The Nature of CaO Produced by Calcite Powder Decomposition in Vacuum and in CO 2

Specially reactive (sr‐) CaO can be produced by decomposing 1‐ to 10‐μm particles of CaCO 3 in vacuum at ∼650°C. Particles of sr‐CaO appear unchanged in dimension from the original CaCO 3 , but have internal pores with surface areas of 92 m 2 /g, more than twice the surface areas reported in a previous study for CaO made from calcite in vacuum at about the same temperature. Weak and broad X‐ray diffraction peaks in normal CaO first become discernible only after about half the CO 2 content of a calcite sample has been lost, but the molar surface areas of samples decomposed in vacuum or in 0.1 torr CO 2 are linear functions of the extent of reaction. The sr‐CaO, therefore, must be a phase with ion positions and spacings like those of normal CaO. Background pressures of CO 2 (up to ∼1 torr) influence the surface area of the CaO by increasing the pore dimensions at the CaCO 3 ‐CaO reaction interface. Sintering of the CaO is negligible at 650°C in CO 2 pressures of≤0.1 torr, but sintering is pronounced in CO 2 , at 9 torr pressure.