Ag/Al2O3 materials may be packaged in a suitable flow-through reactor, close coupled to the exhaust manifold of a diesel engine, and upstream of other exhaust gas treatment devices, such as a diesel oxidation catalyst and a selective reduction catalyst for NOx. The silver/alumina catalyst material uses hydrogen in a cold-start engine exhaust and serves to oxidize NO to NO2 in the relatively low temperature, hydrocarbon-containing, exhaust during a short period following the engine cold start, and to temporarily store NOx during the start-up period. After the exhaust has heated downstream catalytic devices, the silver yields its nitrogen oxides for conversion to nitrogen by the then-operating devices before NOx is discharged to the atmosphere.