Capturing CO2 by ceria and ceria–zirconia nanomaterials of different origin

Abstract

Ceria and ceria–zirconia nanomaterials of different origin were studied in order to elucidate the role of their structural and textural characteristics in controlling the performance towards CO₂ capture. Two commercial cerias and two home-prepared samples, CeO₂ and CeO₂–ZrO₂ (75% CeO₂) mixed oxide, were investigated. The samples were characterized by a number of analytical techniques including XRD, TEM, N₂-adsorption, XPS, H₂-TPR, Raman and FTIR spectroscopy. Static and dynamic CO₂ adsorption experiments were applied to assess the CO₂ capture performance. The type of surface species formed and their thermal stability were evaluated by in situ FTIR spectroscopy and CO₂–TPD analysis. The two commercial ceria samples possessed similar structural and textural characteristics, formed the same types of carbonate-like surface species upon CO₂ adsorption and, consequently, demonstrated almost identical CO₂ capture performance under both static and dynamic conditions. The thermal stability of the adsorbed species increased in the order bidentate (B) carbonates, hydrogen carbonates (HC) and tridentate carbonates (T-III, T-II, T-I). Reduction of CeO₂ increased the relative amount of the most strongly bonded T-I tridentate carbonates. Preadsorbed water led to hydroxylation and enhanced formation of hydrogen carbonates. Although the synthesized CeO₂ sample had a higher surface area (by 30%) it showed a disadvantageous long mass transfer zone in the CO₂-adsorption breakthrough curves. Because of its complex pore structure, this sample probably experiences severe intraparticle CO₂ diffusion resistance. Having the same surface area as the synthesized CeO₂, the mixed CeO₂–ZrO₂ oxide exhibited the highest CO₂ capture capacity of 136 μmol g⁻¹ under dynamic conditions. This was related to the highest concentration of CO₂ adsorption sites (including defects) on this sample. The CeO₂–ZrO₂ system showed the lowest sensitivity to the presence of water vapor in the gas stream due to the lack of dissociative water adsorption on this material.