Phase boundary reaction

For very thin reaction product layers, it is to be expected that electrical effects connected with the electrical double layer at the phase boundary and associated space charge phenomena in the vicinity of the phase boundary will also have an important effect upon the kinetics. No information is presently available regarding these electrical effects for reactions between solid phases. However, in chapter 8 a detailed discussion of such phenomena will be given in connection with another class of solid state reactions. 

Finally, it must be noted that ideal contact between the various phases is not always achieved. If ideal contact does not occur, then the reaction will be affected by gas transport across narrow gaps and fissures. The rate-determining step of the phase boundary reaction could then be evaporation, condensation, or gas transport. 

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The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

Since the catalyst is concentrated and operates in the ionic phase, and also probably at the phase boundary, reaction volumes in the biphasic technology are much lower than in the conventional single-phase Dimersol process, in which the catalyst concentration in the reactor is low. As an example, the Difasol reactor volume can be up to 40 times lower than that classically used in the homogeneous process. 

Probably the only feature common to the mechanism of oxidation of the two groups is that, because of crack or pore formation in the continuous oxide, the rate of transport of oxygen in a molecular form has increased to the point where a phase-boundary reaction has assumed rate control. In... 

As we have seen, a consequence of the formation of porous oxide is that the rate-controlling step reverts to that of a phase boundary reaction and... 

In sulphur dioxide linear kinetics are generally observed due to control by phase boundary reactions, i.e. adsorption of SOj. RahmeF suggested that this is one of the conditions which favours simultaneous nucleation of sulphide and oxide at the gas/scale interface. The main reaction products are NiO, NijSj, Ni-S,j, and NiS04, depending on the temperature and gas pressure for example, according to the following reaction ... 

W. Lorenz and G. Sake, Mechanism of the Electrochemical Phase Boundary Reaction, Z Phys. Chem. (Leipzig) 218 259(1961). 

Figure 5-7. Vacancy concentration distribution (a) and fluxes of h and V" in A O (b) after a change of fi0 at the crystal surface. Phase boundary reaction is y02 = Oq +V +2h. ...

Figure 8-2. Steady state fluxes, component concentrations and phase boundary reactions lor (A.B)O exposed to A/Jq2-...

Simplification of the Model Triple-Phase-Boundary Reactions as Boundary Conditions... 

II Electrochemical syntheses are phase boundary reactions. The boundary problems, which are also familiar from heterogeneous catalysis, must be expected especially during continuous operation. 

The superscript (Oxv) is an abbreviation for oxygen vacancy. Because the newly forming layer i is oxygen-rich relative to the decomposing layer — 1, an additional number of oxygen anions are required in order to utilize all of the cations released in the solid-state decomposition of layer i — 1 at xt = 0 and in the presently considered case, these are provided by a new component of the anion vacancy current generated in layer i by this phase boundary reaction. Since layer i must carry, in addition, the anion vacancy currents generated by the phase boundary reactions at Xi = 0, x2 = 0,. . . , j = 0, we have the identity... 

Thus, the phase boundary reaction at ac,- = 0 utilizes the anion vacancy difference current (J/av) — Jif ) in order to be sustained. This consideration of the anion balance at x( = 0 leads us to the alternative expression... 

In the case of dissolution of a number of semiconducting metal oxides in acid aqueous solutions, it can be calculated from the rate of solution, that a phase boundary reaction must be rate determining. Moreover, it can be shown, that this reaction is electrochemical in nature, for its rate depends on the potential of the dissolving oxide. As an example, Fig. 1 shows the influence of the electrode potential on the rate of dis -solution of FeQ g Oin sulfuric acid and hydrochloric acid. The rate of dissolution is expressed in terms of a dissolution current. A linear relation exists between the electrode potential... 

Phase-boundary reaction of hydrogen with oxygen ions of the magnetite lattice, with formation of water vapor and release of electrons... 

Reactions between Metals. These are essentially phase-boundary reactions between two solids and are of the simplest type. The reaction between copper and... 

The enthalpies of formation of MgS04 and CaS04 have been measured [86] from dissociation equihbria 1173 to 1638 K. There was no evidence for the formation of basic sulfates. Hulbert [87] concluded that magnesium sulfate decomposed by a contracting volume phase-boundary reaction in the range 1193 to 1353 K and, as in the barium salt, was approximately equal to the enthalpy of dissociation (312 kJ mol ). The rate of reaction was decreased by an increase in particle size and by the retention of SO, in the product layer. 

K was well described [80] by the zero-order equation, identified as a onedimensional phase boundary reaction, , = 141 kJ mol . This reaction was studied as a potential energy-storage system. 

Fast mass transfer at the phase boundary Another characteristic feature of microsystems derived from their much greater surface-to-volume ratios is that they make phase boundary reactions such as gasdiquid, liquid/liquid or solid/liquid reactions more efficient. The rate of mass transfer increases with an increase in the phase boundary surface-to-volume ratio. 

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