Transient Studies of CO ₂ Adsorption over CeO ₂ Nanostructures with In Situ DRIFTS and Modulation Excitation

Experiments with in situ DRIFTS combined with modulation excitation (ME) spectroscopy showed a rich surface chemistry associated with the adsorption of CO2 on nanocubes and nanospheres of ceria. The nanocube’s exposed faces have a (100) orientation, with the edges and corners displaying (110) and (111) orientations, respectively. The nanospheres mainly contained ceria (111) and (110) planes. Density functional theory (DFT) calculations showed that CO2 is a multidentate adsorbate on ceria that can undergo changes in its bonding configuration depending on the chemical environment. At 250 °C, a temperature typically used for the conversion of CO2 into oxygenates, alkanes, and olefins, CO2 reacted with O centers or OH groups present on the nanocubes and nanospheres to yield bi- and tridentate carbonates, hydroxycarbonates, and formates. Both nanostructures were highly reactive, and a dynamic equilibrium was established: carbonate species were rapidly generated upon the injection of CO2, and they decomposed upon the removal of CO2 from the gas phase. In the case of the ceria nanocubes, the adsorption/desorption processes were essentially reversible, opening the door to catalytic transformations. A larger concentration of defects in the ceria nanospheres led to strongly bound carbonates and formates that may be spectators, site blockers, or surface modifiers in catalytic processes. In the ME studies, additional intermediates were detected, and it was clear that the response of surface species to the presence or absence of CO2 was highly dependent on the morphology of the ceria nanostructures.