Characterization of the Porous CaO Particles Formed by Decomposition of CaCO ³ and Ca(OH) ² in Vacuum

Abstract

The solid products of decomposing CaCO 3 powder in vacuum at 510°C (sr‐CaO) and of decomposing Ca(OH) 2 powder at 320°C in vacuum (h‐CaO) are particles which have approximately the same exterior dimensions as the parent CaCO 3 or Ca(OH) 2 particles. N 2 adsorption and desorption isotherms show that sr‐and h‐CaO have high internal surface areas which for sr‐CaO have cylindrical symmetry, with the most common diameters being ∼ 10 nm, and for h‐CaO are slit‐shaped, with the most common slit width being ∼ 2.7 nm. The conclusions reached in earlier investigations, i.e that these decomposition reactions in vacuum initially yield a form of CaO which has the same unit cell dimensions as the parent solid, were in error, probably because water vapor converts much of the CaO to poorly crystalline Ca(OH) 2 before XRD measurements can be completed in air. From the volume of N 2 adsorbed by the porous powders, the porosity of h‐CaO is calculated to be 36±5% and of sr‐CaO 41.5±5%. These porosities imply that the linear dimensions of the 1 to 20 μm particles of h‐CaO and sr‐CaO are ∼5% smaller than those of the parent particles. XRD measurements made as a function of time show that particle shrinkage must occur by cooperative, diffusionless movement of crystallites of sr‐CaO or h‐CaO as they form.