MnO_x-CeO₂ mixed oxides prepared by co-precipitation for selective catalytic reduction of NO with NH₃ at low temperatures

A series of manganese-cerium oxide catalysts were prepared by co-precipitation method and used for low temperature selective catalytic reduction (SCR) of NO_x with ammonia in the presence of excess O ₂. These catalysts were characterized by X-ray diffraction (XRD), surface area measurement and FTIR. The experimental results showed that the best Mn-Ce mixed-oxide catalyst yielded 95% NO conversion at 150°C at a space velocity of 42,000 h^-1. As the manganese content was increased from 0 to 40% (i.e. the molar ratio of Mn/(Mn+Ce)), NO conversion increased significantly, but decreased at higher manganese contents. The most active catalyst was obtained with a molar Mn/(Mn+Ce) ratio of 0.4. Only N₂ rather than N₂O was found in the product when the temperature was below 150°C. At higher temperatures, trace amounts of N₂O were detected. A mechanistic pathway for this reaction was proposed based on earlier findings and FTIR results obtained in this work. The initial step was the adsorption of NH₃ on Lewis acid sites of catalyst, followed by reaction with nitrite species to produce N₂ and H₂O. Possible intermediates are proposed and all the intermediates could transform into NH₂NO, which could further react to produce N₂ and H₂O.