Catalytic Soot Oxidation Activity of NiO–CeO(2) Catalysts Prepared by a Coprecipitation Method: Influence of the Preparation pH on the Catalytic Performance

A series of NiO–CeO(2) mixed oxide catalysts have been synthesized by a modified coprecipitation method at three different pH values (pH = 8, 9, and 10). The NiO–CeO(2) mixed oxide samples were characterized by TGA, XRD, inductively coupled plasma atomic emission spectroscopy (ICP-AES), FTIR, Brunauer–Emmett–Teller (BET) surface area, H(2) temperature-programmed reduction (H(2)-TPR), and electron microscopy (high-angle annular dark-field transmission electron microscopy/energy-dispersive X-ray spectroscopy (HAADF-TEM/EDS)). The catalytic activities of the samples for soot oxidation were investigated under loose and tight contact conditions. The catalysts exhibited a high BET surface area with average crystal sizes that varied with the pH values. Electron microscopy results showed the formation of small crystallites (~5 nm) of CeO(2) supported on large plate-shaped particles of NiO (~20 nm thick). XRD showed that a proportion of the Ni(2+) was incorporated into the ceria network, and it appeared that the amount on Ni(2+) that replaced Ce(4+) was higher when the synthesis of the mixed oxides was carried out at a lower pH. Among the synthesized catalysts, Ni-Ce-8 (pH = 8) exhibited the best catalytic performance.