Role of NOδ+ Intermediates in NO Reduction with Propene over NiZSM-5 Zeolite Revealed by EPR and IR Spectroscopic Investigations and DFT Modeling

Abstract

Adsorption at variable temperatures of individual components (NO, NO2, CO, O2, C3H6) and their mixtures simulating the feed of the selective catalytic reduction (SCR) of NOx with propene over NiZSM-5 catalysts was investigated by electron paramagnetic resonance, infrared, and mass spectroscopies to provide direct insights into the nature of the primary reaction intermediates and alternation of the valence state of the nickel centers. The key intermediates ({Ni–NO}2+, {Ni+–C3H6}, {Ni+–(CO)n}, and {Ni2+–O2–}) relevant to the SCR process were isolated and identified, and their structure together with spectroscopic signatures were ascertained by parallel density functional theory molecular modeling. Alternation of the nickel valence state during the SCR reaction, leading to formation of a Ni2+/Ni+ redox couple, was triggered by the reductive adsorption of NO and oxidative adsorption of O2. The sequence at which the reactant molecules were reactively coordinated was dictated by the oxidation state of the nickel centers. It was shown that the SCR process is initiated by chemoselective capture of NO from the reaction mixture by oxidized nickel sites. Concerted spin-pairing and ligand-to-metal charge-transfer events involved in this process lead cooperatively to the formation of nickel-bound nitrosonium (NOδ+) species as the prime intermediate. At SCR temperatures (above 673 K), NOδ+ in the presence of propene and dioxygen is readily converted into N2, COx, and H2O, the final products of the SCR reaction.