Using on-board diesel fuel as a reductant for SCR technology (diesel-SCR) can avoid infrastructure development for reductant storage and delivery, simplifying exhaust after-treatment systems. With this in mind, diesel-SCR can be regarded as the ultimate goal for the reduction of NOx emission from diesel engines. When employing diesel as a reductant, however, the light-off temperature for NOx reduction is still too high to be used for commercial application in diesel vehicles. It is therefore highly desirable to obtaina thorough understanding of the structure-activity relationship of the HC-SCR system for NOx reduction; and then develop new principles and methods for NOx reduction by on-board fuel with high efficiency.
Characteristics of reductants with high efficiency for HC-SCR
ü A surface enolic species (R-CH=CH-O-) produced by the partial oxidation of ethanol, which played a key role in the ethanol-SCR of NOx, was found for the first time on the surface of Ag/Al2O3 and reported.
ü Alcohols containing at least one α-H and one C-C bond are a prerequisite for their partial oxidation to produce active enolic species over Ag/Al2O3, providing an intrinsic criterion for obtaining alcohol-SCR with high efficiency.
ü Employing hydrocarbons containing at least two carbon atoms is necessary for the formation of active enolic species during their partial oxidation over Ag/Al2O3, providing a principle for the promoting effect of H2 on the low-temperature activity of HC-SCR of NOx.
Activity of butanols with different structures for the SCR of NOx and the formation of surface enolic species over Al2O3
Structure-activity relationship of HC-SCR system based on chemical bond analysis
ü Surface enolic species, produced by the partial oxidation of alcohols over Ag/Al2O3, prefer to adsorb on or close to oxidized Ag sites.
ü A mixing of Ag, O, and Al orbitals in the Ag-O-AlⅢ entity significantly decreases the band gap and likely enhances the reactivity of this entity, therefore serving as the active center for ethanol-SCR over Ag/Al2O3.
Principles for approaching diesel-SCR
ü Diesel-SCR technology with high efficiency for diesel engine emission control: diesel reforming coupled with HC-SCR
ü Principles for diesel reforming: ①transformation of diesel to active small hydrocarbons or oxygenated small hydrocarbons containing at least two carbon atoms, beneficial for the formation of enolic species; ② in situ H2 production to enhance low-temperature activity and expand the operating temperature window.
(Chem. Commun., 2014, 50: 8445)
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