Addition of hydrogen to the feed stream during the lean NOx reduction with C3H8 catalyzed by 2 wt.% Ag/gamma-Al2O3 lowers NO light-off to below 100 degrees C. With our reducing mixture NO reaches high conversion in two well-defined zones, the first at low temperature, between 80 and 180 degrees C, and a second one above 180 degrees C, extending to 500 degrees C without a significant variation in conversion. NO conversion drops in the intermediate range. H-2 is responsible for the reaction of NO at low temperature, whereas C3H8 becomes the reductant above about 180 degrees C. N-2 and NO2 are the main competing products, with the highest selectivity to N-2 being obtained around 140 degrees C and above 380 degrees C. NO2 is formed at low temperature and is predominant (77-83 ppm) between 200 and 260 degrees C. N2O is detected in only small amounts (2 ppm) between 160 and 500 degrees C. Operation at high space velocities during H-2-C3H8-SCR led to the formation of about 48 ppm NH3 at 500 degrees C and to an increase in the CO/CO2 ratio. Through UV-Vis spectroscopy we find that Ag moieties are very labile on the Ag/gamma-Al2O3 catalysts even at low temperatures, and it appears that different Ag species intervene and are responsible for the variations in activity and selectivity during SCR. Ex-situ UV-Vis shows that Ag+ and Ag-n(delta+) species are present after C3H8-SCR, and that Ag-n clusters and metallic Ag nanoparticles are also formed when hydrogen is present in the feed. X-ray diffraction confirmed the formation of metallic silver, presumably nanoparticles with size >5 nm, after H-2-C3H8-SCR. Our results are consistent with a mechanism involving competition between O-2 and the reductant for surface species that are precursors of NO2 on one side, but that can also be reduced to N-2. Those intermediates are reduced by H-2 at low temperature and by reaction with C3H8 above 180 degrees C. (C) 2014 Elsevier Ltd. All rights reserved.