Development of skeletal copper-chromia catalysts II. Kinetics of leaching of aluminum and chromia deposition

Skeletal copper-chromia catalysts have been prepared by selective removal o f aluminum from CuAl2 particles and precipitation of chromia in the residua l Cu using a caustic solution containing a hexavalent chromium salt. The ra tes of leaching Al and deposition of chromia were both significantly affect ed by the reaction conditions. The rates of leaching Al and chromia deposit ion both increased with increasing temperature. Increasing sodium chromate concentrations in the leach liquor led to increased chromia deposition but inhibited the rate of leaching Al. Increasing NaOH concentrations reduced t he rate of chromia deposition, while slightly increasing the rate of Al lea ching up to about 6 M NaOH. Higher NaOH concentrations caused the Al leachi ng rate to decrease. Leaching of Al was found to be chemical reaction contr olled and the kinetic data were found to fit a Shrinking Core model. The pr esence of sodium chromate in the caustic directly affected the value of the pre-exponential factor (k(o)) for leaching Al from the alloy. The activati on energies for the leaching reaction in the presence and absence of sodium chromate were found to be 71.9 and 69.1 kJ mol(-1), respectively. The rate of deposition of chromia was fitted to an empirical model based on a funct ion of the leach temperature and of both NaOH and Na2CrO4 concentrations in the leach liquor. The models for leaching of Al and deposition of chromia can be used to determine the conditions for optimizing the preparation of t hese novel Cu-Cr2O3 catalysts. (C) 2001 Elsevier Science B.V. All rights re served.