Using on-board diesel fuel as reductant for SCR technology (diesel-SCR) can avoid infrastructure development for reductant storage and delivering, simplifying exhaust after-treatment systems. With this in mind, diesel-SCR can be regarded as the ultimate goal for the reduction of NOx emissions from diesel engines. When diesel is employed 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 gain a thorough understanding of the structure-activity relationship of HC-SCR system for NOx reduction, and then develop new principles and methods for NOx reduction by on-board fuel with high efficiency.
1.Criteria of reductant with high efficiency for HC-SCR
1.1. Surface enolic species (R-CH=CH-O-), produced by the partial oxidation of ethanol, which play a key role in the ethanol-SCR of NO
x, were found for the first time on the surface of Ag/Al
2O
3 and reported.
1.2. Alcohols containing at least one α-H and one C-C bond are a prerequisite for partial oxidation to produce active enolic species over Ag/Al
2O
3, providing an intrinsic criterion for alcohol-SCR with high efficiency.
1.3. Hydrocarbons containing at least two carbon atoms are necessary for the formation of active enolic species during their partial oxidation over Ag/Al
2O
3, providing a principle for the improvement of the low-temperature activity of HC-SCR of NO
x. (
J. Phys. Chem. B, 2003, 107: 13090;
Appl. Catal. B, 2004, 49: 159;
Catal. Today, 2005, 100: 37;
J. Catal., 2010, 271: 343;
Catal. Sci. Technol., 2014, 4: 1239;
Chem. Commun., 2014, 50: 8445)
Activity of butanols with different structures for the SCR of NOx and the formation of surface enolic species over Al2O3
Relationship between the formation of enolic species triggered by H2 addition and NOx conversion during HC–SCR
2.Structure-activity relationship of HC-SCR system based on chemical bond analysis
2.1. Surface enolic species produced by the partial oxidation of alcohols over Ag/Al
2O
3 preferably adsorb on or close to oxidized Ag sites.
2.2. A mixing of Ag, O, and Al orbitals in the Ag-O-Al
Ⅲ 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/Al
2O
3.
2.3. The synergy of oxidized and metallic silver species enhances the water tolerance of Ag/Al
2O
3 in H
2-assisted HC-SCR.
2.4. The state of silver species affects the mobility of sulfates, and finally governs the sulfur resistance of Ag/Al
2O
3 in HC-SCR. (
Catal. Today, 2005, 100: 37;
J. Catal., 2012, 293: 13;
ACS Catal., 2014, 4: 2776;
Chem. Commun., 2014, 50: 8445;
J. Phys. Chem. C, 2015, 119: 3132;
Appl. Catal. B, 2017, 207: 60;
ACS Catal., 2018, 8: 2699;
Appl. Catal. B, 2019, 244: 909)
The reactive sites for enolic species adsorption and NOx reduction on Ag/Al2O3
The mechanism of water and sulfur tolerance for Ag/Al2O3 in HC-SCR
3.Scheme for the design of diesel-SCR
3.1. Diesel-SCR technology with high efficiency for diesel engine emission control: diesel reforming coupled with HC-SCR
3.2. Principle for diesel reforming: ① transformation 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 H
2 production to enhance low-temperature activity and expand the operating temperature window. (
Chem. Commun.,
2014, 50: 8445)
Design of diesel-SCR technology: Coupling diesel reforming with HC-SCR.
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