No abstract

No abstract

In the present contribution we demonstrate a new testing cell, which allows simultaneous in situ optical and electrochemical investigations of passivation processes on zinc anodes in quiescent alkaline electrolytes. By combination of microscopy and galvanostatic impedance spectroscopy it was possible to detect the starting point of passive film formation, which has not been achieved so far. We found that formation of the so-called type 1 passive film is not dependent on the anodic current density. This passive film appears at electrode overpotentials of <0.15 V after an amount of zincate ions of approx. 8.2 • 10 -4 mol cm -2 has accumulated at the anode in a 30 wt% KOH electrolyte with 2 wt% ZnO at room temperature. Consequently, the passivation of zinc in quiescent electrolyte cannot be avoided even at very low dissolution currents. On the other hand, the so-called type 2 passive film appears below the first film due to direct oxidation of zinc. At overpotentials of ≥0.15 V the direct oxidation of zinc is favored and the potential-dependent type 2 passive film appears before type 1 film is formed.

In recent years, the catalyst pellets made of open‐cell metallic foams have been identified as a promising alternative in fixed‐bed reactors. A reliable modeling tool is necessary to investigate the suitability of different foam properties and the shapes of foam pellets. In this article, a workflow for a detailed computational fluid dynamics (CFD) model is presented, which aims to study the flow characteristics in the slender packed beds made of metal foam pellets. The CFD model accounts for the actual random packing structure and the fluid flow throughout the interstitial regions is fully resolved, whereas flow through the porous foam pellets is represented by the closure equations for the porous media model. The bed structure is generated using rigid body dynamics (RBD) and the influence of the catalyst loading method is also considered. The mean bed voidage and the pressure drop predicted by the simulations show good agreement with the experimental data.