Effect of Surface Structure of TiO₂ Nanoparticles on CO₂ Adsorption and SO₂ Resistance

The effect of surface structure of TiO₂ nanocrystals on the structure, amount, and strength of adsorbed CO₂ and resistance to SO₂ was investigated using in situ IR spectroscopy and mass spectrometric techniques along with first-principles density functional theory (DFT) calculations. TiO₂ nanoshapes, including rods {(010) + (101) + (001)}, disks {(001) + (101)}, and truncated octahedra {(101) + (001)}, were used to represent different TiO₂ structures. Upon CO₂ adsorption, carboxylates and carbonates (bridged, monodentate) are formed on TiO₂ rods and disks, whereas only bidentate and monodentate carbonates are formed on TiO₂ truncated octahedra. In general, the order of thermal stability of the adsorbed CO₂ species is carboxylates ≈ monodentate carbonates > bridged carbonates > bidentate carbonates ≈ bicarbonates. TiO₂ rods and disks adsorb CO₂ more strongly than TiO₂ truncated octahedra, which is explained by the larger number of low coordinated surface oxygen and oxygen vacancies on the rods and disks than the truncated octahedra. Further IR studies showed that the structure and binding strength of the adsorbed CO₂ species are affected by the presence of SO₂. Among the three TiO₂ nanoshapes, CO₂ binding strength for truncated octahedra shows the most decrease due to accumulation of sulfates formed during the SO₂ adsorption cycle. The fundamental understanding obtained here on the effects of the surface structure, oxygen vacancies, and SO₂ on the interaction of CO₂ with TiO₂ may provide insights for the design of more efficient and sulfur-resistant TiO₂-based catalysts involved in CO₂ capture and conversion.