Activated state during diffusion

Figure 7.5 (a) Schematic of activated state during diffusion, (h) Diffusion of an ion down a chemical potential gradient. 

Activation methods can be divided into two groups. Activation by addition of selected metals (a few wt%), mainly transition metals, e.g., fine powders of Fe, Ni, Co, Cr, Pt, Pd, etc. ", or chlorides of these metals when these are reducible to the metal by hydrogen during presintering. The mechanism of activation is not understood (surface tension, surface diffusion, etc.) but is related to the electronic structure of the metal additive. Activation by carbon is also effective. Alternatively, activation utilizes powders in a specially activated state, e.g., very fine (submicronic) powders. ... 

Theoretical descriptions of absolute reaction rates in terms of the rate-limiting formation of an activated complex during the course of a reaction. Transition-state theory (pioneered by Eyring "", Pelzer and Wigner, and Evans and Polanyi ) has been enormously valuable, and beyond its application to chemical reactions, the theory applies to a wider spectrum of rate processes (eg., diffusion, flow of liquids, internal friction in large polymers, eta). Transition state theory assumes (1) that classical mechanics can be used to calculate trajectories over po-... 

A more sophisticated method for automated trajectory analysis and mode of motion detection provides the use of a rolling-window algorithm. The algorithm described by Arcizet et al. [45,46] reliably separates the active and passive states of particles and extracts the velocity during active states as well as the diffusion coefficients during passive states (Fig. 5). It takes into account that active transport by microtubules is characteristically directional over a certain time and measures... 

Enantiomeric excess diminished with time on stream when the modification concentration was 0.34mM, but remained constant when the concentration was 3.4mM (Figure 3). It is known for Pt/silica that alkaloid adsorption can occur on both the metal and the support and in our catalysts there may have been diffusion of alkaloid molecules between the support and the active phase during hydrogenations. In such a case, modification at a higher concentration would have provided more extensive adsorption on the support, and hence a larger reservoir of cinchonidine would have been available to sustain a steady state concentration on the Pt surface or to replace any modifier rendered inactive by partial or complete hydrogenation. 

From the DSA curves measured during the cooling of heat-treated materials, the active state of the powders (the non-equilibrium defect state) can be assessed. The values of the activation energy AH of radon diffusion in samples labeled by radon parent nuclides, and " Ra were used as a parameter characterizing... 

Undesired side reactions occur under starvation conditions, as discussed in detail in Section 20.4. These cell states are caused by an inhomogeneous distribution of reactant gases, temperature, and local water content across the active area during fuel-cell operation. Measurement of the current density distribution allows the detection of these critical conditions. However, gradients of the electrochemical reaction rates are mostly compensated by in-plane currents within the catalyst layer, gas diffusion layer, and bipolar plates. Therefore, for measurement of the current density distribution, the local current flow has to be separated into individual pathways. 

Chemical reactions activated by ultrasounds are conditioned not only by the mechanical forces developed during the dilatation phase but also by the electrochemical or photochemical phenomena that initiate into cavitation bubble, starting with its first stage of formation [1088]. In this stage, suitable conditions for the release of electrical discharges are created, i.e. low pressures and the presence of gases and vaporised substances that by ionisation pass in activated states. Subsequent development of cavitation bubble is accompanied by the increase of internal pressure, followed by decompression, when in the continuous liquid medium the already formed radicals and ions will diffuse. 

Further work has shown that cesium is a better promoter than potassium and that the alkali hydroxides are, surprisingly, reduced to a metallic state during catalyst activation. This seems thermodynamically improbable, but may result from the high heat of adsorption of the alkali metal hydroxides on carbon, which leads to charge transfer to the carbon, and could drive the reduction of the hydroxide to the metal. The alkali promoter may neutralize residual chloride ions and develop catalyst activity dttring reduction as mthenimn ions diffuse from the lattice to form crystallites on the edges of graphite crystals. 

Recently we have found a type of tightly bound ATP which exists even in the presence of saturating concentrations of hexokinase (Aflalo, Shavit, 1982) but is exchangeable with exogenous ATP. This bound ATP appears to arise from a free species of ATP sequestered near the active site during photophosphorylation. The slowed down diffusion of the newly made ATP to the outer medium space allows its binding to proximal non-catalytic sites. Thus, mass transfer of nucleotides between the active site and the bulk medium may limit the steady state rate of ATP synthesis and should be considered in proposals on the mechanism of energy transduction. 

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