Diesel SCR NOx Reduction and Performance on Washcoated SCR Catalysts

This paper describes a study of ternary V₂O₅/WO₃/TiO₂ SCR catalysts coated on standard Celcor^® and new highly porous cordierite substrates. At temperatures below 275°C, where NOx conversion is kinetically limited, high catalyst loadings are required to achieve high conversion efficiencies. In principle there are two ways to achieve high catalyst loadings:

1. 1.
   On standard Celcor^® substrates the washcoat thickness can be increased.
2. 2.
   With new highly porous substrates a high amount of washcoat can be deposited in the walls.

Various catalyst loadings varying from 120g/l to 540 g/l were washcoated on both standard Celcor^® and new high porosity cordierite substrates with standard coating techniques. Simulated laboratory testing of these samples showed that high catalyst loadings improved both low temperature conversion efficiency and high temperature ammonia storage capacity and consequently increased the overall conversion efficiency. The washcoated catalysts retained >95% of original overall SCR activity after aging at 650°C for 200 hours. Ammonia TPD experiments were performed to study the surface acidity of washcoated SCR catalysts

The main disadvantage for high washcoat loadings coated on standard Celcor^® substrates (solution 1) is the strong increase in pressure-drop due to a reduction in the hydraulic channel diameter and hence OFA. Therefore a new high porosity substrate with a high porosity of 62% was developed. Elevated washcoat loadings in combination with this substrate showed enhanced NOx conversion efficiency at low temperatures without the disadvantage of a high backpressure.

The new catalyst system demonstrated better or equal thermal and mechanical properties compared to a washcoated standard wall Celcor^® substrate.