Origin of rate bistability in Mn–O/Al2O3 catalysts for carbon monoxide oxidation: role of the Jahn–Teller effect

Peculiarities in catalytic activity in carbon monoxide oxidation as well as some structure, electronic and magnetic properties of the three oxide catalysts, Mn³⁺–O/Al₂O₃ (1), Mn³⁺–O–Fe/Al₂O₃ (Mn-substituted spinel, 2) and -Fe₂O₃/Al₂O₃ (3), were studied by kinetic measurements and by Mössbauer spectroscopy. The catalysts 1 and 2 showed a kinetic bistability with a sharp transition towards more reactive state at 200°C (ignition point). In contrast, for catalyst 3, at 200–250°C, the behavior of reaction rate against temperature did not display noticeable hysteresis. On cooling the catalysts 1 and 2, extinction was observed at about 170 and 120°C, respectively, i.e., at 30–80°C lower than the corresponding ignition points. Proximity of activation energy for the high and low activity (15–19 kJ/mol) for both Mn-containing catalysts suggests an increase in the number of active sites at high temperature with no changes in the reaction mechanism. The considerable difference between Mn-containing catalysts 1, 2 and Fe-containing catalyst 3 may be caused by Jahn–Teller (JT) type distortions of the oxygen polyhedron around Mn³⁺. A significant spontaneous axial bond stretching within the local polyhedron seems to diminish Mn–O binding energy, facilitate the participation of surface oxygen species, O_S, in the oxidation of CO by a redox mechanism and promote oxygen vacancies at the surface that would cause considerable effect on the activity. An increase in the width of the counterclockwise hysteresis loop for the catalyst 2 compared to the catalyst 1 indicates that clusters of mixed spinel provide more active sites and more labile O_S species than clusters of the binary Mn oxide.     

Low-temperature formation mechanism of double oxides Fe x Zr(Ti)1−0.75x O2−δ prepared from alkoxides and acetylacetonates

As a result of titanium and zirconium alcoholates hydrolysis in the presence of dissolved Fe(acac)₃, amorphous iron-containing gels have been synthesized. Their heat treatment has led to polycrystalline double oxides Fe _x Zr_1–0.75x O_2– (C) and Fe _x Ti_1–0.75x O_2– (T) formation. It has been shown that oxides (C) and (T) are likely to be solid solutions with 0.01<x<0.17 and 0.01<x<0.14, respectively. On the basis of X-ray diffraction and extended X-ray absorption fine structure data, iron-zirconium and iron-titanium crystallite models for gels and oxides have been proposed. It has been found that the crystallization process does not lead to a significant change in interatomic distances typical for local structures detected in gels.     

Superparamagnetic γ-Fe2O3 nanoclusters in silicate matrices

Superparamagnetic γ-Fe₂O₃ nanoclusters have been synthesized on the surfaces of activated layered silica (ALS) and activated mesoporous silica gel (AMS). According to 300-K Mössbauer spectroscopy data and magnetization measurements between 2 and 300 K, the size of the γ-Fe₂O₃ nanoclusters increases from 2–3 to s~6–8 nm as the Fe content of ALS increases from 2.5 and 4.5 to 18.5 wt %. The 2.5 wt % Fe/ALS sample contains superparamagnetic γ-Fe₂O₃ nanoclusters uniform in size and magnetic properties. The nanoclusters produced on the AMS surface are larger and are nonuniform in magnetic properties.     

Photoelectrochemical Activity of Nanosized Titania,Doped with Bismuth and Lead,in Visible Light Region

Protection of Metals and Physical Chemistry of Surfaces - A method for the preparation of film coatings of titania doped with bismuth (Bi3+) and lead (Pb2+) ions, separately and simultaneously, has...     

Photoelectrocatalytic Oxidation of Formic Acid in the Visible Spectral Region on Films of Nanocrystalline Titanium Oxide Doped by Bismuth

Protection of Metals and Physical Chemistry of Surfaces - A method of formation of film coatings of titanium dioxide doped by bismuth ions (Bi3+) is developed on the basis of sol–gel...     

Kinetic description of rapeseed oil conversion into aromatic hydrocarbons on promoted MFI zeolite

An approach to the construction of a kinetic model for the reactions of rapeseed oil hydroconversion to aromatic hydrocarbons has been proposed, which is based on analysis of experimental data obtained using a MFI zeolite promoted with zinc and chromium ions. An empirical mathematical model describing the dynamic behavior of the main products of the decomposition reaction of rapeseed oil as a model feedstock has been developed. It has been shown that an increase in the space time and temperature in the examined range of reaction conditions increase the yield of aromatic hydrocarbons. The influence of hydrogen pressure on the yield of aromatics is nonmonotonic in character, passing through a maximum, with the optimum yield being in the middle of the hydrogen pressure range of 10–20 atm.     

Structure and gas permeability of nanoporous metal oxide coatings produced by the alkoxide method

Peculiarities of the preparation of selective nanoporous metal oxide coatings on the surface of macroporous rutile support (pore size 0.1 μm and mixed aluminum and titanium oxides (pore size 0.05 μm) are presented. Gas selective coatings of empirical formula of P _x Ti_{1 ? 0.5x} O_{2 ± δ} with uniform distribution of nanosized pores have been obtained with the alkoxide method using titanium alkoxide and ettriol phosphite (precursor of the phosphorous-containing component) at their various total concentrations. Surface morphology of the obtained membranes has been investigated and major parameters of gas selective coatings have been determined. It has been shown that the dependences of the He, N₂, CO₂, and C₃H₈ gas permeability on temperature and pressure drop are consistent with the molecular regime of gas flow. The reproducible effect of permeability anisotropy of ～502?60% is observed when the integral vacuum method is used.     

Calculating the viscosity of furnace heavy fuel oil in optimum planning models

A method is proposed for calculating the kinematic viscosity of commercial heavy fuel oil at 100°C on the basis of mixing indices and with allowance for the equivalent kinematic viscosity of tar, which is represented by the product of the kinematic viscosity of the latter at 100°C by a certain coefficient determined from experimental data. There is satisfactory agreement between the calculations and measured data. Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 2, pp. 44–46, March–April, 2009.     

Synthesis,structure, and properties of a Au/MnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>–CeO<Subscript>2</Subscript> nanocatalyst for low-temperature oxidation of carbon monoxide

We have synthesized Au/MnO _x –CeO₂ nanocatalysts for the low-temperature oxidation of carbon monoxide. Gold nanoparticles applied by the deposition precipitation (DP) method were used as an active phase. The composition, structure, and textural characteristics of the materials and the charge state of the components of the catalysts were studied using X-ray diffraction, X-ray photoelectron spectroscopy, highresolution transmission electron microscopy, inductively coupled plasma mass spectrometry, and low-temperature nitrogen adsorption measurements. The carbon monoxide concentration in the catalytic oxidation products was determined by gas chromatography. The influence of calcination temperature on the charge state of the components of the surface layer of Au/ MnO_x–CeO₂ and the catalytic activity of the materials was examined. The catalytic activity of the materials was shown to be determined to a significant degree by the Mn³⁺, Au³⁺, and weakly bound oxygen concentrations in the surface layer.