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.