Catalytic Oxidation of NO over MnO_x-CeO₂ and MnO_x-TiO₂ Catalysts

A series of MnO_x-CeO₂ and MnO_x-TiO₂ catalysts were prepared by a homogeneous precipitation method and their catalytic activities for the NO oxidation in the absence or presence of SO₂ 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-CeO₂ catalyst exhibits higher catalytic activity and better resistance to SO₂ poisoning than the MnO_x-TiO₂ 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-CeO₂ catalyst possesses higher BET surface area and better dispersion of MnO_x over the catalyst than MnO_x-TiO₂ catalyst. X-ray photoelectron spectroscopy (XPS) measurements reveal that MnO_x-CeO₂ catalyst provides the abundance of Mn³⁺ and more surface adsorbed oxygen, and SO₂ might be preferentially adsorbed to the surface of CeO₂ 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-TiO₂ catalyst are easily and quickly sulfated, leading to rapid deactivation of the catalyst for NO oxidation. Furthermore, temperature programmed desorption with NO and O₂ (NO + O₂-TPD) and in situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTS) characterizations results show that the MnO_x-CeO₂ catalyst displays much stronger ability to adsorb NO_x than the MnO_x-TiO₂ catalyst, especially after SO₂ poisoning.