Characterization and FTIR Studies of MnOx−CeO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NO with NH3
A series of high-activity manganese−cerium oxide catalysts for the low-temperature (373−453 K) selective catalytic reduction (SCR) of NO x with ammonia were prepared. They were prepared by using the citric acid method (CA), coprecipitation method (CP), and impregnation method (IM) and were characterized by XRD, ESR, XPS, and FTIR techniques. A sample prepared by the CA method, MnO x (0.3)−CeO 2 (923), showed the highest activity. XRD results showed that the catalyst prepared by the CA method had the smallest particle size and the weakest XRD peak intensity. Using ESR and XPS, Mn 4+, Mn 3+, and Mn 2+ oxide species were found after calcination in air. Three kinds of Mn phases existed in the MnO x −CeO 2 catalysts that were prepared by the CA method: (1) aggregated Mn 2 O 3 on the CeO 2 support, (2) highly dispersed Mn 2 O 3 with strong interactions with CeO 2, and (3) Mn atoms incorporated into the CeO 2 lattice. The distribution of Mn species depends on the preparation methods. Any oxygen vacancy formed in the CeO 2 lattice caused by incorporation of Mn atoms adsorbs and activates molecular oxygen to form active oxygen species. Thus, the high activity of MnO x (0.3)−CeO 2 (923) is attributed to the highly dispersed Mn species and the more active oxygen species that is formed. Ammonia molecules adsorbed onto MnO x (0.3)−CeO 2 (923) to form NH 4 + and coordinated NH 3 . At the same time, NH 2 was observed because of H-abstraction. NO 2, nitrite, and nitrate were formed by oxidation of NO. A mechanistic pathway for this reaction was proposed on the basis of earlier findings and the FTIR results obtained in this work. The initial step was adsorption of NH 3 onto the Lewis acid sites, and then, the NH 2 species was formed, followed by reaction between NH 2 and NO to produce N 2 and H 2 O. Possible intermediates are proposed, and all the intermediates could transform into NH 2 NO, which could further react to produce N 2 and H 2 O.