Increasing the NO2 availability in some aftertreatment systems enhance their performance in reducing pollutants from internal combustion (IC) engines but result in significant fuel economy and CO2 emissions penalties. The presence of NO2 in the engine exhaust gas enhances the regeneration of the Diesel Particulate Filters (DPFs) and can improve the activity of the catalysts in reducing NOx emissions in the selective catalytic reduction (SCRs) process. In this work the production and the role of the increased NO2 concentration in the Ag/Al2O3 catalyst for the SCR process of NOx removal at low exhaust gas temperatures under real engine operation has been investigated. We have increased the NO2 concentration available for the SCR process with (i) the addition of different NH3 and H-2 mixtures upstream the SCR catalyst and/or (ii) by the use of a Pt based Diesel Oxidation Catalyst (DOC) in front of the Ag/Al2O3-SCR catalyst. In the case of NH3 and H-2 mixtures additions, H-2 enhances the NO2 production on the Ag/Al2O3 catalyst, leading in promoting the "Fast-SCR" like reaction by utilising the available NH3 mainly at low reaction temperature. The incorporation of the DOC in front of the Ag/Al2O3 showed the same effect as it enhanced the NO2 availability for the SCR process. (C) 2015 Elsevier B.V. All rights reserved.