Bair S. Bal'zhinimaev

Size effects in ethylene oxidation on silver catalysts. Influence of support and Cs promoter

A new method for preparing supported silver catalysts with a narrow particle size distribution has been developed. For crystals with a diameter ranging from 100 to 1000 A the dependencies of reaction rates on size have been studied for epoxidation and deep oxidation of ethylene. For particles 300–500 A in size the reaction rate was found to increase sharply. The concentration of ethylene π-complexes with Ag+ cations increased proportionally to the reaction rate. These π-complexes are assumed to be the only intermediates directing the reaction towards epoxidation. The ratio of the reaction rate to the concentration of π-complexes was found to remain constant within a wide range of Ag crystal sizes including bulk silver samples. The influence of the support (α-Al2O3, SiO2) on the size effect relates to the secondary processes of ethylene oxide conversion. The factors that cause the catalytic size effect and the promoting effect of Cs are discussed.

Catalysts Based on Fiberglass Support: IV. Platinum Catalysts Based on Fiberglass Support in Oxidation of Hydrocarbons (Propane and n-Butane) and Sulfur Dioxide

Platinum catalysts (0.003–0.52% Pt) based on leached sodium silicate and boron silicate fiberglass supports are studied in the complete oxidation of hydrocarbons (n-butane and propane) and high-temperature SO2 oxidation. It was shown that platinum localized in the bulk of the glass matrix show a higher activity and thermal stability than metal particles supported on the outer surface of fiberglass. The experimental results for hydrocarbon oxidation on platinum-containing fiberglass gauzes at short contact times are discussed.

Transient response and infrared studies of ethylene oxide reactions on silver catalysts and supports

Abstract Secondary reactions of ethylene oxide on silver powder, catalysts Ag/α-Al 2 O 3 , Ag/SiO 2 and support α-Al 2 O 3 have been studied by using transient response technique and IR spectroscopy. Three possible routes of ethylene oxide conversion on the surface of the silver catalysts and supports have been elucidated: (1) ethylene oxide decomposition to ethylene and adsorbed oxygen; (2) ethylene oxide isomerization to acetaldehyde; (3) formation of glycol-like species on the support surface and their further oxidation on silver particles. The first two processes occur mainly on silver powder and catalyst Ag/SiO 2 . Deep ethylene oxide oxidation rates on Ag/α-Al 2 O 3 with small silver particles exceed those on the catalysts with larger silver particles. Ag/α-Al 2 O 3 promotion with cesium or water vapour addition to the reaction mixture decelerate isomerization and accelerate the deep oxidation of ethylene oxide. Formation of glycol-like species on the support surface was found to involve basic hydroxyl groups of the support. The contribution of secondary ethylene oxide reactions via this pathway can be dominating, especially for alumina surface modified with alkali additives and/or water vapour.

Catalysts Based on Fiberglass Supports: V. Absorption and Catalytic Properties of Palladium Catalysts Based on a Leached Silica-Fiberglass Support in the Selective Hydrogenation of an Ethylene–Acetylene Mixture

The absorption and catalytic properties of palladium catalysts (0.01% Pd) based on leached soda–silica fiberglass supports were studied in the selective hydrogenation of acetylene as the constituent of an ethylene–acetylene mixture. It was found that fiberglass catalysts exhibited much higher selectivity than traditional supported Pd catalysts. It was suggested that the high selectivity in the reaction of acetylene hydrogenation resulted from the selective absorption (diffusion) of acetylene in the bulk of fiberglass, where Pd microparticles are localized.

Catalysts Based on Fiberglass Supports: III. Properties of Supported Metals (Pt and Pd) According to Electron-Microscopic and XPS Data

Under various preparation conditions, metals supported on leached borosilicate and soda–silica fiberglass form three types of particles. First, these are metal particles in sizes from tens to hundreds of angstrom distributed over the outer surface of fibers. Second, in the presence of mesopores in fiberglass supports (borosilicate glasses), metal particles of commensurable sizes (15–100 Å) are localized in the cavities. Third, dispersed (<10 Å) particles are present in the bulk of fiberglass; these particles are intercalated into the bulk of leached glasses as deep as several hundreds of angstrom. The amount of a metal and the depth of penetration into the bulk of fiberglass can be controlled by preparation conditions and by the addition of cointercalates, which are responsible for pillaring effects.

Novel statistical lattice model for the supported nanoparticle. Features of the reaction performance influenced by the dynamically changed shape and surface morphology of the supported active particle

The aim of this study is to reveal the mutual influence of the shape and the surface morphology of supported nanoparticles on the reaction kinetics. The analysis has been provided by means of the novel statistical lattice model, which imitates the physicochemical processes that proceed over the supported particles. To simulate the active metal particle the finite Kossel crystal located on the inert support has been chosen. The surface morphology of the particle is defined by distribution of heights of the metal atom columns. The metal atoms attract the nearest neighbour metal atoms and the atoms of the support. The attraction is characterised by interaction energies between the nearest neighbour metal atoms and between the metal atom and the support underneath. The change of morphology is caused by the thermal diffusion of the surface atoms. As a result the equilibrium shape of the particle has been observed to depend on the temperature and the relative ratio of metal–metal and metal–support energies. The model reaction 2Au2006+u2006B2u2006→u20062AB has been studied, taking into account the roughening of the particle surface and the spillover phenomenon of the adsorbed Aads over the support. It has been shown that the kinetics of the roughened nanometer-sized particle can be notably different from those corresponding to the flat homogeneous surface. The shape of the nanoparticle can change under the influence of the adsorbed layer even in the absence of the adsorbate–metal interactions.

Influence of the surface layer of hydrated silicon on the stabilization of Co2+ cations in Zr–Si fiberglass materials according to XPS, UV-Vis DRS, and differential dissolution phase analysis

The stabilization of cobalt cations in zirconium–silica fiberglass materials was studied by X-ray photoelectron spectroscopy, ultraviolet visible diffusion reflectance spectroscopy, and a differential dissolution phase analysis. It was found that the commercial Zr–Si fiberglass material contained a layer of hydrated silicon (the depth of 6 nm) on the surface of the glass fibers. Modification of the fiberglass material with cobalt led to its distribution in the fibers mainly in the Co2+ state. It was shown that 90% of cobalt was stabilized on the surface and in the hydrated silicon layer as Co2+ cations in tetrahedral oxygen coordination, while the remaining 10% was distributed non-uniformly in the bulk of the fibers as Co2+ cations in octahedral oxygen coordination.

Structure of ultra disperse Pt powders and their performance in the partial oxidation of C-H bonds by molecular oxygen

Ultra disperse Pt powders was found to possess of high selectivity and activity at mild temperatures in the partial methane or benzene oxidation, and also in n-heptane isomerization. This is thought to be due to the presence of partially charged Pt sites on the surface, which stabilize molecular oxygen species as peroxide groups.

On the role of oxygen in the reaction of selective NO reduction with methane over Co/ZSM-5 catalyst

In this work the dynamics of adsorbed NO species formation and their reactivity with respect to methane was studied by DRIFT and SSITKA technique. It was shown that NOΔs formed via NO interaction with adsorbed oxygen are the most active species to react with CH4. Basing on the obtained results scheme of the reaction mechanism was proposed.

Modeling of crystallization process in confined melt of sulfuric acid catalyst.

Metropolis Monte Carlo technique has been applied to simulate the crystallization process in the melt of vanadium sulfuric acid supported catalysts. The melt is a lattice binary compound consisting of (V4+)(2) and (V5+)(2) binuclear complexes (dimers) confined by pore walls of cylindrical or slitlike shape. It has been shown that the crystallization process retards significantly as the pore size decreases. This result is in good agreement with the experimental data obtained earlier. The effect of the energy properties of pore walls (attractive, repulsive, or inert) on the crystallization features has been studied as well.