Performance and kinetics study for low-temperature SCR of NO with NH ₃ over MnO_x-CeO₂ catalyst

A series of manganese-cerium oxide catalysts were prepared by different methods and used for low-temperature selective catalytic reduction (SCR) of NO_x with ammonia in the presence of excess O₂. The Mn-Ce-O_x catalysts showed high activities. The experimental results showed that the best Mn-Ce mixed-oxide catalyst yielded nearly 100% NO conversion at 120°C at a high space velocity of 42,000 h^-1. As the manganese content was increased from zero to 30% (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 0.3. The effect of the calcination temperature was also investigated and the optimum calcination temperature was 650°C. These catalysts are substantially more active than all other catalysts reported in the literature. SO₂ and H₂O (at high concentrations) have slight effects on the SCR activity. From a steady-state kinetics study, it was found that the low-temperature SCR reaction was zero order with respect to NH₃ and first order with respect to NO. Compared with the other catalysts reported for low-temperature SCR of NO with ammonia, based on the first-order rate constants, the MnO_x-CeO₂ catalyst was several times more active than other catalysts reported in the literature. Only N₂ rather than N₂O was found in the product when the temperature was below 150°C. At higher temperatures, trace amounts of N ₂O were detected.