Efficient removal of NO_x from exhaust gases of stationary sources is realized by reacting it with NH₃ on a solid catalyst according to the standard selective catalytic reduction (SCR). Metal vanadate-based SCR catalysts exhibit promising advantages over traditional vanadia-based catalysts. Some MeVO₄ were shown to be resistant to deactivation when supported on SiO₂-WO₃-TiO₂ (TWS) and remained active after aging up to 750°C [1]. Nevertheless, it was recently shown that supported FeVO₄ is not stable on TWS and decomposes into Fe₂O₃ and VO_x species above 600°C [2, 3]. The intrinsic activity of the FeVO₄-based SCR catalysts is rather low and is only enhanced by the thermal decomposition. In order to clarify the benefits of vanadates as SCR catalysts, we compared the activity and stability of FeVO₄, CeVO₄ and ErVO₄ on a TWS support.

It was shown that an increase of calcination temperature activates all catalysts (increase in rate constant k_mass, Figure 1) concurrent with the decrease of the metal vanadate peak integral from XRD. The decomposition of the metal vanadate gave rise to the corresponding metal oxides, but no traces of a new vanadium containing phase were observed. However, an estimation of released V₂O₅ could be derived from the loss of the metal vanadate phase (lower panels in Figure 1). The change of released V₂O₅ and k_mass are strongly correlated. While FeVO₄ decomposed completely, the maximum fraction of released V₂O₅ was 1.1 wt% for CeVO₄-TWS and 0.4 wt% for ErVO₄-TWS, hence CeVO₄ and ErVO₄ decomposed only partially. References with 0.5 and 1.0 wt% V₂O₅ on TWS showed very similar activity, indicating that the amount of released V₂O₅ is in fact responsible for the increase in activity. The existence of released V species from metal vanadates was proven using DRIFTS and DRUV, which revealed the predominant formation of extended VO_x units on the surface of TWS. Summarizing, metal vanadate catalysts supported on TWS become active only after their decomposition into metal oxides and free VO_x species.

[1] Casanova M., Schermanz K., Llorca J., Trovarelli A. Catal. Today, 2011 184, 227
[2] Marberger A., M. Elsener M., Ferri D., Sagar A., Schermanz K., Kröcher O. ACS Catal. 2015, 5, 4180  [3] Casanova M., Nodari L., Sagar A., Schermanz K., Trovarelli A. Appl. Catal. B. 2015, 176, 699