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Reaction Mechanism Analysis of Di-Air-Contributions of Hydrocarbons and Intermediates
Abstract:
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.
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