Химия/8. Кинетика и катализ

N. Yu. Masalitina, A. S. Savenkov, V.V. Rossikhin

National Technical University "Kharkiv Polytechnic Institute"

nat_masalitina@ukr.net

SELECTIVE OXIDATION OF AMMONIA TO N₂O
OVER Mn-Bi MIXED OXIDE CATALYST,
MODIFIED WITH CERIUM AND COPPER OXIDES:
CATALYTIC PERFORMANCE AND CHARACTERIZATION

There is an increasing commercial interest in finding alternative ways to produce phenol that overcome the disadvantages of the current cumene process used to synthesize phenol. The drivers for the change are both economic and environmental. A direct oxidation route for producing phenol from benzene is based on using N₂O as an oxidizing agent in the gas phase in the presence of modified Fe-ZSM5 zeolite. Thus, direct phenol synthesis from benzene in a one-step reaction with high benzene conversion and high phenol selectivity is most desirable from viewpoints of environment-friendly green process and economical efficiency.

Selective catalytic oxidation of ammonia (NH₃) with air at low temperatures is an efficient method to produce N₂O as oxidizing agent for organic synthesis. This process has two important parameters: the selectivity and the application temperature. To rationally develop a process for NH₃ oxidation to N₂O over catalysts, the reaction mechanism must be clarified. While several studies have examined the low temperature oxidation process, the mechanism of NH₃ oxidation and N₂O formation is still uncertain. Generally use an imide (NH) mechanism in which the first step yields NH, and then the NH reacts with atomic oxygen (O) to form nitroxyl (HNO) and further conversion to N₂ or nitrous oxide (N₂O), or NH could even react with molecular O₂ to produce nitric oxide (NO) [1].

In the present paper the influence of catalyst composition and some operating variables were evaluated by IR-spectroscopy in terms of N₂O formation, using Mn/Bi/Cu/Ce-oxide catalysts.

Some single and multi-oxides used for active phases were synthesized by sol-gel method. Different from precipitation, solid-state reaction or spray drying, this method is based on the addition of an organic complexation agent (here citric acid) into the precursors. The presence of the organic complexation agent distinguishes this complexation method from the other methods owing to the complexation and gelation steps. These steps are influenced mainly by the atomic ratio of citric acid to metal cations and pH of the solution. Sol-gel method leads to the formation of very pure and homogeneous catalyst powders exhibiting high surface area. Different salts of Mn(NO₃)₂, Cu(NO₃)₂×6H₂O, Ce(NO₃)₃×6H₂O, Bi(NO₃)₂×4H₂O were dissolved in water in order to obtain the solution with the concentration of 0.125M. 10%wt. citric acid solutions prepared from citric acid monohydrate – C₆H₈O₇×H₂O.

MnO₂-Bi₂O₃-CuO-CeO₂ catalyst was synthesized by dropping a suitable amount of Cu(NO₃)₂, Bi(NO₃)₃ and Ce(NO₃)₃ solutions into a suitable volume of Mn(NO₃)₂ solution corresponding to different MnO₂/Bi₂O₃/CuO/CeO₂ molar ratios. If precipitation occurred, concentrated HNO₃ solution was added until the precipitates disappear. A suitable amount of citric acid solution was dropped into the obtained solution with the molar ratio of citric to metals of 2. The obtained solution was stabled within 30 minutes and evaporated at 70–80ºC until the gel was obtained. The gel was then dried at 110ºC for 3 hours. The obtained solid were calcinated at 550ºC for 3 hours with the heating rate is 3ºC/min.

Single metallic oxides, bi-metallic oxides, other triple metallic oxides and tetra metallic oxides were synthesized similarly. The catalysts were characterized by some techniques, such as: X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), BET for detemining specific surface area, Xray Photoelectron Spectrocopy (XPS), TG-DTA, TG-DSC, Infrared Spectrocopy (IR). Catalysis activity of the catalysts were examinied in a micro reactor set up.

The IR spectra of ammonia adsorbed on the catalysts show the bands at 1594 and 1165 cm^–1, which attributed to σ_as and σ_s model of NH₃ coordinated to Lewis acid sites. Another two bands at 1674 and 1445 cm^–1 are attributed to σ_s NH₄⁺ and σ_as NH₄⁺ resulting from ammonia coordinated to Brønsted acid sites [2]. It is indicated by the increase in intensity of band at 1165 cm^–1 that more Lewis acid sites are generated on Mn/Bi/Cu/Ce-oxide by introduction of Ce^x+ which can also serve as Lewis acid sites. Comparison of IR spectra from catalyst treated with 1000 ppm NO, 1000 ppm NO₂, and 1000 ppm NO + 2% O₂ shows the five bands at 1610, 1550, 1466, 1291, and 1030 cm^–1. The bands at 1550, 1291, and 1030 cm^–1 can be assigned to bidentate nitrate; the band at 1466 cm^–1 can be attributed to the monodentate nitrite [3]. The mechanism proposed for N₂O generation at low temperature is based on the formation of surface Ce-ON species which may be produced by the partial oxidation of dissociatively adsorbed ammonia species with NO + O₂ (eventually NO₂) [4]. When these active sites are in close proximity they can interact to form an N₂O molecule.

[1] N.Yu. Masalitina, A.S. Savenkov, O.N. Bliznjuk, A.N. Ogurtsov, Chem. Ind. Ukr. 5 (2014) 24.

[2] W.U. Xiaodong, S.I. Zhichun, L.I. Guo, W. Duan, J. Rare Earth 29 (2011) 64.

[3] G. Qi, R.T. Yang, J. Phys. Chem. B 108 (2004) 15738.

[4] J.A. Martin, M. Yates, P. Avila, S. Suares, J. Blanco, Appl. Catal. B Environ. 70 (2007) 330.