EPR and DFT study of NO interaction with Ni/SiO2 catalyst: insight into mechanistic steps of disproportionation process promoted by tripodal surface nickel complex

The adsorption of NO onto Ni/SiO2 catalyst has been investigated at 77 and 296 K at various pressures by EPR spectroscopy. The principal steps of NO disproportionation involve coordination of NO molecule with concomitant formation of paramagnetic mono {NiII 3c–NO}9/SiO2 and diamagnetic dinitrosyl species {Ni2c–(NO)2}10/SiO2, reduction of nickel through inner-sphere ligand-to-metal electron transfer leading to {NiI 3c– NO+}9/SiO2, formation of {Ni3c–NO2}9/SiO2 complex via metal-oxo intermediate, and spillover of the NO2 ligand onto silica. The key reaction intermediates have been isolated and identified. Their assignment and molecular structure have been ascertained by DFT calculations.