Manganese Oxide Catalysts Supported on TiO2, Al2O3, and SiO2: A Comparison for Low-Temperature SCR of NO with NH3

A series of TiO2-, Al2O3-, and SiO2-supported manganese oxide catalysts were prepared, characterized, and catalytically tested for selective catalytic reduction (SCR) of NO with NH3 in the presence of excess oxygen at low temperatures (373-523 K). Various commercial supports were used in this study to find out the influence of surface area, support nature (acidic, basic), and crystalline phase on SCR activity. XRD studies reveal the presence of anatase and rutile phases for titania supports and the existence of (gamma)alumina in the case of alumina support. Silica support was amorphous. No independent lines corresponding to the crystalline MnO2 were observed on pure anatase and rutile samples. However, the presence of MnO2 was confirmed on other supports by XRD. BET surface area values suggest that specific surface area of the supports was decreased after impregnating with MnO2. The FT-IR and ammonia TPD studies indicate the presence of two types of acid sites on these catalysts, and the acidic strength of the catalysts is higher than the corresponding pure supports. XPS results revealed the presence of two types of manganese oxides, MnO2 (642.4 eV) and Mn2O3 (641.2 eV), on all the samples. The SCR performance of the supported Mn catalysts decreased in the following order: TiO2 (anatase, high surface area) > TiO2 (rutile) > TiO2 (anatase, rutile) > (gamma)-Al2O3 > SiO2 > TiO2 (anatase, low surface area). Quantitative NO conversion with 100% N2 selectivity was achieved at 393 K with Mn supported on TiO2 (anatase). TiO2-supported MnO2 catalysts showed more promising SCR activity than Al2O3- or SiO2-supported manganese oxide catalysts. Various characterization techniques suggest that Lewis acid sites, a high surface concentration of MnO2, and redox properties are important in achieving high catalytic performance at low temperatures.