The Ag/Al2O3, Ag3PO4/Al2O3, Ag2SO4/Al2O3 and AgCl/Al2O3 catalysts showed high activities for the selective catalytic reduction (SCR) of NOx by ethanol. AgCl was highly crystalloid on Al2O3, however, AgCl/Al2O3 gave the highest catalytic turnover frequency (TOF) for NOx reduction among the four catalysts. Calculated Mulliken charges of Ag compounds support the TOF results. The activity test and characterization results confirm that Ag+ is the active silver species for NOx reduction. The in situ FT-IR results of Ag3PO4/Al2O3, Ag2SO4/Al2O3 and AgCl/Al2O3 for NOx reduction further reveal a positive correlation between the high dispersion of Ag+ and the formation of active intermediates (the enolic and -NCO species), indicating a similar reaction pathway to that of the ethanol-SCR over Ag/Al2O3. Using the precipitable silver compounds greatly facilitates the preparation of monolith Ag/Al2O3 catalyst for practical usage. In fact, the high performance of the AgCl/Al2O3 honeycomb catalyst was confirmed by heavy duty diesel engine bench tests. (C) 2010 Elsevier B.V. All rights reserved.