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

• Published in: Applied catalysis. B, Environmental Environmental, 2020-12, Vol.278, p.119337, Article 119337
• Main Authors: Ganjkhanlou, Yadolah, Janssens, Ton V.W., Vennestrøm, Peter N.R., Mino, Lorenzo, Paganini, Maria Cristina, Signorile, Matteo, Bordiga, Silvia, Berlier, Gloria
• Format: Article
• Language: English
• Subjects: Ammonia; Anatase; Catalysis; Catalysts; Catalytic activity; CO adsorption; Electron paramagnetic resonance; Infrared spectroscopy; NH3-SCR; Titanium dioxide; VOx-TiO2 catalyst
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/j.apcatb.2020.119337

NH3-SCR activity dependence of VOx-TiO2 catalyst on V loading. Dispersion of VOx species and their evolution on anatase crystal have been proposed based on the different characterization results. Monomeric VOx species do not have considerable catalytic activity, formation of oligomeric VOx species in medium coverages boost the NH3-SCR activity which is proportional to square of V loadings suggesting the bridged V-O-V as active species. Bulk species in high loading do not change the activity. [Display omitted] •At low coverages, VOx occupies undercoordinated TiO2 sites.•At intermediate coverages, the VOx spreads out over {101} and {100} facets of TiO2 crystals.•Direct contact of VOx with TiO2 enhances reducibility of VOx according to EPR.•Brønsted acidity develops up to V surface density of 6 ay/nm2.•The NH3-SCR activity is proportional to the square of the VOx surface density. 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/nm2. At low coverages, VOx is preferentially located at defect sites, spreading on the main TiO2 {101} and {100} facets at intermediate coverages. Brønsted sites concentration is maximum around 6 V atoms/nm2, 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/nm2. All catalysts show the same activation energy for NH3-SCR of 56.9 ± 1.0 kJ/mol in the temperature range 160−280 °C, indicating that the reaction path does not change in the studied V coverage range. The activity does not follow the observed trends for Brønsted acid sites or reducibility, and it seems that the ability to form V-pairs is the only requirement for catalytic activity.