ABSTRACT
Solid catalysts are generally used as small particles of porous solid with most of the active sites on the internal surfaces. Metal catalysts can be prepared by impregnating porous supports such as Al2O3, SiO2, and TiO2 with an aqueous solution of a metal salt, followed by drying and decomposition or reduction to give tiny crystallites of metal deposited on the support. Examples of this type include Ni/Al2O3, Ag/Al2O3, and Pt/SiO2. The total surface area of the support is often 100-500 m2/g, and almost all of this surface is internal, since the external surface is less than l m2/g. For a spherical particle, the external area per unit mass is
aext ¼ 6
dpp ð4:1Þ
For dp ¼ 0:1 cm and p ¼ 2g=cm3,
aext ¼ 6
0:1 2ð Þ ¼ 30 cm 2=g ¼ 0:003m2=g
Thus, if impregnation leaves metal deposited over the same fraction of external and internal surfaces of the support, a 0.1-cm particle with Sg ¼ 2 surfaces.
