CO2 Adsorption As a Flat-Lying, Tridentate Carbonate on CeO2(100)

Results of an experimental and computational study of CO2 adsorption onto a CeOX(100) thin-film surface are reported. For both oxidized CeO2(100) and reduced CeO1.7(100), a 5 L dose of CO2 at 180 K resulted in mainly carbonate ([CO3](2-)) on the surface with a minute amount of physisorbed CO2 that desorbed by 250 K based on C 1s and O Is photoemission and C k-edge NEXAFS. No evidence for the formation of a carboxylate intermediate was indicated. Angle-dependent C k-edge NEXAFS revealed that the carbonate species was oriented parallel to the surface suggesting a tridentate configuration. Various adsorption geometries were tested using DFT PBE+U calculations. The most stable configuration was a carbonate with its molecular plane parallel to the surface and each O atom bonded to two Ce cations. Through temperature-programmed desorption (TPD), it was determined that CO2 was the sole reaction product. CO was not detected in the TPD for the reduced surface, indicating that reoxidation of a reduced CeO2-X(100) surface by CO2 did not occur. TPD and photoemission indicated that the coverage and the thermal stability of the [CO3](2-) intermediate were greater on partially reduced CeO1.7(100) compared to CeO2(100).