Investigation of leaching kinetics in preparation of raney-silver catalyst

ABSTRACT Raney-Ag is particularly used as an electrocatalyst in producing cathodes for hydrogen- oxygen fuel-cells. Raney-Ag, a Raney-metal catalyst, is prepared by leaching out aluminum from Raney Ag-Al (50:50 % wt) alloy with concentrated sodium hydroxide solution. At the same time, the leaching out of aluminum in Raney Ag-Al alloy with sodium hydroxide solution is a liquid-solid noncatalytic reaction. In this study, the kinetics of leaching of aluminum from Raney Ag-Al alloy had been investigated by following the increase of the amount of aluminum in the reaction mixture at a constant alkaline concentration (sodium hydroxide solution, 25 %). Experiments were carried out in a stirred batch reactor at the different temperatures (i.e., 10, 20, 35 and 35°C) using the different sizes of the alloy particles, i.e. those less than 45ji, 53 to 75 \i, 90 to 125 u and 125 to 150 u. External mass transfer resistance was eliminated by running at high stirring speed. It was observed that the rate of leaching increased with the increasing of temperature and at the initial conditions in which no limitation of external and internal mass transfer were present/The activation energy of the leaching was ranged between 52-60 kJ/mol. At high temperatures, it was observed that particle size did not have any effect on leaching rate. For the average particle size, 90 to 125 u, the fresh surface area of Raney Ag-Al was compared with the surface area of Raney-Ag obtained after leaching at 25 °C, and it was observed that the surface area increased from 0.52 ± 0.05 m2/gr to 7.4 ±0.1 m2/gr. Prout-Tompkins' s model which assumed that chain-branching is the slowest step was applied to the experimental data. By assuming that the reaction rate constant is also changed as inversely proportional to the square of time, it was determined that the model equation is suitable for the leaching out of aluminum from Raney Ag-Al alloy with sodium hydroxide solution, within the experimental mistake. Keywords: Raney-Ag catalyst, Raney Ag-Al alloy, leaching. IX