Table 2, the ratio decreases as
following: Fe0.5Ni0.5 -300,
Fe0.7Ni0.3
Fe0.5Ni0.5
Fe0.3Ni0.7
Fe0.5Ni0.5
Fe0.5Ni0.5
Fe0.5Ni0.5
O
O
O
O
O
O
x
-300
-300
-300
-200
-250
-350
1.13
1.21
0.95
0.95
1.35
1.10
3+
2+
x
x x x
O -300 > Fe0.7Ni0.3O -300 > Fe0.3Ni0.7O
x
exactly the same order as their catalytic activities (Figure 1a).
Figure 7c shows the Ni 2p XPS spectra, where three peaks occur
at 854-862 eV. The peaks at 854.4-854.6 eV and 856.0-856.1 eV
x
63.50
69.85
37.50
36.50
30.15
62.50
present Ni2+ and Ni3+
x
,
respectively, and the peaks around 861.3
[
45, 47]
eV are attributed to the satellite peak.
As shown in Table 2,
the Ni /Ni ratios of Fe0.7Ni0.3Ox-300, Fe0.5Ni0.5 -300 and
-300 samples are 0.90, 1.11 and 0.78, visibly,
-300 is the highest. It is worth noting that, compared
x
2
+
3+
O
x
Fe0.3Ni0.7
Fe0.5Ni0.5
O
x
x
O
2+
3+
with the NiO
x
, the binding energy of Ni , Ni and satellite on
Fe Ni -300 samples are higher, suggesting that the
a
b x
O
introduction of Fe can change the chemical environment of Ni.
Figure 8 displays the O 1s, Fe 2p and Ni 2p of Fe0.5Ni0.5O
x
3+
2+
catalysts calcined at different temperatures. The Fe /Fe ,
2+
3+
Ni /Ni ratios and oxygen species proportions are shown in
Table 2. As a result, with the increase of calcination temperature,
the O
S
proportion increases at first and then decreases,
-250 exhibits the highest O percentage (69.85%).
The ratios of Fe /Fe and Ni /Ni also present similar volcano-
Fe0.5Ni0.5
O
x
S
3+
2+
2+
3+
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Fig 7. XPS spectra of Fe
b) Fe 2p; (c) Ni 2p.
a
Ni
b
O
x
-300 with different Fe/Ni molar ratios. (a) O 1s;
Fig 8. XPS spectra of Fe0.5Ni0.5
O 1s; (b) Fe 2p; (c) Ni 2p.
O
x
with different calcination temperatures. (a)
(
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Catalytic performance of Fe0.5Ni0.5
O
x
-250
investigate the effect of moisture, transmission FTIR was carried
out to inspect the used Fe0.5
conditions. As shown in Figure 9b, the used Fe0.5
catalyst at stage 1 (for 40 h under dry condition) displays a slight
N
0.5
O
x
-250 catalyst under different
Effect of WHSV
0.5 x
N O
-250
Since the optimal catalytic activity is possessed by
-250, we further investigated the effect of weight hour
−
1
Fe0.5
N
0.5
O
x
absorption peak at 1384 cm , this is caused by the antisymmetric
stretching vibration of nitro species.[56-58] The used Fe0.5
N O -
0.5 x
space velocity (WHSV), CO concentration and water due to their
significant influence in actual applications. Figure S2 displays the
250 at stage 2 (reacted for 10 h under dry condition + 20 h under
moist condition) shows a sharper peak at 1384 cm−1 than that of
stage 1, suggesting the vibration of nitro species is more intensive.
As the moisture is removed, the intensity of the absorption peak
at 1384 cm−1 for the used catalyst at stage 3 is back to stage 1.
The experiments imply that the water vapor can aggravate the
accumulation of nitro species on the catalyst surface, but the
accumulation is reversible. According to the results, we consider
NO conversion of Fe0.5
N
0.5
O
x
-250 with varied WHSVs. When the
-1
-1
WHSV is 30000 ml•g •h , the 100% NO conversion temperature
is 140 °C. When the WHSV decreases, the NO conversion of
Fe0.5
0.5 x
N O -250 increase, because the residence time of reactant
gas in the catalyst bed is inversely proportional to WHSV,
indicating that suitably raising temperature is an effective way to
enhance the NO conversion at a higher space velocity.
2
the active sites on the catalyst surface are occupied by H O as
Effect of CO concentration
the moisture added, leading to the less consumption of the nitro
species on the active sites and inhibition of catalytic performance.
The effect of CO concentration on the NO conversion over
Fe0.5
N
0.5
O
x
-250 sample was investigated. As a result in Figure S3,
2
When the moisture is removed, the H O molecules absorbed on
the T100NO is proportional to CO concentration from 4000 to 1000
ppm. Using the reactant gas with CO concentration of 4000 ppm,
the catalyst surface can be rapidly removed by the flowing
reactant gas, thus releasing the active sites. After that, parts of
the surface nitro species are decomposed, resulting in a decrease
of the intensity of nitro species and recovery of the NO elimination
efficiency.
T100NO is 120 °C. When the CO concentration is reduced to 2000
and 1000 ppm, T100NO increases to 130 and 140 °C, respectively.
The results above clearly demonstrate the positive effect of
relatively high CO concentration on NO conversion over the
Fe0.5
0.5 x
N O -250 sample. For the catalytic reduction of NO by CO,
CO can react with surface-capping O2- of catalyst to form surface
oxygen vacancies (SOVs), which promote the dissociation of NO
molecules and enhance the NO conversion [31, 32]. We speculate
that as the concentration of CO increases, more SOVs are
generated on the catalyst surface, resulting in an enhanced NO
elimination performance.
Stability and water resistance
0.5 x
Moreover, the lifetime property of Fe0.5N O -250 was
examined. Figure 9a shows the NO conversion in the time course
at 130 °C. The catalytic stability test result reveals that the
catalytic activity maintains a very high NO conversion of 100%
and no obviously decrease can be observed over 40 h.
Although the Fe0.5
stability under the dry condition of CO-NO-N
problems: oxygen and moisture, should be considered. It is wildly
reported that the O in reactant gas can competitively consume
N
0.5
x
O -250 sample exhibits high catalytic
2
, two intractable
2
CO and thus finally decrease the NO conversion.[52, 53] Fortunately,
recent years, a rotate reactor was invented to solve the problem
2 x
by preventing the mixing of CO and O via a NO adsorption-
reduction process, which could avoid the adverse effect of
oxygen.[54, 55] However, moisture, the other inevitable negative
factor, should be taken into consideration. Figure 8a also shows
the effect of moisture on the catalytic stability of Fe0.5
0.5 x
N O -250
sample at 130 °C. In the nonappearance of H O, the NO can be
2
converted entirely. When ~2.5 vol.% moisture is added, the NO
conversion decreases from 100% to ~40% in 1 h. In addition, after
reacting under the moist condition for 20 h, when moisture is
removed, the NO conversion returns to 100% in 2 h and no
obvious deactivation can be seen in the next 8 h. The results
Fig 9. (a) Stabilities of the Fe0.5Ni0.5
condition. (Reaction conditions: [NO] = 1000 ppm, [CO] = 2000 ppm, [H O] =
x
O -250 catalyst under dry and moisture
2
-
1
-1
2
2.5 vol.%, [N ] = balance; T = 130 °C; WHSV = 15000 ml•g •h ); (b) FTIR
demonstrate that the deactivation of Fe0.5
0.5 x
N O -250 catalyst
spectra of the Fe0.5Ni0.5
x
O -250 catalyst under different stages.
caused by ~2.5 vol.% moisture at 130 °C is reversible. To further
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0.5 x
Additionally, the catalytic activity and stability of Fe0.5N O -
2
50 under moist condition was examined, and the result is shown
in Figure 10. Compared to under dry condition (Figure 1b), when
2.5 vol.% moisture is introduced into the feed gas, T100NO of
-250 increases from 130 °C to 180 °C, indicating the
negative effect of moisture at low temperature. The lifetime study
is also shown in Figure 10, Fe0.5 -250 displays that the
stability of 100% NO conversion is more than 30 h at 180 °C. We
also compare the NO elimination efficiency of Fe0.5 -250 with
many reported catalysts, the results are summarized in Table S1.
The catalytic performance of Fe0.5 -250 is higher than that of
~
Fe0.5
0.5 x
N O
0.5 x
N O
0.5 x
N O
0.5 x
N O
most reported non-precious metals, even comparable to precious
metal catalysts.
Fig 10. Catalytic performance of the Fe0.5Ni0.5
x
O -250 catalyst. (Reaction
conditions: [NO] =1000 ppm, [CO] = 2000 ppm, [H
balance; T = 100 - 300 °C; WHSV = 15000 ml•g •h )
2
O] = 2.5 vol.%, [N ] =
2
-
1
-1
Catalytic mechanism discussion
For the NO + CO reaction, researchers have put forward
many mechanisms to explain the reaction pathway over different
catalysts. From many previous studies,[14, 32, 59, 60 ] a reaction cycle
(cycle 1) can be summarized as the following pathways: 1. The
catalyst surface was reacted with CO to generate surface oxygen
vacancies. 2. The NO molecular was activated by the surface
oxygen vacancies to produce N and O radicals. 3. The N radical
combined with another NO molecule to generate N
combined together to form N . 4. The O radical reacted with CO
to produce CO . In addition to cycle 1, some researchers
proposed another reaction cycle (cycle 2). In cycle 2, pathways 1-
are similar to that of cycle 1, however, after the NO was
2
O or
2
2
3
dissociated and surface oxygen vacancies was formed, the O
radicals dissociated from the NO were more prone to fill in the
oxygen vacancies to generate new surface oxygen species than
2
react with CO to form CO , that is, the reduced catalyst surface
could be re-oxidized by NO.[61, 62]
Fig 11. (a) XPS spectra of Fe 2p; (b) Ni 2p; (c) O 1s
In order to explore the possible reaction mechanism over Fe-
Ni mixed oxide catalysts, XPS analysis was used to investigate
the variation of surface chemical valence states over Fe0.5Ni0.5
50 sample treated by different reactant gases. Figure 11 shows
the XPS spectra of fresh catalyst, CO-pretreated catalyst (purged
x
O -
2
-1
-
by 2000 ppm CO at 250 °C for 200 min, WHSV=15000 mL.g .h
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1)
50 °C for 200 min, then switched to 1000 ppm NO at the same
and NO-adsorbed catalyst (pretreated by 2000 ppm CO at
Ni becomes stronger at 200 °C and then decreases at 250 °C,
+
2
implying that the Ni2 is reduced to form Ni at first and then further
+
+
-1
-1
0
temperature for 200 min to adsorb NO, WHSV=15000 mL.g .h ).
Figure 11a shows the spectra of Fe2p. For the fresh catalyst, the
peaks of Fe 2p3/2 and Fe 2p1/2 appear at 710.88 and 724.08 eV,
respectively. After the treatment of CO, the peaks of Fe 2p3/2 and
Fe 2p1/2 shift to low-valence state, appearing at 709.98 and 722.68
eV. The NO adsorption process does not return the spectrum of
Fe back to the initial position. For the Ni 2p spectra, the Ni 2p3/2
and Ni 2p1/2 peaks shift to the low-valence state after the treatment
of CO in comparison of a fresh one, besides, the valence state of
reduced to Ni , which is in line with the XPS results (Figure 11b).
On the other hand, a new band derived from the coordinated CO
x
with the low-state metal species appears at 1066 cm-1 at 200 and
250 °C, also suggesting that the cations are reduced under NO +
CO atmosphere.[69]
0
Ni can be observed at 852.08 eV and 869.38 eV, after the
adsorption of NO, the positions of Ni species exhibit almost no
3+
change. According to the XPS test, the high-valence ions (Fe ,
Ni3+ and Ni ) on catalyst surface can be reduced to the low-
2+
2+
2+
0
valence states (Fe , Ni and Ni ) under CO atmosphere, but the
low-valence species are difficult to be oxidized back to the initial
valence state by the following treatment of NO. The result also
can be proved by the O 1s spectra, the proportion of surface
oxygen species changes from 69.85%→33.09%→34.60% after
the CO and NO treatment. That is, the catalyst surface can be
reduced by CO to form oxygen vacancies, but the oxygen
vacancies are hard to be re-filled by the O stems from NO.
When the Fe0.5Ni0.5
O
x
-250 was used for 4 h at 150 °C and
50 °C, respectively, the composition of the used catalysts were
characterized by XRD and XPS (see Figure S4). The XRD spectra
of fresh and used catalysts are similar, both NiFe and NiO
2
2
O
4
x
Fig 12. in-situ DRIFTS of CO+NO co-adsorption over Fe0.5Ni0.5O -250.
phases can be observed. However, the XPS spectra of the fresh
and the used are different. For the used catalysts, comparing with
the fresh one, we can see that: (1) The peaks of Fe 2p3/2 and Fe
Based on the XPS and in-situ DRIFTS results, the
mechanism for NO reduction by CO over Fe Ni catalyst should
follow the cycle 1. In detail, when the catalyst is exposed in the
a
b x
O
2
p
1/2 shift to low-valence state after reaction. (2) The valence state
of Ni0 can be observed after reaction. (3) As the reaction
temperature increases, the O proportion decreases gradually.
mixture gas, NO is adsorbed on the catalyst surface to form NO
x
S
species. As the temperature increase, partial adsorbed NO
x
The results above clearly demonstrate that the catalyst surface
was reduced during reaction.
species (bridging bidentate nitrates, chelating bidentate nitrates
and linear nitrites) are desorbed/converted/dissociated. The
In-situ DRIFTS of CO and NO co-adsorption was also utilized
surface oxygen is reacted with CO to form CO
generated CO are absorbed on the surface to produce carbonate
species. Meanwhile, the Fe-O-Ni structure is reduced to generate
2
and some
to explore the reaction process over Fe0.5Ni0.5
in Figure 12，when the sample is exposed in the mixture gas at
0 °C, eight bands can be observed，which are attributed to
x
O -250. As depicted
2
5
2+
2+
2+
0
Fe -□-Ni and Fe -□-Ni species (□ means surface synergetic
oxygen vacancies, SSOVs). Previous literature have reported that
SSOVs is a key factor to weaken the N-O band, which can
dissociate NO to produce O and N radicals. [32, 62, 70] With the
dissociation of NO, the O radicals are reacting with CO to form
-
1 [63]
bridging bidentate nitrates (1025 cm ),
nitrates (1205 cm ), linear nitrites (1276 cm-1),[14] monodentate
chelating bidentate
-
1
[14]
-
1
[64]
(1760 cm-
nitrates (1501 cm ), vibrational modes of Fe-(NO)
2
1)
and 2120 cm ),
00 °C, a band can be observed at 1356 cm , which belongs to
the bidentate formate, resulting from the interaction between CO
,[65] NO adsorbed on Ni (1833 cm-1)[31] and gaseous CO (2178
2+
-
1 [66]
respectively. When the temperature rises to
CO
2
, while the N radicals can combine with another NO molecule
O or combine together to generate N . The
O is further reduced to produce N at higher
-1
1
to generate N
2
2
intermediate N
temperature.
2
2
[
64]
molecular and surface hydroxyls. Meanwhile, gaseous CO
2
can
-
1
be seen in 2250-2400 cm . When the temperature increases to
2
+
-1
150 °C, the band of NO adsorbed on Ni at 1833 cm is replaced
+
-1
2+
by NO adsorbed on Ni at 1815 cm , indicating that the Ni is
+
[67]
reduced to Ni in the reactant gas. In addition, two bands can
be distinguished at 1217 cm-1 and 1252 cm , the former one can
be deemed to the hydrogen carbonates with the δ(C-O…H)
-1
[
68]
vibration mode, and the latter is assigned to the bridged nitrates.
[
41]
As the temperature gradually increases, the band of linear
-1
nitrites (1276 cm ) vanishes because of its poor stability at
00 °C.[68] Besides, the intensity of the band of NO adsorbed on
2
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The Fe-Ni mixed oxide catalysts were prepared through a
3 3 2 3 2 2
sol-gel method. Fe(NO ) •9H O and Ni(NO ) •6H O were utilized
as the salt precursors. In detail, equal moles of metal nitrate (0.05
mol) and citric acid monohydrate (0.05 mol) were dissolved in
deionized water to form a solution and stirred for 6 h at 25 °C.
Then, the solution was evaporated in air at 70 °C for 8 h to form
a sol. The obtained sol was dried at 80 °C overnight to form a dark
red, foam-like solid. The solid samples were ground to powder
and calcined at designed temperature for 4 h in air. The as-
prepared catalysts were denoted as Fe
Fe/Ni molar ratio, T is the calcination temperature in degrees
Celsius. For example, Fe0.5Ni0.5 -300 represents that 0.025 mol
of Fe(NO •9H and 0.025 mol of Ni(NO •6H were
a b x
Ni O -T, where a/b is the
O
x
3
)
3
2
O
3
)
2
2
O
employed, and the calcination temperature was 300 °C. Besides,
Fe-only and Ni-only samples were synthesized by the same
method with the calcination temperature of 300 °C, denoted as
a b x
Fig 13. Possible reaction mechanism of NO reduction by CO over Fe Ni O
catalyst.
x x
FeO and NiO .
Conclusions
Catalyst testing
NO reduction by CO was performed using a fixed bed reactor
In this work, Fe-Ni mixed oxide catalysts were synthesized
via a sol-gel method. Compared with the Fe-only and Ni-only
(
6 mm I.D.). The blind experiment showed no activity from 100 °C
to 300 °C. Before each measurement, 200 mg of sample (20-60
mesh) was pretreated in N flow (20 ml/min) at 100 °C for 1 h to
remove possible impurities. Then, a mixture gas consists of 1000
ppm NO, 2000 ppm CO, 2.5 vol.% H O (when used) and N
a b x
catalysts, Fe Ni O catalysts exhibit a tremendous enhancement
2
of activity in NO + CO reaction. The Fe-Ni mixed oxide catalysts
show higher specific surface area and NO adsorption capacity
2
2
than monometallic ones. For the most active Fe0.5Ni0.5
catalyst, NO can be entirely eliminated at 130 °C, and the 100%
NO conversion is sustained for 40 h. The H -TPR and XPS
x
O -250
balance gas was introduced into the reactor. The total flow rate
was 50 ml/min, corresponding to the weight hour space velocity
2
-1
-1
(
WHSV) of 15000 (ml•g •h ). The concentrations of NO (m/z =30),
analyses reveal that the easier reduction behavior, more surface
oxygen species and stronger electron interaction between Fe and
Ni (Fe2+ + Ni3+ ↔ Fe3+ + Ni ) are the key factors to the NO
elimination. The effect of CO concentration, WHSV and moisture
N
2
O (m/z=44) and CO (m/z=44) were analyzed using a mass
2
spectrometry coupled to gas chromatography (GC-MS). The gas
chromatograph was equipped with a Poropak Q column for
2+
separating N
temperature, the gas concentrations were collected.
The NO conversion, CO conversion and N O selectivity are
calculated by the following equations:
2 2
O and CO . After reaching steady state at each
(
~2.5 vol.%) on the catalytic performance were explored. The
addition of H O can decrease the NO conversion because of the
nitrate deposits, but the deactivation is reversible. Fe0.5Ni0.5
50 also exhibits an inspiring low-temperature catalytic
2
2
x
O -
2
NO conversion (%) = (NO in – NO out) / NO in × 100%
CO conversion (%) = CO2 out / CO in × 100%
performance under moist condition, NO can be converted
completely at 180 °C and the lifetime is more than 30 h in the
presence of ~2.5 vol.% water. The surface composition analyses
2 2
N O selectivity (%) = 2 × N Oout / (NO in – NO out) × 100%
Where in and out indicate the inlet and outlet concentration of the
(
XPS and in-situ DRIFTS) during reaction indicate that the Fe-O-
corresponding gas.
2+
2+
2+
0
Ni structure can be reduced to form Fe -□-Ni and Fe -□-Ni ,
which promote the dissociation of NO. The dissociated O radicals
Catalyst characterization
are prone to react with CO to form CO
oxygen vacancies on the catalyst surface. The N radicals react
with NO molecules to generate N O or combine together to
produce N . Overall, this study demonstrates that Fe-Ni mixed
2
rather than re-fill into the
X-ray diffusion (XRD) spectra were performed using a
RINT2000 vertical goniometer equipped with Cu Kα radiation. X-
ray photoelectron spectroscopy (XPS) experiments were carried
out on a Thermo Scientific ESCALAB 250Xi using a focused
monochromatic Al Kα source. In addition, the C1s peak at 284.6
eV was operated as the reference to calibrate all binding energies.
Fourier transform infrared (FTIR) spectra were acquired with a
Nicolet 380 infrared spectrometer in the wavenumber ranging
2
2
oxide catalysts possess high and steady NO removal
performance in NO + CO reaction at low temperature. The
catalytic process presents potential application value, and can be
applied in locations such as stationary sources gas and
automobile exhaust purification.
−
1
from 500 to 2000 cm . Raman measurements were conducted
using a DXR Raman Microscope scanning with a laser of 532 nm.
2
The N adsorption-desorption isotherms were obtained on a
3
Quantachrome NOVA touch LX instrument. The specific surface
areas, SBET, were calculated by using the Brumauer-Emmett-
Teller (BET) method. Before testing, each catalyst was pretreated
under vacuum at 200 °C for 3 h.
Experimental Section
Catalyst preparation
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H
2
-TPR spectra of the as-prepared samples were obtained
using a Micromeritics AutoChem2090 apparatus. Before each
experiment, the catalyst (50 mg) was heated in N (50mL/min) at
00 °C for 60 min, then, cooled to 50 °C. After that, the N was
[11]B.V. Reddy, S.N. Khanna, Phys. Rev. Lett. 2004, 93, 068301.
[
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Dong, Y. Chen, Catal. Sci. Technol. 2014, 4, 482-493.
−1
2 2
switched to a 10% vol. H /N mixture gas (50 mL min ) and
heated up to 750 °C (heating rate = 10 °C/min).
NO-TPD analysis was operated in
apparatus with a TCD detector. Before each test, 50 mg of sample
was purged in N flow (50mL/min) at 200 °C for 1 h, then cooled
to 50 °C. Then, the gas flow was switched to NO-He mixture gas
2 vol.% NO) and held for 30 min. After that, it was purged by He
14]X. Shi, B. Chu, F. Wang, X. Wei, L. Teng, M. Fan, B. Li, L. Dong, L. Dong,
ACS Appl. Mater. Interfaces. 2018, 10, 40509-40522.
a
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Fe-Ni mixed oxide catalysts show
excellent catalytic performance for NO
reduction by CO at low temperature. The
correlation between physicochemical
properties and catalytic performance
were discussed. A possible mechanism
was proposed.
XX Du, T-L Yao, QL Wei, H Zhang, YM
Huang*
Page No. – Page No.
Investigation on Fe-Ni mixed oxide
catalysts for NO reduction by CO at
low temperature
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