The effect of the silica and alumina ratio (Si/Al) in HY was studied as an important parameter that determines the catalytic activity of zeolite under plasma activation. The Ag loaded on HI was in a more oxidized state at lower Si/Al ratios and in a more metallic state at higher Si/Al ratios. A substantial drop in the relative electrical resistivity occurred when Ag was supported on the HI at a Si/Al ratio of 40. A strong relationship between the propagation of surface streamers and the catalytic activity was observed when the Si/Al ratio was changed in the following order: 2.6 >15 >40. The interactions of oxygen plasma and Ag-loaded HY zeolites were clearly demonstrated via electron spin resonance (ESR) measurements of divalent silver (Ag2+) and Fe3+ cations and plasma-induced fluorescence of Ag-cation clusters. (C) 2014 Elsevier B.V. All rights reserved.