V. A. Matyshak

The reaction mechanism of selective catalytic reduction of nitrogen oxides by hydrocarbons in excess oxygen: Intermediates, their reactivity, and routes of transformation

The main features of the mechanism of selective reduction of nitrogen oxides by hydrocarbons (methane, propane, and propylene) in excess oxygen catalyzed by systems containing transition metal cations are considered. A combination of steady-state and non-steady-state kinetic studies, in situ Fourier-transform infrared (FTIR) spectroscopy, temperature-programmed desorption, and theoretical analysis of bond strengths and spectral data for adsorption complexes made it possible to determine reliably that surface nitrate complexes are key intermediates at real temperatures of catalysis. The rate-limiting step in these reactions includes the interaction of these complexes with hydrocarbons or their activated forms. Factors are considered that determine the structure, bond strength, and routes of nitrate complexes transformations under the action of hydrocarbons. Mechanistic schemes are proposed for the reaction of various types of hydrocarbons in which the determining role belongs to the formation of organic nitro compounds in a rate-limiting step. Their further fast transformation with the participation of surface acid sites resulting in the formation of ammonia, which is a highly efficient reducing agent, though not limiting the whole process, but determines nevertheless both the selectivity to the target product, molecular nitrogen, and the selectivity of hydrocarbon consumption for nitrogen oxide reduction.

The Mechanism of Low-Temperature Ammonia Oxidation on Metal Oxides According to the Data of Spectrokinetic Measurements

Low temperature ammonia oxidation on MoO3, Fe2O3, Cr2O3, and ZnO is studied by the spectrokinetic method. It is shown that the following adsorbed species are intermediates in this reaction: NH3 and N2O on Fe2O3 and ZnO; NH3, N2O, and NO on Cr2O3. All of the detected intermediates are used to construct the mechanism of the process. In the framework of the proposed mechanism, stationary and nonstationary spectral and kinetic data are quantitatively processed. The dependence of the rate constants of the same steps on different oxides on their physicochemical properties is discussed.

Problems of Quantitative Spectroscopic Measurements in Heterogeneous Catalysis: Molar Absorption Coefficients of Vibrations in Adsorbed Substances

Published data on the molar absorption coefficients ε and integral intensities A0 of vibrations in physically and chemically adsorbed molecules are reviewed. Analysis of published data shows that bonds characterized by high values of the dipole momentum derivative with respect to the normal coordinate change during adsorption toward decreasing this derivative, whereas bonds characterized by a low value of the derivative change toward increasing the derivative. Thus, adsorption results in a decrease in the difference in values of the dipole momentum derivative with respect to the normal coordinate of different bonds compared with the same bonds in the individual molecules. In addition, the interval of changing the molar absorption coefficients for the surface complexes are at least two orders of magnitude lower than that for the same bonds in the molecules in the gas phase. A series for the degree of easiness in detecting complexes on the catalyst surface (the series of decreasing the molar absorption coefficient) is proposed.

Formation Mechanism of O2– Radical Anions in the Adsorption of NO + O2 and NO2 + O2 Mixtures on ZrO2 According to EPR and TPD Data

The effects of various factors on the formation of O2– radical anions in the adsorption of an NO + O2 or NO2 + O2 mixture on ZrO2 were studied. It was found that the thermal stability of the O2– species depends on the composition of the adsorbed gas. It was suggested that nitrogen oxide complexes on ZrO2 centers are responsible for the formation of O2–. These centers are formed upon the treatment of the oxide in a vacuum; however, they are different from both coordinatively unsaturated Zr4+ cations (NO adsorption centers at 77 K) and Zr4+–O––O––Zr4+ centers, at which O2– are formed because of the adsorption of H2 + O2. Based on the experimental data, the mechanism of O2– formation in the adsorption of an NO + O2 mixture is discussed.

ESR and TPD Study of the Interaction of Nitromethane and Ammonia with HZSM-5 and CuZSM-5 Zeolites

The properties of complexes formed on HZSM-5 and CuZSM-5 zeolites in the course of ammonia and nitromethane adsorption are studied. Ammonia adsorbs on CuZSM-5 and forms two species, which decompose at different temperatures Tdec. One is due to the formation of the Cu2+(NH3)4 complex (Tdec = 450 K), and the other is assigned to ammonia adsorbed on copper(II) compounds, Cu2+O– and Cu2+–O2––Cu2+, or CuO clusters (Tdec = 650–750 K). Ammonia adsorption on Cu+ and Cu0 is negligible compared with that on the Brönsted acid sites and copper(II). Nitromethane adsorbed on HZSM-5 and CuZSM-5 at 400–500 K transforms into a series of products including ammonia. Ammonia also forms complexes with the Brönsted acid sites and copper(II) similar to those formed in the course of adsorption from the gas phase, but the Cu2+(NH3)4 complexes on CuZSM-5 are not observed. Possible structures of ammonia and nitromethane complexes on Brönsted acid sites and the Cu2+ cations in zeolite channels are discussed. The role of these complexes in selective NOx reduction by hydrocarbons over the zeolites is considered in connection with their thermal stability.

Role of nitrogen dioxide in the oxidation of diesel soot on promoted mixed catalysts with fluorite and perovskite structures

The oxidation of soot on catalysts with the perovskite and fluorite structures (including platinum-promoted catalysts) in the presence and in the absence of NO2 was studied using in situ IR spectroscopy and temperature-programmed techniques (TPR, TPD, and TPO). It was found that, as a rule, the temperature of the onset of soot oxidation considerably decreased upon the addition of NO2 to a flow of O2/N2, whereas the amount of oxygen consumed in soot oxidation considerably increased. To explain these facts, we hypothesized that the initiation of soot combustion in the presence of NO2 was related to the activation of the NO2 molecule through the formation (at a low temperature) and decomposition (at a high temperature) of nitrate structures on the catalyst. Superequilibrium amounts of NO2 resulted from the decomposition of nitrate complexes immediately on the catalyst for soot combustion. Based on a comparison between catalyst activities and data obtained by TPR and the TPD of oxygen, a conclusion was drawn that the presence of labile oxygen in the catalyst is a necessary but insufficient condition for the efficient occurrence of a soot oxidation reaction in the presence of NO2. The introduction of platinum as a constituent of the catalyst increased the amount of labile oxygen and, as a consequence, increased the amount of highly reactive nitrate complexes. As a result, this caused a decrease in the temperature of the onset of soot combustion.

Selective NO reduction by propane over commercial oxide catalysts and their mechanical mixture: I. Synergistic effect

In the reaction of NO reduction by propane in excess oxygen, the activity of a binary mixture of commercial oxide catalysts is higher than should be expected if it were proportional to the activities of separate catalyst components. This synergism reveals itself in the fact that the conversion of NO and C3H8 is higher when the catalyst is a mechanical mixture than the sum of conversions over separate catalysts, all other conditions being the same. Based on the kinetic and thermal desorption measurements, the experimental conditions are found under which the synergism is observed. A hypothetical mechanism is proposed.

The mechanism of selective NOx reduction by hydrocarbons in excess oxygen on oxide catalysts: VI. Spectroscopic and kinetic characteristics of surface complexes on a Ni-Cr oxide catalyst

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.

Selective NO reduction by propane over commercial oxide catalysts and their mechanical mixture: II. Intermediate complexes and reaction mechanism

Based on a spectrokinetic study (simultaneous measurements of the concentrations of surface complexes and reaction rate usingin situ IR spectroscopy), a hypothesis is proposed which explains synergism in the reaction of NO reduction by propane on a mechanical mixture of commercial oxide catalysts.

The mechanism of selective NOx reduction by hydrocarbons in excess oxygen on oxide catalysts: V. Adsorption properties of a commercial Ni-Cr oxide catalyst

According to X-ray diffraction data, the STK catalyst is a mixture of Fe2O3 and Cr2O3. The temperature-programmed reduction spectrum exhibited two reduction peaks: one, with Tmax = 250°C, corresponds to the reduction process Cr2O3 → CrO and the other, with Tmax = 360°C, corresponds to the reduction Fe2O3 → Fe3O4. The results of thermal desorption measurements suggest that the individual adsorption of oxygen on the surface of the STK catalyst is low; in this case (according to IR-spectroscopic data), an atomic form is the main species. Surface nitrite-nitrate complexes are formed upon the adsorption of NO. Nitrite and nitrate complexes desorbed at maximum rates at 105 and 160°C, respectively. Unlike the NTK-10-1 catalyst, the NO species, which desorbed at high temperatures (250–400°C), was absent from the surface of STK. Propane adsorbed at room temperature to form surface compounds containing an acetate group. The interaction of propane with the surface of the STK catalyst at reaction temperatures resulted in strong surface reduction.