Macrokinetic analysis of polarisation characteristics of gas-diffusion electrodes in contact with liquid electrolytes, Part II: Oxygen reduction as example for a higher order reaction

Classical partial oxidation processes often suffer from low selectivities. Promising alternatives are electrochemical processes where the oxidation takes place at a packed-bed anode while an oxygen-consuming gas–liquid membrane is used as cathode. As a basis for the reliable design of such a process, the performance of oxygen-consuming gas-diffusion electrodes (GDE) is investigated experimentally and is analysed based on a rigorous model accounting for the reaction microkinetics and all relevant mass and charge transport phenomena. The results indicate that oxygen is transported in the gas-filled pores by Knudsen diffusion and that the cathodic oxygen reduction follows a parallel reaction scheme forming hydrogen peroxide at the carbon black support and water at the applied platinum catalyst particles.     

Influence of solid phase conductivity on spatial localization of electrochemical processes in flow-through porous electrodes: Part I: Electrodes with uniform conducting matrix

Mathematical simulation of the flow-through porous electrode (PE) operation on the basis of a one-dimensional model with a uniform conducting matrix and a cathodic process involving the main and side reactions (i.e., hydrogen evolution) has been made. The influence of current density and rate and direction of the solution flow on the depth of the main process penetration into the PE has been analysed for different relationships between phase conductivities. It has been shown that when the polarization curve of the side reaction is Tafelian, and the rate of solution circulation is high, there is a limit for the main process penetration into the PE. This limiting value is close to the thickness of the layer, Ld, capable of working at the limiting diffusion current obtained by Sioda's method. The dependence of the Ld layer thickness on phase conductivity has been analysed. In the limiting cases (low fractional conversion, high or identical phase conductivities) analytical expressions for Ld have been obtained. At low flow rates, the depth of the main process penetration increases up to the value of the entire thickness of the PE. It can be concluded that the possibility of increasing the PE efficiency for the uniform matrix by changing phase conductivities is limited.     

Experimental evaluation of a kinetic method for the study of non-catalysed heterogeneous reactions in solid–liquid systems: Leaching of apatite by dilute nitric acid

A method for evaluating the formal reaction order n of a chemical reaction is proposed, based on a kinetic equation describing chemical dissolution of solid particles in a batch reactor. The evaluation is based on the fact that the function expressing the dependence of the conversion, X_B, on the term c_A0ⁿt has the same course for different initial concentrations of the liquid reactant, c_A0 (t denotes time), at constant temperature and ratio of reactants. The measured dependences of the conversion on the time give a function, f_s(X_B), characterizing changes of the solid surface area during the reaction. Minimization of deviations of this function permits the influence of mass transfer between the bulk of the liquid and the solid surface on the measured formal reaction order to be corrected. Knowledge of the value of n and the function f_s(X_B) enables the derived kinetic equation to be integrated numerically and the dependence of the conversion on time to be calculated. A set of experimental data measured during dissolution of natural apatite in dilute nitric acid (0.1–1.5 mol dm⁻³) at 20°C was used for demonstration of the proposed method. © 1998 SCI