Study of La2-xSrxCuO4 (x = 0.0, 0.5, 1.0) catalysts for NO + CO reaction from the measurements of O2-TPD, H2-TPR and cyclic voltammetry

Article abstract of DOI:10.1016/j.molcata.2005.03.036

Catalytic reduction of NO by CO was studied over perovskite-like La _2-xSr_xCuO₄ (x = 0.0, 0.5, 1.0) catalysts prepared by citrate method and calcined at 900 °C. The catalysts were characterized by O₂-TPD, H₂-TPR and cyclic voltammetry (CV) measurements. Results obtained from CV were in well agreement with those obtained from O₂-TPD and H₂-TPR, suggesting that CV is also a powerful means in the study of heterogeneous catalytic reaction carried out at high temperatures. In O₂-TPD experiment, the desorption area of β oxygen, which was contributed by the oxygen adsorbed on the oxygen vacancy, increased with the increase of Sr content and was in the same order as the activity, indicating that the activity depended largely on the oxygen vacancy resulted by Sr addition. In H₂-TPR measurements, the increasing area of the first reduction peak indicating that the oxygen vacancy resulted by Sr addition plays important role in this peak, since the Cu ³⁺ content in La_1.5Sr_o.5CuO₄ and LaSrCuO₄ is the same. And in the CV curves, the area of redox peak, which represents the amount of oxygen vacancy participating in the reaction, has close correlations with the activity of NO + CO reaction, while the symmetry of redox potentials does not contribute much to the activity. Overall, with the link of oxygen vacancy, CV was introduced and has obtained great success in explaining reaction in heterogeneous catalysis. Besides, from the result that none reductive peak was observed in the first CV curves of LaSrCuO₄, it suggested that the first step of a catalytic reaction is reduction, corresponding to the oxidation of catalyst i.e., Mⁿ⁺(Cu²⁺) → M⁽ⁿ⁺¹⁾⁺(Cu³⁺).

Full text of DOI:10.1016/j.molcata.2005.03.036

Journal of Molecular Catalysis A: Chemical 238 (2005) 35–40
Study of La2−xSrxCuO4 (x = 0.0, 0.5, 1.0) catalysts for NO + CO reaction
from the measurements of O2-TPD, H2-TPR and cyclic voltammetry
Junjiang Zhua,b, Zhen Zhaoc, Dehai Xiaoa, Jing Lia, Xiangguang Yanga,∗, Yue Wua
a
Environmental Catalysis Group, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
b
Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
c
State Key Laboratory of Heavy Oil Processing, Beijing University of Petroleum, PR China
Received 17 February 2005; received in revised form 16 March 2005; accepted 23 March 2005
Available online 13 June 2005
Abstract
Catalytic reduction of NO by CO was studied over perovskite-like La2−xSrxCuO4 (x = 0.0, 0.5, 1.0) catalysts prepared by citrate method
and calcined at 900 ◦C. The catalysts were characterized by O2-TPD, H2-TPR and cyclic voltammetry (CV) measurements. Results obtained
from CV were in well agreement with those obtained from O2-TPD and H2-TPR, suggesting that CV is also a powerful means in the study
of heterogeneous catalytic reaction carried out at high temperatures. In O2-TPD experiment, the desorption area of ␤ oxygen, which was
contributed by the oxygen adsorbed on the oxygen vacancy, increased with the increase of Sr content and was in the same order as the activity,
indicating that the activity depended largely on the oxygen vacancy resulted by Sr addition. In H2-TPR measurements, the increasing area of
the first reduction peak indicating that the oxygen vacancy resulted by Sr addition plays important role in this peak, since the Cu3+ content in
La1.5Sro.5CuO4 and LaSrCuO4 is the same. And in the CV curves, the area of redox peak, which represents the amount of oxygen vacancy
participating in the reaction, has close correlations with the activity of NO + CO reaction, while the symmetry of redox potentials does not
contribute much to the activity. Overall, with the link of oxygen vacancy, CV was introduced and has obtained great success in explaining
reaction in heterogeneous catalysis. Besides, from the result that none reductive peak was observed in the first CV curves of LaSrCuO4, it
suggested that the first step of a catalytic reaction is reduction, corresponding to the oxidation of catalyst i.e., Mn+(Cu2+) → M(n+1)+(Cu3+).
© 2005 Elsevier B.V. All rights reserved.
Keywords: NO + CO; La2−xSrxCuO4; Perovskite-like oxides; O2-TPD; H2-TPR; Cyclic voltammetry
1. Introduction
bulk structure and the composition of cations at both A and B
sites can be variously changed without destroying the matrix
structure, they therefore can be very useful as model systems
to investigate the relationships between solid-state properties
and catalytic performance of catalyst [2–4].
Various perovskite(-like) systems [5–9] have been inves-
tigated as catalysts for the reaction related with NO and CO
(NO + CO), both of which are unwanted gas in the atmo-
sphere. Voorhoeve et al. [5] have classified the NO + CO
reaction on perovskites as an intrafacial process involving
oxygen from the perovskite lattice. Pomonis and co-workers
[8] have studied several kinetic models for NO + CO reaction
and found that the oxygen mobility plays important role in
the reaction. Dai et al. [9] investigated a various of catalysts
and correlated the catalytic activity with the structural defect
Compounds with formula A2BO4 generally have the
tetragonal K2NiF4 structure when the radius of the A cation is
˚
˚
1.0 <rA < 1.9 A, the radius of the B cation is 0.5 < rB < 1.2 A.
ThelargerAcationhasnine-foldcoordinationandthesmaller
B cation has octahedral coordination. This structure can
be described as containing alternate layering of perovskite
(ABO3) and rock-salt (AO) units with the nine coordinate
A cation having a surrounding that is the average of what
it would be in perovskite and rock salt [1]. These oxides
(A2BO4), as well as perovskite (ABO3), have a well-defined
∗
Corresponding author. Tel.: +86 431 5262228; fax: +86 431 5685653.
E-mail address: xgyang@ciac.jl.cn (X. Yang).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.03.036

36
J. Zhu et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 35–40
(mainly oxygen vacancies) and the redox (mainly transition
metal ion couples) ability, and concluded that the generation
of oxygen vacancies by A-site replacements favors the acti-
vation of O2 and NOx, while the modification of B-site ion
oxidation states by aliovalent ion substitutions in A and/or
B sites promotes the redox process of the catalyst, both of
which favored the NO + CO reaction.
Previously, we have reported that cyclic voltammetry
(CV) is a powerful means to investigate catalytic reactions
in heterogeneous catalysis [10]. In this work, in addition
to the CV measurement, we also performed the conven-
tional O2-TPD and H2-TPR measurements to investigate the
NO + CO reaction, since O2-TPD measurement is known
as an effective method to investigate reactivity on cata-
lyst surface, while H2-TPR measurement is widely used
to characterize redox properties of the catalyst. The results
obtainedfromCVwereinwellagreementwiththoseobtained
from O2-TPD and H2-TPR, indicating that CV measurement
can be applied to investigate hetergeneous catalytic reaction
occurred at high temperatures. The area of redox peak, which
represents the amount of oxygen vacancy (i.e., “F center”,
Cu2+[ ] → Cu3+[e]) participating in the reaction, has a close
correlations with the activity of NO + CO reaction, due to the
existence of the reducing agent (CO).
the base line on the recorder remained unchanged. Finally,
the sample was heated at a rate of 20 ◦C/min in 8% H2/N2 to
record the TPR spectra.
Cyclic voltammetry (CV) was measured in a conventional
two-compartment, three-electrode cell as described in our
previous work [10].
Steady-state activities of catalysts were evaluated using a
single-pass flow micro-reactor made of quartz, with an inter-
nal diameter of 6 mm. The reactant gas (0.4% NO + 0.4%
CO/He) was passed through 0.5 g catalysts at a rate of
22.5 mL/min. The gas composition was analyzed before and
after the reaction by an online gas chromatography, using
molecular sieve 5A column for separating NO, N2 and Pora-
pork Q for CO2. Before the data were obtained, reactions
were maintained for a period of ∼2 h at each temperature
to ensure the steady-state conditions. The activity was evalu-
ated as follows: NO conversion = ([NO]in − [NO]out)/[NO]in,
where [NO]in and [NO]out are the inlet and outlet concentra-
tion of NO, respectively.
3. Results and discussion
3.1. XRD study of samples
Fig. 1 showed the powder X-ray diffraction patterns of
the samples (also see Table 1). All the samples are in sin-
gle perovskite-like phase with A2BO4 structure. The struc-
ture of La2CuO4 is orthorhombic, while the structure of
La1.5Sr0.5CuO4 and LaSrCuO4 is tetragonal. In this work,
we represented the samples as La2−xSrxCuO4, although the
2. Experimental
The samples, La2−xSrxCuO4 (x = 0.0, 0.5, 1.0), were pre-
pared by citrate combustion method as described elsewhere
[11]. Briefly, to an aqueous solution of La3+, Sr2+, Cu2+
nitrates (all in AR. grade purity) with appropriate stoichiom-
etry, a solution of citric acid 100% in excess of cations was
added. The resulting solution was evaporated to dryness, and
then the precursors were decomposed in air at 300 ◦C, cal-
cined at 600 ◦C for 1 h, and finally pelletized and calcined at
900 ◦C in air for 6 h. The synthesized pellets were pulverized
to ca. 40–80 mesh size to be used.
Powder X-ray diffraction (XRD) data were obtained from
a X-ray diffractometer (type D/MAX-II B Rigaku) over the
ranger 20◦ < 2θ < 80◦ at room temperature, operating at 40 kV
and 10 mA, using Cu K␣ radiation combined with nickel
filter.
actual composition is La2−xSrxCuO4
.
λ
It is known that for perovskite-like oxides with A2BO4
structure, when A-site cations (i.e., La3+) were substituted by
low valence cations (i.e., Sr2+), the average oxidation number
of B-site cations (i.e., Cu2+), as well as the content of oxy-
gen vacancy will increase, in order to satisfy the principle
of electroneutrality. The average oxidation number of B-site
cations was chemically determined by iodometric titration
[14] without treatment of the as-prepared samples. The stoi-
chiometry was calculated on the assumption that the metals
Temperature programmed desorption of oxygen (O2-
TPD) was carried out in a conventional apparatus equipped
with TCD [12,13]. The samples (0.25 g) were first treated
at 850 ◦C for 1 h in oxygen and cooled to room temper-
ature in the same atmosphere, then swept with helium at
a rate of 35 mL/min until the base line on the recorder
remained unchanged. Finally, the sample was heated at a rate
of 35 ◦C/min in helium to record the TPD spectra.
Temperature programmed reduction of hydrogen (H2-
TPR) was carried out on the same apparatus as that in O2-
TPD. The samples (0.1 g) were first treated at 850 ◦C for 1 h
in oxygen and cooled to room temperature in the same atmo-
sphere, then swept with 8% H2/N2 at a rate of 35 mL/min until
Fig. 1. X-ray diffraction patterns of La2−xSrxCuO4 (x = 0.0, 0.5, 1.0).

J. Zhu et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 35–40
37
Table 1
The physical properties and catalytic activities at different temperatures of La2−xSrxCuO4+λ (x = 0.0, 0.5, 1.0)
SSAa (m2/g)
Oxidation no.
of Cub
λc
NO conversion at different temperatures (◦C)/%d
S300–600
◦
Catalysts
Structure
C
250
300
350
400
450
500
550
600
La2CuO4
La1.5Sr0.5CuO4
LaSrCuO4
a
Orthorhombic
Tetragonal
Tetragonal
2.5
2.4
2.6
2.02
2.24
2.24
+0.01
−0.13
−0.38
0
0
0
0
0
0
0
1.8
8.6
1.0
60
96.8
1.3
87.9
97.5
1.8
95.6
97.6
23.3
98.1
98.3
86.5
97.4
98.8
6.1
73.2
168
Specific surface area.
b
c
d
Average oxidation number of Cu.
λ in La2−xSrxCuO4+λ (x = 0.0, 0.5, 1.0).
NO conversion in NO + CO reduction.
were present as either a mixture of Cu2+ and Cu3+ or Cu+
and Cu2+, and other elements were present as La3+, Sr2+ and
O2−, respectively. The results were listed in Table 1.
The amount of non-stoichiometry oxygen (λ) increased
continuously with the substitution of Sr for La, due to the
low valence of Sr2+ (comparing with that of La3+); while for
the average oxidation number of copper, it increased with
the increase of Sr content (x) at the beginning (0 ≤ x ≤ 0.5),
but kept constant (+2.24) at x ≥ 0.5 no matter the amount of
La is replaced by Sr. This suggested that the average oxi-
dation number of copper in this system reached only +2.24,
due to the instability of Cu3+ ions [15,16]. At x ≥ 0.5, the
decreasing charge caused by the excess substitution of Sr2+
for La3+ was compensated totally by increasing the amount
of non-stoichiometry oxygen (λ), since the average oxidation
number of copper cannot be elevated at x > 0.5. The oxida-
tion number is formal, and it is possible that O− is present
instead of Cu3+ [17]. The surface composition, the oxidation
number of copper and the non-stoichiometric oxygen (λ),
which were estimated from X-ray photoelectron spectra (i.e.,
Cu 2p3/2 and O 1s) [12], were close to those of the catalysts
listed in Table 1.
to the lack of Sr2+, which implied that the oxygen vacancy
produced was none or less; while large desorption areas were
observed for La1.5Sr0.5CuO4 and LaSrCuO4, due to the Sr
addition. The desorption area of ␤ oxygen were in the same
sequence as that of the non-stoichiometric oxygen (λ), sug-
gesting that the desorption peaks appeared at this temperature
range were contributed by the non-stoichiometric oxygen (λ).
The desorption peak of ␥ oxygen depended on many factors.
In addition to the structure and the oxygen deficiency, the
oxidability of B-site cation also has large effect on the des-
orption of ␥ oxygen, especially under the condition that the
structure was not destroyed [15]. Therefore, in the present
case, the desorption area of ␥ oxygen depended mainly on
the oxidability of B-site cation. The peak area desorbed at
the range of 300–600 ◦C was calculated and listed in Table 2
3.2. O2-TPD
Fig. 2 showed the O2-TPD profiles of the samples. In gen-
eral, the desorption peaks plotted from the perovskite-like
mixed oxides contain three kinds of oxygen species [18].
The desorption peak appeared at T < 500 ◦C is ascribed to the
oxygen chemically adsorbed on the surface (denoted as: ␣
oxygen); the desorption peak appeared at 500 < T < 800 ◦C
is ascribed to the oxygen chemically adsorbed on the oxy-
gen vacancy (denoted as: ␤ oxygen); and the desorption peak
appeared at T > 800 ◦C is ascribed to the oxygen escaped from
the lattice (denoted as: ␥ oxygen). One should note that in the
samples of La1.5Sr0.5CuO4 and LaSrCuO4, there appeared a
shoulder peak at the range of 500 < T < 800 ◦C, which might
be attributed to the desorption of oxygen caused by the reci-
procity of Cu3+ and O2−, due to the instability of Cu3+ [15].
The peak maximum temperature (TM) and the correspond-
ing desorption area in O2-TPD profile of each catalyst were
calculated and listed in Table 2. No ␣ oxygen desorption peak
was observed in all the three samples. In the ␤ oxygen des-
orption peaks, the desorption area of La2CuO4 was small due
Fig. 2. O2-TPD profiles obtained from La2−xSrxCuO4: (a) La2CuO4; (b)
La1.5Sr0.5CuO4; (c) LaSrCuO4.

38
J. Zhu et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 35–40
Table 2
Data measured and calculated in O2-TPD and H2-TPR measurements of La2−xSrxCuO4+λ (x = 0.0, 0.5, 1.0)
Catalyst
Data obtained from O2-TPD
␣ oxygen ␤ oxygen
T (◦C) S (cm2) T1 (◦C) S (cm2) T2 (◦C) S (cm2) T (◦C) S (cm2)
Data obtained from H2-TPR
◦
␥ oxygen
S300–600
Cu3+ → Cu2+ Cu2+ → Cu2O Cu3+ (%)
C
T1 (◦C)
T2 (◦C)
La2CuO4
La1.5Sr0.5CuO4
LaSrCuO4
–
–
–
–
–
–
501
570
576
6.1
73.2
168.0
–
755
721
–
20.1
67.2
875
948
918
8.2
11.3
44.8
6.1
73.2
168
–
453
450
698
592
601
2.0
24.0
24.0
also, in order to correlate the activity of the reaction, which
was performed in the same temperature range. In this work,
CO was totally oxidized at 600 ◦C over the three catalysts,
and thus it (600 ◦C) was selected to be the upper temperature
of the reaction.
to the reduction of excess oxygen, reaction (1):
La2CuO4+x + (x + y)H2 → La2CuO4−y + (x + y)H2O (1)
The fact that there still existed a large amount of La2CuO4
withorthorhombicstructureinthesampleafterthefirstreduc-
tion peak (which were detected by XRD patterns, not shown
here) certified that reaction (1) is the main reaction occurred
at T < 503 ◦C. The second reduction peak thus was ascribed
to the reduction of Cu2+ to Cu2O, reaction (2):
3.3. H2-TPR
In Fig. 3 the results of samples from H2-TPR measure-
ments were presented. In general, there are two kinds of
reactant species participating in the reaction; one is the non-
stoichiometric oxygen, including the excess oxygen and the
oxygen adsorbed on the oxygen deficiency, the other is B-site
cations with high oxidative state. For La2CuO4, in which no
Sr was added, none oxygen vacancy and Cu3+ ion (only 2%)
was formed and there has excess oxygen (see Table 1), the
first reduction peak (T = 503 ◦C), then, was mainly ascribed
2Cu2+ + H2 + 2O2− → Cu2O + H2O
(2)
This was supported by the fact that the color of sample is red
after the reduction at T = 900 ◦C. Certainly, some samples
with red color were also observed after the first reduction
peak (T = 503 ◦C), but the amount is little. That is to say,
reaction (2) might occur at T < 503 ◦C but it is not the major
reaction.
For La1.5Sr0.5CuO4 andLaSrCuO4, inwhichlargeamount
of Sr were added, lots of oxygen vacancies were produced
and some of Cu2+ ions were oxidized to Cu3+ ions. The
first two split reduction peaks were mainly contributed by
the Cu3+ ions and ␤ oxygen (O−), corresponding to reac-
tions (3) and (4), respectively. (Note: the following equations
were deduced from the XRD patterns after each reduction
peak, and from the number of electrons transferred in the
reaction. The number of electrons were calculated from the
corresponding reduction peak area that were evaluated with
CuOasthereferences, assuggestedbyPatcasetal. [19]. Here,
the dealing procedure were not shown for simplification.)
2Cu3+ + O2− + H2 → 2Cu2+ + H2O
4O− + 2Cu2+ +3H2 → Cu2O + 3H2O
(3)
(4)
The second reduction peak was ascribed to the reduction of
Cu2+ to Cu2O, reaction (5):
2Cu2+ + H2 + 2O2− → Cu2O + H2O
(5)
Table 2 also listed the peak maximum temperature (TMꢀ ) of
each reduction peak and the corresponding reduction species,
which was confirmed by the XRD patterns after each reduc-
tion peak. For La2CuO4, the content of Cu3+ ions is little
(2%) due to the lack of Sr, therefore, the first reduction peak
was mainly contributed by the non-stoichiometric oxygen
(λ). While for La1.5Sr0.5CuO4 and LaSrCuO4, the first reduc-
tion peak was contributed both by the higher oxidative state
Cu3+ ions and by the non-stoichiometric oxygen (λ). The
Fig. 3. H2-TPR profiles obtained from La2−xSrxCuO4: (a) La2CuO4; (b)
La1.5Sr0.5CuO4; (c) LaSrCuO4.

J. Zhu et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 35–40
39
indicating that the Sr addition facilitates NO + CO reaction
to occur. The reason might be that the Sr addition induced
the formation of oxygen vacancy, which is an important
part of the active site of the reaction. Besides, by correlating
◦
the activity with the oxygen desorption area S300–600 C
contributed by the ␤ oxygen (see O2-TPD), it was found
that the activity was in the same sequence as the oxygen
◦
desorption area S300–600 C, suggesting that the oxygen
vacancy (␤ oxygen) do favor the NO + CO reaction.
It is generally accepted that the first step of the reaction
is NO adsorption and dissociation on the active site [20,21],
described as reactions (6) and (7):
M3+ [ ] M2+ + NO = M3+ [NO−] M3+
2M3+ [NO−] M3+ = 2M3+ [O−] M3+ + N2
M3+ [O−] M3+ + CO = M3+ [ ] M2+ + CO2
(6)
(7)
(8)
Fig. 4. Cyclic voltammetry curves of La2−xSrxCuO4 (x = 0.0, 0.5, 1.0).
large reduction peak area of LaSrCuO4 is ascribed to the
more amount of non-stoichiometric oxygen (λ) produced in
it, since the Cu3+ content in the both catalysts is the same.
The TMꢀ of La1.5Sr0.5CuO4 and LaSrCuO4 is similar but is
lower than that of La2CuO4, suggesting that the Sr addi-
tion strengthen the oxidability of the catalyst. For the second
reduction peak of La2CuO4, the peak area is smaller and the
TMꢀ is higher than those of La1.5Sr0.5CuO4 and LaSrCuO4,
suggesting that the Sr addition promotes the mobility of
lattice oxygen or makes the structure unstable, which thus
lowered the TMꢀ and enlarged the peak area.
where M represents Cu; “[ ]” represents the oxygen vacancy.
After NO adsorption and dissociation, the oxygen left on
the oxygen vacancy can be removed easily through the
reaction (8) due to the existence of reducing agent (CO)
and thus, the NO adsorption and dissociation steps became
rate limiting. The more the amount of oxygen vacancies
is, the more the amount of NO being adsorbed. As a
result, the activity depends largely on the oxygen desorp-
◦
tion area (S300–600 C) and the highest activity was obtained
on LaSrCuO4, which possessed the most amount of oxygen
vacancy.
3.4. Cyclic voltammetry
The cyclic voltammetry (CV) curves of the samples
were shown in Fig. 4. For all the samples, the peak area
of the oxidative potential was the same as that of the
reductive potential, and the area of redox peaks, which
represents the number of Cu ions participating in the
reaction, i.e., “F center” (Cu2+[ ] → Cu3+[e]), increased
with the increase of Sr content, suggesting the increase
of the number of oxygen vacancies. This is in accordance
with the non-stoichiometry oxygen (λ) measured in them,
i.e., La2CuO4 < La1.5Sr0.5CuO4 < LaSrCuO4. The reductive
potential decreased with the increase of Sr content, indicat-
ing the enhancement of oxidability of samples. Besides, it
should be noted that the first redox cycle of LaSrCuO4 was
irreversible; no reductive peak appeared in the first cycle,
suggesting that the first step of catalytic reaction is reduction
(corresponding to Cu2+ to Cu3+), since the reduction of Cu3+
to Cu2+ cannot occur in the first cycle.
Results obtained from CV measurements supported the
above conclusions. It has been noted above that no reductive
peak appeared in the first CV curves of LaSrCuO4, which
certifies that the first step of the reaction is NO adsorp-
tion, corresponding to Cu2+ → Cu3+. For LaSrCuO4, the
redox peak area is in the largest while the symmetry of the
redox potentials is the poorest. The redox peak area means
the amount of oxygen vacancy (i.e., “F center”, see above)
participating in the reaction, while the symmetry of redox
potentials is a measure of the transformation of transition
metal, i.e., Mn+ ⇔ Mn+1. The more the symmetry of redox
potentials, the easier the transformation of Mn+ ⇔ Mn+1
,
and the quicker the catalytic cycle running on the catalyst.
However, in NO + CO reduction reaction, because of the
existence of reducing agent (CO), the redox potentials of
catalyst will be largely modulated according to the Nerst
equation: ꢀE = −(RT/nF) ln([oxidizer]/[reducer]), resulting
in the reduction of oxidative potential and the raise of reduc-
tive potential and finally making them become symmetry
[10]. Namely, ꢀE of catalyst is not an intrinsic factor in
deciding the catalytic activity of NO + CO reduction reac-
tion due to the existence of reducing agent. The reversible
active sites or the redox peak area in CV curves of catalyst,
then, became the crucial factor in determining the activity
of NO reduction. The larger the redox area is, the more the
amount of oxygen vacancy participating in the reaction will
be and thus, more amount of NO being adsorbed and disso-
3.5. Catalytic activity
Catalytic activities of samples for NO + CO reduction at
different temperatures were listed in Table 1. The activity
increased with the increase of reaction temperature and
depended largely on the catalysts. For La2CuO4 without Sr
addition, the activity is low (<25%) until 600 ◦C, while for
La1.5Sr0.5CuO4 and LaSrCuO4, high activity was gained
even at 400 ◦C (reached 60 and 96.8%, respectively),

40
J. Zhu et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 35–40
ciated. As a result, the activity increased with the increase of
the non-stoichiometry oxygen (λ) and LaSrCuO4 showed the
highest activity.
Acknowledgements
Financial support from the Ministry of Science and Tech-
nology of China (2001AA 324060) and the Natural Science
Foundation of China (20177022) is gratefully acknowledged.
In all, it can be concluded that the redox peak area, which
represents the amount of oxygen vacancy participating in the
reaction, has close correlations with the activity of NO + CO
reduction, whereas the symmetry of redox potentials does not
relate much to the activity due to the existence of reducing
agent. This is in accordance with the results obtained from
O2-TPD and H2-TPR, indicating that the properties of CV
curves also have close correlations with the activity of het-
erogeneous catalytic reaction occurred at high temperatures
range.
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