Alumina supported manganese oxides for the low-temperature selective catalytic reduction of nitric oxide with ammonia

Alumina supported manganese oxides exhibit a high and selective activity for the catalytic reduction of nitric oxide with ammonia (SCR) between 385 and 575 K. Samples with 3–15 wt.-% manganese were studied at space velocities between 22 000–116 000 h⁻¹ and at standard conditions of 500 ppm NO, 550 ppm NH₃ and 2% O₂. Manganese acetate results in a better dispersion of the manganese oxide on the support and a higher specific catalyst activity than manganese nitrate as precursor, for which crystalline structures could be detected. Temperature-programmed reduction revealed that acetate yields Mn₂O₃ and nitrate mainly MnO₂ on the γ-alumina support. The nitric oxide conversion per amount of manganese is fairly independent of the loading for the catalysts prepared from each precursor. The use of ¹⁵NH₃ reveals that it reacts in a 1:1 molar ratio with nitric oxide towards ¹⁵NN and/or ¹⁵NNO. The SCR activity (to nitrogen) is strongly dependent on the oxygen partial pressure, whereas water inhibits reversibly. Lattice oxygen of the catalyst is not able to maintain the SCR reaction in the absence of oxygen. The nitrous oxide formation is independent of the oxygen partial pressure, but increases with increasing manganese loading and with temperature, resulting in lower selectivities for nitrogen formation. The nitrogen and nitrous oxide formation probably occur at different sites. Above 525 K ¹⁵NH₃ oxidation occurs, yielding mainly ¹⁵N₂O and ¹⁵NO, depending on the temperature. The nitrous oxide is not further reduced by ammonia over this type of catalyst. The addition of tungsten to the catalyst increases the selectivity for nitrogen considerably. The stability of the ex-acetate catalyst is good, for at least 600 h the activity remained constant. The catalysts are sensitive towards sulphur dioxide, the ex-acetate catalysts the least, due to the strong interaction with the alumina support, as is revealed by TPR.