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 <i>in situ</i> 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.