Effect of Zirconia Doping on the Structure and Stability of CaO-Based Sorbents for CO2 Capture during Extended Operating Cycles

A series of CaO-based sorbents are made using flame spray pyrolysis (FSP) and doped in situ by a wide range of zirconia loadings and tested for their CO2 capture during extended operating cycles. Among all these sorbents, the one with a Zr/Ca molar ratio of 5/10 exhibits optimum performance and remarkable stability up to 1200 cycles. That sorbent exhibited excellent resistance toward high temperature sintering under severe conditions. Its mechanical durability is attributed to the formation of well-dispersed CaZrO3 nanoparticles that act as a barrier against sintering, preventing CaO grain growth. X-ray diffraction (XRD) revealed the formation of CaCO3 and CaZrO3 in all sorbents made here. The peak intensity of the perovskite-structured CaZrO3 stabilizes at a molar ratio of Zr/Ca = 5/10. With increasing Zr doping, the CaZrO3 size decreases, the specific surface area and pore volume increase, while the major TPD desorption peak shifts to a lower temperature. Both Ca2p and Zr3d electron binding energies decrease with increasing Zr content up to 5 mols of Zr doping and remained constant beyond that. Also, XPS of the Ols electron binding energies suggests that there are two types of oxygen, one from CaO and another from CaZrO3. Surface enrichment of Zr of the Zr/Ca (6/10) sorbent explains the sharp drop in CO2 capture capacity compared to that of the Zr/Ca (5/10) sorbent. With the reduction of CaZrO3 size and fine-tuning of the Zr/Ca molar ratio, stable sorbents can be obtained for a very large number of cycles, while preserving relatively high CaO molar conversions (as high as 60%).