Catalytic properties of framework Fe³⁺ in MFI-type ferrisilicate: Reaction mechanism studies of CO oxidation using an isotopic tracer technique

The catalytic properties of Fe³⁺ in the framework sites of MFI-type ferrisilicate were studied by comparing with iron-oxide-impregnated silicalite (FeO_x/Sil) for the oxidation of CO with O₂. On FeO_x/Sil the reaction proceeds via a redox mechanism where CO is oxidized with the lattice oxygen of the catalyst (iron oxide), while on ferrisilicate, the CO oxidation occurs by the reaction of CO with the oxygen from the gas phase. These mechanisms were confirmed by kinetic and isotopic tracer results. The CO oxidation with ¹⁸O₂ on ferrisilicate revealed significant incorporation of lattice oxygen into product CO₂ produced. The CO oxidation mechanism on ferrisilicate was explained as follows: CO₂ is produced by reaction between CO and gas phase oxygen; heterophase exchange between lattice oxygen and CO₂ occurs at a significant rate; furthermore, oxygen isotope equilibration in CO₂ occurs simultaneously. On FeO_x/Sil, the amount of lattice oxygen incorporated into CO₂ during CO oxidation with ¹⁸O₂ was very small. The CO oxidation mechanism on FeO_x/Sil was explained as follows: lattice oxygen in the surface layer of Fe₂O₃ crystallite in FeO_x/Sil oxidizes CO into CO₂, and the reduced surface site is rapidly reoxidized with gas phase oxygen. As the reaction proceeds, the surface lattice oxygen is almost replaced by gas phase oxygen because the scrambling of the lattice oxygen between the surface layer and the bulk of Fe₂O₃ occurs much more slowly than CO oxidation.