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.