Physicochemical properties and catalytic performance of Pr2-X SrxCoO4±y mixed oxides for NO reduction by CO

Article abstract of DOI:10.1134/S0023158406030141

Using the polyglycol gel method, a series of Pr_{2 - x} Sr _xCoO_{4 ± y} (0.2 ≤ x ≤ 1.0) mixed oxides were prepared, and their catalytic activities were studied in the test reaction of NO reduction by CO. The solid-state physicochemical properties, including crystal structure, defect structure, IR spectrum, valence state of B-site ions, nonstoichiometry oxygen (y), oxygen species, and redox properties, were characterized by means of XRD, IR, TPD, TPR, XPS, and chemical analysis. The results show that all mixed oxides display a K₂NiF₄ structure. When x = 0.2 and 1.0, the obtained samples still have little uncertain mixed oxides; however, the mixed oxides (x = 0.4, 0.6, 0.8) all represent a single A₂BO₄ phase. With the increase of x, lattice parameters, unit-cell volume, and average crystalline size decrease gradually, whereas microstrain density, the concentration of Co³⁺, the amounts of lattice oxygen released and the concentration of oxygen vacancy increase. The catalytic activities of Pr_{2 - x} Sr_xCoO _{4 ± y} catalysts for NO reduction by CO are closely correlated with oxygen vacancy and the concentration of Co³⁺. MAIK Nauka/Interperiodica , 2006.

Full text of DOI:10.1134/S0023158406030141

ISSN 0023-1584, Kinetics and Catalysis, 2006, Vol. 47, No. 3, pp. 423–429. © MAIK “Nauka /Interperiodica” (Russia), 2006.
Published in Russian in Kinetika i Kataliz, 2006, Vol. 47, No. 3, pp. 431–437.
Physicochemical Properties and Catalytic Performance
of Pr2 – xSrxCoO4 Mixed Oxides for NO Reduction by CO1
y
H. Zhong and X. Zeng
Department of Chemistry, Jinggangshan College, Jian, 343009, P.R. China
e-mail: hua_zh@163.com
Received December 20, 2004
Abstract—Using the polyglycol gel method, a series of Pr2 – xSrxCoO4 y (0.2 ≤ x ≤ 1.0) mixed oxides were
prepared, and their catalytic activities were studied in the test reaction of NO reduction by CO. The solid-state
physicochemical properties, including crystal structure, defect structure, IR spectrum, valence state of B-site
ions, nonstoichiometry oxygen (y), oxygen species, and redox properties, were characterized by means of XRD,
IR, TPD, TPR, XPS, and chemical analysis. The results show that all mixed oxides display a K2NiF4 structure.
When x = 0.2 and 1.0, the obtained samples still have little uncertain mixed oxides; however, the mixed oxides
(x = 0.4, 0.6, 0.8) all represent a single Ä2BO4 phase. With the increase of x, lattice parameters, unit-cell vol-
ume, and average crystalline size decrease gradually, whereas microstrain density, the concentration of Co3+,
the amounts of lattice oxygen released and the concentration of oxygen vacancy increase. The catalytic activi-
ties of Pr2 – xSrxCoO4 y catalysts for NO reduction by CO are closely correlated with oxygen vacancy and the
concentration of Co3+.
DOI: 10.1134/S0023158406030141
1
INTRODUCTION
were prepared and characterized. The catalytic perfor-
mances of all these catalysts were investigated in the
test reaction of NO reduction by CO. In order to reveal
the factors controlling the catalysts’ performance,
XRD, IR, XPS, TPD, and TPR were employed.
Nitrogen oxides (NOx) are major air pollutants that
cause acid rain [1]. The removal of NOx is one of the
key research topics in the protection of the environ-
ment. The method of NOx reduction by NH3 has been
used in practice, however, due to the cost and secondary
pollution [2], it is urgent for us to search for effective
but cheaper catalysts. The perovskite-type A2BO4
mixed oxides of the K2NiF4 structure, consisting of
alternating layers of ABO3 perovskite andAO rock salt,
have recently been studied as new materials for their
low cost, high catalytic activity, and high thermal sta-
bility [3]. Many studies have shown that the catalytic
performance of A2BO4 mixed oxides, to a greater
extent, are associated with the A-site and B-site ions,
their corresponding valences, and the crystal micros-
train of these oxides as well [4]. It is noted that there has
been much literature available, contributing to the
investigations of LnSrCuO4 and LnSrNiO4 catalysts [5,
6]. Recently, CO and C3H8 oxidation-catalyzing reac-
tions of LnSrCoO4 catalysts have been studied [7], but
the studies involving the investigation of LnSrCoO4
catalysts are considerably limited. Though the catalytic
oxidation performance of LnSrCoO4 catalysts is prom-
ising, almost no literature has been dedicated to the cat-
alytic reduction performance in the removal of NOx.
Therefore, further investigation is of great necessity.
EXPERIMENTAL
Catalyst Preparation
Praseodymium nitrate, strontium nitrate, cobalt
nitrate, polyglycol-20000, and citric acid (all in AR
grade purity) were supplied by the Shanghai Chemical
Reagent Company (China), while nitric acid and hydro-
chloric acid were supplied by the Jiangxi Hongdu
Chemical Reagent Company (China). A series of
Pr2 − xSrxCoO4 (x = 0.2, 0.4, 0.6, 0.8, 1.0) samples
were prepared yby the polyglycol gel method in a
desired molar ratio, respectively. For example, the pro-
cedure was as follows for the Pr2 – xSrxCoO4 y (x = 0.2)
sample: Pr(NO3)3 (0.50 mol/l, 36 ml), Sr(NO3)2
(0.50 mol/l, 4 ml), and 6.304 g (0.03 mol) citric acid
were mixed with a solution of Co(NO3)3 (0.50 mol/l,
20 ml). Polyglycol-20000 (20.0 g) was added after the
solution was evaporated to 40 ml at 80°ë with constant
stirring until a viscous gel was formed. Then, the gel
was evaporated to a completely dry state, and the
obtained precursor was calcined at 600°ë for 4 h,
finally pelletized, and calcined again at 1100°C for
10 h. The sintered pellet was crushed and sieved to a
fraction between ~0.25–0.18 mm.
In this paper, the Co-containing mixed oxides
Pr2 − xSrxCoO4 y (0.2 ≤ x ≤ 1.0) with K2NiF4 structures
1
The text was submitted by the authors in English.
423

424
ZHONG, ZENG
XRD Measurement
Micromeritics apparatus model ST-2000, and taking a
value of 0.16 nm2 for the cross-sectional area of the N2
molecule adsorbed at –196°C. Prior to the adsorption
measurements, samples were outgassed at 130°C.
Phase analysis was performed by an X-ray diffrac-
tometer (D8/ADVNCE, Germany), with the operation
conditions as 40 kV/10 mA, CuKα radiation, and nickel
filter. For the evaluations of average crystal size and lat-
tice distortion of Pr2 – xSrxCoO4 y, the Scherrer equa-
tion [8]
TPD and TPR Characterizations
Temperature-programmed desorption (TPD) exper-
iments of catalysts were conducted on an automatic
Micromeritics 3000 apparatus interfaced to a data sta-
tion. Under O2 gas flow, catalyst samples (300 mg)
were heated from ambient temperature to 950°C at a
rate of 8 K/min, keeping this temperature for 1 h. When
the system was cooled down to ambient temperature,
He (carrier gas) was used to remove the O2 in the gas
phase. Subsequently, with the ramp of 8 K/min, the
curves of O2-absorbed desorption were recorded by
a TCD.
L = 0.9λ/βcosθ
(1)
and equation
β2 cos2 θ = 4(λ/L)2/π2 + 32ε2 sin2 θ
(2)
were used. Here, L is the average crystal size, λ is the
wavelength of the X-ray used (0.154 nm), β is the effec-
tive line width of the X-ray reflection, θ is the Bragg
angle, and ε2 is the microstrain density.
IR Spectra
Temperature-programmed reduction (TPR) experi-
ments were conducted on the same apparatus with
TPD. Since water is produced during reduction, the
gases out of the reactor were passed through a cold trap
before entering the TCD. The samples (100 mg) were
first heated to 950°C at a rate of 20 K/min in a flow of
N2 (99.9%) gas and then cooled to ambient tempera-
ture. After being 10% H2/N2 purged for 1 h, with the
ramp of 20 K/min, the curves of H2 reduction were
recorded by the TCD.
The IR spectra were recorded by a Perkin-Elmer
683 spectrophotometer with KBr pressed pellets in the
registration range of 1000–400 cm–1.
X-ray Photoelectron Spectra
The XPS analysis was carried out on a PHI 5000C
ESCA X-ray photoelectron spectrometer using an AlKα
radiation source (hν = 1486.6 eV). A powdered sample
was adpressed to the butt-end of a small stainless-steel
cylinder and outgassed at 10–5 Pa for 1 h before they
were moved into the analysis chamber. The residual
pressure in the ion-pumped analysis chamber was
maintained below the vacuum of 1.3 × 10–7 Pa during
data acquisition. Pretreatments in oxygen were carried
out at 400°C. Energy regions of the photoelectrons were
scanned at a pass energy of 20 eV. Each spectral region
Catalytic Activity Measurements
The catalytic activity tests (0.18–0.25 mm fraction,
250 mg catalyst) were conducted at atmospheric pres-
sures in an automatic Micromeritics flow reactor with
was scanned a number of times to obtain good signal- gaseous mixtures of 2.02% NO, 1.99% CO, and He bal-
ance, at GHSV 8000 h–1. The gas composition was ana-
lyzed before and after the reactor by an online gas chro-
matography with TCD and connected with a computer
integrator system, using a TDX-01 column for N2, CO,
and NO and a Porapak Q column for CO2 and N2O. The
ambient temperatures for the columns and the TCD are
50 and 75°C, respectively. The catalytic activity and the
value of N2 selectivity were calculated with the follow-
ing respective equations:
to-noise ratios. Peak intensities were estimated by cal-
culating the integral of each peak after subtraction of
the S-shaped background and fitting to a curve mixed of
Lorentzian and Gaussian lines of variable proportions.
Although surface charging was observed for all the
samples, accurate binding energies (BE) ( 0.2 eV)
could be determined by charge reference to the ë1s
peak at 285.00 eV.
Analysis of the Valence State of Co
Conversion of NO:
(3)
The total amount of Co was determined by using
complexometric titration; the valence state of Co was
measured by means of the oxidation state of iodine [9].
([NO]in – [NO]out)/[NO]in × 100%,
Selectivity of N2:
(4)
SSA Characterization
2[N2]/([NO]in – [NO]out) × 100%,
The specific surface area (SSA) was calculated by
the BET method from the nitrogen adsorption iso-
where [NO]in, [NO]out are concentrations of NO before
therms, recorded at a liquid nitrogen temperature on a and after the reactor, respectively.
KINETICS AND CATALYSIS Vol. 47 No. 3 2006

PHYSICOCHEMICAL PROPERTIES AND CATALYTIC PERFORMANCE
425
T
T
x = 0.6
T*
T
T
T
T
T*
T
T
T
T
T* T*
TT
T
T
T
T
T
T
x = 1.0
x = 0.4
T
T*T*
T
T
T
T
T
T*
T*
T
T
T
T
T
T
T
T
+
T
T
T
T
T
T
T
T
T
T
T
T
x = 0.8
x = 0.2
T
T
T
T*
T*
T
T
T
T
T
T* T*
T
T
T
T
T
T
TT
*
20
30
40
50
60
70
80 20
2θ, deg
30
40
50
60
70
80
Fig. 1. XRD patterns of the Pr
Sr CoO
catalysts. T, T phase; T*, T* phase; +, SrCO ; *, Pr O .
y 3 2 3
2 – x
x
4
RESULTS AND DISCUSSION
Crystal Structure of Pr2 – xSrxCoO4 y Catalysts
degrees (x = 0.2 and 1.0), diffraction lines other than the
K2NiF4 structure are observed. These new lines of low
intensity are indexed to Pr2O3 or SrCO3 phases, respec-
tively.
As is well-known, A2BO4 mixed oxides with a
K2NiF4 structure can be classified specially into two
forms, i.e., a Fmmm orthorhombic phase and a I4/mmm
tetragonal phase. Also, the I4/mmm tetragonal phase
can be classified further into three phases (T, T*, T')
By calculating and analyzing the correlations of lat-
tice parameters, unit-cell volume, average crystal size,
microstrain density, and x of Pr2 – xSrxCoO4 y, as shown
in Table 1, we find that the lattice parameters, the unit-
cell volume, and the average crystal size decrease with
the increase of x; however, the microstrain density
increases (Table 1).
[10]. XRD patterns of Pr2 – xSrxCoO4 mixed oxides
y
are presented in Fig. 1. The results of phase analysis
obtained by XRD clearly show that all Pr2 – xSrxCoO4
mixed oxides display K2NiF4 structure under the appro-
y
priate condition. When x = 0.2 and 0.4, Pr2 – xSrxCoO4
y
Infrared Spectra of Pr2 – xSrxCoO4 y Catalysts
mixed oxides display K2NiF4 structure, which only
consists of the T phase. When Pr3+ is substituted gradu-
Figure 2 depicts the observed IR spectra of
ally by Sr2+, whose ionic radius (rSr2+ = 0.1310 nm) is Pr2 − xSrxCoO4 catalysts. In the registration range of
1000–400 cm–1y, all samples have the vibrational
larger than that of Pr3+ (rPr3+ = 0.1109 nm), the structure
absorption band around 500 cm–1, which could be
factor (cationic radius ratio rA/rB, 1.7 ≤ rA/rB ≤ 2.4)
increases, which leads to the structure change. In the
range of 0.6 ≤ x ≤ 1.0, Pr2 – xSrxCoO4 y display K2NiF4
assigned to the characteristic absorption band of the
K2NiF4 structure of A2BO4 [11], and the frequency of
this band increases from sample x = 0.4 to sample
structure, which consists of the T phase and a smaller x = 1.0. This result confirms that the series of
quantity of the T* phase. However, for substitution Pr2 − xSrxCoO4 all have the K2NiF4 structure as well.
y
Table 1. Structure parameter of the Pr2 – xSrxCoO4 y catalysts
Cell parameter, nm
L, nm
ε2 × 103
x
Structure
T, Pr2O3
V, nm3
( 0.3%)
( 1%)
a
b
c
0.2
0.4
0.6
0.8
1.0
0.3815
0.3811
0.3802
0.3798
0.3791
0.3809
0.3805
0.3799
0.3795
1.2425
1.2421
1.2418
1.2414
1.2411
0.1806
0.1801
0.1794
0.1789
0.1785
19.5
18.0
16.5
16.2
15.7
3.2
3.4
3.7
3.8
3.9
T
T, T*
T, T*
T, T*, SrCO3
0.3793
2
Note: V, crystal cell volume; L, average crystalline size; ε , microstrain density.
KINETICS AND CATALYSIS Vol. 47 No. 3 2006

426
ZHONG, ZENG
Transmittance, %
Desorption rate, arb. units
α'
β
α
x = 1.0
x = 0.8
x = 1.0
x = 0.8
x = 0.6
x = 0.6
x = 0.4
x = 0.2
x = 0.4
x = 0.2
100
300
500
700
900
T, °C
1000
800
600
400
ν, cm–1
Fig. 2. IR patterns of the Pr
Sr CoO
catalysts.
y
Fig. 3. TPD patterns of the Pr
Sr CoO
catalysts.
y
2 – x
x
4
2 – x
x
4
(Pr3+) at the A-site is replaced by a lower valent cation
(Sr2+), the positive charge reduction can be balanced
either by formation of a higher oxidation state ion at the
B-site, i.e.,
According to [11], the absorption band around
500 cm−1 is attributed to the stretching vibration of
A−O–B (i.e., Pr–O–Co) of A2BO4 belonging to the Ä2u
vibration mode. The vibrational absorption band at
650–680 cm–1 corresponds to the characteristic absorp-
tion band of the B–O (i.e., Co–O) bond in the K2NiF4
structure. The frequency of this band increases from
sample x = 0.4 to sample x = 1.0, indicating that Co–O
bond distance becomes shorter and the band intensity
becomes stronger. The IR spectra of this series of mixed
oxides present weak absorption at 580–640 cm–1,
Pr2 – xSrxCo2+ Co3x+ O4,
1 – x
or by formation of oxygen vacancy (VO):
Pr2 – xSrx(VO)Co2+O4 – y
Pr2 – xSrx(VO)Co2+O4 – y
,
.
which indicates the structure of Pr2 – xSrxCoO4
y
The results of the titration of iodine (Table 2) show
that the two cases exist at the same time; therefore, the
concentration of Co3+ and oxygen vacancy increase
with the increase of x, and the charge of the mixed
oxides is mainly balanced by the increasing of oxygen
vacancy.
becomes deformed and the deformation of coordination
polyhedra are different from x. A low intensity of defor-
mation does not change their original structure. The
absorption band appearing near 850 cm–1 in the sample
Pr2 – xSrxCoO4 (x = 1.0) could be attributed to the
y
characteristic absorption band of SrCO3 [12]. This is in
accordance with the results of XRD.
Redox Properties of Pr2 – xSrxCoO4 y Catalysts
Defect Structure of Pr2 – xSrxCoO4 y Catalysts
TPD study. The results of the é2-TPD investigation
show that the adsorption/desorption behavior of oxy-
The average valence of the Co ion and the concen-
tration of Co3+ in Pr2 – xSrxCoO4 y catalysts were deter-
gen in Pr2 – xSrxCoO4
catalysts varies with x. As
y
mined by the titration of iodine. The nonstoichiometry shown in Fig. 3, peaks α and β appear when x = 0.2 and
oxygen (y) was calculated by using these data, and 0.4; however, when x = 0.6–1.0, peak α' can also be
these calculations are listed in Table 2. According to the seen in addition to peaks α and β. TPD profiles reveal a
principle of electroneutrality [13], as the trivalent ion large oxygen peak at high temperature (β) on the
Table 2. Nonstoichiometry oxygen (y), average valence of Co, concentration of Con+, BET areas, and composition of the
Pr2 – xSrxCoO4 y catalysts
Concentration of Con+, %
Average valence
of Co
SBET, m2/g
( 2%)
x
Composition
y
Co2+ ( 1%)
Co3+ ( 1%)
0.2
0.4
0.6
0.8
1.0
Pr1.8Sr0.2CoO4.12
Pr1.6Sr0.4CoO4.03
Pr1.4Sr0.6CoO3.96
Pr1.2Sr0.8CoO3.91
PrSrCoO3.82
56
53
48
38
37
44
47
52
62
63
2.44
2.47
2.52
2.62
2.63
+0.12
+0.03
–0.04
–0.09
–0.18
2.7
2.8
2.9
3.0
3.2
KINETICS AND CATALYSIS Vol. 47 No. 3 2006

PHYSICOCHEMICAL PROPERTIES AND CATALYTIC PERFORMANCE
427
Hydrogen consumption, arb. units
Pr2 − xSrxCoO4 y catalyst (x = 1.0) (corresponding to a
desorption of 2.3 × 10–6 mol O2/m2) and a desorption
decrease for the other four catalysts. The amount of
oxygen removed in β peak corresponds to ~10% of the
monolayer, so it can be assigned to oxygen bound with
regular cobalt sites in the CoO6 octahedron, such as
β
α
γ
x = 1.0
x = 0.8
x = 0.6
x = 0.4
x = 0.2
Co3+ cations, including those in vicinity of anion vacan-
cies. At lower temperature, the peak (α) (corresponding
to 3.4 × 10–8 mol O2/m2) centered at about 180°C for the
Pr2 – xSrxCoO4 catalyst (x = 1.0) and at ~190–220°C
y
125
325
525
725
925
for the other four catalysts is observed. The amount of
α form is nearly two orders of a magnitude smaller, so
it certainly corresponds to oxygen bound with defect
sites, such as those located at domain boundaries or
associated with microstrains. Generally [14], the peak
β may be due to the result of desorbing the chemically
adsorbed oxygen (O2, O2– ) on the surface of the cata-
lyst, while the peak α' can be related to the chemically
adsorbed oxygen (O–) in the oxygen vacancies of the
catalyst. The release of vacancies oxygen can be due to
the reduction of Co3+:
T, °C
Fig. 4. TPR patterns of the Pr
Sr CoO
catalysts.
y
2 – x
x
4
dition of H2-TPR. As shown in Fig. 4, there are three
peaks (α, β, and γ) for all catalysts, respectively, sug-
gesting the occurrence of a multiple-step reduction.
Conceivably, the partial substitution of Pr3+ by Sr2+
would result in an increase in the concentration of Co3+
and oxygen vacancies due to the charge compensation
accomplished by oxidation from Co2+ to Co3+ and by
the formation of an oxygen-deficient perovskite-type
A2BO4, which would lead to an increase in the reduc-
ibility. The amount of H2 related to the peak β ranges
from 0.22 to 0.31 mol H2/mol M (M in substituted sam-
ples refers to the total amount of transition-metal cat-
ions), the maximum value being given by
Pr2 − xSrxCoO4 y catalyst (x = 1.0). However, the H2 con-
sumption of peak α varies from 1.22 to 1.32 mol H2/mol M
with the increase of x. Thus, the peak α corresponds to
reduction of the chemisorption oxygen. The peak β
could be attributed to the partial reduction of Co3+, i.e.,
2Co3+ + O2–
2Co2+ + 1/2O2 + VO.
(I)
The peak β might be attributed to the release of oxy-
gen (O2–) in the lattice of the catalyst. With the increase
of x, the continuous oxygen desorption (α) at lower
temperatures decreases gradually, whereas the continu-
ous oxygen desorption (β) at high temperatures
increases. The results obtained by the titration of iodine
show that the concentration of Co2+ of Pr2 – xSrxCoO4
y
catalysts decreases with the increase of x and the con-
centration of Co3+ increases. For Pr2 – xSrxCoO4 y cata-
lysts, when x = 0.2 and 0.4, the nonstoichiometry oxy-
gen (y) is positive, indicating that there are no oxygen
vacancies. However, when x = 0.6–1.0, the nonstoichi-
ometry oxygen (y) is negative, indicating that there are
oxygen vacancies, and the concentration of oxygen
vacancies increases with the increase of x. This coincides
with the results obtained from peak α' of the TPD study.
2Co3+ + O2– + H2
2Co2+ + H2O,
(II)
and Pr2 – xSrxCoO4 y catalysts still preserved in the per-
ovskite-type phase structure as a whole. The peak γ most
likely corresponded to the reduction of Co2+ to Co0,
which led to the breakdown of the perovskite-type phase
and formation of the discrete phases Pr2O3, SrO, and Co.
TPR study. The TPR profiles of the catalyst provide
useful information about the reducibility of ëÓn+ spe-
cies in these Pr2 – xSrxCoO4 catalysts, since Pr3+ and
XPS study. The binding energies (Eb) of various
component elements in Pr2 – xSrxCoO4
catalysts
y
Sr2+ of the A-site both are noynreducible under the con- obtained from XPS investigation are shown in Table 3.
Table 3. XPS Eb and surface composition of the Pr2 – xSrxCoO4 y catalysts
Eb, eV*
Surface cation molar percentage ( 2%)
Co/(Pr+Sr),
mol/mol
x
Pr 3d
Sr 3d 5/2 Co 2p 3/2
O1s
Pr
Sr
Co
0.2
0.4
0.6
0.8
1.0
933.4
933.8
933.6
934.0
933.9
954.6
954.3
954.9
955.2
955.1
133.5
133.7
134.1
134.3
133.9
780.7
780.6
780.3
779.8
779.5
528.9
528.7
529.0
529.1
528.8
–
34
30
26
20
14
27
29
30
34
36
39
42
44
46
50
0.65
0.72
0.77
0.86
0.98
–
530.8
531.2
531.1
* Calibrated internally by the carbon deposit C1s at 285.00 eV (E ).
b
KINETICS AND CATALYSIS Vol. 47 No. 3 2006

428
ZHONG, ZENG
Intensity, arb. units
increase of x, which is in accord with the varieties of the
surface concentration of Co2+ and Co3+ and the Co aver-
529 eV
age valence in Table 2. The O1s spectra of Pr2 –
531 eV
x = 1.0
xSrxCoO4 catalysts are shown in Fig. 5. There are
y
x = 0.8
x = 0.6
x = 0.4
x = 0.2
only peaks approaching the BE (529 eV) when x = 0.2
and 0.4; however, the O1s spectra of Pr2 – xSrxCoO4
y
catalysts, when x = 0.6–1.0, included two peaks, and
the peak areas approaching the BE (531 eV) increase
with the increase of x. As commonly accepted [17], the
peak of O1s spectra of Pr2 – xSrxCoO4
catalysts
y
approaching the BE (529 eV) may be ascribed to the
lattice oxygen species, O2– (lattice), and the peak of O1s
curves approaching the BE (531 eV) may originate
526
528
530
532
534
Eb, eV
from the adsorbed oxygen species such as O–ads , i.e., the
Fig. 5. XPS patterns of O1s of Pr
Sr CoO
catalysts.
y
2 – x
x
4
oxygen chemically adsorbed on oxygen vacancies.
According to Wagner et al. [18] for the MgKα source,
the molar percentage of cations of the three kinds of
component elements on the surface is estimated and also
listed in Table 3. Table 3 shows that the Co/(Pr + Sr) sur-
face ratios for the samples are much higher than
expected, considering the nominal compositions, and
increase gradually with the increase of x. These results
suggest that enrichment of the Co cation on the surface
occurred for all samples, and that the surface concentra-
tion of Co increased with the increase of x.
Temperature, °C
450
400
350
300
250
200
2
1
0.2
0.4
0.6
0.8
1.0
Conversion
Catalytic Activity of Pr2 – xSrxCoO4 y Catalysts
Fig. 6. Catalytic activity as a function of x in the
Figure 6 depicts the variations of 50 and 100% con-
version for NO reduction by CO over Pr2 – xSrxCoO4
catalysts versus x. It can be seen that the activity of alyl
samples increases monotonously with the increase of x.
At the order of x = 0.2, 0.4, 0.6, 0.8, and 1.0, their N2
selectivity values are 27, 30, 35, 42, 56% at 250°C and
58, 69, 74, 82, 91% at 300°C, respectively. The results
of N2 selectivity of these catalysts are similar to the
results of catalytic activity. Since the BET area (Table 2)
varies from sample to sample, activity is normalized to
the BET area. Figure 7 shows the strong influence of
the temperature and substitution degree on the intrinsic
activity. The rate is calculated per unit surface area and
increases with the increase of x, for which the oxygen
vacancy increases with the increase of x. The reduction
mechanism over Pr2 – xSrxCoO4 y catalysts, a proposed
mechanism of NO reduction by CO is presented as fol-
lows [19]:
Pr
Sr CoO
catalysts for reduction of NO by CO:
2 − x
x
4
y
(1) 50% conversion and (2) 100% conversion.
Intrinsic activity, mmol/(m2 min)
6
5
4
3
2
1
0
3
2
1
0.2
0.4
0.6
0.8
1.0
x
Fig. 7. Influence of the substitution degree (x) on the intrin-
sic activity of the Pr Sr CoO catalysts for reduction
NO by CO at reaction temperature (1) 220, (2) 250, and (3)
280°C.
Co3+–O–Co3+ + CO
Co2+–VO–Co2+ + NO
Co3+–VO(NO–)–Co2+
2Nads
Co2+–VO–Co2+ + CO2, (III)
Co3+–VO(NO–)–Co2+, (IV)
Co3+–O–Co3+ + Nads, (V)
2 – x
x
4
y
The values of Eb of these component elements are in
good agreement with those reported in the literature
[15, 16]. The results obtained from XPS investigation
show that the measured Eb for Co 2p3/2 deviates grad-
N2.
(VI)
It is found that the catalytic activity is closely corre-
lated with oxygen vacancy for NO reduction by CO
over Pr2 – xSrxCoO4 y catalysts.
ually from the value of Co2+ (Eb = 781.2 eV) with the
KINETICS AND CATALYSIS Vol. 47 No. 3 2006

PHYSICOCHEMICAL PROPERTIES AND CATALYTIC PERFORMANCE
429
4. Liu, Y., Yang, X.G., Liu, Y.M., et al., Acta Phys.-Chim.
Sin., 1999, vol. 15, no. 6, p. 506.
CONCLUSIONS
The perovskite-type Pr2 – xSrxCoO4 y (0.2 ≤ x ≤ 1.0)
mixed oxides of K2NiF4 structure have been prepared
by the polyglycol gel method. All mixed oxides display
K2NiF4 structure; when x = 0.2 and 1.0, the obtained
samples still have a trace of other mixed oxides; how-
ever, the mixed oxides with x = 0.4, 0.6, and 0.8 are the
phase Ä2BO4. With the increase of x, lattice parameters,
unit-cell volume, and average crystalline size decrease
gradually; microstrain density, the concentration of
Co3+, the amounts of lattice oxygen released, and the
concentration of oxygen vacancy increase gradually.
5. Wu, Y., Zhao, Z., Liu, Y., et al., J. Mol. Catal. A: Chem.,
2000, vol. 155, p. 89.
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The Pr2 – xSrxCoO4 catalysts possess different cata-
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vol. 337, p. 345.
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their catalytic activities are closely correlated with oxy-
gen vacancy and the concentration of Co3+.
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ACKNOWLEDGMENTS
p. 88.
This work was supported by the science and tech-
nology bureau of Jiangxi, China (project no. 2006) and
the science and technology department of Jian, Jiangxi
province of China (project no. 20052817) and granted
from the College of Jinggangshan.
14. Zhao, Z., Yang, X.G., and Wu, Y., Appl. Catal., B, 1996,
vol. 8, p. 281.
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