Characterization and reactivity of TiO 2-supported MoO 3 De-No x SCR catalysts

Abstract

TiO 2-supported MoO 3 catalysts, with MoO 3 loading similar to that of commercial DeNO x -SCR catalysts (in the range 0–11% w/w) have been prepared, characterized and tested in the NO reduction by NH 3. The structural and morphological characteristics of the anatase-TiO 2 support are not significantly changed upon MoO 3 addition. For MoO 3 loading below or near 9% (w/w) (corresponding to the formation of the theoretical monolayer), evidence for the presence of isolated molybdenyl surface species have been collected by FT-IR, Laser Raman and EPR spectroscopies. Above this loading, the presence of crystalline MoO 3 is observed. The MoO 3/TiO 2 catalysts are active in the reduction of NO by NH 3: the reactivity of the catalysts increases on increasing the MoO 3 loading, whereas the N 2 selectivity decreases due to the formation of undesired N 2O. The formation of N 2O is primarily ascribed to a reaction between NH 3 and NO, and not to the ammonia oxidation reaction. A comparison with a WO 3/TiO 2 catalyst having similar molar composition indicates that the WO 3- and MoO 3-containing samples exhibit similar structural and morphological characteristics, but different reactivity: the WO 3/TiO 2 sample is less active but more selective in the SCR reaction. On the basis of EPR data, the different catalytic behavior has been tentatively ascribed to the different redox characteristics of the samples.