Advances in Vanadium-Based Catalyst Research for the Selective Catalytic Reduction of NOx by NH₃

The most efficient after-treatment technology for reducing harmful NOx emissions from stationary and mobile sources of diesel exhaust is the selective catalytic reduction (SCR) of NOx with ammonia (NH3). Vanadium-based SCR catalysts reduce NOx selectively between ca. 200 and 500°C. Low temperature activity and high temperature stability are important for the automotive sector because of the large temperature fluctuations in the exhaust gas. Additionally, SCR catalysts need to be resistant to water vapor, sulfur and potentially poi-soning elements. To date, V-based SCR catalysts of type V2O5/WO3/TiO2 (VWT) are the most widespread systems because of their activity over a broad temperature range, high sulfur tolerance and moderate production cost. Draw-backs are the moderate hydrothermal stability, poor low temperature activity and potential vanadium volatility which are the main research topics for V-based SCR catalysts. Exposure to different aging procedures of a V-loading optimized VWT catalyst (2 wt% V2O5) revealed that a hydrothermal environment affects the aging much more severely compared to a dry environment. Before deactivation, VWT catalysts have the capability to activate, which was correlated to changes in V and W surface coverage, the increased fraction of Lewis acid sites and SCR active vanadyl sites. The VWT catalyst with optimized V-loading was further investigated by means of transient experimentation, which revealed important insights into the mechanism of the SCR reaction: The active site was assigned to mono-oxo V5+ Lewis acid species, which were reduced only in presence of both NO and NH3. The formation of the nitrosamide intermediate, which is formed simultaneously to V5+ reduction, was also verified. The addition of 2 - 4 wt% SiO2 during the VWT catalyst synthesis increased the stability up to 650°C because it prevented anatase TiO2 particles from sin-tering by inhibiting their inter-particle contact. Using a SiO2 stabilized TiO2 support material, the potential of FeVO4 as a source of active redox centers was explored as a novel class of hydrothermally stable SCR catalyst. A loading of 4.5 wt% FeVO4 was found to be the optimum trade-off between stability and activity. Remarkable catalyst activation was observed after calcination at 600°C, which was correlated to the decomposition of FeVO4. Similar to FeVO4, it was shown that the catalytic activity of supported CeVO4, AlVO4 and ErVO4 is closely correlated to their degree of decomposition. The decomposition of metal vanadates generates dispersed VOx domains, which resemble those of VWT catalysts. This thesis demonstrates that research on V-based SCR catalysts is still crucial because their modification and optimization can result in enhanced perfor-mance and temperature stability. Metal vanadates can be envisaged as a reser-voir for active V-species as they decompose once supported on TiO2. Transient experimentation allowed in-depth mechanistic studies on V-based catalysts. This encourages further exploration of SCR catalysts in order to improve the fundamental knowledge of such systems that will assist the discovery of highly active and stable catalysts for the abatement of NOx.