The role of platinum on the NOₓ storage and desorption behavior of ceria: An online FT-IR study combined with in situ Raman and UV-Vis spectroscopy

The role of platinum on the room temperature NOₓ storage mechanism and the NOₓ desorption behavior of ceria was investigated by combining online FT-IR gas-phase analysis with in situ Raman and UV-vis spectroscopy. The type of pretreatment, leading to the presence of different platinum states (Pt⁰, and mixed Pt⁰/Pt²⁺), is shown to have a major effect on the NOₓ storage and desorption properties. Upon loading of ceria with platinum (1 wt%), NOₓ storage capacities decrease except for reductively pretreated Pt/CeO₂, enabling new reaction pathways via activation of gas-phase oxygen. In the absence of oxygen, NO is reduced by metallic platinum leading to N₂O and N₂ formation. In situ Raman spectra provide mechanistic information, by monitoring changes in ceria surface and subsurface oxygen, as well as PtOₓ during NOₓ storage. In the presence of gas-phase oxygen, NOₓ storage is related to the consumption of (sub)surface oxygen and PtOₓ, and proposed to involve NO₂ or [NO + O₂] intermediates reacting with surface oxygen. The NOₓ desorption behavior is shown to be strongly related to the stored NOₓ species. Oxidative pretreatment of ceria resulted in the largest amount of stored nitrates, consistent with NOₓ being mostly desorbed at elevated temperatures, i.e., within 300–500 °C. Reductive pretreatment and/or addition of platinum significantly increased the fraction of stored nitrite, thereby shifting the main NOₓ desorption temperature to values <300 °C. Storage and subsequent desorption of NOₓ in PtOₓ/CeO₂ was associated with PtOₓ reduction and reoxidation, as monitored by in situ UV-vis and Raman spectra. Through detailed analysis we were able to elucidate the influence of platinum on NOₓ storage/desorption and demonstrate the participation of different platinum states in room temperature NOₓ storage, with each platinum state opening a distinct new reaction pathway.