74
Z. Gao et al. / Journal of Alloys and Compounds 646 (2015) 73e79
perovskite phase [11]. They proposed that the excess ironic oxide in
the n > 0 samples corresponds to presence of FeeO polyhedra at the
crystallite surface, and in such a way the decrease of La/Fe atomic
ratio does not affect the interior of the structure with a regular
perovskite lattice [11]. They further confirmed by increasing calci-
detector (TCD), where a reducing gas of 10%H
min, temperature was ramped to 900 C at 10 C/min, and sample
powder was fixed at 50.0 mg at each run.
Surface elemental composition of the samples was analyzed on
an X-ray photoelectron spectrometer (PHI Quantera) with mono-
2
/Ar was set at 40 ml/
ꢀ
ꢀ
ꢀ
ꢀ
nation temperature from 600 C to 1200 C that XRD peaks of Fe
phase appear at 800 C first and then increase their intensity at
higher temperatures [11].
2
O
3
chromatized Al K
a
radiation. The base pressure of the instrument
ꢀ
ꢁ9
was 2 ꢂ 10 Torr. Curve-fitting for the XPS peaks, which were
calibrated with the binding energy of adventitious carbon
C1s ¼ 284.8 eV, was performed with a GaussianeLorentzian profile.
It is confirmed that perovskite LaMnO3.158 is not an anion-
excess perovskite and just a conventional written form; it has an
3
actual composition La0.95Mn0.95O with randomly distributed La
2
.3. Catalytic activity for methane oxidation
and Mn vacancies at the cationic lattice sites and a filled sublattice
of anions in the crystal structure [13]. It is reported that the
sample with La/Mn atomic ratio of 0.95 (the La/Mn atomic ratio
Catalytic activity for methane oxidation of the LaeMn oxide
samples as catalysts (200 mg, 40e60 mesh) was evaluated in a
continuous flow fixed-bed quartz tube reactor (8 mm i.d.) at at-
mospheric pressure. Total flow rate of the feed gas was 200 ml/min,
in which methane was 2.0 vol%, oxygen gas 16.8 vol%, nitrogen as
balance gas. The catalytic reaction was carried out at 500 Ce600 C
in temperature rising way. After 1 h stabilization of the reaction at
each the reaction temperatures, reaction products were analyzed
on a gas chromatograph (Shanghai Kechuang, China). Then reaction
ꢀ
was measured by ICP) and calcined at 800 C is single phase
perovskite [14]. Its real composition can be written as
3
þ
4þ
La0.95Mn 0.73Mn 0.27O3.06 in a conventional way, and exactly is
3
þ
4þ
in the form of La0.931Mn 0.716Mn 0.265
O
3
with lattice vacancies
ꢀ
ꢀ
at the A-sites and the B-sites and a filled sublattice of lattice ox-
ygen anions (O2 ) [14]. Besides, the sample with La/Mn atomic
ꢁ
ꢀ
ratio of 0.91 (measured by ICP) and calcined at 800 C is also re-
ported to be single phase perovskite [15]. In the present work, a
series of LaeMn oxide samples with feed La/Mn atomic ratio at
temperature was elevated to the next higher value. CO
sole carbon-containing product.
2
was the
1.03e0.56 (measured by XRF) were prepared. Phase composition
of the samples was estimated by the Rietveld refinement method.
Surface composition was analyzed by XPS. Catalytic activity for
methane oxidation of the samples was also evaluated.
3
. Results and discussion
3.1. Phase composition and crystallite size
2
. Experimental
Fig. 1 shows XRD patterns of the six LaeMn oxide samples. All
samples display clearly characteristic peaks of perovskite phase.
Among the six samples, the four samples with La/Mn atomic ratio
at 1.03, 0.89, 0.81 and 0.72 (measured by XRF) are single phase
perovskites. The other two samples with La/Mn atomic ratio at 0.63
and 0.56 (measured by XRF) are composed of perovskite as the
2
.1. Preparation of LaeMn oxides
Citrate method was adopted to prepare LaeMn oxides from
metal nitrates (Guoyao Chemicals, China). At first, an aqueous so-
lution of lanthanum nitrate and manganese nitrate was added
dropwise into an aqueous citric acid solution (the molar ratio of the
citric acid to the total metallic ions was fixed at 0.5) under rigorous
stirring, and stirring was kept for 30 min after the addition
completed. Then, the citrate solution was heated in a rotary evap-
orator at 80 C and vacuum degree of 0.08 MPa for 25 min, where a
viscous liquid was obtained. The viscous liquid was dried in an oven
at 80 C for 5 h and subsequently at 110 C for 2 h. At last, the dried
sample was calcined in a muffle furnace at 700 C for 5 h. In this
3 4
main phase and Mn O as the minor phase. XRD patterns in Fig. 1
are fitted by the Rietveld refinement method, and the results are
listed in Tables 1 and 2. In the refinement, occupancy of lattice
oxygen anion is fixed at one [13], and occupancies of two of the
3
þ
3þ
4þ
ꢀ
three types of cation (La , Mn , Mn ) are refined with the
confinement of electrical neutrality in the perovskite crystal. Mass
balance on the La/Mn atomic ratios is not considered in the
ꢀ
ꢀ
ꢀ
2 3
refinement, since highly dispersed MnOx and/or La O species may
way six samples were prepared with feed La/Mn atomic ratio at
1.03, 0.89, 0.81, 0.72, 0.63 and 0.56 (measured by X-ray fluorescence
spectroscopy, XRF-1800, Shimadzu), respectively. These samples
are thus denoted as LaxMn, in which the variable x is the La/Mn
atomic ratio of the samples.
2.2. Characterizations
Phase identification of the samples was performed on an X-ray
diffractometer (D8 Advance, Bruker) with Cu Ka irradiation at 40 kV
and 40 mA. The Rietveld refinement method implemented in the
TOPAS software package was used to estimate phase composition
of the samples and crystallite size of the perovskite phase.
Specific surface area (SSA) of the samples was measured on a
SSA instrument (JW-DA, Beijing JWGB, China). At first the samples
ꢀ
were degassed at 150 C for 1 h in high vacuum, and then allowed
ꢀ
to adsorb N
relative pressure of p/p
calculate SSA value.
2
at liquid nitrogen temperature (ꢁ196 C) under a
0
¼ 0.06e0.30. The BET equation was used to
Reducibility of the samples was measured by temperature
programmed reduction (TPR) method on a TPR instrument (PX200,
Tianjin Pengxiang, China) equipped with a thermal conductivity
Fig. 1. XRD patterns of the samples with feed La/Mn atomic ratio at 1.03 (a), 0.89 (b),
0.81 (c), 0.72 (d), 0.63 (e) and 0.56 (f). The black circle denotes Mn phase.
3 4
O