Cerium-zirconium (Ce-Zr) solid solutions have been extensively used in a wide variety of catalytic processes due to their unique catalytic features in conjunction with lower cost compared to noble metal-based systems. A series of Ce-Zr-based catalysts was prepared by the sol-gel method. The structure and morphology of these catalysts were characterized by X-ray diffraction, thermogravimetric-differential scanning calorimetry, scanning electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. Furthermore, investigation on catalytic performance was carried out by constructing a test platform, and the result indicated that the catalysts apparently decreased the soot ignition temperature. These catalysts exhibited higher catalytic activity for soot oxidation under narrow contact conditions. The results revealed that some soot particles could react with adsorbed oxygen, and other part of diesel soot reacted with lattice oxygen. The activity of these catalysts was attributed to synergistic effect arising from the combination of K/Co/Zr and Ce-Zr solid solution, which led to the decrease in the ignition temperature to 294 • C (data from the test platform). The catalyst still keeps good stability and catalytic activity after the cycle oxidation experiment. A reaction pathway was proposed to explain catalytic combustion process of soot, i.e., combination of K/Co/Zr with Ce-Zr solid solution reduced the binding energy of Ce-Zr solid solution, which was conducive to provide more active sites to release the active oxygen (O 2 − ) or lattice oxygen (O 2− ).Catalysts 2019, 9, 646 2 of 18 the filter becomes necessary. Soot burns at 500-600 • C under air conditions, while the diesel exhaust temperature is only 200-400 • C. Therefore, igniting soot combustion under these conditions requires catalytic oxidation. The filter should be able to trap and burn soot particles when it collects them, so that the filter does not saturate. A large number of catalysts has been investigated for controlling the exhaust emissions of diesel engines, and the main objective of most of these studies is to improve the catalytic performance for soot combustion. Numerous articles reporting the use of CeO 2 as an effective catalyst for soot combustion are available [11,12], and a mechanism has been proposed to explain these catalytic processes. The Ce 4+ /Ce 3+ redox cycle confers the ability to absorb gaseous O 2 , thus forming active oxygen at the catalyst surface (O ads ), which can be transferred to the soot catalyst interface by superficial diffusion. For this reason, properties of the Ce 4+ /Ce 3+ couple and the capacity of CeO 2 to exchange oxygen with the gas phase make CeO 2 -based materials one of the most promising catalysts for soot combustion [13,14]. They are widely known to favor the oxidation of soot by increasing the catalyst/soot contact with the high mobility of alkali metals or alkaline-earth metals [15][16][17][18][19][20]. Fan et al. studied the effect of Ba loading on Cu-Ce catalysts, and it was sh...