Location and activity of VOx species on TiO2 particles for NH3-SCR catalysis

The role of anatase TiO2 on the performance of VOx-TiO2 is studied by in situ spectroscopies on a series of catalysts with 1.9-9.7 V at/nm(2). At low coverages, VOx is preferentially located at defect sites, spreading on the main TiO2 {101} and {100} facets at intermediate coverages. Bronsted sites concentration is maximum around 6 V atoms/nm(2), and EPR shows a significant amount of V4+ at intermediate VOx densities, indicating the influence of TiO2 on their reducibility. The activity is proportional to the square of the VOx density up to 5.8 V atoms/nm(2). All catalysts show the same activation energy for NH3-SCR of 56.9 +/- 1.0 kJ/mol in the temperature range 160-280 degrees C, indicating that the reaction path does not change in the studied V coverage range. The activity does not follow the observed trends for Bronsted acid sites or reducibility, and it seems that the ability to form V-pairs is the only requirement for catalytic activity.