TiO2-supported metal oxide catalysts for low-temperature selective catalytic reduction of NO with NH3: I. Evaluation and characterization of first row transition metals

Low-temperature SCR of NO with NH3 in the presence of excess oxygen on the oxides of V, Cr, Mn, Fe, Co, Ni, and Cu supported on anatase TiO2 has been studied. Among the catalysts tested, Mn/TiO2 supported on Hombikat TiO2 provided the best performance with 100% N2 selectivity and complete NO conversion at temperatures as low as 393 K under numerous conditions. The catalytic performance for various transition metal oxides supported on TiO2 decreased in the following order: Mn > Cu ⩾ Cr ⪢ Co > Fe ⪢ V ⋙ Ni. For Mn-based catalysts the activity increases with an increase in Mn loading and the reaction temperature. TiO2 alone did not give any NO conversion at ⩽573 K, and calcination at low temperature (⩽673 K) is preferable. XRD coupled with XPS confirmed the presence of MnO2 as a major phase (peak at 642.2 eV) with Mn2O3, and partially undecomposed Mn-nitrate as the minor phases for supported manganese catalysts. It is proposed that MnO2 contributes to the high activity of Mn/TiO2. XPS results also confirmed a higher concentration of active metal oxides on the surface of Mn/TiO2 compared to the other catalysts. The NH3 FT-IR study showed the presence of Lewis acid sites for the most active catalysts, while the peak corresponding to Brönsted acid sites was weak or absent. This strongly suggests that Brönsted acid sites are not necessary for the reaction to occur at low temperatures. The H2 TPR study indicated the difficulty of reducing Mn oxide when the metal loading is low and/or the catalysts are calcined at temperatures higher than 773 K. It is concluded that lower catalyst calcination temperatures, Lewis acidity, the redox properties of metal oxides and their higher surface concentration are important for very high SCR activity at low temperatures. Mn/TiO2 provided the best performance at 50,000 h−1 when the catalysts were tested in the presence of 11 vol% H2O. Under these conditions, the catalytic activity of the transition metal oxides decreases in the following order: Mn > V ⪢ Co > Cu > Cr > Fe ⪢ Ni.