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.