Mechanistic aspects of model selective catalytic reduction (SCR) of NO with C2H4 over Ni/ZSM-5 zeolite were investigated by means of advanced correlation EPR/HYSCORE and IR time-resolved Rapid-Scan 2D COS techniques combined with DFT/CASSCF calculations. It was shown that two types of the active centers, bare isolated Ni2+ and dual oxo Ni2+-O-2-Ni2+, are involved in this reaction, acting in different ways. NO interaction with the former center results in a Ni2+-NO(delta+)moiety, the unique nature of which was ascertained by HYSCORE studies, and accounted for by the spin-polarized DFT/CASSCF calculations in detail. The electrophilic nature of the bound NO delta+ moiety is crucial for its successful insertion into the C=C< bond of ethylene. This process involves formation of isolated nickel adducts with the coligated C2H4 and NO molecules, and the resultant cyanide and isocyanate intermediates are next hydrolyzed into ammonia or oxidized by NOx. The dual nickel-oxo centers provide the active sites for development of nitrate/nitro species and next mixed-ligand nitrate/nitro-ammonia adducts. Selective catalytic reduction results from the NOx/cyanide and nitrate/nitro-ammonia routes, which both are featured by reversal of the partial charge on the nitrogen atom from positive in N delta+O into negative in -CN delta-/ -N delta-CO or N delta-H3, giving rise to the key nitrogen charge comproportionation step. In both pathways, the nickel(II) mononitrosyls play a pivotal function as the primary source of the N-containing intermediates (-CN/-NCO/NH3). It acts also as a species hindering the undesired oxidation of C2H4 by dioxygen. (c) 2020 Elsevier Inc. All rights reserved.