Precision Calcination Mechanism of CaCO₃ to High‐Porosity Nanoscale CaO CO₂ Sorbent Revealed by Direct In Situ Observations

AbstractDeploying energy storage and carbon capture at scale is hindered by the substantial endothermic penalty of decomposing CaCO3 to CaO and CO2, and the rapid loss of CO2 absorption capacity by CaO sorbent particles due to sintering at the high requisite decomposition temperatures. The decomposition reaction mechanism underlying sorbent deactivation remains unclear at the atomic level and nanoscale due to past reliance on postmortem characterization methods with insufficient spatial and temporal resolution. Thus, elucidating the important CaCO3 decomposition reaction pathway requires direct observation by time‐resolved (sub‐)nanoscale methods. Here, chemical and structural dynamics during the decomposition of CaCO3 nanoparticles to nanoporous CaO particles comprising high‐surface‐area CaO nanocrystallites are examined. Comparing in situ transmission electron microscopy (TEM) and synchrotron X‐ray diffraction experiments gives key insights into the dynamics of nanoparticle calcination, involving anisotropic CaCO3 thermal distortion before conversion to thermally dilated energetically stable CaO crystallites. Time‐resolved TEM uncovered a novel CaO formation mechanism involving heterogeneous nucleation at extended CaCO3 defects followed by sweeping reaction front motion across the initial CaCO3 particle. These observations clarify longstanding, yet incomplete, reaction mechanisms and kinetic models lacking accurate information about (sub‐)nanoscale dynamics, while also demonstrating calcination of CaCO3 without sintering through rapid heating and precise temperature control.