CO2‐Catalyzed Surface Area and Porosity Changes in High‐Surface‐Area CaO Aggregates

The changes in surface area and mesoporosity in aggregates of ∼0.01 μm cross-section CaO particles when heated in CO2at 686°C were determined from N2 adsorption isotherms. Initially, the surface area decreases rapidly with little change in porosity. When the surface area has decreased below ∼90 m2/g, surface area and porosity variations become consistent with expectations for coarsening by grain-boundary or bulk diffusion. The initial rapid decrease in surface area must result from CO2-catalyzed surface diffusion, but the data suggest that surface diffusion is not rate-limiting. The rate-limiting step may be reaction of CO2 to form surface CO32- ions or decomposition of these ions to O2- ions and CO2 gas.