Characterisation and reactivity of TiO2 supported MoO3 de-NOx catalysts

TiO2-supported MoO3 catalysts, with MoO3 loading similar to that of commercial DeNOx-SCR catalysts (in the range 0± 11% w/w) have been prepared, characterized and tested in the NO reduction by NH3. The structural and morphological characteristics of the anatase-TiO2 support are not signi®cantly changed upon MoO3 addition. For MoO3 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 MoO3 is observed. The MoO3/TiO2 catalysts are active in the reduction of NO by NH3: the reactivity of the catalysts increases on increasing the MoO3 loading, whereas the N2 selectivity decreases due to the formation of undesired N2O. The formation of N2O is primarily ascribed to a reaction between NH3 and NO, and not to the ammonia oxidation reaction. A comparison with a WO3/TiO2 catalyst having similar molar composition indicates that the WO3- and MoO3-containing samples exhibit similar structural and morphological characteristics, but different reactivity: the WO3/TiO2 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. # 1998 Elsevier Science B.V. All rights reserved.