Structure and mechanistic relevance of Ni2+–NO adduct in model HC SCR reaction over NiZSM-5 catalyst – Insights from standard and correlation EPR and IR spectroscopic studies corroborated by molecular modeling

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+–O2––Ni2+, are involved in this reaction, acting in different ways. NO interaction with the former center results in a Ni2+–NOδ+ 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 NOδ+ moiety is crucial for its successful insertion into the >C=C< bond of ethylene. This process involves formation of isolated nickel adducts with the co-ligated 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 Nδ+O into negative in –CNδ–/–Nδ–CO or Nδ–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.