Thermally Durable Vanadium-SCR Catalysts for Diesel Applications

The emission regulations for mobile applications will become stricter in Euro 4 - 6 levels and require the use of active aftertreatment methods (deNOx and DPF) in addition to passively operating diesel oxidation catalysts (DOC). Vanadium-SCR (V-Selective Catalytic Reduction) catalysts based on stabilized TiO₂-WO₃ raw materials and tailored preparation methods were first evaluated by the laboratory experiments. Conventional V-SCR catalysts were durable up to about 600°C but the developed catalyst stand hydrothermal ageing up to 700°C without losses of activity. Simultaneously, the performance at 250 - 450°C was about the same as with the traditional V-SCR catalyst and the SCR selectivity at 450 - 600°C was high with a low NH₃ oxidation tendency. Coated V₂O₅/TiO₂-WO₃ catalysts (ceramic and metallic substrates) were evaluated with a 4.9 L engine by engine bench experiments. The new V-SCR catalyst kept the SCR activity also after ageing at 700°C when the conventional V-SCR catalyst lost drastically the activity. NOx conversions (DOC+SCR, 300 cpsi ceramic) without NH₃ slip as 700°C aged were 59 - 96% with the new concept and 21 - 68% with the reference in steady engine points (250 - 530°C, 19.000 - 51.000 h-₁). The surface area and acidity (NH₃ adsorption capacity) measurements showed the improved stability. The use of ammonia slip catalyst (ASC) in system enabled the high NH₃/NOx ratios without NH₃ emissions and extended both low and high temperature SCR performance. The durable V-SCR catalyst is an alternative to zeolite-SCR catalysts in DOC+DPF+SCR systems. In the presence of DPF with active regeneration, exhaust gas is heated up to 700°C, which is very demanding for downstream-located SCR. The integration of this kind V-SCR coating on DPF was also discussed as an opportunity.