Unravelling the mechanism of NO and NO2 storage in ceria: The role of defects and Ce-O surface sites

Ceria is an important NOx storage material often used in combination with barium or zirconia. To elucidate the NO and NO2 storage mechanism in ceria we employed in situ Raman spectroscopy coupled with simultaneous FT-IR gas-phase analysis. The Raman spectra reveal new information about the dynamics of the surface and bulk structure of ceria upon NOx exposure at 30 °C besides the identification of nitrite and nitrate adsorbates. In particular, Raman spectra provide direct evidence of the involvement of Ce-O surface sites so far not accessible by spectroscopic methods. These Ce-O sites play a key role for NOx storage, as their amount strongly influences the NOx storage capacity. A reduction of ceria prior to NOx exposure resulted in a lower NOx storage capacity, as long as no strong oxidizing agent (e.g. NO2) was present to form new Ce-O sites. In the case of NO storage at 30 °C, new reaction pathways are postulated that describe the activation of gas-phase oxygen for ionic nitrite transformation and formation of new NOx adsorption sites. However, it is shown that the activated oxygen species is not a peroxide, as no correlation was found between the peroxides and formed nitrate and nitrite species. Based on our results, a mechanism for NO and NO2 storage in ceria was formulated.