Heterogeneities particle paths

In catalysis active sites are operative that allow for an alternative reaction path. For a satisfactory catalyst this alternative pathway leads to higher rates and higher selectivity. In heterogeneous catalysis reactant molecules adsorb at active sites on the catalyst surface at the surface sites reactions occur and products are desorbed subsequently. After desorption, active sites are again available for reactant molecules and the cycle is closed. In homogeneous catalysis the situation is essentially identical. Here complexation and decomplexation occur. A complication in heterogeneous catalysis is the need for mass transfer into and out of the catalyst particle, which is usually porous with the major part of the active sites at the interior surface. 

The major focus on the effects of exhaust emissions has been on the HC1 component and its role in ozone depletion and on the A1203 particles, which could provide a surface for the heterogeneous conversion of HC1 to active forms of chlorine. It has been proposed that if the HC1 were converted to photochemically active forms relatively rapidly, a mini ozone hole could form in the flight path of the vehicle (Aftergood, 1991 McPeters et al., 1991 Karol et al., 1992). 

Many heterogeneous reactions give rise to an increase or decrease in the total number of moles present in the porous solid due to the reaction stoichiometry. In such cases there will be a pressure difference between the interior and exterior of the particle and forced flow occurs. When the mean free path of the reacting molecules is large compared with the pore diameter, forced flow is indistinguishable from Knudsen flow and is not affected by pressure differentials. When, however, the mean free path is small compared with the pore diameter and a pressure difference exists across the pore, forced flow (Poiseuille flow see Volume 1, Chapter 3) resulting from this pressure difference will be superimposed on molecular flow. The diffusion coefficient Dp for forced flow depends on the square of the pore radius and on the total pressure difference AP ... 

Increased precipitation over the oceans may increase the supply of a more bioavailable, dissolved aeolian Fe via wet deposition (Fig. 35.5 path 8-9-11 direct effect). During long range transport, dust particles undergo heterogeneous reactions at gas-soHd-Hquid interfaces involving poUution-derived substances that may lead to the increased solubility of aeolian Fe (Dentener et al, 1996 Meshidzke et al, 2003 Underwood et al, 2001). Photochemical reduction in more acidic cloud waters and precipitation promote dissolution of Fe in dust, leading to the production... 

The rate of heterogeneous condensation depends on the exchange of matter and heat between a particle and the continuous phase. The extreme cases of a particle much larger or much smaller than the mean free path of the suspending gas are easy to analyze. In the continuum range (dp ip), diffusion theory can be used to calculate the transport rate. For a single sphere in an infinite medium, the steady-state equation of diffusion in spherical coordinates takes the form... 

Most systems in everyday life are not perfectly clean, and dusts, particulates or several substrates provide a template for initial growth of a new phase. If the value of the interfacial tension between the new condensing phase and the particle (or the substrate) surface is low, then this property considerably decreases the energy requirement of nuclei formation (AGT)max so that heterogeneous nucleation occurs as an easier and preferable path for phase transformations. In addition, phase transformations take place at or very close to the equilibrium temperature. 

Packed bed reactors [51] incorporate beads, and the presence of these solid materials in the analytical path also has a beneficial influence on the flow pattern, reducing sample broadening and, hence, sample dispersion. However, hydrodynamic pressure tends to be increased, especially when small particles and/or particles with heterogeneous sizes are used. Single bead string reactors [5] were proposed to circumvent this drawback. These reactors utilise large (particle diameters about 70% or the tube inner... 

Gel permeability Rate constant, fth-order heterogeneous reaction Bubble lubrication film length Mean free path, lattice spacing, or distance between particle collisions Pan length Characteristic length Gel or resin packed bed column length Development length for concentration profile in channel... 

The reduction of unsaturated C-C bonds constitutes one of the largest applications of heterogeneous catalysis [48]. On the surface of a metal particle, hydrogen is either cleaved homolytically to give M-H units or is adsorbed in the intact form. The metal sites that are responsible for either activation path of Hj on the catalyst surface as well as the chemical-physical characteristics of the adsorbed H2 are still a matter of debate. In this respect, molecular systems mimicking the activation of Hj on a single metal site without the cooperation of a solvent are expected to provide valuable information on the mechanism of Hj adsorption/release over heterogeneous metal catalysts. 

This behaviour is characteristic of partial interfacial polarisation induced by conductive paths that are not arranged parallel to the electric field. The same phenomenon has already been observed in heterogeneous combinations of mesoscopic metallic particles in... 

Figure 7.18 shows schematic crack paths through (a) water ice and (b) ice cream. The weakest part of the water ice microstructure is the ice-matrix interface, so the crack follows a path along the edges of the ice crystals as far as possible. In the ice cream, the air bubbles are the weakest links so the crack passes through them. The ice crystals are the most fracture-resistant components so in both cases the crack passes around them. Ice cream is effectively a particulate-reinforced composite material, i. e. a material in which tough particles are embedded in a relatively soft continuous phase in order to increase the overall fracture resistance. Since the microstructure is heterogeneous, it is not possible to predict exactly where the crack will form or precisely what path it... 

Position Dependence. In polymers with heterogeneous structures— for example, semicrystalline polymers and filled elastomers— the transport process is complicated by the generally impermeable dispersed phase. Not only does the crystallite or filler particle create a larger path for the diffusing molecule to traverse, but also the presence of a high area interface within the polymer changes the nature of the continuous phase from that of the pure homogeneous state. These effects are related by the expression (4) ... 

A severe limitation for practical applications is the large pressure difference between single crystal experiments under UHV conditions and the real reaction conditions of heterogeneous catalysis and other surface processes. Most surface science techniques are restricted to pressures below 10 Torr because they require hot filaments and high mean free paths for the probing particles. IR spectroscopy as an optical technique can be used in normal atmosphere as well as with liquid surroundings and is therefore ideally suited for in situ studies of surface processes under real reaction conditions. 

For the heterogeneous catalytic process to be effective, the reactants present in the surrounding fluid phase must be transported to the surfece of the solid catalyst, and after the reaction, the products formed must be carried back from the surface to the bulk fluid. The path of the physical rate processes at the particle scale is divided into two parts, as depicted in the 7-step sequence of the continuous reaction model used in microkinetic analysis ... 

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