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, 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, MnOx(0.3)-CeO2(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, Mn4+, Mn3+, and Mn2+ oxide species were found after calcination in air. Three kinds of Mn phases existed in the MnOx-CeO2 catalysts that were prepared by the CA method: (1) aggregated Mn2O3 on the CeO2 support, (2) highly dispersed Mn2O3 with strong interactions with CeO2, and (3) Mn atoms incorporated into the CeO2 lattice. The distribution of Mn species depends on the preparation methods. Any oxygen vacancy formed in the CeO2 lattice caused by incorporation of Mn atoms adsorbs and activates molecular oxygen to form active oxygen species. Thus, the high activity of MnOx(0.3)-CeO2(923) is attributed to the highly dispersed Mn species and the more active oxygen species that is formed. Ammonia molecules adsorbed onto MnOx(0.3)-CeO2(923) to form NH4+ and coordinated NH3. At the same time, NH2 was observed because of H-abstraction. NO2, 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 NH3 onto the Lewis acid sites, and then, the NH2 species was formed, followed by reaction between NH2 and NO to produce N-2 and H2O. Possible intermediates are proposed, and all the intermediates could transform into NH2NO, which could further react to produce N-2 and H2O.