Investigation of the NO reduction by CO reaction over oxidized and reduced NiOx/CeO2 catalysts

Abstract

CeO₂-supported NiO_x catalysts have been widely studied in various catalytic reactions including NO reduction by CO. This work is mainly focused on investigation of the impact of catalyst synthesis conditions (e.g., oxidation and reduction) on the physicochemical properties of NiO_x/CeO₂ catalysts and the catalytic response for the NO reduction by CO reaction. The oxide NiO_x/CeO₂ sample was prepared by an incipient wetness impregnation (IWI) method and reduced under hydrogen reduction treatment at high temperatures (500 and 700 °C). The physicochemical properties of the synthesized samples were characterized by BET analysis, Raman spectroscopy, XRD, XPS, EELS and high-resolution transmission electron microscopy (HR-TEM). The results showed that higher reduction temperature led to the decrease in specific surface area (SSA), fewer oxygen vacancy/defect site, larger crystallite size of the CeO₂ support, and formation of metallic Ni on the surface. The oxidized NiO_x/CeO₂ catalyst showed the highest catalytic activity, indicating that the presence of oxygen vacancy/defect sites, Ni²⁺ oxidation state, and smaller crystallite size are believed to enhance the catalytic activity. In situ DRIFTS confirmed the generation of several intermediate species, such as nitrate, carbonate, and N₂O. On the basis of in situ DRIFTS and activity results, the possible reaction mechanism of NO reduction by CO over NiO_x/CeO₂ was proposed.