Химия/7.Неорганическая химия

Savenkov A.S. , Masalitina N.Yu.

National Technical University "Kharkiv Polytechnic Institute"

Ukraine, Kharkiv, Frunze str., 21., onbliznyuk@ukr.net

Fe-Ce-Mn-Bi-O MIXED OXIDES CATALYSTS
FOR THE LOW-TEMPERATURE NH₃-SCR

Selective catalytic reduction of NO_x by using ammonia (NH₃-SCR) as the reducing agent, is a consolidate, efficient and widely used method for the abatement of NO_x, especially in stationary applications. As a matter of fact, the reduction of the nitrogen oxides by means of ammonia in an oxidising 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].

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 Bi_1−xCe_xFeO₃, 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, Bi_1−xCe_xFeO₃ 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)O_3-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 behaviour was clearly caused by the competition of the NO_x 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 NO_x 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 MnO_x to BiFeO₃ permitted to obtain higher values of NO_x 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.

References

1. S. Andreoli, F. Deorsola, R. Pirone. MnOx-CeO₂ catalysts synthesized by solution combustion synthesis for the low-temperature NH₃-SCR. – Catalysis Today. – 2015. – V. 253. – Р. 199–206.

2. B. Shen, T. Liu, N. Zhao, X. Yang, L. Deng. Iron-doped Mn-Ce/TiO₂ catalyst for low temperature selective catalytic reduction of NO with NH₃. – Journal of Environmental Sciences. – 2010. – V. 22, №9. – Р. 1447–1454.