Bisaiji, Y., Yoshida, K., Inoue, M., Takagi, N. et al., "Reaction Mechanism Analysis of Di-Air-Contributions of Hydrocarbons and Intermediates," SAE Int. J. Fuels Lubr. 5(3):1310-1316, 2012, doi:10.4271/2012-01-1744.
The details of Di-Air, a new NOx reduction system using continuous short pulse injections of hydrocarbons (HC) in front of a NOx storage and reduction (NSR) catalyst, have already been reported. This paper describes further studies into the deNOx mechanism, mainly from the standpoint of the contribution of HC and intermediates.In the process of a preliminary survey regarding HC oxidation behavior at the moment of injection, it was found that HC have unique advantages as a reductant. The addition of HC lead to the reduction or metallization of platinum group metals (PGM) while keeping the overall gas atmosphere in a lean state due to adsorbed HC. This causes local O₂ inhibition and generates reductive intermediate species such as R-NCO.Therefore, the specific benefits of HC were analyzed from the viewpoints of 1) the impact on the PGM state, 2) the characterization of intermediate species, and 3) Di-Air performance compared to other reductants. As a result, the operando dispersive X-ray adsorption fine structure (DXAFS) method was used to find that HC prolong the metallic state of Pt compared to CO and H₂ during a period of time lasting a number of seconds under a lean atmosphere. Moreover, adsorption species and the catalyst outlet gas compositions during the period when lean gases of NO + O₂ are supplied to the metalized PGM were investigated using FTIR and a mass spectrometer. As a result, under lean conditions, intermediate species such as R-NCO were generated for a number of seconds after the supply of NO + O₂, and these species were changed into N₂.In addition, temperature programmed desorption (TPD) results showed that HC intermediate species were more thermally stable and that their adsorption energy was larger compared to that of CO and bulk nitrates stored in a conventional NSR. This result also suggests that these characteristics of HC intermediate species contribute to high NOx conversion under high temperature conditions. Moreover, it was found that HC is superior to other reductants, such as CO and H₂, only when small amounts of reductant are continuously supplied at short intervals under high temperature conditions. This result is different from past experience, which concluded that H₂ was the best reductant in any case. It was concluded that these observations explain the reason why the Di-Air system can achieve high NOx conversion at high temperatures.