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Synergy study is of great importance to optimize the two-stage plasma-facilitated catalysis system and also necessary to gain a mechanistic understanding of the overall reaction process. In this paper, the effects of the long-lived chemical products of plasma pretreatment on the subsequent selective catalytic reduction (SCR) of NOx at fixed and changed energy density of dielectric-barrier-discharge plasma reactor were investigated, respectively, for the aim of the synergy study of plasma- facilitated C2H4-SCR (PF-SCR) of NOx over Ag/gamma-Al2O3 catalyst at lower temperature. For comparison, C2H4-SCR of NOx over Ag/gamma-Al2O3 catalyst alone was also considered. Results showed that PF-SCR for NOx removal exhibited obvious synergistic effect at 240degC, and the NOx removal efficiency for plasma alone, C2H4-SCR alone, and PF-SCR were 43.6%, 6.7%, and 90.1%, respectively. In the NO/NO2/C2H4/O2/N2 system, the long-lived chemical products of plasma pretreatment for the synergistic link between plasma and catalysis included NO2, aldehyde (formaldehyde and acetaldehyde), and nitrogen-contained organic compounds (methyl nitrate, nitro methyl, and ethyl nitrate). Among the nitrogen-contained chemical products of plasma pretreatment, the major synergistic link between plasma and catalysis was NO2 at lower input energy density, transferred to the mutual action of NO2 and nitrogen-contained organic compounds over catalyst at higher input energy density.