Science in China Series B: Chemistry
© 2007
Science in China Press
Springer-Verlag
Effect of strontium substitution on the activity of
La2−xSrxNiO4 (x = 0.0－1.2) in NO decomposition
ZHU JunJiang1†, YANG XiangGuang2, XU XueLian3 & WEI KeMei1
1
National Research Center of Chemical Fertilizer Catalysts, Fuzhou University, Fuzhou 350002, China;
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China;
3
College of Chemistry & Chemical Engineering, Fuzhou University, Fuzhou 350002, China
2
The aim of this work is to study the effect of Sr substitution on the redox properties and catalytic activity of La2−xSrxNiO4 (x = 0.0－1.2) for NO decomposition. Results suggest that the x = 0.6 sample
shows the highest activity. The characterization (TPD, TPR, etc.) of samples indicates that the x = 0.6
sample possesses suitable abilities in both oxidation and reduction, which facilitates the proceeding of
oxygen desorption and NO adsorption. At temperature below 700℃, the oxygen desorption is difficult,
and is the rate-determining step of NO decomposition. With the increase of reaction temperature (T >
700℃), the oxygen desorption is favorable and, the active adsorption of NO on the active site (NO + Vo +
Ni2+ ⎯⎯
→ NO−-Ni3+) turns out to be the rate-determining step. The existence of oxygen vacancy is the
prerequisite condition for NO decomposition, but its quantity does not relate much to the activity.
NO decomposition, La2−xSrxNiO4, TPD, TPR, oxygen vacancy
1 Introduction
Oxides with the formula A2BO4 generally have the
perovskite-like structure that contains alternated layers
of perovskite (ABO3) and rock-salt (AO)[1,2]. These oxides (ABO4), as well as perovskite (ABO3), have a
well-defined bulk structure and the composition of
cations at both A and B sites can be changed without
destroying the matrix structure, thus the oxidation state
of B-site cations and the amount of oxygen vacancy can
be controlled. Therefore, they are of interest for the
study of solid-state chemistry and catalytic performance[2]. On the other hand, reports suggest that the
perovskite-type oxides, which are composed of base
metals, can show high catalytic activity comparable to
that of platinum catalyst for automotive exhaust[3,4].
Hence, the investigation on the potential application of
perovskite-type oxides for NO removal is worthwhile,
both in science and in practice.
－
Among the oxides for NO removal[5 15], the Ni-containing samples show high activity. Zhao et al.[5] reported
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the excellent performance of LaSrNiO4 for NO decomposition by means of O2-TPD, NO-TPD, XPS, etc., and
found the reaction follows a redox mechanism. Pomonis
et al.[6,7] investigated the NO+CO reaction carried out on
La 2−x -Sr x NiO4 oxides, and found that the reaction
mechanism changed at different reaction temperatures
and Sr content. Zhu et al.[8] investigated the NO decomposition and reduction (NO+CO) reaction over LaSrMO4
(M = Co, Ni, Cu) by means of cyclic voltammetry; the
results indicated that LaSrNiO4 with the most matched
redox potentials showed the highest activity for NO decomposition. All of these results suggest that the series
of La2−xSrxNiO4 samples could be a possible candidate
for NO removal. However, not much literature regarding
the effect of oxygen vacancy on the redox ability, catalytic performance and NO decomposition mechanism
Received February 10, 2006; accepted May 29, 2006
doi: 10.1007/s11426-007-0015-y
†
Corresponding author (email: [email protected])
Supported by the National Hi-Tech Research and Development Program of China
(863 Program)(Grant No. 2004CB 719502) and the National Natural Science Foundation of China (Grant No. 20177022)
Sci China Ser B-Chem | February 2007 | vol. 50 | no. 1 | 41-46
has been reported. Therefore, we believe it is worthwhile
to shed more light on the role and behavior of oxygen
vacancy, which is one important part of the active site,
played on the reaction to illustrate the catalytic nature of
NO decomposition.
In this work, the content of oxygen vacancy and the
redox properties of La2−xSrxNiO4 (x = 0.0－1.2) with
different Sr content were measured through the iodometric titration, O2-TPD and H2-TPR method, and
correlated to the catalytic activity. The study showed
that the mechanism was changed from 700 to 800℃,
corresponding to the transformation of rate-determining
step from oxygen desorption to NO active adsorption.
Also, the study showed that the x = 0.6 sample, which
possesses suitable oxygen vacancy and Ni3+ ions,
showed the highest activity for NO decomposition.
2 Experimental
2.1 Preparation
La2−xSrxNiO4 (x = 0.0－1.2) was prepared by means of
the citrate combustion method[16,17]. Briefly, La3+, Sr2+
and Ni2+ nitrates (all AR purity grade) with appropriate
stoichiometry were first dissolved in a certain amount of
distilled water, then the citric acid solution 50% in excess of stoichiometry (molar ratio) was added to the
above solution, to replace all nitrate groups with citrate
ones. The resulting solution was heated in an evaporating dish to dryness and decomposition; the powders
(precursors) thus obtained were calcined at 600℃ for 1
h, and finally pelletized and calcined at 900℃ in air for
6 h. The synthesized pellets were pulverized to ca. 40－
80 mesh sizes for use.
2.2 Characterization
2.2.1 XRD. Powder X-ray diffraction (XRD) data was
obtained with an X-ray diffractometer (type D/max-IIB,
Rigaku) operated at 40 kV and 20 mA at room temperature, with a scanning speed of 4°/min. Diffraction patterns were recorded in the range of 2θ = 20º－80º, using
Cu Kα radiation combined with nickel filter.
2.2.2 Temperature-programmed desorption of oxygen
(O2-TPD). O2-TPD experiments were carried out on a
conventional apparatus equipped with a thermal conductivity detector (TCD). The samples (0.2 g) were first
treated at 800℃ for 1 h in O2 and cooled to room temperature in the same atmosphere, then swept with helium
42
at a rate of 11.8 mL/min until the base line on the recorder remained unchanged. Finally, the sample was
heated to 900℃ at a rate of 20℃/min in helium to record the spectra.
2.2.3 Temperature-programmed reduction by hydrogen
(H2-TPR). H2-TPR measurements were carried out on
the same apparatus. The samples (0.02 g) were first
treated at 800℃ for 1 h in O2 and cooled to room temperature in the same atmosphere, then swept with 4.88%
H2/N2 at a rate of 23.3 mL/min until the base line on the
recorder remained unchanged. Finally, the sample was
heated to 900℃ at a rate of 20℃/min in 4.88% H2/N2 to
record the spectra.
2.2.4 Average valence of nickel and non-stoichiometry
of oxygen (λ). The average valence of Ni ions in the
oxide systems was chemically determined by iodometric
titration[18] without treating the as-prepared samples. The
oxygen nonstoichiometry (λ) was calculated on the assumption that nickel is present as a mixture of Ni2+ and
Ni3+, and other elements are present as La3+, Sr2+, and
O2−, respectively[19].
2.3 Catalytic activity
Steady-state conversions of the catalysts were evaluated
using a single-pass flow micro-reactor made of quartz
with an internal diameter of 6 mm. The reactant gas
(1.0%(v)NO/He) was passed through 0.5 g of catalyst at
a rate of 25 mL/min. The gas compositions were analyzed before and after the reaction by an online gas
chromatograph, using a molecular sieve 5 A column for
separating NO, N2 and O2. Here, N2O was not detected
due to the difficulty of N2O formation between 500 and
850℃ as reported by Teraoka et al.[20]. Before the data
was obtained, reactions were maintained for a period of
~2 h at each temperature to ensure steady-state conditions. The activity was evaluated according to the equation: N2 yield = 2[N2]out/[NO]in, where [NO]in and [N2]out
are the concentrations of NO and N2 measured before
and after the reaction, respectively.
3 Results and discussion
3.1 Crystal and defect structure
Figure 1 shows the XRD patterns of the samples calcined in air at 900℃. All the samples are perovskite-like
with A2BO4 structure. No impurity was found in the
strontium-free sample (x = 0.0), indicating that the pre-
ZHU JunJiang et al. Sci China Ser B-Chem | February 2007 | vol. 50 | no. 1 | 41-46
sent preparation conditions are enough for the Ni-based
perovskite-like oxides formation. With the increase of Sr
content (x ≥ 0.8), some other diffraction lines with low
intensity, in addition to those of La2NiO4, were observed
and indexed to be SrCO3. Acceptably, SrCO3 phase was
formed in the samples prepared by citrate method, which
could introduce the carbon element to the precursor.
This is similar to the results reported by Rynkowski et
al.[21], except that the SrCO3 phase appeared at x = 0.25
in their work, while it appeared until x = 0.8 in the present case. Besides, results showed that the diffraction
peak was shifted to a high value with the increase of Sr2+,
which might be caused by the various radiuses of Sr2+
and La3+.
0.6, which implies that the change in property before
and after x = 0.6 might be different. As for the nonstoichiometric oxygen (λ), it decreased with the increase
of Sr content at all times. The decrease at 0 ≤ x < 0.6 is
ascribed to the decreasing non-stoichiometric oxygen
chemically adsorbed on catalyst surface (see Figure 2).
The decrease at 0.6 ≤ x ≤ 0.8 is ascribed to the formation of oxygen vacancy caused by the addition of Sr2+,
of which the oxidation state is lower (comparing with
La3+).
Figure 2 Relationships among the average valence of Ni ion, the
non-stoichiometric oxygen (λ), and the Sr content (x) in La2−xSrxNiO4±λ.
■, Average valence of Ni ion; ▲, non-stoichiometric oxygen (λ).
Generally, the substitution of cations with low valence (i.e. Sr2+) for these with high valence (i.e. La3+)
will cause the increase of oxidation state of B-site
Figure 1 XRD patterns of La2−xSrxNiO4±λ (0 ≤ x ≤ 1.2) calcined in air
at 900℃. #, Perovskite-like phase (La2−xSrxNiO4); &, SrCO3 phase.
The average valence of Ni and the non-stoichiometric
oxygen (λ) calculated in the samples are shown in Figure 2. The average valence of Ni increased with the increase of Sr content at the beginning (0 ≤ x ≤ 0.8)
(the difference between x = 0.6 and 0.8 is small), but
then decreased with the further increase of Sr content,
indicating that the average valence of Ni cannot be increased continuously. The large amount of Ni3+ will
cause structure instability by affecting the tolerance factor (t) of perovskite-like oxides. Therefore, to keep the
catalyst in perovskite-like structure, the excess Ni3+ ions
have to be reduced to Ni2+, accompanied by the formation of oxygen vacancy. The compensation of the formation of oxygen vacancy for the decreasing charge caused
by the Sr2+ substitution (for La3+) was observed at x ≥
→ Ni3+), or the increase of oxygen vacations (Ni2+ ⎯⎯
cancy, or both of them. Herein, the substitution effect
could be expressed as follows (Vo, ′ and • represent the
oxygen vacancies, cation holes and anion electrons, respectively)[5,17]:
→ La 2 NiO 4
La 2−x Sr x Ni 2+ 1−x Ni 3+ x O 4 ⎯⎯
3+
2+
+ xLa ⏐ Sr ⏐ ′ + xNi 2+ ⏐ Ni 3+ ⏐ •
(1)
2+
La 2−x Sr x Ni (V o ) λ O 4− λ ⎯⎯
→ La 2 NiO 4
3+
2+
+ xLa ⏐ Sr ⏐ ′ + λ ⏐ O ⏐ •• (x = 2 λ )
(2)
3.2 O2-TPD
Figure 3 shows the O2-TPD profiles of the samples.
Generally, two desorption peaks were observed in the
profiles, the first one is located between 200 and 600℃,
and is attributed to the desorption of oxygen chemically
adsorbed on the catalyst surface/oxygen vacancy (α
oxygen); the second one is located at T > 600℃, and is
ZHU JunJiang et al. Sci China Ser B-Chem | February 2007 | vol. 50 | no. 1 | 41-46
43
attributed to the desorption of lattice oxygen (β oxygen)[22]. The peak area of α oxygen first decreased (0 ≤
x ≤ 0.4), but then increased (x ≥ 0.6) with the increase
of Sr content. Correlating to the non-stoichiometric
oxygen (λ) shown in Figure 2, it can be inferred that the
α oxygen in 0 ≤ x ≤ 0.4 is closely related to the excess non-stoichiometric oxygen chemically adsorbed on
the catalyst surface, while the α oxygen in 0.6 ≤ x ≤
1.2 is to the oxygen chemically adsorbed on the oxygen
vacancy.
The above results show that: (i) the oxygen vacancy
was formed in the 0.6 ≤ x ≤ 1.2 samples; (ii) the lattice oxygen is difficult to be desorbed in the 0.0 ≤ x ≤
0.6 samples; and (iii) the x = 0.6 sample possesses both
behaviors. Namely, the x = 0.6 sample possesses not
only the oxygen vacancy that facilitates oxygen mobility
and desorption (namely, the oxidability), but also the
reducibility that facilitates NO adsorption (i.e. NO +
Ni2+ ⎯⎯
→ NO−-Ni3+). Hence, it has the more suitable
redox ability than the others, and it might exhibit excellent behavior for NO decomposition. This is true as certified below.
3.3 H2-TPR
TPR profiles of the samples are shown in Figure 4. Two
reduction peaks were observed for all the samples. The
first one, located at 150℃≤ T ≤ 550℃, is ascribed to
the consumption of hydrogen used for Ni3+ reduction
→ Ni2+) plus that for oxygen reduction, de(Ni3+ ⎯⎯
scribed as follows:
La2−xSrxNiO4+λ + (δ + λ) H2
⎯⎯
→ La2−xSrxNiO4−δ + (δ + λ) H2O
2Ni
3+
3+
Figure 3 O2-TPD profile of La2−xSrxNiO4±λ (0 ≤ x ≤ 1.2) in the range
of 50℃ ≤ T ≤ 900℃.
The desorption peak of β oxygen is small for 0.0 ≤
x ≤ 0.6 samples, while a large one is observed for 0.8
≤ x ≤ 1.2 samples, indicating that the desorption of
lattice oxygen from the 0.0 ≤ x ≤ 0.6 samples is difficult, while that from the 0.8 ≤ x ≤ 1.2 samples is
favorable. This implies that the reduction of Ni3+ to Ni2+
in 0.0 ≤ x ≤ 0.6 samples is more difficult than that in
0.8 ≤ x ≤ 1.2 samples, since the desorption of lattice
oxygen usually occurred with the reduction of metal
ions[23], i.e. 2Ni3+ + O2− ⎯⎯
→ 2Ni2+ + 1/2O2. As a deduction, it can be inferred that the oxidation of Ni2+ to
Ni3+ in 0.0 ≤ x ≤ 0.6 is more favorable than that in 0.8
≤ x ≤ 1.2.
44
2−
+ H2 ⎯⎯
→ 2Ni
−
2+
+ O
2+
+ H2O
(3)
(4)
Ni + O + H2 ⎯⎯
→ Ni + H2O
(4′)
Reaction (3) occurs mainly in the 0.0 ≤ x ≤ 0.4 samples, of which the oxygen is excess, while reaction (4)
and/or (4′) can occur in all the samples. The reduction
peak area of 0.0 ≤x ≤ 0.4 samples is smaller in comparison with that of the remains. The reason might be
that some of the adsorbed oxygens have desorbed from
the catalyst surface before reacting with the hydrogen,
resulting in less consummation of hydrogen (i.e. peak
area). While for 0.6 ≤ x ≤ 1.2 samples, the oxygen
caused by the thermal desorption is little, and most is
consumed by hydrogen, therefore, it results in large reduction peak area.
The area of the second reduction peak generally is
larger than that of the first one, because the amount of
Ni2+ being reduced in this step includes the Ni2+ reduced
from the first step and that remained in the as-prepared
sample. It should be noted that the second reduction
peak area of 0.0 ≤ x ≤ 0.4 samples, contrary to that of
the first one, is larger compared to that of the remains
(0.6 ≤ x ≤ 1.2). The reason might be that for 0.6 ≤ x
≤ 1.2 samples with high Sr content, many of strontium
ZHU JunJiang et al. Sci China Ser B-Chem | February 2007 | vol. 50 | no. 1 | 41-46
Figure 4 H2-TPR profile of La2−xSrxNiO4±λ (0 ≤ x ≤ 1.2) in the range of 50℃ ≤ T ≤ 900℃.
compounds were collected on the catalyst surface (Sr is
an element that tends to accumulate on the surface),
suppressing the nickel oxides in the inner to be further
reduced, and thus, resulting in less consumption of hydrogen. Comparatively, the reduction of nickel oxides in
the 0.0 ≤ x ≤ 0.4 samples can be carried out adequately due to the low Sr content. The reaction that occurred in this step could be expressed as follows:
Ni2+ + H2 + O2− ⎯⎯
→ Ni0 + H2O
(5)
3.4 Catalytic activity
Activity of the samples in NO decomposition is shown
in Figure 5. The x = 0.8 sample with the highest Ni3+
content (see Figure 2) shows the highest activity at T <
700℃, indicating that the catalyst oxidability that acts
for oxygen desorption plays the important role in NO
decomposition at T < 700℃ (high Ni3+ content helps
with the oxygen desorption). Namely, oxygen desorption
is the rate-determining step of the reaction at T < 700℃.
With the increase of reaction temperature (T > 700℃),
the oxygen desorption is favorable and the NO adsorp-
Figure 5 Activity of La2−xSrxNiO4±λ (0 ≤ x ≤ 1.2) for NO decomposition at different temperatures.
tion (NO + Vo + Ni2+ ⎯⎯
→ Ni3+-NO−) becomes diffi[5]
cult . As a result, the x = 0.6 sample, with suitable ability both to oxidation (for oxygen desorption) and to reduction (NO adsorption), shows the highest activity for
NO decomposition. In all, the results suggest that at low
ZHU JunJiang et al. Sci China Ser B-Chem | February 2007 | vol. 50 | no. 1 | 41-46
45
temperature (T < 700℃), the oxygen desorption is difficult and is the rate determining step of NO decomposition, while at high temperature (T > 700℃), the oxygen
desorption is favorable and the NO adsorption turns out
to be the crucial step.
The above results also imply that NO decomposition
does not relate much to the amount of oxygen vacancy
formed in the catalysts (when oxygen vacancy was produced), since the highest activity is not observed in the
sample with the most oxygen vacancies (i.e. x = 1.2).
However, the fact that catalysts with excess non-stoichiometric oxygen (especially for the x = 0.0 and 0.2
samples) showed minor activity for NO decomposition
indicates that the existence of oxygen vacancy is an indispensable condition for NO decomposition. Therefore,
it might be concluded that the oxygen vacancy is indispensable to NO decomposition, but its amount does not
relate much to the activity.
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4 Conclusion
La2−xSrxNiO4 with different Sr content was investigated
for NO decomposition. With the increase of Sr content,
the non-stoichiometric oxygen of sample changed from
excess to deficiency, and the amount of oxygen vacancy
increased all along. The diffraction peak shifted to a
high place with the increase of Sr2+, of which the radius
is longer than that of La3+. The highest activity for NO
decomposition was observed at x = 0.6, which has the
suitable redox properties comparing with the others, as
deduced from O2-TPD and H2-TPR. The rate-determining step of NO decomposition is oxygen desorption
at T < 700℃, but it turns out to be the active adsorption
of NO with the increase of reaction temperature (T >
700℃).
The support from Fuzhou University is appreciated.
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