The NO reduction by CO over NiOx/CeO2 catalysts with a fixed Ni surface density: pretreatment effects on the catalyst structure and catalytic activity

Abstract

NiO_x/CeO₂ catalysts have shown promising results in various catalytic reactions, including the NO reduction by CO. In this study, we investigated the effect of oxidation and reduction treatments on the NO reduction by CO over pretreated NiO_x/CeO₂ catalysts. A series of oxidized and reduced supported NiO_x catalysts were synthesized in two steps: (1) the pretreatment of CeO₂ supports at different temperatures (400, 500, and 700 °C) under oxidizing and reducing conditions and (2) the synthesis of NiO_x/CeO₂ catalysts by an incipient wetness impregnation method (IWI) with a fixed surface density of 5.3 Ni per nm² under specific conditions (oxidation/reduction) applied during the CeO₂ treatment. The prepared catalysts were characterized via BET, ICP/OES, Raman, XPS, and XRD to understand their physicochemical properties. The results showed that as the pretreatment temperature increased, the physicochemical properties of NiO_x/CeO₂ catalysts were changed: a decreased specific surface area (SSA), decreased oxygen vacancy/defect sites, and an increased crystallite size. The oxidized NiO_x/CeO₂ catalyst at a lower pretreatment temperature displayed a better catalytic activity, indicating that the physicochemical properties of the catalysts were key factors in enhancing the catalytic activity. To understand the intermediate species and reaction mechanism during the NO reduction by CO, an in situ DRIFTS study was performed and a possible reaction mechanism was also discussed.