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.     

Persistence of zero velocity fronts in reaction diffusion systems

Steady, nonpropagating, fronts in reaction diffusion systems usually exist only for special sets of control parameters. When varying one control parameter, the front velocity may become zero only at isolated values (where the Maxwell condition is satisfied, for potential systems). The experimental observation of fronts with a zero velocity over a finite interval of parameters, e.g., in catalytic experiments [Barelko et al., Chem. Eng. Sci., 33, 805 (1978)], therefore, seems paradoxical. We show that the velocity dependence on the control parameter may be such that velocity is very small over a finite interval, and much larger outside. This happens in a class of reaction diffusion systems with two components, with the extra assumptions that (i) the two diffusion coefficients are very different, and that (ii) the slowly diffusing variables has two stable states over a control parameter range. The ratio of the two velocity scales vanishes when the smallest diffusion coefficient goes to zero. A complete study of the effect is carried out in a model of catalytic reaction. (c) 2000 American Institute of Physics.     

Autowave modes of polymerization and other reactions in solid frozen matrixes near of absolute zero and their role in mechanisms of fast chemical transformations of substance in universe

Almost two decades ago there were discovered new autowave selfpropagation phenomena in the cryo‐chemical reactions at investigating of chemical solid phase transformations near absolute zero of temperatures. Such an autowave regime is observed for different classes of chemical reactions (polymerisation, copolymerisation and also hydrocarbon halogenation, hydro‐halogenation etc). The chemical transformations studied at the such low temperatures 4–77 K proceeded with so great rates that they can be compared only with the fastest high‐temperature combustion reactions known in chemistry. It allows to advance a principally new autowave conception of the matter chemical activity in solid state. The essentially non‐Arrenius conception is based on the assumption that a mechanical energy accumulated in solid matrix can be transformated to the chemical forms.     

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: I. Physicochemical Properties of Silica Fiberglass Supports

The physicochemical properties and structure of supports prepared by leaching soda–silica fiberglass materials were studied using a set of physicochemical techniques (BET; IR spectroscopy; transmission electron microscopy; and ²⁹Si, ²³Na, ²⁷Al, ¹³³Cs, and ¹²⁹Xe (of adsorbed molecules) NMR spectroscopy). A matrix that corresponded in chemical composition to SiO₂was formed at high degrees of leaching; however, it was considerably different from ordinary silica gels in properties. The structure and properties of this matrix are most adequately described by the model of a pseudolayer intercalation structure, which includes alternating layers of several silicon–oxygen tetrahedrons separated by narrow (<4 Å) cavities. Considerable amounts of OH groups (5000 mol/g) are contained in these cavities, and these OH groups are different from the surface hydroxyl groups of ordinary globular silica. Although the interlayer spaces are small, comparatively bulky cations can be intercalated into them.     

Propagation and ignition of fast gasless detonation waves of phase or chemical transformation in condensed matter

Fast self sustained waves of chemical or phase transformations, observed in several contexts in condensed matter effectively result in “gasless detonation". The phenomenon is modelled by coupling the reaction diffusion equation, describing chemical or phase transformations, and the wave equation, describing elastic perturbations. The coupling considered in this work involves (i) a dependence of the sound velocity on the chemical (phase) field, and (ii) the destruction of the initial chemical equilibrium when the strain exceeds a critical value (strain induced phase transition). Both the case of an initially unstable state (first order kinetics) and metastable state (second order kinetics) are considered. An exhaustive analytic and numerical study of travelling waves reveals the existence of supersonic modes of transformations. The practically important problem of ignition of fast waves by mechanical perturbation is investigated. With the present model, the critical strain necessary to ignite gasless detonation by local perturbations is determined. Received 18 November 1999     

Stability of active systems with a spatially periodic activity: Analysis of a simple model and application to the boiling crisis problem

We investigate theoretically the possibility to control the transition between two metastable states in reactive systems by imposing a spatial modulation. In particular, we consider the technologically very important case of the transition between the low temperature (nucleate boiling) and the high temperature (film boiling) phases of boiling of a liquid over a heat generating element, also known as the boiling crisis. With the help of a simplified model, we demonstrate that the dangerous regime where the high temperature phase invades the whole system requires a larger heat power in a periodically spatially modulated system, than in a uniform system. The possibility that a local perturbation, such as a small gas bubble, may induce locally a transition to the film boiling state is also considered. We show that the transition to the film boiling regime is hindered in a spatially periodic system. (c) 2002 American Institute of Physics.     

Nonlinear phenomena in heterogeneous catalytic reactions with a branched-chain mechanism of formation of active centers

The branched-chain mechanism of multiplication of active centers on the surface of a catalyst results in a hysteresis temperature dependence of reaction rate. Domain structures appear because of local disturbances in the case of diffusion instability of uniform stationary states under certain conditions for transfer coefficients. Autowave transition processes are characterized by a “plateau“ of zero front velocities with the formation of standing waves.