Characterization of the Porous CaO Particles Formed by Decomposition of CaCO³ and Ca(OH)² in Vacuum

Abstract

The solid products of decomposing CaCO, powder in vacuum at 510°C (sr-CaO) and of decomposing Ca(OH), powder at 320°C in vacuum (h-CaO) are particles which have approximately the same exterior dimensions as the parent CaCO, or Ca(OH), particles. N, adsorption and desorption isotherms show that srand 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), before XRD measurements can be completed in air. From the volume of Nz adsorbed by the porous powders, the porosity of h-CaO is calculated to be 36?5% and of sr-CaO 41.525%. These porosities imply that the linear dimensions of the 1 to 20 pm 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.

Comparative SO2 reactivity of CaO derived from CaCo3 and Ca(OH)2

“…While the distributions themselves were quite similar, the shape of the adsorption/desorption isotherms suggested a difference in the shape of the pores; c-CaO suggested the openings between packed spheres while h-CaO approximated slits or the gaps between plates when interpreted in accordance with Gregg and Sing (1982). This finding was in agreement with the pore structures suggested by Beruto et al (1980). Gullett and Bruce further reported that available porosity was filled by product more slowly for h-CaO, allowing a higher conversion to CaSO,, presumably due to this pore structure difference.…”

Synthesis and morpho-structural characterization of nanostructured magnesium hydroxide obtained by a hydrothermal method

“…4a) in which the degradation of the area of sample can be seen after the high-resolution study. The low stability of hydroxides under the high energy electron beam of the transmission electron microscopy has been reported by some authors [40,41]. Fig.…”

The measurement of meaningful kinetic parameters for solid state decomposition reactions

“…Barret derived this relationship by assuming an adsorbed intermediate layer [4], Benito and Searcy [5] by assuming one of the diffusion steps of the gaseous product to be rate limiting. Cutler et al derived a similar expression [6] by postulating that, for calcium carbonate, CaO is first formed in some special activated form that occupies a constant fraction of the surface. Our derivation shows that no special assumption is necessary beyond that the reaction is reversible.…”