Nonempirical quantum chemical calculations of the vibrational spectra of surface methoxyl groups on aluminum and silicon oxides

Institute of Catalysis, Siberian Branch, Russian Academy of Sciences. Samarkand State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 5, pp. 49–54, September–October, 1992.     

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.     

Electronic and chemical properties of nanostructured cerium dioxide doped with praseodymium

Nanostructured doped ceria is a prospective material for catalytic applications such as the construction of membranes with mixed electronic and ionic conductivity for effective syngas production. In this article, the surface properties of nanostructured ceria doped with praseodymium have been studied by X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and Fourier transform infrared spectroscopy of adsorbed carbon monoxide. The effects of supporting 1.4 wt % Pt as well as structural changes upon the reduction of the samples with methane have been investigated. While in samples without supported platinum, mainly praseodymium cations are reduced in a methane atmosphere; stronger reduction of cerium cations was found in the case of surface modification with Pt. The structural differences correlate with results from temperature-programmed reaction experiments with methane. Explanations are discussed in terms of different reaction mechanisms.     

Synthesis and characterization of the Pt/Al2O3 systems modified with group III (Ga) and Group IV (Ge,Ti, and Zr) elements in the combined conversion of propane and n-Heptane

Under the conditions of a joint reaction of propane and n-heptane at temperatures of 460–520°C and a pressure of 0.35 MPa, the conversion of propane and the concentration of C₇₊ aromatization products on platinum-containing catalysts modified by Group III (Ga) and Group IV (Ge, Ti, and Zr) elements were higher than those on an unmodified Pt/Al₂O₃ sample. This is explained by a change in the aprotic acidity of the catalysts as a result of the support modification. The sample with the addition of gallium was most active. A plausible reason for this is the conversion of hydrocarbons at active sites that consist of Pt and Ga, which were formed upon catalyst activation. It is believed that gallium adjacent to platinum in an ionic form on the support surface acts as an aprotic acid site.     

Structure and acid-base properties of hexaaluminates

Hexaaluminates SrAl₁₂O₁₉, BaAl₁₂O₁₉ and LaAl₁₁O₁₈ were prepared by co-precipitation of soluble nitrates of Sr, Ba, or La and Al using NH₄HCO₃ as a precipitating agent with subsequent calcination at 700–1400°C. The samples were characterized by adsorption methods, thermal analysis (TA and DTG), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy of adsorbed probe molecules (CO and CDCl₃). It was shown that calcination at 1200°C resulted in the formation of single-phase hexaaluminates in all the samples except for LaAl₁₁O₁₈, where an additional phase was found. The specific surface area of the samples was obtained in the range 16–22 m²/g. After the treatment at comparable conditions, the total concentrations of both surface Lewis acid sites and basic sites were found to increase in the series: BaAl₁₂O₁₉ < SrAl₁₂O₁₉ < LaAl₁₁O₁₈. However, the strongest basic sites were detected on the surface of BaAl₁₂O₁₉.     

Steam reforming of dimethoxymethane,methanol and dimethyl ether on CuO–ZnO/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst

The performance of a СuO–ZnO/γ-Al₂O₃ catalyst for the reactions of methanol, dimethyl ether (DME) and dimethoxymethane (DMM) steam reforming (SR) to hydrogen-rich gas was studied. The catalyst was found to be active and selective for methanol and DMM SR producing hydrogen-rich gas with low content of CO (<1 vol %). It provided complete conversion of methanol and DMM at 300°C, and hydrogen productivity of, respectively, 15 and 16.5 L_H2g _cat ^-1 h^-1. With the use of physicochemical methods and catalytic experiments, it was shown that the catalyst surface contained the acid sites typical for γ-Al₂O₃, and CuO–ZnO agglomerates, responsible, respectively, for DMM hydration to methanol and formaldehyde, and SR of these compounds to hydrogen-rich gas.     

Effect of hydrothermal treatment on the composition and structure of Pt(IV) hydroxo complexes as model precursors of the active component in Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

The influence of hydrothermal treatment conditions (temperature, duration, acidity of the medium) on the structural and chemical transformations of the complex H₂[Pt(OH)₆] was studied. The composition and structure of the resulting compounds were determined by several physicochemical methods. Thermal analysis coupled with mass spectrometry showed that, as the hydrothermal treatment temperature is raised from 25 to 120 and 150°C, the product composition in terms of empirical formulas changes as follows: PtO₂ · 4H₂O → PtO₂ · 3.5H₂O → PtO₂ · 1.5H₂O. X-ray diffraction and UV and IR spectroscopy demonstrated that the changes in the chemical composition are accompanied by the amorphization of the structure and Pt-O bond strengthening. X-ray structure determination using the radial electron density distribution method showed that polynuclear species ～10–15 Å in size with a structure similar to that of orthorhombic PtO₂ form in the complexes subjected to “hard” hydrothermal treatment (T ≥ 150°C).     

Joint Conversion of Methane with Pentane on Alumina–Platinum Catalysts Modified with Zirconium Oxide under Nonoxidizing Conditions

Kinetics and Catalysis - The physicochemical properties of alumina–platinum catalysts with supports prepared by mixing aluminum and zirconium hydroxides have been studied. The chemisorption...     

Kinetics of the oxidation of some polycyclic aromatic hydrocarbons by lead tetraacetate in aqueous acetic acid and perchloric acid medium

The state of metal cations in CuCr₂O₄/-Al₂O₃ catalysts under its operation in catalytic heat generators has been examined. IR spectra of adsorbed CO indicate copper ion reduction in the catalyst to Cu⁺, whose surface concentration was determined. Diffuse reflectance spectra have revealed that the initial catalyst contains Cr(VI), disappearing after reaction.

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CuCr₂O₄/-Al₂O₃ (). - CO , Cu⁺. Cu⁺. , Cr(VI), .

     

Determination of the strength of Lewis acid centers via IR spectroscopic measurement of adsorbed pyridine

According to high-temperature IR spectroscopic studies, the isosteric heats of pyridine adsorption on the Lewis centers of Al₂ O₃, ZrO₂ and zeolite HNaY have been estimated. The dependence between the frequency _8a (1580–1630 cm^–1) and the adsorption heat is shown to take place only for centers formed by cations of the same metal.

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, - HNaY. , _8a (1580–1630 cm^–1) , - .