[EN] Ag-exchanged Y zeolites (Si/Al = 2.5; Ag/Al = 0.30-0.95) have been tested in the NH3-SCO reaction, the most promising method for the elimination of ammonia emissions, and deeply characterized before and after reaction by using a variety of techniques (XRD, TEM, UV-Vis, Ag-109 NMR, XAS spectroscopies). The most active centres for the NH3-SCO reaction are Ag-0 nanoparticles (NPs) formed under reduction conditions and both activity and selectivity to N-2 increase with the silver loading. The Ag-0 NPs are dramatically modified under reaction conditions, being most of them dispersed resulting in small clusters and even atomically Ag+ cations, the latter accounting for around half silver atoms. The presence of water into the reaction feed promotes the dispersion and oxidation of silver nanoparticles, but the catalyst performance is only slightly affected. The results are fully consistent with the previously proposed i-SCR mechanism for NH3-SCO reaction on silver catalysts. Financial support by the Ministerio de Ciencia e Innovacion (MICINN) of Spain through the Severo Ochoa (SEV-2016-0683) , RTI2018-101784-B-I00, RTI2018-09639-A-I00 and InnovaXN-26-2019 projects is gratefully acknowledged. The authors also thank the Microscopy Service of the Universitat Politecnica de Valencia for its assistance in microscopy characterization (TEM and FESEM equipment preparation) . C. W. Lopes (Science without Borders Process no. 13191/13-6) thanks CAPES for a predoctoral fellowship and J. Martinez-Ortigosa (SEV-2012-0267-02) is grateful to Severo Ochoa Program for a predoctoral fellowship. The authors also want to thank the ALBA synchrotron and CL AE SS beamline staff for providing beamtime (proposal 2017092477) and for setting the beamline up to perform these studies.