Catalytic Oxidation of NO over MnO(x)–CeO(2) and MnO(x)–TiO(2) Catalysts

A series of MnO(x)–CeO(2) and MnO(x)–TiO(2) catalysts were prepared by a homogeneous precipitation method and their catalytic activities for the NO oxidation in the absence or presence of SO(2) were evaluated. Results show that the optimal molar ratio of Mn/Ce and Mn/Ti are 0.7 and 0.5, respectively. The MnO(x)–CeO(2) catalyst exhibits higher catalytic activity and better resistance to SO(2) poisoning than the MnO(x)–TiO(2) catalyst. On the basis of Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and scanning transmission electron microscope with mapping (STEM-mapping) analyses, it is seen that the MnO(x)–CeO(2) catalyst possesses higher BET surface area and better dispersion of MnO(x) over the catalyst than MnO(x)–TiO(2) catalyst. X-ray photoelectron spectroscopy (XPS) measurements reveal that MnO(x)–CeO(2) catalyst provides the abundance of Mn(3+) and more surface adsorbed oxygen, and SO(2) might be preferentially adsorbed to the surface of CeO(2) to form sulfate species, which provides a protection of MnO(x) active sites from being poisoned. In contrast, MnO(x) active sites over the MnO(x)–TiO(2) catalyst are easily and quickly sulfated, leading to rapid deactivation of the catalyst for NO oxidation. Furthermore, temperature programmed desorption with NO and O(2) (NO + O(2)-TPD) and in situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTS) characterizations results show that the MnO(x)–CeO(2) catalyst displays much stronger ability to adsorb NO(x) than the MnO(x)–TiO(2) catalyst, especially after SO(2) poisoning.