Yolk–shell-type CaO-based sorbents for CO(2) capture: assessing the role of nanostructuring for the stabilization of the cyclic CO(2) uptake

Improving the cyclic CO(2) uptake stability of CaO-based solid sorbents can provide a means to lower CO(2) capture costs. Here, we develop nanostructured yolk(CaO)–shell(ZrO(2)) sorbents with a high cyclic CO(2) uptake stability which outperform benchmark CaO nanoparticles after 20 cycles (0.17 g(CO(2)) g(Sorbent)(−1)) by more than 250% (0.61 g(CO(2)) g(Sorbent)(−1)), even under harsh calcination conditions (i.e. 80 vol% CO(2) at 900 °C). By comparing the yolk–shell sorbents to core–shell sorbents, i.e. structures with an intimate contact between the stabilizing phase and CaO, we are able to identify the main mechanisms behind the stabilization of the CO(2) uptake. While a yolk–shell architecture stabilizes the morphology of single CaO nanoparticles over repeated cycling and minimizes the contact between the yolk and shell materials, core–shell architectures lead to the formation of a thick CaZrO(3)-shell around CaO particles, which limits CO(2) transport to unreacted CaO. Hence, yolk–shell architectures effectively delay CaZrO(3) formation which in turn increases the theoretically possible CO(2) uptake since CaZrO(3) is CO(2)-capture-inert. In addition, we observe that yolk–shell architectures also improved the carbonation kinetics in both the kinetic- and diffusion-controlled regimes leading to a significantly higher cyclic CO(2) uptake for yolk–shell-type sorbents.