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

Abstract

The role of platinum on the room temperature NO_x storage mechanism and the NO_x 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 NO_x storage and desorption properties. Upon loading of ceria with platinum (1 wt%), NO_x 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 PtO_x during NO_x storage. In the presence of gas-phase oxygen, NO_x storage is related to the consumption of (sub)surface oxygen and PtO_x, and proposed to involve NO₂ or [NO + O₂] intermediates reacting with surface oxygen. The NO_x desorption behavior is shown to be strongly related to the stored NO_x species. Oxidative pretreatment of ceria resulted in the largest amount of stored nitrates, consistent with NO_x 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 NO_x desorption temperature to values <300 °C. Storage and subsequent desorption of NO_x in PtO_x/CeO₂ was associated with PtO_x 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 NO_x storage/desorption and demonstrate the participation of different platinum states in room temperature NO_x storage, with each platinum state opening a distinct new reaction pathway.