Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2D-XRD), and transmission electron microscopy coupled with selected area electron diffraction (TEM-SAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO3(s) → CaO(s) + CO2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (1014)calcite display the following topotactic relationships: (1) (1210)calcite//(110)CaO; (2) (1104)calcite⊥ (110)CaO; (3) (1018)calcite//(110)CaO; and (4) (0114)calcite⊥(110)CaO; with [841]calcite//[110]CaO in all four cases. At this stage, the reaction mechanism is independent of P CO2 (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m2/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy E α = 176 ± 9 kJ/mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F1 model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

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Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

http://users.auth.gr/users/8/3/010438/public_html/tdk/Publications/files/Lazaridis_et_al_2003.pdf

Kinetic analysis for the removal of a reactive dye from aqueous solution onto hydrotalcite by adsorption.

Thermal decomposition of calcium carbonate (calcite polymorph) as examined by in-situ high-temperature X-ray powder diffraction

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO2 emissions. Solid materials capable of reversibly absorbing CO2 have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO2 sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO2 absorption both at surfaces and within solid materials.

M. Maciejewski

Papers

Comparison of methods concerning the selection of a function describing the thermal decomposition of solids

Morphological observations on the thermal decomposition of calcium carbonate

The influence of the pressure of the gaseous product on the reversible thermal decomposition of solids

Method of the selection of the g(α) function based on the reduced-time plot

Formation of amorphous CaCO3 during the reaction of CO2 with CaO