CaO Nanoparticles Coated by ZrO2 Layers for Enhanced CO₂ Capture Stability

CaO is an attractive CO₂ acceptor, due to its good kinetics and high capture capacity, even under low CO₂ concentration conditions. The major problem associated with CaO acceptors is the loss of CO₂ capture capacity during cyclic operations. The declining performance of the CaO acceptors is more pronounced in the presence of steam. In this article we propose a method for fabrication of nano-CaO acceptors coated with a ZrO2 layer to improve the stability. The nano-CaO acceptor was prepared by a thermal decomposition method following coating by a sol–gel, incipient wet impregnation, or hydrolysis process. Thermogravimetric analysis of the cyclic process of CO₂ carbonation/decarbonation revealed that the nano-CaO acceptors with sol–gel (CaO-SG) and wet impregnation (CaO-IM) coatings have higher CO₂ capture capacity and longer lifetime than the uncoated ones. The capture capacity of CaO-IM declined after 14 cycles, whereas CaO-SG remained stable up to 20 cycles. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy were used to characterize the coated and uncoated nano-CaO structures. Combined characterization techniques showed that a CaZrO3 layer was formed around the nano-CaO particles by sol–gel coating. Multicycle carbonation/decarbonation processes were performed in the absence and in the presence of steam in CO₂ capture kinetics experiments as well as under the sorption-enhanced reforming conditions, and CaO nanoparticles with a surface CaZrO3 layer showed promising cyclic stability.