Химия и химическое
производство/8. Катализ
Bliznjuk O.N., Ogurtsov A.N., Masalitina N.Yu.
INFLUENCE OF SYNTHESIS PROCEDURE ON THE MORPHOLOGY OF MIXED OXIDE CATALYST FOR SELECTIVE CATALYTIC REDUCTION OF NOX
Nitrogen oxides (NOx) are a group
of air pollutants, including nitrogen oxide, nitrogen dioxide and nitrous
oxide, considered as very dangerous, since they contribute to the greenhouse
effect and participate in photochemical reactions that lead to acid rain, tropospheric ozone and respiratory problems in humans.
Thus, the removal of NOx is becoming a
serious issue because of increasing environmental concerns.
Selective catalytic reduction of NOx by
using ammonia (NH3-SCR) as the reducing agent, is a consolidate,
efficient and widely used method for the abatement of NOx,
especially in stationary applications. As a matter of fact, the reduction of
the nitrogen oxides by means of ammonia in an oxidizing atmosphere has raised
the need of the development of new catalysts, characterized by low cost and
capability of ensuring high conversions, even at relatively low temperatures
[1–4]. Chemical and microstructural properties of
mixed oxide catalyst powders depend on the synthesis route, and for that reason
the researches investigated different routes in the synthesis of oxide
catalyst. In this study, we report the behavior of mixed oxide
as the catalyst prepared by using the sol-gel method, which has the advantages
that include low cost, easy stoichiometric control
and high uniformity.
The
pure BiFeO3, Ce doped Bi1−xCexFeO3, Ce and Mn co-doped BiFeO3
(Fe-Ce-Mn-Bi-O) mixed oxide were fabricated by using
the sol-gel process. Bismuth nitrate Bi(NO3)3×5H2O,
cerium nitrate Ce(NO3)3×6H2O,
manganese nitrate Mn(NO3)2×4H2O and
ferric nitrate Fe(NO3)3×9H2O were
used as the starting materials to prepare the BiFeO3, Bi1−xCexFeO3 and
Fe-Ce-Mn-Bi-O O precursor solution.
Excess 2 mol% Bi(NO3)3×5H2O was
used to attempt to compensate for the expected loss of volatile Bi during the
following heat treatment.
The sol-gel method
employed for the synthesis of the catalysts involved the use of two different
organic surfactants, such as glycine (C2H5NO2) and
citric acid (C6H8O7), with different Fe/Mn/Ce/Bi ratios, in order to
investigate the influence of different compositions of the Fe-Ce-Mn-Bi-O systems
well as of different synthesis procedures on the catalytic activity.
Among all dopants, Ce
should be a promising one since the ionic radii of Ce3+ (1.18
Å) and Ce4+ (1.02 Å) are closer to that of Bi3+ (1.20
Å). Appropriate Ce cations
might be used to substitute for A site Bi3+ in the
BiFeO3 matrix and Ce cations
could stabilize oxygen octahedron that improve the crystallization [1–2]. Moreover,
doping at the Fe site with aliovalent atoms, such as Mn, is found to be advantageous in improving catalytic
properties of BiFeO3. The substitution of Mn
ions is expected to affect the number of oxygen vacancies as well as the
chemical states of Fe ions. All the above arguments motivate the authors to
investigate interaction of the Ce and Mn codoped.
The
crystalline orientation, microstructure of the samples were characterized using
X-ray diffraction (XRD), scanning electron microscopy (SEM). Thermo-gravimetric
curve of Fe-Ce-Mn-Bi-O powder was determined by TGA.
All the peaks at the typical XRD patterns of the BiFeO3 and
Fe-Ce-Mn-Bi-O samples are
indexed according to the standard powder diffraction date of BiFeO3
complied in the JCPDS card which consisted of hexagonal structure (JCPDS №. 20-0169). As Ce and Mn elements doped to BiFeO3 samples, the diffraction
peaks of (111), (200) and (002), (431) appear for CeO2 and Mn-(BiFe)O3-x phase,
respectively. These results indicate that the Ce and Mn elements begin to crystallize in Fe-Ce-Mn-Bi-O samples and
demonstrate that the structure of BiFeO3 is changed. In addition, a peak around 2θ = 46º in the
pattern of the Fe-Ce-Mn-Bi-O shift and was split into three peaks, indexed by tetragonal
symmetry, which results clearly indicated that a phase transition from the
distorted hexagonal structure for the pure BiFeO3 to tetragonal structure for the Fe-Ce-Mn-Bi-O samples was taken
place.
The thermal decomposition process and the DSC curve of Fe-Ce-Mn-Bi-O
powder displayed two exothermic peaks at 220ºC and 400ºC. The first peak could be attributed to
the decomposition of Fe-Ce-Mn-Bi-O from high
molecular polymers to low molecular matters, and significant weight loss was
observed nearly 220ºC. With the increase of temperature, the second
exothermic peak at 400ºC mainly resulted from the burning of carbon decomposed from Fe-Ce-Mn-Bi-O 3 samples. Therefore, on a basis of analyzing the TG
curve, we designed the thermal process that 220ºC and 400ºC were chosen as dry and pyrolyzed temperatures, respectively.
The measurements of the SCR activity were performed by
placing 180 mg of the catalyst in a fixed-bed quartz reactor of
The trend was similar for all samples with a bell shape characterized by
a range of maximum activity and a subsequent decrease. This behavior was
clearly caused by the competition of the NOx reduction
with at least two side reactions: the catalytic oxidation of ammonia to give N2
and the similar one to produce N2O.
The Fe-Ce-Mn-Bi-O sample with glycine
showed the best trend in NOx conversion,
reaching 100% for a wide range at low temperature up to 200ºC. Its
catalytic performance was likely due to the higher BET specific surface area
value and the higher superficial amount of labile oxygen.
The increase of the BiFeO3 content
allowed to enlarge the operating window of temperature in the range 100–225ºC, in which the Fe-Ce-Mn-Bi-O sample with glycine showed a yield higher than
60%. So the addition of ceria oxide and MnOx to BiFeO3 permitted to obtain higher values of NOx conversion as well as higher N2 yield, especially at low
temperatures. In particular, the Fe-Ce-Mn-Bi-O obtained by sol-gel method with glycine as surfactant showed the most promising catalytic
performance for the low-temperature ammonia-SCR of NOx.
In this case, the presence of mixed oxides with a considerable reducibility,
seemed to be the principal factor to get the most active and selective catalyst
in agreement with previous findings over BiFeO3 phase.
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A.S. Savenkov, O.N. Bliznjuk,
A.N. Ogurtsov,
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Earth 29 (2011) 64.
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