Õèìèÿ/7. Íåîðã. õèìèÿ
N.Yu. Masalitina, A.S. Savenkov, V.V. Rossikhin
National Technical
University "Kharkiv Polytechnic Institute"
LOW TEMPERATURE SELECTIVE
CATALYTIC REDUCTION OF NOx OVER Ce AND Mn CO-DOPED CATALYSTS PREPARED BY
SOL-GEL METHOD
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 [1,2].
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 ¹. 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 7 mm under a reacting gas with the following composition: 700 ppm
NO, 700 ppm NH3, 3% O2, and balance He.
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.
[1] Andreoli S., Deorsola F., Pirone R.// Catalysis Today,
2015. – V. 253. – Ð. 199–206.
[2] Shen B., Liu T., Zhao N., Yang X.,
Deng L. // Journal of
Environmental Sciences, 2010. – V. 22, ¹9. – Ð. 1447–1454.