Studies on Indian Ocean manganese nodules .2. Physico-chemical characteristics and catalytic activity of heat-treated marine manganese nodules

Abstract

The effect of calcination temperature on the physico-chemical characteristics and catalytic activity of central Indian Basin manganese nodules have been investigated. TG, XRD, IR, and chemical analysis confirm the presence of amorphous oxyhydroxides of iron and delta-MnO2 or todorokite, which convert to alpha-Fe2O3 and gamma-Mn2O3 or Mn3O4 phases at greater than or equal to 400 degrees C of calcination. The pore volume, average pore diameter, and pore size distribution practically remain unaffected up to 400 degrees C of heat treatment. But in the temperature range 400 to 700 degrees C, smaller pores coalesce to form larger ones and beyond 800 degrees C the material becomes practically nonporous. The surface hydroxyl group and surface acidity are progressively decreased with a rise in calcination temperature. The surface area, surface oxygen, electrical conductivity, as well as catalytic activity for H2O2 decomposition and CO oxidation, however, follow a similar trend: a gradual increase up to 400 degrees C of calcination and then a decrease with further increase in temperature of calcination. (C) 1996 Academic Press,Inc.