Hydrotalcitelike Co-Al and Ni-Al catalysts of different compositions (with the atomic ratio M 2+ /Al 3+ = 0.5–3.0) were studied in the reaction of selective reduction of NO by propane, propylene, and n -decane in the presence of O 2 . The higher activity of the catalysts with M 2+ /Al 3+ = 0.5 is connected with high dispersity of Ni or Co cations stabilized by a significant amount of Al 3+ ions. Propylene was shown to be the most efficient reducing agent for nitrogen oxide. The highest degree of conversion to the extent of 90–99% was attained at 400 and 420–440°C for Ni-Al and Co-Al samples, respectively. When propane and decane were used as reducing agents, the conversion of both catalysts was characterized by the volcano-shaped dependence on temperature due to the fact that the catalyst took part in the concurrent reaction of hydrocarbon (reducing agent) oxidation. Hydrotalcitelike materials are promising representatives of inexpensive bi- and multicomponent systems. The design strategy for new active catalysts for processes of purification of gas exhausts from NO x , that are stable in the presence of water and sulfur oxides, may be based on usage of hydrotalcites with modified ions introduced into them.

Based on a mechanistic study of the selective reduction of NO x by propane on NTK-10-1 and Ni-Cr oxide (NCO) catalysts, the reason for synergism in this process on a mechanical mixture of the catalysts was determined. On the NCO catalyst at temperatures higher than 250°C without NO x activation, C3H8 was oxidized with the formation of a considerable amount of hydrogen. This hydrogen migrated to the surface of NTK-10-1 through a gas phase and reduced this surface. On the reduced surface, H2 reacted with NO x by a mechanism characteristic of supported platinum group metals. In accordance with this mechanism, nitrogen atoms, which were formed by the dissociation of NO on metal atoms reduced by hydrogen, recombined to form nitrogen molecules in a gas phase, whereas oxygen atoms reacted with the hydrocarbon to form CO2 and H2O molecules in a gas phase. The positive effect of H2, which was formed on the NCO surface, on the reduction of NO x on NTK-10-1 is the main reason for synergism. An analysis of the experimental data demonstrated that an effectively working mechanical mixture of catalysts can be obtained if one of the mixture components is responsible for the effective activation of nitrogen oxides and the other is responsible for the activation of hydrocarbons.

C помощью ИК-спектроскопии in situ изучен механизм реакции селективного каталитического восстановления NOx пропаном в присутствии О2 на промышленном Ni–Cr-оксидном катализаторе. Показано, что в условиях реакции существуют нитритные, нитратные и ацетатные поверхностные комплексы. При взаимодействии реакционных смесей NO + C3H8 + O2 или C3H8 + O2 с поверхностью катализатора образуются значительные количества водорода. Измерены скорости превращения поверхностных комплексов, обнаруженных в условиях реакции. Полученные значения сопоставлены со скоростью процесса. Установлено, что при температурах ниже 200°С нитратные комплексы реагируют с углеводородом с образованием ацетатных комплексов, при этом не наблюдается образования продуктов реакции. В области температур выше 250°С реализуется два маршрута. Один из них заключается во взаимодействии ацетатных и нитратных комплексов с образованием продуктов реакции. По второму маршруту происходит разложение NO на восстановленной поверхности. Атомы азота рекомбинируют, а атомы кислорода реокисляют поверхность катализатора и реагируют с активированным углеводородом с образованием СО2 и H2O в газовой фазе.

The reaction mechanism of the selective catalytic reduction of NOx by propane in the presence of O2 on a commercial Ni-Cr oxide catalyst was studied using in situ IR spectroscopy. It was found that nitrite, nitrate, and acetate surface complexes occurred under reaction conditions. Considerable amounts of hydrogen were formed in the interaction of NO + C3H8 + O2 or C3H8 + O2 reaction mixtures with the catalyst surface. The rates of conversion of the surface complexes detected under reaction conditions were measured. The resulting values were compared to the rate of the process. It was found that, at temperatures lower than 200°C, nitrate complexes reacted with the hydrocarbon to form acetate complexes; in this case, the formation of reaction products was not observed. In the temperature region above 250°C, two reaction paths took place. One of them consisted in the interaction of acetate and nitrate complexes with the formation of reaction products. The decomposition of NO on the reduced surface occurred in the second reaction path. Nitrogen atoms underwent recombination, and oxygen atoms reoxidized the catalyst surface and reacted with the activated hydrocarbon to form CO2 and H2O in a gas phase.

The formation of a nematic chromonic mesophase in aqueous solutions of quaternized poly(ethylene imine) has been found using polythermal polarization microscopy. The reaction kinetics of O -( para -nitrophenyl) O,O -dimethyl thiophosphate hydrolysis has been studied by spectrophotometry, and a comparative analysis of the effects of lyotropic liquid crystals constructed as hexagonal and chromonic mesophases on this kinetics has been performed. It has been found that the hydrolysis of the substrate in a nematic chromonic mesophase is accelerated due to the concentration of the reactants.

Investigation of the mechanism of the selective reduction of NOx by propane over the individual samples of commercial catalysts NTK, STK, and Ni–Cr-oxide catalyst and over their binary mechanical mixtures has shown that the synergistic effect observed in the latter case is caused by the oxidative activation of propane on the STK and Ni–Cr-oxide surface which results in the formation of more effective reducing agents, propylene and hydrogen correspondingly. In the case of the Ni–Cr-oxide and NTK catalytic system, hydrogen forms over the former catalyst in propane oxidation, migrates through the gas phase to the latter catalyst, where NOx is activated with the formation of nitrate structures which interact with the said hydrogen giving the products of the overall reaction, N2 and H2O. When the pair of NTK and STK is concerned, the interaction of C3H8 and O2 over the latter catalyst gives stable products of partial propane oxidation and/or oxidative dehydrogenation which are transported due to interphase diffusion to NTK surface. The nature of observed synergistic enhancement of catalysis in the case of binary mixtures is proposed under the terms of “remote control” mechanism described in literature and can serve a useful purpose in the design of catalysts for this reaction.

According to X-ray diffraction analysis data, the test catalyst was a Ni-Cr spinel with an impurity of NiO. With the use of in situ IR spectroscopy, it was found that nitrite, nitrate, and acetate surface complexes occurred under the reaction conditions of the selective catalytic reduction of nitrogen oxides by propane in the presence of oxygen on the nickel-chromium catalyst. As the temperature was increased, the nitrite complexes were converted into nitrate species. The molar absorption coefficient of surface nitrate complexes was determined. According to IR-spectroscopic and TPD data, the nitrate complexes were bound relatively weakly to the surface. The temperature region of their existence was 50–200°C. The temperature region of existence of the surface acetate complexes was 200–400°C. The individual adsorption of oxygen was not observed; however, oxygen-containing surface sites (Cr5+=O) participated in the formation of the surface complexes of reactants.

Formulations of original catalytic compositions have been developed to clear gas emissions of highway transport on the basis of catalysts that are free of precious metals: NTK-10-1, STK, and Ni-Cr oxidic catalyst, the use of which was not made in these processes previously. High activity of the developed catalytic compositions is caused by the effect of synergism that has been discovered and studied for the first time with mechanical mixtures of commercial catalysts. The investigation of the mechanism of the reaction of selective NOx reduction by propane over individual industrial catalysts has demonstrated that the effect of synergism that can be observed with their binary mechanical mixtures is caused by oxidizing activation of propane that proceeds on the surface of STK and Ni-Cr oxidic catalysts with the formation of more efficient reducing agents, namely, propylene and hydrogen respectively. Owing to the interphase diffusion, the last-mentioned fall on the surface of the NTK-10-1 catalyst through the gas phase, and the catalyst activates NOx.

В обзоре проведен анализ работ, посвященных изучению явления синергизма в катализе на механических смесях катализаторов. Рассмотрены методики проведения эксперимента, формулы для расчета величины эффекта синергизма. Приведены гетерогенно-каталитические реакции, широко используемые в нефтехимии и органическом синтезе, в которых ярко выражен синергизм.

Studies concerning the phenomenon of synergism in catalysis over mechanical catalyst mixtures are surveyed. Experimental procedures and formulas for calculating the magnitude of a synergistic effect are considered. Heterogeneous catalytic reactions widely used in petroleum chemistry and organic synthesis that exhibit pronounced synergism are discussed.

Surface nitrite-nitrate and acetate compounds were detected under conditions of the selective reduction of nitrogen oxides by propane on the STK iron-chromium oxide catalyst using spectrokinetic measurements. The rate of conversion of these complexes under reaction conditions was measured. The resulting values were compared to the rate of the process. The results of this comparison indicated that, at low temperatures (to ∼250°C), the rate of reduction of nitrogen oxides was determined by the interaction of surface nitrite-nitrate complexes with the activated hydrocarbon. The amount of acetate complexes on the surface increased with temperature. A reduction of the surface was also observed as the temperature was increased. The reaction of NO decomposition began on the reduced surface. Nitrogen atoms recombined, and oxygen atoms reacted with the hydrocarbon to form CO2 in a gas phase. A distinctive feature of the STK catalyst is its ability to form the products of propane mild oxidation and/or oxidative dehydrogenation. From a synergistic standpoint, the STK catalyst is an effective supplier of the activated hydrocarbon, whereas the NTK-10-1 catalyst well activates NOx. This fact explains the nonadditivity effect observed in the catalytic properties of mechanical mixtures of these catalysts.

Works on catalytic methods of cleaning gas emissions of NOx that appeared within 1996-2001 are surveyed. Basic attention is given to the reduction of NOx with hydrocarbons. Catalyst active sites and the structure of transient surface compounds are discussed. Schemes of reaction mechanisms proposed on the basis of information about surface complexes are presented.