MnOx-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 NOx with ammonia in the presence of excess O2. 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.