Influence of Countercations on the NH₃-SCR of NO Using Supported Vanadium-Containing Polyoxometalate Catalysts

Anatase-supported vanadium-substituted Keggin-type polyoxometalates (POMs) are promising alternative catalysts for the selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures. Generally, alkali poisoning is a major deactivation mechanism in vanadium-catalyzed SCR, but changing the counter cations in the POM catalyst might be a propitious strategy for enhancing the catalyst stability. In this study, the effect and the role of cation variation were investigated in detail by exchanging the protons from the most promising Keggin-type HPA-3 (H₆PV₃Mo₉O₄₀) catalyst dispersed on anatase support with cations such as Na⁺, K⁺, Cs⁺. The synthesized catalysts were characterized in-depth using ICP-OES/AAS, FTIR, TGA, NH₃-TPD, EPR, H₂-TPR, ED (electron diffraction) and XPS. The supported HPA-3/TiO₂ catalyst with a 7.5 wt. % loading was found to be the most active catalyst for the SCR reaction in the temperature range of 200–300 °C, providing nearly complete NO conversion at 300 °C with a relatively high first-order rate constant. In comparison, the analogous alkali-exchanged catalysts possessed lower activities, which by characterization was corroborated to a combined effect of lower acidity and altered redox property. The study reveals new insight into NH₃-SCR catalysts comprising redox-active POMs, which so far mainly have been applied for homogeneously catalyzed reactions.